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A Simple Approach Low-Temperature Solution Process for Preparation of Bismuth Doped-ZnO Nanorods and Its Application in Hybrid Solar Cells Riski Titian Ginting, Hock Beng Lee, Sin Tee Tan, Chun Hui Tan, Mohammad Hafizuddin B. Hj. Jumali, Chi Chin Yap, Jae-Wook Kang, and Muhammad Yahaya J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b11094 • Publication Date (Web): 14 Dec 2015 Downloaded from http://pubs.acs.org on December 15, 2015

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The Journal of Physical Chemistry

A Simple Approach Low-Temperature Solution Process for Preparation of Bismuth Doped-ZnO Nanorods and Its Application in Hybrid Solar Cells Riski Titian Ginting,*a Hock Beng Lee,b Sin Tee Tan,*b Chun Hui Tan,b Mohd. Hafizuddin Hj. Jumalib, Chi Chin Yapb, Jae-Wook Kanga, and Muhammad Yahayab a

Department of Flexible and Printable Electronics, Chonbuk National University, Jeonju 561-756, Republic of Korea

b

School of Applied Physics, Faculty of Science and Technology, Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Selangor, Malaysia *Corresponding author: [email protected]; [email protected]

Abstract In this report, a simple low-temperature solution processed of bismuth-doped ZnO nanorods (NRs) and poly(3-hexylthiophene) (P3HT) were used as electron acceptor and donor, respectively, in a hybrid inorganic-organic photovoltaic system. Control of the Bi precursor concentration via solution processing (hydrothermal method) plays an important role in altering the morphology, structure, and intrinsic defects of ZnO NRs. Interstitial doping of Bi-Bi2O3 into ZnO (BiZO) NRs results in simultaneous improvement of the open circuit voltage and short circuit current density primarily due to prolonged charge carrier recombination lifetime, increased donor-acceptor interfacial areas with efficient exciton dissociation, and charge carrier mobility. As a result, the power conversion efficiency of the 2 wt% BiZO NRs-P3HT device was significantly enhanced by 55 % compared with that of the pristine device. Overall, our study highlighted the immense potential of BiZO NRs as an excellent electron acceptor for fabrication of hybrid optoelectronic devices. Keywords: ZnO nanorods, bismuth, doping, hydrothermal, solar cells, mobility

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Introduction Organic-based photovoltaic cells have attracted tremendous attention over the past few years as nextgeneration solar cells due to their excellent solution processability, low cost, light weight, and large area fabrication.1-2 In particular, the bulk heterojunction structure composed of an interpenetrating network of polymer donors and electron acceptors has been largely developed in organic solar cells (OSCs).3 Despite this progress, a complex synthesis process and tendency towards oxidization in ambient air have hindered the practical application of these devices. Therefore, much effort has been put forth to replace fullerene with use of metal oxides as an electron acceptor in hybrid inorganic-organic solar cells device (hybrid OSCs).4-5 Among the many other metal oxide materials, ZnO exhibits several advantages, including relative ease of processing, environmental friendliness, good chemical stability, and considerably high electron affinity and mobility,6 and possess non-linear optical properties7. In general, the size, length, and density of ZnO NRs can be easily modified via solutionprocessing, thus making them an ideal structure for optimal charge collection in ZnO-poly(3hexylthiophene-2,5-diyl) (P3HT) photovoltaic cell systems. For instance, the vertical alignment of ZnO NRs provides a direct pathway for electron transport with a higher surface area between donor-acceptor interfaces. However, the photovoltaic performance of ZnO NRs-P3HT devices suffers from charge recombination and trapping, mostly due to the presence of intrinsic surface defect states and unfavorable morphology of the nanorods (interfacial area).8 Therefore, much attention has been focused on improving the photovoltaic performance through optimization of nanorod morphology,9 surface treatment via acoustic vibration,4 surface modification,10-11 and self-assembled monolayers12. Nonetheless, the aforementioned methods require the additional steps of chemical modification and external treatment, which further increases the overall complexity. Recently, incorporation of metal cations by substitution into ZnO NRs via solution processes has resulted in promising outcomes; not only does it appear to be an effective approach for control of the morphology but also simultaneously promotes the suppression of oxygen defects.13-15 Several reports are available on pentavalent heavy metal bismuth (Bi)-doped ZnO


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(BiZO), particularly in applications of field-effect transistors,16 nonlinear optical material,17 and photocatalytic activity18. Thus far, the application of BiZO as an electron acceptor in hybrid OSCs device has not yet been explored. In this work, we show for the first time that Bi can be used as a suitable dopant to tune the morphology of ZnO NRs, leading to significant improvement in the photocurrent density and power conversion efficiency (PCE) of an optoelectronic device composed of ZnO-P3HT solar cells. Another standout point of this study is that it involves only low temperature (< 100 °C) synthesis of BiZO NRs via a one-step hydrothermal method (solution processing) and requires no additional treatment. The results show that the morphology, structure, and intrinsic defects of ZnO NRs are highly dependent on the Bi doping concentration. The fabricated device is composed of an FTO bottom electrode, a thin ZnO seed layer that serves as the growth sites of pristine and BiZO NRs (electron acceptors), coating with P3HT (electron donor) and Ag as a top electrode. It was found that interstitial doping of Bi-Bi2O3 into ZnO can simultaneously enhance the open circuit voltage (Voc) and short circuit current density (Jsc) due to the suppression of oxygen defects, efficient exciton dissociation, and increase of charge carrier mobility, with up to 2 wt% BiZO NRs. As a result, the optimum device exhibited 55 % improvement in PCE compared with the pristine device. Experimental details The ZnO sol-gel was prepared by dissolving equimolar 0.2 M zinc acetate dehydrate (Sigma-Aldrich, ≥99.0%) and ethanolamine (Sigma-Aldrich 99.5%) in 2-methoxyethanol (Sigma-Aldrich, 99.8%) under vigorous stirring at 60 °C for 1 h and aging for 12 h. The sol-gel ZnO was spin-casted three times on precleaned FTO (~7 Ω/sq) glass at a speed of 3000 rpm for 30 s and annealed at 300 °C for 1 h in air. The pristine and BiZO NRs were grown via the hydrothermal method from an aqueous solution containing an equimolar solution of 0.04 M zinc nitrate hexahydrate (Alfa Aesar, 99.0%) and hexamethylenetetramine (Sigma-Aldrich, ≥99.5%) in de-ionized water. Subsequently, a small amount of bismuth nitrate



