Efficient bulk heterojunction solar cells based on a low-bandgap polyfluorene copolymers and fullerene derivatives

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Efﬁcient bulk heterojunction solar cells based on a low-bandgap

polyﬂuorene copolymers and fullerene derivatives

Jen-Hsien Huang

a

, Kuang-Chieh Li

c

, Hung-Yu Wei

b

, Po-Yen Chen

a

, Lu-Yin Lin

a

,

Dhananjay Kekuda

d

, Hong-Cheu Lin

c

, Kuo-Chuan Ho

a,b

, Chih-Wei Chu

d,e,*

a

Department of Chemical Engineering, National Taiwan University, Taipei, Taiwan b

Institute of Polymer Science and Engineering, National Taiwan University, Taipei, Taiwan c

Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu, Taiwan d

Research Center for Applied Sciences, Academia Sinica, Taipei, Taiwan e

Department of Photonics National Chiao Tung University, Hsinchu, Taiwan

a r t i c l e

i n f o

Article history: Received 4 May 2009

Received in revised form 28 May 2009 Accepted 28 May 2009

Available online 6 June 2009

PACS: 73.50.Pz 73.61.Ph Keywords: Polymer solar cell Polyﬂuorene

Phenothiazine derivative Narrow bandgap polymers Bulk heterojunction

a b s t r a c t

A low-bandgap polymer (PF-PThCVPTZ) consisted of fluorene and phenothiazine was designed and synthesized. With the donor–acceptor segment, the partial charge transfer can be built in the polymer backbone leading to a wide absorbance. The absorption spec-trum of PF-PThCVPTZ exhibits a peak at 510 nm and an absorption onset at 645 nm in the visible range. As blended with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) as an electron acceptor, narrow bandgap PF-PThCVPTZ as electron donor shows significant solar cell performance. Under AM 1.5 G, 100 mA/cm2_{illumination, a power conversion efficiency} (PCE) of 1.85% was recorded, with a short circuit current (JSC) of 5.37 mA/cm2_{, an open} cir-cuit voltage (VOC) of 0.80 V, and a fill factor (FF) of 43.0%.

1. Introduction

Harvesting energy directly from sunlight using photo-voltaic cells is a very important way to utilize renewable energy of the nature, especially for the development of or-ganic solar cells. Compared with inoror-ganics, oror-ganic mate-rials provide unlimited potential to modify the chemical structure achieving a better opto-electronic characteristic via organic chemistry which allows the devices revealing better performance. Recently, dye-sensitized solar cells

based on nanoporous TiO2thin ﬁlm can reach an efﬁciency

as high as 10%. However, the requirement of liquid electro-lytes and elevated annealing temperature in dye-sensitized solar cells are still a very challenging issue to be overcome for commercial applications. Much research has been fo-cused on development of bulk heterojunction (BHJ) solar cells to resolve the drawback. BHJ solar cells have attracted a great deal of interest recently due to their potential for low-cost, large area, light weight and flexible photovoltaic (PV) devices. The main feature of BHJ solar cells is the sig-nificantly increased in the interface between the donor and acceptor allowing an efficient charge separation compared to the planar interface of bilayer cells leading to a larger photocurrent.

*Corresponding author. Address: Research Center for Applied Sciences, Academia Sinica, Taipei, Taiwan. Tel.: +886 2 27898000x70; fax: +886 2 27826680.

E-mail address:gchu@gate.sinica.edu.tw(C.-W. Chu).

Contents lists available atScienceDirect

Organic Electronics

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Until now, PCE up to 5% has been achieved from the BHJ of poly(3-hexylthiophene) (P3HT) blended with PCBM[1]. Although the transport properties have been dramatically improved by vertical phase separation of both donor and acceptor due to the self-organization effect[2–5], they still lack the absorption in the red and infrared regions. There-fore, great deal efforts have been undertaken to enhance the JSCof solar cells by enhancing the absorption spectra

of active layers. Recently, much work has been focused on synthesis the conjugated polymers[6–13]. Among sev-eral types of organics materials, polymer with electron do-nor–acceptor architectures is one of the most efﬁcient ways to synthesize the narrow bandgap polymers. The ef-fect of intramolecular charge transfer between the electron donor and acceptor units has been found in the polymers with donor–acceptor architectures leading to a narrow bandgap[14–21].

