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Improvements of fill factor in solar cells based on blends of polyfluorene copolymers as electron donors
Thin Solid Films 515 (2007) 3126 – 3131 www.elsevier.com/locate/tsf
Improvements of fill factor in solar cells based on blends of polyfluorene copolymers as electron donors Abay Gadisa a,b,⁎, Fengling Zhang a,b , Deepak Sharma c , Mattias Svensson d , Mats R. Andersson b,d , Olle Inganäs a,b a
d
Department of Physics, Chemistry and Biology (IFM), Linköping University, S-58183 Linköping, Sweden b Center of Organic Electronics (COE), Linköping University, S-58183 Linköping, Sweden c Department of Electrical Engineering, Indian Institute of Technology Kanpur, Kanpur 208016, India Department of Organic Chemistry and Polymer Technology, Chalmers University of Technology, S-41296 Göteborg, Sweden Received 3 February 2006; received in revised form 28 July 2006; accepted 28 August 2006 Available online 11 October 2006
Abstract The photovoltaic characteristics of solar cells based on alternating polyfluorene copolymers, poly(2,7-(9,9-dioctyl-fluorene)-alt-5,5-(4′,7′-di-2-thienyl2′,1′,3′-benzothiadiazole)) (APFO-3), and poly(2,7-(9,9-didodecyl-fluorene)-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)) (APFO-4), blended with an electron acceptor fullerene molecule [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), have been investigated and compared. The two copolymers have the same aromatic backbone structure but differ by the length of their alkyl side chain. The overall photovoltaic performance of the solar cells is comparable irrespective of the copolymer used in the active layer. However, the fill factor (FF) values of the devices are strongly affected by the copolymer type. Higher FF values were realized in solar cells with APFO-4 (with longer alkyl side chain)/PCBM bulk heterojunction active layer. On the other hand, devices with blends of APFO-3/APFO-4/PCBM were found to render fill factor values that are intermediate between the values obtained in solar cells with APFO-3/PCBM and APFO-4/PCBM active film. Upon using APFO-3/APFO-4 blends as electron donors, the cell efficiency can be enhanced by about 16% as compared to cells with either APFO-3 or APFO-4. The transport of holes in each polymer obeys the model of hopping transport in disordered media. However, the degree of energetic barrier against hopping was found to be larger in APFO-3. The tuning of the photovoltaic parameters will be discussed based on studies of hole transport in the pure polymer films, and morphology of blend layers. The effect of bipolar transport in PCBM will also be discussed. © 2006 Elsevier B.V. All rights reserved. Keywords: Solar cells; Photovoltaic characteristics; Charge transport
1. Introduction The performance of solar cells with polymer/fullerene donor/ acceptor interpenetrating networks is limited by several factors, such as narrow optical absorption bands, poor carrier mobility and low stability of the polymeric materials. Other limiting factors may also be introduced during processing, resulting into large-scale phase separation, and poor film morphology. Clearly, optimisation of these devices can be achieved by combing welldesigned molecular structures, better device architecture and careful processing in a clean environment. Promising improve⁎ Corresponding author. Department of Physics, Chemistry and Biology (IFM), Linköping University, S-58183 Linköping, Sweden. Tel.: +46 13 288917; fax: +46 13 288969. E-mail address: [email protected] (A. Gadisa). 0040-6090/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2006.08.043
ments have been achieved through use of new materials with red shifted optical absorption spectrum [1], optimisation of device structure through modification of interfaces at the electrodes [2], improvement of film morphology by appropriate choice of deposition solvents [3,4], controlling evaporation rate of solvents [5], and heat treatments [6]. Polyfluorene copolymers have emerged as a new class of semiconductor materials due to their high charge carrier mobility and good processability [7,8]. It has been observed that several polyfluorenes and their copolymers incorporate liquid crystallinity [9]. In general, polyfluorene copolymers have high band gaps giving blue-shifted optical absorption and hence have limited use in wide band photodiodes. Nevertheless, related copolymers have been synthesized and utilized in light emitting diodes whereby broad emission spectrum in the whole visible range has been realized [8].
