Bifunctional Hydroxylamine Hydrochloride Incorporated Perovskite

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Bifunctional Hydroxylamine Hydrochloride Incorporated Perovskite Films for Efficient and Stable Planar Perovskite Solar Cells Hong Jiang, Zhe Yan, Huan Zhao, Shihao Yuan, Zhou Yang, Juan Li, Bin Liu, Tianqi Niu, Jiangshan Feng, Qiang Wang, Dapeng Wang, Heqing Yang, Zhike Liu, and Shengzhong Frank Liu ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00060 • Publication Date (Web): 06 Feb 2018 Downloaded from http://pubs.acs.org on February 6, 2018

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ACS Applied Energy Materials

Bifunctional Hydroxylamine Hydrochloride Incorporated Perovskite Films for Efficient and Stable Planar Perovskite Solar Cells Hong Jiang, Zhe Yan, Huan Zhao, Shihao Yuan, Zhou Yang, Juan Li, Bin Liu, Tianqi Niu, Jiangshan Feng, Qiang Wang, Dapeng Wang, Heqing Yang, Zhike Liu* and Shengzhong (Frank) Liu* Key Laboratory of Applied Surface and Colloid Chemistry, National Ministry of Education; Shaanxi Key Laboratory for Advanced Energy Devices; Shaanxi Engineering Lab for Advanced Energy Technology, School of Materials Science and Engineering, Shaanxi Normal University, Xi’an 710119, China Keywords: hydroxylamine hydrochloride, perovskite solar cells, hydroxyl group, chloride ion, trap-state density

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ABSTRACT

Research on the addition of suitable materials into perovskite film for improved quality is important to fabricate efficient and stable perovskite solar cells. An attempt to enhance the quality of perovskite is performed by incorporation of a bifunctional hydroxylamine hydrochloride (HaHc) into pristine perovskite solution. On the one hand, the chloride ion in HaHc changes the crystallization kinetic and defect state of the perovskite film and a high-quality perovskite film with larger grain size and lower defect density is obtained. Perovskite solar cell (PSC) with HaHc additive exhibit a power conversion efficiency (PCE) of 18.69% with less hysteresis, which is obviously higher than that of pristine cells (16.85%). On the other hand, the hydroxyl group in HaHc can form a strong hydrogen bond with iodide ion in perovskite film to impede the decomposition of the film when under thermal annealing or storing in air. As a result, the PSCs with HaHc additive show superior thermal and air stability to the pristine devices. These results indicate that the addition of HaHc in perovskite film can greatly improve the performance of PSCs as well as their thermal and air stability.

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Organic-inorganic metal halide perovskites have attracted great attention to photoelectronic

technologies

due

to

their

high-performance

and

low-cost

solution-based fabrication process.1-3 The power conversion efficiency (PCE) of perovskite solar cell (PSC) have reached up to more than 22% within a few years from the initial 3.8% in 2009.4 However, on the one hand, the perovskite films prepared by solution process are polycrystalline, and often possess an exceptional density of defects at surfaces and grain boundaries, which can act as nonradiative recombination centers to reduce the photocurrent and the PCE outputs of PSCs associated with current hysteresis.5 On the other hand, it is found that organic components in perovskite can rather easily migrate out of perovskite film during thermal annealing process and/or storing in high-humid environment,6-7 which can also form a lot of defects in the perovskite film and accelerate the disintegration of perovskite film. Therefore, preparing a high-quality perovskite film with less grain boundaries (large grain size), effectively passivated surface and low organic components migration rate can thus reduce the defect density and decomposition rate of perovskite films, and improve the PCE and long-term stability of PSCs.8

A great many materials have been added into the pristine perovskite solution to obtain high-quality perovskite films with large grain size and low defect density. These materials often have the special groups or ions, such as hydroxyl group (-OH),9 3



