The
role of additives in the performance of CsPbI3 perovskite
solar cells (PSCs) was investigated. Different kinds of cations and
anions were used as additives in a N,N-dimethylformamide
(DMF) solution containing CsI and PbI2 (1:1 molar ratio).
These include HI, HBr, HCl, NH4I, NH4Br, and
NH4Cl. Additive cations (H+ and NH4
+) as well as halide ions (I–, Br–, and Cl–) are important for the
properties of PSCs. Especially, the addition of iodine ion showed
good characteristics compared to Br– and Cl–. Among the CsPbI3 layers prepared with
different kinds of additives and annealed at different temperatures,
the X-ray diffraction peaks of CsPbI3 were clearly found
at 14° and 28° for the sample annealed at 150 °C with
50 μL of HI, suggesting the formation of a cubic structure at
the low temperature of 150 °C. The field emission scanning electron
microscopy images indicate that the surface of the perovskite layer
with hydrogen halide additive (“H+”-based
additive) is more uniform than that with ammonium additive. The roughness
profiles determined by atomic force microscopy indicate that the CsPbI3 film with HI additive shows the least roughness among the
samples with H+-based additives. Therefore, the best power
conversion efficiency (PCE) of 4.72% is obtained for CsPbI3 PSCs annealed at 150 °C with HI (50 μL). The H+-based additives seem to react with PbI2 in DMF solution,
increasing the solubility of PbI2 and thus lowering the
processing temperature. Furthermore, the PCE of CH3NH3PbI3–x
Cl
x
PSCs decreased from 7.45 to 0.23%, whereas that of CsPbI3 PSCs with 50 μL of HI only decreased from 3.55 to 2.78%
after exposing the samples to air for 3 h. These results indicate
that H+-based additives, especially HI, have more impact
on the CsPbI3 PSCs in terms of lowering the processing
temperature and improving the performance.
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