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CuO Homojunction Solar Cells: F-Doped Ntype Thin Film and Highly Improved Efficiency Luo Yu, Liangbin Xiong, and Ying Yu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b06736 • Publication Date (Web): 11 Sep 2015 Downloaded from http://pubs.acs.org on September 17, 2015

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The Journal of Physical Chemistry

Cu2O Homojunction Solar Cells: F-doped N-type Thin Film and Highly Improved Efficiency Luo Yu1, Liangbin Xiong2 and Ying Yu1* 1

Institute of Nanoscience and Nanotechnology, College of Physical Science and Technology, Central China Normal University, Wuhan 430079, China.

2

School of Physics and Electronic-Information Engineering, Hubei Engineering University, Xiaogan, 432000, China

1

ACS Paragon Plus Environment


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ABSTRACT: Herein, F-doped Cu2O thin films with different F content are firstly synthesized on the ITO glass via a simple electrochemical deposition method. The prepared F-doped Cu2O thin films present n-type semiconductor character and show significantly high electronic and optical properties, especially for the one with preparation molar ratio of F/Cu = 1:2. This sample owns a unique net microstructure for a best absorption of visible light and its electron concentration is more than ten times as that of pure Cu2O. Additionally, it has a lowest resistivity, which is beneficial for photogenerated charge transfer and the decrease of electron-hole pair recombination. The F-doped Cu2O films are utilized to fabricate Cu2O homojunction solar cells by consecutive electrochemical depositions. The conversion efficiency of the best homojunction solar cell with the F-doped Cu2O as n-type layer is nearly eight times as that with pure Cu2O as n-type layer. Hence, this study provides a strategy to improve the properties of Cu2O thin films through F ion doping. The application of F-doped Cu2O to homojunction solar cell will shed light on the development of another cheap and environmentally friendly solar cell.

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1. INTRODUCTION The urgent requirement for clean and sustainable energy technologies has invigorated researches on many photovoltaic systems with increasing emphasis on the balance cost and performance, as well as environmental concerns. Cuprous oxide (Cu2O) is a green and abundant semiconductor with low-cost and a high absorption coefficient, 1-5 which is a promising material for cheap solar cells. 6 The Cu2O films prepared in ambient condition usually present p-type semiconductor character due to the presence of Cu vacancies, 6 which are usually used to fabricate Cu2O-based solar cell devices based on p-Cu2O/metal Schottky junctions and p-n heterojunctions, such as p-Cu2O/Cu, 7,8 n-ZnO/p-Cu2O,

9,10

n-CdO/p-Cu2O,

11

and n-TiO2/p-Cu2O, 12 etc.

However, a p-n heterojunction solar cell requires that the p-type and n-type semiconductor own a proper energy level alignment to improve carrier separation and diminish recombination of electrons and holes. 13-15 Such an urgent demand restricts the combination of the two semiconductors. Moreover, the formation of a p-n heterojunction by two kinds of semiconductors normally leads to lattice mismatch, resulting in vast interface states on the interface of the heterojunction, which may act as recombination center to suppress the separation of electrons and holes. 16 However, these problems will be absent for a homojunction. Additionally, the homojunction has no interface strain. 17 These advantages make the homojunction perform better than heterojunction in solar cells. Notably, as early as 1986, the n-type Cu2O thin films have been firstly synthesized by electrodeposition method. 18 In recent years, many methods have been reported to prepare n-type Cu2O on various substrates.

7,16,19

The development of

preparation methods for n-type Cu2O brings a boost in the study on Cu2O homojunction solar cells, and the related work has been published one after another.

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20-23

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Nevertheless, all of the reported Cu2O homojunction solar cells do not show a

satisfactory performance and the efficiency is still far from the theoretical efficiency, 24

which retards a further development for Cu2O homojunction solar cells. Primarily,

the low efficiency is ascribed to the poor electronic properties of Cu2O films, including the short diffusion length of minor carriers

25

and the poor conductivity of

Cu2O films, 16 which make it hard for photoinduced electrons and holes to transfer to electrodes and thus lead to a massive recombination in the bulk. 26 To date, enormous efforts have been made to modify Cu2O for the improvement of its electronic and optical properties. Doping is an effective method among them. The doping of Cu2O thin films by group-IV elements

27

and N ion

28

have been

investigated as early as ten years ago, which has led to a better electronic property for the doped samples. Recently, theoretical studies have demonstrated that the dopants of F, Cl, and Br in Cu2O are stable in the substitution site of the O atoms with low formation energies.

