Jason T. Bloking

Efficient charge generation by relaxed charge-transfer states at organic interfaces

Interfaces between organic electron-donating (D) and electron-accepting (A) materials have the ability to generate charge carriers on illumination. Efficient organic solar cells require a high yield for this process, combined with a minimum of energy losses. Here, we investigate the role of the lowest energy emissive interfacial charge-transfer state (CT1) in the charge generation process. We measure the quantum yield and the electric field dependence of charge generation on excitation of the charge-transfer (CT) state manifold via weakly allowed, low-energy optical transitions. For a wide range of photovoltaic devices based on polymer:fullerene, small-molecule:C60 and polymer:polymer blends, our study reveals that the internal quantum efficiency (IQE) is essentially independent of whether or not D, A or CT states with an energy higher than that of CT1 are excited. The best materials systems show an IQE higher than 90% without the need for excess electronic or vibrational energy.

Hole Transport Materials with Low Glass Transition Temperatures and High Solubility for Application in Solid-State Dye-Sensitized Solar Cells

We present the synthesis and device characterization of new hole transport materials (HTMs) for application in solid-state dye-sensitized solar cells (ssDSSCs). In addition to possessing electrical properties well suited for ssDSSCs, these new HTMs have low glass transition temperatures, low melting points, and high solubility, which make them promising candidates for increased pore filling into mesoporous titania films. Using standard device fabrication methods and Z907 as the sensitizing dye, power conversion efficiencies (PCE) of 2.94% in 2-μm-thick cells were achieved, rivaling the PCE obtained by control devices using the state-of-the-art HTM spiro-OMeTAD. In 6-μm-thick cells, the device performance is shown to be higher than that obtained using spiro-OMeTAD, making these new HTMs promising for preparing high-efficiency ssDSSCs.

Transparent and conductive paper from nanocellulose fibers

Here we report on a novel substrate, nanopaper, made of cellulose nanofibrils, an earth abundant material. Compared with regular paper substrates, nanopaper shows superior optical properties. We have carried out the first study on the optical properties of nanopaper substrates. Since the size of the nanofibrils is much less than the wavelength of visible light, nanopaper is highly transparent with large light scattering in the forward direction. Successful depositions of transparent and conductive materials including tin-doped indium oxide, carbon nanotubes and silver nanowires have been achieved on nanopaper substrates, opening up a wide range of applications in optoelectronics such as displays, touch screens and interactive paper. We have also successfully demonstrated an organic solar cell on the novel substrate.

Electronic Structure and Electrical Conductivity of Undoped LiFePO4

The electronic structure of LiFePO 4 underpins transport properties important to its use as a lithium storage electrode. Here we have calculated the electronic structure of LiFePO 4 in the ordered olivine structure by a first-principles method to determine (i) the effective mass of carriers and (ii) the nature of the band structure. The electrical conductivity in high purity undoped LiFePO 4 has also been measured experimentally. Spin-polarized calculations show a large electron effective mass and a much smaller but highly anisotropic hole effective mass, suggesting that hole-doped compositions should have the greater electronic conductivity. More surprisingly, the calculations show that this polyanion compound is a half-metal with spin-sensitive band structure, like some other oxides being studied for spintronics applications. This previously unappreciated aspect of the LiFePO 4 electronic structure may play a role in determining transport properties including those relevant to electrochemical applications.

Ternary bulk heterojunction solar cells: addition of soluble NIR dyes for photocurrent generation beyond 800 nm.

The incorporation of a tert-butyl-functionalized silicon 2,3-naphthalocyanine bis(trihexylsilyloxide) dye molecule as a third component in a ternary blend bulk heterojunction (BHJ) organic solar cell containing P3HT (donor) and PC60BM (acceptor) results in increased NIR absorption. This absorption yields an increase of up to 40% in the short-circuit current and up to 19% in the power conversion efficiency (PCE) in photovoltaic devices. Two-dimensional grazing incidence wide-angle X-ray scattering (2-D GIWAXS) experiments show that compared to the unfunctionalized dye the tert-butyl functionalization enables an increase in the volume fraction of the dye molecule that can be incorporated before the device performance decreases. Quantum efficiency and absorption spectra also indicate that, at dye concentrations above about 8 wt %, there is an approximately 30 nm red shift in the main silicon naphthalocyanine absorption peak, allowing further dye addition to contribute to added photocurrent. This peak shift is not observed in blends with unfunctionalized dye molecules, however. This simple approach of using ternary blends may be generally applicable for use in other unoptimized BHJ systems towards increasing PCEs beyond current levels. Furthermore, this may offer a new approach towards OPVs that absorb NIR photons without having to design, synthesize, and purify complicated donor-acceptor polymers.