Enhanced Photovoltaic Performance in Polycrystalline BiFeO3 Thin

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Enhanced Photovoltaic Performance in Polycrystalline BiFeO Thin Film/ZnO Nanorods Heterojunction 3

Fen Wu, Yiping Guo, Yangyang Zhang, Huanan Duan, Hua Li, and Hezhou Liu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp5059462 • Publication Date (Web): 30 Jun 2014 Downloaded from http://pubs.acs.org on July 2, 2014

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The Journal of Physical Chemistry

Enhanced Photovoltaic Performance in Polycrystalline BiFeO3 Thin Film/ZnO Nanorods Heterojunction Fen Wu, Yiping Guo*, Yangyang Zhang, Huanan Duan, Hua Li and Hezhou Liu* State Key Lab of MMC, School of Materials Science and Engineering, Shanghai Jiaotong University, Shanghai 200240, China *

Email address: [email protected]

Tel: 86-21-34202549, Fax: 86-21-34202749

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ABSTRACT We demonstrate that the insertion of a n-type ZnO nanorods (NRs) layer between polycrystalline

BiFeO3

(BFO)

thin

film

and

poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) buffer layer in a sandwich-like F-doped SnO2 (FTO)/BFO/PEDOT:PSS/Au capacitor leads to the short-circuit current (Jsc) increasing 64-fold from 16.8 µA/cm2 to 1.08 mA/cm2 under the AM1.5G (100 mW/cm2) illumination. When the semitransparent Au electrode is substituted by a transparent Al-doped ZnO (AZO) electrode, the photovoltaic (PV) output can be further improved, with power conversion efficiency (PCE) reaching 0.17%, which is to our knowledge the highest efficiency that has ever been obtained in the all-solid-state BFO-based capacitor solar devices. It is proposed that the polarization

induced

FTO/BFO

Schottky

barrier,

the

low-resistance

ZnO

NRs/PEDOT:PSS contact plus the carriers transportation characteristics of ZnO NRs contribute to the PV enhancement. A band bending diagram model considering ferroelectric polarization and interface states as well as a schematic energy level diagram of the FTO/BFO/ZnO NRs/PEDOT:PSS/Au device is constructed to illustrate the PV enhancement. Our work may provide a new way to utilize ferroelectrics to improve the power conversion efficiency of heterojunction based solar cells and to produce solar cells with tunable PV response and other novel functions. Keywords: Ferroelectric; Nanostructure; Interface; Solar cells 1. INTRODUCTION 2 ACS Paragon Plus Environment

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Ferroelectric (FE) materials have attracted considerable attentions in recent years as potential photovoltaic (PV) materials both in bulk and in thin film form. Different from the conventional silicon based solar cells which rely on the built-in asymmetry brought by the p-n junction, the FE materials based solar cells provide an alternative driving force — the polarization-induced internal electric field (also known as the depolarization

electric

electron-hole (e-h) pairs.

field 1,2

(EDP)),

to

separate

the

photon-excited

Among the ever reported PV phenomena in FE materials,

multiferroic BiFeO3 (BFO) has a comparatively small band gap (2.3-2.8 eV)

3

and

exhibits a large open-circuit voltage (Voc) 4. Besides, the PV response can be reversed by an external electrical field, which enables BFO to gain potentials in producing solar cells with novel functions. 5 Though, it is still hard for FEs to rival with the mainstream silicon based solar cells due to their small photocurrent densities and limited light-to-electricity conversion efficiencies

6,

the FE polarization can be used to

improve the power conversion efficiency of heterojunction based solar cells 7 and to produce solar cells with tunable PV response thanks to the switchable polarization states by an external electric field 8. 1D nanowires (NWs) or nanorods (NRs) based solar cells hold great promise for the third-generation PVs and for powering nanoscale devices

9,10

due to their cost

reduction by reducing materials use through the fabrication of NW (NR) arrays. 11 NWs (NRs) can be used to improve the charge collection efficiency in polymer-blend and dye-sensitized solar cells (DSSCs). 9 Additionally, they can also be embedded into thin 3 ACS Paragon Plus Environment


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semiconductor films to form heterojunctions, which has been proven to be an efficient way to boost the carrier collection efficiency.

