Potential Substitutes for Replacement of Lead in Perovskite Solar Cells: A Review

Mathematical Modeling and Optimization of Highly Efficient Nontoxic All-Inorganic CsSnGeI3-Based Perovskite Solar Cells with Oxide and Kesterite Charge Transport Layers

Despite exceptional optoelectronic properties and rapidly increasing efficiency of perovskite solar cells (PSCs), the issues of toxicity and device instability have hampered the commercialization of this renewable energy technology. Lead (Pb) being the main culprit creates major environmental risks and therefore must be replaced with a nontoxic material such as tin (Sn), germanium (Ge), etc. Moreover, replacing organic cations in the perovskite's ABX3 structure with inorganic ones like cesium (Cs) helps aid the stability issues. This study uses six different kesterite-based hole transport layers (HTLs) and three different metal oxide-based electron transport layers (ETLs) to numerically simulate and optimize all-inorganic CsSnGeI3 PSCs. Metal oxide ETLs are used in this study due to their large band gap, while kesterite HTLs are used due to their excellent conductive properties. All of the simulations are performed under standard testing conditions. A total of 18 novel planar (n-i-p) PSCs are modeled by the combination of various charge transport layers (CTLs), and the device optimization was done to enhance the power conversion efficiencies (PCEs) of the PSCs. Furthermore, the effect of CTLs on the energy band alignment, electric field, quantum efficiency, light absorption, and recombination rate is analyzed. Additionally, a detailed analysis of the impact of defect density (Nt), interface defects (ETL/Perv, Perv/HTL), temperature, and work function on the functionality of 18 different CsSnGeI3-based PSCs is conducted. The simulation findings demonstrate that SnO2/CsSnGeI3/CNTS is the most efficient optimized PSC among all of the simulated structures, with a PCE of 27.33%, Jsc of 28.04 mA/cm2, FF of 85%, and Voc of 1.14 V.

Exploring optoelectronic and photocatalytic properties of X2AgBiY6 (X = NH4, PH4, AsH4, SbH4 and Y = Cl, Br): a DFT study

Ab initio calculations have been used to investigate lead-free double-perovskites (DPs) X2AgBiY6 (X = NH4, PH4, AsH4, SbH4 and Y = Cl, Br) for solar-cell-based energy sources. The most recent and improved Becke-Johnson potential (TB-mBJ) has been proposed for the computation of optoelectronic properties. Theoretical and calculated values of the lattice constants obtained by applying the Wu-Cohen generalized gradient approximation (WC-GGA) were found to be in good agreement. The computed bandgap values of (NH4)2AgBiBr6 (1.574 eV) and (SbH4)2AgBiBr6 (1.440 eV) revealed their indirect character, demonstrating that they are suitable contenders for visible light solar-cell (SC) technology. Properties like the refractive index, light absorption, reflection, and dielectric constant are all explained in terms of the optical ranges. Within the wavelength range of 620-310 nm, the maximum absorption band has been identified. Additionally, we discover that all chemicals investigated herein have photocatalytic capabilities that can be used to efficiently produce hydrogen at cheap cost using solar water splitting by photocatalysts. In addition, the stability of the compounds was examined using the calculation of mechanical properties.

Multistep optimization for the electrodeposited mixed perovskite FA1-yCsyPbBrxI3-xsolar cells

The electrodeposition method has recently been developed for the fabrication of perovskite solar cells due to its potential advantages in commercial preparation. However, there is few studies on the preparation of perovskite solar cells by the electrodeposition method, especially on the perovskite FAPbI3-based solar cells. Herein, we fabricated the mixed perovskite FA1-yCsyPbBrxI3-xsolar cells by an optimized electrodeposition method, in which the electrodeposited PbO2reacts directly with FAI and an appropriate amount of CsBr dopants. The corresponding solar cells display the best PCE of 4.97%. By regulating the growth temperature in the reaction between PbO2and FAI/CsBr, the efficiency of the mixed perovskite solar cells can be promoted to 10.18%. These results illustrate that the element doping and growth environment regulation can optimize the quality of the perovskite films, thus promoting the efficiency of the perovskite solar cells. With further optimizing the growth process in the electrodeposition method, it is expected to open up a new commercial preparation route for the perovskite solar cells in the near future.

