Manipulating Crystallographic Orientation via Cross-Linkable Ligand for Efficient and Stable Perovskite Solar Cells.

The crystallographic orientation of polycrystalline perovskites is found to be strongly correlated with their intrinsic properties; therefore, it can be used to effectively enhance the performance of perovskite-based devices. Here, a facile way of manipulating the facet orientation of polycrystalline perovskite films in a controllable manner is reported. By incorporating a cross-linkable organic ligand into the perovskite precursor solution, the crystal orientation disorder can be reduced in the resultant perovskite films to exhibit the prominent (001) orientation with a preferred stacking mode. Moreover, the as-formed low-dimensional perovskites (LDPs) between the organic ligand and the excess lead iodide can passivate the defects around the grain boundaries. Consequently, highly efficient p-i-n structured perovskite solar cells (PSCs) can be made in both rigid and flexible forms from modified perovskites to show high power conversion efficiencies (PCE) of 24.12% and 23.23%, respectively. The devices also exhibit superior long-term stability in a humid environment (with T90  > 1000 h) and under thermal stress (retaining 87% of its initial PCE after 1000 h). More importantly, the ligand enables the derived LDPs to be crosslinked (under 254 nm UV illumination) to demonstrate excellent mechanical bending durability in flexible devices.

Co-assembled Monolayers as Hole-Selective Contact for High-Performance Inverted Perovskite Solar Cells with Optimized Recombination Loss and Long-Term Stability.

Self-assembled monolayers (SAMs) have been widely employed as an effective way to modify interfaces of electronic/optoelectronic devices. To achieve a good control of the growth and molecular functionality of SAMs, we develop a co-assembled monolayer (co-SAM) for obtaining efficient hole selection and suppressed recombination at the hole-selective interface in inverted perovskite solar cells (PSCs). By engineering the position of methoxy substituents, an aligned energy level and favorable dipole moment can be obtained in our newly synthesized SAM, ((2,7-dimethoxy-9H-carbazol-9-yl) methyl) phosphonic acid (DC-PA). An alkyl ammonium containing SAM is co-assembled to further optimize the surface functionalization and interaction with perovskite layer on top. A champion device with an excellent power conversion efficiency (PCE) of 23.59 % and improved device stability are achieved. This work demonstrates the advantage of using co-SAM in improving performance and stability of PSCs.

Low-Bandgap Organic Bulk-Heterojunction Enabled Efficient and Flexible Perovskite Solar Cells.

Lead halide perovskite and organic solar cells (PSCs and OSCs) are considered as the prime candidates currently for clean energy applications due to their solution and low-temperature processibility. Nevertheless, the substantial photon loss in near-infrared (NIR) region and relatively large photovoltage deficit need to be improved to enable their uses in high-performance solar cells. To mitigate these disadvantages, low-bandgap organic bulk-heterojunction (BHJ) layer into inverted PSCs to construct facile hybrid solar cells (HSCs) is integrated. By optimizing the BHJ components, an excellent power conversion efficiency (PCE) of 23.80%, with a decent open-circuit voltage (Voc ) of 1.146 V and extended photoresponse over 950 nm for rigid HSCs is achieved. The resultant devices also exhibit superior long-term (over 1000 h) ambient- and photostability compared to those from single-component PSCs and OSCs. More importantly, a champion PCE of 21.73% and excellent mechanical durability can also be achieved in flexible HSCs, which is the highest efficiency reported for flexible solar cells to date. Taking advantage of these impressive device performances, flexible HSCs into a power source for wearable sensors to demonstrate real-time temperature monitoring are successfully integrated.

Pseudo-bilayer architecture enables high-performance organic solar cells with enhanced exciton diffusion length.

Solution-processed organic solar cells (OSCs) are a promising candidate for next-generation photovoltaic technologies. However, the short exciton diffusion length of the bulk heterojunction active layer in OSCs strongly hampers the full potential to be realized in these bulk heterojunction OSCs. Herein, we report high-performance OSCs with a pseudo-bilayer architecture, which possesses longer exciton diffusion length benefited from higher film crystallinity. This feature ensures the synergistic advantages of efficient exciton dissociation and charge transport in OSCs with pseudo-bilayer architecture, enabling a higher power conversion efficiency (17.42%) to be achieved compared to those with bulk heterojunction architecture (16.44%) due to higher short-circuit current density and fill factor. A certified efficiency of 16.31% is also achieved for the ternary OSC with a pseudo-bilayer active layer. Our results demonstrate the excellent potential for pseudo-bilayer architecture to be used for future OSC applications.

