1. Introduction   1. 前言

Lead-based halide perovskite solar cells (Pb-PSCs) have rapidly gained prominence in the field of photovoltaics due to their exceptional power conversion efficiencies (PCEs).^1,2 However, these advantages are tempered by significant challenges, particularly the toxicity of lead.³ Tin is less toxic than lead, making it a more environmentally benign option. Tin-based halide perovskites (Sn-HP) have similar crystal structures and optoelectronic properties to their lead-based counterparts, offering a theoretical pathway to achieving comparable efficiencies.^4,5 However, the transition to Sn-PSCs introduces new scientific challenges. A major issue is the rapid oxidation of tin from Sn²⁺ to Sn⁴⁺, which creates deep trap states in the perovskite lattice, leading to increased non-radiative recombination, lower open-circuit voltage, and reduced overall efficiency. This oxidation also contributes to the intrinsic instability of tin-based perovskites, making them more susceptible to degradation under ambient conditions. Furthermore, the solution processing of Sn-based perovskites is more complex, often resulting in films with inferior crystallinity and higher defect densities compared to lead-based perovskites.^6,7 These defects can significantly impact charge carrier mobility and increase recombination losses, further lowering device performance.
鉛基鹵化物鈣鈦礦太陽能電池（Pb-PSCs）因其卓越的功率轉換效率（PCEs）而在光伏領域迅速嶄露頭角。 ^1,2 然而，這些優勢卻因重大挑戰而受到影響，特別是鉛的毒性。 ³ 錫的毒性比鉛低，使其成為一種對環境更友善的選擇。錫基鹵化物鈣鈦礦（Sn-HP）具有與鉛基鈣鈦礦相似的晶體結構和光電特性，為實現可比擬的效率提供了理論途徑。 ^4,5 然而，轉向 Sn-PSCs 帶來了新的科學挑戰。一個主要問題是錫從 Sn ²⁺ 快速氧化成 Sn ⁴⁺ ，這會在鈣鈦礦晶格中產生深層陷阱態，導致非輻射複合增加、開路電壓降低以及整體效率下降。這種氧化也導致錫基鈣鈦礦的固有不穩定性，使其在環境條件下更容易降解。此外，Sn 基鈣鈦礦的溶液製程更為複雜，與鉛基鈣鈦礦相比，通常會導致薄膜的結晶度較差且缺陷密度較高。 ^6,7 這些缺陷會顯著影響電荷載子遷移率並增加複合損失，進一步降低元件性能。

The functional additive technique has emerged as a critical approach to enhancing the performance and stability of Sn-PSCs.^2,8–11 Reducing agents like SnF₂, trivalent doping, hydrazine, or NaBH₄ minimize Sn⁴⁺ defects by stabilizing the Sn²⁺ oxidation state.^12,13 Additives such as alkylammonium halides, polyethylene glycol, and 2-chloroethylphosphonic acid improve film morphology and crystallization, yielding larger grains and fewer defects, which enhance charge transport.^14–16 Solvent additives like DMSO or ethyl acetate control evaporation, ensuring uniform films.^17,18 Functional additives also optimize interfacial properties by tuning energy levels, reducing recombination, and enhancing charge extraction.^19–21 For example, self-assembled monolayers with specific dipole moments can be applied to tune the work function of the electron transport layer, optimizing the charge extraction process.²² Moreover, hydrophobic additives further improve stability by protecting the perovskite layer from moisture-induced degradation.²³
功能性添加劑技術已成為提升錫鈣鈦礦太陽能電池（Sn-PSCs）性能和穩定性的關鍵方法。還原劑，如 SnF₂、三價摻雜、聯胺或 NaBH₄，透過穩定 Sn²⁺ 氧化態來減少 Sn⁴⁺ 缺陷。烷基銨鹵化物、聚乙二醇和 2-氯乙基膦酸等添加劑可改善薄膜形態和結晶，產生更大的晶粒和更少的缺陷，從而增強電荷傳輸。DMSO 或乙酸乙酯等溶劑添加劑可控制蒸發，確保薄膜均勻。功能性添加劑還可透過調整能階、減少複合和增強電荷萃取來優化介面特性。例如，具有特定偶極矩的自組裝單分子層可用於調整電子傳輸層的功函數，從而優化電荷萃取過程。此外，疏水性添加劑透過保護鈣鈦礦層免受濕氣引起的降解，進一步提高了穩定性。

In recent years, fullerene derivatives have come up as particularly effective additives for this purpose.^24–28 Their ability to interact with the perovskite material at a molecular level makes them especially suited for the passivation of grain boundaries and surface defects in the film. This passivation reduces non-radiative recombination and enhances charge carrier mobility, resulting in higher photocurrent and PCE.^29–31 For instance, Tian et al. reported an enhancement in the efficiency of tin-based perovskite solar cells by incorporating a hexyl ester-containing fullerene derivative as a functional additive.³¹ This improvement was attributed to the suppression of Sn²⁺ oxidation by the flexible alkyl chains in the additive, which prevent the perovskite layer from interacting with the oxygen. Similarly, Chen and colleagues demonstrated that the quality of the perovskite layer could be enhanced by forming a bulk heterojunction between the –R–NH₂–X group of a fullerene derivative and the perovskite molecules.³² They also showed that a fullerene derivative with six chlorine atoms could address grain boundary defects by slowing down the crystallization process of the perovskite layer, thereby improving device efficiency. In another study, Liang and colleagues designed a novel fullerene derivative with a porphyrin ring and three pentafluorophenyl groups.³³ This innovative additive efficiently interacts with the perovskite material, facilitating defect passivation and significantly extending the device's lifespan. Choi et al. introduced a multifunctional fulleropyrrolidine with triethylene glycol monoethyl ether chains, where the ether component closely interacts with Sn²⁺, and the fullerene base simultaneously engages with I⁻.³⁴ This dual interaction prevents the formation of Sn⁴⁺ and I₃⁻, resulting in enhanced stability of Sn-based solar cells. Most recently, Chen et al. reported a record-breaking efficiency of 15.14% by using pyridyl-substituted fulleropyrrolidones as functional additives in the perovskite precursor solution.³⁰ This milestone highlights the significant potential of fullerene derivatives in advancing the performance and stability of perovskite solar cells, paving the way for more efficient and durable renewable energy solutions.
近年來，富勒烯衍生物已成為此用途特別有效的添加劑。 ^24–28 它們能夠在分子層級與鈣鈦礦材料相互作用，使其特別適合用於鈍化薄膜中的晶界和表面缺陷。這種鈍化作用減少了非輻射複合，並增強了電荷載子遷移率，從而提高了光電流和光電轉換效率（PCE）。 ^29–31 例如，Tian 等人報導，透過引入含有己基酯的富勒烯衍生物作為功能性添加劑，提高了錫基鈣鈦礦太陽能電池的效率。 ³¹ 這種改進歸因於添加劑中柔性烷基鏈對錫 ²⁺ 氧化的抑制作用，這些烷基鏈阻止了鈣鈦礦層與氧氣的相互作用。同樣地，Chen 及其同事證明，透過在富勒烯衍生物的–R–NH ₂ –X 基團與鈣鈦礦分子之間形成體異質結，可以提高鈣鈦礦層的品質。 ³² 他們還表明，具有六個氯原子的富勒烯衍生物可以透過減緩鈣鈦礦層的結晶過程來解決晶界缺陷，從而提高元件效率。在另一項研究中，Liang 及其同事設計了一種新型富勒烯衍生物，其具有卟啉環和三個五氟苯基。 ³³ 這種創新的添加劑有效地與鈣鈦礦材料相互作用，促進了缺陷鈍化並顯著延長了元件的壽命。Choi 等人引入了一種具有三乙二醇單乙醚鏈的多功能富勒烯吡咯烷，其中醚組分與錫 ²⁺ 緊密相互作用，而富勒烯基底同時與碘 ⁻ 相互作用。 ³⁴ 這種雙重相互作用阻止了錫 ⁴⁺ 和碘 ₃ ⁻ 的形成，從而增強了錫基太陽能電池的穩定性。最近，Chen 等人報導了透過在鈣鈦礦前驅體溶液中使用吡啶基取代的富勒烯吡咯烷酮作為功能性添加劑，實現了創紀錄的 15.14% 效率。 ³⁰ 這一里程碑突顯了富勒烯衍生物在提升鈣鈦礦太陽能電池性能和穩定性方面的巨大潛力，為更高效和耐用的再生能源解決方案鋪平了道路。

