This article reports an experimental demonstration of photoinduced bulk polarization in hysteresis-free methylammonium (MA) lead-halide perovskite solar cells [ITO/PEDOT:PSS/perovskite/PCBM/PEI/Ag]. An anomalous capacitance–voltage (CV) signal is observed as a broad “shoulder” in the depletion region from −0.5 to +0.5 V under photoexcitation based on CV measurements where a dc bias is gradually scanned to continuously drift mobile ions in order to detect local polarization under a low alternating bias (50 mV, 5 kHz). Essentially, gradually scanning the dc bias and applying a low alternating bias can separately generate continuously drifting ions and a bulk CV signal from local polarization under photoexcitation. Particularly, when the device efficiency is improved from 12.41% to 18.19% upon chlorine incorporation, this anomalous CV signal can be enhanced by a factor of 3. This anomalous CV signal can be assigned as the signature of photoinduced bulk polarization by distinguishing from surface polarization associated with interfacial charge accumulation. Meanwhile, replacing easy-rotational MA+ with difficult-rotational formamidinium (FA+) cations largely minimizes such anomalous CV signal, suggesting that photoinduced bulk polarization relies on the orientational freedom of dipolar organic cations. Furthermore, a Kelvin probe force microscopy study shows that chlorine incorporation can suppress the density of charged defects and thus enhances photoinduced bulk polarization due to the reduced screening effect from charged defects. A bias-dependent photoluminescence study indicates that increasing bulk polarization can suppress carrier recombination by decreasing charge capture probability through the Coulombic screening effect. Clearly, our studies provide an insightful understanding of photoinduced bulk polarization and its effects on photovoltaic actions in perovskite solar cells.Keywords: charge dissociation; chlorine doping; grain boundary passivation; perovskite solar cells; photoinduced bulk polarization;

Organo-metal halide perovskites have become very promising photovoltaic materials with triply nondegenerate spin states due to spin–orbital coupling effects. This paper reports the effects of optically operated spin states on photocurrent (JSC) and photovoltage (VOC) in perovskite (MAPbI3) solar cells with the device architecture of ITO/TiO2 (compact)/TiO2 (mesoporous)/MAPbI3/P3HT/Au. Specifically, we switch the photoexcitation from linear polarization to circular polarization to change the electron–hole pair population of spin singlet φml=±1φ↑e↓h and spin triplet φml=0φ↑e↑h in the perovskite solar cells. Simultaneously, we investigate the photovoltaic actions upon optically shifting the spin population. We find that optically shifting the spin population by switching photoexcitation from linear to circular polarization can cause an increase on both JSC and VOC in the perovskite solar cells under circular photoexcitation. Our studies present the first evidence that the perovskite solar cells are the only type of solar cells where spin states can be optically operated with the consequence of influencing the photovoltaic actions. Our results indicate that switching photoexcitation from linear to circular polarization can increase the population of spin triplet electron–hole pairs available for dissociation and consequently increases the JSC. On the other hand, optically shifting the spin population can decrease the bulk polarization and consequently increases the VOC under circular photoexcitation. Therefore, our studies provide insightful understanding on the effects of optically operating spin states on photovoltaic processes in perovskite solar cells.Keywords: organo-metal halide perovskites; perovskite solar cells; spin states; spin−orbital coupling;

AbstractThe last eight years (2009–2017) have seen an explosive growth of interest in organic–inorganic halide perovskites in the research communities of photovoltaics and light-emitting diodes. In addition, recent advancements have demonstrated that this type of perovskite has a great potential in the technology of light-signal detection with a comparable performance to commercially available crystalline Si and III–V photodetectors. The contemporary growth of state-of-the-art multifunctional perovskites in the field of light-signal detection has benefited from its outstanding intrinsic optoelectronic properties, including photoinduced polarization, high drift mobilities, and effective charge collection, which are excellent for this application. Photoactive perovskite semiconductors combine effective light absorption, allowing detection of a wide range of electromagnetic waves from ultraviolet and visible, to the near-infrared region, with low-cost solution processability and good photon yield. This class of semiconductor might empower breakthrough photodetector technology in the field of imaging, optical communications, and biomedical sensing. Therefore, here, the focus is specifically on the critical understanding of materials synthesis, design, and engineering for the next-stage development of perovskite photodetectors and highlighting the current challenges in the field, which need to be further studied in the future.

•The NPB based organic electronic devices exhibit remarkable magneto-capacitance phenomena upon photo-excitation.•The line-shape of the magneto-capacitance highly relies on the material processing methods through film morphology.•The molecular aggregates with different sizes can change the spin-exchange interaction.Organic semiconductors possess low dielectric property which indeed constrains their applications in organic electronic devices, such as organic photovoltaics. Here, we report an optically tunable magneto-capacitive response from NPB based organic electronic devices at excited states. The devices exhibit some remarkable magneto-capacitance phenomena at room temperature due to the spin-dependent dissociation and charge accumulation at PMMA/NPB interfaces. With two different preparation methods for the NPB layers, such as the thermal evaporation and solution methods, we have found the line-shape of the magneto-capacitance can be significantly changed, and the line-shape narrowing is more favorable for the devices based on the solution method. We have proposed that the effect is due to the formation of some large aggregates, and it eventually leads to the decrease of the spin exchange interaction within intermolecular electron-hole pairs.Download high-res image (246KB)Download full-size image

•A novel look at singlet fission studying a magnetic/organic interface.•The interface studied is between thin films of cobalt and tetracene.•The interface shows spin interactions and electrical polarization effects.•Spin interactions and electrical polarization lead to increases in singlet fission in tetracene.This article reports the magneto-optical effects on the singlet fission of the p-type organic semiconductor, tetracene, from a ferromagnetic/semiconductor interface between thin films of cobalt and tetracene. We experimentally show that this interface has two effects on the thin films of tetracene: spin interactions and electrical polarization. The experimental tools used to study the interface include magnetic field effect photoluminescence (MFEPL), photoluminescence and absorption. Spin interaction effects are shown by MFEPL data, where we observe a large increase in the maximum MFEPL when cobalt is introduced, as well as changes in the hyperfine interactions at low magnetic fields. Electrical polarization is analyzed with photoluminescence and absorption measurements, showing small changes in the energy difference between the HOMO and LUMO levels of tetracene, as well as an increase in the electron-phonon coupling in tetracene. Also, electrical polarization is shown to increase electrical interactions between tetracene molecules. Therefore, we conclude that using spin interactions and electrical polarization from the ferromagnetic/organic semiconductor interface can tune the properties of tetracene, ultimately enhancing singlet fission. This work gives new insight to understand the singlet fission process using a ferromagnetic interface. These changes can be further utilized in photovoltaic applications based on this singlet fission material and be applied to other similar types of singlet fission organic semiconductors.Electrical polarization and spin interactions due to the interface between cobalt and tetracene cause a change in the conversion rate between singlets and triplets. This leads to an increase in triplet state densities, which causes an increase in singlet fission of tetracene.Download high-res image (156KB)Download full-size image

