Fully coated semitransparent organic solar cells with a doctor blade

doctor blade coated composite anode buffer layer of ... blade-coated functional layers and spray-coated Ag nanowire top electrodes showed a highest po...
0 downloads 0 Views 6MB Size
Download PDF
Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 943−954

www.acsami.org

Fully Coated Semitransparent Organic Solar Cells with a DoctorBlade-Coated Composite Anode Buffer Layer of Phosphomolybdic Acid and PEDOT:PSS and a Spray-Coated Silver Nanowire Top Electrode Guoqi Ji,†,ξ Yiling Wang,†,‡ Qun Luo,*,† Kang Han,† Menglan Xie,† Lianping Zhang,† Na Wu,† Jian Lin,† Shugang Xiao,† Yan-Qing Li,∥ Li-Qiang Luo,‡ and 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 ‡ Department of Chemistry, Shanghai University, Shanghai 200444, P. R. China ξ Nano Science and Technology Institute, University of Science and Technology of China, Suzhou, Jiangsu 215123, 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 S Supporting Information *

ABSTRACT: In the aim 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. Through the use of this composite anode buffer layer, fully coated semitransparent devices with doctor-blade-coated functional layers and spray-coated Ag nanowire top electrodes showed the highest power conversion efficiency (PCE) of 5.01% with an excellent average visiblelight transmittance (AVT) of 50.3%, demonstrating superior overall characteristics with a comparable performance to and a 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

1. INTRODUCTION

a nonfullerene acceptor with strong near-infrared absorption as reported by Zhan et al.11 Various semitransparent electrodes, such as ultrathin metals,12,13 high conductive polymers,14 Ag nanowire electrodes,15 carbon nanotubes,16 and graphene17,18 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, Krebs and cow-

Bulk heterojunction organic solar cells have several advantages including that they are lightweight, but they also have high flexibility, low cost, and ease of implementation through largearea fabrication via a roll-to-roll printing process. Currently, bulk heterojunction organic solar cells have achieved great developments1−7 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 high performance of the semitransparent organic solar cells has reached above 9% using © 2017 American Chemical Society

Received: September 4, 2017 Accepted: December 4, 2017 Published: December 4, 2017 943

DOI: 10.1021/acsami.7b13346 ACS Appl. Mater. Interfaces 2018, 10, 943−954

Research Article

ACS Applied Materials & Interfaces

suppress the agglomeration of metal oxides, leading to the advantages of good film forming ability and improved charge transport properties.47−50 As a typical composite ABL, a composite of MoO3 and PEDOT:PSS has been reported in conventional44,51 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 electrode29 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 solutionprocessable POM-based composite materials are 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 first 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 the 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, making it highly compatible with a full-coating process. Through the use of this composite anode buffer layer, fully coated semitransparent devices with doctor-blade-coated functional layers and spray-coated Ag nanowire top electrodes showed the highest power conversion efficiency (PCE) of 5.01% with an excellent average visible-light transmittance (AVT) of 50.3%, demonstrating superior overall characteristics with comparative performance and a much higher AVT 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 a POM:conjugated polymer nanocomposite could serve as an excellent electrode buffer layer in full-coated organic solar cells.

orkers have paid great attention 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 nanowire 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 organic solar cells include highly doped poly(3,4ethylenedioxythiohene):poly(styrenesulfonate) (PEDOT:PSS)31,32 and metal oxides. It is known that PEDOT:PSS forms a core−shell structure with a hydrophobic PEDOT 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. The favorable substitutions of PEDOT:PSS are the solutionprocessed metal oxides, such as MoO 3 ,WO 3 , and V 2 O 5 nanocrystals. 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 use 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,42,43 it is quite difficult to get a condensed POM layer on a polymer surface with a smooth surface.42 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. An 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

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)] (PffBT4T2OD), poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5b′]dithiophene-co-3fluorothieno[3,4-b]thiophene-2-carboxylate] (PTB7-Th), [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) were purchased from Solarmer Energy, Inc. (Beijing). [6,6]-Phenyl-C71butyric 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(styrenesulfonate) (PEDOT:PSS Clevios PVP AI 4083) was obtained from Heraeus Precious Metals GmbH & Co. KG. The Ag nanowire ink that dissolved in isopropanol was provided by Henkel. 2.2. Preparation of PMA:PEDOT:PSS Composite Inks. PMA powder was dispersed in isopropanol with a concentration of 1 mg/mL through continuous stirring for 2 h. Then PMA solution and the PEDOT:PSS aqueous solution were mixed together with a volume ratio of 9:1 to obtain the PMA:PEDOT:PSS composite inks. 944