New Dication-Based Lead-Deficient 3D MAPbI3 and FAPbI3 "d-HPs" Perovskites with Enhanced Stability

Toxicity induced by the presence of lead and the rather poor stability of halide perovskite semiconductors represent the major issues for their large-scale application. We previously reported a new family of lead- and iodide-deficient MAPbI₃ and FAPbI₃ perovskites called d-HPs (for lead- and iodide-deficient halide perovskites) based on two organic cations: hydroxyethylammonium HO-(CH₂)₂-NH₃⁺ (HEA⁺) and thioethylammonium HS-(CH₂)₂-NH₃⁺ (TEA⁺). In this article, we report the use of an organic dication, 2-hydroxypropane-1,3-diaminium (2-propanol 1,3 diammonium), named PDA²⁺, to create new 3D d-HPs based on the MAPbI₃ and FAPbI₃ network with general formulations of (PDA)_0,88x(MA)_1-0,76x[Pb_1-xI_3-x] and (PDA)_1,11x(FA)_1-1,22x[Pb_1-xI_3-x], respectively. These d-HPs have been successfully synthesized as crystals, powders, and thin films and exhibit improved air stability compared to their reference MAPbI₃ and FAPbI₃ perovskite counterparts. PDA²⁺-based deficient MAPbI₃ was also tested in operational perovskite solar cells and exhibited an efficiency of 13.0% with enhanced stability.

Stable Sn-Based Hybrid Perovskite-Related Structures with Tunable Color Coordinates via Organic Cations in Low-Temperature Synthesis

Organic-inorganic Pb-free layered perovskites are efficient broadband emitters and thus are promising materials for lighting applications. However, their synthetic protocols require a controlled atmosphere, high temperature, and long preparation time. This hinders the potential tunability of their emission through organic cations, as is instead common practice in Pb-based structures. Here, we present a set of Sn-Br layered perovskite-related structures that display different chromaticity coordinates and photoluminescence quantum yield (PLQY) up to 80%, depending on the choice of the organic monocation. We first develop a synthetic protocol that is performed under air and at 4 °C, requiring only a few steps. X-ray and 3D electron diffraction analyses show that the structures exhibit diverse octahedra connectivity (disconnected and face-sharing) and thus optical properties, while preserving the organic-inorganic layer intercalation. These results provide key insight into a previously underexplored strategy to tune the color coordinates of Pb-free layered perovskites through organic cations with complex molecular configurations.

Boosting efficiency of eco-friendly perovskite solar cell through optimization of novel charge transport layers

Formamidinium tin triiodide (FASnI₃) is a suitable candidate for the absorber layer in perovskite solar cells (PSC) because of its non-toxicity, narrow band gap, thermal stability and high carrier mobility. This study focuses on the analysis and improvement in the performance of FASnI₃-based PSCs using various inorganic charge transport materials. The copper-based materials such as Cu₂O, CuAlO₂, CuSCN and CuSbS₂ are introduced as hole transport layers due to their earth abundance, ease of manufacturing, high charge mobilities and chemical stability. Similarly, fullerene derivates (PCBM and C₆₀) are deployed as electron transport layers due to their mechanical strength, thermal conductivity and stability. The effect of these materials on optical absorption, quantum efficiency, energy band alignment, band offsets, electric field and recombination are studied in detail. The reasons for the low performance of the cell are identified and improved through design optimization. The PSC performance is analysed in both inverted and conventional architecture. Among all the structures, the best result is achieved through ITO/CuSCN/FASnI₃/C₆₀/Al with an efficiency of 27.26%, V_oc of 1.08 V, J_sc of 29.5 mAcm^-2 and FF of 85.6%.

Effect of aliovalent bismuth substitution on structure and optical properties of CsSnBr3

Aliovalent substitution of the B component in ABX₃ metal halides has often been proposed to modify the band gap and thus the photovoltaic properties, but details about the resulting structure have remained largely unknown. Here, we examine these effects in Bi-substituted CsSnBr₃. Powder X-ray diffraction (XRD) and solid-state ¹¹⁹Sn, ¹³³Cs and ²⁰⁹Bi nuclear magnetic resonance (NMR) spectroscopy were carried out to infer how Bi substitution changes the structure of these compounds. The cubic perovskite structure is preserved upon Bi-substitution, but with disorder in the B site occurring at the atomic level. Bi atoms are randomly distributed as they substitute for Sn atoms with no evidence of Bi segregation. The absorption edge in the optical spectra shifts from 1.8 to 1.2 eV upon Bi-substitution, maintaining a direct band gap according to electronic structure calculations. It is shown that Bi-substitution improves resistance to degradation by inhibiting the oxidation of Sn.

