Writable and patternable organic solar cells and modules inspired by an old Chinese calligraphy tradition

Abstract

Scalable, patternable and affordable thin-film fabrication techniques for solution-processed organic photovoltaics are highly desirable. In this work, we report a new fabrication technique inspired by an old Chinese calligraphy tradition to fabricate organic solar cells and modules. The fabrication tool of “Maobi”, also called “Chinese ink brush”, has over 2000 years of history and has been widely used for writing and painting in old Chinese history. Using Maobi coating, the thickness of the active layers and the polymer electrodes could be tuned by optimizing the coating speed and substrate temperature. Solar cells based on Maobi-coated active layers (P3HT:ICBA, PTB7-Th:PC₇₁BM and PBDB-T:ITIC) display comparable performance to the devices with spin-coated active layers. Among them, the cells with the Maobi-coated PBDB-T:ITIC active layer exhibit high power conversion efficiencies of 10.1%. Furthermore, based on the inherent advantage of the easy patterning of Maobi, we demonstrated Maobi-coated solar modules containing 8 sub-cells that exhibit a high open-circuit voltage (V_OC) of 6.3 V and a high fill factor of 0.71. At the end, large-area solar modules (18 cm²) were demonstrated via a motor-driven computer-controlled automatic Maobi coating. The module displays V_OC of up to 11.6 V and a power conversion efficiency of 6.3%.

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Conceptual insights

Solution-processed and printed electronics are highly attractive to realize low-cost, light-weight, and flexible applications. Processing techniques play a crucial role in this field. Scalable, patternable, and affordable thin-film fabrication techniques are highly desirable to complement existing techniques such as spin coating, doctor blading, inkjet printing, screen-printing and slot-die coating. Here, a solution-processing technique based on Maobi, also known as “Chinese ink brush”, inspired by an old Chinese calligraphy tradition is developed to fabricate organic thin films, solar cells and modules. Motor-driven computer-controlled automatic Maobi coating can yield photoactive layers and conducting polymer electrodes with tunable thickness from nanometers to micrometers, and high-performance solar cells and modules. This technique has unique advantages of easy patterning since Maobi is a writing and drawing tool that can produce non-destructive coatings on curved surfaces even during contact because the Maobi is made from soft animal hairs. The Maobi tool also has the advantage of ink holding due to the nanostructure of the hair surface, which is important for ink control for the precise coating and high ink utilization rate. In addition to organic photovoltaics, this technique is also applicable to other printable devices such as field-effect transistors, photodetectors, thermoelectrics, capacitors and batteries.

1. Introduction

Low-temperature solution processing is the most important advantage of organic solar cells (OSCs) compared to their inorganic counterparts.^1–5 Typically, the organic active layers are prepared by spin coating. This is because the spin coating technique has the following advantages: (1) the technique and processing steps are simple; (2) the cost of the spin coater is affordable; (3) the film thickness is easily tunable and reproducible. However, there are also drawbacks of the spin coating technique: (1) most (>80%, generally) of the solution is spun off the substrate during film formation and is thus wasted; (2) it is challenging to get the layer patterned via spin coating; (3) spin coating is not large-area scalable and is not compatible with roll-to-roll processing.⁶ Other solution-processed techniques, such as doctor blade, inkjet-printing and slot-die coating, are also adopted for the fabrication of organic solar cells.^7–17 These techniques are patternable, large-area scalable and also have a high solution unitization rate but require facilities with higher cost (Table S1, ESI†). Therefore, a new scalable, patternable and affordable solution-processing technique is desirable for the community.

The “Maobi” (Fig. 1a), also called “Chinese ink brush”, is made from animal hairs (mainly goat, weasel and rabbit hairs), and is a Chinese traditional painting/writing tool that was invented over 2000 years ago. For thousands of years, the Maobi has been used by Chinese forefathers to convey artworks and wisdom from generation to generation. Maobi writing and painting is ink-based. When the Maobi is dipped into the ink, the ink is first absorbed and stored inside the Maobi, between hairs. When the hairs are pressed upon the surface of the substrate, the ink slides to the tip and is delivered to the substrate. As the writing direction and pressure change, continuous and patterned handwriting (Fig. 1a) is thus formed under shear stress and capillary force.¹⁸ Furthermore, the hairs’ surface of the Maobi have an oriented micrometer-scale squamae structure as shown in Fig. 1b. This special structure induces a dynamic balance of the Laplace pressure difference, asymmetrical retention force, and gravity.^19–21 The balance enables the ink to be held and stored inside the Maobi steadily. The ink-holding advantage will enhance the ink utilization rate, relieve the need for ink viscosity, and help the control of the coating process. Therefore, Maobi coating has the potential of low cost, ease of patterning formation, ink-holding, good compatibility with large-area production, and high ink utilization rate.

