Progress in Photovoltaics最新文献

The chemical and electronic structure of the front contact i-ZnO/In_xS_y:Na interface for Cu(In,Ga)(S,Se)₂-based thin-film solar cells is investigated using a combination of x-ray and electron spectroscopies. Upon i-ZnO sputter deposition on the In_xS_y:Na buffer layer, we find an intermixed heterojunction and the formation of InO_x and Na₂SO₄. The window layer is shown to consist of a mixture of Zn(OH)₂ and ZnO, with decreasing relative Zn(OH)₂ content for thicker window layers. Moreover, we observe diffusion of sodium to the surface of the window layer. We derive electronic surface band gaps of the i-ZnO and In_xS_y:Na layers of 3.86 ± 0.18 eV and 2.60 ± 0.18 eV, respectively, and find a largely flat conduction band alignment at the i-ZnO/In_xS_y:Na interface.

In the context of offshore floating photovoltaic systems (FPVs), this paper explores the use of bifacial photovoltaic modules installed in the vertical position. The energy harvested from the rear face of vertically configured bifacial PV modules compensates for the reduced production at the front face of the module, and this demonstrates the potential of bifacial technology for offshore applications. By comparison, most existing horizontally tilted bifacial FPV systems gain only a small benefit in production from the rear face of the module due to the minimum radiation received, and what also must be taken into consideration is the negative effect of significant soiling owing to the low tilt angle of the PV modules. Hence, to overcome these drawbacks, we have developed the innovative “PVSail” concept, which explores the deployment of vertical FPV systems on floats, buoys, or poles/minipiles. Floating vertical bifacial PV systems (VBPVs) have huge potential to harness all the energy generation capabilities enhance by reflected light, especially from snow-covered surfaces in northern regions. Our analysis considers a patented mooring and vertical PV system that allows the VBPV structure to align with the prevailing wind direction to shed wind loads, and our numerical analysis explores the potential of VBPV applied to Catania in Italy and Nigg Bay in the United Kingdom. Our analysis study has revealed that across an azimuth angle range (0°–180°), vertical bifacial modules experience roughly a 9% decrease in energy yield at Catania and about a 5% energy yield gain in higher latitude regions like Nigg Bay. Additionally, increasing the latitude of the installation location of VBPV reduces the energy yield sensitivity to the orientation, that is, azimuth angle. The PVSail concept opens the door to novel deployment possibilities in offshore renewable energy projects.

Heavy alkali-metal treatments have been the most recent breakthrough in improving the efficiency of Cu(In,Ga)Se₂ (CIGS) solar cells. Alkali halides are generally evaporated onto the surface of the CIGS thin film by a vacuum process. Here, we report an alternative, low-cost solution process for the surface treatment of CIGS thin films using cesium carbonate (CsCO₃) as a new route to incorporate cesium (Cs) for improving solar cell performance. CIGS thin films were fabricated using pulsed hybrid reactive magnetron sputtering and the surface treatment was performed by spin-coating CsCO₃ solution on the surface of CIGS at room temperature, followed by vacuum annealing at 400°C. The surface chemistry of the CIGS thin film changed after the treatment and the efficiency of respective solar cells improved by more than 30%, mostly driven by an enhancement in open-circuit voltage. X-ray photoelectron spectroscopy revealed the depletion of copper and the presence of Cs on the surface of the CIGS thin film. Ultraviolet photoelectron spectroscopy showed the lowering of the valence band maximum by around 0.25 eV after the treatment, which plays a positive role in reducing interfacial recombination. High-resolution transmission electron microscopy indicates the presence of Cs and depletion of Cu at the grain boundaries of the CIGS thin film. These findings open a low-cost route for improving the performance of CIGS solar cells by surface modification using a solution process.

This paper provides a comprehensive assessment of the up-to-date life-cycle sustainability status of cadmium-telluride based photovoltaic (PV) systems. Current production modules (Series 6 and Series 7) are analyzed in terms of their energy performance and environmental footprint and compared with the older series 4 module production and current single-crystalline Silicon (sc-Si) module production. For fixed-tilt systems with Series 6 modules operating under average US irradiation of 1800 kWh/m²/year, the global warming potential (GWP) is reduced from 16 g CO_2eq/kWh in Series 4 systems to 10 CO_2eq/kWh in Series 6 systems. For operation in US-SW irradiation of 2300 kWh/m²/year, the GWP is reduced from 11 to 8 CO_2eq/kWh and for 1-axis tracking systems operating in Phoenix, Arizona, with point-of array irradiation of 3051 kWh/m²/year the GWP is reduced to 6.5 CO_2eq/kWh. Similar reductions have happened in all environmental indicators. Energy payback times (EPBT) of currently installed systems range from 0.6 years for fixed-tilt ground-mounted installations at average US irradiation at latitude tilt installations to 0.3 years for one-axis trackers at high US-SW irradiation, considering average fossil-fuel dominated electricity grids with fuel to electricity conversion efficiency of 0.3. The resulting energy return on energy investment (EROI) also depends on the conversion efficiency of the electricity grid and on the operation life expectance. For a 30-year operational life and grid conversion efficiency of 0.3, EROI ranges from 50 (at US average irradiation) to 70 for US-SW irradiation. The EROI declines with increased grid conversion efficiency; for CdTe PV operating in south California with grid conversion efficiency of 49%, the EROI is about 50 and is projected to fall to 30 when the state's 2030 target of 80% renewable energy penetration materializes. Material alternatives that show a potential of further reductions in degradation rates and materials for enhanced encapsulation that would enable longer operation lives have also been investigated. A degradation rate of 0.3%/year, which has been verified by accelerated testing, is assumed in 30-year scenarios; this is projected to be reduced to 0.2%/year in the near-term and potentially to 0.1%/year in the longer term. With such low degradation rates and enhanced edge-sealing, modules can last 40- to 50-years. Consequently, all impact indicators will be proportionally reduced while EROI will increase. This detailed LCA was conducted according to ISO standards and IEA PVPS Task 12 guidelines. The study revealed that the choices of system models, methods and temporal system boundaries can significantly impact the results and points out to the need to include assumptions regarding these choices in the “transparency in reporting” requirements listed in the IEA PVPS Task 12 Guidelines.

