Search

Photovoltaics (PV) in onboard vehicle applications adds weight to an electric vehicle (EV), increasing the overall energy consumption. Although the added PV system weight (1.5– 40 kg) is small compared to the vehicle weight (1500–2200 kg), the power generated by PV (55–700 W) is also very small com- pared to the power needed (up to 80–285 kW) to propel an EV, making the effect of additional PV system weight on energy con- sumption a non-trivial topic to analyze. We present a method to study the impact of vehicle onboard PV weight on the energy balance of EVs for different Vehicle-added PV (VAPV) and vehi- cle-integrated PV (VIPV) configurations with eight different PV technologies, using data from vehicle onboard measurement campaigns and simulations. Simulations are carried out for the driving phase of two electric cars (medium and large passenger cars). Our method calculates the energy consumption attributa- ble to the added PV system weight (0.05–1.4 Wh/km) and PV energy yield (0.12–3.12 Wh/km) for a selection of trips. The re- sults of these simulations are expressed through a newly intro- duced parameter called “onboard PV yield factor”, where posi- tive values indicate a net energy gain and negative values indicate a net energy loss of the onboard PV system. Our results show that the onboard PV yield factor for a VAPV configuration can range between -69.1 and 86.9 %, and for a VIPV configuration, between 77.2 and 89.7 %.

The energy conversion efficiency (η) of silicon heterojunction (SHJ) solar cells is limited by the current losses in the layer stack on the illuminated side. To reduce these losses, hydrogenated nanocrystalline silicon oxide (nc-SiOx:H) was implemented as a window layer in SHJ solar cells. However, the integration of nc-SiOx:H in devices without degradation of fill factor (FF) is still a challenge. To optimize the electron performance of devices, the optoelectronic properties and microstructure of nc-SiOx:H were characterized and analyzed systematically. It was found that the PH3 gas fraction (fPH3) plays a big role on the microstructure, oxygen content, and phosphorus (P) doping efficiency of the films. The highest conductivity, 2.84 × 10–1 S/cm, is obtained at a moderate fPH3 with an optical band gap of 2.26 eV. A ternary model was creatively used to show the variation in the composition of nc-SiOx:H as tuning fPH3. The growth of crystalline phase was accelerated by the P dopants when fPH3 is low, but further increasing fPH3 leads to excessive P inactive dopants, causing a phase transition from nanocrystalline silicon to amorphous silicon in nc-SiOx:H. In this work, the best solar cell with an nc-SiOx:H window layer achieves an FF of 81.4%, a short current density (Jsc) of 39.8 mA/cm2, an open-circuit voltage (Voc) of 731 mV, and an η of 23.7% at the moderate fPH3. A decrease in FF and Jsc is shown with higher fPH3, which is the consequence of the increased front contact resistivity and decreased optical band gap of nc-SiOx:H window layer.

Herein, an optical loss analysis of the recently introduced silicon carbide–based transparent passivating contact (TPC) for silicon heterojunction solar cells is presented, the most dominant losses are identified, and the potential for reducing these losses is discussed. Magnesium fluoride is applied as an antireflective coating to reduce the reflective losses by up to 0.8 mA cm−². When applying the magnesium fluoride, the passivation quality of the layer stack degrades, but is restored after annealing on a hot plate in ambient air. Afterwards, a road map for TPC solar cells toward an efficiency of 25% is presented and discussed. The largest part in efficiency gain is achieved by reducing the finger width and by increasing the passivation quality. Furthermore, it is shown that TPC solar cells have the potential to achieve short-circuit current densities above 42 mA cm−² if the finger width is reduced and the front-side indium tin oxide (ITO) layer can be replaced by an ITO silicon nitride double layer. in press

In this work, we propose a route to achieve a certified efficiency of up to 24.51% for silicon heterojunction (SHJ) solar cell on a full-size n-type M2 monocrystalline-silicon Cz wafer (total area, 244.53 cm2) by mainly improving the design of the hydrogenated intrinsic amorphous silicon (a-Si:H) on the rear side of the solar cell and the back reflector. A dense second intrinsic a-Si:H layer with an optimized thickness can improve the vertical carrier transport, resulting in an improved fill factor (FF). In order to reduce the plasmonic absorption at the back reflector, a low-refractive-index magnesium fluoride (MgF2) is deposited before the Ag layer; this leads to an improved gain of short circuit current density (Jsc). In total, together with MgF2 double antireflection coating and other fine optimizations during cell fabrication process, ~1% absolute efficiency enhancement is finally obtained. A detailed loss analysis based on Quokka3 simulation is presented to confirm the design principles, which also gives an outlook of how to improve the efficiency further.