12

Among the ever reported NW (NR)

structures used in solar cells, ZnO has been the most intensively studied one because of its advantages in the ease of crystallization and anisotropic growth, fast electron injection efficiency and better electron transportation, as well as various structural designs and easy control.

13,14

Most of the ever published reports on ZnO NWs or NRs

have been centering on the applications in DSSCs. In DSSCs, light is absorbed by the dyes, which are anchored to the surface of a wide band gap semiconductor. Charge separation takes place at the interface via photon-induced electrons injection from the dyes into the conduction band (CB) of the semiconductor. 15 The ground state of the dyes is regenerated through the reduction by the hole-transporting material (HTM) to give the required charge separation. Charges migrate and are collected at the electrodes. 16

The main function of the 1D ZnO NRs in DSSCs is to offer large active surface

areas for large dye-loading to enhance the light-harvesting and improve the charge-injection. In this work, we present a novel concept of utilizing ZnO NRs arrays in all-solid-state capacitor solar devices by combining the nanorods with polycrystalline BFO thin film to form a BFO/ZnO NRs heterojunction. A highly conductive thin poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) buffer layer was spin-coated on the ZnO NRs surface before the sputtering of the Au electrode to improve the ZnO NRs/Au contact conditions. The results show that the sandwich-like 4 ACS Paragon Plus Environment

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F-doped SnO2 (FTO)/BFO/ZnO NRs/PEDOT:PSS/Au capacitor exhibits a power conversion efficiency (PCE) of 0.086%, 134 times larger than the one without ZnO NRs. When the top Au electrode is substituted by an Al-doped ZnO (AZO) electrode, the PCE further improves to 0.17%, which is to our knowledge the highest efficiency that has ever been reported in the all-solid-state BFO-based capacitor solar devices. 2. EXPERIMENTAL SECTION

Polycrystalline BFO thin films of 120 nm were deposited on commercial FTO glass substrates by the chemical solution deposition (CSD) technique as reported in our previous work. bismuth

17

nitrate

The 0.2 mol/L precursor solutions were synthesized starting from (Bi(NO3)3·5H2O)

and

ferric

nitrate

(Fe(NO3)3·9H2O).

N,N-dimethylmethanamide (C3H7NO) was used as the solvent. The precursor films were spin coated on the FTO glass substrates at 1000 rpm for 10 s and then 3000 rpm for 30 s. After coating each layer, the thin films were dried at 150 °C for 3 min, pre-fired at 350 °C for 5 min in air, and then rapidly thermal annealed at 550 °C for 5 min in oxygen atmosphere. These steps were repeated for 8 times. The ZnO NRs were subsequently grown on the BFO thin film by a two-step seed-growth procedure. 18 The 0.3 mol/L precursor solutions were prepared by dissolving equimolar zinc acetate (Zn(CH3COO)2·2H2O)

and

ethanolamine

(C2H7NO)

in

2-methoxyethanol

(HOCH2CH2OCH3). The precursor films were spin-coated on the BFO layers at 1000 rpm for 10 s and then 3000 rpm for 30 s. After coating each layer, the thin films were dried at 150 °C for 3 min, pre-fired at 350 °C for 5 min in air, and then rapidly thermal 5 ACS Paragon Plus Environment


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annealed at 550 °C for 5 min in oxygen atmosphere. These steps were repeated for 2 times to form uniform and complete film seeds. To grow the ZnO nanorods, the FTO substrates coated with BFO and ZnO seeds were floated facedown in an aqueous bath containing equal volumes of zinc nitrate (Zn(NO3)2·6H2O, 0.05 mol/L) and hexamethylenetetramine (HMTA, 0.05 mol/L) in deionized (DI) water at 90 °C. The diameters and lengths of the ZnO NRs were controlled by the annealing process of the ZnO seeds, the precursor concentration and the growth time of the nanorods. After a growth period of 15-90 min, the substrates were thoroughly rinsed with DI water, dried at 60 °C in air, and followed by rapidly annealed in oxygen atmosphere at 200 °C for 10 min. The crystallographic structure of BFO/ZnO NRs was characterized by x-ray diffraction (XRD) (Rigaku D/max-2550/PC) Cu Kα radiation with a scan speed of 5 °/min. Surface and cross-sectional morphologies studies were carried out by the field emission scanning electron microscopy (FESEM) (FEI QNANTA FEG 250). The PV devices were fabricated with a sandwiched FTO/BFO/ZnO NRs/PEDOT:PSS/Au

structure.

