P-type Molecules Research Articles

(Invited) Non-Polymer Organic Solar Cells: Microscopic Phonon Control to Suppress Non-Radiative Voltage Loss Via Charge-Separated State

Recent remarkable developments on non-fullerene solar cells have reached photoelectric conversion efficiency (PCE) of 18% by tuning the band energy levels in small molecular acceptors. In this respect, development of the small donor molecule is essential. Although several methods have been applied to elucidate the elementary processes of charge carriers in BHJ-OSCs, mechanistic details of non-radiative voltage loss are poorly understood originating from interfacial heterogeneous electron-hole pairs.A recent time-resolved electron paramagnetic resonance (TREPR) study suggested an impact of the recombination process generating the triplet excitons in non-fullerene acceptor solar cells,[1] while geometries and motions of charge-separated (CS) states were elucidated in details by electron spin polarization (ESP).[2-4] Furthermore, although effects of low-frequency nuclear motions (phonon) on the photocarrier generations have been discussed in several literatures,[2,5,6] microscopic origin of the phonon playing a role on the OPV performance is unclear including impact of the phonon assist on primary recombination processes. Here we systematically investigated mechanisms of solar cell performance with diketopyrrolopyrrole (DPP)–tetrabenzoporphyrin (BP) conjugates of C4-DPP–H2BP and C4-DPP–ZnBP where C4 represents butyl group substituted at the DPP unit as small p-type molecules in non-polymer organic solar cells (OSC) with an acceptor of [6,6]-phenyl-C61-buthylic acid methyl ester. We clarified microscopic origins of the photocarrier caused by phonon-assisted 1D electron-hole dissociations at donor:acceptor interface. Using a time-resolved electron paramagnetic resonance, we have characterized controlled charge-recombination by manipulating disorders in π-π donor stacking, ensuring carrier transport through face-on molecular conformations to suppress non-radiative voltage loss via capturing specific interfacial radical pairs separated by 1.8 nm in bulk-heterojunction solar cells. While disordered lattice motions from the π-π stackings via zinc ligation are essential to enhance the entropy for charge-dissociations at the interface, too much ordered crystallinity causes backscattering phonon to reduce OSC performances.

Nonpolymer Organic Solar Cells: Microscopic Phonon Control to Suppress Nonradiative Voltage Loss via Charge-Separated State.

Recent remarkable developments on nonfullerene solar cells have reached a photoelectric conversion efficiency (PCE) of 18% by tuning the band energy levels in small molecular acceptors. In this regard, understanding the impact of small donor molecules on nonpolymer solar cells is essential. Here, we systematically investigated mechanisms of solar cell performance using diketopyrrolopyrrole (DPP)-tetrabenzoporphyrin (BP) conjugates of C4-DPP-H2BP and C4-DPP-ZnBP, where C4 represents the butyl group substituted at the DPP unit as small p-type molecules, while an acceptor of [6,6]-phenyl-C61-buthylic acid methyl ester is employed. We clarified the microscopic origins of the photocarrier caused by phonon-assisted one-dimensional (1D) electron-hole dissociations at the donor-acceptor interface. Using a time-resolved electron paramagnetic resonance, we have characterized controlled charge-recombination by manipulating disorders in π-π donor stacking. This ensures carrier transport through stacking molecular conformations to suppress nonradiative voltage loss capturing specific interfacial radical pairs separated by 1.8 nm in bulk-heterojunction solar cells. We show that, while disordered lattice motions by the π-π stackings via zinc ligation are essential to enhance the entropy for charge dissociations at the interface, too much ordered crystallinity causes the backscattering phonon to reduce the open-circuit voltage by geminate charge-recombination.

Enhancement in the inherent photostability of small molecule-based BHJ device by molecular architecturing

