Regulating Interfacial Strain Improves the Stability and Efficiency of Perovskite Solar Cells

Although perovskite solar cells (PSCs) demonstrated power conversion efficiency (PCE) as high as 26%, stability caused by the hetero-interfaces has been still issued. Since the thermal expansion coefficient of perovskite is about 30 times higher than the SnO₂-coated substrate, this difference can cause in-plane tensile strain at the interface of the perovskite and the SnO₂ layer during the annealing process. It should be therefore treated because the in-plane tensile strain is blamed for causing the instability of PSCs. We report here an effective methodology to regulate the strain via interfacial engineering using a functional molecule. The interfacial engineering changes the strain of the perovskite layer near the SnO₂ layer, which is confirmed by grazing-incidence X-ray diffraction. The strain-released perovskite film shows faster electron extraction from the perovskite to the SnO₂ as confirmed by the time-resolved photoluminescence. In addition, carrier lifetime is increased by a factor of two upon relieving the interfacial strain. Space charge limited current measurement quantifies trap density, where the strain-released perovskite (target) shows lower trap density than the control perovskite. As a result, PCE is increased from 22.47% to 24.33% mainly due to the improvements of open-circuit voltage from 1.14 to 1.17 V and fill factor from 80.3% to 83.8%. Moreover, 90% of initial PCE is observed after 950 hours for the target device, which is higher than that (70%) of the control one.