A

highly

conductive

thin

poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) buffer layer was spin-coated on BFO/ZnO NRs by a mixture of PEDOT:PSS aqueous solution (the content weight ratio is 1.7%) and N,N-dimethylmethanamide (DMF, act as solvent and dopant to improve the conductivity of PEDOT:PSS

19,20

) with a volume ratio of

1:5, and followed by baking at 120 °C on a hot plate. The Au top electrodes of 0.5 6 ACS Paragon Plus Environment

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mm×1 mm were deposited by ion-sputtering, and the Al-doped ZnO (AZO) electrodes were prepared by radio frequency magnetic sputtering 21. The optical properties of the BFO/ZnO NRs heterojunction were investigated using a UV-Vis spectrophotometer in the wavelength range of 300-900 nm. The PV measurements were carried out under the illumination of 100 mW/cm2 (AM1.5G), generated by a solar simulator, and I–V data in the dark and under illumination were collected with the Keithley 2400 instrument. 3. RESULTS AND DISCUSSION XRD patterns of the samples reveal that polycrystalline BFO thin films with perovskite structure are obtained and well aligned ZnO NRs orienting at (002) are successfully grown on the surface of the BFO thin films (Figure 1). The decline of the BFO diffraction peak intensities after the introduction of the ZnO NRs may have something to do with the x-ray scattering loss brought by the ZnO NRs (Figure S1). SEM top-view and cross-sectional images of the FTO/BFO/ZnO NRs structure (Figure 2a) demonstrate that the obtained ZnO NRs with small diameters are uniformly distributed on the BFO surface. It can be inferred that the rapid thermal process (RTP)-obtained ZnO seeds can lead to the growth of ultrathin ZnO NRs with diameters at ~50 nm, far thinner than those obtained by annealing the ZnO seeds at a hot plate. 22,23

Besides, from the SEM images of the FTO/BFO/ZnO NRs/PEDOT:PSS structure

(Figure 2b), we can know that the PEDOT:PSS layer is very thin and caps on the top surface of the nanorods, filling the gaps among the adjacent nanorods and reducing 7 ACS Paragon Plus Environment


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the surface roughness. Optical properties of the BFO/ZnO NRs heterojunction and BFO were carried out to calculate their energy band gap (Eg) (Figure 3). As is shown in Figure 3b, the BFO thin film and the BFO/ZnO NRs heterojunction have almost the same Eg ~2.68 eV, meaning the same absorption edge, ie. the existence of ZnO NRs does not affect the spectral response region of the heterojuction. Besides, from the inset of Figure 3a, it can be found that the heterojunction has higher absorption in the 300-475 nm region than that of BFO. The chemical reactions involved in the growth of ZnO NRs: (CH2)6 + 6H2O → 6HCHO + NH3; NH3 + H2O ↔ NH4+ + OH¯; 2OH¯ + Zn2+ → Zn(OH)2; Zn(OH)2 → ZnO + H2O

24

can barely induce any chemical reactions of BFO in

aqueous solutions at a low temperature of 90 °C. Therefore, the absorption enhancement in the UV-near Vis region may be related to the regularly standing NRs, which possess unique photon management properties due to light trapping. 25 The NRs arrays enhance the light intensity penetrating into the BFO layer, and the incident photons with energy above the band gap will be absorbed to generate e-h pairs. However, compared with the absorption (300-475 nm) properties of the BFO/ZnO planar film heterojunction, it is found that the light trapping of ZnO NRs is not as high as expected (Figure S2). Moreover, after the introduction of the highly conductive thin PEDOT:PSS buffer layer on the ZnO NRs surface, the gaps and voids among the adjacent nanorods are filled in as shown in Figure 2b, the light trapping of the nanorods will consequently be further weakened. 8 ACS Paragon Plus Environment

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It has been reported that the metal/NRs contact is of high resistance due to Fermi-level pinning at the NRs/metal contact interface.

