Deposition Growth and Morphologies of C₆₀ on DTDCTB Surfaces: An Atomistic Insight into the Integrated Impact of Surface Stability, Landscape, and Molecular Orientation

Abstract

residual Coulomb attraction with holes, enabling ultrafast charge separation. [ 21 ] Therefore, it is very important to understand and ultimately control the active layer morphologies and the D/A interface confi gurations at the molecular level. [8][9][10][11][12][13][14][15][17][18][19][20][21][22][23][24] Although modulation of the active layer morphologies has been successfully achieved by several different preparation procedures, [ 1,12,13,[25][26][27][28] it still remains experimentally diffi cult to accurately probe the molecular self-assembly processes and packing structures, [ 29 ] which severely hinders obtaining reliable structure-property relationships and further improving the device performance.Over the past a few years, a series of small-molecule donors with a simple D-A-A structure have been synthesized by Wong and co-workers for high-effi ciency vacuum-deposition OSCs; [30][31][32][33] in conjunction with the C 60 or C 70 acceptor, the highest power conversion effi ciency (PCE) has reached 6.8% for a single junction cell, [ 31 ] 10.0% and 11.1% for a tandem and a triple junction cell, [ 34 ] respectively. Among these donor materials, 2-{[7-(5-N , N -ditolylaminothiophen-2-yl)-2,1,3-benzothiadiazol-4-yl]methylene}malononitrile (DTDCTB, see Figure 1 a) has gained intensive attention. [ 30,31,[34][35][36][37][38][39][40][41][42] Here, interesting to us is the antiparallel packing mode in the DTDCTB crystal (see Figure 1 b), which can provide a variety of model surfaces with different features to systematically probe the infl uence of donor surfaces on the fullerene packing and interface morphologies.In this contribution, we have mimicked vapor-phase deposition and growth of C 60 on different DTDCTB substrate surfaces via nonequilibrium atomistic molecular dynamics (MD) simulations. [ 43,44 ] Using this scheme, Muccioli and co-workers have investigated the crystal growth of pentacene or sexithiophene on the C 60 (001) substrate, while the relationship between different surface properties and thin-fi lm morphologies is yet to be revealed. [ 43,44 ] Our simulated results point to that, both the interfacial and thin-fi lm morphologies substantially depend on a combinational role of the stability, ordering, landscape, and local molecular orientation of the substrate surfaces. This work will be helpful to understand and control the formation of crystalline fullerene in the active layer toward achieving effi cient separation of the photogenerated charges for organic photovoltaics.The initial DTDCTB substrate surfaces with different molecular orientations, i.e., nearly "fl at," "upright," and "edge-on" confi gurations (see Figure S1, Supporting Information) were constructed by cleaving the DTDCTB crystal along the (100), (010), and (001) crystallographic planes, respectively. Each type of confi guration has two opposite facets, labeled as A and B (i.e., obtained by cutting the cell at full and half cell, respectively), because the molecules stack in an antiparallel manner Organic solar cells offer considerab...

From Molecular Packing Structures to Electronic Processes: Theoretical Simulations for Organic Solar Cells

“…Therefore, the ED process in the DTDCTB/C 60 complexes is dominated by the hot CT state, which is favorable for facilitating ultrafast CS. To further confirm this point, we also extracted thousands of DTDCTB/C 60 complexes from more realistic MD simulated interfaces (Figure c) to calculate the charge‐transfer couplings . The average couplings associated with the ED process from S 1 to CT 1A are relatively large and similar to that from S 1 to CT 0 , demonstrating the critical role of hot CT states in ultrafast charge generation for the DTDCTB/C 60 OSCs.…”

“…Copyright 2017, Royal Society of Chemistry. (c) Adapted with permission . Copyright 2015, Wiley‐VCH.…”

“…Therefore, nonequilibrium MD simulations should be considered to model these experimental preparation techniques (Figure c,d). For instance, Muccioli et al simulated the crystal growth of pentacene or α‐sexithiophene (6T) on the C 60 (001) surface, and we investigated the influence of different donor surfaces on the deposition growth and packing morphologies of C 60 , in which the active molecules are one‐by‐one consecutively deposited on the surface to mimic the vapor‐phase deposition process . Brückner et al proposed a modified approach to mimic experimental layer deposition techniques to obtain disordered organic phases, in which the old deposited slices are frozen and the newly added molecular slice is taken from crystal structures but with vacancies created in order to enhance the flexibility of the system .…”

“…c) Equilibrated DTDCTB surfaces and corresponding DTDCTB/C 60 bilayer packing morphologies after deposition of C 60 . Adapted with permission . Copyright 2015, Wiley‐VCH.…”

“…In the following section of “Methodology”, we will introduce the state‐of‐the‐art simulation protocols to reproduce the vapor‐phase deposition and solvent evaporation processes . Different from X‐ray‐based techniques, atomic coordinates of the system can be directly extracted from MD simulations and used for QC calculations to obtain site energies of individual molecules and intermolecular electronic couplings, which are the key parameters to evaluate the intermolecular CT rates .…”