Fully Coated Semitransparent Organic Solar Cells with a Doctor-Blade

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Fully coated semitransparent organic solar cells with a doctor blade coated composite anode buffer layer of phosphomolybdic acid and PEDOT:PSS and a spray coated silver nanowire top electrode JI GUOQI, Yiying Wang, Qun Luo, Kang Han, Menglan Xie, Lianping Zhang, Na Wu, Jian Lin, Shugang Xiao, Yanqing Li, Liqiang Luo, and Chang-Qi Ma ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b13346 • Publication Date (Web): 04 Dec 2017 Downloaded from http://pubs.acs.org on December 4, 2017

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ACS Applied Materials & Interfaces

Fully coated semitransparent organic solar cells with a doctor blade coated composite anode buffer layer of phosphomolybdic acid and PEDOT:PSS and a spray coated silver nanowire top electrode Guoqi Ji,†, Yiling Wang, †,‡ Qun Luo,*,† Kang Han,† Menglan Xie,† Lianping Zhang, † Na Wu, † Jian Lin, † Shugang Xiao, † Yan-Qing Li, ξ Li-Qiang Luo, ‡ Chang-Qi Ma *,† †

Printable Electronic Research Center, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese

Academy of Sciences (CAS), Collaborative Innovation Center of Suzhou Nano Science and Technology, Suzhou, 215123, P. R. China. ‡

ξ

Department of Chemistry, Shanghai University, Shanghai, 200444. P. R. China.

Institute of Functional Nano & Soft Materials, Soochow University, Collaborative Innovation Center of Suzhou Nano Science and Technology, Suzhou, 215123. P. R. China. E-mail: [email protected]; [email protected]

1 ACS Paragon Plus Environment


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ABSTRACT:

Aiming to realize high performance semitransparent fully coated organic solar cells, printable electrode buffer layers and top electrodes are two important key technologies. An ideal ink for the preparation of the electrode buffer layer for printed top electrodes should have good wettability and negligible solvent corrosion to the underlying layer. This work reports a novel organic-inorganic composite of phosphomolybdic acid (PMA) and PEDOT:PSS that features excellent wettability with the active layer and printed top Ag nanowires, and high resistibility to solvent corrosion. This composite buffer layer can be easily deposited on a polymer surface to form a smooth, homogeneous film via spin-coating or doctor blade coating. Using this composite anode buffer layer, fully coated semitransparent devices with doctorblade-coated functional layers and spray-coated Ag nanowire top electrodes showed a highest power conversion efficiency (PCE) of 5.01% with an excellent average visible-light transmittance (AVT) of 50.3%, demonstrating superior overall characteristics with comparable performance to and much higher AVT than cells based on a thermally evaporated MoO3-Ag-MoO3 thin film electrode (with a PCE of 5.77% and AVT of 19.5%). The current work reports the fabrication of fully coated inverted organic solar cells by combining doctor blade coating and spray coating and, more importantly, demonstrates that a nanocomposite of a polyoxometalate and conjugated polymer could be an excellent anode buffer layer for the fully coated polymer solar cells with favorable interfacial contact, hole extraction efficiency, and high comparability with full printing.

Keywords: organic solar cell, solution-processable, phosphomolybdic acid, composite interfacial layer, Ag nanowire top electrode


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1. Introduction: Bulk heterojunction organic solar cells have several advantages, such as high flexibility, light weight, low cost and ease of implementation through large-area fabrication via roll-to-roll printing process. Currently, bulk heterojunction organic solar cells have achieved great developments

1-7

with the highest

power conversion efficiency above 13%.8 With the rapid development of organic photovoltaics, semitransparent organic solar cells have gained much attention due to their foreseeable applications in building integrated photovoltaics (BIPV), power generation windows, rooftops, and green houses.

9-10

Nowadays, the highest performance of the semitransparent organic solar cells has reached to above 9% using a non-fullerene acceptor with strong near infrared absorption reported by Zhan et al. 11 Various semitransparent electrodes, such as ultrathin metals, nanowire electrode,

15

carbon nanotube,

16

graphene

17-18

12-13

high conductive polymers,

14

Ag

have been used as the top electrodes of the

semitransparent solar cells. Among them, one key to realizing full solution-processed coating or printing manufacturing of semitransparent organic solar cells without vacuum technologies is solution-processable interfacial materials and printed top electrodes.

19-25

Previously, Frederik C. Krebs and their co-workers

have paid great attentions on the roll-to-roll coated organic solar cells through slot-die. 26-28 In the case of the printed semitransparent top electrodes, the most promising candidate is the Ag nanowire electrode, which has outstanding light transmittance and conductivity. Previously, several reports from others and our group have reported full-solution fabricated semitransparent organic solar cells with such a printed Ag nanowires network electrode. 15, 29-30 Unlike the vacuum-deposited top metal electrode, the printed top electrode meets the wetting challenges on the top of the photoactive layers and causes serious solvent corrosion to the interfacial and photoactive layers.

30

Therefore, printable electrode buffer layers, such as the anode buffer layer in the

inverted bulk heterojunction solar cells, need to not only minimize the energy barrier and facilitate charge extraction and transportation, but also enable solution-processing for the top electrode and protect the underneath organic photoactive layers. The regular solution-processable anode buffer layers for inverted



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organic solar cells include highly doped poly(3,4-ethylenedioxythiohene):poly(styrenesulfonate) (PEDOT:PSS)

31-32

and metal oxides. It is known that PEDOT:PSS forms a core-shell structure with a

hydrophobic PEDOT:PSS core and a hydrophilic PSS shell.