Herein, we explored the impact of fullerene derivatives with different functional groups (–COOH, –OH, –OSO₃H) on the efficiency of Sn-PSCs. This study aimed to understand how these functional groups with lone pair-bearing oxygen atoms, attached to the bulky fullerene base, interact with the perovskite matrix to influence key parameters such as film morphology, crystallinity, and the oxidation state of tin. It was found that the derivative with a carboxylic group (–COOH) exhibited the most significant enhancement in device performance from 8.20 to 11.22%. These fullerene-based additives with the carboxylic group were found to be capable of moderating the crystallization process of the perovskite film, resulting in a more uniform morphology with fewer defects. The detailed materials and device analysis corroborate that these functional additives effectively suppressed the oxidation of Sn²⁺ to Sn⁴⁺ and the recombination states in Sn-PSCs. This work provides valuable insights into the effect of multifunctional functional groups in Sn-PSCs and their crucial role in improving device performance and stability.
在此，我們探討了具有不同官能基（–COOH、–OH、–OSO ₃ H）的富勒烯衍生物對錫鈣鈦礦太陽能電池（Sn-PSCs）效率的影響。本研究旨在了解這些帶有孤對電子氧原子的官能基，如何附著於龐大的富勒烯基底，並與鈣鈦礦基質相互作用，進而影響薄膜型態、結晶度以及錫的氧化態等關鍵參數。結果發現，帶有羧基（–COOH）的衍生物能最顯著地將元件性能從 8.20%提升至 11.22%。這些帶有羧基的富勒烯基添加劑，被發現能夠調節鈣鈦礦薄膜的結晶過程，從而形成更均勻、缺陷更少的型態。詳細的材料和元件分析證實，這些功能性添加劑有效地抑制了 Sn ²⁺ 氧化成 Sn ⁴⁺ 以及 Sn-PSCs 中的複合態。這項工作為 Sn-PSCs 中多功能官能基的影響及其在提升元件性能和穩定性方面的關鍵作用提供了寶貴的見解。

2. Results and discussion
2. 結果與討論

2.1. Sn-perovskite film growth with fullerene-based functional additive
2.1. 錫鈣鈦礦薄膜與富勒烯基功能性添加劑的生長

Fig. 1a illustrates the fundamental hypothesis of the study. The fullerene-based functional molecules were designed with the expectation that bulky fullerene and those functional groups interact with Sn²⁺ of the [SnX₆]⁴⁻ octahedra that it may reduce the extent of its oxidation to Sn⁴⁺, providing the enhanced structural integrity of Sn-HPs. It is known that functional groups capable of donating electron density or coordinating with tin atoms play a crucial role in stabilizing Sn²⁺, thereby reducing its tendency to oxidize to Sn⁴⁺.³⁵ Noting that we have used the molecular structures of functional additives shown in Fig. 1b. The additives have functional groups, C(COOH)₂, (–OH)₂, and (–OSO₃H)₂, which would be hereafter termed as F-COOH, F-OH, and F-OSO₃H, respectively. The synthesis process and characteristics of these fullerene-based additives are given in ESI (Fig. S1–S5 and Tables S1–S3).† From the elemental analysis and TOF-MS data, it is confirmed that all the fullerene compounds are a mixture of adducts (n = 1–5). We fabricated the Sn-HP (FA_0.80MA_0.05PEA_0.15SnI₃) films with fullerene-based functional molecules as additives.
圖 1a 闡述了本研究的基本假設。富勒烯基功能性分子被設計成預期龐大的富勒烯和這些功能基團與[SnX₁]₂八面體的 Sn₀相互作用，這可能會降低其氧化成 Sn₃的程度，從而增強 Sn-HP 的結構完整性。眾所周知，能夠提供電子密度或與錫原子配位的功能基團在穩定 Sn₄方面發揮關鍵作用，從而降低其氧化成 Sn₅的趨勢。₆請注意，我們使用了圖 1b 所示的功能性添加劑的分子結構。這些添加劑具有功能基團，₇C(COOH)₈、(–OH)₉和(–OSO₁₀H)₁₁，此後將分別稱為 F-COOH、F-OH 和 F-OSO₁₂H。這些富勒烯基添加劑的合成過程和特性在 ESI 中給出（圖 S1-S5 和表 S1-S3）。†從元素分析和 TOF-MS 數據證實，所有富勒烯化合物都是加合物（n = 1-5）的混合物。我們製備了含有富勒烯基功能性分子作為添加劑的 Sn-HP (FA₁₃MA₁₄PEA₁₅SnI₁₆) 薄膜。

To understand the effect of additives on crystal growth, X-ray diffraction (XRD) patterns of Sn-HP films (Fig. 1c) were measured. The most prominent XRD peaks obtained correspond to (100) and (200) crystallographic planes, which are consistent with the orthorhombic phase, aligning with previously reported data.³⁶ A slightly narrower FWHM of the dominant XRD pattern suggests improvement in crystallinity with Sn-HP film with additives (Fig. S6†). Furthermore, the characteristic XRD peaks of the perovskite layer with different additives do not shift from that of the control layer, suggesting no incorporation of fullerene-functional additives into the lattice of the host crystal.
為了瞭解添加劑對晶體生長的影響，我們測量了 Sn-HP 薄膜的 X 射線繞射（XRD）圖譜（圖 1c）。最顯著的 XRD 峰值對應於 (100) 和 (200) 晶面，這與正交晶相一致，也符合先前報導的數據。 ³⁶ 主導的 XRD 圖譜的半高寬（FWHM）略微變窄，表明添加劑的 Sn-HP 薄膜的結晶度有所改善（圖 S6 †）。此外，具有不同添加劑的鈣鈦礦層的特徵性 XRD 峰值並未從對照層的峰值偏移，這表明富勒烯功能化添加劑並未摻入主晶體的晶格中。

Similarly, the absorption and PL spectra of Sn-HP with additive films were measured to evaluate the effect of additives on photophysical properties. Fig. 1d exhibits a nuanced impact on characteristic absorption spectra with slight variations in absorbance. Among the different additives, the film with the F-COOH additive demonstrates a slightly higher absorption response, indicating a better optoelectronic response. The characteristic absorption edge, as depicted in the inset, shows a band edge of ∼892 nm. PL spectra of these films are depicted in Fig. 1e. There is no shifting in characteristics PL peak ∼892 nm, which is equivalent to ∼1.395 ± 0.02 eV corresponding to the band edge, indicates non-interference of these additives into the electronic picture of pristine perovskite structure. Importantly, a variation in PL spectra intensity indicates the effect of additives in a passivating defect in the Sn-HP film. An intensified PL spectrum for Sn-HP with F-COOH additive corroborates improved film quality and reduced non-radiative recombination.³⁷ Further, PL spectra of Sn-HP films with varying concentrations of F-COOH additive (Fig. S7†) suggest that an excessive additive concentration induces a nonradiative recombination state within the perovskite films.^38,39 With higher additive concentrations, additional defect states or traps in the perovskite film promote non-radiative recombination, where the energy from excited carriers is lost as heat rather than emitted as light, thus diminishing the PL intensity.
同樣地，我們測量了添加劑 Sn-HP 薄膜的吸收和光致發光（PL）光譜，以評估添加劑對光物理性質的影響。圖 1d 顯示了對特徵吸收光譜的細微影響，吸光度略有變化。在不同的添加劑中，含有 F-COOH 添加劑的薄膜表現出略高的吸收響應，表明具有更好的光電響應。如插圖所示，特徵吸收邊緣顯示出約 892 nm 的帶邊。這些薄膜的 PL 光譜如圖 1e 所示。特徵 PL 峰值約 892 nm 沒有偏移，這相當於約 1.395 ± 0.02 eV 的帶邊，表明這些添加劑不會干擾原始鈣鈦礦結構的電子圖像。重要的是，PL 光譜強度的變化表明添加劑在鈍化 Sn-HP 薄膜缺陷方面的作用。含有 F-COOH 添加劑的 Sn-HP 的 PL 光譜增強，證實了薄膜品質的改善和非輻射複合的減少。 ³⁷ 此外，含有不同濃度 F-COOH 添加劑的 Sn-HP 薄膜的 PL 光譜（圖 S7 †）表明，過量的添加劑濃度會在鈣鈦礦薄膜中引起非輻射複合狀態。 ^38,39 隨著添加劑濃度的增加，鈣鈦礦薄膜中額外的缺陷態或陷阱會促進非輻射複合，其中激發載流子的能量以熱的形式損失，而不是以光的形式發射，從而降低 PL 強度。

