Pulsed Laser Deposition of Zn(O,Se) Layers for Optoelectronic

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PULSED LASER DEPOSITION OF Zn(O,Se) LAYERS FOR OPTOELECTRONIC APPLICATION Svetlana Polivtseva, Nicolae Spalatu, Akram Abdalla, Olga Volobujeva, Jaan Hiie, and Sergei Bereznev ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b01431 • Publication Date (Web): 16 Oct 2018 Downloaded from http://pubs.acs.org on October 17, 2018

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ACS Applied Energy Materials

PULSED LASER DEPOSITION OF Zn(O,Se) LAYERS FOR OPTOELECTRONIC APPLICATION Svetlana Polivtseva*, Nicolae Spalatu, Akram Abdalla, Olga Volobujeva, Jaan Hiie, and Sergei Bereznev School of Engineering, Department of Materials and Environmental Technology, TalTech University, Ehitajate tee 5, Tallinn, 19086, Estonia

*Corresponding Author Email: [email protected].

ABSTRACT: Zinc oxyselenide – Zn(O,Se) – could become a novel buffer layer in solar cells and a functional layer in different optoelectronic devices. In this study, we systematically investigated the influence of the deposition temperature ranging from room temperature (RT) to 650 °C on the structural and optoelectronic properties of Zn(O,Se) layers grown on PV glass substrates by one-step pulsed laser deposition in a high vacuum. All layers were characterized using energy dispersive X-ray spectroscopy (EDX), scanning electron microscopy (SEM), X-ray

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diffractometry (XRD), UV-vis spectroscopy, and the Hall and Van der Pauw technique. We demonstrated that polycrystalline, uniform and electrically conductive Zn(O,Se) layers were grown at the substrate temperatures of 500–650 °C, while those layers grown at temperatures below 500 °C were characterized as amorphous and exhibiting semi-insulating behavior. According to the XRD data, single-phase layers consisting of a ternary Zn(O,Se) phase were formed only at 500 °C. The lattice parameters monotonously decreased with both increasing deposition temperature and lowering Se concentrations in the films. The electron density increased significantly from 1.0×1014 to 3.2×1018 when changing the substrate temperature from 500 to 550 °C. We attributed these changes to the formation of vacancy-type defects in the Zn(O,Se) system. For the first time, we demonstrated the applicability of Zn(O,Se) as a buffer layer in a complete solar cell structure. We developed a prospective superstrate configuration FTO/Zn(O,Se)/CdTe/Te/Ni solar cell exhibiting a cell efficiency of 7.6 %. Our findings revealed the great potential of Zn(O,Se) to replace conventional CdS buffer layers and to open up new strategies to improve solar cell performance.

KEYWORDS: Zn(O,Se) buffer layer, thin film, XRD, SEM, optical properties, electrical properties, PLD, solar cell


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1. INTRODUCTION Global energy demand has intensified multidisciplinary research activities in the field of clean energy sources such as solar cells (SCs), hydrogen production, etc. [1-3]. During recent decades, scientists have focused on studying new functional materials and their implementation in different SC structures, as the efficiency of the currently dominating and relatively expensive solar cells based on silicon has nearly reached its theoretical efficiency limit [3]. The market still dictates the span and distribution of production costs. This means that the fabrication cost issue determines the development of new high-efficiency and cost-effective SC devices. From this point of view, polycrystalline thin-film SCs utilizing fewer amount absorbers such as CIGS, CZTS and CdTe, along with suitable buffer layers, are an attractive alternative. Even though the theoretical efficiency limit for CIGS-, CZTS- and CdTe-based SCs is beyond that of siliconbased SCs, the record efficiencies are still far from the possible theoretical values [4-7]. Up to now, thin film CIGS technology has reached a record efficiency of 22.9 % [8], CdTe and CZTS technologies have achieved 22.1 % [9] and 12.6 % [7] conversion efficiency, respectively, on small area. Recent research demonstrated that the presence of foreign elements in CIGS, CZTS, and CdTe absorber layers can play a vital role in increasing the cell performance. For instance, the incorporation of rubidium fluoride (RbF) into CIGS films led to SCs with an outstanding record efficiency of 22.6 % in 2016 [5]. The inclusion of selenium into the solution-processed CIGS absorber layers enabled it to achieve a power conversion efficiency of 17.3 % [10]. The selenization of solution-derived CZTS films promoted the improvement of solar cell efficiency from 5.0 to 10.1 % [11]. The approach of selenium band grading has also been advised for CdTe thin-film solar cells as a way to increase the power conversion efficiency [12, 13]. Following this innovative concept, an ultrathin CdSe layer has been incorporated into the cell structure as a


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partner to CdTe, in addition to or instead of CdS [12, 14, 15]. Interdiffusion between CdSe and CdTe occurring during the cell processing yielded the formation of a lower band gap CdTe(1x)Sex

layer, which improved the carrier lifetime and thereby contributed to the enhancement of

the photocurrent [12]. Baines et al. [12] mentioned that the inclusion of CdSe into the cell structure enabled cell processing to occur directly with simple metal oxide layers and without the necessity for CdS, which has side absorption in the region of 300–525 nm [16, 17]. To follow the approach of selenium band grading [12-17], herein we implemented Zn(O,Se) layers in solar cells for the first time. Before we initiated this study, few reports had been available on the fabrication of Se-containing ZnO layers, along with the investigation of their structural, optical, and electrical properties [18-21]. Only one paper covered the investigation of Zn(O,Se) alloy films grown by

pulsed laser deposition (PLD) [18]. Mayer et al. [18]

demonstrated that the optical band gap values of PLD-deposited Zn(O,Se) layers varied between 1.6 eV and 2.8 eV, depending on the variations of Se concentration in films, from 52 % to 1 %. However, the growing conditions for the fabrication of Zn(O,Se) layers by PLD were not clearly provided in [18]. It should be noted that no studies covering the effect of deposition temperature on properties of Zn(O,Se) films grown by PLD as well as no applications for Zn(O,Se), including its application as a buffer layer in solar cells, have been reported to date. This fact motivated us first to deposit Zn(O,Se) films at various substrate temperatures using the one-step PLD method in a high vacuum. We then systematically studied the effect of the deposition temperature on the structural and optoelectronic properties of formed films. We finally implemented Zn(O,Se) as a buffer layer to complete SC structures based on a CdTe photoabsorber and discussed their


