Optimization of back ITO layer as the sandwiched reflector for exploiting longer wavelength lights in thin and flexible (30 µm) single junction c-Si solar cells
Graphical abstract
To meet the fabrication benchmarks as predicted by ITRPV in 2019 regarding the lowering of substrate thickness of single-junction solar cells, and also to achieve the sweet point of the four important factors, viz. cost effectiveness, reliability, efficiency and device flexibility, prototypes of single junction c-Si solar cell based on 30 µm monocrystalline wafer has been fabricated as a pilot scheme. This work also includes detailed explanations on the observed phenomena and theoretical validation of the light trapping mechanism in thin wafers.
Introduction
Cost effectiveness, reliability and efficiency are the three most important factors that should go hand in hand for successful utilization of solar photovoltaics over other renewable energy sources. Nowadays, many reports can be found on perovskite, dye sensitized, organic/polymer, inorganic-organic hybrid type advanced solar cells (Green et al., 2014, Yu et al., 2010, Wang et al., 2016, Wright and Uddin, 2012) with notably high efficiency and flexible structure in some cases. However, there are several predicaments regarding reliability which is restricting major commercialization of such cells. The other kind of solar cell mainly includes the silicon based technology, in which conventional amorphous silicon (a-Si:H) cells are less expensive due to low material consumption and lower thermal budget (Lozac’h et al., 2018, Limodio et al., 2019). However, light induced degradation (LID) is a critically opposing factor for such cells (Inglese et al., 2015, Curtin and Statler, 1975, Hashigami et al., 2003). The advanced variants of amorphous/crystalline solar cells like heterojunction with intrinsic thin layer (HIT), interdigited back contact (IBC), and passivated emitter rear cell (PERC) (Tsunomura et al., 2009, Nordmann et al., 2019, Hamer et al., 2018) include a very complicated process flow involving multi layer of both amorphous and crystalline form of silicon and so, yet to get popularity for wide commercialization. For these reasons, thick (180 µm) wafer based single junction monocrystalline or multicrystalline solar cells hold the lion’s share of the PV industry till date. There are some challenges too in conventional 180 µm silicon cell technology that should be dealt with proper care to make solar PV a prime source for renewable energy. The high capital cost, excessively thick absorber layer and lack of flexibility in the device structure make it less attractive for future technologies, though, the reliability factor in this case is notably higher than the other form of solar cells. Shockley reported (Shockley and Queisser, 1961, Roy et al., 2016) that 30 µm thick c-Si wafer is sufficient enough to fabricate single junction solar cells and this encouraged a group of researchers to develop new technologies for thin crystalline and amorphous kind of solar cells with both homo- and hetero-junctions. An interesting work has been reported by Wang et al. on steel supported thin silicon solar cells grown by reduced pressure chemical vapor deposition (RPCVD) that yielded 16.2% efficiency (Wang et al., 2013). Recently, the Interuniversity Microelectronics Centre (IMEC) of Belgium has reported around 16% efficient epitaxially grown 30 µm silicon solar cells (Kuzma-Filipek et al., 2012). Many PV industries like Solexel, Crystal Solar, Amberwave and Astrowatt, to name a few; have reported some benchmark efficiency on thin silicon solar cells fabricated using epitaxial and exfoliation techniques (Moslehi et al., 2012, https://www.greentechmedia.com, http://www.amberwave.com, Saha et al., 2013, Hilali et al., 2014). Very recently, Lee et al. reported ~50 µm thick “kerfless” wafer based solar cells with 15.2% efficiency (Lee et al., 2018). Using porous silicon layer transfer process, Petermann et al fabricated a 43 µm thick epitaxially grown silicon solar cell that resulted in 19% efficiency (Petermann et al., 2012). Fabrication of silicon micro-cells is another interesting pathway toward the development of high efficiency thin silicon cells. Yao et al fabricated an assembly of silicon micro-cells by micromachining of Si (1 1 1) wafers which came out with a best efficiency of 11.7% under AM 1.5D radiation (Yao et al., 2013). However, to date, there are limited references on such single junction (homo-junction) crystalline wafer based solar cells with notable conversion efficiency that