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Since single junction c-Si solar cells are reaching their practical efficiency limit. Perovskite/c-Si tandem solar cells hold the promise of achieving greater than 30% efficiencies. In this regard, optical simulations can deliver guidelines for reducing the parasitic absorption losses and increasing the photocurrent density of the tandem solar cells. In this work, an optical study of 2, 3 and 4 terminal perovskite/c-Si tandem solar cells with c-Si solar bottom cells passivated by high thermal-budget poly-Si, poly-SiOx and poly-SiCx is performed to evaluate their optical performance with respect to the conventional tandem solar cells employing silicon heterojunction bottom cells. The parasitic absorption in these carrier selective passivating contacts has been quantified. It is shown that they enable greater than 20 mA/cm2 matched implied photocurrent density in un-encapsulated 2T tandem architecture along with being compatible with high temperature production processes. For studying the performance of such tandem devices in real-world irradiance conditions and for different locations of the world, the effect of solar spectrum and angle of incidence on their optical performance is studied. Passing from mono-facial to bi-facial tandem solar cells, the photocurrent density in the bottom cell can be increased, requiring again optical optimization. Here, we analyse the effect of albedo, perovskite thickness and band gap as well as geographical location on the optical performance of these bi-facial perovskite/c-Si tandem solar cells. Our optical study shows that bi-facial 2T tandems, that also convert light incident from the rear, require radically thicker perovskite layers to match the additional current from the c-Si bottom cell. For typical perovskite bandgap and albedo values, even doubling the perovskite thickness is not sufficient. In this respect, lower bandgap perovskites are very interesting for application not only in bi-facial 2T tandems but also in related 3T and 4T tandems.

There are several sensors available in the market to measure the plane-of-array irradiance for photovoltaic applications. The prices of these sensors vary according to the design, calibration procedure, and conducted characterization. In this article, two types of silicon-based sensors with and without temperature correction capabilities are compared with a high-accuracy thermopile pyranometer to check their performance. The obtained results showed that silicon-based sensors deviate from the output of the pyranometers. The tested silicon-based pyranometers overestimate the irradiance with the median bias deviations of around 1.43% (with the average measured irradiance of 256 W/m2). For temperature-corrected silicon pyranometer, the bias deviation is 0.07% with the deviation range of −6.5%–10% (with the average measured irradiance of 257 W/m2). A working-class reference cell was also tested, resulting in a bias deviation of −1.74% and the deviation range of −13%–7% (with the average measured irradiance of 304 W/m2). The effect of air mass on the performance of cost-effective sensors was additionally analyzed. Within the measurement time window, the result also showed that for the silicon-based sensors under tests, the effects of the environmental conditions have the following qualitative order of influence: angle of incidence > red-shift > temperature. The performance of silicon-based sensors also showed seasonal dependence, being more accurate during summertime and wintertime, respectively, for the silicon pyranometer and the working-class reference cell. Finally, using the statistical evaluation, simple linear correction functions are introduced for silicon-based sensors.

At present, electrification and digitalization are two significant trends in the energy sector. Large-scale introduction of variable renewable energy sources, energy storage and power-electronics components, all based on direct current (DC), is fundamentally changing the electrical energy system of today that is based on alternating current (AC). This trend leads to a complex hybrid AC/DC power system with the extensive deployment of information and communication technologies (ICT) to keep the system stable and reliable. Photovoltaics (PV) is a technology that will play an essential role in local generation of clean electricity in expanding urban areas. To take full advantage of PV in the urban environment, PV technology must become intelligent. In this article, we identify, describe, and label a new research field that deals with intelligent PV and its application in components with multiple functionalities. We denote this field photovoltatronics. We review photovoltatronics research areas and introduce new directions for each area. Photovoltatronics brings together disciplines of energy and informatics. Since photons and electrons are carriers of both energy and information, photovoltatronics is the field that designs and delivers autonomous devices for electricity generation and information communication. It introduces a pathway from harvesting energy of photons (hν) to creating bits of information (01) through the energy of photo-generated electrons (eV). We show that ∼10 keV energy is at least needed for transceiving one bit of information in the energy-information chain of the photovoltatronics, while the ultimate efficiency of the chain can reach up to 33.4%. We show that the number of publications related to photovoltatronics is exponentially increasing and the publication rate of combined research areas has been doubled in the present decade and reached 3.4% as a clear sign of its emergence.

This paper describes the core model that lies behind an online modeling environment for photovoltaic (PV) energy generation in the Netherlands, called the Dutch PV Portal 2.0 (PVP 2.0). PVP 2.0 realizes three functionalities: (i) a real-time system efficiency breakdown figure, (ii) an estimate of national solar electricity production, and (iii) an economic analysis for small-scale to large scale PV systems. The unique and novel aspects of this website are the dynamically updated climate database, the rainfall-dependent soiling loss calculation, and the orientation-dependent inverter sizing factor. This paper also justifies the PVP 2.0 capabilities in large scale studies. First, the PVP 2.0 modeling approach was validated by yield comparison with 26 real PV systems distributed over the Netherlands. The validation resulted in −11% to +5.5% deviation from measured data. Then, a back-end sensitivity study found that a rain-free summer in the Netherlands results in an AC energy loss of 3.4% in that period, while a change in wind speed or ambient temperature could lead to losses around 2–4.5 GWh/year (0.58%–1.16%) on province scale. The core of the approach presented in this paper can be used to develop similar websites for other countries of interest with adapted add-ons depending on the target climate condition. Such study tools could eventually provide more quantitative insights about the impact of climate change (by applying probable scenarios) on the renewable energy production of the World.

An interdigitated back-contacted (IBC) configuration is proposed for submicron copper indium gallium (di)selenide (CIGS). In a modelling platform, the structure was opto-electrically optimized for maximum efficiency. The results are compared with a reference front/back-contacted (FBC) solar cell with similar absorber thickness and exhibiting 11.9% efficiency. The electrical passivation at the front side is accomplished by an Al2O3 layer, which is endowed with negative fixed charges. The results indicate that with an optimal geometry and engineered bandgap grading, the efficiency of the new IBC structure can reach 17%. Additionally, with a reasonably low defect density in the absorber layer, efficiencies as high as 19.7% and open-circuit voltage comparable with that of the record solar cell are possible with the IBC structure.