29

Moreover, the doped Cu2O presents an n-type conduction

behavior, which is in good agreement with the very recent experimental results for Cl-doped Cu2O. 30 To the best of our knowledge, there is little study for F-doped Cu2O. Actually, most fluoride compounds are cheap and easy to obtain. Furthermore, the radius of fluorine atom is comparable to oxygen atom, which can case little lattice distortion with F occupying O atom sites. 31 So, F ion doping may be better than Cl ion for the improvement of the property of Cu2O thin film and thus the performance of a homojunction solar cell. In this study, by using electrochemical method, it is the first time that F-doped Cu2O thin films have been prepared by introducing substitutional dopant F on ITO glass substrate. The results show that F ions was incorporated into Cu2O crystalline lattice and the prepared Cu2O had enhanced electronic and optical properties.

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Substantially, the Cu2O homojunction solar cells fabricated with F-doped Cu2O as the n-type layers possessed a much better performance than control solar cell. Additionally, the mechanism of the enhanced performance has been discussed.

2. EXPERIMENTAL SECTION 2.1. Growth of Cu2O Films. All of the chemicals used in this study were purchased from Shanghai Guoyao Chemicals Ltd. Co. They were analytical reagents and used without further purification. ITO glass was purchased from Nippon Sheet Glass (NSG) Group (Tokyo, Japan). Deionized water was used for the preparation of aqueous solutions. The electrodeposition of Cu2O was carried out over a PARSTAT 2273 electrochemical station in a three-electrode system, in which the ITO glass with a size of 2.0 × 5.0 cm2 was used as working electrode, Pt as the counter and Ag/AgCl electrode as the reference. Different electrolytes and experiment conditions were applied to deposit Cu2O. In this study, p-type Cu2O films were electrodeposited at 60°C，and a constant current of -3 mA had been applied for 30 min. The electrolytes contained 0.02 M copper sulfate and 0.4 M lactic acid with a pH value of 11 adjusted by NaOH solution. N-type Cu2O films were electrodeposited in aqueous solutions of 0.02 M copper acetate and 0.08 M acetic acid. Prior to deposition, the pH was adjusted to 4.9 using NaOH solution (4 mol/L). The depositions were conducted at 70°C with +0.02 V (vs. Ag/AgCl) bias. NaF was used as F precursor with three different concentrations of 0, 0.005 and 0.01 mol/L, to which the final obtained Cu2O thin film corresponding was labeled as FC0, FC1:4, FC1:2, respectively, for convenience. In fact, the samples with even higher molar ratios of F/Cu were also prepared but pure metal Cu was formed on the substrate when there was more NaF in the deposition electrolytes, as shown in Fig. S1 (Supporting Information). Cl-doped

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Cu2O films were prepared in 100 mL aqueous solutions containing 0.3M CuSO4 and 4M lactic acid through electrochemical deposition method as well. CuCl2, with the amount of 2, 4 and 6 mmol, was added into the above solution for Cl doping. After that, 4 M NaOH was added into the electrolyte to adjust the pH value of the solution to 9.0. The depositions were carried out at 60°C with -0.4 V (vs. Ag/AgCl) bias. For the growth of different Cu2O homojunctions, p-type Cu2O films were first electrodeposited onto ITO glass substrates, which were then used as cathodes for subsequent n-type film growth by using the above procedure. Subsequently, Au was sputtered on the ITO substrate and Cu2O film as top contacts by using a Leica EM ACE200 sputtering system under an Ar atmosphere. The Au target for top contacts was purchased from ACI Alloys. Finally, the prepared Cu2O homojunction solar cells were ready for characterization. 2.2. Materials and Device Characterizations. The morphology of the prepared samples was detected by scanning electron microscopy (SEM) with the model of JEOL JSM-6700. X-ray powder diffraction (XRD) patterns of the samples were analyzed by a PANalytical diffractometer (D/max 40kv) using Cu Kα radiation (λ= 0.154598 nm) for crystalline phase. The photoelectrochemical experiments, including photocurrent measurements, Mott-Schottky experiments, and electrochemical impedance spectroscopy, were performed through an electrochemical station (PARSTAT 2273, Princeton) in a standard three electrode system with the prepared samples as working electrode, a platinum plate as counter electrode and standard Ag/AgCl electrode as the reference. A 0.02 M Na2SO4 aqueous solution was used as the electrolyte. A 350W Xenon lamp (Lap Pu, XQ) with a 420 nm cutoff filter right above the reactor was employed as the visible light source in the photocurrent measurements. The Mott-Schottky plots were obtained at a frequency of 1.0 KHz, and