Three small molecules of D-A-D architecture were synthesized with benzothiadiazole as basic acceptor unit. It has been showed previously that the fluorination on the benzothiadiazole acceptor moiety and incorporation of fused thienothiophene spacer have increased the photovoltaic properties [1]. Hence the same strategies have applied here on small molecules to study both the photovoltaic and to tailor the inherent photostability of the material. The first small molecule, TBT (4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole) was designed with benzothiadiazole- acceptor, and thiophene moiety as the donor unit. The effect of fluorine substitution (TDFBT- 5,6-difluoro-4,7-di(thiophen-2-yl) benzo [c][1,2,5]thiadiazole) and incorporation of fused donor moiety (TTBTT- 4,7-bis(thieno[3,2-b] thiophene-2-yl) benzo [c][1,2,5]thiadiazole) on the optical electrochemical, photovoltaic and photostability of the molecules were evaluated. The photostability study on the structurally modified molecules and blend with PCBM shows promising results. Structural modifications of the p-type molecules exhibit an improved photostability of neat films by significantly. The photostability of the active layer comprised of small molecule- PCBM blend has increased by >13.8 times on incorporation of fused thienothiophene (TTBTT) and by a factor of >5.2 on fluorination of acceptor (TDFBT). The results suggest that the structural modification p-type materials not only improve the optical, electrochemical and photovoltaic properties, but also the photostability of BHJ active layer. A careful selection of p-type materials with increased electron affinity with a sensible balancing with ambient stability and band edge matching with that of the n-type material would lead to stable photovoltaic devices.

Interfacial Energy Band Alignment Enables the Reduction of Potential Loss for Hole-Conductor-Free Printable Mesoscopic Perovskite Solar Cells.

Perovskite solar cells (PSCs) have achieved high efficiencies with diversified device architectures. In particular, printable mesoscopic PSC has attracted intensive research attention due to its simple fabrication process and superior stability. However, in the absence of hole conductors, the unfavorable energy band alignment between the perovskite and the carbon electrode usually leads to the reduction of device performance, especially the open-circuit voltage (VOC). Here, a p-type molecule, 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ), is utilized to post-treat the perovskite/carbon interface, which benefits the charge transfer and suppresses the charge recombination within the device. As a result, the post-treated device delivers a power conversion efficiency of 18.05% with an enhanced VOC of 1044 mV. This work provides a facile method for tuning the interfacial energy band alignment and improving performance of printable mesoscopic PSCs.

Molecule functionalization to facilitate electrocatalytic oxygen reduction on graphdiyne

Chemical doping is verified to be a promising strategy to regulate local electron distribution and further promote the poor intrinsic catalytic activity of graphdiyne. However, the current doping approach still faces problems such as precise doping for creating active sites and the destruction of graphdiyne skeleton calling for high temperature. Here, we achieved charge redistribution on graphdiyne surface through molecule functionalization. A p-type molecule–F4TCNQ (2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane) was introduced and the site-defined functionalization was accomplished. Theoretical calculations showed that the charge transfer ability is improved and graphdiyne becomes positively charged. The oxygen reduction electrocatalysis was conducted as a proof of principle, where the electronic states of sp hybridized C active site was tuned toward favorable reaction intermediates’ adsorption. Such work from both theoretical prediction and experimental validation, found that molecule functionalization is effective to promote the electrocatalytic oxygen reduction, which creates new possibilities for graphdiyne’s applications in different electrochemical reactions.

Green-Light-Selective Organic Photodiodes with High Detectivity for CMOS Color Image Sensors.

Stacked structures employing wavelength-selective organic photodiodes (OPDs) have been studied as promising alternatives to the conventional Si-based image sensors because of their color constancy. Herein, novel donor (D)-π-acceptor (A) molecules are designed, synthesized, and characterized as green-light-selective absorbers for application in organic-on-Si hybrid complementary metal-oxide-semiconductor (CMOS) color image sensors. The p-type molecules, combined with two fused-type heterocyclic donors and an electron-accepting unit, exhibit cyanine-like properties that are characterized by intense and sharp absorption. This molecular design leads to improved absorption properties, thermal stability, and higher photoelectric conversion compared to those of a molecular design based on a nonfused ring. A maximum external quantum efficiency of 66% (λmax = 550 nm) and high specific detectivity (D*) of 8 × 1013 cm Hz1/2/W are achieved in an OPD consisting of a bulk heterojunction blend with two transparent electrodes on both sides. Finally, the green-light-detection capability of the narrow-band green-selective OPD is demonstrated by the optical simulation of an organic-on-Si hybrid, stacked-type, full-color photodetector comprising the green-light-selective OPD and a bottom Si photodiode with only blue and red color filters. Based on this molecular design, further optimization of the OPDs can allow the development of various optoelectronic sensors including 3D-stacked image sensors with enhanced sensitivities to replace the conventional Si-based CMOS image sensors.

