This is an open access article published under a Creative Commons Non-Commercial No Derivative Works (CC-BY-NC-ND) Attribution License, which permits copying and redistribution of the article, and creation of adaptations, all for non-commercial purposes. Letter http://pubs.acs.org/journal/aelccp Reduction of the Lattice Thermal Conductivity of Polymer Semiconductors by Molecular Doping Osnat Zapata-Arteaga, Aleksandr Perevedentsev, Sara Marina, Jaime Martin, Juan Sebastián Reparaz,* and Mariano Campoy-Quiles* Cite This: ACS Energy Lett. 2020, 5, 2972−2978 Read Online ACCESS Metrics & More Article Recommendations * sı Supporting Information ABSTRACT: Here we show that molecular doping of polymer thermoelectrics increases the electrical conductivity while reducing the thermal conductivity. A high-throughput method- ology based on annealing and doping gradients within individual films is employed to self-consistently analyze and correlate electrical and thermal characteristics for the equivalent of >100 samples. We focus on the benchmark material system poly(2,5- bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT) doped with molecular acceptor 2,3,5,6-tetrafluoro-7,7,8,8-tetra- cyanoquinodimethane (F4TCNQ). The thermal conductivity of neat PBTTT films is dominated by the degree of crystallinity, with thermal percolation observed for annealing temperatures >170 °C. Upon doping the samples with a relatively low amount of F4TCNQ (anion content <1 mol %), the thermal conductivity exhibits a two-fold reduction without compromising the crystalline quality, which resembles the effect of alloy scattering observed in several inorganic systems. The analysis of the relation between thermal and electrical conductivities shows that thermal transport is dominated by a doping-induced reduced lattice contribution. O rganic semiconductors are promising candidates for ubiquitous molecular electronics and energy harvest- ing due to their low cost, abundance of constituent elements, and amenability to scalable low-temperature More particularly, the previously mentioned inter-relation arises from the fact that in electrically conductive systems the total thermal conductivity, κ, is given by the sum of electronic (κe) and lattice (κlatt) contributions. For metals and highly processing. The use of doping to vary the electrical doped semiconductors, κe is proportional to the electrical conductivity by several orders of magnitude1−5 has emerged conductivity following the Wiedemann−Franz law, κe = σLT, as a key enabler for a wide range of organic-semiconductor- where L is the Lorenz number and T is the temperature.11 based devices including transistors, light-emitting diodes, and, Although in such materials the electrical and vibrational in particular, thermoelectric generators.6,7 In the concept of contributions to ZT are inevitably coupled, it cannot be thermoelectricity, heat flux is converted to electrical power concluded that this relation also holds for doped organic with an efficiency that scales with the figure of merit (ZT), semiconductors. In fact, several studies for PEDOT:PSS, which is related to both electrical (σ) and thermal (κ) PEDOT:Tos, and doped polyacetylene suggest that the conductivities as ZT ∝ σκ−1. Whereas the majority of recent correlation between σ and κ is not yet fully understood.12−15 reports have explored doping-induced electrical conductivity as Furthermore, a recent theoretical work based on Monte Carlo the principal parameter for optimizing the characteristics of thermoelectric materials, comparatively few studies have investigated the simultaneous impact on thermal conductiv- Received: July 1, 2020 ity.8,9 Even fewer studies have attempted to unravel the Accepted: August 19, 2020 “Gordian knot” of inter-related electrical and thermal Published: August 19, 2020 conductivities as well as the inevitable dependence of both on the semicrystalline microstructure of the organic semi- conductor.10 © 2020 American Chemical Society https://dx.doi.org/10.1021/acsenergylett.0c01410 2972 ACS Energy Lett. 2020, 5, 2972−2978 ACS Energy Letters http://pubs.acs.org/journal/aelccp Letter Figure 1. (a) Absorption spectra of a PBTTT film featuring a gradient of the degree of crystallinity induced by annealing on a temperature gradient. Data are normalized by the A0−1 transition. (b) Thermal conductivity as a function of the free exciton bandwidth, W, for the annealed PBTTT film; the inset shows the data for W = 40−50 meV. (c) Schematic illustration of the microstructure of a neat PBTTT film for different annealing temperatures, which is rationalized in terms of a series or parallel thermal resistor model. (d) Absorption spectra