Nowadays, an increasing share of photovoltaic (PV) systems makes use of module- or submodule-level power electronics (PE). Furthermore, PE is used in stand-alone devices powered by PV-storage solutions. One way to facilitate further implementation of PE in PV applications is to integrate PE components into crystalline silicon PV cells. Herein, the COSMOS device is introduced, denoting COmbined Solar cell and metal-oxide-semiconductor field-effect transistor (MOSFET). Specifically, the combined manufacturing of lateral power MOSFETs and interdigitated back contact solar cells with tunnel-oxide passivated contacts (TOPCon) on a single wafer is reported. Many steps of the proposed process flow are used for the fabrication of both devices, enabling cost-effective integration of the MOSFET. Both n-type solar cells with integrated p-channel MOSFETs (PMOS) and p-type solar cells with integrated n-channel MOSFETs (NMOS) are successfully manufactured. NMOS devices perform better in achieving low on-resistance, while PMOS devices exhibit lower leakage currents. Furthermore, the study reveals integration challenges where off-state leakage currents of the MOSFET can increase due to illumination and specific configurations of monolithic interconnections between the MOSFET and the solar cell. Nevertheless, for both n-type and p-type solar cells, efficiencies exceeding 20% are achieved, highlighting the potential of the proposed process for COSMOS devices.

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Due to the unique microstructure of hydrogenated nanocrystalline silicon oxide (nc-SiO_x:H), the optoelectronic properties of this material can be tuned over a wide range, which makes it adaptable to different solar cell applications. In this work, the authors review the material properties of nc-SiO_x:H and the versatility of its applications in different types of solar cells. The review starts by introducing the growth principle of doped nc-SiO_x:H layers, the effect of oxygen content on the material properties, and the relationship between optoelectronic properties and its microstructure. A theoretical analysis of charge carrier transport mechanisms in silicon heterojunction (SHJ) solar cells with wide band gap layers is then presented. Afterwards, the authors focus on the recent developments in the implementation of nc-SiO_x:H and hydrogenated amorphous silicon oxide (a-SiO_x:H) films for SHJ, passivating contacts, and perovskite/silicon tandem devices.

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Monolithic perovskite/c-Si tandem solar cells have attracted enormous research attention and have achieved efficiencies above 30%. This work describes the development of monolithic tandem solar cells based on silicon heterojunction (SHJ) bottom- and perovskite top-cells and highlights light management techniques assisted by optical simulation. We first engineered (i)a-Si:H passivating layers for (100)-oriented flat c-Si surfaces and combined them with various (n)a-Si:H, (n)nc-Si:H, and (n)nc-SiOx:H interfacial layers for SHJ bottom-cells. In a symmetrical configuration, a long minority carrier lifetime of 16.9 ms was achieved when combining (i)a-Si:H bilayers with (n)nc-Si:H (extracted at the minority carrier density of 1015 cm-3). The perovskite sub-cell uses a photostable mixed-halide composition and surface passivation strategies to minimize energetic losses at charge-transport interfaces. This allows tandem efficiencies above 23% (a maximum of 24.6%) to be achieved using all three types of (n)-layers. Observations from experimentally prepared devices and optical simulations indicate that both (n)nc-SiOx:H and (n)nc-Si:H are promising for use in high-efficiency tandem solar cells. This is possible due to minimized reflection at the interfaces between the perovskite and SHJ sub-cells by optimized interference effects, demonstrating the applicability of such light management techniques to various tandem structures.

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Herein, the application of a comprehensive modeling framework that can help optimize the design of multilayered optical filters for coloring photovoltaic (PV) modules is presented based on crystalline silicon solar cells. To overcome technical issues related to the implementation of color filters (CFs) on PV modules, like glare and color instability, colorimetry metrics, such as the hue, chroma, luminance color space, and the quantitative concept of difference between two colors are extensively deployed. It is showcased in this work that designing colored modules with high hue and chroma stability is possible by using a front-side texturing with edged geometry, like V-shaped grooves and inverted pyramids, while obtaining colors with relatively high luminance values, indicating good brightness. Furthermore, it is argued that adapting the rear surface of the front glass with a random textured layout where the CF is applied can improve color and luminance stability without significant loss of chroma while eliminating glare. Finally, the models can be used to optimize the number of layers for a given CF, reducing unnecessary optical losses. Compared to a standard PV module, performance simulation of optimized, bright-colored PV modules predicts relative energy yield losses ranging from 7% to 25%.

