Can we see defects in capacitance measurements of thin-film solar cells?

Thermal 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

Vacancy defects in epitaxial thin film CuGaSe2 and CuInSe2

Epitaxial thin film CuGaSe2 and CuInSe2 samples grown on GaAs substrates with varying [Cu]/[Ga,In] ratios were studied using positron annihilation Doppler-broadening spectroscopy and were compared to bulk crystals. We find both Cu monovacancies and Cu-Se divacancies in CuInSe2, whereas, in CuGaSe2, the only observed vacancy defect is the Cu-Se divacancy.Peer reviewe

Electrical Characterisation of CdTe/CdS Photovoltaic Devices

Thin film solar cells based on CdTe/CdS are expected to become the base material for the low-cost and efficient large-scale solar energy conversion devices. The samples have been investigate using current-voltage (I-V) and capacitance-voltage (C-V) measurements in order to define the transport mechanism in heterostructure and basic electronic parameters. Trap-assisted tunneling has been found to dominate carrier transport mechanism in the junction

The Influence of Metastabilities on the Luminescence in the Cu(In,Ga)Se2\text{}_{2} Solar Cells

Photoluminescence and electroluminescence spectra of the absorber layer in ZnO/CdS/Cu(In,Ga)Se$\text{}_{2}$ solar cells were measured. Their dependence on temperature, excitation intensity and applied voltage were studied. Electroluminescence measurements were used to investigate light- and bias-induced metastabilities in the absorber of the cells. We showed that metastable changes of defect distributions, which produce an effect on the electrical characteristics of ZnO/CdS/Cu(In,Ga)Se$\text{}_{2}$ material, affect also the luminescence yield. The dependence of the intensity and shape of the electroluminescence spectra on the state of the sample is observed. These results fit well into the theoretical calculations of Lany and Zunger model showing that divacancy complex (V$\text{}_{Se}$-V$\text{}_{Cu}$) is responsible for metastable changes observed in ZnO/CdS/Cu(In,Ga)Se$\text{}_{2}$-based solar cells. We conclude that during light soaking or/and forward bias the probability of nonradiative recombination is decreased

Persistent phenomena in the electrical characteristics of solar cells based on Cu In,Ga S2

Current voltage and capacitance voltage characteristics of ZnO CdS CuInGaS2 solar cells have been measured in order to investigate persistent changes induced by the reverse bias stress or light soaking. Significant metastable fill factor loss after red illumination under reverse bias condition has been observed. We show a correspondence between relaxation times of the capacitance and photovoltaic parameters after all treatments proving that defects in the absorber are responsible or the changes. Similar phenomena have been observed in Cu In,Ga Se2 based devices, therefore we propose similar explanation negative U defects in Cu In,Ga S2 which create a highly negatively charged layer close to the interface. They are responsible also for a difference between current transport in the dark and under illumination. Moreover, these defects seem to be a source of an limitation of the open circuit voltage in these devices

The Influence of Metastabilities on the Luminescence in the Cu(In,Ga)Se 2

Photoluminescence and electroluminescence spectra of the absorber layer in ZnO/CdS/Cu(In,Ga)Se$\text{}_{2}$ solar cells were measured. Their dependence on temperature, excitation intensity and applied voltage were studied. Electroluminescence measurements were used to investigate light- and bias-induced metastabilities in the absorber of the cells. We showed that metastable changes of defect distributions, which produce an effect on the electrical characteristics of ZnO/CdS/Cu(In,Ga)Se$\text{}_{2}$ material, affect also the luminescence yield. The dependence of the intensity and shape of the electroluminescence spectra on the state of the sample is observed. These results fit well into the theoretical calculations of Lany and Zunger model showing that divacancy complex (V$\text{}_{Se}$-V$\text{}_{Cu}$) is responsible for metastable changes observed in ZnO/CdS/Cu(In,Ga)Se$\text{}_{2}$-based solar cells. We conclude that during light soaking or/and forward bias the probability of nonradiative recombination is decreased

Capacitance spectroscopy of Cu(In,Ga)Se2-based solar cells with a Pt back electrode

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