E
ffect of molecular weight of additives on the
conductivity of PEDOT:PSS and e
fficiency for ITO-free
organic solar cells
†
Desalegn Alemu Mengistie,abcPen-Cheng Wangcand Chih-Wei Chu*bd
We systematically investigated the effect of the molecular weight of additives on the conductivity of
poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) by using different concentrations
and molecular weights of polyethylene glycol (PEG) and ethylene glycol (EG). The conductivity enhancement depends on both the molecular weight and concentration of PEG used. The conductivity
of PEDOT:PSS was enhanced from 0.3 S cm1to 805 S cm1with 2% PEG but to only 640 S cm1with
6% EG. PEGs with molecular weight higher than 400 have too low mobility to impart the required
screening effect, and hence, the conductivity enhancement is less. Through FTIR, XPS and AFM
investigations, the mechanism for the conductivity enhancement is found to be charge screening between PEDOT and PSS followed by phase separation and reorientation of PEDOT chains leading to
bigger and better connected particles. The molecular weight and concentration of PEG also affect solar
cell performances even though the conductivities are the same. Due to their high conductivity and high transmittance, ITO-free organic solar cell devices fabricated using PEDOT:PSS treated with 2% PEG anodes exhibited performance almost equal to that of the ITO counterparts.
1
Introduction
Organic solar cells (OSCs) which use either small molecules or polymers as active solar harvesting materials with promising potential for roll-to-roll and large area processing,exibility and cheap cost of manufacture are considered as next generation green energy sources.1–3A better understanding of the mecha-nism facilitated a rapid increase in power conversion efficiency and the recently reported efficiency of 12% is paving the way for their commercialization.4–6 The next technical challenge is to realize the high speed manufacturing of cells and modules with long lifetimes. OSCs contain bottom and top electrodes, inter-layer materials and active inter-layer materials. As the interinter-layer and active layer materials can be processed through conventional solution processing, the electrodes which are deposited through vacuum deposition are the bottleneck for the roll-to-roll high speed processing of OSCs. At least one of the
electrodes in opto-electronic devices needs to be transparent in order to allow light to be harvested by the active layer or to emit light. However, tin doped indium oxide (ITO) coated on rigid glass which is currently used as the standard transparent elec-trode has many drawbacks. ITO's price is skyrocketing due to its limited availability; moreover, ITO is mechanically brittle and exhibits poor adhesion to organic and polymeric materials.7,8In OSCs, 37–50% of the material cost is from ITO.1Moreover, ITO has additional inherent problems such as release of oxygen and indium into the organic layer, poor transparency in the blue
region, and complete crystallization of ITO lms, which
requires high temperature processing.9 Hence, ITO is a non-ideal transparent electrode for opto-electronic devices and a search for an alternative transparent electrode is inevitable.10
Different materials including metal nanowires,11–13 carbon nanotubes,14–16graphene17–19and conducting polymers20–24are being actively investigated as replacements for ITO. Among them, the conductive polymer poly(3,4-ethylene dioxythio-phene) (PEDOT) doped with poly(styrene sulfonate) (PSS) (chemical structure shown in Fig. 1a) is quite promising as a next-generation transparent electrode material owing to its enormous advantages over other conducting polymers. PEDOT is insoluble in most solvents but can be dispersed in water by using PSS as a counter ion, which also serves as an excellent oxidizing agent, as a charge compensator, and as a template for polymerization.25,26PEDOT:PSSlms have high transparency in the visible range, high mechanical exibility, and excellent aNanoscience and Technology Program, Taiwan International Graduate Program,
Academia Sinica, Taipei 115, Taiwan
bResearch Center for Applied Sciences, Academia Sinica, Taipei 115, Taiwan. E-mail:
gchu@gate.sinica.edu.tw; Fax: +886-2-27896680; Tel: +886-2-27898000 ext. 70
cDepartment of Engineering and Systems Science, National Tsing Hua University,
Hsinchu 30013, Taiwan
dDepartment of Photonics, National Chiao Tung University, Hsinchu 300, Taiwan
† Electronic supplementary information (ESI) available: PEDOT:PSS lm aer PEG400 and methanol treatment, XPS spectra of PEDOT:PSS with PEG400 and J–V curves and device performance tables of OSCs with PEDOT:PSS anodes treated with different concentrations of PEG. See DOI: 10.1039/c3ta11726j Cite this:J. Mater. Chem. A, 2013, 1,
