Investigation of ZnSe and ZnSxSe1-x for application in thin film solar cells

Abstract Successful deposition of ZnSe and ZnS„Sei_x layers has been performed with close-spaced thermal evaporation (CSTE). ZnSe (Eg = 2.67eV) and ZnS,Sei, (Eg to 3.6eV) have the potential to replace CdS (Eg = 2.42 eV) as a buffer layer in solar cell applications, giving a two-fold benefit: (i) increased blue response of the cell, potentially allowing more light to reach the pn junction and contribute to photogeneration and (ii) reduce the toxic Cd element of the buffer layer. CSTE has produced films in which the deposition parameters can be controlled to alter the morphology of the as-deposited coatings. SEM and AFM investigations have shown that pinhole free ZnSe and ZnS„Sei_x films can be produced with this deposition process. In addition, the ZnSxSei, layers show a linear shift in lattice constant and a systematic shift in energy bandgap with alloy composition. XRD data and the steep absorption edges in the transmittance data confirm the good crystallinity of the layers. To partner the ZnSe and ZnS,,Sei_x buffer layers in a thin film heterojunction, CdTe absorber layers were grown in the superstrate configuration. These CdTe layers were deposited in the same deposition chamber, without breaking vacuum, to reduce the risk of interfacial contamination. ZnSe and ZnSxSei_x / CdTe solar cells were fabricated with the best cell producing PV characteristics of: short circuit current 17mAcm-2, open circuit voltage 460mV and efficiency approaching 3%. The spectral response of all ZnSe and ZnSxSei_x / CdTe devices demonstrated a systematic shift to shorter wavelengths with increasing alloy composition, therefore showing the potential of these materials to increase solar cell efficiency. This low cost deposition process has shown excellent potential to be scaled up for commercial applications

Studies of p-type semiconductor photoelectrodes for tandem solar cells

Photoelectrodes and photovoltaic devices have been prepared via multiple thin film deposition methods. Aerosol assisted chemical vapour deposition (AACVD), electrodeposition (ED), chemical bath deposition (CBD) and doctor blade technique (DB) have been used to deposit binary and ternary metal oxide films on FTO glass substrates. The prepared thin films were characterised by a combination of SEM (Scanning Electron Microscopy), powder X-ray diffraction, mechanical strength tests and photochemical measurements. Nickel oxide (NiO) thin films prepared by AACVD were determined to have good mechanical strength . with a photocurrent of 7.6 μA cm-2 at 0 V and an onset potential of about 0.10 V. This contrasted with the dark current density of 0.3 μA cm-2 at 0 V. These NiO samples have very high porosity with crystalline columns evidenced by SEM. In comparison with the AACVD methodology, NiO films prepared using a combination of ED and DB show good mechanical strength but a higher photocurrent of 24 μA cm-2 at 0 V and an onset potential of about 0.10 V with a significantly greater dark current density of 7 μA cm-2 at 0 V. The characteristic features shown in the SEM are smaller pores compared to the AACVD method. Copper (II) oxide (CuO) and copper (I) oxide (Cu2O) films were fabricated by AACVD by varying the annealing temperature between 100-325°C in air using a fixed annealing time of 30 min. It was shown by photocurrent density (J-V) measurements that CuO produced at 325 °C was most stable and provided the highest photocurrent of 173 μA cm-2 at 0 V with an onset potential of about 0.23 V. The alignment of zinc oxide (ZnO) nano-rods and nano-tubes fabricated by CBD have been shown to be strongly affected by the seed layer on the FTO substrate. SEM images showed that AACVD provided the best seed layer for aligning the growth of the nano-rods perpendicular to the surface. Nano-rods were successfully altered into nano-tubes using a potassium chloride bath etching method. NiO prepared by both AACVD and the combined ED/DB method were sensitized to absorb more of the solar spectrum using AACVD to deposit CuO over the NiO. A large increase in the photocurrent was observed for the p-type photoelectrode. These p-type photoelectrode showed a photocurrent density of approximately 100 µA cm-2 at 0 V and an onset potential of 0.3 V. This photocathode was then used as a base to produce a solid state p-type solar cell. For the construction of the solid state solar cells several n-type semiconductors were used, these were ZnO, WO3 and BiVO4. WO3 and BiVO4 were successfully produced with BiVO4 proving to be the optimum choice. This cell was then studied more in depth and optimised by controlling the thickness of each layer and annealing temperatures. The best solid state solar cell produced had a Jsc of 0.541 μA cm-2 (541 nA) and a Voc of 0.14 V, TX146 made up of NiO 20 min, CuFe2O4 50 min, CuO 10 min, BiVO4 27 min, using AACVD and then annealed for 30 min at 600 °C

