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

A Method to Extract Potentials from the Temperature Dependence of Langmuir Constants for Clathrate-Hydrates

It is shown that the temperature dependence of Langmuir constants contains all the information needed to determine spherically averaged intermolecular potentials. An analytical ``inversion'' method based on the standard statistical model of van der Waals and Platteeuw is presented which extracts cell potentials directly from experimental data. The method is applied to ethane and cyclopropane clathrate-hydrates, and the resulting potentials are much simpler and more meaningful than those obtained by the usual method of numerical fitting with Kihara potentials.Comment: 33 pages, 7 figures, to appear in Physica

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

Modelling water vapor permeability through atomic layer deposition coated photovoltaic barrier defects

Transparent barrier films such as Al2O3 used for prevention of oxygen and/or water vapour permeation are the subject of increasing research interest when used for the encapsulation of flexible photovoltaic modules. However, the existence of micro-scale defects in the barrier surface topography has been shown to have the potential to facilitate water vapour ingress, thereby reducing cell efficiency and causing internal electrical shorts. Previous work has shown that small defects (≤ 3 μm lateral dimension) were less significant in determining water vapour ingress. In contrast, larger defects (≥ 3 μm lateral dimension) seem to be more detrimental to the barrier functionality. Experimental results based on surface topography segmentation analysis and a model presented in this paper, will be used to test the hypothesis that the major contributing defects to water vapour transmission rate are small numbers of large defects. The model highlighted in this study has the potential to be used for gaining a better understanding of photovoltaic module efficiency and performance

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

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