Recent progress of doping strategies for organic hole transport materials in perovskite solar cells

Organic-inorganic hybrid perovskite materials demonstrate outstanding optoelectronic properties. including high light absorption capacity, ambipolar charge transport, high defects tolerance. The application of perovskite materials in solar cells shows impressive progress by increasing the power conversion efficiencies (PCEs) from 3.8% to over 25%. Moreover, perovskite solar cells (PSCs) have the advantages of low materials cost and simple fabrication, giving rise to high potentials for future applications. The fast improvement in the photovoltaic performance of PSCs is mainly attributed to the intensive optimization in perovskite composition, device architecture, defects passivation, contact materials and perovskite crystallization kinetics. Contact materials, especially hole transport materials (HTMs), are key components in PSCs, which can not only affect the PCEs of PSCs, but also greatly influence the stability of devices. 2,2',7,7'-Tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9'-spirobifluorerc (spiro-OMeTAD) is one of the most popularly used HTMs in PSCs and widely seen as a standard HTM for comparison. However, the synthetic process of spiro-OMeTAD is tedious and complex, leading to a high material cost. Moreover, spiro-OMeTAD has relatively low mobility, and chemical doping is one of the most efficient ways to improve the hole conductivity. So far different types of inorganic and organic dopants have been reported and their effects on photovoltaic performance are investigated. On the one hand, the introduction of chemical dopants can induce extra positive charges in HTMs and therefore enhance the conductivity. In this respect, doping efficiency is one critical factor for developing new dopants. On the other hand, research shows that chemical dopants can induce side effects in stability of HTMs and interfaces, limiting the lifetime of the PSCs devices. In this review, we first discuss the categories of HTMs and their specific advantages. Compared with inorganic HTMs, organic HTMs attract great attention because of high structural tunability, low cost and solution processability. However, organic molecules generally have low mobility, limiting their charge transport capacity. Therefore, doping by inducing redox reaction is the most effective way to enhance the conductivity of organic materials. Also. the doping mechanism slightly varies with different dopants and doping procedures are different. We focus on the progress of different chemical dopants in PSCs, and their roles and doping efficiencies in HTMs are briefly compared. We use spiro-OMeTAD as a reference HTM and conclude the influences of dopants on the conductivity and photovoltaic parameters in PSCs. The dopants include metal-based salts. ionic liquid and other molecules. Therein dopants based on lithium bis-trifluoromethanesulfonimide (LiTFSI) and 4-tert-butylpyridine (TBP) are widely used in organic HTMs. but recent work shows that these additives have deleterious effects on device stability. Specifically, it is found LiTFSI migrates and accumulates at interfaces under the conditions of bias potentials. TBP has a relatively low boiling point and can easily evaporate at high temperatures, and also causes corrosion at perovskite interfaces. Therefore. developing alternative dopants and investigating their stability are crucial for future scalable applications in PSCs. Finally, we discuss the potentials of dopants in terms of future applications, and believe that developing new dopants with high stability is highly desired. Alternatively, designing efficient dopant-free HTMs is another strategy for obtaining stable HTMs for PSCs.