DOI: 10.1021/acsami.7b13346 ACS Appl. Mater. Interfaces 2018, 10, 943−954

Research Article

ACS Applied Materials & Interfaces

Figure 1. XPS spectra of the (a) Mo 3d, (b) O 1s, (c) P 2p core levels 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 %) recorded by dynamic light scattering measurement (DLS). 2.3. 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 °C 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 was spin-coated on the ZnO layer. Subsequently, the active layers were solvent annealed in 1,2dichlorobenzene for 1.5 h and then thermally annealed at 120 °C for 10 min in an N2 glovebox. For the PTB7-Th:PC71BM devices, the polymer and PC71BM were dissolved in chlorobenzene with a total concentration and weight ratio of 22.5 mg/mL and 1:1.5. And a 3% volume ratio of 1,8-diiodooctane (DIO) was used as the additive. Then, the active layers (about 100 nm) were spin-coated on the ZnO layer. For the preparation of the PffBT4T-2OD:PC71BM films, the PffBT4T-2OD and PC71BM were first dissolved in 1,2-dichlorobenzene with a D:A ratio of 1:1.2 (with the PffBT4T-2OD concentration of 10 mg/mL) with addition of a 3% volume ratio of DIO at 110 °C. The hot solution is dropped on the ITO/ZnO layer, which has been preheated on a hot plate at 100 °C, and this is followed by spin-coating at 800 rpm for 60 s. On the top of photoactive layers, the PMA, or PMA:PEDOT:PSS composite ABL was deposited by spin-coating in an N2 glovebox. Finally, 100 nm Al electrodes were evaporated at a pressure of about 8 × 10−5 Pa. The semitransparent MoO3 (10 nm)/Ag (15nm)/MoO3 (30 nm) top electrodes were fabricated through thermal evaporation. 2.4. Fabrication of Full-Coated Organic Solar Cells. For the fabrication of the full-coated organic solar cells, the ITO glasses were first 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 glovebox. 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 a coating speed of 15 mm/s and a blade−substrate gap of 100 μm. Then, the Ag nanowires top electrodes were deposited on the PMA:PEDOT:PSS composite ABL through spray coating in air at room temperature with a coating speed and distance of 7 mm/s and 35 mm, respectively. After the coating of Ag nanowires, the devices were thermally treated at 50 °C for 5 min in air. 2.5. Film Characterization. The film thicknesses 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 a 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. 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 1 sun operation conditions. The air stability of the devices was carried out at room temperature (∼25 °C) 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 the PEDOT:PSS aqueous solution (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 945

DOI: 10.1021/acsami.7b13346 ACS Appl. Mater. Interfaces 2018, 10, 943−954

Research Article

ACS Applied Materials & Interfaces

indicating less PMA agglomeration in low concentration. Herein, the diameter size of PMA:PEDOT:PSS was also determined to be 6 nm, suggesting that the PMA cluster is suppressed with the addition of PEDOT:PSS aqueous solution 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. 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 (Figures S-2 and 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 rootmean-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 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. Herein, fully coated polymer solar cells involving Ag nanowires electrodes with a device structure of ITO/ZnO/polymerfullerene/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, PTB7-Th, 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 function difference between the cathode and anode.58 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 PC71BM61 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, which might be due to superior film formation ability of the PEDOT:PSS relative to PMA. Due to good film formation ability of PEDOT:PSS, combined with the slight interaction between PEDOT:PSS and PMA, it is reasonable to speculate that PEDOT:PSS would prefer to be located 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

spectra of the (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 the 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 530.544,54 and 531.3 eV55,56 correspond to the O elements in MoO3 and PO43−, respectively. For the PMA:PEDOT:PSS, the O 1s core level was composed of Gaussian-like peaks centered at 530.5, 531.3/531.5, and 532.1 eV, all of which originated from the O elements in MoO3,51,54 PO43−55/PSSH,51 and PEDOT,57 respectively. In the case of the P 2p core level, the binding energy of PMA is 134 eV, which is attributed to the phosphorus elements in the PO43− core of PMA. 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.43 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 can be decreased to 6 nm by reducing the PMA concentration to 0.1 mg/mL (Figure S-1),

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. 946

DOI: 10.1021/acsami.7b13346 ACS Appl. Mater. Interfaces 2018, 10, 943−954

Research Article

ACS Applied Materials & Interfaces

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.

Table 1. Performance Parameters of the Inverted Solar Cells with Different Anode Buffer Layers

a

entry

active layer

ABL

VOC (V)

JSC (mA/cm2)

FF (%)

PCE (%)

aver. PCE ± std. dev. (%)a

1 2 3 4 5 6 7 8 9 10

P3HT:PC61BM P3HT:PC61BM P3HT:PC61BM P3HT:PC61BM PTB7-Th:PC71BM PTB7-Th:PC71BM PTB7-Th:PC71BM PffBT4T-2OD:PC71BM PffBT4T-2OD:PC71BM PffBT4T-2OD:PC71BM

e-MoO3 PMA PEDOT:PSS PMA:PEDOT:PSS e-MoO3 PMA PMA:PEDOT:PSS e-MoO3 PMA PMA:PEDOT:PSS

0.61 0.59 0.59 0.61 0.79 0.75 0.79 0.75 0.69 0.77

9.20 8.30 8.94 9.17 17.18 15.99 17.10 17.76 11.76 18.40

64 58 59 64 68 47 68 67 61 64

3.59 2.84 3.11 3.58 9.22 5.04 9.19 8.92 4.95 9.07

3.47 ± 0.09 2.74 ± 0.13 2.97 ± 0.10 3.44 ± 0.10 8.92 ± 0.25 -----b 8.88 ± 0.22 8.76 ± 0.20 4.71 ± 0.28 8.75 ± 0.12

Standard deviation was calculated over eight individual devices. bThe device performance had a very broad distribution.

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 947

DOI: 10.1021/acsami.7b13346 ACS Appl. Mater. Interfaces 2018, 10, 943−954

Research Article

ACS Applied Materials & Interfaces

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.

