Editor-in-Chief Sean Shaheen reflects on the continued need for higher efficiency photovoltaics. A forthcoming JPE special section on Novel Photovoltaic Device Architectures aims to showcase new device architectures, materials, and photon management strategies to help propel the field into a new era of high-efficiency devices for the future.

The history of quantitative measurements of radiative cooling is briefly reviewed, starting with Count Rumford in 1804. The cooling results from upward emission of thermal infrared radiation (wavelengths of 5 to 50  μm) that is not fully offset by downward atmospheric emission. The downward emission is characterized by the apparent atmospheric (sky) emittance and the surface air temperature. In 1984, an equation was published that describes the clear sky emittance as a function of the surface dew point temperature. At the time, this equation was merely one of many empirical relations. Now that time has passed, experimental and theoretical advances support its validity. Further refinements can include improved corrections for time-of-day and the lower air pressure at elevated locations. Complex computer codes for predicting atmospheric radiation have reached quantitative maturity. Given profiles of air temperature, water vapor, CO₂, O₃, CH₄, N₂O, and aerosols, they can compute spectral radiances with an accuracy of ∼3  %  . The effect of clouds in reducing radiative cooling remains more uncertain.

Daytime radiative cooling has attracted extensive research interest due to its potential impact for energy sustainability. To achieve subambient radiative cooling during the daytime, a white surface that strongly scatters incident solar light is normally desired. However, in many practical applications (e.g., roofing materials and car coatings), colored surfaces are more popular. Because of this, there is a strong desire to develop colorful surfaces for radiative cooling. We summarize the general design criteria of radiative cooling materials with different colors and discuss the limitations in cooling performance. Major efforts on this specific topic are reviewed with some suggested topics for future investigation.

Dielectric microsphere coatings for passive daytime radiative cooling (PDRC) are gaining attention owing to their low cost and potential for mass production. The cooling performance could be further enhanced to effectively reflect solar radiation and emit thermal radiation to the cold sky by designing microspheres suitable for PDRC applications. Hollow dielectric structures were numerically designed to enhance the PDRC performance of dielectric microsphere coatings. The maximum solar reflectance (R¯_solar  =  0.96) was obtained with a fill rate f  =  0.6, outer radius r_out  =  0.5  μm, core–shell rate φ  =  r_in  /  r_out  =  0.3, thickness t  =  300  μm, and thermal infrared emittance ε¯_LWIR  =  0.90. Furthermore, by controlling the multisize sphere distribution within φ  =  0.1 to 0.5, the cooling performance at t  =  300  μm was enhanced to R¯_solar  =  0.98, ε¯_LWIR  =  0.95, and a net cooling power of 77  W  /  m² was achieved at a temperature of 25°C, which was ∼38  %   higher than that achieved with the single-size sphere coating (φ  =  0.3) and ∼64  %   higher than that of the solid SiO₂ sphere coating (φ  =  0). These results indicate that hollow structures can effectively enhance the cooling performance of dielectric microsphere coatings by increasing the number of interfaces between the air and dielectric materials.

Daytime radiative cooling technology can cool objects to sub-ambient temperatures under direct sunlight without energy consumption. The technology relies on high reflectance of solar irradiation and high emissivity in the atmospheric window (infrared emission with 8 to 13 μm wavelengths). We report a bilayer structured coating for passive daytime sub-ambient radiative cooling. The bilayer radiative cooling coating has high solar reflectance (0.94), and high infrared emissivity (0.96) in the atmospheric window. The bilayer coating achieved a sub-ambient temperature of 3.6°C under solar irradiance of 990  W  /  m² at an ambient temperature of 26.6°C, and the averaged sub-ambient cooling temperature of ∼8  °  C during the night. A test with two model rooms shows that the indoor air temperature reached a maximum difference of 9.7°C between the one with the bilayer coating and that with normal white coating.

