Although the power conversion efficiency (PCE) of perovskite solar cells (PSCs) has already reached 26.1%, the interfacial nonradiative recombination losses still limit their development. Self-assembled monolayers (SAMs) as promising hole-selective layers (HSLs) in inverted PSCs enable lossless contacts and minimize interfacial recombination. The development of SAMs based on new functional groups is still in urgent demand. Here we present a novel indolo[3,2-b]carbazole-based SAM with an ethylphosphonic acid anchor, namely 2PICz, as an HSL in inverted PSCs. The use of the indolo[3,2-b]carbazole group distinctly improves the surface wettability of the film, which is beneficial for forming high-coverage perovskite films. Besides, 2PICz shows a favorable energy level alignment with perovskite and reduces the trap density of perovskite film. As a result, PSC employing 2PICz shows a PCE of 25.51% (25.28% certified) with an open-circuit voltage of 1.18?V and good storage stability that maintains 98% PCE in a nitrogen atmosphere for 3000?h and over 87% PCE after heating at 65?°C for 2000?h. In addition, a 1-square centimeter cell shows a PCE of 24.17%. The insights obtained from this study will drive the development of novel SAMs for high-performance solar cells.
Introduction
Perovskite solar cells (PSCs) have received great attention and have been rapidly developed with certified power conversion efficiency (PCE) of 26.1% being achieved to date, comparable to those of silicon solar cells.1–6 The simple solution processing, low manufacturing temperature, and compatibility with flexible and tandem devices make inverted PSCs a promising candidate for commercialization.7–10 Therefore, the development of inverted PSCs is beneficial for the commercial application of PSCs. However, the record PCE of single junction PSCs still lags behind the Shockley–Queisser limit. One of the main reasons is that undesirable defect formation occurs on the surface and grain boundaries of the polycrystalline perovskite during the film processing, leading to numerous nonradiative recombination centers adverse to the device efficiency and stability.11,12 Hole-transporting materials (HTMs) play a critical role in inverted PSCs in terms of hole extraction and perovskite crystal growth.13–15 The commercial HTM, poly[bis(4-phenyl)(2,4,6-trimethylphenyl]amine (PTAA), has disadvantages of energy level mismatch, batch-to-batch variability, and poor wettability.16,17 Therefore, tremendous efforts have been undertaken to develop novel HTMs for inverted PSCs as a substitute for PTAA.18–20
Recently, the PCE of inverted PSCs has been rapidly improved by using self-assembled monolayers (SAMs) as hole-selective layers (HSLs).21–23 SAMs also have advantages of facile processability, low cost, and minimal material consumption. SAMs can bond to the surface of a substrate and spontaneously form ultrathin layers. To date, the most popular SAMs employed in inverted PSCs are [2-(9H-carbazol-9-yl)ethyl]phosphonic acid (2PACz),24 [4-(3,6-dimethyl-9H-carbazol-9-yl)butyl]phosphonic acid (Me-4PACz),25 and [2-(3,6-dimethoxy-9H-carbazol-9-yl)ethyl]phosphonic acid (MeO-2PACz).26 Due to less optical interference, the fluorine-doped tin oxide (FTO) substrate was found to have higher photocurrent in replacing indium tin oxide (ITO) substrate for SAMs adsorption in inverted PSCs.27 However, the cluster formation during the adsorption process of SAMs on FTO substrate easily leads to the inhomogeneity of SAM.28 Several strategies, such as utilizing ultrathin oxide layer,29 co-adsorption strategy,28 and thermal evaporation technique,30 have been proposed to solve this problem. Furthermore, the development of novel SAMs with different functional groups is also an important means.
