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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.

Thanks to the prominent passivating contact structures, silicon heterojunction (SHJ) solar cell has recently achieved revolutionary advancements in the photovoltaic industry. This is, however, bound to further strengthen its contact performance for gaining the competitive edge in the period of technology transformation. Here, we developed SHJ cells with improved rear contact consisting of a p-type doped nanocrystalline silicon and a tailored transparent conductive oxide. Benefiting from the low contact resistance of hole-selective contacts (2), a high power conversion efficiency of 26.74% together with a record filling factor (FF) of 86.48% were certified on industrial-grade silicon wafers (274 cm2, M6 size). The electrical properties of the modified SHJ cells were thoroughly analyzed in comparison with the normal p-type transporting layer counterparts (i.e., amorphous silicon), and the improved charge carrier transport in behind were also fully demonstrated.

Silicon heterojunction (SHJ) solar cells have reached high power conversion efficiency owing to their effective passivating contact structures. Improvements in the optoelectronic properties of these contacts can enable higher device efficiency, thus further consolidating the commercial potential of SHJ technology. Here we increase the efficiency of back junction SHJ solar cells with improved back contacts consisting of p-type doped nanocrystalline silicon and a transparent conductive oxide with a low sheet resistance. The electrical properties of the hole-selective contact are analysed and compared with a p-type doped amorphous silicon contact. We demonstrate improvement in the charge carrier transport and a low contact resistivity (2). Eventually, we report a series of certified power conversion efficiencies of up to 26.81% and fill factors up to 86.59% on industry-grade silicon wafers (274 cm2, M6 size).

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.

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.

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.

Organic-inorganic metal halide perovskites have attracted a considerable interest in the photovoltaic scientific community demonstrating a rapid and unprecedented increase in conversion efficiency in the last decade. Besides the stunning progress in performance, the understanding of the physical mechanisms and limitations that govern perovskite solar cells are far to be completely unravelled. In this work, we study the origin of their hysteretic behaviour from the standpoint of fundamental semiconductor physics by means of technology computer aided design electrical simulations. Our findings identify that the density of shallow interface defects at the interfaces between perovskite and transport layers plays a key role in hysteresis phenomena. Then, by comparing the defect distributions in both spatial and energetic domains for different bias conditions and using fundamental semiconductor equations, we can identify the driving force of hysteresis in terms of slow recombination processes and charge distributions.

In high-efficiency silicon solar cells featuring carrier-selective passivating contacts based on ultrathin SiOx/poly-Si, the appropriate implementation of transparent conductive oxide (TCO) layers is of vital importance. Considerable deterioration in passivation quality occurs for thin poly-Si-based devices owing to the sputtering damage during TCO deposition. Curing treatment at temperatures above 350 °C can recover such degradation, whereas the opto-electrical properties of the TCO are affected as well, and the carrier transport at the poly-Si/TCO contact is widely reported to degrade severely in such a procedure. Here, we propose straightforward approaches, post-deposition annealing at 400 °C in nitrogen, hydrogen, or air ambience, are proposed to tailor material properties of high-mobility hydrogenated fluorine-doped indium oxide (IFO:H) film. Structural, morphological, and opto-electrical properties of the IFO:H films are investigated as well as their inherent electron scattering and doping mechanisms. Hydrogen annealing treatment proves to be the most promising strategy. The resulting layer exhibits both optimal opto-electrical properties (carrier density = 1.5 × 1020 cm-3, electron mobility = 108 cm2 V-1 s-1, and resistivity = 3.9 × 10-4 ω cm) and remarkably low contact resistivities (∼20 mω cm2 for both n- and p-contacts) in poly-Si solar cells. Even though the presented cells are limited by the metallization step, the obtained IFO:H-base solar cell show an efficiency improvement from 20.1 to 20.6% after specific hydrogen treatment, demonstrating the potential of material manipulation and contact engineering strategy in high-efficiency photovoltaic devices endowed with TCOs.

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.

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 VOCs 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 VOC 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.

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.

Thin films of transition metal oxides such as molybdenum oxide (MoOx) 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 MoOx layers must be mastered to obtain an efficient hole collector. Here, we show that the key to control the MoOx layer quality is the interface between the MoOx 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 MoOx by plasma treatments (PTs). PTs act as a barrier to oxygen diffusion/reaction and result in optimal electrical properties of the MoOx hole collector. With this approach, we can thin down the MoOx 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.

