Analysis of buried contact solar cells and some rules for their optimum design

One of the most promising structures for solar cells is the Buried Contact Solar Cell (BCSC), because it offers the possibility of production at low cost, at the same time that it allows the combination of several high efficiency concepts without any design compromise as occurs in conventional cells. Therefore, there is a growing interest for such kind of solar cells. Unfortunately, this structure poses new problems for its design since the buried contact will cause two-dimensional carrier flow effects under both light and dark current conditions. However, as we describe in this paper, it is possible to make a simple analysis of the conversion efficiency as a function of the buried contact depth in the BCSC cell. In this article we will show that the efficiency is minimum for depths in the range between 50 and 70 μm when the assumed parameters are typical for silicon solar cells. In addition, we will show that in order to have good performance for these cells, the buried contact depth should be very small (below 5 μm), or very large (bigger than 150 μm) instead, but not in-between these values, in order to avoid any efficiency degradation caused by the buried contacts. Finally, we shall show that the efficiency can be better than in conventional cells only when the buried contacts have a large depth, whenever they are designed properly such that they allow additional collection of charge carriers generated by sunlight.     

Very high efficiency silicon solar cells-science and technology

Although it has been close to 60 years since the first operational silicon solar cell was demonstrated, the last 15 years have seen large improvements in the technology, with the best confirmed cell efficiency improved by over 50 %. The main drivers have been improved electrical and optical design of the cells. Improvements in the former area include improved passivation of contact and surface regions of the cells and a reduction in the volume of heavily doped material within the cell. Optically, reduced reflection and improved trapping of light within the cell have had a large impact. Such features have increased silicon cell efficiency to a recently confirmed value of 24.7%. Over recent years, good progress has been made in transferring some of the corresponding design improvements into commercial product with commercial cells of 17-18% efficiency now commercially available, record values of a mere 15 years ago. The theory supporting these improvements in bulk cell efficiency shows that thin layers of silicon, only a micron or so in thickness, should be capable of comparably high efficiency     

Applications of Laser Precisely Processing Technology in Solar Cells

According to the design method of laser resonator cavity, we optimized the primary parameters of resonator and utilized LD arrays symmetrically pumping manner to implementing output of the high-brightness laser in our laser cutter, then which was applied to precisely cutting the conductive film of CuInSe2 solar cells, the buried contact silicon solar cells' electrode groove, and perforating in wafer which is used to the emitter wrap through silicon solar cells. Laser processing precision was less than 40 μm, the results have met solar cell's fabrication technology, and made finally the buried cells' conversion efficiency be improved from 18% to 21% .     

Effect of trap location and trap-assisted Auger recombination onsilicon solar cell performance

Model calculations were performed to investigate and quantify the effect of trap location and trap-assisted Auger recombination on silicon solar cell performance. Trap location has a significant influence on the lifetime behavior as a function of doping and injected carrier concentration in silicon. It Is shown in this paper that for a high quality silicon (τ=10 ms at 200 ohm-cm, no intentional doping), high resistivity (⩾200 ohm-cm) is optimum for high efficiency one sun solar cells if the lifetime limiting trap is located near midgap. However, if the trap is shallow (E_t-E_v⩽0.2 eV), the optimum resistivity shifts to about 0.2 ohm-cm. For a low quality silicon material or technology (10 μs at 200 ohm-cm, prior to intentional doping) the optimum base resistivity for one sun solar cells is found to be ~0.2 ohm-cm, regardless of the trap location. It is shown that the presence of a shallow trap can significantly degrade the performance of a concentrator cell fabricated on high-resistivity high-lifetime silicon material because of an undesirable injection level dependence in the carrier lifetime. The effect of trap assisted Auger recombination on the cell performance has also been modelled in this paper. It is found that the trap-assisted Auger recombination does not influence the one sun cell performance appreciably, but can degrade the concentrator cell performance if the trap-assisted Auger recombination coefficient value exceeds 2×10^-14 cm³/s. Therefore, it is necessary to know the starting lifetime as well as trap location in order to specify base resistivity in order to predict or achieve the best cell performance for a given one sun or concentrator cell design     

Investigations of parasitic shunt resistance in n‐type buried contact solar cells

