Preparation And Performance Of Carbon-based Counter Electrodes For Dye Sensitized Solar Cells

Dye-sensitized solar cell (DSSC) is a semiconductor optoelectronic device, which is based on photo-volts principle. Thus, solar energy can directly convert into electrical energy by DSSC. Compared with traditional silicon solar cells, the main advantages of DSSC are low cost, simply manufacturing process and stable performance. For the conventional DSSC counter electrode (CE), transparent conductive glass is used as a substrate and Pt is the most common used as catalyst. Recently, carbon materials attract more and more attention as counter electrodes in order to lower the cost of DSSC. According to previous studies, carbon counter electrodes (CE) were prepared by simply coating of synthesized catalyst particles on the substrate. The conductivity and catalytic activity of carbon counter electrodes in such a configuration is low, and the poor catalytic activity is usually resulted. Moreover, the poor adhesion between the catalyst layer and the substrate will cause many difficulties in the following cell assembly process. Herein, a novel carbon-based CE with integrated structure has been proposed and developed; and the performance of this CE was investigated systematically.The self-made coal-based carbon CEs were used to instead both conductive glass substrate and Pt catalyst to assembly DSSC. The amorphous coal-based carbon CE exhibits high electrical conductivity and excellent catalytic activity toward the I3-reduction reaction. The gas coal-based carbon CE shows good photovoltaic performance with the cell efficiency up to89%of the conventional Pt/FTO electrode. The coal based carbon CE after impregnated demonstrates an integrated structure with a porous top surface and a dense bottom. The solar cell with surface modified coal-based carbon CE shows high solar-to-electricity conversion efficiency (η) of7.16%, which is30%higher than that of the cell with graphite electrode, comparable to that of the cell with Pt electrode (7.21%).The self-made pure carbon CE was also modified with an ordered mesoporous carbon (OMC) catalytic layer. The OMC layer shows a highly ordered, closely packed hexagonal tunnel structure. The pore width of OMC is6-8nm, which is much larger than the size of I3-and I-in the organic electrolyte (0.50nm and0.22nm, respectively). It benefits for I3-quickly accessing to active sites in the pores where reduction reaction occur, meanwhile, I-generated can quickly spread from the pore to the electrolytic fluid. It reveals why OMC catalyst layer of pure carbon CE has a higher optical photovoltaic performance. The influence of carbonization temperature of OMC catalyst layer on the optical and electrical properties of pure carbon CE was investigated. The carbonization temperature affects mesostructure and the specific surface area mainly; the OMC catalyst layer carbonized at800℃has the largest specific surface area (832.46m2/g), and displays the best photovoltaic performance also. The cell efficiency reached8.73%, slightly higher than the conventional Pt electrode cell efficiency (8.64%). The effect of the different mesoporous structures on performance of pure carbon CE was also investigated. Compared to wormlike mesoporous carbon (WMC) catalytic layer, highly ordered mesoporous structure is more benefit to I3-, and I-diffusion, thus the pure carbon CE with OMC catalytic layer has a better cell efficiency. The effect of substrate composite on the conductivity and cell performance was investigated. One can further lower the sheet resistance of substrate by using graphite and carbon black as conductive additives. When the catalytic activity of CE is poor, one can increase the cell efficiency to a certain extent by increasing the electrode conductivity; however, when the CE can provide a good catalytic activity for the I3-reduction, the improvement in the electrode conductivity have less influence on the cell efficiency.The influence of surface chemistry on the cell performance was discussed. Both pure carbon CE and coal-based carbon CE contain certain number and proportion of oxygen-containing functional surface groups, i.e.,-C-OH, O=CO-, and-COOH. Changing surface property can cause varying of the photovoltaic properties of the electrodes to some extent, the electron transfer process at the interface of counter electrode/electrolyte, thus resulting in the change of cell efficiency further.