Co-evaporation as an optimal technique towards compact methylammonium bismuth iodide layers

Abstract

the most studied perovskite-based solar cells reported up to date contain the toxic lead in its composition. photovoltaic research and development towards non-toxic, lead-free perovskite solar cells are critical to finding alternatives to reduce human health concerns associated with them. Bismuth-based perovskite variants, especially in the form of methylammonium bismuth iodide (MBi), is a good candidate for the non-toxic light absorber. However, the reported perovskite variant MBI thin films prepared by the solution process so far suffers from poor morphology and surface coverage. In this work, we investigate for the first time the optoelectronic, crystallographic and morphological properties of MBI thin films prepared via thermal co-evaporation of MAI and BiI 3. We find by modifying the precursor ratio that the layer with pure MBI composition lead to uniform, compact and homogeneous layers, broadening the options of deposition techniques for lead-free based perovskite solar cells. Organic-inorganic metal halide perovskites have emerged as promising candidates for the next generation of photovoltaics (PV) given their unprecedented rise in power conversion efficiency (PCE) achieving 25.2% 1 Beyond their superior optoelectronic properties, low cost and high versatility 2-5 , their instability and lead toxicity are at present major concerns, which greatly limit their incorporation into marketable products. Significant efforts are therefore oriented into the search of stable and non-toxic perovskite materials, yet real alternatives based on lead-free components are still missing. Among the various lead-free perovskites variants proposed in the literature, Sn-based materials have demonstrated promising efficiencies leading to a PCE value of 9.6% 6. However, Sn-based perovskites suffer from inevitable oxidation of Sn 2+ to Sn 4+ , providing a source of inhomogeneity and pinholes that are further enhanced by their fast crystallization 7. As a result, the most straightforward candidates possessing similar electronic properties to Pb 2+ , which are suitable for light-harvesting, are Ge 2+ , Sb 3+ or Bi 3+ cations. While Ge-based perovskites suffer from material instability due to hydrolytic decomposition in humid environment 8-10 , Sb-based perovskites show limited photovoltaic performances in spite of their good chemical stability 8. Bismuth is a heavy metal with a stable oxidation state, little toxicity, and a similar polarizable electron density than Pb 11. Thanks to the formation of BiX 6 3− octahedron, it can also form perovskites with diverse dimensionalities and phases, from which methylammonium bismuth iodide ((CH 3 NH 3) 3 Bi 2 I 9 , MBI) is at the forefront 9. MBI is a wide-bandgap semiconductor with a zero-dimensional structure having reduced toxicity, high stability and promising photovoltaic efficiency 12. Despite such remarkable potential, its use is limited by the low quality of the films resulted from conventional spin-coating deposition, which produces non-compact MBI layers with poor coverage and crysta...

Single- or double A-site cations in A3Bi2I9 bismuth perovskites: What is the suitable choice?

“…Cross-sectional SEM images, as shown in Figure S12, showed a thick perovskite capping layer, however, the device efficiency was significantly reduced. Recently, Momblona et al reported on similar observations in co-evaporated highly uniform MA 3 Bi 2 I 9 thin films, in which a very low PCE of 0.01% was achieved with a thick capping layer [26]. A thick layer of perovskite regardless of uniformity is probably detrimental for efficient charge transport, because of the high exciton binding energies (up to 300 meV [24]) and exciton gap states (see excitonic absorption peaks in Fig.…”

“…In 2015, Park et al reported Cs 3 Bi 2 I 9 and MA 3 Bi 2 I 9 based mesoscopic heterojunction solar cells yielding power conversion efficiency (PCE) of 1.09% for the former and 0.2% for the latter [24]. Subsequently, efforts have been made to further improve the efficiency by employing suitable electron transport layers [25], vapour deposition [26], solvent engineering [27]; however, the efficiency has been limited. Nevertheless, various solution-based and vaporassisted methods were reported with different solvents [27,28] and fabrication processes leading to PCEs between 0.01% [29] and 1.64% [30].…”

Engineering Chemical Vapor Deposition for Lead‐Free Perovskite‐Inspired MA3Bi2I9 Self‐Powered Photodetectors with High Performance and Stability

“…To overcome the film morphology limitations reported in the literature on lead‐free bismuth‐based PIMs grown through vapor‐based strategies, [ 25–32 ] in this study we investigated the deposition of MBI using a modified source delivery CVD system. In CVD, the precursors are brought into the vapor phase by heating and are then carried by an inert gas stream to the vicinity of a heated substrate.…”

“…It is worth noting that the 2T‐CVD MBI films exhibited a morphology that is considerably improved compared to the prior literature, which typically reported MBI films with noncompact surface morphologies, disconnected/isolated grains, and a large number of pinholes. [ 15–20,25–32 ] The enhancement in surface morphology of 2T‐CVD MBI films compared to the 1T‐counterparts can be attributed to the efficient delivery of the precursors to the substrate region, as enabled by the separate delivery system used for the two precursors, which would lead to the formation of a laminar flow regime (Figure S2, Supporting Information). Consequently, a homogenous reaction of the precursors would occur at or in the immediate vicinity of the substrate surface, as reflected by the high quality of the resultant MBI films.…”

“…[ 5,21–24 ] Consequently, various strategies have been developed to improve film quality, both using solution‐based methods (e.g., additive‐assisted spin coating; electric‐field‐assisted spray coating; and two‐stage electric‐field‐assisted reactive deposition) and vapor‐based methods (e.g., vapor‐assisted solution process (VASP); chemical vapor deposition (CVD); and thermal evaporation). [ 15–20,25–32 ] Among these methods, multi‐zone CVD is particularly attractive because it allows a robust control of the film growth conditions through the fine‐tuning of the process parameters (i.e., the pressure, gas flow rate, and temperature of the precursors; and the temperature of the substrate). [ 33 ] For instance, CVD has been successfully used for the large‐area deposition of uniform, compact films of lead‐based perovskites.…”