Structured surfaces are used to reduce reflection and enhance light-trapping in silicon solar cells. In this simulation study, we investigated the relationship between the refractive index of front-side coupling structures on top of planar wafer-based crystalline silicon solar cells and the light-trapping performance of the structures. A crossed diffraction grating with a period of 1 μm and random pyramid structures with varying refractive indices were considered. Simulations were carried out both at the cell level and at the complete module stack level. It is shown that the single pass light path enhancement factor (LPEF) only provides a rough estimate of the light-trapping properties. The light-trapping behavior can only be reliably assessed in the complete system level and these results deviate from the estimated single pass LPEF. It can also be shown that the refractive index of the structure strongly influences the light-trapping behavior.
Structured interfaces to enhance light trapping are standard concepts for silicon solar cells. Within this simulation study we investigated the influence of the refractive index of front side coupling structures on top of a crystalline silicon solar cell on the light trapping performance. Simulations were carried out both at cell level and for the complete module stack. The light trapping behavior can only be reliably assessed if the complete system is investigated. It could be shown that the refractive index of the light trapping structure strongly influences the light trapping behavior.
Jan Christoph Goldschmidt, Patricia S. Schulze, Alexander Bett, Özde Kabakli, Kristina Winkler, Ludmila Cojocaru, Martin Bivour, Benedikt Bläsi, Hubert Hauser, Clarissa Hofmann, Armin Richter, Nico Tucher, Leonard Tutsch, Qinxin Zhang, Martin Hermle, Stefan Glunz
KEYWORDS: Perovskite, Silicon, Tandem solar cells, Solar energy, Energy efficiency, Solar cells, Semiconducting wafers, Silicon solar cells, Photovoltaics, Reflection
Perovskite silicon tandem solar cells can exceed the efficiency limit of 29.4% of single junction silicon solar cells, with comparably low additional costs for depositing the layers for the perovskite solar cell. Hence, they are an attractive option to further decrease the costs of photovoltaic electricity generation.
We present, how by optimizing perovskite absorber composition, choosing adequate carrier selective contact layers, introducing surface passivation and optimizing the individual layer thicknesses solar cell efficiencies above 25% can be realized experimentally.
Furthermore, we discuss different options for reducing front surface reflection by anti-reflection coatings, structured foils and deposition on textured silicon wafers and their impact both on solar cell efficiency as well as on the yearly energy yield. Deposition on textured silicon wafers promises highest energy yield. Hence, we show how perovskite absorbers can be deposited on such substrates by either co-evaporation or hybrid processing combining evaporation and subsequent wet-chemical processing.
By bottom up cost calculations we finally show how and under which conditions perovskite silicon tandem solar cells can yield an economic advantage.
Single-junction Si solar cell efficiencies are intrinsically limited to 29.4%. One common strategy to overcome this fundamental issue is to combine multiple semiconductor materials with different bandgaps in a multi-junction configuration so that light is effectively absorbed over a broad range in the solar spectrum. In particular, the combination of a III-V top cell (GaInP/GaAs) that is wafer-bonded to a planar Si bottom cell led recently to an overall record efficiency of 34.1%. The efficiency of this tandem design could be further improved if absorption in the Si cell near the bandgap (1000-1200 nm) is enhanced.
Here, we present a nanostructured metallodielectric back reflector placed at the rear of the Si cell that selectively steers incoming light to angles outside the escape cone of the tandem cell. The design is composed of a hexagonal array of Ag nanodisks embedded in a PMMA layer at the rear of the Si cell. Using finite-difference time-domain simulations we optimize pitch, radius, and height of the individual Ag scatterer such that we evenly distribute scattered power over the different diffraction orders. We analyze the scattering behavior in terms of plasmon scattering by the Ag disks and Mie scattering in the dielectric PMMA inclusions. To fully optimize light trapping inside the cell, we choose the geometry such that both 0th-order reflection and plasmonic losses in the Ag nanodisks are minimized.