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pentahydrate (Sigma-Aldrich, >99.99%) was added into the above solution with a concentration from 1 to 3 wt% relative to the zinc nitrate concentration. Prior to the growth process (90 °C for 45 min) in a conventional oven, the growth solution was sonicated for 2 min at room temperature, and white-colored precipitates were clearly observed in all solutions with a Bi precursor. The hybrid OSCs devices were fabricated using regio-regular poly(3-hexylthiophene) purchased from Rieke metals (4002-E). P3HT was dissolved in chlorobenzene with a solution concentration of 35 mg/ml and spin-casted onto as-grown pristine and BiZO NRs to obtain an average thickness of 200 nm. All samples were annealed at a temperature of 140 °C for 5 min under a nitrogen atmosphere. Without hole transporting layer, an Ag layer with a thickness of 150 nm was deposited using DC sputtering, and low concentrations of Ar (40 sccm) and O2 gas (1 sccm) were introduced during the sputtering process. The surface morphology of the ZnO NRs with various Bi concentrations ranging from 0 to 3 wt% was characterized by field emission scanning electron microscopy (FESEM, Supra 55V VP). The crystal structures and effective thicknesses of the ZnO NRs were investigated using X-ray diffraction (XRD, Bruker AXS D8 Advance) and surface profilometer (Veeco M6) instruments. The chemical and surface elements of the ZnO NRs were investigated by X-ray photoelectron spectroscopy (Scanning XPS Microprobe, PHI Quantera II) with a monochromatic Al Kα radiation source at room temperature. The steady-state photoluminescence (PL) spectra of the ZnO NRs and ZnO NRs/P3HT films on FTO substrates were collected at excitation wavelengths of 300 and 472 nm, respectively, using an Edinburg FLS920 spectrophotometer. The surface potential images of the ZnO NRs/P3HT films were collected using a Kelvin probe force microscope (KPFM) (NT-MDT Ntegra Prima) under dark and illuminated (10 mWcm-2) conditions via a halogen lamp (Quartzline, 150 W) in ambient air. A similar procedure can be found in our previous report.14 Atomic force microscopy (AFM) topography and conductivity-AFM (CAFM) (NT-MDT Ntegra Prima) measurements were performed in the dark on P3HT film with an underlying layer of pristine and BiZO NRs using tapping-mode CSG10/TiN tips. A low force constant


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was used, in the range of 0.1 - 0.5 N/m, to avoid direct contact between the tips and sample surface, and a relatively low bias voltage of + 0.5 V was applied to the tip for all samples. The J−V characteristics of the devices were measured using a Keithley 237 SM under the illumination of a solar simulator (Newport 96000, 150 W) at 100 mW cm−2 equipped with an AM 1.5 G filter and under dark conditions. The device performance was tested at relatively high RH ~ 80 % conditions. The charge extraction via linearly increasing voltage (CELIV) technique was performed under dark conditions using a Siglent SDG 1020 function generator to apply a ramp voltage pulse with an adjustable bias voltage to obtain different electric field conditions. Simultaneously, a digital oscilloscope (Siglent SDS 1302CFL) was used to record the resulting dynamic response, and the charge carrier extraction of all devices was recorded from the load of a 50 Ω resistor placed in series with the device. A constant offset voltage of 0.4 V was applied to the device in this study to compensate the influence of an internal electric field such that the charge carriers were prevented from leaving the device prior to charge extraction and recombination.19 Furthermore, the RC time constant was held at 80 µs to confirm that the current fully reached the capacitive step in which no carrier injection from the contacts occurs. In addition, transient photovoltage measurements were performed in which the devices were characterized under open circuit condition with illumination from a solar simulator (Newport 96000, 150 W). A small perturbation of Voc (< 20 mV)20 was generated by a pulse from a green LED 505 nm (Thorlabs) with a pulse width of 100 µs, and the decay of the photovoltage was recorded by a high impedance digital oscilloscope. The curves for all samples were fitted with a single exponential decay function to estimate the charge recombination lifetime. All device preparations, fabrications, and measurements were conducted under ambient conditions.

