Analytic Efficiency Optimization of Solar Cells under Light Concentration in the Framework of the Single-Diode Model

Herein, concentrating solar cells are modeled with two recombination active contacts and a recombination active light absorber in the framework of the one-diode model. The two contacts and the absorber contribute to a lumped series resistance and to a lumped recombination current. It is proven that varying the light concentration can be interpreted as iso-selectivity scaling of the cell's resistance and the cell's recombination. As a consequence of that, the optimal efficiency of a concentrator cell is found at maximum combined selectivity of the two contacts and the absorber. Herein, analytic formulas are derived that calculate the optimal contact areas and the optimal light concentration level for achieving an optimum efficiency. The resulting formulas express the efficiency in terms of the selectivities of each contact and the selectivity of the absorber. These equations are used to calculate the optimum contact area fractions and the optimum light concentration level for Si solar cells of various material qualities with screen-printed Al-doped contacts and n-type poly-Si contacts on oxide. The efficiency results of the novel analytic and a conventional numeric optimization agree to the expected level of accuracy

Dynamic photoluminescence lifetime imaging for injectiondependent lifetime measurements

We investigate the impact of an injection-dependent carrier lifetime in crystalline silicon on dynamic photoluminescence lifetime imaging (dynamic PLI). Although the dynamic lifetime approach is a technique that evaluates the time-dependence of a quantity proportional to the excess carrier density, it is only weakly influenced by the injection-level dependence of the lifetime. The reason for the little impact is the fact that the evaluation of dynamic PLI measurements does not only involve the decay of the carrier density, as it is common for photoconductance decay measurements, but also the increase of the carrier density directly after switching on the excitation source. In this contribution, we present injection-dependent lifetime measurements that are acquired with the camera-based dynamic PLI technique. We find that the deviation of the actual steady-state carrier lifetime from the lifetime obtained with dynamic PLI is less than 20 % for a wide range of measurement conditions.State of Lower SaxonyGerman Federal Ministry for the Environment, Nature Conservation, and Nuclear Safet

Fineline printing options for high efficiencies and low Ag paste consumption

In this paper, we investigate and compare three different fine line printing techniques for the silver front side metallization of industrial-type silicon solar cells: single print, dual print and print-on-print. We produce solar cells using the same screen or stencil aperture of 40 μm and about 92 fingers and obtain finger widths below 60 μm for all three approaches. The print-on-print process achieves the highest finger heights of 20 μm after firing but with quite strong finger height variation. In contrast, the dual printed fingers have a very flat surface with a finger height of 14.5 μm which leads to the highest cross-section area of 530 μm2 of the three techniques. The single print shows the lowest cross-section area of 390 μm2 due to the lowest average finger height. The measured finger line resistance correlates with the finger cross-section area. The dual print allows us to use a non-firing through bus bar paste which increases the V oc by 2 mV and hence achieves the highest efficiency of 19.1% using full-area Al-BSF cells. Due to an optimized bus bar screen print in combination with only 30 μm finger aperture, the dual print has the lowest Ag paste consumption of only 75 mg/wafer, one of the lowest Ag paste consumption that has been reported so far. A first batch of PERC solar cells with dual-printed Ag front contacts shows efficiencies up to 19.6%.German Federal Ministry for the Environment, Nature Conservation and Nuclear Safety/0325296SolarWorld Innovations GmbHRENA GmbHSINGULUS TECHNOLOGIES AGHeraeus Precious Metal

Analysis and optimization of the bulk and rear recombination of screen-printed PERC solar cells

In this paper, we investigate the impact of the rear surface passivation, the silicon base material and the local aluminum contacts applied to rear side passivated solar cells with a homogenously doped emitter at the front. We compare different dielectric surface passivation layers (SiO2, Al2O3, SiNx) on a high-efficiency level using 125×125 mm2 and 156×156 mm2 p-type Cz silicon wafers. It turns out that applying an Al2O3/SiN x layer stack outperforms all other surface passivation layers due to its excellent surface passivation as well as optical properties. We determine the impact of the light induced degradation depending on the used Cz base material. We measure an efficiency drop between 0.0 % (Ga-doped) and 0.8 % abs. (B-doped) after 8 hours of illumination under 0.5 Suns. We measure the surface recombination velocity of local screen-printed Al contacts with varying the metallisation fraction frear with the dynamic infrared lifetime mapping technique (dyn-ILM) on lifetime samples. We measure a decrease in contact recombination velocity from above 1100 cm/s for small frear to 400 cm/s for large frear on 1.5 Ωcm p-type FZ-silicon. Microscopy investigations show that this is due to an improved local Al-BSF formation when using higher frear.German Federal Ministry for the Environment, Nature Conservation and Nuclear Safety/0325296Solar Cells BVSolarWorld Innovations GmbHSCHOTT Solar AGRENA GmbHSINGULUS TECHNOLOGIES A

Large-scale spatiotemporal calculation of photovoltaic capacity factors using ray tracing: A case study in urban environments

