Improved environmental stability of HTM free perovskite solar cells by a modified deposition route
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A three-step sequential deposition route (TSDR) is proposed for the growth of perovskite crystals. By this way, a compact perovskite layer containing large crystal grains is formed. The layer is thicker (~ 1.0 µm) with good coverage of substrate compared to those deposited by conventional two-step or sequential deposition route (SDR). Photovoltaic performance of solar cells containing perovskite crystals grown by TSDR-I improves in a longer time (more than 30 days) compared to those fabricated by conventional SDR. In other words, a simple modification (growth of perovskite crystals in two steps) considerably enhances the thickness and compactness of the photo absorber layer containing dominantly large crystals including the lower amount of PbI2 in the core of perovskite grains. Presence of large perfect crystals forming compact perovskite layer inhibits the penetration of the oxygen or humidity deeply inside the film give rise to higher stable performance of the fabricated sample cells. On the other hand in a reverse way (TSDR-II), it is very interesting that dipping the PbI2 layer in high and then low concentration MAI solutions and applying an annealing process between each dipping step results in a very small perovskite crystal grains (~ 70 nm).