Fabrication and photoelectric properties of Er³⁺ and Yb³⁺ co-doped ZnO films

Yb₂O₃ (99.99%), Er₂O₃ (99.99%), zinc acetate [Zn(CH3COO)₂2H₂O, analytical reagent (AR)], ethanol (AR), hydrochloric acid (HCL,GR,40%), are supplied by the Sinopharm Chemical Reagent. All the chemicals are used as received without any further purification. Deionized water is used in all experiments.

The schematic diagram of the ultrasonic spray pyrolysis equipment (CQUTUSP-II) is shown in Fig. 1. The starting solution is atomized into fine and uniform droplets using an ultrasonic atomizer, the atomized particles transport to the space near the silica glass substrate carried by the carrier gas.^22–24) Then the moisture of droplets starts to evaporate under the action of high temperature. After that, the gaseous contacts the substrate. Finally, the film grows on the substrate. The substrate is placed onto a rotatable stage which ensures the uniform growth of the thin film. An upside down mounted substrate and a vertical feeding of the solution is used in order to make use of gravity to screen out the large droplets leaving the nozzle and to concentrate the same size of droplets on the substrate.

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To prepare the Er³⁺ and Yb³⁺ co-doped ZnO films, Zn(CH3COO)₂2H₂O, and ErCl₃, YbCl₃ were mixed in de-ionized water to make a precursor solution. Er³⁺ doping concentrations were set at 2% in molarities, while Yb³⁺ doping concentrations were set at 6%. An appropriate amount of acetic acid was added into the solution to avoid the formation of milky precipitate of hydroxides. Before the experiment, the silica glass substrates, were cleaned successively with hydrochloric acid, ethanol and distilled water in an ultrasonic cleaning system. The distance of source substrate was fixed at 2 cm and the substrates temperatures were fixed at 400 °C. As the precursor droplets arrive on the heated substrate, a pyrolytic decomposition process occurs and high quality EYZO films were produced. The chemical reactions followed during the pyrolytic decomposition process²⁵⁾ are described as follows:

$$\begin{align*} & \text{Zn(CH$_{3}$COO)$_{2}$}\xrightarrow{\text{heat}}\text{4Zn}\text{(CH$_{3}$COO)$_{\text{2(gas)}}$}+\text{H$_{2}$O} \\ &\quad \xrightarrow{\text{adsorption}}\text{Zn$_{4}$O(CH$_{3}$COO)$_{\text{6(adsorption)}}$}+\text{2CH$_{3}$COOH$_{\text{(gas)}}$} \\ & \text{Zn$_{4}$O(CH$_{3}$COO)$_{\text{6(adsorption)}}$}+\text{3H$_{2}$O}\\ &\quad \xrightarrow{\text{heat}}\text{4ZnO$_{\text{(substrate)}}$}+\text{6CH$_{3}$COOH$_{\text{(gas)}}$}{\uparrow} \end{align*}$$

Structures of the samples are investigated by XRD using a X'TRA (ARL) equipment provided with Cu tube with Kα radiation at 1.54056 Å in the range of $25 \leqslant 2\theta \leqslant 80^\circ$. Atomic force microscopy (Veeco Multimode III) is used to analyze the surface morphology of the samples. The AFM images are recorded over a scan area of 10 × 10 µm² of the film surface. Luminescence spectra are obtained by the Omin-λ3007 Spectrophotometer with a photomultiplier tube equipped with a 980 nm laser diode as the excitation source. The photocurrent of the samples are measured by CHI660E when the device is illuminated by a 980 nm laser. All measurements are performed at room temperature.

To investigate the crystalline quality of the EYZO thin films with various annealing temperatures, XRD analysis is carried out and the results are shown in Fig. 2. The XRD patterns of the films indicate the existence of a ZnO single phase with a hexagonal wurtzite structure (JCPDS No. 36-1451). The strong and dominating nature of the peak corresponding to the (002) reflection indicates the preferential c-axis orientation of crystallites which are reported to have the lowest surface energy.²⁶⁾ Moreover, the intensity of the peaks gradually increases with the increase of annealing temperatures, which indicate that the increase in annealing temperature enhances the crystallinity of the films. No impurity peaks can be identified from the XRD pattern, indicating that the sample has been synthesized into pure hexagonal ZnO.

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The surface morphology of the films was characterized by AFM, Fig. 3 shows AFM images of EYZO thin films prepared with different annealing temperatures and deposited at 400 °C for 45 min. The images are recorded on 10 × 10 µm² planar in contact mode. The surface morphology of the film shows uniform and continuous grains without any voids on the surface. Figure 3(d) illustrates the root-mean-square (RMS) surface roughness decreases from 143.67 to 68.22 nm in the investigated thickness range. Close monitoring of the AFM images of films with different annealing temperature further reveals that crystalline quality of films upgrades slightly with increasing the annealing temperature.

