Exciton Relaxation in Highly Rigid Conjugated Polymers: Correlating Radiative Dynamics with Structural Heterogeneity and Wavefunction Delocalization

Conjugated polymers are promising materials for solar cells and other electronic applications due to facile charge and electronic energy migration along the conjugated backbone. Torsional defects due to rotation around single bonds on the backbone are known to decrease the effective conjugation length of these materials, limiting their ability to shuttle charge and electronic energy. We investigated the radiative emission dynamics of a recently synthesized rigid conjugated ladder polymer (<b>LP1</b>) and nonrigid control (<b>CP1</b>) with a similar carbazole backbone moiety. <b>LP1</b> was prepared using a recently reported synthesis under thermodynamic control, leading to a low backbone defect density. We find that the singlet emission lifetime of <b>LP1</b> is longer than that of any previously reported ladder conjugated polymer, which we attribute to its low defect density. Further, the emission contains a large-amplitude long component with a lifetime that lasts as long as 5 ns. Our results imply that careful control of defects at the synthesis level can lead to processable polymers with large electronic wavefunction delocalization and correspondingly long fluorescence lifetimes. This indicates an avenue to further tune the rapid solid-state energy transport rate along the polymer backbone

Energy Transfer in Aqueous Light Harvesting Antennae Based on Brush-like Inter-Conjugated Polyelectrolyte Complexes

Conjugated polyelectrolytes (CPEs) have the potential to serve as building blocks of artificial light-harvesting systems. This is primarily due to their delocalized electronic states and potential for hierarchical self-assembly. We showed previously that inter-CPE complexes composed of oppositely charged exciton-donor and exciton-acceptor CPEs displayed efficient electronic energy transfer. However, near ionic charge equivalence, complexed CPE chains become net-neutral and thus experience a precipitous drop in aqueous solubility. To increase the stability and to rationally manipulate the phase behavior of inter-CPE complexes, we synthesized a series of highly water-soluble exciton-donor CPEs composed of alternating ionic and polar nonionic fluorene monomers. The nonionic monomer contained oligo(ethyleneglycol) sidechains of variable length. We then formed exciton donor-acceptor complexes and investigated their relative energy transfer efficiencies in the presence of a fixed exciton-acceptor CPE. We find that, even when the polar nonionic sidechains become quite long (nine ethyleneglycol units), the energy transfer efficiency is hardly affected so long as the inter-CPE network retains a net polyelectrolyte charge. However, near the onset of spontaneous phase separation, we observe a clear influence of the length of the oligo(ethyleneglycol) sidechains on the photophysics of the complex. Our results have implications for the use of polyelectrolyte phase separation to produce aqueous light-harvesting soft materials

Influence of Molecular Excluded Volume and Connectivity on the Nanoscale Morphology of Conjugated Polymer/Small Molecule Blends

Over the past couple of decades, organic photovoltaics (OPV) based on conjugated polymer/fullerene derivative bulk heterojunctions have been extensively studied, resulting in single-junction efficiencies of order 10%. The need to push the efficiency toward 15% has resulted in the synthesis of a large number of non-fullerene electron acceptors with ever-increasing absorption coefficients in the red. Though some new acceptors have recently begun to be competitive with the fullerene, there is very little systematic understanding of which molecular geometries and spatial frontier orbital extent correlate with improved performance. One of the most important factors that determines the OPV efficiency is the nanoscale, phase-separated morphology of the blend. In this article, using a combination of resonant elastic X-ray scattering and elemental mapping, we investigate the influence of relatively small chemical changes to a nonplanar conjugated small molecule on the nanoscale morphology of the resulting polymer/molecule blend. We find that subtle modifications of the number and placement of peripheral functional groups can have an enormous influence on the length scale of phase separation. We then quantify the extent of phase separation by using the generalized indirect Fourier transform to convert resonant scattering intensities to pair-distance distribution functions. Our results point toward the large influence of the molecular excluded volume as a major morphology determinant. This work has implications for synthetic efforts to create non-fullerene electron acceptors that can substantially outcompete fullerenes in OPV devices

Photoinduced charge carrier generation and decay in sequentially deposited polymer/fullerene layers : bulk heterojunction vs planar interface

In this work, we use the time-resolved microwave conductivity (TRMC) technique to study the dynamics of charge carrier generation in sequentially deposited conjugated polymer/fullerene layers. These layers are either fully solution-processed, using orthogonal solvents for the layers of the polymer poly(3-hexylthiophene) (P3HT) and the fullerene phenyl-C₆₁-butyric acid methyl ester (PCBM), or prepared by thermally evaporating a C₆₀ layer onto P3HT films. Our work is motivated by the remarkable efficiency of organic photovoltaic (OPV) devices using a sequentially processed P3HT/PCBM active layer. Here we use an electrodeless photoconductivity probe, so we can photoexcite the sample either through the polymer or the fullerene layer. We use samples with extremely thick P3HT films (2.4 μm) and show that excitation from either side of both as-cast and thermally annealed sample yields virtually identical results, consistent with mixing of the PCBM into the polymer film. We also compare solution-deposited samples to samples made by thermally evaporating C₆₀ on P3HT, and find that we can distinguish between charge generation in bulk-P3HT and at the polymer/fullerene interface. We show that, despite their morphological differences, the carrier dynamics in the sequentially processed samples resemble those of mixed, bulk heterojunction (BHJ) systems. All of this is consistent with the idea that PCBM readily mixes into the P3HT film in sequentially deposited P3HT/PCBM samples, although the total amount of fullerene mixed into the P3HT appears to be less than that typically used in an optimized BHJ. Finally, we discuss the implications for OPV device architectures prepared by sequential deposition from solution.13 page(s

