Sustainability in Perovskite Solar Cells

Electron Diffusion Length Effect on Direction of Irradiance in Transparent FAPbBr3 Perovskite Solar Cells

Transparent photovoltaics for building integration represent a promising approach for renewable energy deployment. These devices require transparent electrodes to manage transmittance and to ensure proper cell operation. In this study, transparent FAPbBr3-based perovskite solar cells optimized via a passivation treatment were demonstrated with average visible transmittance values above 60% and light utilization efficiencies up to 5.0%. Experiments under varying ultraviolet (UV) irradiance intensities from both front and rear directions revealed performance differences correlated with diffusion-limited transport and open-circuit voltage changes. Combining the UV-radiated experiments and drift-diffusion simulations, an asymmetry between the diffusion lengths of electrons and holes in the perovskite is revealed, with estimated values resulting in less than 50 nm and more than 99 nm, respectively. Our methods not only identify electron-hole diffusion length differences but also introduce a general protocol for characterizing solar cells with transparent electrodes.

Lead Isolation and Capture in Perovskite Photovoltaics toward Eco-Friendly Commercialization

Perovskite solar cells (PSCs) are developed rapidly in efficiency and stability in recent years, which can compete with silicon solar cells. However, an important obstacle to the commercialization of PSCs is the toxicity of lead ions (Pb2+) from water-soluble perovskites. The entry of free Pb2+ into organisms can cause severe harm to humans, such as blood lead poisoning, organ failure, etc. Therefore, this work reports a "lead isolation-capture" dual detoxification strategy with calcium disodium edetate (EDTA Na-Ca), which can inhibit lead leakage from PSCs under extreme conditions. More importantly, leaked lead exists in a nontoxic aggregation state chelated by EDTA. For the first time, in vivo experiments are conducted in mice to systematically prove that this material has a significant inhibitory effect on the toxicity of perovskites. In addition, this strategy can further enhance device performance, enabling the optimized devices to achieve an impressive power conversion efficiency (PCE) of 25.19%. This innovative strategy is a major breakthrough in the research on the prevention of lead toxicity in PSCs.

Synergetic Exterior and Interfacial Approaches by Colloidal Carbon Quantum Dots for More Stable Perovskite Solar Cells Against UV

The achievement of both efficiency and stability in perovskite solar cells (PSCs) remains a challenging and actively researched topic. In particular, among different environmental factors, ultraviolet (UV) photons play a pivotal role in contributing to device degradation. In this work, by harvesting simultaneously both the optical and the structural properties of bottom-up-synthesized colloidal carbon quantum dots (CQDs), a cost-effective means is provided to circumvent the UV-induced degradation in PSCs without scarification on their power conversion efficiencies (PCEs). By exploring and optimizing the number of CQDs and the different locations/interfaces of the solar cells where CQDs are applied, a synergetic configuration is achieved where the photovoltaic performance drop due to optical loss is completely compensated by the increased perovskite crystallinity due to interfacial modification. As a result, on the optimized configurations where CQDs are applied both on the exterior front side as an optical layer and at the interface between the electron transport layer and the perovskite absorber, unencapsulated PSCs with PCEs >20% are fabricated which can maintain up to ≈94% of their initial PCE after 100 h of degradation in ambient air under continuous UV illumination (5 mW cm-2).

Fullerene-C60 and PCBM as interlayers in regular and inverted lead-free PSCs using CH3NH3SnI3: an analysis of device performance and defect density dependence by SCAPS-1D

One of the challenges hindering the commercialization of perovskite solar cells (PSCs) is the presence of toxic metals such as lead in their composition. Simulation studies using SCAPS-1D have already been conducted on lead-free PSCs to find optimized solar cell parameters, having tin as the primary candidate for replacing lead in perovskites. Here, we used fullerene-C60 and its derivative PCBM as interlayers in a lead-free tin-based PSC between the ETL (ZnO) and the perovskite MASI in both regular and inverted configurations of PSCs using SCAPS-1D software. To the best of our knowledge, this is the first simulation study reporting the impact of using fullerene-C60 and PCBM as interlayers in lead-free PSCs. The defect density (Nt) of the perovskite material is varied, allowing us to observe its influence on the power conversion efficiency (PCE). Using an Nt value of 1017 cm-3 without the interlayer, the PCE was 6.90% and 3.72% for regular and inverted devices. Using PCBM as an interlayer improves the efficiency of both simulated PSCs, achieving a maximum PCE of 8.11% and 5.26% for the regular and inverted configurations, respectively. Decreasing the Nt from 1017 cm-3 to 1016 cm-3 caused a significant increase in efficiency, reaching 13.38% (n-i-p) and 10.00% (p-i-n). Finally, using the optimized parameters and an ideal Nt value (1013 cm-3), both PSCs achieved a PCE close to 30%.

