Polarization Energies at Organic-Organic Interfaces: Impact on the Charge Separation Barrier at Donor-Acceptor Interfaces in Organic Solar Cells

Atomic-Scale Insights into the Interfacial Polarization Effect in the InGaN/GaN Heterostructure for Solar Cells

The model system of the InGaN/GaN quantum wells (QWs), based on the first principles calculation, was chosen to understand the underlying mechanism of interfacial polarization and its synergic effect with the built-in electric field (B_ef) at the p-n junction in solar cells (SLs). The polarized electric field (P_ef) was generated due to the redistribution of electrons and holes at the interface; moreover, the P_ef of InGaN/GaN heterostructure on the semipolar (01-11) GaN surface was consistent with that of on the N-polar (000-1) surface, which is on the lines of the B_ef and favors the electron-hole separation efficiency in SLs. Furthermore, the growth of high-quality InGaN/GaN QWs on the semipolar (01-11) GaN surface was achieved. Such an atomic-scale investigation provides a fundamental understanding of the polarization charge-induced P_ef and its interaction coupling with B_ef at the p-n junction, which could be generalized to polar material-based SLs.

Molecular Structure, Quantum Coherence, and Solvent Effects on the Ultrafast Electron Transport in BODIPY- Derivatives

Photoinduced electron transfer in multichromophore molecular systems is defined by a critical interplay between their core unit configuration (donor, molecular bridge, and acceptor) and their system-solvent coupling; these lead to energy and charge transport processes that are key in the design of molecular antennas for efficient light harvesting and organic photovoltaics. Here, we quantify the ultrafast non-Markovian dissipative dynamics of electron transfer in D-π-A molecular photosystems comprising 1,3,5,7-tetramethyl-8-phenyl-4,4-difluoroboradiazaindacene (BODIPY), Zn-porphyrin, fulleropyrrolidine, and fulleroisoxazoline. We find that the stabilization energy of the charge transfer states exhibits a significant variation for different polar (methanol, tetrahydrofuran (THF)) and nonpolar (toluene) environments and determine such sensitivity according to the molecular structure and the electron-vibration couplings that arise at room temperature. For the considered donor-acceptor (D-A) dyads, we show that the stronger the molecule-solvent coupling, the larger the electron transfer rates, regardless of the dyads' electronic coherence properties. We find such coupling strengths to be the largest (lowest) for methanol (toluene), with an electron transfer rate difference of 2 orders of magnitude between the polar and nonpolar solvents. For the considered donor-bridge-acceptor (D-B-A) triads, the molecular bridge introduces an intermediate state that allows the realization of Λ or cascaded-type energy mechanisms. We show that the latter configuration, obtained for BDP-ZnP-[PyrC₆₀] in methanol, exhibits the highest transfer rate of all of the computed triads. Remarkably, and in contrast with the dyads, we show that the larger charge transfer rates are obtained for triads that exhibit prolonged electron coherence and population oscillations.

Barrier-Free Charge Separation Enabled by Electronic Polarization in High-Efficiency Non-fullerene Organic Solar Cells

The separation of charge-transfer states into free charges at the donor/acceptor (D/A) interfaces plays a central role in organic solar cells (OSCs). Because of strong Coulomb attraction, the separation mechanisms are elusive, particularly for the high-efficiency non-fullerene (NF) OSCs with low exciton-dissociation driving forces. Here, we demonstrate that the Coulomb barriers can be substantially overcome by electronic polarization for OSCs based on a series of A-D-A acceptors (ITIC, IT-4F, and Y6). In contrast to fullerene-based D/A heterojunctions, the polarization energies for both donor holes and acceptor electrons are remarkably increased from the interfaces to pure regions in the NF heterojunctions because of strong stabilization on electrons but destabilization on holes by electrostatic interactions in the A-D-A acceptors. In particular, upon incorporation of fluorine substituents and electron-poor cores into ITIC, the increased polarization energies can completely compensate for the Coulomb attraction in the IT-4F- and Y6-based heterojunctions, leading to barrierless charge separation.

Open-circuit-voltage shift of over 0.5 V in organic photovoltaic cells induced by a minor structural difference in alkyl substituents

The recent surge in the efficiency of organic photovoltaic devices (OPVs) largely hinges on the reduction of energy loss (E_loss) that leads to improvements in open-circuit voltage (V_OC). However, there are still many unclarified factors regarding the relationship between the molecular structure and V_OC, hampering the establishment of widely applicable, effective principles for the design of active-layer materials. In this contribution, we examine the origin of the large V_OC shifts induced by minor structural differences in end-alkyl substituents on a series of anthracene-based p-type compounds. The examined p-type compounds are all highly crystalline, thereby enabling detailed investigation of the molecular packing with X-ray diffraction analysis. At the same time, they are strongly aggregating and hardly soluble; therefore, they are deposited with the aid of a photoprecursor approach which we have been employing for controlled deposition of insoluble acene-based organic semiconductors. The resultant OPVs afford the highest V_OC of 0.966 V when the end-alkyl groups are 2-ethylbutyl, and the lowest of 0.419 V when n-butyl is used in replacement of 2-ethylbutyl. X-ray diffraction analyses and density-functional-theory calculations indicate a critical impact of the non-slipped herringbone arrangement on the observed large loss in V_OC. This type of molecular arrangement is prohibited when branched alkyl chains are introduced at the ends of linear π-systems, which we consider an important factor contributing to the relatively high V_OC obtained with the 2-ethylbutyl derivative. These results may serve as a basis of useful molecular-design rules to avoid unnecessary losses in V_OC.

