Electrochimica Acta 282 (2018) 653e661
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A NH4F interface passivation strategy to produce air-processed highperformance planar perovskite solar cells Ziqiu Ren a, Na Wang a, Menghua Zhu a, Xin Li a, *, Jingyao Qi b, ** a b
School of Chemistry and Chemical Engineering, State Key Lab of Urban Water Resource and Environment, Harbin Institute of Technology, 150001, China School of Environment, Harbin Institute of Technology, 150001, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 March 2018 Received in revised form 15 June 2018 Accepted 15 June 2018 Available online 18 June 2018
The simple assembling process and lower manufacturing cost are crucial for wide application of perovskite solar cells. In this work, we report an efficient interface passivation strategy for preparing high-performance planar perovskite solar cells, which does not require well-controlled moisture and oxygen atmosphere. A NH4F solution-based additive is introduced to passivated TiO2 electron transfer layer, which offers reduction of oxygen-induced defects and residual surface hydroxyl groups, and lower trap state level. Using a typical planar structure of FTO/NH4F-TiO2/CH3NH3PbI3/Spiro-OMeTAD/Au, the champion device achieves a power conversion efficiency of 15.61%. After 28-day durability test, the device with NH4F-treated TiO2 storing in air without any encapsulation exhibits a little efficiency loss (<10%). Our work demonstrates a generic approach to develop air-processed efficient planar perovskite solar cells by interface engineering for future commercialization. © 2018 Elsevier Ltd. All rights reserved.
Keywords: Perovskite solar cells TiO2 NH4F solution treatment Oxygen-induced defects Interface passivation
1. Introduction Once that the perovskite solar cell (PSC) based on the inorganicorganic perovskite materials (CH3NH3PbI3) appeared in photovoltaic field, it has attracted extensive attention because of the amazing power conversion efficiency (PCE) [1e5]. Besides an unremitting pursuit of PCE, currently, increasing research efforts have been made to farther improve the device stability and reduce the manufacturing cost of PSCs [6e11]. Nowadays, high-quality perovskite films have to be constructed in the glovebox under inert gas shielding in most studies to avoid contact with the surrounding moisture and oxygen. Obviously, this rigorous preparation condition leads to the increase in manufacturing cost of PSCs. To obtain efficient photovoltaic devices in air, a few attempts, including the thickness regulation on mesoporous TiO2 layer [12], the exploitation of all-inorganic device structure [13], and the preheating procedure for FTO or PbI2 substrate [14,15], have been done. Successively, a new lead source of lead (II) thiocyanate (Pb(SCN)2) was applied into the two-step deposition method, achieving a PCE of 15.12% [16]. Recently, a
* Corresponding author. ** Corresponding author. E-mail addresses:
[email protected] (X. Li),
[email protected] (J. Qi). https://doi.org/10.1016/j.electacta.2018.06.112 0013-4686/© 2018 Elsevier Ltd. All rights reserved.
skillful pressure processing method produced a high-quality perovskite film in ambient air for the large-scale PSCs [17]. In our previous research, we have attempted to apply the isopropanol modification and ultrasonic-assisted method to fabricate efficient PSCs in air [18,19]. Despite making some progresses, so far, the development of an air-processed generic method is still a huge challenge. As a simple, but effective technical means, the interface engineering has been implemented to promote the further development of PSCs [20e23]. In PSCs, there are two interfaces focusing solely on the perovskite light absorption layer, i.e. electron transfer layer (ETL)/perovskite interface and perovskite/hole transfer layer (HTL) interface, which prominently affect carrier separation, transfer, and recombination [24,25]. Particularly, the interfaces of planar PSCs can determine the photovoltaic performance in some degree [26]. As a classical ETL in both the planar and mesoporous PSCs, the TiO2 can effectively transfer photo-generated electrons in perovskite layers [27,28]. However, the TiO2 performs as a photocatalyst under ultraviolet light to extract the electrons in organic groups as well, which can reduce the light resource capture and aggravate the degradation of PSCs [29]. This photocatalytic process of TiO2 is closely related to the surface oxygen vacancies and deep traps of TiO2. Therefore, a few functional interlayers have been inserted between the TiO2 ETL and perovskite layer based on the interface engineering to address this issue. The group of Ito
