in this section , we describe experiments relating to compounds as described above . more specifically , we consider two examples of the above described class of materials ( pi - bt and ni - bt ), and compare their performance to each other and to a reference material that is not an example of the above described class of materials ( ev - bt ). it will be convenient to briefly review the ev - bt story in the following paragraph . here p3ht is short for poly ( 3 - hexylthiophene ), which is a semicrystalline polymeric electron donor used in some of the experiments . in previous work , the synthesis and characterization of a series of n - type conjugated materials based on the heck reaction of 4 , 5 - dicyano - 2 - vinylimidazole ( vinazene ™) with selected dibromoaromatics is described . these materials were easily prepared in high yields from one - step reactions using commercially available materials . the most promising material from these studies was 4 , 7 - bis ( 2 -( 1 -( 2 - ethylhexyl )- 4 , 5 - dicyanoimidazol - 2 - yl ) vinyl ) benzo [ c ][ 1 , 2 , 5 ]- thiadiazole ( ev - bt ), shown in fig4 a . ev - bt , with its favorable ( estimated ) solid - state electron affinity ( 3 . 65 ev ) and strong absorption in the visible spectrum produced , with selected donor polymers , opvs with v oc up to 1 . 4 v and pces of 1 . 4 %. we suspect the lower fill factor ( ff ) and short - circuit current ( j sc ) of these devices ( 50 % and 5 . 7 ma cm − 2 ) compared to p3ht : pc 61 bm ( 67 % and ˜ 10 ma cm − 2 ) can be partially attributed to the low electron mobility of ev - bt [ 10 − 5 cm 2 v − 1 s − 1 , as measured by space charge limited current ( sclc ) methods ] compared to pc 61 bm ( 10 − 3 cm 2 v − 1 s − 1 ). a reason for the low electron mobility of this acceptor may be the asymmetric alkyl - chain direction on the vinyldicyanoimidazole moiety that prevents efficient π - π molecular stacking , as shown in fig4 a . to increase the electron mobility in the acceptor we considered the introduction of electron deficient vinyl imide units as “ a ” building blocks in an a - b - a structure ( fig4 d ) that allows for more symmetric placement of the alkyl chains at the ends of the molecule , as shown in fig4 b ( pi - bt ) and 4 c ( ni - bt ). furthermore , the molecules were designed with extended n - conjugation that helps contribute to significant absorption of visible light , and tunable chemistry to allow for modulation of the ionization potential , electron affinity , and solubility . b1a ) synthesis of ev - bt : 4 , 7 - bis ( 2 -( 1 -( 2 - ethylhexyl )- 4 , 5 - dicyanoimidazol - 2 - yl ) vinyl ) benzo [ c ][ 1 , 2 , 5 ]- thiadiazole ( ev - bt ) was synthesized according to procedures reported in the literature . the alkylated vinazene was synthesized using a modified procedure reported by johnson and rasmussen . the final compound , ev - bt was synthesized according to the procedure reported by ooi et al . b1b ) synthesis of br - ph - im : the imide side group , n - hexyl - 4 - bromo - phthalimide ( br - ph - im ), was first synthesized according to a procedure adapted from dierschke et al . a solution of 2 . 00 g 4 - bromophthalic anhydride , 12 . 0 ml propionic acid , and 1 - hexylamine ( 1 . 5 eq .) was stirred under n 2 at 130 ° c . for three days . the cooled solution was diluted with water , and extracted with dichloromethane three times . the organic layer was dried over magnesium sulfate and concentrated . the compound was purified by column chromatography in silica gel with 50 % ethyl acetate in hexane . after precipitation in 50 % methanol in water the crystals were collected by vacuum filtration and dried overnight . the product was a white solid weighing 2 . 63 g ( 96 % yield ). 1 h nmr ( 300 mhz , cdcl 3 ): δ 0 . 87 ( t , 3h ), 1 . 31 ( m , 6h ), 1 . 65 ( qui , 2h ), 3 . 66 ( t , 2h ), 7 . 71 ( d , 1h ), 7 . 83 ( d , 1h ), 7 . 97 ( s , 1h ). 13 c nmr ( 100 mhz , cdcl 3 ): δ 14 . 