Manipulate Micrometer Surface and Nanometer Bulk Phase

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Organic Electronic Devices

Manipulate Micrometer Surface and Nanometer Bulk Phase Separation Structures in Active Layer of Organic Solar Cell via Synergy of Ultrasonic and High-pressure Gas Spraying Alei Liu, Wenhao Zheng, Xiaolong Yin, Junyu Yang, Yuanbao Lin, Wanzhu Cai, Xiaomin Yu, Quanbin Liang, Zhicai He, Hongbin Wu, Yang Li, Fengling Zhang, and Lintao Hou ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b22215 • Publication Date (Web): 28 Feb 2019 Downloaded from http://pubs.acs.org on February 28, 2019

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ACS Applied Materials & Interfaces

Manipulate Micrometer Surface and Nanometer Bulk Phase Separation Structures in Active Layer of Organic Solar Cell via Synergy of Ultrasonic and High-pressure Gas Spraying Alei Liu,a Wenhao Zheng,a Xiaolong Yin,a Junyu Yang,a Yuanbao Lin,a Wanzhu Cai,*,a Xiaomin Yu,a Quanbin Liang,b Zhicai He,b Hongbin Wu,b Yang Li,*,c Fengling Zhang,*,a,d and Lintao Hou*,a a Guangdong

Provincial Key Laboratory of Optical Fiber Sensing and Communications, Guangzhou Key

Laboratory of Vacuum Coating Technologies and New Energy Materials, Guangdong Provincial Engineering Technology Research Center of Vacuum Coating Technologies and New Energy Materials, Siyuan Laboratory, Department of Physics, Jinan University, Guangzhou 510632, PR China. b

Institute of Polymer Optoelectronic Materials & Devices, State Key Laboratory of Luminescent Materials &

Devices, South China University of Technology, Guangzhou 510640, PR China. c

d

School of Applied Physics and Material, Wuyi University, Jiangmen 529020, China. Biomolecular and Organic Electronics, Department of Physics, Chemistry and Biology (IFM), Linköping

University, SE-58183, Linköping, Sweden.

KEYWORDS: high-pressure gas spraying, ultrasonic spraying, ITO-free, organic solar cells, morphological manipulation 1

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ABSTRACT For organic solar cells (OSCs), the vertical and lateral micro/nanometer-scale structure in the active layer largely determines the device performance. In this work, the surface and bulk domain size of the photoactive layer is successfully manipulated with a facile two-step spraying method, viz. an ultrathin active layer by high-pressure spraying is deliberately stacked on top of the thick active layer by ultrasonic spraying. Thus the morphology is effectively optimized with comprehensive study of optical and electrical characteristics, such as photon absorption, exciton dissociation efficiency and bimolecular recombination. Moreover the novel method can be used not only in the fullerene system but also in the non-fullerene system, demonstrating the remarkable universality through this synergy method. This work provides an easy and reliable strategy to improve photovoltaic device performance in the industrial large-area spray-coating process.

1. INTRODUCTION Organic solar cell (OSC) is an excellent candidate for use in the low-cost green energy generation.1-8 Today, the power conversion efficiency (PCE) of single-junction OSC has reached 14%, which is already beyond the performance benchmark for commercialization.9,10 However, a transition from lab to fab in the roll-to-roll production is still a challenge. Generally the performance of devices processed by 2


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printing techniques is much worse than that processed by spin-coating, which is mainly related to the worse phase separation structure of active layers. Thus during the continuous and high-speed production, precisely controlling the morphology of active layers in conjunction with making full use of the advantages of each printing technology is becoming more and more important. The currently reported fab-scale printing techniques include inkjet printing,11,12 doctor-blading3,4,8, spray-coating13,14, and so on. Till now, the potential ability of these techniques to manipulate the micro/nanoscale structure of photovoltaic layers as required is far from exploration. Compared to other fab-scale techniques, spray coating presents many unique advantages, such as various substrate compatibility, high production speed and less material waste. The deposition process is a multistage method and can be divided into several steps: liquid atomization, droplet flight and evaporation, droplet impact and spreading on the substrate, solidification and drying, leading to more morphological diversification than other fab-scale techniques.15 As we know, high-pressure gas spraying (HS)16,17 and ultrasonic-spraying (US)18,19 are two commonly-used spraying techniques: US technique adopts a high-frequency acoustic wave at the nozzle to break down the liquid droplet into smaller ones; HS technique uses a convergent-divergent nozzle to accelerate the high-pressure gas to a high speed to atomize a liquid solution.20,21 In previous reports, the best PCEs of 8.75% and 7.07% were obtained respectively by using two spraying techniques with the indium tin oxide

