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Development of interdigitated solar cell and module processes for polycrystalline-silicon thin films
Thin Solid Films 511 – 512 (2006) 608 – 612 www.elsevier.com/locate/tsf
Development of interdigitated solar cell and module processes for polycrystalline-silicon thin films I. Gordon *, K. Van Nieuwenhuysen, L. Carnel, D. Van Gestel, G. Beaucarne, J. Poortmans IMEC vzw, Kapeldreef 75, B-3001 Leuven, Belgium Available online 18 January 2006
Abstract Thin-film polycrystalline-silicon (pc-Si) solar cells with a high efficiency could lower the price of photovoltaic electricity substantially. Efficient thin-film solar cells will not only lead to a cost reduction by the use of less silicon material, but will also reduce the module fabrication costs if a monolithic module process is used that integrates cell interconnection with cell contacting. Aluminium-induced crystallization (AIC) of amorphous silicon followed by epitaxial thickening recently proved to be a simple way to obtain large-grained pc-Si thin films with excellent properties for solar cells. However, cell processes different from those for bulk-Si cells have to be implemented to fully exploit the pc-Si material quality and to obtain working solar cells. In this work, we propose a simple monolithic module process for thin-film pc-Si solar cells, in which all contacts are on top of the cells in an interdigitated pattern. As a first step towards implementation of this process, we made single pc-Si solar cells with interdigitated top contacts, using pc-Si layers on ceramic substrates grown by AIC in combination with high-temperature epitaxy. Next, we made pc-Si modules using a simplified metallization scheme. The interdigitated pc-Si cells had much higher efficiencies than mesa cells with base contacts at the periphery of the cells due to a lower series resistance and a higher current density. The maximum obtained cell efficiency was 5.6%, which is the highest efficiency ever achieved with pc-Si solar cells on ceramic substrates where no (re)melting of Si was involved. First module results showed that good cell separation and isolation is crucial to obtain proper working modules. Our interdigitated cell results indicate that monolithic thin-film modules with interdigitated top contacts based on pc-Si layers made by AIC will most likely lead to high efficiencies. D 2005 Elsevier B.V. All rights reserved. Keywords: Silicon solar cell; Thin film; Monolithic module; Aluminium-induced crystallization
1. Introduction The current high price of photovoltaic electricity could be lowered substantially if efficient solar modules could be made from polycrystalline-silicon (pc-Si) thin films on cheap foreign substrates. At present the semiconducting material in standard crystalline-silicon solar modules accounts for roughly 30% of the total device cost. A thin-film Si solar cell technology could lead to much cheaper devices not only by the use of less silicon material, but also by the implementation of monolithic module processes. To lower the photovoltaic electricity price, the thinfilm modules not only need to be cheap but also need to have high efficiencies. Polycrystalline-Si films with grain sizes between 1 –100 Am seem particularly good candidates to obtain low-cost thin-film solar cell modules with high efficiencies. This was recently shown by CSG Solar who developed an advanced * Corresponding author. Tel.: +32 16 28 82 49; fax: +32 16 28 15 01. E-mail address: [email protected] (I. Gordon). 0040-6090/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2005.12.124
module process for pc-Si cells in superstrate configuration, making use of laser grooving, an insulating resin and point contacts created using inkjet technology [1]. Recently, we showed that aluminium-induced crystallization (AIC) of amorphous silicon (a-Si) followed by high-temperature epitaxial thickening is an effective way to obtain good pc-Si solar cells [2]. Ideally, the grains of pc-Si films used for solar cells should have a low aspect ratio. However, small grains with an average size below 1 Am are usually obtained when pc-Si is deposited directly onto foreign substrates. In contrast, aluminium-induced crystallization of amorphous silicon leads to very thin pc-Si layers with grain sizes around 10 – 20 Am [3]. Absorber layers of sufficient thickness but with a low aspect ratio can be obtained by epitaxial thickening of these AIC layers. To obtain efficient pc-Si solar cells, cell processes different from those applied for standard bulk-Si solar cells have to be developed. Because pc-Si layers for solar cells are typically only a few Am thick and contain many defects, light trapping and defect passivation are crucial in any pc-Si solar cell process.