A. Gadisa et al. / Thin Solid Films 515 (2007) 3126–3131
We have reported the synthesis of low band gap polyfluorene copolymers and their performance in solar cells [1,10–13]. In our current report, we demonstrate the enhancement of fill factor (FF) of solar cells upon using blends of two similar polyfluorene copolymers as electron donors. The chemical structure of the copolymers is depicted in Fig. 1. The aromatic back bone structures of the two copolymers are the same while their alkyl side chains have different lengths. Commonly, the main chain of polymers is decorated with side chains for various reasons. Side chains may play a crucial role in determining the conformation and solvent processability of conjugated polymers [14]. It is well understood that long, non-branched hydrocarbon chains can easily pack together and form well ordered solution-processed films [15]. According to a recent report [16], side chain length has a crucial impact on carrier transport. Moreover, the fact that efficient polymer based solar cells rely on polymer/acceptor blends implies that the miscibility of the polymer with acceptor molecule is quite crucial. In this context, the size and shape of the side chains play major roles. In this paper, we report the improvement of photovoltaic parameters of polymer-based solar cells upon using blends of two sim-
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ilar copolymers, poly(2,7-(9,9-dioctyl-fluorene)-alt-5,5-(4′,7′di-2-thienyl-2′,1′,3′-benzothiadiazole)) (APFO-3), and poly (2,7-(9,9-didodecyl-fluorene)-alt-5,5-(4′,7′-di-2-thienyl2′,1′,3′-benzothiadiazole)) (APFO-4), as electron donors, and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) as an electron acceptor and transporting material. 2. Experimental details The procedures for the synthesis of the copolymers were described in Ref. [1]. The chemical structures of the copolymers and PCBM are depicted in Fig. 1. All the devices were constructed in the form of sandwich structure. The anode was formed by spin coating poly (3, 4-ethylene dioxythiophene)-poly (styrene sulphonate) (PEDOT: PSS) (from Bayer AG, EL Grade) on a pre-cleaned indium–tin–oxide (ITO)/glass substrate. The thickness of the PEDOT:PSS film was about 50 nm as measured by DEKTAK 3030 surface profilometer. Solutions of APFO-3/PCBM (1:4 by wt.), APFO-4/PCBM (1:4 by wt.), and APFO-3:APFO-4/PCBM (1:4 by wt.) were prepared in solution of chloroform and spin coated on top of the PEDOT:PSS layer. The thickness of the blend films ranges from 200 to 240 nm. Thin films of lithium fluoride (LiF) (less than 1 nm), and aluminium (Al) (60 nm) were evaporated to form the top electrode, cathode. The thermal evaporation of Al and LiF was done under a shadow mask in a base pressure better than 10− 6 mbar. The active area of the devices thus constructed varies from 4 to 6 mm2. For simplicity, the devices are named according to the type of copolymer they comprise in their active layer: Device 1 and Device 2 comprise APFO-3 and APFO-4, respectively, while Device 3, and Device 4 comprise blends of APFO-3 and APFO-4 in a weight ratio of 1:1, and 3:1, respectively. Sixteen samples were prepared and characterized for each device type. Device preparations and characterizations were done under ambient conditions except the evaporation of Al and LiF. The current–voltage (I–V) measurements under white light illumination (1 sun) were performed with a Keithley 2400 electrometer. Ozone free Xenon lamp, coupled with solar spectrum simulating filters, was used to provide a simulated light of intensity 100 mW/cm2. Photocurrent generated at a specific wavelength was measured using Keithley 485 Picoammeter. The optical absorption spectra were obtained with a Perkin Elmer Spectrophotometer λ9. The film morphologies were studied under tapping mode atomic force microscopy (AFM) (Nanoscope III with J-scanner head, Digital Instruments). Height images were produced by operating the AFM in tapping mode, at a scan rate of 0.4–1.0 Hz, and by collecting 512 × 512 data points. The AFM has a high aspect ratio conical tip that consists of a noncontact “Golden” Silicon cantilever. The tip has a standard size (1.6 × 3.6 mm), and thickness (0.4 mm) of cantilever chip. 3. Results and discussions 3.1. Absorption spectrum
Fig. 1. The chemical structures of the copolymers and PCBM (top panel). The normalized optical absorption spectra of APFO-3, APFO-4, and APFO-3: PCBM (1:4, by wt.) are depicted in the lower panel.
The normalized optical absorption spectra of the polymer films are depicted in Fig. 1. It is observed that, the copolymers have similar optical response in the visible range of the solar
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Fig. 2. Variation of the zero-field mobility (μ0) with temperature. The lines show the fits, which were generated based on the Gaussian correlated disorder model (CDM).
spectrum with an absorption maxima slightly red shifted with increasing side chain length. This shift in absorption with increasing side chain may be an indication of the formation of more planar backbone conformation. Similar phenomenon was observed for other types of polymers of the same family but different side chain length [17]. The absorption spectra of the copolymer/PCBM blend films have similar features; a typical blend absorption spectrum is shown in Fig. 1. 3.2. Hole transport in APFO-3 and APFO-4 thin films Hole transport in thin films (100 nm) of APFO-3 and APFO4 was investigated by analysing the space charge limited (SCL) current–voltage (I–V ) characteristics of hole dominated diodes. The measured I–V characteristics are temperature (T ), and field (E ) dependent, which is a typical signature of hopping transport in disordered materials. Mobility of charge carriers that exhibit hopping transport is often described by the Poole–Frenkel pffiffiffiffiffiffiffiffiffiffiffi equation lðE; T Þ ¼ l0 ðT ÞexpðgðT Þ EðxÞÞ, where μ0(T ) is the zero-field mobility, and γ(T) is the field activation coefficient. Under SCL condition, where drift current dominates over diffusion current, an experimentally measured I–V characteristic of a hole only diode can be analysed by solving the current equation Jp = ep(x)μh(E,T )E(x), and the Poisson's equation dE (x) / dx = (e / ε)p(x) for defined boundary conditions. Here, Jp is hole current, p(x) is hole density at the distance x from the anode, e is electronic charge, and ε is the static dielectric constant of the polymer. Exact analytic solution of the current equation and the Poisson's equation can be given only if the mobility is field independent. However, for carrier transport in a shallow trap environment, the scaling of SCL current with field is attributed to field dependent mobility. Here, we have extracted transport parameters by numerically solving the transport equations for a given hole current. For the devices studied here, the energy barrier at the hole-injecting electrode ITO/PEDOT:PSS is negligible as its work function (5.2 eV) is close to the highest occupied molecular orbital level of the polymers. This gives an ohmic boundary condition with p
(0 ) = Nv where Nv is the effective density of states in the valence band. The internal net Rpotential difference across the device can d be calculated by V ¼ 0 EðxÞdx where d is the film thickness, and E is an electrostatic field. The electrostatic potential V can be approximated by the difference between applied bias and the built-in voltage. The numerical calculations give the zero-field mobility (μ0) (see Fig. 2) and the field activation factor (γ) (see Fig. 3) as a function of temperature. At room temperature, hole mobility (μ0) was found to be (8.8 ± 1) × 10− 10 m2/Vs and (3.4 ± 1) × 10− 9 m2/ Vs for APFO-3, and APFO-4, respectively. Moreover, as can be inferred from Fig. 2, (μ0) scales exponentially with T − 2. The latter is a typical behaviour of hopping transport in disordered materials. Hopping transport in disordered materials takes place among localized states that are subjected to energetic and/or spatial disorder. The hopping site energies are well described by Gaussian distribution of a specific width that can be correlated to charge carrier mobility [18]. Several models have been proposed to describe mobility as a function of both site energy and spatial disorder [19,20]. However, only few models can fit experimental data in wide field range. Here, to further analyse the transport data, we used the so-called correlated disorder model (CDM) that was developed by Novikov et al. [20].