•Chlorine-bearing conjugated polymers exhibit unique properties.•Revealing the structure-property relationship of chlorine-bearing polymers.•Efficient deep-red electroluminescent materials.Since the large steric hindrance caused by chlorine atoms not only suppresses the aggregation but also results in large stokes shift and low self-absorption, a series of donor-acceptor alternating copolymers based on 6,7-dichloroquinoxaline have been synthesized by modifying the structures of oligothiophenes. All the polymers have been well characterized to study the effects of the length of oligothiophenes and the steric hindrance on the optical, electronic and electroluminescent properties. It was observed that the intramolecular charge transfer absorption was weakened by steric hindrance. Unlike non-chloride analogues, prolonged conjugated length resulted in smaller bandgap, given similar steric hindrance. Deep-red emission centered at 678 nm with brightness about 1800 cd m−2 was achieved with external quantum efficiency 1.34% using dopant/host technic.Download high-res image (291KB)Download full-size image

•Using high-dielectric NiOx transport layer increases device built-in field Ebi.•Increasing the device built-in field completely dissociates the e-h pairs.•Enhancing Ebi suppresses radiative recombination to increase dissociation rate.•Decreasing interfacial traps largely decreases non-radiative recombination.•Decreasing interfacial traps increases collection of photogenerated carriers.This article reports the experimental studies on simultaneously enhancing the dissociation and suppressing the recombination in perovskite solar cells by using high-dielectric Nickel Oxide (NiOx) as hole transport layer. Specifically, the magneto-photocurrent, generated by the electron-hole pairs, surprisingly becomes negligible at short-circuit condition when the NiOx is used to replace the poly (3,4-ethylenedioxythiophene) poly (styrene-sulfonate) (PEDOT:PSS). This indicates that the NiOx transport layer leads to a complete dissociation of electron-hole pairs in perovskite layer. On the other hand, the negligible magneto-photocurrent can be recovered to become appreciable when a forward bias is applied towards open-circuit condition to weaken the built-in field. This magneto-photocurrent result suggests that the NiOx transport layer enhances the built-in field, completely dissociating the electron-hole pairs. Furthermore, the photoinduced capacitance studies confirm that the built-in field is enhanced essentially through static and dynamic parameters, by removing the interfacial traps and decreasing the accumulation of photogenerated carriers. The time-resolved photoluminescence shows that the NiOx/CH3NH3PbI3 interface leads to a reduction on non-radiative recombination, increasing the fraction of useful excitons available for photovoltaic actions. Moreover, the field-dependent photoluminescence measured alternatively at short-circuit and open-circuit conditions shows that the NiOx layer can also suppress the radiative recombination within available excitons, boosting the photovoltaic actions. Therefore, our studies reveal that the high-dielectric NiOx transport layer can simultaneously enhance the dissociation of electron-hole pairs and suppress both non-radiative/radiative recombination, leading to the more efficient generation of Jsc and Voc in perovskite solar cells.Download high-res image (296KB)Download full-size image

•SOC plays an important role in determining PV actions in perovskite solar cells.•Replace Pb with Sn then reduce SOC, weaken spin mixing between different spin states.•Parallel spin states are favorable for developing PV actions.•Antiparallel spin states are responsible for developing light-emission actions.•Reducing spin mixing decreases the density of parallel spin states for PV actions.Organo-metal halide perovskites, as emerging photovoltaic materials, have demonstrated interesting spin states due to spin-orbital coupling (SOC) effects. However, replacing the Pb with the Sn can inevitably affect the SOC and consequently changes the internal photovoltaic processes in the development of environmentally friendly perovskite devices. Here, by operating the spin states with circularly polarized photoexcitation we report that the spin-dependent photocurrent (Jsc) becomes much more prominent upon replacing Pb with Sn, increasing the spin dependence from 0.25% to 1.25% by switching the photoexcitation from linear to circular polarization. Essentially, the spin-dependent Jsc is determined by the spin relaxation time, changing with the SOC strength, as compared to the charge dissociation time. On the other hand, our magneto-photocurrent (magneto-Jsc) results show that the internal magnetic parameter decreases from 281 mT to 41 mT upon Sn-Pb replacement, providing an evidence that the SOC is indeed weakened from Pb to Sn based solar cells. Furthermore, the spin-dependent photoluminescence (PL) indicates that weakening the SOC upon the Sn-Pb replacement leads to more antiparallel spin states (singlets) available for PL but less parallel spin states (triplets) available for photovoltaic action. Therefore, SOC plays an important role in the development of photovoltaic actions in Sn-based perovskite solar cells.Download high-res image (334KB)Download full-size image

This article reports on experimental studies on magnetic polarization in the excited state by using magnetic field effects of light scattering (MFELS) together with a photoexcitation beam based on fluorinated multi-layer graphene (FG) particles suspended in an organic solvent. We observe that a magnetic field can change the light scattering of a 532 nm laser beam from the suspended FG particles, generating a MFELS signal with an amplitude of 60% at 900 mT. This phenomenon indicates that the suspended FG particles experience a magnetization force, leading to an orientation of the suspended FG particles in a magnetic field. We find that the magnetization force is a function of a solvent dielectric constant, an analogue phenomenon similar to magneto-electric coupling. More importantly, in the excited state the suspended FG particles exhibit more pronounced MFELS, as compared with the ground state, when the magnetic field effects of light scattering are combined with a photoexcitation beam of 325 nm. Clearly, the FG particles in the excited state possess a stronger magnetization relative to the ground state. This excitation-enhanced magnetization suggests an interaction between the magnetization from the localized spins and the polarization from delocalized π electrons in the FG particles. Therefore, the magnetic field effects of light scattering provide a convenient experimental method to investigate the magnetization of nanoparticles in the excited state.

Organic/inorganic hybrid materials can be promising thermoelectric materials due to the possibilities of achieving high electrical conductivity and low thermal conductivity, which are necessary for their potential application in heat–electricity conversion. In this work, we show that the new n-type organic/inorganic hybrid C6H4NH2CuBr2I can simultaneously exhibit a high electrical conductivity of ∼3.6 × 103 S cm−1 and a Seebeck coefficient of ∼−70 μV K−1 at room temperature in a thin-film design, presenting the highest power factor reaching 1740 μW mK−2. The temperature dependence of both the electrical conductivity and Seebeck coefficient in this C6H4NH2CuBr2I film illustrates that this organic/inorganic hybrid film behaves like a metallic material. The high electrical conductivity of the C6H4NH2CuBr2I film can be attributed to the defect-related heavy doping caused by CuI in this semiconductor, while the relatively large Seebeck coefficient is due to the coexisting entropy difference and polarization difference. The temperature-dependent polarization difference between the hot and the cold side of the film can function as an additional driving force besides the entropy difference and contribute to the Seebeck effect development in C6H4NH2CuBr2I. When applying a temperature difference of 4.8 °C along the in-plane direction, the C6H4NH2CuBr2I film with gold contacts generates a thermoelectric voltage of −0.31 mV and a thermoelectric current of −0.022 mA, suggesting a promising n-type thermoelectric material for heat–electricity conversion in thermoelectric power generators.

