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A comprehensive algorithm for calculating the energy yield (EY) is used as the most important figure-of-merit in determining the capabilities of PV devices in realistic operation. The model is based on advanced optical modeling and extensive opto-thermo-electrical characterization. It takes the realistic environmental and device installation data fully into account, as well as the complete set of device specifications including all relevant temperature-induced variations. A detailed analysis and optimization of two-terminal perovskite-silicon tandem devices are done in terms of long-term electrical energy production at different geographical locations from distinct Köppen–Geiger–Photovoltaic climate zones (KGPV). It is shown that the optimal perovskite bandgap (E$_{G,PK}$) in a tandem device operating under realistic conditions is in all cases higher than the one optimized under STC, yet does not differ significantly from location to location, which is beneficial from the manufacturing point of view. The optimal E$_{G,PK}$ is influenced primarily by the spectral distribution of incident irradiance, and not so much by the operating temperature conditions. Moreover, a clear linear dependency between the optimal E$_{G,PK}$ and the weighted average photon energy of the incident irradiance is demonstrated, which presents an important rule for rapid tandem design.

We demonstrate a monolithic perovskite CIGS tandem solar cell with a certified power conversion efficiency PCE of 24.2 . The tandem solar cell still exhibits photocurrent mismatch between the subcells; thus optical simulations are used to determine the optimal device stack. Results reveal a high optical potential with the optimized device reaching a short circuit current density of 19.9 mA cm 2 and 32 PCE based on semiempirical material properties. To evaluate its energy yield, we first determine the CIGS temperature coefficient, which is at amp; 8722;0.38 K 1 notably higher than the one from the perovskite subcell amp; 8722;0.22 K 1 , favoring perovskite in the field operation at elevated cell temperatures. Both single junction cells, however, are significantly outperformed by the combined tandem device. The enhancement in energy output is more than 50 in the case of CIGS single junction device. The results demonstrate the high potential of perovskite CIGS tandem solar cells, for which we describe optical guidelines toward 30 PCE

A development and calibration of a detailed 3D coupled ray-wave optical model for accurate simulations of laterally varying photovoltaic structures is presented and applied for the analysis of interdigitated back contact cells. Four key aspects, which predominantly contribute to the accuracy of optical simulations are highlighted through the validation of the model. The applicability of the model is demonstrated through an example of electroluminescence simulations.

To develop efficient strategies for mitigating the elevated temperature-induced losses and improving the annual energy yield of solar cells and photovoltaic modules, thermal modelling is of utmost importance. In this contribution, we use rigorous Finite Element Method (FEM) simulations to investigate the steady-state spatial temperature distribution in commercial high-efficiency crystalline silicon PV modules, with particular focus aimed towards studying the impact of various influencing parameters. First, we investigate how heat conduction within an encapsulated solar cell operating at maximum power point is influenced by metallization and surface textures. Then, we study how the operating temperature is affected by the optical power density incident on the PV module and to what extent the natural convection, hence the cooling of the device, is influenced by changing the PV module inclination angle from 0° to 30°. Finally, the forced convection in form of wind is introduced. We demonstrate that forced convection has an even greater beneficial impact at higher wind speeds and larger PV module dimensions, since the transformation of laminar to turbulent wind flow that can occur above the surface of the module contributes to additional cooling.

Hierarchical textures are researched experimentally and by optical simulations in the role of external outcoupling solution for OLEDs. Their full outcoupling potential, limiting factors and pathways for further increase in outcoupling are addressed.

We perform rooftop testing of 6 perovskite solar cells. The devices are tracked under maximum power point and weather data is collected over several weeks. In addition, the changing irradiance and temperature conditions are also investigated under controlled laboratory conditions, where maximum power temperature coefficient of −0.17%/K is extracted. With the help of optical simulations and measured weather data, we are able to model the generated power of our devices. The temperature dependent energy yield can therefore be used to predict energy output or monitor the degradation of devices.

We show a monolithic perovskite/CIGS tandem solar cell with a certified PCE of 24.2% and a high V OC of 1.77 V, enabled by reduction of interface recombination. Using optical simulations, we further optimize the structure of the cell and compare it to perovskite/silicon tandem solar cells. The yearly energy yield of the perovskite/CIGS tandem is only 6% lower than that of the perovskite/silicon tandem, showing great potential of the perovskite/CIGS technology.