9907 Received 2nd May 2013 Accepted 13th June 2013 DOI: 10.1039/c3ta11726j www.rsc.org/MaterialsA
Materials Chemistry A
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thermal stability and can be fabricated through conventional solution processing. PEDOT:PSSlms can also be easily nano-structured to enhance the localized light intensity to the active
layer and generate more power.27 Different grades of
PEDOT:PSS dispersions with different conductivities are commercially available for antistatic coating, hole injection layer and transparent electrode applications. However, pristine PEDOT:PSSlms suffer from a very low conductivity of less than 1 S cm1, which is quite low to be used as standalone electrodes. Several methods are being actively investigated to enhance the conductivity of PEDOT:PSS by more than three orders of magnitude to replace ITO. The methods employed for conduc-tivity enhancement of PEDOT:PSS including the treatment chemicals used have been recently reviewed by Po et al.28The approaches include the addition of organic compounds such as ethylene glycol (EG), dimethyl sulphoxide, dimethyl sulphate, sorbitol, mannitol, ionic liquids, and anionic surfactants into the PEDOT:PSS aqueous solution,29–35treatment of PEDOT:PSS lms with polar organic compound, salt, acid, zwitterions or cosolvents36–40or a combination of both mixing the additive in the PEDOT:PSS dispersion and lm treatment.41 Xia et al. enhanced the conductivity of Clevios PH1000 PEDOT:PSS to 3065 S cm1by treating thelm three times with 1 M sulphuric acid at 160C.42Fabretto et al. also reported PEDOTlms with a conductivity of 3400 S cm1using vacuum vapour phase poly-merization technique.43 But the performances of the OSC devices fabricated with these high conductivity PEDOT:PSS anodes were lower than the ITO counterparts. In addition to high conductivity, high transmittance and otherlm properties should not be impaired duringlm treatment.
Even though such high conductivities are obtained, the fundamental mechanisms of conductivity enhancement are not well understood yet. It has been said that the origin and mechanism of conductivity improvement differ widely and are considered controversial.29,44,45 In our earlier report,46 we showed that the mechanism of conductivity enhancement depends on the treatment chemical, its properties and the method of treatment employed and some of the mechanisms proposed are complementary. We used methanol, which previously was considered as not to signicantly enhance conductivity, to treat PEDOT:PSSlms and the conductivity was enhanced to 1362 S cm1. Morphology changes with phase separated PEDOT and PSS leading to larger grain sizes and lower intergrain hopping,30,47 screening effect by polar solvents,48washing away of the excess insulator PSS from the lm surface,41,46 conformational changes by reorientation of PEDOT:PSS chains leading to better connection between the conducting PEDOT chains,45and even some doping effects are among the mechanisms proposed by different researchers.33,42 Takano et al. used small- and wide-angle X-ray scatterings and found that nanocrystals of PEDOT are formed.49Polar solvents which are secondary dopants added in the PEDOT:PSS aqueous solution bring about morphology changes showing larger grain size and better connected PEDOT chains, whereas thoselm treatments lead to both morphology change and removal of excess PSS from thelm surface. Owing to this, mostly methods employinglm treatment or both lm treatment and additives
in the solution show better conductivity than the mere additives in the PEDOT:PSS aqueous solution.41,42,46
In this study, we showed the effect of the molecular weight/ chain length of the additive on the conductivity and other properties of PH1000 PEDOT:PSSlms using EG and different molecular weight polyethylene glycols (PEGs). While the conductivity of PEDOT:PSS increases with increasing EG concentration and saturates at 6% addition, the conductivity reaches saturation at 2% PEG concentration and even decreases aerwards. The average conductivity was signicantly enhanced from 0.3 S cm1to 805 S cm1 with PEGs having molecular weights from 200 to 400 but to only to 640 S cm1with 6% EG. Even though PEGs with molecular weight higher than 600 still bring about conductivity enhancement, the conductivity is less than that obtained using EG. Phase separation between PEDOT and PSS chains facilitated by the additives leading to better connected and bigger PEDOT chains is the main reason for the conductivity enhancement. FTIR and XPS measurements revealed that PEG still remains in thelms forming hydrogen bonding with PSS. ITO-free OSCs fabricated with PEG treated
PEDOT:PSS lms as standalone transparent electrodes
demonstrate better than EG treated anodes and showed performance almost equal to that of the devices with ITO electrodes.
