Review of CdTe 1 − x Se x Thin Films in Solar Cell Applications

: Recent improvements in CdTe thin film solar cells have been achieved by using CdTe 1 − x Se x as a part of the absorber layer. This review summarizes the published literature concerning the material properties of CdTe 1 − x Se x and its application in current thin film CdTe photovoltaics. One of the important properties of CdTe 1 − x Se x is its band gap bowing, which facilitates a lowering of the CdTe band gap towards the optimum band gap for highest theoretical efficiency. In practice, a CdTe 1 − x Se x gradient is introduced to the front of CdTe, which induces a band gap gradient and allows for the fabrication of solar cells with enhanced short-circuit current while maintaining a high open-circuit voltage. In some device structures, the addition of CdTe 1 − x Se x also allows for a reduction in CdS thickness or its complete elimination, reducing parasitic absorption of low wavelength photons.


Introduction
With a global drive towards renewable energies, photovoltaics is an increasing part of global energy supply. CdTe is the leading technology amongst thin film solar cells for cost effective solar electricity production, due to its high photovoltaic conversion efficiency, long-term performance stability, low fabrication costs and short energy-payback time [1]. The current efficiency record for CdTe cells is 22.1%, held by First Solar [2,3].
In state-of-the-art CdTe solar cells, the JSC is already close to its theoretical limit [4]. According to the Shockley-Queisser thermodynamic limit to solar cell efficiency, the optimum band gap for maximum theoretical solar cell efficiency under AM 1.5 G is 1.34 eV [5,6]; the band gap of CdTe is slightly too wide at 1.5 eV [4]. Strategies to shift the band gap of the CdTe absorber towards the theoretical optimum can help to further increase photocurrent and, consequently, the efficiency. CdTe1−xSex is a promising material for this purpose, as its band gap can be shifted down to 1.4 eV [7,8] by appropriately adjusting the composition. First Solar's world record cell efficiency was improved from 19.5% to 22.1% using an absorber with a lower band gap than CdTe, the CdTe1−xSex alloy [9][10][11].
With an absorber material that has an adjustable band gap, the band gap profile through the absorber layer can be engineered by applying a compositional grading. Different research groups have been investigating the use of a CdTe1−xSex layer towards the front of a CdTe absorber layer, resulting in champion devices with improved JSC due to higher absorption in the low-band gap front layer, while maintaining the VOC from the bulk CdTe [8,[12][13][14][15][16][17].
For traditional CdS/CdTe solar cells, a CdCl2 treatment, generally consisting of deposition of CdCl2 on the absorber and subsequent heating, is needed to recrystallize the absorber layer, resulting in increased CdTe grain size and intermixing of CdS and CdTe as well as modifications in electronic properties leading to enhancement in photovoltaic conversion efficiency [18]. If a CdSe or CdTe1−xSex layer is inserted in between CdS and CdTe, interdiffusion during CdCl2 treatment results in a CdTe1−xSex layer. Because CdTe1−xSex has no miscibility gap, a continuous composition gradient is formed, with an associated band gap gradient [15,19], allowing for absorption of high-wavelength photons. Several research groups have also found that, with the use of a CdTe1−xSex front layer, the CdS window layer thickness can be reduced, or the CdS layer can even be omitted completely, reducing parasitic absorption in the short-wavelength region and further increasing JSC [8,14,20]. A thick CdS layer is generally required to avoid direct contact between the transparent conducting oxide (TCO) and the CdTe absorber, but a CdTe1−xSex absorber seems to be less sensitive to this issue. Further improvement was achieved by replacing CdS with MgZnO, which has a more favorable band alignment with both CdTe and CdTe1−xSex than CdS, and which is more transparent in the shortwavelength region [15,21,22]. Additionally, passivation of defects due to Se has been reported to be responsible for the higher carrier lifetimes in CdTe1−xSex compared to CdTe [19,23].
