Adhesion properties of inverted polymer solarcells: Processing and film structure parameters

We report on the adhesion of weak interfaces in inverted P3HT:PCBM-based polymer solar cells (OPV) with either a conductive polymer, PEDOT:PSS, or a metal oxide, molybdenum trioxide (MoO₃), as the hole transport layer. The PEDOT:PSS OPVs were prepared by spin or spray coating on glass substrates, or slot-die coating on flexible PET substrates. In all cases, we observed adhesive failure at the interface between the P3HT:PCBM with PEDOT:PSS layer. The adhesion energy measured for the solar cells made on glass substrates was about 1.8 J/m², but only 0.5 J/m² for the roll-to-roll processed flexible solar cells. The adhesion energy was insensitive to the PEDOT:PSS layer thickness in the range of 10–40 nm. A marginal increase in adhesion energy was measured with increased O₂ plasma power. Compared to solution processed PEDOT:PSS, we found that thermally evaporated MoO₃ adheres less to the P3HT:PCBM layer, which we attributed to the reduced mixing at the MoO₃/P3HT:PCBM interface during the thermal evaporation process. Insights into the mechanisms of delamination and the effect of different material properties and processing parameters yield general guidelines for the design of more reliable organic photovoltaic devices.     

Energy level alignment and morphology of interfaces between molecular and polymeric organic semiconductors

Ultraviolet photoelectron spectroscopy (UPS) was used to determine the energy level alignment at organic–organic conductor–semiconductor and semiconductor–semiconductor hetero-interfaces that are relevant for organic optoelectronic devices. Such interfaces were formed by in situ vacuum sublimation of small molecular materials [C₆₀ and pentacene (PEN)] and ex situ spin-coating of poly(3-hexylthiophene) (P3HT), all on the common substrate poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT:PSS). We found that the deposition sequence had a significant impact on the interface energetics. The hole injection barrier (HIB) of C₆₀ on PEDOT:PSS could be changed from 1.0 eV (moderate hole injection) to 1.7 eV (good electron injection) by introducing a layer of P3HT. The HIB of P3HT/PEDOT:PSS was increased by 0.35 eV due to an interfacial PEN layer. However, PEN deposited on PEDOT:PSS and P3HT/PEDOT:PSS exhibited the same value. These observations are explained by material-dependent dipoles at the interfaces towards PEDOT:PSS and substrate dependent inter-molecular conformation.     

MoO3/Au/MoO3–PEDOT:PSS multilayer electrodes for ITO-free organic solar cells

The effect of the MoO₃–PEDOT:PSS composite layer in the MoO₃/Au/MoO₃–PEDOT:PSS multilayer electrode on the power conversion efficiency of ITO-free organic solar cells (OSCs) was evaluated. The MoO₃ (30 nm)/Au(12 nm)/MoO₃–PEDOT:PSS (30 nm)/PEDOT:PSS structure showed ~7% more optical transmittance than the MoO₃ (30 nm)/Au (12 nm)/MoO₃(30 nm)/PEDOT:PSS structure at 550 nm wavelength. The OSCs using MoO₃/Au/MoO₃–PEDOT:PSS multilayer electrodes as anodes showed a considerable improvement in power conversion efficiency (PCE), from 1.84% to 2.81%, comparable to ITO based OSCs with PCE of 2.89%. This improvement is attributed to the suppression of MoO₃ dissolution by the acidic hole transport layer (HTL) PEDOT:PSS on the MoO₃/Au/MoO₃–PEDOT:PSS multilayer electrode, resulting in high J_sc, V_oc and FF of the OSCs. This composite based multilayer electrode was shown to be a promising replacement in ITO-free flexible optoelectronic devices.     