Results and discussion Figure 1 presents the top-view FESEM images of the surface morphologies of pristine and Bi-doped ZnO (BiZO) NRs with different doping concentrations. It can be clearly observed that all of the samples



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exhibit hexagonal ZnO NRAs grown vertically on the FTO substrate. The number of nanorods per unit area (density) and average diameter of the sample were estimated from the FESEM images and were found to be highly dependent on the Bi concentration. As the Bi concentration increased, the average diameter of ZnO NRs decreased slightly from 26 to 22 nm. It is also interesting to note that the diameter of nanorods gradually decreased from the bottom to the top end, thus leading to the formation of conelike shaped ZnO NRs with altered growth orientations at higher Bi concentrations. Evidently, the number of cone-like shaped nanorods is more dominant than the nanorod structure itself, with a wider separation between the neighboring top ends of each rod, as shown in Figure 1(d). As a result, the density of ZnO nanorods significantly decreased from 320 (pristine) to 224 rods µm-2 (2 wt% Bi) and further decreased to only 104 rods µm-2 for 3 wt% BiZO NRs. Moreover, as the Bi concentration increased, the average effective thicknesses of ZnO NRs film drastically increased from 180 to 320 nm. This finding suggests that pentavalent heavy metal Bi can facilitate the formation of longer nanorods, which play an important role in the tuning of structural properties and morphologies of ZnO NRs. Although the density of the nanorods decreased, the actual aspect ratio of the nanorods (the ratio between the average effective thickness and diameter) consistently increased with the Bi concentration from 6 to 14, which indicates that the non-polar surface of nanorods (wall side) has a larger area than the polar sides (top of nanorods). Figure 2(a) presents the XRD patterns of ZnO NRs with respect to the Bi doping concentration. The XRD patterns of all pristine and Bi-doped ZnO NRs are in good agreement with the standard datasheet (JCPDS No. 36-1451) of the wurtzite hexagonal ZnO structure. It is important to note that secondary phases such as Bi2O3 or other crystalline phases were absent in the XRD spectra. All samples possess a dominant peak corresponding to the (002) Bragg planes, implying that the preferred growth direction of the ZnO NRs is along the c-axis of the substrate. The relative crystallite size at (002) plane and the intensity ratio between the (002) and (100) planes were calculated and plotted as a function of Bi concentration, as shown in Figure 2 (b). As the Bi concentration increased from 0 to 1 wt%, the (002)/(101) intensity ratio of the ZnO NRs systematically increased. However, the intensity ratio began to


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decrease when the Bi concentration exceeded 1 wt%, indicating that ZnO nanorods began to grow in random orientations, as depicted by the FESEM images. The lattice constant (c) of all samples was calculated from the 2θ of the (002) plane, as presented in Figure 2(b). The lattice constant was found to slightly increase to 5.206 Å from 5.203 Å for Bi doping concentrations greater than 1 wt%. This finding can be ascribed to the larger ionic radius of Bi3+ compared with that of Zn2+, which might have induced lattice distortion during the incorporation of Bi3+ into the ZnO lattice sites.16 Figure 2(c)-(f) shows the HRTEM and selective area electron diffraction (SAED) pattern analyses used to examine the orientation and crystal structure of pristine and BiZO NRs. A single crystalline ZnO NRs consists of a continuous lattice fringe, and 0.25 nm between the adjacent planes was observed. The SAED pattern (inset of Figure 2(c)) reveals that the growth direction of nanorods is parallel to the [0001] direction or c-axis orientation. In detail, the lattice fringes observed in Figure 2(f) were unclear, and certain distortions were observed along the contact area of adjacent particles (Region A). This region is believed to originate from occupation of the larger Bi atom in Zn vacancy sites, leading to a slightly larger lattice fringe of 0.26 nm. Similarly, the SAED diffraction angle is higher than that of the pristine nanorods. This result further confirms the increment of crystallite size, as shown in Figure 2(f). To understand the reason behind the morphological and structural changes of ZnO NRs due to Bi doping, a mechanism is proposed and discussed in the supplementary information based on the above results. XPS analysis was performed to investigate the oxidation states, surface defects, and chemical compositions. Figure 3(a) depicts the survey scan of pristine and BiZnO NRs with different Bi doping concentrations in which only strong elemental peaks of Zn, O, C, Bi were detected. Figure. 3(b) depicts the narrow scan of Bi 4f7/2 and Bi 4f3/2 core level photoemission spectra for BiZO NRs, which clearly demonstrates that Bi metal cations were successfully incorporated into the ZnO lattice. Atomic percentages of 0.7 and 1.2 % Bi were detected for 2 and 3 wt% Bi-doped ZnO NRs, respectively. It can be clearly observed that the Bi 4f spectra are asymmetric, and therefore, de-convolution of the spectra was performed using Gaussian fitting (Casa XPS). The Bi 4f7/2 consisted of two low binding energies



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(B.E.) of 157.2 and 158.9 eV, whereas the higher B.E. values for Bi 4f5/2 were centered at 162.5 and 164.2 eV. The lower B.E. (157.2 – 162.5 eV) can be ascribed as the zero charge state of the Bi (Bi0) element or surface phase of BiO.21 In addition, the higher B.E. (158.9 - 164.2 eV) indicates the presence of Bi3+ in the Bi2O3 phase. The observed B.E. value for Bi2O3 is close to that of a previous report in which the Bi 4f7/2 peak was detected at 159.2eV.22 These findings also revealed that Bi and Bi2O3 are present at the surface of BiZO NRs. However, the specific mechanism of this phenomenon is not well understood thus far. The narrow scans of Zn 2p spectra for the pristine and BiZO NRs are shown in Figure 3 (c). The B.E. values of Zn 2p3/2 and Zn 2p1/2 for the pristine sample were located at approximately 1021 eV and 1044 eV, respectively, with a spin–orbit splitting of 23.1 eV.23 This result confirmed that Zn atoms exist in a +2 oxidation state for the pristine ZnO NRAs sample. It is noteworthy that the Zn2p peaks were slightly shifted toward a lower B.E. by ~0.1 eV for BiZO NRs samples, which could be related to the changes in the Zn electronic structures, as shown in Eq. 10, particularly the increase of ZnO electron density.18 Due to the difference in electron valence states of Bi and Zn, a free electron will be ejected into the electron clouds of ZnO when a Bi atom is interstitially incorporated into the ZnO lattice sites. To achieve stability of the bond length and crystal structure, an electron will be removed to become a free electron. Therefore, increasing the concentration of Bi dopants could increase the free-electron density in ZnO bonding, as evidenced by the decrease of the binding energy between O2- and Zn2+. Figure 3(d) depicts the asymmetric curves of O 1s spectra that can be fitted into two symmetrical peaks centered at 529.9 and 531.36 eV (pristine ZnO NRAs). These two peaks represent different types of oxygen components; the lower B.E. (OL) can be associated with the bonding between O2- and Zn2+ in the ZnO lattice, whereas the higher BE (OH) can be attributed to the O2- ions in the oxygen-deficient regions within the ZnO matrix.24 Additionally, the O 1s peak of BiZO NRs is negatively shifted by ~0.16 eV after the incorporation of the Bi element into the ZnO lattice. The ratio of OH/Ototal was calculated from XPS data and used to determine the relative quantity of oxygen defects in ZnO in which Ototal represents the


