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Understanding device-structure-induced variations in open-circuit voltage for organic photovoltaics
Zhiping Wang, Yu Uemura1, Ying Zhou
1‡Faculty of Engineering, Department of Nanoscience, Sojo University, 4-22-1 Ikeda, Nishi-ku, Kumamoto 860-0082, Japan.
ACS Applied Materials & Interfaces
|May 7, 2015
Summary
Device performance in organic photovoltaic cells is linked to molecular structure. Optimizing squaraine film thickness and molecular orientation enhances open-circuit voltage (V(OC)) by influencing charge recombination and interfacial dipoles.
Area of Science:
- Materials Science
- Organic Electronics
- Photovoltaics
Background:
- Organic photovoltaic (OPV) devices are crucial for renewable energy.
- Controlling device performance, particularly open-circuit voltage (V(OC)), is key for OPV efficiency.
- Squaraine (SQ)/fullerene (C60) systems are promising for OPV applications.
Purpose of the Study:
- To investigate how structural factors, specifically squaraine (SQ) film thickness and molecular orientation, influence the device performance of SQ/fullerene (C60) bilayer cells.
- To elucidate the mechanisms behind the observed variations in open-circuit voltage (V(OC)).
- To explore strategies for enhancing V(OC) through structural control.
Main Methods:
- Fabrication and characterization of SQ/C60 bilayer cells with varying SQ thicknesses.
- Measurement of open-circuit voltage (V(OC)) and ionization potential (IP).
- Polarized infrared-multiple-angle incidence resolution spectroscopy (PIR-MAIRS) to determine SQ molecular orientation.
- Density functional theory (DFT) calculations to understand molecular properties and interfacial effects.
Main Results:
- Open-circuit voltage (V(OC)) varied by up to 40% (0.62–0.86 V) with changes in SQ film thickness.
- Ionization potential (IP), anode recombination, and donor/acceptor (D/A) interface properties were sensitive to film thickness.
- SQ molecular orientation evolved from lying-down (noncentrosymmetric) at low thicknesses to random at high thicknesses.
- Anode recombination was suppressed by using a buffer layer, increasing V(OC).
- Oriented SQ films formed aligned dipole moments at the D/A interface, enhancing V(OC), unlike random structures.
Conclusions:
- Molecular orientation in SQ films critically impacts charge-carrier recombination and interfacial dipole alignment in OPVs.
- These structural factors significantly influence open-circuit voltage (V(OC)).
- Tailoring molecular orientation and controlling interfacial interactions are essential for designing efficient organic photovoltaic molecules and optimizing device performance.
