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Nature Communications volume 16, Article number: 2759 (2025) Cite this article
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An Author Correction to this article was published on 14 April 2025
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While individual perovskite and organic solar cells have demonstrated remarkable performance, achieving similar success in high-efficiency perovskite/organic tandem solar cells (TSCs) has been challenging, primarily due to large voltage deficits and severe non-radiative recombination. By exploring the fundamental mechanisms of carrier losses, we identify that imbalanced carrier transport, particularly inadequate hole transport in the organic subcell significantly limits the overall performance of perovskite/organic TSCs. Herein, we implement a hole transport self-assembled monolayer (SAM) anchored to MoO₃, which converts the inherently n-type MoO₃ to a p-type surface. Further, a SAM/MoO₃/SAM sandwich hole transport configuration is introduced, which significantly enhances hole extraction, facilitating a more balanced carrier transport, and markedly suppressing non-radiative recombination at the interconnection layer (ICL). The resulting perovskite/organic TSCs achieve a power conversion efficiency (PCE) of 26.05%, with an open-circuit voltage of 2.21 V (certified at 2.216 V) and enhanced operation stability.
The theoretical thermodynamic efficiency limit of single-junction solar cells is known to approach ~ 33% under AM 1.5 G illumination, arising from the unavoidable photon spontaneous radiative losses and solar spectra-bandgap mismatch property¹. Multijunction solar cells, which integrate wide-bandgap and narrow-bandgap subcells, offer a promising solution to overcome the efficiency limitations inherent in single-junction cells due to significantly suppressed transmission and thermalization losses^2,3,4. These devices are expected to push the power conversion efficiency (PCE) of photovoltaics devices to 45% for two-junction systems and up to 68% (and even to 80% under concentration) for theoretical infinite junction configurations^2,3,5. Thin-film-based multijunction tandem solar cells (TSCs), including perovskite/copper indium gallium selenide (CIGS)^6,7, perovskite/perovskite^8,9,10, and perovskite/organic TSCs^{11,12,13,14,15,16,17,18,19}, have garnered extensive attention due to their bandgap tunability, cost-effectiveness, and extraordinary optoelectronic performance^12,13,19,20. Perovskite/organic TSCs, in particular, benefit from the use of orthogonal solvents for the key component materials, potentially paving the way for all solution-processing of high-performance and large-area photovoltaics^21,22.
Considerable efforts have been devoted to enhancing the overall performance and stability of perovskite/organic TSCs, including strategies in crystallization control^18,23, strain regulation^24,25, and surface modulation^14,26. These efforts have enabled the achievement of a PCE exceeding 25% for perovskite/organic TSC with the stability of T₉₀ > 1000 h¹². A theoretical prediction also suggests that such types of tandem solar cells have the potential to reach a PCE of over 29%²⁷. Nevertheless, despite significant performance improvements in individual types of subcells, with PCEs exceeding 26% for perovskite solar cells (PSCs) and 20% for organic solar cells (OSCs)^28,29,30, optical and carrier recombination losses, particularly within the interconnection layer (ICL), continue to impede the current performance of perovskite/organic TSCs^{19,20,21,22,27}.
The ICL in TSCs serves as a critical optical and electrical interconnection junction between subcells, profoundly influencing overall tandem device performance. An ideal ICL must meet several key requirements, including chemical stability, high electrical conductivity, and optical transparency. In a typically p-i-n type perovskite/organic TSC, the ICL is comprised of three parts: 1) an electron transport layer (ETL), such as C60, SnO₂^12,18,20,21; 2) an ultra-thin metal layer (~ 1 nm of Ag or Au); 3) a hole transport layer (HTL), such as PEDOT:PSS and MoO₃^15,21. Various ICL structures, such as SnO₂/Au (Ag)/MoO₃^12,13,18, BCP/IZO/MoO₃²⁰, and SnO₂/Au/PEDOT:PSS have been reported¹⁵. For instance, replacing the ultra-thin metal layer with transparent conductive oxides (such as IZO or In₂O₃) can significantly reduce optical losses and establish an effective ohmic contact for smooth carrier transport^19,20. Despite extensive optimization efforts aimed at both the subcells and the ICL, severe non-radiative recombination and large voltage deficits still predominantly limit the current performance of perovskite/organic TSCs. Therefore, understanding the mechanisms behind carrier losses and effectively suppressing non-radiative recombination is greatly necessary and urgent for achieving high-performance perovskite/organic TSCs.
In this study, we investigated the fundamental mechanisms underlying carrier losses in perovskite/organic TSCs. By analyzing the electroluminescence properties, we identified the primary channels of carrier recombination losses. Notably, unbalanced carrier transport, particularly poor hole transport in the organic subcell, significantly hinders TSC performance. To address this challenge, we proposed a SAM/MoO₃/SAM sandwich HTL by introducing a hole transport self-assembled monolayer (SAM) anchored to both the top and bottom surfaces of MoO₃ to effectively tailor the surface potential and energy level of MoO₃, resulting in more balanced carrier transport and effectively suppress non-radiative recombination to achieve a high-performance TSCs. To explore the interactions between the MoO₃ and SAM, we employed density functional theory (DFT) and X-ray photoelectron spectroscopy (XPS) characterization. Our results revealed the formation of hydrogen and coordination bonding interactions between MoO₃ and SAM. Additionally, we conducted transient photovoltage and darkness current measurements to estimate the carrier transport/recombination properties and provided a thorough analysis of voltage loss. After achieving more balanced carrier transport, the constructed perovskite/organic tandem device exhibited a PCE of 26.05% with high operation stability (T₈₀ over 650 h), ranking among the highest performances in the perovskite/organic tandem photovoltaics. Our approach provides a fundamental understanding of carrier balance and transport within the ICL, providing valuable insights for fabricating high-performance TSCs.
