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Multiparametric optimization of polymer solar cells: A route to reproducible high efficiency
ARTICLE IN PRESS Solar Energy Materials & Solar Cells 93 (2009) 508–513
Contents lists available at ScienceDirect
Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat
Multiparametric optimization of polymer solar cells: A route to reproducible high efficiency Joachim A. Renz a, Thomas Keller b, Martin Schneider b, Sviatoslav Shokhovets a, Klaus D. Jandt b, Gerhard Gobsch a, Harald Hoppe a, a b
Institute of Physics, Technical University Ilmenau, Weimarer Str. 32, 98693 Ilmenau, Germany ¨bdergraben 32, 07743 Jena, Germany Institute of Materials Science and Technology (IMT), Friedrich-Schiller-University Jena, Lo
a r t i c l e in f o
a b s t r a c t
Article history: Received 22 September 2008 Received in revised form 5 November 2008 Accepted 6 November 2008 Available online 18 December 2008
We carried out a detailed optimization of P3HT:PCBM polymer solar cells by variation of blending ratio, film thickness and annealing conditions. From our studies it became evident that the film thickness and the fullerene concentration are mutually dependent parameters, what the overall performance concerns. In detail, we revealed a clear relationship between film thickness, PCBM concentration and the blend film morphology. We varied the PCBM concentration in our polymer solar cells between 25% and 50%, and found the best results for 33.3% of PCBM. In this case, the optimum between competing processes like effective charge carrier separation and percolation path establishment was realized. In thicker films, the growth of PCBM aggregates via phase separation leads to formation of percolation paths and therefore improves the photocurrent. In contrast, for thinner films a high PCBM concentration is favourable to achieve optimal efficiencies. & 2008 Elsevier B.V. All rights reserved.
Keywords: Polymer solar cell Morphology Optimization P3HT PCBM
1. Introduction Within the last decade, polymer solar cells have undergone a major development, and their power conversion efficiency has been increased from about 1% to more than 5% to date [1–5]. Among several other material concepts the polymer:fullerene approach to photovoltaic organic bulk heterojunctions remains up to now the most successful one [1,2,5]. During the last few years many reports were presented about optimized P3HT:PCBM (poly(3-hexylthiophene):[6,6]-phenyl C61-butyric acid methyl ester) solar cells [6–12]. Many reports concentrated on understanding the thermal annealing effect in this blends [8,9,13–15]. However, either the reported optimization was rather coarse scale or just a single parameter has been optimized and the mutual connectivity of several production parameters remained unmentioned. For an accountable research many research groups to date require an optimized system to start off, when certain improvements shall be reliably reported. Especially when an improvement of the solar cell is achieved based on the application of certain treatments or changes in the solar cell design, it is important to be sure that the system has been at the optimal point of operation before. Else the seemingly improvement may be based on effects that are not actually controlled by the so-called improvement
Corresponding author.
E-mail address: [email protected] (H. Hoppe). 0927-0248/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2008.11.001
itself, but (at least in part) due to a shift of the preparation parameters in general towards the optimized system. Therefore the knowledge of the efficiency landscape for the system under investigation is of high importance, for this enables also to place the system at a relatively stable point in this landscape, paving the way for reliability in the reproduction of a standardized optimal solar cell. This and to gain knowledge about the morphology, property relations in P3HT:PCBM solar cells triggered us to study the system on a relatively fine scale of the production parameters PCBM concentration and film thickness.
2. Experimental details For preparation of solar cells, we used regio-regular poly(3hexylthiophene) (P3HT) electronic grade from Rieke Metals Inc. (USA) as electron donor, and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) from Nano-C (Canada), as acceptor. The two materials were dissolved in chlorobenzene at a polymer weightpercentage of about 1.2% and corresponding values of PCBM wt%, and stirred at slightly elevated temperatures for several days. The PCBM blending content was varied from 25% to 50%. Bulk heterojunction ITO/PEDOT–PSS/P3HT:PCBM/AL solar cells were produced in the following way: indium tin oxide (ITO)coated glass substrates (purchased from Merck KgA, Germany) were etched at the region of contact to the aluminium and subsequently cleaned in toluene, acetone and iso-propanol. Then a
ARTICLE IN PRESS J.A. Renz et al. / Solar Energy Materials & Solar Cells 93 (2009) 508–513
delivers optimal device efficiencies (compare with Fig. 1), and was therefore not varied for the rest of the study. In the following we concentrate on the blend ratio and the film thickness optimization itself: in all cases we used a P3HT solution concentration of 1.2 wt%; to this the respective PCBM content needs to be added in order to yield the total solution concentration. We chose the PCBM concentrations in the blend to be 25%, 29.4%, 33.3%, 40%, 45.5% and 50% because this enabled a relatively easy weighing of the respective amounts for preparation of the individual chlorobenzene solutions. We varied the spin frequency between 400 and 1800 rpm, which should yield film thicknesses around 150 nm and less. In an independent optical optimization we have already demonstrated earlier the optimum film thickness to be around 90 nm for the identical annealing conditions and a blend ratio of 1:0.8 P3HT:PCBM [16], corresponding to a PCBM concentration of 44.4% in the blend film. Since in this study we did not explicitly determine all film thicknesses, we point out the general relationship between the spinning frequency o and the film thickness d to be proportional to one over the square root of o7 1 d pffiffiffiffiffi .
