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Thin-Film Crystalline Silicon Solar Cells Thin-filmThin-film crystallinecrystalline siliconsilicon solarsolar cellscells Kenji YAMAMOTO A photoelectric conversion efficiency of over 10% has been achieved in thin-film 750 polycrystalline silicon solar cells which consists 700 of a 2 µm thick layer of polycrystalline silicon with a very small grain size (microcrystalline 650 (mV) Astropower Mitsubishi silicon) formed by low-temperature plasma oc V 600 CVD. This has shown that if the recombina- ASE/ISFH ISE tion velocity at grain boundaries can be made 550 Kaneka Sanyo Ti Daido very small, then it is not necessarily important Neuchatel 500 3 10 4 to increase the size of the crystal grains, and 10 ETL 105 that an adequate current can be extracted 450 106 S=107cm/s even from a thin film due to the light trap- BP Tonen open circuit voltage 400 ping effect of silicon with a low absorption coefficient. As a result, this technology may 350 10-2 10-1 100 101 102 103 104 eventually lead to the development of low- grain size � ( µ m) cost solar cells. Also, an initial efficiency as high as 12% has been achieved with a tan- Fig. 1: The relationship between grain size and open circuit voltage (V ) in solar cells. dem solar cell module of microcrystalline sili- oc Voc is correlated to the carrier lifetime (diffusion length). In the figure, S indicates the con and amorphous silicon, which has now recombination velocity at the grain boundaries. In this paper, microcrystalline silicon cells correspond to a grain size of 0.1 µm or less. In the figure, Ti, BP, ASE and ISE are abbre- started to be produced commercially. An even viations of the research facilities from which the associated data came. greater initial efficiency of 14.5% has been achieved in devices with a small size area, and further increases of efficiency can be ex- studies comes from the theoretical finding crystal silicon has become the optimal materi- pected. that if these devices can be constructed so al for the substrate. Accordingly, this research Keywords: solar cell, silicon, light trapping, that they trap sufficient light, it should ideally has shifted toward recycling single-crystal sili- thin film, microcrystalline, plas- be possible to achieve photoelectric conver- con, and at present research is being actively ma chemical vapor deposition, sion efficiencies in excess of 20%, even with performed into single-crystal silicon thin-film recombination velocity solar cells in which the photoelectric layer is solar cells, primarily focusing on porous sili- just a few µm thick.1) con delamination techniques.5) 1. Introduction Historically, the silicon thin-film photo ab- On the other hand, attention has recently With raw materials for crystalline solar sorbed layers in such devices have been pro- been focused on thin-film solar cells made cells in increasingly short supply, renewed ef- duced by high-temperature processes, mainly with crystalline silicon having a small grain forts are being made in the study of ways to chemical vapor deposition (CVD),2) liquid size formed by plasma CVD using inexpensive make practical thin-film solar cells for use in phase epitaxy (LPE),3) and zone melt recrystal- substrate materials such as glass and by using tomorrow’s large-scale solar cell installations. lization (ZMR).4) low temperatures regardless of the type of In particular, since thin-film polycrystalline sili- However, it has become apparent that substrate instead of these high-temperature con solar cells are made of the same materials the properties of the underlying substrate processes.6–11) as bulk silicon solar cells which are currently are the key to obtaining high-performance Figure 1 shows the relationship between in widespread use, they are being enthusiasti- silicon thin-film solar cells. That is, a high- grain size and Voc (open circuit voltage) as cally studied at laboratories all over the world quality silicon thin-film photoelectric layer summarized by J. Werner of Stuttgart Univer- 12) due to the abundance of raw materials for can be obtained by suppressing the diffusion sity. Here, Voc can be regarded as a parame- such devices, their stability, and the character- of impurities from below and using epitaxial ter reflecting the cell characteristics and crys- istics they are expected to possess as silicon deposition that exploits the characteristics of talline properties. As the figure shows, devices. Part of the motivation behind these the underlying material, and evidently single- superior characteristics are obtained with a PV Research Division, Kaneka Corporation (2-1-1, Hieitsuji, Otsu 520-0104) e-mail: [email protected] 12 JSAP International No.7 (January 2003) JSAP