Crystal-structure of active layers of small molecule organic photovoltaics before and after solvent vapor annealing

Abstract

It is demonstrated by a detailed structural analysis that the crystallinity and the efficiency of small molecule based organic photovoltaics can be tuned by solvent vapor annealing (SVA). Blends made of the small molecule donor 2,2′-[(3,3′″,3″″,4′-tetraoctyl[2,2′:5′,2″:5″,2′″:5′″,2″″-quinquethiophene]-5,5″″-diyl)bis[(Z)-methylidyne(3-ethyl-4-oxo-5,2-thiazolidinediylidene)]]bis-propanedinitrile (DRCN5T) and the acceptor [6,6]-phenyl C71 butyric acid methyl ester (PC71BM) were annealed using solvent vapors with either a high solubility for the donor (tetrahydrofuran), the acceptor (carbon disulfide) or both (chloroform). The samples were analyzed by grazing-incidence wide-angle X-ray scattering (GIWAXS), electron diffraction, X-ray pole figures, and time-of-flight secondary ion mass spectrometry (ToF-SIMS). A phase separation of DRCN5T and PC71BM is induced by SVA leading to a crystallization of DRCN5T and the formation of a DRCN5T enriched layer. The DRCN5T crystallites possess the two dimensional oblique crystal system with the lattice parameters a = 19.2 Å, c = 27.1 Å, and β = 111.1° for the chloroform case. No major differences in the crystal structure for the other solvent vapors were observed. However, the solvent choice strongly influences the size of the DRCN5T enriched layer. Missing periodicity in the [010]-direction leads to the extinction of all Bragg reflections with k ≠ 0. The annealed samples are randomly orientated with respect to the normal of the substrate (fiber texture).

Seeing structural evolution of organic molecular nano-crystallites using 4D scanning confocal electron diffraction (4D-SCED)

“…To demonstrate this, the inset in Fig. 3 b shows an indexed SCED pattern extracted from a face-on domain, which agrees well to that obtained from GIWAXS studies 42 . Since the charge carrier transport properties of molecular crystals are highly anisotropic, the orientation of the molecular crystals in the domains and the micrometer scale percolation of the domains are critical to pinpoint the device performance.…”

“…However, these analytical methods do not provide structural/crystalline information and require electron dosage orders-of-magnitude higher than the critical dose of structural damage 7 . In our recent structural analysis of this material system using grazing incidence wide-angle X-ray scattering (GIWAXS), the unit cell structure of the small molecule crystal DRCN5T was determined, and a coexistence of (molecular) face-on and edge-on crystalline domains was deduced 42 . The molecular orientation, particularly the π -stacking orientation, within the nanoscale percolating network is the key to understand the transport properties.…”

Triplet‐Charge Annihilation in a Small Molecule Donor: Acceptor Blend as a Major Loss Mechanism in Organic Photovoltaics

“…Upon thermal annealing of the DRCN5T:PC70BM blend, the structure evolves to a similar extent as the pristine unannealed DRCN5T film. The scattering data presented here for the pristine DRCN5T and DRCN5T:PC70BM blends are consistent with the recent in-depth morphological investigations and resultant structural model presented by Berlinghof et al [48] on DRCN5T:PC70BM blend films.…”

Understanding and Controlling the Evolution of Nanomorphology and Crystallinity of Organic Bulk‐Heterojunction Blends with Solvent Vapor Annealing

“…Furthermore, the crystal orientations in the active layer can be also decisive to the final PCE of the devices since the transport properties of molecular crystals (π‐conjugated systems) are highly anisotropic. This was already demonstrated in previous work by Berlinghof et al [ 38 ] which derives the crystal structure for the DRCN5T electron donor by GIWAXS methods. Apart from the crystal structure also the face‐on and edge‐on orientation of the DRCN5T crystallites could be identified as the two preferred orientations of molecular ordering in the active layer (see Figure 6h).…”