Guanhaojie Zheng

Chemical Reduction of Intrinsic Defects in Thicker Heterojunction Planar Perovskite Solar Cells

Minimization of defects in absorber materials is essential for hybrid perovskite solar cells, especially when constructing thick polycrystalline layers in a planar configuration. Here, a simple methylamine solution-based additive is reported to improve film quality with nearly an order of magnitude reduction in intrinsic defect concentration. In the resultant film, an increase in carrier lifetime as a result of a decrease in shallow electronic disorder is observed. This superior crystalline film quality is further evidenced via a doubled spin relaxation time as compared with other reports. Bearing sufficient carrier diffusion length, a thick absorber layer (≈650 nm) is implemented in planar devices to achieve a champion power conversion efficiency of 20.02% with a stabilized output efficiency of 19.01% under one sun illumination. This work demonstrates a simple approach to improve hybrid perovskite film quality by substantial reduction of intrinsic defects for wide applications in optoelectronics.

CsI Pre-Intercalation in the Inorganic Framework for Efficient and Stable FA1−x CsxPbI3(Cl) Perovskite Solar Cells

Engineering the chemical composition of organic and inorganic hybrid perovskite materials is one of the most feasible methods to boost the efficiency of perovskite solar cells with improved device stability. Among the diverse hybrid perovskite family of ABX3 , formamidinium (FA)-based mixed perovskite (e.g., FA1-x Csx PbI3 ) possesses optimum bandgaps, superior optoelectronic property, as well as thermal- and photostability, which is proven to be the most promising candidate for advanced solar cell. Here, FA0.9 Cs0.1 PbI3 (Cl) is implemented as the light-harvesting layer in planar devices, whereas a low temperature, two-step solution deposition method is employed for the first time in this materials system. This paper comprehensively exploits the role of Cs+ in the FA0.9 Cs0.1 PbI3 (Cl) perovskite that affects the precursor chemistry, film nucleation and grain growth, and defect property via pre-intercalation of CsI in the inorganic framework. In addition, the resultant FA0.9 Cs0.1 PbI3 (Cl) films are demonstrated to exhibit an improved optoelectronic property with an elevated device power conversion efficiency (PCE) of 18.6%, as well as a stable phase with substantial enhancement in humidity and thermal stability, as compared to that of FAPbI3 (Cl). The present method is able to be further extended to a more complicated (FA,MA,Cs)PbX3 material system by delivering a PCE of 19.8%.

Tailored Au@TiO2 nanostructures for the plasmonic effect in planar perovskite solar cells

Among the various methods to advance solar cell technologies, the implementation of nanoparticles with plasmonic effects is an effective way to make better use of incident light and manage carrier dynamics. Herein, for the first time we report the systematic synthesis of gold nanospheres or nanorods coated with a thin layer of titanium oxide (Au@TiO2) and use them to examine the plasmonic effect in planar heterojunction perovskite solar cells. The most efficient assembly mode is to embed the Au@TiO2 nanorods into the electron transport layer (ETL), which elevates the average power conversion efficiency (PCE) from 15.76% to 16.35%, mainly attributed to the short-circuit current enhancement. The optimized device assembled with Au@TiO2 nanorods delivers an efficiency of 20.10%. We further explored the plasmonic enhancement effect of Au@TiO2 nanorods based on the combination of UV-visible absorption spectroscopy, incident photon-to-current efficiency (IPCE), photoluminescence (PL) and transient photocurrent decay (TPC). The results indicate better charge separation/transfer as well as facilitated carrier transport in the presence of plasmonic particles. This work provides an insightful understanding of plasmonic effects in planar perovskite solar cells and also presents a promising approach for simultaneous photon and electron management.

Exploration of Crystallization Kinetics in Quasi Two-Dimensional Perovskite and High Performance Solar Cells

Halide perovskites with reduced-dimensionality (e.g., quasi-2D, Q-2D) have promising stability while retaining their high performance as compared to their three-dimensional counterpart. Generally, they are obtained in (A1)2(A2)n-1PbnI3n+1 thin films by adjusting A site cations, however, the underlying crystallization kinetics mechanism is less explored. In this manuscript, we employed ternary cations halides perovskite (BA)2(MA,FA)3Pb4I13 Q-2D perovskites as an archetypal model, to understand the principles that link the crystal orientation to the carrier behavior in the polycrystalline film. We reveal that appropriate FA+ incorporation can effectively control the perovskite crystallization kinetics, which reduces nonradiative recombination centers to acquire high-quality films with a limited nonorientated phase. We further developed an in situ photoluminescence technique to observe that the Q-2D phase (n = 2, 3, 4) was formed first followed by the generation of n = ∞ perovskite in Q-2D perovskites. These findings substantially benefit the understanding of doping behavior in Q-2D perovskites crystal growth, and ultimately lead to the highest efficiency of 12.81% in (BA)2(MA,FA)3Pb4I13 Q-2D perovskites based photovoltaic devices.

Enhanced physical properties of pulsed laser deposited NiO films via annealing and lithium doping for improving perovskite solar cell efficiency

Pulsed laser deposition (PLD) is a powerful growth technique for thin films, where in situ doping and post-thermal annealing are the most effective ways to tune the crystalline and physical properties of the deposited films. This paper demonstrates that the crystallinity, transparency, and electrical properties of NiO films are well controlled by PLD, which determines the photovoltaic performance of CH3NH3PbI3−xClx-based perovskite solar cells with NiO films as the hole transport layers (HTLs). After post-annealing, the NiO films exhibit enhanced in-plane crystal orientation, high transmittance, and uniform surface morphology, and, accordingly, the power conversion efficiency (PCE) of the perovskite solar cell improves from 5.38% to 12.59%. Moreover, by doping the ablated target with lithium (Li), PLD can produce doped NiO:Li films with significantly enhanced electrical conductivity, which further improves the perovskite cell PCE from 12.59% to 15.51%. These results highlight the importance of optimizing the transporting layer properties toward high-performance inverted perovskite planar solar cells.