We fabricated a typically p-i-n configuration perovskite/organic TSC, with the structure of ITO/SAM/perovskite (E_g ~ 1.85 eV)/C₆₀/SnO₂/Au/MoO₃/organic active layer (E_g ~ 1.37 eV)/PNDIT-F3N/Ag, as depicted in Fig. 1a. The detailed fabrication processes are described in the Experimental Section¹⁸. The bottom narrow-bandgap organic subcell employs PM6:BTP-eC9:L8-BO as the ternary bulk heterojunction (BHJ) active layer with a bandgap of 1.37 eV; the detailed chemical structures of the donor polymer and non-fullerene acceptors used for the BHJ are shown in Supplementary Fig. 1. For the top perovskite subcell, we utilized a triple cation mixed-halide perovskite composition FA_0.8MA_0.1Cs_0.1PbI_0.5Br_0.5 (FA = Formamidinium, MA = Methylammonium, and Cs = Cesium) with a bandgap of 1.85 eV to achieve good photocurrent matching¹⁸. In the perovskite/organic TSCs, the ICL consists of SnO₂ (20 nm)/Au (~ 1 nm)/MoO₃ (8 nm) and acts as the interconnection junction for carrier transport/recombination (Fig. 1a).
a Structure of perovskite/organic TSC featuring the SAM molecule. The ICL consists of SnO₂/Au/MoO₃ and is highlighted with a dashed box. b Energy band diagram of the MoO₃, SAM, SAM/MoO₃, and MoO₃/SAM layer. c, d Schematic diagram of carrier transport in perovskite/organic TSC based on SnO₂/Au/MoO₃ and SnO₂/Au/SAM/MoO₃/SAM ICL, respectively. Carrier transport process and EL spectra under forward bias (e and g) for the control sample, (f and h) for target perovskite/organic TSCs, respectively.
Generally, the performance of MoO₃-based OSCs lags behind those employing other HTL materials such as PEDOT:PSS, which may be attributed to the inadequate hole extraction capacity of MoO₃^31,32. Nevertheless, thermally evaporated MoO₃ is widely used as HTL for organic subcells in perovskite/organic TSCs due to its facile deposition on top of the thin metal recombination layer through subsequent evaporation under a more inert atmosphere²¹. To explore the carrier transport characteristics of the MoO₃ HTL in perovskite/organic TSCs, we performed ultraviolet photoemission spectroscopy (UPS) to elucidate its energy levels (Fig. 1b). The conduction band minimum (E_c) and Fermi-level (E_F) was determined to be at − 4.68 eV and − 4.91 eV respectively, with the valence band maximum (E_V) around − 7.78 eV, indicating n-type doping behavior for the thermally evaporated MoO₃ layer^32,33,34.
To illustrate the dynamics of carrier transport in TSCs with the MoO₃ layer, we present a schematic depicting energy levels alongside a carrier transport diagram in Fig. 1c and Supplementary Fig. 2. Due to the inherent n-doping characteristics of MoO₃ and its deep valence band, direct hole transport from the organic layer to MoO₃ via the valence band is not feasible. Instead, photogenerated holes can be transported from the highest occupied molecular orbital (HOMO) of the organic layer to the localized energy levels of MoO₃ (e.g., defect levels) via a hoping mechanism^35,36. In the meanwhile, undesired electron injection may occur from the lowest unoccupied molecular orbital (LUMO) of the organic layer to MoO₃ due to the low electron transfer barrier. These electrons may recombine with the photogenerated holes within the MoO₃ layer, as depicted in Fig. 1c. This process results in significant interfacial and bulk recombination losses at the MoO₃/organic interface and within MoO₃ itself. In TSCs, incorporating a thin metal film within the ICL is a common strategy to improve carrier recombination efficiency. The high electrical conductivity of metals facilitates the swift recombination of electrons and holes from adjacent subcells, which is particularly beneficial for balancing charge distribution and reducing carrier accumulation in the ICL. This helps minimize voltage losses and enhance overall device efficiency.
In our case, the deposition of 1 nm of Au on the SnO₂ layer leads to the formation of Au nanoclusters rather than a continuous film³⁷. This leaves a significant portion of the SnO₂ surface exposed, directly contacting with the MoO₃. In regions where Au nanoclusters are present within the MoO₃/SnO₂ ICL, the injection of holes from MoO₃ and electrons from SnO₂ can be efficiently recombined (Supplementary Fig. 2b). Conversely, in areas where MoO₃ and SnO₂ directly contacts without the mediation of Au nanoclusters, recombination still occurs either at the interface or within the MoO₃ layer (Supplementary Fig. 2a). However, recombination process tends to be suboptimal, potentially limiting the overall performance of the TSC. In addition, the presence of surface trap states on metal oxides (MoO₃ and SnO₂) may also affect recombination by capturing charge carriers and increasing non-radiative recombination. As a result, the fabricated perovskite/organic TSCs achieve an average PCE of only 22.79%, with a relatively low short-circuit current density (J_sc = 14.11 mA/cm²), open-circuit voltage (V_OC = 2.122 V) and a modest fill factor (FF = 76.46%), as illustrated in Supplementary Fig. 3 and Supplementary Table 1.
To address this challenge, we propose a surface-modification strategy using (4-(3,6-diphenyl-9H-carbazol-9-yl) ethyl) phosphonic acid (Ph-4PACz) self-assembled monolayer (SAM) to regulate the surface properties of MoO₃. The chemical structure of Ph-4PACz is illustrated in Fig. 1a, and the corresponding energy levels for the MoO₃, SAM, SAM/MoO₃, and MoO₃/SAM are shown in Fig. 1b³⁸. The E_F of SAM-anchored MoO₃ shifts to − 4.62 eV, and the E_V moves up to − 5.72 eV, suggesting a n- to p-type conversion of the MoO₃ surface (Fig. 1b). This p-type MoO₃/SAM surface is favorable for hole transport from the organic active layer to the MoO₃ due to aligned energy levels, and also functions as the electron blocking layer to reduce electron injection from LUMO of the organic layer to MoO₃, as displayed in Supplementary Fig. 2c.
