International Symposium on Sustainable Materials II

Cu2ZnSn(S,Se)4 (CZTSSe) solar cells confront formidable challenges in achieving high power conversion efficiency (PCE) due to non-radiative recombination. These limitations are primarily arise from the presence of defects associated with CuZn and SnZn within the bulk of the absorber and at the interface of the heterojunction region. To overcome these challenges, we propose a dual treatment approach, involving Ag alloying in the bulk and an Al2O3 atomic layer deposition (ALD) process at the p-n interface. Extensive characterizations of the fabricated devices revealed that Ag alloying leads to a substantial reduction in deep defects within the bulk, while the Al2O3 ALD process fosters the formation of a well-defined p-n interface region, alongside defect passivation. This synergistic effect led to improvements in PCE, accompanied by enhancements in all device parameters. These advancements, which encompassed both the bulk absorber and the heterojunction, resulted in a PCE exceeding 13%. In conjunction with the development of high-PCE cells, we have utilized various characterization techniques. Furthermore, we will introduce advanced measurement techniques that were essential for enhancing PCE values.

The perovskites are an attractive choice as ideal light absorbing materials for next generation solar cells due to their unique bandgap tunability (1.2-2.5 eV) and excellent optoelectrical properties. As a result, various perovskite-based solar cells have shown many outstanding performances, such as the highest certified efficiency in thin-film criteria (26.1%) and in silicon-based dual-junction tandem device (33.7%). Among the various compositions of perovskites, narrow bandgap perovskite with tin and lead mixed cations used on the B-site of perovskite can extend the light absorption up to the near infrared region. This means that it is theoretically possible to produce higher efficiency single-junction solar cells, and also to extend this to make a solution process-based tandem solar cell to exceed 30% efficiency. As a result, narrow bandgap perovskite has recently become more important in the field of perovskite solar cells.

To realize the high-efficiency Sn-Pb perovskite solar cells, the poly[3,4-ethylene dioxythiophene]-poly[styrene sulfonate] (PEDOT:PSS) has typically been used as the hole transport layer due to its energy alignment. However, there are several disadvantages of PEDOT:PSS layer such as i) acidic condition of PEDOT:PSS, ii) sensitivity to external environment such as moisture, and iii) relatively inferior electrical conductivity for charge transport due to PEDOT surrounded by PSS. These drawbacks of PEDOT:PSS mainly contribute to the defect between PEDOT:PSS/perovskite interface and the performance degradation of the solar cell device.

This study introduces the molecular engineering of the PEDOT:PSS surface region, which can improve the electrical and physical junction properties of the PEDOT:PSS/ perovskite interface. As a result, we have successfully demonstrated the p–i–n narrow bandgap perovskite solar cells with more than 20% efficiency.

Perovskite photovoltaics have attracted a great attention in recent years due to its excellent power conversion efficiency. Interestingly, perovskite thin films often exhibit a complex morphology consisting of various grains with short and long-range order and defects stemming from imperfect chemical composition, local strain and etc, which are in general considered detrimental to the device performance. Therefore, a deep understanding of how charge carriers behave in structurally and morphologically heterogeneous thin films is crucial to not only study fundamental physics of semiconductor but also develop new generation perovskites.   

By utilizing time and space resolved technique, i.e., transient absorption microscopy (TAM), we were able to directly monitor local carrier dynamics of spatially heterogeneous perovskite thin films. In this talk, I will first discuss recent applications of fs-TAM to thin film hybrid metal halide perovskites. Confocal type fs-TAM measurements on a series of MAPI perovskite thin films clearly reveal ballistic transport of non-equilibrium charge carriers with transport length of upto 150 nm. This observation implies that ultrafast long-range transport of non-equilibrium charge carriers plays a key role in enhancing the power conversion efficiency of perovskite photovoltaics. Furthermore, wide-field type fs-TAM study on the alloyed perovskite thin films elucidate how non-equilibrium charge carrier dynamics are dictated by the of nanoscale chemical heterogeneity.

Like any other semiconductors, defects in halide perovskites determine the efficiency and long-term stability of the resulting optoelectronic devices. Understanding the electronic property and the instability mechanism, and the interplay of the two are paramount. To do this, my team uses a combination of electronic structure calculations with reactive molecular dynamics simulations. By determining the electronic energy levels and dynamical properties, we identify defects responsible for recombination losses and chemical degradations. Next, we propose several strategies for mitigating these harmful defects by engineering perovskite compositions and interfaces, the use of passivation agents. Finally, we propose that the ultimate solution for controlling the defects is to optimize the quality of perovskite films by tuning synthetic chemistry and processing parameters. We will show our recent results on the simulations of the nucleation and growth of perovskites, with the aim of synthesizing defect-free/less materials for stable and efficient devices.

Antimony selenide (Sb2Se3) and related sulfoselenide materials have seen a surge of interest for application as solar absorbers for thin film photovoltaic and photoelectrochemical cells with device efficiencies now in excess of 10% [1-3]. This rapid progress has been attributed both to their quasi- one-dimensional crystal structure (consisting of (Sb4Se6)n ribbons oriented along the [001] direction) as well as to features of their electronic structure which suggest they might be intrinsically tolerant to both point and extended defects. 

