Bulk Heterojunction-Assisted Grain Growth for Controllable and Highly

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Bulk Heterojunction-Assisted Grain Growth for Controllable and Highly Crystalline Perovskite Film Yanliang Liu, Insoo Shin, Yongchao Ma, In-Wook Hwang, Yun Kyung Jung, Jae-Won Jang, Jung Hyun Jeong, Sung Heum Park, and Kwang-ho Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 28 Aug 2018 Downloaded from http://pubs.acs.org on August 28, 2018

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ACS Applied Materials & Interfaces

Bulk Heterojunction-Assisted Grain Growth for Controllable and Highly Crystalline Perovskite Film Yanliang Liu1,2, Insoo Shin1,2, Yongchao Ma1,2, In-Wook Hwang3, Yun Kyung Jung4, Jae Won Jang1, Jung Hyun Jeong1, Sung Heum Park1,2* Kwang Ho Kim2* 1

Department of Physics, Pukyong National University, Busan 48513, Republic of Korea

2

Hybrid Interface Materials Global Frontier Research Group, Pusan National University, Busan 46241, Republic of Korea

3

Advanced Photonic Research Institute, Gwangju Institute of Science and Technology, Gwangju 61005, Republic of Korea

4

Department of Biomedical Engineering, Inje University, Gimhae 50834, Republic of Korea

Keywords: Perovskite; perovskite solar cells; perovskite light emitting diode, grain growth， highly crystalline perovskite.

1

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ABSTRACT Perovskite optoelectronic devices are being regarded as future candidates for next-generation optoelectronic devices. Device performance has been shown to be influenced by the perovskite film, which is determined by grain size, surface roughness and film coverage, therefore developing controllable and highly crystalline perovskite films is pivotal for highly efficient devices. In this work, an innovative bulk heterojunction (BHJ)-assisted grain growth technique (BAGG) was developed to accurately control the quality of perovskite films. By a simple modulation of the polymer-to-PC61BM ratio in the BHJ film, the transition to complete-film phase from perovskite precursor was accurately regulated, resulting in a controllable perovskite grain growth and high-quality final perovskite film. Moreover, since the BHJ layer could seep deeply into the perovskite active layer through the grain boundaries in the BAGG process, it facilitated the interface engineering and charge transport. The perovskite solar cells containing an optimized CH3NH3PbI3 film presented a high efficiency of 18.38% and fill factor of 83.71%. The perovskite light emitting diode that contained a nanoscale and uniform CH3NH3PbBr3 film with full coverage presented enhanced emission properties with a brightness value of 1600 cd/m2 at 6.0 V and a luminous efficiency of 0.56 cd/A.

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1. Introduction Perovskite compounds have received a lot of attention as essential materials for future generation optoelectronic devices which include perovskite solar cells (PESC) and perovskite light-emitting diode (PELED).1-6 They provide numerous advantages including high absorbance, desirable bipolar charge transport capability, and large electron-hole diffusion lengths.7-13 In recent years, perovskite materials have been widely studied and successfully utilized as charge generating materials for PESC and light emitting materials for PELED and therefore various technologies for highly efficient and stable devices are continuously being developed.14-18 The quality of a perovskite active layer determined by the grain size, crystallinity, and the extent of coverage is critical for developing a high-performance and highly stable perovskite device.19-23 In PESC, effective charge collection at electrode terminals is demonstrated in large and highly crystalline grains due to minimal or no grain boundaries in the lateral direction of the film,24-27 which contributes to a direct charge transport system in the perovskite film. Compared to PESC, the photovoltaic performance of PELED shows more sensitivity to the morphology of the perovskite film because a high voltage is applied to operate PELED.28-30 The rough and incomplete perovskite film easily creates leak-current loss and causes device failure at a high operating voltage, resulting in an unsatisfactory device performance. 4, 31-33 The sensitivity of the device performance to film morphology has motivated researchers to develop techniques that give a perovskite active layer which is 3