In this article, we develop in parallel two fabrication methods for copper (Cu) electroplated contacts suitable for either silicon nitride or transparent conductive oxide antireflective coatings. We employ alternative seed layers, such as evaporated Ag or Ti, and optimize the Ti–Cu or Ag–Cu contacts with respect to uniformity of plating and aspect ratio of the final plated grid. Moreover, we test plating/deplating sequence instead of a direct current plating or the SiO2 layer approach to solve undesired plating outside the designed contact openings. The main objective of this paper is to explore the physical limit of this contact formation technology keeping the process compatible with industrial needs. In addition, we employ the optimized Cu-plating contacts in three different front/back-contacted crystalline silicon solar cells architectures: 1) silicon heterojunction solar cell with hydrogenated nanocrystalline silicon oxide as doped layers, 2) thin SiO2/doped poly-Si-poly-Si solar cell, and 3) hybrid solar cell endowed with rear thin SiO2/poly-Si contact and front heterojunction contact. To investigate the metallization quality, we compare fabricated devices to reference ones obtained with standard front metallization (Ag screen printing and Al evaporation). We observe a relatively small drop in V OC by 5 to 10 mV by using Cu-plating front grid, whereas fill factor was improved for solar cells with Cu-plated front contact if compared with evaporated Al.

Hydrogenated nanocrystalline silicon oxide (nc-SiOx:H) layers exhibit promising optoelectrical properties for carrier-selective-contacts in silicon heterojunction (SHJ) solar cells. However, achieving high conductivity while preserving crystalline silicon (c-Si) passivation quality is technologically challenging for growing thin layers (less than 20 nm) on the intrinsic hydrogenated amorphous silicon ((i)a-Si:H) layer. Here, we present an evaluation of different strategies to improve optoelectrical parameters of SHJ contact stacks founded on highly transparent nc-SiOx:H layers. Using plasma-enhanced chemical vapor deposition, we firstly investigate the evolution of optoelectrical parameters by varying the main deposition conditions to achieve layers with refractive index below 2.2 and dark conductivity above 1.00 S/cm. Afterwards, we assess the electrical properties with the application of different surface treatments before and after doped layer deposition. Noticeably, we drastically improve the dark conductivity from 0.79 to 2.03 S/cm and 0.02 to 0.07 S/cm for n- and p-contact, respectively. We observe that interface treatments after (i)a-Si:H deposition not only induce prompt nucleation of nanocrystals but also improve c-Si passivation quality. Accordingly, we demonstrate fill factor improvement of 13.5%abs from 65.6% to 79.1% in front/back-contacted solar cells. We achieve conversion efficiency of 21.8% and 22.0% for front and rear junction configurations, respectively. The optical effectiveness of contact stacks based on nc-SiOx:H is demonstrated by averagely 1.5 mA/cm2 higher short-circuit current density thus nearly 1%abs higher cell efficiency as compared with the (n)a-Si:H.

N-type microcrystalline silicon carbide (μc-SiC:H(n)) is a wide bandgap material that is very promising for the use on the front side of crystalline silicon (c-Si) solar cells. It offers a high optical transparency and a suitable refractive index that reduces parasitic absorption and reflection losses, respectively. In this work, we investigate the potential of hot wire chemical vapor deposition (HWCVD)–grown μc-SiC:H(n) for c-Si solar cells with interdigitated back contacts (IBC). We demonstrate outstanding passivation quality of μc-SiC:H(n) on tunnel oxide (SiO2)–passivated c-Si with an implied open-circuit voltage of 742 mV and a saturation current density of 3.6 fA/cm2. This excellent passivation quality is achieved directly after the HWCVD deposition of μc-SiC:H(n) at 250°C heater temperature without any further treatments like recrystallization or hydrogenation. Additionally, we developed magnesium fluoride (MgF2)/silicon nitride (SiNx:H)/silicon carbide antireflection coatings that reduce optical losses on the front side to only 0.47 mA/cm2 with MgF2/SiNx:H/μc-SiC:H(n) and 0.62 mA/cm2 with MgF2/μc-SiC:H(n). Finally, calculations with Sentaurus TCAD simulation using MgF2/μc-SiC:H(n)/SiO2/c-Si as front side layer stack in an IBC solar cell reveal a short-circuit current density of 42.2 mA/cm2, an open-circuit voltage of 738 mV, a fill factor of 85.2% and a maximum power conversion efficiency of 26.6%.

Thin layers of MoOx have been deposited by thermal evaporation followed by post-deposition annealing. The density of states distributions of the MoOx films were extracted deconvoluting the absorption spectra, measured by a photothermal deflection spectroscopy setup, including the small polaron contribution. Results revealed a sub-band defect distribution centered 1.1 eV below the conduction band; the amplitude of this distribution was found to increase with post-deposition annealing temperature and film thickness.

The use of transition metal oxides for the selective carrier contact in the crystalline silicon solar cells technology is rising to interest for the excellent optoelectrical properties of these materials whose implementation, however, can result in lousy performing cells due to an S-shaped electrical characteristic. In this paper, we fabricated solar cells showing S-shaped J-V curve and carried out an analysis of the reasons of such behavior using a model involving the series of a standard cell equivalent circuit with a Schottky junction in order to explain these atypical performances. A good matching between the experimental measurements and the adopted theoretical model was obtained. The extracted parameters are listed and analyzed to shade light on the reasons behind the low-performance cells.

The application of molybdenum oxide in the photovoltaic field is gaining traction as this material can be deployed in doping-free heterojunction solar cells in the role of hole selective contact. For modeling-based optimization of such contact, knowledge of the molybdenum oxide defect density of states (DOS) is crucial. In this paper, we report a method to extract the defect density through nondestructive optical measures, including the contribution given by small polaron optical transitions. The presence of defects related to oxygen-vacancy and of polaron is supported by the results of our opto-electrical characterizations along with the evaluation of previous observations. As part of the study, molybdenum oxide samples have been evaluated after post-deposition thermal treatments. Quantitative results are in agreement with the result of density functional theory showing the presence of a defect band fixed at 1.1 eV below the conduction band edge of the oxide. Moreover, the distribution of defects is affected by post-deposition treatment.

