Organic Photovoltaic Devices Based on a Block Copolymer / Fullerene Blend Richard P. Barber, Jr.+, Romel D. Gomez , Warren N. Herman, Danilo B. Romero* Laboratory for Physical Sciences, University of Maryland, 8050 Greenmead Drive, College Park, Maryland 20740, USA

Abstract Conjugated semiconducting block copolymers are interesting organic materials for possible photovoltaic device applications because of their capability to form self-organized nanostructures and the tunability of their electronic and optical properties as a function of the relative lengths of the constituent monomers. In this work, we report the results of our investigation of polymer bulk heterojunction photovoltaic devices based on the block copolymer [MEH-PPV]–co– [biphenylene vinylene] derived from the widely used poly[2-methoxy-5-[2’-ethyl-hexyloxy]-1,4-phenylenevinylene (MEH-PPV) as an electron donor material in conjunction with the common electron acceptor molecule [6,6]-phenyl C61 butyric acid methyl ester, a soluble derivative of C60. We find that the morphology and photovoltaic characteristics of these devices are strongly influenced by the blend concentration and thickness of the electro-active organic layer.

Our initial results

demonstrate that a power conversion efficiency of 2.4 % is achieved with these materials. PACS: 72.80.Le, 73.50.Pz, 83.80.Uv, 85.65.+h Keywords: block copolymer, organic, photovoltaic, bulk heterojunction, MEH-PPV, C60

*

Corresponding author: Tel.: 301-935-6475 FAX: 301-935-6723 Laboratory for Physical Sciences University of Maryland 8050 Greenmead Drive College Park, MD 20740 (USA) E-mail: [email protected]

+

Permanent address: Department of Physics, Santa Clara University, Santa Clara, CA 95053 (USA).

1. Introduction Organic photovoltaic devices have the potential to revolutionize the production of solar cells by offering an inexpensive and versatile alternative to inorganic semiconductors currently available [1-3]. The allure of lightweight, flexible and mechanically robust devices allows imaginative applications from photovoltaic coated building exteriors to wearable power sources printed onto clothing. However, despite the wide variety of material systems studied to date, the progress in achieving device performance comparable to those of the inorganic photovoltaic devices [4] remains elusive. Challenges such as thermal and chemical instability and the relatively low-power conversion efficiencies represent significant hurdles toward fruition. The device physics of organic photovoltaic devices based on blends of semiconducting conjugated polymers and fullerenes results from the complex interplay of disorder-induced morphological and electronic structure modification in the blend [1-3]. The modification results from the well-established phenomenon that charge separation in these devices occurs at the interface of the heterojunction formed by the donor-acceptor molecular complex [5]. Unlike the conventional silicon-based photovoltaic device associated with a pn-junction formed by a sharply defined interface, the donor-acceptor heterojunction is randomly distributed over the entire electroactive layer of the organic photovoltaic device [1-3,6,7]. The photovoltaic characteristics of these devices will, therefore, be strongly influenced by the relative concentration of the donor and acceptor molecules. This is because the localized fluctuations in the donor-acceptor concentration can lead to variations in the electrical, optical, and structural properties of the device. In this work, we explored the use of a block copolymer poly[2-methoxy-5-[2’-ethylhexyloxy]-1,4-phenylene-vinylene, derived from the widely used MEH-PPV, blended with [6,6]phenyl C61 butyric acid methyl ester (PCBM), a soluble derivative of C60, for fabricating organic

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polymer bulk heterojunction photovoltaic devices. Recently, semiconducting block copolymers have attracted attention for possible organic optoelectronic applications due to their ability to form self-assembled nanostructures and the tunability of their electronic and optical properties as a function of the relative lengths of the constituent monomers [8]. Furthermore, blending C60 with similar polymers has been shown to dramatically increase their photo-oxidative stability [9]. The steady improvement in device characteristics that has accompanied the tuning of a variety of controllable parameters in our measurements suggests that this system might represent a foundation for a viable technology. We investigate the effects of the relative concentration of the donoracceptor molecules and film thickness on the photovoltaic characteristics of devices using blends of the MEH-PPV block copolymer and PCBM as the organic active layer.

