Efficient indoor dye-sensitized solar cells based on concerted companion organic dyes: optimization of the indoline donors

On the basis of our previous report on indoor light energy harvesting by solar cells sensitized with a concerted companion organic dye CCOD-2 which consists of two sub-dye units with triphenylamine and fluorenyl indoline donors in the longer and shorter units, respectively, dye CCOD-5 has been synthesized in this work by replacing the donors in CCOD-2 with two strongly electron-donating and dihexyloxyphenyl substituted fluorenyl indoline donors with the purpose to strengthen the intramolecular charge transfer (ICT) effect and enhance the anti-aggregation capability, and thus simultaneously enhance the short-circuit current density (JSC) and open-circuit voltage (VOC). As a result, CCOD-5 exhibits improved spectral overlap with the emission of the T5 lamps and an enhanced JSC of 0.454 mA·cm-2 under 2500 lux T5 illumination. However, the bulky donor within the short sub-dye unit causes severe steric hindrance and worsens the anchoring of the dye, leading to a lowered VOC of 671 mV and a modest photoelectric conversion efficiency (PCE) of 28.27%. Hence, the bulky fluorenyl indoline donor in the short sub-dye unit of CCOD-5 was replaced by the simple counterpart to afford CCOD-6. As a result, CCOD-6 exhibits a higher loading amount and better anchoring on TiO2 according to DFT calculations, achieving enhanced JSC, VOC and PCE of 0.465 mA·cm-2, 708 mV and 31.34%, respectively, outperforming the efficiencies of both CCOD-5 (28.27%) and CCOD-2 (28.02%). Under AM 1.5 simulated sunlight, CCOD-6 also achieved the highest PCE of 11.14% among the three dyes. These results demonstrate that rational design of CCOD dyes utilizing fluorenyl indoline-based electron donors with optimized sizes is an effective strategy for improving the photovoltaic performance of DSSCs for harvesting energy from various light sources, especially indoor dim light.