Cdte-Based Thin Film Solar Cells: Past, Present and Future
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energies Review CdTe-Based Thin Film Solar Cells: Past, Present and Future Alessandro Romeo * and Elisa Artegiani Laboratory for Photovoltaics and Solid-State Physics (LAPS), Department of Computer Science, University of Verona, Ca’ Vignal 1, Strada Le Grazie 15, 37134 Verona, Italy; [email protected] * Correspondence: [email protected]; Tel.: +39-045-8027-936 Abstract: CdTe is a very robust and chemically stable material and for this reason its related solar cell thin film photovoltaic technology is now the only thin film technology in the first 10 top producers in the world. CdTe has an optimum band gap for the Schockley-Queisser limit and could deliver very high efficiencies as single junction device of more than 32%, with an open circuit voltage of 1 V and a short circuit current density exceeding 30 mA/cm2. CdTe solar cells were introduced at the beginning of the 70s and they have been studied and implemented particularly in the last 30 years. The strong improvement in efficiency in the last 5 years was obtained by a new redesign of the CdTe solar cell device reaching a single solar cell efficiency of 22.1% and a module efficiency of 19%. In this paper we describe the fabrication process following the history of the solar cell as it was developed in the early years up to the latest development and changes. Moreover the paper also presents future possible alternative absorbers and discusses the only apparently controversial environmental impacts of this fantastic technology. Keywords: solar cells; thin films; CdTe 1. Introduction Citation: Romeo, A.; Artegiani, E. CdTe solar cell thin film photovoltaic technology was introduced in the early seventies CdTe-Based Thin Film Solar Cells: of the last century [1] and it is now the only thin film technology in the first 10 top producers Past, Present and Future. Energies in the world [2]. 2021, 14, 1684. https://doi.org/ This is because CdTe is a very robust and highly chemically stable and, also for this 10.3390/en14061684 reason, can be deposited with a large variety of methods available making it very much ideal for production in large area. Academic Editor: Jean-Michel Nunzi CdTe has an optimum band gap near 1.5 eV according to the Schockley-Queisser limit so that it could deliver efficiencies around 32%, with an open circuit voltage of more than Received: 9 February 2021 1 V and a short circuit current density of more than 30 mA/cm2 [3]. Accepted: 12 March 2021 Published: 18 March 2021 CdTe solar cells were introduced for the first time in 1972, by Bonnet and Raben- horst [1], with a CdS/CdTe heterojunction delivering a 6% efficiency. Since then, thin film Publisher’s Note: MDPI stays neutral CdTe based solar cells have been fabricated with this type of heterojunction until it has with regard to jurisdictional claims in been radically changed in the recent years. published maps and institutional affil- A series of improving steps introduced (among them: high substrate temperature iations. during CdTe deposition, CdCl2 activation treatment, copper doping) brought to high conversion factors above 15%. Ferekides et al. [4] demonstrated an efficiency of 15.8%, followed by the 16.5% record which was obtained at the National Renewable Energy Laboratories (NREL) by Wu et al. [5]. The strong improvement in efficiency in the last 5 years was obtained by a new re- Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. design of the CdTe solar cell device, removing CdS and considering new features such as This article is an open access article band gap grading, copper embedding, and more transparent n-layer. With these changes distributed under the terms and First Solar have delivered a 21.5% efficiency device [6] later improved to 22.1% [7], the conditions of the Creative Commons stunning fact is that these results have been gained from materials obtained from the pro- Attribution (CC BY) license (https:// duction line. This demonstrates the high potential in terms of scalability of this technology, creativecommons.org/licenses/by/ in fact a large area module of 19% has been delivered [7]. In the next sections both old and 4.0/). new configuration of the CdTe solar cell will be described. Energies 2021, 14, 1684. https://doi.org/10.3390/en14061684 https://www.mdpi.com/journal/energies Energies 2021, 14, x FOR PEER REVIEW 2 of 25 Energies 2021, 14, 1684 2 of 24 duction line. This demonstrates the high potential in terms of scalability of this technol- ogy, in fact a large area module of 19% has been delivered [7]. In the next sections both old and new configuration of the CdTe solar cell will be described. Like many other thin film film technologies, technologies, CdTe CdTe solar solar cells cells can can be be fabricated fabricated in in a away way wherewhere the sunlight passes passes through through the the supporting supporting glass glass or ornot, not, in inthe the first first case case the thecell cellis isfabricated fabricated in insuperstrate superstrate configuration, configuration, in the in second the second case casein substrate in substrate configuration configuration (see (seeFigure Figure 1). 