Interface properties and band alignment of Cu2S/CdS thin film solar cells

doi:10.1016/S0040-6090(03)00190-1

Thin Solid Films

Volumes 431–432, 1 May 2003, Pages 477-482

https://doi.org/10.1016/S0040-6090(03)00190-1 Get rights and content

Abstract

The stoichiometry and electronic properties of bulk Cu₂S thin films obtained by vacuum evaporation were investigated by optical spectroscopy, X-ray diffraction and photoemission spectroscopy. The Cu₂S/CdS heterojunction interface has been prepared in situ and characterized by photoelectron spectroscopy (X-ray photoemission spectroscopy and ultraviolet photoelectron spectroscopy) after each growth step under ultra high vacuum conditions. The XPS core level spectra as well as valence band spectra of the substrate Cu₂S and overlayer CdS were acquired after each step. From these measurements, a large overall band bending of 0.9 eV is observed. The valence band offset is determined to be ΔE_VB=1.2 eV and the conduction band offset is ΔE_CB=0±0.1 eV.

Introduction

The Cu_xS/CdS cell occupied an almost unique status and a prominent place in photovoltaics, being the most developed and the most efficient thin film solar cells for more than 25 years. Thin film photovoltaic cell of Cu_xS/CdS was the most promising solar energy conversion device for pilot plant production during the past decades due to the reported high conversion efficiency more than 9.1% [1], easy fabrication and low cost. However, the heterojunction structure between p-type Cu_xS and n-type CdS is a rather complicated system because of the interface between two materials with different electron affinities, band gaps and crystal structures. The lattice mismatch and interdiffusion of components cause defect states at or near the interface that strongly affect the junction properties. In particular, there is Cu diffusion into the CdS adjacent to the interface.

The popular method of fabrication of Cu_xS/CdS solar cells is to vacuum evaporate CdS followed by wet dipping to form Cu_xS [2], [3], [4], [5]. Some other growth methods of Cu_xS films include: the dry process [6], vacuum evaporation [7], [8], sputtering [9], spray pyrolysis [10], [11], etc. Although considerable research was done on Cu_xS/CdS cells combined with a number of theoretical models [12], [13], some important questions are still unsettled about the heterojunction formation, electronic properties, the nature of the recombination centers at the interface, and their influence on charge transport mechanism. The question of stability is also unresolved.

The band offset at the interface is one of the most important properties of semiconductor heterostructure, which can be used to design and optimize different contacts to minimize the loss of photogenerated carriers resulting a cell with higher conversion efficiency. The investigation of the electronic structures at the interface with X-ray photoemission spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) after each growth step can be used to determine the interfacial properties and the band offset of the prepared structures with high precision.

In this work, both CdS and Cu₂S films were vacuum evaporated in order to investigate the interface properties in situ under ultra high vacuum (UHV) conditions. To our knowledge, there is no report for experimental determination of band offsets in the Cu₂S/CdS system under UHV conditions. Although the band diagram is often predicted by the electron affinity rule (EAR), the EAR is not precise in many experimentally investigated heterostructures due to the occurrence of interface dipoles [14], [15].

Section snippets

Experimental section

The PES experiments were carried out in an integrated UHV system equipped with multi-technique surface analysis system (Physical Electronics 5700) photoelectron spectrometer (XPS, UPS) which allows in situ characterization of the prepared films and surfaces at a base pressure below 1×10⁻⁹ Torr. An Ar ion gun is available for in situ cleaning of the substrate surface. The XP spectra of the samples were measured using monochromatized Al Kα radiation. The binding energy scale was calibrated

Properties of bulk Cu₂S layer

A variety of Cu_xS phases exist at room temperature depending on preparation conditions used [16], [17]. The optical and electrical properties of Cu_xS vary considerably with the value of x. The structure and stoichiometry of the Cu_xS layer is the major factor determining the number of photogenerated carriers and the stability of the Cu_xS/CdS cell. Furthermore, the quantum efficiency of Cu_xS/CdS cells is strongly affected. It has been proven that chalcocite (for x⩾1.995) is most effective in

Summary and conclusions

In summary, highly p-doped Cu₂S films (VBM=0.1 eV) with well controlled thickness and stoichiometry can be obtained by vacuum evaporation under UHV conditions. The electronic properties at the interface of Cu₂S/CdS system were investigated in situ after each growth step by using X-ray photoelectron spectroscopy (XPS and UPS) measurements. The band lineup between Cu₂S and CdS was experimentally established. At the interface of the heterojunction, a large band bending of 0.9 eV, a valence band

Acknowledgements

We acknowledge financial support of this work by the Alexander von Humboldt Foundation through a fellowship for G. Liu and by the German Ministry for Science and Education within the framework of the ‘Hochspannungsnetz’ (grant no. 01SF0114).

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Interface properties and band alignment of Cu2S/CdS thin film solar cells

Abstract

Introduction

Section snippets

Experimental section

Properties of bulk Cu2S layer

Summary and conclusions

Acknowledgements

Thin Solid Films

Solar Energy Mater.

Thin Solid Films

Solar Energy Mater.

IEEE Trans. Electron. Dev.

J. Phys. D

J. Phys. D

Chem. Mater.

Chem. Mater.

J. Phys. Condens. Matter

Rev. Phys. Appl.

Interface properties and band alignment of Cu₂S/CdS thin film solar cells

Properties of bulk Cu₂S layer