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Abstract
Flexible perovskite solar cells (F-PSCs) prevail in the clean energy field for their light weight, easy fabrication and installation, but the power conversion efficiency of F-PSCs needs further improvement. In this work, we numerically simulate and experimentally demonstrate the effect of the perovskite trap defects density on the power conversion efficiency. The pseudo-halide KBF4 is employed as the additive to passivate the trap defects in the perovskite films. The high electrophilicity of BF4- group ensures its entering into perovskite lattice, optimizing crystallinity and improving the qualities of perovskite films, K+ ions can effectively passivate grain boundaries and inhibit halide anion migrations. After KBF4 passivation, trap defect density of the perovskite film was decreased from 8.0 × 1015cm−3 to 3.9 × 1015cm−3, and also the carrier lifetime increased from 108.52 ns to 234.72 ns. Consequently, the power conversion efficiency (PCE) of the F-PSCs devices increased from 13.99% to 16.04%.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Solar energy has aroused tremendous attention to decelerate the green-house effect induced by the excessive use of fossil fuel in the past decades. Solar energy is a kind of clean, renewability and low cost energy. It is predicted that photovoltaics will dominate the world power market by end of 2030. Recently the demands of portable and wearable devices increase significantly. Perovskite solar cells, representing one of the third generation photovoltaic candidates, attract scientists drastic attention. To date, the certified flexible regular n-i-p perovskite solar cell has gone through a history in efficiency since 2.62% in 2013 to current 22.5% [1,2].
Comparing with traditional rigid perovskite solar cells, F-PSCs usually adopt organic conductive materials (such as PET, PEN) as transparent flexible electrode substrates for their superior bendability [3]. The low-temperature process during fabrication is essential, deformation phenomenon of flexible substrates may occur when the device is overheated. In the fabrication process of F-PSCs, various additives are adopted to improve the performance of perovskite solar cells [4–11]. Methylamine chloride (MACl) and methylamine bromine (MABr) have been widely adopted to enlarge grain size and enhance crystalline of the MAPbI3 perovskite, thus can reduce the trap-assisted recombination in the perovskite solar cell [12–14]. Pseudo-halide anions additives are also used to replace I− or passivate halide vacancy defects to optimize photoelectric properties [6]. Pseudo-halide anions, such as thiocyanate (SCN−) and formate (HCOO−), can improve open-circuit voltage (VOC) and fill factor for the pseudo-halide-based PSCs [15–18]. Both K+ and BF4− have been employed to optimize SCs performance [19–23]. The interstitial occupancy position of K+ can suppress ion migration and hysteresis via passivating grain boundaries [24,25]. The BF4− can be incorporated into perovskite crystal frame, leading to lattice relaxation and enlarge grain size [26,27]. Recently some research groups used KBF4 to improve SCs, where both the alkalis K+ and halide BF4− have the trap passivation effect [4–7]. All the strategies mentioned above are based on rigid PSC structures. Nevertheless, few reports can be found on pseudo-halide additives in MAPbI3 inverted F-PSCs. Although the power conversion efficiency of F-PSCs devices are still lower than the rigid ones [28], it is worth to study the strategies to increase the F-PSCs devices for their tremendous application in the foldable electronic devices.
Herein, we simulate the effect of defect density on the perovskite film solar cell. Also, we develop a convenient approach to improve the performance of inverted FPSCs device by introducing KBF4 to passivate the perovskite thin film, the trap density was decreased from 8.0 × 1015cm−3 to 3.9 × 1015cm−3, and the carrier lifetime is prolonged from 108.52 ns to 234.72 ns. Eventually, a champion inverted device with a PCE of 16.04% is achieved, whereas the control device without KBF4 additive only has the PCE of 13.99%. In addition, the flexible perovskite solar cell can be folded with the angle of 120 degree and the PCE can maintain 70%. This work provides a cheap and convenient way to fabricate inverted F-PSCs with improved performance.
2. Experimental section
2.1 Materials
Methylammonium iodide (CH3NH3I) (MAI, 99.5%), poly (3,4-ethylene dioxy-thiophene)-poly (styrene sulfonate) (PEDOT:PSS) and Lead iodide (PbI2, 99.999%) were purchased from Xi’an Polymer Light Technology Crop. The PET-ITO were purchased from the Fine Chemicals Industry Co. KBF4 was purchased from Aladdin. N, N dimethylformamide (anhydrous DMF, 99.9%) was purchased from Alfa Aesar. Antisolvent Chlorobenzene (CB) is purchased from Shanghai Chemical Industry Co. Ltd. Phenyl-C61-butyric acid methyl ester (PC61BM) and 4,7-Diphenyl-1,10-phenanthroline (Bphen) were purchased from Nichem Fine Technology Co. Ltd. (Taiwan).
