Research Update: Emerging chalcostibite absorbers for thin-film solar cells

Copper antimony chalcogenides CuSbCh2 (Ch==S, Se) are an emerging family of absorbers studied for thin-film solar cells. These non-toxic and Earth-abundant materials show a layered low-dimensional chalcostibite crystal structure, leading to interesting optoelectronic properties for applications in photovoltaic (PV) devices. This research update describes the CuSbCh2 crystallographic structures, synthesis methods, competing phases, band structures, optoelectronic properties, point defects, carrier dynamics, and interface band offsets, based on experimental and theoretical data. Correlations between these absorber properties and PV device performance are discussed, and opportunities for further increase in the efficiency of the chalcostibite PV devices are highlighted. © 2018 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). https://doi.org/10.1063/1.5027862


I. INTRODUCTION
The most studied materials for the thin-film photovoltaic (PV) technologies are Cu(In,Ga)Se 2 (CIGS, 22.6% one-sun energy conversion efficiency) and CdTe (22.1% efficiency). 1 Nevertheless, the future environmental benefits and potential scale of deployment of these solar cells may be limited by their elemental scarcity (In, Ga, Te) and toxicity (Cd). 2 These considerations have led to increased interest in emerging light-absorber materials with less toxic and more abundant elements, including Cu 2 SnS 3 (CTS), 3 Cu 2 O, 4 Sb 2 Se 3 , 5 Cu 3 N, 6,7 SnS, 8,9 ZnSnN 2 , 10 and Cu 2 ZnSn(S,Se) 4 (CZTSSe). 11 Among these, the highest PV efficiency (certified 12.6% 1,12 ) has been reported for the CZTSSe absorber. However, its further improvements may be hindered by chemical and crystallographic complexity, pointing researchers toward less complex materials.
Most of the emerging absorbers, like CZTSSe, have a three-dimensional (3D) crystal structure, where dangling bonds in grain boundaries (GBs) can act as recombination centers, causing efficiency loss. 13,14 In contrast, the low-dimensional crystal structure of some potential absorbers, such as 1D-Sb 2 Se 3, 5,15 1D-SbSeI, 16 2D-CuSbS 2 , 17 2D-CuBiS 2 , 18 and 2D-CuSbSe 2 , 19 can be oriented hypothetically in such a way to decrease the number of dangling bonds, even at the GBs. In turn, this can minimize photoexcited charge-carrier recombination losses, which is one of the major limiting factors for high-efficiency thin-film solar cells.
One class of such non-toxic Earth-abundant less-complex and low-dimensional absorbers is ternary copper chalcogenides, such as CuBiS 2 18 and CuSbCh 2 (where Ch= =S, Se) with a chalcostibite crystal structure. 17,19 Despite a relatively small amount of research, these layered semiconductors have shown promising PV device efficiencies approaching 5%. [19][20][21] The PV-relevant properties of these materials include slightly indirect bandgaps of 1.1-1.6 eV (that is, well-suited for singlejunction terrestrial solar cell applications) and strong optical absorption coefficient >10 5 cm −1 at 0.5 eV above the bandgap (slightly better than CIGSe, CdTe, and CZTSSe, for example). 19,[21][22][23] a Author to whom correspondence should be addressed: andriy.zakutayev@nrel.gov Many other characteristics that make these chalcostibite materials worth exploring will be discussed in this research update.
A good review of the PV applications of CuSbS 2 (CAS) was published three years ago, 24 but much CuSbS 2 research activity has occurred since that time, and there are no review papers about CuSbSe 2 (CASe) as a PV absorber. Thus, in this article, we focus on the most recent research efforts (past three years) on CuSbCh 2 (Ch= =S, Se) as a promising family of absorber materials for thin-film solar cell applications. This paper summarizes both the currently used methods to prepare these materials and their most important PV-relevant properties, as compared to other thin-film PV absorbers. We also discuss the factors that have limited current CuSbCh 2 -based solar cell performance, as well as some strategies that have been used to address them.

