Rediscovering a key interface in dye-sensitized solar cells: guanidinium and iodine competition for binding sites at the dye/electrolyte surface.
We propose a new mechanism by which the common electrolyte additive guanidinium thiocyanate (GdmSCN) improves efficiency in dye-sensitized solar cells (DSSCs). We demonstrate that binding of Gdm(+) to TiO2 is weak and does not passivate recombination sites on the TiO2 surface as has been previously claimed. Instead, we show that Gdm(+) binds strongly to the N719 and D131 dyes and probably to many similar compounds. The binding of Gdm(+) competes with iodine binding to the same molecule, reducing the surface concentration of dye-I2 complexes. This in turn reduces the electron/iodine recombination rate constant, which increases the collection efficiency and thus the photocurrent. We further observe that GdmNO3 can increase efficiency more than the current Gdm(+) source, GdmSCN, at least in some DSSCs. Overall, the results point to an improved paradigm for DSSC operation and development. The TiO2/electrolyte surface has long been held to be the key interface in DSSCs. We now assert that the dye layer/electrolyte interaction is at least, and probably more, important.
Chemistry Department and ‡Physics Department, Imperial College London , 1 Exhibition Way, London SW7 2AZ, United Kingdom.
Xavier A
XA
Jeanbourquin
Xiaoe
X
Li
ChunHung
C
Law
Piers R F
PR
Barnes
Robin
R
Humphry-Baker
Peter
P
Lund
Muhammad I
MI
Asghar
Brian C
BC
O'Regan