Hari M. Upadhyaya

Spray deposited copper zinc tin sulphide (Cu2ZnSnS4) film as a counter electrode in dye sensitized solar cells.

Stoichiometric thin films of Cu2ZnSnS4 (CZTS) were deposited by the spray technique on a FTO coated glass substrate, with post-annealing in a H2S environment to improve the film properties. CZTS films were used as a counter electrode (CE) in Dye-Sensitized Solar Cells (DSCs) with N719 dye and an iodine electrolyte. The DSC of 0.25 cm(2) area using a CE of CZTS film annealed in a H2S environment under AM 1.5G illumination (100 mW cm(-2)) exhibited a short circuit current density (JSC) = 18.63 mA cm(-2), an open circuit voltage (VOC) = 0.65 V and a fill factor (FF) = 0.53, resulting in an overall power conversion efficiency (PCE) = 6.4%. While the DSC using as deposited CZTS film as a CE showed the PCE = 3.7% with JSC = 13.38 mA cm(-2), VOC = 0.57 V and FF = 0.48. Thus, the spray deposited CZTS films can play an important role as a CE in the large area DSC fabrication.

Ethynyl thiophene-appended unsymmetrical zinc porphyrin sensitizers for dye-sensitized solar cells

Four unsymmetrical porphyrins of A2B donor–π–acceptor type have been designed, synthesized, characterized and their photovoltaic properties explored. Polycyclic aromatic hydrocarbons (PAH), such as pyrene or fluorene, act as a donor, the porphyrin is the π-spacer, appended with an ethynyl thiophene linker, and either cyanoacrylic acid or malonic acid acts as the acceptor. All of the compounds were characterized by 1H NMR and mass spectrometry. UV-Vis absorption spectra and B or Soret (λex at 440 nm for the four sensitizers reported) band-excited fluorescence emission spectra were also obtained. The electrochemical properties suggest that the first oxidation is ring-centred, which is supported by in situ spectro-electrochemical and DFT computational studies. The synthesized porphyrins were applied in dye-sensitized solar cells (DSSCs). A conversion efficiency of up to 3.14%was realized for PYR–Por–MA under our experimental conditions.

Thin Film Photovoltaics

Abstract Thin film solar cell technology has recently seen some radical advancement as a result of new materials and innovations in device structures. The increase in the efficiency of thin film solar cells and perovskite into 23% mark has created significant attention in the photovoltaic market, particularly in the integrated photovoltaic (BIPV) field. The efficiency increase in perovskite solar cells has significantly increased the potential thin film PVs, to become a low cost alternative for commercially available solar cell technologies. However, the main challenges for thin film technologies, including perovskite solar cells, are their stability and toxicity involved in the manufacturing process. An attempt has been made to report on the developments into thin film materials and the efficiencies achieved. Key deposition and growth techniques and challenges are also reported in this chapter.

Upconversion and Downconversion Processes for Photovoltaics

Abstract The main efficiency losses of all single threshold solar cells resulting in energy-conversion efficiencies fundamentally constrained by Shockley–Queisser (S-Q) limits to practical values below 30% arises mainly from the photons that are not absorbed due to energy less than the threshold (so-called sub-bandgap or transmission losses) and due to the energy absorbed in excess of the threshold that is converted to heat (so-called lattice thermalization losses). There are many approaches to address these fundamental losses and to exceed the S-Q limits: concentrating the sunlight, restricting cell acceptance angles so that cells convert light only from a limited directional range, by steering different wavelength bands of sunlight to cells of appropriate bandgap for efficient conversion (multi-juction solar cells), multiple-exciton generation in quantum-confined cells, and last but not least via spectral conversion. Of these, the spectral conversion is the most promising cost-effective approach, which does not involve any major modifications in the solar cell architecture or usage of complicated costly optics. The present chapter gives an overview of the spectral conversion processes (upconverison and downconversion) mainly discussing the experimental results on integration with solar cell and resulting enhancements in device characteristics reported in literature.

The optimization of optical properties for increased performance in a monolithic tandem dye-sensitized/Cu(In, Ga)Se 2 solar cell

Tandem solar cells are an attractive solution to increase the performance of one or more low efficiency, low cost technologies into a more efficient device. This has been demonstrated previously using a physically stacked dye-sensitized and Cu(In, Ga)Se2 or CIGS solar cell. The subsequent move to a monolithic design proved to be successful, however only delivering an efficiency of 12.2 % compared to the 15.1 % of the physical stack. This may have been due to optical losses and liquid electrolyte based instabilities on the ZnO:Al layer at the bottom CIGS cell interface. Besides this, shading is known to reduce the photocurrent and voltage in thin film solar cells. In a DSC/CIGS monolithic tandem, the shading caused from measuring the cell accurately with a mask to determine the precise DSC area, reduced the Voc from 1221 mV to 848 mV, with a reduction in efficiency from 12.44 % to 7.75 %. It was discovered that the reduction of the CIGS active area in the tandem cell by altering the fabrication process led to a reduction in the difference between Voc and efficiencies, with the masked tandem producing a Voc of 1221mV and an efficiency of 9.38 % compared to a Voc of 1287 mV and efficiency of 12.30 % when unmasked.

Development of an efficient substrate heating assembly for high efficiency CIGS solar cells over 30 cm × 30 cm-area for an in-line pilot evaporation system

In the current studies, the optimization of the uniformity and temperature control across 30 cm × 30 cm area substrates are identified as crucial parameters to form α-phase CIGS in a desired time period, which is the key to deliver high efficiency solar cells at industrial scale. The following attempts were made to achieve a heating system design using: (i) rapid heating by infra-red (IR) lamps, (ii) IR lamps with a graphite susceptor, (iii) a specially designed graphite heater. A three-dimension heat transfer model using COMSOL Multiphysics 4.2a software has been developed and presented to perform the experimental validation on different substrate heating system used here. The following conclusions were drawn from this study: (a) IR lamps can only be used for rapid space heating purpose; (b) IR lamps with intermediate graphite susceptor can only be used to achieve over 10cm×10cm and thus will suffer from intrinsic limitations; (c) the new designed graphite heater can deliver relatively uniform heating across 30 cm × 30 cm area with non-uniformity of ± 5%, which may be acceptable for industrial purpose.