Understanding the role of organic polar solvent induced nanoscale morphology and electrical evolutions of P3HT:PCBM composite film

We investigated the effect of organic polar solvent on the properties of [6,6]-phenyl-C₇₁-butyric acid methyl ester (PCBM) films and poly(3-hexylthiophene) (P3HT):PCBM blend films employed as active layer in organic photovoltaic. The nanoscale morphology and the electrical characteristics of the P3HT:PCBM film can be controlled through organic polar solvent exposure, which exhibited with a short-circuit current density of 8.64 mA/cm², an open circuit voltage of 0.63 V, and a power conversion efficiency of 3.29% under AM 1.5 illumination with a light intensity of 100 mW/cm². By exposing the active layer films to organic polar solvent a favorable phase separation in the P3HT:PCBM films is obtained. The improved power conversion efficiency can be to the high conductivity and high surface area of the P3HT:PCBM layer treated with organic polar solvent.     

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%.     

Polymer bulk heterojunction photovoltaics employing a squaraine donor additive

We investigate the effects of adding a functionalized squaraine donor 2,4-bis[4-(N,N-diphenylamino)-2,6-dihydroxyphenyl] squaraine (DPSQ) into a conventional poly(3-hexylthiophene)(P3HT):[6,6]-phenyl C₆₁-butyric acid methyl ester (PCBM) polymer bulk heterojunction photovoltaic cell. The near infrared absorption of the blend was enhanced by the DPSQ additive, resulting in an increased power conversion efficiency of the P3HT:PCBM devices by >20%. A maximum power conversion efficiency of 3.4 ± 0.3% and an external quantum efficiency as high as 55% was achieved for a P3HT:PCBM blend that included 5 wt.% DPSQ.     

Long lived photoexcitation dynamics in π-conjugated polymer and fullerene blended films

We used continuous wave photoinduced absorption (PIA) spectroscopy to investigate long lived polarons in blend of [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) and regio-regular poly (3-hexylthiophene) (RR-P3HT), and in blend of PCBM and 2-methoxy-5-(2-ethylhexyloxy) poly(para-phenylenevinylene) (MEH-PPV). In millisecond time regime, delocalized polarons (DP) and localized polarons (LP) in RR-P3HT/PCBM as well as polarons in MEH-PPV/PCBM all exhibit dispersive bimolecular recombination process which was limited by the trap states, with the average lifetimes of those polarons inverse proportional to the square root of pump intensity (I). The recombination in RR-P3HT/PCBM was weak temperature dependence with small thermal activation energy, Δ for DPs and LPs of 25 meV and 13 meV, respectively; in contrast, Δ for polarons in amorphous MEH-PPV/PCBM was ～160 meV. Furthermore, we proved that the values of Δ for both of LP and DP increase, as well as the relatively intensity ratio of DP and LP decreases, in an intentionally degraded RR-P3HT/PCBM film. Overall, it is demonstrated that steady state photomodulation technique with thermal-activated-recombination analysis can be applied to evaluate polymer (dis)order in bulk heterojunction films.     

On the stability of the electrical and photoelectrical properties of P3HT and P3HT:PCBM blends thin films

New photoelectrical properties of poly(3-hexylthiophene-2,5-diyl), highly regioregular (P3HT): Methanofullerene Phenyl-C₆₁-Butyric-Acid-Methyl-Ester [60] PCBM films were putted in evidence. For the first time the electrical conductivity dependencies on temperature in dark and under different illuminations were studied for the P3HT and P3HT:PCBM blend films. These dependencies shows reversible processes and a high sensitivity of the P3HT and P3HT:PCBM to light. The decrease of the resistivity at the exposure to light is of 18% for P3HT films and of 20% for P3HT:PCBM blend films, for a irradiation under 0.5 W/m² white light at room temperature. By adding the fullerene molecules, in the 1:0.8 polymer:fullerene ratio, the electrical resistivity at room temperature of the blend films decrease compared to the polymer film by 40% in dark, and by 68% under 250 W/m² white light irradiance.The decrease of the resistivity with the temperature is more pronounced in the presence of light indicating a photon activated process.The existence of the open circuit voltage was evidenced even for planar geometry photodiodes and the values of the open circuit voltage under 1000 W/m² solar light illumination are coherent with the difference between the work functions of the electrodes.     

