Photovoltaic devices based on PPHT: ZnO and dye-sensitized PPHT: ZnO thin films

Photovoltaic devices based on PPHT: ZnO and dye-sensitized PPHT: ZnO thin films

ARTICLE IN PRESS Solar Energy Materials & Solar Cells 92 (2008) 900– 908 Contents lists available at ScienceDirect Solar Energy Materials & Solar Ce...

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ARTICLE IN PRESS Solar Energy Materials & Solar Cells 92 (2008) 900– 908

Contents lists available at ScienceDirect

Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat

Photovoltaic devices based on PPHT: ZnO and dye-sensitized PPHT: ZnO thin films P. Suresh a, P. Balaraju a, S.K. Sharma a, M.S. Roy b, G.D. Sharma a,,1 a b

Molecular Electronics and Optoelectronic Device Laboratory, Department of Physics, JNV University, Jodhpur 342005, Rajasthan, India Defence Laboratory, Jodhpur 324011, Rajasthan, India

a r t i c l e in f o

a b s t r a c t

Article history: Received 24 September 2007 Received in revised form 8 February 2008 Accepted 17 February 2008 Available online 16 April 2008

We have investigated the optical and photovoltaic properties of hybrid organic solar cells based on the blend of poly-3-phenylhydrazone thiophene (PPHT):ZnO and PPHT:dye:ZnO. In this architecture, ZnO and PPHT were the electron acceptor and donor, respectively, and dye was used both as acceptor as well as sensitizer to enhance the photon absorption in visible region. These results showed that on incorporation of dye in PPHT: ZnO composite the light absorption, exciton separation and photocurrent under white light dramatically enhanced. The dependence of photovoltaic parameters on the weight fraction of ZnO in PPHT: ZnO was also investigated. It is found that the device with 45% of ZnO in both composites exhibits the best photovoltaic performance. The thermal annealing of PPHT: ZnO-based device gives rise to a significant increase in power conversion efficiency as evident from the measurements of incident photon to charge carrier efficiency (IPCE) spectra and current–voltage characteristics under illumination. The absorption band of PPHT: ZnO blend becomes stronger and the absorption peak ascribed to PPHT is shifted towards the longer wavelength region (red shift) upon thermal annealing. The effect of solvent used for thin-film fabrication was also studied. It is also observed that the fluorescence (FL) quenching and solar cell efficiency were found to be strongly dependent on the solvent used for spin coating. & 2008 Elsevier B.V. All rights reserved.

Keywords: Conjugated polymers Nano-particles Photoinduced charge transfer Bulk heterojunction Photovoltaic effect

1. Introduction High charge separation efficiency combined with the reduced fabrication cost associated with solution processing and the potential implementation on flexible substrate make plastic solar cells a competing option for tomorrow’s photovoltaic devices. Organic photovoltaic devices based on both polymeric and small molecular weight semiconductors constitute a subject of intense research activity in recent years, aiming at the production of low-cost and high-efficiency devices [1–6]. Conjugated polymer-based systems have drawn tremendous attention for optoelectronic applications due to their tunable optical and electronic properties [7,8]. Recently, the composites of organic polymers and inorganic nano-particles have attracted great interest due to their potential applications in developing low-cost, large-area, mechanically flexible photovoltaic devices [9,10]. A basic requirement for photovoltaic materials is to generate the free charge carriers produced by the photo-excitation. Subsequently, these carriers are

 Corresponding author. Tel.: +91 291740857; fax: +91 291742495.

E-mail address: [email protected] (G.D. Sharma). MIT, Mandsaur ( MP) 458001, on leave from: JNV University, Jodhpur.

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0927-0248/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2008.02.028

transported through the device to the electrodes without recombining with oppositely charged carriers. Due to the low dielectric constant of the organic materials, the dominant photogenerated species in most of the conjugated polymers is a neutralbound electron–hole pair (exciton). These neutral excitons can be dissociated from coulomb attraction by offering energetically favorable pathways for the electrons from the polymers (donor) to transfer to an electron-accepting species (acceptor). Charge transfer separation in the polymer is often enhanced by inclusion of high electron affinity molecules such as the fullerene derivatives [11–14], organic dyes [15–17] or nano-crystals [18–23]. Although the intermixed (composites) systems can increase dissociation interfaces, transport of carriers remains a challenging problem. Hybrid devices based on the combination of organic and conventional inorganic thin films have attracted much attention, because organic and inorganic materials can take their own advantages in these hybrid devices. Nano-crystals are considered to be more attractive in photovoltaic applications due to their large surface to bulk ratio giving an extension of interfacial area for exciton dissociation into free charge carriers and their transportation, high carrier mobility, good chemical and structural stability. Although the efficiency of conjugated polymer solar cells can be improved upon the introduction of electron-acceptor materials,

