Citation for published version (APA): Lenes, M. (2009). Efficiency enhancement of polymer fullerene solar cells Groningen: s.n.

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1 University of Groningen Efficiency enhancement of polymer fullerene solar cells Lenes, Martijn IMPRTANT NTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite from it. Please check the document version below. Document Version Publisher's PDF, also known as Version of record Publication date: 2009 Link to publication in University of Groningen/UMCG research database Citation for published version (APA): Lenes, M. (2009). Efficiency enhancement of polymer fullerene solar cells Groningen: s.n. Copyright ther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons). Take-down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. Downloaded from the University of Groningen/UMCG research database (Pure): For technical reasons the number of authors shown on this cover page is limited to 10 maximum. Download date:

2 Chapter 5 Higher adduct fullerenes for enhanced open circuit voltage and efficiency in polymer solar cells Abstract The bisadduct analog of, bis, is investigated which has a significant lower electron affinity as compared to the standard acceptor. By this raise of the LUM level the energy loss in the electron transfer from donor to acceptor material is reduced, manifesting itself as an increase of the open circuit voltage of polymer:fullerene bulk heterojunction solar cells. Maintaining high currents and fill factors an externally verified power conversion efficiency of 4.5% is achieved for a P3HT:bis solar cell, 20% higher as compared to the efficiencies of P3HT: cells, clearly showing bis to be the superior acceptor compared to standard. Next to bis, other higher adduct fullerenes are investigated, including C 70 and thienyl based materials. It is shown that the occurrence of a multitude of different isomers results in a decrease in charge carrier transport in single carrier devices for some of the materials. Surprisingly, the solar cell characteristics are very similar for all materials. This apparent discrepancy is explained by a significant amount of shallow trapping occurring in the fullerene phase which does not hamper the solar cell performance due the filling of these shallow traps during illumination. Furthermore, the trisadduct analogue of [60] is investigated which, despite an even further increase in open circuit voltage, results in a significantly reduced device performance due to a strong deterioration of the electron mobility in the fullerene phase. REFERENCES M. Lenes, G. A. H. Wetzelaer, F. B. Kooistra, S. C. Veenstra, J. C. Hummelen, P. W. M. Blom Adv. Mater. 2008, 20, 2116 M. Lenes, S. W. Shelton, A. B. Sieval, D. F. Kronholm J. C. Hummelen, P. W. M. Blom Adv. Funct. Mat. Published nline

3 CHAPTER Introduction In the first chapter of this thesis the prototypical polymer:fullerene system P3HT: was introduced. With a record efficiency of 5.4% 1 this material system is approaching efficiencies warranting large scale commercialization. Already significant research effort is put into developing large area technologies employing these materials. 2 In order for this research effort to be successful it is crucial that materials are available on a relatively large scale. As discussed in the introductory chapter, however, solar cells based on P3HT and are nearing their maximum performance. Three strategies to improve the performance beyond that of P3HT: solar cells are given. Firstly, small bandgap polymers such as the ones used in the previous chapter can be used to enhance the light absorption of the solar cell. Secondly, polymers with lower HM and LUM levels can be used in order to increase the open circuit voltage. Lastly, acceptors with higher LUM levels can be employed to raise the open circuit voltage. In this chapter the third strategy is employed. Thus far, fullerenes have always been the acceptor of choice when making polymer solar cells. Even though polymer n-type materials have a large potential due to the additional absorption in the acceptor, so far efficiencies have been moderate due to problems with charge trapping, dissociation, and phase separation. 3,4 Hybrid solar cells combine polymers with inorganic nanoparticles and are also considered to have great potential, but so far stay behind in performance. 5 Therefore, fullerenes with higher LUM levels are highly desired. Changing the substituent of has shown to result in a slightly higher V oc however, the amount of enhancement using this method is limited. 6 ther fullerenes as reported thus far have not resulted in a significant improvement compared to. 7,8 In this chapter higher adduct fullerenes are investigated as a candidate for acceptors in polymer solar cells. 80