We experimentally demonstrate photonic light trapping by fabricating large scale (2.5×2.5 cm) nanopatterns on untextured Si solar cells. Large-area patterning is performed via Substrate Conformal Imprint Lithography (SCIL) using silica sol-gel as a mask to etch patterns in PMMA, followed by thermal evaporation of Ag. Cross-section SEM shows excellent conformal deposition of Ag inside the patterned nanoholes. Light scattering spectroscopy shows a clearly reduced reflection of the Si cell in the desired wavelength range (1000-1200 nm) due to light trapping, in agreement with simulations. Experimental data of the full nanopatterned III-V/Si tandem geometry will be shown.
KEYWORDS: Silicon, Solar cells, Silver, External quantum efficiency, Tandem solar cells, Etching, Nanoimprint lithography, Scattering, Silicon photonics, Silicon solar cells
Different photonic light trapping structures realized by a combination of interference- and nanoimprint-lithography as well as based on self-organization processes are presented. Their potential as rear side light trapping structures for silicon based tandem solar cells is evaluated based on the comparison of EQE measurements and optical modeling. The photonic structure used in the current world record III-V silicon tandem solar cell is a metallic crossed grating with 1μm period. This structure is shown in detail and acts as benchmark for the comparison of the concepts. Finally, the requirements for a successful implementation of photonic structures in highest efficiency solar cells are shown.
Silicon based multi-junction solar cells are a promising option to overcome the theoretical efficiency limit of a silicon solar cell (29.4%). With III-V semiconductors, high bandgap materials applicable for top cells are available. For the application of such silicon based multi-junction devices, a full integration of all solar cell layers in one 2-terminal device is of great advantage. We realized a triple-junction device by wafer-bonding two III-V-based top cells onto the silicon bottom cell. However, in such a series connected solar cell system, the currents of all sub-cells need to be matched in order to achieve highest efficiencies. To fulfil the current matching condition and maximise the power output, photonic structures were investigated. The reference system without photonic structures, a triple-junction cell with identical GaInP/GaAs top cells, suffered from a current limitation by the weakly absorbing indirect semiconductor silicon bottom cell. Therefore rear side diffraction gratings manufactured by nanoimprint lithography were implemented to trap the infrared light and boost the solar cell current by more than 1 mA/cm2. Since planar passivated surfaces with an additional photonic structure (i.e. electrically planar but optically structured) were used, the optical gain could be realized without deterioration of the electrical cell properties, leading to a strong efficiency increase of 1.9% absolute. With this technology, an efficiency of 33.3% could be achieved.
KEYWORDS: Perovskite, Silicon, Tandem solar cells, Absorption, Solar cells, Reflectivity, Interfaces, Systems modeling, Silicon solar cells, Optical properties
Perovskite silicon tandem solar cells can overcome the efficiency limit of single junction silicon solar cells. Optical modeling plays a crucial role for the device optimization but it becomes complex if optically thin and thick layers as well as interface textures, such as random pyramids, are involved. Within this work, the OPTOS simulation formalism is applied in order to compare perovskite silicon tandem solar cells with planar and textured front side. Modeling the configuration with planar front side and textured rear exhibits a matched photocurrent density of 18.2 mA/cm2. For the system with textured front and planar rear side the reduced reflectance leads to a photocurrent density of 19.6 mA/cm2 although parasitic absorption in the Spiro-OMeTAD and ITO layers increases. Taking into account the full module stack in the OPTOS simulation shows an increased front side reflectance and parasitic absorption in the EVA. The difference between the resulting photocurrent densities (17.8 mA/cm2 and 18.9 mA/cm2) demonstrates the optical superiority of the investigated system with textured front side not only at cell level but also in the full module stack.
Silicon solar cells are typically textured by means of wet chemical etching in order to enhance absorption. Within this work, we apply an optically functional layer onto a planar silicon surface. This layer is made of a high refractive index sol-gel material and can be patterned by nanoimprint lithography (NIL). In first experiments, we investigated various sol-gel based TiO2 precursors and evaluated their refractive index as well as the possibility to apply them in NIL. The refractive index was determined to be up to 2.25 using ellipsometry. This result was achieved with a solution composed of amorphous TiO2 precursors mixed with ethanol and 1,5-pentanediol. The topography of the patterned TiO2 layers were investigated using an atomic force microscope (AFM) and a scanning electron microscope (SEM) revealing a period of 1 μm and a pattern depth of 60 nm after sintering. Furthermore, optical modeling was used to optimize the structure parameters in order to minimize the weighted reflectance of an encapsulated silicon solar cell.