It has been shown that n‐type laser‐grooved buried contact solar cells exhibit a high‐efficiency potential, both on interdigitated backside buried contact (IBBC) and double‐sided buried contact (DSBC) cell structures. As the IBBC solar cell contains heavily doped, compensated regions, the shunt mechanisms are more complicated, and are different from those of the conventional front‐collecting‐junction solar cells. In this paper, several shunting mechanisms hindering the performances of the n‐type buried contact solar cells are investigated and discussed. The main shunting routes in n‐type IBBC solar cells are concluded as follows: (1) the emitter contact metal touching the n‐type substrate, which is either due to nonuniform boron deposition or diffusion‐induced misfit dislocations; (2) the base contact metal touching the p⁺ emitter, attributed to either the phosphorus groove diffusion being unable to compensate for the boron emitter diffusion, or the junction depth located in the diffusion overlap regions not deep enough to prevent nickel from spiking through the groove diffusion. The shunt resistance of the IBBC cells increased by more than two orders of magnitude after eliminating the shunt mechanisms discussed in this study. This led to an improvement in fill factor from 0·71–0·73 to 0·74–0·76, and an increase of average absolute efficiency of more than 0·65%. Copyright © 2005 John Wiley & Sons, Ltd.     

P-Type Versus n-Type Silicon Wafers: Prospects for High-Efficiency Commercial Silicon Solar Cells

Chemical and crystallographic defects are a reality of solar-grade silicon wafers and industrial production processes. Long overlooked, phosphorus as a bulk dopant in silicon wafers is an excellent way to mitigate recombination associated with these defects. This paper details the connection between defect recombination and solar cell terminal characteristics for the specific case of unequal electron and hole lifetimes. It then looks at a detailed case study of the impact of diffusion-induced dislocations on the recombination statistics in n-type and p-type silicon wafers and the terminal characteristics of high-efficiency double-sided buried contact silicon solar cells made on both types of wafers. Several additional short case studies examine the recombination associated with other industrially relevant situations—process-induced dislocations, surface passivation, and unwanted contamination. For the defects studied here, n-type silicon wafers are more tolerant to chemical and crystallographic defects, and as such, they have exceptional potential as a wafer for high-efficiency commercial silicon solar cells.     

Experimental and simulated analysis of front versus all‐back‐contact silicon heterojunction solar cells: effect of interface and doped a‐Si:H layer defects

Front silicon heterojunction and interdigitated all‐back‐contact silicon heterojunction (IBC‐SHJ) solar cells have the potential for high efficiency and low cost because of their good surface passivation, heterojunction contacts, and low temperature fabrication processes. The performance of both heterojunction device structures depends on the interface between the crystalline silicon (c‐Si) and intrinsic amorphous silicon [(i)a‐Si:H] layer, and the defects in doped a‐Si:H emitter or base contact layers. In this paper, effective minority carrier lifetimes of c‐Si using symmetric passivation structures were measured and analyzed using an extended Shockley–Read–Hall formalism to determine the input interface parameters needed for a successful 2D simulation of fabricated baseline solar cells. Subsequently, the performance of front silicon heterojunction and IBC‐SHJ devices was simulated to determine the influence of defects at the (i)a‐Si:H/c‐Si interface and in the doped a‐Si:H layers. For the baseline device parameters, the difference between the two device configurations is caused by the emitter/base contact gap recombination and the back surface geometry of IBC‐SHJ solar cell. This work provides a guide to the optimization of both types of SHJ device performance, predicting an IBC‐SHJ solar cell efficiency of 25% for realistic material parameters. Copyright © 2013 John Wiley & Sons, Ltd.     

Resistivity dependence of silicon solar cell efficiency and its enhancement using a heavily doped back contact region

For the normal solar cell geometry, there is shown to be an upper limit to the cell conversion efficiency for each value of the silicon substrate resistivity. This limit cannot be exceeded regardless of possible improvements in material lifetime properties. It peaks for a value of substrate resistivity of about 0.1 Ω.cm for p-type substrates corresponding to an acceptor concentration of 7 × 10²³m^-3. The limit can be exceeded if the cell structure is modified. A high-low junction incorporated near the ohmic back contact to a suitably designed device not only improves the current collecting properties for a given cell thickness, as has been shown previously, but also increases the ultimate conversion efficiency.     