In the past decades MOS based digital integrated logic circuits have undergone a successful process of miniaturisation eventually leading to dimensions of a few nanometres. With the dimensions in the range of a few atomic radii the end of conventional MOS technology is approaching. Amongst the prospective candidates for sub 10nm logic are integrated logic circuits based on single-electron devices. In our contribution we present the use of MOSES (Monte-Carlo Single-Electronics Simulator) as a method for simulation of complementary single-electron logic circuits based on the orthodox theory. Simulations of single-electron devices including a single-electron box, a single-electron transistor and a complementary single-electron inverter were carried out. Their characteristics were evaluated at different temperatures and compared to measurement results obtained at other institutions. The potential for room-temperature operation was also assessed.

Nanostructures play an important role in state-of-the-art photovoltaic devices. Optical and electrical characteristics of the devices can be improved with properly designed and fabricated nanotextures. In this contribution, we highlight examples of possible fabrication of nanotextures by using UV nanoimprinted lithography, their optical characterization by camera based angular resolved spectroscopy and perform computer-assisted design of nanotextures for best optical performance of heterojunction silicon and ultra-thin chalcopyrite solar cells.

Light management using microtextured foils on the top of planar organic and perovskite solar cells was investigated in detail by means of numerical simulations. Different texture shapes and sizes were analyzed and optimized with respect to the device performance, both under direct and diffuse illumination conditions. Tetrahedral texture with the aspect ratio of 0.78 was indicated as the optimal, enabling J SC boosts of 26.2% and 16.3% in the selected organic and perovskite cells, respectively. Furthermore, light management mechanisms enabled by the microtextured foil were identified, and each of them was studied and evaluated independently. Among them, the ability to internally trap the unabsorbed reflected light coming from the back side of the cell was revealed as the dominant one. Finally, the optical losses associated with a limited area of realistic prototype solar cells enhanced with microtextured foils were investigated. Results showed that in the selected solar cells, an area at least five times larger than the 2 × 2 mm2 area of the incident light beam is required in order to sufficiently suppress these losses and enable error-free experimental characterization of such devices.

Accurate forecasting of yearly energy yield of photovoltaic devices under realistic operating conditions is key for numerous practical applications related to further evolution of photovoltaics. In this work we present development and validation of an energy yield model applied to planar perovskite solar cells. The model is based on the combination of advanced optical modelling and extensive opto-electrical characterization and takes the geographical location and the orientation of the device into account. Using the model, we first perform a detailed sensitivity analysis and indicate the key parameters and their interdependencies that most significantly affect the calculated energy yield values. We show that the results are dominated (within 4% of the final value) by the solar irradiance dependency of the short-circuit current density, and further by the temperature dependency of the open-circuit voltage and the fill factor, whereas the irradiance dependency of the open-circuit voltage and the fill factor are mutually compensated. In the second part of the paper, we investigate how the energy yield of the device is affected by the application of external light management foils. Different texture morphologies, geographical locations, and device orientations are included in the investigation. The results show that even under realistic operating conditions, the overall trends of the light management efficiency of textured foils are not that different from those observed under standard test conditions; the highest energy yield gains in the order of 7–11% are obtained by the tetrahedral texture which is indicated as optimal in all studied cases.

Efficient light management in monolithic perovskite/silicon tandem solar cells is one of the prerequisites for achieving high power conversion efficiencies (PCEs). Textured silicon wafers can be utilized for light management, however, this is typically not compatible with perovskite solution processing. Here, we instead employ a textured light management (LM) foil on the front-side of a tandem solar cell processed on a wafer with a planar front-side and textured back-side. This way the PCE of monolithic, 2-terminal perovskite/silicon-heterojunction tandem solar cells is significantly improved from 23.4% to 25.5%. Furthermore, we validate an advanced numerical model for our fabricated device and use it to optically optimize a number of device designs with textures at different interfaces with respect to the PCE and energy yield. These simulations predict a slightly lower optimal bandgap of the perovskite top cell in a textured device as compared to a flat one and demonstrate strong interdependency between the bandgap and the texture position in the monolithic stack. We estimate the PCE potential for the best performing both-side textured device to be 32.5% for a perovskite bandgap of 1.66 eV. Furthermore, the results show that under perpendicular illumination conditions, for optimized designs, the LM foil on top of the cell performs only slightly better than a flat anti-reflective coating. However, under diffuse illumination, the benefits of the LM foil are much greater. Finally, we calculate the energy yield for the different device designs, based on true weather data for three different locations throughout the year, taking direct as well as diffuse illumination fully into account. The results further confirm the benefits of front-side texture, even more for BIPV applications. Overall, devices built on a both-side textured silicon wafer perform best. However, we show that devices with textured LM foils on the cell's front-side are a highly efficient alternative.

