Diffusion Lengths of Excitons in Polymers in Relation to External

Dec 10, 2013 - Sumitomo Chemical Company Ltd., 5-33 Kitahama 4-chome, Chuo-ku, Osaka 541-8550, Japan. J. Phys. Chem. C , 2014, 118 (1), pp 71–76...
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Diffusion Lengths of Excitons in Polymers in Relation to External Quantum Efficiency of the Photocurrent of Solar Cells Cheahli Leow,† Toshihiro Ohnishi,†,‡ and Michio Matsumura*,† †

Research Center for Solar Energy Chemistry, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531, Japan Sumitomo Chemical Company Ltd., 5-33 Kitahama 4-chome, Chuo-ku, Osaka 541-8550, Japan

‡

ABSTRACT: By measuring quenching of fluorescence from polymers by C60 at a planar C60/polymer junction as a function of thickness of the polymer layers, we determined diffusion lengths of excitons for four kinds of polymers. The exciton diffusion lengths ranged from 5 to 12 nm depending on the polymer. We also fabricated solar cells with a bilayer C60/polymer junction using the polymers. The external quantum efficiency of short-circuit photocurrents was reproduced well using the exciton diffusion lengths, indicating the usefulness of the analytical method. The results also suggest low carrier recombination efficiency at the C60/ polymer interface.

and a fullerene derivative as an electron-accepting material.12 Because the suitable domain size of the two materials is closely related to LD, it is important to determine LD to design the mixed phases and to deepen the understanding of the function of the bulk heterojunction. LD can be measured by different methods including exciton− exciton annihilation,11,13 photovoltaic response of solar cells,6 microwave conductivity,14 and spectrally resolved photoluminescence quenching.15,16 These studies have mostly been carried out for singlet excitons. Concerning triplet excitons, Samiullah et al.17 estimated LD of an organic compound from quenching of the triplet−triplet absorption in the presence of acceptor molecules using a photoinduced absorption spectroscopy. For determination of LD of singlet excitons, the methods based on fluorescence are convenient because the relative density of singlet excitons formed in a film can be easily determined from the observed fluorescence intensity. The steady-state fluorescence quenching method using a planar interface of electron-donating and electron-accepting materials, which is one of the simplest ways, has sometimes been used for the determination of LD.9,18−20 This method typically compares intensity of fluorescence from an electron-donating polymer layer having an interface with an electron-accepting material with that from the same polymer layer without the interface; the comparison gives the degree of fluorescence quenching. If C60 or its derivative is used as the electron-accepting material, fluorescence from a thin polymer layer is effectively quenched because an exciton in a polymer layer is efficiently dissociated into an electron and a hole at the interface.19 However, as the thickness of the polymer layer increases, the quenching of the

1. INTRODUCTION Organic solar cells, which are also called organic photovoltaics (OPVs), have gained increasing attention because of their potential for manufacturing large-scale devices by low-cost processes such as printing methods.1,2 When OPVs are irradiated, photoabsorption materials are photoexcited, leading to the formation of excitons in them. The excitons diffuse in the material to the interface of two materials. One has an electrondonating function and the other has an electron-accepting function. Photoabsorption materials can be either or both of the two materials, although they are usually the electrondonating materials. At the interface of the two materials, excitons dissociate into free carriers. The necessity of the interface (or the two materials) arises from the fact that the electron−hole pair in an exciton is strongly bound together.3 The difference in electronic levels of the two materials provides the driving force for charge separation at the interface.4 Because excitons can only be dissociated at the interface, the exciton diffusion length (LD) in the photoabsorption material plays an important role in the performance of OPVs. Most polymers used in OPVs have been reported to have short LD, normally in the range of 5−20 nm.5−11 If a solar cell is constructed by stacking electron-donating and electron-accepting materials with a planar interface, it is difficult to expect a high photocurrent density because the amount of photons absorbed in a region shorter than 20 nm from the interface is limited; for effective photoabsorption, a photoabsorber layer of about 100 nm in thickness is necessary. Thus, a so-called bulk heterojunction structure is usually introduced into OPVs.3 In these devices, the interface of electron-donating and electronaccepting materials is interpenetrated on a nanometer scale, and most of the excitons are generated at locations close to the interface. This structure is typically introduced into solar cells constructed with a polymer as an electron-donating material © XXXX American Chemical Society

