Zwitterion-Assisted Crystal Growth of 2D Perovskites with Unfavorable Phase Suppression for High-Performance Solar Cells.

Despite two-dimensional (2D) Ruddlesden-Popper-phase layered perovskites (RPLPs) exhibiting excellent environmental stability, most solar cells based on 2D RPLP films are fabricated in a controlled inert atmosphere. Meanwhile, the poor charge transport of 2D RPLP films owing to the unfavorable phase arrangement and defects limits the efficiency of 2D RPLP solar cells. Here, we fabricate high-efficiency 2D RPLP solar cells in ambient air assisted by a zwitterion (ZW) additive. We show that the ZW additive suppresses the formation of the bottom 2D phases (n ≤ 2) and the top 3D-like phases in 2D RPLP films. These 2D phases usually grow parallel to the substrate and act as trap sites that inhibit charge transport in the vertical direction. The 3D-like phases, on the other hand, aggravate the long-term stability due to the intrinsic instability of MA+ cations. With improved phase distribution, crystal orientation, and reduced trap states in 2D RPLP films, efficient charge transport is obtained. Finally, a record-high open-circuit voltage (Voc) of 1.19 V and a power conversion efficiency of 17.04% with an enhanced stability are achieved for (BA0.9PEA0.1)2MA3Pb4I13-based (n = 4) solar cells fabricated under high humidity (∼65% RH).

Efficient Stabilization and Passivation for Low-Temperature-Processed γ-CsPbI3 Solar Cells.

The inorganic CsPbI3 perovskite has attracted tremendous attention in the photovoltaic fields for its chemical stability and suitable band gap. Generally, CsPbI3 solar cells with decent performances adopted high annealing temperature to form high-quality black-phase perovskite films. The high-temperature process hinders its practical application and further development. Hence, fabricating stable black-phase CsPbI3 at low temperature is imperative and necessary. In this work, a new additive p-xylilenediamine bromide (PhDMADBr) is reported to facilitate the synthesis of solution-processed, high-quality, and stable γ-CsPbI3 films at a surprisingly low temperature of 60 °C. The additive with an appropriate content can effectively improve both the film morphology and crystallinity of γ-CsPbI3 perovskite films. PhDMADBr anchors to the perovskite surface or grain boundaries as a protection through hydrogen bonding between its ammonium cations and CsPbI3. In addition, the Br element introduced by the additive passivates I- vacancies in perovskite films, resulting in the improvement of both phase stability and devices' performance. Finally, the PSCs based on the modified γ-CsPbI3 perovskite film achieve a champion efficiency of 12.71%. Moreover, the device retains 85% of its original efficiency after being kept for 1000 h.

Controllable Two-dimensional Perovskite Crystallization via Water Additive for High-performance Solar Cells.

Steering the crystallization of two-dimensional (2D) perovskite film is an important strategy to improve the power conversion efficiency (PCE) of 2D perovskite solar cells (PVSCs). In this paper, the deionized water (H2O) additive is introduced into the perovskite precursor solution to prepare high-quality 2D perovskite films. The 2D perovskite film treated with 3% H2O shows a good surface morphology, increased crystal size, enhanced crystallinity, preferred orientation, and low defect density. The fabricated 2D PVSC with 3% H2O exhibits a higher PCE compared with that without H2O (12.15% vs 2.29%). Furthermore, the shelf stability of unsealed devices with 3% H2O under ambient environment is significantly improved. This work provides a simple method to prepare high-quality 2D perovskite films for efficient and stable 2D PVSCs.

Enhanced Crystallinity of Triple-Cation Perovskite Film via Doping NH4SCN.

The trap-state density in perovskite films largely determines the photovoltaic performance of perovskite solar cells (PSCs). Increasing the crystal grain size in perovskite films is an effective method to reduce the trap-state density. Here, we have added NH4SCN into perovskite precursor solution to obtain perovskite films with an increased crystal grain size. The perovskite with increased crystal grain size shows a much lower trap-state density compared with reference perovskite films, resulting in an improved photovoltaic performance in PSCs. The champion photovoltaic device has achieved a power conversion efficiency of 19.36%. The proposed method may also impact other optoelectronic devices based on perovskite films.

Mini Review on Flexible and Wearable Electronics for Monitoring Human Health Information.

The application potential of wearable electronics in the healthcare field has been of great interest over the past several decades. Flexible and wearable devices based on skin-friendly soft elastic materials can be snugly attached to the surface of human skin, so that a series of vital health information such as wrist pulse, body temperature, and blood glucose can be extracted and analyzed to help the patient maintain physical fitness. Here, we outlined the most common types of wearable electronics for monitoring human health information, including force sensors, temperature sensors, physiological biochemical sensors, and multifunctional sensors. Their general working principles and structural innovations are reviewed. Then, we discussed two functional modules that make the wearable sensors more applicable in real life-self-powered module and signal processing module. The challenges and future research directions are also proposed to develop wearable electronics for monitoring human health information.

Solution-Processed Inorganic Perovskite Flexible Photodetectors with High Performance.

All inorganic CsPbI3-xBrx perovskites have been widely used in photodetectors due to their excellent optoelectronic properties and simple preparation processes. Here, high-performance flexible photodetectors based on inorganic CsPbI3-xBrx perovskites are demonstrated, which are achieved by a modified solution-processed method. When biased at a low voltage of 10 mV, the device yielded fast response speeds (90 µs /110 µs for CsPbI2Br PDs and 100 µs/140 µs for CsPbIBr2 PDs), a high on/off ratio of 104, and a high detectivity about 1012 Jones. Meanwhile, the devices showed outstanding environmental stability and mechanical flexibility. The periodic I-t curves had negligible fluctuation (< 5%) after storing in air atmosphere for 30 days or bending for 100 times. The results indicate that CsPbI3-xBrx perovskites have great potential in photodetection areas and pave the way to achieve high-performance flexible PDs.