Jiaqi Cheng

Pinhole-Free and Surface-Nanostructured NiOx Film by Room-Temperature Solution Process for High-Performance Flexible Perovskite Solar Cells with Good Stability and Reproducibility

Recently, researchers have focused on the design of highly efficient flexible perovskite solar cells (PVSCs), which enables the implementation of portable and roll-to-roll fabrication in large scale. While NiOx is a promising material for hole transport layer (HTL) candidate for fabricating efficient PVSCs on a rigid substrate, the reported NiOx HTLs are formed using different multistep treatments (such as 300-500 °C annealing, O2-plasma, UVO, etc.), which hinders the development of flexible PVSCs based on NiOx. Meanwhile, the features of nanostructured morphology and flawless film quality are very important for the film to function as highly effective HTL of PVSCs. However, it is difficult to have the two features coexist natively, particularly in a solution process that flawless film will usually come with smooth morphology. Here, we demonstrate the flawless and surface-nanostructured NiOx film from a simple and controllable room-temperature solution process for achieving high performance flexible PVSCs with good stability and reproducibility. The power conversion efficiency (PCE) can reaches a promising value of 14.53% with no obvious hysteresis (and a high PCE of 17.60% for PVSC on ITO glass). Furthermore, the NiOx-based PVSCs show markedly improved air stability. Regarding the performance improvement, the flawless and surface-nanostructured NiOx film can make the interfacial recombination and monomolecular Shockley-Read-Hall recombination of PVSC reduce. In addition, the formation of an intimate junction of large interfacial area at NiOx film/the perovskite layer improve the hole extraction and thus PVSC performances. This work contributes to the evolution of flexible PVSCs with simple fabrication process and high device performances.

Post‐treatment‐Free Solution‐Processed Non‐stoichiometric NiOx Nanoparticles for Efficient Hole‐Transport Layers of Organic Optoelectronic Devices

DOI: 10.1002/adma.201405391 In the early stage, Marks and co-workers [ 13 ] reported a striking performance improvement of OSCs by replacing PEDOT:PSS with NiO x fi lm using a pulsed-laser deposition technology. From then on, NiO x HTLs have been reported for organic optoelectronics by various preparation methods, such as thermal evaporation, [ 14 ] sputtering, [ 9a ] and solution process. [ 9d , 15 ] Among them, solution process method is desirable for low-cost, large-scale and roll-to-roll production. Olson and co-workers [ 16 ] proposed a solution-processed NiO x fi lm as highly effi cient HTL in OSCs. The functional NiO x HTL was fabricated through annealing the precursor fi lm at a temperature of 275 °C. So and co-workers [ 17 ] also presented a NiO x fi lm by using monoethanolamine (MEA) to react with Ni in ethanol solution followed by thermally converting (275 °C) coordination complexes ions [Ni(MEA) 2 (OAc)] + into high-quality NiO x . Meanwhile, solution-processed NiO x at 150 °C has also been realized. Ma and co-workers [ 18 ] reported a solution-processed NiO x fi lm for OSCs using oxygen-plasma treatment and annealing treatment simultaneously. Zhang et. al. reported that the colloidal NiO nanoparticles are used as the anode buffer layer in OSCs without high temperature post-annealing to induce decomposition and crystallization. [ 9f ] For a long period, the studies of NiO x HTLs were focused on utilizing sol–gel methods with thermally converting the precursor solution to NiO x thin fi lms. In the process of device fabrications, thermal annealing process and oxygen-plasma treatment may be simultaneously required, which hinders the applications of NiO x in fl exible optoelectronic devices. Instead of precursor method, an approach to signifi cantly reduce the processing temperature of TMO HTLs is to directly use high-quality colloidal nanoparticles (NPs). Jin and co-workers demonstrated a facile and general strategy based on ligand protection for the synthesis of unstable colloidal NiO nanocrystals. [ 19 ] Fattakhova-Rohlfi ng and co-workers described the preparation of ultrasmall, crystalline, and dispersible NiO nanoparticles, which are promising candidates as catalysts for electrochemical oxygen generation. [ 9e ]

Toward All Room-Temperature, Solution- Processed, High-Performance Planar Perovskite Solar Cells: A New Scheme of Pyridine-Promoted Perovskite Formation

A new, all room-temperature solution process is developed to fabricate efficient, low-cost, and stable perovskite solar cells (PVSCs). The PVSCs show high efficiency of 17.10% and 14.19%, with no hysteresis on rigid and flexible substrates, respectively, which are the best efficiencies reported to date for PVSCs fabricated by room-temperature solution-processed techniques. The flexible PVSCs show a remarkable power-per-weight of 23.26 W g-1 .

