P3HT Layer Research Articles

Surface plasmon enhancement of polymer solar cells by penetrating Au/SiO2 core/shell nanoparticles into all organic layers

We report the incorporation of Au@SiO2 core/shell nanoparticles (NPs) into bulk heterojunction polymer solar cell (BHJ-PSC) devices, leading to an obvious efficiency enhancement due to the localized surface plasmon resonance (LSPR) effect. The Au@SiO2 core/shell NPs comprise of large Au NPs with an approximate size of 70nm coated by a ∼50nm thick SiO2 shell. Such NPs were doped into the poly-(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) layer of P3HT:PCBM BHJ-PSCs, resulting in that the large NPs penetrate into all organic layers including the PEDOT:PSS buffer layer and P3HT:PCBM active layer, and are partially embedded in the Al cathode layer. The power conversion efficiency (PCE) of the P3HT:PCBM BHJ-PSC devices incorporated with Au@SiO2 NPs increased from 3.29% to 3.80%, and such a ∼16% efficiency enhancement can be primarily attributed to the light absorption enhancement, originating from the LSPR effect induced by Au NPs as confirmed by the UV–vis absorption spectrocopic study. Contrarily, the analogous P3HT:PCBM BHJ-PSC devices incorporated with bare Au NPs exhibited a lower efficiency enhancement, indicating that the coating of dielectric SiO2 shell is beneficial for the LSPR effect.

Charge transport analysis of poly(3-hexylthiophene) by electroreflectance spectroscopy

The charge transport in organic semiconductors is still not completely understood. We use electroreflectance spectroscopy to investigate charge transport in organic field effect devices in order to obtain optical information on the charges within the charge accumulation layer at the organic semiconductor-insulator interface. Here, the reflectance geometry allows an analysis of devices prepared on opaque substrates. The recorded spectra were analyzed based on optical layer stack simulations to extract quantitatively the change of the dielectric function due to the charge injection of the accumulation layer in poly(3-hexylthiophene) (P3HT). For the simulation the anisotropic optical response of spin-coated P3HT layers is experimentally determined by spectroscopic ellipsometry. Using the developed theoretical approach the characteristic change of three different preparations of the organic semiconductor-insulator interface is analyzed. Laterally resolved measurements close to an injecting contact have revealed characteristic spectral changes in dependence of the preparation conditions, and significant spectral changes within the first $90\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{m}$ from the contact were found, which were attributed to an energetic relaxation of charges during the charge transport process.

Random CNT network and regioregular poly(3-hexylthiophen) FETs for pH sensing applications: A comparison

BackgroundNowadays, there is a tremendous need for cheap disposable sensing devices for medical applications. Materials such as Carbon Nanotubes (CNTs) and regioregular P3HT are proven to offer a huge potential as cost-effective and solution processable semiconductors for (bio)sensing applications. MethodsCNT-based field-effect transistors (CNT-FETs) as well as regioregular P3HT-based ones (P3HT-FETs) are fabricated and operated in the so-called electrolyte-gated configuration. The active layer of the P3HT-FETs consists of a spin-coated regioregular P3HT layer, which serves on one hand as the active sensing element and on the other hand as passivation layer for the transistor's metal contacts. The active layer of the nanotube transistors consists of a randomly distributed single walled CNT-network (>90% semiconducting tubes) deposited from a CNT-ink solution by spin-coating. ResultsWe compare both devices concerning their stability in aqueous environment and their response when exposed to buffers with different pH. We found that even if P3HT shows lower stability its pH sensitivity is reproducible even after long-term measurements. ConclusionCNT-FETs and P3HT-FETs offer different advantages and drawbacks concerning their stability in solution and the ease of fabrication. A discussion of their different sensing mechanisms as well as sensitivity is given here. General SignificanceThis work reports on fast and cost-effective production of solution processable thin-film transistors based on carbon nanotubes and regioregular P3HT and demonstrates their suitability as reliable pH sensors. This article is part of a Special Issue entitled Organic Bioelectronics — Novel Applications in Biomedicine.

