Insights from Transient
Optoelectronic Analyses on
the Open-Circuit Voltage of Organic Solar Cells
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Abstract
In this Perspective, we review recent progress on the
use of transient
optoelectronic techniques to quantify the processes determining the
open-circuit voltage (<i>V</i><sub>OC</sub>) of organic
solar cells. Most theoretical treatments of <i>V</i><sub>OC</sub> include the effects of both material energetics and recombination
dynamics, yet most experimental approaches are based on materials
energetics alone. We show that by direct measurement of charge carrier
dynamics and densities, the rate of nongeminate charge recombination
may be determined within working cells and its impact on achievable <i>V</i><sub>OC</sub> determined. A simple fit-free device model
utilizing these measurements is shown to agree (to within ±5
mV) with experimentally observed open-circuit voltages for devices
comprised of a range of different photoactive layer materials and
different processing conditions, and utilizing both bulk and bilayer
heterojunctions. This agreement is significantly better than that
obtainable from analyzing materials energetics alone, even when employing
an in situ analysis of effective electronic band gap. We go on to
argue that the precision of our <i>V</i><sub>OC</sub> calculations
derives from implicitly including the impact of film microstructure
on open-circuit voltage. We show that this can modulate <i>V</i><sub>OC</sub> by up to 200 mV, and thereby account for the limits
of energy-based models in accurately predicting achievable performance