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Can we see defects in capacitance measurements of thin‐film solar cells?
Author(s) -
Werner Florian,
Babbe Finn,
Elanzeery Hossam,
Siebentritt Susanne
Publication year - 2019
Publication title -
progress in photovoltaics: research and applications
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.286
H-Index - 131
eISSN - 1099-159X
pISSN - 1062-7995
DOI - 10.1002/pip.3196
Subject(s) - capacitance , stack (abstract data type) , admittance , optoelectronics , materials science , capacitive sensing , photovoltaic system , dopant , thin film , spectroscopy , solar cell , electrical engineering , chemistry , doping , physics , nanotechnology , computer science , electrical impedance , engineering , electrode , programming language , quantum mechanics
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)Se 2 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.