Understanding the phase separation evolution in efficient P3HT : IC70BA-based bulk-heterojunction polymer solar cells

The effects of solvent and thermal annealing on the morphology of the active layers and the photovoltaic performance of bulk-heterojunction (BHJ) polymer solar cells (PSCs) are investigated systematically, for PSCs based on a blend of poly(3-hexylthiophene) (P3HT) as a donor and indene-C70 bisadduct (IC70BA) as an acceptor. IC70BA crystallites are found reasonably well dispersed in the P3HT matrix after spin-coating. However, the IC70BA crystallites coarsen in size after annealing, which are clearly evidenced by transmission electron microscopy. Simultaneously, space charge limited current measurements demonstrate that solvent and thermal annealing can improve the hole and electron mobility, which reduces charge-carrier recombination and improves charge-carrier transport in the P3HT and IC70BA blend layers. The corresponding current–voltage curves are measured in quantity and we propose a model to show the variation of the ordered structure of P3HT domains and IC70BA crystallite characteristics in the phase separation process, expressing a viewpoint on the high performance of BHJ PSCs.

Transient photovoltage in poly(3-hexylthiophene)/n-crystalline-silicon heterojunction

Transient photovoltage (PV) phenomena in regioregular poly(3-hexylthiophene) (P3HT)/n-crystalline-silicon (n-Si) heterojunction are studied under pulsed laser illumination. The PV transients of P3HT/n-Si heterojunctions are strongly retarded in time up to about 3 μs. The PV transients of P3HT/n-Si heterojunctions with thinner P3HT layer convert from positive to negative at longer times, which is caused by the long release time of trapped electrons at the P3HT/n-Si interface. Moreover, we find that the response of transient PV is dependent on the thickness of P3HT layer, the temperature and the excitation level.

Effects of bifunctional metal sulfide interlayers on photovoltaic properties of organic–inorganic hybrid solar cells

Bilayer organic–inorganic hybrid solar cells have been fabricated with chemical-bath deposited In2S3, SnS, and Sb2S3 interlayers between the organic P3HT hole-conducting and inorganic TiO2 electron-conducting layers. Sb has the highest electronegativity among the elements investigated, and the ionicity of the metal–sulfur bond decreases in the order In2S3 (15%) > SnS (9%) > Sb2S3 (7%). Thus, with a larger covalent character of the Sb–S bond, Sb may bond to S atoms on P3HT relatively easily compared to In or Sn. Such chemical bonding between Sb2S3 and P3HT may then reduce the overall incompatibility between the TiO2 and P3HT layers, making Sb2S3-based devices more efficient compared to In2S3- and SnS-based devices. Along with strong photon absorption, the presence of the metal–sulfur bond promotes improved wetting between the TiO2 sand P3HT layers, resulting in dual functionality for the metal sulfide interlayers. However, along with differences in metal–sulfur bond ionicity, the efficiency improvements seen with Sb2S3 can further be attributed to factors like differences in crystal structure, morphology, photon absorption, charge carrier recombination rates, and the manner in which the sulfide layer interacts with TiO2. While further studies are required to elucidate the exact reason behind the behavior of Sb2S3, the findings discussed herein can provide guidelines for identifying novel absorber layers for developing efficient and cost-effective organic–inorganic hybrid photovoltaics.

Correlation of diffusion and performance in sequentially processed P3HT/PCBM heterojunction films by time-resolved neutron reflectometry

Control over the structure of donor/acceptor blends is essential for the development of solution processable organic solar cells (OSCs). We have used time-resolved neutron reflectometry (NR) and in situ annealing to investigate the nanoscale structure and interdiffusion of sequentially spin-coated thin films of poly(3-n-hexylthiophene-2,5-diyl) (P3HT)/[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) and correlated the evolving structure with the device performance. While the as-prepared film shows a clear two-layer structure it is evident that (19 wt%) PCBM has percolated throughout the lower P3HT layer. Upon heating, analysis of time-resolved NR data shows that the diffusion process is dependent on the annealing temperature. At temperatures up to 110 °C, the two-layer structure is retained and only a small amount of PCBM diffuses from the interface into the lower layer, increasing the total PCBM content throughout the P3HT layer to 26 wt%. Significantly, this small change in acceptor content leads to a profound increase in device performance; with the power conversion efficiency (PCE) of the OSCs increasing from 0.47% (unannealed, 19 wt% PCBM) to 3.23% (annealed, 26 wt% PCBM) with the latter showing a similar efficiency to devices prepared from a blend containing 50 wt% PCBM. Further annealing at 120 and 130 °C sees rapid interdiffusion between the two layers, along with an overall expansion in the thickness of the bilayer film. Despite the complete intermixing of the PCBM and P3HT to form a structure resembling a bulk heterojunction, essentially no improvement in device performance was observed for annealing at temperatures above 110 °C.