for a PBTTT film with a gradient of doping. (e) Thermal conductivity (left ordinate) as a function of the estimated doping level (green line is a fit to the data) and the fitted free exciton bandwidth as a function of the doping level (orange line, right ordinate). (f) Schematic illustration of the microstructure of a doped PBTTT film for different dedoping temperatures along a proposed a resistor model. simulations questioned the validity of the Wiedemann−Franz fabrication details.) Thereafter, two types of samples were law in conducting polymers.16 One of the reasons leading to fabricated by postprocessing: (i) neat (that is, undoped) this ambiguity is, perhaps, that there are few reports that PBTTT films, which were annealed at spatially varying simultaneously address electrical and thermal conduction.13,17 temperatures using a Kofler bench, and (ii) PBTTT films Here we present a comprehensive study of electrical and (160 nm) with a spatially varying MRD of up to 0.23. The thermal characteristics of poly(2,5-bis(3-alkylthiophen-2-yl)- latter were fabricated by, first, homogeneous vapor-phase thieno[3,2-b]-thiophene) (PBTTT) films doped with 2,3,5,6- doping of “baseline” samples, followed by thermal annealing tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ). For using the Kofler bench (temperatures spanning 130−200 °C), this purpose, we employed a novel high-throughput method- which induced spatially varying degrees of dedoping according ology based on doping gradients and analysis using high- to the local temperature set by the Kofler bench.21 Whereas spatial-resolution noncontact optical methods, which yielded both samples (i) and (ii) were used to study the role of the thermal and electrical conductivity data for the equivalent of dopant in determining the overall thermal conductivity, sample >100 samples with different doping levels while avoiding (ii) was used to address the electrical characteristics. Because uncertainties from sample-to-sample variation. We show that thermal and electrical conductivities were both studied using the key finding is that the presence of a low doping level contactless optical methods, the doping level variation is (hereafter expressed as the molar ratio of F4TCNQ anion per considered to be negligible within the laser spot (typical radius PBTTT monomer, MRD) within PBTTT films leads to a of probed region <5 μm; spatial variation of MRD < 0.004 reduction in thermal conductivity, resembling a similar effect mm−1). An additional set of samples with a homogeneous that arises from the alloy scattering observed in other materials. doping level was specifically fabricated for the grazing- By studying the relation between the thermal and the electrical incidence wide-angle X-ray scattering (GIWAXS) analysis to conductivity, we also demonstrate that the effective lattice correlate the microstructures of the samples with their thermal conductivity decreases by doping within the electrical and thermal properties. Wiedemann−Franz framework. Figure 1a,d presents absorption spectra of films as a function To study the thermal and electrical conductivity of the of the annealing temperature: 70−200 °C for the neat sample PBTTT films as a function of the F4TCNQ anion amount, we (i) and 130−200 °C for doped sample (ii), respectively. The have developed a novel fabrication process based on the spectra for the neat sample (i) exhibit two bands around 530 concept of annealing18−20 and doping gradients.21 The nm (A0−1) and 630 nm (A0−0) belonging to the disordered “baseline” samples comprised PBTTT films (thickness ≈ 160 polymer and crystalline fractions, respectively.22 The intensity nm) deposited onto 75 mm long conventional glass substrates ratio between these bands is related to the free exciton by blade-coating. (See the Supporting Information for bandwidth (W) and is considered to be an indicator of the 2973 https://dx.doi.org/10.1021/acsenergylett.0c01410 ACS Energy Lett. 2020, 5, 2972−2978 ACS Energy Letters http://pubs.acs.org/journal/aelccp Letter Figure 2. GIWAXS patterns for (a) out-of-plane (h00) and (b) in-plane (0k0) integrations. Data are shown for a neat as-deposited PBTTT film and the same film following annealing at T = 100 and 180 °C (as indicated). Also shown are the data for a film annealed at T = 180 °C and subsequently doped and the same film following dedoping. Data are offset for clarity; vertical markers are guides to the eye for peak shifts relative to the data for the neat PBTTT annealed at T = 180 °C. degree of crystallinity, whereby a smaller W implies a higher propose an equivalent thermal resistance circuit that behaves as conjugation length and interchain