Influence of aluminium electrode preparation on PCE values of polymeric solar cells based on P3HT and PCBM

The main goal of the paper was investigation of influence of aluminum electrode preparation via thermal evaporation (TE) and the magnetron sputtering (MS) on power conversion efficiency (PCE) of polymeric solar cells. The photovoltaic properties of such three kinds devices based on poly(3-hexylthiophene-2,5-diyl) (P3HT) as ITO/P3HT/Al, ITO/P3HT:PCBM (1:1, w/w)/Al and ITO/PEDOT:PSS/P3HT:PCBM (1:1, w/w)/Al were investigated. For the constructed devices impedance spectroscopy were analyzed. For devices lack of PEDOT:PSS layer or lack of PCBM, photovoltaic parameters were very low and similar to the parameters obtained for device with Al electrode prepared by magnetron sputtering. The devices comprising PEDOT:PSS with P3HT:PCBM showed the best photovoltaic parameters such as a V_OC of 0.60 V, J_SC of 4.61 mA/cm², FF of 0.21, and PCE of 5.7 × 10^?1%.     

Effects of tetramethylene sulfone solvent additives on conductivity of PEDOT:PSS film and performance of polymer photovoltaic cells

A solvent additive in PEDOT:PSS solution is one of many methods to improve the conductivity of the poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) films. We explore a new type of the solvent additive, namely tetramethylene sulfone (TMS), for the fabrication of the PEDOT:PSS conductive layer in the ITO/PEDOT:PSS/P3HT:PCBM/TiO_x/Al polymer photovoltaic cells, in comparison to a more common dimethyl sulfoxide (DMSO) solvent additive. At optimal conditions, the TMS additive at 10 wt.% has been found to enhance the conductivity of pristine PEDOT:PSS films from 0.04 S/cm up to approximately 189 S/cm, compared with the highest conductivity for the case of the DMSO additive at 15 wt.% of 117 S/cm. Possible mechanisms of this conductivity enhancement, relating to the polymer conformation and the film morphology, have been investigated by Raman spectroscopy, X-ray diffraction, atomic force microscopy, and transmission electron microscopy. The performance of the polymer photovoltaic cells fabricated with the solvent additives PEDOT:PSS films follows a similar trend to the conductivity of the films as a function of the additive concentration. The additives mainly lead to greater short circuit current density (J_sc) of the photovoltaic cells. The highest power conversion efficiency (PCE) of 2.24% of the device has been obtained with the 10 wt.% TMS additive of, compared to the PCE of 1.48% for the standard device without solvent additive.     

Composite film of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) and MoO3 as an efficient hole injection layer for polymer light-emitting diodes

We demonstrate improved performances in polymer light-emitting diodes (PLEDs) using a composite film of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and MoO₃ powder as a hole injection layer. The PLED with the composite film exhibits the current efficiency of 13.5 cd/A, driving voltage of 3.4 V, and half lifetime of 108.1 h, while those values of the PLED with a pristine PEDOT:PSS was 11.3 cd/A, 3.8 V, and 41.5 h, respectively. We also analyze the morphological, optical and electrical properties of the composite films by atomic force microscopy (AFM), UV–Vis-IR absorption, and ultraviolet photoemission spectroscopy (UPS). This work suggests that mixing MoO₃ into PEDOT:PSS is a simple and promising technique for use solution-based devices as an hole injection layer.     

Aqueous solution-processed MoO3 as an effective interfacial layer in polymer/fullerene based organic solar cells

The authors demonstrate an effective anode interfacial layer based on aqueous solution-processed MoO₃ (sMoO₃) in poly (3-hexylthiophene) (P3HT) and indene-C60 bisadduct (ICBA) based bulk-heterojunction organic solar cells (PSCs). Various sMoO₃ concentration (0.03–0.25 wt%) was obtained by dissolving MoO₃ powder into deionized water directly with weak solubility. The characteristics of sMoO₃ films evaluated by atomic force microscope (AFM) and scanning electron microscope (SEM) suggest that the sMoO₃ films continuously cover the entire indium tin oxide (ITO) surface. The sMoO₃ based PSCs exhibit comparable power conversion efficiency with poly (3,4-ethylenedioxythiophene)–polystyrenesulfonic acid (PEDOT:PSS) based devices. However, even more importantly, the stability of sMoO₃ based devices have been greatly improved in air under continual light-illumination at 52 mW/cm². Further evaluations on Mo valence states and work function of sMoO₃ films by X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) demonstrate that the aqueous solution-processed MoO₃ could act as an better anode interfacial layer than the conventional PEDOT:PSS.     

