石墨烯在太阳能电池中的应用

Yin et al. Buffer Layer of PEDOT:PSS/GrapheneComposite for Polymer Solar Cells

photovoltaic device having a structure of ITO/PEDOT:PSSwas transferred to acetone to remove the remaining acid. (40nm)/P3HT:PCBM(1:0.6,100nm)/LiF(1nm)/AlThen, through a drying process, the GO was obtained. (70nm), the energy conversion efﬁciency increased from 2.1to 3.8%,almost by ∼80%.Graphene exhibits a high charge mobility of

2.2. Instruments and Measurements 10,000cm 2V −1s −1at room temperature, long phase

Thermogravimetric analysis (TGA)and differential scan-coherence and elastic scattering lengths, and quantum

ning calorimetry (DSC)analysis were performed using conﬁnementeffects. 14Electrons in graphene obey a linear

a TGA/DSCSTA904at a heating rate of 5 C min −1dispersion relation and behave like massless relativistic

under a nitrogen ﬂow.All current-voltage (J –V ) char-particles, 15resulting in the observation of a number of

acteristics of the photovoltaic devices were measured in very peculiar electronic properties such as the quantum

air using a Keithley SMU 2400unit. A Xenon lamp Hall effect, 16ambipolar electric ﬁeldeffect, 17and pho-with a ﬁlter(broadpasssGRB-3, Beijing Changtuo Sci-tovoltaic acceptor. 18Its one-atom thickness and large

entiﬁclimited company) to simulate AM1.5G condi-two-dimensional plane provide it large speciﬁcarea, and

tions was used as the excitation source with a power of therefore, very large interfaces and low percolation thresh-100mW cm −2white light illumination from the ITO side. old can be achieved when it is added into the polymer

Light source illumination intensity was measured using matrix. Graphene can be prepared using several methods,

a calibrated broadband optical power meter (FZ-A,wave such as micromechanical cleavage, 17epitaxial growth,

length range 400–1000nm, Photoelectric Instrument Co, and chemical oxidation. 20

In view of the aqueous of the PEDOT:PSSsolution, a functionalized Delivered by Ingenta to:

nature

Beijing Normal University, China). All the fabrication and measurements were conducted in (grapheneoxide, GO) was used in this paper prepared University of Houston

graphene

air at room temperature. by

The calculation of the power conversion efﬁciency as a modiﬁedHummers method 20

—anchemical oxidation

IP : 129.7.158.43performed using the following equation:method as described elsewhere, 21

which Tue, 30 Mar 2010 02:56:27can be very

soluble in aqueous solution and, therefore, is expected to

=V oc J sc F F /Pin be well dispersed in the PEDOT:PSSmatrix.

where V oc is the open circuit voltage, J 2. EXPERIMENTAL DETAILS density, F F is the ﬁllfactor, sc is the short cir-cuit curnet and P incident light power. F F is determined according in is the

to F F =

2.1. Preparation of Graphene Oxide

(V current m J m )/(V density oc J sc ), where V in the maximum m and J m are the voltage and the

power point of the J –V GO was prepared by using a modiﬁedHummers method

curve in the fourth quadrant. from ﬂakegraphite (averageparticle diameter 4 m, 99.95%purity, Qingdao Tianhe Graphite Co., Ltd.). 12.3. Fabrication of the Photovoltaic Device Brieﬂy,graphite (5g) and NaNO 3(A.R.,3.75g) were

placed in a ﬂask.Then, H with stirring in an ice water 2SO bath. 4(A.R.,375mL) was added

The photovoltaic device was fabricated by use of KMnO a common process. The active layer was slowly added over about 1h. Cooling 4(A.R.,22.5g)

was prepared was completed

by spin coating a P3HT(10mg mL −1)/PCBM(1:0.6)in 2h, and the mixture was allowed to stand for 5days at

solution in DCB onto an ITO glass substrate that room temperature with vigorous stirring. During the ﬁve

was pre-coated with PEDOT:PSS(BaytronP, Bayer days, the mixture changed from green to dark brown to

