Phase transfer catalyzed LiTFSI doping in Spiro-MeOTAD. The sandwich structure of PSCs with LiTFSI doped Spiro-OMeTAD as the HTL is shown in Fig. 1a. The components in conventional precursor solution include Spiro-OMeTAD, LiTFSI, acetonitrile, 4-tert-Butylpyridine (TBP), and CB, in which TBP assists a uniform dispersion of LiTFSI. The related molecule structures are provided in Fig. 1b. LiTFSI as an ionic compound has low solubility in CB (Fig. 1c). Crown ether possesses a specific charge distribution and cavity size that can bind alkali-metal cation with a fitted size through host-guest assembly. Particularly, 12-crown-4 has an interior cavity diameter that matches well with Li+ ion size.26 In this study, it was chosen to promote the dissociation of LiTFSI and increase the solubility through the following coordination reaction: LiTFSI+12-crown-4→Li(12-crown-4)++TFSI-. After adding 12-crown-4, LiTFSI can be dissolved quickly in CB and we successfully prepared transparent precursor solution without requiring acetonitrile. For simplicity, the precursor solution (HTL or PSCs) prepared with conventional or modified recipe are denoted as control and target precursor (HTL or PSCs), respectively, in the following discussion unless otherwise stated.
According to electrospray ionization-mass spectrometry (ESI-MS), amass/charge ratio (m/e) of 136.1 that is indicative of H(TBP)+ can be detected in the control precursor but the signal of Li(TBP)4+ is not present (Fig. 1d), which implies easy desolvation between Li+ ions and TBP. An extra intense peak with a m/e of 183.1 assigned to Li 12-crown-4)+ is found in the target precursor, confirming the specific coordinating reaction.27 The interaction between 12-crown-4 and Li+ ion was further characterized by nuclear magnetic resonance (NMR) and Fourier transform infrared spectroscopy (FTIR). Compared with the mixed solution without LiTFSI (acetonitrile+12-crown-4), a positive 1H NMR chemical shift of 12-crown-4 after being combined with Li+ ion is identified for the mixture with LiTFSI (Fig. 1e), which suggests a strong Li-O solvation.28,29 Both the 1H NMR spectra have an intense peak at 1.99 ppm characteristic of the H atoms in acetonitrile. A similar shift can also be observed in the FTIR spectra (Supplementary Fig. 1), validating the formation of crown ether-Li+ complex.
Notably, the control and target precursor exhibit a slight different color (Supplementary Fig. 2) and absorption pattern (Fig. 1f).The difference is caused by the change in the Li+ ion solvation after modifying the precursor recipe. A schematic drawing that depicts the Li+ ion solvation shell is shown in the inset. We studied the solvation energy of Li+ ions with first-principal calculations (Supplementary Fig. 3 and Supplementary Note 1). In the control precursor, four TBP molecules surrounding a Li+ ion is most favorable from the perspective of energy reduction and a stable solvation shell consequently forms. The Li+ ion solvation in the target precursor, however, is different. Li+ ion can tightly bond with 12-crown-4 with a strong bonding and the crowned Li+ ion can further interact with a TBP molecule which will be discussed later. As a consequence of less anchoring sites available, there will be more free TBP molecules that can induce Spiro-OMeTAD de-doping because of the chemical reaction with oxidized Spiro-OMeTAD.30
Both the absorption spectra and the conductivity measurement (Fig. 1g and Supplementary Note 2) certify that the modification toward the HTL recipe will not greatly affect the generation of oxidized species and the charge transport. We further monitored the time-dependant change in the PL spectra of unoxidized HTLs in ambient air (Fig. 1h). Pristine Spiro-OMeTAD under UV excitation emits intense blue light with an emission peak around 420 nm while p-type doping results in photoluminescence (PL) quenching.25 The PL intensity of both the HTLs gradually decrease as the exposure time, a phenomenon of doping in progress. The decrease in PL intensity is nearly in step with each other, proving a negligible impact of 12-crown-4 on the oxidation rate of Spiro-OMeTAD. Ultraviolet photoelectron spectroscopy (UPS) of the oxidized HTLs was measured to study the energy band structure (Supplementary Fig. 4). The two HTLs after oxidation show only a small difference in the energy band position and work function, which suggests a similar p-type doping level regardless of the HTL composition.
Elucidation of doping mechanism. To deepen our perception about the role of 12-crown-4 in the doping process, cyclic voltammetry (CV) was conducted to study the oxygen redox couple in LiTFSI-containing electrolyte without or with 12-crown-4. The CV curve without 12-crown-4 has an obvious oxidation peak of O22- at 0.01 V while no oxidation peak of O2- can be found (Fig. 2a and Supplementary Note 3).31 This proves that O2- has converted into O22- during the cathodic sweeping. According to the hard-soft-acid-base theory,32 the hard Li+ ions have a higher affinity for hard Lewis bases such as O22- and O2- than moderately soft base (O2-). LiO2 is quite unstable and it appears as an intermediate phase. However, after adding 12-crown-4, an oxidation peak at -0.40 V increase remarkably and the oxidation peak of O22- declines. The extra oxidation peak is assigned to the oxidation reaction from O2- to O2,33 which confirms the existence of O2- and a strong interaction between O2- and the crown ether-Li+ complexes. The Lewis acidity of Li+ ion is decreased with an increased radius through coordinating with 12-crown-4. As a result, the moderately soft base O2- will have an increased affinity for the Li(12-crown-4)+ complexes.33,34 The addition of 12-crown-4 can substantially improve the stability of the O2- ions and inhibit the formation proportion of Li2O2.
