Surface modifications of CdS/CdSe co-sensitized TiO₂ photoelectrodes for solid-state quantum-dot-sensitized solar cells

Cadmium sulfide (CdS) and cadmium selenide (CdSe) are deposited in situ onto mesoporous TiO₂ films. The CdS/CdSe quantum dot (QD) co-sensitized photoelectrodes were used to fabricate all-solid-state quantum-dot-sensitized solar cells (QDSSCs) by using spiro-OMeTAD as a hole transport material (HTM). Two types of surface modification agents, diisooctyl phosphonic acid (DIOPA) and benzenethiol (BT) derivatives, were used to modify the TiO₂/CdS/CdSe electrode, for inhibiting the charge recombination at the electrode/HTM interface. DIOPA molecules were expected to adsorb onto exposed free TiO₂ adsorption sites on the QD-sensitized TiO ₂ surface, preventing the direct contact between TiO₂ and the HTM. BT molecules on the other hand will attach directly onto the QD surface, impeding interfacial charge recombination by surface passivation effects and/or by molecular dipole effects. The results show that the best efficiency of solid-state QDSSCs achieved by a bare TiO₂/CdS/CdSe electrode is 0.65% (AM 1.5, 100 mW cm^-2). All the electrodes modified by DIOPA and the dipole molecules, 4-methoxythiophenol (BTOMe) and 4-chlorobenzenethiol (BTCl), showed improved cell performance. However, the BTOMe-modified electrode demonstrates the best efficiency (0.76%). Furthermore, simultaneous surface treatment with BTOMe and DIOPA can further inhibit charge recombination, attributed to the synergetic effect of the two inhibition mechanisms. The energy conversion efficiency of the electrode modified by both BTOMe and DIOPA can be elevated further to 0.88% (AM 1.5, 100 mW cm ^-2).