Quantum Dot Solar Cells. Harvesting Light Energy with CdSe Nanocrystals Molecularly Linked to Mesoscopic TiO₂ Films

Abstract

By using bifunctional surface modifiers (SH-R-COOH), CdSe quantum dots (QDs) have been assembled onto mesoscopic TiO(2) films. Upon visible light excitation, CdSe QDs inject electrons into TiO(2) nanocrystallites. Femtosecond transient absorption as well as emission quenching experiments confirm the injection from the excited state of CdSe QDs into TiO(2) nanoparticles. Electron transfer from the thermally relaxed s-state occurs over a wide range of rate constant values between 7.3 x 10(9) and 1.95 x 10(11) s(-1). The injected charge carriers in a CdSe-modified TiO(2) film can be collected at a conducting electrode to generate a photocurrent. The TiO(2)-CdSe composite, when employed as a photoanode in a photoelectrochemical cell, exhibits a photon-to-charge carrier generation efficiency of 12%. Significant loss of electrons occurs due to scattering as well as charge recombination at TiO(2)/CdSe interfaces and internal TiO(2) grain boundaries.

Real-Space Mapping of Surface Trap States in CIGSe Nanocrystals Using 4D Electron Microscopy

“…73 It has been proposed in literature that the observed photocurrent decay during the initial excitation period may arise from: (i) the inability of the hole-transporting materials to quickly scavenge photo-generated holes or (ii) dominant charge recombination/scattering at the NC/TiO 2 heterointerface or at grain boundaries within the mesoporous TiO 2 . 74 As the hole-transporting material in both of the devices remains same, the sharp decay of photocurrent in the CIGSe case suggests the presence of a large number of trap states in the CIGSe NCs, which promote photocarrier recombination at the CIGSe/TiO 2 interface. The slow increase in the photocurrent value after this sharp decay is most likely the signature of trap filling after a sufficient illumination time.…”

High-Efficiency Photochemical Water Splitting of CdZnS/CdZnSe Nanostructures

“…Under white light illumination of 100 mW cm −2 , CdS/CdSe QDssensitized ZnO nanowire electrodes and TiO 2 electrodes had high photocurrent densities of 12 and 14.9 mA cm −2 , respectively [14][15][16][17][18]. Although PECs featuring QDs-sensitized TiO 2 and ZnO electrodes provided greater values for water splitting than corresponding bare electrodes, their values are still less than 4% [11][12][13]. Moreover, an external potential is mandatory to separate the charges efficiently to prevent the recombination of electron-hole pair under illumination [20].…”

“…Quantum dots (QDs) such as CdTe [10], CdS [11][12][13], and CdSe [14][15][16][17][18] have been anchored to TiO 2 and ZnO electrodes to harvest visible light for more efficient water splitting. Photoelectrochemical cells (PECs) incorporating QDssensitized TiO 2 electrodes provide several advantages: (i) ease of fabrication, (ii) generation of multiple electron/hole [19], (iii) high visible light harvesting capability in solar spectrum, and (iv) tunable band gaps due to the quantum size effect of QDs.…”

Polypyrrole/TiO2 composites for the application of photocatalysis

“…Nanostructured TiO 2 is one of the most investigated photoactive materials due to its excellent electronic structure and high stability to photocorrosion under redox conditions [1][2][3]. TiO 2 has drawn much attention due to its potential applications in solar energy conversion (such as photocatalytic water splitting and dye-sensitized solar cell), environmental cleaning (photocatalytic degradation of pollutants, self-cleaning, and water purification), and bio-sensing [4][5][6][7].…”