Advances in Solution‐Processed Multijunction Organic Solar Cells

The efficiency of organic solar cells can benefit from multijunction device architectures, in which energy losses are substantially reduced. Herein, recent developments in the field of solution‐processed multijunction organic solar cells are described. Recently, various strategies have been investigated and implemented to improve the performance of these devices. Next to developing new materials and processing methods for the photoactive and interconnecting layers, specific layers or stacks are designed to increase light absorption and improve the photocurrent by utilizing optical interference effects. These activities have resulted in power conversion efficiencies that approach those of modern thin film photovoltaic technologies. Multijunction cells require more elaborate and intricate characterization procedures to establish their efficiency correctly and a critical view on the results and new insights in this matter are discussed. Application of multijunction cells in photoelectrochemical water splitting and upscaling toward a commercial technology is briefly addressed.     

Efficient Organic Solar Cells with Non‐Fullerene Acceptors

Fullerene‐free OSCs employing n‐type small molecules or polymers as the acceptors have recently experienced a rapid rise with efficiencies exceeding 12%. Owing to the good optoelectronic and morphological tunabilities, non‐fullerene acceptors exhibit great potential for realizing high‐performance and practical OSCs. In this Review, recent exciting progress made in developing highly efficient non‐fullerene acceptors is summarized, mainly correlating factors like absorption, energy loss and morphology of new materials to their correspondent photovoltaic performance.     

Two Well‐Miscible Acceptors Work as One for Efficient Fullerene‐Free Organic Solar Cells

High‐performance ternary organic solar cells are fabricated by using a wide‐bandgap polymer donor (bithienyl‐benzodithiophene‐alt‐fluorobenzotriazole copolymer, J52) and two well‐miscible nonfullerene acceptors, methyl‐modified nonfullerene acceptor (IT‐M) and 2,2′‐((2Z ,2′Z )‐((5,5′‐(4,4,9,9‐tetrakis(4‐hexylphenyl)‐4,9‐dihydros‐indaceno[1,2‐b :5,6‐b ′]dithiophene‐2,7‐diyl)bis(4‐((2‐ethylhexyl)oxy)thiophene‐5,2‐diyl))bis(methanylylidene))bis(3‐oxo‐2,3‐dihydro‐1H ‐indene‐2,1‐diylidene))dimalononitrile (IEICO). The two acceptors with complementary absorption spectra and similar lowest unoccupied molecular orbital levels show excellent compatibility in the blend due to their very similar chemical structures. Consequently, the obtained ternary organic solar cells (OSC) exhibits a high efficiency of 11.1%, with an enhanced short‐circuit current density of 19.7 mA cm^?2 and a fill factor of 0.668. In this ternary system, broadened absorption, similar output voltages, and compatible morphology are achieved simultaneously, demonstrating a promising strategy to further improve the performance of ternary OSCs.     

High‐Performance Organic Bulk‐Heterojunction Solar Cells Based on Multiple‐Donor or Multiple‐Acceptor Components

Organic solar cells (OSCs) based on bulk heterojunction structures are promising candidates for next‐generation solar cells. However, the narrow absorption bandwidth of organic semiconductors is a critical issue resulting in insufficient usage of the energy from the solar spectrum, and as a result, it hinders performance. Devices based on multiple‐donor or multiple‐acceptor components with complementary absorption spectra provide a solution to address this issue. OSCs based on multiple‐donor or multiple‐acceptor systems have achieved power conversion efficiencies over 12%. Moreover, the introduction of an additional component can further facilitate charge transfer and reduce charge recombination through cascade energy structure and optimized morphology. This progress report provides an overview of the recent progress in OSCs based on multiple‐donor (polymer/polymer, polymer/dye, and polymer/small molecule) or multiple‐acceptor (fullerene/fullerene, fullerene/nonfullerene, and nonfullerene/nonfullerene) components.