Special Issue on “Selected Papers from GPVC Conferences”

At the core of the climate crisis lies excessive carbon emissions from the continued use of fossil fuels. Therefore, in order to fight the climate crisis, achieving carbon neutrality is no longer simply a declarative agenda, and there is a call for drastically increased use of renewable energy sources. In 2020, the installation of photovoltaics (PV) was already at 133 GW, and it is expected to reach above 200 GW globally this year. PV technology will obviously become one of the key and core tenets in achieving the worldwide goal of carbon neutrality by 2050.

The purpose of this Special Issue is to select and present the latest research related to solar cells, photodetectors, and PV modules. In total, seven papers were selected in various fields involving solar cells, including the application of plasmonic and luminescent materials, numerical simulation approaches for optimization, adoption of nanotextured window layers, and the implementation of hetero-epitaxial lift-off technology, followed by next-generation PV module technology. Nguyen et al. reported on Schottky junction silicon solar cells with CdSe/ZnS quantum dots (QDs) and gold nanoparticles (AuNPs) [1]. To overcome the weakness of the Schottky junction solar cell, AuNPs were used to enhance solar cell efficiency by reflecting the light, and a QDs layer was deployed as a luminescent downshifter. Khokhar et al. reported on the numerical simulation of a high-efficiency tunnel-oxide-passivated contact (TOPCon) solar cell with a crystalline nanostructured silicon layer [2]. Using with an AFORS-HET simulation tool, the theoretical mechanism of carrier selectivity towards passivated electron contact was examined; they found that, by predicting the efficiency before manufacturing solar cells, high-efficiency TOPCon solar cells are easily achievable. To improve the efficiency of flexible III-V solar cells, Kim et al. adopted a nanotextured window layer of inverted epi-grown GaAs [3]. Because of the omnidirectionality of the nanotextured layer, the refractive index of the effective medium fell off, and the reflectance of the incident light significantly reduced; consequently, the efficiency of the solar cell was increased. Woo et al. demonstrated, for the first time, a thin-film GaAs solar cell, grown on a Si wafer, and transferred to a polyimide film through via wafer bonding and an epitaxial lift-off process [4]. Normally, the mismatch in coefficients of thermal expansion between GaAs and Si cause thermal cracks, but the annealed In_0.1Ga_0.9As layer and the thick GaAs buffer layer prevented thermal crack, achieving low threading dislocation density. Without an anti-reflection layer, an inverted GaAs solar cell proved that using metal wafer bonding and the epitaxial lift-off process is appropriate for III–V solar cells, which are hetero-epitaxially grown on Si substrates. Kim et al. demonstrated the function of a Si-based near-infrared (NIR) photodetector by synthesizing CuInSe₂ [5]. To improve the photo-absorption properties, three-stage co-evaporation processes were used and low-bandgap CISe thin films were grown. By varying the Cu/In ratio, the bandgap energies of thin films were tuned, and the inverse relationship between the Cu/In ratio and the wavelengths was demonstrated; finally, the longest cut-off wavelength of 1300 nm was obtained from a Cu/In ratio of 0.87.

Recently, PV technology for manufacturing high-power, high-efficiency and lightweight modules has been explored. Lim et al. investigated the effects of the reflectance of backsheets and the space between cells in multi-busbars (MBBs) and a shingled PV module [6]. Their results showed gap spaces of 2.5 mm and 6 mm for the MBB and shingled modules, respectively, which improved Isc, leading to the enhancement of both power and efficiency. Because the white backsheet reflects incident light to solar cells, additional absorbed light can increase the power and efficiency; however, considering the effective area of solar cells in PV modules, the gap spaces should be carefully optimized. Park et al. applied a glass-free structure in a shingled PV module to achieve a lightweight and high-power application [7]. Instead of using front cover glass, as is the convention, ethylene tetrafluoroethylene copolymer (ETFE) was used to reduce the weight, and an Al honeycomb structure was applied to ensure rigidity. Through combination with shingled solar strings, the authors achieved a lightweight, high-power, ETFE-applied, Al-honeycomb-structured PV module.

In addition to the technologies presented here, efforts to achieve carbon neutrality are still ongoing around the world; in order to successfully achieve the goal of carbon neutrality, groundbreaking and remarkable research works must continue.