Room-Temperature Solution-Processed NiOx: PbI2 Nanocomposite Structures for Realizing High-Performance Perovskite Photodetectors

While methylammonium lead iodide (MAPbI3) with interesting properties, such as a direct band gap, high and well-balanced electron/hole mobilities, as well as long electron/hole diffusion length, is a potential candidate to become the light absorbers in photodetectors, the challenges for realizing efficient perovskite photodetectors are to suppress dark current, to increase linear dynamic range, and to achieve high specific detectivity and fast response speed. Here, we demonstrate NiOx:PbI2 nanocomposite structures, which can offer dual roles of functioning as an efficient hole extraction layer and favoring the formation of high-quality MAPbI3 to address these challenges. We introduce a room-temperature solution process to form the NiOx:PbI2 nanocomposite structures. The nanocomposite structures facilitate the growth of the compact and ordered MAPbI3 crystalline films, which is essential for efficient photodetectors. Furthermore, the nanocomposite structures work as an effective hole extraction layer, which provides a large electron injection barrier and favorable hole extraction as well as passivates the surface of the perovskite, leading to suppressed dark current and enhanced photocurrent. By optimizing the NiOx:PbI2 nanocomposite structures, a low dark current density of 2 × 10(-10) A/cm(2) at -200 mV and a large linear dynamic range of 112 dB are achieved. Meanwhile, a high responsivity in the visible spectral range of 450-750 nm, a large measured specific detectivity approaching 10(13) Jones, and a fast fall time of 168 ns are demonstrated. The high-performance perovskite photodetectors demonstrated here offer a promising candidate for low-cost and high-performance near-ultraviolet-visible photodetection.

High Efficiency Organic Solar Cells Achieved by the Simultaneous Plasmon-Optical and Plasmon-Electrical Effects from Plasmonic Asymmetric Modes of Gold Nanostars

The plasmon-optical effects have been utilized to optically enhance active layer absorption in organic solar cells (OSCs). The exploited plasmonic resonances of metal nanomaterials are typically from the fundamental dipole/high-order modes with narrow spectral widths for regional OSC absorption improvement. The conventional broadband absorption enhancement (using plasmonic effects) needs linear-superposition of plasmonic resonances. In this work, through strategic incorporation of gold nanostars (Au NSs) in between hole transport layer (HTL) and active layer, the excited plasmonic asymmetric modes offer a new approach toward broadband enhancement. Remarkably, the improvement is explained by energy transfer of plasmonic asymmetric modes of Au NS. In more detail, after incorporation of Au NSs, the optical power in electron transport layer transfers to active layer for improving OSC absorption, which otherwise will become dissipation or leakage as the role of carrier transport layer is not for photon-absorption induced carrier generation. Moreover, Au NSs simultaneously deliver plasmon-electrical effects which shorten transport path length of the typically low-mobility holes and lengthen that of high-mobility electrons for better balanced carrier collection. Meanwhile, the resistance of HTL is reduced by Au NSs. Consequently, power conversion efficiency of 10.5% has been achieved through cooperatively plasmon-optical and plasmon-electrical effects of Au NSs.

Efficient hole transport layers with widely tunable work function for deep HOMO level organic solar cells

Hole transport layers (HTLs) with large work function (WF) tuning ability for good energy level alignment with deep highest occupied molecular orbital (HOMO) level donor materials are desirable for high-performance and high open-circuit voltage (VOC) organic solar cells (OSCs). Here, a novel low-temperature and solution-process approach to achieve WF tuning in HTLs is proposed. Specifically, the HTLs made from 2,3,4,5,6-pentafluorobenzylphosphonic acid (F5BnPA) incorporated graphene oxide (GO) and molybdenum oxide (MoOx) solution (representing two possible classes of HTLs where carriers transport via valence and conduction bands, respectively) offer continuous WF tuning (the tuning range as large as 0.81 eV) by controlling F5BnPAs concentration. By employing a deep HOMO donor material, OSCs using the composite HTLs can achieve improved performances with largely increased VOC (0.92 V for GO:F5BnPA versus 0.65 V for pristine GO; 0.91 V for MoOx:F5BnPA versus 0.88 V for pristine MoOx). The enhanced performance can be experimentally and theoretically explained by the decreased hole injection barrier (HIB) for GO or equivalent HIB (i.e. electron extraction barrier) for MoOx and enhanced surface recombination velocity, which contribute to eliminating S-shaped current–voltage characteristics. Consequently, the incorporation of F5BnPA can efficiently tune HTL WF for high VOC OSCs and extend HTL applications in organic electronics.

