Lead-free perovskite-inspired semiconductors for indoor light-harvesting - the present and the future

G. K. G. thanks Tampere Institute for Advanced Study for the postdoctoral funding. P. V. and V. S. acknowledge the financial support of Jane and Aatos Erkko foundation (SOL-TECH project) and Academy of Finland (Decision No. 347772). B.A-A. thanks Vilho, Yrjö and Kalle Väisälä Fund of the Finnish Academy of Science and Letters for the financial support. This work is part of the Academy of Finland Flagship Programme, Photonics Research and Innovation (PREIN), Decision No. 320165.Are lead-free perovskite-inspired materials (PIMs) the wise choice for efficient yet sustainable indoor light harvesting? This feature article outlines how wide-bandgap PIMs can provide a positive answer to this compelling question. The wide band gaps can hinder sunlight absorption, in turn limiting the solar cell performance. However, PIMs based on group VA of the periodic table can theoretically lead to an outstanding indoor power conversion efficiency up to 60% when their band gap is ∼2 eV. Yet, the research on PIM-based indoor photovoltaics (IPVs) is still in an early stage with highest indoor device efficiencies up to 10%. This article reviews the recent advancements on PIMs for IPVs and identifies the main limiting factors of device performance, thus suggesting effective strategies to address them. We emphasize the poor operational stability of the IPV devices of PIMs being the key bottleneck for the vast adoption of this technology. We believe that this report can provide a solid scaffolding for further researching this fascinating class of materials, ultimately supporting our vision that, upon extensive advancement of the stability and efficiency, PIMs with wide bandgap will become a contender for the next-generation absorbers for sustainable indoor light harvesting.Publisher PDFPeer reviewe

Role of Self-Trapped Excitons in the Broadband Emission of Lead-Free Perovskite-Inspired Cu2AgBiI6

publishedVersionPeer reviewe

Enhancing Charge Transfer in Perovskite-Inspired Silver Iodobismuthate-Based Solar Cells via Cesium Iodide Interlayer

Ag3BiI6 (ABI) is one of the most widely explored lead-free perovskite-inspired materials for eco-friendly solar cell applications. However, despite the intense research efforts, the photovoltaic performance of ABI-based devices remains very modest, primarily due to poor film morphology and ineffective charge extraction. This work aims at investigating the potential benefits of a thermally evaporated cesium iodide (CsI) interlayer on the performance of ABI-based solar cells. Upon the addition of CsI atop the ABI layer in the device stack, the solar cells deliver a power conversion efficiency (PCE) of 2.27%. This is the highest efficiency reported for ABI solar cells employing a similar device architecture. It is found that the enhancement in PCE is largely due to improvement in the ABI|hole transport layer interface upon the introduction of CsI interlayer. The improvement is largely ascribed to enhanced surface coverage upon introduction of CsI interlayer, as evidenced by our comprehensive microscopy studies. Furthermore, impedance spectroscopy analysis is employed to provide further insights into the changes in charge transfer dynamics interlayer that dictate the enhancement of fill factor and short-circuit current density in the devices. The findings indicate that the addition of CsI promotes charge transfer and minimizes recombination losses.Peer reviewe

Halide Engineering in Mixed Halide Perovskite-Inspired Cu2AgBiI6 for Solar Cells with Enhanced Performance

Cu2AgBiI6 (CABI) is a promising perovskite-inspired absorber for solar cells due to its direct band gap and high absorption coefficient. However, the nonradiative recombination caused by the high extrinsic trap density limits the performance of CABI-based solar cells. In this work, we employ halide engineering by doping bromide anions (Br-) in CABI thin films, in turn significantly improving the power conversion efficiency (PCE). By introducing Br- in the synthetic route of CABI thin films, we identify the optimum composition as CABI-10Br (with 10% Br at the halide site). The tailored composition appears to reduce the deep trap density as shown by time-resolved photoluminescence and transient absorption spectroscopy characterizations. This leads to a dramatic increase in the lifetime of charge carriers, which therefore improves both the external quantum efficiency and the integrated short-circuit current. The photovoltaic performance shows a significant boost since the PCE under standard 1 sun illumination increases from 1.32 to 1.69% (∼30% relative enhancement). Systematic theoretical and experimental characterizations were employed to investigate the effect of Br- incorporation on the optoelectronic properties of CABI. Our results highlight the importance of mitigating trap states in lead-free perovskite-inspired materials and that Br- incorporation at the halide site is an effective strategy for improving the device performance.Peer reviewe

