CHEMSUSCHEM FULL PAPERS DOI: 10.1002/cssc.201200950
Polymer Bulk Heterojunction Solar Cells with PEDOT:PSS Bilayer Structure as Hole Extraction Layer Wanjung Kim,[a, b] Namhun Kim,[a] Jung Kyu Kim,[b] Insun Park,[c] Yeong Suk Choi,[c] Dong Hwan Wang,[b] Heeyeop Chae,*[a, b] and Jong Hyeok Park*[a, b] A high current density obtained in a limited, nanometer-thick region is important for high efficiency polymer solar cells (PSCs). The conversion of incident photons to charge carriers only occurs in confined active layers; therefore, charge-carrier extraction from the active layer within the device by using solar light has an important impact on the current density and the related to power conversion efficiency. In this study, we observed a surprising result, that is, extracting the charge carrier generated in the active layer of a PSC device, with a thickness-controlled PEDOT:PSS bilayer that acted as a hole extrac-
tion layer (HEL), yielded a dramatically improved power conversion efficiency in two different model systems (P3HT:PC60BM and PCDTBT:PC70BM). To understand this phenomenon, we conducted optical strength simulation, photocurrent–voltage measurements, incident photon to charge carrier efficiency measurements, ultraviolet photoelectron spectroscopy, and AFM studies. The results revealed that approximately 60 nm was the optimum PEDOT:PSS bilayer HEL thickness in PSCs for producing the maximum power conversion efficiency.
Introduction Polymer solar cells (PSCs) have been studied by many researchers with the aim to increase the power conversion efficiency. In particular, PSCs with bulk heterojunction (BHJ)-type active layers, which contain a blended film of p-conjugated donor material and fullerene derivatives, have been widely studied and reported by many research groups. In recent years, the power conversion efficiencies of PSCs based on BHJs have improved considerably, mainly as a result of the ability to control the nanoscale morphology of BHJs and the introduction of new tailor-made low-band-gap polymers. Recently, the power conversion efficiency of PSCs has reached around 9.2 % in the case of a single cell[1] and 8.6 % for a tandem cell.[2] When newly synthetized photoactive materials are optimized, with the help of interface engineering, to enable efficient electron and hole extraction, even higher PSC power conversion efficiency can be obtained. [a] W. Kim,+ N. Kim,+ Prof. H. Chae, Prof. J. H. Park School of Chemical Engineering Sungkyunkwan University Suwon 440-746 (Republic of Korea) E-mail:
[email protected] [email protected] [b] W. Kim,+ J. K. Kim, Dr. D. H. Wang, Prof. H. Chae, Prof. J. H. Park SKKU Advanced Institute of Nanotechnology (SAINT) Sungkyunkwan University Suwon 440-746 (Republic of Korea) [c] Dr. I. Park, Dr. Y. S. Choi Samsung Advanced Institute of Technology Samsung Electronics Yongin 446-712 (Republic of Korea) [+] These authors are equally contributed to this work. Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cssc.201200950.
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Many materials that could function as hole and electron extraction layers have been studied to increase the power conversion efficiency. However, PEDOT:PSS [poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)] is the conducting polymer that remains to be widely used as the hole extraction layer (HEL) in PSCs. Devices that utilize PEDOT:PSS as the HEL show superior performance in many ways. Firstly, PEDOT:PSS has a well-matched work function for both the highest occupied molecular orbital (HOMO) level of the donor polymer and the work function of tin-doped indium oxide (ITO) glass; therefore, PEDOT:PSS can extract holes from the active layer efficiently. The importance of hole extraction is generally clearer when the donor material in the photoactive layer has a HOMO level that is lower in energy than the work function of ITO. In addition, PEDOT:PSS is able to induce a smooth surface that can help active layers to achieve uniform deposition on the ITO surface.[3] This can be achieved without interfering with the light absorption of the active layer because of their high optical transparency in visible light. Despite its importance in improving device efficiency, the PEDOT:PSS-based HEL has not yet been sufficiently studied. Many reports on the optimization of device performance lack detailed investigations of the mostoften-used polymeric PEDOT:PSS. Therefore, more controlled and optimized hole extraction designs of the PEDOT:PSS bilayer in PSCs are extremely important. Herein, the HEL was controlled in a systematic manner. First, we studied PSCs prepared from a PEDOT:PSS bilayer. Recently, many types of commercialized PEDOT:PSS polymer products have been produced by the Heraeus company, which have different characteristics such as different conductivities and PSS doping concentrations.[4] According to a data sheet of PEDOT:PSS from the Heraeus company, PH500 and PH1000 ChemSusChem 2013, 6, 1070 – 1075
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www.chemsuschem.org ces were similar. To understand this unique phenomenon, we conducted optical strength simulations, photocurrent–voltage (J–V) measurements, incident photon to charge carrier efficiency (IPCE) measurements, ultraviolet photoelectron spectroscopy (UPS), as well as AFM.
