ARTICLE pubs.acs.org/JPCC
Oligo(poly)thiophene Sensitization of CdSe Nanocrystal and TiO2 Polycrystalline Electrodes: A Photoelectrochemical Investigation B. Vercelli and G. Zotti* Istituto CNR per l'Energetica e le Interfasi, C.o Stati Uniti 4, 35127 Padova, Italy
A. Berlin Istituto CNR di Scienze e Tecnologie Molecolari, Via C. Golgi 19, 20133 Milano, Italy
M. Pasini and C. Botta Istituto CNR per lo Studio delle Macromolecole, Via E. Bassini 15, 20133 Milano, Italy
R. Gerbasi Istituto CNR di Chimica Inorganica e delle Superfici, C.o Stati Uniti 4, 35127 Padova, Italy
T. L. Nelson and R. D. McCullough Department of Chemistry, Carnegie Mellon University, 4400 Fifth Ave, Pittsburgh, Pennsylvania 15213, United States
bS Supporting Information ABSTRACT: Oligo(poly)thiophenes were investigated by photoelectrochemical methods as monolayer sensitizers of CdSe nanocrystal (CdSe-NCs) and TiO2 polycrystalline films on ITO electrodes. 7.5 nm diameter CdSe-NCs layers and polycrystalline (anatase, ca. 40 nm crystallite size) TiO2 films were functionalized with an α,ω-dicarboxylic sexithiophene and with two polythiophenes, bearing differently spaced side carboxylic terminals, including regioregular poly(thiophene3-propionic acid). The modified electrodes were investigated in a three-electrode photoelectrochemical cell in the presence of iodide as sacrificial generator of intense and steady oxidative photocurrents. The oligo(poly)thiophenes act in general as moderate hypersensitizers (roughly twice more efficient than calculated). Poly(thiophene-3-propionic acid) monolayers, formed on CdSe-NC structures from ethanol, generate a strong hypersensitization, i.e., a response 5 times higher than expected. From optical and photoluminescence analysis such a result is attributed to a flat disposition of the thiophene planes on the surface which allows a faster phototransport. CdSe-NC sensitization of the ITO and TiO2 surfaces is also reported in detail.
1. INTRODUCTION The search for clean renewable energy sources has recently pushed research toward dye-sensitized solar cells (DSSCs) due to their low cost and high performance.15 The operating principle of DSSCs is to harvest photons from sunlight using a dye with a strong and broad absorption band, linked by electron-withdrawing groups (generally carboxylic acids) to a photoanode (ITO glass covered by a film of nanocrystalline TiO2). The absorbed photons induce excitation of the dye and subsequent electron injection into the TiO2 conduction band. A redox mediator is employed to regenerate the dye in its reduced ground state by electron donation. The oxidized form of the mediator is then restored by reduction at the cathode, thus completing the electric circuit. r 2011 American Chemical Society
The dyes used in the DSSCs fall into two broad categories: (a) all-organic conjugated molecules, such as e.g. perylene, and (b) transition-metal/ligand complexes, such as Ru(dcbpy)2(NCS)2. The latter, also known as the N3 dye, has become the standard model of a heterogeneous charge-transfer sensitizer for mesoporous solar cells owing to its high efficiency and photochemical stability.6 Some ruthenium-based sensitizers achieve in fact power conversion efficiencies in excess of 11%.711 Recently, the solar-cell performances of DSSCs based on allorganic dyes have shown efficiencies in the range 810%.1219 Received: September 19, 2011 Revised: December 13, 2011 Published: December 15, 2011 2033
dx.doi.org/10.1021/jp209042c | J. Phys. Chem. C 2012, 116, 2033–2039
The Journal of Physical Chemistry C Chart 1
The lower efficiency of organic dyes compared with the ruthenium sensitizers are due to the narrow absorption bands in the visible region and formation of dye aggregates on the semiconductor surface.20 Therefore, there is a need of organic dye molecules with higher molar extinction coefficients and broader spectral responses, such as those with donor and acceptor moieties bridged by a π-conjugated linker (D-π-A). To enhance the molar extinction coefficient as well as to realize panchromatic light harvesting, tuning of the length and conjugation of the linker are important. Recently, a successful approach was introduced by the use of a π-conjugated linker such as a thiophene2123 or thienothiophene24,25 derivative. The literature reports several papers on oligo(poly)thiophene sensitization of TiO2. Among these are a number of those with donor and acceptor moieties bridged by a π-conjugated linker (D-π-A), where the bridge is an oligo(poly)thiophene.2630 In a paper terthiophene and pentathiophene α,ω-dicarboxylic acids were used on TiO2 to produce DSSCs with high efficiency.31 Polythiophenes and oligothiophenes can function as both the sensitizer and hole conductor, and in fact polymer-based poly (3-hexylthiophene)(P3HT)/TiO2 systems have been previously reported.32,33 Polythiophene-sensitized nanocrystals of TiO2based photoelectrochemical cells fabricated with poly(3-thiophene acetic acid) has yielded over 2% power conversion efficiency.34 Improvement in the overall efficiency of the DSSCs may be given also by using inorganic quantum dots as a dye. Recently, sensitization of mesoporous TiO2 photoelectrodes with CdSeNCs has attracted much attention.3537 Multiple excitons generation from single photon absorption,38 narrow band gaps, tunability by controlling size to maximize solar absorption, and large extinction coefficient39 are favorable properties of these sensitizers. However, at the present time, these sensitizers show some problems such as (a) low conversion efficiencies (
