Voltage Dependent Carrier Collection in CdTe Solar Cells
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Voltage Dependent Carrier Collection in CdTe Solar Cells D.L. Bätzner1, Guido Agostinelli2, A. Romeo1, H. Zogg1 and A.N. Tiwari1 1 Thin Films Physics Group, Laboratory for Solid State Physics, Swiss Federal Institute of Technology Zurich, Technopark, ETH-Building, Technoparkstr.1, CH-8005 Zurich Tel: +41-1-4451472, Fax: +41-1-4451499; E-mail: [email protected] 2 European Commission, Joint Research Centre, 21020 Ispra (VA), ITALY
ABSTRACT The measurement of quantum efficiency with bias voltage is a powerful tool to characterize CdTe/CdS solar cell. As the quantum efficiency changes drastically with bias it will be referred to as Apparent Quantum Efficiency AQE. The AQE gives insight to the spectral contents of the cell current and therefore resolves the spatial carrier collection in the cell at each working point. So it is possible to understand the influence of the junctions and changing resistances in the cell. The photoconductivity of CdS facilitates AQE well above unity, i.e. up to 100, at high forward bias. The spectral sensitivity of the CdS photoconductivity affects the cell current strongly. This can explain the dependence of fill factor and roll-over of the I-V characteristics on the spectral content of illumination. Further more back contact junction influence and defect related features, such as sub band gap generation, are evident in the AQE for high forward bias.
INTRODUCTION The quantum efficiency QE of a solar cell gives valuable information about the spectral composition of the cells current, which is determined by the carrier generation and collection profiles g(x,λ) and f(x) respectively. The QE is defined at short circuit conditions with bias illumination to keep the cell at a reasonable injection level. QEs measured with applied bias voltage and different injection levels can vary significantly from the defined QE and therefore are referred to as apparent quantum efficiencies AQEs. The AQE(V) is determined by the magnitude of shunt and series resistance, junctions and their space-charge layers widths and blocking (presence of reverse diodes, e.g. back contact) effects. It can be expressed as the sum of two major contributions. The bulk collection and collection connected to a ’gain medium’.
VOLTAGE DEPENDENT QUANTUM EFFICIENCY MEASUREMENTS The quantum efficiencies have been measured with lock-in-technique in the range of 300 nm to 1050 nm using a grid-monochromator. Signal and phase have been monitored with an EG&G lock-in-amplifier. A custom made trans-impedance-amplifier was used to apply bias voltage in order to keep the cell at the adjusted work point during measurements. The AQEs were measured without and with bias illumination of around 200W/m2 to prevent the heating of the cell. I-V measurements under Standard Test Conditions and in the dark completed the characterization of the cells as well as C-V measurements in the dark. H5.17.1 Downloaded from https://www.cambridge.org/core. Iowa State University Library, on 26 Jan 2019 at 08:02:13, subject to the Cambridge Core terms of use, av
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