Solar Energy Materials & Solar Cells 208 (2020) 110435
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Thioacetamide additive assisted crystallization of solution-processed perovskite films for high performance planar heterojunction solar cells Can Cui a, c, *, Danyan Xie a, Ping Lin a, Haihua Hu b, Siyuan Che a, Ke Xiao a, Peng Wang a, Lingbo Xu a, Deren Yang c, Xuegong Yu c, ** a b c
Center for Optoelectronics Materials and Devices, Department of Physics, Zhejiang Sci-Tech University, Hangzhou, 310018, China School of Information Science and Electronic Engineering, Zhejiang University City College, Hangzhou, 310015, China State Key Laboratory of Silicon Materials and School of Material Science and Engineering, Zhejiang University, Hangzhou, 310027, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Lewis base Thioacetamide Thin film growth Large grains Perovskite solar cells
High-quality perovskite films with uniform coverage and large grains are indispensable to enhance the perfor mance of perovskite solar cells with high efficiency and stability. However, solution-processed perovskite films usually possess small grains associated with abundant grain boundaries, which induce high trap state density and then seriously degrade the device performance. In this paper, the volatile Lewis base, thioacetamide (TAA), is employed as an additive to fabricate high-quality methylammonium lead iodide (MAPbI3) films. The average grain size of perovskite films increases continuously with increasing TAA content and reaches a maximum value of 960 nm in the sample with 1.0% TAA. However, the average gain size drops dramatically to the value of samples without TAA when TAA content increases to 2.0%, and then the average gain size keeps nearly un changed upon further increasing TAA content up to 10%. This unusual grain size variation tendency is attributed to the volatility of additive, and a mechanism is proposed based on various characterizations to illustrate how volatile TAA improves perovskite film crystallization. Furthermore, the device based on the MAPbI3 film with 1.0% TAA shows a superior PCE of 18.9% and improved stability that the device with 1.0% TAA retains 88.9% of its initial performance after aging 816 h in the air with 25–35% relative humidity. The results strongly suggest that the TAA-modified MAPbI3 films as absorber layers can significantly enhance the performance of the perovskite solar cell due to large grains, high crystallization and reduced trap state density of the high quality TAA-modified MAPbI3 films.
1. Introduction
carriers and eventually make them annihilate via non-radiative recom bination [14,15]. In addition, the dependence of perovskite grain sta bility on moisture has clearly shown that moisture or oxygen could permeate into perovskite films through defective surface and GBs, and further degrade perovskite performance [16]. Therefore, enlarging the grain size with fewer grain boundaries is expected to be an efficient method to improve device performance. Currently, introducing additives during film preparation is the most convenient and efficient way to fabricate high-quality perovskite films with large grain sizes. So far, several types of additives with different impacts on perovskite films have been adopted, such as metal ions [17–20], Lewis base [21–24], chloride salts [25,26], thiocyanate [27, 28], solvents [29,30] and other dopants (PbS quantum-dot [31], diethyl ether [32], graphene nanofibers [33], 1,8-diiodooctane (DIO) [34], CAN
Organic-inorganic metal halide perovskites are promising semi conductor materials for photovoltaic devices owing to their charming optoelectronic properties of tunable band gap [1], low exciton binding energy [2,3], large charge carrier diffusion length and outstanding op tical absorption efficiency [4,5]. The latest solar cells using perovskites as absorber layers have yielded a power conversion efficiency (PCE) of 25.2% [6,7]. High-quality perovskite absorber layers are highly desir able for higher PCEs. Many efforts have been made to improve perov skite film formation [8–13], however, the polycrystalline perovskite films prepared by common methods usually have small grain sizes accompanied with abundant grain boundaries (GBs). The shallow trap states induced by these GBs would localize a large amount of charge
* Corresponding author. Center for Optoelectronics Materials and Devices, Department of Physics, Zhejiang Sci-Tech University, Hangzhou, 310018, China ** Corresponding author. E-mail addresses:
[email protected] (C. Cui),
[email protected] (X. Yu). https://doi.org/10.1016/j.solmat.2020.110435 Received 23 September 2019; Received in revised form 19 January 2020; Accepted 27 January 2020 Available online 1 February 2020 0927-0248/© 2020 Published by Elsevier B.V.