Results and Discussion By using a PEDOT:PSS bilayer, we fabricated PSCs and measured the corresponding J–V curves under AM 1.5 G solar irradiation at an intensity of 100 mW cm 2, as shown in Figure 2 a. The performances of these devices are summarized in Table 1. First, we optimized the power conversion efficiency of single PEDOT:PSS-based PSCs as a function of PEDOT:PSS thickness. The optimum thickness of Figure 1. Comparison of PSC devices based on a PEDOT:PSS single-layer structure and a PEDOT:PSS bilayer strucPH500 and P VPAI4083 was 40 ture. a) Schematic of PSC devices and b) corresponding energy level diagrams for a conventional device (left): and 25 nm, respectively. To ITO/PEDOT:PSS P VPAI4083/P3HT:PC60BM/TiOx/Al, and a PEDOT:PSS bilayer device (right): ITO/PEDOT:PSS PH500/ study the contribution of the PEDOT:PSS P VPAI4083/P3HT:PC60BM/TiOx/Al. PEDOT:PSS bilayer of the active layer in hole extraction, a layer of PH500 with a thickness of 40 nm was spin coated on top of have higher conductivities than P VP AI4083; these materials an ITO glass substrate. After this, the PEDOT:PSS film was anare all widely used in PSCs by many research groups. Figure 1 a nealed at 115 8C for 15 min on a hotplate, followed by a coating shows the schematic of a PSC containing a PEDOT:PSS bilayer of P VPAI4083 with a thickness of approximately 25 nm. The structure. In this study, we selected two types of PEDOT:PSS polymers, fabricated PEDOT:PSS HEL bilayer had an overall thickness of 60 nm. After the ITO/PEDOT:PSS bilayer was thermally anPH500 and P VP AI4083. Although PH500 and P VP AI4083 are nealed, the P3HT:PC60BM [poly(3-hexylthiophene):[6,6]-phenylboth mainly composed of PEDOT and PSS, they have different work functions. Some studies have reported that a PEDOT:PSS C61-butyric acid methyl ester] photoactive layer (see the Experlayer that is composed of higher surface-enriched PSS has imental Section for details) was spin coated onto the a higher work function. In this case, P VP AI4083 contains more PEDOT:PSS bilayer. As shown in the J–V curves (Figure 2 a), the PSS in PEDOT:PSS than PH500; therefore, P VP AI4083 has PEDOT:PSS-based device containing the bilayer structure exa higher work function than PH500.[5–7] One advantage of the hibited an open-circuit voltage (Voc) of 0.62 V, a short-circuit HEL bilayer structure in PSCs compared to the conventionally current density (Jsc) of 11.33 mA cm 2, a fill factor (FF) of structured device is attributed to the cascade-type hole extrac57.85 %, and a power conversion efficiency (h) of 4.09 %, tion that occurs in the bilayer structure, see Figure 1 b. This rewhereas Voc = 0.59 V, Jsc = 9.99 mA cm 2, FF = 54.31 %, and h = sults in the bilayer HEL exhibiting improved performance, with 3.22 % for the PEDOT:PSS PH500 single layer, and Voc = 0.61 V, regard to charge-carrier transport, than the general PEDOT:PSS Jsc = 8.37 mA cm 2, FF = 60.07 %, and h = 3.09 % for the single-layered device. PEDOT:PSS P VPAI4083 single layer. From these results, we We optimized the device structure by controlling the thickdemonstrated that PSCs based on the PEDOT:PSS bilayer strucness of the PEDOT:PSS bilayer. Generally, there was no signifiture exhibited higher Jsc (11.33 mA cm 2) than both the PH500cant difference in the power conversion efficiency of the cononly device (9.99 mA cm 2) and P VPAI4083-only device ventional structure throughout the PEDOT:PSS layer that was (8.37 mA cm 2). To analyze the improvement of Jsc for the several tens of nanometers thick. However, we were able to PEDOT:PSS bilayer device, we also conducted an IPCE measureobtain the highest power conversion efficiencies for devices ment study on the P VP AI4083-only device, the PH500-only with a PEDOT:PSS bilayer rather than single-layered PEDOT:PSS device, and the PEDOT:PSS bilayer device. As shown in FigHEL, even though the thickness and transmittance of the deviure 2 b, the external quantum efficiency (EQE) of the 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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of PH500 and P VPAI4083 generates a “smooth” pathway for hole extraction to occur from the active layer to the ITO.