2025 年 93 巻 7 号 p. 077001
A series of organic thin-film solar cells using electrodeposited polythiophene films were fabricated and characterized. The cells consisted of an indium-tin oxide transparent electrode/electrodeposited poly(3,4-ethylenedioxythiophene) doped with different anions as a hole transport layer/electrodeposited poly(2,2′-bithiophene) film as a photoexciting p-type semiconductor layer/[6,6]-phenyl-C61-butyric acid methyl ester as an n-type semiconductor layer/aluminum as a top electrode, were fabricated. Obtained solar cells were characterized and photoelectric properties were evaluated. These organic thin-film solar cells generated photovoltages and photocurrents. The photovoltaic property was varied depending on the structure of dopant anions, which seems to be correlated with the surface morphology of each poly(3,4-ethylenedioxythiophene)-dopant anion.
Solar energy conversion is a promising approach for developing clean energy systems. Among these, organic thin-film solar cells have several unique features, including low cost, high transparency, and flexibility compared to silicon-based solar cells.1,2 In organic thin-film solar cells, organic semiconductors act as photo-induced carrier generation moieties. Organic conductive polymers such as polythiophenes are one of the typical p-type organic semiconductors for organic thin-film solar cells.3–7
Basically, a hierarchical organic semiconductor layer is essential for organic thin-film solar cells. As organic conductive polymer is used for the organic semiconductor layer, the conventional fabrication techniques for the preparation of conductive polymer films as organic semiconductors generally involve spin-coating, dip-coating, or spray coating from corresponding solutions.8,9 Achieving a hierarchical structure with these methods requires combinations of materials with different solubilities, which can limit the types of hierarchical structures that can be effectively produced. Electrochemical polymerization offers a solution by enabling the fabrication of insoluble thin films and facilitating the straightforward creation of hierarchical structures.10–15
We have reported on the fabrication and half-photocell application of poly(3,4-ethylenedioxythiophene) (polyEDOT)/poly(2,2′-bithiophene) (polyBiTh) hierarchical structure films through sequential electrochemical polymerization.16–19 Briefly, modified electrode, indium-tin oxide (ITO) transparent electrode/polyEDOT/polyBiTh generated cathodic photocurrent in the presence of sacrificial electron acceptor, whose polyBiTh function as a photoexciting donor and polyEDOT inhibit electron transfer from polyBiTh to ITO.18–25
In the present study, we expanded this approach to fabricate and evaluate an organic thin-film solar cell by applying the sequential electrochemical polymerization technique. A series of organic thin-film solar cells, an indium-tin oxide transparent electrode/electrodeposited polyEDOT–dopant anions film as a hole transport layer/electrodeposited polyBiTh film as a photoexciting p-type semiconductor layer/[6,6]-phenyl-C61-butyric acid methyl ester as an n-type semiconductor layer/aluminum as a top electrode, was fabricated, characterized and their photoelectric properties were evaluated. In particular, we examined the dopant anion for the polyEDOT layer on its photoelectric conversion efficiency.
3,4-ethylenedioxythiophene (EDOT, Tokyo Chemical Industry), 2,2′-bithiophene (BiTh, Tokyo Chemical Industry), [6,6]-phenyl-C61-butyric acid methyl ester (PCBM, Frontier Carbon, nanom spectra E100), sodium p-styrenesulfonate (SS, Wako Pure Chemical Industries), sodium sulfate (Na2SO4, SIGMA-ALDRICH), sodium dodecylbenzenesulfonate (DBS, hard type, mixture, Tokyo Chemical Industry) and other chemicals were used as received. Ultrapure water (18.2 MΩ cm) was used as the aqueous solvent.
