- Email: [email protected]
Biodegradable polymer solar cells
ARTICLE IN PRESS
Solar Energy Materials & Solar Cells 92 (2008) 805–813 www.elsevier.com/locate/solmat
Biodegradable polymer solar cells Marianne Strange, David Plackett, Martin Kaasgaard, Frederik C. Krebs National Laboratory for Sustainable Energy, Technical University of Denmark, Frederiksborgvej 399, DK-4000 Roskilde, Denmark Received 13 November 2007; received in revised form 1 February 2008; accepted 5 February 2008 Available online 20 March 2008
Abstract Polymer photovoltaic devices were prepared using a biodegradable poly-L-lactic acid (PLLA) substrate loaded with nanoclay in an attempt to improve the thermal properties and possibly reduce permeation of water and oxygen. The nanoclay-loaded PLLA substrates were prepared by compounding and extrusion. The substrate thickness was 200 mm and the substrates had good transparency in the range 300–800 nm of, respectively, 480% for PLA and 460% for PLLA loaded with nanoclay. Devices were realized by application of a conducting transparent anode comprising an aluminium grid with a thin overlayer of silver and spin-coated PEDOT:PSS. The active layer consisted of microfibrillar P3HT and PCBM to encompass the low processing temperatures for PLLA and finally an evaporated aluminium cathode. It was found that PLLA as a substrate holds potential, but there are several challenges beyond the photovoltaic itself which must be met before general application within this field can be envisaged. The most important aspects are the planarity of the PLLA surface, the mechanical stresses induced by the extrusion process, the limitation in processing temperature, and the limitation in the available range of solvents for solution processing. r 2008 Elsevier B.V. All rights reserved. Keywords: Nanoclay; Polylactic acid; Solar cells; Biodegradation; Environment; Permeation
1. Introduction Solar cells based on polymers [1–6] are projected to offer renewable energy to niche products within the next few years. The prospects for the use of polymer solar cells for on-grid electrical energy production is also a possibility but is believed to lie a decade or more into the future. The advantages of the polymer solar cell technology over existing solar cell technologies include: flexibility, low cost, low thermal budgets, solution processing, roll-to-roll processing, and light weight. The aspect of roll-to-roll fabrication methods using printing and coating techniques implies that very high throughput speeds of many meters per second can be achieved [7–10]. The consequence of widespread usage of polymer solar cells produced at high volume will pose a challenge to the channels available for disposal of the devices when they stop working. In principle, the polymer substrate used for the solar cells can be recycled efficiently, but in the case of solar cell products for niche market applications that essentially Corresponding author. Tel.: +45 46 77 47 99.
E-mail address: [email protected] (F.C. Krebs). 0927-0248/$ - see front matter r 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2008.02.002
compete with small batteries there is the potential problem of unwanted dispersion and improper disposal in the environment. While there are currently no studies available that have examined the environmental impact of widespread usage of polymer solar cells and their disposal [11], the individual constituents of a typical polymer solar cell could pose a threat to the environment. The constituents of a typical bulk heterojunction device are indium, tin, aluminium, polythiophene, PEDOT:PSS, and a fullerene derivative. Most of these materials have been documented to be harmful to life forms in the environment although this does depend on the conditions and conflicting evidence has been reported. With respect to the metals indium [12], tin [13], and aluminium, the toxicity has been addressed for both soluble and insoluble sources in both organic and inorganic forms. Both indium and tin can in some forms be quite toxic, but the consequences of widespread dispersal in the environment are by no means fully established and have only been considered in the context of case studies on metal discharge close to smelters [14] and in association with the development of lead-free soldering alloys. Aluminium is likely to be the least harmful if released in the environment; however, this is relative to the other
ARTICLE IN PRESS 806
M. Strange et al. / Solar Energy Materials & Solar Cells 92 (2008) 805–813
metals of interest here and it should be noted that an EPA recommendation issued in 1998 suggested that aluminium levels in water should not exceed a 3-day average of 87 mg L1. This average concentration was determined on the basis of fish toxicity studies. A bigger environmental concern with aluminium is generally linked to its production. While the possibility for making solar cells that are free from both indium and tin exists, there is also the issue of the cationic and anionic polyelectrolytes from the transparent electrode PEDOT:PSS that can be harmful to aquatic life. Fullerenes, with their multiple redox states, can also be harmful to organisms. In general, the environmental impact of both fullerenes and polyelectrolytes is considered low, while conflicting reports would suggest that the evaluation of the environmental impact in any given case demands detailed study in the relevant context. Environmental studies of the impact of fullerenes have revealed both harmful and neutral biological consequences [14]. Anionic polyelectrolytes that are actively employed on a large scale for the sequestration of heavy metals and for sewage and waste treatment have been reported as harmless even at high concentrations in soil, although other reports have indicated toxicity towards microorganisms [15]. Cationic polyelectrolytes are considered toxic due to an interaction with cell membranes and this becomes especially problematic in aquatic systems [16]. Cationic electrolytes also show a strong binding to DNA [17]. The environmental impact of the conjugated polymers employed in the active layer is perhaps the least explored and, although no reports are available, it can be anticipated that the environmental impact will be similar to that of an olefinic polymer such as polystyrene, but that the conjugated polymers used in solar cell active layers may photodegrade more rapidly than most polyolefins. Recycling the constituents in the active layer presents a large challenge. In total, the active part of a polymer solar cell is a thin layer that is typically much below 1 mm in thickness, whereas the substrate and potential encapsulation material are typically several hundred mm in thickness. This means that the possible environmental threat of polymer solar cells is not posed by the active part of the polymer solar cell device as it makes up much less than a percent of the device by weight. The largest challenge from an environmental point of view may be how to deal with the substrate and the encapsulant. In this paper, we present solar cell devices prepared on the biodegradable substrate poly-L-lactic acid (PLLA). We demonstrate how PLLA can be filled with nanoclay, which has the potential to improve thermal properties and to reduce water vapour and oxygen permeability.
mol1, and polydispersity index Pd ¼ 3.6. The calculated molar masses were determined by GPC analysis and are based on narrow molar mass polystyrene standards. The nanoclay Cloisite 20A from Southern Clay Products, Inc. (Houston, Texas) is based on a natural montmorillonite modified with a quaternary ammonium salt (N+(CH3)2 (HT)2), where HT is hydrogenated tallow (65% C18, 30% C16, and 5% C14). Cloisite 20A was chosen for blending with PLLA because it is considered to have a relatively hydrophobic surface and this is potentially advantageous in terms of obtaining an intercalated or exfoliated PLLA nanocomposite. Polyethylene glycol (PEG), Mw ¼ 600 g mol1, from Merck was used as a wetting agent to ensure good distribution of the clay with the polymer granulate prior to compounding. 2.2. Substrates
2. Experimental
The raw material for the solar cell substrates was either the pure PLLA or a combination of PLLA and clay obtained by compounding PLLA granulate, PEG (w/w 0.4%), and Cloisites 20A (w/w 2%). The clay/PEG/PLLA mixture was obtained by first adding a small volume of PEG to a weighed amount of PLLA in a closed container and then shaking the contents for a minute or so to achieve coating of the PLLA granulate with PEG. The weighed amount of clay was then added and the closed container placed on rotating rolls for 5 min. The result was a mixture in which most of the clay adhered to the surface of the PEG-coated PLLA granulate. Compounding (Fig. 1) was carried out by feeding either the pure polymer or the clay/ PEG/PLLA mixture into a Haake Rheomex twin-screw extruder with four heating zones. The zone temperatures were set at 150, 180, 190, and 190 1C and the screw speed was set at 50 rpm. The extrudate was pelletized and predried at 40 1C for 2 h and then the pelletized material was typically run through the twin-screw extruder a second time in order to achieve optimum clay dispersion. Compounded pellets of either PLLA or PLLA/PEG/clay pre-dried at 40 1C for 1 h were then fed into a Haake Rheomex single-screw extruder equipped with a film die and extruded into films with a width of 12 cm and a thickness of 200 mm (Fig. 1). The single-screw extruder had five heating zones and temperatures were set to 160, 190, 200, 210, 210 1C from the in-feed to the film die temperature. The as-prepared PLLA films deformed when heated to temperatures above 40 1C due to mechanical stress from the extrusion process. This was efficiently alleviated by lamination through a standard office laminator between sheets of PET (200 mm), which also served to smooth the surface.