A. Crystallographic structure
As mentioned earlier, most of the absorbers used in solar cells have a tetrahedrally bonded lattice, where different atoms are linked to one another in 3D diamond-like structures (e.g., Si, CdTe, CuInS 2 , and Cu 2 ZnSnS 4 ), as can be seen in Fig. 1(a). However, the CuSbCh 2 chalcostibites show a very different 2D-like layered crystal structure, despite the same 1:1:2 stoichiometry as in CuInS 2 chalcopyrites, and similar Sb/In ionic radii. 25 In contrast to the four-fold coordinated chalcopyrite, in the chalcostibite structure the Cu atoms are four-fold coordinated, whereas the Sb atoms are in a distorted three-fold coordination [ Fig. 1(b)]. This difference in the metal coordination number makes chalcostibites "line compounds": namely, the small deviations from the nominal CuSbCh 2 stoichiometry (Cu/Sb = 1) lead to phase impurities, indirectly suggesting a smaller propensity to cation disorder for the stoichiometric material. This is in contrast to the tetrahedrally coordinated CIGS, CTZS, and CTS, which are known to show cation disorder and to tolerate high levels of offstoichiometry (up to ∼20%). 25 The crystallographic differences between the 3D (e.g., CuInS 2 ), 2D (e.g., CuSbS 2 ), and 1D (e.g., Sb 2 Se 3 ) absorbers can be seen in Fig. 1.
As can be seen in Fig. 1(b), the CuSbCh 2 crystal structure is formed by stacking layers composed of SbSe 2 and CuSe 3 motifs along the "c"-axis, with interlayer distances of ∼7.10 Å (for Ch= =S) and ∼7.29 Å (for Ch= =Se). 26 Theoretical calculations reveal that these layers (along with other characteristics) are responsible for the increase in the density of states (DOS) in CuSbCh 2 and that each trivalent Sb atom has a non-bonding electron pair from its 5s orbital. These factors are responsible for the larger optical extinction coefficient of CuSbCh 2 than in CIGS, for example, but they also lead to higher effective masses and less efficient collection of the photogenerated charge carriers. 21,[26][27][28] Another interesting property that results from the low-dimensional structure is the possibility of crystallographically orienting the layers in different ways with respect to the substrate. Such preferential orientation may create chemically inert surfaces in 2D structures and benign grain boundaries (GBs) in 1D structures, both with fewer dangling bonds compared to the 3D structures. The benign surfaces or GBs can reduce the carrier recombination loss, improving the efficiency of thin-film solar cells. The preferential orientation is also important to the charge transport inside CuSbCh 2 , which is easier within the sheets than between them. Within the CuSbCh 2 sheets, the carriers are transported along chemical bonds, whereas in the other case, they are required to hop between the layers, as in Sb 2 Se 3 . 19,21,22 Thus, the crystallographic orientation control is vital for reduced carrier recombination and improved carrier transport in CuSbCh 2 PV devices. 19,21,22

B. Synthesis and secondary phases
A wide variety of techniques has been used to grow CuSbCh 2 and they can be grouped into physical and chemical methods. Physical methods include thermal evaporation 29 and co-sputtering, 19,30,31 whereas chemical methods include chemical bath deposition, 32,33 spray pyrolysis, 34 spin coating, 20,35 solution processing, 36 electrodeposition, 37,38 and solvo-/hydro-thermal synthesis. 39,40 Below, we focus mostly on the physical vapor deposition methods for CuSbCh 2 growth.
The control of the CuSbCh 2 composition during growth is very important because small deviations from the nominal stoichiometry more easily lead to phase impurities than in CIGS and CZTSSe. 25 For this reason, self-regulated growth (a.k.a. adsorption-controlled growth) is the most promising approach to achieve reproducible synthesis of this material, regardless of the deposition method. This kind of CuSbCh 2 growth control has been demonstrated for CuSbCh 2 by radio-frequency (RF) magnetron co-sputtering of Cu 2 Ch and Sb 2 Ch 3 targets in excess of Sb 2 Ch 3 vapor. 19,21,22,41 For CAS, it was found that it is possible to grow highly stoichiometric phase-pure thin films at the substrate temperature of 350 • C in a wide range of Sb 2 S 3 over-flux. 22 The CAS films showed some Sb 2 S 3 impurities at lower temperatures or decomposed to Cu 12 Sb 4 S 13 at higher temperatures.
