Single-Crystalline Nanosheets of Hybrid Perovskite Fabricated by a Vapor-Solution Sequential Deposition Route.

Organic-inorganic hybrid perovskite single crystals have attracted much attention due to their superior optoelectronic properties. Herein, we report a facile vapor-solution sequential route to prepare single-crystalline nanosheets of hybrid lead triiodide perovskite. It is found that this two-step deposition is able to fabricate sizeable high-quality single-crystalline nanosheets with no need of delicate control of crystallization conditions such as concentration or temperature for normal single crystal growth. The resulting perovskite nanosheets show good reproducibility and single crystallinity with bright and uniform photoluminescence. Our study provides a promising strategy for scalable fabrication of perovskite single crystals with great potential in optoelectronic applications.

Polar Solvent Induced Lattice Distortion of Cubic CsPbI3 Nanocubes and Hierarchical Self-Assembly into Orthorhombic Single-Crystalline Nanowires.

Despite the recent surge of interest in inorganic lead halide perovskite nanocrystals, there are still significant gaps in their stability disturbance and the understanding of their destabilization, assembly, and growth processes. Here, we discover that polar solvent molecules can induce the lattice distortion of ligand-stabilized cubic CsPbI3, leading to the phase transition into orthorhombic phase, which is unfavorable for photovoltaic applications. Such lattice distortion triggers the dipole moment on CsPbI3 nanocubes, which subsequently initiates the hierarchical self-assembly of CsPbI3 nanocubes into single-crystalline nanowires. The systematic investigations and in situ monitoring on the kinetics of the self-assembly process disclose that the more amount or the stronger polarity of solvent can induce the more rapid self-assembly and phase transition. These results not only elucidate the destabilization mechanism of cubic CsPbI3 nanocrystals, but also open up opportunities to synthesize and store cubic CsPbI3 for their practical applications in photovoltaics and optoelectronics.

Thermodynamically Stable Orthorhombic γ-CsPbI3 Thin Films for High-Performance Photovoltaics.

All-inorganic lead halide perovskites demonstrate improved thermal stability over the organic-inorganic halide perovskites, but the cubic α-CsPbI3 with the most appropriate bandgap for light harvesting is not structurally stable at room temperature and spontaneously transforms into the undesired orthorhombic δ-CsPbI3. Here, we present a new member of black-phase thin films of all-inorganic perovskites for high-efficiency photovoltaics, the orthorhombic γ-CsPbI3 thin films with intrinsic thermodynamic stability and ideal electronic structure. Exempt from introducing organic ligands or incorporating mixed cations/anions into the crystal lattice, we stabilize the γ-CsPbI3 thin films by a simple solution process in which a small amount of H2O manipulates the size-dependent phase formation through a proton transfer reaction. Theoretical calculations coupled with experiments show that γ-CsPbI3 with a lower surface free energy becomes thermodynamically preferred over δ-CsPbI3 at surface areas greater than 8600 m2/mol and exhibits comparable optoelectronic properties to α-CsPbI3. Consequently, γ-CsPbI3-based solar cells display a highly reproducible efficiency of 11.3%, among the highest records for CsPbI3 thin-film solar cells, with robust stability in ambient atmosphere for months and continuous operating conditions for hours. Our study provides a novel and fundamental perspective to overcome the Achilles' heel of the inorganic lead iodide perovskite and opens it up for high-performance optoelectronic devices.

A Two-Dimensional Hole-Transporting Material for High-Performance Perovskite Solar Cells with 20 % Average Efficiency.

A readily available small molecular hole-transporting material (HTM), OMe-TATPyr, was synthesized and tested in perovskite solar cells (PSCs). OMe-TATPyr is a two-dimensional π-conjugated molecule with a pyrene core and four phenyl-thiophene bridged triarylamine groups. It can be readily synthesized in gram scale with a low lab cost of around US$ 50 g-1 . The incorporation of the phenyl-thiophene units in OMe-TATPyr are beneficial for not only carrier transportation through improved charge delocalization and intermolecular stacking, but also potential trap passivation via Pb-S interaction as supported by depth-profiling XPS, photoluminescence, and electrochemical impedance analysis. As a result, an impressive best power conversion efficiency (PCE) of up to 20.6 % and an average PCE of 20.0 % with good stability has been achieved for mixed-cation PSCs with OMe-TATPyr with an area of 0.09 cm2 . A device with an area of 1.08 cm2 based on OMe-TATPyr demonstrates a PCE of 17.3 %.

General Space-Confined On-Substrate Fabrication of Thickness-Adjustable Hybrid Perovskite Single-Crystalline Thin Films.

Organic-inorganic hybrid perovskite single-crystalline thin films (SCTFs) are promising for enhancing photoelectric device performance due to high carrier mobility, long diffusion length, and carrier lifetime. However, bulk perovskite single crystals available today are not suitable for practical device application due to the unfavorable thickness. Herein, we report a facile space-confined solution-processed strategy to on-substrate grow various hybrid perovskite SCTFs in a size of submillimeter with adjustable thicknesses from nano- to micrometers. These SCTFs exhibit photoelectric properties comparable to bulk single crystals with low defect density and good air stability. The clear thickness-dependent colors allow fast visual selection of SCTFs with a suitable thickness for specific device application. The present substrate-independent growth of perovskite SCTFs opens up opportunities for on-chip fabrication of diverse high-performance devices.

Tuning the Fermi-level of TiO2 mesoporous layer by lanthanum doping towards efficient perovskite solar cells.

