Ultraviolet-ozone-treated PEDOT:PSS as anode buffer layer for organic solar cells

Ultraviolet-ozone-treated poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS)was used as the anode buffer layer in copper phthalocyanine (CuPc)/fullerene-based solar cells. The power conversion efficiency of the cells with appropriated UV-ozone treatment was found to increase about 20% compared to the reference cell. The improved performance is attributed to the increased work function of the PEDOT:PSS layer, which improves the contact condition between PEDOT:PSS and CuPc, hence increasing the extraction efficiency of the photogenerated holes and decreasing the recombination probability of holes and electrons in the active organic layers.


Background
Organic solar cells (OSCs) have attracted significant interests because of their potential for renewable energy source, low-cost and large-scale fabrication, and compatibility with large-area and flexible substrates [1]. In the past two decades, the power conversion efficiency (PCE) of OSCs has been steadily improved, and a PCE exceeding 8% has been demonstrated by using the materials that exhibit a broad absorption with high coefficient in the solar spectrum and by developing new device configurations that provide high exciton dissociation efficiency and charge carrier collection efficiency [2,3]. The mechanism of OSCs involves the formation of excitons under illumination, the diffusion of excitons to the donor-acceptor interface, the dissociation of excitons into electrons and holes, and the collection of electrons and holes at opposite electrodes. One of the most important factors in determining the charge carrier collection efficiency is the interface property of electrode/organic layer. The buffer layer is often adopted in OSCs to improve the device performance. A lot of anode buffer layers have been demonstrated, such as MoO 3 [4], V 2 O 5 [4], NiO [5], WO 3 [6], and graphene oxide [7]. Conductive poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) film presents many advantages, such as high transparency in the visible range, high mechanical flexibility, and excellent thermal stability. These properties make it beneficial to be used as the anode buffer layer in OSCs [8]. However, phase separation of PEDOT and PSS is generally found with the insulating PSS grains atop the as-prepared PEDOT: PSS film cast from aqueous PEDOT:PSS solution, which leads to a low conductivity below 1 S/cm and an improper contact condition between PEDOT:PSS and the following organic layer [9]. Many strategies have been proposed to address such an issue, such as adding sorbitol [10][11][12], glycerol [11], N-methylpyrrolidone [12], isopropanol [12], dimethyl sulfoxide [13,14], N,N-dimethyl formamide [13], tetrahydrofuran [13], ethylene glycol [14], 2-nitroethanol [14], 1-methyl-2-pyrrolidinone [14], mannitol [15], sodium p-toluenesulfonate [16], carbon nanotube [17], and pentacene [18] into PEDOT:PSS aqueous solution and treating the as-prepared PEDOT:PSS film with solvents [14,19], thermal annealing [20], oxygen plasma [21], Ar ion sputtering [22], zwitterions [23], salt solution [24], and H 2 SO 4 [25]. However, these methods make the device construction process more complex and require careful control of the technologies to avoid the deterioration of the PEDOT: PSS film properties.
Increased work function and conductivity of PEDOT: PSS film have been demonstrated by ultraviolet light irradiation [26,27], and the treated PEDOT:PSS has been adopted as the anode buffer layer in OSCs [28,29]. Tengstedt et al. [30] have proposed that the work function of PEDOT:PSS film can be increased while maintaining reasonable conductivity by UV-ozone treatment, which is further confirmed by Helander et al. [31]. Nagata et al. [32] have clarified the respective roles of UV light irradiation and exposure to ozone gas on the PEDOT:PSS film, and they have found that the main role of UV light is to decompose the chemical bonds in the PEDOT:PSS film, resulting in a decrease of the conductivity, while the ozone and atomic oxygen are absorbed and oxidize the surface, leading to an increase of the work function. Thus, the UVozone treatment is capable of controlling the work function and conductivity of PEDOT:PSS film, hence allowing them to be adjusted to the device application. Such UVozone-treated PEDOT:PSS film has been adopted as the anode buffer layer in organic light-emitting diodes, and dramatic improvement of efficiency was observed [33,34]. However, the application of UV-ozone-treated PEDOT: PSS in OSCs has not been exploited. In this paper, UVozone-treated PEDOT:PSS film is adopted as the anode buffer layer in copper phthalocyanine (CuPc)/fullerene (C 60 )-based small molecular OSCs. The power conversion efficiency of the cell was increased by more than 20%, compared with the reference cell without UV-ozone treatment. The improvement is primarily attributed to the increased work function of the PEDOT:PSS film, which improves the contact condition between PEDOT:PSS and CuPc, hence increasing the charge carrier collection efficiency and decreasing the charge carrier recombination probability in the bulk of organic layers.

