Shenghao Wang

High performance perovskite solar cells by hybrid chemical vapor deposition

Organometal halide based perovskites are promising materials for solar cell applications and are rapidly developing with current devices reaching ∼19% efficiency. In this work we introduce a new method of perovskite synthesis by hybrid chemical vapor deposition (HCVD), and demonstrate efficiencies as high as 11.8%. These cells were found to be stable with time, and retained almost the same efficiency after approximately 1100 h storage in dry N2 gas. This method is particularly attractive because of its ability to scale up to industrial levels and the ability to precisely control gas flow rate, temperature, and pressure with high reproducibility. This is the first demonstration of a perovskite solar cell using chemical vapor deposition and there is likely still room for significant optimization in efficiency.

Fabrication of semi-transparent perovskite films with centimeter-scale superior uniformity by the hybrid deposition method

We report the development of instrumentation and methodology for fabricating large area semi-transparent organo-lead-halide perovskite films. In our method, the growth of perovskite films relies on the control of CH3NH3I flow and vapor pressure inside a vacuum chamber. Solar cell devices based on the prepared semi-transparent perovskite films as thin as ∼135 nm achieved an efficiency of 9.9% and a high open circuit voltage of 1.09 V.

Organometal halide perovskite thin films and solar cells by vapor deposition

Organometal halide perovskites (OHPs) are currently under the spotlight as promising materials for new generation low-cost, high-efficiency solar cell technology. Within a few years of intensive research, the solar energy-to-electricity power conversion efficiency (PCE) based on OHP materials has rapidly increased to a level that is on par with that of even the best crystalline silicon solar cells. However, there is plenty of room for further improvements. In particular, the development of protocols to make such a technology applicable to industry is of paramount importance. Vapor based methods show particular potential in fabricating uniform semitransparent perovskite films across large areas. In this article, we review the recent progress of OHP thin-film fabrication based on vapor based deposition techniques. We discuss the instrumentation and specific features of each vapor-based method as well as its corresponding device performance. In the outlook, we outline the vapor deposition related topics that warrant further investigation.

Temperature-dependent hysteresis effects in perovskite-based solar cells

Staircase voltage sweep measurements were performed on a perovskite solar cell at 250 K, 300 K, and 360 K. Time-dependent photocurrent data reveal the complexity of the signal that cannot be described by a simple mono-exponential function, suggesting that multiple charging–discharging processes are responsible for the complex hysteresis behavior.

Smooth perovskite thin films and efficient perovskite solar cells prepared by the hybrid deposition method

We provide details on the development of instrumentation and methodology to overcome the common difficulties that the vacuum-related techniques face for fabrication of perovskite thin films and perovskite solar cells (PSCs). Our methodology relies on precisely controlling the flow of methylammonium iodide (CH3NH3I, MAI), which has a high-vapor pressure nature, and the deposition rate of metal halides (PbCl2 or PbI2). This hybrid deposition method allows the growth of perovskite films with smooth surface, good crystallinity, high surface coverage, uniform chemical composition and semi-transparency. We also systematically investigated the effects of the evaporation source materials (PbCl2 : MAI versus PbI2 : MAI), substrate temperatures, and post-annealing on the properties of perovskite films, as well as device performances based on this method. By employing a thin perovskite film (<200 nm), the power conversion efficiency of PSC can be as high as 11.5%.

Post-annealing of MAPbI3 perovskite films with methylamine for efficient perovskite solar cells

An organo-metal halide perovskite is a promising material for solar cell applications, but the polycrystalline nature of perovskites can cause thin films to be non-uniform with disconnected grains. These grain boundaries make the perovskite film vulnerable to the local chemical environment, or allow unwanted direct contact of the electron transporting layer and the hole transporting layer, increasing carrier recombination. We show that post-annealing with methylamine greatly reduces impurities at perovskite grain boundaries and promotes continuity between adjacent grains. When methylamine post-annealed perovskite films are compared to thermally or solvent-annealed films, the carrier lifetime is increased by 3 times. The recombination resistance for the planar perovskite solar cells with the methylamine post-annealing treatment is increased more than 10 times, and the efficiency is increased by 43.1% and 20.0% with respect to the thermally annealed and solvent-annealed perovskite solar cells, respectively. In addition, we show that methylamine post-annealed, meso-structured perovskite solar cells exhibited a power conversion efficiency of up to 18.4%, with significantly improved stability.