Germany):GO.Then, LiF and Al were vapor deposited brick brown. The liquid obtained was added to an H aqueous solution (700mL, 5wt%)over about 1h 2SO with

on the active layer. The chemical structures of GO and PEDOT:PSSare shown in Figure 1(a).The schematic stirring, and the temperature was kept at 98 C. The resul-representation of the PEDOT:PSS:GO-basedsolar cell tant mixture was further stirred for 2h. Then, the mixture

is shown in Figure 1(b),which has a structure of was cooled to 60 C, H mL, 30wt%aqueous solu-ITO (∼17 sq −1tion), added to the above 2O liquid, 2(15)/PEDOT:PSS:GO(40nm)/P3HT:PCBMand stirred for 2h at room

(1:0.6,100nm)/LiF(1nm)/Al(70nm). The effective area temperature.

of each cell was ∼8mm 2. For comparison, a photovoltaic In order to remove ions of oxidant origin, especially

device based on a buffer layer of pure PEDOT:PSShaving manganese ions, the resultant liquid was puriﬁedby

the same device structure was also prepared. repeating the following procedure cycle at least 15times:centrifugation, removal of the supernatant liquid, addition of mixed aqueous solution (2L) of H SO 3. RESULTS AND DISCUSSION (0.5wt%),and dispersion using vigorous 2stirring 4(3wt%)/Hand bath

2O 2

The experimental results are shown in Figure 2. The per-ultrasonication for 30min at a power of 140W. Then, the

formance details are listed in Table I. procedure was cycled 3times using HCl aqueous solution

The J –V curves of the photovoltaic devices based (3wt%)and 2times using H 2O. The resultant solution on PEDOT:PSSand PEDOT:PSS:GObuffer layers under

J. Nanosci. Nanotechnol. 10, 1934–1938,2010

1935

Buffer Layer of PEDOT:PSS/GrapheneComposite for Polymer Solar Cells Yin et al.

Delivered by Ingenta to:University of HoustonIP : 129.7.158.43

Tue, 30 Mar 2010 02:56:27

Fig. 1. (a)The chemical structures of graphene and PEDOT:PSS.(b)Schematic structure of photovoltaic device based on PEDOT:PSS/graphenebuffer layer.

AM1.5G 100mW cm −2illumination are shown in Figure 2(b).The device based on the pristine PEDOT:PSSbuffer layer gives of 2.1%,J sc of 6.9mA cm −2, V oc of 0.60V , and F F of 0.50. By doping GO, the PEDOT:PSS:GObuffer layer makes the value increase to 2.4%with J sc of 10.2mA cm −2, V oc of 0.56V , and F F of 0.42. The J sc value increases by 48%,and the value increases by ∼14%.Obviously, the improvement in the device performance can be attributed to the addition of GO. The J –V curves of the two devices in dark are shown in Figure 2(a).The dark current at the same bias value increase greatly with the PEDOT:PSS:GObuffer layer, showing an improved charge collection ability of the PEDOT:PSS:GObuffer layer compared with the pristine PEDOT:PSScomposite.

When graphene was introduced with functional groups, such as –COOHand –OHas well as –CO, –C–O–C–,and –CH2–groups, its conductivity and charge carrier mobility were decreased to display an insulating nature, impeding the performance of the photovoltaic device based on the PEDOT:PSS:GObuffer layer. A recent report has 1936

shown that rapid heating of GO results in its expansion and exfoliation to produce conducting single-functionalized graphene sheets. 22In view that the functional groups can be partially removed from the graphene sheet at an ele-vated temperature under an inert atmosphere, and the con-ductivity of the graphene sheet can be recovered, 23we investigated the thermal properties and the weight loss properties of GO using TGA and DSC under an Ar atmo-sphere, as shown in Figure 2(c).It can be seen that there is about 30%weight loss between 180and 250 C from the TGA curve, accompanying which, we can see an obvi-ous exothermic peak in the DSC curve. The weight loss at this temperature range can be attributed to the removal of functional groups from the graphene sheets.

We then carried out an annealing treatment on the PEDOT:PSS/GObuffer layer before the P3HT:PCBMactive layer at 250 C for 10min, the J –V curves of the device in dark and under illumination are also shown in Figures 2(a)and (b),respectively. The device based on the PEDOT:PSS:GObuffer layer after an annealing treatment shows an value of 2.3%,J sc of 10.2mA cm −2, V oc of

J. Nanosci. Nanotechnol. 10, 1934–1938, 2010

Yin et al. Buffer Layer of PEDOT:PSS/GrapheneComposite for Polymer Solar Cells

greatly decreases the overall performance of the photo-(a)

voltaic device. Moreover, the current density from the J –V curve of the device in dark (Fig.2(a))decreases more than that of PEDOT:PSSbuffer layer without annealing treat-ment at the same bias. This indicates the decrease in the charge collection ability of the buffer layer, which may be (b)caused by the decomposition of the PEDOT:PSS.