Diffractive optical elements (DOEs) are widely used in various applications such as material processing, illumination, medical, and sensor applications by providing a shape on demand of laser beams. In contrast to refractive optical elements, the effect of DOEs is based on modulating the phase of the beam locally. This creates an interference pattern of the beam. The more height levels are implemented in a DOE, the higher its diffraction efficiency.
Rapid fabrication and testing in practice of new designs is desirable to shorten the prototyping development cycle of DOEs. High Precision 3D Printing via a two-photon absorption (TPA) process initiating a polymerization reaction allows manufacturing of virtually any 3D-shaped object, thus being the technique of choice to fabricate DOEs at high precision with an arbitrary number of levels within short time periods.
We demonstrate the use of High Precision 3D Printing to fabricate DOEs to be used as beam-shaping and beam-splitting elements, respectively. Different exposure strategies in polymer-like materials are used to fabricate DOEs which have significant impact on the fabrication time. The quality of the fabricated DOEs will be assessed by a variety of characterization methods such as metrology investigations for determination of the surface quality (for example, shape deviations, roughness), AFM, and optical characterization. The impact of different exposure strategies on the final DOEs will be presented and discussed.
Micro- and nanostructures can be used for reflectance reduction or light guidance in applications like photovoltaic solar cells, LEDs or display technology. The combination of interference lithography and nanoimprint lithography enables the fabrication and replication of high resolution structures on large areas. The origination of master structures, seamlessly patterned on areas as large as 1.2 × 1.2 m2 was shown using interference lithography. Within this work we demonstrate our current results on the up-scaling of the replication process chain based on nanoimprint lithography with in-line capable tools. Application examples in the fields of photovoltaics are demonstrated, e.g. the micron-scale patterning of multicrystalline silicon substrates to increase the solar cell efficiency. Furthermore, the lifetime of soft PDMS stamps is investigated. AFM force-distance measurements are introduced as suitable method to quantify the PDMS hardness as a parameter indicating stamp degradation. This technique is subsequently applied to evaluate two different resist materials. Applying the epoxy material (SU-8) with its more complex molecular structure results in a strongly increased stamp lifetime compared to the acrylate resist (Laromer LR 8996). This is a highly valuable result for further developments towards an up-scaled realization of nanoimprint lithography.
We investigate the integration of Al nanoparticle arrays into the anti-reflection coatings (ARCs) of commercial triple-junction GaInP/ In0.01GaAs /Ge space solar cells, and study their effect on the radiation-hardness. It is postulated that the presence of nanoparticle arrays can improve the radiation-hardness of space solar cells by scattering incident photons obliquely into the device, causing charger carriers to be photogenerated closer to the junction, and hence improving the carrier collection efficiency in the irradiation-damaged subcells. The Al nanoparticle arrays were successfully embedded in the ARCs, over large areas, using nanoimprint lithography: a replication technique with the potential for high throughput and low cost. Irradiation testing showed that the presence of the nanoparticles did not improve the radiation-hardness of the solar cells, so the investigated structure has proven not to be ideal in this context. Nonetheless, this paper reports on the details and results of the nanofabrication to inform about future integration of alternative light-scattering structures into multi-junction solar cells or other optoelectronic devices.