2.1. Materials
2.3. Device construction
The biodegradable PLLA Biomers L-9000 was obtained from Biomer, Germany. The PLLA granulate had Mw ¼ 160,000 g mol1, Mn ¼ 45,000 g mol1, Mp ¼ 165,000 g
The devices were prepared by applying a 100-nm-thick layer of aluminium onto the substrates by thermal evaporation at a pressure o1 106 mbar. The aluminium
ARTICLE IN PRESS M. Strange et al. / Solar Energy Materials & Solar Cells 92 (2008) 805–813
807
coating at 2800 rpm. Since PLLA does not support the high temperatures employed for annealing P3HT:PCBM mixtures, a microfibrillar approach was employed [18]. P3HT (30 mg) was thus dissolved in p-xylene (2 mL) by heating to 130 1C. The clear orange solution was allowed to cool slowly to room temperature, whereby the mixture becomes gel-like and dark brown from the precipitated microfibrils [18]. PCBM (30 mg) dissolved in chlorobenzene (1 mL) was then added and the mixture was shaken to give a homogenous solution that was used for spin-coating at 800 rpm. In the final step, the devices were completed by thermal evaporation of an aluminium electrode at a pressure o1 106 mbar. Sheet resistivities were measured using a four-point-contact probe from Jandel (www.jandel.com) in conjunction with a Keithley 2400 source meter. The value was extracted by passing a series of currents from a low to a high level through the film. In order to avoid offsets in the source meter and effects of thermovoltages, the same level of current was passed in both directions. The sheet resistivity was extracted from an intermediate current range in which the resistivity is independent of the current. The sheet resistivity of the front electrode comprising aluminium grid, thin Ag-layer and PEDOT:PSS was 22 O square1. 2.4. Device characterization
Fig 1. Photographs of the compounding (top) and extrusion (bottom) of the PLLA substrates employed in this work.
adhered well to both PLLA and PLLA/PEG/nanoclay. Standard ORDYL 940 photonegative photoresist (4615 from www.megauk.com) was applied by cold lamination onto the PLLA substrate with an evaporated aluminium electrode. The photoresist was illuminated for 45 s through a photonegative mask of the anode grid pattern consisting of parallel lines with a thickness of 250 mm and a spacing of 500 mm. The geometric fill factor of the anode was thus 50%. The resist was developed (developer for 4615 from www.megauk.com) and etched carefully in 10% HCl(aq) containing FeCl3 (5 wt%/V) until the aluminium at the exposed area had dissolved. Since PLLA is soluble in certain organic solvents, in order to maintain optical transmission we avoided using these solvents for removal of the photoresists. The best method to remove the resist was by subjecting the substrates to ultrasound in ethanol, whereby the photoresist detaches efficiently within 5 min. The substrates with the aluminium pattern were washed with ethanol and dried at 25 1C for 10 min before application of a thin semi-transparent silver layer (5 nm) by evaporation followed by the PEDOT:PSS layer by spin
The devices were illuminated in the ambient atmosphere using a solar simulator from Steuernagel lichttechnik, KHS 575. The luminous intensity of the solar simulator approaches AM1.5G and was set to 1000 W m2 using a bolometric radiometer from Eppley Laboratories. No corrections for mismatch were made. I–V curves were recorded from 1 to +1 V in steps of 50 mV with a speed of 0.1 s step1. The devices were not stable for extended periods of time in air and the operational lifetime was short. All illumination experiments were carried out with fan cooling of the devices, keeping the temperature at 30 1C to avoid heating the devices to temperatures above the Tg of PLLA. All manipulations were carried out in ambient air. 2.5. Microscopy Optical microscopy images were obtained using a Zeiss Axioskop with a magnification of 20. Transmission electron microscopy (TEM) images were obtained by analysis of thin sections of the material filled with nanoclay. 3. Results and discussion 3.1. Poly-L-lactic acid, a biodegradable polymer Aliphatic polyesters with their hydrolysable ester bonds have attracted much attention for applications in which biodegradability or compostability is considered
ARTICLE IN PRESS M. Strange et al. / Solar Energy Materials & Solar Cells 92 (2008) 805–813
808
important. A variety of aliphatic biodegradable polyesters have therefore been developed commercially or are under development, including those naturally produced, such as the polyhydroxyalkanoates (PHAs) or those derived synthetically in several steps using monomers derived from renewable resources such as PLLA. The production of PLLA involves bacterial fermentation of starch derived from crops such as corn, sugar cane or sugar beet to obtain lactic acid, which is then catalytically dimerized to the lactide, followed by ring-opening polymerization (ROP). Unlike the direct polycondensation route from lactic acid, which requires removal of water, the ROP method is preferred commercially because it provides a route to relatively high-molecular-weight PLLA [19]. The ester linkages in PLLA are sensitive to both chemical hydrolysis and enzymatic chain cleavage. PLLA is therefore completely biodegradable when composted in humid environments at temperatures above Tg (60 1C), conditions typically found in municipal/industrial composting facilities. The first step in the degradation process is the hydrolysis of the ester bonds, leading to a cleavage of the polymer backbone and the liberation of lactic acid. The lactic acid is composted by microorganisms. PLLA-based materials can therefore be rapidly broken down in nature and their disposal does not pose an environmental threat to the same degree as the disposal of non-biodegradable polymers such as the polyolefins [20]. 3.2. Filling with nanoclay The optimal use of PLLA as a solar cell substrate and encapsulation material requires further development of the material. First, improved mechanical and thermal stability of the substrate is needed to give a robust device. Second, good water and oxygen barrier properties are needed in order to protect the active components of the solar cell and
ensure an adequate shelf and service life. A potentially effective solution to improve material performance in this way is to make a nanocomposite material by dispersing nanoscale silicate clay particles or platelets into the biodegradable polymer. A number of previous researchers have demonstrated this possibility with PLLA [21–24], and there have been recent reviews on the topic of PLLA nanocomposites [25,26]. The challenge in this process, which can be carried out through a melt extrusion process, is to obtain the optimum material through the best possible dispersion of clay in the polymer and the insertion of polymer in the clay galleries to achieve either an intercalated or an exfoliated nanocomposite. A schematic showing the idealized structure of these two forms of polymer/clay nanocomposite is depicted in Fig. 2. Factors that can influence the result of such processes include clay type and surface treatment, pre-drying processes, mixing/compounding methods, and extrusion conditions (e.g., operating temperatures, screw speed, extruder barrel length:diameter ratio). In order to characterize the PLLA substrates filled with nanoclay, both optical and transmission electron microscopy images were obtained as shown in Fig. 3. The optical microscopy images are naturally unable to establish how well dispersed the nanoclay particles are on the nanoscale, but proved very useful when establishing if dispersion had been efficiently achieved at optical length scales and if there were still aggregates present. As seen in the optical images, dispersion had been achieved when using PEG as wetting agent. The TEM images showed that the nanoclay plates were efficiently dispersed at the nanoscale and that probably both exfoliated and intercalated structures are present. It should be mentioned that the relatively low loadings employed here do imply that permeation between the nanoclay plates is possible. Higher loadings
Fig. 2. Idealized schematic of the structure of intercalated or exfoliated polymer/clay nanocomposites.
ARTICLE IN PRESS M. Strange et al. / Solar Energy Materials & Solar Cells 92 (2008) 805–813
809
Fig. 3. Optical images at 20 magnification of dry Cloisite 20A nanoclay (top left), Cloisite 20A nanoclay dispersed in PEG (top middle) and 2% Cloisite 20 A nanoclay in PLA (top right) showing that the particles are well dispersed. TEM images of 2% Cloisite 30B nanoclay in PLA (bottom right and left at two different magnifications) showing mainly exfoliated dispersion at the nanoscale.