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in many cases, the active layer of the solar cells cannot absorb photon over all the solar spectrum and produces low photocurrents. The light absorption in the active layer can be increased by introducing dyes with photoactive layer. Alzarine dyes possess high electron affinity and high absorption coefficient (higher than PPHT). It has been reported that these dyes were used as electron acceptors in conjugated polymer/dye composite photovoltaic devices [24,25] and dye-sensitized solar cells [26,27]. As in energy conversion devices, the power conversion efficiency is a very important parameter. In order to increase the power conversion efficiency, many aspects should be taken into account, such as the absorption coefficient of the materials, light harvesting by the materials, the exciton dissociation rate and the charge carrier mobility. In this article, we report the optical and photovoltaic properties of hybrid organic devices based on poly-3-phenylhydrazone thiophene (PPHT):ZnO and PPHT:dye:ZnO composites. The effect of ZnO concentration on the photovoltaic performance of the device based on PPHT:ZnO blend was also investigated. The incorporation of dye in the PPHT:ZnO blend improves the efficiency of the bulk heterojunction through enhancing the exciton dissociation efficiency and light harvesting. The effect of ZnO fraction in the blends and thermal annealing of the devices were looked via interpreting the measurements of the absorption spectra, incident photon to charge carrier efficiency (IPCE) of photo-generated charge carriers and current–voltage (J– V) characteristics. The effect of solvents used for spin casting of the PPHT:ZnO blend on the fluorescence (FL) quenching and photovoltaic performance was also discussed in detail.

2. Experimental detail ZnO nano-particles were synthesized by hydrolysis and condensation of zinc acetate dihydrate by potassium hydroxide in methanol using a Zn2+:OH ratio as given by Weller et al. [28]. The resulting ZnO nano-particles are insoluble in pure methanol, but by addition of suitable amounts of less polar solvents such as dichloromethane, chloroform or chlorobenzene a stable solution is obtained. The poly-3-phenylhydrazone thiophene (PPHT) was synthesized by the simple condensation of thiophene-3 carboxyaldehyde with phenylhydrazine at low temperatures, i.e., 0–10 1C as reported by us [29]. The blends of PPHT and ZnO were prepared starting by dissolving PPHT in chlorobenzene. The exact amount of ZnO nano-particles solution was added to PPHT solution. The UV–visible spectra of the thin films were recorded on a Perkin Elmer spectrophotometer. FL spectra of the thin films were recorded on Hitachi FL spectrophotometer (F-4500 model). The cyclic voltammetery experiment was performed with a potentiostat (Auto-Lab model), at room temperature using a threeelectrode system. The photovoltaic devices were fabricated by spin casting a mixed solution of PPHT and ZnO on ITO-coated glass substrate. The blend films were spin cast between 500 and 3000 rpm for 2 min to vary the film thickness. All cast films were additionally dried at 2500 rpm for 1 min. The back electrode consisting of 100 nm Al was evaporated in high vacuum. The J– V measurements were performed in air atmosphere at room temperature. In forward bias, the ITO electrode was positively biased. The devices were illuminated through the transparent ITO electrode. The J– V characteristics were measured with Keithley electrometer with the built-in power supply, in dark and under illumination. The IPCE measurements were performed on a set-up utilizing a halogen lamp and a monochromator. The monochromatic light was focused by a lens to create a small light spot. The resultant photocurrent was recorded by Keithley electrometer under short circuit condition. For PPHT:dye blend, a chloroben-

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zene solution of PPHT and dye, i.e. safarnine-O (SAF), were mixed. The films of PPHT:dye:ZnO were grown by similar process as adopted for PPHT:ZnO. The thermal annealing treatment was accomplished by placing the device on a thermoelectric hot plate in a glove box at 120 1C for a period of 10 min under ambient atmosphere, after the Al electrode deposition. After the 10 min period, the devices were left in glove box in order to cool down to room temperature and then the measurements were performed.

