Abstract
A crucial issue facing polymer-based solar cells is how to manage the energetics of the polymer/fullerene blends to maximize short-circuit current density and open-circuit voltage at the same time and thus the power conversion efficiency. Here we demonstrate that the use of a naphthobisoxadiazole-based polymer with a narrow bandgap of 1.52 eV leads to high open-circuit voltages of approximately 1 V and high-power conversion efficiencies of ∼9% in solar cells, resulting in photon energy loss as small as ∼0.5 eV, which is much smaller than that of typical polymer systems (0.7–1.0 eV). This is ascribed to the high external quantum efficiency for the systems with a very small energy offset for charge separation. These unconventional features of the present polymer system will inspire the field of polymer-based solar cells towards further improvement of power conversion efficiencies with both high short-circuit current density and open-circuit voltage.
Highlights
A crucial issue facing polymer-based solar cells is how to manage the energetics of the polymer/fullerene blends to maximize short-circuit current density and open-circuit voltage at the same time and the power conversion efficiency
A key to resolving the issue in Polymer-based bulk-heterojunction solar cells (PSCs) is to reduce the relatively large photon energy loss (Eloss), which is defined by Eg À eVOC, where Eg is the optical bandgap of the semiconducting polymer evaluated from the absorption edge[17,18,19,20,21]
Some particular polymers were reported to have small Eloss in PSCs when used in combination with [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) or [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM)
Summary
A crucial issue facing polymer-based solar cells is how to manage the energetics of the polymer/fullerene blends to maximize short-circuit current density and open-circuit voltage at the same time and the power conversion efficiency. These EQE values are quite high for the polymer/PCBM systems with such a small energy offset, that is, DEL or Eg À ECT.
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