Methods
Devices were fabricated on pattered indium tin oxide (ITO)-coated glass substrates with a sheet resistance of 15 Ω/sq. The substrates were routinely cleaned, followed by UV-ozone treatment for 10 min. The structure of the OSCs used here was ITO/PEDOT:PSS/CuPc (30 nm)/C 60 (40 nm)/4,7-diphenyl-1,10-phenanthroline (8 nm)/Al (100 nm). Two types of PEDOT:PSS (Clevios P VP Al 4083 (H. C. Starck, Clevios GmbH, Leverkusen, Germany) and 483095 (Aldrich, St. Louis, MO, USA) with PEDOT/ PSS mass ratio of 1:6 and 1:1.6, respectively) were used here, and they were spin-coated onto the ITO anode with a speed of 4,000 rad/min, followed by baking in vacuum at 120°C for 1 h, which forms a PEDOT:PSS layer of about 30 nm. The PEDOT:PSS films were then treated in a UVozone environment for different times(0, 2, 4, 6, and 10 min) before loading into a high-vacuum chamber. The other organic layers and the cathode were deposited onto the substrates via thermal evaporation in the vacuum chamber at a pressure of approximately 10 −7 Torr. Deposition rates and thickness of the layers were monitored in situ using oscillating quartz monitors. The evaporation rates were kept at approximately 1 Å/s for organic layers and Al cathode. Current-voltage (I-V) characteristics of the devices were measured with a programmable Keithley 2400 power source (Keithley Instruments, Inc., Cleveland, OH, USA) both in dark and under illumination of a Xe lamp light source with an intensity of 100 mW/cm 2 . The surface characterization of PEDOT:PSS films was performed with a Bruker Multi-Mode 8 atomic force microscope (AFM; BRUKER, Ettlingen, Germany) in tapping mode. All the measurements were carried out at room temperature under ambient conditions.  Figure 1 and Table 1 that the V OC of the cells with PEDOT:PSS anode buffer layer is almost constant regardless of the UV-ozone treatment time. The V OC of the CuPc/C 60 -based OSCs is reported to be determined by the energy offset of the highest occupied CuPc molecular orbital and the lowest unoccupied C 60 molecular orbital [35]. The same V OC of the cells suggests that the UV-ozone treatment on PEDOT: PSS does not alter this energy offset. In contrast, the J SC and FF of the cells first increase and then decrease with further increase of the UV-ozone treatment time. The cell with 6-min UV-ozone-treated PEDOT:PSS shows the maximum J SC and FF of 5.897 mA/cm 2 and 0.495, respectively. Consequently, a PCE of 1.429% was obtained, which was increased by 24% compared to the reference cell.

