• Energy Research

Abstract Physics ruling the temperature sensitivity of photovoltaic (PV) cells is discussed. Dependences with temperature of the fundamental losses for single junction solar cells are examined and fundamental temperature coefficients (TCs) are calculated. Impacts on TCs of the incident spectrum and of variations of the bandgap with temperature are highlighted. It is shown that the unusual behavior of the bandgaps of perovskite semiconductor compounds such as CH 3 NH 3 PbI 3- x Cl x and CsSnI 3 will ultimately, in the radiative limit, give PV cells made of these materials peculiar temperature sensitivities. The different losses limiting the efficiency of present commercial cells are depicted on a p–n junction diagram. This representation provides valuable information on the energy transfer mechanisms within PV cells. In particular, it is shown that an important fraction of the heat generation occurs at the junction. A review of the loss mechanisms driving the temperature coefficients of the different cell parameters (open circuit voltage V oc , short circuit current density J sc , fill factor FF ) is proposed. The temperature sensitivity of open circuit voltage is connected to the balance between generation and recombination of carriers and its variation with temperature. A general expression that relates the temperature sensitivity of V oc to the External Radiative Efficiency (ERE) of a solar cell is provided. Comparisons with experimental data are discussed. The impacts of bandgap temperature dependence and incident spectrum on the temperature sensitivity of short circuit current are demonstrated. Finally, it is argued that if the fill factor temperature sensitivity is ideally closely related to the open circuit voltage temperature sensitivity of the cell, it depends for some cells strongly on technological issues linked to carrier transport such as contact resistances.

The present article provides an overview about photovoltaic/thermal systems categorised by the temperature of the working fluid: Low-temperature (lower than 60º C), medium-temperature (between 60 and 90º C) and hightemperature (higher than 90º C). Concerning photovoltaic/thermal-air systems for low-temperature use, the majority of studies involve building-integrated non-concentrating systems with phase change materials and working-fluid temperatures at around 30-55º C. Concerning low-temperature photovoltaic/thermal-water systems, a large number of studies are about non-concentrating configurations appropriate for building-integrated and, in general, domestic applications with working fluids at approximately 50–60º C. Regarding nonconcentrating photovoltaic/thermal systems for medium-temperature use, a large number of references are appropriate for industrial and domestic applications (working fluids: air; water) with around 60-70º C workingfluid temperatures. The literature review about medium-temperature concentrating photovoltaic/thermal systems shows that the majority of investigations concern photovoltaic/thermal-water systems with concentration ratios up to 190X and working fluids at approximately 62-90º C, appropriate for domestic and waterdesalination applications. As for high-temperature concentrating photovoltaic/thermal systems, most of them have concentration ratios up to 1000X, involve parabolic concentrators and use water (as the working fluid) at around 100-250º C. Moreover, in the field of high-temperature photovoltaic/thermal systems, most of the configurations are appropriate for building and industrial applications, and consist of triple-junction or siliconbased photovoltaic/thermal cells. In light of the issues mentioned above, a critical discussion and key challenges (in terms of materials, efficiencies, technologies, etc.) are presented. The authors would like to thank ’’Ministerio de Economía y Competitividad’’ and “Ministerio de Ciencia e Innovación” of Spain for the funding (grant references ENE2016-81040-R and PID2019-111536RBI00). D. Chemisana thanks ’’Institució Catalana de Recerca i Estudis Avançats (ICREA)’’ for the ICREA Acadèmia award. Chr. Lamnatou is Lecturer of the Serra Húnter programme. Figures 1–6: reproduced with permission.

