Cu (99.99%), Zn (99.99%), S powder (99.95%, Aladdin), 1,2-ethylenediamine (AR), 1,2-ethanedithiol (AR), cadmium sulfate (AR), and thiourea (AR) were purchased from Aladdin, Sn (99.8%) and Se powders (99.5%) were from Alfa Aesar, and ammonium hydroxide (AR) was from Sinopharm Chemical Reagent Co. Ltd. All the chemicals were directly used as received without further purification.

The CZTSSe precursor solution with molar ratios of Cu/(Sn+Zn) = 0.74 and Zn/Sn = 1.25 was prepared in accordance with our previous work.^[25] Cu (1.65 mmol), Zn (1.23 mmol), Sn (0.98 mmol), S (4 mmol), and Se (0.4 mmol) were dissolved in 5-mL 1, 2-ethylenediamine and 0.5-mL 1, 2-ethanedithiol. The mixture was continuously stirred for 1.5 h at 90°C to give a clear yellow solution. The precursor solution was spin-coated at 5000 rpm onto Mo-coated soda lime glass and heated on a hot plate at 380 °C for 1 min. The same process was repeated several times to produce a precursor film of about 1.5-μm thickness which was then selenized under Se/N₂ atmosphere at 550 °C in a graphite box for 15 min to get the required CZTSSe film.

An SLG/Mo/CZTSSe/CdS/ZnO/ITO/Ag solar cell was prepared by successively depositing a CdS buffer by chemical bath method, radio frequency (RF) magnetron sputtered ZnO combined with indium tin oxide (ITO) window layers, and evaporated Ag collection grid. The active area of each device for J–V measurement was 0.18 cm².

For characterization, the as-prepared CZTSSe film or device was labelled as “x d” which represents samples of the same batch stored for x days, for example, “0 d” is the sample without storage and “7 d” is that of the same batch stored in dark conditions for 7 days. These 7-d samples were sealed by a vacuum compressor under a rough vacuum and stored at room temperature. The current density–voltage (J–V) characterization for the solar cells was performed under AM1.5 illumination (1000 W · m⁻²) using an Xe-based light source solar simulator (Zolix SS150A), calibrated by a standard Si reference cell. The x-ray diffraction (XRD) was performed by using an x-ray diffractometer with Cu Kα as the radiation source (Empyrean, PANaltical). Raman spectra were collected by a Raman spectrometer (HR800, Jobin Yvon), using a 532-nm excitation laser with a power of 0.1 mW. The compositional depth profile was obtained by secondary-ion mass spectroscopy (TOF-SIMS 5, Germany ION-TOF GmbH), samples were CZTSSe and standard CdS films on Mo/SLG, 20-kV ion energy and 13.7-nA ion current was employed for analysis throughout the film, 5-kV ion energy and 4-nA ion current was used to analyze a superficial zone of CdS. The capacitance–voltage (C–V) and drive-level capacitance profile (DLCP) characterizations were measured on an electrochemical workstation (Versa STAT3, Princeton). The C–V data was performed at 100 kHz and 50-mV alternating current (AC) excitation source with direct current (DC) bias ranging from 0.5 V to −1.0 V, and the DLCP measurement was performed on 100 kHz while changing the AC perturbation voltage from 20 mV to 140 mV and DC bias from 0.5 V to −1.0 V. The band gap of the samples was determined by UV-Vis-NIR spectra on UV-3600 spectrophotometer, Shimadzu and steady-state PL spectra were obtained from a fluorescence spectrometer (FLS920, Edinburgh Instruments) with the time integration mode. The film was excited by a 535-nm monochromatic light from a xenon lamp. The valence band information was investigated via x-ray photoelectron spectroscopy (XPS) (ESCALAB 250X, Thermo Fisher Scientific) using a monochromatic Al Kα source of energy 1486.6 eV. The samples were charge-neutralized using an in-lens electron source combined with a low-energy (1 eV) Ar+ flood source. The samples for XPS characterization were as follows: “CZTSSe” sample was a fresh absorber layer, “CdS” sample was standard CdS (CBD for 11 min) deposited on CZTSSe base, “CZTSSe/CdS” sample was ultrathin CdS (CBD for 2 min) depositing on CZTSSe, which allowed the x-rays to penetrate the thin CdS to investigate both the thin CdS layer and the CZTSSe layer.

The carrier distribution and electric field inside the cell were theoretically calculated by solving the Poisson’s equations charge conservation and continuity equations in the dark by a general device simulator Analysis of Microelectronic and Photonic Structures (AMPS-1D). Dielectric constant 8.1 for CZTSSe, 9 for CdS, 7.8 for ZnO; defect densities of 10¹⁷ cm⁻³ and 10¹⁶ cm⁻³ were the values used to calculate the band alignments for 0 d and 7 d.