00 , 22 . 48 , 26 . 47 , 28 . 45 , 31 . 30 , 38 . 29 , 124 . 51 , 126 . 53 , 128 . 71 , 130 . 64 , 133 . 78 , 136 . 80 , 167 . 05 , 167 . 59 . c 14 h 16 brno 2 ( 310 . 19 ): calcd . c 54 . 21 , h 5 . 20 , n 4 . 52 , br 25 . 76 ; found c 54 . 07 , h 4 . 97 , n 4 . 46 , br 26 . 28 . b1c ) synthesis of br — n - im : the imide side group , n - hexyl - 4 - bromo - naphthalimide ( br — n - im ), was synthesized using the same general method as described above for pi - bt , except using 4 - bromo - 1 , 8 - naphthalic anhydride . the product was a tan solid weighing 1 . 91 g ( 74 % yield ). 1 h nmr ( 300 mhz , cdcl 3 ): δ 0 . 89 ( t , 3h ), 1 . 34 ( m , 6h ), 1 . 72 ( qui , 2h ), 4 . 16 ( t , 2h ), 7 . 85 ( t , 1h ), 8 . 05 ( d , 1h ), 8 . 40 ( d , 1h ), 8 . 56 ( d , 1h ), 8 . 64 ( d , 1h ). 13 c nmr ( 100 mhz , cdcl 3 ): δ 14 . 04 , 22 . 53 , 26 . 74 , 27 . 98 , 31 . 50 , 40 . 58 , 122 . 20 , 123 . 06 , 128 . 00 , 128 . 87 , 130 . 10 , 130 . 50 , 131 . 00 , 131 . 11 , 131 . 92 , 133 . 10 , 163 . 49 , 163 . 51 . c 18 h 18 br 2 no 2 ( 360 . 25 ): calcd . c 60 . 01 , h 5 . 04 , n 3 . 89 , br 22 . 18 ; found c 59 . 93 , h 4 . 83 , n 3 . 87 , br 26 . 26 . b1d ) synthesis of bt - si : the central group , 4 , 7 - triethoxyvinylsilane - benzo [ c ]- 1 , 2 , 5 - thiadiazole ( bt - si ), was synthesized by adding toluene ( 25 ml ), triethoxyvinylsilane ( 1 . 8 ml , 8 . 50 mmol ), and nitrogen degassed cy 2 nme ( 1 . 8 ml , 8 . 50 mmol ) to a schlenk flask containing 4 , 7 - dibromobenzo [ c ]- 1 , 2 , 5 - thiadiazole ( 1 . 00 g , 3 . 40 mmol ) and pd ( p [ tbu ] 3 ) 2 ( 75 mg , 0 . 15 mmol ) under nitrogen . the solution was stirred at 90 ° c . for 48 hours and then extracted three times with 5 % hcl at 5 ° c . and once with brine . after drying over magnesium sulfate , the solvent was removed through rotoevaporation . the crude product was a very sticky red gel weighing 1 . 35 g and was used without further purification for the next reaction step . 1 h nmr ( 300 mhz , cdcl 3 ): δ 1 . 30 ( t , 18h ), 3 . 92 ( qui , 12h ), 7 . 05 ( d , 2h ), 7 . 68 ( s , 2h ), 7 . 74 ( d , 2h ). b1e ) synthesis of pi - bt : 4 , 7 - bis ( 4 -( n - hexyl - phthalimide ) vinyl ) benzo [ c ] 1 , 2 , 5 - thiadiazole ( pi - bt ), was prepared by adding bt - si ( 0 . 80 g , 1 . 56 mmol ), br - ph - im ( 1 . 06 g , 3 . 43 mmol ), pd ( dba ) 2 ( 90 mg , 0 . 16 mmol ), and ligand ( p ( o - tol ) 3 , 142 mg , 0 . 47 mmol ) to a schlenk flask . the flask was evacuated , and then refilled with n 2 three times . 25 ml toluene was added and the reaction mixture was stirred at 80 ° c . for 20 min . tetrabutyl ammonium fluoride ( 3 . 7 ml , 1 . 0 m in thf , 3 . 7 mmol ) was then added and the reaction mixture was stirred at 80 ° c . for 2 days . the solution was extracted with water and ch 2 cl 2 three times and dried over magnesium sulfate . the compound was purified by column chromatography in silica gel using ch 2 cl 2 . precipitation was done in methanol and the red color solid weighing 0 . 49 g ( 49 % yield ) was collected by vacuum filtration and dried overnight . 1 h nmr ( 300 mhz , cdcl 3 ): δ 0 . 89 ( t , 6h ), 1 . 33 ( m , 12h ), 1 . 71 ( qui , 4h ), 3 . 70 ( t , 4h ), 7 . 75 ( d , 2h ), 7 . 79 ( s , 2h ), 7 . 86 ( d , 2h ), 7 . 89 ( d , 2h ), 8 . 14 ( s , 2h ), 8 . 24 ( d , 2h ). 13 c nmr ( 100 mhz , cdcl 3 ): δ 14 . 02 , 22 . 52 , 26 . 55 , 28 . 58 , 31 . 37 , 38 . 18 , 120 . 51 , 123 . 66 , 128 . 20 , 128 . 53 , 129 . 26 , 130 . 85 , 132 . 07 , 133 . 03 , 143 . 46 , 153 . 65 , 168 . 13 , 168 . 31 . b1f ) synthesis of ni - bt : 4 , 7 - bis ( 4 -( n - hexyl - naphthalimide ) vinyl ) benzo [ c ] 1 , 2 , 5 - thiadiazole ( ni - bt ), was prepared according to the same method as pi - bt but using br — n - im ( 1 . 24 g , 3 . 43 mmol ). the product was a red color solid weighing 0 . 25 g ( 21 %). 1 h nmr ( 300 mhz , cdcl 3 ): δ 0 . 90 ( t , 6h ), 1 . 35 ( m , 12h ), 1 . 75 ( qui , 4h ), 4 . 19 ( t , 4h ), 7 . 