Polycrystalline silicon (poly-Si) carrier-selective passivating contacts (CSPCs), featuring high photoconversion efficiency (PCE) and cost-effectiveness, have emerged as a promising approach for high-efficiency crystalline silicon (c-Si) solar cells. To minimize parasitic absorption losses induced by doped poly-Si window layers, wide bandgap oxygen-alloyed poly-Si (poly-SiO_x) layers are developed. However, challenges persist in achieving excellent surface passivation for boron-doped poly-SiO_x contact stacks, likely caused by boron diffusion during annealing and the reduced doping concentration resulting from lower crystallinity as oxygen content increases. In this study, we investigate the impact on the passivating contact structure and solar cell performance of a 10-nm thick intrinsic hydrogenated amorphous silicon buffer layer with varying oxygen content (a-Si (O_x):H) deposited by plasma-enhanced chemical vapor deposition (PECVD), and placed between the tunneling silicon oxide (SiO_x) and the poly-SiO_x (p⁺). After the hydrogenation step, we obtain both high passivation quality with implied open circuit voltage (iV_oc) of 728.3 mV and low contact resistivity (ρ_c) of 59.18 mΩ cm² on polished surface for oxygen-free a-Si:H buffer layer. These improvements can be attributed to the appropriate thickness of the tunnel oxide and confirmed by transmission electron microscopy (TEM) images, to higher crystallinity of the buffer layer, which facilitates more efficient doping in the buffer layer. This is evidenced by energy dispersive X-ray spectroscopy (EDX), and X-ray photoelectron spectroscopy (XPS) results. At the device level, a front-side textured, rear-side flat, rear junction poly-SiO_x/poly-SiO_x solar cell on n-type c-Si wafer, an efficiency improvement can be observed from 3.55 % without a PECVD buffer layer to 18.9 % with an oxygen-free a-Si:H PECVD buffer layer. The impact of the buffer layer crystallinity on cell performance is further demonstrated by deploying a 10-nm thick LPCVD buffer layer, which facilitates an efficiency of 21.15 % for the same device structure.

@en

Throughout the development of silicon heterojunction (SHJ) solar cells, the transparent conductive oxide has been regarded as an essential component of their front electrode, facilitating lateral charge transport of photogenerated carriers toward the front metal grid fingers. In rear junction (RJ)-SHJ solar cells, the (n)c-Si bulk is known to support the lateral electron transport at maximum power point injection level, provided that the contact resistance of the front contact stack is sufficiently low. This enables experimental RJ-SHJ solar cell architectures featuring a localized front carrier-selective passivating contact exclusively covering the area contacted by the metal grid. Herein, a top-down approach to the synthesis of this type of architecture is studied and its optical and electrical performance applied to different (n)-type contacts are investigated. Additionally, the potential of the localized contact architecture through Cu-plated RJ-SHJ solar cells is demonstrated. These solar cell demonstrators feature high short-circuit current density of 40.5 mA cm⁻², without significantly compromising their open-circuit voltage or fill factor, enabling efficiencies well above 23%, a 2%_abs improvement compared to their state before localization of the front contact.

@en

The fabrication process of interdigitated-back-contacted silicon heterojunction (IBC-SHJ) solar cells has been significantly simplified with the development of the so-called tunnel-IBC architecture. This architecture utilizes a highly conductive (p)-type nanocrystalline silicon (nc-Si:H) layer deposited over the full substrate area comprising pre-patterned (n)-type nc-Si:H fingers. In this context, the (p)-type nc-Si:H layer is referred to as blanket layer. As both electrodes are connected to the same blanket layer, the high lateral conductivity of (p)nc-Si:H layer can potentially lead to relatively low shunt resistance in the device, thus limiting the performance of such solar cells. To overcome such limitation, we introduce a thin (<2 nm) full-area molybdenum oxide (MoO_x) layer as an alternative to the (p)nc-Si:H blanket layer. We demonstrate that the use of such a thin MoO_x minimizes the shunting losses thanks to its low lateral conductivity while preserving the simplified fabrication process. In this process, a novel (n)-type nc-Si:H/MoO_x electron collection contact stack is implemented within the proposed solar cell architecture. We assess its transport mechanisms via electrical simulations showing that electron transport, unlike in the case of tunnel-IBC, occurs in the conduction band fully. Moreover, the proposed contact stack is evaluated in terms of contact resistivity and integrated into a proof-of-concept front/back-contacted (FBC) SHJ solar cells. Contact resistivity as low as 100 mΩcm² is achieved, and fabricated FBC-SHJ solar cells obtain a fill factor above 81.5% and open-circuit voltage above 705 mV. Lastly, the IBC-SHJ solar cells featuring the MoO_x blanket layer are fabricated, exhibiting efficiencies up to 21.14% with high shunt resistances above 150 kΩcm². Further optimizations in terms of layer properties and fabrication process are proposed to improve device performance and realize the efficiency potential of our novel IBC-SHJ solar cell architecture.