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smooth and uniform with pinhole-free grains and full coverage. Some techniques developed so far to enhance the quality of films include, temperature control of annealed devices, use of chemical additives, non-stoichiometric perovskite precursor

solutions,

adjustment

of

the

fabrication

environment,

as

well

as

solvent-induction.34-40 Because the perovskite grain growth is significantly affected by environmental conditions including annealing temperature, treatment time, humidity level, and solvent vapor concentration,20,41-43 most techniques for film fabrication are very complicated. Moreover, precise morphology control of perovskite films remains a major problem, resulting in poor reproducibility and wide distribution of device performance.44-47 Since most perovskite emitting materials with the Br3 component have much faster crystallization than the MAPbI3 charge generating layer used in PESC, the morphology control is more critical in PELED devices.48 Therefore, developing a facile technique for morphology control is urgently required to fabricate high-performance perovskite devices. In our work, we developed a simple but effective bulk-heterojunction (BHJ)-assisted grain growth (BAGG) method which controls precisely, the quality of the perovskite film. We make use of a BHJ layer to regulate the rate of evaporating solvents from the underlying perovskite precursor film. This is simply achieved by adjusting the BHJ blending ratio which enables the precise control of the perovskite morphology including grain size, surface roughness, and film coverage. PESC fabricated by this method exhibited a best efficiency of 18.38% with an open-circuit voltage (Voc) of 0.96 V, a short-circuit current (Jsc) of 22.83 4


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mA/cm2, and a fill factor (FF) of 83.71% with high repeatability. The PELED fabricated using this method also exhibits enhanced device performance with a luminance of 1600 cd/m2 at 6 V and luminous efficiency of 0.56 cd/A. Due to the strong humidity resistance of the top BHJ film, both PESC and PELED present high device stability under different environmental conditions.

2. Experimental 2.1. Materials and sample preparation Synthesis of methylammonium iodide and methylammonium bromide are described in previous

study.1,9

The

45wt%

CH3NH3PbI3

solution

was

prepared

with

MAI:PbI2:dimethylsulfoxide (DMSO) with molecular ration 1:1:1 in dimethyl formamide (DMF) and the 40 wt% CH3NH3PbBr3 precursor solution was prepared with MABr:PbBr2 with 1:0.9 molar ratio in DMSO. 2.2. Device fabrication For the PESC fabrication, the hole transport layer (HTL) of PEDOT:PSS (Baytron PH) was prepared by spin coating on ITO glass substrate and drying at 140 °C for 10 min. The CH3NH3PbI3 precursor solution was spin-coated on substrate (glass/ITO/PEDOT:PSS) at 5000 rpm for 50 s by using solvent engineering method.15 Subsequently, a P3HT-to-PC61BM = x : y (20 mg/ml in chlorobenzene (CB)) blending film was directly spin-coated on unheated

5



film and then the device was heated at 100 °C for 10 min to achieve final CH3NH3PbI3 film. After that, a PC61BM in CB was deposited on P3HT-to-PC61BM film. Finally, a 100 nm of Al layer was thermal evaporated under a pressure of 5×10-7 Torr. For the PELED fabrication, the PEDOT:PSS was prepared and then the CH3NH3PbBr3 precursor solution was spin-coated at 5000 rpm by using solvent engineering method.15 A PFN solution (10 mg/mL in CB) was coated at 3000 rpm on top of CH3NH3PbBr3 precursor film and following heated at 100 °C for 10 min. Then, the Ca/Al cathode with thickness of 10 nm and 100 nm respectively, was thermal evaporated to complete the device. 2.3. Characterization The methods for measurements of UV-Vis absorption spectra, XRD spectra, time-resolved and steady-state PL spectra, SEM and AFM of the perovskite film are same with our earlier reports.49 The contact angle experiments were performed by SMARTFROP (S/N: SDS-TEZD10144). Impedance analyzer (1260 Impedance/Gain-Phase Analyzer, Solartron) was used to confirm the existence of trap state of various samples. The J-V characteristics of the PESC were recorded using a Source Measure Unit (Keithley 2400) under 1.5 Global (AM 1.5 G) solar simulator. The electroluminescence (EL) spectra of the PELED was recorded by using an SR-3AR spectrophotometer. The J-V-L curves of the PELED were obtained by using a Keithley model 2450 power supply combined with a UVIS-50 spot photometer, and were recorded simultaneously during the measurement