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employed the Sb2S3 interlayer to successfully passivate the interface defects and then improve the operational stability [30]. Based on the self-assembly monolayer technique, Snaith and co-authors had favorably arranged the fullerene molecules onto the TiO2 surface by the carboxylic acid anchoring group [31]. The photovoltaic performance of modified PSCs was notably promoted depending on excellent electron transfer ability and defect passivation effect of fullerene. In their further research, the CsBr layer was introduced into the TiO2/perovskite interface to reduce the defect density and suppress the degradation of perovskite materials [32]. Recently, Tan et al. demonstrated that the chlorine locating at the TiO2/perovskite interface could passivate the defects and stabilize the interface [33]. In addition to the interface defects, the solvents especially dimethyl formamide used in the solution assembling process of PSCs had a strong adsorption effect on the TiO2 surface resulting in the degradation of perovskite films and the decrease of photovoltaic performance [34,35]. The residual hydroxyl groups on the TiO2 surface, which were inevitable due to the hydrolysis preparation process of TiO2, could provide favorable adsorption sites for organic solvents. To address this problem, Ma et al. demonstrated a strategy of introducing the (6,6)-phenyl-C61-butyric acid into the TiO2/ perovskite interface to suppress the adsorption of TiO2 [36]. Noteworthily, most of these interface-engineering techniques were performed in the glovebox under inert gas shielding. Therefore, it is desired to develop an efficient ETL using interface engineering and understand corresponding mechanisms for air-processed highperformance PSCs. Herein, we have demonstrated an interface engineering strategy of applying the NH4F solution to modify the TiO2 ETL in PSCs as the schematic diagram in Fig. 1. With a planar structure of FTO/NH4FTiO2/CH3NH3PbI3/Spiro-OMeTAD/Au, all-air assembling fabrication has been exploited to construct highly effective and stable PSCs yielding the highest PCE of 15.61%. Larger perovskite crystalline grains and more compact perovskite films have been prepared onto the NH4F-treated TiO2 ETLs. Additionally, the PSCs with NH4F-treated TiO2 produce a better operational stability, of which the PCE maintained over 90% even after 28-day storing in air without any encapsulation. Due to the treatment of NH4F solution, the oxygen-induced defects and surface hydroxyl groups have been effectively passivated, which accelerate the carrier transport and reduce the recombination in device. Moreover, the low trap state density in NH4F-treated TiO2
ETL has also been evaluated according to the space-charge-limited current results. With a facile but valid solution modification, this strategy demonstrated here has a promising application prospect in PSCs. 2. Experimental 2.1. Reagents and materials The ammonium fluoride (NH4F) and titanium (IV) isopropoxide (TTIP) were provided by Aladdin. All anhydrous solvents were obtained from Alfa Aesar. Lead iodide (PbI2), 4-tert-butypyridine and lithium bis (trifluoromethanesulfonyl) imide (Li-TFSI), methylammonium iodide (CH3NH3I), and 2,20 ,7,7' -tetrakis-(N, N-di-4methoxyphenyl amino)-9,9'spirobifluorene (spiro-OMeTAD) were purchased from Youxuan Trade Co., Ltd. All chemicals and solvents were employed as received without further purification. 2.2. Device fabrication The fluorine-doped tin oxide (FTO) glasses (1.5 cm 1.0 cm) were etched 0.5 cm with Zn powder and 2 M hydrochloric solution along the length and then washed by ultrasonic cleaner in deionized water, detergent solution, ethyl alcohol and acetone respectively. The ultraviolet-ozone (UV-O3) cleaner was used to do the surface cleaning of FTO glasses after drying at 100 C in 30 min. To prepare TiO2 precursor, 738 mL titanium (IV) isopropoxide (TTIP) was slowly added into 5.06 mL isopropanol under stirring denoted as solution-A and 70 mL 2 M hydrochloric was dropped into 5.06 mL isopropanol under stirring denoted as solution-B. Then solution-B was dropwise added into the solution-A under stirring to obtain the TiO2 precursor. The 60 mL as-obtain TiO2 precursor filtered through a 0.45 mm PVDF syringe filter was deposited onto the cleaned FTO substrates through spin-coating (3000 rpm for 30 s). After 10-min air drying at 100 C, the substrates were transferred to a muffle furnace for the annealing at 500 C for 45 min. After cooling at room temperature, the FTO/TiO2 substrates were dipped in the NH4F solution of absolute methanol (0.1 mol L1) for 30 s and washed with absolute methanol, followed by drying at 80 C for 20 min. Then the PbI2 solution (450 mg mL1) in dimethyl formamide was spin-coated onto the FTO/NH4F-TiO2 (FTO/TiO2) substrates at 3000 rpm for 30 s. After drying at 80 C for 30 min, the
Fig. 1. Schematic diagram of the device construction and the carrier extraction in PSCs.