Fullerene (C60) is an important n-type organic semiconductor with high electron mobility and low thermal conductivity. In this work, we report the experimental results on the tunable Seebeck effect of C60 hybrid thin-film devices by adopting different oxide layers. After inserting n-type high-dielectric constant titanium oxide (TiOx) and zinc oxide (ZnO) layers, we observed a significantly enhanced n-type Seebeck effect in oxide/C60 hybrid devices with Seebeck coefficients of −5.8 mV K−1 for TiOx/C60 and −2.08 mV K−1 for ZnO/C60 devices at 100 °C, compared with the value of −400 μV K−1 for the pristine C60 device. However, when a p-type nickel oxide (NiO) layer is inserted, the C60 hybrid devices show a p-type to n-type Seebeck effect transition when the temperature increases. The remarkable Seebeck effect and change in Seebeck coefficient in different oxide/C60 hybrid devices can be attributed to two reasons: the temperature-dependent surface polarization difference and thermally-dependent interface dipoles. Firstly, the surface polarization difference due to temperature-dependent electron–phonon coupling can be enhanced by inserting an oxide layer and functions as an additional driving force for the Seebeck effect development. Secondly, thermally-dependent interface dipoles formed at the electrode/oxide interface play an important role in modifying the density of interface states and affecting the charge diffusion in hybrid devices. The surface polarization difference and interface dipoles function in the same direction in hybrid devices with TiOx and ZnO dielectric layers, leading to enhanced n-type Seebeck effect, while the surface polarization difference and interface dipoles generate the opposite impact on electron diffusion in ITO/NiO/C60/Al, leading to a p-type to n-type transition in the Seebeck effect. Therefore, inserting different oxide layers could effectively modulate the Seebeck effect of C60-based hybrid devices through the surface polarization difference and thermally-dependent interface dipoles, which represents an effective approach to tune the vertical Seebeck effect in organic functional devices.

The power conversion efficiency (PCE) of organic solar cells has been constantly refreshed in the past ten years from 4% up to 11% due to the contribution from the chemists on novel materials and the physicists on device engineering. For practical applications, a single bulk heterojunction structure may be the best candidate due to the cell with a high PCE, easy fabrication and low cost. Recently, ternary solar cells have attracted much attention due to enhanced photon harvesting by using absorption spectral or energy level complementary materials as the second donor or acceptor based on a single bulk heterojunction structure. For better promoting the development of ternary solar cells, we summarize the recent progress of ternary solar cells and try our best to concise out the scientific issues in preparing high performance ternary solar cells.

A giant magnetocurrent (>100%) is observed in the electrochemical system based on tertiary amines at room temperature. This giant magnetocurrent is ascribed to spin-dependent deprotonation during the oxidation of tertiary amines. This presents a new approach of using spin-dependent deprotonation to generate giant magnetocurrent in electrochemical reactions.

•Changing composition can tune Seebeck coefficient between positive and negative values.•Varying composition can tune semiconducting properties between n and p-type regimes.•Internal polarization is responsible for large temperature-dependent Seebeck effects.Organo-metal halide perovskites can exhibit co-existed electrical polarizations and semiconducting properties respectively from organic and inorganic components. Here, we find that the Seebeck coefficient can be changed between positive and negative values when the concentration of chloride ions is varied between single-halide (CH3NH3PbI3) and mixed-halide structures (CH3NH3PbIxCl3−x). This indicates that varying the concentration of chloride ions can tune the semiconducting properties between the n-type and p-type regimes in the organo-metal halide perovskites. Our temperature-dependent capacitance measurement shows that increasing temperature can cause a change on internal electrical polarization. As a result, we can propose that the internal polarization functions as the underlying mechanism responsible for large temperature-dependent Seebeck coefficients in organo-metal halide perovskites operating between n-type and p-type regimes.

•Separately exciting D/A components reveals dipole interaction in MFE measurements.•Photoexcitation establishes dipole-dipole interaction in PTB7:PCBM devices.•Dipole-dipole interaction decreases the e-h binding energy, enhancing Jsc.•Dipole-dipole interaction facilitates charge transport, increasing Voc and FF.Organic materials normally have low dielectric constants and are often formed with high electron-hole binding energy, which are detrimental to the generation of photovoltaic actions in solution-processing thin-film solar cells. Here, we show that optically induced dipole-dipole interaction can largely decrease the electron-hole binding energy at donor:acceptor (D:A) based on the PTB7:PCBM bulk-heterojunctions. Our experimental measurements combine (i) double-beam 325 nm and 532 nm excitations to establish dipole-dipole interaction by selectively exciting the intramolecular charge-transfer donor and optically polarizable acceptor and (ii) magneto-photocurrent to monitor the electron-hole binding energy through charge dissociation at D:A interfaces. We find that the electron-hole binding energy at D:A interfaces can be significantly decreased when the dipole-dipole interaction is optically established in PTB7:PCBM solar cells. Furthermore, the dipole-dipole interaction forms a drifting field to facilitate charge transport, and consequently enhancing the Voc and FF in developing photovoltaic actions in organic solar cells.

Organo-metal halide perovskite solar cells have shown remarkable progress in power conversion efficiencies in the past five years due to some amazing intrinsic properties such as long-range ambipolar transport characteristics, high dielectric constants, low exciton binding energies, and intrinsic ferroelectric polarizations. This review article discusses recent results with the focus on fundamental physics involved in internal photovoltaic processes in perovskite solar cells. The discussion includes charge transport, photoexcited carriers versus excitons, exciton binding energies, ferroelectric properties, and magnetic field effects. The objective of this review article is to provide the critical understanding for materials synthesis and device engineering to further advance photovoltaic actions in the state-of-the-art organo-metal halide perovskite solar cells.

This paper reports experimental studies on the effects of a ferroelectric interface on thermionic cooling in single-heterojunction electrode/medium/electrode thin-film devices by using high-dielectric MoO3 oxide and a ferroelectric P(VDF-TrFE) polymer. We observe a thermionic cooling of 0.10 °C from the single Au/MoO3/ITO device at a low injection current of 0.50 mA cm−2. The experimental studies at different film thicknesses and current densities suggest that this cooling effect is determined by three competing processes: phonon absorption from injected carriers through a thermionic process via charge–phonon interactions, Joule heating from the electrical transport of injected carriers, and heat transfer between charge-injecting and charge-collecting electrodes through phonon conduction. Furthermore, we find that inserting a ferroelectric polymer [P(VDF-TrFE)] interface can largely enhance the thermionic cooling from 0.10 °C to 0.20 °C by a factor of 2 in the Au/P(VDF-TrFE)/MoO3/ITO device at a very low injection current of 0.15 mA cm−2, as compared with the Au/MoO3/ITO device without a ferroelectric interface. Our analysis indicates that the ferroelectric P(VDF-TrFE) interface can decrease the heat transfer between charge-injecting and charge-collecting electrodes due to its low thermal conductivity but still allow a thermionic injection due to its ferroelectric polarization to enhance the cooling effect. Therefore, our work presents a new approach to enhance the thermionic cooling effect by using a ferroelectric interface in organic heterojunction thin-film electronic devices.