Performance analysis and optimization of inverted inorganic CsGeI3 perovskite cells with carbon/copper charge transport materials using SCAPS-1D

Organic-inorganic perovskite solar cells (PSCs) have achieved the power conversion efficiencies (PCEs) of more than 25%. However, the organic compound in the material is causing structural degradation of the PSC owing to heat (thermal instability), humidity and moisture. This has led to the exploration of only inorganic perovskite materials. Inorganic PSCs such as caesium have seen a breakthrough by achieving highly stable PSC with PCE exceeding 15%. In this work, the inorganic non-toxic PSC of caesium germanium tri-iodide (CsGeI₃) is numerically modelled in SCAPS-1D with two carbon-based electron transport layers (ETLs) and two copper-based hole transport layers (HTLs). This study introduces in-depth numerical modelling and analysis of CsGeI₃ through continuity and Poisson equations. Cu HTLs are selected to increase the electric conductivity of the cell, while carbon-based ETL is used to increase the thermal conductivity of the PSC. A total of four unique PSC structures are designed and presented. A systematic approach is adopted to obtain the optimized PSC design parameters for maximum performance. From the optimized results, it is observed that the C₆₀/CsGeI₃/CuSCN structure is the highest performance PSC, with open-circuit voltage (V _oc) of 1.0169 V, short-circuit current density (J _sc) of 19.653 mA cm^-2, fill factor of 88.13% and the PCE of 17.61%. Moreover, the effect of quantum efficiency, electric field, interface recombination, interface defects, layer thickness, defect density, doping concentration, working temperature and reflection coating on the cell performance are studied in detail.

Lead-Free Metal Halide Perovskite Nanocrystals: From Fundamentals to Applications

Lead (Pb) halide perovskite nanocrystals, with the general formula APbX₃ , where A=CH₃ NH³⁺ , CH(NH₂ )²⁺ , or Cs⁺ and X=Cl^- , Br^- , or I^- , have emerged as a class of materials with promising properties due to their remarkable optical properties and solar cell performance. However, important issues still need to be addressed to enable practical applications of these materials, such as instability, mass production, and Pb toxicity. Recent studies have carried out the replacement of Pb by various less-toxic cations as Sn, Ge, Sb, and Bi. This variety of chemical compositions provide Pb-free perovskite and metal halide nanostructures with a wide spectral range, in addition to being considered less toxic, therefore having greater practical applicability. Highlighting the necessity to address and solve the toxicity problems related to Pb-containing perovskite, this review considers the prospects of the Pb-free perovskite, involving synthesis methods, and properties of them, including advantages, disadvantages, and applications.

(C5N2H14)GeBr4: A 2D Organic Germanium Bromide Perovskite with Strong Orange Photoluminescence Properties

Hybrid organic-inorganic metal halide (OIMH) perovskites are regarded as potential photoluminescent (PL) materials and have attracted intensive attention. Here, we select 1-methylpiperazine as an organic component and successfully obtain a two-dimensional (2D) Ge-based OIMH perovskite, (1-mpz)GeBr₄. It features a 2D layered structure composed of distorted [GeBr₆]^4- octahedra with organic (C₅H₁₄N₂)²⁺ located between the layers. (1-mpz)GeBr₄ exhibits strong orange color under ultraviolet (UV) light and possesses good PL stability for over 2 months. The photoluminescence quantum efficiency is measured to be 7.15% at room temperature, which is the largest among all reported low-dimensional Ge-based perovskites. Experimental measurements, combined with first-principles calculations, reveal that its PL property is attributed to self-trapped excitons (STEs) from [GeBr₆]^4- groups. From the deduced structure-property relationship, Ge-based OIMH PL perovskites with good stability and high PL efficiency can be expected.

