Thin film solar cells are favorable because of their minimum material usage and rising efficiencies. The three major thin film solar cell technologies include amorphous silicon (α-Si), copper indium gallium selenide (CIGS), and cadmium telluride (CdTe). In this paper, the evolution of each technology is discussed in both laboratory and commercial settings, and market share and reliability are equally explored. The module efficiencies of CIGS and CdTe technologies almost rival that of crystalline solar cells, which currently possess greater than 55% of the market share. α-Si is plagued with low efficiency and light-induced degradation, so it is almost extinct in terrestrial applications. CIGS and CdTe hold the greatest promise for the future of thin film. Longevity, reliability, consumer confidence and greater investments must be established before thin film solar cells are explored on building integrated photovoltaic systems.

A review of thin film solar cell technologies and challenges

Stability of CdTe/CdS thin-film solar cells

This paper details the laboratory processes used to fabricate CdS/CdTe solar cells at the National Renewable Energy Laboratory. The basic fabrication technique includes low-pressure chemical vapor deposited SnO2 , chemical-bath deposited CdS, close-spaced sublimated CdTe, solution-CdCl2 treatment, and an acid-contact etch, followed by application of a doped-graphite paste. This paper also describes the results of a reproducibility study in which cells were produced by multiple operators with an average AM1·5 efficiency of 12·6%. And finally, this paper discusses process sensitivities and alternative cell fabrication procedures and reports the fabrication of a cell with an AM1·5 efficiency of 15·4%. Copyright © 1999 John Wiley & Sons, Ltd.

Fabrication procedures and process sensitivities for CdS/CdTe solar cells

Cadmium telluride is a promising thin‐film photovoltaic material as shown by the more than 10% efficient CdS/CdTe heterojunction solar cells. In this work, thin‐film CdS/CdTe solar cells have been prepared using CdS films grown from an aqueous solution and p‐CdTe films deposited by close‐spaced sublimation (CSS). The properties of CdS films deposited from an ammonical solution of a Cd‐salt, an ammonium salt, and thiourea have been controlled by optimizing the temperature and composition of the solution. The solution‐grown CdS films have a high photoconductivity ratio, and its optical transmission is superior to that of vacuum evaporated CdS films. The properties of p‐CdTe films deposited by CSS have been optimized by controlling the temperature and composition of the source material, and the substrate temperature. The properties of CdS/CdTe heterojunctions have been studied; junction photovoltage spectroscopy is used for the qualitative comparison of junction characteristics. Solar cells of 1‐cm2 area wit...

13.4% efficient thin‐film CdS/CdTe solar cells

This review centers on the status, and future directions of the cell and module technologies, with emphasis on the research and development aspects. The framework is established with a consideration of the historical parameters of photovoltaics and each particular technology approach. The problems and strengths of the single-crystal, polycrystalline, and amorphous technologies are discussed, compared, and assessed. Single- and multiple junction or tandem cell configurations are evaluated for performance, processing, and engineering criteria. Thin-film technologies are highlighted as emerging, low-cost options for terrestrial applications and markets. Discussions focus on the fundamental building block for the photovoltaic system, the solar cell, but important module developments and issues are cited. Future research and technology directions are examined, including issues that are considered important for the development of the specific materials, cell, and module approaches. Novel technologies and new research areas are surveyed as potential photovoltaic options of the future.

Photovoltaics: A review of cell and module technologies

Heating conditions of carbon electrodes for CdTe and the effect of impurities in the carbon paste on the characteristics of solar cells are investigated. The series resistance (Rs) and the conversion efficiency (ηi) of solar cells exhibit strong dependence on O2 concentration in the heating atmosphere. Addition of Cu to the carbon paste causes Rs and diode factor (n) to decrease, resulting in a remarkable improvement in ηi. As a result of preparation of low resistance contact electrode for CdTe, a cell with 0.78 cm2 active area which was made from 50 ppm Cu-added carbon paste showed Voc=0.754 V, Isc=0.022 A, FF=0.606 and ηi=12.8%.

Preparation of Low Resistance Contact Electrode in Screen Printed CdS/CdTe Solar Cell

The preparation conditions of low-resistance CdS sintered film and the effect of the resistance of CdS film on the photovoltaic properties of screen-printed CdS/CdTe solar cells were studied. CdS films were sintered in an alumina case with a perforated cover, and the rate of evaporation of CdCl2 in a printed CdS film was found to be influenced by the area of the holes in the cover. The resistivity of CdS film was less than 0.5 ?-cm for the samples sintered with the cover (cover area is 102 cm2) which had a hole area between 0.4 and 1.4 cm2. CdS/CdTe solar cells prepared from CdS films with resistivities of about 0.3, 0.7 and 1.4 ?-cm had conversion efficiencies of 8, 6 and 4% respectively. The decrease in efficiency was due to the increase in series resistance, which depended on the sheet resistance of the CdS film.

Effect of Resistivity of CdS Sintered Film on Photovoltaic Properties of Screen-Printed CdS/CdTe Solar Cell

A solar cell module having a CdS/CdTe junction as a photovoltaic portion comprises a glass substrate, a metal foil, a layer of resin bonding the glass substrate and the metal foil together at a peripheral region of the module. For minimizing the reduction in conversion efficiency, an oxygen releasing agent is filled in the space delimited by the glass substrate, the metal foil and the resin layer.

solar cell module

30×30 cm2cds/cdte single substrate module prepared by screen printing method

Some stability studies have been conducted on screen printed CdS/CdTe solar cells Encapsulated CdS/CdTe modules exhibited excellent stabilities in output performance over 206 days when subjected to outdoor tests under various loading conditions Accelerated tests were performed on unencapsulated cells No degradation was observed after 90 days in continuous illumination test, A decrease in conversion efficiency due to an increase in series resistance of the unencapsulated cell took place under high temperature and high humidity tests It is concluded that the increase in series resistance is caused by a decrease in adhesion between CdTe and C layers, and by swelling of the carbon layer with water absorption Improved cells showed no degradation in efficiency over periods of 110 days at 100°C and over periods of 28 days at 65°C in 95% relative humidity

Kiyoshi Kuribayashi

Papers

Preparation of Low Resistance Contact Electrode in Screen Printed CdS/CdTe Solar Cell

Effect of Resistivity of CdS Sintered Film on Photovoltaic Properties of Screen-Printed CdS/CdTe Solar Cell

solar cell module

30×30 cm2cds/cdte single substrate module prepared by screen printing method

Stability of Screen Printed CdS/CdTe Solar Cells