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photovoltaic properties before and after thermal activation in the presence of cadmium chloride (CdCl2). 2. RESULTS AND DISCUSSION 2.1 Elemental composition and morphology. The elemental compositions of all the films produced by PLD at different substrate temperatures were analyzed by energy dispersive X-ray (EDX) spectroscopy. The EDX analysis showed that the composition of the Zn(O,Se) layers is close to the composition of the target when the substrate temperature was varied in the range of RT–300 °C (Table 1). On the other hand, we observed a significant decrease in the Se content from approximately 13 to 5 % with the increase of the substrate temperature from 300 to 650 °C (Table 1). The observed effect can be attributed to the evaporation of Se during the deposition process before the nucleation stage, similarly to that reported for sulfur-based ternary Zn(O,S) films grown by PLD [22]. Table 1 The atomic percentages of zinc, oxygen and selenium according to the EDX data, thicknesses and optical band gap values (Eg) of the Zn(O,Se) films deposited at various substrate temperatures (Ts). Ts (°C) RT 300 400

Elements (at. %) a Zn

O

Se

Thickness (nm)

50 50 50

37 37 39

13 13 11

730 630 560

Eg (eV) 2.91 2.83 2.85

500 50 42 8 500 2.85 550 50 43 7 460 2.90 600 50 44 6 420 2.85 (3.23) 650 50 45 5 410 2.87 (3.23) aThe atomic percentages for elements and film thickness were determined by averaging the values obtained from three measurements for each sample.


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The surface morphology and cross-sectional views of films deposited at substrate temperatures varying from RT to 650 °C are compared in Figure 1. It should be noted that the morphological properties of films fabricated in the temperature region of RT–400 °C were very similar (See Figure 1, Figure S2 in the SI). HR-SEM images show that the films are dense, homogeneous, and well-adherent to the substrate. Films deposited at temperatures below 500 °C are amorphous, while those grown in the temperature range of 500–650 °C exhibit a polycrystalline structure (See also the XRD patterns in Figure 2). According to the cross-sectional views, the film thickness decreases with the increase of the substrate temperature from 730 nm for RT to 410 nm for the film produced at 650 °C. This phenomenon is similar to that reported for the PLD-grown Zn(O,S) layers in our previous work [22]. For instance, the thicknesses of Zn(O,S) films grown at RT and 400 °C have been approximately 650 nm and 580 nm, respectively. This difference in film thicknesses slightly exceeds 10 %, and it has been explained by the formation of a polycrystalline, denser layer of Zn(O,S) [22]. For Se-based materials, the difference in the thicknesses of films grown at similar temperatures of RT and 400 °C is almost 23 %. These changes cannot be explained by the growing crystalline and denser material only, as both films are amorphous. The thickness of the film deposited at 650 °C is almost half of the thickness of the film deposited at RT, 410 and 730 nm, respectively (Table 1). The observed effect might be explained by several reasons. First, it could be speculated that the desorption of ZnSe species from the substrate exceeds their adsorption at higher deposition temperatures, thereby resulting in thinner films. Second, the crystallization and densification of films at higher substrate temperatures (500–650 °C) also contribute to the decrease in the film’s thickness. Finally, the occurrence of side chemical


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reactions in plasma during the film processing (e.g., formation of volatile species such as SeO2) should also be considered.

Figure 1. HR-SEM images of the Zn(O,Se) films deposited onto glass substrate at various substrate temperatures.

2.2 XRD analysis. The structural properties of formed films were characterized using X-ray diffractometry (XRD). The diffractograms of the films grown at temperatures below 500 °C are omitted as they show no peaks, thus suggesting an amorphous structure. Figure 2 shows the XRD patterns of Zn(O,Se) films grown in the temperature range of 500–650 °C. For more reliable analysis of the film phase composition, the layers of ZnO and ZnSe were also deposited at 500 °C and 650 °C using similar experimental conditions. The diffractograms of the PLD ZnO or ZnSe layers were similar irrespective of the deposition temperature, and Figure 2 additionally shows the diffractograms of the ZnO and ZnSe layers grown at 650 °C. According to the XRD results, ZnO films grown at 500 or 650 °C are composed of the cubic ZnO phase (PDF 03-065-


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2880), and ZnSe films deposited at the same temperatures consist of the cubic ZnSe phase (PDF 01-071-4771). Both ZnO and ZnSe layers have a strong preferred orientation along the (111) reflectance plane [23].

Figure 2. XRD patterns of the Zn(O,Se) films deposited in the temperature region of 500–650 °C and ZnO and ZnSe films deposited at 650 °C.

