Electron Transport in Acceptor-Sensitized Polymer–Oxide Solar Cells: The Importance of Surface Dipoles and Electron Cascade Effects

Abstract

Fullerene and acenequinone compounds have been examined as electron mediators between a p-type semiconductive polymer and two n-type oxide semiconductors. Composite interlayer materials and photovoltaic test cells were assembled and studied for their fluorescence quenching, current-voltage, and quantum efficiency behavior to characterize the efficacy of the acceptor-sensitizers as electron-selective interlayers. The sensitizers are generally more effective with titanium dioxide than with zinc oxide, due to the difference in magnitude of dipole-induced vacuum level shifts at the respective oxide interfaces. In titanium dioxide-based solar cells, where dipole effects are weak, photovoltage and fill factor increase in a trend that matches the increase in the first reduction potential of the acceptor-sensitizers. Photosensitization of the oxide semiconductor by the acceptor-sensitizers is observed to operate either in parallel with the polymer as an alternate photosensitizer or in series with the polymer in a two-photon process, according to an acceptor-sensitizer's first reduction potential. In zinc oxide-based solar cells, where dipole effects are stronger, the acceptor-sensitizers impaired most devices, which is attributed to an upward shift of the oxide's conduction band edge caused by dipole-induced vacuum level shifts. These results have broad implications for designing electron-selective interlayers and solid-state photocells using sensitized oxide semiconductors.

Impact of Molecular Dipole Moments on Fermi Level Pinning in Thin Films

“…Merocyanine dyes are molecules consisting of a donor (D) and an acceptor (A) part providing a strong dipolar character and high polarizability to these D–A molecules, enabling high absorption coefficients and tunable electronic character from polyene- to polymethine-type chromophores. − These properties make merocyanines naturally attractive for application in organic photovoltaics and particularly dye-sensitized solar cells (DSSCs). − Despite its importance for efficient charge collection, the energy level alignment at the D−A dye/electrode interfaces has not yet been studied extensively, − and the mechanisms (particularly including the effects due to the molecular dipoles) that determine the interface electronic properties remain to be established, which will then allow correlation with device performances. − To achieve low resistance electrical contacts in organic (opto-) electronic devices, a proper matching of the electrode Fermi level ( E F ) to the charge transport levels of the organic semiconductor is necessary. − Within the Schottky–Mott limit, i.e., assuming vacuum level alignment, one might try estimating the relative position of the energy levels at the interface simply from the values of the electrode work function (Φ) as well as the ionization energy (IE) and electron affinity (EA) of the organic semiconductor. However, for interfaces between organic semiconductors and metals, the invalidity of vacuum level alignment is established meanwhile. , …”

Effect of interfacial dipoles on charge traps in organic–inorganic hybrid solar cells

“…10,13 In fact, the majority of previous efforts that examined the effect of interfacial modiers on device structures drew conclusions by studying congurations in which no electrodes were present or by indirectly evaluating currentvoltage curves of the corresponding photovoltaic devices. 14,15 A fundamental understanding of the fate of charges generated at the complex interface between dissimilar materials is critical to further improve the efficiency of hybrid solar cells. For example, charge trapping can lead to enhanced backward recombination rates, and is, therefore, an important issue affecting device performance.…”

Solid-State Photogalvanic Dye-Sensitized Solar Cells

“…23 We have previously examined thermal electron injection by acceptor-sensitizers such as fullerene and acenequinone dyes into TiO 2 and ZnO at the polymer/oxide interface of hybrid inverted organic solar cells. 24 Despite the lower surface area of our oxide nanorod films compared to mesoporous oxide electrodes, we observed photosensitization by adsorbed acceptor-dye. The polymer/dye/oxide solar cells can be viewed as a solid-state version of a P-DSC, with poly(3-hexylthiophene) acting both as a photoactive primary donor and a holeconducting substitute for electrolyte.…”