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Thin-film solar cells: an overview

Profile image of V. DuttaV. Dutta

2004, Progress in photovoltaics

Abstract

o The different thin film technologies o Market share of thin film technologies o The record efficiencies of thin film technologies o Quantity of needed active material o The manufacturing

Figures (32)
To compare the output of the different photovoltaic technologies, we show in table 2 the record lab efficiencies for cells and modules of different technologies (situation first half of 2008). Please note that these are record lab efficiencies and not commercially guaranteed efficiencies. The progress of the efficiencies in the last 15 years is plotted in figure 2.
To compare the output of the different photovoltaic technologies, we show in table 2 the record lab efficiencies for cells and modules of different technologies (situation first half of 2008). Please note that these are record lab efficiencies and not commercially guaranteed efficiencies. The progress of the efficiencies in the last 15 years is plotted in figure 2.
Figure 2: Fifteen years of progress: confirmed cell efficiencies for crystalline Si (c-Si), multi crystalline Si (m-Si), CIGS, CdTe and amorphous silicon (a-Si) [GREE]. The apparent decrease in performance of the a-Si category arises from a change to reporting stabilised values.
Figure 2: Fifteen years of progress: confirmed cell efficiencies for crystalline Si (c-Si), multi crystalline Si (m-Si), CIGS, CdTe and amorphous silicon (a-Si) [GREE]. The apparent decrease in performance of the a-Si category arises from a change to reporting stabilised values.
Figure 6: Common structural elements of most thin film modules. Thin film photovoltaic technologies use similar encapsulation schemes to construct modules, including lamination of a cover glass to the circuit substrate, a frame, and a junction box or other electrical connection. About half of the total module cost - material, labor, and overhead - originates in the encapsulation scheme. These costs are for the most part independent of the thin film technology itself. A typical module encapsulation scheme is illustrated in figure 6.
Figure 6: Common structural elements of most thin film modules. Thin film photovoltaic technologies use similar encapsulation schemes to construct modules, including lamination of a cover glass to the circuit substrate, a frame, and a junction box or other electrical connection. About half of the total module cost - material, labor, and overhead - originates in the encapsulation scheme. These costs are for the most part independent of the thin film technology itself. A typical module encapsulation scheme is illustrated in figure 6.
Figure 7: Process sequence for manufacturing thin film modules [AVAN1].
Figure 7: Process sequence for manufacturing thin film modules [AVAN1].
ie “RoHS Directive”, restricts the use of certain hazardous substances, including cadmium, | specified products. Other parts of the world are considering adopting similar legislation. urrently, photovoltaic solar modules in general are not subject to the WEEE or RoHS irectives; however, these directives allow for future amendments subjecting additional roducts to their requirements and the scope, applicability and the products included in the JEEE and RoHS Directives are currently being considered and may change [FIRST]. ccording to the European legislation, it is at this time allowed to use cadmium in CdTe and IGS solar cells. However, the conversion from European directives to nationals regulations an lead to a stricter rules. For example, in the Netherlands, the cadmium order of 1999 bids the use of cadmium in solar cells, due to a different interpretation of the definition of irface treatments. This resulted in the situation that solar cells with cadmium were allowed » be produced in the Netherlands, but only when they were intended for export. Further, udmium containing solar cells, legally produced in the | nported into the Netherlands [WENTI]. European Union, could not be Figure 8: Layer sequence for CdTe solar cells [POWAL]
ie “RoHS Directive”, restricts the use of certain hazardous substances, including cadmium, | specified products. Other parts of the world are considering adopting similar legislation. urrently, photovoltaic solar modules in general are not subject to the WEEE or RoHS irectives; however, these directives allow for future amendments subjecting additional roducts to their requirements and the scope, applicability and the products included in the JEEE and RoHS Directives are currently being considered and may change [FIRST]. ccording to the European legislation, it is at this time allowed to use cadmium in CdTe and IGS solar cells. However, the conversion from European directives to nationals regulations an lead to a stricter rules. For example, in the Netherlands, the cadmium order of 1999 bids the use of cadmium in solar cells, due to a different interpretation of the definition of irface treatments. This resulted in the situation that solar cells with cadmium were allowed » be produced in the Netherlands, but only when they were intended for export. Further, udmium containing solar cells, legally produced in the | nported into the Netherlands [WENTI]. European Union, could not be Figure 8: Layer sequence for CdTe solar cells [POWAL]
Figure 10: CdTe module production at Antec. An overview of the CdTe photovoltaic module industrial production line is given (figure 9). In-line coating systems are required for each processing step as well as automated mechanisms to transfer the glass panes from each step to the next one. The production begins with cleaning the substrate glass (1) in a washer. Step (2) is the sputter deposition of the TCO front contact. The initial film is patterned with a laser in step (3). Steps (4), (5), and (6) involve the junction deposition and formation. A single plant with different processing zones can cover the deposition by close-spaced sublimation of CdS and CdTe and the junction activation step. Step (7) is the second patterning step, usually amechanical scribe. Step (8) is the contact deposition; a chemical etch precedes contact deposition for improved current flow. A final mechanical patterning step (9) completes the separation of the individual cells. The edge insulation step involves removing all films in the boundary region around the module for perfect electric insulation from the environment after encapsulation. The contact bands between the first and last cells are applied in step (10) before the module is laminated with a clean cover glass in connected to the con and its classification step (11). The modules are framed—if required—and contact bands are act box in step (12). Step (13) is the final characterization of the module by quality [POWAL].