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[35], 2D MXene [36], etc.). These additives either serve as nucleation sites or interact with precursors to regulate the crystallization for better morphology with larger grain sizes. Particularly, Lewis base can react with Pb (II) halides to form an adduct, which can be easily confirmed by Fourier transform infrared (FTIR) spectrometer. For example, Lee et al. reported that the replacement of DMSO with thiourea during FAPbI3 perovskite layer preparation can greatly increase the grain size and thus lead to decent device performance [21]. Also, Lee et al. introduced bifunctional Lewis base urea into the perovskite precursor solution to regulate film crystallization and passivate GBs, and thus obtained high performance solar cells [22]. In addition, Zheng et al. reported highly efficient and stable solar cells based on acetamide-prepared perovskite layers with large grain sizes [23]. Whereas, the Lewis bases reported so far are all non-volatile, and the role of additives in the crystallization of perovskite films is not yet well explained. In this paper, a volatile Lewis base, thioacetamide (CH3CSNH2), abbreviated as TAA, is employed as an additive added into methyl ammonium lead iodide (MAPbI3) perovskite precursor solution to con trol the growth of MAPbI3 films. Via Lewis Acid-Base reaction, the precursor intermediates MAI�PbI2�DMSO�TAA effectively improve the grain morphology and crystallinity of resulted perovskite films. Partic ularly, compared to the films obtained with the addition of non-volatile Lewis base, the grain size of the as-prepared films changes in an unusual tendency. With increasing TAA content, the average grain size increases continuously to a maximum value of 960 nm in the sample with 1.0% TAA, then in the sample with 2.0% TAA, it dramatically decreases to the value of the control sample without TAA and keeps nearly unchanged upon further increasing TAA content up to 10%. Based on various characterizations, a working mechanism is proposed to demonstrate perovskite nucleation and grain growth with the introduction of volatile Lewis base. Moreover, solar cells of high efficiency and stability were assembled with perovskite films with 1.0% TAA by employing the planar configuration of ITO/SnO2/perovskite/spiro-OMeTAD/Au. The optimal device shows a PCE of 18.9% and retains over 88.9% of its initial PCE after 816 h aging in the air with 25–35% relative humidity.
perovskite-based absorber layers were prepared by an anti-solvent dripping method as described previously. Briefly, the MAPbI3 precur sor solution was dropped on the substrate and then spin-coated in a twostep program at 1000 rpm for 10 s and 5000 rpm for 20 s, respectively. During the second step, 0.6 mL of diethyl ether was added on the spinning substrate 15 s prior to the end of the program. Then, the MAPbI3 film was heated on a hotplate at 100 � C for 10 min. The hole transport layers (HTLs) were formed by spin-coating a spiro-OMeTAD solution at 3000 rpm for 30 s, and the solution was composed of 72.3 mg spiro-OMeTAD, 28.8 μL 4-tert-butylpyridine and 17.5 μL lithium salt acetonitrile solution (520 mg/mL) in 1 mL chlorobenzene. Finally, a layer of 100 nm Au was deposited as the positive electrode by thermal evaporation under a high vacuum. The active area of the device is 0.12 cm2. 2.3. Synthesis of adduct powders To synthesize the MAI�PbI2�DMSO adduct powder, 1 mmol of PbI2, MAI and DMSO were dissolved in 600 mg DMF, then, 5 mL diethyl ether was slowly added with stirring. After 5 min, the precipitate was collected and dried for 5 h under vacuum. The same procedure was repeated after 1 mmol of PbI2, MAI, DMSO and TAA, or 1 mmol of PbI2, MAI and TAA, were fully dissolved in 600 mg DMF to obtain MAI�P bI2�DMSO�TAA and MAI�PbI2�TAA adducts, respectively. 