Nano-structured CdTe, CdS and TiO2 for thin film solar cell applications

https://doi.org/10.1016/j.solmat.2004.02.006Get rights and content

Abstract

Nano-crystalline CdTe and nano-structured CdS films in crystalline, porous and fiber forms were deposited on ITO-coated glass substrates. CdTe films exhibited a typical particle size of 10nm and a blue shift in the absorption with an effective band gap of 2.8eV. CdS films exhibited a typical particle size of 15nm and an effective band gap of 2.98eV. Also, porous CdS and porous TiO2 films were deposited on plastic substrates by a self-assembly method. Typical pore sizes were 80 and 70nm, respectively. These can be used in nano-structured solar cell configuration where the pores are filled with a suitable absorber material. Additionally, CdS fibers and nano-crystalline films of TiO2 on plastic substrates were fabricated. Typical particle size in TiO2 was 10nm and CdS fibers were about 80nm wide and 1.5μm long.

Introduction

Many nano-structured materials are now being investigated for their potential applications in photovoltaic, electro-optical, micromechanical and sensor devices [1], [2], [3]. Our interest lies in taking advantage of the benefits offered by nanotechnology to make inexpensive and efficient solar cells on a large scale. To this end, nano-structured layers in thin film solar cells offer three important advantages. First, due to multiple reflections, the effective optical path for absorption is much larger than the actual film thickness. Second, light generated electrons and holes need to travel over a much shorter path and thus recombination losses are greatly reduced. As a result, the absorber layer thickness in nano-structured solar cells can be as thin as 150nm instead of several micrometers in the traditional thin film solar cells [4]. Third, the energy band gap of various layers can be tailored to the desired design value by varying the size of nano-particles. This allows for more design flexibility in the absorber and window layers in the solar cell. In particular nano-structured CdS, CdTe and TiO2 are of interest as window and absorber layers in thin film solar cells [5], [6], [7], [8], [9], [10]. Fabrication and characterization of these films constitute the focus of this work.

Production of large-area solar cells would also require a process for inexpensive fabrication of large periodic arrays of semiconductor nanostructures that will allow for (a) controlled variations in the size and composition of the nanostructures, (b) encapsulation of the semiconductor nanostructures in a rugged host material, (c) flexibility to use a variety of substrate materials, and, preferably, (d) compatibility with standard silicon fabrication techniques. Self-assembly is such a process. We have used the self-assembly process to fabricate a variety of nano-structured films including CdTe and CdS on ITO-coated glass substrates. In addition nano-porous CdS and TiO2 films were fabricated on a plastic substrate with a view to making devices on a lightweight, flexible substrate. Fabrication techniques and material characteristics of these films and their applications to solar cells are described below. Results on CdTe, CdS and TiO2 are presented in 2 Nano-crystalline CdTe films, 3 Nano-structured CdS films, 4 Nano-structured TiO, respectively.

Section snippets

Nano-crystalline CdTe films

Nano-crystalline CdTe films are of interest because of their potential applications as n-type window layers in a p–n homojunction thin film CdTe solar cell (Fig. 1) and in electroluminescent displays devices [11], [12], [13]. CdTe nanocrystals were prepared by microwave-assisted synthesis and films were cast from colloidal solutions containing nano-CdTe. These films were characterized by optical absorption, photoluminescence spectroscopy, profilometer and scanning electron microscopy on glass

Nano-structured CdS films

Nano-structured CdS was fabricated in crystalline, porous and fiber forms. The fabrication and characterization of these films are described below.

Nano-structured TiO2 films

TiO2 films were deposited on plastic substrates by the sol–gel method. The TiO2 sol was prepared using the standard procedure starting with high-purity reagents, titanium tetra isopropoxide, isopropanol and nitric acid (70% redistilled). This method is based on the hydrolysis of metal alkoxides or ethoxides in alcoholic solutions in the presence of acid catalysts. The procedure for the preparation involved the dissolution of TTIP as precursor in isopropanol as a solvent, followed by adding

Conclusions

Nano-crystalline CdTe films were deposited on ITO coated glass substrates and characterized. The results indicated particle sizes of 10nm and a blue shift in the absorption with an effective band gap of 2.8eV. This opens the possibility of using nano-crystalline n-type CdTe as a window layer in an n-CdTe/p-CdTe homojunction solar cell. Nano-crystalline CdS films on ITO-coated glass substrates exhibited particle sizes of 15nm and an effective band gap of 2.98eV as compared to the 2.4eV value for

Acknowledgements

The authors thank the Kentucky Science and Engineering foundation for support through Grants KSEF-148-502-02-27 and KSEF-148-502-03-68. One of the authors (VKR) thanks Dr. Shaik Jeelani, Director, T-CAM, Tuskegee University for support and encouragement.

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