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Abstract
Zn1−xCdxO thin films spanning the whole composition range have been explored as an active region in photonic devices. The precise control of the Cd concentration, as well as its crystalline phase, allowed to characterize their optoelectronic properties. However, its application as a transparent conducting oxide material in photonics has yet to be unveiled. Here, we fabricated Zn1−xCdxO thin films via the spray pyrolysis method and confirmed their composition via Energy-dispersive X-ray spectroscopy measurements. We obtained their dielectric function through spectroscopy ellipsometry over the 300-3200 nm wavelength range and validated our model performing transmittance measurements. We observed a nonlinear red-shift of the optical bandgap while increasing Cd concentration, from 3.2 eV for ZnO to 2.9 eV for Zn0.10Cd0.90O. We found that the samples with Cd concentration > 50% have sheet resistance as low as 19.8 Ω/Square. The use of alloyed metal oxides on organic solar cells as photonic crystal structures (PhC) was also evaluated by performing finite-difference time-domain simulations. We saw an enhancement in the light absorption leading to a 39.75% increase of the short-circuit current for Zn0.25Cd0.75O PhC when compared to organic solar cells with no PhC structure.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Transparent conducting oxide (TCO) materials have expanded the frontiers of large-area electronics due to their unique properties, including high optical transparency, excellent carrier mobilities even in the amorphous state, mechanical stress tolerance, and compatibility with organic, dielectric and photoactive materials. High-quality TCO thin films are easily fabricated using physical and solution-based deposition methods, making their applicability feasible to solar cells [1], telecommunication devices [2], and flexible organic light-emitting diode (OLED) displays [3].
In the past decades, ZnO re-emerged as an active optical component due to its potential as a replacement of GaN for UV light emission devices, such as light-emitting diodes [4], nanolasers [5], photo-detectors [6], and nanosensors [7]. The advantages of a ZnO-based system relies on its excitonic binding energy of 60 meV, allowing for more efficient excitonic emission, and it is vastly more abundant in natural resources [8]. In this framework, Zn1−xCdxO emerged as an alternative material for engineering the bandgap of towards the visible range of the electromagnetic spectrum [9–11]. However, the structural incompatibility between wurtzite ZnO and rocksalt CdO limited its success in this field [12]. For TCO applications, this is not an issue, and this compound is very promising because it combines the optical transparency of ZnO and the high conductivity of CdO [13–16].
Here, we fabricate Zn1−xCdxO alloys across the full composition range by the spray pyrolysis method, characterize their optical and electronic properties, and evaluate their performance as a photonic crystal (PhC) in an organic solar cell. Using spectroscopic ellipsometry and transmittance measurements over a broad wavelength range (300–3200 nm), we characterize the dielectric function and find that the bandgap is tailored in a nonlinear way by changing the composition fraction between the two elements. Simultaneously, we confirm that alloys with more than 50% Cd concentration presented values for the sheet resistance comparable to ITO. To further illustrate how the chemical composition affects light absorption in a PhC solar cell, we perform finite-difference time-domain (FDTD) simulations. Our results show that the Zn0.25Cd0.75O reveal an enhancement of 20.09% in the Jsc when compared to ZnO, and 39.75% when compared to the organic solar cell with no PhC structure.
2. Methods
2.1 Thin film sample fabrication
All Zn1−xCdxO thin were deposited by the spray pyrolysis method from a 0.01M precursor containing zinc acetate dihydrate and cadmium acetate dihydrate in appropriated ratio. We pulverized them in cycles on top of glass substrates at 330$^\circ{\textrm{C}}$, when the temperature dropped down to 250$^\circ{\textrm{C}}$, we interrupted the flux and annealed the layers up to 330$^\circ{\textrm{C}}$, and restarted the cycle. The formation of Zn1−xCdxO is explained by the dehydration of bound water, the decomposition and oxidation of Zn or Cd (CH3COO)2 with the liberation of CO2 and H2O, as seen in NiO growth [17]. We performed energy-dispersive X-ray spectroscopy (EDX) on three representative areas of 200 µm X 200 µm to confirm the even distribution of elements on the oxide thin films (see Table 2 in the Appendix).
