G R R A Kumara

Dye-sensitized solar cell with the hole collector p-CuSCN deposited from a solution in n-propyl sulphide

Abstract A method is devised for the deposition of CuSCN on ruthenium bipyridyl dye coated nanocrystalline TiO2 films from a solution in n-propyl sulphide. The dye-sensitized solid state photovoltaic cell formed was found to yield higher short-circuit photocurrent, open-circuit voltage and efficiency compared to the cells made with CuSCN by other deposition techniques. Factors affecting the stability of the cell are investigated.

A solid-state photovoltaic cell sensitized with a ruthenium bipyridyl complex

Ruthenium bipyridyl complexes have been used as efficient sensitizers for photoelectrochemical cells based on nano-porous films of . It is found that cis-dithiocyanate-bis(2,2-bipyridyl-4,4-dicarboxylate) ruthenium (II) can be used as the sensitizer in a solid-state photovoltaic cell in which a monolayer of the sensitizer is sandwiched between nano-porous n- and p-CuI. The data on performance of the cell and the problems encountered in construction of dye-sensitized solid-state photovoltaic cells are described.

Enhanced Efficiency of a Dye-Sensitized Solar Cell Made from MgO-Coated Nanocrystalline SnO2

Nanocrystalline SnO2-based dye-sensitized photoelectrochemical solar cells have very low open-circuit voltages of 325–375 mV and efficiencies of ~ 1%. However, on coating the SnO2 crystallites with a thin film of MgO, the voltage and efficiency are increased to 650–700 mV and ~ 6.5%, respectively. Evidence is presented to show that the photoexcited dye on the outer MgO shell could tunnel electrons to SnO2 and that the low probability of reverse tunneling suppresses recombinations, thus increasing the efficiency. An explanation is also given as to understand why dye-sensitized TiO2 cells are more efficient than those made from SnO2 alone.

Dye-sensitized solid state photovoltaic cell based on composite zinc oxide/tin (IV) oxide films

Dye-sensitized fully solid state cells having the structure of rough n-type semiconductor film comprising zinc and tin(IV) oxide/Ru-bipyridyl complex/p-CuI are found to generate high short-circuit photocurrents and open-circuit voltages in contrast to the cells of same type made with a single oxide (tin(IV) or zinc). The mechanism involved is explained as suppression recombinations of photogenerated carriers and dye ions resulting from interparticle charge transfer.

Suppression of recombinations in a dye-sensitized photoelectrochemical cell made from a film of tin IV oxide crystallites coated with a thin layer of aluminium oxide

A dye-sensitized photoelectrochemical cell consisting of a film of SnO2 crystallites coated with ultrafine particles of Al2O3 generates an exceptionally high open-circuit voltage as compared to a cell made only from SnO2. Al2O3 coating on SnO2 improves the efficiency and the fill factor while delivering reasonably high photocurrents. Photoexcited dye molecules on Al2O3 injects electrons into the conduction band of SnO2 via tunnelling through the Al2O3 barrier. Suppression of recombinations of electrons with the dye cations and the acceptors at the electrolytic interface build up the quasi-Fermi level in SnO2 with an impressive increase of the open-circuit voltage.

An efficient dye-sensitized photoelectrochemical solar cell made from oxides of tin and zinc

A photoelectrochemical solar cell made from a porous film consisting of a mixture of tin (IV) and zinc oxides sensitized with a ruthenium bipyridyl complex suppresses recombination of the photogenerated electrons and dye cations, generating a short-circuit photocurrent of ca. 22.8 mA cm–2 and an open-circuit voltage of ca. 670 mV in direct sunlight (900 W m–2) with an efficiency ca. 8 %.

