Highly stable and efficient perovskite solar cells with native-oxide passivation

Abstract

There has been an urgent need to eliminate toxic lead from the prevailing halide perovskite solar cells (PSCs), but the current lead-free PSCs are still plagued with the critical issues of low efficiency and poor stability. This is primarily due to their inadequate photovoltaic properties and chemical stability. Herein we demonstrate the use of the lead-free, all-inorganic cesium tin-germanium triiodide (CsSn_0.5Ge_0.5I₃) solid-solution perovskite as the light absorber in PSCs, delivering promising efficiency of up to 7.11%. More importantly, these PSCs show very high stability, with less than 10% decay in efficiency after 500 h of continuous operation in N₂ atmosphere under one-sun illumination. The key to this striking performance of these PSCs is the formation of a full-coverage, stable native-oxide layer, which fully encapsulates and passivates the perovskite surfaces. The native-oxide passivation approach reported here represents an alternate avenue for boosting the efficiency and stability of lead-free PSCs.

Introduction

The promise of high efficiency and low cost has been propelling perovskite solar cells (PSCs) research over the past decade or so. While the record power conversion efficiency (PCE) of PSCs is now approaching 24% rivaling that of silicon-based solar cells, the state-of-the-art PSCs employ lead-based organic–inorganic halide perovskite absorber materials. The toxicity of lead associated with the lifecycle of these PSCs is a serious concern, and it may prove to be a major hurdle in the path toward their commercialization. Thus, significant effort is being devoted toward the development of low-cost, efficient lead-free PSCs. Several low-toxicity cations have been proposed for replacing Pb(II) in halide perovskites, including Ag(I), Bi(III), Sb(III), Ti(IV), Ge(II), and Sn(II). Among these candidates, halide perovskites based on Sn(II) have shown the highest PCE, and, thus, have attracted the most attention in the PSC field. Typical Sn-based halide perovskites that have been studied include CH₃NH₃SnI₃ (MASnI₃), HC(NH₂)₂SnI₃ (FASnI₃), and CsSnI₃. While PSCs based on MASnI₃ and FASnI₃ perovskites have been shown to deliver high PCE, up to 9%, these materials have intrinsically low stability. This is primarily attributed to the presence of the organic cation, which is prone to facile volatilization. In this context, the all-inorganic lead-free CsSnI₃ perovskite becomes a more attractive candidate. However, the facile oxidation of Sn(II) to Sn(IV), and attendant phase instability in the CsSnI₃ perovskite, results in the rapid degradation of its properties. The most effective strategy that has been proposed for mitigating this issue is to incorporate Sn(II)-halide additive (SnF₂, SnCl₂, and SnI₂), but the resulting PSCs show maximum PCE of only 4.81%, and no operational stability data on these PSCs has been reported. This calls for new stabilization approaches that can boost the stability and PCE of CsSnI₃-based PSCs simultaneously.
Herein, we report the surprising discovery that by simply alloying Ge(II) in CsSnI₃ to form a CsSn_0.5Ge_0.5I₃ composition perovskite, its thin films can become highly stable and air-tolerant. While the favorable Goldschmidt tolerance (0.94) and octahedral (0.4) factors in CsSn_0.5Ge_0.5I₃ contribute to the structural stability of this alloy (Supplementary Fig. ), the extremely high oxidation activity of Ge(II) enables the rapid formation of an ultrathin (<5 nm) uniform native-oxide surface passivating layer on the CsSn_0.5Ge_0.5I₃ perovskite, imparting it with superior stability, even compared with the prototypical MAPbI₃ perovskite. We also demonstrate a facile one-step vapor-processing method for the deposition of CsSn_0.5Ge_0.5I₃ perovskite thin films, leading to PSCs with PCE up to 7.11%. We further show that these CsSn_0.5Ge_0.5I₃ PSCs are highly stable upon continuous operation under 1-sun illumination for over 500 h. The extraordinary stability of the CsSn_0.5Ge_0.5I₃ perovskite is explained using coupled experiments and theory.