amino-group (-NH2)10 and chloride ion (Cl−),11 which could interaction with ions in perovskite to influence their growth kinetic and surface state.12 For example, Zhao et al. added polyethylene glycol (PEG) into perovskite film, and found that PEG could chemically interact with halogen atoms in the perovskite to prevent them from migrating.13 As a result, the electronic property, crystallinity and thermal stability of perovskite film are all improved. More interesting, PEG added PSCs exhibited a self-healing behavior. The devices can rapidly recover their photovoltaic performance after removing from water vapor, which can be attributed the strong binding effect of hydroxyl group in PEG with methyl ammonium iodide (MAI) to protect the perovskite from decomposition. Fu et al. introduced 3-hydroxypyridine (3-HP) into perovskite film, it is found that 3-HP can interact with Pb ions or I ions in perovskite and change the crystallization process of perovskite grains.14 Concomitantly, hydrogen bonds of hydroxyl groups can effectively suppress the migration of ions in perovskite film to reduce the hysteresis and improve the stability of PSCs. Hou et al. incorporated terephthalic acid as an additive into the perovskite precursor solution and found that there is a strong coordination between iodide ion and hydroxyl group of terephthalic acid which can suppress the iodide ion migration of perovskite to obtain a more stable and high efficient PSCs.15 On the other hand, many groups have demonstrated that the chloride ions in the precursor perovskite solution can slow down the nucleation and growth rate of perovskite crystals to obtain a high quality film with longer carrier diffusion length and lower defect density.16 The PSC based on 4


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perovskite film with longer carrier diffusion length and lower defect density often exhibit a higher fill factor (FF) and short circuit current (Jsc). Due to the relatively larger band gap of chloride ions containing perovskite film, the open circuit voltage (Voc) of PSC is also enhanced.17 For example, Liao et al. incorporated chloride ion into perovskite precursor by using PbCl2 and ammonium chloride (MACl).18 Compared to the pristine PSC with a PCE of 15.5%, the chloride ion incorporated PSC exhibits an excellent PCE of 18.2%, which is attributed to the improved perovskite film morphology, prolonged carrier diffusion lengths and preferred orientation of perovskite crystallite after incorporation of chloride ions into perovskite films. Yang et al, have demonstrated that the chloride ions present on surface of grain boundaries of perovskite film, rather than in its crystal lattice19. Through the density functional theory (DFT) calculation and experiment, Tan et al. have proved that the chloride ions can suppress the formation of deep trap states (antisite defects) on the surface of perovskite films, which can lead to the improved surface passivation to reduce interfacial recombination.20 In summary, hydroxyl group and chloride ion containing additive seems to play an important role in determining the electronic properties, morphology and stability of the perovskite films. However, there was no report on using an additive containing both hydroxyl group and chloride ion to improve the performance of PSCs.

In order to obtain a high quality perovskite film, Hydroxylamine hydrochloride (HaHc) (Figure S1) as a bifunctional additive is first incorporated into perovskite 5



precursor solution by us. The chloride ion in HaHc can reduce grain boundary through increasing grain size, and the hydroxyl group in HaHc can passivate the surface of perovskite film by forming a strong binding with iodide. The cooperation of these two effects enables improved quantity of perovskite films. As a result, a high quality perovskite film with larger grain, lower defect states, longer carrier lifetime and higher stability is synthesized by adding a small amount of HaHc into pristine perovskite solution. Compared with the pristine device (16.85%), the PSCs with HaHc exhibit a PCE of 18.69%. More interestingly, the devices with HaHc demonstrate obviously improved thermal and air stability owing to the formation of strong hydrogen binding between the hydroxyl group in HaHc and iodide ion in perovskite film.

The colour of MAPbI3 precursor solution is yellow, which changes into light red after addition of HaHc (0.08 M) (Figure S2). The as-spin-coated MAPbI3 precursor films with and without HaHc exhibit the same colour (Figure S3). When the MAPbI3 film is stored at room temperature for 5 min in glovebox, the colour of the film is changed into light brown. However, the MAPbI3+HaHc (0.08 M) film is colourless after the same processing, and there is only little light brown at marginal area. When the perovskite film was annealed at 100oC for 5s, the colour of the MAPbI3 film changes from light brown to deep brown, MAPbI3+HaHc (0.08 M) film is still colourless in most areas, there is only little deep brown at marginal area, these results

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indicate that the HaHc can slow down the crystallization rate of MAPbI3 film at the beginning of annealing process.