[11–13] This effect resulted in the higher Jsc and EQE values that were obtained for the PEDOT:PSS bilayer-based PSC compared to the PH500-only device. Recently, studies on the manipulation of the optical electric field in PSCs have been reported as a way to enhance the photocurrent, for example by using an optical spacer. [14–17] Generally, a large electric field amplitude in the active layer generated an increased photocurrent in the PSCs. Conversely, a low electric field amplitude could generate less photocurrent.[18] Therefore, many researchers have aimed to maximize the optical electric field in Figure 2. Characterization of the PSC devices comprising a PEDOT:PSS bilayer and P3HT:PCBM compared to devices based on a PEDOT:PSS single layer. a) J–V characteristics of the PSC devices under 1 sun (100 mW cm 2) illumithe active layer of PSCs by connation, b) the EQE of the devices under 1 sun (100 mW cm 2) illumination, c) J–V characteristics of the PSC devices trolling the properties of the 2 with various thicknesses of the PEDOT:PSS bilayer under 1 sun (100 mW cm ) illumination, and d) comparison of electron extraction layer in Jsc and the FF in the PEDOT:PSS bilayer devices at controlled thicknesses. PSCs. We assumed that it was possible to manipulate the optical electric field in the PEDOT:PSS bilayer PSC because this HEL Table 1. J–V characteristics of devices composed of a PEDOT:PSS bilayer compared to devices with a PEDOT:PSS single layer. was composed of two different layers, and hole extraction was still controlled. P VPAI4083 and PH500 consist of the same Jsc [%] FF [%] h [%] Sample Voc [%] components (PEDOT and PSS); however, there are some differP VPAI4083 only 0.61 8.37 60.07 3.09 ences, including the ratio of PEDOT and PSS, which result in PH500 only 0.59 9.99 54.31 3.22 different refractive index profiles and absorption constants for 60 nm bilayer 0.62 11.33 57.85 4.09 the two polymers.[19] This means that the electric field strength generated by illuminated solar light was different in the PEDOT:PSS bilayer devices and the thin PEDOT:PSS single-layPEDOT:PSS bilayer device was enhanced compared to the ered devices. To confirm this hypothesis, we simulated the opP VPAI4083- and PH500-only devices. The EQE of the PEtical electric field distribution at different wavelengths for the DOT:PSS bilayer device was about 13 % higher than the PEDOT:PSS bilayer and PEDOT:PSS single-layered devices; P VPAI4083-only device over the maximum spectral range. Figure 4 shows the results of the simulation.[20] [8, 9] According to previous research, a device containing more Optical parameters of materials (refractive index and absorpconductive PEDOT:PSS showed higher Jsc and h. PH500 has tion constants) for simulation were obtained from previous a higher conductivity by about 105 times compared with studies.[21–24] Through this simulation we discovered an impor[10] P VPAI4083. tant phenomena, that is, compared with the PEDOT:PSS singleTherefore, it was reasonable that the PH500layered device, the device containing the PH500 bilayer enonly and the PEDOT:PSS bilayer devices exhibited higher Jsc hanced the electric field amplitude and absorbed more energy and h values compared to the P VPAI4083-only device. In addiin the active layer of the PSC. This was achieved even though tion, the advantages of the PEDOT:PSS bilayer structure comthe transmittance and thicknesses of the PEDOT:PSS single pared to the PH500-only device, in terms of Jsc, could be exlayer and bilayer were similar, as observed in Figure 4 a–c. plained by the efficient hole extraction of the PEDOT:PSS bilayConsistent with the optical electric field simulation data and J– er. As mentioned in the introduction, P VPAI4083 has a higher V measurements, the Jsc values of the PEDOT:PSS bilayer deviwork function than PH500 because of the PSS doping concentration. As shown in Figure 3 and Table 2, we confirmed the ces were higher than the PEDOT:PSS single-layered devices; difference in the work function between P VP AI4083 and this could be attributed to effects caused by the difference in PH500 through UPS analysis. The difference in work function work function between PH500 and P VPAI4083 as well as by 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Figure 3. UPS spectroscopy data of the PEDOT:PSS layer on ITO versus a Au reference electrode (Fermi level: 0.5 eV). a) ITO/PEDOT:PSS P VPAI4083, b) ITO/PEDOT:PSS PH500, and C) ITO/PEDOT:PSS PH500/PEDOT:PSS P VPAI4083.