@en

Reducing indium consumption in transparent conductive oxide (TCO) layers is crucial for mass production of silicon heterojunction (SHJ) solar cells. In this contribution, optical simulation-assisted design and optimization of SHJ solar cells featuring MoO_x hole collectors with ultra-thin TCO layers is performed. Firstly, bifacial SHJ solar cells with MoO_x as the hole transport layer (HTL) and three types of n-contact as electron transport layer (ETL) are fabricated with 50 nm thick ITO on both sides. It is found that bilayer (nc-Si:H/a-Si:H) and trilayer (nc-SiO_x:H/nc-Si:H/a-Si:H) as n-contacts performed electronically and optically better than monolayer (a-Si:H) in bifacial SHJ cells, respectively. Then, as suggested by optical simulations, the same stack of tungsten-doped indium oxide (IWO) and optimized MgF₂ layers are applied on both sides of front/back-contacted SHJ solar cells. Devices endowed with 10 nm thick IWO and bilayer n-contact exhibit a certified efficiency of 21.66% and 20.66% when measured from MoO_x and n-contact side, respectively. Specifically, when illuminating from the MoO_x side, the short-circuit current density and the fill factor remain well above 40 mA cm⁻² and 77%, respectively. Compared to standard front/rear TCO thicknesses (75 nm/150 nm) deployed in monofacial SHJ solar cells, this represents over 90% TCO reduction. As for bifacial cells featuring 50 nm thick IWO layers, a champion device with a bilayer n-contact as ETL is obtained, which exhibits certified conversion efficiency of 23.25% and 22.75% when characterized from the MoO_x side and the n-layer side, respectively, with a bifaciality factor of 0.98. In general, by utilizing a n-type bilayer stack, bifaciality factor is above 0.96 and it can be further enhanced up to 0.99 by switching to a n-type trilayer stack. Again, compared to the aforementioned standard front/rear TCO thicknesses, this translates to a TCO reduction of more than 67%.

@en

Transition metal oxide (TMO) thin films exhibit large bandgap and hold great potential for enhancing the performance of silicon heterojunction (SHJ) solar cells by increasing the short-circuit current density significantly. On the other hand, achieving precise control over the electrical properties of TMO layers is crucial for optimizing their function as efficient carrier-selective layer. This study demonstrates a general and feasible approach for manipulating the quality of several TMO films, aimed at enhancing their applicability in silicon heterojunction (SHJ) solar cells. The core of our method involves precise engineering of the interface between the TMO film and the underlying hydrogenated intrinsic amorphous silicon passivation layer by managing the reaction of the TMO on the surface. X-ray photoelectron spectroscopy spectra demonstrate that our methods can modify the oxygen content in TMO films, thereby adjusting their electronic properties. By applying this method, we have successfully fabricated WO_x-based SHJ solar cells with 23.30 % conversion efficiency and V₂O_x-based SHJ solar cells with 22.04 % conversion efficiency, while keeping n-type silicon-based electron-transport layer at the rear side. This research paves the way for extending such interface engineering methods to other TMO materials used as hole-transport layers in SHJ solar cells.

@en

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.

@en

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.

@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.

@en

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.

@en

Large-scale deployment of photovoltaic (PV) technology is imperative for realizing a future sustainable and electrified energy system. Over the past decades, technological advancements that enhance the efficiency of PV technologies have been one of the crucial aspects for significantly reducing the cost of PV-generated electricity. Among various crystalline silicon (c-Si) PV technologies, silicon heterojunction (SHJ) solar cells, which have achieved the highest efficiency of single-junction c-Si solar cells, hold great promise for advancing the energy transition facilitated by PV technologies even further. Moreover, notable efficiency enhancements, which are well beyond the theoretical efficiency limit of single-junction c-Si solar cells, have been experimentally demonstrated by combining SHJ solar cells with semi-transparent perovskite solar cells in tandem configurations. This thesis focuses on addressing the challenges of efficient deployments of doped hydrogenated nanocrystalline silicon-based (nc-Si:H-based) layers for high-efficiency front/back-contacted (FBC) SHJ solar cells and applications of FBC-SHJ bottom-cells in two-terminal (2T) and four-terminal (4T) tandem devices with perovskite top-cells, supported by advanced opto-electrical simulations.@en

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

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.

@en

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

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.

@en

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.

@en

Two terminal multi-junction (MJ) photovoltaic (PV) devices are well established concepts to increase the solar-to-electrical power conversion in reference to single PV junctions. In multi-junction PV devices two consecutive sub-cells are interconnected using a tunnel recombination junction (TRJ) in which the light excited holes of one sub-cell recombine with the light excited electrons of the other sub cell. An ideal TRJ is an ohmic contact with non-rectifying behaviour. TRJ's based on p- and n-doped silicon-oxides have been successfully applied in a variety of hybrid multi-junction PV devices in which tunnelling and trap-assisted tunnelling over width of 5–20 nm rules the TRJ's recombination kinetics. In this contribution the qualitative fundamental working principles of tunnel recombination junctions based on p- and n-doped silicon and silicon-oxide alloys are revealed using both electrical modelling and experiments based on a unique set of tandem lab cells (four types based on four different PV materials) combined with structural variations in TRJ architectures. The study results in design rules for the integration of silicon-oxide based TRJ's and provides fundamental insights into the sensitivity of the electrical performance of the TRJ's to doping concentrations, to alignment of the conduction and valence bands of consecutive sub-cells, to the nature of interface defects, to the growth of amorphous and crystalline phases and its dependence on substrate or seed layers and to the nanoscale thicknesses of the TRJ layers.

@en

Solar-assisted water electrolysis is a promising technology for storing the energy of incident solar irradiation into hydrogen as a fuel. Here, an integrated continuous flow electrochemical reactor coupled to a monolithic perovskite-silicon tandem solar cell is demonstrated that provides light-driven electrochemical solar-to-hydrogen conversion with an energy conversion efficiency exceeding 21% at 1-Sun equivalent light intensity and stable operation during three simulated day-night cycles.

@en

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.

@en

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.

@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%.

@en