7Li NMR spectra of the precursor solutions before and after conducting O2-bubbling (Fig. 2b) were measured to further clarify the difference between the two HTLs in the oxidation process. The O2-bubbling process is an effective method to pre-oxidize HTL precursor solution. The control and target HTL precursor without pre-oxidization has a peak around -0.85 ppm and -0.78 ppm, respectively. A positive chemical shift in the 7Li NMR spectra suggests a different coordination condition for Li+ ion due to the formation of crown ether-Li+ complexes in the target HTL precursor solution.26 This phenomenon is in agreement with the result of 1H NMR as discussed above. After O2-bubbling, the dominating peak position remains almost the same for the control precursor and the signal at the positive side slightly increases, which is due to the formation of Li2O2 that contains Li vacancies.33 The 7Li NMR signal of the target precursor shifts to -0.97 ppm after pre-oxidization. We speculate that this change is attributed to the replacement of TBP with O2− as the ligand for the crowned Li+ ion. In lithium oxides, the 7Li NMR peak position follows the order: LiO2<Li2O2<Li2O and the peaks at lower frequencies can be assigned to Li species with a higher coordination number.35,36 The oxidation induced chemical shift is consistent with the enhanced coordination number for Li+ ions after the bonding between O2− and the crowned Li+ ion.
To get an in-depth understanding of the molecular interaction at the nanoscale, we calculated the binding energy of involved configurations (Fig. 2, c-e) by first-principal calculations. The binding energy for Li+ ion and 12-crown-4 complexation is -4.23 eV. Such a high binding energy ensures the stable existence of the Li(12-crown-4)+ complex. The system energy decreases by -1.28 eV when the crowned Li+ ion further interact with a TBP molecule, suggesting Li+ ions in the target precursor exist most likely in the form of Li(12-crown-4)+TPB complex. When the ligand of the crowned Li+ ion is substituted with O2−, the decrease in energy can reach up to -5.15 eV. Therefore, O2− can bond with crowned Li+ ion more tightly than TBP molecule and supports our speculation that the crowned Li+ ion will be finally stabilized with O2− after O2-bubbling.
In the control HTL, lithium oxides including Li2O2 and Li2O eventually form after the oxidative reaction (Fig. 2f, Supplementary Note 4).9 Lithium oxides are extremely hygroscopic and can easily react with moisture to form lithium hydroxide.37, 38 It is one of the main reasons for the instability of Spiro-OMeTAD based HTL and will be discussed later. In the target HTL, Li+ ions are crowned by 12-crown-4 and the direct reaction with oxygen-reduction species is postponed, leaving less opportunity for the generation of lithium oxides. The crowned Li+ ion with a decreased Lewis acidity has a strong interaction with the moderately soft base O2− as confirmed by the calculations and C-V analyses, so they can be stabilized in the final HTL. At this point, the oxidation mechanism of the p-type doping in the target HTL is straightforward and the involved oxidation process is presented in Fig. 2g.
Charge dynamics and photovoltaic performance. Both the control and target HTLs deposited on perovskite films are compact according to the scanning electron microscope (SEM) and atomic force microscope (AFM) images (Supplementary Fig. 5). The perovskite/HTL films have quenched PL intensity compared with the pristine perovskite film and the one with a target HTL display a little stronger PL intensity than that with a control HTL (Fig. 3a). The time-resolved PL (TRPL) spectra (Supplementary Fig. 6) can be divided and fitted into two parts (Supplementary Table 1). With the existence of HTL, the fast process (τ1) relates to the charge extraction and the slow one (τ2) represents the radiative recombination.39 The perovskite film with a target HTL has a lower τ1 and higher τ2. We conclude that the target HTL possesses a better capability in extracting charge carriers from perovskite40 and the charge recombination is decreased for perovskite film with a target HTL. The lifetime decay behavior is in connection with the crown ether-Li+ complexes (Supplementary Fig. 7 and Supplementary Note 5), which can passivate the perovskite and reduce charge recombination.29 The defect passivation will exert a positive influence on the device performance.41 In addition, we measured the space charge limited current (SCLC) of the HTLs (Fig. 3b and Supplementary Note 6) and the defect state density in the target HTL is lower than that in the control HTL.
