Molecular doping principles in organic electronics: fundamentals and recent progress

In the case of conventional inorganic semiconductors, doping is conducted by injecting dopant atoms that have different numbers of valence electrons compared with host atoms (B for p-type doping or P for n-type doping Si). When the incorporated dopant atoms are donor impurities (i.e. n-type doping), the dopant atoms generate donor states near the conduction band of the intrinsic semiconductor, and then extra free electrons can be generated in the conduction band by the excitation of electrons from the donor states. Conversely, free holes can be generated from the incorporation of acceptor impurities via the excitation of electrons from the valence band to the acceptor states formed near the valence band, creating holes in the valence band (i.e. p-type doping).

On the other hand, the doping of OSCs is carried out by molecular dopants rather than atomic dopants like in inorganic semiconductors. When host OSC and molecular dopant are put in proximity, charge transfer occurs driven by the energy level difference (${\rm{\Delta }}{E}_{n}={E}_{\mathrm{HOMO}}^{\mathrm{Dopant}}-{E}_{\mathrm{LUMO}}^{\mathrm{OSC}},$ or ${\rm{\Delta }}{E}_{p}={E}_{\mathrm{HOMO}}^{\mathrm{OSC}}-{E}_{\mathrm{LUMO}}^{\mathrm{Dopant}}$) between the energy levels of the host OSC and those of the dopant molecule as shown in Fig. 2(a). The energy level difference between the dopant and OSC is a good indicator for predicting the charge transfer mechanism, i.e. whether the doping process of the OSC-dopant system occurs via ion pair formation [IP, Fig. 2(b)] or the formation of charge-transfer complexes [CTCs, Fig. 2(c)]. ⁷⁾ However, such indicator based on the energy differences requires careful attention since ${E}_{\mathrm{HOMO}}^{\mathrm{Dopant}},$ ${E}_{\mathrm{LUMO}}^{\mathrm{OSC}},$ ${E}_{\mathrm{HOMO}}^{<mml:mpadded>\mathrm{OSC}</mml:mpadded>},$ and ${E}_{\mathrm{LUMO}}^{\mathrm{Dopant}}$ are obtained from the isolated single molecules, not reflecting accurate conditions that charge transfer occurs (i.e. solid-state). Therefore, more sophisticated modelling of charge transfer mechanism should be established.

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In principle, the LUMO level of p-type molecular dopants should be deeper than the HOMO level of OSC to generate free holes (i.e. ${\rm{\Delta }}{E}_{p}\geqslant 0$), while the n-type molecular dopant should have a HOMO level shallower than the LUMO of the OSC to generate electrons in the OSC (i.e. ${\rm{\Delta }}{E}_{n}\geqslant 0$). In the IP mechanism, a whole integer of charge is transferred between a molecular dopant and the host OSC, which ionizes the dopant during doping and generates a $\pm 1$ charge in OSC, thus forming an ion pair [Fig. 2(b)]. ¹¹⁾ Energetics related to Coulomb interaction between the molecular dopant and the OSC can be considered in describing the charge transfer process by the ion pair energy, E, between the dopant and the OSC in the following form ⁸⁾

$$\begin{eqnarray}&&E\left(\delta \right)=\left({\rm{IE}}-{\rm{EA}}\right)\delta -k\displaystyle \frac{{e}^{2}}{r}{\delta }^{2},\end{eqnarray} \tag{ 1 }$$

where δ is the degree of charge transfer, k is the Coulomb constant, e is the electric charge, r is the distance between the electron and the hole, IE is the ionization energy, and EA is the electron affinity. Herein, Eq. (1) reveals that δ determines the ion pair energy, and the amount of charge transferred through this OSC−dopant interaction is determined at the value of δ that minimizes E. The degree of charge transfer can be quantified through Fourier-transform infrared spectroscopy (FTIR) [Fig. 2(d)]. ¹²⁾ Méndez et al. reported that the degree of charge transfer linearly scale with the observed frequency shift (${\rm{\Delta }}v$) in FTIR as the following ¹³⁾

$$\begin{eqnarray}&&\delta =\displaystyle \frac{2{\rm{\Delta }}v}{{v}_{0}}{\left[1-\displaystyle \frac{{v}_{1}^{2}}{{v}_{0}^{2}}\right]}^{-1},\end{eqnarray} \tag{ 2 }$$

where ${v}_{0}$ and ${v}_{1}\,$denote the vibrational frequencies of the neutral dopant and that of its radical anion, respectively.