In order to reactivate the performance of GO and avoid

the decomposition of the PEDOT:PSScomposite with annealing treatment at same time, the GO was ﬁrstheated at 250 C for 10min, then mixed with PEDOT:PSSsolu-tion by use of ultrasonic dispersion, and ﬁnallyused as the buffer layer of the photovoltaic device. The device J –V curves of this treatment in dark and under illumination are shown in Figures 2(a)and (b),respectively, showing an value of 3.8%,J sc of 14.2mA cm −2, V oc of 0.62V , and (c)

F F of 0.43. It can be seen that the energy conversion efﬁ-ciency based on pre-annealing of GO is ∼1.8times of that Delivered by Ingenta to:of the pristine PEDOT:PSSbuffer layer, especially when

University of Houstonthe J sc value reaches 2times. The higher dark current den-sity of this device compared with the pristine PEDOT:PSS

Tue, 30 Mar 2010 02:56:27IP : 129.7.158.43device at the same bias also indicates that pre-annealing of GO sheets greatly improves the hole collection ability

of the PEDOT:PSSbuffer layer. It is worth noting that the devices based on the PEDOT:PSS/graphenecompos-ite buffer layer show lower ﬁllfactor than those based on the pristine PEDOT:PSSbuffer layer. This indicates that besides the high conductivity of the GO sheet, there may be other factors of GO inﬂuencingthe device performance. Fig. 2. J –V curves of photovoltaic devices based on different buffer

For example, GO in the buffer layer and PCBM also form layer in (a)dark and (b)under a simulated AM1.5G 100mw illumination, and (c)TG and DSC curves of the GO in the range of 35–960

a bilayer heterojunction subcell, besides the bulk hetero-C at a heating rate of 5C min

in nitrogen ﬂow.

junction formed by P3HT:PCBM.24

−1

0.62V , and F F of 0.37, indicating that the overall perfor-4. CONCLUSIONS

mance of the annealed device is almost the same as that

of the un-annealed one, except for a slight decrease in the In summary, a one atom thick carbon material, graphene, and V oc . It has been reported that an annealing treatment was used as a buffer layer compounded with PEDOT:PSS.at 250 C will increase the conductivity of the PEDOT:PSSWe achieved a performance of 3.8%,which is 1.8times of ﬁlm,10Therefore, we investigated the photovoltaic device that of the device based on a pristine PEDOT:PSSbuffer based on the pristine buffer layer of PEDOT:PSSwith an layer. Thus, the composite is a promising buffer layer annealing treatment at 250 C for 10min, the J –V curves for use in photovoltaic applications. Furthermore, such a of the device in dark and under illumination are also shown buffer layer can also be used in other electronic applica-in Figures 2(a)and (b),respectively. This device shows tions. The device performance should be further improved an value of 1.3%,J by optimization in composite fabrication details. sc of 4.6mA cm −2, V oc of 0.60V , and F F of 0.46, indicating that the annealing treatment on the PEDOT:PSSbuffer layer at 250 C for 10min

Acknowledgments:The authors gratefully acknowl-edge the ﬁnancialsupport from the NSFC (60876046,60676051, 20644004, 07JCYBJC03000) of China, Key Table I. Performance details (V oc , J sc , F F , and ) of the photovoltaic Project of Chinese Ministry of Education (209007),MoST devices having different buffer layers. (2006CB0N0702),and the Tianjin Key Discipline of Mate-Buffer layer

V oc (V)J rial Physics and Chemistry.

sc (mAcm −2)

F F (%)PEDOT:PSS0.606.90.502.1PEDOT:PSS/GO

0.5610.20.422.4References and Notes

(PEDOT:PSS/GO)annealing 0.6210.20.372.3PEDOT:PSSannealing 0.64.60.461.31. A. C. Mayer, S. R. Scully, B. E. Hardin, M. W. Rowell, and GO-annealing/PEDOT:PSS

0.62

14.2

0.43

3.8

M. D. McGehee, Mater. Today 10, 28(2007). 2. S. R. Forrest, MRS Bull. 30, 28(2005).

J. Nanosci. Nanotechnol. 10, 1934–1938,20101937

Buffer Layer of PEDOT:PSS/GrapheneComposite for Polymer Solar Cells Yin et al.

