Photoinduced Injection Enhancement in Fullerene-Based Organic

May 8, 2014 - Abstract. Abstract Image. The dependence of photocurrent of a typical copper phthalocyanine (CuPc)/C60 device on external negative bias ...
0 downloads 4 Views 736KB Size
Download PDF
Article pubs.acs.org/JPCC

Photoinduced Injection Enhancement in Fullerene-Based Organic Solar Cell Originates from Exciton−Electron Interaction Wenbin Li, Haomiao Yu, Jiawei Zhang, Yao Yao, Changqin Wu, and Xiaoyuan Hou* Key Laboratory of Micro and Nano Photonic Structures (Ministry of Education) and State Key Laboratory of Surface Physics, Fudan University, Shanghai 200433, China S Supporting Information *

ABSTRACT: The dependence of photocurrent of a typical copper phthalocyanine (CuPc)/C60 device on external negative bias under monochromatic light illumination is reported. With increase of the negative bias, the photocurrent originating from excitons in the CuPc layer is quickly saturated. On the contrary, the photocurrent originating from excitons in C60 layer does not reach saturation but increases continuously with negative bias. This abnormal enhancement shows that the behavior of excitons in C60 layer is completely different from that in the typical donor material CuPc. The triplet exciton−electron interaction in C60 is supposed to cause the photoinduced hole injection enhancement from cathode, resulting in the observed phenomenon. More experiments designed have confirmed this mechanism.

■

INTRODUCTION Recently, in order to improve the efficiency of organic solar cells (OSCs), a large number of electron donor materials have been proposed and widely applied.1−5 Consequently, the power conversion efficiency has increased continuously. Meanwhile, fullerene and its derivatives have been considered as the most suitable acceptor material for a long time.4,6,7 To cause the photocurrent in OSCs, light-induced excitons need to reach the donor/acceptor (D/A) interface where they dissociate into free carriers. Therefore, no matter whether the material is donor or acceptor, the behavior of excitons must have a significant impact on the efficiency of the OSCs.3,8 As fullerene is the most widely used acceptor material, the behavior of the excitons in fullerene should be much concerned. However, unlike general donor materials, fullerenes can hardly emit light at room temperature.9 Therefore, the excitons in fullerene can only be observed by special technique such as triplet−triplet absorption spectroscopy and electron paramagnetic resonance (EPR) spectroscopy.9,10 However, these methods are not suitable to study the behavior of excitons in device under normal operating conditions. Most of the existing researches on the properties of the OSCs photocurrent pay attention to voltage−current characteristics of the device under simulated sunlight, which is related to the total power conversion efficiency. Such kinds of experiments cannot distinguish the photocurrents derived from the excitons in donor material from that in acceptor material. On the other hand, the external quantum efficiency (EQE) spectroscopy, which uses monochromatic light to illuminate the device, can distinguish the contribution of excitons generated in donor from acceptor.2,7,8,11 It has been reported that the behavior of the bias dependence of the photocurrent © 2014 American Chemical Society

would be different when the donor and the fullerene layer were selectively excited.12−14 However, the mechanism involved is still not clear, although different suggestions have been proposed. Reynaert et al. proposed that the photocurrent enhancement is related to the field-modulated tunneling barrier.12 Huang et al. proposed that the photocurrent enhancement results from the interfacial traps that enable a significant increase of electron injection.13 Jeong et al. explained the photocurrent enhancement by the bulk-ionized photoconductivity.14 In order to clarify the mechanism of this phenomenon, monochromatized light with two specific wavelengths were adopted to illuminate the typical copper phthalocyanine (CuPc)/C60 heterojunction devices in this work, to generate excitons in donor and acceptor, respectively. Then the dependence of the photocurrent on the negative bias was investigated. The experimental results show that, with increase of negative bias, the photocurrent originating from the excitons in the CuPc layer is quickly saturated. On the contrary, the photocurrent originating from the excitons in the C60 layer increases continuously and significantly with negative bias. It has been proposed that the triplet exciton−electron interaction in C60 caused photoinduced injection enhancement of holes from cathode.