33

This leads to a hydrophilic PEDOT:PSS

layer, making it difficult for it to be deposited on the hydrophobic organic layers directly. 25, 34 Surfactants or other additives are therefore used to improve the wettability of the PEDOT:PSS solution on an organic surface,

34-35

which results in an additional problem of removing the surfactant or additives. However,

these metal oxide nanoparticles often suffer batch variation, surface charge traps, and relatively low charge carrier densities.

36-38

Polyoxometalate (POM) that consists of multiple metal oxyanions linked

together by shared oxygen atoms is a special kind of metal oxide and has recently been used as an electrode buffer layer in organic electronic devices.

39

Due to its good solubility in water or alcohol, it is

much cheaper, easier, and more environmentally friendly to be used as a polyoxometalate buffer layer than regular metal oxides. Zhu et al. first demonstrated that a Keggin -type phosphomolybdic acid (PMA) could be directly deposited on the polymer surface as an ABL in inverted polymer solar cells. 40 Although an inhomogeneous PMA film formed on the polymer surface owing to the intensive aggregation behavior of the PMA molecules, improved device performance was achieved for the PTB7:PC71BM devices and was attributed to the light scattering effect of the PMA clusters. Jia et al.

41

developed an isopropanol

(IPA) pretreatment procedure to improve the wettability between the PMA solution and polymer and to smooth the polymer-PMA interface. All these results showed that POM could serve as an electrode buffer layer material in organic electronic devices. However, since POM crystallization is very sensitive to the substrate surface as well as the precursor solution, on a polymer surface with a smooth surface.

42

42-43

it is quite difficult to get a condensed POM layer

For the fully printed organic solar cells, this rough and

inhomogeneous morphology is undesirable for the formation of good interface contact, as well as for preventing the active layers from being corroded by solvent. Therefore, reducing the crystallization tendency of POM materials is necessary and highly interesting for fully printed organic solar cells.


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Organic-inorganic composite could be a novel and improved solution-processable electrode buffer layer. 44-46 The organic component could passivate the surface defects and suppress the agglomeration of metal oxides, leading to advantages of good film forming ability and improved charge transport properties.

47-50

conventional,

As a typical composite ABL, composite of MoO3 and PEDOT:PSS has been reported in

44,51

and inverted organic solar cells.

52

Such a composite ink can be easily spin-coated on

the organic layer and presents low thickness dependence for the device performance. More importantly, the composite buffer layers are compact and robust enough to suppress solvent corrosion.

53

Therefore,

we have used the MoO3:PEDOT:PSS composite hole extraction layer to facilitate the printing of the top electrode,29 and have achieved a performance of approximately 2.7% for the semitransparent P3HT:PC61BM device. Since POMs have the advantages of a defined molecular structure, ease of chemical modification, and excellent water solubility, developing solution processable POM - based composite materials is very interesting for use in full- coated organic photovoltaics. In this work, to overcome the aforementioned challenges and achieve fully coated semitransparent organic solar cells, we firstly report a novel, cost-effective nanocomposite ink based on phosphomolybdic acid (PMA) and PEDOT:PSS. In this PMA:PEDOT:PSS composite ink, the weak interaction between PEDOT:PSS and PMA molecules enables suppression of PMA aggregation and improves the lipophilicity of the composite ink. The prepared PMA:PEDOT:PSS composite ink presents good wettability on the organic photoactive layer, and a smooth and homogeneous thin film can be easily deposited on the active layer via spin-coating or doctor - blade coating. Additionally, this composite electrode buffer layer has an outstanding solvent resistance and good wetting properties with printed Ag nanowires, highly compatible with full-printing process. Using this composite anode buffer layer, fully coated semitransparent devices with doctor-blade-coated functional layers and spray-coated Ag nanowire top electrodes showed a highest power conversion efficiency (PCE) of 5.01% with excellent average visible light transmittance (AVT) of 50.3%, demonstrating a superior overall characteristics with comparative performance and much higher AVT



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than cells based on thermally evaporated MoO3-Ag-MoO3 thin film electrodes (with a PCE of 5.77% and AVT of 19.5%). The current work demonstrates that POM: conjugated polymer nanocomposite could serve as an excellent electrode buffer layer in full-coated organic solar cells. 2. Experimental Section: 2.1 Materials: Regioregular poly(3-hexylthiophene) (SMI-P3HT, Mn = 5.0×104 g/mol, PDI=1.7, regioregularity

Rr

=

95%)

and

poly[(5,6-difluoro-2,1,3-benzothiadiazol-4,7-diyl)-alt-(3,3'''-di(2-

octyldodecyl)-2,2';5',2'';5'',2'''-quaterthiophen-5,5'''-diyl)]

(PffBT4T-2OD),

poly[4,8-bis(5-

(2ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b']dithiophene-co-3ﬂuorothieno[3,4-b]thiophene-2carboxylate] (PTB7-Th), [6,6]-Phenyl-C61-butyric acid methyl ester (PC61BM) were purchased from Solarmer Energy, Inc. (Beijing). [6,6]-Phenyl-C71-butyric acid methyl ester (PC71BM) was provided by American Dye Source Inc. Phosphomolybdic acid (PMA, 98%) was purchased from Aladdin. Inc. Poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS Clevios PVP AI 4083) was obtained from Heraeus Precious Metals GmbH & Co. KG. The Ag nanowires ink that dissolved in isopropanol was provided by Henkel. 2.2 The preparation of PMA:PEDOT:PSS composite inks: PMA powder was dispersed in isopropanol with the concentration of 1 mg/mL through continuous stirring for 2 h. Then PMA solution and the PEDOT:PSS aqueous were mixed together with volume ratio of 9:1 to obtain the PMA:PEDOT:PSS composite inks. 2.3 The fabrication of organic solar cells through spin-coating: The inverted solar cells were fabricated with indium tin oxide (ITO) coated glasses as the substrate. First, the substrates were sequentially cleaned by deionized water, acetone, and isopropanol, and finally treated by UV-ozone for 30 min. A 30 nm-thick ZnO cathode buffer layer was deposited through spin coating at 2000 rpm for 60 s and followed by thermal treating at 130 ºC for 10 min in air. For the P3HT:PC61BM devices, P3HT and PC61BM were dissolved in 1,2-dichlorobenzene with ratio of 1:1 (with a total concentration of 40 mg/mL). The ~230 nm thick P3HT:PC61BM film were spin-coated on the ZnO layer. Subsequently, the