4 Current [ A] PLYMER:FULLERENE SLAR CELLS 5.2 The bisadduct analogue of First we introduce bis, which is the bisadduct analogue of [60], as a new fullerene based n-type semiconductor material. Bis is normally obtained as a side product in the preparation of. 9 The material used is obtained by standard chromatographic separation from the other reaction products. The material consists of a number of regio-isomers. The general structure of these isomers (with the second addend at various positions on the fullerene cage) is depicted in Fig Bis has a substantially higher LUM than, 10 as can be seen by cyclo voltametric (CV) comparison of bis and (Fig. 5.1). An increase of the LUM level of ~ 100 mev was found, raising the LUM to 3.7 ev below the vacuum level. Here the pure isomeric mixture of bisadducts (free of monoadduct and higher adducts) was used. The bisadduct isomer mixture is made up of a minimum of 17 isomers, as indicated by LC-MS traces. The 1H- NMR data further indicate that the bisadducts consist of very complex mixture of isomers, showing at least 17 methoxy resonance signals. First, layers of pristine bis were investigated to see whether the additional functionalization of the fullerene, and the fact that the material is made up out of a mixture of isomers, have any negative side effects on the charge transport properties bis Voltage [V] Figure 5.1: Cyclic Voltametry measurement performed on (solid line) and bis (dashed line). Experimental conditions: V vs Fc/Fc+, Bu 4 NPF 6 (0.1 M) as the supporting electrolyte, DCB/acetonitrile (4/1) as the solvent, 10 mv/s scan rate. The inset shows the generalized chemical structure of the bis regio-isomers (i.e. the bottom addend is attached in a cyclopropane manner at various [6,6] positions, relative to the top one). 81

5 J [A/m 2 ] CHAPTER 5 The electron transport through the fullerene was measured by sandwiching a layer of bis between a layer of indium tin oxide (IT) covered with ~70 nm of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDT:PSS) and a samarium(5 nm)/aluminium(100 nm) top electrode. Since the work function of PEDT:PSS (5.2 ev) is significantly lower than the HM of bis (6.1 ev), hole injection into the fullerene can be neglected and only electrons flow at forward bias. Figure 5.2 shows the J-V characteristic of a bis electron only device with a thickness of 182 nm, with the applied voltage corrected for the builtin voltage and series resistance of the contact. The transport through these single carrier devices is space-charge limited, resulting in a low-field electron mobility of 7 x 10-8 m 2 /Vs. Even though the measured electron mobility for bis is lower compared to values reported for normal (2 x 10-7 m 2 /Vs), measured under the same conditions, 11 the observed electron mobility is still expected to result in a balanced charge transport when combined with P3HT Fit Data V-V bi -V rs Figure 5.2: Current density versus voltage, corrected for built in voltage and series resistance of a bis electron only device. Data (symbols) is fitted (solid line) using a space-charge limited current with a field dependent mobility P3HT:bis solar cells Next, bis was used as an acceptor in a polymer:fullerene solar cells using the solvent annealing technique. 12 P3HT and bis were dissolved in 1,2-dichlorobenzene (odcb) by stirring the mixture for 2 days. The blend was spin cast on top of IT covered with PEDT:PSS and left to dry in a closed petri dish for 48 hours. After the solvent annealing a short (5 minute) thermal annealing step 82

