The Role of Morphology in Optically Switchable Transistors Based on a Photochromic Molecule/p‐Type Polymer Semiconductor Blend

The correlation between morphology and optoelectronic performance in organic thin‐film transistors based on blends of photochromic diarylethenes (DAE) and poly(3‐hexylthiophene) (P3HT) is investigated by varying molecular weight (M_w = 20–100 kDa) and regioregularity of the conjugated polymer as well as the temperature of thermal annealing (rt‐160 °C) in thin films. Semicrystalline architectures of P3HT/DAE blends comprise crystalline domains, ensuring efficient charge transport, and less aggregated regions, where DAEs are located as a result of their spontaneous expulsion from the crystalline domains during the self‐assembly. The best compromise between field‐effect mobility (μ) and switching capabilities is observed in blends containing P3HT with M_w = 50 kDa, exhibiting μ as high as 1 × 10^?3 cm² V^?1 s^?1 combined with a >50% photoswitching ratio. Higher or lower M_w than 50 kDa are found to be detrimental for field‐effect mobility and to lead to reduced device current switchability. The microstructure of the regioregular P3HT blend is found to be sensitive to the thermal annealing temperature, with an increase in μ and a decrease in current modulation being observed as a response to the light‐stimulus likely due to an increased P3HT‐DAE segregation, partially hindering DAE photoisomerization. The findings demonstrate the paramount importance of fine tuning the structure and morphology of bicomponent films for leveraging the multifunctional nature of optoelectronic devices.     

Temperature‐Resolved Local and Macroscopic Charge Carrier Transport in Thin P3HT Layers

Previous investigations of the field‐effect mobility in poly(3‐hexylthiophene) (P3HT) layers revealed a strong dependence on molecular weight (MW), which was shown to be closely related to layer morphology. Here, charge carrier mobilities of two P3HT MW fractions (medium‐MW: M_n = 7 200 g mol^?1; high‐MW: M_n = 27 000 g mol^?1) are probed as a function of temperature at a local and a macroscopic length scale, using pulse‐radiolysis time‐resolved microwave conductivity (PR‐TRMC) and organic field‐effect transistor measurements, respectively. In contrast to the macroscopic transport properties, the local intra‐grain mobility depends only weakly on MW (being in the order of 10^?2 cm² V^?1 s^?1) and being thermally activated below the melting temperature for both fractions. The striking differences of charge transport at both length scales are related to the heterogeneity of the layer morphology. The quantitative analysis of temperature‐dependent UV/Vis absorption spectra according to a model of F. C. Spano reveals that a substantial amount of disordered material is present in these P3HT layers. Moreover, the analysis predicts that aggregates in medium‐MW P3HT undergo a “pre‐melting” significantly below the actual melting temperature. The results suggest that macroscopic charge transport in samples of short‐chain P3HT is strongly inhibited by the presence of disordered domains, while in high‐MW P3HT the low‐mobility disordered zones are bridged via inter‐crystalline molecular connections.     

Effect of Alkyl Side‐Chain Length on Photovoltaic Properties of Poly(3‐alkylthiophene)/PCBM Bulk Heterojunctions

The morphological, bipolar charge‐carrier transport, and photovoltaic characteristics of poly(3‐alkylthiophene) (P3AT):[6,6]‐phenyl‐C61‐butyric acid methyl ester (PCBM) blends are studied as a function of alkyl side‐chain length m, where m equals the number of alkyl carbon atoms. The P3ATs studied are poly(3‐butylthiophene) (P3BT, m = 4), poly(3‐pentylthiophene) (P3PT, m = 5), and poly(3‐hexylthiophene) (P3HT, m = 6). Solar cells with these blends deliver similar order of photo‐current yield (exceeding 10 mA cm^?2) irrespective of side‐chain length. Power conversion efficiencies of 3.2, 4.3, and 4.6% are within reach using solar cells with active layers of P3BT:PCBM (1:0.8), P3PT:PCBM (1:1), and P3HT:PCBM (1:1), respectively. A difference in fill factor values is found to be the main source of efficiency difference. Morphological studies reveal an increase in the degree of phase separation with increasing alkyl chain length. Moreover, while P3PT:PCBM and P3HT:PCBM films have similar hole mobility, measured by hole‐only diodes, the hole mobility in P3BT:PCBM lowers by nearly a factor of four. Bipolar measurements made by field‐effect transistor showed a decrease in the hole mobility and an increase in the electron mobility with increasing alkyl chain length. Balanced charge transport is only achieved in the P3HT:PCBM blend. This, together with better processing properties, explains the superior properties of P3HT as a solar cell material. P3PT is proved to be a potentially competitive material. The optoelectronic and charge transport properties observed in the different P3AT:PCBM bulk heterojunction (BHJ) blends provide useful information for understanding the physics of BHJ films and the working principles of the corresponding solar cells.     