Exciton Transfer Between Extended Electronic States in Conjugated Inter-Polyelectrolyte Complexes

Artificial light harvesting, a process that involves converting sunlight into chemical potential energy, is considered to be a promising part of the overall solution to address urgent global energy challenges. Conjugated polyelectrolyte complexes (CPECs) are particularly attractive for this purpose due to their extended electronic states, tunable assembly thermodynamics, and sensitivity to their local environment. Importantly, ionically assembled complexes of conjugated polyelectrolytes can act as efficient donor-acceptor pairs for electronic energy transfer (EET). However, to be of use in material applications, we must understand how modifying the chemical structure of the CPE backbone alters the EET rate beyond spectral overlap considerations. In this report we investigate the dependence of the EET efficiency and rate on the electronic structure and excitonic wave function of the CPE backbone. To do so, we synthesized a series of alternating copolymers where the electronic states are systematically altered by introducing comonomers with electron withdrawing and electron-rich character while keeping the linear ionic charge density nearly fixed. We find evidence that the excitonic coupling may be significantly affected by the exciton delocalization radius, in accordance with analytical models based on the line-dipole approximation and quantum chemistry calculations. Our results imply that care should be taken when selecting CPE components for optimal CPEC EET. These results have implications for using CPECs as key components in water-based light-harvesting materials, either as standalone assemblies or as adsorbates on nanoparticles and thin films

Ultrafast Studies of Exciton Migration and Polaron Formation in Sequentially Solution-Processed Conjugated Polymer/Fullerene Quasi-Bilayer Photovoltaics

We examine the ultrafast dynamics of exciton migration and polaron production in sequentially processed ‘quasi-bilayer’ and preblended ‘bulk heterojunction’ (BHJ) solar cells based on conjugated polymer films that contain the same total amount of fullerene. We find that even though the polaron yields are similar, the dynamics of polaron production are significantly slower in quasi-bilayers than BHJs. We argue that the different polaron production dynamics result from the fact that (1) there is significantly less fullerene inside the polymer in quasi-bilayers than in BHJs and (2) sequential processing yields polymer layers that are significantly more ordered than BHJs. We also argue that thermal annealing improves the performance of quasi-bilayer solar cells not because annealing drives additional fullerene into the polymer but because annealing improves the fullerene crystallinity. All of the results suggest that sequential processing remains a viable alternative for producing polymer/fullerene solar cells with a nanometer-scale architecture that differs from BHJs

Impact of the crystallite orientation distribution on exciton transport in donor-acceptor conjugated polymers

Conjugated polymers are widely used materials in organic photovoltaic devices. Owing to their extended electronic wave functions, they often form semicrystalline thin films. In this work, we aim to understand whether distribution of crystallographic orientations affects exciton diffusion using a low-band-gap polymer backbone motif that is representative of the donor/acceptor copolymer class. Using the fact that the polymer side chain can tune the dominant crystallographic orientation in the thin film, we have measured the quenching of polymer photoluminescence, and thus the extent of exciton dissociation, as a function of crystal orientation with respect to a quenching substrate. We find that the crystallite orientation distribution has little effect on the average exciton diffusion length. We suggest several possibilities for the lack of correlation between crystallographic texture and exciton transport in semicrystalline conjugated polymer films.7 page(s

Ultrafast Electron Transfer at Organic Semiconductor Interfaces: Importance of Molecular Orientation

Much is known about the rate of photoexcited charge generation in at organic donor/acceptor (D/A) heterojunctions overaged over all relative arrangements. However, there has been very little experimental work investigating how the photoexcited electron transfer (ET) rate depends on the precise relative molecular orientation between D and A in thin solid films. This is the question that we address in this work. We find that the ET rate depends strongly on the relative molecular arrangement: The interface where the model donor compound copper phthalocyanine is oriented face-on with respect to the fullerene C₆₀ acceptor yields a rate that is approximately 4 times faster than that of the edge-on oriented interface. Our results suggest that the D/A electronic coupling is significantly enhanced in the face-on case, which agrees well with theoretical predictions, underscoring the importance of controlling the relative interfacial molecular orientation

Thermotropic phase transition of benzodithiophene copolymer thin films and its impact on electrical and photovoltaic characteristics

We observed a thermotropic phase transition in poly[3,4-dihexyl thiophene-2,2′:5,6′-benzo[1,2-b:4,5-b′]dithiophene] (PDHBDT) thin films accompanied by a transition from a random orientation to an ordered lamellar phase via a nearly hexagonal lattice upon annealing. We demonstrate the effect of temperature-dependent molecular packing on charge carrier mobility (μ) in organic field-effect transistors (OFETs) and photovoltaic characteristics, such as exciton diffusion length (LD) and power conversion efficiency (PCE), in organic solar cells (OSCs) using PDHBDT. The μ was continuously improved with increasing annealing temperature and PDHBDT films annealed at 270 °C resulted in a maximum μ up to 0.46 cm²/(V s) (μavg = 0.22 cm²/(V s)), which is attributed to the well-ordered lamellar structure with a closer π–π stacking distance of 3.5 Å as shown by grazing incidence-angle X-ray diffraction (GIXD). On the other hand, PDHBDT films with a random molecular orientation are more effective in photovoltaic devices than films with an ordered hexagonal or lamellar phase based on current–voltage characteristics of PDHBDT/C60 bilayer solar cells. This observation corresponds to an enhanced dark current density (JD) and a decreased LD upon annealing. This study provides insight into the dependence of charge transport and photovoltaic characteristics on molecular packing in polymer semiconductors, which is crucial for the management of charge and energy transport in a range of organic optoelectronic devices.10 page(s