A series of photomultiplication (PM)-type polymer photodetectors (PPDs) were fabricated with polymer poly(3-hexylthiophene)–[6,6]-phenyl-C71-butyric acid methyl ester (P3HT–PC71BM) (100:1, w/w) as the active layers, the only difference being the self-assembly time of the active layers for adjusting the P3HT molecular arrangement. The grazing incidence X-ray diffraction (GIXRD) results exhibit that P3HT molecular arrangement can be adjusted between face-on and edge-on structures by controlling the self-assembly time. The champion EQE value of PPDs, based on the active layers without the self-assembly process, arrives at 6380% under 610 nm light illumination at −10 V bias, corresponding to the face-on molecular arrangement of P3HT in the active layers. The EQE values of PPDs were markedly decreased to 1600%, along with the self-assembly time up to 12 min, which should be attributed to the variation of absorption and hole transport ability of the active layers induced by the change of P3HT molecular arrangement. This finding provides an effective strategy for improving the performance of PM-type PPDs by adjusting the molecular arrangement, in addition to the enhanced trap-assisted charge-carrier tunneling injection.Keywords: mobility; photomultiplication; polymer photodetectors; self-assembly; tunneling injection

A series of polymer solar cells (PSCs) were fabricated with indene-C60 bisadduct (ICBA) or [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) as an electron acceptor and with PBDT-TS1 as an electron donor. The donor/acceptor (D/A) phase separation was adjusted with different solution processing methods, consisting of cool (room temperature, 20 °C) solution, hot (70 °C) solution and the solutions with solvent additive 1,8-diiodideoctane (DIO). The champion PCE of PSCs with ICBA or PC61BM as an electron acceptor is 4.32% or 5.97% for the active layers prepared from hot solution with DIO additive or cool solution with DIO additive, respectively. The improved PCEs should be attributed to the optimized D/A phase separation in the active layers by adjusting the redistribution of PC61BM or the ICBA among the PBDT-TS1 networks. The degree of phase separation of the active layers with different acceptors was evaluated according to the current density–voltage (J–V) curves of hole-only and electron-only devices. The distribution of PC61BM or ICBA molecules in the normal direction can be simply judged from the symmetry degree of J–V curves of electron-only devices measured under the forward and reverse bias.

•Photoinduced proton-transfer states lead to enhancement on Seebeck coefficients.•Photoinduced proton-transfer states lead to enhancement on electrical conductivity.•Temperature-dependent polarization acts as a new driving force for Seebeck effect.This paper reports Seebeck effects from optically-induced intramolecular proton-transfer HPI-Cbz molecules based on vertical electrode/organic film/electrode thin-film devices. We observed large Seebeck coefficients of 428 μV/K and 390 μV/K from HPI-Cbz based thin-film devices at 60 °C when proton-transfer was induced by the photoexcitation of a 325 nm laser with an intensity of 12 mW/cm2 and 6 mW/cm2 respectively. Under dark condition without proton transfer occurring, the Seebeck coefficient was measured to be 342 μV/K at 60 °C. The Seebeck coefficient enhancement by the induced intramolecular charge transfer can be attributed to the enhanced polarization difference between high- and low-temperature surface due to the stronger electron–phonon coupling followed with the proton-transfer in HPI-Cbz under photoexcitation, and the strength of electron–phonon coupling is proportional to the photoexcitation intensity. The enhanced temperature-dependent electrical polarization between the high and low-temperature surfaces acts as an additional driving force to diffuse the majority charge carriers for the development of a large Seebeck effect. Therefore, using intramolecular proton-transfer presents an effective approach of enhancing Seebeck effect in organic materials.

This paper reports Seebeck effects driven by both surface polarization difference and entropy difference by using photoinduced intramolecular charge-transfer states in n-type and p-type conjugated polymers, namely IIDT and IIDDT, respectively, based on vertical conductor/polymer/conductor thin-film devices. We obtain large Seebeck coefficients of −898 μV/K from n-type IIDT and 1300 μV/K from p-type IIDDT when the charge-transfer states are generated by a white light illumination of 100 mW/cm2, compared with the values of 380 and 470 μV/K in dark condition, respectively. Simultaneously, the electrical conductivities are increased from almost insulating state in dark condition to conducting state under photoexcitation in both n-type IIDT and p-type IIDDT based devices. The large Seebeck effects can be attributed to the following two mechanisms. First, the intramolecular charge-transfer states exhibit strong electron–phonon coupling, which leads to a polarization difference between high and low temperature surfaces. This polarization difference essentially forms a temperature-dependent electric field, functioning as a new driving force additional to entropy difference, to drive the energetic carriers for the development of Seebeck effects under a temperature difference. Second, the intramolecular charge-transfer states generate negative or positive majority carriers (electrons or holes) in the n-type IIDT or p-type IIDDT, ready to be driven between high and low temperature surfaces for developing Seebeck effects. On the basis of coexisted polarization difference and entropy difference, the intramolecular charge-transfer states can largely enhance the Seebeck effects in both n-type IIDT and p-type IIDDT devices. Furthermore, we find that changing electrical conductivity can switch the Seebeck effects between polarization and entropy regimes when the charge-transfer states are generated upon applying photoexcitation. Therefore, using intramolecular charge-transfer states presents an approach to develop thermoelectric effects in organic materials-based vertical conductor/polymer/conductor thin-film devices.Keywords: charge-transfer states; entropy difference; n-type organic polymer; p-type organic polymer; Seebeck effects; surface polarization;

Organic solar cells exhibit both electrode/photovoltaic (E/P) interface and donor/acceptor (D/A) interface in controlling the critical photovoltaic processes. Here, on the basis of the efficient PTB7:PC71BM solar cells, our studies on magneto-photocurrent with external bias provide the first evidence that the E/P interface and D/A interface are dynamically coupled at device-operating condition. Specifically, we observe a significant decrease (90%) in the critical bias required to completely dissociate the charge transfer (CT) states at D/A interface by changing the E/P interface with increased interface dipole field from ZnO to the polyelectrolyte (PFN). The experimental results demonstrate that strong dipole field at the E/P interface can effectively decrease the electron–hole binding energy at D/A interface through the enhancement of built-in field applied on CT states. Clearly, our experimental studies provide new insight on the interfacial engineering through addressing the dynamic coupling between E/P interface and D/A interface in organic bulk-heterojunction solar cells toward further improvement on photovoltaic efficiencies.

Two different electrogenerated chemiluminescence (ECL) systems, Ru(bpy)32+/TPrA and Ru(bpy)32+/C2O42–, are chosen to study the relationship between the sign of exchange interaction in radical pairs and magnetic field effects (MFEs) on electrogenerated chemiluminescence intensity (MFEECL). A positive MFEECL up to 210% is observed for the Ru(bpy)32+/TPrA system, while a negative MFEECL of only −33% is observed based on the Ru(bpy)32+/C2O42– system. The significant difference on MFEECL is ascribed to different signs of exchange interaction in radical pairs [Ru(bpy)33+···TPrA•] and [Ru(bpy)33+···CO2–•] because they have a distant and proximate separation distance between two radicals of a pair, which result in different magnetic-field-induced intersystem crossing directions between singlet and triplet states. The experimental results suggest that an applied magnetic field can enhance the singlet → triplet conversion rate in radical pairs [Ru(bpy)33+···TPrA•] while facilitating an inverse conversion of triplet → singlet in radical pairs [Ru(bpy)33+···CO2–•]. The increase/decrease of triplet density in radical pairs stimulated by an applied magnetic field leads to an increase/decrease on the density of light-emitting triplets of Ru(bpy)32+*. As a consequence, we can tune MFEECL between positive and negative values by changing the sign of exchange interaction in radical pairs during an electrochemical reaction.

