Understanding and implementation of rapid thermal technologies forhigh-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 (FF's) 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_eff) of 200 cm/s on 2.3 Ω-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 Ω-cm p-Si surface. The essential feature of the stack passivation scheme is its ability to withstand short 700-850°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_oe) of 40 and 90 Ω/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     

Fabrication and characterization of 18.6% efficientmulticrystalline silicon solar cells

Solar cell efficiencies as high as 18.6%(1 cm² area) have been achieved by a process which involves impurity gettering and effective back surface recombination velocity reduction of 0.65 Ω-cm multicrystalline silicon (mc-Si) grown by the heat exchanger method (HEM). Contactless photoconductance decay (PCD) analysis revealed that the bulk lifetime (τ_b) in HEM samples after phosphorus gettering can exceed 100 μs. At these τ_b levels, the back surface recombination velocity (S_b) resulting from unoptimized back surface field (BSF) design becomes a major limitation to solar cell performance. By implementing an improved aluminum back surface field (Al-BSF), S_b values in this study were lowered from 8000-10000 cm/s range to 2000 cm/s for HEM mc-Si devices. This combination of high τ_b and moderately low S _b resulted in the 18.6% device efficiency. Detailed model calculations indicate that lowering S_b further can raise the efficiency of similar HEM mc-Si devices above 19.0%, thus closing the efficiency gap between good quality, untextured single crystal and mc-Si solar cells. For less efficient devices formed on the same material, the presence of electrically active extended defects have been found to be the main cause for the performance degradation. A combination of light beam induced current (LBIC) scans as well as forward-biased current measurements have been used to analyze the effects of these extended defects on cell performance     

Fabrication and analysis of record high 18.2% efficient solar cellson multicrystalline silicon material

Solar cell efficiencies of 18.2% (1 cm² areas) have been achieved using a process sequence which involves impurity gettering on 0.65 Ω-cm multicrystalline silicon (mc-Si) grown by the heat exchanger method (HEM). This represents the highest reported solar cell efficiency on mc-Si to date. Photoconductive decay (PCD) measurements were used to monitor the lifetime in control samples which underwent the same process sequence as solar cells. PCD analysis reveals that HEM mc-Si material with an average as-grown bulk lifetime (τ_b) of 12 μs can achieve a lifetime as high as 135 μs by process-induced gettering. Internal quantum efficiency (IQE) measurements reveal that the effective diffusion length (L_eff) in the finished devices is 244 μm (or close to 90% of the total device width). Detailed cell analysis shows that for this combination of τ_b and L_eff, the back surface recombination velocity (S_b) of 10 000 cm/s or higher is the dominant efficiency limiting factor in the uniform regions of these mc-Si devices. Lowering S_b can raise the efficiency of untextured HEM mc-Si solar cells above 19.0%, thus closing the efficiency gap between good quality, untextured single crystal and mc-Si solar cells     

Integration of screen-printing and rapid thermal processingtechnologies for silicon solar cell fabrication

For the first time, the potentially cost-effective technologies of rapid thermal processing (RTP) and screen-printing (SP) have been combined into a single process sequence to achieve solar cell efficiencies as high as 14.7% on 0.2 Ω-cm FZ and 14.8% on 3 Ω-cm Cz silicon. These results were achieved without application of a nonhomogeneous (selective) emitter, texturing, or oxide passivation. By tailoring the RTP thermal cycles for emitter diffusion and firing of the screen-printed silver contacts, fill factor values >0.79 were realized on emitters with a sheet resistance (ρ_s ) of ~20 Ω/□ and grid shading <6%. Such high fill factors clearly demonstrate that screen-printed contacts can be fired on extremely shallow RTP emitters (x_j=0.25-0.3 μm) without shunting cells. IQE analysis depicts a strong preference for shallow emitter junction depths to achieve optimal short wavelength response of these unpassivated emitters. In some cases, front contacts were printed through plasma enhanced chemical vapor deposited (PECVD) SiN/SiO₂ dielectrics which prevented the shunting of shallow emitters by serving as partial barriers minimizing the diffusion of metallic species from the contacts. The firing of screen-printed contacts through these PECVD films, achieved the multiple purposes of contact formation, efficient front surface passivation due to annealing of the SiN, and high quality antireflection (AR). Research is presently underway to further optimize the RTP emitter design for screen-printing and develop techniques for implementing selective emitter and oxide passivation technologies for higher efficiency cells     