Charge Transport and Photocurrent Generation in Poly(3‐hexylthiophene): Methanofullerene Bulk‐Heterojunction Solar Cells

The effect of controlled thermal annealing on charge transport and photogeneration in bulk‐heterojunction solar cells made from blend films of regioregular poly(3‐hexylthiophene) (P3HT) and methanofullerene (PCBM) has been studied. With respect to the charge transport, it is demonstrated that the electron mobility dominates the transport of the cell, varying from 10^–8 m² V^–1 s^–1 in as‐cast devices to ≈3 × 10^–7 m² V^–1 s^–1 after thermal annealing. The hole mobility in the P3HT phase of the blend is dramatically affected by thermal annealing. It increases by more than three orders of magnitude, to reach a value of up to ≈ 2 × 10^–8 m² V^–1 s^–1 after the annealing process, as a result of an improved crystallinity of the film. Moreover, upon annealing the absorption spectrum of P3HT:PCBM blends undergo a strong red‐shift, improving the spectral overlap with solar emission, which results in an increase of more than 60 % in the rate of charge‐carrier generation. Subsequently, the experimental electron and hole mobilities are used to study the photocurrent generation in P3HT:PCBM devices as a function of annealing temperature. The results indicate that the most important factor leading to a strong enhancement of the efficiency, compared with non‐annealed devices, is the increase of the hole mobility in the P3HT phase of the blend. Furthermore, numerical simulations indicate that under short‐circuit conditions the dissociation efficiency of bound electron–hole pairs at the donor/acceptor interface is close to 90 %, which explains the large quantum efficiencies measured in P3HT:PCBM blends.     

Self Encapsulated Poly(3‐hexylthiophene)‐poly(fluorinated alkyl methacrylate) Rod‐Coil Block Copolymers with High Field Effect Mobilities on Bare SiO2

Conjugated rod‐coil block copolymers provide an interesting route towards enhancing the properties of the conjugated block due to self‐assembly and the interplay of rod‐rod and rod‐coil interactions. Here, we demonstrate the ability of an attached semi‐fluorinated block to significantly improve upon the charge carrier properties of regioregular poly(3‐hexyl thiophene) (rr‐P3HT) materials on bare SiO₂. The thin film hole mobilities on bare SiO₂ dielectric surfaces of poly (3‐hexyl thiophene)‐block‐polyfluoromethacrylates (P3HT‐b‐PFMAs) can approach up to 0.12 cm² V^?1 s^?1 with only 33 wt% of the P3HT block incorporated in the copolymer, as compared to rr‐P3HT alone which typically has mobilities averaging 0.03 cm² V^?1 s^?1. To our knowledge, this is the highest mobility reported in literature for block copolymers containing a P3HT. More importantly, these high hole mobilities are achieved without multistep OTS treatments, argon protection, or post‐annealing conditions. Grazing incidence wide‐angle x‐ray scattering (GIWAX) data revealed that in the P3HT‐b‐PFMA copolymers, the P3HT rod block self‐assembles into highly ordered lamellar structures, similar to that of the rr‐P3HT homopolymer. Grazing incidence small‐angle x‐ray scattering (GISAXS) data revealed that lamellar structures are only observed in perpendicular direction with short PFMA blocks, while lamellae in both perpendicular and parallel directions are observed in polymers with longer PFMA blocks. AFM, GIWAXS, and contact angle measurements also indicate that PFMA block assembles at the polymer thin film surface and forms an encapsulation layer. The high charge carrier mobilities and the hydrophobic surface of the block copolymer films clearly demonstrates the influence of the coil block segment on device performance by balancing the crystallization and microphase separation in the bulk morphological structure.     

