High-performance perovskite CH3NH3PbI3 thin films for solar cells prepared by single-source physical vapour deposition

Fabricating Planar Perovskite Solar Cells through a Greener Approach

High-quality perovskite thin films are typically produced via solvent engineering, which results in efficient perovskite solar cells (PSCs). Nevertheless, the use of hazardous solvents like precursor solvents (N-Methyl-2-pyrrolidone (NMP), dimethyl sulfoxide (DMSO), dimethylformamide (DMF), gamma-butyrolactone (GBL)) and antisolvents (chlorobenzene (CB), dibutyl ether (DEE), diethyl ether (Et2O), etc.) is crucial to the preparation of perovskite solutions and the control of perovskite thin film crystallization. The consumption of hazardous solvents poses an imminent threat to both the health of manufacturers and the environment. Consequently, before PSCs are commercialized, the current concerns about the toxicity of solvents must be addressed. In this study, we fabricated highly efficient planar PSCs using a novel, environmentally friendly method. Initially, we employed a greener solvent engineering approach that substituted the hazardous precursor solvents with an environmentally friendly solvent called triethyl phosphate (TEP). In the following stage, we fabricated perovskite thin films without the use of an antisolvent by employing a two-step procedure. Of all the greener techniques used to fabricate PSCs, the FTO/SnO2/MAFAPbI3/spiro-OMeTAD planar device configuration yielded the highest PCE of 20.98%. Therefore, this work addresses the toxicity of the solvents used in the perovskite film fabrication procedure and provides a promising universal method for producing PSCs with high efficiency. The aforementioned environmentally friendly approach might allow for PSC fabrication on an industrial scale in the future under sustainable conditions.

Surface Reconstruction with Aprotic Trimethylsulfonium Iodide for Efficient and Stable Perovskite Solar Cells

Organic ammonium salts are widely used for surface passivation to enhance the photovoltaic (PV) performance and stability of perovskite solar cells (PSCs). However, the protic nature of ammonium units results in the quick degradation of perovskites due to the hydrogen bonding interaction with water molecules. Recently, organo-sulfur compounds have attracted growing interest as passivation layers on three-dimensional perovskites due to their moisture-resistive behavior. Herein, trimethylsulfonium iodide (TMSI), an aprotic S-based organic compound, is employed for surface modification of methylammonium lead iodide-based PSCs to impede moisture penetration, improve charge transfer, and passivate surface defects. The TMSI effectively passivates uncoordinated Pb through Pb···S interactions, and the optimized PSC exhibits a power conversion efficiency (PCE) of 21.03% with an open-circuit voltage of ca. 1.13 V under one-sun illumination, while it reached up to 37.58 and 37.69% under low-intensity indoor illuminations, 1000 and 2000 lx with LED 5000 K, respectively. TMSI-treated cells display enhanced device stability by retaining 92.7% of their initial PCE after 50 days of storage in ambient conditions. This study provides a novel and effective surface reconstruction strategy with aprotic materials to improve PV performance and device stability in PSCs.

Sub-Bandgap Sensitization of Perovskite Semiconductors via Colloidal Quantum Dots Incorporation

By taking advantage of the outstanding intrinsic optoelectronic properties of perovskite-based photovoltaic materials, together with the strong near-infrared (NIR) absorption and electronic confinement in PbS quantum dots (QDs), sub-bandgap photocurrent generation is possible, opening the way for solar cell efficiencies surpassing the classical limits. The present study shows an effective methodology for the inclusion of high densities of colloidal PbS QDs in a MAPbI3 (methylammonium lead iodide) perovskite matrix as a means to enhance the spectral window of photon absorption of the perovskite host film and allow photocurrent production below its bandgap. The QDs were introduced in the perovskite matrix in different sizes and concentrations to study the formation of quantum-confined levels within the host bandgap and the potential formation of a delocalized intermediate mini-band (IB). Pronounced sub-bandgap (in NIR) absorption was optically confirmed with the introduction of QDs in the perovskite. The consequent photocurrent generation was demonstrated via photoconductivity measurements, which indicated IB establishment in the films. Despite verifying the reduced crystallinity of the MAPbI3 matrix with a higher concentration and size of the embedded QDs, the nanostructured films showed pronounced enhancement (above 10-fold) in NIR absorption and consequent photocurrent generation at photon energies below the perovskite bandgap.

Influence of Al doping in zinc oxide electron transport layer for the degradation triple-cation-based organometal halide perovskite solar cells

Various strategies have been adapted to fabricate stable organic-inorganic hybrid perovskite (PVT) solar cells (PSCs). The triple-cation (CH₃NH₃⁺ (MA⁺), CH₃(NH₂)²⁺ (FA⁺), and Cs⁺) along with dual-anion (I^- and Br^-)-based PVT (TC-PVT) layer offers better stability than single cation-based PVTs. The deprivation of the PVT absorber is also influenced by the interface of the absorber with the charge transport layer (electron transport layer (ETL) and hole transport layer (HTL)). Here, the degradation of the TC-PVT coated on Al-doped zinc oxide (AZO) as well as FTO/AZO/TC-PVT/HTL structured PSC was examined for various Al to Zn molar ratio (R_Al/Zn) of AZO. The PL decay study of FTO/AZO/TC-PVT revealed that the lowest degradation in the power (35.38%) was observed for the AZO with R_Al/Zn of 5%. Furthermore, the PV cell parameters of the PSCs were analytically determined to explore the losses in the PSCs during degradation. The shunt resistance reduction was maximum (50.32%) for R_Al/Zn = 10%, whereas, minimum shunt loss (7.33%) for R_Al/Zn of 2%. The highest loss due to series resistance was observed for R_Al/Zn of 0%. The changes in diode ideality factor (n) and reverse saturation current density (J₀) were the smallest for R_Al/Znof 10%.