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CH3NH3I solution (60 mg mL1) was spin-coated onto the cooling PbI2 film at 3000 rpm for 30 s. Then the substrates were immediately transferred to the oven at 100 C for 20-min annealing process. 90 mg Spiro-OMeTAD dissolved in 1 mL chlorobenzene, containing 17.5 mL the additives (520 mg Li-TFSI in 1 mL acetonitrile) and 28.8 mL 4-tert-butylpyridine. The 50 mL Spiro-OMeTAD solution covered onto the surface of the perovskite layer by spincoating at 4000 rpm for 20 s. Finally, Au was evaporated under high vacuum (8 104 Pa) as the counter electrodes. The metal mask was used to calibrate the device active area (0.12 cm2). 2.3. Characterization and measurement A Empyrean Panalytical X'Pert Pro X-ray diffractometer with Cu Ka radiation (l ¼ 1.5418 Å) was employed to characterize the crystal structure. A field-emission scanning electron microscope (FEI HELIOS NanoLab 600i) was used to analyze the morphology of the sample. Scanning Kelvin Probe Microscopy (SKPM) test was performed by Bruker Dimension FastScan Scanning Probe Microscope (SPM). X-ray photoelectron spectroscopy (XPS) was carried out using a photoelectron spectrometer (PHI 5400 ESCA System, Al Ka). The current density-voltage (i-V) data were collected by the electrochemical workstation with a scan rate of 0.2 V s1 (VersaSTAT 3, Ametek, USA) under AM1.5G illumination (100 mW cm2, Newport 94021A) standardized by a standard Si solar cell (1218, Newport,
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USA). The monochromatic incident photon-to-electron conversion efficiency (IPCE) was measured by the Crowntech solar cell quantum efficiency measurement system (QTest Station 500AD, USA) containing a monochromator, a chopper, a lock-in amplifier, and a multimeter (Keithley Model 2000). Photoluminescence (PL) spectra were recorded on Fluorescence Spectrometer (LS55 Perkin Elmer, PE) with excitation wavelength of 400 nm. Electrochemical impedance spectroscopy (EIS) was measured under darkness by the electrochemical workstation (VersaSTAT 3, Ametek, USA) with a 10 mV rms amplitude. The frequency range of 100 kHz-0.1 Hz and logarithmic frequency distribution were employed. The EIS was collected after applying a 10 s bias of 0.9 V. The space-chargelimited current (SCLC) curves for the device of FTO/Pristine TiO2 or NH4F-TiO2/Au were measured under the linear sweep method with the voltage range from 5.0 V to 5.0 V by the electrochemical workstation (VersaSTAT 3, Ametek, USA). 3. Results and discussion 3.1. Characterization of TiO2 ETL and perovskite films The superior n-type TiO2 ETL in PSCs should not only extract the photo-generated electrons effectively but also block the holes, moreover, the surface of TiO2 layer has a significant effect on the morphology of upper perovskite film, which is an important part of
Fig. 2. SEM images of (a) TiO2 film with NH4F treatment, (b) pristine TiO2 film, (c) perovskite films spin-coated on NH4F-treated TiO2 film, (d) perovskite films spin-coated on pristine TiO2 film, and (e) XRD patterns of perovskite films on pristine and NH4F-treated TiO2 film.
