Co‐optimization of emitter profile and metal grid of selective emitter silicon solar cells

Co‐optimization of the metallization and emitter dopant profile is fully investigated for selective emitter crystalline silicon solar cells. The simulation parameters for the laser doping selective emitter, metallization by plating, silicon nitride passivation, and aluminum back surface field are identified and reached. Internal light flux reflection is also considered in the model. In particular, the influence of the increased light trapping ability of a textured surface on the optimization results is clarified by comparing a cell with a non‐textured surface. In this paper, the optimization results, including the electrical performances of a solar cell are discussed in detail. On the basis of these simulation results, an optimized metallization and emitter dopant profile is proposed. Copyright © 2011 John Wiley & Sons, Ltd.     

Selective doping of silicon by rapid thermal and laser assisted processes

The selective doping technique, made by the combination of spin-on dopant (SOD) source deposition, rapid thermal annealing (RTA) and laser treatments is proposed as an innovative process for large area devices, like silicon solar cells.Rapid thermal diffusion (RTD) is first carried out from phosphorus SOD layers to form a lightly doped junction followed by pulsed laser irradiation to induce overdoping in selectively chosen regions.Here we present extensive study on the dependence of selective doping efficiency through different working variables, such as dopant source dilution, diffusion temperature and time for RTPs, and power and translation velocity for lasers. Electrical and structural characterizations have been performed by using several techniques: SIMS, stripping-Hall, four-point probe resistivity, SEM and TEM analysis.The combined use of these processes has been applied to the realization of selective emitter structures for silicon solar cells.     

Application of junction capacitance measurements to the characterization of solar cells

The quasi-static capacitance-voltage ( C-V) technique measures the dependence of junction capacitance on the bias voltage by applying a slow, reverse-bias voltage ramp to the solar cell in the dark, using simple circuitry. The resulting C-V curves contain information on the junction area and base dopant concentration, as well as their built-in potential. However, in the case of solar cells made on low to medium resistivity substrates and having thick emitters, the emitter dopant profile has to be taken into account. A simple method can then be used to model the complete C-V curves, which, if the base doping is known, permits one to estimate the emitter doping profile. To illustrate the method experimentally, several silicon solar cells with different base resistivities have been measured. They comprise a wide range of areas, surface faceting conditions and emitter doping profiles. The analysis of the quasi-static capacitance characteristics of the flat surface cells resulted in good agreement with independent data for the wafer resistivity and the emitter doping profile. The capacitance in the case of textured surfaces is a function of the effective junction area, which is otherwise difficult to measure, and is essential to understand the emitter and space charge region recombination currents. The results indicate that the effective area of the junction is not as large as the area of the textured surface.     

丝网印刷制备黑硅太阳电池及其性能表征

采用纳米金颗粒催化腐蚀的方法在硅片表面制备得纳米多孔结构,实现了1.5%（300-1200 nm）的权重反射率。本文采用OPCl3扩散、丝网印刷制备前后电极及共烧等常规太阳电池工艺来制备黑硅太阳电池,对不同腐蚀深度及不同扩散方阻的黑硅太阳电池片的输出电性能进行了分析,并对制备工艺进行了优化,提高了电池的转换效率,实现了丝网印刷制备12.17%的黑硅太阳电池转换效率。     

Visualization of Photoexcited Carrier Responses in a Solar Cell Using Optical Pump—Terahertz Emission Probe Technique

We observed photoexcited carrier responses in solar cells excited by femtosecond laser pulses with spatial and temporal resolution using an optical pump-terahertz emission probe technique. We visualized the ultrafast local variation of the intensity of terahertz emission from a polycrystalline silicon solar cell using this technique and clearly observed the change in signals between a grain boundary and the inside of a grain in the solar cell. Further, the time evolution of the pump–probe signals of the polycrystalline and monocrystalline silicon solar cells was observed, and the relaxation times of photoexcited carriers in the emitter layers of crystalline silicon solar cells were estimated using this technique. The estimated relaxation time was consistent with the lifetime of the Auger recombination process that was dominant in heavily doped silicon used as an emitter layer for the silicon solar cells, which is difficult to obtain with photoluminescence method commonly used for the evaluation of solar cells.     