3. Results and discussion 3.1. Electrochemical and optical properties Cyclic voltammetery (CV) is a useful method for measuring electrochemical behavior, evaluating the relative HOMO and LUMO energy level and the band gap of the materials. The energy bandgap of PPHT and dye (Eg ¼ eDf) have been calculated from the difference between energies of LUMO and HOMO levels, which are estimated from the onset potential of the n-doping (fn) and p-doping (fp) processes, respectively. These levels are estimated with respect to (SCE) saturated calmel electrode) as the reference electrode by adding 4.4 eV to the corresponding measured electrochemical potentials. Thus, ELUMO and EHOMO can be described by following equations [30]: ELUMO ¼ eðfn þ 4:4Þ EHOMO ¼ eðfp þ 4:4Þ

(1)

The CV results of PPHT are shown in Fig. 1. Similar results have also been obtained for dye. The obtained energy levels for LUMO and HOMO and energy gap values for dye and PPHT are displayed in Table 1. The value of energy gap estimated through the analysis of the electrochemical data is quite close to that extrapolated from the absorption onset, i.e. optical band gap, which demonstrates the reliability of the electrochemical evaluation of the LUMO and HOMO levels. In Fig. 2, the n- and p-doping process of PPHT:ZnO nanocrystal-blended film is shown. In reduction cycle, the PPHT blend with ZnO exhibits n-doping and n-doping peak shifted to a higher potential with respect to those found for pure PPHT. In oxidation cycle, an irreversible p-doping is observed, appearing shifted to higher potential as compared with that for the pure PPHT.

Fig. 1. Cyclic voltammetry of PPHT.

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Table 1 Electrochemical evaluation of fn, fp, HOMO energies and LUMO energies for PPHT, dye and PPHT: ZnO blend films and band gap

PPHT Dye PPHT:ZnO

fp (V vs Ag)

fn (V vs Ag)

EHOMO (eV)

ELUMO (eV)

Eg (eV)

1.4 1.0 2.0

0.4 0.9 0.8

4.7 5.3 6.4

2.7 3.5 5.2

2.0 1.8 1.2

Fig. 3. Absorption spectra of ZnO, PPHT and dye thin films.

Fig. 2. Cyclic voltammetry of PPHT:ZnO blend.

The values of fn and fp, HOMO and LUMO energies for the films of PPHT, PPHT:ZnO blend as estimated from the CV data are illustrated in Table 1. The shift to higher potentials in both oxidation and reduction cycles suggests a lower ionic conductivity and/or a slower ionic motion in the blended film with respect to that in the pure polymer. The lower oxidation current can be attributed either to some dissolution of the oxidized form of the film, following the shift toward more positive potentials, or to a decrease in the p-conductivity of the polymer by the presence of the inorganic semiconductor [31]. Figs. 3 and 4 show the UV–visible absorption spectra of PPHT, ZnO and dye and PPHT:ZnO and PPHT:dye:ZnO thin films, respectively. The pure PPHT has absorption edge at around 560 nm corresponding to the lowest energy p to p transition. It can be seen from these figures that PPHT shows a strong absorption in the visible range from 400 to 550 nm, while the ZnO absorbs light in the wavelength range from 200 to 350 nm. The absorption peak of PPHT:ZnO composite shows the overlapping absorption bands of PPHT and ZnO, which indicates that the there is no charge transfer taking place at ground state. The PPHT:dye:ZnO blend shows the broad and strong absorption nearly throughout all of the visible wavelength range resulting enhanced light harvesting. The FL quenching in blend of donor–acceptor (D–A) is a useful indication for the efficient charge transfer between the materials. The FL spectra of PPHT, PPHT:ZnO and PPHT dye:ZnO are shown in Fig. 5. For pure PPHT, there is an emission peak at the wavelength of 620 nm. The FL emission intensity is decreased from 80 to 35 for PPHT:ZnO composite film, and when dye is incorporated into the PPHT:ZnO blend further quenching is observed. The strong FL quenching for PPHT:dye:ZnO composite is an evidence of the more efficient photo-induced charge transfer for this blend than that for PPHT:ZnO blend. In this blend film, not only can the dye

Fig. 4. Absorption spectra of blend films.