Design and synthesis of indole-substituted fullerene derivatives with different side groups for organic photovoltaic devices

A series of indole-substituted fulleropyrrolidine derivatives with different side groups on a pyrrolidine rings, including methyl (OIMC60P), benzyl (OIBC60P), 2,5-difluoroinebenzyl (OIB2FC60P), and 2,3,4,5,6-pentafluoroinebenzyl (OIB5FC60P), have been synthesized and used as electron acceptor in the active layer of polymer-fullerene solar cells to investigate the effect of various substitute groups on the electronic structures, morphologies, and device performances. Optical absorption, electrochemical properties and solubility of the fullerene derivatives have been explored and compared. The inverted photovoltaic devices with the configuration ITO/ZnO/Poly(3-hexylthiophene)(P3HT):[60]fullerene derivatives/MoO₃/Ag have been prepared including the reference cell based on the P3HT: methyl [6,6]-phenyl-C61-butylate (PCBM) blend films. All the devices properties were measured in air without encapsulation. We also investigated the effect of the thermal annealing on the crystallinity and morphology of the active layer and the device performance. The device based on the blend film of P3HT and OIBC60P showed a power conversion efficiency of 2.46% under illumination by AM1.5G (100 mW/cm²) after the annealing treatment at 120 °C for 10 min in air.     

Effective protection of sequential solution-processed polymer/fullerene bilayer solar cell against charge recombination and degradation

Both charge recombination and degradation in sequential solution processed polymer/fullerene bilayer organic photovoltaics (OPV) are effectively reduced by the insertion of a TiO₂ inter-layer between the bilayer and Al electrode. The polymer/fullerene bilayer composed of a poly(3-hexylthiophene) (P3HT) bottom-layer and a [6,6] phenyl C61-butyric acid methyl ester (PCBM) top-layer shows significant change in morphology due to the substantial inter-penetration of P3HT and PCBM during the thermal annealing process. Consequently, the bilayer surface becomes P3HT rich resulting in significant charge recombination at the bilayer/Al interface of the bilayer OPV. The charge recombination rate of the bilayer OPV is reduced by one order of magnitude upon the insertion of a TiO₂ nanoparticle inter-layer between the bilayer and the Al electrode after the thermal annealing process. In contrast, when the thermal annealing process is conducted after insertion of the inter-layer, the effect of the TiO₂ inter-layer becomes insignificant. The V_OC and efficiency of the bilayer OPV is greatly enhanced from 0.37 to 0.66 V and 1.2% to 3.7%, respectively by utilizing the properly constructed TiO₂ inter-layer in the bilayer OPV. Additionally, insertion of the TiO₂ inter-layer significantly improves the stability of the bilayer OPV. The bilayer OPV with a TiO₂ inter-layer maintains 51% of its initial PCE after storage under dark ambient conditions for 700 h without encapsulation, whereas the bilayer OPV without a TiO₂ inter-layer did not show any solar cell performance after 200 h under the same conditions.     

Annealing effects on the electrical properties of spin coated poly (3-hexylthiophene) (P3HT) thin films

Poly (3-hexylthiophene-2, 5-diyl) (P3HT) thin films were prepared using spin coating technique and the effect of annealing on the bias switching for memory applications were studied. Due to annealing, the threshold voltage for switching was reduced considerably. In bias switching, threshold voltage was least for the sample annealed at 100 °C. Addition of phenyl-C₆₁-butyric acid-methyl-ester (PCBM) into P3HT also reduced the threshold voltage. It was also found that the devices with gold (Au) top electrode switched at a lower threshold voltage compared to their aluminium (Al) counterparts.     

Enhanced performance in inverted polymer solar cells via solution process: Morphology controlling of PEDOT:PSS as anode buffer layer by adding surfactants

Inverted polymer solar cells were fabricated by adding the amphiphilic surfactant ‘Surfynol 104 series’ to Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) as a anode buffer layer by solution process. With the introduction of Surfynol 104 series-added PEDOT:PSS, it was able to form a homogeneous film by adjusting the wettability of a hydrophobic poly(3-hexylthiophene) (P3HT):[6,6]-phenyl C₆₁-butyric acid methyl ester (PCBM) film. With decrease in series resistance (R_S) and increase in shunt resistance (R_SH), as a result, the short circuit current density (J_SC), open circuit voltage (V_OC) and fill factor (FF) of the optimized device were 10.2 mA/cm², 0.63 V and 61.3%, respectively, calculated the power conversion efficiency (PCE) was 4.0%. In addition, the air stability of the fabricated device was improved.     

