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Organic electrochemical transistors are believed to face an inherent material design tension between optimizing for ion mobility and for electronic mobility. These devices transduce ion uptake into electrical current, thereby requiring high ion mobility for efficient electrochemical doping and rapid turn-on kinetics and high electronic mobility for the maximum transconductance. Here, we explore a facile route to improve operational kinetics and volumetric capacitance in a high-mobility conjugated polymer (poly[2,5-(2-octyldodecyl)-3,6-diketopyrrolopyrrole-alt-5,5-(2,5-di(thien-2-yl)thieno [3,2-b]thiophene)], DPP-DTT) by employing a nanowire morphology. For equivalent thicknesses, the DPP-DTT nanowire films exhibit consistently faster kinetics (∼6-10× faster) compared to a neat DPP-DTT film. The nanowire architectures show ∼4× higher volumetric capacitance, increasing from 7.1 to 27.7 F/cm3, consistent with the porous structure better enabling ion uptake throughout the film. The nanowires also exhibit a small but energetically favorable shift in a threshold voltage of ∼17 mV, making the nanostructured system both faster and energetically easier to electrochemically dope compared to neat films. We explain the variation using two atomic force microscopy methods: in situ electrochemical strain microscopy and nanoinfrared imaging via photoinduced force microscopy. These data show that the nanowire film's structure allows greater swelling and ion uptake throughout the active layer, indicating that the nanowire architecture exhibits volumetric operation, whereas the neat film is largely operating via the field effect. We propose that for higher-mobility materials, casting the active layer in a nanowire form may offer faster kinetics, enhanced volumetric capacitance, and possibly lower threshold voltage while maintaining desirable device performance.

As easy-to-grow quantum wells with narrow excitonic features at room temperature, two-dimensional (2D) Ruddleson-Popper perovskites are promising for realizing novel nanophotonic devices based on exciton-photon interactions. Here, we demonstrate a distinct hybrid exciton-photon Fano resonance in (C4H9NH3)2PbI4 thin films prepared via spin coating. Using a classical coupled-oscillator model and finite-difference time-domain simulations, we link the Fano interference to the coupling of the exciton with the Rayleigh-like scattering of the film microstructure. Combining colloidal plasmonic cavities with the 2D perovskite films, we demonstrate tuning of the Fano resonance. In combination with silver nanoparticles, the exciton-photon Fano interference couples to the in-plane plasmonic modes with indications of Rabi splitting. By creating a nanoparticle on mirror geometry, we address the out-of-plane excitonic component, reaching an intermediate coupling regime. These structures suggest possible photonic targets for biomolecular self-assembly applications.

Moir\'e superlattices in van der Waals heterostructures are gaining increasing attention because they offer new opportunities to tailor and explore unique electronic phenomena when stacking 2D materials with small twist angles. Here, we reveal local surface potentials associated with stacking domains in twisted double bilayer graphene (TDBG) moir\'e superlattices. Using a combination of both lateral Piezoresponse Force Microscopy (LPFM) and Scanning Kelvin Probe Microscopy (SKPM), we distinguish between Bernal (ABAB) and rhombohedral (ABCA) stacked graphene and directly correlate these stacking configurations with local surface potential. We find that the surface potential of the ABCA domains is ~15 mV higher (smaller work function) than that of the ABAB domains. First-principles calculations based on density functional theory further show that the different work functions between ABCA and ABAB domains arise from the stacking dependent electronic structure. We show that, while the moir\'e superlattice visualized by LPFM can change with time, imaging the surface potential distribution via SKPM appears more stable, enabling the mapping of ABAB and ABCA domains without tip-sample contact-induced effects. Our results provide a new means to visualize and probe local domain stacking in moir\'e superlattices along with its impact on electronic properties., Comment: 15 pages, 4 figures

At present, electric lighting accounts for ~15% of global power consumption and thus the adoption of efficient, low-cost lighting technologies is important. Halide perovskites have been shown to be good emitters of pure red, green and blue light, but an efficient source of broadband white electroluminescence suitable for lighting applications is desirable. Here, we report a white light-emitting diode (LED) strategy based on solution-processed heterophase halide perovskites that, unlike GaN white LEDs, feature only one broadband emissive layer and no phosphor. Our LEDs operate with a peak luminance of 12,200 cd m−2 at a bias of 6.6 V and a maximum external quantum efficiency of 6.5% at a current density of 8.3 mA cm−2. Systematic in situ and ex situ characterizations reveal that the mechanism of efficient electroluminescence is charge injection into the α phase of CsPbI3, α to δ charge transfer and α–δ balanced radiative recombination. Future advances in fabrication technology and mechanistic understanding should lead to further improvements in device efficiency and luminance. Heterophase CsPbI3 perovskite gives rise to bright white phosphor-free LEDs.

Achieving high reproducibility in materials properties of perovskites is critical to the reliable development of optoelectronic device applications such as photovoltaics and light-emitting diodes. ...

Despite rapid advancements in power conversion efficiency in the last decade, perovskite solar cells still perform below their thermodynamic efficiency limits. Non-radiative recombination, in particular, has limited the external radiative efficiency and open circuit voltage in the highest performing devices. We review the historical progress in enhancing perovskite external radiative efficiency and determine key strategies for reaching high optoelectronic quality. Specifically, we focus on non-radiative recombination within the perovskite layer and highlight novel approaches to reduce energy losses at interfaces and through parasitic absorption. By strategically targeting defects, it is likely that the next set of record-performing devices with ultra-low voltage losses will be achieved.

