A Novel Low-Cost Method for Fabricating Bifacial Solar Cells (original) (raw)

Conference Record of the IEEE Photovoltaic Specialists Conference

In this work we propose and demonstrate a novel and cost-effective method to fabricate bifacial cells with conventional homojunction architecture. The method combines benefits of lithography-less, self-aligned patterning during deposition of antireflective coating (ARC) and simultaneous metallization of both surfaces aided by electroplating. We have fabricated a conventional diffused n + pp + junction bifacial solar cell on a monocrystalline silicon (c-Si) substrate using this method. Electrochemically grown nickel is used to simultaneously form front and back electrodes. The bifacial solar cell fabricated with an un-optimized process has a front and rear efficiencies (under AM1.5G one sun illumination) of 12% and 8.66%, respectively. Part of the low performance of the cell is attributed to poor quality of the passivation layer and the post deposition annealing to reduce pinholes in deposited SiN x layer to prevent parasitic plating.

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Laser-doped metal-plated bifacial silicon solar cells

Solar Energy Materials and Solar Cells, 2014

In this paper we report on the fabrication of laser-doped p-type bifacial cells using self-aligned metalplating with energy conversion efficiencies as high as 19.2%. A key fabrication step for these cells is recognising that the p-type silicon regions can be made cathodic by forward biasing the p-n junction in a process which we call here field-induced plating (FIP). Used in conjunction with light-induced plating (LIP) in the same plating apparatus, FIP can be used to form low cost nickel/copper grids on both surfaces of a cell. Furthermore, the simplicity of the FIP process means that it can potentially be performed using the same plating equipment and chemistry as used for LIP. Plating rates similar to LIP were achieved (i.e., $ 10 mm of copper in 10 min), however there is potential to plate at much faster rates with FIP because the junction is forward-biased. This bifacial cell plating method could be adapted to metallise a range of bifacial cells including heterojunction cells.

A Novel Non-photolithographic Patterning Method for Fabricating Solar Cells

— In this work we propose and demonstrate a novel method to fabricate semiconductor devices using a shadow mask that allows selective deposition of PECVD layers and selective exposure of doped silicon regions. The exposed regions then act as seed layers for selective metallization by a process such as electroplating. A device structure taking advantage of this process can be completely done without any photolithographic steps. In semiconductor processes where submicron-feature sizes are generally not required and reducing cost of manufacturing is key (e.g. in solar cells), this proposed process flow could be really useful. We have fabricated a conventional diffused p-n junction monofacial solar cell on a monocrystalline silicon (c-Si) substrate using this method. Nickel is electrochemically grown to form front surface electrode. The monofacial solar cell fabricated with an un-optimized process shows an efficiency of 14.5% under AM1.5G one sun illumination. The feature size of metal electrodes formed in this way is 80µm and could be narrowed down to smaller size-width. Optimization of doping, surface passivation and metallization would improve the efficiency even further to values in the high teens by improving open circuit voltage (V OC), current density (J SC) and Fill Factor (FF).

High efficiency screen printed bifacial solar cells on monocrystalline CZ silicon

We present industrialized bifacial solar cells on large area (149 cm 2 ) 2 cm CZ monocrystalline silicon wafers processed with industrially relevant techniques such as liquid source BBr 3 and POCl 3 open-tube furnace diffusions, plasma enhanced chemical vapor deposition (PECVD) SiN x deposition, and screen printed contacts. The fundamental analysis of the paste using at boron-diffused surface and the bifacial solar cell firing cycle has been investigated. The resulting solar cells have front and rear efficiencies of 16.6 and 12.8%, respectively. The ratio of the rear J SC to front J SC is 76.8%. It increases the bifacial power by 15.4% over a conventional solar cell at 20% of 1-sun rear illumination, which equals to the power of a conventional solar cell with 19.2% efficiency. We also present a bifacial glass-glass photovoltaic (PV) module with 30 bifacial cells with the electrical characteristics.

Bifacial Solar Cells on Multi-Crystalline Silicon with Boron BSF and Open Rear Contact

2006 IEEE 4th World Conference on Photovoltaic Energy Conference, 2006

The standard industrial multi-crystalline silicon (mc-Si) solar cell is monofacial and includes screen printed aluminum back surface field (BSF). A simple approach to increase performance and reduce costs per Wpeak is to collect the albedo on the rear side. In this work a bifacial, screen printed mc-Si solar cell with boron BSF is demonstrated. Rear to front efficiency ratios of up to 0.83 have been reached on 100x100mm² mc-Si wafers with a thickness of about 200mm. The best solar cell processed so far with a boron BSF had an efficiency under front side illumination of h=16.1% and a back to front efficiency ratio of 0.77. The possible gain in performance in later operation was estimated using PC1D simulation and depends on the albedo that is the amount of light that penetrates into the solar cell from the rear side. The simulation was confirmed by outside module tests, leading to an average gain of 19.5% over one day.