Table 2. The work function of the PEDOT:PSS layer on ITO glass measured by using UPS spectroscopy. Sample
Work function [eV]
AI4083 only PH500 only 60 nm bilayer
5.16 5.04 5.07
an enhanced absorbed energy in the active layer from the increased electric field amplitude. Furthermore, the FF as well as Jsc of PEDOT:PSS bilayer devices increased. This was demonstrated through the J–V measurements of the devices; the FF of the bilayer-based PEDOT:PSS device (57.85 %) was higher than the FF of the PH500-only device (54.31 %). In general, FF is strongly dependent on series resistance and shunt resistance. Because PH500 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Figure 4. Simulated distribution of the normalized optical electric field for light of different wavelengths (at 500 nm and 600 nm) inside the PSC devices based on the structure ITO (150 nm)/PEDOT:PSS PH500/PEDOT:PSS P VPAI4083/P3HT:PC60BM (80 nm)/TiOx (3 nm)/Al (100 nm) containing a) PEDOT:PSS single-layered P VPAI4083, b) PEDOT:PSS single-layered PH500, and c) PEDOT:PSS 60 nm bilayer.
has a much higher electrical conductivity, a lower series resistance was observed in both the PH500-based PSC and the bilayer-based PEDOT:PSS PSCs. However, a higher conductivity of the PEDOT:PSS layer could induce random leakage of current, resulting in a low FF. Another possible explanation for the poor FF in the PH500-based PSC is based on morphology characteristics. Generally, roughness measured by using AFM represents the root-mean-square (RMS) value of peak heights. The PEDOT:PSS bilayer film exhibited a RMS peak height of 2.2983 nm. This value was larger than the P VPAI4083 singlelayered film (1.7492 nm) but smaller than the PH500 single-layered film (2.9889 nm), as detailed in Figure S1 and Table S1 (see the Supporting Information). According to a previous study, a device consisting of a smoother PEDOT:PSS layer has a higher FF than one with a rough PEDOT:PSS layer.[25] In the case of bilayer-based PEDOT:PSS PSCs, blocking direct contact between the ITO substrate and the highly conductive PEDOT:PSS could have suppressed the leakage current. ChemSusChem 2013, 6, 1070 – 1075
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Because P VPAI4083 was coated over the PH500, it resulted in a relatively smooth surface morphology, which could minimize the negative effect of the PH500. The FF of the PEDOT:PSS bilayer device was increased to about 3.5 % compared to the FF of the PH500 device, but was decreased to about 2 % against the FF of the P VPAI4083-only device. To optimize the bilayer-based PEDOT:PSS HEL system by controlling the thickness of the PEDOT:PSS bilayers, we changed the spin-coating speed; the thickness of various PEDOT:PSS bilayers is summarized in Table S2. According to data, we observed that the upper PEDOT:PSS layer sparingly dissolved the lower PEDOT:PSS layer. Therefore, only a small amount of erosion occurred when the upper PEDOT:PSS layer was spin coated on the lower PEDOT:PSS layer. Under these conditions, we fabricated PSCs with different PEDOT:PSS bilayer properties. The J–V curves of these devices measured under AM 1.5 G solar irradiation at an intensity of 100 mW cm 2 are shown in Figure 2 c. The results of the device performance tests are summarized in Table 3. From Figure 2 c and Table 3, it is clear that the devices with a PEDOT:PSS bilayer with a spin-coated layer thickness from 50 to 200 nm showed an enhancement in FF from 54.49 % to 62.20 % and in Voc from 0.62 to 0.64 V slightly.
