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.     

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.     

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.     

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.     

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.     

An optimized rapid aluminum back surface field technique forsilicon solar cells

Screen-printing and rapid thermal annealing have been combined to achieve an aluminum-alloyed back surface field (Al-BSF) that lowers the effective back surface recombination velocity (S_eff) to approximately 200 cm/s for solar cells formed on 2.3 Ω-cm Si. Analysis and characterization of the BSF structures show that this formation process satisfies the two main requirements for achieving low S_eff: (1) deep p⁺ regions and (2) uniform junctions. Screen-printing is ideally suited for fast deposition of thick Al films which, upon alloying, result in deep BSF regions. Use of a rapid alloying treatment is shown to significantly improve the BSF junction uniformity and reduce S_eff. The Al-BSFs formed by screen-printing and rapid alloying have been integrated into both laboratory and industrial-type fabrication sequences to achieve solar cell efficiencies in excess of 19.0 and 17.0%, respectively, on planar 2.3 Ω-cm float zone Si. For both process sequences, these cell efficiencies are 1-2% (absolute) higher than analogous cells made with unoptimized Al-BSFs or highly recombinative rear surfaces     

高效单晶硅PERC局部背表面场及接触的优化

钝化发射极和背面电池(PERC)技术可有效提高电池效率,在常规p型电池的背面增加了钝化层,并形成了局部背表面场(LBSF)结构.介绍了PERC结构电池的工艺流程,分析了背场(BSF)的形成机制,主要研究了PERC的LBSF制备工艺及影响要素.通过采用激光消融后清洗方法改善了背表面形貌,平整的背表面形貌有利于BSF的形成.通过优化烧结条件,电池的填充因子得到改善.讨论了激光开槽图形对开路电压以及填充因子的影响.测试结果表明,PERC转换效率绝对值提升了0.9％,达到20.83％,填充因子达到80.7％.     

Influence of wafer thickness on the performance of multicrystalline Si solar cells: an experimental study

The influence of the thickness of silicon solar cells has been investigated using neighbouring multicrystalline silicon wafers with thickness ranging from 150 to 325 μm. For silicon solar cell structures with a high minority‐carrier diffusion length one expects that J_sc would decrease as the wafer becomes thinner due to a shorter optical path length. It was found experimentally that J_sc is nearly independent of the thickness of the solar cell, even when the minority‐carrier diffusion length is about 300 μm. This indicates that the Al rear metallisation acts as a good back surface reflector. A decrease in J_sc is observed only if the wafer thickness becomes less than about 200 μm. The observed trend in V_oc as a function of the wafer thickness has been explained with PC1D modelling by a minority‐carrier diffusion length in the Al‐oped BSF which is small in relation to the thickness of the BSF. This effectively increases the recombination velocity at the rear of the cell. We have shown that the efficiency of solar cells made with standard industrial processing is hardly reduced by reducing the wafer thickness. Solar cell efficiencies might be increased by better rear surface passivation. Copyright © 2002 John Wiley & Sons, Ltd.     

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     

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.     

A current-generating BSF silicon solar cell fabricated through masked ion implantation

A preferentially current-generating back-surface-field (BSF) silicon solar cell with a near-ideal value of the short-circuit current density JSCunder AM1 conditions has been developed through masked ion implantation of the n⁺-p junction. The BSF cells, fabricated in industrial conditions, possess a conversion efficiency η and a fill-factor FF up to 14.2 and 76.8 percent, respectively. The dependence of JSC, η, and FF on technological parameters are outlined in this letter.     

Optimizing back surface field for improving Voc of(Al)GaInP solar cell

GaInP and AlGaInP solar cells were grown by metal organic chemical vapor deposition (MOCVD), and theoretical analysis demonstrated that hetero-interface recombination velocity plays an important role in the optimizing of cell performance, especially the interface between base layer and back surface field (BSF). Measurements including lattice-matched growth and pseudo-BSF were taken to optimize BSF design. Significant improvement of V_oc in GaInP and AlGaInP solar cells imply that the measures we took are effective and promising for performance improvement in the next generation high efficiency solar cells.     