Concentrating photovoltaics is an attractive route for achieving high power output with thin film solar cells, using low-cost optics. In this work, the performance of CdTe:As thin film solar cells on two different transparent conducting oxide (TCO)-coated substrates is investigated and compared under varying concentrated light intensities (1–6.3 Suns). Samples tested had CdZnS/CdTe:As devices deposited atop of either a soda-lime glass with a fluorine-doped tin oxide TCO or an ultra-thin glass (UTG) with an aluminium zinc oxide TCO and ZnO high-resistive transparent (HRT) layer. Device current density was found to increase linearly with increased light intensities, for both sample configurations. Power conversion efficiencies of both device samples decreased with increased light intensity, due to a decrease in fill factor. The fill factor, for both sample configurations, was affected by reducing shunt resistance with increasing illumination intensity. The two device types performed differently at the high illumination intensities due to their series resistance. Light-soaking devices under 6.3 Suns illumination intensity for 90 min showed no significant performance degradation, indicative of relatively stable devices under the highest illumination intensity tested. Efficiency limiting factors are assessed, evaluated and discussed.

Multi-junction photonic power converters (PPCs) are photovoltaic cells used in photonic power transmission systems that convert monochromatic light to electricity at enhanced output voltages. The junctions of a multi-junction PPC have overlapping spectral responsivity, which poses a unique challenge for spectrally resolved external quantum efficiency (EQE) measurements. In this work, we present a novel EQE measurement technique based on a wavelength-tunable laser system and characterize the differential multi-junction device-level EQE (dEQE_MJ) as a function of the monochromatic irradiance over seven orders of magnitude. The irradiance-dependent measurements reveal three distinct irradiance regimes with different dEQE_MJ. For the experimentally studied 2-junction GaAs-based device, at medium irradiance with photocurrent densities between 0.3 and 90 mA/cm², dEQE_MJ is independent of irradiance and follows the expected EQE of the current-limiting subcell across all wavelengths. At higher irradiance, nonlinear device response is observed and attributed to luminescent coupling between the subcells. At lower irradiances, namely, in the range of conventional EQE measurement systems, nonlinear effects appear, which mimic luminescent coupling behavior but are instead attributed to finite shunt resistance artifacts that artificially inflate dEQE_MJ. The results demonstrate the importance of measuring the device-level dEQE_MJ in the relevant irradiance regime. We propose that device-level measurements in the finite shunt artifact regime at low monochromatic irradiance should be avoided.

The reliability of CIGS solar systems in agricultural environments was investigated using an accelerated aging test. Both complete cells and representative stacks of selected layers and interfaces were exposed to humidity and temperature variations for 9 to 14 days with and without ammonium sulfate (NH₄)₂SO₄, an aerosol pollutant representative of agricultural activities. The performance evolution of complete cells was evaluated by J-V curves and EQE measurements. After 9 days, the presence of (NH₄)₂SO₄ led to a performance loss of 58%, significantly higher than the 37% loss observed without pollutants. Using computer calculations based on the two-diode model, it was possible to de-correlate some interactions between J-V parameters. The results of modeling suggested that the pollutant caused optical losses and conductivity loss of electrical contacts, presumably by corrosion. Sheet resistance and Hall effect measurements on the representative stacks of layers confirmed that the conductivity loss of ZnO:Al (AZO) after 14 days of aging strongly impacted the cell performance, this phenomenon being even more severe in the presence of (NH₄)₂SO₄. The conductivity of Mo remained significantly less affected by aging both with and without pollutants. The NiAlNi contacts after aging with (NH₄)₂SO₄ became so resistive that measurement was impossible. Corroborating modeling and experimental results, the drop in J_sc was attributed to the loss of the interference fringes in the AZO rather than to the loss of optical transmittance. Finally, aging without pollutants mostly impacted V_oc and R_sh due to the formation of shunt paths.