are possibly required before the substrate as prepared here becomes impervious to water and oxygen diffusion. 3.3. Substrate properties of PLLA The successful application of a polymer in photovoltaic devices imposes requirements in terms of transparency, mechanical stability, thermal stability, stability towards water/oxygen, and stability towards solvent processing. Fortunately, PLLA meets most of these requirements and the only issues requiring precautions are with respect to organic solvents and high temperature. The optical quality of PLLA films is excellent and good transparency is achieved in the UV–vis range. Even with a loading of 2% nanoclay good transparency without light scattering was observed as shown in Fig. 4. Mechanical properties of PLLA are considered similar to those of polystyrene, with tensile strength in the order of 70 MPa, tensile modulus of 3.6 GPa and elongation at break of 2.4% as reported by Biomer [27]. PLLA does not degrade when heated below the melting point; however, the relatively low Tg of about 60 1C can present problems for certain applications (e.g., transportation of rigid packaging). PLLA is stable in air and will only hydrolyse very slowly when exposed to normal atmospheric conditions. An overview of PLLA properties in relation to composition was prepared by Sodergard and Stolt [28] and is a useful source of further information. Commercial interest in PLLA has developed quite rapidly over the past 2–3 years. The polymer has been used in biomedical applications for some time, but there is also now expanding use of PLLA film in packaging, as well as strong interest in PLLA fibers for textile and other applications. The price of PLLA
remains higher than that of most commodity polymers, but this may change as PLLA production volumes increase in parallel with increases in the prices of other polymers driven by rising oil prices. The distinct disadvantages with PLLA as a substrate for polymer photovoltaics is the low glass transition temperature and the soluble nature. The only solvents that PLLA will sustain are solvents such as water and ethanol, which makes it quite difficult to prepare the active layer using common organic solvents and thermal annealing. 3.4. Indium-free transparent electrodes Aside from the environmental considerations associated with the toxicity of indium, a generally acknowledged problem is that the transparent conductor that is employed almost exclusively in all laboratory trials reported to this date is made of indium tin oxide (ITO). Transparent conductors based on indium are by far the highest performing materials available, and indium is currently used in most flexible displays and flat screens for personal computers and television. Common to these products is that they are of relatively high value. The price of indium has increased nearly 10-fold since 2002, when the price was 100 $ kg1, to 2005, when the price was at 900 $ kg1 [29]. The price has decreased a little in 2007 but remains high at 700 $ kg1. A further complication is that the amount of indium available in the earth’s crust is believed to be too low to satisfy the needs of the future and the only source of indium currently is as a by-product of zinc mining. The current estimate is that indium will make up more than 30% of the cost of polymer solar cell devices comprising transparent electrodes based on indium. Thus,
ARTICLE IN PRESS M. Strange et al. / Solar Energy Materials & Solar Cells 92 (2008) 805–813
810
Transmission (%)
100 80 PLA PLA/PEG/Nanoclay PLA/Nanoclay/Al-grid /Ag-layer
60 40 20 0 200
300
400
500
600
700
800
Wavelength (nm) Fig. 4. Transmission of extruded films based on pure PLLA and PLLA/PEG/nanoclay (left) and a photograph taken through a PLLA film showing that the material is highly transparent while the optical quality is uneven (right).
there is also a considerable motivation in terms of cost reduction for avoiding the use of indium in polymer solar cells. There have been only a few reports where indium-free transparent electrodes have been employed and these have involved either very high-conductivity PEDOT:PTS with a sacrifice in optical transmission [30] or a highly transparent low-conductivity PEDOT:PSS with an underlying metallic silver grid [31,32] that both require very smooth substrates. The PLLA films prepared in this work have a relatively rough surface, making the realization of a fine grid difficult. Instead, a course grid was made by evaporating aluminium onto PLLA substrates that had been smoothed as much as possible by lamination between PET plates followed by evaporation of 100 nm of aluminium. A simple photonegative photoresist was then cold laminated on top of the aluminium. The low temperature during lamination was employed to avoid shrinkage. Subsequent illumination through a mask gave an aluminium grid with a geometric fill factor of 50% (i.e. 50% shadow loss). While controlled under-etching could be employed to increase the geometric fill factor by having grid lines with as little as 180 mm in width after etching, this only improved the geometric fill factor from 50% to 62.5% and must be seen as a marginal improvement. What is needed is a resolution on the 10 mm scale with respect to both line width and position as compared to the current resolution, which is on the 250 mm scale. A current limitation to this technique is the low geometric fill factor, which has its roots in the limit to the definition of the underlying metal grid using this photoresist technology. The layout of the electrode and the devices is shown in Fig. 5. While a finer grid definition is desirable, this is the best we have been able to obtain due to limitations in the PLLA substrate surface and the subsequent resolution of the photoresist. The current limit is around two 250 mm lines per mm, giving the 50% shadow loss. The optical transmission as measured on a complete substrate for polymer solar cells comprising PLLA substrate filled with nanoclay, aluminium grid and a thin silver layer had a transmission of around 20% in the visible range as shown in Fig. 4. The relatively low transmission is a drawback of the substrate
in its current development stage, and further development and improvements are needed (Fig. 5). 3.5. Performance, stability, and lifetime of PLLA devices It may be viewed as a simple exercise to replace the commonly employed rigid glass substrate with a flexible plastic substrate. The general observation, however, is that the performance of polymer solar cells prepared on a flexible plastic substrate such as PET is inferior to those prepared on rigid glass. PET as a substrate has many desirable properties such as insolubility in most organic solvents, good optical quality, relative ease of planarization, and mechanical stability up to temperatures well above 120 1C. PLLA was easily extruded into films, and at room temperature the film is flexible with a rigidity comparable to that of PET. The devices prepared performed relatively poorly, and in spite of many attempts we were not able to even approach the performance of the state-of-the-art. This is ascribed to several factors that are all connected with the PLLA substrate and the anode. Currently, the most severe problem is the incompatibility of PLLA with common organic solvents. PLLA substrates with the simple aluminium anode were completely destroyed when spincoating the polymer fullerene mixture from 1,2-dichlorobenzene or chlorobenzene. Our partial solution to this problem was the evaporation of a thin silver layer and the use of chlorobenzene that evaporates much faster than 1,2-dichlorobenzene. It was then possible to spincoat the film without destruction of the substrate and underlying anode grid, but contact times with solvent had to be kept short. Thermal annealing was also not possible and for this reason the preformed microfibrillar approach [18] was employed. The devices were subjected to degradation in air from the moment they were exposed to the atmosphere and the degradation was quick. The best open circuit voltage that could be obtained was 0.23 V, a short circuit current of 0.27 mA cm2 for the active area and a fill factor of 26% as shown in Fig. 6. The device performance degraded quickly, and to record the I–V curves voltage steps of 50 mV and a
ARTICLE IN PRESS M. Strange et al. / Solar Energy Materials & Solar Cells 92 (2008) 805–813
811
Fig. 5. A drawing of the electrode layout (2 2 cm) (A) and photographs of the PLLA substrate with aluminium electrode and exposed photoresist showing many devices (B), the completed anode (C), and a completed photovoltaic device seen from the front (D).
2
-0.25
0.05
0
0.00
-0.05
-1
-0.10 -0.5
0.0 Voltage (Volts)
0.5
1.0
Fig. 6. I–V curves for a device as shown in Fig. 5 with an active area of 1 cm2 in the dark and under illumination at 1000 W m2 and AM1.5G. The device temperature was 30 1C.
step time of 0.1 s was employed. Many of the devices were short circuited or open and it was difficult to obtain functional devices. The best electrical contact was obtained with silver epoxy. After the I–V curves had been recorded, the lifetime was determined and generally followed the curves shown in Fig. 7. The devices quickly degraded and during the time between I–V-curve measurements (light followed by dark) until the lifetime measurement was started the device performance decreased by as much as a factor of 1000. The time taken for total degradation of the device was of the order of 2 h (only the first 15 min after I–V-curve recordings are shown in Fig. 7). An explanation for the exceptionally poor performance of the devices prepared here is most likely linked to the problems with planarity of the substrate and the poor thermal properties of the PLLA prepared. The addition of the nanoclay was within experimental error, without influence on the performance of the material both in terms
Current density (µA cm-2)
1
Dark current density (mA cm-2)
Current density (mA cm-2)
Dark 1000 W m-2 @ AM1.5G
-2 -1.0
PLA PLA/PEG/Nanoclay
0.10 -0.20
-0.15
-0.10
-0.05
0.00 0
5
10
15
Time (Minutes) Fig. 7. Decay of the short circuit current recorded every second during continuous illumination I–V curves for a device as shown in Fig. 5 with an active area of 1 cm2 under illumination at 1000 W m2 and AM1.5G. The device temperature was 30 1C. Devices prepared on PLA with and without nanoclay are shown.