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Silicon heterojunction (SHJ) solar cells have achieved a record efficiency of 26.81% in a front/back-contacted (FBC) configuration. Moreover, thanks to their advantageous high V_OC and good infrared response, SHJ solar cells can be further combined with wide bandgap perovskite cells forming tandem devices to enable efficiencies well above 33%. In this study, we present strategies to realize high-efficiency SHJ solar cells through combined theoretical and experimental studies, starting from the optimization of Si-based thin-film layers to the implementation of electrodes with reduced indium and silver usage. Advanced opto-electrical simulations, which enable comprehensive theoretical understandings of the main physical mechanisms governing carriers’ collection and light management, provide clear pathways for device designs and experimental optimizations. We present the fabricated FBC-SHJ solar cells in both monofacial and bifacial configurations with the best efficiencies of 24.18% and 23.25%, respectively. We point out that to achieve optimum device performance, the compositional materials should be holistically optimized and evaluated as part of the contact stacks with adjacent layers. As an outlook beyond the classical FBC-SHJ solar cell architecture, we propose various novel SHJ-based solar cell architectures. Their potential performance was assessed and compared via rigorous opto-electrical simulations and a maximal efficiency of 27.60% was simulated for FBC-SHJ solar cells featuring localized contacts.

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Passivating contacts based on poly-Si have enabled record-high c-Si solar cell efficiencies due to their excellent surface passivation quality and carrier selectivity. The eventual existence of pinholes within the ultra-thin SiOx layer is one of the key factors for carrier collection, beside the tunneling mechanism. However, pinholes are usually believed to have negative impact on the passivation quality of poly-Si passivating contacts. This work studied the influence of the pinhole density on the passivation quality of ion-implanted poly-Si passivating contacts by decoupling the pinhole generation from the dopants diffusion process by means of two annealing steps: (1) a pre-annealing step at high temperature after the intrinsic poly-Si deposition to visualize the formation of pinholes and (2) a post-annealing step for dopants activation/diffusion after ion-implantation. The pinhole density is quantified in the range of 1✕106 to 3✕108 cm2 by the TMAH selective etching approach. The passivation quality is discussed with respect to the pinhole density and the post-annealing thermal budget (TB) for dopants diffusion. The study shows that a moderate pinhole density does not induce doping profile variations that can be detectable by the coarse spatial resolution of ECV measurements. It is surprising that the existence of pinholes in a moderate density within our thickness fixed SiOx layer can effectively enhance the passivation qualities for both n+ and p+ poly-Si passivating contacts. We speculate the reason is due to the enhanced field-effect passivation at the pinhole surrounding. In fact, the variation of the passivation quality depends on the balance between a strengthened field-effect passivation and an excessive local Auger recombination, being both effects induced by the higher and deeper level of dopants diffused into the c-Si surface through the pinholes.@en

Passivating contacts are crucial for realizing high-performance crystalline silicon solar cells. Herein, contact formation by plasma-enhanced chemical vapor deposition (PECVD) followed by an annealing step is focused on. Poly-SiO_x passivating contacts by combining plasma-assisted N₂O-based oxidation of silicon (PANO-SiO_x) with a thin film of phosphorus (n⁺) or boron (p⁺)-doped hydrogenated amorphous silicon oxide (a-SiO_x:H) are manufactured. Postannealing is conducted for transitioning a-SiO_x:H into poly-SiO_x. The aim is to achieve a contact with low absorption and high-quality passivation. It is demonstrated that by tuning the plasma oxidation process time and power, the PANO-SiO_x thickness and its passivation quality can be controlled. A higher SiO₂ content is observed in PANO-SiO_x than in the nitric acid oxidation of silicon (NAOS-SiO_x) counterpart. PANO-SiO_x acts as a stronger diffusion barrier for both boron and phosphorus atoms compared to NAOS-SiO_x, affecting the dopant distribution during annealing. Implied open-circuit voltages up to 751 and 710 mV for n⁺ and p⁺ flat symmetric samples, respectively, are demonstrated. With respect to standard thermally grown SiO₂ tunneling oxide combined with (in/ex)situ-doped low-pressure chemical vapor deposition poly-Si, this study presents a simple alternative for manufacturing passivating contact fully based on PECVD processes.

