Improved Photo(electro)chemical Response and Solar Cell Performance of (ThEA)₂PbI₄‑Based Layered Perovskites by Reduced Graphene Oxide (rGO)

Three-dimensional (3D) lead halide perovskites have evolved as a champion light-harvesting material in the race of third-generation solar cells within a decade due to their excellent optoelectronic properties. Instability due to moisture is one of the daunting issues, creating challenges toward the commercialization of 3D perovskite solar cells. On the other hand, pure two-dimensional (2D) layered perovskites show significant potential for photovoltaic applications, exhibiting moisture resistance properties, compared to 3D perovskites. However, these pure 2D layered perovskites exhibit poor device performance due to the long nonconductive carbon chain present inherently in the structure of light-absorbing perovskites. Reduced graphene oxide (rGO) has attracted attention due to its high electrical conductivity in addition to its inexpensive synthesis methods. In this study, we have examined the effect of rGO as an “interlayer” in R2PbI4 (where R = thiophene ethylamine)-based 2D layered perovskite solar cells (conventional n–i–p configuration) and during photo(electro)chemical analysis of R2PbI4 (coated over compact TiO2- and mesoporous TiO2-coated fluorine-doped tin oxide (FTO) photoanodes). The perovskite layer was fabricated by a one-step spin coating method and characterized using X-ray diffraction (XRD), UV–visible, scanning electron microscopy (SEM), and atomic force microscopy (AFM) instruments. The role of rGO as an interlayer was observed in (i) decreasing the charge transfer resistance, (ii) decreasing the photoluminescence (PL) intensity, (iii) enhancing the photocurrent density during photo(electro)chemical analysis, and (iv) improving the efficiencies of solar cells of 2D layered perovskites

CsSnBr₃, A Lead-Free Halide Perovskite for Long-Term Solar Cell Application: Insights on SnF₂ Addition

Solar cells based on “halide perovskites” (HaPs) have demonstrated unprecedented high power conversion efficiencies in recent years. However, the well-known toxicity of lead (Pb), which is used in the most studied cells, may affect its widespread use. We explored an all-inorganic lead-free perovskite option, cesium tin bromide (CsSnBr₃), for optoelectronic applications. CsSnBr₃-based solar cells exhibited photoconversion efficiencies (PCEs) of 2.1%, with a short-circuit current (<i>J</i>_SC) of ∼9 mA cm^–2, an open circuit potential (<i>V</i>_OC) of 0.41 V, and a fill factor (FF) of 58% under 1 sun (100 mW cm^–2) illumination, which, even though meager compared to the Pb analogue-based cells, are among the best reported until now. As reported earlier, addition of tin fluoride (SnF₂) was found to be beneficial for obtaining good device performance, possibly due to reduction of the background carrier density by neutralizing traps, possibly via filling of cation vacancies. The roles of SnF₂ on the properties of the CsSnBr₃ were investigated using ultraviolet photoemission spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS) analysis

Polyvinylbutyral Based Hybrid Organic/Inorganic Films as a Moisture Barrier Material

Flexible and thermally stable, freestanding hybrid organic/inorganic based polymer-composite films have been fabricated using a simple solution casting method. Polyvinylbutyral and amine functionalized mesoporous silica were used to synthesize the composite. An additional polyol“tripentaerythritol”component was also used to increase the −OH group content in the composite matrix. The moisture permeability of the composites was investigated by following a calcium degradation test protocol. This showed a reduction in the moisture permeability with the increase in functionalized silica loadings in the matrix. A reduction in permeability was observed for the composites as compared to the neat polymer film. The thermal and mechanical properties of these composites were also investigated by various techniques like thermogravimetric analysis, differential scanning calorimetry, tensile experiments, and dynamic mechanical analysis. It was observed that these properties detoriate with the increase in the functionalized silica content and hence an optimized loading is required in order to retain critical properties. This deterioration is due to the aggregation of the fillers in the matrix. Furthermore, the films were used to encapsulate P3HT (poly 3 hexyl thiophene) based organic Schottky structured diodes, and the diode characteristics under accelerated aging conditions were studied. The weathered diodes, encapsulated with composite film showed an improvement in the lifetime as compared to neat polymer film. The initial investigation of these films suggests that they can be used as a moisture barrier layer for organic electronics encapsulation application