2.2 Ellipsometry and transmission measurements
Our optical analyses were conducted using Variable Angle Spectroscopic Ellipsometry (VASE) with a white-light source over a wavelength range of 240−3200 nm. Incident angles of 50°, 55°, and 60°, with respect to normal incidence, were used for ellipsometry measurements and 0° for transmission measurements. The ellipsometry data were analyzed by the general oscillator model to determine the wavelength dependent dielectric function of each film. The best fitting parameters were calculated by the nonlinear Levenberg-Marquardt algorithm, which computes the minimum value of the Mean Square Error (MSE), the measure of the fit likelihood. The MSE is defined by:
2.3 Modeling the dielectric function
The dielectric function of each thin film was modeled by the General Oscillator method [18]. Using a combination of 1 to 3 Gaussian, 0 to 1 Drude and 0 to 3 Sellmeier oscillators we obtained the imaginary part of the dielectric function ${\varepsilon _2}$ as a function of wavelength $\lambda $. Then ${\varepsilon _1}$ was calculated using the Kramers−Kronig (KK) relation. We included a graded function to account for the top/bottom dielectric function layered difference due to the deposition method. Also, non-uniformity parameters and roughness were used to improve our model [19]. For all films, the MSE was less than 7.1.
2.4 Numerical simulations
The finite-difference time-domain method was used to simulate the light absorption in a photonic crystal (PhC) organic solar cell. The Zn1−xCdxO PhC had a hexagonal distribution. A normal incident plane wave source was used in the simulation and a spectral range from 300 to 2500 nm where the CW normalized impulse response of the system was multiplied by the solar power spectrum at AM 1.5. Perfect matched layer boundary conditions were used in the z-direction while periodic boundary conditions were used in the x- and y- in-plane directions. The dielectric function for all simulations was modeled through the multi-coefficient material model using the ellipsometry measurements of the alloyed thin films and ITO as the input data. For the Al bottom contact, Palik’s data was used, and for the P3HT:PCBM layer Stelling’s data was used [20]. The absorptance was calculated as a function of the wavelength for each simulation. Here, we used the net power flow out of a squared surface above the solar cell active layer and subtracted the net power flow out of a squared surface below the active layer. Finally, the short-circuit current was calculated, assuming that all the photo-generated electron-hole pairs contribute to the actual photo-current.
3. Results and discussion
We fabricate Zn1−xCdxO thin films through the spray pyrolysis method [21], as described in the Methods Section. Eight thin films are obtained from pure ZnO to Zn0.10Cd0.90O using the same deposition conditions. Figure 1(a)–(h) show the scanning electron microscope images (SEM) of the samples. All films present good adherence to the glass substrate, are continuous and crack free. Also, they are transparent, and a color change towards yellow is present while the Cd concentration increases. To assess if the fabrication method provides an even distribution of both elements on the oxide thin films, we perform EDX on three representative areas of 200 µm X 200 µm (see Table 2 in the Appendix). The amount of Zn and Cd show an average that coincides with the nominal composition of the precursor. An important property of TCOs rely on their surface roughness since this is directly related with the reflection of the incident light; therefore, most of our samples have no substantial change in its surface morphology, except for the Zn0.68Cd0.32O, Fig. 1(d). This characteristic makes the samples suitable to perform ellipsometry measurements to determine their wavelength dependent complex dielectric function, $\tilde{\varepsilon }(\lambda) = {\varepsilon _1} + i{\varepsilon _2}$.
Fig. 1. Scanning electron microscope (SEM) plan-view images of a) ZnCdO, b) Zn0.90Cd0.10O, c) Zn0.75Cd0.25O, d) Zn0.68Cd0.32O, e) Zn0.50Cd0.50O, f) Zn0.40Cd0.60O, g) Zn0.25Cd0.75O, and h) Zn0.10Cd0.90O thin films fabricated through the spray pyrolysis deposition method. Substrate: glass.