Results

Synthesis and processing of CsSn_0.5Ge_0.5I₃ perovskites

Figure 1(a) is a photograph of CsSn_0.5Ge_0.5I₃ perovskite powder synthesized by solid-state reaction (at 450°C) in an evacuated Pyrex glass tube. The indexed X-ray diffraction pattern (XRD) in Supplementary Fig. 1(a) confirms that this powder is a single-phase perovskite, and it appears to crystallize in the space group of R3m. Its comparison with the XRD pattern of reference CsGeI₃ perovskite powder (space group R3m), synthesized using the same procedure, in Supplementary Fig. 1(b) shows peak shifts to lower 2θ angles. This indicates an expansion of the unit cell, as expected with the substitution of the smaller Ge(II) by the larger Sn(II). Reference black phase B-γ-CsSnI₃ perovskite, which was also synthesized using the same procedure, crystallizes in the orthorhombic (space group Pnma) structure; Supplementary Fig. 1(c) shows its XRD pattern. (More detailed crystallographic analyses of CsSn_1–xGe_xI₃ perovskite alloys will be studied in the future to resolve the symmetry transition with the increase of Ge content.)

Fig. 1

Film synthesis and characterization. a Photograph of as-synthesized CsSn_0.5Ge_0.5I₃ perovskite solid using the melt-crystallization method. b Schematic illustration of the single-source evaporation method for the deposition of ultrasmooth CsSn_0.5Ge_0.5I₃ perovskite thin film. c Photograph of an as-synthesized large-area CsSn_0.5Ge_0.5I₃ perovskite thin film on a glass substrate showing dark reddish color. d Indexed XRD pattern of the fresh CsSn_0.5Ge_0.5I₃ perovskite thin film (inset: top-view SEM image of the microstructure). e Absorption and steady-state PL spectra of a fresh CsSn_0.5Ge_0.5I₃ perovskite thin film

The CsSn_0.5Ge_0.5I₃ perovskite powder was used to evaporate thin films onto various substrates, as shown schematically in Fig. 1b. Figure 1c is a photograph of such a film deposited on a 10 × 10-cm² glass substrate showing dark reddish color. The XRD pattern in Fig. 1d confirms the single-phase nature of the CsSn_0.5Ge_0.5I₃ perovskite thin film (on a glass substrate). The inset within Fig. 1d shows scanning electron microscope (SEM) image of the top surface. SEM image of the cross section is presented in Supplementary Fig. 2a, where the typical film thickness is about 200 nm, and the grain size is estimated at about 80 nm. The films appear highly uniform, full-coverage, and ultrasmooth, where the root-mean-square roughness is found to be 2.1 nm, as revealed in the atomic force microscope (AFM) image in Supplementary Fig. 2b.
Figure 1e presents optical absorption and steady-state photoluminescence (PL) spectra of the CsSn_0.5Ge_0.5I₃ perovskite thin film. Good absorption is observed across the visible region, with an edge at about 840 nm, although the estimated Urbach energy is relatively high (37 meV), which could be related to the intrinsic Sn vacancies. The PL emission peak is centered at about 830 nm, which is consistent with the absorption. The PL peak is relatively sharp (FWHM 52 nm), a hallmark of a good light-absorber material. The PL emission was also mapped over a 50 × 50-μm² area at various locations on the thin films (Supplementary Fig. 3), confirming the optical uniformity across the entire film. The Tauc plot of the thin films in Supplementary Fig. 4a indicates an optical bandgap of about 1.50 eV, which is consistent with that predicted in our earlier computational studies. This bandgap lies in-between the bandgaps of CsSnI₃ (1.31 eV) and CsGeI₃ (1.63 eV) perovskites, which is to be expected due to the upshift of the valence band maximum (VBM) with the substitution of Sn(II) for Ge(II) in CsGeI₃. Supplementary Fig. 4b plots the real and the imaginary parts of the refractive index of the CsSn_0.5Ge_0.5I₃ perovskite thin films as a function of the wavelength.

