Thin Film

With the introduction of the alkali post-deposition treatment (PDT) for the absorber layer in Cu(In,Ga)Se2 (CIGS)-based solar cells, new efficiency records approaching 22% have become feasible. After gallium incorporation, sodium doping and the three-stage process, this is the next milestone on the CIGS roadmap. In this paper the current understanding of how PDT alters the CIGS surface and affects device parameters is illustrated. A comparative study of cell device parameters from ZSW and the evolution of efficiencies from other institutes and companies with and without PDT is presented.

Most development work in the laboratory is dedicated to efficiency enhancements at the cell level; improvements in efficiency can lead to higher cost-competitiveness of PV. However, the cost of panel manufacturing is an important aspect as well. For CIGS panels the deposition of the active layer is an important part of the cost, and decreasing the layer thickness can reduce costs. Moreover, cost of ownership calculations can determine how much benefit can be expected from thinner absorber layers from a cost perspective; clearly, a thinner absorber will result in reduced absorption. To avoid losses, modelling can be used to predict the efficiency and viable light management strategies. Other efficiency-enhancing technology is related to the fact that most thin-film solar panels are monolithically interconnected. The area loss involved in this type of interconnection, and the trade-off between conductivity and transmittance of the front contact, impose limits on the maximum efficiency. The impact of improving both of these aspects is demonstrated in this paper. A viable way to improve the front contact is by supplementing the front contact with a metallic pattern. The benefit and the impact of different configurations and dimensions of the cell and metallic pattern are presented.

The current standards (IEC 61646 and IEC 61730-2, and IEC 62804 draft for c-Si only) are clearly insufficient to guarantee satisfactory long-term stability and energy yield for thin-film modules, given that reports from the field, as well as from laboratory test results (beyond IEC testing), in some cases show significant degradation of IEC-certified modules. Accordingly, thin-film modules can also exhibit degradation effects, such as TCO corrosion and power degradation, because of potential-induced degradation (PID). This paper presents the results obtained for thin-film modules subjected to bias and damp-heat (BDH) conditions in both indoor and outdoor tests. In order to assess module lifetimes for different thin-film technologies with respect to PID, indoor- and outdoor-determined leakage currents are compared and analysed, taking into account weather data and results from accelerated ageing tests. Finally, on the basis of simulations and investigations for different installation locations, module lifetimes are estimated and discussed.

During the past two years remarkable performance improvements have been reported for polycrystalline Cu(In,Ga)Se2 (CIGS), CdTe and perovskite thin-film solar cells. In this paper the key breakthroughs in CIGS thin-film technology are reviewed and the scope for further performance improvements by analysing the stillremaining electrical and optical losses in record-efficiency CIGS solar cells is discussed. On the basis of this analysis it is believed that conversion efficiencies up to 25% are achievable with CIGS solar cells in the mid term. Furthermore, the potential for the concept of polycrystalline multi-junction solar cells to push efficiencies even further, towards 30%, is discussed. Finally, a short review of the CIGS market and an outlook from an industrial perspective are presented.

A detailed analysis of state-of-the-art CIGS technology has resulted in a direct cost of ownership (CoO) of €0.44/Wp for this PV module type. However, the reduction in production costs, although impressive, is not sufficient for CIGS to become competitive with today’s c-Si technology. In order to answer the question as to whether CIGS will ever be able to challenge c-Si, the cost-reduction potential of CIGS is investigated. The impact of savings is evaluated in respect of the material segment, production equipment, energy and labour, production yield, device efficiency and absorber thickness. A total cost-reduction potential of around €0.21/ Wp is identified, which would be enough to put CIGS back into the game (the direct CoO will continue to be dominated by material and equipment depreciation, adding up to 68%). These cost reductions, however, cannot be realized immediately: within the next two years, €0.03/Wp is expected to be feasible, while it will take two to four years for the next €0.107/Wp. For the final €0.073/Wp, a time frame of at least five years is predicted, with corresponding costs for the technology developments. Provided that someone is willing to spend the necessary amount of time and money, the second part of the answer regarding CIGS’ competitiveness will depend on how c-Si evolves within this time period.

