Synthetic Metals 158 (2008) 908–911

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Synthetic Metals

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Effects of thermal annealing on polymer photovoltaic cells with buffer layers andin situ formation of interfacial layer for enhancing power conversion efficiency

Yun Zhao, Zhiyuan Xie ∗, Yao Qu, Yanhou Geng, Lixiang WangState Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Graduate School of theChinese 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 5 December 2007Received in revised form 14 April 2008Accepted 18 June 2008

Keywords:

a b s t r a c t

We have investigated the effects of thermal annealing before and after cathode deposition on poly(3-hexylthiophene) (P3HT)/[6,6]–phenyl C61-butyric acid methyl ester (PCBM) blend photovoltaic cells withdifferent cathode buffer layers. The introduction of cathode buffer layer such as lithium fluoride (LiF)and calcium oxide (CaO) in pre-annealing cells can increase the open-circuit voltage (Voc) and the powerconversion efficiency (PCE). Post thermal annealing after cathode deposition further enhanced the PCEof the cells with LiF/Al cathode, whereas the performance of the cell with CaO/Al cathode is decreased.

Photovoltaic cellsConjugated polymerThermal annealingOP

It is found that the Voc and PCE can be largely enhanced when the post-annealed cell with Ca/Al cathodeis exposed in air. This may attribute to the formation of CaO buffer layer, which favors increasing shuntresistance and Voc. Finally, a PCE of 4.60% is calculated under AM1.5G solar illumination at 100 mW/cm2.

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. Introduction

Polymer photovoltaic cells offer great technological advan-ages as a potential source of renewable energy includinghe possibility of low-cost fabrication, low specific weight and

echanical flexibility [1–7]. Recently, the poly(3-hexylthiophene)P3HT)/[6,6]–phenyl C61-butyric acid methyl ester (PCBM) blendhotovoltaic cells with 5% power conversion efficiency (PCE)ere already realized [8]. The fundamental parameters includ-

ng the short-circuit current (Jsc), the open-circuit voltage (Voc),nd the fill factor (FF) are the key factors in determining theCE of the photovoltaic cells by the multiplication of the threearameters under solar illumination. In polymer photovoltaic cellsonsisting of polymer donor and PCBM acceptor, Voc should beroportional to the energy gap between the highest occupiedolecular orbital (HOMO) of the donor and the lowest unoccu-

ied molecular orbital (LUMO) of the acceptor [9,10]. Since thenterfaces between the electrodes and the organic active layersre not perfect ohmic contacts, the work functions of the elec-

rodes and the energy level alignment at organic layer/electrodesave a significant impact on the Voc. For example, the modificationf anode buffer layer of poly(3,4-ethylenedioxythiophene):poly-styrenesulfonate) (PEDOT:PSS) can effectively tune the Voc of the

∗ Corresponding author. Tel.: +86 431 85262819.E-mail address: xiezy n@ciac.jl.cn (Z. Xie).

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379-6779/$ – see front matter © 2008 Elsevier B.V. All rights reserved.oi:10.1016/j.synthmet.2008.06.011

© 2008 Elsevier B.V. All rights reserved.

ell [11]. The introduction of cathode buffer layers such as LiF andoly(ethylene oxide) (PEO) also increases the Voc of the photovoltaicells [12,13].

Over the past few years, the Jsc and PCE of the P3HT/PCBMlend photovoltaic cells have been improved by controllinghe P3HT/PCBM blend nanoscale morphology through variouspproaches [8,14–18]. Thermal annealing is one of the most effec-ive ways to realize high PCE since it results in the improvedanoscale morphology, the increased crystallinity of P3HT and the

mproved contact to the electron-collecting electrode. However, theoc of most P3HT:PCBM blend photovoltaic devices were kept atround 0.6 V, much lower than the theoretical value of P3HT:PCBMlend Photovoltaic cells. The Voc can be improved via interfaceodification. Until now, there are few reports about the effect of

hermal annealing, especially post-annealing, on the P3HT:PCBMlend photovoltaic cells with cathode buffer layers.