Charge carrier mobility and lifetime of organic bulk heterojunctions analyzed by impedance spectroscopy

Charge carrier diffusion and recombination in an absorber blend of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C₆₁-butyric acid methyl ester (PCBM) with indium tin oxide (ITO) and aluminium contacts have been analyzed in the dark by means of impedance spectroscopy. Reverse bias capacitance exhibits Mott–Schottky-like behavior indicating the formation of a Schottky junction (band bending) at the P3H:PCBM-Al contact. Impedance measurements show that minority carrier (electrons) diffuse out of the P3HT:PCBM-Al depletion zone and their accumulation contributes to the capacitive response at forward bias. A diffusion–recombination impedance model accounting for the mobility and lifetime parameters is outlined. Electron mobility results to be 2 × 10⁻³ cm² V⁻¹ s⁻¹ and lifetime lies within the milliseconds timescale.     

All-conjugated diblock copolymer of poly(3-hexylthiophene)-block-poly(3-phenoxymethylthiophene) for field-effect transistor and photovoltaic applications

The electronic properties, morphology and optoelectronic device characteristics of conjugated diblock copolythiophene, poly(3-hexylthiophene)-block -poly(3-phenoxymethylthiophene) (P3HT-b-P3PT), are firstly reported. The polymer properties and structures were explored through different solvent mixtures of chloroform (CHCl₃), dichlorobenzene (DCB), and CHCl₃:DCB (1:1 ratio). The absorption maximum (λ_max) of P3HT-b-P3PT prepared from DCB was around 554 nm with a shoulder peak indicative for the highly crystalline structure around 604 nm while that from CHCl₃ was 516 nm without the clear shoulder peak. The field-effect hole mobility of P3HT-b-P3PT increased from ～6.0 × 10^?3, ～8.0 × 10^?3 to ～2.0 × 10^?2 cm² V^?1 s^?1 as the DCB content in the solvent mixture enhanced. The AFM images suggested that the highly volatile CHCl₃ processing solvent led to the amorphous structure, on the other hand, less volatile DCB resulted in the largely crystalline structure of the P3HT-b-P3PT. Such difference on the polymer structure and hole mobility led to the varied power conversion efficiency (PCE) of the photovoltaic cells fabricated from the blend of P3HT-b-P3PT/[6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) (1:1, w/w): 1.88 (CHCl₃), 2.13 (CHCl₃:DCB (1:1)), and 2.60% (DCB). The PCBM blend ratio also significantly affected the surface structure and the solar cell performance. The PCE of polymer/PCBM could be improved to 2.80% while the ratio of polymer to PCBM went to 1:0.7. The present study suggested that the surface structures and optoelectronic device characteristics of conjugated diblock copolymers could be easily manipulated by the processing solvent, the block segment characteristic, and blend composition.     

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.     

Effective mobility in amorphous organic transistors: Influence of the width of the density of states

The temperature dependence of poly(3-hexylthiophene-2,5-diyl) (P3HT)/polystyrene (PS) blend organic transistor current/voltage (I–V) characteristics has been experimentally and theoretically studied. The planar transistors, realized by drop casting the P3HT/PS ink, exhibit high mobilities (over 5 × 10⁻³ cm² V⁻¹ s⁻¹) and good overall characteristics. A transistor model accounting for transport mechanisms in disordered organic materials was used to fit the measured characteristics. Using a single set of parameters, the measured effective mobility versus gate bias, either increasing or decreasing with the gate bias depending on temperature, is well reproduced over a wide temperature range (130–343 K). A Gaussian density of states width of 0.045 eV was determined for this P3HT/PS blend. The transistor I–V characteristics are very well described considering disordered material properties within a self-consistent transistor model.     

