Investigating hematite nanostructures for photovoltaic applications

Photoelectrochemical solar cells are promising because of their low cost and fabrication process. Iron oxide (hematite phase) can be employed in forming a semiconductor/electrolyte junction to harness solar energy. Given a bandgap of 2.2 eV of Fe2O3 , a maximum of 12-13 mAcm−2 can be extracted from it. But the short hole diffusion lengths which are in the order of few nanometers pose a problem in achieving this goal. This thesis focuses on utilizing iron oxide nanostructures as a tool to resolve this problem. Solution processed method was employed in synthesizing different nanostructures of Fe2O3 on conductive substrates such as FTO. These nanostructures were directly integrated into PEC solar cells using iodine/iodide electrolyte. The limitations of such a system were systematically investigated by measuring the short circuit current with varying light intensity and impedance spectroscopy. Decoupling the absorption properties and charge transport properties in hematite nanostructures was realized by utilizing near IR absorbing organic dye (SQ02). Due to the complimentary nature of absorption of SQ02 dye, absorption and charge transport characteristics in iron oxide were studied by measuring impedance under different spectral excitations. These studies revealed that iron oxide is a good transporter of electrons when it is not excited. The bulk recombination in iron oxide is still a prevalent factor for the poor performance of these solar cells under white light illumination. Ultrathin layers of iron oxide were also deposited on tin oxide which act as host-scaffold. This approach tackles the problem of low diffusion lengths while not compromising on the absorption of iron oxide. Different thicknesses of overlayer were studied. The thickness variation was done in such a way that we could cover all the possibilities such as thickness below the hole diffusion lengths and also thickness above the hole diffusion length. Impedance spectroscopy was performed on PEC solar cells to investigate the root causes of recombination occurring in such systems.Master of Philosophy (MSE

Metal/metal sulfide functionalized single-walled carbon nanotubes : FTO-free counter electrodes for dye sensitized solar cells

The use of single-walled carbon nanotubes (CNT) thin films to replace conventional fluorine-doped tin oxide (FTO) and both FTO and platinum (Pt) as the counter electrode in dye sensitized solar cells (DSSC) requires surface modification due to high sheet resistance and charge transfer resistance. In this paper, we report a simple, solution-based method of preparing FTO-free counter electrodes based on metal (Pt) or metal sulfide (Co8.4S8, Ni3S2) nanoparticles/CNT composite films to improve device performance. Based on electrochemical studies, the relative catalytic activity of the composite films was Pt > Co8.4S8 > Ni3S2. We achieved a maximum efficiency of 3.76% for the device with an FTO-free counter electrode (Pt/CNT). The device with an FTO- and Pt-free (CoS/CNT) counter electrode gives 3.13% efficiency

Modulating the optical and electrical properties of all metal oxide solar cells through nanostructuring and ultrathin interfacial layers

The benefits and drawbacks of nanostructuring in all oxide ZnO/Cu2O solar cells were studied. The solar cells were fabricated on fluorine doped tin oxide substrates, with solution processed deposition methods. Both planar ZnO layer and Cu2O were deposited by electrodeposition while ZnO nanorods were grown by chemical bath deposition technique. It is shown that short circuit current (Jsc) of the devices increases with nanostructuring of ZnO due to electrical and optical gains. Despite improving the photocurrent, nanostructuring decreases the Voc of the device due to carrier recombination. The introduction of a thin TiO2 interfacial layer through atomic layer deposition was able to reduce the recombination

The effect of annealing temperature on the optical properties of In2S3 thin film

Indium Sulfide (In2S3) thin film were deposited on glass substrate through successive ionic layer adsorption and reaction (SILAR) method. The samples were annealed at 250 °C, 350 °C and 450 °C respectively after deposition. The X-ray diffraction (XRD) analysis shows that the sample annealed at 450 °C has the most clear and obvious XRD pattern, which indicates that In2S3 annealed at 450 °C possess the highest crystallinity. Ultraviolet-visible spectroscopy analysis of the samples shows that the band gap of In2S3 is around 2 eV. This study shows that annealing does not induce any change in optical bandgap of In2S3 thin films on glass substrates as reported elsewhere

Perovskite-Hematite Tandem Cells for Efficient Overall Solar Driven Water Splitting