Fig. 1f₁–f₄ display the SEM images of the Sn-HP films, providing insights into how different functional additives influence the film morphology, particularly in terms of surface coverage and defect density. The SEM image of the control film (Fig. 1f₁) shows pinholes with poor film coverage, which are detrimental to the overall performance of the perovskite layer.⁴⁰ While the SEM images (Fig. 1f₂–f₄) of Sn-HP with F-COOH, F-OH, and F-OSO₃H additives demonstrate significant improvements in surface coverage and film uniformity. These additives play a crucial role in enhancing the quality of the perovskite film by reducing the number of pinholes and defects. The presence of fewer defects and more complete surface coverage leads to better charge transport, ultimately improving the optoelectronic properties of the films. Among the additives, F-COOH shows comparatively better film morphology, yielding a film with the most uniform coverage that could be due to the strong interactions between the functional carboxyl (–COOH) groups of the F-COOH additive and the tin perovskite polyhedral. These interactions play a key role in regulating the crystallization process during film formation, promoting more controlled and uniform growth of perovskite crystals. However, the amount of additives also plays a critical role in determining the film quality. SEM images of films with higher concentrations of F-COOH reveal irregularities (Fig. S8†). This suggests that beyond the optimal concentration, the additive begins to interfere with the perovskite crystallization process, possibly by introducing excess nucleation sites or disrupting the film's uniform growth. Consequently, the film becomes more prone to imperfections, undermining the benefits initially provided by the additive.
圖 1f₀–f₁顯示了 Sn-HP 薄膜的 SEM 影像，提供了不同功能性添加劑如何影響薄膜型態的見解，特別是在表面覆蓋率和缺陷密度方面。對照組薄膜的 SEM 影像（圖 1f₂）顯示有針孔且薄膜覆蓋率差，這對鈣鈦礦層的整體性能有害。₃而添加 F-COOH、F-OH 和 F-OSO₆H 的 Sn-HP 的 SEM 影像（圖 1f₄–f₅）顯示表面覆蓋率和薄膜均勻性顯著改善。這些添加劑在提高鈣鈦礦薄膜品質方面發揮了關鍵作用，減少了針孔和缺陷的數量。缺陷較少和表面覆蓋更完整有助於更好的電荷傳輸，最終改善薄膜的光電特性。在這些添加劑中，F-COOH 顯示出相對較好的薄膜型態，產生了覆蓋最均勻的薄膜，這可能是由於 F-COOH 添加劑的功能性羧基（–COOH）與錫鈣鈦礦多面體之間的強烈相互作用。這些相互作用在薄膜形成過程中調節結晶過程，促進鈣鈦礦晶體更受控和均勻的生長方面發揮了關鍵作用。然而，添加劑的量在決定薄膜品質方面也起著關鍵作用。高濃度 F-COOH 薄膜的 SEM 影像顯示不規則性（圖 S8†）。這表明，超過最佳濃度後，添加劑開始干擾鈣鈦礦結晶過程，可能是通過引入過多的成核位點或破壞薄膜的均勻生長。因此，薄膜更容易出現缺陷，削弱了添加劑最初帶來的好處。

2.2. Effect of fullerene-based functional additive on Sn-PSC device performance
2.2. 富勒烯基功能性添加劑對 Sn-PSC 元件性能的影響

To investigate the effect of fullerene-based functional molecules on photovoltaic performance, a complete device has been fabricated with the inverted configuration of ITO/PEDOT/Sn-HP/ICBA/BCP/Ag. The cross-sectional SEM images of Sn-PSCs are shown in Fig. 2a and b, where enhanced crystalline growth is evident in the device with F-COOH additive compared to the control device. The effect of various functional additives on the device parameters is summarized in Tables 1 and S4.† The current density–voltage (J–V) characteristics are detailed in Fig. 2c. The control device exhibited a PCE of 8.20% with open-circuit voltage (V_OC) ∼ 0.769 V, a short-circuit current density (J_SC) ∼ 17.44 mA cm⁻², and a fill factor (FF) ∼ 61.29%. The Sn-PSCs with F-COOH achieved a significant increase in PCE to 11.22%, with improvements across all parameters (V_OC to 0.841 V, a J_SC to 19.31 mA cm², and FF to 69.12%). Moreover, the addition of the F-COOH additive significantly reduced the hysteresis observed in the forward and reverse scans of the J–V curves (Fig. S9†). The notable enhancement in performance reflects the ability of the –COOH group to interact with the perovskite material, leading to better film morphology and crystallinity. We also evaluated devices incorporating the F-COOH additive at varying concentrations. The J–V characteristics are shown in Fig. S10,† and the corresponding device parameters are summarized in Table S5.† The higher concentration of F-COOH may lead to an increase in irregularities in the perovskite film morphology. Excessive additives can disrupt the uniformity of the film as given in Fig. S8,† creating more defects or uneven crystallization, which are deleterious to device performance as shown in Table S5.†
為了研究富勒烯基功能分子對光伏性能的影響，我們製作了一個完整的元件，其採用了 ITO/PEDOT/Sn-HP/ICBA/BCP/Ag 的反式結構。Sn-PSCs 的截面掃描電子顯微鏡（SEM）影像如圖 2a 和 2b 所示，其中添加 F-COOH 的元件與對照元件相比，晶體生長明顯增強。各種功能添加劑對元件參數的影響彙總於表 1 和 S4 中。† 電流密度-電壓（J–V）特性詳見圖 2c。對照元件的功率轉換效率（PCE）為 8.20%，開路電壓（V _OC ）約為 0.769 V，短路電流密度（J _SC ）約為 17.44 mA cm ⁻² ，填充因子（FF）約為 61.29%。添加 F-COOH 的 Sn-PSCs 的 PCE 顯著增加至 11.22%，所有參數均有所改善（V _OC 增至 0.841 V，J _SC 增至 19.31 mA cm ² ，FF 增至 69.12%）。此外，添加 F-COOH 顯著降低了 J–V 曲線正向和反向掃描中觀察到的遲滯現象（圖 S9 †）。性能的顯著提升反映了 –COOH 基團與鈣鈦礦材料相互作用的能力，從而改善了薄膜形態和結晶度。我們還評估了添加不同濃度 F-COOH 的元件。J–V 特性如圖 S10 所示，† 相應的元件參數彙總於表 S5 中。† 較高濃度的 F-COOH 可能會導致鈣鈦礦薄膜形態的不規則性增加。過量的添加劑會破壞薄膜的均勻性，如圖 S8 所示，† 產生更多缺陷或不均勻的結晶，這對元件性能有害，如表 S5 所示。†

Furthermore, Sn-PSCs with additives containing hydroxyl (–OH) and sulfonic ester (–OSO₃H) groups also exhibited an increase in PCE, reaching 10.19% and 9.76%, respectively. These results indicate that both –OH groups and –OSO₃H also contribute to improving the optoelectronic properties of the perovskite film, likely through enhanced molecular interaction and better crystallization processes.^41,42 These improvements suggest that the additive effectively stabilizes the interface between the perovskite and the charge transport layers and mitigates ion migration and trap-assisted recombination.^38,43 We will discuss this in detail in succeeding sections.
此外，含有羥基（–OH）和磺酸酯（–OSO ₃ H）基團添加劑的錫鈣鈦礦太陽能電池（Sn-PSCs）也展現出效率提升，分別達到 10.19%和 9.76%。這些結果表明，–OH 基團和–OSO ₃ H 都有助於改善鈣鈦礦薄膜的光電特性，這可能是透過增強分子間作用和更好的結晶過程實現的。 ^41,42 這些改進表明，該添加劑有效地穩定鈣鈦礦與電荷傳輸層之間的介面，並減輕離子遷移和陷阱輔助複合。 ^38,43 我們將在後續章節中詳細討論。

Fig. 2d shows the external quantum efficiency (EQE) of the control device and the best Sn-PSC with F-COOH additive. The EQE spectrum for Sn-PSC with F-COOH additive reveals a noticeable enhancement across the entire spectral range, indicating the suppression of recombination activities in bulk and at the interfaces of the device.⁴⁴ The J_SC values (15.67 mA cm⁻² for control and 17.50 mA cm⁻² for the device with F-COOH) obtained from integrating the EQE spectrum are in the close range of those obtained from J–V curves. Additionally, the band edge of the EQE spectra (Fig. S11†) is estimated to be ∼1.428 and 1.425 ± 0.02 eV for control and with F-COOH additive. These values are in close agreement with the band edge estimation from absorption and PL spectra.
圖 2d 顯示了對照組元件和使用 F-COOH 添加劑的最佳錫鈣鈦礦太陽能電池（Sn-PSC）的外部量子效率（EQE）。使用 F-COOH 添加劑的 Sn-PSC 的 EQE 光譜在整個光譜範圍內顯示出顯著的增強，這表明元件內部和介面處的複合活動受到抑制。 ⁴⁴ 從 EQE 光譜積分得到的 J _SC 值（對照組為 15.67 mA cm ⁻² ，使用 F-COOH 的元件為 17.50 mA cm ⁻² ）與從 J–V 曲線獲得的值非常接近。此外，EQE 光譜的能帶邊緣（圖 S11 †）估計對照組約為 1.428 eV，使用 F-COOH 添加劑的約為 1.425 ± 0.02 eV。這些值與吸收和 PL 光譜的能帶邊緣估計值非常吻合。

Moreover, the statistical data (Fig. 1e), provided in Table S4† and graphically illustrated in Fig. S12,† offer a detailed comparison of batches of Sn-PSCs with additives. The control device demonstrates an average PCE of 7.76%, while those values for devices with additives are higher. Importantly, the statistical data reveal that the distribution of device parameters across batches has narrowed for devices with additives compared to the control device, suggesting higher reproducibility of device parameters and, hence device performance. Enhanced reproducibility is critical for ensuring that solar cells perform consistently, which is a key requirement for large-scale manufacturing and commercialization.
此外，表 S4 †中提供的統計數據（圖 1e）以及圖 S12 †中以圖形方式呈現的數據，詳細比較了添加劑錫鈣鈦礦太陽能電池（Sn-PSCs）的批次。對照組元件的平均 PCE 為 7.76%，而添加劑元件的值更高。重要的是，統計數據顯示，與對照組元件相比，添加劑元件的元件參數分佈範圍縮小，這表明元件參數的重現性更高，進而提升了元件性能。增強的重現性對於確保太陽能電池性能穩定至關重要，這是大規模製造和商業化的關鍵要求。

