In recent years, there has been a surge in interest and research surrounding organic semiconductor (OSC)-based electronic devices, including organic field-effect transistors (OFETs)^[1−4], organic thermoelectric devices (OTEs)^[5−7], organic photovoltaics (OPVs)^[8−10], organic light-emitting diodes (OLEDs)^[11−13], and so on. As research progresses, the performance of these organic electronic and optoelectronic devices has seen remarkable enhancements. However, compared to inorganic semiconductors such as silicon, OSCs still lag in electrical performance, particularly in terms of electronic conductivity. This limitation significantly impacts the charge carrier transport within OSCs, consequently constraining the overall device performance. To address this challenge, the development of new OSC materials, alongside advances in OSC single crystals and doping techniques, has become a major focus of research aimed at substantially enhancing the electrical properties of OSCs^[14−19]. Similar to the critical role doping which has played in the advancement of inorganic semiconductors, it holds great potential to dramatically improve the electrical characteristics of OSCs. Hence, the development of dopants and doping techniques tailored for OSCs is essential for advancing the technology of OSC-based devices.

Among the various types of dopants, ionic dopants based on organic salts have shown great promise due to their high doping efficiency and broad applicability. These dopants, which operate through mechanisms such as electrophilic attack, have demonstrated superior performance in improving the electrical conductivity of OSCs^{[1, 20−27]}. One such ionic dopant, 4-isopropyl-4′-methyldiphenyliodonium tetrakis(penta-fluorophenyl-borate) (DPI-TPFB), has been previously reported as a p-dopant for hole transport layers (HTLs) in perovskite solar cells (PSCs). For example, Zheng et al. reported that the electrical conductivity of DPI-TPFB-doped PTAA increased by approximately 22 times compared to pristine PTAA films^[28]. This enhancement led to an improvement in the fill factor (FF) of tandem solar cells by about 80%, resulting in a power conversion efficiency (PCE) of 27.8%. Zhang et al. reported a PCE of 21.38% for a perovskite solar cell (PSC) utilizing DPI-TPFB-doped PTPD as the HTL, significantly surpassing the 17.63% PCE of the pristine PTPD HTL-based PSCs^[29]. Hu et al. employed DPI-TPFB-doped Spiro-OMeTAD as the HTL in the fabrication of PSCs, achieving PCE of up to 10.9%, which stands as the highest recorded for FASnI₃-based n−i−p structured PSCs^[30]. Likewise, Risqi et al. reported a PCE of 17.27% for a device using DPI-TPFB-doped spiro-OMeTAD blended with P3HT as the HTL, marking the highest value to date for n−i−p structured Sn-rich mixed lead-tinv halide PSCs^[31]. However, despite these promising results, the doping performance and efficiency of DPI-TPFB in OSCs remain largely unexplored, and its potential applications beyond HTLs have not been systematically investigated.

In this work, we aim to address this gap by performing a comprehensive investigation of the doping performance of DPI-TPFB in OSCs. Using the p-type OSC PBBT-2T as a model system, we evaluate the doping efficiency of DPI-TPFB through quantitative characterization methods, including ultraviolet−visible−near-infrared (UV−vis−NIR) absorption spectroscopy, electron spin resonance (ESR), electronic conductivity measurements, and work function analysis of doped films. Our findings reveal that DPI-TPFB enhances the electronic conductivity of PBBT-2T films by more than four orders of magnitude. Furthermore, we demonstrate that DPI-TPFB can effectively dope a variety of p-type OSCs and even exhibit p-doping effect in the n-type OSC N2200, transforming its intrinsic n-type characteristics into p-type. In addition to its doping performance, we explore the application of DPI-TPFB-doped OSCs in organic thermoelectric devices (OTEs), a field where its potential has not been previously studied. By leveraging the significantly enhanced electronic conductivity of DPI-TPFB-doped PBBT-2T films, we achieve improved thermoelectric performance, demonstrating the feasibility of using this dopant in OTEs. These results underscore the critical importance of developing dopants with both high efficiency and broad applicability to advance OSC-based technologies in optoelectronic devices.

