@article{365dbe0b77874576b567540a3c9e6a25,
title = "Impact of Nonfullerene Acceptor Core Structure on the Photophysics and Efficiency of Polymer Solar Cells",
abstract = "Small-molecule {"}nonfullerene{"} acceptors are promising alternatives to fullerene (PC61/71BM) derivatives often used in bulk heterojunction (BHJ) organic solar cells; yet, the efficiency-limiting processes and their dependence on the acceptor structure are not clearly understood. Here, we investigate the impact of the acceptor core structure (cyclopenta-[2,1-b:3,4-b′]dithiophene (CDT) versus indacenodithiophene (IDTT)) of malononitrile (BM)-terminated acceptors, namely CDTBM and IDTTBM, on the photophysical characteristics of BHJ solar cells. Using PCE10 as donor polymer, the IDTT-based acceptor achieves power conversion efficiencies (8.4%) that are higher than those of the CDT-based acceptor (5.6%) because of a concurrent increase in short-circuit current and open-circuit voltage. Using (ultra)fast transient spectroscopy we demonstrate that reduced geminate recombination in PCE10:IDTTBM blends is the reason for the difference in short-circuit currents. External quantum efficiency measurements indicate that the higher energy of interfacial charge-transfer states observed for the IDTT-based acceptor blends is the origin of the higher open-circuit voltage.",
author = "Alamoudi, {Maha A.} and Khan, {Jafar I.} and Yuliar Firdaus and Kai Wang and Denis Andrienko and Beaujuge, {Pierre M.} and Fr{\'e}d{\'e}ric Laquai",
note = "Funding Information: for a PCE10:CDTBM blend. The TA spectrum consists of the ground-state bleach (GSB) spanning the region from 1.6 to 2.2 eV and a broad photoinduced absorption (PA) in the spectral range of 0.95−1.35 eV peaking 3 ps after excitation. The decay of the PA was found to be faster in the spectral region of 1.2− 1.35 eV compared to the spectral region of 0.95−1.15 eV, indicating that different states contribute to the PA. We assign the fast decay observed in the region between 1.2 and 1.35 eV to quenching and dissociation of CDTBM singlet excitons, while the slower decay is attributed to ps−ns charge recombination. This assignment is supported by TA experiments on thin films of the neat acceptor CDTBM as shown in Figure 4b and by the spectra of neat PCE10 shown in Figure S9. A broad and unstructured ground-state bleach band in the region of the CDTBM absorption is apparent in the neat film, which is different in shape from the GSB observed in the blend; however, it matches the TA observed from neat PCE10. Furthermore, the PA band observed for the neat CDTBM film is significantly narrower compared to the PA of neat PCE10 and the blend and it peaks around 1.22 eV. Here, we assign the PA in the region of 1.2−1.35 eV to exciton-induced absorption of CDTBM. When the spectra of the neat CDTBM film and the PCE10:CDTBM blend are compared, it becomes clear that the faster decaying PA feature in the blend is at the same spectral position as the exciton-induced absorption observed in the neat CDTBM film, hence the assignment of this part of the PA to excitons. We note that in the blend, the excitonic PA feature vanishes after 300−450 ps, and thereafter the remaining PA signal is solely attributed to charge-induced absorption. Overall, 35% of the initial signal amplitude remains after 8 ns in the blend. We also note that a significant contribution to the blend{\textquoteright}s PA stems from PCE10 cations, which is supported by comparing the TA spectra to the absorption spectra of chemically oxidized PCE10 films as shown in Figure S10. In fact, the overall PA band is composed of PCE10 cation-induced absorption and CDTBM anion-induced absorption, as further confirmed from the anion-induced absorption of CDTBM of chemically reduced films of neat CDTBM. Publisher Copyright: {\textcopyright} 2018 American Chemical Society.",
year = "2018",
month = apr,
day = "13",
doi = "10.1021/acsenergylett.8b00045",
language = "English (US)",
volume = "3",
pages = "802--811",
journal = "ACS Energy Letters",
issn = "2380-8195",
publisher = "AMER CHEMICAL SOC",
number = "4",
}