We also performed Kelvin probe force microscopy (KPFM) to measure the surface potentials of the MoO₃ with and without SAM. The surface potential of the MoO₃/SAM film decreases by 466 mV compared to intrinsic MoO₃, indicating that the SAM layer significantly alters the surface properties of MoO₃. As a result, the MoO₃/SAM surface exhibits characteristics similar to those of the SAM layer, as displayed in Supplementary Figs. 4 and 5. In addition, to investigate whether the bulk properties of MoO₃ were affected by SAM modification, we fabricated SAM/MoO₃ films with varying MoO₃ thicknesses ranging from 1 to 8 nm. As shown in Supplementary Fig. 6, the WF of these films is similar to that of intrinsic MoO₃, indicating that SAM modification primarily alters the surface properties of MoO₃, while its bulk properties remain largely unchanged.
When the SAM layer composed of the phosphonic acid functional group are coated on the SnO₂/Au film before depositing the MoO₃ layer, it partially bonds to the exposed SnO₂ surface due to its affinity for metal oxides. This bonding is less likely to occur directly with the Au nanoclusters. The chemical bond formed at the SnO₂ surface enhances its stability by passivating potential surface defects. Moreover, the SAM layer can function as the electron blocking layer to reduce the excess electrons injection from SnO₂ into MoO₃, effectively promoting electron injection into the Au recombination center, as illustrated in Supplementary Fig. 2d.
Leveraging the advantages of SAM modification on distinct surfaces of MoO₃, here we propose a bifacial modification approach that employs SAM modifications both above and below the MoO₃ layer, exemplified by the SnO₂/Au/SAM/MoO₃/SAM ICL structure. At the MoO₃/organic interface, the SAM layer enhances hole extraction and acts as an electron-blocking layer to minimize interfacial recombination. Simultaneously, the SAM layer beneath MoO₃ not only controls excess electron injection from SnO₂ to MoO₃ but also facilitates enhanced hole and electron injection to Au from MoO₃ and SnO₂, respectively, ensuring balanced carrier transport and effective charge recombination, as depicted in Fig. 1d and Supplementary Fig. 2e. These modifications have had a significant impact on the performance and stability of the TSCs, which will be discussed in detail later.
To further explore the benefits of SAM-modified MoO₃ acting as HTL for hole transport efficiency, we measured the photoluminescence (PL) spectra for the polymer donor PM6 and PM6:BTP-eC9:L8-BO blend films based on MoO₃ and MoO₃/SAM substrates (Supplementary Fig. 7). Both organic active layer films based on MoO₃/SAM HTL show significant PL quenching, which is attributed to the improved hole extraction capacity facilitated by better energy level alignment at the interface. This conclusion is supported by the higher photovoltaic performance, enhanced EQE response, and reduced series resistance observed in single-junction OSCs employing SAM/MoO₃, MoO₃/SAM, and SAM/MoO₃/SAM-based HTLs (Supplementary Figs. 8 and 9, and Supplementary Table 2). Importantly, these results indicate that SAM-modified HTLs not only enhance charge extraction but also effectively minimize nonradiative recombination losses. Moreover, the weak absorption of the ultra-thin SAM ensures that it introduces negligible optical losses in TSCs (Supplementary Fig. 10).
To investigate the effect of SAM-modified HTLs on the conductivity of ICL, we fabricated four types of devices with the following configurations: ITO/C₆₀/SnO₂/Au/HTL/Ag, where HTL includes MoO₃, SAM/MoO₃, MoO₃/SAM, and SAM/MoO₃/SAM. Compared to the MoO₃ HTL, the series resistance of the ICL based on SAM/MoO₃ and MoO₃/SAM HTLs shows a significant decrease, indicating reduced non-radiative recombination at the Au/HTL and HTL/organic interfaces after the introduction of SAM layer. Notably, the SAM/MoO₃/SAM HTL further reduces the series resistance of the ICL, suggesting a simultaneous reduction in non-radiative recombination at both heterojunction interfaces. As shown in Supplementary Fig. 11, the SAM/MoO₃/SAM ICL shows the lowest series resistance around 4 ohms, which is significantly lower than that of the MoO₃ HTL (~ 12 ohms). Additionally, the series resistance of the MoO₃/SAM ICL is lower than the SAM/MoO₃ configuration, indicating that the carrier transport barrier is primarily located at the MoO₃/organic interface for the OSC subcells. All ICLs demonstrate linear current-voltage (J-V) characteristics, suggesting quasi-ohmic behavior (see Supplementary Fig. 11).
The photovoltaics performances of TSCs based on SAM/MoO₃, MoO₃/SAM, and SAM/MoO₃/SAM HTL have shown significant improvement, achieving PCEs of 24.29%, 24.64%, and 25.47%, with improved V_OC values of 2.163, 2.195, and 2.205 V, respectively, as illustrated in Supplementary Fig. 3. The detailed performance parameters are summarized in Supplementary Table 1. Compared to the SAM/MoO₃ HTL, the TSCs based on MoO₃/SAM HTL show higher performance and lower series resistance. These results are consistent with the changes in ICL conductivity (Supplementary Fig. 11), indicating that more serious interface recombination occurs at the organic/MoO₃ interface in the TSCs. However, single junction OSCs and TSCs based on SAM/MoO₃ HTL demonstrate better performance reproducibility, which may be attributed to improved interface contact between the Au and MoO₃ and reduced interface recombination, as shown in Supplementary Figs. 3 and 8. Notably, both single-junction and tandem SCs with SAM/MoO₃/SAM sandwich HTL achieve high performance and reproducibility.