1). FigureFigure 1. Structure of a standard superstrate CdTe CdTe solar solar cell cell.. However for CdTe devices, superstrate configuration configuration is is the the best best option option in in terms terms of of efficiency,efficiency, mainlymainly becausebecause thisthis structurestructure allows allows the the necessary necessary post post deposition deposition treatment treatment to beto effective,be effective, more more details details will will be reportedbe reported forward. forward. In this In this case, case, the windowthe window and a absorbernd ab- layerssorber (respectivelylayers (respectively CdS and CdS CdTe) and are CdTe) grown are ongrown the front on the contact, front contact, which is which a degenerately is a de- dopedgenerately semiconductor, doped semiconductor, typically an typically oxide layer, an oxide defined layer, as transparent defined as conducting-oxidetransparent con- (TCO).ducting The-oxide supporting (TCO). The substrate supporting is preferably substrate a low-costis preferably soda-lime a low glass-cost whensoda-lime the growth glass iswhen obtained the growth at substrate is obtained temperatures at substrate below temperatures 450 ◦C, otherwise below a 450 more °C expensive, otherwise alkali-free a more glassexpensive needs alkali to be-free applied glass in needs case of to high be substrateapplied in temperature case of high growth substrate (above temperature 500 ◦C). growthIn substrate(above 500 configuration °C). instead, the first layer that coats the substrate is usually molybdenum,In substrate a stable configuration metal that instead, works the as anfirst efficient layer tha backt coats contact the andsubstrate at the is same usually time avoidsmolybdenum, diffusion a stable of impurities metal that in works the subsequent as an efficient layers. back One contact important and at the benefit same of time this configurationavoids diffusion is theof impurities possibility in of the using subsequent opaque layers. substrates; One important so instead benefit of using of glass,this con- one canfiguration apply is flexible the possibility materials of such using as opaque thin metal substrates; foils or so polymers instead of and using deliver glass, aone flexible can deviceapply flexible [8]. At thematerials moment, such in as substrate thin metal configuration, foils or polymers record and efficiency deliver a for flexible flexible device CdTe cells[8]. At is the 13.6% moment, on a Mo/MoOx in substrate coated configuration, glass substrate record [efficiency9], while for on flexibleflexible CdTe polymer cells foils is with13.6% a MoOon a xMo/MoOx/Te back contact coated is glass 11.5% substrate with copper [9], while and 10.9% on flexible without, polymer if deposited foils with on steel a substratesMoOx/Te back [9]. Incontact superstrate is 11.5% configuration with copper it and is also 10.9% possible without, to perform if deposited flexible on steel CdTe sub- solar cellsstrates either [9]. byIn superstrate the application configuration of a special it flexibleis also possible polymer to or perform ultra-thin flexible flexible CdTe glass solar [10]. cells either by the application of a special flexible polymer or ultra-thin flexible glass [10]. 2. The Past and Present: CdTe/CdS Solar Cell Configuration 2. TheFor Past a very and longPresent period: CdTe/CdS of time Solar up to Cell 2015, Configuration CdTe solar cells have been prepared in superstrateFor a very configuration long period by of depositing time up to subsequently2015, CdTe solar on glasscells have substrate been aprepared transparent in conductivesuperstrate oxide,configuration a CdS layer, by depositing the CdTe assubsequently absorber layer, on andglass finally substrate the back a transparent contact. conductiveFor thin oxide, films a the CdS substrate layer, the represents CdTe as moreabsorber than layer, 98% ofand the finally material the soback it iscontact. necessary to chooseFor thin a low films cost the material substrate such represents as soda lime more glass than in 98% order of the limit material the costs. so it However is necessary soda limeto choose glass a is low limited cost bymaterial the presence such as ofsoda sodium lime glass and potassium in order limit that the can costs. diffuse However into the device degrading it and that make the glass soft at temperatures above 500 ◦C. We now describe all the different layers that compose the standard configuration, the majority of these is still used in the new configuration that will be shown in Section3. Energies 2021, 14, x FOR PEER REVIEW 3 of 25 soda lime glass is limited by the presence of sodium and potassium that can diffuse into the device degrading it and that make the glass soft at temperatures above 500 °C .