The adoption of new thin-film materials in high-end technologies, such as monolithic tandem solar cells and integrated circuits, demands fabrication processes that allow a high level of control over film properties such as thickness, conformality, composition, and crystal structure. Achieving this with traditional optoelectronic materials, such as silicon, indium phosphide, gallium arsenide, silicon nitride, and several metal oxides, has opened the way for applications such as high-efficiency photovoltaics, light emitting devices, and integrated photonics. More recently, halide perovskites have demonstrated huge potential in optoelectronic applications, showing exceptional photovoltaic properties, light emission, and lasing performance. Common growth techniques for these halide perovskites have been solution-based methods. Optimized solution-based processes yield high quality thin films well-suited for applications, such as single-junction solar cells, but remain incompatible with integration into complex devices such as monolithic tandem photovoltaics and photonic circuits. Therefore, new fabrication methods allowing atomic, structural, and compositional precision with the conformal growth of hybrid and multi-compound halide perovskite thin films are of utmost importance for material exploration and for their application in complex devices. This Perspective reviews the progress on synthesis methods of halide perovskite thin films, discusses pressing challenges, and proposes strategies for growth control, versatile film deposition, monolithic device integration, epitaxial growth, and high-throughput synthesis to discover novel and non-toxic stable metal halide compositions.

The recent sky-rocketing performance of perovskite solar cells has triggered a strong interest in further upgrading the fabrication techniques to meet the scalability requirements of the photovoltaic industry. The integration of vapor-deposition into the solution process in a sequential fashion can boost the uniformity and reproducibility of the perovskite solar cells.Besides, mixed-halide perovskites have exhibited outstanding crystallinity as well as higher stability compared with iodide-only perovskite. An extensive study was carried out to identify a reproducible process leading to highly crystalline perovskite films that when integrated into solar cells exhibited high power conversion efficiency (max. 19.8%). This was achieved by optimizing the deposition rate of the PbI2 layer as well as by inserting small amounts of methylammonium (MA) bromide and chloride salts to the primary MAI salt in the solution-based conversion step. 3The optimum MABr/MAI molar ratio leading to the most efficient and stable solar cells was found to be 0.4. Stabilities were in excess of 90 hours for p-i-n type solar cells. This reproducible approach towards the fabrication of triple halide perovskites using a hybrid vapor-solution method is a promising method towards scalable production techniques.Recently, Rafizadeh et. al. used a hybrid vapor-solution method to fabricate planar MAPI-based devices with 18.9% efficiency in the n-i-p structure 26 . In that work, TiO2 was used as the Supporting Information.XRD, AFM, and, SEM of the PbI2 layer deposited at different rates, device statistical data depending on PbI2 deposition rate, XRD of MAPI-BrCl depending on MACl concentration, the effect of MABr/MAI ratio on the absorbance edge and on the Shockley-Queisser limit and average values of J-V parameters, stability analysis of the devices.

coefficient and carrier mobilities, and long minority carrier lifetimes. [1][2][3][4][5] Vast amount of research has been conducted on further improving stability and increasing the efficiency of perovskite solar cells. One way of boosting the efficiency of perovskite solar cells is maximizing the photocurrent generation by light management. Light management in perovskite solar cells can be provided by surface texturing, [6][7][8][9][10][11][12][13] plasmonics, [14][15][16] antireflective films on the glass substrate, [17,18] vertical cavity design, [19][20][21][22][23][24][25][26] and photon recycling. [27,28] Among them, vertical cavity design is popular since it does not require any additional material other than what is needed to fabricate a perovskite solar cell, guaranteeing its low-cost.Ball et al. reported optical simulation of glass/fluorine-doped tin oxide (FTO)/TiO 2 /CH 3 NH 3 PbI 3 (MAPI)/Spiro-OMeTAD/Au solar cell structure based on transfer matrix method (TMM), where they reported local maxima in the modeled short circuit current at MAPI thicknesses of ≈190, ≈320, ≈470, and ≈630 nm thanks to favorable interference conditions. [21] However, they did not extend their simulations to cover transport materials (TLs) with different refractive indices. In a recent study, Grant et al. published a comprehensive optical simulation study on MAPI/silicon tandem solar cells using the finite element method. [25] They divided the ideal refractive index of a front transport layer (FTL) of the perovskite top solar cell into two regions: those larger and smaller than the refractive index of MAPI at 1000 nm of wavelength. However, this separation is incapable of explaining single junction perovskite solar cells targeting shorter wavelengths. Filipic et al. provided vertical cavity designs for 2-and 4-terminal (2T and 4T)-MAPI/ silicon tandem solar cells in which, MAPI solar cell is composed of glass, front indium tin oxide (ITO), Spiro-OMeTAD, CH 3 NH 3 PbI 3 , TiO 2 , and rear ITO layers. [23] It is important to note that optimum thickness of a transport layer changes based on 2T and 4T configurations since in 2T configuration nonoptimum layer thicknesses can lead to a photocurrent reduction in the perovskite top cell and its increase in the silicon bottom cell. Therefore, an optical cavity design of a perovskite solar cell resembles that of perovskite top cell in a 4T tandem cell geometry, yet, the effect of replacing the rear solar cell with a planar metal is optically substantial. Although an FTL refractive index (n FTL ) around that of perovskite is commonly suggested in the literature, [20,25] there is no Organometallic halide perovskite solar cells have emerged as a versatile photovoltaic technology with soaring efficiencies. Planar configuration, in particular, has been a structure of choice thanks to its lower temperature processing, compatibility with tandem solar cells, and potential in commercialization. Despite all the breakthroughs in the field, the optical mechanisms leading to highly efficient perovsk...