We compared the morphology and Raman response of nanoscale shaped surfaces of Si substrates versus monocrystalline Si. Samples were structured by reactive ion etching, and four of them were covered by a RuO2-IrO2 layer. Raman bands, centred at approx. 520 cm–1, belonging to samples processed by etching the Si surface have intensities higher by approximately one order of magnitude than those of reference non-etched samples. For nanostructured samples, the rise in the Raman signal was 12–14 ×, which is in agreement with the model of the electric field at the tips of Si due to their geometry. This phenomenon is related to the high absorption of excitation radiation. Nanostructured surfaces of samples containing a layer of RuO2-IrO2 give rise to the phenomenon of surface enhancement of the Raman response most likely due to the charge transfer at the interface between silicon and conductive oxides. The nanostructured surface of Si without a metal layer behaves as a SERS substrate and detects the analytes at a low concentration.

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.

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.

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.

The study of a two-terminal (2T) perovskite/c-Si tandem solar cell requires accurate and concurrent description of photons absorption and tunnelling-mediated charge transport. By analysing current collection across the device heterointerfaces, we investigated the effect of (i) perovskite thickness on the short-circuit current density (J sc ) of the tandem device and (ii) temperature on devices performance. We deployed an advanced opto-electrical modelling framework based on optical sub-models from GenPro4 and on self-consistent fundamental semiconductor equations implemented in TCAD Sentaurus . Using these simulations of perovskite/c-Si tandem solar cells, an in-depth analysis of the physics of current contribution of supporting layers has been carried out. Solving numerically the fundamental equations of semiconductors, we theoretically show for the first time that electron-hole pairs photo-generated in the TRJ can be collected, effectively boosting J sc values well beyond (photocurrent density) J ph levels. In addition, a temperature-based study of these perovskite/c-Si tandem solar cells has been performed to evaluate the temperature coefficient which is useful for their energy yield simulations.

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.

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%.

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.

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%.

This work presents a detailed analysis of the impact of a rear point-contact (PC) scheme and a selective emitter (SE) design on the performance of a crystalline silicon (c-Si) back contact-back junction (BC-BJ) solar cell featuring narrower highly-doped back surface field (BSF) areas and wider lowly-doped emitter areas. The analysis was performed both through light and dark J-V simulations by using a rigorous full three-dimensional (3D) electro-optical modeling approach to accurately include the 3D effects of several competing physical mechanisms occurring in a PC structure. Simulation results show that the adoption of a relatively thick and wide metallization contacting the silicon at the rear side only via small circular-shaped holes gives an efficiency improvement above 0.3%abs with respect to a conventional BC-BJ solar cell featuring a linear-contact (LC) metallization scheme. Moreover, the introduction of an optimized SE design, featuring a local deeper highly-doped (HDOP) profile underneath rear point contacts and a narrower lowly-doped (LDOP) profile at the non-contacted rear interfaces, can lead to a further efficiency enhancement of about 0.2%abs, which increases with the emitter width.

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 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.

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.

This work presents a study on back contact-back junction solar cells when using a metallization scheme with multiple-emitter contact lines. For this purpose, TCAD-based simulations have been carried out by taking into account realistic parameters and state-of-the-art physical models for silicon solar cells. The analysis has been performed by varying different geometrical parameters related to rear metallization, such as the number of the emitter contacts and the emitter metal coverage. In general, an efficiency improvement has been observed by uniformly placing the multiple contacts along the emitter region and by increasing the number of the emitter metal lines. An optimal emitter metallization fraction has been found for different numbers of contacts as a result of the tradeoff between recombination losses at metal/silicon interfaces and series resistance losses. Moreover, the effect of the multiple-emitter contacts has been evaluated as a function of the finger length, the emitter width, and the rear pitch.

This paper presents a detailed analysis on the impact of the emitter contact geometry on the performance of back contact-back junction (BC- BJ) c-Si solar cells by means of a TCAD-based electro-optical model. BC- BJ c-Si solar cells featuring a conventional linear-contact (LC) scheme and an advanced point-contact (PC) pattern are considered. In addition, the introduction of a selective emitter (SE) design in the PC structure and the effect of including multiple contacts in the emitter region of the considered BC- BJ solar cells are also investigated.

