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.     

Microcavity Structure Provides High‐Performance (>8.1%) Semitransparent and Colorful Organic Photovoltaics

High‐performance colored aesthetic semitransparent organic photovoltaics (OPVs) featuring a silver/indium tin oxide/silver (Ag/ITO/Ag) microcavity structure are prepared. By precisely controlling the thickness of the ITO layer, OPV devices exhibiting high transparency and a wide and high‐purity color gamut are obtained: blue ( B ), green ( G ), yellow‐green ( YG ), yellow ( Y ), orange ( O ), and red ( R ). The power conversion efficiencies (PCEs) of the G , YG , and Y color devices are greater than 8% (AM 1.5G irradiation, 100 mW cm^?2) with maximum transmittances (T_MAX) of greater than 14.5%. An optimized PCE of 8.2% was obtained for the YG OPV [CIE 1931 coordinates: (0.364, 0.542)], with a value of T_MAX of 17.3% (at 561 nm). As far as it is known, this performance is the highest ever reported for a transparent colorful OPV. Such high transparency and desired transmitted colors, which can perspective see the clear images, suggest great potential for use in building‐integrated photovoltaic applications.     

Elucidating the role of current injection on the influence of open-circuit voltage in small-molecule organic photovoltaic devices: From the aspects of charge transfer and electroluminescent spectrum

We demonstrate that the open-circuit voltage (V_OC) of organic photovoltaic (OPV) devices composed of rubrene and C₆₀ can be considerably different when the anode and active layer are changed. Two types of anodes and active layers were compared. In plasma-treated indium-tin-oxide (ITO) OPV devices, the parameter V_OC exhibits an improvement from 0.68 V to 0.76 V when the device structure is varied from a bilayer to a mixed structure. However, in the OPV devices that use ITO/MoO₃ as the anode, a similar V_OC is observed regardless of the device structure. A series of temperature-dependent measurements are conducted to investigate these results. The calculation of barrier height at the rubrene/C₆₀ (or rubrene:C₆₀) interface yields the prediction of V_OC, suggesting that an excess energetic loss occurs in the mixed structures. The electroluminescent (EL) spectra of these devices show that the mixed structure can completely quench the EL of rubrene single layer. A broad band of the charge transfer (CT) emission is observed clearly. A temperature-dependent measurement for the extracting injection barrier is conducted and shows that the mixed structure is favorable for the hole current injection. The CT properties are obtained using the external quantum efficiency and EL spectra of the OPV devices. We find that the nonradiative recombination loss is highly correlated with the injected current; the lower the injection barrier induced the less the nonradiative recombination loss. Therefore, the parameter V_OC can be improved when the injected current is increased.     

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.     

Metal deposition for optoelectronic devices using a low vacuum vapor phase deposition (VPD) system

A vapor phase deposition (VPD) system has been used to deposit magnesium and zinc films and prepare optoelectronic devices under low vacuum conditions, i.e. 1 torr. An analysis of the metal films via SEM, AFM, XRD and four-point probe resistivity measurements revealed comparable characteristics to metal films deposited in a vacuum thermal evaporation (VTE) system. Magnesium cathodes were fabricated for organic light emitting diodes (OLEDs) and organic photovoltaic (OPV) devices. OLEDs were fully made in either the VPD or VTE system employing aluminum tris-(8 hydroxyquinoline) [Alq₃] as the green fluorescent emitter or fac-tris(2-phenylpyridine)iridium [Ir(ppy)₃] as the green emitting phosphor. Analysis of the OLED devices made in the VPD system showed external quantum efficiencies (EQE = 0.9 ± 0.1%) and (EQE = 7.6 ± 0.6%) at a luminance of 100 cd/m² for the fluorescent and phosphorescent devices, respectively. In addition, organic photovoltaics (OPVs) were fully fabricated by both methods employing copper phthalocyanine (CuPc) and C₆₀ as the donor/acceptor materials. Analysis of the OPV devices made in the VPD system showed a power efficiency of 0.5 ± 0.1%, an open circuit voltage of 0.45 ± 0.05% and a fill factor of 0.50 ± 0.05%.     

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.     