Table 3. J–V characteristics of devices containing a PEDOT:PSS bilayer with different thicknesses. Bilayer thickness
Voc [%]
Jsc [%]
FF [%]
h [%]
200 nm 160 nm 60 nm 50 nm
0.64 0.64 0.62 0.62
8.80 9.67 11.33 11.95
62.20 58.97 57.85 54.49
3.53 3.64 4.09 4.06
To further analyze this phenomenon, we measured the roughness of PEDOT:PSS by analyzing layers of different thicknesses on ITO glass through AFM measurements, as shown in Figure S2 and Table S3. The RMS of the PEDOT:PSS bilayer peak height decreased from 2.3296 to 1.6624 nm, although the thickness of the PEDOT:PSS bilayer increased from 50 to 200 nm. As mentioned previously, the smoother the PEDOT:PSS layer in the PSC, the higher the FF of PSC.[25] Because of this effect, the FF of the PEDOT:PSS bilayer-based devices increased despite the increase in the PEDOT:PSS layer thickness. However, the h of the PEDOT:PSS bilayer devices, with controlled PEDOT:PSS thickness, did not follow the observed linear variance of the FF. As the thickness of PEDOT:PSS layer was increased, the FF of the devices increased, whereas Jsc decreased from 11.95 to 8.80 mA cm 2. From the decreased transmittance of the thickest PEDOT:PSS bilayer (Figure S3 and Table S4), we could demonstrate that the Jsc of the devices containing a PEDOT:PSS bilayer decreased according to the thickness of the bilayers. Therefore, we were able to obtain an optimized PEDOT:PSS bilayer thickness to maximize the h of the devices. PCDTBT (poly-[N-9’’-hepta-decanyl]-2,7-carbazole-alt-5,5(4’,7’-di-2-thienyl-2’,1’,3’-benzothiadiazole) is a high-Voc donor material that is well-known for its deep HOMO level, which can 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
be optimized by using PEDOT:PSS (Baytron PH, H.C. Starck) with a layer thickness of approximately 35 nm.[26] Interestingly, the h value for PCDTBT:PC70BM BHJ PSCs (see the Experimental Section for details) was also increased when a PEDOT:PSS bilayer was implemented as the HEL. The h of the device with a single PEDOT:PSS layer showed 6.23 % (Voc = 0.87, Jsc = 10.47, FF = 69 %), whereas the bilayer-based PEDOT:PSS device showed an improved value of 7.07 % (Voc = 0.86, Jsc = 12.27 = FF: 67 %), which could be attributed to efficient hole extraction in the bilayer devices, as shown in Figure 5 and Table 4.
Figure 5. J–V characteristics of PSCs that utilized of the PCDTBT:PC70BM devices. A comparison of devices containing the PEDOT:PSS bilayer and the PEDOT:PSS single layer under 1 sun (100 mW cm 2) illumination.
Table 4. J–V characteristics of devices containing a PEDOT:PSS bilayer compared to devices with a PEDOT:PSS single layer, based on PCDTBT:PC70BM. Sample
Voc [%]
Jsc [%]
FF [%]
h [%]
single layer bilayer
0.87 0.86
10.47 12.27
69.0 67.0
6.23 7.07
Conclusions In this work, we reported unexpected and greatly enhanced h values for PSCs. This was accomplished by using a PEDOT:PSS bilayer as the HEL in P3HT:PC60BM and PCDTBT:PC70BM systems. The h values of the devices composed of the PEDOT:PSS bilayer structure was enhanced from 3.09 % to 4.09 % in P3HT:PC60BM and from 6.23 % to 7.07 % in PCDTBT:PC70BM, under optimized conditions and with a bilayer thickness of approximately 60 nm, compared to the devices that utilized a P VPAI4083 single layer even though there was no significant difference in the layer thickness or the transmittance.