The fabrication processes of the hierarchical polythiophene thin films consisting of polyEDOT and polyBiTh are summarized in Fig. 1. Initially, a polyEDOT thin film was fabricated on an ITO transparent electrode using electrochemical polymerization in a two-electrode cell. The solution was prepared from 0.01 mol/L EDOT, 0.01 mol/L electrolytes (SS, Na2SO4, or DBS), and ultrapure water. The electrodes used were an ITO transparent electrode (anode, 2 × 2 cm2) and a nickel plate (cathode, 3 × 3 cm2). The distance between the two electrodes in the solution was set to 45 mm, and the solution was continuously stirred at 400 rpm during polymerization. A voltage range of 0 to +2 V (ITO electrode to nickel plate electrode) was scanned at a rate of 0.05 V/s for one cycle (initial voltage 0 V, switching voltage +2 V, final voltage 0 V). After polymerization, the ITO electrode was removed from the solution, washed with ultrapure water, dried with nitrogen gas, and annealed at 100 °C for 10 minutes on a hot plate to form the polyEDOT film, denoted as ITO/polyEDOT(SS, Na2SO4, or DBS).
Fabrication route for organic thin-film solar cells using hierarchical electropolymerized polythiophene films.
Next, a polyBiTh thin film was deposited onto the ITO/polyEDOT(SS, Na2SO4, or DBS) using a three-electrode electrochemical cell. The electrolyte solution was prepared with 0.1 mol/L n-Bu4NPF6 and 1 × 10−3 mol/L BiTh in dichloromethane. During polymerization, the solution was stirred continuously at 400 rpm. A voltage range of 0 to +2 V (ITO/polyEDOT electrode to Ag wire) was scanned at a rate of 0.05 V/s for five cycles (initial voltage 0 V, switching voltage +2 V, final voltage 0 V). After polymerization, the ITO/polyEDOT(SS, Na2SO4, or DBS) electrode was removed from the solution, cleaned with dichloromethane, dried with nitrogen gas, and annealed at 150 °C for 10 minutes on a hot plate to form the polyBiTh film (denoted as ITO/polyEDOT(SS, Na2SO4, or DBS)/polyBiTh). The oxidation potential of ferrocene was measured under the same conditions without BiTh and used as an internal standard to calibrate the potentials to the standard hydrogen electrode (SHE) scale.
Subsequently, a PCBM layer was spin-coated onto the ITO/polyEDOT(SS, Na2SO4, or DBS)/polyBiTh structure. The spin-coating solution was prepared by dissolving and filtering PCBM in chlorobenzene at a concentration of 20 mg/mL. The spin-coating was performed by dropping 100 µL of the solution and spinning at 500 rpm for 5 s followed by 700 rpm for 55 s (denoted as ITO/polyEDOT(SS, Na2SO4, or DBS)/polyBiTh/PCBM). Finally, an aluminum electrode was deposited through evaporation using a 3 mm × 3 mm mask to complete the organic thin-film solar cell, denoted as ITO/polyEDOT(SS, Na2SO4, or DBS)/polyBiTh/PCBM/Al.
In addition, to evaluate the differences in the series resistance of polyEDOT layers, sandwich-type cells with ITO/polyEDOT(SS, Na2SO4, or DBS)/Au and ITO/Au were fabricated. The polyEDOT(SS, Na2SO4, or DBS) layers were prepared under the same conditions as described above. A gold electrode was deposited by evaporation using a 3 mm × 3 mm mask. Resistance values were measured between the ITO and Au electrodes.
Electrochemical polymerization was conducted using a potentiostat (ALS, model 650 C). Absorption spectra were measured with a spectrophotometer (JASCO, V-670ST). Film thickness was measured using an atomic force microscope (AFM) (SII, SPA400). To measure the thickness of the films, the polyEDOT layer was partially delaminated to create a distinct step between the film and the ITO electrode. Surface observations of ITO/polyEDOT(SS, Na2SO4, or DBS) were performed with a scanning electron microscope (SEM; HITACHI, S-4500). The current density-voltage (J-V) characteristics of the solar cells were measured under the illumination of 100 mW/cm2 using an AM 1.5 solar simulator (Peccell Technologies, PEC-L01). Irradiation wavelength-dependent photocurrents were measured under monochromatic light irradiation using hypermonolight (Bunko Keiki, SM-25). The photocurrent was recorded using an I-V Curve Analyzer (Peccell Technologies) and a Huso HECS-318C potentiostat. External quantum efficiency (EQE) refers to the ratio of the number of electrons to the number of incident photons, evaluated using the following formula based on the wavelength λ (nm), photocurrent density J (A/cm2), incident light power Φ (W/cm2), the Planck constant h, and the speed of light c.
