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writers set about redefining the nature of writing. Their reflections on this subject are what, by and large, gave the concept of authorship its modern form.
The resulting romantic concept of authorship still strongly shapes many authors’ self-understanding of the work they do.
Writing about authorship, not surprisingly, also encompassed writing laws. In England in 1710, the Statute of Anne established for authors and their assignees an exclusive, fourteen-year right to print books and other writings. The statute’s stated purpose was to promote the occupation of author – to prevent the economic harm that unauthorized printing was causing to authors and their families, and to encourage writing of useful books. In the U.S. in the 1780s, twelve out of the thirteen colonies passed copyright laws literally directed to securing author’s rights. The U.S. Constitution, written in 1787, included a clause empowering congress “[t]o promote the progress of science and useful arts, by securing for limited times, to authors and inventors, the exclusive right to their respective writings and discoveries.” The first U.S. Congress, meeting in 1790, promptly wrote and passed a federal copyright act. In France, the opportunity to receive an exclusive right to print a work was extended to authors in a new royal regulation issued in 1777. Four years after the French Revolution swept away royal power, the French National Convention in 1794 passed a law protecting “property in the production of genius.” The law granted to authors, heirs, and assignees exclusive publication rights for the lifetime of the author plus ten years. All these laws were passed before industrial revolutions dramatically changed the nature of work.
Despite great changes in typical daily work since the eighteenth century, creativity is not usually associated with ordinary daily work. If labor (workers) and capital (machines) are considered to be substitutable inputs in a production process, human work is understood to be like that of machines. Machines might then be described as dead labor; that is, the essential value extracted from workers and re-embodied so that it can continue to work and never complain. In high-income societies today, the distribution of jobs has shifted greatly toward services; relationships among women, men, and children are much different; and information and communication technologies are much more powerful. Yet the eighteenth-century understanding of creativity remains. Creativity is some mysterious blessing from somewhere upon some persons, at some times. Creativity occurs in a realm separate from the work that keeps the lights on, the water running, the lawn neat and green, and puts food in the bowl for the cat and on the table for the children. Promoting creativity depends on a narrow legal field of reproductive rights historically centered on copyright.
The occupational history of photographers and authors in the U.S. suggests a much different understanding of creativity. In 1850, only 82 males claimed the occupation of author in the first occupational census of the U.S. In 1900, self-professed photographers were about ten times as numerous as self-professed authors. Being a photographer was associated with manufacturing and depended only on mastering technical skills and making a living. Being an author, in contrast, was an elite status associated with science and literature. Across the twentieth century, the number of writers and authors grew much more rapidly than the number of photographers. The relative success of writers and authors in creating jobs seems to have depended not on differences in copyright or possibilities for self-production, but on greater occupational innovation. Creativity in organizing daily work is an important form of creativity.
Early commercial practice of photography rapidly and enduringly established “photographers” as an occupational category in government statistics. In 1850, the first U.S. occupational census included “daguerreotypists” as an occupational category. “Daguerreotypists” describes persons using the first widely known photographic process. This process was primarily used in the commercial provision of personal portraits. Other photographic processes developed rapidly. The evolution of nomenclature from “daguerreotypists” to the more general term “photographers” in censuses from 1860 to 1880 reflects this development. All subsequent censuses through to the 2000 census include the occupational category “photographers.” The 2000 census also included a new category, “Television, video, and motion picture camera operators and editors.” The introduction of this new category is an unusual development. The reduction in the technical complexity of making photographs, the development of new image-making technology, and the emergence of new fields, such as product design, advertising, and public relations, has not led to any other new occupational categories classified closely with photographers. What photographers do has been commonly and distinctively understood since the nineteenth century, and it has changed little since then.
The position of photographers in the job classification schema has, however, changed significantly from 1850 to 2000. Through the 1900 census, photographers were classified under manufacturing and mechanical industries. In 1910, photographers moved into the class “professional service.” In 1940, a sub-heading “semiprofessional workers” was established, and photographers were placed under it. In 1950, that subheading was eliminated, and the higher level heading was changed from “Professional and semiprofessional workers” to “Professional, technical.” These classification changes suggest struggles over the boundary of “professional.” By 1980, photographers were being placed under a subheading “writers, artists, entertainers, and athletes.” In 2000 “photographers” came under the subheading “media and communication equipment workers” [italics added]. Thus in the 2000 occupational classification, and in 1900 and earlier ones, photographers have been associated with machine workers.
Over the past hundred and fifty years, authors have become much less distinguished in occupational category and classification. Authors had their own occupational category in 1850 and 1860, while from 1870 to 1900, authors were variously categorized with lecturers, literary persons, and scientists. These elite groups have high barriers to entry. By 1980, authors had been placed within a higher-level category, “Writers, Artists, Entertainers, and Athletes.” “Technical writers,” a category with a large number of workers, was also introduced in 1980 as a category with the same classification as “authors.” The 2000 census included this category, but changed the category “authors” to “writers and authors.” These changes indicate innovation and diversification in jobs associated with authors.
In occupational categorization, reporters and editors have been consistently distinguished from authors and writers, but not from each other. In 1850 and 1860, reporters and editors were separate categories, but from 1870 to 1990 they were grouped in a single category as “journalists” or “editors and reporters.” In 2000, reporters and editors were again placed in two different categories. Originality provides one criterion for attempting to distinguish among authors, writers, reporters, and editors, while authority provides another, different, one. Nonetheless, editors sometimes effectively act as writers, and reporters as authors. Moreover, some authors attempt to act as their own editors, but might be regarded as not succeeding in doing so. In any case, authors, writers, reporters, and editors have been consistently grouped together in the next higher level of occupational classification.
The plaintiff in this case was Napoleon Sarony, who had been commercially and culturally prominent as a photographer for at least two decades. Sarony specialized in celebrity photographs. From his studio on Broadway, he made photographs of most of the stars of New York theatre, including Sarah Barnhart, whom he paid $1500 for the opportunity. Sarony himself did not generally operate the camera; he directed his efforts toward arranging the subject and evoking from her or him the expression that he sought. He was an author of photographs in this way, and also as the owner of the studio that produced photographs.
it is said that…a photograph is the mere mechanical reproduction of the physical features or outlines of some object, animate or inanimate, and involves no originality of thought or any novelty in the intellectual operation connected with its visible reproduction in shape of a picture. … This may be true in regard to the ordinary production of a photograph, and that in such a case copyright is no protection. On the question as thus stated we decide nothing.
The court did not include in its officially published opinion a copy of Sarony’s photograph. Including a copy probably would have advanced significantly appreciation for the opinion’s legal distinction, at the cost of raising practical challenges and additional, important legal questions. Deciding little in a way that limited communication of the decision was a pragmatic judicial strategy. The status of author, then and now, had little practical importance to most photographers’ businesses and most persons’ interest in photographs.
Others are free to copy the original. They are not free to copy the copy. The copy is the personal reaction of an individual upon nature. Personality always contains something unique. It expresses its singularity even in handwriting, and a very modest grade of art has in it something irreducible, which is one man’s alone. That something he may copyright unless there is a restriction in the words of the act.
These sentences, forceful and impressive, proclaim the significance of an author in every persons’ every act. Most persons, regardless of profession, would probably desire the elite status of author, under copyright or more generally. However, how all the effects of persons remain separate from nature and originals, so that a copy of a copy does not contain something personal and unique, “which is one man’s alone,” is unclear. Moreover, it is clearly incorrect that copyright exists “unless there is a restriction in the words of the act.” In the U.S., legal recognition of copyright depends on statutory provision of it.
The above sentences from Bleistein are probably best understood as invoking general appreciation for human creativity in a case that concerned posters advertising a circus. The original defendant described the circus advertisements as “tawdry pictures”  His counsel objected that “one, the Ballet, …is an immoral picture,” and cited a case that invalidated copyright to a song because it used the word “hottest” in a way judged to be “indelicate and vulgar.” Abstracting from the vulgarities of particular images, the Court’s decision argued that every person is special, a unique individual. The effects of every person are necessarily rare, valuable, and worthy of protection. In effect, every person essentially has the elite status of author.
[Henry James], like his brother and the parsons, attaches a kind of transcendental value to personality; whereas my bet is that we have not the kind of cosmic importance that the parsons and philosophers teach. I doubt if a shudder would go through the spheres if the whole ant heap were kerosened. Of course, for man, man is the most important theme.... Man of course has the significance of fact, but so has a grain of sand. I think the attitude of being a little god, even if the great one has vanished, is the sin against the Holy Ghost.
This forceful and impressive language figures human beings as ants and grains of sand, interchangeable and uncountable. Personality is merely vanity – man who wants to believe that she is the most important theme. As for belief that persons have a unique creative power that is an irreducible part of their being, that is mocked with cutting irony as a “sin against the Holy Ghost” in a world in which clear thought and unsentimental rationality has dispelled God. A dominant tendency in the twentieth century has been to assert that human persons are more like ants than like little gods. That everyone under copyright law is an author, a little god, solely by law and without the need for any distinctive acts, can be understood as legal consolation for this intellectual development.
