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Photon management is a key element to optimize the optical and electro-optical performance of solar cells. Photon management concepts and the potential of 3D photonic crystals for photon management in solar cells are discussed.

In contrast to its electronic excellence, optical properties of crystalline Si are rather poor, making it less suitable for optoelectronic and photonic purposes. One promising solution to overcome this hurdle is by making use of nanostructured configurations, such as nanolayers or nanocrystals, where quantum confinement (QC)-induced effects increase the optical activity.

Photon management is a key component in the development of efficient solar cells. Especially light-trapping concepts have a high potential to realize enhanced efficiencies. Here, we give an overview over several light trapping concepts for photon management in solar cells. These include basic as well as advanced light-trapping concepts. The theoretical limits of light path enhancement of the different concepts are given and experimental work on these topics is presented. The potential of 3D photonic crystals is discussed in the context of the corresponding approaches as well.

Photon management is a key component in the development of efficient solar cells. Especially light-trapping concepts have a high potential to realize enhanced efficiencies. Here, we give an overview over several light trapping concepts for photon management in solar cells. These include basic as well as advanced light-trapping concepts. The theoretical limits of light path enhancement of the different concepts are given and experimental work on these topics is resented. The potential of 3D photonic crystals is discussed in the context of the corresponding approaches as well.

The C7 low-concentration PV (LCPV) tracker is presented. This modular advanced technology platform is designed for utility-scale deployment and delivers the lowest levelized cost of electricity available today. Its inherent advantages are detailed.

Light management in single and multi-junction solar cells is becoming increasingly important to optimize the optical and electro-optical properties of solar cells. The limits of light path enhancements are discussed for non-resonant and resonant optical nanostructures as well as for angular-selective and energy-selective approaches. The potential of 3D photonic crystals for photon management in solar cells is discussed for the corresponding concepts and the state of the art is reviewed.

Due to their high specific detectivities and large response speeds, photodetectors based on narrow-bandgap semiconductors like indium-antimonide and mercury cadmium telluride are indispensable for mid- and far-infrared wavelength ranges. Their response is directly proportional to the infrared radiation flux introduced to the detector active area. Thus various photon management techniques are of crucial importance for enhancing their performance. This work analyzes the use of subwavelength nanophotonic structures with antireflective properties. It considers surface-based diffractive optical structure that can be produced by micromachining and nanofabrication. The first part of the consideration is dedicated to all-dielectric subwavelength arrays with 1D and 2D periodicity, which effectively behave as impedance-matching structures with graded effective refractive index. The consideration is then expanded to metal-dielectric structures, especially those belonging to the class of the so-called plasmonic ultra-absorbers. Technologies available in Serbia are then investigated technologies for the fabrication of the dielectric and metaldielectric subwavelength antireflective structures for the infrared, including isotropic and anisotropic etching of surface reliefs and ultrathin-film deposition techniques. It is concluded that diffractive dielectric and plasmonic structures offer a novel degree of freedom in optimization of infrared semiconductor photodetectors.

In the preceding Chap. 4, high energy photons were applied for device fabrication and, also, for the analysis of (photo)electrochemically modified silicon. In addition, the use of soft X-rays in the energetic analysis of metallo-proteins and in materials development was emphasized.

