Improved Performance of Polymer Solar Cells Featuring
One-Dimensional PEDOT Nanorods in a Modified Buffer Layer
Yu-Kai Han,a,zMei-Ying Chang,bWen-Yao Huang,b,
*
Hsin-Yu Pan,b Ko-Shan Ho,aTar-Hwa Hsieh,a and Sin-Yu Panba
Department of Chemical and Materials Engineering, National Kaohsiung University of Applied Sciences, Kaohsiung 807, Taiwan
b
Department of Photonics, National Sun Yat-Sen University, Kaohsiung 804, Taiwan
This paper describes the use of one-dimensional 共1D兲 poly共3,4-ethylenedioxythiophene兲 nanorod 共n-PEDOT兲 and modified poly共3,4-ethylenedioxythiophene兲:poly共styrene sulfonate兲 共PEDOT:PSS兲 films as anode buffer layers in polymer photovoltaic cells based on poly共3-hexylthiophene兲:关6,6兴-phenyl-C61-butyric acid methyl ester. We employed high-speed vibrational milling to
dis-perse the n-PEDOTs into an aqueous PSS medium. Raman spectroscopy measurements revealed quinoid-dominated structures for these aqueous-soluble n-PEDOT:PSS materials. The presence of the 1D n-PEDOTs in the buffer layer improved the photovoltaic performance共AM1.5= 3.10%兲 of the polymer solar cell relative to that of the system prepared using the unmodified PEDOT:PSS
共AM1.5= 2.17%兲. This enhancement was accompanied by an increase in the values of JSC共from 5.85 to 7.62 mA/cm2兲 and the fill
factor共from 0.58 to 0.64兲. The conductivity and polarity of the n-PEDOT–modified buffer layer increased upon increasing the content of n-PEDOT, resulting in increases of the short-circuit current, open-circuit voltage, and power conversion efficiency of the polymer solar cell incorporating the 1D n-PEDOTs.
© 2011 The Electrochemical Society. 关DOI: 10.1149/1.3534201兴 All rights reserved.
Manuscript submitted June 2, 2010; revised manuscript received December 7, 2010. Published January 14, 2011.
Polymer solar cells are attracting much research interest for their application as possible sources of electrical energy because of their attractive combination of flexibility and low-cost fabrication. Many research groups have developed solar cells based on poly 共3-hexylthiophene兲 共P3HT兲 and conjugated-fullerene composites.1-6 Al-though a great progress has been made, with several reported poly-mer solar cell devices providing power conversion efficiencies 共PCEs兲 of 3–6%, there remains much room for improvement.
The injection ability of a charge transporting material depends critically on the nature of its interfaces with the electrodes. If the injection barrier at the electrodes decreases, the contact resistance will decrease accordingly. On the anode side, the injection barrier decreases when the work function of indium tin oxide 共ITO兲 is increased—for example, through plasma treatment. Poly 共3,4-ethylenedioxythiophene兲 共PEDOT兲:poly共styrene sulfonate兲 共PSS兲 layers, which have work functions higher than that of ITO, have been used as anode buffer layers to increase the performance of several organic solar cells.7,8The conductivity of PEDOT:PSS can be increased through the addition of small amounts of glycerol or sorbitol.9,10The surface sheet resistance of a PEDOT:PSS film de-creases when it is fabricated with the addition of glycerol; however, its optical transparency remains constant.11These highly transparent and conductive polymers are ideal candidates for use as anode buffer layers in display devices such as organic light-emitting diodes 共OLEDs兲 or organic solar cells. Kim et al.12
improved the device performance, including the external electroluminescence quantum efficiency and the current–voltage共I-V兲 luminance characteristics, of an OLED fabricated using a highly conductive and transparent PEDOT:PSS doped with glycerol as an anode buffer layer. Fung et al.13fabricated a glycerol-modified PEDOT:PSS anode buffer layer in a polymer light-emitting device 共PLED兲 using poly共9,9-dioctylfluorene兲 共F8兲 as the emitter; this glycerol-modified device exhibited a much larger current density relative to that of the un-modified device.
