Silicon solar cells incorporating double-sided pyramidal texture are capable of superior light tr... more Silicon solar cells incorporating double-sided pyramidal texture are capable of superior light trapping over cells with front-side only texture. However, increased surface area, roughness and exposed <111> crystal planes of textured surfaces not only causes increased recombination, but also makes cells susceptible to shunting through pinholes in the dielectric at the sharp peaks and valleys of the textured pyramids. A polyimide film as an insulating interlayer film is investigated to circumvent the tradeoff between improved light trapping, increased recombination and increased shunt paths. When applied at the rear of the interdigitated back contact silicon solar cell structure, the polyimide film provides an excellent electrical insulation (>1000 MΩ of insulation resistance) and increases photocurrent (~1.1 mA/cm2) owing to an increased rear internal reflectance. The polyimide is also compatible with metal annealing of passivating dielectrics such as silicon nitride. Optical simulation and experimental results are combined in a 3D semiconductor simulation (Quokka) to quantify the possible gain of implementing the double-sided texture in high efficiency silicon solar cells.
—We investigate the light trapping in Si wafers that are textured with conventional random pyrami... more —We investigate the light trapping in Si wafers that are textured with conventional random pyramids on their front surface and rounded random pyramids on their rear. It is well established that rounding the pyramids leads to better surface passivation, but whether or not it improves light trapping depends on the cell structure. In this paper, we apply ray tracing, spectrophotome-try, and photoluminescence spectroscopy (PLS) to understand and quantify how rounding the rear pyramids might affect the light trapping in back-contact solar cells. We describe how rounding the pyramids leads to two competing optical effects: 1) reduced absorption in the rear films and 2) reduced scattering from the rear texture. The first effect improves light trapping whereas the latter degrades it. We show how the influence of each effect depends on wavelength and how they can be discerned (but not easily quantified) in reflectance curves. With PLS measurements, we conclude that for our sample structure and etch solution, the generation current is approximately constant for etch durations less than ∼60 s, and decreases significantly as the etch duration increases. Thus, by limiting the duration of the rounding etch, superior surface passivation can be attained without degrading the light trapping.
Carrier lifetime degradation of reactive ion etch-processed silicon samples are investigated. Two... more Carrier lifetime degradation of reactive ion etch-processed silicon samples are investigated. Two types of carrier recombination: reversible and irreversible degradations induced by reactive ion etching (RIE) are identified. Irreversible carrier recombination is due to surface damage created by the RIE process that propagates a few microns deep into the silicon substrate. Reversible carrier recombination, on the other hand, is found to be caused by radiation damage when RIE etches only into the silicon oxide, and nitrogen annealing can restore the degraded carrier lifetime. A recombination-free RIE process is then developed in combination with a passivation stack consisting of silicon dioxide and silicon nitride layers. This improved RIE process is applied to the development of high efficiency silicon solar cells resulting in a conversion efficiency exceeding 24%.
— Interdigitated back-contact (IBC) solar cells developed in the past two years have efficiencies... more — Interdigitated back-contact (IBC) solar cells developed in the past two years have efficiencies in the range 24.4%–25.6%. As high as these efficiencies are, there are opportunities to increase them further by improving on the light trapping. Silicon solar cells incorporating double-sided pyramidal texture are capable of superior light trapping than cells with texture on just the front. One of the principle losses of double-sided pyramidal texture is the light that escapes after a second pass through the cell when the facet angles are the same on the front and rear. This contribution investigates how this loss might be reduced by changing the facet angle of the rear pyramids. A textured pyramid rounding is introduced to improve the light trapping. The reduction in surface recombination that rounding the facets introduces is also evaluated. With confocal microscopy, spectrophotometry and ray tracing, the rounding etch time required to yield the best light trapping is investigated. With photoconductance lifetime measurements, the surface recombination is found to continue to decrease as the rounding time increases. The spectrophotometry and ray tracing suggests that the double sided textured samples featuring rounded rear pyramids have superior light trapping to the sample with a planar rear surface. The high-efficiency potential of rounded textured pyramids in silicon solar cells is demonstrated by the fabrication of 24% efficient back-contact silicon solar cells.