order.22−24 (See Figure follows: For low annealing temperatures, the thermal transport S3a.) As the annealing temperature increases, W gradually is dominated by the thermal resistance between crystals, which decreases (100 → 50 meV), indicating that the annealing can be modeled by a series resistance model. However, as the process increases the degree of crystallinity within the polymer annealing temperature increases, the size of the crystals grows film, in agreement with the observations of refs 25 and 26. In until thermal percolation is established, which is then the case of the doped sample (ii), we identify integer charge represented by a parallel resistor geometry. We note, however, transfer (ICT) between PBTTT and F4TCNQ by the that this is our current working hypothesis, as we do not have appearance of three subgap transitions that belong to the access to the exact crystalline/amorphous ratio that will be radical anion of the F4TCNQ*− (A1, A2, and A3; see Figure necessary to quantitatively describe the observed behavior 1d).18,27,28 By fitting these features as well as those of the through available models, such as effective medium, neutral polymer, the doping level across the gradient of the percolation theory, or the rule of mixtures, among others.8,31,32 film can be estimated23,28,29 (Figure S3b). The effect of doping on the overall thermal conductivity is The thermal conductivity of both samples is shown in Figure shown in Figure 1e. In this case, the thermal conductivity is 1b,e. To address the influence of the degree of crystallinity of plotted as a function of the estimated doping level present in the PBTTT films upon their thermal conductivity, we plot the the PBTTT thin films, which is obtained via the previously thermal conductivity as a function of W for the neat sample (i) described analysis (Figure 1d). Interestingly, the thermal in Figure 1b, as obtained from the analysis of the conductivity remains essentially constant at ∼0.35 W m−1 K−1 corresponding absorption spectra. It is interesting to note for an MRD greater than ∼0.01, with the few points falling out that the thermal conductivity of the neat sample exhibits an of the trend attributed to dopant aggregates on the film abrupt increase from ∼0.3 to ∼0.7 W m−1 K−1, which surface.21 (See Figure S6 for more details.) Below this value, corresponds to a rather small change in the exciton bandwidth the thermal conductivity of the films approaches the value (∼50 meV) for annealing temperatures above ∼170 °C. We obtained for the neat sample (i) at high annealing temper- ascribe this to the occurrence of thermal percolation; that is, as atures. The latter is expected given that the doped sample (ii) the annealing temperature increases, PBTTT crystallites grow was precrystallized by annealing at 180 °C prior to doping, and larger25,30 within the films, which yields a substantial increase thus its crystalline structure before the doping process matches in thermal conductivity when the crystals are thermally that obtained for the neat sample (i) at annealing temperatures interconnected. These observations are also consistent with >170 °C. Surprisingly, a relatively low MRD (<0.01) already the appearance of a “terraced” phase in PBTTT, as reported induces a two-fold decrease in the thermal conductivity. This elsewhere,26,30 and with our atomic force microscopy measure- resembles the effect of alloy scattering, as observed in inorganic ments. (See Figure 2 and Figure S4 for more details.) Figure 1c materials.33−37 Furthermore, as shown in Figure 1e, the exciton shows a schematic illustration of the influence of the annealing bandwidth remains constant (∼40 meV) across the entire process on the crystalline microstructure of PBTTT films. We range of the doping level, which suggests that the crystalline 2974 https://dx.doi.org/10.1021/acsenergylett.0c01410 ACS Energy Lett. 2020, 5, 2972−2978 ACS Energy Letters http://pubs.acs.org/journal/aelccp Letter Figure 3. (a) Estimated electrical conductivity as a function of the doping level. (b) Thermal conductivity as a function of electrical conductivity. The green line is a fit of the experimental data using κexp = κlatt + σL0T. The black curve is a model of κexp = κlatt + σL0T using κlatt from the fit (0.36 W m−1 K−1) and L0 = 2.44 × 10−9 V2 K−2. Also shown for comparison are literature data for polyaniline doped with hydrochloric acid (solid stars)17 and with a set of distinct acids (open stars).44 Also shown for comparison is PEDOT doped with tosylate or PSS supported on a substrate (open triangles) and suspended on a microdevice (solid triangles).13 quality of the PBTTT thin film does not deteriorate following