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active layers were solvent annealed in 1,2-dichlorobenzene for 1.5 h and then thermal annealed at 120 ºC for 10 min in N2 glove box. For the PTB7-Th:PC71BM devices, the polymer and PC71BM were dissolved in chlorobenzene with total concentration and weight ratio of 22.5 mg/mL and 1:1.5. And 3% volume ratio of 1,8-diiodooctane (DIO) was used as the additive. And then the active layers (about 100 nm) were spin-coated on the ZnO layer. For the preparation of PffBT4T-2OD:PC71BM films, the PffBT4T-2OD and PC71BM were first dissolved in 1,2-dichlorobenzene with D/A ratio of 1:1.2 (with the PffBT4T-2OD concentration of 10 mg/mL) with addition of 3% volume ratio of DIO at 110 ºC. The hot solution is dropped on the ITO/ZnO layer which has been preheated on a hot plate at 100 ºC, and followed by spincoating at 800 rpm for 1 min. On the top of photoactive layers, the PMA, or PMA:PEDOT:PSS composite ABL was deposited by spin-coating in N2 glove box. Finally, 100 nm Al electrodes were evaporated at pressure about 8×10-5 Pa. The semitransparent MoO3 (10 nm)-Ag (15 nm)-MoO3 (30 nm) top electrodes were fabricated through thermal evaporation. 2.4 The fabrication of full-coated organic solar cells: For the fabrication of the full-coated organic solar cells, the ITO glasses were firstly UV-ozone treated for 30 min. After that, the ZnO layer was deposited by doctor-blade coating with a coating speed of 80 mm/s and a blade-substrate gap of 25 µm at room temperature in a nitrogen atmosphere. The PTB7-Th: PC71BM active layers were deposited through doctor-blade coating with a coating speed of 10 mm/s and a blade-substrate gap of 125 µm at room temperature in a nitrogen-filled glove box. The P3HT:PC61BM active layers were deposited with a coating speed and blade-substrate gap of 9 mm/s, and 200 µm, respectively. PMA:PEDOT:PSS layers were deposited through doctor-blade coating on the top of the active layer with coating speed of 15 mm/s and blade-substrate gap of 100 µm. Then the Ag nanowires top electrode was deposited on the PMA:PEDOT:PSS composite ABL through spray coating in air at room temperature with the coating speed and distance of 7 mm/s, and 3.5 cm, respectively. After the coating of Ag nanowires, the devices were thermal treated at 50 oC for 5 min in air.



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2.5 Films characterization: The film thickness were measured using an AlphaStep profilometer (Veeo, Dektak 150). The UV-vis absorption and reflectance spectra of the inverted devices were recorded by the Lambda 750 UV/vis/NIR spectrophotometer (PerkinElmer). Atomic Force Microscopy (AFM) images were recorded by Dimension 3100. The SEM images were recorded by the Quanta 400 FEG. The ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS) measurements of the PMA and PMA:PEDOT:PSS films were recorded by the Kratos Asxis Ultra DLD (Kratos Analytical Shimadzu Group Company). An unfiltered He-I (21.2 eV) discharge lamp and a total instrumental energy resolution of 100 meV was used for UPS measurement. And an Al Kα radiation source was used for the XPS measurement. The diameter of particles was measured by dynamic light scattering (DLS) measurements using a Malvern granulometer (Zatasizer Nano). The current density-voltage (J-V) measurement was carried out with a Keithley 2400 source meter under simulated AM 1.5G solar illumination (100 mW/cm2). External quantum efficiencies (EQE) were measured under simulated one sun operation conditions. The air stability of the devices was carried out at room temperature (~25 oC) with the humidity around 60 %. 3. Results and Discussion In this work, the PMA:PEDOT:PSS composite inks were prepared by mixing the PMA solution (dissolved in isopropanol at 1 mg/mL) and PEDOT:PSS aqueous (Clevios 4083, ~1.5% wt in water) together in a volume ratio of 9:1, leading to a weight ratio of PMA to PEDOT:PSS of 0.6:1 and a total weight concentration of 1.2 mg/mL. X-ray photoelectron spectroscopy (XPS) was measured to analyze the element valences in the PMA:PEDOT:PSS composite layer. The XPS spectra of (a) Mo 3d, (b) O 1s and (c) P 2p core levels in the PMA and PMA:PEDOT:PSS films deposited on the ITO glasses are shown in Figure 1. The Mo 3d core levels of PMA films exhibit two peaks at 232.8 and 235.8 eV, which could be attributed to the 3d orbital doublet of Mo6+.

40, 44

The Mo 3d core levels of PMA:PEDOT:PSS are

located at nearly the same positions as those of the pure PMA. The O 1s core levels of PMA centered at about 530.5 eV

44, 54

and 531.3 eV

55-56

correspond to the O elements in MoO3, and PO43-, respectively.