CdTe technology has shifted in recent years from a traditional CdS/CdTe structure to a device structure that utilizes a CdTe1−xSex layer and a band gap gradient, but no CdS layer, in order to achieve high currents and voltages. In this review paper we discuss the steps of this transformation, from the first introduction of a CdTe1−xSex layer to the complete replacement of the CdS layer by MgZnO and CdTe1−xSex. We discuss the material properties of CdTe1−xSex and their dependence on composition and the application of this material in thin-film solar cells.

Growth of CdTe1−xSex Thin Films and Material Properties
In this section, we will discuss the deposition of CdTe1−xSex films and their structural, optical and electronic properties.
CdTe1−xSex is a solid solution without a miscibility gap; the alloy is stable over the complete composition range 0 ≤ x ≤ 1. Thin films of desired composition can be grown with a variety of methods, such as high-vacuum evaporation of the elements [24] or the alloy CdTe1−xSex [25,26], coevaporation of CdTe and CdSe [8,27], close space sublimation of the alloy CdTe1−xSex [23,28] or cosublimation of the components CdTe and CdSe [29], molecular beam epitaxy [30,31], hot wall deposition [32], electron beam deposition [7,33], or electrodeposition [34,35]. The properties of grown layers depend on the deposition method and the conditions used.
In solar cells, the main methods used for CdTe1-xSex formation are high-vacuum evaporation [8] or radio frequency sputtering [14,16,20] of a CdSe layer next to a CdTe layer, with interdiffusion during CdCl2 treatment, or close space sublimation of a CdTe1−xSex layer next to a CdTe layer [15], which also interdiffuse during CdCl2 treatment. Close space sublimation is performed at higher temperatures than high-vacuum deposition or sputtering, and in CdTe it results in very large grains. The lower deposition temperature of high-vacuum evaporation results in smaller as-deposited grains. However, recrystallization during CdCl2 treatment results in overall comparable grain sizes between different deposition methods [36]. The properties of CdTe1−xSex are therefore expected to depend on deposition parameters as well as CdCl2 treatment conditions.

Crystal Structure
CdTe crystallizes in the cubic zinc blende structure, while CdSe crystallizes in the hexagonal wurzite structure. The two structures are closely related; each Cd 2+ ion is surrounded by a tetrahedron of four Se 2− and/or Te 2− ions. In the solid solution CdTe1−xSex, both the zinc blende and the wurzite structure have been reported [16,34,[37][38][39]. For 0.2 ≤ x ≤ 0.5, cubic zinc blende is the low-temperature structure, with a transition to hexagonal wurzite at temperatures above 800 °C. This transition temperature decreases with increasing x, and at x = 0.6 the hexagonal wurzite structure is formed at 600 °C [16,37]. With appropriate deposition parameters, mainly dependent on substrate temperature, the entire composition range of CdTe1−xSex can be fabricated in the zinc blende structure. In this structure, the lattice constant a decreases with increasing x, following Vegard's law, due to the smaller size of Se 2− compared to Te 2− [7,33,40,41]. The wurzite structure can occur at room temperature for x = 0.3 and higher, sometimes mixed with the zinc blende structure [34,38,39]. In solar cell applications, the zinc blende structure is desired, as it is photoactive, i.e., it can convert light into photovoltaic current, while the wurzite structure is not photoactive [16].

Optical Properties
CdTe1−xSex has a direct band gap with a pronounced bowing behavior. Figure 1 shows reported band gaps of CdTe1−xSex thin films measured with different methods, as well as one band gap bowing derived from first-principles DFT calculations, all showing a similar bowing behavior trend. The bowing parameter b can be extracted from experimental data with a second-degree polynomial fit. The band gap of any composition x can then be calculated using Equation (1) [42]: Band gap bowing parameters from bulk and thin-film CdTe1−xSex, fabricated by different methods and using different measurement techniques, are listed in Table 1. The published bowing parameter values cover a range of 0.56-0.97, and they all agree on the composition of the band gap minimum at 0.3 ≤ x ≤ 0.4, independent of fabrication method. The bowing only marginally depends on the crystal structure [39]. The band gap determination using photoluminescence and cathodoluminescence seem to yield bowing parameters slightly higher than transmittance/reflectance measurements, but they are still in good agreement [31].