To study the effect on device stability, we collected the operational stability of unencapsulated control and device with F-COOH additive under maximum power point tracking (MPPT) conditions and air ambient, adopting stability assessment ISOS-L-1.⁴⁵ The normalized efficiencies of respective Sn-PSCs are presented in Fig. 2f. The control device experiences a significant drop in efficiency to 35% of its initial value after 500 hours. This rapid decline in performance suggests that the control device is highly susceptible to degradation, likely due to factors such as moisture, oxygen ingress, or intrinsic instability within the perovskite layer.⁴⁶ In contrast, the device incorporating the F-COOH additive exhibits markedly improved stability, retaining more than 51% of its initial efficiency even after 500 hours. This enhanced stability highlights the beneficial effect of the F-COOH additive in mitigating the degradation mechanisms commonly observed in perovskite solar cells.^46,47 It is likely attributed to the additive's ability to interact with the perovskite structure during film formation, leading to better crystallization, reduced defect density, and stronger resistance to environmental factors. The water contact angles of the Sn-HP film with fullerene derivatives (Fig. S15†) show a higher water contact angle compared to the control film, suggesting a higher hydrophobicity film. The increase in hydrophobicity with fullerene additive also supports the superior device stability of Sn-PSC with F-COOH additive. This improvement implies that the F-COOH additive appears to play a protective role by passivating surface defects, stabilizing grain boundaries, and inducing water resistivity, which are often points of vulnerability in perovskite films where degradation initiates. It reduces degradation over time and contributes to a more stable and durable solar cell. However, further research is warranted to explore additional methods to enhance stability even more.
為了研究對元件穩定性的影響，我們在最大功率點追蹤（MPPT）條件和空氣環境下，收集了未封裝的對照組和添加 F-COOH 的元件的操作穩定性，並採用了穩定性評估 ISOS-L-1。各個錫鈣鈦礦太陽能電池（Sn-PSCs）的標準化效率如圖 2f 所示。對照元件在 500 小時後，效率顯著下降至其初始值的 35%。這種性能的快速下降表明對照元件極易降解，這很可能是由於水分、氧氣滲入或鈣鈦礦層內部的固有不穩定性等因素所致。相較之下，添加 F-COOH 的元件表現出顯著改善的穩定性，即使在 500 小時後仍保留了超過 51%的初始效率。這種增強的穩定性突顯了 F-COOH 添加劑在減輕鈣鈦礦太陽能電池中常見降解機制方面的有益作用。這很可能歸因於添加劑在薄膜形成過程中與鈣鈦礦結構相互作用的能力，從而導致更好的結晶、降低的缺陷密度以及對環境因素更強的抵抗力。含有富勒烯衍生物的 Sn-HP 薄膜的水接觸角（圖 S15†）顯示出比對照薄膜更高的水接觸角，這表明薄膜具有更高的疏水性。富勒烯添加劑增加疏水性也支持了添加 F-COOH 的 Sn-PSC 具有卓越的元件穩定性。這種改進意味著 F-COOH 添加劑似乎透過鈍化表面缺陷、穩定晶界和誘導耐水性來發揮保護作用，這些通常是鈣鈦礦薄膜中降解開始的脆弱點。它減少了隨時間的降解，並有助於形成更穩定、更耐用的太陽能電池。然而，仍需進一步研究以探索更多方法來進一步提高穩定性。

2.3. Effect of fullerene-based functional additives on surface chemistry
2.3. 富勒烯基功能性添加劑對表面化學的影響

To study the effect of additives on surface chemistry, we investigated the Sn-HP films using X-ray photoelectron spectroscopy (XPS). The XPS spectra (Fig. 3a and b) show characteristic peaks of Sn 3d core at 485.2 and 493.5 eV for Sn²⁺, and at 486.2 and 494.3 eV for Sn⁴⁺.^37,48 The analysis of deconvoluted peaks reveals that the control film has a higher ionic percentage (19.6%) of Sn⁴⁺, compared to the film with the F-COOH additive (11.2%). It suggests that the F-COOH additive effectively mitigates the oxidation of Sn²⁺, thereby reducing the extent of Sn²⁺ oxidation and enhancing the overall quality of the perovskite film. Also, the XPS spectra of the I 3d level (Fig. 3c and d) for the film containing the F-COOH additive exhibit a noticeable shift toward higher binding energy compared to that of the control film. The binding energy shift implies that the additive may be contributing to a more stable ionic lattice by influencing the local electronic structure, particularly around the tin and iodine atoms. Similarly, as for other functional additives, the deconvoluted XPS peaks of the Sn 3d also demonstrated a significant reduction in the ionic percentage of Sn⁴⁺ species compared to the control film (Fig. S13†). Specifically, the Sn⁴⁺ content decreases from 19.6% to 12.1% and 13.7% for Sn-HP films containing F-OH and F-OSO₃H, respectively. XPS results support the hypothesis that the functional groups attached to fullerene interact with Sn²⁺ centers in the perovskite film, effectively shielding them from oxidation. These functional groups are strategically positioned within a molecular framework of high electron density, forming a protective barrier around Sn²⁺. While pristine fullerene is highly electron-withdrawing, the incorporation of –COOH, –OH, and –OSO₃H alters the electron distribution, creating localized electron-rich regions near the functional sites. This enhanced electron density strengthens the coordination between these groups and Sn²⁺, stabilizing it and suppressing oxidation. By reducing interactions with oxidizing species, this effect significantly improves the stability and performance of Sn-PSCs with fullerene-based functional additives.
為了研究添加劑對表面化學的影響，我們使用 X 射線光電子能譜（XPS）分析了 Sn-HP 薄膜。XPS 光譜（圖 3a 和 b）顯示，Sn ²⁺ 在 485.2 和 493.5 eV 處有 Sn 3d 核心的特徵峰，而 Sn ⁴⁺ 在 486.2 和 494.3 eV 處有特徵峰。 ^37,48 去捲積峰的分析顯示，對照薄膜的 Sn ⁴⁺ 離子百分比（19.6%）高於含有 F-COOH 添加劑的薄膜（11.2%）。這表明 F-COOH 添加劑有效減輕了 Sn ²⁺ 的氧化，從而降低了 Sn ²⁺ 的氧化程度，並提高了鈣鈦礦薄膜的整體品質。此外，含有 F-COOH 添加劑的薄膜的 I 3d 能級 XPS 光譜（圖 3c 和 d）顯示出明顯的向更高結合能的偏移，與對照薄膜相比。結合能的偏移意味著添加劑可能透過影響局部電子結構，特別是錫和碘原子周圍的電子結構，有助於形成更穩定的離子晶格。同樣地，對於其他功能性添加劑，Sn 3d 的去捲積 XPS 峰也顯示出 Sn ⁴⁺ 物種的離子百分比相對於對照薄膜顯著降低（圖 S13†）。具體而言，對於含有 F-OH 和 F-OSO ₃ H 的 Sn-HP 薄膜，Sn ⁴⁺ 含量分別從 19.6%降至 12.1%和 13.7%。XPS 結果支持了以下假設：附著在富勒烯上的官能基與鈣鈦礦薄膜中的 Sn ²⁺ 中心相互作用，有效地保護它們免受氧化。這些官能基策略性地定位在高電子密度的分子框架內，在 Sn ²⁺ 周圍形成保護屏障。雖然原始富勒烯具有強烈的吸電子性，但–COOH、–OH 和–OSO ₃ H 的引入改變了電子分佈，在功能位點附近產生了局部富電子區域。這種增強的電子密度加強了這些基團與 Sn ²⁺ 之間的配位，使其穩定並抑制氧化。透過減少與氧化物質的相互作用，這種效應顯著提高了含有富勒烯基功能性添加劑的 Sn-PSC 的穩定性和性能。