We present an optical optimization on a red bottom-emitting OLED with an enhanced cavity in combination with external outcoupling textures – periodically positive and negative hexagonal dome shaped structures. A coupled optical modelling, combining simulations of coherent planar and incoherent structured layers, is verified and employed for optimization of the OLED structure and applied textures. Special attention is paid on the study of optical effects, related to the limited device area. Using optical modelling, we present and quantify main optical losses in the device. We show the importance of considering external outcoupling structures in optimization of thicknesses of organic films, which define the optical cavity. When optimized, an OLED with 50.5% light extraction efficiency (39.4% external quantum efficiency) and improved angular intensity distribution is predicted for devices with positive shaped dome structures. This approach allows for global efficiency optimization of complete OLEDs, comprisi...

Potential gains in short-circuit current density related to improvements in optical confinement in chalcopyrite based solar cells are studied and quantified by means of optical simulations. In the first part idealised optical conditions – anti-reflection at front interfaces, high reflection at back contact and light scattering - are introduced by simulating realistic scenarios of Cu(In, Ga)Se 2 (CIGS) solar cells with 2000 nm thick and 300 nm ultra-thin CIGS absorber, including the encapsulation at the front. For anti-reflection effect at front interfaces simulations revealed that in the photovoltaic module structure the most critical reflectance is the reflectance of the front surface of the protecting glass (possible 4.4% gain in short-circuit current density) and not the one at the front transparent conductive oxide contact, as in the case of non-encapsulated solar cell. Introduction of a highly reflective, highly diffusive back reflector is the most crucial point to improve the short-circuit current density of the ultra-thin devices. Potential for 15.8% gain in short-circuit current density related to ideal reflectance and additional 17.4% related to ideal scattering introduced at the back contact was revealed. A concrete example of light management structure was investigated in the second part by employing fully three-dimensional rigorous optical simulations. A semi-ellipsoidal texture was introduced to the substrate of the ultra-thin device. By using ZrN back reflector in simulations the gains in short-circuit current density related to the optimised size of the texture reach 10%, whereas in the case of an ideal highly reflective contact the gain is > 22% according to simulations.

Inorganic-organic perovskites like methylammonium-lead-iodide have proven to be an effective class of 17 materials for fabricating efficient solar cells. To improve their performance, light management techniques using textured surfaces, similar to those used in established solar cell technologies, should be considered. Here, we apply a light management foil created by UV nanoimprint lithography on the glass side of an inverted (p-i-n) perovskite solar cell with 16.3% efficiency. The obtained 1 mA cm(-2) increase in the short-circuit current density translates to a relative improvement in cell performance of 5%, which results in a power conversion efficiency of 17.1%. Optical 3D simulations based on experimentally obtained parameters were used to support the experimental findings. A good match between the simulated and experimental data was obtained, validating the model. Optical simulations reveal that the main improvement in device performance is due to a reduction in total reflection and that relative improvement in the short-circuit current density of up to 10% is possible for large-area devices. Therefore, our results present the potential of light management foils for improving the device performance of perovskite solar cells and pave the way for further use of optical simulations in the field of perovskite solar cells.

Hierarchical textures (combining 2D periodic large and small micro textures) as an external outcoupling solution for OLEDs have been researched, both experimentally and by optical simulations. For the case of a red bottom emitting OLED, different hierarchical textures were fabricated using laser-based methods and a replication step and applied to the OLED substrate, resulting in an increased light outcoupling. Laboratory-size OLED devices with applied textured foils show a smaller increase in efficiency compared to the final large area devices. The results show that the full exploitation of textured foils in laboratory-size samples is mainly limited by the lateral size of the thin film stack area and by limited light collection area of the measuring equipment. Modeling and simulations are used to further evaluate the full prospective of hierarchical textures in large area OLED devices. Optimization of hierarchical textures is done by simultaneously changing the aspect ratios of the small and large textures and a potential of 57% improvement in EQE compared to devices without applied textures is predicted by simulations. Optimized hierarchical textures show similar outcoupling efficiencies compared to optimized single textures, while on the other hand hierarchical textures require less pronounced features, lower aspect ratios, compared to single textures to achieve the same efficiencies. Hierarchical textures also help in eliminating flat parts that limit outcoupling efficiency. Finally, the limiting factors that prevent higher outcoupling are addressed. We show that the dominant factor is non-ideal reflection from the organic thin film stack due to parasitic absorption. In addition, possible ways to further increase the outcoupling from a thick substrate are indicated.