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3. Results and Discussion Perovskite grain growth is very sensitive to the evaporation rate of solvents from the precursor film during annealing.47 Slower solvent evaporation gives adequate time for self-assembly and leads to the high crystallinity of perovskite.50 Our earlier works demonstrated that the PC61BM layer coated on the perovskite active layer before rather than after annealing acts as a shield material that extends the time taken for residual solvents to evaporate from the perovskite precursor film and consequently produces grains that are of high crystallinity.49 In the BAGG method, BHJ consisting of the electron donating P3HT layer and electron accepting PC61BM is layered on the perovskite precursor film instead of pure PC61BM as a shield material (Figure 1(a)). Using different ratios of P3HT and PC61BM regulates the evaporation speed of the residual solvent due to the differences in the affinity for the perovskite precursor solution dissolved in DMSO. Figure 1(b)–(d) shows the contact angles of precursor solution on pure PC61BM and pure P3HT and their BHJ with the ratio of 3:10, respectively. While the pure PC61BM has a high affinity with a contact angle of 35.6°, P3HT exhibits poor affinity for the perovskite precursor solution with a contact angle of 62.0° and BHJ presents a moderate affinity with a contact angle of 44.9°. PC61BM with a high affinity holds on to the remaining DMSO solvent in the spin-coated precursor film leading to the slow growth of perovskite grains, whereas the P3HT layer with poor affinity forms the complete perovskite film from the perovskite precursor much faster, thus producing smaller grain sizes. Fourier transform infrared (FTIR) spectra for films with P3HT and PC61BM top 7



layer (see the Figure S1) was carried out to confirm this effect. Figure 1(e)–(g) exhibits the annealing time-dependent absorption spectra of perovskite films covered by PC61BM, P3HT, and their BHJ, respectively. For this experiment, PC61BM, P3HT, and BHJ were deposited separately on the perovskite precursor film and then annealed at 100°C with increasing annealing times. It is observed that the PC61BM-perovskite precursor film transitioned to a complete-perovskite phase very slowly (Figure. 1(e)) compared to P3HT (Figure. 1(f)) and the BHJ-perovskite precursor film (Figure. 1(g)); 10, 1, and 2 min for PC61BM-, the P3HT- and the BHJ-perovskite precursor film, respectively, estimated from the time when the absorbance became saturated. The experimental results showed that regulating the BHJ blending ratio can control the growth of grain in the perovskite films. Therefore, the BAGG method is designed to produce controlled grains of the perovskite film with controlled grain sizes.

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Figure 1. (a) Schematics of the BAGG method. The contact angles for perovskite precursor solution dropped on (b) pure PC61BM, (c) pure P3HT, and (d) P3HT-to-PC61BM=3:10 film. The absorbance spectra of perovskite films with different annealing times covered by (e) pure PC61BM, (f) pure P3HT, and (g) P3HT-to-PC61BM= 3:10 film.

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Figure 2(a)–(j) displays top view SEM images of the BAGG-method-prepared CH3NH3PbI3 films. The CH3NH3PbI3 film covered with pure PC61BM shows remarkable grains with sizes of 2.1 µm determined by the image analysis software (Image J), whereas the film covered by P3HT exhibits very small grains with indistinguishable grain boundaries. SEM images in Figure 2(b)–(i) show how the size of grain in CH3NH3PbI3 films is controlled by the quantity of P3HT in the BHJ blend. When P3HT is added to PC61BM at the blending ratio of 1:10 wt%, the average grain size reduced to 1.2 µm with a dense and uniform CH3NH3PbI3 film. Further increasing the amount of P3HT in the BHJ resulted in a gradual decrease in grain sizes with denser and homogeneous film morphology (Figure 2(d)–(i)). Figure 2(k)–(o) displays the corresponding cross-sectional images of CH3NH3PbI3 films with thicknesses of approximately~500 nm. The CH3NH3PbI3 grain size gradually decreased with increased quantities of P3HT in the BHJ, which is well consistent with top-view SEM images. In Figure S2, we see from Atomic Force Microscope (AFM) images, a decrease in root-mean-square (RMS) roughness values for a decreased perovskite grain size. Figures 2(k) and (l) show highly crystalline perovskite grains. The grain boundaries run from the bottom hole-transport layer (HTL) through the top BHJ film, to the electron-transport layer (ETL), creating a direct transport network which allows for charges to be extracted efficiently. The grain size of the CH3NH3PbI3 perovskite film diminished gradually for increased quantities of P3HT in the BHJ solution (Figure 2(m)–(o)). The results show that the grain size in the perovskite film is accurately controlled using the BAGG method. 10


