基于CuPc…C60的混合层异质结光伏器件性能研究

制备了基于CuPc…C60混合层异质结有机光伏器件,将其与CuPc-C60双层结构光伏器件进行对比研究。结果表明混合层结构器件性能得到改善,其开路电压、短路电流密度、填充因子和光电转换效率都有提高,分别从CuPc-C60双层结构器件的0.39V、1.92mA/cm2、0.36%、0.48依次提高到CuPc…C60混合层结构器件的0.48V、2.21mA/cm2、0.54%、0.51。根据整数电荷转移模型来分析光伏器件D/A界面及有机材料-ITO衬底界面特性,认为混合层异质结有机光伏器件给体材料HOMO与受体材料LUMO的能级差增加使得器件开路电压提高。混合层异质结有机光伏器件D/A界面面积增加和给体材料HOMO与受体材料LUMO的能级差增加都提高了激子的分离效率,所以器件的短路电流密度增加。     

Ultraviolet-illuminated fluoropolymer indium–tin-oxide buffer layers for improved power conversion in organic photovoltaics

We demonstrate that the charge carrier extraction in double heterojunction organic photovoltaic(OPV) devices can be enhanced by inserting an UV-illuminated fluoropolymer polytetrafluoroethylene(PTFE) layer between indium–tin-oxide and the thermal evaporated copper–phthalocyanine(CuPc)/buckyball(C₆₀) organic active layers. In this work, we show that the anode work function influences the photocarrier collection characteristics, where the short-circuit current and open-circuit voltage increase from 1.6 to 4.8 mA/cm² and 0.41 to 0.48 V, respectively after the buffer layer insertion associated primary with the barrier decrease in the ITO/CuPc interface. This result shows the potential of UV-illuminated PTFE as a low-cost stable buffer layer for OPV devices.     

Impact of Ground‐State Charge Transfer and Polarization Energy Change on Energy Band Offsets at Donor/Acceptor Interface in Organic Photovoltaics

The fullerene (C₆₀)/copper phthalocyanine (CuPc) interface is one of the widely used donor/acceptor (DA) interfaces for organic photovoltaics (OPVs), and information on the electronic structure at the interface is essential for fully understanding the energetics of excitons and carriers in OPVs. Here, an investigation into the energy levels at the C₆₀/CuPc interface is made using UV photoelectron, X‐ray photoelectron, and inverse photoemission spectroscopies. The vacuum level and core levels rise with C₆₀ deposition on the CuPc film, which indicates that the interfacial dipole is formed with the negative charge on the C₆₀ side. The interfacial dipole can be formed by the electron transfer from CuPc to C₆₀ in the ground state at the interface, which is indicated by the analysis of the UV–vis–NIR absorption spectrum of the CuPc/C₆₀ blended film. On the other hand, the highest occupied and lowest unoccupied molecular orbitals of CuPc and C₆₀ shift in opposite directions at the interface. This is attributed to the changes of the polarization energies of CuPc and C₆₀ at the interface. The formation of the interfacial dipole and the change of the polarization energy result in the anomalous energy band offsets at the C₆₀/CuPc interface, which are entirely different from those in inorganic p–n junctions.     

Enhanced storage/operation stability of small molecule organic photovoltaics using graphene oxide interfacial layer

Graphene and graphene oxide (GO) have been applied in flexible organic electronic devices with enhanced efficiency of polymeric photovoltaic (OPV) devices. In this work, we demonstrate that storage/operation stability of OPV can be substantially enhanced by spin-coating a GO buffer layer on ITO without any further treatment. With a 2 nm GO buffer layer, the power conversion efficiency (PCE) of a standard copper phthalocyanine (CuPc)/fullerene (C₆₀) based OPV device shows about 30% enhancement from 1.5% to 1.9%. More importantly, while the PCE of the standard device drop to 1/1000 of its original value after 60-days of operation-storage cycles; those of GO-buffered device maintained 84% of initial PCE even after 132-days. Atomic force microscopy studies show that CuPc forms larger crystallites on the GO-buffered ITO substrate leading to better optical absorption and thus photon utilization. Stability enhancement is attributed to the diffusion barrier of the GO layer which slow down diffusion of oxygen species from ITO to the active layers.     