(1)
o
In Fig. 2 some examples for P3HT:PCBM blend films having a PCBM concentration of 44.4% and initial total solution concentrations as indicated in the inset of the graph are shown.
400 3.6% CB 2.7% CB 2.2% CB 1.8% CB
3. Results and discussion In general, it is necessary to optimize the four production parameters: blend ratio, film thickness, annealing temperature and time all together. This is, however, beyond the scope of this paper, and we will only report the detailed optimization of blending ratio (PCBM concentration) in combination with the film thickness. The trend of optimal PCBM concentration for reported record P3HT:PCBM solar cell devices is in the range of 50% or somewhat less, while the optimal annealing temperature was reported to be at 150 1C or little more [8–10]. In addition we found 155 1C to be quite optimal in an independent temperature optimization (not shown here). Therefore we decided to choose blending ratios from 50% to 25% PCBM content and an annealing temperature of 155 1C for the present optimization. We were able to show, that in our case the annealing duration around 5 min
film thickness [nm]
350
8.7
200 150
50 0
100
200
300
400
500
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700
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η [%]
8.3
2.7
Isc [mA/cm2]
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2.6
8.1 8.0
2.5 2
4
6 8 annealing time
10
η [%]
8.4
Voc [V]
0.60 2.8
900
Fig. 2. Relation between spin frequency and film thickness for different concentrations of the P3HT:PCBM blend chlorobenzene solution. The experimental data points follow the trend of Eq. (1).
2.9
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100
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thin layer of poly(3,4 ethylendioxythiophene)–poly(styrenesulfonate) (PEDOT–PSS) (H.C. Starck, Germany) was spin cast on top of the cleaned ITO-substrates. After drying the PEDOT:PSS-coated substrates for 15 min on a hot plate a thin layer of the P3HT:PCBM solution was spin coated on top under an inert gas atmosphere inside the glove box at room temperature. Aluminium electrodes were thermally evaporated into the organic active layer under high vacuum conditions at an evaporation rate of 0.5–1 nm/s. The devices were completed by a thermal annealing step on a hot plate at 155 1C for a period of 5 min. Solar cell devices with an active area of 50 mm2 were characterized by current–voltage (I–V) and by external quantum efficiency (EQE) measurements. To determine the solar power conversion efficiency and diode behaviour, I–V measurements were carried out in the dark and at 100 mW/cm2 white light illumination from an AM 1.5 Solar Constant 575 (K.H. Steuernagel, Germany) solar simulator. The reported efficiency values may deviate considerably from those obtained under a ‘‘real’’ AM 1.5 solar spectrum, but the reproducible calibration of the solar simulator by a silicon solar cell allows relative comparisons between the performance of different solar cells made within our study. EQE measurements were recorded under illumination of a monochromatized halogen lamp. The incident light beam was chopped with a mechanical chopper and the photocurrent was detected using a lock-in-amplifier. All atomic force microscopy (AFM) images have been recorded in the tapping mode using a Veeco Instruments NanoScope Dimension from Digital Instruments.
509
0.50 2
4
6 8 annealing time
10
Fig. 1. Solar cell parameters of a P3HT:PCBM blend containing 45.5% PCBM, annealed for different durations. It is evident from the data that between 4- and 8-min annealing time the device efficiency is quite constant.
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Fig. 3 summarizes all solar cell device parameters obtained from the current–voltage (I–V) measurements under 100 mW/cm2 white light illumination of an AM 1.5 solar simulator. Each of the plots shows the combined dependence of the device parameter on PCBM concentration and spin frequency in a three-dimensional overview. From these plots we can summarize the following observations: If significant at all, the open circuit voltage shows the weakest dependence on the preparation parameters: in all cases it is near 600 mV. All other parameters have a pronounced dependence on film thickness and blend ratio. While the fill factor drops from more than 50% to about 30% for thicker devices on increasing the fullerene concentration, it stabilizes for all concentrations to roughly 55% when higher spin frequencies and thus thinner films are produced. Oppositely, the short circuit current density is nearly constant for thick active layers with rather high values; it drops considerably for the thinnest films on lowering the PCBM concentration. However, a slight, less-pronounced drop in the current density is also observed for thick films on increased PCBM concentration. As a consequence, in the case of the lowest shown
PCBM concentration of 33.3%, the rise of the short circuit current for increasing film thickness is most strong. The coordinates for the maximum values of the short circuit current density are lying on a line from small-to-medium spin frequencies on increasing the PCBM concentrations—a feature that is resembled more weakly in the fill factor plot as well. All these dependencies are factors that add up in the resulting power conversion efficiency: The overall maximum line (ridge) is found for smaller to larger PCBM concentrations from 700 rpm up to 1100 rpm. The efficiency–spinning frequency relationship shows an antidromic behaviour: While for thicker films the performance is lowered on increasing PCBM concentrations, the thinnest films exhibit the opposite behaviour: increasing PCBM concentration leads to improved efficiencies. A detailed plot of the efficiency dependence on the film thickness is shown in Fig. 4 for all PCBM concentrations under investigation. On a smaller spin frequency interval it is clearly demonstrated that even lower PCBM contents than 33% result in much lower efficiencies—mainly due to decreased fill factors and short circuit photocurrents. Since the drop in efficiency is rather steep, the local efficiency maximum at 33% is not very stable
Fig. 3. Solar cell device parameters dependent on the PCBM concentration and spin frequency for P3HT:PCBM bulk heterojunctions.
