The optical modelling for optimizing high-efficiency c-Si solar cells endowed with poly-SiO_x or poly-SiC_x carrier-selective passivating contacts (CSPCs) demands a thorough understanding of their optical properties, especially their absorption coefficient. Due to the mixed phase nature of these CSPCs, spectroscopic ellipsometry is unable to accurately detect the weak free carrier absorption (FCA) at long wavelengths. In this work, the absorption coefficient of doped poly-SiO_x and poly-SiC_x layers as function of oxygen and carbon content, respectively, was obtained for wavelengths (300–2000 nm) by means of two alternative techniques. The first approach, photothermal deflection spectroscopy (PDS), was used for layers grown on quartz substrates and is appealing from the point of view of sample fabrication. The second, a novel inverse modelling (IM) approach based on reflectance and transmittance measurements, was instead used for layers grown on textured c-Si wafer substrates to mimic symmetrical samples. Although the absorption coefficients obtained from these two techniques slightly differ due to the different used substrates, we could successfully measure weak FCA in our CSPCs layers. Using an in-house developed multi-optical regime simulator and comparing modelled reflectance and transmittance with measured counterparts from symmetrical samples, we confirmed that with increasing doping concentration FCA increases; and found that the absorption coefficients obtained from IM can now be used to perform optical simulations of these CSPCs in solar cells.

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The coming years will see humanity facing significant challenges to ensure its continued survival. The threat of global warming to humans and the environment – exacerbated by rapidly growing population and energy demand – requires quick and decisive actions. Among them, increasing the generation of electricity from renewable resources is paramount to mitigate the effects of climate change. Photovoltaic (PV) energy conversion can be one of main technologies that propels the transition from fossil fuels towards a more sustainable future. In recent years, the deployment of photovoltaic systems has increased at an astounding pace, with more than 100 GWp of power installed during each of the last three years. However, further expansion of PV installations cannot solely rely on increasing industrial production, but should also be supported by research aimed at increasing the efficiency of PV devices and reduce their manufacturing costs. One of the key aspects of photovoltaic energy conversion is absorption of light. By increasing the amount of solar energy that is absorbed inside PV devices, the efficiency of solar cells can be boosted. This is particularly true for thin-film structures, which due to their limited thickness struggle to effectively absorb photons. Light management indicates all the techniques aimed at maximising light absorption inside photovoltaic devices and is the main topic of this manuscript. The goal of this work is to investigate and optimise light management approaches – based on periodic structures – applied to different thin-film device technologies, and through this analysis provide guidelines for the design of photovoltaic devices and gain an insight into their optical performance. After describing the theoretical background in chapter 1 and the methodology in chapter 2, chapter 3 begins the study of light management approaches by investigating nanowire arrays applied to thin-film nano-crystalline silicon solar cells. A proof-of-concept device was manufactured to ensure the feasibility of the proposed novel approach. Then, simulations were used to optimise the nanowire array structure. Results showed the good anti-reflective and scattering properties of nanowires, which are able to significantly boost absorption in the nano-crystalline silicon active layer. In chapter 4, the analysis shifts to periodic metasurfaces and the achievement of perfect absorption in amorphous silicon solar cells. By tuning the size and arrangement of the dielectric nanostructures that make up the metasurface, near 100% absorption can be achieved in the spectral region where amorphous silicon struggles to efficiently absorb incident photons. With respect to flat devices the performance is significantly increased, despite a reduction of used material of more than 50%. In chapter 5, a thorough investigation of periodic gratings for Cu(In,Ga)Se2 solar cells (CIGS) is carried out, complete with the selection of appropriate supporting materials to reduce the device thickness with a minimal sacrifice in performance. The accuracy of 3-dimensional rigorous modelling in predicting the performance of real CIGS devices was demonstrated for the first time. Then, a full study of 1-D and 2-D gratings was conducted – together with an analysis of device architectures that employ more transparent materials at the front and highly reflective metals at the back side. Results showed a marked increase in light absorption, mostly owing to a lower device reflectance and to reduced parasitic absorption in all supporting layers. The high optical performance was maintained when the thickness of the CIGS layer was reduced by 60%, which is crucial to reduce the utilisation of indium in the device and of the costs associated with it. In chapter 6, the concept of front/back pyramidal textures with different geometries is introduced and fully explored. Its application to (nano-)crystalline silicon absorbers or to supporting layers is compared, showing a preference for the former. After careful study of the decoupled texture geometry, an optimised optical performance beyond the traditional Lambertian scattering limit was achieved. In chapter 7 the concept of double front and back textures is analysed further, by applying it to cheap and abundant barium silicide (BaSi2). The optical potential of of this novel PV material was first characterised with spectroscopy measurements, then assessed in both single- and multi-junction configurations with the aid of rigorous optical simulations. Results showed that BaSi2 outperforms thin-film silicon absorbers, owing to its higher absorption coefficient. This highlights the promise of this novel material, which can be an ideal candidate for both single- and double-junction thin-film devices.@en

