Kësterite thin films for photovoltaics: a review

In the years to come, electricity production is bound to increase, and Cu2ZnSn(S, Se)4 (CZTS) compounds, due to their suitability to thin-film solar cells, could be a means to fulfill the demand. After explaining the reasons of the sudden interest of the CIGS scientific community for CZTS solar cells, this paper reviews recent papers published on the subject of kësterites-based solar cells. After a description of crystallographic and optoelectronic properties, including CZTS crystalline structure, defect formation and metal composition, this review paper focuses on CZTS synthesis processes and device properties. Synthesis strategies, including oneor two-step processes, deposition temperature, binary formation control via atmosphere control and their effect on device properties are discussed. 1 Sustainability of photovoltaic industry In the years to come, electricity production is bound to increase (from 17 PWh in 2009 to 28–34 PWh in 2035 [1]) and as a consequence the increased demand on fossil fuels as well as possible regulations on CO2 emissions are bound to increase the wholesale electricity costs. On the other hand, the price-experience factor of 22% that was observed for the last 4 decades should stay at the same level or slightly below [2, 3], leading to a decrease of photovoltaic (PV) electricity costs [2,4]. A first change of paradigm is now happening, because in some places like South Italy and other sunny/high electricity price regions [5], the price of retail electricity is higher than the cost of PV electricity (grid parity). A second parity, the “fuel parity”, will happen when the cost of PV electricity is lower than the marginal costs of operating fossil fuel based power plants [6]. According to deployment scenarios, this fuel parity could happen between 2020 and 2040. The massive deployment of PV foreseen in years 2020−2040 could lead to 1.5 to 2.5 TWp of worldwide installed PV capacity in 2030, with a demand of 20−150 GWp/y and around 30% of the market taken by thin films [2, 4]. The question of the sustainability of the production of 20−150 GWp/y of PV was raised by a few publications, and in particular the worldwide mining and refining capacity of a few critical elements could be the main problem of the photovoltaic industry in the years to come. Each a e-mail: [email protected] PV technology has one or more limiting elements, such as silver for crystalline silicon, indium and gallium for CIGS, Tellurium for CdTe, Ruthenium for dye-sensitized solar cells, silver and indium for thin-film silicon, germanium, gold, indium for III-V PV [7, 8]. The chalcogenide technologies are particularly vulnerable to In, Ga, Te supply, which has been noted critical or near critical by the US Department of Energy [9] or the European Commission [10]. According to various authors, the production of PV modules could be limited below the demand reported earlier in this paper, as summarized in Table 1. Crystalline silicon PV seems suitable for supplying the demand, but one should keep in mind that the basic assumption used for the third and fourth column of this table is that all worldwide production of silver is used for crystalline PV. The fifth column shows the forecast 2030 demand for Ag, Te, In with respect to the 2008 production. It appears that the demand will grow enormously. If we focus on In, in 2030 the total demand (including PV and other uses) could be 3 to 7 times the 2008 production [10,11], whereas, as In is a byproduct of Zn extraction, the elasticity of the production with respect to demand is low. One author suggests that CIGS and CdTe technologies will not be impeded by resources problems, but he does not take the increase of other sources of demand for In and Te into account [12]. The sustainability of PV production is therefore a real question, and the development of a new PV technology based on abundant and preferably non-toxic elements would alleviate the pressure on all PV technologies in term of resources. This is an Open Access article distributed under the terms of the Creative Commons Attribution-Noncommercial License 3.0, which permits unrestricted use, distribution, and reproduction in any noncommercial medium, provided the original work is properly cited.