Photoconductivity of C60 as an Origin of Bias‐Dependent Photocurrent in Organic Photovoltaics

The bulk‐ionized photoconductivity of C₆₀ is reported as an origin of the bias‐dependent linear change of the photocurrent in copper phthalocyanine (CuPc)/C₆₀ planar heterojunction solar cells, based on the observation of the variation of the bias‐dependent photocurrent on excitation wavelengths and the thickness‐dependent photocurrent of the C₆₀ layer. A theoretical model, which is a combination of the Braun‐Onsager model for the dissociation of excitons at the donor/acceptor interface and the Onsager model for the bulk ionization of excitons in the C₆₀ layer, describes the bias‐dependent photocurrent in the devices very well. The bulk‐ionized photoconductivity of C₆₀ must generally contribute to the photocurrent in organic photovoltaics, since fullerene and fullerene derivatives are widely used in these devices.     

CuPc buffer layer role in OLED performance: a study of the interfacial band energies

We present a UV photoemission (UPS) and topographic study of the indium–tin-oxide (ITO) anode used in organic light emitting diodes (OLEDs). The performance of these devices has been shown to be improved by introducing a thin layer of copper phthalocyanine (CuPc) between the anode and the hole-transport layer (HTL). While the device current at constant driving voltage increases with increasing CuPc thickness, the efficiency is optimized at 12–18 nm. In this article we address the issue of the charge (hole) injection process at the anode interface and demonstrate the importance of directly measuring the vacuum levels in quantitative study of the energy levels of OLED interfaces. As a result of this insight, many calculations relating to the relative energies of the bands at OLED interfaces may have to be revised and corrected. The interface morphologies were also studied using AFM to determine any changes with film growth.     

Improved photovoltaic characteristics of organic cells with heterointerface layer as a hole-extraction layer inserted between ITO anode and donor layer

We have fabricated an improved organic photovoltaic (OPV) cell in which organic heterointerface layer is inserted between indium-tin-oxide (ITO) anode and copper-phthalocyanine (CuPc) donor layer in the conventional OPV cell of ITO/CuPc/fullerene (C₆₀)/bathophenanthroline (Bphen)/Al to enhance the power conversion efficiency (PCE) and fill factor (FF). The inserted ITO-buffer layer consists of electron-transporting layer (ETL) and hole-transporting layer (HTL). We have changed the ETL and HTL materials variously and also changed their layer thickness variously. It is confirmed that ETL materials with higher LUMO level than the work function of ITO give low PCE and FF. All the double layer buffers give higher PCE than a single layer buffer of TAPC. The highest PCE of 1.67% and FF of 0.57% are obtained from an ITO buffer consisted of 3 nm thick ETL of hexadecafkluoro-copper-phthalocyanine (F₁₆CuPc) and 3 nm thick HTL of 1,1-bis-(4-methyl-phenyl)-aminophenylcyclohexane (TAPC). This PCE is 1.64 times higher than PCE of the cell without ITO buffer and 2.98 times higher than PCE of the cell with single layer ITO buffer of TAPC. PCE is found to increase with increasing energy difference (ΔE) between the HOMO level of HTL and LUMO level of F₁₆CuPc in a range of ΔE < 0.6 eV. From the ΔE dependence of PCE, it is suggested that electrons moved from ITO to the LUMO level of the electron-transporting F₁₆CuPc are recombined, at the F₁₆CuPc/HTL-interface, with holes transported from CuPc to the HOMO level of HTL in the double layer ITO buffer ETL, leading to efficient extraction of holes photo-generated in CuPc donor layer.     