— In this paper, a discussion is made of the design of silicon cells to be used in a six-junction... more — In this paper, a discussion is made of the design of silicon cells to be used in a six-junction tandem solar cell structure as part of the Very High Efficiency Solar Cell (VHESC) program. Minority carrier recombination at surfaces and in the volume, internal quantum efficiency, resistance losses, free carrier parasitic absorption, optical reflection, light trapping, and light absorption must be traded off against each other. Modelling was used to analyse the various parameters and produce estimates of short circuit current, fill factor and open-circuit voltage of the cell. In addition, quasi-steady-state phtotoconductance measurements to analyse carrier recombination and emitter saturation current (Joe) as well as to predict the open-circuit voltage of solar cell is presented. For metallisation of such small solar cells, alternate methods of making contact such as light-induced plating and electrolyte plating in addition to evaporating metal on the contacts were explored and employed. Numerical resistive loss modelling was made to calculate the optimum metal thickness achieved by light-induced and electroplating to minimise resistive losses. Experiments were conducted to determine the proper plating rate by light-induced and electrolyte plating. Cells were fabricated by standard silicon processing techniques followed by testing of IV curves using current-voltage flash-tester to achieve the target efficiency.
Reactive Ion Etching (RIE) is used in the fabrication of some types of solar cells to achieve a h... more Reactive Ion Etching (RIE) is used in the fabrication of some types of solar cells to achieve a highly directional etch. However, cells fabricated using RIE have lower than expected efficiency, possibly caused by increased carrier recombination. Characterisation of the carrier lifetime in solar cells was conducted using the quasi steady state photoconductance (QSSPC) measurement technique. Substantial effective lifetime degradation was observed for silicon samples processed by RIE. Lifetime degradation for samples where RIE etches into silicon is found to be permanent, while for samples where RIE etches only on dielectric layers of SiO 2 grown on the wafer, the lifetime degradation is found to be reversible. The reversible degradation in RIE-processed samples is associated with radiation damage. By reducing the proportion of a wafer exposed to RIE, the degradation of the effective lifetime of RIE-etched silicon samples can be minimised, and the performance of silicon solar cells can be improved significantly.
In this paper a detailed discussion of different options of silicon solar cell design meant to be... more In this paper a detailed discussion of different options of silicon solar cell design meant to be used in conjunction with a tandem cell stack is presented. Conventional single junction (SJ), horizontally-stacked (HS) and vertically-stacked (VS) silicon solar cell approaches are discussed. The design considerations of each individual silicon solar cell approach (SJ, HS and VS) are detailed, and benefits and drawbacks of these cells are presented. Expected conversion efficiencies of SJ, HS and VS silicon solar cells under a condition-illuminated with a light spot of 1.9 mm diameter through 2 W/cm2 intensity under the infrared light spectrum-are deduced for the purpose of selecting a suitable design for the tandem stack.
Various types of shunt occur in solar cells, including process errors, defects, tunnel shunts bet... more Various types of shunt occur in solar cells, including process errors, defects, tunnel shunts between adjacent opposing diffusions and pinholes in insulating dielectric layers used to separate opposite-polarity regions. We have found that boron diffusions into small windows in dielectric layers generate pinholes in the layers following the removal of borosilicate glass (BSG) after the diffusion. These " boron-spots " lie close to the edge of the diffusion windows. If a phosphorus diffused region underlies the dielectric then subsequent metallisation can short circuit the two regions.
— The process of making Interdigitated Back Contact (IBC) solar cell is implemented by a novel si... more — The process of making Interdigitated Back Contact (IBC) solar cell is implemented by a novel simplified etch-back technique, while aiming for no compromise on high-efficiency potentials. Simplified etch-back creates localized heavy and light phosphorus and boron diffusions simultaneously. This process also leaves localised heavy diffusions to be approximately a micron higher than neighbouring light diffusion regions. In comparison to the IBC solar cells that ANU developed to date [1], key advantages of this technique feature reduction in cell process steps; requires only two diffusions to create p, p+, n and n+ diffusions; no high-temperature oxidation masking steps required as diffusion barriers; independent optimization of contact recombination, lateral carriers transport and surface passivation; and potential higher silicon bulk lifetime and reduced contamination due to low thermal budget. Based on the etch-back technique, the total saturation current density deduced from the test structures for the IBC cell is below 30 fA/cm 2 .
