Preparations And Properties Of Novel Counter Electrodes For Dye-Sensitized Solar Cells

Since the breakthrough made by Prof. M. Gratzel’s group in1991, dye-sensitized solar cells (DSSCs) have received tremendous scientific and industrial attentions because of their low-cost, ease of fabrication, environmental friendliness of raw materials, and relatively high efficiency. As an important component, counter electrode (CE) fulfils electrocatalyzing reduction of oxidized electrolyte at the CE/electrolyte interface by transferring electrons flowing through the external circuit to the electrolyte. The conductivity, catalytic activity, and chemical stability of CE are responsible for the photovoltaic performance of DSSCs. Traditionally, platinum (Pt) deposited conductive glass is a preferential CE for DSSCs. However, as a noble metal, the high cost and energy comsumption in preparation seriously hinder the large-scale production of DSSCs. Therefore, novel CE materials of low-cost and high-performance, which can be prepared through simple methods, are vital for the practical application. This thesis mainly focuses on different types of electrocatalysts, and some innovative results have been achieved, as summarized as follows:(1) Ultrathin film of PDDA/graphene/PDDA/[PtCl6]2-is fabricated on FTO conductive glass via electrostatic layer-by-layer self-assembly technique, which is then converted to graphene/Pt monolayer electrode by calcination for use as counter electrode in DSSC. As compared to the sputtered Pt counter electrode, the self-assembled graphene/Pt electrode reduces the Pt amount by about1000-fold but exhibits comparable photovoltaic performance (7.66%vs.8.16%). This finding provides a new route to fabrication of cheap and efficient counter electrodes for flow-line production of DSSCs.(2) Graphene/SiO2nanocomposite was prepared by oxidization-reduction method. And the graphene and graphene/SiO2films, which were prepared by drop-casting the suspensions without organic binder or surfactant onto the conductive glass substrates, were used directly as CEs for DSSCs without any post-treatment. When the graphene nanosheets (GNS) were deposited on FTO, a close-packed non-porous GNS film was formed, while a meso-porous film was formed upon the introduction of SiO2nanoparticles into the GNS suspension. As compared to the pure graphene film, the porous graphene/SiO2composite film demonstrates much better electrocatalytic performance for the reduction of triiodide in DSSCs due to the remarkably enhanced contact surface area with electrolyte. The power conversion efficiency of DSSC with graphene/SiO2CE achieves6.82%, which is much better than that for the DSSC with pure graphene CE (4.02%), and comparable to that for the DSSC with Pt CE.(3) Polypyrrole (PPy) nanoparticles were firstly prepared by redox polymerization. And then, reduced graphene oxide (RGO) sheets are successfully introduced into the PPy matrix as conductive channels and co-catalyst, through simple incorporation of graphene oxide (GO) into PPy and subsequent in situ reduction from GO/PPy to RGO/PPy composite film, which was used as CE for DSSCs after doping with HC1. Based on the electrochemical and photovoltaic tests, the incorporated RGO sheets significantly improve short-circuit photocurrent density and fill factor. As a consequence, the power conversion efficiency increased from7.11%to8.14%, which is comparable with that for the cell based on Pt cathode (8.34%).(4) Metal selenides (Co0.85Se and Ni0.85Se) in situ grew onto the conductive glass via a facile one-step low temperature hydrothermal reaction, which were directly utilized as transparent CEs for DSSCs without any post-treatments. As-prepared Co0.85Se CE exhibits better electrocatalytic activity than Pt for the reduction of triiodide, and the DSSC with it generates a power conversion efficiency of9.40%, which is higher than that (8.64%) for the DSSC with Pt CE. To the best of our knowledge, this is the highest power conversion efficiency for I-/I3-redox couple based DSSCs with Pt-free cathodes under AM1.5G simulated solar light (100mW cm-2). The low-cost and high-performance CEs prepared by the extremely simple method may facilitate the large-scale application of DSSCs.(5) Nickel diselenide (NiSe2) was synthesized by one-step hydrothermal reaction, and applied as a CE for DSSC for the first time. A new method for comparing the catalytic activity has also been proposed, which emphasizes that the catalytic performance of electrocatalysts should be compared at the same active surface area. A series of electrochemical characterizations were carried out and the NiSe2displayed remarkable catalytic activity on the reduction of I3-. The DSSC with a NiSe2CE produces a higher power conversion efficiency (8.69%) than that (8.04%) for the cell with a Pt CE under the same conditions.