The Limits to Organic Photovoltaic Cell Efficiency
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The Limits to Organic Photovoltaic Cell Efficiency Stephen R. Forrest Abstract We consider the fundamental limits to organic solar cell efficiency, and the schemes that have been used to overcome many of these limitations. In particular, the use of double and bulk heterojunctions, as well as tandem cells employing materials with high exciton diffusion lengths, is discussed. We show that in the last few years, a combination of strategies has led to a power conversion efficiency of P 5.7% (under AM 1.5 G simulated solar radiation at 1 sun intensity) for tandem cells based on small-molecularweight materials, suggesting that even higher efficiencies are possible. We conclude by considering the ultimate power conversion efficiency that is expected from organic thinfilm solar cells. Keywords: organic photovoltaics, solar cell efficiency.
The Energy Conversion Process To determine the limits to the power conversion efficiency of organic solar cells, we begin by referring to the optical-to-electrical conversion process detailed in Figure 1. The internal quantum efficiency, IQE, is the product of four efficiencies,1 each corresponding to a step in the charge generation process: IQE AEDCTCC ,
(1)
where A is the absorption efficiency of light within the active region of the solar cell; ED is the exciton diffusion efficiency to a dissociation site; CT is the charge transfer efficiency, which is the efficiency for dissociation of an exciton into a free electron and hole pair at that site; and CC is the charge collection efficiency. Taking into consideration the optical losses that occur on coupling light in the device active region, we arrive at the external quantum efficiency: EQE (1 – R) IQE ,
(2)
where R is the reflectivity of the substrate– air interface. Finally, the power conversion efficiency of the cell is given by P
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VOC JSCFF , Pinc
(3)
where FF is the fill factor, VOC is the opencircuit voltage, JSC is the short-circuit current density, and Pinc is the incident power density. Organic photovoltaic (PV) cells confront several limitations that are apparent by examining Equations 1–3. First, there is an inherent tradeoff between the absorption and the exciton diffusion efficiencies. That is, the exciton diffusion length, LD, is typically much less than the optical absorption length, 1/. For an absorbing organic layer of thickness d, we have A (1 – ed),
(4)
whereas, assuming that the arrival of excitons at a dissociation site is independent of electric fields or other extrinsic conditions, the diffusion efficiency is given by ED ed/L . D
(5)
In general, it has been found that the charge collection and charge transfer efficiencies at organic donor/acceptor (DA) interfaces commonly used in thin-film molecular organic semiconductor PV cells approach 100%, in which case the internal quantum efficiency is determined by the product AED. This is shown in Figure 2 as a function of d/LD for LD 0.1, 0.2, and 1.0,
the former two values being more characteristic of organic semicond
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