of thermal properties and with respect to diffusion of oxygen and/or water as evidenced by the poor stability in both cases (Fig. 7). The results from optical microscopy and TEM indicate that the nanoclay was well dispersed and that either an exfoliated or an intercalated nanocomposite was obtained (see Section 3.2 and Figs. 2 and 3). It would, however, seem that higher loadings are required before the nanoclay will significantly increase the diffusion path for permeants. The device performance is limited by the timescale of the degradation, which is comparable with or shorter than the experimental timescale for applying electrical contacts, carrying out measurements of I–V curves and setting up the lifetime test. PLLA has a propensity to absorb humidity from the atmosphere and will as such actively transport water to the active layer of the cell. Unencapsulated devices on flexible substrates have been observed to have shorter lifetimes than on glass, as observed for other ITO-free devices with a PET-PEDOT or a PE-PEDOT substrate [30]. We exclude the active
ARTICLE IN PRESS 812
M. Strange et al. / Solar Energy Materials & Solar Cells 92 (2008) 805–813
materials in this section as playing a part in the poor performance and being the source of the poor stability of these devices as the microfibrillar approach [18] has been shown to give high-efficiency devices without the need for thermal annealing and has been shown to give devices with very good stability when prepared from chlorobenzene. While the use of flexible substrates often leads to slightly inferior performance when compared to rigid glass substrates, this cannot account for the poor performance observed here. We thus ascribe the poor performance observed to the PLLA substrate in the form that we were able to prepare it. The relatively low voltages suggest that the planarity needs to be improved and the stress present in the substrate needs to be removed. The fast degradation is most likely due to fast water and oxygen diffusion through the PLLA. Our results do, however, demonstrate that functional devices could be prepared, and further work on the properties of PLLA as a substrate for polymer photovoltaics seems highly feasible. 3.6. Comparison with batteries and environmental considerations One of the niche applications of polymer solar cells is anticipated to be as power supplies for small electronic devices. A direct competitor to the polymer solar cell is the battery, which has obvious advantages in terms of energy storage and operation in the dark. The polymer solar cells offer flexibility, a thin outline, light weight, and possible environmental benefits in terms of biodegradability as outlined in this paper. A direct comparison between batteries and polymer solar cells is naturally difficult as there is a certain degree of incongruence between the two technologies. The polymer solar cells will naturally only be applicable when light is available as the source of energy and their energy output depends on the incoming light flux. The battery relies on energy stored chemically, and when energy is drawn from a battery chemical reactions inside the battery are the source of the electrical energy. This process continues until the chemicals available inside the battery have been consumed and this therefore places a limit on the capacity of the battery. The energy output is, depending on the battery type, quite constant throughout the service life of the battery. In this respect, a photovoltaic device offers the principal advantage that it will operate as long as it is illuminated. However, polymer photovoltaics degrade and this means that they have a finite service life just like batteries. There is though an environmental benefit of having a biodegradable material that can help to avoid concerns of accidental disposal in nature. The development of a biodegradable substrate for polymer-based solar cells should not be seen as an attempt to encourage disposal in nature, but rather as an attempt to limit the effect of the disposal in nature that evidently takes place. Strong schemes for reuse of the materials and proper disposal should still be the only acceptable practice, and it has been demonstrated that this can be handled efficiently. Efficient
recycling schemes for the various battery technologies have been explored [33,34] and, in the case of lead acid batteries, a very high degree of recycling of 497% between 1997 and 2001 has been established [35]. The problem still remains for small applications where the distribution is vast and responsibility for reuse of materials and recycling is in the hands of the consumer. Further, as the value of the consumable decreases, so does the responsibility that the consumer feels towards the product. 4. Conclusion The use of polymer photovoltaics based on a biodegradable platform was discussed and found to potentially solve some of the issues related to disposal in the environment in a manner that conventional technologies such as batteries have never achieved. We have demonstrated the preparation of PLLA film substrates by extrusion of films based either on PLLA or on PLLA–nanoclay composites and the use of these films to prepare solar cell devices. We found no improvements in terms of lifetime or performance when comparing the performance of devices based on PLLA films and PLLA–nanoclay composite films, but this may be because the clay was inadequately dispersed and/or the chosen clay was not optimal for this process. An indiumfree transparent front electrode was prepared by evaporation of aluminium followed by application of a photoresist, illumination through a photonegative mask, development, and etching. The device lifetime was very short and adversely affected the device performance on the timescale of the measurement and characterization. The devices were not thermally stable due to the low glass transition temperature. Further problems were associated with the thermal stress from the extrusion process and the planarity of the substrate. In conclusion, the main limiting factors are the restricted temperature range available for post processing the device and the soluble nature of PLLA making the range of solvents available for processing limited to water and ethanol. 5. Future work Our research has established the feasibility of using PLLA as a solar cell substrate, and, although improved solar cell performance through utilization of a PLLA/ organoclay composite films was not demonstrated, future plans include the evaluation of alternative true nanocomposite films based on PLLA and nano-scale reinforcing materials targeted at high-performance solar cell applications. Further developments include new polymer materials that can be processed from water, ethanol, or similar environmentally friendly solvents, indium-free front electrodes with a better transparency, and exfoliated nanocomposite structures with higher loading of the nanoclay and true barrier properties towards water and oxygen.
ARTICLE IN PRESS M. Strange et al. / Solar Energy Materials & Solar Cells 92 (2008) 805–813
Acknowledgements This work was supported by the Danish Strategic Research Council (DSF 2104-05-0052). Jan Alstrup is acknowledged for technical assistance. References [1] C.J. Brabec, N.S. Sariciftci, J.C. Hummelen, Adv. Funct. Mater. 11 (2001) 15. [2] H. Spanggaard, F.C. Krebs, Sol. Energy Mater. Sol. Cells 83 (2004) 125. [3] K.M. Coakley, M.D. McGehee, Chem. Mater. 16 (2004) 4533. [4] H. Hoppe, N.S. Sariciftci, J. Mater. Res. 19 (2004) 1924. [5] E. Bundgaard, F.C. Krebs, Sol. Energy Mater. Sol. Cells 91 (2007) 954. [6] S. Gu¨nes, H. Neugebauer, N.S. Sariciftci, Chem. Rev. 107 (2007) 1324. [7] C.J. Brabec, Sol. Energy Mater. Sol. Cells 83 (2004) 273. [8] F.C. Krebs, Refocus 6 (3) (2005) 38. [9] F.C. Krebs, J. Alstrup, H. Spangaard, K. Larsen, E. Kold, Sol. Energy Mater. Sol. Cells 83 (2004) 293. [10] F.C. Krebs, H. Spanggaard, T. Kjær, M. Biancardo, J. Alstrup, Mater. Sci. Eng. B 138 (2007) 106. [11] R.W. Miles, K.M. Hynes, I. Forbes, Prog. Cryst. Growth Charact. Mater. 51 (2005) 1. [12] R.E. Chapin, M.W. Harris, E.S. Hunter, B.J. Davis, B.J. Collins, A.C. Lockhart, Toxicol. Sci. 27 (1995) 140. [13] A. Boughriet, N. Proix, G. Billon, P. Recourt, B. Ouddane, Water Air Soil Pollut. 180 (2007) 83. [14] Z. Tong, M. Bischoff, L. Nies, R.F. Turco, Environ. Sci. Technol. 41 (2007) 2985. [15] C.P. Chu, D.J. Lee, B.V. Chang, C.S. Liao, J. Chem. Tech. Biotech. 76 (2001) 598.
813
[16] A.A. Beim, A.M. Beim, Environ. Technol. 15 (1994) 195. [17] J.W. Hong, W.L. Hemme, G.E. Keller, M.T. Rinke, G.C. Bazan, Adv. Mater. 18 (2006) 878. [18] S. Berson, R. de Bettignies, S. Bailly, S. Guillerez, Adv. Funct. Mater. 17 (2007) 1377. [19] R. Je´roˆme, P. Lecomte, New developments in the synthesis of aliphatic polyesters by ring-opening polymerization, in: R. Smith (Ed.), Chapter 4 in Biodegradable Polymers for Industrial Applications, Woodhead Publishing Ltd., 2005. [20] J.–F. Zhang, X. Sun, Poly(lactic acid)-based bioplastics, in: R. Smith (Ed.), Chapter 10 in Biodegradable Polymers for Industrial Applications, Woodhead Publishing Ltd., 2005. [21] M. Pluta, J. Poly Sci. Part B Polym. Phys. 44 (2006) 3392. [22] S.S. Ray, K. Yamada, A. Ogami, M. Okamoto, K. Ueda, Macromol. Rapid Commun. 23 (2002) 943. [23] M.A. Paul, M. Alexandre, P. Degee, C. Henrist, A. Rulmont, P. Dubois, Polymer 44 (2003) 443. [24] S.S. Ray, M. Bousmina, Prog. Mater. Sci. 50 (2005) 962. [25] K.K. Yang, X.L. Wang, Y.Z. Wang, J. Ind. Eng. Chem. 13 (2007) 485. [26] S.S. Singh, S.S. Ray, J. Nanosci. Nanotechnol. 7 (2007) 2596. [27] http://www.biomer.de/. [28] A. Sodergard, M. Stolt, Prog. Poly. Sci. 27 (2002) 1123. [29] http://geology.com/articles/indium.shtml. [30] B. Winther-Jensen, F.C. Krebs, Sol. Energy Mater. Sol. Cells 90 (2006) 123. [31] T. Aernouts, P. Vanlaeke, W. Geens, J. Poortmans, P. Heremans, S. Borghs, R. Mertens, R. Andriessen, L. Leenders, Thin Solid Films 451–452 (2004) 22. [32] K. Tvingstedt, O. Ingana¨s, Adv. Mater. 19 (2007) 2893–2897, doi:10.1002/adma.200602561 [33] A.M. Bernardes, D.C.R. Espinosa, J.A.S. Teno´rio, J. Power Sourc. 130 (2004) 291. [34] F. Ahmed, J. Power Sourc. 59 (1996) 107. [35] http://www.batterycouncil.org/recycling.html.
