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Intrinsic and doped amorphous silicon carbide films for the surface passivation of silicon solar cells

Intrinsic and doped amorphous silicon carbide films for the surface passivation of silicon solar cells


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SUWITO, Dominik, 2011. Intrinsic and doped amorphous silicon carbide films for the surface passivation of silicon solar cells [Dissertation]. Konstanz: University of Konstanz. Dr. Hut. ISBN 978-3-86853-853-3

@phdthesis{Suwito2011Intri-19346, publisher={Dr. Hut}, title={Intrinsic and doped amorphous silicon carbide films for the surface passivation of silicon solar cells}, year={2011}, author={Suwito, Dominik}, address={Konstanz}, school={Universität Konstanz} }

The amorphous silicon carbide films investigated throughout this work were pro-duced on the basis of plasma enhanced chemical vapor deposition (PECVD). The two different PECVD reactors used for the deposition of the films were presented. The principle development of the material was conducted at the AK400M batch-type reactor from Roth&Rau and the processes were then transferred to an industrial type SINA in-line reactor from the same company. For standard passivating Si-rich a-Si1 xCx layers deposited in the AK400M reactor working with RF power only, SIMS measurements revealed rather low C-atom densities (< 10 at.%) in the matrix. Doping atom densities of 1×1021 cm-3 for phosphorous and of 5×1020 cm-3 for boron were determined for a typical gas flow of 100 sccm (B2H6 or PH3 highly diluted in H2). The a Si1 xCx films fabricated in the SINA reactor exhibit a similar composition as well as similar electrical properties as their batch-type counterparts.<br /><br />A parallel analysis of a-Si1-xCx/c-Si and a-Si1-xCx/c-Ge systems was performed. From literature it is known that the main differences in terms of surface passivation of crystalline silicon and germanium substrates are the instability of (thermally grown) GexOy and the limited practicability of hydrogen for the passivation of Ge dangling bonds. From a compositional viewpoint, the performed transmission electron micros-copy (TEM) studies reveal no difference in the film structure depending on the substrate type. With increasing C-content, the Si-rich a-Si1-xCx films become less dense, a fact which is attributed to an increasing microvoid density. The c-Si surface passivation quality is found to be directly correlated with the Si-H bond density in the film. The onset of Si H bond rupture at around 300°C therefore coincides with the onset of electrical degradation independently of the carbon content and the doping density in the film. The surface passivation of germanium by a-Si1-xCx demonstrates a fairly different behavior. First of all, the thermal stability of intrinsic films is significantly increased. The electrical degradation starts at temperatures as high as 450°C and therefore indicates a “decoupling” of hydrogen present in the film and passivation quality.<br /><br />Regarding the optimum substrate temperature during film deposition, a value of Topt=270°C was found to provide the best overall performance for Si as well as for the Ge substrate. A similar behavior for both substrate types is observed for deposition temperatures below and above this temperature. Post-deposition annealing improves the passivation quality for Tann <Topt and results in a degradation for Tann >Topt. This finding is in accordance with the role of hydrogen for the passivation of c-Si surfaces. As a result of the experiments, the carbon in the a-Si matrix is supposed to inhibit the epitaxial growth observed for a-Si depositions at increased temperatures. The latter is considered to be responsible for the electrical degradation of the film. In the case of c Ge, the level of surface passivation is assumed to be indirectly correlated with the hydrogen incorporation at increased deposition temperatures since the latter is supposed to have a direct impact on the growth kinetics of the film.<br /><br />From isothermal lifetime experiments, an activation energy for the degradation of the passivation quality was extracted. For the c-Si/a-Si1-xCx system this energy amounts to approx. 0.5 eV, for the c-Ge/a-Si1-xCx system a value of approx. 2 eV was found, clearly pointing to different passivation mechanisms involved. Furthermore the results of this type of experiment allow for linking the temperature stability of the electrical properties of the film to the equilibrium between Si-H bond rupture and its inverse process (Si H↔Si + H). The latter is triggered by the availability of free atomic or molecular hydrogen in the matrix.<br /><br />A set of a-Si1-xCx passivated silicon wafers of different thicknesses allowed for the extraction of the “pure” injection dependent effective SRVs. The fitting of the SRV data of p- and n-type silicon substrate with the extended Shockley-Read-Hall (SRH) model reveals a change of sign for the fixed charge density parameter Qf of the film when changing the doping polarity of the substrate. The existence of amphoteric interface (near) states is confirmed by SPV measurements. Several experiments performed with the a-Si1-xCx/c-Ge system point to a direct saturation of Ge dangling-bonds by Si- and/or C-atoms. The latter effectively suppresses carrier recombination at the germanium surface. The passivation of c-Si surfaces on the contrary is inherently linked to the saturation of Si dangling-bonds by hydrogen. Although the incorporation of carbon in the a-Si matrix was shown to clearly enhance the thermal stability of the c-Si passivation in the temperature range up to 500°C (as compared to pure a-Si), the stability during firing processes (750-900°C) could not be verified.