Download files
Abstract
Conventional screen-printed crystalline silicon solar cell technology has dominated the photovoltaic industry for three decades. The key strengths of the technology are its robustness, simple processing and ready availability of the required processing equipment. Its performance is limited by several fundamental factors however. The largest performance losses arise from the inability to reliably print finer lines and the requirement for a heavily-doped emitter, which provides the cell with very poor short wavelength response.
This thesis aims to overcome these limitations, initially through the development of techniques for creating a homogeneous and lightly-diffused emitter with sufficiently deep junction to be compatible with screen-printed contacts. It has been shown that by changing the doping profile of the emitter, excellent short wavelength response is achieved, and contact resistance losses for such an emitter can be reduced to acceptable levels, leading to respectable fill factors. For further improvement, a novel semiconductor finger solar cell design has been proposed and developed that effectively facilitates the formation of a selective emitter for screen-printed contacts but without requiring any alignment or close spacing of metal lines. The semiconductor fingers can be formed by laser grooving the lightly-doped Si surface and then heavily doping the groove walls via high temperature furnace diffusion. The current-carrying ability of these semiconductor fingers eliminates the requirement for a heavily-diffused top surface emitter and corresponding poor short wavelength response. In addition, they simultaneously allow screen-printed metal fingers to be placed further apart, therefore compensating for the inability to print narrower screen-printed lines, thus reducing shading. The addition of semiconductor fingers to the lightly-doped emitter enables fill factors as high as 79% to be achieved with standard screen-printing metallisation. Standard commercially-deposited PECVD silicon nitride has been shown capable of providing excellent surface passivation of the lightly-doped emitter, as demonstrated by the device’s excellent short wavelength response and open circuit voltages close to 640 mV. Cell efficiency of 18.4% has been demonstrated on 155 cm2 solar-grade p-type CZ wafers.
Laser doping has been investigated as a method for simplifying and lowering the cost for forming semiconductor fingers by replacing laser grooves with laser-doped lines. Finally, the potential benefits of using n-type CZ wafers instead of p-type CZ wafers are evaluated through an innovative rear emitter solar cell design and processing techniques. Commercial screen-printed aluminium pastes have been used to form an aluminium-doped p+ emitter on the entire rear surface using a modified firing process. Open circuit voltages close to 650 mV and fill factors approaching 80% demonstrate the success in this cell design in eliminating both junction shunting and resistive losses that commonly limit screen-printed solar cells to much lower efficiencies. Efficiencies as high as 18.7% have been achieved with this cell design using large area solar-grade n-type CZ wafers.