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For the PMA:PEDOT:PSS, the O 1s core level was comprised by Gaussian-like peaks centered at 530.5, 531.3/531.5, and 532.1 eV, which are originated from the O elements in MoO3, and PEDOT,

57

51, 54

, PO43-

55

/PSSH

51

respectively. In the case of the P 2p core level, the binding energy of PMA is 134 eV,

which is attributed to phosphorus elements in the PO43- core of PMA. While for the PMA:PEDOT:PSS blended film, no obvious P 2p peak was found (Figure 1c). Since the weight ratio between PMA and PEDOT:PSS in this blended film is approximately 0.6:1, the phosphorus content in the blended film could be too low to be measured. Based on these results, one would conclude that there is no obvious decomposition of PMA and PEDOT:PSS in such a composite solution, and these two components coexist in their individual chemical forms. To understand the interaction of PMA and PEDOT:PSS, the particle diameter of the composite ink was determined using dynamic light scattering (DLS). As illustrated by Figure 1d, the PMA in isopropanol (IPA, 1 mg/mL) formed large clusters with a size of approximately 255 nm, corresponding well to the atomic force microscopy (AFM) image results (vide infra, Figure 2). However, from the previous study, we know that the isolated PMA particle should be sized about 5-6 nm.56 Thus, we also tried to measure the particle size of PMA in extremely diluted solution. TEM analysis confirmed that the size of the PMA aggregates do can be decreased to 6 nm by reducing the PMA concentration to 0.1 mg/mL (Figure S-1), indicating less PMA agglomeration formed in low concentration. Herein, the diameter size of PMA:PEDOT:PSS was also determined to be 6 nm, suggesting the PMA cluster is suppressed with the addition of PEDOT:PSS aqueous in this blend solution. Although the detailed structure of these PMA:PEDOT:PSS composite inks is not fully understood yet, these results unambiguously confirmed that PMA aggregation was suppressed by the incorporation of PEDOT:PSS, which should be beneficial to thin film formation.



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Figure 1. XPS spectra of the (a) Mo 3d, (b) O 1s, (c) P 2p core level in the PMA and PMA:PEDOT:PSS (0.6:1 w/w) films, and (d) particle diameter of the PMA dispersed in isopropanol (1 mg/mL), PMA:PEDOT:PSS inks (0.6:1 w/w, with total concentration of PMA:PEDOT:PSS as 1.2 wt%) that recorded by dynamic light scattering measurement (DLS). The surface morphologies of the composite layers deposited on the photoactive layer were investigated by atomic force microscopy (AFM) measurements. Since the pristine aqueous PEDOT:PSS can not be successfully deposited on the top of an organic layer (Figure S-2, Figure S-3), the sample with a pristine PEDOT:PSS layer is absent here. As presented in Figure 2, the PTB7-Th:PC71BM photoactive layer is quite smooth with a root-mean-square (RMS) roughness of 1.3 nm. The deposition of the PMA film on the PTB7-Th:PC71BM layers leads to a much rougher surface with a RMS roughness of 4.2 nm. Large local islands and poor surface coverage of PMA on the organic layer are observed (Figure 2b). This observation corresponds well to the previous reports by Zhu et al.

40

For the PMA:PEDOT:PSS

composite film, however, the surface is rather smooth (RMS = 1.9 nm) and uniform without any obvious


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phase separation or agglomeration (Figure 2c). This is clearly indicative of the suppression of PMA agglomeration in such a blended film. Such a smooth and uniform PMA:PEDOT:PSS film should contribute to good interface contact between the organic photoactive layer and anode.

Figure 2. AFM phase and height image of the (a) PTB7-Th:PC71BM, (b) PTB7-Th:PC71BM /PMA, and (c) PTB7-Th:PC71BM /PMA:PEDOT:PSS films. Herein, fully coated polymer solar cells involving Ag nanowires electrodes with a device structure of ITO/ZnO/polymer: fullerene/ABL/Ag nanowires were fabricated. Polymer: PCBM heterojunction systems with three polymers, P3HT, PTB7-Th, and PffBT4T-2OD were investigated. The inverted organic solar cell structure, as well as the chemical structure of the materials used here, i.e. P3HT, PTB7Th, PffBT4T-2OD, PC71BM, PEDOT:PSS, and PMA are illustrated in Figure 3. ZnO and PMA:PEDOT:PSS were used as the cathode buffer layer and the anode buffer layer, respectively. It is known that the VOC of polymer solar cells is theoretically determined by the energy level difference between ELUMO of the acceptor and EHOMO of the donor, while it is in reality influenced by the work



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function difference between the cathode and anode.

58

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Therefore, the work functions of PMA,

PEDOT:PSS and PMA:PEDOT:PSS were measured by ultraviolet photoelectron spectroscopy (UPS, Figure 3b) to estimate the VOC difference. Figure 3c depicts the energy level alignment for the inverted device, among which the values of P3HT,2 PTB7-Th,59 PffBT4T-2OD,60 PC61BM,61 and PC71BM 61 were cited from references. As seen here, the work function of the PMA:PEDOT:PSS composite is determined to be 5.02 eV, which is similar to that of PEDOT:PSS (5.06 eV) but 0.40 eV higher than that of a pristine PMA film (4.62 eV). It is obvious that the work function of this PMA:PEDOT:PSS composite film was mainly influenced by PEDOT:PSS rather than PMA, that might be due to superior films formation ability of the PEDOT:PSS relative to PMA. Due to good film formation ability of PEDOT:PSS, combining with the slight interaction between PEDOT:PSS and PMA, it is reasonable to speculate that PEDOT:PSS would prefer to locate on the top or the outside of PMA. Therefore, PEDOT:PSS contributes more to the work function of the PMA:PEDOT:PSS composite ABL. The increased work function of the composite film reduces the hole injection barrier at the photoactive /ABL interface, which was likely one of the reasons for the higher VOC of the PMA:PEDOT:PSS devices than that of the PMA devices (Table 1). First, we fabricated a series of inverted solar cells with thermally evaporated Al electrodes to investigate the function of this PMA:PEDOT:PSS composite as an anode buffer layer. For comparison, devices with PMA and thermally evaporated deposited MoO3 (e-MoO3) ABLs were also fabricated and tested. All these devices were optimized through regulating the thickness of the anode buffer layers, and the optimal layer thicknesses of the PMA and PMA:PEDOT:PSS ABLs were found to be 20-30 nm. As mentioned above, the IPA-diluted PEDOT:PSS can not be deposited on the top of the photoactive layer. Thus, we tried surface treatments of the active layers before depositing the pure PEDOT:PSS layer. For the P3HT:PC61BM solar cells, three second O2 plasma treatment on P3HT:PC61BM surface was applied to achieve necessary hydrophilic surface for the deposition of pristine PEDOT:PSS layer. The active layer morphology might be slight destroyed by O2, thus presents lower performance. However, such as surface treatment destroyed the PTB7-Th:PC71BM and PffBT4T-2OD:PC71BM blend films totally, yielding very