½
3r l ¼ ll exp − 5kB T
2
þ0:78
r kB T
3=2
!rffiffiffiffiffiffiffiffi eaE −C r
ð1Þ
Here, μ∞ is the mobility as T → 0, σ is the width of the Gaussian Density of States (GDS), a is the intersite spacing, and kB is the Boltzmann's Constant. This model was proposed based on Monte Carlo simulations that take care of long-range interactions such as dipole–charge interactions. The inclusion of correlations reproduces the Poole–Frenkel type of mobility pffiffiffiffi lnl~ E both in the low and high field region while in the uncorrelated model mobility is described by lnμ ∝ E / (KBT ) in the low field interval [18,21]. The presence of dipole–charge
Fig. 3. Variation of the field activation coefficient (γ) with temperature. The lines show the fits, which were generated based on the Gaussian correlated disorder model (CDM).
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interactions in conjugated polymers has been justified due to the fact that morphological variations, and an anisotropy in conjugation length distribution induce strong polarizations [22]. Using the CDM model and fitting the data of Fig. 2, we extracted σ = (64 ± 4) meV for APFO-3, and σ = (56 ± 4) meV for APFO-4. Once σ is known, the value of the intersite distance, a, can be obtained from the plot of γ versus T− 1.5 (Fig. 3). Accordingly, we have calculated a to be (1.55 ± 0.2) nm for APFO-3, and (1.52 ± 0.2) nm for APFO-4. The latter analysis gives a simple picture that relates intrinsic polymer property with transport parameters, such as mobility. We have shown that the difference in hole mobility of the copolymers mainly originates from the difference in energy disorder. The relatively low energy disorder in APFO-4 could be taken as a confirmation of better planar conformation due to its longer side chain. 3.3. Photovoltaic results Fig. 4 shows the external quantum efficiency (EQE) of all the devices. EQE is the measure of the efficiency of a cell to convert incident photons into free charge carriers. Mathematically it is defined as EQE ¼ 1240
Jsc ki Pin
ð2Þ
where Jsc is the short-circuit current density (μA/cm2), λi is the excitation wavelength (nm), and Pin is the incident photon flux (W/m2). As EQE is linearly correlated to Jsc, its value at each wavelength critically scales with the number of photogenerated free charge carriers. Therefore, high EQE values can be achieved by the combined effect of efficient exciton generation in the bulk of the active blend layer, and their subsequent dissociation into free charge carriers. As can be observed from Fig. 4, the EQE values clearly follow a specific trend, with the least value being achieved in the device that comprises APFO-4/PCBM blend (Device 2) as an active layer. Nevertheless, the maximum value of EQE exceeds 40% in all the devices.
Fig. 5. a) Typical I–V characteristics of all the devices as measured under simulated white light of intensity 100 mW/cm2, and (b) the variation of shortcircuit current density as a function of device type. The statistical graph comprises data from sixteen samples of each device type.
The photocurrent characteristics measured under white light illumination of intensity 100 mW/cm2 are depicted in Fig. 5. The short-circuit current (Jsc), open-circuit voltage (Voc), fill factor and power conversion efficiency (η) of the best sample of each device were extracted from their respective I–V curves and summarized in Table 1. As can be observed from the table, the maximum value of η for Device 1 and Device 2 are comparable. However, the solar cells with active layer of blends of both Table 1 Comparison of the maximum values of the photovoltaic parameters of all devices
Fig. 4. The external quantum efficiency (EQE%) of all the devices.
