Experimental and Theoretical Study into Interface Structure and Band Alignment of the Cu2Zn1–xCdxSnS4 Heterointerface for Photovoltaic Applications

To improve the constraints of kesterite Cu2ZnSnS4 (CZTS) solar cell, such as undesirable band alignment at p–n interfaces, bandgap tuning, and fast carrier recombination, cadmium (Cd) is introduced into CZTS nanocrystals forming Cu2Zn1–xCdxSnS4 through cost-effective solution-based method without postannealing or sulfurization treatments. A synergetic experimental–theoretical approach was employed to characterize and assess the optoelectronic properties of Cu2Zn1–xCdxSnS4 materials. Tunable direct band gap energy ranging from 1.51 to 1.03 eV with high absorption coefficient was demonstrated for the Cu2Zn1–xCdxSnS4 nanocrystals with changing Zn/Cd ratio. Such bandgap engineering in Cu2Zn1–xCdxSnS4 helps in effective carrier separation at interface. Ultrafast spectroscopy reveals a longer lifetime and efficient separation of photoexcited charge carriers in Cu2CdSnS4 (CCTS) nanocrystals compared to that of CZTS. We found that there exists a type-II staggered band alignment at the CZTS (CCTS)/CdS interface, from cyclic voltammetric (CV) measurements, corroborated by first-principles density functional theory (DFT) calculations, predicting smaller conduction band offset (CBO) at the CCTS/CdS interface as compared to the CZTS/CdS interface. These results point toward efficient separation of photoexcited carriers across the p–n junction in the ultrafast time scale and highlight a route to improve device performances.


■ INTRODUCTION
The advancement of safe, renewable, and low-cost clean energy technologies to substitute the environmentally unfriendly provider of fossil fuels have attracted significant scientific interest in recent years. Solar cell technology, which harnesses the enormous amount of the Sun's energy to directly produce electricity, is a viable option with obvious socioeconomic and environmental benefits. However, solar devices must be made of inexpensive and earth-abundant materials before photovoltaic (PV) technology can readily provide a significant fraction of the world's energy demands at reduced cost. 1,2 Earth-abundant, nontoxic, and low-cost Cu-based materials Cu 2 MSnX 4 (M = Zn, Ni, Fe, Co, Cd and x = S, Se) are attractive potential alternatives to silicon-based PV technology, due to their wide tunable direct bandgap (1.5−1.0 eV) across the visible region, high absorption coefficient (∼10 4 cm −1 ), and high carrier mobility (0.1−35 cm 2 V −1 s −1 ). 3 Besides, the outstanding theoretical efficiency of 31% (according to Schockley Queisser limit) makes Cu 2 MSnX 4 materials competitive candidates for environmentally friendly, low-cost, and scalable thin-film PV applications. 4 Cu 2 MSnX 4 -based materials are also interesting candidates for a wide variety of other applications, including photodetectors, 5 photocatalysis, 6 hydrogen production, 7 photoelectrochemical cell, 8 and batteries. 9 The first Cu 2 ZnSnS 4 (CZTS) solar cell was made-up of vacuum technique and generated a conversion efficiency of 0.66%. 10 So far the kesterite CZTSSe-based PV cells show the highest conversion efficiency of 12.6% as reported by the group of Mitzi. 11 Currently, many research efforts are focused on cation and anion exchange for the enhancement of the conversion efficiencies of CZTS materials. Wong and coworkers have reported conversion efficiencies of 9% 12 for single cation substitution (Cd for Zn) and 10% for double cation substitution (Cd for Zn and Ag for Cu) in CZTS thin film based solar cells. 13 Despite the recent improvements in Cd-substituted CZTS materials, information regarding the influence of thin film grains, the nature of interfaces between the absorber and buffer layers, and charge carrier dynamics in the Cd-substituted CZTS nanocrystals remained unresolved. The performance of polycrystalline PV cells can be significantly influenced by charge carrier dynamics and grain boundary engineering, 14−17 hence their deep understanding in Cu-based quaternary chalcogenides will assist future optimization of PV devices to achieve improved performance. In earlier investigations, 18−22 we have reported a direct correlation between efficiency and carrier dynamics in solar cells. Solar device performance can be significantly enhanced with elongated charge-carrier lifetimes when the grain size is increased from a few hundred nanometers to the micrometer level. 11 The work of Seto et al. explained how larger grain boundaries can reduce the quantity of defects due to partial atomic bonding. 23 A reduction in the trapping states can thus lead to an increase in the number of free carriers available for electrical conduction. Cation and anion substitution have been employed to tune the bandgap of CZTS materials in progression toward CZTS thinfilm solar cells. 12,13,24 Low efficiencies of CZTS-based PV cells have been attributed to several factors including defects, poor interface chemistry, inefficient charge separation due to poor band offsets, and carrier trapping. However, until now there has been no systematic study dedicated to providing detailed insight into the carrier dynamics and interface chemistry in Cu 2 Zn 1−x Cd x SnS 4 thin films, which makes this investigation timely.