For a solar cell with optimal carrier transport and radiative carrier recombination, it is expected to be a good light-emitting diode (LED) too. We, therefore, also conducted an electroluminescence (EL) study to better evaluate the carrier transport process in the ICL, which can mitigate the influence of illumination-induced current mismatch. With increasing voltage, a large number of electrons and holes are injected into the perovskite and organic active layers, respectively, from the Au layer, which now acts as a charge generation layer, as illustrated in Fig. 1e, f. Unbalanced carrier transport (e.g., J_e ≠ J_h) between electrons (from Au to SnO₂) and hole (from Au to MoO₃) inevitably leads to additional carrier recombination in the ICL (e.g., J_rec = J_e − J_h), resulting in EL intensity quenching compared to the target devices, as shown in Fig. 1g, h. The SAM/MoO₃/SAM heterojunction as HTL facilitates the smooth hole transport, thus, a stronger EL intensity within the perovskite (at ~ 670 nm) and organic (at ~ 910 nm) layers were observed in the target perovskite/organic TSCs. These enhancements are visible in the inserted photos in Fig. 1g, h. As a result, the target devices exhibit higher current densities compared to control devices when subjected to the same external voltage, as clearly shown in Supplementary Fig. 12. Moreover, obvious EL spectra can be observed in the target devices under lower drive voltage (e.g., 2.8 V) compared to the control devices (e.g., 3.2 V), as displayed in Fig. 1g, h. These results provide clear evidence of balanced carrier transport in the target devices, where the J_e closely matches the J_h. This balance effectively minimizes non-radiative recombination losses within the ICL.
Moreover, it is important to highlight that the EL intensity from the perovskite is stronger than that of the organic layer due to the strong photon radiation property of perovskite materials (Supplementary Fig. 13)¹². Furthermore, the electroluminescence quantum efficiency (EQE_EL) of SAM/MoO₃/SAM sandwich HTL-based single-junction OSCs is observed to be 20 times greater than the MoO₃ configuration, underscoring the significant impact of SAM modification, as illustrated in Supplementary Fig. 13.
Molecules functionalized with phosphonic acids are extensively employed to modify the surfaces of metal oxides such as ZnO, NiO, and ITO, which are commonly used in a variety of electronic devices, including perovskite optoelectronic devices^39,40,41,42. These oxides are basic and can interact with phosphonic acids via acid-based condensation reactions, wherein the deprotonated phosphonate groups form covalent bonds with metal cations. This process, resulting in the release of water, modifies the properties of the oxide surface by enhancing its chemical stability and electronic characteristics.
MoO₃ is considered an acidic oxide primarily due to the high oxidation state of the molybdenum (Mo⁶⁺). The acidic nature of MoO₃ is also influenced by its crystal structure, which can have lattice defects like oxygen vacancies that contribute to its reactive surface properties. Given the unique characteristics of MoO₃ as an acidic oxide, its interaction with phosphonic acid could differ markedly from that with basic metal oxides. The high oxidation state in MoO₃ strongly attracts electron pairs, facilitating its function as a Lewis acid. This affinity for electrons means that MoO₃ can effectively coordinate with the oxygen atoms of deprotonated phosphonic acid groups. Specifically, the formation of coordination bonds can occur at sites of oxygen vacancies on the MoO₃ surface, where the phosphonate and diphosphonate ions (arising from the deprotonation of phosphonic acid) can anchor. These ions, possessing lone pairs of electrons, are prone to interact with the electron-deficient areas of the surface, particularly where oxygen atoms are missing, thus forming stable complexes.
This bonding mechanism significantly influences the electronic properties of the MoO₃ surface, which could potentially affect the electron transfer process between the two materials. Understanding these interactions at a molecular level is essential for optimizing the surface characteristics for specific applications. Therefore, we performed Density Functional Theory (DFT) calculations to gain a deeper insight into the nature of these bonds and their stability under various conditions.
We considered three types of representative SAM-treated surfaces, each modeled with both pristine and surface oxygen-defect (V_O) scenarios, as illustrated in Fig. 2a–c. In addition to studying the surface bonding of the (−PO(OH)₂) anchor group, we also focused on various deprotonated phosphonic acids, including the phosphate ion (−PO₂(OH)^–) and diphosphate ion (−PO₃^2-), labeled as [SAM-ate]^– and [SAM-ate]^-2 respectively. These variants simulate the hydration process occurring during annealing, as illustrated in Fig. 2b, c. The binding energy of all the heterojunction surface systems were calculated and are summarized in Fig. 2d. Scenarios without deprotonation in both pristine and the one with surface oxygen-defect yielded the lowest surface binding energies, at − 2.83 eV and − 2.48 eV, respectively, indicating preferred configurations for MoO₃-SAM interaction. Our DFT results also reveal that deprotonated phosphate and diphosphate ions exhibit small or even positive binding energies with MoO₃, suggesting that these ions are less likely to form bonds with MoO₃.
a–c DFT models of the MoO₃/SAM heterojunction interface for SAM with (− PO(OH)₂), (− PO(O₂H)⁻) and (− PO(O₂)⁻²) groups, respectively, under conditions of pristine surface and surface with oxygen vacancies. d Summarized surface binding energies for the DFT models. e–g XPS signals of P 2p, Mo 3d, and O 1s spectra for different samples. h Possible schematic of the chemical interaction between SAM and MoO₃.
To further probe the role of surface chemistry in bonding interactions, we also investigated the ITO/SAM interface, as shown in Supplementary Fig. 14. This analysis revealed significantly stronger interactions, with a binding energy of − 11.08 eV, compared to the MoO₃/SAM interface. The ITO surface promotes a more efficient deprotonation process, reflecting its distinctive electronic and chemical properties conducive to robust acid-base reactions. This comparison underscores the distinct molecular mechanisms at play across different material interfaces, offering crucial insights for optimizing device performance.
Using XPS, we further analyzed the chemical interactions and electron transfer process between the SAM and MoO₃, as detailed in Fig. 2e–g. Analysis of the Mo 3d spectra revealed shifts in the binding energy for hexavalent Mo⁶⁺ in the 3d_3/2 and 3d_5/2 peaks from 232.98 and 236.11 eV to 233.51 and 236.7 eV, respectively. This shift in binding energies suggests an electron transfer from Mo to the (−PO(OH)₂) group, leading to reduced electron density around Mo and consequently increasing the binding energy.
Concurrently, a prominent peak corresponding to pentavalent Mo⁵⁺ at 232.14 eV is observed after SAM modification. This is attributed to the breaking of Mo – O bonds, which leads to the formation of oxygen vacancies and the partial reduction of Mo⁶⁺ to Mo^{5+ 43}. This likely enhances the hole concentrations in MoO₃, thereby improving its conductivity. For the (−PO(OH)₂) group, the binding energies of P 2p electrons at 134.19 and 132.36 eV were assigned to the P − O and P = O bonds, respectively. Both bonds exhibited shifts to lower binding energies, further supporting the notion of electron transfer from MoO₃.