The diffractogram of the Zn(O,Se) film grown at 500 °C exhibits only one peak located at the 2θ value of 33.65°, thus suggesting that the formed film consists of a single phase. This peak cannot be assigned to the (111) plane of the cubic ZnO (2θ value of 34.50°) or the (111) plane of the cubic ZnSe (2θ value of 27.30°) phases [23]. In the literature, this reflection has been attributed to the (002) plane of ZnO(1-x)Sex phase, in which x ranges from 8 to 10 % [18]. Thus,


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we can conclude that our Zn(O,Se) layer grown at 500 °C consists of the Zn(O,Se) phase as the main crystalline phase. With an increase in the deposition temperature from 500 to 550 °C, a new diffraction peak appears at the 2θ value of 27.85°, which could be assigned to the cubic ZnSe phase. A noticeable strengthening of the intensities for both XRD peaks along with their narrowing can be seen with an increase in the deposition temperature up to 650 °C (Figure 2). This effect can probably be attributed to the increase in the crystallite size (See Table 2) and the increased amount of the polycrystalline phase vs. amorphous phase. The crystallite size of films was calculated using the following relation [24]: 𝐷 = 0.94𝜆(𝛽𝑐𝑜𝑠𝜃) ―1, (1) where λ is the wavelength of the X-ray radiation (1.5406 Å), θ is the Bragg angle and β is the full width at the half maximum (FWHM) of the peak located in the 2θ region of 33–35° in radians. By carefully evaluating the XRD patterns of Zn(O,Se) films, we observed the monotonic shift of the main peak that appeared in the 2θ region of 33–35° to the higher diffraction angles with the simultaneous increase of the deposition temperature and reduction of the Se content. The changes in the peak positions could be associated with the variability in the lattice constants. The lattice parameters corresponding to the cubic crystal structure were calculated using the following relation [24]: 𝑎 = 𝑑 ℎ2 + 𝑘2 + 𝑙2 , (2) where (hkl) are the Miller indices, and d is the interplanar distance. The gradual decrease in the lattice parameters with the increase of the deposition temperature from 500 to 650 °C is likely caused by the strain changes. The microstrain (ɛ) was estimated using the following relation: ɛ = 𝛽𝑐𝑜𝑠𝜃/4 , (3)


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The estimated structural characteristics of Zn(O,Se) films are summarized in Table 2. It is clearly seen that the lattice constants and the microstrain in films decrease with the increase in the deposition temperature from 500 to 650 °C. The changes are consistent with the decreasing amount of Se (See Tables 1). The ionic radius of Se2– (1.98 Å) is higher than that of O2– (1.32 Å) [25], and oxygen has a higher electronegativity than selenium. Thus, oxygen partially substitutes for selenium with the increase in the deposition temperature, and the lattice constants of Zn(O,Se) structures become smaller. Considering the data presented above, it can be concluded that the Zn(O,Se) phase formed at 500 °C disproportionates at higher substrate temperatures according to the equation (4) with the formation of the ZnO and ZnSe phases: 𝑍𝑛(𝑂𝑥,𝑆𝑒(1 ― 𝑥))→𝑥𝑍𝑛𝑂 + (1 ― 𝑥)𝑍𝑛𝑆𝑒 , (4) This process is probably more favorable when films are deposited at temperatures above 550 °C. Further studies are needed to confirm this hypothesis. Table 2 The structural parameters of the Zn(O,Se) films deposited in the temperature region of 500–650 °C. Structural parameters a Ts (℃) 500

d (Å)

lattice constant (Å)

Crystallite size (nm)

micro strain (ε)

2.660 ± 0.003

4.605 ± 0.005

23 ± 0.4

1.59×10-3

550 2.640 ± 0.002 4.570 ± 0.004 22 ± 0.5 1.50×10-3 600 2.622 ± 0.002 4.542 ± 0.004 26 ± 0.3 1.40×10-3 650 2.615± 0.002 4.530 ± 0.003 35 ± 0.3 1.04×10-3 d-spacing (d). aEach parameter was determined by averaging the values obtained from three XRD runs for each sample. The error represents the standard deviation.

2.3 Optical properties. The optical transmittance spectra of polycrystalline Zn(O,Se) films grown in the temperature range of 500–650 °C are shown in Figure 3. The total optical


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transparency is ca. 80 % in the main part of the visible region of the spectra. With the increase of the deposition temperature accompanied by a simultaneous decrease in the Se content in the films, the absorption edge shifts from higher to lower wavelengths (blueshift). The optical bandgap values of films were evaluated from the optical transmission data using the Tauc relation, similarly to that reported in [22, 26, 27], and tabulated in Table 1. Although the Tauc plots are not strongly linear, the trend of obtained bandgap values is consistent with the transmission data, and is valid (Figure 3, Figures S3 and S4 in the SI).

Figure 3. Total transmittance spectra of the Zn(O,Se) films deposited in the temperature region of 500–650 °C.

Deposition at 500 °C yields the film with the Eg value of 2.85 eV, and the film was single phase according to XRD (Figure 2). The films deposited at 550 °C demonstrate the slightly higher band gap value of 2.90 eV, as the concentration of Se is decreased in the film (ZnSe Eg = 2.7 eV, ZnO Eg = 3.2 eV). Similar changes in band gap values have been reported elsewhere [18, 19]. Films formed at 600 and 650 °C exhibit two band gap values (Table 1, Figure S4 in the SI). It is in good agreement with the XRD data splitting of the Zn(O,Se) phase to a mixture of ZnSe and ZnO phases (Figure 2).