2
Experimental
2.1 Preparation and characterization of PEDOT:PSSlms
PEDOT:PSS aqueous solution (Clevios PH1000) with a
PEDOT:PSS concentration of 1.0–1.3% by weight was purchased from Heraeus Ltd. and the weight ratio of PSS to PEDOT is 2.5. Glass substrates with an area of 1.5 1.5 cm2were cleaned by sonication successively in detergent water and twice with deionized water for 15 min each time and then dried in an oven aer purging with N2air. Different concentrations (v/v%) of EG and PEG were mixed with PEDOT:PSS aqueous solution. Different molecular weights of PEG were used (PEG200, PEG300, PEG400, PEG600, PEG1000 and PEG6000, the number indicating the molecular weight). PEDOT:PSS mixed with EG and PEGltered through a 0.45 mm PTFE syringe lter was spin coated at 3000 rpm for 60 s on glass substrates which were treated with UV/ozone for 15 min prior to spin coating. The lms were annealed on a hot plate under ambient atmosphere at 130C for 30 min. Thicker PEDOT:PSSlms were prepared by spin coating multiple times and annealing aer each layer. For
some lms, a combined treatment with methanol was
per-formed by immersing the annealedlms in methanol for 10 min and then thelms were again dried at 140C for 5 min. The other combinedlm treatment was done by dropping 120 mL of methanol on thelm at 130C aer annealing for 5 min.
Film thickness was measured using a Veeco Dektak 150 alpha step surface proler. Conductivities were measured using van der Pauw four-point probe technique with a Hall effect
measurement system (Ecopia, HMS-5000). Transmission
spectra of thelms were measured using a Jacobs V-670 UV-Vis-NIR spectrophotometer. The transmittance values reported in this paper are at a wavelength of 550 nm and include the
absorption of the glass substrate. X-ray photoelectron spec-troscopy (XPS) was performed using a PHI 5000 VersaProbe equipped with an Al Ka X-ray source (1486.6 eV). Attenuated
total reectance Fourier transform infrared (ATR-FTIR)
spectroscopy was performed using an FT-IR spectrometer (PerkinElmer, Spectrum 100) for PEDOT:PSS lms coated on glass or by dropping PEG solution on the IR cell. Veeco di Innova was used in the taping mode to take the atomic force microscopy (AFM) images of polymerlms.
2.2 Fabrication and characterization of OSCs
OSCs were fabricated using both ITO-free highly conductive EG and PEG treated PEDOT:PSS lms and ITO (<7 U ,1, RiT display) anodes on glass. A relatively well studied and stabilized donor:acceptor blend of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) was used as the active layer. The OSC devices were fabricated by spin coating a blend solution of P3HT:PCBM, prepared by dissolving 20 mg mL1of each component in 1,2-dichlorobenzene at 70 C for 3 h, at 600 rpm for 60 s in a nitrogenlled glove box on the PEDOT:PSS treated with EG and PEG and the ITO surface. Less conductive PEDOT:PSS (Clevios P VP 4083) was spin coated at 4000 rpm for 60 s on the ITO surface as a buffer layer and annealed in the same way prior to the active layer deposition. The active layer was then solvent annealed by covering in glass Petri dishes for 30 min, and subsequently, the lms were annealed on top of a hotplate at 130C for 30 min. The devices were completed by thermal deposition of 30 and 60 nm thick calcium and aluminium, respectively, at a pressure below 106 Torr through a shadow mask.