The above two models have been shown to accurately predict doping efficiency via IP formation, and therefore the resulting carrier mobility and electrical conductivity of the doped OSC. ⁸⁾ Furthermore, considering the ion pair energy above, the optimal dopant can be selected for the target host OSC. The accuracy of this prediction can be further improved by measuring the exact EA and IE values of the dopant and host OSC at the corresponding doping levels and doping conditions (i.e. in solid-state) since the EA and IE values used for the calculation are often the values theoretically calculated for those of isolated molecules in vacuum state.

Unlike the IP mechanism, if the number of electrons transferred to OSC per dopant molecule is less than $\pm 1,$ the doping between the molecular dopant and the OSC occurs through the formation of CTC rather than IP. In the CTC mechanism, the frontier orbitals of the host OSC and molecular dopant are hybridized to form new sets of hybrid orbitals [Fig. 2(c)]. ¹⁴⁾ The degree of hybridization and the resulting energy splitting of the molecular orbitals depend on the energetics and spatial extent of each orbital. When CTC is formed, free charges are not immediately generated like in the case of IP. Charge transfer proceeds through the hybridized orbital, therefore the amount of charge transfer is limited owing to the higher energy level of the empty hybrid state (several tenths of eV) compared with HOMO of OSC (for p-type doping). This partial charge transfer leads to low doping efficiency and generally low conductivity compared to the OSCs doped by IP formation. ^12,15)

However, it is yet unclear exactly which physical properties of OSCs and dopants lead to doping by IP or CTC, other than the simple energetic considerations outlined above. ^16,17) For instance, in the case of integer charge transfer complex (ICTC) [Fig. 2(e)], it is in the form of ion pairs, however as in the case of CTCs, additional energy is required to overcome the binding energy required to generate a free charge. ^16,17) Tietze et al. proposed that integer-charge-transfer doping is a two-step process (rather than a single step), which involves firstly a single-electron transfer from donor to acceptor molecule forming a ground-state ICTC and secondly the dissociation of the ground-state ICTC via thermal activation to free (i.e. separated) charges [see Fig. 2(e)]. ¹⁷⁾ The doping efficiency is directly linked to the dissociation of ICTCs, which demonstrates that selection criteria for optimal doping are not only related to synthesizing host-dopant molecules with the energetic requirements of ${\rm{\Delta }}{E}_{p,n}\geqslant 0,$ but also optimizing considerable factors such as interface energetics regarding electrostatics and degree of energetic disorder related to dissociation of IPs. ¹⁷⁾

The molecular dopant is a neutral small molecule that can transfer a charge through a thermodynamically reversible reaction without breaking the covalent bond of OSCs. In the case of a p-type molecular dopant, an electron in the HOMO of the OSC is transferred to the LUMO of the dopant, and in the case of an n-type molecular dopant, an electron in the HOMO of the dopant is transferred to the LUMO of the OSC [Fig. 2(a)].

Initial studies on p-type molecular dopants were mostly related to the investigation of quinone molecules ^18,19) (i.e. conjugated molecules derived from aromatic molecules with a cyclic dione structure), owing to the unsaturated aromatic ring structure. This structure increases the EA of p-type molecular dopants, facilitating the charge transfer process and improving the doping efficiency. Tetracyanoquinodimethane (TCNQ) is a representative example of p-type molecular dopants, however, the doping efficiency of TCNQ can be low due to a relatively low EA of TCNQ (LUMO ∼ −4.3 eV) compared to general HOMO levels of p-type OSCs. Méndez et al. studied the influence of fluorination on the aromatic core and showed that fluorination makes TCNQ electron-deficient, increasing the EA of TCNQ and doping efficiency. ^20,21) Consequently, 2,3,5,6-Tetrafluoro-tetracyanoquinodimethane (F₄TCNQ) prepared by replacing 2,3,5,6-hydrogens of the aromatic core with fluorine showed the highest increase in the EA by 0.9 eV, leading to a high doping efficiency. This molecular dopant is one of the most widely used p-type molecular dopants to date. ²²⁾ In addition, 3,5-difluoro-2,5,7,7,8,8-hexacyanoquinodimethane (F₂HCNQ) was designed, which is a TCNQ molecule with 2,3,5,6-hydrogens replaced with two nitrogen and two fluorine atoms, with a further increase in the EA by 0.25 eV compared with F₄TCNQ. ²³⁾ Another structural modification of TCNQ is introducing ester groups to control the solubility of the dopant. Among these F₄TCNQ analogs, dopants with the diester group not only showed the adjusted solubility of the dopant but also stabilized the doping of poly(3-hexylthiophene) (P3HT) by dimerization reaction between diester groups which suppressed the diffusion of the dopants within the P3HT film. ²⁴⁾ Furthermore, new molecular structures were introduced for high electron affinities, such as 1,3,4,5,7,8-hexafluorotetracyanonaphthoquinodimethane (F₆TCNNQ), 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ), Dipyrazino[2,3-f:2',3'-h]quinoxaline-2,3,6,7,10,11-hexacarbonitrile (HAT-CN6), and hexacyano-trimethylene-cyclopropane (CN6-CP), with a planar molecular structure and C₆₀F₃₆, C₆₀F₄₈, molybdenum tris(1,2-bis(trifluoromethyl)ethane-1,2-dithiolene) (Mo(tfd)₃) with a bulky, 3D structure.

According to the aforementioned doping mechanism, the possible doping mechanism of each molecular dopant can be predicted by calculating the ΔE_p between OSC and dopant [Fig. 3]. For a representative p-type OSC, poly[2,5-bis(3-tetradecylthiophene-2-yl)thieno[3,2-b]thiophene] (PBTTT) with HOMO level of ∼ −5 eV, the dopants such as HAT-CN6 and DDQ are more likely to form CTCs, while dopants such as F₃-R-TCNQ, F₄-R-TCNQ, F₄TCNQ, F₆TCNQ, C₆₀F₃₆, F₂HCNQ, Mo(tfd)₃, and CN6-CP are more likely to form IP, although detailed energetic considerations should also involve the Coulomb interaction and the dissociation of the resulting IPs.

n-type molecular dopants usually have a shallower HOMO level (low IP) than the LUMO level of OSCs for efficient electron transfer from the HOMO of dopants to the LUMO of OSCs. Ironically, one of the biggest problems to be solved with n-type doping is that the chemical stability of dopants is low due to the shallow HOMO level, which means n-type dopants are readily oxidized. ²⁵⁾ Therefore, developing an n-type dopant is challenging due to the restrictions set by the LUMO of the OSCs, as well as the oxidation potential. Demand for n-type dopants for efficient and stable doping is increasing since the device applications such as OLEDs, ^26–29) complementary circuits, ^30–32) and thermoelectric generation all simultaneously require both p- and n-doping. ^33–35) Even though various materials and methods were introduced for n-type doping of OSCs such as (i) using alkali metals, ^36–38) (ii) molecular compounds with significantly shallow HOMO, ^39–43) and (iii) air-stable precursor molecules that can donate an electron to the matrix material in the deposited film, ^44–46) we focus here on the latter two due to the doping stability of alkali metals, ⁴⁷⁾

One of the earliest works on n-type molecular dopants with low IE is bis(cyclopentadienyl)-cobalt(ΙΙ) (cobaltocene, CoCp₂), which is a strong reducing agent with an ionization energy of about 4 eV, and an n-type dopant capable of shifting the Fermi level of OSC by 0.5 eV to the conduction band. ⁴³⁾ In addition, interfacial doping of CoCp₂ reduced the charge injection barrier at the Au/PEDOT:PSS interface, demonstrating decent current enhancement that is comparable to that of p-doping of N,N'-diphenyl-N,N'-bis(1-naphthyl)−1,1'-bi-phenyl-4,4'diamine (α-NPD) with F₄TCNQ. Hydride transfer doping is another way for efficient and stable n-doping of OSC because the Fermi level of OSC can be controlled by hydride doping and the main deciding factor for the selection of OSC for a given dopant is hydride affinity, instead of the electron affinity. (4-(1,3-dimethyl-2,3-dihydro-1H-benzimidazole-2-yl)phenyl)dimethylamine (N-DMBI) showed good controllability in the Fermi level of various n-type OSCs through hydride transfer doping. ³⁹⁾ In the case of FETs incorporating C₆₀ or C₇₀, bias-stress measurement results showed that the bias-stress stability is improved by N-DMBI doping compared to the other n-type OSCs owing to the filling of shallow electron traps in the pristine films. ³⁹⁾ Benzyl viologen (BV) and diquat (DQ) are commonly used n-type dopants and have recently been used for doping of bulk heterojunctions (BHJs) as a way to improve the performance of OPVs by improving the film morphology and suppressing the trap density. ^40,41)

According to the aforementioned doping mechanism as in p-type dopant, a similar approach can be taken to predict the possible doping mechanism of each molecular dopant with ${\rm{\Delta }}{E}_{n}.$ For a representative n-type OSC, naphthalene diimide (NDI), which has LUMO of ∼−3.7 eV, dopants such as DQ, CoCP₂, N-DMBI, zinc phthalocyanine (ZnPc) and tetrabutylammonium fluoride (TBAF) are more likely to form CTCs, while tetrakis(hexahydropyrimidinopyrimidine)ditungsten(II) (W₂(hpp)₄), BV and mesitylene pentamethylcyclopentadienyl ruthenium dimer ([RuCp*(mes)]₂) are more likely to form IPs [Fig. 4].

In this section, various implementation examples of doping will be introduced by addressing methodological differences and features based on the aforementioned materials and doping mechanisms [Fig. 5]. Doping methods can be divided into two major types: (i) co-deposition and (ii) sequential deposition. The sequential deposition methods can be further classified into four small categories: solid-state diffusion, ion-exchange, cascade, and modulation. Here, we outline the basic principles and main differences between the above doping methods in order to discuss their implications in device applications in detail in the next section.

Co-deposition method involves doping by dissolving dopants in the same solvent as that of the host semiconductor. Doped films can be deposited by spin coating or blade coating the blended solution with a desired ratio of OSC to dopant. The advantage of this method lies in the controllability of the doping ratio by simply mixing the desired ratio of OSC to dopant in the blended solution. ⁴²⁾ However, the solubility of dopant molecules can be limited in good solvents for the host OSCs, and the solubility of the OSC decreases as doping proceeds due to the ionization of the OSC. Therefore, the limited solubility of both the OSC and the dopant causes aggregation in the solution, leading to undesirable defects in the morphology of the cast, ⁴⁸⁾ which limits the electrical conductivity of the doped OSC.

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Unlike the co-deposition method, the sequential deposition method involves the deposition of dopants onto the pre-deposited OSC film. The deposited dopant molecules infiltrate into the OSC film and dope the OSC. Therefore, it is hard to estimate the doping efficiency because the infiltration of the dopant molecule may not be uniform over the whole OSC film. Therefore, additional analysis techniques such as UV–vis-NIR spectroscopy, photoluminescence quenching, and ultraviolet photoelectron spectroscopy (UPS) should be complemented to quantify how many dopant molecules have penetrated into the OSC film for calculating the doping efficiency. ⁴⁹⁾

