Recent advances in eco-friendly quantum dots-based solar energy conversion applications

Semiconductor quantum dots (QDs) are nanocrystals with three-dimensional confined excitons, showing size/shape/ composition-dependent optical properties with broadband absorption from ultraviolet to near infrared, high quantum yield, large Stokes shift and good photo/chemical stability, which have been widely applied in solar energy conversion applications such as solar cells (SCs), luminescent solar concentrators (LSCs) and photoelectrochemical (PEC) cells. However, most of the high-performance QDs-solar energy conversion devices are still based on QDs containing highly toxic heavy metals (such as Pb, Cd, Hg-based chalcogenides), which can inevitably induce the human health and environmental pollution issues, thus hindering their future commercialization Developing high-efficiency solar energy conversion devices based on environmentally friendly QDs is a promising research direction to promote the practical application. Towards this effort, a new generation of environment-friendly semiconductor QDs (such as carbon QDs, silicon QDs, III-V compound QDs, I-III-VI compound QDs and lead-free perovskite QDs) has recently attracted extensive research interests. Here, notable advances and developments of solar cells, luminescent solar concentrators and photoelectrochemical cells based on these environmentally friendly QDs are summarized. Various strategies including band gap engineering, core/shell structure construction, doping, defect states tuning and alloying of these eco-friendly QDs as well as relevant QDs-based device performance are discussed in detail. Specifically, growing core/shell structure can effectively passivate the surface defect states, inhibit the non-radiative recombination and improve the photoluminescence quantum yield (PLQY) as well as photo-/chemical-stability of QDs. The optical absorption and PL spectra of QDs can be precisely tuned by altering the concentration of dopants and chemical compositions of QDs to match the solar spectrum for high-efficiency utilization of solar energy. Alloying strategy also enables the realization of optimized intrinsic and surface defects. tailored band structure and improved photo-/chemical-stability of QDs, thus enhancing the performance of corresponding QDs-solar conversion devices. Furthermore, the current existing challenges and drawbacks are analyzed, providing guidelines for future developments of eco-friendly QDs-based solar energy conversion devices. In summary, the rational design and synthesis of eco-friendly QDs with broad light absorption, highly efficient charge separation/transfer and outstanding photo-/chemical-stability are beneficial to improve the performance of "green" QDs-solar energy conversion devices. For QD's synthesis, low-temperature, less toxic and large-scale synthetic technology with reduced energy consumption and eliminated toxic organic solvents/surfactants should be developed to achieve future cost-effective and high quality QDs. For QDs-SCs, the best reported power conversion efficiency of state-of-the-art QDs-SCs is still much lower than that of the commercialized silicon SCs. It is of great significance to explore more SCs device performance and stability optimization strategies. For QDs-LSCs, the balance between reabsorption loss and PLQY should be further studied and the standard measurement methodology needs to be refined. For QDs-PEC devices, the long-term durability of QDs under device operation needs to be largely improved for real-life application. Besides, the electrolyte used in most of the current QDs-PEC systems contains sacrificial agents (e.g., Na2S/Na2SO3) with highly corrosive feature, which may induce environmental issues and should be properly replaced by neutral solution for future commercial perspectives.