We determine $\tilde{\varepsilon }(\lambda)$ and thickness of each Zn1−xCdxO thin film through ellipsometry measurements over a broad range of the electromagnetic spectrum, 300-3200 nm wavelength range. Figure 2 display the raw experimental $\mathrm{\Psi }$ and $\Delta $ ellipsometry data (open blue and red circles, respectively), and the modeled curve (black lines). For all samples, the mean square error (MSE) of all fits is found to be less than 7.1. The ellipsometry reflection measurements are acquired at 50°, 55° and 60° angles of incidence using a J. A. Woollam spectroscopic ellipsometer (wavelength range: 240–3200 nm). The general oscillator model is applied to derive the $\tilde{\varepsilon }(\lambda)$ for each oxide thin film. In particular, the model describes ${\varepsilon _2}$ by a set of oscillators, where Gaussian (N) and Sellmeier (P) account for absorptions, and Drude (D) model the metallic behavior [18,19,22]. Next, ${\varepsilon _1}$ is obtained thought the KK consistency. The combination of all oscillators of each sample results in the following relation
Fig. 2. Raw ellipsometry measurements for the Zn1-xCdxO thin films. The Ψ (open blue circles) and Δ (open red circles) data were obtained from reflection measurements at 50, 55 and 60 degrees. The solid black lines correspond to their fit, obtained by the general oscillator method. The mean square error (MSE) of all fits is found to be less than 7.1.
One advantage of modeling the ellipsometry data with the general oscillator approach is that the KK relation between the real ${\varepsilon _1}$ and imaginary ${\varepsilon _2}$ parts of the dielectric function do not need to be enforced [19]. Figure 3 displays ${\varepsilon _1}$ and ${\varepsilon _2}$ as a function of the wavelength of the incident light. For all alloyed samples, we observe a decrease on the value ${\varepsilon _1}$ towards to negative values in the IR range of the spectrum while increasing the Cd content, indicating a decrease in resistivity, Fig. 3(a). Also, we notice a well-defined absorption peak and a nonlinear redshift by increasing the Cd content in the ${\varepsilon _2}$, Fig. 3(b). The ZnO thin films have a high transparency from the visible to the IR region of the spectrum, indicated by the lower value of ${\varepsilon _2}$, with an absorption peak in the UV range, associated with its band gap energy, in agreement with the literature [23–26]. CdO deposited by the spray pyrolysis method has a very rough surface making it not suitable to be characterized by the ellipsometry method (See Fig. 6 in the Appendix); however one expect a lower resistivity compared to ZnO with its interband transition laying in the visible range [23–25]. Another advantage of describing $\tilde{\varepsilon }(\lambda )$ with physical oscillators is that we can infer optical (optical bandgap) and electrical (sheet resistance) properties of the materials.
Fig. 3. Real (ɛ1) and imaginary (ɛ2) part of dielectric function for Zn1-xCdxO thin films as a function of wavelength obtained by ellipsometry.