The Impact of Polymer Regioregularity on Charge Transport and Efficiency of P3HT:PCBM Photovoltaic Devices

The charge transport in pristine poly(3‐hexylthiophene) (P3HT) films and in photovoltaic blends of P3HT with [6,6]‐phenyl C61 butyric acid methyl ester (PCBM) is investigated to study the influence of charge‐carrier transport on photovoltaic efficiency. The field‐ and temperature dependence of the charge‐carrier mobility in P3HT of three different regioregularities, namely, regiorandom, regioregular with medium regioregularity, and regioregular with very high regioregularity are investigated by the time‐of‐flight technique. While medium and very high regioregularity polymers show the typical absorption features of ordered lamellar structures of P3HT in the solid state even without previous annealing, films of regiorandom P3HT are very disordered as indicated by their broad and featureless absorption. This structural difference in the solid state coincides with partially non‐dispersive transport and hole mobilities µ_h of around 10^?4 and 10^?5 cm² V^?1 s^?1 for the high and medium regioregularity P3HT, respectively, and a slow and dispersive charge transport for the regiorandom P3HT. Upon blending the regioregular polymers with PCBM, the hole mobilities are typically reduced by one order of magnitude, but they do not significantly change upon additional post‐spincasting annealing. Only in the case of P3HT with high regioregularity are the electron mobilities similar to the hole mobilities and the charge transport is, thus, balanced. Nonetheless, devices prepared from both materials exhibit similar power conversion efficiencies of 2.5%, indicating that very high regioregularity may not substantially improve order and charge‐carrier transport in P3HT:PCBM and does not lead to significant improvements in the power‐conversion efficiency of photovoltaic devices.     

Measurement of Charge‐Density Dependence of Carrier Mobility in an Organic Semiconductor Blend

Here, a new methodology for analyzing the charge‐density dependence of carrier mobility in organic semiconductors, applicable to the low‐charge‐density regime (10¹⁴–10¹⁷ cm^?3) corresponding to the operation conditions of many organic optoelectronic devices, is reported. For the P3HT/PCBM blend photovoltaic devices studied herein, the hole mobility µ is found to depend on charge density n according to a power law µ(n) ∝ n^δ, where δ = 0.35. This dependence is shown to be consistent with an energetic disorder model based upon an exponential tail of localized intra‐band states.     

Effect of Molecular Weight on the Structure and Morphology of Oriented Thin Films of Regioregular Poly(3‐hexylthiophene) Grown by Directional Epitaxial Solidification

Regioregular head‐to‐tail (HT)‐coupled poly(3‐hexylthiophene‐2,5‐diyl) (P3HT) with a weight‐average molecular weight (M_w) in the 7.3–69.6 kDa range is crystallized by directional epitaxial solidification in 1,3,5‐trichlorobenzene (TCB) to yield highly oriented thin films. An oriented and periodic lamellar structure consisting of crystalline lamellae separated by amorphous interlamellar zones is evidenced by atomic force microscopy (AFM) and transmission electron microscopy (TEM). Both the overall crystallinity as well as the orientation of the crystalline lamellae decrease significantly with increasing M_w. The total lamellar periodicity is close to the length of “fully extended” chains for M_w = 7.3 kDa (polystyrene‐equivalent molecular weight, eq. PS) and it saturates to a value of ca. (25–28) ± 2 nm for M_w ≥ 18.8 kDa (eq. PS). This behavior is attributed to a transition from an oligomeric‐like system, for which P3HT chains are essentially in a fully extended all‐trans conformation and do not fold, to a semicrystalline system that involves a periodic alternation of crystalline lamellae separated by extended amorphous interlamellar zones, which harbor chain folds, chain ends, and tie molecules. For P3HT with M_w of ca. 7.3 kDa (eq. PS), epitaxial crystallization on TCB allows for the growth of both “edge‐on” and “flat‐on” oriented crystalline lamellae on the TCB substrate. The orientation of the lamellae is attributed to 1D epitaxy. Because of the large size of the “flat‐on” crystalline lamellae, a characteristic single‐crystal electron diffraction pattern corresponding to the [001] zone was obtained by selected area electron diffraction (SAED), indicating that P3HT crystallizes in a monoclinic unit cell with a = 16.0 Å, b = 7.8 Å, c = 7.8 Å, and γ = 93.5°.     