Cesium Enhances Long-Term Stability of Lead Bromide Perovskite-Based Solar Cells

Direct comparison between perovskite-structured hybrid organic–inorganic methylammonium lead bromide (MAPbBr₃) and all-inorganic cesium lead bromide (CsPbBr₃), allows identifying possible fundamental differences in their structural, thermal and electronic characteristics. Both materials possess a similar direct optical band gap, but CsPbBr₃ demonstrates a higher thermal stability than MAPbBr₃. In order to compare device properties, we fabricated solar cells, with similarly synthesized MAPbBr₃ or CsPbBr₃, over mesoporous titania scaffolds. Both cell types demonstrated comparable photovoltaic performances under AM1.5 illumination, reaching power conversion efficiencies of ∼6% with a poly aryl amine-based derivative as hole transport material. Further analysis shows that Cs-based devices are as efficient as, and more stable than methylammonium-based ones, after aging (storing the cells for 2 weeks in a dry (relative humidity 15–20%) air atmosphere in the dark) for 2 weeks, under constant illumination (at maximum power), and under electron beam irradiation

Mobility–Lifetime Products in MAPbI₃ Films

Photovoltaic solar cells operate under steady-state conditions that are established during the charge carrier excitation and recombination. However, to date no model of the steady-state recombination scenario in halide perovskites has been proposed. In this Letter we present such a model that is based on a single type of recombination center, which is deduced from our measurements of the illumination intensity dependence of the photoconductivity and the ambipolar diffusion length in those materials. The relation between the present results and those from time-resolved measurements, such as photoluminescence that are commonly reported in the literature, is discussed

What Is the Mechanism of MAPbI₃ p‑Doping by I₂? Insights from Optoelectronic Properties

Obtaining insight into, and ultimately control over, electronic doping of halide perovskites may improve tuning of their remarkable optoelectronic properties, reflected in what appear to be low defect densities and as expressed in various charge transport and optical parameters. Doping is important for charge transport because it determines the electrical field within the semiconducting photoabsorber, which strongly affects collection efficiency of photogenerated charges. Here we report on intrinsic doping of methylammonium lead tri-iodide, MAPbI₃, as thin films of the types used for solar cells and LEDs, by I₂ vapor at a level that does not affect the optical absorption and leads to a small (<20 meV, ∼9 nm) red shift in the photoluminescence peak. This I₂ vapor treatment makes the films 10× more electronically conductive in the dark. We show that this change is due to p-type doping because we find their work function to increase by 150 mV with respect to the ionization energy (valence band maximum), which does not change upon I₂ exposure. The majority carrier (hole) diffusion length increases upon doping, making the material less ambipolar. Our results are well-explained by I₂ exposure decreasing the density of donor defects, likely iodide vacancies (V_I) or defect complexes, containing V_I. Invoking iodide interstitials, which are acceptor defects, seems less likely based on calculations of the formation energies of such defects and is in agreement with a recent report on pressed pellets

Low-Temperature Solution-Grown CsPbBr₃ Single Crystals and Their Characterization

Cesium lead bromide (CsPbBr₃) was recently introduced as a potentially high performance thin-film halide perovskite (HaP) material for optoelectronics, including photovoltaics, significantly more stable than MAPbBr₃ (MA = CH₃NH₃⁺). Because of the importance of single crystals to study relevant material properties per se, crystals grown under conditions comparable to those used for preparing thin films, i.e., low-temperature solution-based growth, are needed. We show here two simple ways, antisolvent-vapor saturation or heating a solution containing retrograde soluble CsPbBr₃, to grow single crystals of CsPbBr₃ from a precursor solution, treated with acetonitrile (MeCN) or methanol (MeOH). The precursor solutions are stable for at least several months. Millimeter-sized crystals are grown without crystal-seeding and can provide a 100% yield of CsPbBr₃ perovskite crystals, avoiding a CsBr-rich (or PbBr₂-rich) composition, which is often present alongside the perovskite phase. Further growth is demonstrated to be possible with crystal seeding. The crystals are characterized in several ways, including first results of charge carrier lifetime (30 ns) and an upper-limit of the Urbach energy (19 meV). As the crystals are grown from a polar aprotic solvent (DMSO), which is similar to those used to grow hybrid organic–inorganic HaP crystals, this may allow growing mixed (organic and inorganic) monovalent cation HaP crystals