Abstract
AbstractThermal admittance spectroscopy and capacitance‐voltage measurements are well established techniques to study recombination‐active deep defect levels and determine the shallow dopant concentration in photovoltaic absorbers. Applied to thin‐film solar cells or any device stack consisting of multiple layers, interpretation of these capacitance‐based techniques is ambiguous at best. We demonstrate how to assess electrical measurements of thin‐film devices and develop a range of criteria that allow to estimate whether deep defects could consistently explain a given capacitance measurement. We show that a broad parameter space, achieved by exploiting bias voltage, time, and illumination as additional experimental parameters in admittance spectroscopy, helps to distinguish between deep defects and capacitive contributions from transport barriers or additional layers in the device stack. On the example of Cu(In,Ga)Se2 thin‐film solar cells, we show that slow trap states are indeed present but cannot be resolved in typical admittance spectra. We explain the common N1 signature by the presence of a capacitive barrier layer and show that the shallow net dopant concentration is not distributed uniformly within the depth of the absorber.
Highlights
An accurate measurement of the net dopant concentration and a quantitative characterization of recombination‐active defects in photovoltaic absorbers are critical for understanding and optimizing solar cell performance
We show that a broad parameter space, achieved by exploiting bias voltage and illumination as additional experimental parameters in admittance spectroscopy, helps to verify whether deep defects can consistently explain features observed in capacitance‐ based measurements
Based on a simple analytic model of band bending in the depletion region, we demonstrated that the voltage‐dependent height of a capacitance step is most conveniently expressed as a change of apparent depth Δx = ε0εr/C and is a helpful measure to identify the physical origin of a capacitance step
Summary
An accurate measurement of the net dopant concentration and a quantitative characterization of recombination‐active defects in photovoltaic absorbers are critical for understanding and optimizing solar cell performance. Using the relation Nd = p = Nvexp(−EF/kT) for the bulk majority carrier concentration, where Nv is the effective density of states at the valence band edge, we find an upper limit of the FIGURE 2 Solid lines show the maximum step height in apparent depth Δx as a function of temperature T, calculated for different deep trap levels given above the graph and assuming εr = 10 and Nv = 1.5 × 1019cm−3 (T/300K)3/2. The reason is that the height of a characteristic peak in the impedance spectrum is proportional to the resistance of that circuit element, which differs drastically between barrier layer and junction, whereas the peak height is proportional to the inverse capacitance in the ωZ( f ) spectrum
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