Native-oxide surface passivation in CsSn_0.5Ge_0.5I₃ perovskite

As soon as the CsSn_0.5Ge_0.5I₃ perovskite thin films are exposed to air, a stable native-oxide layer forms on the surface, within 30 s. Figure 2a plots Ge 3d X-ray photoelectron spectroscopy (XPS) spectra at different incidence angles. At shallow angles, the presence of Ge(IV) is detected (binding energy 33.0 eV). Since the surface roughness is about 2.1 nm and the native-oxide layer is expected to be less than 5-nm thick, the surface layer is sampled primarily at such shallow angles. With increasing incidence angle, the Ge(II) peak (binding energy 31.0 eV) dominates, as the underlying CsSn_0.5Ge_0.5I₃ perovskite thin film is sampled primarily. These spectra are deconvoluted, and the estimated Ge(II) content (relative to total Ge) is plotted in Fig. 2b. A sharp drop in the Ge(II) content at incidence angles 30 to 45° indicates a distinct native-oxide layer comprising Ge(IV) primarily. XPS maps (normal incidence angle) of Ge 3d (33.0 eV) and O 1s (532.0 eV) in Fig. 2c, d, respectively, of the same surface region show a strong correlation, confirming the formation of a Ge(IV)-rich native oxide. Supplementary Fig. 5a plots Sn 3d XPS spectra as a function of incidence angle. Since the binding energies for Sn(II) (486.0 eV) and Sn(IV) (486.6 eV) are very close, it is difficult to discern the oxidation state of Sn. Nevertheless, the Sn:Ge atomic ratio is extracted from the data in Supplementary Fig. 5a and Fig. 2a, and it is plotted in Supplementary Fig. 5b as a function of incidence angle. It can be seen that there is a small amount (<10%) of Sn present in the native oxide. In a related experiment, Ar sputtering (15 s) was utilized in situ to remove the native-oxide layer at the surface. The XPS results (15° incidence angle) after Ar sputtering (Supplementary Figs. 6a and 6b) show Ge(II)-rich and O-lean surface corresponding to the CsSn_0.5Ge_0.5I₃ perovskite, which confirms that the thickness of the oxide layer is within 5 nm. Taken together, the XPS results indicate that a uniform layer of Sn-containing Ge(IV)-rich native oxide (<5 nm) forms on the surface of the CsSn_0.5Ge_0.5I₃ perovskite thin films. As with most native-oxide layers that are less than 5 nm in thickness, this native-oxide layer is expected to be amorphous.

Fig. 2

XPS characterization. a Ge 3d XPS spectra, at different incidence angles, from CsSn_0.5Ge_0.5I₃ perovskite thin film that has been exposed to air. b Corresponding plot of the fraction of Ge(II) vs. the incidence angle. c, d XPS maps of Ge 3d (33 eV) and O 1s (532 eV), respectively, from the same area of the thin film

Native oxide passivated CsSn_0.5Ge_0.5I₃ perovskite stability

The as-synthesized CsSn_0.5Ge_0.5I₃ perovskite powders when exposed to ambient atmosphere (25 °C, 80% RH) for 24 h remain black and maintain their phase purity (Supplementary Fig. 1d). Moreover, it appears that the CsSn_xGe_1–xI₃ perovskite can become quite stable when the x value is in the range of 0.25–0.75 (Supplementary Fig. 8). In contrast, reference pure CsSnI₃ and CsGeI₃ powders turn into yellow non-perovskite phases (Supplementary Fig. 1f and 1e). These results demonstrate clearly the superior air stability of the alloy CsSn_0.5Ge_0.5I₃ perovskite over its pure components. The CsSn_0.5Ge_0.5I₃ perovskite thin films subjected to continuous 1-sun illumination in ambient atmosphere (~45 °C, 80% RH) for up to 72 h are also found to be highly stable. Figure 3a presents XRD patterns from CsSn_0.5Ge_0.5I₃ perovskite thin films exposed for 0, 24, 48, and 72 h showing negligible change. The corresponding relative intensities of the main XRD peak are plotted in Fig. 3e. Comparative experiments on thin films of the popular halide perovskites (CsSnI₃, CsPbI₃, and MAPbI₃) show complete degradation after 24, 48, and 72 h, respectively (Fig. 3b–d). Supplementary Fig. 9 presents conductive AFM (c-AFM) images of the CsSn_0.5Ge_0.5I₃ perovskite thin film samples exposed to ambient atmosphere for 0, 24, and 48 h, where the microstructure and conductivity appear unchanged at the nanoscale.