Synthetic approaches for perovskite thin films and single-crystals

Halide perovskites are compelling candidates for the next generation of photovoltaic technologies owing to an unprecedented increase in power conversion efficiency and their low cost, facile fabrication and outstanding semiconductor properties.

SnO2@ZnO nanocomposites doped polyaniline polymer for high performance of HTM-free perovskite solar cells and carbon-based

Herein, at first, green SnO₂@ZnO nanocomposites were synthesized using Calotropis plant extract as an electron transfer material (ETM) to fabricate low-temperature-processed perovskite solar cells (PSCs). Then, the polyaniline (PANI) polymer was applied as an efficient additive to improve perovskite film quality. Under the effects of the small content of PANI additive, the quality of perovskite films is enhanced, which showed higher crystallinity in (110) crystal plane; also, the perovskite grains were found to be enlarged from 342 to 588 nm. The power conversion efficiency (PCE) of the prepared PSCs with SnO₂@ZnO.PANI nanocomposites electron transfer layer (ETL) increased by 3.12%, compared with the PCE of SnO₂@ZnO nanocomposites. The perovskite devices using SnO₂@ZnO.PANI nanocomposites ETL have shown good stability during 480 h of tests. Furthermore, the optimal PSCs were fabricated by the mp-TiO₂/SnO₂@ZnO.PANI nanocomposites as ETL, which has a power conversion efficiency of 15.45%. We expect that these results will boost the development of low-temperature ETL, which is essential for the commercializing of high-performance, stable, and flexible perovskite solar cells.

Ultraviolet-to-infrared broadband photodetector and imaging application based on a perovskite single crystal

The organic-inorganic hybrid perovskite CH₃NH₃PbBr₃(MAPbBr₃) has been well developed in the X-ray to visible light band due to its superior optoelectronic properties, but this material is rarely studied in the infrared band. In this paper, a UV-NIR broadband optical detector based on MAPbBr₃ single crystal is studied, and the response range can reach the near-infrared region. In the visible light band, the optical response of the device is mainly caused by the photoelectric effect; in the near-infrared band, the optical response of the device is mainly caused by the thermal effect. The carrier response of MAPbBr₃ material under different wavelengths of light was investigated using a non-contact measurement method (optical pump terahertz (THz) probe spectroscopy). This paper also builds a set of photoelectric sensor array components, and successfully realizes the conversion of optical image signals to electrical image signals in the visible light band and infrared band. The experimental results show that MAPbBr₃ crystals provide a new possibility for UV-NIR broadband photodetectors.

Vacuum-Deposited Cesium Tin Iodide Thin Films with Tunable Thermoelectric Properties

Most current thermoelectric materials have important drawbacks, such as toxicity, scarceness, and peak operating temperatures above 300 °C. Herein, we report the thermoelectric properties of different crystalline phases of Sn-based perovskite thin films. The 2D phase, Cs₂SnI₄, is obtained through vacuum thermal deposition and easily converted into the black β phase of CsSnI₃ (B-β CsSnI₃) by annealing at 150 °C. B-β CsSnI₃ is a p-type semiconductor with a figure of merit (ZT) ranging from 0.021 to 0.033 for temperatures below 100 °C, which makes it a promising candidate to power small electronic devices such as wearable sensors which may be interconnected in the so-called Internet of Things. The B-β phase is stable in nitrogen, whereas it spontaneously oxidizes to Cs₂SnI₆ upon exposure to air. Cs₂SnI₆ shows a negative Seebeck coefficient and an ultralow thermal conductivity. However, the ZT values are 1 order of magnitude lower than for B-β CsSnI₃ due to a considerably lower electrical conductivity.

Theoretical investigation of FAPbSnGeX3 efficiency

The use of hybrid lead halide perovskites as light absorbers in photovoltaic cells have gained large interest due to their optoelectronic properties and high efficiency. However, most hybrid perovskites contain toxic lead which has a negative impact on the environment. In this work, we systematically study the structural, electronic, and optical properties of lower lead halide perovskites FAPb_0.5Sn_0.25Ge_0.25X₃ (X = I, Br, Cl), as well as discussing their photovoltaic performance (open circuit voltage (V _oc), the short circuit current density (J _sc), and the power conversion efficiency (η)) using density functional theory (DFT), and we compare these with FAPbX₃ (X = I, Br, Cl) frameworks. The compounds show a suitable band gap for photovoltaic applications, in which iodine has a lower gap value compared to chlorine. It is noteworthy that we found that lead doping by both germanium and tin in the FAPb_0.5Sn_0.25Ge_0.25X₃ (X = I, Br, Cl) materials significantly improves the adsorption coefficient and the stability of these systems compared to the FAPbX₃ (X = I, Br, Cl) systems. The calculated Jsc shows a monotonical decrease from FAPb_0.5Sn_0.25Ge_0.25I₃ to FAPbCl₃, which represents the lowest Jsc. Results reveal that FAPb_0.5Sn_0.25Ge_0.25Cl₃ demonstrates promising potential for photovoltaic application as it shows the highest efficiency. This study can help reduce the toxicity of hybrid lead halide perovskites and also raises their experimental power conversion efficiency.

