ApplPhysLett_98_243308

Performance improvement of polymer solar cells by using a solvent-treated poly …3,4-ethylenedioxythiophene …:poly …styrenesulfonate …buffer layer

Bo Peng,1,2Xia Guo,1Chaohua Cui,1Yingping Zou,2Chunyue Pan,2and Yongfang Li 1,a ?

1

Beijing National Laboratory for Molecular Sciences,CAS Key Laboratory of Organic Solids,Institute of Chemistry,Chinese Academy of Sciences,Beijing 100190,People’s Republic of China 2

College of Chemistry and Chemical Engineering,Central South University,Changsha 410083,People’s Republic of China

?Received 17April 2011;accepted 20May 2011;published online 16June 2011?

Photovoltaic performance of the polymer solar cell ?PSC ?based on poly ?3-hexylthiophene ??P3HT ?as donor and ?6,6?-phenyl-C 61-butyric acid methyl ester ?PCBM ?as acceptor was improved by using the poly ?3,4-ethylenedioxythiophene ?:poly ?styrenesulfonate ??PEDOT:PSS ?modi?cation layer treated by ethanol or 2-propanol.Power conversion ef?ciency ?PCE ?of the PSC based on P3HT:PCBM ?1:1,w/w ?with the 2-propanol-treated PEDOT:PSS modi?cation layer reached 4.74%,in comparison with a PCE of 3.39%for the PSC with the PEDOT:PSS layer without the organic solvent treatment.The enhanced performance of the PSCs is attributed to higher conductivity and optimized surface morphology of the PEDOT:PSS layers treated by the organic solvent.?2011American Institute of Physics .?doi:10.1063/1.3600665?

Bulk-heterojunction ?BHJ ?polymer solar cells ?PSCs ?have attracted considerable attention recently due to their advantages of easy fabrication,low cost,light weight,and ?exibility in large-area applications.1–3The blend system consisting of regioregular poly ?3-hexylthiophene ??P3HT ?and fullerene derivative ?6,6?-phenyl-C 61-butyric acid methyl ester ?PCBM ?has shown power conversion ef?ciencies of about 3%–5%.4,5To improve the device ef?ciency,methods like thermal annealing,5solvent annealing ?or slow-growth ?,4vapor annealing,6and morphology control using mixed solvent mixtures 7or additives 8in the solutions have been studied in P3HT:PCBM-basd PSCs.Additives to poly ?3,4-ethylenedioxythiophene ?:poly ?styrenesulfonate ??PEDOT:PSS ?that enhance performance of PSCs were also evaluated.9–15

PEDOT:PSS ?chemical structure is shown in Fig.1?a ???lm has high transparency in the visible range,high me-chanical ?exibility,and good thermal stability.It has been used extensively as a modi?cation layer on indium-tin-oxide ?ITO ?electrode to improve hole-collection in PSCs.Ouyang et al.9reported that conductivity of PEDOT:PSS ?lm was signi?cantly enhanced after treatment with a cosolvent of water and a common organic solvent,such as ethanol,dim-ethyl sulfoxide,acetonitrile,or tetrahydrofuran.They also demonstrated the application of these highly conductive PEDOT:PSS ?lms as the transparent electrode in PSCs.14,15Shinar et al.10mixed the original PEDOT:PSS solution with ethylene glycol,the treatment improved the power conver-sion ef?ciency ?PCE ?of the PSC based on P3HT/PCBM by up to 27%.The authors suggested that these additives may affect the morphology and enhance the conductivity of PEDOT:PSS ?lm,hence improved performance of the device.

In this work,we fabricated the PSCs based on P3HT:PCBM with the ITO electrode modi?cation layer of PEDOT:PSS treated by common organic solvent of ethanol

or 2-propanol and studied the effect of the organic solvent treatment on the photovoltaic performance of the PSCs.The resulting devices showed enhanced PCEs due to the im-provement of J sc and ?ll factor ?FF ?.The conductivity and morphological changes of the PEDOT:PSS ?lms were ana-lyzed using four-point probe technique and atomic force mi-croscopy ?AFM ?.We found that enhanced conductivity and surface morphology of PEDOT:PSS contributed to the en-hanced photovoltaic performance.