PEDOT:PSS is one of the best hole-conducting buffers because its ionization potential is close to the work function of ITO, whereas its electron affinity 共ca. 2.2 eV兲 is sufficiently low to block electrons.14For this reason, the electronic properties of single-wall nanotubes 共SWNTs兲 dispersed in a PEDOT:PSS buffer layer are interesting because pure SWNTs are p-type semiconductors.15,16 These findings encouraged us to study the performance of
P3HT:关6,6兴-phenyl-C61-butyric acid methyl ester 共PCBM兲-based polymer solar cells by dispersing PEDOT nanorod共n-PEDOT兲 ma-terials into PEDOT:PSS buffer layers. Nanostructures of the con-ducting polymers polypyrrole 共PPy兲 and polyaniline 共PANi兲 have been prepared previously.17-20For this study, we chose to use nano-structured PEDOT21,22 because it possesses several advantageous features: environmental stability, a low redox potential, and a high optical transparency in the doped conducting state.23
When used in photovoltaic共PV兲 devices, PCBM acts as an elec-tron acceptor; it has been intermixed at the nanometer scale with an organic semiconducting polymer共P3HT, electron donor兲 to obtain a high charge separation yields. Following charge transfer, both elec-trons and holes must be transported to the electrodes before recom-bination can occur. In some cases, however, charge transportation is limited by inefficient hopping along poorly formed conduction path-ways. Thus, routes that enhance charge transport are desirable to improve the performance of photovoltaic cells. One-dimensional 共1D兲 n-PEDOTs are favorable materials that offer direct pathways for charge transport;24-27they can be obtained through chemical syn-thesis using a hexane/water reverse microemulsion system consist-ing of sodium bis共2-ethylhexyl兲 sulfosuccinate 共AOT兲 cylindrical micelles as the template and FeCl3 as the oxidant.22 The as-synthesized n-PEDOTs are readily dispersed in common organic solvents and films cast on a variety of substrates, but they do not disperse well in aqueous media. Therefore, to disperse the n-PEDOTs into an aqueous medium, we employed the mecha-nochemical technique of high-speed vibrational milling 共HSVM兲, which had been used previously to prepare organic-soluble, well-ordered 1D SWNTs through the formation of supramolecular complexes.28The approach we adapted in this study is superior to that of conventional sonication for preparing water-dispersed n-PEDOT:PSS blends.
Furthermore, we fabricated polymer solar cells, with and without n-PEDOT modification of the PEDOT:PSS anode buffer layer, in the configuration ITO/PEDOT:PSS/P3HT:PCBM/Al. We constructed the various modified anode buffer layers by adding different amounts of 1D n-PEDOT into a commercialized PEDOT:PSS 共Bay-tron AI 4083兲 buffer layer 共hereafter referred to as mn-PEDOT:PSS兲.
Experimental
The film thickness was measured using a Dektak 6M stylus pro-filometer. The optical spectra were measured using a UV/visible spectrophotometer; the work functions of the films were measured using a Riken Keiki AC-2 surface-analyzer photoelectron
spectrom-*Electrochemical Society Active Member.
z
E-mail: [email protected]
Journal of The Electrochemical Society, 158共3兲 K88-K93 共2011兲 0013-4651/2011/158共3兲/K88/6/$28.00 © The Electrochemical Society K88
resistance reveals that a higher current will flow through the device. TableIIlists the values of RSthat we obtained from the I-V curves of the devices; a significant decrease in RSoccurred when the device incorporated an mn-PEDOT:PSS layer modified with introduced n-PEDOTs. The series resistance, which can be expressed as the sum of the bulk and interfacial resistances, can reflect ohmic loss in solar cells; ohmic loss includes the resistance of the organic/ electrode contacts, the photoactive layer, the electrodes, and the parasitic probe resistance.50It is likely that the two interfaces fea-turing the introduced n-PEDOTs共i.e., the ITO/mn-PEDOT:PSS and mn-PEDOT:PSS/active layer interfaces兲 provided much-lower-magnitude series resistances relative to that of the PEDOT:PSS/ active layer contact. The introduction of the 1D n-PEDOTs de-creased the series resistance and inde-creased the conductivity of the buffer layer in the device. The fill factor共FF兲 also increased, from 0.58 to 0.64, upon increasing the conductivity of the buffer layer.