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evaluating the film quality. The hydrolysis method of TTIP in acidic condition and the NH4F solution have been employed to prepare and modify the TiO2 ETL layer in this work (see Experimental section). As shown in Fig. 2a and b, the morphologies of TiO2 films with and without NH4F treatment have been surveyed by the SEM respectively. Pristine TiO2 layer presents unevenness particle distribution and rough film surface. By comparison, compact and pinhole-free TiO2 ETL layer has been obtained after NH4F treatment. This improvement on TiO2 film morphology may come from the dissolution effect of NH4F solution to the TiO2 nanoparticles, which can remove bulged particles and smooth the film surface [37,38]. On these two different ETL substrates, the perovskite films have been coated through the two-step solution deposition method (see Experimental section). According to the SEM images as shown in Fig. 2c, the perovskite film spin-coated on the NH4F-treated TiO2 substrate demonstrates larger crystalline grains and closer contact among grains. However, it is obvious that the perovskite film on pristine TiO2 substrates is composed of smaller crystalline grains and contains ubiquitous pinholes as shown in Fig. 2d. The existing pinholes lead to the direct contact of the ETL and HTL and provide recombination centers to trap photo-generated carriers. As previously reported, larger crystalline grains of perovskite, which mean fewer bulk defects and easier exciton separation, exert a positive influence on promoting the photovoltaic performance [39]. From the XRD results of perovskite films on these two different TiO2 substrates as Fig. 2e, there are characteristic diffraction peak consistent with the tetragonal phase CH3NH3PbI3. The (110), (211), (202), (220), (310), (224), and (314) planes of tetragonal phase CH3NH3PbI3 at 14.1, 23.4 , 24.5 , 28.5 , 31.6 , 40.4 , and 43.1 have been marked in the diffraction patterns respectively [40,41]. It can
be concluded that same crystal structure has been prepared even on different TiO2 substrates. The surface chemical state and electronic state of the elements existing in TiO2, which can be estimated through XPS measurement, have a significant effect on electron transfer and recombination in PSCs. From the full XPS spectra shown in Fig. 3a, the peaks of Ti and O elements can be found in both samples of pristine TiO2 and NH4F-treated TiO2. The weak characteristic peak of F1s appears in the spectroscopy of NH4F-treated TiO2, this existing is further verified through the detailed survey for F1s at 684.7 eV as shown in Fig. 3b. There is no characteristic peak of F1s in the pristine TiO2 as expected, suggesting that the full coverage TiO2 film has been prepared onto the FTO. As shown in Fig. 3c of Ti 2p peaks, there are no obvious differences for pristine TiO2 and NH4F-treated TiO2 samples. The Ti2p3/2 and Ti2p1/2 peaks for these two kind TiO2 films are both at 458.2 eV and 463.9 eV, respectively, corresponding to tetravalent titanium bonding with oxygen [42]. It is expected that the surface treatment of TiO2 in NH4F solution has no significant effect on the TiO2 bulk structure. According to the XPS-peakdifferentiation-imitating analysis (Gaussian distributions) of O1s spectra shown in Fig. 3d, there are mainly three O chemical states, including the oxygen bonded with titanium (OTi), the oxygen vacancy (OV), and the hydroxyl oxygen (OH) locating at about 529.6 eV, 530.8 eV, and 532.2 eV respectively [43]. That means it can be estimated, with reasonable calculation of each peak area ratio (OV, OH) in total peak area (OTiþOVþOH), the relative content of oxygen vacancy and hydroxyl oxygen. As the peak position, peak area and calculated area ratio of OTi, OV, OH shown in Table 1, the OV existing in the TiO2 has been affected by NH4F solution treatment, resulting in a remarkable decrease of OV relative content from
Fig. 3. (a) Full XPS spectra of pristine TiO2 and NH4F-treated TiO2; (b) F1s, (c) Ti2p, and (d) O1s of pristine TiO2 and NH4F-treated TiO2.
