Light intensity dependent measurements of the open-circuit voltage of a photovoltaic cell, the so called Suns-Voc method, are often used to identify the free carrier recombination mechanism of electrons and holes. Additionally varying the temperature during such a study by performing the measurement in a cryostat allows to access a larger data space and by that a deeper understanding of the processes at play [Tvingstedt et al., Adv. Energy Mater. 2016, 1502230].
Here, we present a modified Suns-Voc method that makes use of a temperature variation which is simply induced by the absorption of the light [Ullbrich et al., submitted]. In this case, the open-circuit voltage continually decreases with respect to the isothermal case as a consequence of the absorption induced temperature rise. An exact mathematical treatment of the problem reveals that the open-circuit voltage can even decrease although the incident light intensity increases. The entire characterization can be done at ambient conditions using a strong light source which sufficiently heats up the sample without the need for an external heating.
We test our model on different photovoltaic cells based on small molecules, polymers, perovskite and silicon. A very good agreement between fit and experimental data proves the general validity and allows us to extract important information. First, the energy gap over which recombination between electron and holes takes place are determined as it is a direct fit parameter. Second, we can demonstrate that the ideality factor can be assumed to remain constant (close to 1 in case of organic semiconductor devices) allover the measured light intensity range and third, the fitted thermal resistance yields the temperature rise of the sample.
A general aspect of our finding is that the often found voltage turnover in Suns-Voc measurements do not necessarily arise from recombination at the contacts, as often believed, but can simply stem from thermal effects.
The turn-around effect can basically be understood by a strong temperature-dependent broadening of the Fermi-Dirac distribution, representing an increasing number of charge carriers even for back-shifting quasi-Fermi levels.
From a mathematical point of view, it is very similar to the electrothermal feedback caused by Joule-self-heating in semiconductor devices with increasing electrical conductivity towards higher temperatures [Fischer et al., Phys. Rev. Lett. 2013, 110, 126621].
Finally, we will shortly discuss that this effect cannot only be found in photovoltaic cells but also all kind of devices which produce an open-circuit voltage under light illumination, e.g. organic light-emitting diodes.