Solar Energy Materials & Solar Cells 92 (2008) 805–813 www.elsevier.com/locate/solmat
Biodegradable polymer solar cells Marianne Strange, David Plackett, Martin Kaasgaard, Frederik C. Krebs National Laboratory for Sustainable Energy, Technical University of Denmark, Frederiksborgvej 399, DK-4000 Roskilde, Denmark Received 13 November 2007; received in revised form 1 February 2008; accepted 5 February 2008 Available online 20 March 2008
Abstract Polymer photovoltaic devices were prepared using a biodegradable poly-L-lactic acid (PLLA) substrate loaded with nanoclay in an attempt to improve the thermal properties and possibly reduce permeation of water and oxygen. The nanoclay-loaded PLLA substrates were prepared by compounding and extrusion. The substrate thickness was 200 mm and the substrates had good transparency in the range 300–800 nm of, respectively, 480% for PLA and 460% for PLLA loaded with nanoclay. Devices were realized by application of a conducting transparent anode comprising an aluminium grid with a thin overlayer of silver and spin-coated PEDOT:PSS. The active layer consisted of microfibrillar P3HT and PCBM to encompass the low processing temperatures for PLLA and finally an evaporated aluminium cathode. It was found that PLLA as a substrate holds potential, but there are several challenges beyond the photovoltaic itself which must be met before general application within this field can be envisaged. The most important aspects are the planarity of the PLLA surface, the mechanical stresses induced by the extrusion process, the limitation in processing temperature, and the limitation in the available range of solvents for solution processing. r 2008 Elsevier B.V. All rights reserved. Keywords: Nanoclay; Polylactic acid; Solar cells; Biodegradation; Environment; Permeation
1. Introduction Solar cells based on polymers [1–6] are projected to offer renewable energy to niche products within the next few years. The prospects for the use of polymer solar cells for on-grid electrical energy production is also a possibility but is believed to lie a decade or more into the future. The advantages of the polymer solar cell technology over existing solar cell technologies include: flexibility, low cost, low thermal budgets, solution processing, roll-to-roll processing, and light weight. The aspect of roll-to-roll fabrication methods using printing and coating techniques implies that very high throughput speeds of many meters per second can be achieved [7–10]. The consequence of widespread usage of polymer solar cells produced at high volume will pose a challenge to the channels available for disposal of the devices when they stop working. In principle, the polymer substrate used for the solar cells can be recycled efficiently, but in the case of solar cell products for niche market applications that essentially Corresponding author. Tel.: +45 46 77 47 99.
E-mail address: [email protected] (F.C. Krebs). 0927-0248/$ - see front matter r 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2008.02.002
compete with small batteries there is the potential problem of unwanted dispersion and improper disposal in the environment. While there are currently no studies available that have examined the environmental impact of widespread usage of polymer solar cells and their disposal [11], the individual constituents of a typical polymer solar cell could pose a threat to the environment. The constituents of a typical bulk heterojunction device are indium, tin, aluminium, polythiophene, PEDOT:PSS, and a fullerene derivative. Most of these materials have been documented to be harmful to life forms in the environment although this does depend on the conditions and conflicting evidence has been reported. With respect to the metals indium [12], tin [13], and aluminium, the toxicity has been addressed for both soluble and insoluble sources in both organic and inorganic forms. Both indium and tin can in some forms be quite toxic, but the consequences of widespread dispersal in the environment are by no means fully established and have only been considered in the context of case studies on metal discharge close to smelters [14] and in association with the development of lead-free soldering alloys. Aluminium is likely to be the least harmful if released in the environment; however, this is relative to the other
ARTICLE IN PRESS 806
M. Strange et al. / Solar Energy Materials & Solar Cells 92 (2008) 805–813
metals of interest here and it should be noted that an EPA recommendation issued in 1998 suggested that aluminium levels in water should not exceed a 3-day average of 87 mg L1. This average concentration was determined on the basis of fish toxicity studies. A bigger environmental concern with aluminium is generally linked to its production. While the possibility for making solar cells that are free from both indium and tin exists, there is also the issue of the cationic and anionic polyelectrolytes from the transparent electrode PEDOT:PSS that can be harmful to aquatic life. Fullerenes, with their multiple redox states, can also be harmful to organisms. In general, the environmental impact of both fullerenes and polyelectrolytes is considered low, while conflicting reports would suggest that the evaluation of the environmental impact in any given case demands detailed study in the relevant context. Environmental studies of the impact of fullerenes have revealed both harmful and neutral biological consequences [14]. Anionic polyelectrolytes that are actively employed on a large scale for the sequestration of heavy metals and for sewage and waste treatment have been reported as harmless even at high concentrations in soil, although other reports have indicated toxicity towards microorganisms [15]. Cationic polyelectrolytes are considered toxic due to an interaction with cell membranes and this becomes especially problematic in aquatic systems [16]. Cationic electrolytes also show a strong binding to DNA [17]. The environmental impact of the conjugated polymers employed in the active layer is perhaps the least explored and, although no reports are available, it can be anticipated that the environmental impact will be similar to that of an olefinic polymer such as polystyrene, but that the conjugated polymers used in solar cell active layers may photodegrade more rapidly than most polyolefins. Recycling the constituents in the active layer presents a large challenge. In total, the active part of a polymer solar cell is a thin layer that is typically much below 1 mm in thickness, whereas the substrate and potential encapsulation material are typically several hundred mm in thickness. This means that the possible environmental threat of polymer solar cells is not posed by the active part of the polymer solar cell device as it makes up much less than a percent of the device by weight. The largest challenge from an environmental point of view may be how to deal with the substrate and the encapsulant. In this paper, we present solar cell devices prepared on the biodegradable substrate poly-L-lactic acid (PLLA). We demonstrate how PLLA can be filled with nanoclay, which has the potential to improve thermal properties and to reduce water vapour and oxygen permeability.
mol1, and polydispersity index Pd ¼ 3.6. The calculated molar masses were determined by GPC analysis and are based on narrow molar mass polystyrene standards. The nanoclay Cloisite 20A from Southern Clay Products, Inc. (Houston, Texas) is based on a natural montmorillonite modified with a quaternary ammonium salt (N+(CH3)2 (HT)2), where HT is hydrogenated tallow (65% C18, 30% C16, and 5% C14). Cloisite 20A was chosen for blending with PLLA because it is considered to have a relatively hydrophobic surface and this is potentially advantageous in terms of obtaining an intercalated or exfoliated PLLA nanocomposite. Polyethylene glycol (PEG), Mw ¼ 600 g mol1, from Merck was used as a wetting agent to ensure good distribution of the clay with the polymer granulate prior to compounding. 2.2. Substrates
2. Experimental
The raw material for the solar cell substrates was either the pure PLLA or a combination of PLLA and clay obtained by compounding PLLA granulate, PEG (w/w 0.4%), and Cloisites 20A (w/w 2%). The clay/PEG/PLLA mixture was obtained by first adding a small volume of PEG to a weighed amount of PLLA in a closed container and then shaking the contents for a minute or so to achieve coating of the PLLA granulate with PEG. The weighed amount of clay was then added and the closed container placed on rotating rolls for 5 min. The result was a mixture in which most of the clay adhered to the surface of the PEG-coated PLLA granulate. Compounding (Fig. 1) was carried out by feeding either the pure polymer or the clay/ PEG/PLLA mixture into a Haake Rheomex twin-screw extruder with four heating zones. The zone temperatures were set at 150, 180, 190, and 190 1C and the screw speed was set at 50 rpm. The extrudate was pelletized and predried at 40 1C for 2 h and then the pelletized material was typically run through the twin-screw extruder a second time in order to achieve optimum clay dispersion. Compounded pellets of either PLLA or PLLA/PEG/clay pre-dried at 40 1C for 1 h were then fed into a Haake Rheomex single-screw extruder equipped with a film die and extruded into films with a width of 12 cm and a thickness of 200 mm (Fig. 1). The single-screw extruder had five heating zones and temperatures were set to 160, 190, 200, 210, 210 1C from the in-feed to the film die temperature. The as-prepared PLLA films deformed when heated to temperatures above 40 1C due to mechanical stress from the extrusion process. This was efficiently alleviated by lamination through a standard office laminator between sheets of PET (200 mm), which also served to smooth the surface.
