Molecular-Scale Characterization of Photoinduced Charge Separation in Mixed-Dimensional InSe-Organic van der Waals Heterostructures.

Layered indium selenide (InSe) is an emerging two-dimensional semiconductor that has shown significant promise for high-performance transistors and photodetectors. The range of optoelectronic applications for InSe can potentially be broadened by forming mixed-dimensional van der Waals heterostructures with zero-dimensional molecular systems that are widely employed in organic electronics and photovoltaics. Here, we report the spatially resolved investigation of photoinduced charge separation between InSe and two molecules (C70 and C8-BTBT) using scanning tunneling microscopy combined with laser illumination. We experimentally and computationally show that InSe forms type-II and type-I heterojunctions with C70 and C8-BTBT, respectively, due to an interplay of charge transfer and dielectric screening at the interface. Laser-excited scanning tunneling spectroscopy reveals a ∼0.25 eV decrease in the energy of the lowest unoccupied molecular orbital of C70 with optical illumination. Furthermore, photoluminescence spectroscopy and Kelvin probe force microscopy indicate that electron transfer from InSe to C70 in the type-II heterojunction induces a photovoltage that quantitatively matches the observed downshift in the tunneling spectra. In contrast, no significant changes are observed upon optical illumination in the type-I heterojunction formed between InSe and C8-BTBT. Density functional theory calculations further show that, despite the weak coupling between the molecular species and InSe, the band alignment of these mixed-dimensional heterostructures strongly differs from the one suggested by the ionization potential and electronic affinities of the isolated components. Self-energy-corrected density functional theory indicates that these effects are the result of the combination of charge redistribution at the interface and heterogeneous dielectric screening of the electron-electron interactions in the heterostructure. In addition to providing specific insight for mixed-dimensional InSe-organic van der Waals heterostructures, this work establishes a general experimental methodology for studying localized charge transfer at the molecular scale that is applicable to other photoactive nanoscale systems.

Hot Carrier and Surface Recombination Dynamics in Layered InSe Crystals.

Layered indium selenide (InSe) is a van der Waals solid that has emerged as a promising material for high-performance ultrathin solar cells. The optoelectronic parameters that are critical to photoconversion efficiencies, such as hot carrier lifetime and surface recombination velocity, are however largely unexplored in InSe. Here, these key photophysical properties of layered InSe are measured with femtosecond transient reflection spectroscopy. The hot carrier cooling process is found to occur through phonon scattering. The surface recombination velocity and ambipolar diffusion coefficient are extracted from fits to the pump energy-dependent transient reflection kinetics using a free carrier diffusion model. The extracted surface recombination velocity is approximately an order of magnitude larger than that for methylammonium lead-iodide perovskites, suggesting that surface recombination is a principal source of photocarrier loss in InSe. The extracted ambipolar diffusion coefficient is consistent with previously reported values of InSe carrier mobility.

Mechanisms of Ultrafast Charge Separation in a PTB7/Monolayer MoS2 van der Waals Heterojunction.

Mixed-dimensional van der Waals heterojunctions comprising polymer and two-dimensional (2D) semiconductors have many characteristics of an ideal charge separation interface for optoelectronic and photonic applications. However, the photoelectron dynamics at polymer-2D semiconductor heterojunction interfaces are currently not sufficiently understood to guide the optimization of devices for these applications. This Letter reports a systematic exploration of the time-dependent photophysical processes that occur upon photoexcitation of a type-II heterojunction between the polymer PTB7 and monolayer MoS2. In particular, photoinduced electron transfer from PTB7 to electronically hot states of MoS2 occurs in less than 250 fs. This process is followed by a 1-5 ps exciton diffusion-limited electron transfer from PTB7 to MoS2 and a sub-3 ps photoinduced hole transfer from MoS2 to PTB7. The equilibrium between excitons and polaron pairs in PTB7 determines the charge separation yield, whereas the 3-4 ns lifetime of photogenerated carriers is probably limited by MoS2 defects.

High-Performance Solution-Processed Non-Fullerene Organic Solar Cells Based on Selenophene-Containing Perylene Bisimide Acceptor.