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poor device performance. Figure 3 shows the J-V characteristics and EQE spectra of the inverted P3HT:PC61BM, PTBT7-Th:PC71BM, and PffBT4T-2OD:PC71BM devices. The performance parameters of these devices are listed in Table 1. As seen from Table 1, the champion PCEs of the P3HT:PC61BM, PTB7-Th:PC71BM, and PffBT4T-2OD:PC71BM devices with PMA:PEDOT:PSS ABLs were 3.58%, 9.19%, and 9.07%, respectively, which were comparable to or slight higher than the e-MoO3 ABL control device. The PMA:PEDOT:PSS ABL- based devices exhibited considerably better device performance than the pure PMA - based P3HT, PTB7-Th and PffBT4T-2OD devices. As seen in Table 1, the composite ABL- based P3HT:PC61BM, PTB7-Th:PC71BM and PffBT4T-2OD:PC71BM cells showed 26%, 82% and 85% improvements over the corresponding PMA based devices, which originated from the simultaneous improvement of the VOC, JSC and FF. The improved VOC might benefit from the increased work function of PMA:PEDOT:PSS relative to pure PMA. The higher JSC of the PMA:PEDOT:PSS based device relative to the pure PMA based device could be on one side due to the improved hole transport properties, and on the other side to the good film quality and interface contact with the organic active layer. To check the difference of the hole transport properties, the hole-only devices with structure of ITO/HTL/active layer/MoO3/Al were fabricated. From the J-V curves (Figure S-4) that taken from these hole-only device, it was found that the MoO3 ABL based device gave highest current density, and the PMA:PEDOT:PSS ABL based device presented higher current density than the pure PMA based device. The increased current density indicated enhanced hole transport characteristics by using this PMA:PEDOT:PSS composite ABL. The increased FF in the PMA:PEDOT:PSS employing device indicated improved charge selectivity at the anode interface due to the higher work function, which could be evidenced by the dark J-V characteristics. Taking the PTB7-Th: PC71BM device for example, the dark J-V curve of the PMA: PEDOT:PSS ABL employing device presented a higher rectification ratio than the PMA device. (Figure S-5). All these results confirmed that the PMA:PEDOT:PSS composite is an excellent solution- processable anode buffer material for the organic solar cells.



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Meanwhile, the air stability of these inverted solar cells with the PMA, PMA:PEDOT:PSS, and e-MoO3 anode buffer layers were investigated at about 25 oC with room humidity of 60%. Herein, the P3HT:PC61BM based devices were chosen as the model devices for studying the influence of the ABL on device stability due to its relative higher stability. From the performance evolution illustrated in Figure S6, it was found that the PMA:PEDOT:PSS ABL based device was not as stable as the pure PMA and the e-MoO3 ABL based device, which could be due to the hygroscopicity of PEDOT:PSS. However, the PMA:PEDOT:PSS ABL based presented improved long-term stability when stored in the N2-filled glove box (Figure S-7). This result indicated the two key interfaces of PMA:PEDOT:PSS/organic layer and PMA:PEDOT:PSS/Al were intrinsically stable with the exclusion of water and oxygen. Thus, the longterm stability of the PMA:PEDOT:PSS ABL based device could be greatly improved through proper encapsulation.

Figure 3. (a) Device structure and molecular structure. (b) UPS spectra of the PMA, PEDOT:PSS, and PMA:PEDOT:PSS films. (c) The energy level diagram of the P3HT:PC61BM, PTB7-Th:PC71BM, and PffBT4T-2OD:PC71BM inverted solar cells with e-MoO3, PMA and PMA:PEDOT:PSS ABLs. 14 ACS Paragon Plus Environment

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Figure 4. J-V curves and EQE curves of the (a, b) P3HT:PC61BM, (c, d) PTB7-Th:PC71BM, and (e, f) PffBT4T-2OD:PC71BM devices with different ABLs.



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Table 1. The performance parameters of the inverted solar cells with different anode buffer layers.

VOC Entry

Active layer

JSC

FF

PCE

(V)

(mA/cm )

(%)

(%)

Aver. PCE ± std. dev. (%)a)

ABL

2

1

P3HT:PC61BM

e-MoO3

0.61

9.20

64

3.59

3.47±0.09

2

P3HT:PC61BM

PMA

0.59

8.30

58

2.84

2.74±0.13

3

P3HT:PC61BM

PEDOT:PSS

0.59

8.94

59

3.11

2.97±0.10

4

P3HT:PC61BM

PMA:PEDOT:PSS

0.61

9.17

64

3.58

3.44±0.10

5

PTB7-Th:PC71BM

e-MoO3

0.79

17.18

68

9.22

8.92±0.25

6

PTB7-Th:PC71BM

PMA

0.75

15.99

47

5.04

----- b

7

PTB7-Th:PC71BM

PMA:PEDOT:PSS

0.79

17.10

68

9.19

8.88±0.22

8

PffBT4T-2OD:PC71BM

e-MoO3

0.75

17.76

67

8.92

8.76±0.20

9

PffBT4T-2OD:PC71BM

PMA

0.69

11.76

61

4.95

4.71±0.28

10

PffBT4T-2OD:PC71BM

PMA:PEDOT:PSS

0.77

18.40

64

9.07

8.75±0.12

a)

Standard deviation was calculated over 8 individual devices. b) The device performance distributed very wildly.

