Development of high efficiency monocrystalline si solar cells through improved optical and electrical confinement
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The objective of this thesis is to understand and improve optical and electrical confinement to achieve cost-effective high-efficiency thin p-type Si solar cells. Optical confinement is achieved by front surface texturing in conjunction with an internal reflective layer on the back surface. Electrical confinement is obtained through the use of a high-lifetime material coupled with high-quality passivation on both surfaces. This research is divided into five tasks. In the first task, Ga-doped Cz Si was investigated to achieve a high and stable lifetime. It was found that for 1 ohm-cm nominal-resistivity screen-printed Al-back surface filed (BSF) cells, the Ga-doped ingot gave ~1.5% higher absolute efficiency after light-soaking relative to the B-doped counterpart. The benefit of using Ga is therefore quite explicit. In the second task, the screen-printed Al-BSF was investigated to explore its potential and limitations for achieving high-efficiency cells. It was found that there exists a critical alloying temperature for a given Al-thickness, above which the Al-BSF becomes non-uniform and cell performance starts to degrade. This puts a limit on the quality of the Al-BSF that can be achieved. An alternative way of back passivation involving dielectric/metal layers was therefore explored. In Task three, two key requirements for achieving high-efficiency dielectric back-passivated cells were established through device modeling. These are (1) a formation of a high-quality BSF underneath the local back contact through vias in the dielectric and (2) a high-quality dielectric passivation with either a moderate positive charge density or a high negative charge density. Task four involved the development of a metallization technique through vias in the dielectric to achieve a high-quality contact and an efficient internal reflector in conjunction with a high-quality local BSF. Further, a novel dielectric system composed of a spin-on SiO<SUB>2</SUB> layer capped with SiN<SUB>x</SUB> was developed that exhibited excellent passivation and a moderate positive charge density. The final task involved fabrication and analysis of dielectric back-passivated cells. The new dielectric and process sequence developed in this thesis resulted in screen-printed solar cells with efficiency as high as 19% with the potential for 20% efficient cells on 100-µm thick Si substrates.