Jianhua Zhang, Xufeng Liao, Weisheng Li, Yutian Tian, Qinyang Huang, Yitong Ji, Guotang Hu, Qingguo Du, Wenchao Huang, Donghoe Kim, Yi-Bing Cheng, Jinhui Tong. Minimizing tin (Ⅱ) oxidation using ethylhydrazine oxalate for high-performance all-perovskite tandem solar cells[J]. Journal of Semiconductors, 2025, 46(5): 052802
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【AIGC One Sentence Reading】:EDO enhances Sn-Pb perovskite solar cell performance by inhibiting Sn2+ oxidation, achieving 27.58% efficient ATSCs.
【AIGC Short Abstract】:We enhanced mixed tin-lead perovskite solar cells for all-perovskite tandem solar cells by using ethylhydrazine oxalate to inhibit Sn2+ oxidation and improve crystallinity. This led to a 21.96% efficient PSC and a 27.58% efficient ATSC, overcoming challenges in performance and stability.
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Abstract
All-perovskite tandem solar cells (ATSCs) have the potential to surpass the Shockley?Queisser efficiency limit of conventional single-junction devices. However, the performance and stability of mixed tin–lead (Sn–Pb) perovskite solar cells (PSCs), which are crucial components of ATSCs, are much lower than those of lead-based perovskites. The primary challenges include the high crystallization rate of perovskite materials and the susceptibility of Sn2+ oxidation, which leads to rough morphology and unfavorable p-type self-doping. To address these issues, we introduced ethylhydrazine oxalate (EDO) at the perovskite interface, which effectively inhibits the oxidation of Sn2+ and simultaneously enhances the crystallinity of the perovskite. Consequently, the EDO-modified mixed tin?lead PSCs reached a power conversion efficiency (PCE) of 21.96% with high reproducibility. We further achieved a 27.58% efficient ATSCs by using EDO as interfacial passivator in the Sn?Pb PSCs.
Metal halide perovskite solar cells (PSCs) have attracted considerable interest because of their exceptional optical and electrical properties, positioning them as a promising technology for the future of the solar energy industry[1−4]. After more than a decade of development, lead-based cells have reached an efficiency of 26.7%[5]. Mixed tin−lead PSCs with bandgaps as low as 1.2 eV can be achieved by substituting part of the lead (Pb) with tin (Sn). The Shockley−Queisser limit[6] indicates that the optimal bandgap range for solar cells lies between 1.2 and 1.4 eV, a range that can be tuned in Sn−Pb perovskite materials[7, 8]. Moreover, Sn−Pb perovskite can be paired with wide bandgap (WBG) perovskites to form ATSCs, which offer a higher theoretical efficiency limit[9−11]. However, incorporating Sn into Sn−Pb perovskite has adverse effects. Sn2+ ions are easily oxidized to Sn4+[12], leading to the formation of Sn vacancies. These vacancies induce p-type self-doping and act as recombination centers for carriers, which shortens the carrier lifetime[13, 14]. The quick crystallization of tin-based perovskite causes uneven formation and low quality films[15−17]. These problems make the device work less effectively and steadily, which prevents it from improving.
The perovskite structure, shown as ABX₃, can be changed by altering the ratio of different elements at each position, which helps to inhibit oxidation, modify the band gap, and enhance device stability[18−20]. The performance of PSCs can be significantly enhanced through the strategic use of additives. For instance, incorporating Sn and SnF₂ powder helps prevent the oxidation of Sn2+[21, 22]. The addition of formamidine sulfonic acid can inhibit the oxidation of Sn2+ and passivate surface defects of the perovskite grains[15]. The use of octylammonium tetrafluoroborate surpresses the formation of the iodine vacancies[23]. The use of these additives has time and again resulted in the highest efficiencies for mixed Sn−Pb PSCs. However, the defect density of perovskite interfaces is much higher compared to the bulk phase. In addition, interfaces play a key role in facilitating interlayer carrier transport, making it crucial to optimize their properties. Despite the promising potential of Sn−Pb perovskites, there is a limited amount of research focused on optimizing their interfaces, with most focusing on organic ammonium salts such as ethylenediammonium diiodide[24], phenethylammonium iodide[25], and ethylenediamine[26]. The use of these materials reduces the formation of interfacial defects and promotes charge extraction. However, there is an urgent need for more and better interfacially modified materials to improve the performance of PSCs.
