A roadmap for PERC cell efficiency towards 22%, focused on technology-related constraints

Presently, the crystalline silicon (c-Si) photovoltaic (PV) industry is switching from standard cells to PERC cells to increase cell efficiency from about 18% to about 20%. This paper gives a roadmap for increasing PERC cell efficiency further towards 22%. Which equipment and which process conditions are feasible to go beyond 20% efficiency? To help answer this as generally as possible, we conduct state-of-the-art modelling in which we sweep the inputs that represent major technology-related constraints, such as diffusion depth, metal finger width and height, alignment tolerances, etc. (these are assigned to the x- And y-axes of our graphs). We then predict the optimum device parameters resulting from these restrictions (shown as contour lines). There are many different ways to achieve 22%. Our modelling predicts, for example, that 60 μm wide screen-printed metal fingers are sufficiently narrow if the alignment tolerance (width of the n++ region) is below 90 μm. The rear may be contacted with 30 μm wide openings of the Al2O3/SiNx stack and with local J0,BSF values as high as 900 fA/cm2. If these requirements cannot be met, they may be compensated by improvements in other device parts. Regardless of this, the wafer material requires a SRH lifetime of at least 1 ms at excess carrier densities near 10(14) cm(-3)

Prediction of a double-antireflection coating made solely with SiN x in a single, directional deposition step

Silicon solar cell modules, where the EVA layer is replaced by an air gap, are able to produce the same electric power as standard modules with EVA only if their anti-reflective properties are enhanced. We propose a method to do this by exploiting the fact that, on Si surfaces textured with random pyramids, light incident from near normal angle always hits at least two pyramidal faces before being reflected back toward the sun. If these two faces are covered with an anti-reflective coating (ARC) made of one and the same material but with two different thicknesses, the coating acts as a double ARC. Such a coating can be produced by depositing the SiNx layer from an oblique angle, optimally from 14.7°. Our detailed raytracing analysis predicts that J sc can then be improved by 0.2 mA/cm2 for normal incident sunlight and AM1.5g standard illumination, and is improved for all angles within a cone with an apex angle of approximately 64°. Furthermore, the coating can be optimized for modules in vertical mounting, where a Jsc gain of 0.1 mA/cm2 is predicted for an angle of incidence of 40°

Conceptual comparison between standard Si solar cells and back contacted cells

It is often stated that one of the main advantages of back-contacted (BC) Si solar cells over standard cells is that shading due to the front-finger metallization is avoided - while it is also often stated that BC cells are prone to "electronic shadowing", which means that the carrier collection efficiency in front of the back-surface-field (BSF) may be reduced. We compare these two cell concepts in two ways: by means of an extensive collection of measured IV data from literature, and by interpreting as well as quantifying the differences with the aid of numerical device simulation. Both literature data and simulations indicate that BC cells have a Jsc-advantage of maximally about 1 mA/cm2, but in Voc and FF there is no clear advantage or disadvantage over standard cells. With a parameter study, we reveal the main design advantages and weaknesses in each cell type. Our numerical device modeling indicates that one of the most crucial design advantages of BC over standard cells is that the collection of minority carriers in the emitter is rather unimportant, which leaves greater flexibility in emitter design than in standard cells

Evaluating the quality of selective emitter structures by imaging the emitter saturation current density

A method to derive the emitter saturation current density J0e with lateral resolution is applied to investigate selective emitter structures. The method uses PL lifetime imaging at several injection densities to laterally evaluate J0e by applying the method of Kane and Swanson [1] pixel by pixel. Samples with two-sided diffused emitters on lowlydoped Cz wafers were used to produce selective emitter structures by laser doping of the phosphorus-rich glass (LDSE). By comparison of experimental and numerical simulation results of J0e linescans, a limited resolution of a feature size of an inhomogeneous emitter is determined to be theoretically between 0.5-1.0 mm and experimentally about 2 mm. The method was successfully applied to investigate the dependence of J0e on the laser power of a selective emitter structure. The expected behaviour of a maximum J0e for medium laser intensities is observed. The method is suitable to evaluate the selective emitter process and its optimization.BMU/032520

Assessing the Device-performance Impacts of Structural Defects with TCAD Modeling

Advanced solar cell architectures like passivated emitter and rear (PERC) and heterojunction with intrinsic thin layer (HIT) are increasingly sensitive to bulk recombination. Present device models consider homogeneous bulk lifetime, which does not accurately reflect the effects of heterogeneously distributed defects. To determine the efficiency potential of multicrystalline silicon (mc-Si) in next-generation architectures, we present a higher-dimensional numerical simulation study of the impacts of structural defects on solar cell performance. We simulate these defects as an interfacial density of traps with a single mid-gap energy level using Shockley-Read-Hall (SRH) statistics. To account for enhanced recombination at the structural defects, we apply a linear scaling to the majority-carrier capture cross-section and scale the minority-carrier capture cross-section with the inverse of the line density of traps. At 300 K, our simulations of carrier occupation and recombination rate match literature electron-beam-induced current (EBIC) data and first-principles calculations of carrier capture, emission, and recombination for all the energy levels associated with dislocations decorated with metal impurities. We implement our model in Sentaurus Device, determining the losses across different device architectures for varying impurity decoration of grain boundaries.American Society for Engineering Education. National Defense Science and Engineering Graduate FellowshipNational Science Foundation (U.S.). Engineering Research Centers Program (Cooperative Agreement EEC-1041895

Assessing the Device-performance Impacts of Structural Defects with TCAD Modeling