Surface textures can significantly improve anti-reflective and light trapping properties of silicon solar cells. Combining standard pyramidal front side textures with scattering or diffractive rear side textures has the potential to further increase the light path length inside the silicon and thereby increase the solar cell efficiency. In this work we introduce the OPTOS (Optical Properties of Textured Optical Sheets) simulation formalism and apply it to the modelling of silicon solar cells with different surface textures at front and rear side. OPTOS is a matrix-based method that allows for the computationally-efficient calculation of non-coherent light propagation within textured solar cells, featuring multiple textures that may operate in different optical regimes. After calculating redistribution matrices for each individual surface texture with the most appropriate technique, optical properties like angle dependent reflectance, transmittance or absorptance can be determined via matrix multiplications. Using OPTOS, we demonstrate for example that the integration of a diffractive grating at the rear side of solar cells with random pyramids at the front results in an absorptance gain that corresponds to a photocurrent density enhancement of 0.73 mA/cm2 for a 250 μm thick cell. The re-usability of matrices enables the investigation of different solar cell thicknesses within minutes. For thicknesses down to 50 μm the simulated gain increases up to 1.22 mA/cm2. The OPTOS formalism is furthermore not restricted with respect to the number of textured interfaces. By combining two or more textured sheets to effective interfaces, it is possible to optically model a complete photovoltaic module including EVA and potentially textured glass layers with one calculation tool.
After more than 20 years of research on rear side gratings for light trapping in solar cells, we have been able to demonstrate enhanced efficiencies for crystalline silicon solar cells with two different grating concepts and solar cell architectures. In both cases planar front sides have been used. With hexagonal sphere gratings and the tunnel oxide passivated contact (TOPCon) concept, a grating induced Jsc increase of 1.4 mA/cm2 and an efficiency increase of 0.8%absolute could be achieved. With binary crossed gratings fabricated by a nanoimprint based process chain, a grating induced Jsc gain of 1.2 mA/cm2 and an efficiency gain of 0.7% absolute could be achieved. For the binary grating concept, cell thickness variations have also been performed. The increasing importance of the light trapping properties towards low solar cell thicknesses could be confirmed by an enhanced EQE in the long wavelength region (Jsc increase: 1.6 mA/cm2 for 150 μm and 1.8 mA/cm2 for 100 μm thick solar cells). The results are in very good agreement with simulations using the OPTOS modeling formalism. OPTOS enables the further analysis and optimization of grating concepts in silicon solar cells and modules. So a grating induced Jsc gain of 0.8 mA/cm2 is forecast for solar cells with pyramidal front side texture. On module level, still a grating induced Jsc gain of 0.6 mA/cm2 can be expected.
Interference lithography (IL) is the best suited technology for the origination of large area master structures with high resolution. In prior works, we seamlessly pattern areas of up to 1.2 x 1.2 m2 with periodic features, i.e. a diffraction grating with a period in the micron range. For this process we use an argon ion laser emitting at 363.8 nm. Thus, feasible periods are in the range of 100 μm to 200 nm. Edge-defined techniques or also called (self-aligned) double patterning processes can be used to double the spatial frequency of such structures. This way, we aim to reduce achievable periods further down to 100 nm. In order to replicate master structures, we make use of nanoimprint lithography (NIL) processes. In this work, we present results using IL as mastering and NIL as replication technology in the fields of photovoltaics as well as display and lighting applications. In photovoltaics different concepts like the micron-scale patterning of the front side as well as the realization of rear side diffraction gratings are presented. The benefit for each is shown on final device level. In the context of display and lighting applications, we realized various structures ranging from designed, symmetric or asymmetric, diffusers, antireflective and/or antiglare structures, polarization optical elements (wire grid polarizers), light guidance and light outcoupling structures.
A large variety of optical systems incorporate multiple textured surfaces for reflectance reduction, light redirection or absorptance enhancement. One example for such a system is a textured silicon wafer solar cell. We introduce the OPTOS (Optical Properties of Textured Optical Sheets) formalism for the modelling of light propagation and absorption in optically thick sheets with two arbitrary surface textures at the front and rear side, and demonstrate applications.
In contrast to many optical simulation techniques, which are tailored to specific surface morphologies, the OPTOS formalism is a matrix-based method that allows including textures that are described by different optical modelling techniques (e.g. ray optical or wave optical) within one simulation tool. It offers the computationally efficient simulation of light redistribution and non-coherent propagation inside thick sheets. After calculating redistribution matrices for each individual surface texture with the most appropriate technique, optical properties of the complete textured sheet, like e. g. angle dependent reflectance, transmittance or depth resolved absorptance, can be determined via iterative matrix multiplications (for propagation and redistribution) with low computational effort.