In this communication, we present a synergetic experimental and computational evidence of the beneficial role of cation exchange (Cd for Zn) in Cu 2 Zn 1−x Cd x SnS 4 . High quality and crystalline thin films (RMS roughness below 20 nm) with large grain boundaries were obtained from a unique cost-effective, benchtop solution-processed method, at room temperature without postannealing or sulfurization, which is important in reducing cost of device fabrication. X-ray diffraction and Raman spectroscopy confirmed the nanocrystals without impurity phases. We expect that there is a huge opportunity to improve the device performance with band alignment optimization and monitoring of charge transfer dynamics. Bandgap modulation from 1.51 to 1.03 eV was achieved with growing Cd content in Cu 2 Zn 1−x Cd x SnS 4 from UV−visible− NIR spectroscopy. The photoexcited charge carrier dynamics, which dictates the photovoltaic performance of Cu 2 Zn 1−x Cd x SnS 4 , was demonstrated with the help of transient absorption spectroscopy where we observed longer charge carrier lifetime in CCTS compared to the CZTS nanocrystals. A type-II band alignment was demonstrated at the CZTS/CdS and CCTS/CdS heterointerfaces through CV measurements and validated by first-principles DFT calculations. Smaller conduction offsets of 0.15 eV (0.11 from DFT) were observed at CCTS/CdS compared to 0.23 eV (0.21 from DFT) at CZTS/CdS interface, suggesting lesser resistance to electron transport across the CCTS/CdS interface. Our results highlight a route to improved solar cell efficiencies through cation substitution and interface band offset optimization. Herein, the colloidal synthesis of Cu 2 Zn 1−x Cd x SnS 4 (x = 0 corresponds to CZTS and x = 1 corresponds to CCTS) nanocrystals was carried out as per our previous report using a homemade hot injection setup. 25 In a typical synthesis of CZTS nanocrystals, all three metal (copper, zinc, and tin) acetylacetonate precursors with 2:1:1 ratio are dissolved in 15 mL of oleylamine under high purity nitrogen in a three neck flask. The CCTS nanocrystals were similarly synthesized as mentioned above where instead of zinc, cadmium acetate is used in a separate three neck flask. The solutions for both CZTS and CCTS were heated under nitrogen with constant stirring and degassed for 1 h at ∼135°C. The temperature was set at 240°C for the injection of sulfur precursor. Fresh 3 mL of sulfuroleylamine (1 M) solution was injected in the three neck flask and maintained at 260°C for 1 h for nanocrystal growth. The reaction was quenched by injecting toluene and propanol mixture (1:8), followed by purification of the nanocrystals from unreacted precursors by centrifugation. 25 The intermediate compositions of Cu 2 Zn 1−x Cd x SnS 4 nanocrystals with x = 0.2, 0.4, 0.6, and 0.8 mmol of Zn/Cd were carried out by similar method discussed in earlier section.
Characterization. Different properties of Cu 2 Zn 1−x Cd x SnS 4 were characterized using various experimental techniques. X-ray diffraction (XRD) spectra were recorded by X-ray diffractometer (Bruker D8 Advance, Germany). The phase characterization of the as-synthesized samples was obtained by Raman spectroscopy with "Renishaw Raman Microscope" using a wavelength of 532 nm of Nd:YAG laser, in the backscattering geometry. High-resolution transmission electron microscopy (HR-TEM) (TECNAI G2-20-TWIN) was used to obtain high-resolution micrographs and selected area electron diffraction (SAED) patterns. Atomic force microscopy (NC-AFM; JEOL, JSPM-5200) provided micrographs for the investigation of the surface topology in noncontact mode. The compositional details of Cu 2 Zn 1−x Cd x SnS 4 nanocrystals were obtained from energy dispersive X-ray spectroscopy (EDS) measurements done using a Hitachi, S-4800. The optical band gap was determined using a JASCO, V-670 UV−visible−NIR spectrophotometer. The cyclic voltammetric (CV) measurements for determining the band structure parameters were carried out using Metrohm potentiostat/galvanostat, Autolab PGSTAT 100 as per our recent studies. 3,25 The femtosecond transient absorption spectroscopy was carried out using HELIOS ultrafast setup. 26 ■ RESULTS AND DISCUSSION XRD spectra of the Cu 2 Zn 1−x Cd x SnS 4 nanocrystals as shown in  12,13,24 Substitution of Zn 2+ cation with Cd 2+ resulted in a monotonic shift of the diffraction peaks toward small 2θ values, which indicate increasing lattice constants and unit cell volume with increasing Cd content. The increased unit cell parameters can be rationalized by considering the larger Cd 2+ covalent radius (1.48 Å) compared to that of Zn 2+ (1.25 Å). Bragg's equation was used to calculate the structural parameters, including the lattice parameters (a, b, c) and the interplanar spacing (d hkl ) (Table S1, Supporting Information).