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 hand due to the improved hole transport properties and on the other hand due 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 the structure of ITO/HTL/active layer/MoO3/Al were fabricated. From the J−V curves (Figure S4) taken from these hole-only devices, it was found that the MoO3 ABL-based device gave the highest current density, and the PMA:PEDOT:PSS ABL-based device presented a 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

depositing the pure PEDOT:PSS layer. For the P3HT:PC61BM solar cells, a 3 s O2 plasma treatment on the P3HT:PC61BM surface was applied to achieve the necessary hydrophilic surface for the deposition of the pristine PEDOT:PSS layer. The active layer morphology might be slightly destroyed by O2, thus presenting lower performance. Such surface treatment destroyed the PTB7-Th:PC71BM and PffBT4T-2OD:PC71BM blend films totally, yielding very poor device performance. Figure 4 shows the J−V characteristics and EQE spectra of the inverted P3HT:PC 6 1 BM, PTBT7-Th:PC 7 1 BM, and PffBT4T2OD:PC71BM devices. The performance parameters of these devices are listed in Table 1. As seen from Table 1, the highest PCEs of the P3HT:PC61BM, PTB7-Th:PC71BM, and PffBT4T2OD:PC71BM devices with PMA:PEDOT:PSS ABLs were 3.59, 9.19, and 9.07%, respectively, which were comparable to or slight higher than that of 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 948

DOI: 10.1021/acsami.7b13346 ACS Appl. Mater. Interfaces 2018, 10, 943−954

Research Article

ACS Applied Materials & Interfaces

Table 2. Performance Parameters of Inverted Solar Cells with Doctor-Blade-Coated PMA:PEDOT:PSS Layers and Thermally Vacuum-Deposited Al Electrode

a

entry

active layer

anode

VOC (V)

JSC (mA/cm2)

FF (%)

PCE (%)

aver. PCE ± std. dev. (%)a)

11 12

P3HT:PC61BM PTB7-Th:PC71BM

e-Al e-Al

0.61 0.79

9.23 17.12

60 64

3.38 8.66

3.20 ± 0.19 8.41 ± 0.20

Standard deviation was calculated over 12 individual devices.

Figure 5. (a−c) Picture of a drop of Ag nanowire ink on the top of the pristine PTB7-Th:PC71BM, PTB7-Th:PC71BM/PMA, and PTB7-Th:PC71BM/ PMA:PEDOT:PSS films. (d−f) The SEM image of the Ag nanowire network on the top of PTB7-Th:PC71BM, PTB7-Th:PC71BM/PMA, and PTB7Th:PC71BM/PMA:PEDOT:PSS films.

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 that of the PMA device. (Figure S-5). All these results confirmed that the PMA:PEDOT:PSS composite is an excellent solutionprocessable anode buffer material for the organic solar cells. 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 °C with a 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 S-6, 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 device presented improved long-term stability when stored in the N2-filled glovebox (Figure S-7). This result indicated that the two key interfaces of the PMA:PEDOT:PSS/organic layer and PMA:PEDOT:PSS/Al were intrinsically stable with the exclusion of water and oxygen. Thus, the long-term stability of the PMA:PEDOT:PSS ABLbased device could be greatly improved through proper encapsulation. Then, the printability and thickness optimization of this PMA:PEDOT:PSS composite anode buffer layer were investigated through doctor-blade coating. A series of inverted organic solar cells with doctor-blade-coated PMA:PEDOT:PSS ABL electrodes 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, entrys 4 and 7), we can find that

these devices with doctor-blade-coated ABLs present nearly the same performance parameter, indicating that this PMA:PEDOT:PSS ABL is suitable for doctor-blade coating. 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 an effective protecting effect of 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 5a shows the contact angle between the PMA:PEDOT:PSS layer and Ag nanowire 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 nanowire 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 was dispersed more homogeneously on the surface of PMA:PEDOT:PSS than on the pristine PTB7-Th:PC71BM or PTB7Th:PC71BM/PMA films. Therefore, the insertion of this PMA:PEDOT:PSS ABL can enable the coating process of Ag nanowires on the 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 proven by the device performance (vide infra) . 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 6a−d illustrates the photographs of the P3HT:PC61BM and PTB7-Th:PC71BM semitransparent devices with Ag NWs (p-Ag) and a MoO3/Ag/ 949

DOI: 10.1021/acsami.7b13346 ACS Appl. Mater. Interfaces 2018, 10, 943−954

Research Article

ACS Applied Materials & Interfaces

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, the full-coated semitransparent device with a doctor-blade-coated photoactive layer and PMA:PEDOT:PSS layer, and the spray-coated Ag nanowire top electrode. (h) The histogram of fully printed PTB7-Th:PC71BM semitransparent devices performance obtained from 12 devices.

Table 3. Performance Parameters of the Semitransparent Inverted Solar Cells entry

active layer

13 14

P3HT:PC61BM

15 16 a

PTB7-Th:PC71BM

anode

AVT (%)

illuminated side

VOC (V)

JSC (mA/cm2)

FF (%)

PCE (%)

aver. PCE ± std. dev. (%)a

p-Ag

45.5

MoO3/Ag/MoO3

27.3

p-Ag

50.3

MoO3/Ag/MoO3

19.5

bottom top bottom top bottom top bottom top

0.61 0.61 0.60 0.60 0.78 0.76 0.77 0.72

7.56 5.61 7.19 3.34 11.28 8.60 12.70 2.97

0.63 0.63 0.65 0.60 0.57 0.55 0.59 0.58

2.91 2.16 2.80 1.20 5.01 3.59 5.77 1.24

2.78 ± 0.12 2.03 ± 0.17 2.74 ± 0.11 1.09 ± 0.10 4.93 ± 0.13 3.33 ± 0.18 5.53 ± 0.25 1.20 ± 0.11

Standard deviation was calculated over 12 individual devices.

found that the optimized thickness of the inner MoO3, the thin Ag layer, and the MoO3 capping layer are 10, 15, and 30 nm, respectively. 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 a sheet resistance of 12.5 Ω/□ and a average visible transmittance of 86.1%. Therefore, the semitransparent devices using a MoO3/Ag/MoO3 top electrode showed a lower average visible-light transmittance with AVTs of approximately 27.3 and 19.5% for the P3HT: PC61BM and

MoO3 top electrode. For the Ag nanowire-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 50.3% 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 of the dependence of the photoelectrical properties of the semitransparent electrode on the electrode parameter, we 950