Recently, much BHJ polymer solar cells using polyfluo-rene based alternating copolymers as donor have been re-ported due to their high charge carrier mobility[22,17]. However, polyfluorene copolymers in general have high bandgaps giving blue-shift optical absorption and hence have limited the photocurrent generation. For example, poly[9,9’-dioctyl-fluorene-co-bithiophene] (F8T2) only ab-sorbs light at wavelengths less than 500 nm [23]. Even though F8T2 exhibits excellent thermo tropic liquid crys-tallinity allowing better chain packing via self-assembly

[24], the poor absorption still depresses the extraction of photocurrent dramatically. In this article, the BHJ device performance of a low-bandgap polyﬂuorene copolymer which comprises the phenothiazine units is reported. The copolymer used for here is referred to as PF-PThCVPTZ. With the incorporation of phenothiazine units into PF,

the absorption can be stretched due to the donor–acceptor structure.

2. Experimental

2.1. Synthetic procedures 2.1.1. Synthesis

Monomer was synthesized according to a reported pro-cedure [25], and the characterization is described as fol-lows. The copolymer PF-PThCVPTZ is shown inScheme 1, and this synthetic procedure is described as follows. 2.1.2. Monomer Yield: 53%.1_{H NMR (CDCl} 3, ppm), d: 7.72 (dd, J = 8.7, 1.8 Hz, 1H), 7.45–7.36 (m, 3H), 7.29–7.18 (m, 3H), 7.07– 7.01 (m, 5H), 6.79 (d, J = 8.7 Hz, 2H), 3.80 (t, J = 7.2 Hz, 2H), 1.80 (m, 2H), 1.45–1.14 (m, 6H), 0.89 (m, 3H). 13_C NMR (CDCl3, ppm), d: 148.11, 146.00, 143.66, 140.67, 140.14, 138.08, 135.80, 134.54, 132.05, 131.00, 130.89, 130.67, 128.54, 128.11, 127.49, 126.56, 125.31, 124.32, 123.88, 123.53, 122.87, 116.56, 116.49, 115.43, 114.99, 112.64, 112.62, 102.51, 100.94, 48.00, 31.35, 29.67, 26.49, 22.59, 13.98. MS (EI): m/z [M+_{] 788.94, calcd m/z [M}+_]

789.0. Anal. Calcd for C36H27Br2N3S4: C, 54.75; H, 3.45; N,

5.32. Found: C, 55.19; H, 3.90; N, 4.82.

2.1.3. General procedure for the synthesis of PF-PThCVPTZ The synthetic route of polymers is shown inScheme 1. The polymerizations was carried out through the palla-dium(0)-catalyzed Suzuki coupling reactions. Into 50 mL of two-neck ﬂask, 1 equiv of monomer and 1 equiv of

2,7-bis-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-B

B

O

1 C

6

H

13

C

6

H

13

Pd(PPh

3

)

4

, Aliquat 336

toluene, K

2

CO

3

(aq. 2M)

reflux

Phenylboronic acid

Iodobenzene

+

N

S

C

6

H

13

S

NC

CN

S

Br

C

6

H

13

C

6

H

13

N

S

C

6

H

13

S

NC

CN

S

*

PF-PThCVPTZ

Monomer

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dihexylﬂuorene (1) were added in 10 mL of anhydrous tol-uene. The Pd(0) complex, Pd{P(p-tolyl)3}3 (1 mol%), was

transferred into the mixture in a dry environment. Then, 2 M aqueous potassium carbonate and the phase transfer catalyst, i.e., aliquat 336 (several drops), were subse-quently transferred via cannula into the previous mixture under nitrogen. The reaction mixture was stirred at 90 °C for 2 days, and then the excess amount of iodobenzene and phenylboronic acid, the end-capper, dissolved in 1 mL of anhydrous toluene was added and stirring for 4 h, respectively. The reaction mixture was cooled to 50 °C and added slowly into a vigorously stirred mixture of 300 mL of methanol. The polymers were collected by ﬁl-tration and reprecipitation from methanol. The crude poly-mers were further puriﬁed by washing with acetone for 3 days in a Soxhlet apparatus to remove oligomers and cat-alyst residues. The resulting polymers were soluble in common organic solvents.