As the employment of halide perovskite films in single-junction and tandem solar cells continues to soar, there is a strong drive -from academia to industry-to produce these films using dry processes, avoiding the use of toxic solvents. Vapor deposition methods such as co-evaporation have shown advantages of solvent-free approaches to produce high-efficiency solar cells. However, co-evaporation requires the use of multiple sources that challenge the deposition rate control of complex halide perovskite compositions. Here, Pulsed Laser Deposition (PLD) is proposed as an alternative method to deposit hybrid halide perovskites films from a single-source and following a fully dry approach. We use the archetypical methylammonium lead iodide (MAPbI3) to demonstrate the formation of high-quality films with optimal optoelectronic properties by PLD on various substrates for single-junction and tandem devices. Furthermore, the important role of the PLD target composition and deposition parameters to achieve control over film microstructure and optoelectronic properties is discussed. The controlled conformal growth provided by PLD demonstrated in this work with MAPbI3 on device-relevant substrates will broaden opportunities to explore PLD of more complex hybrid halide perovskite compositions for efficient, stable, and scalable solar cell devices.

Vapor deposition of halide perovskites presents high potential for scalability and industrial processing of perovskite solar cells. It prevents the use of toxic solvents, allows thickness control, and yields conformal and uniform coating over large areas. However, the distinct volatility of the perovskite organic and inorganic components currently requires the use of multiple thermal sources or two‐step deposition to achieve the perovskite phase. In this work, single‐source, single‐step MA1–xFAxPbI3 thin film deposition with tunable stoichiometry by pulsed laser deposition is demostrated. By controlling the laser ablation of a solid target containing adjustable MAI:FAI:PbI2 ratios, the room temperature formation of cubic α‐phase MA1–xFAxPbI3 thin films is demonstrated. The target‐to‐film transfer of the ablated species, including the integrity of the organic molecules and the desired MA+:FA+ ratio, is confirmed by x‐ray photoelectron spectroscopy and solid‐state NMR. Photoluminescence analysis further confirms the shift of the bandgap with varying the MA+:FA+ ratio. Finally, proof‐of‐concept n‐i‐p solar cells with 14% efficiency are demonstrated with as‐deposited non‐passivated pulsed laser deposition (PLD)‐MA1–xFAxPbI3. This study opens the path for future developments in industry‐compatible vapor‐deposition methods for perovskite solar cells.

Chloride is extensively used in the preparation of metal halide perovskites such as methylammonium lead iodide (MAPbI3–xClx), but its persistence and role in solution‐processed materials has not yet been rationalized. Multiple‐source vacuum deposition of perovskites enables a fine control over thin‐film stoichiometry and allows the incorporation of chemical species irrespective of their solubility. Herein, the first example of mixed MAPbI3–xClx thin films prepared by three‐source vacuum deposition is presented using methylammonium iodide (MAI), PbI2, and PbCl2 as precursors. The optoelectronic properties of the material are evaluated through photovoltaic and electro‐/photoluminescent characterizations. Besides the very similar structural and optical properties of MAPbI3 and MAPbI3–xClx, an increased electroluminescence efficiency, longer photoluminescence lifetimes, as well as larger photovoltage, are observed in the presence of chloride, suggesting a reduction of nonradiative charge recombination.

Electron transport layers (ETL) based on tin(IV) oxide (SnO 2 ) are recurrently employed in perovskite solar cells (PSCs) by many deposition techniques. Pulsed laser deposition (PLD) offers a few advantages for the fabrication of such layers, such as being compatible with large scale, patternable, and allowing deposition at fast rates. However, a precise understanding of how the deposition parameters can affect the SnO 2 film, and as a consequence the solar cell performance, is needed. Herein, we use a PLD tool equipped with a droplet trap to minimize the number of excess particles (originated from debris) reaching the substrate, and we show how to control the PLD chamber pressure to obtain surfaces with very low roughness and how the concentration of oxygen in the background gas can affect the number of oxygen vacancies in the film. Using optimized deposition conditions, we obtained solar cells in the n−i−p configuration employing methylammonium lead iodide perovskite as the absorber layer with power conversion efficiencies exceeding 18% and identical performance to devices having the more typical atomic layer deposited SnO 2 ETL.

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