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Figure 2. Top-view SEM images of BAGG-method prepared CH3NH3PbI3 layers using various

top

shield

films,

(a)

pure

PC61BM,

P3HT-to-PC61BM=1:10,

(d)

P3HT-to-PC61BM =1:5,

P3HT-to-PC61BM=1:2,

(g)

P3HT-to-PC61BM=3:5,

(b)


(c)

(e) P3HT-to-PC61BM=3:10,

(f)

(h)


(i)

P3HT-to-PC61BM=1:1, (j) pure P3HT. Corresponding cross-sectional SEM images of CH3NH3PbI3 films, (k) pure PC61BM, (l) P3HT-to-PC61BM=1:10, (m) P3HT-to-PC61BM=3:10, (n) P3HT-to-PC61BM=7:10, (o) P3HT-to-PC61BM=1:1.

11



The BAGG method provides an enhanced contact between the top BHJ layer and the CH3NH3PbI3 film with no chemical reactions. Figures 3(a) and (b) exhibit the surface morphology of the perovskite and BHJ films, respectively. For this experiment, the BHJ layer was simply rinsed with the organic solvent and the surface morphology of perovskite layer underneath was monitored using SEM and AFM images. The BHJ film was separated from the CH3NH3PbI3 film by dipping the device (ITO/PEDOT:PSS/ CH3NH3PbI3/BHJ) into hot water of 40 0C to dissolve the CH3NH3PbI3 film selectively. The bottom surface morphology of the BHJ film was then measured using SEM and AFM, the schematic and details of this experiment process was provided in Figure S3. In this process, the morphology of the BHJ film was not noticeably changed (see the Figure S4). Surface morphology of both the CH3NH3PbI3 and BHJ layers is similar to each other with highly crystalline and enlarged grains (Figure 3(a) and (b)). An insert of the AFM image in Figure 3(b) exhibits the network structure and shape of BHJ, which resembles the shape of the corresponding perovskite grain boundaries in Figure 3(a). Moreover, the protruding parts on the BHJ surface in Figure 3(c) of the three-dimensional (3D) AFM image exactly matched the hollow parts on the perovskite surface. This indicates that the CH3NH3PbI3 film is deeply infiltrated by the top BHJ layer through the grain boundaries thus facilitating the interface engineering between the perovskite and the ETL, leading to further enhancement in electron transport.

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Figure 3. SEM and AFM images of (a) the CH3NH3PbI3 film and (b) the top BHJ layer (P3HT:PC61BM=3:10) in the perovskite preparation process. (c) 3D AFM images of the CH3NH3PbI3 layer with a top BHJ layer.

The difference in the affinity towards the perovskite precursor solution provides different perovskite film morphology. Therefore, using a blend of two or more materials with different affinities to the perovskite precursor controls the perovskite grain growth. Figures S5(a)-(d) and (e)-(h) exhibit the morphology variations by controlling the blending ratio of PMMA:PC61BM and DT-PDPP2T-TT:PC61BM, respectively. The sizes of grain are easily controlled by changing the material blends of PMMA:PC61BM and DT-PDPP2T-TT:PC61BM and the SEM images of both exhibit the same tendency in morphology variation as that of P3HT:PC61BM. These results imply that various polymer materials including semiconductors or insulators can be widely utilized as top shielding materials in perovskite film fabrication for morphology modulation and the techniques can be applicable in various devices.