功能层界面制备方法对OLED性能的影响

在功能层界面处采用各功能材料共蒸的方法,制备了典型的绿光有机发光器件(OLED)。器件的结构为ITO/NPB(37nm)/(NPB:Alq3)(3nm)/Alq3(27nm):C545T(3%)/Alq3(20nm)/LiF(1nm)/Al(100nm),并与传统的制备方法进行了比较。结果发现,起亮电压从4.5V降低到2.5V,最高耐压从16V提高到21V,最大亮度从13 940cd/m2提高到24 630cd/m2,发光效率由7.0cd/A提高到11.4cd/A。结果表明,本文方法有利于载流子传输,可以有效提高激子形成概率,提高了OLED发光效率。     

Organic/inorganic F8T2/GaN light emitting heterojunction

An organic/inorganic white-light emitting F8T2 (9,9-dioctylfluorene-co-bithiophene)/GaN heterojunction is reported. The white-light emission is produced by hybridizing the blue light (464 nm) emitted from the GaN MQWs and the yellow/green light (500–650 nm) emitted at the F8T2/p-GaN interface by electroluminescence (EL). The yellow/green light emission in the F8T2 layer is resulted from the carrier accumulation and Frenkel excitons at the F8T2/p-GaN junction interface. It is concluded that the energy barrier and large mobility discrepancy at the F8T2/p-GaN junction interface cause carriers accumulating in the F8T2 side near the F8T2/p-GaN interface. The accumulated carriers at the F8T2/p-GaN interface form Frenkel excitons by Coulombic interaction. Then, the Frenkel excitons recombine to radiate the yellow/green emission in the F8T2 layer. The International Commission on Illumination (CIE) coordinate of the white-light emitted from the present device is at (0.28, 0.30), which is very close to the standard white light (0.33, 0.33).     

Using Self‐Assembling Dipole Molecules to Improve Charge Collection in Molecular Solar Cells

Surface modification of indium tin oxide (ITO)‐coated substrates through the use of self‐assembled monolayers (SAMs) of molecules with permanent dipole moments has been used to control the anode work function and device performance in molecular solar cells based on a CuPc:C₆₀ (CuPc: copper phthalocyanine) heterojunction. Use of SAMs increases both the short‐circuit current density (J_sc) and fill factor, increasing the power‐conversion efficiency by up to an order of magnitude. This improvement is attributed primarily to an enhanced interfacial charge transfer rate at the anode, due to both a decrease in the interfacial energy step between the anode work function and the highest occupied molecular orbital (HOMO) level of the organic layer, and a better compatibility of the SAM‐modified electrodes with the initial CuPc layers, which leads to a higher density of active sites for charge transfer. An additional factor may be the influence of increasing electric field at the heterojunction on the exciton‐dissociation efficiency. This is supported by calculations of the electric potential distribution for the structures. Work‐function modification has virtually no effect on the open‐circuit voltage (V_oc), in accordance with the idea that V_oc is controlled primarily by the energy levels of the donor and acceptor materials.     

Enhancement of hole injection with an ultra-thin Ag2O modified anode in organic light-emitting diodes

The interface between the organic layer and the Indium Tin Oxide (ITO) layer of an organic light-emitting diode (OLED) is crucial to the performance of the device. An ultra-thin Ag₂O film, used as an anode modification layer, has been employed on ITO surface through the UV-ozone treatment of Ag films. The insertion of this thin film with higher work function enhances the hole injection in the organic light-emitting diode and improves the performance of the devices effectively. The maximum electroluminescence (EL) efficiency of the device with the Ag₂O film is 4.95 cd/A, it is about 60% higher than that of the device without it.     

Photomultiplication in Disordered Unipolar Organic Materials

Photomultiplication in conventional inorganic semiconductors has been known and used for decades, the underlying mechanism being multiplication by impact ionization triggered by hot carriers. Since neither carrier heating by an electric field nor avalanche multiplication are possible in strongly disordered organic solids, charge multiplication seems to be highly unlikely in these materials. However, here the photomultiplication observed in the bulk of a unipolar disordered organic semiconductor is reported. The proportion of extracted carriers to incident photons is experimentally determined to be in excess of 3000 % in a single‐layer device of the air‐stable, n‐type organic semiconductor F₁₆CuPc (Pc: phthalocyanine). This effect is explained in terms of exciton quenching by localized charges, the subsequent promotion of these detrapped charges to the high‐mobility energy band of the density‐of‐states (DOS) distribution, and subsequent slow equilibration within this broad intrinsic DOS. Such a mechanism allows multiple replenishment of the optically released charge by mobile carriers injected from an Ohmic electrode. Also shown is photomultiplication in double‐layer devices composed of layers of donor and acceptor small‐molecule materials. This result implies that, apart from exciton dissociation at a donor/acceptor interface, exciton energy transfer to trapped carriers is a complementary photoconductivity process in organic solar cells. This new insight paves the way to cheap, highly efficient organic photodetectors on flexible substrates for numerous applications.     