DPI-TPFB (from Aladdin Reagent Co., Ltd.) and PBBT-2T were both dissolved in chlorobenzene (CB, from Sigma Aldrich, Inc.) to form the solutions. The concentrations of DPI-TPFB solution were 0.5, 1, 2.5, 5, and 10 g∙L⁻¹, respectively. The concentrations of PBBT-2T and N2200 solutions were 5 g∙L⁻¹, while the concentrations of P3HT were 5, 10, and 20 g∙L⁻¹, respectively. All solutions were filtered through a filter with a 0.45 μm filtration membrane prior to utilization. PBBT-2T solution needs to be preheated at 60 °C for 2 h before use. The doping solutions were prepared by blending the DPI-TPFB and PBBT-2T solutions according to the desired doping ratios.

The preparation of PBBT-2T, P3HT, and N2200 films was carried out by spin-coating doping solutions on the substrate at a speed of 1500 r/min for 30 s. Then the films were annealed at 100 °C for 10 min for PBBT-2T, 130 °C for 5 min for P3HT and at 150 °C for 20 min for N2200. Bottom-gate bottom-contact (BGBC) OFET devices were fabricated on Si wafers with a 300 nm SiO₂ layer, featuring pre-patterned electrodes (Cr/Au: 3/50 nm). For conductivity measurement devices, the doped PBBT-2T films were prepared on Si wafers with 300 nm SiO₂, on which patterned electrodes (Cr/Au: 2/30 nm) were prepared through photolithography and thermal evaporation. For the devices measuring thermoelectric performance, doped PBBT-2T films were prepared on glass substrates with preprepared patterned electrodes (Cr/Au: 10/15 nm).

The UV−vis−NIR absorption spectra of PBBT-2T doped solutions were obtained by UV-3600PLUS (SHIMADZU). The doped OSC films were prepared on glass substrates and sealed in quartz tubes for ESR measurement at room temperature using JEOL JES-FA200 ESR spectrometer. The morphology of doped PBBT-2T films was obtained by atomic force microscope (AFM) measurement in the Park XE-7 System.

The electrical conductivity of PBBT-2T films doped with DPI-TPFB was determined using the four-point probe technique on a Keithley 4200 semiconductor parameter analyzer. The thermoelectric properties of doped PBBT-2T films were measured using Keithley 2182A and Janis ST-100 at 300 K in a high vacuum (<10⁻⁵ mbar). Characterization of OFET performance was conducted on a Keithley B2912A source meter. The work function of doped PBBT-2T films was measured by Kelvin probe (KP020) system.

DPI-TPFB, an organic salt composed of one anion and one cation, is depicted in its molecular structure in Fig. 1(a). It exhibits excellent solubility in common organic solvents like chlorobenzene and chloroform, which is ideal for its application as a dopant in OSCs. The host OCS used in this study is PBBT-2T, a polymer OSC distinguished by its hole-transporting characteristics and high intrinsic mobility, which is illustrated in its molecular structure in Fig. 1(b). To evaluate the hole transport capabilities of PBBT-2T, we fabricated OFETs with the structure of BGBC (as shown in Fig. 1(c) illustration) using PBBT-2T as the active layer and proceeded to characterize their performance. Fig. 1(c) illustrates the transfer characteristics of PBBT-2T OFETs in the saturation region at gate-source voltage V_GS = −60 V, confirming that PBBT-2T predominantly transports holes with a mobility of 0.24 ± 0.0089 cm²∙V⁻¹∙s⁻¹. Fig. 1(d) presents the output characteristics of PBBT-2T OFETs, depicting the variation of drain−source current (I_DS) with drain−source voltage (V_DS) at V_GS from 0 to −60 V in steps of −20 V.

Subsequently, we assess the doping effect and efficiency of DPI-TPFB on PBBT-2T, thereby gauging its doping capability on OSCs to a certain extent. Fig. 2(a) displays the UV−vis−NIR absorption spectra of PBBT-2T in CB solutions with varying doping ratios from 5−100 mol% of DPI-TPFB. As depicted in Fig. 2(a), the pristine PBBT-2T solution exhibits three distinct intrinsic absorption peaks, which are centered at approximately 360, 500, and 1150 nm, respectively. Upon comparing the spectra of the PBBT-2T with the pristine state and those with varying doping ratios, it was observed that the intensities of the three primary intrinsic absorption peaks consistently diminished as the doping ratios increased, which suggests that the pristine PBBT-2T is progressively being replaced by DPI-TPFB-doped PBBT-2T. Correspondingly, the absorption peak intensity in the near-infrared region escalates with increasing doping ratios, a phenomenon attributed to the emergence of polarons within the DPI-TPFB-doped PBBT-2T system^[23]. To further assess the quantity of polarons induced by DPI-TPFB doping, the peak intensity at 1700 nm (about 0.73 eV) was extracted and presented in Fig. 2(b). As depicted in Fig. 2(b), the absorption intensity associated with polarons escalates in tandem with the rise in doping ratio, signifying a proportional increase in the number of polarons formed.