13



Figure 4(a) presents X-ray diffraction (XRD) spectra for CH3NH3PbI3 films fabricated by the BAGG method with varying P3HT-to-PC61BM blending ratios. The BHJ layers were rinsed with the organic solvent for XRD measurements after annealing. All films exhibited two main peaks of CH3NH3PbI3 assigned to (110) and (220) plains, consistent with the previous reports.11 The film covered by pure PC61BM exhibited the strongest XRD intensity at (110) and (220) peaks, while the peaks became weaker when the quantity of P3HT increased in the BHJ blend. After adding the P3HT to the PC61BM shield film (P3HT-to-PC61BM=1:10), the intensity of the peaks related to (110) and (220) simultaneously decreased. Further increasing the P3HT content in the top BHJ film sequentially reduced the (110) and (220) peak intensity, consistent with the SEM images. The intensity of (112), (211), (202), and (312) peaks remained unchanged with different P3HT-to-PC61BM shield films. This indicates that the top BHJ layer only confines the formation of (110) and (220) plains in the CH3NH3PbI3 crystal. For the (110) peak in the XRD spectra, its full-width-half-maximum (FWHM) does not exhibit the changes in different shield films. This suggests that the crystal structures for films with different grain sizes are identical. However, the absorbance of perovskite films in Figure 4(b) present slightly different optical density values in the range 800–650 nm for small quantities of residual BHJ which is present in the grain boundaries even after rinsing.

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Figure 4. (a) XRD patterns and (b) absorption spectra of CH3NH3PbI3 films prepared by the BAGG method. We used the BAGG technique to fabricate a one-step-solution processable inverted PESC and examined their performance. Figure 5(a), shows the J-V curves for PESC with an ITO/PEDOT:PSS/CH3NH3PbI3/BHJ/PC61BM/Al structure. The BHJ layer was introduced as a shield material to control the grain growth and easily rinsed after annealing. Summarized photovoltaic values are displayed in Table 1. PESC with pure PC61BM exhibited a high efficiency of 17.36%, with a Voc of 0.95 V, a Jsc of 22.54 mA/cm2, and a FF of 81.23%, comparable to our previous results.50 The best PESC efficiency was achieved by the device with the BHJ of P3HT-to-PC61BM=1:10 (efficiency of 18.34%, Jsc of 22.85 mA/cm2, Voc of 0.96 V, and an extremely high FF of 83.57%). The significantly enhanced FF is justified by the dense and uniform CH3NH3PbI3 having size-controllable grain (Figure 2). However, a

15



further decrease in the grain size of CH3NH3PbI3 films obtained by increasing P3HT quantities in the BHJ blend, resulted in a gradual loss of Jsc and efficiency. As displayed in Figure 5(b), the incident photon-to-current efficiency (IPCE) measurement was further performed to confirm the loss in Jsc. PESC having smaller grain sizes exhibited a lower IPCE intensity over the entire wavelength for increased amounts of P3HT in the BHJ. A likely reason for the decrease in Jsc is an increase in the number of trap sites present in the perovskite active layer. Because small grains with lower crystallinity and more grain boundaries than highly crystalline large grains should have more trap sites, the generated electrons can be trapped and prevented from collection at the electrode. On the other hand, there is a constant collection of free electrons at the electrode because the perovskite active layer has high carrier mobility when the electric field is applied during the J-V measurements. Consequently, it provides a constant value of charges collected at the electrodes resulting in lower Jsc but higher FF in J−V curves. Further investigating the performance of the devices, impedance spectroscopy was used to observe the internal resistance of the devices. Since PESC with a type of sandwich formed from two different metal electrodes are dominantly capacitive due to the existence of energy barriers in devices under short-circuit conditions, the capacitance of the device can be measured precisely. Figure 5(c) displays Nyquist plots of PESC devices for different BHJ blending ratios. As the amount of P3HT in the BHJ increased, the diameter of the semicircle became larger representing the increase in the resistance of devices. Therefore, the resistance 16


