Phosphorus Back Surface Field Research Articles

Investigation of phosphorus diffused back surface field (BSF) in bifacial nFAB solar cells

Recently bifacial n-type passivated emitter and rear totally diffused (PERT) monocrystalline silicon solar cells have become one hot spot in photovoltaic (PV) industries, due to good bifaciality, high and stabilised conversion efficiency. Unlike passivated emitter and rear contacts (PERC) structure, n-PERT solar cells require the use of a thin and uniform back surface field (BSF) layer, usually achieved by phosphorus doping. In this study, we optimised the phosphorus diffusion process in terms of surface concentration, junction depth and carrier lifetime. The effects of different phosphorus BSF were investigated by fabricating n-type front and back contact (nFAB) PERT solar cells using industrial feasible approaches and M2 size Czochralski (Cz) monocrystalline Si wafers (6 in., 244.32 cm2). Good phosphorus profiles were developed, which gives low parasitic absorption, low contact resistance and low J0 value when passivated with silicon nitride (SiNx) layer. The optimised champion cell shows a high Voc of 666.5 mV, Jsc of 40.2 mA/cm2, fill factor of 79.9%, efficiency of 21.43% from front side-illumination, together with a good bifaciality factor of 93.0%.

Screen-printed contacts with H-patterned n-type passivated emitter rear totally diffused solar cell and front-side boron selective emitter formed by wet chemical etching

This paper presents the efficiency distribution of an n-type bifacial solar cell. In this work, the solar cells were produced by our standard cell fabrication process, which is very similar to the industrial line process. The best cell efficiency was 20.4%, and the average efficiency was 20.06%. The cell efficiency ranged between 19.9 and 20.4% and showed a narrow efficiency distribution. This paper also presents the development of a boron selective emitter (p+/p++) n-type bifacial solar cell. In this work, the cells were fabricated using the standard cell fabrication process based on tube-furnace thermal diffusion employing liquid sources: BBr3 for the front-side boron emitter and POCl3 for the rear-side phosphorus back-surface field (BSF). The p+/p++ structure was formed by screen-printing resist masking and wet chemical etching technology. Both the front- and rear-side electrodes were obtained by using screen-printed contacts with H-patterns. The cell efficiency with the selective boron emitter was almost the same as that with the homogeneous emitter; however, the Voc of the selective emitter solar cell was higher than that of the homogeneous emitter solar cell by 4.4 mV. Finally, we performed the recombination analysis of the completed cell.

Printable liquid silicon for local doping of solar cells

We demonstrate the application of a liquid-processed doped silicon precursor as a doping source for the fabrication of interdigitated back contact solar cells. We integrate phosphorus- as well as boron-doped liquid silicon in our n-type interdigitated back contact cell process based on laser-structuring. The cell with the phosphorus back surface field from liquid silicon has an efficiency of 20.9% and the cell with the boron emitter from liquid silicon has an efficiency of 21.9%. We measure saturation current densities of 34fAcm−2 on phosphorus-doped layers with a sheet resistance of 108Ω/sq and 18fAcm−2 on boron-doped layers with a sheet resistance of 140Ω/sq using passivated test samples.

Internal quantum efficiency mapping analysis for a >20%-efficiency n-type bifacial solar cell with front-side emitter formed by BBr3 thermal diffusion

This paper presents a large-area (239-cm2) high-efficiency n-type bifacial solar cell that is processed using tube-furnace thermal diffusion employing liquid sources BBr3 for the front-side boron emitter and POCl3 for the rear-side phosphorus back surface field (BSF). The SiNx/Al2O3 stack was applied to the front-side boron emitter as a passivation layer. Both the front and rear-side electrodes are obtained using screen-printed contacts with H-patterns. The resulting highest-efficiency solar cell has front- and rear-side efficiencies of 20.3 and 18.7%, respectively, while the corresponding bifaciality is up to 92%. Finally, the passivation quality of the SiNx/Al2O3 stack on the front-side boron emitter and rear-side phosphorus BSF is investigated and visualized by measuring the internal quantum efficiency mapping of the bifacial solar cell.