International No.7 (January 2003) 13 grain size of 100 µm, but the characteristics to light. The very first reports on the char- ease with which integrated structures can be are worse with a grain size of a few tens of µ acteristics of an n-i-p cell were made by the formed, which is a characteristic of thin-film m. Conversely, favorable characteristics have PV Research Division using the light trapping solar cells, an advantage of p-i-n cells is that been obtained experimentally both at Neuchâtel structure described below. These cells had an they can be formed as super straight modules University and at Kaneka Corporation’s PV Re- intrinsic conversion efficiency of 10.7% and using integration techniques similar to those search Division by using thin-film polycrystalline an apparent efficiency of 10.1% for a film used for amorphous silicon as described be- silicon formed at low temperatures by plasma thickness of 2 µm (surface area 1 cm2, mea- low. On the other hand, it should be pos- CVD with a submicron grain size (this is generally sured by the JQA (Japan Quality Assurance sible to make integrated structures of n-i-p referred to as microcrystalline silicon due to the Organization)) (Fig. 2).8–11) Also, by subject- cells by methods equivalent to those used for 13) small grain size). This is an interesting discovery ing the silicon film in the photoelectric layer Cu(InGa)Se2-based solar cells. At the pres- which has caused recent research to shift toward of this cell to XRD (x-ray diffraction) measure- ent time it is difficult to determine which is both extremes. ments, it was found to have a preferential better, but it should become possible to arrive This report discusses these thin-film poly- (110) orientation. The p-i-n and n-i-p cells at a conclusion in terms of cost, performance crystalline silicon solar cells formed at low have different characteristics due to their dif- and applications through the production of temperature (referred to as microcrystalline ferent fabrication sequences. A large differ- sub-modules in the future. silicon solar cells below) which have been the ence is that the underlying layer of a p-i-n cell subject of studies conducted by the PV Re- is the transparent p-type electrode, where- 2.2 Carrier transport in microcrys- search Division using thin-film polycrystalline as the underlying layer of a n-i-p cell is the talline silicon thin-film solar cells silicon with a very small grain size formed at n-type back electrode. To improve the efficiency of these solar low temperature. It will also introduce the re- As a general rule, transparent electrodes cells in the future, it is absolutely essential to sults of the research and development of sili- are made of oxides, and since there is a risk gain some understanding of the relationship con hybrid solar cells with a tandem structure of these oxides being reduced by the hy- between the crystalline microstructure includ- comprising these solar cells and amorphous drogen atoms that are needed to form mi- ing grain boundaries and the carrier transport solar cells, which were put to practical use for crocrystalline cells, there is a smaller process process, in particular the carrier lifetime (dif- the first time in 2001. window in the cell formation conditions for fusion length) that determines the solar cell the p-i-n type. From the viewpoint of the characteristics and the recombination velocity 2. Thin-Film Polycrystalline (Microcrystalline) Silicon Solar Cells 2.1 Microcrystalline silicon thin- 0.035 film solar cells formed at low tem- 0.030 perature 0.020 2 Microcrystalline silicon solar cells formed Jsc : 25.8 (mA/cm ) 0.025 : 24.35 (mA/cm2) (AP) by plasma CVD at low temperature are as- Voc : 0.539 (V) F.F. : 76.8 (%) 0.015 sumed to have a shorter lifetime than single- ) ) 0.020 Eff AP : 10.1(%) crystal cells, and it is common to employ a Eff IN : 10.7 (%) p-i-n structure including an internal electric 0.015 field in the same way as an amorphous so- 0.010 POWER (W POWER lar cell. A p-i-n type microcrystalline silicon (A CURRENT solar cell is formed by a process fairly similar 0.010 0.005 to that of an amorphous solar cell. Strictly speaking, these cells can be divided into p-i-n 0.005 and n-i-p types according to the film deposi- 0 0 tion order, although the window layer of the 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 solar cell is the p-type layer in both cases. VOLTAGE(V) The characteristics of a cell having a p-i-n structure were first reported by Neuchâtel 6,7) Fig. 2: Current-voltage characteristics of an n-i-p type microcrystalline silicon cell (film thickness University. Unlike an amorphous solar cell, 2 2 µm, area 1 cm , measured by JQA).
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