Power generation from photovoltaic (PV) modules suffers from heat losses that are a function of the temperature coefficient of maximum power ( γ ) of the PV module. Another loss mechanism is shading, where the power output of the module and ultimately the system can be severely compromised. Various approaches such as passive, active, or smart bypasses and module level power electronics have been employed to minimize the effects of shading on a PV module. So far, the performance of a PV module under shading had only been addressed vaguely and was generally described qualitatively. Therefore, a probability-based parameter called shading tolerability (ST) was recently developed to quantify the behavior of PV modules under all kinds of shade. However, the observations pertaining to ST were only carried out at 25 °C ambient temperature. Hence, this paper aims to: 1) investigate whether ST is an innate property of a PV module by formulating a correlation between ST and ambient temperature, 2) classify various PV technologies based on ST and γ, and 3) develop a PV module selection map. Theoretical result shows that thermal features of a PV module (nominal operational cell temperature and γ) can influence the PV module ST by 12.25%. Also, an analytical limit is derived for ST variation with respect to ambient temperature change $(\Delta \text{ST}/ \Delta T_{a})$ . The proposed selection map, which is similar to the Ragone plot for energy-storing devices, is validated using the data of 27 PV systems installed in the Netherlands. The map can be used for optimal selection of PV modules for the design of PV systems simply by knowing the specifications of the module along with the meteorological conditions of the installation location.

This paper presents an analysis of physical mechanisms related to operation and optimization of interdigitated back contact (IBC) poly-silicon-based devices. Concepts of carrier selectivity and tunneling are used to identify the parameters that impact on the fill factor. Then, based on technology computer-aided design (TCAD) numerical simulations, we describe the device performance in terms of transport and passivation. A validation of the model is performed by matching measured and simulated R, T, and external quantum efficiency spectra and electrical parameters. As result of such process, the opto-electrical losses of the reference device are identified. Then, we execute a study of the impact of process parameters on the performance of the IBC device under analysis. Assuming a uniform SiO 2 layer, simulation results reveal that both n-type and p-type poly-Si contacts can be theoretically perfect (i.e., approx. lossless), if assuming no interface recombination but considering tunneling of both carrier types. In other words, there exists an optimum oxide thickness (1 nm) for which majority carriers tunneling works already very well, and minority tunneling is still low enough to not result in significant recombination. Moreover, SiO 2 thickness up to maximum 1.6 nm is crucial to achieve high efficiency. Regarding rear geometry analysis, the efficiency curve as a function of emitter width peaks at 70% of pitch coverage. Further, it is shown that diffused dopants inside crystalline silicon make the device resilient to passivation quality. Finally, the calibrated model is used to perform an optimization study aiming at calculating the performance limit. The estimated performance limit is 27.3% for a 100-μm-thick bulk, 20-nm-thick poly-silicon layers, silver as rear contact, and double ARC.

Perovskite/silicon tandem solar cells (TSCs) have the potential to achieve power conversion efficiency exceeding 30%. To be compatible with high-efficiency solution-deposited perovskite top cell, a planar front surface for silicon bottom cell is generally required. However, flat front surfaces result in large light reflection losses and thus reduce the performance of tandem device. To boost light absorption, we design light management antireflective foils made from polydimethylsiloxane (PDMS) polymer carrying random-pyramidal textures with three different pyramid size ranges (1–3 µm, 3–8 µm, 8–15 µm). The optical properties, together with the reflection behavior applied to perovskite/silicon tandem solar cells have been systematically studied. One of the PDMS layer exhibited a relatively strong light-scattering property with a high average haze ratio originated from synergistic effect of the appropriate pyramid size and the uneven random pyramid distribution. Consequently, the short-circuit current density of the tandem device was improved by 1.72 mA/cm2 and thus its efficiency increased from 19.38% to 21.93%, after laminating the PDMS-based antireflection coating (ARC) onto the front surface of tandem device. Furthermore, this work provides a facile and cost-effective way to introduce light-management foils and indicates a broad strategy to enhance the performance of solar cells with planar front surface.

Dynamic photovoltaic (PV) greenhouses integrate sustainable energy generation with plant cultivation, offering more possibilities of energy production and microclimate control by adjusting the sun-tracking angles. Previous studies on PV greenhouses barely paid attention to the PV partial shading effects, and rarely recorded the performance across the full range of rotation angles. In this study, we first build computer simulation models of typical greenhouses with high-density (1/2 roof area) and low-density (1/3 and 1/4 roof area) PV layouts. Then four special sun-tracking positions are found in the model of equivalent global irradiance, which is defined as the quotient of the total input power divided by the area of PV module under partial diffuse shadows. Simulation models are also built in terms of PV modules and interior irradiance. Simulations are conducted using the climate data of Delft, the Netherlands (52.01°N,4.36°E). Results show that high-density PVs under no-shading sun tracking generate 6.91% more energy than that under conventional (quasi-perpendicular) sun-tracking. Meanwhile, no-shading sun tracking allows more diffuse sunlight to enter the greenhouse mounted with high-density PV panels, resulting in 10.96% and 10.68% improvement on the annual average global irradiance and uniformity on the target plane compared to the fixed PV panels in the closed position. Regarding low-density PV layouts, which barely suffer from partial shading problems, quasi-perpendicular sun tracking improves the annual energy generation by 7.40% relative to the closed position. However, the average global irradiance reaches the minimum in this position because more sunlight is blocked by PVs. Meanwhile, the average uniformity of global irradiance reveals good (but not the best) performance, resulting in up to 9.80% (1/3 coverage) and 4.70% (1/4 coverage) improvement respectively compared to the closed position. The proposed methods and simulation results provide guidelines for the initial design and daily operation of PV greenhouses, aiming to balance the PV power generation and food production.

