The effect of side-chain substitution and hot processing on diketopyrrolopyrrole-based polymers for organic solar cells† †Electronic supplementary information (ESI) available. See DOI: 10.1039/c7ta01740e

The effects of cold and hot processing on the performance of polymer–fullerene solar cells are investigated for polymers designed to exhibit temperature-dependent aggregation in solution via second-position branched alkyl side chains.


S3
tungsten-halogen lamp. The 1 sun conditions were provided by the use of a 730 nm LED (Thorlabs) at different intensities for appropriate bias illumination. A calibrated silicon cell was used as reference prior to the J−V and EQE measurements. Thermal evaporation of the back electrode and J−V measurements were both performed inside a nitrogen filled glovebox. For the EQE measurements, the photovoltaic devices were encapsulated in a nitrogen filled box with a quartz window. The active layer thickness was determined on a Veeco Dektak150 profilometer.
Hole-only devices with an active area of 0.09 and 0.16 cm² were fabricated in air on patterned indium tin oxide (ITO) glass substrates (Naranjo Substrates). The substrates were cleaned with the same procedure as mentioned before for the solar cell devices. Poly(ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) (Clevios P, VP Al4083) was then spin coated at 3000 rpm to form a 40 nm layer. The active layers were then cast under the same conditions as for the solar cell devices. By varying the spin speed, thicknesses from 120 to 250 nm were obtained. 10 nm of MoO3 and 100 nm of Ag were then deposited by thermal evaporation under high vacuum (~3 × 10 −7 mbar). Current density -voltage (J−V ) characteristics were measured with a Keithley 2400 source meter, after illumination of the cell with UV light to dope the MoO3 for at least 10 minutes, sweeping from 0 to 6 V. For each type of active layer, three layers of different thicknesses were cast and measured. No thickness dependence of the mobility was observed. The mobility was determined by fitting the J−V curves with the Mott-Gurney square law, using an empirical series resistance correction, which varied between 5 and 20 Ω. The published hole mobility is the average of 12 individual devices using three active layer thicknesses.

Synthesis
Scheme S1 shows the synthesis route to the four DPP polymers. In the subsequent sections the synthetic procedures and molecular characterization are provided.

D-PDDP3T-EH
Freshly recrystallized 9 (43.0 mg, 105 μmol), monomer 8a (107 mg, 105 μmol), recrystallized triphenylphosphine (1.65 mg, 6.30 μmol), and Pd2(dba)3 (1.44 mg, 1.57 μmol) were placed inside a dried Schlenk tube and placed under argon. Toluene (1.8 mL) and DMF (0.2 mL) were added and the mixture was degassed with argon at 40 °C. The mixture was then reacted at 115 °C overnight. The viscous polymer solution was dissolved in warm CHCl3 and precipitated in methanol. The polymer was dissolved in chloroform with ethylenediaminetetraacetic acid (EDTA) and refluxed during one hour. Water was added, refluxed for one hour and subsequently the organic layer was washed with water. The organic layer was concentrated under reduced pressure and the polymer precipitated in methanol. The polymer was then subjected to Soxhlet extraction with acetone, hexane, dichloromethane, and chloroform. Finally, the purified polymer was precipitated in methanol. CHCl3 fraction: 98 mg, yield: 99%. GPC (o-DCB, 140 °C): Mn = 39.0 kDa, Mw = 90.0 kDa, PDI = 2.31.

D-PDDP4T-EH
Freshly recrystallized 10 (33.8 mg, 68.7 μmol), 8b (70.0 mg, 68.7 μmol), recrystallized triphenylphosphine (1.08 mg, 4.12 μmol)) and Pd2(dba)3 (0.943 mg, 1.03 μmol) were placed under Ar in a dried Schlenk tube. Toluene (1.8 mL) and DMF (0.2 mL) were added and the mixture was degassed with argon at 40 °C. The mixture was then reacted at 115 °C overnight. The viscous polymer solution was dissolved in warm 1,1,2,2-tetrachloroethane (TCE) and precipitated in methanol. The polymer was dissolved in chloroform with ethylenediaminetetraacetic acid (EDTA) and refluxed during one hour. Water was added, refluxed for one hour and subsequently the organic layer was washed with water. The organic layer was concentrated under reduced pressure and the polymer precipitated in methanol. The precipitated polymer was then subjected to Soxhlet extraction with acetone, hexane, dichloromethane and chloroform. Swollen polymer remaining inside the Soxhlet thimble was dissolved in hot TCE and the solution was filtrated over a heated cellulose filter. Finally, the TCE solution was concentrated in vacuo and the purified polymer was precipitated in acetone.

UV-vis-NIR measurements in o-dichlorobenzene (o-OCB)
UV-vis-NIR spectra were recorded for the four polymers in o-OCB at different temperatures ( Figure S3). o-OCB is a less good solvent for these polymers than chloroform or TCE. With increasing temperature of o-OCB D-PDPP3T-EH, D-PDPP4T-EH, and D-PDPP4T-HD show a more or less gradual loss of the long wavelength peak and its vibronic structure. At ~60 °C D-PDPP3T-EH, D-PDPP4T-HD, reach a state in which the polymer chains are truly molecularly dissolved. For D-PDPP4T-EH a small shoulder around 800 nm remained until 100 °C, which agrees with the lower solubility of this material. For EH-PDPP4T-EH the results ( Figure S3d) were different from the other three polymers. At room temperature the material is sparsely soluble in o-DCB and when conducting the experiment from high to low temperature the polymer precipitated in its aggregated state in the cuvette, resulting in a loss of signal at lower temperatures.   Table 2 of the main text are based on 12 individual devices using three different active layer thicknesses.   The Jsc values were based on integration of the EQE (measured with 1 sun bias illumination) with the AM1.5G spectrum. b The PCE is determined from the EQE integrated Jsc. All devices were made on the same day from the same solutions.