Influence of the dopant density profile on minority-carrier current in shallow,heavily doped emitters of silicon bipolar devices

Experimental determination of the dependence of recombination current in p⁺ and n⁺ regions on the dopant profile for shallow emitters of ion-implanted silicon solar cells is described. The results are analyzed by extending a previous analytical model for the transport of minority carriers in heavily doped regions. The extension accounts for an effective electric field, defined by heavy-doping effects at the surface, and suggests that the energy-gap narrowing for p⁺ silicon is slightly smaller than that for n⁺ silicon and/or that minority-carrier diffusivities are substantially lower than the majority-carrier ones at comparable dopant densities. The very high dopant densities achieved with the ion implantation/laser annealing technique provide an in situ surface passivation that supresses surface recombination and minimizes the emitter recombination current.     

Analysis of the EWT‐DGB solar cell at low and medium concentration and comparison with a PESC architecture

Silicon represents an interesting material to fabricate low‐cost and relatively simple and high‐efficient solar cells in the low and medium concentration range. In this paper, we discuss a novel cell scheme conceived for concentrating photovoltaic, named emitter wrap through with deep grooved base (EWT‐DGB), and compare it with the simpler passivated emitter solar cell. Both cells have been fabricated by means of a complementary metal–oxide–semiconductor‐compatible process in our laboratory. The experimental characterization of both cells is reported in the range 1–200 suns in terms of conversion efficiency, open circuit voltage, short circuit current density and fill factor. In particular, for the EWT‐DGB solar cells, we obtain an encouraging 21.4% maximum conversion efficiency at 44 suns. By using a calibrated finite‐element numerical electro‐optical simulation tool, validated by a comparison with experimental data, we study the potentials of the two architectures for concentrated light conditions considering possible realistic improvements with respect to the fabricated devices. We compare the solar cell figures of merit with those of the state‐of‐the‐art silicon back‐contact back‐junction solar cell holding the conversion efficiency record for concentrator photovoltaic silicon. Simulation results predict a 24.8% efficiency at 50 suns for the EWT‐DGB cell and up to 23.9% at 100 suns for the passivated emitter solar cell, thus confirming the good potential of the proposed architectures for low to medium light concentration. Finally, simulations are exploited to provide additional analysis of the EWT‐DGB scheme under concentrated light. Copyright © 2017 John Wiley & Sons, Ltd.     

Genuine wide-bandgap microcrystalline emitter Si-HBT with enhancedcurrent gain by suppressing homocrystallization

The energy bandgap of microcrystalline silicon (μc-Si) emitter prepared by the plasma CVD method for Si-HBTs was investigated. The μc-Si films directly deposited on c-Si substrates were confirmed to have almost the same energy bandgap as c-Si because of μc-Si crystallization, resulting in formation of a homojunction. In order to suppress such a homojunction formation, a c-Si surface modification method using an a-SiC thin layer was proposed. The a-SiC layer was confirmed to have the effect of producing an abrupt and uniform heterojunction. A current gain as high as 523 was obtained by using the a-SiC thin layer, which was 24 times larger than that without the a-SiC layer     

一种新的测量硅太阳电池前电极穿透p-n结所引起的漏电电阻率的方法

漏电电阻可以大幅度地降低太阳电池的转换效率,目前对它的测量一般是电池中所有的漏电所造成的总效果。为了测得太阳电池前电极穿透发射区所引起的漏电电阻值,本文制备了一种具有梁桥电极的新型结构太阳电池。结果表明,电池前电极穿透发射区所引起的漏电电阻率值比其他发射区域的漏电要严重的多。这些样品制备有与生产工艺兼容的特点,同时使用它能够为电池结构、材料组成和工艺参数提供许多有用的信息。     

Phosphorus diffusion from a spin-on doped glass (SOD) source during rapid thermal annealing

Limiting thermal exposure time using rapid thermal processing (RTP) has emerged as a promising simplified process for microelectronics applications and for manufacturing of terrestrial solar cells in a continuous way. Especially, rapid thermal diffusion (RTD) of phosphorus from doped oxide films (SOD) was extensively used for the emitter formation purpose but few work concerned the diffusion mechanism.Here we investigate more in details the diffusion kinetics of phosphorus after rapid thermal annealing of P-SOD coated silicon samples. The observed enhanced distribution of phosphorus after RTD is discussed based on the dopant sources and processing conditions. Comparisons between experimental profiles and simulation results using up to date phosphorus diffusion models allow us to discriminate between various possible enhancement mechanisms.     