The average crystallite size (D hkl ) is calculated using the Scherrer equation (D hkl = kλ/β cos θ), 27 where k is the shape factor, λ is the X-ray wavelength, β is the line broadening at half-maximum intensity, and θ is the Bragg's angle. The crystallite size of Cu 2 Zn 1−x Cd x SnS 4 is found to increase from 13.01 to 19.5 nm (Figure 1c) and the strain during cation substitution (Table S1, Supporting Information) decreases from 2.66 × 10 −3 to 1.78 × 10 −3 . Raman spectra (Figure 1b) confirm the phase purity of the Cu 2 Zn 1−x Cd x SnS 4 nanocrystals as no secondary phase was present. We have observed a linear shift in the Raman spectra ( Figure 1b) with increasing Cd content toward lower wavenumbers, which confirms a gradual structural change from kesterite to stannite phase. Transmission electron microscopy pictures and the corresponding selected area electron diffraction (SAED) patterns recorded for CZTS and CCTS nanocrystals are shown in Figure 1c(i−vi). The TEM images (Figure 1c(i,iv)) show that the CZTS and CCTS nanocrystals are spherical in shape with diameter ranging from 15 to 25 nm. The HRTEM images ( Figure  1c(ii,v)) specify CZTS and CCTS nanocrystals have lattice fringes with interplanar distance (d) of 0.312 and 0.319 nm, respectively, which are in good agreement with those determined from the diffraction peak in the XRD pattern (Table S1, Supporting Information). The SAED analysis of the CZTS and CCTS nanocrystals in Figure 1c(iii,vi) show concentric rings reflecting the polycrystalline nature of these materials. The room-temperature absorption spectra of the Cu 2 Zn 1−x Cd x SnS 4 nanocrystals ( Figure 1d) disclose absorption in the entire visible area with a tail extending to the near-IR (NIR) region, giving rise to the black color of the samples. The inset in Figure 1d shows the subplot of (αhν) 2 versus (hν) for the Cu 2 Zn 1−x Cd x SnS 4 nanocrystals. We have observed a systematic decrease in the band gap from 1.51 to 1.03 eV for x = 0.0 to x = 1.0, as shown in Figure 1e and Table S2, Supporting Information. These results are consistent with a number of previous reports. 28 32 The differences reported in bandgaps by the various authors suggest that the bandgap of Cu 2 Zn 1−x Cd x SnS 4 is influenced by several factors including but not limited to film thickness, grain size, elemental composition, and defects. 3,10,[15][16][17]25 The compositional analysis of the Cu 2 Zn 1−x Cd x SnS 4 nanocrystals was carried out using energy dispersive X-ray (EDS) analysis technique. The EDS data recorded in binding energy regions of 0−10 keV for the Cu 2 Zn 1−x Cd x SnS 4 nanocrystals (Table S3, Supporting Information) reveal that the Cu:Zn/Cd−Sn−S is close to the desired 2:1:1:4 ratio, hence our synthesis technique presents an accurate approach to carry out a controlled composition dependent synthesis, which is usually rather very challenging to achieve.