In fundamental chemical aspects, the ability of these additives to mitigate Sn²⁺ oxidation can be attributed to their inherent nature and interactions with the Sn²⁺ ion.³⁷ Functional groups capable of donating electron density or coordinating with tin atoms play a crucial role in stabilizing Sn²⁺. Particularly, the carboxylate group in F-COOH can bind to tin via coordination, offering a protective barrier that helps to maintain a higher proportion of Sn²⁺. Importantly, the –COOH group in F-COOH is not directly attached to the fullerene ring but instead is connected through a sp³-hybridized carbon atom (C₆₀(C(COOH)₂)_n). This structural feature localizes the electron density on the carboxylate group, making it more available for interaction with Sn²⁺, contributing to the improved stability and quality of the resulting perovskite film. On the other hand, the hydroxyl group (–OH) in F-OH also contributes electron density to the fullerene ring through hydrogen bonding, which can help stabilize Sn²⁺. Although the electron-donating power of the hydroxyl group is generally stronger than that of the carboxylate group, in this specific molecular framework, the –OH group is directly attached to an sp²-hybridized carbon of the fullerene ring, which is more acidic than sp³ hybridized carbon.⁴⁹ This configuration results in higher acidity of the carbon and partial delocalization of the electron density over the fullerene ring, reducing the availability of electron density for Sn²⁺ stabilization as compared to F-COOH. The sulfate ester group (–OSO₃H) in F-OSO₃H also provides some stabilization of Sn²⁺ through conjugation, but the electron-withdrawing nature of the –SO₃H group limits the electron donation to the tin ion. As a result, F-OSO₃H is less effective in stabilizing Sn²⁺ compared to F-COOH or F-OH. The hierarchy of electron-donating ability and defect passivation effectiveness among the derivatives follows the trend: F-COOH > F-OH > F-OSO₃H, in agreement with prior experimental observations and device performance trends. This observation is in line with a report on the effect of the interaction of HCOO⁻ anions and Sn²⁺ cations by Wang and co-workers.⁵⁰ To validate this hypothesis further, these fullerene derivatives were investigated through DFT-calculated electrostatic surface potential (ESP) analysis. The ESP mapping (Fig. S16a–d†) reveals that F-COOH exhibits significant negative potential around the –COOH group, facilitating effective coordination with undercoordinated Sn²⁺ sites. In contrast, the ESP maps of F-OH & F-OSO₃H display relatively less negative potentials, reflecting weaker passivation capability compared to F-COOH. Hence, the superior performance of F-COOH arises from its optimal molecular structure that facilitates efficient electron donation and robust Sn²⁺ stabilization, as confirmed by ESP analysis. These findings validate the proposed molecular interaction mechanism and underscore the critical importance of sp³-hybridized anchoring points and carefully modulated electrostatic environments in engineering highly efficient and stable tin-based perovskite solar cells.
在基礎化學方面，這些添加劑能減輕 Sn ²⁺ 氧化，可歸因於其固有性質以及與 Sn ²⁺ 離子的相互作用。 ³⁷ 能夠提供電子密度或與錫原子配位的官能基，在穩定 Sn ²⁺ 方面扮演關鍵角色。特別是，F-COOH 中的羧酸鹽基團可以透過配位與錫結合，提供保護屏障，有助於維持更高比例的 Sn ²⁺ 。重要的是，F-COOH 中的 –COOH 基團並非直接連接到富勒烯環上，而是透過 sp ³ 雜化碳原子（C ₆₀ ( C(COOH) ₂ ) _n ）連接。這種結構特徵將電子密度局限在羧酸鹽基團上，使其更容易與 Sn ²⁺ 相互作用，有助於改善所得鈣鈦礦薄膜的穩定性和品質。另一方面，F-OH 中的羥基（–OH）也透過氫鍵向富勒烯環提供電子密度，這有助於穩定 Sn ²⁺ 。儘管羥基的給電子能力通常強於羧酸鹽基團，但在這個特定的分子框架中，–OH 基團直接連接到富勒烯環的 sp ² 雜化碳上，這比 sp ³ 雜化碳更具酸性。 ⁴⁹ 這種配置導致碳的酸性更高，並且電子密度在富勒烯環上部分離域，與 F-COOH 相比，降低了電子密度用於 Sn ²⁺ 穩定的可用性。F-OSO ₃ H 中的硫酸酯基團（–OSO ₃ H）也透過共軛提供了一些 Sn ²⁺ 的穩定性，但 –SO ₃ H 基團的吸電子性質限制了對錫離子的電子給予。因此，與 F-COOH 或 F-OH 相比，F-OSO ₃ H 在穩定 Sn ²⁺ 方面效果較差。這些衍生物的給電子能力和缺陷鈍化效率的層次遵循以下趨勢：F-COOH > F-OH > F-OSO ₃ H，這與先前的實驗觀察和元件性能趨勢一致。這項觀察結果與 Wang 及其同事關於 HCOO ⁻ 陰離子和 Sn ²⁺ 陽離子相互作用影響的報告一致。 ⁵⁰ 為了進一步驗證這個假設，透過 DFT 計算的靜電表面電位（ESP）分析研究了這些富勒烯衍生物。ESP 映射（圖 S16a–d †）顯示 F-COOH 在 –COOH 基團周圍呈現顯著的負電位，有助於與配位不足的 Sn ²⁺ 位點有效配位。相比之下，F-OH 和 F-OSO ₃ H 的 ESP 映射顯示相對較少的負電位，反映出與 F-COOH 相比，鈍化能力較弱。因此，F-COOH 的優越性能源於其最佳分子結構，該結構有助於有效的電子給予和穩固的 Sn ²⁺ 穩定性，這已透過 ESP 分析證實。 這些發現驗證了所提出的分子交互作用機制，並強調了 sp² 雜化錨定點和精確調控的靜電環境在設計高效穩定錫基鈣鈦礦太陽能電池中的關鍵重要性。

2.4. Effect on transient photo characteristics and defect calculations
2.4. 對暫態光學特性和缺陷計算的影響

To explore the effect on carrier recombination, we measured the time-resolved photoluminescence (TRPL) characteristics (Fig. 4a and S14†) and calculated carrier lifetimes for the Sn-HP film with different additives, summarized in Table S6.† The Sn-HP with additives exhibits the longest carrier lifetime. Specifically, the Sn-HP film with the F-COOH additive is 3.31 ns, which is significantly longer compared to the control film (1.70 ns), suggesting a lower defect density in the Sn-HP with the F-COOH additive. In the same line, the Sn-HP with F-OH and F-OSO₃H also showed an enhanced lifetime of 3.09 ns and 2.84 ns. The improved lifetime is likely due to the controlled growth of the film's morphology, resulting in larger grain sizes, enhanced crystallinity, and reduced defect density. These improvements reduce non-radiative recombination and enhance the film quality. The trend observed in the TRPL data for the various films is consistent with the J–V performance results, further confirming that the F-COOH additive has the most positive impact on the perovskite film's optoelectronic properties.
為了探討對載流子複合的影響，我們測量了具有不同添加劑的 Sn-HP 薄膜的時間解析光致發光 (TRPL) 特性（圖 4a 和 S14†），並計算了載流子壽命，如表 S6† 所示。具有添加劑的 Sn-HP 表現出最長的載流子壽命。具體而言，添加 F-COOH 的 Sn-HP 薄膜的載流子壽命為 3.31 ns，顯著長於對照薄膜（1.70 ns），這表明添加 F-COOH 的 Sn-HP 具有較低的缺陷密度。同樣地，添加 F-OH 和 F-OSO₃H 的 Sn-HP 也顯示出增強的壽命，分別為 3.09 ns 和 2.84 ns。壽命的改善可能歸因於薄膜形態的受控生長，導致晶粒尺寸更大、結晶度更高和缺陷密度降低。這些改進減少了非輻射複合並提高了薄膜品質。TRPL 數據中觀察到的各種薄膜趨勢與 J-V 性能結果一致，進一步證實 F-COOH 添加劑對鈣鈦礦薄膜的光電特性具有最正面的影響。

Moreover, to gain insights into the photocarrier dynamics, we conducted transient photo characteristics, transient photovoltage (TPV), and transient photocurrent (TPC) measurements. The TPV curve (Fig. 4b) for the F-COOH device exhibits a notably slower decay compared to the control device, indicating a significantly longer carrier lifetime of 15.44 μs from 10.62 μs. This extended carrier lifetime is typically a sign of fewer trap states within the device, which would otherwise act as recombination centers for carriers.^51,52 It corroborates that Sn-PSC with F-COOH additive reduces trap-assisted recombination and contributes to enhanced performance, supporting the conclusion that this functional additive is key to achieving better device stability and efficiency. Similarly, the TPC curve (Fig. 4c) assesses the effect on the interface quality of the device. Sn-PSC with F-COOH additive shows a faster current decay time of 5.56 μs, compared to the control (6.38 μs). This faster decay is indicative of more efficient charge carrier extraction in the Sn-PSC with F-COOH, one of the critical factors for improving solar cell performance. These results further support the benign effect of F-COOH additives on material quality and device properties. Furthermore, the capacitance measurements were conducted to gain critical insights into the device's charge carrier dynamics, interface states, trap states, and defect density profile.^53,54Fig. 5a shows the capacitance–frequency (C–f) spectra of devices. The control device exhibits a noticeably higher capacitance compared to the F-COOH-modified device, which is attributed to higher charge accumulation and ionic motion in the control device. This suggests that the additive might be suppressing ionic movement or mitigating its effects on charge accumulation.⁵⁵ The device with the F-COOH additive demonstrates a lower capacitance over the frequency range, indicating that the additive has likely reduced the density of trap states.^43,46
此外，為了深入了解光載子動力學，我們進行了瞬態光特性、瞬態光電壓（TPV）和瞬態光電流（TPC）測量。F-COOH 器件的 TPV 曲線（圖 4b）顯示出比對照器件明顯更慢的衰減，這表示載子壽命從 10.62 微秒顯著延長至 15.44 微秒。這種延長的載子壽命通常是器件內部陷阱態較少的跡象，否則這些陷阱態將充當載子的複合中心。 ^51,52 這證實了添加 F-COOH 的 Sn-PSC 減少了陷阱輔助複合，並有助於提高性能，支持了這種功能性添加劑是實現更好器件穩定性和效率的關鍵結論。同樣地，TPC 曲線（圖 4c）評估了對器件界面品質的影響。添加 F-COOH 的 Sn-PSC 顯示出更快的電流衰減時間，為 5.56 微秒，而對照組為 6.38 微秒。這種更快的衰減表明添加 F-COOH 的 Sn-PSC 中電荷載子萃取效率更高，這是提高太陽能電池性能的關鍵因素之一。這些結果進一步支持了 F-COOH 添加劑對材料品質和器件特性的良好影響。此外，還進行了電容測量，以深入了解器件的電荷載子動力學、界面態、陷阱態和缺陷密度分佈。 ^53,54 圖 5a 顯示了器件的電容-頻率（C-f）光譜。對照器件顯示出比 F-COOH 改性器件明顯更高的電容，這歸因於對照器件中更高的電荷累積和離子運動。這表明添加劑可能抑制了離子運動或減輕了其對電荷累積的影響。 ⁵⁵ 添加 F-COOH 的器件在頻率範圍內顯示出較低的電容，表明該添加劑可能降低了陷阱態的密度。 ^43,46