Theoretical calculations using different methods are also shown in Table 1, and they seem to be in good agreement with experimental data [42][43][44]. The band gap minima in all experiments and calculations are around 1.4 eV, confirming that CdTe1−xSex is a good candidate for band-gap engineering in CdTe solar cells for the purpose of increasing JSC.
In CdTe1−xSex, the valence band edge is dominated by Se and Te p-orbitals, and the conduction band edge is dominated by Cd s-and p-states. Therefore, the anion substitution would be expected to primarily affect the valence band edge [43,45]. However, first-principles calculations show that the bowing of the band gap is caused by the bowing of both band edges due to strong intra-band coupling in both the valence and the conduction band [44].
The refractive index of CdTe1−xSex is almost constant over a large range of wavelengths, as well as with changes in composition and film thickness. Use of a CdTe1−xSex layer is therefore not expected to change the overall reflectance of a solar cell, and the introduction of a CdTe1−xSex gradient does not introduce a gradient in the refractive index [7,25,32].

Electronic Properties
The electronic properties of CdTe1−xSex have not been studied extensively. As-deposited CdTe1−xSex is an n-type semiconductor with carrier densities between 10 13 cm −3 (CdTe) and 10 18 cm −3 (CdSe) [40]. The Hall mobility increases by less than an order of magnitude between CdTe and CdSe. Both carrier density and mobility are dependent on deposition temperature, with higher values at higher deposition temperatures due to increased grain size and possibly a reduction in structural defects [26,40]. In solar cell processing, the CdTe1−xSex layer is subjected to a CdCl2 treatment, during which it recrystallizes. In CdTe, the recrystallization results in larger grains, reduced structural defects, and reduced recombination [18,47]. A similar effect is expected for CdTe1−xSex. In absorbers consisting of a CdTe1−xSex layer in front of CdTe, recrystallization during CdCl2 treatment has been shown to be responsible for a reduction in stacking faults, combined with interdiffusion of the CdTe1−xSex and CdTe layers [19,28].
In solar cells that use CdTe1−xSex as part of the absorber layer, dopants are generally added to the whole CdTe1−xSex/CdTe stack and permeate the CdTe1−xSex layer via diffusion. A commonly used extrinsic dopant is copper, which forms deep acceptors as Cu on a Cd site (CuCd) [48,49]. Firstprinciples calculations indicate that the formation energy of this acceptor defect exhibits a bowing, with lower formation energies in the CdTe1−xSex solid solution than in either parent compound [44]. In the mixed alloy, the bonding Te orbitals are partially replaced with Se orbitals, which have a lower energy. The electronic effects of this change in bonding orbitals are one reason for the bowing behavior. Another reason is the smaller lattice constant with increasing x leading to reduced compressive strain when Cd is replaced by the much smaller Cu [44]. The reduced formation energy indicates better dopability of CdTe1−xSex with copper compared to CdTe. However, Hall effect measurements of CdTe1−xSex with 0 ≤ x ≤ 0.2, deposited by high-vacuum evaporation and subjected to a CdCl2 treatment, revealed that the achievable doping concentration is lower in Se-containing samples, with more than an order of magnitude difference between CdTe and x = 0.2 [8]. A possible explanation is that compensating donors, such as copper on interstitial sites (Cui), are formed as well, and that the formation energy of these defects is also dependent on x. If the electronic and/or structural effects of CdTe1−xSex alloying facilitate the formation of compensating donors more than the formation of acceptor defects, the result is a limit to the achievable hole density [8,49,50].