6 E.Q.E. [%] PLYMER:FULLERENE SLAR CELLS was done at 110 o C. To finish the devices a samarium(5 nm)/aluminium(100 nm) top contact was evaporated. Since bis has a lower electron mobility and higher molecular weight compared to normal, also a different optimal composition of the blend was anticipated. Indeed, optimization showed a polymer fullerene weight ratio of (1:1.2) to give the highest efficiencies. The optimal active layer thickness for P3HT:bis was found to be ~ nm. After fabrication the samples were evaluated and the best cells were shipped inside a nitrogen filled container to the Energy research Centre of the Netherlands (ECN), to accurately determine the device performance. As a reference, P3HT cells with normal in a 1:1 weight ratio were made with the same fabrication procedure. The optimal thickness of these cells was somewhat higher than for bis, around 350 nm Wavelength [nm] bis Figure 5.3: External quantum efficiency of a P3HT: and P3HT:bis solar cell. Figure 5.3 shows the external quantum efficiency (EQE) determined at ECN for P3HT:bis and P3HT: solar cells. Even though similar in shape normal devices result in slightly higher external quantum efficiencies, probably due to a thicker active layer. From the EQE measurements the short circuit current density under AM 1.5 conditions was estimated to be 96 A/m 2 for P3HT:bis versus 104 A/m 2 for P3HT:. Figure 5.4 shows the J-V characteristics of the cells measured using a halogen lamp with a light output equivalent to an AM1.5 light source with an intensity of 1.16 kw/m 2. The open circuit voltage of the P3HT:bis cell amounted to 0.73 V, which is 0.15 V higher than the cell with P3HT:. As predicted by the EQE measurements the short circuit current is only slightly lower for P3HT:bis. Due to the enhanced V oc, bis is clearly the superior acceptor in combination with P3HT. In order to accurately quote efficiencies, calibrated measurements are needed. ur 83

7 Current Density [A/m 2 ] CHAPTER 5 best cell was measured under a 1000 W/m 2, simulated AM1.5 illumination from a WXS-300S-50 solar simulator (WACM Electric Co). These externally verified measurements resulted in an open circuit voltage of V, fill factor of 68% and a short circuit current of 91.4 A/m 2. The resulting power conversion efficiency amounts to 4.5% for the P3HT:bis solar cell with an active area of 0.16 cm 2. Devices with larger active areas of 1 cm 2 showed a small decrease in fill factor to 62%, resulting in efficiencies of 4.1%. The discrepancy between the calculated short circuit current from the EQE measurements and the AM 1.5 current is probably due to the absence of a bias illumination during the EQE measurement. The efficiency of 4.5% is about a factor 1.2 larger as compared to the efficiencies of our best P3HT: cells of 3.8%. This improvement is entirely due to the increase of V oc bis Voltage [V] Figure 5.4: Current density versus voltage of P3HT: and P3HT:bis solar cells under illumination of a halogen lamp with an intensity equivalent to 1.16 sun. 84

8 PLYMER:FULLERENE SLAR CELLS 5.3 ther higher adduct fullerenes In the previous paragraph, the bisadduct analog of was introduced as a new acceptor for use in polymer solar cells. Next, other higher adducts analogues were investigated in order to see whether the concept can be extended to other fullerenes. Furthermore, the charge transport of the various higher adducts was studied in more detail to explain the peculiar feature that the bisadduct mixture of isomers can be used to produce high performance solar cell. It is noted that in this follow-up study the device performance is somewhat lower compared to the one described above. Furthermore, a larger difference in short circuit current between mono and bisadducts is observed. The reason for this behaviour is the different fabrication technique used here, based on chloroform as solvent and thermal annealing. This fabrication technique is chosen above spin coating from orthodichlorobenzene and solvent annealing, due to the much larger spread in device performance of the latter, making a comparison of single carrier devices and solar cells difficult. Furthermore, other than accounting for the difference in weight ratio, all cells were fabricated using identical procedures and no optimisation was done for the individual materials. C 6 H 13 S n S S S S Figure 5.5: Materials used in this chapter. From top left to bottom right, regioregular poly[3- hexylthiophene] (P3HT), [60], and highly generalized structures for the isomeric mixtures of the bisadducts bis[60], bis[70], bis[60]thcbm, bis[70]thcbm, and the trisadduct tris[60]. 85

