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(FBTTh2)2:PC71BM 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(FBTTh2)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.
Highlights
Organic photovoltaics (OPVs) are promising light conversion technologies expecting to meet the demands of low manufacturing costs, green technology, and efficient operation under low-light conditions[1,2]
The results obtained by single crystal x-ray diffraction (SCXRD) and grazing incidence wide angle x-ray scattering (GIWAXS) indicated that p-DTS(FBTTh2)[2] molecules grew into two-dimensional (2D) columnar arrays with increased π –π overlapping, which leads to improved intermolecular charge transport, and enhanced value of PCE13
We report that the thermodynamically stable p-DTS(FBTTh2)2:PC71BM bulk heterojunction (BHJ) obtained by thermal annealing are different in lateral and depth phase morphology and crystallinity from BHJs obtained with a DIO additive
Summary
Organic photovoltaics (OPVs) are promising light conversion technologies expecting to meet the demands of low manufacturing costs, green technology, and efficient operation under low-light conditions[1,2]. Low band gap conjugated polymer/fullerene[21,22] and small molecule/fullerene[23,24,25,26,27,28,29,30,31] blends have shown more promising results The latter system is known to be easier to purify, and the devices show consistently better batch-to-batch reproducibility, higher crystallinity and better performance[28,32,33]. We use neutron reflectometry[43] to unfold the depth phase morphology of p-DTS(FBTTh2)[2] and PC71BM blend and complemented with the use of absorption and photoluminescence spectroscopy, atomic force microscopy (AFM) and X-ray diffraction (XRD) characterizations to correlate the morphology to the OPV device performance after thermal annealing and DIO additive processing. Our combined experimental and theoretical approach allows us to demonstrate correlations between the morphology and device performance, as well as to explain origin of different morphologies
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