Non-fullerene acceptors have recently attracted tremendous interest because of their potential as alternatives to fullerene derivatives in bulk heterojunction organic solar cells. However, the power conversion efficiencies (PCEs) have lagged far behind those of the polymer/fullerene system, mainly because of the low fill factor (FF) and photocurrent. Here we report a novel perylene bisimide (PBI) acceptor, SdiPBI-Se, in which selenium atoms were introduced into the perylene core. With a well-established wide-band-gap polymer (PDBT-T1) as the donor, a high efficiency of 8.4% with an unprecedented high FF of 70.2% is achieved for solution-processed non-fullerene organic solar cells. Efficient photon absorption, high and balanced charge carrier mobility, and ultrafast charge generation processes in PDBT-T1:SdiPBI-Se films account for the high photovoltaic performance. Our results suggest that non-fullerene acceptors have enormous potential to rival or even surpass the performance of their fullerene counterparts.

Ultrafast charge transfer in operating bulk heterojunction solar cells.

The ultrafast charge generation process in organic solar cell devices is investigated by transient reflection spectroscopy on five state-of-the-art bulk heterojunction systems. The charge generation process in operating devices is found to be a combination of an ultrafast generation mechanism over several hundred femto-seconds and a slow process from pico-seconds to nanoseconds, limited by exciton diffusion dynamics. In addition, the lack of electric field dependence in the charge dynamics rules out geminate recombination as an important loss mechanism.

The influence of binary processing additives on the performance of polymer solar cells.

In this study, we report the investigation of the influence of binary processing additives, 1,8-octanedithiol (ODT) and 1-chloronaphthalene (CN) on the performance of polymer solar cells (PSCs). It was found that the power conversion efficiency (PCE) can be enhanced to 8.55% from the PSCs processed with binary processing additives as compared with ∼6.50% from the PSCs processed with either ODT or CN processing additives. With binary processing additives, the crystallinity of the electron donor 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]], was elevated, which in turn facilitated charge transport within the bulk heterojunction (BHJ) layer, resulting in a high short-circuit current and large fill factor. By photophysical studies, we further found that the high PCE is majorly attributed to the minimized nongeminate recombination by controlling the kinetic film morphologies of the BHJ composite by binary solvent processing additives.

Highly efficient inverted polymer solar cells based on a cross-linkable water-/alcohol-soluble conjugated polymer interlayer.

A cross-linkable water/alcohol soluble conjugated polymer (WSCP) material poly[9,9-bis(6'-(N,N-diethylamino)propyl)-fluorene-alt-9,9-bis(3-ethyl(oxetane-3-ethyloxy)-hexyl) fluorene] (PFN-OX) was designed. The cross-linkable nature of PFN-OX is good for fabricating inverted polymer solar cells (PSCs) with well-defined interface and investigating the detailed working mechanism of high-efficiency inverted PSCs based on poly[4,8-bis(2-ethylhexyloxyl)benzo[1,2-b:4,5-b']dithio-phene-2,6-diyl-alt-ethylhexyl-3-fluorothithieno[3,4-b]thiophene-2-carboxylate-4,6-diyl] (PTB7) and (6,6)-phenyl-C71-butyric acid methyl ester (PC71BM) blend active layer. The detailed working mechanism of WSCP materials in high-efficiency PSCs were studied and can be summarized into the following three effects: a) PFN-OX tunes cathode work function to enhance open-circuit voltage (Voc); b) PFN-OX dopes PC71BM at interface to facilitate electron extraction; and c) PFN-OX extracts electrons and blocks holes to enhance fill factor (FF). On the basis of this understanding, the hole-blocking function of the PFN-OX interlayer was further improved with addition of a ZnO layer between ITO and PFN-OX, which led to inverted PSCs with a power conversion efficiency of 9.28% and fill factor high up to 74.4%.

Ultrafast Charge Generation in an Organic Bilayer Film.

The dynamics of charge generation in a high performing molecular photovoltaic system, p-SIDT(FBTTh2)2 (see Figure 1 ) is studied with transient absorption. The optimized bulk heterojunction material shows behavior observed in many other systems; the majority of charges are generated at short time scales (<150 fs), and a slower contribution from incoherently diffusing excitons is observed at low pump fluence. In a separate experiment, the role of bulk heterojunction material morphology on the process of ultrafast charge generation was investigated with bilayers made with solution processed donor molecules on a photopolymerized C60 layer. The majority of carriers are again produced at short time scales, ruling out the idea that subpicosecond charge generation can be understood wholly in terms of localized excitons. We evaluate possible causes of this behavior and propose that the excited state is highly delocalized on short time scales, providing ample probability density at the charge generating interface.

