Abstract
Herein, the long‐term stability of vacuum‐deposited methylammonium lead iodide (MAPbI3) perovskite solar cells (PSCs) with power conversion efficiencies (PCEs) of around 19% is evaluated. A low‐temperature atomic layer deposition (ALD) Al2O3 coating is developed and used to protect the MAPbI3 layers and the solar cells from environmental agents. The ALD encapsulation enables the MAPbI3 to be exposed to temperatures as high as 150 °C for several hours without change in color. It also improves the thermal stability of the solar cells, which maintain 80% of the initial PCEs after aging for ≈40 and 37 days at 65 and 85 °C, respectively. However, room‐temperature operation of the solar cells under 1 sun illumination leads to a loss of 20% of their initial PCE in 230 h. Due to the very thin ALD Al2O3 encapsulation, X‐ray diffraction can be performed on the MAPbI3 films and completed solar cells before and after the different stress conditions. Surprisingly, it is found that the main effect of light soaking and thermal stress is a crystal reorientation with respect to the substrate from (002) to (202) of the perovskite layer, and that this reorientation is accelerated under illumination.
Highlights
(ALD) Al2O3 coating is developed and used to protect the MAPbI3 layers and the solar cells from environmental agents
It improves the thermal stability of the solar cells, Organic–inorganic lead halide perovskite solar cells (PCSs) have attracted considerable scientific and technological attention over the past decade due to simple fabrication processes and a tremendous increase in power conversion efficiency (PCE) since the first report in 2009.[1]. A typical PSC
Due to the very thin atomic layer deposition (ALD) Al2O3 encapsulation, X-ray diffraction can be performed on the MAPbI3 films and completed solar cells before and after the different stress consists of a transparent conductive oxide (TCO), electron and hole transporting layers (ETL and HTL, respectively), a perovskite film, and a metal electrode.[2,3]
Summary
Thin MAPbI3 films were formed by dual-source vacuum deposition of MAI and PbI2 (with deposition rates of 1 and 0.6 Å sÀ1, respectively) following the procedure used to obtain high-efficiency solar cells reported elsewhere[18,63] (see the Experimental Section for more details). The small amount of residual PbI2 impurity present in our films has been observed by others and is considered advantageous to the performance of the PSCs due to its passivation effect on the grain boundaries.[64] While the calculated cell parameters for MAPbI3 (see Table S1, Supporting Information) are in very close agreement to reference data,[65] the relative intensities of the diffraction peaks are indicative of a preferential orientation along the c-axis (see Figure 1A and Table S2, Supporting Information) This is the most common orientation of MAPbI3 thin films prepared either by solution or by vacuum processes.[66,67]. The bandgap value of the MAPbI3 film obtained in this study is well matched to the reported values for the tetragonal MAPbI3 in the literature.[69,70]
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