Results and discussion
Performance of the cells under illumination with a PEDOT:PSS (Clevios P VP Al 4083) anode buffer layer   Table 2. The V OC , J SC , FF, and PCE of the reference cell are 0.493 V, 5.334 mA/cm 2 , 0.483, and 1.271%, respectively. The superior performance of the cell with Aldrich 483095 PEDOT:PSS compared with Baytron P VP Al 4083 PEDOT:PSS is attributed to the higher conductivity of the former, which may increase the extraction efficiency of the photogenerated charge carriers. Similarly, the V OC of the cells are unaffected by the UV-ozone treatment on the PEDOT:PSS layer. In contrast, the J SC and FF are increased with UV-ozone treatment time. The maximum J SC and FF are found with 4-min UV-ozone treatment time, which are 6.099 mA/cm 2 and 0.499, respectively. Correspondingly, the PCE reaches up to 1.529%, which is increased by 20% compared to the reference cell. These findings suggest that the improved performance of the CuPc/C 60 -based OSCs can be observed with UV-ozone-treated PEDOT:PSS as the anode buffer layer, and this effect is valid for the PEDOT:PSS film with different conductivities.
Performance of the cells under illumination with a PEDOT:PSS (Aldrich 483095) anode buffer layer treated with UV-ozone for various times. FF, fill factor; PCE, power conversion efficiency.
The improvement of the device performance by UVozone treatment may come from two factors: improved interface contact condition in PEDOT:PSS/CuPc and increased conductivity of the PEDOT:PSS film. Figure 3 displays the AFM image of the PEDOT:PSS (Clevios P VP Al 4083) films on ITO anode treated with UV-ozone for various times. The morphology of the PEDOT:PSS film without UV-ozone treatment is quite smooth with a root mean square (RMS) roughness of 1.06 nm, while the RMS roughnesses of the 2-, 4-, 6-, and 10-min UVozone-treated PEDOT:PSS films are 1.15, 1.10, 1.08, and 1.23 nm, respectively. This finding indicates that the UV-ozone treatment has little effect on the morphology of the PEDOT:PSS films. Thus, the PEDOT:PSS morphology change-induced alterations of the optical field distribution in the active organic layers, the CuPc molecule stacking mode, and the crystallinity of CuPc layer could be ruled out for the improved performance of the OSCs. Tengstedt et al. [30] have found that both the PEDOT and PSS moistures of the PEDOT:PSS film could be oxidized under UV-ozone treatment, which results in an increase of the work function of the PEDOT:PSS film. The effect was further confirmed by Helander et al. [31] and Nagata et al. [32]. Thus, increased work function of PEDOT:PSS film with UV-ozone treatment can be expected in this study. Such an effect improves the contact condition between PEDOT:PSS and CuPc, which increases the extraction efficiency of the photogenerated holes and decreases the recombination probability of holes and electrons in the active organic layers. As a result, both the J SC and FF and, hence, the PCE are enhanced in the optimized cells.
To exploit the UV-ozone treatment on the conductivity of the PEDOT:PSS film, the dark current of the cells with a PEDOT:PSS (Clevios P VP Al 4083) anode buffer layer treated with UV-ozone for various times was investigated, as shown in Figure 4. All five cells present   under UV-ozone treatment, which increases the extraction efficiency of the photogenerated holes and decreases the recombination probability of holes and electrons in the active organic layers. The decreased performance of the cells with prolonged UV-ozone treatment time may result from the saturation in the change of the work function, decomposition of the chemical bonds, and/or formation of ping hole defects in the PEDOT:PSS layer [32,33], which would decrease the extraction efficiency of the photogenerated charge carriers.

Conclusions
In summary, UV-ozone-treated PEDOT:PSS film was used as the anode buffer layer in CuPc/C 60 -based OSCs. The morphology of the PEDOT:PSS film is unaffected by the UV-ozone treatment. However, the PCE is found to increase about 20% compared to the reference cell without UV-ozone treatment. The improved performance is attributed to the increased work function of the PEDOT:PSS layer, which increases the extraction efficiency of the photogenerated holes and decreases the recombination probability of holes and electrons in the active organic layers. This work provides a facile and cost-effective method to improve the performance of OSCs. Besides, such a strategy may have potential applications to improve the contact condition between PEDOT:PSS and metal anode in inverted OSCs where a PEDOT:PSS/metal bilayer anode is adopted.