To Greatly Reduce Defects via Photoannealing for High-Quality

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To Greatly Reduce Defect via PhotoAnnealing for High Quality Perovskite Film Duo Wang, Cuncun Wu, Wei Luo, Xuan Guo, Xin Qi, Yuqing Zhang, Zehao Zhang, Ning Zhu, Bo Qu, Lixin Xiao, and Zhijian Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 23 May 2019 Downloaded from http://pubs.acs.org on May 23, 2019

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ACS Applied Materials & Interfaces

To Greatly Reduce Defect via Photo-Annealing for High Quality Perovskite Film Duo Wang, Cuncun Wu, Wei Luo, Xuan Guo, Xin Qi, Yuqing Zhang, Zehao Zhang, Ning Zhu, Bo Qu, Lixin Xiao,* and Zhijian Chen * State Key Laboratory for Mesoscopic Physics and Department of Physics, Peking University, Beijing 100871, P.R. China.

Keywords: Photo-annealing, Crystallization, Defect density, Perovskite, Solar cell

Abstract The performances of perovskite solar cells (PSCs) depend on the crystallization of perovskite layer. Herein, we demonstrate an effective photo-annealing (PA) process by a halogen lamp. During PA process, in one hand, the lower energy photon, i.e., near IR up to ~1015 nm photon drives the crystallization of perovskite film as the conventional thermal annealing (TA). In another hand, the higher energy photon of PA can provide excite the trapped carriers and release the space charges, thus leading to an ideal property perovskite layer with better crystallinity and lower density of defect than that of TA. A maximum power conversion efficiency (PCE) has been obtained to be

20.41% in the

CH3NH3PbI3-based planar PSCs based on PA due to the increase of Jsc and Voc, much higher

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than the control device based on the conventional TA with a maximum PCE of 18.08%. Therefore, this result demonstrates that PA is an effective method to promote the device performances and reduce fabrication cost, which provides a potential approach for the commercial application of perovskite device.

1. Introduction Photoelectronic devices based on perovskite have become an appealing research focus owing to its fascinating optoelectronic properties1-7. The PCE of PSCs soared to today’s certified 23.7% within 10 years8-10. For the sake of obtaining a perovskite film with excellent crystallization and morphology, the commonly used method of annealing is TA at ~100 °C for ~10 min for CH3NH3PbI3 (denoted as MAPbI3) 11-14. But the defect density of the resulted film is still high, therefore new methods of annealing is required to improve the quality of perovskite layer. Laser-induced crystallization is a very effective method for rapid crystallization, reported by Feinleib et al. in 197115. It has been used in a wide variety of materials due to the unique photothermic and photochemical effect occurring when the laser irradiates the films16. Li et al. applied laser irradiation assisted TA to promote the crystallinity of MAPbI3 layer, which could effectively increase the Voc of PSCs16. However, thus it is difficult to achieve large-area radiation with uniform light intensity distribution by laser irradiation. In our previous work, Luo et al. have developed a method for preparing perovskite films in large area by electric current annealing, and reduced the defect density11. To further reduce the defect density for large area perovskite film, an effective photo-annealing (PA) is proposed in this study. PA, based on incoherent light source of halogen lamp, has the merits including large and uniform irradiation area, and its emissive spectrum and the absorption sperctrum of 2 ACS Paragon Plus Environment

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MAPbI3 have well overlap. Usually in the case of TA film, the carriers are trapped in defect state and form space charge region, where Coulomb repulsive force (Fc) would hinder further crystallization of perovskite. By changing from TA to PA process, the lower energy photon, i.e., near IR up to ~1015 nm photon drives the crystallization of perovskite film as the conventional TA. Moreover, the higher energy photon of PA may excite the trapped carriers and release the space charges, thus leading to a high quality perovskite film with halved defect density and higher crystallinity than that of TA. In this work, PSCs based on PA had higher Voc, Jsc and PCE (20.41%), while PCE of the devices based on TA was 18.08%. 2. Results and Discussion We manufactured PSCs by TA (as control devices) and PA processes, respectively. The device architecture is shown in Fig. 1(a) and the energy-level graph is shown in Fig. 1(c). The spectrum of the halogen lamp (Philips QVF135) with the power of 500 W is shown in Fig. S1 (a). The spectrum of the halogen lamp covers from near ultraviolet to near infrared, mainly including 364.82 nm, 404.19 nm, 435.81 nm, 546.21 nm, 578.49 nm, 691.03 nm, 1013.7 nm, and so on, which are matched with the absoption of MAPbI3 film. It is an ideal condition for the perovskite film to crystallize. In addition, the corresponding light intensity of the halogen lamp is 300 mW/cm2, about three times AM 1.5 irradiation. The distance between sample and halogen lamp remains at 5 cm durring annealling, and the temperature is measured to be 75 ℃ by a thermocouple. In order to explore the optimal annealing time for PA-induced crystallization of the perovskite film, PSCs were prepared by different PA time of 5 min, 10 min, and 15 min, respectively. Fig. 2 shows the average values of Voc, Jsc, FF and PCE , which were measured under 100mW/cm2 AM 1.5 simulated illumination. The best photovoltaic parameters for each group are listed in parentheses in Table 1. Each group has thirty devices. The rate of the J-V 3 ACS Paragon Plus Environment


