Doping Dependent In‐Plane and Cross‐Plane Thermoelectric Performance of Thin n‐Type Polymer P(NDI2OD‐T2) Films

Thermoelectric generators pose a promising approach in renewable energies as they can convert waste heat into electricity. In order to build high efficiency devices, suitable thermoelectric materials, both n‐ and p‐type, are needed. Here, the n‐type high‐mobility polymer poly[N,N′‐bis(2‐octyldodecyl)naphthalene‐1,4,5,8‐bis(dicarboximide)‐2,6‐diyl]‐alt‐5,5′‐(2,2′‐bithiophene) (P(NDI2OD‐T2)) is focused upon. Via solution doping with 4‐(1,3‐dimethyl‐2,3‐dihydro‐1H‐benzoimidazol‐2‐yl)‐N,N‐diphenylaniline (N‐DPBI), a maximum power factor of (1.84 ± 0.13) µW K^?2 m^?1 is achieved in an in‐plane geometry for 5 wt% dopant concentration. Additionally, UV–vis spectroscopy and grazing‐incidence wide‐angle X‐ray scattering are applied to elucidate the mechanisms of the doping process and to explain the discrepancy in thermoelectric performance depending on the charge carriers being either transported in‐plane or cross‐plane. Morphological changes are found such that the crystallites, built‐up by extended polymer chains interacting via lamellar and π–π stacking, re‐arrange from face‐ to edge‐on orientation upon doping. At high doping concentrations, dopant molecules disturb the crystallinity of the polymer, hindering charge transport and leading to a decreased power factor at high dopant concentrations. These observations explain why an intermediate doping concentration of N‐DPBI leads to an optimized thermoelectric performance of P(NDI2OD‐T2) in an in‐plane geometry as compared to the cross‐plane case.     

Controllable Molecular Doping and Charge Transport in Solution‐Processed Polymer Semiconducting Layers

Here, controlled p‐type doping of poly(2‐methoxy‐5‐(2′‐ethylhexyloxy)‐p‐phenylene vinylene) (MEH‐PPV) deposited from solution using tetrafluoro‐tetracyanoquinodimethane (F4‐TCNQ) as a dopant is presented. By using a co‐solvent, aggregation in solution can be prevented and doped films can be deposited. Upon doping the current–voltage characteristics of MEH‐PPV‐based hole‐only devices are increased by several orders of magnitude and a clear Ohmic behavior is observed at low bias. Taking the density dependence of the hole mobility into account the free hole concentration due to doping can be derived. It is found that a molar doping ratio of 1 F4‐TCNQ dopant per 600 repeat units of MEH‐PPV leads to a free carrier density of 4 × 10²² m^?3. Neglecting the density‐dependent mobility would lead to an overestimation of the free hole density by an order of magnitude. The free hole densities are further confirmed by impedance measurements on Schottky diodes based on F4‐TCNQ doped MEH‐PPV and a silver electrode.     

Anisotropy of Charge Transport in a Uniaxially Aligned and Chain‐Extended,High‐Mobility,Conjugated Polymer Semiconductor

Charge transport in the ribbon phase of poly(2,5‐bis(3‐alkylthiophen‐2‐yl)thieno[3,2‐b]thiophene) (PBTTT)—one of the most highly ordered, chain‐extended crystalline microstructures available in a conjugated polymer semiconductor—is studied. Ribbon‐phase PBTTT has previously been found not to exhibit high carrier mobilities, but it is shown here that field‐effect mobilities depend strongly on the device architecture and active interface. When devices are constructed such that the ribbon‐phase films are in contact with either a polymer gate dielectric or an SiO₂ gate dielectric modified by a hydrophobic, self‐assembled monolayer, high mobilities of up to 0.4 cm² V^?1 s^?1 can be achieved, which is comparable to those observed previously in terrace‐phase PBTTT. In uniaxially aligned, zone‐cast films of ribbon‐phase PBTTT the mobility anisotropy is measured for transport both parallel and perpendicular to the polymer chain direction. The mobility anisotropy is relatively small, with the mobility along the polymer chain direction being higher by a factor of 3–5, consistent with the grain size encountered in the two transport directions.     