Effects of UV light-irradiated buffer layer on the performance of polymer solar cells

We investigate the effect of a UV-irradiated poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) buffer layer on the performance of polymer photovoltaic cells based on poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) blends. It was found that UV irradiation can reduce the bulk and contact resistance of PEDOT:PSS films, improving the power conversion efficiency from (3.05 ± 0.04)% to (3.50 ± 0.03)% due to the lower device series resistance under an illumination of AM1.5G, 100 mW/cm². The work function change after UV irradiation and negligible surface morphology change was noticed.     

MoO3–Au composite interfacial layer for high efficiency and air-stable organic solar cells

Efficient and stable polymer bulk-heterojunction solar cells based on regioregular poly(3-hexylthiophene):[6,6]-phenyl-C₆₁-butyric acid methyl ester (P3HT:PC₆₁BM) blend active layer have been fabricated with a MoO₃–Au co-evaporation composite film as the anode interfacial layer (AIL). The optical and electrical properties of the composite MoO₃–Au film can be tuned by altering the concentration of Au. A composite film with 30% (weight ratio) Au was used as the AIL and showed a better performance than both pure MoO₃ and PEDOT:PSS as AIL. The surface morphology of the MoO₃–Au composite film was investigated by atomic force microscopy (AFM) and showed that the originally rough ITO substrate became smooth after depositing the composite film, with the root mean square roughness (RMS) decreased from 4.08 nm to 1.81 nm. The smooth surface reduced the bias-dependent carrier recombination, resulting in a large shunt resistance and thus improving the fill factor and efficiency of the devices. Additionally, the air stability of devices with different AILs (MoO₃–Au composite, MoO₃ and PEDOT:PSS) were studied and it was found that the MoO₃–Au composite layer remarkably improved the stability of the solar cells with shelf life-time enhanced by more than 3 and 40 times compared with pure MoO₃ layer and PEDOT:PSS layer, respectively. We argue that the stability improvement might be related with the defect states in MoO₃ component.     

Charge injection and transport studies of poly(2,7-carbazole) copolymer PCDTBT and their relationship to solar cell performance

The charge injection and transport properties of a high performance semiconducting polymer for organic photovoltaic (OPV) applications, poly[N-9″-hepta-decanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT), are investigated by time-of-flight (TOF) and dark-injection space-charge-limited current (DI-SCLC) techniques. OPV cells employing PCDTBT are known to possess power conversion efficiency (PCE) exceeding 6% [1], [2]. While TOF probes only the hole mobilities of a thick film, DI-SCLC is shown to be useful down to a sample thickness of ～200 nm, which is comparable to thicknesses used in OPV cells. We show that for pristine PCDTBT, the hole mobilities for both thick used in TOF and thin films for DI-SCLC are essentially the same, and they are in the range of 0.4–3.0 × 10^?4 cm²/Vs at room temperature. Both poly(3,4-ethylene dioxythioplene) doped with poly(strenesulfonate) (PEDOT:PSS) and molybdenum (VI) oxide (MoO₃) form quasi-Ohmic contacts to PCDTBT with better hole injection from MoO₃. Furthermore, the Gaussian Disorder Model (GDM) was employed to analyze the hopping transport of PCDTBT thin films. We show that PCDTBT possesses a relatively large energetic disorder (σ) of ～129 meV, which is significantly higher than the σ of poly(3-hexylthiophene) (P3HT) processed under similar conditions. The correlation between σ and OPV device performance is addressed.     