A new approach is introduced and illustrated for optimizing the performances of photovoltaic cells. A thermal criterion, the minimization of the internal heat sources, is added to the usual criteria that consist of minimizing the optical and electrical losses. A proof of concept is delivered by means of modeling in the case of a standard crystalline silicon (cSi) cell for which the dependence on temperature of optical, electrical, and thermal properties is well known. A numerical code named TASC-1D-cSi simulating the optical-radiative, electrical, and thermal behaviors of cSi solar cells is used. Besides the current-voltage characteristics, this simulation tool provides the spectral variations of the thermalization, recombination, and radiative internal heat sources as well as the equilibrium temperature of the cell which depends on the outdoor conditions. The cell or the anti-reflection coating thickness is varied while the other parameters are prescribed. It is demonstrated in given outdoor conditions that considering the minimization of the internal total heat source in addition to the minimization of the optical and electrical losses modifies significantly the value of each of these parameters that maximizes the output power of the cell. For example, in a thermal condition with natural convection at the front and insulation at the back of the cell, the optimum cell thickness is found to be 55 μm and the optimum antireflection coating thickness 87 nm, instead of, respectively, 75 μm and 80 nm when the cell is maintained at 25 °C. These results suggest that a thermal design rule involving the internal heat source might be included in the improvement of solar cells at use and in the development of the next generation photovoltaics.

Operating a solar cell under thermal stress at temperatures >100°C and up to 500°C seems counterintuitive because conversion efficiency drops dramatically. Even so, there are cases in which solar cells are in high-illumination high-temperature conditions, for near-the-sun space missions and in various terrestrial hybrid systems involving solar-to-thermal energy conversion. This review analyzes the progress of solar cells tested in the laboratory under thermal stress. The fundamental physics governing the thermal sensitivity of solar cells and the main criteria determining the ability of semiconductor materials to survive high temperatures are recalled. Materials and architectures of a selection of the solar cells tested so far are examined. Deviation from the Shockley-Queisser limit at each temperature is used for a fair assessment of the performances. Our analysis reveals the strengths and weaknesses of the existing technologies and the gaps to be filled to develop new classes of solar cells capable of withstanding high temperatures. This work was developed in the frame of the French program Investments for the Future managed by the National Agency for Research under contract ANR-10-LABX-22-01-SOLSTICE. C.L. is lecturer of the Serra Húnter programme. D.C. thanks the Institució Catalana de Recerca i Estudis Avançats (ICREA) for the ICREA Acadèmia, and the Ministerio de Ciencia e Innovación (project PID2019-111536RB-I00).

The thermal impacts on the performance of nanoscale-gap thermophotovoltaic (nano-TPV) power generators are investigated using a coupled near-field thermal radiation, charge, and heat transport formulation. A nano-TPV device consisting of a tungsten radiator, maintained at 2000 K, and cells made of indium gallium antimonide (In0.18Ga0.82Sb) are considered; the thermal management system is modeled assuming a convective boundary with a fluid temperature fixed at 293 K. Results reveal that nano-TPV performance characteristics are closely related to the temperature of the cell. When the radiator and the junction are separated by a 20 nm vacuum gap, the power output and the conversion efficiency of the system are respectively 5.83 × 105 Wm-2 and 24.8% at 300 K, whereas these values drop to 8.09 × 104 Wm-2 and 3.2% at 500 K. In order to maintain the cell at room temperature, a heat transfer coefficient as high as 105 Wm-2 K-1 is required for nanometer-size vacuum gaps. The reason for this is that thermal radiation since thermal radiation enhancement beyond the blackbody from a bulk radiator of tungsten is broadband in nature, while only a certain part of the spectrum is useful for maximizing nano-TPV performance. In future studies, near-field radiation spectral conditions leading to optimal performance characteristics of the device will be investigated.

Indium antimonide photovoltaic cells are specifically designed and fabricated for use in a near-field thermophotovoltaic device demonstrator. The optimum conditions for growing the p-n junction stack of the cell by means of solid-source molecular beam epitaxy are investigated. Then processing of circular micron-sized mesa structures, including passivation of the side walls, is described. The resulting photovoltaic cells, cooled down to around 77 K in order to operate optimally, exhibit excellent performances in the dark and under far-field illumination by thermal sources in the [600-1000] °C temperature range. A short-circuit current beyond 10 µA, open-circuit voltage reaching almost 85 mV, fill factor of 0.64 and electrical power at the maximum power point larger than 0.5 W are measured for the cell with the largest mesa diameter under the highest illumination. These results demonstrate that these photovoltaic cells will be suitable for measuring a near-field enhancement of the generated electrical power.