9 CHAPTER 5 Due to its more asymmetric shape the C 70 based [70] has a higher absorption coefficient compared to [60] which has shown to be useful for complementing the absorption of small bandgap polymers. 13 The thienyl based [6,6]-thienyl-C 61 -butyric acid methyl ester (ThCBM) has been developed to provide a better conformity between polymer and fullerene in P3HT based devices. 14 The generalized chemical structures of these materials are shown in figure 5.5. All fullerenes were synthesised according to a procedure reported in our previous work. Next to the standard p-type polymer P3HT and n-type molecule [60], the bisadduct analogues of [60], [70], [60]ThCBM, [70]ThCBM and the trisadduct analogue of [60] have been investigated. In order to study the effect of the additional fuctionalisation and the fact that a mixture of isomers is used, electron transport measurements have been performed on blends of P3HT and fullerenes. In figure 5.6 the J-V characteristics of electron-only devices (using an Alx electrode) of all P3HT:fullerene blends at room temperature are shown. The device currents of the bisadducts are all lower compared to P3HT- blends where the biggest difference occurs for the blend based on [70]ThCBM. For the trisadduct the electron current is even further decreased by 3 orders of magnitude. A possible explanation for this drop in device current can be an increase in disorder in the fullerene phase, due to the presence of a multitude of isomers of the fullerenes. In order to determine the amount of disorder in the materials the temperature dependence of the zero-field mobility as determined from the various electron only devices is studied. According to the Gaussian disorder model this temperature dependence is governed by the width of a Gaussian density of states σ following 15 86

10 J [A/m 2 ] PLYMER:FULLERENE SLAR CELLS E-3 [60] tris bis[60] bis[60]thcbm bis[70] bis[70]thcbm 1E V [V] Figure 5.6: J-V characteristics of P3HT:methanofullerene blend electron single carrier devices. exp 3 5k B T k B T 3 / 2 2 eae (5.3) where μ is the mobility as the temperature goes to infinity, a is the intersite spacing and k B is Bolzmann s constant. Figure 5.7 shows the temperature dependence of the zero-field mobility as determined from the electron only devices. According to Equation 5.3 the amount of disorder σ can be calculated from the slope of the (log) mobility versus 1/T 2. For a σ of 68 mev is determined, which agrees with the previously reported value. 11 For the bisadducts the magnitude of the disorder is significantly larger as given in the inset of figure 5.7. Note that for tris[60] σ could not be determined. At low temperatures the electron current decreased below the leakage current of the devices due to local shorts, so that the electron mobility could not be measured in this material. Looking at the room temperature zero-field mobilities, as compared to, 87

11 CHAPTER 5 1E-7 1E-8 1E-9 1E-10 0 [m2 /Vs] 1E-11 [mev] 1E bis[60] 94 1E-13 bis[70] 100 bis[60]thcbm 88 1E-14 bis[70]thcbm 129 tris 1E /T 2 Figure 5.7: Temperature dependence of the zero field mobility of P3HT:fullerene single carrier devices. The Gaussian disorder model is used to determine the disorder parameter σ for the various methanofullerenes in the blend. a decrease in mobility of up to 2 orders of magnitude is seen for the bisadducts, and an even higher decrease for the trisadduct. It is expected that the device performance of solar cells based on these materials will suffer significantly from the much lower mobility due to space-charge formation, additional recombination losses and a lower dissociation probability of the bound electron-hole pairs. 16,17,18 Using the numerical program, we have performed simulations in order to analyze the effect such a lowering of the mobility has on the device performance. In figure 5.8 the J-V characteristics of a P3HT: reference device is shown. Using the electron mobility for determined above and typical simulation parameters as previously reported, 19 the J-V characteristics are described adequately. After accounting for the increase in V oc of the bisadducts and trisadducts the J-V characteristic is then calculated using the mobilities determined using the electrononly devices. As expected, due tothe lower electron mobilities the calculated short circuit current and fill factor decrease dramatically resulting in a predicted drop in efficiency of up to 60% for bis[70]thcbm. 88