Fig. 5. Fluorescence spectra of PPHT, PPHT:ZnO and PPHT:dye:ZnO films.

absorb more photons from the light but also the interface area between donor and acceptor in the blend is also increased. As a result of increased interface area, the excitons can be more

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effectively separated into free electrons and holes, which cause the FL quenching. The FL quenching in the presence of ZnO nano-crystals can be attributed to either energy or charge transfer from polymer to the inorganic semiconductor. In a blended device the photo-induced charge transfer depends on the difference in the position of LUMO levels of donor and acceptor components. Since the difference in the position of LUMO of PPHT and conduction band of ZnO is about 1.5 eV, which is greater than the exciton binding energy (0.5 eV) in PPHT. This favors the photoinduced charge transfer process in the blend. The energy transfer mechanism is ruled out in this system due to the lack of overlap between the ZnO absorption of PPHT emission spectra. On the basis of the relative positions of PPHT and ZnO energy levels, the dissociation of the exciton at the interface between the two materials is energetically allowed. Additionally, the transfer of PPHT photo-generated electrons to ZnO conduction band should be responsible for the polymer emission quenching as shown in Fig. 5. Fig. 6 shows the variation of photoconductance and FL quenching as a function of weight fraction of ZnO (WZnO) in the blend. It is observed that the dependence of FL quenching on WZnO is much less pronounced than the variation of photoconductance with WZnO. This may be due to the fact that for low WZnO quenching of excitons already occurs efficiently at the interfaces with individual ZnO nano-particles, while for higher WZnO the amount of interfacial exciton (and FL) quenching does not increase very much. However, the fraction of excitons that are dissociated at the interface with a cluster of ZnO particle rather than individual nano-particles will increase with WZnO. Therefore, photoconductance depends more strongly on ZnO concentration than on the FL quenching.

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Fig. 7. Current–voltage characteristics of ITO/PPHT:ZnO (45 wt%) /Al device.

3.2. Electrical and photovoltaic properties of ITO/PPHT:ZnO/Al device We have measured the J– V characteristics of the Al/PPHT:ZnO/ ITO devices in dark and under illumination as a function of concentration of ZnO in the blend, and the J– V characteristics of a device with 45 wt% ZnO are shown in Fig. 7. From the analysis of J– V characteristics under illumination we have estimated the photovoltaic parameters, i.e. short circuit photocurrent (Jsc), open Fig. 8. Variation of short circuit current and open circuit voltage with ZnO concentration in the PPHT:ZnO blend for Al/PPHT:ZnO/ITO device.

Fig. 6. Variation of fluorescence quenching and photoconductance with fraction of ZnO in the PPHT:ZnO blend.

circuit voltage (Voc), fill factor (FF) and power conversion efficiency (Z%). Figs. 8 and 9 show the variation of these parameters with concentration of ZnO. The Voc of the device decreases with increasing amount of ZnO; this may be related with the increase in reverse saturation current in dark due to the formation of easy conducting pathways of ZnO particles from the electrode to electrode. The Jsc initially increases when the amount of ZnO in the blend is increased. This variation can be explained by the increased formation of charges as the FL intensity decreases and as the contents of ZnO in the blend increase (Fig. 6). Additionally, the formation of more percolation pathways of ZnO takes place, which improves the transport of electrons, resulting in an increase in photocurrent. A higher concentration of ZnO in the blend beyond 45% leads to a decrease in Jsc, which may be due to a decrease of absorption of light by PPHT. Similar results have been reported for the devices based on the blend consisting of ZnO:MDMO-PPV [32]. The reason may be the occurrence of completely integrated band energy level pathways in the composite films. With the formation of the continuous pathways, there will be many mid gap-states on the surface of ZnO, which pin the Fermi level.

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contacts is provided by the built-in potential, which decreases at higher concentration of the ZnO, the collection of charge carrier is weakened and Jsc also decreases. 3.3. Effect of thermal annealing

Fig. 9. Variation of power conversion efficiency and fill factor with ZnO concentration in the blend for AI/PPHT; ZnO/ITO device.