Cell name
Maximum open-circuit voltage (V)
Maximum short-circuit current (mA/cm2)
Maximum fill factor
Maximum power conversion efficiency (%)
Device 1 Device 2 Device 3 Device 4
1.003 0.971 0.997 0.989
4.1 3.6 4.6 4.4
0.56 0.65 0.62 0.57
2.26 2.24 2.62 2.45
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Fig. 6. Atomic force microscopy (AFM) images of, (a) APFO-3/PCBM (1:4, by wt.), and (b) APFO-4/PCBM (1:4, by wt.) blend films.
copolymers (Device 3 and Device 4), in general, have better over all efficiencies as compared to Device 1 and Device 2. Closer investigation of this data set also shows a larger fill factor value being associated with Device 2. The fill factor of a solar cell is the measure of the power that can be extracted from the cell and is defined as FF ¼
ðJVÞmax Jsc Voc
strated that Voc for polymer/fullerene solar cells is affected by the morphology of the active layer [23], as well as the oxidation potential of the polymer [24]. The blend film quality was studied under AFM by spin coating the blend films on a substrate. The height images of APFO-3/PCBM (1:4, by wt.), and APFO-4/ PCBM (1:4, by wt.) are displayed in Fig. 6. The AFM images clearly reveal the difference in the degree of phase separation in the films; the larger phase is associated with the mixture of APFO-4 with PCBM. In the polymer/PCBM mixed phase, the transfer of electron from the excited polymer to the PCBM is highly dependent on the distance between the polymer and the molecule. Larger distances may result into loss of the photogenerated electron through recombination and/or trapping before it is transferred to the acceptor material. As discussed by several authors, the degree of the phase separation in a blend film is a deterministic factor for both Voc and Jsc [3–5]. The difference in morphology of APFO-3/PCBM and APFO-4/PCBM films is reflected in the variation of the photocurrent and Voc of the solar cells (Device 1 and Device 2). The most interesting and important observation in this study is rather the performance of the solar cells that comprises the blends of both APFO-3 and APFO-4 as electron donors (Device 3 and Device 4). As depicted in Table 1, the photocurrent output of Device 3 and Device 4 is better than that of Device 1 and Device 2. The enhancement of photocurrent can be attributed to increase in absorption spectrum, enhancement of exciton dissociation and/or enhanced charge carriers transport. In particular, the fill factor values of Device 3 and Device 4 (Fig. 7 and Table 1) are intermediate between that of Device 1 and Device 2. The latter supports the occurrence of better charge transport in the APFO-3/APFO-4/PCBM film as compared to APFO-3/ PCBM based solar cells. For solar cells based on blends of two polymers, no such observation has been reported so far. The variation of photovoltaic properties as a function of the electron donor polymers can also be considered as a tuning of the interaction between PCBM and the polymers thereby limiting the hole mobility in the blend phase. Several authors have shown that hole transport in a polymer is enhanced when the
ð3Þ
where the (JV)max represents the maximum power that can be extracted from the cell. The scaling of the fill factor with the copolymer type can be taken as an indication of varying hole transport in the active blend layers. As FF is totally dependent on the quality of the I–V characteristics (rectangular shape gives higher FF), it is crucial to have high mobility of free charge carriers (balance of electron and hole transport), low traps, and hence a negligible space charge effect. In other words, effective collection of free charge carriers gives better output power. As can be observed in Fig. 5, comparison of the I–V curves reveals the best quality (highest FF) being achieved in Device 2. On the other hand, the best value of the open-circuit voltage has been achieved in Device 1 for all the measurements. It was demon-
Fig. 7. Variation of fill factor values as extracted from the I–V curves of sixteen solar cells of a particular device type.
A. Gadisa et al. / Thin Solid Films 515 (2007) 3126–3131
polymer is mixed with PCBM [25–29]. The reason for enhancement of hole mobility in blend phase is not resolved yet. However, based on the transport studies in polymer/PCBM phase, some authors have argued that PCBM participates in hole transport [29]. Ambipolar transport in PCBM film was demonstrated in field effect transistor measurements [30,31]. In fact, PCBM was found to transport holes and electrons with comparable mobility [30]. We have recently confirmed the ambipolar transport behaviour of PCBM through investigation of its electroluminescence property in an ITO/PEDOT:PSS/PCBM/ LiF/Al configuration. Amazingly, light emission was realized with a low onset potential close to 1 V [25]. Thus, it is most plausible that PCBM participates in hole transport in blend layers. In this context, the miscibility of PCBM with polymers is quite a limiting factor. In general, the studies presented here show that using mixture of copolymers, which have similar chemical properties, we can enhance photovoltaic properties of polyfluorene copolymer based solar cells. The mixtures may enhance exciton production through broadening of absorption spectrum. Moreover, the enhancement of fill factor in the presence of APFO-4 might be due to the enhanced hole transport in the APFO-4/PCBM mixed phase. 4. Conclusion The photovoltaic property of solar cells based on polyfluorene copolymers as electron donors and PCBM as electron acceptor was studied and compared. It has been observed that high quality I–V curves (high fill factor values) are recorded for devices that comprise the copolymer with longer side chain length. This was attributed to the enhancement of hole conduction. Hole transport studies in pure polymer films were presented and compared. On the other hand, surface morphology studies have shown that the blend of APFO-3/PCBM has relatively small phase separation as compared to that of APFO4/PCBM mixture. This increase in phase separation limits electron transfer rate from the excited polymer to the electrontransporting molecule, PCBM. Hence, it is concluded that the interplay between morphology and photocurrent generation was found to be a strong limiting step towards production of photocurrent in APFO-4/PCBM based solar cell. Using blends of the two copolymers as an electron donor was found to improve device performance. Such an improvement is attributed to the better charge generation through enhanced absorption and/or improved free photogenerated carrier transport. Acknowledgments One of the authors (A. Gadisa) gratefully acknowledges the financial support from the International Program in the Physical Sciences (IPPS) of Uppsala University, Sweden. These investigations were financially supported by the Center of Organic Electronics (COE) at Linköping University, Sweden, financed by the Strategic Research Foundation SSF.