PEDOT:PSS-Free Polymer Non-Fullerene Polymer Solar Cells with Efficiency up to 18.60% Employing a Binary-Solvent-Chlorinated ITO Anode

Despite the tremendous development of different high-performing photovoltaic systems in non-fullerene polymer solar cells (PSCs), improving their performance is still highly demanding. Herein, an effective and compatible strategy, i.e., binary-solvent-chlorinated indium tin oxide (ITO) anode, is presented to improve the device performance of the state-of-the-art photoactive systems. Although both ODCB (1,2-dichlorobenzene) solvent- and ODCB:H₂O₂ (hydrogen peroxide) co-solvent-chlorinated ITO (ITO-Cl_-ODCB and ITO-Cl_-ODCB:H2O2) show similar optical transmittance, electrical conductivities, and work function values, ITO-Cl_-ODCB:H2O2 exhibits higher Cl surface coverage and more suitable surface free energy close to the poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)-buffered ITO anode (ITO/PEDOT:PSS). As a direct consequence, the performance of ITO-Cl_-ODCB-based PBDB-T-2F:BTP-eC9:PC₇₁BM PSCs is comparable as the bare ITO-based devices. In contrast, the performance of ITO-Cl_-ODCB:H2O2-based devices with both small and the scaled-up areas significantly surpass the ITO/PEDOT:PSS-based devices. Furthermore, detailed experimental studies are conducted linking optical property, blend morphology, and physical dynamics to find the reasons for the performance difference. By applying the ITO-Cl_-ODCB:H2O2 anode to six other photovoltaic systems, the device efficiencies are enhanced by 3.6–6.2% relative to those of the ITO/PEDOT:PSS-based control devices, which validates its great application potential of co-solvent-modified ITO anode employed into PEDOT:PSS-free PSCs.     

Work‐Function Engineering of Graphene Anode by Bis(trifluoromethanesulfonyl)amide Doping for Efficient Polymer Light‐Emitting Diodes

Graphene has been considered to be a potential alternative transparent and flexible electrode for replacing commercially available indium tin oxide (ITO) anode. However, the relatively high sheet resistance and low work function of graphene compared with ITO limit the application of graphene as an anode for organic or polymer light‐emitting diodes (OLEDs or PLEDs). Here, flexible PLEDs made by using bis(trifluoromethanesulfonyl)amide (TFSA, [CF₃SO₂]₂NH) doped graphene anodes are demonstrated to have low sheet resistance and high work function. The graphene is easily doped with TFSA by means of a simple spin‐coating process. After TFSA doping, the sheet resistance of the TFSA‐doped five‐layer graphene, with optical transmittance of ≈88%, is as low as ≈90 Ω sq^?1. The maximum current efficiency and power efficiency of the PLED fabricated on the TFSA‐doped graphene anode are 9.6 cd A^?1 and 10.5 lm W^?1, respectively; these values are markedly higher than those of the PLED fabricated on pristine graphene anode and comparable to those of an ITO anode.     

Degradation of self-assembled monolayers in organic photovoltaic devices

Self-assembled monolayers (SAMs) based on n-octylphosphonic acid (C8PA) and 1H,1H,2H,2H-perfluorooctanephosphonic acid (PFOPA) were investigated for application as an anode buffer layer in C₆₀-based organic photovoltaic (OPV) devices. We found that the degradation of the OPV efficiency with respect to air exposure was significantly reduced with the perfluorinated PFOPA compared to the aliphatic C8PA. We attribute the OPV degradation to moisture diffusion from the top aluminum electrode and the lowering of the anode work function as a result of hydrolysis of the SAM buffer layer.     