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2.4 Electrical properties. As we propose the PLD-grown Zn(O,Se) thin films as a potential n-type buffer layer for p-type absorbers such as CdTe, SnS, CIGS or CZTS, the electrical properties of these films depending on the PLD processing conditions are considered to be extremely important. The four probe method at RT was used to correlate the analyses with the resistivity (ρ), Hall mobility (µ) and carrier concentration (n) of Zn(O,Se) films deposited in the substrate temperature range of 500–650 ºC. All films grown at temperatures below 500 ºC were characterized as having high dark resistivity, which was attributable to the amorphous semiinsulating Zn(O,Se) materials. Table 3 displays the obtained results for the substrate temperatures ranging from 500 to 650 ºC. It was indicated from the Hall effect measurements that electrons are the majority carriers in PLD-grown Zn(O,Se) films, similar to the data reported in [18]. Table 3 The electrical parameters of the Zn(O,Se) films deposited in the temperature region of 500–650 °C. Electrical parameters a Ts (℃)

ρ (Ω•cm)

µ (cm2/V.s )

n (cm-3)

500 1.5×104 4.0 1.0×1014 550 6.8×10-1 3.0 3.2×1018 600 6.8×10-2 3.0 2.7×1019 650 1.1×10-1 4.0 1.4×1019 aThe resistivity (ρ); Hall mobility (µ); carrier concentration (n). The changes in the electrical properties of Zn(O,Se) films deposited in the temperature range of 500–650 °C can be divided into two stages. The first stage (500–550 °C) is characterized by a sharp decrease of the film resistivity by 5 orders of magnitude from 1.5×104 to 6.8×10–1 Ω·cm and the abrupt increase of electron concentration from 1014 to the level of 1018 cm–3. In the


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second stage (600–650 °C), the electron density further increases to the level of 1019 cm–3, thus reaching a plateau of stable electron concentrations. The reduction of Zn(O,Se) film resistivity by 5–6 orders of magnitude with the creation of high electron density of approximately 1019 cm–3 at higher deposition temperatures represents the most striking result. For the explanation of this phenomenon, we analyzed some thermodynamic aspects in the Zn(O,Se) system. Namely, we compared the Gibbs energies of the formation for ZnO, ZnSe, and SeO2 at elevated temperatures. 1

𝑍𝑛(𝑔) + 2𝑂2(𝑔)↔𝑍𝑛𝑂(𝑠) (∆G500 °C = –74.76 kcal/mole), (5) 1

𝑍𝑛(𝑔) + 2𝑆𝑒2(𝑔)↔𝑍𝑛𝑆𝑒(𝑠) (∆G500 °C = –51.33 kcal/mole), (6) 1 2𝑆𝑒2(𝑔)

+ 𝑂2(𝑔)↔𝑆𝑒𝑂2(𝑔) (∆G500 °C = –30.97 kcal/mole), (7)

Scheme 1 represents the dissociation reactions of ZnSe, ZnO and Zn(O,Se) in the gas phase and the formation of Frenkel defects (equations 8-13) in the system at the equilibrium between solid and gas phase. Scheme 1. The dissociation reactions of ZnSe, ZnO and Zn(O,Se) in the gas phase and the formation of Frenkel defects (equations 8-13) in the system.


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According to the thermodynamically favored reactions (5-7), ZnO is the most stable phase in the Zn(O,Se) system since it exhibits the highest value of the Gibbs free energy of the formation (See also Table S1 in the SI). Thus, the partial pressure of zinc above ZnO is lower than the partial pressure of zinc above ZnSe. Moreover, the formation of SeO2 also decreases the partial pressures of oxygen and selenium above ZnO and ZnSe, respectively, and consequently generates Zn-rich composition in the vapor phase of the Zn(O,Se) system. Under these conditions, the increased partial pressure of zinc vapor generates more selenium and oxygen vacancies (n-type intrinsic defects) and as a result increases the overall electron density in Zn(O,Se) polycrystalline films. This is in good agreement with the XRD and EDX data. The XRD data show that the lattice parameter decreases with the increase of the substrate temperature, thus indicating the enhanced concentration of vacancy type defects. The EDX data demonstrate a monotonic decrease in selenium concentrations with the increase in the substrate temperature, thus indicating the formation of volatile se-containing species (e.g., vapors of Se and SeO2). 2.5 Application of Zn(O,Se) films in a Zn(O,Se)/CdTe solar cell. For a proof-of-concept demonstration, we applied Zn(O,Se) films deposited by PLD at 500 °C as a buffer layer in a conventional superstrate configuration FTO/Zn(O,Se)/CdTe/Te/Ni solar cell. Figure 4 shows the current density–voltage (J–V) characteristics for the device measured under AM1.5 conditions before and after the CdCl2 activation treatment. The corresponding photovoltaic parameters are included in Table 4. The untreated solar cell showed a very low efficiency of 0.1 %. For this solar cell, the J–V curve (Figure 4a) exhibited a very pronounced roll-over effect (photocurrent saturation at high forward bias). Historically, such an anomaly in the J–V response of CdTe solar cells with CdS as a buffer layer was explained as resulting from the interaction of three effects:


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the low density of the electronically active p-type dopant, the similar densities of the compensating recombination centers, and the Schottky barrier between CdTe and the metal back contact [28, 29]. It is well known that the untreated CdTe films deposited by CSS are highly resistive p-type polycrystalline layers [30] due to the self-compensation of intrinsic defects such as cadmium and tellurium vacancies [31, 32].

Figure 4. Comparison of (a) current density–voltage (J–V) characteristics and (b) external quantum efficiency spectra of complete CdTe/Zn(O,Se) solar cells before and after CdCl2 activation treatment.