I‑V measurements are sensitive to the number and positioning of current and voltage sensing contacts. For busbarless solar cells, measurement setups have been developed using current collection wires and separate voltage sense contacts. Placing the latter at a defined position enables a grid resistance neglecting measurement and thus I‑V characteristics independent from the contacting system. This technique has been developed for solar cells having a finger grid and good conductivity in the direction of the fingers. The optimal position of the sense contact in case of finger-free silicon heterojunction solar cells has not yet been studied. Here, the lateral charge carrier transport occurs in a transparent conductive oxide layer resulting in a higher lateral resistance. We perform finite difference method simulations of HJT solar cells without front metallization to investigate the impact of high lateral resistances on the I-V measurement of solar cells. We show the high sensitivity on the number of used wires for contacting as well as the position of the sense contact for the voltage measurement. Using the simulations, we are able to explain the high difference of up to 7.5% in fill factor measurements of metal free solar cells with varying TCO sheet resistances between two measurement systems using different contacting setups. We propose a method to compensate for the contacting system to achieve a grid-resistance neglecting measurement with both systems allowing a reduction of the FF difference to below 1.5%.

A silicon heterojunction solar cell using silicon carbide as front contact is presented, which features the main advantage of high transparency. To enhance this advantage, an optical loss analysis is performed. It is found that reflection losses play an important role for the solar cell, which can easily be reduced by applying an additional MgF2 coating. The deposition of the coating degrades the passivation quality of the contact but can be cured, eventually leading to a certified short circuit current density of 40.9 mA/cm² and efficiency of 23.99%. Afterwards, a roadmap to a theoretical efficiency of 25% is presented.

To optimize the electrical performance of silicon heterojunction solar cell devices, the electronic properties and microstructure of n-type nc-Si:H were characterized and analyzed. It was found that higher conductivity and crystalline volume fraction (F c ) of nc-Si:H can be obtained at lower silane gas fraction (f SiH4 ), lower power and higher phosphorous gas fraction (f PH3 ). In our case, there is a decline of the passivation for the devices with nc-Si:H after sputtering process. By increasing the phosphine flow fraction, the sputter damage can be reduced and 3% abs gain of FF as well as 0.7% abs gain of efficiency is reached compared with reference. The best solar cell exhibits the V oc of 733.3 mV, FF of 79.7%, J sc of 39.00 mA/cm2 and η of 22.79% at the M2 size wafer.

Silicon heterojunction with nc-SiC(n)/SiO 2 based front transparent passivating contact (TPC) is numerically modeled. The model is then used to study the effect of active dopant concentration at the front and rear contact of the solar cell. A potential of power conversion efficiency above 25 % can be achieved with a suitable acceptor dopant concentration of p-type amorphous silicon at the rear side. Improving fill factor via SiC dopant concentration can enhance the cell power conversion efficiency within a narrow range of active dopant concentration. However, very high doping of SiC can affect the cell performance negatively.

Mitigating the adverse effect of transparent conductive oxide (TCO) sputtering onthe passivation quality of the heterojunction contact is very critical for achievinghigh-efficiency silicon heterojunction (SHJ) solar cells. Herein, n-type ultra-thin(5 nm) nanocrystalline silicon (nc-Si:H) is utilized as a contact layer in rear-junctionSHJ solar cells. It is revealed that the TCO sputtering damage on the contact layer iscaused by the ion bombardment during the sputtering process and cannot be fullyrecovered by low-temperature annealing. A more severe passivation deteriorationis observed for the devices with nc-Si:H(n) having a higher microstructure factor.This result is explained by an increased ion penetration depth in the porous nc-Si:Hfilm. By applying denser nc-Si:H(n) in SHJ solar cells, the front silicon film stackshows more resilience to sputter damage, yielding a remarkable cell performanceon the M2-size wafer with certificated power conversion efficiency (η) of 23.87%.The sputter damage on nc-Si:H by TCO in SHJ solar cells is explored in depth forthe first time from the perspective of silicon thin films.

