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High efficiency, broadband solar cell architectures based on arrays of volumetrically distributed narrowband photovoltaic fibers

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Abstract

We propose a novel solar cell architecture consisting of multiple fiber-based photovoltaic (PV) cells. Each PV fiber element is designed to maximize the power conversion efficiency within a narrow band of the incident solar spectrum, while reflecting other spectral components through the use of optical microcavity effects and distributed Bragg reflector (DBR) coatings. Combining PV fibers with complementary absorption and reflection characteristics into volume-filling arrays enables spectrally tuned modules having an effective dispersion element intrinsic to the architecture, resulting in high external quantum efficiency over the incident spectrum. While this new reflective tandem architecture is not limited to one particular material system, here we apply the concept to organic PV (OPV) cells that use a metal-organic-metal-dielectric layer structure, and calculate the expected performance of such arrays. Using realistic material properties for organic absorbers, transport layers, metallic electrodes, and DBR coatings, 17% power conversion efficiency can be reached.

©2010 Optical Society of America

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Figures (7)

Fig. 1
Fig. 1 Tandem solar cell designs including (a) a traditional transmissive solar cell design, (b) a reflective tandem solar cell in a V-shape configuration, and (c) an example of a reflective fiber based tandem cell design consisting of three rows of three spectrally-tuned photovoltaic sub-cells. The fiber OPV cells consist of a distributed Bragg reflector (DBR), a thick spacer layer, a transparent top electrode, the active organic layers, and finally an optically thick center electrode. Note that the fibers are not drawn to scale and are expected to be no less than 50 μm in diameter.
Fig. 2
Fig. 2 Device structures modeled for a single solar cell design: (a) planar metal-organic-metal solar cell, (b) fiber OPV cell geometry, (c) row of fibers, and (d) matrix of fiber cells. (e) Qualitative view of the energy band structure for the solar cell in all configurations.
Fig. 3
Fig. 3 Output of the ray-tracing program that is used to analyze periodic multi-fiber OPV systems. Sample rays are traced to visually inspect the performance of the bundled fiber OPV system. Rays that leave the system are shown in green, and rays from the emitter and those that are incident on at least two bodies are shown in blue.
Fig. 4
Fig. 4 (a-d) 2-dimensional coordinates for the best-performing fiber-OPV bundles for 1, 2, 3, and 10-row systems. These geometries are determined through a non-exhaustive search and further optimization is likely possible. The coordinates are given in units of fiber diameters.
Fig. 5
Fig. 5 (a) Predicted short circuit current for the fiber bundles ranging from a single fiber, to a fiber system consisting of 20 rows. Coordinates for the 1, 2, 3 and 10 row systems are given in Fig. 4. The number of sub-cells is varied from 1 to 4 designs with details of these designs provided in Table 1. Results for a similar OPV cell based on a planar heterojunction structure with CuPc and C60 as the donor-acceptor materials are shown for comparison.
Fig. 6
Fig. 6 Performance parameters for the 10-row, 4 color-tuned OPV fiber bundle. (a) External quantum efficiencies (EQEs) of the planar counterparts of the 4 microcavity tuned fiber OPV cells under normal illuminations. The reflectivity of one of the cells is also given to illustrate the high reflectivity for off-resonant wavelengths. (b) Total EQE along with the contributions of the separate color-tuned fibers in the 10-row, 4 color-tuned bundle. Predicted open circuit voltage is also given for each sub-cell.
Fig. 7
Fig. 7 The angular dependence of a planar metal-organic-metal OPV device and a 2-row fiber bundle having the same cell design. Also plotted is the performance of the 10-row, 3 color-tuned fiber bundle with the layout given in Fig. 4. The variation in the incident angle for the bundle is illustrated in Fig. 1c. The 10-row fiber bundle is asymmetric and the performance is therefore given for 3-angle variations. The relative responsivity is a measure of performance assuming the intensity on the top surface of the solar cell is constant with angle.

Tables (1)

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Table 1 Optical absorption band, expected VOC, and device structure of the fiber sub-cells used in the multi-fiber tandem OPV cells modeled in Fig. 4 a

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