High levels of airborne dust, frequent dust storms and infrequent rain events are some of the reasons why soiling can drastically reduce the energy yield of photovoltaic modules in desert areas. There are ongoing and increasing efforts to identify appropriate and economically feasible strategies that can be used to mitigate soiling in deserts. Both innovative tracking with adapted resting positions during night and anti-soiling coatings (ASC) are considered as potential solutions to reduce soiling. In this study, the individual mitigation potential of both ASC and tracking routines as well as the combination of the two approaches are investigated. For this, outdoor exposure tests were carried out in desert region of Saudi Arabia. Coated and uncoated glass samples were tested in different tilt configurations: fixed, 1-axis tracking with horizontal stowage (facing the sky) and 1-axis tracking with vertical stowage during the night. Both methods indicate significant soiling reductions, especially for the combined solution of ASC and tracking with vertical night stowage, where soiling losses can be reduced by up to 85%. In addition, it has been shown that by adapting tracking, the relative ASC performance can be improved compared to fixed tilt or standard 1-axis tracking scenarios.
STC power control of PV modules supply requires testing large samples of modules with low uncertainty. This paper analyses the feasibility of outdoor measurements with the modules kept at their operating positions. The classical procedure of recording I-V curves and translating them to STC in accordance with IEC 60891 using the cell temperature directly observed at a few points of the rear of the module entails uncertainties larger than 3% (k=2), which is too much for this procedure being accepted in quality controls with contractual consequences. A convenient procedure for overcoming this barrier consists in comparing the I-V curves of a tested and a reference module of the same type, both working under the same operating conditions. The latter is mostly secured if they are in adjacent positions. However, when the procedure is applied to large samples of PV modules kept in their operating position, the distance between both modules can reach tens of meters and significant inter-module temperature differences can arise. An artifice for counterbalancing these differences consists of estimating the temperature of the tested module and the “true” temperature of the reference module, as deduced from the V OC measurement, by the temperature difference observed at their respective back-sheets in a central position. This allows the measured power values to be corrected and provides clues to estimating the uncertainty of the results. This procedure has been applied in seven testing campaigns, carried out at commercial PV plants. Dedicated instrumentation, based on two radio linked I-V tracers, allowing the simultaneous measurement of the I-V curves and of the temperature at the centres of the reference and the tested modules, has been developed for that. The resulting uncertainties are slightly larger than those corresponding to high-quality solar simulators, but still low enough for dealing, in practice, with strict quality control requirements.
Multijunction solar cells offer a path to very high conversion efficiency, exceeding 60% in theory. Under ideal conditions, efficiency increases monotonically with the number of junctions. In this study, we explore technical and economic mechanisms acting on tandem solar cells. We find that these mechanisms produce limitations that are the more pronounced the greater the number of junction is and, hence, limit the ideal number of junctions, as well as the corresponding efficiencies. Spectral variations induce current losses in series-connected tandem solar cells. For Denver, we find that these losses reduce achievable harvesting efficiencies to 51% for non-concentrated light, and that they restrict the ideal number of junctions to less than nine. Independently operated solar cells suffer from optical losses with similar consequences. Optical efficiencies of 99% restrict the ideal number of junctions to below ten, and reduce achievable efficiencies by more than 10%. Only architectures with a sequential cell illumination are more resilient to these losses. Restricting available materials reveals that a sufficiently low band gap for the bottom cell of 0.9 eV or below is expedient to realize high efficiencies. Economic considerations show that five junctions or less are economically ideal for most conceivable applications.
The present article discusses the investigation of CuIn 1-xGa xS 2 (CIGS) thin films for photovoltaic applications. For decades, a Cu-rich composition has been used to create solar cells with efficiencies of up to 13.5%; however, interest in chalcopyrite sulfide has recently been revived due to its high and adjustable bandgap, making it a serious candidate as a top cell in tandem configurations. Although chalcopyrite selenides share many properties with CIGS thin films, crucial differences have been reported. To further understand these materials, we studied more than 500 samples of absorbers and resulting solar cells. First, we found that the compositional window for obtaining single-phase CIGS thin films with a 3-stage co-evaporation process is very narrow. Second, we reported that a combination of low copper content and sodium addition during growth is required to maximize the Photoluminescence intensity ( i.e. to minimize the absorber-related open-circuit voltage losses). Finally, we showed that solar cell performance and stability depend not only on absorber quality but also on phenomena at interfaces (absorber/buffer and grain boundaries). Altogether, we formulate growth recommendations for the manufacture of stable CIGS/CdS solar cells with state-of-the-art efficiency.
Single junction crystalline silicon (c-Si) solar cells are reaching their practical efficiency limit while perovskite/c-Si tandem solar cells have achieved efficiencies above the theoretical limit of single junction c-Si solar cells. Next to low-thermal budget silicon heterojunction architecture, high-thermal budget carrier-selective passivating contacts (CSPCs) based on polycrystalline-SiO x (poly-SiO x) also constitute a promising architecture for high efficiency perovskite/c-Si tandem solar cells. In this work, we present the development of c-Si bottom cells based on high-temperature poly-SiO x CSPCs and demonstrate novel high-efficiency four-terminal (4T) and two-terminal (2T) perovskite/c-Si tandem solar cells. First, we tuned the ultra-thin, thermally grown SiO x. Then we optimized the passivation properties of p-type and n-type doped poly-SiO x CSPCs. Here, we have optimized the p-type doped poly-SiO x CSPC on textured interfaces via a two-step annealing process. Finally, we integrated such bottom solar cells in both 4T and 2T tandems, achieving 28.1% and 23.2% conversion efficiency, respectively.
In recent years, developing dopant-free carrier-selective contacts, instead of heavily doped Si layer (either externally or internally), for crystalline silicon (c-Si) solar cells have attracted considerable interests with the aims to reduce parasitic light absorption and fabrication cost. However, the stability still remains a big challenge for dopant-free contacts, especially when thermal treatment is involved, which limits their industrial adoption. In this study, a perovskite material ZnTiO 3 combining with an ultrathin (~1 nm) SiO 2 film and Al layer is used as an electron-selective contact, forming an isotype heterojunction with n-type c-Si. The perovskite/c-Si heterojunction solar cells exhibit a performance-enhanced effect by post-metallization annealing when the annealing temperature is 200-350 °C. Thanks to the post-annealing treatment, an impressive efficiency of 22.0% has been demonstrated, which is 3.5% in absolute value higher than that of the as-fabricated solar cell. A detailed material and device characterization reveal that post annealing leads to the diffusion of Al into ZnTiO 3 film, thus doping the film and reducing its work function. Besides, the coverage of SiO 2 is also improved. Both these two factors contribute to the enhanced passivation effect and electron selectivity of the ZnTiO 3-based contact, and hence improve the cell performance.