2.1. Materials
2.3. Device construction
The biodegradable PLLA Biomers L-9000 was obtained from Biomer, Germany. The PLLA granulate had Mw ¼ 160,000 g mol1, Mn ¼ 45,000 g mol1, Mp ¼ 165,000 g
The devices were prepared by applying a 100-nm-thick layer of aluminium onto the substrates by thermal evaporation at a pressure o1 106 mbar. The aluminium
ARTICLE IN PRESS M. Strange et al. / Solar Energy Materials & Solar Cells 92 (2008) 805–813
807
coating at 2800 rpm. Since PLLA does not support the high temperatures employed for annealing P3HT:PCBM mixtures, a microfibrillar approach was employed [18]. P3HT (30 mg) was thus dissolved in p-xylene (2 mL) by heating to 130 1C. The clear orange solution was allowed to cool slowly to room temperature, whereby the mixture becomes gel-like and dark brown from the precipitated microfibrils [18]. PCBM (30 mg) dissolved in chlorobenzene (1 mL) was then added and the mixture was shaken to give a homogenous solution that was used for spin-coating at 800 rpm. In the final step, the devices were completed by thermal evaporation of an aluminium electrode at a pressure o1 106 mbar. Sheet resistivities were measured using a four-point-contact probe from Jandel (www.jandel.com) in conjunction with a Keithley 2400 source meter. The value was extracted by passing a series of currents from a low to a high level through the film. In order to avoid offsets in the source meter and effects of thermovoltages, the same level of current was passed in both directions. The sheet resistivity was extracted from an intermediate current range in which the resistivity is independent of the current. The sheet resistivity of the front electrode comprising aluminium grid, thin Ag-layer and PEDOT:PSS was 22 O square1. 2.4. Device characterization
Fig 1. Photographs of the compounding (top) and extrusion (bottom) of the PLLA substrates employed in this work.
adhered well to both PLLA and PLLA/PEG/nanoclay. Standard ORDYL 940 photonegative photoresist (4615 from www.megauk.com) was applied by cold lamination onto the PLLA substrate with an evaporated aluminium electrode. The photoresist was illuminated for 45 s through a photonegative mask of the anode grid pattern consisting of parallel lines with a thickness of 250 mm and a spacing of 500 mm. The geometric fill factor of the anode was thus 50%. The resist was developed (developer for 4615 from www.megauk.com) and etched carefully in 10% HCl(aq) containing FeCl3 (5 wt%/V) until the aluminium at the exposed area had dissolved. Since PLLA is soluble in certain organic solvents, in order to maintain optical transmission we avoided using these solvents for removal of the photoresists. The best method to remove the resist was by subjecting the substrates to ultrasound in ethanol, whereby the photoresist detaches efficiently within 5 min. The substrates with the aluminium pattern were washed with ethanol and dried at 25 1C for 10 min before application of a thin semi-transparent silver layer (5 nm) by evaporation followed by the PEDOT:PSS layer by spin
The devices were illuminated in the ambient atmosphere using a solar simulator from Steuernagel lichttechnik, KHS 575. The luminous intensity of the solar simulator approaches AM1.5G and was set to 1000 W m2 using a bolometric radiometer from Eppley Laboratories. No corrections for mismatch were made. I–V curves were recorded from 1 to +1 V in steps of 50 mV with a speed of 0.1 s step1. The devices were not stable for extended periods of time in air and the operational lifetime was short. All illumination experiments were carried out with fan cooling of the devices, keeping the temperature at 30 1C to avoid heating the devices to temperatures above the Tg of PLLA. All manipulations were carried out in ambient air. 2.5. Microscopy Optical microscopy images were obtained using a Zeiss Axioskop with a magnification of 20. Transmission electron microscopy (TEM) images were obtained by analysis of thin sections of the material filled with nanoclay. 3. Results and discussion 3.1. Poly-L-lactic acid, a biodegradable polymer Aliphatic polyesters with their hydrolysable ester bonds have attracted much attention for applications in which biodegradability or compostability is considered
ARTICLE IN PRESS M. Strange et al. / Solar Energy Materials & Solar Cells 92 (2008) 805–813
808
important. A variety of aliphatic biodegradable polyesters have therefore been developed commercially or are under development, including those naturally produced, such as the polyhydroxyalkanoates (PHAs) or those derived synthetically in several steps using monomers derived from renewable resources such as PLLA. The production of PLLA involves bacterial fermentation of starch derived from crops such as corn, sugar cane or sugar beet to obtain lactic acid, which is then catalytically dimerized to the lactide, followed by ring-opening polymerization (ROP). Unlike the direct polycondensation route from lactic acid, which requires removal of water, the ROP method is preferred commercially because it provides a route to relatively high-molecular-weight PLLA [19]. The ester linkages in PLLA are sensitive to both chemical hydrolysis and enzymatic chain cleavage. PLLA is therefore completely biodegradable when composted in humid environments at temperatures above Tg (60 1C), conditions typically found in municipal/industrial composting facilities. The first step in the degradation process is the hydrolysis of the ester bonds, leading to a cleavage of the polymer backbone and the liberation of lactic acid. The lactic acid is composted by microorganisms. PLLA-based materials can therefore be rapidly broken down in nature and their disposal does not pose an environmental threat to the same degree as the disposal of non-biodegradable polymers such as the polyolefins [20]. 3.2. Filling with nanoclay The optimal use of PLLA as a solar cell substrate and encapsulation material requires further development of the material. First, improved mechanical and thermal stability of the substrate is needed to give a robust device. Second, good water and oxygen barrier properties are needed in order to protect the active components of the solar cell and
ensure an adequate shelf and service life. A potentially effective solution to improve material performance in this way is to make a nanocomposite material by dispersing nanoscale silicate clay particles or platelets into the biodegradable polymer. A number of previous researchers have demonstrated this possibility with PLLA [21–24], and there have been recent reviews on the topic of PLLA nanocomposites [25,26]. The challenge in this process, which can be carried out through a melt extrusion process, is to obtain the optimum material through the best possible dispersion of clay in the polymer and the insertion of polymer in the clay galleries to achieve either an intercalated or an exfoliated nanocomposite. A schematic showing the idealized structure of these two forms of polymer/clay nanocomposite is depicted in Fig. 2. Factors that can influence the result of such processes include clay type and surface treatment, pre-drying processes, mixing/compounding methods, and extrusion conditions (e.g., operating temperatures, screw speed, extruder barrel length:diameter ratio). In order to characterize the PLLA substrates filled with nanoclay, both optical and transmission electron microscopy images were obtained as shown in Fig. 3. The optical microscopy images are naturally unable to establish how well dispersed the nanoclay particles are on the nanoscale, but proved very useful when establishing if dispersion had been efficiently achieved at optical length scales and if there were still aggregates present. As seen in the optical images, dispersion had been achieved when using PEG as wetting agent. The TEM images showed that the nanoclay plates were efficiently dispersed at the nanoscale and that probably both exfoliated and intercalated structures are present. It should be mentioned that the relatively low loadings employed here do imply that permeation between the nanoclay plates is possible. Higher loadings
Fig. 2. Idealized schematic of the structure of intercalated or exfoliated polymer/clay nanocomposites.
ARTICLE IN PRESS M. Strange et al. / Solar Energy Materials & Solar Cells 92 (2008) 805–813
809
Fig. 3. Optical images at 20 magnification of dry Cloisite 20A nanoclay (top left), Cloisite 20A nanoclay dispersed in PEG (top middle) and 2% Cloisite 20 A nanoclay in PLA (top right) showing that the particles are well dispersed. TEM images of 2% Cloisite 30B nanoclay in PLA (bottom right and left at two different magnifications) showing mainly exfoliated dispersion at the nanoscale.
