Dye-sensitized solar cells with titanium dioxide (TiO2) nanostructures have excellent charge-collection capabilities, high open-circuit voltages, and good fill-factors. However, they do not absorb all visible and near-IR photons and consequently have lower short-circuit photocurrent densities than conventional inorganic photovoltaics. Increasing the short-circuit current density of DSSCs is a key factor in improving the optical-to-electrical efficiency of these devices.
Options include the development of new dyes that absorb photons from a wider solar spectral range, and tailoring the TiO2 nanostructures to offer more efficient charge transport. A number of different schemes regarding these two factors have been suggested and shown to enhance the efficiency. Nevertheless, DSSC efficiencies are still low compared with inorganic devices. One of the main reasons is that the developed methods can be difficult to combine with other schemes to cumulatively improve the efficiency of the device.
Simple, fast, and additive
Researchers from the Department of Materials Science and Engineering at Yonsei University have demonstrated that the inter-electrode contact resistance arising from poor interfacial adhesion is responsible for a considerable portion of the total resistance in the DSSC. The group has shown that the current flow can be greatly improved by welding the interface with a laser.
Titanium dioxide films formed on transparent conducting oxide (TCO)-coated glass substrates are irradiated with a pulsed UV laser beam at 355 nm, which transmits through TCO and glass, but is strongly absorbed by TiO2. It was found that a thin continuous TiO2 layer is formed at the interface as a result of the local melting of TiO2 nanoparticles. This layer completely bridges the gap between the two electrodes and improves current flow by reducing the contact resistance.
Using the process, the team could improve the efficiency of devices by 35% to 65%. For example, DSSC cells fabricated with and without the interface welding exhibited efficiencies of 11.2% and 8.2%, respectively. The laser-welding technique is simple, fast and, more importantly, additive to any other efficiency-enhancing schemes.
1. Jinsoo Kim et al., 2010 Nanotechnology 21 345203; doi: 10.1088/0957-4484/21/34/345203
Posted by John Wallace
Subscribe now to Laser Focus World magazine; It’s free!