Influence of the layer parameters on the performance of the CdTe solar cells

Influence of the layer parameters on the performances of the CdTe solar cells is analyzed by SCAPS-1D. The ZnO:Al film shows a high efficiency than SnO2:F. Moreover, the thinner window layer and lower defect density of CdS films are the factor in the enhancement of the short-circuit current density. As well, to increase the open-circuit voltage, the responsible factors are low defect density of the absorbing layer CdTe and high metal work function. For the low cost of cell production, ultrathin film CdTe cells are used with a back surface field (BSF) between CdTe and back contact, such as PbTe. Further, the simulation results show that the conversion efficiency of 19.28% can be obtained for the cell with 1-μm-thick CdTe, 0.1-μm-thick PbTe and 30-nm-thick CdS.     

Secondary phase formation in Zn‐rich Cu2ZnSnSe4‐based solar cells annealed in low pressure and temperature conditions

Zn‐rich Cu₂ZnSnSe₄ (CZTSe) films were prepared by a two‐step process consisting in the DC‐magnetron sputtering deposition of a metallic stack precursor followed by a reactive anneal under a Se + Sn containing atmosphere. Precursor composition and annealing temperature were varied in order to analyze their effects on the morphological, structural, and optoelectronic properties of the films and solar cell devices. Raman scattering measurements show the presence of ZnSe as the main secondary phase in the films, as well as the presence of SnSe at the back absorber region of the films processed with lower Zn‐excess values and annealing temperatures. The ZnSe phase is found to accumulate more towards the surface of the absorber in samples with lower Zn‐excess and lower temperature annealing, while increasing Zn‐excess and annealing temperature promote its aggregation towards the back absorber region of the devices. These measurements indicate a strong dependence of these process variables in secondary phase formation and accumulation. In a preliminary optimization of both the composition and reactive annealing process, a solar cell with 4.8% efficiency has been fabricated, and potential mechanisms limiting device efficiency in these devices are discussed. Copyright © 2014 John Wiley & Sons, Ltd.     

Numerical Analysis of Characterized Back Interface Traps of SOI Devices by R-G Current

Characterized back interface traps of SOI devices by the Recombination-Generation (R-G) curren: has been analyzed numerically with an advanced semiconductor simulation tool,namely DESSiS-ISE. The basis of the principle for the R-G current's characterizing the back interface traps of SOI lateral p+p-n+ diode has been demonstrated. The dependence of R-G cur rent on interface trap characteristics has been examined, such as the state density, surface recombination velocity and the trap energy level. The R-G current proves to be an effective tool for monitoring the back interface of SOI devices.     

Determination of minority carrier lifetime and effective back surface recombination velocity in BSF silicon solar cells from transient measurements

The method of determining the base lifetime ?B and the effective surface recombination velocity Seff in a BSF solar cell from the transient decay of open-circuit voltage and short-circuit current is extended to include emitter recombinations. If the emitter recombinations in modern Si solar cells are neglected in interpreting the experimental data, the experimental value of Seff is found to be in large error.     

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.     

Surface recombination effects on the performance of n+p step and diffused junction silicon solar cells

Effect of front and back surface recombination velocity (SRV) on the performance of n⁺p step and diffused junction Si solar cells has been investigated. Assuming the impurity profile to be Gaussian in the diffused region, the influence of built-in field and field gradient on the cell performance parameters have been discussed. It has been found that relatively higher front SRV can be tolerated for terrestrial utilisation than for space application. The improvement in conversion efficiency due to the reduction in back SRV is more if the cell thickness is smaller and the junction is shallower. The benefits of a lower back SRV are limited by the value of front SRV and recombination losses in the front region. The built-in field in the front region counteracts the recombination losses at the top surface and in the front region provided the field gradient is small.