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Table 1 The peak position, peak area and calculated area ratio of OTi, OV, OH in whole O1s peak area for the pristine and NH4F-treated TiO2 film. O1s OTi Ov OH
position (eV) area position (eV) area position (eV) area
OTi area ratio (%) Ov area ratio (%) OH area ratio (%)
24.31% to 17.73%. This exciting result indicates that the NH4F solution treatment may passivate the oxygen-induced defects in TiO2, which lead to trapping the photon-generated carriers and providing recombination centers in photovoltaic process. Moreover, the relative content of OH inevitably existing in TiO2 synthesized with hydrolysis reaction in acidic medium has also been reduced from 13.55% to 6.16%. These hydroxyl groups at interface of TiO2/perovskite may provide bonding sites for the organic solvents and then accelerate the degeneration of perovskite materials [44,45]. The decrease of hydroxyl groups is beneficial to the operational stability of photovoltaic device. 3.2. Photovoltaic performance of planar PSCs fabricated in air The planar PSCs with the architecture of FTO/Pristine TiO2 or NH4F-treated TiO2/CH3NH3PbI3/Spiro-OMeTAD/Au have been constructed in air successfully. The forward and reverse scan i-V curves
TiO2
NH4F-TiO2
529.6 2314.8 530.7 905.6 532.2 504.9 62.14 24.31 13.55
529.7 2475.8 530.9 576.8 532.1 200.4 76.11 17.73 6.16
of PSCs with pristine TiO2 and NH4F-treated TiO2 (denoted as NH4FPSCs) are shown in Fig. 4a to evaluate photovoltaic parameters. After NH4F solution treatment, the champion device yields a 15.61% PCE under reverse scan, which is far better than the 12.17% PCE of device with pristine TiO2. These improvements for NH4F-PSCs are embodied in all photovoltaic parameters including the isc from 19.07 to 21.37 mA cm2, Voc from 0.96 to 1.04 V, and FF from 66.48 to 70.24% (Table 2), which can be attributed to the smooth TiO2 ETL layer and larger crystalline grains as observed above through SEM images. Except the high-quality perovskite films prepared onto the NH4F-treated TiO2, the superior interface of NH4F-treated TiO2 and perovskite, owning fewer oxygen-induced defects, can also contribute to this promotion in photovoltaic performance due to faster electron extraction and transfer. In addition, the hysteresis effect of i-V curves under forward and reverse scan has been suppressed dramatically as the treatment for TiO2 ETL with NH4F solution. According to the changes of surface potential of pristine TiO2
Fig. 4. (a) The i-V curves with forward and reverse scan under the 0.2 V s1 scan rate (black line, reverse scan curve for NH4F-PSCs; red line, forward scan curve for NH4F-PSCs; green line, reverse scan curve for PSCs; blue line, forward scan curve for NH4F-PSCs), (b) IPCE spectra, (c) stabilized output of isc and PCE, and (d) PCE histograms of PSCs with pristine TiO2 and NH4F-treated TiO2 (denoted as NH4F-PSCs). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
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Table 2 The photovoltaic parameters (isc, Voc, FF and PCE) of champion devices with the pristine and NH4F-treated TiO2 ETL under forward and reverse scan. Devices PSCs NH4F-PSCs
Forward Reverse Forward Reverse
isc [mA cm2]
Voc [V]
FF [%]
PCE [%]
Average PCE [%]
18.95 19.07 21.30 21.37
0.90 0.96 1.02 1.04
63.61 66.48 66.65 70.24
10.85 12.17 14.48 15.61
11.51
and NH4F-treated TiO2 films obtained from SKPM test, the variation of work function can be indicated. Based on the results in Fig. S1, the NH4F-treated TiO2 films demonstrate lower work function of 4.40 eV (4.45 eV for pristine TiO2), which corresponds to an enhanced built-in potential. The promotion in Voc of NH4F-PSCs can be attributed to the increase of built-in potential [46]. The PCE of champion NH4F-PSC with forward scan slightly decreases to 14.48% with a isc of 21.30 mA cm2, a Voc of 1.02 V, and a FF of 66.65%, by contrast, the forward PCE for the device with pristine TiO2 drops sharply to 10.85% with a isc of 18.95 mA cm2, a Voc of 0.90 V, and a FF of 63.61%. The existing hysteresis in i-V measurement leads to unstable PCE outputs, which is a severe obstruction for the PSC application [47,48]. To ensure the objectivity and integrity of the results, we defined a parameter of Delta PCE to evaluate the suppression for the hysteresis effect, indicating the difference of average PCE values under forward and reverse scan. According to the 50 PCE values under forward and reverse scan for each kind of device respectively as shown in Table S1, Delta PCE can be calculated. The Delta PCE obtained from 50 devices with NH4F-treated TiO2 ETL is 0.97% point, which is much lower than the value of 1.47 calculated from the devices with pristine TiO2 