Ni/Cu electroplating,a worthwhile alternative to use instead of Ag screen-printed front side metallization of conventional solar cells

For commercial purposes, it is necessary to manufacture high-efficiency and low-cost solar cells using simple processes. The front contact formation is one of the most critical steps in solar cell processing. Although silver paste screen-printed solar cells are the most widespread on the photovoltaic market, their efficiency is strongly limited as a result of shading and resistive losses, or more precisely the high contact resistance. Cu metallization for crystalline Si solar cells has attracted much attention as an alternative to the screen-printing technology. The low-cost Ni/Cu metal contact is regarded as the next generation of metallization processes to still improve the efficiency with a low specific contact resistance; it is formed using low-cost electroless plating and electroplating. A diffusion barrier should be placed between Cu and Si, to prevent Cu diffusion. Ni is shown to be an adequate barrier to Cu diffusion. For these reasons, geometry optimization of metal contacts of the front face, deposited by commercial processes, is investigated in this paper, in order to improve the spectral response of conventional multicrystalline mc-Si silicon solar cells. Their efficiency variation is analyzed as a function of changes in cell parameters (finger separation distance, height and width of finger, sheet resistance emitter...) using simulation programs in MATLAB, using contours to represent the efficiency evolution in terms of two variables. Efficiency gain of more than 0.7% has been achieved in this study. The simulation results were then compared with experimental data in order to be validated.     

局部背接触结构单晶Si太阳电池的研究

报道了采用局部背接触结构的激光刻槽埋栅太阳电池的研究结果.模拟分析了局部背接触结构的作用,设计了合理的电池结构.通过工艺优化,得到了转换效率达到17.28%(大气质量AM=1.5 G,VOC=650.4mV,JSC=33.15 mA/cm2,FF=0.8014,电池面积为4 cm2)的太阳电池.     

Optimizing the diffused regions of interdigitated backside buried contact solar cells

Recombination and a number of other important factors must be considered in the optimization of the diffused regions of high‐efficiency silicon solar cells. In this paper, we examine issues related to the four types of diffusions used in rear‐junction, interdigitated backside buried contact solar cells made on n‐type silicon wafers: the phosphorus‐diffused front‐surface field (FSF), the boron‐diffused emitter, and the boron and the phosphorus diffused contact regions. Dark saturation current density, effective lifetime, implied open‐circuit voltage and sheet resistance are characterized for the optimization of the above‐mentioned diffused regions. Diffusion uniformity and the avoidance of the diffusion‐induced dislocations are also discussed for the heavily diffused, metal coated contact diffusions. It is found that the optimal sheet resistances of the FSF for planar and textured surfaces are 120 Ω/□ and 105 Ω/□ respectively, whereas the optimal post‐processing sheet resistance for the boron emitter is approximately 100 Ω/□. Moreover, sheet resistance as heavy as 10–20 Ω/□ for the boron groove diffusion and 5–10 Ω/□ for the phosphorus groove diffusion have been achieved without introducing the diffusion‐induced misfit dislocations. Careful consideration of the issues discussed here led to an absolute efficiency improvement on the planar n‐type IBBC solar cell of more than 0·6%. Copyright © 2006 John Wiley & Sons, Ltd.     

Boron‐doped selective silicon epitaxy: high efficiency and process simplification in interdigitated back contact cells

The present research and development activities in crystalline silicon photovoltaics include the exploration of doping technologies alternative to the mainstream diffusion process. The goal is to identify those technologies with potential to increase the solar cell efficiency and reduce the cost per watt peak. In that respect, this work presents the selective epitaxial growth of silicon as a candidate for boron doping; showing the results of the evaluation of boron‐doped silicon epitaxial emitters on slurry and diamond‐coated wire‐sliced Czochralski material, their integration in interdigitated back contact solar cells, and the development of a novel process sequence to create the interdigitated rear junction of these devices using selective epitaxial growth. Boron‐doped silicon epitaxy is demonstrated to perform in the high efficiency range (>22%), and the use of selective epitaxial growth is proposed as a route for the simplification of the interdigitated back contact solar cell flow. Copyright © 2013 John Wiley & Sons, Ltd.     

