Optical and Electrical Studies of Silicon Nanowires in Photovoltaic Applications

Abstract

Recently, there has been increasing research interest in the application of silicon nanowires (SiNWs) in photovoltaic (PV) cells. SiNW may emerge as a more viable choice over conventional bulk Si structure in future PV devices because of its unique optical and electrical properties. In this work, features and working principles of conventional planar Si solar cell and novel SiNW solar cell have been studied and compared, highlighting the advantages and promising prospect of SiNWs in the design and fabrication of third generation solar cells. In previous works, SiNWs were fabricated using a variety of methods, which mainly fall into two categories: ?bottom-up? growth and ?top-down? etching. ?Bottom-up? method generally involves Vapour-Liquid-Solid (VLS) growth of crystalline silicon on cheap substrate in the presence of gold or other metal catalysts. ?Top-down? method usually refers to etching of starting silicon wafer in ionized plasma (reactive ion etch/plasma etch) or chemical electrolyte (wet etch). Performances of these SiNW based PV devices generally do not exceed 3%, which is significantly lower than that of existing commercial Si solar cells (~20%). This implies that despite the theoretical advantages of SiNWs in solar applications, there exist unsolved technical issues which hinders SiNW PV device from attaining its theoretical efficiency. Therefore, the research emphasis in the community has always been the improvement of device design and experimental techniques, in order to increase the overall power conversion efficiency (PCE) of the devices. In this work, optical lithography patterned plasma etch was utilised in fabricating highly ordered, vertical SiNWs from single-crystalline Si (100) starting wafer. Several different designs have been explored, including buried p-n junction SiNW solar cell, buried p-n junction silicon nanowall solar cell and core-shell p-n junction SiNW solar cell. Planar Si control devices have been fabricated as well for comparative analysis. Optical and electrical characterisation demonstrates significant suppression in surface reflection and prominent enhancement of light generated current in SiNW devices. Buried-junction SiNW and nanowall solar cells demonstrate 33% and 42% increase in short circuit current comparing to Si planar device, owing to effective light trapping and anti-reflection property of SiNWs. Core-shell SiNW device displays a higher increase of 52% in Jsc, as a result of larger junction area from the radial p-n junction. An overall PCE of 8.2% and 4.2% are attained for buried-junction and core-shell junction SiNW devices respectively, surpassing the efficiencies obtained by previous groups with similarly structured SiNW devices. Factors which limit the device performance are also analyzed, revealing the impact of series resistance (Rs) on fill factor (FF) and PCE of the device. Significant improvement of performance could be expected by eliminating the effect of Rs. In addition, as a promising and highly efficient route of enhancing PCEs in semiconductor PV devices, multiple exciton generation (MEG) has been studied, including its mechanism and experimental detection methods. Photoluminescence (PL) signals from some SiNW samples demonstrate substantial light-emitting property in SiNWs, confirming the validity of time-resolved PL (TRPL) as an effective MEG detection method in SiNWs. Lastly, a proposal of future device design has been raised. The new structure aims at integrating the effect of MEG with buried or core-shell junction SiNW PV device, opening a possibility of further enhancement in PCEs.