Influence of Grain Size on the Band-gap of Annealed SnS Thin Films

The manuscript reports the variation in optical band-gap of vacuum annealed SnS thin films. The samples were characterized by using X-Ray Diffraction, UV-visible Spectroscopy and Raman Analysis. Results show that while annealing does not effect the nano-crystalline sample's lattice structure or unit cell size it does control the grain size. The band-gap (Eg) decreases with increase in grain size. Eg values were found to be very high (1.8-2.5 eV) for samples studied

Process and Post-annealing Optimisation of SnS Thin Films with Alternative Buffer layers

Tin sulphide (SnS) is an environmentally friendly, Earth abundant and easy to fabricate thin film solar absorber for photovoltaic solar cell application. This work examines the properties of thermally evaporated SnS thin films, as a function of deposition parameters. Films were also subjected to a range of post-deposition treatments in vacuum, atmospheric pressure, chlorine and selenium ambient. SnS solar absorber layers were successfully deposited at low temperature (100 oC) to a thickness range from 100 to 3500 nm using thermal evaporation. Grain growth was partly dependent on the layer thickness where a progressive increase in grain size was noticed with increasing film thickness from 100 to 1500 nm; above 1500 nm thickness no further visible increase in the grains could be seen. Films grown to a thickness of 800 nm are found to be near stoichiometry with optimum energy bandgap compared to the thinner or thicker films. However, the SnS thin films showed strong dependence on substrate temperature. The temperature dependent study reveals that higher substrate temperatures lead to an increase in adatoms mobility, thereby promoting coalescences of smaller grains to form bigger grains. The increase in grain size with substrate temperature however stagnates after 350 oC such that further increasing the temperature does not induce further grain growth. Samples deposited at 350 oC substrate temperature were stoichiometric (Sn/S = 1.00) and with energy bandgap of 1.37 eV. Texture coefficient calculations showed that (111) orientation is more likely associated with the substrate temperatures 300 oC while, the (040) diffraction plane is related to higher temperatures (350 oC). Photoluminescence measurements demonstrated that controlling the film composition and optical bandgap is critical to produce a film that will luminesce, a requisite for any implementation in solar devices. On the other hand, the type of susbtrate material was found to significantly influence the properties of the SnS absorber films.The substrates studied include soda lime glass (SLG), quartz (Q), indium tin oxide (ITO) and fluorine-doped tin oxide (FTO) coated glass, molybdenum (Mo) coated SLG and quartz. ii Films composition remains stoichiometric (Sn/S = 1.00 0.01) across the range of substrates. For the Na-free samples, reduction in micro-strain followed an increase in grain size. Unlike kesterite or chalcopyrite materials, the absence of Na in the substrate induces a significant grain growth with the average grain size increasing from 0.14 μm on SLG to 0.32 μm on quartz, ITO and FTO. SnS absorber layers deposited at 350 oC (thickness of 800 nm) were subjected to heat treatment in diverse environments such as vacuum (P = 10-6 mbar, 60 min), nitrogen (P=1000 mbar, 60 min) and selenium (20 min under 10 mbar argon pressure) for temperatures greater than the growth temperature (400-500 oC). Vacuum annealing was ineffective in both inducing grain growth and achieving recrystallisation. Nitrogen ambient revealed a recrystallised structure with slight increase in grain sizes and ~6% decrease in the bandgap compared to the reference 1.37 eV for the as-grown layer due to loss of sulphur (Sn/S ratio increased from 1.00 to 1.27 following anneal). The incorporation of Se led to substantial increase in grains with an average grain size of ~2.0 µm compared to 0.14 µm for as-grown films, with a nearly complete sulphur substitution by selenium. In addition, Se incorporation minimised voids while reducing the bandgap to 1.28 eV, improving photoluminescence yield and the open circuit voltage. Finally, this thesis explores a range of n-type buffer layers in order to fabricate devices. Numerical simulations show that ZnS buffer layer has potential to replace conventional CdS in fabricating SnS-based solar cells as it offers the most appropriate band alignment. Working devices could only be fabricated when combining the selenium heat treatment and the ZnS buffer layer

Deposition and Characterisation of SnS Thin Films for Application in Photovoltaic Solar Cell Devices