Very Low Degree of Energetic Disorder as the Origin of High Mobility in an n‐channel Polymer Semiconductor

Charge transport is investigated in high‐mobility n‐channel organic field‐effect transistors (OFETs) based on poly{[N,N′‐bis(2‐octyldodecyl)‐naphthalene‐1,4,5,8‐bis(dicarboximide)‐2,6‐diyl]‐alt‐5,5′‐(2,2′‐bithiophene)} (P(NDI2OD‐T2), Polyera ActivInk? N2200) with variable‐temperature electrical measurements and charge‐modulation spectroscopy. Results indicate an unusually uniform energetic landscape of sites for charge‐carrier transport along the channel of the transistor as the main reason for the observed high‐electron mobility. Consistent with a lateral field‐independent transport at temperatures down to 10 K, the reorganization energy is proposed to play an important role in determining the activation energy for the mobility. Quantum chemical calculations, which show an efficient electronic coupling between adjacent units and a reorganization energy of a few hundred meV, are consistent with these findings.     

Higher Molecular Weight Leads to Improved Photoresponsivity,Charge Transport and Interfacial Ordering in a Narrow Bandgap Semiconducting Polymer

Increasing the molecular weight of the low‐bandgap semiconducting copolymer, poly[(4,4‐didoecyldithieno[3,2‐b:2′,3′‐d]silole)‐2,6‐diyl‐alt‐(2,1,3‐benzothiadiazole)‐4,7‐diyl], Si‐PDTBT, from 9 kDa to 38 kDa improves both photoresponsivity and charge transport properties dramatically. The photocurrent measured under steady state conditions is 20 times larger in the higher molecular weight polymer (HM_n Si‐PDTBT). Different decays of polarization memory in transient photoinduced spectroscopy measurements are consistent with more mobile photoexcitations in HM_n Si‐PDTBT relative to the lower molecular weight counterpart (LM_n Si‐PDTBT). Analysis of the current‐voltage characteristics of field effect transistors reveals an increase in the mobility by a factor of 700 for HM_n Si‐PDTBT. Near edge X‐ray absorption fine structure (NEXAFS) spectroscopy and grazing incidence small angle X‐ray scattering (GISAXS) measurements demonstrate that LM_n Si‐PDTBT forms a disordered morphology throughout the depth of the film, whereas HM_n Si‐PDTBT exhibits pronounced π‐π stacking in an edge‐on configuration near the substrate interface. Increased interchain overlap between polymers in the edge‐on configuration in HM_n Si‐PDTBT results in the higher carrier mobility. The improved optical response, transport mobility, and interfacial ordering highlight the subtle role that the degree of polymerization plays on the optoelectronic properties of conjugated polymer based organic semiconductors.     

Accurate Extraction of Charge Carrier Mobility in 4‐Probe Field‐Effect Transistors

Charge carrier mobility is an important characteristic of organic field‐effect transistors (OFETs) and other semiconductor devices. However, accurate mobility determination in FETs is frequently compromised by issues related to Schottky‐barrier contact resistance, that can be efficiently addressed by measurements in 4‐probe/Hall‐bar contact geometry. Here, it is shown that this technique, widely used in materials science, can still lead to significant mobility overestimation due to longitudinal channel shunting caused by voltage probes in 4‐probe structures. This effect is investigated numerically and experimentally in specially designed multiterminal OFETs based on optimized novel organic‐semiconductor blends and bulk single crystals. Numerical simulations reveal that 4‐probe FETs with long but narrow channels and wide voltage probes are especially prone to channel shunting, that can lead to mobilities overestimated by as much as 350%. In addition, the first Hall effect measurements in blended OFETs are reported and how Hall mobility can be affected by channel shunting is shown. As a solution to this problem, a numerical correction factor is introduced that can be used to obtain much more accurate experimental mobilities. This methodology is relevant to characterization of a variety of materials, including organic semiconductors, inorganic oxides, monolayer materials, as well as carbon nanotube and semiconductor nanocrystal arrays.     