19% efficient all-back-contact silicon wafer solar cells developed on n-type FZ material at the A... more 19% efficient all-back-contact silicon wafer solar cells developed on n-type FZ material at the Australian National University (ANU) are reported as part of a collaboration between the PV manufacturer Trina Solar and the Solar Energy Research Institute of Singapore (SERIS). The cells, having an area of 4×4 cm 2 , incorporate a heavy phosphorus diffusion (n + BSF) and boron emitter (p +) at the rear, and a light phosphorus diffusion (FSF) at the front. The boron emitter covers 75% of the rear surface, while the phosphorus BSF covers 25% of the rear surface. On the front, a 2-layer stack of thin thermal oxide and LPCVD silicon nitride is used to provide good surface passivation and anti-reflection properties. No texturing was used in this fabrication process. The innovative features of all-back-contact (ABC) cells developed at ANU involve uninterrupted p-n junctions with no gap in between them for simplified processing and a single-step diffusion enabling both light phosphorus diffusion at the front and heavy phosphorus diffusion at the rear. The cell process incorporating the above features has already led to solar cells with efficiency of over 19%, despite the small number of cells processed as yet.
N-type all-back-contact (ABC) silicon solar cells incorporating a simple oxide-nitride passivatio... more N-type all-back-contact (ABC) silicon solar cells incorporating a simple oxide-nitride passivation scheme are presently being developed at the Australian National University. Having already achieved promising efficiencies with planar ABC cells [1], this work analyses the cell performance after integrating a surface texturing step into the process flow. Although the textured cells have significantly lower front surface reflection, the measured short-circuit current density is actually lower than that of the planar cells. Photoconductance decay data indicate the presence of high carrier recombination at the textured surface of the ABC cells, which are deposited with a stack of thermal oxide and LPCVD nitride. Further examination confirmed that high carrier recombination is due to stress induced by the LPCVD nitride on the peaks and valleys of the textured surface. Use of PECVD nitride instead of LPCVD nitride as an antireflection layer avoids the degraded carrier lifetime caused by the textured surface. Therefore, PECVD nitride should be a good substitute for constructing the oxide-nitride stacks of our future ABC cells.
Development of all-back-contact (ABC) silicon solar cells at the Australian National University (... more Development of all-back-contact (ABC) silicon solar cells at the Australian National University (ANU), as part of a collaboration between Trina Solar and the Solar Energy Research Institute of Singapore (SERIS), is progressing, and 22.6% efficient ABC cells, based on the aperture-area of 13 cm 2 that excludes busbars, were recently fabricated at ANU. When measured using the 16 cm 2 aperture-area that includes busbars, the cells are 21.7% efficient. In this paper, we demonstrate the technique of removing shunts, associated in the development of ABC cells, by laser-assisted means. The laser that we use for the shunt removal is 532 nm diode pump solid state (DPSS) laser. The shunts are caused by residual boron (p +) diffusion within the phosphorus (n +) diffused region following the trench etch that separates the p and n regions. Photoluminescence (PL) imaging showed that apparent shunt regions were removed following this process. Analysis of ABC solar cells by dark IV characterisation further confirmed that the shunt resistance was increased by about 30-fold (350 to 11500 Ω.cm 2). The effective removal of shunts has increased the cell efficiency by 0.5% absolute. Carrier recombination induced by laser damage appears to be minimal since open-circuit voltage of the ABC cells barely changes for pre-and post-laser ablation, although more detailed investigations are required.
Silicon remains the material of choice for photovoltaics because of its abundance, non-toxicity, ... more Silicon remains the material of choice for photovoltaics because of its abundance, non-toxicity, high and stable cell efficiencies, the maturity of production infrastructure and the deep and widespread level of skill available in relation to silicon devices. Rapidly decreasing module prices mean that area-related balance of systems costs are an increasing proportion of photovoltaic systems price. This places a premium on efficient cells. In recent years there have been large improvements in mass production of high quality wafers, the ability to handle thin wafers, maintenance of high minority carrier lifetimes, surface passivation, minimisation of optical losses, device characterisation and in other areas. Many of these improvements are viable in mass production. The upper limit of silicon solar cell efficiency is 29%, which is substantially higher than the best laboratory (25%) [1] and large-area commercial (24%) [2, 3] cells. Cell efficiencies above 25% appear to be feasible in both a laboratory and commercial environment. Such a cell will have minimal bulk recombination due to a combination of a thin substrate with a very high minority carrier lifetime; superb surface passivation; small-area electrical contacts consistent with low contact recombination, free carrier absorption and contact resistance; excellent optical control through the use of texturing, antireflection coatings and rear surface reflectors; low edge recombination assisted by the use of thinner wafers, larger cells and edge passivation; and sufficient metal coverage to minimise resistive losses. This paper will survey current work in high-performance silicon solar cell design and fabrication, and discuss approaches to efficiency improvements.
