Visualizing the Structure-Property Nexus of Wide-Bandgap Perovskite Solar Cells under Thermal Stress

Abstract

Wide-bandgap perovskite solar cells (PSCs) toward tandem photovoltaic applications are confronted with the challenge of device thermal stability, which motivates to figure out a thorough cognition of wide-bandgap PSCs under thermal stress, using in situ atomic-resolved transmission electron microscopy (TEM) tools combing with photovoltaic performance characterizations of these devices. The in situ dynamic process of morphology-dependent defects formation at initial thermal stage and their proliferations in perovskites as the temperature increased are captured. Meanwhile, considerable iodine enables to diffuse into the hole-transport-layer along the damaged perovskite surface, which significantly degrade device performance and stability. With more intense thermal treatment, atomistic phase transition reveals the perovskite transform to PbI₂ along the topo-coherent interface of PbI₂/perovskite. In conjunction with density functional theory calculations, a mutual inducement mechanism of perovskite surface damage and iodide diffusion is proposed to account for the structure-property nexus of wide-bandgap PSCs under thermal stress. The entire interpretation also guided to develop a thermal-stable monolithic perovskite/silicon tandem solar cell.

Keywords: in situ TEM; perovskite/silicon tandem; structure‐property nexus; thermal degradation mechanisms; wide‐bandgap perovskite.

Figure 1

Planar PSCs device structure and photovoltaic performance under thermal operating: V _OC, J _SC, FF, and PCE. a) Schematic of the full device structure of planar PSCs. b) Cross‐sectional STEM image and corresponding line EDX intensity of the relevant elements in perovskite. c–e) J–V curve and performance parameters (V _OC, J _SC, FF, and PCE) of Spiro‐OMeTAD‐based PSCs under stepwise thermal operating. The photovoltaic performance values are normalized by the initial values obtained at room temperature for the cell. f–h) J–V curve and corresponding parameters of PTAA‐based PSCs performance under same operated conditions with Spiro‐OMeTAD‐based PSCs.

Figure 2

Morphology‐dependent thermal degradation and component analysis of planar perovskites. a–d) Surface defects formation of GBs‐free and e–h) GBs‐rich perovskite film under different thermal condition from room temperature to 100 °C. Scale bar: 200 nm. i) The line scanning EDX analysis of the pin‐holes showed the main element loss in perovskite. Left panel: STEM image of pin‐hole region, white arrow is line scanning direction. Right panel: corresponding line profile intensity of elements in perovskite and the concave peak shape toward the left means the element intensity is reduced. j) The content changes of different elements in perovskite from quantitative analysis of EDX mapping. k) Schematic illustration of thermal degradation of GBs‐rich perovskite film.

Figure 3

Iodide ions diffusion at the interface of perovskite/HTL and effects on HTL electronic structures. a,b) ADF‐STEM images showing cross‐sectional views of the perovskite/Spiro‐OMeTAD and d,e) perovskite/PTAA interface before and after heating. c) The line profile showing the intensity difference between pristine and heating at perovskite/HTL interface. f) Time sequential ADF‐STEM images with false‐color of the diffusion region expanding at elevated temperature of 200 °C. g) The corresponding statistical analysis of the thickness of four different regions in f. h,i) Side view of fully relaxed atomic models of perovskite slabs and HTL (Spiro‐OMeTAD as an example). j) Density of states of the pristine Spiro‐OMeTAD molecule on perfect perovskite surface. k) Density of states of the iodide ions diffused Spiro‐OMeTAD molecule on perovskite surface, iodide ion concentration is 16%.

Figure 4

In situ TEM investigation the decomposition pathway. a) Time‐sequenced HRTEM images of perovskite grain evolution under continuous electron beam illumination. The edge regions are zoomed‐in rectangle framework images. Scale bar: 10 nm. b) ADF‐STEM image of planar perovskite grain with nearly triple‐symmetric grain boundary. c) Time‐lapsed ADF‐STEM images of PbI₂ formation and growth at grain boundary and edge of perovskite. d) Atomic‐resolved ADF‐STEM image of topo‐coherent interface of perovskite/PbI₂, and e) its corresponding FFT image and f) atomic models.

Figure 5

PTAA‐based Perovskite/ silicon tandem solar cells. a) Cross‐sectional sketch of the perovskite/c‐Si bifacial tandem device and b) corresponding SEM image of the tandem realized on a both‐sides textured c‐Si bottom cell. c) Representative J–V curve and performance parameters (PCE, V _oc, FF, and J _sc) of PTAA‐based tandem PSCs. d) EQE curves of the tandem PSCs. e) Performance parameters under stepwise thermal operating. The photovoltaic performance values are normalized by the initial values obtained at room temperature for the cell. f) MPP tracking stability tests of the tandem PSCs.