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the electrochemical impedance spectroscopy was conducted at a frequency from 100 KHz to 0.1 Hz at a potential of 0 V vs Ag/AgCl under dark conditions. The surface chemical analysis of the samples was achieved by using X-ray Photoelectron Spectroscopy (XPS) (VG Multiab-2000) using a PHI Quantum 2000 XPS system with a monochromatic Al Kα source and charge neutralizer. All of the spectra were calibrated to C1s peak at 284.6 eV. The UV-vis diffuse reflectance spectra (UV-vis DRS) of the samples were recorded with PerkinElmer Lambda 35 Spectrophotometer in the range of 400-800 nm. The performance of the solar cell devices was detected using a Newport Oriel Sol2A solar simulator with a light source of standard AM 1.5G (100 mW/cm2). I-V data were collected with a CHI660E electrochemical station. Raman spectra were recorded at room temperature using a LabRAMHR Raman system under Ar+ (532 nm) laser excitation.

3. RESULTS AND DISCUSSION Figure 1a shows the schematic preparation process of p-n Cu2O homojunction films by a simple two-step electrodeposition method. Figure 1b displays the typical surface morphology of as-prepared p-type Cu2O film, which exhibited a good coverage of the substrate with Cu2O crystals that uniformly had cubic surface morphology. The thickness of the prepared p-type film was around 1 µm observed from the side-view SEM image presented in Figure 1c. The X-ray diffraction peaks of p-type Cu2O film were well-indexed to cubic Cu2O (JCPDS 78-2076) (Figure S1 in Supporting Information). Mott-Schottky plot and photocurrent data indicate that the samples prepared in basic condition were p-type semiconductors (Figure S2 in Supporting Information). The SEM images of F-doped Cu2O samples prepared on ITO substrate by one-step

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electrodeposition method are shown in Figures. 1e~1g. We can see that pure Cu2O sample had a typical dendritic structure (Figure 1e). When NaF was present in the electrolyte, the Cu2O samples turned to a net structure, especially for Sample FC1:2, which exhibited a unique net microstructure, just like a spider web. This unique net structure may be beneficial for sunlight absorption and photoinduced charge separation.32 In fact, the samples with even higher molar ratios of F/Cu were prepared as well, but metal Cu was formed on the substrate, which was verified by the XRD patterns as shown in Figure S3 (Supporting Information). It may be explained by the inhibition effect of F ions during the electrodeposition due to the coverage of the surface with almost insoluble CuF. 19 Moreover, the kinetics and mechanisms of Cu+1 deposition was changed when there was too much NaF in the deposition electrolytes. The typical cross-section SEM image of the Cu2O homojunction assembled is displayed in Figure 1d, from which we can observe that the p-Cu2O layer was about 900 nm and n-type layer 600 nm thick. In addition, there was no void between the interfaces, indicating that good gap-filling was achieved. The SEM images of F-doped Cu2O grown on p-Cu2O film are presented in Figures. 2b and 2c compared with that of pure Cu2O (Figure 2a), from which it can be found that the net morphology was mostly preserved for Sample FC1:2 (Fig. 2c). Compared with the SEM images of F-doped samples prepared on ITO substrate, the apparent difference is mainly attributable to the distinction of substrate. 33 Figure 3 shows XRD patterns of F-doped Cu2O samples with different preparation molar ratio of F/Cu. From Figure 3a, it can be found that the crystalline phase of all the samples was a pure phase without any impurities. Figure 3b displayed the enlarged peaks corresponding to (111) for F-doped Cu2O samples. Compared with pure Cu2O, the diffraction peak (111) shifted toward the region of large theta with the increase of F

