Cheng Bi

Efficient, high yield perovskite photovoltaic devices grown by interdiffusion of solution-processed precursor stacking layers

We report on an interdiffusion method to fabricate pin-hole free perovskite films using a low temperature (<105 °C) solution process. A high efficiency of 15.4%, with a fill factor of ∼80%, was achieved for the devices under one sun illumination. The interdiffusion method results in high device yield, with an efficiency of above 14.5% for more than 85% of the devices.

Origin and elimination of photocurrent hysteresis by fullerene passivation in CH3NH3PbI3 planar heterojunction solar cells

The large photocurrent hysteresis observed in many organometal trihalide perovskite solar cells has become a major hindrance impairing the ultimate performance and stability of these devices, while its origin was unknown. Here we demonstrate the trap states on the surface and grain boundaries of the perovskite materials to be the origin of photocurrent hysteresis and that the fullerene layers deposited on perovskites can effectively passivate these charge trap states and eliminate the notorious photocurrent hysteresis. Fullerenes deposited on the top of the perovskites reduce the trap density by two orders of magnitude and double the power conversion efficiency of CH(3)NH(3)PbI(3) solar cells. The elucidation of the origin of photocurrent hysteresis and its elimination by trap passivation in perovskite solar cells provides important directions for future enhancements to device efficiency.

Solvent Annealing of Perovskite-Induced Crystal Growth for Photovoltaic-Device Efficiency Enhancement

Solvent-annealing is found to be an effective method to increase the grain size and carrier diffusion lengths of trihalide perovskite materials. The carrier diffusion length of MAPbI3 is increased to over 1 μm. The efficiency remains above 14.5% when the MAPbI3 thickness changes from 250 nm to 1 μm, with the highest efficiency reaching 15.6%.

Giant switchable photovoltaic effect in organometal trihalide perovskite devices

Organolead trihalide perovskite (OTP) materials are emerging as naturally abundant materials for low-cost, solution-processed and highly efficient solar cells. Here, we show that, in OTP-based photovoltaic devices with vertical and lateral cell configurations, the photocurrent direction can be switched repeatedly by applying a small electric field of <1 V μm(-1). The switchable photocurrent, generally observed in devices based on ferroelectric materials, reached 20.1 mA cm(-2) under one sun illumination in OTP devices with a vertical architecture, which is four orders of magnitude larger than that measured in other ferroelectric photovoltaic devices. This field-switchable photovoltaic effect can be explained by the formation of reversible p-i-n structures induced by ion drift in the perovskite layer. The demonstration of switchable OTP photovoltaics and electric-field-manipulated doping paves the way for innovative solar cell designs and for the exploitation of OTP materials in electrically and optically readable memristors and circuits.

Non-wetting surface-driven high-aspect-ratio crystalline grain growth for efficient hybrid perovskite solar cells

Large-aspect-ratio grains are needed in polycrystalline thin-film solar cells for reduced charge recombination at grain boundaries; however, the grain size in organolead trihalide perovskite (OTP) films is generally limited by the film thickness. Here we report the growth of OTP grains with high average aspect ratio of 2.3–7.9 on a wide range of non-wetting hole transport layers (HTLs), which increase nucleus spacing by suppressing heterogeneous nucleation and facilitate grain boundary migration in grain growth by imposing less drag force. The reduced grain boundary area and improved crystallinity dramatically reduce the charge recombination in OTP thin films to the level in OTP single crystals. Combining the high work function of several HTLs, a high stabilized device efficiency of 18.3% in low-temperature-processed planar-heterojunction OTP devices under 1 sun illumination is achieved. This simple method in enhancing OTP morphology paves the way for its application in other optoelectronic devices for enhanced performance.

Understanding the formation and evolution of interdiffusion grown organolead halide perovskite thin films by thermal annealing

Organolead trihalide perovskites are emerging excellent active materials for thin film solar cells. Here, the formation and evolution of methylammonium lead iodide perovskite thin films grown by the low-temperature thermal annealing induced interdiffusion process are investigated. It is found that thermal annealing not only drives the formation of perovskite but also affects the morphology, optoelectronic properties and correlated device performance. Thermal annealing at 105 °C quickly drives the formation of phase-pure perovskite in a short time of 15 min, and continued thermal annealing up to two hours continuously increases the perovskite crystallinity and grain size without losing film continuity or coverage. The measured Hall mobility increases monotonically to 36.0 cm2 V−1 s−1 upon annealing, which is correlated with the increased crystallinity and grain size. Device efficiencies increase with an increased short circuit current density and fill factor with a longer annealing time up to two hours, and the highest device efficiency of 13.4% is achieved. It is found that the reduction of work function in the perovskite films, caused by the increased annealing duration, is linearly correlated with the open circuit voltage loss, which points out a path for the further increase of the device efficiency.

Interfacial electronic structure at the CH3NH3PbI3/MoOx interface

Interfacial electronic properties of the CH3NH3PbI3 (MAPbI3)/MoOx interface are investigated using ultraviolet photoemission spectroscopy and X-ray photoemission spectroscopy. It is found that the pristine MAPbI3 film coated onto the substrate of poly (3,4-ethylenedioxythiophene) poly(styrenesulfonate)/indium tin oxide by two-step method behaves as an n-type semiconductor, with a band gap of ∼1.7 eV and a valence band edge of 1.40 eV below the Fermi energy (EF). With the MoOx deposition of 64 A upon MAPbI3, the energy levels of MAPbI3 shift toward higher binding energy by 0.25 eV due to electron transfer from MAPbI3 to MoOx. Its conduction band edge is observed to almost pin to the EF, indicating a significant enhancement of conductivity. Meanwhile, the energy levels of MoOx shift toward lower binding energy by ∼0.30 eV, and an interface dipole of 2.13 eV is observed at the interface of MAPbI3/MoOx. Most importantly, the chemical reaction taking place at this interface results in unfavorable interface ene...

Stabilized wide bandgap MAPbBrxI3-x perovskite by enhanced grain size and improved crystallinity

The light instability of CH3NH3PbIxBr3–x is one of the biggest challenges for its application in tandem solar cells. Here we show that an improved crystallinity and grain size of CH3NH3PbIxBr3–x films could stabilize these materials under one sun illumination, improving both the efficiency and stability of the wide‐bandgap perovskite solar cells.

Distinct exciton dissociation behavior of organolead trihalide perovskite and excitonic semiconductors studied in the same system.

The nonexcitonic character for organometal trihalide perovskites is demonstrated by examining the field-dependent exciton dissociation behavior. It is found that photogenerated excitons can be effectively dissociated into free charges inside perovskite without the assistance of charge extraction layer or external field, which is a stark contrast to the charge-separation behavior in excitonic materials in the same photovoltaic operation system.

Electronic structure evolution of fullerene on CH3NH3PbI3

The thickness dependence of fullerene on CH3NH3PbI3 perovskite film surface has been investigated by using ultraviolet photoemission spectroscopy (UPS), X-ray photoemission spectroscopy (XPS), and inverse photoemission spectroscopy (IPES). The lowest unoccupied molecular orbital and highest occupied molecular orbital (HOMO) can be observed directly with IPES and UPS. It is observed that the HOMO level in fullerene shifts to lower binding energy. The XPS results show a strong initial shift of core levels to lower binding energy in the perovskite, which indicates that electrons transfer from the perovskite film to fullerene molecules. Further deposition of fullerene forms C60 solid, accompanied by the reduction of the electron transfer. The strongest electron transfer happened at 1/4 monolayer of fullerene.