The business history of writers and photographers offers a different perspective on authors and values. Photographers’ business model was much different from that of writer-authors. Photographers saw directly their end-customers and transacted directly with them Writers, in contrast, had to make sense of many different readers. Writers confronted the risk that editors and printers would mangle the writer’s work, and that publishers would capture almost all the monetary value of books, journals, magazines, and newspapers. In addition, photographers predominately generated revenue from products custom-produced to evoke a specific sense of presence. Writers generally produced a single product that readers customized in interpretation.
no encouragement whatsoever is given to the unfortunate author. The votary of the Muses, the instructor and improver of mankind, is permitted to saunter around the streets with his elbows peeping out of a more than thread-bare coat.
Without relying on their status as authors under copyright law, in the nineteenth century photographers created a much more profitable business model than writer-authors.
Creating new patterns of valued activity is an important aspect of creativity in work. In the U.S. in 1860, self-professed daguerreotypists and photographers were fifteen times as numerous as authors, and the former had much better earning opportunities than the latter. In 2000, photographers and “writers and authors” were about equally numerous,  and photographers’ straight-time gross annual pay was only 58% that of writers and authors. Growth in self-production of photographs contributed to the decline in the fortune of photographers. But many persons have long been able to self-produce writing. Promoting progress in science and useful arts depends not just on copyright law and the economics of self-production, but also on recognizing and encouraging workers’ creativity across a wide range of ordinary jobs.
 Over the past two decades scholars have done much work on the history of authorship and copyright. See, e.g. Woodmansee (1984), Boyle (1988), Ginsburg (1990), Jaszi (1991), Hesse (1991), Saunders (1992), Rose (1993), Woodmansee and Jaszi (1994), and Rice (1997). This literature has largely failed to explore over time how many persons actually made their living as authors.
 The title is from Ernst Martin Gräff’s book, cited in Woodmansee (1984) p. 440. As a follow-up to relatively important work on footnotes, scholars not interested in pressing contemporary problems in communications policy might consider the history of titles, the style of which has changed significantly over time. For some preliminary research, see Darnton (1990) pp. 98-103.
 Cf. Hepzibah’s thoughts and actions in Hawthorne (1851) pp. 31-49, 241.
 Art I., § 8, cl. 8. On Aug. 18, 1787, James Madison and Charles Pinckney each submitted to the Constitutional Convention a list of additional, enumerated federal powers. Madison’s list included, “To secure to literary authors their copyright for limited time.” Pinckney’s list included, “To secure to authors exclusive rights for a limited time.” See Patterson (1968) pp. 192-3. The distinction between “literary authors” and “authors” is significant. In 1810, the French government established a separate regime of prepublication review for publications of twenty or fewer pages. More generally, early French literary law fostered the professionalization of writing and the transformation of the republic of letters into a republic of books. See Hesse (1991) pp. 245-6.
 Id. pp. 118-21. As id. emphasizes, the law also abolished existing royal privileges for printing works and gave all citizens the right to print works formerly under royal privilege.
 The scholarly literature on communications tends to divide by sensory mode or media. This division obscures important aspects of communication. See main text, infra. Hawthorne (1851) recognizes commonalities in the occupations of writer and photographer (daguerreotypists) by having Holgrave both write stories and make daguerreotypes.
 In 1853, perhaps 420 new books, not reprinted or translated from foreign books, were published in the U.S. Tebbel (1972) v. II, p. 23, cf. id. v. I, p. 222. Reconciling this figure with the census figure for authors requires more scholarly work. Woodmansee (1984) p. 443, ft. 17 cites a “contemporary catalog of German writers” that estimated the number of writers in Germany in 1800 as 10,650. Id p. 431 also notes, “The professional writer emerged considerably later in Germany than in England and France.” Reprinting of foreign works was prevalent in the U.S. in nineteenth century. See McGill (2003). About 1850, about 40% of new titles printed in the U.S. were reprintings of foreign works. See Tebbel (1972) v. II, p. 23; id. v. I, pp 221-2; cf. McGill (2003) p. 279, n. 2. Reprinting, however, cannot account for the huge difference in estimates of the number of authors in the U.S. and Germany. Rather than focusing on a few prominent authors, the history of authorship might focus more comprehensively on the number, background, and scope of activities of real persons involved in writing books.
 The number of authors (and closely related occupations) grew much faster than the overall population. Cf. Nunberg (1996) pp. 23-4.
 These objects were known as “cartes de visite.” See Leggat (1999) and Stratford Hall Plantation (2002).
 Copyright Act, ch. 126, 13 Stat. 540-41 (1865).
 111 U.S. 53, 59, 60.
 In the last third of the nineteenth century, Sarony was the best-known portrait photographer in the U.S. In 1897, a photographic journal lauded him as “the father of artistic photography in America.” Sarony, “an inveterate joiner and a gadabout in New York City’s literary and artistic circles,” often wore a tasseled red fez. The reception room of his studio featured a stuffed crocodile hanging from the ceiling. Bassham (1978) pp. 4-6, 13, 16-7.
 Id. Sarony paid Lillie Langtry, called in her time “the world’s most beautiful woman,” $5000 for the exclusive right to make and sell portraits of her. For one such portrait, see id. p. 73. I’m not impressed. I think that many persons have known women more beautiful than this image.
 Id. at 55 mentions “the picture in suit, Exhibit A,” but the official report of the case does not reproduce that picture. Photolithography was just becoming practical late in the nineteenth century, and photographs were not commonly reproduced in magazines until the twentieth century. If the photograph could have been printed in court reporters, doing so would have required advancing doctrines of use context as a limit on copyright. Legal disputes concerning the printing of judicial opinions have been important in shaping U.S. copyright law. See, e.g. Wheaton v. Peters, 33 U.S. 591 (1834). For a pioneering and endearingly unsophisticated discussion of U.S. judges’ use of non-verbal material in printed opinions, see Dellinger (1997).
 188 U.S. 239, 249-50 (Feb. 2, 1903). Justice Oliver Wendell Holmes, who took his judicial oath for the Supreme Court on Dec. 8, 1902, delivered the opinion of the court. It was the second Supreme Court opinion that he wrote.
 The phrase “the personal reaction of an individual upon nature” might mean a person’s reaction to nature, where that reaction might be fixed only in some realm separate from nature. Alternatively, “the personal reaction of an individual upon nature” might mean a person acting upon nature so as to augment it in a way uniquely attributable to that particular person. Neither interpretation seems consistent with the modern scientific view that human beings are part of created nature and continually transform the natural world. For discussion of related issues and interesting French case law, see Edelman (1994).
 Personal letter from Oliver Wendell Holmes to Lewis Einstein, Aug. 19, 1909, printed in part in Posner (1992) pp. xxv-vi.
 For some outstanding and highly influential scholarly work on ants and other insects, see Wilson (1971), Hölldobler and Wilson (1994), and Eisner (2003).
 Distinctive acts necessary for legal recognition as an author of work under copyright, such as affixing a copyright notice in publication, were greatly lessened in 1978 and then eliminated in 1989.
 Id., quoting the New York Minerva.
 Including “technical writers” with “writers and authors” would tip the balance away from photographers by about 50%. See Tables C1 and C2, above.
 Bureau of Labor Statistics (2002) Table 1.
 Tools such as word processors, the Internet, and blogs have further increased opportunities for self-production of writing. The effect of these technologies will depend greatly on the adaptability of organizations and jobs.

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Massachusetts Supreme Court interprets state statutory requirements for when criminal charges must be dismissed against a defendant who is incompetent to stand trial, and upholds the statutes against claims of Due Process and Equal Protection violations.
The Supreme Court of California found that the prosecution was permitted to dismiss prior charges and refile identical charges where the defendant had been committed for the duration of the statutory period while being evaluated for competence to stand trial; provided, however, that if found still incompetent to stand trial, the defendant could not be re-committed for another statutory period.
The Ninth Circuit Court of Appeals has held that a juvenile charged with murder under the Federal Juvenile Delinquency Act whose competency to stand trial is in doubt must be committed under the juvenile provisions of 18 U.S.C. § 5037(e) and not the provisions related generally to all commitments under 18 U.S.C. § 4241(d).
Tribal authorities with the Tohono O’odham nation charged 17-year old LKAV with murder in May 2009. He was found incompetent and remained in tribal custody but without being sent to a treatment facility for restoration to competency. In November 2011, the United States filed its own charge against LKAV as an alleged juvenile delinquent under the Federal Juvenile Delinquency Act. The United States then obtained a writ of habeas corpus to remove him from tribal custody and moved to commit him for a psychiatric evaluation pursuant to the provision pertaining in general to all federal criminal cases under 18 U.S.C. § 4241(d). The presiding magistrate judge granted LKAV’s request for a local evaluation in Phoenix, Arizona.