The photovoltaic performance of the devices was measured inside a glove box lled with N2 under simulated AM 1.5G illumination (100 W cm2) using a xenon lamp based solar simulator (Thermo Oriel 1000 W). The light intensity was cali-brated by a mono-silicon photodiode with a KG-5 colorlter (Hamamatsu, Inc.). Devices were illuminated under mask and the active area of the device was controlled to be 0.1 cm2. The external quantum efficiency (EQE) spectra were obtained under short-circuit conditions. Devices were encapsulated before they were taken out for the EQE measurement. The light source was a 450 W Xe lamp (Oriel Instruments, Model 6123NS). The light output from the monochromator (Oriel Instruments, Model 74100) was focused on the photovoltaic cell being tested.
3
Results and discussion
3.1 Conductivity and opto-electronic properties of treated
PEDOT:PSSlms
It is well known that adding polyols in an aqueous solution of
PEDOT:PSS or lm treatment with them can signicantly
enhance its conductivity. The physical properties of the addi-tives like hydrophilicity and dielectric constant affect the conductivity. However, there is no systematic study on the effect of the molecular weight/chain length of the additives on the conductivity and nally on the device performance especially when the additive is an oligomer or a polymer. Here, we studied
systematically the effect of the additive molecular weight on the conductivity of PEDOT:PSS using EG and different molecular weight PEGs. Fig. 1 shows the conductivities of PH1000 PEDOT:PSSlms prepared by using EG and different molecular weight PEGs with different concentrations. As PEG is a polymer of EG, conductivity enhancement is expected. PEGs with molecular weights 200, 300 and 400 showed almost the same conductivity and the average conductivity was tremendously improved from 0.3 S cm1to 805 S cm1using 2% PEG additive, and then it slightly decreased, whereas the conductivity was only 640 S cm1when 6% EG was used. PEGs with molecular weights of 600 and higher showed lower conductivity than EG, but the trend is similar to that of the other lower molecular
weight PEGs. As reported elsewhere,41 the conductivity
increases gradually from 1% to 4% EG, reaches maximum at 6% EG and then levels off aerwards, whereas even the 1% PEG additive tremendously enhanced the conductivity. Wang et al. used PEG400, PEG800 and higher molecular weight PEGs to enhance the conductivity of PEDOT:PSS but the conductivity was enhanced to only 17.7 S cm1using PEG400,50even though they did not mention the grade of PEDOT:PSS used and also did not compare it with EG. They observed a similar conductivity enhancement trend with increasing concentration and molec-ular weight of PEG. We also noted that the viscosity of the
PEDOT:PSS solution increased with increasing PEG
Fig. 1 (a) Chemical structures of PEDOT:PSS, EG and PEG. (b) Conductivities of PEDOT:PSS treated with different molecular weight PEGs with different concentrations.
concentration andlm uniformity was decreasing by shrinking at the edges especially for higher molecular weight PEGs. The decrease in conductivity with increasing PEG concentration may be attributed to the increase in the insulator PEG on the lm surface as the high boiling point PEG will not evaporate during annealing.
In an effort to further enhance the conductivity, we applied lm treatment either by dropping methanol on the lm or immersing in methanol solution as reported in our previous work.46The conductivity was further enhanced to more than 1100 S cm1aer methanol treatment by either method for EG and PEG200 treatedlms with all concentrations. Additionally, methanol treatment brought betterlm uniformity. For lms prepared using 1% and 2% PEG300 additives, conductivity could be enhanced the same as EG and PEG200 by methanol treatment but for higher concentrations of the PEG300lm, it was disrupted by methanol. Methanol treatment disrupted all PEDOT:PSSlms prepared with all concentrations of PEG400 and higher as shown in Fig. S1 (ESI†). Disruption of the PEDOT:PSSlm with a higher molecular weight PEG indicates that both PEG and PSS have formed some kind of interaction and are removed by methanol as both are soluble in methanol. The relationship between conductivity, mobility and bulk concentration is given by:51
s ¼ emN
wheres is the conductivity, e is the elementary charge, m is the carrier mobility and N is the bulk concentration. As measured by the Hall effect measurement system, the bulk concentration of PEDOT:PSSlms increased from an order of 17 cm3to an order of 21 cm3aer lm treatment and mobility was of the same order varying between 1 and 9 cm2 V1s1before and aer treatment. Here, it is clear that the conductivity enhance-ment is due to an increase in carrier concentration in contrast to Ouyang et al.'s23,52claim that conductivity enhancement is due to the charge carrier mobility resulting from conforma-tional changes of PEDOT chains. This carrier concentration is equal or even a little higher than that of ITO. The bulk concentration for semimetals is in the order of 18 to 21 cm3 and that of metals more than the order of 22 cm3.
