Ligang Yuan

Advanced Materials, EarlyView.

Advanced Energy Materials, July 2018.

Advanced Energy Materials, Volume 8, Issue 25, September 5, 2018.

Advanced Functional Materials, EarlyView.

Advanced Functional Materials, EarlyView.

Advanced Science, Volume 5, Issue 7, July 2018.

Solar RRL, Volume 2, Issue 9, September 2018.

Composition transformation is found mutually taking place among 2D and 3D organolead halide perovskites through alkyl amine‐bound intermediates with well‐defined compositions and unexpected good solubility in alcohol. This enables the use of ethanol as main processing solvent to prepare high quality perovskite films and solar cell devices.

Post‐deposition processing of organolead halide films, by exposure to alkyl amine vapor and composition transformation, is an attractive approach for the formation of high‐quality and large‐scale perovskite films of desired compositions. However, the generalization of such composition transformation requires further understanding of the mechanism of the process. In this work MAPbI₃, BA₂PbI₄, and OA₂PbI₄ (MA⁺: CH₃NH₃ ⁺, BA⁺: C₄H₉NH₃ ⁺, and OA⁺: C₈H₁₇NH₃ ⁺) are chosen as typical representatives of 3D and 2D organolead halide perovskites, and their transformations are systematically studied. It is found that the transformation can universally take place among these organolead iodides, irrespective of either from 3D to 2D or from 2D to 3D form. The important intermediates in these transformations have been identified to be alkyl amine‐bound lead iodides of well‐defined chemical composition and crystalline nature in solid state. Furthermore, such intermediates have unexpected good solubility in alcohol, enabling the use of ethanol as a processing solvent for fabrication of high‐quality perovskite films and solar cells. The so‐prepared perovskite solar cells display an average power conversion efficiency of 14.08% with the best one of 15.79%. These findings not only gain new insights on perovskite transformation mechanism, but also lead to developing an environmentally benign processing method for perovskite films and devices.

Two novel wide‐bandgap small molecular acceptors (SMAs) based on 1,3‐diethyl‐2‐thiobarbituric acid (TBA) as building blocks have designed and synthesized for fullerene‐free polymer solar cells. Power conversion efficiencies (PCEs) of 6.5% for IDT‐TBA and 7.5% for IDDT‐TBA, respectively, were achieved with using PBDB‐T as the donor polymers. Time‐delay collection field (TDCF) experiments suggest that both IDT‐TBA and IDDT‐TBA based cells exhibit field‐independent charge generation with external charge generation efficiencies exceeding 90%, implying negligible geminate recombination losses.

Narrow‐bandgap small molecular acceptors (SMAs) with absorption extending into the near‐infrared spectral region such as ITIC derivatives are widely investigated, while the development of their wide‐bandgap counterparts remains largely unexplored. Wide‐bandgap non‐fullerene acceptors (NFAs) are highly desirable and beneficial for constructing efficient device layouts such as ternary blend and tandem solar cells that require multiple light‐harvesting materials with different regions of absorption. In this contribution, the design and synthesis of two wide‐bandgap SMAs (IDT‐TBA and IDDT‐TBA), consisting of a weak electron‐withdrawing moiety (1,3‐diethyl‐2‐thiobarbituric acid, TBA) is presented. Compared to ITIC, this molecular design strategy results in energetically down‐shifted HOMO levels and hence much enlarged bandgaps of 1.91 eV for IDT‐TBA and 1.78 eV for IDDT‐TBA, respectively. Further photovoltaic performance evaluation demonstrates power conversion efficiencies (PCEs) of 6.5% for IDT‐TBA and 7.5% for IDDT‐TBA, respectively, when using PBDB‐T as the electron donor polymer. In addition, time‐delayed collection field (TDCF) experiments suggest that both IDT‐TBA and IDDT‐TBA based cells exhibit field‐independent charge generation with external charge generation efficiencies exceeding 90%, implying negligible geminate recombination losses. The results demonstrate that TBA units are promising and attractive building blocks as weak electron‐withdrawing acceptors to construct wide‐bandgap high‐efficiency SMAs for efficient organic photovoltaic devices.

A comprehensive optoelectronic simulation of the perovskite solar cells (PSCs) under various system configurations, including the complete (with ETL and HTL), free‐HTL, and free‐ETL systems, which thoroughly reveals the detailed optical and carrier transport dynamics under solar illumination. The energy diagrams, built‐in electrical fields and recombination losses are analyzed. Manipulation strategies are given to optimally design a high‐performance perovskite solar cell.