■

EXPERIMENTAL SECTION In the present work, all the devices were fabricated on precleaned glass substrates coated with transparent conducting Received: July 19, 2013 Revised: April 16, 2014 Published: May 8, 2014 11928

dx.doi.org/10.1021/jp501078f | J. Phys. Chem. C 2014, 118, 11928−11934

The Journal of Physical Chemistry C

Article

indium tin oxide (ITO) with sheet resistance of about 17 Ω/□. The substrate was heated to 150 °C in air after solvent cleaning and then transferred into a vacuum chamber. The typical structure of devices was ITO/CuPc/C60/4,7-diphenyl-1,10phenanthroline (Bphen)/Al, in which the Bphen layer was used as buffer layer. All the organic materials were purchased from Luminescence Technology Corp. The organic layer and cathode were deposited at a base pressure better than 10−4 Pa with deposition rates of ∼1 and ∼2 Å/s, respectively. The deposition rate was monitored by using an in situ quartz crystal. In the experiments, a 150 W xenon lamp was used as light source to illuminate the devices. The emitted light was monochromatized by narrow band-pass filters, of which the full width at half-maximum transmission is 10 nm, and modulated by a chopper with the frequency of 173 Hz. The bias on the device was provided by a Keithley 2440 source meter. The Al cathode was grounded in the experiment, so that when the positive bias was applied the ITO electrode was at high potential. The photocurrent of the device was measured by the SR830 lock-in amplifier. The leakage current through the devices was not detected by the lock-in amplifier and would not influence the photocurrent measurement. The photocurrent Jph produced by the modulated incident light was expressed by Jph = π/ 2 × Vsignal/RL, considering the inverse Fourier transformation of a block-wave periodic signal, where Vsignal and RL are voltage signal and sampling resistor. In order to study the behavior of excitons in the donor and acceptor separately, two monochromatic wavelengths of incident light, i.e. 480 and 620 nm, were used to illuminate the device, respectively. Figure 1 shows the absorption spectra of both C60

Figure 2. Photocurrent of (a) device A and (b) device B under the monochromatic light illumination at the wavelength of 480 nm (corresponding to C60 absorption) and 620 nm (corresponding to the CuPc absorption) versus applied bias. The insets show the photocurrent ratio of 480 nm (corresponding to C60 absorption) to that of 620 nm (corresponding to the CuPc absorption) versus applied bias.

The monochromatic light with the wavelength of 490 and 650 nm was also used to selectively excite the C60 and CuPc layer. The results showed the same behavior of bias dependence of photocurrent as that under the illumination of wavelength 480 and 620 nm, respectively. All the measurements were carried out in the air without any encapsulation.

■

RESULTS AND DISCUSSION Figure 2a shows the photocurrent of the planar heterojunction device A under the monochromatic light illumination of the two different wavelengths as a function of applied bias. The structure of device A is ITO/CuPc(24 nm)/C60(40 nm)/ BPhen(10 nm)/Al. The solid squares represent the photocurrent of the device under 480 nm monochromatic light illumination, which originates from the excitons in C60 layer, while the filled circles represent the photocurrent of the device under 620 nm monochromatic light illumination, which originates from the excitons in CuPc layer. With the increase of positive bias across the device, both the photocurrents for the two monochromatic light decrease and change the polarity from negative to positive. On the other hand, with increase of the negative bias, the photocurrent produced by the 620 nm light illumination corresponding to the exciton in CuPc layer is quickly saturated. When the applied negative bias is increased to −5 V, the photocurrent increases only by 28.5% compared with that at 0 V. However, the photocurrent produced by the 480 nm light corresponding to the excitons in C60 layer increases continuously and significantly with the negative bias without saturation. When the applied negative bias is increased to −5 V, the photocurrent increases by 170.9% compared with that at 0 V. In this experiment the light-independent dark leakage current injected from the electrodes was not detected; therefore, it was

Figure 1. Normalized absorption spectra of CuPc and C60. The vertical black and blue dashed line indicate the wavelengths of monochromatic light (480 and 620 nm) used in this work.

and CuPc illuminated with the light of wavelengths 480 and 620 nm, including the nonabsorbing region and absorption region of C60, and the CuPc absorption peak and absorption valley, respectively. It was reported that Bphen is nearly transparent in the visible range.15 The spectra suggest that under the illumination of wavelength 480 or 620 nm the majority of the excitons in the device were generated in either C60 or CuPc layer, respectively. Therefore, the photocurrent of the device resulting from the illumination at these two wavelengths can show the behavior of excitons in the C60 and CuPc layer separately. The intensity of both the 480 and 620 nm incident light at the very beginning of this work was 3 mW/cm2. The results under this intensity illumination are shown in Figure 2b. Then the experimental setup had been modified to increase the intensity of the incident light to 13 mW/cm2 (480 nm) and 15 mW/cm2 (620 nm), respectively. 11929