As shown in Fig. 1(a), after being stored at room temperature for a week, the average efficiency of the CZTSSe solar cell exhibits an obvious increase indicating that the solar cells have been self-healed. Figure 1(b) shows that the solar cell efficiency does not decrease even after 110 days, suggesting that the self-healing effect is stable and irreversible. In order to find the origin of this phenomenon, systematic experiments were performed. At first, the influence of different atmospheres on the efficiency evolution was studied. As shown in Fig. 1(c), whether in air or Ar atmosphere, the average efficiency of the cells increased similarly by about 0.7%, thus suggesting that oxygen is not responsible for the self-healing process.^[26] Moreover, a similar phenomenon is observed when the cell is stored in vacuum. This performance enhancement is thus suggested to be attributed to the device itself.

Fig. 1.

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Fig. 1. (color online) (a) Photoelectric conversion efficiency evolution of baseline devices. (b) Device efficiency stability from initial fabrication to 110-days remeasurement. (c) Photoelectric conversion efficiency changes of baseline devices by seven-day storage in different conditions: rough vacuum, argon atmosphere and air. (d) Time-dependent light current–voltage (J–V) of the baseline SLG base sample and quartz base sample.

It is widely reported that the Na diffusion within the CZTSSe film, usually introduced by post-annealing,^[27–29] could help to improve the device performance. Relevant contrast experiments were also carried out to resolve this influence. For comparison, the CZTSSe films were fabricated on the quartz substrate (without Na).^[30] Time-dependent current–voltage (J–V) results of the quartz substrate-based cell as well as that of the general soda-lime glass (SLG) are given in Fig. 1(d). It can be seen that, the standard SLG-based cell exhibits an obvious improvement in efficiency after seven days of storage, and a high efficiency up to 10.45% is obtained. Although the efficiency of the quartz-based cells is much lower than that of the SLG one, an improvement is observed in its performance as well. The quartzbased solar cell exhibits a level of performance growth similar to the SLG solar cell, indicating that Na diffusion is not the origin for self-healing. Thus, in the following sections, we mainly focus on the SLG-based cell.

Table 1 gives the statistical results of the performance parameters from 65 cells (the corresponding statistical histogram is shown in Fig. S1 in Appendix A). We can see that, after 7 days, the average efficiency of the devices increases from 8.03% to 9.08%, by about 13%. Obviously, this improvement mainly comes from the increase in V_oc (from 432.2 mV to 461.2 mV) and the FF (from 0.58 to 0.64), whereas the short-circuit current densities (J_sc) are similar to each other. As the energy bandgap E_g, (Fig. 3(b)) remains unchanged in the storage process, this improvement in V_oc directly indicates a decrease in the V_oc deficit. This implies that the performance enhancement could arise from the suppression of charge recombination.^[31] According to the charge transfer model of a heterojunction, the electrical properties of these cells have been further derived from their J–V curves.^[31] The series resistance (R_S), slightly increases from 0.98 Ω·cm² to 1.11 Ω·cm², implying that the charge transport capability of the CZTSSe film and the interfaces, especially of the CZTSSe/Mo interface, has not improved. On the other hand, the shunt resistance (R_SH) is significantly enhanced from 249 kΩ·cm² to 472 kΩ·cm², which in theory could directly lead to the increase in V_oc and FF. This improvement may arise from (i) partial elimination of high-conductive secondary phases, such as Cu₂(S, Se) and Cu₂Sn(S, Se)₃, within the CZTSSe film or (ii) the charge recombination suppression.^[5,32] The reduction in the charge recombination is further supported by the decreased ideality factor (A) of the heterojunction. For the as-prepared cell, A is about 2.08, almost the same as that of the theoretical value (A = 2) indicating the recombination mechanism dominating in the junction region. After 7 days, the value of A is reduced to 1.76, suggesting that the charge diffusion mechanism also participates in determining the charge transfer within the cell, which is mainly caused by the decrease in the charge recombination. The calculation of the inverse saturated current density (J) directly supports this result. As shown in Table 1, after 7 days, the J of the cell decreases from 2.1 × 10⁻² mA·cm⁻² to 2.2 × 10⁻³ mA·cm⁻², by about one order of magnitude. It can be thus verified that the charge recombination within the CZTSSe absorber, especially in the junction region, has been significantly suppressed.

Fig. 3.

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	Fig. 3. (color online) (a) Comparison of the time-dependent charge carrier densities in the CZTSSe absorber film solar cells at 300 K determined by C–V (Ncv) and DLCP (Ndl). The dashed line indicates the depletion width at 0-V bias. (b) Photoluminescence (PL) spectrum and Tauc plots calculated from ultraviolet–visible measurements of CZTSSe absorber films before and after storage.

Table 1.

Table 1.

