Surface Passivation of Perovskite Solar Cells Toward Improved Efficiency and Stability
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The TTC layer was efficiently deposited at the grain boundary of the perovskite, which passivated the grain surface and grain boundary, thereby decreasing the interfacial recombination of the perovskite solar cells.
The hydrophobic small molecule TTC on the perovskite films forms a water-resistant layer that protects the perovskite from water damage.
KeywordsPerovskite solar cells Surface passivation Charge transport Surface defect
Organic lead halide perovskite materials such as methylammonium lead iodide (CH3NH3PbI3) have been revitalizing worldwide photovoltaic research due to the superb photoelectric properties [1, 2, 3, 4]. More encouragingly, the hybrid perovskite solar cells (PVSCs) possess the potential to be highly scalable and manufactured by low-cost solution routes [5, 6, 7]. Today, the certificated efficiency of PVSCs has already reached 24.2%, rivaling their contemporary inorganic counterparts . However, the PVSCs still suffer from instability, especially when exposed to moisture. Therefore, their sensitivity against moisture needs to be rationally addressed before they can be considered as commercially viable .
The decomposition of perovskite in humid ambience is mainly ascribed to the hydrolysis of CH3NH3PbI3. The combination of H2O and CH3NH3PbI3 forms PbI2 and CH3NH3I, and the latter further degrades into the HI and CH3NH2 . PbI2 dissolves out of the perovskite, resulting in a porous structure that in turn accelerates H2O and O2 absorption, HI reacts with O2 to form I2 and H2O, which can further drive the degradation process and decrease device efficiency [11, 12, 13].Therefore, to overcome the moisture instabilities, numerous strategies have been proposed, such as additive promoted crystallization , mixed-dimensional perovskite preparation , modification of the electron/hole transport layer [16, 17], and device encapsulation . Recently, many groups have succeeded in utilizing the interface engineering strategy to minimize the influence of moisture and improve the stability. Likewise, many interfacial materials have been investigated (Table S1), including metal oxides , polymers [20, 21, 22, 23], and carbon-based materials . These hydrophobic interfaces can not only substantially limit the permeation of atmospheric moisture but also enhance the device performance in terms of decreasing surface recombination, tuning band energy offsets, and optimizing interfacial contact . Among these, small organic molecules have attracted interest for their easy synthesis, purification, and reproducible property [26, 27]. A slight chemical modification of the molecules’ structure enables further fine-tuning of the interfacial properties according to the research requirements . Additionally, organic molecules can be deposited by solution processing and vacuum evaporation, making this a scalable technique in future . Considering the above advantages, interface strategy of small molecule is essential to enhance the performance and stability of the PVSCs.
In this work, we developed an organic small molecule tetratetracontane (TTC, CH3(CH2)42CH3) as an interlayer for planar p-i-n PVSCs. Based on the device structure of ITO/poly(triarylamine)(PTAA):2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ)/CH3NH3PbI3/TTC/fullerene(C60)/BCP/Ag, we achieved a high power conversion efficiency (PCE) of 20.05% with a high fill factor (FF) of 79.4%, compared with that of 17.38% achieved by the control device. The TTC passivation layer reduces the defects at the perovskite surface, which suppresses electron recombination and facilitates electron extraction. Moreover, the hydrophobic TTC can function as a water-resistant layer and protect the device from water damage, leading to highly stable perovskite devices.
2 Experimental Section
2.1 Materials and Sample Preparation
PbI2, methylammonium iodide (MAI), and PTAA were purchased from Xi’an Polymer Light Technology Corp. N,N-Dimethylformamide (DMF) (99.8%), dimethyl sulfoxide (DMSO) (99.8%), and F4-TCNQ were received from Sigma-Aldrich. C60 and BCP were purchased from American Dye Source Inc. All materials mentioned above were used as received without further purification.
2.2 PTAA Precursor Preparation
The PTAA solution was prepared by dissolving PTAA in toluene (Sigma-Aldrich) with a concentration of 2 mg mL−1 and stirred overnight.
