Sivanarayanamoorthy Sivoththaman

Advanced manufacturing concepts for crystalline silicon solar cells

An overview is given concerning current industrial technologies, near future improvements and medium term developments in the field of industrially implementable crystalline silicon solar cell fabrication. The paper proves that considerable improvements are still possible, both in efficiency and in production cost. The paper also proves that a lot of effort is being put worldwide on thinner substrates and on thin-film crystalline silicon cells deposited on cheap carriers, in order to save in substrate cost and in order to gain more independence from availability problems of silicon feedback.

Advanced cost-effective crystalline silicon solar cell technologies

An overview is given concerning current industrial technologies, near future improvements and medium-term developments in the field of industrially viable crystalline silicon terrestrial solar cell fabrication (without concentration).

Fabrication of large area silicon solar cells by rapid thermal processing

Large area n+pp+ solar cells have been fabricated on 10 cm×10 cm pseudo‐quasi‐square CZ silicon wafers (1 Ω cm, p‐type) predominantly used by the photovoltaic (PV) industry. All the high‐temperature steps have been performed by rapid thermal processing (RTP). Emitter formation, back surface field (BSF) formation, and surface oxidation have been performed in just two RTP steps each lasting 50 s. Solar cells of 15% efficiency have been fabricated this way, demonstrating the applicability of this low thermal budget technology to large area, modulable size, industrial quality Si wafers. Furthermore, the rapid thermal oxidation (RTO) is shown to result in good quality thin oxides with Si/SiO2 interface trap densities (Dit)<1011 cm−3 eV−1 near‐midgap.

Selective emitters in Si by single step rapid thermal diffusion for photovoltaic devices

Selective phosphorous diffusion is performed in Si to simultaneously form shallow n/sup +/p junctions of different depths in the submicron range by rapid thermal annealing (RTA). Low temperature (400/spl deg/C) atmospheric pressure chemical vapor deposited (APCVD) phosphosilicate glass (PSG) is used as diffusion source. A wide range of n/sup +/p junctions could be tailored with the same thermal budget by changing only the APCVD-PSG composition. This allows the formation of selectively diffused emitters in different regions of the wafer in one RTA step. 10 cm/spl times/10 cm Cz-Si selective emitter photovoltaic (PV) devices are fabricated this way with high energy conversion efficiencies in the range of 17% to 18%.

Improving low-temperature APCVD SiO 2 passivation by rapid thermal annealing for Si devices

The quality of low-temperature (/spl ap/400/spl deg/C) atmospheric pressure chemical vapor deposited (APCVD) silicon dioxide (SiO/sub 2/) films has been improved by a short time rapid thermal annealing (RTA) step. The RTA step followed by a low temperature (400/spl deg/C) forming gas anneal (FGA) results in a well-passivated Si-SiO/sub 2/ interface, comparable to thermally grown conventional oxides. Efficient and stable surface passivation is obtained by this technique on virgin silicon as well as on photovoltaic devices with diffused (n/sup +/p) emitter surface while maintaining a very low thermal budget. Device parameters are improved by this APCVD/RTA/FGA passivation process.

Rapid thermal annealing of spin-coated phosphoric acid films for shallow junction formation

Rapid thermal annealing (RTA) of spin-coated phosphoric acid (H3PO4) films on silicon substrates has been studied for the formation of shallow junctions. The junctions are characterized by spreading resistance profiling. Device quality, shallow ( 750 °C), below which absorption bands originating from water species are noted. More than 15% efficient, shallow emitter, large-area (10 cm×10 cm) n+pp+ silicon solar cells are fabricated with a short-time processing using this rapid thermal processing technique.