This article presents our experimental studies to unravel the dynamic photovoltaic processes occurring at donor:acceptor (D:A) and electrode:active layer (E:A) interfaces under device-operating conditions by using two unique magneto-optical measurements, namely photo-induced capacitance and magnetic field effect measurement. First, we have found that a higher surface polarization of dielectric thin film can decrease the surface charge accumulation at E:A interface. The photo-induced capacitance results indicate that dielectric thin film plays a crucial role in the charge collection in generating photocurrent in organic solar cells. Second, our experimental results from magnetic field effect show that the binding energies of charge transfer (CT) states at D:A interface can be evaluated by using the critical bias required to completely dissociate the CT states. This is the first experimental demonstration that the binding energies of CT states can be measured under deviceoperating conditions. Furthermore, we use our measurement of magnetic field effect to investigate the most popular organic photovoltaic solar cells, organometal halide perovskite photovoltaic devices. The results of magneto-photoluminescence show that the photogenerated electrons and holes are inevitably recombined into electron-hole pairs through a spin-dependent process in the perovskites. Therefore, using spin polarizations can present a new design to control the photovoltaic loss in perovskites-based photovoltaic devices. Also, we found that introducing D:A interface can largely affect the bulk charge dissociation and recombination in perovskite solar cells. This indicates that the interfacial and bulk photovoltaic processes are internally coupled in developing photovoltaic actions in perovskite devices. Clearly, these magneto-optical measurements show a great potential to unravel the deeper photovoltaic processes occurring at D:A and E:A interfaces in both organic bulk-heterojunction and perovskite solar cells under device-operating conditions.

This article reports the magneto-dielectric studies on the coupling between optically generated CT states and magnetized CT states based on thin-film devices with the architecture of ITO/TPD:BBOT/TPD/Co/Al. The magnetized CT states are generated at the Co/TPD interface, generating a magneto-dielectric response with a broad, non-Lorentzian line-shape. The optically generated CT states are formed at the TPD:BBOT interfaces in the heterojunction under photoexcitation, leading to a magneto-dielectric signal with a narrow, Lorentzian line-shape. We find that combining the optically generated CT states and magnetized CT states yields a new magneto-dielectric signal with distinctive line-shape and amplitude in the ITO/TPD:BBOT/TPD/Co/Al device. The magneto-dielectric analysis indicates that there exists a coupling between optically generated CT states and magnetized CT states through the interactions between the magnetic Co/TPD interface and the optically excited TPD:BBOT heterojunction. Furthermore, we show that the coupling between optically generated CT states and magnetized CT states experiences Coulomb interactions and spin–orbital interaction by changing (i) the density of optically generated CT states and (ii) the separation distance between optically generated CT states and magnetized CT states. Clearly, this coupling provides a new approach to mutually tune magnetic and electronic properties through thin-film engineering by combining magnetic and organic materials.

We explore a new mechanism to develop Seebeck effects by using temperature-dependent surface polarization based on vertical multi-layer Al–P3HT:PCBM–Al thin-film devices. Here, the temperature-dependent surface polarization functions as an additional driving force, as compared with the traditional driving force from the entropy difference, to diffuse the charge carriers under a temperature gradient towards the development of Seebeck effects. The temperature-dependent surface polarization is essentially generated by both the thermally dependent polarization through the charge–phonon coupling mechanism and the thermally modulated interface dipoles by Fermi electrons. It is noted that the entropy difference often causes an inverse relationship between the Seebeck coefficient and electrical conductivity in thermoelectric developments. However, this temperature-dependent surface polarization provides a mechanism allowing a co-operative relationship between the Seebeck coefficient and electrical conductivity. We demonstrate simultaneously the enhanced Seebeck coefficient and electrical conductivity by using the dielectric interface through the temperature-dependent surface polarization to diffuse charge carriers in the Al–MoO3–P3HT:PCBM–Al thin-film device.

The accumulation of dissociated charge carriers plays an important role in reducing the loss occurring in organic solar cells. We find from light-assisted capacitance measurements that the charge accumulation inevitably occurred at the electrode and photovoltaic layer interface for bulk-heterojunction ITO/PEDOT:PSS/P3HT:PCBM/Ca/Al solar cells. Our results indicate, for the first time through impedance measurements, that the charge accumulation exists at the anode side of the device, and more importantly, we successfully identify the type of charge accumulated. Further study shows that the charge accumulation can significantly affect open circuit voltage and short circuit current. As a result, our experimental results from light assisted capacitance measurements provide a new understanding of the loss in open-circuit voltage and short-circuit photocurrent based on charge accumulation. Clearly, controlling charge accumulation presents a new mechanism to improve the photovoltaic performance of organic solar cells.

•Charge extraction limited processes can be distinguished by transient technology.•Interfacial dipole can directly affect interfacial charge extraction time.•Interfacial dipole can influence the charge accumulation within the bulk layer.•Accumulated charge can induce charge recombination and loss in charge carriers.In organic solar cells, the interfacial and bulk photovoltaic processes are typically coupled based on charge transport and accumulation. In this article, we demonstrated that the in situ transient photocurrent measurements can be a powerful approach to separately investigate the interfacial effects on interfacial and bulk photovoltaic process. Based on this method, the effects of interfacial dipoles on charge extraction, accumulation, and recombination are solely studied by comparing Ca and Al devices with standard architecture of ITO/PEDOT/P3HT:PCBM/cathode. We observe that stronger interfacial dipoles can significantly decrease the charge extraction time and consequently increase the charge extraction efficiency. More importantly, stronger interfacial dipoles can also decrease the charge accumulation within the bulk photovoltaic layer. Furthermore, our experimental results indicate that the bulk-accumulated charges can act as recombination centers under device-operating condition, resulting in the recombination loss in photogenerated carriers. Clearly, our studies of transient photocurrents elucidated the charge extraction, accumulation, and recombination in OSCs.Graphical abstract

•Spin interaction between triplets determine the singlet fission process.•Low magnetic field can enhance the dephasing of the spin precessions.•High magnetic can facilitate the correlated spin precessions.This article reports the experimental studies on the effects of inter-triplet spin interaction on singlet fission by using magnetic field effects of photoluminescence (MFEPL) based on tetracene. The MFEPL are compared for three different morphological states based on polycrystalline solid powder, amorphous solid film, and liquid solution. It is observed that the polycrystalline solid powder gives stronger MFEPL than that of amorphous solid film, while the liquid solution exhibits no detectable MFEPL. In essence, the MFEPL are determined by the inter-conversion between different spin states initiated by inter-triplet spin interaction through spin mixing in intermediate triplet–triplet pairs towards the singlet fission. The different MFEPL amplitudes suggest that the polycrystalline solid powder possesses an enhanced inter-triplet spin interaction in intermediate triplet–triplet pairs as compared to amorphous solid film. As a result, the enhanced inter-triplet spin interaction can cause a larger inter-conversion between different spin states in intermediate triplet–triplet pairs and consequently increases the singlet fission within polycrystalline structures. The absorption spectral characteristics and X-ray diffraction data confirm that the polycrystalline solid powder can indeed exhibits stronger intermolecular electronic interaction relative to amorphous solid film. Here, the stronger intermolecular electronic interaction provides an evidence for the enhanced inter-triplet spin interaction occurring within polycrystalline structures in the solid powder. Our experimental results indicate that increasing the inter-triplet spin interaction can boost the inter-conversion between different spin states in intermediate triplet–triplet pairs and consequently facilitates the singlet fission.Graphical abstract