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.

This paper shows the application of carrier-selective passivating contacts (CSPCs) in c-Si front-back contacted (FBC) and interdigitated back-contacted (IBC) solar cells with different thermal budgets. From lifetime analysis of our poly-Si and a-Si:H CSPCs, three FBC and one IBC architectures are devised to progressively increase efficiency (c) and achieve record short-circuit current density (JSC): (i) a poly-poly cell ($\eta $ = 19.6%); (ii) a selective emitter structure known as PeRFeCT (Passivated Emitter Rear and Front ConTacts, $\eta $ = 20.0%); (iii) a so-called hybrid solar cell with poly-Si and a-Si:H CSPCs at rear and front, respectively ($\eta $ = 21.0%); and (iv) an IBC solar cell with poly-Si CSPC ($\eta $ = 23.0%, JSC = 42.2 mA/cm2).

The poly-Si carrier-selective passivating contacts (CSPCs) parasitically absorb a substantial amount of light, especially in the form of free carrier absorption. To minimize these losses, we developed CSPCs based on oxygen-alloyed poly-Si (poly-SiOx) and deployed them in c-Si solar cells. Transmission electron microscopy analysis indicates the presence of nanometer-scale silicon crystals within such poly-SiOx layers. By varying the O content during material deposition, we can manipulate the crystallinity of the poly-SiOx material and its absorption coefficient. Also, depending on the O content, the bandgap of the poly-SiOx material can be widened, making it transparent for longer wavelength light. Thus, we optimized the O alloying, doping, annealing, and hydrogenation conditions. As a result, an extremely high passivation quality for both n-type poly-SiOx (J0 = 3.0 fA/cm2 and iVoc = 740 mV) and p-type poly-SiOx (J0 = 17.0 fA/cm2 and iVoc = 700 mV) is obtained. A fill factor of 83.5% is measured in front/back-contacted solar cells with both polarities made up of poly-SiOx. This indicates that the carrier transport through the junction between poly-SiOx and c-Si is sufficiently efficient. To demonstrate the merit of poly-SiOx layers' high transparency at long wavelengths, they are deployed at the back side of interdigitated back-contacted (IBC) solar cells. A preliminary cell efficiency of 19.7% is obtained with much room for further improvement. Compared to an IBC solar cell with poly-Si CSPCs, a higher internal quantum efficiency at long wavelengths is observed for the IBC solar cell with poly-SiOx CSPCs, thus demonstrating the potential of poly-SiOx in enabling higher JSC.

In this work we present a theoretical analysis of charge carriers transport mechanisms in IBC-SHJ solar cells. The concepts of contact and transport selectivity are correlated through the band bending at c-Si interface and are used to identify thin-film silicon parameters affecting fill factor (FF) and open-circuit voltage (VOC). Additionally, the transport of carriers is associated to energy barriers at the conduction band for electrons and at the valence band for holes. In case of p-type contact, the transport of holes is mainly affected by activation energy and band gap of the p-type layer and work function of the TCO. In case of n-type contact, the activation energy and work function of the doped layer impact the most on transport of electrons. Selective transport is improved by maximizing the collection of majority carrier in each doped contact stack while blocking minority carriers. In particular, low activation energy values of doped layers are crucial to minimize energy barriers for majority carriers and increase the band bending at c-Si interface. Simulation results based on TCAD Sentaurus reveal that the FF increases as the activation energy of the doped layers is reduced. Also, for the p-type contact, the bandgap of p-type layer strongly affects the band bending at c-Si interface. Particularly, widening the bandgap of p-type layer enhances passivation and transport in terms of VOC and FF but work function mismatch between the p-type layer and the related transparent conductive oxide (TCO) strongly increases as bandgap increases. This possibly makes the device less performant because it is more sensitive to activation energy of the p-layer in combination with the choice of the proper TCO. Considering realistic deposited layers, a wide bandgap p-type layer, in combination with low activation energy, potentially improves hole collection leading to maximal simulated FF = 86.8% and VOC = 754 mV for a conversion efficiency η = 27.2%.