57th Annual Report on Research 2012 Under Sponsorship of the ACS Petroleum Research Fund

In the past one year we have focused on the synthesis of pyrite iron disulfide (FeS₂) nanomaterials, investigating the structural, electronic and optical properties of synthesized nanocrystals (NCs), and fabricated hybrid photovoltaic (PV) devices using pyrite NCs. The major accomplishments are summarized below.

1. Synthesis and characterization of pyrite nanomaterials

   Amorphous FeS₂ Nanowires. We synthesized FeS₂ nanowires via a solvothermal process. We found that higher concentrations of iron and sulfur agents resulted in shorter and wider nanowires about 2 μm in length and 500 nm in diameter. By reducing the concentrations to ¼ and 1/8, nanowires are longer and thinner with about several microns in length and about 200 nm in diameter. The energy dispersion spectroscopy (EDS) was used to determine the element ratio of the nanowires and a S:Fe ratio of 1.98:1 was obtained. However, powder X-ray diffraction (XRD) experiments showed no diffraction peaks, indicating an amorphous structure of nanowires. The thermogravimetric analysis (TGA) showed that these nanowires started to lose weight at ~180ºC, much lower than the naturally occurred bulk FeS₂ sample. After annealing nanowires in N₂at 700ºC for 10 min, nanowires showed crystalline structure of iron monosulfide (FeS). Therefore, our results showed that only amorphous nanowires were formed in a solvothermal process and annealing can turn them into FeS crystalline structures. 

   Pyrite FeS₂ cubes and octahedrons.When we changed the solvent to water and added polyvinyl alcohol (PVA), polyvinyl pyrrolidone (PVP), and sodium hydroxide (NaOH) via a hydrothermal process, either nice cubes with (100) facets or octahedrons with (111) facets can be synthesized by changing the concentration of NaOH. XRD patterns confirmed that they are pyrite phase. The particles are typically in the range of 200-500 nm, which are large for making hybrid PV devices.

   Pyrite FeS₂ hierarchical particles. We further investigated the pyrite crystallization mechanisms and the means to control the morphology and phase of pyrite hierarchical particles. The roles of surfactants in the synthesis process were elucidated by the investigation of morphology and crystalline phases of the products under different reaction conditions and by the analysis of atomic structures of the pyrite (100) and (111) surfaces with different termination layers. It was also found that the oxidation state of iron ions and the species of anions in the iron reagents played important roles in the formation of crystalline phase and morphology of the products.

   Pyrite FeS₂ nanocrystals. Using the hot injection method, we successfully synthesized pyrite NCs with dot, rod, and kinked rod shapes. Transmission electron microscope (TEM), scanning TEM (STEM) and dynamic light scattering (DLS) were used to characterize the size of synthesized pyrite NCs. Typical size is in the range of 10-30 nm. XRD, Raman vibrational spectroscopy and high resolution TEM confirmed the pyrite phase. The UV-vis absorption spectra demonstrate the absorption of light up to ~1000 nm. Different solvents and ligands were used in the synthesis in an attempt to control the size, shape and facets of NCs as well as surface states, which could have impact to the PV device performance.

   In summary, we have conducted a thorough investigation of synthesis pyrite nanomaterials using hydrothermal methods. Although we can control the morphology, size and phase of the products, the pyrite particles are typically larger than those to be used in hybrid PV devices. Therefore, we turned our focus onto the hot injection method and successfully synthesized size, shape controlled pyrite NCs which are suitable for making hybrid PV devices.

2. Pyrite-based hybrid PV devices

   Band gap and band edges. In order to design appropriate PV device architectures, we first determined the band gap and valence and conduction band edges. UV-vis spectra were used to obtain the estimated band gap of as-synthesized pyrite NCs which is in the range of 1.1-1.3 eV, larger than reported value from thin films. We also used electrochemistry method, cyclic voltammetry (CV), to determine the band edges and average E_CB of -3.9 eV and E_VBof -5.6 eV were obtained.

   Ternary inverted hybrid PV devices. We mixed pyrite NCs with P3HT:PCBM to form bulk heterojunction (BHJ) as an active layer and built an inverted device that has a thin layer of ZnO layer on ITO glass as an electron collected layer and PEDOT:PSS on top of the active layer for collecting holes and finally a silver cathode. We systematically studied the effect of adding FeS₂ NC’s in the ternary inverted BHJ to observe their effect on film morphology and charge transport.  By varying the concentration between 0 and 4 wt% FeS₂ in a P3HT:PCBM matrix, we observed the device performance transition between three distinct regimes. With a pyrite loading lower than 0.5 wt%, the J_SC increased by ~10-20% while Voc and Fill Factor (FF) decreased a little, resulting a similar power conversion efficiency (PCE) as P3HT:PCBM system (~2.03%). When the pyrite loading was between 0.8 and 3.0 wt%, the J_SC continued increase while Voc and FF decreased, resulting in a PCE less than 1%. Further increasing the pyrite loading above 3 wt%, rectification is almost totally lost and the V_OCshrinks to less than 1V. We also performed photoluminescence (PL), time-resolved PL and external quantum efficiency (EQE) measurements in order to understand the charge separation and transport mechanisms.

   Other PV devices. We also fabricated other architecture PV devices utilizing pyrite NCs, including conventional binary hybrid BHJ P3HT:FeS₂ NCs, biheterojunction P3HT/FeS₂NCs, and Schottky devices. However, no rectification was observed. This leads us to further investigate the surface states of pyrite and morphology of active layers.

3. Broader impact.

   Three graduate students were partially supported by this grant and participated in synthesis and making PV devices. Two female undergraduate students and one female high school student were involved in the project. Two papers have been published and two manuscripts will be submitted within two weeks. Six oral and five poster presentations were given in the 2011 AIChE and MRS Fall conferences and 2012 SPIE Optics + Photonics meeting.