Jan Kupec

Light absorption and emission in nanowire array solar cells

Inorganic nanowires are under intense research for large scale solar power generation intended to ultimately contribute a substantial fraction to the overall power mix. Their unique feature is to allow different pathways for the light absorption and carrier transport. In this publication we investigate the properties of a nanowire array acting as a photonic device governed by wave-optical phenomena. We solve the Maxwell equations and calculate the light absorption efficiency for the AM1.5d spectrum and give recommendations on the design. Due to concentration of the incident sunlight at a microscopic level the absorptivity of nanowire solar cells can exceed the absorptivity of an equal amount of material used in thin-film devices. We compute the local density of photon states to assess the effect of emission enhancement, which influences the radiative lifetime of excess carriers. This allows us to compute the efficiency limit within the framework of detailed balance. The efficiency is highly sensitive with respect to the diameter and distance of the nanowires. Designs featuring nanowires below a certain diameter will intrinsically feature low short-circuit current that cannot be compensated even by increasing the nanowire density. Optimum efficiency is not achieved in densely packed arrays, in fact spacing the nanowires further apart (simultaneously decreasing the material use) can even improve efficiency in certain scenarios. We observe absorption enhancement reducing the material use. In terms of carrier generation per material use, nanowire devices can outperform thin-film devices by far.

Dispersion, Wave Propagation and Efficiency Analysis of Nanowire Solar Cells

We analyze the electromagnetic properties of InP/InAs nanowire solar cells for different geometries. We address both eigenvalue calculations to determine the wave propagation as well as source problems to simulate direct perpendicular illumination by three-dimensional finite element calculations. We demonstrate the validity of a 2D waveguide modal analysis as a method of estimating the results of the computationally far more demanding 3D analysis. The resulting data is employed in a detailed balance analysis in order to determine the optimum set of bandgap energies for a single-junction and dual-junction cell as well as the corresponding efficiency limit. The efficiency of the nanowire design can approach the efficiency of conventional thin-film designs despite the low volume fill-factor.

Analysis of photonic crystal defect modes by maximal symmetrization and reduction

We analyze in depth the eigenmodes symmetry of the vectorial electromagnetic wave equation with discrete symmetry, using a recently developed maximal symmetrization and reduction scheme leading to an automatic technique which decomposes every mode into its most fundamental internal geometrical components carrying independent symmetries, the ultimately reduced component functions (URCFs). Using URCFs, geometrical properties of photonic crystal defect modes can be analyzed in great details. In particular we analytically identify the kind of modes that display non-vanishing transverse electric or transverse magnetic amplitude at the cavity center in C2v, C3v, C4v, and C6v symmetries, and their degeneracies. We also build a postprocessing tool able to extract and identify URCFs out of the modes whether from experimental or numerical origin. In the latter case it is independent of the eigenmode computation method. In another variant the whole eigenmode computation can be systematically reduced to a minimal domain, without any need for applying specific non-trivial boundary conditions. The approach leads to strong analytical predictions which are illustrated for specific H1 and L3 cavities using the postprocessing tool on full three-dimensional computed modes. It not only constitutes an unprecedented check of the symmetry of the computational results, but it is shown to also deliver a deep geometrical and physical insight into the structure of the modes of photonic bandgap microcavities, which is of direct use for most modern applications in quantum photonics.

Electro-optical modeling of InP nanowire solar cells: Core-shell vs. axial structure

We present an investigation of the photovoltaic effect in InP nanowire arrays for both axial and core-shell structures. A microscopic electro-optical model is used to analyze the currentvoltage relationships and cell efficiencies. It is found that coreshell structures provide a larger photocurrent and efficiency yet lower open circuit voltage than an axial structure. The analysis of the p-n junction operation explains this difference and gives design guidelines for nano-photovoltaic devices.

Zonal efficiency limit calculation for nanostructured solar cells

We extend the well-known Shockley-Queisser detailed balance calculation for determining the efficiency limit of a solar cell to the case of strong local deviations of the optical power absorption as present in nano-structured photovoltaic devices. In addition, the simple assumption of perfect absorption of all incident light exceeding the bandgap is refined. We present a modified Shockley-Queisser efficiency limit calculation for nano-structured photovoltaic devices, it incorporates a rigorous wave optics calculation and spatially resolved generation of electron-hole pairs. We apply this method to core-shell single-junction InP nanowire array for the use in concentrator solar cells. We investigate the efficiency limits regarding the arrangement of the active regions within the wire. Our results indicate that in a nanowire array solar cell with low volume fill factor the efficiency limit can approach the values of planar thin-film devices. This observation indicates the occurrence of micro-concentration and underlines the necessity of a wave optics approach. The spatially and spectrally resolved analysis shows that generation on the surface of the nanowires is considerable, particularly with regard to high energy photons. Therefore, it is necessary to efficiently extract those carriers.

Analysis of semiconductor nanowire arrays for photovoltaics

The electro-optical properties of nanowire array solar cells are analyzed with a microscopic simulation model. For the electromagnetic part, the vectorial Helmholtz equation is solved for the three-dimensional structure, and a modal analysis reveals the absorption mechanisms. The electronic part is analyzed with a drift-diffusion approach. It is shown that with a proper design, a III–V nanowire array solar cell can approach the efficiency of a thin-film solar cell, while only a fraction of the active material volume is used.

Nanowire Arrays for Photovoltaics and Lighting: Electronic and Optical Properties

Semiconductor nanowire arrays possess both subwavelength electromagnetic as well as quantum electronic features. As active elements, they are attractive device concepts for optoelectronics. In this paper, the properties of nanowire arrays are discussed for their use as light emitting diodes and photovoltaic devices.