73 ( d , 2h ), 7 . 80 ( d , 2h ), 7 . 86 ( d , 2h ), 8 . 16 ( d , 2h ), 8 . 63 ( d , 2h ), 8 . 66 ( d , 2h ), 8 . 72 ( d , 2h ), 9 . 12 ( d , 2h ). 13 c nmr ( 100 mhz , cdcl 3 ): δ 14 . 08 , 22 . 58 , 26 . 82 , 28 . 07 , 31 . 56 , 40 . 54 , 121 . 99 , 123 . 15 , 123 . 68 , 126 . 97 , 128 . 72 , 129 . 61 , 129 . 63 , 129 . 69 , 129 . 76 , 130 . 71 , 130 . 97 , 131 . 28 , 134 . 89 , 141 . 20 , 153 . 75 , 163 . 97 , 164 . 19 . 1 h and 13 c nmr spectra were recorded using a varian inova 300 or 400 in cdcl 3 at 293 k . thermal gravimetric analyses ( tga ) were carried out using a mettler toledo tga / sdta 851e at a heating rate of 10 ° c . min − 1 under nitrogen flow of 20 ml min − 1 . differential scanning calorimetry ( dsc ) analyses were performed on a dsc q100 ( ta instruments ). dsc curves were recorded at a scanning rate of 10 ° c . min − 1 under nitrogen flow . uv - vis absorption spectra in tetrahydrofuran ( thf ) solution were recorded in a uv - vis spectrophotometer ( cary 6000i ) at room temperature using a glass cuvette with a path length of 1 cm . photoluminescence ( pl ) spectra were recorded on a horiba jobin yvon fl3 - 2ihr fluorometer . photo - electron spectra in air ( pesa ) were recorded in air on a riken keiki ac - 2 ultraviolet photoelectron spectrophotometer . cyclic voltammetry ( cv ) measurements were carried out on a chi411 electrochemical workstation , using a concentration of a few millimolar in dichloromethane containing approximately 0 . 05 m supporting electrolyte of tetrabutylammonium phosphorous hexafluoride in a three - electrode cell , where the saturated calomel electrode ( sce ) was used as the reference electrode and platinum wire as the working electrode . the scanning rate was 0 . 1 v s − 1 . solutions of p3ht ( rieke metals , ee - grade ) and powder of either pi - bt or ni - bt were prepared in two methods . first , weighed amounts of each material in a 1 : 1 ratio ( by weight ) were added into an amber glass vial to which solvent was added to yield the desired concentration . solvents used consisted of chloroform , chlorobenzene and 1 , 2 - dichlorobenzene ( aldrich ). solutions were then diluted as needed to test the effects of solution concentration on device performance . the second solution preparation method consisted of making separate stock solutions of p3ht and acceptor material in chlorobenzene at concentrations of 20 mg ml − 1 . these stock solutions were blended together in various ratios in order to test the effect of donor : acceptor blend ratio on device performance . all solutions were prepared in a dry nitrogen glove box to avoid exposure of all materials to oxygen and moisture and were stirred overnight at temperatures of at least 90 ° c . to ensure complete dissolution . bulk heterojunction solar cells were prepared on glass substrates with tin - doped indium oxide ( ito , 15 ωsq − 1 ) patterned on the surface . substrates were first scrubbed with dilute extran 300 detergent solution to remove organic residues before immersing in an ultrasonic bath of dilute extran 300 for 15 minutes . samples were rinsed in flowing deionized water for 5 minutes before being sonicated for 15 minutes each in successive baths of acetone and isopropanol . after a final 5 minute rinse in flowing deionized water , samples were dried with pressurized nitrogen before being exposed to a uv - ozone plasma for 15 minutes . immediately after uv - ozone exposure , an aqueous solution of poly ( 3 , 4 - ethylenedioxythiophene ) poly ( styrenesulfonate ) ( pedot : pss , clevios ™ p vp ai 4083 ) was deposited via spin coating at a speed of 4000 rpm for 30 s , resulting in a film approximately 25 nm in thickness . the samples were then dried on a hot plate at 150 ° c . for 15 minutes to remove residual water . all steps above were performed in a laminar flow hood to prevent adsorption of dust particles on the substrates . samples were then transferred