I. Gordon et al. / Thin Solid Films 511 – 512 (2006) 608 – 612
Furthermore, new metal contact schemes have to be developed because most foreign substrates suited for pc-Si solar cells are insulating, preventing the use of a backcontact. Finally, in designing new cell concepts for pc-Si thin films, one should aim at integrating the cell interconnection with the cell fabrication to ensure a maximum cost reduction in module fabrication. In this paper we report on the development of a monolithic module process for thin-film pc-Si solar cells in substrate configuration, in which the cell interconnection is combined with the cell contacting. In this process, all contacts are made on top of the cells in an interdigitated pattern. As a first step towards implementation of this process, we made single pc-Si solar cells with interdigitated top contacts and compared these to mesa cells that had base contacts at the periphery of the cells. We also made some first pc-Si modules to gain experience in cell isolation. We used large-grained thin pc-Si films on ceramic substrates made by AIC in combination with thermal chemical vapour deposition (CVD) to carry out our research. 2. Monolithic module process In this section we propose a monolithic module process for poly-Si thin films in which the cell interconnection is combined with the cell contacting. Fig. 1 shows a schematic top view of the resulting interconnection of three adjacent cells. The individual cells are separated by laser grooving or alternatively by dry or wet chemical etching. The cells have an elongated shape and are connected in series by an alternating overlap of the contact fingers of one polarity with the contact fingers of opposite polarity of the next cell. To contact the base of the cells, the emitter has to be locally removed by, e.g., etching. Busbars for both polarities are formed at the edges of the module. The contact spacing between fingers of the same polarity has to be optimized by taking resistive and optical losses into account. Different process sequences can be used to achieve the module structure that is shown in Fig. 1. We propose a simple laboratory process in which the metallisation is done in two steps by photolithography in combination with metal evaporation. In an industrial process however, the metallisation
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should ideally be performed in one single step by screenprinting or evaporation through a shadow mask [4]. Our laboratory process consists of five steps: Step 1: An emitter is formed over the whole pc-Si film by phosphorus diffusion. The surface of the layers can be textured in advance to obtain light trapping. Step 2: The layers are passivated by plasma hydrogenation. This step is crucial to get good working pc-Si solar cells because as-grown pc-Si layers typically contain a lot of defects [5]. After the defect passivation, SiNx is deposited as an anti-reflective coating (ARC). Step 3: Grooves are formed by laser grooving or by etching to divide the layer into individual cells. At the same time, the parasitic junction at the edges of the sample is removed. Next, to prevent shunting the grooves are partially filled with an insulating material by using, e.g., flowable dielectrics. Step 4: Base contacts are formed by photolithography in combination with aluminium evaporation. To contact the base layer, the emitter layer is locally removed by etching prior to the metal evaporation. After evaporation, the sample is annealed for a short time at 400 -C to obtain good ohmic contacts. Step 5: Emitter contacts are formed by photolithography and evaporation of Ti/Pd/Ag stacks. The SiNx layer is locally removed by etching to contact the emitter layer. A careful alignment is necessary to ensure that the emitter fingers are located at the right spots with respect to the base fingers. The base fingers are broader than the emitter fingers to reduce resistive losses (see Fig. 1). To implement the above module process on our pc-Si layers, we started by making single cells with interdigitated top contacts using the same steps as described above except for step 3. Simultaneously, we did some experiments on making modules with a simplified (non-interdigitated) metallisation scheme to gain experience with the groove formation and groove filling of step 3. Section 3 describes the experimental details while Section 4 deals with the results obtained on our interdigitated pc-Si cells and on our first module tests. 3. Experimental
Fig. 1. Schematic top view of the cell interconnection in our proposed monolithic module process. Busbars are at the two outer sides of the module (not shown in the figure).
We made pc-Si films on alumina substrates by epitaxial thickening of AIC seed layers. The substrates were covered by a spin-on flowable oxide (Fox-25 from Dow Corning) to reduce their surface roughness, prior to the seed layer formation. Double layers of Al and a-Si were deposited on these substrates in an electron-beam high-vacuum evaporator. In between the two depositions, the aluminium was oxidized by exposure to air for two minutes. The thickness of the Al and a-Si layers was fixed at 200 and 250 nm, respectively. After deposition, the samples were annealed in a tube furnace under nitrogen ambient at 500 -C for 4 h. During this annealing, the a-Si crystallized into pc-Si and both layers exchanged places [3]. Finally, the top aluminium layer was removed by selective chemical etching.
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it briefly characterizes the grain size distribution of the pc-Si layers used in the cells and modules. Secondly, it compares solar cells with interdigitated top contacts to solar cells with base contacts at the periphery of the cells. Finally, it describes our first results obtained on thin-film pc-Si modules. 4.1. Layer characterization The average grain size of the pc-Si layers used in this work was 5 Am, as determined by optical microscopy [6]. The surface of the pc-Si films was too rough after epitaxial deposition for individual grains to be distinguishable. However, after mechanical polishing of the layers followed by a secco etch, grain boundaries became visible. The standard deviation on the average grain size was around 2.5 Am, while the largest grain size observed in these layers was about 12 Am. These values indicate that there is a large variation in grain size in our pc-Si layers with the presence of some very large grains but also of a lot of small grains. 4.2. Interdigitated solar cells
Fig. 2. Schematic structure of pc-Si solar cells with mesa contacts (a) and interdigitated contacts (b). The different layers are not drawn to scale.