We optically designed and investigated two deterministic light-trapping concepts named “Hutong” (wafer thickness dependent, patch-like arrangement of “V” grooves with alternating orientations) and “VOSTBAT” (one directional “V” grooves at the front and saw-tooth like structures at the back) for the application in emerging thin silicon heterojunction (SHJ) solar cells. Calculated photocurrent density (Jph) (by weighting the spectrally resolved absorptance with AM1.5g spectrum and integrating over the wavelength) showed that both the Hutong and VOSTBAT structures exceed the Lambertian reference and achieved Jph of 41.72 mA/cm2 and 41.86 mA/cm2, respectively, on 60 μm thin wafers in the case of directional, normal incidence.

Summary In order to compensate the insufficient conductance of heterojunction thin films, transparent conductive oxides (TCO) have been used for decades in both sides of contacted crystalline silicon heterojunction (SHJ) solar cells to provide lateral conduction for carrier collection. In this work, we substitute the TCO layers by utilizing the lateral conduction of c-Si absorber, thereby enabling a TCO-free design for SHJ solar cells achieving a low series resistivity of 0.32 Ωcm2 and a good fill factor of 80.7% with a conventional finger pitch of 1.8 mm. Achieving high efficiencies in TCO-free SHJ solar cells requires suppressing deterioration of the passivation quality induced by the direct metal-to-a-Si:H contacts. We show that an ozone treatment at the a-Si:H/metal interface suppresses the metal diffusion into the a-Si:H layer and improves the passivation without increasing the contact resistivity. SHJ solar cells with TCO-free front contacts and ozone treatment achieve efficiencies of >22%.

The color produced by visible light that reflects from the photovoltaic modules can influence visual aesthetics for colored photovoltaic applications, such as the building integrated photo-voltaic and the vehicles integrated photovoltaic. How two colors lying close together can be perceived by the human eye is important for aesthetic design. In this article, we investigate the reflectance spectra variation caused by the variation of indium tin oxide thickness and incidence angle of sunlight based on the well-known silicon heterojunction solar cells and modules. By converting the reflectance spectra into the Delta E 2000 value, we quantify whether differences in color can be perceived. The colors are also predicted based on the standard red, green, and blue color space. The results show that the reflectance variation because of an ITO thickness deviation of 5 nm in SHJ solar cells leads to a perceptible color difference, which can be suppressed after encapsulation but is still perceptible on close observation. The ITO thickness deviation should be controlled within 3 nm to produce a nearly imperceptible visual appearance. The color difference of SHJ modules with an ITO thickness of 70 nm is nearly imperceptible if the incidence angle is below 70°. For comparison, the color differences of the passivated emitter and rear contact solar cells using SiNx as an antireflection layer is also investigated

This work presents a review of life-cycle assessment (LCA) studies of hydrogen electrolysis using power from photovoltaic (PV) systems. The paper discusses the assumptions, strengths and weaknesses of 13 LCA studies and identifies the causes of the environmental impact. Differences in assumptions of system boundaries, system sizes, evaluation methods, and functional units make it challenging to directly compare the Global Warming Potential (GWP) resulting from different studies. To simplify this process, 13 selected LCA studies on PV-powered hydrogen production have been harmonized following a consistent framework described by this paper. The harmonized GWP values vary from 0.7 to 6.6 kg CO2-eq/kg H2 which can be considered a wide range. The maximum absolute difference between the original and harmonized GWP results of a study is 1.5 kg CO2-eq/kg H2. Yet even the highest GWP of this study is over four times lower than the GWP of grid-powered electrolysis in Germany. Due to the lack of transparency of most LCAs included in this review, full identification of the sources of discrepancies (methods applied, assumed production conditions) is not possible. Overall it can be concluded that the environmental impact of the electrolytic hydrogen production process is mainly caused by the GWP of the electricity supply. For future environmental impact studies on hydrogen production systems, it is highly recommended to 1) divide the whole system into well-defined subsystems using compression as the final stage of the LCA and 2) to provide energy inputs/GWP results for the different subsystems.