All-polymer field-effect transistors using a brush gate dielectric

Interfaces between a poly(3-hexylthiophene) [P3HT] and an end-grafted (brush) layer of poly(methyl methacrylate) [PMMA] are shown using neutron reflectometry to be dependent on heat treatment. Annealing the samples allows part of the brush layer to cross into the P3HT layer creating a very asymmetric interface. We suggest that the P3HT rearrangement occurs, creating space for movement of the brush into the film. This interpenetration was observed with two different molecular weight (17.5 and 28 kg mol−1) P3HT films. Output characteristics of devices made from P3HT layers on PMMA brushes show that different amounts of heat treatment do not significantly change the device performance. Saturated hole mobilities are dependent on heat treatment, with devices made from a smaller molecular weight P3HT (22 kg mol−1) demonstrating larger mobilities than devices created using 48 kg mol−1 P3HT, but only after heat treatment.

Spectrum Response Enhancement of Organic Solar Cell Using a Poly(3-hexylthiophene) Photosensitizing Layer

We report here the photovoltaic performance of a solar cell using poly-{[4,8-bis[(2-ethylhexyl)oxy]-benzo[1,2-b:4,5-b']dithiophene-2,6-diyl]-alt-[3-fluore-2-(octyloxy)carbonyl-thieno[3,4-b]thiophene-4,6-diyl]} (PBDT-TT-F):[6,6]-phenyl-C-61-butyric acid methyl ester (PC61BM) as the photoactive layer in combination with a thin poly(3-hexylthiophene) (P3HT) film as a photosensitizing layer. PBDT-TT-F strongly absorbs light in the wavelength range of 550-700 nm, but it absorbs relatively weakly in the range of 350-550 nm. In contrast, a P3HT film shows intensive absorption ability in the 450-600 nm region, suggesting that the absorptions of these two materials complement each other to give a broad range. An organic solar cell with the structure indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene)-polystyrenesulfonic acid (PEDOT:PSS)/P3HT/PBDT-TT-F:[6,6]-phenyl C61-butyric acid methyl ester (PC61BM)/LiF/Al was fabricated in which P3HT was deposited between the hole transporting layer (PEDOT:PSS) and bulk heterojunction (BHJ) photoactive layer (PBDT-TT-F:PC61BM). This solar cell exhibited improved spectrum response in the wavelength range of 450-600 nm compared with one without a P3HT layer, which is attributed to the photosensitizing effect of the P3HT film. Optimization of the thickness of P3HT led to an increase of short-circuit current density (J(SC)) from 11.42 to 12.15 mA.cm(-2).

An energy-harvesting scheme employing CuGaSe2 quantum dot-modified ZnO buffer layers for drastic conversion efficiency enhancement in inorganic–organic hybrid solar cells

We demonstrated a promising route to enhance the performance of inverted organic photovoltaic (OPV) devices by the incorporation of CuGaSe2 (CGS) quantum dots (QDs) into the ZnO buffer layer of P3HT:PCBM-based devices. The modification of QDs provides better band alignment between the organic/cathode interface, improves ZnO crystal quality, and increases photon absorption, leading to more effective carrier transport/collection. By employing this energy-harvesting scheme, short-circuit current density, open-circuit voltage, and fill factor of the OPV device after CGS QD modification are improved by 9.43%, 7.02% and 6.31%, respectively, giving rise to a 23.8% enhancement in the power conversion efficiency.

A poly(3-hexylthiophene) modified CdS@TiO2shell-core nanorod array solar cell

In this paper, a novel solar cell device was fabricated based on poly(3-hexylthiophene) (P3HT) modified CdS@TiO2 shell-core nanorod array as the photoanode. To prepare the photoanode, rutile TiO2 nanorod arrays were first grown directly on fluorine doped transparent conductive glass (FTO) by a simple hydrothermal method. Then the CdS nanoparticles and a thin layer of P3HT were deposited onto the surface of the TiO2 nanorods sequentially through an electrochemical route. It was demonstrated that the surface modification of the TiO2 nanorods with CdS nanoparticles and P3HT could enhance the light absorption in the visible region. An energy conversion efficiency up to 1.38% was obtained with the designed solar cell.

