Employing Cascaded Energy Structures in Next-Generation Photovoltaic Devices
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Next generation solar cells have the potential to be cost effective without compromising efficiency. Stacks of semiconducting thin films comprised of organic polymers or small molecules have superior absorbance and fabrication capabilities to crystalline silicon solar cells. Recently, researchers have achieved increased efficiency by employing multiple cascading energy levels, forming a series of donor/acceptor heterojunctions where exciton dissociation may occur. However, the origin of the open-circuit voltage (Voc) of these cascaded structures is ambiguous since conventional theories are based on single heterojunction devices. Voc is believed to be limited by the energy difference between the highest occupied molecular orbital (HOMO) of the donor and the lowest unoccupied molecular orbital (LUMO) of the acceptor; a direct result of the energy losses at the donor/acceptor heterojunction. To investigate Voc in cascaded devices, we employ wide band gap, hole transporting triarylamine-based derivatives such as 4,4,4-Tris[phenyl(m-tolyl)amino]triphenylamine (m-MTDATA), phthalocyanines such as copper phthalocyanine (CuPc), chloroaluminum phthalocyanine (ClAlPc), and boron subphthalocyanine chloride (SubPc) for absorption, and buckminsterfullerene (C60) for efficient electron collection. The final device architecture is ITO/m-MTDATA/x/SubPc/C60/BCP/Ag, where x refers to either CuPc or ClAlPc as a base material. We found that CuPc-based cascaded structures do not impact Voc, yet Voc increases from 0.601V to 0.940V for ClAlPc-based cascades, achieving Voc that is 0.34V greater than the limit set by its highest donor-HOMO and lowest acceptor-LUMO levels. Analysis of current-voltage characterization reveals Voc as great as 1.6V is obtainable. Our results demonstrate that the Voc of cascaded energy structures is limited by the individual donor/acceptor heterojunctions, rather than the cumulative energy losses occurring across the cascaded energy levels.