The collaboration between the Solar Energy Research Institute of Singapore (SERIS), Trina Solar a... more The collaboration between the Solar Energy Research Institute of Singapore (SERIS), Trina Solar and ANU is progressing well, and ANU has already developed all-back-contacted (ABC) silicon wafer cells with best one-sun efficiencies of 21.2% and 22.1% on FZ material, when measured with the aperture areas of 16 cm 2 (includes busbars) and 13 cm 2 (excludes busbars) respectively. This paper presents the continuing development of ABC cells targeting the efficiency of 23.5% on 16-cm 2 cell area. Further developments such as optimising front surface field (FSF), rear diffusion, anti-reflection coating (ARC), and incorporation of lithographically aligned metal contacts were sity (J oe) of the FSF by 22 fA/cm 2. The optimised thickness of anti-reflection coating (ARC) PECVD SiN x further reduces the average reflectance across the wavelength range of 300 to 1200 nm by about 4%. Incorporation of aligned metal contacts and heavier rear 2. The above optimised improvements have increased the efficiency of the champion ABC cell by 0.5% absolute. In addition, we present further refinements in areas of texturing; FSF passivation; electrical shading loss in terms of cell pitch, bus-bar and base doping; and metallisation to aim for the 16-cm 2 ABC cells with the conversion efficiency > 22% in the near term.
— We demonstrate enhanced absorption in silicon wafers when plasmonic nanoparticles are added to ... more — We demonstrate enhanced absorption in silicon wafers when plasmonic nanoparticles are added to a conventional rear contact structure. A rear side light trapping with plasmonic nanoparticles and various thicknesses of Si 3 N 4 layer is studied and compared to a structure with combined plasmonics and diffused paint. Photoluminescence is applied to extract the absorptivity in order to exclude free carrier and parasitic absorption. Modeling shows that the absorption within a cell structure with plasmonic nanoparticles and optimum capping layer is expected to be enhanced by 53% of value for an ideal Lambertian reflector.
The interdigitated back contact (IBC) solar cells developed at the Australian National University... more The interdigitated back contact (IBC) solar cells developed at the Australian National University have resulted in an independently confirmed (Fraunhofer Institut für Solare Energiesysteme (ISE) CalLab) designated-area efficiency of 24.4 ± 0.7%, featuring short-circuit current density of 41.95 mA/cm 2 , open-circuit voltage of 703 mV and 82.7% fill factor. The cell, 2 × 2 cm 2 in area, was fabricated on a 230 μm thick 1.5 Ω cm n-type Czochralski wafer, utilising plasma-enhanced chemical vapour deposition (CVD) SiN x front-surface passivation without front-surface diffusion, rear-side thermal oxide/low-pressure CVD Si 3 N 4 passivation stack and evaporated aluminium contacts with a finger-to-finger pitch of 500 μm. This paper describes the design and fabrication of lab-scale high-efficiency IBC cells. Characterisation of optical and electronic properties of the best produced cell is made, with subsequent incorporation into 3D device modelling used to accurately quantify all losses. Loss analysis demonstrates that bulk and emitter recombination, bulk resistive and optical losses are dominant and suggests a clear route to efficiency values in excess of 25%. Additionally, laser processing is explored as a means to simplify the manufacture of IBC cells, with a confirmed efficiency value of 23.5% recorded for cells fabricated using damage-free deep UV laser ablation for contact formation. Meanwhile all-laser-doped cells, where every doping and patterning step is performed by lasers, are demonstrated with a preliminary result of 19.1% conversion efficiency recorded.
—The contact resistivity of evaporated Al on doped silicon is examined for a range of process con... more —The contact resistivity of evaporated Al on doped silicon is examined for a range of process conditions common to the fabrication of laboratory silicon solar cells. The effects of silicon surface preparation prior to evaporation, sintering temperature, the use of a shutter, and evaporation power are investigated. The presented evaporation conditions yielded the lowest published contact resistivity between Al and phosphorus doped Si over a large range of doping concentration. It is also demonstrated that a contact resistivity below 10-6 Ω·cm 2 can be achieved without sintering. Three-dimensional simulations are utilized to compare the obtained results for evaporated Al contacts with those for passivated contacts.