Fig. 1 (a) Demonstration of traditional Chinese calligraphy written using Maobi (also called “Chinese ink brush”). (b) SEM image of a single hair of the Maobi. (c) Chemical structures of the active layer, interfacial layer (PEI) and polymer electrode (PEDOT:PSS) used for the Maobi-coated solar cells and modules.

In this paper, we report the use of Maobi for the fabrication of organic thin layers and ultimately Maobi-coated organic solar cells as well as solar modules. The film thicknesses of the organic active layers and conducting polymer electrodes can be tuned and controlled ranging from a few tens of nanometers to one micrometer with the Maobi coating by optimizing the coating speed and substrate temperature. Solar cells with a Maobi-coated PBDB-T:ITIC active layer exhibit a power conversion efficiency (PCE) of 10.1%. Moreover, based on the advantage of patterning of the Maobi coating, we demonstrate Maobi-coated solar modules containing 8 sub-cells that exhibit a high open-circuit voltage of 6.30 V and a high fill factor of 0.71. Finally, large-area (18 cm²) solar modules were demonstrated via motor-driven, computer-controlled, automatic Maobi coating. The module displays a V_OC of up to 11.6 V and a power conversion efficiency of 6.3%.

2. Results and discussion

2.1 Optimization of the active layers using Maobi coating and their photovoltaic performance

The solution-processed photoactive layers are the core components of organic solar cells.^22,23 Because of the intrinsic properties of organic semiconductors, i.e., short exciton diffusion length and low charge carrier mobility, the film thickness of the active layer is critical to the device’s performance. Typically, the optimized film thickness of the active layer is about 100–300 nm. Thicker films would result in charge recombination inside the active layer before reaching the electrodes and would lead to reduced photovoltaic performance.^24,25

We start the Maobi coating of the active layer with a classic polymer poly(3-hexylthiophene) (P3HT) and indene-C₆₀ bis-adduct (ICBA) blend (Fig. 1c) dissolved in chlorobenzene. Different coating speeds and substrate (ITO glass) temperatures were tried to optimize the film thickness. As shown in Fig. S1 (ESI†), both the coating speed and substrate temperature strongly influence the film thickness. For the P3HT:ICBA active layer, a faster coating speed results in thicker films. The phenomenon that the thickness of the P3HT-based active layer increases with the coating speed has also been observed in slot-die coated active layers.²⁶ It was believed that when the solution was in contact with the substrate, the substrate dragged the solution by surface tension and adhesive forces. A fast movement of the coating head does not provide enough time for the adhered solutions to flow back, which yields larger film thicknesses. The thickness first decreases and then increases as the substrate temperature increases from 30 to 100 °C. Overall, film thicknesses between 50 nm and 1 μm can be obtained by changing the coating speed and the substrate temperature. Therefore, Maobi coating could meet the requirement of the active layer thickness (100–300 nm, as mentioned above) for efficient polymer solar cells.

In addition to the film thickness, the coating temperature is also important for the film’s morphology, particularly for polymers with strong interactions between chains. As shown in Fig. 2a and Fig. S2 (ESI†), a uniform film could be obtained when the substrate temperature increased to 50 °C. UV-vis absorption spectra and X-ray diffraction patterns of the Maobi-coated (at 50 °C) and spin coated P3HT:ICBA films are shown in Fig. S3 (ESI†). The Maobi-coated films display a stronger absorption shoulder at 600 nm and a more intense diffraction peak at 5.2° than those fabricated with spin coating, due to P3HT interchain stacking. Maobi coating is more beneficial for the formation of ordered structures for the P3HT:ICBA films.^27,28 The shear stress during Maobi coating is possibly the main reason for the improved crystallinity, which improves the absorption and hole mobility.^29–32 Therefore, solar cells with Maobi-coated P3HT:ICBA result in improved performance (V_OC = 0.86 V, FF = 0.67, J_SC = 8.3 mA cm⁻², and PCE = 4.8%, Fig. 2d) in relation to devices with spin-coated active layers (V_OC = 0.88 V, FF = 0.65, J_SC = 7.5 mA cm⁻², and PCE = 4.3%, Fig. S4a, ESI†) where both the P3HT:ICBA active layers have similar thicknesses of 200 nm. EQE spectra and integrated J_SC from the EQE were calculated and are shown in Fig. S5 (ESI†).

