Enhanced transient photovoltaic characteristics of core–shell ZnSe/ZnS/L-Cys quantum-dot-sensitized TiO ₂ thin-film

Abstract

Photoanodic properties greatly determine the overall performance of quantum-dot-sensitized solar cells (QDSCs). In the present report, the microdynamic behaviors of carriers in the nanocomposite thin-film, a ZnSe QD-sensitized mesoporous La-doped nano-TiO 2 thin-film, as a potential candidate for photoanode, are probed via nanosecond transient photovoltaic (TPV) spectroscopy. The results confirm that the L-Cys ligand has a dual function serving as a stabilizer and molecular linker. Large quantities of interface states are located at the energy level with a photoelectric threshold of 1.58 eV and a quantum well (QW) depth of 0.67 eV. This QW depth is approximately 0.14 eV deeper than the depth of QW buried in the ZnSe QDs, and a deeper QW results in a higher quantum confinement energy. A strong quantum confinement effect of the interface state may be responsible for the excellent TPV characteristics of the photoanode. For example, the peak intensity of the TPV response of the QD-sensitized thin-film lasts a long time, from 9.40 × 10 −7 s to 2.96 × 10 −4 s, and the end time of the PTV response of the QD-sensitized thin-film is extended by approximately an order of magnitude compared with those of the TiO 2 substrate and the QDs. The TPV characteristics of the QD-sensitized thin-film change from p-type to n-type for the QDs before and after sensitizing. These properties strongly depend on the extended diffusion length of the photogenerated carries and the reduced recombination rate of photogenerated electron-hole pairs, resulting in prolonged carrier lifetime and an increased level of electron injection into the TiO 2 thin-film substrate.

Improved carrier transport in Mn:ZnSe quantum dots sensitized La-doped nano-TiO2 thin film*

“…The La-doped nano-TiO 2 substrate with the three-layer structure of approximately 21 µm was chosen according to our previous work. [50] The aggregation of the La-doped nano-TiO 2 particles at the substrate surface displayed the average size of 0.5 µm as shown in Fig. 2(a).…”

The effect of Mn-doped ZnSe passivation layer on the performance of CdS/CdSe quantum dot-sensitized solar cells*

“…[7,8] It is an effective method to improve the PCE of QDSSCs by preparing a passivation layer on the surface of quantum dots to reduce surface defects. [9][10][11][12] Meanwhile, Huang et al pointed out that both the conduction band position and the valence band position of ZnSe were higher than CdS/CdSe quantum dots. [13] Therefore, the type-II core-shell structure with quantum dots not only can effectively prevent the reverse transmission of photoelectrons to the electrolyte, but also facilitate the rapid transport of holes from the quantum dots to the electrolyte after the exciton separation.…”