Figure 10: CdTe module production at Antec. An overview of the CdTe photovoltaic module industrial production line is given (figure 9). In-line coating systems are required for each processing step as well as automated mechanisms to transfer the glass panes from each step to the next one. The production begins with cleaning the substrate glass (1) in a washer. Step (2) is the sputter deposition of the TCO front contact. The initial film is patterned with a laser in step (3). Steps (4), (5), and (6) involve the junction deposition and formation. A single plant with different processing zones can cover the deposition by close-spaced sublimation of CdS and CdTe and the junction activation step. Step (7) is the second patterning step, usually amechanical scribe. Step (8) is the contact deposition; a chemical etch precedes contact deposition for improved current flow. A final mechanical patterning step (9) completes the separation of the individual cells. The edge insulation step involves removing all films in the boundary region around the module for perfect electric insulation from the environment after encapsulation. The contact bands between the first and last cells are applied in step (10) before the module is laminated with a clean cover glass in connected to the con and its classification step (11). The modules are framed—if required—and contact bands are act box in step (12). Step (13) is the final characterization of the module by quality [POWAL].
Figure 12: Thin-Film CdTe manufacturing capacity and cost reduction of First Solar versus calendar year [ULLAL].
Figure 12: Thin-Film CdTe manufacturing capacity and cost reduction of First Solar versus calendar year [ULLAL].
Manufacturing of CIGS solar cells The layer sequence for CIGS solar cells is illustrated in figure 13. CIGS cells are processed freedom for optimization, for example, by in oxic Cd can however already be successfully ransparent conductive ZnO:Al window layer egrating gradients. CIGS solar cells typica employ an extremely thin CdS film deposited in a chemical bath. CIGS solar cells without t produced (good candidates for Cd-replacement are ZnSe, ZnIn2Se, and MgZnO [MILES]. Finally, a sputtered intrinsic ZnO film and t are also deposited by sputtering. As already from back to front, so the molybdenum (Mo) back contact does not need to be optically ransparent. The Mo is coated by sputtering. The CIGS absorber film follows. CIGS can either be deposited directly, for example, by thermal coevaporation of the elements, or indirectly by first depositing more simple precursor layers which then react in a subsequent processing step o form the compound semiconductor. The CIGS materials system is more complex than CdTe and has higher demands on the process control systems. However, it also offers more ly ne ne mentioned, CIGS devices have achieved the [POWAL]. highest efficiencies of all thin-film solar ce Is
Manufacturing of CIGS solar cells The layer sequence for CIGS solar cells is illustrated in figure 13. CIGS cells are processed freedom for optimization, for example, by in oxic Cd can however already be successfully ransparent conductive ZnO:Al window layer egrating gradients. CIGS solar cells typica employ an extremely thin CdS film deposited in a chemical bath. CIGS solar cells without t produced (good candidates for Cd-replacement are ZnSe, ZnIn2Se, and MgZnO [MILES]. Finally, a sputtered intrinsic ZnO film and t are also deposited by sputtering. As already from back to front, so the molybdenum (Mo) back contact does not need to be optically ransparent. The Mo is coated by sputtering. The CIGS absorber film follows. CIGS can either be deposited directly, for example, by thermal coevaporation of the elements, or indirectly by first depositing more simple precursor layers which then react in a subsequent processing step o form the compound semiconductor. The CIGS materials system is more complex than CdTe and has higher demands on the process control systems. However, it also offers more ly ne ne mentioned, CIGS devices have achieved the [POWAL]. highest efficiencies of all thin-film solar ce Is
Figure 15: The CIG module of Sulfurcel.
