Bart E. Pieters

Design of nanostructured plasmonic back contacts for thin-film silicon solar cells

We report on a plasmonic light-trapping concept based on localized surface plasmon polariton induced light scattering at nanostructured Ag back contacts of thin-film silicon solar cells. The electromagnetic interaction between incident light and localized surface plasmon polariton resonances in nanostructured Ag back contacts was simulated with a three-dimensional numerical solver of Maxwells equations. Geometrical parameters as well as the embedding material of single and periodic nanostructures on Ag layers were varied. The design of the nanostructures was analyzed regarding their ability to scatter incident light at low optical losses into large angles in the silicon absorber layers of the thin-film silicon solar cells.

Determination of the mobility gap of intrinsic μc-Si:H in p-i-n solar cells

Microcrystalline silicon (?c-Si:H) is a promising material for application in multijunction thin-film solar cells. A detailed analysis of the optoelectronic properties is impeded by its complex microstructural properties. In this work we will focus on determining the mobility gap of ?c-Si:H material. Commonly a value of 1.1?eV is found, similar to the bandgap of crystalline silicon. However, in other studies mobility gap values have been reported to be in the range of 1.48–1.59?eV, depending on crystalline volume fraction. Indeed, for the accurate modeling of ?c-Si:H solar cells, it is paramount that key parameters such as the mobility gap are accurately determined. A method is presented to determine the mobility gap of the intrinsic layer in a p-i-n device from the voltage-dependent dark current activation energy. We thus determined a value of 1.19?eV for the mobility gap of the intrinsic layer of an ?c-Si:H p-i-n device. We analyze the obtained results in detail through numerical simulations of the ?c-Si:H p-i-n device. The applicability of the method for other than the investigated devices is discussed with the aid of numerical simulations.

Electro-optical modeling of bulk heterojunction solar cells

We introduce a model for charge separation in bulk heterojunction solar cells that combines exciton transport to the interface between donor and acceptor phases with the dissociation of the bound electron/hole pair. We implement this model into a standard semiconductor device simulator, thereby creating a convenient method to simulate the optical and electrical characteristics of a bulk heterojunction solar cell with a commercially available program. By taking into account different collection probabilities for the excitons in the polymer and the fullerene, we are able to reproduce absorptance, internal and external quantum efficiency, as well as current/voltage curves of bulk heterojunction solar cells. We further investigate the influence of mobilities of the free excitons as well as the mobilities of the free charge carriers on the performance of bulk heterojunction solar cells. We find that, in general, the highest efficiencies are achieved with the highest mobilities. However, an optimum finite mobil...

Photoresponse enhancement in the near infrared wavelength range of ultrathin amorphous silicon photosensitive devices by integration of silver nanoparticles

We have investigated the contribution of localized surface plasmon polaritons (LSPPs) in silver nanoparticles with radii smaller than 20 nm to the photocurrent of ultrathin photosensitive devices based on amorphous silicon. An increased light absorption and an enhanced photocurrent are found for wavelengths between 600 nm and 1150 nm in presence of nanoparticles. As amorphous silicon absorbs light efficiently only at wavelengths up to 750 nm, the increased photocurrent in the near infrared range is explained in terms of LSPP-induced photoemission of electrons within and in close vicinity of the nanoparticles.

Spatial Modeling of Thin-Film Solar Modules Using the Network Simulation Method and SPICE

The use of the network simulation method (NSM) for the modeling of thin-film solar modules is discussed. It will be shown that for the accurate modeling of small defects using the NSM, a variable mesh is required to keep the computation time within reasonable limits. A simulation tool for thin-film solar modules is developed that implements the NSM with a variable, adaptive mesh. The results of this model will be compared with electroluminescence experiments on a Cu(In,Ga)Se2 solar module with a defect (shunt). Furthermore, it is demonstrated that the program can handle complex-shaped inhomogeneities such as local variations in solar cell properties, illumination, and contact layer resistance.