For some years CIGS was seen as the great white hope of the PV industry, until c-Si revealed its true competitiveness in mass production. Most companies dedicated to the commercialization of CIGS, many of which were VC financed, did not survive this development. Nonetheless, the industry has recently seen new corporate entrants with impressive plans for the roll-out of CIGS. The motives for these strategic actions are of interest, so a cost-of-ownership calculation was performed for a state-of-the-art CIGS production: the result is that current production cost for a CIGS module is €0.44/Wp, with material and depreciation being the main cost drivers. Although significant progress has been made in the last few years, this is still higher than the production costs for standard c-Si modules. However, the costs for CIGS coating materials, which correspond to the wafer in a c-Si module, are significantly lower than those for a wafer. Could this be a motive for the actions that have been witnessed in the CIGS industry? The next task would be to evaluate the further costreduction potential of CIGS and the likelihood of its realization.

This paper describes the degradation of sputtered aluminium-doped zinc oxide (ZnO:Al) layers which were exposed to damp heat (85°C/85% relative humidity). The ZnO:Al samples were characterized by electrical, compositional and optical measurements before, during and after damp heat exposure. Hall measurements showed that the carrier concentration stayed constant, while the mobility decreased and the overall resistivity thus increased. This mobility decrease can be explained by the enhancement of the potential barriers at the grain boundaries because of the occurrence of additional electron-trapping sites. X-ray diffraction (XRD) and optical measurements demonstrated that the crystal structure and transmission in the range 300 –1100nm did not change, thereby confirming that the bulk structure stayed constant. Depth profiling showed that the increase of the potential barriers was caused by the diffusion of H2O/OH- through the grain boundaries, leading to adsorption of these species or to the formation of Zn(OH)2 or similar species. Depth profiling also revealed the presence of carbon, chloride and sulphide in the top layer, which indicates the possible presence of Zn5(CO3)2(OH)6, Zn5(OH)8Cl2•H2O and Zn4SO4(OH)6•nH2O. Furthermore, white spots appeared on the ZnO:Al surface during damp heat exposure. The spots contained elements, such as silicon and calcium, which might have migrated from the glass and which reacted with species from the environment, including oxygen, carbon and chlorine.

A critical failure mechanism of PV modules is the degradation in performance as a result of exposure to temperature and humidity during a typical product lifetime of over 25 years. The time to failure of a PV module attributable to moisture ingress under given field conditions involves multiple factors, including encapsulant and edge seal moisture barrier performance as well as the degradation rate of particular solar cells when exposed to moisture. The aim of the work presented here is to establish a conservative estimate of field lifetime by examining the time to breakthrough of moisture across the edge seal. Establishing a lifetime model for the edge seal independent of the characteristics of the encapsulant and solar cells facilitates the design optimization of the cells and encapsulant. For the accelerated testing of edge seal materials in standard temperature- and humidity-controlled chambers, a novel test configuration is proposed that is amenable to varying dimensions of the edge seal and is decoupled from encapsulated components. A theoretical framework that accounts for the presence of desiccants is developed for analyzing the moisture ingress performance of the edge seal. Also developed is an approach to analyzing test data from accelerated testing which incorporates temperature dependence of the material properties of the edge seal. The proposed equations and functional forms have been validated by demonstrating fits to experimental test data. These functional forms and equations allow the prediction of edge seal performance in field conditions characterized by historical meteorological data. In the specific case of the edge seal used in certain MiaSolé glass–glass modules, this work has confirmed that the edge seal can prevent moisture ingress well beyond the intended service lifetime in the most aggressive climate conditions evaluated.

Recent advances in cadmium telluride (CdTe) research and development have improved the long-term power output degradation and extended reliability test performance of First Solar’s thin-film CdTe PV modules. This paper reviews the characterization results of the new First Solar cell structure with improved back-contact design that better manages the fundamental power-output degradation mechanism. First Solar's proprietary ‘Black’ series module construction significantly enhances the long-term durability and extended test performance of the modules. The accelerated lab-testing methods, field testing and associated analyses are discussed. These advances in the solar cell performance, coupled with upgraded module materials, further substantiate the long-term power-generating capability of First Solar's CdTe PV modules in harsh operating conditions.

Economic issues are the driving forces behind PV adoption. Even technological advances are measured against their impacts on cost per watt, levelized cost of energy (LCOE), and total cost of ownership for energy (TCOe™). This sixth paper in a series covering business analysis for PV processes looks at two approaches to manufacturing thin-film copper-indium-gallium-diselenide (CIGS) PV – sputtering and co-evaporation – and their potential areas for cost improvement.