ARTICLE IN PRESS J.A. Renz et al. / Solar Energy Materials & Solar Cells 93 (2009) 508–513
70
3.5
60
600 rpm 1100 rpm 1300 rpm 1500 rpm 1800 rpm
50.0%
50
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EQE [%]
η [%]
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1.0 300
600
900 1200 1500 spin frequency [rpm]
1800
10
Fig. 4. Solar cell power conversion efficiencies dependent on the PCBM concentration. For concentrations smaller than 33%, the efficiency drops significantly.
10
4.0
9
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8
0 300
400
500 600 wavelength [nm]
700
800
Fig. 7. External quantum efficiencies spectra of P3HT:PCBM solar cells prepared with different spin frequencies and thus film thicknesses for a PCBM concentration of 33.3%.
3.0
7
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6
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5 4
η [%]
3
Jsc [mA/cm ]
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511
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2
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2
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1 0 25
30
35
40
45
50
0.0
PCBM-concentration [%] Fig. 5. Maximum energy conversion efficiencies and short circuit photocurrent densities over the PCBM concentration for the respective optimum film thickness are shown. The efficiency peaks for devices with a PCBM weight percentage between 33.3% and 40%.
70 60
EQE [%]
50 50.0% 45.5% 40.0% 33.3% 29.4% 25.0%
40 30 20 10 0 300
350
400
450 500 550 wavelength [nm]
600
650
700
Fig. 6. External quantum efficiency spectra of P3HT:PCBM solar cells with varying PCBM concentrations, as indicated in the legend. The largest integrated photocurrent is observed for 33.3%, as the spectrum is nearly box-like shaped between 400 and 600 nm.
Fig. 8. AFM images of PEDOT:PSS/P3HT:PCBM films on glass substrates for different weight percentages of PCBM 50%, 45.5%, 40%, 33.3%, 29.4% and 25% (all annealed at 155 1C) and three different spin frequencies (400 rpm, ‘‘optimal efficiency’’ spin frequency, 1800 rpm).
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against blending variations; therefore a more reliable blending ratio useful for a reproducible production of solar cells lies somewhere in between 33% and 40%. Taking only the maximum efficiencies into account, these maxima can be plotted against the PCBM concentration of the photoactive layer. Fig. 5 demonstrates the relationship between maximum efficiency and PCBM concentration to closely follow the short circuit current density dependency. A clear maximum exists at a PCBM concentration between 33% and 40%. The EQF of photo-generated charge carriers is given by the ratio between the number of electrons produced in the outer circuit and that of incident photons to the device. Fig. 6 compares EQE spectra of P3HT:PCBM solar cells with the different concentrations of PCBM, as discussed above. The best overall EQE spectra is obtained for 33.3%, which is assigned to the nearly box-like shape of the spectrum between 400 and 600 nm having an average EQE of 60%. The EQE spectra vary in the height and width. The highest monochromatic value of 66% is obtained for a PCBM concentration of 40% at a wavelength of 490 nm. In general the maximum EQE value is increased on going from 50% to 40% PCBM content. For lowering the fullerene concentration further down to 29%, the spectrum widens and lowers the peak height. Fig. 7 points out the relationship between EQF and film thickness of the photoactive layer at the optimal PCBM concentration. In correlation to the progression of the short current density curve, a clear rise in the EQE value with decreasing spin frequency is evident. In addition to the I–V characterization we investigated the topography of active layers by tapping mode scanning AFM. For this the blend films were prepared on glass substrates for PCBM concentration of 50%, 45.5%, 40%, 33.3%, 29.4% and 25% and annealed under the same conditions as the solar cells. For each PCBM concentration we chose three different spin frequencies: 400, 1800 rpm and the spin frequency at optimal efficiency.