The device ITO/PPHT:ZnO/Al (optimized condition) exhibited a Voc of 0.67 V, Jsc of 0.30 mA/cm2 and FF of 0.52 and conversion efficiency of about 0.12%. The theoretical value of Voc for devices with blend photoactive layer is given by following expression: qV theo ¼ ðLUMOÞacceptor  ðHOMOÞdonor oc

(2)

In the case of PPHT:ZnO, the theoretical bandgap estimated from Eq. (2) is approximately 0.8 eV. The estimated value of Voc from the J– V characteristics under illumination is less than that estimated from Eq. (2). This can be understood as follows: The work function of ITO (fITO ¼ 4.8 eV) matches with the HOMO of PPHT, resulting in a nearly ohmic contact for holes in the bulk heterojunction photovoltaic device, under forward bias. On the other hand, Al makes a nearly ohmic contact for electron injection to the conduction band of ZnO (4.4 eV). Due to a slight mismatch of the work functions of the electrodes with the HOMO of PPHT and the conduction band of ZnO, band bending occurs at both interfaces. For this, Eq. (2) modifies to Eq. (3): qðV theo oc þ DV b Þ ¼ ðLUMOÞacceptor  ðHOMOÞdonor

The efficiency of the composite devices is strongly dependent on thermal annealing. The overall effect of thermal annealing at 120 1C is that Jsc increases and Voc is almost constant. Table 2 summarizes the effect of thermal annealing and concentration of ZnO on the photovoltaic parameters of ITO/PPHT:ZnO/Al device. It is observed that the improvement in Jsc is more pronounced for devices with lower concentration of ZnO. This is an indication that annealing mainly affects the PPHT phase. The improved chain ordering of PPHT chains caused by the thermal treatment results in enhanced hole transport. This improved charge transport is consistent with the increased FF upon thermal annealing. The absorption spectra of PPHT:ZnO blend were investigated before and after thermal annealing as shown in Fig. 10. It is observed that the peak due to ZnO remains unchanged, whereas the region of PPHT absorption is strongly affected by thermal annealing. The peak at 480 nm in the case of non-treated blend shifts to 540 nm after 1 h of thermal annealing at 120 1C. The absorption band edge shift is even larger. The apparent modification of the PPHT:ZnO absorption spectra can be attributed to the molecular diffusion of ZnO in the polymer matrix. After 1 h of thermal annealing, the overall absorption spectra of PPHT:ZnO film looks different compared to the untreated case, i.e. the region Table 2 The effect of thermal annealing (Temperature 120 1C for 10 min) on the performance ITO/PPHT:ZnO/Al device for different concentration of ZnO in the blend film ZnO (%Wt)

10 20 30 40 45 55

Jsc (mA/cm2)

Voc (V)

un-annl

annl

un-annl

annl

un-annl

annl

un-annl

annl

0.002 0.08 0.16 0.33 0.38 0.35

0.006 0.12 0.21 0.37 0.54 0.23

0.76 0.70 0.68 0.64 0.64 0.64

0.61 0.61 0.64 0.61 0.61 0.60

0.37 0.41 0.48 0.53 0.54 0.50

0.44 0.45 0.50 0.55 0.57 0.54

0.01 0.02 0.05 0.12 0.12 0.11

0.12 0.23 0.34 0.42 0.53 0.95

FF

Z (%)

(3)

where DVb is the sum of the voltage losses at each contact due to band bending. Hence, we can conclude that the lower value of Voc is due to the voltage losses occurred due to the band bending at the interfaces. The other possible reason for the lower measured Voc includes the presence of mid-gap states on the surface of ZnO that pin the Fermi level, or electron mobility of the ZnO could be actually too high, resulting in increased carrier recombination at ZnO/PPHT interface resulting in a reduced Voc. The higher value of Jsc for PPHT: ZnO blend than pure PPHT is due to the fact that spacing between ZnO nano-crystal is substantially larger than the typical exciton diffusion length in PPHT. This suggests that closer spacing of nano-crystal yields larger photocurrent. Therefore, with increasing concentration of the ZnO nanoparticles, carrier transport characteristics of the composite film change from dominating by holes to dominating by electrons gradually. This result shows that the electron mobility of the ZnO nano-crystal is too high, resulting in increase of carrier recombination at the ZnO/polymer interface that causes decrease of Voc. Since the driving force of charge carrier transport to respective

Fig. 10. Effect thermal annealing (120 1C for 1 h) on absorption spectra of PPHT:ZnO blend.