Photovoltaic (PV) technology has the potential to be integrated on many surfaces in various environments, even on water. Modeling, design, and realization of a floating PV system have more challenges than conventional rooftop or freestanding PV system. In this work, we introduce two innovative concepts for floating bifacial PV systems, describing their modeling, design, and performance monitoring. The developed concepts are retractable and enable maximum energy production through tracking the Sun. Various floating PV systems (monofacial, bifacial with and without reflectors) with different tilts and tracking capabilities are installed on a Dutch pond and are being monitored. Results of the thermal study showed that partially soaking the frame of PV modules into water does not bring a considerable additional yield (+0.17%) and revealed that floating PV modules experience higher temperature special variance compared with land-based systems. Observations showed that the birds' presence has a severe effect on floating PV performance in the short term. Electrical yield investigation concluded that due to low albedo of inland water areas (~6.5%), bifacial PV systems must have reflectors. One-year monitoring showed that a bifacial PV system with reflector and horizontal tracking delivers ~17.3% more specific yield (up to 29% in a clear-sky month) compared with a monofacial PV system installed on land. Ecological monitoring showed no discernable impacts on the water quality in weekly samplings but did show significant impacts on the aquatic plant biomass and periods of low oxygen concentrations.

Nature energy (2021). doi:10.1038/s41560-021-00806-9, Published by Nature Publishing Group, London

The window layers limit the performance of silicon heterojunction (SHJ) solar cells with front and back contacts. Here, we optimized tungsten-doped indium oxide (IWO) film deposited by radio frequency magnetron sputtering at room temperature. The opto-electrical properties of the IWO were manipulated when deposited on top of thin-film silicon layers. The optimal IWO on glass shows carrier density and mobility of 2.1 × 1020 cm−3 and 34 cm2 V−1s−1, respectively, which were tuned to 2.0 × 1020 cm−3 and 47 cm2 V−1s−1, as well as 1.9 × 1020 cm−3 and 42 cm2 V−1s−1, after treated on i/n/glass and i/p/glass substrates, respectively. Using the more realistic TCO data that were obtained on thin-film silicon stacks, optical simulation indicates a promising visible-to-near-infrared optical response in IWO-based SHJ device structure, which was demonstrated in fabricated devices. Additionally, by adding an additional magnesium fluoride layer on device, the champion IWO-based SHJ device showed an active area cell efficiency of 22.92%, which is an absolute 0.98% efficiency gain compared to the ITO counterpart, mainly due to its current gain of 1.48 mA/cm2.

We performed advanced opto-electrical simulations on thin-film BaSi2 solar cells. First, absorption spectra of BaSi2-pn homojunction solar cells on Si substrate were calculated based on flat and/or pyramidally-textured surfaces, wherein 20-nm-thick n+-BaSi2 was the topmost electron transport layer. By changing the front surface structure from flat to texture, the reflectance decreased in the wavelength (λ) range 700–1200 nm and the photocurrent density (Jph) delivered by the photogenerated carriers in the 500-nm thick p-BaSi2 layer increased by 1.2 mA/cm2. Simulations revealed that the key factor inhibiting light absorption in the p-BaSi2 layer was parasitic absorption in the n+-BaSi2 and in the c-Si substrate. To solve these optical issues, we propose a new device structure, Al-doped n+-ZnO (AZO, 50 nm)/i-ZnO (20 nm)/p-BaSi2 (500 nm) heterojunction solar cell (HJSC). In this device structure, the parasitic absorption reduced drastically, and Jph reached 30.23 mA/cm2. Furthermore, by replacing the Si substrate with a glass substrate, the light trapping worked more effectively, and the absorber layer thickness required for Jph to saturate was reduced to 1 μm, yielding 32.06 mA/cm2. Based on these simulation results, we manufactured n+-AZO/p-BaSi2 HJSC. The internal quantum efficiency exceeded 30% at λ = 600 nm, meaning that we demonstrated the operation of n+-AZO/p-BaSi2 HJSC for the first time. We investigated origins of small efficiencies compared to those simulated, and found that the passivation of defects in the p-BaSi2 layer and the reduction of carrier recombination at the i-ZnO/p-BaSi2 interface would significantly improve the solar cell performance.

Low activation energy (Ea) and wide bandgap (Eg) are essential for (p)-contacts to achieve effective hole collection in silicon heterojunction (SHJ) solar cells. In this work, we study Plasma-Enhanced Chemical Vapor Deposition p-type hydrogenated nanocrystalline silicon oxide, (p)nc-SiOx:H, combined with (p)nc-Si:H as (p)-contact in front/back-contacted SHJ solar cells. We firstly determine the effect of a plasma treatment at the (i)a-Si:H/(p)-contact interface on the thickness-dependent Ea of (p)-contacts. Notably, when the (p)nc-Si:H layer is thinner than 20 nm, the Ea decreases by applying a hydrogen plasma treatment and a very-high-frequency (i)nc-Si:H treatment. Such an interface treatment also significantly reduces the contact resistivity of the (p)-contact stacks (ρc,p), resulting in an improvement of 6.1%abs in fill factor (FF) of the completed cells. Thinning down the (i)a-Si:H passivating layer to 5 nm leads to a low ρc,p (144 mΩ⋅cm2) for (p)-contact stacks. Interestingly, we observe an increment of FF from 72.9% to 78.3% by using (p)nc-SiOx:H layers featuring larger differences between their optical gap (E04) and Ea, which tend to enhance the built-in potential at the c-Si/(i)a-Si:H interface. Furthermore, we observe clear impacts on ρc,p, open-circuit voltage, and FF by optimizing the thicknesses of (p)-contact that influence its Ea. In front junction cells, the vertical and lateral collection of holes is affected by ρc,p of (p)-contact stacks. This observation is also supported by TCAD simulations which reveal different components of lateral contributions. Lastly, we obtain both front and rear junction cells with certified FF well-above 80% and the best efficiency of 22.47%.