into a dry nitrogen glove box (& lt ; 1 ppm o 2 ) for active layer deposition . blended solutions of p3ht and acceptor molecules were deposited by spin - coating ( 1500 rpm for 45 s unless otherwise noted ) using a laurell technologies ws - 650 - npp - lite programmable spin coater . after spinning , samples were placed in a covered petri dish overnight to dry completely . after drying , the edges of samples were scraped with a razor blade to remove a part of the active layer in order to make contact to the underlying ito layer and then placed in a thermal evaporator for evaporation of electrodes . two cathode material stacks were used : lif ( 1 nm )/ al ( 200 nm ) for p3ht : pi - bt bulk heterojunction cells and ca ( 7 nm )/ al ( 200 nm ) for p3ht : ni - bt cells . lif ( cerac technologies ) was evaporated at a rate no higher than 0 . 1 å s − 1 to ensure accurate film thickness . calcium was evaporated at a rate of 1 å s − 1 and aluminum evaporated at 10 å s − 1 . following cathode deposition , samples underwent i - v testing inside the glove box prior to and at several stages during a thermal annealing step to determine optimal annealing time and temperature . high - efficiency devices were then selected for external quantum efficiency ( eqe ) testing . eqe testing was performed inside the glove box using monochromatic light from a tungsten lamp and the photocurrent spectrum was calibrated to a nist traceable silicon calibration photodiode . two - dimensional grazing incidence x - ray scattering ( gixs ) spectra were conducted on both blended and pure films ( p3ht and acceptor ) prepared identically to solar cells but on silicon substrates and without the pedot : pss and metal electrode layers . gixs experiments were conducted on beamline 11 - 3 at the stanford synchrotron radiation laboratory ( ssrl ). calculations for the ( gas - phase ) molecular series in the neutral , radical - cation , and radical - anion states were carried out with density functional theory ( dft ) using the becke &# 39 ; s three - parameter exchange functional , and the lee - yang - parr correlation functional ( b3lyp ) in conjunction with a 6 - 31g ( d , p ) pople basis set . low - lying excited state properties were evaluated at the neutral ground state geometries using time dependent dft ( tddft ) at the b3lyp / 6 - 31g ( d , p ) level . all calculations were performed using the gaussian ( 03 revision e . 01 ) suite of programs . initial quantum - chemical calculations using density functional theory ( dft ) of methyl analogs show that pi - bt , like ev - bt , maintains a planar geometry in the ground state that , together with the position of the alkyl chains , we propose will favor efficient π - π stacking of the acceptor molecules and enhance the electron mobility in the π - π direction . however , in the case of ni - bt , steric interactions between neighboring hydrogen atoms in the naphthyl and vinyl moieties induce a 27 . 3 ° twist in the ground state of the isolated molecule . this twist may prohibit efficient packing and crystal formation between neighboring molecules , and thus lower the electron mobility in ni - bt compared with pi - bt . based on the quantum - chemical studies , we designed pi - bt and ni - bt as electron - accepting molecules for application in bhj solar cells . the molecules were synthesized through the sequential modified heck coupling reactions shown in scheme 1 ( shown on fig5 ). the isolated , silicon - based intermediate ( bt - si ) was selected as a safer and more environmentally friendly analogue to the organotin counterparts using stille chemistry . bis ( tri - tert - butylphosphine ) palladium ( 0 ) ( pd [ p ( tbu ) 3 ] 2 ) was used as a catalyst for the first heck reaction step of 4 , 7 - dibromobenzothiadiazole with vinyltriethoxysilane to form bt - si as a red - colored gel in 77 % yield . in step two , tetrabutylammonium fluoride ( tbaf ) was used