Remnant PbI₂, an unforeseen necessity in high-efficiency hybrid perovskite-based solar cells?

Perovskite-containing solar cells were fabricated in a two-step procedure in which PbI₂is deposited via spin-coating and subsequently converted to the CH₃NH₃PbI₃perovskite by dipping in a solution of CH₃NH₃I. By varying the dipping time from 5 s to 2 h, we observe that the device performance shows an unexpectedly remarkable trend. At dipping times below 15 min the current density and voltage of the device are enhanced from 10.1 mA/cm²and 933 mV (5 s) to 15.1 mA/cm²and 1036 mV (15 min). However, upon further conversion, the current density decreases to 9.7 mA/cm²and 846 mV after 2 h. Based on X-ray diffraction data, we determined that remnant PbI₂is always present in these devices. Work function and dark current measurements showed that the remnant PbI₂has a beneficial effect and acts as a blocking layer between the TiO₂semiconductor and the perovskite itself reducing the probability of back electron transfer (charge recombination). Furthermore, we find that increased dipping time leads to an increase in the size of perovskite crystals at the perovskite-hole-transporting material interface. Overall, approximately 15 min dipping time (∼ 2% unconverted PbI₂) is necessary for achieving optimal device efficiency.