Cesium-containing all-inorganic perovskites have received considerable interest in photovoltaics research because of their potential for improved stability compared to their organic–inorganic hybri...

The success of halide perovskites in a host of optoelectronic applications is often attributed to their long photoexcited carrier lifetimes, which has led to charge-carrier recombination processes being described as unique compared to other semiconductors. Here, we integrate recent literature findings to provide a critical assessment of the factors we believe are most likely controlling recombination in the most widely studied halide perovskite systems. We focus on four mechanisms that have been proposed to affect measured charge carrier recombination lifetimes, namely: (1) recombination via trap states, (2) polaron formation, (3) the indirect nature of the bandgap (e.g., Rashba effect), and (4) photon recycling. We scrutinize the evidence for each case and the implications of each process on carrier recombination dynamics. Although they have attracted considerable speculation, we conclude that multiple trapping or hopping in shallow trap states, and the possible indirect nature of the bandgap (e.g., Rashba effect), seem to be less likely given the combined evidence, at least in high-quality samples most relevant to solar cells and light-emitting diodes. On the other hand, photon recycling appears to play a clear role in increasing apparent lifetime for samples with high photoluminescence quantum yields. We conclude that polaron dynamics are intriguing and deserving of further study. We highlight potential interdependencies of these processes and suggest future experiments to better decouple their relative contributions. A more complete understanding of the recombination processes could allow us to rationally tailor the properties of these fascinating semiconductors and will aid the discovery of other materials exhibiting similarly exceptional optoelectronic properties.

We describe an open-source and widely adaptable Python library that recognizes morphological features and domains in images collected via scanning probe microscopy. π-Conjugated polymers (CPs) are ideal for evaluating the Materials Morphology Python (m2py) library because of their wide range of morphologies and feature sizes. Using thin films of nanostructured CPs, we demonstrate the functionality of a general m2py workflow. We apply numerical methods to enhance the signals collected by the scanning probe, followed by Principal Component Analysis (PCA) to reduce the dimensionality of the data. Then, a Gaussian Mixture Model segments every pixel in the image into phases, which have similar material-property signals. Finally, the phase-labeled pixels are grouped and labeled as morphological domains using either connected components labeling or persistence watershed segmentation. These tools are adaptable to any scanning probe measurement, so the labels that m2py generates will allow researchers to individually address and analyze the identified domains in the image. This level of control, allows one to describe the morphology of the system using quantitative and statistical descriptors such as the size, distribution, and shape of the domains. Such descriptors will enable researchers to quantitatively track and compare differences within and between samples.

Perovskite light-emitting diodes (PeLEDs) have undergone rapid development in the last several years with external quantum efficiencies (EQEs) reaching over 21%. However, most PeLEDs still suffer from severe efficiency roll-off (droop) at high injection current densities, thus limiting their achievable brightness and presenting a challenge to their use in laser diode applications. In this work, we show that the roll-off characteristics of PeLEDs are affected by a combination of charge injection imbalance, nonradiative Auger recombination, and Joule heating. To realize ultrabright and efficient PeLEDs, several strategies have been applied. First, we designed an energy ladder to balance the electron and hole transport. Second, we optimized perovskite materials to possess reduced Auger recombination rates and improved carrier mobility. Third, we replaced glass substrates with sapphire substrates to better dissipate joule heat. Finally, by applying a current-focusing architecture, we achieved PeLEDs with a record luminance of 7.6 Mcd/m2. The devices can be operated at very high current densities (J) up to ∼1 kA/cm2. Our work suggests a broad application prospect of perovskite materials for high-brightness LEDs and ultimately a potential for solution-processed electrically pumped laser diodes.

We explore the effects of nonradiative recombination at the extracting contacts on the achievable performance of halide perovskite photovoltaic cells. First, we perform device simulations using standard drift-diffusion models with experimental semiconductor parameters matching those of methylammonium lead triiodide (MAPbI3). We quantify the range of surface recombination velocities (SRVs) that would allow this archetypal perovskite to reach power conversion efficiencies of 27%. In particular, for contacts with well-aligned energy levels, SRVs of ∼1–10 cm/s should enable open-circuit voltages of 1.30 V, within 96% of the Shockley–Queisser limit. Next, we use time-resolved photoluminescence to experimentally determine the SRVs on 14 different common electron- and hole-extracting contacts, including TiO2, SnO2, ZnO, PCBM, ITIC, ICBA, TPBi, PEDOT:PSS, PTAA, PVK, NiO, MoO3, WO3, and spiro-OMeTAD. These results point the way to the selection and rational engineering of better contacts as a means to achieve high...

Quasi-2D perovskites are attractive because of their improved stability compared to 3D counterparts, but they suffer from reduced performance. Here we report an efficient quasi-2D perovskite (PEA)2(MA)4Pb5I16-based optoelectronic device processed with NH4SCN and NH4Cl additives, showing a stabilized photovoltaic power conversion efficiency as high as 14.1% (average value 12.9 ± 0.8%), which is among the highest-performing quasi-2D perovskite solar cells. These additives increase the perovskite crystallinity and induce a preferred orientation with the (0k0) planes perpendicular to the substrate, resulting in improved transport properties and hence increased short-circuit current density. Furthermore, the NH4Cl treatment enriches the Cl– concentration near the PEDOT:PSS/perovskite interface, which passivates the electron traps, leading to an enhanced electroluminescence external quantum efficiency (0.68% at +2.5 V bias). As a result, high open-circuit voltages of 1.21 ± 0.01 V with a record low nonradiative...

