Assembly, charge-transfer and solar cell performance with porphyrin-C 60 on NiO for p-type dye-sensitized solar cells.

A series of zinc tetraphenylporphyrin photosensitizers furnished with three different anchoring groups, benzoic acid, phenylphosphonate and coumarin-3-carboxylic acid, were prepared using 'click' methodology. All three gave modest performances in liquid junction devices with I-3(-)/I- as the electrolyte. The distinct spectroscopic properties of the porphyrins allowed a detailed investigation of the adsorption behaviour and kinetics for charge transfer at the NiO vertical bar porphyrin interface. The adsorption behaviour was modelled using the Langmuir isotherm model and the phosphonate anchoring group was found to have the highest affinity for NiO (6.65 x 10(4) M-1) and the fastest rate of adsorption (2.46 x 10(7) cm(2) mol(-1) min(-1)). The photocurrent of the p-type dye-sensitized solar cells increased with increasing dye loading and corresponding light harvesting efficiency of the electrodes. Coordinating the zinc to a pyridylfunctionalized fullerene (C60PPy) extended the charge-separated state lifetime from ca 200 ps to 4 ns and a positive improvement in the absorbed photon to current conversion efficiency was observed. Finally, we confirmed the viability of electron transfer from the appended C60PPy to phenyl-C61-butyric acid methyl ester, a typical electron transporting layer in organic photovoltaics. This has implications for assembling efficient solid-state tandem solar cells in the future. This article is part of a discussion meeting issue 'Energy materials for a low carbon future'.