Deposition Of Perovskite Films Research Articles

Direct Evidence of Chlorine-Induced Preferential Crystalline Orientation in Methylammonium Lead Iodide Perovskites Grown on TiO2

Mixed halide hybrid perovskites CH3NH3PbI3–xClx (MAPICl) show higher charge carrier diffusion lengths, improved solar cell characteristics, and enhanced stability as compared to their single halide counterparts CH3NH3PbI3 (MAPI). This is surprising in view of the fact that the actually observed Cl content is very low (x < 0.1) as the incorporation of large amounts of Cl in the MAPI lattice is crystallographically not possible. Understanding the role of chlorine in these systems is therefore of the utmost importance and has been the subject of several experimental and theoretical studies. The main conclusions are that Cl addition leads to larger grain sizes and preferential crystalline orientation; however, so far a more quantitative description of these aspects is lacking. Synchrotron X-ray diffraction microscopy is a very promising technique in this respect, as it allows for the direct imaging of crystalline domains oriented in a selected direction with a resolution of some hundreds of nanometers. When imaging perovskite thin films deposited on an FTO-covered glass substrate containing a dense polycrystalline TiO2 layer at the perovskite (220) reflection, diffraction micrographs show an increase in surface coverage of oriented grains from 2.5% (MAPI) to 5.5% (MAPICl; x = 0.07). This is a clear evidence of the increased preferential orientation in the mixed perovskite. At the same time the integrated intensity increases by a factor of 4, which can be related to a larger perovskite grain size. When a single crystalline TiO2(001) substrate is used for the perovskite film deposition, both effects are dramatically enhanced in the case of MAPICl, leading to a surface coverage of 80%, a 55-fold increase in integrated diffracted intensity with respect to MAPI, and a mean crystallite size of 2.5 μm. In contrast, MAPI does not show any differences when grown on TiO2(001), which demonstrates that chlorine locates preferentially at the TiO2–perovskite interface and induces the growth of larger crystallites showing preferential orientation along the (110) direction.

Perovskite ink with wide processing window for scalable high-efficiency solar cells

Perovskite solar cells have made tremendous progress using laboratory-scale spin-coating methods in the past few years owing to advances in controls of perovskite film deposition. However, devices made via scalable methods are still lagging behind state-of-the-art spin-coated devices because of the complicated nature of perovskite crystallization from a precursor state. Here we demonstrate a chlorine-containing methylammonium lead iodide precursor formulation along with solvent tuning to enable a wide precursor-processing window (up to ∼8 min) and a rapid grain growth rate (as short as ∼1 min). Coupled with antisolvent extraction, this precursor ink delivers high-quality perovskite films with large-scale uniformity. The ink can be used by both spin-coating and blade-coating methods with indistinguishable film morphology and device performance. Using a blade-coated absorber, devices with 0.12-cm2 and 1.2-cm2 areas yield average efficiencies of 18.55% and 17.33%, respectively. We further demonstrate a 12.6-cm2 four-cell module (88% geometric fill factor) with 13.3% stabilized active-area efficiency output. Perovskite-based solar cells are often fabricated by methods that are not industrially scalable. Here, Yang et al. develop an ink formulation which gives similar devices by spin coating, the lab-scale standard, and blade coating, which is a more scalable, industry-relevant deposition method.

Crystallization of a perovskite film for higher performance solar cells by controlling water concentration in methyl ammonium iodide precursor solution.

An optimal small amount of water added into methyl ammonium iodide (MAI) solution in isopropyl alcohol (IPA) helps perovskite crystallization and leads to larger grain size from sequential deposition of perovskite films. The concentration of water was varied from 1% to 7% (vol% of IPA) in MAI solution and optical absorption, crystallization, morphology of perovskite films and their photovoltaic performance were studied in perovskite solar cells. 5% by volume was found to lead to preferential crystallization in the (110) plane with grain size about three times that of perovskite films prepared without adding water into the MAI solution. The optimal water concentration of 5% by volume in the MAI solution led to average perovskite grain size of ∼600 nm and solar cell efficiency of 12.42% at forward scan with a rate of 0.5 V s(-1). Device performance decreases after increasing water concentration beyond 5% in the MAI solution due to formation of the PbI2 phase. Transient photocurrent and photovoltage measurements show the shortest charge transport time at 0.99 μs and the longest charge carrier life time at 13.6 μs for perovskite films prepared from 5% water in MAI solution, which improved perovskite solar cell efficiency from 9.04% to 12.42%.