| \begin{equation} EQE\ (\% ) = \frac{J \times h \times c}{q \times \varPhi \times \lambda} \end{equation} | (1) |
Figure 2 shows scanning electron microscope (SEM) images of polyEDOT(SS, Na2SO4, or DBS), which reveal the surface morphology of the polyEDOT layer considerably varies depending on electrolytes (SS, Na2SO4, or DBS). The top image of polyEDOT(SS) exhibited a granular morphology with numerous particles on the order of tens of nanometers. In contrast, in cases of polyEDOT(Na2SO4) and polyEDOT(DBS), relatively smooth surface morphologies are observed compared to that of polyEDOT(SS). Therefore, the electronic active surface area of polyEDOT(SS) might be higher than that of polyEDOT(Na2SO4) and polyEDOT(DBS). In the case of electrochemical polymerization using SS, the π–π interactions between the aromatic styrene moiety and EDOT are considered to induce localized aggregation, resulting in a granular morphology. In contrast, for Na2SO4, the small size of the anion is likely to facilitate uniform doping, leading to the formation of a smooth film. For DBS, the long alkyl chain imparts hydrophobicity, which is presumed to inhibit polymerization and promote a more gradual film formation, resulting in a smooth morphology.
SEM images of ITO/polyEDOT(SS, Na2SO4, or DBS).
The thicknesses of polyEDOT(SS, Na2SO4, or DBS) are approximately 10 nm, obtained from AFM measurement of the samples (Fig. S1). The results indicate that the kinds of electrolytes during the electrochemical polymerization of polyEDOT have no significant impact on the film thickness. The thickness of each layer in the ITO/polyEDOT(SS, Na2SO4, or DBS)/polyBiTh/PCBM/Al structure are summarized in Fig. S2. The thicknesses of the polyBiTh layer on polyEDOT(SS, Na2SO4, or DBS) were 54, 75, and 76 nm, respectively. A relatively thinner polyBiTh layer was observed on polyEDOT(SS), which may be due to differences in the surface morphology of polyEDOT(SS) compared to polyEDOT(Na2SO4 or DBS). The thicknesses of the PCBM layer and the Al electrode were 60–69 nm and 34 nm, respectively.
Figures 3a and 3b show the cyclic voltammograms recorded during the electrochemical polymerization of polyEDOT(SS, Na2SO4, or DBS) layers, and polyBiTh on polyEDOT(SS, Na2SO4, or DBS) layers, respectively. In Fig. 3a, the cyclic voltammogram for the deposition of polyEDOT(SS, Na2SO4, or DBS) indicates the polymerization of EDOT and formation of polyEDOT(SS, Na2SO4, or DBS) through the oxidation currents observed in the range of +1 to +2 V.
Cyclic voltammograms of the electrochemical polymerization of (a) polyEDOT(SS, Na2SO4, or DBS) film on ITO, and (b) polyBiTh film on ITO/polyEDOT(SS, Na2SO4, or DBS) at 5th cycle for electropolymerization of BiTh.