Light-incoupling and light-trapping are major prerequisites for high efficiencies in solar cells and modules. This is especially the case for silicon-based thin film solar cells, because the absorber thickness must be smaller than the absorption length of sunlight, for both physical and economic reasons. This thesis investigates the directional and spectral dependence of light-incoupling and light-trapping in solar cells. The directional dependence of light-incoupling for glass-covered solar cells is dominated by the reduced transmittance under angles exceeding the Brewster angle. The reduced transmittance results in a loss in the annual yield of solar cells of about 2 %. The light-trapping does not notably change under increased angles of incidence. To enhance the incoupling at the front of the solar cell, the effects of a textured surface structure on the cover glass of the solar cell are investigated. The texture reduces the reflectance at the air-glass interface and, additionally, reduces the reflection losses originating at the interface between the glass and the transparent conductive oxide (TCO) as well as the TCO and the silicon (Si) absorber due to the randomization of light. On samples without a textured TCO/Si interface, the textured foil induces additional light-trapping in the photovoltaically active absorber material. This effect is not observed for samples with a textured TCO/Si interface. In this case, using tandem solar cells, a redistribution of light absorption in the top and bottom subcells is detected. The anti-reflective texture increases the short circuit current density in thin film silicon tandem solar cells by up to 1 mA/cm², and the conversion efficiency by up to 0.7 % absolute. The increase in the annual yield of solar cells is estimated to be up to 10 %. Further, the spectral dependence of the efficiency and annual yield of a tandem solar cell was investigated. The daily variation of the incident spectrum causes a change in the current matching of the serial connected subcells. Simulations determine the optimum subcell layer thicknesses of tandem solar cells. The thicknesses optimized in respect to the annual yield overlap in a wide range for both investigated locations with those for the AM1.5g standard spectrum. Though, a slight top limitation is favorable. The daily variation is more crucial than the geographic location. Technologically it is not straight forward to design a tandem solar cell with optimum subcell thicknesses for maximum efficiency. Matching the short circuit currents of the subcells maximizes the overall current, but minimizes the fill factor. This thesis introduces a new definition for the matching condition of tandem solar cells. This definition optimizes the power output of the solar cell, instead of the short circuit current density. A new method is proposed to quantify the power mismatch of a given solar cell by varying the composition of the incident spectrum. Experimentally, it is shown exemplary on a silicon thin film tandem solar cell that the efficiency can be increased by 0.5 % absolute by adopting the layer thicknesses to the power matching instead of the current matching. Moreover, the efficiency under standard conditions can be interpolated from the measurements without the need for time-consuming calibrations. It is further shown in this thesis that an angle and energy selectivity of the surface of a solar cell can enhance the light-trapping, and lead to efficiencies in very thin cells above the theoretical limit for non-concentrating solar cells. Simulations determine a theoretically achievable gain of up to 33 % in the annual yield if the cell is tracked to the solar path. For non-tracked solar cells, simulations show that there is a wide angle and energy range of acceptable restrictions that lead to only minor losses but also not to gains. In experiments on hydrogenated amorphous silicon thin film solar cells an enhancement of the overall path length in the device up to a factor of 3.5 was demonstrated using a Bragg-like angle selective filter. This result emphasizes the potential for directional selectivity to improve light-trapping. The total benefit of this potential is, however, limited by parasitic absorption in the adjacent doped layers, in the transparent conductive oxide and at the back reflector. The effective path length enhancement was a maximum factor of about 1.5, in the wavelength regime close to the band gap. Nonetheless, this thesis demonstrates an improvement in the short circuit current density of hydrogenated amorphous silicon solar cells by 0.2 mA/cm². This result is the first experimental proof that the concept of directional selectivity is able to improve solar cell performance.

Photon management in thin‐film solar cells is nowadays indispensable for enhancing their efficiency. We describe various strategies where modern concepts from the field of micro‐ and nanooptics are exploited for this purpose. While concentrating on single and tandem solid state solar cells, we detail how plasmonic structures, photonic crystals, randomly textured interfaces, or combinations thereof, can be beneficially integrated into solar cells. Peculiar to our theoretical and numerical work is the consideration of structures with geometrical details taken from fabricated devices. We detail how state‐of‐the‐art computational photonics provides insight into the evolution of light in solar cells and show how explicit guidelines for an optimization of the photon management can be derived from adapted simulations.

Rigorous diffraction theory is used to reveal the peculiarities of randomly textured surfaces as used for photon management. The effect of such surfaces in solar cells with an increasing complexity is analyzed to provide unprecedented insights.

