Electron transport properties in ZnO nanowires/poly(3-hexylthiophene) hybrid nanostructure

Materials Chemistry and Physics 124 (2010) 1239–1242

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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys

Electron transport properties in ZnO nanowires/poly(3-hexylthiophene) hybrid nanostructure Ke Cheng a,b , Gang Cheng a , Shujie Wang a , Dongwei Fu a , Bingsuo Zou b , Zuliang Du a,∗ a b

Key Lab for Special Functional Materials of Ministry of Education, Henan University, Kaifeng 475004, People’s Republic of China Institute of Physics, Chinese Academy of Sciences, Beijing, 100190, People’s Republic of China

a r t i c l e

i n f o

Article history: Received 14 January 2010 Received in revised form 28 April 2010 Accepted 18 August 2010 Keywords: Semiconductors Electrical properties Nanostructures Transport properties

a b s t r a c t The ZnO nanowires (NWs) array/poly(3-hexylthiophene) (P3HT) hybrid prototype device was fabricated. An ultraviolet (UV) light of  = 350 nm is used to investigate the photo-electric properties of the ZnO NWs array and hybrid structure. In this way, we can avoid the excitation of P3HT, which can give us a real electron transport ability of ZnO NWs itself. Our results demonstrated a higher and faster photo-electric response of 3 s for the hybrid structure while 9 s for the ZnO NWs array. The surface states related slow photo-electric response was also observed for them. The charge transfer mechanism and the influence of surface states were discussed. The current work provides us profound understandings on the electron transport ability of ZnO NWs array in a working hybrid polymer solar cell, which is crucial for optimizing the device performance. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Polymer solar cells have attracted a great interest in developing the low-cost, large-area, mechanically flexible photovoltaic devices [1–3]. However, the electron mobilities are extremely low for many conjugated polymers, typically below 10−4 cm2 V−1 s−1 , due to the presence of ubiquitous electrons traps such as oxygen [4]. This disadvantage is partly resolved by introducing inorganic semiconductor nanocrystals with higher electron mobility, such as CdSe or TiO2 . This strategy in polymer solar cells is so-called bulk heterojunction (BHJ). In this way, excitons do not need to travel long distances to reach the donor/acceptor interface, and charge separation can take place throughout the whole depth of the photoactive layer. After the charge separation, the electrons can be extracted adopting an interparticle hopping mechanism. However, the hopping charge transport is inefficient due to the increased recombination risks [5]. Therefore, a device structure using nanorods or nanowires (NWs) array to provide direct and ordered pathways for photogenerated electrons has been proposed [6]. Recently, Olson et al. fabricated a ZnO nanorods/P3HT BHJ solar cell show a power conversion efficiency of 0.28% [7]. Greene and Yang et al. built an ordered ZnO–TiO2 core–shell nanorods array/P3HT solar cell with an efficiency of 0.29% [8]. Soon afterward, Kuo et al. fabricated a TiO2 nanorods array/P3HT BHJ solar

∗ Corresponding author. Tel.: +86 378 3881358; fax: +86 378 3881358. E-mail addresses: [email protected], [email protected] (Z. Du). 0254-0584/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2010.08.064

cell with a well controlled diameter, which demonstrated a power conversion efficiency of 0.51% [6]. As can be seen, the efficiency is improved steadily through optimizing the device structure and the P3HT infiltration process. Nevertheless, the devices perform poorly contrast to the P3HT/(6,6)-phenyl C61 butyric acid methyl ester (PCBM) BHJ solar cells with an efficiency exceeding 5% [9]. This is mainly due to the lack of profound understandings on the electron transport of ZnO NWs array, which is crucial for optimizing the device performance. Despite the abundant electron transport researches on single ZnO NW [10–13], experimental data of the electron transport in a working hybrid polymer solar cell is not available. In this paper, we fabricated the P3HT/ZnO NWs hybrid prototype device. The I–V and time-depend photocurrent was measured for both the pristine ZnO NWs array and the hybrid structure. An ultraviolet (UV) light of  = 350 nm was used to investigate the photo-electric properties of the pristine ZnO NWs array and the hybrid structure in our measurement. In this way, we can avoid the excitation of P3HT, which can give us a real electron transport ability of ZnO NWs itself in a working hybrid polymer solar cell. 2. Experimental details The vertical aligned ZnO NWs arrays were grown on the indium tin oxide coated glass (ITO) using a simple two-step process [14,15]. The P3HT was spin-coated (800 rmp, 10 s) on the surface of the ZnO NWs from chloroform solution with a concentration of 1 g L−1 . The sample was then heated at a temperature of 150 ◦ C under vacuum for 20 min to facilitate the infiltration of the P3HT. The sample was allowed to cool naturally for about 2 h to help recrystallize the P3HT. The morphologies of the ZnO NWs array and the hybrid structure were characterized by scanning electron microscopy (SEM, JSM-5600). The I–V curves were measured by

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Fig. 1. The schematic diagram for our I–V measurements.

semiconductor characteristic measurement system (Keithley 4200 SCS). The indium tin oxide coated glass was covered directly on the top of the samples used as the upper electrode during the I–V measurement as shown in Fig. 1. And both ends were tightly fixed by two crocodile clip to ensure a good electric contact. I–V and PL measurements were made at 300 K and in air. An ultraviolet (UV) light of  = 350 nm is used for our photocurrent measurements.