2.2. Fabrication of photovoltaic devices

The PV devices in this study consists of a layer of PF-PThCVPTZ:PCBM blend thin ﬁlm sandwiched between transparent anode indium tin oxide (ITO) and metal cath-ode. Before device fabrication, the ITO glasses (1.5 1.5 cm2_{) were ultrasonically cleaned in detergent,}

de-ionized water, acetone and isopropyl alcohol before the deposition. After routine solvent cleaning, the sub-strates were treated with UV ozone for 15 min. Then, a modified ITO surface was obtained by spin-coating a layer of poly(ethylene dioxythiophene): polystyrenesulfonate (PEDOT:PSS) (30 nm). After baking at 130 °C for 1 h, the substrates were then transferred into a nitrogen-filled glove box. The polymer PV devices were fabricated by spin-coating blend of PF-PThCVPTZ:PCBM on the PED-OT:PSS modified ITO surface. Subsequently, a 30 and 100 nm thick of calcium and aluminum was thermally evaporated under vacuum at a pressure below 6 106_{Torr through a shadow mask. The active area of}

the device was 0.12 cm2_{. In the hole-only devices, the}

MoO3 was used to replace Ca with higher work function

(U¼ 5:3 eV), which is a good hole injection contact for PF-PThCVPTZ:PCBM[26]. The MoO3was thermally

evapo-rated with a thickness of 20 nm and then capped with 50 nm of Al. For the electron-only devices, PEDOT:PSS layer was replaced with CsCO3(U¼ 2:9 eV) which has been used

as an efﬁcient electron injection layer[27]. The Cs2CO3was

thermally evaporated with a thickness of 2 nm. The active layers were annealed at 130 °C for 20 min before the hole and electron-only devices were fabricated.

2.3. Characterization of polymer ﬁlms and PV devices Cyclic voltammetry (CV) studies were performed with a three-electrode cell with 0.1 M LiClO4/ACN using ITO as the

working electrode, a platinum sheet as the counter elec-trode, and nonaqueous Ag/Ag+_{(containing 0.01 M AgNO}

3

and 0.1 M TBAClO4 in ACN) as the reference electrode.

For measuring absorption and photoluminescence (PL) emission properties of polymer ﬁlms, samples were fabri-cated on a glass substrate. The UV–visible absorption

spec-tra were measured using a Jasco-V-670 UV–visible spectrophotometer. PL spectra were obtained using a Hit-achi F-4500 photoluminescence. Surface morphologies were observed by an atomic force microscopy (AFM, Digi-tal instrument NS 3a controller with D3100 stage). The thickness of all polymer films was measured using a sur-face profiler (Alpha-step IQ, KLA Tencor). Current–voltage (J–V) characteristics were measured in the glove box under nitrogen atmosphere with simulated AM 1.5 G irradiation at 100 mW/cm2using a xenon lamp based solar simulator (Thermal Oriel 1000 W). The light intensity was calibrated by a mono-silicon photodiode with KG-5 color filter (Ham-amatsu, Inc.). The external quantum efficiency (EQE) action spectrum was obtained at short-circuit condition. The light source was a 450 W Xe lamp (Oriel Instrument, model 6266) equipped with a water-based IR filter (Oriel Instru-ment, model 6123NS). The light output from the mono-chromator (Oriel Instrument, model 74100) was focused onto the photovoltaic cell under test.

3. Results and discussion

The CV was performed to investigate the electronic state of the PF-PThCVPTZ. As shown inFig. 1a, the CV of PF-PThCVPTZ presents an oxidation process (E1=2ox ¼ 0:89 V

vs. Ag/Ag+_{) and a reduction process (E}1=2

red¼ 1:08 V).

Fur-thermore, the Eonset ox and E

onset

red are 0.78 and 0.81 V,

respec-tively, which allows us to calculate the highest molecular orbital (HOMO) and lowest molecular orbital (LUMO) according to following equation [28]: EHOMO=LUMO¼

½ðEonset 0:45Þ 4:8 eV. The factor of 0.45 and 4.8 are

de-rived from the formal potential for ferrocene vs. Ag/Ag+

and the energy level of ferrocene below the vaccum. On the basis of these electrochemical data, the HOMO and LUMO levels can be deﬁned as 5.13 and 3.54 eV. There-fore, the bandgap offset between the LUMOs of PF-PThCVPTZ and PCBM is enough for electrons to be driven forward. Moreover, the polymer also reveals a multiply

-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 -0.04 -0.02 0.00 0.02 0.04 0.06 Potential (V vs. Ag/Ag+) Current ( m A/cm 2) Neutral Re-1 Re-2

Fig. 1. Cyclic voltammogram of PF-PThCVPTZ ﬁlm cast on a platinum wire in 0.1 M LiClO4/acetonitrile at 50 mV/s. (Inset: picture of the polymer ﬁlm on the ITO electrode at different coloration states.)

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colored electrochromic property.Fig. 1a shows the photo-graphs of PF-PThCVPTZ ﬁlms in uncharged (neutral, red), half (Re-1, green) and full (Re-2, blue) reduction states. The ﬁlm colors are homogenerously distributed across the electrode surface and the color changes are easily de-tected by the naked eye.