The cause of a large shift in open-circuit voltage induced by a minor difference in end-alkyl groups of p-type small molecules is examined via X-ray diffraction and computation, revealing a critical impact of molecular packing.

Exploring Alkyl Chains in Benzobisthiazole-Naphthobisthiadiazole Polymers: Impact on Solar-Cell Performance, Crystalline Structures, and Optoelectronics

The shapes and lengths of the alkyl chains of conjugated polymers greatly affect the efficiencies of organic photovoltaic devices. This often results in a trade-off between solubility and self-organizing behavior; however, each material has specific optimal chains. Here we report on the effect of alkyl side chains on the film morphologies, crystallinities, and optoelectronic properties of new benzobisthiazole-naphthobisthiadiazole (PBBT-NTz) polymers. The power conversion efficiencies (PCEs) of linear-branched and all-branched polymers range from 2.5% to 6.6%; the variations in these PCEs are investigated by atomic force microscopy, two-dimensional X-ray diffraction (2D-GIXRD), and transient photoconductivity techniques. The best-performing linear-branched polymer, bearing dodecyl and decyltetradecyl chains (C12-DT), exhibits nanometer-scale fibers along with the highest crystallinity, comprising predominant edge-on and partial face-on orientations. This morphology leads to the highest photoconductivity and the longest carrier lifetime. These results highlight the importance of long alkyl chains for inducing intermolecular stacking, which is in contrast to observations made for analogous previously reported polymers.

Effect of Infrared Pulse Excitation on the Bound Charge-Transfer State of Photovoltaic Interfaces

The nature and dynamics of the bound charge-transfer (CT) state in the exciton dissociation process in organic solar cells are of critical importance for the understanding of these devices. It was recently demonstrated that this state can be probed by a new experiment in which an infrared (IR) push-pulse is used to dissociate charges from the bound excited state. Here we proposed a simple quantum dynamics model to simulate the excitation of the IR pulse on the bound CT state with model parameters extracted from quantum chemical calculations. We show that the pulse dissociates the CT state following two different mechanisms: one, fairly expected, is the direct excitation of higher energy CT states leading to charge separation; the other, proposed here for the first time, is a rebound mechanism in which the negative charge is transferred in the opposite direction to form the neutral Frenkel exciton state from where it dissociates.

Intercalated vs Nonintercalated Morphologies in Donor-Acceptor Bulk Heterojunction Solar Cells: PBTTT:Fullerene Charge Generation and Recombination Revisited

In this Letter, we study the role of the donor:acceptor interface nanostructure upon charge separation and recombination in organic photovoltaic devices and blend films, using mixtures of PBTTT and two different fullerene derivatives (PC₇₀BM and ICTA) as models for intercalated and nonintercalated morphologies, respectively. Thermodynamic simulations show that while the completely intercalated system exhibits a large free-energy barrier for charge separation, this barrier is significantly lower in the nonintercalated system and almost vanishes when energetic disorder is included in the model. Despite these differences, both femtosecond-resolved transient absorption spectroscopy (TAS) and time-delayed collection field (TDCF) exhibit extensive first-order losses in both systems, suggesting that geminate pairs are the primary product of photoexcitation. In contrast, the system that comprises a combination of fully intercalated polymer:fullerene areas and fullerene-aggregated domains (1:4 PBTTT:PC₇₀BM) is the only one that shows slow, second-order recombination of free charges, resulting in devices with an overall higher short-circuit current and fill factor. This study therefore provides a novel consideration of the role of the interfacial nanostructure and the nature of bound charges and their impact upon charge generation and recombination.

Efficient Charge Separation of Cold Charge-Transfer States in Organic Solar Cells Through Incoherent Hopping

We demonstrate that efficient and nearly field-independent charge separation of electron-hole pairs in organic planar heterojunction solar cells can be described by an incoherent hopping mechanism. Using kinetic Monte Carlo simulations that include the effect of on-chain delocalization as well as entropic contributions, we simulate the dissociation of the charge-transfer state in polymer-fullerene bilayer solar cells. The model further explains experimental results of almost field independent charge separation in bilayers of molecular systems with fullerenes and provides important guidelines at the molecular level for maximizing the efficiencies of organic solar cells. Thus, utilizing coherent phenomena is not necessarily required for highly efficient charge separation in organic solar cells.

Photovoltaic concepts inspired by coherence effects in photosynthetic systems

The past decade has seen rapid advances in our understanding of how coherent and vibronic phenomena in biological photosynthetic systems aid in the efficient transport of energy from light-harvesting antennas to photosynthetic reaction centres. Such coherence effects suggest strategies to increase transport lengths even in the presence of structural disorder. Here we explore how these principles could be exploited in making improved solar cells. We investigate in depth the case of organic materials, systems in which energy and charge transport stand to be improved by overcoming challenges that arise from the effects of static and dynamic disorder - structural and energetic - and from inherently strong electron-vibration couplings. We discuss how solar-cell device architectures can evolve to use coherence-exploiting materials, and we speculate as to the prospects for a coherent energy conversion system. We conclude with a survey of the impacts of coherence and bioinspiration on diverse solar-energy harvesting solutions, including artificial photosynthetic systems.