58th Annual Report on Research 2013 Under Sponsorship of the ACS Petroleum Research Fund

Over this two-year period of research we successfully synthesized a new semiconductor nanomaterial, pyrite iron disulfide (FeS₂), which is from the earth-abundant elements and non-toxic. We investigated the structural, optical and electronic properties and explored its application in hybrid photovoltaic (PV) devices. The effort in the first year has been reported. Here, we focus on summarizing our effort in the second year (09/2012 - 08/2013). In the second year, we focused on the investigation of the conditions controlling the morphology and the mechanisms for controlling phase purity and the growth of pyrite FeS₂nanocrystals (NCs). We explored the methods to make the electron transport layer, i.e. ZnO for using in the inverted hybrid PV devices and conducted electromagnetic and optical simulations to optimize device structures. We fabricated hybrid PV devices and investigated the effect of hole and electron transport layers on the overall device performance. The major accomplishments are summarized below.

 

1. 1.    Controlling phase purity and growth mechanism of pyrite nanomaterials

            Controlling phase purity. Earth-abundant and nontoxic pyrite FeS₂ is very promising for photovoltaic applications but the phase purity and the morphology of iron pyrite NCs have a significant impact on the solar cell performance. We systematically investigated reaction conditions and local chemical environment on phase purity and morphology of iron pyrite NCs synthesized via the hot injection method. Our experimental results show that the molar ratio of sulfur to iron and the reaction temperature are two critical factors in determining the crystalline phase of the synthesized materials. By reducing the molar ratio of sulfur to iron from 6:1 to 4:1, a trace amount of the greigite (Fe₃S₄) phase exists in the product besides the pyrite phase. Further reducing this ratio to 2:1, only Fe₃S₄ and pyrrhotite (Fe_1-xS) phases are in the product and no pyrite phase. By increasing and lowering the reaction temperature from 220 to 250°C and 120°C, respectively, the product has Fe₃S₄ crystalline phase with a trace amount of pyrite at the higher reaction temperature while an amorphous phase at the lower reaction temperature. A mechanism involving the generation of H₂S is proposed to explain the phase purity observed.

            Controlling morphology.Oleylamine (OLA) and octadecylamine (ODA) have been widely used in the hot injection synthesis of nanomaterials. They can provide three functions: solvent, reducing agent, and/or stabilizing ligand attached to the nanoparticles (NPs). The double bond in the middle of OLA could cause different packing density due to the steric effect and provide additional reducing effect compared to ODA. We used ODA or OLA to dissolve the iron agent and DPE or OLA to dissolve the sulfur agent. By using ODA-DPE, OLA-DPE, and OLA-only as solvents, varying reactant concentrations, and adding trioctylphosphine oxide (TOPO) or 1, 2-hexadecanediol (Diol) in the reaction solution, we are able to control the morphology of pyrite iron NCs. In general, high concentrations produce NCs with short, kinked, branched or chromosome-like rods with the diameter of ~10 nm and the length of ~20-30 nm. Low concentration or adding TOPO or Diol leads to quasi-cubic NC agglomerations with the size of ~200 nm.

            Orientation attachment (OA) crystal growth mechanism. In order to understand the formation of the unique morphology of pyrite NCs, we performed HRTEM studies to reveal the detail crystalline structures. Most of the curved and branched NCs are single crystals without defects but some of them have defects along the attached line of two particles. The “irregular” edges of the NCs suggested the growth following the OA instead of the Ostwald ripening (OR) growth mechanism. The most favorable attachment directions are along the <100> and <210>. We examined the molecular structures of (100), (210) and (111) surfaces with different terminations as well as their surface energies and suggested that the attachment occurs most likely between the surfaces of (001)-Fe and (001)-2S and between the surface of (210)-Fe and (210)-S.

            The as-synthesized iron pyrite NCs can be dispersed well in chloroform, chlorobenzene, toluene, and hexane and thus are promising in solution-processable photovoltaic applications.

           

1. 2.    Inverted hybrid PV devices

            Hybrid solar cells have attracted considerable attention as a promising third generation solar cell technologies. It combines the solution processing feasibility of polymer semiconductors and environmentally stable inorganic parts with high electron mobilities and tunable band gaps and energy levels. We investigated the inverted hybrid PV device performance with the focus on the roles of hole and electron transport layers. We constructed inverted P3HT:CdSe hybrid solar cells using ZnO colloid NPs or sol-gel grown ZnO NPs as the electron transport layer (ETL) and MoO₃, PEDOT:PSS, or combined MoO₃ and PEDOT:PSS as the hole transport layer (HTL). We optimized the structure and thickness for ZnO layer and the active layers using finite-difference time-domain (FDTD) and transfer matrix method (TMM). Shockley equation was used to obtain the shunt resistance and series resistance and to analyze the device. The relationship between V_oc, FF, J_scand light intensity indicates a bimolecular recombination mechanism. The optimized ETL and HTL could be extended to other polymer hybrid solar cells.

 

1. 3.    Broader impact

            Two graduate students were partially supported by this grant and participated in synthesis and making PV devices. One master graduate student, three undergraduate students (two are female), one female high school student (admitted to MIT in 2013), and one visiting graduate student from Tsinghua University in China were involved in the project. Four papers have been published and two manuscripts will be submitted within two months. Seven oral and seven poster presentations were given in the AIChE, MRS, and SPIE conferences. Two abstract will be submitted to the 2014 MRS Spring conference.

            This ACS PRF ND grant made a significant impact on the PI’s career and allowed the PI to conduct the research in a new direction that is nanomaterials and its application in solar energy. The capacities built and the experiences gained through this project will enable us to receive more supports to continue the research along this direction and explore new ideas.