Role of Intrinsic Defects in Enhancing the Photoabsorption Capability of CuZn2AlSe4.

As a promising candidate for low-cost and eco-friendly thin-film photovoltaics, the emerging quaternary chalcogenide based solar cells have experienced rapid advances over the past decade. Here, we propose quaternary semiconducting chalcogenides CuZn2AlSe4 (CZASe) through cross-substitutions (cation mutations). The nonexistence of imaginary modes in the entire Brillouin zone of CZASe represents the inherent dynamic stability of the system. The electronic, optical, and defect properties of stannite CZASe quaternary semiconducting material was systematically investigated using density functional theory calculations. We have found that the chemical-potential control is very important for growing good-quality crystals and also to avoid secondary-phase formations such as ZnSe, Al2ZnSe4, and Cu3Se2. The observed p-type conductivity is mainly due to antisite defect CuZn, which has the lowest formation energy with a relatively deeper acceptor level than that of the Cu vacant site (VCu). The electronic band structures of vacancies and antisite defects by means of hybrid functional calculations show energy band shifting and energy band narrowing or broadening, which eventually tunes the optical band gap and improves the solar energy-conversion performance of semiconducting CZASe. Our results suggest that the stannite CZASe quaternary chalcogenides could be promising candidates for the efficient earth-abundant thin-film solar cells.

Introduction

First-generation solar cells, namely, single crystalline silicon-based solar technology, have dominated the photovoltaic industry by converting solar energy into electricity with a record efficiency of 25%.[1] However, a very low photon-to-electron conversion efficiency and relatively high expenditure of silicon solar cells has forced the search for new solutions.[2] Taking into consideration the advantages of cost-effectiveness, low toxicity, and earth abundant elements, the family of quaternary semiconducting chalcogenides, especially Cu2ZnSn(S/Se)4 (CZTS/Se), has been considered as the most useful materials for producing solar electricity.[3,4] The crystal structure and bandgap of CZTS/Se are similar to commercially available Cu(InGa1–)S/Se2 (CIGS) based solar cells,[5] despite the fact that the formation of a homogeneous and single-phase system is very difficult without the presence of secondary phases, which limits the absorption of solar spectrum and thereby reduces solar energy harvesting.[6] At this juncture, the quest for the design and synthesis of sustainable and affordable energy generation materials remains.[7−9]

To further enhance the efficiency and stability of semiconducting materials, new multicomponent quaternary chalcogenides can be designed by a series of cross-substitutions.[10,11] Using this structural and chemical freedom approach, various quaternary chalcogenides I–II2–III–VI4 (I = Cu, Ag; II = Zn, Cd; III = Si, Ge, Sn; and VI = S, Se, Te) were designed by cation mutations between binary II–VI and ternary I–III–VI2 compounds (see Figure a), including Cu2FeSn(S/Se)4,[12] Cu2BaSnS4,[13] Ag2ZnSn(S/Se)4,[14] and CuFe2InSe4,[15] to improve the power conversion efficiency of absorber materials. Table S1 represents the electrical performance parameters of recently developed multicomponent quaternary semiconductors. It is clearly exemplified that the modification of the chemical structure with various elements improves the performance of the absorber materials. For instance, the Cu2ZnGeS4 system exhibits power conversion efficiency (PCE) of 5.5%, while the replacement of Zn with Cd in Cu2CdGeS4 material further improved the PCE to 7.67%. Despite this exponential improvement, the presence of multiple elements in quaternary chalcogenides makes it challenging to design single-crystal samples with good quality. Typically, such nonstoichiometry represents the high population of intrinsic defects, and their equilibrium concentration is determined by their electronic chemical potential as well as the formation energy (growth conditions). These defects can act as recombination centers due to deep states in the electronic band gap. Nagoya et al. reported the formation energies of vacancies (VCu and VZn) and antisite defects (CuZn, ZnCu, CuZn, and ZnSn) and found that the CuZn antisite as most stable defect.[16] The clear understanding of the concentration of lattice defects, such as vacancies and antisites, are essential for elucidating the compositional reliance of the photovoltaic efficiency.