Regarded as a promising candidate for absorber material in photovoltaic applications, $\mathrm{Ba}{\mathrm{Si}}_{2}$ confronts the challenge of high-quality material synthesis via low-cost processes. Here, we fabricated $\mathrm{Ba}{\mathrm{Si}}_{2}$ thin films through the industrially applicable sputtering technique with the face-to-face annealing (FTFA) approach. The employment of the FTFA approach leads to an improvement of the sputtered $\mathrm{Ba}{\mathrm{Si}}_{2}$ from perspectives of surface homogeneity and crystal quality. Various covers are applied in the FTFA, including $\mathrm{Ba}{\mathrm{Si}}_{2}$, glass, and Si, which causes alterations in the film's electrical and optical properties. These impacts of the FTFA method on sputtered $\mathrm{Ba}{\mathrm{Si}}_{2}$ films stem from two aspects, i.e., heat redistributions caused by the variation of thermal networks, and interfacial interactions within the confined space between the cover and the film. The FTFA approach provides a facile strategy for minimizing the impacts of $\mathrm{Ba}{\mathrm{Si}}_{2}$ surface oxidation during high-temperature processes. These results and findings can push forward the material development of $\mathrm{Ba}{\mathrm{Si}}_{2}$ and its photovoltaic applications.

If electric vehicles have to be truly sustainable, it is essential to charge them from sustainable sources of electricity, such as solar or wind energy. In this paper, the design of solar powered e-bike charging station that provides AC, DC and wireless charging of e-bikes is investigated. The charging station has integrated battery storage that enables for both grid-connected and off-grid operation. The DC charging uses the DC power from the photovoltaic panels directly for charging the e-bike battery without the use of an AC charging adapter. For the wireless charging, the e-bike can be charged through inductive power transfer via the bike kickstand (receiver) and a specially designed tile (transmitter) at the charging station, which provides maximum convenience to the user.

Electrical simulations show that the dipole formed at (i)a-Si:H/MoOx interface can explain electrical performance degradation. We experimentally manipulate this interface by a plasma treatment (PT) to mitigate the dipole strength without harming the optical response. The optimal PT + MoOx stack results in strongly improved electrical parameters as compared to the one featuring only MoOx and to the silicon heterojunction reference cell. Optical simulations and experimentally measured currents suggest that the additional PT is responsible of very limited parasitic absorption overcompensated by the thinner MoOx used (3.5 nm) and by the lower losses in the (i)a-Si:H layer underneath.

The knowledge of the structural and compositional details of Si/BaSi2/Si heterostructure annealed at high temperature is a prerequisite for BaSi2 application in heterojunction thin-film solar cells. For this purpose, Si/BaSi2/Si heterostructures deposited by magnetron sputtering with different Si layer thickness are submitted to systematic structural and compositional characterizations. Results reveal a BaSi2/Si heterointerfacial variation caused by surface oxidation and Ba diffusion at the high temperature. Its effects on the optical and electrical properties of Si/BaSi2/Si heterostructure are presented. The outcomes of this work can be extended to BaSi2 deposited by other techniques, and generate substantial advantages in BaSi2 development ranging from improvement on material qualities and eventual deployment in thin-film solar cells.

A quick-scan yield prediction method has been developed to assess rooftop photovoltaic (PV) potential. The method has three main parts. For each roof, first (i) virtual 3D roof segments were reconstructed using aerial imagery, then, (ii) PV modules were automatically fitted onto roof segments using a fitting algorithm and finally, (iii) expected annual yield was calculated. For each roof, the annual yield was calculated by three different quick yield calculation approaches. Two approaches are commercial software packages of Solar Monkey (SM) and Photovoltaic Geographical Information System (PVGIS) whereas the other one is the simplified skyline-based approach developed in photovoltaic material and devices (PVMD) group of Delft University of Technology. To validate the quick-scan method, a set of 145 roofs and 215 roof segments were chosen in urban areas in the Netherlands. For the chosen roofs, the number of fitted modules and calculated yield were compared with the actual modular layout and the measured yield of existing PV systems. Results showed a satisfactory agreement between the quick-scan yield prediction and measured annual yield per roof, with relative standard deviations of 7.2%, 9.1%, and 7.5% respectively for SM, PVGIS, and PVMD approaches. It was concluded that the obstacle-including approaches (e.g. SM and PVMD) outperformed the approaches which neglect the shading by surrounding obstacles (e.g. PVGIS). Results also showed that 3D roof segments had added value as input for the quick-scan PV yield prediction methods since the precision of yield prediction was significantly lower using only 2D land register data of buildings.

In this work, we develop SiOx/poly-Si carrier-selective contacts grown by low-pressure chemical vapor deposition and boron or phosphorus doped by ion implantation. We investigate their passivation properties on symmetric structures while varying the thickness of poly-Si in a wide range (20-250 nm). Dose and energy of implantation as well as temperature and time of annealing were optimized, achieving implied open-circuit voltage well above 700 mV for electron-selective contacts regardless the poly-Si layer thickness. In case of hole-selective contacts, the passivation quality decreases by thinning the poly-Si layer. For both poly-Si doping types, forming gas annealing helps to augment the passivation quality. The optimized doped poly-Si layers are then implemented in c-Si solar cells featuring SiO2/poly-Si contacts with different polarities on both front and rear sides in a lean manufacturing process free from transparent conductive oxide (TCO). At cell level, open-circuit voltage degrades when thinner p-type poly-Si layer is employed, while a consistent gain in short circuit current is measured when front poly-Si thickness is thinned down from 250 to 35 nm (up to +4 mA/cm2). We circumvent this limitation by decoupling front and rear layer thickness obtaining, on one hand, reasonably high current (JSC-EQE = 38.2 mA/cm2) and, on the other hand, relatively high VOC of approximately 690 mV. The best TCO-free device using Ti-seeded Cu-plated front contact exhibits a fill factor of 75.2% and conversion efficiency of 19.6%.

Photovoltaic systems in urban environments are usually partially shaded, thus the need for shade tolerant solar panels in the urban environment. We present the design of a series-parallel reconfigurable photovoltaic module. Given a specific irradiance distribution on its surface, it can change the interconnections between its solar cells to maximize the output power. First, we analyze the main trade-offs involved in the design of such module; then we propose an algorithm to choose the optimal module configuration; finally, we simulate the performance of different solar module architectures inferring the potential gain in annual energy yield.

