Photovoltaic Devices: Opto-Electro-Thermal Physics and Modeling.

An opto-electro-thermal simulation of solar cells (SCs) is presented by addressing optoelectronic and thermodynamic responses simultaneously. The photocurrent losses due to carrier recombinations and the intrinsic heat generation (thermalization/Joule/Peltier/recombination heat) and dissipation (convective/radiative cooling) processes in the SCs are investigated quantitatively.

Broadband and wide-angle light harvesting by ultra-thin silicon solar cells with partially embedded dielectric spheres.

We propose a design of crystalline silicon thin-film solar cells (c-Si TFSCs, 2 µm-thick) configured with partially embedded dielectric spheres on the light-injecting side. The intrinsic light trapping and photoconversion are simulated by the complete optoelectronic simulation. It shows that the embedding depth of the spheres provides an effective way to modulate and significantly enhance the optical absorption. Compared to the conventional planar and front sphere systems, the optimized partially embedded sphere design enables a broadband, wide-angle, and strong optical absorption and efficient carrier transportation. Optoelectronic simulation predicts that a 2 µm-thick c-Si TFSC with half-embedded spheres shows an increment of more than 10 mA/cm2 in short-circuit current density and an enhancement ratio of more than 56% in light-conversion efficiency, compared to the conventional planar counterparts.

Enhanced Photoelectrical Response of Hydrogenated Amorphous Silicon Single-Nanowire Solar Cells by Front-Opening Crescent Design.

We report an approach for substantially enhancing the light-trapping and photoconversion efficiency of hydrogenated amorphous silicon (a-Si:H) single-nanowire solar cells (SNSCs) by engineering the cross section of the nanowire from circular into a front-opening crescent shape. The proposed SNSCs show a broadband and highly tunable optical absorption compared to the conventional circular counterparts under both transverse electric and transverse magnetic incidences, enabling an enhancement ratio of over 40 % in both the photocurrent density and the photoconversion efficiency in a-Si:H SNSCs with a diameter of 200 nm. We further show that the superior performance can be well maintained under a wide range of incident angle and is robust to the blunt crescent edges.

Enhanced photoabsorption in front-tapered single-nanowire solar cells.

Vertically aligned single-nanowire is verified to be a unique building block to realize the high-efficiency solar cell beyond Schockley-Queisser limit. We proposed a front-tapered vertically aligned single-nanowire solar cell (V-SNSC) and investigated numerically the possibility of this configuration to improve the photoabsorption efficiency compared to the conventional designs, by using 2.5D full-wave finite-element method. The influences of the feature sizes of aspect ratio, bottom radius, and nanowire length on the light-trapping properties were explored; the detailed field distribution and carrier generation rate were revealed as well based on the theory of dielectric resonant antenna, in order to elucidate the underlying physical mechanism. Results showed that, compared with the cylindrical counterparts, the absorption capability of V-SNSCs could be greatly enhanced by using a front-tapered configuration with less material utilized, and that such a positive effect can be further strengthened by increasing the nanowire length. The proposed configuration provides a promising approach to engineer the photoabsorption in the photovoltaic and other optoelectronic devices.

Design of dual-diameter nanoholes for efficient solar-light harvesting.

A dual-diameter nanohole (DNH) photovoltaic system is proposed, where a top (bottom) layer with large (small) nanoholes is used to improve the absorption for the short-wavelength (long-wavelength) solar incidence, leading to a broadband light absorption enhancement. Through three-dimensional finite-element simulation, the core device parameters, including the lattice constant, nanohole diameters, and nanohole depths, are engineered in order to realize the best light-matter coupling between nanostructured silicon and solar spectrum. The designed bare DNH system exhibits an outstanding absorption capability with a photocurrent density (under perfect internal quantum process) predicted to be 27.93 mA/cm(2), which is 17.39%, 26.17%, and over 100% higher than the best single-nanohole (SNH) system, SNH system with an identical Si volume, and equivalent planar configuration, respectively. Considering the fabrication feasibility, a modified DNH system with an anti-reflection coating and back silver reflector is examined by simulating both optical absorption and carrier transport in a coupled way in frequency and three-dimensional spatial domains, achieving a light-conversion efficiency of 13.72%. PACS: 85.60.-q; Optoelectronic device; 84.60.Jt; Photovoltaic conversion.

Performance-improved thin-film a-Si:H/µc-Si:H tandem solar cells by two-dimensionally nanopatterning photoactive layer.

Tandem solar cells consisting of amorphous and microcrystalline silicon junctions with the top junction nanopatterned as a two-dimensional photonic crystal are studied. Broadband light trapping, detailed electron/hole transport, and photocurrent matching modulation are considered. It is found that the absorptances of both junctions can be significantly increased by properly engineering the duty cycles and pitches of the photonic crystal; however, the photocurrent enhancement is always unevenly distributed in the junctions, leading to a relatively high photocurrent mismatch. Further considering an optimized intermediate layer and device resistances, the optimally matched photocurrent approximately 12.74 mA/cm2 is achieved with a light-conversion efficiency predicted to be 12.67%, exhibiting an enhancement of over 27.72% compared to conventional planar configuration.

Design of µc-Si:H/a-Si:H coaxial tandem single-nanowire solar cells considering photocurrent matching.

The single nanowire solar cells (SNSCs) with radial junctions are expected to show the superiority in efficient carrier collection benefited from the largely shortened junction length. Considering that the conversion efficiency of the existing SNSCs is still limited due to the low operation voltage, we design µc-Si:H(core)/a-Si:H(shell) radial tandem SNSCs, giving much attention to the intrinsic optical and electrical properties. The core and shell cells are carefully engineered in order to realize the photocurrent matching. It is found that under matching condition the radius of the entire cell (R) shows linear dependence on the radius of the core cell (r), i.e., R ~1.2r. Under an optimal design of the tandem cell, the open-circuit voltage (photoconversion efficiency) is increased by 160% (34% relative) compared to the equivalent-size µc-Si:H SNSCs.

Ultra-broadband performance enhancement of thin-film amorphous silicon solar cells with conformal zig-zag configuration.

An ultrathin amorphous silicon solar cell with conformal zig-zag nanoconfiguration is studied from both light-trapping and light-conversion perspectives. The design improves the front antireflection property, optimizes the rear metallic reflector, and elongates the optical path inside the photoactive layer. Compared to conventional nanoconfigurations, this system shows significant absorption enhancement in the whole amorphous silicon band and exhibits extremely low sensitivity to light polarization. The nano-optimization indicates that the short-circuit current density (light-conversion efficiency) of the 200-nm-thick solar cell can be 16.88 mA/cm² (13.38%), showing an enhancement factor of 32.90% (33.53%) from the planar system.

Broadband, polarization-insensitive and wide-angle absorption enhancement of a-Si:H/µc-Si:H tandem solar cells by nanopatterning a-Si:H layer.

A photonic crystal design that significantly enhances the absorption of tandem thin-film solar cells composed by amorphous and microcrystalline silicon (i.e., a-Si:H/µc-Si:H tandem cell) is proposed. The top junction with a-Si:H is nanopatterned as a one-dimensional photonic crystal. Considering the photocurrent matching, we optimally design the junction thickness and the configuration of the nanopattern; moreover, both transverse electric and magnetic incidences with various illuminating angles are taken into account. Calculations by rigorous coupled-wave approach and finite-element method show that the nanophotonic crystal design can improve the absorption and output photocurrent by over 20%, which shows very low sensitivity to the incident polarization. Moreover, the proposed structure is able to sustain the performance for a very wide angle ranges from 0° to ~80°.