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CdTe is one of the leading absorbing materials for thin film solar cells [1], showing conversion efficiencies above 16% [2]. In the solar cells, polycrystalline CdTe thin films are grown with a thickness of a few micrometers onto polycrystalline CdS substrates with grain size ~100 nm. The lattice mismatch between the CdS buffer material and the CdTe absorber is 10% [3]. For such a large lattice mismatch, dislocation networks are present at atomically abrupt interfaces [3], which can act as electronic defects and cause recombination of photoexcited charge carriers.CdTe usually grows with a preferred (111) orientation on the polycrystalline CdS substrates [4]. The solar cell efficiency at this stage is rather bad [5,6] and a subsequent annealing step in a chlorine atmosphere (activation) is necessary to reach highest efficiencies. The activation leads to an interdiffused CdS/CdTe interface [7,8] and a recrystallized CdTe film with randomly oriented grains [4, 6, 7], leading to a strong reduction of carrier recombination [6,9].It is not yet known how the polycrystalline CdS film influences the growth, the texture and the properties of CdTe. A microscopic model of the growth can hardly be determined on polycrystalline samples. We have therefore investigated the growth of CdTe on single crystalline CdS surfaces with hexagonal (0001) and rectangular (10 10) structure with the emphasis on structure and chemical bonding at the interface.CdS single crystals with (0001) and (10 10) orientation where cleaned by sputtering with 400 eV argon ions for 20 minutes and subsequent annealing at 460 °C for 10 h. The low energy electron diffraction (LEED) pattern of the (0001) surface shows a hexagonal structure with a (2 × 2) reconstruction as typically observed for the cation terminated (111) surface of zincblende compounds [10]. The LEED pattern of the (10 10) surface exhibits a rectangular symmetry without reconstruction in good agreement with literature.CdTe was deposited by thermal evaporation of the compound from a home made effusion cell equipped with a resistively heated Al 2 O 3 crucible using a source temperature of 485 °C. The substrates were held either at T S = 220 °C or 340 °C during deposition. In order to obtain reasonable and comparable growth rates the distance between source and substrate was set to d = 6 cm and d = 3 cm for 220 °C and 340 °C substrate temperature, respectively. The layers were characterized by LEED and photoelectron spectroscopy using synchrotron radiation as supplied by the U49/2-PGM2 beamline of the BESSY storage ring.LEED patterns recorded from thick CdTe films grown at 340 °C (not shown) exhibit a diffuse background and re-The growth of CdTe on (0001) and (10 10) surfaces of CdS was studied using low energy electron diffraction and photoelectron spectroscopy. The results indicate oriented growth of the CdTe films with a faster nucleation of CdTe on CdS(0001), explaining the preferred (111) orientation of CdTe films on polycrystalline substrates. The faster nucleation on the (0001) surface is ...

We have investigated CdTe thin film solar cells without activation treatment and with CdCl2activation treatment at temperatures between 370 and 430 C using a constant activation time of25 min. For this purpose, CdS/CdTe layers were deposited by closed-space-sublimation on FTO coated float glass. The solar cells were characterized by measurements of the JV characteristics and quantum efficiencies. In addition, ion polished cross sections of the solar cells were prepared for high-resolution FE-SEM imaging of the microstructure and the simultaneous registration of electron beam induced current (EBIC) signal distribution. By measurement of the EBIC signal distribution, it can be shown that without activation treatment the CdTe grain boundaries itself and grain boundary near regions exhibit no EBIC signal, whereas centres of some singular grains already show a distinct EBIC signal. In contrast, after the chlorine activation treatment, the grain boundary near regions exhibit a significant higher EBIC signal than the centre of the grains. The results can be discussed as a direct evidence for defect passivation of grain boundary near regions by the chlorine activation treatment. At activation temperature of 430 C, additionally, a significant grain growth and agglomeration of the CdS layer can be recognized, which is linked with the formation of voids within the CdS layer and a deterioration of pn junction properties

In this paper a promising solution for the notorious problem of manufacturing a stable low ohmic back contact of a CdTe thin film superstrate solar cell is presented without using elemental copper. Instead we have used a Cu2O layer inserted between the CdTe absorber and metal contact (Au). In contrast to the barrier free band alignment gained by using the transitivity rules, XPS measurements show a barrier in the valence band of the Cu2O layers directly after deposition, which results in a low performing JV curve. The contact can be improved by a short thermal treatment resulting in efficiencies superior to copper based contacts for standard CdS/CdTe hetero junction solar cells prepared on commercial glass/FTO substrates. By replacing the CdS window layer with a CdS:O buffer layer efficiencies of >15% could be achieved. Copyright © 2016 John Wiley & Sons, Ltd.

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