Beyond Efficiency: the Challenge of Stability in Mesoscopic Perovskite Solar Cells

Over the past five years, the rapid emergence of a new class of solar cell based on mixed organic–inorganic halide perovskite semiconductors has captured the attention of scientists and researchers in the field of energy conversion. Benefiting from the optimization of perovskite film deposition approaches, the design of new material systems, and the diversity of device concepts, the efficiency of perovskite solar cells (PSCs) has increased from 2.19% in 2006 to a certified 20.1% in 2014, making this the fastest‐advancing solar cell technology to date. However, as a photovoltaic technology, which needs to meet the requirements of working under long‐term sunlight, PSCs suffer stability concerns for both materials and devices. Evolved from dye‐sensitized solar cells (DSSCs), PSCs usually contain a mesoporous electron transporting layer or scaffold layer, a perovskite active layer, a hole transporting layer and a back contact to construct a mesoscopic‐structured device. Using interface engineering, mesoscopic PSCs (MPSCs) have obtained exciting stability with a hole‐conductor‐free printable triple‐layer architecture or conventional heterojunction version. Herein, the achievements of mesoscopic solar cells from solid‐state DSSCs to MPSCs are outlined and summary of recent progress in the stability of MPSCs is presented. Possible degradation mechanism and solutions are presented and, finally, challenges for the commercialization of this photovoltaic technology are discussed.

(Invited) Uniform Nanostructures for Highly Efficient and Reproducible Perovskite Solar Cells

Organo-lead trihalide perovskites (CH3NH3PbX3, X = I, Br or Cl) as light absorption materials have attracted great attention recently in photovoltaic research. By virtue of their high absorptivity and long carrier diffusion length, the power conversion efficiency (PCE) was rapidly increased from 3.8% to above 17% within the short five years. In this presentation, I will report some our latest researches in this field. Firstly, the hole-blocking layer is required to prevent holes formed in the perovskie or HTL layer from reaching the FTO electrode, which would otherwise short-circuit the cell. Optimization of the blocking layer is a key factor for improving the device performance of perovskite solar cells. We systematically examined the different blocking layer fabricated by three different methods. The surface morphology and film resistance shows that atomic layer deposition (ALD) based TiO2 compact layer contains much lower density of nano-scaled pinholes with respect to spin-coating and spray pyrolysis ones. The ALD-TiO2 layer acts as an efficient hole-blocking layer in perovskite solar cells, which offers a large shunt resistance and enables a high power conversion efficiency of 12.56%. Secondly, uniform, highly crystalline perovskite film is important for sequential deposition processed perovskite solar cells. We develop a new method to enable an efficient deposition of perovskite film with enhanced conversion rate and surface morphology. By this approach, the energy conversion efficiency of mesoporous-free perovskite solar cells was twice as high as that of using traditional method. Figure 1

Laser Processing of Novel Functional Materials

Laser processing and laser ablation are the subject of fascinating fundamental studies and are used in various industrial processes. The applications of laser ablation range from microstructuring of nearly all materials, to the production of thin crystalline films with special properties, and micro-spectroscopy. Our projects in the field of laser ablation encompass both applied and fundamental research. In detail, we are working on the design and structuring of novel photopolymers for laser applications, on the development of advanced methods for the microstructuring of glassy carbon, and the pulsed laser deposition of perovskite films used in electrocatalysis. Additionally, emission or plasma spectroscopy is used to study the pulsed laser deposition process, and as tool for trace analysis of heterogeneous samples.