Silicon solar cells incorporating double-sided pyramidal texture are capable of superior light tr... more Silicon solar cells incorporating double-sided pyramidal texture are capable of superior light trapping over cells with front-side only texture. However, increased surface area, roughness and exposed <111> crystal planes of textured surfaces not only causes increased recombination, but also makes cells susceptible to shunting through pinholes in the dielectric at the sharp peaks and valleys of the textured pyramids. A polyimide film as an insulating interlayer film is investigated to circumvent the tradeoff between improved light trapping, increased recombination and increased shunt paths. When applied at the rear of the interdigitated back contact silicon solar cell structure, the polyimide film provides an excellent electrical insulation (>1000 MΩ of insulation resistance) and increases photocurrent (~1.1 mA/cm2) owing to an increased rear internal reflectance. The polyimide is also compatible with metal annealing of passivating dielectrics such as silicon nitride. Optical simulation and experimental results are combined in a 3D semiconductor simulation (Quokka) to quantify the possible gain of implementing the double-sided texture in high efficiency silicon solar cells.
—We investigate the light trapping in Si wafers that are textured with conventional random pyrami... more —We investigate the light trapping in Si wafers that are textured with conventional random pyramids on their front surface and rounded random pyramids on their rear. It is well established that rounding the pyramids leads to better surface passivation, but whether or not it improves light trapping depends on the cell structure. In this paper, we apply ray tracing, spectrophotome-try, and photoluminescence spectroscopy (PLS) to understand and quantify how rounding the rear pyramids might affect the light trapping in back-contact solar cells. We describe how rounding the pyramids leads to two competing optical effects: 1) reduced absorption in the rear films and 2) reduced scattering from the rear texture. The first effect improves light trapping whereas the latter degrades it. We show how the influence of each effect depends on wavelength and how they can be discerned (but not easily quantified) in reflectance curves. With PLS measurements, we conclude that for our sample structure and etch solution, the generation current is approximately constant for etch durations less than ∼60 s, and decreases significantly as the etch duration increases. Thus, by limiting the duration of the rounding etch, superior surface passivation can be attained without degrading the light trapping.
Carrier lifetime degradation of reactive ion etch-processed silicon samples are investigated. Two... more Carrier lifetime degradation of reactive ion etch-processed silicon samples are investigated. Two types of carrier recombination: reversible and irreversible degradations induced by reactive ion etching (RIE) are identified. Irreversible carrier recombination is due to surface damage created by the RIE process that propagates a few microns deep into the silicon substrate. Reversible carrier recombination, on the other hand, is found to be caused by radiation damage when RIE etches only into the silicon oxide, and nitrogen annealing can restore the degraded carrier lifetime. A recombination-free RIE process is then developed in combination with a passivation stack consisting of silicon dioxide and silicon nitride layers. This improved RIE process is applied to the development of high efficiency silicon solar cells resulting in a conversion efficiency exceeding 24%.
— Interdigitated back-contact (IBC) solar cells developed in the past two years have efficiencies... more — Interdigitated back-contact (IBC) solar cells developed in the past two years have efficiencies in the range 24.4%–25.6%. As high as these efficiencies are, there are opportunities to increase them further by improving on the light trapping. Silicon solar cells incorporating double-sided pyramidal texture are capable of superior light trapping than cells with texture on just the front. One of the principle losses of double-sided pyramidal texture is the light that escapes after a second pass through the cell when the facet angles are the same on the front and rear. This contribution investigates how this loss might be reduced by changing the facet angle of the rear pyramids. A textured pyramid rounding is introduced to improve the light trapping. The reduction in surface recombination that rounding the facets introduces is also evaluated. With confocal microscopy, spectrophotometry and ray tracing, the rounding etch time required to yield the best light trapping is investigated. With photoconductance lifetime measurements, the surface recombination is found to continue to decrease as the rounding time increases. The spectrophotometry and ray tracing suggests that the double sided textured samples featuring rounded rear pyramids have superior light trapping to the sample with a planar rear surface. The high-efficiency potential of rounded textured pyramids in silicon solar cells is demonstrated by the fabrication of 24% efficient back-contact silicon solar cells.