2.4. Characterizations The ultraviolet–visible (UV–Vis) absorption spectra of the perovskite films were measured by using a spectrophotometer (U-4100, HITACHI). X-ray diffraction (XRD) patterns of films were conducted on an X-ray diffractometer (D8 discover, Bruker). Fourier transform infrared (FTIR) spectroscopic data were recorded using Nicolet 5700 Spectrometer (Thermo Fisher) in KBr pellet at room temperature. Thermogravimetric (TGA) analysis was executed using Pyris 1-TGA (Perkin–Elmer). The samples were heated from 25 � C to 400 � C under dry air with a heating rate of 10 � C/min. Surface and cross-sectional microscopic images of the films and devices were obtained by scanning electron microscopy (SEM, S-4800, HITACHI) linked to an energy dispersive spectrometer (EDS). EDS analysis was conducted to analyze the elemental compositions of the perovskite films. The atomic force microscope (AFM) images were taken by using Dimension Edge AFM (Bruker). The steady-state photo luminescence (PL) spectra and time-resolved photoluminescence (TRPL) spectra were investigated by a PL spectrometer (FLS 920, Edinburgh Instruments), in which a 405 pulsed laser was employed as an excitation fluorescence source. Electrochemical impedance spectroscopy (EIS) was tested by a potentiostat (VersaSTAT 4, Princeton Applied Research) with the frequency ranging from 1 Hz to 1 MHz in dark. The current densi ty voltage (J V) analysis of perovskite solar cells were measured by a Keithley 2400 source meter with a solar simulator (Newport, 94022A) under one sun condition (AM 1.5 G, 100 mW cm 2), calibrated by a standard Si solar cell (PVM937, Newport). The external quantum effi ciency (EQE) was obtained by an EQE measurement system (Model QEX10, PV Measurements, Inc.). Both J-V and EQE measurements of devices were carried out in an ambient atmosphere without encapsulation.
2. Experimental 2.1. Materials Thioacetamide (TAA, 99%), Tin (II) chloride dehydrate (SnCl2⋅2H2O, 98%), dimethyl sulfoxide (DMSO, 99.9%), dimethylformamide (DMF, 99%), 4-tertbutylpyridine (TBP, 96%) and Bis(trifluoromethane)sulfo nimide lithium salt (99.95%) were purchased from Sigma-Aldrich. Spiro-OMeTAD (99.8%), lead iodide (PbI2, 99.99%) and CH3NH3I (MAI, 99.5%) were obtained from Xi’an Polymer Light Technology Corporation. Chlorobenzene (CB, 99.8%), acetonitrile (99%) were or dered from Aladdin. Diethyl ether was purchased from Sinopharm Chemical Reagent Co., Ltd. All the chemicals were used as received without any further purification. 2.2. Perovskite solar cells fabrication The device was assembled on patterned ITO substrates which were cleaned under sonication for 5 min by deionized water, detergent, acetone and ethanol in sequence. Then the SnO2 electron transport layers (ETLs) were deposited on the cleaned ITO substrates by spincoating the precursor solution of 19 mg/mL SnCl2⋅2H2O in ethanol at 3000 rpm for 30 s. Then the prepared substrates were annealed at 100 � C for 5 min on a hotplate and at 230 � C for 1 h in a muffle furnace, respectively. The thickness of the obtained SnO2 layers is about 40 nm. Before coating MAPbI3, the substrates were treated with UV-ozone for 10 min. The MAPbI3 precursor solution was synthesized by dissolving 1 mmol of PbI2, MAI and DMSO in 600 mg DMF, while solutions with various TAA contents were prepared by adding 0.5%, 1.0% and 2.0% (molar ratio to PbI2) of TAA into the former solution respectively. The
3. Results and discussions The XRD spectra have been measured to study the effect of TAA on the phase and crystallization of perovskite films. Fig. 1a compares the XRD patterns of perovskite films deposited on glass substrates without and with 0.5%, 1.0%, 2.0% (molar ratio to PbI2) TAA. The main char acteristic peaks at 13.98� , 28.33� and 31.73� , which are associated with (110), (220), (310) crystal planes for the tetragonal phase of MAPbI3 perovskite [37], are in identical positions for all the perovskite films with and without TAA. Meanwhile, no additional peaks are detected. 2
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Fig. 1. (a) XRD patterns of perovskite films with various TAA contents (0, 0.5%, 1.0% and 2.0%). (b) FTIR spectra of TAA powder, DMSO liquid, MAI�PbI2�DMSO adducts, MAI�PbI2�DMSO�TAA adducts and MAI�PbI2�TAA adducts. (c, d) The fingerprint regions for C¼ S stretch and S– –O stretch. The arrows in different colors in – S stretch from 698 cm 1 to 710 cm 1 when TAA is sharing the Lewis acid of Pb2þ with DMSO to form MAI�PbI �DM Fig. 1c indicate the reduced redshift of C– 2 SO�TAA adduct. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