curve measured was 0.1V/s. The devices with PA held higher Voc and larger Jsc than those fabricated by TA. The devices with different PA time about 5 min, 10 min, and 15 min, were obtained with higher average PCE of 18.26%, 19.52% and 18.71%, respectively. In contrast, the control devices with TA showed an average PCE of 17.25%. In addition, the devices based on TA 75 ℃ were fabricated with average PCE of 14.82%, while the photovoltaic parameters also were shown in Table 1, and it shows that the best perovskite films cannot be obtained under TA 75 ℃.

Fig. 1. (a) The perovskite film fabrication process diagram. (i) TA. (ii) PA. (b) Device architecture. (c) Energy levels of the each functional layer.


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The performance improvement of devices can not ascribed to the change of absorption of the perovskite films based on two different annealing methods. The UV−vis spectra were measured to investigate the light absorbing abilities for the perovskite films (Fig. S1 (b)), where shows a similar absorption between the perovskite films obtained by PA and TA. The improvement is estimated to be higher quality of perovskite film based on PA.

Fig. 2. Average values of Voc (a), Jsc (b), FF (c) and PCE (d) for PSCs by TA and PSCs by PA with different illuminating time.

Table 1. Average photovoltaic parameters of PSCs based on TA and PA. Annealing condition

Jsc (mA·cm-2)

Voc (V)

TA 75 ℃

22.01

1.05


FF 0.675

PCE (%) 14.82±0.79 (15.61)


Control

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22.20

1.05

0.740

17.25±0.83 （18.08）

PA 5 min

22.84

1.06

0.754

18.26±0.82 （19.08）

PA 10 min

22.88

1.10

0.776

19.52±0.89 （20.41）

PA 15 min

22.29

1.09

0.770

18.71±0.63 （19.32）

(TA 100 ℃)

Fig. 3. (a) XRD patterns of Glass/ ITO/ SnO2/ MAPbI3 layers based on TA and PA . (b) Schematic diagram with crystallization process of perovskite under illumination with PA.

The morphology of ITO/SnO2/MAPbI3 films based on TA and PA was investigated by the scanning electron microscope (SEM) (Hitachi S-4800) (Fig. S2), severally. The surfaces of all 6 ACS Paragon Plus Environment

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the perovskite layers are uniform and smooth, without any pinholes. By comparison, we can find that the perovskite films based on PA has a clearer and more ordered domain within the grain than the perovskite films based on TA. Fig. S3 (a) (b) showed the cross-section SEM of the device based on TA and PA, respectively. The cross-section SEM incidates that the perovskite layer grew more uniformly and compactly based on PA than that based on TA. In addition, the morphology of MAPbI3 films based on TA and PA applied with AFM as seen in Fig. S3 (c) (d). The root mean square roughness of film by TA is 21.8 nm, and that of the film by PA is 12.7 nm, which indicated that PA led the perovskite film to a smaller roughness. The AFM pictures exhibit similar results to SEM image (Fig. S2). The AFM and SEM results indicate that PA drives the crystal of the perovskite film more compact and dense, which is consistent with the results of the powder X-ray diffraction (XRD) in Fig. 3 (a).

The XRD patterns of the MAPbI3 films based on TA and PA were carried out (Fig. 3 (a)). The control MAPbI3 films showed two major diffraction peaks of the MAPbI3 perovskite phase at 14.12° (half of the maximum intensity (FWHM) of 0.13°)and 28.43° (FWHM of 0.12°) which correspond to (110) and (220), respectively. Meanwhile, the MAPbI3 films based on PA showed two major diffraction peaks at 14.14° (FWHM of 0.11°) and 28.45° (FWHM of 0.10°) which also correspond to (110) and (220), respectively. The diffraction peaks of the MAPbI3 film based on PA moved 0.02 degrees toward a large angle compared to the films based on TA. According to Bragg’s law of 7 ACS Paragon Plus Environment


2dsinθ = 𝑘λ

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(1)

where d is the distance bewtween crystal lattice plane , λ wavelength of incident x-ray beam and θ diffraction angle. There was a decrease in d, indicating that MAPbI3 lattice cell based on PA is more dense. The FWHM of the diffraction peaks in perovskite layers based on PA decrease by over 15%. The Debye–Scherrer equation gives the average crystallite size (D) as follow17,18: D=

