Abstract
Structural characteristics of the active layers in organic photovoltaic (OPV) devices play a critical role in charge generation, separation and transport. Here we report on morphology and structural control of p-DTS(FBTTh 2) 2:PC 71 BM films by means of thermal annealing and 1,8-diiodooctane (DIO) solvent additive processing, and correlate it to the device performance. By combining surface imaging with nanoscale depth-sensitive neutron reflectometry (NR) and X-ray diffraction, three-dimensional morphologies of the films are reconstituted with information extending length scales from nanometers to microns. DIO promotes the formation of a well-mixed donor-acceptor vertical phase morphology with a large population of small p-DTS(FBTTh 2) 2 nanocrystals arranged in an elongated domain network of the film, thereby enhancing the device performance. In contrast, films without DIO exhibit three-sublayer vertical phase morphology with phase separation in agglomerated domains. Our findings are supported by thermodynamic description based on the Flory-Huggins theory with quantitative evaluation of pairwise interaction parameters that explain the morphological changes resulting from thermal and solvent treatments. Our study reveals that vertical phase morphology of small-molecule based OPVs is significantly different from polymer-based systems. The significant enhancement of morphology and information obtained from theoretical modeling may aid in developing an optimized morphology to enhance device performance for OPVs.
| Original language | English |
|---|---|
| Article number | 13407 |
| Journal | Scientific Reports |
| Volume | 5 |
| DOIs | |
| State | Published - Aug 28 2015 |
Funding
N.H., J.Z. P.J. acknowledge support of ORNL Laboratory Research and Development Program (LDRD) project. This research was conducted at the Center for Nanophase Materials Sciences and the Spallation Neutron Source, which are sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy. N.H. thanks Artur Glavic, Haile Ambaye and Richard Goyette for a partial assistance during the NR measurements. This manuscript has been authored by UT-Battelle, LLC under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy. The Department of Energy will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan).