Improvement of organic solar cell efficiency by solution-processed doping of pentacene in PEDOT:PSS layer

In the current research, organic solar cells (OSCs) with various concentrations of pentacene in Poly(ethylenedioxythiopene):Poly(styrenesulfonate) (PEDOT:PSS) interface layer were investigated for better hole extraction. The ITO/Pentacene?+?PEDOT:PSS/P3HT:PCBM/Al-fabricated solar cell fabricated via brush coating provides superior photovoltaic, electrical and optical characteristics when compared with the ITO/PEDOT:PSS/P3HT:PCBM/Al solar cell. The ITO/Pentacene?+?PEDOT:PSS/P3HT:PCBM/Al solar cells deliver a V_OC ~350?mV and 2.57% efficiency. It is observed that the optimized concentration of pentacene doping in PEDOT:PSS layer, along with an active layer of P3HT and PC₆₀BM, doubles the efficiency of the device, when compared with pristine PEDOT:PSS layer. The degradation studies of the fabricated bulk heterojunction OSCs reveal that the degrading abilities of ITO/Pentacene?+?PEDOT:PSS/P3HT:PCBM/Al solar cells are 60% more better than those of ITO/PEDOT:PSS/P3HT:PCBM/Al devices. Thus, this work will ultimately contribute toward fully solution processed painted device, which will provide low-cost manufacturing and improved stability of pentacene-based organic photovoltaics.     

Nanoimprint of dehydrated PEDOT:PSS for organic photovoltaics

We demonstrate the fabrication of poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) nanogratings by a dehydration-assisted nanoimprint lithographic technique. Dehydration of PEDOT:PSS increases its cohesion to protect the nanostructures formed by nanoimprinting during demolding, resulting in the formation of high quality nanogratings of 60?nm in height, 70?nm in width and 70?nm in spacing (aspect ratio of 0.86). PEDOT:PSS nanogratings are used as hole transport and an electron blocking layer in blended poly(3-hexylthiophene-2,5-diyl) (P3HT):[6,6]-penyl-C61-butyric-acid-methyl-ester (PCBM) organic photovoltaic devices (OPV), showing enhancement of photocurrent and power efficiency in comparison to OPV devices with non-patterned PEDOT:PSS films.     

Efficient TCO-free organic solar cells with modified poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) anodes

Organic photovoltaic cells (OPVs) with a highly conductive poly 3,4-ethylenedioxythiophene:poly styrenesulfonate (PEDOT:PSS) layer as an anode and that were modified with the addition of some organic solvents such as sorbitol (So), dimethyl sulfoxide (DMSO), N-methyl-pyrrolidone (NMP), dimethylformamide (DMF), and ethylene glycol (EG) were fabricated without the use of transparent conducting oxide (TCO). The conductivity of the PEDOT:PSS film that was modified with each additive was enhanced by three orders of magnitude. According to the atomic force microscopy (AFM) study, the conductivity enhancement might have been related to the better connections between the conducting PEDOT chains. The TCO-free solar cells with a modified PEDOT:PSS layer and an active layer composed of poly (3-hexylthiophene) (P3HT) and phenyl [6, 6] C61 butyric acid methyl ester (PCBM) performed as well as the indium-tin-oxide (ITO)-based organic solar cells. The power conversion efficiency (PCE) of the organic solar cells with a DMSO-, So + DMSO-, and EG-modified PEDOT:PSS layer reached 3.51, 3.64, and 3.77%, respectively, under an illumination of AM 1.5 (100 mW/cm2).     

Enhanced performance of polymer solar cells with a monolayer of assembled gold nanoparticle films fabricated by Langmuir–Blodgett technique

We reported the enhanced performance of polymer solar cells with the blend of poly (2-methoxy-5(2′-ethylhexyloxy)-1,4-phenylene-vinylene) (MEH-PPV) and [6,6]-phenyl C61 butyric acid methyl ester (PCBM) as active layer by incorporation of an assembled gold nanoparticle (Au NP) monolayer. The dense Au NP monolayer has been fabricated by Langmuir–Blodgett (LB) assembly and positioned between the transparent electrode ITO and the anode-modification PEDOT:PSS [poly (3,4-ethylenedioxythiophene):poly(styrene sulfonate)] layer, resulting in the device architecture of ITO/Au/PEDOT:PSS/MEH-PPV:PCBM/Al. We attribute the performance improvement to the localized surface plasmon resonance (LSPR) effect of Au NP films, which could lead to the increased absorption of the active layer. The parameters (nanoparticle size and interparticle distance) that govern this SPR effect have been optimized by selecting various sizes of Au NPs and controlling the LB assembly conditions. We observed ～10–20% enhancement in power conversion efficiency for all the devices with the Au NP monolayer.     

Enhancement of the power conversion efficiency of polymer solar cells by functionalized single-walled carbon nanotubes decorated with CdSe/ZnS core–shell colloidal quantum dots

In this paper, we demonstrated an enhanced performance of polymer solar cells by incorporating functionalized single-walled carbon nanotubes (SWCNTs) decorated with CdSe/ZnS core–shell colloidal quantum dots (CQDs) into copolymers of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) as active layer. Short-circuit current density and power conversion efficiency of the ITO/PEDOT:PSS/P3HT:PCBM:(CdSe/ZnS-SWCNTs)/Al solar cells can be enhanced by more than 31 and 23 %, respectively, as compared with the control device ITO/PEDOT:PSS/P3HT:PCBM/Al. This enhancement is due to the high electron-transporting ability of SWCNTs and the increased absorption of CdSe/ZnS CQD in visible region. It shows an applicable way to improve the efficiency of polymer solar cells by incorporating suitable quantity of CQDs-decorated SWCNTs with suitable kinds of CQDs and suitable acid treatment to the SWCNTs.     

Composition depth profile analysis of bulk heterojunction layer by time-of-flight secondary ion mass spectrometry with gradient shaving preparation

Composition depth profile analysis of bulk heterojunction (BHJ) layer was performed by time-of-flight secondary ion mass spectrometry with gradient shaving preparation. The BHJ layer comprised of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61butyric acid methyl ester (PCBM) was formed on the substrate coated with poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) followed by annealing. The P3HT component increased toward the top surface in the BHJ layer. In addition, C₈H₇SO₃⁻ was detected inside the BHJ layer, suggesting penetration of PSS. P3HT was uniformly distributed in the BHJ layer without PEDOT:PSS. The P3HT-rich distribution in the top surface may be attributed to PSS penetration.     

Effect of cadmium sulphide quantum dot processing and post thermal annealing on P3HT/PCBM photovoltaic device

The present study demonstrates the effect on photovoltaic performance of poly(3-hexylthiophene) (P3HT) on doping of cadmium sulphide (CdS) quantum dots (QDs). The P3HT/CdS nanocomposite shows a 10 nm blue shift in the UV-vis absorption relative to the pristine P3HT. The blue shift in the absorption of the P3HT/CdS nanocomposite can be assigned to the quantum confinement effect from the CdS nanoparticles. Significant PL quenching was observed for the nanocomposite films, attributed to additional decaying paths of the excited electrons through the CdS. Solar cell performance of pure P3HT and dispersed with CdS QDs have been studied in the device configuration viz indium tin oxide (ITO)/poly(3,4-ethylendioxythiophene)-poly(styrene sulfonate) (PEDOT:PSS)/P3HT:PCBM/Al and ITO/PEDOT:PSS/ P3HT:CdS:PCBM/Al, respectively. Incorporation of CdS QDs in the P3HT matrix results in the enhancement in the device efficiency (?) of the solar cell from 0.45 to 0.87%. Postproduction thermal annealing at 150 °C for 30 min improves device performance due to enhancement in the device parameters like FF, V_OC and improvement in contact between active layer and Al.     

Efficient organic photovoltaic devices by using PEDOT:PSSs with excellent hole extraction ability

We have demonstrated the poly(3-hexyl-thiophene-1,5-diyl) (P3HT):[6,6]-phenyl C61-butyric acid methyl ester (PCBM) bulk heterojunction (BHJ) organic photovoltaic (OPV) devices on various poly(3,4-ethylene-dioxythiophene):poly(styrenesulfonate) (PEDOT:PSSs). The device with PEDOT:PSS of PH 500 adding 1% dimethyl sulfoxide (DMSO) showed the best performances in term of the fill factor and power conversion efficiency (PCE) than others. The hole extraction ability of PEDOT:PSS is very important to balance between holes and electrons mobility because the carrier mobility of PCBM (approximately 10(-4) cm2/Vs) is higher than that of P3HT (approximately 10(-6) cm2/Vs) in P3HT:PCBM BHJ structure. The optimized BHJ OPV with PEDOT:PSS of PH 500 adding 1% DMSO showed a short-circuit current density of 8.92 mA/cm2 and a PCE of 2.97%, which was nearly increased to 2.5 times than that of control device with PEDOT:PSS of P VP Al 4083.     

Composite inks of poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate)/silver nanoparticles and electric/optical properties of inkjet-printed thin films

Poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate)/silver nanoparticles composite inks have been prepared through in situ synthesis and ultrasonic dispersion. The developed inks were proved to be suitable for various inkjet printing trials to deposit the thin films which were subsequently characterized to assess their electric and optical properties. The results have indicated that the dedoping of PSS from PEDOT during the in situ synthesis can be detrimental to the conductivity of the deposited composite films. However, the addition of silver nanoparticles to pristine PEDOT:PSS has significantly enhanced the conductivity of the thin films, with an inevitable loss in transparency. The various factors that can influence the properties of the thin films have also been analyzed and discussed. This study provides an insight into the effect of silver nanoparticles on PEDOT:PSS thin films deposited using inkjet printing process, and their properties due to the methods of ink formulation.     

Indium tin oxide nanopillar electrodes in polymer/fullerene solar cells

Using high surface area nanostructured electrodes in organic photovoltaic (OPV) devices is a route to enhanced power conversion efficiency. In this paper, indium tin oxide (ITO) and hybrid ITO/SiO(2) nanopillars are employed as three-dimensional high surface area transparent electrodes in OPVs. The nanopillar arrays are fabricated via glancing angle deposition (GLAD) and electrochemically modified with nanofibrous PEDOT:PSS (poly(3,4-ethylenedioxythiophene):poly(p-styrenesulfonate)). The structures are found to have increased surface area as characterized by porosimetry. When applied as anodes in polymer/fullerene OPVs (architecture: commercial ITO/GLAD ITO/PEDOT:PSS/P3HT:PCBM/Al, where P3HT is 2,5-diyl-poly(3-hexylthiophene) and PCBM is [6,6]-phenyl-C(61)-butyric acid methyl ester), the air-processed solar cells incorporating high surface area, PEDOT:PSS-modified ITO nanoelectrode arrays operate with improved performance relative to devices processed identically on unstructured, commercial ITO substrates. The resulting power conversion efficiency is 2.2% which is a third greater than for devices prepared on commercial ITO. To further refine the structure, insulating SiO(2) caps are added above the GLAD ITO nanopillars to produce a hybrid ITO/SiO(2) nanoelectrode. OPV devices based on this system show reduced electrical shorting and series resistance, and as a consequence, a further improved power conversion efficiency of 2.5% is recorded.     

