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.     

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.     

The working mechanism of organic photovoltaic cell by using copper phthalocyanine as exciton blocking layer

We demonstrate the working mechanism of organic photovoltaic (OPV) cell with copper phthalocyanine (CuPc) as exciton blocking layer (EBL). The new EBL material CuPc, commonly has been used as electron donor in the organic solar cells due to its electron-donating and hole-transporting properties. But here we proves that the α-polymorph CuPc layer can transfer electrons to Al cathode through the half-filled b_1g level, this mechanism is different from that of general EBL material with larger band gap and electron-transporting property, which is based on damage states induced by the heat of evaporating Al.     

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.     

Heterojunction Ambipolar Organic Transistors Fabricated by a Two‐Step Vacuum‐Deposition Process

Ambipolar organic field‐effect transistors (OFETs) are produced, based on organic heterojunctions fabricated by a two‐step vacuum‐deposition process. Copper phthalocyanine (CuPc) deposited at a high temperature (250 °C) acts as the first (p‐type component) layer, and hexadecafluorophthalocyaninatocopper (F₁₆CuPc) deposited at room temperature (25 °C) acts as the second (n‐type component) layer. A heterojunction with an interpenetrating network is obtained as the active layer for the OFETs. These heterojunction devices display significant ambipolar charge transport with symmetric electron and hole mobilities of the order of 10^–4 cm² V^–1 s^–1 in air. Conductive channels are at the interface between the F₁₆CuPc and CuPc domains in the interpenetrating networks. Electrons are transported in the F₁₆CuPc regions, and holes in the CuPc regions. The molecular arrangement in the heterojunction is well ordered, resulting in a balance of the two carrier densities responsible for the ambipolar electrical characteristics. The thin‐film morphology of the organic heterojunction with its interpenetrating network structure can be controlled well by the vacuum‐deposition process. The structure of interpenetrating networks is similar to that of the bulk heterojunction used in organic photovoltaic cells, therefore, it may be helpful in understanding the process of charge collection in organic photovoltaic cells.     

Infrared organic photovoltaic device based on charge transfer interaction between organic materials

We found that a 1:1 volume ratio mixed film formed by co-evaporation of 4,4,4-tris(N-3-methylphenyl-N-phenylamino) triphenylamine (m-MTDATA) and copper hexadecafluorophthalocyanine (F₁₆CuPc) showed a board optical absorption band from 900 to 1500 nm not observed from the two constituting materials. The new IR absorption band is attributed to the charge-transfer complex formed by electron transfer from m-MTDATA to F₁₆CuPc upon intimate contact. By using the mixed film as an active layer, we demonstrate an organic photovoltaic device (OPV) which can generate electric power from photons with longer than 1300 nm wavelength.     

Structure analysis and property improvements of the computer-simulated fullerene-based ultralow-k dielectrics

We have recently proposed new ultralow-k dielectric materials using a theoretical approach called molecular design. This approach requires the application of complementary theoretical methods to describe the complex problems. The methods include classical, continuum theoretical, and quantum-chemical approximations. The advantage of the present approach is that various possible candidates for ultralow-k dielectrics can be tested theoretically without performing expensive and time-consuming experiments. In this study, we analyze the way to connect linker molecules to the node molecules, in order to improve mechanical and dielectric properties of generated ultralow-k structures. Two different types of bonding linker molecules to the cage C₆₀ molecule with the >CC< and >CCH₂CH₂C< linker molecules are possible. It is shown that at the present improvement step it is possible to get property combinations with dielectric constant of k = 2.2 and bulk modulus of B = 33 GPa for the simple cubic topology.     

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。     

利用MoO3为电极修饰层的F16CuPc/ CuPc异质结的双极性有机场效应晶体管

在本文中,我们利用钛青铜(CuPc)和氟化钛青铜(F16CuPc)作为空穴传输层和电子传输层的制备了具有异质结结构的有机场效应晶体管(OFETs)。与单层的F16CuPc晶体管相比,异质结结构的晶体管的电子迁移率从3.1×10-3cm2/Vs提高至8.7×10-3cm2/vs,然而,空穴的传输行为却没有被观测到。为了提高空穴的注入能力,我们利用MoO3对源-漏电极进行了修饰,有效地改善了空穴注入。并进一步证实了MoO3的引入使得器件的接触电阻变小,平衡了电子和空穴的注入,从而最终实现了器件的双极性传输。     

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.     

