Dye-sensitized solar cells (DSSCs) are low-cost photovoltaic devices that have been intensively studied in the last decade.1, 2 Although a high efficiency of 11% has been reported for DSSCs with conventional liquid electrolytes, it remains a problem to fabricate large-area modules with high efficiency and good stability. Conventional liquid electrolyte contains iodine and organic solvents, which leads to difficulties of electrode corrosion and electrolyte leakage. In recent years, researchers have developed several types of hole-transporting materials (HTMs) to replace iodine-based liquid electrolytes.
Most research on HTM-based DSSCs is focused on inorganic HTMs and organic small-molecule HTMs. CuI and CuSCN are the most widely used inorganic HTMs in DSSCs.3–5 By using CuI as the HTM and Ru complex dye N3 as the sensitizer, the highest efficiency for inorganic HTM-based DSSCs is reported to be 4.7%.3 Up to now, the most effective organic small-molecule HTM for DSSCs is spiro-OMeTAD, which gives the best efficiency of ∼5% under full sun illumination.6–12Conjugated polymers as low-cost organic semiconductors have also been used as HTMs in DSSCs. However, this type of device often gives low efficiency.13–15
In HTM-based DSSCs, the degree of HTM penetration into the TiO2 porous electrodes and the photoelectron recombination at TiO2/dye/HTM interface are two important factors that affect the device efficiency.16–18 As polymers have a relatively large molecular size, these factors are remarkable for DSSCs with polymers as the HTM. Recently, in situ polymerization of pre-penetrated monomers has been developed to avoid the difficulty of polymer penetration into the TiO2 porous electrodes.19 By using Ru complex dye Z907 as the sensitizer and in situ polymerized poly(3,4-ethylenedioxythiophene) (PEDOT) as the HTM, the light-to-electrical conversion efficiency for polymer-HTM-based DSSCs has been improved to 2.8%.20 However, this efficiency is still much lower as compared to that of DSSCs based on small-molecule HTMs or inorganic HTMs.3, 8 To further improve the efficiency of polymer-HTM-based DSSCs, good polymer penetration and low photoelectron recombination are highly desirable.
For in situ polymerized HTM-based DSSCs, the polymerization is initiated by the photocreated holes at the HOMO level of the dyes. Therefore, dye sensitizer plays an important role in the polymerization process, which determines the properties of the TiO2/dye/polymer interface and the penetration of polymer in the TiO2 layer. Recently, several highly efficient organic dyes have been developed.21–23 Our preliminary study shows that some organic dyes facilitate the in situ polymerization of 2,2′-bis(3,4-ethylenedioxythiophene) (bis-EDOT) as compared to that with Z907,24 which motivates us to study in situ polymerized HTM-based DSSCs with organic dyes as sensitizers.
In this Communication, we report an indoline D149 dye-sensitized solar cell with in situ polymerized PEDOT prepared in a specially designed thin-layer electrolytic cell to serve as the polymer HTM. These devices give an average light-to-electrical efficiency of 6.1% under full sun illumination (AM1.5, 100 mW cm−2), which represents a remarkable improvement for polymer-HTM-based DSSCs. A similar device with Z907 as the sensitizer was also fabricated for comparison. The photoelectrical properties of both devices were studied, and the photoelectron recombination and polymer penetration in each device are discussed.
The chemical structures of D149 dye and Z907 dye are shown in Figure 1a and b, respectively. Z907 is a hydrophobic Ru complex dye, which has been widely used in HTM-based DSSCs.20 D149 dye belongs to the indoline dye series, which is a highly efficient organic dye for DSSCs.21 Figure 1c shows the specially designed thin-layer electrolytic cell for in situ polymerization of PEDOT. This cell only requires several tens of microliters of monomer solution per square centimeter for polymerization. In addition, it can be easily sealed to separate air and prevent the electrolyte from evaporation, which facilitates in situ polymerization. Figure 1d shows the device structure. In these devices, the iodine-based electrolyte is replaced by in situ polymerized PEDOT, which works as the HTM. Detailed polymerization and device fabrication procedures are shown in the experimental section.
Figure 2a and b show the surface morphology of the dye-sensitized TiO2 layer before and after in situ polymerization of bis-EDOT. A porous layer of PEDOT is clearly observed on the surface of the dye-sensitized TiO2 film in Figure 2b. Such a porous layer structure favors the post treatment of the TiO2/dye/PEDOT layer with Li salt/propylene carbonate solution. The performance of DSSCs with different TiO2 layer thicknesses is shown in Tables 1 (D149) and 2 (Z907). The current–voltage (I–V) curves of D149- and Z907-sensitized DSSCs with optimized TiO2 layer thickness (5.8 and 4.2 µm, respectively) are shown in Figure 2c. Under 100 mW cm−2 AM1.5 illumination, the D149-sensitized devices have a short-circuit photocurrent of 9.3 mA cm−2 and efficiency of 6.1%, which are much higher than for devices with Z907 as the sensitizer (3.6 mA cm−2, 1.7%). This result even outperforms the best record for HTM-based DSSCs.8 The performance of both DSSCs under different light intensity is shown in Tables S1 and S2 in the Supporting Information, respectively.
