Since the last time that I wrote on transport resistance as a voltage loss mechanism in organic solar cells, due to low active layer conductivities, we have continued working on it. 
While we learnt from our valued colleague Prof. Chang-Qi Ma that MoOx diffusion contributes strongly to fill factor losses by thermal degradation – they had published their convincing results in [Qin 2023], under our radar – we also understand better how to recognise and comprehend transport resistance losses.
The simplest way to characterise transport resistance is to determine the difference between a normal current density–voltage curve under 1 sun, and the suns-Voc curve. The latter is the pair-wise combination of generation current density and open-circuit voltage that is giving one current density-voltage point per light intensity: doing this for a wide range of light-intensities results in a pseudo-JV curve. To compare this suns-Voc curve to the current density–voltage curve under 1 sun, it has to downshifted so that the open-circuit voltage of both curves coincide on one point: 
. The result could look like the scheme shown in the figure below. I have adapted this figure from Maria’s new publication on transport resistance losses in organic solar cells [Saladina 2025] (the title is Transport resistance strikes back: unveiling its impact on fill factor losses in organic solar cells ;-). The 
curve (blue) is the normal JV curve under illumination, where 
is the applied voltage. The down-shifted suns-Voc curve corresponds to the 
(red); the implied voltage is the voltage without any series-resistance imposed drops. In other words, the externally applied voltage drops over the diode and all series resistances, the external series resistance as well as the transport resistance that comes from a low active layer (or transport layer) conductivity. The implied voltage, as measured by the open-circuit voltage (at zero current, where series resistances do not play a role), corresponds to the voltage without drops over series and transport resistance. 
If a solar cell has a low active layer conductivity and is transport resistance limited, than the measurement of the suns-Voc curve to construct the -curve allows to evaluate the performance of that solar cell as if it did not have any transport resistance losses. One could also say: while the measured curve is transport resistance limited, the pseudo- curve corresponds to the case of infinite conductivity, but contains the same recombination as the measured curve.
Continue reading “Transport resistance strikes back”