In addition, as depicted in the O 1s spectra in Fig. 2g, the binding energy at 532.00 eV was attributed to the P = O functional group of the SAM, while the Mo – O bond was observed at 530.85 eV for the MoO₃ sample. Another peak emerged at 532.58 eV in the MoO₃/SAM spectrum, positioned between the P = O and Mo – O peaks, indicating an interaction between the P = O group and Mo. Integrating these XPS findings with DFT calculations, we propose the formation of hydrogen bonds (P − OH···O) and a coordination bond (P = O → Mo⁵⁺/Mo⁶⁺) between the SAM and MoO₃ surface. The proposed mechanisms are illustrated in Fig. 2h.
Balanced carrier transport within the ICL is crucial for minimizing non-radiative recombination and improving device performance in perovskite/organic TSCs. To access the recombination mechanisms and junction quality of TSCs, we analyzed the dark current characteristics of both control and target devices, as depicted in Fig. 3a. Notably, the target devices exhibited a lower dark current density (J₀) of 1.8 × 10⁻⁷ mA/cm², which is two orders of magnitude lower than that of the control devices (1.2 × 10⁻⁵ mA/cm²). This substantial reduction in J₀ verifies the enhanced efficiency of the heterojunction and the reduced non-radiative recombination in the target devices. The non-zero J₀ value under zero bias is likely due to interfacial recombination at the organic/MoO₃ interface and delayed re-emission at defect states in MoO₃⁴⁴. The larger interfacial recombination in the control devices resulted in higher J₀ and larger non-zero bias compared to the target devices. Additionally, the ideality factor of single-junction solar cells was deduced by fitting the slope of the semilogarithmic J-V curve in the diffusion current-dominated region (Supplementary Fig. 15). The dark J-V characteristics of single-junction PSCs revealed an ideality factor of approximately 4.4, which is higher than that of the OSCs with the SAM/MoO₃/SAM HTL (e.g., 3.4), as shown in Supplementary Fig. 15. These results demonstrate that the organic subcell is not the primary factor limiting the performance of TSCs after optimizing the hole transport process.
a Current density under dark conditions for TSCs. b Normalized photovoltage of TSCs. c EQE_FTPS of single-junction OSC and PSC with highlighted Urbach energy. d V_OC loss analysis for the single-junction and tandem solar cells.
Further investigation into charge dynamics, including the charge extraction properties and charge recombination kinetics, was conducted using transient photocurrent (TPC) and transient photovoltage (TPV) decay measurements. As illustrated in Fig. 3b, the fitted TPV decay lifetime of the target TSCs was measured to be 12.9 μs, which is longer than the 5.9 μs observed in the control devices, indicating significant suppressed non-radiative carrier recombination at the junction interface. Additionally, the TPC lifetime in target devices decreased from 0.50 µs to 0.38 µs, as shown in Supplementary Fig. 16, suggesting that the advanced heterojunction structure facilitates better charge extraction.
To quantitatively estimate the total energy loss in perovskite/organic TSCs, we conducted a detailed analysis of the optical and V_OC losses. Supplementary Fig. 17 presents the absorption spectra for each layer and the reflection for the entire TSC devices, alongside the corresponding equivalent current densities. Notably, reflection was identified as the primary optical loss channel in the perovskite/organic TSCs, accounting for 8.65 mA/cm², while losses attributed to the electron/hole transport layers were negligible, at under 0.4 mA/cm². For a more detailed analysis of the V_OC losses in the two subcells, highly sensitive Fourier-transform photocurrent spectroscopy was employed to measure the EQE responses (EQE_FTPS) in the low photon energy region for single-junction PSC and OSCs. The SAM/MoO₃/SAM-based OSC (OSC_target) obtained a higher EQE_FTPS response compared to MoO₃-based (OSC_control) devices, particularly at the photon energies near the bandgap edge, as shown in Fig. 3c. This improved EQE_FTPS response correlates well with the improved J-V characteristics observed in the OSC_target devices, which led to an increase in PCE from 12.81% to 17.73% (Supplementary Fig. 9). These results suggest that the OSC_target devices achieved better carrier transport and a high-quality SAM/MoO₃/SAM heterojunction. To further investigate the band edge disorder in the active layer film, we extracted the Urbach energy (E_U) from the EQE_FTPS spectra. The E_U was calculated to be 17.8 meV for PSC, 22.3 meV for OSC_target, and 24.6 meV for the OSC_control devices, indicating a reduction in band edge disorder for the OSC_target devices. This improvement may be linked to the increased contact angle, which shifted from 19 degrees from the MoO₃ to 76.7 degrees of the MoO₃/SAM sample, as depicted in Supplementary Fig. 18. This notable increase in contact angle leads to a significant reduction in surface energy, thereby minimizing the effects of surface tension during the preparation of the BHJ layer. Consequently, this better matching of surface energy between the HTL and the hydrophobic BHJ layer likely facilitates the formation of a smoother and defect-free interface. Such an improved interface quality is expected to reduce energetic mismatches and promote more efficient charge transfer and extraction at the organic/HTL interface.
To further delineate V_OC loss mechanisms, we considered three typical channels:
where q is the electron charge, ΔV is the total voltage loss, ({V}_{{OC}}^{{SQ}}) is the Shockley-Queisser radiative limiting of V_OC, ({V}_{{OC}}^{{rad}}) is the V_OC considering only the radiative recombination losses.
