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Abstract
Inverted perovskite solar cells (PSCs) attract researchers’ attention for their potential application due to the low-temperature fabrication, negligible hysteresis and compatibility with multi-junction cells. However, the low-temperature fabricated perovskite films containing excessive undesired defects are not benefit for improving the performance of the inverted PSCs. In this work, we used a simple and effective passivation strategy that Poly(ethylene oxide) (PEO) polymer as an antisolvent additive to modify the perovskite films. The experiments and simulations have shown that the PEO polymer can effectively passivate the interface defects of the perovskite films. The defect passivation by PEO polymers suppressed non-radiative recombination, resulting in an increase in power conversion efficiency (PCE) of the inverted devices from 16.07% to 19.35%. In addition, the PCE of unencapsulated PSCs after PEO treatment maintains 97% of its original stored in a nitrogen atmosphere for 1000 h.
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1. Introduction
Organic-inorganic metal halide materials have shown great potential in the photovoltaic devices due to their excellent optoelectronic properties, easy fabrication and low cost [1–6]. Attributing to the composition engineering of the perovskite light-absorbing layer and interface engineering [7–12], the power conversion efficiency (PCE) of perovskite solar cells (PSCs) increased from 3.8% to 25.7%, which is comparable to the commercial monocrystalline silicon solar cells. However, the low-temperature solution preparation of perovskite film easily forms a polycrystalline perovskite with excessive defects, and most of the defects mainly exist on the surface and grain boundaries of the perovskite film [13,14]. High defect density will lead to serious carrier recombination in PSCs devices, which hinders the improvement of device performance [15–17]. Meanwhile, the defects in the perovskite film provide the channels for ion migration, resulting in aggravated device hysteresis effect [18–21]. Therefore, it is crucial to improve the device performance of PSCs by optimizing the perovskite film quality and reducing defect formation. Therefore, many passivation strategies have been adopted to improve the perovskite quality in PSCs [22–27]. Effective additive can regulate the growth of perovskite crystals, improve the quality of perovskite film, and passivate the bulk defects of perovskite films. Interface engineering mainly promotes the secondary growth of perovskite films and passivates the defects on the perovskite film surface. Recently, antisolvent additive engineering presents good passivation effect on surface and grain boundaries defects in perovskite films [28–32]. Poly(ethylene oxide) (PEO) is a polymer electrolyte material with a linear structure, which can be dissolved in a variety of organic solvents. It contains ether-oxygen unshared electron pairs with a strong affinity for hydrogen bonding [33,34]. Therefore, PEO was found as an ideal material for passivating the perovskite film in the traditional n-i-p solar cells. For example, Wang et al. used PEO to modify TiOx/perovskite, the formed interface dipoles effectively hindered the electron-hole recombination and improved the filling factor (FF) of the device [35]. Similarly, Gang et al. introduced PEO at the SnO2 QD/perovskite interface. The cross-linking behavior of PEO during the annealing process caused the heterogeneous nucleation of the perovskite precursor film, which can effectively improve the film morphology, passivate the bulk and interface defects of the perovskite and effectively improve electron transport capacity. At the same time, the hygroscopic PEO film can prevent water from entering the perovskite film and improve the stability of the device [34]. It was also found that PEO can be directly added into the perovskite precursor solution to control the film morphology and reduce the bulk defect density [36], and the PCE of inverted perovskite solar cells was increased from 16.53% to 19.15% [37]. However, PEO was rarely used in the p-i-n perovskite solar cells. Therefore, it is worth to use PEO polymer to passivate defects to improve the performance of inverted perovskite solar cells.
In this work, we propose an anti-solvent addition strategy to passivate perovskite interfacial defects by PEO. The PEO introduced by the anti-solvent promotes the heterogeneous nucleation of the perovskite film. Although the grain size of the perovskite film becomes smaller, it is denser and pinhole-free, the roughness of the perovskite film is reduced, and the interfacial contact is improved. At the same time, the ether bond and hydroxyl of PEO interact with perovskite to passivate the interfacial defects of perovskite films, reducing the electron-hole recombination at the perovskite/PCBM interface. Unlike insulating poly(methyl methacrylate) (PMMA) polymer, the polymer electrolyte material PEO has good charge transport ability so that PEO treatment would also facilitate the carrier separation. Therefore, the PCE of our fabricated perovskite solar cells is increased from 16.07% to 19.35%, and the FF is increased from 71.40% to 76.93%. In addition, the stability of the device is also significantly improved after PEO treatment.