Rapid thermal processing of conventionally and electromagnetically cast 100 cm/sup 2/ multicrystalline silicon

100 cm/sup 2/ n/sup +/pp/sup +/ solar cells have been fabricated by rapid thermal processing (RTP) on conventionally cast (CC) and electromagnetically cast (EMC) multicrystalline silicon (mc-Si). All thermal steps were carried out by fast-ramp (>30/spl deg/C/s) RTP using tungsten-halogen lamps. Emitter and BSF were simultaneously formed by RTP co-diffusion of phosphorous and boron/or aluminum (50-60 seconds) and surface passivation by rapid thermal oxidation, RTO (40-50 seconds). The all-RTP process resulted in 14.1% and 13.3% efficient cells on CC and EMC mc-Si respectively. The EMC cells, when subjected to an additional plasma hydrogen treatment, improved to give the same efficiency as the CC mc-Si cells. Systematic lifetime measurements performed on these materials show that the degradation in the EMC mc-Si is mainly due to the activated crystallographic defects, responding favourably to hydrogenation treatments but poorly to RT-gettering treatments. On the other hand, significant gettering effects are observed in CC mc-Si.

Low temperature formation of emitter and BSF by rapid thermal co-diffusion of P, Al or B

Rapid thermal processing (RTP) is now emerging as a promising simplified process for manufacturing of terrestrial solar cells in a continuous way. In earlier works, we have shown that RTP can advantageously replace the classical furnace for performing emitter, BSF and surface passivation in addition to an efficient gettering of metallic impurities during these steps. In this work, we present results about co-diffusion of two dopant elements in a single rapid thermal cycle at low temperature in order to form emitter and BSF simultaneously. We have, in particular, found that the diffusion kinetics of dopant elements are enhanced during rapid thermal co-diffusion of P, Al or B and lead to the optimal dopant activity and profile for high efficiency silicon solar cells at low temperature. Additionally, the gettering effect induced by the co-diffusion of P and Al is more efficient than can be achieved by a single diffusion of these two elements. Finally, test solar cells were performed on these structures and gave, for a low processing temperature (800/spl deg/C/60 s), an efficiency of 11% on polished FZ monocrystalline substrates without antireflecting coating.

636 mV open circuit voltage multicrystalline silicon solar cells on Polix material: trade-off between short circuit current and open circuit voltage

The trade-off between the open circuit voltage and the short circuit current i.f.o. the base resistivity of multicrystalline silicon (Polix) solar cells (4 cm/sup 2/) is investigated. Very thin substrates (180 /spl mu/m) with base resistivities between 0.2 /spl Omega/.cm and 2.5 /spl Omega/.cm are used in a laboratory high efficiency process (including back surface field, hydrogen and oxide passivation, selective and homogeneous emitters) to test the limits, in terms of open circuit voltage and short circuit current, that can be reached on this material. The variation of V/sub OC/ and J/sub SC/ with base doping is established for Polix multicrystalline silicon. On low resistivity substrates (0.2 /spl Omega/.cm) a record value of 636 mV is achieved for the open circuit voltage which largely compensates for the relatively small deficit in J/sub SC/. The efficiencies of solar cells fabricated on the lower resistivity substrates tend to be greater than those achieved on the higher resistivity material.<<ETX>>

640 mV open-circuit voltage multicrystalline silicon solar cells: role of base doping on device parameters

Abstract The compensatory behaviour of open-circuit voltage ( V OC ) and short-circuit current ( J SC ) in function of the base resistivity of multicrystalline silicon (Polix) solar cells (4 cm 2 ) is investigated. Thin substrates (200 μm) with base resistivities between 0.2 Ω.cm and 2.5 Ω.cm (boron doped) are used in a laboratory high efficiency process (including Back Surface Field, hydrogen and oxide passivation, selective and homogeneous emitters) to test the limits, in terms of V OC and J SC that can be reached on this material. On low resistivity substrates (0.2 Ω.cm) a record value of 640 mV (independently confirmed by NREL) is achieved for the open-circuit voltage which largely compensates for the relatively small decrease in J SC . This high value of V OC resulted in a cell efficiency of 16.8%. It is found that provided efficient hydrogen passivation treatments are performed on the solar cells, the efficiencies of solar cells fabricated on the lower resistivity substrates are larger than those achieved on the higher resistivity material.