The normalized optical absorption spectra of the PF-PThCVPTZ in solution and solid film and PL emission spec-tra of pristine polymer and their blend films with PCBM are depicted inFig. 2. The solid film shows similar absorption pattern compared with the one of solution. However, slight red-shift still can be observed due to the interchain associ-ation and aggregassoci-ation in the solid state. The PF-PThCVPTZ

ﬁlm reveals a spectral absorption with a peak at 510 nm and an absorption onset at 645 nm. Furthermore, the cor-responding PL emission maxima kem of PF-PThCVPTZ is

centered at 660 nm. The PL emission is signiﬁcantly quenched by the addition of 50 wt% PCBM. This highly efﬁ-cient photoluminescence quenching is the consequence of ultrafast photoinduced charge transfer from the polymer to PCBM. Based on these characterizations, the copolymer is an excellent candidate to fabricate the photovoltaic devices.

The current density for the devices with varying active layer thickness are plotted inFig. 3. The devices were fab-ricated by spin coating a solution of PF-PThCVPTZ/PCBM (1:1 in weight, 4 mg/mL) dissolved in 1,2-dichlorobenzene (DCB). The VOC remains unchanged at about 0.8 V on

changing the active layer thickness as shown in the inset. However, the JSCand the PCE of the devices vary

signiﬁ-cantly with thickness. The JSC varies from as low as

1.20 mA/cm2_{(for the device with active layer thickness,}

0.0 0.2 0.4 0.6 0.8 -4 -3 -2 -1 0 1 50 100 150 200 250 300 0 1 2 3 4 64 nm 93 nm 139 nm 169 nm 203 nm 255 nm Voltage (V) Current density ( mA /c m 2)

Current density (

m

A/cm

2

)

Film thickness (nm)

Fig. 3. JSCplotted as a function of the thickness of active layer for the PF-PThCVPTZ:PCBM solar cells. (Inset: corresponding J–V characteristics.)

2.5 0.0 0.5 1.0 1.5 2.0 3.0 3.5 4.0 4.5 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 0.55 0.60 0.65 0.70 0.75 0.80 0.85 0.90 0.95 JSC ( mA/cm 2 ) [PCBM]/[PF-PThCVPTZ]

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VOC (V) 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 20 25 30 35 40 45 50 55 60 PCE (%) [PCBM]/[PF-PThCVPTZ]

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FF (%)

Fig. 4. JSC, VOC, FF and PCE plotted as a function of the composition for the PF-PThCVPTZ:PCBM solar cells. 400 500 600 700 800 0.2 0.4 0.6 0.8 1.0 1.2 PF-PThCVPTZ in DCB PF-PThCVPTZ in film Photoluminescence (a.u.) Wavelength (nm) Absorbance (normalized) PF-PThCVPTZ film PF-PThCVPTZ :PCBM (1:1)

Fig. 2. UV–vis spectrum in DCB (dash line) and in the solid state (solid line), photoluminescence in solid state (h) and photoluminescence quenching for PF-PThCVPTZ:PCBM (1:1) (s).

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t = 64 nm) to as high as 3.67 mA/cm2_{(t = 170 nm) and back}

to 3.0 mA/cm2 _{(t = 255 nm). The increased J}

SC is

contrib-uted from the larger absorbance with thicker active layer. However, the JSCdecreases with a thickness larger

than 169 nm due to the large series resistance and poor charge transport leading to a serious recombination[29]. An efﬁciency of 1.23% is achieved for the device with t = 170 nm.

Fig. 4shows the performance parameters of the mea-sured devices, namely JSC, VOC, FF and PCE as a function

of PCBM weight ratios. First, the JSCincreases from 2.98

for the device with containing 33 wt% PCBM to 3.87, and 4.93 mA/cm2 _{for those containing 50 and 66 wt% PCBM.}

However, the JSCdecreases with a larger PCBM weight ratio

as shown in Fig. 3a which means that a higher PCBM weight ratio leads to an imbalanced mobility between do-nor and acceptor, thus impeding the charge transport. Sec-ond, the VOC and the FF strongly decrease with PCBM

loading, resulting in a doubling of the calculated PCE when increasing the PCBM concentration from 33 to 80 wt%. According to our previous work[30], we have found that the values for VOCand FF based on polyﬂuorene copolymer

decrease monotonically with increasing fullerene content in the BHJ ﬁlms. This can be rationalized by the incomplete charge generation and transport resulted from the aggre-gation of the large amount of fullerenes[31]. The best cell has an active layer thickness of about 180 nm, containing 66 wt% of PCBM with a JSC of 4.93 mA/cm2, a VOC of