2.3 Perovskite Precursor Preparation
The perovskite precursor solution was prepared via mixing 462 mg of MAI, 159 mg of PbI2, and 78 μL of DMSO (a molar ratio of 1:1:1) powder in 600 μL of anhydrous DMF. The solution was stirred overnight at room temperature and filtered with 0.22 μm PVDF filters before device fabrication.
2.4 Fabrication of Perovskite Solar Cells
The configuration of the fabricated devices was ITO/PTAA:F4TCNQ/CH3NH3PbI3/TTA/C60/BCP/Ag. ITO (15 O sq−1) glass substrates were cleaned sequentially with detergent, deionized water, acetone, and isopropanol followed by drying with N2 stream and UV–ozone treatment for 10 min. The PTAA:F4-TCNQ hole transport layers were formed by spin coating onto the cleaned ITO substrates at 4000 rpm for 30 s and annealed on the hot plate at 100 °C for 15 min in air. Then, the perovskite precursor solution was spin-coated onto the top of the prepared substrates at 4000 rpm for 30 s and annealed at 100 °C for 10 min. After the CH3NH3PbI3 film was formed and cooled to room temperature, TTA layers of different thicknesses were deposited by thermal evaporation on the top of the perovskite. Afterward, 25 nm C60 and 6 nm BCP were sequentially deposited by thermal evaporation under a vacuum of 5 × 10−4 Pa. Finally, a Ag electrode of 100 nm thickness was evaporated through a shadow mask. The device area was defined as 4 mm2 for each solar cell discussed in this work, and all of the above processes were executed completely in air at room temperature.
2.5 Device Characterization
The device photocurrent was recorded using a Keithley 2400 Source Meter unit under AM1.5 illumination condition at an intensity of 100 mW cm−2 in air. The illumination intensity of the light source was accurately calibrated with a standard Si solar cell. The incident photon-to-electron conversion efficiency (IPCE) was measured using a Newport Oriel IPCE measurement kit. The light intensity was calibrated using a single-crystal Si photovoltaic cell. The scanning electron microscopy (SEM) images were taken on a ZEISS Sigma field-emission scanning electron microscopy (FE-SEM). The X-ray diffraction (XRD) patterns were recorded on a Rigaku Ultima IV diffractometer using Cu Kα radiation. The UV–Vis absorption spectra were measured using a UV-1700 spectrometer.
3 Results and Discussion
We carried out XRD of the four perovskite/organic combinations (Fig. 1c), including the pristine perovskite, perovskite/TTC, perovskite/C60, and perovskite/TTC/C60, to show the phase of TTC on the surface. We found that the position of the diffraction peaks for the perovskite with TTC, C60, and TTC/C60 is almost as same as the pristine perovskite. The similar half-widths of all the perovskites suggest that organic combinations have little impact on the crystal size. Interestingly, we observed a significant decrease in the intensity of the diffraction peak for TTC/C60-coated perovskite, which might be a result of the improved coverage of the perovskite surface . The UV–Vis spectra of these perovskite films (Fig. 1d) were collected to explore the role of TTC on light absorption. Compared to the pristine perovskite, we observed a similar absorbance behavior of the films containing the various passivation layers. The thickness of the normal perovskite film is 450 nm, as estimated from the cross-sectional SEM (Fig. 1b). The passivation layer with a negligible thickness affords a very little effect on light harvesting.