The photoluminescence decay of CdTe1−xSex measured by time-resolved photoluminescence (TRPL) is generally dominated by surface recombination, resulting in short apparent lifetimes [23,51]. In order to get a reliable measure for the bulk minority carrier lifetime, the surface has to be passivated, e.g., using Al2O3, which provides field-effect passivation [52]. Another approach is twophoton excitation (2PE) TRPL, which allows for lifetime measurements at a selected depth of the absorber [51,53]. In CdTe1−xSex layers (x = 0.2, deposited by close space sublimation and subjected to CdCl2 treatment) with both surfaces passivated by sputtered Al2O3, lifetimes of up to 430 ns were measured, an order of magnitude higher than lifetimes in similarly passivated CdTe [23]. The reason seems to be that Se passivates a deep defect in the bulk of the absorber, even at low concentrations [19]. This is also correlated with longer diffusion length in material with higher x [13,19]. Two-photon excitation TRPL measurements performed on an interdiffused CdTe1−xSex/CdTe absorber (close space sublimation, CdCl2 treated, resulting maximum Se content x ≈ 0.16) have shown lifetimes of around an order of magnitude higher in the Se-rich front layer compared to the CdTe back layer. This lifetime difference within a single absorber was linked to reduced recombination at the grain boundaries in the Se-containing region [51,53]. Se therefore, at least up to x ≈ 0.2, seems to provide both passivation of a bulk defect and of the grain boundaries, resulting in an overall increased lifetime. Further studies are needed to accurately determine the lifetime dependence with x, and to determine the steepness of the lifetime gradient in an interdiffused CdTe1−xSex/CdTe absorber.
The various deposition methods used for CdSe or CdTe1−xSex apply a variety of substrate temperatures and deposition rates. We expect that the reported properties of CdTe1−xSex may depend on deposition method as well as on the parameters of the CdCl2 treatment, but further investigations are needed to determine the extent.

CdTe1−xSex in Solar Cells
In recent years, CdTe1−xSex has been used as a layer at the front of a predominantly CdTe absorber layer (see Figure 2), either deposited as CdSe or as CdTe1−xSex, which interdiffuse with CdTe during CdCl2 treatment to form a continuous composition gradient. This results in an absorber layer with a low band gap, increasing the JSC, and it allows for a reduction in CdS thickness, both in superstrate and substrate configuration. As a further step, Munshi et al. [15] replaced the CdS with MgZnO for the highest device efficiencies. Schematics of the different device configurations in which CdTe1−xSex layers have been used are shown in Figure 2, from CdS/CdTe1−xSex/CdTe (Figure 2b), to CdTe1−xSex/CdTe (Figure 2c), to MgZnO/CdTe1−xSex/CdTe ( Figure 2d). As an alternative to the conventional superstrate configuration, a device in substrate configuration (Figure 2e) is discussed as well.
The best reported devices for each of the different structures in Figure 2 are listed in Table 2, as well as the CdTe1−xSex-containing record device by First Solar. By removing the CdS layer, a small increase in JSC and FF has been shown, due to reduced parasitic absorption of CdS, at the cost of some losses in VOC, due to interface recombination at the TCO/CdTe1−xSex interface [20]. With a MgZnO window layer, all device parameters could be improved: high JSC due to the absence of parasitic absorption in the window layer, and high VOC and FF due to a good band alignment between MgZnO and CdTe1−xSex, resulting in a power conversion efficiency of 19.1% [15]. For the substrate configuration device, while the FF is comparable to superstrate devices, the VOC and JSC are lower. The loss in VOC may be due to interface recombination at the CdS/CdTe1−xSex interface; the loss in JSC due to parasitic absorption in the remaining CdS layer [8]. The world record device from First Solar surpasses all other devices in terms of VOC and JSC, and only the MgZnO/CdTe1−xSex device can exceed its FF. The exact structure and therefore the reasons for this outstanding performance are not public knowledge, but the high JSC and VOC indicate the use of MgZnO or a similar window layer [3,9,10].
In the following section, we will first discuss the deposition methods used to form a CdTe1−xSex/CdTe solar cell absorber, and in subsequent subsections we will discuss the details of how CdTe1−xSex has been used to improve performance in devices with different configurations. (c) a device with CdSe replacing CdS [14,16]; (d) a device with MgZnO and CdSe replacing CdS [15]; and (e) a device in substrate configuration with a CdSe layer between CdTe and CdS [8]. Table 2. J-V parameters of the best CdTe1−xSex-containing devices for each configuration shown in Figure 2.
