Equivalent Circuit
A particularly attractive feature of such geometries is that it is possible to fabricate an efficient cell using ultra-thin organic thin films (lower than 50 nm), and thereby lead to high filling factors due to decreased recombination (Reprinted with perm
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Equivalent Circuit
2.1
Equivalent Circuit Model
A particularly attractive feature of such geometries is that it is possible to fabricate an efficient cell using ultra-thin organic thin films (lower than 50 nm), and thereby lead to high filling factors due to decreased recombination (Reprinted with permission from [1], Copyright (2011) by The American Physical Society), [2–4] Further, due to the well-defined mode structure of many of these geometries, there is the possibility of effectively utilizing frequency conversion schemes. Because of the long optical path in the fiber, a frequency convertor can play an effective role in doubling high energy photons to improve current. However, there are still several problems that must be overcome. We have reported that the open-circuit voltage (Voc) tends to decrease in Optical Confinement Geometry Organic Photovoltaics (OCGOPV) geometries [5, 6]. In that earlier work [5], we defined two “active” areas of the general three dimensional geometry as Fig. 2.1a shows: the “Current Active Area” (CAA) is the area from which current is collected and the “Illumination Active Area” (IAA) is the area of illumination of the structure [5]. These are a distinctive feature of any OCGOPV. Essentially for the planar cell, IAA equals CAA, but in the OCGOPV the CAA is much greater than the IAA. In other words, the flux entering the aperture (IAA) is spread over a much larger area within the cell (the CAA) leading to a lower optical intensity on CAA like an inverse concentrator. Because light is generally partitioned into modes of the “confining cavity,” the optical intensity in an OCGOPV is typically heterogeneously distributed across the CAA (HeOI), differing from the homogeneous optical intensity (HoOI) in planar OPV. The heterogeneous distribution in OCGOPVs can be simulated by the ray tracing model we reported [7] as shown in Fig. 2.1b. If this power heterogeneity becomes too great, a loss in Voc will occur for the device. In this section, we examine the effects of optical heterogeneity on a model OCGOPV using a composite equivalent circuit analysis. To understand the performance of an OCGOPV, it is necessary to know the connections between their electrical and optical characteristics. First, as shown in Y. Li, Three Dimensional Solar Cells Based on Optical Confinement Geometries, Springer Theses, DOI 10.1007/978-1-4614-5699-5_2, # Springer Science+Business Media New York 2013
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2 Equivalent Circuit
Fig. 2.1 (a) IAA and CAA are represented as pink area at the top and blue area round the fiber. (b) Heterogeneous absorption distribution through inner surface in OCGOPVs is simulated by a ray tracing model [7]. The legend at the right represents the absorption level at inner surface
Fig. 2.2a, we take a small piece from a whole OCGOPV (a.1) as one subunit (a.2) and treat it like a planar solar cell (a.3). For each subunit in (a.2), when very small, we may assume it fits planar cell theory. In Fig. 2.2b, the equivalent circuit of planar cell is described by J-V characteristics expr
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