2.2 Device fabrication
Substrate treatment: The PET-ITO substrate was cleaned by ultra-sonication with detergent, deionized water, acetone and anhydrous ethanol in sequence for 15 min and dried in oven working at 70 °C for 30 min afterwards. After drying, the substrate was moved into an ultraviolet ozone chamber and treated for 15 min.
Solutions Preparation: The perovskite precursor solution was prepared by dissolving 1.25 M PbI2 and MAI into DMF and stirred at 70 °C in a N2 atmosphere for 12 h. For KBF4-treated solution, 1, 2 and 3 mg of KBF4 were dissolved in the perovskite precursor solution, respectively. PEDOT:PSS solution is filtered with a 0.45 µm PES filter, then the PEDOT:PSS layer is spin-coated on the substrates. 20 mg PC61BM powder is dissolved in 1 ml chlorobenzene (CB) solution. 0.5 mg Bphen is obtained in 1 ml ethanol.
Solar cells fabrication: The PEDOT:PSS solution is spin-coated on the PET-ITO substrate for 60s at 4000 rpm at room temperature with an annealing process at 100 °C for 30 min. Then samples are transferred into a nitrogen glovebox. The filtered MAPbI3 precursor solution is deposited on the surface of hole transport layer (HTL) by one-step spin-coating method at 1000 rpm for 15s and 5000 rpm for 25 s, 150 µl chlorobenzene was dripped 20 seconds prior to the end of spin-coating, followed by annealing at 100 °C for 10 min. PC61BM is spin-coated on the perovskite layer at 4000 rpm for 30 s. The solution of Bphen is spin-coated at a speed for 4000 rpm for 60 s and annealed for 10 min at 60 °C.
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 cm−2. X-ray diffraction (XRD) pattern of as-prepared perovskite film was recorded by using a Rigaku Ultima IV X-ray Diffractometer with Cu K radiation (λ=1.54060 Å) source. The XPS measurement was performed with hermo Scientific K-Alpha. The absorption spectra were 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). The steady-state 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
SCAPS-1D (Solar Cell Capacitance Simulator) software was used to simulate the performance of the perovskite solar devices at room temperature (300 K), AM 1.5 G solar spectrum and light irradiation with an incident power of 100 mW/cm2. Device simulation parameters are shown in Table 1.
Table 1. The inverted MAPbI3 F-PSC simulation parameters.
In our inverted p-i-n MAPbI3 F-PSC solar cell devices, PEDOT:PSS was adopted as hole transport material and PCBM as electron transport material. We simulated the changes of perovskite film defect state density between 1 × 1014 cm−2 and 1 × 1017 cm−2 to evaluate the influence of perovskite films defect density on the photovoltaic performance of solar cells. The J-V curves and simulation values are shown in Fig. 1 and Table 2. According to the simulation results, FF increased from 0.55 to 0.82 and PCE increased from 11.79% to 19.06% when trap density decreased from 1 × 1017 cm−2 to 1 × 1014 cm−2. This indicates that the reduction of defect density in perovskite films is beneficial to improve FF and therefore modify photovoltaic performance, which is in consistence with our experimental results.
Table 2. Simulation values with different defect density of perovskite films
The structure of PET/PEDOT:PSS/KBF4-MAPbI3/PCBM/Bphen/Ag is shown in Fig. 2. A one-step spin-coating method is adopted to prepare the MAPbI3 films with various concentrations of KBF4 additives, then the perovskite thin film was treated by chlorobenzene antisolvent to accelerate the crystallization process. To examine the surface morphology of perovskite films, the plane-view scanning electron microscope (SEM) images were performed. As shown in Fig. 3(a-d), enlarged grain size is achieved with the increasing amount of KBF4 additives, indicating the improvement in crystallization of perovskite film. The distribution of perovskite particle size is shown in Fig. 4(a-d), the distribution range of perovskite particle size without the KBF4 additives is 150-200 nm. When the KBF4 additive concentration is 2 mg/ml, the most probable particle size of 350-400 nm is larger than other samples. The result demonstrates that 2 mg/ml KBF4 additives provide the best-optimized grain size along with crystallization of perovskite films.