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of the device increased as the amount of P3HT increased in the BHJ. Since the perovskite film with a higher ratio of P3HT produced more grains and grain boundaries (Figure 3(a)–(j)), the decreased grain size inhibited the charge collection at the electrodes and resulted in the decreased photocurrent. In addition, from the J-V curve of best performing device, no hysteresis is exhibited despite repeated application of bias voltage (Figure S6), which is an advantage associated with inverted planar PESC. To investigate the reproducibility of the device, fifty devices with P3HT-to-PC61BM BHJ blend ratios of 1:10 were fabricated and the distribution of PCE was plotted in Figure 5d. All devices showed high PCE of over 17.00%. Moreover, most samples had extremely high FF of over 82.00%, with a maximum of 84.38% (Figure 5(e)). This high FF was also achieved in flexible PESC using polyethylene naphthalate two formic acid glycol ester (PEN)-ITO substrates (Figure 5(f)) and the flexible device exhibited a reasonable PCE of 13.86% with high FF of 82.01% indicating the quality and reproducibility of our device fabricated using the BAGG technique. The extremely high FF value is attributed to the well optimized CH3NH3PbI3 film morphology which results from the BAGG method and penetration of BHJ film into CH3NH3PbI3 layer through grain boundaries. This in turn is advantageous for charge generation and extraction in PESC, leading to a higher device performance.

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Figure 5. (a) J–V curve and (b) IPCE (c) impedance spectra of the solar cells fabricated by the BAGG method. (d) Distribution of perovskite solar cell efficiencies for optimized CH3NH3PbI3 films. (e) J-V curve for the perovskite solar cells with highest FF of 84.38%. (f) J-V curve for the flexible perovskite solar cell. Table 1. Photovoltaic Parameters of solar cells prepared by BAGG method.

P3HT:PC61BM

Jsc (mA/cm2)

Voc (V)

FF (%)

PC61BM

22.54

0.95

81.23

17.36

1:10

22.85

0.96

83.57

18.34

3:10

19.18

0.92

81.54

14.33

7:10

12.41

0.91

81.27

9.15

1:1

7.72

0.92

76.85

5.46

18


PCE

(%)

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Since the perovskite film morphology is modulated by selecting the ratio of the BHJ or shielding materials, the method can be utilized in other optoelectronic device fabrication applications. For example, in PELED, smaller grains with full coverage are more favorable for high device performance.4-5, 51-52 Therefore, using a single polymer as a shielding material may

provide

CH3NH3PbBr3

smaller

grains

films

annealed

with

full

with

coverage. no

The

shielding

SEM material,

images

of

PC61BM,

poly[(9,9-bis(3'-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9–dioctylfluorene)] (PFN), and their BHJ shielding materials are shown in Figure S7. PFN which is a widely used electron transport material, is frequently spin-coated on the perovskite film in PELEDs. Therefore, we selected PFN as a top layer to shield the perovskite layer before annealing. After spinning the CH3NH3PbBr3 precursor on the PEDOT:PSS layer, we directly deposited PFN on top of the CH3NH3PbBr3 precursor film and annealed them together to attain a complete CH3NH3PbBr3 film (Figure 6(a)). We call this device the PFN-film or device. For the comparison, we also prepared the CH3NH3PbBr3 film and annealed it before the PFN deposition using the conventional annealing method. After annealing, we deposited the PFN layer on top of the complete perovskite film. We call that device the conventional film or device. The photoluminescence (PL) and time resolved-PL (TR-PL) for both CH3NH3PbBr3 films can be seen in Figures 6(b) and (c). The PFN-CH3NH3PbBr3 film showed significantly higher PL intensity and longer PL lifetime than the conventional film, indicating a low

trap

density

in

the

CH3NH3PbBr3

film.

