**1. Introduction**

274 Solar Cells – Thin-Film Technologies

[30] Y. Seunghyup, Y. Changhun, H. Seung-Chan and C. Hyunsoo "*Flexible/ ITO-free* 

*organic optoelectronic devices based on versatile multilayer electrodes*" – Raport Integrated Organic Electronics Lab (IOEL)Dept. of Electrical EngineeringKorea Advanced Institute of Science and Technology (KAIST), Daejeon, Korea 2009.

> Despite significant progress in research, the energy provided by photovoltaic cells is still a small fraction of the world energy needs. This fraction could be considerably increased by lowering solar cell costs. To achieve this aim, we need to economize on the material and thermal budgets, as well as increase cell efficiency. The silicon "**H**eterojunction with **I**intrinsic **T**hin layer (HIT)" solar cell is one of the promising options for a cost effective, high efficiency photovoltaic system. This is because in "HIT" cells the P/N junction and the back surface field (BSF) layer formation steps take place at a relatively low temperature (~200°C) using hydrogenated amorphous silicon (a-Si:H) deposition technology, whereas in normal crystalline silicon (c-Si) cells the wafer has to be raised to ~800°C for junction and BSF layer formation by diffusion. This means not only a lower thermal budget, but also cost reduction from thinner wafers, since the danger of the latter becoming brittle is strongly reduced at lower (~200°C) temperatures. Thin intrinsic layers on either face of the c-Si substrate, effectively passivate c-Si surface defects, which would otherwise degrade cell performance. Moreover it has been demonstrated that carriers can pass through the passivating layers without significant loss.

> In this chapter, we use detailed electrical-optical modeling to understand carrier transport in these structures and the sensitivity of the solar cell output to various material and device parameters. The global electrical - optical model "Amorphous Semiconductor Device Modeling Program (ASDMP)", originally conceived to simulate the characteristics of solar cells based on disordered thin films, and later extended to model also mono-crystalline silicon and "HIT" solar cells (Nath et al, 2008), has been used for all simulations in this chapter. The model takes account of specular interference effects, when polished c-Si wafers are used, as well as of light-trapping when HIT cells are depositd on textured c-Si.