Here, we introduced a reducing agent, ethylhydrazine oxalate (EDO) as the surface passivator to optimize the film quality of perovskites. The results demonstrate that EDO forms new bonds through interactions with metal ions in the perovskite, inhibiting the oxidation of Sn2+. In addition, EDO increases the crystallinity of perovskite films, improves film quality and passivates interfacial defects. Consequently, we successfully fabricated stable Sn−Pb PSCs with an efficiency of 21.96%, which maintained 83.9% of its efficiency after 1000 h of light exposure, showcasing excellent stability. By combining EDO-modified Sn−Pb PSCs with WBG PSCs, we achieved a 2-terminal ATSCs with an efficiency of 27.58%.
Results and discussion
Characterization of mixed Sn−Pb perovskite films
In this work, we utilized a Sn−Pb narrow bandgap (NBG) perovskite with the composition Cs0.1MA0.3FA0.6Pb0.5Sn0.5I3 as the light absorbing material[24]. The chemical structure of EDO is presented in Fig. 1(a). and Supplementary material Fig. S1. EDO contains two functional groups: oxalic acid and ethylhydrazine. The carboxylic acid groups (−COOH) in oxalic acid function as effective Lewis bases, forming cross-links with uncoordinated Sn2+, which enhances the perovskite crystallinity[27, 28]. We think that the main factor that plays a reducing role is the hydrazine group in ethylhydrazine. The hydrazine group has multiple lone pairs of electrons (each N atom in hydrazine has a lone pair electron) which has a strong electron donor property and thus can donate electrons to Sn2+ to prevent its oxidation. And ethylhydrazine can interact with Sn2+/Pb2+ ions to regulate crystallization[29]. This interaction efficiently suppresses the oxidation of Sn2+ while further improving the crystallinity of the perovskite. Details regarding the specific application and dosage of EDO can be found in the Supplementary material. In this study, samples without EDO were designated as controls, while those treated with EDO were referred to as target samples. To verify the interaction between EDO and the perovskite, we conducted Fourier transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS) measurements. The FTIR results showed that when SnI₂ was added to EDO, the peak corresponding to the C=O bond in the carboxylate group shifted significantly, and new peaks appeared. This suggests the formation of a new bond between EDO and Sn2+. Similarly, the peak value of the C=O bond shifted significantly to a lower wavenumber after the addition of PbI2 to EDO, from 1657 cm−1 in the control sample to 1650 cm−1 in the target sample. However, the telescopic vibration peaks of the N−H bond in ethylhydrazine showed significant shifts upon the addition of both SnI₂ and PbI₂, suggesting that the hydrazine group in ethylhydrazine strongly interacts with Sn2+ and Pb2+ in the perovskite structure (Figs. 1(b) and 1(c)). Additionally, XPS analysis revealed that the binding energies of both metal cations and halide ions in the perovskite films shifted to lower values after EDO treatment (Figs. 1(d)−1(f)), further confirming the interaction between EDO and the perovskite material. To visually illustrate this interaction, a schematic diagram is presented in Fig. 1(g).
Figure 1.(Color online) (a) Three-dimensional chemical structure of EDO. FTIR spectra of (b) C=O bonds and (c) N−H bonds of Pb−Sn perovskite precursor solutions. Control and target perovskite XPS spectra of (d) Sn 3d, (e) Pb 4f, and (f) I 3d . (g) Schematic illustration of the interaction of perovskite with EDO.
To study how EDO changes the crystal structure of perovskite, we did X-ray diffraction (XRD) tests on both the control samples and the EDO-modified samples. As shown in the Fig. 2(a), we saw peaks at 14° and 28°, which match the (100) and (200) planes of Sn−Pb perovskite[30]. Notably, the EDO-modified perovskite film exhibited significantly stronger peak intensities compared to the control, indicating a marked enhancement in crystallinity. Additionally, the PbI₂ peak at 12.6° was reduced, suggesting that EDO treatment effectively decreases residual PbI₂. We employed scanning electron microscopy (SEM) to gain deeper insights into the impact of EDO on crystallization. As shown in Figs. 2(b) and 2(c), the cross-section of the modified sample is smoother and more uniform compared to the control. Furthermore, the EDO-modified sample displayed larger grain sizes and a denser morphology, as illustrated in Figs. 2(d) and 2(e). The small grain sizes of the perovskite are due to the rapidly crystallization rate of the Sn−containing perovskite materials. Atomic force microscopy (AFM) measurements are displayed in Figs. 2(f) and 2(g). The results showed that the root-mean-square (RMS) surface roughness dropped from 59.1 nm in the control to 45.1 nm in the modified sample. This means that the surface got smoother when EDO was added. Kelvin probe force microscopy (KPFM) results presented in Figs. 2(h) and 2(i). Compared with the standard film, there is no significant color change in the target film, which indicates that it has a more uniform surface potential distribution, and the surface potential fluctuation is reduced, which promotes efficient charge transport, and suggests that the perovskite film has a lower surface defect density. As a result, the RMS surface potential decreases from 39.5 to 21.7 mV, suggesting that EDO treatment promotes carrier extraction and minimizes non-radiative recombination[28, 30, 31].
