Abstract
Using scanning tunneling microscopy (STM) and spectroscopy (STS) at the liquid/solid interface, morphology evolution process and energetic level alignment of very thin solid films (thickness: <700 pm), of the low molecular weight molecule DRCN5T and DRCN5T:[70]PCBM blend are analyzed after applying thermal annealing at different temperatures. These films exhibit a worm-like pattern without thermal annealing (amorphous shape); however, after applying thermal annealing at 120 °C, the small molecule film domains crystallize verified by X-ray diffraction: structural geometry becomes a well-defined organized array. By using STS, the energy band diagrams of the semiconductor bulk heterojunction (blended film) at the donor-acceptor interface are determined; morphology and energy characteristics can be correlated with the organic solar cells (OSC) performance. When combining thermal treatment and solvent vapor annealing processes as described in previous literature by using other techniques, OSC devices based on DRCN5T show a very acceptable power conversion efficiency of 9.0%.
Highlights
In the last two decades, bulk heterojunction (BHJ) organic solar cells (OSCs) have received great attention due to their advantages, including low cost, flexibility, lightweight, and roll-to-roll processing compatibility [1,2]
OSCs based on small molecules possess prominent advantages because the low molecular weight molecules, unlike polymers, have well-defined chemical structures, reduced batch-to-batch variability, fine tunable energy level and absorption, intrinsic monodispersity, etc
Among various approaches for device optimization, morphological control over the active layer plays an extremely important role in order to increase the photovoltaic characteristics of OSCs [6]
Summary
In the last two decades, bulk heterojunction (BHJ) organic solar cells (OSCs) have received great attention due to their advantages, including low cost, flexibility, lightweight, and roll-to-roll processing compatibility [1,2].
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