X-ray Diffraction (XRD) patterns are recorded to determine the composition of different precursors (Figure 1a, 1b). The film MAPbI3 and MAPbI3+HaHc (0.08 M) precursor films without annealing show the diffraction peaks at 6.55° and 9.25°, which can be indexed to MA2Pb3I8(DMSO)2 intermediate phase.21 As HaHc was added into precursor films, the content of MA2Pb3I8(DMSO)2 is dramatically increased. This kind of intermediate phase based on DMSO could retard the rapid reaction of MAI and PbI222, which enables the formation of a highly uniform and dense perovskite film. It is interesting to found that MAPbI3+HaHc (0.08 M) precursor films is completely changed into MAPbI3 phase after only one minute high temperature annealing treatment, however, a week peak at 6.55° can also be detected for MAPbI3 precursor films even after 3 min. In order to further investigate the effect of HaHc on the crystallization of MAPbI3 film, as shown in Figure 1c, 1d, XRD patterns of MAPbI3 with different concentrations of HaHc addition were carried out. The intensity of the main diffraction (110) peaks is gradually enhanced by increasing the HaHc content, while the full width at half maxima (FWHM) of perovskite (110) peaks is gradually decreased, both of which indicate a higher crystallinity of the perovskite with a larger crystal size as will be discussed later.

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To evaluate the impact of HaHc on the photoluminescence property of the perovskite film, steady photoluminescence (PL) is conducted. The perovskite films with and without HaHc are directly grown on glass substrates for measurement. As shown in Figure 2a, compared with the MAPbI3 film, the MAPbI3+HaHc (0.08 M) film has a sevenfold stronger PL intensity and a strong blue PL shift. As confirmed in the literature, both of them are indicative of a suppressed non-radiative recombination and a lower trap density, which is probably the origin of the enhanced Voc of PSCs due to the reduced band bending of the perovskite caused by trap states.23-25 The steady PL result means that the HaHc can interact or react with components of MAPbI3 film.12 The time-resolved photoluminescence (TRPL) spectra are also used to assess the lifetime of the carrier and defect state in perovskite films with and without HaHc (Figure 2b), The measured PL decay times and amplitudes are determined by fitting TRPL curves with a bi-exponential equation (1)26: f (t ) = ∑ Ai exp( −t / τ i ) + B

(1)

i

where τi, Ai and B are the decay time, decay amplitude and a constant, respectively. As listed in Table S1, the slow decay process (τ1) is originated from the direct recommbination of free carriers and the fast decay process (τ2) is originated from the Shockley-Read-Hall (SRH) recombination.27-28 The lifetimes of MAPbI3+HaHc (0.08 M) film (τ1 = 108.80 ns, τ2 = 45.03 ns) show largely improved τ1 and τ2 compared to

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those of the MAPbI3 film (τ1 = 35.86 ns, τ2 = 12.04 ns). Therefore, the lifetimes of the direct recombination and the SRH recombination are all increased after addition of HaHc, indicating the simultaneous decrease in the recombination center and the defect state density. The average PL decay times (τave) is calculated according to the fitted Ai and τi values (Table S2) by equation (2): 2

τ ave =

∑ Aiτ i ∑ Aiτ i

(2)

Compared with MAPbI3 film, the τave of MAPbI3+HaHc (0.08 M) film is significantly increased from 28.75 ns to 98.00 ns, which implies that the increased intensity in steady PL results from a decrease in trap-assisted recombination.29 These results further verified the markedly improved

perovskite film quality after the addition of

HaHc.30 To investigate the effects of HaHc addition on the electronic property of perovskite film, the space-charge-limited current of perovskite films under different bias voltages is applied to assess their trap density. The electron-only devices with a structure of FTO/TiO2/MAPbI3 or MAPbI3+HaHc/PCBM/Ag) are fabricated to measure the electron trap-state density. Figure 2c shows the current-voltage curves of the perovskite with and without HaHc. There are two kink points on the two curves where the current quickly increases, corresponding to trap-states filled point. The trap-state density can be calculated by the trap-filled bias voltage (VTFL) using equation (3)31:

VTFL =

entL2 2εε0

(3)

