Trap-Filling Magnetoconductance as an Initialization and Readout Mechanism of Triplet Exciton Spins

Photoexcited triplet states are promising candidates for hybrid qubit systems, as they can be used as a controlling gate for nuclear spins. But microwave readout schemes do not generally offer the sensitivity needed to approach the single-molecule limit or the scope to integrate such systems into devices. Here, we demonstrate the possibility of electrical readout of triplet spins at room temperature through a specific mechanism of magnetoconductance (MC) in polycrystalline pentacene. We show that hole-only pentacene devices exhibit a positive photoinduced MC response that is consistent with a trap-filling mechanism. Spin and magnetic-field-dependent quenching of photogenerated triplets by holes quantitatively explains the MC response we observe. These results are distinct in both sign and proposed mechanism compared to previous reports on polyacene materials and provide clear design rules for future spintronic devices based on this spin-sensing mechanism

Delocalization Drives Free Charge Generation in Conjugated Polymer Films

We demonstrate that the product of photoinduced electron transfer between a conjugated polymer host and a dilute molecular sensitizer is controlled by the structural state of the polymer. Ordered semicrystalline solids exhibit free charge generation, while disordered polymers in the melt phase do not. We use photoluminescence (PL) and time-resolved microwave conductivity (TRMC) measurements to sweep through polymer melt transitions in situ. Free charge generation measured by TRMC turns off upon melting, whereas PL quenching of the molecular sensitizers remains constant, implying unchanged electron transfer efficiency. The key difference is the intermolecular order of the polymer host in the solid state compared to the melt. We propose that this order–disorder transition modulates the localization length of the initial charge-transfer state, which controls the probability of free charge formation

Imaging the Evolution of Nanoscale Photocurrent Collection and Transport Networks during Annealing of Polythiophene/Fullerene Solar Cells

We use photoconductive atomic force microscopy to image nanoscale spatial variations in photocurrent across the surfaces of photovoltaic cells made from blends of the conjugated polymer regioregular poly(3-hexylthiopene) (P3HT) with phenyl-C61-butyric acid methyl ester (PCBM). We study how the spatial variations in photocurrent evolve with thermal annealing, and we correlate these changes with the evolution of macroscopic film and device properties such as external quantum efficiency and carrier mobility. We use conductive atomic force microscopy to examine the development of injection and transport networks for both electrons and holes as a function of annealing. We find that the hole transport, electron transport, and photocurrent collection networks become increasingly heterogeneous with thermal annealing and remain heterogeneous on the 10−100 nm length scale even in the most efficient P3HT/PCBM devices. After annealing, the regions of the greatest dark hole currents, greatest dark electron currents, and greatest photocurrents are each associated with different regions of the nanostructured films. These results suggest spatial heterogeneity can contribute to the imperfect internal quantum efficiency even in relatively efficient organic photovoltaics and that fully 3D modeling is needed to describe the devices physics of polymer blend solar cells

Missing Excitons: How Energy Transfer Competes with Free Charge Generation in Dilute-Donor/Acceptor Systems

Energy transfer across the donor–acceptor interface in organic photovoltaics is usually beneficial to device performance, as it assists energy transport to the site of free charge generation. Here, we present a case where the opposite is true: dilute donor molecules in an acceptor host matrix exhibit ultrafast excitation energy transfer (EET) to the host, which suppresses the free charge yield. We observe an optimal photochemical driving force for free charge generation, as detected via time-resolved microwave conductivity (TRMC), but with a low yield when the sensitizer is excited. Meanwhile, transient absorption shows that transferred excitons efficiently produce charge-transfer states. This behavior is well described by a competition for the excited state between long-range electron transfer that produces free charge and EET that ultimately produces only localized charge-transfer states. It cannot be explained if the most localized CT states are the intermediate between excitons and the free charge in this system

Concerted Emission and Local Potentiometry of Light-Emitting Electrochemical Cells

