Photonics for Energy IV

We will present a universal close space annealing (CSA) strategy that increases grain size, enhances crystallinity, and prolongs carrier lifetimes in low-bandgap (~1.25 eV) and wide-bandgap (~1.75-1.80 eV) perovskite films. High-quality perovskite absorber layers are obtained with slowed solvent releasing process, enabling fabrication of efficient single-junction perovskite solar cells and all-perovskite tandem solar cells. We will also present a new self-assembly monolayer (SAM) material for efficient wide-bandgap perovskite subcells with a centimeter-scale area, leading to 26.4% certified record effciency of 1 cm2 all-perovskite tandem solar cells. We will also discuss the design and optimization of interconneting layers for all-perovskite tandems.

The tunable band gaps and facile fabrication of perovskites make them attractive for multi-junction photovoltaics. However, light-induced phase segregation limits their efficiency and stability. This report will show that lattice distortion in I/Br mixed perovskites is correlated with the suppression of phase segregation, generating an increased ion migration energy barrier arising from the decreased average interatomic distance between A-site cation and iodide. Using a ~2.0 eV Rb/Cs mixed-cation inorganic perovskite with large lattice distortion in the top subcell, we fabricated all-perovskite triple-junction solar cells and achieved an efficiency of 24.3% (23.3% certified quasi-steady-state efficiency) with an open-circuit voltage of 3.21 V. This is the first reported certified efficiency for perovskite-based triple-junction solar cells. The triple-junction devices retain 80% of their initial efficiency following 420 hours of operation at the maximum power point. To further resolve the challenges of phase segregation in multiple junctions, we further develop the all-perovskite quadruple-junction solar cells based on a light-stable pure-Br perovskite in the front subcell.

Perovskite tandem solar cells promise performance beyond the detailed balance limit of single-junction solar cells; however, non-radiative recombination and its progressive worsening with time, especially in the mixed Sn–Pb low-bandgap layer, currently limit performance and stability. We find that mixed Sn–Pb perovskite thin films exhibit a compositional gradient, with an excess of Sn on the surface – and we show this gradient exacerbates oxidation and increases the recombination rate. We find that diamines preferentially chelate Sn atoms, removing them from the film surface and achieving a more balanced Sn:Pb stoichiometry, making the surface of the film resistive to the oxidation of Sn. The process forms an electrically resistive low-dimensional barrier layer, passivating defects and reducing interface recombination. Tandems achieve a power conversion efficiency of 28.8% with improved stability.

Here we report a chemically stable and multifunctional buffer material, ytterbium oxide (YbOx) by scalable thermal vaporation deposition.

Halide perovskite represents a promising new class of materials for the next generation of optoelectronic devices, spanning from solar cells to light-emitting diodes. However, achieving both high device efficiencies and stabilities faces sizable challenges due to non-ideal interface contacts. Therefore, developing rational strategies for engineering interfaces in perovskite optoelectronic devices plays a key role in enhancing overall performance. We conducted studies on the chemical reaction kinetics involved in surface passivation of halide perovskite semiconductors, and elucidated fundamental physical principles dictating band alignment at perovskite/organic interfaces. Upon a deeper understanding of interface physics in perovskite optoelectronic devices, we devised effective approaches to tackle interface-related issues that impede device performance improvement, resulting in improved device performance and the realization of all-in-one perovskite devices. Lastly, we introduced an amorphous rare metal oxide (YbOx) buffer layer with quantum localized states, considerably boosting device efficiencies and stabilities of inverted perovskite solar cells.

The integration of multiple functionalities into a single device holds significant value for the advancement of miniaturized and integrated devices. Eli Yablonovitch's assertion that "A great solar cell has to be a great LED" underscores the potential for combining light emission and sensing capabilities within a single device. However, the realization of devices capable of both emitting and sensing light is seldom reported, likely due to the challenge of achieving satisfactory performance in both functions simultaneously with current optoelectronic devices. In this presentation, I will discuss our progress in developing an efficient solution-processed perovskite diode capable of operating in both emission and detection modes. Our device can seamlessly transition between modes by adjusting the bias direction and demonstrates exceptional performance in both light emission, with high external quantum efficiency and brightness, and light detection, showcasing outstanding photovoltaic capabilities. Furthermore, I will showcase their intriguing applications in optical communication, biomedical sensing, multifunctional displays, and building-integrated photovoltaics.

Organic single-crystalline semiconductors constructed by π-conjugated molecules have attracted great attention due to their unique and superior features for optoelectronic applications. Their inherent long-range molecular ordering provides a great opportunity for fundamental research on the basic molecular interactions and structure-property relationships. The regular and dense packing of molecules contributes to the high mobility of charge carriers, and oriented alignment of transition dipole moments, which is conducive to optoelectronic devices achieving fast exciton formation and high light outcoupling. Via the rational molecular design and crystal engineering, the luminescence efficiency of organic single-crystalline semiconductors has exceeded 80%. Over the past decade, great efforts have been devoted by our group to the development of single-crystal organic light-emitting devices (OLEDs) for realizing bright and efficient EL emission. The realization of three-primary-color and white single-crystal OLEDs with improved EL performances constituted a major step toward practical flat-panel display and solid-state lighting applications.

