The Essence and Efficiency Limits of Bulk-Heterostructure Organic Solar Cells
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The Essence and Efficiency Limits of Bulk-Heterostructure Organic Solar Cells
M. Alam*, B. Ray, M. Khan, and S. Dongaonkar School of Electrical and Computer Engineering, Purdue University, West Lafayette, IN 47906
Abstract: Since its introduction in early 1990s, bulk-heterojunction organic photovoltaic solar cell (BHJ-OPV) has promised high-efficiency at ultra-low cost and weight, with potential for nontraditional applications such as building-integrated PV. There is a widespread presumption, however, that the complexity of morphology makes carrier transport in OPV irreducibly complicated, and possibly, beyond predictive modeling. In this paper, we use elementary and intuitive arguments to derive the fundamental thermodynamic as well as morphology-specific practical limits of BHJ-OPV efficiency. We find that constraints of the percolation threshold and trade-off among short-circuit current, open circuit voltage, and fill factor make substantial improvement in OPV efficiency difficult. We posit that future improvement in OPV will rely not on morphology engineering, or reducing the polymer bandgap, but on increasing both the effective product and the cross-gap between donor/acceptors. Even if the OPV fails to achieve the highest efficiency anticipated by the thermodynamic limit, its novel form factor, lightweight, and transparency can make it a commercially viable option for many applications.
Keywords: Modeling/Simulation, Organic Solar Cell, Plastic Solar Cell, Spinodal Decompositon, PV Reliability, Percolation, Computational Physics
Introduction and Background A bulk heterojunction organic solar cell (BHJ-OPV) 1,2,3 consists of two demixed, spinodally phase-segregated, bicontinuous, donor-acceptor polymers capped by a transparent anode and a metallic cathode, see Fig. 1. Although a wide variety of polymer-pairs have been analyzed, we will consider p-type P3HT and n-type PCBM (red and blue regions in Fig. 1a) as an illustrative example1,2,3. The operation of BHJ-OPV is often explained as follows: excitons generated by sunlight in such polymer composite are localized in single conjugated unit of few nm and held together by undiluted Coulomb attraction between the charges, typical of low dielectric-constant materials. Therefore, if the excitons are not dissociated into free electrons and holes by atomically-sharp quasi-electric field of donor/acceptor interface (see Fig. 1b-d), they would be lost to self-recombination. If the phase-segregated morphology is viewed as being entangled regions of donor-rich and acceptor-rich polymers, as in Fig. 1a, one finds that the effective cross-sectional ‘diameter’ of each phase evolves continuously with anneal time, i.e., . During the initial transient of phase separation, the donor/acceptor (D/A) interface is
diffuse, and the quasi-field may not be strong enough for exciton dissociation4. However, once becomes comparable to the exciton diffusion length, , and the D/A interface becomes sharply defined, the excitons can reach and be dissociated by the heterojunction with high
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