Bilayer–Ternary Polymer Solar Cells Fabricated Using Spontaneous Spreading on Water

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Abstract
A new method is presented to fabricate bilayer organic solar cells via sequential deposition of bulk‐heterojunction layers obtained using spontaneous spreading of polymer–fullerene blends on a water surface. Using two layers of a small bandgap diketopyrrolopyrrole polymer–fullerene blend, a small improvement in power conversion efficiency (PCE) from 4.9% to 5.1% is obtained compared to spin‐coated devices of similar thickness. Next, bilayer–ternary cells are fabricated by first spin coating a wide bandgap thiophene polymer–fullerene blend, followed by depositing a small bandgap diketopyrrolopyrrole polymer–fullerene layer by transfer from a water surface. These novel bilayer–ternary devices feature a PCE of 5.9%, higher than that of the individual layers. Remarkable, external quantum efficiencies (EQEs) over 100% are measured for the wide bandgap layer under near‐infrared bias light illumination. Drift‐diffusion calculations confirm that near‐infrared bias illumination can result in a significant increase in EQE as a result of a change in the internal electric field in the device, but cannot yet account for the magnitude of the effect. The experimental results indicate that the high EQEs over 100% under bias illumination are related to a barrier for electron transport over the interface between the two blends.
Funding Information
  • Nederlandse Organisatie voor Wetenschappelijk Onderzoek
  • Seventh Framework Programme (FP/2007-2013)
  • European Research Council (339031)
  • Ministerie van Onderwijs, Cultuur en Wetenschap (024.001.035)
  • National Natural Science Foundation of China (21574138, 91633301)
  • Stichting voor de Technische Wetenschappen (VIDI 13476)

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