The solid-state diffusion method is a sequential doping method that deposits the dopant molecules on OSC film by the evaporation of dopant molecules. The dopant molecules diffuse into the OSC film and dope OSC molecules in solid-state without employing any solvents during the doping process. Kang et al. studied the solid-state diffusion of F₄TCNQ in highly crystallized PBTTT polymer thin film. ⁹⁾ This study shows that the doping of PBTTT was effectively conducted by solid-state diffusion of F₄TCNQ while maintaining the local crystalline π–π stacked structure of PBTTT that facilitated coherent charge transport properties similar to that of free electrons. The resulting doping concentration was maximised by bulk-incorporating dopant molecules in PBTTT film and the electrical conductivity that can be achieved with this doping method was more than 200 times (∼250 S cm⁻¹) larger than that of the solution co-deposition method. ⁹⁾

Maximising doping ratio can be achieved not only by processing but selecting the right OSC-dopant combination. Ideally, doping ratio can be more effective if one could transfer more than one charge per dopant molecule since this could lead to lowering of the dissociation energy and enhancement of the doping efficiency. ⁵⁰⁾ Ionization efficiency could be enhanced by using a dimer dopant which transfers two charges per dopant molecule. However, ionized dimer dopants are split into two separate entities, instead, that act as counterions inducing structural disorder. ^51–53) Kiefer et al. demonstrated double-doping using a common p-type dopant, F₄TCNQ that could accept two electrons per dopant molecule from the host semiconductor even without dimerization. Double-doping occurs through a two-step process; (i) neutral dopant molecule which has the EA deeper than IE of the host semiconductor generates a hole in the host OSC and the neutral dopant is ionized. (ii) Even ionized dopant anion still has the EA deeper than the IE of the oxidized OSC, and therefore an additional hole can be transferred from the dopant anion to the host OSC. ⁵⁰⁾ By investigating different semiconductor−dopant combinations, the effect of dopant concentration, and doping efficiency, this study suggests that the double-doping effect is a generic principle that could lead to ionization efficiency of up to 200%.

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The ion-exchange doping method is a two-step method consisting of a molecular doping process and an ion-exchange process. The first step is a general molecular doping process, which involves incorporation of dopants in the host semiconductor film for inducing charge transfer between the dopant and OSC. After that, the doped OSC film is brought in contact with an ionic liquid in which an ion-exchange occurs between the dopant ion in the OSC film and an ion in the ionic liquid. The above method was implemented in the form of p-type doping of PBTTT, and it was reported that the doping occurs through the replacement of F₄TCNQ radical anion with a Y⁻anion of an ionic liquid (existing in X⁺Y⁻ type) [Fig. 6]. ⁵⁴⁾ This eventually leads to the IP formation of a PBTTT radical cation and a Y⁻anion. The ion-exchange method has been found to be particularly effective for a certain combination of ionic liquid and OSCs, and the ion conversion rate was discovered to reach up to 100%. ¹⁰⁾

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The cascade method is a type of ion-exchange doping method improving the doping efficiency by mixing multiple dopant molecules. ⁵⁵⁾ In a recent study on cascade doping, F₄TCNQ and NOBF₄ were used as dopant molecules, in which neutral F₄TCNQ molecules were found to be regenerated to engage in doping after anion-exchange occurred with NOBF₄ ⁻ in PIDF-BF polymer, which led to improved doping efficiency with a high conductivity value exceeding 610 S cm⁻¹.

The modulation doping method is widely used in inorganic semiconductors where a heavily doped wide bandgap semiconductor is brought in contact with a narrow bandgap semiconductor. ^56–58) Generation of free charge carriers from the spatially separated dopants from the channel can minimize ionized impurity scattering, and therefore maximising the carrier mobility in the channel. ^56–58) Wang et al. demonstrated an efficient modulation doping effect with a high-mobility crystalline OSC film, rubrene. ⁵⁹⁾ This study shows that high doping efficiencies (over 20%) can be reached through the modulation doping of vacuum-deposited rubrene thin-film crystals even for high dopant loadings.