DOI: 10.1021/acsami.7b13346 ACS Appl. Mater. Interfaces 2018, 10, 943−954

Research Article

ACS Applied Materials & Interfaces PTB7-Th:PC71BM devices, respectively. It is worth noticing that these device transmittances were among the same level for the MoO3/Ag/MoO3 electrode-based devices.12,13 As shown in Figure 6f, Table 3, and Figures S-9 and S-10, the semitransparent P3HT:PC61BM and PTBT7-Th:PC71BM devices with a MoO3/ Ag/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-Agbased 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 that 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 illuminationintensity-dependent VOC, as weaker illumination intensity was apparent in the top illumination case.62 On the other hand, 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 acceptor using ultrathin hybrid-metal top electrodes is about 7%.63−65 While for a semitransparent single organic solar cell with a solutionprocessable top electrode, the highest performance is approximately 6.0%.15,66 The lower performance for the semitransparent device with the solution-processed top electrode is attributed to both the lower FF caused by moisture and air effects during the printing and postannealing processes and the lower JSC due to high light transmittance.30 A PCE of 5.01% in this work is among one of the highest photovoltaic performances 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 6h).

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.

4. 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 homogeneous 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:PC 61 BM, PTB7-Th:PC 71 BM, and

■

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b13346. Figure S-1. TEM image of PMA nanoparticles dispersed in isopropanol with a concentration of 0.1 mg/mL. Figure S2. 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 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 stored in glovebox. 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) 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 (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Q.L.) *E-mail: [email protected] (C.-Q.M.) ORCID

Qun Luo: 0000-0002-7527-460X Chang-Qi Ma: 0000-0002-9293-5027 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The work is financially supported by the National Natural Science Foundation of China (51773224), the Ministry of Science and Technology of China (No 2016YFA0200700), the Suzhou Science and Technology Project (SYG201735), the 951

DOI: 10.1021/acsami.7b13346 ACS Appl. Mater. Interfaces 2018, 10, 943−954

Research Article

ACS Applied Materials & Interfaces

Highly Efficient Reflective and Semitransparent Polymer Solar Cells. Nano Energy 2016, 28, 277−287. (16) Kim, Y. H.; Muller-Meskamp, L.; Zakhidov, A. A.; Sachse, C.; Meiss, J.; Bikova, J.; Cook, A.; Zakhidov, A. A.; Leo, K. SemiTransparent Small Molecule Organic Solar Cells with Laminated FreeStanding Carbon Nanotube Top Electrodes. Sol. Energy Mater. Sol. Cells 2012, 96, 244−250. (17) Liu, Z. K.; Li, J. H.; Sun, Z. H.; Tai, G. A.; Lau, S. P.; Yan, F. The Application of Highly Doped Single-Layer Graphene as the Top Electrodes of Semitransparent Organic Solar Cells. ACS Nano 2012, 6, 810−818. (18) Liu, Z. K.; You, P.; Liu, S. H.; Yan, F. Neutral-Color Semitransparent Organic Solar Cells with All-Graphene Electrodes. ACS Nano 2015, 9, 12026−12034. (19) Chueh, C. C.; Li, C. Z.; Jen, A. K. Y. Recent Progress and Perspective in Solution-Processed Interfacial Materials for Efficient and Stable Polymer and Organometal Perovskite Solar Cells. Energy Environ. Sci. 2015, 8, 1160−1189. (20) Chen, L. M.; Xu, Z.; Hong, Z. R.; Yang, Y. Interface Investigation and Engineering - Achieving High Performance Polymer Photovoltaic Devices. J. Mater. Chem. 2010, 20, 2575−2598. (21) Turak, A. Interfacial Degradation in Organic Optoelectronics. RSC Adv. 2013, 3, 6188−6225. (22) Zeng, H.; Zhu, X. C.; Liang, Y. Y.; Guo, X. G. Interfacial Layer Engineering for Performance Enhancement in Polymer Solar Cells. Polymers 2015, 7, 333−372. (23) Andersen, T. R.; Almyahi, F.; Cooling, N. A.; Elkington, D.; Wiggins, L.; Fahy, A.; Feron, K.; Vaughan, B.; Griffith, M. J.; Mozer, A. J.; Sae-Kung, C.; Wallace, G. G.; Belcher, W. J.; Dastoor, P. C. Comparison of Inorganic Electron Transport Layers in Fully Roll-to-Roll Coated/ Printed Organic Photovoltaics in Normal Geometry. J. Mater. Chem. A 2016, 4, 15986−15996. (24) Zhang, H.; Tan, W. Y.; Fladischer, S.; Ke, L. L.; Ameri, T.; Li, N.; Turbiez, M.; Spiecker, E.; Zhu, X. H.; Cao, Y.; Brabec, C. J. Roll to Roll Compatible Fabrication of Inverted Organic Solar Cells with a SelfOrganized Charge Selective Cathode Interfacial Layer. J. Mater. Chem. A 2016, 4, 5032−5038. (25) Voigt, M. M.; Mackenzie, R. C. I.; Yau, C. P.; Atienzar, P.; Dane, J.; Keivanidis, P. E.; Bradley, D. D. C.; Nelson, J. Gravure Printing for Three Subsequent Solar Cell Layers of Inverted Structures on Flexible Substrates. Sol. Energy Mater. Sol. Cells 2011, 95, 731−734. (26) Cheng, P.; Lin, Y. Z.; Zawacka, N. K.; Andersen, T. R.; Liu, W. Q.; Bundgaard, E.; Jorgensen, M.; Chen, H. Z.; Krebs, F. C.; Zhan, X. W. Comparison of Additive Amount Used in Spin-Coated and Roll-Coated Organic Solar Cells. J. Mater. Chem. A 2014, 2, 19542−19549. (27) Liu, K.; Larsen-Olsen, T. T.; Lin, Y. Z.; Beliatis, M.; Bundgaard, E.; Jorgensen, M.; Krebs, F. C.; Zhan, X. W. Roll-Coating Fabrication of Flexible Organic Solar Cells: Comparison of Fullerene and FullereneFree Systems. J. Mater. Chem. A 2016, 4, 1044−1051. (28) Liu, Y.; Larsen-Olsen, T. T.; Zhao, X. G.; Andreasen, B.; Sondergaard, R. R.; Helgesen, M.; Norrman, K.; Jorgensen, M.; Krebs, F. C.; Zhan, X. W. All Polymer Photovoltaics: From Small Inverted Devices to Large Roll-to-Roll Coated and Printed Solar Cells. Sol. Energy Mater. Sol. Cells 2013, 112, 157−162. (29) Lu, H.; Lin, J.; Wu, N.; Nie, S. H.; Luo, Q.; Ma, C. Q.; Cui, Z. Inkjet Printed Silver Nanowire Network as Top Electrode for SemiTransparent Organic Photovoltaic Devices. Appl. Phys. Lett. 2015, 106, 093302. (30) Min, J.; Bronnbauer, C.; Zhang, Z. G.; Cui, C. H.; Luponosov, Y. N.; Ata, I.; Schweizer, P.; Przybilla, T.; Guo, F.; Ameri, T.; Forberich, K.; Spiecker, E.; Bauerle, P.; Ponomarenko, S. A.; Li, Y. F.; Brabec, C. J. Fully Solution-Processed Small Molecule Semitransparent Solar Cells: Optimization of Transparent Cathode Architecture and Four Absorbing Layers. Adv. Funct. Mater. 2016, 26, 4543−4550. (31) Shi, H.; Liu, C. C.; Jiang, Q. L.; Xu, J. K. Effective Approaches to Improve the Electrical Conductivity of PEDOT:PSS: A Review. Adv. Electron. Mater. 2015, 1, 1500017. (32) Steim, R.; Kogler, F. R.; Brabec, C. J. Interface Materials for Organic Solar Cells. J. Mater. Chem. 2010, 20, 2499−2512.