Reducing non-radiative recombination in semiconducting materials is a prerequisite for achieving the highest performance in light-emitting and photovoltaic applications. Here, we characterize both external and internal photoluminescence quantum efficiency and quasi-Fermi-level splitting of surface-treated hybrid perovskite (CH3NH3PbI3) thin films. With respect to the material bandgap, these passivated films exhibit the highest quasi-Fermi-level splitting measured to date, reaching 97.1 ± 0.7% of the radiative limit, approaching that of the highest performing GaAs solar cells. We confirm these values with independent measurements of internal photoluminescence quantum efficiency of 91.9 ± 2.7% under 1 Sun illumination intensity, setting a new benchmark for these materials. These results suggest hybrid perovskite solar cells are inherently capable of further increases in power conversion efficiency if surface passivation can be combined with optimized charge carrier selective interfaces. Surface treatment is shown to yield passivated perovskite films with very high quasi-Fermi level splitting and internal photoluminescence quantum efficiency, indicating that further improvements in the performance of perovskite optoelectronics should be feasible.

Ionic transport phenomena in organic semiconductor materials underpin emerging technologies ranging from bioelectronics to energy storage. The performance of these systems is affected by an interplay of film morphology, ionic transport and electronic transport that is unique to organic semiconductors yet poorly understood. Using in situ electrochemical strain microscopy (ESM), we demonstrate that we can directly probe local variations in ion transport in polymer devices by measuring subnanometre volumetric expansion due to ion uptake following electrochemical oxidation of the semiconductor. The ESM data show that poly(3-hexylthiophene) electrochemical devices exhibit voltage-dependent heterogeneous swelling consistent with device operation and electrochromism. Our data show that polymer semiconductors can simultaneously exhibit field-effect and electrochemical operation regimes, with the operation modality and its distribution varying locally as a function of nanoscale film morphology, ion concentration and potential. Importantly, we provide a direct test of structure-function relationships by correlating strain heterogeneity with local stiffness maps. These data indicate that nanoscale variations in ion uptake are associated with local changes in polymer packing that may impede ion transport to different extents within the same macroscopic film and can inform future materials optimization.

We use transient absorption (TA) spectroscopy to study the origin of photon-energy dependent hole transfer yields in blends of PbS quantum dots with the conjugated polymer poly(3-hexylthiophene-2,5-diyl) (P3HT). We selectively excite only the quantum dots at two different wavelengths and measure the polymer ground state bleach resulting from the transfer of photoexcited holes. The higher photon-energy pump shows a greater prompt yield of hole transfer compared to the lower photon-energy excitation, on time scales sufficient to out-compete hot carrier cooling in lead chalcogenide quantum dots. We interpret the results as evidence that the excess energy of nonthermalized, or "hot," excitons resulting from higher photon-energy excitation allows more efficient charge transfer to the polymer in these systems. The data also demonstrate slow charge transfer rates, up to ∼1 ns, of the relaxed excitations on the PbS dots. These findings help to clarify the role of excess photon energy and carrier relaxation dynamics on free carrier generation in donor/acceptor solar cells.

From hybrid perovskites to semiconducting polymer/fullerene blends for organic photovoltaics, many new materials being explored for energy harvesting and storage exhibit performance characteristics that depend sensitively on their nanoscale morphology. At the same time, rapid advances in the capability and accessibility of scanning probe microscopy methods over the past decade have made it possible to study processing/structure/function relationships ranging from photocurrent collection to photocarrier lifetimes with resolutions on the scale of tens of nanometers or better. Importantly, such scanning probe methods offer the potential to combine measurements of local structure with local function, and they can be implemented to study materials in situ or devices in operando to better understand how materials evolve in time in response to an external stimulus or environmental perturbation. This Account highlights recent advances in the development and application of scanning probe microscopy methods that can help address such questions while filling key gaps between the capabilities of conventional electron microscopy and newer super-resolution optical methods. Focusing on semiconductor materials for solar energy applications, we highlight a range of electrical and optoelectronic scanning probe microscopy methods that exploit the local dynamics of an atomic force microscope tip to probe key properties of the solar cell material or device structure. We discuss how it is possible to extract relevant device properties using noncontact scanning probe methods as well as how these properties guide materials development. Specifically, we discuss intensity-modulated scanning Kelvin probe microscopy (IM-SKPM), time-resolved electrostatic force microscopy (trEFM), frequency-modulated electrostatic force microscopy (FM-EFM), and cantilever ringdown imaging. We explain these developments in the context of classic atomic force microscopy (AFM) methods that exploit the physics of cantilever motion and photocarrier generation to provide robust, nanoscale measurements of materials physics that are correlated with device operation. We predict that the multidimensional data sets made possible by these types of methods will become increasingly important as advances in data science expand capabilities and opportunities for image correlation and discovery.

We use dissipation imaging to probe local changes in electronic properties of nanostructured semiconductor films due to local photochemistry. We make quantitative maps of electrostatic dissipation due to photogenerated carriers by measuring the ringdown time of an oscillating atomic force microscope cantilever. Using organic photovoltaic materials as a testbed, we study macroscopic device degradation as a function of photooxidation for three different film morphologies comprising the conjugated polymer poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7) and the fullerene derivative [6,6]-phenyl-C71 butyric acid methyl ester (PC71BM). We find that, judged by device performance, the stability of the macroscopic devices is sensitive to processing conditions, with films processed with the solvent additive 1,8-diiodooctane being the most stable. At the microscopic level, we compare the evolution of cantilever power diss...

