Enhancing the Microstructure of Perovskite-Inspired Cu-Ag-Bi-I Absorber for Efficient Indoor Photovoltaics

Lead-free perovskite-inspired materials (PIMs) are gaining attention in optoelectronics due to their low toxicity and inherent air stability. Their wide bandgaps (≈2 eV) make them ideal for indoor light harvesting. However, the investigation of PIMs for indoor photovoltaics (IPVs) is still in its infancy. Herein, the IPV potential of a quaternary PIM, Cu2AgBiI6 (CABI), is demonstrated upon controlling the film crystallization dynamics via additive engineering. The addition of 1.5 vol% hydroiodic acid (HI) leads to films with improved surface coverage and large crystalline domains. The morphologically-enhanced CABI+HI absorber leads to photovoltaic cells with a power conversion efficiency of 1.3% under 1 sun illumination-the highest efficiency ever reported for CABI cells and of 4.7% under indoor white light-emitting diode lighting-that is, within the same range of commercial IPVs. This work highlights the great potential of CABI for IPVs and paves the way for future performance improvements through effective passivation strategies.</p

Surface and optical properties of phase-pure silver iodobismuthate nanocrystals

The study of surface defects is one of the forefronts of halide perovskite research. In the nanoscale regime, where the surface-to-volume ratio is high, the surface plays a key role in determining the electronic properties of perovskites. Perovskite-inspired silver iodobismuthates are promising photovoltaic absorbers. Herein, we demonstrate the colloidal synthesis of phase pure and highly crystalline AgBiI4 nanocrystals (NCs). Surface-sensitive spectroscopic techniques reveal the rich surface features of the NCs that enable their impressive long-term environmental and thermal stabilities. Notably, the surface termination and its passivation effects on the electronic properties of AgBiI4 are investigated. Our atomistic simulations suggest that a bismuth iodide-rich surface, as in the case of AgBiI4 NCs, does not introduce surface trap states within the band gap region of AgBiI4, unlike a silver iodide-rich surface. These findings may encourage the investigation of surfaces of other lead-free perovskite-inspired materials.publishedVersionPeer reviewe

Advances in the Stability of Halide Perovskite Nanocrystals

Colloidal halide perovskite nanocrystals are promising candidates for next-generation optoelectronics because of their facile synthesis and their outstanding and size-tunable properties. However, these materials suffer from rapid degradation, similarly to their bulk perovskite counterparts. Here, we survey the most recent strategies to boost perovskite nanocrystals stability, with a special focus on the intrinsic chemical- and compositional-factors at synthetic and post-synthetic stage. Finally, we review the most promising approaches to address the environmental extrinsic stability of perovskite nanocrystals (PNCs). Our final goal is to outline the most promising research directions to enhance PNCs&rsquo; lifetime, bringing them a step closer to their commercialization

Advances in the Stability of Halide Perovskite Nanocrystals

Colloidal halide perovskite nanocrystals are promising candidates for next-generation optoelectronics because of their facile synthesis and their outstanding and size-tunable properties. However, these materials suffer from rapid degradation, similarly to their bulk perovskite counterparts. Here, we survey the most recent strategies to boost perovskite nanocrystals stability, with a special focus on the intrinsic chemical- and compositional-factors at synthetic and post-synthetic stage. Finally, we review the most promising approaches to address the environmental extrinsic stability of perovskite nanocrystals (PNCs). Our final goal is to outline the most promising research directions to enhance PNCs’ lifetime, bringing them a step closer to their commercialization.publishedVersionPeer reviewe

Influence of surface ligands on the electronic properties of AgBiI4 rudorffites nanocrystals