Received: August 8, 2013 Revised: December 3, 2013

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fluorescence becomes less because more excitons cannot reach the interface. Hence, from the analysis of the relationship between the degree of fluorescence quenching vs the thickness of the polymer layer, LD can be determined. There has been difficulty in the determination of LD because the density of excitons photogenerated in the polymer films has a distribution depending on their photoabsorption properties. In this study, we tried to improve the analytical method for determining LD values by considering the distribution of photogenerated excitons in the polymer films. In the experiments, we used a bilayer structure with a planar junction of a polymer film and a vacuum-deposited C60 layer, which provides a well-defined interface.9,19−21 We report here the LD values obtained by using this method for four different polymers, which showed LD values in the range of 5−12 nm. In addition, external quantum efficiencies of OPVs made from the polymers are discussed on the basis of the LD values.

Figure 2. Structures of samples for measuring (a) fluorescence and (b) solar cell properties. The optical paths are shown by arrows.

coating, which were measured with a laser microscope (KEYENCE VK9710). Fluorescence from the polymer-only and polymer/C60stacked samples was measured using a fluorescence spectrofluorometer (JASCO, PF-6500) in air immediately after they were prepared. The samples were photoexcited in a manner shown in Figure 2a at wavelengths suited to the polymers. Absorption spectra of the samples were obtained with a UV− vis spectrometer (Hitachi U4100). The highest occupied molecular orbital (HOMO) of the polymers was determined by photoelectron spectroscopy measurement (AC2, Riken Keiki Co., Ltd.) Solar cells were prepared using indium−tin-oxide (ITO)coated glass plates (Asahi Glass, 15 Ω/sq) as substrates. On the surface of the ITO-coated glass plate, a PEDOT:PSS layer was deposited by spin coating a PEDOT:PSS solution at 3000 rpm for 60 s, which was heat-treated at 120 °C for 15 min for drying. Then a polymer layer and a 50 nm thick C60 layer were deposited sequentially in the same manner as that for the polymer/C60-stacked layers for fluorescence measurement. Finally, a 100 nm thick aluminum (Al) layer as an electrode was deposited on top of the C60 layer by vacuum deposition at a pressure lower than 6 × 10−3 Pa. The device structure is shown in Figure 2b. The effective area of the device was about 2 mm2. Current density−voltage (J−V) characteristics were measured under simulated solar radiation (AM 1.5, 100 mW/ cm2) with a solar cell evaluation system (Bunko Keiki, CEP015). External quantum efficiencies of photocurrents were also measured with the system.

2. EXPERIMENTAL SECTION Regioregular poly(3-hexylthiophene) (P3HT; 99.995%) and fullerene (C60; 99.9%) were purchased from Sigma-Aldrich. Poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-bithiophene] (F8T2), poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-(benzo[2,1,3]thiadiazol4,8-diyl)] (F8BT), and a copolymer composed of 9,9didodecylfluorene and 4,7-di(thiophen-2-yl)benzo[1,2,5]thiadiazole (F12TBT) were supplied by Sumitomo Chemical Co. Structures of the polymers used as photoactive layers in this study are shown in Figure 1. Poly(3,4-

Figure 1. Chemical structures of polymers used as photoabsorbers in this study.