2. Experimental section
2.1 Materials
The indium-doped tin oxide (ITO) glass substrates (9 ohm/square) were purchased from Advanced Election Technology Co., Ltd. The MAI (99.5%), PbI2 (99.999%), PTAA (Mn = 6,000∼15,000), PC61BM (99%), and Bphen (99%) were purchased from Xi'an Polymer Light Technology Crop. The DMF (99.8%) and PEO (average Mv = 200,000) were purchased from Sigma-Aldrich. All the reagents were directly used as received without any further purification.
2.2 Device fabrication
The ITO glass was cleaned with detergent, deionized water, acetone, and anhydrous ethanol sequentially for 15 min. The clean ITO glass was placed in an oven working at 100 °C for 30 min to remove residual solvent. The pretreated ITO glass was treated with UV-ozone for 15 min. PTAA with a concentration of 5 mg/mL dissolved in chlorobenzene was spin-coated on the substrate at 4500 rpm for 30 s and annealed at 105 °C for 10 min. To improve the wettability of PTAA, we used toluene for surface modification of PTAA. The perovskite precursor solution was prepared by dissolving 1.25 M MAI and PbI2 into DMF, and stirred at 70 °C under a nitrogen atmosphere for 12 h. The perovskite precursor solution was spin-coated onto the substrate at 1000 rpm for 15 s and 5000 rpm for 25 s. 150 µL chlorobenzene or chlorobenzene solution with different concentrations of PEO was dripped 22 s prior to the end of spin-coating, followed by annealing at 100 °C for 10 min. PC61BM (20 mg/mL in chlorobenzene) was spin-coated on the perovskite layer at 3000 rpm for 30 s. 0.5 mg/mL Bphen in ethanol was spin-coated on PC61BM layer at 6000 rpm for 40 s and annealed at 60 °C for 10 min. Finally, the silver electrodes were thermally evaporated in an evaporation chamber of 4 × 10−4 Pa.
2.3 Device and film characterization
The current density-voltage (J-V) curves were measured using a solar simulator (Newport, Oriel Class AAA) with a source-measure unit (Keithley 2400) at 100 mW/cm2, and a certified AM 1.5 G illumination source was calibrated by a standard Si-reference cell system from the NREL. And the active area of perovskite solar cells is 0.0625 cm2. XRD of as-prepared perovskite film was recorded by using a Rigaku Ultima IV X-ray Diffractometer with Cu Kα radiation (λ=1.54060 Å) at an acquisition rate of 5 ◦/min to measure the crystal structure properties. The valence state of the perovskite film was studied by X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250Xi). The absorption spectra was measured by a Hitachi UV-Vis spectrophotometer U-4100 at room temperature. The perovskite film morphology was characterized by a scanning electron microscope (SEM, ZEISS, Supra 55). Atomic force microscopy (AFM) images were measured by Bruker Dimension Icon. The steady-state photoluminescence (PL) spectra was obtained by using a 325 nm spectrometer in a dark condition. Time-resolved photoluminescence (TRPL) was measured by an Edinburgh FLS1000 spectrometer.
