Rational design of self-assembled monolayer composition for efficient perovskite/Si tandem solar cells

Abstract

Efficient charge extraction at the interface is essential for achieving high-performance perovskite/Si tandem solar cells. Here, we demonstrate a highly efficient wide-bandgap perovskite single-junction solar cell using a co-adsorption strategy of commercial self-assembled monolayers (SAMs), specifically MeO-4PACz and Br-4PACz. Mixed-SAMs, with electron-withdrawing Br functional groups, induce a larger dipole moment and a downward shift in the HOMO level, thereby promoting favorable energy level alignment with a wide-bandgap perovskite. It further passivates interfacial defects, suppressing non-radiative recombination. This combined effect enables effective hole transport at the SAM/perovskite interface, enhancing the open-circuit voltage and fill factor of the perovskite device. Accordingly, the optimal single-junction perovskite solar cell exhibits an efficiency of 19.72%. By integrating this cell with a Si bottom cell, we achieve a tandem solar cell efficiency of 28.02%. This study provides a universal strategy to design SAM-based interfacial layers in p-i-n perovskite and tandem solar cells.

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Introduction

Perovskite/Si tandem solar cells have emerged as a leading candidate for next-generation photovoltaics, capable of surpassing the Shockley–Queisser limit of single-junction solar cells.^1–4 Significant advances have been achieved in recent years, with the certified efficiency of perovskite/Si tandem solar cells reaching 34.9%.⁵ Within these tandem structures, carbazole-based self-assembled monolayers (SAMs) have been widely employed as hole-selective materials due to their superior hole-transporting properties, minimal parasitic absorption, and tunable energy levels.^6–9 To fully utilize these advantages, further optimization is required for wide-bandgap perovskites in tandem solar cells. Previous studies have employed molecular and compositional engineering of SAMs to passivate interfacial defects and achieve favorable energy level alignment, thereby enabling efficient charge transport at the SAM/perovskite interface.^10–12

Since Albrecht et al. introduced [2-(9H-carbazol-9-yl)ethyl]phosphonic acid (2PACz) featuring a carbazole core and a phosphonic acid anchoring group,¹³ methoxy- and methyl-functionalized derivatives have been developed to enhance hole extraction at the SAM/perovskite interface.¹⁴ In addition, tailoring the anchoring and functional groups of SAM molecules altered their dipole moment, energy level, and wettability, thereby mitigating interfacial defects and improving energy level alignment.^15–17 Although such modifications of molecular structures are effective in further enhancing the device performance, they often require complex synthesis processes, imposing considerable challenges for practical applications. To overcome the inherent limitations of single-component approaches, a co-adsorption strategy incorporating additional molecular species has been employed. Zhao et al. blended a non-planar molecule, 3,3-(4-amino-4H-1,2,4-triazole-3,5-diyl)-dibenzo acid (ABT) with [4-(3,6-dimethyl-9H-carbazol-9-yl)butyl]phosphonic acid (Me-4PACz), forming a densely packed layer that improves interfacial bonding and reduces tensile strain.¹⁸ Harter et al. achieved over 30% stabilized efficiency in perovskite/Si tandem solar cells by co-adsorbing functionalized phosphonic acids to improve the wettability of the Me-4PACz layer.¹⁹ However, these additive incorporation strategies can also induce trade-offs, including slower hole extraction and unfavorable shifts in the work function of tandem solar cells.²⁰ Therefore, a facile and versatile approach is needed to realize superior interfacial properties while maintaining compatibility with tandem solar cells.

In this study, we develop a feasible and efficient co-adsorption strategy that utilizes a mixture of commercial SAM molecules (4-(3,6-dimethoxy-9H-carbazol-9-yl)butyl)phosphonic acid (MeO-4PACz) and (4-(3,6-dibromo-9H-carbazol-9-yl)butyl)phosphonic acid (Br-4PACz). By introducing a SAM functionalized with electron-withdrawing Br groups, the work function (WF) was increased and the highest occupied molecular orbital (HOMO) level was lowered, enabling favorable energy level alignment with the wide-bandgap perovskite absorber layer. Furthermore, the introduction of the Br groups suppressed trap-assisted recombination. As a result, hole extraction at the SAM/perovskite interface was enhanced, leading to improvements in both V_OC and FF in perovskite single-junction devices and recording a conversion efficiency of 19.72%. By integrating the perovskite cell on top of a Si bottom cell, a monolithic perovskite/Si tandem solar cell with an efficiency of 28.02% could be achieved.