We study the operation of polymer light-emitting electrochemical cells (LECs) by combining scanning Kelvin probe microscopy with in situ imaging of the electroluminescence and photoluminescence on planar LECs. By combining these techniques on the same device in the same apparatus we directly map the relationship between the spatial distribution of electroluminescence and the local potential profile across the device. We find that the electroluminescence is always associated with a region of potential drop in LECs made with poly[2-methoxy-5-(3′,7′-dimethyl-octyloxy)-p-phenylenevinylene] (MDMO-PPV), poly(ethylene oxide)(PEO), and potassium trifluoromethanesulfonate. Nevertheless, depending on the electrode metal used, we also find significant potential drops at or near the electrode/organic interfaces. We study the effects of using different electrodes and show that both the electroluminescence and potential profiles are strongly dependent on the electrode work function for thin junctions operated at low potentials. These results indicate injection barriers can affect the operation of LECs even in the presence of doping

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

Reconciling the Driving Force and the Barrier to Charge Separation in Donor–Nonfullerene Acceptor Films

We investigate the dependence of charge yields on the driving force for photoinduced electron transfer in a series of all-small-molecule, semiconducting films made from indacenodithiophene nonfullerene acceptors (IDT NFAs). In contrast to reports of efficient, barrierless charge separation at near zero driving force for NFA-containing organic photovoltaics, we find that barrierless charge separation occurs only if the driving force is sufficient to overcome the Coulomb potential, ≥300 meV, based on time-resolved microwave conductivity and PL quenching measurements of 5 mmolal sensitized and 700 mmolal blended films comprising IDT NFAs paired with three different donors. This discrepancy with recent literature is caused by a difference in the way that we calculate driving force. Far from being semantic, the driving force calculation is crucial because its value controls the mechanisms needed to explain experimental observations. We provide a candid assessment of the uncertainties for our methods and popular ones used in the literature and emphasize the importance of standardizing methods across this field

Probing Exciton Diffusion and Dissociation in Single-Walled Carbon Nanotube–C₆₀ Heterojunctions

The efficiency of thin-film organic photovoltaic (OPV) devices relies heavily upon the transport of excitons to type-II heterojunction interfaces, where there is sufficient driving force for exciton dissociation and ultimately the formation of charge carriers. Semiconducting single-walled carbon nanotubes (SWCNTs) are strong near-infrared absorbers that form type-II heterojunctions with fullerenes such as C60. Although the efficiencies of SWCNT–fullerene OPV devices have climbed over the past few years, questions remain regarding the fundamental factors that currently limit their performance. In this study, we determine the exciton diffusion length in the C60 layer of SWCNT–C60 bilayer active layers using femtosecond transient absorption measurements. We demonstrate that hole transfer from photoexcited C60 molecules to SWCNTs can be tracked by the growth of narrow spectroscopic signatures of holes in the SWCNT “reporter layer”. In bilayers with thick C60 layers, the SWCNT charge-related signatures display a slow rise over hundreds of picoseconds, reflecting exciton diffusion through the C60 layer to the interface. A model based on exciton diffusion with a Beer–Lambert excitation profile, as well as Monte Carlo simulations, gives the best fit to the data as a function of C60 layer thickness using an exciton diffusion length of approximately 5 nm

Polymer Nanowire/Fullerene Bulk Heterojunction Solar Cells: How Nanostructure Determines Photovoltaic Properties

We report studies of bulk heterojunction solar cells composed of self-assembled poly(3-butylthiophene) nanowires (P3BT-nw) as the donor component with a fullerene acceptor. We show that the nanostructure of these devices is the single most important variable determining their performance, and we use a combination of solvent and thermal annealing to control it. A combination of conductive and photoconductive atomic force microscopy provides direct connections between local nanostructure and overall device performance. Films with a dense random web of nanowires cause the fullerene to aggregate in the interstices, giving a quasi-ordered interpenetrating heterojunction with high short-circuit current density (10.58 mA/cm2), but relatively low open circuit voltage (520 mV). Films with a low density of nanowires result in a random bulk heterojunction composed of small crystalline PCBM and P3BT phases. Fewer nanowires result in higher open circuit voltage (650 mV) but lower current density (6.02 mA/cm2). An average power conversion efficiency of 3.35% was achieved in a structure which balances these factors, with intermediate nanowire density. The best photovoltaic performance would be realized in a material structure which maintains the interpenetrating network of nanowires and fullerene phases (high current density), but avoids the device bridging we observe, and the recombination and shunt losses associated with it (high open-circuit voltage)