19


PELED

with

an


ITO/PEDOT:PSS/CH3NH3PbBr3/PFN/Ca/Al configuration were fabricated completely after the deposition of the Ca/Al electrode (PFN-PELED). We also prepared the same PELED with an identical configuration but annealed it without a shielding material (conventional PELED). PEDOT:PSS, PFN, and CH3NH3PbBr3 were used as the HTL, ETL and light emitting layer, respectively. The energy level diagram of the PELED is shown in Figure S8. When the devices are biased by externally applied voltage, the electrons are injected from the Al/Ca cathode to the CH3NH3PbBr3 emitting layer via PFN layer, whereas the holes are injected from the ITO anode to the CH3NH3PbBr3 emitter via the PEDOT:PSS layer.

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Figure 6. (a) Schematic of PELED fabrication by the BAGG method. (b) Photoluminescence and (c) time resolved-PL spectra of conventional and PFN assisted CH3NH3PbBr3 film. (d) Electroluminescence, (e) current density and luminance versus applied voltage spectra of conventional and PFN-assisted PELED.

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Figure 6(d) exhibits the electroluminescence (EL) spectra of the PELED fabricated from the conventional and PFN layer-assisted thermal annealing processes. The PFN-PELED exhibits a stronger green EL intensity than the conventional device with the emission peak at 530 nm and FWHM of 20 nm. Figure 6(e) displays the current-voltage-luminance (J-V-L) characteristics for the PELED under forward bias condition. The J-V-L curves showed typical characteristics of PELED. The turn-on voltages for the current injection of both devices were around 4 V with a similar current density. However, the operating voltage for light emissions for the PFN-device was 4 V lower and the EL intensity was much higher than that of the conventional device. We also see that the EL intensity of the PFN-device linearly increased with enhanced current density, a luminous efficiency of 0.56 cd/A and maximum brightness of device reached 1600 cd/m2 at 6 V. We conclude therefore that the modified BAGG method works well in PELED applications and significantly enhances the perovskite film morphology as well as its optoelectronic properties therefore improving the photovoltaic performance of PELED.

4. Conclusion In summary, we developed a simple but effective BAGG method for a controllable and highly crystalline perovskite active layer. By varying the polymer content in the BHJ layer, we successfully demonstrated the controlled grain morphology with controlled crystallinity and grain size. Blending two materials with different affinities with the perovskite precursor 22


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regulates the grain growth of the perovskite precursor and provides a controllable perovskite grain growth. This results in accurately controllable properties of the perovskite film including grain size, surface roughness, and film coverage. Moreover, the BHJ layer can deeply infiltrate the perovskite film through grain boundaries during grain growth, which enhances interface engineering between the charge-transporting layer and perovskite film. PESC prepared by this method contains dense and micro-sized single-crystal CH3NH3PbI3 grains and exhibits a high PCE of 18.34% with an FF of 83.57%. In addition, PELED fabricated by the top PFN layer-assisted technique contains uniform and full-coverage CH3NH3PbBr3 films and exhibits a bright green emission light intensity of 1600 cd/m2 at 6.0 V. Therefore, the BAGG method proposed in this research can significantly impact the future development of perovskite solar cells and LEDs.

ASSOCIATED CONTENT

Supporting information The Supporting Information is available free of charge on the ACS Publications website. Additional data for FTIR spectra of CH3NH3PbI3 films (Figure S1), AFM images of CH3NH3PbI3 films (Figure S2), schematic of the BHJ film separation from the perovskite film (Figure S3), TEM image of P3HT:PC61BM(1:1) BHJ film treated in the water (Figure S4), SEM images of CH3NH3PbI3 films (Figure S5), J-V curves of champion perovskite 23



solar cells (Figure S6), SEM images of CH3NH3PbBr3 films (Figure S7), energy level diagram of PELED (Figure S8) and chemical structure of P3HT, PC61BM and PFN (Figure S9).

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

*E-mail: [email protected]

ACKNOWLEDGMENT The research was mainly supported by the Global Frontier Program through the Global Frontier Hybrid Interface Materials (GFHIM) of the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (2013M3A6 B1078874). The research was also supported by the NRF Grant funded by the Korea Government (NRF-2016R1A2B4011474)

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Bulk Heterojunction-Assisted Grain Growth for Controllable and Highly

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