Efficiency Improvement of Semitransparent Polymer Solar Cells with Invariable Color Render Index

The color render index (CRI) and the power conversion efficiency (PCE) are two critical parameters of semitransparent polymer solar cells (PSCs) which are contradictory. The CRI is strongly dependent on the absorption of the polymer:fullerene active layer. The PCE not only relies on absorption but also on a bulk heterojunction structure. Here, the 1,8-diiodooctane (DIO) additive has been added to PCDTBT:PC71BM blend PSCs to improve the PCE with almost unchanged CRI. Based on the higher boiling point than the host solvent o-dichlorobenzene (ODB) and the better solubility of PC71BM, the device photovoltaic properties with DIO additive were obviously changed. The PCE improvement is attributed to the charge recombination and transportation process which means that the utilization of the absorbed photons is improved. However, this improvement cannot affect the transmitted light. Hence, the CRI of transmitted light is nearly invariable. With 3% v/v DIO, the short-circuit current density (Jsc), open circuit voltage (Voc), and fill factor (FF) are all increased. Correspondingly, a highest PCE is achieved of 6.15%, while the reference device without DIO only has a PCE of 5.23%. At the same time, the CRI is slightly changed from 82 to 81.