Youyu Liu

Bio: Youyu Liu is an academic researcher from Chinese Academy of Sciences. The author has contributed to research in topics: Luminescence & Nanocrystal. The author has an hindex of 2, co-authored 3 publications receiving 74 citations. Previous affiliations of Youyu Liu include Fuzhou University.

Achieving high-performance planar perovskite solar cells with co-sputtered Co-doping NiOx hole transport layers by efficient extraction and enhanced mobility

Abstract: Perovskite solar cells are some of the most promising photovoltaic devices and they have experienced extraordinary progress in efficiency and fabricating technologies. Herein, we explore the effect of Co-doped NiOx hole transport layers on the electronic structure and photovoltaic properties of PSCs, which were deposited onto FTO substrates via DC magnetron sputtering at room temperature. Appropriate Co-doping can slightly regulate the optical band gap and the Fermi level position, leading to an increased potential cell performance. By virtue of continuously adjusting the power loaded onto the Co target, we can obtain the optimal atomic ratio of the Co:NiOx hole transport layer, and the PSC based on Co:NiOx exhibited a 25% higher efficiency than its undoped counterparts (from 9.46% to 12.61%). Therefore, these results demonstrate that Co is an appropriate dopant and the PSCs based on Co:NiOx layers have a good performance.

A general strategy via charge transfer sensitization to achieve efficient NIR luminescence in lanthanide-doped NaGdS2 nanocrystals

Abstract: Lanthanide (Ln3+)-doped near-infrared (NIR) luminescent nanocrystals (NCs) have shown great promise in many technological fields. However, because of the parity-forbidden 4f → 4f electronic transitions of Ln3+, these NCs normally suffer from a low absorption and luminescence efficiency. Herein, we report a novel strategy for sensitizing the NIR luminescence of Ln3+ in NaGdS2 NCs based on the ligand-to-metal charge transfer transition from S2− to Yb3+. Through sensitization by the allowed S2−–Yb3+ charge transfer transition, intense NIR luminescence from Yb3+ and Er3+ with quantum yields up to 21.2% and 25.0% is achieved in Yb3+ singly-doped and Yb3+/Er3+ co-doped NaGdS2 NCs, respectively. These findings provide a general approach for the development of highly efficient Ln3+-doped NIR luminescent NCs through charge transfer sensitization, and also lay a foundation for future design of sulfide-based NIR luminescent nanomaterials towards various applications.

Blue-LED-excitable NIR-II luminescent lanthanide-doped SrS nanoprobes for ratiometric thermal sensing

Abstract: Lanthanide (Ln3+)-doped near infrared (NIR)-II luminescent nanoprobes have shown great promise in many technological fields, but are currently limited by the low absorption efficiency of Ln3+ due to the forbidden 4f→4f transition. Herein, we report a novel NIR-II luminescent nanoprobe based on efficient energy transfer from Ce3+ to Er3+ and Nd3+ in sub-10 nm SrS nanocrystals (NCs), which are excitable by using a commercial blue light-emitting diode (LED). Through sensitization by the allowed 4f→5d transition of Ce3+, the NCs exhibit strong NIR-II luminescence from Er3+ and Nd3+ with quantum yields of 2.9% and 2.3%, respectively. Furthermore, by utilizing the intense NIR-II luminescence of Er3+ from the thermally coupled Stark sublevels of 4I13/2, we demonstrate the application of SrS:Ce3+/Er3+ NCs as blue-LED-excitable NIR-II luminescent nanoprobes for ratiometric thermal sensing. These findings reveal the unique advantages of SrS:Ln3+ NCs in NIR-II luminescence, which may open up a new avenue for exploring novel and versatile luminescent nanoprobes based on Ln3+-doped sulphide NCs.

NiOx Hole Transport Layer for Perovskite Solar Cells with Improved Stability and Reproducibility

Abstract: In this study, highly stable, low-temperature-processed planar lead halide perovskite (MAPbI3–xClx) solar cells with NiOx interfaces have been developed. Our solar cells maintain over 85% of the initial efficiency for more than 670 h, at the maximum power point tracking (MPPT) under 1 sun illumination (no UV-light filtering) at 30 °C, and over 73% of the initial efficiency for more than 1000 h, at the accelerating aging test (85 °C) under the same MPPT condition. Storing the encapsulated devices at 85 °C in dark over 1000 h revealed no performance degradation. The key factor for the prolonged lifetime of the devices was the sputter-deposited polycrystalline NiOx hole transport layer (HTL). We observed that the properties of NiOx are dependent on its composition. At a higher Ni3+/Ni2+ ratio, the conductivity of NiOx is higher, but at the expense of optical transmittance. We obtained the highest power conversion efficiency of 15.2% at the optimized NiOx condition. The sputtered NiOx films were used to fabri...

Metal Oxides as Efficient Charge Transporters in Perovskite Solar Cells

Abstract: Over the past few years, hybrid halide perovskites have emerged as a highly promising class of materials for photovoltaic technology, and the power conversion efficiency of perovskite solar cells (PSCs) has accelerated at an unprecedented pace, reaching a record value of over 22%. In the context of PSC research, wide-bandgap semiconducting metal oxides have been extensively studied because of their exceptional performance for injection and extraction of photo-generated carriers. In this comprehensive review, we focus on the synthesis and applications of metal oxides as electron and hole transporters in efficient PSCs with both mesoporous and planar architectures. Metal oxides and their doped variants with proper energy band alignment with halide perovskites, in the form of nanostructured layers and compact thin films, can not only assist with charge transport but also improve the stability of PSCs under ambient conditions. Strategies for the implementation of metal oxides with tailored compositions and structures, and for the engineering of their interfaces with perovskites will be critical for the future development and commercialization of PSCs.

Nickel Oxide as Efficient Hole Transport Materials for Perovskite Solar Cells

Abstract: Organic–inorganic halide perovskite solar cells (PSCs) have achieved great success in recent years with a demonstrated power conversion efficiency (PCE) increasing rapidly from 3.8% to 22.3% for single junction devices. Most high-performance PSCs consist of a perovskite absorber sandwiched between an electron transport layer (ETL) and a hole transport layer (HTL), which extracts electrons (holes) and blocks holes (electrons) from the absorber efficiently. Inorganic hole transport materials have extracted extensive attention due to their higher mobility and better stability. Particularly, the excellent hole selective transport property of nickel oxide (NiOx) has been highlighted by its recent application in organometallic halide PSCs, due to the favorable band alignment formed between the halide perovskite absorber and NiOx HTL. This comprehensive review summarizes the recent progress in the fabrication of NiOx films and their application in PSCs. Special attention is paid to the optoelectronic properties of NiOx films, which strongly depend on the synthesis methods and post-treatment conditions, as well as the resulting photovoltaic device performance. Surface modification and doping strategies that are used to improve the optoelectronic properties of NiOx films and the resulting device performance are discussed with emphasis. Finally, a short perspective of NiOx-based PSCs is also provided.