Interfacial dipole in organic p–n junction to realize write-once–read-many-times memory

A new approach is exploited to realize nonvolatile organic write-once–read-many-times (WORM) memory based on copper phthalocyanine (CuPc)/hexadecafluoro-copper-phthalocyanine (F₁₆CuPc) p–n junction. The as-fabricated device is found to be at its ON state and can be programmed irreversibly to the OFF state by applying a negative bias. The WORM device exhibits a high ON/OFF current ratio of up to 2.6 × 10⁴. An interfacial dipole layer is testified to be formed and destructed at the p–n junction interface for the ON and OFF states, respectively. The ON state at positive voltage region is attributed to the efficient hole and electron injection from the respective electrodes and then recombination at the CuPc/F₁₆CuPc interface, and the transition of the device to the OFF state results from the destruction of the interfacial dipole layer and formation of an insulating layer which restricts charge carrier recombination at the interface.     

以ZnO为空穴缓冲层的高效率有机电致发光器件

研制了在传统双层有机电致发光器件(OLED) ITO/NPB/AlQ/Al的阳极与空穴传输层间加入ZnO缓冲层的新型器件.研究了加入缓冲层后对OLED性能的影响,并比较了新型与传统OLED的性能,结果表明,新型器件比传统器件的耐压能力有了显著提高;当电压达到7 V时,发光效率提高了35%.分析认为,ZnO缓冲层的加入,改善了界面, 减少了漏电流,并且阻碍了空穴的注入,有利于改善空穴和电子的注入平衡,提高复合效率.     

Au-ITO anode for efficient polymer light-emitting device operation

A thick gold layer is deposited on indium tin oxide (ITO) to improve the interface quality between the ITO anode and the organic layer in organic light-emitting diodes (LEDs). With this improvement, the device with structure ITO/Au/hole-transport-layer(HTL)/poly(p-phenylenevinylene)/Ca/Ag, achieved a lower turn-on voltage from 4 V to about 1.6 V and an increase in luminescence intensity by more than a factor of two at the same voltage. The work function of the Au facilitates the formation of an ohmic contact and good mechanical adhesion to the HTL. The experimental results suggest that the ITO contact limits the supply of current for radiative recombination. The improvement of the device performance is due to the smoother Au surface and the matching of the Au work function with the highest occupied molecular orbital level of adjacent HTL layer.     

A reduced electron-extraction barrier at an interface between a polymer poly(3-hexylthiophene) layer and an indium tin oxide layer

Roles of the buried interface between polymer poly(3-hexylthiophene) (P3HT) layer and indium tin oxide (ITO) on the glass substrate have been characterized by transient photovoltage (TPV). Since P3HT is the hole-transporting material, from intuitiveness, ITO/P3HT contact (IPcontact) tends to be hole extracting. However, in this letter, the negative TPV of ITO/P3HT/Al demonstrates that IPcontact dominates the reversed built in electric field, namely pointing from ITO to Al, and is confirmed to be electron extracting. Meanwhile, an interesting biphasic feature of TPV is demonstrated in a device of ITO/P3HT:[6,6]-phenyl-C61-butyric acid methyl ester/Al. The negative component in biphasic TPV shows that IPcontact is one reason resulting in the leakage current for P3HT based solar cells in normal structures. The theoretical study is conducted, and reveals that the interaction between P3HT and ITO reduces electron barrier by 0.5 eV for IPcontact. Band bending and dipole formation are two possible reasons to reduce the electron barrier. By taking advantage of the electron extraction, IPcontact is employed as a composite cathode in an inverted solar cell by pre-coating a pristine P3HT buffer layer between a blended layer and ITO. The study paves a way to characterize the buried interface in solution processable optoelectronics by observing polarity change of TPV, and to fabricate the simplified inverted organic solar cell employing IPcontact to extract electrons.     

