Khulna University of Engineering & Technology

Photovoltaic (PV) power generation is becoming widespread as a clean and gentle energy source for the earth. The main drawback of currently used photovoltaic cell is its low conversion efficiency and materials with the appropriate band gaps that can perfectly match the broad range of solar radiation. Recently it has been shown that the energy gap of InxGa1-xN alloys potentially can be continuously varied from 0.7 to 3.4 eV, providing a perfect matching to the full-solar-spectrum. Therefore, InxGa1-xN becomes a promising material for very high efficiency multijunction solar cell. Any desired value of bandgap can be obtained from this material choosing the appropriate composition. In this work, InxGa1-xN-based multijunction solar cells have been designed theoretically for high efficiency and the performance of the designed solar cells are evaluated with various parameters. The theoretical design and performance evaluation are done by developing a simulation model. The developed mode optimized the solar cell design for high efficiency at different junction numbers. The efficiency is found to be varied from 24.49 to 45.35 % for single junction to eight junction solar cells. The current mismatches of multijunction solar cell are kept within 0.29%. The lattice mismatches between different cells were found to be varied from 0.86 to 3.15%. The increase in surface recombination velocity and emitter thickness decreases the efficiency. On the other hand, the increase in minority carrier lifetime of emitter and base, and doping density increase the efficiency. In order to get more accurate results the effect of depletion width was taken into account. However, no significant change is observed between the results without and with considering depletion width. The performance of InGaN-based MJ solar cells under concentrator is studied. The efficiency is found to be varied from 24.49 to 39.28 (%) for single junction and 45.35 to 72.79 (%) for eight junction, without and with concentrator, respectively. This model can be applied to design the solar cells at any number of junctions.

Photovoltaic (PV) power generation is becoming widespread as a clean and gentle energy source for the earth. The main drawback of currently used PV cell is its low conversion efficiency and materials with the appropriate band gaps that can perfectly match the broad range of solar radiation. InxGa1-xN solar cells have the potential to span 90% of the solar spectrum as the band gap can be continuously varied from 0.64 to 3.4 eV. The purpose of the proposed research is to establish the technology for material growth by metalorganic vapor phase epitaxy (MOVPE) and fabrication of InxGa1-xN solar cells. The InGaN material system poses many challenges which are extended into the performance of the devices. In this dissertation, the challenges are overcome individually towards the fabrication of solar cell. The InGaN film with an indium contents up to 0.4 are successfully grown by controlling the fundamental growth parameters such as the precursor gas flow rates and temperature using MOVPE. The variation of TMI/(TMI+TEG) molar ratio shows a significant effect on the growing films. The formation of metallic In originates from the higher value of TMI/(TMI+TEG) molar ratio with low V/III ratio while the lower value of TMI/(TMI+TEG) causes the phase separation. It is also necessary to control the growth rate and epitaxial film thickness to suppress the phase separation in the material. Crystalline quality of grown films is markedly deteriorated with increasing the In composition. The lattice as well as the thermal expansion coefficient mismatch between GaN template and InGaN epi-layer is primarily considered as the reasons to deteriorate the film quality for higher In composition. Maintaining the material quality in InGaN at higher In compositions is a fundamental challenge. The step-graded inter layer technique has been investigated in order to improve the InGaN epi-layer quality for the first time. Multilayer epitaxial structures consisting of InxGa1-xN layers with various compositions have been successfully grown. In the step graded inter layer with step by step increases of In content decreases the relative lattice and thermal mismatch between the GaN template and InGaN epi-layer. High resolution X-ray diffraction (HRXRD) measurements reveal that the insertion of step-graded inter layer significantly reduces the twist while little effects are observed on tilt. Decreasing of twist with increasing the number of inter layers is found to be remarkable up to a certain value. It is also found that more inter layers are necessary to reduce the twist for the samples with higher In composition. Different types of dislocation density have also been studied using the tilt and twist. Dislocation densities, particularly the edge dislocation density, decrease considerably with the insertion of inter layers for higher In composition. An edge dislocation of 3E10 cm-2 is obtained for a sample with 0.25 In composition, most of the edge dislocations seem to come from the GaN layer beneath the step-graded inter layers. The probable mechanism for the reduction of dislocation due to step graded inter layer has been explained. InxGa1-xN solar cells require p-type doping to create the p-n junction. Magnesium (Mg) has been proven to be the most successful p-type dopant. The Mg doping behavior such as Mg doping effect, controllability of Mg concentration, n-p conversion and crystalline quality of the films are studied. The high residual donors (1019-1020 cm-3) in InGaN with an In content from 0.05 to 0.37 can be compensated by Mg doping. The p-type conduction is obtained for those with an In content up to 0.25. It is found that a higher Cp2Mg flow rate is needed to get p-type conduction in InGaN with a higher In content x; for example, Cp2Mg/(TEG+TMI) ≈ 0.5% for x=0 (GaN), ≈ 2 % for x=0.05 and ≈ 4% for x=0.25. Such a high Cp2Mg flow rate is needed due to the high residual donor concentration (1019-1020 cm-3) of InGaN films and the low activation efficiency of Mg. The crystalline quality of InGaN is deteriorated with increasing the Mg doping level. To achieve a p-type InGaN with a lower Mg doping, it is essential to improve the crystalline quality of non-doped InGaN. By using In0.25Ga0.75N films, an n+-p homo-junction structure is fabricated on GaN template. For such a device, the response to the light illumination (AM1.5) is observed an open circuit voltage of 1.5 V and the short circuit current density of 0.5 mA/cm2. This is the p-n homo-junction InGaN solar cell with highest In composition ever reported by MOVPE. The n+-p structure with step graded inter layers has also been fabricated. The difficulties of formation of p-n junction has been understood by SIMS depth profile of the dopant. It is found that the diffusion and delay effect of Mg is a barrier to form the p-n junction. Finally, further directions to InGaN solar cell research have been proposed.