Absorber layers were deposited on the AIC layers by thermal CVD. The depositions were performed in a singlewafer epitaxial reactor (ASM Epsilon 2000) under atmospheric pressure, at a temperature of 1130 -C. The growth rate was around 1.4 Am/min. Double layers of p+ and p silicon with variable thickness ratios were made. The p+ layer acts as a back surface field (BSF) while the p layer is the actual absorber layer. The total layer thickness was always between 2 and 6 Am. Solar cells with different metal contact schemes were made from these absorber layers by emitter formation followed by defect passivation and metallization. After epitaxial deposition, n-type emitters were formed by phosphorus diffusion from a doped pyrolithic oxide. Defect passivation of the layers was performed by plasma hydrogenation in a PECVD system, directly followed by the deposition in the same system of a SiNx layer that is used as an anti-reflective coating. The details of this process are described in Ref. [5]. Next, metal contacts were formed by photolithography and wet chemical etching in combination with metal evaporation. Different contact schemes were used to obtain cells with all contacts on top of the cell (interdigitated cells) or with emitter contacts on top and base contacts at the periphery of the cell (mesa cells). All cells were separated by wet chemical etching and had an active area of 1 cm2, defined as the sample area exposed to light. 4. Results and discussion This section presents the interdigitated solar cell results and module results obtained so far on our pc-Si layers. First,
As a first step towards implementation of our monolithic module process, we made single pc-Si solar cells with interdigitated top contacts and compared these to mesa cells that had base contacts at the periphery of the cells (see Fig. 2). We tested our interdigitated cell process on samples with a homojunction emitter, but the same process could also be applied on samples with a heterojunction emitter [7]. To compare the two cell types, we always processed both on pc-Si layers with identical parameters. Shadowing losses caused by the metal top contacts amounted to around 6% for the mesa cells and around 8% for the interdigitated cells. Interdigitated solar cells always showed larger fill factors and larger current densities compared to mesa cells. We investigated three layer types with different thickness (see Table 1). Samples A to D had a base doping of 3 1016 cm 3 while samples E and F had a base doping of 1 1017 cm 3. The open-circuit voltages (Voc) of the interdigitated cells were almost the same as those of the corresponding mesa cells for all three layer types. The short-circuit densities ( J sc) and the fill factors (FF) of the interdigitated cells however were always substantially larger than those of their mesa counterparts. The Table 1 Comparison between thin-film pc-Si solar cells with different layer thickness and different metal contact schemes Sample
t Base (Am)
t BSF (Am)
Cell type
J sc (mA/cm2)
Voc (mV)
Fill factor (%)
Eff. (%)
A B C D E F
1.5 1.5 1.5 1.5 2.0 2.0
0.5 0.5 1.0 1.0 2.0 2.0
Mesa Interdigitated Mesa Interdigitated Mesa Interdigitated
16.4 17.9 14.1 15.2 14.7 15.7
460 455 450 455 438 431
46.6 69.0 56.1 69.2 61.3 69.6
3.5 5.6 3.6 4.8 3.9 4.7
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substrates where no (re)melting of Si was involved. The corresponding mesa cell (sample A) had an efficiency of only 3.5% due to the very thin BSF layer. Interdigitated cells generally led to higher efficiencies than mesa cells independent of the cell thickness (see Table 1). Although sample B was the thinnest of the interdigitated cells, it had the highest J sc and Voc values. As reported earlier, the Voc values of our pc-Si cells increase with decreasing total thickness [8]. This probably arises from a partial breakdown of the epitaxial growth due to the build-up of stress in the growing layer. This breakdown of the layer quality might also explain the larger current densities observed for the thinnest cells. 4.3. First module results
Fig. 3. Illuminated current – voltage characteristics of mesa (sample A) and interdigitated (sample B) cells on very thin pc-Si layers.