Hybrid Organic-Inorganic Perovskite Halide Materials for Photovoltaics towards Their Commercialization

Hybrid organic-inorganic perovskite (HOIP) photovoltaics have emerged as a promising new technology for the next generation of photovoltaics since their first development 10 years ago, and show a high-power conversion efficiency (PCE) of about 29.3%. The power-conversion efficiency of these perovskite photovoltaics depends on the base materials used in their development, and methylammonium lead iodide is generally used as the main component. Perovskite materials have been further explored to increase their efficiency, as they are cheaper and easier to fabricate than silicon photovoltaics, which will lead to better commercialization. Even with these advantages, perovskite photovoltaics have a few drawbacks, such as their stability when in contact with heat and humidity, which pales in comparison to the 25-year stability of silicon, even with improvements are made when exploring new materials. To expand the benefits and address the drawbacks of perovskite photovoltaics, perovskite-silicon tandem photovoltaics have been suggested as a solution in the commercialization of perovskite photovoltaics. This tandem photovoltaic results in an increased PCE value by presenting a better total absorption wavelength for both perovskite and silicon photovoltaics. In this work, we summarized the advances in HOIP photovoltaics in the contact of new material developments, enhanced device fabrication, and innovative approaches to the commercialization of large-scale devices.

Methylammonium Compensation Effects in MAPbI3 Perovskite Solar Cells for High-Quality Inorganic CuSCN Hole Transport Layers

Recent studies have demonstrated that copper (I) thiocyanate (CuSCN) has huge potential as a hole extraction material (HEM) for perovskite solar cells. Here, we used CuSCN as a HEM and analyzed its relationships with a methylammonium lead iodide (MAPbI₃) perovskite layer. The CuSCN dissolved in diethyl sulfide (DES) was spin-coated on the MAPbI₃ layer. For high-quality and dense CuSCN layers, post-annealing was carried out at various temperatures and times. However, the unwanted dissociation of MAPbI₃ to PbI₂ was observed due to the post-annealing for a long time at elevated temperatures. In addition, DES, which is used as a CuSCN solvent, is a polar solvent that damages the surface of MAPbI₃ perovskites and causes poor interfacial properties between the perovskite layer and HEM. To solve this problem, the effect of the molar ratio of methylammonium iodide (MAI) and PbI₂ in the MAPbI₃ precursor solution was investigated. The excess MAI molar ratio in the MAPbI₃ precursor solution reduced MAPbI₃ surface damage despite using DES polar solvent for CuSCN solution. In addition, dissociation of MAPbI₃ to PbI₂ following an adequate post-annealing process was well suppressed. The excess MAI molar ratio in the MAPbI₃ precursor could be compensated for the MA loss and effectively suppress phase separation from MAPbI₃ to MAI + PbI₂ during post-annealing. The efficiency based on the normal planar structure of CuSCN/MAPbI₃ (using excess MAI)/TiO₂ was approximately 17%. The CuSCN-based MAPbI₃ device shows more optimized stability than the conventional spiro-OMeTAD under damp heat (85 °C and 85% relative humidity) conditions because of the robust inorganic HEM.

Investigation of Cation Exchange Behaviors of FAxMA1-xPbI3 Films Using Dynamic Spin-Coating

In this study, we fabricated and characterized uniform multi-cation perovskite FA_xMA_1−xPbI₃ films. We used the dynamic spin-coating method to control the cation ratio of the film by gradually increasing the FA+, which replaced the MA+ in the films. When the FA+ concentration was lower than xFA ~0.415 in the films, the stability of the multi-cation perovskite improved. Above this concentration, the film exhibited δ-phase FAPbI₃ in the FA_xMA_1−xPbI₃ films. The formation of δ-phase FAPbI₃ disturbed the homogeneity of the photoluminescence spatial distribution and suppressed the absorption spectral bandwidth with the increasing bandgap. The precise control of the cation ratio of multi-cation perovskite films is necessary to optimize the energy-harvesting performance.

Long-Range Interfacial Charge Carrier Trapping in Halide Perovskite-C60 and Halide Perovskite-TiO2 Donor-Acceptor Films

Interfacial electron transfer across perovskite-electron acceptor heterojunctions plays a significant role in the power-conversion efficiency of perovskite solar cells. Thus, electron donor-acceptor thin films of halide perovskite nanocrystals receive considerable attention. Nevertheless, understanding and optimizing distance- and thickness-dependent electron transfer in perovskite-electron acceptor heterojunctions are important. We reveal the distance-dependent and diffusion-controlled interfacial electron transfer across donor-acceptor heterojunction films formed by formamidinium or cesium lead bromide (FAPbBr₃/CsPbBr₃) perovskite nanocrystals with TiO₂/C₆₀. Self-assembled nanocrystal films prepared from FAPbBr₃ show a longer photoluminescence lifetime than a solution, showing a long-range carrier migration. The acceptors quench the photoluminescence intensity but not the lifetime in a solution, revealing a static electron transfer. Conversely, the electron transfer in the films changes from dynamic to static by moving toward the donor-acceptor interface. While radiative recombination dominates the electron transfer at 800 μm or farther, the acceptors scavenge the photogenerated carriers within 100 μm. This research highlights the significance of interfacial electron transfer in perovskite films.