To determine the stability region for the deposition of stoichiometric phase-pure CAS films, the phase diagram has been calculated as a function of temperature and Sb 2 S 3 partial pressure [ Fig. 2(a)]. 22 It was concluded that for a given substrate temperature, phase-pure CuSbS 2 can be grown in a ∼10 2 Torr dynamic range of Sb 2 S 3 partial pressure. In turn, for a given Sb 2 S 3 partial pressure, the region of stability extends ∼70 • C of substrate temperature in agreement with experimental data. Overall, these results suggest that no fine control of the Sb 2 S 3 precursor flux or substrate temperature is required within this region of CuSbS 2 self-regulated growth. Similar conclusions hold for CuSbSe 2 adsorption-controlled growth in an Sb 2 Se 3 -rich atmosphere, but at slightly higher temperatures (∼380 • C), and with somewhat different competing phases [Sb 2 Se 3 , Cu 3 SbSe 3 , and Cu 2 Se in Fig. 2 It is worth mentioning that the CuSbCh 2 films grown by different methods often show small grains and/or low crystallinity, encouraging the use of post-deposition thermal treatments (TTs) to improve film quality. Such a treatment has been demonstrated under atmospheric conditions, 30,35,42 H 2 S gas and S vapor, 20 or Sb 2 S 3 vapor. 30,31,43,44 For example, taking into account the calculated stability of the CuSbCh 2 phase under an Sb 2 Ch 3 -rich environment [ Fig. 2(a)], it has been shown that the TT of the sputtered CAS thin films under Sb 2 S 3 vapor increases their grain size without affecting the composition or phase purity. This TT also improved the structural quality and optoelectronic properties of the films, leading to more reproducible and efficient TT-CuSbS 2 PV devices. 44 In this context, it is also important to note that CuSbCh 2 has a lower melting point of 480 • C (for Ch= =Se) and 535 • C (for Ch= =S), compared to CIGS, CZTSSe, and CdTe (∼1000 • C 21,45 ). This should lead to grain growth at a lower temperature, making CuSbCh 2 a suitable absorber for the fabrication of flexible thin-film solar cells on polymer substrates.
Based on the experimental and calculated data shown in Fig. 2, the most likely impurities found in CuSbCh 2 (Ch= =S or Se) are the following phases: Sb 2 Ch 3 , Cu 1.8-2 Ch, Cu 12 Sb 4 S 13 (for the Cu-Sb-S system), and Cu 3 SbSe 3 (for the Cu− −Sb− −Se system). As shown by the simulated X-ray diffraction (XRD) reference patterns in Fig. 3, there are several regions of superposition between the diffraction peaks of the CuSbCh 2 and impurity phases. For CAS [ Fig. 3(a)], the most difficult impurity to identify by XRD is Cu 12 Sb 4 S 13 (12-4-13 phase) because its main peak is at the same position as the CAS (200)/(013) peaks. Thus, a common mistake made in the literature is attributing an increase in the (200)/(013) peaks compared to the (111)/(104) peaks to preferential orientation of CAS, instead of the actual presence of the very stable 12-4-13 secondary phase.
The Raman spectroscopy can be useful for determining the presence of this impurity because its main peak is ∼20 cm −1 higher compared to the CuSbS 2 main peak at 332 cm −1 . 47 Also, the CuSbS 2 thin film with the Cu 12 Sb 4 S 13 phase impurity often has higher conductivity and higher hole density (10 −1 to 10 2 S/cm and 10 18 -10 21 cm −3 ) than phase-pure CuSbS 2 (10 −3 to 10 −2 S/cm and 10 16 -10 17 cm −3 ). Thus, it should be possible to combine electric measurements with XRD and Raman spectroscopy characterizations to rule out the existence of the 12-4-13 phase. 22,41 The phase purity is equally important but less difficult to determine in CuSbSe 2 [ Fig. 3 and Cu 1.8 Se secondary phases can be more easily identified by XRD, despite some overlap with the weaker CuSbSe 2 (113)/(105)/(203) peaks.