26

A low resistance Ohmic

contact to the collection electrode is thus required to promote e-h pairs separation and carriers transportation. It has been pointed out that the barrier height between ZnO and Au can range from 0 to 1.2 eV, depending on the crystal structure, surface preparation and the conditions under which the contact is formed.

27

PEDOT:PSS is reported to

have comparable conductivity as that of the In2O3-doped SnO2 (ITO) electrode. 20,28 By the introduction of a PEDOT:PSS layer between the ZnO NRs and Ag electrode, Manekkathodi has obtained an Ohmic ZnO NRs/Ag contact.

29

In our work, the

PEDOT:PSS is introduced as a buffer layer to fill in the gaps among the adjacent ZnO NRs, which not only will reduce the series resistance due to the improved NRs/electrode contact but also will reduce the barrier height of the NRs/electrode contact (Figure S3-Figure S5). After optimizing the thickness of BFO and the ZnO NRs arrays, greatly enhanced PV output has been achieved in our FTO/BFO/ZnO NRs/PEDOT:PSS/Au devices (Figure S6). The obtained optimum thickness of BFO is ~120 nm (8 layers) and that of ZnO NRs is ~150nm (growth time of 30 min). Figure 4 is the J-V curves in the dark and under white light illumination of the optimized FTO/BFO/ZnO NRs/PEDOT:PSS/Au device, and the inset image is the time dependence of zero bias photocurrent density with light ON and OFF. We have measured the light ON/OFF response behaviors in a duration period of ~2500 s (Figure S7), result shows that the 9 ACS Paragon Plus Environment


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device exhibits stable and repeatable light ON/OFF response. Figure 5a is the J-V curves for the devices without ZnO NRs and Figure 5b is the J-V curves for the devices without BFO. (In our measurements, the bottom FTO electrodes were grounded, and the current was defined as positive when it moved from FTO to Au.) It is clear that the heterojunction composed of BFO and ZnO NRs illustrates a remarkable enhancement in photocurrent when compared with the one without ZnO NRs. (Figure 4 and Figure 5a). By inserting ZnO NRs, the short-circuit current density (Jsc) increases from 16.8 µA/cm2 to 1.08 mA/cm2 (~64 fold), and PCE from 0.00064% to 0.086% (~134 fold). The fill factor (FF) of the FTO/BFO/ZnO NRs/PEDOT:PSS/Au capacitor can be calculated by the following equation as FF=PCE/(Jsc×Voc)=0.25. Such low FF may have something to do with the large surface roughness of the ZnO NRs

30

as well as the large series resistance. Moreover,

in a FTO/ZnO NRs/PEDOT:PSS/Au device without the BFO film, there is a negligible PV effect (Figure 5b), and it is therefore reasonable that the enhanced PV output in the FTO/BFO/ZnO NRs/PEDOT:PSS/Au device cannot be attributed to the extra PV generation effect of ZnO NRs. Furthermore, it can be inferred from the symmetric liner dark J-V curves in Figure 5b that an Ohmic ZnO NRs/Au contact has been obtained with the introduction of PEDOT:PSS. It is known that the 1D nanorods (nanowires) geometry not only provides superior light trapping and electrons transportation but also dramatically increases the surface recombination. 30 Therefore, we may gain further insight to the effect of the ZnO NRs on the PV performance by 10 ACS Paragon Plus Environment

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preparing different lengths of ZnO NRs arrays with the nanorods’ growth time being controlled. From the obtained results shown in Figure 6, we can see that Voc decreases steadily with the increasing growth time, which may have something to do with the formed defects caused by ion-depletion when the growth time extends,