Fabrication and characteristics of photodiode based on ZnO nanowire arrays and spray-coated regioregular P3HT layers

A photodiode based on well-aligned ZnO nanowire arrays (ZNAs) and spray-coated regioregular poly(3-hexylthiophene-2,5-diyl) (P3HT) layers hybrid hetero junctions was fabricated, and its electrical characteristics in dark and under illumination with a solar simulator were investigated. Current–voltage (I–V) data of ITO/ZNAs/P3HT/Ag device in dark and under illumination showed typical diode characteristics. A rectification ratio (RR) of 22.7 at 1.7V and a low turn-on voltage of 0.4V in dark were obtained. Also, the photodiode with high photo-response in the order of 0.31A/W at -2V using 80mW/cm2 illumination power was observed. Upon increasing illumination power from 40 to 100mW/cm2, the RR value for the photodiode continuously was improved with a highest value of 12.5.

Improvement of the power conversion efficiency of organic photovoltaic cells with a P3HT layer fabricated by using a sonication process and having a vertically modulated nanoscale morphology

In organic photovoltaic (OPV) cells, the nanoscale morphology of the organic layer is an essential parameter governing their charge-transport properties. Optimization of the vertical composition gradient in the poly(3-hexylthiophene) (P3HT) active layer in an OPV cell with an enhanced efficiency was achieved by using different sonication times of 10, 15, 20, 25, and 30min. Atomic force microscopy images confirmed that the sonication process increased the roughness of the P3HT layer in a vertically modulated nanoscale morphology. Photoluminescence spectra exhibited a strong peak corresponding to the P3HT particles. The power conversion efficiency of the OPV cells with a vertically modulated P3HT nanostructural layer sonicated at 15min was enhanced by 1.04% due to an increase in the interfacial region between the acceptor and the donor materials.

Effects of interfacial oxide layer in P3HT/n-Si organic/inorganic heterojunction diodes on their carrier transport properties

Current density–voltage (J–V) characteristics of poly(3-hexylthiophene) (P3HT)/n-silicon heterojunctions are investigated to improve the electrical characteristics of organic/inorganic heterojunction diodes. The diodes are fabricated by spin coating P3HT with SiC or post-HF treatment. These treatments are effective in suppressing the interfacial oxide between the P3HT and Si layers, where the forward current density is improved effectively. The thickness of the interfacial oxide layer correlates with improvements of not only the forward-bias current but also the ideality factor. The interfacial oxide layer effect on the carrier transport properties is discussed using a metal–insulator–semiconductor junction model.

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 1h, 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.

Instability and Pattern Formation Induced in Thin Crystalline Layers of a Conducting Polymer P3HT by Unstable Carrier Films of an Insulating Polymer

We investigate the stability, self-organized morphology, and kinetics of dewetting in a thin polymer bilayer (∼100 nm) of P3HT/polystyrene (PS) supported on Si wafers. The thin film of the conducting polymer, P3HT or regioregular poly(3-hexylthiophene), is driven to microstructure formation by dewetting of the unstable PS underlayer. Eventually, dewetting results in the formation of an undulating P3HT conducting skin covering the isolated islands of PS. The characteristic length scale of the structure is linearly proportional to the thickness of the PS underlayer. The kinetics of hole growth shows that the slippage behavior of the P3HT/PS bilayer which is different from the no-slip seen without the presence of the P3HT over layer. The bending elasticity and slippage induced by confinement of the PS layer by the P3HT layer strongly influence the length scale and kinetics of the dewetting process.

Poly(3-hexylthiophene) and [6,6]-Phenyl-C61-butyric Acid Methyl Ester Mixing in Organic Solar Cells