Influence of chemical interactions on the electronic properties of BiOI/organic semiconductor heterojunctions for application in solution-processed electronics

Bismuth oxide iodide (BiOI) has been viewed as a suitable environmentally-friendly alternative to lead-halide perovskites for low-cost (opto-)electronic applications such as photodetectors, phototransistors and sensors. To enable its incorporation in these devices in a convenient, scalable, and economical way, BiOI thin films were investigated as part of heterojunctions with various p-type organic semiconductors (OSCs) and tested in a field-effect transistor (FET) configuration. The hybrid heterojunctions, which combine the respective functionalities of BiOI and the OSCs were processed from solution under ambient atmosphere. The characteristics of each of these hybrid systems were correlated with the physical and chemical properties of the respective materials using a concept based on heteropolar chemical interactions at the interface. Systems suitable for application in lateral transport devices were identified and it was demonstrated how materials in the hybrids interact to provide improved and synergistic properties. These indentified heterojunction FETs are a first instance of successful incorporation of solution-processed BiOI thin films in a three-terminal device. They show a significant threshold voltage shift and retained carrier mobility compared to pristine OSC devices and open up possibilities for future optoelectronic applications.

Solvent-antisolvent interactions in metal halide perovskites

The fabrication of metal halide perovskite films using the solvent-engineering method is increasingly common. In this method, the crystallisation of the perovskite layer is triggered by the application of an antisolvent during the spin-coating of a perovskite precursor solution. Herein, we introduce the current state of understanding of the processes involved in the crystallisation of perovskite layers formed by solvent engineering, focusing in particular on the role of antisolvent properties and solvent-antisolvent interactions. By considering the impact of the Hansen solubility parameters, we propose guidelines for selecting the appropriate antisolvent and outline open questions and future research directions for the fabrication of perovskite films by this method.

Soil adsorption and transport of lead in the presence of perovskite solar cell-derived organic cations

Perovskite photovoltaics offer a highly efficient and low-cost solar energy harvesting technology. However, the presence of lead (Pb) cations in photovoltaic halide perovskite (HaPs) materials is concerning, and quantifying the environmental hazard of accidental Pb²⁺ leaching into the soil is crucial for assessing the sustainability of this technology. Pb²⁺ from inorganic salts was previously found to remain in the upper soil layers due to adsorption. However, Pb-HaPs contain additional organic and inorganic cations, and competitive cation adsorption may affect Pb²⁺ retention in soils. Therefore, we measured, analyzed by simulations and report the depths to which Pb²⁺ from HaPs penetrates into 3 types of agricultural soil. Most of the HaP-leached Pb²⁺ is found to be retained already in the first cm of the soil columns, and subsequent rain events do not induce Pb²⁺ penetration below the first few cm of soil surface. Surprisingly, organic co-cations from the dissolved HaP are found to enhance the Pb²⁺ adsorption capacity in clay-rich soil, compared to non-HaP-based Pb²⁺ sources. Our results imply that installation over soil types with improved Pb²⁺ adsorption, and removal of only the contaminated topsoil, are sufficient means to prevent ground water contamination by HaP-leached Pb²⁺.

Maskless lithography for large area patterning of three-dimensional microstructures with application on a light guiding plate

This paper presents a maskless lithography system that can perform three-dimensional (3D) ultraviolet (UV) patterning on a photoresist (PR) layer. After PR developing processes, patterned 3D PR microstructures over a large area are obtained. This maskless lithography system utilizes an UV light source, a digital micromirror device (DMD), and an image projection lens to project a digital UV image on the PR layer. The projected UV image is then mechanically scanned over the PR layer. An UV patterning scheme based on the idea of obliquely scanning and step strobe lighting (OS³L) is developed to precisely control the spatial distribution of projected UV dose, such that desired 3D PR microstructures can be obtained after PR development. Two types of concave microstructures with truncated conical and nuzzle-shaped cross-sectional profiles are experimentally obtained over a patterning area of 160 ×115 mm². These patterned microstructures are then used for replicating nickel molds and for mass-production of light-guiding plates used in back-lighting and display industry. Potential improvements and advancements of the proposed 3D maskless lithography technique for future applications will be addressed.