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Single junction crystalline silicon (c-Si) solar cells are reaching their practical efficiency limit whereas perovskite/c-Si tandem solar cells have achieved efficiencies above the theoretical limit of single junction c-Si solar cells. Next to low-thermal budget silicon heterojunction architecture, high-thermal budget carrier-selective passivating contacts (CSPCs) based on polycrystalline-SiO_x (poly-SiO_x) also constitute a promising architecture for high efficiency perovskite/c-Si tandem solar cells. In this work, we present the development of c-Si bottom cells based on high temperature poly-SiO_x CSPCs and demonstrate novel high efficiency four-terminal (4T) and two-terminal (2T) perovskite/c-Si tandem solar cells. First, we tuned the ultra-thin, thermally grown SiO_x. Then we optimized the passivation properties of p-type and n-type doped poly-SiO_x CSPCs. Here, we have optimized the p-type doped poly-SiO_x CSPC on textured interfaces via a two-step annealing process. Finally, we integrated such bottom solar cells in both 4T and 2T tandems, achieving 28.1% and 23.2% conversion efficiency, respectively.

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Crystalline silicon solar cells with passivating contacts based on doped poly-Si exhibit high optical parasitic losses. Aiming at minimizing these losses, we developed the oxygen-alloyed poly-Si (poly-SiOx) as suitable material for passivating contacts. From passivation point of view, poly-SiOx layers show excellent passivation quality and carrier selectivity for both n-type (iVOC,flat = 740 mV, contact resistance ρc = 0.7 mΩ/cm2, iVOC,textured = 723 mV) and p-type (iVOC,flat = 709 mV, ρc = 0.5 mΩ/cm2). Optically, due to the incorporation of oxygen, the absorption coefficient of poly-SiOx becomes much lower than that of doped poly-Si at long wavelength. Both n-type and p-type poly-SiOx layers are concurrently deployed in front/back-contacted (FBC) solar cells with a front indium tin oxide (ITO) layer to facilitate the lateral transport of carriers and minimize cell's reflection. A high cell FF of 83.5% obtained in double-side flat FBC solar cell indicates an efficient carrier collection by these passivating contacts. An active-area cell efficiency of 21.0% featuring JSC,EQE = 39.7 mA/cm2 is obtained in front-side textured poly-SiOx FBC cell, with the potential of further improvement in both VOC and FF. The optical advantage of poly-SiOx over poly-Si as passivating contact is also observed with a 19.7% interdigitated back-contacted (IBC) solar cell endowed with poly-SiOx emitter and back surface field. Compared to the reference 23.0% IBC solar cell with poly-Si passivating contacts, the one based on poly-SiOx passivating contacts shows higher IQE at wavelengths above 1100 nm. This indicates that for both FBC and IBC cells, poly-SiOx passivating contacts hold potential in enhancing the cell JSC by maximizing the cell spectral response.@en

Excellent surface passivation induced by (i)a-Si:H is critical to achieve high-efficiency silicon heterojunction (SHJ) solar cells. This is key for conventional single-junction cell applications but also for bottom cell application in tandem devices. In this study, we investigated the effects of (i)a-Si:H deposition temperature on passivation quality and SHJ solar cell performance. At the lower end of temperatures ranging from 140°C to 200°C, it was observed with Fourier-transform infrared spectroscopy (FTIR) that (i)a-Si:H films are less dense, thus hindering their surface passivation capabilities. However, with additional hydrogen plasma treatments (HPTs), those (i)a-Si:H layers deposited at lower temperatures exhibited significant improvements and better passivation qualities than their counterparts deposited at higher temperatures. On the other hand, even though we observed the highest V_OCs for cells with (i)a-Si:H deposited at the lowest temperature (140°C), the related FFs are poorer as compared to their higher temperature counterparts. The optimum trade-off between V_OC and FF for the SHJ cells was found with temperatures ranging from 160°C to 180°C, which delivered independently certified efficiencies of 23.71%. With a further improved p-layer that enables a FF of 83.3%, an efficiency of 24.18% was achieved. Thus, our study reveals two critical requirements for optimizing the (i)a-Si:H layers in high-efficiency SHJ solar cells: (i) excellent surface passivation quality to reduce losses induced by interface recombination and simultaneously (ii) less-defective (i)a-Si:H bulk to not disrupt the charge carrier collections.