Thus we received AFM images of thin, thick and medium film thicknesses for each blend solution. Based on the images, displayed in Fig. 8, it is obvious that the feature size is gradually increasing with both lower PCBM concentrations and thicker films. For example, the films prepared with 50% PCBM show little feature sizes for all films, while films containing only 25% PCBM show more coarse features. The blend films at 25%, 29.4% and 33.3% PCBM exhibit, at low spin frequency and thus large film thickness, a pronounced increased feature size. The AFM image of the film prepared under optimal solar cell efficiency conditions (33.3% PCBM, 900 rpm) is highlighted by a blue frame. From the feature sizes and roughness of the prepared and annealed sample films, some conclusions on the underlying infilm phase separation of the system can be drawn [17]. Id est, once the feature size exceeds the film thickness considerably, it becomes visible by surface undulations [18]. From the device I–V characterization we conclude that for a higher PCBM content, the film thickness of the active layer needs to be reduced to reach maximal efficiency. At first view this result may appear unreasonable, because the P3HT content responsible for absorption in the film is inevitably reduced at a smaller film thickness. Apparently, in the case of thicker films, the lossmechanisms accompanying higher PCBM loads overbalance the increase in absorption due to an increased absorption volume. For a larger scale of phase separation, the establishment of percolation paths is more probable. By comparing the roughness/ feature size, the film thickness and the efficiency at the edges of the efficiency diagram, we suggest a model to explain the observed performances with the help of Fig. 9. Good efficiencies are reached by thick films, low PCBM (33.3%) content and coarse scale of phase separation as well as by thin films, relatively high or rather balanced PCBM (50%) content and small scale of phase separation.
Fig. 9. Relationship between efficiency, spin frequency and film morphology for several P3HT:PCBM bulk heterojunction solar cells with varying PCBM concentration. The AFM images at the high-efficiency border of the parameter variation are shown, to connect both data sets.
ARTICLE IN PRESS J.A. Renz et al. / Solar Energy Materials & Solar Cells 93 (2009) 508–513
Thick films necessitate a coarse phase separation to guarantee percolation up to the electrodes. At low PCBM concentration only thick films allow for the development of coarse enough phase separation for sufficient percolation. A thinner film does not require such a coarse phase separation. In this case a finer phase separation with a larger interfaces leads to maximum efficiencies. At thin films, more balanced PCBM contents (45–50%) enable sufficient percolation for transport of the charge carriers.
4. Conclusion In conclusion, we systematically determined the combined influence of PCBM blend ratio and spin frequency on P3HT:PCBM bulk heterojunction solar cell parameters. We found antidromic behaviour for the combined influences: thinner films require higher or more balanced PCBM concentrations in the blend, while thicker films lead to higher efficiencies in the case of lower PCBM loads. We were able to motivate these dependencies by the help of tapping mode AFM topography measurements, exploiting the estimation of phase separation scale based on surface undulations. For a good reproducibility of high device efficiencies a region with a rather shallow curvature in the efficiency landscape is beneficial. In case of unwanted variations of one of the underlying production parameters the overall efficiency will not vary much. As often predicted and expected, an intermediate scale of phase separation of the active layer yields the best performing device.
Acknowledgements H.H. gratefully acknowledges financial support from the Fonds der Chemischen Industrie. We gratefully acknowledge financial support from the Thuringian Ministry of Culture (contract numbers B507-04010 and B514-07028). We thank S. Sensfuss from TITK Rudolstadt for kind support in the I–V measurements.
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References [1] M.A. Green, K. Emery, Y. Hishikawa, W. Warta, Solar cell efficiency tables (Version 31), Progress in Photovoltaics: Research and Applications 16 (2008) 61–67. [2] H. Hoppe, N.S. Sariciftci, Organic solar cells: an overview, Journal of Materials Research 19 (2004) 1924–1945. [3] K.M. Coakley, M.D. McGehee, Conjugated polymer photovoltaic cells, Chemistry of Materials 16 (2004) 4533–4542. [4] H. Spanggaard, F.C. Krebs, A brief history of the development of organic and polymeric photovoltaics, Solar Energy Materials and Solar Cells 83 (2004) 125–146. [5] H. Hoppe, N.S. Sariciftci, Polymer solar cells, in: S.R. Marder, K.