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.     

Photophysics of Molecular‐Weight‐Induced Losses in Indacenodithienothiophene‐Based Solar Cells

The photovoltaic performance and optoelectronic properties of a donor–acceptor copolymer are reported based on indacenodithienothiophene (IDTT) and 2,3‐bis(3‐(octyloxy)phenyl)quinoxaline moieties (PIDTTQ) as a function of the number‐average molecular weight (M_n). Current–voltage measurements and photoinduced charge carrier extraction by linear increasing voltage (photo‐CELIV) reveal improved charge generation and charge transport properties in these high band gap systems with increasing M_n, while polymers with low molecular weight suffer from diminished charge carrier extraction because of low mobility–lifetime (μτ) product. By combining Fourier‐transform photocurrent spectroscopy (FTPS) with electroluminscence spectroscopy, it is demonstrate that increasing M_n reduces the nonradiative recombination losses. Solar cells based on PIDTTQ with M_n = 58 kD feature a power conversion efficiency of 6.0% and a charge carrier mobility of 2.1 × 10^?4 cm² V^?1 s^?1 when doctor bladed in air, without the need for thermal treatment. This study exhibits the strong correlations between polymer fractionation and its optoelectronics characteristics, which informs the polymer design rules toward highly efficient organic solar cells.     

High Electron Mobility and Ambipolar Charge Transport in Binary Blends of Donor and Acceptor Conjugated Polymers

High electron mobility and ambipolar charge transport are observed in phase‐separated binary blends of n‐type poly(benzobisimidazobenzophenanthroline) (BBL) with p‐type polymer semiconductors, poly[(thiophene‐2,5‐diyl)‐alt‐(2,3‐diheptylquinoxaline‐5,8‐diyl)] (PTHQx) and poly(10‐hexylphenoxazine‐3,7‐diyl‐alt‐3‐hexyl‐2,5‐thiophene) (POT). Atomic force microscopy (AFM) and transmission electron microscopy (TEM) show phase‐separated domains of 50–300 nm in the binary blend thin films. The TEM images and electron diffraction of BBL/PTHQx blends show the growth of single‐crystalline phases of PTHQx within the BBL matrix. A relatively high electron mobility (1.0 × 10^–3 cm² V^–1 s^–1) that is constant over a wide blend‐composition range is observed in the PTHQx blend field‐effect transistors (FETs). Ambipolar charge transport is observed in both blend systems at a very high concentration of the p‐type semiconductor (≥90 wt % PTHQx or ≥80 wt % POT). Ambipolar charge transport is exemplified by an electron mobility of 1.4 × 10^–5 cm² V^–1 s^–1 and a hole mobility of 1.0 × 10^–4 cm² V^–1 s^–1 observed in the 98 wt % PTHQx blend FETs. These results show that ambipolar charge transport and the associated carrier mobilities in blends of conjugated polymer semiconductors have a complex dependence on the blend composition and the phase‐separated morphology.     

Hybrid Solar Cells from a Blend of Poly(3‐hexylthiophene) and Ligand‐Capped TiO2 Nanorods