Further insights into the structural parameters, thermodynamic, and mechanical stabilities of the Cu 2 Zn 1−  Figure S2. The calculated lattice constants and lattice spacing are found to increase with increasing Cd concentration, which is in good agreement with the experimentally measured ones, as shown in Table S4 (Supporting Information). The cohesive energy, which gives a measure of the stability of the studied materials, has been evaluated for the different Cd compositions in Cu 2 Zn 1−x Cd x SnS 4 . The cohesive energy of CZTS is found to be 4.036 eV/atom, which closely agrees with the experimentally reported 33 value of 3.9 eV/atom and is comparable to that of bulk silicon (expt., 4.68 eV/atom). A gradual decrease in cohesive energy with increasing Cd content in the unit cell is observed ( To examine the result of Cd substitution on electronic structures of the Cu 2 Zn 1−x Cd x SnS 4 materials, we have calculated the electronic band structures and partial density of states (PDOS) of the different compositions using the hybrid exchange-correlation functional (HSE06), which has been shown to give a good description of the electronic properties of CZTS. 35 Figure 2(c,f) indicates that the valence band maximum (VBM) of both materials is composed mainly of Cu-3d and S-3p states. The conduction band minimum (CBM) on the other hand is composed primarily of Sn-5s and S-3p states. Considering that the optical absorption properties of a semiconductor are of great importance in its photovoltaic activity, we have simulated the absorption spectra of the Cu 2 Zn 1−x Cd x SnS 4 materials, as shown in Supporting Information in Figure S4. A high absorption coefficient (α abs > 10 5 cm −1 ) is predicted for the Cu 2 Zn 1−x Cd x SnS 4 materials, which agrees very well with experimental results. The red shift in the absorption peak with increasing Cd concentration is consistent with the trends observed in the experimental UV−visible−NIR optical absorption spectra. These properties show great promise for Cu 2 Zn 1−x Cd x SnS 4 in multijunction tandem solar cells. Next, to investigate the topological properties, we have developed a unique and simple electrophoretic deposition method to successfully deposit highly dense and uniform Cu 2 Zn 1−x Cd x SnS 4 thin films. Shown in Figure 3a, is the schematic diagram illustrating the deposition process for Cu 2 Zn 1−x Cd x SnS 4 thin films. In the setup, DC power was used to apply the potential to two FTO electrodes dipped into Cu 2 Zn 1−x Cd x SnS 4 nanocrystals solution. The working and counter bare FTO electrodes were introduced in parallel with ∼7 mm gap in 5 mg/mL Cu 2 Zn 1−x Cd x SnS 4 nanocrystal solution. After systematic optimization of the applied voltage to 50 V and increasing the deposition time to 45 min, the suspended nanocrystals in the solution moved toward the positive electrode to form highly dense and uniform thin films. The surface properties of the Cu 2 Zn 1−x Cd x SnS 4 films were examined by atomic force microscopy (AFM). Two-dimensional (2D) AFM micrographs (Figure 3b) were obtained without any additional or postannealing treatment. The threedimensional (3D) surface topology of the Cu 2 Zn 1−x Cd x SnS 4 films were evaluated for 1 μm 2 surface areas (Supporting Information Figure S1), which revealed a smooth and highly homogeneous thin film with low surface roughness within 10− 20 nm range (Table S7, 40 employed a similar deposition procedure with an applied voltage of 100 V to achieve the deposition of larger linear rods (∼500 nm in length) of cesium lead halide perovskite nanocrystals without postannealing. The effect of the annealing temperature on the formation of Cu 2 ZnSnS 4 thin films has been investigated by several authors who have all achieved large grain (micron-sized grains) with sulfurization treatments. 41−46 The obtained large grains in the present study may facilitate carrier transport and easy collection, thus improving device performance via minimizing trapping and recombination of photogenerated carriers. In Figure 3c, we provide a schematic diagram showing how larger grain can enhance charge carrier transport and reduce carrier trapping at grain boundaries in thin films compared to smaller grain size. With smaller grain sizes ( Figure  3c(i)), the probability of recombination will be high at grain boundaries whereas with larger grain sizes (Figure 3c(i)), the recombination of charge carries can be significantly minimized.