Persistent Ion Accumulation at Interfaces Improves the Performance of Perovskite Solar Cells

The mixed ionic-electronic nature of lead halide perovskites makes their performance in solar cells complex in nature. Ion migration is often associated with negative impacts-such as hysteresis or device degradation-leading to significant efforts to suppress ionic movement in perovskite solar cells. In this work, we demonstrate that ion trapping at the perovskite/electron transport layer interface induces band bending, thus increasing the built-in potential and open-circuit voltage of the device. Quantum chemical calculations reveal that iodine interstitials are stabilized at that interface, effectively trapping them at a remarkably high density of ∼10²¹ cm^-3 which causes the band bending. Despite the presence of this high density of ionic defects, the electronic structure calculations show no sub-band-gap states (electronic traps) are formed due to a pronounced perovskite lattice reorganization. Our work demonstrates that ionic traps can have a positive impact on device performance of perovskite solar cells.

Highly Efficient Perovskite Solar Cell Based on PVK Hole Transport Layer

A π-conjugated small molecule N, N'-bis(naphthalen-1-yl)-N, N'-bis(phenyl)benzidine (NPB) was introduced into poly(9-vinylcarbazole) (PVK) as a hole transport layer (HTL) in inverted perovskite solar cells (PSCs). The NPB doping induces a better perovskite crystal growth, resulting in perovskite with a larger grain size and less defect density. Thus, the V_OC, J_SC, and FF of the PSC were all enhanced. Experimental results show that it can be ascribed to the reduction of surface roughness and improved hydrophilicity of the HTL. The effect of NPB on the aggregation of PVK was also discussed. This work demonstrates the great potential of PVK as the HTL of PSCs and provides an attractive alternative for HTL to realize high-efficiency PSCs.

Quantifying Efficiency Limitations in All-Inorganic Halide Perovskite Solar Cells

While halide perovskites have excellent optoelectronic properties, their poor stability is a major obstacle toward commercialization. There is a strong interest to move away from organic A-site cations such as methylammonium and formamidinium toward Cs with the aim of improving thermal stability of the perovskite layers. While the optoelectronic properties and the device performance of Cs-based all-inorganic lead-halide perovskites are very good, they are still trailing behind those of perovskites that use organic cations. Here, the state-of-the-art of all-inorganic perovskites for photovoltaic applications is reviewed by performing detailed meta-analyses of key performance parameters on the cell and material level. Key material properties such as carrier mobilities, external photoluminescence quantum efficiency, and photoluminescence lifetime are discussed and what is known about defect tolerance in all-inorganic is compared relative to hybrid (organic-inorganic) perovskites. Subsequently, a unified approach is adopted for analyzing performance losses in perovskite solar cells based on breaking down the losses into several figures of merit representing recombination losses, resistive losses, and optical losses. Based on this detailed loss analysis, guidelines are eventually developed for future performance improvement of all-inorganic perovskite solar cells.

Simple-Structured Low-Cost Dopant-Free Hole-Transporting Polymers for High-Stability CsPbI2Br Perovskite Solar Cells

Among the solution-processed devices, perovskite solar cells (PSCs) exhibit the highest power conversion efficiency (PCE) of over 25%; tremendous efforts are being undertaken to improve their stability. Recently, all-inorganic CsPbI₂Br-based PSCs were reported to exhibit a significantly improved device stability, with a promising PCE of up to 16.79%. In this study, we report stable all-inorganic PSCs by incorporating novel dopant-free hole-transporting materials (HTMs). The synthesis strategy of the newly synthesized polymeric HTMs was similar to that of 2,2',7,7'-tetrakis(N,N-di-p-methoxyphenylamine)-9,9'-spirobifluorene (spiro-OMeTAD), with the exception that they were designed to exhibit dopant-free characteristics. In particular, their polymeric backbone structure was significantly simpler than that of spiro-OMeTADs, and they were easily synthesized in two steps from commercially available chemicals, with an overall yield of ∼50%. The cost of synthesis at the laboratory scale was calculated to be at least 2.4 times cheaper than that of spiro-OMeTADs. The PCE of dopant-free HTM-based PSCs was 11.01%, which is 1.5 times higher than that of the dopant-free spiro-OMeTAD-based devices (7.52%) and comparable to that of the doped spiro-OMeTAD-based devices (12.22%). Notably, the stability of the device based on our dopant-free HTM to atmospheric oxygen and moisture as well as heat and light irradiation was superior to that of devices based on doped and dopant-free spiro-OMeTAD HTMs. On consideration of the synthesis cost, device efficiency, and device stability, our dopant-free HTM is highly promising for all-inorganic PSCs.

