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

Dissecting the Role of the Hole-Transport Layer in Cu2 AgBiI6 Solar Cells: An Integrated Experimental and Theoretical Study

Perovskite-inspired materials (PIMs) provide low-toxicity and air-stable photo-absorbers for several possible optoelectronic devices. In this context, the pnictogen-based halides Cu2AgBiI6 (CABI) are receiving increasing attention in photovoltaics. Despite extensive studies on power conversion efficiency and shelf-life stability, nearly no attention has been given to the physicochemical properties of the interface between CABI and the hole transport layer (HTL), which can strongly impact overall cell operations. Here, we address this specific interface with three polymeric HTLs: poly(N,N'-bis(4-butylphenyl)-N,N'-bis(phenyl)benzidine) (poly-TPD), thiophene-(poly(3-hexylthiophene)) (P3HT), and poly(bis(4-phenyl)(2,4,6-trimethylphenyl)amine) (PTAA). Our findings reveal that devices fabricated with poly-TPD and P3HT outperform the commonly used Spiro-OMeTAD in terms of device operational stability, while PTAA exhibits worse performances. Density functional theory calculations unveil the electronic and chemical interactions at the CABI-HTL interfaces, providing new insights into observed experimental behaviors. Our study highlights the importance of addressing the buried interfaces in PIM-based devices to enhance their overall performance and stability.

Unraveling the Role of 2D Ti3C2Tx MXene Nanosheets in Cu-Based Double Perovskite Active Layer for Enhanced Photovoltaic Performance

Although the atmospheric stability of lead-free inorganic double perovskite (DP) solar cells (PSCs) looks promising, their further development is hampered by inadequate film quality and non-radiative carrier recombination at the interfaces. Herein, the incorporation of a newly developed intriguing class of 2D material Ti3C2Tx MXene nanosheets with the photo-absorbing Cu2AgBiI6 (CABI) active layer of a fully inorganic solar cell is reported. The highly conductive Ti3C2Tx nanosheets work as a multi-functional additive by tuning the band gap, reducing the non-radiative carrier recombination, and inhibiting carrier accumulation. In addition, the presence of Ti3C2Tx MXene increases the surface free energy of the perovskite film, which elevates the energy barrier for nucleation and realizes a highly crystalline CABI perovskite film. Primarily, the MXene modification accelerates the charge extraction and transport at the interfaces of the active layer, utilizing energy level alignment with the charge transport layers. Consequently, the photo-conversion efficiency (PCE) of the device with MXene is substantially enhanced to 1.50%. Moreover, the 2D Ti3C2Tx nanosheets increased the long-term stability of the devices by retaining 70% of the initial PCE after 1680 h. With regard to relieving the severe carrier recombination at the interfaces, this work sets a new paradigm toward imminent solar energy conversion.

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.

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

The perovskite-inspired Cu2 AgBiI6 (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.

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

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.

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

The perovskite-inspired Cu₂AgBiI₆ (CABI) absorber shows promise for low-toxicity indoor photovoltaics. However, the carrier self-trapping in this material limits its photovoltaic performance. Herein, we examine the self-trapping mechanism in CABI by analyzing the excited-state dynamics of its absorption band at 425 nm, which is responsible for the self-trapped exciton emission, using a combination of photoluminescence and ultrafast transient absorption spectroscopies. Photoexcitation in CABI rapidly generates charge carriers in the silver iodide lattice sites, which localize into the self-trapped states and luminesce. Furthermore, a Cu-Ag-I-rich phase that exhibits similar spectral responses as CABI is synthesized, and a comprehensive structural and photophysical study of this phase provides insights into the nature of the excited states of CABI. Overall, this work explains the origin of self-trapping in CABI. This understanding will play a crucial role in optimizing its optoelectronic properties. It also encourages compositional engineering as the key to suppressing self-trapping in CABI.

One-Step Gas-Solid-Phase Diffusion-Induced Elemental Reaction for Bandgap-Tunable CuaAgm1Bim2In/CuI Thin Film Solar Cells

Lead-free inorganic copper-silver-bismuth-halide materials have attracted more and more attention due to their environmental friendliness, high element abundance, and low cost. Here, we developed a strategy of one-step gas-solid-phase diffusion-induced reaction to fabricate a series of bandgap-tunable CuaAgm1Bim2In/CuI bilayer films due to the atomic diffusion effect for the first time. By designing and regulating the sputtered Cu/Ag/Bi metal film thickness, the bandgap of CuaAgm1Bim2In could be reduced from 2.06 to 1.78 eV. Solar cells with the structure of FTO/TiO2/CuaAgm1Bim2In/CuI/carbon were constructed, yielding a champion power conversion efficiency of 2.76%, which is the highest reported for this class of materials owing to the bandgap reduction and the peculiar bilayer structure. The current work provides a practical path for developing the next generation of efficient, stable, and environmentally friendly photovoltaic materials.

2.11 - Accurate characterization of indoor photovoltaic performance

Ambient energy harvesting has great potential to contribute to sustainable development and address growing environmental challenges. Converting waste energy from energy-intensive processes and systems (e.g. combustion engines and furnaces) is crucial to reducing their environmental impact and achieving net-zero emissions. Compact energy harvesters will also be key to powering the exponentially growing smart devices ecosystem that is part of the Internet of Things, thus enabling futuristic applications that can improve our quality of life (e.g. smart homes, smart cities, smart manufacturing, and smart healthcare). To achieve these goals, innovative materials are needed to efficiently convert ambient energy into electricity through various physical mechanisms, such as the photovoltaic effect, thermoelectricity, piezoelectricity, triboelectricity, and radiofrequency wireless power transfer. By bringing together the perspectives of experts in various types of energy harvesting materials, this Roadmap provides extensive insights into recent advances and present challenges in the field. Additionally, the Roadmap analyses the key performance metrics of these technologies in relation to their ultimate energy conversion limits. Building on these insights, the Roadmap outlines promising directions for future research to fully harness the potential of energy harvesting materials for green energy anytime, anywhere.