Gapless point back surface field for the counter doping of large‐area interdigitated back contact solar cells using a blanket shadow mask implantation process

AbstractGapless interdigitated back contact (IBC) solar cells were fabricated with phosphorous back surface field on a boron emitter, using an ion implantation process. Boron emitter (boron ion implantation) is counter doped by the phosphorus back surface field (BSF) (phosphorus ion implantation) without gap. The gapless process step between the emitter and BSF was compared to existing IBC solar cell with gaps between emitters and BSFs obtained using diffusion processes. We optimized the doping process in the phosphorous BSF and boron emitter region, and the implied Voc and contact resistance relationship of the phosphorous and boron implantation dose in the counter doped region was analyzed. We confirmed the shunt resistance of the gapless IBC solar cells and the possibility of shunt behavior in gapless IBC solar cells. The highly doped counter doped BSF led to a controlled junction breakdown at high reverse bias voltages of around 7.5 V. After the doping region was optimized with the counter doped BSF and emitter, a large‐area (5 inch pseudo square) gapless IBC solar cell with a power conversion efficiency of 22.9% was made.

Universal Passivation for p++ and n++ Areas on IBC Solar Cells

We present a universal passivation for both, phosphorus and boron doped surfaces of interdigitated back contact solar cells. The amorphous silicon layer shows excellent passivation on boron emitter and phosphorus back surface field as long as the deposition temperature is below Tdep = 200°C. The amorphous silicon enables saturation current densities Jo,em = 46 fA/cm2 for laser doped p++ boron emitters with sheet resistance Rsh,em = 90Ω/sq as well as Jo,bsf = 73 fA/cm2 for laser doped n++ phosphorus back surface fields with Rsh,bsf°=°36°Ω/sq. Integration of the amorphous silicon low temperature passivation into our laser processed IBC solar cells yields a mean efficiency η = 22.8%. The open circuit voltage gain ΔVoc amounts to ΔVoc = 5mV compared to cells with thermal silicon dioxide rear passivation. The best solar cells achieve a confirmed efficiency η = 23.24%.

20.7% efficient ion‐implanted large area n‐type front junction silicon solar cells with rear point contacts formed by laser opening and physical vapor deposition

ABSTRACTIn this work, we report on ion‐implanted, high‐efficiency n‐type silicon solar cells fabricated on large area pseudosquare Czochralski wafers. The sputtering of aluminum (Al) via physical vapor deposition (PVD) in combination with a laser‐patterned dielectric stack was used on the rear side to produce front junction cells with an implanted boron emitter and a phosphorus back surface field. Front and back surface passivation was achieved by thin thermally grown oxide during the implant anneal. Both front and back oxides were capped with SiNx, followed by screen‐printed metal grid formation on the front side. An ultraviolet laser was used to selectively ablate the SiO2/SiNx passivation stack on the back to form the pattern for metal–Si contact. The laser pulse energy had to be optimized to fully open the SiO2/SiNx passivation layers, without inducing appreciable damage or defects on the surface of the n+ back surface field layer. It was also found that a low temperature annealing for less than 3 min after PVD Al provided an excellent charge collecting contact on the back. In order to obtain high fill factor of ~80%, an in situ plasma etching in an inert ambient prior to PVD was found to be essential for etching the native oxide formed in the rear vias during the front contact firing. Finally, through optimization of the size and pitch of the rear point contacts, an efficiency of 20.7% was achieved for the large area n‐type passivated emitter, rear totally diffused cell. Copyright © 2014 John Wiley & Sons, Ltd.

High efficiency large area n -type front junction silicon solar cells with boron emitter formed by screen printing technology

In this paper, we report on commercially viable screen printing (SP) technology to form boron emitters. A screen-printed boron emitter and ion-implanted phosphorus back surface field were formed simultaneously by a co-annealing process. Front and back surfaces were passivated by chemically grown oxide capped with plasma-enhanced chemical vapor deposition silicon nitride stack. Front and back contacts were formed by traditional SP and firing processes with silver/aluminum grid on front and local silver back contacts on the rear. This resulted in 19.6% efficient large area (239 cm2) n-type solar cells with an open-circuit voltage Voc of 645 mV, short-circuit current density Jsc of 38.6 mA/cm2, and fill factor of 78.6%. This demonstrates the potential of this novel technology for production of low-cost high-efficiency n-type silicon solar cells. Copyright © 2014 John Wiley & Sons, Ltd.