to remove the triethoxysilyl group and generate 4 , 7 - divinylbenzo [ c ][ 1 , 2 , 5 ] thiadiazole in - situ . bis ( dibenzylideneacetone ) palladium ( 0 ) ( pd ( dba ) 2 )/( p ( o - tol ) 3 ) was then used for the heck coupling of the in - situ formed 4 , 7 - divinylbenzo [ c ][ 1 , 2 , 5 ] thiadiazole with n - hexyl - 4 - bromo - phthalimide ( br - ph - im ) and n - hexyl - 4 - bromo - naphthalimide ( br — n - im ) to form pi - bt and ni - bt respectively in yields of 49 and 21 %, scheme 1 . we confirmed the chemical structures with 1 h and 13 c nmr . pi - bt shows two 1 h nmr resonance peaks at 3 . 70 ( 4h ) and 7 . 6 - 8 . 2 ( 12h ), which are assigned to the methylene protons adjacent to the n of the phthalimide units and the aromatic protons , respectively . the composition of pi - bt was determined in part from the ratio ( 1 : 3 ) of the integrals of these two sets of peaks . similarly , ni - bt produced two 1 h nmr resonance sets of peaks at 4 . 19 ( 4h ) and 7 . 7 - 9 . 4 ( 16h ), which are assigned to the methylene protons adjacent to the n of the naphthalimide units and the aromatic protons , respectively . the composition of ni - bt was also determined in part from the ratio ( 1 : 4 ) of the integrals of these two sets of peaks . 13 c nmr of pi - bt shows six resonance peaks , ranging from 14 . 02 to 38 . 18 ppm , that correspond to the hexyl side chains on the phthalimide units . the remaining thirteen peaks , ranging from 120 . 51 to 168 . 31 ppm , correspond to the aromatic carbons of the benzothiadiazole , vinyl , and phthalimide units . the 13 c nmr of ni - bt also gave six resonance peaks , ranging from 14 . 08 to 40 . 54 ppm , that correspond to the hexyl side chains on the naphthalimide units . the remaining seventeen peaks , ranging from 121 . 99 to 164 . 19 ppm , correspond to the aromatic carbons of the benzothiadiazole , vinyl , and naphthalimide units . the optical , electrical , and thermal properties of ev - bt ( provided here as a comparison ), pi - bt , and ni - bt were characterized using ultraviolet - visible ( uv - vis ) spectroscopy , photoluminescence ( pl ) spectroscopy , photo - electron spectroscopy in air ( pesa ), cyclic voltammetry ( cv ), thermal gravimetric analysis ( tga ), and differential scanning calorimetry ( dsc ) and are reported in table 1 . the uv - vis absorption ( fig6 a ) and pl spectra ( fig6 b ) of pi - bt and ni - bt in dilute thf solutions are shown in fig6 a - b . thin - film absorption data of ev - bt and pi - bt are also shown in fig6 c . ev - bt has two main absorption peaks at 335 and 442 nm . the lower wavelength main absorption peak originates from the vinazene unit . the absorption onset for ev - bt of 510 nm corresponds to an estimated optical gap of 2 . 43 ev . the maximum absorption wavelengths ( λ max , abs ) of pi - bt and ni - bt , which contain the same “ b ” building block ( benzothiadiazole ), but different imide - based “ a ” building blocks , are 448 and 467 nm , respectively . the absorption onset of ni - bt ( 575 nm , 2 . 16 ev ) is red - shifted by 45 nm ( 0 . 18 ev ) compared to pi - bt ( 530 nm , 2 . 34 ev ) due to the extended conjugation of the naphthalimide unit . results obtained using time - dependent dft ( td - dft ) at the b3lyp / 6 - 31g ** level are consistent with the empirical results . the first optical transitions are principally homo → lumo electronic excitations . thus , the optical gap of the acceptor can be tuned using different “ a ” building blocks to complement the absorption of the donor material . the maximum emission wavelengths ( λ max , em ) of ev - bt , pi - bt and ni - bt in solution are at 516 , 525 and 547 nm , respectively . the absorption coefficients for ev - bt and pi - bt in solution were measured in the range of 2 . 3 - 3 . 9 × 10 5 m − 1 cm − 1 , and for thin films the absorption coefficients were 0 . 4 - 1 . 