3. B. C. Thompson and J. M. J. Frechet, Angew. Chem. Int. Ed. 47, 5814. C. Berger, Z. M. Song, X. B. Li, X. S. Wu, N. Brown, C. Naud, (2008). D. Mayou, T. B. Li, J. Hass, A. N. Marchenkov, E. H. Conrad, 4. C. J. Brabec, N. S. Sariciftci, and J. C. Hummelen, Adv. Funct. P. N. First, and W. A. de Heer, Science 312, 1191(2006). Mater. 11, 15(2001). 15. M. I. Katsnelson, Mater. Today 10, 20(2007). 5. L. S. Roman, W. Mammo, L. A. A. Pettersson, M. R. Andersson, 16. Y . B. Zhang, Y . W. Tan, H. L. Stormer, and P. Kim, Nature 438, 201and O. Inganas, Adv. Mater. 10, 774(1998). (2005). 6. Y . F. Zhou, Y . B. Yuan, L. F. Cao, J. Zhang, H. Q. Pang, J. R. Lian, 17. K. S. Novoselov, A. K. Geim, S. V . Morozov, D. Jiang, Y . Zhang, and X. Zhou, J. Lumin. 122, 602(2007). S. V . Dubonos, I. V . Grigorieva, and A. A. Firsov, Science 306, 6667. S. Admassie, F. L. Zhang, A. G. Manoj, M. Svensson, (2004). M. R. Andersson, and O. Inganas, Sol. Energy Mater. Sol. Cells 18. Q. Liu, Z. F. Liu, X. Y . Zhang, N. Zhang, L. Y . Yang, S. G. Yin, 90, 133(2006). and Y . S. Chen, Appl. Phys. Lett. 92, 223303(2008). 8. S. H. Park, J. U. Kim, J. K. Lee, and M. R. Kim, Molecular Crystals 19. A. K. Geim and K. S. Novoselov, Nat. Mater. 6, 183(2007). and Liquid Crystals 471, 113(2007). 20. W. S. Hummers and R. E. Offeman, J. Am. Chem. Soc. 80, 13399. B. Winther-Jensen and K. West, Macromolecules 37, 4538(2004). (1958). 10. J. Huang, P. F. Miller, J. C. de Mello, A. J. de Mello, and 21. H. A. Becerril, J. Mao, Z. Liu, R. M. Stoltenberg, Z. Bao, and

D. D. C. Bradley, Synthetic Metals 139, 569(2003). Y . Chen, Acs Nano 2, 463(2008). 11. B. L. Groenendaal, F. Jonas, D. Freitag, H. Pielartzik, and 22. H. C. Schniepp, J. L. Li, M. J. McAllister, H. Sai,

J. R. Reynolds, Adv. Mater. 12, 481(2000). M. Herrera-Alonso, D. H. Adamson, R. K. Prud’homme,R. Car, 12. E. Kymakis, G. Klapsis, E. Koudoumas, E. Stratakis, N. Kornilios, D. A. Saville, and I. A. Aksay, J. Phys. Chem. B 110, 8535

N. Vidakis, and Y . Franghiadakis, Eur. Phys. J. Appl. Phys. 36, 257(2006). (2006). 23. X. Wang, L. J. Zhi, and K. Mullen, Nano Lett. 8, 323(2008). 13. H. T. Ham, Y . S. Choi, M. G. Chee, M. H. Cha, and I. J. Chung, 24. Z. Chunfu, T. Shi Wun, J. Changyun, E. T. Kang, D. S. H. Chan,

Polym. Eng. Sci. 48, 1(2008). and Z. Chunxiang, Appl. Phys. Lett. 92, 083310(2008). Delivered by Ingenta to:

University of Houston

IP : 129.7.158.43Received:9November 2008. Accepted:23April 2009. Tue, 30 Mar 2010 02:56:27

1938J. Nanosci. Nanotechnol. 10, 1934–1938, 2010