Pursuing grid parity with photovoltaic technology has steadily pushed down the cost of ownership of solar module manufacturing. Silicon thin film has been recognized to play a role in either reducing manufacturing cost or increasing conversion efficiency, which are Si-TF solar modules and Si-HJ solar cell, respectively. Table 1 shows different market introduction of these two solar cell/modules.
In case of Si-TF solar module manufacturing, many tool suppliers have already demonstrated their PECVD systems for this application. The experienced tool suppliers from the LCD-TFT industry have modified the systems to handle rather thick glass substrates with ease. The required high deposition rate for 1~1.5 µm thickness microcrystalline bottom cell has been attempted to be overcome by applying higher- frequency, large-substrate or batch-type system configuration. By contrast, the detail manufacturing process of Si-HJ solar cells is not disclosed by Sanyo, the frontier of HIT (heterojunction with intrinsic thin layer) solar cells. Recently many institutes have started to investigate Si-HJ cells, based on their accumulated process know-how in fabricating Si-TF solar cells, resulting in a steep efficiency increase in small-size cells. Sanyo’s original patent expiration has triggered interest on high-volume production-ready PECVD systems.
Device Structure and Performance
Figure 1 shows stacked layers of two solar cells. The device structures can be described simply as P-I-N and P-N junction for Si-TF and -HJ solar cells, respectively. The main difference comes from an absorption layer, where both light absorption and carrier generation take place.
In Si-TF cells, deposited intrinsic amorphous (or/and microcrystalline) silicon film is used as the absorption layer. Photo-generated charge carriers in this layer are collected by an internal electric field exerted by p- and n-doped layers deposited on its both sides. The carrier drift movement is schematically shown in Figure 2. Interestingly, heterojunction also exists in this kind of solar cell.[§] As a p-layer at the sunny side, hydrogenated silicon carbide has been introduced because of its high energy band gap (1.9 eV), resulting in an increase of current density, though some carriers cannot reach the metal electrode due to this defective window layer. As a common practice, carbon content is gradually reduced down to zero, resulting in continuous interconnection of energy bands to a-Si(i) layer (1.7 eV). With tandem cell structure (a-Si/µc-Si double junction), attainable conversion efficiency is speculated to be around 11 percent in a practical module size. A recent manufacturing boom during a period of high wafer price shows certain merits of fully integrated solar module production with rather a low price of raw materials, ironically by using very expensive vacuum systems, including PECVD. Eventually, Si-TF modules are expected to find appropriate manufacturing and installation places, such as tropical regions and the Middle East.
In Si-HJ cells, n-type crystalline silicon wafers absorb light (Eg = 1.1 eV) and the generated charge carriers are diffused to the space-charge region formed at the junction. Some charge carriers are reflected by the front- and back-surface fields formed by an amorphous doped layer with the same polarity (Eg = 1.6~1.7 eV): Relatively high band gap