Regulating Surface Termination for Efficient Inverted Perovskite Solar Cells with Greater Than 23% Efficiency.

Passivating surface and bulk defects of perovskite films has been proven to be an effective way to minimize nonradiative recombination losses in perovskite solar cells (PVSCs). The lattice interference and perturbation of atomic periodicity at the perovskite surfaces often significantly affect the material properties and device efficiencies. By tailoring the terminal groups on the perovskite surface and modifying the surface chemical environment, the defects can be reduced to enhance the photovoltaic performance and stability of derived PVSCs. Here, we report a rationally designed bifunctional molecule, piperazinium iodide (PI), containing both R2NH and R2NH2+ groups on the same six-membered ring, behaving both as an electron donor and an electron acceptor to react with different surface-terminating ends on perovskite films. The resulting perovskite films after defect passivation show released surface residual stress, suppressed nonradiative recombination loss, and more n-type characteristics for sufficient energy transfer. Consequently, charge recombination is significantly suppressed to result in a high open-circuit voltage (VOC) of 1.17 V and a reduced VOC loss of 0.33 V. A very high power conversion efficiency (PCE) of 23.37% (with 22.75% certified) could be achieved, which is the highest value reported for inverted PVSCs. Our work reveals a very effective way of using rationally designed bifunctional molecules to simultaneously enhance the device performance and stability.

2D metal-organic framework for stable perovskite solar cells with minimized lead leakage.

Despite the notable progress in perovskite solar cells, maintaining long-term operational stability and minimizing potentially leaked lead (Pb2+) ions are two challenges that are yet to be resolved. Here we address these issues using a thiol-functionalized 2D conjugated metal-organic framework as an electron-extraction layer at the perovskite/cathode interface. The resultant devices exhibit high power conversion efficiency (22.02%) along with a substantially improved long-term operational stability. The perovskite solar cell modified with a metal-organic framework could retain more than 90% of its initial efficiency under accelerated testing conditions, that is continuous light irradiation at maximum power point tracking for 1,000 h at 85 °C. More importantly, the functionalized metal-organic framework could capture most of the Pb2+ leaked from the degraded perovskite solar cells by forming water-insoluble solids. Therefore, this method that simultaneously tackles the operational stability and lead contamination issues in perovskite solar cells could greatly improve the feasibility of large-scale deployment of perovskite photovoltaic technology.

Water-resistant perovskite nanodots enable robust two-photon lasing in aqueous environment.

Owing to their large absorption cross-sections and high photoluminescence quantum yields, lead halide perovskite quantum dots (PQDs) are regarded as a promising candidate for various optoelectronics applications. However, easy degradation of PQDs in water and in a humid environment is a critical hindrance for applications. Here we develop a Pb-S bonding approach to synthesize water-resistant perovskite@silica nanodots keeping their emission in water for over six weeks. A two-photon whispering-gallery mode laser device made of these ultra-stable nanodots retain 80% of its initial emission quantum yield when immersed in water for 13 h, and a two-photon random laser based on the perovskite@silica nanodots powder could still operate after the nanodots were dispersed in water for up to 15 days. Our synthetic approach opens up an entirely new avenue for utilizing PQDs in aqueous environment, which will significantly broaden their applications not only in optoelectronics but also in bioimaging and biosensing.

Enhanced Near-Infrared Photoresponse of Inverted Perovskite Solar Cells Through Rational Design of Bulk-Heterojunction Electron-Transporting Layers.

How to extend the photoresponse of perovskite solar cells (PVSCs) to the region of near-infrared (NIR)/infrared light has become an appealing research subject in this field since it can better harness the solar irradiation. Herein, the typical fullerene electron-transporting layer (ETL) of an inverted PVSC is systematically engineered to enhance device's NIR photoresponse. A low bandgap nonfullerene acceptor (NFA) is incorporated into the fullerene ETL aiming to intercept the NIR light passing through the device. However, despite forming type II charge transfer with fullerene, the blended NFA cannot enhance the device's NIR photoresponse, as limited by the poor dissociation of photoexciton induced by NIR light. Fortunately, it can be addressed by adding a p-type polymer. The ternary bulk-heterojunction (BHJ) ETL is demonstrated to effectively enhance the device's NIR photoresponse due to the better cascade-energy-level alignment and increased hole mobility. By further optimizing the morphology of such a BHJ ETL, the derived PVSC is finally demonstrated to possess a 40% external quantum efficiency at 800 nm with photoresponse extended to the NIR region (to 950 nm), contributing ≈9% of the overall photocurrent. This study unveils an effective and simple approach for enhancing the NIR photoresponse of inverted PVSCs.