These results reveal that the tetragonal phase of MAPbI3 perovskite is retained without any new phase generated by adding up to 2.0% TAA in the perovskite films. Notably, compared to the perovskite films without TAA, the films with TAA, especially with 1.0% TAA, display much stronger diffraction peak intensity. The (110) peak of perovskite layer with 1.0% TAA is twice that of the sample without TAA. The results indicate that the crystallinity is significantly enhanced in the films with 1.0% TAA, even though the diffraction peak intensity decreases when the TAA content increases to 2.0%. The XRD results demonstrate that appropriate TAA addition is beneficial for the crystallization and growth of perovskite thin films. In addition, as displayed in the UV–Vis ab sorption spectra in Fig. S1, the perovskite films with various TAA con tents nearly show identical curves, indicating that TAA does not affect the band gap of MAPbI3, which is consistent with XRD characterization. The intermolecular interactions upon TAA addition were confirmed by comparing the FTIR spectra of as-prepared adducts without and with TAA (MAI�PbI2�DMSO, MAI�PbI2�DMSO�TAA and MAI�PbI2�TAA), pure DMSO and TAA powder, as shown in Fig. 1b, and the photograph of MAI�PbI2�DMSO and MAI�PbI2�DMSO�TAA adducts is shown in – S stretch and S– – O stretch are Fig. S2. The finger print regions of C– magnified in Fig. 1c and d, respectively. The peaks at around 715 and – S stretch, and 1031 cm 1 in the TAA powder are originated from C– obviously these two peaks are redshifted to 698 and 1018 cm 1 in MAI�PbI2�TAA, respectively. The clear observed peak redshifts in – S stretch caused by MAI�PbI2�TAA are attributed to the weakened C– strong interaction of TAA with Pb2þ, indicating the formation of an – O stretch from DMSO is shifted from 1040 adduct. Similarly, the S– cm 1 to 1020 cm 1 upon MAI�PbI2�DMSO formation. When adding TAA in MAI�PbI2�DMSO, because TAA is sharing the Lewis acid of Pb2þ – S stretch peak at 715 cm 1 is slightly shifted to 710 with DMSO, the C– 1 cm , rather than 698 cm 1 as in MAI�PbI2�TAA. It verifies the strong interaction of Pb2þ with TAA and DMSO, and confirms the existence of MAI�PbI2�DMSO�TAA rather than the mixture of MAI�PbI2�DMSO and MAI�PbI2�TAA upon TAA addition. Furthermore, in the magnified XRD patterns of adducts with various TAA contents (Fig. S3), upon TAA addition, the (021) peak of MAI�PbI2�DMSO is significantly shifted to smaller angle, meanwhile, the (002) peak intensity increases with increasing TAA content. Both the significant shift of (021) peak and the obvious increase of (002) peak intensity are resulted from the expanded lattice of MAI�PbI2�DMSO with the TAA molecule embedding, further confirming the formation of MAI�PbI2�DMSO�TAA. In addition, the – S stretch peak at 1031 cm 1 is significantly shifted to 1018 cm 1 in C– the MAI�PbI2�DMSO�TAA, which might be attributed to the
– S stretch from TAA and the S– – O stretch from combination of the C– DMSO, which is shifted from 1040 cm 1 to 1020 cm 1 upon MAI�P bI2�DMSO formation. Based on the above discussion, it is evident that TAA strongly interacts with Pb2þ and further strengthens the intermo lecular interactions by forming MAI�PbI2�DMSO�TAA adducts. SEM images were recorded to investigate the effect of TAA on the surface morphology of perovskite films, as shown in Fig. 2a–d. Notably, as the TAA content increases from 0 to 1.0%, the maximum grain size increases significantly from 300 nm to 1500 nm. However, the grain size dramatically decreases with TAA content increasing to 2.0% (shown in Fig. 2d), and then it keeps unchanged upon further increasing TAA content to 5.0% and 10.0% (shown in Fig. S4). The dependence of average grain size on TAA content was displayed in Fig. S5. Such an unusual grain size change tendency is completely different from those reported in literatures, in which, the grain size of perovskite films in creases with increasing additives content, resulting in isolated grains under large additive content [21–23]. The grain size distribution his tograms with different TAA ratios are summarized in Fig. S6. The sam ples without TAA and with 2.0% TAA show similar grain size distribution with average grain size of 249 nm and 230 nm, respectively. On the other hand, the films with 0.5% and 1.0% TAA present wider distribution with some small grains, which are inevitable because small grains are essential to fill in the gap between large gains during dense film formation. However, an abnormally rough surface with uneven grain size is observed in the film with 0.5% TAA, as shown in Fig. S7, because insufficient TAA in the precursor solution cannot control uni form growth of perovskite crystals. The perovskite film with 