Limits to Electrical Mobility in Lead-Halide Perovskite Semiconductors

Semiconducting polycrystalline thin films are cheap to produce and can be deposited on flexible substrates, yet high-performance electronic devices usually utilize single-crystal semiconductors, owing to their superior charge-carrier mobilities and longer diffusion lengths. Here we show that the electrical performance of polycrystalline films of metal-halide perovskites (MHPs) approaches that of single crystals at room temperature. Combining temperature-dependent terahertz conductivity measurements and ab initio calculations we uncover a complete picture of the origins of charge-carrier scattering in single crystals and polycrystalline films of CH₃NH₃PbI₃. We show that Fröhlich scattering of charge carriers with multiple phonon modes is the dominant mechanism limiting mobility, with grain-boundary scattering further reducing mobility in polycrystalline films. We reconcile the large discrepancy in charge-carrier diffusion lengths between single crystals and films by considering photon reabsorption. Thus, polycrystalline films of MHPs offer great promise for devices beyond solar cells, including light-emitting diodes and modulators.

Solvent Engineering of the Precursor Solution toward Large-Area Production of Perovskite Solar Cells

Solar cells based on emerging organic-inorganic hybrid perovskite materials have reached certified power conversion efficiency as high as 25.5%, showing great potential in the next generation of photovoltaics toward large-scale industrialization. The most competitive feature of perovskite solar cells (PSCs) is that the perovskite light absorber can be fabricated by a low-cost solution method. For the solution method, the characteristics of the solvent play a key role in determining the crystallization kinetics, growth orientation, and optoelectronic properties of the perovskite film. Although significant progress has been made in the field of solvent engineering in PSCs, it is still challenging for the solution method to sustainably produce industrial-scale PSCs for future commercialization applications. Herein, the advanced progress of solvent engineering of precursor solution in terms of coordination regulation and toxicity reduction is highlighted. The physical and chemical characteristics of different solvents in reducing the toxicity of the solvent system, regulating the coordination property of the precursor solution, controlling the film-forming process of the perovskite film, and adjusting the photovoltaic performance of the PSC are systematically discussed. Lastly, important perspectives on solvent engineering of the perovskite precursor solution toward future industrial production of high-performance PSCs are provided.

Crystal Engineering Approach for Fabrication of Inverted Perovskite Solar Cell in Ambient Conditions

In this paper, we demonstrate the high potentialities of pristine single-cation and mixed cation/anion perovskite solar cells (PSC) fabricated by sequential method deposition in p-i-n planar architecture (ITO/NiOX/Perovskite/PCBM/BCP/Ag) in ambient conditions. We applied the crystal engineering approach for perovskite deposition to control the quality and crystallinity of the light-harvesting film. The formation of a full converted and uniform perovskite absorber layer from poriferous pre-film on a planar hole transporting layer (HTL) is one of the crucial factors for the fabrication of high-performance PSCs. We show that the in-air sequential deposited MAPbI3-based PSCs on planar nickel oxide (NiOX) permitted to obtain a Power Conversion Efficiency (PCE) exceeding 14% while the (FA,MA,Cs)Pb(I,Br)3-based PSC achieved 15.6%. In this paper we also compared the influence of transporting layers on the cell performance by testing material depositions quantity and thickness (for hole transporting layer), and conditions of deposition processes (for electron transporting layer). Moreover, we optimized second step of perovskite deposition by varying the dipping time of substrates into the MA(I,Br) solution. We have shown that the layer by layer deposition of the NiOx is the key point to improve the efficiency for inverted perovskite solar cell out of glove-box using sequential deposition method, increasing the relative efficiency of +26% with respect to reference cells.

Impact of absorber layer thickness, defect density, and operating temperature on the performance of MAPbI3 solar cells based on ZnO electron transporting material

Hybrid organic-inorganic perovskite solar cells (PSCs) are the novel fourth-generation solar cells, with impressive progress in the last few years. MAPbI₃ is a cost-effective material used as an absorber layer in PSCs. Due to the different diffusion length of carriers, the electron transporting material (ETM) plays a vital role in PSCs' performance. ZnO ETM is a promising candidate for low-cost and high-efficiency photovoltaic technology. In this work, the normal n-i-p planar heterojunction structure has been simulated using SCAPS-1D. The influence of various parameters such as the defect density, the thickness of the MAPbI₃ layer, the temperature on fill factor, the open-circuit voltage, the short circuit current density, and the power conversion efficiency are investigated and discussed in detail. We found that a 21.42% efficiency can be obtained under a thickness of around 0.5 μm, and a total defect of 10¹³ cm⁻³ at ambient temperature. These simulation results will help fabricate low-cost, high-efficiency, and low-temperature PSCs.

