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Reduced bimolecular charge recombination in efficient organic solar cells comprising non-fullerene acceptors.

ABSTRACT: Bimolecular charge recombination is one of the most important loss processes in organic solar cells. However, the bimolecular recombination rate in solar cells based on novel non-fullerene acceptors is mostly unclear. Moreover, the origin of the reduced-Langevin recombination rate in bulk heterojunction solar cells in general is still poorly understood. Here, we investigate the bimolecular recombination rate and charge transport in a series of high-performance organic solar cells based on non-fullerene acceptors. From steady-state dark injection measurements and drift-diffusion simulations of the current-voltage characteristics under illumination, Langevin reduction factors of up to over two orders of magnitude are observed. The reduced recombination is essential for the high fill factors of these solar cells. The Langevin reduction factors are observed to correlate with the quadrupole moment of the acceptors, which is responsible for band bending at the donor-acceptor interface, forming a barrier for charge recombination. Overall these results therefore show that suppressed bimolecular recombination is essential for the performance of organic solar cells and provide design rules for novel materials.

SUBMITTER: Wu Y

PROVIDER: S-EPMC10033508 | biostudies-literature | 2023 Mar

REPOSITORIES: biostudies-literature

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Reduced bimolecular charge recombination in efficient organic solar cells comprising non-fullerene acceptors.

Wu Yue Y Li Yungui Y van der Zee Bas B Liu Wenlan W Markina Anastasia A Fan Hongyu H Yang Hang H Cui Chaohua C Li Yongfang Y Blom Paul W M PWM Andrienko Denis D Wetzelaer Gert-Jan A H GAH

Scientific reports 20230322 1

Bimolecular charge recombination is one of the most important loss processes in organic solar cells. However, the bimolecular recombination rate in solar cells based on novel non-fullerene acceptors is mostly unclear. Moreover, the origin of the reduced-Langevin recombination rate in bulk heterojunction solar cells in general is still poorly understood. Here, we investigate the bimolecular recombination rate and charge transport in a series of high-performance organic solar cells based on non-fu ...[more]

PMID: 36949087

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Project description:The active layer in a solution processed organic photovoltaic device comprises a light absorbing electron donor semiconductor, typically a polymer, and an electron accepting fullerene acceptor. Although there has been huge effort targeted to optimize the absorbing, energetic, and transport properties of the donor material, fullerenes remain as the exclusive electron acceptor in all high performance devices. Very recently, some new non-fullerene acceptors have been demonstrated to outperform fullerenes in comparative devices. This Account describes this progress, discussing molecular design considerations and the structure-property relationships that are emerging. The motivation to replace fullerene acceptors stems from their synthetic inflexibility, leading to constraints in manipulating frontier energy levels, as well as poor absorption in the solar spectrum range, and an inherent tendency to undergo postfabrication crystallization, resulting in device instability. New acceptors have to address these limitations, providing tunable absorption with high extinction coefficients, thus contributing to device photocurrent. The ability to vary and optimize the lowest unoccupied molecular orbital (LUMO) energy level for a specific donor polymer is also an important requirement, ensuring minimal energy loss on electron transfer and as high an internal voltage as possible. Initially perylene diimide acceptors were evaluated as promising acceptor materials. These electron deficient aromatic molecules can exhibit good electron transport, facilitated by close packed herringbone crystal motifs, and their energy levels can be synthetically tuned. The principal drawback of this class of materials, their tendency to crystallize on too large a length scale for an optimal heterojunction nanostructure, has been shown to be overcome through introduction of conformation twisting through steric effects. This has been primarily achieved by coupling two units together, forming dimers with a large intramolecular twist, which suppresses both nucleation and crystal growth. The generic design concept of rotationally symmetrical aromatic small molecules with extended π orbital delocalization, including polyaromatic hydrocarbons, phthalocyanines, etc., has also provided some excellent small molecule acceptors. In most cases, additional electron withdrawing functionality, such as imide or ester groups, can be incorporated to stabilize the LUMO and improve properties. New calamitic acceptors have been developed, where molecular orbital hybridization of electron rich and poor segments can be judiciously employed to precisely control energy levels. Conformation and intermolecular associations can be controlled by peripheral functionalization leading to optimization of crystallization length scales. In particular, the use of rhodanine end groups, coupled electronically through short bridged aromatic chains, has been a successful strategy, with promising device efficiencies attributed to high lying LUMO energy levels and subsequently large open circuit voltages.

Dataset Information

Reduced bimolecular charge recombination in efficient organic solar cells comprising non-fullerene acceptors.

Publications

Reduced bimolecular charge recombination in efficient organic solar cells comprising non-fullerene acceptors.

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