all peaks shift toward shorter scattering vectors, indicating an the doping process. In fact, the simultaneous observation of a expansion of the lamella by ∼0.24 nm and simultaneously small exciton bandwidth and a small thermal conductivity can introducing some disorder relative to the pristine film, as seen be rationalized by considering that the dopant is probably in Figure S7. In the in-plane direction, the (010) scattering located within PBTTT crystals. Figure 1f displays a schematic peak exhibits the opposite behavior; that is, it contracts by illustration of the microstructure of doped samples. In this ∼0.01 nm and induces a relative broadening of the (003) case, the microstructure of the samples before the doping scattering peak compared with that annealed at 180 °C. This process is dictated by the initial annealing process. Further behavior suggests that the dopant induces a certain uniaxial doping increases the thermal resistance between crystalline strain on the PBTTT microcrystals, which is compensated by domains, thus leading to a smaller average thermal the distortion of the lattice, as given by its Poisson ratio. conductivity of the films. The present observations suggest Strikingly, after the dedoping process, all diffraction peaks from that the primary thermal transport mechanism in this system is both crystalline planes shift to, essentially, their original given by the lattice and not by the free electrons arising from position. This is consistent with the increase in the thermal doping. conductivity observed for MRD < 0.01, demonstrating that in To get further insights into the mechanisms controlling the the doped samples, thermal transport is dominated by the thermal conductivity, we have performed a detailed structural physical presence of the dopant. analysis of the samples by means of GIWAXS. Figure 2a,b To evaluate this point further, we investigated the thermal shows the evolution of the (h00) and (0k0) crystalline planes properties of a different polymer, PTB7-Th, mixed with associated with the lamellar packing and π−π stacking in F4TCNQ. Reassuringly, we observed a similar evolution of PBTTT, respectively.18,25,27,38−40 For the case of the neat thermal conductivity with the doping level (cf. Figure S8 and samples, two main observations should be highlighted. As the Figure 1e) as for the PBTTT case. These experiments suggest annealing temperature increases, the width of the out-of-plane that the alloying effect constitutes a general behavior for diffraction peaks (Figure 2a) progressively narrows, thus polymers and molecular dopants (not just a particular effect indicating an improved ordering of the lamellar packing.25,41 associated with dopants inhibiting a particular phase in This observation is consistent with the corresponding PBTTT). Moreover, this experiment also indicates that the evolution of the exciton bandwidth and thermal conductivity effect is associated with the physical presence of the dopant; (Figure 1a,b respectively); that is, as the annealing temperature the reduction of thermal conductivity cannot be ascribed to increases, the exciton bandwidth decreases, leading to the electronic interactions such as the formation of the charge- observed two-fold increase in the thermal conductivity. We transfer complex because PTB7-Th is not effectively doped by note that typically, an increased degree of crystallinity or F4TCNQ due to the unfavorable energy level alignment. crystal coherence length is responsible for higher thermal Finally, we study the relationship between thermal and conductivities. On the contrary, the peak attributed to π−π electrical conductivity to test the applicability of the stacked planes along the in-plane integration, that is, the (010) Wiedemann−Franz law. We have already shown in Figure 1e reflection shown in Figure 2b, shifts toward lower scattering that the thermal conductivity of PBTTT films is independent, vectors, Q. This suggests that increasing the annealing to a large extent, of the MRD for values >0.01. The local temperatures gradually leads to an expansion of the lattice in electrical conductivity is estimated using an indirect analysis the (010) direction by ∼0.01 nm. No appreciable effect on the based on spectroscopic Raman mapping, previously shown to thermal conductivity is expected for this rather small lattice be effective for spatially mapping conductivity in PBTTT distortion. doped with the Lewis acid tris(pentafluorophenyl)borane.42,43 After the doping process was conducted on the sample The method relies on there being continuous variation of the annealed at 180 °C, more significant changes in both intensity ratio rR of the ∼1393 and ∼1493 cm−1 Raman peaks crystalline planes were observed. In the out-of-plane direction, of PBTTT with electrical conductivity (that is, the MRD). 2975 https://dx.doi.org/10.1021/acsenergylett.0c01410 ACS Energy Lett. 2020, 5, 2972−2978 ACS Energy Letters http://pubs.acs.org/journal/aelccp Letter Hence, by obtaining the “calibration” data of rR as a function of Juan Sebastián Reparaz − Institute of Materials Science of electrical conductivity for reference samples (see the Methods Barcelona (ICMAB-CSIC), 08193 Bellaterra, Spain; in the Supporting Information), the local electrical con- Email: jsreparaz@icmab.es ductivity was estimated across the entire sample along the doping gradient. Authors Electrical conductivity as a function of the F4TCNQ molar Osnat Zapata-Arteaga − Institute of Materials Science of fraction is shown in Figure 3a. We note that a rather small Barcelona (ICMAB-CSIC), 08193 Bellaterra, Spain; (MRD < 0.03) doping level already increases the electrical orcid.org/0000-0002-0844-2773 conductivity by several orders of magnitude, finally saturating Aleksandr Perevedentsev − Institute of Materials Science of at σ ≈ 103 S m−1 and reaching values as high as 6800 S m−1. Barcelona (ICMAB-CSIC), 08193 Bellaterra, Spain These values are in reasonable agreement with those reported Sara Marina − POLYMAT and Polymer Science and Technology for similar materials and processing.18,40 Figure 3b shows the Department, Faculty of Chemistry, University of the Basque thermal conductivity as a function of the electrical conductivity Country UPV/EHU, 20018 Donostia-San Sebastián, Spain for PBTTT films. We have also included the observations of Jaime Martin − POLYMAT and Polymer Science and refs 17, 44, and 13 in two conducting polymers for comparison, Technology Department, Faculty of Chemistry, University of the that is, PEDOT and PANI. Our data were modeled using the Basque Country UPV/EHU, 20018 Donostia-San Sebastián, expression κexp = κlatt + σL0T, where the second term accounts Spain; Ikerbasque, Basque Foundation for Science, E-48011 for the electronic contribution according to the Wiedemann− Bilbao, Spain; orcid.org/0000-0002-9669-7273 Franz law (κe = σLT), and using the Sommerfeld value (L0) as Complete contact information is available at: an approximation for the Lorenz number. The fit suggests a https://pubs.acs.org/10.1021/acsenergylett.0c01410 lattice contribution of 0.36 W m−1 K−1, which is much smaller than that of the undoped polymer. This, again, provides Notes evidence that the presence of the dopants reduces the thermal The authors declare no competing financial interest. ■ conductivity. Indeed, the deviations observed for the lower electrical conductivity values arise from the higher thermal ACKNOWLEDGMENTS conductivity exhibited by the neat PBTTT samples, as shown by the previously described analysis (Figure 1). Judging by the We acknowledge financial support from the Spanish Ministry compiled data for different systems, the electronic contribution of Science and Innovation through projects PGC2018-095411- to the overall thermal conductivity becomes apparent only at B-I00 and MAT2017-90024-P (TANGENTS)-EI/FEDER, UE very high electrical conductivities. projects; the Generalitat de Catalunya through grants In conclusion, we have shown that the thermal conductivity 2017SGR488 and AGAUR 2018 PROD 00191; and from of PBTTT thin films can be tailored between 0.3 and 0.7 W the European Research Council (ERC) under grant agreement m−1 K−1 by increasing the annealing temperature, which no. 648901. O.Z.-A. acknowledges CONACYT-SENER for his originates from the formation of percolating crystals within the Ph.D. scholarship (no. 472571). J.M. thanks MCIU for the films. Furthermore, the addition of small doping levels (MRD < Ramón y Cajal contract and grant PGC2018-094620-A-I00. 0.01) is sufficient to reduce the thermal conductivity to a We thank Dr. Agustiń Mihi for the access to and support with minimum of 0.38 W m−1 K−1. This effect resembles the the FTIR equipment. We acknowledge the technical and observation in alloy systems, where thermal transport is human support provided by SGIker of UPV/EHU and dominated by disorder. Studying the relation between the European funding (ERDF and ESF). We thank Andrés ́ Gómez Rodriguez from the Scanning Probe Microscopy thermal and electrical conductivity, we also show that the electronic contribution to the overall thermal conductivity is Laboratory (ICMAB-CSIC) for a set of AFM measurements. negligible up to electrical conductivities greater than σ ≈ 6.8 × We acknowledge support of the publication fee by the CSIC 103 S m−1. 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