After an extensive evaluation, the examining psychologist determined LKAV was incompetent to stand trial. LKAV then moved to proceed with commitment under the juvenile act. The United States maintained that LKAV should be committed to an adult facility under § 4241(d). The magistrate judge granted the United States’ motion and committed LKAV to the custody of the Attorney General for hospitalization for a period not to exceed four months to determine whether he could be restored to competency. LKAC filed a timely appeal, but in the interim was transported to the Federal Medical Center in Butner, North Carolina. FMC-Butner completed its competency evaluation in January 2013 and advised the court that with an additional period of hospitalization and treatment, LKAV could be restored to competency. It requested an additional 120-day extension of the commitment order, which the district court granted. LKAV appealed.
The Ninth Circuit heard LKAV’s appeal under the collateral order doctrine finding that the commitment order conclusively determines LKAV’s rights as to his pre-adjudication commitment; his commitment is a completely separate issue from the ultimate issue of his delinquency; and delay until a final decision of his delinquency on the merits would render the commitment order effectively unreviewable.
By contrast, the commitment scheme generally applicable to all defendants contained in § 4241(d) requires mandatory commitment for determination of the defendant’s potential for restoration to competency. The United States had argued that § 5037(e) does not mention competency and therefore the mandatory competency evaluation and commitment procedures in § 4241(d), which are more explicit and comprehensive and apply to all federal criminal proceedings applies.
The Court found that because § 5037(e) expressly provides for commitment, study, and observation of alleged juvenile delinquents, and specifically references a study of any mental or physical defect, it controls over conflicting provisions in § 4241(d) that apply to federal criminal defendants generally. The Court pointed out, however, that the United States could have sought to have LKAV transferred for trial as an adult and therefore all of the provisions in § 4241(d) would have applied, but for some reason chose not to do so. The Court recognized that because LKAV has now turned 21 and is no longer a juvenile, his further treatment and custody may cause the United States some incidental inconvenience because he cannot be housed with other juveniles or adults. Nonetheless the Court held that the purpose of Federal Juvenile Delinquency Act is to provide for the preferential and protective care and treatment of juvenile delinquents who are significantly different from adult offenders, and its provisions must therefore control.
Cruz was arrested and convicted on two counts of threatening a federal law enforcement officer. After the court received the pre-sentence investigation report, the prosecution successfully moved for a determination of competency. A Federal Bureau of Prisons forensic psychologist concluded that Cruz was mentally incompetent and suffered from schizophrenic disorder, bipolar type. After a hearing, the court concluded that Cruz was incompetent and found that he could not proceed with sentencing.
A second report concurred with the diagnosis, noted Cruz’s ongoing refusal to take antipsychotic medication recommended by BOP personnel, concluded that without medication Cruz would remain incompetent, and stated that “there is a substantial probability that [his] competency can be restored with a period of forced medication.” The prosecution obtained an order authorizing the BOP to medicate Cruz against his will.
On appeal, the issue was whether “the Government, pursuant to the Supreme Court's decision in Sell v. United States, 539 U.S. 166, 123 S.Ct. 2174, 156 L.Ed.2d 197 (2003), can have a sufficiently important interest in forcibly medicating a defendant to restore his mental competency and render him fit to proceed with sentencing.” In affirming the decision of the federal district court, the Third Circuit held that the government could have a sufficiently important interest in sentencing a defendant for serious crimes to justify involuntary medication. Relying on the stated concern in Sell that “memories may fade or evidence may be lost,” the Third Circuit held the same concern applies with equal force in the sentencing context (the guilt phase was at issue in Sell) because it means that it may be “difficult or impossible to sentence a defendant who regains competence after years of commitment.” Additionally, while it may be cognizable that some crimes are not “serious” enough to justify forcible medication at the sentencing stage, Cruz’s offense was certainly “serious” enough.
The United States Supreme Court denied certiorari in January 2015.
The Supreme Court of Virginia upheld the Court of Appeals’ denial of Dang’s petition. The Supreme Court found that the circuit court had given adequate weight to the new information acquired by defense counsel and had focused on the proper issue at hand— Dang’s “present ability to understand the proceedings and assist his counsel.” Given the first evaluator’s opinion that Dang’s shifting focus was representative of “situational anxiety” and the “wide latitude” offered to circuit courts in light of their “first-hand interactions with, and observations of, the defendant and the attorneys at bar” the Supreme Court of Virginia found that the circuit court did not abuse its discretion in denying the request for a second evaluation.
The Seventh Circuit held that the trial court and state supreme court failed to follow the due process competence to stand trial standard set out in Pate v. Robinson, 383 U.S. 375 (1966) and Dusky v. United States, 362 U.S. 402 (1960). During the trial, McManus suffered several panic attacks and had to be transported to the emergency room where he was treated with several psychotropic drugs, including both opioid painkillers and ones that affected memory. The Seventh Circuit held that the “powerful effect of the medications alone created substantial doubt about McManus’ mental fitness for trial” and faulted the state judge for not ordering a competency evaluation and instead focusing on “getting McManus ‘fixed-up’ enough to complete the trial.” This course of action violated not only the due process standard set out in Dusky but also the Indiana Code, which requires a trial court to appoint a team of medical experts with expertise in determining competency and to hold a hearing any time there are bona fide doubts about a defendant’s competency. See Ind. Code § 35-36-3-1. The Seventh Circuit reversed and remanded the case to the district court with instructions to grant the writ of habeas corpus unless Indiana gave notice of its intent to retry McManus within a reasonable time.
Background: After Jesse James Warren was indicted on four counts of murder in connection with a mass shooting, he was found incompetent to stand trial and placed in the custody of the Department of Behavioral Health and Development Disabilities. The state of Georgia filed a petition under Sell to medicate Warren involuntarily in an attempt to restore his competency. The Superior Court for Cobb County granted the state’s motion and the defendant appealed.


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Solution-processed semiconductor quantum dot solar cells offer a path towards both reduced fabrication cost and higher efficiency enabled by novel processes such as hot-electron extraction and carrier multiplication. Here we use a new class of low-cost, low-toxicity CuInSexS2−x quantum dots to demonstrate sensitized solar cells with certified efficiencies exceeding 5%. Among other material and device design improvements studied, use of a methanol-based polysulfide electrolyte results in a particularly dramatic enhancement in photocurrent and reduced series resistance. Despite the high vapour pressure of methanol, the solar cells are stable for months under ambient conditions, which is much longer than any previously reported quantum dot sensitized solar cell. This study demonstrates the large potential of CuInSexS2−x quantum dots as active materials for the realization of low-cost, robust and efficient photovoltaics as well as a platform for investigating various advanced concepts derived from the unique physics of the nanoscale size regime.
Solar cells utilizing colloidal quantum dots (QDs) as the light-absorbing material have seen rapid advances in recent years1 from the first certified power conversion efficiency (PCE) of 3% reported in 2010 (ref. 2) to >7% last year3. These high-performance certified devices have used PbS QDs, and are notable for their low cost of fabrication and ability to efficiently harvest the near-infrared portion of the solar spectrum. Furthermore, the measurement of external quantum efficiencies (EQEs) above 100% using PbSe QDs4 demonstrates the potential of these devices for surpassing the Shockley–Queisser limit5 by employing carrier multiplication. Although recent progress in QD photovoltaics (PVs) based on PbX (X=S, Se) QDs is promising, major challenges to reaching commercializable efficiencies remain. The primary factor limiting performance of these devices is high recombination losses in the active layer due to presence of charge traps that originate from surface states. This results in short diffusion lengths for minority carriers6 in QD solids and ultimately limits the thickness of the active layer to <500 nm (Fig. 1a), which is insufficient for complete absorption of incident photons. Furthermore, the p–n heterojunction architecture requires fine control over carrier doping in order to tune the depletion width, which is another outstanding challenge7,8,9. The high atomic fraction of toxic lead atoms and the generally poor stability of PbX QDs under ambient conditions further complicate commercialization of this emerging technology, although PbS QD solar cells have been demonstrated with 1,000 h stability under continuous illumination in air2.
Figure 1: Working principles of colloidal QD solar cells.
(a) Schematic of a typical thin-film p–n junction QD PV (PbS QDs are purple circles, the contacts are omitted); high recombination losses due to the long paths for minority carriers to reach the electrode ultimately limit the thickness of the QD layer. (b) Schematic of the QDSSC architecture (CISeS QDs are red triangles, the contacts are omitted) that avoids QD-to-QD carrier transport completely, and benefits from more complete light absorption and a modular design. See Methods and Supplementary Information for the details of device design and fabrication. (c) QDSSC photoanode (SEM (scanning electron microscope) cross-sectional image) along with a schematic band structure (VB and CB are valence and conduction bands, respectively) and the depiction of various steps of the light conversion process: (1) generation of an excited electron (e−)—hole (h+) pair by an absorbed solar photon, (2) electron transfer (ET) to TiO2, (3) electron drift followed by diffusion within the TiO2 anode, (4) hole transfer to (QD reduction by) polysulfides in the electrolyte (not necessarily after steps 2 or 3) and (5) hole collection by (oxidation of) the CuxS electrode (not shown); EF is the Fermi level of the TiO2 anode and Eredox is the electrolyte redox potential. Scale bar, 10 μm.
