Effect of GeI2 and GeBr2 incorporation on perovskite properties and performance of carbon-based perovskite solar cells

CH 3 NH 3 PbI 3 (MAPbI 3 ) film was prepared by a convective method, and its X-ray diffraction spectrum displays the tetragonal perovskite structure. GeI 2 and GeBr 2 powders dissolve poorly in a mixed dimethylformamide-dimethylsulfoxide (DMF-DMSO) solvent, but the introduction of 5 wt% 5-ammonium valeric acid iodide (5-AVAI) into the mixed DMF-DMSO solvent greatly promotes GeI 2 and GeBr 2 solubility. XRD spectra of the (5-AVAI)MAPbI 3 , MAGe x Pb 1– x I 3 , and MAGe x Pb 1– x Br 2 x I 3–2 x ( x = 0.0625 and 0.125) films exhibit a tetragonal perovskite structure, but the film morphologies become rougher than that of the pristine MAPbI 3 film. Discontinuous islands are formed on MAGe x Pb 1– x I 3 and MAGe x Pb 1– x Br 2 x I 3–2 x ( x = 0.125) films. X-ray photoemission spectroscopy (XPS) analysis detected the Ge element on all Ge-doped films. The best performance levels of the carbon-based hole transport layer-free MAGe x Pb 1 –x I 3– x ( x = 0.0625, η = 3.63%) and MAGe x Pb 1– x Br 2 x I 3–2 x ( x = 0.0625, η = 2.95%) perovskite solar cells are lower than that of the pristine MAPbI 3 –based perovskite solar cell ( η = 5.28%). This is likely due to the increased surface roughness, pin-holes, isolated islands, and the decreased light absorbance of the Ge-doped films in comparison with the pristine MAPbI 3 film.