Two-terminal all-perovskite tandem structures are promising as low-cost yet highly efficient solar cells, but their development is limited by the poor quality of the low bandgap absorber layer. Now, a processing method has been shown to enable the production of uniform, thick tin–lead perovskite layers, which translate into improved photovoltaic parameters.
The method relies on the incorporation of small amounts of chlorine in the precursor solution of the low-bandgap perovskite by mixing lead chloride and lead iodide in the correct ratios before combining it with the tin–iodide solution. The final solution is then processed into perovskite films by spin-coating on a suitable substrate. The uniform crystallization of the perovskite film is facilitated by the deposition of a small amount of diethyl ether during the spin-coating process, followed by a short annealing step at 100 °C. By tuning the concentration and composition of the precursors, the researchers fabricate rather thick (750 nm) Sn–Pb perovskite films with large grains (approximately 750 nm). When these films are employed in single-junction solar cells, they lead to power conversion efficiencies of 18.4%, the best performing low-bandgap perovskite solar cells reported so far. The corresponding open circuit voltage of 0.85 V is close to the radiative limit for a 1.25 eV bandgap semiconductor, indicating that there are very few non-radiative losses. The researchers attribute this to the role of chlorine ions in the perovskite film that passivate potential charge trapping or recombination sites. Zhao, Yan and colleagues then prepared two terminal (2T) tandem cells using a 1.75 eV wide-bandgap perovskite as the front-cell absorber and the optimised low-bandgap Sn–Pb perovskite as the back-cell absorber. To prevent intermixing of the materials during solution-processing, a charge recombination tri-layer, consisting of gold, molybdenum trioxide and indium tin oxide (ITO) is deposited by vacuum methods on top of the wide-bandgap front cell. This layer effectively protects the front cell during the processing of the back cell. The tandem solar cells are finished by depositing an electron extraction layer and a reflective electrode. In this way, monolithic tandem solar cells are obtained with a record power conversion efficiency of 21%. High-quality low-bandgap perovskite absorber layers are crucial to the deployment of all-perovskite multi-junction devices, although other factors are also important. However, even if remarkable, the efficiency obtained by Zhao, Yan and colleagues is still below the record value obtained for perovskite single-junction cells. This is partially due to losses in the short circuit current density caused by parasitic absorption in the front cell and in the charge recombination layer. Processing of the Sn–Pb perovskite using orthogonal solvents or by vacuum deposition would remove the necessity of the protective transparent conductor between the sub-cells, allowing for an increase in the current density. However, the main hurdle that prevents a higher efficiency is the low open-circuit voltage delivered by the wide-bandgap sub-cell. To tune the bandgap, iodide ions are partly substituted for bromine in wide-bandgap perovskites. However, these substituted perovskites have a tendency to partially phase separate into bromide and iodide-rich domains8. The iodide-rich regions determine the effective bandgap of the material and as such the maximum achievable open-circuit voltage9. Hence, avenues to prevent halide segregation, or routes towards single-halide wide-bandgap perovskites, need to be identified to further advance perovskite–perovskite tandem cells. Passivation methods, such as those reported by Zhao, Yan and colleagues, can effectively improve the quality of the low bandgap layer, a step forwards in the realization of highly efficient all-perovskite 2T tandem solar cells.
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