As an interesting layered material, molybdenum disulfide (MoS2) has been extensively studied in recent years due to its exciting properties. However, the applications of MoS2 in optoelectronic devices are impeded by the lack of high-quality p–n junction, low light absorption for mono-/multilayers, and the difficulty for large-scale monolayer growth. Here, it is demonstrated that MoS2 films with vertically standing layered structure can be deposited on silicon substrate with a scalable sputtering method, forming the heterojunction-type photodetectors. Molecular layers of the MoS2 films are perpendicular to the substrate, offering high-speed paths for the separation and transportation of photo-generated carriers. Owing to the strong light absorption of the relatively thick MoS2 film and the unique vertically standing layered structure, MoS2/Si heterojunction photodetectors with unprecedented performance are actualized. The self-driven MoS2/Si heterojunction photodetector is sensitive to a broadband wavelength from visible light to near-infrared light, showing an extremely high detectivity up to ≈1013 Jones (Jones = cm Hz1/2 W−1), and an ultrafast response speed of ≈3 μs. The performance is significantly better than the photodetectors based on mono-/multilayer MoS2 nanosheets. Additionally, the MoS2/Si photodetectors exhibit excellent stability in air for a month. This work unveils the great potential of MoS2/Si heterojunction for optoelectronic applications.

MoS2/Si Heterojunction with Vertically Standing Layered Structure for Ultrafast, High‐Detectivity, Self‐Driven Visible–Near Infrared Photodetectors

Laser contact openings for local poly-Si-metal contacts enabling 26.1%-efficient POLO-IBC solar cells

The global photovoltaic (PV) market is dominated by crystalline silicon (c-Si) based technologies with heavily doped, directly metallized contacts. Recombination of photo-generated electrons and holes at the contact regions is increasingly constraining the power conversion efficiencies of these devices as other performance-limiting energy losses are overcome. To move forward, c-Si PV technologies must implement alternative contacting approaches. Passivating contacts, which incorporate thin films within the contact structure that simultaneously supress recombination and promote charge-carrier selectivity, are a promising next step for the mainstream c-Si PV industry. In this work, we review the fundamental physical processes governing contact formation in c-Si. In doing so we identify the role passivating contacts play in increasing c-Si solar cell efficiencies beyond the limitations imposed by heavy doping and direct metallization. Strategies towards the implementation of passivating contacts in industrial environments are discussed. The development of passivating contacts holds great potential for enhancing the power conversion efficiency of silicon photovoltaics. Here, De Wolf et al. review recent advances in material design and device architecture, and discuss technical challenges to industrial fabrication.

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Passivating contacts for crystalline silicon solar cells

A wafer-based monocrystalline silicon photovoltaics road map: Utilizing known technology improvement opportunities for further reductions in manufacturing costs

This letter examines the application of transparent MoOx (x < 3) films deposited by thermal evaporation directly onto crystalline silicon (c-Si) to create hole-conducting contacts for silicon solar cells. The carrier-selectivity of MoOx based contacts on both n- and p-type surfaces is evaluated via simultaneous consideration of the contact recombination parameter J0c and the contact resistivity ρc. Contacts made to p-type wafers and p+ diffused regions achieve optimum ρc values of 1 and 0.2 mΩ·cm2, respectively, and both result in a J0c of ∼200 fA/cm2. These values suggest that significant gains can be made over conventional hole contacts to p-type material. Similar MoOx contacts made to n-type silicon result in higher J0c and ρc with optimum values of ∼300 fA/cm2 and 30 mΩ·cm2 but still offer significant advantages over conventional approaches in terms of contact passivation, optical properties, and device fabrication.

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Molybdenum oxide MoOx: A versatile hole contact for silicon solar cells

Working principle of carrier selective poly-Si/c-Si junctions: Is tunnelling the whole story?

Parasitic Absorption in Polycrystalline Si-layers for Carrier-selective Front Junctions

The surface passivation properties of thermal-SiO2/plasma-enhanced chemical vapor deposition SiN stacks on 2.5 Ω cm n-type Czochralski silicon substrates have been investigated. By annealing these stacks in air we achieve surface recombination velocities (SRV) lower than 2.4 cm/s for thin SiO2 layers. We find a clear correlation between the thickness of the oxide layers and the annealing duration. We also show that the absolute passivation quality of the SiO2/SiN stacks correlates to the SiO2 thickness. We find that the SRV increases with increasing oxide thickness. We also present data of the surface passivation of these SiO2/SiN stacks after storage in the dark for several weeks.

Surface passivation of n-type Czochralski silicon substrates by thermal-SiO2/plasma-enhanced chemical vapor deposition SiN stacks

In the pursuit of ever higher conversion efficiencies for silicon photovoltaic cells, polycrystalline silicon (poly-Si) layers on thin silicon oxide films were shown to form excellent carrier-selective junctions on crystalline silicon substrates. Investigating the pinhole formation that is induced in the thermal processing of the poly-Si on oxide (POLO) junctions is essential for optimizing their electronic performance. We observe the pinholes in the oxide layer by selective etching of the underlying crystalline silicon. The originally nm-sized pinholes are thus readily detected using simple optical and scanning electron microscopy. The resulting pinhole densities are in the range of 6.6 × 106 cm−2 to 1.6 × 108 cm−2 for POLO junctions with selectivities close to S10 = 16, i.e., saturation current density J0c below 10 fA/cm2 and contact resistivity ρc below 10 mΩcm2. The measured pinhole densities agree with values deduced by a pinhole-mediated current transport model. Thus, we conclude pinhole-mediated current transport to be the dominating transport mechanism in the POLO junctions investigated here.

Pinhole density and contact resistivity of carrier selective junctions with polycrystalline silicon on oxide

We investigate the passivation quality of hole-collecting junctions consisting of thermally or wet-chemically grown interfacial oxides, sandwiched between a monocrystalline-Si substrate and a p-type polycrystalline-silicon (Si) layer. The three different approaches for polycrystalline-Si preparation are compared: the plasma-enhanced chemical vapor deposition (PECVD) of in situ p+-type boron-doped amorphous Si layers, the low pressure chemical vapor deposition (LPCVD) of in situ p+-type B-doped polycrystalline Si layers, and the LPCVD of intrinsic amorphous Si, subsequently ion-implanted with boron. We observe the lowest J0e values of 3.8 fA cm−2 on thermally grown interfacial oxide on planar surfaces for the case of intrinsic amorphous Si deposited by LPCVD and subsequently implanted with boron. Also, we obtain a similar high passivation of p+-type poly-Si junctions on wet-chemically grown oxides as well as for all the investigated polycrystalline-Si deposition approaches. Conversely, on alkaline-textured surfaces, J0e is at least 4 times higher compared to planar surfaces. This finding holds for all the junction preparation methods investigated. We show that the higher J0e on textured surfaces can be attributed to a poorer passivation of the p+ poly/c-Si stacks on (111) when compared to (100) surfaces.

Yevgeniya Larionova

Papers

Working principle of carrier selective poly-Si/c-Si junctions: Is tunnelling the whole story?

Parasitic Absorption in Polycrystalline Si-layers for Carrier-selective Front Junctions

Surface passivation of n-type Czochralski silicon substrates by thermal-SiO2/plasma-enhanced chemical vapor deposition SiN stacks

Pinhole density and contact resistivity of carrier selective junctions with polycrystalline silicon on oxide

On the recombination behavior of p+‐type polysilicon on oxide junctions deposited by different methods on textured and planar surfaces