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In this chapter, we have reviewed candidates for further enhancement of cell efficiencies beyond those of today's mainstream PERC cells, with a focus on technological aspects rather than, e.g. cost. Regarding silicon single junctions, the prevalent theme is the use of carrier-selective passivating contacts, CSPCs. Of these, silicon heterojunction and polysilicon-on-silicon oxide (TOPCon/POLO) are most advanced and have enabled record high efficiencies above and close to 26%, respectively, on n-type silicon wafers. Further important topics are bifacial cell designs, which can be applied to different PV technologies. Single-side efficiencies above 25% have been achieved on bifacial TOPCon and bifacial SHJ solar cells. With proven bankability, bifacial PV products can be expected to gain more momentum in future development. In contrast, contacts based on metal compounds have yielded remarkable results in the last decade, yet failing to clearly evidence a significant advantage compared to the ones based on silicon. Further research is needed to unravel the material combination that would enable the long-awaited ultimate passivating contact for Si solar cells.

The second major topic are tandem and multijunction cells. This is the technology to move beyond the ultimate efficiency barrier of 29.4% for silicon PV and indeed, efficiencies well above 29% have been demonstrated in the lab for Si-based tandems. We have reviewed the current state of the art in lead halide perovskite-silicon tandems as well as III-V/silicon tandems. The former have reached a record PCE of 32.5% in monolithically integrated 2-terminal tandems, while III-V/Si 2T tandems currently stand at 23.4%. However, in III-V-Si devices, the number of absorbers has already been increased further, to three: in triple junction III-V/III-V/Si cells, PCEs of 35.9% have been realized with both 2T and 4T architectures. With a substantially higher cost for the III-V technology as compared to perovskites, but still inferior long-term stability in perovskites, as well as challenges in upscaling for both technologies, it remains to be seen which one of these technologies will gain an advantage. It should be mentioned that an important difference between reported silicon single junction and tandem/multijunction record devices is the cell area: while the single junction Si record devices have "industrial-size" active areas of several tens of cm2 or even full wafers, record tandem cells are lab-scale 1-4 cm2. Thus, up-scaling of tandem cells will remain an important topic in the near future.

At any rate, it can be expected that the exponential growth of PV as well as the diversity of applications (utility, rooftop and BIPV, agri-PV, etc.) will create ample opportunity for the market entry of quite a few of the mentioned technologies, and even for entirely new concepts such as three-terminal tandems or, at the module level, integrated PV and storage systems.@en

Thin films of transition metal oxides such as molybdenum oxide (MoO_x) are attractive for application in silicon heterojunction solar cells for their potential to yield large short-circuit current density. However, full control of electrical properties of thin MoO_x layers must be mastered to obtain an efficient hole collector. Here, we show that the key to control the MoO_x layer quality is the interface between the MoO_x and the hydrogenated intrinsic amorphous silicon passivation layer underneath. By means of ab initio modelling, we demonstrate a dipole at such interface and study its minimization in terms of work function variation to enable high performance hole transport. We apply this knowledge to experimentally tailor the oxygen content in MoO_x by plasma treatments (PTs). PTs act as a barrier to oxygen diffusion/reaction and result in optimal electrical properties of the MoO_x hole collector. With this approach, we can thin down the MoO_x thickness to 1.7 nm and demonstrate short-circuit current density well above 40 mA/cm² and a champion device exhibiting 23.83% conversion efficiency.