-S. Lee (Eds.), Photoresponsive Polymers II, Advances in Polymer Science, Publ., Springer, Berlin-Heidelberg, 2008, pp. 1–86. [6] D. Chirvase, J. Parisi, J.C. Hummelen, V. Dyakonov, Influence of nanomorphology on the photovoltaic action of polymer–fullerene composites, Nanotechnology 15 (2004) 1317–1323. [7] M. Al-Ibrahim, O. Ambacher, S. Sensfuss, G. Gobsch, Effects of solvent and annealing on the improved performance of solar cells based on poly (3-hexylthiophene): fullerene, Applied Physics Letters 86 (2005) 201120. [8] W. Ma, C. Yang, X. Gong, K. Lee, A.J. Heeger, Thermally stable, efficient polymer solar cells with nanoscale control of the interpenetrating network morphology, Advanced Functional Materials 15 (2005) 1617–1622. [9] M. Reyes-Reyes, K. Kim, D.L. Carroll, High-efficiency photovoltaic devices based on annealed poly(3-hexylthiophene) and 1-(3-methoxycarbonyl)propyl-1-phenyl-(6,6)C61 blends, Applied Physics Letters 87 (2005) 083506. [10] G. Li, V. Shrotriya, J. Huang, Y. Yao, T. Moriarty, K. Emery, Y. Yang, Highefficiency solution processable polymer photovoltaic cells by self-organization of polymer blends, Nature Materials 4 (2005) 864–868. [11] P. Schilinsky, U. Asawapirom, U. Scherf, M. Biele, C.J. Brabec, Influence of the molecular weight of poly(3-hexylthiophene) on the performance of bulk heterojunction solar cells, Chemistry of Materials 17 (2005) 2175–2180. [12] Y. Kim, S. Cook, S.M. Tuladhar, S.A. Choulis, J. Nelson, J.R. Durrant, D.D.C. Bradley, M. Giles, I. McCulloch, C.-S. Ha, M. Ree, A strong regioregularity effect in self-organizing conjugated polymer films and high-efficiency polythiophene: fullerene solar cells, Nature Materials 5 (2006) 197–203. [13] F. Padinger, R.S. Rittberger, N.S. Sariciftci, Effects of postproduction treatment on plastic solar cells, Advanced Functional Materials 13 (2003) 1–4. [14] M. Reyes-Reyes, K. Kim, J. Dewald, R. Lopez-Sandoval, A. Avadhanula, S. Curran, D.L. Carroll, Meso-structure formation for enhanced organic photovoltaic cells, Organic Letters 7 (2005) 5749–5752. [15] G. Li, V. Shrotriya, Y. Yao, Y. Yang, Investigation of annealing effects and film thickness dependence of polymer solar cells based on poly(3-hexylthiophene), Journal of Applied Physics 98 (2005) 043704. [16] H. Hoppe, S. Shokhovets, G. Gobsch, Inverse relation between photocurrent and absorption layer thickness in polymer solar cells, Physica Status Solidi Rapid Research Letters 1 (2007) R40–R42. [17] H. Hoppe, N.S. Sariciftci, Morphology of polymer/fullerene bulk heterojunction solar cells, Journal of Materials Chemistry 16 (2006) 45–61. [18] H. Hoppe, M. Heuberger, J. Klein, Self-similarity and pattern selection in the roughening of binary liquid films, Physical Review Letters 86 (2001) 4863–4866.
Contents lists available at ScienceDirect
Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat
Multiparametric optimization of polymer solar cells: A route to reproducible high efficiency Joachim A. Renz a, Thomas Keller b, Martin Schneider b, Sviatoslav Shokhovets a, Klaus D. Jandt b, Gerhard Gobsch a, Harald Hoppe a, a b
Institute of Physics, Technical University Ilmenau, Weimarer Str. 32, 98693 Ilmenau, Germany ¨bdergraben 32, 07743 Jena, Germany Institute of Materials Science and Technology (IMT), Friedrich-Schiller-University Jena, Lo
a r t i c l e in f o
a b s t r a c t
Article history: Received 22 September 2008 Received in revised form 5 November 2008 Accepted 6 November 2008 Available online 18 December 2008
We carried out a detailed optimization of P3HT:PCBM polymer solar cells by variation of blending ratio, film thickness and annealing conditions. From our studies it became evident that the film thickness and the fullerene concentration are mutually dependent parameters, what the overall performance concerns. In detail, we revealed a clear relationship between film thickness, PCBM concentration and the blend film morphology. We varied the PCBM concentration in our polymer solar cells between 25% and 50%, and found the best results for 33.3% of PCBM. In this case, the optimum between competing processes like effective charge carrier separation and percolation path establishment was realized. In thicker films, the growth of PCBM aggregates via phase separation leads to formation of percolation paths and therefore improves the photocurrent. In contrast, for thinner films a high PCBM concentration is favourable to achieve optimal efficiencies. & 2008 Elsevier B.V. All rights reserved.
Keywords: Polymer solar cell Morphology Optimization P3HT PCBM
1. Introduction Within the last decade, polymer solar cells have undergone a major development, and their power conversion efficiency has been increased from about 1% to more than 5% to date [1–5]. Among several other material concepts the polymer:fullerene approach to photovoltaic organic bulk heterojunctions remains up to now the most successful one [1,2,5]. During the last few years many reports were presented about optimized P3HT:PCBM (poly(3-hexylthiophene):[6,6]-phenyl C61-butyric acid methyl ester) solar cells [6–12]. Many reports concentrated on understanding the thermal annealing effect in this blends [8,9,13–15]. However, either the reported optimization was rather coarse scale or just a single parameter has been optimized and the mutual connectivity of several production parameters remained unmentioned. For an accountable research many research groups to date require an optimized system to start off, when certain improvements shall be reliably reported. Especially when an improvement of the solar cell is achieved based on the application of certain treatments or changes in the solar cell design, it is important to be sure that the system has been at the optimal point of operation before. Else the seemingly improvement may be based on effects that are not actually controlled by the so-called improvement
Corresponding author.