The term qΔV₁ represents the photon energy loss due to temperature-dependent black-body radiation. At an ambient temperature of 298.15 K, ΔV₁ of single-junction OSC with a bandgap of 1.371 eV is calculated to be around 0.249 eV, and 0.288 eV for a PSC with a bandgap of 1.85 eV, as shown in Fig. 3d and Table 1. The radiative recombination losses, represented by ΔV₂, were calculated to be 0.156 eV, 0.109 eV, and 0.158 eV for PSC, OSC_control, and OSC_target, respectively, based on the EQE_FTPS studies (Fig. 3a). The V_OC loss resulting from the non-radiative recombination, ΔV₃, was calculated from the EQE_EL (Supplementary Fig. 13) using the equation⁴⁵:
The calculated ΔV₃ values for PSC, OSC_control, and OSC_target are 0.045 eV, 0.192 eV, and 0.110 eV, respectively, indicating that single-junction PSC devices show lower non-radiative recombination compared to their OSC counterparts (Fig. 3d). In addition, the OSC with SAM-modified MoO₃ achieved a higher V_OC, which can be attributed to efficient hole extraction that suppresses the non-radiative recombination at the interface. It is notable that V_OC losses in tandem devices often exceed the sum of the V_OC loss from the subcells. These additional V_OC losses primarily stem from the interface recombination within the ICL due to current mismatch or unbalanced carrier transport. In the perovskite/organic TSCs, ΔV₃ was derived from the non-radiative recombination losses in each subcell (({Delta V}_{3}^{{subcell}})) and the ICL (({Delta V}_{3}^{{ICL}})). Apparently, the calculated ({,Delta V}_{3}^{ICL}) of target and control tandem devices were 0.01 eV and 0.06 eV, respectively, demonstrating significantly suppressed carrier recombination in the ICL for the target devices, leading to higher V_OC.
Based on the high-performance single-junction perovskite (Supplementary Fig. 19 and Supplementary Table 3) and organic solar cells, our perovskite/organic TSCs achieve a remarkable champion PCE of 26.05%, with a J_SC of 14.42 mA/cm², FF of 81.75%, and V_OC of 2.21 V, as shown in Fig. 4a. Furthermore, negligible hysteresis was observed, with PCE of 25.23% and 25.60% under forward and reverse scans, respectively, as shown in Fig. 4b and Supplementary Table 4. The balanced carrier transport in ICL ensures good current matching, where the J_sc of 14.24 mA/cm² and 14.1 mA/cm² were integrated from the EQE spectra of the perovskite and organic subcells, respectively (Fig. 4c). We sent an unencapsulated TSC to the third-party certification institute, Enli Tech Optoelectronic Calibration Lab, where it achieved a certified efficiency of 24.53% and a V_OC of 2.216 V, as shown in Supplementary Fig. 20. To the best of our knowledge, this represents one of the highest PCEs, and highest certified V_OC among the reported perovskite/organic TSCs, as displayed in Fig. 4d and Supplementary Table 5. The stabilized power output test was also conducted by tracking at the maximum power point (MPP) for 1200 s, confirming stable PCE output (Supplementary Fig. 21). Lastly, we evaluated the long-term stability of unencapsulated devices under 1-sun illumination in an N₂-filled condition. As shown in Fig. 4e, the control device experienced significant degradation, retaining only 80% of its original PCE after 200 h. On the contrary, the target device demonstrates commendable operational stability, retaining over 84% of its PCE after 650 h at an operating temperature of approximately 40 ± 5 °C. To verify the thermal stability of the target TSCs, we conducted temperature cycling tests between 25 and 85 °C (ISOS-T-1). The results indicate that the target TSCs retained 84.6% of their initial PCE after 500 h. Furthermore, when the TSCs were aged in an ambient environment (relative humidity ~ 65–70%) at 85 °C (ISOS-V-2), the encapsulated TSCs maintained 86% of their initial PCE after 300 h, as shown in Supplementary Fig. 22. These results demonstrate that our TSCs exhibit better light and thermal stability.
a The champion PCE of single-junction subcells and perovskite/organic TSC. b J-V characteristics of TSCs under forward and reverse scans. c EQE spectra for perovskite and organic subcells within the TSC. d Comparative summary of the PCEs for state-of-the-art perovskite/organic TSCs. e Long-term stability tests conducted under AM 1.5 illumination in an N₂ atmosphere for both control and target TSCs.
We have successfully implemented a SAM-anchored MoO₃ strategy to enhance carrier balance and suppress non-radiative recombination within ICL of high-performance perovskite/organic TSCs. Our results reveal that while thermally evaporated MoO₃ exhibits n-doping properties with non-ideal hole extraction capacity. SAM-anchored MoO₃ effectively optimizes the surface potential, energy level alignment, and carrier transport process. Consequently, a SAM/MoO₃/SAM sandwich HTL significantly improves the hole extraction capacity to effectively reduce the interface non-radiative recombination and reach balance carrier transport in ICL. We further substantiated our findings through DFT calculations and XPS analysis, which highlighted the hydrogen and coordination bonds in the interactions between MoO₃ and SAM. This strategic approach has proven effective in drastically reducing non-radiative recombination within both the subcell and ICL, culminating in our perovskite/organic TSCs achieving a remarkable PCE of 26.05% (certified 24.53%), coupled with enhanced operational stability (T₈₀ > 650 h). This work not only elucidates the fundamental mechanisms of carrier balance transport with the ICL but also underscores the critical interactions between SAM and MoO₃. These insights pave the way for the future development of advanced perovskite/organic TSC, marking a significant advancement in tandem photovoltaic technology.
For the materials of perovskite, formamidinium iodide (FAI), methylammonium iodide (MAI), and Cesium iodide (CsI), were purchased from Greatcell Solar Materials Pty. Ltd; and lead iodide (PbI₂) and lead bromide (PbBr₂) were purchased from TCI; C₆₀, was purchased from Xi’an Yuri Solar Co., Ltd. The (4-(3,6-diphenyl-9H-carbazol-9-yl) ethyl) phosphonic acid (Ph-4PACz) and (4-(7H-dibenzo[c,g]carbazol-7-yl)butyl)phosphonic acid (CbzNaph) SAM materials were synthesized from our group. For the OSC materials, PM6, BTP-eC9, L8-BO, and PNDIT-F3N were purchased from Solarmer Materials Inc, MoO₃ was purchased from Sigma-Aldrich. All the solvents, including dimethylformamide (DMF), dimethyl sulfoxide (DMSO), isopropanol, and chlorobenzene were purchased from J&K Scientific. All solvents and chemicals were applied as received from commercial sources without further purification.