The influence of the organic/inorganic interface on the organic-inorganic hybrid solar cells

Organic-inorganic hybrid solar cells based on poly(3-hexylthiophene) (P3HT) and (6,6)-phenyl C61 butyric acid methyl ester (PCBM) hybridized with ZnO nanorods were fabricated by growing vertical ZnO nanorods on indium tin oxide (ITO) substrates and filling with bulk heterojunction polymers (P3HT:PCBM). The interface between the organic and inorganic nanostructures influences the performance of the organic-inorganic hybrid solar cells. In this paper, the influence of the state of the P3HT:PCBM/ZnO interface on the performance of organic-inorganic hybrid solar cells is examined. The solar cell performance was high when the P3HT:PCBM/ZnO junction area was large. The charge separation is effective when the active layer/electron transport layer junction area is large, resulting in increasing photocurrent and a high conversion efficiency. The bulk-heterojunction polymer concentration was kept low to infiltrate into the ZnO nanorods, resulting in a large active layer/electron transport layer junction area.     

Dithiapyrannylidenes as efficient hole collection interfacial layers in organic solar cells

One inherent limitation to the efficiency of photovoltaic solar cells based on polymer/fullerene bulk heterojunctions (BHJs) is the accumulation of positive charges at the anodic interface. The unsymmetrical charge collection of holes and electrons dramatically decreases the short-circuit current. Interfacial layers (IFLs) such as poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) have no effect on the unbalanced electron/hole transport across the BHJ. We report here on the use of dithiapyrannylidenes (DITPY), a new class of planar quinoid compounds, as efficient hole-transporting/electron-blocking layers in organic solar cells based on poly(3-hexylthiophene)/[6,6]-phenyl-C(61)-butyric acid methyl ester (P3HT:PCBM) BHJs. Inserting a 15-nm-thick IFL of 4,4'-bis(diphenyl-2,6-thiapyrannylidene) (DITPY-Ph(4)) between the indium-tin oxide electrode and the P3HT:PCBM BHJ prevents detrimental space-charge effects and favors recombination-limited currents. Current-sensing atomic force microscopy reveals a drastic increase of the hole-carrying pathways in DITPY-Ph(4) compared to PEDOT:PSS. In ambient conditions, photovoltaic cells using DITPY-Ph(4) exhibit an 8% increase in the current density, although the conversion efficiency remains slightly lower compared to PEDOT:PSS-based devices. Finally, we present a detailed analysis of the photocurrent generation, showing that DITPY-Ph(4) IFLs induce a transition from unproductive space-charge-limited currents to recombination-limited currents.     

The effect of the functionalization of multiple carrier transporting interlayers on the performance and stability of bulk heterojunction organic solar cells

In this paper, we describe how the functionalization of carrier transporting interlayers affects the performance and stability of poly(3-hexylthiophene): poly(3-hexylthiophene): 3′H-cyclopropa [8,25] [5,6] fullerene-C60-D5h(6)-3′-butanoic acid 3′-phenyl methyl ester (P3HT:PCBM)-based organic solar cells. Composites of graphene oxide (GO) with zinc oxide (ZnO), (GO+ZnO) and with poly(3,4-ethylenedioxythiophene): poly (styrene sulfonate) (PEDOT:PSS) (GO+PEDOT:PSS) were produced by grafting the ZnO and PEDOT:PSS onto GO sheets using a molecular level functionalization approach. This molecular level attachment was confirmed based on X-ray diffraction patterns, an X-ray photoelectron spectroscopic analysis, and Raman spectroscopy. The functionalization interlayer helped to attach the PEDOT:PSS and ZnO firmly to the GO layer, thereby forming an air-resistant layer and also supporting the backflow of free carrier transfer from the photoactive layer to their respective electrodes. Consequently, the device fabricated with the ZnO+GO composite as an electron transport layer and the GO+PEDOT:PSS composite as a hole transport layer demonstrated a significant improvement in PCE (4.88%), reproducibility, and environmental stability (40% after 432 h). Thus, we confirmed that these air-resistant and fast carriers transporting composite layers will probably contribute significantly to the widespread commercialization of low-cost and easily fabricated organic solar cells.     

