Achieving a high open-circuit voltage of 1.339 V in 1.77 eV wide-bandgap perovskite solar cells via self-assembled monolayers

Abstract

Severe open-circuit voltage (V_OC) loss significantly hinders the performance improvement of wide-bandgap (WBG) perovskite solar cells (PSCs) and their application in perovskite-based tandem devices. Herein, we develop a novel self-assembled monolayer of (4-(5,9-dibromo-7H-dibenzo[c,g]carbazol-7-yl)butyl)phosphonic acid (DCB-BPA) as the hole-selective layer for WBG PSCs with a 1.77 eV perovskite absorber. DCB-BPA facilitates the subsequent growth of WBG perovskite with improved buried-interface quality. Compared with that of poly(triarylamine) PTAA-based control devices, a substantially enhanced average V_OC from 1.18 V to 1.31 V of DCB-BPA-based devices has been realized due to reduced interfacial nonradiative recombination and enhanced energy level alignment. Our certified device delivers an impressive V_OC of up to 1.339 V and a power conversion efficiency (PCE) of 18.88%, corresponding to a very low V_OC loss of 431 mV (with respect to the bandgap). This enables us to fabricate efficient 4-terminal all-perovskite tandem solar cells with a PCE of 26.9% by combining with a 1.25 eV low-bandgap PSC, demonstrating the promising application of DCB-BPA in tandem devices.

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Broader context

Perovskite-based tandem solar cells (TSCs) hold the promise of surpassing the single-junction efficiency limits by reducing the thermalization losses. Nevertheless, the current wide-bandgap (WBG) subcells using polymeric poly(triarylamine) (PTAA) still suffer from severe open-circuit voltage (V_OC) loss due to considerable nonradiative recombination in the perovskite film and at the perovskite/hole transport layer (HTL) interface. The choice of appropriate HTLs that simultaneously accelerate charge extraction, passivate interfacial defects and regulate the upper perovskite growth is thus an important priority to unite the device efficiency and stability. Among various types of HTLs, self-assembled monolayers (SAMs) show great promise for practical applications owing to their facile synthesis, tunable energy levels, and strong capability for large-area device fabrication. In this work, we demonstrate that replacing PTAA with a well-designed SAM can significantly increase the V_OC of WBG subcells and show how this regulation affects the interfacial characteristics. Our work provides a valuable prospect of rationally engineering perfect charge-selective contacts for WBG subcells, beneficially unveiling a possibility for further promoting the performances of TSCs.

Introduction

The theoretical power conversion efficiency (PCE) of single-junction solar cells is limited to ∼33%,¹ due to inevitable thermal relaxation losses of photon energy above the bandgap. Constructing tandem solar cells (TSCs) with theoretical efficiency above 40% by reducing the thermalization losses and maximizing the usage of the solar spectrum is considered effective to break such a limit,² and great progress has been achieved.^3–7 In TSCs, wide-bandgap (WBG) subcells play an essential role in harvesting high-energy photons and providing high open-circuit voltage (V_OC) for the whole tandem devices.^2,8 Wide bandgap tunability and low-temperature solution processes make the WBG perovskite solar cells (PSCs) the most suitable for top subcells in TSCs. Though many efforts have been proven effective for better device performance in PSCs,^9,10 WBG PSCs used in TSCs still suffer from large V_OC losses, preventing further performance improvement of tandem devices. Even in state-of-the-art tandem devices, the V_OC losses in WBG (1.75–1.80 eV) PSCs used for all-perovskite tandems are still much larger than that in normal-bandgap (1.50–1.60 eV) ones.¹¹ For example, a V_OC of 1.19 V was obtained in FAPbI₃ (the bandgap is 1.51 eV) PSCs with a record efficiency of 25.5%, approaching 96.0% of its V_OC,SQ (V_OC of Shockley–Queisser limit) of 1.24 V,¹² while the V_OC of a 1.78 eV WBG perovskite top cell used in the 28% efficiency all-perovskite TSCs was only 1.274 V (85.3% of its V_OC,SQ).¹³ Finding effective strategies to minimize severe V_OC losses of WBG PSCs is thus of great significance to the development of high-efficiency tandem devices.