In this work, we focus on textured wafer-based silicon solar cells as application examples for the OPTOS formalism. The simulation enables us to investigate and optimize combinations of front and rear textures on solar cells in order to increase the photocurrent generation. A solar cell with inverted pyramid front side and a diffractive grating at the rear is found to show similar light trapping properties as one with Lambertian scattering at the rear.
Hubert Hauser, Nico Tucher, Katharina Tokai, Patrick Schneider, Christine Wellens, Anne Volk, Sonja Seitz, Jan Benick, Simon Barke, Frank Dimroth, Claas Müller, Thomas Glinsner, Benedikt Bläsi
Due to its high resolution and applicability for large area patterning, nanoimprint lithography (NIL) is a promising technology for photovoltaic (PV) applications. However, a successful industrial application of NIL processes is only possible if large-area processing on thin, brittle, and potentially rough substrates can be achieved in a high-throughput process. The development of NIL processes using the SmartNIL technology from EV Group with a focus on PV applications is described. The authors applied this tooling to realize a honeycomb texture (8 μm period) on the front side of multicrystalline silicon solar cells, leading to an improvement in optical efficiency of 7% relative and a total efficiency gain of 0.5% absolute compared to the industrial standard texture (isotexture). On the rear side of monocrystalline silicon solar cells, the authors realized diffraction gratings to make use of light trapping effects. An absorption enhancement of up to 35% absolute at a wavelength of 1100 nm is demonstrated. Furthermore, photolithography was combined with NIL processes to introduce features for metal contacts into honeycomb master structures, which were initially realized using interference lithography. As a final application, the authors investigated the realization of very fine contact fingers with prismatic shape in order to minimize reflection losses.
Due to its high resolution and applicability for large area patterning, Nanoimprint Lithography (NIL) is a promising technology for photovoltaic (PV) applications. However, a successful industrial application of NIL processes is only possible if large-area processing on thin, brittle and potentially rough substrates can be achieved in a high-throughput process. In this work, the development of NIL processes using the novel SmartNILTM technology from EV Group with a focus on PV applications is described. We applied this tooling to realize a honeycomb texture (8 μm period) on the front side of multicrystalline silicon solar cells leading to an improvement in optical efficiency of 7% relative and a total efficiency gain of 0.5% absolute compared to the industrial standard texture (isotexture). On the rear side of monocrystalline silicon solar cells, we realized diffraction gratings to make use of light trapping effects. An absorption enhancement of up to 35% absolute at a wavelength of 1100 nm is demonstrated. Furthermore, we combined photolithography and NIL processes to introduce features for metal contacts into honeycomb master structures, which initially were realized using interference lithography. As final application, we investigated the realization of very fine contact fingers with prismatic shape in order to minimize reflection losses.
Crystalline silicon solar cells absorb light in the near infrared only weakly. To utilize also the infrared light of the solar spectrum with energies still greater than the band gap of silicon, the effective path of the light inside the solar cell has to be enhanced. Light paths can be manipulated at the front side as well as at the rear side of a solar cell. For the front side, pyramidal textures that also show anti-reflection properties are widely used. These anti-reflection properties, however, can also be achieved with planar dielectric coatings or nanostructured surfaces. In this case, the path length enhancement can be achieved with rear side structures that are especially optimized for this purpose, thus de-coupling anti-reflection and path-length enhancement functionalities. This de-coupling creates leeway to optimize not only the optical properties but also the electrical properties of the optically active structures, and to realize structures that are compatible with very thin silicon wafers. To this end, this paper investigates two kinds of diffractive rear side structures, both, theoretically and experimentally. First, hexagonal sphere gratings that are produced by a self-organized growth process using spin coating, and second, binary gratings produced via nano-imprint lithography. Both process chains are potentially scalable to large areas. In optical measurements we determined potential photocurrent density gains of over 1 mA/cm2 for 250 μm thick wafers for both structures. Furthermore, we developed a process for contact formation as one key step to fully processed solar cells with diffractive rear side structures.
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