3. Results and discussion
We introduced PEO polymer with antisolvent to passivate perovskite surface and grain boundaries defects. Figure 1 shows the spin-coating process for the preparation of perovskite films by introducing the PEO polymer into an anti-solvent. The MAPbI3 precursor solution was firstly spin-coated at 1000 rpm for 15 s and then at 5000 rpm for 25 s. At 3rd second of the second step, 150 uL chlorobenzene with PEO polymer was dripped on the perovskite films, then annealed at 100 °C for 10 min to obtain a black perovskite film. The morphology of the MAPbI3 perovskite films was investigated by SEM, as shown in Fig. 2(a-b). It can be observed that the grain size of the perovskite film decreased with the addition of PEO polymer. However, compared with the control perovskite film, the PEO-treated perovskite film is denser. It is possible that the PEO polymer triggering heterogeneous nucleation of the perovskite films, which enhanced the density of nuclei and led to the formation of an ultrafine grain structure [27,34]. In addition, The PEO polymer treatment eliminated the appearance of pinholes in the perovskite films and passivated the perovskite grain boundary defects, which is consistent with previous reports [24,27]. Therefore, we believe that although the PEO treatment reduces the grain size, the PEO introduced by the antisolvent can cross-link with the perovskite wet film, and the polymer can enter the grain boundary and fill the grain boundary. The interaction between PEO and perovskite can eliminate the effect of increased defects at the grain boundary due to grain size reduction. As shown in Fig. 2(c-d), the perovskite film with PEO treatment displays a reduced root-mean-square (RMS) surface roughness of 10.8 nm, compared to that of control perovskite film (RMS = 21.2 nm). It suggests that the cross-linking between PEO and perovskite particles resulting in smoother film surface. The smooth surface is a key for obtaining favorable interface contact between perovskite and PCBM, which helps to improve the FF in the device.
Fig. 2. Top-view SEM images of perovskite film (a) without PEO (b) with PEO. AFM images of perovskite film (c) without and (d) with PEO.
XRD was used to investigate the crystalline structure of perovskite films, as shown in Fig. 3(a). The XRD patterns of the perovskite films after PEO treatment showed identical peak positions. The diffraction peaks at 14.28°, 20.18°, 23.66°, 24.64°, 28.60°, and 32.02° were ascribed to the crystal planes of (110), (112), (211), (202), (220) and (310), respectively [38]. The results indicated that the incorporation of PEO did not change the crystalline phase of MAPbI3. The residual PbI2, which has the characteristic peak at 2θ=12.7° corresponding to the (001) plane, was observed for the control perovskite film [39]. The peak intensity of PbI2 in the perovskite film decreases with PEO treatment, which could be due to the crosslinking reaction of PbI2 with PEO polymer resulting in the reduction of the residual PbI2 [34].
Fig. 3. (a) XRD pattern of the perovskite films, where “*” denotes the diffraction peaks of PbI2. XPS spectra of corresponding perovskite films: (b) O 1s spectra, (c) Pb 4f spectra, and (d) I 3d spectra.
Furthermore, XPS measurements were carried out to investigate the surface composition of our perovskite film after PEO treatment, as shown in Fig. 3(b-d). The peak at the binding energy of 531.43 eV can be assigned to the O 1s core level, which originates from PEO, indicating that PEO in the antisolvent is successfully introduced into the perovskite film. Figure 3(c) shows that the Pb 4f7/2 and the Pb 4f5/2 peaks of control perovskite films are observed at 136.9 eV and 141.8 eV, respectively. Their positions slightly shift toward the lower binding energy side after PEO treatment. Similar peak shifts are observed for the I 3d core level peaks. The control perovskite film shows I 3d5/2 and I 3d3/2 peaks at 617.9 and 629.4 eV, respectively. The peak positions of both Pb and I are shifted, which indicates a change in the electronic states of Pb and I [40]. These results demonstrate that the effectively introduced PEO interacts with perovskite films.
The effect of anti-solvent introduced PEO on the optical properties and carrier dynamics of perovskite film was investigated. The UV-vis absorption spectra of the perovskite films were measured, as shown in Fig. 4(a). All the samples are fabricated on the ITO/PTAA to screen the effect of charge injection. Compared to the control films, the perovskite films with PEO treatment show a slight improvement in light absorption. In the Tauc plot, there is a slight reduction in the bandgap of the PEO-treated perovskite films. In addition, the steady-state PL spectroscopy is investigated in Fig. 4(c). We observed a significant weakening of the PL intensity of the perovskite films with PEO polymer compared to the control films. Previous studies have shown that PEO polymers possessing polar groups can be used as an electrolyte material for efficient charge transport [34]. Therefore, the reduced PL intensity of the perovskite films is related to the quenching effect, which suggests that the carrier can be extracted more efficiently after PEO polymer treatment. This result is consistent with previous studies [41]. Meanwhile, there is a 2 nm blueshift in the PL peak of the perovskite films after PEO treatment. This blueshift of PL peak is attributed to a decrease of spontaneous radiative recombination induced by defect state, indicating that PEO treatment passivates the defect state of the perovskite films [32,42]. The time-resolved PL (TRPL) measurements for the perovskite films after PEO treatment are shown in Fig. 4(d). The TRPL curves of both samples exhibit a commonly observed bi-exponential decay [32]:
where τ1 is the fast decay time, τ2 is the slow decay time, A1 and A2 are the corresponding relative amplitudes, and y0 is a constant. The average carrier decay lifetime (τavg) is estimated according to the following formula:Fig. 4. (a) UV-Vis absorption spectra, (b) Tauc plot, (c) Steady-state PL spectra and (d) Time-resolved PL spectra of the perovskite films after PEO treatment.