0.9𝜆 𝛽 cos 𝜃

(2)

where the 0.9 is shape factor, β is the instrumental corrected line broadening at FWHM, increase of diffraction angle θ and decrease of FWHM indicated that the larger average crystallite size D was obtained in MAPbI3 film based on PA. Meanwhile, Fig. 3 (a) shows that the major diffraction peaks of the perovskite (MAPbI3) phase obtained by PA is about 10 times stronger than that obtained by TA, which also indicated that the MAPbI3 perovskite film had higher quality of crystallization, and it was helpful to promote the property of PSCs. The improvement of the crystalline properties of the MAPbI3 perovskite film is estimated to PA could provide matching photon energy to excite the trapped carriers and release the space charges. Fig. 3 (b) shows the schematic diagram. The carriers are trapped in defect state and form space charge region. Unfortunately, Coulomb repulsive force (Fc) among space charges would hinder further crystallization of perovskite. PA can provide enough energy to make the trapped carriers transition from the defect state level to the LUMO ( or HOMO), which drive the crystal of the perovskite film more compact and dense. Moreover, the high-energy photon could provide a sufficiently high activation energy for the reactive atoms, which caused that it was easy to combine metal atoms into a suitable lattice point19. The space charge limited current (SCLC) measurement was carried out to obtain the defect density of MAPbI3 films20-21. A single carrier device of ITO/ SnO2/ perovskite/ PCBM/ Ag was prepared for SCLC measurement, and the J–V characteristics of the devices by different 8 ACS Paragon Plus Environment

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annealing methods were obtained (Fig. 4). The abrupt rise of J–V curve relating with a trapfilled limit, and the corresponding voltage is denoted as VTEF. The following formula is used to estimate the defect density (Ndefects): 𝑁𝑑𝑒𝑓𝑒𝑐𝑡𝑠 =

2𝜀𝜀0 𝑉𝑇𝐸𝐹 𝑒𝐿2

(3)

where ε is the relative dielectric constants of MAPbI3 and 𝜀0 the vacuum permittivity, L the thickness of MAPbI3 film (about 400 nm), and e the elementary charge22. Ndefects is estimated to be 1.57 × 1015, 6.71 × 1014, 6.17 × 1014 and 1.23 × 1015 cm−3 for the control, PA 5 min, PA 10 min, and PA 15 min samples, respectively. The defect density of perovskite film based on PA is significantly reduced comparing with that of control sample. The experimental results show that the optimal time of PA is 10 min, which are consistent with the trends of photovoltaic parameters of devices in Table 1.



Fig. 4. J-V characteristics of devices appearance used to estimate the defect density. (a) Control, (b) PA 5 min, (c) PA 10 min, (d) PA 15 min.

Electrochemical impedance spectroscopy (EIS) by the Zahner Zennium electrochemical workstation under dark was carried out to illustrate insights into charge transport, accumulation, and recombination of charges in PSCs13, 23. Fig. S4 showed the Nyquist plots for D1 and D2 at a bias of 1.0 V over the frequency range of 1 MHz to 1 kHz under dark. The semicircle was assigned to charge transport resistance (Rct) and recombination resistance (Rrec). The D2 had a lower Rct and Rrec, which leads to reduced recombination losses. The results implied the perovskite films were more compact and lower defect density based on PA than TA.

Fig. 5. (a) PL and (b) TRPL of perovskite layers on glass.

To further investigate nonradioactive and radiative recombination channels in perovskite films, we carried out the steady-state photoluminescence (PL) and time-resolved photoluminescence (TRPL) measurements. The steady PL spectra of Fig. 5 (a) gives the PL intensity of the perovskite film PA 10 min annealing process was enhanced by over ten times compared with that of based on TA on glass. This indicates that the non-radiative composite channels in perovskite films were reduced because of lower defect density. Besides, there is