For the wide-bandgap (WBG) PSCs, the precursor solution of FA_0.8MA_0.1Cs_0.1Pb(I_0.5Br_0.5)₃ was prepared by dissolving mmol FAI: CsI: MAI: PbI₂: PbBr₂ with the stoichiometric ratio of 0.8: 0.1: 0.1: 0.25: 0.75 in 1 mL of a mixed solvent consisting of DMF and DMSO with a volume ratio of 4:1. A concentration of the precursor solution of 1.1 M was prepared. The perovskite precursor solution was thoroughly dissolved and aged over 2 h before use. The 2.5% PEAAc as the additive was added to the prepared perovskite precursor before use. For preparing the perovskite passivation solution, 0.5 mg of EDADI was added to 1 mL of IPA. The solution of the self-assembled monolayer was prepared by dissolving 1 mg CbzNaph and 0.5 mg Ph-4PACz in 1 mL IPA, respectively. All the solutions were filtered through PTFE syringe filters (0.45 μm) before use.
To construct the tandem cells, the WBG subcells were first prepared and then integrated with the narrow-bandgap organic subcells. The CbzNaph SAM layer was prepared by spin coating the SAM solution on precleaned glass/ITO substrates at 3000 rpm for 30 s followed by 10 min annealing at 100 °C on a hot plate. When the sample cooled to room temperature, the SAM layer was washed using the IPA at 3000 rpm for 30 s, followed by 5 min annealing at 100 °C. Based on a two-step spin-coating process, the perovskite layers were prepared on top of the SAM layer at the first step at 1000 rpm for 10 s and the second step at 4000 rpm for 40 s. In the spin-coating process, 180 μL CB as antisolvent was dripped at 25 s of the second step before the end of the spin-coating program. Then, the sample was quickly transferred to the hot stage to anneal within 10 min. The passivation layer was done by spin-coating EDADI at 5000 rpm followed by 10 min of annealing at 100 °C. Then, the samples were transferred to thermal evaporation equipment to deposit the electron-transport layers (C₆₀~ 20 nm). Further, an interconnection junction consisting of SnO₂/Au/MoO₃ was prepared. A 20 nm SnO₂ layer was prepared by atomic layer deposition (ALD) equipment, and 1 nm Au and 8 nm of MoO₃ were deposited sequentially by thermal evaporation, both at a rate of 0.1 Å/s under a base pressure of ~ 1 × 10⁻⁷Torr. For the target tandem device, the Ph-4PACz was first spin-coated on the Au followed by thermal annealing at 100 °C for 5 min, and then the MoO₃ was evaporated on the SAM. The Ph-4PACz was spin-coated on the MoO₃ at 3000 rpm for 30 s followed by 10 min annealing at 100 °C on a hot plate. When the sample cooled to room temperature, the SAM layer was washed using the IPA at 3000 rpm for 30 s, followed by 5 min annealing at 100 °C. However, for the control tandem device, the active layer was directly spin-coated on the MoO₃. The active layer of the organic subcell with a blend of components (PM6:BTP-eC9:L8-BO with a weight ratio of 1:0.96:0.24, PM6 = 8 mg/mL in chloroform solution) was prepared at 3500 rpm for 30 s, followed by thermal annealing at 80 °C for 10 min on a hot plate. After the active layer solution deposition, a methanol solution (0.5 mg mL⁻¹) of PNDIT-F3N with 0.5 wt% acetic acid was spin-coated on the active layer at 2000 rpm for 25 as the electron transport layer. Finally, an Ag (100 nm) electrode was thermally evaporated on PNDIT-F3N via a customized mask with an active area of 4 mm². Finally, a 140 nm MgF₂ layer was thermally evaporated onto the glass substrate as an antireflection layer. An additional anti-reflection coating was applied for the certificated TSC. For the fabrication of single-junction perovskite and organic subcells, all processes were performed in an N₂-filled glove box. The ALD deposition process was prepared in the ambient atmosphere.
The J-V characteristics were tested in an N₂-filled glovebox and recorded by a Keithley 2400 source meter under AM 1.5 G (100 mW cm⁻²) irradiation using an Enlitech SS-F5 solar simulator. The light intensity was calibrated using a silicon solar cell (with a KG-2 filter) from the National Renewable Energy Laboratory. For the single-junction SCs, EQE spectra were collected by an Enlitech EQE measurement system. For the TSCs, a white light chopper was used and cooperated with the 550 and 850 nm filters for the bottom and top subcells. Directly depositing the PM6 and PM6:BTP-eC9:L8-BO on the glass substrate, PL signals were measured by an FLS980 spectrofluorometer (Edinburgh) with a pulsed excitation laser of 485 nm using a flashing light system (FLS980, Edinburgh Inc.). The XPS and UPS characterization measurements were conducted using a VG ESCALAB 220i XL surface analysis system with a He-discharge lamp (hv = 21.22 eV) and a monochromatic Al Kα X-ray gun (hv = 1486.68 eV), respectively. The MoO₃ sample was exposed to ambient air for several minutes during the transfer process. To minimize any further environmental effects, the sample was then stored in a low-pressure helium gas chamber overnight prior to WF measurements, which were conducted the following day. TPV and TPC decay were measured by the Paios (FLUXiM AG, Switzerland) characterization platform. KPFM images were conducted on Dimension Icon AFM (Bruker) with the tapping mode. Fourier transform photocurrent spectrometry-EQE (EQE_FTPS) was detected using Bruker Optics (Vertex 70) equipped with an external detector. The photocurrent generated by the devices under illumination was amplified with a low-noise current amplifier (SR570), while the light source was modulated by the Fourier transform infrared (FTIR) spectroscope. The electroluminescence (EL) spectra were detected using a Shamrock SR-303i spectrometer (Andor Tech) with a Newton EM-CCD Silicon at − 60 °C. A Keithley 2400 Source Meter was employed to apply bias to the devices. The electroluminescence external quantum efficiency (EQE_EL) values were obtained from a custom-built system consisting of a Hamamatsu silicon photodiode (1010B), a Keithley 2400 Source Meter for voltage application and current capture, and a Keithley 485 Picoammeter for detecting the emitted light intensity. The stability was evaluated by the JINGHE solar cell lifetime testing system (Guangzhou Crysco Equipment Co., Ltd.). The JV curves were continuously captured in a forward scan direction under an LED light source that well matches the AM1.5 G spectrum in the N₂ atmosphere.
