The Role of a “Schottky Barrier” at an Electron‐Collection Electrode in Solid‐State Dye‐Sensitized Solar Cells

Abstract

The field of polymer, molecular, and hybrid optoelectronics is progressing at a phenomenal pace with new records for material properties and nanostructured composites being reported regularly. [1][2][3][4][5] The promise of cheap, large-area, flexible solar cells, displays, and printable electronics is close to becoming a reality. However, connecting the functional components to the "outside world" can limit the end performance of the systems. The characteristics of the functional materials are often completely governed by the nature of the interfaces between the electrodes and these materials. [6,7] Here, we show that a "Schottky barrier" present at an electron-collecting electrode governs the performance of a solid-state dye-sensitized solar cell. This is a hybrid system comprising of light-absorbing sensitizer molecules at the interface between a nanostructured inorganic (n-type) and an organic (p-type) semiconductor. There has been much work concentrating on the dynamics, kinetics, and physical processes occurring at the dye interface. [8][9][10][11] However, very few studies address the nature of the contacts between the semiconductors and the electrodes. [12][13][14][15] We show that the size of a Schottky barrier, present at the anode interface, is dramatically reduced under UV illumination. We attribute this to the generation of surface states, resulting in a "pinning" of the Fermi level (E F ) in the semiconductor at this junction. We tune the size of the barrier by controlling the "doping level" in the inorganic semiconductor; reduction of the barrier height results in significant improvements in device performance. The solid-state dye-sensitized solar cell comprises of a fluorine-doped tin-oxide (FTO) semitransparent anode, a nanoporous TiO 2 film coated with a monolayer of dye molecules, infiltrated with the organic hole transporter 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene (Spiro-MeOTAD) and caped with a gold cathode.[16] A compact TiO 2 layer is inserted between the FTO and the nanoporous TiO 2 in order to reduce losses through recombination between holes in the hole transporter and electrons in the FTO. A compact TiO 2 layer is also occasionally employed in the "liquid cell" where a liquid electrolyte is used in place of Spiro-MeOTAD. [17,18] A phenomenon associated with dye-sensitized solar cells is the "UV effect", where the solar-cell performance is observed to improve dramatically when exposed to UV light. [19,20] The observed behavior has been explained by the generation of surface states on the TiO 2 nanoparticles. There is uncertainty as to the exact nature of these surface states, whether they are neutral or charged.[21] However, a consistency throughout is that an apparent positive shift in the conduction band is always observed under UV illumination (in this work positive is with respect to the normal hydrogen electrode (NHE), which is more negative with respect to the vacuum energy). This has the effect of improving electron injection from the lowest unoccupied...

Controlling the Electrical Properties of Undoped and Ta‐Doped TiO2 Polycrystalline Films via Ultra‐Fast‐Annealing Treatments

“…This reveals the possibility of fabricating an all-TaTO electrode, i.e., a TaTO as TCO fi lm with a top selective layer created by the UFA process, in which it is possible to tune the thickness of the selective layer as a function of the chosen device architecture in a single deposition followed by the annealing process. [ 18 ] This capability could reduce the number of sharp interfaces among different materials (e.g., FTO-TiO 2 ) in several solar cell confi gurations and consequently the possible presence of recombination centers [ 43 ] or energy barriers [ 11 ] for a more effi cient electron collection.…”

Recent Developments in Solid‐State Dye‐Sensitized Solar Cells

“…The reduced barrier in turn improves the performance of the solid-state DSSC (e.g. from 2.1 % to 3.7 % [49,50] ).…”

Advances in Liquid‐Electrolyte and Solid‐State Dye‐Sensitized Solar Cells

“…It has recently been demonstrated that there is a Schottky barrier present at this interface, which inhibits charge collection in the solid-state DSC. [89] The size of the Schottky barrier appears to be dramatically reduced under UV illumination. This is attributed to the generation of surface states, resulting in a "pinning" of the Fermi level in the TiO 2 at this junction.…”