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sum of the OH and OL areas. As the Bi concentration increased, the OH/Ototal decreased from 0.31 to 0.28, as shown in Table S1, suggesting a reduced oxygen defect concentration. To confirm whether Bi-Bi2O3 is incorporated into the ZnO lattice by means of substitution or interstitials, a relative quantitative analysis was carried out to determine the stoichiometric ratio between O and Zn according to the equation reported by Jing et al.25 Based on the increasing trend of O/Zn with Bi concentration, it is more likely that Bi3+ is incorporated into the ZnO lattice interstitially and forms Bi2O3 rather than via substitution because the atomic radius of Bi3+ (0.96 Å) is much larger than that of Zn2+ (0.60 Å). Figure 4 shows the normalized PL spectra for pristine and BiZO NRs samples with different Bi concentrations, and the combined spectra of all samples are shown in Figure S2. Generally, the PL spectra of all samples consist of two main emission peaks that correspond to the near band-edge emission (NBE) and deep level emission (DLE). To investigate the defect emission and compare the relative changes in the defect states of each sample, the PL spectra were normalized with the respective NBE intensity peak. All of the PL spectra were de-convoluted into several regions using Gaussian fitting and are denoted separately as P1 (378 nm), P2 (410 nm), P3 (555 nm), P4 (626 nm) and P5 (715 nm). P1 emission can be attributed to the NBE of ZnO, which arises from excitonic recombination.26 Compared with that of the pristine sample (3.28 eV), the NBE peak was obviously red-shifted to 381 (3.25 eV) and 383 nm (3.24 eV) for 2 and 3 wt% BiZO NRs, respectively, as shown in the Figure 4. Although P2 emission (violet) is believed to originate from the interstitial recombination of an electron in the defect state of Zn (Zni) with a hole in the valence band,27 the P3 emission (green) can be related to the presence of oxygen vacancy (Ov)28 and surface defects29, and P4 emission (yellow-orange) is commonly assigned to oxygen interstitials (Oi) and surface hydroxyl groups (OH-) It can be clearly observed that both Ov and Oi related emissions decreased systematically with increasing Bi concentration. This finding agrees well with previous XPS analysis in which Bi doping could lead to the suppression of surface dangling oxygen and OH- bonds on the surface of ZnO. An additional weak emission peak located at ~464 nm was observed with concentrations greater than 1 wt%



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Bi, which could arise from Bi-Bi2O3 dopants and/or Zn vacancy defects.30-31 This observation also reveals that more Bi3+ was incorporated interstitially with oxygen in the ZnO lattice instead of substituting the Zn2+ ions. The suppression of oxygen defects can be ascribed to the presence of Bi dopants in which free Bi3+ interacted with O2- anions or filled in existing Ov in the ZnO lattice during the growth process of ZnO NRs. In addition, the stronger Bi3+ emission with increasing Bi concentration clearly shows the formation of the Bi2O3 phase in the ZnO lattice, which might be due to the formation of bonds between Bi3+ and O2- interstitials (Oi). This observation also substantiates the decrease in binding energy between O2- and Zn2+, as shown in the XPS analysis. The inset in Figure 4(d) illustrates the possible energy level diagram of Bi3+ in which the emissions are based on the allowed energy transition from 3P1 to 1S0. In addition, this diagram also provides insights on the recombination process from the conduction band of ZnO to the deep-trap state.32 It is well known that red emission can be related to the defects associated with an excess of oxygen on the ZnO surface.33-34 In this study, the red emission vanished when the Bi concentration exceeded 1 wt%, as shown in (Fig. S2). We believe that this emission arises from the FTO layer (Fig. S3). The formation of thicker BiZO NRs at higher Bi doping concentrations completely prevents the background emission of FTO substrates. The current-density voltage (J-V) characteristics of pristine and BiZO NRs with different Bi concentrations under an illumination intensity of 100 mWcm−2 (AM1.5G) are shown in Figure 5(a), and the extracted photovoltaic parameter data of the devices are summarized in Table 1. The series and shunt resistances (Rs and Rsh) were calculated from the J-V curves under illumination. Incorporation of Bi into ZnO NRs resulted in a systematic increase in the short-circuit density (Jsc) from 1.83 mA cm-2 for the pristine device to as high as 2.87 mAcm-2 for 2 wt% Bi. However, the Jsc of the device dropped to 2.18 mAcm-2 with further increases in the Bi concentration. In contrast, the Voc increases with Bi concentration from 0.42 up to 0.46 V, whereas the fill factor (FF) of the devices gradually decreased from 52 to 46 %. As a result, the 2 wt% Bi device exhibited the highest power conversion efficiency (PCE) of ~0.6%, primarily due to the higher Jsc and Voc compared with those of the pristine devices, albeit a lower FF.