Strategic Priority Research Program of the Chinese Academy of Sciences (Grant no. XDA09020201). G.Q. Ji and Y.L. 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.

■

REFERENCES

(1) Deng, D.; Zhang, Y. J.; Zhang, J. Q.; Wang, Z. Y.; Zhu, L. Y.; Fang, J.; Xia, B. Z.; Wang, Z.; Lu, K.; Ma, W.; Wei, Z. X. Fluorination-Enabled Optimal Morphology Leads to over 11% Efficiency for Inverted SmallMolecule Organic Solar Cells. Nat. Commun. 2016, 7, 13740. (2) Li, M.; Gao, K.; Wan, X.; Zhang, Q.; Kan, B.; Xia, R.; Liu, F.; Yang, X.; Feng, H.; Ni, W.; Wang, Y.; Peng, J.; Zhang, H.; Liang, Z.; Yip, H.-L.; Peng, X.; Cao, Y.; Chen, Y. Solution-Processed Organic Tandem Solar Cells with Power Conversion Efficiencies > 12%. Nat. Photonics 2016, 11, 85. (3) Nian, L.; Gao, K.; Liu, F.; Kan, Y. Y.; Jiang, X. F.; Liu, L. L.; Xie, Z. Q.; Peng, X. B.; Russell, T. P.; Ma, Y. G. 11% Efficient Ternary Organic Solar Cells with High Composition Tolerance Via Integrated near-IR Sensitization and Interface Engineering. Adv. Mater. 2016, 28, 8184− 8190. (4) Yang, Y. K.; Zhang, Z. G.; Bin, H. J.; Chen, S. S.; Gao, L.; Xue, L. W.; Yang, C.; Li, Y. F. Side-Chain Isomerization on an N-Type Organic Semiconductor ITIC Acceptor Makes 11.77% High Efficiency Polymer Solar Cells. J. Am. Chem. Soc. 2016, 138, 15011−15018. (5) Zhang, K.; Gao, K.; Xia, R. X.; Wu, Z. H.; Sun, C.; Cao, J. M.; Qian, L.; Li, W. Q.; Liu, S. Y.; Huang, F.; Peng, X. B.; Ding, L. M.; Yip, H. L.; Cao, Y. High-Performance Polymer Tandem Solar Cells Employing a New N-Type Conjugated Polymer as an Interconnecting Layer. Adv. Mater. 2016, 28, 4817−4823. (6) Zhao, W. C.; Qian, D. P.; Zhang, S. Q.; Li, S. S.; Inganas, O.; Gao, F.; Hou, J. H. Fullerene-Free Polymer Solar Cells with over 11% Efficiency and Excellent Thermal Stability. Adv. Mater. 2016, 28, 4734− 4739. (7) Zhao, J.; Li, Y.; Yang, G.; Jiang, K.; Lin, H.; Ade, H.; Ma, W.; Yan, H. Efficient Organic Solar Cells Processed from Hydrocarbon Solvents. Nat. Energy 2016, 1, 15027. (8) Cui, Y.; Yao, H. F.; Gao, B. W.; Qin, Y. P.; Zhang, S. Q.; Yang, B.; He, C.; Xu, B. W.; Hou, J. H. Fine Tuned Photoactive and Interconnection Layers for Achieving Over 13% Efficiency in a Fullerene-Free Tandem Organic Solar Cell. J. Am. Chem. Soc. 2017, 139, 7302−7309. (9) Quesada, G.; Rousse, D.; Dutil, Y.; Badache, M.; Halle, S. A Comprehensive Review of Solar Facades. Transparent and Translucent Solar Facades. Renewable Sustainable Energy Rev. 2012, 16, 2643−2651. (10) Romero-Gomez, P.; Pastorelli, F.; Mantilla-Perez, P.; Mariano, M.; Martinez-Otero, A.; Elias, X.; Betancur, R.; Martorell, J. SemiTransparent Polymer Solar Cells. J. Photonics Energy 2015, 5, 057212. (11) Wang, W.; Yan, C. Q.; Lau, T. K.; Wang, J. Y.; Liu, K.; Fan, Y.; Lu, X. H.; Zhan, X. W. Fused Hexacyclic Nonfullerene Acceptor with Strong near Infrared Absorption for Semitransparent Organic Solar Cells with 9.77% Efficiency. Adv. Mater. 2017, 29, 1701308. (12) Xu, G. Y.; Shen, L.; Cui, C. H.; Wen, S. P.; Xue, R. M.; Chen, W. J.; Chen, H. Y.; Zhang, J. W.; Li, H. K.; Li, Y. W.; Li, Y. F. HighPerformance Colorful Semitransparent Polymer Solar Cells with Ultrathin Hybrid-Metal Electrodes and Fine-Tuned Dielectric Mirrors. Adv. Funct. Mater. 2017, 27, 1605908. (13) Cho, J. M.; Lee, S. K.; Moon, S. J.; Jo, J.; Shin, W. S. MoO3/Ag/ MoO3 Top Anode Structure for Semitransparent Inverted Organic Solar Cells. Curr. Appl. Phys. 2014, 14, 1144−1148. (14) Wang, Y. F.; Jia, B. Y.; Qin, F.; Wu, Y.; Meng, W.; Dai, S. X.; Zhou, Y. H.; Zhan, X. W. Semitransparent, Non-Fullerene and Flexible AllPlastic Solar Cells. Polymer 2016, 107, 108−112. (15) Jagadamma, L. K.; Hu, H. L.; Kim, T.; Ndjawa, G. O. N.; Mansour, A. E.; El Labban, A.; Faria, J. C. D.; Munir, R.; Anjum, D. H.; McLachlan, M. A.; Amassian, A. Solution-Processable MoOx Nanocrystals Enable 952