C. Band structure and optoelectronic properties
The knowledge of the band structure and DOS helps us to understand the optoelectronic properties of CuSbCh 2 compared with the more studied absorber materials such as CIS, as shown in Fig. 4. It has been predicted that chalcostibite CuSbS 2 and CuSbSe 2 have slightly indirect bandgaps [E g dir -E g ind ≈ 0.1 eV, Figs. 4(a) and 4(b)] in contrast with the well-defined direct bandgap of chalcopyrite CuInSe 2 [ Fig. 4(c)]. Chalcostibites also present higher DOS compared to chalcopyrites. This band-structure feature not only results in a larger optical extinction coefficient (more effective absorption of photons) but also in higher effective masses (less efficient charge-carrier collection for the PV device). 28 Theoretical calculations in combination with experimentally measured X-ray photoemission spectra show that the top of the valence band (VB) of CAS is mainly formed by strongly antibonding Cu 3d and S 3p states. In contrast, Sb 5p and S 3p states are main contributions for the bottom of the conduction band (CB). 28,48 It is also interesting to note that Sb 5s electrons in CuSbCh 2 are not fully inert and localized lone pairs. Instead, these Sb 5s states contribute somewhat to the formation of bonding states in the VB and antibonding states in the CB. These states are also partially associated with the rising of the energy levels of the bands, 48 which has important implications for the selection of contacts to CuSbCh 2 absorbers.
In agreement with the theoretical observations, the CuSbCh 2 materials feature strong experimental optical absorption coefficients of >10 5 cm −1 for photon energies 0.5 eV higher than their bandgaps, 19,21-23 as shown in Fig. 5(a), and have slightly indirect bandgaps of 1.1-1.2 eV 19,21 (for Ch= =Se) and 1.4-1.6 eV (for Ch= =S) 44 [Figs. 5(b) and 5(c)]. In Fig. 5(b), the presence of an electronic defect 0.22 eV below the bandgap energy can also be observed (i.e., 0.22 eV close to VB or CB); the different defects likely present in CuSbCh 2 samples will be discussed in Sec. II D.
CuSbCh 2 possesses p-type conductivity and presents a tunable hole concentration in a range of 10 15 -10 18 cm −3 suitable for PV application. 22 The hole mobility (µ h ) has been measured by the Hall effect, showing values of 0.1-49 cm 2 V −1 s −1 (for Ch= =S) 35 and 12 cm 2 V −1 s −1 (for Ch= =Se). 21 The large variation in the values of µ h reported in the literature may be related to differences in both the concentration of defects and anisotropic crystal structures in combination with different crystallographic textures of the films. Based on optical-pump THz probe (OPTP) transient reflection spectroscopy, it has been observed that the hole and electron mobilities in 2D-like CuSbS 2 are similar to each other, in contrast to the significantly different hole and electron mobilities in 3D-bonded materials such as CIGS, CZTS, and CTS. 44

D. Defects and carrier dynamics
Some of the most important characteristics of the PV absorbers that should be considered for their device performance are defect properties and photoexcited charge-carrier dynamics. Thus,  act as donors or as acceptors, depending on the position of the Fermi level (E F ). These data are in agreement with the previously reported properties of defects in CuSbS 2 . 35 Similar conclusions for CuSbSe 2 19,26 can be made, as can be seen in Fig. 6(b). From an experimental point of view, defects in different CuSbS 2 samples (as-deposited, TT, and micro-crystals) have been studied by AS, PL, and DLTS. 49 Three acceptor defects were found at about 0.08, 0.17, and 0.24 eV above the VBM by the capacitance-based methods, in agreement with free-to-bound radiative photoluminescent transitions observed in these thin films. Comparing these results to the theoretical calculation [ Fig. 6(a)], the shallower defect was associated with V Cu , whereas the two deeper ones were attributed to Cu Sb and/or V S , as can be seen in the diagram of Fig. 7(a). No investigation of the experimental behavior of the CuSbSe 2 defects can be found in the literature.