In ultra-thin chalcopyrite solar cells and photovoltaic modules, efficient light management is required to increase the photocurrent and to gain in conversion efficiency. In this work we employ optical modelling to investigate different optical approaches and quantify their potential improvements in the short-circuit current density of Cu(In, Ga)Se2 (CIGS) devices. For structures with an ultra-thin (500 nm) CIGS absorber, we study the improvements related to the introduction of (i) highly reflective metal back reflectors, (ii) internal nano-textures applied to the substrate and (iii) external micro-textures by using a light management foil. In the analysis we use CIGS devices in a PV module configuration, thus, solar cell structure including encapsulation and front glass. A thin Al2O3 layer was considered in the structure at the rear side of CIGS for passivation and diffusion barrier for metal reflectors. We show that not any individual aforementioned approach is sufficient to compensate for the short circuit drop related to ultra-thin absorber, but a combination of a highly reflective back contact and textures (internal or external) is needed to obtain and also exceed the short-circuit current density of a thick (1800 nm) CIGS absorber.

Numerical simulations were employed to investigate the impact of micro-textured light management foils applied on top of an example planar perovskite solar cell, both under direct and diffuse illumination conditions. A detailed optimization of texture features revealed that the optimized Tetrahedral texture enables the greatest enhancement of the device performance, reaching a 16.3 % boost of JSC compared to the non-textured cell. Further on, by studying the influence of the individual light management mechanisms, we observed that this enhancement is primarily enabled by efficient trapping of the light reflected from the back side of the cell and by redirecting it back towards the perovskite layer where it can be further absorbed. Therefore, this internal light trapping effect has been indicated as the dominant light management mechanism of the investigated micro-textured foils.

We present improvements in light outcoupling for the example of red, bottom-emitting, ITO free OLEDs. As an optimization tool we use experimentally verified coupled modelling approach, where we simulate a complete OLED device, including thin-film coherent stacks as well as thick microtextured incoherent layers (substrate). We calibrate the combined model on a fabricated small sample OLED. The research of lateral limitations and limited integrating sphere opening effects show that small area effects can lead to large deviations in outcoupling efficiency with respect to the large area devices commonly used in lighting applications. On the large area devices, we focus on the optimization of the thinfilm stack cavity in the OLED by tuning the thicknesses of thin layers. We show the importance of including the complete device in the optimization process, including the thin-film stack and the thick substrate with the outcoupling textures. We show that an OLED with an optimized planar cavity and applied external positive shaped dome texture can reach up to 50.5 % light extraction efficiency according to simulations.

A variety of light management structures have been introduced in solar cells to improve light harvesting and further boost their conversion efficiency. Reliable and accurate simulation tools are required to design and optimize the individual structures and complete devices. In the first part of this paper, we analyze the performance of rigorous coupled-wave analysis (RCWA) for accurate three-dimensional optical simulation of solar cells, in particular heterojunction silicon (HJ Si) solar cells. The structure of HJ Si solar cells consists of thin and thick layers, and additionally, micro- and nano-textures are also introduced to further exploit the potential of light trapping. The RCWA was tested on the front substructure of the solar cell, including the texture, thin passivation and contact layers. Inverted pyramidal textures of different sizes were included in the simulations. The simulations rapidly converge as long as the textures are small (in the (sub)micrometer range), while for larger microscale textures (feature sizes of a few micrometers), this is not the case. Small textures were optimized to decrease the reflectance, and consequently, increase the absorption in the active layers of the solar cell. Decreasing the flat parts of the texture was shown to improve performance. For simulations of structures with microtextures, and for simulations of complete HJ Si cells, we propose a coupled modeling approach (CMA), where the RCWA is coupled with raytracing and the transfer matrix method. By means of CMA and nanotexture optimization, we show the possible benefits of nanotextures at the front interface of HJ Si solar cells, demonstrating a 13.4% improvement in the short-circuit current density with respect to the flat cell and 1.4% with respect to the cell with double-sided random micropyramids. We additionally demonstrate the ability to simulate a combination of nano- and microtextures at a single interface, although the considered structure did not show an improvement over the pyramidal textures.

We present an optical model that was developed for the purpose of simulation and optimization of luminescent down-shifting layers based on phosphor particles. The model combines three-dimensional ray tracing with a novel effective scattering approach based on the Mie scattering theory. Verification of the model with experimental results shows that the model accurately takes various layer parameters into account, such as the layer thickness, the phosphor volume concentration, and the phosphor particle size distribution. Finally, using the verified model, we investigate the concept of luminescent down-shifting in organic solar cells and discuss the optimal layer parameters. Preliminary simulation results show that in the case of the studied solar cell, an improvement in the short-circuit current density of more than 6% can be obtained.