F4-TCNQ doped strategy of nickel oxide as high-efficient hole transporting materials for invert perovskite solar cell

Nickel oxide (NiOx) as a promising hole transport material based on inverted perovskite solar cells has advantages of low cost, high photo stability and small hysteresis. However, low carrier transport capacity and rough surface morphology leading to serious interface recombination are limited the improvement of photoelectric conversion efficiency. In this work, 2,3,5,6-Tetrafluoro-7,7,8,8 tetracyanoquinodimethane (F4-TCNQ), a high temperature resistant small molecule material, was doped into the NiOx hole transport layer (HTL) by sol-gel method in order to achieve band adjustment and optimize the surface contact performance of perovskite substrate. As a result, a higher recombination resistance (18804Ω) is obtained comparing the reference device (5349Ω). And the conversion efficiency of the optimized device with doping transport layer increased to 15.7%, which was 16.2% higher than the undoped transport layer. The charge separation and hole mobility derived from the perovskite/HTL interface are enhanced. The rational design of the p-type molecule doped NiOx substrate strategy provides desired solution for future development of inverted based perovskite solar cells.

Room-Temperature Operation of a p-Type Molecular Spin Photovoltaic Device on a Transparent Substrate.

The coupling of diverse degrees of freedom opens the door to physical effects that go beyond each of them individually, making multifunctionality a much sought-after attribute for high-performance devices. Here, the multifunctional operation of a single-layer p-type organic device, displaying both spin transport and photovoltaic effect at the room temperature on a transparent substrate, is shown. The generated photovoltage is almost three times larger than the applied bias to the device which facilitates the modulation of the magnetic response of the device with both bias and light. The device shows an increase in power conversion efficiency under magnetic field, an ability to invert the current with magnetic field and under certain conditions it can act as a spin photodetector with zero power consumption in the standby mode. The room-temperature exploitation of the interplay among light, bias, and magnetic field in the single device with a p-type molecule opens a way toward the development of efficient high-performance spin photovoltaic cells.

Green-light-selective organic photodiodes for full-color imaging.

In this work, organic photodiodes (OPDs) based on two newly synthesized p-type dipolar small molecules are reported for application to green-light-selective OPDs. In order to reduce the blue-color absorption induced by the use of C60 as the n-type material in a bulk heterojunction (BHJ), the electron donor:electron acceptor composition ratio is tuned in the BHJ. With this light manipulation approach, the blue-wavelength external quantum efficiency (EQE) is minimized to 18% after reducing the C60 concentration in the center part of the BHJ. The two p-type molecules get a cyanine-like character with intense and sharp absorption in the green color by adjusting the strength of their donating and accepting parts and by choosing a selenophene unit as a π-linker. When combined to C60, the green-wavelength EQE reaches 70% in a complete device composed of two transparent electrodes. Finally, the optical simulation shows the good color-balance performance of hybrid full-color image sensor without an additional filter by using the developed green OPD as the top-layer in stacked device architecture.

Electrostatic Polarization Energies of Charge Carriers in Organic Molecular Crystals: A Comparative Study with Explicit State-Specific Atomic Polarizability Based AMOEBA Force Field and Implicit Solvent Method.

The electrostatic polarization plays an important role in determining the energy levels of charge carriers in organic solids, which is controlled by the atomic polarizability in AMOEBA polarizable force field. QTAIM-based space partitioning of molecular polarizability is utilized to uniformly parametrize the state-specific atomic polarizability (SSAP) of π-conjugated organic small molecules to avoid fitting molecular polarizability of some artificial training set. Herein, the SSAPs are applied to explicitly extrapolate the electrostatic polarization energy ( E pol) of the charge carriers of nine π-conjugated organic crystals including six p-type transfer materials, oligoacenes and TIPS-substituted oligoacenes, and three n-type transfer materials, F-substituted oligoacenes and TCNQ. Our results demonstrate that the electrostatic polarization energies of the hole carrier ( E+pol) are smaller than that of the electron carrier ( E-pol) for p-type molecules while E+pol are larger than E-pol for n-type molecules. SSAP-based E pol values of oligoacenes behave as a nearly unvaried feature with the increase of conjugation length which is similar to implicit polarizable continuum model (PCM) results, while E pol obtained from the default atomic polarizability behaves with a notable decrease. Implicit PCM can correctly capture most of electrostatic polarization of ions in bulk system although it slightly underestimates the gap between the electrostatic polarization of hole and electron carriers in oligoacene crystals. Our results demonstrate that this unified parametrized SSAP provides a reliable and cheap tool to estimate the energy landscape of charge carriers in condensed-phase organic solids.