Vertical space bears great potential of solar energy especially for congested urban areas, where photovoltaic (PV) windows in high-rise buildings can contribute to both power generation and daylight harvest. Previous studies on sun-tracking PV windows strayed into the trade-off between tracking performance and mutual shading, failing to achieve the maximum energy generation. Here we first build integrated models which couple the performance of sun-tracking PV windows to the rotation angles. Secondly, one-degree-of-freedom (DOF) and two-DOF sun tracking are mathematically proven to be not able to gain either maximum power generation or non-glare daylighting under reasonable assumptions. Then we derive the optimum rotation angles of the variable-pivot-three-degree-of-freedom (VP-3-DOF) sun-tracking elements and demonstrate that the optimum VP-3-DOF sun tracking can achieve the aforementioned goals. When the restriction of the proposed model is relaxed, the same performance can be achieved by the optimum one-DOF sun tracking with extended PV slats and particular design of cell layout, requiring less complicated mechanical structures. Simulation results of nine global cities show that the annual energy generation and average module efficiency are improved respectively by 27.40% and 19.17% via the optimum VP-3-DOF sun tracking over the conventional perpendicular sun tracking. The proposed optimum sun-tracking methods also reveal better protection against sun glare. The optimum VP-3-DOF sun tracking is also demonstrated to be applicable to horizontal PV windows, as those applied in the sun roof of a glass greenhouse.

Imaging techniques, like electroluminescence and dark lock-in thermography, are valuable quality control tools as they yield quantitative and spatially resolved information about the device. In this paper, we isolated some of the conductive foil–cell interconnections of back-contact solar cells to study the appearance of these intentional failures in electroluminescence, dark lock-in thermography, and series resistance images. It has been found that isolated emitter-to-foil contacts are clearly visible in the three imaging techniques, as they show characteristic features that deviate from the features typical of functioning emitter-to-foil dots. Isolated base-to-foil contacts are instead invisible in the images obtained by electroluminescence and only hardly visible in the images obtained by the other two techniques. Only after a large amount of contacts are isolated, a local current redistribution or drastic series resistance increase is noticeable. Two graphical methods for the automatic identification of isolated emitter-to-foil contacts in electroluminescence, dark lock-in thermography, and series resistance images were also designed, showing a success rate of 97% in the investigated cells. Such techniques could represent useful tools for implementation in inline quality control processes. Moreover, the techniques and conclusions drawn in this paper can be extended to a large number of other conventional and emerging photovoltaic technologies.

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.

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.

Highest conversion efficiency in crystalline silicon (c-Si) solar cells can be enabled by quenching minority carriers' recombination at c-Si/contact interface owing to carrier-selective passivating contacts. With the semi-insulating poly-crystalline silicon (SIPOS, poly-Si) a very good passivation of c-Si surfaces was obtained. We have explored these passivating structures on IBC solar cells and obtained over 22% efficiency with over 23% within reach on the short term. We present in detail the passivation quality of p-type and n-type ion-implanted LPCVD poly-crystalline silicon (poly-Si) and its relation to the doping profile. Optimized poly-Si layers in the role of emitter and BSF showed excellent passivation (J0,emitter = 11.5 fA/cm2 and J0,BSF = 4.5 fA/cm2) and have been deployed in FSF-based IBC c-Si solar cells using a simple self-aligned patterning process. Applying an optimized passivation of FSF by PECVD a-Si:H/SiNx layer (J0,FSF = 6.5 fA/cm2) leads to a cell with efficiency of 22.1% (VOC = 709 mV, JSC = 40.7 mA/cm2, FF = 76.6%). Since over 83% FF has been reached with adjusted metallization technology on similar IBC structures, we believe 23% efficiency is within reach on the short term. Further improvement, especially at JSC level, is expected by deploying less absorbing carrier-selective passivating contacts based on poly-Si or wide bandgap poly-SiOx layers (J0 ~ 10 fA/cm2).

A major limitation in current liquid-phase crystallized (LPC) silicon thin-film record solar cells is optical losses caused by their planar glass–silicon interface. In this study, silicon is grown on nanoimprinted periodically, as well as randomly textured glass substrates, and successfully implemented into state-of-the-art LPC silicon thin-film solar cells. Compared with an optimized planar reference device, both textures enhance absorption of light. Interlayer and process optimization allowed achieving a material quality comparable with the planar reference device. On the random texture, an open-circuit voltage above 630 mV was obtained, as well as an external quantum efficiency exceeding the planar reference device by +3 mA/cm2.

This work presents the concept of a photovoltaic (PV)-powered solar chimney. We modeled and experimentally studied the integration of a PV system within a naturally ventilated façade (NVF), attempting to use the inherent cavity as a ventilation channel to transfer heat. Thermodynamic models were created to study the thermal and, therefore, the electrical performance of a PV system installed at different positions within the cavity of the NVF. An experimental setup of the PV chimney was manufactured to validate the computational models. Results show low root mean square error (RMSE) values for the prediction of the mass flow and the temperature of the different materials considered in the chimney. A basic sensitivity analysis was performed to find the best position of the PV modules within the chimney for a three-story household in the Netherlands. Optimization showed that with a cavity depth of 0.2 m with PV modules located at the front layer, the electric annual yield is maximized. For the same cavity depth, placing the modules in the middle significantly increases heat flow production, albeit with a reduction on electrical performance.

The concept of view factor has various applications in engineering problems, ranging from heat management to bifacial photovoltaics. Analytical solutions for view factor calculations are difficult to obtain and normally numerical methods are used. For complex geometries, when several surfaces are arbitrarily arranged in a threedimensional environment, conventional numerical approaches such as Monte Carlo method will take a lot of simulation time. To tackle this challenge, we have developed a simple and yet accurate view factor estimationmethod based on ray-casting. In our method, the view factor is determined by sending out rays, evenly distributed in all directions from the target surface, and consequently counting the number of rays intercepted by each of the other surfaces present in the environment under study. Then, a simple algebraic procedure enables the estimation of a large number of view factors simultaneously. The results have been compared with exact and numerical solutions, proving that we have devised a fast and accurate view factor estimation method. This can be used to determine the view factors in environments generated via Light Detection And Ranging (LiDAR). Themethod has the potential to be applied in several scientific researches and engineering studies including heat transfer and solar energy.

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.

