Hydrogen Generation Using Integrated Photovoltaic and Photoelectrochemical Cells

Investigation into integrated PEC/PVC for water splitting and light harvesting has focused on doping, QD sensitization and nanomaterials morphology. We have demonstrated, for the first time, PVC based on combined use of nitrogen doping and CdSe QD sensitization of nanocrystalline TiO2 thin films with promising results for solar energy conversion. Such TiO2:N/CdSe PVC produced an incident-photon-to-current efficiency (IPCE) of 95% at 300 nm, a 27.7% fill factor (FF) and a power conversion efficiency of 0.84%. Many aspects of that system are still not optimized including nitrogen doping level, properties of CdSe QDs, and TiO2 film morphology. PEC based on ZnO nanostructures produced via pulsed laser deposition (PLD), oblique angle deposition (OAD) and electron beam glancing angle deposition (GLAD) show differing morphological, photophysical and PEC characteristics due to each technique. A combination of HRSEM, UV-vis spectroscopy, XRD and photoelectrochemistry has been used to characterize the fundamental properties of the ZnO PEC. Hybrid nanorod structures of WO3 and TiO2 in a stacked configuration via the GLAD technique have also been studied for their light harvesting and water splitting abilities. Progress Report To maximize the ability of TiO2 to harvest photons and produce usable electrical current there have been two main strategies. Firstly, the most widely probed pathway is to exploit nanoporous TiO2 by sensitizing the nanostructures with dyes to act as electron acceptor/injector pairs (Gratzel cell). Doping has also been quite successful in narrowing the bandgap of metal oxides, most notably TiO2 with N. A modification of the dye sensitization has been to replace the organic dye with tunable semiconductor QDs. A merger of both N doping and CdSe QD sensitization was found to be very advantageous and showed promising light harvesting PV characteristics. While extensive wet chemical modifications of the TiO2:N thin films were performed, the use of nitric acid, hexamethyltetramine (HMT) and titanium iso-propoxide was the most efficient combination. TiO2:N thin films were thoroughly sensitized by CdSe QDs via the linking molecule thioglycolic acid (TGA). Nanocrystalline hybrid thin films of only 1.1 μm produced impressive initial results. TiO2:N-TGA-CdSe thin films in this initial study showed a short circuit photocurrent (Isc) of 682 μA/cm, an open circuit voltage (Voc) of -1.2V and a fill factor (FF) of 27.7%. With these characteristics it was found the hybrid thin films (1.1 μm) attained a 0.84% overall power conversion efficiency. Nitrogen doping level was determined using XPA to be in the range of 0.6-0.8% with HMT as the nitrogen precursor. Absorption of the TiO2:N was significantly red shifted from ~ 390 nm to an absorption onset of 600 nm, a bandgap narrowing of 1.14 eV (Figure 1). Due to the high extinction coefficient of CdSe QDs in the visible range, the majority of increased light harvesting was due to QD sensitization. Comparisons of power conversion efficiencies leads to the conclusion that the combination of doping and sensitization is greater than the simple sum of nitrogen doping (0.01%) alone or CdSe QD sensitization (0.73%) alone. Our assertion is that the valence hole recombination of CdSe QDs is aided, by the filled nitrogen doping energy level as viewed on the normal hydrogen electrode (NHE) scale at pH=0 (Figure 1). The two main processes are the electron injection from CdSe to TiO2:N, and the movement of nitrogen level electrons into the valence band of CdSe. Solid state TiO2:N-TGA-CdSe devices showed increased photo response and an IPCE of ~4.3% at the excitonic peak of the CdSe QDs at 600 nm (Figure 2). Light harvesting in the solid state device directly matched that of the 4.6 nm CdSe QDs utilized as electron donors. Immediate conversion optimization parameters include increasing nitrogen doping within the TiO2 nanoparticles, and modifying the capping agent of CdSe from tetradecylphosphonic acid (TDPA) to pyridine. Added doping should increase hole mobility and light harvesting at the 600 nm absorption onset, and the smaller tunneling barrier of a pyridine coated CdSe QD will enhance the injection rate greatly by decreasing the tunneling barrier as indicated in previous studies. In a comparative PEC study of ZnO thin films, three deposition techniques, including PLD, OAD and GLAD, were utilized to produce ZnO films with different morphologies,. ZnO has been extensively studied for both PEC and PV applications using various wet chemical and deposition techniques. A general illustration of PLD/OAD is shown wherein a high energy pulsed laser produces an adatom plume which deposits onto a substrate with a varying deposition angle α (Figure 3). GLAD works in a similar fashion in that the substrate is tilted, but the adatom plume is produced by an accelerated electron stream instead and the substrate is also rotating at a specific rpm. All three techniques were found to produce very different morphologies and porosity as seen in HRSEM images (Figure 4). PLD produced the densest films with a deposition angle α =0o (Figure 4, top) and this resulted in a light brownish hue to the film that is indicative of defects in the crystal lattice of ZnO. Sintering of PLD thin films at 550 Co for 2 hours did not affect the overall color of these films. OAD ZnO thin films revealed a porous scale-like morphology at low resolution and a collection of interconnected nanoparticles of various spheroid shapes at increased magnification (Figure 4, middle). E-beam GLAD produced vastly different thin films made of 20-60 nm ZnO nanoparticles in a more classic nanoporous structure similar to TiO2. PEC measurement revealed good photoresponse for all ZnO films at AM 1.5 (~100 mW/cm). The onset of photocurrent begins at 0V in comparison to the flat band potential of -0.28 V (VFB) indicating inefficient electron-hole separation at negative potentials (not shown). Flat band potentials of both OAD and GLAD ZnO were found by the potential intercepts of Mott-Schottky plots. The VFB of GLAD ZnO Figure 1. Illustration of the band structure of TiO2:N and 3.5 nm CdSe QDs versus NHE. The two arrows represent electron injection from CdSe to TiO2:N (top),, and hole recombination from the nitrogen doping level to the CdSe valence band (bottom). 400 500 600 700 0 50 100 150 200 250 300