Optical modelling and simulations present an indispensable tool in the design, analysis and optimisation of thin-film solar cells of different technologies. In this paper highlights and challenges of different numerical modelling approaches are reviewed, from one-dimensional to rigorous two- or three-dimensional optical modelling. A concept of Coupled Modelling Approach (CMA) is proposed to be used, in which different optical models are coupled together to achieve best performance in speed and accuracy of simulations. A Combined Ray-Optics Wave-optics Model (CROWM) is presented as a simple example of the CMA. In the second part two examples of modelling and simulations are presented. To demonstrate the applicability of 3-D rigorous optical modelling, results of optimisation of periodic substrate surface texture in a micromorph silicon solar cell are shown. Furthermore, a model of non-conformal layer growth is employed to determine morphologies of internal interfaces and to make a selection of suitable textures for defect-less thin-film silicon layer growth. The CROWM simulator was employed to demonstrate the usability of coupled modelling on the example of optimisation of macro-textures in organic solar cells.

To improve light absorption in organic solar cells, microscale surface-textured light-management (LM) films are applied on top of the front glass substrate. In this study, numerical simulations are employed to determine the optimal texture of the LM films that would result in the highest short-circuit current density of the solar cells in perpendicular, as well as oblique, illumination conditions. Different types of 2-D periodic surface textures are analyzed (pyramidal, parabolic, sinusoidal), and the effects of the period and groove height sizes are investigated. Numerical simulations are based on a model that combines geometric optics and wave optics and, thus, enables simulation of light propagation through the thick microtextured LM film and glass, as well as thin layers of the device, respectively. Results show that parabolic textures are the most advantageous for the solar cells to achieve high performance operating in changing illumination conditions. When properly optimized, they enable over 14% boost of the short-circuit current density in a broad range of illumination incident angles, with the maximum of 22% for perpendicular incidence, with respect to that of the nontextured cell.

While p-n junction solar cells have long been established as the dominant solar-cell technology in the market, the origin of the charge-carrier separation in these devices remains open to debate. It is often attributed to the built-in electric field that exists across the junction in thermodynamic equilibrium, although this interpretation can lead to physical inconsistencies. In this work we present an interpretation approach based on the analogy between a solar cell and a generalized electric source model. Our interpretation is given through a detailed analysis of the electric potential and the non-electric chemical potential across each device, which are plotted together in complete potential diagrams introduced for this purpose. We demonstrate that the driving force separating the free charge carriers in both devices originates from the change of the non-electric chemical voltage that happens once the device is brought out of thermodynamic equilibrium. This change, therefore, can be interpreted as the driving force that triggers the selective motion of charge carriers and, thus, induces the electric voltage at the terminals of the device.While p-n junction solar cells have long been established as the dominant solar-cell technology in the market, the origin of the charge-carrier separation in these devices remains open to debate. It is often attributed to the built-in electric field that exists across the junction in thermodynamic equilibrium, although this interpretation can lead to physical inconsistencies. In this work we present an interpretation approach based on the analogy between a solar cell and a generalized electric source model. Our interpretation is given through a detailed analysis of the electric potential and the non-electric chemical potential across each device, which are plotted together in complete potential diagrams introduced for this purpose. We demonstrate that the driving force separating the free charge carriers in both devices originates from the change of the non-electric chemical voltage that happens once the device is brought out of thermodynamic equilibrium. This change, therefore, can be interpreted as the ...

The rear side of a silicon solar cell is often designed to minimize surface recombination, series resistance, and cost, but not necessarily parasitic absorption. We present a comprehensive study of parasitic absorption in the metal layer of solar cells with dielectric/metal rear reflectors. The sub-bandgap reflectance of a solar cell or test structure is proposed as an experimentally accessible probe of parasitic absorption, and it is correlated with short-circuit current density. The influence of surface texture, dielectric refractive index and thickness, and metal refractive index on sub-bandgap reflectance-and thus current is then both calculated and measured. From the results, we formulate design rules that promote optimum infrared response in a wide variety of silicon solar cells. (C) 2013 Elsevier B.V. All rights reserved.

The perovskite / silicon-heterojunction (SHJ) tandem solar cell can theoretically overcome the efficiencies of the single junction solar cells with experimental results not far behind. Here, we use optical simulations to analyze the potential of such currently feasible monolithic perovskite / SHJ tandem devices. We consider three different device designs in the optical simulations: planar device, device built on back-side textured Si wafer, and device with textured UV Nanoimprint Lithography light management foil. For each of these three designs, the current matching point is simulated to evaluate device efficiencies. While the device with back-side textured Si wafer causes light trapping and therefore enhanced photocurrent generation for longer wavelengths, the anti-reflection foils prove to be overall more efficient as the foil significantly reduces the reflection in almost full wavelength range.