3. Results and discussion Fig. 2a and b shows the top-view and cross-sectional SEM images of ZnO NWs array. We can see that the synthesized ZnO NWs have a tendency to grow perpendicular to the substrate surface, which are 5 ␮m long and 60–100 nm in diameter. Fig. 3 is the XRD pattern of ZnO NWs array, which showed a remarkably enhanced (0 0 0 2) reflection peak with a hexagonal crystal structure. This indicates the ZnO NWs are grown along the c-axis [16]. To elucidate the structure of the ZnO NWs array/P3HT hybrid film, top-view and cross-sectional SEM images are also taken as shown in Fig. 2c and d. As clearly seen from the cross-sectional SEM image, the diameter of ZnO NW increased slightly, which illustrate the ZnO NWs are coated by a thin layer of P3HT. Fig. 4 shows the typical I–V characteristic of the ZnO NWs array and the hybrid structure. I–V measurements are made after illumination 120 s by 350 nm light. The difference between the dark current and photocurrent can be clearly observed as shown in Fig. 4. There is a low dark current for both the pristine ZnO NWs array and the hybrid structure, which implies a low carrier concentration under dark. As we know, the charge carrier mobility in organic

Fig. 3. The XRD pattern of ZnO NWs array.

semiconductors is generally much lower than that of their inorganic counterpart. So we can observe the dark current of hybrid structure is slightly lower than the pristine ZnO NW array. This is due to the influence of P3HT, which coated on the surface of ZnO NWs as seen from the SEM image shown in Fig. 2d. Upon the illumination of the UV light, a significant increase of the current is observed for both them. The current increases 2.5 times of magnitude for pristine ZnO NWs array. Meanwhile, the current increases about 10 times of magnitude for the hybrid structure. In fact, the photocurrent enhancement has been observed previously when blend the P3HT with inorganic semiconductor. Usually, the photocurrent enhancement for such system under visible light illumination is attributed to the charge transfer process between P3HT and the inorganic semiconductor [5,17]. In this situation, visible light is absorbed by the P3HT. The free electrons can inject to the inorganic semiconductor causing a photocurrent increase. However, an UV light of  = 350 nm is used for the photocurrent measurement

Fig. 2. The SEM images of ZnO NWs array structure. (a) Top-view image of ZnO NWs array. (b) Cross-sectional image of ZnO NWs array. An inset image is the PL spectrum of ZnO NWs array. (c) Top-view image of hybrid structure. (d) Cross-sectional image of hybrid structure.

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Fig. 4. The typical I–V curves under dark and illumination for the ZnO NWs array and the hybrid structure.

in our case. And we know that there is almost no absorption for the P3HT at the UV region [4]. Therefore, the larger photocurrent increase in our P3HT/ZnO NWs array hybrid structure cannot be attributed to the mechanism described above. Fig. 5a and b shows the photo-electric response under continuous illumination and the photocurrent decay process when remove the light for the ZnO NWs array and the hybrid structure. The applied bias voltage is 1 V. We observed that there is a fast photocurrent increase within 0.2 s as shown in the inset in Fig. 5a, and then reached the maximum in 120 s for the pristine ZnO NWs. While for the hybrid structure, there is a fast increase within 0.1 s as can be seen from the inset in Fig. 5b, and then reached the maximum in 40 s. This indicates that there are two different photo-electric response processes for them. As we know, the electron–hole pair generation by photon energy is the fast process, this would be responsible for the rapid photo-electric response [18]. While, the slow process can be attributed to the desorption of oxygen [19]. In fact, similar observations of the surface states related slower photoelectric response have been reported previously [20–23]. Actually, there are abundant surface states on the surface of sol–gel derived ZnO NWs, which can be seen from the PL spectrum shown in the inset in Fig. 2b and our previous work [15]. The UV emission band in Fig. 2b can be assigned to the well-known recombination of excitons. While, the broad green emission related to the PL of surface states. Under UV illumination, the absorbed oxygen will be photodesorbed by capturing photogenerated holes and increase the conductivity of ZnO NWs: h+ + O2 − (ad) → O2 (g) Since the photodesorbed process is slower than the bulk related photo-electric response [19,24], then a slow increase of photocurrent observed under the continuous illumination. On the other hand, we should notice that the photo-electric response of hybrid structure is faster than that of pristine ZnO NWs. We think the fast photo-electric response for the hybrid structure has a close relation to the charge transfer process between the P3HT and ZnO NWs. Electron–hole pairs can be produced in the ZnO NWs when illuminated by the UV light. They can diffuse to the P3HT/ZnO interface and separate into free electron and hole. Since the P3HT is favorable for the holes injection and transport. So the holes can inject to the P3HT which leads to a fast photo-electric response for the hybrid structure. In other aspects, the holes captured process can also increase the conductivity of ZnO NWs. This would be responsible