difference in J sc was between 1.0 and 1.5 mA cm 2 for all layers while the increase in FF depended strongly on the layer thickness. All interdigitated cells had fill factors around 69% independent of the layer thickness. The fill factors of the mesa cells however strongly depended on the BSF thickness with values ranging from 47% for a BSF thickness of 0.5 Am to 61% for a BSF thickness of 2.0 Am. The larger fill factor of interdigitated cells compared to mesa cells arises from a much lower series resistance as can be seen in Fig. 3 for the thinnest samples A and B. These samples had BSF layers of only 0.5 Am thickness. The mesa cell had a very high series resistance of 8.5 V cm2 and a low fill factor of 47%. This high series resistance results from the long distance holes have to travel through the thin, highly resistive BSF layer to reach the base contacts. In contrast, the series resistance of the interdigitated cell was below 1 V cm2, yielding a much higher fill factor of 69%. While the series resistance of the interdigitated cells was almost independent of the BSF thickness, the series resistance of the mesa cells strongly depended on the BSF thickness. Nevertheless, the series resistance of the mesa cells always stayed above 2.5 V cm2 even for BSF layers of 3 Am thickness, which is well above the 1 V cm2 series resistance of the interdigitated cells. The larger current density of the interdigitated cells compared to the mesa cells arises from a much better spectral response of the interdigitated cells in the visible part of the light spectrum (see Fig. 4). Short-circuit densities calculated from the spectral response curves correspond well to the J sc values obtained from the current– voltage curves of Fig. 3. Up to now, it is not clear why the interdigitated cells lead to a better response in the visible part of the light spectrum. The highest energy conversion efficiency obtained so far on our interdigitated pc-Si cells was 5.6% (sample B). For comparison, the highest efficiencies we obtained on pc-Si mesa cells were 5.0% for cells with a homojunction emitter [6] and 5.3% for cells with a heterojunction emitter [7]. To our knowledge, the 5.6% efficiency of sample B is the highest efficiency ever achieved with pc-Si solar cells on ceramic
We tested a simplified (non-interdigitated) module process on our poly-Si layers to gain more experience with the separation and isolation of individual cells (step 3 in our module process). In this simplified process we used dry etching through a polymeric mask to separate the cells. The modules had an active area of 7 8 cm2 consisting of 26 cells of dimensions 2.7 80 mm2. The best module had an open-circuit voltage around 6.7 V (corresponding to 255 mV per cell), but the current density was only 8 AA cm 2. The main reason for these low voltage and current density values was a poor adhesion of the insulating material (e.g., silicones) to the grooves, resulting in local shunting and a local breakdown of the series connection of the cells. In the near future, we will apply the interdigitated module process we proposed in Section 2 on our pc-Si layers. In a first stage, modules with a total active area of 4 4 cm2 that consist of four cells in series will be processed. In a later stage, the process will be upscaled to larger module areas. Laser grooving will be used for cell isolation. The crucial step to obtain good working modules will be the proper separation and isolation of the individual cells, as indicated by our preliminary module results described above.
Fig. 4. External quantum efficiency measurements of mesa (sample A) and interdigitated (sample B) cells on very thin pc-Si layers.
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5. Conclusions and outlook
Acknowledgements
We presented a monolithic module process for thin-film pcSi solar cells, in which cell interconnection is combined with cell contacting. The metal contacts in this process are arranged in an interdigitated pattern on top of the cells. Preliminary module results showed that cell isolation is the most crucial step to obtain proper working pc-Si modules using our process. As a first step towards implementation of our module process we made thin-film pc-Si solar cells with interdigitated top contacts and achieved energy conversion efficiencies up to 5.6%. This is the highest efficiency ever achieved with pc-Si solar cells on ceramic substrates where no (re)melting of Si was involved. Interdigitated cells had much higher efficiencies than mesa cells with base contacts at the periphery of the cells due to lower series resistances and higher current densities. The higher current density arises from a much better spectral response of the interdigitated cells in the visible part of the light spectrum. Our cell results indicate that a monolithic module process with interdigitated metal top contacts can probably lead to efficient thin-film modules based on pc-Si layers made by AIC in combination with epitaxial deposition. To be cost-effective however, our solar cells and modules still need much higher efficiencies. In the future, we will focus both on improving the pc-Si layer quality [9] and on implementing novel process features like heterojunction emitters [7] to obtain interdigitated cells and modules of better quality.
This work was partly funded by the European Commission under contract numbers ENK5-CT-2001-00543 (FMETEOR_) and ENK6-CT-2002-00640 (FLATECS_). We thank J. Irigoyen for his investigation of the grain size distribution of our pc-Si layers. References [1] P.A. Basore, in: Proceedings 19th European Photovoltaic Solar Energy Conference, vol. I, 2004, p. 455. [2] G. Beaucarne, D. Van Gestel, I. Gordon, L. Carnel, K. Van Nieuwenhuysen, C. Ornaghi, J. Poortmans, M. Sto¨ger-Pollach, P. Schattschneider, in: Proceedings 19th European Photovoltaic Solar Energy Conference, vol. I, 2004, p. 467. [3] O. Nast, T. Puzzer, L.M. Koschier, A.B. Sproul, S.R. Wenham, Appl. Phys. Lett. 73 (1998) 3214. [4] B. Terheiden, R. Horbelt, A. Hammud, R. Auer, R. Brendel, in: Proceedings 19th European Photovoltaic Solar Energy Conference, vol. I, 2004, p. 463. [5] L. Carnel, I. Gordon, K. Van Nieuwenhuysen, D. Van Gestel, G. Beaucarne, J. Poortmans, Thin Solid Films 487 (2005) 147. [6] I. Gordon, D. Van Gestel, L. Carnel, K. Van Nieuwenhuysen, J. Irigoyen, G. Beaucarne, J. Poortmans, Proceedings 20th European Photovoltaic Solar Energy Conference, 2005, p. 972. [7] L. Carnel, I. Gordon, D. Van Gestel, K. Van Nieuwenhuysen, G. Agostinelli, G. Beaucarne, J. Poortmans, Thin Solid Films in press. [8] I. Gordon, D. Van Gestel, K. Van Nieuwenhuysen, L. Carnel, G. Beaucarne, J. Poortmans, Thin Solid Films 487 (2005) 113. [9] D. Van Gestel, I. Gordon, L. Carnel, K. Van Nieuwenhuysen, J. D’Haen, J. Irigoyen, G. Beaucarne, J. Poortmans, Thin Solid Films in press.