Fig. 3

Thin-film stability. XRD patterns of perovskite thin films before and after exposure for 24, 48, and 72 h to light-soaking (1 sun) at approximately 45 ˚C and 80% RH: a CsSn_0.5Ge_0.5I₃, b CsSnI₃, c CsPbI₃, and d MAPbI₃. e Plots of relative XRD peak intensities vs. time from a to d

Physical properties of CsSn_0.5Ge_0.5I₃ perovskite thin films

The time-resolved PL (TRPL) spectroscopy data from CsSn_0.5Ge_0.5I₃ perovskite thin films (on glass substrates) in Supplementary Fig. 10a reveal a promising lifetime of 10.92 ns, compared with only 510 ps for the reference CsSnI₃ thin film. Supplementary Fig. 10b shows TRPL results for CsSn_0.5Ge_0.5I₃ perovskite thin films coated with either phenyl-C₆₁-butyric acid methyl ester:C₆₀ (PCBM:C₆₀) or spiro-OMeTAD as electron- or hole-quenching layers, respectively. Based on the PL decay dynamics, the photogenerated carrier diffusion coefficients are estimated at 0.85 cm² s⁻¹ and 0.39 cm² s⁻¹ for electrons and holes, respectively. These correspond to diffusion lengths of 963 nm and 653 nm for electrons and holes, respectively, which are sufficiently long for planar thin-film solar cells.
The carrier mobilities in the CsSn_0.5Ge_0.5I₃ perovskite thin films were determined by the space-charge-limited-current (SCLC) method using symmetric capacitor-like devices shown schematically in Supplementary Fig. 11a and 11b insets. For determining the electron mobility (μ_e) and the hole mobility (μ_h) in the dark, the Ga/PCBM:C₆₀/CsSn_0.5Ge_0.5I₃/PCBM:C₆₀/Ga and the Au/spiro-OMeTAD/CsSn_0.5Ge_0.5I₃/spiro-OMeTAD/Au device structures, respectively, were used, in conjunction with the following equation^:

JSCL=9εε0μV28L3

(1)

Here, J_SCL is the measured current density, L is the film thickness (=1 μm), μ is the carrier mobility, ε is the dielectric constant (=28), and ε_o is the permittivity of free space. The μ_e and μ_h are estimated at 974 cm² V⁻¹ s⁻¹ and 213 cm² V⁻¹ s⁻¹, respectively, where the latter is consistent with the μ_h value of 298 cm² V⁻¹ s⁻¹ determined using the Hall-effect measurements. The electron- and hole-trap densities (n_t) were calculated using the following equation:

nt=VTFL2εε0eL2

(2)

where V_TFL is the trap-filled limit (TFL) voltage from the plots in Supplementary Fig. 11a and 11b; the electron- and hole-trap densities are estimated to be both as low as about 10¹⁶ cm^–3. All of the above results indicate that the properties of CsSn_0.5Ge_0.5I₃ perovskite thin films are highly suitable for thin-film planar solar cells.