Figure 15: The CIG module of Sulfurcel.
Figure 16: Wiirth Solar’s thin-film manufacturing plant in Schwabisch Hall, Germany.
Figure 16: Wiirth Solar’s thin-film manufacturing plant in Schwabisch Hall, Germany.
Long-term outdoor testing is especially important for a new PV technology to prove itself in the existing market. The consumer expects high performance for a period of 20 years or more in order to justify the investment. Provided that the modules are properly encapsulated, neither fundamental considerations of the materials chemistry, nor accelerated and real outdoor testing indicate that long-term stability issues will be of concern for either CdTe or CIS modules. Due to the strong ionicity of the CdTe and CIS semiconductor compounds, the energy of the chemical bonds between the constituting elements is quite high, leading to an extremely high chemical and thermal stability and reducing the risk of performance degradation over time. Furthermore, the energy of any photon in the solar spectrum is lower than the binding energies so that the compounds will not degrade as a result of solar irradiation [POWAL]. Figure 17: Grain silo “Schapfenmihle” on the outskirts of Ulm, Germany. Integrated into the fagade are 1306 CIGS modules from the Wirth Solar pilot plant with a nominal installed power of 98 kWp.
Long-term outdoor testing is especially important for a new PV technology to prove itself in the existing market. The consumer expects high performance for a period of 20 years or more in order to justify the investment. Provided that the modules are properly encapsulated, neither fundamental considerations of the materials chemistry, nor accelerated and real outdoor testing indicate that long-term stability issues will be of concern for either CdTe or CIS modules. Due to the strong ionicity of the CdTe and CIS semiconductor compounds, the energy of the chemical bonds between the constituting elements is quite high, leading to an extremely high chemical and thermal stability and reducing the risk of performance degradation over time. Furthermore, the energy of any photon in the solar spectrum is lower than the binding energies so that the compounds will not degrade as a result of solar irradiation [POWAL]. Figure 17: Grain silo “Schapfenmihle” on the outskirts of Ulm, Germany. Integrated into the fagade are 1306 CIGS modules from the Wirth Solar pilot plant with a nominal installed power of 98 kWp.
Figure 18: A 287 kW IBC Solar Installation of CSG-80 modules in Eurishofen, Germany [KEEVE]. Today’s terrestrial PV market is dominated by crystalline silicon wafers. However, due to its material intensiveness, it appears unlikely that wafer-based technology will ever reach cost levels (€/W) required for a widespread application of PV [ABERL]. The solution to this problem might be polycrystalline silicon thin film solar cells on supporting materials. Per watt of output power, such devices require as little as 1 % or less of Si compared to today’s wafer- based technology. Glass is a particularly promising supporting material as it is cheap, transparent, electrically insulating (simplifying cell interconnection), long-term stable, readily available and already a standard component in today’s PV modules. The drawback with glass sheets is their limited thermal stability, excluding the use of lengthy high-temperature (> 650 °C) processing steps during solar cell fabrication.
Figure 18: A 287 kW IBC Solar Installation of CSG-80 modules in Eurishofen, Germany [KEEVE]. Today’s terrestrial PV market is dominated by crystalline silicon wafers. However, due to its material intensiveness, it appears unlikely that wafer-based technology will ever reach cost levels (€/W) required for a widespread application of PV [ABERL]. The solution to this problem might be polycrystalline silicon thin film solar cells on supporting materials. Per watt of output power, such devices require as little as 1 % or less of Si compared to today’s wafer- based technology. Glass is a particularly promising supporting material as it is cheap, transparent, electrically insulating (simplifying cell interconnection), long-term stable, readily available and already a standard component in today’s PV modules. The drawback with glass sheets is their limited thermal stability, excluding the use of lengthy high-temperature (> 650 °C) processing steps during solar cell fabrication.