Deposition and characterization of PEDOT/ZnO layers onto PET substrates

ZnO thin films of different thicknesses were deposited by pulsed direct-current magnetron sputtering onto poly(ethylene terephthalate) (PET) substrates and afterwards poly 3, 4-ethylenedioxythiophene:polystyrenesulfonate (PEDOT:PSS) was spin-coated onto the ZnO film. Spectroscopic ellipsometry in the Vis–fUV energy range (1.5–6.5 eV), X-ray diffraction and atomic force microscopy were used to reveal the properties of the deposited films. The size of crystallites increased from 5.1 to 7.4 nm, whereas the crystallinity of the ZnO films has been improved. The influence of different ZnO thickness on the optical properties of the PEDOT:PSS layer was studied as well. As the thickness of ZnO films increased, the surface roughness increased but the energy gap decreased after a critical thickness. Concerning the consequences to the PEDOT:PSS optical properties, no major changes occurred in the transition energies.     

Application of poly(3,4-ethylenedioxythiophene):polystyrenesulfonate in polymer heterojunction solar cells

In this study, p-type semiconducting polymer of acid, poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS), has been employed as a hole-transporting electrode to fabricate organic polymer heterojunction photovoltaic cells. The results showed that the resultant poly(3-hexylthiophene): C₆₀ derivatives [6]-phenyl-C₆₁-butyric acid methyl ester (P3HT:PCBM)/PEDOT:PSS can significantly expand the light absorption range which was expected to enhance the sunlight excitation. The influences of annealing conditions and barrier layer on the photoelectric performances were investigated in detail, giving an optimized synthesis conditions: annealed temperature was at 120 °C for 90 min, the thickness of PEDOT:PSS film was approximately 3–4 μm, and the ratio of PCBM and P3HT was 1:2. The blended heterojunction consisting of PCBM and P3HT was used as charge carrier-transferring medium to replace I₃ ^?/I^? redox electrolyte, showing a short-circuit current of 4.30 mA cm^?2, an open-circuit voltage of 0.83 V, and a light-to-electric energy conversion efficiency of 2.37 % under a simulated solar light irradiation of 100 mW cm^?2. In addition, a solid-state polymer heterojunction photovoltaic cells with a short-circuit current of 3.59 mA cm^?2, an open-circuit voltage of 0.80 V, and a light-to-electric energy conversion efficiency of 1.9 % was successfully fabricated by simplifying the process.     

Solvent effect on the thin film formation of polymeric solar cells

This research work investigates the influence of the solvent used upon the formation of polymeric photoactive layer. This photoactive layer consists of a blend of poly (3-hexylthiophene) (P3HT) and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM), to produce P3HT:PCBM (at 1:1 ratio) blend thin films, in which the material solutions are deposited onto clean substrates via spin-coating technique. Three types of solvents were used namely: 100% dichlorobenzene (DCB), 100% chloroform (CH) and 50%:50% mixture of DCB and chloroform. The results reveal that the mixture of solvent produces a better solubility of the polymeric materials compared to that of the pure single solvent.     

Preparation of poly(3-hexylthiophene)/[6,6]-phenyl-C61-butyric acid methyl ester Langmuir-Blodgett bulk hetero junctions on indium tin oxide

Mono-layers of aggregated Poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) molecules were obtained by using solutions of P3HT, PCBM and P3HT-PCBM mixture without stabilizers such as stearates in chloroform at an air-water interface. 1 to 10 cycle-lifted LB films of P3HT and PCBM were successfully transferred to cleaned bare indium-tin-oxide coated glass substrate by vertical lifting method excluding the first 1 to 2 cycle layer. The dependence of P3HT and PCBM film thickness on the transfer cycles has been explained by the molecular sizes, where four edge-on P3HT molecular and six PCBM molecular stacking which result in thickness was taken into account. Work functions of deposited LB-layers were consistent with those of the ordinary casted films. P3HT and PCBM LB-layers showed optical activity in both infra-red (IR) and visible absorption regions of the spectrum. P-polarized IR absorption owing to C=C and C=O stretching vibrations observed in LB-layered films clearly indicate the enhancement of the orientation of these bonds perpendicular to the substrate surface in contrast to the spin-coated one. Visible optical absorption intensity was increased well in proportion with the lift cycle-numbers of both P3HT and PCBM LB films. The photovoltaic characteristics have been observed in the devices fabricated with P3HT (5 cycles-layer)/PCBM (5 cycles-layer) LB hetero structure as an active layer of the solar cells. The surface pressure of LB compression for the mixture of P3HT and PCBM, that is, bulk hetero mixtures, has also been well built up to 30 mN/m.     