Fig. 2a shows the transmittance spectra of ITO and
PEDOT:PSS lms prepared using 6% EG and 2% PEGs. All
PEDOT:PSS lms have better transmittance than ITO; the
transmittance of PEDOT:PSSlms treated with 6% EG and 2% PEG200 is 93% at 550 nm but that of ITO is only 88% including the glass substrate. The lower transmittance of ITO than PEDOT:PSS lms is well noticeable in the violet and blue regions of the spectrum. Film thickness increased with increasing PEG molecular weight as well as increasing the concentration of PEG added in the PEDOT:PSS solution. The
thicknesses of PEDOT:PSSlms were 70 nm with 6% EG and
95 nm with 2% PEG200. Fig. 2b shows the variation of lm thickness, transmittance and sheet resistance with PEG200 concentration. The transmittance value is above 92% till 2% PEG200 concentration and then goes down with increasing concentration in line with the increase inlm thickness from
90 nm to 120 nm for 1% and 6% PEG200 concentrations, respectively. The sheet resistance decreased from 195U ,1to 113U ,1and then increased because of low conductivity. The sheet resistance can further be decreased by preparing multiple layers of PEDOT:PSS. Two layers of the PEDOT:PSSlm fulll the minimum optical and electrical requirements for trans-parent electrodes which are transmittance higher than 90% and sheet resistance less than 100U ,1ensuring that PEDOT:PSS lms are promising replacements for ITO electrodes.
3.2 Mechanism of conductivity enhancement
To further explore the effect of PEG and the mechanism of conductivity enhancement, we utilized various chemical and physical characterizations. The FT-IR spectra of PEG400, PEDOT:PSS and PEDOT:PSS with 2% PEG400 are shown in Fig. 3. As the boiling point of PEG is more than 250C, it is expected that it will not evaporate by annealing at 130C for 30 min. The presence of the peaks at 2875 cm1(C–H stretch-ing) and at 1645 cm1(C–O–H bending) both in PEG400 and in PEDOT:PSSlms treated with PEG conrm the presence of PEG in thelm. The FTIR spectra (not shown here) of PEDOT:PSS lms treated with EG didn't show the presence of EG inside the lm, which is in agreement with the previous reports.23
Fig. 2 (a) Transmittance spectra of ITO and PEDOT:PSSfilms treated with 6% EG and 2% PEGs. The inset compares the transmittances of bare glass (1), PEDOT:PSS treated with EG (2), PEG200 (3) and ITO (4). (b) Variation of transmittance,film thickness and sheet resistance with PEG200 concentration.