Previous studies have investigated the fundamental mechanisms for the large V_OC losses of WBG PSCs and provided some insights. One is attributed to the changed crystallization dynamics in the presence of higher bromine (Br) content, which is prone to induce more defects.¹⁴ The lack of proper charge transport layers is another important reason. In principle, an ideal hole transport layer (HTL) should help to form a compact perovskite absorber with a void-free buried interface, manifesting a simplified process without additional interfacial modifications. However, for the conventional poly(triarylamine) (PTAA) HTL, it is very challenging to achieve high-efficiency WBG PSCs (especially high V_OCs) without an additional pre-treatment such as poly((9,9-bis(3′-((N,N-dimethyl)-N-ethylammonium)-propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene))dibromide (PFNBr), due to its poor wettability for the perovskite precursor and relatively shallow highest occupied molecular orbital (HOMO) energy level (∼−5.1 eV).^15–18 In terms of device fabrication cost, the high price (2000 $ per g) of PTAA also limits its large-scale application.¹⁹ Thus, it is of great importance and necessity to finely design effective and low-cost HTLs for better device performance.

Recently, self-assembled monolayers (SAMs) have become promising HTLs to fabricate efficient inverted PSCs benefitting from their facile synthesis, tunable energy levels, and strong capability for large-area device fabrication.^20–22 By modulating the work function (WF) of the metal oxide substrates while minimizing the HTL/perovskite interfacial V_OC losses, SAMs have gained impressive performance in both WBG and low-bandgap (LBG) PSCs.^23–25 Al-Ashouri et al. used (2-(9H-carbazol-9-yl)ethyl)phosphonic acid in a 1.60 eV perovskite subcell for monolithic perovskite/CIGS TSCs, and realized an average V_OC enhancement of 60 mV and a certified efficiency of 23.26%.²⁶ In our recent work, we found that a spatially twisted SAM can effectively improve the V_OC of 1.77 eV WBG PSCs, enabling 26.4% certified efficiency of 1.044 cm² monolithic all-perovskite TSCs.²⁷ These show the great potential of SAMs to construct highly efficient TSCs, while comprehensive mechanism studies are desired to provide for further molecular design and device engineering.

In this work, we employ a rationally designed SAM, e.g., (4-(5,9-dibromo-7H-dibenzo[c,g]carbazol-7-yl)butyl)phosphonic acid (DCB-BPA) in 1.77 eV WBG PSCs, benefiting from its matched energy levels and defect passivation ability. DCB-BPA enables the growth of high-quality WBG perovskite with an improved buried interface and reduced trap state density, which effectively suppresses interfacial nonradiative recombination losses. We then obtain a substantial average V_OC enhancement from 1.18 V to 1.31 V. Our certified device yields a high PCE of 18.88% with an impressive V_OC of 1.339 V. Further EQE_EL (external quantum efficiency of electroluminescence) measurements quantified the reduced nonradiative V_OC loss of 155 mV in the DCB-BPA device from that of 272 mV in the PTAA control device. We also achieve a 26.9%-efficiency 4-terminal (4-T) all-perovskite tandem device by stacking a DCB-BPA-based semitransparent 1.77 eV WBG top subcell and a 1.25 eV LBG bottom subcell. Our work suggests the promise and significance of the novel SAMs in reducing V_OC losses for WBG PSCs and perovskite-based tandem devices.