Development of interdigitated solar cell and module processes for polycrystalline-silicon thin films I. Gordon *, K. Van Nieuwenhuysen, L. Carnel, D. Van Gestel, G. Beaucarne, J. Poortmans IMEC vzw, Kapeldreef 75, B-3001 Leuven, Belgium Available online 18 January 2006
Abstract Thin-film polycrystalline-silicon (pc-Si) solar cells with a high efficiency could lower the price of photovoltaic electricity substantially. Efficient thin-film solar cells will not only lead to a cost reduction by the use of less silicon material, but will also reduce the module fabrication costs if a monolithic module process is used that integrates cell interconnection with cell contacting. Aluminium-induced crystallization (AIC) of amorphous silicon followed by epitaxial thickening recently proved to be a simple way to obtain large-grained pc-Si thin films with excellent properties for solar cells. However, cell processes different from those for bulk-Si cells have to be implemented to fully exploit the pc-Si material quality and to obtain working solar cells. In this work, we propose a simple monolithic module process for thin-film pc-Si solar cells, in which all contacts are on top of the cells in an interdigitated pattern. As a first step towards implementation of this process, we made single pc-Si solar cells with interdigitated top contacts, using pc-Si layers on ceramic substrates grown by AIC in combination with high-temperature epitaxy. Next, we made pc-Si modules using a simplified metallization scheme. The interdigitated pc-Si cells had much higher efficiencies than mesa cells with base contacts at the periphery of the cells due to a lower series resistance and a higher current density. The maximum obtained cell efficiency was 5.6%, which is the highest efficiency ever achieved with pc-Si solar cells on ceramic substrates where no (re)melting of Si was involved. First module results showed that good cell separation and isolation is crucial to obtain proper working modules. Our interdigitated cell results indicate that monolithic thin-film modules with interdigitated top contacts based on pc-Si layers made by AIC will most likely lead to high efficiencies. D 2005 Elsevier B.V. All rights reserved. Keywords: Silicon solar cell; Thin film; Monolithic module; Aluminium-induced crystallization
1. Introduction The current high price of photovoltaic electricity could be lowered substantially if efficient solar modules could be made from polycrystalline-silicon (pc-Si) thin films on cheap foreign substrates. At present the semiconducting material in standard crystalline-silicon solar modules accounts for roughly 30% of the total device cost. A thin-film Si solar cell technology could lead to much cheaper devices not only by the use of less silicon material, but also by the implementation of monolithic module processes. To lower the photovoltaic electricity price, the thinfilm modules not only need to be cheap but also need to have high efficiencies. Polycrystalline-Si films with grain sizes between 1 –100 Am seem particularly good candidates to obtain low-cost thin-film solar cell modules with high efficiencies. This was recently shown by CSG Solar who developed an advanced * Corresponding author. Tel.: +32 16 28 82 49; fax: +32 16 28 15 01. E-mail address: [email protected] (I. Gordon). 0040-6090/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2005.12.124
module process for pc-Si cells in superstrate configuration, making use of laser grooving, an insulating resin and point contacts created using inkjet technology [1]. Recently, we showed that aluminium-induced crystallization (AIC) of amorphous silicon (a-Si) followed by high-temperature epitaxial thickening is an effective way to obtain good pc-Si solar cells [2]. Ideally, the grains of pc-Si films used for solar cells should have a low aspect ratio. However, small grains with an average size below 1 Am are usually obtained when pc-Si is deposited directly onto foreign substrates. In contrast, aluminium-induced crystallization of amorphous silicon leads to very thin pc-Si layers with grain sizes around 10 – 20 Am [3]. Absorber layers of sufficient thickness but with a low aspect ratio can be obtained by epitaxial thickening of these AIC layers. To obtain efficient pc-Si solar cells, cell processes different from those applied for standard bulk-Si solar cells have to be developed. Because pc-Si layers for solar cells are typically only a few Am thick and contain many defects, light trapping and defect passivation are crucial in any pc-Si solar cell process.