Investigation of aged organic solar cell stacks by cross-sectional transmission electron microscopy coupled with elemental analysis

Polymer solar cells are of great interest as candidates for future low-cost and lightweight energy sources. One of the major reliability problems of these devices is the thermal instability of the blend morphology typically composed of poly(3-hexylthiophene) and [6,6]-phenyl-C₆₁-butyric acid methyl ester (P3HT and PCBM, respectively). Phase segregation of the blend has been extensively investigated by transmission electron microscopy (TEM) on free-standing films. In this study, we investigate in cross-section the morphology reorganization of P3HT:PCBM layers confined between poly(3,4-ethylenedioxythiophene)poly-(styrenesulfonate) (PEDOT:PSS) and a metal electrode similar to functional solar cell devices. The strengths of different TEM imaging and compositional analysis modes for the investigation of organic solar cells is illustrated by studying the evolution of the material stack with ageing conditions. Combining TEM imaging of the layer stack with energy-dispersive X-ray and energy loss electron spectroscopy, we not only gain insight into the phase segregation process but also explore the interdiffusion in the layer stack. More than 100 °C annealing leads to the formation of elongated protrusions ranging 100–500 nm. Thinning of the neighboring areas indicates lateral diffusion in the stack. Interestingly, the metal cathode remains still conformal over these large aggregates. The particles protrude through the metal layer only after prolonged (>100 h) annealing at higher temperatures when they reach several micrometer in height and are identified as crystalline PCBM-like material. Hence, almost full phase separation occurs by PCBM agglomeration and diffusion over large distances. Elemental analysis confirms that diffusion of the electrode materials (In, Sn and Yb) into the P3HT:PCBM stack remains below the detection limit.     

Efficiency enhancement of polymer solar cells with Ag nanoparticles incorporated into PEDOT:PSS layer

In this study, large-sized silver nanoparticles (Ag NPs) (average size: 80 nm) have been introduced into the anodic buffer poly-(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) layer (thickness: about 55 nm) of poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester bulk heterojunction polymer solar cells. The results showed that the short-circuit current density can increase from 8.73 to 11.36 mA/cm², and power conversion efficiency increases from 2.28 to 2.65 % when 0.1 wt% Ag NPs was incorporated in PEDOT:PSS layer, corresponding to an efficiency improvement of 16.2 %. Absorption spectrums of the active layers indicate that large-sized Ag NPs have no clear contribution to optical absorption improvement. By measuring the conductivity of PEDOT:PSS films without and with Ag NPs and analyzing device structure of this polymer solar cell, it was founded that the improvements in power conversion efficiency was originated from higher conductivity of PEDOT:PSS layer incorporated with Ag NPs and the shorter routes for holes to travel to the anode.     

Conductivity enhancement of PEDOT:PSS thin film using roll to plate technique and its characterization as a Schottky diode

The conductivity of poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) was improved by pressing the PEDOT:PSS thin film using roll to plate system. PEDOT:PSS thin film was deposited on polyethylene terephthalate using electrohydrodynamics atomization technique. The physico-chemical properties of the pressed thin film at different loads were compared with an un-pressed sample. The electrical properties show that the film conductivity has been increased by four times. An optimized pressing load was found to have good conductivity and transmittance of the thin film. A hybrid device (PEDOT:PSS/F8BT/ZnO/Ag) was fabricated using layer by layer method with PEDOT:PSS as anode. The I–V characterization showed that the device with pressed PEDOT:PSS showed higher current densities. The results give a promising future of PEDOT:PSS in electronics device applications using printed electronics techniques.