ARTICLE IN PRESS

Solar Energy Materials & Solar Cells 93 (2009) 604–608

Contents lists available at ScienceDirect

Solar Energy Materials & Solar Cells

0927-02

doi:10.1

� Corr

E-m

journal homepage: www.elsevier.com/locate/solmat

Enhanced charge collection in polymer photovoltaic cells by using anethanol-soluble conjugated polyfluorene as cathode buffer layer

Yun Zhao, Zhiyuan Xie �, Chuanjiang Qin, Yao Qu, Yanhou Geng, Lixiang Wang

State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Graduate School of the Chinese Academy of Sciences, Chinese Academy of Sciences,

Changchun 130022, PR China

a r t i c l e i n f o

Article history:

Received 12 September 2008

Received in revised form

1 December 2008

Accepted 3 December 2008Available online 14 January 2009

Keywords:

Photovoltaic cells

Conjugated polymer

Buffer layer

Charge collection

48/$ - see front matter & 2008 Elsevier B.V. A

016/j.solmat.2008.12.007

esponding author. Tel./fax: +86 4318526 281

ail address: xiezy_n@ciac.jl.cn (Z. Xie).

a b s t r a c t

We report enhanced polymer photovoltaic (PV) cells by utilizing ethanol-soluble conjugated poly

(9, 9-bis (60-diethoxylphosphorylhexyl) fluorene) (PF-EP) as a buffer layer between the active layer

consisting of poly(3-hexylthiophene)/[6, 6]-phenyl C61-butyric acid methyl ester blend and the Al

cathode. Compared to the control PV cell with Al cathode, the introduction of PF-EP effectively increases

the shunt resistance and improves the photo-generated charge collection since the slightly thicker

semi-conducting PF-EP layer may restrain the penetration of Al atoms into the active layer that may

result in increased leakage current and quench photo-generated excitons. The power conversion

efficiency is increased ca. 8% compared to the post-annealed cell with Al cathode.

& 2008 Elsevier B.V. All rights reserved.

1. Introduction

As a potential alternative to silicon-based photovoltaic (PV)cells, polymer PV cells offer the combined attraction of low cost,light weight, mechanical flexibility, and amenability to manufac-ture by high-throughput and large-area roll-to-roll coatingprocesses. Thereby polymer PV cells have become a highlightresearch area in recent years [1–10]. To date, the high powerconversion efficiency (PCE) polymer PV cells have been fabricatedwith an active layer composed of a blend of regioregular(3-hexylthiophene) (P3HT) and the fullerene derivative [6, 6]-phenyl C61-butyric acid methyl ester (PCBM) [11–14]. In additionto the high PCE of the PV cells, stability and large-area processingtechnology of polymer PV cells are the other two criteria thatmust be solved for their future application. The investigationabout the stability [15–20] and the large-area processingtechnology [21,22] of the polymer PV cells have been conductedin recent years.

In common polymer PV cells, the active layer is directly incontact with thermally deposited cathode such as Al; the possiblemetal migration into the active layer during thermal depositionprocess would result in increased leakage current and low shuntresistance, and therefore limits the enhancement of the PV cells[23]. In addition, the strong dipole effect at PCBM/cathodeinterface may cause an increase of the work function of metalcathode, resulting in a decrease of the open-circuit voltage (VOC)

ll rights reserved.

9.

[24]. Hence, it is necessary to modify the active layer/cathodeinterface to enhance the PV performance [25–28]. For example,the sub-monolayer LiF has been successfully utilized in polymerPV cells to modify the work function of the cathode and increasethe VOC [26]. However, LiF is vapor deposited in vacuum and isimpossible to be used in printing techniques, which are thebiggest advantage of polymer PV cells compared to their inorganiccounterparts. Moreover, the sub-monolayer of LiF cannot effec-tively protect metal atoms from migrating into the active layerduring metal deposition process; the thicker LiF increases theseries resistance and reduces charge collection due to itsinsulating property. We also reported an efficient PV cell withCaO buffer layer [27]. However, all of these thin layers have to bedeposited via high-vacuum thermal evaporation, and are im-possible in an entire roll-to-roll process. Recently, Zhang et al. [28]have successfully enhanced the VOC of polymer PV cells by using athin layer of insulating poly(ethylene oxide) (PEO) through spincoating from aqueous solution to modify the cathode.