Cyclic voltammograms for 1st–5th cycles of ITO/polyEDOT(SS, Na2SO4, or DBS)/polyBiTh (Fig. S3), whose oxidation and re-reduction currents increased by increasing scanning cycles of applied potentials. Figure 3b shows the cyclic voltammogram for the deposition of polyBiTh on polyEDOT(SS, Na2SO4, or DBS) layers at the fifth cycle. In all samples, the oxidation currents were observed between +0.5 and +2.0 V and the re-reduction current was observed between +1.3 and 0 V are corresponding to the electrochemical polymerized polythiophenes. It was observed that electrochemical polymerization of polyBiTh on polyEDOT(SS) results in obviously higher oxidation and re-reduction currents compared to those of polyEDOT(Na2SO4) and polyEDOT(DBS). While, the color intensity of polyBiTh observed in the photographs (Fig. S4) and the absorbance of the absorption band originating from polyBiTh (as discussed later, Fig. 4) show little difference among the samples. This suggests that, in the cyclic voltammetry, the morphology of the polyBiTh film in ITO/polyEDOT(SS)/polyBiTh differs significantly compared to the other samples. This seems to be consistent with the expected higher surface area of polyEDOT film in ITO/polyEDOT(SS) than the others in corresponding SEM images (Fig. 2).
Absorption spectra of ITO/polyEDOT(SS, Na2SO4, or DBS)/polyBiTh, ITO/polyEDOT(SS, Na2SO4, or DBS) and ITO.
Figure 4 shows the absorption spectra of a bare ITO electrode, ITO/polyEDOT(SS, Na2SO4, or DBS), and ITO/polyEDOT(SS, Na2SO4, or DBS)/polyBiTh. In the polyEDOT(SS, Na2SO4, or DBS), a near-infrared band primarily associated with polaron absorption from polyEDOT is observed. The differences in absorption properties of the polyEDOT layer depending on the electrolyte (SS, Na2SO4, or DBS) are not significantly distinct. The absorption spectra of ITO/polyEDOT(SS, Na2SO4, or DBS)/polyBiTh exhibits a broad absorption band in the visible region (400–620 nm) attributed to polyBiTh. These spectral characteristics correspond to the photoimages of ITO/polyEDOT(SS, Na2SO4, or DBS), and ITO/polyEDOT(SS, Na2SO4, or DBS)/polyBiTh, as shown in Fig. S3. Figure S3 also indicates that no obvious color change was observed after PCBM modification on ITO/polyEDOT(SS, Na2SO4, or DBS)/polyBiTh, and the Al electrode had a mirrored surface in all solar cells.
Figure 5 shows the J-V curves of the present solar cells, ITO/polyEDOT(SS, Na2SO4, or DBS)/polyBiTh/PCBM/Al. The photoelectric conversion performances, the short-circuit photocurrent density (Jsc), the open-circuit voltage (Voc), the fill factor (FF), and the power conversion efficiency (η) of these solar cells are summarized in Table 1. All present solar cells generated photocurrents and photovoltages under illumination. The highest photovoltaic performances of all parameters (Jsc, Voc, FF, and η) were observed in ITO/polyEDOT(SS)/polyBiTh/PCBM/Al. The second best was ITO/polyEDOT(Na2SO4)/polyBiTh/PCBM/Al, followed by ITO/polyEDOT(DBS)/polyBiTh/PCBM/Al as the third. The values of Jsc and FF for the solar cells differ considerably, whereas the Voc of each solar cell is not so different from each other, and is varied within 60 mV.
J–V characteristics of ITO/polyEDOT(SS, Na2SO4, or DBS)/polyBiTh/PCBM/Al.
| Jsc/ mA cm−2 |
Voc/ V |
FF | η/ % |
|
|---|---|---|---|---|
| ITO/polyEDOT(SS) /polyBiTh/PCBM/Al |
0.76 | 0.72 | 0.49 | 0.27 |
| ITO/polyEDOT(Na2SO4) /polyBiTh/PCBM/Al |
0.47 | 0.69 | 0.43 | 0.15 |
| ITO/polyEDOT(DBS) /polyBiTh/PCBM/Al |
0.25 | 0.66 | 0.28 | 0.06 |
These characteristics suggest that the essential conduction band potentials of polyEDOT(SS, Na2SO4, or DBS) are close to each other. On the other hand, measurements of the series resistance of sandwich-type devices ITO/polyEDOT(SS, Na2SO4, or DBS)/Au and ITO/Au were 7.3, 8.9, 10.0, and 6.9 ohms, respectively. This result suggests that the series resistance of the polyEDOT layer varies depending on the anion species (SS, Na2SO4, or DBS). Therefore, the notable variations in Jsc and FF are considered to originate from a combination of differences in the series resistance of the polyEDOT(SS, Na2SO4, or DBS) layer and interfacial conditions or morphological differences of hierarchical organic layers.