(ITO)-based

device

structure.19,17

It

should

be

noted

that

the

morphology-property relationship for spray coating is not clearly clarified till now. 3



Astoundingly, contradictory conclusions are demonstrated in many reports. For example, some groups claimed non-uniform disk-like grains or pancake boundaries formed on the active layer surface are advantageous for enhancing the device performance, whereas other groups suggested more uniform film morphology is beneficial for the improvement of device performance.16,17,22,23 Thus further study is needed for understanding different spraying techniques and new solutions should be proposed for maximizing the advantages of them. In addition, in the past decades ITO is widely used as the transparent conductive electrode for OSCs, which hinders the commercial application of OSCs due to the brittle flaws and high cost of deficient indium. Therefore, replacing ITO is necessary for large-scale OSCs processed by printing. In this work, by controlling the atomization process and drying process of HS and US, the micro/nanometer scale structure of the active layer can be effectively manipulated with a comprehensive comparison of morphology-performance relationship. Combining with the photoelectric characterization measurements, it shows that US-processed device has better carrier generation and transport properties due to a well-mixed donor/acceptor (D/A) phase separation with a fine crystallization in nanoscale, and HS-processed device has an enhanced absorption because of the unique microscale topography on the surface. To make full use of the advantages of both US and HS, a facile two-step spraying method is creatively proposed to further optimize the morphology of active layers. By inserting a textured HS-coated active layer between the anode and the pin-free active layer with US, the device performance 4


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is greatly improved. The efficiency can be increased by 33% and 22% for fullerene and non-fullerene OSCs, respectively. The encouraging PCE of 8.06% is the highest value ever reported among ITO-free non-fullerene sprayed OSCs. This work demonstrates that through reasonable design of spraying scheme, high-quality active layer with fine micrometer and nanometer structures can be produced, illustrating spraying coating, which is usually underestimated compared to other fab-scale methods, as a viable and effective method in high throughput process.

2. EXPERIMENTAL SECTION 2.1 Materials. Polymer donor PTB7 (thieno[3,4-b]thiophene/benzo-dithiophene) and

interface

material

PFN

(poly[(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluore ne)])

are

purchased

from

1-Material

Inc.

Fullerene

acceptor

PC71BM

([6,6]-phenyl-C71-butyric acid methyl ester) is purchased from Solenne Inc. The other polymer

donor

PBDB-T

(poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene)-co(1,3-di(5-thiophene-2-yl)-5,7-bis(2-ethylhexyl)benzo[1,2-c:4,5-c′]dithiophene-4,8-dio ne)))

and

non-fullerene

acceptor

IT-M

((3-(1,1-dicyanomethylene)-1-methyl-indanone)-5,5,11,11-tetrakis(4-hexylphenyl)-dit hieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]-dithiophene)) are both purchased from Solarmer Materials Inc. The blending ratio of PTB7:PC71BM is 1:1.5 (w/w) with a polymer concentration of 10 mg mL−1 in a ternary solvent system CB: DIO: CN 5