Background: Elongated nanostructures, such as nanowires, have attracted significant attention for application in silicon-based solar cells. The high aspect ratio and characteristic radial junction configuration can lead to higher device performance, by increasing light absorption and, at the same time, improving the collection efficiency of photo-generated charge carriers. This work investigates the performance of ultra-thin solar cells characterised by nanowire arrays on a crystalline silicon bulk. Results: Proof-of-concept devices on a p-type mono-crystalline silicon wafer were manufactured and compared to flat references, showing improved absorption of light, while the final 11.8% (best-device) efficiency was hindered by sub-optimal passivation of the nanowire array. A modelling analysis of the optical performance of the proposed solar cell architecture was also carried out. Results showed that nanowires act as resonators, amplifying interference resonances and exciting additional wave-guided modes. The optimisation of the array geometrical dimensions highlighted a strong dependence of absorption on the nanowire cross section, a weaker effect of the nanowire height and good resilience for angles of incidence of light up to 60°. Conclusion: The presence of a nanowire array increases the optical performance of ultra-thin crystalline silicon solar cells in a wide range of illumination conditions, by exciting resonances inside the absorber layer. However, passivation of nanowires is critical to further improve the efficiency of such devices.

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The high-index all-dielectric nanoantenna system is a platform recently used for multiple applications, from metalenses to light management. These systems usually exhibit low absorption/scattering ratios and are not efficient photon harvesters. Nevertheless, by exploiting far-field interference, all-dielectric nanostructures can be engineered to achieve near-perfect absorption in specific wavelength ranges. Here, we propose – based on electrodynamics simulations – that a metasurface composed of an array of hydrogenated amorphous silicon nanoparticles on a mirror can achieve nearly complete light absorption close to the bandgap. We apply this concept to a realistic device, predicting a boost of optical performance of thin-film solar cells made of such nanostructures. In the proposed device, high-index dielectric nanoparticles act not only as nanoatennas able to concentrate light but also as the solar cell active medium, contacted at its top and bottom by transparent electrodes. By optimization of the exact geometrical parameters, we predict a system that could achieve initial conversion efficiency values well beyond 9% – using only the equivalent of a 75-nm thick active material. The device absorption enhancement is 50% compared to an unstructured device in the 400 nm − 550 nm range and more than 300% in the 650 nm − 700 nm spectral region. We demonstrate that such large values are related to the metasurface properties and to the perfect absorption mechanism.

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Periodic texturing is one of the main techniques to enhance light absorption in thin-film solar cells. The presence of periodicity, such as grating, allows the excitation of guided modes in the structure, thus enhancing absorption. However, grating efficiency in exciting guided modes is highly dependent on the wavelength and incident angle of light. This is relevant especially in solar cells application, where the light source - the sun - is broadband and largely angle-dependent. Nevertheless, most of literature only focuses on the frequency response of periodic texturing, thus neglecting the effect of angular movement of the sun. In this work we use Fourier expansion to calculate the absorption of each type of mode (guided and non-guided) in an absorptive periodic waveguide. The structure is illuminated with TM and TE polarized light and under three different incident angles. Using this method, we are able to calculate the contribution of a guided resonance to total absorption for different angles of incidence. The work here developed and supported by rigorous numerical calculations can be used to better understand light propagation in a periodic waveguide structure, such as thin-film solar cells.

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Surface texturing is one of the main techniques to enhance light absorption in solar cells. In thin film devices, periodic texturing can be used to excite the guided resonances supported by the structure. Therefore, total absorption is enhanced largely due to the excitation of these resonances. Although the maximum absorption enhancement limit in both bulk and photonic structures is known already, the weight of each resonance type in this limit is not yet clear. In this contribution, we extend the temporal couple-mode theory, deriving a closed formula to distinguish the contribution of Fabry-Perot and wave-guided modes within the absorption limit for 1-D grating structures. Secondly, using this analytical approach, we can clearly address cases of bulk and thin absorber thicknesses. Our results, supported by rigorous electromagnetic calculation, show that absorption enhancement in a 1-D grating structure can be much higher than the nano-photonic limit (2πn) reported by Yu et al. Thirdly, beyond the framework put forward by Yu et al., we extended our theory to describe the absorption enhancement in double side textured slabs. We have found that when the periods of top and bottom gratings are aliquant, absorption is enhanced in a wider frequency range. We provide rigorous numerical calculations to support our theoretical approach.

