E. A. Schiff

Determining the locus for photocarrier recombination in dye-sensitized solar cells

We present intensity-modulated photocurrent and infrared transmittance measurements on dye-sensitized solar cells based on a mesoporous titania (TiO2) matrix immersed in an iodine-based electrolyte. Under short-circuit conditions, we show that an elementary analysis accurately relates the two measurements. Under open-circuit conditions, infrared transmittance, and photovoltage measurements yield information on the characteristic depth at which electrons recombine with ions (the “locus of recombination”). For one particular series of samples recombination occurred near the substrate supporting the titania film, as opposed to homogeneously throughout the film.

Conducting polymer and hydrogenated amorphous silicon hybrid solar cells

An organic-inorganic hybrid solar cell with a p-i-n stack structure has been investigated. The p-layer was a spin coated film of PEDOT:PSS [poly(3,4-ethylenedioxythiophene) poly (styrenesulfonate)]. The i-layer was hydrogenated amorphous silicon (a-Si:H), and the n-layer was microcrystalline silicon (μc-Si). The inorganic layers were deposited on top of the organic layer by the hot-wire chemical vapor deposition technique at 200°C. These hybrid devices exhibited open circuit voltages (VOC) as large as 0.88V and solar conversion efficiencies as large as 2.1%. Comparison of these devices with those incorporating a-SiC:H:B p-layers indicates that the organic layer is acting as an electrically ideal p-layer.

Polyaniline on crystalline silicon heterojunction solar cells

Organic/inorganic heterojunction solar cells were fabricated on the (100) face of n-type silicon crystals using acid-doped polyaniline (PANI) with widely varying conductivities. For films with conductivities below 10−1S∕cm, the open-circuit voltage VOC increases with increasing film conductivity as expected when VOC is limited by the work function of the film. Extrapolation of these results to the higher conductivity films indicates that PANI could support VOC of 0.7V or larger. VOC measurements for the cells with higher conductivity PANI saturated at 0.51V. We speculate that uncontrolled surface states at the PANI/Si interface are reducing these values.

Thermodynamic Limit to Photonic-Plasmonic Light-Trapping in Thin Films on Metals

We calculate the maximum optical absorptance enhancements in thin semiconductor films on metals due to structures that diffuse light and couple it to surface plasmon polaritons. The calculations can be used to estimate plasmonic effects on light-trapping in solar cells. The calculations are based on the statistical distribution of energy in the electromagnetic modes of the structure, which include surface plasmon polariton modes at the metal interface as well as the trapped waveguide modes in the film. The enhancement has the form 4n2+nλ/h (n – film refractive index, λ – optical wavelength, h – film thickness), which is an increase beyond the non-plasmonic “classical” enhancement 4n2. Larger resonant enhancements occur for wavelengths near the surface plasmon frequency; these add up to 2 mA/cm2 to the photocurrent of a solar cell based on a 500 nm film of crystalline silicon. We also calculated the effects of plasmon dissipation in the metal. Dissipation rates typical of silver reverse the resonant enhanc...

Hole drift-mobility measurements in microcrystalline silicon

We have measured transient photocurrents on several p‐i‐n solar cells based on microcrystalline silicon. For two of these samples, we were able to obtain conclusive hole drift-mobility measurements. Despite the predominant crystallinity of these samples, temperature-dependent measurements were consistent with an exponential-bandtail trapping model for transport, which is usually associated with noncrystalline materials. We estimated valence bandtail widths of about 31meV and hole band mobilities of 1–2cm2∕Vs. The measurements support mobility-edge transport for holes in these microcrystalline materials, and broaden the range of materials for which mobility-edge transport corresponds to an apparently universal band mobility of order 1cm2∕Vs.

Hole-mobility limit of amorphous silicon solar cells

We present temperature-dependentmeasurements and modeling for a thickness series of hydrogenated amorphous siliconnipsolar cells. The comparison indicates that the maximum power density ( P MAX ) from the as-deposited cells has achieved the hole-mobility limit established by valence bandtail trapping, and P MAX is thus not significantly limited by intrinsic-layer dangling bonds or by the doped layers and interfaces.Measurements of the temperature-dependent properties of light-soaked cells show that the properties of as-deposited and light-soaked cells converge below 250 K; a model perturbing the valence band tail traps with a density of dangling bonds accounts adequately for the convergence effect.

Hole drift mobility measurements in amorphous silicon‐carbon alloys

Hole drift mobilities have been measured using photocarrier time‐of‐flight for several hydrogenated amorphous silicon‐carbon alloy specimens. We find that, as the band gap increases, the hole drift mobility remains essentially constant. The temperature and dispersion properties were broadly consistent with hole multiple trapping in the valence bandtail. In conjunction with previous drift mobility measurements in hydrogenated amorphous silicon‐carbon alloys and hydrogenated amorphous silicon‐germanium alloys, these hole measurements complete a simple pattern for the effects of band gap modification on drift mobilities: electron mobilities decline as the band gap is increased or decreased from 1.75 eV, but hole mobilities are relatively unaffected.

Open-circuit voltage physics in amorphous silicon solar cells

We have performed computer calculations to explore effects of the p/i interface on the open-circuit voltage in a-Si:H based pin solar cells. The principal conclusions are that interface limitation can occur for values of V OC significantly below the built-in potential V BI of a cell, and that the effects can be understood in terms of thermionic emission of electrons from the intrinsic layer into the p -layer. We compare measurements of V OC and electroabsorption estimates of V BI with the model calculations. We conclude that p/i interface limitation is important for current a-Si:H based cells, and that the conduction band offset between the p and i layers is as important as the built-in potential for future improvements to V OC .

Picosecond electron drift mobility measurements in hydrogenated amorphous silicon

The electron photocarrier drift mobility in undoped, hydrogenated amorphous silicon was measured in the picosecond domain using optically detected time‐of‐flight techniques based on the electroabsorption effect. The electron drift mobility at room temperature was approximately 3 cm2 /V s between 50 and 500 ps at electric fields of (1–3)×105 V/cm, essentially the same as the mobility measured in the nanosecond domain. The picosecond measurement constrains transport models postulating larger electron mobilities shortly after photogeneration; the mobility lifetime product for a short‐time process must be less than 3×10−11 cm2 /V. A slightly superlinear dependence of drift velocity upon an electric field was also found which could not be explained by dispersion effects.

Drift-mobility measurements and mobility edges in disordered silicons

Published electron and hole drift-mobility measurements in hydrogenated amorphous silicon (a-Si:H), amorphous silicon alloys (a-SiGe:H and a-SiC:H), and microcrystalline silicon (μc-Si:H) are analysed in terms of the exponential bandtail trapping model. A three-parameter model was employed using an exponential bandtail width ΔE, the band mobility μ 0 , and the attempt-to-escape frequency ν. Low-temperature measurements indicate a value around μ 0 = 1 cm 2 V -1 s -1 for both the conduction and valence bands over the entire range of materials. High temperature-measurements for electrons in a-Si:H suggest a larger value of 7 cm 2 V -1 s -1 . These properties and those of the frequency v are discussed as possible attributes of a mobility edge.