Measuring Photoexcited Free Charge Carriers in Mono- to Few-Layer Transition-Metal Dichalcogenides with Steady-State Microwave Conductivity.

Photoinduced generation of mobile charge carriers is the fundamental process underlying many applications, such as solar energy harvesting, solar fuel production, and efficient photodetectors. Monolayer transition-metal dichalcogenides (TMDCs) are an attractive model system for studying photoinduced carrier generation mechanisms in low-dimensional materials because they possess strong direct band gap absorption, large exciton binding energies, and are only a few atoms thick. While a number of studies have observed charge generation in neat TMDCs for photoexcitation at, above, or even below the optical band gap, the role of nonlinear processes (resulting from high photon fluences), defect states, excess charges, and layer interactions remains unclear. In this study, we introduce steady-state microwave conductivity (SSMC) spectroscopy for measuring charge generation action spectra in a model WS2 mono- to few-layer TMDC system at fluences that coincide with the terrestrial solar flux. Despite utilizing photon fluences well below those used in previous pump-probe measurements, the SSMC technique is sensitive enough to easily resolve the photoconductivity spectrum arising in mono- to few-layer WS2. By correlating SSMC with other spectroscopy and microscopy experiments, we find that photoconductivity is observed predominantly for excitation wavelengths resonant with the excitonic transition of the multilayer portions of the sample, the density of which can be controlled by the synthesis conditions. These results highlight the potential of layer engineering as a route toward achieving high yields of photoinduced charge carriers in neat TMDCs, with implications for a broad range of optoelectronic applications.

Scanning Kelvin probe imaging of the potential profiles in fixed and dynamic planar LECs.

We measure the potential profiles of both dynamic and fixed junction planar light-emitting electrochemical cells (LECs) using Scanning Kelvin Probe Microscopy (SKPM) and compare the results against models of LEC operation. We find that, in conventional dynamic junction LECs formed using lithium trifluoromethanesulfonate (LiTf), poly(ethylene oxide) (PEO), and the soluble alkoxy-PPV derivative poly[2-methoxy-5-(3',7'-dimethyl-octyloxy)-p-phenylenevinylene (MDMO-PPV), the majority (>90%) of the potential is dropped near the cathode with little potential drop across either the film or the anode/polymer interface. In contrast, when examining fixed junction LECs where the LiTf is replaced with [2-(methacryloyloxy)ethyl] trimethylammonium 2-(methacryloyloxy)ethane-sulfonate (METMA/MES), the potential is dropped at both contacts during the initial poling. The potential profile evolves over a period of approximately 60 min under bias to achieve a final profile similar to that obtained in the LiTf systems. In addition to elucidating the differences between conventional dynamic LECs and fixed LECs incorporating cross-linkable ion pair monomers, the results on both systems provide direct evidence for a primarily "p-type" LEC consistent with the emitting junction near the cathode and relatively small electric fields across the bulk of the device for these two material systems.

Nucleating pattern formation in spin-coated polymer blend films with nanoscale surface templates.

We use Dip-Pen Nanolithography (DPN) to generate monolayer surface templates for guiding pattern formation in spin-coated polymer blend films. We study template-directed pattern formation in blends of polystyrene/poly(2-vinylpyridine) (PS/P2VP) as well as blends of PS and the semiconducting conjugated polymer poly(3-hexylthiophene) (P3HT). We show that acid-terminated monolayers can be used to template pattern formation in PS/P3HT blends, while hydrophobic monolayers can be used to template pattern formation in PS/P2VP blends. In both blends, the polymer patterns comprise laterally-phase separated regions surrounded by vertically separated bilayers. We hypothesize that the observed patterns are formed by template-induced dewetting of the bottom layer of a polymer bilayer during the spin-coating process. We compare the effects of template feature size and spacing on the resulting polymer patterns with predictions from published models of template-directed dewetting in thin films and find the data in good agreement. For both blends we observe that a minimum feature size is required to nucleate dewetting/phase separation. We find this minimum template diameter to be approximately 180 nm in 50/50 PS/P2VP blends, and approximately 100 nm in 50/50 PS/P3HT blends. For larger template diameters, PS/P2VP blends show evidence for pattern formation beginning at the template boundaries, while PS/P3HT blends rupture randomly across the template features.

Imaging Charge Transfer State Excitations in Polymer/Fullerene Solar Cells with Time-Resolved Electrostatic Force Microscopy.

We demonstrate nanoscale imaging of charge transfer state photoexcitations in polymer/fullerene bulk heterojunction solar cells using time-resolved electrostatic force microscopy (trEFM). We compare local trEFM charging rates and external quantum efficiencies (EQE) for both above-gap and below-gap excitation of the model system poly[2-methoxy-5-(3',7'-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) and [6,6]-phenyl C61 butyric acid methyl ester (PCBM). We show that the local trEFM charging rate correlates with device EQE for both above-gap and below-gap photoexcitation, demonstrating that EFM methods have sufficient sensitivity to detect the low EQEs associated with CT state formation, a result that could be useful for probing weak subgap excitations in nanostructured materials such as quantum dot and organometal halide perovskite solar cells. Further, we use trEFM to map spatial variations in EQE arising from subgap CT excitation in organic photovoltaics (OPVs) and find that the local distribution of photocurrent arising from these states is nearly identical to the spatial variation in EQE from above-gap singlet excitation. These results are consistent with recent work showing that both above-gap and below-gap excitation have similar internal quantum efficiency.

