Role of the linker molecule and of the counter electrode

Speaker:

Ivan Mora-Sero Researcher, University Jaume I

Co-Author(s):

Sixto Gimenez, Thomas Moehl, Francisco Fabregat-Santiago, Teresa Lana-Villareal, Roberto Gomez and Juan Bisquert

The use of quantum dots (QDs) as photosensitizers has been spurred in the past years due to the demonstration of extremely high efficient carrier multiplication in colloidal QDs with internal quantum efficiency higher than 100%. One of the possible solar cell configurations to take advantage of this phenomenon is the quantum dot sensitized solar cell (QDSC), where a nanostructured metaloxides as TiO2 is photosensitized with QDs. However, it has not been shown so far that multiple carriers could be injected in external media. Therefore, an improved understanding of QDSCs is needed in order to design the appropriate cell configuration in which this effect could be used. Part of the work realized in the last two decades on conventional dye sensitized solar cells (DSCs) can be applied to the development of QDSCs, although some new parameters or reformulated ones with respect to conventional DSCs have to be taken into account to optimize QDSCs. In particular, CdSe QDs have received a special attention due to their relatively easy synthesis and also because the tuneable band edge offers the opportunity to harvest light energy in most of the visible region of the solar spectra. In this sense, they are appropiate sensitizers to check the QDSCs concepts. This work will focus on two important aspects of CdSe-QDSC, namely, the role of the molecular linkers to improve photocarrier harvesting from QDs and, the comparison of the characteristics of photoanodes and complete solar cells. Colloidal CdSe quantum dots (QDs) with different sizes prepared by a solvothermal route have been employed as sensitizers of nanostructured TiO2 electrode-based solar cells. Three different bifunctional linker molecules have been used to attach colloidal QDs to the TiO2 surface: mercaptopropionic acid (MPA), thioglycolic acid (TGA) and cysteine. The linker molecule plays a determinant role in the solar cell performance as illustrated by the fact that the incident photon to charge carrier generation efficiency (IPCE) could be improved by a factor 5-6 using cysteine with respect to MPA, see Figure A. Photovoltaic properties of QDs sensitized electrodes have been characterized for both three-electrode and closed two-electrode solar cell configuration. For three-electrode measurement a maximum power conversion efficiency near 1% can be deduced, but this efficiency is halved in closed cell configuration mainly due to the decrease of the fill factor (FF).