Among the armoury of photovoltaic materials, thin film heterojunction photovoltaics continue to be a promising candidate for solar energy conversion delivering a vast scope in terms of device design and fabrication. Their production does not require expensive semiconductor substrates and high temperature device processing, which allows reduced cost per unit area while maintaining reasonable efficiency. In this regard, superstrate CdTe/CdS solar cells are extensively investigated because of their suitable bandgap alignments, cost effective methods of production at large scales and stability against proton/electron irradiation. The conversion efficiencies in the range of 6–20% are achieved by structuring the device by varying the absorber/window layer thickness, junction activation/annealing steps, with more suitable front/back contacts, preparation techniques, doping with foreign ions, etc. This review focuses on fundamental and critical aspects like: (a) choice of CdS window layer and CdTe absorber layer; (b) drawbacks associated with the device including environmental problems, optical absorption losses and back contact barriers; (c) structural dynamics at CdS–CdTe interface; (d) influence of junction activation process by CdCl2 or HCF2Cl treatment; (e) interface and grain boundary passivation effects; (f) device degradation due to impurity diffusion and stress; (g) fabrication with suitable front and back contacts; (h) chemical processes occurring at various interfaces; (i) strategies and modifications developed to improve their efficiency. The complexity involved in understanding the multiple aspects of tuning the solar cell efficiency is reviewed in detail by considering the individual contribution from each component of the device. It is expected that this review article will enrich the materials aspects of CdTe/CdS devices for solar energy conversion and stimulate further innovative research interest on this intriguing topic.

Physics and chemistry of CdTe/CdS thin film heterojunction photovoltaic devices: fundamental and critical aspects

Flexible CIGS, CdTe and a-Si:H based thin film solar cells: A review

Cadmium Telluride Solar Cells

Scarcity of indium and tellurium has been highlighted as a potential barrier for cadmium telluride (CdTe) and copper indium gallium (di)selenide (CIGS) thin film technologies to achieve both large‐scale deployment and reduction in production costs. With the aim of assessing the relative weight of the aforementioned concerns, this paper looks at indium and tellurium availability and implications for CIGS and CdTe production costs of their price increases. A breakdown of CdTe and CIGS module production costs is provided, accounting for the relative share of each component materials over total module cost. Conditions affecting usage of the key materials in CdTe and CIGS technologies are weighted against potential future indium and tellurium price escalation to provide a fuller picture of production cost implications for the thin film photovoltaics technologies. This includes exploring and quantifying the potential for innovation and technological developments (including efficiency increases, reduced absorber layer, higher material utilisation during deposition process) to ease the impact of possible future indium and tellurium supply constraints and price increases. Copyright © 2012 John Wiley & Sons, Ltd.

Implications for CdTe and CIGS technologies production costs of indium and tellurium scarcity

Photovoltaic (PV) technologies have experienced considerable growth rates of up to 70% in the last years. This has been possible because of low total CO 2 emissions and a positive overall energy balance for PV. Several institutions have developed future scenarios which show an increase in global electricity demand from 17 000 TWh in 2005 to some 60 000 TWh by 2050. A significant part of this amount should be supplied by PV installations. Based on selected scenarios material demand is calculated for four different PV technologies: crystalline silicon (c-Si), amorphous silicon (a-Si) in tandem configuration, cadmium tellurium (CdTe) and copper indium gallium diselenide (CIGS). As these technologies use rare metals it is shown, that particular scenarios are unlikely to be realized because of supply constraints and scarcity phenomena. Critical materials are silver, tellurium and indium. We consider photovoltaics as an appropriate example for the implementation of resource availability considerations into technology development strategies.

Considerations of resource availability in technology development strategies: The case study of photovoltaics

Thin-film CdTe cells: Reducing the CdTe

Fabricating high-efficiency CdS/CdTe solar cells with ultra-thin (below 1µm) absorber layers is a challenging yet highly desirable step in improving CdTe technology. Typically solar cell performance decreases due to shunting, incomplete absorption (deep penetration loss), fully depleted CdTe layers or interference between the main and the back contact junction when the CdTe layer thickness approaches a certain limit. While some of these losses are fundamental, others can be minimized by careful optimization of the fabrication steps. We present the results of such optimization.

10% efficiency solar cells with 0.5 µm of CdTe

Photovoltaic devices have been increasingly relied upon for distributed power generation at various levels (portable power, residential, commercial and municipal). One particularly appealing option for PV devices is their incorporation as building-integrated or vehicle-integrated structures which would serve both electrical generation and aesthetic requirements. Two particular embodiments of this application are semitransparent PV windows and curtain walls for residential/commercial buildings as well as PV sunroofs for vehicles. The technology of semitransparent PV modules with good photo-conversion efficiency and electrical performance is non-trivial. This report discusses the use of ultrathin devices based upon CdS/CdTe films augmented with color shifting technologies to meet these requirements.

Semitransparent PV windows with sputtered CdS/CdTe thin films

We have studied the effects on cell stability of the amount of copper in the back contact and the dependence on CdCl 2 activation treatment for sputtered CdS/CdTe cells. Unencapsulated cells were tested in the dark and at one sun at 85 °C and higher. We find that cells with > 3 nm of evaporated Cu (covered with 20 nm Au) had initially higher short-circuit current but were less stable than cells with limited Cu (1–3 nm) and that cells without intentional Cu in the back contact had neither better stability nor better initial performance. For cells with Cu in the back contact, a window of CdCl 2 treatment time was found for optimal stability. The cells without back-contact Cu were not very sensitive to the treatment time. With optimized CdCl2 treatment and Cu thickness, cells have shown very good stability. For cells with unoptimized Cu thickness or activation treatment, the decreases in fill factor (FF) and open-circuit voltage (Voc) were significant. We believe these changes may arise from possible formation of an oxide or telluride layer involving copper at the back contact and the migration of Cu from the back contact.

Effects of Cu and CdCl 2 treatment on the stability of sputtered CdS/CdTe solar cells

............................................................................................................................ iii Acknowledgements............................................................................................................v Table of

Studies of Sputtered CdTe and CdSe Solar Cells

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