**1. Introduction**

Among the semiconductor materials with suitable optoelectronic properties for photovoltaic applications, silicon has been the most widely accepted and used in the current production of photovoltaic modules. The basic advantage of silicon is its abundance in nature and mastered silicon wafer fabrication, as well as the compatibility of the technological processes of solar cells with the microelectronics industry. The increasing cost of processed crystalline silicon ingots in the past years became a driving force decreasing the wafer thickness for solar cell

© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

fabrication [1]. However, this trend was stopped due to the bending of thin wafers during high temperature processing of standard silicon solar cells, which results into the increasing efforts focused on the technologies with lower silicon usage. Among them, the silicon heterojunction solar cells (SHJ) provide both high performance together with a perspective of low-cost fabrication and decrease of silicon wafers thickness bellow 100 µm [2]. The advantages of heterojunction between amorphous and crystalline silicon were first introduced into the socalled HIT concept (Hetero-junction with Intrinsic Thin-layer) by former company SANYO (currently SANYO is part of the company Panasonic) in 1992 [3]. The SHJ HIT solar cell is composed of a single thin crystalline silicon wafer, c-Si surrounded by ultra-thin intrinsic silicon layers, a-Si:H(i) and n-type and p-type doped amorphous silicon layers, a-Si:H (**Figure 1**), which can be deposited at temperature below 200°C and so can be used in processing of thin wafers. On the two doped layers, transparent conducting oxide (TCO) layers and metal electrodes are formed with sputtering and screen-printing methods, respectively. The TCO layer on the top also works as an anti-reflection layer.

**Figure 1.** Silicon heterojunction solar cells with on n-type silicon (SHJn) and n-type silicon (SHJp) hetero-junction with intrinsic thin-layer (HIT) solar cell.

Since the first introduction, the HIT solar cells have been the subject of extensive research. Recently, the record efficiency *η* = 25.6% with open-circuit voltage *V*OC = 0.74 V, short-circuit current *J*SC = 41.8 mA/cm2 and fill factor *FF* = 82.7% were achieved on the rear junction HIT solar cell by Panasonic, which makes this technology currently the most efficient among siliconbased solar cells [4]. Current strong interest in SHJ concept is motivated by the high conversion efficiency as well as further possibilities for decreasing the fabrication cost. SHJ can be prepared by simple and low temperature fabrication processes, which decreases the thermal budget and thus the cost of the cell. Since the base material of the structure is crystalline silicon, the typical degradation due to the Staebler-Wronski effect observed in amorphous silicon solar cells does not take place in SHJ solar cells, where the base material of the structure is crystalline silicon [5]. Moreover, the HIT cell shows a better temperature coefficient (<–0.25%/K) compared to standard c-Si solar cells (–0.45%/K), which means more power generated in outdoor conditions for the same nominal conversion efficiency [6]. Since the SHJ HIT has symmetrical front and back structures, the possibility to use it for the bifacial solar module is feasible. The experiments show that bifacial use of the HIT structure brings a performance higher by more than 10% compared to the conventional structures with light incident only from one side [7].