We fabricated PSCs with a regular-type structure of ITO/SnO2/perovskite/HTL/Ag to investigate the photovoltaic performance (Fig. 3c). The perovskite layer has a bandgap of 1.56 eV. The control PSCs exhibit a PCE of 22.28% with an open-circuit voltage (VOC) of 1.10 V, a short-circuit current density (JSC) of 24.76 mA cm−2, and a fill factor (FF) of 81.74%. In contrast, the target solar cell reached a VOC of 1.10 V, a JSC of 24.98 mA cm−2, and an FF of 83.20%, yielding a PEC of 22.90%. The statistic distributions of photovoltaic parameters based on 25 individual devices for each condition (Fig. 3d, Supplementary Fig. 8) claim that JSC and VOC are slightly influenced by the changed HTL, and the improvement in PCE is mainly attributed to the enhancement in FF. We further evaluated the hysteresis behavior of the PSCs using the two types of HTLs and both the devices suffered from a similar degree of hysteresis (Supplementary Fig. 9). We also systematically investigated the effect of different types of crown ether and the concentration of 12-crown-4 as well as LiTFSI on the photovoltaic performance. The results can be found in the Supplementary Table 2-4.
Crowning lithium ions improves device stability. To evaluate the stability of the perovskite films at the presence of HTL, as-prepared perovskite films with different HTLs were stored in an ambient condition with a humid of around 40%. The fresh samples are dark in color (Fig. 4a). After aging for 550 h, the perovskite film with a control HTL turns yellow while the one with a target HTL still keeps a dark color. To obtain more insights into the degradation, we measured the XRD patterns of the sample before and after aging (Fig. 4b). Regarding the perovskite film with a control HTL, the signal from PbI2 increases markedly and an extra peak appears at 11.6o resulting from δ-phase perovskite,42 which indicates a severe degradation of perovskite caused by the moisture. Conversely, the PbI2 content of the perovskite film with a target HTL holds constant and the δ-phase has not formed during aging, suggesting a better resistance toward the moisture. The change in nanoscale morphology by comparing the cross-sectional SEM images between the fresh and aged samples43 and the water contact angle of the HTLs (Supplementary Fig. 10, 11) further illustrate the better moisture stability of the target HTL.
We then explored the Li+ ion distribution in the PSCs by measuring time-of-flight secondary ion mass spectrometry (ToF-SIMS). ToF-SIMS depth profiles and corresponding 3D mapping are provided in Fig. 4c-4f. In both the PSCs Li+ ion concentration does not obey a simple Fickian diffusion distribution and there is a higher concentration in the SnO2 layer than that in the perovskite layer. This suggests a lower affinity of Li+ ions to perovskite than SnO2. The same trend of Li+ ion distribution had been reported in previous literature.44,45 For the control device, a much higher Li+ ion signal at the SnO2 layer than that at the HTL can be found, suggesting a rather serious Li+ ion migration. In sharp contrast, Li+ ions primarily located within the target HTL and a relatively small portion of Li+ ions are diffused out and detected at the SnO2 layer, indicating the ion migration has been greatly suppressed.17 The Li+ ion signal intensity over the perovskite range in the target device is a little higher than that for the control, which is due to the migration of Li(12-crown-4)+ complexes into the inner part of perovskite film where the 12-crown-4 has a high affinity to Li+ ions.
To assess the moisture stability of the resultant PSCs, we traced the photovoltaic performance during storing the unencapsulated devices in the dark at 25 oC and 30% relative humidity. The target device maintains 95% of initial PCE after aging over 2000 h, whereas a significant degradation in PCE with only 67% initial value being preserved after 1400 h occurs for the control device (Fig. 4g). The thermal stability of control and target PSCs were compared with unencapsulated devices aged at 60 oC in a glove box. The target device shows a much slower decrease in PCE than the control device and 90% of initial PCE can be maintained after thermal aging for 700 h (Supplementary Fig. 12). The lowered Li+ ion diffusivity in the target HTL due to a strong complexation of the Li+ ion with 12-crown-4 contributes to a higher degree of thermal stability. These results fully demonstrate that the stability of the target solar cell is better than that of the control device, which is ascribed to the use of high-quality HTL.
Phase transfer catalyzed doping in π-conjugated polymer. π-conjugated polymers can be also used as HTL for PSCs in spite of less satisfactory performance compared with Spiro-OMeTAD. Finally, we performed phase transfer catalyzed LiTFSI doping in poly(3-hexylthiophene-2,5-diyl) (P3HT), a well-known π-conjugated conductive polymer. The addition of 12-crown-4 into the P3HT based HTL precursor also avoid the use of acetonitrile and convert the mobile Li+ ions into the crown ether-Li+ complexes. PSCs fabricated using P3HT as the HTL with a phase transfer catalyzed LiTFSI doping exhibit comparable PCE compared with conventional P3HT HTL based device (Supplementary Fig. 13). The P3HT layer with phase transfer catalyzed LiTFSI doping protects the perovskite film from moisture attacks in humid conditions (Supplementary Fig. 14). The improvement in stability is primarily attributed to the formation of the crown ether-Li+ complexes in the HTL with an enhanced resistance against the moisture.