Autodiffusion: a novel method for emitter formation in crystalline silicon thin‐film solar cells

The in situ formation of an emitter in monocrystalline silicon thin‐film solar cells by solid‐state diffusion of dopants from the growth substrate during epitaxy is demonstrated. This approach, that we denote autodiffusion, combines the epitaxy and the diffusion into one single process. Layer‐transfer with porous silicon (PSI process) is used to fabricate n‐type silicon thin‐film solar cells. The cells feature a boron emitter on the cell rear side that is formed by autodiffusion. The sheet resistance of this autodiffused emitter is 330 Ω/□. An independently confirmed conversion efficiency of (14·5 ± 0·4)% with a high short circuit current density of (33·3 ± 0·8) mA/cm² is achieved for a 2 × 2 cm² large cell with a thickness of (24 ± 1) µm. Transferred n‐type silicon thin films made from the same run as the cells show effective carrier lifetimes exceeding 13 µs. From these samples a bulk diffusion length L > 111 µm is deduced. Amorphous silicon is used to passivate the rear surface of these samples after the layer‐transfer resulting in a surface recombination velocity lower than 38 cm/s. Copyright © 2006 John Wiley & Sons, Ltd.     

Auger recombination in heavily doped shallow-emitter silicon p-n-junction solar cells, diodes, and transistors

A rigorous analytic evaluation of an emitter model that includes Auger recombination but excludes bandgap narrowing is presented. It is shown that such a model cannot explain the experimentally observed values of the open-circuit voltage VOCin p-n-junction silicon solar cells. Thus physical mechanisms in addition to Auger recombination are responsible for the experimentally observed values of VOCin silicon solar cells and the common-emitter current gain in bipolar transistors.     

Analytical models for the series resistance of selective emitters in silicon solar cells including the effect of busbars

A theoretical analysis of the power loss and series resistance of the front side emitter in silicon solar cells is presented. Existing 1D models (infinitely long finger) and 2D models (including the effect of busbars) of emitter series resistance contribution are extended to the case of selective emitters. The general case of different current densities for both emitters in the selective emitter scheme is considered in these extensions. The resulting models depend on the individual sheet resistances and current densities in both emitters and the device's overall grid geometry. The models are corroborated by finite element simulation of the potential in the emitter. An excellent agreement is found between the analytical models, and the simulations for a wide range of sheet resistances typically encountered in silicon solar cells. Grid simulations using the 2D model are applied to solar cells with selective emitters, where the width of the low‐resistive emitter was varied. The simulations demonstrate that the 2D model can explain the absolute change in fill factor observed in these cells. Copyright © 2013 John Wiley & Sons, Ltd.     

A novel strategy for preparing a selective back surface field of n-type emitter wrap through solar cells

In the present study, we have developed a novel mixed co-diffusion (MCD) process by which to prepare a selective back surface field (BSF) of n-type emitter wrap through (EWT) solar cells, which combines a plasma enhanced chemical vapor deposition (PECVD) phosphorus-doped n-type microcrystalline silicon as the dopant source, with a low temperature thermal oxidation (LTO) process. Through comparison between our MCD process and a standard co-diffusion (SCD) process, a BSF with a shallow doping depth of 0.56 µm and high doping concentration of 1.9×10²⁰ at/cm³ is easily obtained by the MCD process under the low temperature of 750 ℃. Therefore, the MCD process is shown to reduce the number of high temperature processes, which cannot produce dopant redistribution, and can accurately control the doping concentrations and depths of the BSF and emitter. In addition, the novel method also eliminates the boron-rich layer, which induces misfit dislocations and bulk lifetime degradation, without extra chemical treatment. Therefore, the MCD process' open circuit voltage, short circuit current density, conversion efficiency and fill factor of the solar cells are respectively increased by 7 mV, 6 mA/cm², 2% and 2%. These results indicate that the MCD process is a novel and potential agent for the SCD process.     

Analysis of the p+/p window layer of thin film solar cells by simulation

正The application of a p~+/p configuration in the window layer of hydrogenated amorphous silicon thin film solar cells is simulated and analyzed utilizing an AMPS-ID program.The differences between p~+-p-i-n configuration solar cells and p-i-n configuration solar cells are pointed out.The effects of dopant concentration, thickness of p~+-layer,contact barrier height and defect density on solar cells are analyzed.Our results indicate that solar cells with a p~+-p-i-n configuration have a better performance.The open circuit voltage and short circuit current were improved by increasing the dopant concentration of the p~+ layer and lowering the front contact barrier height.The defect density at the p/i interface which exceeds two orders of magnitude in the intrinsic layer will deteriorate the cell property.     