Figure 1

(a) Correlation between binary, ternary, and quaternary chalcogenides to module CZASe, beginning from II–VI parent material. (b,c) Crystal structure of CZASe along two different crystallographic directions. (d,e) Calculated phonon dispersion and phonon density of states of CZASe, (f) the 2D stability diagram of CZASe (for μCu = 0 eV).

Theoretical and experimental work related to cross-substituted quaternary systems is quite limited, and very little work has been reported on CuZn2AlS4 (CZAS). The stannite phase CuZn2AlS4 (CZAS) is an interesting material having a direct band gap (1.61 eV) with a suitable position of valence (5.4 eV) and conduction band (3.8 eV) and a relatively high absorption coefficient (≥105 cm–1).[17] Recently, Ghosh and co-workers[11,18] have synthesized CuZn2AS4 and Cu2ZnAS4– (A = Al, Ga, In) semiconductors in which stannite phase is found to be more stable than zinc blende-, orthorhombic-, wurtzite-, and kesterite-type structures. Their results highlight the importance of low-cost absorber photovoltaic materials in solar energy applications due to their direct band gap (1.20–1.72 eV) and high optical absorption (>104 cm–1). Subsequently, Yalcin[19] studied the ground-state properties of CuZn2AS4 (CZAS) (A = Al, Ga and In) nanocrystals using hybrid functional calculations. Recently, Wencong et al.[20] explored the phase stability for a series of quaternary chalcogenides with I–II2–III–VI4 chemical composition and found that the CuZn2AlSe4 (CZASe) structure is energetically stable in the stannite phase. This unexplored semiconducting chalcogenide CZASe can be modified by inducing defects that will allow relatively complex electronic and optical properties, which might bring further improvements in their use as solar harvesting materials.

Here, the formation-energy and the transition-energy levels were systematically investigated for a series of vacancies and antisite defects of quaternary chalcogenide CuZn2AlSe4 (CZASe) using first-principles calculations. The electronic band structure of defect-induced CZASe show energy band shifting and energy band narrowing or broadening. The absorption spectra demonstrate the significance of defects by introducing low energy excitations in the visible-light region. Since the knowledge of current density and upper limit of the theoretical power conversion efficiency is essential for the design and development of solar harvesting material, the corresponding values are also reported and the results highlight the importance of vacancies and antisite defects.

Computational Details

All electronic structure calculations for the designed CZASe material were performed under periodic boundary conditions using density functional theory via Vienna Ab initio Simulation Package (VASP)[21] within the framework of pseudopotential projector augmented-wave (PAW) method.[22] Geometry optimization was performed using the conjugate–gradient algorithm. The sampling of k-points in reciprocal space and the value of kinetic energy cutoff (size of the basis set) were chosen to be 7 × 7 × 7 and 450 eV, respectively. The generalized gradient approximation (GGA) developed by Perdew, Burke, and Ernzerhof was implemented to include the exchange-correlation potential.[23] The convergence criterion for self-consistent iterations and the maximal ionic Hellmann–Feynman forces were set to be 10–5 eV per atom and less than 0.01 eV Å–1, respectively. During geometry optimization, the symmetry of crystals as well as the size of the unit cells were conserved to preserve the crystallinity of investigated chalcogenide structures. In order to obtain accurate electronic and optical properties, the Heyd Scuseria Ernzerhof 06 (HSE06)[24,25] hybrid functional that screens the Coulomb potential for the Hartree–Fock (HF) exchange is implemented using a 0.25 mining parameter. The dynamic dielectric function calculated through the HSE06 functional is used to evaluate the absorption spectrum. The power conversion efficiency and current density were measured according to our previously developed methodology.[26] In order to check the dynamical stability, phonon dispersion calculations were performed within the harmonic approximation through VASP using Phonopy code that utilizes the finite displacement method[27] via a supercell approach. To obtain accurate forces, the convergence criterion of the total energy was set to 10–10 au. The default PHONOPY displacement of 0.01 Å was employed for all the calculations. To correct possibly lost symmetries due to numerical inaccuracies, the obtained harmonic force constants were symmetrized with PHONOPY’s internal subroutines.