Fig. 4 shows the XPS spectra of pristine PEDOT:PSS and those treated with 2% PEG and 6% EG. The S(2p) peak at the binding energy of 167.8 eV corresponds to the sulphur signal of PSS and those at 164 eV and 163 eV correspond to the sulphur signal of PEDOT (Fig. 4a).53The PEDOT:PSS ratio increased aer treatment with EG and PEG which is due to the depletion of the
PSS layer from the lm surface. The PEDOT:PSS ratio of
PEG200, 300 and 400 treatedlms is more than the one treated with EG, which is in agreement with the conductivity enhancement. PEG and EG induce a screening effect between the positively charged PEDOT chains and negatively charged PSS chains, thus reducing the coulombic interaction between them.48 The reduction of the coulombic interaction between PEDOT and PSS chains facilitates phase separation on the nanometer scale. Eventually, by segregation of the excess PSS, there will be higher PEDOT domains on thelm surface which leads to higher conductivity. The sulphur signal could not be detected from PEDOT:PSSlms treated with PEG600 and 1000. Thelm should be covered with a thin layer of PEG. Longer chain PEG has low mobility and some chains will remain on the lm which again is the reason for the low conductivity with
higher molecular weight PEGs. The sulphur signal was also
weak orlms were covered with PEG400 beyond 2%
concen-tration (Fig. S2, ESI†). The C(1s) and O(1s) core level spectra (Fig. 4b and c) of pristine and EG and PEG treated PEDOT:PSS are different. The C(1s) peak is at 283.6 eV for pristine and EG treated lms but it is shied to 285.4 eV for PEG treated
PEDOT:PSSlms.54PEDOT:PSS lms with 2% PEG200 have a
main peak at 283.6 eV and a small peak at 285.4 eV, indicating that only a small amount of PEG200 is present in thelm. Films with 2% PEG300 show a main peak at 285.4 eV and a small peak at 283.6 eV. Pristine and EG treated PEDOT:PSSlms have the same O(1s) spectra with a peak at 531 eV but it is shied to 531.8 eV for PEG treatedlms. The shiing of the C(1s) and O(1s) peaks to higher energy for PEG treated PEDOT:PSSlms conrms the presence of PEG in the lm. As the boiling point of PEG is more than 250C, the presence of PEG in the PEDOT:PSS lm will not affect its thermal stability.
Conductivity is also related to lm morphology and the
changes inlm morphology before and aer lm treatment
were investigated by taking AFM images (Fig. 5). The phase image is homogeneous with disconnected PEDOT chains and weak phase separation between PEDOT and PSS for the pristine lms, whereas there is a good phase separation between PEDOT and PSS chains with moreber like interconnected conductive PEDOT chains aer lm treatment with both EG and PEG. The
Fig. 3 ATR-FTIR spectra of PEDOT:PSSfilm, PEG400 solution and the PEDOT:PSS film treated with 2% PEG400.
Fig. 4 XPS spectra of pristine PEDOT:PSS and PEDOT:PSSfilms treated with 6% EG and 2% PEG. (a) S(2p), (b) C(1s) and (c) O(1s) core-level spectra.
Fig. 5 AFM images of PEDOT:PSSfilms: pristine (a and d), treated with 6% EG (b and e) and treated with 2% PEG200 (c and f). (a), (b) and (c) are phase images, while (d), (e) and (f) are topographic images. All the images are 1mm 1 mm.
depletion of insulating PSS leads to a 3D conducting network of highly conductive PEDOT, resulting in an increase in the conductivity. The topographic AFM images indicate that bigger particles are formed aer lm treatment and those treated with PEG have a more elliptical shape which are bigger than those treated with EG. The rms roughnesses of thelms are 1.2 nm, 1.35 nm and 1.46 nm for the pristine, 6% EG and 2% PEG200 treated PEDOT:PSS lms, respectively. AFM images on lms prepared with a higher molecular weight and a higher concentration of PEG are difficult to obtain because the lms are covered with PEG; the same phenomenon was observed by Badre et al. when they used ionic liquids.34In these cases, the lms were seen to be smoother than the pristine ones.
Xia and Ouyang55revealed that PEDOT:PSS with a higher average molecular weight of PEDOT chains will have bigger gel particles and increased conductivity aer lm treatment than the one with a low molecular weight. As particles grow, the total number of particle boundaries in a given volume or area decreases, and there will be few energy barriers for charge
conduction.47 Hence, charge hopping among the polymer
chains which is believed to be the dominant conduction mechanism in conducting polymers will be easier.56
Theber like phase separated interconnected PEDOT chains in the AFM phase images suggest that the conformation of PEDOT will be changed from a coiled to a linear/extended-coil structure owing to the depletion of PSS from thelm surface. PEDOT and PSS are held by coulombic attractions and have a coiled or core–shell structure due to repulsion between long PSS chains.57 The PEG/EG treatment screens the ionic interaction between PEDOT and PSS by forming hydrogen bonding with both PSSand PSSH. This will lead to better phase separation between PEDOT and PSS, linearly oriented PEDOT chains as proposed in Scheme 1 and bigger and more aggregated PEDOT chains on thelm surface. Ouyang et al. also claimed that the EG treatment of PEDOT:PSS changes the resonant structure of PEDOT chains from a benzoid with a preferred coiled structure to a quinoid with a preferred linear or expanded-coil structure.45 The reorientation of the PEDOT polymer chains from coiled to linear or extended-coil structure allows more chain inter-action between the conducting polymers; hence, the energy barrier for inter-chain and inter domain charge hopping will be
lowered leading to better charge transfer among the PEDOT chains. PEDOT-rich chains with linear structure, larger grain size and lower intergrain hopping promote the charge hopping and eventually the conductivity is tremendously enhanced.