Machine Learning Approach to Delineate the Impact of Material Properties on Solar Cell Device Physics

In this research, solar cell capacitance simulator-one-dimensional (SCAPS-1D) software was used to build and probe nontoxic Cs-based perovskite solar devices and investigate modulations of key material parameters on ultimate power conversion efficiency (PCE). The input material parameters of the absorber Cs-perovskite layer were incrementally changed, and with the various resulting combinations, 63,500 unique devices were formed and probed to produce device PCE. Versatile and well-established machine learning algorithms were thereafter utilized to train, test, and evaluate the output dataset with a focused goal to delineate and rank the input material parameters for their impact on ultimate device performance and PCE. The most impactful parameters were then tuned to showcase unique ranges that would ultimately lead to higher device PCE values. As a validation step, the predicted results were confirmed against SCAPS simulated results as well, highlighting high accuracy and low error metrics. Further optimization of intrinsic material parameters was conducted through modulation of absorber layer thickness, back contact metal, and bulk defect concentration, resulting in an improvement in the PCE of the device from 13.29 to 16.68%. Overall, the results from this investigation provide much-needed insight and guidance for researchers at large, and experimentalists in particular, toward fabricating commercially viable nontoxic inorganic perovskite alternatives for the burgeoning solar industry.

Numerical Simulation of 30% Efficient Lead-Free Perovskite CsSnGeI3-Based Solar Cells

A cesium tin−germanium triiodide (CsSnGeI3) perovskite-based solar cell (PSC) has been reported to achieve a high-power-conversion efficiency (PCE > 7%) and extreme air stability. A thorough understanding of the role of the interfaces in the perovskite solar cell, along with the optimization of different parameters, is still required for further improvement in PCE. In this study, lead-free CsSnGeI3 PSC has been quantitatively analyzed using a solar cell capacitance simulator (SCAPS−1D). Five electron transport layers (ETL) were comparatively studied, while keeping other layers fixed. The use of SnO2 as an ETL, which has the best band alignment with the perovskite layer, can increase the power conversion efficiency (PCE) of PSC by up to 30%. The defect density and thickness of the absorber layer has been thoroughly investigated. Results show that the device efficiency is highly governed by the defect density of the absorber layer. All the PSCs with a different ETL exhibit PCE exceeding 20% when the defect density of the absorber layer is in the range of 1014 cm−3−1016 cm−3, and degrade dramatically at higher values. With the optimized structure, the simulation found the highest PCE of CsSnGeI3-based PSCs to be 30.98%, with an open circuit voltage (Voc) of 1.22 V, short-circuit current density (Jsc) of 28.18 mA·cm−2, and fill factor (FF) of 89.52%. Our unprecedented results clearly demonstrate that CsSnGeI3-based PSC is an excellent candidate to become the most efficient single-junction solar cell technology soon.

Tunable deep-blue luminescence from ball-milled chlorine-rich Csx(NH4)1-xPbCl2Br nanocrystals by ammonium modulation

For the first time, a novel class of deep-blue (DB)-emitting Cs_x(NH₄)_1-xPbCl₂Br (0.3 ≤ x ≤ 1) perovskite nanocrystals (PNCs) were prepared by a facile ligand-assisted one-step ball milling method. The resulted PNCs are characterized by high chlorine content (66.7%) and excellent color purity. Their photoluminescence position can be finely modulated from 434 nm to 447 nm, which extends notably beyond the current Rec. 2020 color standard, by the NH₄⁺ content. Among them, Cs_0.3(NH₄)_0.7PbCl₂Br shows the highest quantum yield close to 40%. The PNCs exhibit high phase and optical stability under ambient conditions and UV light according to the NH₄⁺ content. This work offers a new avenue to produce DB perovskites for future full-color displays and optoelectronics.

A Review of Recent Developments in Preparation Methods for Large-Area Perovskite Solar Cells

The recent rapid development in perovskite solar cells (PSCs) has led to significant research interest due to their notable photovoltaic performance, currently exceeding 25% power conversion efficiency for small-area PSCs. The materials used to fabricate PSCs dominate the current photovoltaic market, especially with the rapid increase in efficiency and performance. The present work reviews recent developments in PSCs’ preparation and fabrication methods, the associated advantages and disadvantages, and methods for improving the efficiency of large-area perovskite films for commercial application. The work is structured in three parts. First is a brief overview of large-area PSCs, followed by a discussion of the preparation methods and methods to improve PSC efficiency, quality, and stability. Envisioned future perspectives on the synthesis and commercialization of large-area PSCs are discussed last. Most of the growth in commercial PSC applications is likely to be in building integrated photovoltaics and electric vehicle battery charging solutions. This review concludes that blade coating, slot-die coating, and ink-jet printing carry the highest potential for the scalable manufacture of large-area PSCs with moderate-to-high PCEs. More research and development are key to improving PSC stability and, in the long-term, closing the chasm in lifespan between PSCs and conventional photovoltaic cells.