Room temperature formation of organic–inorganic lead halide perovskites: design of nanostructured and highly reactive intermediates

Recently, organic–inorganic lead halide perovskites have been intensively studied for use in solar cells because of their low cost and high performance. Most of the efficient perovskite solar cells (PVSCs) need layer-dependent high-temperature treatment for each layer of the multilayered structures, increasing the fabrication complexity. In addition, high temperature processes hinder their applications in flexible devices. Therefore, it is highly desirable to develop room-temperature processed methods for controllably forming perovskite films which can simplify the complicated device process and promote emerging flexible device technologies. In this work, we propose a room-temperature scheme of ligand-promoted formation of high quality perovskite films through the judicious design of nanostructured PbI2·(L)x intermediates, where L denotes the ligand. The high quality perovskite films are free of pinholes and impurities, and have high crystallinity. Using our room-temperature crystallization of perovskite films, we have fabricated highly efficient room-temperature solution-processed PVSCs with a power conversion efficiency (PCE) of 17.21% (the current best PVSCs based on perovskite fabricated at room temperature have a PCE of <16%). Meanwhile, we have experimentally investigated the effects of different ligands on building the nanostructured PbI2·(L)x intermediates, and thus the purity, morphology, and optoelectronic performances of the resultant perovskite films. Through thermodynamic and kinetic studies, we theoretically study the reactivity of nanostructured PbI2·(L)x intermediates, thus elucidating the possible mechanism of ligand-promoted perovskite formation. Furthermore, with the experimental and theoretical studies, we establish the selection rules for identifying ideal ligands for the formation of high-quality perovskite films. This work offers a fundamental understanding of ligand effects on the formation of perovskite films for the future design of high-performance and low-cost perovskite-based optoelectronic devices.

Effects of Self-Assembled Monolayer Modification of Nickel Oxide Nanoparticles Layer on the Performance and Application of Inverted Perovskite Solar Cells

Entirely low-temperature solution-processed (≤100 °C) planar p-i-n perovskite solar cells (PSCs) offer great potential for commercialization of roll-to-roll fabricated photovoltaic devices. However, the stable inorganic hole-transporting layer (HTL) in PSCs is usually processed at high temperature (200-500 °C), which is far beyond the tolerant temperature (≤150 °C) of roll-to-roll fabrication. In this context, inorganic NiOx nanoparticles (NPs) are an excellent candidate to serve as the HTL in PSCs, owing to their excellent solution processability at room temperature. However, the low-temperature processing condition is usually accompanied with defect formation, which deteriorates the film quality and device efficiency to a large extent. To suppress this setback, we used a series of benzoic acid selfassembled monolayers (SAMs) to passivate the surface defects of the NiOx NPs and found that 4-bromobenzoic acid could effectively play the role of the surface passivation. This SAM layer reduces the trap-assisted recombination, minimizes the energy offset between the NiOx NPs and perovskite, and changes the HTL surface wettability, thus enhancing the perovskite crystallization, resulting in more stable PSCs with enhanced power conversion efficiency (PCE) of 18.4 %, exceeding the control device PCE (15.5 %). Also, we incorporated the above-mentioned SAMs into flexible PSCs (F-PSCs) and achieved one of the highest PCE of 16.2 % on a polyethylene terephthalate (PET) substrate with a remarkable power-per-weight of 26.9 W g-1 . This facile interfacial engineering method offers great potential for the large-scale manufacturing and commercialization of PSCs.

Improving the stability and performance of perovskite solar cells via off-the-shelf post-device ligand treatment

While metal halide perovskite solar cells (PVSCs) have drawn intense attention due to their high solar-to-power conversion efficiency (PCE), their practical application is hampered by their poor long-term stability against moisture. Although strategies have been reported to solve this issue, these methods are introduced during core-device fabrication processes which will increase the risk of introducing unexpected impurities during the fabrication. Herein, we introduce the first kind of simple post-device ligand (PDL) treatment to significantly improve the PCE of completely fabricated PVSCs from 18.7% to 20.13%. Meanwhile, the stability of the treated devices without any encapsulation remarkably improves, with 70% PCE maintained under ambient conditions after a 500-hour maximum-power-point tracking test, while the control unencapsulated device will completely break down within 100 hours. Equally important is that this post-device treatment shows a special ‘stitching effect’, namely repairing the as-fabricated ‘poor devices’ by healing the defects of the perovskite active region, and can improve the PCE by over 900%. We also experimentally and theoretically study the fundamental mechanism of the improvement. Consequently, our approach greatly improves the production yield of high-quality PVSCs and their module performances as well as the reduction of lead-waste. Additionally, the treatment is an off-the-shelf post-device approach that can be integrated into any existing perovskite-device fabrication, offering a general strategy to improve the stability and performance of perovskite optoelectronic devices.