CH 3 NH 3 PbI 3 and HC(NH 2 ) 2 PbI 3 Powders Synthesized from Low-Grade PbI 2 : Single Precursor for High-Efficiency Perovskite Solar Cells.

High-efficiency perovskite solar cells are generally fabricated by using highly pure (>99.99 %) PbI2 mixed with an organic iodide in polar aprotic solvents. However, the use of such an expensive chemical may impede progress toward large-scale industrial applications. Here, we report on the synthesis of perovskite powders by using inexpensive low-grade (99 %) PbI2 and on the photovoltaic performance of perovskite solar cells prepared from a powder-based single precursor. Pure APbI3 [A=methylammonium (MA) or formamidinium (FA)] perovskite powders were synthesized by treating low-grade PbI2 with MAI or FAI in acetonitrile at ambient temperature. The structural phase purity was confirmed by X-ray diffraction. The solar cell with a MAPbI3 film prepared from the synthesized perovskite powder demonstrated a power conversion efficiency (PCE) of 17.14 %, which is higher than the PCE of MAPbI3 films prepared by using both MAI and PbI2 as precursors (PCE=13.09 % for 99 % pure PbI2 and PCE=16.39 % for 99.9985 % pure PbI2 ). The synthesized powder showed better absorption and photoluminescence, which were responsible for the better photovoltaic performance. For the FAPbI3 powder, a solution with a yellow non-perovskite δ-FAPbI3 powder synthesized at room temperature was found to lead to a black perovskite film, whereas a solution with the black perovskite α-FAPbI3 powder synthesized at 150 °C was not transformed into a black perovskite film. The α↔δ transition between the powder and film was assumed to correlate with the difference in the iodoplumbates in the powder-dissolved solution. An average PCE of 17.21 % along with a smaller hysteresis [ΔPCE=PCEreverse -PCEforward )=1.53 %] was demonstrated from the perovskite solar cell prepared by using δ-FAPbI3 powder; this PCE is higher than the average PCE of 17.05 % with a larger hysteresis (ΔPCE=2.71 %) for a device based on a conventional precursor solution dissolving MAI with high-purity PbI2 . The smaller hysteresis was indicative of fewer defects in the resulting FAPbI3 film prepared by using the δ-FAPbI3 powder.