Results and discussion

Tailoring SAM composition

Fig. 1A presents the molecular structures and co-adsorption strategy of SAM molecules with different substituents, including MeO– and Br– groups. The preferential orientation of SAM terminal groups toward the perovskites affects the interfacial characteristics. While the MeO-substituent is known to passivate defects at SAM/perovskite interfaces, symmetrically positioned terminal groups can reduce dipole moment.^13,14 This reduced dipole moment decreases the WF, impeding hole extraction between the perovskite and SAM.²¹ On the other hand, the electron-withdrawing Br-substituent alters electron cloud density on the carbazole core, resulting in an increased dipole moment and a downward shift in the HOMO level.^22–25

Fig. 1 Electrical properties of the SAMs. (A) Schematic diagram of the co-adsorption strategy and the molecular structures of MeO-4PACz and Br-4PACz. (B) UPS spectra representing the secondary electron cut-off and valence band off-set regions. (C) Contact potential difference histogram of the various SAMs obtained from KPFM. (D) Energy band diagram of the various SAMs and perovskite absorber layers.²⁶

To investigate the effect of the Br-substituent on energy level alignment, we varied the mixing ratio of MeO- and Br-substituents. The detailed conditions of the mixing ratio are provided in the Experimental section (SI). A representative mixed-SAM with a Br : MeO mixing ratio of 4 : 1 was selected because a noticeable work function (WF) shift emerged at this ratio, and results for other ratios are shown in Fig. S1. Fig. 1B shows the ultraviolet photoelectron spectroscopy (UPS) spectra in the valence band onset and secondary cut-off regions, comparing the WF and valence band maximum level (E_VB) of MeO-4PACz, mixed-SAM, and Br-4PACz. The WFs of MeO-4PACz, mixed-SAM, and Br-4PACz are 4.72, 4.92, and 4.97 eV, respectively, revealing that the WF of SAM molecules increases with the increasing Br ratio. More negative contact potential difference (CPD) values with higher Br ratios, as measured by Kelvin probe force microscopy (KPFM), further confirm the increase in WF (Fig. 1C).

Fig. 1D shows the energy band diagram of various SAM molecules and perovskites measured by UPS. The electron-withdrawing nature of the Br-substituent reduces the electron cloud density,^25,27 resulting in the lower HOMO levels of Br-4PACz (−5.76 eV) and mixed-SAM (−5.69 eV) compared to MeO-4PACz (−5.4 eV). This trend is clearly observed for various mixing ratios (Fig. S2), indicating that favorable alignment with perovskites can be achieved by adjusting the SAM composition.

Perovskite cell performance

To examine the influence of the Br-substituent on device performance, we fabricated single-junction wide-bandgap perovskite solar cells consisting of an ITO/SAM/perovskite/C₆₀/PEIE/Ag structure. The composition of the wide-bandgap (E_g = 1.67 eV) perovskite is Cs_0.05(FA_0.77MA_0.23)_0.95Pb(I_0.77Br_0.23)₃ (Fig. S3), which is suitable for application in perovskite/Si tandem solar cells. Fig. 2A shows the current density–voltage (J–V) curves of perovskite devices for various Br : MeO ratios. The photovoltaic and diode parameters derived from the dark J–V curves are summarized in Table S1. According to our preliminary experiments (Fig. S4), the optimal Br : MeO ratio was demonstrated to be 4 : 1, as noted for the mixed-SAM. The device incorporating a pure Br-4PACz SAM did not function properly, likely due to poor junction properties characterized by high series resistance (R_S = 635.79 Ω cm²) and a low FF. One possible origin of the high R_S is inappropriate energy band alignment with the perovskite layer.^28–30 A direct comparison between MeO-4PACz and mixed-SAM reveals that Br substitution enhances power conversion efficiency (PCE), notably increasing the open-circuit voltage (V_OC) and FF, owing to improved energy level alignment at the SAM/perovskite interface. However, there are no significant changes in the short-circuit current density (J_SC). This tendency is further demonstrated in the statistical photovoltaic parameters shown in Fig. 2B. Consequently, the best-performing mixed-SAM-based device achieved the best PCE of 19.72% with a J_SC of 20.41 mA cm⁻², a V_OC of 1.20 V, and a FF of 0.81. Fig. 2C shows the external quantum efficiency (EQE) curves and integrated J_SC of the perovskite device, with no considerable difference between the MeO-4PACz and mixed-SAM, which is consistent with the J_SC obtained by J–V curves. The Br functional groups were found to have a negligible effect on the absorbance, grain morphology, and crystallinity of the perovskite absorber layers (Fig. S5–7), yielding comparable J_SC for both SAMs.²⁸

Fig. 2 Single-junction device characteristics and charge dynamics (A) J–V curves of the champion perovskite single-junction solar cells. (B) Statistical photovoltaic parameters of the perovskite device with MeO-4PACz and a mixed-SAM. This statistical representation shows the minimum and maximum values as crosses, the mean value as a circle, and the box indicating the 25th to 75th percentile range. (C) EQE and integrated J_SC curves. (D) Lifetime calculated from EIS results using the equation τ = 1/(2πf_max), where f_max is the characteristic frequency in the Bode plot. (E and F) TR-PL decay curves of the perovskite layers. The schematic of (E) shows the results from perovskite-side excitation of the ITO/SAM/perovskite film using a 520 nm laser, while the schematic of (F) shows the spectra from SAM-side excitation of the ITO/SAM/perovskite/C₆₀ film with a 405 nm laser.