To determine the optical bandgap (${E_g}$), we use the Tauc relation, ${({\alpha E} )^{{\raise0.7ex\hbox{$1$} \!\mathord{\left/ {\vphantom {1 n}} \right.}\!\lower0.7ex\hbox{$n$}}}} = C({E - {E_g}} )$, where E is the energy of the incident photon, C is the band tailing parameter, $\alpha $ is the absorption coefficient of the material, and n is the index corresponding to the nature of the transition (i.e., indirect allowed, indirect forbidden, direct allowed or direct forbidden) [27]. The absorption coefficients are calculated using the dielectric function, $\alpha = 2\sqrt 2 \pi {\lambda ^{ - 1}}\sqrt {{{({{\varepsilon_1}^2 + {\varepsilon_2}^2} )}^{1/2}} - {\varepsilon _1}} $. The best linear fit of the Tauc relation is obtained for $n = \frac{1}{2}$ (see Fig. 7 in the appendix), indicating a direct bandgap transition. Figure 4(a) show the estimated ${E_g}$ of the Zn1-xCdxO thin films. For ZnO, the value found is 3.2 eV, agreeing with the reported values [23,25,26,28]. As expected, increasing the Cd content lead to a nonlinear decrease in the optical bandgap due to the difference in the crystal structure between pure ZnO (wurtzite) and CdO (cubic) [12]. The Zn0.90Cd0.10O, Zn0.75Cd0.25O, Zn0.50Cd0.50O, Zn0.40Cd0.60O, and Zn0.10Cd0.90O present ${E_g}$ around 2.9 eV [25]. Surprisingly, Zn0.63Cd0.37O and Zn0.25Cd0.75 have their ${E_g} = 3.15$ eV, and 2.34 eV, respectively. We hypostasize that the crystal structure in the latter case is found to be dominant by the wurtzite type. As one increases the Cd concentration in the ZnO, a phase transition in Zn1−xCdxO occurs, which may cause a phase separation within the film. Hence, the thin film will either behave like one or the other [26,29]. To validate our optical bandgap model, we carry transmittance measurements. In Fig. 4(b) a sudden increase in transmittance, characterizing the optical ${E_g}$, takes place in a nonlinear manner, agreeing with the estimated band gap. Figure 4(c) presents the sheet resistance of the alloyed thin films. Increasing the Cd content of the samples a decrease in the sheet resistance is noticed. For thin films containing Cd concentration ≥ 50%, the sheet resistance value is around 20 Ω/Square, making them transparent conducting oxide films. On the other hand, ZnO, Zn0.75Cd0.25, and Zn0.63Cd0.37 are poor conductors with a relatively high sheet resistance. Likewise, Zn0.9Cd0.1 does not present any signs of conductivity, where the optical model employed does not require a Drude oscillator, that account for the metallic behavior of the samples, since an increase in the $\mathrm{\Psi }$ data in the IR region is not observed. Table 1 summarizes the obtained values for the film thickness, roughness, resistivity, conductivity, and sheet resistance. For samples with Cd concentration above 50%, we find its sheet resistance in the same order of magnitude of the ITO samples, making them suitable for applications such as gas [30] and biochemical [31] sensors, and liquid crystal displays (LCD) [32].
Fig. 4. (a) The optical bandgap (Eg) as a function of the Cd concentration. We estimated Eg by the Tauc equation, that relates the absorption coefficient (α) and the incident photon energy (E), (see Appendix 4). The absorption coefficients were calculated using the dielectric function obtained from the ellipsometry measurements. (b) Measured transmittance as a function of wavelength. (c) Sheet resistance versus Cd concentration.
Table 1. Film thickness, roughness, electrical resistivity, electrical conductivity, and sheet resistance of Zn1-xCdxO thin films obtained through spectroscopic ellipsometry measurements
Motivated by the optoelectronic behavior of the Zn1−xCdxO thin films, we assess their potential to enhanced light absorption in a hexagonal lattice photonic crystal (PhC) organic solar cell. PhCs are periodic dielectric structures that can enhance light absorption in thin solar cells at a certain wavelength. We perform FDTD simulations to calculate the absorptance of the incident light in the system as well as one figure of merit, the short-circuit current (Jsc). Figure 5(a-b) shows the schematics of the system which consist of an Al bottom contact, a photoactive layer composed by poly-3-hexylthiophene/[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) and Zn1−xCdxO PhC structure, a highly transparent conductive polymer poly(3,4ethylenedioxythiophene)-poly(styrene sulfonate) (PEDOT:PSS), and an ITO top contact [33,34]. The wavelength dependence of the absorptance spectra in the photoactive layer as we increase Cd concentration is displayed in Fig. 5(c). We observe two main peaks at the visible range, around 400 nm and 600 nm, respectively. The first absorptance peak increases and red-shifts as we increase Cd concentration, while the second decreases. The absorptance in the NIR-range is the one most benefited by the inclusion of the PhC in the photoactive layer of the organic solar cell, especially by the structures based on the alloys. We find that Zn0.25Cd0.75O is the best material among the alloys, outperforming ZnO, see Fig. 5(c). Figure 5(d) shows the calculated Jsc for each system, considering that all the photo-generated electron-hole pairs contribute to the actual photo-current. By comparison, our data show that a flat device presents Jsc = 8.90 mA/cm2 while the simple introduction of a ZnO PhC layer increases the Jsc to 10.35 mA/cm2. This tendency of increased Jsc continues for the whole concentration of Cd, and we found that Zn0.25Cd0.75O carries the optimal short-circuit current at 12.43 mA/cm2. These values correspond to a 20.09% increase in the Jsc when compared to ZnO, and a 39.75% when compared to the organic solar cell with no PhC structure.