Results and discussion

Fig. 1a shows the molecular structure of DCB-BPA and the detailed synthesis and characterization can be found in our previous work.²⁸ Featuring a phosphonic acid anchoring group, DCB-BPA can chemically self-absorb on the indium tin oxide (ITO) surface to form extremely thin and stable layers.²⁹ Due to the steric repulsive interaction between the terminal aromatic rings, the 7H-dibenzo[c,g]carbazole skeleton in DCB-BPA exhibits a unique non-coplanar screw-shaped configuration (Fig. S1, ESI†), which effectively hinders intermolecular π–π stacking to improve the solubility of the resultant molecules.²⁷ The electron-withdrawing Br substituents have the function of lowering the HOMO level of the SAMs while endowing the potential of the SAMs to passivate interfacial defects,³⁰ due to the relatively high negative electrostatic potential in Br atoms illustrated by the electrostatic potential analysis (in red color, Fig. S1, ESI†). Besides, the ultrathin nature and excellent optical transparency of SAMs enable DCB-BPA to exhibit high transmittance over the entire visible region (Fig. S2, ESI†), which is beneficial for obtaining a higher J_SC.

Fig. 1 Molecular structure of DCB-BPA and characterizations of the HTLs on the ITO substrate and perovskite films grown on PTAA and DCB-BPA. (a) Molecular structure of DCB-BPA. (b) and (c) Water contact angles of PTAA (b) and DCB-BPA (c) on the ITO substrate. (d) Energy level diagram between different HTLs and the perovskite. (e) Schematic of minimal interfacial losses enabled by improved buried-interface quality.

We compared the wettability of two HTLs via contact angle measurements. DCB-BPA shows a significantly reduced contact angle of 67°, compared with that of 89° for PTAA (Fig. 1b and c). Better wettability of DCB-BPA than PTAA could help to reduce pinholes in the perovskite films and improve the reproducibility, while a recent work shows that improved wettability of SAMs helps to reduce the void at the buried interface, demonstrating the significance of the good wettability of SAMs.³¹ As the energetic property of the HTL exerts a great influence on the interfacial charge extraction and transport, we thus conducted ultra-violet photoelectron spectroscopy (UPS) measurements to investigate the energy level alignment between DCB-BPA and perovskite (Fig. S3, ESI†). Fig. 1d plots the energy level diagram of the perovskite and different HTLs. A valence band offset of 0.57 eV exists between PTAA and the WBG perovskite, larger than that of 0.12 eV between DCB-BPA and the WBG perovskite. Some works have shown that improved energy level alignment of the HOMO between the HTLs and perovskite benefits the enhancement of the V_OC,³² while SAMs with the HOMO lower than the perovskite layer could lead to decreased V_OC and J_SC.³³ So, the favorable energy level alignment enabled by DCB-BPA may play an important role in enhancing the device V_OC.

To study the impacts of different HTLs on carrier kinetics, we conducted time-resolved photoluminescence (TRPL) measurements on HTL/1.77 eV FA_0.8Cs_0.2PbI_1.8Br_1.2 perovskite. Notably, perovskite film grown on DCB-BPA exhibits a much longer average carrier lifetime than that of the PTAA sample, which even surpasses that of perovskite film grown on glass. The TRPL decays of perovskite film grown on HTL can be roughly divided into two regimes, according to the carrier extraction regime where carrier extraction and carrier trapping occur, and the carrier recombination regime where carrier recombination occurs.^34,35 For the DCB-BPA sample, we see a fast initial decay in early times compared with the perovskite film on glass (Fig. S4, ESI†), demonstrating the carrier transfer from perovskite to DCB-BPA.^26,34 When carrier recombination dominates the TRPL curve, we can see a slower decay of the DCB-BPA sample in the carrier recombination regime, corresponding to a much longer τ₂ of 584 ns for the DCB-BPA sample than 401 ns for the glass sample, which is related with nonradiative recombination in the perovskite layer,²⁶ indicating that the DCB-BPA HTL effectively suppresses nonradiative recombination losses in the perovskite films (Fig. S4 and Table S1, ESI†). We attribute the prolonged carrier lifetime to improved buried-interface quality in the DCB-BPA sample (Fig. 1e), considering that no notable change in crystallinity (Fig. S5, ESI†) and surface morphology (Fig. 2a and d) of WBG perovskite films grown on different HTLs is observed, indicating that DCB-BPA does not significantly change the crystallization of the WBG perovskite. We thus exfoliated the perovskite films from ITO substrates to visualize their buried-interface microstructures. Notably, several nano-sized voids appear at the buried interface of the PTAA sample, while in the DCB-BPA sample, a more homogeneous morphology with no voids is obtained (Fig. 2b and e). Moreover, the PL mapping of the buried interface of the DCB-BPA sample also shows notably improved uniformity and intensity (Fig. 2c and f). All these results confirm much improved homogeneity of the perovskite films on DCB-BPA, which helps to suppress the interfacial non-radiative recombination and enhance the device stability.