In order to compensate the insufficient conductance of heterojunction thin films, transparent conductive oxides (TCO) have been used for decades in both-sides contacted crystalline silicon heterojunction (SHJ) solar cells to provide lateral conduction for efficient carrier collection. In this work, we substitute the TCO layers by utilizing the lateral conduction of c-Si absorber, thereby enabling a TCO-free design. A series resistance of 0.32 Ωcm2 and a fill factor of 80.7% were measured for a TCO-free back-junction SHJ solar cell with a conventional finger pitch of 1.8 mm, thereby proving that relying on lateral conduction in the c-Si bulk is compatible with low series resistances. Achieving high efficiencies in SHJ solar cells with TCO-free front contacts requires suppressing deterioration of the passivation quality induced by direct metal-a-Si:H contacts and in-diffusion of metal into the a-Si:H layer. We show that an ozone treatment at the a-Si:H/metal interface suppresses the metal diffusion and improves the passivation without increasing the contact resistivity. SHJ solar cells with TCO-free front contacts and ozone treatment achieve efficiencies of > 22%.

In this work, we have improved the absorption properties of thin film solar cells by introducing light trapping reflectors deposited onto self-assembled nanostructures. The latter consist of a disordered array of nanopillars and are fabricated by polymer blend lithography. Their broadband light scattering properties are exploited to enhance the photocurrent density of thin film devices, here based on hydrogenated amorphous silicon active layers. We demonstrate that these light scattering nanopillars yield a short-circuit current density increase of +33%rel with respect to equivalent solar cells processed on a planar reflector. Moreover, we experimentally show that they outperform randomly textured substrates that are commonly used for achieving efficient light trapping. Complementary optical simulations are conducted on an accurate 3D model to analyze the superior light harvesting properties of the nanopillar array and to derive general design rules. Our approach allows one to easily tune the morphology of the self-assembled nanostructures, is up-scalable and operated at room temperature, and is applicable to other photovoltaic technologies.

Mixed ionic-electronic-conducting perovskites such as SrTiO3 are promising materials to be employed in efficient energy conversion or information processing. These materials exhibit a self-doping effect related to the formation of oxygen vacancies and electronic charge carriers upon reduction. It has been found that dislocations play a prominent role in this self-doping process, serving as easy reduction sites, which result in the formation of conducting filaments along the dislocations. While this effect has been investigated in detail with theoretical calculations and direct observations using local-conductivity atomic force microscopy, the present work highlights the optical properties of dislocations in SrTiO3 single crystals. Using the change in optical absorption upon reduction as an indicator, two well-defined arrangements of dislocations, namely a bicrystal boundary and a slip band induced by mechanical deformation, are investigated by means of scanning near-field optical microscopy. In both cases, the regions with enhanced dislocation density can be clearly identified as regions with higher optical absorption. Assisted by ab initio calculations, confirming that the agglomeration of oxygen vacancies significantly change the local dielectric constants of the material, the results provide direct evidence that reduced dislocations can be classified as alien matter embedded in the SrTiO3 matrix.

Incomplete photon absorption is one big drawback in solar cells with thin absorbers. Therefore, the development of advanced light-trapping schemes is one major task to reduce the optical losses. The so-called Lambertian scattering is often used as a benchmark for the application of light-trapping schemes with features that are treated by geometrical optics. In our optical simulation study, we introduce two different deterministic light-trapping approaches. We show that in both cases, the absorptance in 60 μm thin c-Si absorbers exceeds the value for Lambertian scattering in a broad spectral range.