Reducing detrimental defects in metal halide perovskites is critical for the improvement of the performance of perovskite-based optoelectronic devices including solar cells, detectors, and light-emitting devices. However, resolving the defects including their spatial and energy distributions as well as their chemical nature remains a huge challenge for the perovskite community. Here, we develop techniques including drive-level capacitance profiling (DLCP) and confocal PL-carrier diffusion mapping to reveal the defect distributions in perovskites and the charge recombination velocity at the grain boundaries. We further identified the chemical nature of the dominant defects in both iodine- and bromide-based perovskites.

Organic-inorganic hybrid perovskite solar cells (PSCs) have shown great progress over the past decade. However, their long-term stability still lags behind silicon-based competitors, which has significantly decelerated their commercialization progress. Faced with this challenge, major efforts have been made to unravel instability mechanisms, develop long-term performance-tracking protocols, and explore various approaches for enhancing the stability of PSCs. Among these strategies, interface manipulation is the most critical as it could simultaneously passivate or anchor the perovskite surface defects and acts as a barrier to protect the bulk of perovskite from the external environment. Recently, the development of diversified interface modification molecules and interfacial regulation strategies has greatly contributed to the improvement of PSC stability. This presentation will elucidate the factors influencing the stability of PSCs, provide an in-depth understanding of device degradation mechanisms, and present advanced approaches to reveal them. Additionally, we will disc

The low density of deep trapping defects in metal halide perovskites (MHPs) is essential for the high-performance optoelectronic devices of energy conversion and light emission. However, it is unknown about the shallow trap chemical nature and distribution, and their impact on MHPs solar cell operation. Herein, we report that shallow traps are much richer in MHPs than traditional semiconductors, and the shallow trap density can be further enhanced by >100 times by introducing local surface strain. The surface strain is introduced by anchoring the two ends of a molecule terminated with amine groups onto formamidinium cations. Density function theory calculation shows that the shallow traps are formed by the band edge downshifting toward defect levels at the strained surface. The high-density shallow traps temporarily hold one type of charges and increased concentration of the other type of free carrier in working solar cells by keeping photogenerated charges from bimolecular recombination, resulting in reduced open circuit voltage loss of a very stable formamidinium-caesium-perovskite to 317 mV.

Perovskite solar cells (PSCs) have inspired burgeoning interests for making skyrocketing revolution in new-generation photovoltaic field. However, the polarity mismatch between the highly nonpolar/hydrophobic organic hole transport layers (HTLs) in inverted PSCs and the highly polar solvents used in perovskite precursor solution, such as N-N’-dimethyl formamide (DMF) and dimethylsulfoxide (DMSO), leads to high contact angles and non-ideal wetting issues. As a result, the uniformity and crystallization of polycrystalline perovskite films and the carrier transport dynamics at the buried interface will be severely affected based on the bottom-up growth process. In addition, the buried interface of the polycrystalline perovskites suffers from the high-density defect, unfavorable strain and deleterious heterogeneity. Therefore, a multidimensional and efficacious method is desired to construct a favorable buried interface conducive to perovskite crystallinity and interfacial contact.

Conventional photovoltaic photodetectors (OPDs) are not suitable for direct detection of weak light signals as external quantum efficiency (EQE) less than 100%, and Photomultiplication(PM)-type OPDs with EQE larger than 100%, making them powerful low-intensity photodetectors candidate. However, the pronounced sensitivity commonly operating with high operating bias and low response time, impeding their widespread applications for PM-OPDs. In this work, one lead-free Cs3Cu2I5 nanocrystals with self-trapping exciton nature was introduced as interfacial layer adjacent to bulk and layer-by-layer heterojunction structure. The fabricated device exhibits low operating bias (0 V for PV mode and 0.8 V for PM mode), high specific detectivity (~1013Jones), fast response speed as low as 1.59 µs, large bandwidth over 0.2 MHz and long-term operational stability last for several months in ambient condition. This synergy strategy also validated in different materials and device architectures, providing a convenient and scalable production process to develop highly efficient bias-switchable multi-functional organic optoelectrical applications.

Kesterite Cu2ZnSn(S, Se)4 (CZTSSe) solar cells are highly promising low-cost thin-film photovoltaics. However, the efficiency of these solar cells is challenged by severe charge losses and complex defects. Here, we firstly reveal through a data-driven correlation analysis that the dominant deep defect in CZTSSe exhibits a donor character. We further propose that incomplete cation exchange in the multi-step crystallization reactions of CZTSSe is the kinetic mechanism responsible for the defect formation. To facilitate the cation exchange, we introduce a multi-elemental alloying approach aimed at weakening the metal-chalcogen bond strength and the stability of intermediate phases. This strategy leads to a significant reduction in charge losses within the CZTSSe absorber. Overall, these results not only present a significant advancement for kesterite solar cells but could also help identify and regulate defects in photovoltaic materials.

