Electroluminescence from Cu(In,Ga)Se 2 Thin-film Solar Cells
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1012-Y03-08
Electroluminescence from Cu(In,Ga)Se2 Thin-Film Solar Cells Thomas Kirchartz1, Julian Mattheis2, and Uwe Rau3 1 Institut f¸r Physikalische Elektronik, Universit‰t Stuttgart, Pfaffenwaldring 47, Stuttgart, 70569, Germany 2 Q-Cells AG, Guardianstrafle 16, Thalheim, 06766, Germany 3 IEF-5, Photovoltaik, Forschungszentrum J¸lich, J¸lich, 52428, Germany
ABSTRACT We compare the electroluminescence (EL) of three polycrystalline ZnO/CdS/Cu(In,Ga)Se2 heterojunction solar cells with similar bandgaps but different open circuit voltages, indicating a difference in the electronic quality of the absorber. Temperature dependent electroluminescence measurements reveal that all cells feature transitions from donor-acceptor pair recombination at lower temperatures to band to band recombination at higher temperatures. However, the less efficient cells show a longer transition range with donor-acceptor pair recombination still apparent at room temperature. The thus broadened room temperature luminescence is one effect which reduces the open circuit voltage of the devices below the Shockley-Queisser-limit. The other effect is the existence of non-radiative recombination currents, which determine the efficiency of the device as light emitting diode. To quantify the open circuit voltage losses, we use reciprocity relations between electroluminescent and photovoltaic action of solar cells, which allow us to predict the light emitting diode efficiency. Measurements support the theory and show that Cu(In,Ga)Se2 solar cells reach external LED efficiencies approaching QLED = 0.1 % . INTRODUCTION Recent work [1,2] showed both theoretically and experimentally the connections between the two complementary optoelectronic situations of a pn-junction diode ñ namely the ëlightemitting-diode-situationí and the ësolar-cell-situationí. Although the requirements for a perfect solar cell and a perfect LED lead to the same device, the requirements differ in real devices. To identify connections between imperfect LEDs and solar cells, a detailed analysis was done, which revealed two reciprocities, one at short circuit and one at open circuit conditions of the solar cell: (i) The equivalence of the spectral shape of the solar cell quantum efficiency close to the band gap and the electroluminescence of the same device operated as LED. (ii) The link between the open circuit voltage of a device operated as a solar cell and the LED quantum efficiency. The present contribution applies these newly found relations to the case of Cu(In,Ga)Se2 solar cells. We make a comparative study of the electroluminescence of three different Cu(In,Ga)Se2 solar cells at room temperature and for lower temperatures down to T = 80 K to evaluate how the dominant recombination mechanism changes over temperature. The Cu(In,Ga)Se2 solar cells are prepared by a three-stage co-evaporation process [3] from elemental sources on Mo coated glass substrate. The samples are equipped with a CdS buffer and a ZnO window layer. Due to process optimization, these Cu(In,Ga)Se2 devices pr
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