Considering this fact, the most evident rollover (Figure 4a) would likely be due to a reduced acceptor density in the CdTe absorber and the formation of a low-quality p-n heterojunction. All photovoltaic parameters including the open-circuit voltage (VOC), fill factor (FF), short-circuit current density (Jsc), and efficiency were improved substantially after the CdCl2 activation treatment (Table 4). This is in good agreement with the observed natural occurrences in CdTe/CdS devices after the same processing step [33]. It is widely accepted that the CdCl2 activation treatment aids grain growth and sintering by recrystallization in the CdTe and CdS layers. It passivates and reduces the density of GBs, and increases the p-type conductivity (acceptor density reaching 1014∕cm3 level) and the minority carrier lifetimes (>3 ns) in the CdTe


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films based on the AFM, SEM, XRD, and time-resolved photoluminescence measurements [29, 30, 34-36]. In particular, we demonstrated previously that this activation step induces the formation of a liquid flux and mass transport through the melted phase, thereby promoting grain growth, doping and intermixing at the CdTe/CdS interface by recrystallization and sintering [33, 37]. Table 4 Photovoltaic parameters of CdTe/Zn(O,Se) solar devices before and after CdCl2 activation treatment. Sample

VOC (mV)

JSC (mA/cm2)

FF (%)

PCE (%) a

SC as dep

150 (145 ± 2)

3.2 (2.8 ± 0.3)

28 (20 ± 5)

0.1 (0.1 ± 0.05)

SC+CdCl2

620 (620 ± 3)

23.5 (22.8 ± 0.5)

55 (50 ± 3)

8 (7 ± 0.6)

Short-circuit current density (JSC); open-circuit voltage (VOC); fill factor (FF); photocurrent conversion efficiency (PCE). aEach parameter was determined by averaging the values for 5 solar devices. Table presents the peak value and average ± standard deviation (in brackets) of device parameters.

Interestingly, for the device with a 500 nm thick Zn(O,Se) film, we obtained a significantly lower VOC of 620 mV compared to 820 mV for the 150 nm thick CSS CdS-based device [33], although the latter has a lower band gap. For the CdS/CdTe superstrate configuration device, the efficiency of 11.6 % has been reported by authors [33]. The loss in VOC could be due to a number of reasons: (i) the lattice mismatch at the interface of CdTe-Zn(O,Se) leading to high interfacial recombination and thus a reduced VOC, or (ii) the formation of the lower band gap CdTe(1-x)Se(x) phases following the Se diffusion that induces band gap grading in CdTe, which in turn means the reduced maximum attainable VOC. The latter hypothesis is well accepted in CdSe/CdTe heterojunctions. It has been demonstrated that during cell processing, the CdSe diffuses into the CdTe, thus converting it from a photoinactive CdSe (wurtzite) phase to a photoactive CdTe(1-


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x)Sex

(zincblende) structure (Eg≈1.36 eV) and resulting in photocurrent enhancement [12, 15, 38].

It has also been suggested that there is a bandgap grading within the CdTe(1-x)Sex layer resulting in a subsequent increase in carrier lifetime [13]. As the third reason (iii), the CdCl2 activation treatment might change the concentration of charge carries in the Zn(O,Se) layer, which affects the width of the depletion layer in the p-n heterojunction and hence the attainable Voc. Further studies are needed to prove this hypothesis. The above assumptions are supported by simultaneous changes in the short and long-wavelength regions of the external quantum efficiency (EQE) for the CdTe-Zn(O,Se) device (Figure 4b). The EQE showed an enhanced response in both the short and long-wavelength regions in the result of the CdCl2 activation treatment. Such phenomena can be explained as being due to the alloy formation at the Zn(O,Se)-CdTe

interface.

Similar

effects

have

been

observed

in

CdS(CdSe)/CdTe

heterostructures in the results of CdCl2 activation treatment [12, 15, 33, 38]. It is also known that alloy formation has both beneficial and detrimental effects. The interdiffusion can narrow the absorber-layer bandgap, resulting in higher long-wavelength quantum efficiency. In the shortwavelength, the interdiffusion can decrease the parasitic absorption of the window layer. Although this enhancement is beneficial for window transmission, the non-uniform material consumption may occur leading to the formation of parallel junctions between the absorber and the front contact, and hence to the reduction of solar-cell performance [33]. Our results support these well-studied processes. The increased absorption at long wavelengths indicates the formation of a CdTe(1-x)Sex alloy phase at the Zn(O,Se)-CdTe interface with a band gap value of ≈1.4 eV. At the same time, the enhanced EQE response in the short-wavelength region (350–580 nm) suggests the thinning of Zn(O,Se) as a result of the same intermixing effect at the Zn(O,Se)CdTe interface, which is also beneficial for window transmission.


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Following the achieved 7.6 % efficiency of the FTO/Zn(O,Se)/CdTe/Te/Ni solar cell, there remains substantial headroom for improvement in this device, which may allow for the VOC to be increased. Such an improvement may involve the optimization of the Zn(O,Se) thickness by controlling the parameters in the PLD process, the optimization of the selenium content in the target, and postdeposition treatments. Overall, we demonstrates the feasibility of Zn(O,Se) as a buffer layer for CSS CdTe, but significant further work is required for the optimization of highefficiency solar cells. 3. CONCLUSIONS In this study, we showed that dense, uniform, polycrystalline and conductive films consisting of Zn(O,Se) as a main crystalline phase can be grown at 500 °C using a one-stage PLD in a high vacuum. According to XRD and UV-Vis data, films grown at higher substrate temperatures of 600 and 650 °C were composed of a mixture of ZnO and ZnSe phases, and it was likely that the formed Zn(O,Se) phase was metastable and decomposed at those temperatures. Optical studies revealed that Zn(O,Se) films obtained at 500 °C have high transmittance in the visible light region and exhibited an optical band gap value of 2.85 eV, which is suitable for application as a buffer layer in solar cells. It was found that the resistivity of Zn(O,Se) films dramatically decreased from the practically insulating state to 1.5×104 at the substrate temperature of 500 °C and then to 6.8×10–1 Ω·cm along with the increase of the electron density from 1014 to 1018 cm–3 when the substrate temperature was changed from 500 to 550 °C. A plateau of stable electron concentrations of approximately 1019 cm–3 was achieved when the deposition temperature was equal to 600 °C or higher. The reason for these changes can be connected to the possible formation of selenium and


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oxygen vacancies, thus resulting in the enhancement of the overall electron density in Zn(O,Se) films. For the first time, we demonstrated that Zn(O,Se) layers can be used as a buffer layer in complete solar cell structures instead of CdS, and a prepared FTO/Zn(O,Se)/CdTe/Te/Ni solar cell with a 500 nm thick Zn(O,Se) layer achieves the efficiency of 7.6 %. Our initial study showed that there remains substantial headroom for improvement in this device, which may allow for the VOC to be increased. Such an improvement may involve altering the Zn(O,Se) thicknesses, changing selenium contents in Zn(O,Se) layers, or applying postdeposition treatments. Overall, we demonstrated the feasibility of Zn(O,Se) as a buffer layer for CdTe, but significant further work is required to reach high-efficiency solar cells.