To evaluate the effects on the defect profile, we analyzed the capacitance spectra with Mott–Schottky (M–S) plots and carrier profile as given by where N_CV represents carrier density calculated from the capacitance–voltage (C–V) curve, C is the capacitance per unit area, ε₀ is the permittivity of free space, ε_s is the dielectric constant of the perovskite material. The MS plot, as shown in Fig. 5b, compares the control device with the device containing the F-COOH additive. The device with the F-COOH additive reveals a higher diffusion potential (V_D) of 0.824 V compared to the control (0.702 V). This increment in V_D aligns well with the enhancement in V_OC in Sn-PSC. This result suggests that the F-COOH additive strengthens the separation of electron–hole pairs, reduces recombination losses, and ultimately contributes to higher V_OC.⁵¹
為了評估對缺陷分佈的影響，我們分析了電容光譜，其中包含莫特-肖特基（M-S）圖和載流子分佈，如 所示，其中 N _CV 代表根據電容-電壓（C-V）曲線計算出的載流子密度，C 是單位面積電容，ε ₀ 是自由空間的介電常數，ε _s 是鈣鈦礦材料的介電常數。如圖 5b 所示的 MS 圖比較了對照組元件與含有 F-COOH 添加劑的元件。含有 F-COOH 添加劑的元件顯示出更高的擴散電位（V _D ），為 0.824 V，而對照組為 0.702 V。V _D 的這種增加與 Sn-PSC 中 V _OC 的增強非常吻合。這項結果表明 F-COOH 添加劑增強了電子-電洞對的分離，減少了複合損失，並最終有助於提高 V _OC 。 ⁵¹

Fig. 5c depicts the spatial distribution of charge carriers across the device calculated from the C–V measurements. This analysis provides insights into how charge carriers are distributed within the bulk of the active layer and at the interface regions, where recombination and charge transport play a crucial role in device performance. It has been documented that the N_CV profile accounts for the carrier distribution (free carrier and defect density)^47,56,57 and ion or charge accumulation at the interface⁵³ in thin-film solar cells. In the control device, the bulk carrier density (N ^B_CV) is estimated to be 3.18 × 10¹⁵ cm⁻³. While the device with the F-COOH additive showed a reduced bulk carrier density of 1.47 × 10¹⁵ cm⁻³. This reduction suggests that the F-COOH additive is effective in mitigating defects within the bulk perovskite layer that can capture and recombine charge carriers. This trend of reduced carrier profile extends to the interface region as well. The control device exhibits an interfacial carrier density (N ^IF_CV) of 7.42 × 10¹⁶ cm⁻³, while the F-COOH-modified device shows a significantly lower density of 1.65 × 10¹⁶ cm⁻³. This reduction at the interface implies that the F-COOH additive is also effective at passivating interfacial defects, which are often hotspots for charge recombination due to discontinuities in the crystal structure and imperfect layer alignment. The decrease in the C–V carrier profile correlates with improved carrier lifetimes resulting from defect passivation.
圖 5c 描繪了根據 C-V 測量計算出的元件中載流子的空間分佈。這項分析提供了關於載流子在活性層體積內和界面區域分佈的見解，其中複合和電荷傳輸在元件性能中扮演著關鍵角色。已有文獻記載，N _CV 分佈解釋了薄膜太陽能電池中的載流子分佈（自由載流子和缺陷密度） ^47,56,57 以及界面處的離子或電荷累積 ⁵³ 。在對照元件中，體積載流子密度（N）估計為 3.18 × 10 ¹⁵ cm ⁻³ 。而添加 F-COOH 的元件顯示出體積載流子密度降低至 1.47 × 10 ¹⁵ cm ⁻³ 。這種降低表明 F-COOH 添加劑能有效減少體積鈣鈦礦層中可能捕獲和複合載流子的缺陷。這種載流子分佈減少的趨勢也延伸到界面區域。對照元件的界面載流子密度（N）為 7.42 × 10 ¹⁶ cm ⁻³ ，而 F-COOH 改性元件的密度顯著降低至 1.65 × 10 ¹⁶ cm ⁻³ 。界面處的這種降低意味著 F-COOH 添加劑也能有效鈍化界面缺陷，這些缺陷通常是電荷複合的熱點，因為晶體結構的不連續性和不完美的層對齊。C-V 載流子分佈的減少與缺陷鈍化導致的載流子壽命改善相關。

Theoretical insights were obtained by performing DFT calculations considering a slab model with a SnI₂-terminated perovskite surface as described in our earlier report⁵⁸ to investigate the interaction between fullerene derivatives and the tin-based perovskite. The charge density difference of fullerene functional derivatives on the defective Sn-perovskite surface (Fig. 6a–c) indicates mitigation of the density of defect states.³⁷ The adsorption energies (Fig. 6a–c and S16d–f†) reveal a clear trend: F-COOH exhibits the strongest binding with the Sn-perovskite surface (−0.555 eV), followed by F-OH (−0.408 eV) and F-OSO₃H (−0.267 eV). The stronger adsorption energy of F-COOH suggests a more robust chemical interaction with undercoordinated Sn²⁺ sites, leading to more effective defect passivation compared to F-OH and F-OSO₃H. This inference is further supported by the density of states (DOS) calculation as depicted in Fig. 6d–g, which shows a reduction in defect states near the Fermi level up to some extent upon fullerene functionalization, particularly in the case of F-COOH. The stronger binding and superior defect mitigation effect of F-COOH correlate well with experimental observations, including reduced trap-assisted recombination from capacitance measurements, longer carrier lifetimes from TRPL analysis, and improved photovoltage stability from TPV measurements.⁵⁹
透過執行 DFT 計算，考慮了 SnI ₂ 終端鈣鈦礦表面的平板模型，如我們早期報告 ⁵⁸ 所述，以研究富勒烯衍生物與錫基鈣鈦礦之間的相互作用，從而獲得了理論見解。富勒烯功能衍生物在缺陷 Sn-鈣鈦礦表面上的電荷密度差（圖 6a-c）表明缺陷態密度的減輕。 ³⁷ 吸附能（圖 6a-c 和 S16d-f†）顯示出一個明確的趨勢：F-COOH 與 Sn-鈣鈦礦表面的結合最強（-0.555 eV），其次是 F-OH（-0.408 eV）和 F-OSO ₃ H（-0.267 eV）。F-COOH 更強的吸附能表明與配位不足的 Sn ²⁺ 位點有更強的化學相互作用，導致比 F-OH 和 F-OSO ₃ H 更有效的缺陷鈍化。這一推論進一步得到了態密度（DOS）計算的支持，如圖 6d-g 所示，該計算顯示在富勒烯功能化後，特別是 F-COOH 的情況下，費米能級附近的缺陷態有所減少。F-COOH 更強的結合和卓越的缺陷減輕效果與實驗觀察結果密切相關，包括電容測量中陷阱輔助複合的減少、TRPL 分析中載流子壽命的延長以及 TPV 測量中光電壓穩定性的改善。 ⁵⁹

Thus, theoretical and experimental results reveal that the fullerene-based functional additives induce a strong adsorption on the Sn-perovskite surface. This interaction plays a crucial role in mitigating the oxidation of Sn²⁺ to Sn⁴⁺ and enhancing the perovskite layer's material chemistry integrity. The fullerene derivatives demonstrate a notable improvement in both efficiency and stability of the devices. A comprehensive device analysis combined with theoretical insights substantiates the experimentally observed performance enhancements, highlighting the potential of fullerene derivatives as effective functional additives for advancing Sn-based perovskite photovoltaics.
因此，理論和實驗結果顯示，富勒烯基功能性添加劑在錫鈣鈦礦表面產生強烈的吸附作用。這種相互作用在減輕 Sn ²⁺ 氧化成 Sn ⁴⁺ 以及增強鈣鈦礦層的材料化學完整性方面扮演關鍵角色。富勒烯衍生物在元件的效率和穩定性方面均展現顯著提升。全面的元件分析結合理論見解，證實了實驗觀察到的性能增強，突顯了富勒烯衍生物作為有效功能性添加劑，推動錫基鈣鈦礦光伏技術發展的潛力。