An alternative device architecture for utilizing QD absorbers is the liquid-junction-sensitized solar cell pioneered by O’Regan and Grätzel10,11. In sensitized PVs, a meso-porous (mp) TiO2 anode decorated with a narrow-gap absorber is contacted by an electrolyte solution that provides a conductive pathway to a metal or semiconductor cathode (Fig. 1b). This architecture avoids inefficient QD-to-QD carrier transport by employing discrete interfacial charge-transfer steps in which a photo-excited electron transfers from the QD to TiO2, while the remaining hole is reduced by the electrolyte (Fig. 1c).
Traditionally this device architecture has utilized dye molecules as light absorbers/sensitizers10. The best dye-sensitized solar cells (DSSCs) reach efficiencies in excess of 10%, with a current record of 12.3%12. However, despite the large promise, widespread application of DSSCs is complicated by inherent limitations of the dyes themselves, which usually contain low-abundance elements such as ruthenium, have limited absorption bandwidth and suffer from photo-bleaching and poor long-term chemical stability, particularly under strong illumination13.
To circumvent the drawbacks of dyes, several groups have explored replacing them with QDs14,15,16,17,18. The investigated approaches include infusion of pre-synthesized colloidal QDs into the mp-TiO2 (refs 17, 19, 20, 21) and direct growth of semiconductor clusters within the matrix14,15,16,22. This latter method was used to achieve the highest reported (yet uncertified) efficiency of 5.6% in structures comprising PbS:Hg QDs deposited with a successive ionic layer adsorption reaction (SILAR)23. These devices however were highly unstable and degraded over a period of minutes, meaning that accurate characterization of their performance was extremely difficult due to transient changes in device characteristics.
In principle, colloidal QD sensitizers can have several advantages over SILAR-deposited QDs that arise from the ability to synthesize them under optimal conditions. This allows for much better control over size, composition20, shape and heterostructuring, and generally results in QDs with improved crystallinity and stability. Indeed, we recently reported proof-of-principle sensitized solar cells utilizing colloidally synthesized CuInSexS2−x (CISeS) QDs24. This previous work was a synthesis-focused effort on optimizing QD structural parameters for achieving more complete coverage of the solar spectrum, enhancing QD surface chemistry for multi-day stability and maximizing the loading of QDs into the pores of the TiO2 matrix. In the course of this study, we determined that the optimal QD parameters corresponded to the CuInSexS2−x alloy with x=1.4 and an averaged QD size of ~4.5 nm (in these pyramidal QDs, size is the measured length from an apex to the midpoint of the opposing edge of the triangular projections observed in TEM images). Without exploring the parameter space extensively, we also noted that exposing the QDs to Cd-oleate could be beneficial to QD-sensitized solar cell (QDSSC) performance. With an energy gap (Eg) of ~1.3 eV, the optimized CISeS QDs allowed for ~100% absorption and a maximum external quantum efficiency (EQE) of ~45% (at 3 eV) and an overall uncertified PCE of 3.5% in the best devices.24 QDSSCs based on CuInS2 QDs (without Se) have also been reported in recent years with uncertified efficiencies as high as 4.7%25,26,27; however, the maximum photocurrent achievable for this material is roughly the same as for CdSe QDs due to the relatively large bulk band-gap.
In the present report we reveal a crucial effect of CISeS QD surface properties on device performance. Specifically, by utilizing inorganic passivation of the QDs with partial cation exchange using Zn2+ or Cd2+ ions prior to sensitization, we suppress non-radiative recombination, which otherwise can outcompete fairly slow (>5 ns) electron transfer into the mp-TiO2 photoanode. Further, we demonstrate the importance of recapping with short ligands in order to achieve efficient sensitization via direct attachment28. In addition, we optimize the electrolyte composition and reveal the critical importance of improved wetting through methanol addition for achieving high performance in these liquid-junction solar cells. We find that in addition to improving short-circuit current, tuning electrolyte composition allows us to reduce series resistance of the devices and thus increase the fill factor (FF). A further improvement in FF is obtained via the utilization of a meso-textured, high-surface-area CuxS cathode. These coordinated efforts to understand and optimize the various stages of the photoconversion process have allowed us to achieve a record-long stability of several months under ambient conditions, the first-ever certified efficiency of 5.1% and a champion efficiency of 5.5%, which are record values for this emerging class of PV devices and among the highest reported for QD PVs29.
The synthesis of colloidal CISeS QDs uses 1-dodecanethiol (DDT) as an effective yet low-cost solvent, ligand and sulphur precursor; controlled amounts of trioctylphosphine selenide (TOP-Se) are added to produce an alloy of reduced band-gap24,30. This synthetic approach results in pyramidally shaped CuInSexS2−x QDs (inset in Fig. 2a) with x as high as ~1.6 (that is 80% Se anions, as measured by inductively coupled plasma optical emission spectroscopy (ICP-OES)). Utilizing as-synthesized CISeS QDs results in relatively poor PV performance24, which is primarily due to significant non-radiative losses associated with surface traps30. To address this issue, following the synthesis of the QDs, we treat them with a zinc oleate (Zn-oleate) solution, which results in temperature-controllable Zn2+ exchange with surface Cu1+ and In3+, cations (Supplementary Fig. S3). This procedure leads to a strong reduction in surface-related trap states as evidenced by an increase in the PL intensity (Fig. 2a) and a lengthening of the PL lifetime31,32. The suppression of non-radiative recombination also manifests as increased open-circuit voltage (Voc, from 0.33 V for untreated QDs to as high as 0.54 V in the case of the optimal treatment with Zn2+ at 100 °C) and short-circuit current (Jsc, from 2.7–6.0 mA cm−2) in devices (Fig. 2b; details can be found in Supplementary Table S2). However, as the degree of cation exchange increases, a maximum in device performance is quickly reached, and Jsc then decreases even as the PL efficiency of QDs in solution increases (Fig. 2a). We attribute this phenomenon to partial suppression of electron transfer to TiO2 by the increasingly thick Zn-rich surface layer, which serves as a barrier due to its significantly higher-lying conduction band32. This observation suggests that electron transfer to TiO2 is fairly slow, especially after cation exchange with Zn2+, and is likely characterized by timescales comparable to those of exciton recombination in the QDs (tens of nanoseconds)32.
Figure 2: Passivation of QD surfaces via cation exchange.
(a) Absorption and photoluminescence (PL) spectra of the QDs before and after exposure to Zn-oleate at various temperatures after recapping with tert-butylamine (tBA). Inset: TEM image of 4.5 nm CuInSe1.4S0.6 QDs. Scale bar, 5 nm. (b) Current density versus voltage characteristics for QDs with varying degrees of Zn-cation exchange, or with ZnS SILAR post-treatment of the QD-infused anode (orange line). (c) Absorption and PL spectra of the QDs before and after exposure to Cd-oleate and recapping with tBA. (d) Current density versus voltage characteristics for QDs with varying degrees of Cd-cation exchange controlled by reaction temperature. The photovoltaic performance properties related to b and d can be found in Supplementary Tables S2 and S3.
In principle, treating the QDs in a colloidal suspension by cation exchange should be preferable to the commonly applied approach of depositing a thin layer of ZnS over the entire sensitized electrode by SILAR, since it enables greater flexibility in fabrication and characterization. On the other hand, ZnS deposited by SILAR may provide the benefits of inorganic passivation but without creating a barrier at the QD/TiO2 interface33. In a comparison study, when a ZnS SILAR post-treatment (two monolayers) is applied to non-cation-exchanged CISeS QD-sensitized TiO2 (Fig. 2b, orange curve), the enhancement is similar yet slightly less pronounced than the colloidal/cation-exchange method.
While we have observed real performance enhancement using cation exchange with Zn2+, the tendency of thicker Zn-rich surface layers to act as barriers to charge extraction is ultimately limiting. An isovalent alternative to Zn2+ is Cd2+; importantly, Cd(Se, S) has a lower-lying conduction band than Zn(Se, S) and therefore it might present a smaller (if any) barrier to electron transfer. We observe that the effect of cadmium oleate (Cd-oleate) exposure on CISeS QDs’ optical properties is approximately the same as with Zn-oleate (Fig. 2a)24: absorption remains essentially unchanged, while the PL intensities (Fig. 2c) and lifetimes dramatically increase. However, analysis of the composition with ICP-OES (Supplementary Fig. S3) indicates that Cd-oleate is more reactive towards CISeS QDs at low temperature and results in a more aggressive exchange of cations. Comparing the performance of QDSSCs made from QDs treated with Zn-oleate (Fig. 2b) and Cd-oleate (Fig. 2d) at various temperatures, we find that while Zn-treated QDs (PCE of 1.91%) show a significant improvement over untreated QDs (0.38%), Cd treatment produces even a greater enhancement in the PV performance (3.34%). The best devices incorporated CISeS QDs treated at 50 °C, which corresponded to the mildest exposure to Cd2+ ions with a resulting overall amount of cadmium in the sensitized film of <0.5 atomic %. The extremely low content of Cd in these devices significantly reduces concerns of potential toxicity relative to the already widely used CdTe thin-film solar cells.