1.0% TAA exhibits high surface coverage and more excellent crystallites, with an average size of 960 nm, demonstrating that adding appropriate TAA is beneficial for MAPbI3 morphology and the optimal TAA content is 1.0%. As shown in Fig. 2e, the perovskite solar cell device with 1.0% TAA exhibits clear layers in the cross-sectional SEM image, and the thickness of MAPbI3 with 1% TAA is about 400 nm, the same as that of the MAPbI3 without TAA (Fig. S8). The schematic structure is depicted accordingly. Based on above characterization results, a working mechanism is proposed to demonstrate how volatile TAA affect perovskite film crys tallization. As illustrated in Fig. 3, the precursor solution with 1.0% TAA was dropped and spin coated onto the substrate. After washing by ether, the obtained transparent film appears as mixed adducts including MAI�PbI2�DMSO and MAI�PbI2�DMSO�TAA [9]. The molecular struc tures of MAI�PbI2�DMSO and MAI�PbI2�DMSO�TAA are presented in Fig. S9. As shown in Fig. S10a, the decomposition point of MAI�P bI2�DMSO at 70.1 � C is lower than that of MAI�PbI2�DMSO�TAA at 3
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Fig. 2. (a)-(d) Top view SEM images of perovskite films with various TAA contents (0, 0.5%, 1.0% and 2.0%). (e) Cross-sectional SEM image and the schematic structure of the perovskite solar cell device with 1.0% TAA.
reduce perovskite solar cells performance. Other Lewis bases, which are volatile at much higher temperatures than the annealing temperature of MAPbI3 films, for example, urea (volatile at 130 � C) or thiourea (volatile at 180 � C), would remain in perovskite film during annealing and continuously regulate the crystallization of perovskite [21,22]. How ever, excessive non-volatile Lewis base would be residual in the GBs and slow down charge carrier transport, thus degrade the device performance. The PL spectra and TRPL spectra of MAPbI3 films without and with 1.0% TAA were measured to investigate the influence of TAA on the properties of charge-carrier in MAPbI3 films. As shown in Fig. 4a, compared to the normalized PL spectra of MAPbI3 films without TAA, the PL peak at 773 nm of the films with 1.0% TAA displays a blueshift with slightly narrower half peak width, indicating reduced band-edge trap states in the films with 1.0% TAA owing to enlarged grains with fewer GBs [40,41]. Fig. 4b presents the TRPL decay spectra of the films without and with 1.0% TAA, which can be well fitted with a mono exponential formula, I(t) ¼ A exp( t/τ) þ y0, where A is the relative amplitude, τ is the slow decay lifetime from radiative recombination of trapped charges in the bulk of MAPbI3 film. The characteristic param eters of time constants τ are extrapolated and listed in the inset of Fig. 4b. By adding 1.0% TAA, the lifetime τ increases significantly from 104.4 to 171.4 ns, which confirms that adding TAA can reduce defects and result in high-quality MAPbI3 films. The EIS of devices were measured under dark condition at 0.8 V bias. Fig. 4c shows the Nyquist plot and the equivalent circuit, which reveals the process of charge transfer [23,31]. The charge transport resistance (Rtr), represented by the radius of the arc, greatly decreases from 2831 to 2198 Ω with TAA addition, suggesting much more efficient charge transfer in the devices using MAPbI3 with 1.0% TAA as absorbers. The detailed numerical fitting parameters are listed in Table S1. In order to investigate the impact of TAA on the performance of solar cells, the devices were assembled with perovskite films with and without TAA by employing the n-i-p structure (Fig. 2e), where SnO2 and spiroOMeTAD were used as electron and hole transport layer respectively. The J-V characteristics of devices are shown in Fig. 5a and the detailed performance parameters are summarized in Table 1, which indicate enhanced photovoltaic performance with TAA addition. The device based on the film with 1.0% TAA exhibits an optimal PCE of 18.9%, with a Voc of 1.109 V, a Jsc of 22.91 mA/cm 2, an FF of 74.36%, as well as small hysteresis as shown in Fig. 5c, and the detailed photovoltaic pa rameters under reverse and forward scan directions are summarized in Table S2. On the other hand, the device without TAA shows a PCE of 17.01%, with a Voc of 1.086 V, a Jsc of 22.70 mA/cm 2, an FF of 68.96%
Fig. 3. Schematic process of perovskite crystal growth and film formation with 1.0% TAA.
