Sensitivity of Sub-Bandgap External Quantum Efficiency Measurements of Solar Cells under Electrical and Light Bias

The measurement of the external quantum efficiency (EQE) for photocurrent generation at photon energies below the bandgap of semiconductors has always been an important tool for understanding phenomena such as charge photogeneration via tail and trap states. The shape of the subgap EQE can also reveal the subtle but important physics of inter- and intramolecular states that lay at the heart of charge photogeneration in molecular systems such as organic semiconductors. In this work, we examine the influence of optical and electrical noise on the sensitivity of EQE measurements under different electrical and optical bias conditions and demonstrate how to enhance the dynamic range to an unprecedented >100 dB. We identify and study several apparatus-and-device-related factors limiting the sensitivity including: the electrical noise floor of the measurement system; flicker and pick-up noise; probe light source stray light; the photon noise of the light bias source; the electrical noise of the voltage bias source; and the shunt-resistance-limited thermal and electrical shot noise of the device. By understanding and minimizing the influence of these factors we are able to detect EQE signals derived from weak subgap absorption features in both organic and inorganic solar cell systems at photon energies well below their bandgaps.