12 J L [A/m 2 ] PLYMER:FULLERENE SLAR CELLS Data bis[60]thcbm bis[60] bis[70] bis[70]thcbm trsi V [V] Figure 5.8: Predictions of solar cell characteristics for all fullerenes. First the standard P3HT: J-V curve (symbols) is fitted using the numerical program (solid line). Next the J-V curve for the other fullerenes is calculated taking into account the lower mobility as determined in figure 3 and the increase in Voc of the bis and tris adducts Solar cells based on higher adduct fullerenes Next, bulk heterojunction solar cells were fabricated using the methanofullerenes introduced above. Figure 5.9 shows the J-V characteristics of all solar cells at room temperature under simulated AM1.5 illumination. As discussed in the first part of this chapter, the raised LUM level of the bisadducts result in a significant enhancement of the open circuit voltage of the devices. What is very surprising however, is that all bisadducts show an almost identical device performance in contradiction to the predicted performance shown in Fig Apparently, the lower electron currents as seen in the electron-only devices are not at all reflected in the performance of the solar cells. For the trisadduct however, despite the even further enhanced open circuit voltage(which is among the highest reported for a P3HT based device) 20 the power conversion efficiency drops dramatically as predicted from the deteriorated electron transport. Another remarkable feature, as can be seen in Figure 5.10, is that a difference in device performance of the solar 89

13 J L [A/m 2 ] CHAPTER 5 cells between monoadducts and bisadducts reappears when cooling the samples below room temperature. Furthermore, the solar cells which show a stronger temperature dependence of their performance are those which gave low device currents in the electron-only devices. These observations strongly suggest that the transport in the fullerene phase is hampered by a large amount of shallow trapping. When shallow traps are present the J-V characteristics at low voltages are also described by a quadratic dependence on voltage, given by bis[60] bis[70] bis[60]thcbm bis[70]thcbm tris V [V] Figure 5.9: J-V characteristics of P3HT:fullerene solar cells under illumination of a simulated AM1.5 irradiation with an equivalent of 1.4kW/m 2. J r 2 int 3 V L (5.1) with N c exp N t Et k T B (5.2) and N c the effective density of states, N t the amount of traps and E t the trapdepth. In this case θμ represents an effective mobility, that contains the ratio of free and trapped charges. The relatively low electron-only currents for a 90

14 PLYMER:FULLERENE SLAR CELLS number of bisadducts are in that case due to the fact that many electrons are immobile because they are trapped in shallow traps. For the solar cells, during illumination a number of trap states will be filled, leading to an enhanced transport and the device operation then approaches the one of the trap-free device. Such an illumination dependent transport has recently been observed in n-type polymers.. 21 Further evidence for charge trapping in solar cells can be obtained from the intensity dependence of the open circuit voltage of the devices. 22 For trap-free polymer:fullerene solar cells, when plotting the V oc versus the natural logarithm of the light intensity, the slope of the V oc follows S=(k B T/q), where k B is the Bolzmann constant, T is the temperature and q is the elementary charge. In the case of recombination with trapped charges, however, the intensity dependence of the V oc is enhanced. Figure 5.11 shows the V oc dependence on light intensity for our devices. Again, the fullerenes which exhibit lower electron currents in the electron only devices and a stronger temperature dependence in the solar cells, show a larger dependence of the Vo c versus light intensity. 91

15 pen Circuit Voltage [V] Fill Factor Maximum Power Pint [W/m 2 ] Short Circuit Current [A/m 2 ] CHAPTER bis[60] bis[70] bis[60]thcbm bis[70]thcbm Temperature [K] bis[60] 0.2 bis[70] 0.1 bis[60]thcbm bis[70]thcbm Temperature [K] bis[60] bis[70] bis[60]thcbm bis[70]thcbm Temperature [K] bis[60] 0.4 bis[70] bis[60]thcbm bis[70]thcbm Temperature [K] Figure 5.10: Solar cell parameters; maximum power point (MPP), short circuit current density (J sc ), open circuit voltage (V oc ), and fill factor (FF) of P3HT:fullerene solar cells as a function of temperature. 92