A novel back-contacted solar cell based on a submicron copper indium gallium (di)selenide (CIGS) absorber is proposed and optically investigated. First, charge carrier collection feasibility is studied by band diagram analysis. Then, two back-contacted configurations are suggested and optimized for maximum current production. The results are compared with a reference front/back-contacted CIGS solar cell with a 750-nm-thick absorber. Current density production of 38.84 mA/cm 2 is predicted according to our simulations for a realistic front-side texturing. This shows more than 38% improvement in optical performance compared to the reference cell and only 7.7% deviation from the theoretical Green absorption benchmark.

This work shows an alternative surface cleaning method for c-Si wafers to replace the standard chemical procedures as RCA or HNO 3 which involve hazardous chemicals or unstable processes. The method consists in a high-temperature oxidation treatment (HTO) performed in a classical tube furnace that incorporates organic and metal particles present on the c-Si surfaces in the growing SiO 2 layer. The result is as a reliable pre-treatment method for obtaining less defective c-Si surfaces ready for solar cell fabrication after SiO 2 removal. To test the surface passivation quality obtained with our alternative cleaning method, we grow amorphous silicon (a-Si:H) layers by plasma enhanced chemical vapor deposition on both sides of the c-Si wafer and systematically compare the effective carrier lifetime (τ eff ) and implied V OC (iV oc ) to the wafer treated with the standard cleaning in our laboratory. We optimize HTO treatment time reaching τ eff of ∼6 ms and iV oc of 721 mV for the best sample. We ascribe the improved passivation quality using HTO to two concurrent factors. Firstly, the encapsulation of defects into SiO 2 layer that is then etched prior a-Si:H deposition and secondly, to modification of the pyramids’ morphology that facilitates the surface passivation. SEM pictures and reflection measurements support the latter hypothesis.

The deposition of dust, soil, and microfibers resulting from the surroundings, as well as the growth of minute pollens like moss and fungi, contributes toward photovoltaic (PV) module soiling. Soiling is a widely recognized factor that significantly reduces the power production by acting as a barrier for effective light absorption by the module. The estimated loss in the irradiance and power can be determined with the help of a soiling ratio (SR) parameter, which is the ratio of the short-circuit current (Isc) or the maximum power produced (Pmax) by a soiled module to a clean one. The measured SR is normally not constant throughout a day but changes with the position of the Sun and the amount of dust on the module. This paper proposes an empirical equation to determine the SR at any instant of time of the day based on the Sun's angle of incidence on the module and a single SR value measured at the mid of the day. First, an indoor experiment was done to examine the angular loss dependence of two totally different dust colors for the same SR at normal light incidence. Next, in an outdoor experiment, the SR of an artificially soiled module was measured over the course of the day for three conditions of high, medium, and low daily average irradiance due to variation in cloudiness. Then, an empirical equation is introduced based on an incident angle modifier for soiled and cleaned PV modules. The proposed equation was further used to determine the SR. Finally, the average residuals between the measured and the modeled SRs were determined with the help of root-mean-square deviation. The results showed that the modeled SR was determined with a deviation of ±0.21% and ±0.28%, respectively, for high-and medium-irradiance days, whereas the deviation increased to ±1.04% in the case of low irradiance due to clouds.

Reducing the absorber layer thickness below 1 μm for a regular copperindiumgalliumdi-selenide (CIGS)solar celllowers the minimum quality requirements for the absorber layer due to shorter electron diffusion length. Additionally, it reduces material costs and production time. Yet, having such a thin absorber reduces the cell efficiency significantly. This is due to incompletelight absorptionand high Molybdenum/CIGS rear-surface recombination [1]. The aim of this research is to implement some innovative rearsurface modificationson a 430 nm thick CIGS absorber layer to reduce both these affects: analuminium oxidepassivationlayer to reduce the back-surface recombination and point contact openings usingnano-particlesfor electrical contact. The impact of the implementation of all these rear-surface modifications on the opto-electrical properties of the CIGS solar cell will be discussed and analyzed in this paper.

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.

In this work we develop a rear emitter silicon solar cell integrating carrier-selective passivating contacts (CSPCs) with different thermal budget in the same device. The solar cell consists of a B-doped poly-Si/SiOx hole collector and an i/n hydrogenated amorphous silicon (a-Si:H) stack acting as electron collector placed on the planar rear and textured front side, respectively. We investigate the passivation properties of both CSPCs on symmetric structures by optimizing the interdependency among annealing temperature, time and environment. The optimized B-doped poly-Si/SiOx reaches a saturation current density of ~10 fA/cm2 on n-type wafers and an implied open circuit voltage (iVOC) of 716 mV. Furthermore, the i/n a-Si:H stack shows an effective carrier lifetime above 4 ms and iVOC of ~705 mV for cell-relevant layers thickness. After a post-deposition annealing in H2, lifetime is above 10 ms and iVOC = 708 mV. Finally, we optimize the optoelectronic properties of indium-based transparent conductive oxide (Indium Tin Oxide ITO and hydrogenated indium oxide IO:H) to reduce parasitic absorption with a gain in short circuit current density of 0.23 mA/cm2. In conclusion, the optimized layer stacks are implemented at device level obtaining a device with VOC = 704 mV, fill factor of 73.8%, a short circuit current of 39.7 mA/cm2 and 21.0% aperture-area conversion efficiency.

Solar cells based on black silicon (b-Si) are proven to be promising in photovoltaics (PVs) by exceeding 22% efficiency. To reach high efficiencies with b-Si surfaces, the most crucial step is the effective surface passivation. Up to now, the highest effective minority carrier lifetimes are achieved with atomic layer-deposited Al2O3 or thermal SiO2. Plasma-enhanced chemical vapor deposition (PECVD)-grown hydrogenated amorphous silicon (a-Si:H) passivation of b-Si is seldom reported due to conformality problems. In this current study, b-Si surfaces superposed on standard pyramidal textures, also known as modulated surface textures (MSTs), are successfully passivated by PECVD-grown conformal layers of a-Si:H. It is shown that under proper plasma-processing conditions, the effective minority carrier lifetimes of samples endowed with front MST and rear standard pyramidal textures can reach up to 2.3 ms. A route to the conformal growth is described and developed by transmission electron microscopic (TEM) images. Passivated MST samples exhibit less than 4% reflection in a wide spectral range from 430 to 1020 nm.