— In this paper, a discussion is made of the design of silicon cells to be used in a six-junction... more — In this paper, a discussion is made of the design of silicon cells to be used in a six-junction tandem solar cell structure as part of the Very High Efficiency Solar Cell (VHESC) program. Minority carrier recombination at surfaces and in the volume, internal quantum efficiency, resistance losses, free carrier parasitic absorption, optical reflection, light trapping, and light absorption must be traded off against each other. Modelling was used to analyse the various parameters and produce estimates of short circuit current, fill factor and open-circuit voltage of the cell. In addition, quasi-steady-state phtotoconductance measurements to analyse carrier recombination and emitter saturation current (Joe) as well as to predict the open-circuit voltage of solar cell is presented. For metallisation of such small solar cells, alternate methods of making contact such as light-induced plating and electrolyte plating in addition to evaporating metal on the contacts were explored and employed. Numerical resistive loss modelling was made to calculate the optimum metal thickness achieved by light-induced and electroplating to minimise resistive losses. Experiments were conducted to determine the proper plating rate by light-induced and electrolyte plating. Cells were fabricated by standard silicon processing techniques followed by testing of IV curves using current-voltage flash-tester to achieve the target efficiency.
Reactive Ion Etching (RIE) is used in the fabrication of some types of solar cells to achieve a h... more Reactive Ion Etching (RIE) is used in the fabrication of some types of solar cells to achieve a highly directional etch. However, cells fabricated using RIE have lower than expected efficiency, possibly caused by increased carrier recombination. Characterisation of the carrier lifetime in solar cells was conducted using the quasi steady state photoconductance (QSSPC) measurement technique. Substantial effective lifetime degradation was observed for silicon samples processed by RIE. Lifetime degradation for samples where RIE etches into silicon is found to be permanent, while for samples where RIE etches only on dielectric layers of SiO 2 grown on the wafer, the lifetime degradation is found to be reversible. The reversible degradation in RIE-processed samples is associated with radiation damage. By reducing the proportion of a wafer exposed to RIE, the degradation of the effective lifetime of RIE-etched silicon samples can be minimised, and the performance of silicon solar cells can be improved significantly.
In this paper a detailed discussion of different options of silicon solar cell design meant to be... more In this paper a detailed discussion of different options of silicon solar cell design meant to be used in conjunction with a tandem cell stack is presented. Conventional single junction (SJ), horizontally-stacked (HS) and vertically-stacked (VS) silicon solar cell approaches are discussed. The design considerations of each individual silicon solar cell approach (SJ, HS and VS) are detailed, and benefits and drawbacks of these cells are presented. Expected conversion efficiencies of SJ, HS and VS silicon solar cells under a condition-illuminated with a light spot of 1.9 mm diameter through 2 W/cm2 intensity under the infrared light spectrum-are deduced for the purpose of selecting a suitable design for the tandem stack.
Various types of shunt occur in solar cells, including process errors, defects, tunnel shunts bet... more Various types of shunt occur in solar cells, including process errors, defects, tunnel shunts between adjacent opposing diffusions and pinholes in insulating dielectric layers used to separate opposite-polarity regions. We have found that boron diffusions into small windows in dielectric layers generate pinholes in the layers following the removal of borosilicate glass (BSG) after the diffusion. These " boron-spots " lie close to the edge of the diffusion windows. If a phosphorus diffused region underlies the dielectric then subsequent metallisation can short circuit the two regions.
— The process of making Interdigitated Back Contact (IBC) solar cell is implemented by a novel si... more — The process of making Interdigitated Back Contact (IBC) solar cell is implemented by a novel simplified etch-back technique, while aiming for no compromise on high-efficiency potentials. Simplified etch-back creates localized heavy and light phosphorus and boron diffusions simultaneously. This process also leaves localised heavy diffusions to be approximately a micron higher than neighbouring light diffusion regions. In comparison to the IBC solar cells that ANU developed to date [1], key advantages of this technique feature reduction in cell process steps; requires only two diffusions to create p, p+, n and n+ diffusions; no high-temperature oxidation masking steps required as diffusion barriers; independent optimization of contact recombination, lateral carriers transport and surface passivation; and potential higher silicon bulk lifetime and reduced contamination due to low thermal budget. Based on the etch-back technique, the total saturation current density deduced from the test structures for the IBC cell is below 30 fA/cm 2 .