ESR is commonly used to detect the doping effect of dopants through the appearance of unpaired electrons^[32]. Quantitative ESR analysis was conducted on PBBT-2T films doped with varying ratios of DPI-TPFB. Fig. 2(c) demonstrates that the ESR signal intensity increases in correlation with the doping ratio, signifying a strengthened doping effect, which implies an increase in the number of polarons produced through doping. It is noteworthy that the pristine PBBT-2T film exhibited a faint ESR signal, likely due to the unintentional doping of PBBT-2T by oxygen from the air, even though it was sealed within a quartz tube before measurement. Furthermore, the quantity of polarons in the doped films was deduced by calculating the number of spins via the quadratic integration of the ESR signal^[24]. The polaron generation efficiency of DPI-TPFB-doped PBBT-2T films with varying doping ratios was assessed by calculating the ratio of the number of polarons to dopants present in the system. As shown in Fig. 2(d), the polaron generation efficiency for DPI-TPFB-doped PBBT-2T films was found to be consistently around 50% across doping ratios ranging from 0.5 to 2 mol%.

Generally, the electrical conductivity of OSC films is significantly enhanced after doping, owing to the increased carrier concentration and improved mobility^[23]. To mitigate the impact of contact resistance between the probes and the electrodes and accurately determine the electronic conductivity of PBBT-2T-doped films, the four-point probe technique is employed. Fig. 3(a) illustrates the schematic diagram of the device structure employed for measuring the electronic conductivity of the DPI-TPFB-doped PBBT-2T films. A constant current is applied to the electrodes at both ends of the device, and the voltage across the central pair of electrodes is measured. The electronic conductivity of the doped films is then determined using the equation σ=ILΔVWd, where σ is the electronic conductivity, I is the current, ΔV is the voltage across the central pair of electrodes, d is the thickness of the doped film, L and W are the length and width of the central pair of electrodes, respectively. Fig. 3(b) depicts the electronic conductivity of DPI-TPFB-doped PBBT-2T films as a function of doping ratios. The electronic conductivity of the pristine PBBT-2T film is about 0.02 ± 0.01 S∙cm^–1, and this value escalates swiftly in conjunction with increasing doping ratios. The rate of increase in electronic conductivity begins to decelerate once the doping ratio surpasses 30 mol%, reaching a peak value of 21.37 ± 0.28 S∙cm^–1 (about 4 orders of magnitude for the pristine PBBT-2T) at a doping ratio of 60 mol%. Beyond this point, the electronic conductivity commences to diminish with further increments in the doping ratio. This phenomenon could be attributed to two primary factors: Firstly, an excess of dopant may not contribute effectively to the doping process once the density of carrier induced by doping reaches saturation; and secondly, the surplus dopant might disrupt the orderly structure/morphology of PBBT-2T films, thereby interfering with the charge carrier transport pathways and leading to reduced carrier mobility^[23]. These two factors together lead to the decrease of electronic conductivity (σ = nqμ, with n and μ being the charge carrier density and mobility, respectively).

The work function of the doped films serves as an indicator of the doping effect, which is influenced by the substantial increase in carrier density attributable to the doping process. As shown in Fig. 3(c), the work function of DPI-TPFB-doped PBBT-2T films obtained by Kelvin probe rapidly decreases from approximately –4.4 eV as the doping ratio increases, until it reaches 30 mol%, beyond which the rate of decrease slows. This pattern aligns with the observed changes in the electronic conductivity of the PBBT-2T film in response to varying doping ratios. Subsequently, the work function gradually declined and stabilized at around –5.3 eV.