Nanoporous n-/selenium/p-CuCNS photovoltaic cell

On illumination, selenium deposited on nanoporous n- transfers photogenerated electrons into . When p-CuCNS is coated on top of the selenium deposited on nanoporous n-, holes are directed into the p-CuCNS. A photovoltaic cell of nanoporous n-/selenium/p-CuCNS based on the above charge transfer process generates a photocurrent of and a photovoltage of mV at simulated sunlight. The efficiency of the cell seems to be limited by surface recombination and the presence of voids in the film. Photoelectrochemical experiments also indicate that when selenium is deposited on nanoporous n- photogenerated electrons in selenium are efficiently transferred to .

Dye-sensitized composite semiconductor nanostructures

Understanding of the charge transport and recombination mechanisms of dye-sensitized solar cells based on semiconductor nanostructures is essential for the improvement of their performance. A great deal of information on these systems have been obtained from studies on a single material (mostly TiO2 and to a lesser extent ZnO and SnO2). We have conducted extensive measurements on composite dye-sensitized nanosturctures and found that the composite systems possess unusual properties. Dye-sensitized photoelectrochemical cells made from nanocrystalline 7lms of some materials (e.g., SnO2) yield comparatively small open-circuit voltages and energy and quantum conversion e9ciencies, despite excellent dye-semiconductor interaction. However, on deposition of ultra-thin shells of insulators or high band gap semiconductors on the crystallites, a dramatic increase in the above parameters is observed. Outer shells were found to have insigni7cant or in most cases a negative e;ect on TiO2 7lms. We explain the above 7ndings on the basis of vast di;erences in the leakage rates of trapped electrons in di;erent materials which is sensitive to the e;ective electron mass. Electrons injected to the conduction band in dye-sensitization enter into shallow traps from which they get thermally reemitted to the conduction band. The building up of the electron quasi-fermi level and transport depends on this process. The spread of the hydrogenic wave function of a trapped electron increases inverse exponentially with the e;ective mass so that the electron leakage and their recombination with acceptors ‘outside’ become severe when the crystallite size is comparable to the Bohr radius of the trapped electron. Such recombinations are e;ectively suppressed by deposition of thin 7lms on the crystallites. Excited dye molecules anchored to the outer shell injects electrons to the conduction band via tunneling. ? 2002 Elsevier Science B.V. All rights reserved.

Efficient dye-sensitized photoelectrochemical cells made from nanocrystalline tin(IV) oxide–zinc oxide composite films

Dye-sensitized photoelectrochemical cells based on nanocrystalline films of TiO2 yield energy conversion efficiencies ~10%. The efficiencies of similar cells with films of other oxide materials (SnO2, ZnO) are well below the above value. However, the cells made from SnO2–ZnO composite films give efficiencies comparable to TiO2 cells. Two types of composite systems with SnO2 and ZnO are possible. In the first type, SnO2 crystallites are covered with an ultra-thin (<1 nm) outer shell of ZnO2 and in the second type, the film comprises SnO2 crystallites (~10 nm) with a thin ZnO outer shell and larger ZnO particles (~100 nm). The short-circuit photocurrent and efficiency of these cells are ~17 mA cm−2, 19 mA cm−2 and 7%, 8% respectively. This paper explains in detail how a thin shell of ZnO on SnO2 could effectively counteract recombinations of electrons with acceptors in the electrolyte (e.g., I3−) and increase the efficiency although SnO2 and ZnO are individually not good materials for dye-sensitized photoelectrochemical cells. In the second type, larger ZnO crystallites reduce the rate of geminate recombinations, in addition to the effect of the outer shell.

Continuous flow photochemical reactor for solar decontamination of water using immobilized TiO2

A photochemical reactor is designed for solar decontamination of organic pollutants in water, where the nanocrystalline photocatalyst TiO 2 is immobilized on glass. The reactor modules could be connected in series and/or parallel to achieve desired #ow rates under di!erent conditions of illumination and degree of contamination. Methyl violet and phenol was found to completely degrade under solor irradiation and #ow rates of 102}138 ml/h. ( 1999 Elsevier Science B.V. All rights reserved.