Donor-to-Donor vs Donor-to-Acceptor Interfacial Charge Transfer States in the Phthalocyanine–Fullerene Organic Photovoltaic System

Abstract

Charge transfer (CT) states formed at the donor/acceptor heterointerface are key for photocurrent generation in organic photovoltaics (OPV). Our calculations show that interfacial donor-to-donor CT states in the phthalocyanine-fullerene OPV system may be more stable than donor-to-acceptor CT states and that they may rapidly recombine, thereby constituting a potentially critical and thus far overlooked loss mechanism. Our results provide new insight into processes that may compete with charge separation, and suggest that the efficiency for charge separation may be improved by destabilizing donor-to-donor CT states or decoupling them from other states.

Charge-Transfer States in Organic Solar Cells: Understanding the Impact of Polarization, Delocalization, and Disorder

“…have been recently performed 14,16,18,25,34 to investigate the effect of system size on the CT states. However, the interplay between electron delocalization and polarization effects on the nature of CT states remains to be addressed in detail.…”

Theoretical investigations on enhancing the performance of terminally diketopyrrolopyrrole-based small-molecular donors in organic solar cell applications

“…After electronic excitation in the small-molecular donor upon absorption of sunlight, the generated exciton will migrate to the donor-acceptor (D-A) interface and dissociate to free charge in the charge transfer (CT) state [45,49,50]. To examine the CT absorptions of the D-A blends, the transition energies and oscillator strengths in the singlet excited-state were obtained from TD-PBE0/6-311G (d,p) calculations as shown in Table 2.…”

“…Here, expectation of a large charge-dissociation rate could ensure high electrontransfer efficiency from small-molecular donor to PCBM. It is generally accepted that electron-transfer rates between donor and acceptor are described by Marcus' theory [45,[49][50][51]. Three parameters-electronic coupling (V DA ), reorganization energy (λ) and free enthalpy (ΔG)-control the electrontransfer rate of the charge transfer reaction in the Marcus equation.…”

Equilibrium Fermi’s Golden Rule Charge Transfer Rate Constants in the Condensed Phase: The Linearized Semiclassical Method vs Classical Marcus Theory

“…They can occur spontaneously, as in redox reactions, 6,7,11,18,19 or can be induced by electron injection, as in electrochemical devices, or by photoexcitation, as in photochemical reactions and photovoltaic devices. 14,[24][25][26][27][28][29][30][31][32][33] The rates of CT processes can vary on an extremely wide dynamical range and can be very sensitive to the underlying molecular structure, molecular environment, temperature, etc. Molecular level untem is assumed to start out with the nuclear degrees of freedom (DOF) at thermal equilibrium on the diabatic donor state potential energy surface (PES) and when the electronic coupling between the donor and acceptor electronic states can be assumed weak.…”

“…Another example corresponds to the inverted region, where the enhanced overlap between nuclear wavefunctions can make nuclear tunneling, as opposed to the classical activated process underlying classical Marcus theory, into the dominant CT pathway. 9,[31][32][33]40 This scenario is particularly relevant to organic photovoltaic devices, where the rigid solid state environment and low dielectric constant of the organic host conspire to give rise to relatively small reorganization energies. [31][32][33] A third example corresponds to cases where the electronic coupling autocorrelation function decays on sufficiently slow time scales so as to make it sensitive to the underlying nuclear dynamics.…”