Figure 2.(Color online) (a) XRD patterns of control and EDO-modified films. Cross-section SEM images of (b) control and (c) target perovskite films, Top-view SEM images of (d) control and (e) target perovskite films, AFM images of (f) control and (g) EDO-modified films. KPFM images of (h) control and (i) EDO-modified films.
To verify the reducing properties of EDO, we exposed the perovskite precursor solutions with and without EDO to air. As displayed in Fig. S2, the color of the perovskite. The solution without EDO turned red after three days, which means that Sn2+ was oxidized to Sn4+. In contrast, the solution of perovskite precursor with added EDO remained yellow, demonstrating that EDO exhibits strong reducing properties. To further investigate the impact of EDO on the oxidation of Sn2+ at the perovskite surface, we conducted XPS measurements. The XPS spectra of the samples can be divided into two sub-peaks: Sn2+ and Sn4+ (Figs. 3(a) and 3(b)). The analysis clearly reveals that the EDO-modified perovskite films have a lower Sn4+ to Sn2+ ratio compared to the control films, suggesting that EDO effectively prevents the oxidation of Sn2+. We hypothesize that the presence of EDO hinders the oxidation process, likely due to the enhanced electron density around the Sn2+ ions.
Figure 3.(Color online) XPS spectra of (a) control and (b) EDO-modified films. (c) PL spectra and (d) TRPL spectra of Pb–Sn perovskite films without (control) and with EDO (target) treatment. (e) Dark J−V curves of representative control and target. (f) Mott−Schottky plots of control and target. (g) Dependence of Voc on light intensity for control and target. (h) Electronic impedance spectroscopy (EIS) plots of control and target. (i) SCLC curves of control and target hole-only devices.
To investigate how EDO treatment affects the way the charge recombines, we spin coat perovskite films onto bare glass and performed steady state photoluminescence (PL) and time resolved photoluminescence (TRPL) tests, as shown in the Figs. 3(c) and 3(d). The EDO-treated film has stronger peaks than the control sample. This means that the films have less non-radiative complexes and lower defects. Additionally, the measured carrier lifetime has improved significantly, from 170 ns for the control film to 332 ns after introduce the EDO. It is worth noting that the addition of EDO did not affect the bandgap of the Sn−Pb perovskite, as shown in Fig. S3.
To further assess the impact of EDO modification, we carried out various analyses on the control and modified devices. Initially, J−V curves were measured under dark conditions (Fig. 3(e)). The target device demonstrated a lower leakage current compared to the control, indicating that EDO treatment effectively reduces defect density[32]. Next, Mott−Schottky (M−S) analysis was performed using capacitance voltage (C−V) measurements. As shown in Fig. 3(f), the built in potential (Vbi) of the target device was higher, which could strengthen the internal driving force for carrier transport and thereby enhance the Voc of the device[33, 34]. Additionally, the dependence of Voc on light intensity was analyzed to calculate the ideality factors (n), which were found to be 1.50 and 1.14 kT/q for the control and EDO-modified devices, respectively (Fig. 3(g)). The lower n value observed in the target device suggests a significant suppression of trap-assisted non-radiative recombination, consistent with the findings from the dark J−V measurements[35]. To gain deeper insight into the influence of EDO modification on carrier dynamics, electrical impedance spectroscopy (EIS) was conducted (Fig. 3(h)). Both the control and EDO-treated devices exhibited a prominent semicircle in the low frequency region, with the target device displaying a considerably larger circle radius. This indicates that the target device has higher recombination resistance (Rrec) and lower sheet resistance (Rs) compared to the control. The results we saw suggest that EDO treatment is very important for reducing charge recombination at the perovskite/ETL interface. This helps to get more charge carriers from the perovskite layer to the ETL[36]. To quantify the defect state density in the perovskite films, SCLC measurements were conducted (Fig. 3(i)), with the trap density determined using the following equation:
Here, represents the trap state density, while ε, ε0, e, and L denote the relative dielectric constant of the perovskite material, vacuum permittivity, electron charge, and the thickness of the perovskite layer, respectively. Following EDO treatment, the VTFL of the device decreased notably from 0.56 to 0.36 V when compared to the untreated control. This reduction was accompanied by a drop in the calculated value, which fell from 3.05 × 1015 to 1.96 × 1015 cm−3. These findings indicate a significant enhancement in the film quality of the device, accompanied by a marked reduction in defect occurrences[37, 38]. Next, we performed PLQY measurements on the films on glass (Fig. S4). The PLQY for the control and EDO-modified perovskite were 0.85% and 1.71%, respectively. A higher PLQY indicates a lower incidence of carrier non-radiative composite events[39].