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Improvements of fill factor in solar cells based on blends of polyfluorene copolymers as electron donors Abay Gadisa a,b,⁎, Fengling Zhang a,b , Deepak Sharma c , Mattias Svensson d , Mats R. Andersson b,d , Olle Inganäs a,b a
d
Department of Physics, Chemistry and Biology (IFM), Linköping University, S-58183 Linköping, Sweden b Center of Organic Electronics (COE), Linköping University, S-58183 Linköping, Sweden c Department of Electrical Engineering, Indian Institute of Technology Kanpur, Kanpur 208016, India Department of Organic Chemistry and Polymer Technology, Chalmers University of Technology, S-41296 Göteborg, Sweden Received 3 February 2006; received in revised form 28 July 2006; accepted 28 August 2006 Available online 11 October 2006
Abstract The photovoltaic characteristics of solar cells based on alternating polyfluorene copolymers, poly(2,7-(9,9-dioctyl-fluorene)-alt-5,5-(4′,7′-di-2-thienyl2′,1′,3′-benzothiadiazole)) (APFO-3), and poly(2,7-(9,9-didodecyl-fluorene)-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)) (APFO-4), blended with an electron acceptor fullerene molecule [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), have been investigated and compared. The two copolymers have the same aromatic backbone structure but differ by the length of their alkyl side chain. The overall photovoltaic performance of the solar cells is comparable irrespective of the copolymer used in the active layer. However, the fill factor (FF) values of the devices are strongly affected by the copolymer type. Higher FF values were realized in solar cells with APFO-4 (with longer alkyl side chain)/PCBM bulk heterojunction active layer. On the other hand, devices with blends of APFO-3/APFO-4/PCBM were found to render fill factor values that are intermediate between the values obtained in solar cells with APFO-3/PCBM and APFO-4/PCBM active film. Upon using APFO-3/APFO-4 blends as electron donors, the cell efficiency can be enhanced by about 16% as compared to cells with either APFO-3 or APFO-4. The transport of holes in each polymer obeys the model of hopping transport in disordered media. However, the degree of energetic barrier against hopping was found to be larger in APFO-3. The tuning of the photovoltaic parameters will be discussed based on studies of hole transport in the pure polymer films, and morphology of blend layers. The effect of bipolar transport in PCBM will also be discussed. © 2006 Elsevier B.V. All rights reserved. Keywords: Solar cells; Photovoltaic characteristics; Charge transport
1. Introduction The performance of solar cells with polymer/fullerene donor/ acceptor interpenetrating networks is limited by several factors, such as narrow optical absorption bands, poor carrier mobility and low stability of the polymeric materials. Other limiting factors may also be introduced during processing, resulting into large-scale phase separation, and poor film morphology. Clearly, optimisation of these devices can be achieved by combing welldesigned molecular structures, better device architecture and careful processing in a clean environment. Promising improve⁎ Corresponding author. Department of Physics, Chemistry and Biology (IFM), Linköping University, S-58183 Linköping, Sweden. Tel.: +46 13 288917; fax: +46 13 288969. E-mail address: [email protected] (A. Gadisa). 0040-6090/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2006.08.043
ments have been achieved through use of new materials with red shifted optical absorption spectrum [1], optimisation of device structure through modification of interfaces at the electrodes [2], improvement of film morphology by appropriate choice of deposition solvents [3,4], controlling evaporation rate of solvents [5], and heat treatments [6]. Polyfluorene copolymers have emerged as a new class of semiconductor materials due to their high charge carrier mobility and good processability [7,8]. It has been observed that several polyfluorenes and their copolymers incorporate liquid crystallinity [9]. In general, polyfluorene copolymers have high band gaps giving blue-shifted optical absorption and hence have limited use in wide band photodiodes. Nevertheless, related copolymers have been synthesized and utilized in light emitting diodes whereby broad emission spectrum in the whole visible range has been realized [8].