|Thickness [µm]||Isc [mA cm−2]||Voc [V]||Fill factor||Efficiency [%]|
|3.3||7.5 ± 0.5||0.83 ± 0.02||0.78 ± 0.01||4.9 ± 0.2|
|4.2||8.9 ± 0.2||0.85 ± 0.01||0.76 ± 0.01||5.7 ± 0.1|
|5.8||9.3 ± 0.2||0.86 ± 0.01||0.75 ± 0.02||6.1 ± 0.1|
|9.4||8.6 ± 0.4||0.87 ± 0.02||0.73 ± 0.03||5.5 ± 0.3|
|Thickness [µm]||Isc [mA cm−2]||Voc [V]||Fill factor||Efficiency [%]|
|3.3||3.1 ± 0.2||0.66 ± 0.01||0.69 ± 0.01||1.4 ± 0.1|
|4.2||3.6 ± 0.3||0.69 ± 0.02||0.70 ± 0.01||1.7 ± 0.1|
|5.8||1.4 ± 0.2||0.66 ± 0.02||0.58 ± 0.03||0.5 ± 0.1|
|9.4||0.9 ± 0.1||0.64 ± 0.01||0.52 ± 0.02||0.3 ± 0.1|
Figure 2d shows the corresponding incident photon-to-current efficiency (IPCE) spectra for devices used in Figure 2c. IPCE spectra reflect the light response of photovoltaic devices, which is directly related to the short-circuit current. For both devices, the IPCE values decrease at the long wavelength region (550–700 nm). The wavelength at the half maximum of the IPCE value is thus used to reflect the light-response range of the solar cells. For Z907-sensitized devices, the half maximum of the IPCE value occurs at 597 nm, which is 39-nm shorter as compared to that for D149-sensitized devices. Therefore, D149-sensitized devices have a broader light response than that of Z907-sensitized ones in the red-light region. On the other hand, the IPCE value of D149-sensitized devices (65.5% at 500 nm) is more than twice that of Z907-sensitized ones (25.7% at 500 nm). These data indicate that D149-sensitized devices not only absorb more solar energy, but also utilize solar energy more effectively relative to those devices with Z907 as the sensitizer. As only the absorbed photons can generate photoelectrons, the light absorption is directly related to the IPCE, which is reflected by the transmittance spectra (1 – T%) of dye-sensitized TiO2 layers shown in Figure 2d. The half-maximum value of transmittance for D149-sensitized TiO2 layer is at 655 nm, which is 41-nm longer than that for Z907-sensitized ones. This agrees with the IPCE spectra and, therefore, the broad light-response range of the IPCE spectrum for D149-sensitized devices can be attributed to the broad light-absorption range of the D149-dye-sensitized TiO2 layer. Although the light absorption for D149- and Z907-sensitized TiO2 layers at 500 nm is similar (99.8% and 90.0%, respectively), the IPCE value of D149-sensitized devices at 500 nm is ∼150% higher than that for Z907-sensitized ones. We also note that the competing light absorption from PEDOT is less than 15% and 6% at 500 nm for Z907- and D149-sensitized DSSCs, respectively, based on the light absorption for PEDOT (35%) and that for both dyes at 500 nm. This implies that some other factors exist that should account for the difference in the IPCE spectra.
The main possible recombination and charge-collection processes in a typical DSSC are illustrated in Figure 3a.25 Electrochemical impedance is a widely used method to study the recombination in DSSCs. The impedance for PEDOT-based DSSCs with 4.2 µM TiO2 thickness was measured under different bias voltages.10, 12 The spectra were analyzed by fitting with equivalent circuit in the literature to yield chemical capacitance (Cµ) and charge transfer resistance (Rct) for each device.20 Figure 3b shows the Cµ as a function of bias voltage, which indicates the distribution of electron state density in the TiO2 electrode. The Cµ of D149-sensitized DSSCs is lower than that of Z907-sensitized ones, indicating that the energy gap between TiO2 conduction-band edge and PEDOT Fermi level is larger for D149-sensitized DSSCs than that for Z907-sensitized ones. On the other hand, the sheet resistance of the in situ polymerized PEDOT layers was measured to be 6.57 kΩ sq−1 for D149-sensitized DSSCs and 6.45 kΩ sq−1 for Z907-sensitized ones. The resistivity of the PEDOT layers in both devices is similar, which implies that the PEDOT layers have a similar Fermi level position. Therefore, the TiO2 conduction band is upward shifted (∼0.1 V) for D149-sensitized DSSCs relative to that for Z907-sensitized ones. The upward shift of TiO2 conduction band is beneficial to the device performance for increased photovoltage and decreased recombination.12, 26 Figure 3c shows the Rct as a function of bias voltage. The Rct in D149-sensitized DSSCs is higher than that in Z907-sensitized ones at each bias voltage. As Rct at the TiO2/HTM interface is a widely used parameter to evaluate the recombination velocity at the TiO2/HTM (or electrolyte) interface, this result indicates that the recombination velocity in D149-sensitized DSSCs is lower than that in Z907-sensitized ones under the same bias voltage.