Modeling and characterization of high-efficiency silicon solarcells fabricated by rapid thermal processing, screen printing, andplasma-enhanced chemical vapor deposition

This paper presents, for the first time, the successful integration of three rapid, low-cost, high-throughput technologies for silicon solar cell fabrication, namely: rapid thermal processing (RTP) for simultaneous diffusion of a phosphorus emitter and aluminum back surface field; screen printing (SP) for the front grid contact; and low-temperature plasma-enhanced chemical vapor deposition (PECVD) of SiN for antireflection coating and surface passivation. This combination has resulted in 4 cm² cells with efficiencies of 16.3% and 15.9% on 2 Ω-cm FZ and Cz, respectively, as well as 15.4% efficient, 25-cm² FZ cells. Despite the respectable RTP/SP/PECVD efficiencies, cells formed by conventional furnace processing and photolithography (CFP/PL) give ~2% (absolute) greater efficiencies. Through in-depth modeling and characterization, this efficiency difference is quantified on the basis of emitter design and front surface passivation, grid shading, and quality of contacts. Detailed analysis reveals that the difference is primarily due to the requirements of screen printing and not RTP     

Rapid thermal processing of next generation silicon solar cells

Rapid and potentially low‐cost process techniques are analyzed and successfully applied towards the fabrication of high‐efficiency mono‐ and multicrystalline Si solar cells. First, a novel dielectric passivation scheme (formed by stacking a plasma silicon nitride film on top of a rapid thermal oxide layer) is developed that serves as antireflection coating and reduces the surface recombination velocity (S_eff) of the 1˙3 Ω‐cm p‐Si surface to approximately 10 cm/s. The essential feature of the stack passivation scheme is its ability to withstand short 700 – 850°C anneal treatments used to fire screen printed (SP) contacts, without degradation in S_oeff. The stack also lowers the emitter saturation current density (J_oe) of 40 and 90 Ω/□ emitters by a factor of three and 10, respectively, compared to no passivation. Next, rapid emitter formation is accomplished by diffusion under tungsten halogen lamps in both belt line and rapid thermal processing (RTP) systems (instead of in a conventional infrared furnace) . Third, a combination of SP aluminium and RTP is used to form an excellent back surface field (BSF) in 2 min to achieve an effective back surface recombination velocity (S_eff) of 200 cm/s on 2˙3 Ω‐cm Si. 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 photolithography, (2) 17% efficient manufacturable cells with emitter and Al‐BSF formed in a belt line furnace and contacts formed by SP. Copyright © 2000 John Wiley & Sons, Ltd.     

Study of injection effects on BSF silicon solar cells

The behavior of p⁺-n-n⁺ and n⁺-p-p⁺ silicon solar cells in terms of short-circuit current, open-circuit voltage, fill factor, and efficiency is studied as a function of base doping and illumination levels. A theoretical model that is valid for any injection level in the base region is used. Experimental results for cells of n-type base (in the range of 0.3 to 1000 Ω-cm) and a p-type base (0.4 to 300 Ω-cm) are presented. The theoretical model is able to explain phenomena such as the superlinearity of I_sc with concentration and the degradation of short-circuit current and efficiency at very high concentrations. These effects are seen as connected with the ohmic electric field in the base region. For the emitter saturation currents considered here, it can be concluded that, for p-type substrates, low base resistivities (≅1 Ω-cm) are necessary to achieve high efficiencies under concentrated light (≅100 suns), while for flat-array cells a particular resistivity is not required. For n-type substrates, it is found that any resistivity level can be used for both flat-array and concentrator cells     

