The Optical Origin Of Near-Unity External Quantum Efficiencies In Perovskite Solar Cells

With the emergence of highly efficient perovskite solar cells in both single- and multijunction architectures, there is an abundance of reports of extremely high external quantum efficiencies (EQE) up to 98%. Typically, the spectral maximum of the EQE is found in the range between 400 and 500 nm, which is even more surprising, as the transmittance of typically used indium tin oxide (ITO)/glass substrates does not exceed 90% in this wavelength range. Herein, the root cause of the high EQE values by a combination of experimental data and optical simulations is analyzed and explained. It is shown that the high refractive index of the perovskite absorber is strongly increasing the transmittance of incident light into the active perovskite layer, while the spectral distribution and ultimately the spectral position of the peak in the transmittance spectrum are strongly affected by the thickness and optical properties of the underlying transparent electrode.