2. Experimental The chemical structures of the molecules investigated in this work are shown in the inset to Fig. 1. Chlorobenzene was used as a solvent for these organic materials because an earlier report [10] showed that highly homogeneous films can be spun-cast using this solvent. Since the relative concentration of the MEH-PPV block copolymer : PCBM is a critical parameter in our investigation, we developed a simple procedure that allows for an accurate determination of the molar fraction of C60 in the blend. Separate solutions of the MEH-PPV copolymer and PCBM in chlorobenzene were prepared and stirred at 80 oC for a few days. The solutions were then filtered using a 1

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was weighed to estimate the weight of the solvent. The solvent was then allowed to evaporate and the weight of the remnant organic material was measured. This procedure yielded approximately 0.45% for the concentration of both filtered solutions. Blends with different relative concentrations

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of MEH-PPV copolymer : PCBM were prepared from appropriate combinations of the filtered solutions. We chose to express the relative concentration of C60 and copolymer as the molar fraction x of C60. Film thickness variations were determined using a similar technique. In this case, a solution with molar fraction 0.69 was diluted with additional solvent to make thinner films or subjected to further solvent evaporation to make thicker films. These solutions with identical molar fraction were then weighed before and after the chlorobenzene was evaporated, yielding weight percents between 0.4 and 2.5. We expressed the thickness in terms of concentration of the organic molecules in solution. Typical values for all of these experiments were of order 1%. The devices were fabricated using indium-tin-oxide (ITO) coated glass substrates purchased from Delta Technologies with sheet resistance of ~ 2 ohms per square as the transparent conducting electrode for hole-injection. The ITO was photo-lithographically patterned on the glass substrate by means of wet chemical-etching techniques. A layer (~ 100 nm) of PLED grade Baytron poly[3,4ethylenedioxythiophene]:poly[styrenesulfonate] (PEDOT:PSS) followed by the organic active layer (~ 150 nm) was spin-cast onto the ITO glass substrate in an inert nitrogen atmosphere inside a glove box. Both the PCBM and the MEH-PPV block copolymer are commercially available products from American Dye Source. The electron-injecting contact consisted of a patterned layer of ~ 2 nm thick lithium-flouride (LiF) followed by ~ 100 nm of aluminum (Al) deposited on top of the organic layer by shadow mask thermal evaporation. The electrically active area of the fabricated devices is 2 mm x 2 mm. In the initial experiments that included the stoichiometric data, the samples were covered with an aluminum foil to minimize photo-oxidative degradation and moved to an external evaporator for top electrode fabrication.

Later samples were prepared using an evaporator

contained within the glove box. This upgrade resulted in a significant improvement in the device characteristics.

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The photovoltaic characteristics of the devices were extracted from the current versus voltage characteristics under illumination from an AM1.5 direct solar simulator with ~ 92 mW/cm2 intensity.

3. Results and Discussion Figure 2 shows representative Atomic Force Microscopy scans of devices with varying blend composition. The MEH-PPV copolymer : PCBM blend exhibits a granular morphology that changes systematically with the relative concentration of the two species. The pure C60 device (x = 1) shows the appearance of amorphous large clusters (~ 1 µm). These clusters transform into relatively uniform fine grains (~ 100 nm) at intermediate values of x. This relatively ordered clustering disappears and is replaced by an inhomogeneous morphology in the pure polymer film (x = 0). Similarly, it is apparent that connected islands also form in the blends at intermediate molar fractions and increase in density as x is decreased. Such complex morphology is an inherent characteristic of polymer blends [11] and earlier works [10,12] have indicated improvements in the optical and electrical characteristics of organic electronic devices as a function of the morphology. We examine the effects of this morphological variation on the photovoltaic characteristics of organic photovoltaic devices fabricated from the MEH-PPV copolymer / C60 blends. The spectral dependence of the incident photon to current conversion efficiency (IPCE) is shown in Fig. 3 for three representative devices. The spectra are compared to the absorbance spectrum of the pure MEH-PPV copolymer. For each device, the peaks in the IPCE spectrum are shifted to shorter wavelengths when compared to the MEH-PPV copolymer absorbance peak. The x = 0.6 device displays a single peak near λ = 486 nm. The peak for the x = 0.8 device appears to

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move to shorter wavelength while the x = 0.1 device manifests two peaks, one at shorter wavelengths and the other close to the absorbance peak of the MEH-PPV copolymer. These results suggest the formation of a donor-acceptor complex in the copolymer / C60 blend that is responsible for the IPCE maximum near λ = 486 nm. The density of this donor-acceptor complex is optimum around x = 0.4 to 0.6, the range of C60 molar fractions in which the IPCE is maximum as depicted in the inset to Fig. 3.