After Cd-treated QDs are recapped with short ligands (chosen from those in Fig. 3a), they are incorporated into a mp-TiO2 photoanode via soaking in dilute QD–octane or methanol solutions. We found that the identity of the ligand had a major impact on device performance. We observe that tert-butylamine (tBA), n-butylamine (nBA) and sec-butylamine (sBA) are equally effective at achieving high QD-loading according to optical absorption measurements (Fig. 3b). Unlike most QDSSC reports, we achieve this high degree of loading (using butylamine) by direct attachment without the assistance of a high electric field (electrophoretic deposition)27 or bifunctional linkers (for example, MPA)25. On the other hand, the use of the native ligands (oleylamine and DDT), mercaptoproprionic acid (MPA) or pyridine resulted in much poorer loading. Accordingly, we found that the choice of the ligand did not affect Voc, but did have a significant effect on Jsc (Fig. 3d and Supplementary Table S4). Typically, improvement in only Jsc would indicate that enhanced light-harvesting is the primary reason for the benefit of using the butylamines, rather than a reduction in recombination losses, which would be expected to impact Voc as well. However, in liquid-junction cells the absorber band-gap (that is, QD band-gap) can be well in excess of the maximum attainable Voc, which is determined by the energy difference between the TiO2 Fermi level and the redox potential of the electrolyte. In this case, the relationship between recombination, open-circuit voltage and short-circuit current is not as trivial as with solid-state solar cells, and so we can’t fully rule out differences in recombination using these ligands. In fact, we do observe differences in the absorption-normalized PL intensity (Fig. 3c) from QDs recapped with these ligands in solution (prior to sensitization), which indicates differences in non-radiative recombination related to surface traps. Therefore, in addition to differences in loading, Jsc may also be impacted by non-radiative recombination within the QD absorbers. At this time we do not have a complete understanding of why sBA and nBA show poorer performance than tBA, but we note that the lower PL intensity (Fig. 3c) from QDs recapped with these isomers may implicate recombination as a factor. Furthermore, we find that devices made using QDs recapped with nBA show a surprisingly poor stability (blue curves on Fig. 3e) compared with sBA and tBA, which could also be related to the particular ineffectiveness of nBA at surface-trap passivation.
Figure 3: Organic passivation of QDs.
(a) Molecular structures of ligands investigated in this work. (b) The absorbed fraction of incident radiation (as 1−T, where T is film transmittance in %) of sensitized films using identical QDs recapped with various ligands. Contributions from absorption/scattering by the glass/FTO/mp-TiO2 substrate have been subtracted in each case; a scattering layer was not used for these measurements. (c) Absorption-normalized PL in solution of as-synthesized CISeS QDs (purple) and cation exchanged (Cd-OA 50 °C treatment) after recapping with each ligand. (d) Current density versus voltage characteristics for each ligand 5 days after fabrication. (e) Performance of the QDSSCs after 60 days. Two devices are shown for the nBA (blue) and sBA (green) cases to demonstrate the reproducibility of the result. tBA is shown to have the highest Jsc initially and after a period of months. These devices did not have the mp-TiO2 scattering layer nor a ZnS SILAR coating. The devices fabricated with pyridine-capped QDs failed after roughly 2 weeks. The photovoltaic performance properties related to d and e can be found in Supplementary Tables S4 and S5.
The overall device performance (Fig. 3d) and longevity (Fig. 3e) were optimal with tBA. Relatively high band-edge PL quantum yield (QY) from tBA-capped QDs in solution (~20%, Supplementary Table S1) suggests that recapping with tBA is also effective at passivating surface traps, in addition to improving QD-loading and stability. Indeed, our observations confirm what others have reported: that effective organic passivation in combination with robust inorganic passivation is critical for high-performance QD PVs3,34. At the same time, the small length of the tBA ligand enables efficient electron extraction into TiO2 by allowing relatively intimate contact between QDs and the TiO2 surface (Fig. 4a). Full understanding of the mechanism of initial attachment requires further study, but may be related to QD charging (electrostatic attraction to induced charges); however, once contact is made, Van-der-Waals forces and insolubility of the QDs in the electrolyte are likely to be the primary reasons for their robust and prolonged attachment to the mp-TiO2.
(a) TEM image of a QD/TiO2 heterojunction showing the intimate contact between them when tBA is used as a ligand. Scale bar, 5 nm. (b) SEM cross-sectional image of our champion device. The substrate used was fluorinated tin oxide (FTO)-coated glass. Scale bar, 10 μm. (c) EDX spectroscopy line scan of the cross-section showing the elemental composition of the sensitized film. Atomic percentages are of the total composition excluding oxygen (not shown) and tin (shown in orange, but arbitrarily scaled), and the titanium atomic percent is scaled down by a factor of 10 for ease of comparison.
As inferred from energy-dispersive X-ray (EDX) spectroscopy measurements of photoanode cross-sections (Fig. 4b), the use of this ligand results in QD/TiO2 films with highly uniform loading throughout the mp-TiO2 (Fig. 4c). By comparing QD constituents (for example, Cu, In, Se, S) to measured meso-porous support content (Ti), we estimate that ~3% of the solid material (by atomic %) in the film is QD material. The fraction of QDs is reduced near the top of the film where a scattering layer of 400 nm TiO2 particles (presenting a lower surface area for attachment28) is added prior to sensitization to improve light harvesting, which is consistent with SEM cross-section images showing the larger pores in the scattering layer are not filled with aggregated QDs.
Following electron transfer from a QD to the TiO2 (Figs 1c and 4a), the photogenerated hole is reduced by the electrolyte, which also serves as a hole transporting medium. In order to elucidate the importance of electrolyte parameters, we explored dilution of the electrolyte with methanol26,35. Zewdu et al.35 reported a synergistic effect of methanol in combination with a ZnS SILAR treatment of CdSe/CdS-sensitized TiO2, which was preliminarily attributed to improved wetting of the photoanode by the electrolyte. In our experiments, we observe a significantly greater improvement in the PV performance upon addition of methanol than previously seen. Specifically, as the aqueous polysulfide electrolyte (2 M Na2S and 2 M S) is diluted with methanol (Fig. 5a), the measurements of current–voltage (J–V) characteristics indicate a greatly increased photocurrent and a reduced series resistance (Rseries, determined as the inverse of the slope of the J–V curve at the Voc), which leads to a higher FF. Reduced Rseries is likely due to the lower viscosity of methanol (0.54 mPa s) relative to water (0.89 mPa s), enabling faster ionic transport (ionic mobility increases with decreasing solvent viscosity). The enhancement in the photocurrent however can be caused by several effects. In our studies, we have considered three possibilities: (1) methanol can potentially act as a sacrificial electron donor aiding in the reduction of photogenerated holes26,27; (2) it could improve electron injection efficiency by lowering the Fermi level of the TiO2 film36 and (3) it can lead to better contact between the QDs and the electrolyte due to improved wetting35.
Figure 5: Dependence of device performance on the electrolyte composition.
(a) Current density versus voltage characteristics under simulated sunlight for QDSSCs with varying amounts of methanol in the electrolyte (reducing the polysulfide concentration by dilution). A ZnS SILAR coating was not used. A table of the photovoltaic performance properties can be found in Supplementary Table S6. (b) QD population decay monitored by probing the temporal evolution of the band-edge bleach with transient absorption. The traces in the figure correspond to isolated QDs dispersed in octane (black square), and QDs attached to the TiO2 matrix exposed to either methanol (red triangle) or electrolyte (blue circle).
The first possibility can be excluded based on the prolonged stability of our device performance achieved without replenishing methanol in the electrolyte, which would be impossible if methanol served as a sacrificial donor. Furthermore, we observe in a control study that the PL QY from QD-sensitized films immersed in water or methanol without electrolyte is more than two orders of magnitude lower than that of the same films with electrolyte (Supplementary Table S1). Similar observations have been made in DSSCs where dye molecules remain oxidized for orders of magnitude longer timescales in the absence of a redox electrolyte37,38. This result suggests that in pure methanol, holes are not removed from the QDs, which leads to a build-up of a net positive charge in photo-excited QDs and associated activation of a fast non-radiative decay channel via Auger recombination of charged excitons39,40.
This assessment is confirmed by transient absorption (TA) studies of carrier dynamics in QD-sensitized films upon their exposure to various solvents (Fig. 5b). In the case of dry methanol, the decay is dominated by a fast 20–50 ps component, which is consistent with Auger recombination of a positively charged exciton (positive trion), a process in which the electron-hole recombination energy is transferred to a pre-existing hole39,41 (left inset in Fig. 5b). This suggests that the hole remaining in the QD following electron transfer to TiO2 is not reduced by methanol in the absence of electrolyte and still remains in the QD when it absorbs the next photon. On the other hand, the QD hole is efficiently regenerated in the presence of electrolyte: the measured dynamics are nearly identical to those of isolated QDs dispersed in solution, which is dominated by recombination of neutral excitons (right inset in Fig. 5b). We also observe a less pronounced ~400 ps decay component in methanol that we attribute to surface traps that are effectively passivated by the electrolyte. Further, these TA measurements conducted on samples exposed to electrolyte also provide a lower limit for the characteristic time of electron transfer of >5 ns, which is surprisingly long compared with some previous reports on photo-excited dynamics at QD-metal oxide interfaces27,42. However, this long time scale is consistent with our observation that various types of QD surface modification lead to correlated changes in QD PL emission efficiency and their performance in PV devices. These correlations would not have been observed if electron transfer was significantly faster than competing recombination mechanisms in isolated QDs.