The obtained fitting parameters are shown in Table 1. Compared to the control films, the τavg of the perovskite films decreased from 96.29 ns to 25.14 ns after PEO treatment. It is attributed to the fact that the passivation of PEO reduces the defects and facilitates the charge transport [43,44]. This result is consistent with the PL quenching effect.
Table 1. Summary of the TRPL parameters of the perovskite films after PEO treatment
To experimentally verify the passivated perovskite, we evaluated the defect density (Ntrap) by the hole-only device (ITO/PTAA/MAPbI3/(PEO)/PTAA/Ag) using the space charge limited current (SCLC) measurement, as shown in Fig. 5(a-b). The defect density was determined by the trap-filled limit voltage (VTFL) using the following equation [45]:
where ε0 is the vacuum permittivity, εr is the relative dielectric constant of the perovskite film, VTFL is the onset voltage of the trap-filled limit region, e is the elementary charge, and L is the thickness of the perovskite layer. VTFL of hole-only devices is reduced from 1.006 V to 0.764 V, indicating that the defect density in the perovskite film is reduced. The relatively lower defect density for perovskite film with PEO treatment indicates that the PEO treatment could effectively passivate the defects due to the interaction between the PEO and the undercoordinated Pb2+/I- sites. The reduction of defect states in perovskite films facilitates the improvement of open circuit voltage (Voc) and FF of PSCs devices.In order to explain the passivation mechanism between PEO and perovskite, it is necessary to know what defects are present in the pristine perovskite films. The MAPbI3 films have various defects both in the film and also on the surface and grain boundaries, including undercoordinated Pb2+/I- ion, MA/I vacancies, and Pb-I antisite substitution. However, the MA/I vacancies in the bulk are reported to form shallow level defects that contribute negligibly to the nonradiative recombination, while Pb-I antisite substitution rarely occurs [46,47]. Therefore, the defects at the surface and grain boundaries of the perovskite films are mainly undercoordinated Pb2+ or I-. When the antisolvent acts on the perovskite precursor film, most of the polymer PEO in the antisolvent is located on the surface of the perovskite films, and a small portion of PEO enters the perovskite grain boundaries. The ether bond and hydroxyl in PEO interacts with uncoordinated Pb2+ and I- on the surface and grain boundaries of the perovskite films to passivate Pb2+ and I- defects, as shown in Fig. 6(a) [48]. Meanwhile, the defect passivation of PEO reduces nonradiative recombination which facilitates carrier transport. In addition, PEO can act as an electron transport channel to accelerate electron transport to the PCBM layer.
Fig. 6. (a) Possible passivation mechanism of the PEO for the perovskite film. (b) Schematic diagram of the structure of the inverted device. (c) J-V curves of different concentrations of PEO treated devices. (d) J-V curves of the devices with and without PEO treatment under reverse and forward scans. (e) Statistics of the PCE distribution of control and PEO-treated devices under reverse scan. (f) Stability evolution of devices without encapsulation in a glove box.