(94:3:3 by volume). PBDB-T and IT-M blends (1:1 weight ratio) are dissolved in CB: DIO mixture (99:1 by volume) with a total concentration of 5 mg mL−1 and stirred at 100 °C overnight. PFN is dissolved in methanol (0.5 mg mL−1) with a few drops of acetic acid. PEDOT:PSS (Clevios PH1000) solution is mixed with 5% dimethyl sulfoxide (DMSO) (Alfa Aesar 99.9%) and 0.5% surfactant (FS-30) for higher conductivity and better surface wettability. 2.2 Device Fabrication. The glass substrates are cleaned with acetone, alkaline lotion, deionized water, and isopropanol in order. Ag (100 nm) used as the cathode is thermally evaporated under a pressure of 3×10−4 Pa on top of glass substrates and PFN (10 nm) is spin-coated onto the Ag cathode. Then PTB7:PC71BM ink or PBDB-T: IT-M ink is deposited on top of the PFN layer with US or HS in air. For US coating, a commercial spray nozzle (D12) is used under a constant ultrasonic power (2 W) with an ideal nozzle-to-substrate distance of 5 cm; the PTB7:PC71BM or PBDB-T:IT-M solution is delivered by a syringe pump at a constant liquid flow rate of 0.33 mL min−1; a low-pressure nitrogen gas (N2) is attached to the ultrasonic nozzle for achieving a downward focusing flow. For high-pressure gas-spraying, the 0.3 mm diameter nozzle orifice (MA-S) with an constant nozzle-to-substrate distance of 12 cm is connected to the active solution and high-pressure N2;16 the gas flow rate is 16.67 L min−1, and the liquid flow rate is 5.98 mL min−1. PEDOT:PSS is spin-coated onto the top of the active layer at 1000 rpm and annealed at 60 °C for 30 s to remove the residual water. Finally, the devices are encapsulated with clean glasses. The device area is 4 mm2, determined by a patterned mask. The substrate temperature in 6


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each spraying process is controlled by a hot plate. 2.3 Measurements and Characterization. The device current density-voltage (J−V) characteristics are tested using a Keithley 2400 source meter. Standard solar radiation (AM 1.5G) is provided by the solar simulator (Sun 2000, Abet). External quantum efficiency (EQE) spectrum is measured using a QE-R test system from Enli Technology Company. Transient photovoltage (TPV) measurement is performed under the open-circuit condition and illumination of a 100 W tungsten halogen lamp. The devices are connected to an oscilloscope (Tektronix DPO4104, 1 GHz) with an input impedance of 1 MΩ to keep the device at the open-circuit condition. The excitation pulse is generated using a pulsed laser with a wavelength of 532 nm (OPOTEK Vibrant 355, with a 5 ns optical pulse). The perturbation light intensity is attenuated by a set of neutral density filters so that the amplitude of TPV is much lower than the open-circuit voltage (VOC). Transient photocurrent (TPC) measurement is performed with the device held under a short-circuit condition by connecting the device to the ground through a small resistor. The resulting transient current is measured using an oscilloscope in parallel with a small resistor of 50 Ω. Laser-beam-induced current maps (LBIC) is imaged using a pulsed laser-diode beam at 405 nm that scanned the cell's surface with an image resolution of 50 µm (LSD4, Enli). The impedance spectroscopy (IS) is performed using an electrochemical workstation (Princeton Applied Research) with an AC amplitude of 10 mV and a frequency range between 100 kHz and 1 Hz as well as an applied bias of 0.7 V at room temperature (RT). The film thickness is measured with a surface profiler 7



(XP-2). The film topography is investigated using atomic force microscopy (AFM) (Bioscope Catalyst Nanoscope-V) and transmission electron microscopy (TEM) (JEM-2100F). The cross-sectional image is measured by scanning electron microscopy (SEM) (Apreo, FEI). UV-vis absorption and reflectance spectra are recorded on a Shimadzu UV-2550 UV-vis spectrophotometer with an integrating sphere. A transient drying monitoring technique (TDMT) is utilized to track the film specular reflection excited by a blue LED and recorded by a CMOS camera (IDS uEye). Grazing incidence wide-angle X-ray scattering (GIWAXS) is conducted at the Advanced Light Source. The wavelength of the X-ray radiation is 0.134144 nm; Beam Size = 0.8×0.8 mm2; Beam Center = (269.983, 1027.982); Pixel Size: 0.172×0.172 mm2; Exposure Time: 1800s × 2; Incident Angle: 0.2 degree.

3. RESULTS AND DISCUSSION PTB7, PC71BM, and PFN are used as the donor, acceptor and interface modification materials respectively (Figure 1a). Figure 1b illustrates the schematic diagrams of the US and HS processes. The US method applies ultrasonic atomization, while the HS method applies the high-pressure gas blast atomization. For both cases, the active film thickness is accurately controlled by spraying time. For obtaining an optimal substrate temperature, different substrate temperatures are employed with the supplementary device J−V and photovoltaic parameters shown in Figure S1 and Table S1. It is interesting that the optimal substrate temperature for US is RT, whereas it is 60 °C for

8


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HS, indicating that ultrasonic spraying is more energy-saving.

a)

PFN

b)

PTB7

Ultrasonic-spraying

PC 71BM

High-pressure gas-spraying

Electrical connector High-pressure N2 gas

Low-pressure N2 gas

Figure 1. (a) The molecular structures of PFN, PTB7, and PC71BM. (b) Schematic diagrams of US and HS processes.

