Tailoring the Nature of Interface States in Efficient and Stable Bilayer Organic Solar Cells by a Transfer‐Printing Technique

Abstract

to planar bilayers [2] and further on to the bulk-heterojunction (BHJ) structure. [3] In recent years, BHJ OSCs based on nonfullerene acceptors (NFAs) have made great progress in device efficiency and exceeded 19%. [4,5] However, the use of blended electron-donating and electronaccepting semiconductors in the BHJ concept leads to the most complex challenge of managing microstructure disorder in nanocrystalline semiconductor composites. [6,7] A precise control of separated domains and intermixed regions in the BHJ architecture remains a challenge, even though various strategies were developed to tune the BHJ microstructure. [8][9][10] Moreover, the formed microstructure in the BHJ blend is typically optimized for achieving high efficiency, which is generally away from its equilibrium state. The diffusion and re-organization of molecular acceptors in the blend films are detrimental factors limiting both, charge generation as well as inherent stability of BHJ devices. [8] Thus, it is imperative to redefine the ideal morphology of an active layer at the molecular scale, given that the exciton diffusion length (L D ) Bulk heterojunction organic solar cells (BHJ-OSCs) are designed to overcome inefficient charge generation and limited exciton diffusion of organic semiconductors. However, there are some disadvantages involving inherent voltage losses, microstructure control, and operational instability. Bilayer solar cells (BL-OSCs) without mixed regions are free from these problems and offer a platform to explore the photophysical processes at the donoracceptor interface. Here, a breakthrough for n-i-p BL-OSCs in charge generation efficiency is reported. A gradual adjustment of the molecular interface orientation of the polymer donor (PM6) is accompanied by the evolution of charge-transfer states and Forster energy transfer. Besides, less recombination losses and superior morphological stability of BL-OSCs are achieved at a PCE comparable to that of BHJ-OSCs with similar layer thickness. This investigation confirms the feasibility of manufacturing BL-OSCs by a transfer printing method and provides a versatile architecture to study fundamental interface phenomena in OSCs independent from microstructure disorder.The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/admi.202200342.

Organic Planar Heterojunction Solar Cells and Photodetectors Tailored to the Exciton Diffusion Length Scale of a Non‐Fullerene Acceptor

“…The efficiency of BHJ solar cells now approaches 19%, [ 5a ] and quasi‐PHJ designs by sequential D/A deposition approach 18% power conversion efficiency. [ 20b ] To the best of our knowledge, efficient bilayer PHJ solar cells with NFAs have only been reported with our previous IT4F/PM6 PHJs, [ 12a,16 ] except as vacuum‐deposited bilayers. [ 14,32 ] BHJ and quasi‐PHJ (double spin coating) [ 28a,33 ] organic photodetectors have been reported with a detectivity ≈10 13 cm Hz 1/2 W −1 , [ 34 ] but with an intentionally thick active layer to achieve a lower dark current; [ 28b ] such devices showed lower extracted photocurrent current densities due to their thicker active layer compared to the optimized thickness for photogeneration.…”

“…Herein, we exploit a recently reported “film transfer” technique, [ 12a ] to fabricate sharp organic PHJs, which have been demonstrated to be more stable than equivalent BHJs under thermal stress, [ 16 ] and investigate their performance as organic solar cells and photodetectors. The study herein focuses on organic PHJ devices tailored to the exciton diffusion length scale of a state‐of‐the‐art NFA: IT4F.…”

“…[14] Studies of solution processed sharp PHJs have to date been very limited due to the difficulty in preventing intermixing during the deposition of the second layer. [4b,c,12a,15] Herein, we exploit a recently reported "film transfer" technique, [12a] to fabricate sharp organic PHJs, which have been demonstrated to be more stable than equivalent BHJs under thermal stress, [16] and investigate their performance as organic solar cells and photodetectors. The study herein focuses on organic PHJ devices tailored to the exciton diffusion length scale of a state-of-the-art NFA: IT4F.…”

Maximizing Performance and Stability of Organic Solar Cells at Low Driving Force for Charge Separation

“…Wang and co‐workers have systematically varied the donor morphology of BL devices of PM6:IT‐4F, [ 100 ] as observed by optical spectroscopy and GIWAXS measurements, comparing the resulting Fourier Transform Photocurrent spectra (FTPS). The additive 1,8‐diiodooctane (DIO) was used to control the order in the PM6 phase, while the order of the IT‐4F phase was controlled by thermal annealing prior to transferring the PM6 layer and kept constant for both films.…”

“…Position and evolution of photocurrent generation by the interfacial CT state is indicated by a dashed arrow. Reproduced with permission [100]. Copyright 2022, Wiley-VCH.…”