Photoelectrochemieal water splitting half reactions on semiconducting photoelectrodes have received much attention but efficient overall water splitting driven by a single photoelectrode has remained elusive due to stringent electronic and thermodynamic property requirements. Utilizing a tandem configuration wherein the total photovoltage is generated by complementary optical absorption across different semiconducting electrodes is a possible pathway to unassisted overall light-induced water splitting. Because of the low photovoltages generated by conventional photovoltaic materials (e.g., Si, CIGS), such systems typically consist of triple junction design that increases the complexity due to riptoelectrical trade-offs and are also not cost-effective. Here, we show that a single solution processed organic inorganic halide perovskite (CH3NH3PI3) solar cell in tandem with a Fe2O3 photoanode can achieve overall, unassisted water splitting with a solar-to-hydrogen conversion efficiency of 2.4%. Systematic dectro-optical studies were performed to investigate the performance of tandem device. It was found that the overall efficiency was limited by the hematite's photocurrent and onset potential. To understand these limitations, we have estimated the intrinsic solar to chemical conversion efficiency of the doped and undoped Fe2O3 photoanodes. The total photopotentid generated by our tandem system (1.87 V) exceeds both the thermodynamic and kinetic requirements (1.6 V), resulting in overall water splitting without the assistance of an electrical bias

Flexible, low-temperature, solution processed ZnO-based perovskite solid state solar cells

A ZnO compact layer formed by electrodeposition and ZnO nanorods grown by chemical bath deposition (CBD) allow the processing of low-temperature, solution based and flexible solid state perovskite CH3NH3PbI3 solar cells. Conversion efficiencies of 8.90% were achieved on rigid substrates while the flexible ones yielded 2.62%

Limitations of Cs3Bi2I9 as Lead-Free Photovoltaic Absorber Materials

Lead (Pb) halide perovskites have attracted tremendous attention in recent years because of their rich optoelectronic properties, which have resulted in more than 22% power conversion efficient photovoltaics (PVs). Nevertheless, Pb-metal toxicity remains a huge hurdle for extensive applications of these compounds. Thus, alternative compounds with similar optoelectronic properties need to be developed. Bismuth possesses electronic structure similar to that of lead with the presence of ns2 electrons that exhibit rich structural variety as well as interesting optical and electronic properties. Herein, we critically assess Cs3Bi2I9 as a candidate for thin-film solar cell absorber. Despite a reasonable optical band gap (∼2 eV) and absorption coefficient, the power conversion efficiency of the Cs3Bi2I9 mesoscopic solar cells was found to be severely lacking, limited by the poor photocurrent density. The efficiency of the Cs3Bi2I9 solar cell can be slightly improved by changing the stoichiometry of the precursor solutions, which is most probably due to the reduction in nonradiative defects as evident from our single-crystal photoluminescence spectroscopy. However, detailed investigations on pristine Cs3Bi2I9 reveal that zero-dimensional molecular crystal structure remains one of the main bottlenecks in achieving high performance. On the basis of our comprehensive studies, we have proposed that a continuous network of three-dimensional crystal structure should be another major criterion in addition to proper band gap and suitable optical properties of the future PV compounds.This research was supported by the National Research Foundation, Prime Minister’s Office, Singapore under its Competitive Research Programme (CRP award no. NRFCRP14-2014-03) and through the Singapore−Berkeley Research Initiative for Sustainable Energy (SinBeRISE) CREATE Program. T.C.S. would like to acknowledge the financial support from the Ministry of Education Academic Research Fund Tier 1 Grants RG101/15 and RG173/16 and Tier 2 Grants MOE2015-T2-2-015 and MOE2016-T2-1-034

Decoupling light absorption and charge transport properties in near IR-sensitized Fe2O3 regenerative cells

We report a regenerative iron oxide nanorod solar cell sensitized with squarine (SQ02) dye, which improves the overall light harvesting due to the dye's complementary absorption. The use of illumination sources which enabled selective excitation of Fe2O3 and the dye independently allowed the decoupling of transport and light absorption in Fe2O3. From absorbed photon-to-current conversion efficiency (APCE) calculations, it was found out that about 80% (at lambda = 680 nm) of the absorbed photons in SQ02 dye are converted to electrons and transported without any further loss into the FTO electrode. Impedance spectroscopy revealed that bulk recombination is still a prominent problem while utilizing hematite as a photoanode in a dye sensitized configuration