Figure 19: Schematic cross section of an CSG module. There is very limited data on reliability and durability of thin-film polysilicon modules because the technology is so new. The expectation is that, based on the fully crystalline nature of the active layer and the absence of a degradation prone TCO, very little degradation will occur. The scarce experimental data available, indeed, give very positive indications [BEAUC]. CSG technology uses the relatively simple device structure shown in figure 19. It consists of a extured ayer pat borosilicate glass superstrate, silicon nitride anti-reflection layer, a thin layer of poly- Si (~1.4 um thick) with ‘grooves’ separating adjacent cells, a white reflective insulating resin erned to produce n-type ‘crater’ and p-type ‘dimple’ contacts, and Al metallisation patterned to connect adjacent cells in series. Good light trapping is provided by the combination of textured glass and rear reflector. Adequate lateral conductance is provided by he heavi y doped n+ and p+ poly-Si layers so there is no need for any TCO layers [KEEVE].
Figure 19: Schematic cross section of an CSG module. There is very limited data on reliability and durability of thin-film polysilicon modules because the technology is so new. The expectation is that, based on the fully crystalline nature of the active layer and the absence of a degradation prone TCO, very little degradation will occur. The scarce experimental data available, indeed, give very positive indications [BEAUC]. CSG technology uses the relatively simple device structure shown in figure 19. It consists of a extured ayer pat borosilicate glass superstrate, silicon nitride anti-reflection layer, a thin layer of poly- Si (~1.4 um thick) with ‘grooves’ separating adjacent cells, a white reflective insulating resin erned to produce n-type ‘crater’ and p-type ‘dimple’ contacts, and Al metallisation patterned to connect adjacent cells in series. Good light trapping is provided by the combination of textured glass and rear reflector. Adequate lateral conductance is provided by he heavi y doped n+ and p+ poly-Si layers so there is no need for any TCO layers [KEEVE].
Figure 21: Factory flow of CSG modules in CSG Solar, Thalheim, Germany. Manufacturing of CSG solar cells
Figure 21: Factory flow of CSG modules in CSG Solar, Thalheim, Germany. Manufacturing of CSG solar cells
The crystallisation step produces a polycrystalline silicon material having an average grain size of about 1 um. The silicon as-crystallized contains many crystallographic defects. These are annealed in a furnace shown in figure 23 by heating the glass briefly to over 900°C [BASO1]. Like all polycrystalline Si materials, CSG requires hydrogen passivation to maximize performance. The amount of hydrogen required is much greater than for cast-ingot silicon. The equipment developed for this step was especially developed at CSG Solar. It accomplishes the required intense exposure to atomic hydrogen at temperatures above 600°C using an in-line process for high throughput [BASO1].
The crystallisation step produces a polycrystalline silicon material having an average grain size of about 1 um. The silicon as-crystallized contains many crystallographic defects. These are annealed in a furnace shown in figure 23 by heating the glass briefly to over 900°C [BASO1]. Like all polycrystalline Si materials, CSG requires hydrogen passivation to maximize performance. The amount of hydrogen required is much greater than for cast-ingot silicon. The equipment developed for this step was especially developed at CSG Solar. It accomplishes the required intense exposure to atomic hydrogen at temperatures above 600°C using an in-line process for high throughput [BASO1].
The panel is then exposed to a solvent vapor that causes the resin to reflow just enough to protect the walls of the etched ‘craters’. A second pass through the ink-jet creates openings in the resin where contact is to be made to the surface p+ layer. Another etch is used to remove damaged silicon from the surface within these ‘dimple’ openings, then a thin layer of aluminum is sputtered onto the surface. The metal contacts the n+ and p+ layers at the contact openings, but is otherwise isolated from the silicon by the resin layer. The pulsed infrared laser is used to remove the metal in an intricate pattern that separates the n+ and p+ contacts within each cell while connecting all of the p+ contacts in one cell to the n+ contacts in the adjacent cell (figure 25). This step obtains high throughput despite the complexity of the pattern by using scanned pattern-projection optics while the panel is moving below. Bead blasting removes all layers from the perimeter of the module to ensure safe operation up to 1000 volts [BASO1].