Table 1. Statistical result of the electrical properties of the cells (65 cells). RS, RSH, A, and J0 are the series resistance, shunt resistance, ideality factor, and reverse saturation current determined from light J–V data referenced Steven S Hegedus method.[31] . Efficiency/% Jsc/mA·cm−2 Voc/mV FF RS/Ω·cm2 RSH/kΩ·cm2 A J0/mA·cm−2 0 d 8.03 ± 0.93 32.04 ± 1.40 432.2 ± 24.8 0.58 ± 0.05 0.98 ± 0.52 249 ± 140 2.08 ± 0.24 (2.1 ± 2.8)×10−2 7 d 9.08 ± 0.65 32.26 ± 1.10 461.2 ± 19.6 0.64 ± 0.02 1.11 ± 0.44 472 ± 240 1.76 ± 0.20 (2.2 ± 1.7)×10−3

Table 1. Statistical result of the electrical properties of the cells (65 cells). RS, RSH, A, and J0 are the series resistance, shunt resistance, ideality factor, and reverse saturation current determined from light J–V data referenced Steven S Hegedus method.[31] .

For a CZTSSe solar cell, its charge recombination is influenced by many aspects, such as the crystallization, secondary phase, element distribution and defect density.^[5,32,33] First, the XRD pattern and Raman spectra are used to study the possible phase evolution during the storage. As seen in Fig. 2(a), almost no difference in XRD patterns can be distinguished even after 7 days’ storage. The diffraction peaks at 17.5°, 27.4°, 36.4°, 45.6°, and 54.1° are assigned to the [011], [112], [211], [220]/[204], and [312]/[116] planes of the CZTSSe lattice,^[34,35] respectively, and no obvious secondary phase peak is observed. As illustrated in Fig. 2(b), the Raman spectra shows a strong peak at 198 cm⁻¹, which is close to the A mode of kesterite CZTSe at 196 cm⁻¹.^[36] Being similar to the results of XRD, no minor phases like Cu₂(S, Se) or Cu₂Sn(S, Se)₃ are observed and the spectra are unchanged after storage. In addition, top-view scanning electron microscope (SEM) images in Figs. S2(a) and S2(b) of Appendix A also show no difference with time. All these results infer that the crystallization and phase of the CZTSSe film has not changed with time. Besides, the Raman spectrum and band gap have also been used to characterize the Cu/Zn disorder within the CZTSSe lattice.^[37,38] The identical Raman spectrum and unchanged band gap (Fig. 3(b)) indicate that the Cu/Zn disorder also experiences no change with time. The element distribution within the cell is probed by a TOF-SIMS measurement. The depth of it is calibrated by the Mo signal and film thickness acquired from a cross-sectional SEM image as shown in Fig. S2(c). We can see from Fig. 2(c) that the distribution of the metal elements does not change with time, and the distribution of S and Se in Fig. S3 of Appendix A show little change as well. Therefore, the possibility of an exhausted Cu surface is also excluded.^[39,40] For the CZTSSe, it is generally considered that the diffusion of the impurity atoms, such as the Na from the SLG substrate and Cd from the buffer layer, could help to passivate some defects and thus promote the device performance.^[29,41,42] However, as shown in Figs. 2(c) and 2(d), the Na and Cd distribution within the CZTSSe film is unchanged, suggesting that further longitudinal diffusion of these elements does not happen in the self-healing process. Based on the above measurements, we can conclude that the performance enhancement of the cell after storage is not due to the change in the phase, Cu/Zn ordering, crystalline or element distribution.

Fig. 2.

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	Fig. 2. (color online) (a) Time-dependent XRD pattern of CZTSSe absorber film. (b) Normalized Raman spectra of CZTSSe absorber films. SIMS depth profile of (c) Na, Cu, Zn, Sn content throughout the film (d) Cd content of superficial area, the 0-d sample intensity multiplied by 2.4 times.

The DLCP and C–V measurements^[43,44] have been conducted to evaluate defect properties of the CZTSSe absorber. To exclude the possible influence from other layers, the same batch of Mo/CZTSSe films is selected for the following measurement. In this batch, the samples are classified into two parts, defined as 0-d and 7-d samples, respectively, and one part was directly fabricated into the device without any storage while the other was stored for 7 days and then fabricated into the devices by following the same process. As shown in Fig. 3(a), after 7 days, the charge-carrier density within the film is significantly reduced by about one order of magnitude (from 10¹⁷ cm⁻³ to 10¹⁶ cm⁻³), indicating an obvious reduction in defects, even though it cannot be determined whether these are shallow or deep defects. For 0-d sample, a high charge density of about 10¹⁷ cm⁻³ is estimated by both the DLCP and C–V measurements, suggesting that a large number of bulk defects could exist within the as-prepared CZTSSe film, thus resulting in a narrow charge depletion region with a width (W_D) of only about 140 nm. For the 7-d cell, as the charge-carrier density decreased, the W_D is subsequently extended to 240 nm, which could effectively facilitate photocarrier transportation.

