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Register a new userA study of SnS thin films and its suitability for photovoltaic application based on the existence of persistent photocurrent
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Tin Sulphide is a layered compound which retains its structure when deposited as thin films by thermal evaporation. The films were found to have oriented growth with the direction of orientation changing with film thickness. The films morphology was found to change with orientation. The poor conductivity of the thicker samples made it difficult to make photocunductivity characterisation. However, unlike reported the thinner samples showed photo-sensitivity to be independent of film thickness and grain size with a high persistent photocurrent. With their absorption, photosensitivty, optimum band-gap and traps within the band-gap giving the charge carriers a longer life-time, thin samples of tin sulphide gives adequate scope designing efficient photovoltaics. The refractive index was modeled using Sellmeirs model, while most of the previous studies talk of Wemple-DiDomenico single oscillator model or Cauchys dispersion relation. The Sellmeirs fitting parameters are reported which can be of use in ellipsometric studies.
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Tin sulphide thin films of p-type conductivity were grown on glass substrates. The refractive index of the as grown films, calculated using both Transmission and ellipsometry data were found to follow the Sellmeier dispersion model. The improvement in the dispersion data obtained using ellipsometry was validated by Wemple-Dedomenico (WDD) single oscillator model fitting. The optical properties of the films were found to be closely related to the structural properties of the films. The band-gap, its spread and appearance of defect levels within the band-gap intimately controls the refractive index of the films.
Tin monosulfide (SnS) usually exhibits p-type conduction due to the low formation enthalpy of acceptor-type defects, and as a result n-type SnS thin films have never been obtained. This study realizes n-type conduction in SnS thin films for the first time by using RF-magnetron sputtering with Cl doping and sulfur plasma source during deposition. N-type SnS thin films are obtained at all the substrate temperatures employed in this study (221-341 C), exhibiting carrier concentrations and Hall mobilities of ~2 x 10 18 cm-3 and 0.1-1 cm V-1s-1, respectively. The films prepared without sulfur plasma source, on the other hand, exhibit p-type conduction despite containing a comparable amount of Cl donors. This is likely due to a significant amount of acceptor-type defects originating from sulfur deficiency in p-type films, which appears as a broad optical absorption within the band gap. The demonstration of n-type SnS thin films in this study is a breakthrough for the realization of SnS homojunction solar cells, which are expected to have a higher conversion efficiency than the conventional heterojunction SnS solar cells.
The parameters influencing the band gap of tin sulphide thin nano-crystalline films have been investigated. Both grain size and lattice parameters are known to influence the band gap. The present study initially investigates each contribution individually. The experimentally determined dependency on lattice parameter is verified by theoretical calculations. We also suggest how to treat the variation of band gap as a two variable problem. The results allow us to show dependency of effective mass (reduced) on lattice unit volume.
Van der Waals junctions of two-dimensional materials with an atomically sharp interface open up unprecedented opportunities to design and study functional heterostructures. Semiconducting transition metal dichalcogenides have shown tremendous potential for future applications due to their unique electronic properties and strong light-matter interaction. However, many important optoelectronic applications, such as broadband photodetection, are severely hindered by their limited spectral range and reduced light absorption. Here, we present a p-g-n heterostructure formed by sandwiching graphene with a gapless bandstructure and wide absorption spectrum in an atomically thin p-n junction to overcome these major limitations. We have successfully demonstrated a MoS2-graphene-WSe2 heterostructure for broadband photodetection in the visible to short-wavelength infrared range at room temperature that exhibits competitive device performance, including a specific detectivity of up to 1011 Jones in the near-infrared region. Our results pave the way toward the implementation of atomically thin heterostructures for broadband and sensitive optoelectronic applications.
Neutron reflectometry is a powerful tool used for studies of surfaces and interfaces. In general the absorption in the typical studied materials can be neglected and this technique is limited to the measurement of the reflectivity only. In the case of strongly absorbing nuclei the number of neutrons is not conserved and the absorption can be directly measured by using the neutron-induced fluorescence technique which exploits the prompt particle emission of absorbing isotopes. This technique is emerging from soft matter and biology where highly absorbing nuclei, generally in very small quantities, are used as a label for buried layers. Nowadays the importance of highly absorbing layers is rapidly increasing, partially because of their application in neutron detection; a field that has become more and more active also due to the 3He-shortage. In this manuscript we extend the neutron-induced fluorescence technique to the study of thick layers of highly absorbing materials; in particular 10B4C. The theory of neutron reflectometry is a commonly studied topic, however the subtle relationship between the reflection and the absorption of neutrons is not widely known, in particular when a strong absorption is present. The theory for a general stack of absorbing layers has been developed and compared to measurements. This new technique has potential as a tool for characterization of highly absorbing layers. We also report on the requirements that a 10B4C layer must fulfill in order to be employed as a converter in neutron detection.
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