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We have reported the synthesis of low band gap polyfluorene copolymers and their performance in solar cells [1,10–13]. In our current report, we demonstrate the enhancement of fill factor (FF) of solar cells upon using blends of two similar polyfluorene copolymers as electron donors. The chemical structure of the copolymers is depicted in Fig. 1. The aromatic back bone structures of the two copolymers are the same while their alkyl side chains have different lengths. Commonly, the main chain of polymers is decorated with side chains for various reasons. Side chains may play a crucial role in determining the conformation and solvent processability of conjugated polymers [14]. It is well understood that long, non-branched hydrocarbon chains can easily pack together and form well ordered solution-processed films [15]. According to a recent report [16], side chain length has a crucial impact on carrier transport. Moreover, the fact that efficient polymer based solar cells rely on polymer/acceptor blends implies that the miscibility of the polymer with acceptor molecule is quite crucial. In this context, the size and shape of the side chains play major roles. In this paper, we report the improvement of photovoltaic parameters of polymer-based solar cells upon using blends of two sim-
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ilar copolymers, poly(2,7-(9,9-dioctyl-fluorene)-alt-5,5-(4′,7′di-2-thienyl-2′,1′,3′-benzothiadiazole)) (APFO-3), and poly (2,7-(9,9-didodecyl-fluorene)-alt-5,5-(4′,7′-di-2-thienyl2′,1′,3′-benzothiadiazole)) (APFO-4), as electron donors, and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) as an electron acceptor and transporting material. 2. Experimental details The procedures for the synthesis of the copolymers were described in Ref. [1]. The chemical structures of the copolymers and PCBM are depicted in Fig. 1. All the devices were constructed in the form of sandwich structure. The anode was formed by spin coating poly (3, 4-ethylene dioxythiophene)-poly (styrene sulphonate) (PEDOT: PSS) (from Bayer AG, EL Grade) on a pre-cleaned indium–tin–oxide (ITO)/glass substrate. The thickness of the PEDOT:PSS film was about 50 nm as measured by DEKTAK 3030 surface profilometer. Solutions of APFO-3/PCBM (1:4 by wt.), APFO-4/PCBM (1:4 by wt.), and APFO-3:APFO-4/PCBM (1:4 by wt.) were prepared in solution of chloroform and spin coated on top of the PEDOT:PSS layer. The thickness of the blend films ranges from 200 to 240 nm. Thin films of lithium fluoride (LiF) (less than 1 nm), and aluminium (Al) (60 nm) were evaporated to form the top electrode, cathode. The thermal evaporation of Al and LiF was done under a shadow mask in a base pressure better than 10− 6 mbar. The active area of the devices thus constructed varies from 4 to 6 mm2. For simplicity, the devices are named according to the type of copolymer they comprise in their active layer: Device 1 and Device 2 comprise APFO-3 and APFO-4, respectively, while Device 3, and Device 4 comprise blends of APFO-3 and APFO-4 in a weight ratio of 1:1, and 3:1, respectively. Sixteen samples were prepared and characterized for each device type. Device preparations and characterizations were done under ambient conditions except the evaporation of Al and LiF. The current–voltage (I–V) measurements under white light illumination (1 sun) were performed with a Keithley 2400 electrometer. Ozone free Xenon lamp, coupled with solar spectrum simulating filters, was used to provide a simulated light of intensity 100 mW/cm2. Photocurrent generated at a specific wavelength was measured using Keithley 485 Picoammeter. The optical absorption spectra were obtained with a Perkin Elmer Spectrophotometer λ9. The film morphologies were studied under tapping mode atomic force microscopy (AFM) (Nanoscope III with J-scanner head, Digital Instruments). Height images were produced by operating the AFM in tapping mode, at a scan rate of 0.4–1.0 Hz, and by collecting 512 × 512 data points. The AFM has a high aspect ratio conical tip that consists of a noncontact “Golden” Silicon cantilever. The tip has a standard size (1.6 × 3.6 mm), and thickness (0.4 mm) of cantilever chip. 3. Results and discussions 3.1. Absorption spectrum
Fig. 1. The chemical structures of the copolymers and PCBM (top panel). The normalized optical absorption spectra of APFO-3, APFO-4, and APFO-3: PCBM (1:4, by wt.) are depicted in the lower panel.
The normalized optical absorption spectra of the polymer films are depicted in Fig. 1. It is observed that, the copolymers have similar optical response in the visible range of the solar
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Fig. 2. Variation of the zero-field mobility (μ0) with temperature. The lines show the fits, which were generated based on the Gaussian correlated disorder model (CDM).
spectrum with an absorption maxima slightly red shifted with increasing side chain length. This shift in absorption with increasing side chain may be an indication of the formation of more planar backbone conformation. Similar phenomenon was observed for other types of polymers of the same family but different side chain length [17]. The absorption spectra of the copolymer/PCBM blend films have similar features; a typical blend absorption spectrum is shown in Fig. 1. 3.2. Hole transport in APFO-3 and APFO-4 thin films Hole transport in thin films (100 nm) of APFO-3 and APFO4 was investigated by analysing the space charge limited (SCL) current–voltage (I–V ) characteristics of hole dominated diodes. The measured I–V characteristics are temperature (T ), and field (E ) dependent, which is a typical signature of hopping transport in disordered materials. Mobility of charge carriers that exhibit hopping transport is often described by the Poole–Frenkel pffiffiffiffiffiffiffiffiffiffiffi equation lðE; T Þ ¼ l0 ðT ÞexpðgðT Þ EðxÞÞ, where μ0(T ) is the zero-field mobility, and γ(T) is the field activation coefficient. Under SCL condition, where drift current dominates over diffusion current, an experimentally measured I–V characteristic of a hole only diode can be analysed by solving the current equation Jp = ep(x)μh(E,T )E(x), and the Poisson's equation dE (x) / dx = (e / ε)p(x) for defined boundary conditions. Here, Jp is hole current, p(x) is hole density at the distance x from the anode, e is electronic charge, and ε is the static dielectric constant of the polymer. Exact analytic solution of the current equation and the Poisson's equation can be given only if the mobility is field independent. However, for carrier transport in a shallow trap environment, the scaling of SCL current with field is attributed to field dependent mobility. Here, we have extracted transport parameters by numerically solving the transport equations for a given hole current. For the devices studied here, the energy barrier at the hole-injecting electrode ITO/PEDOT:PSS is negligible as its work function (5.2 eV) is close to the highest occupied molecular orbital level of the polymers. This gives an ohmic boundary condition with p
(0 ) = Nv where Nv is the effective density of states in the valence band. The internal net Rpotential difference across the device can d be calculated by V ¼ 0 EðxÞdx where d is the film thickness, and E is an electrostatic field. The electrostatic potential V can be approximated by the difference between applied bias and the built-in voltage. The numerical calculations give the zero-field mobility (μ0) (see Fig. 2) and the field activation factor (γ) (see Fig. 3) as a function of temperature. At room temperature, hole mobility (μ0) was found to be (8.8 ± 1) × 10− 10 m2/Vs and (3.4 ± 1) × 10− 9 m2/ Vs for APFO-3, and APFO-4, respectively. Moreover, as can be inferred from Fig. 2, (μ0) scales exponentially with T − 2. The latter is a typical behaviour of hopping transport in disordered materials. Hopping transport in disordered materials takes place among localized states that are subjected to energetic and/or spatial disorder. The hopping site energies are well described by Gaussian distribution of a specific width that can be correlated to charge carrier mobility [18]. Several models have been proposed to describe mobility as a function of both site energy and spatial disorder [19,20]. However, only few models can fit experimental data in wide field range. Here, to further analyse the transport data, we used the so-called correlated disorder model (CDM) that was developed by Novikov et al. [20].