The rate of solvent evaporation has a big effect on the film morphology.24 Here, in situ TDMT is applied to study the drying process of the different sprayed active layers. Figure 2 shows the variation of the thicknesses of different sprayed films with the increase of time. Here the drying time (tdry) is determined by the intersection of the tangent line and the straight line in transient film reflectance curves, and the drying velocity (vdry) is decided by the interval time of adjacent peaks. The transient wet film thickness during the drying process can also be back-calculated with the peak-counting method after achieving the final dry film thickness. It is found that tdry for US films is 375 s at RT and 159 s at 60 °C, respectively. In comparison, for HS films, tdry is even prolonged to 1050 s at RT and 712 s at 60 °C, respectively. The ultrasonic-sprayed film presents shorter tdry than that of the high-pressure gas sprayed 9



films at the same substrate temperature mainly due to the smaller atomized droplet sizes. Correspondingly, the vdry for US films is increased from 3.62 to 7.58 nm s−1 when increasing the substrate temperature from RT to 60 °C, which are much higher than vdry for HS films (1.01 to 1.54 nm s−1). The slow solvent evaporation rate for HS films will produce more aggregates and bigger phase separation compared to US films. In this study, the drying time of the active layer is longer than that of some other deposition techniques due to the lower solution concentration used in the process. But to cater to the large scale and fast industrial process, drying time control should be considered as well as the performance optimization in the future.

a)

US at RT

1600 800

375 s

400 0 1600

US at 60 oC

1200

159 s

800 400

0

100

200

Time (s)

300

10


0 400

Thickness (nm)

1200

Reflectance (arb)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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b)

HS at RT

1600 800

1050 s

400 0 1600

HS at 60 oC 712 s

1200

Thickness (nm)

1200

Reflectance (arb)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


800 400 0

300

600

900

Time (s)

1200

0 1500

Figure 2. The transient drying processes of (a) US films and (b) HS films at RT and 60 °C, respectively.

As we know, the topographic and the bulk morphology are greatly influenced by the deposition condition. For US films formed at RT (Figure 3a), the surface are quite smooth from AFM observation, since a continuous small droplet network is coalesced in sufficient time.19,21 When the substrate temperature is elevated to 60 °C (Figure 3b), the root mean square (RMS) roughness of US film is slightly increased from 14.9 to 15.4 nm. The mean max height of profile (Rz) value of US film is increased from 14.7 to 15.8 nm. The HS film formed at RT presents the RMS roughness of 19.3 nm and Rz of 24.8 nm, and the RMS roughness and Rz are greatly increased to 22.5 nm and 47.2 nm at 60 °C respectively (Figure 3c, d). The topography of the latter HS film shows disk-like grains with pancake boundary, which should be attributed mainly to the insufficient mixing of big droplets in a relatively fast solvent drying process compared to that at RT.16,22,25 This information is verified again in larger dimensions 11



seen from the optical microscope images, as shown in Figure S2. The corresponding internal bulk morphology is characterized with TEM images. In Figure 3e-h, the dark area corresponds to PC71BM-rich domains, and the light one corresponds to PTB7-rich domains.26,27 Smaller phase separation, which is advantageous for exciton diffusion and separation, is found inside the US film (RT) compared with that inside the HS film (60 °C). Also there is almost no difference of phase separation in TEM between the US films sprayed at RT and 60 °C mainly owing to the original small US-sprayed particles from the nozzle. For HS films, the film formed under RT shows much bigger bulk domain size than the film formed at 60 °C since the processing de-mixing of donor and acceptor at RT might happen due to the longer drying time. The micrometer surface with a finer bulk D/A intermixing for HS-processed film at 60 °C is very different from most morphology characteristics of spin-coated films that a small D/A phase separation in bulk always corresponds to a smooth film surface. It should be pointed out that the grain boundaries probed by AFM result from the droplet boundaries when they merge, whereas the boundaries probed by TEM are those separating polymer-rich and PCBM-rich domains in the bulk. This observation demonstrated that the surface topography of sprayed active layers could be manipulated independently, without bringing a side effect of changing the internal phase separation degree.