The indolo[3,2-b]carbazole (ICZ) with rigid conjugated electron-rich aromatic ring has been widely employed in organic light-emitting diodes,31,32 organic thin-film transistors,33 and solar cells,34,35 due to its structural symmetry and high planarity. Sun and coworkers36 first reported ICZ core-based dopant-free HTMs in PSCs and obtained an efficiency of 17.7%. Recently, Ge and coworkers37 attained a champion PCE of 25.15% by improving the molecular dipole moments and strengthening the π–π interactions between the indolocarbazole units by means of the modulation of the position of two nitrogen atoms. However, the application of ICZ-based SAMs in inverted PSCs is relatively limited. Here, we present an ICZ-based SAM, {2-(11H-indolo[3,2-b]carbazol-5-yl)ethyl}phosphonic acid (abbreviated as 2PICz), with an ethylphosphonic acid anchor. The use of indolo[3,2-b]carbazole group is beneficial for improving the surface wettability of the film, leading to the growth of perovskite films with high coverage. Besides, 2PICz helps to reduce the defect density of perovskite and shows a favorable energy level alignment with perovskite. At the same time, we also synthesized a SAM containing two ethylphosphonic acid anchor, (indolo[3,2-b]carbazole-5,11-diylbis(ethane-2,1-diyl))bis(phosphonic acid) (abbreviated as 2BPICz), similar to the reported molecule (IDCz-1).37 Unfortunately, compared to 2PICz, 2BPICz shows poor solubility in ethanol and dimethylformamide (DMF) ( Supporting Information Figure S1). The PSC based on 2PICz presents a champion PCE of 25.51% (certified 25.28%) in small-area (0.071?cm2). Meanwhile, the 2PICz-based PSCs exhibit high stability, maintaining 98% and 87% of their initial PCE in a nitrogen atmosphere for 3000?h and at 65?°C for 2000?h.
Experimental Methods
Synthesis of diethyl{2-(11H-indolo[3,2-b]carbazol-5-yl)ethyl}phosphonate (1)
To a solution of 5H,11H-indolo[3,2-b]carbazole (256?mg, 1.0?mmol) in dry DMF (10?mL),
NaH (40?mg, 1.0?mmol) was added and stirred for 10?min at room temperature under N2. Then, diethyl 2-bromoethylphosphonate (300?mg, 1.2?mmol) was added and the mixture
was heated at 70?°C overnight. After cooling down to room temperature, the reaction
mixture was poured into water, extracted with dichloromethane, and washed with brine.
The organic phase was dried over MgSO4, and concentrated under reduced pressure to give the crude product. The obtained
crude product was purified by silica gel column chromatography (ethyl acetate: methanol?=?100:2)
to give 270?mg of
Synthesis of 2PICz
Compound
Device fabrication
The prepatterned transparent conducting oxide glass substrates (FTO) were cleaned by sonication with detergent, deionized water, acetone, and isopropanol for 20?min each. Then the clean glasses were dried with nitrogen and treated with ultraviolet ozone for 20?min. A 0.5?mg·mL?1 2PICz SAM solution dissolved in ethanol was spin-coated on substrates at 3000?rpm for 30?s in a nitrogen glovebox, followed by annealing at 100?°C for 10?min. The perovskite composition is Rb0.05Cs0.05MA0.05FA0.85Pb(I0.98Br0.02)3. The perovskite precursor was prepared in mixed solvents of DMF:DMSO (volume ratio 4:1) and the concentration was 1.7?M. The perovskite precursor was spin-coated at 1000?rpm for 10?s and 6000?rpm for 40?s. Two hundred microliters of chlorobenzene was dropped onto the film 10?s before the end of the spin-coating process. The film was annealed at 100?°C for 10?min. A solution of 1?mg·mL?1 1,3-diaminopropane dihydroiodide (PDADI) in isopropanol (IPA) was spin-coated on the perovskite surface at 4000?rpm for 30?s, followed by annealing at 100?°C for 5?min. Afterwards, [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) (20?mg·mL?1) solution in chlorobenzene (CB) was spin-coated on the film at 1500?rpm for 30s. After that, bathocuproine (BCP) (2?mg·mL?1) solution in IPA was dropped on the film at 5000?rpm for 30s, followed by annealing at 90?°C for 5?min. All the spin-coating procedures were conducted in an N2-filled glovebox with the contents of O2 and H2O?>?5?ppm. Finally, 70?nm silver was thermally evaporated at a rate of 1?? s?1 as an electrode. All solution was filtered through a 0.22?μm pore poly(tetrafluoroethylene) filter before use. Unless otherwise stated, the devices were masked with metal aperture masks (0.071?cm2) during the J–V measurement.
More experimental details are available in the Supporting Information.
Results and Discussion
The synthetic pathway of 2PICz is shown in Figure?1a. The phosphonic acid ethyl ester derivative (
Figure 1 | (a)?Synthetic route of 2PICz. (b)?Absorption spectra of 2PACz and 2PICz in THF solution.
(c)?HOMO image of 2PACz and 2PICz.