Furthermore, we evaluated the quality of these films by surveying the charge dynamics using time-resolved photoluminescence (TRPL). Figure 2e shows representative TRPL traces of the perovskite films without and with the respective passivation layer. The pristine perovskite exhibits a long fluorescent lifetime and a long-lived tail with a fast-decay lifetime (τ1) of 3.7 ns and slow-decay lifetime (τ2) of 37.7 ns. The long lifetime reflects the high quality of the perovskite film. The TRPL lifetime vastly reduces after capping the top surface of the perovskite with C60 (τ1 of 1.7 ns, τ2 of 33.6 ns), which is attributed to the fast electron transfer from the perovskite to the fullerene layer because of the matched energy offset . However, the TRPL lifetime of the TTC-coated perovskite is almost the same as the original perovskite, which indicates that electrons cannot transfer from the perovskite to TTC because of the insulativity of TTC. (TTC-only has no effect.) Interestingly, inserting the TTC interlayer into the perovskite and C60 increases the PL lifetimes. The perovskite/TTC/C60 films decay to background level on timescales longer (τ1 of 3.1 ns, τ2 of 36.2 ns) than those of the perovskite/C60 samples, although still faster than the pristine perovskite films. To understand the reason for the increased PL lifetimes, we further measured the steady-state PL of all samples. As shown in Fig. 2f, the pristine perovskite shows a strong PL intensity with a peak centered at 774 nm. The perovskite passivated by C60 has a decreased PL intensity with a slightly blueshifted peak to 771 nm, which is attributed to the passivated trap states on the surface and/or along the grain boundaries of the perovskite. As expected, the steady-state PL of the TTC passivation sample exhibits a weaker PL intensity. The result shows that the alkyl chain axes of TTC can fill the interface states such as defect states or metal-induced gap states and passivate perovskite film .
Average performance parameters of perovskite devices with different thicknesses of TTC tunneling layers
Jsc (mA cm−2)
It is also worth mentioning that the devices with TTC exhibit a negligible hysteresis compared to the C60-only devices, which suggests that the TTC passivation layer can block the ion migration channel at the grain boundary; moreover, ion migration at grain boundaries plays a dominant role in the photocurrent hysteresis. The absence of photocurrent hysteresis is confirmed by altering the photocurrent scanning direction (Fig. 3d, e). Besides, we also performed the steady-state photocurrent measurement at the maximum power output point. The control device shows a photocurrent of 20.3 mA cm−2 and a PCE of 17.2%. The modified device indicates a photocurrent of 22.0 mA cm−2 and a PCE of 20.0% (Fig. 3f), which is consistent with the J–V results .
We also illustrated the function of TTC passivation layer on transporting electrons and blocking holes using the proposed energy diagram in Fig. 3g. The C–C–C plane of TTC can form a typical organic ultrathin insulator film, which would adjust the band alignment of the devices. We measured the conductivity of the ITO/PTAA:F4-TCNQ/TTC/C60/BCP/Ag and ITO/PTAA:F4-TCNQ/C60/BCP/Ag films via the current–voltage (I–V) curve measurement and show this in Fig. S1. It can be seen that their conductivity has little variation, suggesting that TTC has little impact on the electron transport inside the devices because of the small thickness of TTC. The photogenerated electrons at the conduction band of the perovskite can tunnel into the lowest unoccupied molecular orbital (LUMO) of C60, because there are energy-matching unoccupied states in C60 for electrons to tunnel into. On the contrary, the tunneling rate of the holes will be very low because there are no unoccupied states for the holes to tunnel into. Therefore, the TTC layer allows electron transfer from the perovskite to C60 layer by tunneling and blocks the holes, which reduces their recombination at the interface . The dark current density of the TTC-inserted device was suppressed at negative bias (Fig. 3h), suggesting a reduced leakage current density and an increased shunt resistance. To further prove the effect of the electron injection efficiency at the perovskite/TTC/C60 interface, we fabricated electron-only devices with the configuration of ITO/C60/perovskite TTC/C60/Ag and ITO/C60/perovskite/C60/Ag. As shown in Fig. 3i, the TTC-modified device exhibits a smaller trap-filled limited voltage, implying that lower electron trap density occurs at the perovskite/TTC/C60 interface [38, 39, 40].
In summary, a hydrophobic small molecular TTC was demonstrated as an effective interlayer for planar p-i-n PVSCs. The insertion of TTC reduces the interface trap density and enhances the electron extraction at the perovskite/C60 interface. With the device structure of ITO/PTAA:F4TCNQ/CH3NH3PbI3/TTC/C60/BCP/Ag, we achieved a PCE of 20.05% with a high FF of 79.41%. Moreover, the hydrophobic TTC successfully protected the perovskite film from water damage as a water-resistant layer on the perovskite films. Thus, PVSCs with better long-term operation stability were realized. This study provides an efficient method using small molecular to improve the efficiency and stability of the PVSCs.
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