To probe the performance of perovskite memory devices: defects property and hysteresis

Hybrid organic–inorganic perovskite materials offer a range of interesting characteristics that are suitable for optoelectronic devices, such as photovoltaics. Along with the fast rise in device performance, a current density–voltage (J–V) hysteresis originating from defects and their movement has attracted intense attention, which renders challenges regarding the stability and reliability of the novel materials. Here, we carefully probe the effects of defects in perovskite materials and across interfaces within the device, in which bistable conductive states are achieved for the next generation of nonvolatile memory. The memory device shows an operating voltage as low as 0.25 V, and a decent ON/OFF ratio. More importantly, we correlate the defect density and hysteresis-index of different perovskite films with the corresponding memory device performance. The findings enrich our understanding of the working mechanism of perovskite memory devices, which will also benefit other organic–inorganic hybrid perovskite optoelectronics.

High-Mobility p-Type Organic Semiconducting Interlayer Enhancing Efficiency and Stability of Perovskite Solar Cells

A high‐mobility p‐type organic semiconductor based on benzodithiophene and diketopyrrolopyrrole with linear alkylthio substituents (BDTS‐2DPP) is used as a dual function interfacial layer to modify the interface of perovskite/2,2′,7,7′‐tetrakis(N,N′‐di‐p‐methoxyphenylamine)‐9,9′‐spirobifluorene in planar perovskite solar cells. The BDTS‐2DPP layer can remarkably passivate the surface defects of perovskite through the formation of Lewis adduct between the under‐coordinated Pb atoms in perovskite and S atoms in BDTS‐2DPP, and also shows efficient hole extraction and transfer properties. The devices with BDTS‐2DPP interlayer show a peak power conversion efficiency of 18.2%, which is higher than that of reference devices without the BDTS‐2DPP interlayer (16.9%). Moreover, the hydrophobic BDTS‐2DPP interlayer effectively protects the perovskite against moisture, leading to enhanced device stability.

An amino-substituted perylene diimide polymer for conventional perovskite solar cells

We design and synthesize an amino-functionalized conjugated polymer (PPDI-F3N) based on perylene diimide and use it as a multifunctional interfacial layer of TiO2/perovskite in conventional planar perovskite solar cells. The work function of TiO2 is modulated by PPDI-F3N to better align with the conduction band of the perovskite, leading to efficient charge extraction. PPDI-F3N can passivate the TiO2 surface to reduce the severe recombination loss and rapid degradation caused by oxygen vacancies on the UV-sensitive TiO2 surface. Moreover, modulated polarity of PPDI-F3N is beneficial to optimal perovskite crystallization and morphology. All these features contribute to a higher efficiency (18.3%) of the PSCs with the PPDI-F3N interlayer relative to the control devices without the interlayer (16.7%) as well as improved stability and a reduced hysteresis effect.

The investigation of an amidine-based additive in the perovskite films and solar cells*

Here, we introduced acetamidine (C2H3N2H3, Aa)-based salt as an additive in the fabrication of perovskite (CH3NH3PbI3) layer for perovskite solar cells. It was found that as an amidine-based salt, this additive successfully enhanced the crystallinity of CH3NH3PbI3 and helped to form smooth and uniform films with comparable grain size and full coverage. Besides, perovskite film with additive showed a much longer carrier lifetime and an obviously enhanced open-circuit voltage in the corresponding devices, indicating that the acetamidine-based salt can reduce the carrier recombination in both the film and device. We further demonstrate a promising perovskite device based on acetamidine salt by using a configuration of ITO/TiO2/Perovskite/Spiro-OMeTAD/Au under < 150℃ fabrication condition. A power conversion efficiency (PCE) of 16.54% was achieved, which is much higher than the control device without acetamidine salt. These results present a simple method for film quality optimization of perovskite to further improve photovoltaic performances of perovskite solar cells, which may also benefit the exploration of A cation in perovskite materials.

Manipulation of facet orientation in hybrid perovskite polycrystalline films by cation cascade

Crystal orientations in multiple orders correlate to the properties of polycrystalline materials, and it is critical to manipulate these microstructural arrangements to enhance device performance. Herein, we report a controllable approach to manipulate the facet orientation within the ABX3 hybrid perovskites polycrystalline films by cation cascade doping at A-site. Two-dimensional synchrotron radiation grazing incidence wide-angle X-ray scattering is employed to probe the crystal orientations in multiple orders in mixed perovskites thin films, revealing a general pattern to guide crystal planes stacking upon extrinsic doping during crystallization. Different from previous studies, this method enables to adjust the crystal stacking mode of certain crystallographic planes in polycrystalline perovskites. Moreover, the preferred facet orientation is found to facilitate photocarrier transport across the absorber and pertaining interface in the resultant PV device, which provides an exemplary paradigm for further explorations that relate to the microstructures of hybrid perovskite materials and relevant optoelectronics.Crystal facet orientations of the polycrystalline hybrid lead halide perovskite thin films play a crucial role in determining the device performance. Here Zheng et al. demonstrate effective control of the crystal stacking mode by cation cascade doping, which promotes the charge transport in the photovoltaic device.