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Reducing indium consumption, which is related to the transparent conductive oxide (TCO) use, is a key challenge for scaling up silicon heterojunction (SHJ) solar cell technology to terawatt level. In this work, we developed bifacial SHJ solar cells with reduced TCO thickness. We present three types of In₂O₃-based TCOs, tin-, fluorine-, and tungsten-doped In₂O₃ (ITO, IFO, and IWO), whose thickness has been optimally minimized. These are promising TCOs, respectively, from post-transition metal doping, anionic doping, and transition metal doping and exhibit different opto-electrical properties. We performed optical simulations and electrical investigations with varied TCO thicknesses. The results indicate that (i) reducing TCO thickness could yield larger current in both monofacial and bifacial SHJ devices; (ii) our IWO and IFO are favorable for n-contact and p-contact, respectively; and (iii) our ITO could serve well for both n-contact and p-contact. Interestingly, for the p-contact, with the ITO thickness reducing from 75 nm to 25 nm, the average contact resistivity values show a decreasing trend from 390 mΩ cm² to 114 mΩ cm². With applying 25-nm-thick front IWO in n-contact, and 25-nm-thick rear ITO use in p-contact, we obtained front side efficiencies above 22% in bifacial SHJ solar cells. This represents a 67% TCO reduction with respect to a reference bifacial solar cell with 75-nm-thick TCO on both sides.

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Bifacial (BF) copper-plated crystalline silicon solar cell is an attractive topic to concurrently reduce silver consumption and maintain good device performance. However, it is still challenging to realize a high aspect ratio (AR) of the metal fingers. Herein, a new type of hybrid-shaped Cu finger is electromagnetically fabricated in a BF plating process. Cyclic voltammetry is employed to disclose the electrochemical behaviors of cupric ions in monofacial and simultaneous BF Cu-plating processes, such that the controllability of the plating process could be assessed. The optimal hybrid Cu finger is composed of a rectangular bottom part and a round top part, such that an utmost effective AR value of 1.73 is reached. In BF Cu-plating, two sub-three-electrode electrochemical cells are employed to realize equal metal finger heights on both sides of the wafer. Compared to our low thermal-budget screen-printing metallization, the Cu-plated silicon heterojunction devices show both optical and electrical advantages (based on lab-scale tests). The champion BF Cu-plated device shows a front-side efficiency of 22.1% and a bifaciality factor of 0.99.

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Hydrogenation of polycrystalline silicon (poly-Si) passivating contacts is crucial for maximizing their passivation performance. This work presents the application of Al2O3 prepared by atomic layer deposition as a hydrogenating capping layer. Several important questions related to this application of Al2O3 are addressed by comparing results from Al2O3 single layers, SiNx single layers, and Al2O3/SiNx double layers to different poly-Si types. We investigate the effect of the Al2O3 thickness, the poly-Si thickness, the poly-Si doping type, and the postdeposition annealing treatment on the passivation quality of poly-Si passivating contacts. Especially, the Al2O3/SiNx stack greatly enhances the passivation quality of both n+ and p+ doped as well as intrinsic poly-Si layers. The Al2O3 layer thickness is crucial for the single-layer approach, whereas the Al2O3/SiNx stack is less sensitive to the thickness of the Al2O3 layer. A thicker Al2O3 layer is needed for effectively hydrogenating p+ compared to n+ poly-Si passivating contact. The capping layers can hydrogenate poly-Si layers with thicknesses up to at least 600 nm. The hydrogenation-enhanced passivation for n+ poly-Si is found to be more thermally stable in comparison to p+ poly-Si. These results provide guidelines on the use of Al2O3 capping layers for poly-Si contacts to significantly improve their passivation performance.

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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.@en

Low activation energy (E_a) and wide bandgap (E_g) 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-SiO_x: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 E_a of (p)-contacts. Notably, when the (p)nc-Si:H layer is thinner than 20 nm, the E_a 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 (ρ_c,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 ρ_c,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-SiO_x:H layers featuring larger differences between their optical gap (E₀₄) and E_a, which tend to enhance the built-in potential at the c-Si/(i)a-Si:H interface. Furthermore, we observe clear impacts on ρ_c,p, open-circuit voltage, and FF by optimizing the thicknesses of (p)-contact that influence its E_a. In front junction cells, the vertical and lateral collection of holes is affected by ρ_c,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%.