Transparent conducting oxides (TCOs), such as indium tin oxide and zinc oxide, play an important role as electrode materials in organic-semiconductor devices. The properties of the inorganic-organic interface-the offset between the TCO Fermi level and the relevant transport level, the extent to which the organic semiconductor can wet the oxide surface, and the influence of the surface on semiconductor morphology-significantly affect device performance. This review surveys the literature on TCO modification with phosphonic acids (PAs), which has increasingly been used to engineer these interfacial properties. The first part outlines the relevance of TCO surface modification to organic electronics, surveys methods for the synthesis of PAs, discusses the modes by which they can bind to TCO surfaces, and compares PAs to alternative organic surface modifiers. The next section discusses methods of PA monolayer deposition, the kinetics of monolayer formation, and structural evidence regarding molecular orientation on TCOs. The next sections discuss TCO work-function modification using PAs, tuning of TCO surface energy using PAs, and initiation of polymerizations from TCO-tethered PAs. Finally, studies that examine the use of PA-modified TCOs in organic light-emitting diodes and organic photovoltaics are compared.

A better understanding of how interfacial structure affects charge carrier recombination would benefit the development of highly efficient organic photovoltaic (OPV) devices. In this paper, transient photovoltage (TPV) and charge extraction (CE) measurements are used in combination with synchrotron radiation photoemission spectroscopy (SRPES) to gain insight into the correlation between interfacial properties and device performance. OPV devices based on PCDTBT/PC71BM with a Ca interlayer were studied as a reference system to investigate the interfacial effects on device performance. Devices with a Ca interlayer exhibit a lower recombination than devices with only an Al cathode at a given charge carrier density (n). In addition, the interfacial band structures indicate that the strong dipole moment produced by the Ca interlayer can facilitate the extraction of electrons and drive holes away from the cathode/polymer interface, resulting in beneficial reduction in interfacial recombination losses. These results help explain the higher efficiencies of devices made with Ca interlayers compared to that without the Ca interlayer.

While organo-metal halide perovskite photovoltaics have seen rapid development, growth of high quality material remains a challenge. Herein, we report a facile post deposition treatment utilizing coordination between methylamine (CH3NH2) vapor and CH3NH3PbI3 perovskite that rapidly improves film quality, enhancing power conversion efficiency (PCE) by ∼9%. We further comprehensively analyze the physical impact of this process with regard to the material's optoelectronic properties and its detailed microstructural changes. Connecting this with an analysis of the source of organo-metal halide perovskite reactivity toward the vapor as well as phase behavior as a function of CH3NH2 vapor pressure and time, we provide design rules for the broad, rational extension of this process to new systems and scales.

We study the photoinduced degradation of hybrid organometal perovskite photovoltaics under illumination and ambient atmosphere using UV–vis absorption, atomic force microscopy, and device performance. We correlate the structural changes in the surface of the perovskite film with changes in the optical and electronic properties of the devices. The photodecomposition of the methylammonium lead triiodide perovskite layer itself proceeds much more slowly than the photodegradation of the performance of devices with fullerene/bathocuproine/aluminum top contacts, indicating that the active layer alone is more stable than the interface with the electrodes in this geometry. The evolution of the perovskite active layer performance proceeded through several phases: (1) an initial improvement in device characteristics, (2) a plateau with very slow degradation, and (3) a catastrophic decline in material performance accompanied by marked changes in film morphology. The rapid increase in surface roughness of the active ...

Moisture, in the form of ambient humidity, has a significant impact on methylammonium lead halide perovskite films. In particular, due to the hygroscopic nature of the methylammonium component, moisture plays a significant role during film formation. This issue has so far not been well understood and neither has the impact of moisture on the physical properties of resultant films. Herein, we carry out a comprehensive and well-controlled study of the effect of moisture exposure on methylammonium lead halide perovskite film formation and properties. We find that films formed in higher humidity atmospheres have a less continuous morphology but significantly improved photoluminescence, and that film formation is faster. In photovoltaic devices, we find that exposure to moisture, either in the precursor solution or in the atmosphere during formation, results in significantly improved open-circuit voltages and hence overall device performance. We then find that by post-treating dry films with moisture exposure, we can enhance photovoltaic performance and photoluminescence in a similar way. The enhanced photoluminescence and open-circuit voltage imply that the material quality is improved in films that have been exposed to moisture. We determine that this improvement stems from a reduction in trap density in the films, which we postulate to be due to the partial solvation of the methylammonium component and "self-healing" of the perovskite lattice. This work highlights the importance of controlled moisture exposure when fabricating high-performance perovskite devices and provides guidelines for the optimum environment for fabrication. Moreover, we note that often an unintentional water exposure is likely responsible for the high performance of solar cells produced in some laboratories, whereas careful synthesis and fabrication in a dry environment will lead to lower-performing devices.