We investigate the influence of surface texture on the effectiveness of reflectance by intermediate reflectors in thin-film tandem solar cells. Two distinct angular reflection regimes are identified. For large surfaces with large aspect ratios, frustrated total internal reflection or photon tunneling is found to be the dominating mechanism. We show that the parasitic reflection losses of commonly used randomly textured surfaces are explained by these distinct regimes and introduce a spectral reflectance parameter, which serves as a guideline for designing optimized spectrally selective intermediate reflectors.

Nanophotonic gratings can be used for efficient light trapping in crystalline silicon solar cells. A common challenge is the high quality surface passivation, when applying such gratings. In this article, we report on the prototyping of the application of nanophotonic grating back contacts to planar silicon heterojunction solar cells for improved light trapping. These back contacts are processed after the Si wafer passivation, omitting any direct texturing of the absorber layer. Thereby, maintaining the integrity of the planar passivation layer of high quality and simultaneous light trapping is realized. The induced light-trapping effect is further studied by photoluminescence imaging, external quantum efficiency measurements as well as absorptance measurements of solar cells applying nanophotonic grating back contacts of different grating periods. A clear correlation is found for the three characterization methods which allows the optimal period for efficient light trapping in the investigated solar cells to be identified. For the optimized grating period of the nanophotonic grating back contact an increased short-circuit current density is obtained and the power conversion efficiency is enhanced by 0.4%, absolute.

To improve the infrared (IR) response, a high-refractive-index intrinsic amorphous silicon (a-Si:H) layer is introduced after metallization of bifacial silicon heterojunction (SHJ) solar cells, resulting in a transparent conductive oxide (TCO)/a-Si:H back reflector, which functions like distributed Bragg reflector (DBR). This concept is demonstrated by both Sentaurus Technology Computer-Aided Design (TCAD) simulation and experimental methods. The TCO/a-Si:H back reflector can increase rear internal reflectance by reducing the transmission loss, thus improving the IR external quantum efficiency. The using of Sn-doped In2O3 (ITO)/a-Si:H back reflector in >23.5% efficiency SHJ solar cells can improve short-circuit current density by 0.4 mA cm2 which is quite similar as using the more expensive ITO/Ag back reflector, while keeping a cell bifaciality of 55%. This brings its advantage for monofacial application case. Future studies would be nice to work on higher transparent back reflectors to broaden the application in bifacial case. This back-reflector design promotes IR response of SHJ solar cells with transferring to a wide variety of TCOs.

In this work, ultra-thin n-type hydrogenated nanocrystalline silicon oxide [(nc-SiOx:H (n)] film was used to replace amorphous silicon [a-Si:H (n)] as electron transport layer (ETL) in rear-junction silicon heterojunction (SHJ) solar cell to reduce front parasitic absorption. The contact resistivity between the transparent conductive oxide (TCO) and ultra-thin ETL interface plays an important role on the cell performance. A nanocrystalline silicon (nc-Si:H) contact or seed layer was introduced in the solar cell with ultra-thin nc-SiOx:H and the impact of the nc-Si:H thickness on the cell performance was investigated. To demonstrate scalability, bifacial solar cells with 10 nm ETL were fabricated on the M2 (244 cm2) wafer. The best cell performance is obtained by the solar cell with 5 nm nc-SiOx:H (n) and 5 nm nc-Si:H (n) contact layer and it exhibits open-circuit voltage (Voc) of 738 mV, fill factor (FF) of 80.4%, short-circuit current density (Jsc) of 39.0 mA/cm2 and power conversion efficiency (η) of 23.1% on M2 wafer. Compared to the one with nc-SiOx:H (n), an increase of 3%abs of FF and 0.5%abs of η and lower front contact resistivity is demonstrated for the solar cells with nc-Si:H (n) / nc-SiOx:H (n) double layer, which is caused by the lower energy barrier for electrons, according to the band diagram calculated by the AFORS-HET simulator. A simulation on the solar cell optical and electrical losses was done by the Quokka 3 simulator and shows much lower electrical transport loss and a bit higher front surface transmission loss for the one with double layer than nc-SiOx:H (n) single layer.