dx.doi.org/10.1021/jp501078f | J. Phys. Chem. C 2014, 118, 11928−11934

The Journal of Physical Chemistry C

Article

originating from the excitons in CuPc and C60 molecules on the external bias has no significant difference. This is different from that in planar heterojunction devices. In planar heterojunction device, as shown in Figure 2a, the photocurrent originating from excitons in the C60 layer increases significantly with negative bias and does not tend to be saturated. Therefore, these results indicate that this abnormal enhancement effect only occurs in planar device. The reason for the inserted MoOx layer in the coevaporated device is to make the efficiency of coevaporated device B comparable to that of device A. The MoOx layer in the coevaporated device could increase the open circuit voltage which can be comparable to planar device. The photocurrent of planar devices with MoOx layer is shown in the Supporting Information. It can be seen that the difference between the planar devices with and without MoOx layer is that the efficiency of device with MoOx layer is a little bit higher than that of device without MoOx layer (device A). For the CuPc/C60 planar heterojunction devices, the only difference that results in the different dependence of the photocurrent excited by monochromatic light illumination at two wavelengths on external bias is that the light-induced excitons are formed in different layer, i.e., either CuPc or C60 layer. Therefore, the different dependence of photocurrent is inevitably derived from the different behavior of the excitons in the CuPc and C60 layer. In OSCs, three processes are involved for the exciton-produced photocurrent (as shown in Figure 3):

not included in the photocurrent measured. Thus, the photocurrent consists of photoinduced charge carriers only. In the case of CuPc/C60 heterojuction device A, photoinduced carriers are mainly generated by the dissociation of excitons at the D/A (donor/acceptor) interface. Since the intensity of the incident light was unchanged for the duration of the applied bias measurement on a single device, the generation rate of excitons was invariable throughout the measurement. Thus, the number of excitons reaching the D/A interface per unit time and the rate of charge carriers generation at the D/A interface were also invariable. The electrons and holes generated at the D/A interface were driven by the electric field to the cathode and anode, respectively. With increase of the negative bias, the electrons and holes were drifted to the electrodes with increasing speed and this would result in increasing carrier collection. For any time interval, however, once the negative bias increased to such a value that all the carriers were collected by the electrodes, further increasing the negative bias cannot result in any more carriers collected. Therefore, the photocurrent will eventually reach saturation. In brief, the photocurrent originating from the interfacial dissociation of excitons at the D/A interface will eventually reach saturation with increase of negative bias. This result has been widely reported2,4,7,8,16 and widely adopted as the basic assumption for the existing OSCs circuit model.6,17−19 The present experiments show that the photocurrent originating from the excitons in the CuPc layer is quickly saturated, consistent with the previous experimental and theoretical results reported. But the photocurrent originating from the excitons in the C60 layer is very sensitive to the external electric bias. With increase of negative bias, the photocurrent increases continuously and significantly. The inset in Figure 2a shows the photocurrent ratio of that produced by 480 nm incident light (corresponding to C60 absorption) to that produced by 620 nm incident light (corresponding to the CuPc absorption) as a function of applied bias. It can be seen that the ratio increases continuously and significantly with increase of negative bias from 0.85 at 0 V to 1.80 at −5 V. This result clearly shows the photocurrent of 480 nm incident light increases significantly compared to that of 620 nm incident light. Such abnormal behavior of excitons in C60 is completely different from that reported previously. This indicates that the excitons in CuPc and C60 layers would behave differently under external bias. In order to further study the different behaviors of excitons in the two different materials under negative bias, a coevaporated device, device B, was fabricated. The structure of device B is ITO/MoOx(4 nm)/CuPc:C60 (60 nm)/BPhen(6 nm)/Al, in which the mixed CuPc/C60 layer was coevaporated with the weight ratio of 1:1. In Figure 2b, the solid squares and filled circles represent the photocurrent of the device under 480 and 620 nm light illumination as a function of external bias, respectively. It can be seen that with increase of negative bias both of the photocurrents increase similarly and finally approach saturation. No obvious difference can be seen for the general trend of the two curves. The inset in Figure 2b shows the photocurrent ratio of that produced by 480 nm incident light (corresponding to C60 absorption) to that produced by 620 nm incident light (corresponding to the CuPc absorption) as a function of applied bias. It can be seen that the ratio nearly keeps unchanged with increase of negative bias from 0.56 at 0 V to 0.58 at −4 V. This indicates that in the coevaporated device the dependence of the photocurrent

Figure 3. Three processes involved in the photocurrent generation from excitons. Black arrows represent the processes relevant to excitons in the CuPc and C60 layers, respectively, and red arrows represent the steps irrelevant to where the excitons are generated.

