High-Performance Ternary Organic Solar Cell Enabled by a Thick Active Layer Containing a Liquid Crystalline Small Molecule Donor

Ternary organic solar cells (OSCs) have attracted much research attention in the past few years, as ternary organic blends can broaden the absorption range of OSCs without the use of complicated tandem cell structures. Despite their broadened absorption range, the light harvesting capability of ternary OSCs is still limited because most ternary OSCs use thin active layers of about 100 nm in thickness, which is not sufficient to absorb all photons in their spectral range and may also cause problems for future roll-to-roll mass production that requires thick active layers. In this paper, we report a highly efficient ternary OSC (11.40%) obtained by incorporating a nematic liquid crystalline small molecule (named benzodithiophene terthiophene rhodanine (BTR)) into a state-of-the-art PTB7-Th:PC71BM binary system. The addition of BTR into PTB7-Th:PC71BM was found to improve the morphology of the blend film with decreased π–π stacking distance, enlarged coherence length, and enhanced domain purity. This resulted in more efficient charge separation, faster charge transport, and less bimolecular recombination, which, when combined, led to better device performance even with thick active layers. Our results show that the introduction of highly crystalline small molecule donors into ternary OSCs is an effective means to enhance the charge transport and thus increase the active layer thickness of ternary OSCs to make them more suitable for roll-to-roll production than previous thinner devices

Enhanced Photovoltaic Performance of Ternary Polymer Solar Cells by Incorporation of a Narrow-Bandgap Nonfullerene Acceptor

We developed a novel nonfullerene electron acceptor, IffBR, that consists of electron-rich indaceno­[1,2-<i>b</i>:5,6-<i>b</i>′]­dithiophene as the central unit and an electron-deficient 5,6-difluorobenzo­[<i>c</i>]­[1,2,5]­thiadiazole unit flanked with rhodanine as the peripheral group. IffBR exhibits peak UV–vis absorbance at 658 nm, which is complementary with the absorption profiles of the wide-bandgap conjugated polymers poly­[4,8-bis­(4,5-dihexylthiophen-2-yl)­benzo­[1,2-<i>b</i>:4,5-<i>b</i>′]-dithiophene-<i>alt</i>-2-(2-butyloctyl)-5,6-difluoro-4,7-di­(thiophen-2-yl)-2<i>H</i>-benzo­[<i>d</i>]­[1,2,3]­triazole] (PBTA-BO) and the fullerene acceptor [6,6]-phenyl-C71-butyric acid methyl ester (PC₇₁BM). The ternary device constructed with PBTA-BO/PC₇₁BM/IffBR as the light-absorption layer exhibited significantly better photovoltaic performance than those obtained from devices based on a bulk-heterojunction layer comprised of binary components. This improvement was attributed to the broadened absorbance, formation of cascade charge-transfer pathways, reduced nongeminate recombination, enhanced charge extraction, and more favorable morphologies of the bulk-heterojunction films. The optimized ternary device exhibited a power conversion efficiency of 9.06%, which is significantly higher than those of binary devices based on either PBTA-BO/IffBR (6.24%) or PBTA-BO/PC₇₁BM (4.73%). These results indicate that IffBR is an outstanding electron acceptor, suitable for the fabrication of nonfullerene or multicomponent-blend polymer solar cells