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...