31

and the

defects can act as trapping centers for the separated carriers, leading to carriers loss. The Jsc and PCE, however, first increase and then decrease with increasing nanorods’ growth time, showing peak values with the nanorods’ growth time being 30 min. It is well accepted that the photovoltage of a solar cell is closely related to the intensity of the incident light. 32 Our results have shown that the existence of ZnO NRs can to some degree increase the light intensity penetrating into the BFO film. In order to figure out how the length of ZnO NRs influences their light enhancement ability, we measured the optical properties of the BFO/ZnO NRs (with the growth time of the ZnO NRs respectively being 30 min and 90 min) heterojunctions (Figure 7). From Figure 7, it can be observed that longer ZnO NRs only lead to a slightly enhanced absorption in the active spectral response region. Thus if the PV enhancement of the heterojuntion is merely caused by the enhanced light utilization brought by ZnO NRs, the device with longer ZnO NRs should exhibit a larger PV output. It can hence be inferred that the enhanced light utilization brought by ZnO NRs is not the main reason for the improved PV performance, and there are other factors influencing the PV output. In previous studies concerning the ZnO NRs (NWs) based DSSCs

33,34

,

researchers have mainly ascribed the large PV output to the high aspect ratio of the 11 ACS Paragon Plus Environment


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NRs (NWs), which offers larger dye-molecules loading and better light-harvesting. However, this theory is not suitable for explaining the length-dependent Jsc variation trend in our case concerning the divergent PV mechanisms of the two types of solar devices (Figure S8). In this paper, we therefore highlight the two-sided electric characteristics of ZnO NRs to discuss the length dependence of the Jsc. On the one hand, the ZnO NRs have high electron mobility due to the single crystalline morphology, thereby avoiding the grain boundary scattering of electrons, and at the same time they can also serve as direct pathways for the transportation of photon-induced electrons, reducing recombination loss. On the other hand, it is known that the native defects at the ZnO NRs surface can serve as binding sites for the chemisorptions processes, such as the formation of charged oxygen molecule complexes (O2 + e¯ → O2¯), which will become the scattering and trapping centers for carriers, lowering the carrier mobility. Moreover, the O2¯ complexes can form depletion regions at the NRs surface, reducing the devices conductivity.

29,35

Hence

with the increasing growth time of NRs, the active surface areas of the NRs increase, meaning the binding sites for the chemisorptions processes increase, resultantly, more O2¯ complexes will form, which undoubtedly will have negative effect on the Jsc and PCE output. Thus the observed variation trend of Jsc and PCE in our case is mainly induced by the competition between the two forces brought by the ZnO NRs—electron transportation convenience against conductivity reduction. A transparent AZO electrode is adopted as the top electrode to substitute the Au 12 ACS Paragon Plus Environment

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electrode. J-V curves in the dark and under white light illumination of the FTO/BFO/ZnO NRs/PEDOT:PSS/AZO device are shown in Figure 8. Compared with the device using Au as the top electrode, the one using AZO exhibits an elevated PV output, with Voc increasing from -0.32 V to -0.52 V, Jsc increasing from 1.08 mA/cm2 to 1.34 mA/cm2, and PCE increasing from 0.086% to 0.17%. The boosted PV output mainly attributes to the higher transparence of the AZO electrode than that of the Au electrode. 21 To illustrate the PV mechanism of the FTO/BFO/ZnO NRs/PEDOT:PSS/Au device, a band bending diagram taken into consideration of

the ferroelectric

polarization and interface states, as well as a energy level diagram, is depicted (Figure 9). From the findings of the Klein’s group,

36,37

we can derive that the valence band

maximum (VBM) of BFO: EVBM(BFO) is -6.82 eV (with reference to the absolute vacuum energy (E0)). The band gap (Eg) of our BFO sample is calculated to be 2.68 eV, thus the conduction band minimum (CBM) of BFO: ECBM(BFO) is estimated to be -4.14 eV. The energy levels of ZnO NRs can refer to the Ravirajan’s report 38. BFO and ZnO are believed to be both n-type semiconductors due to the naturally produced oxygen vacancies and zinc interstitials during preparation.