•High molecular weight and non-crosslinking chlorine-bearing polymers have been straight forward synthesized.•Chlorine-bearing polymers exhibit lower LUMO, larger Stokes shift and lower self-absorption.•These polymers bearing chlorine atoms on the backbone can be used as efficient light-emitting materials.High molecular weight and non-crosslinking conjugated donor–acceptor copolymers with chlorine on the backbones were straight forward synthesized for the first time under Stille condensation reaction by using the different reaction activities between chlorine and bromine atoms. The chlorine-bearing polymers exhibited much lower LUMO, larger Stokes shift and lower self-absorption compared with the non-chloride analogue, for the electron affinity and large steric hindrance of chlorine atoms. Deep red to NIR emission centered at 698 nm was obtained with brightness about 1500 cd/m2 and centered at 708 nm was obtained with brightness over 400 cd/m2 based on dopant/host system.Graphical abstract

A triplet can annihilate with a charge or a triplet, generating triplet–charge annihilation (TCA) or triplet–triplet annihilation (TTA) in organic semiconductors. On one hand, the TCA and TTA are critical issues to improve optoelectronic responses by using triplet states. On the other hand, the TCA and TTA are important spin-dependent processes to generate magneto-optoelectronic responses. Our experimental studies find that the TCA is a dominant process over TTA in organic semiconductors. Specifically, we separately confine triplets with charges or with triplets towards the generation of TCA and TTA by adjusting triplet density, charge confinement, and charge/exciton ratio based on organic light-emitting diodes. We then use magnetic field effects of electroluminescence (MFEEL) as an experimental tool to study the generation of TCA and TTA. We observe that the electroluminescence can show a negative response to an applied magnetic field, generating a negative MFEEL, when triplets and charges are simultaneously confined within close proximity by using interfacial confinement with unbalanced charge/exciton ratio. In contrast, the electroluminescence only exhibits a positive MFEEL when triplets are confined within close proximity by using interfacial confinement without unbalanced charge/exciton ratio. Therefore, it can be concluded from our MFEEL results that the TCA is a dominant process to annihilate triplets over TTA. Clearly, this experimental finding provides a new understanding on controlling triplets-related optoelectronic and magneto-optoelectronic processes in organic semiconductors.

This article reports the experimental studies on using photoexcitation-generated excited states as a new approach to increase Seebeck coefficient as well as electrical conductivity in an organic semiconducting polymer MEH-PPV (poly(2-methoxy-5-(2′-ethylhexyloxy)-p-phenylene vinylene) with multilayer electrode/polymer/electrode thin-film architecture. Our experimental results show that a photoexcitation can lead to a large Seebeck coefficient of 305 μV/K in the ITO/MEH-PPV/Au device under the light intensity of 16 mW/cm2. Simultaneously, the electrical conductivity is increased to 8.2 × 10–5 S/cm from the dark conductivity of 3.6 × 10–6 S/cm. Clearly, using excited states can lead to a simultaneous increase on Seebeck coefficient and electrical conductivity, as compared to opposite phenomenon induced by doping: increasing electrical conductivity but decreasing Seebeck coefficient. Our experimental studies suggest that excited states can increase both the entropy difference between high and low-temperature surfaces through electron–phonon coupling and the charge density through dissociation to simultaneously enhance Seebeck coefficient and electrical conductivity in the ITO/MEH-PPV/Au device. The underlying mechanism can be attributed to the fact that excited states can largely affect the dominating electrical-transporting factor, namely, interchain electrical transport, but negligibly changes the dominating thermal-transporting factor, namely, interchain thermal transport in an organic semiconducting polymer. As a result, using photogenerated excited states presents a promising approach to develop high Seebeck effects in organic semiconducting materials based on multilayer electrode/polymer/electrode thin-film design.

This article reports the magnetocapacitance effect (MFC) based on both pristine polymer MEH-PPV and its composite system doped with spin radicals (6R-BDTSCSB). We observed that a photoexcitation leads to a significant positive MFC in the pristine MEH-PPV. Moreover, we found that a low doping of spin radicals in polymer MEH-PPV causes a significant change on the MFC signal: an amplitude increase and a line-shape narrowing under light illumination at room temperature. However, no MFC signal was observed under dark conditions in either the pristine MEH-PPV or the radical-doped MEH-PPV. Furthermore, the magnitude increase and line-shape narrowing caused by the doped spin radicals are very similar to the phenomena induced by increasing the photoexcitation intensity. Our studies suggest that the MFC is essentially originated from the intermolecular excited states, namely, intermolecular electron–hole pairs, generated by a photoexcitation in the MEH-PPV. More importantly, by comparing the effects of spin radicals and electrically polar molecules on the MFC magnitude and line shape, we concluded that the doped spin radicals can have the spin interaction with intermolecular excited states and consequently affect the internal spin-exchange interaction within intermolecular excited states in the development of MFC. Clearly, our experimental results indicate that dispersing spin radicals forms a convenient method to enhance the magnetocapacitance effect in organic semiconducting materials.

We demonstrated the strategy of a nanocomposite design by the incorporation of both a delocalized π-electrons system in a closely bound acceptor–donor analogue chromophore, based on charge-polarizable C60(>DPAF-C9) nanostructure 1, and spin-polarized d-electrons in the form of γ-FeOx nanoparticles. Facile intramolecular electron transfer from the DPAF-C9 donor moiety to the C60 acceptor cage of 1 upon activation to the excited state with a long lifetime of the charge-separated state forms a possible mechanism to integrate semiconducting and magnetic properties in a single system. We observed an appreciable magnetocurrent (MC) of C60(>DPAF-C9)-encapsulated magnetic γ-FeOx nanoparticles in PMMA matrix upon applying a magnetic field from 0 to 300 mT at either 77 K (12% MC) or 300 K (4.5% MC). Interestingly, the detailed analysis of magnetocurrent curve profiles taken at 77 K allowed us to conclude that the measured magnetocurrent may be attributed to the contributions from magnetic field-dependent excited-state populations in semiconducting structure (density-based MC), magnetism from magnetic structure (mobility-based MC), and product of density and mobility-based MC components (π–d electronic coupling). At the higher temperature region up to 300 K, the semiconducting mechanism dominated the determining factor of measured magnetocurrent. This experimental observation indicated the feasibility of combining delocalized π electrons and spin-polarized d electrons through charge transfer to induce internally coupled dual mobility- and density-based MC through the modulation of spin polarization and excited states in semiconducting/magnetic hybrid materials.

TiO2 is widely available as a chemically stable and harmless material. Recently, it has received a great deal of attention as a practical thermoelectric material. In this work, we studied the effects of aluminum (Al) doping concentration and lattice parameter on the thermoelectric properties of TiO2 at both room and high temperatures (475 °C). We found that the Al doping leads to a compression in the lattice constant and an increase in carrier concentration and consequently increases the electrical conductivity in the TiO2. We observed that the Al-doped TiO2 thin film shows a negative Seebeck coefficient and its value linearly decreases with increasing electrical conductivity. This result indicates that the Seebeck effect is developed by entropy-driven thermally assisted charge diffusion between low- and high-temperature surfaces. Further studies found that doping-induced reduction on entropy difference through electrical conductivity is accountable for the decrease of the Seebeck effect in the Al-doped TiO2. However, the Al doping can lead to phonon scattering at grain boundary interfaces and consequently decreases thermal conductivity. Therefore, the Al doping can essentially function as a mechanism to separately adjust electrical and thermal conductivities in the Al-doped TiO2. With Al doping, we obtained a maximal figure of merit (Z value) to be 1.30 at 475 °C and 0.48 At 23 °C from the Al-doped TiO2 at the doping concentration of 3%. The comparison of Z values between room and high temperatures confirms that thermally assisted charge diffusion and phonon scattering are two critical parameters for the development of efficient thermoelectric function in the Al-doped TiO2.