4. EXPERIMENTAL METHODS Targets. A commercial hot-pressed target containing 75 at.% of ZnO and 25 at.% of ZnSe (p.a. > 99.99 %, Testbourne Ltd.) was used for the deposition of Zn(O,Se) layers using the PLD method. ZnO and ZnSe (both having p.a. > 99.99 %, Testbourne Ltd.) targets were used for the deposition of ZnO and ZnSe layers. The dimensions of all targets that were used were 25.4 mm dia. × 6 mm thickness. Films deposition. Zn(O,Se) layers were deposited onto PV glass substrates with dimensions of 36 mm × 18 mm × 1 mm. The substrate temperature (Ts) was varied from RT to 650 °C. All glass substrates were ultrasonically cleaned in 20 % Decon 90 at RT for 15 min and followed by washing in deionized water at 50 °C for 15 min. Then, they were dried under a filtered air flow and subjected to treatments in a NovaScan Digital UV-Ozone cleaning system for 15 min. The deposition was carried out using a Neocera Pioneer PLD system equipped with a 248 nm KrF


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excimer laser (Coherent Compex 102 F). The pressure in the vacuum chamber during the deposition of layers was 3 × 10−6 Torr. Irrespective of the deposition temperature, all layers were fabricated using the following conditions: the distance between the target and the substrate was 9 cm, the number of laser pulses was 40000, the pulse energy was 200 mJ, the repetition rate was 10 Hz, and the laser beam was focused on the target with a spot size of ~5 mm2. Both the target and substrate were rotated during the fabrication process for the uniform distribution of the layer material. For comparison, ZnO and ZnSe layers were deposited at 500 and 650 ºC using similar deposition conditions. Film characterization. X-ray diffractograms (XRDs) of all the films were recorded using a Rigaku Ultima IV diffractometer with monochromatic Cu Kα1 radiation (λ = 1.5406 Å) at 40 kV and 40 mA using the silicon strip detector D/teX Ultra. Samples were studied in a 2θ range of 10–60° with a scan step of 0.02°. The surface morphologies and cross sections of the films were investigated using a high resolution scanning electron microscopy (HR-SEM), and the elemental compositions of the films were studied by energy-dispersive X-ray (EDX) analysis using a Zeiss Merlin scanning electron microscope equipped with the Bruker EDX-XFlash6/30 detector. The SEM and EDX measurements were performed at acceleration voltages of 2 and 10 kV, respectively. The optical transmittance and reflectance spectra of films were recorded in the wavelength range of 300–1000 nm at room temperature using a Shimadzu UV-1800 UV-Vis spectrophotometer. Based on the Tauc relation (equation 14), the band gap (Eg) values were found by extrapolating the straight-line portion of the (αhν)2 versus hν graph to a zero absorption coefficient value: (𝛼·ℎ𝜈)𝑛 = 𝐴(ℎ𝜈 ― 𝐸𝑔) , (14)


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where α is the absorption coefficient, h is the Planck constant, A is the constant which is independent from the photon energy, Eg is a bandgap energy, hν is the incident photon energy, and n=2 for the direct transitions. The electrical parameters of the formed Zn(O,Se) layers were investigated at RT using the MMR’s variable temperature Hall System and a Hall Van der Pauw controller H-50. Indium was used as a contact material on the Zn(O,Se) layers using the four-point contact Van der Pauw geometry. The changes in the Gibbs free energies for the reactions were calculated using the Database of HSC Chemistry Ver. 6.0. by Outoukumpu Research Oy, Pori, Finland. Solar cell fabrication and characterization. For the fabrication of CdTe/Zn(O,Se) solar cells in a superstrate configuration, Zn(O,Se) films were deposited at 500 ºC onto FTO-coated soda-lime glass substrates with dimensions of 20 mm × 20 mm × 1 mm. The typical sheet resistance of the FTO layer was 15 Ω/sq, with a nominal film thickness of 200 nm. All other deposition parameters were fixed as described in the section of the films’ deposition. The CdTe absorber layers were grown by the close space sublimation (CSS) method at 500 ºC using 5N CdTe source materials. The detailed experimental parameters for the deposition of CdTe layers are described in [31, 32]. The procedure for the CdCl2 activation treatment along with the description of all further steps necessary to complete solar cell structures are presented in [31, See the SI]. The complete solar cells were characterized using the current–voltage characteristics (J–V) and the external quantum efficiency (EQE) measurements. The J–V curves were measured under a white light with an illumination intensity of 100 mW/cm2 (AM1.5) using the AUTOLAB PGSTAT 30. The EQE was measured in the spectral region of 300–1000 nm using a computercontrolled SPM-2 monochromator (Carl Zeiss-Jena) and a 300 W Xe lamp as the excitation light


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source. The dispersed light from the Xe lamp incident on the solar cell as monochromatic light was optically chopped at 30 Hz.

ACKNOWLEDGMENT This research was supported by the institutional research funding IUT19-28 and IUT19-4 of the Estonian Ministry of Education and Research, TalTech base finance project B54, PUT1495 Project of the Estonian Ministry of Education and Research, the European Union through the European Regional Development Fund project “Center of Excellence” TK141, “Advanced materials and high-technology devices for sustainable energetics, sensorics and nanoelectronics.”

