Wide Band Gap Electron Research Articles

Improving the performance of colloidal quantum-dot-sensitized solar cells

Solar cells based on a mesoporous structure ofTiO2 and the polysulfide redox electrolyte were prepared by direct adsorption of colloidal CdSequantum dot light absorbers onto the oxide without any particular linker. Severalfactors cooperate to improve the performance of quantum-dot-sensitized solarcells: an open structure of the wide bandgap electron collector, which facilitates ahigher covering of the internal surface with the sensitizer, a surface passivation ofTiO2 to reduce recombination and improved counter electrode materials. As aresult, solar cells of 1.83% efficiency under full 1 sun illumination intensityhave been obtained. Despite a relatively large short circuit current (Jsc = 7.13 mA cm−2) and opencircuit voltage (Voc = 0.53 V), the colloidal quantum dot solar cell performance is still limited by a low fill factor of0.50, which is believed to arise from charge transfer of photogenerated electrons to theaqueous electrolyte.

Low voltage efficient simple p-i-n type electrophosphorescent green organic light-emitting devices

We present simple p-i-n structures with double-emitting and mixed-emitting layers for highly efficient phosphorescent green devices. Using a wide band-gap hole transporting material of 4,4′4″-tris(N-carbazolyl)-triphenylamine and a wide band-gap electron transporting material of bis[2-(2-hydroxyphenyl)-pyridine]beryllium, the bilayered p-i-n structure with no heterointerface barriers has been realized. A very low onset voltage value of 2.4 V corresponding to the energy of 2.4 eV of green electroluminescence, which is close to the photon energy of dopant emitting molecules (2.3–2.4 eV), is achieved in this simple p-i-n device configuration. Maximum current- and power-efficiency values of 53.3 cd/A and 61.4 lm/W and low rolloff of current efficiency (6%) are demonstrated in the simple p-i-n green phosphorescent devices, promising for the practical and economical high brightness applications.

Surface Photovoltage Spectroscopy of Dye-Sensitized Solar Cells with TiO2, Nb2O5, and SrTiO3Nanocrystalline Photoanodes: Indication for Electron Injection from Higher Excited Dye States

The onset wavelengths of the surface photovoltage (SPV) in dye-sensitized solar cells (DSSCs) with different mesoporous, wide-band gap electron conductor anode materials, viz., TiO2 (anatase), Nb2O5 (amorphous and crystalline), and SrTiO3, using the same Ru bis-bipyridyl dye for all experiments, are different. We find a clear dependence of these onset wavelengths on the conduction band edge energies (ECB) of these oxides. This is manifested in a blue-shift for cells with Nb2O5 and SrTiO3 compared to those with TiO2. The ECB levels of Nb2O5 and SrTiO3 are known to be some 200−250 meV closer to the vacuum level than that of our anatase films, while there is no significant difference between the optical absorption spectra of the dye on the various films. We, therefore, suggest that the blue shift is due to electron injection from excited-state dye levels above the LUMO into Nb2O5 and SrTiO3. Such injection comes about because, in contrast to what is the case for anatase, the LUMO of the adsorbed dye in the solution is below the ECB of these semiconductors, necessitating the involvement of higher vibrational and/or electronic levels of the dye, with the former being more likely than the latter. While for Nb2O5 hot electron injection has been proposed earlier, on the basis of flash photolysis experiments, this is the first evidence for such ballistic electron-transfer involving SrTiO3, a material very similar to anatase but with a significantly smaller electron affinity. Additional features in the SPV spectra of SrTiO3 and amorphous Nb2O5 (but not in those of crystalline Nb2O5) can be understood in terms of hole injection from the dye into the oxide via intraband gap surface states.