12


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a) US at RT

b) US at 60 oC

c) HS at RT

d) HS at 60 oC

RMS~14.9 nm Rz~14.7 nm




e) US at RT

f) US at 60 oC

g) HS at RT

h) HS at 60 oC

100 nm

100 nm

100 nm

100 nm

60 nm

-60 nm

Figure 3. AFM images of the films processed with (a) US at RT, (b) US at 60 °C, (c) HS at RT, and (d) HS at 60 °C. TEM images of the films processed with (e) US at RT, (f) US at 60 °C, (g) HS at RT, and (h) HS at 60 °C. (AFM image size: 30 µm × 30 µm, TEM bar = 100 nm)

The relationship between the surface structure and the photon absorption property is of most concern. As we know, enhanced optical absorption in active layers can be achieved with many methods, such as metal nanoparticle plasmonic enhancement28 and nanopatterned light manipulation29-31 as well as photonic crystal microcavity adjustment.32 Figure 4a shows the absorption spectra of US (RT) (95 nm) and HS (60 °C) (110 nm) films with the reference of the spin-coated film (100 nm). The absorption intensity of the HS film is greatly improved compared with the US film and the control film in the wavelength range from 350 to 800 nm, indicating that the surface micrometer structure formed by HS has better optical absorption ability among three films. Furthermore, the reflectance intensity of the spin-coated device shows the strongest reflection in comparison with the sprayed devices in a wide 13



wavelength range (Figure 4b). A clear trend is concluded that the device with the roughest active layer surface shows the lowest reflectance intensity, attributing to effective light scattering and trapping effect. The randomly textured surface of the HS-sprayed film can also be reflected in LBIC measurement as the HS device at 60 °C exhibits an inhomogeneous distribution of photocurrent compared to the US device at RT with a comparatively flat surface structure (inset of Figure 4b). In other words, some higher photocurrent generation spots are embedded in a relatively low photocurrent generation area for the HS device at 60 °C. In our case, the control of the HS conditions can easily generate different-dimensional micro/nano 3D structures, endowing a desirable strategy to further improve performance of large-area organic solar panel in manufacturing.

a) Absorbance (a.u.)

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0.8 US at RT HS at 60 oC Spin at RT

0.6 0.4 0.2 300

400

500

600

Wavelength (nm)

14


700

800

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b) Reflectance (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


100

US at RT

HS at 60 oC

60 µA

80 60

0

40 20 300

400

500 600 700 Wavelength (nm)

800

Figure 4. (a) UV-vis absorption spectra of the US and HS films in comparison to the spin-coated one. (b) Reflectance spectra of the corresponding ITO-free OSCs with a structure of Ag/PFN/active layer/PEDOT:PSS. The inset images are LBIC results of the US device at RT and the HS device at 60 °C.

Despite the above morphological characteristics differences, the molecular orientations and crystallographic properties are also investigated by GIWAXS measurements (Figure 5). As seen from in-plane direction images, for the US film at RT, a weak and broad peak at lower q value of 0.34 Å-1 (Table 1) arises from the PTB7 (100) Bragg diffraction, resulting in the corresponding lamellar distance of 18.33 Å (d(100)=2π/q(100)).33,34 For the HS film formed at 60 °C, a diffused peak at 0.34 Å-1 appeared with a similar lamellar distance of 18.37 Å, due to the aggregation of PTB7 and PC71BM. Therefore, a better crystallinity of PTB7 is found in the US film, which means US has better control over the nucleation and phase purity of PTB7 than HS. The evident π-π stacking of PTB7 (010) can be found in the out-of-plane 15