Supporting Information Additional Figures (S1-S5), Table S1 and details on additional steps for solar cell fabrication are included in the Supporting Information and referring to: Photographs and additional HR-SEM images of Zn(O,Se) films, Optical data, Schematic of CSS system, Details on calculated Gibbs energies, Details on methodology for postdeposition CdCl2 treatment and contacting.

REFERENCES 1. Zhang, K.; Ma, M.; Li, P.; Hwan Wang, D.; Hyeok Park, J. Water Splitting Progress in Tandem Devices: Moving Photolysis beyond Electrolysis. Adv. Energ. Mater. 2016, 6, 1600602. 2. Walter, M. G.; Warren, E. L.; McKone, J. R.; Boettcher, S. W.; Mi, Q. X.; Santori, E. A.; Lewis, N. S. Solar Water Splitting Cells. Chem. Rev. 2010, 110, 6446–6473. 3. Yoshikawa, K.; Kawasaki, H.; Yoshida, W.; Irie, T.; Konishi, K.; Nakano, K.; Uto, T.; Adachi, D.; Kanematsu, M.; Uzu, H.; Yamamoto, K. Silicon heterojunction solar cell with


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12. Baines, T.; Zoppi, G.; Bowen, L.; Shalvey, T. P.; Mariotti, S.; Durose, K.; Major, J. D. Incorporation of CdSe layers into CdTe thin film solar cells. Sol. Energy Mater. Sol. Cells 2018, 180, 196–204. 13. Munshi, A.; Kephart, J.; Abbas, A.; Raguse, J.; Beaudry, J.; Barth, K.; Walls, J.; Sampath, W. Polycrystalline CdSeTe/CdTe absorber cells with 28 mA/cm−2 short-circuit current. In: IEEE Journal of Photovoltaics, 2018, 8. 14. Swanson, D. E.; Sites, J. R.; Sampath, W. S. Co-sublimation of CdSexTe(1-x) layers for CdTe solar cells. Sol. Energy Mater. Sol. Cells 2017, 159, 389–394. 15. Poplawsky, J. D.; Guo, W.; Paudel, N.; Ng, A.; More, K.; Leonard, D.; Yan, Y. Structural and compositional dependence of the CdTexSe(1−x) alloy layer photoactivity in CdTe-based solar cells, Nat. Commun. 2016, 7. 16. Romeo, N.; Bosio, A.; Canevari, V.; Podestà, A. Recent progress on CdTe/CdS thin film solar cells. Sol. Energy 2004, 77, 795–801. 17. Kephart, J. M.; McCamy, J. W.; Ma, Z.; Ganjoo, A.; Alamgir, F. M.; Sampath, W. S. Band alignment of front contact layers for high-efficiency CdTe solar cells. Sol. Energy Mater. Sol. Cells 2016, 157, 266–275. 18. Mayer, M. A.; Speaks, D. T.; Yu, K. M.; Mao, S. S.; Haller E. E.; Walukiewicz, W. Band structure engineering of ZnO1−xSex alloys. Appl. Phys. Lett. 2010, 97, 022104. 19. Polity, A.; Meyer, B. K.; Krämer, T.; Wang, C.; Haboeck, U.; Hoffmann, A. ZnO based ternary transparent conductors. Phys. Status Solidi A 2006, 203, 2867–2872. 20. Shan, W.; Walukiewicz, W.; Ager III, J. W.; Yu, K. M.; Wu, J. Effect of Oxygen on the Electronic Band Structure in ZnOxSe1-x Alloys. PACS numbers: 71.20.-b, 71.20.Nr, 72.80.Ey, 78.20.-e


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21. Gromyko, I.; Dedova, T.; Polivtseva, S.; Kois, J.; Puust, L.; Sildos, I.; Mere, A.; Krunks, M. Electrodeposited ZnO morphology transformations under the influence of SeO2 additive: Rods, disks, nanosheets network. Thin Solid Films 2018, 652, 10−15. 22. Bereznev, S.; Kocharyan, H.; Maticiuc, N.; Naidu, R.; Volobujeva, O.; Tverjanovich A.; Kois. J. One-stage pulsed laser deposition of conductive zinc oxysulfide layers. Appl. Surf. Sci. 2017, 425, 722–727. 23. International Centre for Diffraction Data (ICDD), Powder Diffraction File (PDF 03-0652880), PDF-2 Release 2008. 24. Srinivasa Reddy T.; Santhosh Kumar, M. C. Co-evaporated SnS thin films for visible light photodetector applications. RSC Adv. 2016, 6, 95680. 25. Cordero, B.; Gómez, V.; Platero-Prats, A. E.; Revés, M.; Echeverría, J.; Cremades, E.; Barragán, F.; Alvarez, S. Covalent radii revisited. Dalton Trans. 2008, 21, 2832–2838. 26. Polivtseva, S.; Oja Acik, I.; Katerski, A.; Mere, A.; Mikli, V.; Krunks, M. Tin sulfide films by spray pyrolysis technique using L-cysteine as a novel sulfur source. Phys. Status Solidi C 2016, 13, 18–23. 27. Polivtseva, S.; Katerski, A.; Kärber, E.; Oja Acik, I.; Mere, A.; Mikli, V.; Krunks, M. Postdeposition thermal treatment of sprayed SnS films. Thin Solid Films 2017, 633, 179–184. 28. McMahon, T. J.; Fahrenbruch, A. L. Insights into the nonideal behavior of CdS/CdTe solar cells. In: Conference Record of the 28th IEEE Photovoltaic Specialists Conference 2000, 539– 542. 29. McCandless, B. E.; Sites, J. R. Cadmium telluride solar cells. In Handbook of Photovoltaic Science and Engineering; A. Luque and S. Hegedus, Eds., West Sussex, England: John Wiley & Sons Ltd.; 2003, 617–662.