Supervised Machine Learning-Aided SCAPS-Based Quantitative Analysis for the Discovery of Optimum Bromine Doping in Methylammonium Tin-Based Perovskite (MASnI3-xBrx)

In this investigation, supervised machine learning (ML) was utilized to accurately predict the optimum bromine doping concentration in single-junction MASnI_3-xBr_x devices. Data-driven optimizations were carried out on 42 000 unique devices built utilizing a solar cell capacitance simulator (SCAPS). The devices were investigated through variations of bromine doping %, bandgap, electron affinity, series resistance, back-contact metal, and acceptor concentration─parameters that were specifically chosen because of their tunable nature and ability to be modified through facile experimental fabrication techniques of the device. Five different algorithms were utilized to explore feature engineering. The first step before bromine doping within the device included validation studies of a pure tin-based system, MASnI₃: a power conversion efficiency (PCE) of 6.71% was achieved, having close congruence with experimental data. ML analyses for optimal bromine doping resulted in the discovery of two devices with bromine concentrations of 22.43% (Br22) and 25.63% (Br25), with the latter being a more fine-tuned value obtained through extra rigorous analysis. To understand the total and relative impact of each feature on power conversion efficiency (PCE), Br22 and Br25 were analyzed with a state-of-the-art algorithm, namely, the SHapley Additive exPlanations (SHAP) algorithm. Focusing on the two discovered devices, further device optimizations were carried out utilizing SCAPS. Modulations of absorber thickness, bulk and interfacial defect density, and choice of electron transport layer (ETL) and hole transport layer (HTL) materials were tried. Device stability was analyzed through carrier lifetime studies. Following these optimization steps, Br22 and Br25 demonstrated final high PCE values of 20.72 and 17.37%, respectively. The ML-assisted quantitative analysis of the current work provides significant confidence for optimal bromine-doped tin-based devices to be considered as viable and competitive nontoxic alternatives to traditional technologies.

Progress and Perspectives on Covalent-organic Frameworks (COFs) and Composites for Various Energy Applications

Covalent-organic frameworks (COFs), being a new member of the crystalline porous materials family, have emerged as important materials for energy storage/conversion/generation devices. They possess high surface areas, ordered micro/mesopores, designable structures and an ability to precisely control electro-active groups in their pores, which broaden their application window. Thanks to their low weight density, long range crystallinity, reticular nature and tunable synthesis approach towards two and three dimensional (2D and 3D) networks, they have been found suitable for a range of challenging electrochemical applications. Our review focuses on the progress made on the design, synthesis and structure of COFs and their composites for various energy applications, such as metal-ion batteries, supercapacitors, water-splitting and solar cells. Additionally, attempts have been made to correlate the structural and mechanistic characteristics of COFs with their applications.

Low-Toxicity Perovskite Applications in Carbon Electrode Perovskite Solar Cells—A Review

Perovskite solar cells (PSCs) with earth-abundant carbon as an effective replacer for unstable hole-transporting materials and expensive electrodes is a recently proposed structure promising better air and moisture stability. In this review paper, we report on the latest advances and state of the art of Pb-free and low-Pb-content perovskites, used as absorbers in carbon-based perovskite solar cells. The focus is on the implementation of these, environmentally friendly and non-toxic, structures in PSCs with a carbon electrode as a replacement of the noble metal electrode typically used (C-PSCs). The motivation for this study has been the great potential that C-PSCs have shown for the leap towards the commercialization of PSCs. Some of their outstanding properties include low cost, high-stability, ambient processability and compatibility with most up-scaling methods (e.g., printing). By surpassing the key obstacle of toxicity, caused by the Pb content of the highest-performing perovskites, and by combining the advantages of C-PSCs with the Pb-free perovskites low toxicity, this technology will move one step further; this review summarizes the most promising routes that have been reported so far towards that direction.