This section is relevant to relating molecular doping of OSCs for their applications in electronic devices and energy applications. In particular, doping effects in field effect transistors, thermoelectrics, and photovoltaics are addressed because the doping of active channel in these devices is critical to determining the device performance. In OFETs, doping can be applied to either the active channel region or the contact region to modulate the device characteristics and improve the carrier injection. For thermoelectrics and photovoltaics, tuning the charge carrier concentration is the key to maximising the power conversion efficiency (PCE) by improving the charge transport and charge extraction. All of the above benefits of doping can be better understood by discussing the effects on the charge transport of OSCs. In this section, the effect of doping in charge transport of OSCs is discussed first, then representative works of doping applications in OFETs and organic thermoelectrics in the hope of demonstrating the potential of doping strategies in organic electronics.

2.15.1. Doping effects in charge transport

2.15.2. OFET

2.15.3. Charge injection

The charge injection problem at the interface of OSC and metal electrode has been one of the critical obstacles that lower the performance of OFETs. ⁶¹⁾ Contact doping (i.e. local doping of OSC near the contact region) can be an effective solution by either (1) reducing the energetic barrier for charge injection by bringing the Fermi level of OSC closer to the work function of the metal electrode, ^77,78) or (2) narrowing the depletion layer formed at the Schottky barrier at the interface. However, contact doping could also interrupt molecular arrangement near the contact and increase trap density as a result, leading to poor charge injection. ^79,80) Therefore, careful selection of dopants and doping method are required to improve the overall OFET performance.

As an example of effective contact doping, Kim et al. introduced "molecular implantation doping" method for extensively doping OSC selectively near the contact region, improving the charge injection of the bottom-gate bottom-contact PBTTT OFETs by solid-state diffusion of F₄TCNQ [Fig. 7(a)]. ⁷⁵⁾ The OFETs fabricated using the contact doping technique showed a markedly improved charge injection from the contact resistance lowered by approximate factor of 5 [Fig. 7(e)]. The large energetic barrier [∼0.87 eV, Fig. 7(c)] at the metal/semiconductor interface is reduced after locally doping OSC near the contact due to the reduced depletion width induced by large doping concentration induced from the effective solid-state diffusion of F₄TCNQ in PBTTT [Fig. 7(d)].

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In addition, molecular doping can be directly applied to the active channel of OFETs. For high-mobility donor–acceptor (D−A) copolymers, ⁸¹⁾ their narrow HOMO-LUMO gap has been demonstrated as high-performance OFET channel material for ambipolar transport. However, only staggered device structures have been commonly utilized due to poor charge injection in planar structures [Fig. 7(f)]. Xu et al. demonstrated that reliable high-performance p-type OFETs could be made with a DA polymer, diketopyrrolopyrrole-thieno[3,2-b]thiophene (DPPT-TT), by employing contact doping in planar structure (top-gate top-contact). Contact doping with a p-type dopant, iron chloride (FeCl₃), exhibited a lower contact resistance due to a simultaneous lowering of hole injection barrier and raising of an electron injection barrier [Fig. 7(g)]. Therefore, the enhanced hole injection and blocked electron injection led to a dramatic enhancement in hole mobility, showing ideal unipolar p-type FET transfer characteristics. ⁷⁶⁾

In the aforementioned contact doping works, however, the stability of contact doping was poor as the dopant molecules diffused toward the bulk of the channel. To overcome this problem, the dopant-blockade method was developed. ⁸²⁾ Kim and coworkers used F₄TCNQ and TCNQ as a dopant and blockade molecule, respectively [Fig. 8(a)]. The blockade molecules were deposited in the middle of the OFET channel first, and then dopant molecules were deposited at the contact region. By comparing the stability of two devices with doped-contact stabilized by the dopant-blockade method (DB/DC) FET and without the blockade molecule deposition doped-contact (DC) FET, it is found that the doping stability reflected by the on/off ratio of the OFETs with the dopant blockade molecules was retained over 2 months, whereas that of DC-FET decreased significantly, which reflects an enhanced doping stability for with the proposed dopant-blockade method [Figs. 8(b)–8(d)]. The experimental results were also supported by numerical simulation of dopant diffusion in PBTTT channel and its suppression by site-occupying TCNQ molecules [Fig. 8(e)].