Fig. 2 Characterizations of perovskite films grown on PTAA and DCB-BPA. (a) and (d) Top-view SEM images of perovskite films deposited on PTAA (a) and DCB-BPA (d). (b) and (e) SEM images of the buried interface of perovskite films deposited on PTAA (b) and DCB-BPA (e). (c) and (f) PL mapping of the buried interface of the perovskite films deposited on PTAA (c) and DCB-BPA (f).

Encouraged by the aforementioned improvements, we fabricated complete WBG PSCs to demonstrate the potential of DCB-BPA as the HTL in WBG PSCs, as depicted in Fig. 3a. The deposition of DCB-BPA onto ITO was optimized from varying the DCB-BPA concentration, and the corresponding statistics of photovoltaic parameters are summarized in Fig. S6 (ESI†). We note that DCB-BPA shows poor solubility when the concentration exceeds 0.4 mg mL⁻¹, so we failed to try higher concentrations. At an optimal concentration of 0.4 mg mL⁻¹, DCB-BPA-based devices show a notable improvement in average V_OC from ∼1.18 V to ∼1.31 V, compared with PTAA ones (Fig. 3b). Coupled with simultaneous improvements in average fill factor (FF) and short-circuit current density (J_SC) (Fig. S7, ESI†), the average PCE of the DCB-BPA-based devices is 19.18%, higher than that of the PTAA counterpart (16.65%).

Fig. 3 Photovoltaic performance and characterizations of complete WBG devices with different HTLs. (a) Device structure of a complete WBG PSC. (b) and (c) Statistics of V_OC (b) and PCE (c) of WBG PSCs using PTAA and DCB-BPA as HTLs. (d)–(f) J–V curves (d), EQE spectra (e), and MPP tracking under continuous illumination (f) of PTAA and DCB-BPA devices.

Fig. 3d shows the J–V curves of our champion devices measured under different voltage scans, and the detailed photovoltaic parameters are listed in Table S2 (ESI†). The PTAA device delivers a PCE of 17.22% (17.08%) with a V_OC of 1.20 (1.20) V, an FF of 81.28% (80.69%), and a J_SC of 17.67 (17.67) mA cm⁻² under reverse (forward) scan. However, the DCB-BPA device yields a PCE of 19.53% (19.52%), with a V_OC of 1.33 (1.33) V, an FF of 82.70% (82.42%), and a J_SC of 17.75 (17.80) mA cm⁻² under reverse (forward) scan. Both devices exhibit negligible hysteresis. The EQE-integrated J_SCs values are 17.07 and 17.25 mA cm⁻² for PTAA and DCB-BPA devices, respectively, matching well with the J_SCs from the J–V scans (Fig. 3e). We also sent an encapsulated DCB-BPA device to Shanghai Institute of Microsystem and Information Technology for certification (Fig. S8, ESI†), obtaining an efficiency of 18.88% with a V_OC of 1.339 V. Notably, the V_OC (1.339 V) of the certified DCB-BPA device achieved 90% of its V_OC,SQ (1.485 V) (Fig. S9, ESI†), among the highest values of WBG PSCs with a bandgap higher than 1.75 eV (Table S3, ESI†), indicating that DCB-BPA as the HTL significantly mitigates the severe V_OC losses in WBG PSCs. To better understand the origins of the improved V_OC, we compared the performance of the devices with DCB-BPA and 2PACz as HTLs in Figure S10 (ESI†). Although 2PACz shows a favorable HOMO level of −5.6 eV,²⁶ the DCB-BPA device achieves increased V_OC and FF, implying that the 7H-dibenzo[c,g]carbazole core in DCB-BPA benefits the performance improvement as reported in recent works,^27,36 and the modified Br atoms enable favorable energy level alignment and passivation of interfacial defects. To assess the operational stability of the DCB-BPA device, we recorded the efficiency evolution of encapsulated devices by tracking the maximum power point (MPP) at room temperature with ∼50% humidity. In Fig. 3f, the DCB-BPA device shows superior operational stability with over 80% of its initial efficiency maintained after continuous illumination for 427 h. On the contrary, the PCE of the PTAA device decays rapidly to only 64% of its initial value after 131 h.

