Ultraviolet-ozone-treated PEDOT:PSS as anode buffer layer for organic solar cells
© Su et al.; licensee Springer. 2012
Received: 2 July 2012
Accepted: 25 July 2012
Published: 17 August 2012
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.
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 . 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 MoO3, V2O5, NiO , WO3, and graphene oxide . 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 . 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 . Many strategies have been proposed to address such an issue, such as adding sorbitol [10–12], glycerol , N-methylpyrrolidone , isopropanol , dimethyl sulfoxide [13, 14], N,N-dimethyl formamide , tetrahydrofuran , ethylene glycol , 2-nitroethanol , 1-methyl-2-pyrrolidinone , mannitol , sodium p-toluenesulfonate , carbon nanotube , and pentacene  into PEDOT:PSS aqueous solution and treating the as-prepared PEDOT:PSS film with solvents [14, 19], thermal annealing , oxygen plasma , Ar ion sputtering , zwitterions , salt solution , and H2SO4. 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.  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. . Nagata et al.  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 UV-ozone 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 UV-ozone-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, UV-ozone-treated PEDOT:PSS film is adopted as the anode buffer layer in copper phthalocyanine (CuPc)/fullerene (C60)-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.
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)/C60(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 UV-ozone 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/cm2. The surface characterization of PEDOT:PSS films was performed with a Bruker MultiMode 8 atomic force microscope (AFM; BRUKER, Ettlingen, Germany) in tapping mode. All the measurements were carried out at room temperature under ambient conditions.
Results and discussion
Performance of cells with Clevios P VP Al 4083 PEDOT:PSS anode buffer layer
UV treatment time (min)
Performance of the cells under illumination with a PEDOT:PSS (Clevios P VP Al 4083) anode buffer layer treated with UV-ozone for various times. FF, fill factor; PCE, power conversion efficiency.
Performance of the cells with Aldrich 483095PEDOT:PSS anode buffer layer It is appropriate
UV treatment time (min)
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.
In summary, UV-ozone-treated PEDOT:PSS film was used as the anode buffer layer in CuPc/C60-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.
This work was supported by the National Natural Science Foundation of China grant numbers 11004187, 61076047, 61107082, and 60877027.
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