The treatment of organic solvent on PEDOT:PSS was performed by mixing the aqueous solution of PEDOT:PSS ?Clevious P VP AI 4083from H.C.Stark ?with organic solvent of ethanol or 2-propanol.We optimized the volume ratio ?1:1,1:2,and 1:3?of the PEDOT:PSS aqueous solu-tion and the organic solvent according to the photovoltaic performance of the PSCs with the organic solvent-treated PEDOT:PSS layer,and found that the optimized volume ra-tio is 1:2for PEDOT:PSS aqueous solution to the added organic solvent ?ethanol or 2-propanol ?.A common BHJ PSC device structure of ITO/PEDOT:PSS/P3HT:PCBM/Ca/Al ?as shown in Fig.1?b ??was used in the studies,and the optimized volume ratio of 1:2was used in the organic solvent treatment to the PEDOT:PSS.16For the convenience of discussion,the PSCs with the untreated PEDOT:PSS layer,ethanol-treated PEDOT:PSS layer,and 2-propanol-treated PEDOT:PSS layer were named as device A,device B,and device C respectively in the following.

a ?

Author to whom correspondence should be addressed.Electronic mail:

liyf@https://www.wendangku.net/doc/1d10380774.html,.FIG.1.?Color online ??a ?Chemical structure of PEDOT:PSS,ethanol,and 2-propanol and ?b ?the device structure of the PSC.

APPLIED PHYSICS LETTERS 98,243308?2011?

0003-6951/2011/98?24?/243308/3/$30.00?2011American Institute of Physics

98,243308-1

Figure 2?a ?displays the current density-voltage ?J -V ?curves of the PSCs under the illumination of AM 1.5G ?Air Mass 1.5Global ?,100mW /cm 2,and Table I summarizes the device performance results.It can be seen that the pho-tovoltaic performance was considerably improved for the PSCs ?device B and device C ?with the organic solvent-treated PEDOT:PSS layer compared to the device with the PEDOT-PSS layer without the organic solvent treatment ?device A ?.Device A demonstrated a PCE of 3.39%with a short-circuit current ?J sc ?of 9.22mA /cm 2,an open-circuit voltage ?V oc ?of 0.61V ,and a FF of 60.3%.With the treat-ment by ethanol in device B,J sc increased from 9.22to 9.62mA /cm 2,FF increased from 60.3%to 70.3%,and PCE increased from 3.39%to 4.06%,respectively.When we used 2-propanol to treat the PEDOT:PSS layer in device C,J sc and PCE increased further to 11.07mA /cm 2and 4.74%,respec-tively.The PCE of 4.74%is among the highest values for the PSCs based on P3HT:PCBM reported in literatures so far.4–7

Figure 2?b ?shows the external quantum ef?ciency ?EQE ?plots of the PSC devices.Device C shows the highest EQE values with a maximum value of approximately 70%at around 550nm,which is consistent with the higher J sc value of device C.

In order to understand the origin of the performance improvement of the PSCs with the organic solvent-treated PEDOT:PSS layer,we investigated the effect of the organic solvent-treatment on the transparency,conductivity,and sur-face morphology of the PEDOT:PSS ?lms.Figure 3shows the transmittance of the ITO electrodes with PEDOT:PSS modi?cation layer spin-coated from PEDOT:PSS aqueous solutions with different treatment conditions.It can be seen from Fig.3that the transmission of the ITO/PEDOT:PSS electrodes in the range of 600–800nm is similar,no matter the PEDOT:PSS layer was treated by the organic solvent or not.In the range of 375–600nm,the transmission of the ITO/PEDOT:PSS layer treated by organic solvent is slightly lower than that of the electrode without the organic solvent treatment.Hence,the improved photovoltaic performance of the PSC devices with the PEDOT:PSS layer treated by or-ganic solvent is not directly related to the transmission of the different PEDOT:PSS layers.