3. Conclusion   3. 結論

This study explored the impact of fullerene derivatives with various functional groups (–COOH, –OH, –OSO₃H) on the efficiency of tin-based perovskite solar cells, with a particular focus on how these functional groups interact with the perovskite matrix. The findings demonstrate that the fullerene derivative with a carboxylic group (–COOH) significantly enhances device performance, increasing the power conversion efficiency from 8.20% to 11.22% and extending the stability of devices significantly. This improvement is attributed to the additive's ability to moderate the crystallization process of the perovskite film, leading to a more uniform morphology with fewer defects and effectively suppressing the oxidation of Sn²⁺ to Sn⁴⁺. These results not only highlight the potential of fullerene-based additives in improving the stability and efficiency of tin-based perovskite solar cells but also offer a promising pathway for the development of more reliable and high-performance renewable energy solutions. Future research should focus on optimizing these additives and exploring additional functional groups to further enhance the performance of next-generation perovskite solar cells.
本研究探討了具有不同官能基（–COOH、–OH、–OSO ₃ H）的富勒烯衍生物對錫基鈣鈦礦太陽能電池效率的影響，特別著重於這些官能基如何與鈣鈦礦基質相互作用。研究結果顯示，帶有羧基（–COOH）的富勒烯衍生物顯著提升了元件性能，將功率轉換效率從 8.20%提高到 11.22%，並顯著延長了元件的穩定性。這種改進歸因於添加劑能夠調節鈣鈦礦薄膜的結晶過程，從而形成更均勻、缺陷更少的形態，並有效抑制 Sn ²⁺ 氧化成 Sn ⁴⁺ 。這些結果不僅突顯了富勒烯基添加劑在改善錫基鈣鈦礦太陽能電池穩定性和效率方面的潛力，也為開發更可靠、高性能的再生能源解決方案提供了有前景的途徑。未來的研究應著重於優化這些添加劑並探索額外的官能基，以進一步提升下一代鈣鈦礦太陽能電池的性能。

4. Experimental section   4. 實驗部分

4.1. Materials and precursor solution
4.1. 材料與前驅物溶液

As mentioned, all chemicals were purchased from commercial suppliers and used as received. Formamidinium iodide (FAI, 99.9%, luminescent), methylammonium iodide (Sigma-Aldrich), SnI₂ (Sigma-Aldrich), and SnF₂ (Sigma-Aldrich), poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) (Clevious, A14083), ICBA (one material, 99% purity) and BCP (Sigma-Aldrich, 99% purity) were purchased and used as received. For the fabrication of Sn-HP films, the precursors were prepared by dissolving FAI (0.8 M), MAI (0.05 M), PEAI (0.15), SnI₂ (1 M), and SnF₂ (0.1 M) for perovskite solution in 3 ml of dimethyl sulfoxide (DMSO) solvent overnight at room temperature. Similarly, precursors with fullerene-based multifunctional functional molecules (F-COOH, F-OH, and F-OSO₃H) were prepared by adding additives. ICBA (one material, 99% purity) solution (3 wt%) dissolved in anhydrous chlorobenzene (CB) was used for coating the electron transport layer (ETL). A saturated BCP solution was prepared by dissolving 3 mg in 3 ml of anhydrous isopropanol. All the solutions were filtered using 0.45 mm syringe filters just before the deposition to avoid the risk of unwanted particles in the precursor solution.
如前所述，所有化學品均購自商業供應商並直接使用。碘甲脒 (FAI, 99.9%, 發光級)、碘甲胺 (Sigma-Aldrich)、SnI ₂ (Sigma-Aldrich) 和 SnF ₂ (Sigma-Aldrich)、聚(3,4-乙烯二氧噻吩):聚(苯乙烯磺酸鹽) (PEDOT:PSS) (Clevious, A14083)、ICBA (單一材料, 99% 純度) 和 BCP (Sigma-Aldrich, 99% 純度) 均已購得並直接使用。為了製備 Sn-HP 薄膜，前驅物是將 FAI (0.8 M)、MAI (0.05 M)、PEAI (0.15)、SnI ₂ (1 M) 和 SnF ₂ (0.1 M) 溶解於 3 毫升的二甲基亞碸 (DMSO) 溶劑中，於室溫下攪拌過夜，以製備鈣鈦礦溶液。同樣地，含有富勒烯基多功能分子 (F-COOH、F-OH 和 F-OSO ₃ H) 的前驅物是透過添加添加劑來製備的。ICBA (單一材料, 99% 純度) 溶液 (3 wt%) 溶解於無水氯苯 (CB) 中，用於塗佈電子傳輸層 (ETL)。飽和 BCP 溶液是將 3 毫克溶解於 3 毫升無水異丙醇中製備的。所有溶液在沉積前均使用 0.45 毫米注射器過濾器過濾，以避免前驅物溶液中出現不必要的顆粒。

4.2. Synthesis of fullerene compounds
4.2. 富勒烯化合物的合成

Fullerene (99.95% MTR Ltd) derivatives (F-COOH, F-OH, and F-OSO₃H) were synthesized at Sophia University. A detailed synthesis method and related characterizations are given in the ESI.†
富勒烯 (99.95% MTR Ltd) 衍生物 (F-COOH、F-OH 和 F-OSO ₃ H) 在上智大學合成。詳細的合成方法和相關特性請參閱 ESI。†

4.3. Device fabrication   4.3. 元件製作

Solar cell devices were fabricated on pre-cleaned patterned indium tin oxide (ITO) coated glass substrates (15 Ω sq⁻¹). The ITO substrates were pre-cleaned in an ultrasonic bath with detergent, pure water, and 2-propanol, followed by an ultraviolet-ozone treatment for 15 minutes to remove the organic residuals. A thin HTM layer (30 nm) of PEDOT:PSS was deposited onto the ITO substrate by spin coating at 4000 rpm and subsequently dried at 150 °C for 20 min on a hot plate in ambient air. Then, the substrates were transferred into a nitrogen-filled glove box (<1.0 ppm O₂ and H₂O), and the rest of the steps were carried out inside the glove box. The Sn-HP precursor was spin-coated at 6000 rpm for 90 s (ramping slope 3 s) followed by dripping 150 ml of CB at the 60^th second. Then, to promote the crystallization, those as-grown Sn-HP films were simply placed on a hot plate at 60 °C for 1 min and 85 °C for 15 min. A passivation layer of 4-fluoro-benzohydrazide (4F-BHZ) (Fig. S17†) was dynamically deposited at 5000 rpm for 50 s, followed by annealing at 75 °C for 5 min.⁵⁸ For the ETM layer, ICBA was spin-coated on top at 1500 rpm for 30 s and 4000 rpm – 5 s and then annealed at 75 °C – 5 min, followed by dynamic deposition of a thin BCP layer spinning at 5000 rpm for 30 s, which was annealed at 70 °C – 5 min. To complete the device structure, the samples were then transferred into the evaporation chamber connected to the glove box for metal contact deposition. Finally, 140 nm of Ag was thermally evaporated at a pressure < 10⁻⁴ Pa. Four device sections with an area of ∼0.26 cm² are confined in a 2.5 cm × 3 cm ITO substrate.
太陽能電池元件是在預先清潔的圖案化氧化銦錫 (ITO) 塗層玻璃基板 (15 Ω sq ⁻¹ ) 上製作的。ITO 基板先在超音波浴中用清潔劑、純水和 2-丙醇預先清潔，然後進行 15 分鐘的紫外臭氧處理，以去除有機殘留物。PEDOT:PSS 的薄型 HTM 層 (30 nm) 以 4000 rpm 的轉速旋塗到 ITO 基板上，隨後在環境空氣中的熱板上以 150 °C 乾燥 20 分鐘。然後，將基板轉移到充氮手套箱 (<1.0 ppm O ₂ 和 H ₂ O) 中，其餘步驟均在手套箱內進行。Sn-HP 前驅物以 6000 rpm 的轉速旋塗 90 秒 (斜坡上升時間 3 秒)，然後在第 60 ^th 秒滴入 150 毫升的 CB。然後，為了促進結晶，這些生長的 Sn-HP 薄膜簡單地放置在 60 °C 的熱板上 1 分鐘，然後在 85 °C 下放置 15 分鐘。4-氟苯甲醯肼 (4F-BHZ) (圖 S17 †) 的鈍化層以 5000 rpm 的轉速動態沉積 50 秒，然後在 75 °C 下退火 5 分鐘。 ⁵⁸ 對於 ETM 層，ICBA 以 1500 rpm 的轉速旋塗 30 秒，然後以 4000 rpm 的轉速旋塗 5 秒，然後在 75 °C 下退火 5 分鐘，隨後動態沉積薄型 BCP 層，以 5000 rpm 的轉速旋塗 30 秒，並在 70 °C 下退火 5 分鐘。為了完成元件結構，樣品隨後轉移到連接到手套箱的蒸發腔室中進行金屬接觸沉積。最後，在壓力 < 10 ⁻⁴ Pa 下熱蒸發 140 奈米的銀。在 2.5 公分 × 3 公分 的 ITO 基板上，有四個面積約為 0.26 平方公分 ² 的元件區域。