Next, we analyse the potential effect of methanol on the Fermi level of TiO2. It is known that methanol can modify TiO2 surface structure43 or undergo photochemical reactions in the presence of a catalyst44. Reactions between TiO2 and methanol could increase photocurrent yield if they lead to a lowering of the TiO2 Fermi level (that is, if TiO2 is oxidized as a result of the reaction), as this would make electron injection from QDs even more favourable. In order to see the effects of intentional oxidation of a TiO2 anode, we use Li+ ions that are known to effectively remove electrons from the surface of TiO2 (ref. 37). This approach has been utilized to realize ~100% electron injection efficiency in CdSe-QDSSCs36. When we add Li2S to the electrolyte at 50% methanol concentration, we do indeed observe an increase in Jsc; however, it is accompanied by a reduction in the Voc (Supplementary Fig. S6 and Supplementary Table S7). Both of these are anticipated results of a lower TiO2 Fermi level, which in addition to increasing Jsc should also reduce the energy offset between the anode Fermi level and the electrolyte redox potential, decreasing Voc. If methanol were also oxidizing TiO2, then we would expect Voc to be reduced with increasing methanol content, which is not observed; in fact, increasing amounts of methanol, if anything, lead to higher Voc (Fig. 5a).
Finally, we analyse the third possibility, namely that improved wetting of the QD/TiO2 surface by the methanolic electrolyte solution can facilitate hole extraction from the QDs and thus improve Jsc. The degree of wetting of a solid material by a liquid depends on the relative surface tensions of the three joining interfaces (that is, γgas–liquid, γgas–solid, γliquid–solid). As the electrolyte solvent changes from water to methanol, only the two tensions involving the liquid will be modified. Estimating the surface tension of mp-TiO2/QDs with various solvents (that is, γliquid–solid) in an actual device is complicated by the high surface/volume ratio45, the hydrophobicity of the ligand-passivated QD surfaces within the sensitized film, and by dramatic changes in wetting behaviour (TiO2 becomes amphiphilic) under ultra-violet irradiation46. However, a comparison of the well-established surface tensions between air and most solvents (that is, γgas–liquid), can be instructional in this case. The three-fold greater surface tension at a water–gas interface (γair–water=72 mN m−1) than at a methanol–gas interface (γair–methanol=23 mN m−1) means that methanol wets most surfaces better than water. In a simple test of placing a droplet of both solvents on a mp-TiO2 film, methanol clearly demonstrated a greater tendency to wet the surface (Supplementary Fig. S5). As photocurrent relies directly on contact between the electrolyte and the QD surface (for hole extraction), our observations of increased Jsc are consistent with improved wetting by the electrolyte.
Another critical element of a sensitized solar cell is the cathode, which has a significant effect on the overall device performance through its influence on Rseries and thus FF. In our solar cells we employ a CuxS cathode (Fig. 6), which allows us to obtain Rseries of 10 Ω·cm2 and FF of ~0.60 in the best devices, among the highest for QDPVs. Our simple method for making the cathode is by chemical transformation of 100-nm thick thermally evaporated Cu (on FTO) by exposure to the polysulfide electrolyte. Based on EDX measurements, the CuxS has an x=1.0 stoichiometry upon fabrication (that is, covellite, a p-type metal), but it becomes sulphur enriched (x of ca. 0.8) after being used in a device. The observed level of relative copper deficiency corresponds to an even higher degree of p-type doping, which should greatly improve cathode conductivity. Further, after sulphurisation our cathodes develop a meso-textured structure, characterized by a large surface area (Fig. 6b inset), which persists after many months of testing (Supplementary Fig. S4) and facilitates charge collection from the electrolyte. Together with improved conductivity, this is likely a second major factor in reduced Rseries and improved performance in our devices compared with previously published studies typically with Cu2S, which is a semiconductor. To better understand the contribution of the cathode and other factors (for example, charge-transfer resistance, electrolyte, mp-TiO2) to the aggregate series resistance of the device (measured by the inverse slope of the J–V curve), impendence spectroscopy may be insightful.
(a) SEM image of a cross-section of a fresh glass/FTO/CuxS cathode where x=1.0 according to EDX analysis. Scale bar, 1 μm. (b) SEM image from the top-down perspective of the cathode. Scale bar, 10 μm. Higher magnification inset shows the flower-like structures with high surface area. Scale bar, 1 μm.
A common and critical issue with QDSSCs is their poor stability resulting from electrochemical corrosion of many candidate sensitizers23,47. As our QDs are synthesized under excess sulphur conditions, we expected that our CISeS QDs would be chemically stable in the presence of the sulphur-based electrolyte. However, as we added more Se to reduce the band-gap of the QDs, they became less stable in the electrolyte, probably due to the replacement of Se by S anions at the surface over time24. The chemical incompatibility problem is typically addressed in the QDSSCs by the application of a thin insulating ZnS layer deposited by SILAR. The partial cation exchange (with Cd2+ or Zn2+) of QDs in our devices prior to sensitization (discussed earlier) helps solve the stability issue, and allows for stable operation of the devices for 1–2 months instead of 1–2 days24. To further improve the stability, we deposit two monolayers of ZnS using SILAR on photoanodes after they are sensitized with Cd-treated and tBA-recapped CISeS QDs. Following this procedure, the initial performance of a device is unchanged. However, the thin layer of ZnS seems to act as a good chemical barrier, which complements the existing barrier due to cation exchange without appreciably suppressing hole extraction into the electrolyte (ZnS deposited by SILAR is not expected to influence electron extraction, as the QD-TiO2 close contacts are established prior to ZnS application). Devices with the ZnS SILAR consistently showed prolonged stability under air exposure (Fig. 7a), although it is worth noting that even without the ZnS layer, some devices survived for up to 2 months (Fig. 3e). This is in drastic contrast to previously reported high-performance QDSSCs that were stable for just a few minutes23. The eventual catastrophic failure of the devices with a ZnS SILAR coating was associated with electrode delamination (for example, CuxS would peel off the substrate), which can be addressed by establishing better adhesion between layers during electrode fabrication. In order to better elucidate the true long-term stability of these devices, extended light-soaking tests should be conducted. Studies of the effect of light-soaking for 30 h indicate that the performance of our devices improves over the first ~15 min, which we attribute to improved ionic transport due to heating of the electrolyte, and then stays unchanged once the device temperature stabilizes under the light source (Supplementary Fig. S8).
Figure 7: Certified PV performance and stability.
(a) Current density versus voltage characteristics of a QDSSC certified by NREL. (b) EQE spectra for the device measured at NREL. (c) Performance of our champion device (different from the one in a and b) over time, including the certification data from NREL. The device was tested further (for example, day 1, day 4, day 6, and so on), but these measurements are not included in for plot clarity. (d) Sequential measurements of our champion device at 71 days under constant illumination for ~15 min. Each measurement takes 90 s, there was a 1 min delay between J–V measurements. The photovoltaic performance properties related to c and d can be found in Supplementary Tables S8 and S9.
The stability of our devices enabled us to have two of them (one with and one without a ZnS SILAR coating) certified by the PV Characterization Team at the National Renewable Energy Laboratory (NREL) 1 week after fabrication. Both devices were certified at 5.1% and the details of the certified performance can be found in Fig. 7a (device without ZnS) and in the Supplementary Fig. S2 (device with ZnS). As far as we know, these are the only certified performances reported for any QDSSC, which is important for a variety of reasons (see Supplementary Discussion). We continued testing the certified device, which had a ZnS SILAR coating periodically, with storage under ambient but dark conditions (Fig. 7c). This device survived 71 days, and maintained an efficiency of 5.5% through the final measurement using our in-house solar simulator under continuous illumination (Fig. 7d and Supplementary Table S9), which is the highest reported for any colloidal (that is, non-SILAR) QDSSC. Interestingly, the NREL-certified photocurrent was ~10% (relative) higher in this device than what we measured with our solar simulator, indicating that our 5.5% record measurement may be slightly underestimating the performance. Compared with a maximum possible EQE of ~80% (considering absorption and reflection losses due to the fluorinated tin oxide on glass electrode), our best devices achieve a peak EQE of ~60% at ~3.0 eV, although it is considerably lower for lower-energy photons (Fig. 7b). The relatively low and non-flat EQE of our devices suggests that there is significant room for improvement of Jsc by enhancing absorption (for example, via improved QD-loading) and/or by optimizing internal charge collection efficiencies (for example, via faster charge extraction). Although we observe relatively high QD-loading by simply soaking the electrode in a dilute solution of (recapped) QDs, loading might be further improved by the use of electrophoretic deposition27,48 or by attachment using alternative bifunctional linkers25,49.