Here we report enhanced PV cells based on P3HT:PCBM bulkheterojunction by introducing a conjugated polymer between theactive layer and the cathode via spin coating from ethanolsolution. It is found that both the shunt resistance (Rsh) andexternal quantum efficiencies (EQEs) are enhanced. Unlike thethin insulating buffer layers such as PEO or LiF, the increasedthickness of semi-conducting conjugated polymer buffer layermay protect more effectively metal atom from penetrating intothe active layer and hence the Rsh is increased and the EQE isenhanced. Moreover, the conjugated polymer/Al contact can beimproved via thermal annealing, which results in a furtherenhanced charge collection and the PCE.

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Y. Zhao et al. / Solar Energy Materials & Solar Cells 93 (2009) 604–608 605

2. Experimental

Poly(9, 9-bis (60-diethoxylphosphorylhexyl) fluorene) (PF-EP),shown in the inset of Fig. 1, is used as the cathode buffer layer. Theintroduction of phosphonate groups as side chains into conju-gated polyfluorene makes PF-EP possess good solubility in ethanoland does not influence the charge-transporting property of themain chain. PF-EP was synthesized in our group and its synthesisis described in Ref. [29]. PF-EP has been successfully used inefficient polymer light-emitting diodes as an electron-injectionlayer due to its efficient electron injection with Al cathode [30].P3HT was synthesized in our group with purity over 99%. PCBMwith purity over 99% was purchased from the Solenne Companyand used as received.

In this study, a P3HT:PCBM blend at the ratio of 1:0.8 is used asthe active layer. The bulk-heterojunction PV cells have a structureof indium tin oxide (ITO)/PEDOT:PSS/P3HT:PCBM (1:0.8)/PF-EP/Al. The pre-cleaned ITO substrate was spin coated with a30-nm-thick PEDOT: PSS and baked at 120 1C for 30 min. TheP3HT:PCBM film was deposited from chlorobenzene solution ofP3HT:PCBM blend (1:0.8) onto the PEDOT:PSS layer to produce a100-nm-thick active layer. PF-EP was spin coated onto the top ofthe P3HT:PCBM blend layer at a speed of 3000 rpm from ethanolsolution and its thickness was adjusted by changing the PF-EPconcentration in ethanol solution. An Al layer of 100-nm thicknesswas thermally deposited to produce an active area of 0.12 cm2 foreach cell. The thermal annealing of the samples was carried out at150 1C for 3 min on a hot plate inside a nitrogen-filled glove boxand the cells were encapsulated for measurement. Currentdensity–voltage (J–V) characteristics of the PV cells were mea-sured using a computer-controlled Keithley 236 source meter inthe dark and under white light (CHF-XM 500 W Xenon lamp)

0.3

0.2

0.1

0.0300 400 500 600 700 800

Wavelength (nm)

Abs

orpt

ion

(a.u

.)

PF-EP

n

Fig. 1. Absorption spectra of pristine PF-EP solid film (filled triangles), the pristine

P3HT:PCBM blend film (filled squares), and the pristine P3HT:PCBM blend film

overlaid by a thin layer of PF-EP via spin coating from ethanol solution (open

circles). The inset shows the chemical structure of PF-EP.

Table 1The performance of P3HT: PCBM blend PV cells with different PF-EP thicknesses.