Going toward the grains Great strides have been made in improving the efficiency of organic-inorganic perovskite solar cells. Further improvements are likely to depend on understanding the role of film morphology on charge-carrier dynamics. de Quilettes et al. correlated confocal fluorescence microscopy images with those from scanning electron microscopy to spatially resolve the photoluminescence and carrier decay dynamics from films of organic-inorganic perovskites. Carrier lifetimes varied widely even between grains, and chemical treatments could improve lifetimes Science , this issue p. 683

We present a synergistic approach to modulate organic–perovskite interfaces and their photovoltaic behaviors by tuning the properties of n-contact fullerenes layered atop of perovskite. Fullerenes with excited charge transfer are found to not only suppress fullerene photoluminescence, but also enhance molecular polarization and transport capabilities. This results in optimized perovskite–fullerene contact.

We use correlated confocal and wide-field fluorescence microscopy to probe the interplay between local variations in charge carrier recombination and charge carrier transport in methylammonium lead triiodide perovskite thin films. We find that local photoluminescence variations present in confocal imaging are also observed in wide-field imaging, while intensity-dependent confocal measurements show that the heterogeneity in nonradiative losses observed at low excitation powers becomes less pronounced at higher excitation powers. Both confocal and wide-field images show that carriers undergo anisotropic diffusion due to differences in intergrain connectivity. These data are all qualitatively consistent with trap-dominated variations in local photoluminescence intensity and with grain boundaries that exhibit varying degrees of opacity to carrier transport. We use a two-dimensional kinetic model to simulate and compare confocal time-resolved photoluminescence decay traces with experimental data. The simulations further support the assignment of local variations in nonradiative recombination as the primary cause of photoluminescence heterogeneity in the films studied herein. These results point to surface passivation and intergrain connectivity as areas that could yield improvements in perovskite solar cells and optoelectronic device performance.

The UCLA team has successfully passivated perovskite film by controlling the film growth with functional polymers as additive., The solution processing of polycrystalline perovskite films introduces trap states that can adversely affect their optoelectronic properties. Motivated by the use of small-molecule surfactants to improve the optoelectronic performance of perovskites, we demonstrate the use of polymers with coordinating groups to improve the performance of solution-processed semiconductor films. The use of these polymer modifiers results in a marked change in the electronic properties of the films, as measured by both carrier dynamics and overall device performance. The devices grown with the polymer poly(4-vinylpyridine) (PVP) show significantly enhanced power conversion efficiency from 16.9 ± 0.7% to 18.8 ± 0.8% (champion efficiency, 20.2%) from a reverse scan and stabilized champion efficiency from 17.5 to 19.1% [under a bias of 0.94 V and AM (air mass) 1.5-G, 1-sun illumination over 30 min] compared to controls without any passivation. Treating the perovskite film with PVP enables a VOC of up to 1.16 V, which is among the best reported for a CH3NH3PbI3 perovskite solar cell and one of the lowest voltage deficits reported for any perovskite to date. In addition, perovskite solar cells treated with PVP show a long shelf lifetime of up to 90 days (retaining 85% of the initial efficiency) and increased by a factor of more than 20 compared to those without any polymer (degrading to 85% after ~4 days). Our work opens up a new class of chemical additives for improving perovskite performance and should pave the way toward improving perovskite solar cells for high efficiency and stability.

We investigate the effect of quantum dot size on photocurrent and photoinduced charge transfer yields in blends of the conjugated polymer, poly((4,8-bis(octyloxy)benzo(1,2-b:4,5-b′)dithiophene-2,6-diyl)(2-((dodecyloxy)carbonyl)thieno(3,4-b)thiophenediyl)) (PTB1), with PbS nanocrystal quantum dots (QDs). These hybrid solar cells exhibit external quantum efficiencies of over 70% and power conversion efficiencies of up to 2.8%. We use photoinduced absorption (PIA) spectroscopy and device EQE measurements to probe long-lived charge transfer at the polymer/QD interface as a function of QD size. We observe that both the PIA signal associated with charge formation on the polymer, as well as the external quantum efficiency of the hybrid photovoltaic devices decrease in magnitude with increasing quantum dot size, despite the broader absorption spectrum of the larger dots. We interpret these results as evidence that PTB1/PbS blends behave at least partially as bulk heterojunction (BHJ) solar cells, and conclude tha...

We use dipolar phosphonic acid self-assembled monolayers (PA SAMs) to modify the work function of the hole-extracting contact in polymer/fullerene bulk heterojunction solar cells. We observe a linear dependence of the open-circuit voltage (VOC) of these organic photovoltaic devices on the modified indium tin oxide (ITO) work function when using a donor polymer with a deep-lying ionization energy. With specific SAMs, we can obtain VOC values exceeding those obtained with the common poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) hole-extraction layer. We measure charge-carrier lifetimes and densities using transient photovoltage and charge extraction in a series of devices with SAM-modified contacts. As expected, these measurements show systematically longer carrier lifetimes in devices with higher VOC values; however, the trends provide useful distinctions between different hypotheses of how transient photovoltage decays might be controlled by surface chemistry. We interpret our result...