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ion amount, owing to the crystalline lattice contraction. As F- (133 pm) is just smaller than O2- (142 pm), 34 F ions may substitute O ions in Cu2O crystalline lattice. As a consequence, the crystalline lattice of Cu2O became shrinked, leading to the decrease of the lattice plane distance. According to Bragger equation: 2dsinθ = nλ (where d is the lattice plane distance value, θ the value of diffraction theta, and λ the wavelength of X-ray), it is obvious that as d turns to be smaller, θ will become larger accordingly. Thus, we can make a preliminary conclusion that the F ions may be incorporated into the lattice of Cu2O. To further confirm that F ions substitute O ions in Cu2O crystalline lattice, Raman spectra and X-ray photoemission spectroscopic (XPS) spectra were collected as shown in Figure 4 and Figure 5, respectively. All of the characteristic peaks in the Raman spectra were consistent with the vibration mode of Cu2O, which is identical with previous report. 35 It is evident that the peaks at 148 and 626 cm-1 were very weak for the pure Cu2O sample, but those became much stronger for the F-doped Cu2O samples. Actually, the peaks at 148 and 626 cm-1 were attributed to the intrinsic IR active modes. 35

In general, the IR active modes are ascribed to the change of the molecular dipole

moment resulted from the vibration of polar groups and molecular unsymmetrical vibration. 36 It is well known that F atom owns a very strong electronegativity, which can change the charge distribution of Cu and lead to the asymmetric distribution of positive and negative charge, 37 and thus form strong polar groups with Cu. So, the very strong peaks at 148 and 626 cm-1 in the F-doped Cu2O samples indicate that F ions may been incorporated into Cu2O crystalline lattice and form polar bonds with Cu. XPS experiments were performed to analyze the composition of the samples and the chemical state of the atoms. An apparent peak at 685 eV, as expected, was observed in the survey for Sample FC1:2 (Figure S4 in Supporting Information),

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which can be attributed to F 1s. The high resolution XPS regional spectra of the main constituents F 1s and Cu 2p for Samples FC0 and FC1:2 in Figure 5 were recorded for a more detailed analysis. Figure 5a shows the high resolution XPS spectra of F1s with different sputtering time during the measurement for Sample FC1:2. It can be seen that there were two peaks before the sample was sputtered to remove surface layers. Generally, the binding energy at 687.3 eV corresponded to the F ions adsorbed on the sample, and that at 685 eV was assigned to those in the lattice. 38 After the sample had been sputtered for 300 and 600 s before XPS detection, the peak at 687.3 eV disappeared and only the one at 685 eV was left. This further validates that F ions have been doped into Cu2O crystalline lattice, and the nearly identical peak shape after sputtered for 300 and 600 s indicated that a homogeneous doping was achieved. 39

In Figure 5b, for pure Cu2O sample of FC0, the binding energy of Cu 2p3/2 was

932.7 eV and that of Cu 2p1/2 was 952.7 eV, which is identical to the reported results. 40

However, for the F-doped Cu2O sample of FC1:2, the binding energy of Cu 2p3/2

and Cu 2p1/2 shifted to be 934.4 and 954.4eV respectively, which was 1.7 eV higher than that for pure Cu2O sample. This may be attributed to the fact that the F ions substitute O ions in Cu2O crystalline lattice and the strong electronegativity of F atom alters the charge distribution of Cu. 37 Furthermore, from the XPS spectra, the atomic ratio of Cu and F was around 4.2:1 (Figure S5 in Supporting Information) for the FC1:2 sample with preparation molar ratio of Cu/F = 2:1, which pertained to be heavy doping, since CuF, CuF2 and other impurity were not detected from the XRD and Raman data. 41 To determine the conduction type and carrier concentrations of the F-doped Cu2O samples, Mott-Schottky measurements were carried out. The results are shown in Figure 6. The positive slope indicated that the samples were n-type semiconductors

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with electron conduction. The donor concentration of the F-doped n-type Cu2O can be quantified by the Mott-Schottky equation 42 1/C2 = (2/eε0εNd)[(V-VFB)-kT/e],

(1)

where C represents the capacitance of the space charge region, e the electron charge, ε0 the vacuum permittivity, ε the dielectric constant of material (for Cu2O, ε = 6.3 20), V the electrode applied potential, k the Boltzmann constant, T the absolute temperature and Nd the donor concentration. At room temperature, the term of kT/e is calculated to be 25.693 meV, which is exceedingly small and can be neglected. Then the slope determined from the analysis of Mott-Schottky plot can be used to estimate the donor concentration using the equation 43 Nd = (2/eε0ε)[d(1/C2)/dV]-1 ,