In the present work, two types of defective systems were investigated: first, cation vacancies were created in the pure structure of CZASe by removing copper, zinc, and aluminum atoms and denoted as VCu, VZn, and VAl, respectively. Second, the positions of zinc and copper (ZnCu and CuZn), aluminum and copper (AlCu and CuAl), aluminum and zinc (AlZn and ZnAl) were interchanged.

Formation Energy

The formation energy ΔH(α,q) of a defect α in the charge state q and the corresponding charge transition level ε(α,q/q′) calculations were performed using supercell approach with 64 atoms of the CZASe systemwhere E(host) and E(α,q) represent the total energies of perfect and defect supercells, respectively. EF is the Fermi energy, and εVBM is the energy of valence band maximum of the host material. n and μ are the number of atoms and atomic chemical potential of element i in CZASe. It should be noted that the concentration of “pure” CZASe is exclusively dependent on chemical potentials and is thermodynamically governed by several conditions. The defect transition-energy level ε(α,q/q′) is the adiabatic transition energy between two defect charge states which is defined as the formation energy ΔH(α,q) of the α defect with charge q equal to ΔH(α,q′) with a distinct charge q′. The complete description and implementation about the defect methodology can be found in refs (28) and (29).

Results and Discussion

The quaternary chalcogenide CuZn2AlSe4 (CZASe) crystallizes in stannite structure (space group I4̅2m) with lattice parameters a = 11.53 Å and c = 11.49 Å. Here, the cations Zn, Cu, and Al have +2, +1, and +3 oxidation states, while chalcogen anion Se possesses a −2 oxidation state. The weighted sum of the oxidation states in CZASe system is neutralized, which is a necessary condition for a material to be a semiconductor. Selenium atom is coordinated to two zinc, one copper, and one aluminum atom, forming a tetrahedron and thereby following the Lew’s octet rule. Full geometry optimization was performed for the CZASe crystal structure (see Figure b,c), and the relaxed structural parameters are given in the Table .

Table 1

Calculated Lattice Parameters, Bond Distance, and Formation Energies of Pure and Defect Induced CZASe

	lattice parameters (Å)
structure	a	c	bond distance (Å)	formation energy (eV)
pure	11.39	11.34	Cu 2.45
			Zn 2.49
			Al 2.43
VCu	11.40	11.27		0.32
VZn	11.36	11.29		4.95
VAl	11.35	11.34		7.43
ZnCu	11.44	11.38	2.51	2.16
AlCu	11.47	11.35	2.62	0.63
CuZn	11.39	11.31	2.44	–0.48
AlZn	11.43	11.36	2.46	0.39
CuAl	11.37	11.33	2.42	2.57
ZnAl	11.40	11.33	2.47	3.73

Dynamical and Phase Stability

If a structure is a maximum on the potential-energy surface, then the frequencies of one or more vibrational motions will reduce, leading to dynamic instability of a crystal, while the minimum potential-energy structure has ω ≥ 0 (at the center of the Brillouin zone, the three acoustic modes have ω = 0). Therefore, the vibrational spectra allow us to validate the stability of the material. The phonon dispersion spectrum and the phonon density of the states for CZASe structure are presented in Figure d,e. The band dispersion and phonon density of states does not show any imaginary frequencies in the entire Brillouin zone, indicating CZASe as a stable material.