0.77 V, a FF of 40%, and a PCE of 1.51%.

In order to control the morphology of the blending ﬁlms, we investigated the effect of solvents with different boiling points.Fig. 5compares the morphology of the ﬁlms cast from chloroform (CF), chlorobenzene (CB), DCB and 1,2,4-trichlorobenzene (TCB) with the boiling points of 60, 132, 180 and 218 °C, respectively. The images are ob-tained in tapping mode for a 2 2

l

m2 _{surface area. It}

can be found the ﬁlms spin-cast from CB, DCB and TCB show a coarse chainlike feature stretching across the sur-faces. With higher boiling point, the chainlike feature is more obvious and the root mean square roughness (RMS) also increases from 4.3 (for the ﬁlm cast from CB) to 7.8 (TCB). These chainlike features are originated from the strong

p

–

p

stacking of the polymer chains which is be-lieved to enhance the charge transport[32,33]. This feature

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is well known and easily reproducible for the P3HT:PCBM blends using slow growth with high boiling point solvent such as TCB or DCB. In contrast, the ﬁlm prepared from CF solution appears a very smooth surface without appar-ent nanostructure. Although CF is a good solvappar-ent for PF-PThCVPTZ, however, its low boiling point and rapid evapo-ration limit the time for crystallization during the spin-coating process.

To further understand the dependence of charge trans-fer properties on morphologies that were introduced by varying the solvents, we have performed dark current measurements on hole-only and electron-only devices based on the blend ﬁlms. The charge mobilities were calcu-lated by space-charge limited current (SCLC)[34]. The dark current is given by J = 9

e

0

e

r

l

V2/8L3, where

e

0

e

ris the

per-mittivity of the polymer,

l

is the carrier mobility, and L is the device thickness. The hole and electron mobilities of the blend ﬁlms preapard from different solvents are summarized inTable 1. For the device made by CF have the lowest hole and electron mobility. Overall, the largest mobility can be found in the devices fabricated from TCB. The hole and electron mobility up to 1.93 108 _and

2.41 108_m2

V1_s1 _{is observed. Furthermore, we}

ob-served more balanced charge mobility (

l

h/

l

e= 0.8) in the

film cast from TCB which can enhance the photocur-rent.[35,36] For both DCB and TCB (which has a higher boiling point) it takes 10–15 min for the film to dry, while films cast from CF would dry within a few seconds of spin-ning. Therefore, the molecules in TCB can self-organization over a long time to form a thermodynamically favored structure leading to higher charge mobility[37,38].

Fig. 6shows the J–V curves and EQE of the PV devices fabricated from various solvents. The JSCvalues of the

de-vices fabricated from CF, CB, DCB and TCB are 2.45, 3.58, 4.73 and 5.37 mA/cm2_{, and the FF values are 45.4, 41.4,}

38.3 and 43.0%, respectively. The VOCvalues of all cases

are almost the same (0.80 V). As a result, the PCE of 1.84% is the highest among these devices which is obtained from TCB. The highest JSCof device fabricated from TCB can

be realized by the slow growth effect which provides a charge transport channel formed from self-organization leading to a higher photocurrent. This is supported by EQE characterization. After measuring the J–V curves, the devices were encapsulated in a nitrogen-ﬁlled glove box, and the EQE of the devices were measured in air. As shown in the inset, the devices exhibit a photoresponse covering from 350 to 650 nm. It can be seen that the EQE values in-crease by using solvents with higher boiling points. These results are in good agreement with the values of JSC. The

EQE for the device fabricated from CF shows a maximum

of 20.7% at a wavelength of 370 nm. On the other hand, for the device fabricated from TCB, the EQE maximum in-creases to 39.8 at 450 nm leading to a larger JSC.

4. Conclusion

An alternating PF copolymer, PF-PThCVPTZ, containing a low-bandgap donor–acceptor segment has been de-signed and synthesized for use in PV devices. A broad absorption spectrum in PF-PThCVPTZ ﬁlm covers the visi-ble solar spectrum, resulting in an extended photocurrent response. Under AM 1.5 G 100 mA/cm2 _{illumination, the}

devices fabricated from TCB with 67 wt% PCBM reveal a PCE of 1.84%. Based on these ﬁndings, PF-PThCVPTZ is a po-tential candidate for application in polymer solar cells.

Acknowledgements

The authors are also grateful to the National Science Council (NSC), Taiwan, (NSC 96-2221-E-001-017-MY2 and NSC 96-2628-E-007-030-MY2) and Academia Sinica, Taiwan for ﬁnancial support.

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