3. RESULTS AND DISCUSSION 3.1. Quenching of Fluorescence from a Polymer Layer by a Polymer/Electron-Acceptor Interface and Estimation of Diffusion Length of Excitons. We explain the method for determining the LD value by taking P3HT as a representative of the four kinds of polymers. A P3HT film shows an absorption band peaking at 534 nm and a fluorescence band peaking at 634 nm, as can be seen in Figure 3. These bands are assigned to a π−π* transition of the P3HT backbone.22−24 Fluorescence of the 43 nm thick P3HT layer is quenched by about 30% by depositing a C60 layer on the P3HT film, as shown by the broken line in Figure 3b. The quenching of fluorescence is attributed to charge separation from the excitons at the C60/P3HT interface. The absorption of incident photons by the C60 layer can decrease the fluorescence from the P3HT layer. The absorption by a 10 nm thick C60 film at wavelengths used in this study was about 3%, which was corrected in the latter analyses of fluorescence quenching by C60. Figure 4a shows peak intensities of fluorescence of P3HT films and peak intensities of fluorescence of P3HT/C60-stacked

ethylenedioxthiophene):poly(styrenesulfonate) (PEDOT:PSS; CLEVIOS P VP.Al 4083) purchased from Heraeus was used as the hole extraction layer of solar cells. All of the chemicals were used without further purification. For determination of LD, two types of polymer samples were prepared on glass substrates: one type was polymer-only films and the other type was polymer/C60-stacked films. The two types of samples were prepared on the same substrate, as shown in Figure 2a. The samples were prepared on glass substrates, which had been ultrasonically cleaned in water, chemical detergent, and acetone for 15 min successively and then subjected to UV−O3 treatment for 20 min prior to use. Polymer films were formed on them by spin coating from polymer solutions dissolved in chloroform. A 10 nm thick C60 layer was deposited on half of the polymer film by vacuum deposition at a pressure lower than 10−3 Pa to make the polymer/C60-stacked films. The thickness of the polymer films was varied in the range of 5−80 nm by controlling the concentration of the polymer solution and the speed of spin B

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Figure 3. (a) Absorption spectra of P3HT and C60 films and (b) fluorescence spectra of P3HT and P3HT/C60 films. The thickness of the P3HT layer was about 43 nm.

Figure 4. Intensity of fluorescence from polymer films (solid lines) and polymer/C60 films (broken lines) as a function of the thickness of the polymer layer. The polymers are (a) P3HT, (b) F8T2, (c) F12TBT, and (d) F8BT.

films by a solid line and a broken line, respectively, as a function of thickness, L. The fluorescence intensity of the P3HT films without a C60 layer increases with an increase in L and shows a tendency to approach a limiting value. This profile is determined by the photoabsorptivity of the P3HT film at the wavelength of photoexcitation. When a C60 layer was deposited on them, the fluorescence intensity was weakened and the profiles shifted in the larger L direction due to the quenching of excitons at the C60/P3HT interface, as discussed above. It is believed that in polymer solar cells made of a polymer and C60 or its derivative, the charge generation efficiency from an exciton reaching the interface is very high.25,26 This is consistent with the almost complete quenching of fluorescence of the P3HT films with a thickness of about 8 nm. However, as the P3HT layer becomes thicker, fluorescence from P3HT is quenched only partly by a C60 layer. This means that some of

the excitons formed in the P3HT layer did not reach the C60/ P3HT interface during their lifetime. The intensity ratio of fluorescence of the P3HT and C60/P3HT films was used to determine the exciton diffusion length, as discussed later. In addition to the samples with P3HT, we measured the fluorescence of F8T2, F12TBT, and F8BT films, with and without a C60 layer, as a function of their thickness. The results are shown in Figure 4b−d. The profiles are similar to those of the P3HT films, although the degree of quenching is dependent on the kind of polymer. For determination of the exciton diffusion length, we introduce quenching efficiency of fluorescence of a polymer film with a thickness of L by a C60 layer, Q(L), which is given by C

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PL intensity from a sample with a C60 layer PL intensity from a sample without a C60 layer

Q (L ) =

∫0

L

2L D x tanh × e−αL dx ÷ (1 − e−αL) x 2L D

(1)

(3)

For more precise determination of LD, the effect of photoabsorption by the C60 layer at the wavelengths of photoexcitation was corrected; the photoabsorption by the C60 layer was about 3−12%. Q(L) values for the four kinds of polymers are shown in Figure 5, which are plotted using the data shown in Figure 4.