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direction images with the qz peaks at 1.60 Å-1 for the US film (RT) and 1.65 Å-1 for the HS film (60 °C) (Figure S3), indicating PTB7 prefers a face-on orientation, which is similar to other reports about qz of the films processed by spin-coating.33,34 The nearly equal qz (3.92 vs. 3.81 Å) means the similar π-π stacking spacing distance for the US and HS films. On the other hand, the crystal coherence length (CCL=2πK/FWHM), viz. crystallite size, is also obtained from the fits.35,36 The US sprayed film shows higher CCL value than the HS film (33.77 vs. 30.69 Å). Therefore, the π-π stacking interaction should be more pronounced for the US film compared to the HS film, suggesting a better charge transport. Moreover, in order to make use of the good optical absorption of the HS film and the good film morphology in bulk of the US film, the two-step sprayed film processed with the combination of US and HS (US+HS) was also fabricated. For the US+HS film, PTB7 (100) diffraction peak is seen in in-plane direction at 0.35 Å−1 with a CCL of 35.03 Å, and PTB7 (010) peak is seen in out-of-plane direction at 1.65 Å−1 with a CCL of 33.45 Å. It is evident that the crystallinity extent of the US+HS active layer is located between US and HS films by comparing their CCL, meaning that the active layer processed by US+HS should produce the best device performance. Table 1. Fitting results for GIWAXS. Reflection

Sample

Position (Å-1)

d (Å)

FWHM (Å-1)

CCL (Å)a

PTB7 (100)

US HS US+HS US HS US+HS

0.34 0.34 0.35 1.60 1.65 1.65

18.33 18.37 17.99 3.92 3.81 3.81

0.14 0.19 0.16 0.17 0.18 0.17

41.27 30.38 35.03 33.77 30.69 33.45

PTB7 (010)

aCalculated

from Scherer equation: CCL = 2πK/FWHM, where K is a shape factor. 16


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b)

a) US

PTB7 (010)

PTB7 (100)

US

HS

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


HS

US+HS

US+HS

0.2

0.4 0.60.8 1 1.4 2

Q vector (A-1)

Figure 5. (a) Contrastive 2D GIWAXS patterns of the US film formed at RT and HS film formed at 60 °C as well as the US+HS film. (b) In-plane (dotted line) and out-of-plane (solid line) cut-line profiles of the US film and HS film as well as the US+HS film.

The different extents of photon absorption and D/A phase separation as well as crystallinity will in turn influence the device performance. The sprayed devices with the structures of Ag/PFN/active layer (95 nm by US; or 110 nm by HS; or 70 nm+30 nm by US+HS)/PEDOT:PSS are constructed and evaluated. As shown in Figure 6, the US device at RT exhibits a short circuit current density (JSC) of 13.96 ± 0.12 mA cm−2 while the HS device at 60 °C obtains a higher JSC of 14.24 ± 0.23 mA cm−2 due to the notable photon absorption (Figure 4). However, the US device with the smaller 17



D/A phase separation shows a higher PCE of 6.20% than 4.87% of the HS device, contributing to its higher fill factor (FF) due to the efficient charge transport. It is found that the US+HS device obtains the highest PCE of 6.48% with the highest JSC of 14.92 mA cm-2 and a relatively high FF of 60.07%, which is comparable to the spin-coated one. The EQE spectral intensities of three different types of sprayed devices correspond well to their measured JSC values. The concrete photovoltaic parameters are summarized in Table 2. Here we emphasize that the spraying sequence of HS and US has a big effect on the device performance. When the bulk active layer is processed with HS and the surface active layer is processed with US, a very low device performance is achieved (Figure S4). Combined with the excellent phase separation and crystallinity in the bottom of the active layer processed by US and the great photon absorption boost produced by HS, the two-step sprayed US+HS device performance is greatly improved. Moreover, the preliminary good device performance with a large area (1 cm2) processed by US+HS verifies that the facile two-step spraying method is very effective in large-scale spraying production of solar cells (Figure S5). For the development of large-scale flexible OSCs, ITO-free device configuration is more promising. We adopt ITO-free configuration during the whole work, to prove our approach can be an efficient strategy to optimize the microscale and nanoscale structure for printed flexible devices.

Table 2. Photovoltaic parameters of the optimized ITO-free devices by US and HS as well as US+HS. Over 10 individual devices were measured for each averaged value. 18


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Device

JSC (mA cm−2)

VOC (V)

FF (%)

PCEmaxa [PCEaveb] (%)

US

13.96 ± 0.12

0.72 ± 0.01

60.9 ± 1.13

6.20[6.16]

HS

14.24 ± 0.23

0.67 ± 0.01

49.3 ± 1.19

4.87[4.68]

US+HS

14.71 ± 0.21

0.72 ± 0.01

59.1 ± 1.34

6.48[6.26]

US+HSc

16.66 ± 0.17

0.75 ± 0.01

63.0 ± 1.38

8.06[7.88]

Spin coating

14.15 ± 0.11

0.73 ± 0.01

64.9 ± 1.05

6.72[6.68]

aMaximum

power conversion efficiency; bAverage power conversion efficiency;

cNonfullerene

ITO-free OSC based on PBDB-T:IT-M.