(1) diffusion of excitons toward the D/A interface, (2) dissociation of excitons into free carriers at the D/A interface, and (3) collection of carriers by electrodes. The dependence of photocurrent on external electric bias must be an effect of the electric field on these processes. On the other hand, any influence of the electric field on these processes will be reflected by the photocurrent. In the case of constant illumination intensity, the total rate of exciton generation in the device is constant. Because of the electric neutrality of exciton, applied external electric field cannot directly drive the exciton movement. This means the process of the exciton diffusion toward the D/A interface is not dependent on the applied electric field. Thus, the applied electric field cannot directly change the number of excitons reaching the D/A interface per unit time and cannot directly affect process 1. Process 2 (interfacial dissociation) and process 3 (carrier collection) are directly affected by the external electric field. Process 2 consists of two steps: when an exciton reaches the D/ A interface, an electron−hole bound state, called charge transfer (CT) state, will be formed at a very fast rate and then dissociate into free electron and hole by the electric 11930

dx.doi.org/10.1021/jp501078f | J. Phys. Chem. C 2014, 118, 11928−11934

The Journal of Physical Chemistry C

Article

field.3,20 It should be noted that regardless of where the exciton comes from, the formation of CT state means an electron is located on acceptor and a hole on donor eventually. Excitons from different organic layers would form same CT state and dissociate into free electrons and holes. Therefore, the electrons and holes generated by the interfacial dissociation are identically the same, no matter where the excitons come from, either donor or acceptor. That means, in process 2, it cannot be distinguished that the electrons and holes originating from the excitons in CuPc and that in C60. In process 3, these electrons and holes are driven by the electric field and collected by the electrodes, respectively. Since the interfacial dissociations are the same, the collection of charge carriers would depend on the electric field but would not depend on where the excitons are generated. Processes 2 and 3 are indeed dependent on the electric field but do not depend on where the exciton is generated. In summary, wherever the excitons are generated with increase of negative bias the photocurrent originating from excitons in the CuPc and C60 layers will eventually reach saturation with the same trend. The experimental results shown in Figure 2b are completely consistent with the above analysis. With increase of negative bias both the photocurrents reach saturation eventually with the same trend. However, in Figure 2a, the photocurrent originating from the excitons in CuPc layer is quickly saturated, but the photocurrent originating from the excitons in C60 layer increases abnormally. Above analysis applies only to the behavior of the photocurrent originating from the excitons in CuPc layer but could not explain that in the C60 layer. The only difference between these two photocurrents in Figure 2a is where the excitons are generated. Once the excitons reach the D/A interface, the dissociation of the excitons either from donor or from acceptor is identically the same. The difference of these two photocurrents cannot be explained by the above three processes. Therefore, the different results must be derived from other physical processes of excitons in CuPc and C60 layers before reaching the D/A interface. It also indicates that the behavior of the excitons in the C60 layer is significantly different from that in the CuPc layer. In order to explain the photocurrent enhancement originating from the excitons in C60 layer, a mechanism has been suggest that exciton−electron interaction causes injection of holes from cathode, as shown in Figure 4. The energy level diagram of device A is shown in Figure 4a. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of C60 are 6.4 and 4.1 eV, respectively.21 The work function of Al is 4.2 eV. Although there is a 2.2 eV difference between HOMO of C60 and work function of Al, it has been reported that the C60 layer can significantly enhance the hole injection ability of electrode.22−25 The dark current of devices A and C without illumination is shown in Figure 5. The black solid squares and red filled circles represent the dark current of devices A and C, respectively. The structure of device C is ITO/CuPc (20 nm)/C60 (40 nm)/Al. The leakage of device can be seen from the reverse current in Figure 5. The values of reverse current of devices A and C at −1 V are −0.030 and −11.14 mA/cm2, respectively. Compared with device C, the leakage current of device A is rather small. Since the main difference between these two devices is that device C has no Bphen layer at the C60 /Al interface, the leakage current of device C originates from the hole injection at C60 /Al interface. This result clearly shows that holes can be injected from Al to C60 and the inserting of a buffer Bphen layer

Figure 4. Mechanism of hole injection from cathode based on the energy level realignment and tunneling model. Parts a and b show the energy level diagram of device A under negative bias without and with illumination. Parts c and d show the energy level diagram of device D and E under negative bias with illumination. The tunneling barrier of hole is represented by the shaded area.

Figure 5. Dark current of device A and device C. The structure of device C is ITO/CuPc/C60/Al. The main difference between these two devices is that device C has no Bphen layer.