78.5 � C. During the annealing process of the obtained transparent film at 100 � C on hotplate, MAI�PbI2�DMSO decomposes and nucleates first, while the MAI�PbI2�DMSO�TAA still exists around the nucleation, making the MAPbI3 film grow slowly and uniformly. Therefore, appro priate TAA addition retards the nucleation rate and grain growth rate, thus reduces the supersaturation, and results in large grain sizes during the crystallization and growth of TAA-modified films, as reported by Liu et al. [38]. On the other hand, an Ostwald ripening effect might further enlarge the grain size during the growth of TAA-modified films, ac cording to Cao’s recent report [39]. However, very low TAA content may result in uneven grain sizes, as shown in the perovskite film with 0.5% TAA, because insufficient TAA additive cannot generate even distribution of MAI�PbI2�DMSO�TAA. On the other hand, excessive TAA additive would generate a large amount of MAI�PbI2�DMSO�TAA adducts but most of which would simultaneously decompose at 78.5 � C (as shown in Fig. S10a). Therefore, excessive TAA does not help prevent supersaturation, and it cannot enhance the film crystallization. As a result, the grain size decreases with 2.0% TAA and then keeps un changed with further increasing. The detailed schematic process of film formation by adding various TAA contents are shown in Figs. S11 and S12. Furthermore, compared to other Lewis bases, TAA has negligible impact on perovskite solar cells performance. The volatile temperate of TAA is about 100 � C (as indicated in Fig. S10b), the same as the annealing temperature of MAPbI3 films, eventually TAA does not exist in the perovskite film after annealing at 100 � C, which is confirmed by EDS in Fig. S13. Therefore, adding TAA during the film growth would not 4
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Fig. 4. (a) PL spectra and (b) TRPL decay spectra and (c) Nyquist plots of the EIS spectra of perovskite films without and with 1.0% TAA. Fig. 5. (a) J-V curves of perovskite solar cells with various TAA contents (0, 0.5%, 1.0%, and 2.0%). (b) EQE spectra of perov skite solar cells without and with 1.0% TAA. (c) J-V curves of perovskite solar cells without and with 1.0% TAA under reverse and forward scan directions. (d) The statis tical distribution of PCEs for the devices with various TAA contents (0, 0.5%, 1.0%, and 2.0%), each group counts 20 devices. (e) Maximum-power-point current and PCE outputs measured at bias voltage of 0.845 V for the device without TAA and 0.894 V for the device with 1.0% TAA. (f) Normalized PCE aging curves of unsealed perovskite devices without and with 1.0% TAA under ambient condition with relative humidity about 25–35%.