Lending Triarylphosphine Oxide to Phenanthroline: a Facile Approach to High‐Performance Organic Small‐Molecule Cathode Interfacial Material for Organic Photovoltaics utilizing Air‐Stable Cathodes

Cathode interfacial material (CIM) is critical to improving the power conversion efficiency (PCE) and long‐term stability of an organic photovoltaic cell that utilizes a high work function cathode. In this contribution, a novel CIM is reported through an effective and yet simple combination of triarylphosphine oxide with a 1,10‐phenanthrolinyl unit. The resulting CIM possesses easy synthesis and purification, a high T _g of 116 °C and attractive electron‐transport properties. The characterization of photovoltaic devices involving Ag or Al cathodes shows that this thermally deposited interlayer can considerably improve the PCE, due largely to a simultaneous increase in V _oc and FF relative to the reference devices without a CIM. Notably, a PCE of 7.51% is obtained for the CIM/Ag device utilizing the active layer PTB7:PC₇₁BM, which far exceeds that of the reference Ag device and compares well to that of the Ca/Al device. The PCE is further increased to 8.56% for the CIM/Al device (with J _sc = 16.81 mA cm^?2, V _oc = 0.75 V, FF = 0.68). Ultraviolet photoemission spectroscopy studies reveal that this promising CIM can significantly lower the work function of the Ag metal as well as ITO and HOPG, and facilitate electron extraction in OPV devices.     

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.     

Combined alternative electrodes for semi-transparent and ITO-free small molecule organic solar cells

We investigate various electrode combinations of bottom and top contacts for organic photovoltaic (OPV) cells. Silver (Ag), indium tin oxide (ITO), poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), and silver nanowires (AgNW) are used as bottom electrodes. As top electrodes, thin silver layers (t-Ag) and free-standing carbon nanotube (f-CNT) sheets are employed. The manufactured zinc phthalocyanine (ZnPc): fullerene C₆₀ small molecule bulk heterojunction OPV cells with different kinds of bottom electrodes show efficiencies of 1.9∼2.2% and 1.1∼1.5%, when comprised of t-Ag and f-CNT top contacts, respectively. We demonstrate alternative electrodes beyond ITO, silver, and aluminum, which can be readily used for organic photovoltaics technology.     

Effect of Al:Ag alloy cathode on the performance of transparent organic light-emitting devices

Transparent organic light-emitting devices (TOLEDs) based on a stacked alloy cathode of LiF/Al:Ag are investigated. The devices have a structure of indium-tin-oxide (ITO)/4,4′,4′′-Tris[2-naphthyl(phenyl)amino]triphenylamine (2T-NATA) (25 nm)/N,N''-Di-[(1-naphthyl)-N,N''-diphenyl]-1,1''-biphenyl-4,4''-diamine (NPB) (40 nm)/tris-(8-hydroxyquinoline) aluminum (Alq3) (50 nm)/LiF (1 nm)/Al:Ag (1:3) (x), where the thicknesses of cathode metal layers (Al:Ag) are adjusted, respectively, from 70 nm to 100 nm. In the experiment, it is found that the LiF (1 nm)/Al:Ag (1:3) (75 nm) has good electron injection efficiency. Compared with an Al-only cathode, the turn-on voltage is lowered. At the voltage of 10 V, the luminances for bottom emission from ITO anode side and top emission from metal cathode side are 2 459 cd/m2 and 1 729 cd/m2, respectively. Thanks to electron injection enhancement by using Al:Ag cathode, we can obtain a better energy level matching between the cathode and the organic layer, thus the devices have lower turn-on voltage and higher luminance. The total transmittance of the devices can achieve about 40% at the wavelength of 550 nm.     

Highly efficient two component phosphorescent organic light-emitting diodes based on direct hole injection into dopant and gradient doping

A series of two component phosphorescent organic light-emitting diodes (PHOLEDs) combing the direct hole injection into dopant strategy with a gradient doping profile were demonstrated. The dopant, host, as well as molybdenum oxide (MoO₃)-modified indium tin oxide (ITO) anode were investigated. It is found that the devices ITO/MoO₃ (0 or 1 nm)/fac-tris(2-phenylpyridine)iridium [Ir(ppy)₃]:1,3,5-tris(N-phenylbenzimidazole-2-yl)benzene (TPBi) (30 → 0 wt%, 105 nm)/LiF (1 nm)/Al (100 nm) show maximum external quantum efficiency (EQE) over 20%, which are comparable to multi-layered PHOLEDs. Moreover, the systematic variation of the host from TPBi to 4,7-diphenyl-1,10-phenanthroline (Bphen), dopant from Ir(ppy)₃ to bis(2-phenylpyridine)(acetylacetonate)iridium [Ir(ppy)₂(acac)], and anodes between ITO and ITO/MoO₃ indicates that balancing the charge as well as controlling the charge recombination zone play critical roles in the design of highly efficient two component PHOLEDs.     