Carrier-selective passivating contacts have been demonstrated to be crucial to reach the practical efficiency limit of single junction, crystalline silicon (c-Si) based solar cells. Yet, the electrical transport losses affecting the collection of photogenerated carriers remain to be addressed. To this aim, different methodologies and characterization techniques are currently used. In this contribution, we propose the concept of shell as a new terminology to describe carrier-selective passivating contacts. Then, we present a novel characterization methodology using transfer length method (TLM) measurement under variable illumination to investigate the charge-carrier transport in amorphous/crystalline silicon heterojunction (SHJ) n-type contact stacks. We use technology computer-aided design simulation to model a TLM structure and to identify the physical phenomena and the key parameters affecting the contact resistivity ( ρ c ) and the charge carrier accumulation of such contact stacks. Then, the simulation results are compared with experimental data by performing variable-illumination TLM measurements of actual SHJ n-type contact stacks. Specifically, we demonstrate that illumination has a strong impact on the measured ρ c value, highlighting the importance of measuring ρ c under maximum power point conditions for a relevant characterization of solar cell transport losses. In addition, we investigate the dependence of ρ c to a change in the injected carrier density within the c-Si bulk to compare the illumination responses of different SHJ n-type contact stacks. In the quest for maximal efficiency, this method may insightfully complete other characterization techniques to further understand and study the electrical transport in solar cells.

Front and rear contacted wafer-based c-Si solar cells characterized by P-diffused emitter and Al-based back surface field currently constitute the dominant solar cell architecture in the photovoltaic market. The key success of this technology is based on the simple and cost effective fabrication process. However, its conversion efficiency is limited. High-efficiency c-Si solar cells architectures have been demonstrated at laboratory and industrial scale with the aim of decreasing the levelized cost of electricity (LCOE) by increasing efficiency. For this reason, high-efficiency solar cells are expected to increase their market share in next decade. In particular, interdigitated back contacted (IBC) c-Si solar cell architecture, which the current world record efficiency is based on, is expected to gain shortly relevance at industrial level. In this work, activities at TUDelft on the fabrication of IBC c-Si solar cells are reported. In particular, a novel method for realizing high-efficiency IBC c-Si solar cells based on single-side and (relatively) low-temperature doping techniques is demonstrated. In particular, epitaxial growth of B-doped Si is used to form the emitter, while P-ion implantation is deployed to form both front and back surface fields. To pattern the rear junction, a self-aligned process based on one lithographic step has been developed. In addition, metal lift-off is used to define the metal contacts of both polarities. By using this process, efficiency higher than 20% has been demonstrated.

The photovoltaic (PV) industry has recently become more oriented toward n-type c-Si solar cells. Among the different n-type solar cell architectures, bifacial cells are quickly emerging. The open-rear configuration of a bifacial device results in high transmittance (T) losses at long wavelengths (>1000 nm). This limitation is usually overcome at the module level either by using a bifacial encapsulation or by placing a reflective foil on the rear side. In this paper, we have investigated the application of a distributed Bragg reflector (DBR) and TiO2-based white paint (WP) as alternative metal-free back-reflector options applied to the textured open-rear of bifacial n-Pasha cells. Because of the high T losses at long wavelengths of the DBR applied on textured surface, its design and fabrication is studied in detail. The dielectric (DBR and WP) and optimized Ag back-reflectors, which are used as a reference, are applied to bifacial n-Pasha cells, and their performance is evaluated. In particular, we demonstrate T below 20% at 1200 nm by optimizing the DBR thickness for textured surfaces. In addition, the optimized DBR and WP show performance comparable with a state-of-the-art Ag back-reflector. The highest increase of the conversion efficiency is measured for the WP back-reflector: +0.34% absolute compared with n-Pasha measured with no-additional back-reflector.

The optical modelling for optimizing high-efficiency c-Si solar cells endowed with poly-SiOx or poly-SiCx 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-SiOx and poly-SiCx 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.

Barium di-silicide (BaSi2) is a very promising absorber material for high-efficiency thin-film solar cells, due to its suitable bandgap, high light absorption coefficient, and long minority-carrier lifetime. In this study, we compare the nanostructure, layer composition, and point defects of BaSi2 thin films deposited by Radio Frequency (RF) sputtering, Thermal Evaporation (TE), and Molecular Beam Epitaxy (MBE), using Doppler Broadening Positron Annihilation Spectroscopy (DB-PAS) depth profiling, Raman spectroscopy, and x-ray diffraction. Our DB-PAS study on thermally annealed RF-sputter deposited and on TE-deposited BaSi2 layers, in a comparison with high quality BaSi2 films produced by MBE, points to the presence of vacancy-oxygen complexes and Si or Ba mono-vacancies, respectively, in the (poly)crystalline BaSi2 films. The degree of near-surface oxidation increases, going from MBE and TE to the industrially applicable RF-sputtered deposition synthesis. The use of a-Si capping layers on the thermally annealed RF-sputtered BaSi2 films leads to a clear reduction in sub-surface oxidation and improves the quality of the BaSi2 films, as judged from DB-PAS.

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.

Amorphous silicon carbide (a-SiC:H) is a promising material for photoelectrochemical water splitting owing to its relatively small band-gap energy and high chemical and optoelectrical stability. This work studies the interplay between charge-carrier separation and collection, and their injection into the electrolyte, when modifying the semiconductor/electrolyte interface. By introducing an n-doped nanocrystaline silicon oxide layer into a p-doped/intrinsic a-SiC:H photocathode, the photovoltage and photocurrent of the device can be significantly improved, reaching values higher than 0.8V. This results from enhancing the internal electric field of the photocathode, reducing the Shockley-Read-Hall recombination at the crucial interfaces because of better charge-carrier separation. In addition, the charge-carrier injection into the electrolyte is enhanced by introducing a TiO2 protective layer owing to better band alignment at the interface. Finally, the photocurrent was further enhanced by tuning the absorber layer thickness, arriving at a thickness of 150nm, after which the current saturates to 10mAcm-2 at 0V vs. the reversible hydrogen electrode in a 0.2m aqueous potassium hydrogen phthalate (KPH) electrolyte at pH4.