and much more serious hysteresis. The improved FF of the device with 1.0% TAA is resulted from the enhanced charge transfer that is evi denced by the reduced Rtr in Fig. 4c, while the higher Voc is attributed to the effective suppression of charge carrier recombination (Fig. 4a and b), both of which are resulted from high-quality TAA-modified MAPbI3 films with larger grains, fewer boundary defects, enhanced crystalliza tion and reduced trap state density. The decrease of hysteresis in the device with 1.0% TAA is also attributed to the enlarged grain size and reduced trap state density in MAPbI3 films. In addition, the device with 2.0% TAA exhibits similar performance to the device without TAA, owing to nearly non-improved MAPbI3 films morphology compared to pristine perovskite. Fig. 5b presents the EQE spectra of devices
Table 1 Photovoltaic parameters extrapolated from J-V tests of devices with various TAA contents (0, 0.5, 1.0 and 2.0%). w/o TAA 0.5% TAA 1.0% TAA 2.0% TAA
Jsc (mA/cm2)
Voc(V)
FF(%)
PCE(%)
22.70 22.79 22.91 22.75
1.086 1.106 1.109 1.091
68.96 72.43 74.36 69.65
17.01 18.26 18.91 17.29
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assembled by MAPbI3 films without and with 1.0% TAA. Compared to the device without TAA, the spectral response from 400 to 775 nm is slightly enhanced in the device with 1.0% TAA, reflecting higher effi ciency of incident photon conversion into electric current, which con tributes to the enhancement of Jsc in J-V measurement. The statistical distribution of PCEs for the devices with various TAA contents is shown in Fig. 5d, indicating that all the devices exhibit excellent repeatability. Upon increasing TAA content, the average PCE first increases to a peak value at 1.0% TAA content and then decreases, which is in consistence with the optimal PCE results. The steady out measurement (MPP) characterizations of the devices without and with 1.0% TAA were per formed, and the results are shown in Fig. 5e. Compared to the device without TAA, the device with 1.0% TAA exhibits faster response and reaches its maximal PCE of 18.6%, which is consistent with the PCE extracted from the J-V curve. These results further confirm the reduced hysteresis behavior of the device with 1.0% TAA, which is in agreement with the results in Fig. 5c. Furthermore, the aging test was carried out by storing the devices without and with 1.0% TAA under ambient condition with relative humidity (RH) about 25–35%, and the result is shown in Fig. 5f. Compared to the device without TAA, the device with 1.0% TAA exhibits much slower PCE degradation and retains over 88.9% of its initial PCE after 816 h aging, because the perovskite films with TAA modification exhibit larger grains and reduced trap states, which effectively inhibit oxygen and moisture permeating into the perovskite films through GBs. As a result, the stability of perovskite solar cells is strongly enhanced by employing TAA as additive to fabricate high quality MAPbI3 films.
Acknowledgement This work was supported by Natural Science Foundation of Zhejiang Province (No. LY17F040005), National Natural Science Foundation of China (No. 61704154, 61604131), Science Foundation of Zhejiang SciTech University (ZSTU) under Grant No. 15062021-Y, Visiting Scholar Foundation of State Key Laboratory of Silicon Materials (No. SKL201809). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.solmat.2020.110435. References [1] T.J. Jacobsson, J.P. Correa-Baen, M. Pazoki, M. Saliba, K. Schenk, M. Gratzel, A. Hagfeldt, Exploration of the compositional space for mixed lead halogen perovskites for high efficiency solar cells, Energy Environ. Sci. 9 (2016) 1706–1724. [2] A. Miyata, A. Mitioglu, P. Plochocka, O. Portugall, J.T.W. Wang, S.D. Stranks, H. J. Snaith, R.J. Nicholas, Direct measurement of the exciton binding energy and effective masses for charge carriers in organic-inorganic tri-halide perovskites, Nat. 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4. Conclusions In conclusion, we have successfully fabricated high quality perov skite films by introducing TAA as an additive and assembled perovskite solar cells based on the films with various TAA contents. With increasing TAA content, the average grain size of perovskite films increases continuously to a maximum value of 960 nm in the sample with 1.0% TAA, but the grain size in the sample with 2.0% TAA decreases dramatically to the value of the sample without TAA, and then the average gain size keeps nearly unchanged upon further increasing TAA content up to 10%. This unusual grain size variation tendency is attributed to the volatility of TAA, suggesting additive volatility as a potential criterion to choose appropriate additives. Furthermore, a mechanism is proposed based on various characterizations to demon strate how volatile TAA works during perovskite crystallization. Finally, a PCE of 18.9% and improved stability were obtained in the perovskite solar cell assembled by the MAPbI3 film with 1.0% TAA. The results strongly suggest that the photovoltaic performance of perovskite solar cells can be significantly improved by large grains, high crystallization and reduced trap state density of the high quality MAPbI3 films by TAA modification. Author contributions Can Cui and Xuegong Yu supervised the whole project. Can Cui and Danyan Xie designed the experiments and drafted the manuscript. Danyan Xie and Siyuan Che conducted most of experiments. Ping Lin contributed to the PL and TRPL test. Ke Xiao and Haihua Hu contributed to the XRD and SEM measurements. Peng Wang, Lingbo Xu and Deren Yang revised the manuscript. All authors contributed to the discussion of the paper Declaration of competing interest We declare that we have no conflict of interest.
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