Harnessing plasmonics for solar cells

Due to the rising power demand and substantial interest in acquiring green energy from sunlight, there has been rapid development in the science and technology of photovoltaics (PV) in the last few decades. Furthermore, the synergy of the fields of metrology and fabrication has paved the way to acquire improved light collecting ability for solar cells. Based on recent studies, the performance of solar cell can improve due to the application of subwavelength nano-structures which results in smaller reflection losses and better light manipulation and/or trapping at subwavelength scale. In this paper, we propose a numerical optimization technique to analyze the reflection losses on an optimized GaAs-based solar cell which is covered with nano-structured features from the same material. Using the finite difference time domain (FDTD) method, we have designed, modelled, and analyzed the performance of three different arrangements of periodic nano-structures with different pitches and heights. The simulated results confirmed that different geometries of nano-structures behave uniquely towards the impinging light.

Chapter 6 - Nanostructured silicon for antireflection and light trapping in crystalline silicon solar cells

Plasmonic enhanced solar cells: Summary of possible strategies and recent results

Organic solar cells (OSCs) have attracted intense attention in recent years due to their advantages of low cost, easy fabrication, and high flexibility compared to its inorganic counterparts. However, due to the conflicts between the short diffusion length of excitons and long absorption length of incident photons, the thickness of OSCs is typically thin, and thus power conversion efficiency (PCE) is generally lower than traditional silicon solar cells. Therefore, an exquisite design of light trapping schemes is essential to the PCE improvement. Generally, physical guideline of light trapping involves two main approaches: geometric optics methods and wave optics methods. The former aims at elongating optical path inside the photoactive layer and thus enhancing photon absorption. For organic thin film solar cells with typical active layer thickness of 100 nm-200 nm, which is in subwavelength scale, we cannot investigate light harvesting mechanism simply by the geometric optics methods and instead wave optics properties should be considered. 
\nIn this thesis, two different light trapping enhancement designs are proposed. In order to simulate these structures, we built up programs for absorption power calculation based on scattering matrix method (SMM) by rigorously solving Maxwell’s equations. It is worth to point out that, different from the widely-used calculation method by Absorption = 1-Transmission-Reflection, our algorithm can extract the net optical absorption of the active layer rather than the whole OSCs. This improvement is very important because metal absorption, which does not contribute to exciton generation, can be excluded from the result. 
\nIn Chapter 3, design of organic solar cell incorporating periodically arranged gradient type active layer is presented. This design can enhance light harvesting with patterned organic materials themselves (i.e. self-enhanced active layer design) to avoid degrading electrical performance in contrast to introducing inorganic concentrators into the active layers such as silicon and metallic nanostructures. Our numerical results show that the OSC with a self-enhanced active layer, compared with the conventional planar active layer configuration, has broadband and wide-angle range absorption enhancement due to better geometric impedance matching and prolonged optical path. 
\nIn Chapter 4, OSC with interstitial lattice patterned metal nanoparticles (NPs) is proposed, which can improve the light blocking of traditional square lattice patterned NPs structure and achieve broadband absorption enhancement. Compared to square lattice design, the plasmonic mode couplings between individual NPs in the interstitial lattice are more versatile and much stronger. Moreover, plasmonic modes can couple to the guided modes, resulting in large enhancement factor at some wavelengths. These works provide a theoretical foundation and engineering reference for high performance OSC designs.

The unique optical properties of nanometallic structures can be exploited to confine light at subwavelength scales. This excellent light trapping is critical to improve light absorption efficiency in nanoscale photovoltaic devices. Here, we apply a multiscale quantum mechanics/electromagnetics (QM/EM) method to model the current-voltage characteristics and optical properties of plasmonic nanowire-based solar cells. The QM/EM method features a combination of first-principles quantum mechanical treatment of the photoactive component and classical description of electromagnetic environment. The coupled optical-electrical QM/EM simulations demonstrate a dramatic enhancement for power conversion efficiency of nanowire solar cells due to the surface plasmon effect of nanometallic structures. The improvement is attributed to the enhanced scattering of light into the photoactive layer. We further investigate the optimal configuration of the nanostructured solar cell. Our QM/EM simulation result demonstrates that a further increase of internal quantum efficiency can be achieved by scattering light into the n-doped region of the device.

Near-field study of optical modes in randomly textured ZnO thin films

Conventional light trapping techniques are inefficient at the sub-wavelength scale. This is the main limitation for the thickness reduction of thin-film solar cells below 500nm. We propose a novel architecture for broadband light absorption in ultra-thin active layers based on plasmonic nano-cavities and multi-resonant mechanism. Strong light enhancement will be shown numerically for photovoltaic materials such as CIGSe and GaAs. First experiments on ultrathin nano-patterned CIGSe solar cells will be presented.

This study addresses the potential of different approaches to improve the generated current density in ultrathin Cu(In,Ga)Se₂ (CIGSe) based solar cells down to 0.1 μm. Advanced photon management, involving both absorption enhancement and reflection reduction in the absorber, is studied. In this contribution, the three main approaches used are: - The reduction of the CIGSe thickness by chemical etching which combines thickness reduction and smoothing effect on the absorber. - Optical management by front contact engineering and by the replacement of the back contact by the "lift-off" of CIGSe layer from the Mo layer and the deposition of a new reflective back contact. - Application of plasmonic structures to CIGSe solar cells enabling light confinement at the subwavelength scale.