Figure 3d is the intensity-modulated photocurrent spectra (IMPS) of PEDOT-based DSSCs with D149 and Z907 as sensitizers. The intensity of the IMPS shows that the D149-sensitized devices have a larger light response than that for Z907-sensitized ones in the measured frequency range, which agrees with the IPCE measurement shown in Figure 2d. The inverse of the frequency (1/2πfmin) at the minimum of the IMPS arch represents the typical time interval from photoelectron injection to photoelectron arrival at fluorinated tin oxide (FTO) glass.16 The calculated time interval is 0.15 ms for D149-sensitized devices, which is faster than that for Z907-based ones (0.32 ms). Therefore, the photoelectron collection process (process 5 in Fig. 3a) in D149-sensitized devices is faster than that in Z907-based ones. Since the recombination fraction of photoelectrons in the TiO2 electrode comes from the competition between the electron collection process and the recombination process, the slow photoelectron recombination (Fig. 3c) and fast photoelectron collection (Fig. 3d) in D149-sensitized devices should result in a relatively lower recombination fraction of photoelectrons as compared to that for Z907-based devices.
On the other hand, the recombination of photoelectrons in TiO2 with the oxidized dye on TiO2 could also occur (process 4 in Fig. 3a). Since reduction of oxidized dyes by HTM (process 3 in Fig. 3a) is much faster than that by electrons in TiO2,25, 27, 28 the recombination from process 4 is generally small. In this case, the short-circuit current (ISC) of HTM-based DSSCs should increase with increased TiO2 layer thickness, because a thicker TiO2 layer can absorb more dye molecules. However, if the TiO2 layer is too thick to be well penetrated by the HTM, the holes will have difficulties to be transported to the counter electrode and the oxidized dye will not be reduced in time by the HTM.17, 18 In this case, the recombination from process 4 will increase remarkably, and the performance of DSSCs will decrease with increased TiO2 layer thickness. The TiO2-thickness-dependent ISC for both devices is shown in Figure 4a. The ISC reaches the maximum at a TiO2 film thickness of ∼5.8 and 4.2 µm for D149- and Z907-sensitized devices, respectively. Further increasing the TiO2 film thickness leads to decreased device performance for both dyes. The TiO2-film-thickness-dependent device performance indicates that the sensitizers could affect the polymer penetration depth and the effective penetration depth in D149-sensitized devices should be longer than that in Z907-sensitized ones. As the optimized thickness of the TiO2 layer for spin-coating HTM-based DSSCs is usually 1.5–2.5 µm,18, 29, 30 these results indicate that in situ polymerization is a promising method to improve polymer penetration.
To further understand the difference in polymer penetration depth, the impedance spectra of PEDOT-based DSSCs in the absence of a TiO2 dense film at 0 V bias are studied, and the results are shown in Figure 4b. In the absence of a dense film, the electrons are not blocked and the PEDOT is able to make direct contact with FTO glass. At zero-bias voltage, the resistance of PEDOT is much smaller than that of TiO2, and as a consequence, the impedance is mainly determined by the distribution of PEDOT in the TiO2 porous layer. For both D149-and Z907-sensitized devices, the impedance increases with increased TiO2 layer thickness. The slope of the increase is faster for Z907-sensitized devices than for D149-sensitized ones, which indicates that in situ polymerization of bis-EDOT on D149-sensitized TiO2 layers gives better penetration than on Z907-sensitized ones. This agrees with the ISC–TiO2 thickness curves in Figure 4a. Figure 4b shows the relationship between the resistance obtained from the impedance spectra and the TiO2 layer thickness for D149- and Z907-sensitized solar cells. The resistances of PEDOT from the foremost of the TiO2 layer (FTO side) throughout the device to the Au electrode are 36 ohm cm−2 for a 5.8-µm D149-sensitized TiO2 layer and 25 ohm cm−2 for a 4.2-µm Z907-sensitized TiO2 layer. These values are sufficiently low for hole transportation in the devices.
In conclusion, we fabricated efficient polymer-HTM-based DSSCs by in situ polymerization of bis-EDOT in a thin-layer electrolytic cell using D149 dye as the sensitizer. The devices have shown an average efficiency of 6.1%, which represents a remarkable improvement for polymer-HTM-based DSSCs. These results indicate that polymer-HTM-based DSSCs are a promising generation of HTM-based DSSCs, which deserve further studies. By comparing with Z907-sensitized devices, the excellent light response of D149-sensitized devices is attributed to the broad light absorption, low photoelectron recombination, and good polymer penetration. It is important to note that for in situ polymerized HTM-based devices, the dye sensitizer determines not only the light absorption, but also the polymerization process, which could greatly affect the properties of the TiO2/dye/polymer interface and the polymer penetration depth in the TiO2 porous layer. Further improvement in dye sensitizers and in situ polymerization methods is likely to yield devices with further improved efficiency.