Fig. 5. (a) Time-dependent photocurrent of ZnO NWs array under continuous illumination and the photocurrent decay process when the UV light is removed. The inset is the magnified plot of the time interval between −0.5 and 0.5 s for ZnO NWs array under continuous illumination. (b) Time-dependent photocurrent of hybrid structure under continuous illumination and the photocurrent decay process when the UV light is removed. The inset is the magnified plot of the time interval between −0.5 and 0.5 s for hybrid structure under continuous illumination.

for the larger photocurrent enhancement for the hybrid structure as observed in Fig. 4. The two different photo-electric response processes can also be observed from the photocurrent decay measurements as shown in Fig. 5a and b. When the UV illumination was removed, the photocurrent fell from the maximum fast at first stage, and then fall slowly to the initial current value before illumination. The fast decrease indicates the rapid loss of electrons in the conduction band via retrapping the electrons [11]. Meanwhile, the slow decay process can be attributed to the reabsorbed oxygen on the ZnO NWs. The reabsorbed oxygen will trap free electrons from ZnO NWs and decrease its conductivity, and then a slow current decay process can be observed: O2 (g) + e− → O2 − (ad) We should notice that the photocurrent increase time is much shorter than the decay time of the pristine ZnO NWs from. The decay time  d =  f +  s can be obtained by fitting to the two exponentially decaying function in decaying stage, which given by In = A1 exp( − (t/ f )) + A2 exp( − (t/ s )) [25]. This equation reflects the two different decay mechanisms with time constant  f and  s . The values of A1 and A2 , where A1 + A2 = 1, represent weight factors that quantify the relative contribution of fast and

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slow decay to the whole decay process. It can be seen in Fig. 5a and b that the model equation (black dot curve) fits fairly well with the experimental data. For the pristine ZnO NWs, we can get the values of A1 = 0.55 and A2 = 0.45 from Fig. 5a. We can obtain  f-ZnO NWs = 9 s,  s-ZnO NWs = 450 s by fitting the equation. We should notice that the total photocurrent decay time is much longer than the photocurrent increase time for the pristine ZnO NWs, which indicates the chemical readsorption of O2 process is slower than the photodesorption process [24,26]. While for the hybrid structure, the values of A1 and A2 are 0.6 and 0.4, then we can obtain  f-hybrid = 3 s,  s-hybrid = 140 s respectively. Clearly, the photo-electric response of hybrid structure is about thrice faster than the pristine ZnO NWs, which is mainly benefit from the charge transfer process between P3HT and ZnO NWs as discussed above. On the other hand, the P3HT coating process can eliminating the influence of absorbed oxygen, which is also contribute to the rapid photo-electric response. Despite this, we also should notice that the surface states related slow photo-electric response still can be observed for the hybrid structure. These surface states can capture the photogenerated carriers, which is unfavorable for the application of hybrid polymer solar cell. Then, we deduce that further optimization of the sol–gel derived ZnO NWs array before the polymer coating process could improve the electron transport ability of ZnO NWs array, which is crucial for the final performance of the hybrid polymer/nanowires array based solar cells. 4. Conclusions The ZnO NWs array/P3HT Hybrid prototype device was fabricated and the electron transport properties of the pristine ZnO NW array and the hybrid structures were measured. Our results showed that the P3HT coating process resulted in a higher and faster photoelectric response for the hybrid structure, which is benefit from the charge transfer process and the eliminating of adsorbed oxygen. The present work provides us profound understandings on the electron transport of ZnO NWs array in a working hybrid polymer solar cell. In other aspects, we find the surface states are unfavorable to the electron transport of ZnO NWs array. So eliminating the surface states of inorganic counterpart before fabricating into a real inorganic/organic hybrid solar cells is necessary. Acknowledgements This work was supported by National Natural Science Foundation of China (Grant Nos. 10874040, 60906056 and 20773103),

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