Figure 6 shows the EQE profile of the Jsc of ITO/polyEDOT(SS)/polyBiTh/PCBM/Al under irradiation of simulated sunlight (AM1.5, 100 mW/cm2). A broad EQE peak was observed in the 400–650 nm region, which corresponds well to the absorption band of polyBiTh. Therefore, the photocurrent of this solar cell is generated via the excited state of polyBiTh, mainly.
EQE profile of the cathodic photocurrent of Al/PCBM/polyBiTh/polyEDOT(SS)/ITO.
Based on the aforementioned photovoltaic properties, the energy diagram of the present organic thin-film solar cells fabricated via sequential electrochemical polymerization, with the structure ITO/polyEDOT(SS, Na2SO4, or DBS)/polyBiTh/PCBM/Al, is proposed as shown in Fig. 7. This diagram is essentially the same as a typical polythiophene: fullerene-derivative based organic thin-film solar cell.
Energy diagram.
In organic thin-film solar cells using hierarchically structured polythiophene electrodeposited films, the chemical species of the dopant anion in the hole transport polythiophene layer prepared by electrodeposition is critical for photoelectric conversion efficiency, as found experimentally in this research. This experimental finding is expected to contribute to the development of organic electronic devices.
In contrast to conventional organic thin-film solar cells, which typically utilize a bulk heterojunction structure composed of a P3HT:PCBM blend, our system employs a heterojunction structure comprising electrochemically polymerized poly(2,2′-bithiophene) from unsubstituted bithiophene as the photoactive layer, followed by spin-coating of PCBM. This structural difference is believed to be a performance-limiting factor. In the future, taking advantage of the unique capabilities of electrochemical polymerization, such as copolymerization or mixed monomer polymerization, could enable the construction of photoactive layers with morphologies closer to those of bulk heterojunctions. Furthermore, optimization of electrochemical polymerization conditions to achieve highly oriented film growth, similar to that of P3HT, is expected to enhance device performance.
The hierarchical electrodeposited polythiophene films, which function as a hole transport layer and a photoexciting p-type semiconductor, were fabricated on a transparent electrode. Organic thin-film solar cells incorporating these hierarchical thin films were constructed. Three types of dopant anions were used to prepare the hole transport polythiophene layer. The chemical species of the dopant anions had a significant effect on the photoelectric conversion efficiency. The morphology of the hole transport polythiophene layer varied depending on the dopant anion, which may explain the observed differences in photoelectric conversion efficiency.
The data that support the findings of this study are openly available under the terms of the designated Creative Commons License in J-STAGE Data at https://doi.org/10.50892/data.electrochemistry.29194484.
Kengo Kanbe: Conceptualization (Equal), Data curation (Lead), Investigation (Lead), Validation (Lead), Visualization (Lead), Writing – original draft (Lead), Writing – review & editing (Equal)
Takeo Oku: Conceptualization (Supporting), Data curation (Supporting), Investigation (Supporting), Visualization (Supporting), Writing – review & editing (Equal)
Tsuyoshi Akiyama: Conceptualization (Equal), Data curation (Equal), Investigation (Equal), Methodology (Equal), Supervision (Lead), Validation (Equal), Visualization (Supporting), Writing – review & editing (Equal)
The authors declare no conflict of interest in the manuscript.
K. Kanbe: ECSJ Student Member