E-mail address: [email protected] (H. Hoppe). 0927-0248/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2008.11.001
itself, but (at least in part) due to a shift of the preparation parameters in general towards the optimized system. Therefore the knowledge of the efficiency landscape for the system under investigation is of high importance, for this enables also to place the system at a relatively stable point in this landscape, paving the way for reliability in the reproduction of a standardized optimal solar cell. This and to gain knowledge about the morphology, property relations in P3HT:PCBM solar cells triggered us to study the system on a relatively fine scale of the production parameters PCBM concentration and film thickness.
2. Experimental details For preparation of solar cells, we used regio-regular poly(3hexylthiophene) (P3HT) electronic grade from Rieke Metals Inc. (USA) as electron donor, and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) from Nano-C (Canada), as acceptor. The two materials were dissolved in chlorobenzene at a polymer weightpercentage of about 1.2% and corresponding values of PCBM wt%, and stirred at slightly elevated temperatures for several days. The PCBM blending content was varied from 25% to 50%. Bulk heterojunction ITO/PEDOT–PSS/P3HT:PCBM/AL solar cells were produced in the following way: indium tin oxide (ITO)coated glass substrates (purchased from Merck KgA, Germany) were etched at the region of contact to the aluminium and subsequently cleaned in toluene, acetone and iso-propanol. Then a
ARTICLE IN PRESS J.A. Renz et al. / Solar Energy Materials & Solar Cells 93 (2009) 508–513
delivers optimal device efficiencies (compare with Fig. 1), and was therefore not varied for the rest of the study. In the following we concentrate on the blend ratio and the film thickness optimization itself: in all cases we used a P3HT solution concentration of 1.2 wt%; to this the respective PCBM content needs to be added in order to yield the total solution concentration. We chose the PCBM concentrations in the blend to be 25%, 29.4%, 33.3%, 40%, 45.5% and 50% because this enabled a relatively easy weighing of the respective amounts for preparation of the individual chlorobenzene solutions. We varied the spin frequency between 400 and 1800 rpm, which should yield film thicknesses around 150 nm and less. In an independent optical optimization we have already demonstrated earlier the optimum film thickness to be around 90 nm for the identical annealing conditions and a blend ratio of 1:0.8 P3HT:PCBM [16], corresponding to a PCBM concentration of 44.4% in the blend film. Since in this study we did not explicitly determine all film thicknesses, we point out the general relationship between the spinning frequency o and the film thickness d to be proportional to one over the square root of o7 1 d pffiffiffiffiffi .
(1)
o
In Fig. 2 some examples for P3HT:PCBM blend films having a PCBM concentration of 44.4% and initial total solution concentrations as indicated in the inset of the graph are shown.
400 3.6% CB 2.7% CB 2.2% CB 1.8% CB
3. Results and discussion In general, it is necessary to optimize the four production parameters: blend ratio, film thickness, annealing temperature and time all together. This is, however, beyond the scope of this paper, and we will only report the detailed optimization of blending ratio (PCBM concentration) in combination with the film thickness. The trend of optimal PCBM concentration for reported record P3HT:PCBM solar cell devices is in the range of 50% or somewhat less, while the optimal annealing temperature was reported to be at 150 1C or little more [8–10]. In addition we found 155 1C to be quite optimal in an independent temperature optimization (not shown here). Therefore we decided to choose blending ratios from 50% to 25% PCBM content and an annealing temperature of 155 1C for the present optimization. We were able to show, that in our case the annealing duration around 5 min
film thickness [nm]
350
8.7
200 150
50 0
100
200
300
400
500
600
700
0.64
0.64
0.62
0.62 0.60
η [%]
8.3
2.7
Isc [mA/cm2]
8.2
2.6
8.1 8.0
2.5 2
4
6 8 annealing time
10
η [%]
8.4
Voc [V]
0.60 2.8
900
Fig. 2. Relation between spin frequency and film thickness for different concentrations of the P3HT:PCBM blend chlorobenzene solution. The experimental data points follow the trend of Eq. (1).