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around the absorption peak of PPHT extends towards the longer wavelengths, and also the height of the peak increases. After the thermal annealing, the part of the absorption spectra originating from the ordered phase not only increases but is also redshifted. This is in agreement with the crystal growth theory in polymers, which states that with increasing temperature the size of crystallites increases. As the polymer chains are extended through the crystallites, thicker crystallites would give a longer conjugation length [33]. Additionally, the observed shift in the band gap of blend absorption spectrum can be attributed to the particular thermo-chromatic properties of PPHT to a possible molecular diffusion of nc-ZnO to polymer matrix [34]. The processes that may occur upon the photo-excitation of the polymer in blend with ZnO include the following: hv

P ! P

(A)

P ! P

(B)

P þ ZnO ! Pþ þ ½ZnO

(C)

P þ ½ZnOn ! Pþ þ ½ZnO n

(D)

Pþ þ ½ZnO ! P þ ½ZnO

(E)

Pþ þ ½ZnO n ! P þ ½ZnOn

(F)

 ½ZnO n ðefree Þ ! ½ZnOn ðetrap Þ

(G)

In the above process, an individual ZnO nano-particle is denoted as ZnO, while [ZnO]n refers to a cluster of n-ZnO nano-particles. Photo-excitation of the polymer results in the formation of excitons (A), which can decay back to the ground state by FL or a non-radiative transition (B). During their lifetime, the excitons may diffuse to an interface with an individual ZnO nano-particle (C) or reach a cluster consisting of ZnO nano-particles (D). At the interface with ZnO, excitons can dissociate into charge carriers, leading to electron injection into the ZnO and production of a hole in the polymer. Opposite charges can recombine to each other (E) and (F), while electrons in a ZnO cluster may also become trapped at a defect site. Annealing of photoactive blend layer results in the diffusion of nc-ZnO within PPHT film with an enhanced crystallinity, which implies that after photo-induced charge separation the chance of recombination is reduced. This can be expressed by the following processes: þ  Pþ þ ½ZnO n ! P þ ½ZnOn

(H)

½Pþ þ ½ZnO n ! D þ ½ZnOn

(I)

Since, after thermal annealing, process (H) effectively completes with process (E) and (F), the formation of mobile positive charge carriers in PPHT is enhanced, which leads to an increase in photoconducting quantum efficiency. The diffusional escape from the recombination and possible subsequent trapping of electron in ZnO-rich parts will reduce the charge recombination (process (G)), resulting in a higher quantum efficiency in annealed films as compared to that in untreated films. 3.4. Effect of solvent In order to investigate the influence of the film morphology on the FL quenching and photovoltaic performance, we prepared PPHT:ZnO film by spin casting using different solvents i.e. THF, chloroform, chlorobenzene and xylene. Fig. 11 shows the FL spectra of PPHT:ZnO thin films with 45% ZnO concentration fabricated using different solvents. The photovoltaic performance

Fig. 11. Fluorescence spectra from PPHT:ZnO layers spin coated from different solvents.

Table 3 Comparison of solar cells with PPHT: ZnO (45 wt%) layer prepared from different solvents Solvent

Voc (V)

Jsc (mA/cm2)

FF

Z (%)

THF Chloroform Chlorobenzene Xylene

0.12 0.23 0.38 0.67

0.65 0.66 0.64 0.62

0.45 0.48 0.54 0.59

0.034 072 0.12 0.24

of the devices is compiled in Table 3. It is observed that the strongest FL quenching and highest power conversion efficiency are observed for xylene. Therefore, it can be concluded that the solvent used for deposition of thin film affects both FL quenching and recombination of separated charges, which is likely due to the different film morphologies caused by the different solvents. It is known that the solvent and spin-coating conditions can affect the aggregation of the polymer chain [35], as well as the quality of the polymer electrode contacts [36]. Power conversion efficiency of the device depends upon the solvent used, which indicates that in spite of thermal annealing after spin coating the solvent used affects the quality of the PPHT and its interface with ITO. The results obtained are similar to those reported for MEHPPV-C60 [37] and P3HT:TiO2 [38] devices, where the difference in the photovoltaic performance of the devices fabricated from xylene, dichlorobenzene, chlorobenzene, THF and chloroform was explained by the preferential solvation of p-electron-conjugated segments in aromatic solvents. Unlike the results reported by Liu et al. [37], our results did not show any significant variation in the pure PPHT devices on the solvent used for the film fabrication. Therefore, the mechanism proposed by Liu et al. [37] is probably not a significant factor in determining the performance of PPHT:ZnO solar cells. The main difference between the films prepared from different solvents is the degree of mixing between PPHT and ZnO, which is dependent on the solvent power of the solvent used and on the solvent evaporation rate. It is known that the solvent evaporation rates also influence the surface morphology of the polymer film [39]. THF and chloroform have one order of magnitude higher vapor pressure compared to xylene and hence evaporate significantly faster than xylene and chlorobenzene. Additionally, the solvent power of aromatic solvents like

