Laser Annealed Boron Implanted Silicon Solar Cells (original) (raw)
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Laser-Annealed, Implanted Boron Emitters for B-BSF Silicon Solar Cells
Energy Procedia, 2012
Boron (B) emitters are required in an increasing number of silicon solar cells technologies, each of them requires an appropriate emitter profile. This work aims at investigating laser thermal annealing (LTA) from implanted B as a versatile approach for B-emitter processing compared to standard furnace annealing. Symmetrical p + /n/p + structures featuring various LTA B-emitters were characterized using the QssPC technique. Experimental results show that in contrast to thermally-diffused B-emitters, implied V oc of LTA emitters increases when their sheet resistance decreases. Numerical simulations suggest that the observed trend could be attributed to a reduction in the surface recombination velocity. LTA emitters were then integrated as B-BSF into silicon solar cells. Gains of 2 mV in V oc and 0.8 mA/cm² in J sc compared to Al-BSF were obtained, leading to an overall efficiency enhancement of 0.3 % abs .
Excimer laser annealing for boron implanted local BSF for PERC crystalline silicon solar cells
In this work, the activation of an implanted boron (B) junction by an Excimer Laser Annealing (ELA) process is investigated. The objective is to substitute the standard Aluminium Back Surface Field (Al-BSF) normally used at the rear side of PERC cells by a Boron-BSF, taking advantage of the higher solubility of B in Si compared to Al. The long annealing step used to re-crystallize the amorphized region after implantation is usually performed in an oven at high temperature. In this study, we replaced it by the ELA based on a system from Excico. The laser has a 150 ns pulse duration, a wavelength of 308 nm, a top-hat beam profile with excellent energy uniformity (±2%) and an annealing area up to 400 mm 2 . The samples are analysed by sheet resistance, SIMS profiles and lifetime/J 0e for different laser energy densities and implantation conditions (dose and energy), suggesting process conditions for a B-BSF implementation. The B-BSF formed by implantation and subsequent ELA shows sheet resistance values as low as 20 Ohm/sq, J 0e values down to 200 fA/cm 2 and dopant profiles with surface concentration higher than 10 20 at/cm 3 .
High-efficiency n-type silicon solar cells with front side boron emitter
2009
High-efficiency n-type PERL solar cells with a front side boron emitter passivated by ALD Al 2 O 3 are presented within this work. For the applied PERL cell design two variations have been employed: i) different boron emitters (deep / shallow) and ii) different dielectric layers for rear side passivation (thermal grown SiO 2 and PECVD SiN x ). Both, thermal grown SiO 2 as well as PECVD SiN x provide an effective passivation of the n-type rear surface with effective surface recombination velocities of 4 cm/s and 7 cm/s respectively. If the metalized rear side point contacts (with BSF) together with the recombination of the 1 Ω cm FZ base silicon are taken into account this results in saturation current densities of 30 fA/cm 2 and 37 fA/cm 2 respectively, limiting the open-circuit voltage (all recombination losses due to the front side are neglected) to 717 mV and 712 mV. The passivation of the boron emitter with ALD Al 2 O 3 results in an emitter saturation current density as low as 11 fA/cm 2 . Together with the losses at the rear side as well as the front side contacts this allows for an open-circuit voltage of the applied PERL solar cell design of ~700 mV. For n-type PERL solar cells featuring a lowly doped boron emitter as well as a SiO 2 passivated rear such a high open-circuit voltage (up to 703.6 mV) could be reached also at the device level, resulting in a conversion efficiency of 23.4%. Also for the PERL solar cells featuring a high surface concentration boron emitter with a PECVD SiN x passivated rear, i.e. first steps towards an industrial structure, still a high conversion efficiency of 21.8% could be achieved. All cells have been shown to be perfectly stable under illumination at 1 sun.
Plasma immersion ion implantation of boron for ribbon silicon solar cells
EPJ Photovoltaics, 2013
In this work, we report for the first time on the solar cell fabrication on n-type silicon RST (for Ribbon on Sacrificial Template) using plasma immersion ion implantation. The experiments were also carried out on FZ silicon as a reference. Boron was implanted at energies from 10 to 15 kV and doses from 10 15 to 10 16 cm −2 , then activated by a thermal annealing in a conventional furnace at 900 and 950 • C for 30 min. The n + region acting as a back surface field was achieved by phosphorus spin-coating. The frontside boron emitter was passivated either by applying a 10 nm deposited SiOX plasma-enhanced chemical vapor deposition (PECVD) or with a 10 nm grown thermal oxide. The anti-reflection coating layer formed a 60 nm thick SiNX layer. We show that energies less than 15 kV and doses around 5×10 15 cm −2 are appropriate to achieve open circuit voltage higher than 590 mV and efficiency around 16.7% on FZ-Si. The photovoltaic performances on ribbon silicon are so far limited by the bulk quality of the material and by the quality of the junction through the presence of silicon carbide precipitates at the surface. Nevertheless, we demonstrate that plasma immersion ion implantation is very promising for solar cell fabrication on ultrathin silicon wafers such as ribbons.
