The conversion efficiency of solar energy in semiconductors is fundamentally limited by ultrafast hot-carrier relaxation processes, and slowing down these processes is critical for improved energy harvesting. Here we report formamidinium tin iodide (FASnI₃) nanocrystals where quantum confinement effects yield an evolution from a continuous band structure to separate energy states with decreasing nanocrystal size, as observed by transient absorption spectroscopy. The appearance of separate energy levels slows down the relaxation of hot carriers by two orders of magnitude at low injected carrier densities (<1 carrier pair per nanoparticle). The observed build up time of the ground-state bleach at the band edge is two orders of magnitude slower in FASnI₃ nanocrystals than in lead halide perovskite bulk and nanocrystals, which we attribute to a phonon bottleneck effect. Our results highlight the promise of lead-free perovskite nanocrystals for high-efficiency photovoltaic applications operating above the Shockley–Queisser limit.

半导体中太阳能的转换效率从根本上受到超快热载流子弛豫过程的限制，而减慢这些过程对于改进能量收集至关重要。在这里，我们报告了甲脒碘化锡 (FASnI ₃ ) 纳米晶体，其中量子限制效应产生了从连续带结构到随着纳米晶体尺寸减小而分离的能态的演变，正如瞬态吸收光谱所观察到的那样。在低注入载流子密度（每个纳米粒子<1个载流子对）下，单独能级的出现将热载流子的弛豫减慢了两个数量级。_{在 FASnI 3}中观察到的基态漂白剂在带边缘的形成时间慢了两个数量级纳米晶体而不是卤化铅钙钛矿块体和纳米晶体，我们将其归因于声子瓶颈效应。我们的结果突出了无铅钙钛矿纳米晶体在 Shockley-Queisser 极限以上运行的高效光伏应用的前景。