Chemical nature of the passivation layer depending on the oxidizing agent in Gd2O3/GeO2/Ge stacks grown by molecular beam deposition

In Ge-based metal oxide semiconductor technology, the insertion of a passivation layer seems to be crucial in unpinning the Fermi level at the interface and in reducing the amount of interface defects. GeO₂ was obtained by atomic oxygen (AO), molecular oxygen or ozone chemisorption. Atomic or molecular oxygen was used in the deposition of Gd₂O₃. Gd₂O₃ thin films were grown by molecular beam deposition directly on (1 0 0) Ge or on a GeO₂ interlayer. The chemical nature of the Gd₂O₃/Ge interface was characterized by time-of-flight secondary ion mass spectrometry depth profiles. Without GeO₂ layer Gd and Ge interdiffusion is observed and the concomitant formation of GeOGd bonds is also supported by X-ray photoelectron spectroscopy energy shift at the Ge 3d peak and by a singularity in the interface defect energy distribution at ∼0.48 eV. Further, depending on the GeO₂ formation process, the profile shape of Ge and O related secondary ions at the GeO₂/Ge interface can be related with a defective Ge region close to the GeO₂/Ge. In particular, considering the ratio between Ge and GeO₂ related secondary ion signals, the interlayer passivated using AO turns out to be comparatively enriched in Ge, while the use of ozone for GeO₂ formation leads to a Ge deficient layer.     

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.     

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.     

Photovoltaic Function and Exciton/Charge Transfer Dynamics in a Highly Efficient Semiconducting Copolymer

Exciton dissociation is a key step for the light energy conversion to electricity in organic photovoltaic (OPV) devices. Here, excitonic dissociation pathways in the high‐performance, low bandgap “in‐chain donor–acceptor” polymer PTB7 by transient optical absorption (TA) spectroscopy in solutions, neat films, and bulk heterojunction (BHJ) PTB7:PC₇₁BM (phenyl‐C₇₁‐butyric acid methyl ester) films are investigated. The dynamics and energetics of the exciton and intra‐/intermolecular charge separated states are characterized. A distinct, dynamic, spectral red‐shift of the polymer cation is observed in the BHJ films in TA spectra following electron transfer from the polymer to PC₇₁BM, which can be attributed to the time evolution of the hole–electron spatial separation after exciton splitting. Effects of film morphology are also investigated and compared to those of conjugated homopolymers. The enhanced charge separation along the PTB7 alternating donor–acceptor backbone is understood by intramolecular charge separation through polarized, delocalized excitons that lower the exciton binding energy. Consequently, ultrafast charge separation and transport along these polymer backbones reduce carrier recombination in these largely amorphous films. This charge separation mechanism explains why higher degrees of PCBM intercalation within BHJ matrices enhances exciton splitting and charge transport, and thus increase OPV performance. This study proposes new guidelines for OPV materials development.     

Formation of Bulk Heterojunctions by Alternative Thermal Deposition and Its Structure Analysis for High Efficiency Small Molecular Organic Photovoltaics

We propose a new method to form small‐molecule based bulk heterojunctions (SM‐BHJs) through alternative thermal deposition (ATD), which is a simple modification of conventional thermal evaporation. By ATD, the thicknesses of alternative donor and acceptor layers are precisely controlled down to 0.1 nm, which is critical to form BHJs. The formation of a BHJ in copper(II) phthalocyanine (CuPc) and fullerene (C₆₀) systems is confirmed by atomic force microscopy (AFM), grazing incidence X‐ray small angle scattering (GISAXS), and absorption measurements. From analysis of the data, we find that the CuPc|C₆₀ films fabricated by ATD are composed of nanometer sized disk‐shaped CuPc nano grains and aggregated C₆₀, which explains the phase separation of CuPc and C₆₀. On the other hand, the co‐deposited CuPc:C₆₀ films do not show the existence of separated CuPc nano grains in the CuPc:C₆₀ matrix. The OPV cells fabricated using the ATD method show significantly enhanced power conversion efficiency compared to the co‐deposited OPV cells with the same composition.