ETL. As reported, ionic migration and defects especially at interface in the device, accumulating imbalance charge carriers, are the two primary origins of hysteresis [49,50]. In NH4F-PSCs, the reduction of oxygeninduced defects and the hydroxyl oxygen at the TiO2/perovskite interface can accelerate interfacial electron transfer and decrease recombination, undoubtedly resulting in lower photocurrent loss and hysteresis effect. Finally, the champion NH4F-PSC achieves an higher average PCE of 15.05% irrespective of scanning direction than the pristine PSC of 11.51% as shown in Table 2. The IPCE spectra for both samples shown in Fig. 4b reveal same light response range from 300 to 800 nm, while the NH4F-PSC exhibits a stronger response value about 75e85% than the pristine PSC only giving an IPCE value of 60e70%. According to the IPCE spectra, the integrated iscs of 17.64 mA cm2 for pristine PSC and 20.50 mA cm2 for NH4F-PSC can be calculated, in good agreement with the iscs obtained from the i-V results. The 200 s operating stabilities of iscs and PCEs with a bias voltage of 0.75 V for pristine PSC and 0.81 V for NH4F-PSC, which correspond to their maximum power points determined from the above i-V curves, are continuously monitor under AM1.5G illumination shown in Fig. 4c. The stable iscs and PCEs for both samples indicate the successful assembly of PSCs in air. Based on the above results at the maximum power point, the differential curves of isc and PCE versus time (d(isc)/dt vs t, d(PCE)/dt) for each kind of device can be plotted respectively in Fig. S2 (a) and (b) to reflect the variation of isc and PCE. Obviously, isc and PCE values of the NH4F-PSC reach to saturation faster in the differential curves. According to the previous work, the time to approach saturation of isc (PCE) during light soaking for the PSCs indicates the device stability [51,52]. As is wellknown, the stability of PSC is closely related to interface defects, which could be passivated by the NH4F treatment as demonstrated above by the XPS and SCLC results. Moreover, the efficiency of 50 devices with pristine TiO2 or NH4F-treated TiO2 are collected independently to form the histograms shown in Fig. 4d. The PCE values of NH4F-PSCs distribute in a range of 14e16%, which are
15.05
better than the pristine PSCs in 9e11%. 3.3. Interface defects and Carrier transport in air-processed PSCs Based on the quenching intensities of perovskite films onto FTO, TiO2/FTO, and NH4F-TiO2/FTO in PL spectra (Fig. 5a), the electron extraction ability of ETL in PSCs can be evaluated, where lower intensity of perovskite film on ETL reveals stronger electron extraction of ETL [53,54]. Comparing with sole perovskite films onto FTO substrates, perovskite films attached onto TiO2 ETLs demonstrate the distinct quenching effects in PL spectra. Notably, the NH4F-TiO2 as the ETL exhibits a stronger quenching effect compared with the pristine TiO2, meaning a faster extraction of photo-induced electrons. To investigate the recombination of carriers in photovoltaic devices, the Nyquist plots have been acquired from the dark EIS measurement with the 0.1 Hz-100 kHz frequency range (Fig. 5b), in which one semicircle reveals the recombination resistance as reported in literature [55,56]. The NH4F-PSC shows a
Fig. 5. (a) PL spectra of perovskite films onto FTO, TiO2/FTO, and NH4F-TiO2/FTO, (b) Nyquist plots of pristine PSC and NH4F-PSC acquired from the dark EIS measurement under a bias of 0.9 V.
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larger recombination resistance resulting decline in carrier recombination. This increase in recombination resistance can also be proved by the dark i-V curves as shown in Fig. S3. As demonstrated above, the defect passivation of TiO2/perovskite interface due to the NH4F treatment reduces the recombination potential of carrier transporting process. To gain further insight into the trap density in TiO2 ETL, the space-charge-limited current (SCLC) measurement is employed for the device of FTO/Pristine TiO2 or NH4F-TiO2/Au shown in Fig. 6a. In the SCLC curves, the current versus low bias voltage expresses a linear relation of ohmic response, however, this linear relation is broken with increasing voltage to a specific value [57,58]. According to this rapid non-linear increase of current in middle bias voltage, the trap-filled limit voltages (VTFLs) of 0.61 V for pristine TiO2 and 0.29 V for NH4F-TiO2 can be estimated [59,60]. Based on the VTFL, the trap state density (ntrap) can be calculated by the following equation (1) [61,62],
VTFL ¼
Fig. 6. (a) SCLC curves for the device of FTO/Pristine TiO2 or NH4F-TiO2/Au, (b) the linear scanning for Ti, Pb, and I elements of NH4F-PSC cross section.