2 × 10 5 cm − 1 , indicating that the molecules have strong light absorption in the visible region of the spectrum , which is desired for contribution to the photocurrent generation in photovoltaic devices . for comparison , the peak absorption coefficients in the visible ( 350 - 600 nm ) for pc 71 bm and p3ht 46 thin films are 0 . 6 × 10 5 cm − 1 and 2 . 5 × 10 5 cm − 1 . the ionization energies of ev - bt , pi - bt and ni - bt in solution as estimated by cyclic voltammetry are 5 . 97 , 5 . 77 and 5 . 66 ev , respectively . using photoelectron spectroscopy in air ( pesa ), the solid - state ionization energies for ev - bt , pi - bt and ni - bt were 6 . 10 , 6 . 05 and 5 . 99 ev , respectively . for ev - bt thin films , however , the value of 6 . 10 ev is a lower limit as the instrument is not able to detect a clear photoelectron signal at excitations above 6 . 1 ev . the differences in ionization potential measurements between the two techniques have been attributed to solvation effects in the cyclic voltammetry setup . however , a linear relationship between the two measurements has been found experimentally , and this is in qualitative agreement with our data . in addition , dft calculations on the isolated molecules point to the ionization potential for ev - bt as being considerably larger than that for the other two compounds . the intramolecular reorganization energies for hole transport are of the order of 250 - 300 mev . the electron affinities of ev - bt ( 3 . 43 ev ), pi - bt ( 3 . 30 ev ) and ni - bt ( 3 . 35 ev ) were also estimated using cyclic voltammetry , and are consistent with electron affinities determined with dft that indicate ev - bt ( 2 . 56 ev ) is the easiest to reduce , followed by ni - bt ( 2 . 24 ev ) and pi - bt ( 2 . 03 ev ). as with oxidation , the intramolecular reorganization energies for electron - transport are similar for ev - bt and pi - bt , and about 70 mev larger for ni - bt — a consequence of the more twisted geometric structure . pi - bt and ni - bt do not thermally decompose under nitrogen until 360 - 370 ° c . in the tga measurements , indicating these acceptors have good thermal stability under n 2 . the melting points were measured using dsc to be in the range of 156 - 241 ° c . the relatively high melting points of pi - bt and ni - bt indicate the upper temperature limits of thermal annealing during processing . bhj solar cells using p3ht as the donor and ev - bt , pi - bt , or ni - bt as the acceptor were fabricated and characterized in a preliminary device study . p3ht : ev - bt devices were prepared according to a previously published procedure . p3ht : pi - bt bhj devices were prepared from solutions of p3ht ( rieke , ee - grade ) and pi - bt in various blending ratios and of various concentrations in chloroform , chlorobenzene , and 1 , 2 - dichlorobenzene ( aldrich ). optimal p3ht : pi - bt devices were obtained when using chlorobenzene - based solutions at a total solute concentration of about 20 mg / ml and a blending ratio of 1 : 1 . 4 by weight p3ht : pi - bt . device stacks included ito / pedot : pss ( 25 nm )/ p3ht : pi - bt ( ca . 80 nm )/ lif ( 1 nm )/ al ( 200 nm ). j - v curves under am1 . 5g illumination of one sun intensity are shown on fig7 a . j - v curves of optimized p3ht : ni - bt bhj devices prepared from chloroform solutions of 12 mg / ml total concentration of p3ht and ni - bt in a 1 : 2 ( by weight ) ratio are also shown on fig7 a . optimized device stacks included ito / pedot : pss ( 25 nm )/ p3ht : ni - bt ( ca . 120 nm )/ ca ( 7 nm )/ al ( 200 nm ). optimized pi - bt samples were annealed after cathode deposition at 110 ° c . for 3 minutes , whereas optimized ni - bt samples were annealed prior to cathode deposition at 110 ° c . for 10 minutes . external quantum efficiency ( eqe ) curves of the optimal p3ht : pi - bt devices are shown in fig7 b . a collection of relevant solar cell parameters is given in table 2 for optimized p3ht - based bhj devices based on ev - bt , pi - bt and ni - bt . the highest power conversion efficiency obtained was 2 . 