Fig. 2 Optical images of the active layer films prepared using Maobi coating: (a) P3HT:ICBA; (b) PTB7-Th:PCB₇₁M; and (c) PBDB-T:ITIC. J–V characteristics of the cells based on the Maobi-coated films: (d) P3HT:ICBA; (e) PTB7-Th:PCB₇₁M; and (f) PBDB-T:ITIC. The insets correspond to the J–V characteristics in a semi-log scale.

Furthermore, we also Maobi-coated another two donor–acceptor combinations, i.e., poly(thieno[3,4-b]thiophene-alt-benzodithiophene) derivative (PTB7-Th):[6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM), and poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)-benzo[1,2-b:4,5-b′]dithiophene))-alt-(5,5-(1′,3′-di-2-thienyl-5′,7′-bis(2-ethylhexyl)benzo[1′,2′-c:4′,5′-c′]dithiophene-4,8-dione))] (PBDB-T):3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone)-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]dithiophene) (ITIC). Their molecular structures are shown in Fig. 1c. PTB7-Th has been widely used as a benchmark low-bandgap material for high-efficient OSCs with PCEs that range from 7–10%.^33–37 PBDB-T:ITIC has recently been developed as a new type of active layer containing a non-fullerene acceptor that could yield PCEs of over 10%.^38–42 The optimized thicknesses of the PTB7-Th:PC₇₁BM and PBDB-T:ITIC active layers are both around 80–100 nm. To obtain these thicknesses, we optimized the temperature of the substrate to 50 °C, and the coating speed to around 1 cm s⁻¹ for the PTB7-Th:PC₇₁BM films and 1.5 cm s⁻¹ for the PBDB-T:ITIC films. Under these coating conditions, the obtained film thickness is about 80 nm for PTB7-Th:PC₇₁BM and 100 nm for PBDB-T:ITIC. Fig. 2b and c are optical images of the Maobi-coated PTB7-Th:PC₇₁BM and PBDB-T:ITIC films. The edge of the films appears different to the center of the films. The non-uniformity of the edge of the films will not affect the device performance since the edge area is not included in the effective area of the cells and modules.

With the desired thicknesses of these active layers, we fabricated OSCs with structures of ITO/ZnO/Maobi-coated active layer/MoO₃/Ag for the Maobi-coated PTB7-Th:PC₇₁BM and PBDB-T:ITIC active layers. For the PTB7-Th:PC₇₁BM layer, the devices exhibit the following photovoltaic performance: V_OC = 0.79 ± 0.01 V, FF = 0.65 ± 0.02, J_SC = 15.1 ± 0.5 mA cm⁻², and PCE = 8.0 ± 0.4% (Fig. 2e). For the PBDB-T:ITIC layer, the devices show: V_OC = 0.85 ± 0.01 V, FF = 0.66 ± 0.02, J_SC = 16.7 ± 0.6 mA cm⁻², and PCE = 9.7 ± 0.4% (Fig. 2f). EQE spectra and integrated J_SC from the EQE were calculated and are shown in Fig. S5 (ESI†). For comparison, solar cells with spin coated active layers were also fabricated. As shown in Fig. S4b and c (ESI†), the cells with the Maobi-coated active layers display a similar performance to the reference cells. This confirms that the Maobi is applicable to the processing of active layers to achieve high-performance organic solar cells.

