S. Geißendörfer

Laser perforated ultrathin metal films for transparent electrode applications

Transmittance and conductivity are the key requirements for transparent electrodes. Many optoelectronic applications require additional features such as mechanical flexibility and cost-efficient fabrication at low temperatures. Here we demonstrate a simple method to fabricate high performance transparent electrodes that is based on perforation of thin silver layers using picosecond laser pulses. Transparent electrodes have been characterized optically and electrically in order to determine the influence of specific surface coverage. Special attention was paid to maintaining sufficient conductivity in the metal-free areas. As a result, transmittance of a much higher bandwidth was achieved as compared to unpatterned metal films. Transparent electrodes have been fabricated on glass and plastic foil, as well as wafer-based silicon heterojunction solar cells, demonstrating their applicability for most relevant cases.

Optimizing Folded Silicon Thin-Film Solar Cells on ZnO Honeycomb Electrodes

A promising approach for low-cost nanostructured thin-film solar cells with enhanced absorption is the fabrication of zinc oxide (ZnO) honeycomb electrodes in a combined bottom-up process of nanosphere lithography and electrochemical deposition. To optimize the honeycomb structures, we investigate thin hydrogenated amorphous silicon (a-Si:H) solar cells (with 100 nm absorber thickness) on honeycomb electrodes with different periodicities in optical and electrical simulations; whereas the electrical performance is not significantly affected with changing periodicity, the short-circuit current density is reduced for increasing honeycomb diameter due to increased parasitic absorption of the electrochemically deposited ZnO. Furthermore, we demonstrate that for micromorph tandem solar cells with an intrinsic layer thickness of hydrogenated microcrystalline silicon (μc-Si:H) of >500 nm, a focusing effect occurs, which leads to a strong enhancement in the quantum efficiency in the microcrystalline bottom solar cell.

Cost-effective nanostructured thin-film solar cell with enhanced absorption

Nanostructured transparent conductive electrodes are highly interesting for efficient light management in thin-film solar cells, but they are often costly to manufacture and limited to small scales. This work reports on a low-cost and scalable bottom-up approach to fabricate nanostructured thin-film solar cells. A folded solar cell with increased optical absorber volume was deposited on honeycomb patterned zinc oxide nanostructures, fabricated in a combined process of nanosphere lithography and electrochemical deposition. The periodicity of the honeycomb pattern can be easily varied in the fabrication process, which allows structural optimization for different absorber materials. The implementation of this concept in amorphous silicon thin-film solar cells with only 100 nm absorber layer was demonstrated. The nanostructured solar cell showed approximately 10% increase in the short circuit current density compared to a cell on an optimized commercial textured reference electrode. The concept presented here...

Laser textured substrates for light in-coupling in thin-film solar cells

Abstract. In this work, we investigate the use of a picosecond (ps) laser used for monolithic connection to texture three commercially available and frequently used multicomponent glasses, Corning EAGLE XG®, Schott BOROFLOAT® 33 and Saint-Gobain SGG DIAMANT®. The results show that the ablated crater profile and degree of texturing are glass composition dependent. This might be attributed to the different laser-induced electron collision times and recombination rates, and thus the critical electron density evolution leading to ablation. The surface texture is altered from periodic to random with decreasing scribing speed. The transmission of the textured substrates gradually decreases, whereas the multireflection on the surface increases as a consequence of the topological and morphological changes. The angular resolved measurements illustrate that the textured glass substrates scatter the light toward greater angles, which is necessary to increase the effective optical path in the absorber layer. Simulation results show that textured glass increases the absorption in the absorber material and the slightly modified refractive index region around the crater does not counteract the light in-coupling effect. The results suggest that these substrates can be used in various photovoltaic technologies and show potential for the application of alternative front contacts, such as carbon nanotubes.

Textured substrates for light in-coupling in thin-film solar cells

The current world population of 7.2 billion is predicted to increase to 9.6 billion in 2050,1 and world energy consumption is projected to grow by 56% between 2010 and 2040 to 820 quadrillion British thermal units (Btu).2 Although fossil fuels constitute almost 80% of energy use,2 they are depleting, and it is becoming more urgent to ensure that energy supply is sustainable. Renewable energies already belong to the world’s fastest-growing energy sources and for electricity generation are increasing by 2.8% per year.2 The most important renewable energy sources are hydro, wind, and photovoltaic (PV).3 Silicon wafer-based modules dominate the PV market with an 86% market share. The other 14% belongs to thin-film PV,4 some of which is based on hydrogenated amorphous silicon (a-Si:H) and hydrogenated microcrystalline silicon ( c-Si:H), which share the advantage of silicon’s abundance as a raw material. An additional advantage is that less silicon is required than for conventional wafer silicon solar cells. Thus, silicon-based thin-film technology has the potential for low-cost, rapid, largearea industrial production and low energy-payback time.5 Current solar cell efficiencies are 10.1% for single-junction a-Si:H in the p-i-n configuration (meaning a combination of positively doped, intrinsic, and negatively doped silicon),6 10.8% for single-junction c-Si:H p-i-n,7 and 12.3% for tandem a-Si:H/ c-Si:H (micromorph).8 Light trapping has proven a useful tool for increasing efficiency. The concept of light trapping, also termed light in-coupling, is illustrated in Figure 1(a). Incorporating a textured surface increases the effective light path and hence absorption in the silicon absorber material.9 Consequently, even thinner absorber layers can be used, an advantage for amorphous silicon, which suffers from a light-induced Figure 1. Schematic of a silicon thin-film p-i-n (a combination of positively doped, intrinsic, and negatively doped silicon) solar cell with (a) a textured transparent conductive oxide (TCO) front contact9 and (b) a textured glass substrate. c-Si: Microcrystalline silicon. a-Si: Amorphous silicon.

Fourier transform based interface roughness analysis for flexible thin-film silicon solar cells

Abstract. Analysis of randomly textured interface in solar cells is an ambitious effort considering the numerous structures and variation in the texture. We use the Fourier transform based method, which takes randomly textured interface as initial input and manipulates its frequency spectrum to synthesize interface texture with the desired texture behavior. Afterward, the synthesized interfaces are applied to simulate flexible thin-film silicon tandem (aSi/μcSi) solar cells. Using the simulation, we have shown how interface between the back and front areas of the solar cell evolves, and the effect of the back contact; the front area roughness is analyzed and interference fringes observed on experiments are discussed. For the simulations, a finite integration technique with time-harmonic inverse iterative method is used. All simulations are performed on high-performance computers, which allows simulation of a big simulation domain (>5  μm×5  μm) with fine mesh size (<10  nm). The analysis performed shows that the interface roughness at the front contact remains similar to the initial back contact roughness. Furthermore, a solar cell with flat back contact can be as efficient as a solar cell with rough back contact when the front area roughness is well optimized.

Analytical energy-barrier-dependent Voc model for amorphous silicon solar cells

We show that the open circuit voltage (Voc) in hydrogenated amorphous silicon (a-Si:H) solar cells can be described by an analytical energy-barrier-dependent equation, considering thermionic emission as the physical mechanism determining the recombination current. For this purpose, the current-voltage characteristics of two device structures, i.e., a-Si:H(n)/a-Si:H(i)/a-Si:H(p)/AZO p-i-n solar cells with different p-doping concentrations and a-Si:H(n)/a-Si:H(i)/AZO Schottky structures with different intrinsic layer thicknesses, were analyzed in dark and under illumination, respectively. The calculated barrier in the p-i-n devices is consistent with the difference between the work function of the p-layer and the conduction band edge of the i-layer at the interface in thermal equilibrium.