Here we present density functional theory calculations for a range of extended defects (surfaces and grain boundaries) in Sb2Se3 and Sb2S3 to provide direct insight into their atomic structure and electronic properties. While most extended defects (besides those parallel to [001]) cut (Sb4Se6)n ribbons and introduce deep gap states, we show that such defects readily reconstruct through rebonding of undercoordinated atoms leading to the effective elimination of all associated defect states within the band gap (termed ‘self-healing’). As a result, unlike other chalcogenide semiconductors such as Cu(In,Ga)Se2 and CdTe, extended defects in Sb2Se3 and Sb2S3 are predicted to exhibit very similar properties to the bulk crystal. This unexpected prediction is consistent with recent experimental results obtained by deep level transient spectroscopy and Kelvin probe force microscopy.

While similar self-healing effects have been observed previously for the (110) surfaces of III–V semiconductors and also CdS nanoparticles, here the reconstructions seem to be unusually effective for all extended defects rather than a restricted subset. Therefore, providing deleterious point defects can be brought under control polycrystalline antimony sulfoselenides hold the promise of exceptional performance without the need for any special grain boundary treatments.

Cu2ZnSn(S,Se)4 (CZTSSe) solar cells confront formidable challenges in achieving high power conversion efficiency (PCE) due to non-radiative recombination. These limitations are primarily arise from the presence of defects associated with CuZn and SnZn within the bulk of the absorber and at the interface of the heterojunction region. To overcome these challenges, we propose a dual treatment approach, involving Ag alloying in the bulk and an Al2O3 atomic layer deposition (ALD) process at the p-n interface. Extensive characterizations of the fabricated devices revealed that Ag alloying leads to a substantial reduction in deep defects within the bulk, while the Al2O3 ALD process fosters the formation of a well-defined p-n interface region, alongside defect passivation. This synergistic effect led to improvements in PCE, accompanied by enhancements in all device parameters. These advancements, which encompassed both the bulk absorber and the heterojunction, resulted in a PCE exceeding 13%. In conjunction with the development of high-PCE cells, we have utilized various characterization techniques. Furthermore, we will introduce advanced measurement techniques that were essential for enhancing PCE values.

Catalytic water splitting, which uses solar-generated electrical energy to produce hydrogen and oxygen molecules, is a crucial step towards achieving sustainable and clean energy. However, the electro-kinetics and efficiency of hydrogen production are limited by the four-electron-proton coupling process for the oxygen evolution reaction (OER). Despite many efforts to enhance the catalytic performance of OER electrocatalysts, the overpotential is still too high for cost-effective production. We use quantum mechanics calculations to investigate the OER mechanism on nickel oxyhydroxide based catalysts and identify the stabilization of radical character on the oxygen of the metal-oxyl bond as a critical step for OER. Then we further verify these predictions by evaluating a series of metal-doped nickel oxyhydroxide electrocatalysts and establish a relationship between OER activity, electronic configuration of the doped transition metal ions, and in situ conductivity of the electrocatalysts. This work provides guidance for designing more efficient OER catalysts.

Perovskite solar cells (PSCs) have been promptly raised to a strong candidate, exhibiting a certified power conversion efficiency (PCE) of 26.1%. Intensive efforts have been still made to reach an extremely high PCE on one hand and to ensure the long-term stability of PSCs on the other hand, where the interface of PSCs plays a critical role. Interface engineering of perovskite layer was pursued by implementing the heteroepitaxial growth on the bottom and the surface recrystallization on the top. The heteroepitaxial growth of perovskite film enabled the residual lattice tensile strain to be tuned, affecting the interface recombination and the crystal phase stability as well. Furthermore, the recrystallization not only reduced the trap density but also promoted the favorable crystal orientation of the top surface, encouraging the efficient charge extraction at the interface. The effect of interface engineering was well reflected in both the photovoltaic parameters, particularly in open-circuit voltage and fill factor, and the long-term stability, which highlights the importance of the interface engineering for perovskite-employed electronic devices.

Hybrid organic-inorganic perovskites have emerged as one of the leading materials for solarto-electric energy conversion in photovoltaics. However, their instability under operating conditions poses an obstacle to practical applications. While this can, to an extent, be overcome by incorporating organic moieties within hybrid perovskite frameworks that form low dimensional architectures with superior operational stabilities, their insulating character often compromises the resulting photovoltaic performances. We demonstrate the capacity to rely on supramolecular engineering of (photo) electroactive organic species to enhance the functionality of layered hybrid perovskites by enabling control of their properties in response to external stimuli, such as voltage bias, light, and pressure, opening a path toward multifunctional materials in new generation photovoltaics.

The discovery and design of new materials is critical for advancing carbon-emission reducing technologies such as renewable energy and electric vehicles. Experimental discovery of new materials is typically slow and costly, quantum mechanics (QM) calculations have brought computational materials design within reach. However, QM calculations are often limited to relatively small sets of materials, as their computational costs are too great for large-scale screening, this is the case for calculating properties required for new energy materials. New methods in machine learning (ML) and deep learning (DL) have emerged as a powerful complementary tool to QM calculations – learning rules from data calculated from QM and applying cheap, efficient models to explore large chemical spaces. However, several challenges still exist for example, learning from small and limited datasets, obtaining measures of confidence in models and understanding the results of DL models. All these challenges must be addressed to fully realise the power of DL for design of new sustainable materials. In this talk I will give examples of recent work in our group to address these issues, including using unsupervised learning to accelerate the characterisation of battery materials without requiring labelled data,  using interpretability methods to explain the results of ML/DL models used for predicting and characterising materials properties and building models with reliable uncertainty quantification, capable of learning on significantly smaller datasets than regular DL models. Finally, I will also discuss how the latest exciting developments in large language models could help to solve the challenges of crystal structure prediction.