entrap L2 2ε0 ε
(1)
where 3 is the relative dielectric constant of TiO2 (3 ¼ 48) [43], 3 0 is vacuum permittivity (3 0 ¼ 8.854 1012 F/m), e is the elementary charge (e ¼ 1.60 1019 C), L is the thickness of TiO2 film. According to the linear scanning for Ti, Pb, and I elements of the device cross section (Fig. 6b), the thickness of TiO2 ETL in PSCs can be estimated (L ¼ 50 nm). After NH4F solution treatment, the VTFL drops from 0.61 V to 0.29 V, consequentially, the trap state density is prominently decreased from 1.30 1016 cm3 to 0.62 1016 cm3. These characterization results above all demonstrate that the defects in TiO2 have been passivated effectively as the NH4F solution
Fig. 7. Durability test of pristine PSCs and NH4F-PSCs, (a) normalized PCE, (b) normalized isc, (c) normalized Voc, (d) normalized FF as a function of storage time (days) in air.
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treatment, leading to less recombination and faster transfer of photon-generated carriers in NH4F-PSCs. The passivation of defects in TiO2 reduces the obstacles during electron transport process, alleviating some of interfacial charge accumulation.
Acknowledgement This work is supported by the National Natural Science Foundation of China (51579057, 51779065) and the State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology (2016DX07).
3.4. Stability test of PSCs fabricated in air The durability of photovoltaic device is an important evaluation indicator for practical application, which is one of the greatest challenges in PSCs owing to fragile organic components in perovskite materials [63,64]. Therefore, 28-day durability tests of representative pristine PSC and NH4F-PSC, which are stored in air without any encapsulation and collected the i-V data every fourth day (T ¼ 20e30 C, RH ¼ 30e50%), are performed. To ensure assessment reliability, the photovoltaic parameter (average isc, Voc, FF and PCE) variations of three respective NH4F-PSCs and pristine PSCs have been surveyed (Table S2 and Table S3). Based on the results, the mean PCEs of three devices are presented in Fig. 7a, where the NH4F-PSCs exhibit over 90% original PCE after 28 days. However, in comparison, the PCE of pristine PSC sharply degenerates after 8 days falling to less than 80% original PCE after 28day storing in air. The normalized iscs, normalized Vocs, and normalized FFs of both kind devices as a function of storage time are presented in Fig. 7bed respectively. It can be indicated that the declines of isc and Voc for NH4F-PSC have a far greater impact on the PCE. According to the degeneration curves, the photovoltaic parameters of pristine PSC, i.e. isc, and Voc decrease dramatically from the eighth day inevitably leading to the sharp decline of PCE. On one hand, the superior perovskite films with larger grains spincoated onto the NH4F-treated TiO2 are beneficial for the stability improvement. Additionally, the previous results indicated that the perovskite crystallization process was retarded and unfavorable perovskite phase transformation could be suppressed by employing a proper additive of NH4Cl in perovskite precursor [65e67]. Similarly, interfacial NH4F is expected to have a positive effect on the decrease of interface trap states and inhibition of the perovskite film degeneration in air. Eventually, the highly efficient and stable planar PSCs can be fabricated in air by means of this effective interfacial modification.
4. Conclusions In summary, we have developed a facile NH4F solution processing strategy for the TiO2 ETL in PSCs. The higher quality perovskite films with larger crystalline grain have been successfully prepared onto the NH4F-treated TiO2 layers, and succeedingly the planar architecture of FTO/NH4F-TiO2/CH3NH3PbI3/Spiro-OMeTAD/ Au have been assembled in air. The champion device with a NH4Ftreated TiO2 ETL yields a higher PCE of 15.61% than the pristine device only with a PCE of 12.17%. Furthermore, the NH4F-PSCs exhibit a superior environmental stability, which can still maintain over 90% original PCE even after 28-day storing in air without any encapsulation. The improvements in PCE and stability of the NH4FPSCs can be attributed to the oxygen-induced defect suppression and residual surface hydroxyl elimination existing at the TiO2/ perovskite interface. Similarly, the trap state density calculated in NH4F-treated TiO2 has been prominently reduced in comparison with that in pristine TiO2. Although the PCEs of our devices are inferior to that of PSCs obtained with controlled atmosphere, such interface passivation can provide a simple and practical route for efficient air-processed planar PSCs. Further development of devices is expected by comprehensively understanding work mechanisms and manipulating carrier transport behaviors of air-processed PSCs.
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