高方块电阻发射区单晶硅太阳电池的性能优化

通过提高发射区的方块电阻和优化发射区的磷杂质浓度纵向分布,制备了性能优良的单晶硅太阳电池。I-V测量分析表明:高表面活性磷杂质浓度浅结发射区太阳电池短路电流密度、开路电压和填充因子分别提高了0.32mA/cm2,1.19mV和0.22%,因此转换效率提高了0.22%。内量子效率分析表明:高表面活性磷杂质浓度浅结发射区太阳电池短路电流密度的提高是由于短波光谱响应增强了。SEM分析表明:高表面活性磷杂质浓度浅结发射区太阳电池在发射区硅表面沉积的Ag晶粒分布数量更多、一致性更好,从而更容易收集光生电流传输到Ag栅线,改善了太阳电池的性能。     

High efficiency polycrystalline silicon solar cells using lowtemperature PECVD process

Conventionally directionally solidified (DS) and silicon film (SF) polycrystalline silicon solar cells are fabricated using gettering and low temperature plasma enhanced chemical vapor deposition (PECVD) passivation. Thin layer (~10 nm) of PECVD SiO₂ is used to passivate the emitter of the solar cell, while direct hydrogen rf plasma and PECVD silicon nitride (Si₃N₄) are implemented to provide emitter and bulk passivation. It is found in this work that hydrogen rf plasma can significantly improve the solar cell blue and long wavelength responses when it is performed through a thin layer of PECVD Si₃N₄. High efficiency DS and SF polycrystalline silicon solar cells have been achieved using a simple solar cell process with uniform emitter, Al/POCl₃ gettering, hydrogen rf plasma/PECVD Si₃N₄ and PECVD SiO₂ passivation. On the other hand, a comprehensive experimental study of the characteristics of the PECVD Si₃N₄ layer and its role in improving the efficiency of polycrystalline silicon solar cells is carried out in this paper. For the polycrystalline silicon used in this investigation, it is found that the PECVD Si₃N₄ layer doesn't provide a sufficient cap for the out diffusion of hydrogen at temperatures higher than 500°C. Low temperature (⩽400°C) annealing of the PECVD Si₃N ₄ provides efficient hydrogen bulk passivation, while higher temperature annealing relaxes the deposition induced stress and improves mainly the short wavelength (blue) response of the solar cells     

Diffusion‐free high efficiency silicon solar cells

Traditional POCl₃ diffusion is performed in large diffusion furnaces heated to ~850 C and takes an hour long. This may be replaced by an implant and subsequent 90‐s rapid thermal annealing step (in a firing furnace) for the fabrication of p‐type passivated emitter rear contacted silicon solar cells. Implantation has long been deemed a technology too expensive for fabrication of silicon solar cells, but if coupled with innovative process flows as that which is mentioned in this paper, implantation has a fighting chance. An SiOx/SiN_y rear side passivated p‐type wafer is implanted at the front with phosphorus. The implantation creates an inactive amorphous layer and a region of silicon full of interstitials and vacancies. The front side is then passivated using a plasma‐enhanced chemical vapor deposited SiN_xH_y. The wafer is placed in a firing furnace to achieve dopant activation. The hydrogen‐rich silicon nitride releases hydrogen that is diffused into the Si, the defect rich amorphous front side is immediately passivated by the readily available hydrogen; all the while, the amorphous silicon recrystallizes and dopants become electrically active. It is shown in this paper that the combination of this particular process flow leads to an efficient Si solar cell. Cell results on 160‐µm thick, 148.25‐cm² Cz Si wafers with the use of the proposed traditional diffusion‐free process flow are up to 18.8% with a V_oc of 638 mV, J_sc of 38.5 mA/cm², and a fill factor of 76.6%. Copyright © 2012 John Wiley & Sons, Ltd.     

A Polymer/Carbon‐Nanotube Ink as a Boron‐Dopant/Inorganic‐Passivation Free Carrier Selective Contact for Silicon Solar Cells with over 21% Efficiency

Traditional silicon solar cells extract holes and achieve interface passivation with the use of a boron dopant and dielectric thin films such as silicon oxide or hydrogenated amorphous silicon. Without these two key components, few technologies have realized power conversion efficiencies above 20%. Here, a carbon nanotube ink is spin coated directly onto a silicon wafer to serve simultaneously as a hole extraction layer, but also to passivate interfacial defects. This enables a low‐cost fabrication process that is absent of vacuum equipment and high‐temperatures. Power conversion efficiencies of 21.4% on an device area of 4.8 cm² and 20% on an industrial size (245.71 cm²) wafer are obtained. Additionally, the high quality of this passivated carrier selective contact affords a fill factor of 82%, which is a record for silicon solar cells with dopant‐free contacts. The combination of low‐dimensional materials with an organic passivation is a new strategy to high performance photovoltaics.