The surface patterning of Si heterojunction solar cells is of key importance to enhance the light trapping properties and enable high efficiencies of the emerging thin crystalline silicon solar cells. Herein, periodic inverted pyramids with different periodicity patterns were fabricated by nanoimprint lithography in combination with dry- and wetetching techniques. The impact of their periodicity on the light trapping properties was studied by measuring their reflectance and photoluminescence properties. The inverted pyramids with 700 nm periodicity show excellent anti-reflection and selective light-trapping properties.

International audience; The numerical modeling of light interaction with nanostructured materials is at the heart of many computational photonics studies. A typical example of interest to the present work is the simulation of light trapping in complex photovoltaic devices. This can be a challenging task when the underlying material layers are textured in a very general way. Very often, such studies rely on the Finite Difference Time-Domain (FDTD) method. The FDTD method is a widely used approach for solving the system of time-domain Maxwell equations in the presence of heterogenous media and complex three-dimensional structures. In the classical formulation of the this method, the whole computational domain is discretized using a uniform structured (Cartesian) grid. In this work, we consider an alternative approach by adapting and exploiting a particular finite element method, which is able to deal with topography conforming geometrical models based on non-uniform discretization meshes. The underlying modeling method is known as the Discontinuous Galerkin Time-Domain (DGTD) method. It is a discontinuous finite element type that relies on a high order interpolation of the electromagnetic field components within each cell of an unstructured tetrahedral mesh.

Aluminum-doped zinc oxide (ZnO:Al) layers deposited by sputtering and boron-doped zinc oxide (ZnO:B) layers deposited by low-pressure chemical vapor deposition (LPCVD) are well-established materials for front electrodes in thin-film silicon solar cells. In this study, both types of front electrodes are evaluated with respect to their inherent properties and their surface textures in micromorph tandem solar cells in the superstrate configuration. The silicon layer stack investigated here consists of a 220-nm-thick amorphous silicon top cell, a 40-nm-thick intermediate reflector and a 1.1-mu m-thick microcrystalline silicon bottom cell; for this specific silicon layer stack, the LPCVD ZnO:B provides higher power conversion efficiency than its sputtered ZnO:Al counterpart. The growth-friendly surface topography of ZnO:Al yields better electrical performance. However, tandem cells on ZnO:Al suffer from a fundamental optical limitation in terms of light trapping in the top cell. They also show a higher parasitic absorption linked to the relatively high doping concentration of the ZnO:Al layers used here. Detailed analysis of the experimental results allows us to clearly understand the opto-electrical behavior of both types of cells and envisage several possible upgrades to further improve their performance. (C) 2015 Published by Elsevier B.V.

Light trapping in crystalline silicon (c-Si) solar cells is an essential building block for high efficiency solar cells targeting low material consumption and low costs. In this study, we present the successful implementation of highly efficient light-trapping back contacts, subsequent to the passivation of Si heterojunction solar cells. The back contacts are realized by texturing an amorphous silicon layer with a refractive index close to the one of crystalline silicon at the back side of the silicon wafer. As a result, decoupling of optically active and electrically active layers is introduced. In the long run, the presented concept has the potential to improve light trapping in monolithic Si multijunction solar cells as well as solar cell configurations where texturing of the Si absorber surfaces usually results in a deterioration of the electrical properties. As part of this study, different light-trapping textures were applied to prototype silicon heterojunction solar cells. The best path length enhancement factors, at high passivation quality, were obtained with light-trapping textures based on randomly distributed craters. Comparing a planar reference solar cell with an absorber thickness of 280 μm and additional anti-reflection coating, the short-circuit current density (JSC) improves for a similar solar cell with light-trapping back contact. Due to the light trapping back contact, the JSC is enhanced around 1.8 mA cm−2 to 38.5 mA cm−2 due to light trapping in the wavelength range between 1000 nm and 1150 nm.