39,40

The work function

value of a semiconductor is sensitive to the surface conditions, such as surface charges, surface defects and surface contaminations, it generally varies in a certain range. 41 The work function of the well aligned ZnO NRs arrays is reported to be 4.84 eV, which shows an obvious dependence on the arrangement of the ZnO NRs arrays. 41

The Fermi-level of BFO is about 0.5 eV below the CBM

42

. PEDOT:PSS is a

highly conductive polymer with a constant work function of 5.0 eV,

43

and the work

function of Au is 5.4 eV 44. The work function of FTO is commonly cited as 4.4 eV. 45 13 ACS Paragon Plus Environment


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The Fermi-level of BFO is higher than that of ZnO, therefore when BFO contacts with ZnO NRs, electrons will thermally migrate from the CB of BFO to that of ZnO to achieve consistency in the Fermi-level, resulting in an upward (downward) energy band bending in the depletion region of BFO (ZnO) (Figure S9), leading to the formation of a built-in electric field: En-n. However, as is reported in our previous work, self-polarization (Pself) with an upward state (from FTO to Au) exists in the as-prepared BFO thin films, and the polarization will modify the energy band bending of the device. 4 The original Ohmic FTO/BFO contact will become a Schottky one, while the original upward band bending in the depletion region of BFO will become a downward one. The polarization charges on the interface will bring in a huge electric field called the depolarization electric field (EDP) inside the FE layer if they are not screened.

32

A corresponding schematic energy band bending diagram can thereby be

depicted (Figure 9a). As can be seen from the band bending diagram, the large EDP will offset the countered En-n, and acts the main built-in potential for the separation of the photon-induced e-h pairs. Holes move to the bottom FTO electrode and electrons to the top Au electrode by the built-in potential. Electrons transport to the top electrode along the pathways offered by ZnO NRs, reducing the possibilities of the carriers recombination during the transportation. Additionally, the ZnO NRs in the device can result in substantial photoconductivity by prolonging the photocarriers lifetime and shortening the carriers transit time, which can enhance the charge collection efficiency. Moreover, from the energy level diagram of the device shown in Figure 9b, the energy level alignment in the BFO/ZnO NRs heterojunction is favorable for the transportation

of

the

photon-induced

e-h

pairs

NRs/PEDOT:PSS/Au device. 4. CONCLUSION 14 ACS Paragon Plus Environment

in

the

FTO/BFO/ZnO

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The FTO/BFO/ZnO NRs/PEDOT:PSS/Au device was fabricated. The PV device based on the BFO thin films combining with the ZnO NRs shows power conversion efficiency over ~134 times larger than that for a similar device without the ZnO NRs. The realization of the remarkably enhanced PV performance is based on the special architecture of the device. The introduction of ZnO NRs improves the PV performance by enhancing the efficiency of carriers transportation and charge collection, with Jsc increasing from 16.8 µA/cm2 to 1.08 mA/cm2 (~ 64 fold), Voc from -0.17 V to -0.32 V (~2 fold), PCE from 0.00064% to 0.086% (~134 fold). When the semitransparent Au electrode is replaced with the transparent AZO electrode, further improvement in the PV output has been obtained, with Voc climbing to -0.52 V, Jsc to 1.34 mA/cm2, PCE to 0.17%. These results represent the highest conversion efficiency among the all-solid-state BFO-based capacitor solar devices in previous reports. It is believed that the polarization induced FTO/BFO Schottky barrier and the low-resistance ZnO NRs/PEDOT:PSS contact plus the superior carriers transportation characteristics of the ZnO NRs contribute to the PV enhancement of the device. This work sheds light on a new approach to enhance the power conversion efficiency of all-solid-state FE thin film based capacitor solar devices by means of the insertion of ZnO NRs, which has the potential for applying in novel optoelectronic and tunable solar energy devices. ASSOCIATED CONTENTS Supporting information 15 ACS Paragon Plus Environment