Cyclic voltammetry measurements were conducted to study the electrochemical properties
of 2PACz and 2PICz and the HOMO energy levels were estimated to be ?5.36 and ?5.08?eV,
respectively ( Supporting Information Figure S4). To determine the energy levels of 2PICz and 2PACz on FTO, ultraviolet photoelectron
spectroscopy (UPS) was performed ( Supporting Information Figure S5). The HOMO energy level of 2PICz was estimated to be ?5.18?eV, which was distinctly
up-shifted compared to that of 2PACz (?5.83?eV) (Figure?2a), further corroborated by the DFT calculations results above (Figure?1c), which show a predicted HOMO level of ?4.93 and ?5.42?eV, for 2PICz and 2PACz, respectively.
The perovskite material used in this work is Rb0.05Cs0.05MA0.05FA0.85Pb(I0.98Br0.02)3 (RbCsMAFA), which has a valence band of ?5.68?eV. This indicates that 2PICz would
be favorable for hole transfer from perovskite, while there is an energy alignment
mismatch between 2PACz and RbCsMAFA. Based on their optical bandgaps, the lowest unoccupied
molecular orbitals of 2PICz and 2PACz were calculated to be ?2.12 and ?2.31?eV, respectively.
Figure 2 | (a)?Schematic diagram of energy level arrangement. (b)?Sn 3d core level XPS spectra
of the bare FTO and FTO/2PICz films. (c)?The contact angles of perovskite precursor
solutions on FTO/2PICz (left) and FTO/2PACz (right). (d)?Conductive atomic force microscopy
images of the FTO/2PICz (left) and FTO/2PACz (right) films.
The Sn 3d X-ray photoelectron spectroscopy (XPS) spectra of the bare FTO, FTO/2PACz, and FTO/2PICz samples are shown in Figure?2b and Supporting Information Figure S6. The shift of the Sn 3d peak of FTO after molecular modification toward the higher binding energy region suggests that there is an interaction between the phosphonic acids and FTO, which is key to SAM formation.39 As for the active hydrogen in 2PICz, XPS spectra illustrate that it has no interaction with either Pb or I of perovskite ( Supporting Information Figure S7). Contact angle measurements were employed to assess the surface wettability of the SAM-modified FTO substrates. The contact angles of perovskite precursor solutions on 2PICz- and 2PACz-modified substrates were measured to be 7.1° and 13.4°, respectively (Figure?2c). The perovskite solution was fully spread on the FTO/2PICz substrate after 30?s, while the contact angle of the perovskite solution on the FTO/2PACz substrate remained unchanged after the same period of time. The quick solution spreading validates that the 2PICz-modified substrate shows an enhanced affinity for perovskite precursor solution. As shown in Supporting Information Figure S8, the perovskite films fabricated on 2PICz displayed full coverage of the substrate, but could not be achieved on 2PACz. We measured the electrical properties of the SAM films using conductive atomic force microscopy (Figure?2d and Supporting Information Figure S9). Under a 1?V bias voltage, FTO/2PICz had a higher current than that of the FTO/2PACz film, suggesting that the 2PICz film has better conductivity.
X-ray diffraction (XRD), scanning electron microscopy (SEM), and atomic force microscope measurements were performed to characterize the crystallinity and morphology of the perovskite films deposited on different SAMs. XRD spectra in Supporting Information Figure S10 show that the crystallinity of perovskite films is not significantly influenced by different SAMs. In addition, the perovskite films grown on different SAMs do not show notable morphological changes in the top-view SEM images ( Supporting Information Figure S11). As shown in Supporting Information Figure S12, the difference between optical band gaps of the perovskite films deposited on different SAMs is negligible, with both band gaps close to 1.54?eV. The root mean square roughness of the perovskite film deposited on 2PICz (23.3?nm) is nearly the same for the perovskite deposited on 2PACz (22.1?nm) ( Supporting Information Figure S13).