I. Gordon et al. / Thin Solid Films 511 – 512 (2006) 608 – 612
Furthermore, new metal contact schemes have to be developed because most foreign substrates suited for pc-Si solar cells are insulating, preventing the use of a backcontact. Finally, in designing new cell concepts for pc-Si thin films, one should aim at integrating the cell interconnection with the cell fabrication to ensure a maximum cost reduction in module fabrication. In this paper we report on the development of a monolithic module process for thin-film pc-Si solar cells in substrate configuration, in which the cell interconnection is combined with the cell contacting. In this process, all contacts are made on top of the cells in an interdigitated pattern. As a first step towards implementation of this process, we made single pc-Si solar cells with interdigitated top contacts and compared these to mesa cells that had base contacts at the periphery of the cells. We also made some first pc-Si modules to gain experience in cell isolation. We used large-grained thin pc-Si films on ceramic substrates made by AIC in combination with thermal chemical vapour deposition (CVD) to carry out our research. 2. Monolithic module process In this section we propose a monolithic module process for poly-Si thin films in which the cell interconnection is combined with the cell contacting. Fig. 1 shows a schematic top view of the resulting interconnection of three adjacent cells. The individual cells are separated by laser grooving or alternatively by dry or wet chemical etching. The cells have an elongated shape and are connected in series by an alternating overlap of the contact fingers of one polarity with the contact fingers of opposite polarity of the next cell. To contact the base of the cells, the emitter has to be locally removed by, e.g., etching. Busbars for both polarities are formed at the edges of the module. The contact spacing between fingers of the same polarity has to be optimized by taking resistive and optical losses into account. Different process sequences can be used to achieve the module structure that is shown in Fig. 1. We propose a simple laboratory process in which the metallisation is done in two steps by photolithography in combination with metal evaporation. In an industrial process however, the metallisation
609
should ideally be performed in one single step by screenprinting or evaporation through a shadow mask [4]. Our laboratory process consists of five steps: Step 1: An emitter is formed over the whole pc-Si film by phosphorus diffusion. The surface of the layers can be textured in advance to obtain light trapping. Step 2: The layers are passivated by plasma hydrogenation. This step is crucial to get good working pc-Si solar cells because as-grown pc-Si layers typically contain a lot of defects [5]. After the defect passivation, SiNx is deposited as an anti-reflective coating (ARC). Step 3: Grooves are formed by laser grooving or by etching to divide the layer into individual cells. At the same time, the parasitic junction at the edges of the sample is removed. Next, to prevent shunting the grooves are partially filled with an insulating material by using, e.g., flowable dielectrics. Step 4: Base contacts are formed by photolithography in combination with aluminium evaporation. To contact the base layer, the emitter layer is locally removed by etching prior to the metal evaporation. After evaporation, the sample is annealed for a short time at 400 -C to obtain good ohmic contacts. Step 5: Emitter contacts are formed by photolithography and evaporation of Ti/Pd/Ag stacks. The SiNx layer is locally removed by etching to contact the emitter layer. A careful alignment is necessary to ensure that the emitter fingers are located at the right spots with respect to the base fingers. The base fingers are broader than the emitter fingers to reduce resistive losses (see Fig. 1). To implement the above module process on our pc-Si layers, we started by making single cells with interdigitated top contacts using the same steps as described above except for step 3. Simultaneously, we did some experiments on making modules with a simplified (non-interdigitated) metallisation scheme to gain experience with the groove formation and groove filling of step 3. Section 3 describes the experimental details while Section 4 deals with the results obtained on our interdigitated pc-Si cells and on our first module tests. 3. Experimental
Fig. 1. Schematic top view of the cell interconnection in our proposed monolithic module process. Busbars are at the two outer sides of the module (not shown in the figure).
We made pc-Si films on alumina substrates by epitaxial thickening of AIC seed layers. The substrates were covered by a spin-on flowable oxide (Fox-25 from Dow Corning) to reduce their surface roughness, prior to the seed layer formation. Double layers of Al and a-Si were deposited on these substrates in an electron-beam high-vacuum evaporator. In between the two depositions, the aluminium was oxidized by exposure to air for two minutes. The thickness of the Al and a-Si layers was fixed at 200 and 250 nm, respectively. After deposition, the samples were annealed in a tube furnace under nitrogen ambient at 500 -C for 4 h. During this annealing, the a-Si crystallized into pc-Si and both layers exchanged places [3]. Finally, the top aluminium layer was removed by selective chemical etching.
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it briefly characterizes the grain size distribution of the pc-Si layers used in the cells and modules. Secondly, it compares solar cells with interdigitated top contacts to solar cells with base contacts at the periphery of the cells. Finally, it describes our first results obtained on thin-film pc-Si modules. 4.1. Layer characterization The average grain size of the pc-Si layers used in this work was 5 Am, as determined by optical microscopy [6]. The surface of the pc-Si films was too rough after epitaxial deposition for individual grains to be distinguishable. However, after mechanical polishing of the layers followed by a secco etch, grain boundaries became visible. The standard deviation on the average grain size was around 2.5 Am, while the largest grain size observed in these layers was about 12 Am. These values indicate that there is a large variation in grain size in our pc-Si layers with the presence of some very large grains but also of a lot of small grains. 4.2. Interdigitated solar cells
Fig. 2. Schematic structure of pc-Si solar cells with mesa contacts (a) and interdigitated contacts (b). The different layers are not drawn to scale.