Spiral inductors on Si p/p+ substrates with resonantfrequency of 20 GHz

Porous Si of up to 200 μm in thickness has been used to fabricate high-performance spiral inductors on heavily doped Si substrates (0.007 Ω-cm). Spiral inductors with L~5.7 nH are fabricated demonstrating Q_max~29 at 7 GHz and f_r>20 GHz. The resonant frequency (f_r) increases with increasing porous Si thickness and saturates beyond 120 μm. A corresponding decrease in total capacitance is observed. Q_max increases monotonically with porous Si layer thickness to beyond 200 μm. For inductors with a smaller footprint, Q_max begins to saturate at less than 100-μm thick porous Si     

Comprehensive study of rapid, low-cost silicon surface passivationtechnologies

A comprehensive and systematic investigation of low-cost surface passivation technologies is presented for achieving high-performance silicon devices such as solar cells. Most commercial solar cells today lack adequate surface passivation, while laboratory cells use conventional furnace oxides (CFO) for high-quality surface passivation involving an expensive and lengthy high-temperature step. This investigation tries to bridge the gap between commercial and laboratory cells by providing fast, low-cost methods for effective surface passivation. This paper demonstrates for the first time, the efficacy of TiO₂, thin (<10 nm) rapid thermal oxide (RTO), and PECVD SiN individually and in combination for (phosphorus diffused) emitter and (undiffused) back surface passivation. The effects of emitter sheet resistance, surface texture, and three different SiN depositions (two direct PECVD systems and one remote plasma system) were investigated. The effects of post-growth/deposition treatments such as forming gas anneal (FGA) and firing of screen printed contacts were also examined. This study reveals that the optimum passivation scheme consisting of a thin RTO with a SiN cap followed by a very short 730°C anneal can 1) reduce the emitter saturation current density, J_0e, by a factor of >15 for a 90 Ω/sq. emitter, 2) reduce J_0e by a factor of >3 for a 40 Ω/sq, emitter, and 3) reduce S_back below 20 cm/s on 1.3 Ωcm p-Si. Furthermore, this double-layer RTO+SiN passivation is relatively independent of the deposition conditions (direct or remote) of the SiN film and is more stable under heat treatment than SiN or RTO alone. Model calculations are also performed to show that the RTO+SiN surface passivation scheme may lead to 17%-efficient thin screen-printed cells even with a low bulk lifetime of 20 μs     

High efficiency screen‐printed EFG Si solar cells through rapid thermal processing‐induced bulk lifetime enhancement

This paper shows that one second (1 s) firing of Si solar cells with screen‐printed Al on the back and SiN _x anti‐reflection coating on the front can produce a high quality Al‐doped back‐surface‐field (Al‐BSF) and significantly enhance SiN _x ‐induced defect hydrogenation in the bulk Si. Open‐circuit voltage, internal quantum efficiency measurements, and cross‐sectional scanning electron microscopy pictures on float‐zone silicon cells revealed that 1 s firing in rapid thermal processing at 750°C produces just as good a BSF as 60 s firing, indicating that the quality of Al‐BSF region is not a strong function of RTP firing time at 750°C. Analysis of edge‐defined film‐fed grown (EFG) Si cells showed that short‐term firing is much more effective in improving the hydrogen passivation of bulk defects in EFG Si. Average minority‐carrier lifetime in EFG wafers improved from ∼3 to ∼33 μs by 60 s firing but reached as high as 95μs with 1 s firing, resulting in 15·6% efficient screen‐printed cells on EFG Si. Copyright © 2004 John Wiley & Sons, Ltd.     

Simulation analysis of the effects of a back surface field on a p-a-Si:H/n-c-Si/n+-a-Si:H heterojunction solar cell

In order to investigate the effects of a back surface field (BSF) on the performance of a p-doped amorphous silicon (p-a-Si:H)/n-doped crystalline silicon (n-c-Si) solar cell, a heterojunction solar cell with a p-a-Si:H/nc-Si/n+-a-Si:H structure was designed. An n+-a-Si:H film was deposited on the back of an n-c-Si wafer as the BSF.The photovoltaic performance of p-a-Si:H/n-c-Si/n+-a-Si:H solar cells were simulated. It was shown that the BSF of the p-a-Si:H/n-c-Si/n+-a-Si:H solar cells could effectively inhibit the decrease of the cell performance caused by interface states.     

