KEYWORDS: Lithium, Polymers, Control systems, External quantum efficiency, Photovoltaics, Dye sensitized solar cells, Interfaces, Ions, Solar cells, Sun
Modification of the interface of titanium dioxide/poly[2-(2-ethylhexyloxy)-5-methoxy-1,4,-phenylenevinylene] (TiO2/MEH-PPV) nanocomposite photovoltaic devices with a lithium salt, Li[CF3SO2]2N, is shown to result in a twofold increase in device efficiency. The devices are of the type ITO/TiO2/MEH-PPV/Au. The TiO2 layer is deposited by doctor blading a colloidal anatase paste, and the polymer is then spin-coated on top followed by thermal evaporation of gold contacts. Careful control of manufacturing conditions and use of a 35 nm polymer layer leads to a device efficiency of 0.48% for un-modified devices. The increased efficiency following Li treatment is the result of a 40% increase in both the short-circuit current and fill factor, while the open-circuit voltage remains unchanged. A maximum efficiency of 1.05% has been achieved under 80% sun illumination. This represents a record efficiency for this type of cell. Photoconductivity experiments show a substantial increase in conductivity of the TiO2 layer following Li modification. Interfacial modification is done via a simple soaking procedure, and the effect of varying the concentration of Li[CF3SO2]2N is discussed. We report investigations into optimization and the mechanism of such improvement, for example by varying processing parameters of the modification procedure or the ionic species themselves.
We present data on the initial period of operation of Gilch-route MEH-PPV/TiO2 composite solar cells (CSCs) which show that during this period the CSCs operate in a non-steady state regime. The behavior is complex and may include a gradual rise of the open circuit voltage (Voc) and of the short-circuit current density (Jsc) with time, a passage through a maximum of either or both parameters, and even a sign reversal. The mechanisms most probably contributing to the transient processes are: i) diffusion driven redistribution of charges resulting in the build up of a quasi steady state charge density profile across the device; ii) photo-doping resulting in a relatively slow increase of the average charge carrier concentration and consequently of the conductivity of the device. The latter is responsible for a strong decrease in Voc, and is evidenced by the significant increase in dark current after device illumination.
Nanocomposite titanium dioxide/polymer photovoltaic cells have been fabricated using poly[2-(2-ethylhexyloxy)-5-methoxy-1,4-phenylenevinylene] (MEHPPV). Two different types of titanium dioxide were used, one synthesized using a sol-gel method, the other was a commercial paste. The crystal structure, porosity and absorption spectra of the titanium dioxide layers were measured, and the titanium dioxide synthesized using the sol-gel method had a much lower level of anatase. The photovoltaic properties of the ITO/TiO2/MEHPPV/Au cells, which were similar for both types of TiO2, were measured as a function of illumination power and compared with equivalent circuit models. A simple equivalent circuit model incorporating a diode, two resistances and a light induced current was inconsistent with the illumination -- dependent data and was improved by adding an illumination dependent shunt resistance. A very long lived, photo-induced increase in dark current was observed, which could not be explained by a polymer degradation mechanism or an increase in temperature under illumination, but was more likely to be due to trapped charge.
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