Advanced solar cell architectures like passivated emitter and rear (PERC) and heterojunction with intrinsic thin layer (HIT) are increasingly sensitive to bulk recombination. Present device models consider homogeneous bulk lifetime, which does not accurately reflect the effects of heterogeneously distributed defects. To determine the efficiency potential of multicrystalline silicon (mc-Si) in next-generation architectures, we present a higher-dimensional numerical simulation study of the impacts of structural defects on solar cell performance. We simulate these defects as an interfacial density of traps with a single mid-gap energy level using Shockley-Read-Hall (SRH) statistics. To account for enhanced recombination at the structural defects, we apply a linear scaling to the majority-carrier capture cross-section and scale the minority-carrier capture cross-section with the inverse of the line density of traps. At 300 K, our simulations of carrier occupation and recombination rate match literature electron-beam-induced current (EBIC) data and first-principles calculations of carrier capture, emission, and recombination for all the energy levels associated with dislocations decorated with metal impurities. We implement our model in Sentaurus Device, determining the losses across different device architectures for varying impurity decoration of grain boundaries.DoD/National Defense Science & Engineering Graduate Fellowship (NDSEG

Device Architecture and Lifetime Requirements for High Efficiency Multicrystalline Silicon Solar Cells

We present a numerical simulation study of different multicrystalline silicon materials and solar cell architectures to understand today's efficiency limitations and future efficiency possibilities. We compare conventional full-area BSF and PERC solar cells to future cell designs with a gallium phosphide heteroemitter. For all designs, mc-Si materials with different excess carrier lifetime distributions are used as simulation input parameters to capture a broad range of materials. The results show that conventional solar cell designs are sufficient for generalized mean lifetimes between 40 – 90 μs, but do not give a clear advantage in terms of efficiency for higher mean lifetime mc-Si material because they are often limited by recombination in the phosphorus diffused emitter region. Heteroemitter designs instead increase in cell efficiency considerable up to generalized mean lifetimes of 380 μs because they are significantly less limited by recombination in the emitter and the bulk lifetime becomes more important. In conclusion, to benefit from increasing mc-Si lifetime, new cell designs, especially heteroemitter, are desirable.United States. Department of Energy. Office of Energy Efficiency and Renewable Energy (Award DE-EE0006335)Australian Renewable Energy Agency (Postdoctoral Fellowship

Optimized stencil print for low Ag paste consumption and high conversion efficiencies

We evaluate industrial-type PERC solar cells applying a dual printed front grid with stencil printed Ag fingers. We vary the Ag paste consumption for the finger print between 8.4 mg and 120.4 mg per 156 x 156 mm(2) wafer (weighted after printing before drying) by using polyurethane squeegees with different shore hardness as well as a metal squeegee and by varying the printing pressure to obtain different finger heights. The busbar consumes additional 19.5 mg Ag paste. We obtain average finger heights from 5.9 mu m up to 24.3 mu m for 55 mu m to 65 mu m wide fingers. The resulting PERC solar cells show an average efficiency of 20.2% for finger paste consumptions above 60 mg. In contrast, a strong reduction of the conversion efficiency with less than 60 mg finger paste consumption is observed since the increased series resistance reduces the FF. By analytical modelling, we compare the calculated series resistance to the experimental data and observe a good accordance for more than 40 mg finger paste consumption whereas the experimental series resistance slightly exceed the modelled values below 40 mg. In addition, we use numerical simulations to investigate the series resistance dependence on the finger height which shows higher experimental values for finger height below 10 mu m. The deviation of the measured series resistance and the two modelled cases is mostly due to inhomogeneous distribution of finger height profiles and finger interruptions on the solar cells with front finger paste consumption of less than 40 mg. For finger paste consumption below 60 mg, we find that also the specific contact resistance increases. A physical model of the root cause for this dependence still has to be found

Numerical Modeling of c-Si PV Modules by Coupling the Semiconductor with the Thermal Conduction, Convection and Radiation Equations

Commonly, the thermal behavior of solar cell modules is calculated with analytical approaches using non wavelength-dependent optical data. Here, we employ ray tracing of entire solar modules at wavelengths of 300-2500 nm to calculate heat sources. Subsequently, finite element method (FEM) simulations are used to solve the semiconductor equations coupled with the thermal conduction, thermal convection, and thermal radiation equations. The implemented model is validated with measurements from an outdoor test over the period of an entire year. Our ray tracing analysis of different solar modules under the AM.15G spectrum shows that, for a standard module about 18.9% of the sun's intensity becomes parasitically absorbed. A loss analysis shows that the biggest parasitic heat source is the cell's full-area rear side metallization. We hence propose the use of a SiNx layer as rear side mirror to reduce the parasitic absorption to 11.7%. This change can lead to a 3.2 °C lower module operating temperature, which results in an about 5 W higher electrical power output when considering a typical 260 W module

Relationships between diffusion parameters and phosphorus precipitation during the POCl3 diffusion process

The POCl3 diffusion process is still a common way to create the pn-junction of Si solar cells. Concerning the screen-printing process, it is necessary to find a compromise between low emitter recombination, low contact resistance and high lateral conductivity. The formation of a homogeneous emitter during the POCl3 diffusion process depends on several diffusion parameters, including duration, temperature and gas flow. This primarily controls the growth of the highly doped phosphosilicate glass (PSG) layer, which acts as a dopant source during the diffusion process. Detailed investigations of the PSG layer have shown a distinct correlation between the process gas flows and the composition of the PSG layer. Specifically, in this research we examine the influence of phosphorus precipitation at the PSG/Si interface. Furthermore, we show the influence of phosphorus precipitation during the pre-deposition phase on the passivation quality of the corresponding emitter. In a second step, we use the results to create emitters with a reduced density of phosphorus precipitates. In a last step, the optimized emitter structure was transferred to screen-printed solar cell processes, whereby efficiencies up to 19.4% abs. were achieved on monocrystalline p-type Cz material with full area Al-BSF rear side