Efficient Approach for Improving the Performance of Nonhalogenated Green Solvent-Processed Polymer Solar Cells via Ternary-Blend Strategy.

The ternary-blend approach has the potential to enhance the power conversion efficiencies (PCEs) of polymer solar cells (PSCs) by providing complementary absorption and efficient charge generation. Unfortunately, most PSCs are processed with toxic halogenated solvents, which are harmful to human health and the environment. Herein, we report the addition of a nonfullerene electron acceptor 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3- d:2',3'- d']- s-indaceno[1,2- b:5,6- b']dithiophene (ITIC) to a binary blend (poly[4,8-bis(2-(4-(2-ethylhexyloxy)3-fluorophenyl)-5-thienyl)benzo[1,2- b:4,5- b']dithiophene- alt-1,3-bis(4-octylthien-2-yl)-5-(2-ethylhexyl)thieno[3,4- c]pyrrole-4,6-dione] (P1):[6,6]-phenyl-C71-butyric acid methyl ester (PC71BM), PCE = 8.07%) to produce an efficient nonhalogenated green solvent-processed ternary PSC system with a high PCE of 10.11%. The estimated wetting coefficient value (0.086) for the ternary blend suggests that ITIC could be located at the P1:PC71BM interface, resulting in efficient charge generation and charge transport. In addition, the improved current density, sustained open-circuit voltage and PCE of the optimized ternary PSCs were highly correlated with their better external quantum efficiency response and flat-band potential value obtained from the Mott-Schottky analysis. In addition, the ternary PSCs also showed excellent ambient stability over 720 h. Therefore, our results demonstrate the combination of fullerene and nonfullerene acceptors in ternary blend as an efficient approach to improve the performance of eco-friendly solvent-processed PSCs with long-term stability.

Triazine-based Polyelectrolyte as an Efficient Cathode Interfacial Material for Polymer Solar Cells.

A novel polyelectrolyte containing triazine (TAZ) and benzodithiophene (BDT) scaffolds with polar phosphine oxide (P═O) and quaternary ammonium ions as pendant groups, respectively, in the polymer backbone (PBTAZPOBr) was synthesized to use it as a cathode interfacial layer (CIL) for polymer solar cell (PSC) application. Owing to the high electron affinity of the TAZ unit and P═O group, PBTAZPOBr could behave as an effective electron transport material. Due to the polar quaternary ammonium and P═O groups, the interfacial dipole moment created by PBTAZPOBr substantially reduced the work function of the metal cathode to afford better energy alignment in the device, thus enabling electron extraction and reducing recombination of excitons at the photoactive layer/cathode interface. Consequently, the PSC devices based on the poly[4,8-bis(2-ethylhexyloxyl)benzo[1,2-b:4,5-b']dithiophene-2,6-diyl-alt-ethylhexyl-3-fluorothithieno[3,4-b]thiophene-2-carboxylate-4,6-diyl]:[6,6]-phenyl-C71-butyric acid methyl ester (PTB7:PC71BM) system with PBTAZPOBr as CIL displayed simultaneously enhanced open-circuit voltage, short-circuit current density, and fill factor, whereas the power conversion efficiency increased from 5.42% to 8.04% compared to that of the pristine Al device. The outstanding performance of PBTAZPOBr is attributed not only to the polar pendant groups of BDT unit but also to the TAZ unit linked with the P═O group of PBTAZPOBr, demonstrating that functionalized TAZ building blocks are very promising cathode interfacial materials (CIMs). The design strategy proposed in this work will be helpful to develop more efficient CIMs for high performance PSCs in the future.