Recent advances in water/alcohol-soluble π-conjugated materials: new materials and growing applications in solar cells.

Water/alcohol-soluble conjugated polymers (WSCPs) and small molecules (WSCSs) are materials that can be processed from water or other polar solvents. They provide good opportunities to fabricate multilayer organic optoelectronic devices without interface mixing by solution processing, and exhibit a promising interface modification ability for metal or metal oxide electrodes to greatly enhance the device performance of solar cells. Moreover, owing to their intriguing processability, WSCPs and WSCSs have great potential for applying environmentally friendly processing technologies to fabricate solar cells. In this review, the authors give an overview of recent developments in WSCPs and WSCSs, including their molecular design, material synthesis, functional principles and application as interface modification layers and photoactive components in emerging photovoltaic technologies such as organic/polymer solar cells, organic-inorganic hybrid solar cells and dye-sensitised solar cells.

Electrochemical route to fabricate film-like conjugated microporous polymers and application for organic electronics.

Film-like conjugated microporous polymers (CMPs) are fabricated by the novel strategy of carbazole-based electropolymerization. The CMP film storing a mass of counterions acting as an anode interlayer provides a significant power-conversion efficiency of 7.56% in polymer solar cells and 20.7 cd A(-1) in polymer light-emitting diodes, demonstrating its universality and potential as an electrode interlayer in organic electronics.

Ordered polymer nanofibers enhance output brightness in bilayer light-emitting field-effect transistors.

Polymer light emitting field effect transistors are a class of light emitting devices that reveal interesting device physics. Device performance can be directly correlated to the most fundamental polymer science. Control over surface properties of the transistor dielectric can dramatically change the polymer morphology, introducing ordered phase. Electronic properties such as carrier mobility and injection efficiency on the interface can be promoted by ordered nanofibers in the polymer. Moreover, by controlling space charge in the polymer interface, the recombination zone can be spatially extended and thereby enhance the optical output.

In situ electrochemical deposition and doping of C60 films applied to high-performance inverted organic photovoltaics.

Novel C(60)-based cross-linked films formed by electrodeposition are produced and used as the electron-collection layer in inverted polymer solar cells (PSCs). The electrodeposited films exhibit a low work function of 4.2 eV and the PSCs perform well, with power conversion efficiencies of up to 6.31%. This new kind of electrodeposited film affords more opportunities to develop modified electrodes with a low work function.

Solution-processed bulk heterojunction photovoltaic cells from gradient pi-conjugated thienylene vinylene dendrimers.

A series of gradient pi-conjugated dendrimers and their corresponding models based on 5,5,10,10,15,15-hexahexyltruxene moieties as nodes and oligo(thienylene vinylene) (OTVs) units with different lengths as branching arms are synthesized in good yields through Wittig-Horner reactions. All new compounds are fully characterized by (1)H and (13)C NMR spectroscopy, elemental analysis, and MALDI-TOF MS or ESI-MS. Investigation of their photophysical properties reveals that the gradient dendritic scaffold not only results in a higher molar absorption coefficient and broader absorption region than those of their corresponding model compounds, but also improves the PL quantum yields relative to the corresponding OTVs. The suitable HOMO and LUMO levels as well as excellent film forming properties make these molecules potential candidates for organic solar cells. Solution-processed bulk heterojunction solar cells using these dendrimers as donor and [6,6]-phenyl-C(61) butyric acid methyl ester as acceptor are prepared and tested. The power conversion efficiency of the devices based on G0-4-2 is 0.40 % under illumination of air mass 1.5 and 100 mW cm(-2). This is the highest record value for OTV-based materials to date. Although the absorption band of dendrimer G0-4-2 is much narrower than that of poly(3-hexylthienylene vinylene) (P3HTV), the efficiency of its solar cell device is almost twice that of the device based on P3HTV. This result shows clearly the advantage of gradient dendritic structures as active materials for photovoltaic cells.