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Comparison of the XRD patterns, absorption spectrums of the BFO/ZnO NRs and the BFO/ZnO planar film, in-the-dark and under-white-light illumination J-V curves of the devices without and with the PEDOT:PSS buffer layer, AC impedance spectroscopy, J-V curves in-the-dark and under white light illumination of the devices with the buffer layer respectively being BCP and PCBM and their corresponding energy level diagrams, factors influencing the PV output, time dependence of zero bias photocurrent density with light ON and OFF in a longer duration period, sketch maps of two different types of solar cell devices, band bending diagrams of the device considering the impact of FE self-polarization. The related material is available free of charge via the Internet at http://pubs.acs.org. ACKNOWLEDGEMENT This work is supported by the National Natural Science Foundation of China (NO.11074165 and NO.51332009). REFERENCE (1) Choi, T.; Lee, S.; Choi, Y. J.; Kiryukhin, V.; Cheong, S. W. Switchable Ferroelectric Diode and Photovoltaic Effect in BiFeO3. Science 2009, 324, 63-66. (2) Ji, W.; Yao, K.; Liang, Y. C. Bulk Photovoltaic Effect at Visible Wavelength in Epitaxial Ferroelectric BiFeO3 Thin Films. Adv. Mater. 2010, 22, 1763-1766. (3) Catalan, G.; Scott, J. F. Physics and Applications of Bismuth Ferrite. Adv. Mater. 2009, 21, 2463-2485. (4) Guo, Y.; Guo, B.; Dong, W.; Li, H.; Liu, H. Evidence for Oxygen Vacancy or 16 ACS Paragon Plus Environment

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(42) Zhu, A.; Zhao, Q.; Li, X.; Shi, Y. BiFeO3/TiO2 Nanotube Arrays Composite Electrode: Construction, Characterization, and Enhanced Photoelectrochemical Properties. ACS Appl. Mater. Interfaces 2014, 6, 671-679. (43) Weickert, J.; Sun, H.; Palumbiny, C.; Hesse, H. C.; Schmidt-Mende, L. Spray-Deposited PEDOT: PSS for Inverted Organic Solar Cells. Solar Ener. Mater. Solar Cells 2010, 94, 2371-2374. (44) Uda, M.; Nakamura, A.; Yamamoto, T.; Fujimoto, Y., Work Function of Polycrystalline Ag, Au and Al. J. Electron Spectrosc. 1998, 88, 643-648. (45) Dong, W.; Guo, Y.; Guo, B.; Li, H.; Liu, H.; Joel, T. W. Enhanced Photovoltaic Effect in BiVO4 Semiconductor by Incorporation with an Ultrathin BiFeO3 Ferroelectric Layer. ACS Appl. Mater. Interfaces 2013, 5, 6925-6929. Figure captions Figure 1. XRD patterns of BFO/ZnO seed and BFO/ZnO NRs grown on the FTO glass substrates; Figure 2. SEM top-view and cross-sectional images of: (a) the FTO/BFO/ZnO NRs structure; (b) the FTO/BFO/ZnO NRs/PEDOT:PSS structure; Figure 3. Optical measurements of BFO and BFO/ZnO NRs: (a) Transmittance curves of BFO and BFO/ZnO NRs at the wavelength range of 300-900nm (inset is the partially enlarged view of the transmittance curves in the wavelength range of 300-475nm); (b) (αhν)2 versus hν curves of BFO and BFO/ZnO NRs heterojunction. Figure 4. J-V curves in the dark and under white light illumination of the 22 ACS Paragon Plus Environment

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FTO/BFO/ZnO NRs/PEDOT:PSS/Au device (inset is the time dependence of zero bias photocurrent density with light ON and OFF). Figure 5. J-V curves in the dark and under white light illumination of: (a) the FTO/BFO/PEDOT:PSS/Au device; (b) the FTO/ZnO NRs/PEDOT:PSS/Au device. Figure 6. Effect of the length of ZnO NRs (with reference to the growth time of the ZnO NRs) on the key photovoltaic performance parameters

of FTO/BFO/ZnO

NRs/PEDOT:PSS/Au solar cells. Figure 7. Effect of the length of ZnO NRs (with reference to the growth time of the ZnO NRs) on the optical properties of FTO/BFO/ZnO NRs/PEDOT:PSS/Au solar cells. Figure 8. J-V curves in the dark and under white light illumination of the FTO/BFO/ZnO NRs/PEDOT:PSS/AZO device (inset is the time dependence of zero bias photocurrent density with light ON and OFF). Figure 9. Schematic diagrams showing the migration of photon-induced e-h pairs inside the FTO/BFO/ZnO NRs/PEDOT:PSS/Au device: (a) band bending diagram; (b) energy level diagram. Figures



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Enhanced Photovoltaic Performance in Polycrystalline BiFeO3 Thin

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