The compatibility of methylammonium and formamidinium in mixed cation perovskite: the optoelectronic and stability properties

The methylammonium (MA) and formamidinium (FA) are the most commonly used organic cations in perovskite solar cells (PSCs), whereas the impact of size and polarity differences between these two on the photovoltaic performances has been rarely revealed. Herein, we systematically investigated the phase distribution, optoelectronic and stability properties of FA-MA mixed perovskites. To identify the phase homogeneity, depth-dependent grazing-incidence wide-angle x-ray scattering measurements were employed, which demonstrates that the mixed cation perovskite possesses a FA-rich phase on the film surface and the bottom is comprised of MA-rich phase. Additionally, upon long-time illumination, a new PL peak is appeared at 778 nm, representing the generation of MA-rich phase induced by ion migration. It is worth noting that the phase splitting and inhomogeneous phase distribution would not bring any obvious detrimental effects to the photovoltaic performances and stability properties. Through judiciously tuning the cation proportion in pure-iodide perovskite, the additive-free PSCs achieve an efficiency as high as 20.7%. Furthermore, the PSCs with a broad range of FA/MA ratios show improved humidity/thermal/light stability despite the phase inhomogeneity. Therefore, the work shows that the MA and FA cations have a high compatibility in perovskite structure and the precise ratio control can further improve the performances.

Anisotropic Performance of High-Quality MAPbBr3 Single-Crystal Wafers

It has been proved that bulk single crystals of a halide perovskite behave much better than its polycrystalline counterparts in multiple application scenarios. Thus, the growth of large-sized and high-quality single crystals is significant to guarantee their ultimate device performances. Here, based on our recently invented settled temperature and controlled antisolvent diffusion system, improvements achieved in this work include the following: (1) We modified the growth system to optimize the control over both mass and heat transport to alleviate defect formation. State-of-the-art-quality MAPbBr₃ crystals were grown, and from the bulk crystals, differently oriented crystalline wafers were fabricated with the full width at half-maximum of X-ray rocking curves of 40-86 arcsec. (2) The optical band gaps revealed no anisotropy on differently oriented wafers, whereas the refractive index and extinction coefficient exhibited obvious anisotropy. (3) Angle-resolved polarized Raman spectra demonstrate distinct in-plane anisotropy on (100) and (110) wafers but not on the (111) wafer. The equilibrium MA⁺ orientations are deduced to adopt the <111> direction with the antiparallel MA⁺ orientation between adjacent domains. (4) Radiation detectors fabricated on differently oriented wafers proved photoresponse anisotropy to both visible and X-ray radiation, following a general order of (100) > (110) > (111). Because anisotropy is an inevitable issue for various applications employing crystalline materials, this study, based on the clarification of the debatable intrinsic dipole configuration in the pseudocubic crystal lattice, will provide quantitative information on physicochemical property anisotropy and subsequently facilitate optimization of device performance referring to crystal orientations of halide perovskite crystals.

Transparent, flexible MAPbI3 perovskite microwire arrays passivated with ultra-hydrophobic supramolecular self-assembly for stable and high-performance photodetectors

The emergence of organic-inorganic hybrid perovskites (OHPs) has revolutionised the potential performance of optoelectronic devices; most perovskites are opaque and hence incompatible with transparent optoelectronics and sensitive to environmental degradation. Here, we have reported a single-step fabrication of ultra-long MAPbI₃ perovskite microwire arrays over a large area using stencil lithography based on sequential vacuum sublimation. The environmental stability of MAPbI₃ is empowered with a newly designed and synthesized transparent supramolecular self-assembly based on a mixture of two tripodal l-Phe-C₁₁H₂₃/C₇F₁₅ molecules, which showed a contact angle of 105° and served as ultra-hydrophobic passivation layers for more than 45 days in an ambient atmosphere. The MAPbI₃ microwire arrays passivated with the supramolecular self-assembly demonstrated for the first time both excellent transparency of ∼89% at 550 nm and a remarkable photoresponse with a photo-switching ratio of ∼10⁴, responsivity of 789 A W^-1, detectivity of 10¹⁴ Jones, linear dynamic range of ∼122 dB, and rise time of 432 μs. Furthermore, the photodetector fabricated on a flexible PET substrate demonstrated robust mechanical flexibility even beyond 1200 bending cycles. Therefore, the scalable stencil lithography and supramolecular passivation approaches have the potential to deliver next-generation transparent, flexible, and stable optoelectronic devices.

All-Inorganic CsPbBr3 Perovskite Films Prepared by Single Source Thermal Ablation

Hybrid organo-lead halide perovskites are becoming the benchmark material for next generation photovoltaics and a very important player for other applications such as photodetectors and light emitting diodes. Nevertheless, the most important issue hindering the large-scale application of these materials remains their intrinsic instability due to the organic cation. Although the substitution with inorganic cesium (Cs) enhances stability, in most cases solution deposition methods of fully inorganic perovskites result in high surface roughness and poor surface coverage. This work reports on the evaporation of the CsPbBr₃ precursor by Single Source Thermal Ablation, showing that just after deposition films consist of a mixture of CsPbBr₃, CsPb₂Br₅, and Cs₄PbBr₆ due to a vertical composition gradient. We point out that mild post deposition treatments lead to the conversion of CsPb₂Br₅ and Cs₄PbBr₆ into CsPbBr₃ due to its higher thermodynamic stability. Conversion results into smooth and pinhole-free CsPbBr₃ films with good light absorption and emission properties. We demonstrate the suitability of obtained films for planar devices by preparing perovskite-based pure-green light emitting diodes, thus promoting Single Source Thermal Ablation as a promising alternative deposition technique for all-inorganic perovskite-based devices.