between C60 and electrode has a great effect on the hole injection. With the inserting of the buffer layer, the structure organic layer/buffer/electrode was formed, as shown in Figure 4. According to the interfacial tunneling model, the holes must tunnel through the barrier represented by the shaded area in Figure 4. Therefore, the injection current depends on the height of tunneling barrier.26−28 The energy diagram of device A without illumination is shown in Figure 4a. Because of the large tunneling barrier, the hole injection current from cathode of device A without illumination is very small. But reducing the height of tunneling barrier will enhance the injection current. According to the interfacial tunneling model, the height of tunneling barrier depends on three factors: charge accumulation in the organic layer (shown in Figure 4b),26 thickness of the buffer layer (shown in Figure 4c),27 and work function of the electrode (shown in Figure 4d).28 This indicates that when the device A is illuminated, the charges in C60 could also have an effect on the hole injection at cathode interface. Therefore, the hole injection from cathode in device A under illumination should not be ignored. The special spherical structure of fullerene may make the σ−π orbital hybridization stronger than that in general organic 11931

dx.doi.org/10.1021/jp501078f | J. Phys. Chem. C 2014, 118, 11928−11934

The Journal of Physical Chemistry C

Article

materials.29 Nearly all of the photoinduced singlet excitons in C60 will transfer into triplet excitons through intersystem crossing.9,30,31 It is reported that the lifetime of the triplet exciton in C60 is 0.41 ms, about 4 orders of magnitude longer than that of the triplet exciton in CuPc.10,32,33 Therefore, when device A is illuminated by 480 nm light, a large number of triplet excitons will be accumulated in the C60 layer. The electrons are mainly generated at the D/A interface and transport through the C60 layer before collected by the cathode. During this transport process, the interaction between triplet excitons and electrons should not be ignored. The reduction of the electron mobility due to the triplet excitons−electrons interaction has been reported.34,35 This effect is significantly enhanced due to the large number of the accumulated triplet excitons in C60 layer. According to the space-charge-limited current (SCLC) theory, the reduction of the electron mobility will cause that more electrons accumulate in C60 layer,36,37 as shown in Figure 4b. Therefore, the C60 layer will be negatively charged, leading to band bending due to these accumulated electrons.36,37 According to the interfacial tunneling model, this electron accumulation in C60 layer will reduce the injection barrier and enhance the ability of hole injection from Al cathode into C60 layer.26,38 Therefore, the number of injected holes will also increase with the negative bias, which causes the photocurrent enhancement originating from excitons in the C60 layer. On the other hand, the lifetime of triplet exciton in CuPc is about 35 ns, about 4 orders of magnitude shorter than that of the triplet exciton in C60.33 According to the exponential decay, if it is assumed that the generation rate of triplet excitons in CuPc and C60 layers are same, the concentration of triplet excitons in CuPc should be about 4 orders of magnitude smaller than that in C60 due to the long lifetime of triplet in C60. Thus, the triplet excitons−electrons interaction effect in CuPc layer can be ignored, compared with that in C60 layer, leading to that no enhancement of electron injection occurs at anode/ CuPc interface. Therefore, the photocurrent originating from the excitons in CuPc layer is quickly saturated with increase of negative bias. For device B where CuPc and C60 molecules are mixed together, the excitons on C60 molecules can easily be dissociated by adjacent CuPc molecules. Therefore, no triplet exciton accumulation and no triplet exciton−electron interaction occur in device B. Thus, there would be no difference between the general varying trends of the photocurrent with bias under 480 and 620 nm light illumination. In summary, the experimental results in Figure 2 can be explained by this mechanism. According to the interfacial tunneling model, the injection current is sensitive to the thickness of buffer layer.27 To confirm the photoinduced injection enhancement mechanism, device D has been prepared with the structure ITO/CuPc (25 nm)/C60 (40 nm)/Bphen (20 nm)/Al, where the thickness of Bphen layer is increased to 20 nm. The energy diagram of device D is shown in Figure 4c. The ratio of the photocurrent under 480 and 620 nm illumination of device A and device D varying with the applied voltage is shown in Figure 6, wherein the black solid squares and red filled circles represent the results of devices A and D, respectively. These curves represent the enhancement effect of photocurrent originating from the excitons in the C60 layer compared with that in CuPc layer. It can be seen that the ratio of devices A and D increases with increasing negative voltage. For device A, the ratio increase from 0.85 at 0 V bias to 1.80 at −5 V bias. For device D, the ratio increase from 0.82 at

Figure 6. Ratio of the photocurrent under 480 and 620 nm illumination versus applied voltage. The only difference between device A (black solid squares) and device D (red filled circles) is the thickness of Bphen layer with the thickness of 10 and 20 nm, respectively. The inset shows the photocurrent of device D versus applied bias.

