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.@en

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.@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-SiOx passivating contacts by combining plasma-assisted N2O-based oxidation of silicon (PANO-SiOx) with a thin film of phosphorus (n+) or boron (p+)-doped hydrogenated amorphous silicon oxide (a-SiOx:H) are manufactured. Postannealing is conducted for transitioning a-SiOx:H into poly-SiOx. 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-SiOx thickness and its passivation quality can be controlled. A higher SiO2 content is observed in PANO-SiOx than in the nitric acid oxidation of silicon (NAOS-SiOx) counterpart. PANO-SiOx acts as a stronger diffusion barrier for both boron and phosphorus atoms compared to NAOS-SiOx, 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

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 MoOx hole collectors with ultra-thin TCO layers is performed. Firstly, bifacial SHJ solar cells with MoOx 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-SiOx: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 MoOx and n-contact side, respectively. Specifically, when illuminating from the MoOx 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 MoOx 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

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 MoOx hole collectors with ultra-thin TCO layers is performed. Firstly, bifacial SHJ solar cells with MoOx 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-SiOx: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 MoOx and n-contact side, respectively. Specifically, when illuminating from the MoOx 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 MoOx 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

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 (MoOx) layer as an alternative to the (p)nc-Si:H blanket layer. We demonstrate that the use of such a thin MoOx 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/MoOx 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 MoOx 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

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 VOC 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