(2)

so the electron concentration of the F-doped Cu2O was then calculated to be 6.18 × 1016, 1.49 × 1017 and 9.25 × 1017 cm-3 for Sample FC0, FC1:4 and FC1:2, respectively. The electron concentration for FC1:2 was more than ten times as that of the pure Cu2O sample of FC0. The relatively high carrier concentration of the F-doped Cu2O may be beneficial to the performance of a Cu2O homojunction solar cell. The electrochemical impedance spectra (EIS) shown in Figure 7a were used to analyze the interfacial charge transfer process for the Cu2O electrodes in electrolyte. The measurement was carried out in the frequency range of 100 kHz to 0.1 Hz at a potential of 0 V vs Ag/AgCl under dark conditions. The diameter of the semicircle is indicative

of

the

interfacial

charge

transfer

resistance,

Rct,

across

the

electrode/electrolyte interface. 44 The F-doped Cu2O samples showed much lower Rct value than the pure Cu2O, and FC1:2 owned a lowest Rct. This was in accordance with the photocurrent results shown in Figure 7b that the F-doped Cu2O samples exhibit much higher photocurrent response under visible-light irradiation. The lower

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interfacial charge transfer resistance can be related to faster charge transfer and lower rate of charge recombination , 45 which will definitely improve the performance of a Cu2O homojunction solar cell. UV-vis diffuse reflectance spectrum is used to probe the optical absorption property, and the band structure of materials. Figure 8 showed the diffuse reflectance absorption spectra of the F-doped Cu2O samples. All of the samples demonstrated excellent visible-light absorption in the range of less than about 630 nm. Apparently, the absorption increase for F-doped Cu2O films was observed compared with that for the pure Cu2O. Especially for Sample FC1:2, it exhibited a much stronger absorption in the visible-light range, which availed the utilization of solar light for a homojunction solar cell. The stronger absorption of visible-light was mostly attributed to the unique net structure of FC1:2, which is beneficial for sunlight harvesting. 35 For a crystalline semiconductor, it is known that the optical absorption near the band edge follows the following equation 46 αhν = A(hν − Eg)1/2 ,

(3)

where α, ν, Eg, and A are the absorption coefficient, light frequency, band gap and constant, respectively. According to this equation, the band gap of the F-doped Cu2O samples were determined to be 1.923, 1.900 and 1.882 eV for Sample FC0, FC1:4 and FC1:2, respectively, which is close to the values in previous reports. 47, 48 It can be seen that with the increase of F ion doping amount, the band gap of the corresponding F-doped Cu2O turned to be narrower gradually. Figures 9a and 9c are the schematic representation of device structure containing a single n-Cu2O layer and p-n Cu2O homojunction respectively. Dark I-V measurement results for F-doped n-type Cu2O electrodes are shown in Figure 9b. Substantially, the I-V curves manifested ideal Ohmic behavior, and the slopes of the

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lines verified that the F-doped Cu2O layers possessed lower resistivity compared to the pure Cu2O layer. As indicated in Figure 9d, the I-V curve for p-Cu2O/n-Cu2O (FC1:2) homojunction film showed a typical rectification effect of a p-n junction, which represents the successful formation of a p-n homojunction in the film. 49 To further test the performance of F-doped Cu2O as n-type semiconductor in solar cells, we fabricated three homojunction solar cell devices with the structure of ITO/p-Cu2O/F-doped Cu2O/Au. The three solar cells are named as follows, Cell 01 for p-n Cu2O (FC0), Cell 02 for p-n Cu2O (FC1:4), Cell 03 for p-n Cu2O (FC1:2). Figure 10a is the schematic assembly of p-n Cu2O homojunction solar cell in this study. Figure 10b shows the current density-voltage (I-V) curves, measured for these devices under AM 1.5G illumination, and the corresponding calculated characteristic parameters of different cells are listed in Table 1. It is pointed out that the I-V tests of Cell 01 and Cell 03 were conducted every other day for several times during ambient storage, and the data that represented the average level for Cell 01 and Cell 03 were shown here. The specific I-V data of Cell 01 and Cell 03 for six times are displayed in Figure S6 and Table S1 (Supporting Information). The decay of power conversion efficiency (PCE) for Cell 01 and Cell 03 was attributed to the formation of Cu during photo-etching, 50 since Cu was identified from XRD patterns for the samples after I-V tests (Figure S7). As can be seen, Cell 02 and Cell 03 with F-doped Cu2O as n-type semiconductors had better performance as expected. Cell 03 with the best performance exhibited an open-circuit voltage (Voc) of 0.191 V, short-circuit current density (Jsc) of 6.67 mA/cm2, fill factor (FF) of 0.263, and power conversion efficiency (PCE) of 0.335%. This performance was much better than that of Cell 01 with pure Cu2O as n-type layer, and the PCE of Cell 03 was nearly eight times as high as that for Cell 01. As revealed above, all of the high carrier density, low resistivity