A simple method is developed to determine the light trapping properties of arbitrarily textured solar cells with high accuracy. The method allows for determining the quantum efficiency and short circuit current density of thin film solar cells prepared on randomly nanotextured surfaces. The light trapping of the randomly textured solar cell is described by the area weighted superposition of periodically textured solar cells. The necessary input parameters for the calculations are determined by analyzing the randomly textured surfaces of the solar cells using atomic force microscopy and image processing. The analysis of the atomic force microscope images and the calculation of the quantum efficiency and short circuit current can be determined from current maps, without complex and time-consuming calculations. The calculated solar cell parameters exhibit excellent agreement with experimentally measured quantum efficiencies and short circuit current densities for amorphous and microcrystalline silicon thin film solar cells prepared on randomly textured substrates. Finally, the work contributes to a comparison of random and periodic light trapping structures.

Nanophotonic light management concepts are essential building blocks of advanced thin-film solar cells. These concepts make use of light coupling to waveguide modes that are supported by the photoactive absorber material of the solar cell. In a recent study, we presented a new method based on scanning near-field optical microscopy that enables the direct nanoscale investigation of light coupling to an individual waveguide mode in a nanophotonic thin-film silicon solar cell. Making use of this method, we investigate in this contribution the polarization dependence of the light coupling to a waveguide mode. Based on this polarization dependence, we can attribute the investigated waveguide mode to a transverse electric mode. Moreover, we identify the grating vector, which is responsible for the light coupling to the investigated waveguide mode in the nanopatterned thin-film silicon solar cell.

An aperture-type scanning near-field optical microscope (a-SNOM) is readily used for the optical and optoelectronic characterizations of a wide variety of chemical, biological and optoelectronic samples with sub-wavelength optical resolution. These samples mostly exhibit nanoscale topographic variations, which are related to local material inhomogeneity probed either by an optical contrast or by secondary effects such as photoconductivity or photoluminescence. To date, in the interpretation and evaluation of the measurement results from a-SNOM or derived methods, often only the local material inhomogeneity is taken into account. A possible influence of the optical interaction between the scanning probe and the surface topography is rarely discussed. In this paper, we present experimental and theoretical investigation of the effects of nanoscale topographic features on a-SNOM measurement results. We conduct local photocurrent measurements on a thin-film solar cell with an a-SNOM as the illumination source. A clear correlation between the photocurrent response and local topography is observed in all measurements with a signal contrast of up to ∼30%, although the sample features homogeneous permittivity and electrical properties. With the help of finite-difference time-domain (FDTD) simulations, this correlation is reproduced and local light coupling is identified as the mechanism which determines the local photocurrent response. Our results suggest that a-SNOM-based measurements of any sample with material inhomogeneity will be superimposed by the local light-coupling effect if surface topography variation exists. This effect should always be taken into consideration for an accurate interpretation of the measurement results.

High-efficiency thin-film silicon solar cells require advanced textures at the front contacts for light management. In this contribution, the influence of the texture of various transparent conductive oxides (TCO) on the effectiveness of an intermediate reflector layer (IRL) in a-Si:H/μc-Si:H tandem solar cells is investigated. The employed front side TCOs include several types of sputter-etched ZnO:Al, LPCVD ZnO:B and APCVD SnO 2 :F. The topographies after different stages of the deposition process of the tandem solar cell, at the front TCO, after deposition of the amorphous top cell and after the deposition of the microcrystalline bottom cell, were characterized by atomic force microscopy at precisely the same spot. The external quantum efficiency of the fabricated solar cells were measured and successfully reproduced by a finite-difference time-domain method applying the measured topographies at each interface of the solar cell. With these simulations, the impact of structure type and feature size on the effectiveness of the IRL is investigated. The highest IRL effectiveness in a tandem solar cell was found for double-textured ZnO:Al. In this contribution, we study the interplay between interface textures and parasitic losses. Our findings are relevant for the design of topography for optimized IRL performance.

