Metal halide perovskites (MHPs) have recently emerged as an ideal semiconductor for photovoltaic application. Much of the advantageous properties of perovskite is dominated by its large charge carrier mobility (μ)...

Lead-free halide perovskites, as environment-friendly materials, have received critical interest in photovoltaic applications. In this regard, the bismuth halide perovskites demonstrate better stability under ambient conditions than lead halide perovskites and consequently remain one of the critical areas for the development of lead-free absorber materials. The steady-state optical properties are widely investigated in these bismuth halide perovskites, but excited-state charge carrier dynamics such as hot carrier relaxation remain elusive. However, it is crucial to investigate the rapid relaxation of above band gap “hot” carriers as it restricts the fundamental efficiency limit in the perovskite solar cells. Here, we demonstrate the cation-dependent hot carrier cooling in the lead-free A3Bi2I9 [A = FA (formamidinium), MA (methylammonium), and Cs (cesium)] perovskite by using femtosecond transient absorption spectroscopy. These lead-free perovskites were fabricated from gamma-butyrolactone (γ-GBL) solvent to ensure uniformity and continuity of the as-grown film and were well characterized by XRD, SEM, and steady-state absorption and photoluminescence spectroscopy. With varying A-cations, we observe that the hot-hole relaxation is slowest in the all-inorganic perovskite Cs3Bi2I9 (12.83 ps) and hot electron relaxation is slowest in the hybrid MA3Bi2I9 perovskite (6.42 ps) at the same excitation energy. The observed strong dependence of carrier cooling on cation composition is explained by the interaction between the different organic cations (A = FA, MA, and Cs) with the Pb–Br frameworks. Our study provides an opportunity to understand the effect of cations on the excited-state carrier dynamics, especially the hot carrier relaxation in the bismuth halide perovskites. This will pave the way for designing hot carrier-based high-efficient lead-free perovskite photovoltaic devices.

Organic photovoltaics have received active research interest during the past 30 years due to their low cost, flexibility, easy scalability, and robustness. Recently, several efforts have been made to enhance their power conversion efficiency (PCE) and stability by considering advanced photon harvesting technology, utilization of novel donor–acceptor materials, and optimizing device design strategy. Specifically, the photon multiplication process like singlet fission (SF) and design of novel materials, including low‐bandgap conjugated polymers and non‐fullerene acceptors (NFA), have led to the development of advanced organic photovoltaics with PCE close to theoretical Shockley–Queisser (SQ) limit. Here, an up‐to‐date overview of the recent progress during the last five years in advanced organic photovoltaics with a special focus on emerging techniques and materials was reported. Further, various designing and deployment strategies for these processes and materials were explored along with their properties, challenges, and achievements. Finally, a strategy for the next‐stage research directions was analyzed and proposed that could drive this field even further beyond laboratory research to reach the final goal of commercialization.

There is currently substantial interest in commercializing perovskite solar cells as they offer superior properties over silicon-based solar cells, such as ability for bandgap tuning, higher absorption coefficients, and potentially lower manufacturing costs. However, trap states originating from ionic vacancies, imperfect interfaces, and grain boundaries have hampered their performance and long-term stability during operation. Identifying and quantifying defects in perovskite solar cells becomes inevitable to address these challenges and mitigate the deteriorating effects of these defects. This Review focuses on recent developments in optical and electrical characterization techniques employed for the investigation of defects in halide perovskites and the techniques to understand ion migration in devices. We focus on sample preparation, advantages, limitations, and the nature of information obtained from each of the spectroscopic techniques. This Review will enable the researchers to understand and identify suitable characterization techniques for characterizing defect concentrations and their energetic and spatial distribution in perovskite solar cells.

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