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Up-conversion photoluminescence excitation was performed through a 980 nm laser. As we can see from Fig. 4, the up-conversion luminescence can be enhanced by increasing the annealing temperature. And when the annealing temperature reached 900 °C, there is no emission peak observed. The observed peaks located at 527, 546, and 660 nm are exactly attributed to the ²H_11/2 → ⁴I_15/2, ⁴S_3/2 → ⁴I_15/2, ⁴F_9/2 → ⁴F_15/2 transitions of Er³⁺ ion. It is well known, the main absorption wavelength of the amorphous silicon solar cell is from 400 to 800 nm, which indicates the EYZO films convert the "waste light" from 980 nm laser to visible light which can be utilized by the amorphous silicon solar cell. Yb³⁺ ion is usually co-doped as a sensitizer so as to enhance up-conversion luminescence efficiency for its large absorption cross-section in the 900 nm NIR region, correspond to the transition of ²F_5/2 → ²F_7/2.

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The energy level of Er³⁺ and Yb³⁺ ions as well as the proposed up-conversion mechanism are shown in Fig. 5. It is obvious that, for the single 980 nm excitation corresponding to the ²F_7/2 → ²F_5/2 transition of Yb³⁺ ion and successive energy transfers from Yb³⁺ to Er³⁺, the green emission is ascribed to ²H_11/2 → ⁴I_15/2, ⁴S_3/2 → ⁴I_15/2 transitions of Er³⁺, and the red emission is ascribed to ⁴F_9/2 → ⁴I_15/2.²⁷⁾ In upconversion emission mechanisms, excited-state absorption and energy-transfer excitation have been reported.^28–30) Green emission can be explained as follows: a 980 nm laser excites from the ground state ⁴I_15/2 to the excited state ⁴I_11/2. In the excited-state absorption, the photon energy of a 980 nm laser excites from the ⁴I_11/2 level to the ⁴F_9/2 in the Er³⁺ level. In the energy-transfer excitation, the photon energy transfers from Yb³⁺(²F_5/2 → ²F_7/2) to the Er³⁺(⁴I_11/2 → ⁴F_7/2). Subsequently, a nonradioactive relaxation occurs from the ⁴F_7/2 level to the ²H_11/2 level or the ⁴S_3/2 level. Finally, the green emission is observed in ²H_11/2 → ⁴I_15/2, ⁴S_3/2 → ⁴I_15/2 transition. Conversely, red emission can be explained as follows: a 980 nm laser excites Er³⁺ from the ground state ⁴I_15/2 to the excited state ⁴I_11/2. Subsequently, a nonradioactive relaxation occurs from the ⁴I_11/2 level to the ⁴I_13/2. In the excited-state absorption, the photon energy of a 980 nm laser excites from the ⁴I_13/2 level to the ⁴F_9/2 level. In the energy-transfer excitation, the photon energy transfers from Yb³⁺(²F_5/2 → ²F_7/2) to the Er³⁺(⁴I_13/2 → ⁴F_9/2). Finally, the red emission is observed from the ⁴F_9/2 → ⁴I_15/2 transition in the Er³⁺.

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To study the efficiency of up-conversion for this material, we measure the photovoltaic parameters of the amorphous silicon solar cell containing the Er³⁺ and Yb³⁺ co-doped ZnO films. According to photovoltaic properties of the materials shown in Table I and Fig. 6, the ZnO film annealed at 800 °C yielded the best performance among these films with an overall efficiency (η) of 4.27% under the simulated solar irradiation of 100 mA·cm⁻². The open-circuit voltage (V_oc) and the short-circuit current density (I_sc) of sample d is 2.9 V and 2.51 mA·cm⁻². Compared with the conversion efficiency (4.12%) of amorphous silicon with pure ZnO structure, the increased conversion efficiency (η, ∼3.6% increase) in the amorphous silicon solar cell containing the EYZO film indicates that this material is better than La–ZnO, Ce–ZnO, Nd–ZnO, and Sm–ZnO materials.³¹⁾ The improvement in the conversion efficiency is closed to the theoretical estimation (5%) of the reference.¹²⁾

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In order to demonstrate unequivocally that introducing the EYZO film on the solar cell leads to photocurrent generation via the upconversion of NIR light, we put the films on the solar cell as showed in Fig. 7. We use 980 nm infrared laser as the excitation source because the amorphous silicon solar cell cannot absorb at the wavelength.³²⁾ When the laser turning off, the photocurrent decreases quickly while the photocurrent increase significantly when the laser turning on. No photoelectric effect is observed without excitation from the 980 nm laser. Thus, the generated photocurrent can be solely contributed to the NIR light harvesting via the EYZO film modified the solar cell. The photocurrent is found to increase with the increase of annealing temperature from 600 to 800 °C. Figure 8 shows that transient current of the cells containing the EYZO films increase with increasing excitation power of the 980 nm laser from 53.6 to 180.0 mW. The photocurrent origion of the amorphous silicon solar cell containing the EYZO films under 980 nm excitation can be explained as follows: The Yb³⁺ ions absorbed the energy from 980 nm laser and the energy transferred from Yb³⁺ to Er³⁺, then generated the red light, finally the red light absorbed by the amorphous silicon solar cell. It should be noted that although the up-converting luminescence is emitted in all spatial directions, most of the visible photons are absorbed by the amorphous silicon solar cell because of their strong absorption in the visible-light region.

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