External quantum efficiency measurements used to study the stability of differently deposited perovskite solar cells

The instability exhibited by perovskite solar cells when exposed to the environment under illumination is one of the major obstacles for the entry of perovskite technology in the photovoltaic market. In this work, we use the external quantum efficiency (EQE) technique to study the photoinduced degradation of two types of solar cells having CH₃NH₃PbI₃ as an absorber layer: one deposited by spin coating with an n-i-p architecture and the other deposited by evaporation with an inverted p-i-n structure. We also study the effect of different encapsulants to protect the cells against atmospheric agents. We find that EQE provides information regarding the areas of the cell most susceptible to degradation, in addition to providing an estimate of the optical gap and the Urbach energy of the absorbent material. We confirm that the combined action of illumination and the environment markedly accelerate the degradation, which is reflected in the deterioration of all the parameters of the cell. The rear part of the cell is the first region to suffer the light-induced degradation. On the other hand, the cells deposited by evaporation and with a good encapsulation process are highly stable since after 30 h of exposure just a small spectral change is noticed in the red/infrared region of the EQE spectrum.