Simulation analysis of the effects of a back surface field on a p-a-Si:H/n-c-Si/n+-a-Si:H heterojunction solar cell

In order to investigate the effects of a back surface field（BSF） on the performance of a p-doped amorphous silicon（p-a-Si：H）/n-doped crystalline silicon（n-c-Si） solar cell,a heterojunction solar cell with a p-a-Si：H/n-c-Si/n^＋-a-Si：H structure was designed.An n^＋-a-Si：H film was deposited on the back of an n-c-Si wafer as the BSF.The photovoltaic performance of p-a-Si：H/n-c-Si/n^＋-a-Si：H solar cells were simulated.It was shown that the BSF of the p-a-Si：H/n-c-Si/n^＋-a-Si：H solar cells could effectively inhibit the decrease of the cell performance caused by interface states.     

Highest efficiency rapid thermal processed multicrystalline silicon solar cells

The formation of pn junctions and surface passivation by rapid thermal processing is being proved as a new and competitive method for silicon solar cell production. As the main process mechanisms are enhanced, the total process time at high temperature can be kept in the minute range, for the realization of emitter, back surface field (BSF) and surface passivation. In this work, we demonstrate for the first time that this knowledge, avoiding any in‐situ annealing step acquired on the sc‐Si, can also be applied on industrial mc‐Si (Polix_©) without bulk degradation, leading to a record conversion efficiency of 16·7%. Copyright © 2001 John Wiley & Sons, Ltd.     

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

Gapless 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 V_oc 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.     

Diffusion‐based model of local Al back surface field formation for industrial passivated emitter and rear cell solar cells

In this work, the back surface field (BSF) formation of locally alloyed Al‐paste contacts employed in recent industrial passivated emitter and rear cell solar cell designs is discussed. A predictive model for resulting local BSF thickness and doping profile is proposed that is based on the time‐dependent Si distribution in the molten Al paste during the firing step. Diffusion of Si in liquid Al away from the contact points is identified as the main differentiator to a full‐area Al‐BSF; therefore, a diffusion‐based solution to the involved differential equation is pursued. Data on the Si distribution in the Al and the resulting BSF structures are experimentally obtained by firing samples with different metal contact geometries, peak temperature times and pastes as well as by investigating them by means of scanning electron microscopy and energy dispersive X‐ray spectroscopy. The Si diffusivity in the Al paste is then calculated from these results. It is found that the diffusivity is strongly dependent on the paste composition. Furthermore, the local BSF doping profiles and thicknesses resulting from different contact geometries and paste parameters are calculated from the Si concentration at the contact sites, the diffusivity and solubility data. These profiles are then used in a finite element device simulator to evaluate their performance on solar cell level. With this approach, a beneficial paste composition for any given rear contact geometry can be determined. Two line widths are investigated, and the effects of the different paste properties are discussed in the light of the solar cell results obtained by simulation. Copyright © 2013 John Wiley & Sons, Ltd.     

Bulk resistivity optimization for low‐bulk‐lifetime silicon solar cells

Guidelines are presented which are designed to achieve planar solar cell efficiencies as high as 17.5% using existing fabrication technologies and silicon substrates with lifetimes as low as 20 μs. Device simulations are performed to elucidate the need and impact of base doping optimization for different back‐surface passivation schemes, cell thicknesses, emitter profiles, and degrees of dopant–defect interaction. Results indicate that optimal resistivity is a function of back‐surface passivation, with the aluminum back‐surface field (BSF) requiring the highest resistivity, the oxide/nitride stack passivation excelling at an intermediate resistivity, and the ohmic contact needing the lowest resistivity. A comparison of simulated 300 and 100 μm cells shows that thinner cells magnify the differences in optimal resistivity for the three back‐surface passivation schemes. A lifetime model is used to account for dopant–defect interaction that can lower bulk lifetime at higher doping levels. It is demonstrated that cell efficiency decreases and optimal resistivity increases at higher levels of dopant–defect interaction. Simulated devices with an optimized base doping showed an efficiency improvement of as much as 2% (absolute) compared with identical devices with a typical base doping level (1.6 or 1.8 Ω cm) and bulk lifetime of 20 μs. Copyright © 2001 John Wiley & Sons, Ltd.     