J (mA cm-2)

a)

0

US HS US + HS

-3 -6 -9 -12 -15 0.0

b)

0.2 0.4 0.6 Voltage (V)

0.8

500 600 700 Wavelength (nm)

800

80 60

EQE (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


40 20 0 300

400

Figure 6. (a) J−V and (b) EQE curves of the optimal US and HS as well as US+HS devices.

19



To shed further light on the charge transport and collection characteristics in device, TPV and TPC measurements are performed to probe the charge generation and decay dynamics. Figure 7a shows the TPV results of the US+HS device compared with the US device and HS device. The extracted carrier lifetime is 3.95 µs for the US+HS device while it is 4.98 µs for the US device and 2.38 µs for the HS device. Generally the short carrier lifetime is related to a pronounced bimolecular recombination. Thus the nongeminate recombination in the US+HS device is significantly reduced compared to the HS device. In this case, the long carrier lifetime should result from the high crystallinity degree of the donor. In Figure 7b, TPC shows the charge carrier density of the US+HS device is higher than other devices, which is consistent with the measured J−V curves. IS is also employed to analyze the electrical properties of three different kinds of sprayed devices. According to the device structure, a circuit model is defined as shown in the inset of Figure 7c. R0, R1, and C1, R2 and C2 correspond to the electrode resistance, the resistance and capacitance of the bulk heterojunction layer as well as the two interfaces, respectively. Figure 7c shows the Cole−Cole plot with the fitting curve based on the equivalent model. The fitting parameter values of each element are listed in Table 3. The R1 of the US+HS device is smaller than that of the HS device and close to the US device, indicating a better charge transportability originated from a high crystallinity degree. The average carrier transition time (τavg) of this equivalent circuit can be calculated from the following equation: C1=τavg/R1. The long τavg means the low charge recombination.37 The τavg of the US+HS device is longer than that of the HS device and close to the US device, 20


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which is consistent with the carrier lifetime obtained from TPV (Figure 7a). The similar charge carrier lifetimes from IS and TPV illustrate the validity of two different measurements (4.98 µs vs. 5.21 µs for the US device, 2.38 µs vs. 2.56 µs for the HS device as well as 3.95 µs vs. 4.23 µs for the US+HS device). The R2 of the HS device and US+HS device is smaller than that of the US device because of less considerable contact resistance coming from the more contacting area between the rough surface active layer and PEDOT:PSS electrode. However, for the HS and US+HS devices, the very thin PEDOT:PSS layer located at some protruding spots of the rough active layer surface would result in a high R0 compared to the US-processed one, assuming that the cathode resistance is the same for three types of sprayed devices.

Table 3. The fitting element parameters based on the equivalent model with the free carrier lifetime extracted from IS and TPV for the optimized US and HS as well as US+HS devices. Device Power US (W) HS Ta (°C) US+HS aCarrier

τa (s)

τb (s)

R0 (Ω)

R1 (Ω)

C1 (F)

R2 (Ω)

C2 (F)

54.17

432.9

1.20E-08

128.8

2.20E-08

5.21E-06 4.98E-06

62.39

671.3

3.82E-09

107.1

1.20E-08

2.56E-06 2.38E-06

60.87

503.8

8.39E-09

112.5

2.51E-08

4.23E-06 3.95E-06

lifetime extracted from IS; bCarrier lifetime extracted from TPV.

21



Figure 7. (a) TPV of the optimal US, HS and US+HS devices measured under 1 sun illumination and VOC conditions. (b) TPC of the optimal US, HS and US+HS devices measured under a short-circuit condition using the same intensity laser pulse. (c) Cole−Cole plots of the optimal US, HS and US+HS devices with the fitting curves. (d) A comparison of this work with previous sprayed ITO-free and ITO-based OSCs reported to date.