Then, the printability and thickness optimization of this PMA:PEDOT:PSS composite anode buffer layers were investigated through doctor-blade coating. A series of inverted organic solar cells with doctor-blade- coated PMA:PEDOT:PSS ABL and vacuum-deposited Al electrodes were fabricated. As shown in Table 2, the device with a thermally-evaporated Al electrode presented a reasonable performance with PCEs of 3.38%, and 8.66% for the P3HT:PC61BM, and PTB7-Th:PC71BM devices, respectively. Compared to the regular devices with spin-coated ABLs (Table 1, Entry 4 and Entry 7), we can find that these devices with doctor blade coated ABLs present nearly the same performance parameter, indicating this PMA:PEDOT:PSS ABL is suitable for doctor blade coating.


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Table 2. The performance parameters of inverted solar cells with doctor blade coated PMA: PEDOT:PSS layers and thermally vacuum deposited Al electrode. VOC Entry

Active layer

JSC

FF

PCE

(V)

(mA/cm )

(%)

(%)

Aver. PCE ± std. dev. (%)a)

Anode

2

11

P3HT:PC61BM

e-Al

0.61

9.23

60

3.38

3.20±0.19

12

PTB7-Th:PC71BM

e-Al

0.79

17.12

64

8.66

8.41±0.20

a. Standard deviation was calculated over 12 individual devices.

As aforementioned, the fully printed compatible electrode buffer layer should meet two requirements: suitable surface chemistry and surface energy to enable the printing/coating of the top electrode, and effectively protecting effect to the underneath active layer. Therefore, the compatibility of these PMA:PEDOT:PSS composite layers with full-coating manufacturing was studied before we moved forward to the fabrication of fully coated devices. Figure 5 (a) shows the contact angle between the PMA:PEDOT:PSS layer and Ag nanowires inks, and the SEM images of the Ag nanowires network on the top of the PMA:PEDOT:PSS layers. From these images, one can see that the Ag nanowires inks show excellent wettability both on the PMA and the PMA:PEDOT:PSS surface. However, the SEM images show that the spray coated Ag nanowire network dispersed more homogeneously on the surface of PMA:PEDOT:PSS than on the pristine PTB7-Th:PC71BM or PTB7-Th:PC71BM/PMA films. Therefore, the insertion of this PMA:PEDOT:PSS ABL can enable the coating process of Ag nanowires top electrodes due to good interface compatibility between the Ag nanowires and PMA:PEDOT:PSS composite ABLs. Then, the protective effect of this PMA:PEDOT:PSS composite layer for the underneath active layer could be positively proved by the device performance (vide infra) .



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Figure 5. (a-c) Picture of a drop of Ag nanowire ink on the top of the pristine PTB7-Th:PC71BM film, PTB7-Th:PC71BM /PMA, and PTB7-Th:PC71BM /PMA:PEDOT:PSS films. (d-f) the SEM image of the Ag nanowires network on the top of PTB7-Th:PC71BM, PTB7-Th:PC71BM/ PMA, and PTB7Th:PC71BM/PMA:PEDOT:PSS films. Finally, fully coated semitransparent organic solar cells were fabricated by combining a doctor-blade coated photoactive layer and PMA:PEDOT:PSS ABL, with a spray- printed Ag nanowire top electrode. Meanwhile, semitransparent devices with a thermally-evaporated MoO3-Ag-MoO3 semitransparent electrode were also fabricated for comparison. The device performances are shown in Figure 6 and Table 3. Figure 6(a)-(d) illustrate the photographs of the P3HT:PC61BM and PTB7-Th:PC71BM semitransparent devices with Ag NWs (p-Ag) and a MoO3-Ag-MoO3 top electrode. For the Ag nanowires based devices, the relative white square regions are the Ag NW semitransparent electrodes. Because of the high transparency of the Ag NW electrode, the devices show high optical transparency over the visible light range with average visible light transmittances (AVTs, 400-780 nm) of 45.5%, and 50.1% for the P3HT:PC61BM and PTB7-Th: PC71BM devices, respectively. In contrast, the thickness of each layer in the MoO3-Ag-MoO3 semitransparent electrode should be strictly controlled to achieve balance between conductivity and transparency.

13

After comprehensive studying the dependence of the photoelectrical

properties of the semitransparent electrode on the electrode parameter, we found the optimized thickness of the inner MoO3, the thin Ag layer, and the MoO3 capping layer are 10, 15, and 30 nm, respectively. 18 ACS Paragon Plus Environment

