Abasifreke Ebong

685 mV open-circuit voltage laser grooved silicon solar cell

Abstract The recombination limiting the voltage of the present buried contact solar cell (BCSC) can be reduced by replacing the present high recombination sintered aluminium back with a floating rear junction for passivation, heavy boron diffusion below the rear contact, and by limiting the rear surface contact area. Analysis of these implementations in the double sided laser grooved (DSLG) structure shows that the floating junction passivation is effective in reducing the recombination component at the rear surface and that the boron diffusion in the rear groove comprises up to half of the total saturation current. Limiting the area of the heavily diffused boron grooves allows open-circuit voltages of 685 mV while maintaining the simplicity of the BCSC processing sequence. An open-circuit voltage of 685 mV represents nearly a 50 mV increase over the conventional BCSC.

Enhanced silicon solar cell performance by rapid thermal firing of screen-printed metals

Rapid thermal processing (RTP) of screen-printed (SP) Al on the back and silver (Ag) grid on the front produced significant improvement in back surface field (BSF) of n/sup +/-p-p/sup +/ float-zone (FZ) Si solar cells. Two-step firing was found to form more effective BSF than co-firing, resulting in 0.6-1.0% increase in absolute cell efficiency. In addition, RTP was found to be more effective than the beltline processing (BLP), resulting in 0.5-1.0% increase in absolute cell efficiency. Although the Al-BSF formed by the BLP was inferior to the RTP, the difference between the two is virtually eliminated during the subsequent RTP contact firing. Internal quantum efficiency (IQE) analysis of the solar cells gave effective back surface recombination velocities (S/sub eff/) of >5000 cm/s and /spl sim/1500 cm/s for co-firing in the BLP and the RTP, respectively. Two-step firing produced S/sub eff/ of /spl sim/1500 cm/s and /spl sim/700 cm/s in the BLP and the RTP, respectively. However, S/sub eff/ for the two-step firing, involving BLP BSF formation followed by RTP contact firing, was found to be /spl sim/700 cm/s, which indicates that RTP contact firing with a faster ramp-up (100/spl deg/C/s) restores the poor-quality BLP BSF. On the other hand, BLP contact firing with a slow ramp-up (<10/spl deg/C/s) degrades the high-quality RTP BSF, increasing S/sub eff/ from /spl sim/700 cm/s to /spl sim/1500 cm/s.

Understanding and implementation of rapid thermal technologies for high-efficiency silicon solar cells

Rapid and potentially low-cost process techniques are analyzed and successfully applied toward the fabrication of high-efficiency monocrystalline Si solar cells. First, a methodology for achieving high-quality screen-printed (SP) contacts is developed to achieve fill factors (FFs) of 0.785-0.795 on monocrystalline Si. Second, rapid emitter formation is accomplished by diffusion under tungsten halogen lamps in both beltline and rapid thermal processing (RTP) systems (instead of in a conventional infrared furnace). Third, a combination of SP aluminum and RTP is used to form an excellent back surface field (BSF) in 2 min to achieve an effective back surface recombination velocity (S/sub eff/) of 200 cm/s on 2.3 /spl Omega/-cm Si. Next, a novel dielectric passivation scheme (formed by stacking a plasma silicon nitride film on top of a rapid thermal oxide layer) is developed that reduces the surface recombination velocity (S) to approximately 10 cm/s on the 1.3 /spl Omega/-cm p-Si surface. The essential feature of the stack passivation scheme is its ability to withstand short 700-850/spl deg/C anneal treatments (like the ones used to fire SP contacts) without degradation in S. The stack also lowers the emitter saturation current density (J/sub oe/) of 40 and 90 /spl Omega//sq emitters by a factor of three and ten, respectively, compared to no passivation. Finally, the above individual processes are integrated to achieve (1) >19% efficient solar cells with emitter and Al-BSF formed by RTP and contacts formed by vacuum evaporation and lift-off, (2) 17% efficient manufacturable cells with emitter and Al-BSF formed in a beltline furnace and contacts formed by SP, and (3) 17% efficient gridded-back contact (bifacial) cells with surface passivation accomplished by the stack and gridded front and back contacts formed by SP and cofiring.

Investigation of radiative tunneling in GaN/InGaN single quantum well light-emitting diodes

Abstract The mechanisms of carrier injection and recombination in a GaN/InGaN single quantum well light-emitting diodes have been studied. Strong defect-assisted tunneling behavior has been observed in both forward and reverse current–voltage characteristics. In addition to band-edge emission at 400 nm, the electroluminescence has also been attributed to radiative tunneling from band-to-deep level states and band-to-band tail states. The approximately current-squared dependence of light intensity at 400 nm even at high currents indicates dominant nonradiative recombination through deep-lying states within the space-charge region. Inhomogeneous avalanche breakdown luminescence, which is primarily caused by deep-level recombination, suggests a nonuniform spatial distribution of reverse leakage in these diodes.

