CH3NH3PbI3 and HC(NH2)2PbI3 Powders Synthesized from Low-Grade PbI2: 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 APbI(3) [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 MAPbI(3) film prepared from the synthesized perovskite powder demonstrated a power conversion efficiency (PCE) of 17.14%, which is higher than the PCE of MAPbI(3) 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 FAPbI(3) powder, a solution with a yellow non-perovskite delta-FAPbI(3) powder synthesized at room temperature was found to lead to a black perovskite film, whereas a solution with the black perovskite alpha-FAPbI(3) powder synthesized at 150 degrees C was not transformed into a black perovskite film. The alpha <->delta 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 [Delta PCE = PCEreverse-PCEforward)=1.53%] was demonstrated from the perovskite solar cell prepared by using delta-FAPbI(3) powder; this PCE is higher than the average PCE of 17.05% with a larger hysteresis (Delta PCE = 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 FAPbI(3) film prepared by using the delta-FAPbI(3) powder.