Ion-implanted and screen-printed large area 20% efficient N-type front junction Si solar cells

This paper reports on the fabrication of high efficiency (~20%) front junction n-type Si solar cells on 239cm2 Cz using ion implanted boron emitter and phosphorus back surface field (BSF) in combination with screen printed metallization. Cell efficiencies of 19.8% and 20.0% were achieved with SiO2/SiNx and Al2O3/SiNx passivation of boron implanted emitter, respectively, supporting the superiority of Al2O3 passivation. This is consistent with low boron emitter saturation current densities of 76 and 45fA/cm2 achieved for boron emitter passivated with SiO2/SiNx and Al2O3/SiNx stacks, respectively. Saturation current density in metal contact area of boron emitter and phosphorus BSF was measured directly by varying the metal contact coverage. Detailed analysis of saturation current density showed that the performance of our 20% is largely limited by saturation current density associated with recombination on metal contact area of boron emitter and bulk of phosphorus BSF, which accounted for almost ~50% of the total saturation current density.

Analysis of PV Modules and N-type Silicon Solar Cells with Different Rear Metal Grid

N-type silicon solar cells are being investigated due to the potential to obtain high efficiency. In this solar cell, the front p+ emitter is performed with boron doping and the Al or Al/Ag screen printing pastes don’t reach the development of Ag pastes used to form the front contact in n+pp+ solar cells. Moreover, the rear area covered by metal grid on the phosphorus back surface field may be optimized to increase the silicon solar cell efficiency and reduce the cost production. The goal of this paper is to present the analysis of PV modules and silicon solar cells developed on phosphorus-doped Czochralski wafers with different metal grid on the rear face. The solar cells were processed by using spin-on dopant to obtain the boron emitter and the back surface field was formed by phosphorus diffusion. The front metal grid was formed with Al/Ag paste and the rear area covered by the Ag metal paste was varied from 9% to 53%. Solar cells were manufactured, characterized and sorted taking into account the short-circuit current. Eight PV modules were fabricated using a standard process. Solar cells presented low fill factor. When the Ag/Sn/Cu ribbon was soldered on the front busbars of a solar cell, the fill factor rose from 0.70 to 0.75, causing an increase of 13.2% to 15.0% in the efficiency. Therefore, the fill factor of the PV modules was higher than that of solar cells. The reduction of the metal grid area on the rear face of solar cells did not affect the fill factor and solar cells with low rear metal coverage presented higher short-circuit current. The infrared thermography analysis was performed under a cloudless sky day with modules in the short-circuit condition. All modules presented hot spots and this result was not related to the different metallized rear area. The temperature difference between the solar cells of each photovoltaic module varied from 9°C to 18°C.

Codiffused Bifacial n-type Solar Cells (CoBiN)

We present codiffused bifacial n-type (CoBiN) solar cells on 156 mm Czochralski grown (Cz) Si wafers with peak efficiencies of 19.6 % fabricated using a lean industrial process. Simultaneous diffusion of phosphorus back surface field (BSF) and boron emitter in one single tube furnace process, the so called codiffusion, leads to a significant process simplification. Manipulation of the borosilicate glass (BSG) layer, deposited by atmospheric pressure chemical vapor deposition (APCVD) prior to the POCl3 based codiffusion process, allows for emitter profile tuning, without influencing the phosphorus doped BSF. Analytical simulations concerning the BSF identify the dark saturation current density of the passivated part of the BSF J0pass, BSF as the parameter that allows for maximum improvement of cell efficiency.