DOI: 10.1021/acsami.7b13346 ACS Appl. Mater. Interfaces 2018, 10, 943−954

Research Article

ACS Applied Materials & Interfaces

Improved Efficiency in Inverted Organic Solar Cells. Appl. Phys. Lett. 2014, 104, 041114. (50) Liu, J.; Wu, J.; Shao, S.; Deng, Y.; Meng, B.; Xie, Z.; Geng, Y.; Wang, L.; Zhang, F. Printable Highly Conductive Conjugated Polymer Sensitized ZnO NCs as Cathode Interfacial Layer for Efficient Polymer Solar Cells. ACS Appl. Mater. Interfaces 2014, 6, 8237−8245. (51) Lee, S. J.; Kim, B. S.; Kim, J. Y.; Yusoff, A. B.; Jang, J. Stable Organic Photovoltaic with PEDOT:PSS and MoOx Mixture Anode Interfacial Layer without Encapsulation. Org. Electron. 2015, 19, 140− 146. (52) Kim, G.; Kong, J.; Kim, J.; Kang, H.; Back, H.; Kim, H.; Lee, K. Overcoming the Light-Soaking Problem in Inverted Polymer Solar Cells by Introducing a Heavily Doped Titanium Sub-Oxide Functional Layer. Adv. Energy Mater. 2015, 5, 1401298. (53) Wang, Y. L.; Luo, Q.; Wu, N.; Wang, Q. K.; Zhu, H. F.; Chen, L. W.; Li, Y. Q.; Luo, L. Q.; Ma, C. Q. Solution-Processed MoO3:PEDOT:PSS Hybrid Hole Transporting Layer for Inverted Polymer Solar Cells. ACS Appl. Mater. Interfaces 2015, 7, 7170−7179. (54) Liu, J.; Shao, S. Y.; Fang, G.; Meng, B.; Xie, Z. Y.; Wang, L. X. High-Efficiency Inverted Polymer Solar Cells with Transparent and Work-Function Tunable MoO3-Al Composite Film as Cathode Buffer Layer. Adv. Mater. 2012, 24, 2774−2779. (55) Boukhvalov, D. W.; Korotin, D. M.; Efremov, A. V.; Kurmaev, E. Z.; Borchers, C.; Zhidkov, I. S.; Gunderov, D. V.; Valiev, R. Z.; Gavrilov, N. V.; Cholakh, S. O. Modification of Titanum and Titanium Dioxide Surfaces by Ion Implantation: Combined XPS and DFT Study. Phys. Status Solidi B 2015, 252, 748−754. (56) Huang, B.; Zheng, X. D.; Lu, M.; Dong, S.; Qiao, Y. Novel Spherical Li3V2(PO4)3/C Cathode Material for Application in High Power Lithium Ion Battery. Int. J. Electrochem. Sci. 2012, 7, 437−444. (57) Wang, J.; Cai, K. F.; Shen, S. Enhanced Thermoelectric Properties of Poly(3,4-Ethylenedioxythiophene) Thin Films Treated with H2SO4. Org. Electron. 2014, 15, 3087−3095. (58) Ma, H.; Yip, H. L.; Huang, F.; Jen, A. K. Y. Interface Engineering for Organic Electronics. Adv. Funct. Mater. 2010, 20, 1371−1388. (59) Liao, S. H.; Jhuo, H. J.; Cheng, Y. S.; Chen, S. A. Fullerene Derivative-Doped Zinc Oxide Nanofilm as the Cathode of Inverted Polymer Solar Cells with Low-Bandgap Polymer (PTB7-Th) for High Performance. Adv. Mater. 2013, 25, 4766−4771. (60) Hu, H. W.; Jiang, K.; Yang, G. F.; Liu, J.; Li, Z. K.; Lin, H. R.; Liu, Y. H.; Zhao, J. B.; Zhang, J.; Huang, F.; Qu, Y. Q.; Ma, W.; Yan, H. Terthiophene-Based D-A Polymer with an Asymmetric Arrangement of Alkyl Chains That Enables Efficient Polymer Solar Cells. J. Am. Chem. Soc. 2015, 137, 14149−14157. (61) Liu, H. M.; Hu, L.; Wu, F. Y.; Chen, L.; Chen, Y. W. Polyfluorene Electrolytes Interfacial Layer for Efficient Polymer Solar Cells: Controllably Interfacial Dipoles by Regulation of Polar Groups. ACS Appl. Mater. Interfaces 2016, 8, 9821−9828. (62) Koster, L. J. A.; Mihailetchi, V. D.; Ramaker, R.; Blom, P. W. M. Light Intensity Dependence of Open-Circuit Voltage of Polymer: Fullerene Solar Cells. Appl. Phys. Lett. 2005, 86, 123509. (63) Xu, G. Y.; Shen, L.; Cui, C. H.; Wen, S. P.; Xue, R. M.; Chen, W. J.; Chen, H. Y.; Zhang, J. W.; Li, H. K.; Li, Y. W.; Li, Y. F. HighPerformance Colorful Semitransparent Polymer Solar Cells with Ultrathin Hybrid-Metal Electrodes and Fine-Tuned Dielectric Mirrors. Adv. Funct. Mater. 2017, 27, 1605908. (64) Liu, F.; Zhou, Z. C.; Zhang, C.; Zhang, J. Y.; Hu, Q.; Vergote, T.; Liu, F.; Russell, T. P.; Zhu, X. Z. Efficient Semitransparent Solar Cells with High NIR Responsiveness Enabled by a Small-Bandgap Electron Acceptor. Adv. Mater. 2017, 29, 1606574. (65) Yin, Z. G.; Wei, J. J.; Chen, S. C.; Cai, D. D.; Ma, Y. L.; Wang, M.; Zheng, Q. D. Long Lifetime Stable and Efficient Semitransparent Organic Solar Cells Using a ZnMgO-Modified Cathode Combined with a Thin MoO3/Ag Anode. J. Mater. Chem. A 2017, 5, 3888−3899. (66) Czolk, J.; Landerer, D.; Koppitz, M.; Nass, D.; Colsmann, A. Highly Efficient, Mechanically Flexible, Semi-Transparent Organic Solar Cells Doctor Bladed from Non-Halogenated Solvents. Adv. Mater. Technol. 2016, 1, 1600184.