Photovoltaics (PVs) on building facades, either building-integrated or building-attached, offer a large energy yield potential especially in densely populated urban areas. Targeting this potential requires the availability of planning tools such as insolation forecasts. However, calculating the PV potential of facade surfaces in an urban environment is challenging. Complex time-dependent shadowing and light reflections must be considered. In this contribution, we present fast ray tracing calculations for insolation forecasts in large urban environments using clustering of Sun positions into typical days. We use our approach to determine time resolved PV capacity factors for rooftops and facades in a wide variety of environments, which is particularly useful for energy system analyses. The advantage of our approach is that the determined capacity factors for one geographic location can be easily extended to larger geographic regions. In this contribution, we perform calculations in three exemplary environments and extend the results globally. Especially for facade surfaces, we find that there is a pronounced intra-day and also seasonal distribution of PV potentials that strongly depends on the degree of latitude. The consideration of light reflections in our ray tracing approach causes an increase in calculated full load hours for facade surfaces between 10% and 25% for most geographical locations

Ni:Si as Barrier Material for a Solderable PVD Metallization of Silicon Solar Cells

AbstractWe study Ni:Si as a barrier material for the PVD metallization of silicon solar cells and investigate the long term solderability of Al/Ni:Si/Ag metal stacks in terms of peel forces and contact resistances. For this purpose, solar cell connectors are soldered on the Al/Ni:Si/Ag stacks in three different aging states: directly after metallization, after accelerated storage and after storage for six months. The thickness of the Ni:Si layer is varied in these tests. Furthermore we measure the contact resistance between cell interconnect ribbons and the test stack. To assess possible contamination of the Si by the metals we measure the effective lifetime of electron hole pairs during a regularly interrupted thermal treatment procedure. The samples with 200nm or thicker Ni:Si layers soldered with the lead-containing solder and the flux 952S perform best and pass all tests

Layer selective laser ablation for local contacts to thin emitters

High efficiency solar cells require high generation and low recombination rates. High bulk lifetime, well passivated surfaces, and lowly doped thin emitters allow for low recombination rates. Thin passivated emitters should be contacted locally in order to avoid excessive contact recombination. This is common practice for front junction solar cells but is also advantageous for back junction cells. We analyze a novel layer selective laser ablation process. From a passivating stack composed of 70 nm silicon nitride that we deposit on top of 35 nm of amorphous silicon we selectively ablate the silicon nitride layer. Transmission electron microscopy investigations confirm the full ablation of the silicon nitride layer. After the ablation process, a 17 nm-thick amorphous silicon layer remains on the substrate. The crystalline silicon substrate shows no dislocations after the process. Evaporating aluminum on top of the locally ablated nitride layers forms local contacts of the aluminum to the silicon

Multilayer etching for kerf-free solar cells from macroporous silicon

Kerf-free techniques for subdividing a single thick crystalline Si wafer into a multitude of thin Si layers have a large potential for cost reductions. In this paper, we explore pore formation in Si for separating many 18 μm-thick surface-textured layers from a thick wafer with a single etching process. We demonstrate the fabrication and separation of four macroporous Si layers in a single etching step. Generating many instead of single macroporous layers per etching step improves the economics of the macroporous Si process. We present our etching process that maintains the pore pattern defined by photolithography even after etching many absorber and separation layers.Federal Ministry for Environment, Nature Conservation, and Nuclear Safety/FKZ 032514

Failure stress of epitaxial silicon thin films

Ultra-thin silicon wafer have to withstand forces and stresses during handling procedures without breakage. Here we investigate the failure stresses of ?30 ?m thick monocrystalline silicon films produced with the porous silicon process by use of a three line bending setup. We use a finite element simulation in order to evaluate the experiments and conclude that the porous silicon layers break at stresses comparable to those of silicon wafers with standard thickness. The edge preparation has a large impact on the failure stress. For samples with manually cleaved edges the failure stress surpasses 600 MPa, which is the largest stress that is accessible with our testing setup

Lift-off of free-standing layers in the kerfless porous silicon process

We discuss the lift-off of free-standing epitaxially grown silicon layers from the porous silicon (PSI) process, which is a kerfless wafering technology. The lift-off is a crucial step in the PSI cycle. A high-porosity layer serves as a mechanically weak layer for lift-off and consists of widely spaced silicon bridges with thicknesses of 40-100 nm. The low width leads to a 33-fold stress enhancement in the bridges, making them break when a force is applied while the epitaxial layer and the substrate remain intact. We perform the free-standing lift-off with a curved vacuum chuck. A vacuum pressure of 0.2 bar is sufficient for controlled peeling off of the 30-50 um thick silicon layers. We simulate the stresses and the displacements of the epitaxial layer in the lift-off process close to the first non-broken bridge. We demonstrate the defect-free lift-off of 8 of 9 of 9 × 9 cm2 layers from 6" substrates.Renewable Energy Corporatio