0 V bias to 1.22 at −5 V bias. It can be seen that the increment of the ratio of device D is less than device A. The only difference between the device D and device A is the thickness of the Bphen layer. The thickness of Bphen layer of device A and D is 10 and 20 nm, respectively; i.e., the Bphen layer of device D is thicker than that of device A. According to the tunneling model, the increase of the thickness of Bphen layer enlarges the tunneling barrier of hole injection. Therefore, increase of the thickness of Bphen layer can weaken the effect of the photocurrent enhancement originating from the excitons in the C60 layer. Thus, it can be concluded that this photocurrent enhancement effect is an interfacial effect associated with the cathode, rather than an inherent bulk effect. According to the interfacial tunneling model, the injection current is also sensitive to the work function of the cathode.28 In order to further confirm the photoinduced injection enhancement mechanism, device E has been prepared with the structure ITO/CuPc (25 nm)/C60 (40 nm)/Bphen (20 nm)/MoOx(15 nm)/Al. The energy diagram of device D is shown in Figure 4d. Figure 7 shows the photocurrent of device

Figure 7. Photocurrent of devices A and E under the 480 nm light illumination versus bias voltage. The black solid squares and blue filled circles represent the photocurrent of devices A and E, respectively.

A and E under the 480 nm light illumination as a function of bias voltage. The black solid squares and blue filled circles represent the photocurrent of device A and E, respectively. As the bias voltage turns from positive to negative, both of the photocurrent increase rapidly and both curves show an inflection point where the slope significantly changes. The inflection point of device A and E appears at 0 and −0.5 V, respectively. Both the photocurrents of devices A and E 11932

dx.doi.org/10.1021/jp501078f | J. Phys. Chem. C 2014, 118, 11928−11934

The Journal of Physical Chemistry C

■

increase continuously with the increasing of negative bias voltage beyond the inflection point, but the slope of the curve of device E is greater than that of device A. The absolute value of the photocurrent of device E increases more rapidly and surpasses that of device A with the increase of negative bias voltage. The only difference between device A and E is a MoOx layer inserted between Bphen and Al in device E. Compared with the 4.2 eV work function of Al, the work function and electron affinity of MoOx are 6.86 and 6.7 eV, respectively.39 According to the tunneling model, increase the work function of cathode will reduce the tunneling barrier.28 The inserted MoOx layer will greatly enhance the capability of hole injection from the Al cathode. If the main reason for the photocurrent enhancement effect is indeed the hole injection from the cathode, the photocurrent enhancement effect will be further enhanced due to the inserting MoOx layer. The results in Figure 7 show that the slope of the curve of device E is greater than that of device A and the absolute value of the photocurrent of device E increases more rapidly and eventually surpasses that of device A, when the bias voltage passes over the inflection point. This result indicates that the inserted MoOx layer effectively enhance the photocurrent enhancement effect. Therefore, this result further confirms that the photocurrent enhancement effect indeed results from the hole injection from cathode, rather than a bulk effect. In summary, the main reason for the photocurrent enhancement effect originating from excitons in C60 layer is the triplet exciton−electron interaction induced hole injection from cathode.

CONCLUSION In present experiments, the photocurrent originating from the excitons in CuPc layer is quickly saturated with increase of negative bias. On the contrary, the photocurrent originating from the excitons in the C60 layer will not be saturated but increase continuously and significantly with negative bias. This abnormal enhancement shows that the behavior of excitons in the C60 layer is completely different from that in the typical donor material CuPc. The main reason for this effect is suggested as the triplet exciton−electron interaction in C60 causes injection enhancement of holes from cathode. More experiments designed have confirmed this mechanism. This result also suggests that performing the EQE measurement should be more careful because changing the applied bias voltage will significantly affect the shape of EQE spectra. ASSOCIATED CONTENT

S Supporting Information *

Photocurrent of planar device with MoOx under the monochromatic light illumination at the wavelength of 480 and 620 nm versus applied bias. This material is available free of charge via the Internet at http://pubs.acs.org.

