Laser doped selective emitter solar cells

Abstract

To reach the goal of grid parity, the cost of crystalline solar cells need to be reduced considerably. This can be done by using cheaper materials, such as thinner or lower quality wafers, or by reducing manufacturing costs, both process and material costs. Increasing the efficiencies of the devices while maintaining low costs via simple fabrication schemes is an ideal combination. The benefits of selective emitter have been known and quantified for many years. However, the adaptation of selective emitter solar cell designs into mass-production has been quite slow due to the relatively complex processing steps involved in applying such a design. This thesis begins with a review of various existing selective emitter solar cell technologies. A method of creating a selective emitter is then selected to be the main focus of the thesis due to its simple yet powerful features. This method is laser-doping through a dielectric layer. A new solar cell structure that employs the laser-doping process combined with a self-aligned metallisation method is then introduced, termed Laser-Doped Selective Emitter (LDSE) solar cell. A study is then presented to help further understanding of the laser-doping through dielectric process. Investigative work combines the use of defect Yang etch, SEM and EBIC analysis, light JV curve measurements and local ideality factor derived from dark JV curve measurements. These investigations help to identify several challenges associated with the laser-doping process. Theories on the causes of these challenges are then presented along with possible solutions. One of the most critical findings is the use of a stacked layer consisting of a thin layer of thermally grown SiO2 and PECVD SiNx to prevent laser-induced defects. Next, several characterisation methods that include four point-probes, Secondary Ion Mass Spectrometry (SIMS), spreading sheet resistance, Electron Beam Induced Current (EBIC) analysis and photoluminescence imaging are utilised to measure the sheet resistances, doping profiles and lifetime of the laser-doped region. These measurement techniques are used to optimise the laser parameters and other processing parameters involved in the fabrication of LDSE solar cells. In the final chapter, a novel self-aligned metallisation method that utilises the solar cell’s ability to produce voltage and current is investigated. It is found that this photoplating method is well-suited for fabrication of LDSE cells and potentially suitable in mass-production. Finally, the integration of the optimised process flow and parameters combined with the photoplating method is demonstrated through the fabrication and characterisation of a batch of LDSE solar cells made of commercial 125mm 1 Ω.cm p-type wafers. Efficiencies of up to 19% with VOC in excess of 635 mV and JSC close to 38 mA/cm2 are reported on 125 mm commercial grade CZ p-type wafers. The use of typical production line equipments for most of the fabrication steps is also demonstrated.