can go in-line with main stream fabrication flow line. On the other hand, in 2019, the international technology roadmap for photovoltaic (ITRPV) predicted that there would be a considerable lowering in the substrate thickness for wafer based mono and multicrystalline solar cells within the 2029. Here, we have started with an extreme case, i.e., 30 µm wafer thickness. In an attempt to achieve the sweet point of these four factors, viz. cost effectiveness, reliability, efficiency and device flexibility, this work has been taken as a pilot scheme to make a single junction c-Si solar cell prototype based on 30 µm wafer obtained by scaling down technique due to the lack of commercially available thin wafers. 30 µm thick c-Si wafer is known to pass through a portion of light in the longer wavelengths (>700 nm) owing to the fact of its absorption coefficient in the order 103/cm at this point (Carotta et al., 1992, Thorp et al., 1996). So, a thin indium tin oxide (ITO) layer has been introduced in between the aluminum back contact and p-Si layer for proper management of lights with longer wavelengths (Das et al., 2017) as well as to block the dangling bonds on the silicon surface to minimize the defects. The influence of thickness of this ITO layer on the efficiency of the cell has been studied thoroughly and validated through theoretical modeling. Starting from the basic p-n junction device, four cells with different architecture were fabricated in batches using the thin (~30 µm) wafer for stepwise improvement in efficiency. If solar cells can be made using 1/6th of the material than the conventional system without compromising much with reliability and efficiency, a drastic cost reduction can be expected along with the additional advantage of flexibility of the device. This will also help us to adhere to the ITRPV predictions. The objective of the present work includes (i) development of thin c-Si wafer based single junction solar cells with flexible structure to facilitate the concept of future roll-to-roll electronics, (ii) use of some optical management that is required to overcome the light losses (mainly longer wavelengths) in thin wafers without compromising with the conventional Al BSF structure, and (iii) fabrication of the proposed device in-line with presently available fabrication technology for the ease of commercialization.
Section snippets
Device fabrication
Commercially available (Just Solar, China) p-type and 180 µm thick 〈1 0 0〉 monocrystalline Si wafers were taken to produce 30 µm wafers by a scaling down technique through alkali etching. 3 in. × 3 in. of such wafers were vertically mounted in a beaker containing 10% aqueous AR grade NaOH solution at a temperature of 85 °C and quickly removed after 55 min and washed thoroughly with deionized water followed by isopropanol. This gives a 30 µm thin c-Si substrate. Improvement in throughput will be
Structural properties of thin wafers
The thickness of the wafer after scaling down was established by taking a cross-sectional micrograph which is presented in Fig. 2a. This indicates the formation of uniform 30 µm wafer by alkali etching (indicated between two arrows in Fig. 2a). A topographical atomic force microscopic (AFM) image was also captured (Fig. 2b) to check for any deformities that might be present in the surface due to alkali etching, and it has been found that the surface of the thin wafer does not contain any
Conclusion
In an attempt to meet the sweet point of four major criteria of modern photovoltaics, i.e. cost effectiveness, reliability, efficiency and flexibility, a new concept of single junction monocrystalline silicon solar cells based on 30 µm thick wafer has been introduced. The fabricated prototype showed a state-of-the-art efficiency of 12.23% under AM 1.5 simulated radiation. To manage the incident photons with longer wavelengths (>700 nm) that has a tendency to pass through the thin wafer, an ITO
Acknowledgement
The authors would like to acknowledge DST (GoI) for providing the fabrication facility to the Centre of Excellence for Green Energy and Sensor Systems, Indian Institute of Engineering Science and Technology under its DST-Solar HUB 2nd Phase project (DST/TMD/SERI/HUB/2C) and CGCRI-CSIR, Kolkata for the XRD measurements. The authors would also like to thank Mr. Pritam Banerjee of CEGESS, IIEST, Shibpur for his untiring help in simulation studies.
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The author is currently the Director, Indian Institute of Engineering Science and Technology, Shibpur, Howrah 711103, India.