Photonic structures can be used to eliminate the main loss mechanism in fluorescent concentrators. Simulation routines have been established to investigate the optical characteristic of different photonic crystals. Especially two kinds of structures with an appropriate characteristic have been examined closely. The first is the rugate filter, a one-dimensional photonic structure. In the rugate filter the refractive index is varied sinusoidally over the thickness of the filter. The second is the opal, a three-dimensional photonic crystals made of spheres that are arranged in a self organization process. Filters from these structures have been designed and optimized for the application and fluorescent concentrators and have been optimized. Additional aspects of the structures like angular effects have been examined.

High efficiency photovoltaic (PV) cells represent a clean and efficient method to utilize the abundant solar energy in both space and terrestrial applications. Advances in solar cell technology and associated light management systems are the primary determinants of improvements in efficiency. Single junction (SJ) solar cells are already near theoretical efficiency limits defined by thermalization losses and sub-bandgap transparency. Devices that incorporate multiple junctions (i.e. sub-cells) in monolithic stacks, known as multijunction (MJ) cells, provide one of the most attractive routes to ultrahigh efficiency. Over the last decade, increases in the efficiency of MJ cells correspond to nearly 1% (absolute) per year, reaching values that are presently ~44%. Further improvements, however, will require solutions to daunting challenges in achieving lattice-matched or metamorphic epitaxial growth in complex stacks and in maintaining current matched outputs from each of the sub-cells. Here we present materials and strategies to make printed multijunction solar cell structures, to bypass the challenges in conventional multijunction cell design. In the first part, I will talk about printing-based assembly of microscale, quadruple junction, four-terminal solar cells with measured efficiencies of 43.9% at concentrations exceeding 1000 suns, and modules with efficiencies of 36.5%. Secondly, I will introduce an alternative device architecture, which uses low refractive index interface materials for enhanced photon recycling in printed multijunction solar cell stacks. These results establish routes to ultrahigh efficiency cells and modules, with potential to approach thermodynamic efficiency limits and realize large-scale photovoltaic energy production.

Optical systems for photon management, that is the generation of tailored electromagnetic fields, constitute one of the keys for innovation through photonics. An important subfield of photon management deals with the transformation of an incident light field into a field of specified intensity distribution. In this paper we consider some basic aspects of the nature of systems for those light transformations. It turns out, that the transversal redistribution of energy (TRE) is of central concern to achieve systems with high transformation efficiency. Besides established techniques nanostructured optical elements (NOE) are demanded to implement transversal energy redistribution. That builds a bridge between the needs of photon management, optical engineering, and nanooptics.

In wave-optical engineering the propagation of light through an optical system can be simulated by using several physical approximations. Independent of the used method it is necessary to have full access to the complete electromagnetic field information in the desired regions. In this article we firstly go into the question what the required information is to get access to the whole electromagnetic field and as a second step we give some insights into physical modeling accuracy and numerical accuracy which is of high importance to evaluate the quality of the calculated results.

Conference Name:Conference on Photon Management. Conference Address: Strasbourg, FRANCE. Time:APR 27-28, 2004.

Conference Name:Conference on Photon Management. Conference Address: Strasbourg, FRANCE. Time:APR 27-28, 2004.

Data and analysis are presented employing several new methods to address carrier dynamics and photon management issues in order to improve internal quantum efficiency (IQE) and light-extraction efficiency (LEE) of GaN-based light-emitting diodes (LEDs). First, the roles of a new InAlN electron blocking layer (EBL) and hole transport among quantum wells (QWs) are discussed with a focus on the mitigation of efficiency droop at high current densities. While the origins of the efficiency droop remain to be controversial, currently suggested origins are all associated with carrier dynamics: the effect of the electron leakage being related to carrier confinement; hole transport to active region being pertinent to carrier injection and concentration in each QW, and Auger recombination becoming more dominant with increasing carrier density in each QW. The strategy for the improvement, therefore, includes (1) confining electrons in the active region as much as possible, (2) injecting holes into the active region as much as possible, (3) distributing the both carriers among QWs as uniformly as possible. In order to implement the strategies, we especially studied EBLs and hole injection and transport in the multiple QW (MQW) active region. The effects of the various EBLs on the efficiency droop was compared (Figure 1), showing the lowest efficiency droop ratio of ~18% for the LEDs with InAlN EBL due to better electron confining/blocking effect than the standard EBLs. Hole injection (into active region) and transport (across MQWs) are as important as electron confinement. Also, they are not inter-related. Uniform distributions of electrons and holes with the same concentration in each QW are a goal to effectively mitigate the efficiency droop. Whereas the uniform distribution of electrons among QWs is achieved without difficulties, that of hole is quite challenging. We employed triple-wavelength-emitting MQWs with different bandgap energies for the experimental evaluation of the hole distribution among MQWs (Figure 2), showing hole transport and resulting distribution of holes can be engineered by p-type layers. Secondly, new way of surface patterning is demonstrated. The periodic surface patterns having 2-dimensional hexagonal array were directly achieved by three-beam interference laser ablation technique without photolithography processes (Figure 3), resulting in improvement of LEE by ~20%. This new direct laser patterning also maintained ohmic behavior of contacts on p-type surface, unlike the case of the patterning using plasma dry etching.