The prediction/synthesis of homogeneous quaternary semiconducting material without formation of unintentional secondary phases is the prime concern to achieve high-performance solar cells.[28−30] To obtain stable pristine CZASe, all possible secondary selenium phases formed by Cu, Zn, and Al atoms are taken into account, which may coexist at different growth conditions. Moreover, the formation of elemental solids should be avoided by considering atomic chemical potentials in CZASe smaller than that of corresponding elemental solids (i.e., ΔμCu < 0, ΔμZn < 0, ΔμAl < 0, ΔμSe < 0). Thus, the stable stoichiometric CZASe is obtained by maintaining the chemical potentials of constituent elements μi:Here, ΔHf(CuZn2AlSe4) is the formation energy. Each chemical potential diagram corresponds to a plane cut at distinct values of ΔμCu; the diagonal, height, and abscissa of every diagram are bounded by ΔH(CuZn2AlSe4/nα(α = Zn, Al and Se). The relations for ΔHf(CuAlSe4), ΔHf(CuSe), ΔHf(CuSe2), ΔHf(Cu3Se2), ΔHf(ZnSe), ΔHf(Al2Se3), ΔHf(Al2AnSe4), and ΔHf(Al5CuSe8) are given in the Supporting Information. These sets of conditions confine the stable CZASe chemical potential region in a polyhedron of ΔμCu, ΔμZn, and ΔμAl three-dimensional space. The chemical potential-based stability diagram along with the other existing secondary phases are shown in Figure f, where the homogeneous and single-phase CZASe is clearly represented by gray regions ABCD. Depending upon growth environment, each line on the chemical potential diagram indicates the boundary of secondary phase that forms during the synthesis of CZASe. Moreover, the nonuniform control of chemical potential in the stable region of CZASe may lead to the formation of Cu3Se2, ZnSe, and Al2ZnSe4 phases, which indicates that these secondary compounds grow easily than other phases. Among three cations, μZn shows relatively lower value (∼−0.7 eV) than μCu (−0.55 eV < μCu < 0 eV) and μAl (−1.01 eV < μAl < −2.5 eV) due to strong binding between Al and Zn and thus, the well-defined stable pristine CZASe is limited only to the corresponding region. Any slight deviation of Δμα (α = Zn, Al and Se) at ΔμCu = −0.35 eV may force CuZn2AlSe4 to coexist with the secondary phases, which indicates P as one of the end points of the stable three-dimensional polyhedron (see Figure S1).

Therefore, the complex stable region at various (μZn, μAl) planes demonstrate that the quality of the CZASe crystal growth is only possible by controlling the chemical-potential.

Typically, the formation energies are related to the formation ability of defects in the materials: The low formation energies usually affect the properties of host materials, while the high formation energies that exist in smaller amounts do not show much impact on the behavior of materials. As mentioned, the cation vacancies are created by removing copper, zinc, and aluminum atoms in the chalcogenide CZASe, represented by VCu, VZn, and VAl. After creating Cu vacant site, the nearest Cu–Se bond lengths are compressed by about 0.039 Å when compared to its pure material. For VZn and VAl, we could observe a decrease in the neighboring Zn–Se and Al–Se bond lengths of about 0.059 and 0.066 Å, respectively. Among three types of vacant sites in CZASe, the size mismatch of Al atom is higher such that the displacement of four nearest neighbor Se atoms around the VAl vacant site is relatively large when compared with that of VCu and VZn vacant sites. On the other hand, Al in CZASe can replace Zn and Cu from their lattice sites and can create AlCu and AlZn antisite point defects. The ZnCu, ZnAl, CuZn, and CuAl antisite defects are also considered in CZASe, and the corresponding defect bond lengths are presented in Table . It should be noted that the Cu and Zn disorders possess similar chemical structures and form deep trap states, midgap states, shallow acceptor levels, and shallow donor level within the band gap.