Enhancing the short-circuit current,efficiency of inverted organic solar cells using tetra sulfonic copper phthalocyanine (TS-CuPc) as electron transporting layer

Efficient bulk-heterojunction polymer solar cells based on poly(3-hexylthiophene) (P3HT) blended with a fullerene derivative, [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) were fabricated in inverted configuration by using copper phthalocyanine-3,4′,4′′,4′′′tetra-sulfonated acid tetrasodium salt (TS-CuPc) as the electron collecting layer and MoO₃ as hole collecting layer. TS-CuPc is observed to be critical for the device performance, significantly enhancing the Jsc and the PCE compared to devices based on TiOx. The optimal thicknesses of MoO₃ and TS-CuPc were 10 nm and 15 nm, respectively. Based on these optimal parameters, the PCE of 3.6% was obtained compared to 3.4% for the reference TiOx/P3HT:PCBM/MoO₃/Ag.     

Photo stability enhancement of Poly(3-hexylthiophene)-PCBM nanocomposites by addition of multi walled carbon nanotubes under ambient conditions

Poly(3-hexylthiophene)-Phenyl-C61-butyric acid methyl ester (P3HT–PCBM) composites find wide application in optoelectronic devices, especially bulk-hetero junction (BHJ) solar cells. These composites, even though could give efficient polymer solar cells with ∼4–5% power conversion efficiencies (PCE), a major problem of photo stability is associated with it and remains unsolved. P3HT–PCBM composite was found to be degrading on irradiation with ultraviolet radiation or a solar simulator providing AM1.5G illumination (1000 W m^–2, 72 ± 2 °C or 330 W m⁻², 25 °C), in presence of oxygen and moisture. Here, we have studied the photo stability of P3HT–PCBM under ambient conditions and showed that a new ternary composite, P3HT–PCBM–MWCNT (multi walled carbon nanotube) has superior photo stability even on extended UV–Vis exposure. A total of 7% (w/w) PCBM and 3% (w/w) MWCNT with respect to P3HT resulted in optimum stability. UV–Visible and fluorescence spectral analysis have been used to study the photo stability, both in solution state and solid/film state. Transmission electron micrograph (TEM) along with selected area electron diffraction (SAED) pattern and Field Emission Scanning Electron Microscopy (FE-SEM) micrographs have been used to show the well coating of MWCNT on P3HT–PCBM composite. Since MWCNT is one of the very important carbon based nanomaterial with several supreme characteristics, this new ternary composite has great importance for optoelectronic applications.     

Effect of UV light irradiation on photovoltaic characteristics of inverted polymer solar cells containing sol–gel zinc oxide electron collection layer

An inverted organic bulk-heterojunction solar cell containing a zinc oxide (ZnO) based electron collection layer with a structure of ITO/ZnO/[6,6]-phenyl C₆₁ butyric acid methyl ester (PCBM): regioregular poly(3-hexylthiophene) (P3HT)/poly(3,4-ethylenedioxylenethiophene): poly(4-styrene sulfonic acid)/Au (ZnO cell) was fabricated. We examined the relationship between the heating temperature of the ZnO layer and the device performance under irradiation by simulated sunlight while cutting the UV light. The effects of the UV light contained in simulated sunlight were investigated by photocurrent–voltage (I–V) and alternating current impedance spectroscopy (IS) measurements. When the ZnO cells were irradiated with simulated sunlight, they exhibited a maximum power conversion efficiency (PCE) of over 3%, which hardly varied with the heating temperature of ZnO layers treated at 250 °C, 350 °C, and 450 °C. In contrast, when the ZnO cells were irradiated with simulated sunlight without UV content, their photovoltaic characteristics were very different. In the case of the cell with ZnO prepared by heating at 250 °C, PCE of 2.7% was maintained even under continuous irradiation with simulated sunlight without UV. However, for the cells with ZnO prepared by heating at 350 °C and 450 °C, the shapes of the I–V curves changed with the UV-cut light irradiation time, accompanying an increase in their series resistance. Overall, after UV-cut light irradiation for 1 h, the PCE of the cell with ZnO prepared by heating at 350 °C decreased to 1.80%, while that of the cell with ZnO prepared by heating at 450 °C fell to 1.35%. The photo IS investigations suggested that this performance change was responsible for the formation of charge-trapping sites at the ZnO/PCBM:P3HT interface which act as recombination centers for photo-produced charges in the PCBM:P3HT layer.     