The electrochemical impedance spectroscopy (EIS) reveals that the charge carrier lifetime of the mixed-SAM (5.458 μs) is longer than that of MeO-4PACz (0.928 μs), as shown in Fig. 2D,^29,30 which is consistent with the transient photovoltage decay (TPVD) curves (Fig. S8) showing an extended charge carrier lifetime of the mixed-SAM. These observations can be ascribed to the effective defect passivation at the SAM/perovskite interface by the Br substituent.³¹ The semicircle in the low frequency region of the Nyquist plot (Fig. S9), corresponding to the charge recombination resistance (R_rec), reveals that the mixed-SAM has a higher R_rec (126.41 Ω for the mixed-SAM and 21.5 Ω for MeO-4PACz), further supporting suppressed charge recombination by Br substitution.^32,33 These results suggest that both interfacial energy alignment and defect suppression contribute to the enhancement of V_OC. To investigate the charge transfer characteristics at the SAM/perovskite interfaces, we conducted steady-state (SS) and time-resolved (TR) photoluminescence (PL) measurements. Fig. 2E and F present TR-PL curves, while the inset illustrates the sample structure, the excitation wavelength, and the laser irradiation direction. The penetration depth varies with the excitation wavelength (Fig. S10). For instance, the 405 nm laser probes the surface (∼27 nm), whereas the 520 nm laser extends into bulk (∼217 nm).^10,17,34,35 Under 520 nm excitation from the perovskite side, the mixed-SAM exhibits a longer average carrier lifetime (τ_avg = 34.8 μs) than MeO-4PACz (τ_avg = 22.3 μs). This is consistent with higher SS-PL intensity observed in the mixed-SAM, indicating reduced non-radiative recombination (Fig. S11).^36,37 Meanwhile, the mixed-SAM shows a faster decay under 405 nm excitation from the SAM side, resulting from improved interfacial charge extraction.^38,39 Under these excitation conditions, both MeO-4PACz and mixed-SAM samples exhibit a comparable degree of quenching (Fig. S12).³⁴ Therefore, the enhanced FF by the Br substitution can be attributed to the facilitated hole extraction at the SAM/perovskite interface.

Tandem cell performance

We fabricated two-terminal (2T) perovskite/Si tandem solar cells using the best wide-bandgap perovskite solar cell incorporating the mixed-SAM layer. Further details about the tandem device fabrication are provided in the Experimental section. Prior to the tandem integration, the single-junction transparent perovskite solar cell was optimized to enable light illumination from the top electrode rather than the glass substrate. The resulting semi-transparent perovskite solar cell exhibits a J_SC of 19.37 mA cm⁻², a V_OC of 1.20 V, a FF of 76.7%, and a PCE of 17.81%, with an aperture size of 0.1875 cm² (Fig. S13). Fig. 3A shows the J–V curve of the best performing perovskite/Si tandem solar cell consisting of a silicon heterojunction (SHJ) bottom cell and a mixed-SAM-based perovskite top cell. This device exhibits a J_SC of 19.33 mA cm⁻², a V_OC of 1.90 V, a FF of 76.3%, and a PCE of 28.02%, with an aperture size of 1 cm². The steady-state power output (SPO) curve yields a stabilized PCE of 27.56% (Fig. 3B). Fig. 3C presents the J_SC of each sub-cell, demonstrating effective current matching between the top and bottom cells, with values comparable to those derived from the J–V curve.

Fig. 3 Tandem device performance. (A) J–V curves, (B) SPO measured at the maximum power point, and (C) EQE spectra of each subcell of the champion 2T perovskite/Si tandem solar cells.

Conclusion

We examined the effects of the functional groups in SAM molecules by varying the mixing ratios between MeO-4PACz and Br-4PACz. Incorporation of the electron-withdrawing Br-substituent induced a downward shift in the HOMO level of SAM molecules, leading to a favorable energy band alignment with the wide-bandgap perovskite absorber. It also enabled efficient hole extraction at the SAM/perovskite interface. Furthermore, the Br-substituent alleviated interfacial defects, effectively reducing charge recombination losses. The combined effects of accelerated charge extraction and suppressed non-radiative recombination at the SAM/perovskite interface significantly enhanced the V_OC and FF of the single-junction perovskite solar cell. By integrating the optimized perovskite top cell with a SHJ bottom cell, we achieved a 2T perovskite/Si tandem solar cell with a PCE of 28.02%. The effect of mixing SAM molecules investigated in this study offers a simple yet effective strategy to tune the electronic properties of SAMs for highly efficient p-i-n perovskite solar cells and tandem devices.

Author contributions

M. S. K. carried out the overall experiments, including device fabrication and characterization. P. S. J. contributed to the conceptual development of the study, provided critical feedback throughout the research, and fabricated the tandem solar cells. G. P. H. assisted with the stability analysis of the tandem devices. J. Y. K. contributed to conceptual discussions and managed and supervised the research project.

Conflicts of interest

The authors declare no competing interests.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information is available. See DOI: https://doi.org/10.1039/d5nr04517g.

Acknowledgements

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIT) (RS-2023-00273532) and the Korea Institute of Energy Technology Evaluation and Planning (KETEP), and the Ministry of Trade, Industry and Energy (MOTIE) of the Republic of Korea (RS-2023-00302107). This work was suppoted by Mid-Career Bridging Program through Seoul National University.

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Footnote

† These authors contributed equally to this work.

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