Novel symmetrical bifacial flexible CZTSSe thin film solar cells for indoor photovoltaic applications

Polytypic metal chalcogenide nanocrystals

By engineering chemically identical but structurally distinct materials into intricate and sophisticated polytypic nanostructures, which often surpass their pure phase objects and even produce novel physical and chemical properties, exciting applications in the fields of photovoltaics, electronics and photocatalysis can be achieved. In recent decades, various methods have been developed for synthesizing a library of polytypic nanocrystals encompassing IV, III-V and II-VI polytypic semiconductors. The exceptional performances of polytypic metal chalcogenide nanocrystals have been observed, making them highly promising candidates for applications in photonics and electronics. However, achieving high-precision control over the morphology, composition, crystal structure, size, homojunctions, and periodicity of polytypic metal chalcogenide nanostructures remains a significant synthetic challenge. This review article offers a comprehensive overview of recent progress in the synthesis and control of polytypic metal chalcogenide nanocrystals using colloidal synthetic strategies. Starting from a concise introduction on the crystal structures of metal chalcogenides, the subsequent discussion delves into the colloidal synthesis of polytypic metal chalcogenide nanocrystals, followed by an in-depth exploration of the key factors governing polytypic structure construction. Subsequently, we provide comprehensive insights into the physical properties of polytypic metal chalcogenide nanocrystals, which exhibit strong correlations with their applications. Thereafter, we emphasize the significance of polytypic nanostructures in various applications, such as photovoltaics, photocatalysis, transistors, thermoelectrics, stress sensors, and the electrocatalytic hydrogen evolution. Finally, we present a summary of the recent advancements in this research field and provide insightful perspectives on the forthcoming challenges, opportunities, and future research directions.

Defect Synergistic Regulations of Li&Na Co-Doped Flexible Cu2 ZnSn(S,Se)4 Solar Cells Achieving over 10% Certified Efficiency

Ion doping is an effective strategy for achieving high-performance flexible Cu2 ZnSn(S,Se)4 (CZTSSe) solar cells by defect regulations. Here, a Li&Na co-doped strategy is applied to synergistically regulate defects in CZTSSe bulks. The quality absorbers with the uniformly distributed Li and Na elements are obtained using the solution method, where the acetates (LiAc and NaAc) are as additives. The concentration of the harmful CuZn anti-site defects is decreased by 8.13% after Li incorporation, and that of the benign NaZn defects is increased by 36.91% after Na incorporation. Synergistic Li&Na co-doping enhances the carrier concentration and reduces the interfacial defects concentration by one order of magnitude. As a result, the flexible CZTSSe solar cell achieves a power conversion efficiency (PCE) of 10.53% with certified 10.12%. Because of the high PCE and the homogeneous property, the Li&Na co-doped device is fabricated to a large area (2.38 cm2 ) and obtains 9.41% PCE. The co-doping investigation to synergistically regulate defects provides a new perspective for efficient flexible CZTSSe solar cells.

Bifacial and Semitransparent Sb2 (S,Se)3 Solar Cells for Single-Junction and Tandem Photovoltaic Applications

Thin-film solar cells are expected to play a significant role in the space industry, building integrated photovoltaic (BIPV), indoor applications, and tandem solar cells, where bifaciality and semitransparency are highly desired. Sb2 (S,Se)3 has emerged as a promising new photovoltaic (PV) material for its high absorption coefficient, tunable bandgap, and nontoxic and earth-abundant constituents. However, high-efficiency Sb2 (S,Se)3 solar cells exclusively employ monofacial architectures, leaving a considerable gap toward large-scale application in aforementioned fields. Here, a bifacial and semitransparent Sb2 (S,Se)3 solar cell and its extended application in tandem solar cells are reported. The transparent conductive oxides (TCOs) and the ultrathin inner n-i-p structure provide high long-wavelength transmittance. Despite the MnS/ITO Schottky junction, power conversion efficiencies (PCEs) of 7.41% and 6.36% are achieved with front and rear illumination, respectively, contributing to a great bifaciality of 0.86. Consequently, the reported device gains great enhancement in PV performance by exploiting albedo of surroundings and shows exceptional capability in absorbing tilt incident light. Moreover, an Sb2 (S,Se)3 /Si tandem solar cell with a PCE of 11.66% is achieved in preliminary trials. These exciting findings imply that bifacial and semitransparent Sb2 (S,Se)3 solar cells possess tremendous potential in practical applications based on their unique characteristics.

2.11 - Accurate characterization of indoor photovoltaic performance

Ambient energy harvesting has great potential to contribute to sustainable development and address growing environmental challenges. Converting waste energy from energy-intensive processes and systems (e.g. combustion engines and furnaces) is crucial to reducing their environmental impact and achieving net-zero emissions. Compact energy harvesters will also be key to powering the exponentially growing smart devices ecosystem that is part of the Internet of Things, thus enabling futuristic applications that can improve our quality of life (e.g. smart homes, smart cities, smart manufacturing, and smart healthcare). To achieve these goals, innovative materials are needed to efficiently convert ambient energy into electricity through various physical mechanisms, such as the photovoltaic effect, thermoelectricity, piezoelectricity, triboelectricity, and radiofrequency wireless power transfer. By bringing together the perspectives of experts in various types of energy harvesting materials, this Roadmap provides extensive insights into recent advances and present challenges in the field. Additionally, the Roadmap analyses the key performance metrics of these technologies in relation to their ultimate energy conversion limits. Building on these insights, the Roadmap outlines promising directions for future research to fully harness the potential of energy harvesting materials for green energy anytime, anywhere.

