Thermal Assisted Oxygen Annealing for High Efficiency Planar CH3NH3PbI3 Perovskite Solar Cells

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Crystal Engineering for Low Defect Density and High Efficiency Hybrid Chemical Vapor Deposition Grown Perovskite Solar Cells

Synthesis of high quality perovskite absorber is a key factor in determining the performance of the solar cells. We demonstrate that hybrid chemical vapor deposition (HCVD) growth technique can provide high level of versatility and repeatability to ensure the optimal conditions for the growth of the perovskite films as well as potential for batch processing. It is found that the growth ambient and degree of crystallization of CH3NH3PbI3 (MAPI) have strong impact on the defect density of MAPI. We demonstrate that HCVD process with slow postdeposition cooling rate can significantly reduce the density of shallow and deep traps in the MAPI due to enhanced material crystallization, while a mixed O2/N2 carrier gas is effective in passivating both shallow and deep traps. By careful control of the perovskite growth process, a champion device with power conversion efficiency of 17.6% is achieved. Our work complements the existing theoretical studies on different types of trap states in MAPI and fills the gap on the theoretical analysis of the interaction between deep levels and oxygen. The experimental results are consistent with the theoretical predictions.Department of Electronic and Information Engineerin

Enhanced Performance of PTB7:PC71BM Solar Cells via Different Morphologies of Gold Nanoparticles

The effects of gold nanoparticles (AuNPs) incorporated in the hole transporting layer (HTL) of poly[[4,8-bis[(2-ethylhexyl)oxy] benzo[1,2-b:4,5-b'] dithiophene-2, 6-diyl] [3-fluoro-2-[(2-ethylhexy)carbonyl]thieno[3,4-b]thiophened iyl]] (PTB7): [6,6]-phenyl C-71 butyric acid methyl ester (PC71BM) based solar cells are being systematically investigated in terms of the optical properties, electrical properties, and photovoltaic performance. The impacts of AuNPs on the optical response of the devices are modeled by finite-difference time-domain (FDTD) simulation. The size of the AuNPs used in this work is around 50-70 nm, so that 10-20 nm penetrated from the HTL into the active layer. We found that the power conversion efficiencies (PCEs) of the devices with AuNPs are significantly enhanced from 7.5%, for the control device, to 8.0%, 8.1%, and 8.2% for Au nanosphere-, nanorod-, and nanocube-incorporated devices, respectively. Among the photovoltaic parameters of the AuNP devices, the short circuit current density (JSC) exhibits the largest improvement, which can be attributed to the improved optical properties of the devices. On the basis of the calculation results, the scattering cross section for the samples in the presence of AuNPs can be enhanced by a factor of similar to 10(10)-10(13) and Au nanocubes exhibit superior scattering cross section compared to the Au nanospheres and nanorods with the same linear dimension. From the experimental impedance spectroscopy results, we found that the addition of AuNPs had little effect on the electrical properties of the device. The device performance is also found to be sensitive to the concentration and morphology of the AuNPs.Department of Electronic and Information Engineerin

Thermal assisted oxygen annealing for high efficiency planar CH3NH3PbI3 Perovskite solar cells

2014-2015 > Academic research: refereed > Publication in refereed journalVersion of RecordPublishe

Efficiency enhancement by defect engineering in perovskite photovoltaic cells prepared using evaporated PbI2/CH3NH3I multilayers

We report, for the first time, on the synthesis of perovskite films by thermal annealing of evaporated lead(ii) iodide (PbI2)/methylammonium iodide (CH3NH3I) multilayers. Detailed characterization of the resulting films is presented. Our work demonstrates that compact, high quality and uniform perovskite films can be grown using this technique. Optimization of the device structure was achieved by careful design of the layer thickness and the number of PbI2/CH3NH3I pairs used in the formation of the absorber layer. Utilizing additional annealing steps in a controlled atmosphere was shown to result in significant improvement in the device performance. Our experimental data indicate that O2 treatments may result in substantial reduction in the trap density of the device and thereby significant improvement in the lifetimes of the carriers. A high power conversion efficiency (PCE) of 12.5% was recorded for the champion device.Department of Applied PhysicsDepartment of Electronic and Information EngineeringDepartment of Mechanical Engineerin

Age-related hyperkyphosis: update of its potential causes and clinical impacts—narrative review

The present study aims to qualitatively review the contributing factors and health implications of age-related hyperkyphosis. We conducted a narrative review of observational and cohort studies describing the risk factors and epidemiology of hyperkyphosis from 1955 to 2016 using the following key words: kyphosis, hyperkyphosis, posture, age-related hyperkyphosis, kyphotic posture, aetiology and causes. This review included 77 studies. Approximately 60–70 % of the most severe hyperkyphosis cases have no evidence of underlying vertebral compression fractures. Other proposed factors contributing to hyperkyphosis are degenerative disc disease, weakness of back extensor muscles and genetic predisposition. Strength and endurance of back extensor muscles are very important for maintaining normal postural alignment. Recent evidence suggests that age-related hyperkyphosis is not equivalent to spinal osteoporosis. Due to the negative impact of hyperkyphosis on physical function, quality of life and mortality rates, physicians should focus not only on osteoporosis, but also on age-related postural changes. More research about the relationship between spinal morphology and modifiable factors, especially the structural and functional parameters of trunk muscles, could further illuminate our understanding and treatment options for hyperkyphosis