International audienceIn the last decade, owing to a combination of outstanding optical and electronic properties, lead halide perovskites (LHP) have prevailed as promising low-cost materials for optoelectronic applications going from photovoltaics to photocatalysis. However, within a time when ecology is the keystone of concerns, the presence of lead at the forefront of their structures and properties, as well as their operational instability, hindered their commercial applications.In the sake of non-toxic alternatives, Ag-Bi-I Rudorffites have recently emerged as potential candidates. Indeed, in 2016, solar cells made from these materials showed a power conversion efficiency of 1.2%, which has now increased to 5.4%. With edges-shared AgI6 and BiI6 octahedra, Rudorffites have well-positioned band gap and high absorbance in the entire visible range, along with greater stability in ambient conditions than LHP.In this study, AgBiI4 nanocrystals (NCs) have been synthesized via the hot-injection route. Interestingly, they show a quasi-direct band gap value closely matching the bulk material one. Alongside, surface characterization has shown the presence of ligands passivating the NCs’ surface, suggesting an effect of the surface coverage on the electronic properties. Recent theoretical developments based on density functional theory (DFT), have uncovered the influence of surface dipoles on work functions in LHP and similar mechanisms might occur here. Up to date, this has not been demonstrated yet for Ag-Bi-I Rudorffites NCs. Therefore, expending recent theoretical descriptions of Ag-Bi-I Rudorffite bulk properties, we propose to combine DFT surface calculations with experimental results, to clarify the fundamental mechanisms behind the bandgap closing observed in these nanocrystals

Lead-free cesium titanium bromide double perovskite nanocrystals

Double perovskites are a promising family of lead-free materials that not only replace lead but also enable new optoelectronic applications beyond photovoltaics. Recently, a titanium (Ti)-based vacancy-ordered double perovskite, Cs2 TiBr6, has been reported as an example of truly sustainable and earth-abundant perovskite with controversial results in terms of photoluminescence and environmental stability. Our work looks at this material from a new perspective, i.e., at the nanoscale. We demonstrate the first colloidal synthesis of Cs2 TiX6 nanocrystals (X = Br, Cl) and observe tunable morphology and size of the nanocrystals according to the set reaction temperature. The Cs2 TiBr6 nanocrystals synthesized at 185◦ C show a bandgap of 1.9 eV and are relatively stable up to 8 weeks in suspensions. However, they do not display notable photoluminescence. The centrosymmetric crystal structure of Cs2 TiBr6 suggests that this material could enable third-harmonic generation (THG) responses. Indeed, we provide a clear evidence of THG signals detected by the THG microscopy technique. As only a few THG-active halide perovskite materials are known to date and they are all lead-based, our findings promote future research on Cs2 TiBr6 as well as on other lead-free double perovskites, with stronger focus on currently unexplored nonlinear optical applications.publishedVersionPeer reviewe

Moisture-assisted Near-UV Emission Enhancement of Lead-free Cs4CuIn2Cl12 Double Perovskite Nanocrystals

Lead-based halide perovskite nanocrystals (NCs) are recognized as emerging emissive materials with superior photoluminescence (PL) properties. However, the toxicity of lead and the swift chemical decomposition under atmospheric moisture severely hinder their commercialization process. Herein, we report the first colloidal synthesis of lead-free Cs4CuIn2Cl12 layered double perovskite NCs via a facile moisture-assisted hot-injection method stemming from relatively nontoxic precursors. While moisture is typically detrimental to NC synthesis, we demonstrate that the presence of water molecules in Cs4CuIn2Cl12 synthesis enhances the PL quantum yield (mainly in the near-UV range), induces a morphological transformation from 3D nanocubes to 2D nanoplatelets, and converts the dark transitions to radiative transitions for the observed self-trapped excitons relaxation. This work paves the way for further studies on the moisture-assisted synthesis of novel lead-free halide perovskite NCs for a wide range of applications

Synthetic Control over the Surface Chemistry of Blue-Emitting Perovskite Nanocrystals for Photocatalysis