Optimizing Lignosulfonic Acid-Grafted Polyaniline as a Hole-Transport Layer for Inverted CH3NH3PbI3 Perovskite Solar Cells

A conducting polymer of lignosulfonic acid-grafted, polyaniline-doped camphorsulfonic acid (LS-PANI-CSA), created via a low-temperature solution process, has been explored as an efficient hole-transport layer (HTL) for inverted single cation-anion CH₃NH₃PbI₃ perovskite solar cells. The performance of the solar cell was optimized in this study by tuning the morphology and work function of LS-PANI-CSA films using dimethylsulfoxide (DMSO) as a solvent in treatment. Results showed that DMSO washing enhanced the electronic properties of the LS-PANI-CSA film and increased its hydrophobicity, which is very important for perovskite growth. The perovskite active layer deposited onto the DMSO-treated LS-PANI-CSA layer had higher crystallinity with large grain sizes (>5 μm), more uniform and complete surface coverage, and very low pinhole density and PbI₂ residues compared to untreated LS-PANI-CSA. These enhancements result in higher device performance and stability. Using DMSO-treated LS-PANI-CSA as an HTL at 15 nm of thickness, a maximum 10.8% power conversion efficiency was obtained in ITO/LS-PANI-CSA/MAPbI₃/PCBM/BCP/Ag inverted-device configurations. This was a significant improvement compared to 5.18% for devices based on untreated LS-PANI-CSA and a slight improvement over PEDOT:PSS-based devices with 9.48%. Furthermore, the perovskite based on treated LS-PANI-CSA showed the higher stability compared to both untreated LS-PANI-CSA and PEDOT:PSS HTL-based devices.

Single-Source Vapor-Deposited Cs2AgBiBr6 Thin Films for Lead-Free Perovskite Solar Cells

Lead-free double perovskites have been considered as a potential environmentally friendly photovoltaic material for substituting the hybrid lead halide perovskites due to their high stability and nontoxicity. Here, lead-free double perovskite Cs2AgBiBr6 films are initially fabricated by single-source evaporation deposition under high vacuum condition. X-ray diffraction and scanning electron microscopy characterization show that the high crystallinity, flat, and pinhole-free double perovskite Cs2AgBiBr6 films were obtained after post-annealing at 300 °C for 15 min. By changing the annealing temperature, annealing time, and film thickness, perovskite Cs2AgBiBr6 solar cells with planar heterojunction structure of FTO/TiO2/Cs2AgBiBr6/Spiro-OMeTAD/Ag achieve an encouraging power conversion efficiency of 0.70%. Our preliminary work opens a feasible approach for preparing high-quality double perovskite Cs2AgBiBr6 films wielding considerable potential for photovoltaic application.

Single Source Thermal Evaporation of Two-dimensional Perovskite Thin Films for Photovoltaic Applications

Hybrid two-dimensional (2D) halide perovskites has been widely studied due to its potential application for high performance perovskite solar cells. Understanding the relationship between microstructural and opto-electronic properties is very important for fabricating high-performance 2D perovskite solar cell. In this work, the effect of solvent annealing on grain growth was investigated to enhance the efficiency of photovoltaic devices with 2D perovskite films based on (BA)₂(MA)₃Pb₄I₁₃ prepared by single-source thermal evaporation. Results show that solvent annealing with the introduction of solvent vapor can effectively enhance the crystallization of the (BA)₂(MA)₃Pb₄I₁₃ thin films and produce denser, larger-crystal grains. The thin films also display a favorable band gap of 1.896 eV, which benefits for increasing the charge-diffusion lengths. The solvent-annealed (BA)₂(MA)₃Pb₄I₁₃ thin-film solar cell prepared by single-source thermal evaporation shows an efficiency range of 2.54-4.67%. Thus, the proposed method can be used to prepare efficient large-area 2D perovskite solar cells.

Extremely Robust Gas-Quenching Deposition of Halide Perovskites on Top of Hydrophobic Hole Transport Materials for Inverted (p-i-n) Solar Cells by Targeting the Precursor Wetting Issue

Lead halide perovskite solar cells afford high power conversion efficiencies, even though the photoactive layer is formed in a solution process. At the same time, solution processing may impose some severe dewetting issues, especially if organic, hydrophobic charge transport layers are considered. Ultimately, very narrow processing windows with a relatively large spread in device performance and a considerable lab-to-lab variation result. Here, we unambiguously identify dimethylsulfoxide (DMSO), which is commonly used as a co-solvent and complexing agent, to be the main reason for dewetting of the precursor solution on hydrophobic hole transport layers, such as polytriarylamine, in a gas-quenching-assisted deposition process. In striking contrast, we will show that N-methyl-2-pyrrolidon (NMP), which has a lower hydrophilic-lipophilic-balance, can be favorably used instead of DMSO to strongly mitigate these dewetting issues. The resulting high-quality perovskite layers are extremely tolerant with respect to the mixing ratio (NMP/dimethylformamide) and other process parameters. Thus, our findings afford an outstandingly robust, easy to use, and fail-safe deposition technique, yielding single (MAPbI₃) and double (FA_0.94Cs_0.06PbI₃) cation perovskite solar cells with high efficiencies (∼18.5%). Most notably, the statistical variation of the devices is significantly reduced, even if the deposition process is performed by different persons. We foresee that our results will further the reliable preparation of perovskite thin films and mitigate process-to-process variations that still hinder the prospects of upscaling perovskite solar technology.