An Indium‐Free Anode for Large‐Area Flexible OLEDs: Defect‐Free Transparent Conductive Zinc Tin Oxide

Flexible large‐area organic light‐emitting diodes (OLEDs) require highly conductive and transparent anodes for efficient and uniform light emission. Tin‐doped indium oxide (ITO) is the standard anode in industry. However, due to the scarcity of indium, alternative anodes that eliminate its use are highly desired. Here an indium‐free anode is developed by a combinatorial study of zinc oxide (ZnO) and tin oxide (SnO₂), both composed of earth‐abundant elements. The optimized Zn–Sn–O (ZTO) films have electron mobilities of up to 21 cm² V^?1 s^?1, a conductivity of 245 S cm^?1, and <5% absorptance in the visible range of the spectrum. The high electron mobilities and low surface roughness (<0.2 nm) are achieved by producing dense and void‐free amorphous layers as confirmed by transmission electron microscopy. These ZTO layers are evaluated for OLEDs in two anode configurations: i) 10 cm² devices with ZTO/Ag/ZTO and ii) 41 cm² devices with ZTO plus a metal grid. The ZTO layers are compatible with OLED processing steps and large‐area white OLEDs fabricated with the ZTO/grid anode show better performance than those with ITO/grid anodes. These results confirm that ZTO has the potential as an In‐free and Earth‐abundant alternative to ITO for large‐area flexible OLEDs.     

Polymer solar cells with NiO hole-collecting interlayers processed by atomic layer deposition

We report on the photovoltaic properties of polymer solar cells that use NiO-coated indium tin oxide (ITO) as the hole-collecting electrode. The NiO films were prepared by atomic layer deposition (ALD) on top of ITO with thicknesses varying from 6 to 25 nm. The NiO films increase the work function (WF) of the ITO, allowing NiO-coated ITO to act as an efficient hole-collecting electrode. Devices made with pristine NiO showed poor current–voltage characteristics. However, subsequent O₂-plasma treatment further increased the WF of NiO, tuning NiO-coated ITO into an efficient hole-collecting electrode for polymer solar cells based on the donor poly(3-hexylthiophene-2,5-diyl) (P3HT). The polymer solar cells with the O₂-plasma treated NiO-coated ITO hole-collecting electrodes yield a power conversion efficiency of 4.1 ± 0.2% under simulated air mass 1.5 G 100 mW/cm² illumination, which is comparable to reference devices with poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS)-coated ITO hole-collecting electrodes.     

Fine tuning hole injection for high-performance polyfluorene-based blue emitting device by adjusting work function of anode via deposition of CFx layer with proper ionization potential on indium tin oxide

By introducing CF_x thin film as hole injection layer on top of indium tin oxide (ITO) anode via plasma polymerization of CHF₃, the device with poly(9,9-dioctylfluorene) (PFO) as emitting layer, ITO/CF_x(35 W)/PFO/CsF/Ca/Al, is prepared. At the optimal C/F atom ratio using the radio frequency power 35 W, the device performance is optimal having the maximum current efficiency 3.1 cd/A and maximum brightness 8400 cd/m². This is attributed to a better balance between hole and electron fluxes, resulting from a decrease in hole injection barrier as manifested by ultraviolet photoelectron spectroscopy and scanning surface potential microscopy.     

Spraycoating of silver nanoparticle electrodes for inverted polymer solar cells

Inverted polymer solar cells using non-vacuum processed spraycoated silver nanoparticles (Ag-NPs) as the anode electrode were compared to vacuum deposited Ag electrodes. The number of spray coating layers can affect its final device performance showing with a higher number of coating layers, a better nanoparticle interconnectivity and morphology is achieved which reduces the sheet resistance and transparency of the Ag electrode leading to improved fill factor and device performances (～3.0%). These devices can be fabricated onto flexible ITO substrates devices showing a performance of ～1.40%. Using a non-vacuum technique to deposit the electrode is important for low-cost polymer solar cells.