To realize reliably high efficiency of perovskite solar cells (PSCs), material synthesis, interface manipulation, and device realization have been widely studied. Nevertheless, deeply understanding the fundamental optics and physics which regulate the multi‐domain optoelectronic responses is crucial. Here, the authors present an optoelectronic study of PSCs under various configurations. It combines electromagnetic response and carrier electrodynamics inside PSCs so that the microscopic response in frequency/spatial domains and the device output can be obtained. The effects of perovskite doping type/concentration/thickness and doping concentrations of electron (hole) transport layer (i.e., ETL [HTL]) on the optoelectronic response of the complete, free‐HTL, and free‐ETL PSCs are studied. The energy diagrams addressing the band bending, the built‐in electrical field addressing the carrier separation, and the surface/bulk recombination addressing the current losses are analyzed. It is found that the doping type of perovskite greatly affects the cell performance, for example, for ETL‐side illumination, p‐perovskite enables higher efficiency than n‐doping due to a much reduced bulk recombination. Achieving good agreements with existing experiments, the model is used for the design of PSCs. It shows that the photoconversion efficiencies of complete, free‐HTL, and free‐ETL PSCs can be enhanced from 16.6%, 10.7%, and 13.3% to 19.0%, 15.9%, and 18.5%, respectively.

Solar RRL, Volume 2, Issue 9, September 2018.

Advanced Materials, EarlyView.

Advanced Materials, EarlyView.

Advanced Materials, EarlyView.

Advanced Materials, EarlyView.

Solar RRL, Volume 2, Issue 9, September 2018.

Solar RRL, Volume 2, Issue 7, July 2018.

Solar RRL, Volume 2, Issue 7, July 2018.

Advanced Energy Materials, Volume 8, Issue 25, September 5, 2018.

Opportunities and challenges for tandem solar cells using metal halide perovskite semiconductors

Opportunities and challenges for tandem solar cells using metal halide perovskite semiconductors, Published online: 09 July 2018; doi:10.1038/s41560-018-0190-4

Perovskites, with their wide bandgap range, are good partners for both commercial and novel photovoltaic technologies in multijunction solar cells. Here, McGehee and co-workers review recent material and device developments and highlight future challenges and opportunities for perovskite-based tandems.

Tunable low‐bandgap Sn–Pb binary perovskites can work in ideal‐bandgap perovskite photovoltaics approaching the Shockley–Queisser‐efficiency limit, low‐bandgap perovskite‐based bottom subcells in tandem devices, and near‐infrared photodetection. Here, Zhu and Choi systematically review crystallization, properties, and challenges of low‐bandgap Sn–Pb binary perovskites and associated applications in single‐junction, 2‐terminal, 4‐terminal photovoltaics, and photodetectors. Potential prospects for advancing low‐bandgap Sn–Pb binary perovskite‐based optoelectronic devices are also discussed.

Solution‐process and low‐temperature perovskites have motivated a broad range of interests and intensive studies for applications in solar cells (SCs) and photodetectors (PDs). Perovskite SCs with the bandgap of ≈1.5 eV currently exhibit the certified efficiency over 22% comparable with those of established thin film technologies. Meanwhile, perovskite PDs achieve superb performances in the visible region compared with commercial Si PDs. Partial substitution of Sn into Pb‐based perovskites can tune the absorption to near‐infrared (NIR) region, which would achieve an ideal‐bandgap perovskite approaching the Shockley–Queisser‐efficiency limit, low‐bandgap perovskite‐based bottom subcells in tandem devices (≈1.2 eV), and NIR photodetection. Here, various crystallization methods for growing low‐bandgap Sn–Pb binary perovskites are presented. Their impacts on morphology, crystallinity, preferred orientation, carrier lifetimes, Urbach energy, and stability of the resultant Sn–Pb binary perovskites are highlighted. Then, a description is given of single‐junction, 2‐terminal, and 4‐terminal SCs using these perovskites as absorbers, which achieve up‐to‐date efficiencies of 17.8%, 18.4%, and 21.2%, respectively. The current development of ultraviolet–visible–NIR PDs using these perovskites is also discussed. Furthermore, the challenges in controlling inter‐grain Sn/Pb element distributions and perovskite stability, which will influence performance and stability of Sn–Pb perovskite‐based devices, are presented. Finally, potential prospects are discussed for advancing low‐bandgap Sn–Pb binary perovskite‐based optoelectronic devices.

A fluorene-terminated hole-transporting material for highly efficient and stable perovskite solar cells

A fluorene-terminated hole-transporting material for highly efficient and stable perovskite solar cells, Published online: 09 July 2018; doi:10.1038/s41560-018-0200-6

Interfacial losses between device layers play a key role in determining characteristics of solar cells. Jeon et al. address this in perovskite solar cells by synthesizing a hole-transporting layer that is better matched to the surrounding layers, and show high-efficiency and high-stability devices.

Advanced Energy Materials, July 2018.