are possibly required before the substrate as prepared here becomes impervious to water and oxygen diffusion. 3.3. Substrate properties of PLLA The successful application of a polymer in photovoltaic devices imposes requirements in terms of transparency, mechanical stability, thermal stability, stability towards water/oxygen, and stability towards solvent processing. Fortunately, PLLA meets most of these requirements and the only issues requiring precautions are with respect to organic solvents and high temperature. The optical quality of PLLA films is excellent and good transparency is achieved in the UV–vis range. Even with a loading of 2% nanoclay good transparency without light scattering was observed as shown in Fig. 4. Mechanical properties of PLLA are considered similar to those of polystyrene, with tensile strength in the order of 70 MPa, tensile modulus of 3.6 GPa and elongation at break of 2.4% as reported by Biomer [27]. PLLA does not degrade when heated below the melting point; however, the relatively low Tg of about 60 1C can present problems for certain applications (e.g., transportation of rigid packaging). PLLA is stable in air and will only hydrolyse very slowly when exposed to normal atmospheric conditions. An overview of PLLA properties in relation to composition was prepared by Sodergard and Stolt [28] and is a useful source of further information. Commercial interest in PLLA has developed quite rapidly over the past 2–3 years. The polymer has been used in biomedical applications for some time, but there is also now expanding use of PLLA film in packaging, as well as strong interest in PLLA fibers for textile and other applications. The price of PLLA
remains higher than that of most commodity polymers, but this may change as PLLA production volumes increase in parallel with increases in the prices of other polymers driven by rising oil prices. The distinct disadvantages with PLLA as a substrate for polymer photovoltaics is the low glass transition temperature and the soluble nature. The only solvents that PLLA will sustain are solvents such as water and ethanol, which makes it quite difficult to prepare the active layer using common organic solvents and thermal annealing. 3.4. Indium-free transparent electrodes Aside from the environmental considerations associated with the toxicity of indium, a generally acknowledged problem is that the transparent conductor that is employed almost exclusively in all laboratory trials reported to this date is made of indium tin oxide (ITO). Transparent conductors based on indium are by far the highest performing materials available, and indium is currently used in most flexible displays and flat screens for personal computers and television. Common to these products is that they are of relatively high value. The price of indium has increased nearly 10-fold since 2002, when the price was 100 $ kg1, to 2005, when the price was at 900 $ kg1 [29]. The price has decreased a little in 2007 but remains high at 700 $ kg1. A further complication is that the amount of indium available in the earth’s crust is believed to be too low to satisfy the needs of the future and the only source of indium currently is as a by-product of zinc mining. The current estimate is that indium will make up more than 30% of the cost of polymer solar cell devices comprising transparent electrodes based on indium. Thus,
ARTICLE IN PRESS M. Strange et al. / Solar Energy Materials & Solar Cells 92 (2008) 805–813
810
Transmission (%)
100 80 PLA PLA/PEG/Nanoclay PLA/Nanoclay/Al-grid /Ag-layer
60 40 20 0 200
300
400
500
600
700
800
Wavelength (nm) Fig. 4. Transmission of extruded films based on pure PLLA and PLLA/PEG/nanoclay (left) and a photograph taken through a PLLA film showing that the material is highly transparent while the optical quality is uneven (right).
there is also a considerable motivation in terms of cost reduction for avoiding the use of indium in polymer solar cells. There have been only a few reports where indium-free transparent electrodes have been employed and these have involved either very high-conductivity PEDOT:PTS with a sacrifice in optical transmission [30] or a highly transparent low-conductivity PEDOT:PSS with an underlying metallic silver grid [31,32] that both require very smooth substrates. The PLLA films prepared in this work have a relatively rough surface, making the realization of a fine grid difficult. Instead, a course grid was made by evaporating aluminium onto PLLA substrates that had been smoothed as much as possible by lamination between PET plates followed by evaporation of 100 nm of aluminium. A simple photonegative photoresist was then cold laminated on top of the aluminium. The low temperature during lamination was employed to avoid shrinkage. Subsequent illumination through a mask gave an aluminium grid with a geometric fill factor of 50% (i.e. 50% shadow loss). While controlled under-etching could be employed to increase the geometric fill factor by having grid lines with as little as 180 mm in width after etching, this only improved the geometric fill factor from 50% to 62.5% and must be seen as a marginal improvement. What is needed is a resolution on the 10 mm scale with respect to both line width and position as compared to the current resolution, which is on the 250 mm scale. A current limitation to this technique is the low geometric fill factor, which has its roots in the limit to the definition of the underlying metal grid using this photoresist technology. The layout of the electrode and the devices is shown in Fig. 5. While a finer grid definition is desirable, this is the best we have been able to obtain due to limitations in the PLLA substrate surface and the subsequent resolution of the photoresist. The current limit is around two 250 mm lines per mm, giving the 50% shadow loss. The optical transmission as measured on a complete substrate for polymer solar cells comprising PLLA substrate filled with nanoclay, aluminium grid and a thin silver layer had a transmission of around 20% in the visible range as shown in Fig. 4. The relatively low transmission is a drawback of the substrate
in its current development stage, and further development and improvements are needed (Fig. 5). 3.5. Performance, stability, and lifetime of PLLA devices It may be viewed as a simple exercise to replace the commonly employed rigid glass substrate with a flexible plastic substrate. The general observation, however, is that the performance of polymer solar cells prepared on a flexible plastic substrate such as PET is inferior to those prepared on rigid glass. PET as a substrate has many desirable properties such as insolubility in most organic solvents, good optical quality, relative ease of planarization, and mechanical stability up to temperatures well above 120 1C. PLLA was easily extruded into films, and at room temperature the film is flexible with a rigidity comparable to that of PET. The devices prepared performed relatively poorly, and in spite of many attempts we were not able to even approach the performance of the state-of-the-art. This is ascribed to several factors that are all connected with the PLLA substrate and the anode. Currently, the most severe problem is the incompatibility of PLLA with common organic solvents. PLLA substrates with the simple aluminium anode were completely destroyed when spincoating the polymer fullerene mixture from 1,2-dichlorobenzene or chlorobenzene. Our partial solution to this problem was the evaporation of a thin silver layer and the use of chlorobenzene that evaporates much faster than 1,2-dichlorobenzene. It was then possible to spincoat the film without destruction of the substrate and underlying anode grid, but contact times with solvent had to be kept short. Thermal annealing was also not possible and for this reason the preformed microfibrillar approach [18] was employed. The devices were subjected to degradation in air from the moment they were exposed to the atmosphere and the degradation was quick. The best open circuit voltage that could be obtained was 0.23 V, a short circuit current of 0.27 mA cm2 for the active area and a fill factor of 26% as shown in Fig. 6. The device performance degraded quickly, and to record the I–V curves voltage steps of 50 mV and a
ARTICLE IN PRESS M. Strange et al. / Solar Energy Materials & Solar Cells 92 (2008) 805–813
811
Fig. 5. A drawing of the electrode layout (2 2 cm) (A) and photographs of the PLLA substrate with aluminium electrode and exposed photoresist showing many devices (B), the completed anode (C), and a completed photovoltaic device seen from the front (D).
2
-0.25
0.05
0
0.00
-0.05
-1
-0.10 -0.5
0.0 Voltage (Volts)
0.5
1.0
Fig. 6. I–V curves for a device as shown in Fig. 5 with an active area of 1 cm2 in the dark and under illumination at 1000 W m2 and AM1.5G. The device temperature was 30 1C.
step time of 0.1 s was employed. Many of the devices were short circuited or open and it was difficult to obtain functional devices. The best electrical contact was obtained with silver epoxy. After the I–V curves had been recorded, the lifetime was determined and generally followed the curves shown in Fig. 7. The devices quickly degraded and during the time between I–V-curve measurements (light followed by dark) until the lifetime measurement was started the device performance decreased by as much as a factor of 1000. The time taken for total degradation of the device was of the order of 2 h (only the first 15 min after I–V-curve recordings are shown in Fig. 7). An explanation for the exceptionally poor performance of the devices prepared here is most likely linked to the problems with planarity of the substrate and the poor thermal properties of the PLLA prepared. The addition of the nanoclay was within experimental error, without influence on the performance of the material both in terms
Current density (µA cm-2)
1
Dark current density (mA cm-2)
Current density (mA cm-2)
Dark 1000 W m-2 @ AM1.5G
-2 -1.0
PLA PLA/PEG/Nanoclay
0.10 -0.20
-0.15
-0.10
-0.05
0.00 0
5
10
15
Time (Minutes) Fig. 7. Decay of the short circuit current recorded every second during continuous illumination I–V curves for a device as shown in Fig. 5 with an active area of 1 cm2 under illumination at 1000 W m2 and AM1.5G. The device temperature was 30 1C. Devices prepared on PLA with and without nanoclay are shown.