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Figure 5(b) demonstrates a comparison of the IPCE spectra of devices with various Bi concentrations. The maximum IPCE value was achieved by device C, approximately 23 % (520 nm), which is in accordance with the Jsc trend measured in the J-V characteristics, as shown in Table 1. Based on Figure 5(c), it is apparent that both the forward and reverse bias (leakage) currents increase with increasing Bi concentration. Noticeably, the forward bias and leakage current displayed a similar trend against Bi concentration with the Rs and Rsh values, respectively. The increment of Jsc can be attributed to the decrease of the Rs value. With a lower Rsh value, the Voc values of the device were found to improve linearly with the Bi concentration. The high leakage current might be associated with the increase of the average effective thickness of the nanorods, thus leading to the probability of current shunting between the polymer layer and the FTO.14 Therefore, it is unlikely that the improvement of Voc is related to the increase of the leakage current. To further clarify the original improvement of Jsc with increasing Bi concentration, KPFM measurement was carried out. Figure 6(a) and (b) depicts a comparison of the surface potential (SP) images between the pristine and 2 wt% BiZO NRs coated with thin layer of P3HT. The histogram plot (inset of Figure 6(a-b)) is based on the extraction of SP values under dark and illuminated conditions. The surface potential shift (SP shift = SPillumination − SPdark) value of the pristine sample is 29.0 ± 0.1 mV and increases remarkably, and a positive value as high as 64.7 ± 0.3 mV was achieved for 2 wt% Bi. The positive shift implies that the photo-generated excitons are efficiently separated at the BiZO NRs/P3HT interface, leading to the accumulation of holes at the surface of the active layer, whereas negatively charged electrons were transported effectively to the FTO substrate through the nanorods.14 These findings are in good agreement with the trend of PL quenching, which reveals efficient exciton dissociation at a higher Bi doping concentration of 2 wt%, as shown in Figure S4(a). The efficient exciton dissociation could be correlated with the larger donor-acceptor interfacial area, as proven by the actual aspect ratio calculation (FESEM analysis). Figure 6(c) and (d) demonstrates the tapping-mode topography AFM images with a significant increase of rms roughness from 9.8 to 13.8 nm for pristine and 2 wt%



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BiZO NRs/P3HT films. This result might be due to the random orientation of BiZO NRs located underneath the P3HT layer, in agreement with the FESEM results. Furthermore, the C-AFM measurements were performed under dark conditions to determine the electronic conductivities of the active layers. The mean current of P3HT slightly increased from 2.2 up to 2.6 pA, suggesting that the electronic conductivity slightly improved due to the enhanced crystallinity of the P3HT, thus promoting better charge transport and resulting in the improvement of Jsc.35 These finding are clear manifestations of a more pronounced P3HT vibronic peak, as shown in Figure S4(b). Although a small increment of dark current was obtained, it could not be the only factor that contributes to the improvement of Jsc for the 2 wt% BiZO NRs device. In general, the Voc can be determine by the difference between the quasi-Fermi levels of the donor and acceptor, and therefore, KPFM measurement was used to gain further insight into the interface between the ZnO NRs and P3HT to investigate the reason behind the improvement of Voc with increasing Bi concentration. Figure 7(a) and (c) presents the KPFM images of pristine and 2 wt% BiZO NRs/P3HT interfaces. The histograms of the surface potential differences (∆SP) between ZnO NRs (negative value) and P3HT (positive value) are shown in Figure 7(b) and (d). The measured ∆SP values for pristine ZnO/P3HT and 2 wt% BiZnO/P3HT are 82 mV and 40 mV, respectively. This result indicates that the Fermi level of ZnO shifts downward due to the Bi doping, as described in the schematic band-diagram in Figure 7(e), and is correlated with the red-shift of NBE emission in the PL spectra (Figure 4). Nevertheless, based on the J-V analysis, Voc increases with increasing Bi doping concentration, and therefore, the improvement of Voc does not relate to the narrowing band-gap of BiZO NRs. Accordingly, it is essential to find an intrinsic reason behind the Voc improvement using TPV measurements.

The recombination dynamics of free charge carriers in the devices as a function of Bi concentration were characterized by transient photovoltage (TPV). Figure 8(a) depicts the normalized photovoltage decay curves of the devices with various Bi concentrations. It was found that the


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recombination lifetime of the devices were greatly enhanced with increasing Bi doping concentration, from 88 up to 149 µs. The longer lifetime of the 3 wt% Bi device provides lower charge recombination and consequently enhances the charge transfer at the interface between the ZnO and the active layer. These observations are primarily due to suppression of charge trapping due to oxygen defects, which reduces the probability of charge recombination at the interface between ZnO and P3HT. Generally, the device Voc not only depends on the energy band offset between the donor and acceptor and electrode work functions but also relies on charge-carrier recombination rates.36-37 Therefore, the increasing trend of Voc with Bi-doping concentration clearly indicates that the prolonged recombination lifetime could compensate the leakage current and small energy band-offset between the lowest unoccupied molecular orbital (LUMO) of the P3HT and the conduction band edge of ZnO.