Native-oxide-passivated CsSn_0.5Ge_0.5I₃ perovskite solar cells

The PV performance of CsSn_0.5Ge_0.5I₃ perovskite thin films with the native-oxide layer is evaluated by incorporating them into PSC devices of the architecture shown in Fig. 4a. The CsSn_0.5Ge_0.5I₃ perovskite thin film (about 200-nm thickness) is sandwiched between PCBM electron-transport layer (ETL) and spiro-OMeTAD hole-transport layer (HTL), with the thin native-oxide layer serving as a wide-bandgap interfacial layer between the CsSn_0.5Ge_0.5I₃ perovskite thin film and the HTL. FTO and Au are used as the conducting electrodes. Figure 4b shows the energy-level diagrams for this device architecture. The VBM and the apparent bandgap of the amorphous native oxide were determined using the ultraviolet photoemission spectroscopy (UPS) results presented in Supplementary Fig. 12. The VBM of the CsSn_0.5Ge_0.5I₃ thin film is determined by performing UPS measurement after the removal of the native-oxide layer using Ar sputtering (Supplementary Fig. 13). Figure 4c plots the current density (J)–voltage (V) responses of the “champion” PSC in reverse and forward scans, showing negligible hysteresis. High overall PCE of 7.11% is achieved with open-circuit voltage (V_OC) of 0.63 V, short-circuit current density (J_SC) of 18.61 mA cm^–2, and fill factor (FF) of 0.606. The J_SC value is consistent with the integrated J of 18.1 mA cm^–2 from the external quantum efficiency (EQE) spectrum in Fig. 4f. The PCE output at the maximum power point (0.45 V) shows 7.03% in Fig. 4e, which is very close to the extracted PCE from J–V curves. The V_OC reported here is much higher than that in other reported PSCs based on all-inorganic Sn-based halide perovskites, and it appears to be stable (Supplementary Fig. 14). The evaluation of PV performance of about 40 PSC devices shows good reproducibility (Fig. 4d), with average PCE of 6.48%. More detailed performance statistics are included in Supplementary Fig. 15, and the device performance parameters are included in Supplementary Table 1.

Fig. 4

Device architecture and performance of CsSn_0.5Ge_0.5I₃ thin-film PSCs. a Schematic illustration showing the planar PSC device structure used here. b Corresponding energy-level diagram. c J–V responses of the “champion” PSC device. d PCE statistics. e Stabilized power output and f EQE spectrum of the “champion” PSC device

In order to study the effect of the native-oxide layer on the PSC performance, control PSCs based on CsSn_0.5Ge_0.5I₃ perovskite thin films, without the native oxide, were also fabricated. This was accomplished by performing all the processing steps inside a N₂-filled glovebox (O₂ and H₂O levels below 0.1 ppm). Note that the processing of the PSCs described earlier was performed outside of the glovebox. The control PSC shows much lower V_OC of 0.48 V (reverse scan) and PCE of 3.72% (Supplementary Fig. 16a). By comparison, a PSC with the native-oxide layer shows significantly higher V_OC of 0.62 V (reverse scan) and PCE of 6.52% (Supplementary Fig. 16b). Also, a control PSC based on CsSnI₃ perovskite was fabricated using the same evaporation method, showing low PCE of 1.7% (Supplementary Fig. 17). This clearly demonstrates the advantage of the mixed CsSn_0.5Ge_0.5I₃ perovskite composition, and the beneficial effect of the native oxide. The corresponding dark J–V responses are also plotted in Supplementary Figs. 16a and 16b, showing lower current leakage when the native-oxide layer is present.

Long-term device stability

A typical unencapsulated CsSn_0.5Ge_0.5I₃ thin-film PSC with 6.79% PCE (Fig. 5b) was chosen for stability testing where it was operated continuously under continuous 1-sun illumination in N₂ atmosphere (45 °C). The continuous monitoring of the PV performance shows that the PCE has degraded to only 6.23% (92% of the initial PCE) after 500 h of continuous operation (Fig. 5a, b). Figure 5b also shows that there is no degradation in the V_OC. This demonstrates clearly the excellent operational stability of PSCs based on CsSn_0.5Ge_0.5I₃ perovskite thin films with the native-oxide layer. Regarding the stability in air, we have periodically measured the J–V performance of a CsSn_0.5Ge_0.5I₃-based PSC that was stored under 1-sun illumination. As shown in Supplementary Fig. 18, after 100-h storage, the PSC promisingly maintains 91% of the initial PCE. Overall, to the best of our knowledge, such stability is the highest of all lead-free perovskite PSCs reported so far. Note that, while it is practically challenging to perform a direct performance comparison between the CsSn_0.5Ge_0.5I₃-based and Pb-based PSCs with the same device configuration, the stability of the CsSn_0.5Ge_0.5I₃-based PSCs is reasonably comparable to that of Pb-based PSCs.

Fig. 5

Device stability of CsSn_0.5Ge_0.5I₃ thin-film PSCs. a PCE evolution of a typical unencapsulated PSC in continuous operation under 1-sun illumination at 45 °C in N₂ atmosphere. b Initial J–V curves (reverse and forward scans) of the PSC and ones at the 500-h mark during continuous operation

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