Effects of single walled carbon nanotubes (SWNTs) on the electroluminescent performance of organic light-emitting diodes (OLEDs) have been investigated by mixing them in a hole-conducting layer and in a light-emitting layer in OLEDs. We found that SWNTs play different roles when used as polymer:SWNT composites in OLEDs. When used in a hole-conducting layer, SWNTs facilitate the charge transport in the transport layer and on the other hand they also act as the exciton quenching centers at the transporting/emitting interface provided their concentration is high enough. When used in a light-emitting layer, SWNTs act as an n-type dopant to increase electron transport in p-type electroluminescent film and subsequently improve the balancing degree of bipolar injection, leading to an enhancement in the electroluminescence efficiency.Graphical abstractThis paper report the experimental studies on the effects of single walled carbon nanotubes (SWNTs) on the electroluminescent performance of organic light-emitting diodes (OLEDs). We found that SWNTs play different roles when used as polymer:SWNT composites in OLEDs. When used in a hole-conducting layer, SWNTs facilitate the charge transport in the transport layer and on the other hand they also act as the exciton quenching centers at the transporting/emitting interface provided their concentration is high enough. When used in a light-emitting layer, SWNTs act as an n-type dopant to increase electron transport in p-type electroluminescent film and subsequently improve the balancing degree of bipolar injection, leading to an enhancement in the electroluminescence efficiency.Highlights► In hole-conducting layer the SWNTS can facilitate the charge transport. ► In light-emitting layer the SWNTs can balance electron and hole injection. ► The SWNTs can have multi-functions to enhance organic EL in OLEDs.

The electron−hole pairs can be formed in intermolecular charge-transfer (CT) states between two adjacent molecules due to Coulomb interaction in organic semiconducting materials. In general, the exciton dissociation can experience the intermediate states: intermolecular CT states at the donor−acceptor interfaces to generate a photocurrent in organic solar cells. This article reports the magneto-optical studies on intermolecular CT states in the generation of photocurrent by using magnetic field effects of photocurrent (MFEPC) and light-assisted dielectric response (LADR). The MFEPC and LADR studies reveal that internal electrical drifting and local Coulomb interaction can largely change the formation and dissociation of CT states by changing internal charge-transport channels and local Coulomb interaction through morphological development upon thermal annealing. Therefore, the MFEPC and LADR can be used as effective magneto-optical tools to investigate charge recombination, separation, and transport in organic solar cells.

It has been found that non-magnetic organic semiconducting materials can exhibit magnetic responses in electrical current, electroluminescence, photoluminescence, and photocurrent when an external magnetic field is applied. These intrinsic magnetic responses can be naturally attributed to (i) magnetic field-dependent singlet/triplet ratio and (ii) singlet/triplet ratio-dependent excited processes. We observe that inter-molecular and intra-molecular excited states are sensitive and insensitive, respectively, to external magnetic field in magnetic field dependences of electroluminescence, photocurrent, and photoluminescence. This experimental phenomenon suggests that the electron–hole separation distance essentially determines whether magnetic field effects can be activated through the competition between spin-exchange interaction-induced singlet–triplet energy difference and magnetic Zeeman splitting when applied magnetic field is stronger than spin–orbital coupling. Furthermore, the dissociation in inter-molecular excited states and exciton–charge reaction in intra-molecular excited states have positive and negative responses to external magnetic field. As a result, controlling the dissociation in inter-molecular excited states and the exciton–charge reaction in intra-molecular excited states provide an effective methodology to tune the magnetic field effects between positive and negative values in organic semiconducting materials.

First, we have to clarify that in our work1 the different dissociation and charge reaction rates (not the recombination rates, as Lupton and Boehme questioned) between singlets and triplets are responsible for the magnetic-field-dependent generation of secondary charge carriers and of injection current. The different contributions to the dissociation and charge reaction from singlets and triplets have been shown by both theoretical calculations and experimental results2, 3.

Organic semiconducting materials have demonstrated attractive light-absorption and photocurrent-generation functions due to their delocalized π electrons as well as intra-molecular and inter-molecular charge separation processes. On the other hand, organic semiconducting materials have easy property tuning, are mechanically flexible, and have large-area thin film formation properties. As a result, organic materials have become potential candidates in solar energy applications. This article will review critical energy-conversion processes in organic solar cells with the focus on singlet and triplet photovoltaic responses.

Organo-metal halide perovskite solar cells have shown remarkable progress in power conversion efficiencies in the past five years due to some amazing intrinsic properties such as long-range ambipolar transport characteristics, high dielectric constants, low exciton binding energies, and intrinsic ferroelectric polarizations. This review article discusses recent results with the focus on fundamental physics involved in internal photovoltaic processes in perovskite solar cells. The discussion includes charge transport, photoexcited carriers versus excitons, exciton binding energies, ferroelectric properties, and magnetic field effects. The objective of this review article is to provide the critical understanding for materials synthesis and device engineering to further advance photovoltaic actions in the state-of-the-art organo-metal halide perovskite solar cells.

A triplet can annihilate with a charge or a triplet, generating triplet–charge annihilation (TCA) or triplet–triplet annihilation (TTA) in organic semiconductors. On one hand, the TCA and TTA are critical issues to improve optoelectronic responses by using triplet states. On the other hand, the TCA and TTA are important spin-dependent processes to generate magneto-optoelectronic responses. Our experimental studies find that the TCA is a dominant process over TTA in organic semiconductors. Specifically, we separately confine triplets with charges or with triplets towards the generation of TCA and TTA by adjusting triplet density, charge confinement, and charge/exciton ratio based on organic light-emitting diodes. We then use magnetic field effects of electroluminescence (MFEEL) as an experimental tool to study the generation of TCA and TTA. We observe that the electroluminescence can show a negative response to an applied magnetic field, generating a negative MFEEL, when triplets and charges are simultaneously confined within close proximity by using interfacial confinement with unbalanced charge/exciton ratio. In contrast, the electroluminescence only exhibits a positive MFEEL when triplets are confined within close proximity by using interfacial confinement without unbalanced charge/exciton ratio. Therefore, it can be concluded from our MFEEL results that the TCA is a dominant process to annihilate triplets over TTA. Clearly, this experimental finding provides a new understanding on controlling triplets-related optoelectronic and magneto-optoelectronic processes in organic semiconductors.

A giant magnetocurrent (>100%) is observed in the electrochemical system based on tertiary amines at room temperature. This giant magnetocurrent is ascribed to spin-dependent deprotonation during the oxidation of tertiary amines. This presents a new approach of using spin-dependent deprotonation to generate giant magnetocurrent in electrochemical reactions.

