Effect of Alkali on the Efficiency and Reliability of Cu(In,Ga)Se2 Solar Cells

The incorporation of alkali metal has contributed tremendously in a bid to realize greater than 20% efficient Cu(In,Ga)Se2(CIGS) solar cells. Achieving high efficiency is one key parameter for the success of a photovoltaic technology but so is its long-term stability. In this thesis, the relationship between the performance of alkali treated Cu(In,Ga)Se2 solar cells and their physicochemical, electronic and structural properties are explored through a comparative study between standard devices and alkali (K, Rb) treated devices. The alkali treated devices tend to have a lower concentration of Ev+0.98 eV trap, higher majority carrier concentration and improved minority carrier lifetime, contributing to the experimentally observed improvement in open circuit voltage. Critical changes in alkali elemental profile occur throughout the film, while no other major physicochemical or structural properties are modified. Furthermore, we explored the long-term stability of CIGS solar cells due to damp heat treatment. We specifically study the influence on the molybdenum back contact and the CIGS absorber layer itself, with an emphasis on the role played by sodium. Molybdenum thin films showed drastic micro-structural, surface morphology, electrical and optical properties deterioration leading to the degradation of solar cell performance. In the case of bare CIGS thin films, we observed surface oxidation and degraded electronic properties, also leading to degradation of the solar cell performance. In both cases, alkali migration is responsible for the most part of the degradation, along with surface oxidation

Bandgap Profiling in CIGS Solar Cells Via Valence Electron Energy-Loss Spectroscopy

A robust, reproducible method for the extraction of relative bandgap trends from scanning transmission electron microscopy (STEM) based electron energy-loss spectroscopy (EELS) is described. The effectiveness of the approach is demonstrated by profiling the bandgap through a CuIn1-xGaxSe2 solar cell that possesses intentional Ga/(In + Ga) composition variation. The EELS-determined bandgap profile is compared to the nominal profile calculated from compositional data collected via STEM-based energy dispersive X-ray spectroscopy. The EELS based profile is found to closely track the calculated bandgap trends, with only a small, fixed offset difference. This method, which is particularly advantageous for relatively narrow bandgap materials and/or STEM systems with modest resolution capabilities (i.e., 100 meV), compromises absolute accuracy to provide a straightforward route for the correlation of local electronic structure trends with nanoscale chemical and physical structure/microstructure within semiconductor materials and devices. Published by AIP Publishing

Effect of heat stress on crossbred dairy cattle in tropical Nepal: Impact on blood parameters

Chitwan district of Nepal has been known as the dairy kingdom of the country considering its strategic location andpotential to easily provision inputs and produce as well as market quality milk across the country. However, theclimate especially during summer has been a challenge to farmers that potentially compromise the daily milk yieldof cows. Therefore, sixteen crossbred dairy cattle were placed in a RCBD fashion to assess the impacts of cold waterbathing in ameliorating effect of heat stress on their performance and body physiology. Routine assessment of themicroclimate within the experimental shed and blood parameters was made. The results from the experimentinferred that frequency of bathing (none, once, twice or thrice a day) did not have any significant impact onHaemoglobin, Haematocrit, Sodium, Potassium, Chloride and Bicarbonate levels in the blood plasma(p>0.05). Inaddition, the animals did not exhibit any signs of physiological distress clinically either. A detailed study scopingtemperature humidity index and over a number of other milk and blood parameters are to be tested across a numberof other available breeds too in order for the researchers to come to a meaningful strategy to beat the heat stress

Real-Time Optimization of Anti-Reflective Coatings for CIGS Solar Cells

A new method combining in-situ real-time spectroscopic ellipsometry and optical modeling to optimize the thickness of an anti-reflective (AR) coating for Cu(In,Ga)Se2 (CIGS) solar cells is described and applied directly to fabricate devices. The model is based on transfer matrix theory with input from the accurate measurement of complex dielectric function spectra and thickness of each layer in the solar cell by spectroscopic ellipsometry. The AR coating thickness is optimized in real time to optically enhance device performance with varying thickness and properties of the constituent layers. Among the parameters studied, we notably demonstrate how changes in thickness of the CIGS absorber layer, buffer layers, and transparent contact layer of higher performance solar cells affect the optimized AR coating thickness. An increase in the device performance of up to 6% with the optimized AR layer is demonstrated, emphasizing the importance of designing the AR coating based on the properties of the device structure

Characterization and Analysis of Ultrathin CIGS Films and Solar Cells Deposited by 3-Stage Process

In view of the large-scale utilization of Cu(In,Ga)Se2 (CIGS) solar cells for photovoltaic application, it is of interest not only to enhance the conversion efficiency but also to reduce the thickness of the CIGS absorber layer in order to reduce the cost and improve the solar cell manufacturing throughput. In situ and real-time spectroscopic ellipsometry (RTSE) has been used conjointly with ex situ characterizations to understand the properties of ultrathin CIGS films. This enables monitoring the growth process, analyzing the optical properties of the CIGS films during deposition, and extracting composition, film thickness, grain size, and surface roughness which can be corroborated with ex situ measurements. The fabricated devices were characterized using current voltage and quantum efficiency measurements and modeled using a 1-dimensional solar cell device simulator. An analysis of the diode parameters indicates that the efficiency of the thinnest cells was restricted not only by limited light absorption, as expected, but also by a low fill factor and open-circuit voltage, explained by an increased series resistance, reverse saturation current, and diode quality factor, associated with an increased trap density

Assessment of Cu(In, Ga)Se₂ Solar Cells Degradation Due to Water Ingress Effect on the CdS Buffer Layer

The effect of water ingress on the surface of the buffer layer of a Cu(In, Ga)Se2 (CIGS) solar cell was studied. Such degradation can occur either during the fabrication process, if it involves a chemical bath as is often the case for CdS, or while the modules are in the field and encapsulants degrade. To simulate the impact of this moisture ingress, devices with a structure sodalime glass/Mo/CIGS/CdS were immersed in deionized water. The thin films were then analyzed both pre and post water soaking. Dynamic secondary ion mass spectroscopy (SIMS) was performed on completed devices to analyze impurity diffusion (predominantly sodium and potassium) and to assess potential degradation mechanisms. The results were compared to device measurements, which indicate a degradation of all device parameters due to an increase in the total and peak trap densities, as shown by simulation. This is potentially due to a modification of the sodium profile in the bulk CIGS, with a decrease content after water soaking or because the oxygen profile increased in the bulk CIGS after water soaking

Analysis of Post-Deposition Recrystallization Processing via Indium Bromide of Cu(In,Ga)Se\u3csub\u3e2\u3c/sub\u3e Thin Films

Cu(In,Ga)Se2 (CIGS) thin films were deposited at low temperature (350 °C) and high rate (10 µm/h) by a single stage process. The effect of post-deposition treatments at 400 °C and 500 °C by indium bromide vapor were studied and compared to the effect of a simple annealing under selenium. Structural, electrical, and chemical analyses demonstrate that there is a drastic difference between the different types of annealing, with the ones under indium bromide leading to much larger grains and higher conductivity. These properties are associated with a modification of the elemental profiles, specifically for gallium and sodium