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

Low-pressure Chemical Vapour Deposition of Silicon Nanoparticles:Synthesis and Characterisation

emiconductor nanostructures such as quantum wells, quantum wires or quantum dots exhibit superior properties in comparison to their bulk forms. Quantum dots are described aszero-dimensional electron gas system, as carriers are confined in all the three directions. Densityof states is discrete function of energy. Allowed energy spectrum is discrete like in an atom.Energy band gap is broadened due to carriers confinement. Semiconductor quantum dots exhibittypical coulomb blockade characteristic which is exploited for development of new generationof nanoelectronic devices namely single-electron transistor, memories, etc, whose operationdepends on quantum mechanical tunneling of carriers through energy barriers. Thesesemiconductor nanostructures emit light in visible range upon excitation by optical means. Inrecent years,  research  has been focused on different nano-scale materials; metals (Au, Ag, Fe,Mn, Ni), metal oxides (SnO2, ZnO2), compound semiconductors (GaAs, GaAlAs, CdSe, CdS,GaN), and elemental semiconductors (silicon and germanium). As silicon is the most favouredmaterial in the established integrated circuits manufacturing technology, research is being donefor controlled synthesis and characterisation of Si nanoparticles. The Si nanoparticles havebeen synthesised on oxide and nitride layers over  Si substrate by IC technology compatiblelow-pressure chemical vapour deposition technique. Atomic force microscopy (AFM)characterisation has been extensively carried out on the samples. It is shown that the tip radiusand shape of tip lead to less accurate estimate of the actual size. The AFM images have been evaluated based on the real surface topography and shape of the tip. Photolumine scence (PL) studies have been performed to characterise the samples. The PL measurements showed visiblelight emission from synthesised silicon nanoparticles.Defence Science Journal, 2008, 58(4), pp.550-558, DOI:http://dx.doi.org/10.14429/dsj.58.167

Development of high efficiency dye sensitized solar cells: novel conducting oxides, tandem devices and flexible solar cells

Photovoltaic technologies use light from the sun to create electricity, using a wide range of materials and mechanisms. The generation of clean, renewable energy using this technology must become price competitive with conventional power generation if it is to succeed on a large scale. The field of photovoltaics can be split into many sub-groups, however the overall aim of each is to reduce the cost per watt of the produced electricity. One such solar cell which has potential to reduce the cost significantly is the dye sensitised solar cell (DSC), which utilises cheap materials and processing methods. The reduction in cost of the generated electricity is largely dependent on two parameters. Firstly, the efficiency that the solar cell can convert light into electricity and secondly, the cost to deposit the solar cell. This thesis aims to address both factors, specifically looking at altering the transparent conducting oxide (TCO) and substrate in the solar cell. One method to improve the overall conversion efficiency of the device is to implement the DSC as the top cell in a tandem structure, with a bottom infra-red absorbing solar cell. The top solar cell in such a structure must not needlessly absorb photons which the bottom solar cell can utilise, which can be the case in solar cells utilising standard transparent contacts such as fluorine-doped tin oxide. In this work, transparent conducting oxides with high mobility such as titanium-doped indium oxide (ITiO) have been used to successfully increase the amount of photons through a DSC, available for a bottom infra-red sensitive solar cell such as Cu(In,Ga)Se2 (CIGS). Although electrically and optically of very high quality, the production of DSCs on this material is difficult due to the heat and chemical instability of the film, as well as the poor adhesion of TiO2 on the ITiO surface. Deposition of a interfacial SnO2 layer and a post-deposition annealing treatment in vacuum aided the deposition process, and transparent DSCs of 7.4% have been fabricated. The deposition of a high quality TCO utilising cheap materials is another method to improve the cost/watt ratio. Aluminium-doped zinc oxide (AZO) is a TCO which offers very high optical and electronic quality, whilst avoiding the high cost of indium based TCOs. The chemical and thermal instability of AZO films though present a problem due to the processing steps used in DSC fabrication. Such films etch very easily in slightly acidic environments, and are susceptible to a loss of conductivity upon annealing in air, so some steps have to be taken to fabricate intact devices. In this work, thick layers of SnO2 have been used to reduce the amount of etching on the surface of the film, whilst careful control of the deposition parameters can produce AZO films of high stability. High efficiency devices close to 9% have been fabricated using these stacked layers. Finally, transferring solar cells from rigid to flexible substrates offers cost advantages, since the price of the glass substrate is a significant part of the final cost of the cell. Also, the savings associated with roll to roll deposition of solar cells is large since the production doesn't rely on a batch process, using heavy glass substrates, but a fast, continuous process. This work has explored using the high temperature stable polymer, polyimide, commonly used in CIGS and CdTe solar cells. AZO thin films have been deposited on 7.5um thick polyimide foils, and DSCs of efficiency over 4% have been fabricated on the substrates, using standard processing methods