Lead halide perovskite nanocrystals (NCs) are particularly suitable for light-emitting and photocatalysis applications, where their potential can be maximized by controlling the surface composition of their organic shell. In this study, the preparation of CsPbClxBr3-x NCs at room temperature in toluene is described. Three differently structured surfactants are utilized for the synthesis, each with a specific function, namely, the solubilization of the lead precursor (n- Hept4NBr), the surface passivation with halide modification (dimethyldioctade-cylammonium chloride), and the protection of the surface-active sites (octanoic acid) for photocatalysis. Under these conditions, nearly monodispersed blue-emitting nanocubes are selectively obtained in a one-pot synthesis by combining specific amounts of the perovskite precursors. As supported by thermogravimetric analysis (TGA) and Fourier transform infrared (FT-IR) spectroscopy investigations, the organic shell of the obtained NCs is composed of electrostatically bound dimethyldioctadecylammonium ions, granting robustness to the corresponding NCs, and octanoic acid molecules, interacting with the nanoparticle surface through weaker secondary bonds. The obtained NCs exhibit a high photoluminescence quantum yield (PLQY = 72 +/- 3%) notwithstanding multiexponential recombination dynamics of the excited state, resulting from the different passivation modes at the NC surface. Moreover, the NCs show a remarkable optical stability after exposure to high temperatures and to water contact due to the high surface density of the multifunctional organic ligands. The introduction of 4-tert-butylphenyl thiol promotes a charge transfer process at the NC/thiol interface formed upon removal of the labile ligands (octanoic acid) at the NC surface. In these conditions, the NCs are prone to the photoinduced conversion of the aromatic thiol into the corresponding disulfide without varying the optical properties of the perovskite photocatalyst upon the substrate conversion. Therefore, the obtained results cast light on the versatility of the surface engineering of lead halide perovskite NCs for efficient blue emission and photocatalysis with improved stability

Moisture-Assisted near-UV Emission Enhancement of Lead-Free Cs4CuIn2Cl12 Double Perovskite Nanocrystals

Lead-based halide perovskite nanocrystals (NCs) are recognized as emerging emissive materials with superior photoluminescence (PL) properties. However, the toxicity of lead and the swift chemical decomposition under atmospheric moisture severely hinder their commercialization process. Herein, we report the first colloidal synthesis of lead-free Cs4CuIn2Cl12 layered double perovskite NCs via a facile moisture-assisted hot-injection method stemming from relatively nontoxic precursors. Although moisture is typically detrimental to NC synthesis, we demonstrate that the presence of water molecules in Cs4CuIn2Cl12 synthesis enhances the PL quantum yield (mainly in the near-UV range), induces a morphological transformation from 3D nanocubes to 2D nanoplatelets, and converts the dark transitions to radiative transitions for the observed self-trapped exciton relaxation. This work paves the way for further studies on the moisture-assisted synthesis of novel lead-free halide perovskite NCs for a wide range of applications.publishedVersionPeer reviewe

Octahedral distortion driven by CsPbI3 nanocrystal reaction temperature – the effects on phase stability and beyond

Cesium lead iodide (CsPbI3) perovskite nanocrystals (NCs) suffer from a known transformation at room temperature from their red-emitting (black) to non-emitting (yellow) phase, induced by the tilting of PbI6 octahedra. While the reported attempts to stabilize CsPbI3 NCs mainly involve Pb2+-site doping as well as compositional and/or NC surface engineering, the black phase stability in relation only to the variation of the reaction temperature of CsPbI3 NCs is surprisingly overlooked. We report a holistic study of the phase stability of CsPbI3 NCs, encompassing dispersions, films, and even devices by tuning the hot-injection temperature between 120-170 °C. Our findings suggest that the transition from the black to the yellow phase occurs after over a month for NCs synthesized at 150 °C (150@NCs). Structural refinement studies attribute the enhanced stability of 150@NCs to their observed lowest octahedral distortion. The 150@NCs also lead to stable unencapsulated solar cells with unchanged performance upon 26 days of shelf storage in dry air. Our study underlines the importance of scrutinizing synthesis parameters for designing stable perovskite NCs towards long-lasting optoelectronic devices.publishedVersionPeer reviewe