Antimony-Bismuth Alloying : The Key to a Major Boost in the Efficiency of Lead-Free Perovskite-Inspired Photovoltaics

The perovskite-inspired Cu2AgBiI6 (CABI) material has been gaining increasing momentum as photovoltaic (PV) absorber due to its low toxicity, intrinsic air stability, direct bandgap, and a high absorption coefficient in the range of 105 cm−1. However, the power conversion efficiency (PCE) of existing CABI-based PVs is still seriously constrained by the presence of both intrinsic and surface defects. Herein, antimony (III) (Sb3+) is introduced into the octahedral lattice sites of the CABI structure, leading to CABI-Sb with larger crystalline domains than CABI. The alloying of Sb3+ with bismuth (III) (Bi3+) induces changes in the local structural symmetry that dramatically increase the formation energy of intrinsic defects. Light-intensity dependence and electron impedance spectroscopic studies show reduced trap-assisted recombination in the CABI-Sb PV devices. CABI-Sb solar cells feature a nearly 40% PCE enhancement (from 1.31% to 1.82%) with respect to the CABI devices mainly due to improvement in short-circuit current density. This work will promote future compositional design studies to enhance the intrinsic defect tolerance of next-generation wide-bandgap absorbers for high-performance and stable PVs.Peer reviewe

Bismuth based perovskite inspired absorbers for optoelectronic applications

Ever increasing population and the dependence on the depleting resources to meet the energy needs is becoming a serious concern worldwide. The dependence on conventional fossil fuels has grave bearings on the environment. Solar energy is hence considered a viable alternative to meet the energy equirements in an environment friendly sustainable manner. Among the various available technologies, the silicon- based solar cells are considered the most promising ones, to potentially serve the energy needs of the entire world. However, the heavy financial constraints involved in production and purification of silicon and the allied implication on environment remains a major concern. Hence, numerous thin film technologies have dominated the research arena for a long time now, with an aim to develop much more sustainable alternatives. The organic-inorganic hybrid halide perovskite with a general structure of ABX3 (where ‘A’ is mostly organic cation, X = halide anion and Pb being the characteristic B site cation), is among the most researched candidates due to their favourable optoelectronic properties. Despite the promises they offer, the halide perovskites are not yet commercialized due to the high ambient instability and toxicity of Pb which is a characteristic component in the same. The stability concerns have been addressed to some extent, but there is a need for extensive research for elimination of lead and making the Perovskites commercially competent. Bismuth-based, lead-free halide double perovskites (LFHDPs) have been considered as a viable less toxic alternative, attracting abundant research attention. Cs2AgBiBr6 being the most explored candidate among them. However, they exhibit wide bandgaps making them potentially less applicable in solar cells. In my research, I employ doping or dilute alloying methods to tune the bandgap of these materials to improve the optoelectronic performances. We introduce a set of novel double perovskite materials, developed by alloying Cs2AgBiBr6, with lead (Pb) and gold (Au) and study the respective properties. Dilute alloying with Pb helped to change the nature of the bandgap from indirect to direct, along with reduction in bandgap, making them promising for solar cell applications. Though the introduction of lead makes it toxic to some extent, its percentage is significantly reduced as compared to the conventional perovskites. This trade-of may be acceptable considering that it helps in improving the optoelectronic properties. Au doping helped to reduce the bandgap to a competitive value, making them potentially applicable as absorbers in solar cells. The emissive properties of the same double perovskite are explored and the role of carrier- lattice interaction and associated self-trapped excitons (STE) on the same is studied. I, develop a better understanding on optoelectronic properties and electron-phonon interaction in Potassium (K+) alloyed Cs2AgBiBr6, while evaluating its potential as a single source tunable light emitter. At low concentrations K+ partially occupying Cs+ site allowing blue-light emissions, while white-light emission occur at higher alloying concentrations of K+. This phenomenon is explained by the Frohlich mechanism. The increased density of STEs and the free exciton (FE) assisted emissions together lead to the PL broadening and white-light emissions. AgBiI4 perovskite inspired materials (PIM) for solar cell application is widely studied, but poor power conversion efficiencies (PCEs) limited their applications in solar cells. These poor performances can be attributed to the poor fill factors due to poor charge transfer. Here I propose the addition of Sn atoms in the precursor solution, that helps in modifying the film morphologies in solution processed thin films, leading to the formation of uniform, and ordered films. Sn is incorporated at the grain boundaries, helping better charge transfer and thus the fill factors improved from 61% to about 75.6%, thus helping to boost the performance of the solar cells. The devices thus developed showed high reproducibility and consistency. The progress of the PIMs is largely limited by the complex and non-uniform morphologies of the thin films. Using the conventional deposition methods, it is difficult to obtain quality films that are free of surface defects and hence the optoelectronic performances of these materials are highly constrained. In my research I introduce a novel two-step modified synthesis method to obtain large area uniform films of Ag3BiI6. Ag and Bi metals are sequentially evaporated and then treated with sublimed iodine vapors in a vacuum chamber. High-quality films with excellent coverage were obtained and applied in perovskite-inspired solar cells. The devices thus fabricated, yielded a power conversion efficiency (PCE) of ~0.7% for large areas, thus, demonstrating the potential to be used in commercial solar cell fabrication. Bismuth sulphoiodide (BiSI) is another material explored for application in for toxicity-free photovoltaics (PV). Its applications in thin-film is however restricted by the lack of methods to obtain pure phases. Here, I design a novel precursor composition to fabricate pure phase BiSI thin-films by simple solution processed spin-coating method. The as-prepared thin-films are characterized by X-ray diffraction studies, optical absorption spectroscopy, X-ray photoelectron spectroscopy, and transmission electron spectroscopy to confirm the phase purity. The films thus developed were p-type and exhibited excellent photoconductivity, presenting their aplicability in PV absorbers. In summary the thesis work emphases on the various limitations in bismuth-based perovskite inspired materials and attempts to improve on the same in order to contribute towards developing commercially viable thin film solar cells.Doctor of Philosoph

