Perovskite solar cells prepared by a new 3-step method including a PbI 2 scavenging step

Abstract

CH3NH3PbI3 films prepared by the 2-step method for perovskite solar cells generally have problems of residual unreacted PbI2 and rough surface structure. Here, we report a new 3step method, based on the 2-step method with an additional spin-coating of CH3NH3(I,Br) solution on the CH3NH3PbI3 film to scavenge remnant PbI2. The 3-step method improved light absorption of the film by converting the residual PbI2 into CH3NH3PbI3−xBrx. Morphological improvements such as a network formation among perovskite grains and a decrease of intergranular voids were also observed. The additional CH3NH3I spin-coating resulted in the increases of both JSC and VOC, while that of CH3NH3Br solution showed a slight decrease of JSC and large increase of VOC due to the enlarged bandgaps. The maximum power conversion efficiency was improved from 12.9 % (2-step) to 14.4 % (3-step). Furthermore, the 3-step cells retained ~85 % of the original PCE values after storing in air for 700 h, which indicates an improved stability.

Perovskite Solar Cells Prepared by Advanced Three-Step Method Using Additional HC(NH2)2I Spin-Coating: Efficiency Improvement with Multiple Bandgap Structure

“…Morphology of perovskite active layer Figures 5a-d show top-view SEM images of the 3-step prepared perovskite active layers on ETL. The perovskite particles with the diameter of ~0.5-1 μm were observed in all the samples.As we reported in the previous study,22 some bridge-like networks between perovskite particles were observed in MAI: 0.13 M, which were formed by the surface dissolution of perovskite particles during the additional spin-coating (see alsoFigure S1as enlargement). On the other hand, the bridge-like networks were not frequently observed in the additionally FAI spin-coated cells.…”

“…For the 3-step samples, the peak intensity of unreacted PbI2 significantly decreased due to a conversion of the unreacted PbI2, similarly to our previous work. 22 Although a small peak of unreacted PbI2 at ~12.8° still remained in the 3-step films, the peak intensity effectively decreased with increasing the FAI concentration. Furthermore, the peaks for the perovskite shifted to smaller angles by the additional FAI spin-coating, and the peak shift became larger with increasing the FAI concentration as seen in the enlarged XRD patterns at 13°-15°.…”

“…To solve this problem, some improved processes were proposed, e.g., solvent selection and PbI 2 morphology control. − Instead of a conventional N , N -dimethylformamide (DMF) solvent, dimethyl sulfoxide (DMSO) effectively suppressed the crystallization of PbI 2 via the complex formation. , Another strategy is to form a porous PbI 2 film using a molecular level pore-forming agent, which is effective at increasing the reactivity between PbI 2 and MAI . Recently, we have reported an alternative method, viz., the three-step method, to reduce the unreacted PbI 2 , which is realized by the additional MA­(I,Br) spin-coating . The three-step method resulted in the obvious enhancement of the PCE from 12.9% (two-step) to 14.4% (three-step) and of the stability in air.…”

Perovskite Solar Cells with >19% Efficiency Achieved by an Advanced Three-Step Method Using Additional HC(NH2)2I–NaI Spin-Coating

“…By forming a compositional gradation (FA/MA/Cs cations or I/Br anions) in the perovskite layer, a band gradient structure can be obtained. − Because the band gradient structure is a driving force for the carrier transportation, it is effective to suppress the carrier recombination and to improve the photovoltaic performance. The band gradient structures in the literatures were usually formed at the top side of the perovskite layer by post-treatments. − However, we recently reported a formation of multiple bandgap structure at the bottom side using a 3-step method, i.e., by an additional spin-coating of MAI or FAI solution on a 2-step prepared MAPbI 3 layer containing an unreacted PbI 2 next to the TiO 2 mesoporous layer. , The additional FAI spin-coating converted MAPbI 3 into FA x MA 1– x PbI 3 and also converted the unreacted PbI 2 into a FA-rich FA y MA 1– y PbI 3 layer ( y > x ) with a smaller bandgap at the bottom side, which improved the PCE.…”

“…28−30 However, we recently reported a formation of multiple bandgap structure at the bottom side using a 3-step method, i.e., by an additional spin-coating of MAI or FAI solution on a 2-step prepared MAPbI 3 layer containing an unreacted PbI 2 next to the TiO 2 mesoporous layer. 31,32 The additional FAI spin-coating converted MAPbI 3 into FA x MA 1−x PbI 3 and also converted the unreacted PbI 2 into a FA-rich FA y MA 1−y PbI 3 layer (y > x) with a smaller bandgap at the bottom side, which improved the PCE.…”