Drawing from the characterization data, which includes UV−vis−NIR absorption spectra, ESR, electronic conductivity, and work function measurements, it is evident that DPI-TPFB displays pronounced doping effects for PBBT-2T. However, its doping efficiency is lower than that of similar dopants TrTPFB, which may be due to the weaker electrophilic ability of DPI⁺ compared to Tr^{+[23, 24]}. Doping with heterogeneous substances commonly influences the organization and spatial architecture of OSC molecules, thereby inducing alterations in the morphology of OSC films^[33]. Hence, we investigated the morphological changes in PBBT-2T films doped with DPI-TPFB at various doping ratios by AFM.

Fig. 4 presents the AFM images of the surface morphology for both pristine PBBT-2T and PBBT-2T films doped with 10–100 mol% DPI-TPFB. As shown in Fig. 4(a), the pristine PBBT-2T film exhibits a uniformly flat and smooth surface, with a root mean square (RMS) roughness value of just 0.809 nm. Upon the addition of the dopant DPI-TPFB, the surface roughness of the PBBT-2T film escalates in tandem with the rising doping ratio. As depicted in Fig. 4(b), the RMS roughness of the film with a 10 mol% DPI-TPFB doping level modestly rises to 0.873 nm. In contrast, Fig. 4(c) reveals a substantial increase in the RMS roughness to 2.551 nm for the film doped with 30 mol% DPI-TPFB, accompanied by a noticeable trend towards pore formation. Following the increase in doping ratio, the PBBT-2T films doped with DPI-TPFB began to exhibit a pronounced porous structure, with the pore size progressively enlarging, particularly in films doped at 50 and 70 mol% doping ratios (as shown in Figs. 4(d) and 4(e)). Concurrently, the RMS roughness of them surged to 5.243 and 8.841 nm, respectively. Nevertheless, the RMS roughness of the PBBT-2T film doped with 100 mol% DPI-TPFB dropped to 6.574 nm, concurrent with the dissolution of the porous structure, as shown in Fig. 4(f). This pattern is likely due to the addition of an optimal amount of dopants that promote the orderly alignment of PBBT-2T molecules, leading to the formation of a porous structure. Conversely, an overabundance of dopants can disrupt the orderly arrangement of PBBT-2T molecules, thereby undermining the porous structure. Additionally, the trend in the porous structure as a function of doping ratios mirrors the trends in electronic conductivity and work function of the doped PBBT-2T films, indicating a link between the porous structure and the orderly arrangement of PBBT-2T molecules. In essence, the enhanced electronic conductivity of the doped films is a result of both the doping efficiency of DPI-TPFB and the structural regularity, as elaborated in our earlier work^[23].

In the preceding section, we demonstrated the doping effect and efficiency of DPI-TPFB with PBBT-2T, demonstrating a substantial enhancement in the electronic conductivity of PBBT-2T films by over 4 orders of magnitude, which underscores its effective doping effect. However, for a dopant to be truly impactful, it must not only exhibit high doping efficiency but also broad applicability across various OSCs, a factor that is often overlooked. The doping applicability of dopants is typically associated with their specific doping mechanisms. For instance, molecular dopants involving redox reaction mechanisms, like F₄TCNQ, can effectively dope OSCs with matching energy levels^{[34, 35]}. In contrast, dopants that operate through electrophilic-attack mechanisms (such as TrTPFB) exhibit broad doping applicability for OSCs. This is attributed to the easier occurrence of electrophilic-attack reaction between the electrophiles and the electron-rich aromatic units within the OSCs.

Considering that DPI-TPFB is a highly reactive iodine compound that can be used as a source of electrophilic iodine in a variety of chemical reactions, the doping mechanism of DPI-TPFB in OSCs is believed to be involved an electrophilic-attack mechanism^{[23, 24]}. Consequently, we hypothesize that DPI-TPFB possesses broad applicability across a wide range of OSCs. To validate this assumption, we conducted further investigations using the classic p-type OSC P3HT and the n-type OSC N2200 as model systems.

Fig. 5(a) shows the molecular structure of P3HT and N2200. ESR was employed to assess the doping effect of DPI-TPFB on P3HT, revealing that even at a doping ratio as low as 1 mol%, the P3HT film doped with DPI-TPFB displayed pronounced ESR signals, while the pristine P3HT film exhibited ESR silence, as shown in Fig. 5(b). Fig. 5(c) illustrates the relationship between the electronic conductivity of P3HT films doped with DPI-TPFB and the doping ratios. The electronic conductivity initially increases with the doping ratio, reaching a peak value of over 5 S∙cm⁻¹, before declining at higher doping concentrations.