Optically selective and thermally insulating (OSTI) covers, such as polyethylene aerogels (PEAs), have recently been proposed to improve subambient daytime radiative cooling performance by minimizing parasitic solar absorption and heat gain at the emitter. We investigate the addition of zinc sulfide (ZnS) nanoparticles inside PEAs to improve their optical selectivity. Solving for multiple scattering effects using the radiative transfer equation and Mie theory, we model the optical properties of PEA covers with different ZnS concentrations and particle diameters. Our theoretical and experimental results show that ZnS particles inside PEAs can significantly reduce solar transmittance (<0.01) while maintaining high infrared transmittance (>0.8). Our modeling also demonstrates that ZnS-pigmented PEA covers enable higher subambient cooling powers (up to 53.8  W  /  m² higher) and lower stagnation temperatures than conventional PEA under direct solar radiation (1000  W  /  m²). Finally, we investigate spatial distribution of ZnS within the cover and show that confining the ZnS near the air–cover boundary can reduce the total ZnS mass required by 59% or can enable 4% higher cooling power. Our findings promise to improve the performance of subambient radiative coolers and enable their application in passive cooling of buildings and refrigeration of food produce.

We demonstrate switchable polarized thermal emission from VO₂ nanofin stacks fabricated by co-deposition, etching, and oxidation. We find that reverse switching of the thermal emission is enabled by a reflecting underlayer and induced by either short oxidation time or additional deposition of a reflecting underlayer. Observed thermal emission is well explained by a biaxial Bruggeman effective medium model, which predicts the strong polarization change for aligned fin layers in the micron thickness range. The dominant polarization of the emission is modulated by the presence of a reflector, oxidation of the fins, fin fill-factor, and structural anisotropy. Normal incidence polarized emittance change of up to 0.6 is theoretically possible, and we were able to demonstrate a change of 0.34, similar to that predicted by the model.

The efficiency and performance of solar cells and modules are typically evaluated and reported at normal incidence under peak solar radiation. We present a simple clear-sky model for solar irradiance that can be used to study the angular and annual performance of new photovoltaic materials. Using this model, we study the effect of solar module orientation for fixed-tilt module installations and different types of tracking (seasonal, 1D, and 2D) as a function of latitude. For fixed-tilt modules, the optimum tilt as a fraction of latitude varies from 0.83 at 1 deg to 0.73 at 60 deg. The effect of tilt misorientation for panels at the optimum azimuth is not very strong as the solar irradiance is about 94.5% of its optimum at ±20  deg mistilt. Both azimuth misorientation and tilt misorientation are studied. Optimized tilts and times of year for tilting are also obtained for modules that are seasonally adjusted twice and three times a year. The annual solar insolation of fixed modules is compared with modules that are seasonally adjusted twice and three times a year, continuously tracked in the north/south direction, continuously tracked in the east/west direction, and continuously tracked in two directions. The use of single-axis tracking in the east/west direction is preferable to north/south tracking and potentially improves overall energy collection by 16.2% to 31.0%. Continuous dual-axis tracking enhances overall annual energy collection by 36.0% to 45.5%. The model and provision of open source code provides for a way to assess the performance of new materials.

The quality of the perovskite film highly affects the performance of perovskite solar cells (PSCs). To deposit uniform and dense perovskite films, the fabrication processes are usually conducted in a controlled environment such as a glove box, which limits the future commercial development. High-quality perovskite films were fabricated in high humidity using mixed antisolvent consisting of ethyl acetate (EA) and diethyl ether (DE). It is found that the speed of perovskite crystallization could be controlled by adjusting the volume ratio of EA and DE in the mixed antisolvent, enhancing the grain size of perovskite films. The PSC fabricated in a humidity of 70% treated with an optimal ratio EA and DE exhibits an enhanced efficiency of 18.77% from 11.07%. Our work provides a convenient strategy for fabricating highly efficient PSCs in a high-humidity environment.

The erratum corrects the affiliations listed in the originally published article.