Steady-state photoluminescence spectroscopy (PL) has been further employed to investigate bulk defects in the CZTSSe film. The CZTSSe film was covered by a PMMA layer (∼ 200 nm) to prevent the surface from the external atmosphere (such as oxygen and water). As shown in Fig. 3(b), for the as-prepared 0-d CZTSSe film, two irradiative emission bands, a continuum-band emission at 1.08 eV (∼ 1149 nm) and a defect emission at below 0.85 eV, can be distinguished. The position of the continuum-band emission agrees well with the E_g (1.1 eV) derived from the light absorption curve, implying that no obvious emission red-shift was caused by band-tail states in the CZTSSe system.^[45] Owing to limited measurement range of the instrument, we could not obtain its full spectrum. However, it could still be ascertained that this defect was deeper than Cu_Zn or V_Cu which could provide free carriers in CZTSe, and further, these deep defects may be Sn-related or S/Se vacancy defects.^[6] For the 7-d CZTSSe film, the E_g and continuum-band emission are similar to that of the 0-d one, while the deep-defect emission is significantly suppressed. This implies that these defect states have been significantly passivated, in line with the DLCP and C–V results. In view of the above observations, it is suggested that the performance improvement of the cell with extended storage time is mainly due to the reduction of these deep defects.

To further confirm the above assumption, the influence of the reduction in the deep-defect density on the electric properties of the cell has been studied by theoretical simulation on the band alignment.^[46,47] As shown in Fig. 4(a), when the defect density decreases from 10¹⁷ cm⁻³ to 10¹⁶ cm⁻³, the W_D is obviously extended as expected. Moreover, the simulation indicates that the band bending properties at the interface are different for these two conditions. For the low-defect-density cell, the energy difference between the Fermi energy level and the valence band of the CZTSSe is obviously increased, implying that band bending is enhanced. Experimentally, this energy difference has been probed by the XPS valence-band spectroscopy. Figure 4(b) gives the XPS valence-band spectra of the 0-d and 7-d CdS/CZTSSe samples, where the CdS is ultrathin to make the CZTSSe detectable. The core levels of both Sn^{3 d} and Cd^{3 d} in the CdS/CZTSSe sample are shown in Fig. S4 of Appendix A. The spectra of CdS/CZTSSe sample can be separated into two parts, as depicted in Fig. 4(b). The low-energy tail (part I) is ascribed to the CZTSSe while the high-energy shoulder (part II) to both the CdS and CZTSSe. It can be found that the valence-band edge variation of part I (from 0.39 eV to 0.54 eV) is close to that of part II (from 1.50 eV to 1.66 eV), suggesting that the change of the CdS/CZTSSe sample is caused by the common CZTSSe layer. No obvious change in the valence-band spectrum of CdS sample after storage is found to support this result, as shown in Fig. S4(c). Besides, the 150-meV down-shift of the valence band is in good agreement with the theoretical prediction, which signifies an increased built-in electric field.

Fig. 4.

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Fig. 4. (color online) (a) Band alignments theoretically simulated by AMPS-1D with different defect densities (1 × 1017 cm−3 for “0 d”, 1 × 1016 cm−3 for “7 d”). (b) XPS measured valence band spectrum of CZTSSe sample, CdS sample and the 7-days change of several nanometers coated CZTSSe film (CdS/CZTSSe sample). The black dot lines are only guides for the eye and indicate the approximate VBM positions, according to the linear extrapolation method. The spectra of CdS/CZTSSe sample is divided by the zero point of CdS sample’s density of states, named part I and part II.

In view of the above discussion, it is suggested that the deep defects, like Sn-related or S/Se vacancy defects, in the CZTSSe absorber layer are spontaneously reduced after storage at room temperature. This reduction could help to suppress charge recombination, and furthermore lead to a wider depletion and a stronger built-in electric field. Therefore, V_oc and FF significantly increase after the self-healing and lead to better device performance. This improvement may be attributed to: i) the atomic relaxation in grain boundary (GB) regions.^[48] The dangling bonds at the GB create localized defect states,^[49,50] and the nanoscale horizontal diffusion of atoms is undetectable by SIMS, but can break or weaken the detrimental dangling bonds thus eliminating the deep gap states; ii) the release of internal stress within the CZTSSe film. The compressive internal stress in the asprepared film^[51] is spontaneously released at room temperature,^[52] which helps to optimize the spatial energy band distribution and thus reduce the charge localization centers. This self-healing effect presents a valuable opportunity to recognize the hidden mechanism that was limiting the device performance.