The panel is then exposed to a solvent vapor that causes the resin to reflow just enough to protect the walls of the etched ‘craters’. A second pass through the ink-jet creates openings in the resin where contact is to be made to the surface p+ layer. Another etch is used to remove damaged silicon from the surface within these ‘dimple’ openings, then a thin layer of aluminum is sputtered onto the surface. The metal contacts the n+ and p+ layers at the contact openings, but is otherwise isolated from the silicon by the resin layer. The pulsed infrared laser is used to remove the metal in an intricate pattern that separates the n+ and p+ contacts within each cell while connecting all of the p+ contacts in one cell to the n+ contacts in the adjacent cell (figure 25). This step obtains high throughput despite the complexity of the pattern by using scanned pattern-projection optics while the panel is moving below. Bead blasting removes all layers from the perimeter of the module to ensure safe operation up to 1000 volts [BASO1].
Figure 25: Exitech four-head scanning laser
Figure 25: Exitech four-head scanning laser
Figure 26: Typical structure for an a-Si triple-junction device. The silver layer is used as metal reflector. The ITO and ZnO layer are respectively the front and back contact. The i-1, 1- 2 and i-3 a-Si alloys can e.g. be respectively wc-Si:H, a-SiGe:H and a-Si:H (i.e. from lowest to highest bandgap).
Figure 26: Typical structure for an a-Si triple-junction device. The silver layer is used as metal reflector. The ITO and ZnO layer are respectively the front and back contact. The i-1, 1- 2 and i-3 a-Si alloys can e.g. be respectively wc-Si:H, a-SiGe:H and a-Si:H (i.e. from lowest to highest bandgap).
The degradation of single junction a-Si:H devices are ~ 25% and the degradation of multijunction devices is between 12%-18%. Figure 27 shows the degradation of a triple junction device from Uni-Solar which stabilizes at 88% of its initial efficiency [ARYA]. Amorphous silicon panels have functioned well in outdoor testing and applications. Long- term outdoor reliability is a function of the “back-end” processes, such as, tabbing, lamination, wiring and junction termination. These “back-end processes” are different for the two technology configurations. In the superstrate configuration, generally, the solar cells are interconnected to form the module by monolithic laser or mechanical scribing during various stages of the solar panel production. The solar panel is then laminated with EVA (Ethyl Vinyl Acetate) or a similar material and another piece of glass or TAP (Tedlar Aluminum Polymer). In the substrate configuration, the solar cells slabs are tabbed and laminated with EVA and Tefzel or glass. In both configurations, junction boxes and frames can be added [ARYA].
The degradation of single junction a-Si:H devices are ~ 25% and the degradation of multijunction devices is between 12%-18%. Figure 27 shows the degradation of a triple junction device from Uni-Solar which stabilizes at 88% of its initial efficiency [ARYA]. Amorphous silicon panels have functioned well in outdoor testing and applications. Long- term outdoor reliability is a function of the “back-end” processes, such as, tabbing, lamination, wiring and junction termination. These “back-end processes” are different for the two technology configurations. In the superstrate configuration, generally, the solar cells are interconnected to form the module by monolithic laser or mechanical scribing during various stages of the solar panel production. The solar panel is then laminated with EVA (Ethyl Vinyl Acetate) or a similar material and another piece of glass or TAP (Tedlar Aluminum Polymer). In the substrate configuration, the solar cells slabs are tabbed and laminated with EVA and Tefzel or glass. In both configurations, junction boxes and frames can be added [ARYA].
modes of failure. Many organizations have developed packaging schemes that pass the testing requirements for 20 year outdoor applications [ARYA]. Fig. 28: a-Si solar cells from Kaneka
modes of failure. Many organizations have developed packaging schemes that pass the testing requirements for 20 year outdoor applications [ARYA]. Fig. 28: a-Si solar cells from Kaneka

Key takeaways

  • The remainder is based on thin film solar cell technologies and consists mainly of 5 % based on thin film amorphous Si solar cells and 1 % CdTe (CIGS and thin film silicon, although yet commercially available, take up (for now) a negligible part of the market).
  • Obviously, the complexity of the manufacturing of the solar modules strongly influences cost.
  • According to the European legislation, it is at this time allowed to use cadmium in CdTe and CIGS solar cells.
  • The solution to this problem might be polycrystalline silicon thin film solar cells on supporting materials.
  • This low thickness is a clear advantage for manufacturing, as one of the most significant costs in amorphous silicon modules is the deposition cost.

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