We note that this range of concentration corresponds to the case of

approximately one C60 molecule bound to the polymer repeat unit. This is highly suggestive of significant photodoping at intermediate blend concentration. The electron transfer from the polymer to the C60 leads to the creation of mobile hole polarons that enhances the charge transport along the polymer chain. Figure 1 displays the dark and illuminated current versus voltage (I-V) characteristic from a device with x = 0.57. From these curves, we extract the photovoltaic characteristics of the device. The open-circuit voltage, VOC, is the voltage intercept at zero current. The short-circuit current density, JSC, is the current intercept at zero voltage. The fill factor, FF, is calculated as

FF = (VJ )max (VOC J SC ) where (VJ )max is the maximum electrical power density that is generated by the device [2]. A practical figure of merit for photovoltaic devices is the external power P conversion efficiency, ηext , defined as the ratio of the maximum electrical power density generated

in the device to the incident light intensity [2], P ηext =

(VJ )max Pin

=

VOC J SC FF . Pin

P Figure 3 summarizes the values of VOC, JSC, FF, and ηext derived from the devices with P different x. The plot shows the strong dependence of ηext on x. The pure MEH-PPV copolymer

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device (x = 0) manifests a weak photovoltaic effect. While there is significant VOC (~ 0.82 V) in this device, JSC (~ 0.1 mA/cm2) is very small.

The low value of JSC is due to radiative

recombination dominating over the charge separation process with much of the latter taking place near the PEDOT:PSS / MEH-PPV copolymer interface. On the other extreme, pure PCBM device P increases (x = 1), similarly shows no photovoltaic effect in either VOC or JSC. At intermediate x, ηext P significantly with a maximum of 1% near x = 0.6. We note that the maximum ηext does not appear

at x = 0.5 which corresponds to the case of one C60 molecule for each repeat unit of the MEH-PPV copolymer in which charge separation is expected to be optimal. This suggests that a higher concentration of C60 molecules is necessary for better electron transport in the blend to achieve P maximum ηext .

In general, the open-circuit voltage is determined by the built-in voltage associated with the band-offset at the electrode / organic layer interface as well as the diffusion of the separated charge carriers in the device [13]. In Fig. 3 we find that VOC is essentially constant (VOC ~ 0.85 V) for x < 0.7 then decreases to VOC = 0 V for the pure C60 device. The fact that VOC is nearly the same for a wide-range of blend concentration indicates that the contribution of charge diffusion to the opencircuit voltage is negligible in organic photovoltaic devices. Therefore, proper engineering of the interfaces can lead to higher VOC in these devices. P Figure 3 shows that the x dependence of ηext mimics that of JSC. The short-circuit current is

determined by a three step process: 1) exciton creation, 2) exciton diffusion towards the donoracceptor junction for charge separation, and 3) charge transport to and collection at the electrodes [1]. These processes are expected to be strongly dependent on the thickness of the organic layer in the photovoltaic devices. Fig. 4 shows the photovoltaic device parameters for a series of films of

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different weight percent prepared from the same x=0.69 mixture. We note the dramatic effect of P varying this parameter producing devices with ηext up to 1.7%. Measurements of the resultant

thickness range with a Dektak stylus profiler indicated that these samples varied in thickness by nearly a factor of two from 120 nm to 210 nm over the range of weight percents from 0.4 to 2.25. Note that in polymer-based bulk heterojunction photovoltaic devices, the charge separation efficiency is close to unity since the excitons are created close to the heterojunction and the P concomitant charge-transfer process is very fast [1,5]. Thus, the thickness dependence of ηext that

we observe is most likely a result of the competition between the exciton creation and charge collection efficiencies in the photovoltaic devices. For the devices with a thin organic layer, a larger fraction of the incident light is not absorbed, which results in a reduced production of charge carriers. On the other hand, poor transport in the devices with thick active layer inhibits the separated holes and electrons from reaching the electrodes in order to represent a measurable current. Thus, the optimum thickness seen in Fig. 4 represents a compromise between these two mechanisms to maximum power conversion efficiency in the devices.