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and strong absorbance of the F-doped Cu2O contributed to the excellent device performance. Furthermore, Cu2O homojunction solar cells with Cl-doped Cu2O as n-type layers were fabricated in comparison with the F-doped Cu2O homojunction solar cells. The I-V data are shown in Figure S8 and Table S2 (Supporting Information). Notably, the performance of the Cl-doped Cu2O homojunction solar cells was far behind that of the F-doped homojunction solar cells. Finally, we explore the mechanism of the enhanced performance in detail. A first-principle study about n-type doping in Cu2O with F has shown that the dopant of F is stable in the site of the substitution of the O atom with low formation energy,

29

which is in good agreement with our experimental results. More importantly, the first-principle study has figured out that a shallow donor level, below the conduction band bottom of pure Cu2O, can be formed due to F doping. 29 So, for the F-doped Cu2O, the electrons on the valence band can be excited to the conduction band as well as the donor level upon visible light, thus broadening the absorption range of visible light. This is consistent with the band gap decrease. In addition, the electrons on the shallow donor level can be easily excited by thermal energy to the conduction band bottom, which then enhances the carrier concentrations and the n-type conductivity. 51 Figure 11a is the schematic diagram of the generation and transfer of electrons and holes for F-doped Cu2O under visible light irradiation. Based on the knowledge of p-n junction, the energy level diagram and charge transfer process for Cu2O homojunction are proposed in Figure 11b. When p-type Cu2O and n-type Cu2O present a homojunction, electrons diffuse from the n-type side, which has a higher Fermi level than p-type side as electrons are abundant in n-type Cu2O. The similar process occurs for holes in the p-type Cu2O layer. And then the Fermi level will reach a balance and a built-in electric field (Vbi) can be formed in the

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space charge region resulted from the charge transfer. The built-in electric field (Vbi) drives electrons from p-type Cu2O to the n-type, and holes from n-type Cu2O to the p-type. For the F-doped Cu2O, higher carrier concentrations contributed to a larger built-in electric field (Vbi) so that the Voc increased from Cell 01 to Cell 03. 16,48 In addition, the F-doped Cu2O had a much stronger absorption in visible-light range and thus the solar light utilization for the Cu2O homojunction solar cell was improved, which is in favour of a larger Voc as well. However, the Voc of 0.191V is still lower than the best reported value of about 0.6 V. 7 One possible reason is that structural discontinuity may be present at the junction of p-Cu2O and n-Cu2O, which will generate many interface states. 22 And the photoelectrons can be captured by interface states and recombine with holes. As a result, the interface states offset the effect of the built-in potential and lower the Voc of the homojunction solar cells. For Cell 02 and Cell 03, the increase of Jsc was attributed to the low resistivity and fast charge transfer of F-doped Cu2O. Therefore, the photovoltaic performance from Cell 01 to Cell 03 is improved as shown in Table 1.

4. CONCLUSIONS In summary, F-doped n-type Cu2O on the ITO glass electrode has been successfully synthesized by using a simple electrochemical deposition method. The F-doped Cu2O exhibited high carrier concentration, low resistivity and strong absorption in visible-light range. Additionally, the Cu2O homojunction solar cell devices fabricated with the F-doped thin film as n-type layer showed a much better performance compared with that assembled with pure Cu2O layer. The excellent performance for this special Cu2O homojunction solar cell device was attributed to the significantly high electronic and optical properties of the F-doped Cu2O layer.

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Although the PCE of 0.335% for Cell 03 was still far from the theoretical efficiency for a Cu2O homojunction solar cell, the PCE for the best cell was nearly eight times as high as that for Cell 01 in our study. Consequently, this study proposes a simple and cheap strategy to improve the properties of Cu2O thin films, thus representing a good starting point for the development of the very low cost Cu2O homojunction solar cells.