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30. Spalatu, N.; Krunks, M.; Hiie, J. Structural and optoelectronic properties of CdCl2 activated CdTe thin films modified by multiple thermal annealing. Thin Solid Films 2017, 633, 106–111. 31. Marfaing, Y. Fundamental studies on compensation mechanisms in II–VI compounds, J. Cryst. Growth 1996, 161, 205–213. 32. Desnica, U.V. Doping limits in II-VI compounds-challenges, problems and solutions. Prog. Cryst. Growth and Charact. 1998, 36, 291–357. 33. Spalatu, N. Development of CdTe absorber layer for thin-film solar cells. Ph.D Thesis, Tallinn University of Technology, Tallinn, Estonia, 2017. 34. Spalatu, N.; Hiie, J.; Mikli, V.; Krunks, M.; Valdna, V.; Maticiuc, N.; Raadik, T.; Caraman, M. Effect of CdCl2 annealing treatment on structural and optoelectronic properties of close spaced sublimation CdTe/CdS thin film solar cells vs deposition conditions. Thin Solid Films 2015, 582, 128–133. 35. Moutinho, H. R.; Dhere, R. G.; Al-Jassim, M. M.; Levi, D. H.; Kazmerski, L. L. Investigation of induced recrystallization and stress in close-spaced sublimated and radiofrequency magnetron sputtered CdTe thin films. J. Vac. Sci. Technol. 1999, 17, 1793–1798. 36. Kumar, S. G.; Koteswara Rao, K. S. R. Physics and chemistry of CdTe/CdS thin film heterojunction photovoltaic devices: fundamental and critical aspects. Energy Environ. Sci. 2014, 7, 45–102. 37. Hiie, J. CdTe:CdCl2:O2 annealing process. Thin Solid Films 2003, 431–432, 90–93. 38. Paudel, N. R.; Yan, Y. Enhancing the photo-currents of CdTe thin-film solar cells in both short and long wavelength regions. Appl. Phys. Lett. 2014, 105, 183510–183515.


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For Table of Contents Only


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Table 1 The atomic percentages of zinc, oxygen and selenium according to the EDX data, thicknesses and optical band gap values (Eg) of the Zn(O,Se) films deposited at various substrate temperatures (Ts). Ts (°C) RT 300 400

Elements (at. %) a Zn

O

Se

Thickness (nm)

50 50 50

37 37 39

13 13 11

730 630 560

Eg (eV) 2.91 2.83 2.85

500 50 42 8 500 2.85 550 50 43 7 460 2.90 600 50 44 6 420 2.85 (3.23) 650 50 45 5 410 2.87 (3.23) a The atomic percentages for elements and film thickness were determined by averaging the values obtained from three measurements for each sample.


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Table 2 The structural parameters of the Zn(O,Se) films deposited in the temperature region of 500–650 °C. Structural parameters a Ts (℃) 500

d (Å)

lattice constant (Å)

Crystallite size (nm)

micro strain (ε)

2.660 ± 0.003

4.605 ± 0.005

23 ± 0.4

1.59×10-3

550 2.640 ± 0.002 4.570 ± 0.004 22 ± 0.5 1.50×10-3 600 2.622 ± 0.002 4.542 ± 0.004 26 ± 0.3 1.40×10-3 650 2.615± 0.002 4.530 ± 0.003 35 ± 0.3 1.04×10-3 d-spacing (d). aEach parameter was determined by averaging the values obtained from three XRD runs for each sample. The error represents the standard deviation.



Table 3 The electrical parameters of the Zn(O,Se) films deposited in the temperature region of 500–650 °C. Ts (℃)

Electrical parameters a ρ (Ω•cm)

µ (cm2/V.s)

n (cm-3)

500 1.5×104 4.0 1.0×1014 550 6.8×10-1 3.0 3.2×1018 600 6.8×10-2 3.0 2.7×1019 650 1.1×10-1 4.0 1.4×1019 a The resistivity (ρ); Hall mobility (µ); carrier concentration (n).


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Table 4 Photovoltaic parameters of CdTe/Zn(O,Se) solar devices before and after CdCl 2 activation treatment. Sample

VOC (mV)

JSC (mA/cm2)

FF (%)

PCE (%) a

SC as dep

150 (145 ± 2)

3.2 (2.8 ± 0.3)

28 (20 ± 5)

0.1 (0.1 ± 0.05)

SC+CdCl2

620 (620 ± 3)

23.5 (22.8 ± 0.5)

55 (50 ± 3)

8 (7 ± 0.6)

Short-circuit current density (JSC); open-circuit voltage (VOC); fill factor (FF); photocurrent conversion efficiency (PCE). aEach parameter was determined by averaging the values for 5 solar devices. Table presents the peak value and average ± standard deviation (in brackets) of device parameters.



Figure 1. HR-SEM images of the Zn(O,Se) films deposited onto glass substrate at various substrate temperatures.


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Figure 2. XRD patterns of the Zn(O,Se) films deposited in the temperature region of 500–650 °C and ZnO and ZnSe films deposited at 650 °C.



Figure 3. Total transmittance spectra of the Zn(O,Se) films deposited in the temperature region of 500–650 °C.


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Scheme 1. The dissociation reactions of ZnSe, ZnO and Zn(O,Se) in the gas phase and the formation of Frenkel defects (equations A-F) in the system.



Figure 4. Comparison of (a) current density–voltage (J–V) characteristics and (b) external quantum efficiency spectra of complete CdTe/Zn(O,Se) solar cells before and after CdCl 2 activation treatment.


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Pulsed Laser Deposition of Zn(O,Se) Layers for Optoelectronic

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