4. Conclusion In summary, we have investigated the dependence of the photovoltaic properties of conjugated block copolymer : C60 based photovoltaic devices on the relative concentration of donoracceptor heterojunctions. We find that the best performance is exhibited by devices with a C60 molar fraction of 0.6. This concentration is slightly higher than the ideal concentration for optimal charge separation which corresponds to one C60 per polymer molecular repeat unit. Furthermore, control of the active layer thickness also produced significant gains in power conversion efficiency.

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These results by no means represent an exhaustive investigation of a rather large phase space of controllable device parameters. The potential to “tune” the peak absorption wavelength by varying the composition of the copolymer provides an additional method for adjusting device performance. We expect that further refinement of this system will lead to continued improvement and perhaps even a technologically usable design.

Acknowledgements This work is supported by the Polymer Program at the Laboratory for Physical Sciences under the Organic Polymer Photocell Technology project . References [1] C. Brabec, V. Dyakonov, J. Parisi, N.S. Sariciftici (Eds.), Organic Photovoltaics: Concepts and Realization, Springer Series in Materials Science, 2003. [2] H. Hoppe and N.S. Sariciftci, J. Mater. Res. 19 (7) (2004) 1924. [3] S.E. Shaheen, D.S. Ginley, G.E. Jabbour, Materials Research Society Bulletin 30 (1) (2005) 10. [4] S.F. Baldwin, Physics Today 55 (2002) 62. [5] N.S. Sariciftci, L. Smilowits, A.J. Heeger, F. Wudl, Science 258 (5087) (1992) 1474. [6] G. Yu, J. Gao, J.C. Hummelen, F. Wudl, A.J. Heeger, Science 270 (5243) (1995) 1789. [7] J.J.M. Halls, C.A. Walsh, N.C. Greenham, E.A. Marseglia, R.H. Friend, S.C. Moratti, A.B. Holmes, Nature 376 (6540) (1995) 498. [8] G. Hadziioannou, Materials Research Society Bulletin 27 (6) (2002) 456. [9] H. W. Sarkas, W. Kwan, S. R. Flom, C. D. Merritt, Z. H. Kafafi , J. Phys. Chem. 100 (13) (1996) 5169. [10] S.E. Shaheen, C.J. Brabec, N.S. Sariciftci, F. Padinger, T. Fromherz, J.C. Hummelen, Appl. Phys. Lett. 78 (6) (2001) 841. [11] G. Malliaras, R. Friend, Physics Today 58 (2005) 53. [12] T. Nguyen, B.J. Schwartz, J. Chem. Phys. 116 (18) (2002) 8198. [13] G.G. Malliaras, J.R. Salem, P.J. Brock, J.C. Scott, J. Appl. Phys. 84 (3) (1998) 1583.

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Figure Captions

Fig. 1: Comparison of the dark and illuminated current versus voltage characteristics for a device with x = 0.57 C60 molar fraction. The photovoltaic characteristics of this device are: VOC = 0.88 V, P JSC = 4.92 mA/cm2, FF = 0.47, ηext = 2.4 % using AM1.5 direct solar simulator with Pin=92

mW/cm2. The inset shows the chemical structures of MEH-PPV–co– biphenylene vinylene and PCBM molecules investigated in this work.

Fig. 2: Representative Atomic Force Microscopy images as a function of C60 molar fraction for four different photovoltaic devices: a) x=1, b) x=0.43, c) x=0.26, d) x=0. The scale of all four images is shown in the right frame.

Fig. 3: Spectral dependence of the incident photon to current conversion efficiency (IPCE) for three representative devices. The absorbance for the pure MEH-PPV copolymer film is also shown for comparison. The inset plots the IPCE at λ = 486 nm as a function of the C60 molar fraction.

Fig. 4: Dependence of the photovoltaic device characteristics on the C60 molar fraction. (a) opencircuit voltage and short-circuit current density, (b) power conversion efficiency and fill factor.

Fig. 5: Dependence of the photovoltaic device characteristics on the organic solution weight percentage. (a) open-circuit voltage and short-circuit current density, (b) power conversion efficiency and fill factor.

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Fig. 1

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Organic photovoltaic devices based on a block ...

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