½
3r l ¼ ll exp − 5kB T
2
þ0:78
r kB T
3=2
!rffiffiffiffiffiffiffiffi eaE −C r
ð1Þ
Here, μ∞ is the mobility as T → 0, σ is the width of the Gaussian Density of States (GDS), a is the intersite spacing, and kB is the Boltzmann's Constant. This model was proposed based on Monte Carlo simulations that take care of long-range interactions such as dipole–charge interactions. The inclusion of correlations reproduces the Poole–Frenkel type of mobility pffiffiffiffi lnl~ E both in the low and high field region while in the uncorrelated model mobility is described by lnμ ∝ E / (KBT ) in the low field interval [18,21]. The presence of dipole–charge
Fig. 3. Variation of the field activation coefficient (γ) with temperature. The lines show the fits, which were generated based on the Gaussian correlated disorder model (CDM).
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interactions in conjugated polymers has been justified due to the fact that morphological variations, and an anisotropy in conjugation length distribution induce strong polarizations [22]. Using the CDM model and fitting the data of Fig. 2, we extracted σ = (64 ± 4) meV for APFO-3, and σ = (56 ± 4) meV for APFO-4. Once σ is known, the value of the intersite distance, a, can be obtained from the plot of γ versus T− 1.5 (Fig. 3). Accordingly, we have calculated a to be (1.55 ± 0.2) nm for APFO-3, and (1.52 ± 0.2) nm for APFO-4. The latter analysis gives a simple picture that relates intrinsic polymer property with transport parameters, such as mobility. We have shown that the difference in hole mobility of the copolymers mainly originates from the difference in energy disorder. The relatively low energy disorder in APFO-4 could be taken as a confirmation of better planar conformation due to its longer side chain. 3.3. Photovoltaic results Fig. 4 shows the external quantum efficiency (EQE) of all the devices. EQE is the measure of the efficiency of a cell to convert incident photons into free charge carriers. Mathematically it is defined as EQE ¼ 1240
Jsc ki Pin
ð2Þ
where Jsc is the short-circuit current density (μA/cm2), λi is the excitation wavelength (nm), and Pin is the incident photon flux (W/m2). As EQE is linearly correlated to Jsc, its value at each wavelength critically scales with the number of photogenerated free charge carriers. Therefore, high EQE values can be achieved by the combined effect of efficient exciton generation in the bulk of the active blend layer, and their subsequent dissociation into free charge carriers. As can be observed from Fig. 4, the EQE values clearly follow a specific trend, with the least value being achieved in the device that comprises APFO-4/PCBM blend (Device 2) as an active layer. Nevertheless, the maximum value of EQE exceeds 40% in all the devices.
Fig. 5. a) Typical I–V characteristics of all the devices as measured under simulated white light of intensity 100 mW/cm2, and (b) the variation of shortcircuit current density as a function of device type. The statistical graph comprises data from sixteen samples of each device type.
The photocurrent characteristics measured under white light illumination of intensity 100 mW/cm2 are depicted in Fig. 5. The short-circuit current (Jsc), open-circuit voltage (Voc), fill factor and power conversion efficiency (η) of the best sample of each device were extracted from their respective I–V curves and summarized in Table 1. As can be observed from the table, the maximum value of η for Device 1 and Device 2 are comparable. However, the solar cells with active layer of blends of both Table 1 Comparison of the maximum values of the photovoltaic parameters of all devices
Fig. 4. The external quantum efficiency (EQE%) of all the devices.