Derivation and external validation of a model to predict 2-year mortality risk of patients with advanced schistosomiasis after discharge.

To date, no risk prediction tools have been developed to identify high mortality risk of patients with advanced schistosomiasis within 2 years after discharge. We aim to derive and validate a risk prediction model to be applied in clinical practice. The risk prediction model was derived from 1487 patients from Jingzhou and externally validated by 723 patients of Huangshi, two prefecture-level cities in Hubei province, China (from September 2014 to January 2015, with follow-up to January 2017). The baseline variables were collected. The mean age [SD] was 62.89 [10.38] years for the derivation cohort and 62.95 [12.22] years for the external validation cohort. The females accounted for 36.3% and 43.7% of the derivation and validation cohorts, respectively. 8.27% patients (123/1487) in the derivation cohort and 7.75% patients (56/723) in the external validation cohort died within 2 years after discharge. We constructed 4 models based on the 7 selected variables: age, clinical classification, serum direct bilirubin (DBil), aspartate aminotransferase (AST), alkaline phosphatase (ALP), hepatitis B surface antigen (HBsAg), alpha fetoprotein (AFP) at admission. In the external validation cohort, the multivariate model including 7 variables had a C statistic of 0.717 (95% CI, 0.646-0.788) and improved integrated discrimination improvement (IDI) value and net reclassification improvement (NRI) value compared to the other reduced models. Therefore, a multivariate model was developed to predict the 2-year mortality risk for patients with advanced schistosomiasis after discharge. It could also help guide follow-up, aid prognostic assessment and inform resource allocation.

Efficient large guanidinium mixed perovskite solar cells with enhanced photovoltage and low energy losses.

We present a strategy for suppressing the open-circuit voltage (Voc) loss of perovskite solar cells by incorporating large guanidinium cations (Gua+) into a perovskite lattice, leading to a significantly improved Voc of 1.19 V and an impressive power conversion efficiency of >21%. Our results manifest the critical role of Gua in passivating defects and highlight the importance of compositional engineering in promoting the performance of perovskites.

Donor-σ-Acceptor Molecules for Green Thermally Activated Delayed Fluorescence by Spatially Approaching Spiro Conformation.

By separating donor/acceptor with a σ linker while keeping them in contact through space interactions, new oxygen-bridged triphenylamine/fluorene-based donor-σ-acceptor (D-σ-A) type thermally activated delayed fluorescence (TADF) emitters are developed. X-ray structural analyses and time-dependent density functional theory reveal that tilted configuration of spiro skeleton, extended delocalization of the highest occupied molecular orbital (HOMO), and lowest triplet state of charge transfer property (3CT) play key roles in the TADF mechanism. OLEDs fabricated with these D-σ-A emitters achieved good external quantum efficiency of 20.4% and long operating durability of 18000 h at 100 cd m-2.

Highly Efficient White Organic Light-Emitting Diodes with Ultrathin Emissive Layers and a Spacer-Free Structure.

Ultrathin emissive layers (UEMLs) of phosphorescent materials with a layer thickness of less than 0.3 nm were introduced for high-efficiency organic light-emitting diodes (OLEDs). All the UEMLs for white OLEDs can be prepared without the use of interlayers or spacers. Compared with devices fabricated with interlayers inserted in-between the UEMLs, our spacer-free structure not only significantly improves device efficiency, but also simplifies the fabrication process, thus it has a great potential in lowering the cost of OLED panels. In addition, its spacer-free structure decreases the number of interfaces which often introduce unnecessary energy barriers in these devices. In the present work, UEMLs of red, green and blue-emitting phosphorescent materials and yellow and blue phosphorescent emitters are utilized for the demonstration of spacer-free white OLEDs. Upon optimization of the device structure, we demonstrated spacer-free and simple-structured white-emitting OLEDs with a good device performance. The current and power efficiencies of our white-emitting devices are as high as 56.0 cd/A and 55.5 lm/W, respectively. These efficiencies are the highest ever reported for OLEDs fabricated with the UEML approach.