Impact of solid-phase crystallization of amorphous silicon on the chemical structure of the buried Si/ZnO thin film solar cell interface

Abstract

͑͒The chemical interface structure between phosphorus-doped hydrogenated amorphous silicon and aluminum-doped zinc oxide thin films is investigated with soft x-ray emission spectroscopy ͑XES͒ before and after solid-phase crystallization ͑SPC͒ at 600°C. In addition to the expected SPC-induced phase transition from amorphous to polycrystalline silicon, our XES data indicates a pronounced chemical interaction at the buried Si/ZnO interface. In particular, we find an SPC-enhanced formation of Si-O bonds and the accumulation of Zn in close proximity to the interface. For an assumed closed and homogeneous SiO 2 interlayer, an effective thickness of ͑5 Ϯ 2͒ nm after SPC could be estimated. ͓͔ Polycrystalline silicon ͑poly-Si͒ is a promising absorber material candidate for low-cost, high efficiency thin film solar cells. An attractive approach to obtain poly-Si at relatively low process temperatures is solid-phase crystallization ͑SPC͒ of hydrogenated amorphous silicon ͑a-Si:H͒. 1 Solar cell mini modules ͑94 cm 2 ͒ based on poly-Si prepared by the SPC process have already demonstrated efficiencies of up to 10.4% by utilizing a complex point contact design. 2 If this contact structure could be replaced by a transparent conducting oxide ͑TCO͒, the manufacturing process would be greatly simplified and an easy series interconnection scheme using laser scribing would become possible; both would aid large-scale mass production needed for commercialization. In addition, by texturing the TCO, one could easily improve light trapping. For these reasons, an Al/ a-Si: H͑p + ͒ / poly-Si͑p͒ / poly-Si͑n + ͒ / ZnO: Al/ glass device structure was recently suggested. 3 This design, however, involves a high-temperature annealing step ͑SPC at 600°C͒, which leads to significantly higher electron mobilities in the capped ZnO:Al layer 4 but could also induce chemical interaction/diffusion processes at/across the Si/ZnO interface. Hence, this letter presents a soft x-ray emission spectroscopy ͑XES͒ investigation of the buried Si/ZnO interface before and after SPC.For our experiments we used Borofloat ® borosilicate glass covered with an approximately 80 nm thick Si 3 N 4 layer ͑as diffusion barrier͒ as the substrate. As TCO a 900 nm ZnO:Al layer rf-sputtered onto the substrate was used ͓sample ͑i͔͒. Subsequently, 50 nm thin films of phosphorusdoped a-Si: H͑n + ͒ were deposited by plasma-enhanced chemical vapor deposition ͑PECVD͒ at approx. 200°C ͓sample ͑ii͔͒. For more details about sample preparation see Ref.2. After a-Si: H͑n + ͒ deposition, some samples underwent SPC at 600°C for 24 or 72 h in a tube furnace under N 2 -flow ͓samples ͑iii 24 ͒ and ͑iii 72 ͔͒. A schematic presentation of the investigated samples is shown in Fig. 1. To identify and eliminate spectral contributions of native silicon oxides at the Si thin film surfaces, XES measurements were performed before and after sample etching in hydrofluoric ͑HF͒ acid ͑30 s in 5% aqueous HF-solution͒. Reference materials ͑single crystalline Si as well as SiO 2 and ZnO powders͒ were charac...

Chemical speciation at buried interfaces in high-temperature processed polycrystalline silicon thin-film solar cells on ZnO:Al

“…However, it has been reported elsewhere that already after solid phase crystallization at 600 C, corresponding to the "no RTA" results shown here, the formation of Si-O bonds and the accumulation of Zn in close proximity to the interface occurs. 21 With increasing RTA-temperature, the shape of the curve is more and more fitting to the Zn 2 SiO 4 -spectrum illustrating the modification of the chemical bonding of the Zn close to the interface. The integration of a SiN barrier layer between poly-Si and ZnO:Al has no considerable influence on this behavior (grey curves in Fig.…”

“…Synchrotron based X-ray spectrometry is an emerging method for the non-destructive investigation of diffusion and the determination of the chemical species at buried interfaces of stacked thin layers, because of which it is particularly suited for the characterization of high-temperature processes in complex thin-film solar cell devices. [21][22][23] Here, we applied grazing incidence X-ray fluorescence combined with near edge X-ray absorption fine structure spectroscopy (GIXRF-NEXAFS) 24 at the Plane Grating Monochromator (PGM) beamline for undulator radiation 25 in the PTB laboratory at BESSY II for the analysis of buried ZnO:Al/poly-Si interfaces after high-temperature RTA processing.…”

Hard x-ray photoelectron spectroscopy study of the buried Si/ZnO thin-film solar cell interface: Direct evidence for the formation of Si–O at the expense of Zn-O bonds

“…Nevertheless, it was observed that the electrical properties of the ZnO:Al improved due to the heat treatment. 4 First, x-ray spectroscopy experiments 6,7 indeed indicate that the SPC treatment has an impact on the chemical Si/ZnO interface structure, but could not completely clarify the underlying reaction mechanism. In this letter, we employ photoemission and its unique capability in chemical speciation combined with hard x-ray excitation allowing for the study of relevant buried Si/ZnO interfaces i.e., the Si top layer is sufficiently thick to form the same interface structure as present in a real-world Si thin-film solar cell layer stack.…”

“…These findings are in good agreement with results of our earlier studies. 7 In order to shed more light on the chemical structure of the Si/ZnO interface and how it changes upon SPC annealing, we analyze the Si 1s and O 1s spectra at hm ¼ 6030 eV in more detail (Fig. 3).…”

The silicon/zinc oxide interface in amorphous silicon-based thin-film solar cells: Understanding an empirically optimized contact

“…Note that both thin Si samples exhibit a higher Si-O x contribution, presumably due to the higher surface/bulk ratio and, potentially, the previously observed oxidation at the Si/ZnO:Al interface. 6,33,34 The IIBB is determined by subtracting the binding energy difference between the core levels of the capping layer and the substrate of the thin silicon sample [e.g., E Si 2s Fig. 4.…”