Unraveling the Effect of Crystal Structure on Degradation of Methylammonium Lead Halide Perovskite

Despite the remarkable efficiencies of perovskite solar cells, moisture instability has still been the major constraint in the technology deployment. Although, some research groups have discussed the possible mechanisms involved in the perovskite degradation, no broader understanding has been developed so far. Here, we demonstrate that the crystal orientation of perovskite film plays a major role in its degradation. We observed that the films fabricated via different routes led to different degradation behaviors and unraveled that diversity in the degradation rate arises due to the difference in crystallographic characteristics of the films. Using optical and electrical measurements, we show that the film prepared via a single-step (lead chloride precursor based) route undergoes a much faster degradation rate as compared with films prepared using single step (acetate precursor based) and two-step (or sequential deposition) routes. Although the resulting film is methylammonium lead iodide (MAPbI₃) regardless of processing via different routes, their respective crystal orientation is different. In this manuscript, we correlate crystal orientation of MAPbI₃ with their degradation pattern. Our studies also suggest a possible way to make stable perovskite film.

Effective Control of Chlorine Contents in MAPbI3- xCl x Perovskite Solar Cells Using a Single-Source Vapor Deposition and Anion-Exchange Technique

In this study, a new method is developed to control the Cl-to-I ratio in MAPbI_{3- x}Cl _x perovskite solar cells (PSCs) more easily and precisely using single-source vapor deposition of MAPbCl₃ thin films and a subsequent anion exchange by repeated spin-coatings of methylammonium iodide (MAI) solution. This method can overcome the problems of previous vapor-deposition techniques for PSCs such as the occurrence of morphological defects in the films and difficulty in controlling the stoichiometry of the elements. The repetitive MAI treatments gradually fill the interstitial voids in the perovskite film and increase the average grain size up to 1.2 μm, which improves the charge-transfer property of the cells. The atomic Cl content, i.e., the x value, of the MAPbI_{3- x}Cl _x film can also be simply controlled by changing the number of MAI treatments. The energy levels and resistive elements of the cells are strongly dependent on the x value of the MAPbI_{3- x}Cl _x film. A maximum power conversion efficiency of 19.1% is achieved at x = 0.005.

High-Quality Perovskite CH₃NH₃PbI₃ Thin Films for Solar Cells Prepared by Single-Source Thermal Evaporation Combined with Solvent Treatment

In this work, solvent annealing process for CH₃NH₃PbI₃ thin film prepared by single source evaporation was reported. Characterized by the scanning electron microscope (SEM), X-ray diffractometer (XRD), energy dispersive spectroscope (EDS), ultraviolet-visible (UV) spectrophotometer, and the photoluminescence (PL) spectrometer, our method ensured higher quality film with crystallinity, composition, well-defined grain structure, and reproducibility. The optimized solar cell device based on the structure of ITO/PEDOT:PSS/CH₃NH₃PbI₃/PCBM/Ag achieved better performance in power conversion efficiency from 2.64% to 9.92%, providing an effective method to optimize the quality of perovskite film for solar cell application.

Direct synthesis of cubic phase CsPbI3 nanowires

One-dimensional all-inorganic halide perovskites have emerged as one of the most prominent materials in the application of optoelectronic devices due to their remarkable properties such as a low number of defects, morphological anisotropy, mechanical flexibility and fast charge transfer capability.

Highly Uniform Large-Area (100 cm2) Perovskite CH3NH3PbI3 Thin-Films Prepared by Single-Source Thermal Evaporation

In this work, we report the reproducible preparation method of highly uniform large-area perovskite CH3NH3PbI3 thin films by scalable single-source thermal evaporation with the area of 100 cm2. The microstructural and optical properties of large-area CH3NH3PbI3 thin films were investigated. The dense, uniform, smooth, high crystallinity of large-area perovskite thin film was obtained. The element ratio of Pb/I was close to the ideal stoichiometric ratio of CH3NH3PbI3 thin film. These films show a favorable bandgap of 1.58 eV, long and balanced carrier-diffusion lengths. The CH3NH3PbI3 thin film perovskite solar cell shows a stable efficiency of 7.73% with almost no hysteresis, indicating a single-source thermal evaporation that is suitable for a large area perovskite solar cell.

High-performance p-type inorganic-organic hybrid thermoelectric thin films

The performance of organic-inorganic hybrid thermoelectric thin films can be dramatically enhanced by optimizing energy filtering and carrier transport states at the organic-inorganic interfaces. In this work, p-type "Sb2Te3/CH3NH3I/Sb2Te3" multilayer thin films were firstly fabricated with varied contents of CH3NH3I, and then an annealing process was used in order to form homogeneous organic-inorganic hybrid thin films. The results revealed that the introduced organic component can promote thin film growth and develop a dense nanostructure with improved crystallinity, thus resulting in a significantly increased Seebeck coefficient and a reduced thermal conductivity as a result of the optimized electronic transport characteristics and enhanced effects of phonon scattering. As is expected, the thermoelectric performance of the hybrid-nanocomposite films is enhanced, achieving the maximum ZT value of 1.55 at a temperature of 413 K, which is several times higher than that of the as-fabricated film, thereby suggesting that the proposed strategy can be applied as an efficient method for the preparation of high-performance thermoelectric thin films.