Tuning the band alignment is proved to be an effective way to facilitate carrier transportation and thus enhance the power conversion efficiency (PCE) of solar cells. Doping the compact layer with metal ions or modifying the interfaces among functional layers in perovskite solar cells (PSCs) can appreciably improve the PCE of PSCs. Inspired by the rare earth elemental doping of TiO2, which has witnessed the success in photocatalysis and dye-sensitized solar cells, we firstly demonstrated here that La3+ doping in the mesoporous TiO2 layer of a mesostructured PSC can tune its Fermi level and thus significantly enhance the device PCE. Systematic analysis reveals that doping La3+ into TiO2 raises the Fermi level of TiO2 through scavenging oxygen and inducing vacancies, which subsequently increases the open circuit voltage and the fill factor while reducing the series resistance of the PSC using La3+-doped TiO2 as a mesoporous layer. As a result, a PCE of 15.42% is achieved, which is appreciably higher than the PCE of a device with undoped TiO2 (12.11%).

Solvent-Assisted Preparation of High-Performance Mesoporous CH3NH3Pbl3 Perovskite Solar Cells.

Organometal trihalide perovskite based solar cells have attracted great attention worldwide since their power conversion efficiency (PCE) have risen to over 15% within only 3 years of development. Comparing with other types of perovskite solar cells, mesostructured perovskite solar cells based on CH3NH3Pbl3 as light harvesting material have already demonstrated remarkable advance in performance and reproducibility. Here, we reported a mesoscopic TiO2/CH3NH3Pbl3 heterojunction solar cell with uniform perovskite thin film prepared via solvent-assisted solution processing method. The best performing device delivered photocurrent density of 20.11 mA cm⁻², open-circuit voltage of 1.02 V, and fill factor of 0.70, leading to a PCE of 14.41%. A small anomalous hysteresis in the J-V curves was observed, where the PCE at forward scan was measured to be 84% of the PCE at reverse scan. Based on a statistical analysis, the perovskite solar cells prepared by the reported method exhibited reproducible and high PCE, indicating its promising application in the fabrication of low-cost and high-efficiency perovskite solar cells.

Microscopic Investigation of Grain Boundaries in Organolead Halide Perovskite Solar Cells.

Grain boundaries (GBs) play an important role in organic-inorganic halide perovskite solar cells, which have generally been recognized as a new class of materials for photovoltaic applications. To definitely understand the electrical structure and behavior of GBs, here we present Kelvin probe force microscopy and conductive atomic force microscopy (c-AFM) measurements of both typical and inverted planar organolead halide perovskite solar cells. By comparing the contact potential difference (CPD) of these two devices in the dark and under illumination, we found that a downward band bending exists in GBs that predominantly attract photoinduced electrons. The c-AFM measurements observed that higher photocurrents flow through GBs when a low bias overcomes the barrier created by the band bending, indicating that GBs act as effective charge dissociation interfaces and photocurrent transduction pathways rather than recombination sites.

Engineering the interfaces of ITO@Cu2S nanowire arrays toward efficient and stable counter electrodes for quantum-dot-sensitized solar cells.

Among the issues that restrict the power conversion efficiency (PCE) of quantum-dot-sensitized solar cells (QDSSCs), insufficient catalytic activity and stability of counter electrodes (CEs) are critical but challenging ones. The state-of-the-art Cu/Cu2S CEs still suffer from mechanical instability and uncertainty due to the reaction of copper and electrolyte. Herein, ITO@Cu2S core-shell nanowire arrays were developed to fabricate CEs for QDSSCs, which have no such issues in Cu/Cu2S CEs. These nanowire arrays exhibited small charge transfer resistance and sheet resistance, and provided more active catalytic sites and easy accessibility for electrolyte due to the three-dimensional structure upon use as CEs. More interestingly, it was found that the interface of ITO/Cu2S significantly affected the performance of ITO@Cu2S nanowire array CEs. By varying synthetic methods, a series of ITO@Cu2S nanowire arrays were prepared to investigate the influence of ITO/Cu2S interface on their performance. The results showed that ITO@Cu2S nanowire array CEs with a continuous Cu2S nanocrystal shell fabricated via an improved cation exchange route exhibited excellent and thickness-dependent performance. The PCE of corresponding QDSSCs increased by 11.6 and 16.5% compared to that with the discrete Cu2S nanocrystal and the classic Cu/Cu2S CE, respectively, indicating its promising potential as a new type of CE for QDSSCs.

ITO@Cu2S tunnel junction nanowire arrays as efficient counter electrode for quantum-dot-sensitized solar cells.

Quantum-dot-sensitized solar cell (QDSSC) has been considered as an alternative to new generation photovoltaics, but it still presents very low power conversion efficiency. Besides the continuous effort on improving photoanodes and electrolytes, the focused investigation on charge transfer at interfaces and the rational design for counter electrodes (CEs) are recently receiving much attention. Herein, core-shell nanowire arrays with tin-doped indium oxide (ITO) nanowire core and Cu2S nanocrystal shell (ITO@Cu2S) were dedicatedly designed and fabricated as new efficient CEs for QDSSCs in order to improve charge collection and transport and to avoid the intrinsic issue of copper dissolution in popular and most efficient Cu/Cu2S CEs. The high-quality tunnel junctions formed between n-type ITO nanowires and p-type Cu2S nanocrystals led to the considerable decrease in sheet resistance and charge transfer resistance and thus facilitated the electron transport during the operation of QDSSCs. The three-dimensional structure of nanowire arrays provided high surface area for more active catalytic sites and easy accessibility for an electrolyte. As a result, the power conversion efficiency of QDSSCs with the designed ITO@Cu2S CEs increased by 84.5 and 33.5% compared to that with planar Au and Cu2S CEs, respectively.