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On the other hand, molecular doping has been shown not only effective in enhancing charge injection in OFETs but for improving charge extraction at the metal/OSC interface in OPVs. ^41,83–85) Recently, Yan et al. improved PCE by more than 30% using the solid-state diffusion method, minimizing structural disorder even in the condition of relatively high doping concentrations. ⁴³⁾ This work demonstrated the feasibility of heavy doping in OPVs, enlarging the doping range applicable to OPVs. Another doping approach attempted has been focused on enhancing the charge transport within the BHJ of OPVs. ⁴¹⁾ BV is incorporated in BHJ as an n-type dopant and synergistically improved the microstructure and reduced the trap density which led to increasing the power conversion efficiency from 13.2% to 14.4% upon the addition of the optimal concentration of 0.004 wt% of BV for inducing better molecular stacking.

2.15.4. Charge transport polarity modulation

Due to oxidative stability of the OSC channel, OSCs with high work function have been predominantly used for p-type FETs and most OSCs typically have higher hole mobility compared with electron mobility. However, for CMOS applications, both p- and n-type FETs are required, and therefore efficient hole and electron injection from the metal electrodes to the OSC channel, and retaining high on/off ratio values are demanded. Khim et al. doped initially ambipolar methanofullerene [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) OFETs with an n-type dopant cesium fluoride (CsF) and a p-type dopant F₄TCNQ, respectively by blending or inkjet printing of a dopant layer on the pre-deposited semiconductor film [Fig. 9(a)]. ³⁰⁾ Figures 9(b) and 9(c) show that 1.0 wt% doping ratio of CsF and F4-TCNQ drastically decrease the μ_FET,h, and μ_FET,e, respectively, suggesting three different operation regimes; (i) equivalent ambipolar transport with pristine PCBM, (ii) n-channel predominant operation with n-doped PCBM, and (iii) p-channel predominant operation with p-doped PCBM. Additionally, the turn-on voltage, V_on, shift plot with p-doping and n-doping indicates an effective control of V_on by appropriate doping level. Finally, the authors implemented a complementary ambipolar inverter with a selective molecular doping method using inkjet printing [Fig. 9(d)].

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Another practical implementation of polarity modulation by molecular doping of OSC was the development of inversion-mode OFETs, which was hard to realize due to the inefficient injection of minority charge carriers into the inversion channel. ⁸⁶⁾ Lüssem et al. demonstrated both organic depletion and inversion transistors by setting up the transistors consisting of Al₂O₃ as a gate dielectric, a thin doped pentacene layer as the OFET channel, an intrinsic pentacene layer and doped source and drain contact electrodes [Fig. 9(e)]. ⁸⁷⁾ F₆TCNNQ and W₂(hpp)₄ were used as a p-type dopant and an n-type dopant, respectively. The fabricated depletion-mode transistor showed a positive shift in the threshold voltage, V_th, according to appropriate doping conditions without sacrificing the on/off ratio, suggesting that the doped layer can be fully depleted with gate voltage. A linear shift of V_th with the thickness of the doped layer clearly supports generation of free charges into the channel [Fig. 9(f)]. As seen in Fig. 9(g), the n-doped transistors clearly show a p-type characteristic, and the threshold voltage increases with increasing n-doping concentration. This shift of threshold voltage is attributed to the formation of an inversion layer (i.e. the n-doped layer) at the oxide/pentacene interface, which is a clear demonstration of the inversion-mode transistor.