Experimental Section Chemical preparation: To prepare the active solution (P3HT:PC60BM), P3HT (Rieke metals) and PC60BM (Nano-C) were dissolved in chlorobenzene at a ratio of P3HT/PC60BM = 1:0.6 by weight at 25 8C for 24 h in an argon-filled glove box. Also, a mixture ChemSusChem 2013, 6, 1070 – 1075
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CHEMSUSCHEM FULL PAPERS containing PCDTBT:PC70BM was prepared at a ratio of PCDTBT/ PC70BM = 1:4 by weight and dissolved in a mixed solvent (dichlorobenzene/chlorobenzene = 3:1 by volume ratio). Before making the device, PEDOT:PSS was mixed with PH500 and Clevios P VPAI4083 (Heraeus). Dimethyl sulfoxide was added to PEDOT:PSS PH500 at a ratio of PH500/dimethyl sulfoxide = 9:1 (by volume). PEDOT:PSS P VPAI4083 was diluted with methanol at a ratio of 1:1 (by volume). Fabrication of organic photovoltaic devices: ITO glass was cleaned in the sequence acetone, 2-propnaol, acetone, and 2-propanol in a sonic bath; each step was performed for 10 min. Following this, UV/ozone treatment (Altech LTS) was performed for 10 min on the ITO glass. After thorough cleaning, PEDOT:PSS PH500 solution was spin coated on the ITO glass at various speeds. The samples were then baked at 115 8C for 15 min on a hotplate. After baking the samples, PEDOT:PSS P VPAI4083 solution was spin coated over the PH500 film at the same speed at which the PH500 layer was spin coated. After this, the samples were baked again at 115 8C for 15 min on a hotplate. The PEDOT:PSS layers of the reference devices (P VPAI4083- and PH500-only devices) were spin coated to achieve an optimum thickness and baked. On the top of the PEDOT:PSS layer, we spin coated the P3HT:PC60BM layer. The thickness of the P3HT:PC60BM layer for all devices was the same at 80 nm. For the device with PCDTBT:PC70BM, we spin coated the PCDTBT:PC70BM layer over the PEDOT:PSS bilayer, which was optimized in the thickness variance experiment with P3HT:PC60BM. The active layer was subsequently dried. After spin-coating, the active polymeric TiOx layer was spin coated onto the active layer and baked at 80 8C.[27] A 100 nm aluminum cathode was then deposited under 2 10 6 Torr vacuum in a thermal evaporator. Finally, for the devices with P3HT:PC60BM, a post-annealing step was conducted on a hot plate at 150 8C for 30 min in a glove box. Film characterization: The thickness of the layers in the organic photovoltaic devices was measured by using an Alpha-step analyzer (KLA-Tencor, Alpha-Step IQ Surface Profiler). The morphology and roughness were investigated by using AFM (Veeco diINNOVA 840-012-711, tapping mode). UV/Vis absorption spectra were obtained by using a UV/Vis spectrophotometer (UV-2401 PC, Shimadzu). Device measurement: The areas of the device active layer were confined by using a video microscope (Sometech, SV-35), and an aperture with an area of 9.00 mm2 was used on top of the cell to confirm the accuracy of the device area. The J–V characteristics of the PSCs were measured by using a Keithley model 2400 source measuring unit under AM 1.5 G white light (100 mW cm 2) illumination through a solar simulator (Oriel Sol 3 A, class AAA), based on a filtered 450W Xe lamp light source adjusted with a Si reference cell (VLSI standards, Oriel P/N 91150 V), for 1 sunlight intensity of 100 mW cm 2. The IPCE was measured as a function of wavelength ranging from 300 to 900 nm (PV Measurement, Inc.).
Acknowledgements W.K. and N.K. contributed equally to this work. This work was supported by an NRF grant funded by the Korea Ministry of Education, Science and Technology (MEST) (2011-0030254, 2011-
2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemsuschem.org 0012279, 2011-0030254), the NCRC program (2011-0006268), and Future-based Technology Development program (2010-0029321). Keywords: electrochemistry · hole extraction layer · photochemistry · polymer solar cells · power conversion efficiency
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