16 V oc [V] PLYMER:FULLERENE SLAR CELLS 0,95 0,90 0,85 0,80 0,75 0,70 0,65 bis[60] bis[70] bis[60]thcbm bis[70]thcbm tris[60] slope [kt/q] ,60 0,55 0,50 0,45 7, , , , , , ,95799 Intensity [W/m 2 ] Figure 5.11: pen circuit voltage versus the natural logarithm of the intensity of P3HT:fullerene solar cells. The slope of the V oc vs. intensity in units of [kt/q] is given in the legend Device simulations using charge trapping In order to quantitatively describe the electron only devices and solar cells the effect of charge trapping on the simulations is incorporated. Figure 5.12 shows the J-V characteristics of a P3HT:bis[70] electron-only device on a double log scale. When modelling these electron currents, it is assumed that the mobility of bis[70] is identical to reference [60]. We note that field effect transport studies have shown the mobility of [60] and [70] to be equal within experimental error. 23 Next, we introduce shallow traps that are exponentially distributed in energy. We observe that a relatively narrow distribution in energy, as expected for (random) disorder, gives better results than only a discrete trap level. The width of the distribution is governed by a trap temperature T trap =340K. 22 For such an exponential trap distribution the effective number of traps has been shown to vary with temperature with exp{-[1/2(σ 2 /kt]/kt trap }. Using this temperature dependence of the effective number of traps (assuming σ to be 68meV as determined from the devices) we can describe the whole temperature range. For the simulations of the photocurrent of the P3HT: and P3HT:bis[70] solar cells in figure 5.12, we again start with our description of the P3HT: reference device. Using the same set of parameters, we subsequently add the enhanced open circuit voltage, and the trap distribution as determined from the electron-only device of the P3HT:bis[70] blend. As can be seen in figure 5.13, the incorporated trap distribution indeed does not lower the device performance and we can describe the J-V characteristics adequately. As a 93

17 J [A/m 2 ] CHAPTER 5 result the occurrence of shallow traps simultaneously explains the reduced electron currents and the relatively good solar cell performance, due to filling of these traps under illumination. The nature of the shallow trapping is likely to be one or more specific bisadduct isomers with lower lying LUM s. Although we expect that certain single isomers of bisadducts can show improved performance, the fact that the mixture of isomers can be used as such, and that it still results in a proper device operation is a great benefit for commercialisation of polymer:fullerene solar cells T [K] V-V bi -V rs Figure 5.12: J-V characteristics, corrected for built-in voltage and series resistance of a P3HT:bis[70] electron single carrier at different temperatures. The J-V curves are fitted using mobilities and an exponential trap distribution. 94

18 J L [A/m 2 ] PLYMER:FULLERENE SLAR CELLS Data bis[70] Data -100 Fit bis[70] Fit V [V] Figure 5.13: J-V characteristics of a P3HT: and P3HT:bis[70] solar cell. The J-V curve (symbols) are fitted with our numerical program where for the P3HT:bis[70] cell an exponential trap distribution is introduced. 95

19 CHAPTER Conclusions A novel type of fullerene, bis, with a higher LUM level compared to that of, is used in order to minimize the energy loss in the electron transfer from the donor to the acceptor material in bulk heterojunction solar cells. The additional functionalization of the fullerene cage in bis was shown to have little negative influence on the charge-carrier properties of the fullerene. As predicted, the higher LUM resulted in a significantly enhanced open-circuit voltage when used in combination with P3HT, while maintaining a high short-circuit current and fill factor. An externally verified power-conversion efficiency of 4.5% was reported for a P3HT:bis solar cell. We showed that the bisadduct isomer mixture, free of monoadduct and higher adducts, can be used without further separation of the individual isomers, resulting in an enhanced cell performance compared to that of. Furthermore, several other higher adduct fullerenes are investigated in combination with P3HT. At first sight the higher adduct fullerenes show signs of an enhanced disorder, reflected by a reduced current in electron-only devices. Such an enhanced disorder however does not comply with the temperature and intensity dependence of the solar cells. Instead, a substantial amount of shallow trapping is likely to be the cause of the reduced currents in the electron-only devices. Under illumination these trap states are filled and normal solar cell operation is observed. An exponential trap distribution has been shown to adequately describe both electron only and solar cell. The nature of the shallow trapping is likely to be specific bisadduct isomers with lower lying LUM s. The fact that the mixture of isomers can be used as such, and still results in a proper device operation is a great benefit for commercialisation of polymer:fullerene solar cells. The trisadduct analogue of however, despite leading to a high open circuit voltage of 813 mv, results in a significantly reduced device performance due to a deterioration of the charge transport in the fullerene. 96