Impact of Morphology on Charge Carrier Transport and Thermoelectric Properties of N‐Type FBDOPV‐Based Polymers

The impact of the chemical structure and molecular order on the charge transport properties of two donor–acceptor copolymers in their neutral and doped states is investigated. Both polymers comprise 3,7‐bis((E)‐7‐fluoro‐1‐(2‐octyl‐dodecyl)‐2‐oxoindolin‐3‐ylidene)‐3,7‐dihydrobenzo[1,2‐b:4,5‐b′]difuran‐2,6‐dione (FBDOPV) as electron‐accepting unit, copolymerized with 9,9‐dioctyl‐fluorene (P(FBDOPV‐F)) or with 3‐dodecyl‐2,2′‐bithiophene (P(FBDOPV‐2T‐C₁₂)). These copolymers possess an amorphous and semi‐crystalline nature, respectively, and exhibit remarkable electron mobilities of 0.065 and 0.25 cm² V^–1 s^–1 in field effect transistors. However, after chemical n‐doping with 4‐(1,3‐dimethyl‐2,3‐dihydro‐1H‐benzoimidazol‐2‐yl)phenyl)dimethylamine (N‐DMBI), electrical conductivities four orders of magnitude higher can be achieved for P(FBDOPV‐2T‐C₁₂) (σ = 0.042 S cm^?1). More charge‐transfer complexes are formed between P(FBDOPV‐F) and N‐DMBI, but the highly localized polaronic states poorly contribute to the charge transport. Doped P(FBDOPV‐2T‐C₁₂) exhibits a negative Seebeck coefficient of –265 µV K^?1 and a thermoelectric power factor (PF) of 0.30 µW m^?1 K^?2 at 303 K which increases to 0.72 µW m^?1 K^?2 at 388 K. The in‐plane thermal conductivity (κ_|| = 0.53 W m^?1 K^?1) on the same micrometer‐thick solution‐processed film is measured, resulting in a figure of merit (ZT) of 5.0 × 10^?4 at 388 K. The results provide important design guidelines to improve the doping efficiency and thermoelectric properties of n‐type organic semiconductors.     

Competitive Absorption and Inefficient Exciton Harvesting: Lessons Learned from Bulk Heterojunction Organic Photovoltaics Utilizing the Polymer Acceptor P(NDI2OD‐T2)

Organic solar cells utilizing the small molecule donor 7,7′‐(4,4‐bis(2‐ethylhexyl)‐4H‐silolo[3,2‐b:4,5‐b′]dithiophene‐2,6‐diyl)bis(6‐fluoro‐4‐(5′‐hexyl‐[2,2′‐bithiophen]‐5‐yl)benzo[c][1,2,5] thiadiazole) (p‐DTS(FBTTh₂)₂ and the polymer acceptor poly{[N,N′‐bis(2‐octyldodecyl)‐1,4,5,8‐naphthalenedicarboximide‐2,6‐diyl]‐alt‐5,5′‐(2,2′‐bithiophene)}(P(NDI2OD‐T2)) are investigated and a power conversion efficiency of 2.1% is achieved. By systematic study of bulk heterojunction (BHJ) organic photovoltaic (OPV) quantum efficiency, film morphology, charge transport and extraction and exciton diffusion, the loss processes in this blend is revealed compared to the blend of [6,6]‐phenyl‐C71‐butyric acid methyl ester (PC₇₁BM) and the same donor. An exciton diffussion study using Förster resonant energy transfer (FRET) shows the upper limit of the P(NDI2OD‐T2) exciton diffusion length to be only 1.1 nm. The extremely low exciton diffusion length of P(NDI2OD‐T2), in combination with the overlap in donor and acceptor absorption, is then found to significantly limit device performance. These results suggest that BHJ OPV devices utilizing P(NDI2OD‐T2) as an acceptor material will likely be limited by its low exciton diffusion length compared to devices utilizing functionalized fullerene acceptors, especially when P(NDI2OD‐T2) significantly competes with the donor molecule for photon absorption.     