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Building Integrated Photovoltaic systems can produce a significant portion of the energy demand of urban areas. Despite their potential, they remain a niche technology that architects and project engineers still find esthetically limited. The dark blue or black color of standard photovoltaic panels is considered inappropriate for restoration projects of historic buildings and represents a major constraint on the development of new projects. This work will provide insight into how the use of optic filters can offer new pathways for architectural acceptance of photovoltaic panels. Optic filters selectively reflect or transmit light by interference and can be designed and fabricated using cost-effective and industrially compatible processes. By using in-house developed ray tracing software coupled with TCAD Sentaurus, more than 400 colors were obtained, and their impact on the opto-electrical performance of interdigitated back-contacted solar cells was studied. Results show a maximum efficiency loss of 1.6% absolute at the perpendicular incidence of light on the range of obtained colors when compared with a standard dark blue solar cell. Simulations for different angles of incidence showed that the current reduction on the standard device could be modeled using a cosine relationship. The colored cells, however, deviated significantly from this relationship. We propose that the angular behavior of any cell (colored or standard) could be simulated by modifying the effective irradiance with scaling factors equal to the ratios of the photogenerated current at any angle with respect to the value at normal incidence. We demonstrate that this approach accurately models the effect of the color filter and allows for an easy transition from a bare cell to an encapsulated one. Due to the spectral effect of the filter, we developed both a spectrally resolved optical model and a two-dimensional finite volume transient thermal model. In case of the optical model, we demonstrate an accuracy in the prediction of the reflectance produced by the color with values of mean bias error (MBE) between 2.0% and 3.9%. As for the thermal model, it was validated by first analyzing a standard model under conditions of nominal operating cell temperature and then comparing its results with published scientific literature. Later, we compare its prediction against 2 weeks of measurements. In both cases the thermal model proves an adequate accuracy, yielding differences below 1.5°C with respect to other scientific works and an MBE value of 0.89°C as well as a root-mean-square error value of 2.10°C for the entire measurement period. With the validated models, we studied the effect of the encapsulation on the color perception. We present two options of color filters. The first one produces relatively low reflectance losses and presents relative annual direct current (DC) energy losses of up to 6.4% for Delft, in the Netherlands, and up to 5.9% for Alice Springs in Australia. However, this first option has very poor color brightness. The second studied filter produces highly saturated bright colors. Improving brightness can increase the annual DC relative losses up to 13.7% and 13.5% for Delft and Alice Springs, respectively. Overall, we demonstrate that colored filters based on multilayer optical stacks are a versatile option for coloring cells that allow a good compromise between esthetics and performance.

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Low parasitic absorption and high conductivity enable (n)-type hydrogenated nanocrystalline silicon [(n)nc-Si:H], eventually alloyed with oxygen [(n)nc-SiO_x:H], to be deployed as window layer in high-efficiency silicon heterojunction (SHJ) solar cells. Besides the appropriate opto-electrical properties of these nanocrystalline films, reduction of their thickness is sought for minimizing parasitic absorption losses. Many strategies proposed so far reveal practical limits of the minimum (n)-layer thickness that we address and overcome in this manuscript. We demonstrated the successful application of an ultra-thin layer of only 3-nm-thick based on (n)nc-Si:H PECVD plasma growth conditions without the use of additional contact or buffer layers. For simplicity, we still name (n)nc-Si:H this ultra-thin layer and the solar cell endowed with it delivers a certified efficiency η of 22.20%. This cell shows a 0.61 mA/cm² overall J_SC gain over the (n)a-Si:H counterpart mainly owing to the higher transparency of (n)nc-Si:H, while maintaining comparable V_OC > 714 mV and FF > 80%. Our optimized (n)nc-Si:H layer yields low absorption losses that are commonly measured for (n)nc-SiO_x:H films. In this way, we are able to avoid the detrimental effect that oxygen incorporation has on the electrical parameters of these functional layers. Further, by applying a MgF₂/ITO double-layer anti-reflection coating, a cell with 3-nm-thick (n)nc-Si:H exhibits a J_SC,EQE up to 40.0 mA/cm². By means of EDX elemental mapping, we additionally identified the (n)nc-Si:H/ITO interface as critical for electron transport due to unexpected oxidation. To avoid this interfacial oxidation, insertion of a 2-nm-thick (n)a-Si:H on the 3-nm-thick (n)nc-Si:H contributes to FF gains of 1.4%_abs. (FF increased from 78.6% to 80.0%), and showing further room for improvements.

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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 SiO ₂ 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 SiO ₂/doped poly-Si-poly-Si solar cell, and 3) hybrid solar cell endowed with rear thin SiO ₂/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.

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