The conductivity of the PEDOT:PSS ?lms was measured by the four-point probe technique,and the results are also listed in Table I .As expected,the conductivity of the organic solvent-treated PEDOT:PSS layers ?9.55?10?4S /cm for ethanol-treated PEDOT:PSS layer and 2.42?10?3S /cm for 2-propanol-treated PEDOT:PSS layer ?increased signi?-cantly than that ?5.18?10?4S /cm ?without the organic solvent-treatment.The conductivity of the PEDOT:PSS layer treated by 2-propanol was approximately ?vefold larger than that of the untreated PEDOT:PSS ?lm.The tendency of the conductivity change is consistent with the tendency of the J sc and PCE changes in the PSCs with different PEDOT:PSS modi?cation layer.Obviously,the improved photovoltaic performance of the PSC with the organic solvent-treated PEDOT:PSS layer resulted from the higher conductivity of the modi?cation layer treated by the organic solvent.

The surface morphology of the PEDOT:PSS modi?ca-tion layer could also in?uence the photovoltaic performance of the PSCs.Therefore,The effect of the organic

solvent

FIG.2.?Color online ??a ?J -V curves and ?b ?EQE plots of the PSCs based on P3HT/PCBM under illumination of AM 1.5G,100mW /cm 2.

TABLE I.Conductivity,roughness,and photovoltaic performance of the devices with and without the organic solvent treatment.

PEDOT:PSS

Thickness ?nm ?

Conductivity ?S/cm ?Roughness ?nm ?V oc ?V ?J sc

?mA /cm 2?FF ?%?PCE ?%?Without organic solvent treatment 82 5.18?10?4 1.120.619.2260.3 3.39Ethanol-treated 799.55?10?4 1.490.609.6270.3 4.062-propanol-treated

81

2.42?10?3

1.58

0.61

11.07

70.4

4.74

FIG.3.?Color online ?Transmittance of the ITO electrodes with PEDOT:PSS ?lm spin-coated from PEDOT:PSS aqueous solution ?4083?,4083with addition of 200%ethanol,and 4083with addition of 200%2-propanol.

treatment on the surface morphology of the PEDOT:PSS layer was examined by atomic force microscope ?AFM ?technique in tapping mode,as displayed in Fig.4.The bright regions in an AFM image are hard domains corresponding to PEDOT,whereas the dark regions denote the soft structure arising from excess PSS.17The bright PEDOT-rich regions increased and the dark excess PSS regions decreased after the organic solvent treatment,and the 2-propanol-treated

PEDOT:PSS layer showed the most bright regions and the less dark regions.The AFM results agree with that the cosol-vent treatment lowers the energy barrier for the charge trans-port across the PEDOT chains and increases the localization length.9The roughness of the PEDOT:PSS layer measured by AFM was also listed in Table I .The rms increased slightly from 1.12nm for the untreated layer to 1.49nm for the ethanol-treated layer and 1.58nm for the 2-propanol-treated layer.The increased roughness likely increase the contact area between the PEDOT:PSS and the active layer,improv-ing hole extraction to the anode.On the other hand,rough surfaces may increase scattering of the incident light back into the active layer and hence lead to increased absorption.Thus,the enhanced conductivity and better surface morphol-ogy of the organic solvent-treated PEDOT:PSS layer should play an important role in the improvement of the photovol-taic performance of the PSCs with the organic solvent-treated PEDOT:PSS modi?cation layer.

In summary,we studied the effect of the organic solvent treatment of the PEDOT:PSS buffer layer on the photovoltaic performance of the PSCs.The ethanol or 2-propanol treat-ment improved the conductivity and surface morphology of the PEDOT:PSS layer,so that increased the J sc ,FF,and PCE of the PSCs with the organic solvent-treated PEDOT:PSS buffer layer.The PCE of the P3HT/PCBM-based PSC with 2-propanol treated PEDOT:PSS buffer layer reached 4.74%with a higher J sc of 11.07mA /cm 2and a higher FF of 70.4%.These results indicate that the 2-propanol treatment for PEDOT:PSS layer could be an attractive approach for improving the PCEs.

This work was supported by the NSFC ?Grant Nos.20821120293,50933003,and 20976199?,the Ministry of Science and Technology of China and the Chinese Academy of Sciences.

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FIG.4.?Color online ?AFM topographic images of PEDOT:PSS ?lms ?a ?without organic solvent treatment,?b ?with ethanol treatment,and ?c ?with 2-propanol treatment.