Wong Yuen-fai, Dick. / Includes bibliographical references / Thesis (M.Ph.)--Chinese University of Hong Kong, 1985

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Graduation date: 1991

It is widely known that the element sodium plays a vital role in providing highefficiency CIGS solar cells and that when cells are built on sodium free substrates theyneed an alternative (a substitute) sodium source. In this study a molybdenum-sodiumcompound has been deposited, investigated and evaluated as an alternative backcontact layer containing sodium. The compound had a 5 at % sodium concentrationand it was manufactured by an Austrian company called Plansee. The aim of the studywas to create an equivalent back contact in the sense of sodium delivery, conductivityand adhesion compared to a normal molybdenum back contact on a soda lime glass. The experimental part of the study started with the construction of complete cells,which were fabricated and measured. This work took place at the ÅngströmLaboratory, Uppsala University, Sweden. The characteristics of the layer and the cellswere analyzed by current voltage measurements, quantum efficiency measurementsand secondary ion mass spectrometry analysis. Cell manufacturing involved sputtering,co evaporation and chemical deposition processes. Results show that the molybdenum-sodium compound increases the efficiency of acell built on a sodium-free substrate. Efficiencies reached 8 % for cells without sodiumin the molybdenum and these cells produced 67 % efficiency and 80 % open circuitvoltage of the reference value. Cells with sodium in the back contact layer produced90 % of the efficiency and 95% of the open circuit voltage relative to the references.The best cell with the molybdenum-sodium compound reached an efficiency of 13.3%. This implies that the new back contact layer acts as a sodium source but the cellshave 1-2 % lower efficiency than the reference cells built on soda lime glass. Othercharacteristics of the layer as conductivity and adhesion show no significant differenceto an ordinary molybdenum back contact. Measurements also indicate that the sodium is probably located inside themolybdenum grains and just a small amount is found at the boundaries and in betweenthe grains. Sodium inside the molybdenum grains is difficult to extract and thereforenot enough sodium will diffuse into the CIGS layer. The conclusions drawn from this study are that the molybdenum-sodium compoundhelps to increase the efficiency of a CIGS solar cell built on a sodium-free substrate,but it does not deliver enough sodium to constitute a substitute sodium source.

We had demonstrated that, by controlling and changing the temperature, reaction time and washing agents, the morphology and powder size of CuInSe2 can be altered considerably. CuInSe2 was synthesized by a solvothermal route as described by Y. Qian et al. By synthesizing nanorods, we can control the distance which the electrons are transported in hybrid solar cell with conjugated polymers. With the tuning of the band gap by controlling the nanorod length, we can increase the opportunity of orbits overlap between the CuInSe2 material and the PBO polymer matrix.. The synthetic temperature 1800C and reaction time 48 hrs are the best condition in our experiment. By using D.I water and ethanol as washing agents, we can find different morphology of spherical and nanowire. However, we fabricate hybrid solar cell by using these CuInSe2 powders in PBO conjugated polymers. By preparing hybrid solution in different concentration and controlling spin coating rate, we fabricate solar cells device successfully. From the I-V characteristics, we can get noticeable characteristics of diode. Then, we calculate the fill factor (F.F.) with the open-circuit voltage¡]Voc¡^and short-circuit current¡]Isc¡^

Widespread commercialization of photovoltaics (PVs) requires both higher power conversion efficiencies and low-cost, high throughput manufacturing. High efficiencies have been achieved in devices made from materials such as CuIn[subscript x]Ga₁₋[subscript x]Se₂ (CIGS). However, processing of these solar cells still requires high temperature and vacuum, driving up cost. A reduction in manufacturing costs can be achieved by utilizing colloidal nanocrystals. Semiconductor nanocrystals can be dispersed in solvents and deposited via simple and scalable methods under ambient conditions to form the absorber layer in low-cost solar cells. Efficiencies of ~3% have been achieved with CIGS nanocrystal PVs, but this must be improved substantially for commercialization. These devices suffer from poor charge transport in the nanocrystal layer. Here, the synthesis of nanowires and their utilization in solar cells was explored as a way to improve charge transport. CuInSe₂ (CIS) nanowires were synthesized via the solution-liquid-solid method. PV devices were fabricated using the nanowires as the light absorbing layer, and were found to exhibit a measureable power output. Earth-abundant materials were also explored, motivated by the material availability concerns associated with CIGS. Pyrite FeS₂ nanocrystals were synthesized via an arrested precipitation reaction to produce phase-pure particles 15 nm in size. These nanocrystals were spray coated to form the active layer in several different common device architectures. These devices failed to produce any power output. The material was determined to be slightly sulfur deficient, leading to a high carrier concentration and metallic behavior in the thin films, with conductivities measured to be ~5 S/nm. A nanocrystal synthesis of Cu₂ZnSnS₄ (CZTS) was also developed to produce highly dispersible crystalline particles ~11 nm in size. These nanocrystals were spray coated onto glass substrates to form the absorber layer in test PV devices, and an efficiency of 0.23% was achieved without high-temperature or chemical post-processing. Additional studies included the synthesis of CZTS nanorods and their incorporation into functioning solar cells. The selenization of CZTS nanocrystal films was also studied as a way to improve solar cell performance. High temperature annealing in a Se atmosphere was found to produce CZTS(Se) layers, which could be used in working PV devices. / text

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