where x stands for the distance from the interface of polymer/ C60 to a point in the polymer film and α stands for the absorption coefficient (nm−1) of the polymer at the wavelength of photoexcitation. Although we cannot analytically determine the exact values of eq 3, we can numerically integrate eq 3 by dividing the whole film thickness into small sections, for example every 0.1 nm, and summing the values calculated for all of the sections. The lines in Figure 5 are drawn by numerical integration for all of the sections over the whole film thickness by choosing the LD values to best fit the experimental data. In the calculation, the following α values were used for the polymers: 0.0127 nm−1 (P3HT, at 550 nm), 0.0136 nm−1 (F8T2, at 430 nm), 0.0115 nm−1 (F12TBT, at 550 nm), and 0.0073 nm−1 (F8BT, at 440 nm), which were multiplied by √2 in eq 3 because the angle of incidence of the excitation light for the fluorescence measurements was 45°. The LD values which gave the best fitting to the experimental data were 5, 8, 11, and 12 nm for P3HT, F8T2, F12TBT, and F8BT, respectively. The results indicate that the LD value depends considerably on the polymer. The L D value determined for P3HT, 5 nm, falls in the range of the values commonly reported so far, i.e., 3−8.5 nm;5−8,11 the values for other polymers have not been reported. The short LD of P3HT films compared with those of the other polymer films is attributed to the fact that P3HT has a rather large Stokes shift (about 3470 cm−1) and the fact that P3HT shows a rather low quantum efficiency of fluorescence of about 5%. Both of these properties impede the diffusion of excitons by the Foerster mechanism. Incidentally, the other three polymers show a Stokes shift of about 2130−2770 cm−1 and fluorescence quantum efficiency of about 30−40% from our measurements. 3.2. Relationship between Diffusion Lengths of Excitons and External Quantum Efficiencies of the Photocurrent of OPVs. Solar cells with a structure of glass/ ITO/PEDOT:PSS/polymer/C60/Al were fabricated using the four polymers, each of which had a planar junction of C60/ polymer. Several devices were fabricated for each of the polymers by changing the thickness of the polymer layer in the range of 10−150 nm. Except for F8BT, OPVs made of the polymers showed J−V characteristics typical of solar cells under photoirradiation. The poor solar cell properties for the device with an F8BT layer may be because it lacks the thiophene unit, which gives hole conductivity to the polymers. Representative J−V curves for the devices with the other three kinds of polymers (30 nm in thickness) are shown in Figure 6. Voc of the device with P3HT is low, possibly due to the fact that the difference between LUMO of C60 (4.5 V)27 and HOMO of P3HT (4.7 V) is smaller than the differences at the junctions with other polymers; i.e., HOMO of F12TBT and F8T2 are 5.3 and 5.5 V, respectively. Figure 7 shows external quantum efficiency of photocurrent measured at V = 0 for the devices with the 3 kinds of polymers as a function of thickness of the polymer layer, which were photoirradiated at wavelengths suited to the polymers. The devices with F12TBT show a maximum external quantum efficiencey (EQE) at the film thickness of about 30 nm. The peak appears because the number of excitons formed in the polymer films increases with increase in polymer thickness,27

Figure 5. Fluorescence quenching efficiency of polymer (P3HT, F8T2, F12TBT, and F8BT) layers by stacking a C60 layer on them as a function of the thickness of the polymer layer. The lines are drawn using eq 3 by inserting LD values chosen to best fit the experimental data: LD values were 5 nm for P3HT, 8 nm for F8T2, 11 nm for F12TBT, and 12 nm for F8BT.

Q(L) decreases as the thickness of the polymer films increases. This is attributed to the fact that the probability for the excitons to reach the interface decreases with an increase in the thickness of the polymer layer. It is important to notice that the profiles of the Q(L)−L curves depend on the polymer. Q(L) was larger at any L in the order of P3HT < F8T2 < F12TBT < F8BT. In other words, the quenching is more significant in this order, suggesting that LD increases in the order of P3HT < F8T2 < F12TBT < F8BT. However, for more precise discussion, we have to take into account the distribution of exciton generation in the films, which is dependent on the photoabsorptivity at the wavelength of photoexcitation. The relationship between L and LD is sometimes expressed on the basis of the assumption that excitons are homogeneously photoproduced in the polymer film, by9 Q (L ) =

