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Schematic of a nanowire solar cell with a PHS. Organic molecules are chemically attached to the surface of the nanowires. The organic shell absorbs light and transfers the energy to the SiNWs via resonant energy transfer. The SiNWs have radial junctions which selectively separate and transport the charges to the respective electrodes (shown in the isolated nanowire to the right).
(a) Dye A absorption (blue) and PL (blue dashed) with dye B absorption (red) and PL (red dashed) for two hypothetical dyes. Dyes with molar extinction coefficients of greater than that absorb between 700 and 1000 nm are available commercially, although modifications for proper attachment to are required. (b) ETE as a function of dye PL efficiency for Dyes A (blue) and B (red). The solid and dashed lines are for values of 2.7 nm and 1.7 nm, respectively. From the much weaker variation of the dashed curves compared to the solid curves, it is clear that it is important to carefully control the dye–silicon separation to achieve a high ETE for arbitrary dyes.
Fraction of photons above the bandgap of silicon that are absorbed with bare SiNWs (gray) and with dyes A and B attached to the nanowires (red) as a function of nanowire length. The nanowires are 50 nm in diameter with a center-to-center distance of 70 nm. Dyes A and B are assumed to have equal surface coverage with a molar absorptivity of and surface concentration on the wires of .
(a) Diagram of sample used to determine the Förster radius experimentally. The MEHPPV layer was kept constant at 5 nm while the oxide thickness was varied from . (b) Experimental PL data (black) and best fit (red) for MEHPPV on . Fitting yields and are in good agreement with predicted values.
Device architecture used for measuring the external quantum efficiency spectrum of bare silicon and silicon with a 5 nm layer of MEHPPV. 80-nm-thick palladium (Pd) contacts were deposited on 50 nm of silicon, which was insulated from the bulk wafer by a 150 nm buried oxide layer. The spacing between contacts is .
(a) Photoresponse of device under 1 V bias with (solid red) and without (solid blue) MEHPPV. The device with MEHPPV provides an increase in photocurrent. Shown also are the calculated percent absorptions for the device with (red dashed) and without (blue dashed) a 5 nm layer of MEHPPV. The absorption was modeled using the transfer-matrix method and tabulated optical constant data for the materials. The absorption profile due to interference effects explains the shape of the measured photoresponse spectrum. (b) The absorption spectrum of a 5-nm-thick layer of MEHPPV on quartz.
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