Thin films of SnS have been deposited onto heated glass substrates using the thermal evaporation method and the chemical and physical properties of the layers determined and correlated to the deposition conditions and to post-deposition heat treatments. In particular scanning electron microscopy, energy dispersive X-ray analysis, X-ray di.ractrometry and Raman studies were used to determine the material properties, transmittance and reflectance spectroscopy to determine the optical constants and 4-probe and van der Pauw measurements to determine the electrical properties. The results indicate that for a wide range of deposition conditions it is possible to produce high quality layers of SnS that are free from pin-holes and cracks, that are made of densely packed grains, and that adhere strongly to the substrate. For substrate temperatures between 280°C to 360°C it is possible to produce single phase SnS layers. The energy bandgap of these layers was in the range 1.3eV to 1.35eV, was direct, and had an optical absorption coefficient α > 105 cm-1 for photons with energies greater than the energy bandgap. The electrical properties indicate that all the layers are p-conductivity type with resistivities in the range 40Ωcm to 100Ωcm. Solar cell devices were fabricated in the superstrate and substrate configurations using n-type cadmium sulphide (CdS) and zinc indium diselenide (ZIS) buffer layers to partner the p-type SnS. The devices were investigated by measuring the I-V characteristics in the dark, to determine the predominant conduction mechanisms, the I-V characteristics under illumination to determine the open-circuit voltage V, the short circuit current density Jsc, the fill factor FF and solar conversion efficiency of the devices, C-V studies to determine the doping profile in the SnS and the built-in voltage at the junction and spectral response measurements to determine the minority carrier diffusion length in the p-SnS. Devices made with CdS as the n-type partner had a high density of interface states (1.36 x 1011 F C-1cm-2) with low photovoltaic parameters and a negative band offset of -0.36 eV obtained (as measured using x-ray photoelectron spectroscopy). The best devices made were substrate configuration solar cells in which the back contact on glass was molybdenum and the bu.er layer was ZIS. These devices have Voc = 472 mV, Jsc = 16.1 mA/cm2 , FF = 0.38 and a solar conversion efficiency of 2.9%. This is a world record efficiency for SnS-based solar cells at the time of submission of this PhD thesis

Cu2ZnSnS4 thin film solar cells grown by fast thermal evaporation and thermal treatment

Cu2ZnSnS4 thin films have been produced via rapid thermal evaporation of off-stoichiometric kesterite powder followed by annealing in an Ar atmosphere. Different heating rates were applied during the thermal treatments. The chemical composition and structural properties of the deposited layers as well as the distribution of the elements through the kesterite thin film have been investigated. The initial growth of a SnS secondary phase during evaporation led to the formation of this secondary phase next to the Mo back contact. Solar cell power conversion efficiencies were limited to values about 3 % due to this secondary phase. Furthermore, an increased open circuit voltage was demonstrated by using a Zn(O,S) buffer layerThis work was supported by DAAD project (INTERKEST, Ref: 57050358), Marie Curie-ITN (KESTCELLS, GA: 316488) and MINECO project (SUNBEAM, ENE2013-49136-C4-3-R). RC and ES acknowledge financial support from Spanish MINECO within the Ramón y Cajal program (RYC-2011-08521) and (RYC-2011-09212) respectively. SG also thanks the Government of Spain for the FPI fellowship (BES-2014-068533)

The Effect of Chlorine Concentration on the Optical Constants of SnS Thin Films

Chlorine doped SnS have been prepared utilizing chemical spray pyrolysis. The effects of chlorine concentration on the optical constants were studied. It was seen that the transmittance decreased with doping, while reflectance, refractive index, extinction coefficient, real and imaginary parts of dielectric constant were increased as the doping percentage increased. The results show also that the skin depth decrease as the chlorine percentage increased which could be assure that it is transmittance related

The impact of sodium contamination in tin sulfide thin-film solar cells

Through empirical observations, sodium (Na) has been identified as a benign contaminant in some thin-film solar cells. Here, we intentionally contaminate thermally evaporated tin sulfide (SnS) thin-films with sodium and measure the SnS absorber properties and solar cell characteristics. The carrier concentration increases from 2 × 10[superscript 16] cm[superscript −3] to 4.3 × 10[superscript17] cm[superscript−3] in Na-doped SnS thin-films, when using a 13 nm NaCl seed layer, which is detrimental for SnS photovoltaic applications but could make Na-doped SnS an attractive candidate in thermoelectrics. The observed trend in carrier concentration is in good agreement with density functional theory calculations, which predict an acceptor-type Na[subscriptSn] defect with low formation energy.United States. Department of Energy (SunShot Initiative, Contract No. DE-EE0005329)National Science Foundation (U.S.) (Grant No. CHE-11115577)Alexander von Humboldt FoundationNational Science Foundation (U.S.). Graduate Research Fellowship ProgramMIT Energy Initiative (Fellowship)United States. Department of Energy. Office of Energy Efficiency and Renewable Energy (Postdoctoral Research Award)National Science Foundation (U.S.) (Award No. DMR-08-19762)National Science Foundation (U.S.). Center for Nanoscale Systems (Award No. ECS-0335765