Advances on tin-based perovskite solar cells

Abstract

<p indent=0mm>Organic–inorganic hybrid perovskites have been a focus in the photovoltaic research field due to their remarkable advantages such as tunable direct bandgap, low exciton binding energy, high light harvesting, and so on. The power conversion efficiency (PCE) of lead (Pb)-based perovskite solar cells (PSCs) has been rapidly raised from 3.8% to 25.5% over the decades. With the advantages of a simple preparation process and low production cost, PSCs have great potential in commercialization. However, the toxicity of lead in traditional PSCs limits their large-scale production. Among all lead-free perovskites, the tin-based one inherits the majority of the excellent photoelectric properties of lead-based perovskites. However, the maximum PCE of tin-based PSCs has only exceeded 13%. Although the electronic structure of Sn²⁺ is similar to that of Pb²⁺, it is easy to be oxidized to Sn⁴⁺, and the formation energy of Sn vacancy defects is low, resulting in heavy p-doping in tin-based perovskite films. The background carrier concentrations of tin-based perovskites are as high as <sc>10¹⁸–10²⁰ cm^–3,</sc> which makes photo-generated carriers easy to be recombined. Tin-based perovskites usually experience single-molecule recombination, accompanied by a small amount of bi-molecular recombination, and Auger recombination is negligible. Therefore, reducing recombination in tin-based perovskites is to suppress the single-molecule recombination. At present, additive engineering and component modification strategies are mainly adopted to inhibit carrier recombination in tin-based PSCs. Additive engineering includes tin compensating additives, reducing additives and some other additives which interact with perovskites. Theories and experiments show that adding tin compensating additives in precursor solution can effectively improve the chemical potential of Sn and increase the formation energy of tin vacancies. Although the work mechanism of stannous halide as an additive is not fully understood, it has become an indispensable part to prepare high performance tin-based PSCs. The introduction of reducing additives in the precursor can effectively reduce Sn⁴⁺ and inhibit the oxidation of Sn²⁺, thereby inhibiting the generation of tin vacancies, optimizing the morphology of the film, and improving the efficiency and stability of the device. In addition, some additives which interact with perovskite through coordination bonds, ionic bonds and hydrogen bonds can optimize the nucleation rate of tin-based perovskites and passivate the unsaturated coordination of Sn²⁺ at the grain boundaries. The composition modification of tin-based perovskites mainly includes A-site and X-site substitutions. Since the band structure of tin-based perovskites is mainly determined by Sn and X ions, A-site cation substitution only fine-tunes the bandgap of the perovskite. However, A-site cation substitution can tune the tolerance factor of perovskite, thereby improving the efficiency and stability of perovskite. As the alkyl chain of the A-site cation gradually increases, low-dimensional perovskites will be formed. The low-dimensional perovskite can inhibit the self-doping and ion migration, improve the efficiency and stability of the PSCs. It is worth noting that the current efficient tin-based PSCs are mainly focused on low-dimensional perovskites. In addition, since the np² electrons of halogens directly participate in the construction of the electronic structure, the bandgap of tin-based perovskites can be directly changed by adjusting the X-site anions. Compared with lead-based perovskites, tin-based ones have more suitable bandgap and higher theoretical efficiency. At the same time, the low toxicity of tin-based perovskites is also conducive to their large-scale industrial production. It is believed that with the in-depth research, tin-based perovskite will surely exert its full potential in the photovoltaic field.