To study the impact of PEO treatment on the photovoltaic performance, we fabricated planar inverted PSCs with the configuration of ITO/PTAA/MAPbI3/PCBM/Bphen/Ag by PEO treatment, as shown in Fig. 6(b). The J-V curve of the devices are shown in Fig. 6(c), and the photovoltaic parameters of champion devices are summarized in Table 2. The champion control device exhibits a Voc of 1.042 V, a Jsc of 21.58 mA/cm2, and an FF of 71.40%, yielding a PCE of 16.07% in the reverse scan direction. After PEO treatment, the champion device at the optimal PEO concentration of 0.2 mg/mL exhibits a Voc of 1.059 V, a Jsc of 23.73 mA/cm2, and an FF of 76.93%, yielding the highest PCE of 19.35%. The improved device performance is due to the PEO passivation of defects on the surface and grain boundaries of the perovskite films and the reduction of nonradiative recombination. Figure 6(d) shows the J-V curves of inverted PSCs measured using both the reverse and forward scans directions. The hysteresis effect is also suppressed in the devices, manifested by the reduction of hysteresis index (HI) from 0.056 (without PEO treatment) to 0.031 after PEO treatment, according to the following equation [25]:
Table 2. Photovoltaic parameters of PSCs modified with different concentrations of PEO
The mitigation of the hysteresis effect may originate from the fact that PEO passivates the defects in the perovskite and hinders the ion migration [49]. The device reproducibility was assessed by fabricating more than 90 cells for the control and PEO treatment, and the histograms of the average PCE are shown in Fig. 6(e). The average PCE for the control and PEO treated devices is 14.58% and 16.39%, respectively. It indicates that the PEO passivation strategy has high reproducibility. In addition to the enhanced efficiency, we also investigated the effect of PEO treatment on device stability. All the devices were unencapsulated. For the long-term stability test, the devices were stored in a glove box filled with N2 at room temperature, and the result is shown in Fig. 6(f). It is observed that the PEO treated devices retained 97% efficiency up 1000 h storage duration, while the control devices retained only 86% of its initial PCE. It indicates that the PEO passivation strategy could effectively enhance the stability of the devices.
To further confirm that antisolvent introduction of PEO can passivate interfacial defects, the simulation software of Silvaco is utilized to clarify the enhancement achieved in the J-V characteristics of the fabricated PSCs structure. Figure 7(a) shows the schematic view of the simulated MAPbI3 PSCs. The J-V characteristics with altering the interface defect density from 1 × 1012 cm-2 to 1 × 1016 cm-2 are calculated at ideality factor ($l$) of 4. These ideality factors are corresponding to characteristic temperatures (Tc) of 1200 K. The detailed physical model, carrier transport process and simulated device parameters are shown in the Supplement 1. In this regard, the perovskite/ETL interface defect density is changed in the simulation process. Two ideality factor parameters for electrons(n)/holes(p), ${l_{(n/p)}} = {T_{c(n/p)}}/T$ are introduced [50]. Figure 7(b) shows the J-V characteristics of the simulated PSCs with different values of interface defect densities at a fixed characteristic temperature of 1200 K, and the photovoltaic parameters of the simulated devices are summarized in Table 3. If the defect density is reduced to 1 × 1014 cm-2, 1 × 1013 cm-2 and 1 × 1012 cm-2, the PCE is increased to 17.91%, 18.67 and 19.05%, respectively. The enhancement is due to the increase of the FF and Jsc [51]. It is worth noting that at an ideality factor of 4 and interface defects density of 5 × 1015 cm-2, the simulated results are in good agreement with the measured results. Figure 8(a) shows the J-V characteristics of the fabricated solar cell devices along with the simulated curves. At the high defect density of 5 × 1015 cm-2, the Jsc, Voc, FF and PCE of the simulated/fabricated PSCs are equal to 21.9/21.6 mA/cm2, 1.044/1.042 V, 71.4/71.4 and 16.3/16.1%, respectively. By the PEO treatment, the PCE of the passivated PSCs is increased to 19.4% with an enhancement of 20% compared to the control device. Such J-V curve of passivated PSCs nearly matches the simulated J-V characteristics at interface defect density of 1 × 1012 cm-2.
Fig. 7. (a) Schematic diagram of the simulated MAPbI3 PSCs and (b) the J-V characteristics of the simulated PSCs by decreasing the perovskite interface defect density from 1 × 1016 cm-2 to 1 × 1012 cm-2 with ideality-factor electron characteristics temperature of 4.
