Nanostructures with their unique physical chemical and properties show immense potential in the design and fabrication of low cost photovoltaics. To explore the various possibilities for designing an improved low cost solar cell with nanowire as the building block this research work has been carried out. In this work a single silicon nanowire structure has been simulated to study the effect of varying doping density on the solar cell parameters so as to optimise the overall quantum efficiency. Three case studies have been carried out in the process by using Silvaco TCAD’s Atlas tool-kit and an optimum set of open-circuit voltage (Voc), Short-circuit current density (Jsc), Fill-factor (FF) and efficiency(??) obtained. It has been shown that the efficiency of nanowire solar cell improves with increase in the doping density. A single nanowire provides an efficiency of 8.25% thus indicating that when many such nanowires are integrated to form an array, a much improved overall quantum efficiency can be obtained.
The sun is ever present and its rays covering the earth surface has enough power to meet more than half of the global energy demand. Although the potentiality of solar cell as a means for energy production was realised way back in 1977 but till now the technology has only been able to cross its nascent stage. This photovoltaic device is faced with certain challenges that have kept this renewable source of energy out of large scale commercial use. The difficulties include high cost and low efficiencies compared to the conventional power generation strategies like fossil fuels and nuclear. Silicon, the most preferred material for the fabrication of solar cell cannot be used in its raw form and converting the metallurgical grade silicon to solar grade silicon is highly cost intensive. The low efficiencies are due to incomplete light absorption, carrier recombination and thermalization losses. To overcome the above obstacles a lot of research is being carried out to explore new and improved avenues for solar power generation. In this regard nanostructured devices are promising candidates that could break the impasse and lead towards high efficiency and low cost solar cells.
Within the nano-world domain lies the possible source of next-generation solar cells, the ‘nanowires’. These are one-dimensional structures with a large aspect ratio and 1-100nm in diameter and offers unique advantages. Firstly due to the small diameter the radial dimension of nanowires is comparable to wavelength of light leading to efficient light absorption. Secondly the large surface-to-volume ratio results in enhanced chemical reactivity. Thirdly the inherently short charge separation distance in NWs with coaxial p-n junctions reduces the recombination losses thereby increasing the efficiency. Finally the less complex and more economic synthesis techniques for fabrication of nanowires will have significant effect on the overall cost reduction of solar cells.
3. DEVICE STRUCTURE
The device consists of a coaxial p+-n- n+ structure of length 3.1??m, simulated using three-dimensional cylindrical co-ordinate system. In this structure three different regions are defined each made up of silicon but with different doping levels. The innermost region is the doped p+ region and outermost region is the n+ region while the middle region is an intrinsic n region.
Incident solar rays fall vertically on the top surface and have an intensity of one sun. For simulating the device all the important carrier generation and recombination models namely Shockley-Read Hall recombination, Auger recombination and surface recombination are incorporated. The Atlas toolkit under TCAD calculates the solar cell parameters using ray tracing model and couples it with electrical simulation using Poisson’ s and Continuity equations.
3. WORKING PRINCIPLE
The TCAD simulation carried out is divided into three case studies-:
In the first case study the doping concentration of the intrinsic layer is kept constant while the concentration of p-type and n-type regions is varied. TCAD evaluates the short circuit current density (Jsc) open circuit voltage (Voc), fill factor (FF) and conversion efficiency (??) with varying doping densities. The efficiency is calculated using the following relation
Where Pm is the maximum power obtained Win is the input optical intensity multiplied by area of the device under illumination.
In this case study the NW structure is optimized by keeping the doping concentrations of p-type and n-type region constant and varying the doping concentration of intrinsic region .The parameters are evaluated and the efficiency is recorded.
In this case study the doping concentration of intrinsic layer is kept at a constant value and also the concentration of p-type and n-type regions is kept constant and equal. The corresponding changes in the parameter results in changed conversion efficiency.
4. RESULTS AND DISCUSSION
From the first case study we find that open circuit voltage (Voc) and short circuit current density (Jsc) both increases with doping density. While the fill factor shows a decline due decrease in maximum current obtained (Im) and simultaneous increase in Voc and Jsc. The efficiency, however, continues to increase and reaches a maximum value of 8.25%.
From the second and the third case study the results obtained showed that varying the doping density of the middle intrinsic n-type region has almost negligible effect on the parameters. The maximum voltage obtained remains constant while the fill factor and efficiency shows a slight reduction in their values.
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