Amorphous silicon passivation applied to the front side boron emitter of n-type silicon solar cells
Hydrogenated amorphous silicon (a-Si:H) was used to passivated the front side boron emitter of n-type silicon solar cells. The main aspect was to investigate the parasitic absorption behavior of the a-Si:H layers on the solar cells front side with different thicknesses between 5 nm and 20 nm. Therefore planar solar cells featuring a diffused boron emitter and a diffused back surface field were fabricated. The cells reached energy conversion efficiencies up to 17.8% with an open- circuit voltage VOC of 641 mV for an a-Si:H thickness of 15 nm. Analogous processed reference cells, with an Al2O3 passivated emitter achieved an efficiency of 19% and a VOC of 658 mV.
First solar cells on silicon wafers doped using sprayed boric acid
Semiconductor …, 2010
A new method for boron bulk doping of silicon ribbons is developed. The method is based on the spraying of the ribbons with a boric acid solution and is particularly suited for silicon ribbons that require a zone-melting recrystallization step. To analyse the quality of the material thus obtained, multicrystalline silicon samples doped with this doping process were used as substrate for solar cells and compared with solar cells made on commercial multicrystalline silicon wafers. The values obtained for the diffusion length and the IV curve parameters show that the method of doping with the boric acid solution is suitable to produce p-doped silicon ribbons for solar cell applications.
Nonmelt laser annealing of 5-KeV and 1-KeV boron-implanted silicon
IEEE Transactions on Electron Devices, 2002
Nonmelt laser annealing has been investigated for the formation of ultrashallow, heavily doped regions. With the correct lasing and implant conditions, the process can be used to form ultrashallow, heavily doped junctions in boron-implanted silicon. Laser energy in the nonmelt regime has been supplied to the silicon surface at a ramp rate greater than 10 C/s. This rapid ramp rate will help decrease dopant diffusion while supplying enough energy to the surface to produce dopant activation. High-dose, nonamorphizing boron implants at a dose of 10 ions/cm and energies of 5 KeV and 1 KeV are annealed with a 308-nm excimer laser. Subsequent rapid thermal anneals are used to study the effect of laser annealing as a pretreatment. SIMS, sheet resistance and mobility data have been measured for all annealing and implant conditions. For the 5-KeV implants, the 308-nm nonmelt laser preanneal results in increased diffusion. However, for the 1-KeV implant processed with ten laser pulses, the SIMS profile shows that no measurable diffusion has occurred, yet a sheet resistance of 420 /sq was produced.
EPJ Photovoltaics
A few microns thick silicon films on glass coated with a dielectric intermediate layer can be crystallised by a single pass of a line-focused diode laser beam. Under favorable process conditions relatively large linear grains with low defect density are formed. Most grain boundaries are defect-free low-energy twin-boundaries. Boron-doped laser crystallised films are processed into solar cells by diffusing an emitter from a phosphorous spin-on-dopant source, measuring up to 539 mV open-circuit voltage prior to metallisation. After applying a point-contact metallisation the best cell achieves 7.8% energy conversion efficiency, open-circuit voltage of 526 mV and short-circuit current of 26 mA/cm 2 . The efficiency is significantly limited by a low fill-factor of 56% due to the simplified metallisation approach. The internal quantum efficiency of laser crystallised cells is consistent with low front surface recombination. By improving cell metallisation and enhancing light-trapping the efficiencies of above 13% can be achieved.
High-Throughput Ion-Implantation for Low-Cost High-Efficiency Silicon Solar Cells
Energy Procedia, 2012
This paper presents the use of ion-implantation for high-volume manufacturing of silicon solar cells. Ion-implantation provides a unique opportunity to obtain grid-parity because it simplifies the fabrication of advanced cell structures. It is shown in production that a streamlined ion-implantation process with homogeneous phosphorus doped emitter can raise the efficiency of 239 cm 2 p-base Cz cells by 0.8 % absolute, from 18.3 % to 19.1 %, while reducing the process sequence by one step relative to traditional POCl 3 process. Average production cell efficiency is about 18.6 % with maximum exceeding 19 %. Several advanced cell structures were fabricated in R&D using ion-implantation and screen printed contacts. The advanced p-base structure with ion implanted selective emitter and local Al-BSF resulted in an efficiency of 19.6 %. In addition, three different n-base cell structures were fabricated using boron (B) and phosphorus (P) implantation followed by in-situ front and back passivation during the implant anneal: the n-base cell with B emitter, passivated P-BSF with local contact and full metal back gave 19.2 % efficiency, the implanted n-base bifacial cell was 19 % efficient, and the n-base back junction cell with B emitter in the rear and P front surface field resulted in 19 % efficiency.
Lattice location of boron implanted silicon after laser annealing
Lettere Al Nuovo Cimento, 1978
Boron implanted (100)Si specimens were irradiated with Q-swicthed ruby laser pulse of (40+50) ns duration and of 0.7 or 2.7 J/cm 2 energy. The ion implanted amorphous layer becomes single crystal as measured by channeling effect with 1.6 MeV He + baekscattering after 2.7 J/cm ~ laser pulse irradiation, and the boron atoms are almost substitutionally located as determined by the liB(p, ~)SBe nuclear reaction. The 0.7 J/cm 2 laser pulse does not induce the amorphous to single-crystal transition no lattice location of boron was found by channeling, and the implanted ion concentration profile is the same as the as implanted one. Changes in the profile were found instead after the 2.7 J/cm 2 laser pulse irradiation.