This paper reports experimental studies on the effects of a ferroelectric interface on thermionic cooling in single-heterojunction electrode/medium/electrode thin-film devices by using high-dielectric MoO3 oxide and a ferroelectric P(VDF-TrFE) polymer. We observe a thermionic cooling of 0.10 °C from the single Au/MoO3/ITO device at a low injection current of 0.50 mA cm−2. The experimental studies at different film thicknesses and current densities suggest that this cooling effect is determined by three competing processes: phonon absorption from injected carriers through a thermionic process via charge–phonon interactions, Joule heating from the electrical transport of injected carriers, and heat transfer between charge-injecting and charge-collecting electrodes through phonon conduction. Furthermore, we find that inserting a ferroelectric polymer [P(VDF-TrFE)] interface can largely enhance the thermionic cooling from 0.10 °C to 0.20 °C by a factor of 2 in the Au/P(VDF-TrFE)/MoO3/ITO device at a very low injection current of 0.15 mA cm−2, as compared with the Au/MoO3/ITO device without a ferroelectric interface. Our analysis indicates that the ferroelectric P(VDF-TrFE) interface can decrease the heat transfer between charge-injecting and charge-collecting electrodes due to its low thermal conductivity but still allow a thermionic injection due to its ferroelectric polarization to enhance the cooling effect. Therefore, our work presents a new approach to enhance the thermionic cooling effect by using a ferroelectric interface in organic heterojunction thin-film electronic devices.

We explore a new mechanism to develop Seebeck effects by using temperature-dependent surface polarization based on vertical multi-layer Al–P3HT:PCBM–Al thin-film devices. Here, the temperature-dependent surface polarization functions as an additional driving force, as compared with the traditional driving force from the entropy difference, to diffuse the charge carriers under a temperature gradient towards the development of Seebeck effects. The temperature-dependent surface polarization is essentially generated by both the thermally dependent polarization through the charge–phonon coupling mechanism and the thermally modulated interface dipoles by Fermi electrons. It is noted that the entropy difference often causes an inverse relationship between the Seebeck coefficient and electrical conductivity in thermoelectric developments. However, this temperature-dependent surface polarization provides a mechanism allowing a co-operative relationship between the Seebeck coefficient and electrical conductivity. We demonstrate simultaneously the enhanced Seebeck coefficient and electrical conductivity by using the dielectric interface through the temperature-dependent surface polarization to diffuse charge carriers in the Al–MoO3–P3HT:PCBM–Al thin-film device.

Fullerene (C60) is an important n-type organic semiconductor with high electron mobility and low thermal conductivity. In this work, we report the experimental results on the tunable Seebeck effect of C60 hybrid thin-film devices by adopting different oxide layers. After inserting n-type high-dielectric constant titanium oxide (TiOx) and zinc oxide (ZnO) layers, we observed a significantly enhanced n-type Seebeck effect in oxide/C60 hybrid devices with Seebeck coefficients of −5.8 mV K−1 for TiOx/C60 and −2.08 mV K−1 for ZnO/C60 devices at 100 °C, compared with the value of −400 μV K−1 for the pristine C60 device. However, when a p-type nickel oxide (NiO) layer is inserted, the C60 hybrid devices show a p-type to n-type Seebeck effect transition when the temperature increases. The remarkable Seebeck effect and change in Seebeck coefficient in different oxide/C60 hybrid devices can be attributed to two reasons: the temperature-dependent surface polarization difference and thermally-dependent interface dipoles. Firstly, the surface polarization difference due to temperature-dependent electron–phonon coupling can be enhanced by inserting an oxide layer and functions as an additional driving force for the Seebeck effect development. Secondly, thermally-dependent interface dipoles formed at the electrode/oxide interface play an important role in modifying the density of interface states and affecting the charge diffusion in hybrid devices. The surface polarization difference and interface dipoles function in the same direction in hybrid devices with TiOx and ZnO dielectric layers, leading to enhanced n-type Seebeck effect, while the surface polarization difference and interface dipoles generate the opposite impact on electron diffusion in ITO/NiO/C60/Al, leading to a p-type to n-type transition in the Seebeck effect. Therefore, inserting different oxide layers could effectively modulate the Seebeck effect of C60-based hybrid devices through the surface polarization difference and thermally-dependent interface dipoles, which represents an effective approach to tune the vertical Seebeck effect in organic functional devices.

A series of polymer solar cells (PSCs) were fabricated with indene-C60 bisadduct (ICBA) or [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) as an electron acceptor and with PBDT-TS1 as an electron donor. The donor/acceptor (D/A) phase separation was adjusted with different solution processing methods, consisting of cool (room temperature, 20 °C) solution, hot (70 °C) solution and the solutions with solvent additive 1,8-diiodideoctane (DIO). The champion PCE of PSCs with ICBA or PC61BM as an electron acceptor is 4.32% or 5.97% for the active layers prepared from hot solution with DIO additive or cool solution with DIO additive, respectively. The improved PCEs should be attributed to the optimized D/A phase separation in the active layers by adjusting the redistribution of PC61BM or the ICBA among the PBDT-TS1 networks. The degree of phase separation of the active layers with different acceptors was evaluated according to the current density–voltage (J–V) curves of hole-only and electron-only devices. The distribution of PC61BM or ICBA molecules in the normal direction can be simply judged from the symmetry degree of J–V curves of electron-only devices measured under the forward and reverse bias.

The accumulation of dissociated charge carriers plays an important role in reducing the loss occurring in organic solar cells. We find from light-assisted capacitance measurements that the charge accumulation inevitably occurred at the electrode and photovoltaic layer interface for bulk-heterojunction ITO/PEDOT:PSS/P3HT:PCBM/Ca/Al solar cells. Our results indicate, for the first time through impedance measurements, that the charge accumulation exists at the anode side of the device, and more importantly, we successfully identify the type of charge accumulated. Further study shows that the charge accumulation can significantly affect open circuit voltage and short circuit current. As a result, our experimental results from light assisted capacitance measurements provide a new understanding of the loss in open-circuit voltage and short-circuit photocurrent based on charge accumulation. Clearly, controlling charge accumulation presents a new mechanism to improve the photovoltaic performance of organic solar cells.

Organic/inorganic hybrid materials can be promising thermoelectric materials due to the possibilities of achieving high electrical conductivity and low thermal conductivity, which are necessary for their potential application in heat–electricity conversion. In this work, we show that the new n-type organic/inorganic hybrid C6H4NH2CuBr2I can simultaneously exhibit a high electrical conductivity of ∼3.6 × 103 S cm−1 and a Seebeck coefficient of ∼−70 μV K−1 at room temperature in a thin-film design, presenting the highest power factor reaching 1740 μW mK−2. The temperature dependence of both the electrical conductivity and Seebeck coefficient in this C6H4NH2CuBr2I film illustrates that this organic/inorganic hybrid film behaves like a metallic material. The high electrical conductivity of the C6H4NH2CuBr2I film can be attributed to the defect-related heavy doping caused by CuI in this semiconductor, while the relatively large Seebeck coefficient is due to the coexisting entropy difference and polarization difference. The temperature-dependent polarization difference between the hot and the cold side of the film can function as an additional driving force besides the entropy difference and contribute to the Seebeck effect development in C6H4NH2CuBr2I. When applying a temperature difference of 4.8 °C along the in-plane direction, the C6H4NH2CuBr2I film with gold contacts generates a thermoelectric voltage of −0.31 mV and a thermoelectric current of −0.022 mA, suggesting a promising n-type thermoelectric material for heat–electricity conversion in thermoelectric power generators.