Interface Engineering of Chalcogenide Semiconductors in Thin Film Solar Cells: CdTe as an Example

Abstract

In this paper the electronic properties of the different interfaces of CdTe thin film solar cells will be analysed by using a surface science approach. Experimental basis for the experiments is an integrated UHV systems which allows to prepare and analyse real solar cells as well as appropriate model interfaces. Recently obtained data on the ITO surface, the ITO/SnO2/CdS front contact, the CdS/CdTe heterojunction and the CdTe/Te back contact will be presented. In addition, bulk properties as doping and lateral inhomogeneities will be addressed. For all these interfaces experimentally determined band energy diagrams will be given and discussed in relation to solar cell performance. Finally, the sum of the results will be used to propose a modified band energy diagram of the complete CdTe thin film solar cell and its implication for further cell improvement will be presented.

Thin-Film Photovoltaics as a Mainstream of Solar Power Engineering

“…It is known that in CdS/CdTe solar cells only the CdTe layer contributes to the light-toelectric energy conversion, while the CdS "window" layer only absorbs light in the range λ < 500-520 nm thereby reducing the photocurrent. Therefore in numerous papers a band bending (and hence a depletion layer) in CdS is not depicted on the energy diagram (see, for example, Birkmire & Eser, 1997;Fritsche et al, 2001;Goetzberger et al, 2003), i.e. the depletion layer of the CdS/CdTe diode structure is virtually located in the p-CdTe layer (Fig.…”

Interface Engineering of Inorganic Thin‐Film Solar Cells – Materials‐Science Challenges for Advanced Physical Concepts

“…Depending on the given interface properties, defect levels may be formed, which, even in the sub-monolayer regime, are sufficient to lead to Fermi level pinning effects, inferring contact formation (for reviews on fundamental issues of contact formation in relation to solar cells, see for example ref. [24,26,27] ), and also to fast surface recombination. For this reason, preparation of the films and the junctions must be possible with maximum control, avoiding unwanted contamination.…”

“…In the literature, very often, the high ionization potential of CdTe of 5.9 eV, given by the position of the valence-band edge, is considered to be a problem as metal contacts do not form an Ohmic contact for hole transport. [27] However, this argument is probably not valid, as for most well-defined metal contacts prepared with covalent semiconductors, the contact formation is not governed by the Schottky limit. [50] In the Schottky limit, the barrier height for holes is given by the difference between the metal work function and the semiconductor ionization potential.…”

Efficiency of Thin-Film CdS/CdTe Solar Cells

“…Therefore in numerous papers where the energy band diagram of a CdS/CdTe junction is discussed a band bending in the CdS layer (and hence a depletion layer) is not depicted (see, for example, Goetzberger et al, 2003;Birkmire & Eser, 1997;Fritsche et al, 2001). Analyzing the efficiency of CdS/CdTe solar cells, however, one is forced to assume the concentration of uncompensated acceptors in the CdTe layer to be 10 16 -10 17 cm -3 and even higher (a narrow depletion layer is assumed).…”