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Total electric field in a periodic thin-film structure is described by its Fourier coefficients. These coefficients can be used to calculate the share of different resonances in total absorption in the structure.@en

We present a modelling study of thin silicon based solar cells endowed with periodic and decoupled front/back textures. After careful optimization, the proposed device models exhibit absorption beyond the Lambertian light trapping limit for a wide range of light angles of incidence. The advanced light management scheme is applied to (nano)crystalline silicon solar cells, where the benefits of texturing the absorber rather than the supporting layers is clear and to barium (di)silicide solar cells, which could achieve an implied photocurrent densityof41.1 mA/cm2 for a thickness of only 2 \mum.

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We present the study of an advanced light trapping scheme applied to thin-film silicon-based solar cells, overcoming the broadband Green absorption limit, that is the generalized case of the 4n² classical absorption limit for all wavelengths. This result is achieved by the 3-dimensional optical modelling of a fully functional thin-film hydrogenated nano-crystalline silicon (nc-Si:H) solar cell endowed with decoupled front and back textures. Our results stem from rigorously characterized optical properties of state-of-the-art materials, optimized geometric nano-features on the front and rear surfaces of the solar cell, and thickness optimization of the front transparent oxide. The simulated improvements derive from a gain in light absorption, especially in the near-infrared part of the spectrum close to the band gap of nc-Si:H. In this wavelength region, the material is weakly absorbing, whereas we now find significant absorptance peaks that can only be explained by the concurrent excitation of guided resonances by front and rear textures. This insight indicates the need to modify the temporal coupled-mode theory, which fails to predict the absorption enhancement achieved in this work, extending its validity to the case of decoupled front/back texturing. Our approach results in substantially high photocurrent density (>36 mA/cm²), creating a platform suitable for high efficiency single and multi-junction thin-film solar cells based either on typical silicon alloys or on the novel and promising barium (di)silicide (BaSi₂) absorber. In the latter case, using the same advanced light trapping employed for nc-Si:H, we demonstrate a very high implied photocurrent density of 41.1 mA/cm², for a device endowed with 2-μm thick absorber.

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Barium disilicide (BaSi2) has been regarded as a promising absorber material for high-efficiency thin-film solar cells. However, it has confronted issues related to material synthesis and quality control. Here, we fabricate BaSi2 thin films via an industrially applicable sputtering process and uncovered the mechanism of structure transformation. Polycrystalline BaSi2 thin films are obtained through the sputtering process followed by a postannealing treatment. The crystalline quality and phase composition of sputtered BaSi2 are characterized by Raman spectroscopy and X-ray diffraction (XRD). A higher annealing temperature can promote crystallization of BaSi2, but also causes an intensive surface oxidation and BaSi2/SiO2 interfacial diffusion. As a consequence, an inhomogeneous and layered structure of BaSi2 is revealed by Auger electron spectroscopy (AES) and transmission electron microscopy (TEM). The thick oxide layer in such an inhomogeneous structure hinders further both optical and electrical characterizations of sputtered BaSi2. The structural transformation process of sputtered BaSi2 films then is studied by the Raman depth-profiling method, and all of the above observations come to an oxidation-induced structure transformation mechanism. It interprets interfacial phenomena including surface oxidation and BaSi2/SiO2 interdiffusion, which lead to the inhomogeneous and layered structure of sputtered BaSi2. The mechanism can also be extended to epitaxial and evaporated BaSi2 films. In addition, a glimpse toward future developments in both material and device levels is presented. Such fundamental knowledge on structural transformations and complex interfacial activities is significant for further quality control and interface engineering on BaSi2 films toward high-efficiency solar cells.@en