Determination of the minority carrier lifetime in crystalline silicon thin‐film material

The effective minority carrier lifetimes on epitaxial silicon thin‐film material have been measured successfully using two independent microwave‐detected photoconductivity decay setups. Both measurement setups are found to be equally suited to determine the minority carrier lifetime of crystalline silicon thin‐film (cSiTF) material. The different measurement conditions to which the sample under investigation is exposed are critically analyzed by both simulations and measurements on a large number of lifetime samples. No systematic deviation between the lifetime results from different measurement setups could be observed, underlining the accuracy of the determined lifetime value. Subsequently, a method to separate the epitaxial bulk lifetime and the total recombination velocity, consisting of front surface and interface recombination between the epitaxial layer and the substrate, is presented. The method, based on different thicknesses of the epitaxial layer, is applied to all batches of this investigation. Each batch consists of samples with the same material quality but different epitaxial layer thicknesses whereas different batches differ in their material quality. In addition, the same method is also successfully applied on individual cSiTF samples. From the results, it can be concluded that the limiting factor of the effective minority carrier lifetime for the investigated solar‐grade cSiTF material is the elevated recombination velocity at the interface between epitaxial layer and the substrate compared with microelectronic‐grade material. In contrast, the samples cannot be classified into different material qualities by their epitaxial bulk lifetimes. Even on multicrystalline substrate, solar‐grade material can exhibit high epitaxial bulk lifetimes comparable to microelectronic‐grade material. Copyright © 2012 John Wiley & Sons, Ltd.     

Ultrathin and micro‐sized solar cell performance optimization via simulations

Back‐contacted, ultrathin (<10 µm), and submillimeter‐sized solar cells made with microsystem tools are a new type of cell that has not been optimized for performance. The literature reports efficiencies up to 15% using thicknesses of 14 µm and cell sizes of 250 µm. In this paper, we present the design, conditions, and fabrication parameters necessary to optimize these devices. The optimization was performed using commercial simulation tools from the microsystems arena. A systematic variation of the different parameters that influence the performance of the cell was accomplished. The researched parameters were resistance, Shockley–Read–Hall (SRH) lifetime, contact separation, implant characteristics (size, dosage, energy, and ratio between the species), contact size, substrate thickness, surface recombination, and light concentration. The performance of the cell was measured with efficiency, open‐circuit voltage, and short‐circuit current. Among all the parameters investigated, surface recombination and SRH lifetime proved to be the most important. Through completing the simulations, an optimized concept solar cell design was introduced for two scenarios: high and low quality materials/passivation. Simulated efficiencies up to 23.4% (1 sun) and 26.7% (100 suns) were attained for 20‐µm‐thick devices. Copyright © 2012 John Wiley & Sons, Ltd.     

Designing III–V multijunction solar cells on silicon

Single junction Si solar cells dominate photovoltaics but are close to their efficiency limits. This paper presents ideal limiting efficiencies for tandem and triple junction multijunction solar cells featuring a Si subcell also serving as substrate. Subject to this Si bandgap constraint, we design optimum cell structures that we show depart from the unconstrained ideal. In order to progress to manufacturable designs, the use of III–V materials is considered, using a novel growth method capable of yielding low defect density III–V layers on Si. In order to evaluate the real potential of these proposed multijunction designs, a quantitative model is presented, the strength of which is the joint modelling of external quantum efficiency and current–voltage characteristics using the same parameters. The method yields a single‐parameter fit in terms of the Shockley–Read–Hall lifetime. This model is validated by fitting experimental data of external quantum efficiency, dark current and conversion efficiency of world record tandem and triple junction cells under terrestrial solar spectra without concentration. We apply this quantitative model to the design of tandem and triple junction solar cells, yielding cell designs capable of reaching efficiencies without concentration of 32% for the best tandem cell and 36% for the best triple junction cell. This demonstrates that efficiencies within a few per cent of world records are realistically achievable without the use of concentrating optics, with growth methods being developed for multijunction cells combining III–V and Si materials. Copyright © 2014 John Wiley & Sons, Ltd.     