To understand the effects of point defects on CuSbCh 2 photoexcited charge carriers, transient OPTP reflection spectroscopy [a.k.a time-resolved terahertz spectroscopy (TRTS)] has been performed. An experimental study of the carrier dynamics in the solution-processed CuSbS 2 nanoplates 33 indicates a minority lifetime (τ) of up to ∼1 ns and a mobility (µ) of ∼1 cm 2 V −1 s −1 , which result in a diffusion length (L d ) estimate of ∼60 nm [L d = (kTµτ/e) 1/2 Ref. 21]. Similar results have been TABLE I. Summary of optoelectronic properties of CuSbCh 2 (CAS and CASe), Cu 2 ZnSn(S,Se) 4 (CZTSSe), and Cu(In,Ga)Se 2 (CIGS). p = Hole concentration, and µ h = majority-carrier mobility measured by the Hall effect. m * = "e" or "h" effective masses, m 0 = electron rest mass (9.1 × 10 31 kg). τ = minority-carrier lifetime measured by # transient opticalpump THz probe (OPTP) reflection spectroscopy or $ time-resolved photoluminescence (TRPL). α = absorption coefficient for photon energy 0.5 eV higher than the bandgap energy (E g ). shown using the OPTP for the thermally treated CuSbS 2 films (τ of 0.7 ns, µ of ∼4 cm 2 V −1 s −1 , and estimated L d of ∼90 nm). 44 The photoexcited charge-carrier dynamics in CuSbSe 2 thin films studied using the OPTP indicate higher µ (∼12 cm 2 V −1 s −1 ), but smaller τ (0.2 ns), and hence similar L d (∼80 nm). Figure 7 shows the time-domain OPTP spectra for CuSbS 2 and CuSbSe 2 thin films. Finally, a summary of the important optoelectronic properties of the chalcostibite CuSbCh 2 is shown in Table I, in comparison with the chalcopyrite Cu(In,Ga)Se 2 and kesterite Cu 2 ZnSn(S,Se) 4 . Overall, both chalcostibites show similar properties to one another but somewhat different if compared to chalcopyrites and kesterites. Despite the higher absorption coefficient shown by the 2D chalcostibite materials, they show larger effective masses of charge carriers and lower photoexcited charge-carrier lifetime. However, the carrier concentrations and the bandgaps of chalcostibites and chalcopyrites/kesterites are quite similar.

A. Band offsets and contacts
Advancing from intrinsic properties of the CuSbCh 2 material to CuSbCh 2 device integration, the development of suitable front contacts (also known as "buffers") and back contacts is an important direction of research. This is related to the anisotropic properties of the layered CuSbCh 2 structure, which makes the surface energies, densities of surface states, and energy band positions [ionization potentials (IP) and electron affinities (χ)] different than in CIGS and related absorbers. First-principles calculations indicate that CuSbS 2 has two lower-energy surfaces: the (001)-plane with a surface energy of 12.4 meV Å −1 [(a) and (b) planes in Fig. 1(c)] and the (010)-plane with 14.6 meV Å −1 . As shown in Fig. 8(a), the (001) surface has a lower density of surface states than the (010) surface, which is consistent with the weak bonding of the (001) planes. Figure 8(b) shows the calculated energy band positions for [001]-and [010]-oriented CuSbS 2 absorber surfaces compared to the commonly used CdS contact layer. These results suggest a cliff-type CuSbS 2 /CdS band offset of 0.85-1.43 eV, depending on the surface orientation. 41 Experimental measurements of IP and work function (Φ) on CAS thin films have been performed by X-ray photoemission spectroscopy (XPS) 48 and ultraviolet photoemission spectroscopy (UPS). 35 XPS for the as-deposited and thermally treated CuSbS 2 is shown in Fig. 9(a)  also indicative of a 0.7-0.9 eV cliff-type band offset at the CuSbS 2 /CdS interface. 44 In contrast, the Fermi level and VBM of CuSbSe 2 measured by UPS are −4.63 eV and −4.88 eV with respect to vacuum level (0 eV), respectively. 26 Thus, CuSbSe 2 may have a less detrimental CB offset with CdS, compared to CuSbS 2 . Motivated by the high CB offset between the CuSbCh 2 absorber and the commonly used CdS contact, some research has been directed at alternative buffer layers, such as undoped and Ga-doped Cd 1−x Zn x S deposited by atomic layer deposition. 57 The CuSbS 2 devices made with Zn-rich undoped buffers (Cd 0.61 Zn 0.39 S and Cd 0.14 Zn 0.86 S) showed high series resistance and very low photoresponse, but 2.4%-5.5% Ga doping in Cd 0.6 Zn 0.4 S partially addressed these problems. Comparing the performance of the standard chemical-bath-deposited CdS, V OC improved from 211 mV to 449 mV, the short-circuit current density (J SC ) improved from 3.82 to 6.24 mA cm −2 , and the efficiency increased from ∼0.3% to 1%. Addressing the issue of Cd toxicity, the CdS/ZnO layers were replaced with GaN/In 0.15 Ga 0.85 N in CAS devices, achieving the efficiency of ∼3% and the highest value of J SC (∼34 mA cm −2 ) reported in the literature, 31 which is above the theoretical limit for this 1.5 eV bandgap material.