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xylene and chlorobenzene for PPHT leads to a more extended polymer chain in solid state and thus to a different morphology. It should be noted that while best efficiencies are generally obtained for the nano-scale phase separation, the solvent resulting in the nano-scale phase separation might be different for different materials used in the systems. Therefore, the general guideline for improving the efficiency of composite or blend polymer solar cells is to achieve the exciton efficiency, which would otherwise be poor due to the short diffusion length. The morphology of the composite or blend layer is strongly solvent dependent, but the optimal solvent for the small-scale phase separation may be different for different materials used. 3.5. Effect of dye incorporation in the PPHT:ZnO blend Although we have observed that the efficiency of polymer solar cells can be improved upon introduction of nano-particles of inorganic semiconductors, the blend of PPHT:ZnO does not absorb the photons of the whole solar spectrum to produce high photocurrent. The light absorption in the PPHT:ZnO photoactive layer can be increased by incorporation of dye in the PPHT:ZnO blend. The dye has high electron affinity and high absorption coefficient than PPHT. The photovoltaic device PPHT:dye:ZnO photoactive layers were prepared to study the photoelectric performance of the materials. The configuration of the PPHT:dye:ZnO solar cell is given in Fig. 12(a). All the devices used for comparison have similar layout. The phase morphology of the PPHT:dye:ZnO bulk heterojunction film is shown in Fig. 12(b). The J– V characteristics of photovoltaic device consisting of PPHT:dye:ZnO as active material in dark and under illumination are shown in Fig. 13. The photovoltaic parameters calculated from the J– V characteristics of the device are summarized in Table 4. Blending with either dye or ZnO improves the Jsc of the device. In the case when the photoactive material is PPHT:dye:ZnO, the photocurrent is the highest, of about 1.2 mA/cm2, which is ten times higher than that of PPHT:ZnO device. As shown in Fig. 5, FL is strongly quenched in the PPHT:dye:ZnO composites films, which indicates a high efficiency of charge separation. Therefore, the increase in the photocurrent can be attributed to the increased light absorption and efficient charge separation. The incorporation of dye into PPHT results in efficient exciton dissociation and improvement in Jsc. This result suggests that the efficient electron transfer from dye to ZnO is taking place as there is no direct pathway for electrons to be transferred from dye to ITO electrode. The effectiveness of the photovoltaic device to convert incident photons of a given wavelength into photocurrent is measured by

Fig. 12. (a) Layout of PPHT:dye:ZnO photovoltaic device, active layer is sandwiched between ITO and aluminum top electrode. (b) Schematic morphology of bulk heterojunction layer.

Fig. 13. Current–voltage characterisitics of ITO/PPHT:dye:ZnO(45 wt%)AI device in dark and under illumination.

Table 4 Photovoltaic parameters for Al/PPHT: ZnO(45 wt%)/ITO and Al/PPHT: dye: ZnO (45 wt%)/ITO device Device

Jsc (mA/cm2)

Voc (V)

FF

Z (%)

Al/PPHT: ZnO/ITO Al/PPHT: dye: ZnO/ITO

0.38 1.45

0.64 0.62

0.54 0.61

0.12 0.55

the IPCE as given by IPCE ¼ 1240J sc =lP in

(4)

where Jsc is the short circuit photocurrent at incident light, l is the wavelength of the light and Pin is the intensity of the light. The IPCE spectra of Al/PPHT:ZnO/ITO and Al/PPHT:dye:ZnO/ITO devices are shown in Fig. 14. The IPCE spectra of both devices closely resemble the absorption spectra of the respective photoactive blend, indicating the formation of bulk heterojunction. The device ITO/PPHT:ZnO/Al shows a maximum IPCE of 40% at 490 nm, with a clear contribution to the photocurrent from both PPHT and ZnO counterparts. The peak in IPCE spectra at 360 nm results from the absorption of ZnO and holes transfer to PPHT. When a blend of PPHT:dye is added to ZnO, a dramatic increase in the IPCE is observed to a value of 72% at 620 nm. This increase in the IPCE is likely the result of enhanced exciton dissociation in the film, where the blend effectively reduces the distance between donor and acceptor materials. It is observed that both the absorption of photoactive layer and IPCE spectra of the device are broadened over that of the device without dye, which is likely due to the strong absorption of dye in the visible part of the spectrum. The FF of the device is slightly reduced with the incorporation of dye, which may be related to the blend morphology. The role of dye is as follows: (a) in addition to PPHT/ZnO interface, PPHT/dye interface also acts as a site for exciton dissociation. Hence, the effective volumes of exciton dissociation centers are much higher in the dye-blended device (b) dye in PPHT matrix, which provides efficient pathways for hole transport to the ITO. This dual contribution has resulted in higher photocurrent in the dyeblended device. The energy levels of the materials used in the present investigation are shown in Fig. 15. The energy gap of the dye lies between the LUMO of the PPHT and the conduction band of ZnO.