Device modelling and characterization are indispensable tools in the design of photovoltaic devices. In the contribution we present two challenging issues related to accurate modelling and efficient characterization of light scattering at nanotextured interfaces or other nanophotonic structures used in solar cell technologies. The model based on finite element method, which is upgraded with the Huygens’ expansion theorem is presented. It enables to calculate the angular distribution function of scattered light in the near and far field. It accounts also for the antireflection effects originating from nanoroughnesses. To characterize scattered light efficiently a camera based angular resolved spectroscopy system is presented. It captures the spatial angular distribution function in broad angular range at one shot.

Absorption filters based on hydrogenated amorphous silicon–carbon alloys are developed for application as fluorescence filters in microarray and lab-on-a-chip systems. The carbon content of the thin film is varied by changing the amount of ethylene added to the ethylene/silane gas mixture during plasma-enhanced chemical vapour deposition. The optical properties of the films are characterised by transmittance measurements to obtain the refractive index, the optical bandgap, and the energy E 04 of the films as a function of their carbon content. A set of filters that is appropriate for the detection of naturally fluorescent biomolecules is indicated. The system is used to demonstrate the detection of one of these naturally fluorescent biomolecules, the reduced form of nicotinamide adenine dinucleotide (NADH).

The potential of three advanced optical concepts in chalcopyrite-based solar cells is investigated by means of simulations with realistic optical and electrical parameters of state-of-the-art CGS and CIGS cells. First, a monolithically stacked tandem CGS/CIGS structure is analysed, achieving efficiencies up to 20.3% under standard test conditions. Then, in order to reduce the effects of the parasitic sub-bandgap absorption, a wavelength-selective intermediate reflector is incorporated in the tandem, leading to efficiencies up to 20.8%. Finally, the concept of spectrum splitting is tested in a hybrid four-terminal configuration of dislocated CGS and CIGS cells, and efficiencies up to 22.5% are simulated. To indicate further possible improvements, all three concepts are also tested in optically and electrically idealised CGS/CIGS structures, indicating the potential of efficiencies up to 28%.

In this work, we investigate the concept of transparent conductive oxide (TCO)/white paint (WP) back reflectors as an alternative to the conventional textured metal-based reflectors in thin-film silicon solar cells. The optical characterisation results show that WP back reflectors exhibit good reflectivity and excellent light scattering properties which can lead to enhanced light confinement within the thin absorber layers of the cell. To confirm this, ZnO:Al/WP reflectors with WP films of different pigment volume concentration and thickness are tested in thin-film amorphous silicon solar cells. The external quantum efficiency measurements indicate that by employing ZnO:Al/WP reflectors, good solar cell performance can be achieved, which is comparable to the performance of the cell with the conventional TCO/Ag reflector. Finally, optical simulations are employed to study the potential for further improvements in cell performance as a result of the optimisation of TCO/WP reflectors. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

A microdevice based on a thin-film hydrogenated amorphous silicon (a -Si:H) p-i-n photodiode is used to quantify the density of DNA oligonucleotides labelled with a fluorophore and the density of E. coli cells expressing GFP (green fluorescent protein) by fluorescence. An absorption filter is integrated in the device which filters the excitation light and transmits the emitted light to the photodetector. This filter is an amorphous silicon carbon (a-SiC:H) alloy in which the bandgap can be selected according to the excitation and emission wavelengths of the fluorophores used by varying the carbon content of the film. Control of the thickness of the a-SiC:H film allows the modulation of the transmission ratio between the excitation wavelength and the emission wavelength. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Potential improvements in the performance of tandem amorphous silicon/microcrystalline silicon (a-Si:H/μc-Si:H) solar cells, related to the TCO superstrates with enhanced scattering properties are studied. In particular, optical effects of a high haze double textured (W-textured) SnO2:F TCO superstrate are analyzed and compared to the properties of the pyramidal type SnO2:F TCO superstrate. Solar cell with W-textured superstrate exhibits higher long-wavelength external quantum efficiency of the bottom μc-Si:H cell than the one with pyramidal type TCO superstrate. Optical simulations are employed to study the potential improvements of the solar cell performance if ideal haze parameter (H = 1) and/or a broad angular distribution function (Lambertian) of scattered light are applied to textured interfaces in the solar cell structure. Simulations reveal significant improvements in long-wavelength quantum efficiencies if a broad angular distribution function of scattered light is applied. Optical losses in the cells with enhanced scattering properties are analysed and evaluated in terms of short-circuit current losses in the supporting layers and losses due to reflected light.