C_(60)作激子阻挡层的有机发光二极管

制备了结构为ITO/CuPc(25nm)/NPB(40nm)/Alq3(xnm)/C60(ynm)/LiF(1nm)/Al(100nm)的有机发光二极管(OLEDs),研究了C60插入层对器件性能的影响。结果表明,在无C60的器件中,当Alq3层较厚时,器件的电流密度-电压(J-V)曲线右移,不利于获得高功率效率;当Alq3层较薄时,又会导致激子在LiF/Al阴极的严重淬灭。实验优化得出,在无C60的器件中,Alq3厚为45nm的器件可获得最高的功率效率。在Alq3与LiF之间插入15nmC60层后,对器件的J-V曲线几乎没有影响,但C60层阻挡了激子向阴极扩散,减少了淬灭。当在Alq3厚度为45nm的器件的Alq3和LiF间插入15nmC60层后,可使器件获得更高的功率效率,尤其是插入15nmC并将Alq厚度降至30nm,获得了最大的功率效率。     

Cover Picture: n‐Channel,Ambipolar, and p‐Channel Organic Heterojunction Transistors Fabricated with Various Film Morphologies (Adv. Funct. Mater. 3/2007)

Simultaneous introduction of short‐range repulsive interactions between dissimilar colloidal particles and attractive interactions between like particles provides a general new route to fabricating self‐organizing bipolar devices. By identifying combinations of conductive device materials between which short‐range repulsive forces exist in the presence of an intervening liquid, electrochemical junctions can be self‐formed, as reported by Chiang and co‐workers on p. 379. The relationship between the performance characteristics of organic field‐effect transistors (OFETs) with 2,5‐bis(4‐biphenylyl)bithiophene/copper hexadecafluorophthalocyanine (BP2T/F₁₆CuPc) heterojunctions and the thickness of the BP2T bottom layer is investigated. Three operating modes (n‐channel, ambipolar, and p‐channel) are obtained by varying the thickness of the organic semiconductor layer. The changes in operating mode are attributable to the morphology of the film and the heterojunction effect, which also leads to an evolution of the field‐effect mobility with increasing film thickness. In BP2T/F₁₆CuPc heterojunctions the mobile charge carriers accumulate at both sides of the heterojunction interface, with an accumulation layer thickness of ca. 10 nm. High field‐effect mobility values can be achieved in continuous and flat films that exhibit the heterojunction effect.     

n‐Channel,Ambipolar, and p‐Channel Organic Heterojunction Transistors Fabricated with Various Film Morphologies

The relationship between the performance characteristics of organic field‐effect transistors (OFETs) with 2,5‐bis(4‐biphenylyl)bithiophene/copper hexadecafluorophthalocyanine (BP2T/F₁₆CuPc) heterojunctions and the thickness of the BP2T bottom layer is investigated. Three operating modes (n‐channel, ambipolar, and p‐channel) are obtained by varying the thickness of the organic semiconductor layer. The changes in operating mode are attributable to the morphology of the film and the heterojunction effect, which also leads to an evolution of the field‐effect mobility with increasing film thickness. In BP2T/F₁₆CuPc heterojunctions the mobile charge carriers accumulate at both sides of the heterojunction interface, with an accumulation layer thickness of ca. 10 nm. High field‐effect mobility values can be achieved in continuous and flat films that exhibit the heterojunction effect.     

Interplay of cleaning and de-doping in oxygen plasma treated high work function indium tin oxide (ITO)

Indium tin oxide (ITO) is extensively used as a transparent electrode in photovoltaic cells and organic light emitting diodes. High surface work function (WF) of ITO is a crucial parameter for enhanced device performance. The ITO WF is usually around 4.3 eV without any surface treatment. With surface treatments ITO WF, as high as 5.4 eV has been reported. We designed a surface treatment method with which we achieved substantially high ITO surface work function of over 6.1 eV. We observed changes in valence electronic structure and core levels, apart from surface cleaning. We also investigated interface formation of copper phthalocyanine (CuPc) on the high WF ITO. In the proximity of the interface the highest occupied energy level of CuPc was observed to be almost pinned to the Fermi level. We fabricated three simple devices with no to high treatment. The device results were observed to be consistent with the findings of electronic energy level alignment.