The first principles DFT simulations were performed with the Vienna Ab Initio Simulation Package (VASP 6.4) to study the geometric and electronic structures of all the bare and SAM-treated metal oxide surface (MoO₃ and In₂O₃) structures. Unless otherwise specified, the generalized gradient approximation exchange-correlation functional of Perdew-Burke-Ernzerhof (PBE) was adopted in the DFT calculations. The electronic constituents are 2 s 2p for O C and N, 3 s 3p for P, 3 d 4 s for In, 4 d 5 s for Mo, and 1 s for H. For all the bare and SAM-treated metal oxide surface structures, we adopted 2 × 2 × 1 k-point mesh for 2D vacuum surfaces, generated by the Monkhorst-Pack scheme, for detailed properties obtained with PBE functional. For all the isolated structures in large vacuum boxes, we adopt the Γ-only k-point mesh. The projector augmented wave pseudopotentials with the cut-off energy of 600 eV were employed. Considering the Van der Waals interaction between the hydrogen atoms and high-electronegativity groups, the PBE with the DFT-D3 dispersion correction of Grimme with zero-damping was applied to optimize the geometric structures. During the optimization of the geometries, all structures were allowed to relax to ensure that each atom was in mechanical equilibrium without any residual force larger than 10⁻⁴ eV/Å. A 15-Å vacuum layer is adopted on the surface structures to prevent the interaction between the fixed terminal layers. For surface binding energy, we adopt the equation:
in which ε represents the surface binding energy value, E_slab is the optimized energy of the surface structure while E_i(comp) means the optimized energy of every component of the heterostructure in independent vacuum boxes, the S term on the denominator represents the surface area of the structure, this work negates the area term S as 1 in identical surfaces.
The optical model of TSC is developed by addressing Maxwell’s equations based on the COMSOL Multiphysics soft platform. By the simulation, we can obtain the optical characteristics of TSC, including spatial distribution of electromagnetic distribution, reflection, absorption, and transmittance. By checking the position-dependent absorption distribution, the optical absorption percentage of each layer can be quantitatively analyzed. The solved Maxwell’s equations are written as:
where E is the frequency and spatially dependent electric, k₀ is the wave vector, ε_c is the wavelength-dependent relative permittivity of each layer material.
Further information on research design is available in the Nature Portfolio Reporting Summary linked to this article.
The data generated in this study are provided in the Supplementary Information/Source Data file. Source data are provided in this paper.
A Correction to this paper has been published: https://doi.org/10.1038/s41467-025-58944-7
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We thank S. Liu and K. Zhang from the South China University of Technology for providing the TPC and TPV measurement equipment. H.L.Y. acknowledges financial support from the Research Grant Council (RGC) of Hong Kong (GRF No. 11307323), the NSFC/RGC Collaborative Research Scheme (CRS_CityU104/23), the Innovation Technology Fund (GHP/394/22GD and MRP/040/21X), the Green Technology Fund (GTF202020164), and the Seed Collaborative Research Fund (No. SCRF/0069) provided by the State Key Laboratory of Marine Pollution at City University of Hong Kong. Open Access was made possible with partial support from the Open Access Publishing Fund of the City University of Hong Kong.
These authors contributed equally: Yidan An, Nan Zhang.
Department of Materials Science and Engineering, City University of Hong Kong, Kowloon, Hong Kong, China
Yidan An, Nan Zhang, Qi Liu, Gengxin Du, Desui Chen, Ming Liu, Tingfeng Lei, Quanrun Qiu, Xiao Cheng Zeng, Alex K.-Y. Jen & Hin-Lap Yip
Center of Super-Diamond and Advanced Films, City University of Hong Kong, Kowloon, Hong Kong, China
Nan Zhang, Gengxin Du, Tingfeng Lei, Quanrun Qiu & Hin-Lap Yip
Department of Chemistry, City University of Hong Kong, Kowloon, Hong Kong, China
Wenlin Jiang, Xiaofeng Huang, Francis R. Lin & Alex K.-Y. Jen
Hong Kong Institute for Clean Energy, City University of Hong Kong, Kowloon, Hong Kong, China
Francis R. Lin, Alex K.-Y. Jen & Hin-Lap Yip
School of Energy and Environment, City University of Hong Kong, Kowloon, Hong Kong, China
Hin-Lap Yip
State Key Laboratory for Marine Pollution, City University of Hong Kong, Kowloon, Hong Kong, China
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Y.A. and H.L.Y. conceived the idea for the study and designed the experiments. Y.A. fabricated the PSC and ICL, together with N.Z. fabricated the TSCs. Y.A. and N.Z. contributed equally to this work. Q.L. carried out the DFT calculations under the supervision of X.C.Z.; W.J. and G.D. provided the SAM solution under the supervision of F.R.L., and A.K.Y.J. Y.A., N.Z., G.D., D.C., M.L., X.H., Q.Q., and T.L. carried out the EL and other characterization measurements. Y.A., N.Z., and H.L.Y. analyzed the data and wrote the manuscript. All authors contributed to the manuscript.
Correspondence to Hin-Lap Yip.
The authors declare no competing interests.
Nature Communications thanks Fei Zhang, Arafat Mahmud, and the other anonymous reviewer(s) for their contribution to the peer review of this work. A peer review file is available.
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An, Y., Zhang, N., Liu, Q. et al. Balancing carrier transport in interconnection layer for efficient perovskite/organic tandem solar cells. Nat Commun 16, 2759 (2025). https://doi.org/10.1038/s41467-025-58047-3
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Received: 11 September 2024
Accepted: 11 March 2025
Published: 20 March 2025
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DOI: https://doi.org/10.1038/s41467-025-58047-3
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