A library of polytypic copper-based quaternary sulfide nanocrystals enables efficient solar-to-hydrogen conversion

Designing polytypic homojunction is an efficient way to regulate photogenerated electrons and holes, thereafter bringing desired physical and chemical properties and being attractive photocatalysts for solar-to-hydrogen conversion. However, the high-yield and controllable synthesis of well-defined polytypes especially for multinary chalcogenide - the fundamental factor favoring highly efficient solar-to-hydrogen conversion - has yet to be achieved. Here, we report a general colloidal method to construct a library of polytypic copper-based quaternary sulfide nanocrystals, including Cu₂ZnSnS₄, Cu₂CdSnS₄, Cu₂CoSnS₄, Cu₂MnSnS₄, Cu₂FeSnS₄, Cu₃InSnS₅ and Cu₃GaSnS₅, which can be synthesized by selective epitaxial growth of kesterite phase on wurtzite structure. Besides, this colloidal method allows the precise controlling of the homojunction number corresponding to the photocatalytic performance. The single-homojunction and double-homojunction polytypic Cu₂ZnSnS₄ nanocrystal photocatalysts show 2.8-fold and 3.9-fold improvement in photocatalytic hydrogen evolution rates relative to the kesterite nanocrystals, respectively. This homojunction existed in the polytypic structure opens another way to engineer photocatalysts.

While polytypic semiconductors are promising for solar-to-fuel applications, preparing homojunction nanomaterials has proven challenging. Here, authors obtain a library of polytypic copper-based quaternary sulfide nanocrystals by selective epitaxial growth of kesterite phase on wurtzite structure.

10.24% Efficiency of Flexible Cu2 ZnSn(S,Se)4 Solar Cells by Pre-Evaporation Selenization Technique

Flexible Cu₂ ZnSn(S,Se)₄ (CZTSSe) solar cells show great potential due to non-toxicity and low cost. The quality of CZTSSe absorber suffering from the high-temperature selenization process is the key to overcoming open-circuit voltage (V_OC ) deficit and obtaining high efficiency. In this work, the authors develop a selenization technique to improve the quality of the CZTSSe layer by pre-evaporation selenization. This method provides saturated selenium vapor at the beginning of the selenization process to promote the crystallization process. The oversaturated selenium source greatly shortens the annealing time at high temperatures. The prepared CZTSSe films have larger grains and fewer voids. The measurement of the space charge limited current shows that the defect density of the CZTSSe absorber is significantly reduced from 4.43 × 10¹³  cm^-3 to 3.46 × 10¹³ cm^-3 . Electrical tests show that the devices have better charge separation at the interface, resulting in a charge recombination lifetime, increasing from 133 to 197 µs. The power conversion efficiency of the flexible CZTSSe solar cell reaches 10.24% with V_OC and the fill factor enhances to 463 mV and 62%, respectively. The safe non-toxicity and efficient selenization method is expected to provide a new strategy for high-efficiency flexible CZTSSe solar cells.

Dependence of the Heterogeneity of Grain Boundaries on Adjacent Grains in Perovskites and Its Impact on Photovoltage

In polycrystalline perovskites, grain boundaries (GBs) that isolate grains determine the optoelectronic properties of a semiconductor, and hence affect the photovoltaic performance of a solar cell. Photocurrent and photovoltage are affected by the microscopic structure of perovskites but are difficult to quantify on the intragrain length scale and are often treated as homogeneous within the photoactive layer. Here, the nanoscale through-film and lateral photoresponse of large-grained perovskite are studied by photoconductive atomic force microscopy. Photocurrent collection along GBs relies on the formation of adjacent grains, exhibiting GB to GB heterogeneity. Regarding to the spatially correlated heterogeneity, the photovoltage of grains deduced from the photoresponse curves at specific positions is larger than that of GBs by up to 0.4 V, suggesting that the photovoltage loss mainly originates from the shunting of GBs through the whole perovskite layer. These spatial heterogeneities are alleviated by depositing a capping layer onto the perovskite layer, highlighting the role of the inserted layer between the perovskite and electrode in real solar cells. This research reveals the heterogeneity of GBs and its influence on photovoltage that actually occurs in virtual solar cells, which is crucial for optimizing perovskite-based solar cells.

Toward a Scalable Fabrication of Perovskite Solar Cells under Fully Ambient Air Atmosphere: From Spin-Coating to Inkjet-Printing of Perovskite Absorbent Layer

Up until now, the vast majority of perovskite solar cells (PSCs) have relied on the spin-coating of perovskite precursor solution under inert fully controlled conditions, with the performance of solar cells that are developed by alternative techniques and under an ambient atmosphere to lag far behind. This impedes the technology transfer from the laboratory to industrial large-scale production; thus, the investigation of new scalable techniques should be thoroughly considered. The present work constitutes one of the few investigations on the application of inkjet-printing as an advanced alternative technique to the conventional spin-coating technique used for the fabrication of fully ambient air-processed perovskite absorbent layers for carbon-based hole transport layer-free PSCs. A systematic study of the characteristics of the perovskite material and solar cells indicated that the coffee-ring effect combined with poor ink penetration into the mesoporous network of the anode semiconductor were the main reasons for obtaining poor perovskite structure morphology and lower PSC performance by inkjet-printing, which arises from a lower internal quantum efficiency and an increased charge transfer and recombination rate. On the other hand, the crystallinity and optical characteristics of the materials obtained by the compared techniques did not differ considerably, while small differences were observed in the hysteretic behavior and long-term stability of the solar cells.