Hybrid bulk heterojunction solar cells based on nanocrystalline TiO₂ (nc‐TiO₂) nanorods capped with trioctylphosphine oxide (TOPO) and regioregular poly(3‐hexylthiophene) (P3HT) are processed from solution and characterized in order to relate the device function (optical absorption, charge separation, and transport and photovoltaic properties) to active‐layer properties and device parameters. Annealing the blend films is found to greatly improve the polymer–metal oxide interaction at the nc‐TiO₂/P3HT interface, resulting in a six‐fold increase of the charge separation yield and improved photovoltaic device performance under simulated solar illumination. In addition, the influence of the organic ligand at the nc‐TiO₂ particle surface is found to be crucial for charge separation. Ligand‐exchange procedures applied on the TOPO‐capped nc‐TiO₂ nanorods with an amphiphilic ruthenium‐based dye are found to further improve the charge‐separation yield at the polymer–nanocrystal interface. However, the poor photocurrents generated in the hybrid blend devices, before and after ligand exchange, suggest that transport within or between nanoparticles limits performance. By comparison with other donor–acceptor bulk heterojunction systems, we conclude that charge transport in the nc‐TiO₂:P3HT blend films is limited by the presence of an intrinsic trap distribution mainly associated with the nc‐TiO₂ particles.     

The Effects of Crystallinity on Charge Transport and the Structure of Sequentially Processed F4TCNQ‐Doped Conjugated Polymer Films

The properties of molecularly doped films of conjugated polymers are explored as the crystallinity of the polymer is systematically varied. Solution sequential processing (SqP) was used to introduce 2,3,5,6‐tetrafluoro‐7,7,8,8‐tetracyanoquinodimethane (F₄TCNQ) into poly(3‐hexylthiophene‐2,5‐diyl) (P3HT) while preserving the pristine polymer's degree of crystallinity. X‐ray data suggest that F₄TCNQ anions reside primarily in the amorphous regions of the film as well as in the P3HT lamellae between the side chains, but do not π‐stack within the polymer crystallites. Optical spectroscopy shows that the polaron absorption redshifts with increasing polymer crystallinity and increases in cross section. Theoretical modeling suggests that the polaron spectrum is inhomogeneously broadened by the presence of the anions, which reside on average 6–8 Å from the polymer backbone. Electrical measurements show that the conductivity of P3HT films doped by F₄TCNQ via SqP can be improved by increasing the polymer crystallinity. AC magnetic field Hall measurements show that the increased conductivity results from improved mobility of the carriers with increasing crystallinity, reaching over 0.1 cm² V^?1 s^?1 in the most crystalline P3HT samples. Temperature‐dependent conductivity measurements show that polaron mobility in SqP‐doped P3HT is still dominated by hopping transport, but that more crystalline samples are on the edge of a transition to diffusive transport at room temperature.     

Donor–Acceptor Poly(3‐hexylthiophene)‐block‐Pendent Poly(isoindigo) with Dual Roles of Charge Transporting and Storage Layer for High‐Performance Transistor‐Type Memory Applications

Field‐effect transistor memories usually require one additional charge storage layer between the gate contact and organic semiconductor channel. To avoid such complication, new donor–acceptor rod–coil diblock copolymers (P3HT₄₄‐b‐Piso_n) of poly(3‐hexylthiophene) (P3HT)‐block‐poly(pendent isoindigo) (Piso) are designed, which exhibit high performance transistor memory characteristics without additional charge storage layer. The P3HT and Piso blocks are acted as the charge transporting and storage elements, respectively. The prepared P3HT₄₄‐b‐Piso_n can be self‐assembled into fibrillar‐like nanostructures after the thermal annealing process, confirmed by atomic force microscopy and grazing‐incidence X‐ray diffraction. The lowest‐unoccupied molecular orbital levels of the studied polymers are significantly lowered as the block length of Piso increases, leading to a stronger electron affinity as well as charge storage capability. The field‐effect transistors (FETs) fabricated from P3HT₄₄‐b‐Piso_n possess p‐type mobilities up to 4.56 × 10^?2 cm² V^?1 s^?1, similar to that of the regioregular P3HT. More interestingly, the FET memory devices fabricated from P3HT₄₄‐b‐Piso_n exhibit a memory window ranging from 26 to 79 V by manipulating the block length of Piso, and showed stable long‐term data endurance. The results suggest that the FET characteristics and data storage capability can be effectively tuned simultaneously through donor/acceptor ratio and thin film morphology in the block copolymer system.     

