Advanced surface texturing for silicon solar cells
Thesis DisciplineElectrical Engineering
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
The multi-crystalline silicon (me-Si) solar cell is considered to be one of the most promising cells capable of achieving high efficiency at low cost and high reliability. Improving solar cells efficiency using low cost materials requires careful design considerations aiming to minimise the optical and electrical losses. In this work plasma texturing was employed to reduce optical reflections from silicon surfaces well below 1%. Plasma texturing is used to form light trapping structures suitable for silicon solar cells. Several plasma texturing methods are investigated and associated defects are analysed. Masked as well as mask-less texturing techniques are investigated. Conventional parallel plate Reactive Ion Elching (RIE), Inductively Coupled Plasma (ICP) and Electron Cyclotron Resonance (ECR) plasma system are used to compare the plasma induced defects in silicon. The influence of various plasma etch parameters on plasma induced defect is investigated. A correlation between the minority carriers lifetime and surface area increased by texturing is established. Effective lifetime measurements using Quasi Steady State Photo Conductive (QSSPC) technique is mainly used to estimate the plasma induced defect in textured silicon substrates. Sinton lifetime tester is used to measure the effective lifetime of the substrates. The implied open circuit voltage is calculated from the lifetime data for textured substrates. In this work low temperature photoluminescence spectroscopy is also used to analyse the defect caused by plasma on me-Si substrates. Photoluminescence (PL) data is obtained using the 514.5 nm line of an Ar⁺ laser as an excitation source. The luminescence is dispersed with SPEX 1700 spectrometer with a liquid nitrogen cooled Germanium detector. Reflectance measurements are performed on textured surfaces usmg a purpose built integrating sphere attachment of a high accuracy spectrophotometer. Modelling is also performed using PV-optics software to compare the experimental and theoretical results. Finally, silicon solar cells are fabricated with measured efficiency around 18% . The efficiency is estimated from the I-V characteristics data obtained using a calibrated halogen lamp and a HP semiconductor parameter analyser. Spin-on-dopant source as well as solid diffusion source is used to form the ewitter junction of the solar cells fabricated on p-type silicon wafers. Multicrystalline silicon, CZ- silicon and FZ silicon wafers are used to fabricate solar cells in this thesis. The effect of single and double layer antireflection coatings on diffused reflections is also investigated.