Charge generation measured for fullerene-helical nanofilament liquid crystal heterojunctions.

The helical nanofilament (HNF) liquid crystal phase is an ordered architecture exhibiting interesting properties for charge transport. It is a small molecule self-assembly of stacked and twisted crystalline layers, which form alignable organic nanorods with half the surface area of the filaments consisting of aromatic sublayer edges. HNFs mixed with an electron acceptor generate an intriguing network for photoinduced electron transfer (PET). In this work, we characterize the structure of the HNF phase as processed into thin films with transmission electron microscopy (TEM) and X-ray diffraction (XRD). Additionally, we measure the flash-photolysis time-resolved microwave conductivity (TRMC) in samples where the HNF phase is fabricated into heterojunctions with the fullerenes C60 and PC60BM, prototypical electron acceptors for organic photovoltaics. Two distinct microstructures of the thin films were identified and compared for PET. A near-unity charge generation yield is observed in a bilayer of HNFs with C60. Moreover, the HNF phase is shown to be 10× better at charge generation than a lamellar structuring of the same components. Thus, the HNF phase is shown to be a good charge-generation interface.

Photovoltaic charge generation in organic semiconductors based on long-range energy transfer.

For efficient charge generation in organic solar cells, photogenerated excitons must migrate to a donor/acceptor interface where they can be dissociated. This migration is traditionally presumed to be based on diffusion through the absorber material. Herein we study an alternative migration route--two-step exciton dissociation--whereby the exciton jumps from the donor to acceptor before charge creation takes place. We study this process in a series of multilayer donor/barrier/acceptor samples, where either poly(3-hexylthiophene) (P3HT) or copper phthalocyanine (CuPc) is the donor, fullerene (C60) is the acceptor, and N,N-diphenyl-N,N-bis(3-methylphenyl)-[1,1-bisphenyl]-4,4-diamine (TPD) acts as a barrier to energy transfer. By varying the thickness of the barrier layer, we find that energy transfer from P3HT to C60 proceeds over large distances (∼50% probability of transfer across a 11 nm barrier), and that this process is consistent with long-range Förster resonance energy transfer (FRET). Finally, we demonstrate a fundamentally different architecture concept that utilizes the two-step mechanism to enhance performance in a series of P3HT/CuPc/C60 devices.

Mapping local photocurrents in polymer/fullerene solar cells with photoconductive atomic force microscopy.

The performance of organic solar cells is highly dependent on film morphology. However, directly correlating local film structures with device performance remains challenging. We demonstrate that photoconductive atomic force microscopy (pcAFM) can be used to map local photocurrents with 20 nm resolution in donor/acceptor blend solar cells of the conjugated polymer poly[2-methoxy-5-(3',7'-dimethyloctyl-oxy)-1,4-phenylene vinylene] (MDMO-PPV) with the fullerene (6,6)-phenyl-C61-butyric acid methyl ester (PCBM) spin-coated from various solvents. We present photocurrent maps under short-circuit conditions (zero applied bias) as well as under various applied voltages. We find significant variation in the short-circuit current between regions that appear identical in AFM topography. These variations occur from one domain to another as well as on larger length scales incorporating multiple domains. These results suggest that the performance of polymer-fullerene blends can still be improved through better control of morphology.

Time-resolved electrostatic force microscopy of polymer solar cells.

Blends of conjugated polymers with fullerenes, polymers, or nanocrystals make promising materials for low-cost photovoltaic applications. Different processing conditions affect the efficiencies of these solar cells by creating a variety of nanostructured morphologies, however, the relationship between film structure and device efficiency is not fully understood. We introduce time-resolved electrostatic force microscopy (EFM) as a means to measure photoexcited charge in polymer films with a resolution of 100 nm and 100 micros. These EFM measurements correlate well with the external quantum efficiencies measured for a series of polymer photodiodes, providing a direct link between local morphology, local optoelectronic properties and device performance. The data show that the domain centres account for the majority of the photoinduced charge collected in polyfluorene blend devices. These results underscore the importance of controlling not only the length scale of phase separation, but also the composition of the domains when optimizing nanostructured solar cells.

Patterning phase separation in polymer films with dip-pen nanolithography.

We report a rapid-prototyping method for controlling nanoscale phase separation and pattern formation in conjugated polymer blend films using Dip-Pen Nanolithography (DPN). We use DPN to generate patterned alkylthiol monolayers with feature sizes down to 50 nm on gold surfaces and show how such patterns can nucleate the formation of lateral domains in blends of poly-3-hexylthiophene (P3HT) and polystyrene (PS) cast from solution. We show that this process can be used to probe phase nucleation at heterogeneous surface sites ranging in size from 50 to 750 nm, and that polymer features smaller than 150 nm in diameter can be achieved. We anticipate this method will be useful for studying polymer film responses to nanoscale surface fluctuations as well as for correlating nanoscale phase separation with optoelectronic processes in organic films used in light-emitting diode and photovoltaic devices.