In this work, we studied the influence of thermal annealing onhe performance of P3HT:PCBM photovoltaic cells with differentuffer layers including LiF and CaO. It is found that post thermalnnealing process is not suitable for the photovoltaic cells withaO buffer layer and the formation of CaO via Ca oxidation afternnealing can effectively improve the Voc and PCE of the photo-oltaic cells. For the P3HT:PCBM blend photovoltaic cells with Al

athode, there exists a dipole layer at PCBM/Al interface with elec-ric field pointing from Al to PCBM [10,19–21], resulting in a smalloc. The formation of a thin CaO layer between P3HT:PCBM blendnd Al prohibits the dipole formation at PCBM/Al interface result-

Y. Zhao et al. / Synthetic Metals 158 (2008) 908–911 909

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ipeeacathode buffer layers. Fig. 3 shows the changes of Voc, Jsc, FF and PCEfor the post-annealed P3HT:PCBM photovoltaic cells with differentbuffer layers. The data for the pre-annealed cells were also includedfor the convenience of comparison. It can be seen that post thermalannealing is more effective than pre-annealing to enhance the per-

Table 1Changes of the shunt resistances of the photovoltaic cells prepared with P3HT:PCBM

Fig. 1. The molecular structures of P3HT and PCBM.

ng in high shunt resistance (Rsh) and Voc that favor enhancing theerformance of the photovoltaic cells.

. Experimental details

In our experiments, the bulk heterojunction photovoltaic cellsave a structure of indium tin oxide (ITO)/PEDOT:PSS/P3HT:PCBM1:0.8)/buffer layer/Al. The pre-cleaned ITO substrate was spinoated with a 30-nm-thick PEDOT:PSS (Baytron P 4083) and bakedt 120 ◦C for 30 min. The P3HT:PCBM film is deposited fromhlorobenzene solution containing P3HT:PCBM blend (1:0.8) ontohe PEDOT:PSS layer to produce a 100-nm-thick active layer in aitrogen-filled glove box (<0.1 ppm of O2 and H2O). P3HT was syn-hesized in our lab and PCBM with a purity of 99% was purchasedrom Solenne Company and used as received, and their moleculartructures are shown in Fig. 1. The 1-nm-thick buffer layers includ-ng LiF and calcium oxide (CaO) were thermally evaporated with aate of 1–2 Å/min in vacuum. Finally, 100-nm-thick Al is thermallyeposited to produce an active area of 0.12 cm2 for each cell. Thehotovoltaic cells with Ca (1 nm)/Al cathodes are also fabricated.he thermal annealing of the samples was carried out at 140 ◦Cor 3 min on a hot plate inside a nitrogen-filled glove box. Currentensity–voltage (J–V) characteristics of the encapsulated devicesnder white light illumination (Xenon lamp) were measured usingcomputer-controlled Keithley 236 source meter. The photo sen-

itivity was measured at a chopping frequency of 280 Hz with aock-in amplifier (Stanford, SR830) during illumination with the

onochromatic light from a Xenon lamp.

. Results and discussion

Fig. 2 shows the dark (a) and illuminated (b) J–V curves of thehotovoltaic cells without or with cathode buffer layers. For thesehotovoltaic cells, the P3HT:PCBM blend films are subject to ther-al annealing before buffer layer and cathode deposition. It can be

een from Fig. 2(a) that the dark current of the devices was loweredy inserting cathode buffer layers compared to the device withoutathode buffer layer. The rectification ratio (±1 V) of the diodes ismproved from 103 of the diode without buffer layer to 104–105

f the diodes with LiF or CaO buffer layers. It indicates that thentroduction of the cathode buffer layers can reduce the leakageurrent. The diode with CaO buffer layer shows the best perfor-ance. The illuminated J–V curves of the photovoltaic cells without

r with cathode buffer layers under 100 mW/cm2 white light illu-ination are shown in Fig. 2(b). The diode with Al cathode has a Voc

f 0.49 V, a Jsc of 8.35 mA/cm2 and a calculated FF of 0.52. The over-ll PCE for this diode is therefore 2.14%. When a thin layer of LiF or

aO buffer layer is introduced between the P3HT:PCBM active layernd Al cathode, the Voc is increased to 0.57 V. The Jsc of the diodesith LiF and CaO buffer layers are 8.07 and 8.53 mA/cm2, respec-

ively, comparable to 8.35 mA/cm2 of the diode with Al cathode.he FF is also increased to 0.63 and 0.59 for the photovoltaic cells

