Yongli Gao

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.

Work function of indium tin oxide transparent conductor measured by photoelectron spectroscopy

We used ultraviolet and x‐ray photoelectron spectroscopy (XPS) and (UPS) techniques to directly measure absolute values of vacuum work function of indium tin oxide (ITO) thin films. We obtained a work function of 4.4–4.5 eV which is lower than the commonly cited value. These values do not change substantially by heating and Ar ion sputtering. The atomic concentrations of each element in ITO, measured with XPS, are also quite stable under heat treatment and ion sputtering.

Interfacial chemistry of Alq3 and LiF with reactive metals

The electronic structure and chemistry of interfaces between tris-(8-hydroxyquinoline) aluminum (Alq3) and representative group IA and IIA metals, Al, and Al/LiF have been studied by x-ray and ultraviolet photoelectron spectroscopies. Quantum-chemical calculations at the density functional theory level predict that the Alq3 radical anion is formed upon reaction with the alkali metals. In this case, up to three metal atoms can react with a given Alq3 molecule to form the trivalent anion. The anion formation results in a splitting of the N 1 s core level and formation of a new feature in the previously forbidden energy gap. Virtually identical spectra are observed in the Al/LiF/Alq3 system, leading to the conclusion that the radical anion is also formed when all three of these constituents are present. This is support by a simple thermodynamic model based on bulk heats of formation. In the absence of LiF or similar material, the reaction of Al with Alq3 appears to be destructive, with the deposited Al reacting directly with the quinolate oxygen. We proposed that in those circumstances where the radical anion is formed, it and not the cathode metal are responsible for the electron injection properties. This is borne out by producing excellent injecting contacts when Ag and Au are used as the metallic component of the cathode structure.

High Performance All-Polymer Solar Cell via Polymer Side-Chain Engineering

Acknowledge support from the Office of Naval Research (N00014-14-1-0142), KAUST Center for Advanced Molecular Photovoltaics at Stanford and the Stanford Global Climate and Energy Program, NSF DMR-1303742 and the National Natural Science Foundation of China (Projects 21174004 and 21222403). Soft X-ray characterization and analysis by NCSU supported by the U.S. Department of Energy, Office of Science, Basic Energy Science, Division of Materials Science and Engineering under Contract DE-FG02-98ER45737. Soft X-ray data was acquired at beamlines 11.0.1.2 at the Advanced Light Source, which is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U. S. Department of Energy under Contract No. DE-AC02-05CH11231. We thank Professor Michael D. McGehee, Dr. George F. Burkhard and Dr. Eric T. Hoke for their help in discussion of the recombination mechanism.

A photoelectron spectroscopy study on the indium tin oxide treatment by acids and bases

We report on the chemical adsorption of acids and bases on indium tin oxide (ITO). Ultraviolet photoelectron spectroscopy was used to measure the work function of the treated ITO and atomic surface concentrations were determined by x-ray photoelectron spectroscopy. The acid treatments yield work-function shifts as high as 0.7 eV compared to the nontreated ITO. Huge shifts in the work function are also obtained for the treatments with bases and are opposite to those obtained with the acids. These dramatic shifts are indicative of a double ionic surface layer. The importance of an appropriate plasma treatment prior to the chemical adsorption of acids or bases is discussed in terms of surface acido-basicity.

Electronic structure symmetry of interfaces between pentacene and metals

We examined the electronic structure of the interfaces formed between pentacene and various metals: Au, Ag, and Ca, using photoemission spectroscopy. For all interfaces, we observed the formation of an interface dipole that varied linearly with the measured metal work function. We observed that the behavior of the interfaces of Ca with pentacene was symmetric with respect to whether Ca was deposited on pentacene or vice versa. On the other hand, the interfaces of Au and Ag with pentacene were asymmetric. For both Au and Ag we saw indications that upon metal deposition, metal penetration resulted in a metallic surface with a work function significantly different from that of pure Au or Ag. This also resulted in different interface dipoles for these interfaces.

Qualifying composition dependent p and n self-doping in CH3NH3PbI3

We report the observation of self-doping in perovskite. CH3NH3PbI3 was found to be either n- or p-doped by changing the ratio of methylammonium halide (MAI) and lead iodine (PbI2) which are the two precursors for perovskite formation. MAI-rich and PbI2-rich perovskite films are p and n self-doped, respectively. Thermal annealing can convert the p-type perovskite to n-type by removing MAI. The carrier concentration varied as much as six orders of magnitude. A clear correlation between doping level and device performance was also observed.

The effect of molybdenum oxide interlayer on organic photovoltaic cells

Both small molecule and polymer photovoltaic cells were fabricated with molybdenum oxide interlayer at the indium tin oxide electrode. Enhancement in power efficiencies was observed in both small molecule and polymer cells. Specifically, the power conversion efficiencies of small molecule cells with the molybdenum oxide interlayer were enhanced by a maximum of 38% due to a significant enhancement in the fill factor. The improved fill factor is attributed to the reduction in series resistance. Our ultraviolet photoemission spectroscopy data indicate that the formation of band bending and the built-in field at the interface due to the interlayer leads to enhancement in hole extraction from the photoactive layer toward the anode.

Photoemission study of aluminum/tris-(8-hydroxyquinoline) aluminum and aluminum/LiF/tris-(8-hydroxyquinoline) aluminum interfaces

We have investigated the interfaces of aluminum on tris-(8-hydroxyquinoline) aluminum (Alq3) and aluminum on LiF/Alq3, using x-ray and ultraviolet photoemission spectroscopy (UPS). Aluminum appears to react destructively with Alq3 causing significant modification of the oxygen, nitrogen, and aluminum spectra. The well-defined UPS spectrum of Alq3 is quickly destroyed by very low coverages of aluminum. With only a 5 A layer of LiF on the Alq3, the reaction with aluminum is significantly suppressed. The Alq3 molecular orbital features in the UPS shift to higher binding energy but remain easily recognizable. In addition, a well-defined gap-state forms which is significantly different from that produced without LiF. Both the core-level spectra and the gap-state suggest that the Alq3 anion is formed in the presence of aluminum and LiF.

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.