Impact of sunlight spectral shifts on bifacial perovskite-silicon tandem solar cells

The ASTM G173 Standard Spectral Distribution of Sunlight represents a standard, reference solar spectrum that is used for solar photovoltaics testing and calibration purposes. It was defined in the seventies and is derived from measurements at a location with clear atmospheric conditions and an Air Mass of 1.5 (AM 1.5), which is typical of sunlight at mid-latitudes (like the Midwest USA) when the sun is at a moderate angle. The spectral content of sunlight in sunbelt countries, where more than 60% of the global population lives and where the uptake of PV is growing at a fast pace, is quite different from the standard spectrum, and these differences influence the output of emerging PV technologies such as tandem perovskite-Si, pure perovskite and CdTe thin film technologies.

The well-proven and over 70-year-old crystalline silicon PV technology represents 98% the solar photovoltaic market, and while it gets closer to its maximum theoretical efficiency, emerging technologies such as tandem perovskite-Si have already reached close to 35% conversion efficiencies, and are projected to gain market share in the coming decade. Solar cell conversion efficiencies are measured at Standard Test Conditions (STC), which include the standard spectrum, but which are never simultaneously occurring in the field. Despite efforts to introduce the concept of Realistic Reporting Conditions in the early nineties, STC remains the preferred and convenient set of power rating conditions for the solar industry. However, what the consumer buys when purchasing a solar PV module rated a certain power (Wp) at STC is the corresponding energy output (kWh/year), which will depend on where it is operating. This energy output will depend mostly on the PV cell operating temperature (very seldom the standard 25 C, especially under high irradiances in warm climates), the irradiance integrated over the year, and the spectral distribution of this irradiance (very seldom the standard AM 1.5).

Spectral shifts from the ASTM G-173 Standard Distribution of Sunlight, which was defined for the 1976 USA Standard Atmosphere with a rural aerosol loading, under AM 1.5 conditions, representing a specific solar zenith angle of approximately 48.2 degrees, are common. Key atmospheric parameters for this standard spectrum include a total column water vapour of 1.42 cm, a total column ozone of 0.34 atm-cm, a CO₂ concentration of 370 ppmv (current CO₂ levels at time of writing are 426 ppmv), and an Aerosol Optical Depth (AOD) of 0.084. Variations in these atmospheric parameters lead to spectra that are blue-light rich and red-light poor, as the ones shown in the figure below, for four different locations in Brazil.

Deviations (in %) from the ASTM G-173 standard spectrum of the spectral content of sunlight at four different locations in Brazil. Brazilian skies are much bluer than the standard (Adapted from Rafael Haag PhD Thesis).Image: ISES

These spectral variations have different consequences in PV device output performance, depending on their spectral response. The figures below show simulations carried out in Brazil for the Spectral Factors (SF) of these shifts in spectral content of sunlight on the performance of silicon HBC (Heterojunction Back Contact), CdTe (Cadmium Telluride) and perovskite thin film, as well as tandem perovskite-silicon solar cell devices. In Brazil, thin film technologies have shown important performance gains (up to 4%) caused by the characteristic spectral distribution of light. For single-junction silicon devices, the impacts of spectral distribution are negligible (less than 0.5%), due to their wide spectral response. For tandem perovskite-silicon, significant spectral losses (up to 6%) are expected due to the current mismatch between the top perovskite and the bottom silicon cell. This happens because the top and bottom cells operate in series in a 2-terminal tandem and must have the same current. However, the silicon bottom only absorbs the red side of the spectrum, and the local spectrum is very blue-rich, making it so the top perovskite cell can have a much higher current, but is limited by the bottom silicon cell.

These losses can be compensated in bifacial tandem perovskite-silicon over high-albedo ground covers. When a tandem cell is made bifacial, the bottom cell current is boosted with rear-side illumination, decreasing the current mismatch, thus unlocking the full potential of the top perovskite cell, which benefits from the local blue-shifted spectral distribution. An albedo value of 50% is needed to compensate for the top/bottom cell current mismatch under Brazilian atmospheric conditions. These results show the importance of high-reflectance ground covers in unleashing the full potential of perovskite/silicon PV devices.

Maps of annual estimated spectral factors (SF) for different PV technologies across Brazil: monocrystalline n-type silicon heterojunction interdigitated back contact (Si HBC); 2T perovskite/silicon tandem (Tandem); thin film cadmium telluride (CdTe), and thin film perovskite. Black dots indicate the locations of measurement stations used to extract TMY data. Interpolation and extrapolation were applied in data-sparse regions to provide a complete spatial representation of spectral effects. Different colour scales were used across subplots to highlight the variations for spectral effects within each technology, which should be considered when interpreting the maps and comparing PV technologies (Adapted from Marilia Braga PhD Thesis).Image: ISES

Spectral albedo curves measured under clear-sky conditions for eight ground surface types (solid lines, left vertical axis) in Florianópolis, Brazil (27°S, 48°W). Relative rear side spectral response (SR) of five PV technologies is also shown (dashed lines, right vertical axis): Si PERC, Si IBC, CdTe, perovskite, and 2-terminal perovskite/silicon tandem (Adapted from Marilia Braga PhD Thesis).Image: ISES

Relative current mismatch between top perovskite and bottom silicon sub-cells in a two-terminal (2T) tandem device calculated based on front and rear spectral irradiance measurements captured under various albedo conditions in Florianópolis, Brazil (27°S, 48°W) (Adapted from Marilia Braga PhD Thesis).Image: ISES

Authors: Prof. Ricardo Rüther (UFSC), rruther@gmail.com; Prof. Andrew Blakers /ANU, Andrew.blakers@anu.edu.au

ISES, the International Solar Energy Society is a UN-accredited membership NGO founded in 1954 working towards a world with 100% renewable energy for all, used efficiently and wisely.

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