■

REFERENCES

(1) Vandewal1, K.; Tvingstedt, K.; Gadisa, A.; Inganäs, O.; Manca1, J. V. On the Origin of the Open-Circuit Voltage of Polymer-Fullerene Solar Cells. Nat. Mater. 2009, 8, 904−909. (2) Terao, Y.; Sasabe, H.; Adachi, C. Correlation of Hole Mobility, Exciton Eiffusion Length, and Solar Cell Characteristics in Phthalocyanine/Fullerene Organic Solar Cells. Appl. Phys. Lett. 2007, 90, 103515 1−3. (3) Bakulin, A. A.; Rao, A.; Pavelyev, V. G.; van Loosdrecht, P. H. M.; Pshenichnikov, M. S.; Niedzialek, D.; Cornil, J.; Beljonne, D.; Friend, R. H. The Role of Driving Energy and Delocalized States for Charge Separation in Organic Semiconductors. Science 2012, 335, 1340−1344. (4) Kim, J. Y.; Lee, K.; Coates, N. E.; Moses, D.; Nguyen, T.; Dante, M.; Heeger, A. J. Efficient Tandem Polymer Solar Cells Fabricated by All-Solution Processing. Science 2007, 317, 222−225. (5) Chen, G.; Sasabe, H.; Wang, Z.; Wang, X.; Hong, Z.; Yang, Y.; Kido, J. Co-Evaporated Bulk Heterojunction Solar Cells with > 6.0% Efficiency. Adv. Mater. 2012, 24, 2768−2773. (6) Xue, J.; Uchida, S.; Rand, B. P.; Forrest, S. R. Asymmetric Tandem Organic Photovoltaic Cells with Hybrid Planar-Mixed Molecular Heterojunctions. Appl. Phys. Lett. 2004, 85, 5757−5759. (7) Luhman, W. A.; Holmes, R. J. Investigation of Energy Transfer in Organic Photovoltaic Cells and Impact on Exciton Diffusion Length Measurements. Adv. Funct. Mater. 2011, 21, 764−771. (8) Peumans, P.; Yakimov, A.; Forrest, S. R. Small Molecular Weight Organic Thin-Film Photodetectors and Solar Cells. J. Appl. Phys. 2003, 93, 3693−3723. (9) Arbogast, J. W.; Darmanyan, A. P.; Foote, C. S.; Rubin, Y.; Diederich, F. N.; Alvarez, M. M.; Anz, S. J.; Whetten, R. L. Photophysical Properties of C60. J. Phys. Chem. 1991, 95, 11−12. (10) Wasielewski, M. R.; O’Nei, M. P.; Lykke, K. R.; Pellin, M. J.; Gruen, D. M. Triplet States of Fullerenes C60 and C70: Electron Paramagnetic Resonance Spectra, Photophysics, and Electronic Structures. J. Am. Chem. Soc. 1991, 113, 2774−2776. (11) Stübinger, T.; Brütting, W. Exciton Diffusion and Optical Interference in Organic Donor-Acceptor Photovoltaic Cells. J. Appl. Phys. 2001, 90, 3632−3641. (12) Reynaert, J.; Arkhipov, V.; Heremans, P.; Poortmans, J. hotomultiplication in Disordered Unipolar Organic Materials. Adv. Funct. Mater. 2006, 16, 784−790. (13) Huang, J.; Yang, Y. Origin of Photomultiplication in C60 Based Devices. Appl. Phys. Lett. 2007, 91, 203505 1−3. (14) Jeong, W.; Lee, Y.; Shim, H.; Kim, T.; Kim, S.; Kim, J. Photoconductivity of C60 as an Origin of Bias-Dependent Photocurrent in Organic Photovoltaics. Adv. Funct. Mater. 2012, 22, 3089− 3094. (15) Chan, M. Y.; Lai, S. L.; Lau, K. M.; Lee, C. S.; Lee, S. T. Application of Metal-Doped Organic Layer Both as Exciton Blocker and Optical Spacer for Organic Photovoltaic Devices. Appl. Phys. Lett. 2006, 89, 63515 1−3. (16) Pandey, R.; Holmes, R. J. Characterizing the Charge Collection Efficiency in Bulk Heterojunction Organic Photovoltaic Cells. Appl. Phys. Lett. 2012, 100, 083303 1−3. (17) Rand, B. P.; Burk, D. P. Offset Energies at Organic Semiconductor Heterojunctions and Their Influence on the OpenCircuit Voltage of Thin-Film Solar Cells. Phys. Rev. B 2007, 75, 115327 1−11. (18) Waldauf, C.; Scharber, M. C.; Schilinsky, P.; Hauch, J. A.; Brabec, C. J. Physics of Organic Bulk Heterojunction Devices for Photovoltaic Applications. J. Appl. Phys. 2006, 99, 104503 1−6. (19) Mazhari, B. An Improved Solar Cell Circuit Model for Organic Solar Cells. Sol. Energy Mater. Sol. Cells 2006, 90, 1021−1033. (20) Sariciftci, N. S.; Smilowitz, L.; Heeger, A. J.; Wudi, F. Photoinduced Electron Transfer from a Conducting Polymer to Buckminsterfullerene. Science 1992, 258, 1474−1476. (21) Park, S. H.; Jeong, J. G.; Kim, H. J.; Park, S. H.; Cho, M. H.; Cho, S. W.; Yi, Y.; Heo, M. Y.; Sohn, H. The Electronic Structure of C60/ZnPc Interface for Organic Photovoltaic Device with Blended Layer Architecture. Appl. Phys. Lett. 2010, 96, 013302 1−3.