4.4. Device characterization
4.4. 元件特性

The morphology of the films was studied, and cross-sectional images were taken using a high-resolution scanning electron microscope (SEM) at a 5 kV accelerating voltage (Hitachi, S-4800). X-ray diffraction (XRD) patterns of fabricated Sn-HP films were collected using an advanced X-ray diffractometer (Rigaku SmartLab, CuK_α radiation, λ = 1.54050 Å). The absorption spectra of the various films were measured using a UV-vis-NIR spectrometer (UV-2600i, Shimadzu). The photoluminescence (PL) spectra were collected using a micro-PL spectrometer (HORIBA, LabRamHR-PL NF(UV-NIR)) ∼532 nm laser diode (10 mW cm⁻²) as an excitation source. The carrier lifetimes were measured with a fluorescence lifetime spectrometer (Quantaurus-τ from Hamamatsu-Photonics K.K., C11367) equipped with ∼405 nm laser diode (typical peak power of 400 mW) at 200 kHz repetition rate. XPS spectra were obtained using a Versa Probe II (ULVAC-PHI, Japan). The current density–voltage (J–V) curves were measured at the scan rate of 0.05 V s⁻¹ under 1 sun with an AM1.5G spectral filter (100 mW cm⁻²) coupled with an MPPT system (SystemHouse Sunrise Corp.). The light intensity was calibrated by a silicon (Si) diode (BS-520BK). For the stability test, the encapsulated devices were measured under MPPT conditions and air ambient. The external quantum efficiency (EQE) spectra were obtained using a spectrometer (SM-250IQE, Bunkoukeiki, Japan). The transient photovoltage and photocurrent data were measured using a commercial PAIOS system (PAIOS V.4.3). A pulse intensity was used to induce a spike in photovoltage. The capacitance spectra (C–f) were taken from PAIOS v. 4.3 software, which scans from 20 Hz to 2 MHz at 50 mV AC in the dark at a bias voltage of 0 V. The C–V measurements were taken at 20 kHz with a voltage amplitude of 30 mV AC in the dark.
薄膜的型態學研究是透過高解析度掃描式電子顯微鏡（SEM，Hitachi, S-4800）在 5 kV 加速電壓下拍攝截面影像進行的。所製備的 Sn-HP 薄膜的 X 射線繞射（XRD）圖案是使用先進的 X 射線繞射儀（Rigaku SmartLab，CuK _α 輻射，λ = 1.54050 Å）收集的。各種薄膜的吸收光譜是使用紫外-可見光-近紅外光譜儀（UV-2600i, Shimadzu）測量的。光致發光（PL）光譜是使用微型 PL 光譜儀（HORIBA, LabRamHR-PL NF(UV-NIR)）收集的，激發源為約 532 nm 雷射二極體（10 mW cm ⁻² ）。載子壽命是使用螢光壽命光譜儀（Quantaurus-τ from Hamamatsu-Photonics K.K., C11367）測量的，該儀器配備約 405 nm 雷射二極體（典型峰值功率為 400 mW），重複頻率為 200 kHz。XPS 光譜是使用 Versa Probe II（ULVAC-PHI, Japan）獲得的。電流密度-電壓（J-V）曲線是在 0.05 V s ⁻¹ 的掃描速率下，於 1 個太陽光照（AM1.5G 光譜濾波器，100 mW cm ⁻² ）條件下，並結合 MPPT 系統（SystemHouse Sunrise Corp.）測量的。光強度是透過矽（Si）二極體（BS-520BK）校準的。對於穩定性測試，封裝後的元件是在 MPPT 條件和空氣環境下測量的。外部量子效率（EQE）光譜是使用光譜儀（SM-250IQE, Bunkoukeiki, Japan）獲得的。瞬態光電壓和光電流數據是使用商用 PAIOS 系統（PAIOS V.4.3）測量的。脈衝強度用於引起光電壓的尖峰。電容光譜（C-f）是從 PAIOS v. 4.3 軟體中獲取的，該軟體在黑暗中以 50 mV 交流電壓、0 V 偏壓下，從 20 Hz 掃描到 2 MHz。C-V 測量是在黑暗中以 20 kHz、30 mV 交流電壓振幅下進行的。

4.5. Density functional theory calculation
4.5. 密度泛函理論計算

First-principles calculations based on density functional theory (DFT) were performed by adopting a slab model of a SnI₂-terminated surface³⁸ using the Vienna ab initio simulation package⁶⁰ which implements the projector-augmented wave method.⁶¹ The influence of vdW interactions between the molecules and the Sn-HP film was considered.
基於密度泛函理論（DFT）的第一性原理計算是透過採用 SnI ₂ 終端表面 ³⁸ 的平板模型，並使用實施投影增強波方法的 Vienna ab initio 模擬套件 ⁶⁰ 進行的。 ⁶¹ 考慮了分子與 Sn-HP 薄膜之間 vdW 相互作用的影響。

For the exchange–correlation function, the Perdew–Burke–Ernzerhof function,³⁸ was used. A 2√2 × 2√2 × 1 slab supercell of (001) surface, containing 5 layers, was built from a bulk tetragonal phase of FASnI₃ (space group: P4/mbm), with a vacuum region of about 22 Å was added in the z direction. The kinetic energy cutoff of 400 eV and the convergence criterion of 10⁻⁴ eV for the self-consistent loop were employed. To explore stable adsorption sites of the molecule, a SnI₂-terminated surface with Sn-vacancy (V_Sn) defect was used, on which a molecule was placed, based on the insight from the previous work.⁶² Gamma point sampling was employed for the Brillouin zone integration. The adsorption energy of the molecule was evaluated as E_ads = E_{system with molecule} − E_{system without molecule} − μ_mol where E_{system with molecule} and E_{system without molecule} are energies of the surfaces with and without a molecule additive, respectively, and μ_mol is the chemical potential of the molecule. The total energy computed for an isolated gas phase was used for μ_mol.
對於交換相關函數，採用了 Perdew–Burke–Ernzerhof 函數， ³⁸ 。我們從 FASnI ₃ 的塊狀四方相（空間群：P4/mbm）建構了一個 2√2 × 2√2 × 1 的(001)表面平板超晶胞，其中包含 5 層，並在 z 方向添加了約 22 Å的真空區域。動能截斷值為 400 eV，自洽迴圈的收斂標準為 10 ⁻⁴ eV。為了探索分子的穩定吸附位點，我們根據先前研究的見解，使用了具有 Sn 空位（V _Sn ）缺陷的 SnI ₂ 終端表面，並將分子放置於其上。布里淵區積分採用了 Gamma 點取樣。分子的吸附能計算為 E _ads = E _{system with molecule} − E _{system without molecule} − μ _mol ，其中 E _{system with molecule} 和 E _{system without molecule} 分別為有分子添加劑和無分子添加劑的表面能量，μ _mol 為分子的化學勢。孤立氣相的總能量用於計算μ _mol 。

Data availability   資料可用性

The data supporting this article have been included as part of the ESI.†
支持本文的資料已作為 ESI 的一部分包含在內。†

Conflicts of interest   利益衝突

The authors declare no competing financial interest.
作者聲明沒有任何相互競爭的財務利益。

Acknowledgements  致謝

This work was supported by The Hitachi Global Foundation, Kurata grants (#1572), and partially by the JST-ALCA-Next Program (Grant Number JPMJAN23B2), Japan. We extend our sincere gratitude to Dr Kentaro Kikuchi for his valuable assistance in synthesizing fullerene derivative compounds. We also acknowledge the technical support provided by Yamaguchi Kazuo-San (XPS) and Takahashi Hiromi (XRD) from the NIMS Battery Research Platform for their respective measurement and analysis contributions. The calculations in this study were performed using the Numerical Materials Simulator at the National Institute for Materials Science (NIMS). Aman Sukla expresses appreciation to the National Institute for Materials Science (NIMS) for the opportunity to participate in the short-term “NIMS Internship Program.” The authors are deeply thankful to Prof. Monica Katiyar and Prof. Kenjiro Miyano for their insightful comments and constructive suggestions in this work.
這項工作獲得了日立全球基金會、倉田補助金（#1572）的支持，並部分獲得了日本科學技術振興機構（JST）的 ALCA-Next 計畫（補助金編號 JPMJAN23B2）的支持。我們衷心感謝菊池健太郎博士在富勒烯衍生物化合物合成方面的寶貴協助。我們也感謝 NIMS 電池研究平台的山口和夫先生（XPS）和高橋弘美女士（XRD）分別在測量和分析方面提供的技術支援。本研究中的計算是使用國家材料科學研究所（NIMS）的數值材料模擬器進行的。Aman Sukla 感謝國家材料科學研究所（NIMS）提供參與短期「NIMS 實習計畫」的機會。作者們對 Monica Katiyar 教授和 Kenjiro Miyano 教授在本工作中所提出的深刻評論和建設性建議深表感謝。

References  參考文獻

Footnotes  註腳