For devices in substrate configuration, a CdSe layer is deposited onto CdTe, followed by CdCl2 and doping treatments and window layer/front contact deposition (see Figure 2e) [8].
In all of these device structures, the absorber and window layer are recrystallized and the CdSe/CdTe1−xSex interdiffuse with the CdTe during CdCl2 treatment. In an as-deposited CdTe1−xSex and CdTe absorber, a large number of small grains towards the front (CdTe1−xSex) and increasing grain size towards the back are observed, but after the CdCl2 treatment, the grains are uniform in size and no distinct boundary is visible between CdTe1−xSex and CdTe [28]. Scanning transmission electron microscopy (STEM) / energy-dispersive x-ray spectroscopy (EDS) mapping reveals interdiffusion of CdTe1−xSex and CdTe, with Se diffusing more than a µm into the CdTe layer [15,28]. Secondary ion mass spectrometry (SIMS) reveals accumulation of Se at grain boundaries in the CdTe layer, and a relative loss of Se at grain boundaries in the CdTe1−xSex layer, which is a strong indication for a combination of fast diffusion of Se along the grain boundaries and slow diffusion into the grains [19].
Any photons with energies above the CdTe band gap (≥1.5 eV) are absorbed in the CdTe1−xSex layer or in the CdTe absorber underneath. CdTe1−xSex has a lower band gap than CdTe (down to 1.4 eV), and it therefore absorbs photons that cannot be absorbed by CdTe. To maximize JSC gain, the CdTe1−xSex layer has to be sufficiently thick to absorb the majority of these photons with energies below the CdTe band gap. If it is formed from CdSe, the composition of the CdTe1−xSex layer after interdiffusion depends on the thickness of the deposited CdSe layer as well as on the annealing conditions. Yan et al. found that for CdSe layers up to 100 nm thick, the interdiffusion is sufficient to result in a completely photoactive zinc blende absorber with compositions of x ≤ 0.65. For thicker layers, a non-photoactive wurzite layer with x > 0.65 remains at the front of the absorber, which is undesirable because of parasitic absorption. With this method, it is difficult to achieve a thick photoactive layer for full absorption without formation of a non-photoactive layer [16,20,47]. Forming the CdTe1−xSex layer by depositing CdTe1−xSex with x ≤ 0.65 instead of CdSe allows for a higher overall Se content in the absorber because the risk of forming a non-photoactive wurzite layer is minimized, and thick photoactive low-band gap layers can be formed [15].
The interdiffusion of CdTe1−xSex and CdTe creates a compositional gradient, which is expected to be correlated to the gradient in the material properties of CdTe1−xSex, such as band gap, lattice constant, charge carrier concentration, and minority carrier lifetime. In particular, Fiducia et al. [19] reported gradients in both band gap and diffusion length (determined from cathodoluminescence measurements) that are directly correlated with the Se concentration gradient in a CdTe1−xSex/CdTe device.

Overview of Device Performances with CdTe1−xSex
Different research groups have had varying success in improving device performance using CdSe. Published device parameters are listed in Table 3, where the parameters are compared with reference devices without CdSe. Generally, the JSC could be improved by the reduced band gap and by reducing or removing the CdS layer. VOC losses are partially due to the reduced band gap. Further VOC losses together with fill factor losses have been mostly attributed to interface recombination, but they were not further investigated. The highest VOC was achieved by use of a MgZnO window layer instead of CdS. The deposition methods are also listed as they may have an influence on the material properties and device performance. The different approaches and results are discussed in the following subsections. Table 3. J-V parameters of devices containing CdSe compared with non-CdSe devices. Deposition methods are abbreviated as RFS (radio frequency sputtering), CSS (close space sublimation), and HVE (high vacuum evaporation).

CdSe/CdTe Devices without a Window Layer
The performance of devices where the CdS window layer is completely substituted by a CdSe layer (Figure 2c) depends strongly on the thickness of the CdSe layer. Up to 100 nm thick CdSe layers completely interdiffuse with the CdTe during CdCl2 treatment, forming photoactive zinc blende CdTe1−xSex. This results in increased JSC compared to CdS/CdTe devices (see Figure 3) on the one hand because of increased absorption in the long-wavelength region due to a reduced band gap of the absorber, and on the other hand because the absorbing CdS layer is eliminated (see Figure 4). The overall efficiency however is still reduced because of losses in VOC and FF.