Bright monolayer tungsten disulfide via exciton and trion chemical modulations

Atomically thin transition metal dichalcogenides (TMDCs) with exceptional electrical and optical properties have drawn tremendous attention for use in novel optoelectronic applications as photodetectors, transistors, light emitters, etc. However, electron bound trions formed through the combination of neutral excitons and electrons significantly decrease the photoluminescence (PL) efficiency of TMDCs. In this study, we report a simple yet efficient chemical doping strategy to modulate the optical properties of monolayer tungsten disulfide (WS2). As a demonstrative example, a chemically doped monolayer of WS2 exhibits remarkable PL enhancement of about one order of magnitude higher than that of pristine WS2. This outstanding PL enhancement is attributed to the fact that excess electrons, which promote the formation of electron-bound trions, are reduced in number through charge transfer from WS2 to the chemical dopant. Furthermore, an improved degree of circular polarization from ∼9.0% to ∼41.5% was also observed in the chemically doped WS2 monolayer. This work describes a feasible strategy to manipulate the optical properties of TMDCs via exciton modulation, making TMDCs promising candidates for versatile semiconductor-based photonic devices.

Charge transport network dynamics in molecular aggregates

Due to the nonperiodic nature of charge transport in disordered systems, generating insight into static charge transport networks, as well as analyzing the network dynamics, can be challenging. Here, we apply time-dependent network analysis to scrutinize the charge transport networks of two representative molecular semiconductors: a rigid n-type molecule, perylenediimide, and a flexible p-type molecule, [Formula: see text] Simulations reveal the relevant timescale for local transfer integral decorrelation to be [Formula: see text]100 fs, which is shown to be faster than that of a crystalline morphology of the same molecule. Using a simple graph metric, global network changes are observed over timescales competitive with charge carrier lifetimes. These insights demonstrate that static charge transport networks are qualitatively inadequate, whereas average networks often overestimate network connectivity. Finally, a simple methodology for tracking dynamic charge transport properties is proposed.

Strong n-type molecule as low bias negative differential resistance device predicted by first-principles study

A first-principles study of the transport properties of two thiolated pentacenes sandwiching ethyl is performed. The thiolated pentacene molecule shows strong n-type characteristics when contact Ag lead because of low work function about metal Ag. A strong negative differential resistance (NDR) effect with large peak-to-valley ratio of 758% is present under low bias. Our investigations indicate that strong n- or p-type molecules can be used as low bias molecular NDR devices and that the molecular NDR effect based on molecular-level leaving not on molecular-level crossing has no hysteresis.

A new method to induce molecular low bias negative differential resistance with multi-peaks.

According to a first-principles study of the transport properties of two thiolated anthracene-9,10-diono molecules sandwiching ethyl, a new method to induce molecular low bias negative differential resistance with multi-peaks for strong n- or p-type molecules is proposed. The anthracene-9,10-diono molecule shows strong n-type characteristics when in contact with Au and Ag electrodes via a thiolate. The multiple negative differential resistance effect originated from the molecule-electrode couple is different between Ag and Au electrodes. Our investigations may promise potential for applications in molecular devices with low power dissipation and multifunction in the future.

Horizontally oriented molecular thin films for application in organic solar cells

The authors investigate the influence of molecular orientation of p-type molecules of alpha-sexithiophene (α-6T) and n-type molecules of 3,4,9,10-perylene tetracarboxylic bisbenzimidazole (PTCBI) on organic solar cell (OSC) performance. Deposition of α-6T and subsequently PTCBI on an α-6T buffer surface rubbed with a nylon cloth allows their horizontal orientations to be formed in separate layers. Power conversion efficiency and operation stability of a rubbed OSC are markedly improved when compared with an unrubbed OSC. The improved OSC performance is confirmed to be due to increases in light absorption, exciton diffusion length, energy difference between the p-type and n-type layers, and carrier collection by electrodes, which are caused by the rubbing-induced double horizontal orientations.