<br /><br />The performance of intrinsic and doped a-Si1-x¬Cx as rear side passivation (in im-mediate contact to the solar cell base) was investigated. Intrinsic films preserve their excellent passivation quality at the solar cell level, in particular no indication of inversion layer shunting comparable to the phenomenon observed in SiNx is found. The performance of its doped counterparts as passivating media strongly depends on the doping level and on the applied rear contacting scheme. However, the major benefit of doped Si-rich a Si1 xCx layers is unveiled in combination with (thin) intrinsic films. This is demonstrated by a newly developed rear passivation and contacting scheme based on intrinsic and doped a Si1 xCx films in conjunction with an adequate laser process (PassDop). The fulfilled demands of the amorphous silicon carbide system on the rear of the solar cell are threefold: very low SRVs are yielded by the intrinsic film next to the crystalline silicon interface, an overlying highly doped Si-rich a Si1 xCx layer acts as dopant source during the laser process and a C-rich layer enhances the optical con-finement at the rear of the cell. On high-efficiency n-type solar cells, this approach evidenced a very high process stability and led to efficiencies of up to 22.4 % (Voc =701 mV, FF =80.1 %). On p-type cells, the equivalent process yielded efficiencies of up to 21.6 % (Voc =683 mV, FF =80.7 %) being therefore directly comparable to passivation schemes featuring high quality thermal SiO2 and to atomic-layer deposited Al2O3 in combination with the LFC process.<br /><br />The use of a highly doped a Si1 xCx layer as a doping source during the solar cell fabrication can result in a thermally stable high-low passivation scheme. A simplified process sequence for n-type solar cells based on the in-situ diffusion of a n+ BSF from phosphorous doped a Si1 xCx was introduced. The diffusion of the rear high-low junc-tion takes place during the high-temperature process for the formation of the boron emitter. The rear contacting is done by a laser process equal to the one presented in the PassDop approach. The introduction of an additional C-rich film on the rear makes this approach optically superior to diffused full area contacted BSFs. In combination with a well passivated front side, this concept was found to have an efficiency potential of up to 20 %.<br /><br />Layer structures based on amorphous silicon carbide were also applied to the solar cell front side. The former necessarily comprise at least a two layer system. A thin Si-rich film (≈ 10 nm) next to the interface is needed for the electrical surface passivation and a C-rich film of adjusted layer thickness serves as anti-reflection coating (ARC).<br /><br />Si-rich a-Si1-x¬Cx films exhibit good passivation properties on planar (shiny etched) phosphorous diffused n+-surfaces (J0e ≈ 40 fA/cm2), however a strongly deteriorated performance is observed on textured (random pyramids) surfaces (J0e ≈ 400 fA/cm2). Boron diffused p+-surfaces could not be satisfactorily passivated by a-Si1-x¬Cx. (J0e ≈ 900 fA/cm2). The typical procedure for the extraction of the emitter saturation current density (J0e) from lifetime measurements was found to be unfeasible for the passivation with a Si1 xCx. This finding is ascribed to the strongly injection dependent surface recombination velocity of a Si1 xCx at low injection levels independently of the surface doping polarity.<br /><br />C-rich a SiyC1-y (y >0.5) layers were optimized regarding their transparency, yielding anti-reflection coating properties slightly inferior to the ones of typical ARC-SiNx. A low temperature approach for the front side passivation and front contact formation (SE-PassDop) was presented which basically consists of a laser process for opening of the silicon carbide and of subsequent front metallization by Ni/Ag or Ni/Cu plating. During the laser process, additional dopant atoms incorporated in the front passivation scheme are driven into the surface and allow for the formation of a selective emitter.<br /><br />First p-type solar cells featuring the PassDop rear and the SE-PassDop front side approach for the passivation and contacting of respective surfaces were fabricated with planar (shiny etched) and textured (random pyramids) front side. A strong parasitic absorption on the front side is mainly attributed to a non-optimized C-rich a SiyC1-y layer. Open-circuit voltages of up to 672 mV and 623 mV evidence a high level of surface passivation on planar emitters and a strongly deteriorated performance on textured surfaces, respectively. The feasibility of the metallization concept including the laser process is impressively demonstrated by fill factors of up to 79 %. The incor-poration of a doped, Si-rich a Si1 xCx film in the emitter passivation scheme proved to be beneficial in terms of series (contact) resistance as well as in terms of front surface recombination.<br /><br />While the parasitic absorption in the front a-SiCx layer stack should be strongly re-duced by application of an optimized C-rich a SiyC1-y layer and by further reduction of the thickness of the passivating/dopant containing films, it is not yet clear how to over-come the problems related to the poor electrical passivation of surfaces featuring random pyramids.<br /><br />As a final, practical remark, passivating a Si1 xCx is more closely related to amor-phous silicon than to SiNx – from a chemical as well as from an electrical viewpoint. However, “carbon doping” of the amorphous silicon matrix clearly exhibits a beneficial impact on the (low temperature) thermal stability of the film (< 500°C) while maintaining an excellent level of electrical properties. Suwito, Dominik Dr. Hut 2011 eng 2012-06-05T09:24:54Z 978-3-86853-853-3 terms-of-use 2012-06-05T09:24:54Z Intrinsic and doped amorphous silicon carbide films for the surface passivation of silicon solar cells Suwito, Dominik

Dateiabrufe seit 01.10.2014 (Informationen über die Zugriffsstatistik)

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