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ig. 2. Dark (a) and illuminated (b) J–V curves of the P3HT:PCBM photovoltaic cellsith Al, LiF/Al and CaO/Al cathodes under 100 mW/cm2 white light illumination.

he P3HT:PCBM blend is subject to thermal annealing before cathode deposition.

ith LiF or CaO buffer layers. Finally, the PCE is enhanced to 2.91%r 2.85% by inserting a thin layer of LiF or CaO buffer layers. Thesh of the photovoltaic cells is summarized in Table 1, which wereerived from the slope of J–V characteristics close to 0 V under darkonditions. The Rsh of the diodes increases by almost two ordersf magnitude by inserting a thin layer of LiF or CaO buffer layer.wo issues contribute to enhancing Rsh. The introduction of a thinuffer layer between P3HT:PCBM blend and Al prohibits the dipoleormation at PCBM/Al interface [19–21] resulting in a high Voc thatowers the recombination of photo-generated electrons and holes10]. The insulating LiF or CaO layer blocks exciton quenching byl cathode.

Ma et al. reported that post thermal annealing could furthermprove the PCE of the P3HT:PCBM blend photovoltaic cells com-ared to pre-annealed devices due to the improved contact to thelectron-collecting electrode facilitating charge collection at thelectrodes [8]. It is necessary to understand how post thermalnnealing influences the performance of the photovoltaic cells with

lend subject to pre- or post-annealing

sh (� cm2) Al LiF/Al CaO/Al

re-annealing 1.86 × 104 0.54 × 106 2.67 × 106

ost-annealing 1.70 × 105 4.62 × 106 2.16 × 104

910 Y. Zhao et al. / Synthetic Metals 158 (2008) 908–911

F ith Alb

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mpaptaoitomdfor the post-annealed cells with Al or Ca/Al cathode. Most impor-tantly, a calculated FF as high as 0.66 was achieved by exposing thediode with Ca/Al cathode to the air. These improvements led to aPCE of 5.19%, much higher than 3.83%, 4.06% or 4.87% of the post-

ig. 3. Comparison of the Voc (a), Jsc (b), PCE (c), and FF (d) of the photovoltaic cells wefore or after cathode deposition.

ormance of the P3HT:PCBM photovoltaic cell with Al cathode. Forhe diode with Al cathode, the Voc, Jsc and FF are increased from 0.49o 0.63 V, from 8.35 to 10.23 mA/cm2 and from 0.52 to 0.59, respec-ively, via post thermal annealing. These improvement leads to aCE of 3.83%, 80% increase compared to the pre-annealed cell withl cathode. Post thermal annealing also improves the performancef the cells with LiF buffer layer. Compared to the post-annealediode with Al cathode, the insertion of LiF buffer layer increases the

sc from 10.23 to 10.46 mA/cm2 and the Voc from 0.63 to 0.71 V. TheF is also increased from 0.59 to 0.66. Therefore, the PCE of the cellith LiF buffer layer can reach 4.87%. The increased Jsc for the post-

nnealed diode may be attributed to the improved contact at theathode interface and therefore the charge collection is improved.s for the increase of Voc, it is due to the energy level realign-ent at P3HT:PCBM/Al interface and the improvement of Rsh of

he diode via post-annealing as shown in Table 1. However, posthermal annealing is not applicable to all photovoltaic cells withifferent buffer layers. The performance of the photovoltaic cellith CaO buffer layer is reduced by post-annealing, although the

nsertion of CaO can improve the performance of the pre-annealedhotovoltaic cells as shown in Fig. 3. The Voc and Jsc are comparableo the pre-annealed diode, however, the FF and PCE are lowered. Itas reported that either orientation of the LiF or chemical reaction

hat formats the strong dipole moment across the junction was theechanism for the improvement [12]. The dipole or chemical reac-

ions may become stronger when the diode is post-annealed andherefore the performance is further improved. In the case of CaO,ost-annealing may cause a discontinuous CaO film and destroy

ts connection between the P3HT:PCBM layer and Al cathode, andherefore the performance is not further improved.