Fig. 3. Plane-view SEM images of perovskite films with concentrations of (a) 0 mg/ml (b) 1 mg/ml (c) 2 mg/ml (d) 3 mg/ml KBF4.
X-ray diffraction patterns (XRD) was employed to determine the crystal lattice. As is shown in Fig. 5(a), no extra peaks can be observed in the XRD patterns after adding KBF4 into the perovskite film, which means that the introduction of KBF4 does not affect perovskite phase. The diffraction peak intensity of the (110) crystal plane is enhanced with KBF4 additives due to an improvement on the crystal orientation of perovskite. Meanwhile, the FWHM of the best-performed sample is 0.0849 while the control one is 0.0959. The sharper diffraction peak indicates a better crystallinity of perovskite films on the basis of Scherrer formula [29,30], which is in consistent with the above SEM results. As seen in the Fig. 5(b), the peaks of 110 shifts to smaller angles for 0.08 degree (from 14.19° to 14.11°), indicating an expanded crystal lattice. We speculate the expanded lattice structure is attributed to the substitution of I− with BF4−. BF4− may enter into the perovskite lattice to replace I− ions since BF4− (218pm) has the similar ionic radius to that of I− (220 pm) [31]. While the existence of the longer covalent bond between Pb2+ and BF4− accounts for the expanded lattice [6].
Fig. 5. (a) XRD patterns of the perovskite films with different concentrations of KBF4 additives. (b) Zoom-in to the (110) peak of the XRD patterns.
Figure 6(a) shows UV-Vis absorption spectra of the perovskite thin film with different amounts of KBF4. Within the range of 400-540 nm, the absorption of perovskite films increased slightly with the additive of KBF4 and the absorption of perovskite films reaches peak with 2 mg/ml concentration of KBF4 additives. This is attribute to the enhanced crystallinity. Tauc plot is shown in Fig. 6(b) to calculate the optical bandgap Eg:
α, h, ν represent the absorption coefficient, Plank constant and photo frequency respectively. B is a constant. The Eg is around 1.60 eV according to the calculation, indicating that KBF4 treatment has negligible impact to the bandgap of perovskite. The steady-state PL spectrum in Fig. 6(c) shows that the PL intensity is highly enhanced when the concentration of KBF4 additives is 2 mg/ml. Whereas, when extra amount of KBF4 is added, the peak intensity drops. All of these can be attributed to suppressed nonradiative recombination caused by defect passivation.Fig. 6. Perovskite films treated with different concentrations of KBF4 additives (a) UV-Vis absorption spectrum; (b) Tauc plot; (c) Steady-state PL spectra.
Then the F-PSCs performance is characterized, the parameters of the champion and average devices with different concentrations are shown in Fig. 7 and Table 3. The control devices yielding the maximum PCE of 13.99%, VOC of 1.02 V, JSC of 19.04 mA/cm2 and FF of 0.71, while the KBF4-modified devices yielding the maximum PCE of 16.04% and 1.04 V of VOC, 21.73 mA/cm2 of JSC, 0.70 of FF when the concentration is 2 mg/ml. The results indicate significant improvement in JSC and FF which can be attributed to the influence of BF4−. The BF4− can be incorporated into perovskite crystal frame, resulting lattice relaxation together with optimized crystallization. Additionally, passivation with K+ ions on grain boundaries is also beneficial to devices [32–34].
Table 3. The parameters of control device, champion device with different ratios of KBF4.
Figure 8(a-d) shows the box plots of the distribution of performance parameters with different concentrations of KBF4 additives. Conclusions can be drawn that JSC and FF are significantly improved, which is responsible for reduced defects of the perovskite films and grain boundaries.
To investigate the change with chemical states, X-ray photoelectron spectroscopy (XPS) spectra was performed. In the XPS spectra (Fig. 9(a)), the characteristic peak of F 1s can be observed at 687.71 eV in the perovskite film with KBF4 additives. Meanwhile, the characteristic peaks of Pb 4f at 137.83 eV(4f7/2) and 142.70 eV(4f5/2) are observed to shift to higher binding energy with KBF4 additives due to F atoms’ high electronegativity, which suggests a potential interaction between Pb2+ and BF4− (Fig. 9(b)). The tetrahedral BF4− weakly hybridizes with the atomic orbitals of Pb2+ and there exists longer covalent bond between Pb2+ and BF4−. The interaction between Pb2+ and BF4− is weaker than that of I- due to the higher polarizability of I−, which will facilitate the hybridization with Pb2+ and lead to a stronger interaction.
