Sungeun Park

UV Degradation and Recovery of Perovskite Solar Cells

Although the power conversion efficiency of perovskite solar cells has increased from 3.81% to 22.1% in just 7 years, they still suffer from stability issues, as they degrade upon exposure to moisture, UV light, heat, and bias voltage. We herein examined the degradation of perovskite solar cells in the presence of UV light alone. The cells were exposed to 365 nm UV light for over 1,000 h under inert gas at <0.5 ppm humidity without encapsulation. 1-sun illumination after UV degradation resulted in recovery of the fill factor and power conversion efficiency. Furthermore, during exposure to consecutive UV light, the diminished short circuit current density (Jsc) and EQE continuously restored. 1-sun light soaking induced recovery is considered to be caused by resolving of stacked charges and defect state neutralization. The Jsc and EQE bounce-back phenomenon is attributed to the beneficial effects of PbI2 which is generated by the decomposition of perovskite material.

Electric-Field-Induced Degradation of Methylammonium Lead Iodide Perovskite Solar Cells

Perovskite solar cells have great potential for high efficiency generation but are subject to the impact of external environmental conditions such as humidity, UV and sun light, temperature, and electric fields. The long-term stability of perovskite solar cells is an important issue for their commercialization. Various studies on the stability of perovskite solar cells are currently being performed; however, the stability related to electric fields is rarely discussed. Here the electrical stability of perovskite solar cells is studied. Ion migration is confirmed using the temperature-dependent dark current decay. Changes in the power conversion efficiency according to the amount of the external bias are measured in the dark, and a significant drop is observed only at an applied voltage greater than 0.8 V. We demonstrate that perovskite solar cells are stable under an electric field up to the operating voltage.

Relationship between ion migration and interfacial degradation of CH 3 NH 3 PbI 3 perovskite solar cells under thermal conditions

Organic-inorganic hybrid perovskite solar cells (PSCs) have been extensively studied because of their outstanding performance: a power conversion efficiency exceeding 22% has been achieved. The most commonly used PSCs consist of CH3NH3PbI3 (MAPbI3) with a hole-selective contact, such as 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9-spiro-bifluorene (spiro-OMeTAD), for collecting holes. From the perspective of long-term operation of solar cells, the cell performance and constituent layers (MAPbI3, spiro-OMeTAD, etc.) may be influenced by external conditions like temperature, light, etc. Herein, we report the effects of temperature on spiro-OMeTAD and the interface between MAPbI3 and spiro-OMeTAD in a solar cell. It was confirmed that, at high temperatures (85 °C), I− and CH3NH3+ (MA+) diffused into the spiro-OMeTAD layer in the form of CH3NH3I (MAI). The diffused I− ions prevented oxidation of spiro-OMeTAD, thereby degrading the electrical properties of spiro-OMeTAD. Since ion diffusion can occur during outdoor operation, the structural design of PSCs must be considered to achieve long-term stability.

Effect of high-temperature annealing on ion-implanted silicon solar cells

P-type and n-type wafers were implanted with phosphorus and boron, respectively, for emitter formation and were annealed subsequently at 950~1050 for 30~90 min for activation. Boron emitters were activated at or higher, while phosphorus emitters were activated at . QSSPC measurements show that the implied of boron emitters increases about 15 mV and the decreases by deep junction annealing even after the activation due to the reduced recombination in the emitter. However, for phosphorus emitters the implied decreases from 622 mV to 560 mV and the increases with deep junction annealing. This is due to the abrupt decrease in the bulk lifetime of the p-type wafer itself from 178 μs to 14 μs. PC1D simulation based on these results shows that, for p-type implanted solar cells, increasing the annealing temperature and time abruptly decreases the efficiency (%), while, for n-type implanted solar cells, deep junction annealing increases the efficiency and , especially (%) for backside emitter solar cells.

Estimation of Interfacial Fixed Charge at Al2O3/SiO2 Using Slant-Etched Wafer for Solar Cell Application

Al2O3 with a negative fixed charge prepared by atomic layer deposition has been reported to improve surface passivation properties for Si-solar cell applications. The high negative fixed charge at the SiO2/Al2O3 interface facilitates a low surface recombination velocity and a high effective lifetime, which result in greater performance. In this study, we adopted an effective method using a slant-etched sample with various thicknesses of SiO2 to estimate the charge densities of both the bulk and interface of Al2O3 deposited by atomic layer deposition. We found a direct correlation between lifetime and total charge density in Al2O3, which are strong functions of film thickness and annealing condition.

Phase Transformation of Single Crystalline Silicon by Scratching

The relationship between the phase transformation and the stress applied on single-crystalline silicon wafer was studied by performing a scratch test. The cracks and phases were observed by optical microscopy and micro Raman spectroscopy, respectively. Various silicon polymorphs and crack shapes were observed as functions of speed and direction of the scratching, respectively. Atomic force microscopy was also carried out to analyze the morphology of the scratch grooves.

Identification of lifetime limiting defects by temperature- and injection-dependent photoluminescence imaging

Identification of the lifetime limiting defects in silicon plays a key role in systematically optimizing the efficiency potential of material for solar cells. We present a technique based on temperature and injection dependent photoluminescence imaging to determine the energy levels and capture cross section ratios of Shockley–Read–Hall defects. This allows us to identify homogeneously and inhomogeneously distributed defects limiting the charge carrier lifetime in any silicon wafer. The technique is demonstrated on an n-type wafer grown with the non-contact crucible (NOC) method and an industrial Czochralski (Cz) wafer prone to defect formation during high temperature processing. We find that the energy levels for the circular distributed defects in the Cz wafer are in good agreement with literature data for homogeneously grown oxide precipitates. In contrast, the circular distributed defects found in NOC Si have significantly deeper trap levels, despite their similar appearance.

Improvement of electrical properties in screen-printed crystalline silicon solar cells by contact treatment of the grid edge

In this study, the influence of HF treatment of Ag pastes after a firing process was investigated. It was shown that the HF treatment can improve the fill factors and efficiencies of various cells including those with high initial specific contact resistances. SEM images showed that this improvement is due to the etching of the thin glass layer at the Ag-Si boundary, which exposes the Ag crystallites and colloids. These colloids electrically connect the bulk Ag to the Si through a direct contact, which reduces both the transfer length and the specific contact resistance. A model of the current path was proposed to explain the effect of HF treatment on the edge of the Ag grid.

Effects of rapid thermal process on the junction properties of aluminum rear emitter solar cells

N-type silicon with aluminum emitters for rear junctions was studied; aluminum back surface fields were replaced with n-type silicon wafers. Aluminum rear emitters for n-type silicon solar cells were studied with various rapid thermal processing conditions. With fast ramping-up and fast cooling, an aluminum rear junction was formed uniformly with low emitter recombination current. The effects of junction quality on solar cell efficiency were investigated.

A Study on the Optimization of the SiN x :H Film for Crystalline Silicon Sloar Cells

The Hydrogenated silicon nitride (SiNx:H) using plasma enhanced chemical vapor deposition is widely used in photovoltaic industry as an antireflection coating and passivation layer. In the high temperature firing process, the film should not change the properties for its use as high quality surface layer in crystalline silicon solar cells. Initially PECVD- film trends were investigated by varying the deposition parameters (temperature, electrode gap, RF power, gas flow rate etc.) to optimize the process parameter conditions. Then by varying gas ratios (), the hydrogenated silicon nitride films were analyzed for its optical, electrical, chemical and surface passivation properties. The films of refractive indices 1.90~2.20 were obtained. The film deposited with the gas ratio of 3.6 (Refractive index