With thicker CdSe layers, the interdiffusion is not complete, and the JSC is reduced due to parasitic absorption in residual non-photoactive wurzite CdTe1−xSex with high Se content (x > 0.65). With optimized CdSe thickness, the JSC can be increased by more than 3 mA/cm 2 , however, this is not sufficient to offset the losses in VOC, which are attributed to high interface recombination at the direct contact between CdTe1−xSex and TCO [16,20].
The TCO used in CdTe and CdTe1−xSex/CdTe devices is usually a bilayer of a low-resistivity material, such as SnO2:F (FTO), and a high-resistivity material, such as SnO2. The highly resistive transparent (HRT) layer is used to prevent direct contact between the absorber and the FTO in the case of incomplete CdS coverage [55]. Baines et al. [12] investigated different oxides as HRT layers for CdTe1−xSex absorbers and found SnO2 to be the best. It reduces VOC losses that occur from direct contact of CdTe1−xSex with FTO, but the interface recombination is still higher than with a CdS layer [12,20]. In fact, the devices listed in Table 3 all use a SnO2 HRT layer. With electron-beam induced current (EBIC) measurements the position of the junction can be determined: for up to 100 nm CdSe, the junction is located at the CdTe1−xSex/TCO interface. This reveals that the CdTe1−xSex does not act as a window layer, but as part of the p-side of the junction. For thicker CdSe layers, the junction is shifted into the CdTe1−xSex layer, to the interface between photoactive and non-photoactive CdTe1−xSex [16]. The VOC and JSC decrease for thicker layers, confirming that the presence of the non-photoactive wurzite CdTe1−xSex is not beneficial for the device performance. Deposition methods and parameters therefore have to be chosen to minimize the formation of non-photoactive CdTe1−xSex.

CdSe/CdTe Devices with a CdS Window Layer
With a combined CdS/CdSe window layer (Figure 2b), device performance can be increased by combining the upsides of both layers while minimizing the drawbacks (see Table 3). By reducing the CdS thickness from 130 nm in a CdS/CdTe device to 15 nm in a CdS/CdSe/CdTe device, the parasitic absorption in the short-wavelength region is minimized (region 1 in Figure 4a) and the JSC is increased. Further JSC increase is due to a shift in band gap with CdTe1−xSex (region 2 in Figure 4a). The JSC is therefore very similar to a CdSe/CdTe device, while the VOC is improved with the CdS layer in between the CdTe1−xSex and the TCO, which reduces the interface recombination (see Figure 4b) [20,54]. A CdS/CdSe/CdTe structure also works in substrate configuration solar cells (Figure 2e). Substrate configuration CdTe solar cells require a thick CdS layer in order to mitigate the formation of pinholes. With a CdTe1−xSex layer between CdTe and CdS, the CdS layer thickness can be reduced from 120 to 30 nm with only small losses in VOC and with a 7.1 mA/cm 2 increase in JSC due to the reduction in CdS thickness and the reduction in band gap, resulting in an overall improvement in efficiency (see Table 3) [8].

CdTe1-xSex/CdTe Devices with a MgZnO Window Layer
Recent high-efficiency CdTe devices have been achieved by replacing the CdS window layer with MgZnO (see Table 3) [15,22]. MgZnO has a band gap of 3.7 eV, higher than CdS, thereby reducing parasitic absorption, and it has a better band alignment with CdTe than TCO/CdS, reducing interface recombination and allowing for a higher VOC [21,22]. However, introduction of a MgZnO window layer into a device structure can be problematic. The MgZnO/CdTe interface is sensitive to barriers originating from oxides, e.g., MgO, that can be formed during deposition. To obtain highefficiency MgZnO/CdTe devices, deposition parameters, such as temperature and oxygen partial pressure during deposition, and CdCl2 treatment, have to be carefully optimized [56].