Antimony-bismuth alloying: the key to a major boost in the efficiency of lead-free perovskite-inspired indoor photovoltaics

Perovskite-inspired Cu2AgBiI6 (CABI) absorber has recently gained increased popularity due to its low toxicity, intrinsic air stability, and wide bandgap ≈ 2 eV, which makes it ideal for indoor photovoltaics (IPVs). However, the considerable presence of both intrinsic and surface defects is responsible of the still modest indoor power conversion efficiency (PCE(i)) of CABI- based IPVs, with the short-circuit current density (JSC) being nearly half of the theoretical limit. Herein, we introduce antimony (III) (Sb3+) into the octahedral lattice sites of CABI structure, leading to CABI-Sb with substantially larger crystalline domains than CABI. The alloying of Sb3+ with bismuth (III) (Bi3+) induces changes in the local structural symmetry, in turn causing a remarkably increased formation energy of intrinsic defects. This accounts for the overall reduced defect density in CABI-Sb. CABI-Sb IPVs feature an outstanding PCE(i) of nearly 10% (9.53%) at 1000 lux, which represents an almost double PCE(i) compared to that of CABI devices (5.52%) mainly due to an improvement in JSC. This work will promote future compositional design studies to reduce the intrinsic defect tolerance of next-generation wide- bandgap absorbers for high-performance and stable IPVs

Solution-processed, highly crystalline, and oriented MAPbI₃ thin films by engineering crystal-growth kinetics

Growing oriented and monocrystalline layers of lead halide perovskites over device substrates helps to harness their outstanding optoelectronic properties. Epitaxial growth of lead halide perovskites for device fabrication is limited by the lack of lattice-matched substrates and the requirement of compact pinhole-free films. Most optoelectronic devices use amorphous substrates, hindering oriented epitaxial growth. Here, we demonstrate highly crystalline methylammonium lead iodide (MAPbI3) thin films over amorphous substrates by meticulously optimizing the nucleation and growth kinetics in spin coating. The "epitaxial-like" films enable large-area crystalline layer fabrication, with larger than 100 μm spherulitic grains oriented along [200] and [224] planes. The compact, highly crystalline, and oriented films of MAPbI3 formed over ITO/SnO2 are used to fabricate perovskite solar cells (PSCs) with an area of 1 cm2. Despite the perovskite films being highly oriented and crystalline, the PSCs’ performances highlight the critical role the interfaces play in photovoltaic cells.Ministry of Education (MOE)Submitted/Accepted versionThe authors would like to acknowledge that this research is supported bythe Ministry of Education (MOE) under the grant no. MOE-T2EP50221-003