55 %, which we believe is the highest for a single - junction bhj solar cell using any organic donor material and a non - fullerene acceptor . an average value of 2 . 3 % was determined for a large data set of more than 40 devices . a very large v oc of 0 . 96 v is observed in the j - v curve of the p3ht : pi - bt device . these higher voltages were expected compared to p3ht : pc 61 bm devices ( typically 0 . 65v ) given the smaller electron affinity of pi - bt ( 3 . 3 ev ) as compared to pc 61 bm ( 3 . 7 ev ). the eqe curves shown in fig7 b demonstrate that while only 30 to 40 % of the incident photons between 375 and 600 nm result in photocurrent , a significant portion of that photocurrent is due to absorption by the acceptor phase followed by hole transfer to the donor . the ( normalized ) absorption spectra of p3ht and pi - bt thin films are provided as dashed curves for reference . with significant contributions to photocurrent from the acceptor material , there is increased capability to improve solar spectrum absorption via complementary selection of donor / acceptor blend systems . solar cells made with ni - bt as the acceptor performed considerably worse in all metrics compared to those made with pi - bt and ev - bt . while the electron affinity and chemical structure of ni - bt are similar to pi - bt and would suggest similar performance , we believe that the tendency of p3ht : ni - bt bhj cells not to crystallize upon deposition or annealing , as well as the larger intramolecular reorganization energy for electron transport , leads to very poor electron transport through the device . this could allow significant recombination losses across the donor / acceptor interface that would result in lower j sc , v oc , and fill factor values . two - dimensional grazing incidence x - ray scattering images ( gixs ) performed at the stanford synchrotron radiation laboratory ( ssrl ) showed significant crystallization of the pi - bt and ev - bt based films , as well as the lack thereof in the case of the ni - bt based films . fig8 a - b show further experimental results relating to pi - bt opv cells . fig8 a shows current density ( j ) vs . voltage ( v ) curves for bulk heterojunction solar cells using pi - bt as acceptor material and either commercially available p3ht ( rieke metals , ee - grade ) or grignard metathesis method ( grim ) prepared p3ht as the donor material . power conversion efficiency of the best rieke p3ht : pi - bt device is 3 . 03 %. power conversion efficiency of the best grim p3ht : pi - bt device is 3 . 36 %. fig8 b shows external and internal quantum efficiency spectra of rieke p3ht : pi - bt bulk heterojunction solar cells . further results are given in table 3 below . in conclusion , a new series of novel acceptor molecules for organic photovoltaics bearing vinylimide and benzothiadiazole units was successfully designed and synthesized . these units created extended electron - deficient a - b - a π - conjugated systems that can be tuned to vary the homo and lumo levels of the acceptor molecules . by varying the moieties used to build an acceptor molecule ( i . e . imide or vinazene - bearing “ a ” building blocks ), the transport and optical gaps of the acceptor can be tuned to fit a specific donor material and optimize electron transfer and efficiency during photocurrent generation . since the electron affinities of pi - bt and ni - bt ( 3 . 30 and 3 . 35 ev ) are smaller than that of pc 61 bm ( 3 . 7 ev ), a larger v oc for p3ht / pi - bt devices ( up to 0 . 96 v ) is observed . the new acceptors also show strong absorption in the visible spectrum with molar extinction coefficients greater than 10 5 m − 1 cm − 1 . the best p3ht : pi - bt - based opv device yields a record high pce for a bhj utilizing a non - fullerene acceptor material of 3 . 36 %.