The unique geometry and intriguing physical properties of nanostructure-based solar cells gives them great potential to achieve the goals of cost-effectiveness and high-efficiency. With nanostructured solar cells it is expected to be possible to break the Shockley–Queisser limit. This potential has driven widespread research and development in photon management to enhance light absorption over the past decade. However, efficiency is not proportional to light absorption. Nowadays, researchers are starting to address this issue. A thorough understanding of the advantages and the scope of the application of each photon management scheme is critical to finding a breakthrough for this predicament. In this review, we present the theorems and describe recent progresses in primary photon management schemes for nanostructures, including antireflection, light scattering, and resonance (e.g., metallic resonance, dielectric resonance, and photonic crystals). The antireflection effect allows more light to enter the solar cell. Light scattering enhances the interaction between the light and the nanostructure, extending the light propagation paths in the devices. Resonance effects can redirect and precisely confine the light to the region where efficient photoelectric conversion efficiency occurs. Finally, we discuss the challenges of nanostructured solar cells, and indicate potential routes to overcome the performance-limiting problems.

Light absorption in a photovoltaic device becomes critical as the thickness of an absorber layer is decreased to reduce cost. To enhance light absorption, photon management at the nanoscale has been studied because conventional methods, which are based on micrometer-sized structure, do not work well for thinner solar cells. This article reviews recent progress in photon management on the nanoscale for increasing light absorption in Si solar cells. The methodology for the absorption enhancement will be discussed, followed by advances in nanofabrication techniques that make the methodology a scalable and viable solution. The authors conclude with a discussion of the challenge of photon management schemes and future directions for light trapping in ultra-thin Si solar cells.

In tandem solar cells, intermediate reflectors are employed to increase light absorption in the top cell. Thus far, the use of photonic crystals for this purpose was not optimal since side-lobes in the reflection spectrum reduced the absorption in the bottom cell. To compensate this reduction, the bottom cell thickness had to be excessively increased; nullifying the main advantage of thin-film solar cells. Here, we suggest to solve this issue by reducing the impedance mismatch between photonic crystal and bottom cell using anti-reflection layers. The concept is even validated for solar cells comprising random textures.

Light management in single and tandem solar cells is becoming increasingly important to optimize the optical and electro-optical properties of solar cells. After a short introduction to state-of-the-art light management approaches, different applications of photonic crystals for photon management in solar cells are reviewed and discussed concerning their applicability. Results on direction- and energy-selective filters for ultra-light-trapping, intermediate reflectors for optimal current matching in tandem cells, and photonic crystal coating for fluorescence collectors will be presented and discussed.

Photon management by means of random textured surfaces is known to be a promising route to increase the light absorption in a solar cell. To date this randomness was only a posteriori assessed and related to the absorption. Here, we will outline a meaningful strategy for a priori and purposely tailoring the randomness. By defining appropriate angular scattering functions and optimizing the surface profiles, it is shown that the number of absorbed photons can be enhanced by 55% compared to flat-surface solar cells.

This report paves the way to optimize the nanostructured optoelectronic devices with efficient light management by controlling structure profile of nanostructures.