Thin-film silicon solar cells often apply a metal back reflector (BR) separated from the silicon layers by a thin rear dielectric of thickness around 80 nm or a white paint combined with a thick rear dielectric of several micrometers. In this work, we investigate the optical performance of microcrystalline silicon (μc-Si:H) solar cells applying BRs of various topographies. In contrast to a standard 80 nm-ZnO/Ag BR design, for which the BR nearly strictly follows the texture of the underlying μc-Si:H layers, placing the Ag BR far from the μc-Si:H layers allows for a variation of the BR topography. Irrespective of the investigated BR topographies and also for a conventional white paint BR, long distances (of several micrometers) between the BR and the μc-Si:H layers are found to be detrimental for the light trapping. Optical simulations based on both rigorous and scalar scattering theory have been performed to understand the impact of the diverse BR designs on the optical cell performance.

Photon recycling is a fundamental physical process that becomes especially important for photovoltaic devices that operate close to the radiative limit. This implies that the externally measured radiative decay rate deviates from the internal radiative recombination rate of the material. In the present Letter, the probability of photon recycling in organic lead halide perovskite films is manipulated by modifying the underlying layer stacks. We observe recombination kinetics by time-resolved photoluminescence that is controlled by the optical design of the chosen layer structure. Quantitative simulations of decay rates and emission spectra show excellent agreement with experimental results if we assume that the internal bimolecular recombination coefficient is ~ 66 % radiative.

Impact of periodicity was studied for micro- and nano-pyramids fabricated by low-cost nanoimprint lithography process. Investigations by photoluminescence imaging displayed excellent anti-reflection and wavelength-selective light-trapping properties.

Light scattering OLED substrates relying on disordered self-assemblies are fabricated by microsphere and polymer blend lithography and used for light extraction. We report on a device efficiency enhancement of up to 50 %.

Thin-film silicon solar cells often rely on a metal back reflector separated from the silicon layers by a thin rear dielectric as a back reflector (BR) design. In this work, we aim to obtain a better insight into the influence of the rear-dielectric/Ag BR design on the optical performance of hydrogenated microcrystalline silicon (µc-Si:H) solar cells. To allow the application of a large variety of rear dielectrics combined with Ag BRs of diverse topographies, the solar cell is equipped with a local electrical contact scheme that enables the use of non-conductive rear dielectrics such as air or transparent liquids of various refractive indices n. With this approach, detached Ag BRs having the desire surface texture can be placed behind the same solar cell, yielding a direct and precise evaluation of their impact on the optical cell performance. The experiments show that both the external quantum efficiency and the device absorptance are improved with decreasing n and increasing roughness of the BR. Calculations of the angular intensity distribution of the scattered light in the µc-Si:H are presented. They allow for establishing a consistent picture of the light trapping in the solar cell. Copyright © 2013 John Wiley & Sons, Ltd.

We present three different working modes of scanning near-field optical microscopy which provide complementary information about the electro-optical properties of solar cells. Those working modes allows to study the local light scattering and trapping in textured solar cells, the local light propagation inside the cell and the impact of lateral inhomogeneity on a nanoscopic scale. Due to the presence of evanescent light modes, the presented techniques give access to important physical effects which is only possible in the optical near-field.

In the present work, we investigate light-management concepts applied subsequent to the passivation of the Si wafer of planar Si heterojunction solar cells. As a first concept, we apply amorphous silicon based nanophotonic textures at the back side of the Si wafer to realize efficient light trapping. As a second concept, we use a nanoimprint lithography based front side anti-reflection coating to improve the incoupling of light into the solar cell absorber. Both concepts allow for efficient improvements in the short-circuit current densities without degrading the planar passivation layers. As the highlight of this work, we demonstrate planar silicon heterojunction solar cells (thickness ∼ 280 μm) with high passivation quality as well as a short-circuit current density of 38.8 mA/cm2.

The optical properties of textured surfaces in the micro and nanometer range are of interest in manifold topics, such as thin-film silicon photovoltaics. Light scattering models, which are based on Fourier transform techniques, are applied to calculate both, the angularly resolved scattering and the haze. Therein, topography, measured by atomic force microscopy, and the refractive index are used as input data. In this study, these models are applied to zinc oxide/air interfaces and to zinc oxide/hydrogenated amorphous silicon interfaces. Results obtained from zinc oxide/air interfaces are compared to the measured scattering properties.