18% efficient silicon photovoltaic devices by rapid thermaldiffusion and oxidation

For the first time, cells formed by rapid thermal processing (RTP) have resulted in 18%-efficient 1 and 4 cm² single-crystal silicon solar cells. Front surface passivation by rapid thermal oxidation (RTO) significantly enhanced the short wavelength response and decreased the effective front surface recombination velocity (including contact effects) from 7.5×10⁵ to about 2×10⁴ ×10⁴ cm/s. This improvement resulted in an increase of about 1% (absolute) in energy conversion efficiency, up to 20 mV in V_ot, and about 1 mA/cm² in J_sc. These RTO-induced enhancements are shown to be consistent with model calculations. Since only 3 to 4 min are required to simultaneously form the phosphorus emitter and aluminum back-surface-field (BSF) and 5 to 6 min are required for growing the RTO, this RTP/RTO process represents the fastest technology for diffusing and oxidizing ⩾18%-efficient solar cells. Both cycles incorporate an in situ anneal lasting about 1.5 min to preserve the minority carrier lifetime of lower quality materials such as dendritic-web and multicrystalline silicon. These high-efficiency cells confirmed that RTP results in equivalent performance to cells fabricated by conventional furnace processing (CFP). Detailed characterization and modeling reveals that because of RTO passivation of the front surface (which reduced J_0c by nearly a factor of ten), these RTP/RTO cells have become base dominated (J_0b≫J_0c), and further improvement in cell efficiency is possible by a reduction in back surface recombination velocity (BSRV). Based upon model calculations, decreasing the BSRV to 200 cm/s is expected to give 20%-efficient RTP/RTO cells     

Low-resistance bandgap-engineeredW/Si1-xGex/Si contacts

Bandgap-engineered W/Si_1-xGe_x/Si junctions (p⁺ and n⁺) with ultra-low contact resistivity and low leakage have been fabricated and characterized. The junctions are formed via outdiffusion from a selectively deposited Si_0.7Ge _0.3 layer which is implanted and annealed using RTA. The Si _1-xGe_x layer can then be selectively thinned using NH₄OH/H₂O₂/H₂O at 75°C with little change in characteristics or left as-deposited. Leakage currents were better than 1.6×10^-9 A/cm² (areal), 7.45×10^-12 A/cm (peripheral) for p⁺/n and 3.5×10^-10 A/cm² (peripheral) for n⁺/p. W contacts were formed using selective LPCVD on Si_1-xGe_x. A specific contact resistivity of better than 3.2×10^-8 Ω cm² for p ⁺/n and 2.2×10^-8 Ω cm² for n⁺/p is demonstrated-an order of magnitude n⁺ better than current TiSi₂ technology. W/Si_1-xGe _x/Si junctions show great potential for ULSI applications     

Towards thinner and low bowing silicon solar cells: form the boron and aluminum co‐doped back surface field with thinner metallization film

Using thinner wafers can largely reduce the cost of silicon solar cells. One obstacle of using thinner wafers is that few methods can provide good dopant concentration for the back surface field (BSF) and good ohmic contact while generated only in low bowing. In this paper, we have demonstrated the screening–printing B and Al (B/Al) mixture metallization film technique, making use of the screen‐printing technique and the higher solubility of B in silicon to form a B/Al‐BSF. This technique can raise the carrier concentration in the BSF by more than one order of magnitude and reduce the back surface recombination at a low firing temperature (≤800 °C). We have also shown that through the new technique, the metallization paste thickness at the rear could be reduced largely, which however did not degrade the solar cell efficiency. All these efforts are aiming for pushing forward the application of thinner wafers. Copyright © 2011 John Wiley & Sons, Ltd.     

Assessing material qualities and efficiency limits of III–V on silicon solar cells using external radiative efficiency

The paper presents a quantitative approach to the investigation and comparison of the material qualities of III–V on silicon (III–V/Si) solar cells by using external radiative efficiencies. We use this analysis to predict the limiting efficiencies and evaluate the criteria of material quality in order to achieve high‐efficiency III–V/Si solar cells. This result yields several implications for the design of high‐efficiency III–V/Si solar cells. Copyright © 2016 John Wiley & Sons, Ltd.