Absorber layers were deposited on the AIC layers by thermal CVD. The depositions were performed in a singlewafer epitaxial reactor (ASM Epsilon 2000) under atmospheric pressure, at a temperature of 1130 -C. The growth rate was around 1.4 Am/min. Double layers of p+ and p silicon with variable thickness ratios were made. The p+ layer acts as a back surface field (BSF) while the p layer is the actual absorber layer. The total layer thickness was always between 2 and 6 Am. Solar cells with different metal contact schemes were made from these absorber layers by emitter formation followed by defect passivation and metallization. After epitaxial deposition, n-type emitters were formed by phosphorus diffusion from a doped pyrolithic oxide. Defect passivation of the layers was performed by plasma hydrogenation in a PECVD system, directly followed by the deposition in the same system of a SiNx layer that is used as an anti-reflective coating. The details of this process are described in Ref. [5]. Next, metal contacts were formed by photolithography and wet chemical etching in combination with metal evaporation. Different contact schemes were used to obtain cells with all contacts on top of the cell (interdigitated cells) or with emitter contacts on top and base contacts at the periphery of the cell (mesa cells). All cells were separated by wet chemical etching and had an active area of 1 cm2, defined as the sample area exposed to light. 4. Results and discussion This section presents the interdigitated solar cell results and module results obtained so far on our pc-Si layers. First,
As a first step towards implementation of our monolithic module process, we made single pc-Si solar cells with interdigitated top contacts and compared these to mesa cells that had base contacts at the periphery of the cells (see Fig. 2). We tested our interdigitated cell process on samples with a homojunction emitter, but the same process could also be applied on samples with a heterojunction emitter [7]. To compare the two cell types, we always processed both on pc-Si layers with identical parameters. Shadowing losses caused by the metal top contacts amounted to around 6% for the mesa cells and around 8% for the interdigitated cells. Interdigitated solar cells always showed larger fill factors and larger current densities compared to mesa cells. We investigated three layer types with different thickness (see Table 1). Samples A to D had a base doping of 3 1016 cm 3 while samples E and F had a base doping of 1 1017 cm 3. The open-circuit voltages (Voc) of the interdigitated cells were almost the same as those of the corresponding mesa cells for all three layer types. The short-circuit densities ( J sc) and the fill factors (FF) of the interdigitated cells however were always substantially larger than those of their mesa counterparts. The Table 1 Comparison between thin-film pc-Si solar cells with different layer thickness and different metal contact schemes Sample
t Base (Am)
t BSF (Am)
Cell type
J sc (mA/cm2)
Voc (mV)
Fill factor (%)
Eff. (%)
A B C D E F
1.5 1.5 1.5 1.5 2.0 2.0
0.5 0.5 1.0 1.0 2.0 2.0
Mesa Interdigitated Mesa Interdigitated Mesa Interdigitated
16.4 17.9 14.1 15.2 14.7 15.7
460 455 450 455 438 431
46.6 69.0 56.1 69.2 61.3 69.6
3.5 5.6 3.6 4.8 3.9 4.7
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substrates where no (re)melting of Si was involved. The corresponding mesa cell (sample A) had an efficiency of only 3.5% due to the very thin BSF layer. Interdigitated cells generally led to higher efficiencies than mesa cells independent of the cell thickness (see Table 1). Although sample B was the thinnest of the interdigitated cells, it had the highest J sc and Voc values. As reported earlier, the Voc values of our pc-Si cells increase with decreasing total thickness [8]. This probably arises from a partial breakdown of the epitaxial growth due to the build-up of stress in the growing layer. This breakdown of the layer quality might also explain the larger current densities observed for the thinnest cells. 4.3. First module results
Fig. 3. Illuminated current – voltage characteristics of mesa (sample A) and interdigitated (sample B) cells on very thin pc-Si layers.
difference in J sc was between 1.0 and 1.5 mA cm 2 for all layers while the increase in FF depended strongly on the layer thickness. All interdigitated cells had fill factors around 69% independent of the layer thickness. The fill factors of the mesa cells however strongly depended on the BSF thickness with values ranging from 47% for a BSF thickness of 0.5 Am to 61% for a BSF thickness of 2.0 Am. The larger fill factor of interdigitated cells compared to mesa cells arises from a much lower series resistance as can be seen in Fig. 3 for the thinnest samples A and B. These samples had BSF layers of only 0.5 Am thickness. The mesa cell had a very high series resistance of 8.5 V cm2 and a low fill factor of 47%. This high series resistance results from the long distance holes have to travel through the thin, highly resistive BSF layer to reach the base contacts. In contrast, the series resistance of the interdigitated cell was below 1 V cm2, yielding a much higher fill factor of 69%. While the series resistance of the interdigitated cells was almost independent of the BSF thickness, the series resistance of the mesa cells strongly depended on the BSF thickness. Nevertheless, the series resistance of the mesa cells always stayed above 2.5 V cm2 even for BSF layers of 3 Am thickness, which is well above the 1 V cm2 series resistance of the interdigitated cells. The larger current density of the interdigitated cells compared to the mesa cells arises from a much better spectral response of the interdigitated cells in the visible part of the light spectrum (see Fig. 4). Short-circuit densities calculated from the spectral response curves correspond well to the J sc values obtained from the current– voltage curves of Fig. 3. Up to now, it is not clear why the interdigitated cells lead to a better response in the visible part of the light spectrum. The highest energy conversion efficiency obtained so far on our interdigitated pc-Si cells was 5.6% (sample B). For comparison, the highest efficiencies we obtained on pc-Si mesa cells were 5.0% for cells with a homojunction emitter [6] and 5.3% for cells with a heterojunction emitter [7]. To our knowledge, the 5.6% efficiency of sample B is the highest efficiency ever achieved with pc-Si solar cells on ceramic
We tested a simplified (non-interdigitated) module process on our poly-Si layers to gain more experience with the separation and isolation of individual cells (step 3 in our module process). In this simplified process we used dry etching through a polymeric mask to separate the cells. The modules had an active area of 7 8 cm2 consisting of 26 cells of dimensions 2.7 80 mm2. The best module had an open-circuit voltage around 6.7 V (corresponding to 255 mV per cell), but the current density was only 8 AA cm 2. The main reason for these low voltage and current density values was a poor adhesion of the insulating material (e.g., silicones) to the grooves, resulting in local shunting and a local breakdown of the series connection of the cells. In the near future, we will apply the interdigitated module process we proposed in Section 2 on our pc-Si layers. In a first stage, modules with a total active area of 4 4 cm2 that consist of four cells in series will be processed. In a later stage, the process will be upscaled to larger module areas. Laser grooving will be used for cell isolation. The crucial step to obtain good working modules will be the proper separation and isolation of the individual cells, as indicated by our preliminary module results described above.
