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The design and synthesis of a stable and efficient hole-transport material (HTM) for perovskite solar cells (PSCs) are one of the most demanding research areas. At present, 2,2′,7,7′-tetrakis­[N,N-di­(4-methoxyphenyl)­amino]-9,9′-spirobifluorene (spiro-MeOTAD) is a commonly used HTM in the fabrication of high-efficiency PSCs; however, its complicated synthesis, addition of a dopant in order to realize the best efficiency, and high cost are major challenges for the further development of PSCs. Herein, various diketopyrrolopyrrole-based small molecules were synthesized with the same backbone but distinct alkyl side-chain substituents (i.e., 2-ethylhexyl-, n-hexyl-, ((methoxyethoxy)­ethoxy)­ethyl-, and (2-((2-methoxyethoxy)­ethoxy)­ethyl)­acetamide, designated as D-1, D-2, D-3, and D-4, respectively) as HTMs. The variation in the alkyl chain has shown obvious effects on the optical and electrochemical properties as well as on the molecular packing and film-forming ability. Consequently, the power conversion efficiency (PCE) of the PSC under one sun illumination (100 mW cm–2) is shown to increase in the order of D-1 (8.32%) < D-2 (11.12%) < D-3 (12.05%) < D-4 (17.64%). Various characterization techniques reveal that the superior performance of D-4 can be ascribed to the well-aligned highest occupied molecular orbital energy level with the counter electrode, the more compact π–π stacking with a higher coherence length, and the excellent hole mobility of 1.09 × 10–3 cm2 V–1 s–1, thus providing excellent energetics for effective charge transport with minimal charge-carrier recombination. Furthermore, the addition of the dopant Li-TFSI in D-4 is shown to deliver a remarkable PCE of 20.19%, along with a short-circuit current density (J SC), open-circuit voltage (V OC), and fill factor (FF) of 22.94 mA cm–2, 1.14 V, and 73.87%, respectively, and superior stability compared to that of other HTMs. These results demonstrate the effectiveness of side-chain engineering for tailoring the properties of HTMs, thus offering new design tactics to fabricate for the synthesis of highly efficient and stable HTMs for PSCs.

Various organic cations (e.g., methylammonium (MA+), butylammonium (BA+), formamidinium (FA+), etc.) have been studied and used in organometallic halide perovskite solar cells (PSCs). Most of the currently used organic cations are protic in nature, which can induce acid–base reactions and, thus, lead to degradation of the perovskites. So far, the role of aprotic cations in PSCs has not been studied much. In the present study, two aprotic cations, namely, trimethylsulfonium (TMS+) and trimethylsulfoxonium (TMSO+), are introduced into lead-based PSCs to form one-dimensional/three-dimensional (TMSPbI3) x ­(MAPbI3)100‑x and (TMSOPbI3)x(MAPbI3)100‑x perovskite structures, respectively. This is shown to provide enhanced performance and moisture resistance, thus, increasing the stability and lifespan of the PSCs. The power conversion efficiencies of the (TMSPbI3) x (MAPbI3)100‑x and (TMSOPbI3) x (MAPbI3)100‑x devices are found to be 19.34 and 19.94%, respectively, compared to 17.11% for the pristine MAPbI3 PSC, along with enhanced open-circuit voltages (V OC) of 1.14 and 1.12 V, respectively, compared to 1.07 V for the pristine MAPbI3 PSC. Furthermore, the effects of TMS+ and TMSO+ upon the perovskite structure, absorption, recombination, and film morphology are discussed in detail. The results of this study will be helpful in the exploration of sulfur-based cations for the development of more stable PSCs.

The presence of hysteresis in perovskite solar cells (PSCs) complicates the reliable evaluation of cell performance for practical applications. Numerous efforts have been made to figure out the reasons behind this phenomenon and to resolve the hysteresis, but it still needs to be explored for better understanding. This chapter is mainly focused on theoretical and experimental studies to reveal the origin of the hysteresis and discuss the remedies to eliminate the hysteric behavior in PSCs. In the last few years, the PSC has emerged as one of the fastest growing photovoltaic technologies that achieved high-power conversion efficiency (&gt;25%) in a short span of time. Despite the high efficiency attained, PSCs suffer from current density-voltage (J-V) hysteresis when J-V characteristics were traced in forward and reverse scans. The presence of hysteresis in PSCs significantly influences the photovoltaic (PV) properties and most importantly device stability. Generally, the hysteric behavior in a PSC arises due to ferroelectric polarization, charge carrier trapping/detrapping, and ion migration in the perovskite materials. A systematic discussion on the key factors involved in the hysteresis generation and how it can be eliminated from PSCs, which includes improvement in morphology by either increasing grain sizes, additive doping, interface engineering, device architecture, etc. On the other hand, the hysteresis can also be positively utilized in other applications such as memristive switching devices.