9



where ε0 is the vacuum permittivity, e is the elementary charge, ε, L and nt are the relative dielectric constant, the thickness and the trap-state density of perovskite film, respectively. The VTFL of the MAPbI3 with and without HaHc are 0.34 V and 0.60 V, the corresponding electron trap density is 1.2 x1016 and 2.1 x1016 cm-3, respectively. The trap-state density deviation of 17 MAPbI3 devices and 17 MAPbI3+HaHc devices is shown in Figure S6. These results indicate that HaHc additive can effectively decrease the trap-state density of perovskite film. In order to further investigate the influence of HaHc additive on the interfacial charge transfer and the carrier recombination of PSCs, the electrical impedance spectroscopy (EIS) is conducted on the PSCs with a structure of FTO/TiO2/MAPbI3 or MAPbI3+HaHc/Spiro-OMeTAD/Au (Figure S4). Figure 2d shows the Nyquist curves of the PSCs measured at an applied bias of 0.85 V under dark condition. Based on the equivalent circuit model (Figure S5), the Nyquist plot can be fitted in a series resistance (Rs) connected with a contact resistance (Rco) and a recombination resistance (Rrec).32 The Rco represents the charge transfer at the interface of TiO2/perovskite and Spiro-OMeTAD/perovskite, the Rrec represents the charge recombination at the interface. Given no change in main device structure, the Rs and Rco in the PSC remain almost no change (Table S2). The Rrec of the MAPbI3+HaHc (0.08 M) solar cell is 1172 Ω, which is much higher than that of the MAPbI3 device (715 Ω), which refers to an effectively suppressed charge recombination as a result of the reduced defect density.30 The high Rrec would also be help for achieving high charge collection efficiency as well as high fill factor for PSCs.

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To investigate the effect of HaHc on the photovoltaic performance of PSCs, each 30 planar-PSCs employing 0, 0.02, 0.04, 0.08 or 0.12 M HaHc addition were prepared, and the results are shown in Figure 3. In general, the photovoltaic parameters of the devices are increased with increasing HaHc content, and the PSC employing 0.08 M HaHc exhibits the best performance with the highest PCE of up to 18.69%, which is about 11% increase compared with the device without HaHc (PCE = 16.85%). The average PCE on a basis of 30 devices is 17.53% for PSCs with 0.08M HaHc and 16.12% for PSCs without HaHc, respectively, unambiguously indicating that HaHc addition appreciably improves the PSC performance. When 0.12 M HaHc is added into the pristine solution, the annealed perovskite films show a rough surface along with low PCE for PSC. The J-V characteristics and detail parameters of the best device with and without HaHc addition are shown in Figure 4a and Table 1. For PSC with HaHc (0.08 M), a reverse PCE of 18.69% and forward PCE of 17.93% can be obtained with small hysteresis. In contrast, the PSC without HaHc shows a reverse PCE of 16.85% and forward PCE of 12.04%, exhibiting a significant hysteresis. The hysteresis ratio defined as |PCEreverse−PCEforward|/PCEreverse26 is calculated to be around 4.1% and 28.5% for the PSCs with and without HaHc, respectively. The external quantum efficiency (EQE) spectra of PSCs with and without HaHc are shown in

Figure 4b. The HaHc enhances the light-harvesting capacity of the device in large spectral range.

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To confirm whether both the MAPbI3 and MAPbI3+HaHc (0.08 M) solar cells have saturated/stable power conversion efficiency, as shown in Figure 4c, 4d, the current densities at the maximum power condition (Jmax) are measured under light soaking with an applied optimum bias voltage. After short light soaking times, Jmax and efficiency (η) decreased and became saturated, the efficiency is similar to the η value measured in the reverse scan (Figure 4a). The saturation time of MAPbI3+HaHc (0.08 M) solar cell (~ 15.4 s) is much shorter than that of MAPbI3 device (~ 45.2s), which may be the main reason for serious hysteresis in MAPbI3 devices.33

Figure 5a, 5b show the top-view SEM images of the prepared perovskite films without and with HaHc, respectively. It can be seen that the average grain size of the MAPbI3 film is about 150 nm.34 In contrast, the MAPbI3+HaHc (0.08 M) film with an average grain size of 350 nm. Figure 5c, 5d show the cross sectional SEM images of PSCs based on perovskite films without HaHc and with HaHc, respectively. It is clearly displays that the perovskite film without HaHc in device is multi-layers, some small perovskite grains are stacked together to form film. In contrast, the MAPbI3+HaHc (0.08 M) film in device contains only one grain with large size in the thickness direction, indicating that the HaHc additive plays an important role in controlling the morphology and quality of perovskite film. The SEM results are consistent with that of XRD results.