19% efficient all-back-contact silicon wafer solar cells developed on n-type FZ material at the A... more 19% efficient all-back-contact silicon wafer solar cells developed on n-type FZ material at the Australian National University (ANU) are reported as part of a collaboration between the PV manufacturer Trina Solar and the Solar Energy Research Institute of Singapore (SERIS). The cells, having an area of 4×4 cm 2 , incorporate a heavy phosphorus diffusion (n + BSF) and boron emitter (p +) at the rear, and a light phosphorus diffusion (FSF) at the front. The boron emitter covers 75% of the rear surface, while the phosphorus BSF covers 25% of the rear surface. On the front, a 2-layer stack of thin thermal oxide and LPCVD silicon nitride is used to provide good surface passivation and anti-reflection properties. No texturing was used in this fabrication process. The innovative features of all-back-contact (ABC) cells developed at ANU involve uninterrupted p-n junctions with no gap in between them for simplified processing and a single-step diffusion enabling both light phosphorus diffusion at the front and heavy phosphorus diffusion at the rear. The cell process incorporating the above features has already led to solar cells with efficiency of over 19%, despite the small number of cells processed as yet.
N-type all-back-contact (ABC) silicon solar cells incorporating a simple oxide-nitride passivatio... more N-type all-back-contact (ABC) silicon solar cells incorporating a simple oxide-nitride passivation scheme are presently being developed at the Australian National University. Having already achieved promising efficiencies with planar ABC cells [1], this work analyses the cell performance after integrating a surface texturing step into the process flow. Although the textured cells have significantly lower front surface reflection, the measured short-circuit current density is actually lower than that of the planar cells. Photoconductance decay data indicate the presence of high carrier recombination at the textured surface of the ABC cells, which are deposited with a stack of thermal oxide and LPCVD nitride. Further examination confirmed that high carrier recombination is due to stress induced by the LPCVD nitride on the peaks and valleys of the textured surface. Use of PECVD nitride instead of LPCVD nitride as an antireflection layer avoids the degraded carrier lifetime caused by the textured surface. Therefore, PECVD nitride should be a good substitute for constructing the oxide-nitride stacks of our future ABC cells.
Development of all-back-contact (ABC) silicon solar cells at the Australian National University (... more Development of all-back-contact (ABC) silicon solar cells at the Australian National University (ANU), as part of a collaboration between Trina Solar and the Solar Energy Research Institute of Singapore (SERIS), is progressing, and 22.6% efficient ABC cells, based on the aperture-area of 13 cm 2 that excludes busbars, were recently fabricated at ANU. When measured using the 16 cm 2 aperture-area that includes busbars, the cells are 21.7% efficient. In this paper, we demonstrate the technique of removing shunts, associated in the development of ABC cells, by laser-assisted means. The laser that we use for the shunt removal is 532 nm diode pump solid state (DPSS) laser. The shunts are caused by residual boron (p +) diffusion within the phosphorus (n +) diffused region following the trench etch that separates the p and n regions. Photoluminescence (PL) imaging showed that apparent shunt regions were removed following this process. Analysis of ABC solar cells by dark IV characterisation further confirmed that the shunt resistance was increased by about 30-fold (350 to 11500 Ω.cm 2). The effective removal of shunts has increased the cell efficiency by 0.5% absolute. Carrier recombination induced by laser damage appears to be minimal since open-circuit voltage of the ABC cells barely changes for pre-and post-laser ablation, although more detailed investigations are required.
Silicon remains the material of choice for photovoltaics because of its abundance, non-toxicity, ... more Silicon remains the material of choice for photovoltaics because of its abundance, non-toxicity, high and stable cell efficiencies, the maturity of production infrastructure and the deep and widespread level of skill available in relation to silicon devices. Rapidly decreasing module prices mean that area-related balance of systems costs are an increasing proportion of photovoltaic systems price. This places a premium on efficient cells. In recent years there have been large improvements in mass production of high quality wafers, the ability to handle thin wafers, maintenance of high minority carrier lifetimes, surface passivation, minimisation of optical losses, device characterisation and in other areas. Many of these improvements are viable in mass production. The upper limit of silicon solar cell efficiency is 29%, which is substantially higher than the best laboratory (25%) [1] and large-area commercial (24%) [2, 3] cells. Cell efficiencies above 25% appear to be feasible in both a laboratory and commercial environment. Such a cell will have minimal bulk recombination due to a combination of a thin substrate with a very high minority carrier lifetime; superb surface passivation; small-area electrical contacts consistent with low contact recombination, free carrier absorption and contact resistance; excellent optical control through the use of texturing, antireflection coatings and rear surface reflectors; low edge recombination assisted by the use of thinner wafers, larger cells and edge passivation; and sufficient metal coverage to minimise resistive losses. This paper will survey current work in high-performance silicon solar cell design and fabrication, and discuss approaches to efficiency improvements.