We studied the optical properties of polymer layers filled with phosphor particles in two aspects. First, we used two different polymer binders with refractive indices n = 1.46 and n = 1.61 (λ = 600 nm) to decrease Δn with the phosphor particles (n = 1.81). Second, we prepared two particle size distributions D50 = 12 μm and D50 = 19 μm. The particles were dispersed in both polymer binders in several volume concentrations and coated onto glass with thicknesses of 150 - 600 μm. We present further a newly developed optical model for simulation and optimization of such luminescent down-shifting (LDS) layers. The model is developed within the ray tracing framework of the existing optical simulator CROWM (Combined Ray Optics / Wave Optics Model), which enables simulation of standalone LDS layers as well as complete solar cells (including thick and thin layers) enhanced by the LDS layers for an improved solar spectrum harvesting. Experimental results and numerical simulations show that the layers of the higher refractive index binder with larger particles result in the highest optical transmittance in the visible light spectrum. Finally we proved that scattering of the phosphor particles in the LDS layers may increase the overall light harvesting in the solar cell. We used numerical simulations to determine optimal layer composition for application in realistic thin-film photovoltaic devices. Surprisingly LDS layers with lower measured optical transmittance are more efficient when applied onto the solar cells due to graded refractive index and efficient light scattering. Therefore, our phosphor-filled LDS layers could possibly complement other light-coupling techniques in photovoltaics.

We developed an optical model for simulation and optimization of luminescent down-shifting (LDS) layers for photovoltaics. These layers consist of micron-sized phosphor particles embedded in a polymer binder. The model is based on ray tracing and employs an effective approach to scattering and photoluminescence modelling. Experimental verification of the model shows that the model accurately takes all the structural parameters and material properties of the LDS layers into account, including the layer thickness, phosphor particle volume concentration, and phosphor particle size distribution. Finally, using the verified model, complete organic solar cells on glass substrate covered with the LDS layers are simulated. Simulations reveal that an optimized LDS layer can result in more than 6% larger short-circuit current of the solar cell.

Efficient transparent light converters have received lately a growing interest from optical device industries (LEDs, PV, etc.). While organic luminescent dyes were tested in PV light-converting application, such restrictions as small Stokes shifts, short lifetimes, and relatively high costs must yet be overcome. Alternatively, use of phosphors in transparent matrix materials would mean a major breakthrough for this technology, as phosphors exhibit long-term stability and are widely available. For the fabrication of phosphor-filled layers tailored specifically for the desired application, it is of great importance to gain deep understanding of light propagation through the layers, including the detailed optical interplay between the phosphor particles and the matrix material. Our measurements show that absorption and luminescent behavior of the phosphors and especially the scattering of light by the phosphor particles play an important role. In this contribution we have investigated refractive index difference between transparent binder and phosphors. Commercially available highly luminescent UV and near-UV absorbing μm-sized powder is chosen for the fabrication of phosphor-filled layers with varied refractive index of transparent polymer matrix, and well-defined particle size distributions. Solution-processed thick layers on glass substrates are optically analyzed and compared with simulation results acquired from CROWM, a combined wave optics/ray optics home-built software. The results demonstrate the inter-dependence of the layer parameters, prove the importance of careful optimization steps required for fabrication of efficient light converting layers, and, thus, show a path into the future of this promising approach.

To enhance light harvesting in organic solar cells, micro-scale surface-textured plastic foils laminated to the front side of the cells present an efficient low-cost solution. In this work, numerical modelling based on a combined geometric-optics/wave-optics model is used to investigate the potential of different surface texture profiles of light-management foils and to determine the optimal texture parameters. Results indicate that properly optimized parabolic textures are most advantageous for this purpose and enable a more than 15% boost of the short-circuit current density in a broad range of illumination incident angles, with the peak of 22% at perpendicular incidence, compared to that of the nontextured solar cell.

Limitations in performance of single-junction thin film solar cells are reviewed. Conversion efficiency in single junction solar cells is systematically analyzed in terms of energy conversion efficiency, electrical efficiency and optical efficiency. The analysis reveals a strong dependence of limitations in single junction solar cells on the band-gap of the absorber. In the case of CIGS solar cells, the band gap can be varied from 1.04 ev to 1.7 eV, which allows considerable opportunity to optimize for both band gap and the associated temperature coefficient.