Photovoltaic performance of single junction Sn−Pb PSCs
Given the above improved optoelectronic properties of EDO modification perovskite films, we fabricated inverted PSCs to explore their photovoltaic performance. The device design is shown in Fig. 4(a), and there's a picture of the device shown in Fig. S5. The J−V parameters of the PSCs with EDO modification demonstrated substantial improvement, as shown in Fig. 4(b). The PCE increased from 17.79% to 21.96%, driven by enhancements in open circuit voltage (Voc) from 0.816 to 0.860 V, short circuit current density (Jsc) from 30.91 to 31.26 mA·cm−2, and fill factor (FF) from 70.51% to 81.67%. Furthermore, Jsc values derived from external quantum efficiency (EQE) measurements also improved, rising from 30.35 to 31.15 mA·cm−2 after EDO treatment (Fig. 4(c)). Stabilized power outputs (SPO) were recorded at 21.3% for the EDO-modified device and 16.7% for the control (Fig. 4(d)). Long term performance under continuous 1 sun light irradiation in an N₂ glovebox (Fig. 4(e)) revealed that the EDO-treated device kept 83. 9% of its original PCE after 1000 h, while the control only kept 65.2%. Similarly, in the dark N₂ glove box (Fig. S6), the modified device maintained 88.8% of its original efficiency compared to 77.1% for the control. These results show that the addition of EDO can significantly improve the performance and stability of Sn−Pb PSCs.
Figure 4.(Color online) (a) Device architecture of single junction NBG PSCs. (b) J–V curves and (c) EQE curves. (d) Steady-state power outputs. (e) Stability performance of PSCs under continuous light illumination.
Given the excellent performance of a single junction cell, we have combined a 1.25 eV NBG subcell with a 1.75 eV WBG subcell to create an ATSCs (Fig. 5(a)). The J−V parameters for the wide bandgap cells are provided in Fig. S7. Fig. 5(b) presents the cross-sectional SEM image of the ATSCs. The ATSCs incorporating an EDO-modified Sn−Pb narrow bandgap subcell achieved a PCE of 27.41%, with a Voc of 2.12 V, Jsc of 15.90 mA·cm−2, and an FF of 81.34% under forward voltage scanning. In reverse scanning, the PCE slightly improved to 27.58%, maintaining the same Voc of 2.12 V, a Jsc of 15.85 mA·cm−2, and an FF of 82.03% (Fig. 5(c)). The EQE measurements for the top WBG and bottom NBG subcells showed current levels of 15.82 and 15.67 mA·cm−2, respectively (Fig. 5(d)), which closely match the Jsc values found in the J−V curves. The SPO showed a consistent PCE of up to 27.2% (Fig. 5(e)). To confirm the reproducibility of the ATSCs, 10 individual tandem solar cells were fabricated, resulting in an average PCE of 27.35% ± 0.42% (Fig. 5(f)), detailed data are provided in Table S2. The findings indicate that the inclusion of EDO enhances both the performance and reliability of ATSCs.
Figure 5.(Color online) (a) Device architecture of 2T all-perovskite tandem solar cells. (b) Cross-sectional SEM image, (c) J−V curves, (d) EQE curves, (e) steady-state power outputs. (f) Histogram of PCEs for 10 tandem cells.
In short, this study showed that using a special EDO material helped make better and more stable ATSCs. This material can prevent Sn2+ from getting oxidized and improve the quality of the perovskite film. The EDO-modified tin−lead solar cells reached a top efficiency of 21.96% with good reproducibility. In addition, the constructed ATSCs achieved the highest PCE of 27.58% by integrating NBG PSCs with a WBG PSCs top cell. Our strategy provides an effect solution to the Sn2+ oxidation challenges faced in ATSCs, which should be an important step for perovskite technology commercialization.
Jianhua Zhang, Xufeng Liao, Weisheng Li, Yutian Tian, Qinyang Huang, Yitong Ji, Guotang Hu, Qingguo Du, Wenchao Huang, Donghoe Kim, Yi-Bing Cheng, Jinhui Tong. Minimizing tin (Ⅱ) oxidation using ethylhydrazine oxalate for high-performance all-perovskite tandem solar cells[J]. Journal of Semiconductors, 2025, 46(5): 052802