Theoretical investigations on electronic structures and photophysical properties of N-heteroaryl carbazole derivatives as host materials

The carbazole-endcapped host molecules with tailoring different heteroaryl core and meta-position linkage mode have great potential on phosphorescent organic light-emitting diodes. To provide a profound view on structure–property relationships, new linear-shaped counterparts have been designed based on the existing molecular composition and the linkage at para-position (p-type molecules). A series of studies about the influence of the linkage mode on optical and electronic properties of these carbazole derivatives have carried out via density functional theory and time-dependent density functional theory calculations. The geometric and the electronic structure of these molecules in the ground states, ions states, and lowest triplet states have been calculated especially focusing on the analysis of highest occupied molecular orbitals, lowest unoccupied molecular orbitals, energy gaps, triplet energies, ionization potentials, electron affinities, reorganization energies, triplet exciton-formation fraction, and absorption spectra. These optoelectronic properties can be effectively tuned by the chemical modifications of different linkage pattern. The good coordination between our calculated results and the available experimental data has been observed. The study reveals that the designed p-type molecules show great promise as new high-performance red host materials with large triplet energy, narrow energy gap, good electron and hole-transport properties, and high triplet exciton-formation fraction.

Assembling p-type molecules on single wall carbon nanotubes for photovoltaic devices

We report the design and synthesis of an oligothiophene molecule that noncovalently functionalizes carbon nanotubes to create a hybrid material for photovoltaic devices.

Enhancing hole transports and generating hole traps by doping organic hole-transport layers with p-type molecules of 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane

We investigated the relationship between the hole-transport and hole-trap characteristics of N, N′-diphenyl- N, N′-bis(1-naphthyl)-1,1′-biphenyl-4,4′-diamine (α-NPD) doped with p-type molecules of 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F 4-TCNQ) at various concentrations. The results of our current density–voltage, field-effect transistor, and thermally stimulated current studies revealed that the current densities of hole-only α-NPD devices at a certain driving voltage increased in the F 4-TCNQ concentration region between 0 mol% and 3 mol% due to the generation of free holes while the hole mobilities of the α-NPD layers decreased due to an increase in hole-trap concentration and deepened hole-trap energy levels, which were caused by the F 4-TCNQ doping. The optimized doping concentration of F 4-TCNQ was 3 mol%, which provided the highest current density for the hole-only device. On the other hand, since the increase in free-hole concentration was overcome by the decrease in hole mobility, the current density of the hole-only device decreased at the F 4-TCNQ concentration of 4 mol% when compared with the optimized concentration.

Molecular Interface Effect of Heterogeneous Molecular Junctions Consisting of Nonfluorinated and Fluorinated Phthalocyanines

We report the tunneling behavior of homogeneous and heterogeneous molecular junctions using p-type molecules of iron phthalocyanine (FePc), phthalocyanine (H(2)Pc), and copper(II) octaalkoxyl substituted phthalocyanine (CuPcOC8) and n-type molecule of copper hexadecafluorophthalocyanine (F(16)CuPc). The molecular films formed on the electrode surfaces were inspected by X-ray photoelectron spectroscopy (XPS). The measured characteristic tunneling curves of single-component phthalocyanines revealed comparable energy gaps for homogeneous tunneling junctions using the photoemission method. In contrast, for the heterogeneous tunnel junctions of mixed phthalocyanines including fluorinated phthalocyanine a distinctive offset of the energy gaps to the positive bias voltage direction can be clearly identified. It is suggested that the substitution of phthalocyanines and surface affinity of phthalocyanines could contribute to the controlled phase separation within the heterogeneous tunneling junctions. The apparent shift of the tunneling spectra is attributed to the existence of an internal electric field originated with the phase separation of the binary mixture of p-type and n-type phthalocyanines within the tunneling junction.

Field Effect Transistors of BTQBT and its Derivatives

AbstractWe have prepared and characterized thin film field effect transistors (FETs) of bis-(1, 2, 5-thiadiazolo)-p-quinobis(1, 3-dithiole) (BTQBT) and its derivatives. Preparation and characterization of the films were carried out under ultrahigh vacuum condition. Most materials examined showed p-type semiconducting behaviors. Among p-type molecules, BTQBT films deposited at room temperature showed the highest mobility and on/off ratio of 0.2 cm2/Vs and 108, respectively, at optimal film growth conditions. These performances are almost comparable to those of pentacene and polythiophene thin films, indicating that BTQBT molecule is a prominent semiconducting material for high-speed organic transistors. It was also found that a tetracyanoquinodimethane (TCNQ) derivative showed an n-type semiconducting behavior with an electron mobility of 8.9 x 10-4 cm2/Vs.