(33) Takano, T.; Masunaga, H.; Fujiwara, A.; Okuzaki, H.; Sasaki, T. PEDOT Nanocrystal in Highly Conductive PEDOT:PSS Polymer Films. Macromolecules 2012, 45, 3859−3865. (34) Lim, F. J.; Ananthanarayanan, K.; Luther, J.; Ho, G. W. Influence of a Novel Fluorosurfactant Modified PEDOT:PSS Hole Transport Layer on the Performance of Inverted Organic Solar Cells. J. Mater. Chem. 2012, 22, 25057−25064. (35) Heo, S. W.; Baek, K. H.; Lee, T. H.; Lee, J. Y.; Moon, D. K. Enhanced Performance in Inverted Polymer Solar Cells Via Solution Process: Morphology Controlling of PEDOT:PSS as Anode Buffer Layer by Adding Surfactants. Org. Electron. 2013, 14, 1629−1635. (36) Lee, Y. J.; Yi, J.; Gao, G. F.; Koerner, H.; Park, K.; Wang, J.; Luo, K. Y.; Vaia, R. A.; Hsu, J. W. P. Low-Temperature Solution-Processed Molybdenum Oxide Nanoparticle Hole Transport Layers for Organic Photovoltaic Devices. Adv. Energy Mater. 2012, 2, 1193−1197. (37) Xie, F. X.; Choy, W. C. H.; Wang, C. D.; Li, X. C.; Zhang, S. Q.; Hou, J. H. Low-Temperature Solution-Processed Hydrogen Molybdenum and Vanadium Bronzes for an Efficient Hole-Transport Layer in Organic Electronics. Adv. Mater. 2013, 25, 2051−2055. (38) Wong, K. H.; Ananthanarayanan, K.; Luther, J.; Balaya, P. Origin of Hole Selectivity and the Role of Defects in Low-Temperature Solution-Processed Molybdenum Oxide Interfacial Layer for Organic Solar Cells. J. Phys. Chem. C 2012, 116, 16346−16351. (39) Vasilopoulou, M.; Douvas, A. M.; Palilis, L. C.; Kennou, S.; Argitis, P. Old Metal Oxide Clusters in New Applications: Spontaneous Reduction of Keggin and Dawson Polyoxometalate Layers by a Metallic Electrode for Improving Efficiency in Organic Optoelectronics. J. Am. Chem. Soc. 2015, 137, 6844−6856. (40) Zhu, Y. W.; Yuan, Z. C.; Cui, W.; Wu, Z. W.; Sun, Q. J.; Wang, S. D.; Kang, Z. H.; Sun, B. Q. A Cost-Effective Commercial Soluble Oxide Cluster for Highly Efficient and Stable Organic Solar Cells. J. Mater. Chem. A 2014, 2, 1436−1442. (41) Jia, X.; Shen, l.; Yao, M. N.; Liu, Y.; Yu, W. J.; Guo, W.; Ruan, S. P. Highly Efficient Low-Bandgap Polymer Solar Cells with SolutionProcessed and Annealing-Free Phosphomolybdic Acid as Hole Transport Layers. ACS Appl. Mater. Interfaces 2015, 7, 5367−5372. (42) Vasilopoulou, M.; Polydorou, E.; Douvas, A. M.; Palilis, L. C.; Kennou, S.; Argitis, P. Annealing-Free Highly Crystalline SolutionProcessed Molecular Metal Oxides for Efficient Single-Junction and Tandem Polymer Solar Cells. Energy Environ. Sci. 2015, 8, 2448−2463. (43) Alaaeddine, M.; Zhu, Q.; Fichou, D.; Izzet, G.; Rault, J. E.; Barrett, N.; Proust, A.; Tortech, L. Enhancement of Photovoltaic Efficiency by Insertion of a Polyoxometalate Layer at the Anode of an Organic Solar Cell. Inorg. Chem. Front. 2014, 1, 682−688. (44) Shao, S. Y.; Liu, J.; Bergqvist, J.; Shi, S. W.; Veit, C.; Wurfel, U.; Xie, Z. Y.; Zhang, F. L. In Situ Formation of MoO3 in PEDOT:PSS Matrix: A Facile Way to Produce a Smooth and Less Hygroscopic Hole Transport Layer for Highly Stable Polymer Bulk Heterojunction Solar Cells. Adv. Energy Mater. 2013, 3, 349−355. (45) Chen, H. C.; Lin, S. W.; Jiang, J. M.; Su, Y. W.; Wei, K. H. Solution-Processed Zinc Oxide/Polyethylenimine Nanocomposites as Tunable Electron Transport Layers for Highly Efficient Bulk Heterojunction Polymer Solar Cells. ACS Appl. Mater. Interfaces 2015, 7, 6273−6281. (46) Wu, N.; Luo, Q.; Bao, Z. M.; Lin, J.; Li, Y.-Q.; Ma, C.-Q. Zinc Oxide:Conjugated Polymer Nanocomposite as Cathode Buffer Layer for Solution Processed Inverted Organic Solar Cells. Sol. Energy Mater. Sol. Cells 2015, 141, 248−259. (47) Small, C. E.; Chen, S.; Subbiah, J.; Amb, C. M.; Tsang, S. W.; Lai, T. H.; Reynolds, J. R.; So, F. High-Efficiency Inverted DithienogermoleThienopyrrolodione-Based Polymer Solar Cells. Nat. Photonics 2011, 6, 115−120. (48) Jin, T. M.; Rui, W.; Shu, Z.; Yee, S. K.; Jun, L.; Zhikuan, C.; Chellappan, V.; Wei, C. Zno:Polymer Composite Material to Eliminate Kink in J-V Curves of Inverted Polymer Solar Cells. ECS Solid State Lett. 2014, 3, Q9−Q12. (49) Tiwari, J. P.; Pillai, S.; Parakh, S.; Ali, F.; Sharma, A.; Chand, S. A Futuristic Approach Towards Interface Layer Modifications for 953