The collaboration between the Solar Energy Research Institute of Singapore (SERIS), Trina Solar a... more The collaboration between the Solar Energy Research Institute of Singapore (SERIS), Trina Solar and ANU is progressing well, and ANU has already developed all-back-contacted (ABC) silicon wafer cells with best one-sun efficiencies of 21.2% and 22.1% on FZ material, when measured with the aperture areas of 16 cm 2 (includes busbars) and 13 cm 2 (excludes busbars) respectively. This paper presents the continuing development of ABC cells targeting the efficiency of 23.5% on 16-cm 2 cell area. Further developments such as optimising front surface field (FSF), rear diffusion, anti-reflection coating (ARC), and incorporation of lithographically aligned metal contacts were sity (J oe) of the FSF by 22 fA/cm 2. The optimised thickness of anti-reflection coating (ARC) PECVD SiN x further reduces the average reflectance across the wavelength range of 300 to 1200 nm by about 4%. Incorporation of aligned metal contacts and heavier rear 2. The above optimised improvements have increased the efficiency of the champion ABC cell by 0.5% absolute. In addition, we present further refinements in areas of texturing; FSF passivation; electrical shading loss in terms of cell pitch, bus-bar and base doping; and metallisation to aim for the 16-cm 2 ABC cells with the conversion efficiency > 22% in the near term.
— We demonstrate enhanced absorption in silicon wafers when plasmonic nanoparticles are added to ... more — We demonstrate enhanced absorption in silicon wafers when plasmonic nanoparticles are added to a conventional rear contact structure. A rear side light trapping with plasmonic nanoparticles and various thicknesses of Si 3 N 4 layer is studied and compared to a structure with combined plasmonics and diffused paint. Photoluminescence is applied to extract the absorptivity in order to exclude free carrier and parasitic absorption. Modeling shows that the absorption within a cell structure with plasmonic nanoparticles and optimum capping layer is expected to be enhanced by 53% of value for an ideal Lambertian reflector.
The interdigitated back contact (IBC) solar cells developed at the Australian National University... more The interdigitated back contact (IBC) solar cells developed at the Australian National University have resulted in an independently confirmed (Fraunhofer Institut für Solare Energiesysteme (ISE) CalLab) designated-area efficiency of 24.4 ± 0.7%, featuring short-circuit current density of 41.95 mA/cm 2 , open-circuit voltage of 703 mV and 82.7% fill factor. The cell, 2 × 2 cm 2 in area, was fabricated on a 230 μm thick 1.5 Ω cm n-type Czochralski wafer, utilising plasma-enhanced chemical vapour deposition (CVD) SiN x front-surface passivation without front-surface diffusion, rear-side thermal oxide/low-pressure CVD Si 3 N 4 passivation stack and evaporated aluminium contacts with a finger-to-finger pitch of 500 μm. This paper describes the design and fabrication of lab-scale high-efficiency IBC cells. Characterisation of optical and electronic properties of the best produced cell is made, with subsequent incorporation into 3D device modelling used to accurately quantify all losses. Loss analysis demonstrates that bulk and emitter recombination, bulk resistive and optical losses are dominant and suggests a clear route to efficiency values in excess of 25%. Additionally, laser processing is explored as a means to simplify the manufacture of IBC cells, with a confirmed efficiency value of 23.5% recorded for cells fabricated using damage-free deep UV laser ablation for contact formation. Meanwhile all-laser-doped cells, where every doping and patterning step is performed by lasers, are demonstrated with a preliminary result of 19.1% conversion efficiency recorded.
—The contact resistivity of evaporated Al on doped silicon is examined for a range of process con... more —The contact resistivity of evaporated Al on doped silicon is examined for a range of process conditions common to the fabrication of laboratory silicon solar cells. The effects of silicon surface preparation prior to evaporation, sintering temperature, the use of a shutter, and evaporation power are investigated. The presented evaporation conditions yielded the lowest published contact resistivity between Al and phosphorus doped Si over a large range of doping concentration. It is also demonstrated that a contact resistivity below 10-6 Ω·cm 2 can be achieved without sintering. Three-dimensional simulations are utilized to compare the obtained results for evaporated Al contacts with those for passivated contacts.
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