Application of artificial neural network for accelerated optimization of ultra thin organic solar cells

Conventional light trapping techniques are inefficient at the sub-wavelength scale. This is one of the main limitation for the thickness reduction of thin-film solar cells. We propose a novel architecture for broadband light absorption in a 100 nm-thick CIGS active layer. We show that this ultra-thin CIGS solar cell could preserve a constant short-circuit current while decreasing the active layer thickness by at least one order of magnitude. We focus on numerical optimisation to achieve a good short-circuit current with tunable multiple resonances. Fabrication issues are discussed.

The evolution of nanotechnology has provided a better understanding of light-matter interaction at a subwavelength scale and has led to the development of new devices that can possibly play an important role in future applications. Nanoantennas are an example of such devices, having gained interest in recent years for their application in the field of photovoltaic technology at visible and infrared wavelengths, due to their ability to capture and confine energy of free-propagating waves. This property results from a unique phenomenon called extraordinary optical transmission (EOT) where, due to resonant behavior, light passing through subwavelength apertures in a metal film can be transmitted in greater orders of magnitude than that predicted by classical theories. During this study, 2D and 3D models featuring a metallic nanoantenna array with subwavelength holes coupled to a photovoltaic cell are simulated using a Finite Element Tool. These models present with slight variations between them, such as the position of the nanoantenna within the structure, the holes’ geometry and the type of cell, in order to verify how its optical response is affected. The results demonstrate that the coupling of nanoantennas to solar cells can be advantageous and improve the capture and absorption of radiation. It is concluded that aperture nanoantennas may concentrate radiation, meaning that is possible to tune the electric field peak and adjust absorption on the main layers. This may be important because it might be possible to adjust solar cell performance to the global regions’ solar spectrum by only adjusting the nanoantenna parameters.

Emerging high-index all-dielectric nanostructures, capable of manipulating light on the subwavelength scale, empower designing and implementing novel antireflection and light-trapping layers in many photonic and optoelectronic devices. However, their performance and practicality are compromised by relatively narrow bandwidths and highly sophisticated fabrications. In this paper, we demonstrate an ultra-broadband (300-1200 nm) directional light scattering strategy using high-index surface silicon oligomer resonators fabricated by a facile, scalable, and low-cost colloidal lithography technique. The exceptional broadband forward scattering stems from a combined effect of strongly intercoupled Mie resonances within the oligomers composed of randomly positioned nanodisks in the visible region and a strong electric mode coupling between the oligomers and the high-index substrate in the red-to-near-infrared region. By implementing this efficient approach in silicon solar cells, the integrated optical reflection loss across the wavelength range 300-1200 nm can be as low as 7%. Consequently, the short-circuit current density determined from the external quantum efficiency of solar cells can be increased to 35.1 from 25.1 mA/cm2, representing an enhancement of 40%, with a demonstrated energy conversion efficiency exceeding 15.0%. The insights in this paper hold great potentials for new classes of light management and steering photonic devices with drastically improved practicality.

Nanoimprint lithography (NIL) has emerged as a powerful tool for patterning nanoscale structures with high precision, low-cost, and large-scale manufacturing. In photonics, NIL enables the creation of complex optical structures such as gratings, metamaterials, photonic crystals, and waveguides. These nanoscale features are critical for manipulating light at sub-wavelength scales, offering enhanced control over optical properties such as dispersion, polarization, and transmission. NIL's high-resolution patterning capability makes it particularly attractive for fabricating large-area photonic devices with high precision and repeatability. A wide range of applications, including integrated photonic circuits, optical sensors, and advanced light management in displays and solar cells, are now the focus of extensive research and discussion. By enabling the precise engineering of refractive index profiles and light-matter interactions, NIL continues to play a crucial role in advancing the performance and functionality of next-generation photonic systems. This review explores the fundamental principles of NIL and its recent developments. In addition, other patterning techniques related to photonics patterning and fabrication are briefly discussed. We will then focus on the applications in photonics and the advantages and challenges associated with this technique.

Within the UltraCIS project we have started to explore the possibility of reducing down the thickness of the Cu(In, Ga)Se <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> (CIGSe) layer to the sub-micron level (0.1μm) while maintaining a high efficiency level of solar cells. The three main approaches we used are: - Reducing the CIGSe thickness by chemical etching combining the thickness reduction and smoothing effect of the absorber. Efficiency higher than 10 % on small area cells with an absorber thickness of 0.5 μm are obtained. Losses were attributed exclusively to the reduced photocurrent and the loss on texturation of the absorber - Optical management by front contact texturation or by replacement of the back contact by the “lift-off” of CIGSe layer from the Mo layer and deposition of a new reflective back contact. - Application of plasmonic structures to CIGSe solar cells enabling light confinement at the subwavelength scale.

Surrogate based modeling and optimization of plasmonic thin film organic solar cells