In order to develop highly efficient and multi-colored dye-sensitized solar cells, the following important scientific discoveries has been made through innovations in the chemical structure of dyes and the modulation of molecular micro-/nano-structure: (1) developed an anthracene-based polycyclic aromatic hydrocarbons blue dye through innovations in the chemical structures of dyes, and simultaneously realized highly efficient and stable blue solar cells and green solar cells using the exact blue dye, (2) proposed the strategy of using preadsorber to modulate the microstructure of dye molecules on the surface of nanocrystal TiO2 to obtain a more dense and orderly molecular packing, achieving a solar cell with world record power conversion efficiency.

Laser power transmission (LPT) technology has gained significant attention in recent years due to its potential to revolutionize energy transfer in a more efficient, safe, and eco-friendly manner. However, China's LPT technology is currently behind the international standards in terms of component development and system integration, with core technologies and key processes still in need of further development. To address challenges such as limited theoretical research on innovative laser cells, low energy conversion efficiency, inadequate environmental adaptability, and high manufacturing costs, the applicant conducted groundbreaking research at the component level, focusing on enhancing the power conversion efficiency, improving long-term stability, and reducing fabrication costs. In this report, we will share our recent development in LPT system, and focusing on using perovskite materials to fabricate next-generation highly efficient laser power converter, and make a blueprint aiming to space and underwater applications.

Recently, lead-free double perovskite Cs2SnI6 has emerged as one of the most promising alternatives of organic–inorganic hybrid halide perovskites due to its favorable optoelectronic properties, intrinsic stability, and environmental friendliness. However, the poor photoelectric conversion efficiency of such material has seriously hindered its widespread application. In this work, based on the preparation of Cs2SnI6 powders by using a facile and efficient solvothermal method, Ti3C2Tx-Cs2SnI6 hybrid films have been synthesized by spray-coating. It is found that the incorporated Ti3C2Tx can effectively passivate the defects locating at the grain boundaries of Cs2SnI6, indicating improved absorption performance and suppressed charge recombination can be achieved. And eventually, the photocurrent density of Ti3C2Tx-Cs2SnI6 hybrid film can reach around 3 times higher than that of Cs2SnI6 film. This work explored a promising strategy for the application of the double perovskite Cs2SnI6 in high-efficiency optoelectronic devices.

Broadband photodetectors are of great importance for numerous optoelectronic applications. Two-dimensional (2D) transition-metal dichalcogenides (TMDs), have shown great potential for high-sensitivity photodetection due to its extraordinary properties. A new strategy for phase-controlled van der Waals (vdW) epitaxial growth of wafer-scale photoelectric materials has been proposed. With a built-in potential at the heterojunction interface to effectively manipulate the carrier transport properties and then enhance device performance, the novel vdW heterojunctions were assembled with different operating mechanisms to extend the detection range. Moreover, the large-scale vdW epitaxial growth technique allowed the fabrication of on-chip arrayed devices with high-level integration and miniaturization for high-resolution mid-infrared (MIR) image sensing applications at room temperature. This breakthrough overcomes the current bottleneck of MIR image sensors that require low-temperature refrigeration.

Wafer-based on-chip integration is critical for current semiconductor electronics. However, as a promising optoelectronic semiconductor, metal halide perovskites are mainly developed by the type of thin-film polycrystalline structure, which is not suitable for integration optoelectronics and lacks long-term stability. Here, we have developed a series of approaches based on perovskite single crystals from crystal preparation, scaling up, and on-chip microfabrication to device integration, showing their potential application in stable and efficient Micro-LED, Focal-Plane-Array detector and wearable photovoltaics.

Single-photon up-conversion (SPUC) photoluminescence is a remarkable phenomenon observed in lead halide perovskite quantum dots (PQDs), where the intragap trap states play a crucial role as the intermediate for subgap light absorption. The energetic distribution of trap states directly determines the efficiency of SPUC, however, chemical regulation of trap state distribution remains highly challenging. In this study, we demonstrate that deep-level trap states can be eliminated by surface ligand engineering, thereby reduce the nonradiative exciton recombination. In the meantime, shallow-level trap states can be introduced by crystallization kinetics engineering, which enhances the intragap electron transition. As a synergistic effect, the SPUC efficiency of PQDs is boosted by more than 40%, with an optical refrigeration gain surpassing that of the prototypical II-VI quantum dots.

Silver bismuth iodide (Ag2BiI5), as an all-inorganic lead-free perovskite material, has become one of the most promising photoelectric conversion materials thanks to its some great advantages, such as non-toxic, stable, and high photoelectric conversion efficiency and so on. However, Ag2BiI5 films are currently mainly prepared by a one-step solution spinning method, where toxic dimethylsulfoxide (DMSO) and N-N dimethylformamide (DMF) are usually used as the mixed solvent with high Ag2BiI5 concentration (0.1-1 M). In this study, Ag2BiI5 films with excellent optoelectronic performance have been sprayed with low concentration (0.01-0.03M) slurry, which contains Ag2BiI5 powders made by a low-cost green solvothermal method without using toxic solvent. This work could pave the way for the development of the green and efficient photoelectric conversion materials industry.