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From Figure S-8, it is revealed that such a semitransparent MoO3 (10 nm) –Ag (15 nm) -MoO3 (30 nm) semitransparent electrode presents a sheet resistance and average visible transparency of 7.7 Ω/□ and 55.2%, respectively. In contrast, the Ag nanowire semitransparent electrode gave better comprehensive properties with the sheet resistance of 12.5 Ω/□, and the average visible transmittance of 86.1%. Therefore, the semitransparent devices using a MoO3-Ag-MoO3 top electrode showed lower average visible light transmittance with AVTs of approximately 27.3% and 19.5 % for the P3HT: PC61BM and PTBT-Th:PC71BM devices, respectively. It is worth noticing that these device transmittances were among the same level for MoO3-Ag-MoO3 electrode- based devices.12-13 As shown in Figure 6(f), Table 3, Figure S-9 and Figure S-10, the semitransparent P3HT:PC61BM, and PTBT7-Th: PC71BM devices with a MoO3Ag-MoO3 top electrode and printed Ag nanowire top electrode have similar performances under bottom illumination. However, the performance of the MoO3-Ag-MoO3 device determined from top illumination was much lower than that of the p-Ag- based device due to the poor AVT. These results demonstrated that printed Ag nanowires have overwhelming advantages as top electrodes for semitransparent devices due to the roll-to-roll printing compatibility and high transmittance. In addition, it is worthy to note the slightly lower VOC of these semitransparent devices compared to the opaque devices might be due to the usage of a mask during J-V testing to accurately confirm the semitransparent device areas. And lower VOC values of PTB7-Th:PC71BM for top illumination than for bottom illumination could mostly be explained by the illumination -intensity -dependent VOC, as weaker illumination intensity was apparent in the top illumination case. 62 On the other side, it is known that spray coating a top electrode on the top of organic photoactive layer is highly rigorous since the corrosion of photoactive layers and interface layers by solvent should be strictly avoided during printing.

29-30

Therefore, the successful fabrication of a spray coated Ag top

electrode on the top of this PMA:PEDOT:PSS indicated this anode buffer layer is robust and compact enough to protect the underneath organic photoactive layer from corrosion by solvent. To the best of our knowledge, the highest performance of a single semitransparent organic solar cell with a fullerene



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acceptor using ultrathin hybrid-metal top electrodes is about 7%. 63-65 While for a semitransparent single organic solar cell with a solution-processable top electrode, the highest performance is approximately 6.0%. 15, 66 The lower performance for the semitransparent device with solution-processed top electrode is attributed to both the lower FF caused by moisture and air effects during printing and post-annealing process, and the lower JSC due to high light transmittance.30 A PCE of 5.01% in this work is among the highest photovoltaic performance for the fully coated semitransparent organic solar cells.

67-71

Finally, it

is worth noting that the fully coated semitransparent devices present narrow PCE distribution, demonstrating excellent film uniformity (Figure 6(h)).

Figure 6. (a)-(d) The photograph of the P3HT:PC61BM (MoO3-Ag-MoO3 top electrode), P3HT:PC61BM (Ag nanowires), PTB7-Th:PC71BM (MoO3-Ag-MoO3 top electrode), and PTB7-Th:PC71BM (Ag 20 ACS Paragon Plus Environment

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nanowires top electrode) semitransparent solar cells. (e) The transmittance spectra of the semitransparent devices. (f) J-V characteristics and (g) EQE spectra of the P3HT:PC61BM, PTB7-Th: PC71BM semitransparent devices with the MoO3-Ag-MoO3 top electrode, and full-coated semitransparent device with doctor-blade coated photoactive layer and PMA:PEDOT:PSS layer, and spray coated Ag nanowire top electrode. (h) The histogram of fully printed PTB7-Th:PC71BM semitransparent devices performance obtained from 12 devices.



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Table 3. The performance parameters of the semitransparent inverted solar cells.

Entry

Active layer

13

Anode

p-Ag

AVT (%)

JSC

FF

PCE

Aver. PCE ±

side

VOC (V)

(mA/cm2)

(%)

(%)

std. dev. (%)a)

bottom

0.61

7.56

0.63

2.91

2.78±0.12

top

0.61

5.61

0.63

2.16

2.03±0.17

bottom

0.60

7.19

0.65

2.80

2.74±0.11

top

0.60

3.34

0.60

1.20

1.09±0.10

bottom

0.78

11.28

0.57

5.01

4.93±0.13

top

0.76

8.60

0.55

3.59

3.33±0.18

bottom

0.77

12.70

0.59

5.77

5.53±0.25

top

0.72

2.97

0.58

1.24

1.20±0.11

Illuminated

45.5

P3HT:PC61BM

MoO3/Ag/MoO3

14

15

p-Ag

27.3

50.3

PTB7-Th:PC71BM

16

a.

MoO3/Ag/MoO3

19.5

Standard deviation was calculated over 12 individual devices. 22

ACS Paragon Plus Environment

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3 . Conclusions In summary, a printable composite buffer layer based on phosphomolybdic (PMA) and PEDOT:PSS was developed for fully coated semitransparent organic solar cells. In such a composite solution, PMA aggregation was suppressed by PEDOT:PSS, which leads to a smooth and uniform layer after deposition. And the wettability of PEDOT:PSS was improved due to the incorporation of PMA, which leads to excellent and homogenous ABL films on the top of the organic active layers. In this PMA:PEDOT:PSS composite anode buffer layer, PEDOT:PSS contributes more to the electrical properties of the composite layer and final device performance. By using this composite anode transporting layer, an improved photoelectrical conversion efficiency relative to a pristine PMA-based reference device was found, which was mainly due to the enhanced interfacial contact and hole transport efficiency. High power conversion efficiencies of 3.58%, 9.19%, and 9.07% were achieved for the optimized P3HT:PC61BM, PTB7-Th:PC71BM, and PffBT4T-2OD:PC71BM based devices, respectively. Meanwhile, the PMA:PEDOT:PSS composite anode buffer layer effectively protected the underneath photoactive layer and enabled the printing process of the Ag nanowires top electrode. Thus, a fully coated semitransparent organic solar cell with an efficiency of 5.01 % and an average visible light transmittance of 50.3% was fabricated by combining the doctor-blade coated photoactive layer, PMA:PEDOT:PSS ABL, and a spray coated Ag nanowire top electrode, showing the overwhelming potential application for building integrated photovoltaics. Acknowledgements The work is financially supported by the National Natural Science Foundation of China (51773224), Ministry of Science and Technology of China (No 2016YFA0200700), Suzhou Science and Technology Project (SYG201735), Strategic Priority Research Program of the


23


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Page 24 of 43

Chinese Academy of Sciences (Grant no. XDA09020201). Guoqi Ji and Yiling Wang contributed equally to this work. Thanks to Dr. Linpeng Yan from Taiyuan University of Technology for the help in stability testing, and Mr. Yuzhi Wei from Zhejiang University for the help for the deposition of MoO3-Ag-MoO3 electrode.

