Expanded experimental space for luminescence studies of thin film CdS/CdTe solar cells
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Expanded experimental space for luminescence studies of thin film CdS/CdTe solar cells Scott Feldman, Tim Ohno, Victor Kaydanov, and Reuben Collins Physics Department, Colorado School of Mines Golden, CO 80401 ABSTRACT We have explored a large range of experimental space for photoluminescence (PL) and electroluminescence (EL) measurements of CdS/CdTe solar cells. This space includes changes in temperature, injection intensity (laser power for PL, current for EL), electrical bias for PL, and laser energy for PL. Measurements were resolved both spectrally and spatially (2D for EL, 1D for PL). Combination of EL and PL measurements revealed that most spatial inhomogeneity was the result of non-uniform current transport rather than local variations in recombination rate. The greatest spectral resolution was obtained with low temperature EL at low injection rates. High injection EL as well as high forward biased PL suppressed band-edge emission at low temperatures. Spectral structure was found to be greater in EL than in PL. These effects likely originated from preferential current transport along grain boundaries and/or certain grains.
INTRODUCTION CdS/CdTe photovoltaics represents one of the leading technologies for manufacture of polycrystalline thin film solar cells. State of the art CdTe devices have reached efficiencies of 16.5% [1]. Yet much is still not understood about the basic properties of the material and device structure, limiting further advances in conversion efficiency. One factor that hinders cell performance is the non-uniform nature of the devices [2]. Though there are many techniques capable of probing on the scale of individual grains, such microscopic techniques are difficult and time-consuming to perform, disallowing the study of large sample sets. However, several simpler techniques have been employed to quantify mesoscale (larger than grain size, smaller than cell size) non-uniformities including laser beam induced current [3], photoluminescence (PL) mapping [4], and electroluminescence (EL) [5]. Besides studying spatial non-uniformities, EL and PL can also be resolved spectrally to study defects. In this paper, we use EL and PL with simultaneous spatial and spectral resolution. We utilize a large range of experimental variation including injection level, temperature, bias (light for EL, electrical for PL), and laser energy for PL. EL and PL are techniques that are complimentary to each other. Both methods utilize the detection of radiative recombination of excess carriers. However, the method of carrier injection differs. With EL, carriers are injected electrically with forward bias. With PL, carriers are injected optically with a laser. The electrical nature of EL makes it very sensitive to transport properties of the material. Because of the direct optical injection, PL is less sensitive to electrical transport. Combination of both techniques allows one to isolate effects due to transport. In order to observe EL, one needs a complete device whereas PL is also applicable to films and s
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