Potential lead toxicity and leakage issues on lead halide perovskite photovoltaics

Recently, lead halide perovskite solar cells have become a promising next-generation photovoltaics candidate for large-scale application to realize low-cost renewable electricity generation. Although perovskite solar cells have tremendous advantages such as high photovoltaic performance, low cost and facile solution-based fabrication, the issues involving lead could be one of the main obstacles for its commercialization and large-scale applications. Lead has been widely used in photovoltaics industry, yielding its environmental and health issues of vital importance because of the widespread application of photovoltaics. When the solar cell panels especially perovskite solar cells are damaged, lead would possibly leak into the surrounding environment, causing air, soil and groundwater contamination. Therefore, lots of research efforts have been put into evaluating the lead toxicity and potential leakage issues, as well as studying the encapsulation of lead to deal with leakage issue during fire hazard and precipitation in photovoltaics. In this review, we summarize the latest progress on investigating the lead safety issue on photovoltaics, especially lead halide perovskite solar cells, and the corresponding solutions. We also outlook the future development towards solving the lead safety issues from different aspects.

Perovskite Solar Cells Go Bifacial-Mutual Benefits for Efficiency and Durability

Bifacial solar cells hold the potential to achieve a higher power output per unit area than conventional monofacial devices without significantly increasing manufacturing costs. However, efficient bifacial designs are challenging to implement in inorganic thin-film solar cells because of their short carrier lifetimes and high rear surface recombination. The emergence of perovskite photovoltaic (PV) technology creates a golden opportunity to realize efficient bifacial thin-film solar cells, owing to their outstanding optoelectronic properties and unique features of device physics. More importantly, transparent conducting oxide electrodes can prevent electrode corrosion by halide ions, mitigating one major instability issue of the perovskite devices. Here, the theory of bifacial PV devices is summarized and the advantages of bifacial perovskite solar cells, such as high power output, enhanced device durability, and low economic and environmental costs, are reviewed. The limitations and challenges for bifacial perovskite solar cells are also discussed. Finally, the awareness of bifacial solar cells as a feasible commercialization pathway of perovskite PV for mainstream solar power generation and building-integrated PV is advocated and future research directions are suggested.

The critical issue of using lead for sustainable massive production of perovskite solar cells: a review of relevant literature

This work aims to review the most significant studies dealing with the environmental issues of the use of lead in perovskite solar cells (PSCs). A careful discussion and rationalization of the environmental and human health toxicity impacts, evaluated by life cycle assessment and risk assessment studies, is presented. The results of this analysis are prospectively related to the possible future massive production of PSC technology.

A general approach to high-efficiency perovskite solar cells by any antisolvent

Deposition of perovskite films by antisolvent engineering is a highly common method employed in perovskite photovoltaics research. Herein, we report on a general method that allows for the fabrication of highly efficient perovskite solar cells by any antisolvent via manipulation of the antisolvent application rate. Through detailed structural, compositional, and microstructural characterization of perovskite layers fabricated by 14 different antisolvents, we identify two key factors that influence the quality of the perovskite layer: the solubility of the organic precursors in the antisolvent and its miscibility with the host solvent(s) of the perovskite precursor solution, which combine to produce rate-dependent behavior during the antisolvent application step. Leveraging this, we produce devices with power conversion efficiencies (PCEs) that exceed 21% using a wide range of antisolvents. Moreover, we demonstrate that employing the optimal antisolvent application procedure allows for highly efficient solar cells to be fabricated from a broad range of precursor stoichiometries.