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about 4.5 nm red-shift in peak of PA relative to TA (from 768.12 nm to 772.72 nm), which dues to the perovskite film of PA was more compact and dense. TRPL in Fig. 5 (b) was deteced with the peak emission at 770 nm. A double exponential equation was used to fit the data (Table S1), and two PL lifetime constants (τ1 and τ2) were obtained. For control film, τ1 is 30.53 ns with a proportion of 72.75%, and τ2 is 98.95 ns with a proportion of 27.25%, leading an average PL lifetime of 49.17 ns. Meanwhile, for PA 10min film, τ1 is 73.36ns with a proportion of 11.32%, and τ2 is 90.32 ns with a proportion of 88.68%, leading an average PL lifetime of 85.01 ns. The increase in lifetime of perovskite film indicates that the defect density is decreased by PA effectively. The champion device was obtained with 10 min illumination (denoted as D1), generated PCE of 20.41%, along with JSC of 22.97 mA/cm2, VOC of 1.14 V, and FF of 0.779, and the best control device (denoted as D2) showed PCE of 18.08%, along with JSC of 22.31 mA/cm2, VOC of 1.07 V, and FF of 0.757, which was shown in Table 2. The J−V characteristics, incident photon-to-current efficiency (IPCE) figure and the value of corresponding integrated Jsc of D1 and D2 are seen in Fig. 6. D2 has a significantly higher Voc and a slightly larger JSC than D1(Fig. 6 (a)). In addition , the dark J-V characteristics for D1and D2 have been inserted in Fig. 6 (a), it shows that D2 has a higher voltage corresponding to the starting current than D1 in the dark, this corresponds to a higher Voc for D2 than D1 under AM 1.5 irradiation. The IPCE (Fig. 6 (b)) was used to verify the reliability of Jsc and shows an integrated photocurrent of 20.83 mA/cm2 and 21.57 mA/cm2 for D1 and D2 , which were closed to JSC of the J−V measurement. As shown in Fig. 6 (c), the steady-state output of the devices for 30 min under AM 1.5 irradiation were measured. In the first 350 s, D2 showed a stabilized PCE of 19.7% and Jsc of 20.8 mA/cm2 at a bias of 0.95 V, while D1 showed PCE of 17.0% and Jsc of 19.8 mA/cm2 at a bias of 0.87 V. After 350 s, the performance of both devices began to decay. After 30 minutes of the steady-state output measurement, D2 and D1 retained approximately 87.35% and 11 ACS Paragon Plus Environment


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approximately 75.10% of their initial PCE and Jsc, respectively. The results indicate the devices of PA have a better stable light output performance than those of TA. The stability of D1 and D2 were assessed without PSCs encapsulation. As illustrated in the normalized PCE in Fig. 6 (d), after keeping in a dark atmosphere with room temperature (25oC) and about 40% humidity for 30 days, D1 and D2 retained approximately 67.1% and approximately 53.9% of their initial PCEs, respectively, which indicates that the devices of PA have a better device stability than those of TA.

Table 2. The best photovoltaic parameters of D1 and D2.

Device

Jsc (mA·cm-2)

Voc (V)

D1

22.31

1.07

0.757

18.08

D2

22.97

1.14

0.779

20.41


FF

PCE (%)

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Fig. 6. (a) J–V characteristics (Inset of is dark J-V characteristics), (b) IPCE spectra, (c) The steady-state output, and (d) Stabilities for D1 (blue) and D2 (red)

Fig. S5 show the typical J−V curve and Table S2 shows the photovoltaic performance values of the D1 and D2 under both reverse scan and forward scan. We define δ as the hysteresis rate of PSCs, δ can be described as δ=

𝑃𝐶𝐸𝑅𝑒𝑣𝑒𝑟𝑠𝑒 ― 𝑃𝐶𝐸𝐹𝑜𝑟𝑤𝑎𝑟𝑑 𝑃𝐶𝐸𝑅𝑒𝑣𝑒𝑟𝑠𝑒 + 𝑃𝐶𝐸𝐹𝑜𝑟𝑤𝑎𝑟𝑑

(4)

We estimated the δ to be 0.030 and 0.022 for D1 and D2, respectively. Comparing with control D1, D2 has a significantly reduced hysteresis, due to a better crystal quality for the MAPbI3 perovskite film.

3. Conclusion We demonstrate a competent PA method to drive the crystallization of perovskite layer using a halogen lamp. By PA process, the lower energy photon, i.e., near IR up to ~1015 nm photon drives the crystallization of perovskite film, and the higher energy photon of PA may excite the trapped carriers and release the space charges, thus leading to an ideal perovskite layer with higher crystallinity and lower defect density than that of TA. The device with PA 10 min achieves a maximum PCE of 20.41% for CH3NH3PbI3-based planar PSCs. The experimental results indicate that PA is an effective method to achieve a more outstanding performance for perovskite devices.

4. Experimental Section Experimental Section was shown in Supporting information11, 24-25.



ASSOCIATED CONTENT Supporting Information. Additional Spectrogram of the halogen lamp (Philips QVF135), UV-Vis spectra, SEM images, AFM images, the best photovoltaic parameters of D1 and D2, EIS spectra, J-V curves, the Transient PL spectroscopy results and the Experimental Section (PDF)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected], [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT NNSFC (11574009, U1605244, 61575005, 11574013, 61775004) and NKBRDPC (Grant No.2016YFB041003).

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To Greatly Reduce Defects via Photoannealing for High-Quality

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