To gain more insights into the improved V_OC of DCB-BPA-based devices, we performed the Mott–Schottky analysis to evaluate the built-in potential (V_bi) and charge distribution at the HTL/perovskite interface. As shown in Fig. 4a, the V_bi value of the DCB-BPA device is estimated to be 1.05 V, higher than that of the PTAA one (0.94 V). A higher V_bi could provide a stronger driving force for photogenerated carrier separation, facilitating the carrier transfer and resulting in high V_OC. In addition, the DCB-BPA device also possesses a higher slope of the Mott–Schottky plots, which signifies faster charge transfer at the HTL/perovskite interface with reduced carrier accumulation.³⁷ Then, we tested the dark current under bias from −0.1 to 1.2 V to further elucidate the leakage current formed by carrier recombination (Fig. 4b). It is obvious that the DCB-BPA device exhibits a lower leakage current in contrast to the PTAA case, suggesting that DCB-BPA provides a more ideal rectification effect to block the recombination current and allow more photocurrent flow through the device. We examined the ideal factor (n_id) of different devices by light intensity dependent V_OC measurements. In Fig. 4c, the PTAA device shows a n_id of 1.74, while the DCB-BPA device has a reduced n_id of 1.47, indicating reduced SRH recombination in the DCB-BPA device.³⁸ Besides, we also performed electrochemical impedance spectroscopy (EIS) measurements. A higher recombination resistance of the DCB-BPA device shown in Fig. S11 (ESI†) can further demonstrate the suppressed nonradiative recombination.

Fig. 4 Characterizations of WBG devices with different HTLs. (a)–(d) C–V curves (a), dark J–V curves (b), light intensity dependent V_OC (c), and highly-sensitive EQE spectra (d) of PTAA and DCB-BPA devices. (e) EOE_EL of PTAA and DCB-BPA devices recorded under different voltage biases; the inset is the EL image of a DCB-BPA device working as the LED. (f) Nonradiative recombination V_OC losses of PTAA and DCB-BPA devices calculated by the EQE_EL.

To gain a more intuitive comparison of defect-related information, we measured highly-sensitive EQE spectra for PTAA and DCB-BPA devices.³⁹ As shown in Fig. 4d, the EQE curve of the PTAA device reveals the presence of sub gap states (at very low EQEs around 10⁻⁷). Importantly, for the DCB-BPA device, we observed the reduced EQE response belonging to sub gap states, validating the reduced defect state density in the DCB-BPA device.⁴⁰ We further quantitatively evaluated the defect state densities via space-charge limited current measurements.⁴¹ In Fig. S12 (ESI†), the trap-filled limit voltage (V_TFL) decreases from 0.87 to 0.64 V for PTAA and DCB-BPA devices, respectively, corresponding to the decreased defect state densities from 2.43 × 10¹⁶ cm⁻³ for the PTAA device to 1.79 × 10¹⁶ cm⁻³ for the DCB-BPA device,⁴² implying that DBC-BPA HTL could substantially reduce the interfacial nonradiative recombination losses.