of thermal properties and with respect to diffusion of oxygen and/or water as evidenced by the poor stability in both cases (Fig. 7). The results from optical microscopy and TEM indicate that the nanoclay was well dispersed and that either an exfoliated or an intercalated nanocomposite was obtained (see Section 3.2 and Figs. 2 and 3). It would, however, seem that higher loadings are required before the nanoclay will significantly increase the diffusion path for permeants. The device performance is limited by the timescale of the degradation, which is comparable with or shorter than the experimental timescale for applying electrical contacts, carrying out measurements of I–V curves and setting up the lifetime test. PLLA has a propensity to absorb humidity from the atmosphere and will as such actively transport water to the active layer of the cell. Unencapsulated devices on flexible substrates have been observed to have shorter lifetimes than on glass, as observed for other ITO-free devices with a PET-PEDOT or a PE-PEDOT substrate [30]. We exclude the active
ARTICLE IN PRESS 812
M. Strange et al. / Solar Energy Materials & Solar Cells 92 (2008) 805–813
materials in this section as playing a part in the poor performance and being the source of the poor stability of these devices as the microfibrillar approach [18] has been shown to give high-efficiency devices without the need for thermal annealing and has been shown to give devices with very good stability when prepared from chlorobenzene. While the use of flexible substrates often leads to slightly inferior performance when compared to rigid glass substrates, this cannot account for the poor performance observed here. We thus ascribe the poor performance observed to the PLLA substrate in the form that we were able to prepare it. The relatively low voltages suggest that the planarity needs to be improved and the stress present in the substrate needs to be removed. The fast degradation is most likely due to fast water and oxygen diffusion through the PLLA. Our results do, however, demonstrate that functional devices could be prepared, and further work on the properties of PLLA as a substrate for polymer photovoltaics seems highly feasible. 3.6. Comparison with batteries and environmental considerations One of the niche applications of polymer solar cells is anticipated to be as power supplies for small electronic devices. A direct competitor to the polymer solar cell is the battery, which has obvious advantages in terms of energy storage and operation in the dark. The polymer solar cells offer flexibility, a thin outline, light weight, and possible environmental benefits in terms of biodegradability as outlined in this paper. A direct comparison between batteries and polymer solar cells is naturally difficult as there is a certain degree of incongruence between the two technologies. The polymer solar cells will naturally only be applicable when light is available as the source of energy and their energy output depends on the incoming light flux. The battery relies on energy stored chemically, and when energy is drawn from a battery chemical reactions inside the battery are the source of the electrical energy. This process continues until the chemicals available inside the battery have been consumed and this therefore places a limit on the capacity of the battery. The energy output is, depending on the battery type, quite constant throughout the service life of the battery. In this respect, a photovoltaic device offers the principal advantage that it will operate as long as it is illuminated. However, polymer photovoltaics degrade and this means that they have a finite service life just like batteries. There is though an environmental benefit of having a biodegradable material that can help to avoid concerns of accidental disposal in nature. The development of a biodegradable substrate for polymer-based solar cells should not be seen as an attempt to encourage disposal in nature, but rather as an attempt to limit the effect of the disposal in nature that evidently takes place. Strong schemes for reuse of the materials and proper disposal should still be the only acceptable practice, and it has been demonstrated that this can be handled efficiently. Efficient
recycling schemes for the various battery technologies have been explored [33,34] and, in the case of lead acid batteries, a very high degree of recycling of 497% between 1997 and 2001 has been established [35]. The problem still remains for small applications where the distribution is vast and responsibility for reuse of materials and recycling is in the hands of the consumer. Further, as the value of the consumable decreases, so does the responsibility that the consumer feels towards the product. 4. Conclusion The use of polymer photovoltaics based on a biodegradable platform was discussed and found to potentially solve some of the issues related to disposal in the environment in a manner that conventional technologies such as batteries have never achieved. We have demonstrated the preparation of PLLA film substrates by extrusion of films based either on PLLA or on PLLA–nanoclay composites and the use of these films to prepare solar cell devices. We found no improvements in terms of lifetime or performance when comparing the performance of devices based on PLLA films and PLLA–nanoclay composite films, but this may be because the clay was inadequately dispersed and/or the chosen clay was not optimal for this process. An indiumfree transparent front electrode was prepared by evaporation of aluminium followed by application of a photoresist, illumination through a photonegative mask, development, and etching. The device lifetime was very short and adversely affected the device performance on the timescale of the measurement and characterization. The devices were not thermally stable due to the low glass transition temperature. Further problems were associated with the thermal stress from the extrusion process and the planarity of the substrate. In conclusion, the main limiting factors are the restricted temperature range available for post processing the device and the soluble nature of PLLA making the range of solvents available for processing limited to water and ethanol. 5. Future work Our research has established the feasibility of using PLLA as a solar cell substrate, and, although improved solar cell performance through utilization of a PLLA/ organoclay composite films was not demonstrated, future plans include the evaluation of alternative true nanocomposite films based on PLLA and nano-scale reinforcing materials targeted at high-performance solar cell applications. Further developments include new polymer materials that can be processed from water, ethanol, or similar environmentally friendly solvents, indium-free front electrodes with a better transparency, and exfoliated nanocomposite structures with higher loading of the nanoclay and true barrier properties towards water and oxygen.
ARTICLE IN PRESS M. Strange et al. / Solar Energy Materials & Solar Cells 92 (2008) 805–813
Acknowledgements This work was supported by the Danish Strategic Research Council (DSF 2104-05-0052). Jan Alstrup is acknowledged for technical assistance. References [1] C.J. Brabec, N.S. Sariciftci, J.C. Hummelen, Adv. Funct. Mater. 11 (2001) 15. [2] H. Spanggaard, F.C. Krebs, Sol. Energy Mater. Sol. Cells 83 (2004) 125. [3] K.M. Coakley, M.D. McGehee, Chem. Mater. 16 (2004) 4533. [4] H. Hoppe, N.S. Sariciftci, J. Mater. Res. 19 (2004) 1924. [5] E. Bundgaard, F.C. Krebs, Sol. Energy Mater. Sol. Cells 91 (2007) 954. [6] S. Gu¨nes, H. Neugebauer, N.S. Sariciftci, Chem. Rev. 107 (2007) 1324. [7] C.J. Brabec, Sol. Energy Mater. Sol. Cells 83 (2004) 273. [8] F.C. Krebs, Refocus 6 (3) (2005) 38. [9] F.C. Krebs, J. Alstrup, H. Spangaard, K. Larsen, E. Kold, Sol. Energy Mater. Sol. Cells 83 (2004) 293. [10] F.C. Krebs, H. Spanggaard, T. Kjær, M. Biancardo, J. Alstrup, Mater. Sci. Eng. B 138 (2007) 106. [11] R.W. Miles, K.M. Hynes, I. Forbes, Prog. Cryst. Growth Charact. Mater. 51 (2005) 1. [12] R.E. Chapin, M.W. Harris, E.S. Hunter, B.J. Davis, B.J. Collins, A.C. Lockhart, Toxicol. Sci. 27 (1995) 140. [13] A. Boughriet, N. Proix, G. Billon, P. Recourt, B. Ouddane, Water Air Soil Pollut. 180 (2007) 83. [14] Z. Tong, M. Bischoff, L. Nies, R.F. Turco, Environ. Sci. Technol. 41 (2007) 2985. [15] C.P. Chu, D.J. Lee, B.V. Chang, C.S. Liao, J. Chem. Tech. Biotech. 76 (2001) 598.
813
[16] A.A. Beim, A.M. Beim, Environ. Technol. 15 (1994) 195. [17] J.W. Hong, W.L. Hemme, G.E. Keller, M.T. Rinke, G.C. Bazan, Adv. Mater. 18 (2006) 878. [18] S. Berson, R. de Bettignies, S. Bailly, S. Guillerez, Adv. Funct. Mater. 17 (2007) 1377. [19] R. Je´roˆme, P. Lecomte, New developments in the synthesis of aliphatic polyesters by ring-opening polymerization, in: R. Smith (Ed.), Chapter 4 in Biodegradable Polymers for Industrial Applications, Woodhead Publishing Ltd., 2005. [20] J.–F. Zhang, X. Sun, Poly(lactic acid)-based bioplastics, in: R. Smith (Ed.), Chapter 10 in Biodegradable Polymers for Industrial Applications, Woodhead Publishing Ltd., 2005. [21] M. Pluta, J. Poly Sci. Part B Polym. Phys. 44 (2006) 3392. [22] S.S. Ray, K. Yamada, A. Ogami, M. Okamoto, K. Ueda, Macromol. Rapid Commun. 23 (2002) 943. [23] M.A. Paul, M. Alexandre, P. Degee, C. Henrist, A. Rulmont, P. Dubois, Polymer 44 (2003) 443. [24] S.S. Ray, M. Bousmina, Prog. Mater. Sci. 50 (2005) 962. [25] K.K. Yang, X.L. Wang, Y.Z. Wang, J. Ind. Eng. Chem. 13 (2007) 485. [26] S.S. Singh, S.S. Ray, J. Nanosci. Nanotechnol. 7 (2007) 2596. [27] http://www.biomer.de/. [28] A. Sodergard, M. Stolt, Prog. Poly. Sci. 27 (2002) 1123. [29] http://geology.com/articles/indium.shtml. [30] B. Winther-Jensen, F.C. Krebs, Sol. Energy Mater. Sol. Cells 90 (2006) 123. [31] T. Aernouts, P. Vanlaeke, W. Geens, J. Poortmans, P. Heremans, S. Borghs, R. Mertens, R. Andriessen, L. Leenders, Thin Solid Films 451–452 (2004) 22. [32] K. Tvingstedt, O. Ingana¨s, Adv. Mater. 19 (2007) 2893–2897, doi:10.1002/adma.200602561 [33] A.M. Bernardes, D.C.R. Espinosa, J.A.S. Teno´rio, J. Power Sourc. 130 (2004) 291. [34] F. Ahmed, J. Power Sourc. 59 (1996) 107. [35] http://www.batterycouncil.org/recycling.html.