Realizing both improved luminance and stability in organic light-emitting devices based on a solution-processed inter-layer composed of MoOX and Au nanoparticles mixture

We report on the solution-processed mixture of Au nanoparticles (NPs) and MoO_X as an inter-layer in organic light-emitting devices (OLEDs), leading to the enhanced light emission and good stability. An impressive enhancement in current efficiency and power efficiency is achieved up to 70% and 100%, respectively. A systematic study to the effect of the Au NPs:MoO_X inter-layer on OLEDs demonstrates that the improved electrical properties is mainly ascribed to the enhanced hole injection due to the high work function of MoO_X and the good conductivity of Au NPs, and the enhanced optical properties is mainly attributed to the localized surface plasmon induced by Au NPs, which makes a great contribution to the improved efficiency. Besides, Au NPs:MoO_X inter-layer also behaves superior to the frequently-used polyethylene dioxythiophene:polystyrene sulfonate (PEDOT:PSS) in device stability. The decay ratio for Au NPs:MoO_X based device is 60% after 80 h, while it is nearly dying out for the device with PEDOT:PSS inter-layer.     

Efficient small molecule-based bulk heterojunction photovoltaic cells with reduced exciton quenching in fullerene

Most highly efficient small molecule-based bulk heterojunction (BHJ) photovoltaic cells contain a large concentration of fullerene in their blend active layers. However, the excitons generated in fullerene can seriously quench at the surface of the commonly used MoO₃ buffer layer, becoming a key limitation to the photovoltaic performance of these cells. In this study, we’ve investigated various anode buffer layers in the thermally evaporated tetraphenyldibenzoperiflanthene (DBP) and C₇₀-based BHJ cells with high C₇₀ concentration. It’s been found that obviously enhanced power conversion efficiency (PCE) of up to 6.26% can be obtained in DBP and C₇₀-based BHJ cells via simply replacing the MoO₃ buffer by poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT: PSS), which is also a commonly used anode buffer material in polymer-based BHJ cells. Photoluminescence spectra results have confirmed the suppression of exciton quenching at the anode interface by inserting this PEDOT: PSS buffer. Moreover, after adding a C₇₀ interlayer for better electron extraction and the further suppression of exciton quenching, the DBP and C₇₀-based M-i-n photovoltaic cells show a remarkable PCE of 7.04% under illumination with 100 mW/cm², AM 1.5G simulated solar light.     

Detailed investigation of dependencies of photovoltaic performances of P3HT:PC61BM based solar cells on anodic work function modified by surface treatment of indium-tin-oxide electrode with benzenesulfonyl chloride derivatives

The photovoltaic (PV) characteristics of bulk-heterojunction (BHJ) solar cells based on poly(3-hexylthiophene) and [6,6]-phenyl-C₆₁-butyric acid methyl ester (P3HT:PC₆₁BM) were improved using indium-tin-oxide (ITO) anode electrodes modified chemically with CH₃O-, H-, Cl-, CF₃-, and NO₂-terminated benzenesulfonyl chlorides as a self-assembled monolayer (SAM). The ITO electrode surfaces were easily treated through the chemical modification of the reactive –SO₂Cl binding group, and the work function (WF) of the modified ITO was effectively changed depending on the permanent dipole moments introduced in the para-position of benzenesulfonyl chloride. We examined the correlation between the ITO WFs corrected by the change in the contact potential difference and the calculated dipole moments of the SAM models. Moreover, we examined the PV characteristics of the P3HT:PC₆₁BM based BHJ organic PV cells using the SAMs or poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS)-treated ITOs with different WFs lying within ±0.2 eV from the highest occupied molecular orbital (HOMO) level of P3HT. We found that the enhancement effect of the SAMs on the power-conversion efficiency (η_P) reached a maximum with Cl (η_P = 3.72%), and became larger than that of PEDOT:PSS (η_P = 3.62%). Two distinct J_sc dependencies, increasing and decreasing with the increasing WF of the anode ITO, were observed at higher and lower WFs than the HOMO level of the donor, respectively. Almost constant V_oc values (around 0.6 V) were observed with different SAM-modified ITOs, which suggested that Fermi level pinning was achieved by aligning the anode Fermi level and positive polaronic level of the donor polymer.     