AUTHOR INFORMATION Corresponding author * Email: [email protected], tel: 86-27-67867037. Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENT This work was financially supported by National Science Foundation of China (Nos. 21377044 and 21573085), Wuhan Planning Project of Science and Technology (No. 2014010101010023), the Key Project of Natural Science Foundation of Hubei Province (No. 2015CFA037) and self-determined research funds of CCNU from the colleges’ basic research and operation of MOE (Nos. CCNU15ZD007 and CCNU15KFY005). Dr. Xiong also appreciated the supports from China Postdoctoral Science Foundation (2015M572187) and Hubei Provincial Department of Education (D20152702)

ASSOCIATED CONTENT Supporting Information XRD pattern, Mott-Schottky plot and photocurrent of p-type Cu2O, XRD patterns 16


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of F-doped Cu2O prepared with high concentrations of NaF, XPS spectrum of FC1:2 sample before sputtering, atomic contents of Cu, O and F varying with different etching time for Sample FC1:2, I-V curves of Cell 01 and Cell 03 during ambient storage and corresponding photovoltaic properties, XRD patterns of FC0 and FC1:2 before and after photo-voltage performance testing, I-V curves and photovoltaic properties of Cu2O homojunction solar cells with Cl-doped Cu2O as n-type layers. This information is available free of charge via the Internet at http://pubs.acs.org.

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Figure captions Figure 1. (a) Schematic preparation process for p-n Cu2O homojunction film by two-step electrodeposition method; (b) top-view and (c) side-view SEM images of p-type Cu2O film; (d) side-view SEM image of p-Cu2O/n-Cu2O (FC1:2) homojunction; and top-view SEM images of F-doped Cu2O samples deposited on ITO substrate for (e) FC0, (f) FC1:4 and (g) FC1:2. Figure 2. Top-view SEM images of F-doped Cu2O samples grown on p-Cu2O films for (a) FC0, (b) FC1:4 and (c) FC1:2. Figure 3. (a) XRD patterns of F-doped Cu2O samples and (b) corresponding enlarged (111) peaks. Figure 4. Raman spectra of F-doped Cu2O samples. Figure 5. High resolution XPS spectra of (a) F 1s for FC1:2 with different sputtering time and (b) Cu 2p for FC0 and FC1:2. Figure 6. Mott-Schottky plots of F-doped Cu2O electrodes measured in an aqueous solution of Na2SO4 (0.02 M). Figure 7. (a) Electrochemical impedance spectra of F-doped Cu2O electrodes measured in an aqueous solution of Na2SO4 (0.02M) under dark conditions; and (b) photocurrent density of F-doped Cu2O samples with light on/off cycles under visible-light irradiation. Figure 8. UV-vis diffuse reflectance spectra of F-doped Cu2O samples with various

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molar ratio of F/Cu. The insert is the plots of [F(R)]2 versus photo energy. Figure 9. Schematic representations of device structure containing (a) a single n-Cu2O layer and (c) p-n Cu2O homojunction; and I-V curve(s) of (b) F-doped Cu2O layers and (d) p-Cu2O/n-Cu2O (FC1:2) in the dark. Figure 10. (a) Schematic assembly of p-n Cu2O homojunction solar cell used in this study; and (b) I-V curves of three p-n Cu2O homojunction solar cells under 1 Sun, AM 1.5 illumination. Figure 11. (a) Schematic diagram of generation and transfer of electrons and holes for F-doped Cu2O under visible light irradiation; and (b) energy level diagram and charge transfer process for Cu2O homojunction.

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Table 1 Photovoltaic property of the three p-n Cu2O homojunction solar cells.

Voc (V)

Jsc (mA cm-2)

1.49

FF (%)

25.8

ŋ (%)

Cell 01

0.112

0.043

Cell 02

0.150

5.51

24.3

0.192

Cell 03

0.191

6.67

26.3

0.335

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Yu et al. Figure 1.

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Yu et al. Figure 2.

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Yu et al. Figure 3.

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Yu et al. Figure 4.

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Yu et al. Figure 5.

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Yu et al. Figure 6.

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Yu et al. Figure 7.

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Yu et al. Figure 8.

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Yu et al. Figure 9.

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Yu et al. Figure 10.

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(a)

V/SH E


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(b) e-

-2.0

Vbi

CB

-1.0

0.0

EC

e-

Donor level VB h+

1.0

EF EV

h+

visible light

n-Cu2O

p-Cu2O

Yu et al. Figure 11.

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Graphical abstract V/SH E

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e-2 . 0

CB

Donor level

-1 . 0

0.0

VB

visible light

h+ 1.0

EC

eVbi

EF EV

h+

n-Cu2O


p-Cu2O

Cu2O Homojunction Solar Cells: F ... - ACS Publications

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