2.9
8.5
800
spin frequency [min-1]
8.6 Isc [mA/cm2]
250
100
3.0
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300
Voc [V]
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0.58
FF
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0.56
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thin layer of poly(3,4 ethylendioxythiophene)–poly(styrenesulfonate) (PEDOT–PSS) (H.C. Starck, Germany) was spin cast on top of the cleaned ITO-substrates. After drying the PEDOT:PSS-coated substrates for 15 min on a hot plate a thin layer of the P3HT:PCBM solution was spin coated on top under an inert gas atmosphere inside the glove box at room temperature. Aluminium electrodes were thermally evaporated into the organic active layer under high vacuum conditions at an evaporation rate of 0.5–1 nm/s. The devices were completed by a thermal annealing step on a hot plate at 155 1C for a period of 5 min. Solar cell devices with an active area of 50 mm2 were characterized by current–voltage (I–V) and by external quantum efficiency (EQE) measurements. To determine the solar power conversion efficiency and diode behaviour, I–V measurements were carried out in the dark and at 100 mW/cm2 white light illumination from an AM 1.5 Solar Constant 575 (K.H. Steuernagel, Germany) solar simulator. The reported efficiency values may deviate considerably from those obtained under a ‘‘real’’ AM 1.5 solar spectrum, but the reproducible calibration of the solar simulator by a silicon solar cell allows relative comparisons between the performance of different solar cells made within our study. EQE measurements were recorded under illumination of a monochromatized halogen lamp. The incident light beam was chopped with a mechanical chopper and the photocurrent was detected using a lock-in-amplifier. All atomic force microscopy (AFM) images have been recorded in the tapping mode using a Veeco Instruments NanoScope Dimension from Digital Instruments.
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Fig. 1. Solar cell parameters of a P3HT:PCBM blend containing 45.5% PCBM, annealed for different durations. It is evident from the data that between 4- and 8-min annealing time the device efficiency is quite constant.
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Fig. 3 summarizes all solar cell device parameters obtained from the current–voltage (I–V) measurements under 100 mW/cm2 white light illumination of an AM 1.5 solar simulator. Each of the plots shows the combined dependence of the device parameter on PCBM concentration and spin frequency in a three-dimensional overview. From these plots we can summarize the following observations: If significant at all, the open circuit voltage shows the weakest dependence on the preparation parameters: in all cases it is near 600 mV. All other parameters have a pronounced dependence on film thickness and blend ratio. While the fill factor drops from more than 50% to about 30% for thicker devices on increasing the fullerene concentration, it stabilizes for all concentrations to roughly 55% when higher spin frequencies and thus thinner films are produced. Oppositely, the short circuit current density is nearly constant for thick active layers with rather high values; it drops considerably for the thinnest films on lowering the PCBM concentration. However, a slight, less-pronounced drop in the current density is also observed for thick films on increased PCBM concentration. As a consequence, in the case of the lowest shown
PCBM concentration of 33.3%, the rise of the short circuit current for increasing film thickness is most strong. The coordinates for the maximum values of the short circuit current density are lying on a line from small-to-medium spin frequencies on increasing the PCBM concentrations—a feature that is resembled more weakly in the fill factor plot as well. All these dependencies are factors that add up in the resulting power conversion efficiency: The overall maximum line (ridge) is found for smaller to larger PCBM concentrations from 700 rpm up to 1100 rpm. The efficiency–spinning frequency relationship shows an antidromic behaviour: While for thicker films the performance is lowered on increasing PCBM concentrations, the thinnest films exhibit the opposite behaviour: increasing PCBM concentration leads to improved efficiencies. A detailed plot of the efficiency dependence on the film thickness is shown in Fig. 4 for all PCBM concentrations under investigation. On a smaller spin frequency interval it is clearly demonstrated that even lower PCBM contents than 33% result in much lower efficiencies—mainly due to decreased fill factors and short circuit photocurrents. Since the drop in efficiency is rather steep, the local efficiency maximum at 33% is not very stable
Fig. 3. Solar cell device parameters dependent on the PCBM concentration and spin frequency for P3HT:PCBM bulk heterojunctions.
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Fig. 4. Solar cell power conversion efficiencies dependent on the PCBM concentration. For concentrations smaller than 33%, the efficiency drops significantly.
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Fig. 7. External quantum efficiencies spectra of P3HT:PCBM solar cells prepared with different spin frequencies and thus film thicknesses for a PCBM concentration of 33.3%.
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PCBM-concentration [%] Fig. 5. Maximum energy conversion efficiencies and short circuit photocurrent densities over the PCBM concentration for the respective optimum film thickness are shown. The efficiency peaks for devices with a PCBM weight percentage between 33.3% and 40%.
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Fig. 6. External quantum efficiency spectra of P3HT:PCBM solar cells with varying PCBM concentrations, as indicated in the legend. The largest integrated photocurrent is observed for 33.3%, as the spectrum is nearly box-like shaped between 400 and 600 nm.
Fig. 8. AFM images of PEDOT:PSS/P3HT:PCBM films on glass substrates for different weight percentages of PCBM 50%, 45.5%, 40%, 33.3%, 29.4% and 25% (all annealed at 155 1C) and three different spin frequencies (400 rpm, ‘‘optimal efficiency’’ spin frequency, 1800 rpm).