In biological complexes, cascade structures promote the spatial separation of photogenerated electrons and holes, preventing their recombination. In contrast, the photogenerated excitons in organic photovoltaic cells are dissociated at a single donor-acceptor heterojunction formed within a de-mixed blend of the donor and acceptor semiconductors. The nanoscale morphology and high charge densities give a high rate of electron-hole encounters, which should in principle result in the formation of spin-triplet excitons, as in organic light-emitting diodes. Although organic photovoltaic cells would have poor quantum efficiencies if every encounter led to recombination, state-of-the-art examples nevertheless demonstrate near-unity quantum efficiency. Here we show that this suppression of recombination arises through the interplay between spin, energetics and delocalization of electronic excitations in organic semiconductors. We use time-resolved spectroscopy to study a series of model high-efficiency polymer-fullerene systems in which the lowest-energy molecular triplet exciton (T1) for the polymer is lower in energy than the intermolecular charge transfer state. We observe the formation of T1 states following bimolecular recombination, indicating that encounters of spin-uncorrelated electrons and holes generate charge transfer states with both spin-singlet ((1)CT) and spin-triplet ((3)CT) characters. We show that the formation of triplet excitons can be the main loss mechanism in organic photovoltaic cells. But we also find that, even when energetically favoured, the relaxation of (3)CT states to T1 states can be strongly suppressed by wavefunction delocalization, allowing for the dissociation of (3)CT states back to free charges, thereby reducing recombination and enhancing device performance. Our results point towards new design rules both for photoconversion systems, enabling the suppression of electron-hole recombination, and for organic light-emitting diodes, avoiding the formation of triplet excitons and enhancing fluorescence efficiency.

We show that local structural variation affects the rate of aging in nanostructured polymer solar cells by comparing time-resolved electrostatic force microscopy (trEFM) and conventional device measurements on model polymer blends. Specifically, we study photovoltaic devices made from 1:1 blends of the polyfluorene copolymers poly(9,9'-dioctylfluorene-co-bis-N,N'-(4-butylphenyl)-bis-N,N'-phenyl-1,4-phenylene-diamine) (PFB) and poly(9,9'-dioctylfluorene-co-benzothiadiazole) (F8BT). We photooxidize these films in situ using 365, 405, and 455 nm illumination under ambient conditions, with the wavelengths chosen to preferentially excite the different components. During photooxidation, we observe a faster loss of photocurrent generation from F8BT-rich domains, leaving the PFB-rich phases to show higher photoresponse even at wavelengths absorbed predominantly by F8BT. We propose that this effect is due to the more rapid degradation of PFB hole-transport pathways in the F8BT-rich regions, resulting in a loss of percolation pathways for hole transport in the F8BT-rich phase.

Realizing the theoretical limiting power conversion efficiency (PCE) in perovskite solar cells requires a better understanding and control over the fundamental loss processes occurring in the bulk of the perovskite layer and at the internal semiconductor interfaces in devices. One of the main challenges is to eliminate the presence of charge recombination centres throughout the film which have been observed to be most densely located at regions near the grain boundaries. Here, we introduce aluminium acetylacetonate to the perovskite precursor solution, which improves the crystal quality by reducing the microstrain in the polycrystalline film. At the same time, we achieve a reduction in the non-radiative recombination rate, a remarkable improvement in the photoluminescence quantum efficiency (PLQE) and a reduction in the electronic disorder deduced from an Urbach energy of only 12.6 meV in complete devices. As a result, we demonstrate a PCE of 19.1% with negligible hysteresis in planar heterojunction solar cells comprising all organic p and n-type charge collection layers. Our work shows that an additional level of control of perovskite thin film quality is possible via impurity cation doping, and further demonstrates the continuing importance of improving the electronic quality of the perovskite absorber and the nature of the heterojunctions to further improve the solar cell performance.

The ability to measure microsecond- and nanosecond-scale local dynamics below the diffraction limit with widely available atomic force microscopy hardware would enable new scientific studies in fields ranging from biology to semiconductor physics. However, commercially available scanning-probe instruments typically offer the ability to measure dynamics only on time scales of milliseconds to seconds. Here, we describe in detail the implementation of fast time-resolved electrostatic force microscopy using an oscillating cantilever as a means to measure fast local dynamics following a perturbation to a sample. We show how the phase of the oscillating cantilever relative to the perturbation event is critical to achieving reliable sub-cycle time resolution. We explore how noise affects the achievable time resolution and present empirical guidelines for reducing noise and optimizing experimental parameters. Specifically, we show that reducing the noise on the cantilever by using photothermal excitation instead of piezoacoustic excitation further improves time resolution. We demonstrate the discrimination of signal rise times with time constants as fast as 10 ns, and simultaneous data acquisition and analysis for dramatically improved image acquisition times.

Plasmonic metal nanoparticles have been used to enhance the performance of thin-film devices such as organic photovoltaics based on polymer/fullerene blends. We show that silver nanoprisms accumulate long-lived negative charges when they are in contact with a photoexcited bulk heterojunction blend composed of poly(3-hexylthiophene)/phenyl-C61-butyric acid methyl ester (P3HT/PCBM). We report both the charge modulation and electroabsorption spectra of silver nanoprisms in solid-state devices and compare these spectra with the photoinduced absorption spectra of P3HT/PCBM blends containing silver nanoprisms. We assign a previously unidentified peak in the photoinduced absorption spectra to the presence of photoinduced electrons on the silver nanoprisms. We show that coating the nanoprisms with a 2.5 nm thick insulating layer can completely inhibit this charging. These results may inform methods for limiting metal-mediated losses in plasmonic solar cells.

We use photoinduced absorption (PIA) spectroscopy to study charge generation and recombination in a series of bulk heterojunction blends relevant to organic and hybrid solar cells. We compare both organic and inorganic electron acceptors, including the fullerene, phenyl-C61-butyric acid methyl ester (PCBM), oxides such as ZnO and TiO2, and colloidal quantum dots, including CdSe and PbS nanocrystals. We use a variety of donor host polymers, including poly(3-hexylthiophene) (P3HT), poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-p-phenylenevinylene] (MDMO-PPV), and poly(2,3-bis(2-hexyldecyl)quinoxaline-5,8-diyl-alt-N-(2-hexyldecyl)-dithieno[3,2-b:2′,3′-d]pyrrole) (PDTPQx-HD). In every case, we measure longer average carrier lifetimes in blends with the inorganic acceptors as compared to blends with PCBM. The PIA data also suggest that the internal electric fields are attenuated in the inorganic blends, consistent with increased screening between the photogenerated carriers due to the higher dielectric constants o...