An additional measurement of dark CELIV was performed to reveal the underlying reason behind the improvement of Jsc. The current density transients as a function of time for pristine and 2 wt% Bi device are shown in Figure S5. Figure 8(b) demonstrates the device mobility (µ) values for all devices extracted at each electric field (E1/2) using the equation from a previous report for a system with moderate conductivity.38 All devices showed good agreement with log µ ~ β(E)1/2, and the positive electric field was dependent on the mobility compared with the negative field dependence β reported for pure P3HT films.39 Furthermore, the device mobility improved remarkably, by more than a factor of 4 from 0.6 (pristine) up to 2.6·10-4 cm2V-1s-1 (2 wt% Bi) at respective E1/2 values of 286 and 276 V1/2cm-1/2. This trend is similar to that of the Jsc value, as shown in Table 1. It has been reported previously that the random orientation and low density of nanorods results in enhancement of photogenerated exciton dissociation and light scattering, thus improving Jsc.9, 40 Accordingly, in this study, the actual aspect ratio increases with Bi concentration; however, based on the C-AFM results and evidence, the slightly enhanced vibronic peak of P3HT further confirms that the crystallinity of P3HT does not significantly contribute to improvement of Jsc. Additionally, the increment of Jsc could be attributed to the



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improvement of charge carrier mobility due to the interstitial doping of Bi-Bi2O3 into ZnO NRs. However, at 3 wt% Bi concentration, the charge carrier mobility is similar to that of the pristine device, as shown in Figure 8 (b). This result is primarily caused by the presence of a pronounced deep-level state of Bi3+, which might promote a direct electron-hole recombination process despite the large interfacial area.

Conclusions In summary, the pentavalent Bi interstitially doped into ZnO plays an important role in tuning the morphology, crystallinity, and intrinsic surface defects of the ZnO NRs. The large ionic radii of Bi compared with that of Zn showed interstitial doping into ZnO at higher Bi concentrations. The photovoltaic performance was investigated using Bi-doped ZnO NRs as an electron acceptor layer. The result showed that the PCE was significantly improved from 0.38 % for a device using pristine NRs to ~0.6% for another device with 2 wt% BiZO NRs. The increment of Voc with increasing Bi concentration is primarily attributed to the prolonged recombination lifetime due to the suppression of oxygen defects of ZnO, which might compensate for the downward shift of the BiZnO NRs energy offset and high leakage current. The improved Jsc for up to 2 wt% Bi mostly originates from the decreased Rs due to the increase in the donor/acceptor interfacial area, followed by the increase of free electron density and enhancement of the charge carrier mobility. Moreover, this result is further supported by evidence of efficient exciton dissociation and charge separation. In contrast, the higher Bi concentration leads to a significant decrease of the photocurrent and to lower PCE. This finding suggests that proper control of Bi doping concentration is essential to obtaining optimum performance. In addition, the photovoltaic performance could be further improved with the use of a low band-gap polymer capable of absorbing a broad wavelength of the light spectrum.


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Acknowledgement This research was conducted under the financial support from Universiti Kebangsaan Malaysia (UKM), under Research Grant GUP-2014-012 and Ministry of Higher Education Malaysia, under Research Grant FRGS/1/2013/SG02/UKM/01/1. The authors are grateful for the assistance offered by UKM Centre for Research and Instrumentation (CRIM) in various characterizations of samples throughout this work. References 1.

Huang, J.-S.; Goh, T.; Li, X.; Sfeir, M. Y.; Bielinski, E. A.; Tomasulo, S.; Lee, M. L.; Hazari, N.; Taylor, A. D., Polymer Bulk Heterojunction Solar Cells Employing Forster Resonance Energy Transfer. Nat. Photon. 2013, 7, 479-485.

2.

Peet, J.; Kim, J.; Coates, N. E.; Ma, W. L.; Moses, D.; Heeger, A. J.; Bazan, G. C., Efficiency Enhancement in Low-Bandgap Polymer Solar Cells by Processing with Alkane Dithiols. Nat. Mater. 2007, 6, 497-500.

3.

Green, M. A.; Emery, K.; Hishikawa, Y.; Warta, W.; Dunlop, E. D., Solar Cell Efficiency Tables (Version 45). Prog. Photovoltaics Res. Appl. 2015, 23, 1-9.

4.

Shoaee, S.; Briscoe, J.; Durrant, J. R.; Dunn, S., Acoustic Enhancement of Polymer/Zno Nanorod Photovoltaic Device Performance. Adv. Mater. 2014, 26, 263-268.

5.

Nahor, O.; Segal-Peretz, T.; Neeman, L.; Oron, D.; Frey, G. L., Controlling Morphology and Charge Transfer in Zno/Polythiophene Photovoltaic Films. J. Mater. Chem. C 2014, 2, 4167-4176.

6.

Huang, J.; Yin, Z.; Zheng, Q., Applications of Zno in Organic and Hybrid Solar Cells. Energy Environ. Sci. 2011, 4, 3861-3877.

7.

Kulyk, B.; Sahraoui, B.; Figà, V.; Turko, B.; Rudyk, V.; Kapustianyk, V., Influence of Ag, Cu Dopants on the Second and Third Harmonic Response of Zno Films. J. Alloys Compd. 2009, 481, 819-825.

8.

Olson, D. C.; Piris, J.; Collins, R. T.; Shaheen, S. E.; Ginley, D. S., Hybrid Photovoltaic Devices of Polymer and Zno Nanofiber Composites. Thin Solid Films 2006, 496, 26-29.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

9.

Page 16 of 29

Conings, B.; Baeten, L.; Boyen, H.-G.; D’Haen, J.; Van Bael, M. K.; Manca, J. V., Relation between Morphology and Recombination Kinetics in Nanostructured Hybrid Solar Cells. J. Phys. Chem. C 2012, 116, 14237-14242.

