Thick and Efficient Perovskite Solar Cells Enabled by Fast Co-Evaporation

Conformal deposition of perovskite on textured silicon for perovskite/silicon tandem solar cells remains challenging using the widely-used solution spin-coating route. In contrast, perovskite thermal evaporation allows highly-controlled thin film deposition, ensuring conformality on large areas, regardless on the surface texture. To take advantage of the full photocurrent generating capabilities of the perovskite front cell, rather thick films are required. Currently, most perovskites are sublimed in a single approach by co-sublimation of the precursors. This deposition technique is usually done under high vacuum and the deposition rates of each precursor are monitored using quartz crystal microbalances (QCMs). However, due to the slow deposition rates (&lt; 1 Å/s) it is challenging to deposit sufficiently thick perovskite films that are required for highly efficient perovskite/silicon tandem devices.<br/>Herein, we explore the co-evaporation of CH₃NH₃PbI₃ (MAPI) perovskite using faster deposition rates (&gt; 2 Å/s), by simultaneously subliming CH₃NH₃I and PbI₂. These rates are deduced by scaling up linearly the initial speed by a factor of up to 8 times, leading to quick and reproducible evaporation processes. Moving to faster deposition speeds, a change in crystallite orientation is observed by XRD and GIWAXS, showing a preferential growth along the (202) family of planes, as opposed to the (110) orientation.<br/>Thanks to these faster deposition speeds, 1 µm-thick MAPI films can be deposited in a short amount of time. This allows a noticeable J_sc gain in planar glass based single junction solar cells (~0.8 mA), due to an enhanced absorption of the infrared light, as revealed by EQE. On the other hand, the FF drops to lower values, possibly due to a diffusion length problem.