Different back-contact materials have been also evaluated for CAS PV devices, such as Au, W, Ni, Pd, Pt, FTO, and Mo. 41 Many of these back contacts did not result in functioning PV devices due to delamination, pinholes, interface reaction, or other reasons. Among these back contacts, Mo provided the best current collection, even though its work function (4.35-4.90 eV 58 ) was not favorable for charge extraction from the CuSbS 2 absorber. Thus, for improving the back contact, the effect of the addition of charge-selective layers (CSLs) on Mo was also studied. 41 The MoO x CSL-based device showed a significant increase in efficiency (from 0.49% to 0.86%) because of the improvement in J SC (from 3.53 to 8.91 mA cm −2 ). This effect on J SC was explained by the deeper (6.6 eV 59 ) work function of the MoO x layer, which promotes an upward band bending in the absorber, reflecting the photogenerated electrons. 41 A similar device with a sulfurized hybrid ink/spin-coated CuSbS 2 thin-film absorber has shown a record efficiency of 3.22% and J SC of 15 mA cm −2 (see Table II), albeit without statistical histograms or certification results reported in the paper. 20 This high performance may be related to the carbon-containing layer at the Mo/CuSbS 2 interface measured by Auger electron spectroscopy (AES) depth profiling, which may also aid charge selection.

B. Photovoltaic devices
For the most part, CuSbCh 2 research has so far focused on the intrinsic optoelectronic properties of the materials (e.g., absorption coefficient, bandgap, carrier concentration). However, significant  challenges arise on integrating absorber thin films into PV device prototypes, 19 such as the choice of adequate back/front contacts, growth of pinhole-free absorber layers, optimization of absorber thickness, and possible chemical reactions or inter-diffusion of the absorber with contacts. All of these parameters are very time-consuming to evaluate using the traditional experimental approach.
To address this challenge, high-throughput experimental (HTE) combinatorial research methods 60,61 can be extended from materials studies 62,63 to accelerate PV device research. 64,65 The first attempt to apply the HTE approach to CuSbCh 2 devices has focused on studying the effects of the crystallographic orientation, phase purity, composition, morphology, and thickness of the CuSbS 2 absorber on the PV performance of devices with the SiO 2 /Mo/CuSbS 2 /CdS/i-ZnO/Al-ZnO/Al architecture and on evaluating different back contacts. 41 In this particular research, a <1%efficient device was obtained with the optimal absorber thickness of 0.8 µm and MoO X CSL (device #3 in Table II). 41 Further, an improvement in CuSbS 2 PV device performance (1%) and reproducibility was achieved after absorber thermochemical treatment (TT) under Sb 2 S 3 vapor followed by selective KOH chemical etch of a likely Sb 2 S 3 surface layer, prior to the deposition of front contacts (device #4 in Table II). 44 A similar HTE approach was also applied in the accelerated development of 3%-5%-efficient CuSbSe 2 devices (device #12 in Table II) and to understand the tradeoffs of light absorption and charge transport in these layered materials. 19,21 In an effort to suppress the tradeoff between J SC and V OC observed in this drift PV device, the Na incorporation in the absorber was investigated. The Na incorporation has enhanced V OC by ∼50 mV (comparing device #13 with #12 in Table II), but it also caused a decrease in J SC and efficiency. 21 Several other adjustments in the device fabrication process were attempted, compared to the baseline device (#14 in Table II). 19 Some improvement of the efficiency resulted from using an MgF anti-reflection coating and from better design of the front-contact collector grids. 