Furthermore, the time-resolved photoluminescence was measured to explore the carrier dynamics. The TRPL spectrum of the perovskite films in Fig. 10 demonstrates typical double exponential decay process:
The fitting results are demonstrated in Table 4. The fast decay time increase from 14.21 ns to 35.12 ns, and the slow decay time increase from 141.88 ns to 272.93 ns and the average lifetime increased from 108.52 ns to 234.72 ns. Generally the prolonged PL lifetime indicate that the non-radiative recombination is effectively suppressed. This should be attributed to enlarged grain size and reduced grain boundaries according to the SEM result.
Table 4. Summary of the TRPL parameters with/without KBF4 additives
To quantitatively assess the defect density of perovskite films, we performed space charge limited current (SCLC) measurements based on hole-only device, as shown in Fig. 11(a). The dark J-V curve mainly shows the Ohmic region where the current increases slowly and linearly at low voltage, and in the trap-filled limit region the current increases rapidly. The voltage at the intersection of the Ohmic and trap-filled limit stages is named as the trap-filled limit voltage (VTFL). The defect density (Ntrap) can be calculated from the trap-filling limit voltage (VTFL) by:
where L is the thickness of perovskite films, q is the elementary charge, ɛ and ɛ0 are vacuum permittivity and relative dielectric constant of the perovskite layer respectively. The VTFL is the trap-filled limit voltage and decreased from 1.502 V to 0.726 V. Consequently, the trap density of the perovskite film is reduced from 8.0 × 1015cm−3 to 3.9 × 1015cm−3 when KBF4 is added. This result agrees well with the prolonged lifetime measured by TRPL, indicating that the incorporation of KBF4 can effectively reduce the defects in perovskite films. Moreover, devices with KBF4 additives show less influence by hysteresis effect as is presented by Fig. 11(b) and Table 5. References show that K+ can enter the grain boundaries and modify the interface by forming KI with uncoordinated halides to passivate the grain boundaries, and therefore suppress ion migrations and the hysteresis effect [7–42].Fig. 11. (a) Dark J-V curve of the hole-only devices with and without KBF4 additives. (b) J-V curves of the devices with and without KBF4 additives under reverse and forward scans.
Table 5. Photovoltaic performance of KBF4-modified and pristine device
The stability of the perovskite films under ambient conditions with a relative humidity of 30-50% is investigated. As is shown in Fig. 12(a), two unencapsulated devices were stored in the glove box filled with N2. Subsequently, after 30 days of storage the KBF4-modified device still maintained 79% of its initial PCE while the pristine one decreased to 72%. The improved stability might be attributed to the optimized quality of perovskite films and decreased defect.
Fig. 12. (a) The stability measurement of perovskite cells without encapsulation in a glove box. (b) Bending tests with different flexible devices.
To testify the performance of the flexible KBF4-modified devices, we carried on tests with different bending angle. Angles of 0°, 30°, 60°, 90° and 120° were performed for 100 times in turn and tested afterwards. As is shown in Fig. 12(b) and Table 6, after test with bending angle of 30° the device still remained nearly 90% of its initial PCE. While the device with bending angle of 120° only have 74% of its initial PCE left. The decrease in PCE is obviously associated with the drastically drop in FF and it should be attributed to interface damage in FPSCs.
Table 6. The parameters of bending tests with different flexible devices
4. Conclusions
In summary, we demonstrated that the incorporation of KBF4 can effectively optimize the performance of inverted F-PSCs devices. We performed simulation with trap density variations on perovskite films and found that the fill factor improved with the decrease of trap density, which is in consistence with our experimental result. The champion device delivers a PCE of 16.04% and VOC of 1.04 V, JSC of 21.73 mA/cm2, FF of 0.70. The introduction of BF4− can enhance crystallinity, enlarge grain size, passivate defects and eventually optimize the quality of perovskite films. Also, K+ ions can effectively passivate grain boundaries and inhibit ion migrations. In addition, the carrier lifetime is prolonged and nonradiative recombination is suppressed. The devices with KBF4-modified device still maintained 79% of its initial PCE after stored in a N2 filled glove box for 30 days. This work provides a convenient strategy to achieve high-performance F-PSCs with pseudo-halide additive.
Funding
Foreign Expert Project of Ministry of Science and Technology (NO. DL2023014015L).
Acknowledgment
This work is supported by the Foreign Expert Project of Ministry of Science and Technology (NO. DL2023014015 L).
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.
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