PF-EP thickness (nm) Voc (V) Jsc (mA cm�2) P

0.0 0.45 8.57 1

2.0 0.53 8.49 2

5.0 0.64 9.01 3

10.0 0.58 8.64 1

The PV cells were measured under 100 mW cm�2 white light illumination.

illumination. The illumination intensity was measured with acalibrated reference cell. The PCEs of the cells under AM 1.5Gsimulated solar light were calculated by convoluting their photo-sensitivity curves with the tabulated AM1.5 spectrum. The EQEwas measured at a chopping frequency of 280 Hz with a lock-inamplifier (Stanford, SR830) during illumination with the mono-chromatic light from a Xenon lamp. The atomic force microscopy(AFM) measurements were performed on SPA300HV with anSPI3800 controller, Seiko Instruments Industry, Co., Ltd. Thethickness of organic and metal layer is determined by DEKTAK6M Stylus profiler.

3. Results and discussion

Fig. 1 shows the absorption spectra of pristine PF-EP film spincoated from ethanol solution, the pristine P3HT:PCBM blend filmspin coated from chlorobenzene solution, and the pristineP3HT:PCBM blend layer overlaid by a thin layer of PF-EP spincoated from ethanol solution. The absorption of P3HT:PCBM/PF-EP bilayer film is a simple superposition of the individualP3HT:PCBM and PF-EP absorption. Since these films are measuredunder the same condition, the almost identical absorption fromP3HT:PCBM in both shape and intensities for P3HT:PCBM andP3HT:PCBM/PF-EP films indicates that the PF-EP layer can be welldeposited on the P3HT:PCBM film through spin coating fromethanol solution and does not destroy the underlying P3HT:PCBMblend layer morphology.

The thickness of thin PF-EP layer is determined by comparingthe absorption of thin films with that of thick films, whichthickness is measured by a Stylus profiler. The influence of PF-EPthickness on P3HT:PCBM blend PV cells is studied and theparameters of these PV cells are summarized in Table 1. TheP3HT:PCBM blend film is subject to thermal annealing beforedepositing PF-EP layer and Al cathode in order to construct theinterpenetrating network of P3HT and PCBM for realizing efficientexciton dissociation and charge collection. It can be seen fromTable 1 that the PV cell without PF-EP buffer layer has a VOC of0.45 V, a short-circuit current (JSC) of 8.57 mA cm�2, and acalculated fill factor (FF) of 0.51. The overall PCE for this cell istherefore 1.98% (1.74% under 100 mW cm�2 AM1.5G simulatedsolar light). When the PF-EP buffer layer is introduced betweenthe P3HT:PCBM blend layer and Al cathode, VOC increases from0.45 to 0.6070.05 V and the corresponding Rsh of the PV cellsincreases more than two orders of magnitude than in the cellwithout PF-EP. When the shunt resistance Rsh of a solar cell is toosmall, the VOC of the cell is decreased. Here, the increase of VOC

may be possibly due to the introduction of PF-EP, which canrestrain Al atoms from migrating into the P3HT:PCBM blend activelayer and therefore the leakage current is reduced and Rsh isincreased. It should be noted that for the polymer PV cells withfullerene derivatives (such as PCBM) as electron acceptor, thestrong dipole effect at PCBM/cathode interface may cause anincrease of the work function of metal cathode, resulting in adecrease of VOC [24]. In the case of the cell with Al cathode, there

CE (%) FF Rs (O cm2) Rsh (�104O cm2)

