Investigations on the charge transfer mechanism at donor/acceptor interfaces in the quest for descriptors of organic solar cell performance

Abstract

Herein, we theoretically and experimentally investigated the mechanisms of charge separation processes of organic thin-film solar cells. PTB7, PTB1, and PTBF2 have been chosen as donors and PCBM has been chosen as an acceptor considering that effective charge generation depends on the difference between the material combinations. Experimental results of transient absorption spectroscopy show that the hot process is a key step for determining external quantum efficiency (EQE) in these systems. From the quantum chemistry calculations, it has been found that EQE tends to increase as the transferred charge, charge transfer distance, and variation of dipole moments between the ground and excited states of the donor/acceptor complexes increase; this indicates that these physical quantities are a good descriptor to assess the donor-acceptor charge transfer quality contributing to the solar cell performance. We propose that designing donor/acceptor interfaces with large values of charge transfer distance and variation of dipole moments of the donor/acceptor complexes is a prerequisite for developing high-efficiency polymer/PCBM solar cells.

Influence of Intramolecular Charge-Transfer Characteristics of Excitons on Polaron Generation at the Donor/Acceptor Interface in Polymer Solar Cells

“…The CT state, consisting of Coulombically bound electron and hole pair, is the result of photoinduced CT at the interface of donor and acceptor materials. , Recently, it has been proposed that charge separation is greatly influenced by the energetics of CT states, leading to different charge separation pathways through hot or relaxed (cold) CT states. Several researchers have reported efficient charge separation through the electronically and/or vibrationally excited and delocalized CT state. − The hot CT state refers to a loosely bound and delocalized electron–hole pair, facilitating the charge separation at the D/A interface. Numerous experiments have suggested that the sufficient energy of the hot CT state is beneficial for overcoming the Coulomb barrier and avoiding geminate recombination. ,,− For instance, Grancini et al reported that efficient charge separation occurs before the relaxation within CT states in the PCPDTBT/PC 60 BM blend with the aid of a hot and delocalized CT state, and Bakulin et al also offered clear evidence of exciton dissociation through hot CT states, enabling long-range charge separation. , The hot charge separation process was also supported by the temperature-independent polaron generation dynamics. , However, another scenario for charge separation in PSCs is also proposed stating that the lowest (cold) CT state acts as a precursor for the charge separation because of the fast internal conversion/relaxation process, where the hot CT state is no longer important.…”

“…Several researchers have reported efficient charge separation through the electronically and/or vibrationally excited and delocalized CT state. − The hot CT state refers to a loosely bound and delocalized electron–hole pair, facilitating the charge separation at the D/A interface. Numerous experiments have suggested that the sufficient energy of the hot CT state is beneficial for overcoming the Coulomb barrier and avoiding geminate recombination. ,,− For instance, Grancini et al reported that efficient charge separation occurs before the relaxation within CT states in the PCPDTBT/PC 60 BM blend with the aid of a hot and delocalized CT state, and Bakulin et al also offered clear evidence of exciton dissociation through hot CT states, enabling long-range charge separation. , The hot charge separation process was also supported by the temperature-independent polaron generation dynamics. , However, another scenario for charge separation in PSCs is also proposed stating that the lowest (cold) CT state acts as a precursor for the charge separation because of the fast internal conversion/relaxation process, where the hot CT state is no longer important. Excitation energy-independent internal quantum efficiency (IQE) of the photovoltaic device supports the hypothesis that excess energy is not necessary for efficient charge separation and emphasizes the importance of cold CT state dissociation. − Thus, the role of the hot CT state in charge separation is still unclear and a subject of the ongoing debate.…”

“…9,12 The hot charge separation process was also supported by the temperature-independent polaron generation dynamics. 4,10 However, another scenario for charge separation in PSCs is also proposed stating that the lowest (cold) CT state acts as a precursor for the charge separation because of the fast internal conversion/relaxation process, where the hot CT state is no longer important. Excitation energy-independent internal quantum efficiency (IQE) of the photovoltaic device supports the hypothesis that excess energy is not necessary for efficient charge separation and emphasizes the importance of cold CT state dissociation.…”

Nanoscale Mapping of Photo‐Induced Charge Carriers Generated at Interfaces of a Donor/Acceptor 2D‐Assembly by Light‐Assisted‐Scanning Tunneling Microscopy

“…On the one hand, the observation of the electroluminescent phenomenon under the tip of a scanning tunneling Bulk-heterojunction-OPV-cells based on the PTB7/PCBM couple are known for achieving high power conversion efficiencies. [18] As the interfacial area of the donor/acceptor domains has been pointed out as one of the key factors to improve the efficiency of organic photovoltaic devices, [19][20][21][22][23][24] we have first carried out a systematic analysis of the morphology of the bicomponent film that we obtained from a solution deposition approach. Then, we have compared the behavior of the PTB7/ PC 71 BM system under light irradiation with that of a non-photoactive self-assembled monolayer (SAM) of adamantanethiol (AT).…”

Nanoscale and Real-Time Nuclear–Electronic Dynamics Simulation Study of Charge Transfer at the Donor–Acceptor Interface in Organic Photovoltaics

“…Because CT occurs on an ultrafast time scale, experimental characterization of the process has been attempted with various subpicosecond spectroscopic techniques, such as the optical pump–probe, − time-resolved electron paramagnetic resonance, − and time-resolved terahertz spectroscopies. , Furthermore, the CT process has been addressed by computational studies that provide microscopic insights to complement the experimental results from both static and dynamic aspects. Static quantum chemical calculations have revealed the charge carrier energetics in donor–acceptor interface systems. − From a dynamic perspective, the CT process has been investigated via real-time time-dependent (TD) density-functional theory (DFT), , quantum dynamics simulations on parametrized Hamiltonians, , and nonadiabatic molecular dynamics (MD). , …”