Quantifying the Absorption Onset in the Quantum Efficiency of Emerging Photovoltaic Devices

The external quantum efficiency (EQE), also known as incident-photon-to-collected-electron spectra are typically used to access the energy dependent photocurrent losses for photovoltaic devices. The integral over the EQE spectrum results in the theoretical short-circuit current under a given incident illumination spectrum. Additionally, one can also estimate the photovoltaic bandgap energy (E-g) from the inflection point in the absorption threshold region. The latter has recently been implemented in the "Emerging PV reports," where the highest power conversion efficiencies are listed for different application categories, as a function of E-g. Furthermore, the device performance is put into perspective thereby relating it to the corresponding theoretical limit in the Shockley-Queisser (SQ) model. Here, the evaluation of the EQE spectrum through the sigmoid function is discussed and proven to effectively report the E-g value and the sigmoid wavelength range lambda(s), which quantifies the steepness of the absorption onset. It is also shown how EQE spectra with large lambda(s) indicate significant photovoltage losses and present the corresponding implications on the photocurrent SQ model. Similarly, the difference between the photovoltaic and optical bandgap is analyzed in terms of lambda(s).