Cell name
Maximum open-circuit voltage (V)
Maximum short-circuit current (mA/cm2)
Maximum fill factor
Maximum power conversion efficiency (%)
Device 1 Device 2 Device 3 Device 4
1.003 0.971 0.997 0.989
4.1 3.6 4.6 4.4
0.56 0.65 0.62 0.57
2.26 2.24 2.62 2.45
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Fig. 6. Atomic force microscopy (AFM) images of, (a) APFO-3/PCBM (1:4, by wt.), and (b) APFO-4/PCBM (1:4, by wt.) blend films.
copolymers (Device 3 and Device 4), in general, have better over all efficiencies as compared to Device 1 and Device 2. Closer investigation of this data set also shows a larger fill factor value being associated with Device 2. The fill factor of a solar cell is the measure of the power that can be extracted from the cell and is defined as FF ¼
ðJVÞmax Jsc Voc
strated that Voc for polymer/fullerene solar cells is affected by the morphology of the active layer [23], as well as the oxidation potential of the polymer [24]. The blend film quality was studied under AFM by spin coating the blend films on a substrate. The height images of APFO-3/PCBM (1:4, by wt.), and APFO-4/ PCBM (1:4, by wt.) are displayed in Fig. 6. The AFM images clearly reveal the difference in the degree of phase separation in the films; the larger phase is associated with the mixture of APFO-4 with PCBM. In the polymer/PCBM mixed phase, the transfer of electron from the excited polymer to the PCBM is highly dependent on the distance between the polymer and the molecule. Larger distances may result into loss of the photogenerated electron through recombination and/or trapping before it is transferred to the acceptor material. As discussed by several authors, the degree of the phase separation in a blend film is a deterministic factor for both Voc and Jsc [3–5]. The difference in morphology of APFO-3/PCBM and APFO-4/PCBM films is reflected in the variation of the photocurrent and Voc of the solar cells (Device 1 and Device 2). The most interesting and important observation in this study is rather the performance of the solar cells that comprises the blends of both APFO-3 and APFO-4 as electron donors (Device 3 and Device 4). As depicted in Table 1, the photocurrent output of Device 3 and Device 4 is better than that of Device 1 and Device 2. The enhancement of photocurrent can be attributed to increase in absorption spectrum, enhancement of exciton dissociation and/or enhanced charge carriers transport. In particular, the fill factor values of Device 3 and Device 4 (Fig. 7 and Table 1) are intermediate between that of Device 1 and Device 2. The latter supports the occurrence of better charge transport in the APFO-3/APFO-4/PCBM film as compared to APFO-3/ PCBM based solar cells. For solar cells based on blends of two polymers, no such observation has been reported so far. The variation of photovoltaic properties as a function of the electron donor polymers can also be considered as a tuning of the interaction between PCBM and the polymers thereby limiting the hole mobility in the blend phase. Several authors have shown that hole transport in a polymer is enhanced when the
ð3Þ
where the (JV)max represents the maximum power that can be extracted from the cell. The scaling of the fill factor with the copolymer type can be taken as an indication of varying hole transport in the active blend layers. As FF is totally dependent on the quality of the I–V characteristics (rectangular shape gives higher FF), it is crucial to have high mobility of free charge carriers (balance of electron and hole transport), low traps, and hence a negligible space charge effect. In other words, effective collection of free charge carriers gives better output power. As can be observed in Fig. 5, comparison of the I–V curves reveals the best quality (highest FF) being achieved in Device 2. On the other hand, the best value of the open-circuit voltage has been achieved in Device 1 for all the measurements. It was demon-
Fig. 7. Variation of fill factor values as extracted from the I–V curves of sixteen solar cells of a particular device type.
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polymer is mixed with PCBM [25–29]. The reason for enhancement of hole mobility in blend phase is not resolved yet. However, based on the transport studies in polymer/PCBM phase, some authors have argued that PCBM participates in hole transport [29]. Ambipolar transport in PCBM film was demonstrated in field effect transistor measurements [30,31]. In fact, PCBM was found to transport holes and electrons with comparable mobility [30]. We have recently confirmed the ambipolar transport behaviour of PCBM through investigation of its electroluminescence property in an ITO/PEDOT:PSS/PCBM/ LiF/Al configuration. Amazingly, light emission was realized with a low onset potential close to 1 V [25]. Thus, it is most plausible that PCBM participates in hole transport in blend layers. In this context, the miscibility of PCBM with polymers is quite a limiting factor. In general, the studies presented here show that using mixture of copolymers, which have similar chemical properties, we can enhance photovoltaic properties of polyfluorene copolymer based solar cells. The mixtures may enhance exciton production through broadening of absorption spectrum. Moreover, the enhancement of fill factor in the presence of APFO-4 might be due to the enhanced hole transport in the APFO-4/PCBM mixed phase. 4. Conclusion The photovoltaic property of solar cells based on polyfluorene copolymers as electron donors and PCBM as electron acceptor was studied and compared. It has been observed that high quality I–V curves (high fill factor values) are recorded for devices that comprise the copolymer with longer side chain length. This was attributed to the enhancement of hole conduction. Hole transport studies in pure polymer films were presented and compared. On the other hand, surface morphology studies have shown that the blend of APFO-3/PCBM has relatively small phase separation as compared to that of APFO4/PCBM mixture. This increase in phase separation limits electron transfer rate from the excited polymer to the electrontransporting molecule, PCBM. Hence, it is concluded that the interplay between morphology and photocurrent generation was found to be a strong limiting step towards production of photocurrent in APFO-4/PCBM based solar cell. Using blends of the two copolymers as an electron donor was found to improve device performance. Such an improvement is attributed to the better charge generation through enhanced absorption and/or improved free photogenerated carrier transport. Acknowledgments One of the authors (A. Gadisa) gratefully acknowledges the financial support from the International Program in the Physical Sciences (IPPS) of Uppsala University, Sweden. These investigations were financially supported by the Center of Organic Electronics (COE) at Linköping University, Sweden, financed by the Strategic Research Foundation SSF.
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