On the Metallization Losses of Bifacial n-Type Silicon Solar Cells

The efficiency of industrial bifacial n-type cells is mostly limited by the loss in VOC after firing of screen printed metal pastes. We have investigated the mechanism responsible for this loss by varying the metal fraction on either side of the cell and determined the actual influence of surface geometry, diffusion layers and pastes. The experimental results indicate a more detrimental influence of metal pastes on the boron emitter side. This can be attributed to more aggressive pastes and less shielded contacts as in the case of phosphorous diffused emitters.

Analysis of thin bifacial silicon solar cells with locally diffused and selective back surface field

Materials Research, 2014

The aim of this work is to present the development and comparison of thin n + pp + industrial bifacial silicon solar cells produced with the local screen-printed Al back surface field (BSF) to those with the selective BSF doped with aluminum-boron. To produce solar cells with selective BSF, the boron diffusion based on spin-on dopant was introduced in the process sequence. The thin SiO 2 layer (10 nm) thermally grown did not produce good passivation on the rear face and wafers were contaminated during aluminum diffusion in the belt furnace. The implementation of selectively doped BSF improved the efficiency by reflecting minority charge carriers and the wafer contamination by belt furnace was compensated by boron diffusion. The bifacial solar cells with B-Al selective BSF achieved an efficiency of 13.7% / 8.9% (front / rear illumination) and presented lower sensitivity to the belt furnace processing and to the quality of the rear surface passivation.

Realization of Thin MC-Silicon Pert-Type Bifacial Solar Cells in Industrial Environments

2006

We present bifacial solar cells processed with a sequence suitable for industrial production. This method uses the LPCVD silicon nitride deposition based on DCS (dichlorosilane) or BTBAS (bis-(tertiary butyl amino)-silane). The bifacial solar cell process on wafers of 200 mum thickness has the following steps: (1) boron doped BSF of Rsheet =60 ohm/sq; (2) POCl3 emitter (on front side) of Rsheet= 50-55 ohm/sq; (3) thermal oxidation of the wafer surfaces; (4) the deposition of DCS or BTBAS based LPCVD silicon nitride on either sides of the wafer (5); finger grid printing on both sides and firing; (6) edge isolation. The solar cells produced with the DCS based silicon nitride process exhibit fill factor (FF) values of 76% on p-type and 75% on n-type solar cells with a rear to front efficiency ratio etarear/etafront of 67% for the p-type solar cells and 43% for the n-type solar cells. The solar cells with the BTBAS silicon nitride show FF values close to 72% and etarear/etafront 68% for p-type solar cells

High Efficiency Fully Implanted and Co-annealed Bifacial N-type Solar Cells

Energy Procedia, 2013

The aim of the study was to develop a very simple process for the fabrication of large area n-type PERT cells by means of ion implantation. We showed an improvement of the implanted boron activation rate with the annealing temperature by comparing boron SIMS and ECV concentration profiles. A direct positive impact on the boron emitter saturation current density (J 0e ) was measured. We also investigated the effect of varying the oxidation conditions during the annealing on the implanted boron emitter and the phosphorus BSF quality. Low emitter saturation current density (J 0e ) of 131 fA/cm 2 was measured on textured surfaces, close to the value obtained with diffused B-emitters. A process flow was developed leading to an average efficiency of 19% on 239 cm 2 bifacial solar cells, using only eight processing steps with two implantations and one activation annealing.

n-Si Bifacial Concentrator Solar Cell

Various approaches have been developed for reducing the cost of the photoelectricity produced by silicon solar cells (SCs). Of highest priority among these approaches are improvement of the efficiency of the SCs, transition from pSi to nSi, light concentration, and use of bifacial SCs. In the present study, an SC combining all these approaches has been developed. In this SC, transparent conducting oxides serve as antireflection and passivating electrodes in an indium–tin–oxide/(p+nn+)Si/indium–fluorine–oxide structure fabricated from CzSi with wire contacts (Laminated Grid Cell design). The SC has front/rear efficiencies of 16.5–16.7/15.1–15.3% X (under 1–3 suns). This result is unique because the combination of bifaciality and concentrator operation has no analogs and the SC compares well with the world standard among both bifacial and concentrator SCs.

REFLECTS – a European CRAFT project for the development of a bifacial silicon solar cell module

The objective of the REFLECTS project (Novel bifacial single substrate solar cells utilising reflected solar radiation) is the development of a cost effective, high efficiency bifacial silicon solar module exploiting the light reflected from behind the cells for energy generation. A bifacial rear contact silicon solar cell using a simple processing sequence is developed. Mechanical grinding is deployed for the definition of the emitter and base contact regions. The rear surface structure is then utilised as a mask for the self-aligned metallization of the bifacial rear contact cells applying the OECO (obliquely evaporated contacts) -technique. We achieved an efficiency of 20.5 % under illumination from the non-metallised cell side whereas the efficiency is as high as 17.0 % when illuminated from the metallised side. A string of 5 bifacial cells is mounted and encapsulated on a non-concentrating reflector developed within the project. Applying the reflector enhances the short circuit current by 38 %.

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