The calculated formation energies as a function of the Fermi energy level for the defect charge states of CZASe is shown in Figure . Depending on defect formation energies and corresponding charge-state transition levels, the vacancies and antisite defects can form either donor or acceptor levels in CZASe. The acceptor defect CuZn has the lowest formation energy when compared to other acceptor and donor defects, indicating acceptor CuZn as a dominant defect in CZASe with intrinsic p-type semiconducting nature, similar to that of Cu2ZnSnS4. However, Cu vacancy (VCu) is the dominant p-type acceptor in ideal chalcopyrite CuInSe2. The major variation between CuInSe2 and CuZn2AlSe4 is that the huge difference between In and Cu in ternary CuInSe2 forces the antisite defect CuIn to possess higher formation energy than Cu vacancy (VCu), while the three types of cations in quaternary compounds give rise to more antisite defects due to the small valence difference between Cu and Zn, making the CuZn antisite to have lower formation energy compared to that of VCu. Further, the formation energies of acceptors VAl and VZn are much higher than the other defects.

Figure 2

Calculated formation energies as a function of the Fermi energy level for the defect charge states. The negative (positive) slope of the line represents that the defect is an acceptor (donor). The defect is not ionized (or neutral) if the slope is equal to zero.

The calculated charge-state transition levels for all of the studied vacancies and antisite defects in CZASe are shown in Figure . Among all studied defects, VCu (0/−) behaves like shallow acceptor level just ∼0.11 eV above the VBM, which is beneficial to enhance the p-type behavior of the CZASe material. The antibonding coupling between the high-lying Se 4p orbitals and Cu 3d orbitals is responsible for th eshallow nature of VCu, which is similar to other semiconducting Cu-based absorbers like CZTS and CIGSe.[31,32] The calculated acceptor transition energy level for VZn (0/2−) is located at 0.39 eV above the VBM, whereas the transitions (0/1−), (1–/2−), and (2–/3−) of VAl are observed at 0.11, 0.28, and 0.63 eV, respectively. The antisite acceptor defects CuAl, CuZn, and ZnAl has transitions (0/2−), (0/−), and (0/−) at 0.36, 0.18, and 0.2 eV above the VBM, respectively, while the charge-state transitions (0/+) for all of the antisite donor defects AlCu, AlZn, and ZnCu are located outside the bandgap region.

Figure 3

Calculated charge-state transition energy levels of vacancies and antisite defects in the band gap of CZASe.

Electronic Properties

The open-circuit voltage defines the device performance and mainly depends on the band gap of absorber materials. Typically, the standard DFT functionals (i.e., LDA and GGA) has a limitation in predicting the correct band gap of semiconducting materials. To overcome this problem, the electronic band structures were estimated using Heyd–Scuseria–Ernzerhof method. The band structures of pure and defect-induced CZASe materials are present in Figure . It is clearly seen that the highest occupied and lowest unoccupied energy bands of pure CZASe lie on the Γ-point, indicating a direct band gap of 2.08 eV. Here, the lowest energy conduction band appears to be isolated from the higher energy conduction bands. If the lower energy conduction band is fully unoccupied, then the optical excitation will be weak. For VCu, the top of the valence band does not show any variations because of the Cu vacant site when compared with the pure counterpart. It is noteworthy that the valence band edges of VZn, VAl, CuZn, and CuAl defects are up-shifted, while the low energy conduction band is down-shifted for AlCu. When compared with pure CZASe, the energy band gaps of VZn, VAl, CuZn, AlCu, CuAl, and ZnAl are reduced, while the band gaps of VCu, AlZn, and ZnCu are increased. Overall, the maximum band gap corresponds to AlZn (2.74 eV), while the minimum one is for VAl (1.56 eV). It should be noted that the defect induced states are determinantal to the performance of solar cells, as they act as recombination centers of photogenerated electron–hole pairs, thereby decreasing the effective optimal band gap and thus the open-circuit voltage (Voc). Here, the effect of defects on CZASe induces energy band shifting and energy band narrowing or broadening. This tuning of band gap to achieve appropriate bandwidth can eventually improve the performance of the studied CZASe material.

