Nitrogen-doped and CdSe quantum-dot-sensitized nanocrystalline TiO2 films for solar energy conversion applications

Nitrogen-doped titanium dioxide (TiO2/N) nanoparticle thin films have been produced by a sol-gel method with hexamethylenetetramine (HMT) as the dopant source. The synthesized TiO2/N thin films have been sensitized with CdSe quantum dots (QDs) via a linking molecule, thioglycolic acid (TGA). Optical, morphological, structural, and photocurrent properties of the thin films with and without QD sensitization have been characterized by AFM, TEM, XPS, Raman spectroscopy, UV-visible spectroscopy, and photoelectrochemistry techniques. AFM measurements reveals that films with thicknesses of 150 and 1100 nm can be readily. prepared, with an average TiO2 particle size of 100 nm. TEM shows a uniform size distribution of CdSe QDs utilized in sensitizing the TiO2/N films. Doping of the TiO2 crystal lattice by HMT was confirmed to be 0.6-0.8% by XPS. Differences in crystal phase caused by the precursors HMT, nitric acid, and poly(ethylene glycol) (PEG) are elucidated using XRD and Raman spectroscopy. The resultant crystal phase of TiO2/N varies but is a mixture of anatase, brookite, and rutile phases. UV-visible absorption spectra show that N doping of TiO2 causes a red-shifted absorption into the visible region, with an onset around 600 nm. Nitrogen doping is also responsible for the enhanced photocurrent response of the TiO2/N nanoparticle films in the visible region relative to undoped TiO2 films. In addition, CdSe QDs linked to TiO2/N nanoparticles using TGA were found to significantly increase the photocurrent and power conversion of the films compared to standard TiO2/N films without QD sensitization. The incident photon-to-current conversion efficiency (IPCE) is 6% at 400 nm for TiO2/N-TGA-CdSe solid-state solar cells and 95% for TiO2/N-TGA-CdSe films near 300 nm in a Na2S electrolyte, which is much higher than that of undoped TiO2 With QD sensitization or TiO2/N without QD sensitization. The power conversion efficiency (q) was found to be 0.84% with a fill factor (FF%) of 27.7% with 1100 nm thick TiO2/N-TGA-CdSe thin films. The results show that combining nitrogen doping with the QD sensitization of TiO2 thin films is an effective and promising way to enhance the photoresponse in the near-UV and visible region, which is important for potential photovoltaic (PV) and photoelectrochemical applications.