■

■

Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (X.H.). Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work is supported by the Ministry of Science and Technology of China and the National Natural Science Foundation of China. 11933

dx.doi.org/10.1021/jp501078f | J. Phys. Chem. C 2014, 118, 11928−11934

The Journal of Physical Chemistry C

Article

(22) Lee, J. Efficient Hole Injection in Organic Light-Emitting Diodes Using C60 as a Buffer Layer for Al Reflective Anodes. Appl. Phys. Lett. 2006, 88, 073512 1−3. (23) Day, S.; Hatton, R.; Chesters, M.; Willis, M. The Use of Charge Transfer Interlayers to Control Hole Injection in Molecular Organic Light Emitting Diodes. Thin Solid Films 2002, 410, 159−166. (24) Wang, M. L.; Song, Q. L.; Wu, H. R.; Ding, B. F.; Gao, X. D.; Sun, X. Y.; Ding, X. M.; Hou, X. Y. Small-Molecular Organic Solar Cells with C60/Al Composite Anode. Org. Electron. 2007, 8, 445−449. (25) Song, Q. L.; Li, C. M.; Wang, M. L.; Sun, X. Y.; Hou, X. Y. Role of Buffer in Organic Solar Cells Using C60 as an Acceptor. Appl. Phys. Lett. 2007, 90, 071109 1−3. (26) Zhang, S. T.; Zhou, Y. C.; Zhao, J. M.; Zhan, Y. Q.; Wang, Z. J.; Wu, Y.; Ding, X. M.; Hou, X. Y. Role of Hole Playing in Improving Performance of Organic Light-Emitting Devices with an Al2O3 Layer Inserted at the Cathode-Organic Interface. Appl. Phys. Lett. 2006, 89, 043502 1−3. (27) Sun, Z.; Ding, B.; Wu, B.; You, Y.; Ding, X.; Hou, X. LiF Layer at the Interface of Au Cathode in Organic Light-Emitting Devices: A Nonchemical Induced Carrier Injection Enhancement. J. Phys. Chem. C 2012, 116, 2543−2547. (28) Zhao, J.; Zhan, Y.; Zhang, S.; Wang, X.; Zhou, Y.; Wu, Y.; Wang, Z.; Ding, X.; Hou, X. Mechanisms of Injection Enhancement in Organic Light-Emitting Diodes through Insulating Buffer. Appl. Phys. Lett. 2004, 84, 5377−5379. (29) Huertas-Hernando, D.; Guinea, F.; Brataas, A. Spin-Orbit Coupling in Curved Graphene, Fullerenes, Nanotubes, and Nanotube Caps. Phys. Rev. B 2006, 74, 155426 1−15. (30) Guldi, D. M.; Prato, M. Excited-State Properties of C60 Fullerene Derivatives. Acc. Chem. Res. 2000, 33, 695−703. (31) Arbogast, J. W.; Foote, C. S. Photophysical Properties of C70. J. Am. Chem. Soc. 1991, 113, 8886−8889. (32) Cho, H. S.; Yang, S. I.; Kim, S. K.; joo Shin, E.; Kim, D. Comparative Study of Exciton Relaxation Dynamics in C60 and C70 Solid. J. Phys. Chem. B 1999, 103, 6504−6508. (33) Mcvie, J.; Sinclair, R.; Truscott, T. Triplet-States of Copper and Metal-Free Phthalocyanines. J. Chem. Soc., Faraday Trans. 1978, 74, 1870−1879. (34) Desai, P.; Shakya, P.; Kreouzis, T.; Gillin, W. P. Magnetoresistance in Organic Light-Emitting Diode Structures under Illumination. Phys. Rev. B 2007, 76, 235202 1−5. (35) Wagemans, W.; Koopmans, B. Spin Transport and Magnetoresistance in Organic Semiconductors. Phys. Status Solidi B 2011, 248, 1029−1041. (36) ROSE, A. Space-Charge-Limited Currents in Solids. Phys. Rev. 1955, 97, 1538−1544. (37) Blom, P. W. M.; Mihailetchi, V. D.; Koster, L. J. A.; Markov, D. E. Device Physics of Polymer: Fullerene Bulk Heterojunction Solar Cells. Adv. Mater. 2007, 19, 1551−1566. (38) Zhang, S. T.; Ding, X. M.; Zhao, J. M.; Shi, H. Z.; He, J.; Xiong, Z. H.; Ding, H. J.; Obbard, E. G.; Zhan, Y. Q.; Huang, W.; Hou, X. Y. Buffer-Layer-Induced Barrier Reduction: Role of Tunneling in Organic Light-Emitting Devices. Appl. Phys. Lett. 2004, 84, 425−427. (39) Kröger, M.; Hamwi, S.; Meyer, J.; Riedl, T.; Kowalsky, W.; Kahn, A. Role of the Deep-Lying Electronic States of MoO3 in the Enhancement of Hole-Injection in Organic Thin Films. Appl. Phys. Lett. 2009, 95, 123301 1−3.

11934

dx.doi.org/10.1021/jp501078f | J. Phys. Chem. C 2014, 118, 11928−11934