Solution processed WO3 layer for the replacement of PEDOT:PSS layer in organic photovoltaic cells

Tungsten oxide layer is formed uniformly by a sol–gel technique on top of indium tin oxide as a neutral and photo-stable hole extraction layer (HEL). The solution processed tungsten oxide layer (sWO₃) is fully characterized by UV–Vis, XPS, UPS, XRD, AFM, and TEM. Optical transmission of ITO/sWO₃ substrates is nearly identical to ITOs. In addition, the sWO₃ layer induces nearly ohmic contact to P3HT as PEDOT:PSS layer does, which is determined by UPS measurement. In case that an optimized thickness (～10 nm) of the sWO₃ layer is incorporated in the organic photovoltaic devices (OPVs) with a structure of ITO/sWO₃/P3HT:PCBM/Al, the power conversion efficiency (PCE) is 3.4%, comparable to that of devices utilizing PEDOT:PSS as HEL. Furthermore, the stability of OPV utilizing sWO₃ is significantly enhanced due to the air- and photo-stability of the sWO₃ layer itself. PCEs are decreased to 40% and 0% of initial values, when PEDOT:PSS layers are exposed to air and light for 192 h, respectively. In contrast, PCEs are maintained to 90% and 87% of initial PCEs respectively, when sWO₃ layers are exposed to the same conditions. Conclusively, we find that solution processed tungsten oxide layers can be prepared easily, act as an efficient hole extraction layer, and afford a much higher stability than PEDOT:PSS layers.     

Uniform Aerosol Jet printed polymer lines with 30 μm width for 140 ppi resolution RGB organic light emitting diodes

We demonstrate the patterning of top emitting organic light emitting diodes (OLEDs) by direct-write Aerosol Jet printing in air from non-halogenated solvents. Indane was determined to be a suitable single solvent to dissolve the archetypal host poly(N-vinylcarbazole) and guest emitting dopants complexes for red, green, and blue OLEDs, and to print on both PEDOT:PSS and MoO₃ transport layers. The Aerosol Jet deposition parameters were studied, including: atomization flow rate, exhaust flow rate, focusing sheath flow rate, nozzle-to-substrate distance, nozzle speed, and substrate temperature. A line width of 30 μm and thickness of 30 nm was achieved, resulting in RGB OLEDs fabricated side-by-side with a pixel density of 140 ppi.     

The enhancement of electrical and optical properties of PEDOT:PSS using one-step dynamic etching for flexible application

The conductivity enhancement of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) by dynamic etching process was investigated to introduce the outstanding and simplest method for soft electronics. Four different samples which were pristine PEDOT:PSS, PEDOT:PSS doped with 5 wt.% DMSO, PEDOT:PSS with dipping process, and PEDOT:PSS with dynamic etching process were prepared to compare the properties such as conductivity, morphology, relative atomic percentage, and topography. All samples were characterized by four point probe, current atomic force microscopy (C-AFM), X-ray photoelectron spectroscopy (XPS), and UV–visible spectroscopy. The conductivity of the sample with dynamic etching process showed the highest value as 1299 S/cm among four samples. We proved that the dynamic etching process is superior to remove PSS phase from PEDOT:PSS film, to flow strong current through entire surface of PEDOT:PSS, and to show the smoothest surface (RMS 2.28 nm). XPS analysis was conducted for accurate chemical and structural surface environments of four samples and the relative atomic percentage of PEDOT in the sample with dynamic etching was the highest as 29.5%. The device performance of the sample with the dynamic etching process was outstanding as 10.31 mA/cm² of J_sc, 0.75 eV of V_oc, 0.46 of FF, and 3.53% of PCE. All properties and the device performance for PEDOT:PSS film by dynamic etching process were the most excellent among the samples.     