To examine the impact of HaHc addition upon the device thermal stability, the PSCs with and without HaHc are annealed on a hotplate at 100 oC for different time 12


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periods. As shown in Figure 6a, the PCEs of PSCs with HaHc addition can maintain 75% after annealing on a hotplate at 100oC for 20 h. However, the PCEs of PSCs without HaHc decrease to 27% of their initial values under the same processing conditions. To understand the changes of perovskite films, the XRD of the MAPbI3 films with and without HaHc after thermal annealing for 20 h have been conducted. As shown in Figure 6b, there is no obvious peak of PbI2 for MAPbI3 film with HaHc after thermal annealing. In contrast, for the MAPbI3 film without HaHc, the intensity of (110) peak for MAPbI3 is decreased, while the (100) peak for PbI2 is increased, which is attribute to the serious degradation from MAPbI3 to PbI2.32,35 These results indicate HaHc can effectively enhance the thermal stability of perovskite film by effectively impede the decomposition of MAPbI3 into PbI2 and other components. The stability of devices without encapsulation is also measured in ambient condition at 30% relative humidity for 540 h (Figure 6c). The PSCs with HaHc retain ~95% of their initial efficiencies. In contrast, the efficiencies of the PSCs without HaHc decay to less than 85%, indicating that the PSCs with HaHc show much stronger resistance to degradation in air than the PSCs without HaHc. The enhanced stability of the PSCs with HaHc could arise from the enhanced quality and reduced defects of perovskite film by chloride ions in HaHc and the strong hydrogen bonds between hydroxyl groups in HaHc and iodide ions on surface of perovskite film. On the one hand, the loss of iodide ions by evaporation of MAI during ageing process (thermal annealing or storing in air) can be compensated by chloride ions in HaHc.9 On the other hand, 13



the strong hydrogen bond between hydroxyl groups and iodide ions could block the MAI extraction from perovskite film to impede the decomposition of perovskite.36

Figure 7a, 7b show the two main peaks of Pb (Pb 4f7/2, Pb 4f5/2) and I (I 3d5/2, 3d3/2), respectively. It is found that the binding energies of Pb and I valence electrons are shifted to a higher position, suggesting an electronic passivation after added HaHc into perovskite film.37 Oxygen and chloride are the characteristic elements for the molecule HaHc. XPS is performed to detect the existence of HaHc molecules in the perovskite film. Signals of O1s and Cl2p are shown in Figure 7c, 7d, respectively. The area ratio of the peaks is proportional to the amounts of elements. It is estimated the value of the O/Pb ratio to be about 2:1 and 1:1 for the sample with and without HaHc, respectively. In other words, the content of O has doubled after adding HaHc in pristine perovskite solution. For the sample without HaHc, the oxygen is also detected due to the unavoidably absorbed molecules of oxygen gas and moisture from the air. If it is continuing to increase the HaHc concentration, the intensity and area ratio of O 1s are increased gradually (Figure S7). This result strongly supports the existence of hydroxyl groups in the perovskite film. The hydroxyl groups/lead ratio is about 1:1, which is much higher than the ratio in the pristine solution (0.08 M/1.2 M). Therefore, hydroxyl groups are expected to stay at the film surface and/or grain boundaries of perovskite film rather than incorporate into the perovskite lattice. The Cl 2p spectra of the perovskite layer with and without HaHc are shown in Figure 7d, the result clearly suggests that there is small amount of Cl atoms in the perovskite film, demonstrating 14


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the incorporation of Cl into perovskite film with adding the HaHc, which can be explained by the stronger Pb-Cl bonding than Pb-I bonding.20 It is known that Cl− incorporated perovskite films possess enhanced crystallinity, longer carrier diffusion length and lower trap-state density as compared to pure MAPbI3,38-40 which have been demonstrated by the results exhibited before.

As confirmed in the literature, the hydroxyl groups undergo hydrogen bonding with the iodide ions exposed at the surface of perovskite film, and the formation is illustrated in Figure 8a. To probe the formation of hydrogen bonds, the liquid-state 1