We further used EL measurement to quantify nonradiative V_OC losses induced by defect recombination in PTAA and DCB-BPA devices. As shown in Fig. 4e, under the testing voltage range, the DCB-BPA device shows obviously higher EQE_EL. The DCB-BPA device yields an EQE_EL of 0.256% when the injection current equals the J_SC. In contrast, a considerably low EQE_EL of 0.00268% was obtained for the PTAA device. The enhanced EQE_EL value by two orders of magnitude demonstrates the substantially reduced nonradiative recombination losses, agreeing well with the improved V_OC obtained in the DCB-BPA devices. Moreover, we further quantify the nonradiative V_OC losses by the following equation.^43,44

in which the ΔV_OC,nonrad is the V_OC loss of nonradiative recombination, k_B is the Boltzmann constant, and T represents kelvin temperature. According to this equation, a higher EQE_EL means fewer nonradiative recombination losses. We calculated a ΔV_OC,nonrad value of 272 mV for the PTAA device and a much smaller value of 155 mV for the DCB-BPA device (Fig. 4f), indicating that the employment of DCB-BPA can effectively reduce the nonradiative recombination at the DCB-BPA/perovskite interface.

To show the application potential of DCB-BPA in TSCs, we fabricated semitransparent devices by replacing the Cu back electrode with a sputtered indium zinc oxide (IZO) transparent electrode to construct 4-T all-perovskite TSCs with 1.25 eV LBG perovskite bottom cells (Fig. 5a). Our champion semitransparent solar cell yields a PCE of 17.87% with a V_OC of 1.30 V. The LBG solar cell has a PCE of 20.53% and a filtered PCE of 9.03% (Fig. 5b). The detailed photovoltaic parameters of the devices are summarized in Table S4 (ESI†). Eventually, a PCE of 26.90% is achieved, which is among the highest values reported for 4-T all-perovskite TSCs. We note that our 4-T tandem device offers more potential if improved performance of LBG PSCs could be obtained, and the effective light management such as antireflection layers and strategies of parasite absorption reduction is adopted.

Fig. 5 (a) Schematic device configuration of a 4-T all-perovskite TSC. (b) J–V curves of the WBG top cell, original LBG bottom cell, and filtered LBG bottom cell. (c) EQE spectra of the WBG top cell and filtered LBG bottom cell.

Conclusion

We have designed a novel SAM of DCB-BPA to substitute the conventional PTAA HTL to suppress the V_OC loss in 1.77 eV WBG PSCs. DCB-BPA effectively reduces the defect state density at the HTL/perovskite interface, leading to the formation of a high-quality buried interface with minimal nonradiative recombination losses. As revealed by EL, the nonradiative recombination losses induced by DCB-BPA are far less than that induced by PTAA. Moreover, combined with the favorable energy level alignment between DCB-BPA and the perovskite, we achieve a certified high V_OC of up to 1.339 V with a PCE of 18.88%, which is among the highest values reported in WBG PSCs. The DCB-BPA device also exhibits better operational stability, maintaining 80% of its initial efficiency after continuous illumination for 427 h, while only 64% of the PTAA device for 131 h. We also fabricated semitransparent devices to construct a 4-T all-perovskite tandem cell by mechanically stacking with a 1.25 eV LBG PSC, which yields a high PCE of 26.9%. Our work suggests the feasibility and the huge potential of the well-designed SAM as an HTL in WBG PSCs.

Conflicts of interest

The authors declare no competing interests.

Acknowledgements

This work was financially supported by the National Key R&D Program of China (no. 2022YFB4200304), the Fundamental Research Funds for the Central Universities (no. YJ201955 and YJ2021157), the National Natural Science Foundation of China (no. 22375170, 21875111, 51861145401), the Research Funds from Tan Kah Kee Innovation Laboratory (HRTP-[2022]-45), and the Key R&D Program of Natural Science Foundation of Jiangsu Province (BE2019733).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3ee02839a

‡ These authors contributed equally.

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