As discussed above, post thermal annealing process can

ffectively improve the performance of the P3HT:PCBM based pho-ovoltaic cells. However, post-annealing cannot improve the diodeith CaO buffer layer, though the insertion of CaO enhanced the

oc and PCE of the pre-annealed cell. If the CaO buffer layer cane formed after the photovoltaic cell is post-annealed, the perfor-

Fa

, LiF/Al and CaO/Al cathodes, where the P3HT:PCBM blend were thermally annealed

ance of the cell may be further enhanced. So we fabricated thehotovoltaic cells with Ca/Al cathode and these devices were post-nnealed. Fig. 4 shows the illuminated J–V characteristics of thehotovoltaic cells with different cathode structures. It can be seenhat the post-annealed cell with Ca/Al cathode has a Voc of 0.61 V,Jsc of 10.39 mA/cm2, and a calculated FF of 0.64 leading to a PCEf 4.06%. Compared to the post-annealed cell with Al cathode, thensertion of low work-function Ca did not increase the Voc due tohe Fermi level pinning. When the photovoltaic cell with Ca/Al cath-de was exposed in air for 60 min, the Voc was increased to 0.78 V,uch higher than the cells with Al or Ca/Al cathode. The Jsc of the

iode was 10.08 mA/cm2, a little lower than 10.23 or 10.39 mA/cm2

ig. 4. The illuminated J–V curves of the P3HT:PCBM photovoltaic cells with Al, Ca/Alnd Ca/Al (exposed to the air for 60 min) cathodes.

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ig. 5. Photo sensitivity of the post-annealed P3HT:PCBM photovoltaic cell witha/Al (exposed to the air for 60 min) cathode as a function of wavelength.

nnealed cells with Al, Ca/Al or LiF/Al cathodes, respectively. Themprovement may attribute to the formation of CaO after expos-ng the post-annealed diode with Ca/Al cathode to the air sincexygen can penetrate into the device through pinholes inside thel cathode or from the edge of the cathode. The in situ formedaO layer realigned the energy level at P3HT:PCBM/Al interface and

ncreased the Rsh of the cell and therefore the performance of thehotovoltaic cells was enhanced. However, the exposing time muste controlled in an appropriate range. A long exposing time wouldause the formation of traps since the oxygen and water penetratento the P3HT:PCBM blend active layer, and the performance espe-ially the Jsc is reduced. Since the above PCEs of these photovoltaicells are measured under an uncalibrated Xe lamp, the PCE valuesre not comparable to the values measured under AM1.5G simu-ated solar light. We calculate the PCE of the post-annealed cell withn situ formed CaO buffer layer by convoluting its photo sensitivityurve that is shown in Fig. 5 with the tabulated AM1.5 spectrum,nd a PCE of 4.60% is calculated under AM1.5G solar illuminationt 100 mW/cm2.

. Conclusion

In summary, the thermal annealing effect on P3HT:PCBMlend photovoltaic cells with cathode buffer layers is investigated.lthough post thermal annealing is an effective way to enhance theCE of the P3HT:PCBM blend photovoltaic cells with Al cathode due

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ls 158 (2008) 908–911 911

o the improved nanoscale morphology, the increased crystallinityf P3HT and most importantly the improved contact to the cath-de facilitating the charge collection, it is not applicable to somehotovoltaic cells with buffer layers such as CaO. This conflict cane solved by exposing the post-annealed diodes with Ca/Al cath-de to the air to form CaO buffer layer and the performance of thehotovoltaic cell is enhanced.

cknowledgements

This work has been partially supported by the National Nat-ral Science Foundation of China (no. 50573076, no. 20621401).he financial support from National Key Lab of Integratedptoelectronics, Jilin University (2006-JLU-01) is also acknowl-dged.

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