Figure 4

Calculated band structures of pure and defect-induced CuZn2AlSe4 materials.

To present a comprehensive analysis of electronic structure for all the model geometries with vacancies and defect, we examine the partial density of states as shown in Figure S2. The PDOS indicates that the valence band maximum (VBM) is mainly dominated by Zn and Se atoms, while the conduction band minimum (CBM) carries a mixture of Cu, Zn, Al, and Se atoms. The low energy CB around 2 eV is separated from other high energy CBs with a width of 0.7 eV. According to Luque and Martí,[33,34] the solar materials can achieve optimum power conversion efficiency when the valence to low energy CB gap is about 1.2 eV and the low to high energy CB gap is around 0.7 eV, which indicates the necessity of tuning the former gap, while the latter is of appropriate length. As discussed above, the introduction of vacancies and antisite defects in CZASe tunes the valence–low energy CB gap and thereby provides suitable bandwidth required for the model. This is clearly exemplified in the PDOS plot of VZn and VAl, where the additional sharp peaks observed above the Fermi level in the spin-down channel of VZn and VAl are due to the effect of defects in CZASe. Further, the spin down state above 0 eV for VAl is more pronounced than VZn, CuZn, and CuAl, which is in accord with the band structure, showing higher peaks for the corresponding defects. The energy bands of AlZn and ZnCu are shifted toward lower energy levels, and the effect of defects does not show any variations in the low energy CB and higher energy CB’s gap.

Optical Properties

With the purpose of understanding the light absorption mechanism of defect-induced CZASe, the absorption spectra of CZASe in pure form and with vacancies and antisite defects are estimated using the HSE functional (see Figure ). The absorption spectra of pure CZASe show the strongest absorption peak at around 3.9 eV, while the lowest band is observed at 2.2 eV. The introduction of vacancies expands the absorption spectrum and enhances the absorption intensity, thereby exhibiting blue shift in the spectrum from 2.2 eV for conventional CZASe to 2.3, 2.3, and 2.4 eV for VAl, VZn, and VCu, respectively, while the intensities of the strong absorption peak are lowered and moved toward higher energies, indicating a blue shift in the absorption spectrum due to creation of vacancies.

Figure 5

Calculated absorption spectrum of pure and defect-induced CZASe.

More importantly, we could observe low energy excitations in the visible region of VAl and VZn that can increase the solar absorbance of the material. In case of antisite defects, the band profile of the simulated spectrum shows marked variations when compared to that of pristine CZASe. For CuZn, the intensity of the peak located around 2.2 eV is enhanced, while the strong intense peak at 3.9 eV is slightly lowered. In case of CuAl, we could observe a blue shift in the absorption spectrum along with a decrease in the peak intensity. The ZnAl antisite defect does not show any prominent enhancement in the visible light region, while new energy bands are created in the visible light region of AlCu, CuAl, and CuZn defects.

Performance Parameters

For better understanding the performance of semiconducting CZASe material, we have calculated the current density and power conversion efficiency of pure and defect induced CZASe. Typically, the short-circuit current density (Jsc) is equivalent to the absorbed photon flux (Jabs) and can be predicted using the equationwhere Eg, E, A, and Jph represent the band gap, photon energy, absorbance, and incident photon flux, respectively, while the overlap between the solar spectrum and the absorbance upper limit of the theoretical energy conversion efficiency is

Here, λmax and λ represent the longest wavelength and the photon wavelength that can be absorbed by a material, respectively, while the solar spectral irradiance is denoted by W(λ). A detailed description regarding the methodology can be found in our earlier report.[26]