In summary, we have shown that QDSSCs can exhibit both high performance and high stability using low-toxicity materials and low-cost fabrication methods. We have identified optimal conditions for the sensitization of mp-TiO2 with CISeS QDs using a very mild cation exchange with Zn2+ or Cd2+ followed by washing/recapping with tBA. Addition of a ZnS layer by SILAR does not lead to enhanced efficiency but further improves device stability. Adding methanol to the electrolyte significantly enhances the device performance by increasing photocurrent and lowering series resistance. We demonstrated a nanostructured CuxS (where x<1.0) cathode can be made by a simple low-cost method, and acts as an extremely effective reducing catalyst for the polysulfide electrolyte and enabled us to achieve series resistances on par with the best DSSCs that use platinum. Recognizing the challenges in accurately characterizing PVs, and thus the difficulties in comparing between various published results, we report the first certified performance of a QDSSC at 5.1% efficiency. Our champion efficiency approaches the highest reported values for any type of QD solar cells, yet our devices do not degrade for months even if stored in air: this is orders of magnitude longer than in any previous report on QD PVs. Given the fairly low EQEs even in our record devices, we expect that with further optimization, the performance of these novel PV devices can be enhanced considerably, making them competitive with well-established dye-sensitized solar cells.
CISeS QDs were synthesized via a modification of the Li et al.30 method24. Typically, 1 mmol of copper iodide and 1 mmol of indium acetate are dissolved in 5 ml of 1-dodecanethiol and 1 ml of oleylamine, and the mixture is degassed under vacuum in a 50 ml flask at 100 °C for 30 min. The solution was heated to 130 °C until it became yellow and transparent, and then degassed again for 30 min at 100 °C. The flask is then slowly heated to a growth temperature of 230 °C, but at a temperature of 220 °C, slow injection of 6 mmol of 2 M TOP-Se (~12 ml h−1) begins. After 30 min the reaction was cooled and the QDs were cleaned by dissolving in chloroform and precipitation with methanol. The precipitated QDs were collected by centrifugation and the supernatant was discarded. The QDs were stored in 5 ml of octane or ODE following cleaning. The synthesis typically results >90% chemical yield of QDs (relative to the copper and indium precursors).
For cation exchange with Cd2+ or Zn2+, a stock solution of 0.5 M cadmium or zinc oleate was prepared with 3:1 oleic acid:Cd/Zn dissolved in ODE. 4 ml of the cleaned QDs in ODE solution (~50 mg ml−1) were added to 4 ml of 0.5 M Cd or Zn-oleate solution and the temperature was set to 50–150 °C for 10 min depending on the desired degree of cation exchange. Following cation exchange, the reaction solution was dissolved in chloroform, and then acetone was added to precipitate the QDs. The precipitated QDs were collected by centrifugation, and then dissolved in chloroform.
Following cation exchange and purification of QDs, methanol was added to precipitate the QDs from chloroform. Precipitated QDs were collected by centrifugation and the supernatant was discarded.
For tBA, nBA, sBA and pyridine recapping, the ligand was used as a solvent to dissolve the precipitated QDs with sonication used to speed up dissolution and recapping. Then, methanol was added to precipitate the QDs, which were collected by centrifugation and the supernatant was discarded. The ligand was again used as a solvent to dissolve the precipitated QDs and these solutions were sonicated for a few minutes at room temperature. Methanol was added to precipitate the QDs, which were collected by centrifugation. The above process was optimized to give the highest QD-loading in the mp-TiO2 film for tBA-capped QDs. The optimal conditions may be different for the other ligands. Longer exposure of the QDs to the new ligands or heating during exposure might give more complete recapping, but resulted in poorer loading, probably due to partial aggregation of QDs during recapping. The recapped QDs were dissolved in octane (~0.05 g ml−1), and the solution was centrifuged to remove large aggregates that occasionally formed during recapping. Any precipitate was discarded. The QD–octane supernatant was diluted with octane to an optical density of ~0.2 (~0.01 g ml−1) at the 1S absorption peak (typically 850 nm), and used to prepare QD-sensitized mp-TiO2 films.
For MPA recapping, the QDs were recapped by dissolving precipitated QDs in a mixture of 1:1 chloroform and MPA, then precipitating by adding methanol, and centrifuged. The supernatant was discarded, and the QDs were dissolved in the same total volume of MPA and methanol (1:1). The solution was sonicated for a few minutes. Chloroform was added to precipitate the MPA-capped QDs, and the QDs were collected by centrifugation. The MPA-capped QDs were dissolved in methanol (~0.05–0.1 g ml−1), and the solution was centrifuged to remove aggregates that formed during recapping and to remove partially recapped QDs. The QD–methanol supernatant was diluted with methanol to an absorbance of about 0.2 (~0.01 g ml−1) at the 1S absorption peak (typically 850 nm), and used to prepare QD-sensitized mp-TiO2 films.
Scattering and non-scattering mp-TiO2 films are prepared by automated, iterative screen-printing (LS-150, New Long Seimitu Kogyo Co.) of TiO2 pastes containing ~20 nm and ~400 nm TiO2 particles, respectively, on fluorinated tin oxide (FTO)-coated glass. A dense blocking layer of TiO2 is not used. The α-terpineol based pastes are fired at 500 °C for 1 h under air. The 8 μm (or 13 μm with the scattering layer) thick mp-TiO2 films are sensitized by soaking in dilute QD–octane solutions for 36 h. The films were then rinsed with octane (or methanol for MPA-capped QDs) to remove unattached QDs. The cathode is fabricated by thermally evaporating 100 nm of Cu onto a FTO-coated glass substrate, and subsequent immersion in an aqueous polysulfide electrolyte (aqueous 1 M Na2S plus 1 M S) for 15 min to form CuxS/FTO. The device is completed by sandwiching the electrodes around a 40–50 μm Surlyn spacer (DuPont) and sealing by heating the polymer frame. Unless otherwise noted, the devices are filled with the polysulfide electrolyte composed of 1 M Na2S and 1 M S in methanol and water (1:1). For champion devices, silver paint and Cu wires was added to the contacts. The device active area is defined by a 0.47 cm by 0.47 cm (that is, 0.2209, cm2) black metal aperture, which is slightly smaller than the mp-TiO2 area. The mp-TiO2 scattering layer was only used for champion devices that were certified. As our device active area is relatively large we do not see significant device to device variations. However, we fabricated two devices for each case to confirm that any trends we observe are real. Duplicates of all devices were fabricated and tested for consistency and if the performance varied by >5% we repeated the study.
TEM samples were prepared from a dilute solution of NCs in chloroform drop-cast onto Cu grids (obtained from Ted Pella) with a thin carbon film on a holey carbon support. TEM analysis was carried out with a JEOL 2010 TEM operating at 200 kV.
SEM samples were prepared by breaking a QDSSC anode or cathode in half. The SEM used a Quanta 400 field emission gun made by FEI Company. The 20 kV electron beam was scanned across the cross-section, and then the energy and the count of X-rays were measured. Elemental analysis was conducted using Genesis EDX software.
All optical measurements of QDs in solution were performed in either chloroform or octane using a quartz cuvette. UV–vis absorption spectra were obtained with Agilent 8453 photodiode array spectrometer. PL measurements were performed using home-build spectrofluorimeter consisting or 808 nm excitation diode laser, a liquid nitrogen-cooled InSb detector and a grating monochromator. The excitation beam was mechanically chopped at 3 kHz and the signal was measured using a lock-in amplifier. For PL quantum yield (QY) measurements, optically dilute samples were magnetically stirred and the QY values were measured using relative technique by comparing integrated PL intensities to an IR-26 dye standard. The spectra were corrected for the wavelength-dependence sensitivity of the system.
A class ABA solar simulator (AM 1.5 G), built by PV Measurements, calibrated using a Newport-certified single crystal Si solar cell, was used to irradiate the QDSSCs during I–V measurement (in-house). The voltage was swept from −0.1 to 0.6 V at 0.01 V per step with a 1 s delay-time prior to measurement and 200 ms collection time at each point. A square black mask (0.2209, cm2) was attached to the solar cells to precisely define the active area of the device. An additional black cardboard mask (~5 cm by 5 cm outer edges) was placed on top of the contacted device (masking area outside the 0.2209, cm2 mask) immediately prior to current–voltage measurement to prevent scattering of light into the device by the contacting alligator clips. For continuous illumination testing, devices were shorted in between measurements.
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H.M., N.S.M., J.M.P. and V.I.K. acknowledge the support of the Center for Advanced Solar Photophysics, an Energy Frontier Research Center (EFRC) funded by the US Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences. N.F. was supported by Sharp Corporation under the Sharp-Los Alamos National Laboratory CRADA LA11C10656 PTS—001. The simulated sunlight current-voltage, SEM and EDX measurements were performed at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the DOE Office of Science. We thank P.E. Heil for insightful discussions on reducing contact resistances. We thank A. Koposov for assistance with characterization of devices under continuous illumination. We thank K. Emery, P. Ciszek and the rest of the PV Characterization Team at NREL for performing the certifications.
