Tuning Open-Circuit Voltage in Organic Solar Cells with Molecular Orientation

Published

June 24, 2015

Author(s)

Brent Kitchen, Omar Awartani, Regis J. Kline, Terry McAfee, Harald Ade, Brendan T. O'Connor

Abstract

The role of molecular orientation of a polar conjugated polymer in a polymer-fullerene organic photovoltaic (OPV) cells is investigated. A planar heterojunction (PHJ) OPV cell composed of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) is used as a model system to isolate the effect of the interfacial orientation on the photovoltaic properties. The molecular orientation of the aggregate P3HT relative to the PCBM layer is varied from highly edge-on (conjugated ring plane perpendicular to the interface plane) to appreciably face-on (ring plane parallel to the interface). It is found that as the P3HT stacking becomes more face-on there is a positive correlation to the OPV open circuit voltage (VOC), attributed to a shift in the highest occupied molecular orbital (HOMO) energy level of P3HT that is predicted from Density Functional Theory. In addition, the PHJ OPV cell with a broad P3HT stacking orientation distribution has a VOC comparable to an archetypal bulk heterojunction (BHJ) device. These results suggest that in the BHJ OPV cell, the hole energy level in the charge transfer state is defined in-part by the broad orientation distribution of the P3HT at the interface with PCBM. Finally, the spectral photoresponse of the devices are measured and also shown to have a dependence on P3HT stacking orientation.

Citation

Advanced Energy Materials

Pub Type

Journals

Citation

Kitchen, B. , Awartani, O. , Kline, R. , McAfee, T. , Ade, H. and O'Connor, B. (2015), Tuning Open-Circuit Voltage in Organic Solar Cells with Molecular Orientation, Advanced Energy Materials (Accessed May 4, 2026)

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