Fig. 4. External quantum efficiency measurements of mesa (sample A) and interdigitated (sample B) cells on very thin pc-Si layers.
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5. Conclusions and outlook
Acknowledgements
We presented a monolithic module process for thin-film pcSi solar cells, in which cell interconnection is combined with cell contacting. The metal contacts in this process are arranged in an interdigitated pattern on top of the cells. Preliminary module results showed that cell isolation is the most crucial step to obtain proper working pc-Si modules using our process. As a first step towards implementation of our module process we made thin-film pc-Si solar cells with interdigitated top contacts and achieved energy conversion efficiencies up to 5.6%. This is the highest efficiency ever achieved with pc-Si solar cells on ceramic substrates where no (re)melting of Si was involved. Interdigitated cells had much higher efficiencies than mesa cells with base contacts at the periphery of the cells due to lower series resistances and higher current densities. The higher current density arises from a much better spectral response of the interdigitated cells in the visible part of the light spectrum. Our cell results indicate that a monolithic module process with interdigitated metal top contacts can probably lead to efficient thin-film modules based on pc-Si layers made by AIC in combination with epitaxial deposition. To be cost-effective however, our solar cells and modules still need much higher efficiencies. In the future, we will focus both on improving the pc-Si layer quality [9] and on implementing novel process features like heterojunction emitters [7] to obtain interdigitated cells and modules of better quality.
This work was partly funded by the European Commission under contract numbers ENK5-CT-2001-00543 (FMETEOR_) and ENK6-CT-2002-00640 (FLATECS_). We thank J. Irigoyen for his investigation of the grain size distribution of our pc-Si layers. References [1] P.A. Basore, in: Proceedings 19th European Photovoltaic Solar Energy Conference, vol. I, 2004, p. 455. [2] G. Beaucarne, D. Van Gestel, I. Gordon, L. Carnel, K. Van Nieuwenhuysen, C. Ornaghi, J. Poortmans, M. Sto¨ger-Pollach, P. Schattschneider, in: Proceedings 19th European Photovoltaic Solar Energy Conference, vol. I, 2004, p. 467. [3] O. Nast, T. Puzzer, L.M. Koschier, A.B. Sproul, S.R. Wenham, Appl. Phys. Lett. 73 (1998) 3214. [4] B. Terheiden, R. Horbelt, A. Hammud, R. Auer, R. Brendel, in: Proceedings 19th European Photovoltaic Solar Energy Conference, vol. I, 2004, p. 463. [5] L. Carnel, I. Gordon, K. Van Nieuwenhuysen, D. Van Gestel, G. Beaucarne, J. Poortmans, Thin Solid Films 487 (2005) 147. [6] I. Gordon, D. Van Gestel, L. Carnel, K. Van Nieuwenhuysen, J. Irigoyen, G. Beaucarne, J. Poortmans, Proceedings 20th European Photovoltaic Solar Energy Conference, 2005, p. 972. [7] L. Carnel, I. Gordon, D. Van Gestel, K. Van Nieuwenhuysen, G. Agostinelli, G. Beaucarne, J. Poortmans, Thin Solid Films in press. [8] I. Gordon, D. Van Gestel, K. Van Nieuwenhuysen, L. Carnel, G. Beaucarne, J. Poortmans, Thin Solid Films 487 (2005) 113. [9] D. Van Gestel, I. Gordon, L. Carnel, K. Van Nieuwenhuysen, J. D’Haen, J. Irigoyen, G. Beaucarne, J. Poortmans, Thin Solid Films in press.