ASSOCIATED CONTENT Support Information Figure S-1. TEM image of PMA nanoparticles dispersed in isopropanol with a concentration of 0.1 mg/mL. Figure S-2. Absorption spectra of the pristine P3HT:PC61BM films, P3HT:PC61BM/isopropanol

diluted

PEDOT:PSS

(0.7

wt%),

and

(b)

P3HT:PC61BM/PMA:PEDOT:PSS (0.6:1, w/w) films. (c) The absorption spectra of PMA, PEDOT:PSS, and PMA:PEDOT:PSS (0.6:1, w/w) films. Figure S-3. Picture of a drop of (a) PMA, (b) PEDOT:PSS, (c) IPA-diluted PEDOT:PSS (0.7 wt.%), and (d) PMA:PEDOT:PSS composite inks on the photoactive layer. Figure S-4. The J-V characteristics of the hole-only devices with the structure of ITO/HTL/P3HT:PC61BM/MoO3/Al. Figure S-5. Dark J-V characteristics of the PTB7-Th:PC71BM devices with e-MoO3, PMA, and PMA:PEDOT:PSS anode buffer layers. Figure S-6. Normalized (a) VOC, (b) JSC, (c) FF, and (d) PCE decay of the P3HT:PC61BM inverted solar cells using PMA, or PMA:PEDOT:PSS ABL that stored in air. Figure S-7. Normalized (a) VOC, (b) JSC, (c) FF, and (d) PCE decay of the P3HT:PC61BM inverted solar cells using PMA, or PMA:PEDOT:PSS ABL that stored in glove box. Figure S-8. The transmittance spectra of the Ag NWs and MoO3/Ag/MoO3 semitransparent electrode. Figure S-9. (a) J-V characteristics and (b) EQE spectra of the P3HT:PC61BM device with printed Ag nanowires and MoO3-Ag-MoO3 top electrodes from bottom and top illumination. Figure S-10. (a)


24

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J-V characteristics and (b) EQE spectra of the PTB7-Th:PC71BM devices with printed Ag nanowires and MoO3-Ag-MoO3 top electrodes from bottom and top illumination.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected] Notes The authors declare no competing financial interest.

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TOC GRAPHICS


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Figure 1. XPS spectra of the (a) Mo 3d, (b) O 1s, (c) P 2p core level in the PMA and PMA:PEDOT:PSS (0.6:1 w/w) films, respectively, and (d) particle diameter of the PMA dispersed in isopropanol (1 mg/mL), PMA:PEDOT:PSS inks (0.6:1 w/w, with total concentration of PMA:PEDOT:PSS as 1.2 wt%) that recorded by dynamic light scattering measurement (DLS). 182x144mm (72 x 72 DPI)



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Figure 2. AFM phase and height image of the (a) PTB7-Th:PC71BM, (b) PTB7-Th:PC71BM /PMA, and (c) PTB7-Th:PC71BM /PMA:PEDOT:PSS films. 171x246mm (96 x 96 DPI)


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Figure 3. (a) Device structure and molecular structure. (b) UPS spectra of the PMA, and PMA:PEDOT:PSS films. (c) The energy level diagram of the P3HT:PC61BM, PTB7-Th:PC71BM, and PffBT4T-2OD:PC71BM inverted solar cells with e-MoO3, PMA and PMA:PEDOT:PSS ABLs. 252x264mm (96 x 96 DPI)



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Figure 4. J-V curves and EQE curves of the (a, b) P3HT:PC61BM, (c, d) PTB7-Th:PC71BM, and (e, f) PffBT4T-2OD:PC71BM devices with different ABLs. 274x313mm (72 x 72 DPI)


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Figure 5. (a-c) Picture of a drop of Ag nanowire ink on the top of the pristine PTB7-Th:PC71BM film, PTB7Th:PC71BM /PMA, and PTB7-Th:PC71BM /PMA:PEDOT:PSS films. (d-f) the SEM image of the Ag nanowires network on the top of PTB7-Th:PC71BM, PTB7-Th:PC71BM/ PMA, and PTB7-Th:PC71BM/PMA:PEDOT:PSS films. 435x164mm (72 x 72 DPI)



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Figure 6. (a)-(d) The photograph of the P3HT:PC61BM (MoO3-Ag-MoO3 top electrode), P3HT:PC61BM (Ag nanowires), PTB7-Th:PC71BM (MoO3-Ag-MoO3 top electrode), and PTB7-Th:PC71BM (Ag nanowires top electrode) semitransparent solar cells. (e) The transmittance spectra of the semitransparent devices. (f) J-V characteristics and (g) EQE spectra of the P3HT:PC61BM, PTB7-Th: PC71BM semitransparent devices with the MoO3-Ag-MoO3 top electrode, and full printed semitransparent device with doctor-blade coated photoactive layer and PMA:PEDOT:PSS layer, and spray coated Ag nanowire top electrode. (h) The histogram of fully printed PTB7-Th:PC71BM semitransparent devices performance obtained from 12 devices. 270x232mm (96 x 96 DPI)


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Abstract graphic 398x170mm (72 x 72 DPI)


Fully Coated Semitransparent Organic Solar Cells with a Doctor-Blade

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