To examine the effectiveness of 2PICz as an HSL in PSCs referenced to 2PACz, we fabricated
inverted PSCs with a configuration of FTO/HSL/perovskite (PVK)/PCBM/BCP/Ag. The concentration
of 2PICz has been determined to be 0.5?mg/mL by preliminary experimentation ( Supporting Information Figure S14). The cross-section SEM image of the device is shown in Supporting Information Figure S15. Small-area (0.071?cm2) PSCs with 2PICz showed a PCE of 25.51% with an open-circuit voltage (Voc) of 1.179?V, a short-circuit current density (Jsc) of 25.69?mA cm?2, and a fill factor (FF) of 84.24% (Figure?3a and Supporting Information Table S1). This PCE significantly surpassed that of PSCs based on 2PACz (PCE?=?17.48%, Jsc?=?23.47?mA cm?2, Voc?=?1.140?V, and FF?=?65.26%). Considering that 2PACz-based devices exhibit poor performance, devices
based on MeO-2PACz were also fabricated ( Supporting Information Figure S16), which show comparable high efficiencies as the 2PICz-based devices. This suggests
that the device fabrication process has been well optimized. To further reveal the
reasons for the low FF and Jsc of 2PACz-based devices, XPS mapping was performed to investigate the homogeneity
of the deposited SAMs. As shown in Supporting Information Figures S17 and S18, the distribution of P on the FTO/2PICz surface exhibits good uniformity in different
surface zones. In contrast, the three zones of the FTO/2PACz sample appear to have
quite different distributions of P, illustrating the heterogeneity of 2PACz deposited
on FTO. As the shown in UPS measurements (Figure?2a), the Fermi level of anode surfaces is ?5.05?eV for FTO/2PACz and ?4.66?eV for FTO/2PICz,
respectively. Although 2PACz has a deeper Fermi level, its device performance is much
poorer with respect to 2PICz. The poor performance of 2PACz is ascribed to its inhomogeneous
film formation under the same fabrication conditions. The certified efficiency of
the unencapsulated 2PICz-based device by the third-party organization (National Institute
of Metrology, China) was 25.28% (with FF?=?84.15%, Jsc?=?25.43?mA cm?2, and Voc?=?1.181?V) ( Supporting Information Figure S19). The external quantum efficiency (EQE) spectra of the corresponding devices were
measured to ensure the reliability of data (Figure?3b). The integrated Jsc of the 2PICz- and 2PACz-based devices are 25.49 and 23.13?mA cm?2, in good agreement with the J–V measurements. The bandgap of the devices was estimated to be 1.54?eV by EQE spectra,
consistent with the optical bandgap calculated from UV–vis spectra. The stabilized
power output of the PSCs was measured at the maximum power point (MPP) with an applied
voltage of 1.02 and 0.86?V for the 2PICz- and 2PACz-based devices (Figure?3c), respectively. The 2PACz-based device obtained a PCE of 15.3%, whereas the 2PICz-based
device achieved a superior efficiency of 24.8% with minor fluctuation. When the active
area is scaled up to centimeter-scale (1.0?cm2) (Figure?3d), the 2PICz devices achieved a PCE of 24.17% (with Voc?=?1.155?V, Jsc?=?26.12?mA cm?2, and FF?=?80.10%). The integrated Jsc of the 1-cm2 device based on 2PICz calculated from the EQE spectra is 25.90?mA cm?2 (Figure?3e). The statistical distributions of the photovoltaic parameters of the devices are
shown in Figure?3f and Supporting Information Figure S20, indicating a high reproducibility of the device performance with 2PICz.
Figure 3 | (a)?J–V curves and (b)?EQE spectra of the champion devices employing the 2PICz and
2PACz HSLs. (c)?Steady-state power output curves of the 2PICz- and 2PACz-based PSCs
tracked at MPP under AM 1.5G illumination. (d)?J–V curves and (e)?EQE spectrum of
the champion devices with 2PICz with an aperture area of 1?cm2. (f)?PCE distribution of the devices employing the 2PICz and 2PACz HSLs.
Space charge limited current measurements were used to determine the trap density
(Nt) of the perovskite with a device architecture of FTO/SAMs/perovskite/MoO3/Ag to better understand the origin of this improvement.40 The Nt can be obtained from the equation: Nt?=?2VTFL??0/qL2, where VTFL is the trap-filled limit voltage, and L represents the thickness of perovskite film (780?nm). The VTFL values determined from the dark J–V curves were extracted to be 0.70 and 0.86?V for the 2PICz- and 2PACz-based devices,
respectively (Figure?4a,b). The corresponding Nt values were calculated to be 3.3?×?1015?cm?3 and 4.06?×?1015?cm?3 for the 2PICz and 2PACz, respectively. The decreased trap density with the use of
2PICz is expected to improve the Voc and FF of corresponding solar cells. We also evaluated the interfacial recombination properties
of the SAM/perovskite films by performing light intensity-dependent Voc measurements (Figure?4c). The ideality factors (n) can be measured from the slope of the dependence of Voc on the logarithm of the light intensity according to the equation n?=?q/kBT·dVoc/d(ln(L)).41 The ideality factors for PSCs were 1.22 for 2PICz-based devices and 1.55 for 2PACz-based
devices. The lower ideality factor for the 2PICz-based device is in connection with
its improved reduced trap density and hole extraction, leading to a higher Voc and FF. The dark currents of devices based on different SAMs were analyzed in Figure?4d, which shows lower current leakage for 2PICz-based devices.