10. Han, J.; Liu, Z.; Zheng, X.; Guo, K.; Zhang, X.; Hong, T.; Wang, B.; Liu, J., Trilaminar Zno/Zns/Sb2S3 Nanotube Arrays for Efficient Inorganic-Organic Hybrid Solar Cells. RSC Adv. 2014, 4, 23807-23814. 11. Zheng, Y.-Z.; Ding, H.; Liu, Y.; Tao, X.; Cao, G.; Chen, J.-F., In-Situ Hydrothermal Growth of BiHierarchical ZnO Nanoarchitecture with Surface Modification for Efficient Hybrid Solar Cells. Electrochim. Acta 2014, 145, 116-122. 12. Whittaker-Brooks, L.; McClain, W. E.; Schwartz, J.; Loo, Y.-L., Donor-Acceptor Interfacial Interactions Dominate Device Performance in Hybrid P3HT-ZnO Nanowire-Array Solar Cells. Adv. Energy Mater. 2014, 4, 1400585. 13. Ginting, R. T.; Yap, C. C.; Yahaya, M.; Mat Salleh, M., Improvement of Inverted Type Organic Solar Cells Performance by Incorporating Mg Dopant into Hydrothermally Grown ZnO Nanorod Arrays. J. Alloy Compd. 2014, 585, 696-702. 14. Ginting, R. T.; Yap, C. C.; Yahaya, M.; Mat Salleh, M., Solution-Processed Ga-Doped ZnO Nanorod Arrays as Electron Acceptors in Organic Solar Cells. ACS Appl. Mater. Interfaces 2014, 6, 5308-5318. 15. Ruankham, P.; Sagawa, T.; Sakaguchi, H.; Yoshikawa, S., Vertically Aligned Zno Nanorods Doped with Lithium for Polymer Solar Cells: Defect Related Photovoltaic Properties. Journal of Materials Chemistry 2011, 21, 9710-9715. 16. Xu, C.; Chun, J.; Kim, D. E.; Kim, J.-J.; Chon, B.; Joo, T., Electrical Properties and near Band Edge Emission of Bi-Doped Zno Nanowires. Appl. Phys. Lett. 2007, 90, 083113.


Page 17 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


17. Abed, S.; Bougharraf, H.; Bouchouit, K.; Sofiani, Z.; Derkowska-Zielinska, B.; Aida, M. S.; Sahraoui, B., Influence of Bi Doping on the Electrical and Optical Properties of ZnO Thin Films. Superlattices Microstruct. 2015, 85, 370-378. 18. Zhong, J. b.; Li, J. z.; Lu, Y.; He, X. y.; Zeng, J.; Hu, W.; Shen, Y. c., Fabrication of Bi3+-Doped ZnO with Enhanced Photocatalytic Performance. Appl. Surf. Sci. 2012, 258, 4929-4933. 19. Baumann, A.; Lorrmann, J.; Rauh, D.; Deibel, C.; Dyakonov, V., A New Approach for Probing the Mobility and Lifetime of Photogenerated Charge Carriers in Organic Solar Cells under Real Operating Conditions. Adv. Mater. 2012, 24, 4381-4386. 20. Shuttle, C. G.; O’Regan, B.; Ballantyne, A. M.; Nelson, J.; Bradley, D. D. C.; de Mello, J.; Durrant, J. R., Experimental Determination of the Rate Law for Charge Carrier Decay in a Polythiophene: Fullerene Solar Cell. Appl. Phys. Lett. 2008, 92, 093311-3. 21. Gulino, A.; Fragala, I., Deposition and Characterization of Transparent Thin Films of Zinc Oxide Doped with Bi and Sb. Chem. Mater. 2002, 14, 116-121. 22. Zhu, B. L.; Xie, C. S.; Wu, J.; Zeng, D. W.; Wang, A. H.; Zhao, X. Z., Influence of Sb, in and Bi Dopants on the Response of Zno Thick Films to Vocs. Mater. Chem. Phys. 2006, 96, 459-465. 23. Islam, M. N.; Ghosh, T.; Chopra, K.; Acharya, H., Xps and X-Ray Diffraction Studies of Aluminum-Doped Zinc Oxide Transparent Conducting Films. Thin Solid Films 1996, 280, 20-25. 24. Kim, Y.-H.; Heo, J.-S.; Kim, T.-H.; Park, S.; Yoon, M.-H.; Kim, J.; Oh, M. S.; Yi, G.-R.; Noh, Y.Y.; Park, S. K., Flexible Metal-Oxide Devices Made by Room-Temperature Photochemical Activation of Sol-Gel Films. Nature 2012, 489, 128-132. 25. Jing, L.; Xu, Z.; Shang, J.; Sun, X.; Cai, W.; Guo, H., The Preparation and Characterization of Zno Ultrafine Particles. Mater. Sci. Eng. A 2002, 332, 356-361. 26. Djurišić, A. B.; Leung, Y. H., Optical Properties of ZnO Nanostructures. Small 2006, 2, 944-961. 27. Lin, B.; Fu, Z.; Jia, Y., Green Luminescent Center in Undoped Zinc Oxide Films Deposited on Silicon Substrates. Appl. Phys. Lett. 2001, 79, 943-945.