H.M. and N.F. designed this study and analysed the experimental results. H.M. synthesized and modified the surfaces of the QDs. N.F. fabricated the QDSSCs. H.M. and N.F. characterized the QDSSCs. N.S.M. measured the PL, calculated the QY and measured the TA. J.M.P. and V.I.K. directed this study and provided guidance. H.M. and V.I.K. wrote the manuscript.
Correspondence to Hunter McDaniel or Victor I. Klimov.

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WALLEM PHILIPPINES SERVICES, INC. AND WALLEM SHIP MANAGEMENT, LTD., Petitioners, v. HEIRS OF THE LATE PETER PADRONES, Respondents.
Before the Court is a petition for review on certiorari assailing the Decision1 and Resolution2 of the Court of Appeals (CA), dated August 17, 2007 and May 19, 2008, respectively, in CA-G.R. SPNo. 94357.
Peter Padrones (Padrones) was employed as a "motorman" by petitioners on board the vessel M/V "Spirit" from December 30, 1998 to November 23, 1999.3 He finished his contract and was repatriated to the Philippines after completion thereof.
On April 25, 2001, Padrones died of cardio-respiratory arrest brought about by complications of lung cancer.4 Thereafter, or on July 18, 2001, herein respondents, filed with the National Labor Relations Commission (NLRC) a Complaint5 against herein petitioners for recovery of death benefits, exemplary and moral damages, child allowance, burial expenses and attorney's fees arising from the death of Padrones. In their Position Paper,6 respondents alleged that Padrones' death is compensable because the cause of such death was aggravated by tuberculosis, an illness which he acquired during the existence of his contract.
WHEREFORE, premises considered, judgment is hereby rendered directing respondents Wallem Maritime Services, Inc. and Wallem Ship Management Ltd. to pay the complainants heirs of Peter Padrones, jointly and solidarity the sum of US$65,000.00 equivalent to death benefits under the POEA Standard Employment Contract, and attorney's fees [equivalent] to ten percent (10%) of the award as well as dismissing the prayer for damages for lack of merit.
The award is payable in Philippine peso at the rate of exchange prevailing at the time of payment.
x x x "as over emphasized by appellant [herein petitioners] the seafarer [Padrones] passed away one year and five months from the time he finished his employment contract. The employment contract expired on October 30, 1999, but was repatriated on November 23, 1999; Mr. Padrones, the seafarer died on April 25, 2001. Clearly, the employment contract was no longer in force when the seafarer died. Applying the POEA SEC [Standard Employment Contract], complainants are not entitled to death benefits.
Respondents-appellants were able to belie appellee's allegation that complainant was repatriated due to medical reasons. Appellants submitted in evidence a copy of the CLAIM FORM filled up by Mr. Padrones when he reported to respondent's office after sign off from the vessel. In the said claim form, the deceased was asked of his claims, including claims for illness or injury. Mr. Padrones affixed, N/A or "not applicable."
Respondents filed a Motion for Reconsideration,12 but the NLRC denied it in its Resolution13 dated February 24, 2006.
WHEREFORE, premises considered, finding grave abuse of discretion amounting to lack or excess of jurisdiction on the part of the NLRC in dismissing the complaint filed by petitioners, the instant petition for certiorari is GRANTED. The Decision dated April 18, 2005 and the Resolution dated February 24, 2006, respectively, of the NLRC are SET ASIDE. The Decision dated October 30, 2003 of the Labor Arbiter is hereby REINSTATED with a MODIFICATION that respondents should pay to Peter Padrones, herein represented by his heirs, the sum of US$60,000.00 as disability benefits to be paid in Philippine currency equivalent at the exchange rate prevailing during the time of payment. The award of attorney's fees is likewise maintained.
Petitioners filed a Motion for Reconsideration,17 but the CA denied it in its Resolution dated May 19, 2008.
The basic issue in the present case is whether the CA erred in awarding disability benefits in favor of respondents who were asking for death benefits.
Preliminarily, the Court agrees with the CA and the NLRC that herein respondents are not entitled to death benefits.
Thus, it is clear that for the death of a seafarer to be compensable, the same must occur during the term of his contract of employment.20 If the seaman dies after the termination of his contract, his beneficiaries are not entitled to death benefits.21 In the instant case, Padrones' employment contract ended on November 23, 1999. He died on April 25, 2001, more than one (1) year and five (5) months from the time his employment contract expired. It, therefore, follows that respondents, who are the beneficiaries of Padrones, are not entitled to death benefits.
Even if the Court were to consider the possibility of compensation for the death of Padrones after the termination of his employment contract on account of a work-related illness, respondents, nonetheless, did not present evidence to prove that he acquired lung cancer during his employment and that the said disease, which caused his death, was the reason for the termination of his contract. On the contrary, respondents claimed that Padrones was afflicted only with tuberculosis during his employment. In fact, they even failed to present substantial evidence to show that Padrones acquired this illness while he was employed nor were they able to prove their contention that it contributed to his death.
Not being entitled to death benefits, the question that follows now is whether respondents are, instead, entitled to the disability benefits awarded by the CA. The answer is no.
In their Comment to petitioners' Motion for Reconsideration of the CA Decision, as well as in their Comment to the present petition, respondents abandoned their claim for death benefits and focused solely on Padrones' supposed entitlement to disability benefits. However, nowhere in respondents' Comment did they refute petitioners' basic contention that they are not entitled to disability benefits on the ground that this issue was never litigated before the lower tribunals. Respondents argue as if the issue of their entitlement to disability benefits was a matter which was raised at the first instance. Respondents have, in effect, changed their theory of the case.
Settled is the rule that, in this jurisdiction, a party cannot change his theory of the case or his cause of action on appeal.31 It affirms that courts of justice have no jurisdiction or power to decide a question not in issue.32 Thus, a judgment that goes beyond the issues and purports to adjudicate something on which the court did not hear the parties, is not only irregular but also extrajudicial and invalid.33 The rule rests on the fundamental tenets of fair play.34 The exception to this rule is when the factual bases thereof would not require presentation of any further evidence by the adverse party in order to enable it to properly meet the issue raised in the new theory.35 In such a case, the court may give due course to the petition and resolve the principal issues raised therein.36 The instant case does not fall under this exception. To stress, the issue of whether or not Padrones or respondents, as his heirs, are entitled to disability benefits is a factual question that was never alleged, let alone proven before the LA, the NLRC and the CA. Understandably, petitioners did not present evidence before the lower tribunals to refute respondents' alleged entitlement to disability benefits because this was never an issue. It was only after the CA has awarded them disability benefits that respondents changed their theory by claiming that they are indeed entitled to such benefits instead of death benefits. Thus, respondents' belated change of their theory of the case should be disallowed and the instant petition granted.
WHEREFORE, the instant petition is GRANTED. The Decision and Resolution of the Court of Appeals, dated August 17, 2007 and May 19, 2008, respectively, in CA-G.R. SP No. 94357 are REVERSED and SET ASIDE. The Decision of the National Labor Relations Commission, dated April 18, 2005, which dismissed respondents' Complaint, is REINSTATED.
1 Penned by Associate Justice Rosmari D. Carandang, with Associate Justices Marina L. Buzon and Mariflor P. Punzalan Castillo, concurring; Annex "A" to Petition, rollo, pp. 31-43.
2 Penned by Associate Justice Rosmari D. Carandang, with Associate Justices Amelita G. Tolentino and Mariflor P. Punzalan Castillo, concurring; Annex "B" to Petition, id. at 44-47.
3 See Annexes "E," "F," and "G" to Petition, id. at 54-56.
4 See Annex "H" to Petition, id. at 57.
5 Annex "I" to Petition, id at 58-59.
6 CA rollo, pp. 43-58.
17 CA rollo, pp. 276-292.
19Yap v. Rover Maritime Services, Corp., G.R. No. 198342, August 13, 2014.
20Medline Management, Inc., et. al. v. Roslinda, et. al., 645 Phil., 34, 51 (2010); Ortega v. Court of Appeals, 576 Phil. 601, 605 (2008).
21Klmeness Maritime Agency, Inc. v. Beneficiaries of the Late Second Officer Anthony S. Alias, 566 Phil. 579, 586 (2008); Prudential Shipping and Management Corp. v. Sta. Rita, 544 Phil. 94, 106 (2007).
24 CA rollo, pp. 64-71.
27 See CA rollo, p. 66.
29Hermogenes v. Osco Shipping Services, Inc., 504 Phil. 564, 572 (2005).
30 See rollo, p. 22.
31Bote v. Veloso, G.R. No. 194270, December 3, 2012, 686 SCRA 758, 768.
37Panganiban v. Tara Trading Shipmanagement Inc., el. al., 647 Phil. 675, 691 (2010).

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