.98 0.51 9.25 3.07

.27 0.51 11.26 289

.38 0.59 10.19 114

.90 0.38 17.85 135

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Y. Zhao et al. / Solar Energy Materials & Solar Cells 93 (2009) 604–608606

may exist dipoles at the PCBM/Al interface with the electric fieldpointing from Al to PCBM due to the strong electron-drawingcapability of PCBM, which would increase the work function of Aland result in a small VOC. This dipole effect is prohibited in thecase of the polymer PV cells with a thin PF-EP buffer layer. Ourprevious work verified that the interaction of phosphonate groupsin PF-EP with Al favors reducing electron-injection barrier at thePF-EP/Al interface in polymer light-emitting diodes, which meansthat the work function of Al is lowered when contacted with PF-EP[31]. Hence, another possibility for the VOC enhancement may beattributed to the energy level realignment at the P3HT:PCBM/Alinterface when a thin layer of PF-EP is inserted. When thethickness of PF-EP is about 5.0 nm, the PV cell exhibits morepronounced improvement with a VOC of 0.64 V, a JSC of 9.01 mAcm�2, and a FF of 0.59. The calculated PCE is about 3.38% (3.07%under 100 mW cm�2 AM1.5G simulated solar light), which is 70%increase in comparison with that of the control device. In additionto VOC, both JSC and FF are enhanced. The slightly increased JSC maybe attributed to the restraining of exciton quenching by Al cathodeand reduced optical ‘‘dead zone’’ near the metal cathode due tothe introduction of PF-EP buffer layer between the active layer andAl cathode. When the PF-EP layer is further increased to 10 nm, JSC

still keeps at 8.64 mA cm�2, which is comparable to 8.57 mA cm�2

of the control device. However, the increased series resistance (Rs)from 9.25O cm2 of the control device to 17.85O cm2 decreases theFF and PCE. LiF has been successfully used in polymer PV cells tomodify the work function of the cathode and increase the VOC. Wealso fabricated pre-annealed P3HT:PCBM blend PV cells with 1-nm-thick LiF buffer layer, whose VOC, JSC, and FF are 0.57 V,8.07 mA cm�2, and 0.63, respectively. The overall PCE for the cellwith LiF buffer layer is up to 2.95% [26]. The PV cell with5-nm-thick PF-EP buffer layer shows better device performancethan the PV cell with LiF buffer layer. This may be attributedto the fact that the sub-monolayer LiF may not effectively protectmetal atoms from migrating into the active layer duringmetal deposition process. The 5-nm-thick PF-EP buffer layermay protect more effectively metal atom from penetrating intothe active layer.

Fig. 2. AFM height images of the pristine (a) and annealed (b) P3HT:PCBM blend films a

EP via spin coating from ethanol solution. All image sizes are 5mm�5mm surface area

Fig. 2 shows the AFM height images for the pristine andannealed P3HT:PCBM blend films with and without overlaid PF-EPlayer. The pristine P3HT:PCBM blend surface has a rms roughnessof 3.99 nm. When a 5-nm-thick PF-EP is overlaid, the surfaceroughness decreases to 1.60 nm. The annealed films show similarchanges, the surface roughness decreasing from 3.15 to 2.74 nm byoverlying 5 nm PF-EP. This indicates that the overlaid 5.0-nm-thick PF-EP smoothens to some extent the P3HT:PCBM surfaceand may therefore improve the contact to the cathode andenhance charge collection.

The increased Rs for the cells with PF-EP buffer layer may comefrom bulk resistance of PE-EP or interfacial resistance at PF-EP/Alinterface. Since the PF-EP buffer layer is as thin as severalnanometers, the interfacial resistance at the PF-EP/Al contact maybe dominant. Since post-thermal annealing can improve success-fully the active layer/cathode contact and facilitate chargecollection [13], the PV cell with 5-nm-thick PF-EP buffer layer isannealed after depositing Al cathode and its dark and illuminatedJ–V curves are shown in Fig. 3. For the convenience of comparison,the dark and illuminated J–V curves of the pre-annealedP3HT:PCBM blend cells without and with 5.0 nm PF-EP bufferlayer are also shown in Fig. 3 and the EQEs of the three PV cells aregiven in Fig. 4. It can be seen from Fig. 3(a) that the reversecurrent was reduced by two orders of magnitude due to reducedleakage current by inserting 5.0-nm-thick PF-EP. More impor-tantly, post-thermal annealing of the PV cell reduces effectivelythe Rs from 10.19 to 6.32O cm2 and results in high chargecollection with JSC increasing from 9.01 to 10.28 mA cm�2. Theseresults are also better than the Rs of 7.33O cm2 and JSC of9.77 mA cm�2 of the post-annealed cell with Al cathode. Theimproved Rs and Rsh result in a FF as high as 0.66. Finally, a PCE of4.33% (3.78% under 100 mW cm�2 AM1.5G simulated solar light) isachieved, 8% increase compared to 4.03% (3.55% under 100 mWcm�2 AM1.5G simulated solar light) of the post-annealed cell withAl cathode. As can be seen in Fig. 4, the EQE at 500 nm is increasedfrom 55% to 58% when the PF-EP buffer layer is introduced,indicating that the charge collection is enhanced. When the PVcell is subject to thermal annealing, the EQE is increased to 63%

