Insights from Transient Optoelectronic Analyses on the Open-Circuit Voltage of Organic Solar Cells

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>_OC) of organic solar cells. Most theoretical treatments of <i>V</i>_OC 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>_OC 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>_OC calculations derives from implicitly including the impact of film microstructure on open-circuit voltage. We show that this can modulate <i>V</i>_OC by up to 200 mV, and thereby account for the limits of energy-based models in accurately predicting achievable performance

In Situ Measurement of Energy Level Shifts and Recombination Rates in Subphthalocyanine/C₆₀ Bilayer Solar Cells

Understanding the nature and impact of internal interfaces is critical to understanding the operation of nanostructured organic devices, such as organic photovoltaics. Here, we use transient optoelectronic analyses to quantify in situ the HOMO level shifts and changes in interfacial recombination rate that occur within thermally evaporated subphthalocyanine (SubPc)/C₆₀ bilayer solar cells as the SubPc evaporation source is varied. We show how such measurements can complement ex situ optical and physical techniques to access the functional impact of device modification, particularly with respect to the resulting device open-circuit voltage (<i>V</i>_OC). We are able to explain how subtle changes in SubPc deposition conditions lead to significant modification of interfacial energetics and recombination dynamics, which in turn cause substantial changes in <i>V</i>_OC

Analysis of the Relationship between Linearity of Corrected Photocurrent and the Order of Recombination in Organic Solar Cells

We address the claim that the dependence of the “corrected photocurrent” (defined as the difference between the light and dark currents) upon light intensity can be used to determine the charge recombination mechanism in an organic solar cell. We analyze a poly(3-hexylthiophene):[6,6]-phenyl C61-butyric acid methyl ester (P3HT:PCBM) device using corrected photocurrent and transient photovoltage experiments and show that whereas the corrected photocurrent is linear in light intensity the charge recombination rate scales superlinearly with charge carrier density. We explain this apparent discrepancy by measuring the charge carrier densities at different applied voltages and light intensities. We show that it is only safe to infer a linear recombination mechanism from a linear dependence of corrected photocurrent on light intensity under the following special conditions: (i) the photogenerated charge carrier density is much larger than the dark carrier density and (ii) the photogenerated carrier density is proportional to the photogeneration rate

Analysis of Recombination Losses in a Pentacene/C₆₀ Organic Bilayer Solar Cell

Transient photovoltage and charge extraction analyses are used to quantify the rate of nongeminate recombination loss within a pentacene/C₆₀ bilayer solar cell across the power-generating quadrant of the device. Employing these data, a simple model of cell function, based on field-independent generation and a charge-dependent nongeminate loss current without the use of any adjustable fitting parameters, is shown to be in good agreement with the experimental current/voltage behavior of the device both in the dark and under illumination

Charge Dynamics in Solution-Processed Nanocrystalline CuInS₂ Solar Cells

We investigate charge dynamics in solar cells constructed using solution-processed layers of CuInS₂ (CIS) nanocrystals (NCs) as the electron donor and CdS as the electron acceptor. By using time-resolved spectroscopic techniques, we are able to observe photoinduced absorptions that we attribute to the mobile hole carriers in the NC film. In combination with transient photocurrent and photovoltage measurements, we monitor charge dynamics on time scales from 300 fs to 1 ms. Carrier dynamics are investigated for devices with CIS layers composed of either colloidally synthesized 1,3-benzenedithiol-capped nanocrystals or <i>in situ</i> sol–gel synthesized thin films as the active layer. We find that deep trapping of holes in the colloidal NC cells is responsible for decreases in the open-circuit voltage and fill factor as compared to those of the sol–gel synthesized CIS/CdS cell