Absence of Charge Transfer State Enables Very Low VOC Losses in SWCNT:Fullerene Solar Cells

Current state‐of‐the‐art organic solar cells (OSCs) still suffer from high losses of open‐circuit voltage (V_OC). Conventional polymer:fullerene solar cells usually exhibit bandgap to V_OC losses greater than 0.8 V. Here a detailed investigation of V_OC is presented for solution‐processed OSCs based on (6,5) single‐walled carbon nanotube (SWCNT): 6,6]‐phenyl‐C₇₁‐butyric acid methyl ester active layers. Considering the very small optical bandgap of only 1.22 eV of (6,5) SWCNTs, a high V_OC of 0.59 V leading to a low E_gap/q ? V_OC = 0.63 V loss is observed. The low voltage losses are partly due to the lack of a measurable charge transfer state and partly due to the narrow absorption edge of SWCNTs. Consequently, V_OC losses attributed to a broadening of the band edge are very small, resulting in V_OC,SQ ? V_OC,rad = 0.12 V. Interestingly, this loss is mainly caused by minor amounts of SWCNTs with smaller bandgaps as well as (6,5) SWCNT trions, all of which are experimentally well resolved employing Fourier transform photocurrent spectroscopy. In addition, the low losses due to band edge broadening, a very low voltage loss are also found due to nonradiative recombination, ΔV_OC,nonrad = 0.26 V, which is exceptional for fullerene‐based OSCs.