nd the pristine (c) and annealed (d) P3HT:PCBM blend films overlaid by 5.0 nm PF-

s.

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101

10-1

10-3

10-5

10-7

-0.8

12

8

4

0

-4

-8

-12

-0.4 0.0 0.4 0.8

P3HT:PCBM (pre-annealed)P3HT:PCBM (pre-annealed)/PF-EPP3HT:PCBM/PF-EP (post-annealed)

Voltage (V)

-0.8 -0.4 0.0 0.4 0.8

Voltage (V)

Cur

rent

Den

sity

(m

A/c

m2 )

Cur

rent

Den

sity

(m

A/c

m2 )

P3HT:PCBM (pre-annealed)P3HT:PCBM (pre-annealed)/PF-EPP3HT:PCBM/PF-EP (post-annealed)

Fig. 3. Dark (a) and illuminated (b) J–V curves of three kinds of P3HT:PCBM blend

PV cells under 100 mW cm�2 white light illumination: P3HT:PCBM (annealed)/Al

(open squares); P3HT:PCBM (annealed)/5.0 nm PF-EP/Al (open circles);

P3HT:PCBM/5.0 nm PF-EP/Al (post-annealed, open triangles).

1.0

0.8

0.6

0.0

0.4

0.2

300 400 500 600 700

Wavelength (nm)

Ext

erna

l qua

ntum

eff

icie

ncy

P3HT:PCBM (pre-annealed)

P3HT:PCBM (pre-annealed)/PF-EP

P3HT:PCBM/PF-EP (post-annealed)

Fig. 4. The EQEs of three kinds of P3HT:PCBM blend PV cells: P3HT:PCBM

(annealed)/Al (open squares), P3HT:PCBM (annealed)/5.0 nm PF-EP/Al (open

circles), and P3HT:PCBM/5.0 nm PF-EP/Al (post-annealed, open triangles).

Y. Zhao et al. / Solar Energy Materials & Solar Cells 93 (2009) 604–608 607

due to the improved PF-EP/Al contact. The above results show thatthe introduction of conjugated PF-EP buffer layer can effectivelyimprove charge collection and finally the PCE of the polymer PVcell and its solution-processing technique is also suitable to thefuture roll-to-roll printing techniques for large-area deposition.

4. Conclusion

We enhanced the polymer PV cells by utilizing an ethanol-soluble conjugated PF-EP as cathode buffer layer. Solubilitydifference for the active layer material and the buffer material iscompatible with the solution-processing techniques. The intro-duction of the PF-EP improves charge collection efficiency sincethe slightly thicker PF-EP may be more favorable to protect metalatoms from migrating into the active layer during metal deposi-tion process than the insulating LiF, which is commonly used atabout 1 nm in PV cells.

Acknowledgements

This work has been supported by the National NaturalScience Foundation of China (Nos. 50873100, 20834005 and20621401). The financial support from National Key Lab ofIntegrated Optoelectronics, Jilin University (2006-JLU-01) is alsoacknowledged.

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