The depletion of the insulator hygroscopic PSS from thelm surface will not only increase the conductivity of thelm but will also improve its long term stability as PSS is the prime reason for the degradation of organic solar cells.41,46 The conductivity stability of the PEDOT:PSSlms was assessed by keeping them in ambient atmosphere at room temperature and humidity higher than 75%. PEDOT:PSSlms treated with 2% PEG200, 300 and 400 maintained up to 66% of the original conductivity, while the lm treated with 6% EG maintained
52% and the pristine lm maintained only 32% in 20 days
(Fig. 6). One reason for the conductivity loss through time is the absorption of moisture by hygroscopic PSS. As conrmed by FTIR and XPS measurements, PEG still remains in thelm by forming hydrogen bonding with both PSSand PSSH. Hence,
the tendency of the lm to absorb moisture by forming
hydrogen bonding will be less forlms treated with PEG. For PEG600 and 1000, the conductivity loss is higher than the other PEGs due to the presence of PEG on thelm surface which itself will absorb moisture.
3.3 ITO-free OSCs using PEDOT:PSS treated with PEG/EG anodes
To evaluate the device performances of PEG treated PEDOT:PSS samples, solar cell devices using PEDOT:PSS anodes were fabricated. The chemical structures of the active layer chemicals (P3HT and PC61BM) and the device architecture are shown in Fig. 7a and b. The effect of PEG concentration on the device performance was assessed for PEDOT:PSS anodes treated with different molecular weight PEGs and the current density ( J)– voltage (V) curves of the devices are shown in Fig. S3 (ESI†), and the power conversion efficiency (PCE), short-circuit current density ( JSC), open-circuit voltage (VOC) and ll factor (FF)
Scheme 1 Schematic illustration of the mechanism of conductivity enhance-ment of PEDOT:PSS with PEG.
Fig. 6 Conductivity stabilities of pristine, 6% EG and 2% PEG treated PEDOT:PSS films in ambient atmosphere.
values extracted from the J–V curves are given in Tables S1–S3 (ESI†). The PCE increased from 1% to 2% PEG200 concentra-tion and then decreased gradually but the minimum PCE is more than 3.12% for 6% PEG200 (Table S1, ESI†). For PEG300 and 400, the PCE is maximum at 2% PEG concentration and then decreased rapidly, the PCE being 1.62% for 6% PEG300 and 1.37% for 6% PEG400. The same trend is observed for PEG600 and 1000 (not shown here). In all cases, the decrease in
PCE with increasing PEG concentration is due to the decrease in JSC. The reason for the rapid decrease in JSC should not be conductivity as the conductivity only changes slightly from 2% to 6% PEG concentration. The reason should be the presence of a thin layer of insulator PEG on the PEDOT:PSSlm. In the case of PEG200, nil or negligible amount of PEG is present in the PEDOT:PSSlm (as can be seen from the XPS spectra) and this should be well mixed with PEDOT:PSS and hence the JSCdoes not decrease rapidly with increasing PEG concentration.