To verify the effectiveness of this facile two-step spraying method on non-fullerene system, sprayed ITO-free OSCs with PBDB-T:IT-M as the active layer are also explored. The results show that the non-fullerene US+HS device has the highest PCE of 8.06%, compared to 6.60% for the non-fullerene HS device and 7.64% for the non-fullerene US device, indicating that the two-step spray coating method is also validating in improving the non-fullerene device performance (Figure 22


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S6 and Table S2). The RMS difference is more obvious for non-fullerene PBDB-T:IT-M system (Figure S7), illustrating there is more room for device optimization in non-fullerene systems. Thus, manipulating both the micrometer surface and nanometer bulk domain sizes by two-step spraying is very prospective in both fullerene and non-fullerene systems. Compared with literature, the performance of the device with non-fullerene structure has lower VOC, similar JSC and FF.38,39 The lower VOC is caused by the usage of PEDOT:PSS. So, adding a hole transport layer between PEDOT:PSS and active layer may be an effective way to further improve device performance. The PCEs of ITO-free and ITO-based fullerene/non-fullerene OSCs fabricated by spraying in previous reports are presented in Figure 7d.16-19,40–45 It can be seen that our two-step sprayed OSCs with an ITO-free device structure show the highest PCEs among fullerene/non-fullerene ITO-free sprayed devices and their PCEs are comparable to those of the ITO-based OSCs reported to date, due to the maximized integrated optical and electrical properties of active layers through two-step spraying.

4. CONCLUSIONS In this work, the micrometer/nanometer structures of active layers are effectively manipulated through a hybrid spraying process control. A systematic study of the relationship of the morphology structure and film/device property reveals that the micrometer scale rough surface topography with a condense nanometer bulk phase separation can improve the photon absorption and charge transport simultaneously, leading to less biomolecular recombination in device. As a result, the fullerene and 23



non-fullerene based devices present decent performance. This work demonstrates the bright future of the employment of different spraying techniques in optimizing the micro/nanometer scale film structures in different active regions with complete adaptation to match the industrial production of large-scale OSCs and apparent simplicity to operation.

ASSOCIATED CONTENT Supporting Information J−V curves and photovoltaic parameters of devices prepared by US and HS with various substrate temperatures; Optical microscope images of the US-sprayed and HS-sprayed PTB7:PC71BM films; Fitting results of GIWAXS data; J−V and Jph versus Veff curves of sprayed OSCs with device structures of Ag/PFN/PTB7:PC71BM (HS+US or US+HS)/PEDOT:PSS; J−V curve of sprayed fullerene OSC based on PTB7:PC71BM with the device area of 1 cm2; J−V curves, EQE, and photovoltaic parameters of sprayed nonfullerene OSCs based on PBDB-T:IT-M; AFM and TEM images of PBDB-T:IT-M films by US and HS. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (W.C.). *E-mail: [email protected] (Y.L.). *E-mail: [email protected] (F.Z.). *E-mail: [email protected] (L.H.). ORCID Wanzhu Cai: 0000-0002-3209-4115 Hongbin Wu: 0000-0003-2770-6188 24


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Fengling Zhang: 0000-0002-1717-6307 Lintao Hou: 0000-0002-4358-3862 Notes There are no conflicts to declare.

ACKNNOWLEDGMENTS The authors are grateful to the NSFC Project (61774077, 21733005, 51511130077, 61274062, 11204106 and 61804065), the Guangzhou Science and Technology Plan Project (201804010295) and the Fundamental Research Funds for the Central Universities for financial support. F.Z. acknowledges funding from Swedish Research Council (VR 621-2013-5561) and financial support to visiting professor from Jinan University.

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Photovoltaics. IEEE J. Photovoltaics 2014, 4, 1538−1544.

TOC Ultrasonic spray at 25 °C

Reflectance (arb)

375 s

RMS ~ 14.9 nm

100 nm

RMS ~ 22.5 nm

100 nm

100 200 300 400 High-pressure gas spray at 60 °C 712 s

0

300

600 900 Time (s)

1200 1500 3.0

PEDOT:PSS Interfacial active layer (HS) Bulk active layer (US) PFN

Photocurrent (mA)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2.0 1.5 1.0 0.5 0.0 -0.5

Ag

US HS US + HS

2.5

0.0

0.5

1.0

1.5

Time (μs)

29


2.0

2.5

3.0

Manipulate Micrometer Surface and Nanometer Bulk Phase

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