21 Comparing the performances of the CuSbS 2 and CuSbSe 2 devices with similar structure (e.g., devices #4 and #12 in Fig. 10 and Table II), it is seen that V OC for both materials is 300-400 mV [ Fig. 10(a)], which is very low compared to their bandgaps (1.1-1.5 eV). This effect may be directly related to the low quasi Fermi-level splitting due to insufficient absorber quality, and to large CB offset at the interface between these absorbers and the commonly used CdS front contacts. However, the CuSbSe 2 devices have shown higher efficiencies, mainly caused by higher J SC values. The higher J SC may be related to the better overlap of the CuSbSe 2 absorption with the solar spectrum indicated by better external quantum efficiency (EQE) [ Fig. 10(b)] and by lower recombination of the photogenerated charge carriers at the front interface. To represent this comparison, Fig. 10 shows comparative graphs of current density-voltage (J-V) and EQE for these CuSbCh 2 PV devices. The poor carrier collection for photon energies near the band edge can be observed in both CuSbS 2 and CuSbSe 2 EQE graphs [ Fig. 10(b)], which is mainly indicative of the short transport lengths of the photogenerated charge carriers.
In addition to the traditional substrate architecture (CIGS-like) of the PV devices described earlier, several other devices in substrate architectures and one in superstrate architectures (CdTelike) have been attempted. These diverse CuSbCh 2 PV device configurations and performances are summarized in Table II. Most of the CuSbCh 2 PV device prototypes fabricated in the substrate configuration have the glass/back-contact/CuSbCh 2 /buffer/TCO/front-contact architecture, where TCO is a transparent conductive oxide such as Al:ZnO or Sn:In 2 O 3 . Only one of the PV devices has a superstrate configuration with the glass/TCO/CuSbCh 2 /buffer/back-contact architecture (i.e., device #6 in Table II), where TCO is F:SnO 2 . So far, substrate devices had higher efficiencies than this superstrate device, further supporting the analogy of CuSbCh 2 materials with CIGS. However, in both cases, more research and development toward higher-quality absorbers and suitable contacts is needed to improve the efficiencies of CuSbCh 2 PV devices.

IV. SUMMARY
This article provides a research update on the emerging chalcostibite family of absorber materials for thin-film photovoltaic solar cells, with particular focus on the most recent research efforts (past 3 years). The CuSbCh 2 (Ch= =S, Se) features layered the 2D-like chalcostibite crystallographic structure, in contrast to the 3D-like chalcopyrite structure of CuInSe 2 and CuGaSe 2 . Because of the narrow composition phase width of CuSbCh 2 , these absorber materials require a self-regulated adsorptionlimited synthesis method to avoid detrimental Cu-rich competing phases such as Cu 12 Sb 4 S 3 or Cu 2 Se. The band structure and optoelectronic properties of CuSbCh 2 feature higher density of states and optical absorption, but heavier effective masses and worse charge transport, compared to CIGS. The moderate hole density is set by compensation between copper vacancies and interstitials, whereas the charge recombination and carrier dynamics are determined by chalcogen vacancies and Cu-on-Sb antisites. The PV-relevant optoelectronic properties of CuSbS 2 and CuSbSe 2 are summarized in Table I, where they are also compared with the more studied CIGS and CZTS absorbers.
Both CuSbS 2 and CuSbSe 2 have large cliff-like interface band offsets with CdS, calling for further development of the chalcostibite contacts. The chalcostibite photovoltaic device efficiencies are currently limited to 1%-3% for CuSbS 2 and 3%-5% for CuSbSe 2 by short transport lengths of the photogenerated charge carriers and by detrimental voltage and current effects of the contact band offset. Hence, more research is needed on the chalcostibite absorber quality and contact selection. Table II summarizes device architectures and the resulting device performances of different CuSbCh 2 solar cells reported in the literature. In summary, chalcostibite CuSbCh 2 materials are a promising non-toxic and Earth-abundant family of absorbers for applications in thin-film photovoltaic solar cell devices.