Fig. 8. (a) The J-V curve of inverted devices with and without PEO treatment versus the simulated interface trap density. (b) The J-V curve of inverted devices with PEO treatment versus the simulated interface trap density at ideality factor of 1. (c) The J-V characteristics versus the carrier mobility at an interface defect density from 1 × 1014 cm-2.
Table 3. The photovoltaic parameters of the simulated PSCs at different interface defects with ideality factor electron characteristics temperature of 4
However, the practical trap density of the simulated PSCs is still in the order of 1015 cm-2 far way [51] from the value obtained from the simulation with an order of 1012 cm-2. Therefore, the trap density for the PSCs with and without PEO is very close and we believe that there is another parameter affecting the J-V characteristics. This parameter may be the characteristic temperature which may be changed also during the passivation process. In this regard, the J-V characteristics are examined by altering the trap density at the lowest ideality factor of 1. Figure 8(b) shows the performance of the PSCs by reducing the trap density from 1 × 1016 cm-2 to 1 × 1013 cm-2. It is worth noting that the J-V characteristic is appreciably enhanced at a high trap density of an order 1014 cm-2 compared to 1012 cm-2. The Jsc is slightly decreased from 23.47 mA/cm2 to 23.38 mA/cm2 with the aforementioned trap density at the small ideality factor of 1. However, the FF is increased from 74.44% to 79.1% by reducing the trap density from 1 × 1016 cm-2 to 1 × 1013 cm-2. We can also notice that the corresponding Voc is slightly decreased from 1.05 to 1.03. However, such a result is different from the measured data from the fabricated PSCs where the Voc is enhanced by the PEO treatment.
In past research, the Voc was significantly reduced with increasing carrier mobility [52]. This is back to the highly serious dark carrier recombination by increasing the carrier mobility. The influence of the carrier mobility of the active material on the J-V characteristics is shown in Fig. 8(c). In this study, we consider the PSCs is passivated and the corresponding trap density is fixed at 1 × 1014 cm-2. The carrier mobility is increase from 2 cm2/(V⋅s) to 10 cm2/(V⋅s) and the J-V characteristic parameters are presented in Table 4. It may be noticed that the Jsc is slightly increased with the carrier mobility due to the increase of the dark serious recombination. This is back to the Langevin recombination coefficient which is directly dependent on the carrier mobility as revealed in Eq. (S7) in Supplement 1 [53]. However, the FF is noticeably increased from 76.9% to 79.18% with carrier mobility. On the other hand, the Voc is reduced with increasing the carrier mobility. At small carrier mobility of 2 cm2/(V⋅s), the Voc is increased to 1.07 V and the FF is increased to 76.9%. This result is highly appreciated as it matches the J-V characteristics of the fabricated PSCs with PEO treatment at 0.2 mg/mL shown in Table 2. This means that the passivation process not only reduces the surface trap density but also the carrier mobility that compensates the decrease in the Voc due to the decrease in the trap density shown in Fig. 8(a). Consequently, the PEO material is able to passivate the PSCs surface and assures its ability to reduce the carrier recombination on the perovskite/PCBM interface. Therefore, we can speculate that the PEO passivation can effectively reduce the interface defect density and improve the solar cell performance.
Table 4. The J-V characteristic parameters of Jsc, Voc, FF and PCE versus the carrier mobility at an interface defect density from 1 × 1014 cm-2
4. Conclusions
In summary, we introduced a simple passivation strategy to improve perovskite solar cell by the additive PEO in the antisolvent spinning process, the addition of PEO resulted in a reduction in the grain size of the perovskite films, the films were denser and flatter. In addition, PEO interacts with uncoordinated Pb2+ and I- on the surface and grain boundary of the perovskite films, thus effectively passivates the defects in perovskite films. On the other hand, the simulation results also show that the antisolvent introduced PEO passivates perovskite interface defects. Through optimizing the concentration of PEO polymer in antisolvent, an improved PCE of 19.35% was achieved. Meanwhile, the PEO treated inverted PSCs could maintain 97% of its initial PCE after 1000 h without encapsulation, whereas the PCE decreased to 86% of its initial values for the control devices.
Funding
National Natural Science Foundation of China (11874185).
Disclosures
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
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