Angular resolved scattering by a nano-textured ZnO/silicon interface

Textured interfaces in thin-film silicon solar cells improve the efficiency by light scattering. A technique to get experimental access to the angular intensity distribution (AID) at textured interfaces of the transparent conductive oxide (TCO) and silicon is introduced. Measurements are performed on a sample with polished microcrystalline silicon layer deposited onto a rough TCO layer. The AID determined from the experiment is used to validate the AID obtained by a rigorous solution of Maxwell’s equations. Furthermore, the applicability of other theoretical approaches based on scalar scattering theory and ray tracing is discussed with respect to the solution of Maxwell’s equations.

Optical simulations of microcrystalline silicon solar cells applying plasmonic reflection grating back contacts

Light trapping is a key issue for high efficiency thin-film silicon solar cells. The authors present three-dimensional electromagnetic simulations of an n-i-p substrate-type micro- crystalline silicon solar cell applying a plasmonic reflection grating back contact as a novel light- trapping structure. The plasmonic reflection grating back contact consists of half-ellipsoidal silver nanostructures arranged in square lattice at the back contact of thin-film silicon solar cells. Experimental results of prototypes of microcrystalline silicon thin-film solar cells showed significantly enhanced short-circuit current densities in comparison to flat solar cells and, for an optimized period of the plasmonic reflection grating back contact, even a small enhancement of the short-circuit current density in comparison to the reference cells applying the conventional random texture light-trapping structure. The authors demonstrate a very good agreement between the simulated and experimental spectral response data when taking experimental variations into account. This agreement forms an excellent basis for future simulation based optimizations of the light-trapping by plasmonic reflection grating back contacts. Furthermore, from the simulated three-dimensional electromagnetic field distributions detailed absorption profiles were calculated allowing a spatially resolved evaluation of parasitic losses inside the solar cell.

Photocurrent collection efficiency mapping of a silicon solar cell by a differential luminescence imaging technique

A differential electroluminescence imaging method for solar cells which yields local photocurrent collection efficiency maps is introduced. These maps attribute a value between zero and unity to each location on the cell. This value corresponds to the ratio between the current at the cell terminals and the locally generated photocurrent. The method is demonstrated for a multicrystalline silicon solar cell under constant illumination. If the point of maximum power output of the cell is chosen as the bias point, the method yields quantitative information on the local contribution to the maximum output power of the solar cell.

Plasmon-induced photoexcitation of “hot” electrons and “hot” holes in amorphous silicon photosensitive devices containing silver nanoparticles

We report on a plasmon-induced photocurrent in photosensitive devices based on hydrogenated amorphous silicon (a-Si:H) containing silver nanoparticles (NPs). The photocurrent is measured in a spectral region corresponding to optical transitions below the band gap of a-Si:H. Photoexcitation of “hot” electrons in the NPs or in defect states present in the vicinity of the NPs, resulting from plasmon decay in the NPs, is often cited as being responsible for this effect. In this study, we demonstrate that plasmon induced photogeneration of “hot” holes is also able to contribute to a photocurrent. A bifacial symmetrical transparent device was prepared in order to compare the internal quantum efficiency of both processes, the first based on the photogeneration of “hot” electrons and the second based on the photogeneration of “hot” holes.

Effect of light soaking on the electro- and photoluminescence of Cu(In,Ga)Se2 solar cells

ZnO/CdS/Cu(In,Ga)Se2 solar cells are investigated by spectrally resolved electroluminescence and electro-modulated photoluminescence. The results agree well with the reciprocity relation between luminescence emission and photovoltaic quantum efficiency. In contrast, the superposition of photoluminescence and electroluminescence emission is warranted only in a limited injection range. At higher injection levels, we observe a characteristic discrepancy between electroluminescence and electro-modulated photoluminescence which is reduced by light soaking. We attribute this anomaly to a potential barrier close to the CdS/Cu(In,Ga)Se2 interface. Hole injection into the space charge region during light soaking reduces this barrier and enhances the luminescence efficiency by a factor of 2.5.