To gain insight into the native photophysics in CZTS and CCTS systems, we have employed ultrafast pump−probe spectroscopy with 420 nm pump excitation and probed the transients in the visible region of the spectrum. The transient spectra (TA) manifest a broad photoinduced absorption band throughout the visible region (450−760 nm), represented in Figure 4a. This positive feature in the case of CZTS persists along the temporal evolution of the spectra for the entire time range until it completely decays. Interestingly, the TA spectra of CCTS nanocrystals show positive absorption band until 200 ps, but beyond 250 ps a negative absorption band (bleach)  and τ 3 = >2 ns (+100%), respectively. We observed that the majority of the CZTS photoinduced feature depletes at a much faster rate (τ 1 CZTS = 0.38 ps) compared to CCTS (τ 1 CCTS = 3.48 ps). One may attribute this faster transient decay of CZTS due to quicker trapping of photoexcited charge carriers in the CZTS system compared to that of CCTS. It has also been observed that the transient positive signal of CZTS completely decays within 2 ns. However, in the case of CCTS at longer time scale (beyond 250 ps onward), the bleach signal appears which matches with optical absorption band of CCTS ( Figure  4b). The existence of the bleach signal beyond 2 ns clearly suggests that the charge carriers generated in CCTS are much longer lived as compared to that of CZTS. Our results are similar to those obtained by Pundsack et al. who reported short excited state lifetime (<20 ps) of the photoexcited charge carrier for CZTS nanocrystal 47 and unfavorable nonradioactive recombinations of the carrier's limit efficiency in solar cells. Our spectroscopic measurement indicates the more efficient separation of optically excited charge carriers leading into slower electron−hole recombination in CCTS as compared to CZTS nanocrystals. Consistent with our spectroscopic study, previous reports have observed enhanced current density and solar power conversion efficiency for Cu 2 Zn 1−x Cd x SnS 4 device as compare to pure CZTS. 12,13,24 The mean effective mass of charge carriers influences their transport properties; hence, we have calculated the hole and electron mean effective masses for CZTS and CCTS. A quadratic polynomial was fitted to the energy of the corresponding bands in the reciprocal lattice vector k for this calculation. The calculated electron and hole effective masses for CZTS and CCTS NC's in some selective directions of the Brillouin zone are shown in Figure 4d. Such light mass charge carriers can be easily separated due to photoexcitation and can enhance the efficiency of solar cell devices. The slightly smaller electron and hole effective masses calculated for CCTS in almost all directions than CZTS suggest higher charge carrier mobility (electrical conductivity). In line with this, recent studies by Wong and co-workers have reported efficiency improvement for Cd-substitution with Zn (η = 9%) 12 and double cation (Cd for Zn and Ag for Cu) substitution (η = 10%) 13 for CZTS solar cells.
The band edges (VBM and CBM) of the CZTS and CCTS were determined through cyclic voltammetry measurements (scan rate, 100 mV s −1 ). This measurement permits the estimation of the electrochemical bandgap and the electron affinity (EA) of CZTS and CCTS. Figure 4c shows the CV data for CZTS with two peaks marked as A 1 and C 1 at 0.85 V (−5.35 V vs V ac ) and at −0.68 V (−3.82 V vs V ac ) over repeated cycles, which correspond to the anodic and cathodic peaks, respectively. For CCTS, the anodic A 2 and cathodic C 2 peaks are observed at 0.51 V (−5.01 V vs V ac ) and at −0.6 V (−3.9 V vs V ac ), respectively. The potential difference between the anodic and cathodic peaks of CZTS (1.53 V) and CCTS (1.11 V) corresponds to their bandgap values. CdS, which is commonly used as a buffer layer for CZTS PV devices has a band gap of 2.42 eV and electron affinity of 4.0 eV. 48 The exact positions of the conduction and valence bands with respect to the vacuum level are calculated using the relations E VBM = −IP = −(E peak-oxidation + E ref ) and E CBM = −EA = −(E peak-reduction + E ref ), where IP is the ionization potential and EA is the electron affinity. 1 Ferrocene was used as the internal reference E ref (4.5 eV vs NHE), and E peak-oxidation and E peak-reduction energies corresponds to A 1 and C 1 peaks, respectively, in Figure 4c.  In a working PV device, the electron and the hole have to be dissociated to obtain free charge carriers. The exciton is characterized by its binding energy (E b ) and it has to be as low as possible to facilitate the dissociation. If we assume that the exciton will be dissociated by the thermal energy, E b should be lower than k B T (around 25 meV at room temperature). After the free charge carriers are created from the exciton dissociation step, they have to diffuse to the back electrodes in a photovoltaic system before their recombination. The discontinuities at the absorber/buffer interface act as barriers to electrical transport across the interface. Hence the magnitude of the discontinuities (band offsets) controls the transport phenomena at these interfaces and characteristics of devices. The smaller the CBO and the VBO between the absorber/buffer layers the less resistance the free charge carriers experience when moving across the interfaces. This often results in efficient free charge carrier separation and minimized recombination rates. In light of this, the smaller CB offset predicted for the CCTS/CdS compared to the CZTS/ CdS indicates that free electrons generated in CCTS will diffuse much faster and be collected before their recombination with free holes in the VB. Consistent with our results, recent studies have reported higher efficiencies for Cu 2 Zn 1−x Cd x SnS 4 based devices than pure CZTS. 12,13,24 ■ CONCLUSIONS In summary, we have employed combined experimental and theoretical approaches to unravel the interface chemistry and energy band alignment of Cu 2 Zn 1−x Cd x SnS 4 based heterointerface. Insight into the charge carrier dynamics of the CZTS and CCTS nanocrystal are also provided. Without any postannealing treatment, highly dense and uniform Cu 2 Zn 1−x Cd x SnS 4 thin films with large gains were obtained using electrophoretic  The authors declare no competing financial interest.