PC1D方法对铝背场钝化技术的分析

降低单晶硅原材料成本,采用更薄的硅片作为太阳电池的原料是晶体硅太阳电池产业发展的趋势之一。对薄片化的太阳电池,铝背场的背表面钝化工艺显得愈加重要。采用PC1D太阳电池软件模拟的方法,对以商业用p型硅为衬底的单晶硅125×125太阳电池的铝背场的背表面钝化技术进行了模拟,分析得出,对一定厚度的电池片来说,尤其是当少数载流...     

Contact fault characterisation of complex silicon solar cells: a guideline based on current voltage characteristics and luminescence imaging

Highest efficiency solar cells in industrial and R&D environments are increasingly sensitive to local performance limiting processing faults, which are best characterised by spatially resolved characterisation techniques. This work contains a discussion on the processing faults related to contact resistance and finger interruptions in interdigitated back contact silicon solar cells, which are prime example for a complex cell structure. Using experimental and simulated current–voltage measurements and luminescence images, we explore the strongly non‐linear effect of poor local contact resistances on the global series resistance, fill factor, short circuit current density and efficiency. A good agreement between global and spatially resolved characterisation of faults is found, and potential artefacts are discussed. In conclusion, we present seven cases of contacting faults in interdigitated back contact cells with distinct characteristics that can be identified using a flow chart of experiments. The resulting guideline should assist silicon solar cell manufacturers in localising and quantifying local contacting faults that reduce the cells efficiency in manufacturing of complex solar cells. Copyright © 2015 John Wiley & Sons, Ltd.     

Impact of the injection‐level‐dependent lifetime on Voc,FF, ideality m,J02, and the dim light response in a commercial PERC cell

The injection‐level‐dependent (ILD) lifetime of the silicon wafer impacts many characteristics of the final photovoltaic cell. While efficiency is commonly understood to be impacted by the silicon bulk lifetime (at the maximum power point injection level), this work demonstrates the wide ranging impacts of ILD lifetime on the V_oc, the fill factor (FF), the diode ideality factor m, and the dim light response. Instead of a two‐diode model, we utilize a boundary + ILD bulk lifetime model to analyze a commercial passivated emitter rear contact (PERC) cell featuring an AlO_x dielectric rear passivation. The ILD lifetime is directly measured and used to calculate the bulk recombination current across injection levels. With this boundary + ILD lifetime model, we demonstrate the role of the ILD lifetime on many cell parameters in this PERC cell. For most high efficiency commercial p‐type monocrystalline solar cells, the typically lower bulk lifetime at the maximum power point versus the lifetime at the open circuit point reduces the measured FF and pseudo‐FF. This work illustrates that for a commercial PERC cell with AlO_x rear passivation, the ILD lifetime is the primary mechanism behind reduced FF, ideality factors greater than 1, and the source of the J₀₂ term in the two‐diode model. The crucial implications of this work are not only to better understand commercial PERC cell loss mechanisms but also to encourage a focus on different metrics in cell diagnostics. One such metric is the V_oc at 0.1 or 0.05 suns. Copyright © 2016 John Wiley & Sons, Ltd.     

Polycrystalline silicon solar cells from recrystallized plasma deposited thin films

Large grain polycrystalline silicon films are produced by a two step process involving plasma deposition of microcrystalline silicon films on a substrate, separation from the substrate, and subsequent grain enhancement of the silicon films. The effects of doping and substrate temperature during deposition on the solar cell conversion efficiency are investigated. Effects of ppm level molybdenum contamination from the substrate, and silicon microstructure after grain enhancement, on solar cell efficiency parameters are also investigated. Solar cells with efficiencies of up to 10.1% under AM1 illumination, were fabricated on these silicon films.     

高效率n型Si太阳电池技术现状及发展趋势

提高太阳电池光电转换效率、降低光伏发电成本已成为全球光伏领域的研究热点。由于n型晶体Si具有体少子寿命长、光致衰减小等优点,非常适于制作低成本高效率太阳电池,近年来高效率n型Si太阳电池引起了人们广泛的关注。文章在论述n型Si特性的基础上,介绍了IBC结构、PERT结构、HIT结构、PERL结构和常规电池结构n晶体Si太阳电池的研究进展及产业化水平,给出了n型Si电池今后的研究方向。