Interestingly, the n-type OSC N2200, whose molecular structure is presented in Fig. 5(a), is amenable to doping with DPI-TPFB. As depicted in Fig. 5(d), the ESR spectrum of the pristine N2200 film was silent; however, the incorporation of 20 mol% DPI-TPFB induced a detectable ESR signal. To further verify that N2200 can be effectively doped with DPI-TPFB, we fabricated the OFETs with bottom-gate bottom-contact (BGBC) structure (as shown in Fig. 5(e) illustration) based on both undoped and 10 mol% DPI-TPFB-doped N2200 films. As depicted in Fig. 5(e), the transfer characteristics in the saturation region of N2200 OFETs display typical n-type features. In contrast, N2200 OFETs doped with DPI-TPFB exhibit p-type characteristics (as shown in Fig. 5(f)), confirming that DPI-TPFB can effectively dope N2200. These experimental findings validate our hypothesis that DPI-TPFB is a versatile ionic dopant with broad applicability across a wide range of OSCs. This broad applicability as a dopant, combined with its high efficiency, highlights the potential of DPI-TPFB for advancing the performance and functionality of OSC-based devices.

The doping of DPI-TPFB endows PBBT-2T films with higher electronic conductivity, rendering them ideal for organic thermoelectric applications. According to the equation PF=S2σ, where PF is the power factor, S is the Seebeck coefficient, and σ is the electronic conductivity, an increase in S or σ leads to a higher PF value. The PF is a crucial metric for evaluating the performance of thermoelectric devices, with a higher PF value indicating superior thermoelectric performance. To evaluate the thermoelectric performance of PBBT-2T films doped with DPI-TPFB, we constructed the thermoelectric devices based on the doped films with varying doping ratios, and the structural schematic is shown in Fig. 6(a). Fig. 6(b) illustrates the correlation between the thermoelectric potential difference (ΔV) and the temperature gradient (ΔT) across the cold end and hot end within the thermoelectric devices based on the pristine PBBT-2T film and PBBT-2T films doped with different doping ratios. According to the equation S=ΔVΔT, the S for both the pristine PBBT-2T film and PBBT-2T films doped by DPI-TPFB with varying doping ratios were derived from the data shown in Fig. 6(b). Fig. 6(c) presents the relationship between the S, σ, and PF with respect to the doping ratios for the thermoelectric devices fabricated from DPI-TPFB-doped PBBT-2T films. The S of the pristine PBBT-2T film is approximately 390 μV∙K^–1. With the increment of the doping ratio, S initially experiences a sharp decline, followed by a gradual deceleration in the rate of decrease, ultimately stabilizing at around 50 μV∙K^–1. However, the σ of the films initially increased rapidly and then more gradually as the doping ratio increased. Consequently, following the equation PF=S2σ, the PF exhibits an initial increase followed by a decrease with the rising doping ratios, peaking at approximately 10 μW∙m^–1∙K^–2 around the 10 mol% doping level. Fig. 6(d) illustrates the correlation between S and σ, revealing that S gradually diminishes as σ increases, a trend that is in line with previous literature^{[36, 37]}. These results underscore the significant importance of developing efficient dopants, for the advancement of OSC applications, including thermoelectric devices and other related technologies.

In conclusion, this study provides a comprehensive investigation into the doping performance and applicability of the ionic dopant DPI-TPFB for OSCs and its application in OTEs. DPI-TPFB demonstrated exceptional doping efficiency, enhancing the electronic conductivity of PBBT-2T films by over four orders of magnitude. Beyond its high efficiency, DPI-TPFB exhibited broad doping applicability, effectively doping various p-type OSCs and, notably, transforming the n-type OSC N2200 into a p-type material. This versatility underscores the potential of DPI-TPFB as a universal dopant for diverse OSCs. Furthermore, the application of DPI-TPFB-doped PBBT-2T films in OTEs yielded impressive thermoelectric performance, achieving a power factor of approximately 10 μW∙m⁻¹∙K⁻². These results emphasize the importance of developing dopants with both high efficiency and broad applicability to address the challenges of doping OSCs and unlock their full potential in optoelectronic and thermoelectric applications.