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Fig. 14. Photoaction spectra of devices illuminating through ITO electrode.

Fig. 16. Intensity dependence of short circuit current (Jsc) for the devices.

devices is shown in Fig. 16. The Jsc shows power law dependence with illumination intensity with exponent about 0.94 and 1.1 for ITO/PPHT:ZnO/Al and ITO/PPHT:dye:ZnO/Al devices, respectively. This increase in exponent for former devices as compared to ITO/ PPHT:dye/Al device is due to the higher photogenerated excitons and consequently more dissociated carriers at higher intensity. A higher value of slope than unity could be due to the increased mobility at high density of carriers [42,43] and field-enhanced dissociation of excitons [44,45]. This also indicates that the incorporation of dye in the PPHT:ZnO blend reduces the recombination significantly. Fig. 15. Energy-level diagram for PPHT, dye and ZnO relative to vacuum.

4. Conclusions After illumination, first the excitons are generated in the active layer by absorbing incident light, then excitons diffuse in the active layer and dissociate at the D–A interface. The dye can transfer electrons to the ZnO and holes to PPHT in the PPHT:dye:ZnO films. The PPHT can also absorb photons and transfer excited electrons to the adjacent dye or ZnO. Both ZnO and dye act as electron donors to PPHT. The nano-structured ZnO phase can provide a large interfacial area. As the dye absorbs light more effectively in a longer wavelength region, the power conversion efficiency improves. The band diagram of PPHT:dye:ZnO PV device with ITO and Al electrodes is shown in Fig. 15. It shows that this device would allow exciton dissociation and electron and holes transport without any bias. The dye in polymer layer provides an extended region for exciton dissociation at PPHT/dye interfaces. Additionally, hole transport to ITO electrode was energetically favorable through the dye sites. Without dye in polymer matrix, the excitons dissociation would occur only at PPHT/ZnO interface and hence hole transport across the PPHT layer would be restricted due to the low mobility in polymer. Additionally, the dye may serve as a conducting bridge connecting polymer chains [40]. We have also studied the intensity dependence of Jsc and Voc for the device. It is observed that, with the increase in light intensity, the Voc showed a slow increase, which can be explained by considering a strong voltage dependence of photogenerated current [41]. The variation of short circuit current with incident light intensity for ITO/PPHT:ZnO/Al and ITO/PPHT:dye:ZnO/Al

We have studied the optical and electrochemical properties of PPHT and PPHT:ZnO thin films. The photovoltaic performance of PPHT:ZnO blend solar cells as a function of ZnO concentration and the solvent used for the thin-film preparation has been investigated. Significant FL quenching of the PPHT emission is obtained, when the concentration of ZnO is about 45% in the composite. We also found that the FL quenching and power conversion efficiency of the device exhibit strong dependence on the solvent used, which was attributed to the difference in the degree of mixing between PPHT and ZnO. The thermal annealing of the spin-casted PPHT:ZnO blend improves charge transport and power conversion efficiency, which is attributed to the increased carrier mobility and red shift in absorption peak. In PPHT:dye:ZnO composite, the dye acts as sensitizer and enhances the exciton dissociation and transfers the excited electrons and holes to PPHT and ZnO, respectively. The incorporation of dye in PPHT:ZnO composite dramatically increases both the absorption and the FL quenching. The inclusion of dye into the PPHT:ZnO blend film significantly increased the photocurrent in comparison to PPHT:ZnO device, leading to an enhancement in power conversion efficiency.

Acknowledgments We are grateful to the Department of Science and Technology (DST), Government of India, New Delhi, and the Council for

ARTICLE IN PRESS 908

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