Surface textures in thin-film silicon multi-junction solar cells play an important role in gaining the photocurrent of the devices. In this paper, a design of the textures is carried out for the case of amorphous silicon/micro-crystalline silicon (a-Si:H/mu c-Si:H) solar cells, employing advanced modelling to determine the textures for defect-free silicon layer growth and to increase the photocurrent. A model of non-conformal layer growth and a hybrid optical modelling approach are used to perform realistic 3D simulations of the structures. The hybrid optical modelling includes rigorous modelling based on the finite element method and geometrical optics models. This enables us to examine the surface texture scaling from nano- to macro-sized (several tens or hundreds of micrometers) texturisation features. First, selected random and periodic nanotextures are examined with respect to critical positions of defect-region formation in Si layers. We show that despite careful selection of a well-suited semi-ellipsoidal periodic texture for defect-free layer growth, defective regions in Si layers of a-Si: H/mu c-Si: H cell cannot be avoided if the lateral and vertical dimensions of the nano features are optimised only for high gain in photocurrent. Macro features are favourable for defect-free layer growth, but do not render the photocurrent gains as achieved with light-scattering properties of the optimised nanotextures. Simulation results show that from the optical point of view the semi-ellipsoidal periodic nanotextures with lateral features smaller than 0.4 mu m and vertical peak-to-peak heights around or above 0.3 mu m are required to achieve a gain in short-circuit current of the top cell with respect to the state-of-the-art random texture (>16% increase), whereas lateral dimensions around 0.8 mu m and heights around 0.6 mu m lead to a > 6% gain in short-circuit current of the bottom cell.

Employment of advanced light management techniques presents an important aspect in the design of thin-film solar cells. In this chapter, we highlight a number of light management approaches leading towards higher cell performances. Efficient light scattering within the cell, which can boost the photocurrent generation, can be achieved by optimised surface textures. Random textures and periodic textures for efficient light scattering are addressed. Besides surface texturing, the role of metal nanoparticles in thin-film solar cell structures is investigated in the scope of improved light trapping. To minimise optical losses, antireflective coatings and advanced back reflectors are employed. As examples, photonic crystal structures and diffusive dielectric materials are presented. And finally, better utilisation of the solar spectrum can be achieved by multi-bandgap multi-junction cells. For efficient spectrum harvesting, a concept of wavelength-selective intermediate reflectors is investigated. Further on, a concept of spectrum splitting and dislocated cells connected in a multi-terminal configuration is presented.

Diffusive dielectric materials such as white paint have been demonstrated as effective back reflectors in the photovoltaic technology. In this work, a one-dimensional (1D) optical modeling approach for simulation of white paint films is developed and implemented in a 1D optical simulator for thin-film solar cells. The parameters of white paint, such as the paint film thickness, the pigment volume concentration (PVC), and the pigment/binder refractive index ratio (RIR), are examined and optimized to achieve the required optical properties for back reflector application. The simulation trends indicate that white paint back reflectors with sufficient film thickness and higher PVC and RIR values exhibit improved reflectivity characteristics which results in an increased long-wavelength quantum efficiency of thin-film silicon solar cells. The simulation results based on the 1D model agree very well with the experimental data obtained from reflectance measurements of various white paint compositions and quantum efficiency measurements of amorphous silicon solar cells with white paint back reflectors.

This publisher’s note amends the author list and Acknowledgments of [Opt. Mater. Express5, 1295 (2015]. The author names were corrected online as of June 24, 2015: https://www.osapublishing.org/ome/abstract.cfm?uri=ome-5-6-1296.

The detection of biomolecules based on fluorescence measurements is a powerful diagnostic tool for the acquisition of genetic, proteomic and cellular information. One key performance limiting factor remains the integrated optical filter, which is designed to reject strong excitation light while transmitting weak emission (fluorescent) light to the photodetector. Conventional filters have several disadvantages. For instance absorbing filters, like those made from amorphous silicon carbide, exhibit low rejection ratios, especially in the case of small Stokes' shift fluorophores (e.g. green fluorescent protein GFP with λ exc = 480 nm and λ em = 510 nm), whereas interference filters comprising many layers require complex fabrication. This paper describes an alternative solution based on dielectric diffraction gratings. These filters are not only highly efficient but require a smaller number of manufacturing steps. Using FEM-based optical modelling as a design optimization tool, three filtering concepts are explored: (i) a diffraction grating fabricated on the surface of an absorbing filter, (ii) a diffraction grating embedded in a host material with a low refractive index, and (iii) a combination of an embedded grating and an absorbing filter. Both concepts involving an embedded grating show high rejection ratios (over 100,000) for the case of GFP, but also high sensitivity to manufacturing errors and variations in the incident angle of the excitation light. Despite this, simulations show that a 60 times improvement in the rejection ratio relative to a conventional flat absorbing filter can be obtained using an optimized embedded diffraction grating fabricated on top of an absorbing filter.