Energy band bending induced charge accumulation at fullerene/bathocuproine heterojunction interface

The electronic properties of fullerene (C60)/bathocuproine (BCP)/Ag heterostructures were studied as a function of the BCP layer thickness by photoemission spectroscopy. For the thin BCP layer, the energy levels are flat and gap states exist at the interface. In contrast, energy band bending occurs at the C60/BCP interface when the BCP layer is thick, resulting in a considerable barrier for electron transport and therefore causing charge accumulation in organic solar cells. The results reveal that a thin BCP layer gives a much more favorable energy level structure and conform that charge accumulation is responsible to the anomalous current-voltage (I-V) curve.

The presence of CH3NH2 neutral species in organometal halide perovskite films

We report the presence of CH3NH2 neutral species not only on the surface but also at grain boundaries in the interior of thin polycrystalline films of methylammonium lead iodide perovskite CH3NH3PbI3 (thickness ∼ 50 nm) that were prepared using a standard solution method. Different chemical states for C K-edge were observed at the surfaces and in the interiors of perovskite films. Salient features of σ*(CH3-NH3+: methylammonium cation) and σ*(CH3-NH2: methylamine neutral species) were observed at 290.3 and 292.8 eV in both partial (surface-sensitive) and total (bulk) electron yield modes by near-edge x-ray absorption fine structure measurements. Consistently, two chemical states originated from CH3NH3+ and CH3NH2 in C 1s and N 1s core-level spectra were observed using high-resolution x-ray photoelectron spectroscopy. Using density functional theory calculations, we show that CH3NH2 cannot reside stably in the MAPbI3 perovskite crystal structure. Therefore, we propose that these CH3NH2 neutral species are ...

Engineering Interface Structure to Improve Efficiency and Stability of Organometal Halide Perovskite Solar Cells

The rapid rise of power conversion efficiency (PCE) of low cost organometal halide perovskite solar cells suggests that these cells are a promising alternative to conventional photovoltaic technology. However, anomalous hysteresis and unsatisfactory stability hinder the industrialization of perovskite solar cells. Interface engineering is of importance for the fabrication of highly stable and hysteresis free perovskite solar cells. Here we report that a surface modification of the widely used TiO2 compact layer can give insight into interface interaction in perovskite solar cells. A highest PCE of 18.5% is obtained using anatase TiO2, but the device is not stable and degrades rapidly. With an amorphous TiO2 compact layer, the devices show a prolonged lifetime but a lower PCE and more pronounced hysteresis. To achieve a high PCE and long lifetime simultaneously, an insulating polymer interface layer is deposited on top of TiO2. Three polymers, each with a different functional group (hydroxyl, amino, or aromatic group), are investigated to further understand the relation of interface structure and device PCE as well as stability. We show that it is necessary to consider not only the band alignment at the interface, but also interface chemical interactions between the thin interface layer and the perovskite film. The hydroxyl and amino groups interact with CH3NH3PbI3 leading to poor PCEs. In contrast, deposition of a thin layer of polymer consisting of an aromatic group to prevent the direct contact of TiO2 and CH3NH3PbI3 can significantly enhance the device stability, while the same time maintaining a high PCE. The fact that a polymer interface layer on top of TiO2 can enhance device stability, strongly suggests that the interface interaction between TiO2 and CH3NH3PbI3 plays a crucial role. Our work highlights the importance of interface structure and paves the way for further optimization of PCEs and stability of perovskite solar cells.

Improvement of Stability for Small Molecule Organic Solar Cells by Suppressing the Trap Mediated Recombination

To understand the degradation mechanism of organic solar cells (OSCs), the charge dynamics of conventional and inverted planar heterojunction OSCs based on boron subthalocyanine chloride (SubPc) and fullerene (C60) with identical buffers during the air exposure were investigated. The results of light intensity dependent open circuit voltage show that the bimolecular recombination is dominated in the fresh devices, regardless of the device structure. The appearance of transient peak in photocurrent after turn-on and the light intensity independent turn-off traces in transient photocurrent suggest that the rapid degradation of conventional device is due to the energy loss originated from the aggravated trap mediated recombination. In contrast, the half-lifetime of inverted device is ∼25 times longer than the conventional one. The improvement of stability is ascribed to the decrease of the trap generation possibility and the suppression of trap mediated recombination in the case of inverted structure, where the penetration of oxygen and water through buffer layer is avoided.