Influence of SCN- moiety on CH3NH3PbI3 perovskite film properties and the performance of carbon-based hole-transport-layer-free perovskite solar cells

CH3NH3PbI3 perovskite films were prepared via a hot-casting method using six different CH3NH3I, PbI2 and Pb(SCN)(2) solutions. Surface morphology of perovskite films with low SCN- dopant levels (0.0625 M and 0.125 M Pb(SCN)(2)) showed smooth surfaces and large grain sizes. However, with the high SCN- dopant levels (0.1875 M and 0.25 M Pb(SCN)(2)), rough surfaces were produced with pinholes. The crystal of pure CH3NH3PbI3 (0 M Pb(SCN)(2)) film is a tetragonal perovskite structure. XRD spectra of all five Pb(SCN)(2) added films show the present of CH3NH3PbI3 films and the additional peak at 12.66 degrees. Rietveld refinement analysis reveals that the Pb(SCN)(2) addition causes the second phase PbI2 formation along with the tetragonal MAPbI(3) perovskite film rather than the CH3NH3Pb(SCN)(x)I3-x perovskite formation. The carbon-based hole-transport-layer (HTL)-free perovskite (from 0.0625 M Pb(SCN)(2) dopant) solar cell is the optimal ratio in generating a promising cell efficiency, 6.34%, with a good efficiency retention of 79.43% after 30 days of testing in comparison to a pure CH3NH3PbI3 (0 M Pb(SCN)(2) dopant) perovskite solar cell with an efficiency retention of only 26.92%. The great stability of the Pb(SCN)(2) added perovskite solar cells is attributed to the PbI2 layer covered MAPbI(3) grains blocking oxygen and/or water molecules from degrading MAPbI(3) perovskite.