Perovskite Thin Film Synthesised from Sputtered Lead Sulphide

In the last few years, research on dye-sensitised devices has been focused on the development of solar cells, based on CH₃NH₃PbX₃ (X = I⁻, Br⁻, Cl⁻) composites with perovskite structure. The deposition of perovskite thin films is usually carried out by solution-based processes using spin-coating techniques that result in the production of high quality films. Solar cells made by this method exceed 20% efficiency, with the potential for use in large scale production through ink print or screen printing techniques. As an alternative route, perovskite thin films can be deposited through thermal evaporation. A new method is proposed to produce CH₃NH₃PbI₃, based on a radio-frequency (rf) -sputtering technique that results in a high reproducibility of the films and is compatible with roll-to-roll processes. We deposited thin films of lead-sulphide (PbS) and converted them into perovskite by placing the films in an iodine atmosphere, followed by dipping in a solution of methylammonium iodide (CH₃NH₃I). The conversions to PbI₂ and CH₃NH₃PbI₃ were confirmed by elemental analyses, absorption, and photoluminescence spectroscopy. Structural properties were revealed by X-ray diffraction and infrared and Raman spectroscopy.

Sequentially Vapor-Grown Hybrid Perovskite for Planar Heterojunction Solar Cells

High-quality and reproducible perovskite layer fabrication routes are essential for the implementation of efficient planar solar cells. Here, we introduce a sequential vapor-processing route based on physical vacuum evaporation of a PbCl2 layer followed by chemical reaction with methyl-ammonium iodide vapor. The demonstrated vapor-grown perovskite layers show compact, pinhole-free, and uniform microstructure with the average grain size of ~ 320 nm. Planar heterojunction perovskite solar cells are fabricated using TiO2 and spiro-OMeTAD charge transporting layers in regular n-i-p form. The devices exhibit the best efficiency of 11.5% with small deviation indicating the high uniformity and reproducibility of the perovskite layers formed by this route.

Mechanism and effect of γ-butyrolactone solvent vapor post-annealing on the performance of a mesoporous perovskite solar cell

In this paper, γ-butyrolactone (GBL) solvent vapor post-annealing (SVPA) on CH3NH3PbI3 thin films is reported, aiming to improve the complete transformation of PbI2 and increase the grain size of the CH3NH3PbI3 crystal, thus boosting the performance of mesoporous CH3NH3PbI3 perovskite solar cells (PSCs). The influence of GBL SVPA on the microstructure of perovskite layers and performance of PSCs was studied. The short circuit current density (J sc) of the devices significantly increased, yielding a high efficiency of 16.58%, which was 27.05% higher than that of thermally annealed films. A model was derived to explain the effect of GBL SVPA on PSCs. The perovskite films prepared by this method present several advantages such as complete transformation of PbI2 to CH3NH3PbI3, high crystallinity, large grain size, and fewer grain boundaries than those prepared without GBL SVPA. This improvement is beneficial for charge dissociation and transport in hybrid photovoltaic devices.

Recent Advances in Synthesis and Properties of Hybrid Halide Perovskites for Photovoltaics

The progress made by the scientific community in emerging photovoltaic technologies over the past two decades has been outstanding. Numerous methods have been developed for the preparation of hybrid organic-inorganic perovskite solar cells. The power conversion efficiency has been up to 14% by a one-step vacuum deposition technique. A serious concern is the toxicity of the materials. In this review, several methods aimed at resolving these problems to some extent have been compiled, including eco-friendly synthesis. Further efficiency enhancements are expected following optimization, and a better fundamental understanding of the internal electron charge transfer, electron-hole diffusion to the corresponding layers, flexibility, and stability-dependent bandgaps is reported. This paper explores the green synthesis of organic-inorganic perovskites for industrialization. Concerning the above facts, a simple low-cost model called "dispersed photovoltaic cells" is presented.

Enhanced Performance of Perovskite Solar Cells with Zinc Chloride Additives

Perovskite solar cells (PSCs) have attracted extensive attention due to their impressive photovoltaic performance. The quality of the perovskite layer is very critical to achieve high device performance. Here, we explore the partial substitution of PbI₂ by ZnCl₂ in the preparation of CH₃NH₃PbI₃ and its effects on perovskite morphology, optical properties, and photovoltaic performance. Consequently, the device with 3% ZnCl₂ shows great improvement in power conversion efficiency (PCE) from 16.4 to 18.2% compared to that of the control device. Moreover, the device is more stable than the control device, with only 7% degradation after aging for 30 days. These results are attributed to the increased grain size, improved film morphology, and reduced recombination loss after the partial substitution of PbI₂ by ZnCl₂ in the perovskite film. This work develops a new approach for morphology control through rational additives in the perovskite film, and paves the way toward further enhancing the device performances of PSCs including PCE and stability.

The interface degradation of planar organic–inorganic perovskite solar cell traced by light beam induced current (LBIC)

The light beam induced current (LBIC) method was adopted to nondestructively map the photoresponse of real planar organic–inorganic hybrid perovskite solar cells (PSCs).