Figure 4 | J–V characteristics of hole-only devices with 2PICz (a)?and 2PICz (b)?HSLs. (c)?Voc as a function of illumination intensity of the PSCs fabricated on 2PACz and 2PICz
HSLs. (d)?Dark J–V curves of the devices with 2PACz and 2PICz HSLs. (e)?Steady-state
and (f)?TRPL spectra of perovskite/HSLs.
The steady-state photoluminescence (PL) and time-resolved PL (TRPL) measurements were performed to investigate the hole extraction ability of SAMs. Compared to 2PACz, the emission intensity is significantly reduced for the perovskite deposited on 2PICz (Figure?4e), indicating that 2PICz exhibits more efficient hole extraction than 2PACz. This also further illustrates the energy level mismatch between 2PACz and perovskite. Moreover, the perovskite film on 2PICz shows a shorter lifetime (0.48?μs) (Figure?4f and Supporting Information Table S2). The longer PL lifetime of perovskite film on 2PACz is attributed to its poor hole extraction at the bottom interface and the energy level mismatch between 2PACz and perovskite. To support that, an insulating layer of Al2O3 was added between the SAMs and FTO substrate. The samples with Al2O3 display a much longer PL lifetime, illustrating that the decreased charge transfer at the buried interface increases the PL lifetime ( Supporting Information Figure S21).
To further evaluate the long-term stability of PSCs, the performance of the unencapsulated
device was tracked in a nitrogen atmosphere. The 2PICz-based PSC maintained 98.6%
of the initial efficiency after 3000?h shelf storage whereas the 2PACz-based device
remained at 83.9% in the same condition (Figure?5a). Then, we investigated the high-temperature stability (65?°C, N2 atmosphere) for the unencapsulated 2PICz-based device (Figure?5b). The device retained 87.5% of its initial efficiency after 2000?h continuous heating,
exhibiting good thermal stability. Finally, we also performed MPP tracking of unencapsulated
PSCs with 2PICz in a N2 glovebox (Figure?5c). Operated under 1-sun illumination (achieved by a white-light light-emitting diode
(LED) array) at the MPP, the 2PICz-based PSC maintained 80% of its initial efficiency
after 650?h. This suggests that 2PICz improves the stability of PSCs.
Figure 5 | Stability test in the inert atmosphere at 25?°C (a)?and 65?°C (b)?for the unencapsulated
devices. (c)?MPP stability tracking of the unencapsulated 2PICz-based device with
simulated 1 sun illumination (white LED).
Conclusion
In summary, we designed an indolo[3,2-b]carbazole-based SAM (2PICz) with an ethylphosphonic acid anchor, which enables improved surface wettability of the film. The incorporation of the additional indole unit in 2PICz is beneficial for not only better energy level alignment with perovskite by elevating the HOMO level, but also for reducing the trap density of perovskite film. Furthermore, the π-conjugated indolo[3,2-b]carbazole in 2PICz facilitates hole extraction and transport. As a result, the champion PSCs using 2PICz as the HSL achieved an excellent PCE of 25.51% (0.071?cm2) with an open-circuit voltage of 1.18?V. The certified PSC shows a PCE of 25.28%. More importantly, a 1-square centimeter cell shows a PCE of 24.17%. Unencapsulated devices exhibited high thermal and operational stability. This work suggests that the extension of conjugation may provide a simple and useful means for designing more efficient SAM molecules for high-performance solar cells.
Supporting Information
Supporting Information is available and includes experimental procedures, Figures S1–S24 and Tables S1–S2.
Conflict of Interest
There is no conflict of interest to report.
Funding Information
We thank the funding supported by the National Natural Science Foundation of China (grant nos. 21975264, 21925112, and 22090021). J.-Y. Shao acknowledges the support of the Youth Innovation Promotion Association Chinese Academy of Sciences.
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