Performance and lifetime improvement of polymer/fullerene blend photovoltaic cells with a C60 interlayer

Polymer solar cell based on the blend of poly[2-methoxy-5-(2′-ethylhexoxy)-1,4-phenylene vinylene] (MEH-PPV) and [6,6]-phenyl C₆₁-butyric acid methyl ester (PCBM) with C₆₀ insertion layer were fabricated. The solar cell structure was ITO/poly(3,4-ethylenedioxythiopene) (PEDOT):poly(styrenesulfonate) (PSS)/MEH-PPV:PCBM/C₆₀/Al. It was found that the C₆₀ inserting layer could increase the device performance and lifetime. The energy conversion efficiency (ECE) of the solar cell with C₆₀ layer reached about 1.89% under Air Mass 1.5, 100 mW cm⁻² illumination, which is enhanced in comparison with that of the device without C₆₀ layer. Mechanisms of the solar cell performance and lifetime dependence on the C₆₀ layer are discussed.     

Solution processable organic p–n junction bilayer vertical photodiodes

Organic p–n bilayer photodiodes were produced by solution casting poly(3-hexylthiophene) (P3HT) from chlorobenzene and phenyl-C₆₁-butyric acid methyl ester (PCBM):poly(4-chlorostyrene) (PClS) blends from the nearly orthogonal solvent dichloromethane onto flexible indium tin oxide (ITO)/polyester as a substrate. This is the first demonstration of PCBM–inert polymer blends for such a device. The electron mobility of a 90% PCBM–10% PClS blend was 3.5 × 10^?3 cm²/V s in a field-effect transistor. The diodes showed a rectification ratio of 2.0 × 10³ at ±2.0 V with a forward bias current density as high as 340 μA/cm² at 2.0 V in the dark. Irradiation with various light sources (0.013–291 mW/cm²) under ambient atmosphere generated a linear increase in photocurrent. Photodiodes with thinner active layers showed larger photocurrent and relative photoresponse, probably because of lower series resistance and lower recombination probability. The reverse bias response was less dependent on device area than the forward bias response. Photocurrents from multiple devices in parallel were additive as expected. The results demonstrate a simple fabrication route to light detectors compatible with solution processes and flexible substrates.     

Charge transport properties and microstructure of polythiophene/polyfluorene blends

We present a combined charge transport and X-ray diffraction study of blends based on regioregular poly(3-hexylthiophene) (P3HT) and the polyfluorene co-polymer poly((9,9-dioctylfluorene)-2,7-diyl-alt-[4,7-bis(3-hexylthien-5-yl)-2,1,3-benzothiadiazole]-2′,2′′-diyl) (F8TBT) that are used in efficient all-polymer solar cells. Hole mobility is observed to increase by nearly two orders of magnitude from less than 10^?7 cm² V^?1 s^?1 for as spin-coated blends to 6 × 10^?6 cm² V^?1 s^?1 for blends annealed at 453 K at a field of 2.7 × 10⁵ V/cm, but still significantly below the time-of-flight mobility of unblended P3HT of 1.7 × 10^?4 cm² V^?1 s^?1. The hole mobility of the blends also show a strong negative electric-field dependence, compared with a relatively flat electric-field dependence of unblended P3HT, suggestive of increased spatial disorder in the blends. X-ray diffraction measurements reveal that P3HT/F8TBT blends show a phase separation of the two components with a crystalline part attributed to P3HT and an amorphous part attributed to F8TBT. In as-spun and mildly annealed blends, the measured d-values and relative intensities of the 100, 200 and 300 P3HT peaks are noticeably different to unblended P3HT indicating an incorporation of F8TBT in P3HT crystallites that distorts the crystal structure. At higher anneal temperatures the blend d-values approach that of unblended P3HT suggesting a well separated blend with pure P3HT crystallites. P3HT crystallite size in the blend is also observed to increase with annealing from 3.3 to 6.1 nm, however similar changes in crystallite size are observed in unblended P3HT films with annealing. The lower mobility of P3HT/F8TBT blends is attributed not only to increased P3HT structural disorder in the blend, but also due to the blend morphology (increased spatial disorder). Changes in hole mobility with annealing are interpreted in terms of the need to form percolation networks of P3HT crystallites within an F8TBT matrix, with a possible contribution due to the intercalation of F8TBT in P3HT crystallites acting as defects in the as-prepared state.