Tuning the Molecular Weight of the Electron Accepting Polymer in All‐Polymer Solar Cells: Impact on Morphology and Charge Generation

Molecular weight is an important factor determining the morphology and performance of all‐polymer solar cells. Through the application of direct arylation polycondention, a series of batches of a fluorinated naphthalene diimide‐based acceptor polymer are prepared with molecular weight varying from M_n = 20 to 167 kDa. Used in conjunction with a common low bandgap donor polymer, the effect of acceptor molecular weight on solar cell performance, morphology, charge generation, and transport is explored. Increasing the molecular weight of the acceptor from M_n = 20 to 87 kDa is found to increase cell efficiency from 2.3% to 5.4% due to improved charge separation and transport. Further increasing the molecular weight to M_n = 167 kDa however is found to produce a drop in performance to 3% due to liquid–liquid phase separation which produces coarse domains, poor charge generation, and collection. In addition to device studies, a systematic investigation of the microstructure and photophysics of this system is presented using a combination of transmission electron microscopy, grazing‐incidence wide‐angle X‐ray scattering, near‐edge X‐ray absorption fine‐structure spectroscopy, photoluminescence quenching, and transient absorption spectroscopy to provide a comprehensive understanding of the interplay between morphology, photophysics, and photovoltaic performance.     

Effect of Alkyl Chain Branching Point on 3D Crystallinity in High N‐Type Mobility Indolonaphthyridine Polymers

Herein, this study investigates the impact of branching‐point‐extended alkyl chains on the charge transport properties of three ultrahigh n‐type mobility conjugated polymers. Using grazing incidence wide‐angle X‐ray scattering, analysis of the crystallinity of the series shows that while π–π interactions are increased for all three polymers as expected, the impact of the side‐chain engineering on polymer backbone crystallinity is unique to each polymer and correlates to the observed changes in charge transport. With the three polymers exhibiting n‐type mobilities between 0.63 and 1.04 cm² V^?1 s^?1, these results ratify that the indolonaphthyridine building block has an unprecedented intrinsic ability to furnish high‐performance n‐type organic semiconductors.     

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.     

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.     

Shift of the Branching Point of the Side‐Chain in Naphthalenediimide (NDI)‐Based Polymer for Enhanced Electron Mobility and All‐Polymer Solar Cell Performance

The branching point of the side‐chain of naphthalenediimide (NDI)‐based conjugated polymers is systematically controlled by incorporating four different side‐chains, i.e., 2‐hexyloctyl (P(NDI1‐T)), 3‐hexylnonyl (P(NDI2‐T)), 4‐hexyldecyl (P(NDI3‐T)), and 5‐hexylundecyl (P(NDI4‐T)). When the branching point is located farther away from the conjugated backbones, steric hindrance around the backbone is relaxed and the intermolecular interactions between the polymer chains become stronger, which promotes the formation of crystalline structures in thin film state. In particular, thermally annealed films of P(NDI3‐T) and P(NDI4‐T), which have branching points far away from the backbone, possess more‐developed bimodal structure along both the face‐on and edge‐on orientations. Consequently, the field‐effect electron mobilities of P(NDIm‐T) polymers are monotonically increased from 0.03 cm² V⁻¹ s⁻¹ in P(NDI1‐T) to 0.22 cm² V⁻¹ s⁻¹ in P(NDI4‐T), accompanied by reduced activation energy and contact resistance of the thin films. In addition, when the series of P(NDIm‐T) polymers is applied in all‐polymer solar cells (all‐PSCs) as electron acceptor, remarkably high‐power conversion efficiency of 7.1% is achieved along with enhanced current density in P(NDI3‐T)‐based all‐PSCs, which is mainly attributed to red‐shifted light absorption and enhanced electron‐transporting ability.     