Periodic texturing is one of the main techniques for light-trapping in thin-film solar cells. Periodicity allows for the excitation of guided modes in the structure and, thus, largely enhances absorption. Understanding how much a guided resonance can increase the absorption is therefore of great importance. There is a common method to understand if an absorption peak is due to the excitation of a guided mode, using dispersion diagrams. In such graphs, a resonance is identified as the intersection of a guided-mode-line of a uniform waveguide (with the same optical thickness as the grating structure) with the center of a Brillouin zone of the grating. This method is unfortunately not reliable when the grating height is comparable with the thickness of the wave-guide, or when the thickness of the wave-guide is much larger than the wavelength. In this work, we provide a novel approach to calculate the contribution of a guided resonance to the total absorption in a periodic waveguide, without using the dispersion diagram. In this method, the total electric field in the periodic structure is described by its spatial frequencies, using a Fourier expansion. Fourier coefficients of the electric field were used to calculate the absorption of each diffraction order of the grating. Rigorous numerical calculations are provided to support our theoretical approach. This work paves the way for a deeper understanding of light behavior inside a periodic structure and, consequently, for developing more efficient light-trapping techniques for solar cells applications.@en

We present an optical investigation of nanopillar thin-film solar cells based on amorphous silicon, showing implied photocurrent density values > 14 mA/cm² for a volumetric equivalent thicknesses of 85 nm.

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We present the optical investigation of a novel back-contacted architecture for solar cells based on a thin barium (di)silicide (BaSi₂) absorber. First, through the analysis of absorption limits of different semiconducting materials, we show the potential of BaSi₂ for photovoltaic applications. Then, the proposed back contacted BaSi₂ solar cell design is investigated and optimized. An implied photocurrent density of 40.3 mA/cm² in a 1-μm thick absorber was achieved, paving the way for novel BaSi₂-based thin-film solar cells.

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The optical analysis of optically-textured and electrically-flat ultra-thin crystalline silicon (c-Si) slabs is presented. These slabs were endowed with decoupled front titanium-dioxide (TiO2) / back silicon-dioxide (SiO2) dielectric textures and were studied as function of two types of back reflectors: standard silver (Ag) and dielectric modulated distributed Bragg reflector (MDBR). The optical performance of such systems was compared to that of state-of-the-art flat c-Si slabs endowed with so-called front Mie resonators and to those of similar optical systems still endowed with the same back reflectors and decoupled front/back texturing but based on textured c-Si and dielectric coatings (front TiO2 and back SiO2). Our optimized front dielectric textured design on 2-µm thick flat c-Si slab with MDBR resulted in more photo-generated current density in c-Si with respect to the same optical system but featuring state-of-the-art Mie resonators ( + 6.4%), mainly due to an improved light in-coupling between 400 and 700 nm and light scattering between 700 and 1050 nm. On the other hand, the adoption of textured dielectric layers resulted in less photo-generated current density in c-Si up to −20.6% with respect to textured c-Si, depending on the type of back reflector taken into account.@en

We have used 3-D optical modelling to investigate light management concepts based on periodic textures and material optimization for photovoltaic devices based on Cu(In,Ga)Se2 (CIGS) absorber material. At first, calibration of the software based on the characterization of a reference (1500-nm thick) CIGS device was carried out. The effects of 1-D and 2-D symmetric gratings on the cell were then investigated, showing significant improvement in anti-reflection effect and in absorptance in the active layer, achieved by excitation of guided modes in the absorber. In addition, device configurations endowed with alternative back reflector and front transparent conductive oxide (TCO) were tested with the goal to quench parasitic absorption losses at front and back side. The use of In2O3:H (IOH) as front and back TCO, combined with an optimized 2-D grating structure, led to a 25% increase of the optical performance with respect to an equally-thick flat device. Most of the performance increase was kept when the absorber thickness was reduced from 1500 nm to 600 nm.@en

Barium di-silicide (BaSi2) is an abundant and inexpensive semiconductor with appealing opto-electrical properties. In this work we show that a 2-μm thick BaSi2-based thin-film solar cell can exhibit an implied photo-current density equal to 41.1 mA/cm2, which is higher than that of a state-of-the-art wafer-based c-Si hetero-junction solar cell. This performance makes BaSi2 an attractive absorber for high-performing thin-film and multi-junction solar cells. In particular, to assess the potential of barium di-silicide, we propose a thin-film double-junction solar cell based on organometallic halide perovskite (CH3NH3PbI3) as top absorber and BaSi2 as bottom absorber. The resulting modelled ultra-thin double-junction CH3NH3PbI3 / BaSi2 (< 2 μm) exhibits an implied total photo-current density equal to 38.65 mA/cm2 (19.84 mA/cm2 top cell, 18.81 mA/cm2 bottom cell) and conversion efficiencies up to 28%. © (2016) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.@en

We present an investigation of the optical performance of back contacted BaSi2 solar cells. We determine the optimal geometrical configuration and analyze benefits and drawback of such architectures.@en