On the Relation between Morphology and FET Mobility of Poly(3‐alkylthiophene)s at the Polymer/SiO2 and Polymer/Air Interface

The influence of the interface of the dielectric SiO₂ on the performance of bottom‐contact, bottom‐gate poly(3‐alkylthiophene) (P3AT) field‐effect transistors (FETs) is investigated. In particular, the operation of transistors where the active polythiophene layer is directly spin‐coated from chlorobenzene (CB) onto the bare SiO₂ dielectric is compared to those where the active layer is first spin‐coated then laminated via a wet transfer process such that the film/air interface of this film contacts the SiO₂ surface. While an apparent alkyl side‐chain length dependent mobility is observed for films directly spin‐coated onto the SiO₂ dielectric (with mobilities of ≈10^?3 cm² V^?1 s^?1 or less) for laminated films mobilities of 0.14 ± 0.03 cm² V^?1 s^?1 independent of alkyl chain length are recorded. Surface‐sensitive near edge X‐ray absorption fine structure (NEXAFS) spectroscopy measurements indicate a strong out‐of‐plane orientation of the polymer backbone at the original air/film interface while much lower average tilt angles of the polymer backbone are observed at the SiO₂/film interface. A comparison with NEXAFS on crystalline P3AT nanofibers, as well as molecular mechanics and electronic structure calculations on ideal P3AT crystals suggest a close to crystalline polymer organization at the P3AT/air interface of films from CB. These results emphasize the negative influence of wrongly oriented polymer on charge carrier mobility and highlight the potential of the polymer/air interface in achieving excellent “out‐of‐plane” orientation and high FET mobilities.     

Organic Field‐Effect Devices as Tool to Characterize the Bipolar Transport in Polymer‐Fullerene Blends: The Case of P3HT‐PCBM

In organic bulk heterojunction solar cells (oBHJ) the blend morphology in combination with the charge transport properties of the individual components controls the extracted photocurrent. The organic field‐effect transistor (OFET) has been proved as a powerful instrument to evaluate the unipolar carrier transport properties in a wide range of cases. In our work we extend the OFET concept to the evaluation of the bipolar transport properties in polymer‐fullerenes blends and propose a method to improve the accuracy of the evaluation. The method is based on capacitance–voltage (C–V) measurements on MOS structures prepared on the same blends and delivers complementary information on the bulk heterojunction to the one obtained with FETs. The relevance for photovoltaic applications is investigated through the correlation between the current–voltage behavior of solar cells and the bipolar mobility for composites with varying polymer molecular weight and processed from different solvents. In particular the transport features of solar cells produced from o‐Xylene (oX), a non chlorinated solvent more suitable to production requirements, have been compared to the one of devices cast from Chlorobenzene (CB) solution. For the P3HT‐PCBM blend a consistent correlation between the mobility and the electrical fill factor and power performance was found. A significant asymmetry in the bipolar carrier mobility, together with low electron mobility dependent on the M_w value, affects the performances of thick o‐Xylene cast devices. In the case of devices processed from Chlorobenzene the slower carrier has higher mobility and the small electrical losses detected are eventually more related to the formation of space‐charge and eventually to surface recombination. This results in an efficient charge collection that is almost thickness independent. We report a dependence of the slow‐carrier type (electrons or holes) and their mobility on the specific combination of molecular weight and solvent. The mobility data and the solar cell performance coherently fit to the prediction of a device model only based on the drift of carriers under the built‐in electric field originated in the donor‐acceptor oBHJ.     

The Origin of the High Voltage in DPM12/P3HT Organic Solar Cells

Organic solar cells made using a blend of DPM12 and P3HT are studied. The results show that higher V_oc can be obtained when using DPM12 in comparison to the usual mono‐substituted PCBM electron acceptor. Moreover, better device performances are also registered when the cells are irradiated with sun‐simulated light of 10–50 mW cm^?2 intensity. Electrochemical and time‐resolved spectroscopic measurements are compared for both devices and a 100‐mV shift in the density of states (DOS) is observed for DPM12/P3HT devices with respect to PCBM/P3HT solar cells and slow polaron‐recombination dynamics are found for the DPM12/P3HT devices. These observations can be directly correlated with the observed increase in V_oc, which is in contrast with previous results that correlated the higher V_oc with different ideality factors obtained using dark‐diode measurements. The origin for the shift in the DOS can be correlated to the crystallinity of the blend that is influenced by the properties of the included fullerene.     