DOI: 10.1021/acsami.7b13346 ACS Appl. Mater. Interfaces 2018, 10, 943−954

Research Article

ACS Applied Materials & Interfaces (67) Lin, Y.; Dam, H. F.; Andersen, T. R.; Bundgaard, E.; Fu, W.; Chen, H.; Krebs, F. C.; Zhan, X. Ambient Roll-to-Roll Fabrication of Flexible Solar Cells Based on Small Molecules. J. Mater. Chem. C 2013, 1, 8007− 8010. (68) Eggenhuisen, T. M.; Galagan, Y.; Biezemans, A. F. K. V.; Slaats, T. M. W. L.; Voorthuijzen, W. P.; Kommeren, S.; Shanmugam, S.; Teunissen, J. P.; Hadipour, A.; Verhees, W. J. H.; Veenstra, S. C.; Coenen, M. J. J.; Gilot, J.; Andriessen, R.; Groen, W. A. High Efficiency, Fully Inkjet Printed Organic Solar Cells with Freedom of Design. J. Mater. Chem. A 2015, 3, 7255−7262. (69) Kapnopoulos, C.; Mekeridis, E. D.; Tzounis, L.; Polyzoidis, C.; Zachariadis, A.; Tsimikli, S.; Gravalidis, C.; Laskarakis, A.; Vouroutzis, N.; Logothetidis, S. Fully Gravure Printed Organic Photovoltaic Modules: A Straightforward Process with a High Potential for Large Scale Production. Sol. Energy Mater. Sol. Cells 2016, 144, 724−731. (70) Cheng, P.; Bai, H.; Zawacka, N. K.; Andersen, T. R.; Liu, W.; Bundgaard, E.; Jorgensen, M.; Chen, H.; Krebs, F. C.; Zhan, X. RollCoated Fabrication of Fullerene-Free Organic Solar Cells with Improved Stability. Adv. Sci. 2015, 2, 1500096. (71) Liu, W.; Liu, S.; Zawacka, N. K.; Andersen, T. R.; Cheng, P.; Fu, L.; Chen, M.; Fu, W.; Bundgaard, E.; Jørgensen, M.; Zhan, X.; Krebs, F. C.; Chen, H. Roll-Coating Fabrication of Flexible Large Area Small Molecule Solar Cells with Power Conversion Efficiency Exceeding 1%. J. Mater. Chem. A 2014, 2, 19809−19814.

954

DOI: 10.1021/acsami.7b13346 ACS Appl. Mater. Interfaces 2018, 10, 943−954