Yabing Qi

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.

Thermal degradation of CH3NH3PbI3 perovskite into NH3 and CH3I gases observed by coupled thermogravimetry–mass spectrometry analysis

Thermal gravimetric and differential thermal analysis (TG-DTA) coupled with quadrupole mass spectrometry (MS) and first principles calculations were employed to elucidate the chemical nature of released gases during the thermal decomposition of CH3NH3PbI3. In contrast to the common wisdom that CH3NH3PbI3 is decomposed into CH3NH2 and HI, the major gases were methyliodide (CH3I) and ammonia (NH3). We anticipate that our findings will provide new insights into further formulations of the perovskite active material and device design that can prevent methylammonium decomposition and thus increase the long-term stability of perovskite-based optoelectronic devices.

n-Doping of organic electronic materials using air-stable organometallics.

Air-stable dimers of sandwich compounds including rhodocene and (pentamethylcyclopentadienyl)(arene)ruthenium and iron derivatives can be used for n-doping electron-transport materials with electron affinities as small as 2.8 eV. A p-i-n homojunction diode based on copper phthalocyanine and using rhodocene dimer as n-dopant shows a rectification ratio of greater than 10(6) at 4 V.

Universal energy level tailoring of self-organized hole extraction layers in organic solar cells and organic–inorganic hybrid perovskite solar cells

Tailoring the interface energetics between a polymeric hole extraction layer (HEL) and a photoactive layer (PAL) in organic photovoltaics (OPVs) and organic–inorganic hybrid perovskite solar cells (PrSCs) is very important to maximize open circuit voltage (Voc), power conversion efficiency (PCE), and device lifetime. In principle, when Fermi-level pinning and a vacuum level shift take place between the HEL and PAL, they give rise to an energy level offset between the HEL and the valence band maximum (VBM) (or the highly occupied molecular orbital (HOMO) in the case of organic photoactive materials) of the PAL and then Voc loss. However, here we show that the Voc loss at the interface can be overcome by universal energy level tailoring of a self-organized HEL (SOHEL) between the HEL and PAL irrespective of photoactive materials. A SOHEL composed of a conducting polymer and a perfluorinated ionomer (PFI) is effectively used to study the interface energetics in OPVs and PrSCs. We systematically tailored the interface energy level of the SOHEL to remove the energy offset at the interface and understand clearly the universal energy level alignment with the diverse photoactive materials of OPVs and PrSCs. The Fermi-level of the HEL is pinned to the midgap state of photoactive materials, which is about 0.6–0.7 eV above the VBM or HOMO. However, the interface energy state of the PFI-enriched surface layer of the SOHEL can be formed deeper below the Fermi-level by self-organized molecules so that it can match the top of the valence band of the photoactive materials. As a result, the energy offset at the interface between photoactive materials and the SOHEL can be significantly decreased to achieve high Voc and PCE. Furthermore, our SOHEL significantly prolonged the stability of OPVs (half lifetime: 2.84 year) compared with pristine PEDOT:PSS (half lifetime: 0.2 year) under continuous irradiation of air mass-1.5 global simulated sunlight at 100 mW cm−2 due to the diffusion-blocking ability of the self-organized PFI at the surface of SOHELs for impurities from indium tin oxide.

Large formamidinium lead trihalide perovskite solar cells using chemical vapor deposition with high reproducibility and tunable chlorine concentrations

Chemical vapor deposition is an inexpensive way to batch-process solar cells with good uniformity and facilitates low-cost production. Formamidinium lead iodide perovskite has a smaller energy band gap and greater potential efficiency than the widely studied methylammonium lead iodide perovskite and better temperature stability. This work is the first demonstration of vapor deposition of formamidinium-based perovskite. A self-limiting perovskite formation process is recommended, with efficiencies as high as 14.2% and stability up to 155 days after fabrication. Using this process, a batch of semi-transparent solar cells with a large area of 1 cm2 was fabricated. We monitored the growth of perovskite in real time and provide insight that may not be accessible for a solution based process. We directly measured chlorine in perovskite films and correlated the concentration of chlorine with efficiency and stability.

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.

Solution doping of organic semiconductors using air-stable n-dopants

Solution-based n-doping of the polymer poly{[N,N′-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-bithiophene)} [P(NDI2OD-T2)] and the small molecule 6,13-bis(tri(isopropyl)silylethynyl)pentacene (TIPS-pentacene) is realized with the air-stable dimers of rhodocene, [RhCp2]2, and ruthenium(pentamethylcyclopentdienyl)(1,3,5-triethylbenzene), [Cp*Ru(TEB)]2. Fermi level shifts, measured by direct and inverse photoemission spectroscopy, and orders of magnitude increase in current density and film conductivity point to strong n-doping in both materials. The strong reducing power of these air-stable dopants is demonstrated through the n-doping of TIPS-pentacene, a material with low electron affinity (3.0 eV). Doping-induced reduction of the hopping transport activation energy indicates that the increase in film conductivity is due in part to the filling of deep gap states by carriers released by the dopants.

Pinhole-free hole transport layers significantly improve the stability of MAPbI3-based perovskite solar cells under operating conditions

Pinhole-free 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene (spiro-MeOTAD) hole transport layers (HTLs) were deposited on perovskite films. MAPbI3-based perovskite solar cells employing the pinhole-free HTL showed a prolonged lifetime under one sun and was operated at the maximum power point. The solar cell architecture (planar versus mesoporous-layers) was also observed to strongly influence the cells stability.