Co‐diffusion from solid sources for bifacial n‐type solar cells

AbstractWe present a simplified process sequence for the fabrication of large area n‐type silicon solar cells. The boron emitter and full area phosphorus back surface field are formed in one single high temperature step using doped glasses deposited by plasma enhanced chemical vapour deposition (PECVD) as diffusion sources. By optimizing the gas composition during the PECVD process, we not only prevent the formation of a boron rich layer (BRL), but also achieve doping profiles that exhibit a low dark saturation current density while allowing for contact formation by screen printing. The presented co‐diffusion process allows for major process simplification compared to the state of the art diffusion process relying on multiple high temperature processes, masking and wet chemistry steps.magnified imageSolar cell based on n‐type silicon featuring a co‐diffused boron emitter and phosphorus back‐surface field (BSF).(© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Co-diffusion from APCVD BSG and POCl3 for Industrial n-type Solar Cells

We present a simple industrially upscaleable process for the fabrication of bifacial n-type silicon solar cells. A process simplification is achieved by simultaneous diffusion of phosphorus back surface field (BSF) and boron emitter in one single high temperature process, the so called co-diffusion. A borosilicate glass (BSG) layer, deposited by atmospheric pressure chemical vapor deposition (APCVD) and an atmosphere containing POCl3 in an industrial tube furnace serve as dopant sources. We show that the oxygen concentration in the furnace during diffusion affects the boron surface concentration. This is a simple method to avoid boron rich layer formation and adjust the emitter sheet resistance. The doping profile of the phosphorous diffused BSF is controlled by adjusting the N2-POCl3 gas flow during diffusion allowing for independent manipulation of profile depth and surface concentration. For very high concentrations of POCl3 in the atmosphere during diffusion significant diffusion of phosphorus into the boron emitter, the so called cross doping, is observed. By lowering the POCl3 concentration cross doping can be reduced below the measurement accuracy.

Back surface field effects in the 17.3% efficient n-type dendritic web silicon solar cells

A solar cell efficiency of 17.3% (4 cm 2 area) has been achieved on 11 Ω cm, n-type dendritic web silicon. This is the highest reported efficiency to date on any silicon ribbon material. The minority carrier lifetime in the bulk material was determined to be at least 150 μs in spite of the fact that dopant diffusions were done at high temperature in a rapid thermal processing unit (1000°C for 30 s with cooling to 825°C at 50°C/min). Detailed characterization and modeling show that due to the reduced substrate thickness (100 μm) and long minority carrier diffusion length (>400 μm), device performance is strongly dependent on the back surface recombination velocity ( S b). In this study, an n + phosphorus back surface field (BSF) was implemented to reduce the effective S b of a fully metallized rear surface to approximately 20 cm/s. This effective S b value was determined through measurement and numerical analysis of the BSF doping profile. Model calculations reveal that the n +–n BSF served to increase the device efficiency by nearly 4% (absolute) above the case of infinite S b. By extending these model calculations to “mirror” solar cells (analogously doped n +–p–p + and p +–n–n + devices with equivalent lifetimes), it is shown that substrate type plays only a minor role in determining the overall device efficiency.

Book Reviews

We present a simple industrially upscaleable process for the fabrication of bifacial n-type silicon solar cells. A process simplification is achieved by simultaneous diffusion of phosphorus back surface field (BSF) and boron emitter in one single high temperature process, the so called co-diffusion. A borosilicate glass (BSG) layer, deposited by atmospheric pressure chemical vapor deposition (APCVD) and an atmosphere containing POCl3 in an industrial tube furnace serve as dopant sources. We show that the oxygen concentration in the furnace during diffusion affects the boron surface concentration. This is a simple method to avoid boron rich layer formation and adjust the emitter sheet resistance. The doping profile of the phosphorous diffused BSF is controlled by adjusting the N2-POCl3 gas flow during diffusion allowing for independent manipulation of profile depth and surface concentration. For very high concentrations of POCl3 in the atmosphere during diffusion significant diffusion of phosphorus into the boron emitter, the so called cross doping, is observed. By lowering the POCl3 concentration cross doping can be reduced below the measurement accuracy.