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against blending variations; therefore a more reliable blending ratio useful for a reproducible production of solar cells lies somewhere in between 33% and 40%. Taking only the maximum efficiencies into account, these maxima can be plotted against the PCBM concentration of the photoactive layer. Fig. 5 demonstrates the relationship between maximum efficiency and PCBM concentration to closely follow the short circuit current density dependency. A clear maximum exists at a PCBM concentration between 33% and 40%. The EQF of photo-generated charge carriers is given by the ratio between the number of electrons produced in the outer circuit and that of incident photons to the device. Fig. 6 compares EQE spectra of P3HT:PCBM solar cells with the different concentrations of PCBM, as discussed above. The best overall EQE spectra is obtained for 33.3%, which is assigned to the nearly box-like shape of the spectrum between 400 and 600 nm having an average EQE of 60%. The EQE spectra vary in the height and width. The highest monochromatic value of 66% is obtained for a PCBM concentration of 40% at a wavelength of 490 nm. In general the maximum EQE value is increased on going from 50% to 40% PCBM content. For lowering the fullerene concentration further down to 29%, the spectrum widens and lowers the peak height. Fig. 7 points out the relationship between EQF and film thickness of the photoactive layer at the optimal PCBM concentration. In correlation to the progression of the short current density curve, a clear rise in the EQE value with decreasing spin frequency is evident. In addition to the I–V characterization we investigated the topography of active layers by tapping mode scanning AFM. For this the blend films were prepared on glass substrates for PCBM concentration of 50%, 45.5%, 40%, 33.3%, 29.4% and 25% and annealed under the same conditions as the solar cells. For each PCBM concentration we chose three different spin frequencies: 400, 1800 rpm and the spin frequency at optimal efficiency.
Thus we received AFM images of thin, thick and medium film thicknesses for each blend solution. Based on the images, displayed in Fig. 8, it is obvious that the feature size is gradually increasing with both lower PCBM concentrations and thicker films. For example, the films prepared with 50% PCBM show little feature sizes for all films, while films containing only 25% PCBM show more coarse features. The blend films at 25%, 29.4% and 33.3% PCBM exhibit, at low spin frequency and thus large film thickness, a pronounced increased feature size. The AFM image of the film prepared under optimal solar cell efficiency conditions (33.3% PCBM, 900 rpm) is highlighted by a blue frame. From the feature sizes and roughness of the prepared and annealed sample films, some conclusions on the underlying infilm phase separation of the system can be drawn [17]. Id est, once the feature size exceeds the film thickness considerably, it becomes visible by surface undulations [18]. From the device I–V characterization we conclude that for a higher PCBM content, the film thickness of the active layer needs to be reduced to reach maximal efficiency. At first view this result may appear unreasonable, because the P3HT content responsible for absorption in the film is inevitably reduced at a smaller film thickness. Apparently, in the case of thicker films, the lossmechanisms accompanying higher PCBM loads overbalance the increase in absorption due to an increased absorption volume. For a larger scale of phase separation, the establishment of percolation paths is more probable. By comparing the roughness/ feature size, the film thickness and the efficiency at the edges of the efficiency diagram, we suggest a model to explain the observed performances with the help of Fig. 9. Good efficiencies are reached by thick films, low PCBM (33.3%) content and coarse scale of phase separation as well as by thin films, relatively high or rather balanced PCBM (50%) content and small scale of phase separation.
Fig. 9. Relationship between efficiency, spin frequency and film morphology for several P3HT:PCBM bulk heterojunction solar cells with varying PCBM concentration. The AFM images at the high-efficiency border of the parameter variation are shown, to connect both data sets.
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Thick films necessitate a coarse phase separation to guarantee percolation up to the electrodes. At low PCBM concentration only thick films allow for the development of coarse enough phase separation for sufficient percolation. A thinner film does not require such a coarse phase separation. In this case a finer phase separation with a larger interfaces leads to maximum efficiencies. At thin films, more balanced PCBM contents (45–50%) enable sufficient percolation for transport of the charge carriers.
4. Conclusion In conclusion, we systematically determined the combined influence of PCBM blend ratio and spin frequency on P3HT:PCBM bulk heterojunction solar cell parameters. We found antidromic behaviour for the combined influences: thinner films require higher or more balanced PCBM concentrations in the blend, while thicker films lead to higher efficiencies in the case of lower PCBM loads. We were able to motivate these dependencies by the help of tapping mode AFM topography measurements, exploiting the estimation of phase separation scale based on surface undulations. For a good reproducibility of high device efficiencies a region with a rather shallow curvature in the efficiency landscape is beneficial. In case of unwanted variations of one of the underlying production parameters the overall efficiency will not vary much. As often predicted and expected, an intermediate scale of phase separation of the active layer yields the best performing device.
Acknowledgements H.H. gratefully acknowledges financial support from the Fonds der Chemischen Industrie. We gratefully acknowledge financial support from the Thuringian Ministry of Culture (contract numbers B507-04010 and B514-07028). We thank S. Sensfuss from TITK Rudolstadt for kind support in the I–V measurements.
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