New Photovoltaic Devices Based on the Sensitization of p-type Semiconductors: Challenges and Opportunities

Abstract

Because solar energy is the most abundant renewable energy resource, the clear connection between human activity and global warming has strengthened the interest in photovoltaic science. Dye-sensitized solar cells (DSSCs) provide a promising low-cost technology for harnessing this energy source. Until recently, much of the research surrounding DSSCs had been focused on the sensitization of n-type semiconductors, such as titanium dioxide (Gratzel cells). In an n-type dye-sensitized solar cell (n-DSSC), an electron is injected into the conduction band of an n-type semiconductor (n-SC) from the excited state of the sensitizer. Comparatively few studies have examined the sensitization of wide bandgap p-type semiconductors. In a p-type DSSC (p-DSSC), the photoexcited sensitizer is reductively quenched by hole injection into the valence band of a p-type semiconductor (p-SC). The study of p-DSSCs is important both to understand the factors that control the rate of hole photoinjection and to aid the rational design of efficient p-DSSCs. In theory, p-DSSCs should be able to work as efficiently as n-DSSCs. In addition, this research provides a method for preparing tandem DSSCs consisting of a TiO(2)-photosensitized anode and a photosensitized p-type SC as a cathode. Tandem DSSCs are particularly important because they represent low-cost photovoltaic devices whose photoconversion efficiencies could exceed 15%. This Account describes recent research results on p-DSSCs. Because these photoelectrochemical devices are the mirror images of conventional n-DSSCs, they share some structural similarities, but they use different materials and have different charge transfer kinetics. In this technology, nickel oxide is the predominant p-SC material used, but much higher photoconversion efficiencies could be achieved with new p-SCs materials with deeper valence band potential. Currently, iodide/triiodide is the main redox mediator of electron transport within these devices, but we expect that this material could be advantageously replaced with more efficient redox couples. We also discuss valuable information obtained by ultrafast transient absorption spectroscopy, which sheds some light on the factors that govern the efficiency of the cell. Notably, we demonstrate that ultrafast hole injection generally occurs between the sensitizer and the SC, but the resulting charge-separated state (e.g. electron on the sensitizer and hole in the VB) is short-lived and recombines quickly. So far, the only effective strategy for slowing the back recombination reaction relies on a bimolecular system consisting of the sensitizer linked to an electron acceptor, which increases the separation distance between the charges. A photoconversion efficiency of 0.41% under AM 1.5 was recently measured with a p-type DSSC using this strategy.

New materials for tunable plasmonic colloidal nanocrystals

“…Among them, nickel oxide, a p-type transparent semiconductor, has been one of the most investigated members. [135][136][137] In the class of metal oxides, A 2 BO 4 spinels are certainly important members. It has been recently demonstrated by Paudel et al that neither oxygen vacancies nor cation vacancies can be the leading source of conductive behaviour in spinels.…”

Electrochemical characterization of NiO electrodes deposited via a scalable powder microblasting technique

“…5 Because of this interesting combination of electrical and optical properties, NiO x has been considered for energy storage applications, [6][7][8] in electrochromic devices as transparent electrode, 9-17 in optoelectronic devices as electron barrier, 18,19 for gas sensing, 20,21 and, more recently, in dye-sensitized solar cells (DSCs) as photoactive cathode. [22][23][24][25][26][27][28][29][30][31][32][33][34][35] The utilization of NiO x in such diverse applications has been accompanied by the development of various preparation methods and deposition techniques, aimed at producing NiO x -based materials with variable chemical composition, electrical resistivity, compactness and morphology. Examples include electrochemical deposition, 36 chemical vapor 37 and bath deposition, 38 spray pyrolysis, 39,40 sol-gel method, 14,23,25,41 hydrothermal synthesis, 29,42 and sputtering.…”

Inorganic p-Type Semiconductors: Their Applications and Progress in Dye-Sensitized Solar Cells and Perovskite Solar Cells

“…The working principle is very similar to an n-type DSC, the difference being that the excited state of the sensitizer is now reductively quenched by the semiconductor, which is the sensitizer excited state injects holes into the valence band of the p-type semiconductor [25][26][27]. While the general design of dyes consists of electron-donor (D) and electron-acceptor (A) linked with π conjugation (D-π-A), the main difference between n-type dye and p-type dye is position of the acceptor: the anchoring group is attached on the electron-acceptor part for n-type dyes while the group is attached on the electron-donor part for p-type dyes.…”

“…Since He et al first demonstrated the generation of a cathodic photocurrent from organic dye sensitized nanostructured NiO cathode [68], many sophisticatedly designed dyes have been reported (details about their molecular structures can be found in [26]). Among them, a donor-acceptor type dye, comprising a perylene monoimide (PMI) as the acceptor and an oligothiophene coupled to triphenylamine as the donor holds the record efficiency of 0.41% [27].…”