2.2 Optimization of the conducting polymer electrode using Maobi coating

To realize Maobi-coated solar cells, we then continued to fabricate the electrodes via the Maobi coating technique. Conducting polymer poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is used as the electrode material since it is solution-processable. PEDOT:PSS has a high conductivity of over 1000 S cm⁻¹ and a high optical transmittance in the visible and near infrared spectral region. It has been widely used as a transparent electrode of OSCs.^43–47 The thickness of PEDOT:PSS is also critical to the OSC's performance because the transmittance and sheet resistance are influenced by the film thickness. The coating speed and substrate temperature are changed to optimize the film thickness. As shown in Fig. S6 (ESI†), the increase of the coating speed and substrate temperature led to a thinner film, which is different to the active layer coating case. This might be related to the surface tension and wetting properties of the PEDOT:PSS aqueous dispersion on the substrates. The viscosity of aqueous PH1000 mixed with EG and the surfactant is about 15–60 mPa s, significantly higher than that of the P3HT:ICBA solution (4 mPa s). The film thickness could vary between 20 and 500 nm by varying the coating conditions. When coating at 30 °C, the aqueous dispersion tends to shrink and results in a non-uniform morphology (Fig. S7, ESI†). This phenomenon is relieved when the coating temperature increases to 50 °C.

To evaluate the possibility of the Maobi-coated PEDOT:PSS films being used as the top electrodes, we applied the Maobi-coated films on both single-junction and tandem solar cells with the structure ITO/PEI/P3HT:ICBA/MC-PEDOT:PSS and ITO/PEI/P3HT:ICBA/PEDOT:PSS/PEI/P3HT:ICBA/MC-PEDOT:PSS, respectively (MC denotes Maobi-coated). The device performance is shown in Fig. S8 (ESI†). The single-junction P3HT:ICBA-based device shows a V_OC of 0.84 V and a FF of 0.65, and the tandem device shows a V_OC of 1.66 V and a FF of 0.66. We also fabricated solar cells with structures of glass/ITO/ZnO/PTB7-Th:PCBM/MC-PEDOT:PSS and glass/ITO/ZnO/PBDB-T:ITIC/MC-PEDOT:PSS. Their J–V characteristics are shown in Fig. S9 (ESI†). The PBDB-T:ITIC based cells with the PEDOT:PSS electrode exhibit the following photovoltaic performance: V_OC = 0.86 V, FF = 0.66, J_SC = 13.0 mA cm⁻², and PCE = 7.38%. The V_OC and FF of the cells with the PEDOT:PSS electrode are comparable to the device with the MoO₃/Ag electrode (Fig. 2). The J_SC is lower than that of the reference cells with MoO₃/Ag mainly because of the absence of the silver reflectance. For the PTB7-Th:PCBM active layer, the devices with the PEDOT:PSS electrode show lower J_SC, FF and PCE (V_OC = 0.77 V, FF = 0.50, J_SC = 10.8 mA cm⁻², and PCE = 4.16%) than the reference cells with MoO₃/Ag (Fig. 2). This is possibly because the PTb7-Th:PCBM layer is sensitive to the humidity present during the deposition of the PEDOT:PSS electrode. The high performance indicates that Maobi coating is a reliable and effective fabrication technique for the processing of the PEDOT:PSS electrode. Meanwhile, the deposition of the top electrode without using a mask indicates the patterning advantage of the Maobi coating technique. It should be noted that the Maobi is made from animal hairs that are soft. This is a very important feature because the pre-deposited organic layers (underneath the top electrode) are also soft. The softness of the Maobi hairs protects the layers underneath from being damaged by the contact of the Maobi hairs during coating. To confirm whether the contact between the Maobi tool and the pre-deposited active layer or transporting layer could destroy these layers and influence the solar cell performance, we intentionally brushed (contact with moving) the transporting layer or active layer with the Maobi before the top layer coating. As shown in Fig. S10 (ESI†), the cells with PEI or ZnO or active layer brushed with the Maobi show similar device performance to the cells without intentional contact with the Maobi. This shows that contact by the Maobi tool during coating will not influence the solar cell performance.