H NMR titration measurement is performed, as shown in Figure 8b. In neat HaHc

sample, the resonance signal appears at δ = 10.31 ppm is attributed to the hydroxyl groups. An upfield chemical shift of ∆δ ~ -0.26 ppm can be observed in MAPbI3+HaHc (0.08 M) solution and explained by the hydrogen bond between hydroxyl group and iodide ion in MAI. The proton resonance signal of –NH3+ in MAI (peak at δ = 7.49 ppm.) shifts towards upfield with ∆δ ~ -0.05 ppm after the addition of HaHc, which can also be attributed to the influence of hydrogen bonds discussed above.13 1H NMR data provide a solid evidence for the presence of strong hydrogen bonding between hydroxyl group in HaHc and iodide ion in MAI. As shown in XPS result, the molar ratio of hydroxyl groups on perovskite film is much higher than that in pristine perovskite solution, so majority of hydrogen bonds anchored with iodide ions on the surface of the perovskite film. The strong hydrogen bonds could prevent the migration of the iodide ions in perovskite film and impede decomposition of 15



perovskite to improve the thermal stability of perovskite solar cells. Combining the results from XRD, XPS, and NMR, it is confirm that hydroxyl groups in the HaHc can form a strong interaction with perovskite to impede the decomposition of perovskite, and the chloride in the HaHc can improve the crystallinity of perovskite film and remain a small of mount on perovskite surface to passivate the trap states.

In summary, it has demonstrated that HaHc can be used as an effective precursor additive to tune the crystallization, morphology, electronic property and stability of perovskite films. As a result, perovskite film with HaHc showed marked increase in photoluminescence and carrier lifetime, decrease in defect density which are attributed to the presence of chloride ions and hydroxy groups on surface of peovskite film. The PSCs with HaHc produce a considerable increase in both photovoltaic performance and thermal stability when compared to pristine PSC. The introduction of such bifunctional additive as perovskite growth-controlling and surface-passivating agent provides an avenue for preparing other more effective perovskites, such as FAPbI3, for more efficient and stable PSCs.

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EXPERIMENTAL SECTION Fabrication of PSCs. FTO/glass

substrates

were

ultrasonic

cleaned

by

ultra-sonication in isopropanol, acetone and DI water for 10 min. Then, the substrates were dried by nitrogen flow and treated by an ultraviolet for 15 min before the deposition of TiO2. TiO2 layers (40 nm) were grown by chemical bath deposition on washed FTO substrates, as reported by our groups before.26,41 To prepare the MAPbI3 perovskite film with or without HaHc, the commercial PbI2 (99.99%, Alfar Aesar) and prepared CH3NH3I for the MAPbI3 solution (1.25 M) with or without added different content of commercial HaHc (98.5%, Sinopharm Chemical Reagent Co., Ltd) were dissolved in mixed solvent of dimethyl sulfoxide (DMSO, 99.9%, Sigma-Aldrich) andγ-butyrolactone (GBL, 99.0%, Sigma-Aldrich), the volume ratio of GBL/DMSO is 4 : 6. The mixed solution was spin-coated on top of TiO2 as perovskite layer. Then the substrates were heated at 100 °C for 10 min to obtain dark perovskite

films.

The

4-tert-butylpyridine

(TBP,

Sigma

Aldrich)

and

bis(trifluoromethylsulfonyl)imidelithium salt (Li-TFSI, Sigma Aldrich) doped spiro-OMeTAD (70 mM) solution in chlorobenzene was coated on perovskite layer as hole transport layer. Finally, a gold top electrode (~ 100 nm) was deposited on spiro-OMeTAD by thermal evaporation under high vacuum.

Characterization. The surface morphologies and cross-section images of the perovskite films and PSCs were characterized by SEM (HITACHI, SU-8020). The XPS was carried out by using a photoelectron spectrometer (ESCALAB250Xi, Thermo Fisher Scientific). PL spectra (excitation at 532 nm) were obtained with an Edinburgh Instruments Ltd, FLS980 spectrometer. The absorption was taken out by UV/Vis NIR spectrophotometer (PerkinElmer, Lambda 950). The current density 17



versus voltage (J-V) characteristics of the PSCs were obtained by using a digital source meter (Keithley Model 2400) under an illumination of a AM 1.5 solar simulator (100 mW/cm2, SAN-EIELECTRIC XES-40S2-CE), as calibrated by a NREL-traceable KG5-filtered silicon reference cell. The active area of solar cell was defined by a 9 mm2 metal mask. All devices scanned with reverse and forward under standard test procedure at a scan rate of 30 mV/s. The external quantum efficiency (EQE) spectra of the PSCs were measured by a QTest Station 500TI system (Crowntech. Inc., USA). The monochromatic light intensity for EQE was calibrated using a reference silicon detector. Impedance spectroscopic measurements (EIS) were performed using an electrochemical workstation (IM6ex, Zahner, Germany) with a frequency range from 1 Hz to 4 MHz under 0.85 V in the dark. NMR was performed by using JNM-ECZ400S/L1 with a frequency of 400 MHz, and deuterated DMSO was used as solvent to dissolve MAPbI3, HaHc and MAPbI3+HaHc.