Electrical characterization of inorganic-on-organic diode based InP and poly(3,4-ethylenedioxithiophene)/poly(styrenesulfonate) (PEDOT:PSS)

The electronic properties of metal–organic semiconductor-inorganic semiconductor diode between InP and poly(3,4-ethylenedioxithiophene)/poly(styrenesulfonate) (PEDOT:PSS) polymeric organic semiconductor film have been investigated via current–voltage and capacitance–voltage methods. The Al/PEDOT:PSS/p-InP contact exhibits a rectification behavior with the barrier height value of 0.98 eV and with the ideality factor value of 2.6 obtained from their forward bias current voltage (I–V) characteristics at the room temperature greater than the conventional Al/p-InP (0.83 eV, n = 1.13). This increase in barrier height and ideality factor can be attributed to PEDOT:PSS film formed at Al/p-InP interface.     

Energy level alignment in PCDTBT:PC70BM solar cells: Solution processed NiOx for improved hole collection and efficiency

Solution-based NiO_x outperforms PEDOT:PSS in device performance and stability when used as a hole-collection layer in bulk-heterojunction (BHJ) solar cells formed with poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole) (PCDTBT) and PC₇₀BM. The origin of the enhancement is clarified by studying the interfacial energy level alignment between PCDTBT or the 1:4 blended heterojunctions and PEDOT:PSS or NiO_x using ultraviolet and inverse photoemission spectroscopies. The 1.6 eV electronic gap of PEDOT:PSS and energy level alignment with the BHJ result in poor hole selectivity of PEDOT:PSS and allows electron recombination at the PEDOT:PSS/BHJ interface. Conversely, the large band gap (3.7 eV) of NiO_x and interfacial dipole (?0.6 eV) with the organic active layer leads to a hole-selective interface. This interfacial dipole yields enhanced electron blocking properties by increasing the barrier to electron injection. The presence of such a strong dipole is predicted to further promote hole collection from the organic layer into the oxide, resulting in increased fill factor and short circuit current. An overall decrease in recombination is manifested in an increase in open circuit voltage and power conversion efficiency of the device on NiO_x versus PEDOT:PSS interlayers.     

Metal salt modified PEDOT:PSS as anode buffer layer and its effect on power conversion efficiency of organic solar cells

The effects of metal chlorides such as LiCl, NaCl, CdCl₂ and CuCl₂ on optical transmittance, electrical conductivity as well as morphology of PEDOT:PSS films have been investigated. Transmittance spectra of spun PEDOT:PSS layers were improved by more than 6% to a maximum of 94% in LiCl doped PEDOT:PSS film. The surface of the PEDOT:PSS films has exhibited higher roughness associated with an increase in the electrical conductivity after doping with metal salts. The improvement in the physical properties of PEDOT:PSS as the hole transport layer proved to be key factors towards enhancing the P3HT:PCBM bulk heterojunction (BHJ) solar cells. These improvements include significantly improved power conversion efficiency with values as high as 6.82% associated with high fill factor (61%) and larger short circuit current density (∼18 mA cm⁻²).     

Pt–free counter electrodes based on modified screen–printed PEDOT:PSS catalytic layers for dye–sensitized solar cells

One of the most highlighted advantages of dye-sensitized solar cells (DSSCs) consists in the possibility to apply simple and low-cost printing techniques and solution processable materials for their assembling. Here, we report on screen-printed Pt–free counter electrodes (CEs) based on poly(3,4–ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) dispersions with different content of rheological agent – hydroxyethyl cellulose (HEC). These PEDOT:PSS dispersions, having measured pseudoplastic and thixotropic rheological behaviour, were screen–printed onto FTO glasses. The content of rheological agent in PEDOT:PSS catalytic layers showed an effect on measured thickness, electrochemical properties, specific conductivity and subsequently on the evaluated photovoltaic performance of DSSCs. The PEDOT:PSS CE with the 0.03 wt% of HEC achieved the best electrochemical performance and specific conductivity (80 S cm⁻¹), the lowest thickness of 200 nm and transparency in VIS light spectrum over 60%. DSSCs based on this PEDOT:PSS CE reached the highest conversion efficiency of 4.2% which is only approximately 40% lower value than η=6.9% evaluated for Pt CE.