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Page 18 of 29

28. Vanheusden, K.; Seager, C.; Warren, W. t.; Tallant, D.; Voigt, J., Correlation between Photoluminescence and Oxygen Vacancies in Zno Phosphors. Appl. Phys. Lett. 1996, 68, 403-405. 29. Shalish, I.; Temkin, H.; Narayanamurti, V., Size-Dependent Surface Luminescence in ZnO Nanowires. Phys. Rev. B 2004, 69, 245401. 30. Wu, Y.; Lu, G., The Roles of Density-Tunable Surface Oxygen Vacancy over Bouquet-Like Bi2O3 in Enhancing Photocatalytic Activity. Phys. Chem. Chem. Phys. 2014, 16, 4165-4175. 31. Djurišić, A. B.; Ng, A. M. C.; Chen, X. Y., Zno Nanostructures for Optoelectronics: Material Properties and Device Applications. Prog. Quant. Electron. 2010, 34, 191-259. 32. Dong, W.; Zhu, C., Optical Properties of Surface-Modified Bi2O3 Nanoparticles. J.Phys. Chem. Solids 2003, 64, 265-271. 33. Djurišić, A.; Leung, Y.; Tam, K.; Ding, L.; Ge, W.; Chen, H.; Gwo, S., Green, Yellow, and Orange Defect Emission from ZnO Nanostructures: Influence of Excitation Wavelength. Appl. Phys. Lett. 2006, 88, 103107. 34. Djurišić, A.; Leung, Y.; Tam, K.; Hsu, Y.; Ding, L.; Ge, W.; Zhong, Y.; Wong, K.; Chan, W.; Tam, H., Defect Emissions in ZnO Nanostructures. Nanotechnology 2007, 18, 095702. 35. Yuan, K.; Chen, L.; Chen, Y., Photovoltaic Performance Enhancement of P3HT/PCBM Solar Cells Driven by Incorporation of Conjugated Liquid Crystalline Rod-Coil Block Copolymers. J. Mater. Chem. C 2014, 2, 3835-3845. 36. Blakesley, J. C.; Neher, D., Relationship between Energetic Disorder and Open-Circuit Voltage in Bulk Heterojunction Organic Solar Cells. Phys. Rev. B 2011, 84, 075210. 37. Vandewal, K.; Tvingstedt, K.; Gadisa, A.; Inganäs, O.; Manca, J. V., On the Origin of the OpenCircuit Voltage of Polymer–Fullerene Solar Cells. Nat. Mater. 2009, 8, 904-909. 38. Pivrikas, A.; Sariciftci, N.; Juška, G.; Österbacka, R., A Review of Charge Transport and Recombination in Polymer/Fullerene Organic Solar Cells. Prog. Photovoltaics Res. Appl. 2007, 15, 677-696.


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39. Mozer, A. J.; Sariciftci, N. S.; Pivrikas, A.; Österbacka, R.; Juška, G.; Brassat, L.; Bässler, H., Charge Carrier Mobility in Regioregular Poly(3-Hexylthiophene) Probed by Transient Conductivity Techniques: A Comparative Study. Phys. Rev. B 2005, 71, 1-9. 40. Lee, Y.-J.; Lloyd, M. T.; Olson, D. C.; Grubbs, R. K.; Lu, P.; Davis, R. J.; Voigt, J. A.; Hsu, J. W. P., Optimization of ZnO Nanorod Array Morphology for Hybrid Photovoltaic Devices. J. Phys. Chem. C 2009, 113, 15778-15782.



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Table 1. Average value of photovoltaic parameters with different Bi concentrations under illumination of AM1.5G. Bi

Jsc

Voc

PCE

FF

Rs

Rsh

Jsc (cal.)

concentration

(mAcm-2)

(V)

(%)

(%)

(Ωcm2)

(Ωcm2)

(mAcm-2)

pristine

1.83

0.4

0.38

52

100

4000

1.61

1 wt%

2.40

0.42

0.52

51

48

1987

2.14

2 wt%

2.87

0.44

0.59

47

42

1436

2.44

3 wt%

2.18

0.46

0.47

46

53

530

1.95


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Figure 1. Top-view FESEM images of the ZnO NRs with a Bi concentration of (a) 0 wt%, (b) 1 wt%, (c) 2 wt%, and (d) 3 wt%. Scale bars = 100 nm.



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A

Figure 2. (a) XRD spectra of the pristine and BiZO NRs (1 to 3 wt%), (b) intensity ratio of (002)/(101) and crystalline size as a function of Bi concentration, (c, d) HRTEM images of pristine and (e, f) 2 wt% BiZO NRs. The inset in (c) and (e) shows the corresponding SAED patterns.


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Figure 3. (a) Survey scan of pristine and BiZO NRs, (b) narrow scan of Bi 4f, (c) Zn 2p, and (d) O 1s with different Bi concentration.



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Figure 4. Normalized room temperature PL spectra of (a) pristine, (b) 1 wt%, (c) 2 wt%, and (d) 3 wt% BiZO NRs de-convoluted into five different emission regions. The inset (d) shows a schematic energy level diagram of the Bi3+ electronic levels in the ZnO bandgap.


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Figure 5. (a) J-V characteristics under illumination (RH ~ 80%), (b) IPCE spectra, and (c) Dark J-V curve of the devices with different Bi concentrations.



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Figure 6. (a, b) KPFM 3-D (5 x 5 µm) images under dark and light condition, (c-d) tapping-mode AFM images and (e, f) C-AFM current images (5 x 5 µm) of P3HT of pristine and 2 wt% BiZO NRs/P3HT films. Insets (a) and (b) show the histogram plot of the corresponding extracted SP from KPFM.


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Figure 7. KPFM images of the interface between the P3HT and (a) pristine and (c) 2 wt% BiZO NRs (20 x 20 µm), (b) and (d) histogram analysis of extracted KPFM data, (e) schematic energy level diagram of ZnO and P3HT. Inset shows the topography via AFM images of the interface between ZnO and P3HT.



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Figure 8. (a) Normalized photovoltage as a function of time and (b) charge carrier mobility as a function of electric field for devices with different concentrations of Bi.


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Graphical Abstract


A Simple Approach Low-Temperature Solution ... - ACS Publications

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