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ASSOCIATED CONTENT

Supporting Information

Chemical structure of the hydroxylamine hydrochloride; Photographs of pristine MAPbI3 films with and without HaHc; The equivalent circuit model for PSCs in EIS; Trap-state density box charts of PSCs without and with HaHc addition; O 1s peaks in XPS spectrum for perovskite films with different concentrations of HaHc addition. AUTHOR INFORMATION

Corresponding Author * Prof. Zhike Liu, [email protected]. * Prof. Shengzhong (Frank) Liu, [email protected].

ORCID Zhike Liu: 0000-0001-5681-3930 Shengzhong Frank Liu: 0000-0002-6338-852X

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENT

The authors acknowledge all support from the National Key Research and Development Program of China (2016YFA0202403), the National Natural Science Foundation of China (61704101), the Natural Science Foundation of Shaanxi 19



Province (2017JM6020), the Fundamental Research Funds for the Central Universities (GK201702003), the Natural Science Foundation of Shaanxi Provincial Department of Education (2017KW-023) and the Fundamental Research Funds for the Central Universities (GK201603053)

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Table 1. Characteristic photovoltaic parameters of MAPbI3 and MAPbI3+HaHc (0.08 M) solar cells.

Samples

Direction

Voc (V)

Jsc (mA/cm2)

FF (%)

PCE (%)

Reverse

1.05

22.01

74.80

16.85

Forward

1.02

22.01

53.41

12.04

Reverse

1.10

22.42

76.07

18.69

Forward

1.07

22.03

76.06

17.93

MAPbI3

MAPbI3 +HaHc

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Figures:

Figure 1. XRD patterns of the perovskite films (a) without and (b) with HaHc annealed for different times. (c) XRD patterns of the perovskite films with different concentration of HaHc (0, 0.02, 0.04, 0.08, 0.12 M) as additive in the pristine solution. (d) Intensity and FWHM of (110) peaks of perovskite films depending on concentration of HaHc.

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Figure 2. (a) Steady-state and (b) time-resolved photoluminescence spectra of the perovskite film with and without HaHc on the glass substrates. (c) The space charge limited current vs voltage of devices with the structure of FTO/TiO2/MAPbI3 or MAPbI3+HaHc/PCBM/Ag. (d) Nyquist curves of the PSCs with and without HaHc addition under applied bias in darkness.

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Figure 3. Box charts of (a) open-circuit voltage, (b) short circuit current, (c) FF and (d) PCE of PSCs with various concentrations of HaHc (0/0.02/0.04/0.08/0.12 M) additives in the precursor solution.

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Figure 4. (a) Reverse and forward current density-voltage curves and (b) the external quantum efficiency (EQE) spectrum of PSCs based on MAPbI3 without and with HaHc. Stabilized short-circuit photocurrent density and efficiency of PSCs based on MAPbI3 (c) without and (d) with HaHc addition.

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Figure 5. Top-view SEM images of perovskite films without (a) HaHc and with (c) HaHc; Cross-sectional SEM images of PSCs based on perovskite films without (b) HaHc and with (d) HaHc.

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Figure 6. (a) The stability of PSCs with and without HaHc under thermal annealing at 100oC in the glove box for various durations. (b) XRD patterns of MAPbI3 and MAPbI3+HaHc films deposited on TiO2/FTO substrates after thermal annealing for 20 h. Asterisks denote the major reflection peaks from PbI2. (c) The stability of PSCs with and without HaHc under air exposure (humidity: ∼30%) for different time periods.

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Figure 7. X-ray photoelectron spectroscopy (XPS) spectra of (a) Pb 4f, (b) I 3d, (c) O 1s and (d) Cl 2p peaks of perovskite films with and without HaHc addition.

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Figure 8. (a) Schematic illustration of hydrogen-bonding interactions (O–H…I) between the iodide ions from perovskite and hydroxyl groups of HaHc. (b) 1H NMR spectra showing changes in the resonance signals of –OH– and –NH3+ protons in the MAPbI3-HaHc solution.

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