Coupled plasmonic/chromophore systems are of interest in applications ranging from fluorescent biosensors to solar photovoltaics and photoelectrochemical cells because near-field coupling to metal nanostructures can dramatically alter the optical performance of nearby materials. We show that CdSe quantum dots (QDs) near single silver nanoprisms can exhibit photoluminescence lifetimes and quantum yields that depend on the excitation wavelength, in apparent violation of the Kasha-Vavilov rule. We attribute the variation in QD lifetime with excitation wavelength to the wavelength-dependent coupling of higher-order plasmon modes to different spatial subpopulations of nearby QDs. At the QD emission wavelength, these subpopulations are coupled to far-field radiation with varying efficiency by the nanoprism dipolar resonance. These results offer an easily accessible new route to design metachromophores with tailored optical properties.

We study local photooxidation and trap formation in all-polymer bulk-heterojunction organic photovoltaics (OPVs) using both time-resolved electrostatic force microscopy (trEFM) and conventional scanning Kelvin probe microscopy (SKPM). We create electron-trapping defects at known locations by locally photooxidizing blends of poly[(9,9′-dioctylfluorene-alt-(bis(N,N′-(4-butylphenyl))-bis(N,N′-phenyl-1,4-phenylenediamine)] and poly[9,9′-dioctylfluorene-alt-1,4-benzothiadiazole]. We then compare the local surface photovoltage shifts measured via SKPM and the changes in local photoinduced charging rates measured via trEFM with changes in the performance of macroscopic photodiodes that have been exposed to similar photooxidation. We find that the trEFM charging rate images can identify local photooxidation and trap formation with much better sensitivity than conventional SKPM images. In addition, the changes in the trEFM charging rates correlate well with the external quantum efficiencies of the macroscopic phot...

Organic solar cells hold promise as an economical means of harvesting solar energy due to their ease of production and processing. However, the efficiency of such organic photovoltaic (OPV) devices is currently below that required for widespread adoption. The efficiency of an OPV is inextricably linked to its nanoscale morphology. High-resolution metrology can play a key role in the discovery and optimization of new organic semiconductors in the lab, as well as assist the transition of OPVs from the lab to mass production. We review the instrumental issues associated with the application of scanning probe microscopy (SPM) techniques such as photoconductive atomic force microscopy and time-resolved electrostatic force microscopy that have been shown to be useful in the study of nanostructured organic solar cells. These techniques offer unique insight into the underlying heterogeneity of OPV devices and provide a nanoscale basis for understanding how morphology directly affects OPV operation. Finally, we discuss opportunities for further improvements in scanning probe microscopy to contribute to OPV development.

The near-field effects of plasmonic optical antennas are being explored in applications ranging from biosensors to solar cells. We demonstrate that photoluminescence emission enhancement from CdSe quantum dots (QDs) can be obtained in the absence of any excitation enhancement near single silver nanoprisms. The spectral dependence of the radiative and nonradiative decay rate of the QDs closely follows the silver nanoparticle plasmon scattering spectrum. Using both experiment and theory we show that, in the absence of excitation enhancement, the ratio of radiative to nonradiative decay rate enhancement is proportional to the silver nanoparticle scattering efficiency. These results provide guidelines both for separating excitation and emission enhancement effects in sensing and device applications and for tailoring emission enhancement effects using plasmonic nanostructures.

Organic semiconductors are an alternative to inorganic materials in solar cell applications. While the efficiencies of organic photovoltaics (OPVs) have been improving rapidly, they are currently below that required for widespread power generation. OPV performance is sensitive to the nanoscale texture, or film morphology, in the photovoltaic active layer, particularly in bulk heterojunction (BHJ) devices, and characterizing morphology across many length scales is currently a major experimental challenge. Here, we discuss several different experimental approaches for characterizing morphology in BHJ systems. These include techniques ranging from X-ray diffraction and spectroscopy to electron microscopy and scanning probe microscopy. These methods provide complementary information to guide future materials design and device optimization efforts.

We use photoinduced absorption spectroscopy to measure long-lived photogenerated charge carriers in optically thin donor/acceptor conjugated polymer blend films near plasmon-resonant silver nanoprisms. We measure up to 3 times more charge generation, as judged by the magnitude of the polaron absorption signal, in 35 nm thin blend films of poly(3-hexylthiophene)/phenyl-C(61)-butyric acid methyl ester on top of films of silver nanoprisms (approximately 40-100 nm edge length). We find that the polaron yields increase linearly with the total sample extinction. These excitation enhancements could in principle be used to increase photocurrents in thin organic solar cells.

