Metallocenium salts as tunable dopants for enhanced efficiency of perovskite solar cells

Abstract

The generation of free carriers through extrinsic doping is essential for transforming the electronic properties of organic semiconductors (OSCs). Doped OSCs play a crucial role in the successful operation of a wide range of electrical and optoelectronic devices, but challenges remain associated with dopant design, such as processability, stability and efficacy. Herein, we introduce a class of versatile p-type dopants based on metallocenium salts with the general formula ([M(C10H10−n)(X)n]+[Y]−) that meet these requirements. Critical to this approach is the ability to independently tune the cation via the redox-active metal cation (M) and the functionality (X) on the cyclopentadiene ring, allowing control over the oxidation strength. Simultaneously, the ability to tune the counter-anion (Y) allows control over the doping efficacy and stability of the resultant doped OSC+ salt. In this study, we systematically investigate the effect of cation and anion structures on the doping of OSCs and elucidate structure–property relationships for dopant design. We unravel the doping mechanism and demonstrate that such dopants can be used to enhance the hole extraction yield by 45% at perovskite/OSC heterojunctions. Perovskite/OSC photoactive layers using metallocenium dopants show significantly increased tolerance to moisture induced degradation as compared to films using conventional LiTFSI based dopants. Finally, we showcase the use of our optimised ferrocenium dopant in n–i–p configuration perovskite solar cells, demonstrating LiTFSI-free and additive-free devices with impressive solar-light to electrical power conversion efficiencies reaching 25.30%.