Fig. 5. (a) Cross-section and (b) top view of the hexagonal lattice photonic crystal (PhC) organic solar cell. (c) Calculated wavelength dependence of the absorptance spectra in the photoactive layer. (d) Calculated short-circuit current (Jsc) for the flat (red triangle) and PhC (grey dot) solar cell varying the Cd concentration.
4. Conclusion
In conclusion, we have shown the correlation between the chemical composition of Zn1−xCdxO alloyed thin films and their wavelength dependent dielectric function, indicating that it can be tailored by tuning the composition fraction between the two components. Our ellipsometry and transmission measurements are in excellent agreement, validating our general oscillator ellipsometry analysis. From our model, we obtained the optical bandgap and the sheet resistance of the films. The band gap varied in a nonlinear fashion from 3.2 eV, pure ZnO, to 2.6 eV, Zn0.10Cd0.90O. Alloyed thin films with more than 50% Cd concentration presented a lower sheet resistance on the same order of magnitude as ITO films. Additionally, we have shown that for the deposition conditions implemented here the optoelectronic of some alloyed mixtures allowed for the rational design of photonic crystal structures applied to organic solar cells with a higher absorptance and Jsc than its pure counterpart. Further, the combination of transparency and electrical conductivity of the Zn0.25Cd0.75O could likely eliminate the need of the Al bottom contact once the work function of those alloys is reduced with increasing Cd concentration [35]. Our approach to fabricate highly transparent and conducting Zn1−xCdxO thin films with tunable optoelectronic properties can pave the way to the realization of more efficient photonic devices.
Appendix
Table 2. Chemical composition obtained from EDX measurements of 3 representative areas, showing a good agreement between the nominal composition and the average.
Fig. 6. SEM plan-view images of a) ZnO, b) Zn0.90Cd0.10O, c) Zn0.75Cd0.25O, d) Zn0.68Cd0.32O, e) Zn0.50Cd0.50O, f) Zn0.40Cd0.60O, g) Zn0.25Cd0.75O, h) Zn0.10Cd0.90O, and i) CdO thin films fabricated through the spray pyrolysis deposition method. Substrate: glass.
Table 3. General oscillator parameters for the Zn1-xCdxO thin films.
Fig. 7. (αE)2 as a function of the photon energy. The optical bandgaps were estimated by the Tauc equation, that relate the absorption coefficient (α) and the incident photon energy (E). The absorption coefficients were calculated using the dielectric function obtained from the ellipsometry measurements.
Funding
Fundação de Amparo à Pesquisa do Estado de São Paulo (2016/10973-4); University of Richmond.
Acknowledgments
The authors thank T. Tiwald and J. Sun (from J.A. Woollam) for fruitful discussions, and the Nanomaterials Characterization Core at the Virginia Commonwealth University for their technical support. M.R.S.D. thanks the support from C. Mayer for the SEM and EDX measurement. The authors acknowledge the financial support from the UR Arts & Sciences Summer Research Fellowships.
Disclosures
The authors declare that there are no conflicts of interest related to this article.
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