Defect Study and Modelling of SnX3-Based Perovskite Solar Cells with SCAPS-1D

Recent achievements, based on lead (Pb) halide perovskites, have prompted comprehensive research on low-cost photovoltaics, in order to avoid the major challenges that arise in this respect: Stability and toxicity. In this study, device modelling of lead (Pb)-free perovskite solar cells has been carried out considering methyl ammonium tin bromide (CH3NH3SnBr3) as perovskite absorber layer. The perovskite structure has been justified theoretically by Goldschmidt tolerance factor and the octahedral factor. Numerical modelling tools were used to investigate the effects of amphoteric defect and interface defect states on the photovoltaic parameters of CH3NH3SnBr3-based perovskite solar cell. The study identifies the density of defect tolerance in the absorber layer, and that both the interfaces are 1015 cm-3, and 1014 cm-3, respectively. Furthermore, the simulation evaluates the influences of metal work function, uniform donor density in the electron transport layer and the impact of series resistance on the photovoltaic parameters of proposed n-TiO2/i-CH3NH3SnBr3/p-NiO solar cell. Considering all the optimization parameters, CH3NH3SnBr3-based perovskite solar cell exhibits the highest efficiency of 21.66% with the Voc of 0.80 V, Jsc of 31.88 mA/cm2 and Fill Factor of 84.89%. These results divulge the development of environmentally friendly methyl ammonium tin bromide perovskite solar cell.

Recent Progress and Challenges in A3Sb2X9-Based Perovskite Solar Cells

The recent trends and current state of perovskite solar cells (PSCs) suggested their potential for practical applications. Since their origin, organic-inorganic lead halide (MAPbX₃) perovskite material-based PSCs have been widely attractive to the scientific community due to their simple manufacturing process, high performance, and cost effectiveness. In spite of the high performance, the lead halide perovskite solar cells are still agonizing due to the long-term stability and toxic nature of Pb. In the last 4 years or so, many alternative perovskite or perovskite-like materials were explored for the development of Pb-free PSCs. However, antimony (Sb)-based perovskite-like materials have shown enhanced stability and average photovoltaic performance. In this mini-review, we discuss the fabrication, recent trends, and current state of the Sb-based PSCs.

Highly Efficient Blue Emission from Self-Trapped Excitons in Stable Sb3+-Doped Cs2NaInCl6 Double Perovskites

Highly efficient blue-emitting three-dimensional (3D) lead-free halide perovskites with excellent stability have attracted worldwide attention. Herein, a doping route was adopted to incorporate Sb³⁺ ions into the Cs₂NaInCl₆ for decorating the electronic band structure. Due to the moderate electron-phonon coupling, the Sb³⁺-doped Cs₂NaInCl₆ double perovskites showed a narrow and relatively unusual blue emission of self-trapped excitons (STEs). Density functional theory (DFT) calculation indicated that the doped Sb³⁺ ions could break the parity-forbidden transition rule and modulate the density of state (DOS) population effectively to boost the PLQY of STEs drastically. The optimized Sb³⁺:Cs₂NaInCl₆ exhibited a PLQY of up to 75.89% and excellent stability under the consecutive illumination of 365 nm UV light for 1000 h. This kind of highly efficient lead-free Sb³⁺-doped Cs₂NaInCl₆ double perovskites may overcome the bottlenecks of severe toxicity and insufficient stability and therefore have an extensive application in the scarce blue photonic and optoelectronic fields.

Effects of the methylammonium ion substitution by 5-ammoniumvaleric acid in lead trihalide perovskite solar cells: a combined experimental and theoretical investigation

In the last decade, lead triiodide perovskite (APbI₃) (A: organic cation) solar cells (PSCs) have been broadly studied due to their promising features related to the low cost, easy manufacturing process, and stability.

Cs2NaGaBr6: a new lead-free and direct band gap halide double perovskite

In this work, we have studied new double perovskite materials, A₂¹⁺B²⁺B³⁺X₆¹⁻, where A₂¹⁺ = Cs, B²⁺ = Li, Na, B³⁺ = Al, Ga, In, and X₆¹⁻.