2L D L tanh L 2L D

(2)

However, in a polymer film used for solar cells, it cannot be assumed that the polymer is photoexcited homogeneously throughout the film. On the contrary, the photon density in the polymer film decreases exponentially as the distance increases from the polymer/C60 interface due to photoabsorption by the polymer. Instead of introducing a totally new function about the relationship between fluorescence quenching efficiency and polymer thickness, we tried to modify eq 2 by taking into account the profile of photon density in polymer films, which is given by D

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where x stands for the distance from the interface of polymer/ PEDOT:PSS to a point in the polymer film. The first term of eq 4 represents the contribution from excitons generated by the incident light and the second term represents the contribution from excitons generated by light reflected at the Al electrode. By numerically integrating eq 4 for every 0.1 nm over the whole film thickness, ECE(L) values for devices with P3HT, F8T2, and F12TBT were calculated, as shown by lines in Figure 7. In the calculation, the LD values for these polymers determined from the analyses of fluorescence quenching and the α values of the polymers at the corresponding wavelengths were used. As shown in Figure 7, for devices with F12TBT and F8T2, the theoretical lines fit well with the experimental data. However, for devices with P3HT, the theoretical line shows higher EQE values than experimental data by about 40%, which is attributed to the fact that the photocurrent of the devices with P3HT does not reach the limiting value at 0 V, as seen in Figure 6. These results indicate that carriers generated at the C60/polymer interface contribute to the photocurrent at efficiency close to 100% at 0 V, at least for the devices with F12TBT and F8T2, or very low charge recombination at the C60/polymer interface, which forms the basis for highly efficient OPVs made of polymers and fullerene compounds. As discussed above, our method for determining LD values seems to be applicable to the junctions with a variety of materials, including low-molecular-weight donor materials. For further improvement of the method, additional aspects including the optical interference effect of thin films,28,29 morphologies of the C60/polymer interface,18 and contribution of excitons of C60 to the photocurrent30 need to be addressed.

Figure 6. J−V characteristics of solar cells using P3HT, F12TBT and F8T2 layers as photoabsorbers. The thicknesses of the polymer films were about 30 nm.

4. CONCLUSIONS We analyzed the diffusion lengths of excitons of polymers based on steady-state photoluminescence quenching at a conjugated C60/polymer interface taking into account the distribution of excitons photogenerated in the polymers. The exciton diffusion length of the polymers tested ranged from 5 to 12 nm. The validity of the approach is proved by the fact that EQE of shortcircuit photocurrent of polymer-based solar cells was reproduced using the exciton diffusion lengths determined.

Figure 7. EQE of the short-circuit photocurrent of solar cells using P3HT, F8T2, and F12TBT as photoabsorbers as a function of the thickness of the polymer layer, measured at wavelengths of 550, 430, and 550 nm, respectively. The lines were drawn by numerically integrating eq 4 using the corresponding LD values determined from the fluorescence quenching experiments: 5 nm for P3HT, 8 nm for F8T2, and 11 nm for F12TBT.

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and the number of excitons reaching the C60/polymer interface decrease with increase in polymer thickness (see Figure 2b). We could not observe peaks of EQE for the devices with P3HT and F8T2 because it was difficult to fabricate devices, which were free from current leakage, with a polymer layer thinner than 20 nm. We tried to analyze the EQE curve by, in principle, the same method as that used for the analysis of fluorescence quenching by C60. However, in this case, we have to take into account the direction of incident light, which is opposite to the direction for fluorescence measurements (see Figure 2). Reflection of light at the surface of the Al electrode must also be considered (see Figure 2b). Taking into account these properties of light paths and assuming that all of the excitons reaching the interface contribute to currents, the exciton collecting efficiency, ECE(L), at the C60/polymer interface is given by ECE(L) =

Corresponding Author

*E-mail: [email protected]. Phone: +81-6-68506695. Fax: +81-6-6850-6699. Notes

The authors declare no competing financial interest.

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