Effect of Mesoscale Crystalline Structure on the Field‐Effect Mobility of Regioregular Poly(3‐hexyl thiophene) in Thin‐Film Transistors

Regioregular poly(3‐hexyl thiophene) (RR P3HT) is drop‐cast to fabricate field‐effect transistor (FET) devices from different solvents with different boiling points and solubilities for RR P3HT, such as methylene chloride, toluene, tetrahydrofuran, and chloroform. A Petri dish is used to cover the solution, and it takes less than 30 min for the solvents to evaporate at room temperature. The mesoscale crystalline morphology of RR P3HT thin films can be manipulated from well‐dispersed nanofibrils to well‐developed spherulites by changing solution processing conditions. The morphological correlation with the charge‐carrier mobility in RR P3HT thin‐film transistor (TFT) devices is investigated. The TFT devices show charge‐carrier mobilities in the range of 10^–4 ～ 10^–2 cm² V^–1 s^–1 depending on the solvent used, although grazing‐incidence X‐ray diffraction (GIXD) reveals that all films develop the same π–π‐stacking orientation, where the <100>‐axis is normal to the polymer films. By combining results from atomic force microscopy (AFM) and GIXD, it is found that the morphological connectivity of crystalline nanofibrils and the <100>‐axis orientation distribution of the π–π‐stacking plane with respect to the film normal play important roles on the charge‐carrier mobility of RR P3HT for TFT applications.     

High resolution electrical characterisation of organic photovoltaic blends

In this work, electrostatic force microscopy (EFM) and conductive atomic force microscopy (C-AFM) are applied to perform high-resolution electrical characterisation of organic photovoltaic films. These films are composed of the C₆₀-derivative PCBM blended with hole conductive conjugated polymers PPV derivatives or P3HT. It is demonstrated that both EFM and C-AFM are able to electrically evidence phase separation in the blends, suggesting in addition higher density of carriers along interfaces. Correlation between the EFM contrast and the photovoltaic properties of the blends was observed. Local spectroscopy (I-V curves) completes the C-AFM investigations, analysing charge transport mechanisms in the P3HT:PCBM blend. Significant modifications of the local electrical properties of P3HT are shown to occur upon blending. Space charge limited current is evidenced in the blend and a hole mobility of 1.7 × 10⁻² cm² V⁻¹ s⁻¹ is determined for P3HT.     

The Effect of Thermal Treatment on the Morphology and Charge Carrier Dynamics in a Polythiophene–Fullerene Bulk Heterojunction

The influence of various thermal treatment steps on the morphology and the photoconductive properties of a non‐contacted, 50 nm thick blend (50:50 wt.‐%) of [6,6]‐phenyl C₆₁‐butyric acid methyl ester (PCBM) and poly(3‐hexyl thiophene) (P3HT) spin‐coated from chloroform has been studied using transmission electron microscopy (TEM) and the electrodeless time‐resolved microwave conductivity technique. After annealing the film for 5 min at 80 °C, TEM images show the formation of crystalline fibrils of P3HT due to a more ordered packing of the polymer chains. The thermal treatment results in a large increase of the photoconductivity, due to an enhancement of the hole mobility in these crystalline P3HT domains from 0.0056 cm² V^–1 s ^–1 for the non‐annealed sample to 0.044 cm² V^–1 s ^–1 for the sample annealed at 80 °C. In contrast, the temporal shape of the photoconductivity, with typical decay half‐times, τ_1/2, of 1 μs for the lowest excitation intensities, is unaffected by the temperature treatment. Further annealing of the sample at 130 °C results in the formation of three different substructures within the heterojunction: a PCBM:P3HT blend with PCBM‐rich clusters, a region depleted of PCBM, and large PCBM single crystals. Only a minor increase in the amplitude, but a tenfold rise of the decay time of the photoconductivity, is observed. This is explained by the formation of PCBM‐rich clusters and large PCBM single crystals, resulting in an increased diffusional escape probability for mobile charge carriers and hence reduced recombination.