Further improvement was achieved by using a CdTe1−xSex layer at the front of the CdTe absorber. With an 800 nm thick CdTe1−xSex layer in front of a CdTe absorber, Munshi et al. showed an improvement in device efficiency from 17.9% for a MgZnO/CdTe device to 19.1% in a MgZnO/CdTe1−xSex/CdTe device (see Table 3) [15]. They improved the JSC by 2.1 mA/cm 2 while retaining the high VOC (see Figure 5a). As the device contains no CdS, the EQE in the short-wavelength region is improved (region 1 in Figure 5a).
The benefit of depositing CdTe1−xSex instead of CdSe in front of CdTe is the low possibility that non-photoactive wurzite CdTe1−xSex is formed. As a consequence, higher amounts of Se can be introduced to the device for better fine-tuning of the band gap gradient [29]. This results in a thicker photoactive low-band gap CdTe1−xSex layer in the finished device. The low-band gap region appears sufficiently thick to absorb most photons with energies below the band gap of CdTe, down to about 1.42 eV, as evidenced by the shift in absorption edge in the EQE curve in region 2 in Figure 5a. The difference in absorption between the thick photoactive layer achieved with CdTe1−xSex deposition and the thin photoactive layer achieved with CdSe deposition can be seen in the steepness of the EQE curve in region 2 in Figure 5a. The conclusion can be drawn that for maximized JSC gain from CdTe1−xSex, deposition of a CdTe1−xSex layer instead of a CdSe layer is necessary.
The high VOC in the MgZnO/CdTe1−xSex/CdTe device was in part attributed to a high lifetime of 22 ns measured by TRPL, four times higher than in the MgZnO/CdTe device, due to improved interface quality between MgZnO and CdTe1−xSex, and also due to the passivation of a recombination center by Se [13,15,19]. The MgZnO/CdTe1−xSex/CdTe device has a much higher VOC than the CdS/CdTe1−xSex/CdTe devices. This indicates that the VOC in these structures is limited by the TCO/CdS/CdTe1−xSex interfaces, but further investigations into the VOC and FF loss of CdTe1−xSex/CdTe devices is needed to confirm this.  [15]) compared with a CdS/CdSe/CdTe device (adapted from [20]). Figure 6 compares the EQE curve of the high-efficiency MgZnO/CdTe1−xSex/CdTe device with the record device from First Solar. The First Solar device uses a CdTe1−xSex layer and some kind of buffer layer instead of CdS [11]. It has a higher VOC and higher JSC than the MgZnO/CdTe1−xSex/CdTe device (see Table 3) [3,11]. The increase in JSC is primarily due to a very good absorption at high wavelengths (region 2 in Figure 6). The very straight absorption edge indicates the presence of a sufficiently thick CdTe1−xSex layer with a band gap of 1.4 eV for complete absorption of photons in this region. Some further improvement of JSC is achieved by an overall higher EQE, probably due to an antireflection coating. In the short-wavelength region (region 1 in Figure 6), the curves are very similar. This is an indication that First Solar must have used MgZnO or a similar material as a buffer layer, eliminating the influence of parasitic absorption. The increase in VOC might be due to better interface engineering and therefore reduced recombination, but without more information on the First Solar device structure no conclusions can be drawn.  [15]) and the First Solar record device (adapted from [9]).

Conclusion
CdTe1−xSex has been successfully introduced into CdTe devices by several different groups. It has been shown that a band gap gradient can be achieved by several different techniques and deposition methods. For better control and a higher limit to the amount of Se put into the devices, use of a CdTe1−xSex layer is preferable to a CdSe layer in order to avoid formation of non-photoactive wurzite CdTeSe. In devices with a CdTe1−xSex layer at the front of the CdTe absorber, the reduced band gap and possibility of reduced CdS window layer thickness result in increased short-circuit current. The open circuit voltage can be maintained at the level of CdTe devices due to Se passivating a recombination center in the absorber. The best reported devices use a MgZnO window layer instead of CdS, which provides a better band alignment with the absorber and eliminates parasitic absorption from the window layer.