Understanding and Use of IR Belt Furnace for Rapid Thermal Firing of Screen-Printed Contacts to Si Solar Cells

We have simulated the rapid thermal firing process using a high-throughput conveyor belt furnace to study the physics of solar cell contact formation in mass production. We show that as sinter dwell time decreases, a lower Ag finger contact resistance is observed. Scanning electron micrographs reveal a correlation between glass thickness at the Ag/Si finger interface and Ag finger contact resistance. Secondary ion mass spectrometry shows that glass-frit and Ag emitter penetration are controlled by sinter dwell time. The observed trends in contact formation lead to lower series resistance, higher fill factors, and greater efficiencies with rapid firing.

The Study of Silane-Free SiC x N y Film for Crystalline Silicon Solar Cells

We deposited plasma-enhanced chemical vapor deposition silicon carbon nitride (SiC x N y ) antireflection coating and passivation layers using a silane-free process. We used a solid polymer source developed at SiXtron Advanced Materials to eliminate the storage and handling of dangerous pyrophoric silane gas. We used ammonia flow rate as a control for the chemical and optical properties in the silane-free process. As NH 3 flow rate increases, the carbon content, refractive index, extinction coefficient, and surface charge density of the film decrease. At an ammonia flow rate of 3000 sccm, which is similar to the conventional SiN x , the extinction coefficients for the two films were similar. This led to an emitter dark saturation current density (J oe ) of 404 fA/cm 2 for the two films on 45 Ω/□ emitters. However, a stack passivation of SiO 2 /SiC x N y on an 80 Ω/□ emitter resulted in an emitter dark saturation current density of 95 fA/cm 2 , which is enough to provide a good surface passivation for high efficiency solar cells. An energy conversion efficiency of 17.4% was obtained for a 149 cm 2 textured Czochralski screen-printed solar cell with this stack passivation. For a 156 cm 2 nontextured multicrystalline silicon, with only SiC x N y and a 45 Ω/□ emitter, we obtained 14.9% efficiency.

Fundamental understanding and implementation of Al-enhanced PECVD SiNx hydrogenation in silicon ribbons

Presented at the 12th International Photovoltaic Science and Engineering Conference; Jeju Island, Korea; June 11-15, 2001.

Rear surface effects in high efficiency silicon solar cells

Rear surface effects in PERL solar cells can lead not only to degradation in the short circuit current and open circuit voltage, but also fill factor. Three mechanisms capable of changing the effective rear surface recombination velocity with injection level are identified, two associated with oxidised p-type surfaces, and the third with two dimensional effects associated with a rear floating junction. Each of these will degrade the fill factor if the range of junction biases corresponding to the rear surface transition, coincides with the maximum power point. Despite the identified nonidealities, PERL cells with rear floating junctions (PERF cells) have achieved record open circuit voltages for silicon solar cells, while simultaneously achieving fill factor improvements relative to standard PERL solar cells. Without optimisation, a record efficiency of 22% has been demonstrated for a cell with a rear floating junction. The results of both theoretical and experimental studies are provided.

Capitalizing on the Glass-Etching Effect of Silver Plating Chemistry to Contact Si Solar Cells With Homogeneous 100–110

Homogeneous high-sheet-resistance emitter (HHSE), excellent surface passivation, and high-quality contacts, along with narrow gridlines, are needed for high-efficiency solar cells. However, HHSE in conjunction with screen-printed (SP) contacts often gives low fill factor (FF) because of high contact resistance. We capitalized on the glass-etching property of light-induced plating of silver to decrease the contact resistance and formed high-quality contacts to 100-110 Ω/sq HHSE. This led to the achievement of 78.5% FF, 38.3 mA/cm2 short-circuit current density (JSC) due to narrow line widths (65 μm), and efficiency of 18.7%.

\Omega/\hbox{sq}

Two-dimensional numerical simulations were performed to derive design rules for low-cost, high-efficiency interdigitated back contact (IBC) solar cells on a low-cost substrate. The IBC solar cells were designed to be fabricated using either the conventional screen printing or photolithography metallization processes. Bulk lifetime, bulk resistivity, contact spacing (pitch), contact opening width, recombination in the gap between the p+ BSF and n+ emitter, and the ratio of emitter width to pitch have been used as key variables in the simulations. It is found that short circuit current density (Jsc) is not only a strong function of the bulk lifetime but also the emitter coverage of the rear surface. Fill factor (FF) decreases as the emitter coverage increases because the majority carriers need to travel a longer distance through the substrate for longer emitter width. The simulated IBC results were compared with those for conventional screen printed solar cells. It was found that the IBC solar cell outperforms the screen printed (SP) solar cell when the bulk lifetime is above 50 mus due to higher Voc and Jsc , which suggests that higher performance can be realized on low-cost substrates with the IBC structure