2.15.5. Thermoelectrics

A thermoelectric device capable of converting thermal energy into electrical energy is one of the most popular application fields of OSC doping. The figure of merit associated with the thermoelectric energy conversion, ZT, is represented by a combination of transport parameters

$$\begin{eqnarray}&&ZT=\sigma {S}^{2}T/k,\end{eqnarray} \tag{ 3 }$$

where σ is electrical conductivity, S is the Seebeck coefficient, k is thermal conductivity, and T is the absolute temperature. OSCs for thermoelectrics have been thoroughly studied due to their low thermal conductivity (in the order of 0.1 W mK⁻¹), ⁸⁸⁾ which is advantageous for high ZT values. Additionally, organic materials hold promises for their mechanical flexibility, cost-effectiveness, and eco-friendly processing. ⁸⁹⁾ In terms of the commercialization of thermoelectric devices, it is known that there is practical significance when the value of ZT is higher than 1. There have been intensive efforts in improving the ZT value of organic thermoelectric devices which is generally lower than that of inorganic materials. ⁹⁰⁾ However, the above transport parameters are correlated to each other, and independent improvement of one parameter is not possible to realize. ⁹¹⁾ For instance, increasing electrical conductivity accompanies an increase in thermal conductivity (Wiedemann–Franz law), however, the Seebeck coefficient is inversely proportional to the electrical conductivity, compromising the ZT value as a result [Fig. 10(a)]. Therefore, to optimize ZT to develop high-performance thermoelectric devices, it is essential to discern physical relationships and develop dopant and active materials ⁹²⁾ for increasing S, σ, and k simultaneously. ^93–95) In particular, most organic thermoelectric material research has focused on increasing the value of the thermoelectric power factor, σS², which is the maximum thermal energy conversion output of the material. ⁹⁶⁾

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The most effective doping method in OSC is controlling charge carrier concentration without compromising carrier mobility. Considering this, it is important to develop a doping method that provides free electrons and holes efficiently and retains the molecular arrangement of an OSC designed for high charge mobility. From this point-of-view, the solid-state diffusion method discussed above is one of the best doping methods. It has been reported that solid-state diffusion methods can maintain the morphology of the PBTTT while minimizing the structural disorder caused by the dopants and maximising conductivity through high doping efficiency, leading to a high power factor. ⁹²⁾ By comparing with the solution-doped sample (i.e. co-deposition method), the retained orientational correlation length (OCL) was discovered to be the key to improving electrical conductivity and charge mobility without loss of the Seebeck coefficient [Figs. 10(e), 10(f)]. ^92,96)

Recently, Wang et al. demonstrated that modulation doping of orthorhombic rubrene can be realised for achieving superior doping efficiency even at high doping density, which leads to thermoelectric power factors over 20 μWm⁻¹ K⁻² at 80 °C. ⁵⁹⁾ The advantage of avoiding physically separating ionized dopant molecules from the transport channel for minimizing Coulomb scattering can be attributed as the origin for achieving both a high carrier concentration and mobility, which leads to higher power factor values [Figs. 10(g) and 10(h)]. Orthorhombic rubrene showed 2.7 to 7.6 μWm⁻¹ K⁻² of power factor at the doping concentration range from 13 to 39 mol%, and the power factor of triclinic rubrene crystals was increased from 0.48 to 0.99 μWm⁻¹ K⁻² by increasing doping concentration from 13 to 39 mol%. These power factor values obtained by modulation doping are much higher than those of bulk-doped 9,9-bis[4-(N,N-bis-biphenyl-4-yl-amino)phenyl]-9H-fluorene (BPAPF) with the value of 3.5 × 10⁻³ μWm⁻¹ K⁻² even at high doping levels (∼39 mol%). The authors also investigated the charge transport mechanism by measuring temperature-dependent Seebeck coefficient of modulation-doped rubrene crystals. Triclinic rubrene crystals showed an unusual temperature dependence expected for Seebeck coefficient, with the Seebeck coefficient decreasing with increasing temperature, while the Seebeck coefficient of orthorhombic rubrene crystals increased as the temperature increased. This unexpected behaviour can be attributed to the different energetic bandwidth and density of states (DOS) between the doped triclinic and orthorhombic rubrene crystals [Figs. 10(i)], indicating that an enhanced electronic coupling (broader bandwidth and smaller width of a local minimum in DOS) leads to the increase in the Seebeck coefficient with increasing temperature.