Effect of Quencher, Geometry, and Light Outcoupling on the Determination of Exciton Diffusion Length in Nonfullerene Acceptors

The correct determination of the exciton diffusion length (LD) in novel organic photovoltaics (OPV) materials is an important, albeit challenging, task required to understand these systems. Herein, a high-throughput approach to probe LD in nonfullerene acceptors (NFAs) is reported, that builds upon the conventional photoluminescence (PL) surface quenching method using NFA layers with a graded thickness variation in combination with spectroscopic PL mapping. The method is explored for two archetypal NFAs, namely, ITIC and IT-4F, using PEDOT:PSS and the donor polymer PM6 as two distinct and practically relevant quencher materials. Interestingly, conventional analysis of quenching efficiency as a function of acceptor layer thickness results in a threefold difference in LD values depending on the specific quencher. This discrepancy can be reconciled by accounting for the differences in light in- and outcoupling efficiency for different multilayer architectures. In particular, it is shown that the analysis of glass/acceptor/PM6 structures results in a major overestimation of LD, whereas glass/acceptor/PEDOT:PSS structures give no significant contribution to outcoupling, yielding LD values of 6−12 and 8−18 nm for ITIC and IT-4F, respectively. Hence, practical guidelines for quencher choice, sample geometries, and analysis approach for the accurate assessment of LD are provided.V.B., A.P., and J.G. contributed equally to this work. The authors acknowledge that this research was financially supported by the European Research Council (ERC) under grant agreement no. 648901. The authors also acknowledge financial support from the Spanish Ministry of Science and Innovation through the Severo Ochoa Program for Centers of Excellence in R&D (CEX2019-000917-S) and project PGC2018-095411-B-I00. This publication was based upon work supported by the King Abdullah University of Science and Technology (KAUST) Office of Sponsored Research (OSR) under award no: OSR-2018-CARF/CCF-3079 and award no. OSR-CRG2018-3746. The authors thank Anastasia Ragulskaya (The University of Tübingen) for contributing to the development of the computational model.Peer reviewe

Efficient Hybrid Amorphous Silicon/Organic Tandem Solar Cells Enabled by Near-Infrared Absorbing Nonfullerene Acceptors

Monolithically stacked tandem solar cells present opportunities to absorb more of the sun's radiation while reducing the degree of energetic loss through thermalization. In these applications, the bandgap of the tandem's constituent subcells must be carefully adjusted so as to avoid competition for photons. Organic photovoltaics based on nonfullerene acceptors (NFAs) have recently exploded in popularity owing to the ease with which their electrical and optical properties can be tuned through chemistry. Here, highly complementary and efficient 2-terminal tandem solar cells are reported based on a wide bandgap amorphous silicon absorber, and a narrow bandgap NFA bulk-heterojunction with power conversion efficiencies (PCEs) exceeding 15%. Interface engineering of this tandem device allows for high PCEs across a wide range of light intensities both above and below “1 sun.” Furthermore, the addition of an inorganic silicon subcell enhances the operational stability of the tandem by reducing the light-stress experienced by the bulk heterojunction, resolving a long-standing stumbling block in organic photovoltaic research

Rationalizing the influence of tunable energy levels on quantum efficiency to design optimal non-fullerene acceptor-based ternary organic solar cells

Non-fullerene acceptor (NFA)-based ternary bulk heterojunction solar cells (TSC) are the most efficient organic solar cells (OSCs) today due to their broader absorption and quantum efficiencies (QE) often surpassing those of corresponding binary blends. We study how the energetics driving charge transfer at the electron donor:electron acceptor (D/A) interfaces impact the QE in blends of PBDB-T-2F donor with several pairs of lower bandgap NFAs. As in binary blends, the ionization energy offset between donor and acceptor ({\Delta}IE) controls the QE and maximizes for {\Delta}IE > 0.5 eV. However, {\Delta}IE is not controlled by the individual NFAs IEs but by their average, weighted for their blending ratio. Using this property, we improved the QE of a PBDB-T-2F:IEICO binary blend that had an insufficient {\Delta}IE for charge generation by adding a deep IE third component: IT-4F. Combining two NFAs enables to optimize the D/A energy alignment and cells' QE without molecular engineering.Comment: S Karuthedath and S H K Paleti contributed equally. MS: 35 pages, 9 figures. SI: 21 pages, 23 figures - updates: added a model scheme as Fig1, updated T1 EQE and blends PL in Fig 2 (former 1). Added Figure (5): charge generation upon acceptor excitation. Corrected {\Delta}IEs in fig7 and 9 (a few tens of meV off). Added more blends to show generality (2 groups, 10 blends compositions