Fig. 7c and d show the J–V curves and EQE spectra of the OSCs with PEDOT:PSS treated with 2% PEG anodes. OSCs using ITO with less conductive PEDOT:PSS (Clevios P VP Al 4083) buffer layer and PEDOT:PSS treated with 6% EG anodes were also fabricated as control devices. The PCE, JSC, VOC and FF values extracted from the J–V curves of the OSCs are given in Table 1. Generally, PEDOT:PSS anodes treated with EG and PEG200 and PEG300 showed higher JSC values than the ITO anode owing to their high transmittance. The PEDOT:PSS anode treated with 2% PEG200 showed a JSCof 9.8 mA cm2, a VOCof 0.58 V and a PCE of 3.62% with a FF of 63.69%. The device with the reference ITO electrode has a PCE of 3.73% with a little higher FF of 68.56% and a lower JSCof 9.38 mA cm2. PEDOT:PSS treated with 6% EG has lower performance compared to PEG200 and 300 due to lower conductivity and higher sheet resistance. The lower PCE and JSCof PEDOT:PSS anodes treated with 2% PEG400 are attributed to the presence of PEG in thelm as the lm conductivity and transmittance are comparable to those of PEG200 and 300. Interestingly, all PEDOT:PSS electrodes showed the same VOCvalue as that of the ITO counterpart and their FF is higher than 61%. The EQE values are in agreement with the J–V values of the devices. For PEDOT:PSS electrodes, there is usually power loss along the lateral distance (shown in Fig. 7b) due to high sheet resistance which decreases the FF.58Since our PEG treated PEDOT:PSS electrodes have lower sheet resistance, the FF was falling from 64% for the rst nger to only 58% for the fourth nger. This fall was further decreased without affecting the PCE by using multiple layer PEDOT:PSS anodes.
4
Conclusions
In conclusion, we have shown that the molecular weight of additives affect the degree of conductivity enhancement of PEDOT:PSS. The conductivity of PEDOT:PSS was enhanced to 805 S cm1with 2% PEG200, 300 and 400 while to only 640 S
Fig. 7 (a) Chemical structures of active layer chemicals. (b) Device architecture of the ITO-free OSC. (c)J–V curves of OSCs with ITO and PEDOT:PSS treated with 6% EG and 2% PEG anodes. (d) EQE spectra of OSCs with ITO and PEDOT:PSS treated with 6% EG and 2% PEG anodes.
Table 1 Photovoltaic performances of OSCs with ITO and PEDOT:PSS treated with 6% EG and 2% PEG anodes extracted fromJ–V curves
Anode JSC(mA cm2) VOC(V) FF (%) PCE (%)
ITO 9.38 0.58 68.56 3.73 PEDOT:PSS–EG 9.85 0.58 61.44 3.51 PEDOT:PSS–PEG200 9.80 0.58 63.69 3.62 PEDOT:PSS–PEG300 9.73 0.58 63.61 3.59 PEDOT:PSS–PEG400 9.22 0.58 64.14 3.43 PEDOT:PSS–PEG600 8.32 0.58 66.11 3.19 PEDOT:PSS–PEG1000 8.37 0.58 62.21 3.02
cm1with 6% EG. The increase in carrier concentration by three orders of magnitude aer the PEG and EG treatments is the main reason for the conductivity enhancement. PEG treated lms also have transmittances as high as 93%. The mechanism of conductivity enhancement is similar to EG. FTIR and XPS measurements showed that PEG remains in thelm. PEG and EG give screening between coulombically attracted PEDOT and PSS and lead to phase separation and reorientation of the PEDOT chains and bigger particle aggregates. For PEGs with molecular weights above 400, the mobility of the PEG chains is less and hence conductivity enhancement by PEG is less. PEG forms hydrogen bonding with hygroscopic PSS and enhances the conductivity stability. The presence of PEG on the lm surface affects the OSC device performance and 2% PEG concentration gave the best efficiency for all molecular weight PEGs. OSC devices with PEG treated anodes showed a PCE of 3.62%, while those treated with EG showed 3.51% and the ITO counterpart showed 3.73%. Our work claries the mechanism of conductivity enhancement associated with the additives in
PEDOT:PSSlms.
Acknowledgements
This work wasnancially supported by the Thematic Project of Academia Sinica (AS-100-TP-A05), and the National Science Counsel (NSC 101-2221-E-001-010), Taiwan.
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