Researchers from the Applied Nano Tech & Science Lab at Korea Advanced Institute of Science and Technology, and the Laser Thermal Lab at UC Berkeley, US, have teamed up to enhance the efficiency of dye-sensitized solar cells.

Dye-sensitized solar cells

In the late 1960s it was discovered that illuminated organic dyes can generate electricity at oxide electrodes in electrochemical cells. In an effort to understand and simulate the primary processes in photosynthesis the phenomenon was studied at the University of California at Berkeley with chlorophyll extracted from spinach (bio-mimetic or bionic approach). On the basis of such experiments electric power generation via the dye sensitization solar cell (DSSC) principle was demonstrated in 1972.

The dye-sensitized solar cell belonging to the group of thin film solar cells and s based on a semiconductor formed between a photo-sensitized anode and an electrolyte, a photo electrochemical system. The dye molecules are incorporated are of nanometer sized, but in order to capture a reasonable amount of the incoming light the layer of dye molecules needs to be made fairly thick, much thicker than the molecules themselves. To address this problem, a nanomaterial is used as a scaffold to hold large numbers of the dye molecules in a 3-D matrix, increasing the number of molecules for any given surface area of cell.

Features

The DSSC has a number of attractive features; it is simple to make using conventional roll-printing techniques, is semi-flexible and semi-transparent which offers a variety of uses not applicable to glass-based systems, and most of the materials used are low-cost. Although its conversion efficiency is less than the best thin-film cells, in theory its price/performance ratio should be good enough to allow them to compete with fossil fuel electrical generation. Presently, dye-sensitized cells can reach about 11%, whereas the normal traditional silicon cell is more than 15%.

Branching technique

In nature trees have hierarchical multi-generation branching for maximizing the capture of sunlight. Inspired by this fact the researchers have developed a simple hydrothermal approach combined with polymer removal and seed deposition for the synthesis of ZnO nanowires (NWs) that resembled tiny trees with long branches to develop efficient dye-sensitized solar cells.

Principle

The researchers claim that short-circuit current density and the overall light conversion efficiency are almost four times higher than dye-sensitized solar cells fabricated with branched ZnO NWs compared with devices based on vertically grown ZnO NWs. The efficiency increase is due to the increase in surface area for higher dye loading and light harvesting and also to reduced charge recombination through direct conduction along the crystalline ZnO branches.

The hierarchical nano-tree structures will be also useful for high-capacity energy storage and high-efficiency energy consumption devices.