Polymer- and small-molecule-based organic electronic devices are being developed for applications including electroluminescent displays, transistors, and solar cells due to the promise of low-cost manufacturing. It has become clear that these materials exhibit nanoscale heterogeneities in their optical and electrical properties that affect device performance, and that this nanoscale structure varies as a function of film processing and device-fabrication conditions. Thus, there is a need for high-resolution measurements that directly correlate both electronic and optical properties with local film structure in organic semiconductor films. In this article, we highlight the use of electrical scanning probe microscopy techniques, such as conductive atomic force microscopy (c-AFM), electrostatic force microscopy (EFM), scanning Kelvin probe microscopy (SKPM), and similar variants to elucidate charge injection/extraction, transport, trapping, and generation/recombination in organic devices. We discuss the use of these tools to probe device structures ranging from light-emitting diodes (LEDs) and thin-film transistors (TFT), to light-emitting electrochemical cells (LECs) and organic photovoltaics.

Poly(3,4-ethylenedioxythiophene) polystyrenesulfonate (PEDOT:PSS) is widely used as a semitransparent anode layer in organic light-emitting diodes and polymer photovoltaics. We use conductive atomic force microscopy (c-AFM) to map the electronic properties of PEDOT:PSS films during a variety of processing steps to better explain how the observed changes in macroscopic electronic properties arise from local changes in charge transport. We observe only small conductive regions, ∼20 nm in size, surrounded by more insulating regions in all of the films studied. We confirm that these features dominate the c-AFM measurements, independent of the substrate. We observe a marked increase in the density of the conductive regions with increasing annealing times, increasing applied bias (independent of polarity), and decreasing PSS concentration (achieved by altering PEDOT:PSS grades). We also find an increase in current flow following a chlorobenzene wash, suggesting the solvents used in processing the active semicon...

We combined the synthesis of new polymers and organic-inorganic hybrid materials with new experimental characterization tools to investigate bulk heterojunction (BHJ) polymer solar cells and hybrid organic-inorganic solar cells during the 2007-2010 period (phase I) of this project. We showed that the bulk morphology of polymer/fullerene blend solar cells could be controlled by using either self-assembled polymer semiconductor nanowires or diblock poly(3-alkylthiophenes) as the light-absorbing and hole transport component. We developed new characterization tools in-house, including photoinduced absorption (PIA) spectroscopy, time-resolved electrostatic force microscopy (TR-EFM) and conductive and photoconductive atomic force microscopy (c-AFM and pc-AFM), and used them to investigate charge transfer and recombination dynamics in polymer/fullerene BHJ solar cells, hybrid polymer-nanocrystal (PbSe) devices, and dye-sensitized solar cells (DSSCs); we thus showed in detail how the bulk photovoltaic properties are connected to the nanoscale structure of the BHJ polymer solar cells. We created various oxide semiconductor (ZnO, TiO2) nanostructures by solution processing routes, including hierarchical aggregates and nanorods/nanotubes, and showed that the nanostructured photoanodes resulted in substantially enhanced light-harvesting and charge transport, leading to enhanced power conversion efficiency of dye-sensitized solar cells.

We study the homogeneous line width of the plasmon resonance scattering peaks of anisotropic silver nanoparticles using single particle darkfield scattering spectroscopy. We correlate the scattering spectra with scanning electron microscopy images of the particles to investigate the effects of nanoparticle size on the plasmon line width. We observe that the scattering line widths of silver nanoprisms increase both as the particle volume increases and as the plasmon resonance energy increases. We attribute the size dependence to radiation damping and compare our data both to simple dipole-limit approximations for the radiative and nonradiative decay rates and with finite-difference time-domain (FDTD) calculations of the scattering line width. We find the data and calculations in good agreement for an appropriate choice of the bulk optical constants of silver and find that the dipole-limit approximations capture the observed size and energy-dependence of the plasmon line widths.

We examine the effects of surface ligand exchange on the performance of hybrid organic/inorganic light emitting diodes (LEDs) that use colloidal nanocrystal quantum dots as emissive centers. Using a series of primary alkylamines with different alkane chain lengths, we exchange the native surface ligands on a series of CdSe/CdZnS/ZnS core/shell/shell nanocrystal quantum dots and compare the differences in photoluminescence and electroluminescence efficiency of the emissive quantum dot layer. We fabricate LEDs made with octadecylamine-, octylamine-, and butylamine-exchanged quantum dots. We find that the differences in electroluminescence efficiency of the devices are not always proportional to the photoluminescence quantum efficiency of the quantum dots. We discuss this trend both in terms of the competing needs of high photoluminescence efficiency and good charge injection and energy transfer, and also in terms of the different processability and film morphology arising from the use of nanoparticles passivated with shorter ligands.

Passive grain boundaries (GBs) are essential for polycrystalline solar cells to reach high efficiency. However, the GBs in Cu2ZnSn(S,Se)4 have less favorable defect chemistry compared to CuInGaSe2. Here, using scanning probe microscopy we show that lithium doping of Cu2ZnSn(S,Se)4 changes the polarity of the electric field at the GB such that minority carrier electrons are repelled from the GB. Solar cells with lithium-doping show improved performance and yield a new efficiency record of 11.8% for hydrazine-free solution-processed Cu2ZnSn(S,Se)4. We propose that lithium competes for copper vacancies (forming benign isoelectronic LiCu defects) decreasing the concentration of ZnCu donors and competes for zinc vacancies (forming a LiZn acceptor that is likely shallower than CuZn). Both phenomena may explain the order of magnitude increase in conductivity. Further, the effects of lithium doping reported here establish that extrinsic species are able to alter the nanoscale electric fields near the GBs in Cu2ZnSn(S,Se)4. This will be essential for this low-cost Earth abundant element semiconductor to achieve efficiencies that compete with CuInGaSe2 and CdTe.