Effect of Spacer Length of Siloxane‐Terminated Side Chains on Charge Transport in Isoindigo‐Based Polymer Semiconductor Thin Films

A series of isoindigo‐based conjugated polymers (PII2F‐C_mSi, m = 3–11) with alkyl siloxane‐terminated side chains are prepared, in which the branching point is systematically “moved away” from the conjugated backbone by one carbon atom. To investigate the structure–property relationship, the polymer thin film is subsequently tested in top‐contact field‐effect transistors, and further characterized by both grazing incidence X‐ray diffraction and atomic force microscopy. Hole mobilities over 1 cm² V^?1 s^?1 is exhibited for all soluble PII2F‐C_mSi (m = 5–11) polymers, which is 10 times higher than the reference polymer with same polymer backbone. PII2F‐C₉Si shows the highest mobility of 4.8 cm² V^?1 s^?1, even though PII2F‐C₁₁Si exhibits the smallest π–π stacking distance at 3.379 Å. In specific, when the branching point is at, or beyond, the third carbon atoms, the contribution to charge transport arising from π–π stacking distance shortening becomes less significant. Other factors, such as thin‐film microstructure, crystallinity, domain size, become more important in affecting the resulting device's charge transport.     

Mechanism of Crystallization and Implications for Charge Transport in Poly(3‐ethylhexylthiophene) Thin Films

In this work, crystallization kinetics and aggregate growth of poly(3‐ethylhexylthiophene) (P3EHT) thin films are studied as a function of film thickness. X‐ray diffraction and optical absorption show that individual aggregates and crystallites grow anisotropically and mostly along only two packing directions: the alkyl stacking and the polymer chain backbone direction. Further, it is also determined that crystallization kinetics is limited by the reorganization of polymer chains and depends strongly on the film thickness and average molecular weight. Time‐dependent, field‐effect hole mobilities in thin films reveal a percolation threshold for both low and high molecular weight P3EHT. Structural analysis reveals that charge percolation requires bridged aggregates separated by a distance of ≈2–3 nm, which is on the order of the polymer persistence length. These results thus highlight the importance of tie molecules and inter‐aggregate distance in supporting charge percolation in semiconducting polymer thin films. The study as a whole also demonstrates that P3EHT is an ideal model system for polythiophenes and should prove to be useful for future investigations into crystallization kinetics.     

Molecular Weight‐Induced Structural Transition of Liquid‐Crystalline Polymer Semiconductor for High‐Stability Organic Transistor

In order to fabricate polymer field‐effect transistors (PFETs) with high electrical stability under bias‐stress, it is crucial to minimize the density of charge trapping sites caused by the disordered regions. Here we report PFETs with excellent electrical stability comparable to that of single‐crystalline organic semiconductors by specifically controlling the molecular weight (MW) of the donor‐acceptor type copolymer semiconductors, poly (didodecylquaterthiophene‐alt‐didodecylbithiazole). We found that MW‐induced thermally structural transition from liquid‐crystalline to semi‐crystalline phases strongly affects the device performance (charge‐carrier mobility and electrical bias‐stability) as well as the nanostructures such as the molecular ordering and the morphological feature. In particular, for the polymer with a MW of 22 kDa, the transfer curves varied little (ΔV_th = 3～4 V) during a period of prolonged bias stress (about 50 000 s) under ambient conditions. This enhancement of the electrical bias‐stability can be attributed to highly ordered liquid‐crystalline nanostructure of copolymer semiconductors on dielectric surface via the optimization of molecular weights.     

The Effect of Poly(3‐hexylthiophene) Molecular Weight on Charge Transport and the Performance of Polymer:Fullerene Solar Cells

The time‐of‐flight method has been used to study the effect of P3HT molecular weight (M_n = 13–121 kDa) on charge mobility in pristine and PCBM blend films using highly regioregular P3HT. Hole mobility was observed to remain constant at 10^?4 cm²V^?1s^?1 as molecular weight was increased from 13–18 kDa, but then decreased by one order of magnitude as molecular weight was further increased from 34–121 kDa. The decrease in charge mobility observed in blend films is accompanied by a change in surface morphology, and leads to a decrease in the performance of photovoltaic devices made from these blend films.