Figure5cpresents plots of the open-circuit voltages共VOC兲 and FFs with respect to the n-PEDOTs content. The value of VOCof the cells remained relatively constant, in the range ca. 0.62–0.64 V, suggesting an equal energy difference between the HOMO of the donor and the lowest unoccupied molecular orbital共LUMO兲 of the acceptor in the various systems.
Figure5d displays the values of JSCand the efficiency charac-teristics of the devices incorporating effective n-PEDOT contents ranging from 0 to 3 wt %. The values of JSCare highly dependent on the conductivity of the buffer layer, increasing from 5.85 to 7.62 mA/cm2 when the buffer layer conductivity increased from 8.9⫻ 10−5to 8.5⫻ 10−4S/cm; in contrast, the value of VOC remained relatively constant共ca. 0.64 V兲.
Incorporating a suitable amount of n-PEODTs into the buffer layer increased the film’s conductivity from 8.9⫻ 10−5to 8.5 ⫻ 10−4S/cm. The increased carrier transport enhanced the circuit current density of the devices; likewise, the increased short-circuit current density and FF enhanced the PCE. As a result, the maximum PCE of the cell 共3.10%兲 occurred when the n-PEDOT
concentration was 3 wt %; the corresponding values of JSC, VOC, and FF were 7.62 mA/cm2, 0.64V, and 0.65, respectively. The PCE increased from ca. 2.17 to ca. 3.10% after incorporating the 1D PEDOT nanorods into the buffer layer, thereby increasing the PV cell characteristics.
Scheme2depicts our proposed mechanism for carrier transport through the modified buffer layer: 共i兲 The BHJ photoactive layer generates excitons after accepting photons from the sunlight illumi-nator.共ii兲 The internal electric field separates the excitons at the BHJ interfaces and causes them to be swept to their respective electrodes. 共iii兲 The photoinduced holes hop into the nanoscale interface be-tween the mn-PEDOT:PSS and the active layer and are swept into the modified buffer layer.共iv兲 The partial photogenerated holes cross the buffer layer via the high-conductivity n-PEDOT pathways sur-rounding the PDEOT:PSS conductive amorphous phase, decreasing the degrees of interference from impurities and recombination with electrons. 共v兲 The surviving holes and electrons pass through the energy barriers of the ITO and Al films to generate the electric current.
Conclusion
We have fabricated a polymer solar cell incorporating n-PEDOT–modified mn-PEDOT:PSS as the anode buffer layer. Relative to the polymer solar cell prepared using unmodified PE-DOT:PSS共AM1.5= 2.17%兲, the presence of the 1D n-PEDOTs im-proved the PCE共AM1.5= 3.10%兲. This enhancement was accom-panied by an increase in the values of JSC 共from 5.85 to 7.62 mA/cm2兲 and the FF 共from 0.58 to 0.64兲. This superior device performance arose from the increased conductivity and po-larity of the n-PEDOT–modified buffer layer, which provided more-efficient pathways for holes than those in the more frequently used PEDOT:PSS buffer layers. As a result, the hole collection barrier height was reduced, and the photovoltaic response was improved.
Acknowledgment
We thank the National Science Council of Taiwan 共grant no. NSC-95-2113-M-151-001-MY3兲 for financial support.
National Kaohsiung University of Applied Sciences assisted in meeting the publication costs of this article.
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Sample Composition HOMO 共eV兲 Conductivity a 共S/cm兲 共⍀ cmRS 2兲 V共V兲OC 共mA/cmJSC 2兲 FF 共%兲AM1.5 1 PEDOT:PSS 5.15 8.9⫻ 10−5 29.4 0.64 5.85 0.58 2.17 2 0.5 wt % 5.13 1.3⫻ 10−4 27.2 0.62 6.23 0.63 2.44 3 1 wt % 5.10 4.9⫻ 10−4 18.0 0.64 6.71 0.62 2.67 4 2 wt % 5.08 5.6⫻ 10−4 14.5 0.63 7.41 0.63 2.94 5 3 wt % 5.08 8.5⫻ 10−4 11.8 0.64 7.62 0.64 3.12 6 n-PEDOT 4.90 2–3 — — — — —
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