The predicted band gap of pure CZASe is found to be 2.08 eV, and the corresponding current density and upper limit of the theoretical energy conversion efficiency are 9.9 mA/cm2 and 8.13%, respectively. The calculated band gap, photocurrent density, and theoretical power conversion efficiency of pure and defect-induced CZASe are presented in Table . It can be clearly seen that the effect of vacancies and antisite defects show remarkable influence on both power conversion efficiency and current density. The Al vacant site (VAl) shows an increment of 45% in current density when compared to pure CZASe, and the corresponding P is increased to 13.0%. It should be noted that the structure of Al vacant site has an appropriate band gap of 1.56 eV and thus produces maximum performance when compared to other defects. Similarly, the photocurrent densities of CuAl (16.8), CuZn (16.6), AlCu (12.7), and VZn (13.1 mA/cm2) are also increased when compared to its pure counterpart, and the corresponding P (%) is enhanced to 11.7%, 12.2%, 9.0%, and 10.0%, respectively. Interestingly, the valence band edges of these defects are shifted upward, indicating the decrease in the band gap. On the other hand, the ZnAl, VCu, ZnCu, and AlZn suppress the current density from 9.9 mA/cm2 to 5.8, 4.9, 3.4, and 3.4 mA/cm2, and thus, the corresponding P is minimized to 4.2%, 3.82%, 2.71%, and 2.78%, respectively. These defect structures show larger band gaps (except ZnAl) when compared to pure CZASe (see Figure ). In addition, the P curves show sharp enhancement with increasing film thickness and the theoretical energy conversion efficiency approaches the saturation limit when the thickness of the film exceeds 1 μm. As shown in Figure a,b, the theoretical energy conversion efficiency (current density) for VAl, CuZn, CuAl, and AlCu are beyond 30% (30 mA/cm2) when film thickness exceeds 1 μm, which is due to their band gap values being closer to the Shockley–Queisser limit, and thus, the corresponding systems have the ability to absorb large number of photons in the visible light spectrum.[28] Overall, VAl and CuZn defect systems with strong variations in the electronic band structure manifest the maximum theoretical energy conversion efficiency with respect to pure and other defected CZASe structures.

Table 2

Calculated Band Gap, Current Density (mA/cm2), and Upper Limit of the Theoretical Energy Conversion Efficiency P (%) Considering 100 nm Film Thickness

structure	band gap (eV)	current density (mA/cm2)	P (%)
pure	2.08	9.9	8.13
VCu	2.17	4.9	3.82
VZn	1.9	13.1	10.01
VAl	1.56	18.0	13.0
ZnCu	2.71	3.4	2.76
AlCu	1.81	12.7	9.0
CuZn	1.7	16.6	12.2
AlZn	2.74	3.4	2.78
CuAl	1.65	16.8	11.7
ZnAl	2.0	5.8	4.2

Figure 6

Graphical illustration of theoretical energy conversion efficiency P (%) and current density (mA/cm2) plotted against the corresponding band gaps (eV).

Figure 7

Computed (a) theoretical energy conversion efficiency and (b) current density of pure and defect-induced CZASe as functions of film thicknesses.

Conclusion

In summary, we have proposed a novel quaternary semiconducting CuZn2AlSe4 chalcogenide through distant-atom mutation concept. The phonon dispersion spectrum confirms the dynamic stability of stannite CZASe material. Using first-principles HSE06 hybrid calculations, the vacancies and antisite defects as well as electronic and optical properties of CZASe were quantitatively analyzed. The narrow and complex region of CZASe indicates the importance of controlling chemical potential in developing good quality crystals. The antisite defect CuZn has the lowest formation energy with a relatively deeper acceptor level than that of the Cu vacant site (VCu), which is mainly responsible for p-type conductivity. The electronic properties of vacancies and antisite defects show energy band shifting and energy band narrowing or broadening; in particular, the strong variations in the VAl and CuZn band structures manifest the maximum theoretical energy conversion efficiency with respect to pure and other defected CZASe structures. The introduction of low energy excitations in the visible light region of absorption spectrum for defect induced CZASe can eventually enhances the solar energy harvesting. Overall, the concept of cross-substitutions (cation mutations) can provide insights for modeling new solar light absorber materials with desirable photovoltaic applications.