Credible evidence for the passivation effect of remnant PbI₂ in CH₃NH₃PbI₃ films in improving the performance of perovskite solar cells

Abstract

The role of remnant PbI2 in CH3NH3PbI3 films is still controversial, some investigations have revealed that the remnant PbI2 plays a passivation role, reduces the charge recombination in perovskite solar cells (PSCs), and improves the performance of PSCs, but the opposing views state that remnant PbI2 has no passivation effect and it would deteriorate the stability of the devices. In our investigation, the CH3NH3PbI3 films have been prepared by a two-step spin-coating method and the content of the remnant PbI2 in CH3NH3PbI3 films has been tuned by varying the preparation temperature. It has been found that increasing the heating temperature could increase the coverage of spin-coated PbI2 films, which has led to high coverage CH3NH3PbI3 films and more remnant PbI2 in CH3NH3PbI3 films, and as a result, the performance of PSCs was enhanced obviously and the maximum power conversion efficiency of 14.32 ± 0.28% was achieved by the PSCs prepared at 130/120 °C (PbI2 films were heated at 130 °C and CH3NH3PbI3 films were heated at 120 °C). Furthermore, the dark current, electrochemical impedance spectroscopy and time-resolved fluorescence emission decay measurements revealed that the charge recombination in PSCs has been gradually suppressed and the fluorescence emission lifetime has gradually increased with the content of remnant PbI2 increasing. Thus, the passivation effects of the unreacted and decomposed PbI2 in improving the performance of PSCs have been confirmed unquestionably.

Investigation into the Advantages of Pure Perovskite Film without PbI2 for High Performance Solar Cell

“…In contrast, the small trace of PbI 2 peak was still detected from the XRD patterns of the two-step spin-casting-based perovskite film (Fig. 2a), similar to the literatures11121314. SEM images in Fig.…”

“…Figure 2d confirmed that pure perovskite crystal phase without PbI 2 was successfully prepared by printing-based process using 10 mg·ml −1 MAI solution, of which the existence could be identified by the peak at 2 θ  = 12.6°, attributed to the (001) lattice planes of hexagonal (2H polytype) PbI 2 567891011121314. In contrast, the small trace of PbI 2 peak was still detected from the XRD patterns of the two-step spin-casting-based perovskite film (Fig.…”

“…However, in MAPbI 3 -based PSC showing high efficiency, tiny amount of residual PbI 2 impurity was often found with the perovskite crystal phase, even though the equimolar composition of organic (MAI) and inorganic (PbI 2 ) components was utilized to fully convert them to perovskite crystal3567891011121314. Therefore, various approaches have been reported to find out if perovskite crystal with PbI 2 impurity would be advantageous to the performance of PSC or not.…”

“…individually showed that PSCs having remnant PbI 2 in two-step solution-based photoactive layers produced higher PCE, but the inferior performances, obtained from the pure perovskite phase-based devices, mainly originated in the non-uniformly grown perovskite films having over-grown needle-like structures due to longer dipping time to force the remaining PbI 2 converted12 or coarse nanomorphology having a lot of pinholes due to the recrystallization during their film formation11. Similarly, in the work by Wang et al ., the perovskite film without PbI 2 , which provided inferior solar cell performances, was prepared by casting MAI on the coarse PbI 2 layer, consequently inducing the low surface coverage perovskite film13. Therefore, to clearly verify the effect of PbI 2 in the two-step process-based PSC, the morphological discrepancy between perovskites with and without PbI 2 should be eliminated, first.…”

A controllable fabrication of grain boundary PbI2 nanoplates passivated lead halide perovskites for high performance solar cells

“…The crystal size and morphology of perovskite films have been one of most important factors to achieve the high performance perovskite solar cells. Recently the PbI 2 has been demonstrated as another important factor for the perovskite based devices' performance because the PbI 2 can either passivate lead halide perovskite or slow down the electron transfer or even deteriorate the stability in the device [35][36][37][38][39][40][41][42]. In all, it is a important to achieve a balance between passivating perovskite and retarding electron transfer for realization of PbI 2 beneficial effect.…”