Ultrafast charge-transfer in organic photovoltaic interfaces: geometrical and functionalization effects

Abstract

Understanding the microscopic mechanisms of electronic excitation in organic photovoltaic cells is a challenging problem in the design of efficient devices capable of performing sunlight harvesting. Here we develop and apply an ab initio approach based on time-dependent density functional theory and Ehrenfest dynamics to investigate photoinduced charge transfer in small organic molecules. Our calculations include mixed quantum-classical dynamics with ions moving classically and electrons quantum mechanically, where no experimental external parameter other than the material geometry is required. We show that the behavior of photocarriers in zinc phthalocyanine (ZnPc) and C60 systems, an effective prototype system for organic solar cells, is sensitive to the atomic orientation of the donor and the acceptor units as well as the functionalization of covalent molecules at the interface. In particular, configurations with the ZnPc molecules facing on C60 facilitate charge transfer between substrate and molecules that occurs within 200 fs. In contrast, configurations where ZnPc is tilted above C60 present extremely low carrier injection efficiency even at longer times as an effect of the larger interfacial potential level offset and higher energetic barrier between the donor and acceptor molecules. An enhancement of charge injection into C60 at shorter times is observed as binding groups connect ZnPc and C60 in a dyad system. Our results demonstrate a promising way of designing and controlling photoinduced charge transfer on the atomic level in organic devices that would lead to efficient carrier separation and maximize device performance.

Rotational superstructure in van der Waals heterostructure of self-assembled C60 monolayer on the WSe2 surface

“…This is in accordance with the good acceptor characteristics of C 60 due to its high electron affinity, which is advantageous in organic solar cells. 57,58 This also agrees with the spectroscopic observation of C 60 causing p-doping in WSe 2 . 29 We now address the different relative orientations between the molecules in the 2×2 superlattice measured above (Fig.…”

Ultrafast Interlayer Charge Separation, Enhanced Visible‐Light Absorption, and Tunable Overpotential in Twisted Graphitic Carbon Nitride Bilayers for Water Splitting

“…As more clearly shown in Figure 4b and Figure S4b, Supporting Information, in both structures, the photogenerated electron transfers to another layer very fast (≈120 fs), which are at the ultrafast time scale (50 fs to 1 ps) [34] and in the time range as experiments previously reported. [35][36][37] This ultrafast interlayer charge transfer suggests that the photogenerated electron-hole pairs will be separated into different layers very quickly. Recently, Prezhdo and co-workers discussed the charge carrier dynamics in monolayer g-C 3 N 4 using NAMD.…”

A Multidimensional View of Charge Transfer Excitons at Organic Donor–Acceptor Interfaces

“…10,29 This time constant also agrees with a recent simulation on the ZnPc−C 60 interface, in which a transfer time of ∼0.2 ps is found. 52 The slower decay time constant (2.2 ps), which corresponds to the diminishing of the CT h shoulder, is assigned to the depopulation of the CT h state via the relaxation to the lower-lying CT 1 and CT 2 states. The localization process (CT h → CT 1 /CT 2 ) is rather slow, and the observed time constant is about 2 times the hot CT relaxation time observed in a previous study on CuPc−C 60 in which a ML-thick C 60 layer was used.…”