(557b) Optimizing the Fabrication of All-Organic Solar Cells: A Kinetic Monte Carlo Study of Fullerene Adsorption in Phthalocyanine-Based Covalent Organic Frameworks | AIChE

(557b) Optimizing the Fabrication of All-Organic Solar Cells: A Kinetic Monte Carlo Study of Fullerene Adsorption in Phthalocyanine-Based Covalent Organic Frameworks

Authors 

Koo, B. - Presenter, Cornell University
Clancy, P., Cornell University



Optimizing the Fabrication of All-Organic Solar
Cells: A Kinetic Monte Carlo Study of Fullerene Adsorption in Phthalocyanine-based
Covalent Organic Frameworks

Brian Koo, Paulette
Clancy

Chemical and Biomolecular Engineering, Cornell
University

The
class of materials known as covalent organic frameworks (COFs)1
has gained traction recently in the field of photovoltaics. These
two-dimensional polymers, originally studied extensively for small gas storage,
have high surface area to volume ratios that rival the capacities of metal-organic
frameworks and zeolites. Since the incorporation of chromophores
into the framework, specifically phthalocyanine groups,2
optoelectronic applications of COFs became viable. The advancement of
synthesizing 2D COFs as thin films on graphene-covered substrates3 enabled
the processing of COFs as a hole-conductor for organic photovoltaic cells. Indeed,
recently, a solar cell was made from a combination of PCBM (electron carrier)
and a thienothiophene-based COF (hole carrier) but
displayed a disappointingly low solar efficiency.4  Our study explores how to improve this
efficiency by studying the filling of thin film phthalocyanine COFs with
fullerene molecules (C60) to create prototypical ordered heterojunction solar
cells. COFs are a desirable material since they allow, in principle, the
ability to transport free charge carriers perpendicular to its stacked layers,
and its inherent ability to direct the structural characteristics of
semiconducting guest molecules within its pores, such as fullerene.

Many
complementary computational studies have probed the nature of stacking of 2D
COF layers, given the importance of angstrom-sized offsets on the hole/electron
mobility. Adjacent layers adopt a 1.0-1.7 angstrom offset5 in weakly
interacting layers, leading to a stack of 2D layers. This, in turn, induces a
large potential energy barrier on the pore surface, which limits molecular
dynamics (MD) studies of the adsorption of guest molecules within the pores,
since transport within micropores occurs purely by
surface diffusion. In a previous study, we predicted that layers are offset
with no preferred stacking direction; therefore layers are equally likely to
adopt stacking patterns that only have local 3-layer persistence, e.g., helical,
zigzag, and staircase stacking, each with locally defined lattice sites. Here,
we calculate the rates of diffusion between these lattice sites from the free
energy barriers extracted from steered molecular dynamics simulations, and find
the location and stability of lattice sites using a minimization procedure. From
these data, we use Kinetic Monte Carlo to access far longer timescales of
adsorption and diffusion than are possible with MD to find the equilibrated
packing structure of fullerene as prototypical n-type guest molecules. We also
explore the role of solvent and the effect of pore size on the lattice mismatch
between COF and C60. Finally, we predict a theoretical maximum
packing density and compare it to experiment4 to estimate the amount
of improvement possible with this arrangement of solar cell component
materials.

1.      
Côté, A. P. et
al.
Porous, crystalline, covalent organic frameworks. Science 310,
1166?70 (2005).

2.      
Spitler, E. L. &
Dichtel, W. R. Lewis acid-catalysed formation of
two-dimensional phthalocyanine covalent organic frameworks. Nat. Chem. 2,
672?677 (2010).

3.      
Colson,
J. W. et al. Oriented 2D Covalent Organic Framework Thin Films on
Single-Layer Graphene. Science 332, 228?231 (2011).

4.      
Dogru, M. et al. A Photoconductive Thienothiophene-Based Covalent Organic Framework Showing
Charge Transfer Towards Included Fullerene. Angew. Chem. 125, 2992?2996
(2013).

5.      
Koo,
B. T., Dichtel, W. R. & Clancy, P. A classification scheme for the stacking
of two-dimensional boronate ester-linked covalent
organic frameworks. J. Mater. Chem. 22, 17460-17469 (2012).