The mixing behavior of the hole- and electron-transporting materials in bulk heterojunction (BHJ) organic photovoltaic (OPV) blends plays a key role in determining the nanoscale morphology, which is believed to be a decisive factor in determining device performance. We present a systematic investigation of the mixing behavior between poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) in model multilayer structures. The bilayer structures are composed of amorphous PCBM that is mechanically laminated to different P3HT layers with varying degrees of crystallinity. We find that mixing is significantly decreased as the crystallinity of P3HT is increased. The mixing behavior can be explained as resulting from (1) nearly complete miscibility of PCBM with amorphous P3HT (based on our results from regiorandom P3HT) and (2) the existence of tie chains between crystalline P3HT domains that restrain the swelling of the P3HT layer by PCBM. We also introduce a unique PCBM–P3HT–PCBM trilayer structure where one of the PCBM layers is highly crystalline. The crystalline PCBM dramatically alters the mixing behavior. Initial mixing of the amorphous PCBM into P3HT is followed by rapid cold crystallization at the crystalline PCBM layer, which depletes the PCBM in the P3HT layer. These bilayer and trilayer experiments illustrate that mixing of P3HT and PCBM is influenced by multiple factors, such as the semicrystalline nature of P3HT (overall crystallinity, characteristics of amorphous chains) and phase (amorphous or crystalline) of the PCBM.

Dual-layer thermal nanoimprint lithography without dry etching

We report a novel dual-layer thermal nanoimprint with non-compatible polymer layers—poly(methyl methacrylate) (PMMA) on poly(3-hexylthiophene) (P3HT). Depending on mold depth, pattern size and polymer layer thickness, the dual-layer thermal nanoimprint can yield two different patterning results—single-layer patterning and double-layer patterning. The single-layer patterning is successfully performed with a delicate PMMA volume control and a very thin P3HT layer. Double-layer patterning is achieved when a thick and smooth P3HT layer is formed under a thin PMMA layer. Based on the mixing temperature as well as the glass transition temperatures of both polymers, the optimal nanoimprint temperature range is determined for dual-layer thermal nanoimprint. Because the pattern definition is completed without a dry etching step, this patterning technique can be applied to organic functional materials whose electrical properties can be critically degraded when exposed to oxygen dry etching in conventional thermal nanoimprint. The dual-layer thermal nanoimprint is expected to find important applications in the fabrication of high-performance organic electronic devices.

Enhanced Vertical Concentration Gradient in Rubbed P3HT:PCBM Graded Bilayer Solar Cells.

Graded bilayer solar cells have proven to be at least as efficient as the bulk heterojunctions when it comes to the Poly(3-hexylthiophene) (P3HT) - [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) donor-acceptor system. However, control of the vertical concentration gradient using simple techniques has never been reported. We demonstrate that rubbing the P3HT layer prior to PCBM deposition induces major morphological changes in the active layer. Using the newly introduced energy-dispersive X-ray spectroscopy element mapping technique, we found that rubbing P3HT induces the formation of an ideal vertical donor-acceptor concentration gradient. Furthermore, the P3HT crystallites undergo a molecular reorientation from edge-on to face-on configuration inducing a better charge transport in the vertical direction. The combination of these two major morphological changes leads to the fabrication of high-performance solar cells that exhibit, to date, the record efficiencies for spin-coated graded bilayers solar cells.

Electrical characterization of poly(amide-imide) for application in organic field effect devices

The admittance spectra and current–voltage (I–V) characteristics are reported of metal–insulator–metal (MIM) and metal–insulator–semiconductor (MIS) capacitors employing cross-linked poly(amide–imide) (c-PAI) as the insulator and poly(3-hexylthiophene) (P3HT) as the active semiconductor. The capacitance of the MIM devices are constant in the frequency range from 10Hz to 100kHz, with tanδ values as low as 7×10−3 over most of the range. Except at the lowest voltages, the I–V characteristics are well-described by the Schottky equation for thermal emission of electrons from the electrodes into the insulator. The admittance spectra of the MIS devices displayed a classic Maxwell–Wagner frequency response from which the transverse bulk hole mobility was estimated to be ∼2×10−5cm2V−1s−1 or ∼5×10−8cm2V−1s−1 depending on whether or not the surface of the insulator had been treated with hexamethyldisilazane (HMDS) prior to deposition of the P3HT. From the maximum loss observed in admittance-voltage plots, the interface trap density was estimated to be ∼5×1010cm−2eV−1 or ∼9×1010cm−2eV−1 again depending whether or not the insulator was treated with HMDS. We conclude, therefore, that HMDS plays a useful role in promoting order in the P3HT film as well as reducing the density of interface trap states. Although interposing the P3HT layer between the insulator and the gold electrode degrades the insulating properties of the c-PAI, nevertheless, they remain sufficiently good for use in organic electronic devices.