2.3 Writable and patternable organic solar cells and modules

To realize the Maobi-coated organic solar cells, in addition to the active layers and electrodes, the charge-collecting layers or surface modification layers also have to be Maobi-coated. A thin layer of polyethylenimine (PEI) is commonly used to lower the work function of the bottom electrode for electron extraction.^48,49 The thickness of PEI is generally no more than 10 nm due to its insulating nature. To skip the coating of the PEI thin layer, we added a small amount of PEI into the P3HT:ICBA mixture and then coated P3HT:ICBA:PEI directly onto the ITO bottom electrode. PEI spontaneously moves towards the bottom ITO to form an ITO/PEI cathode with a low work function, which has been reported in the literature.^35,50 Thus, the solar cell structure with ITO/PEI/P3HT:ICBA/PEDOT:PSS is simplified as ITO/P3HT:ICBA:PEI/PEDOT:PSS. Based on this simplified structure, we fabricated Maobi-coated solar cells using a two-step coating process: P3HT:ICBA:PEI and PEDOT:PSS. The J–V characteristics are shown in Fig. 3a. The written cell displays the following performance: V_OC = 0.83 ± 0.01 V, FF = 0.64 ± 0.03, J_SC = 7.9 ± 0.3 mA cm⁻² and PCE = 4.4 ± 0.2%, (Table 1). Furthermore, we also fabricated flexible all-plastic fully-Maobi coated OSCs with the structure PES/PEDOT:PSS/P3HT:ICBA:PEI/PEDOT:PSS, where ITO on the glass substrates was replaced by PEDOT:PSS on the polyethersulfone (PES) substrate. The device shows the following performance: V_OC = 0.78 V, FF = 0.57, J_SC = 7.4 mA cm⁻² and PCE = 3.3% (Fig. S11, ESI†).

Fig. 3 (a) J–V characteristics of the Maobi coated solar cells (in blue) and solar modules with 4 (in red) and 8 sub-cells (in black). The inset illustrates the fabrication process of the solar module: the two-step coating of the active layer and the top electrode. (b) Schematic structure of the module (cross-sectional view) and an optical image (top view) of two adjacent cells (part of eight sub-cells) in the module.

Table 1 Photovoltaic parameters of solar cells and solar modules with multiple layers processed with Maobi. The data were averaged over 20 samples for each type of cell and over 10 samples for each type of module

Device structure	V OC (V)	FF	J SC (mA cm^−2)	PCE (%)
a Coated with Maobi. b Transfer laminated. c Evaporated MoO3 and silver. d Written cell and modules are consistently in the ITO/P3HT:ICBA:PEI/PEDOT:PSS structure. Data in parentheses are the highest PCEs.
ITO/PEI/P3HT:ICBA^a/PEDOT:PSS^b	0.85 ± 0.01	0.64 ± 0.03	7.9 ± 0.4	4.6 ± 0.2 (4.8)
ITO/ZnO/PTB7-Th:PC71BM^a/MoO3/Ag^c	0.79 ± 0.01	0.65 ± 0.02	15.1 ± 0.5	8.0 ± 0.4 (8.4)
ITO/ZnO/PBDB-T:ITIC^a/MoO3/Ag^c	0.85 ± 0.01	0.66 ± 0.02	16.7 ± 0.6	9.7 ± 0.4 (10.1)
Written cell^d	0.83 ± 0.01	0.64 ± 0.03	7.9 ± 0.3	4.4 ± 0.2 (4.6)
Written module with four sub-cells^d	3.19 ± 0.03	0.69 ± 0.02	1.7 ± 0.2	4.0 ± 0.2 (4.2)
Written module with eight sub-cells^d	6.25 ± 0.05	0.69 ± 0.02	0.88 ± 0.05	3.9 ± 0.2 (4.1)

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The air stability of the fully-Maobi coated devices with a structure of glass/ITO/MC-ZnO/MC-PBDB-T:ITIC/MC-PEDOT:PSS was evaluated via air exposure in the dark for different lengths of time (relative humidity: 50%). Cells with the structures glass/ITO/ZnO/PBDB-T:ITIC/MoO₃/Ag and glass/ITO/ZnO/PBDB-T:ITIC/MC-PEDOT:PSS were also fabricated and studied as references, where the PBDB-T:ITIC active layers were prepared by spin coating. Fig. S12 (ESI†) shows the evolution of V_OC, J_SC, FF and PCE as a function of air exposure time. The fully-Maobi coated cells show similar efficiency decay to the reference cells with spin-coated layers during air exposure of up to 10 days. This indicates that the processing technique of Maobi coating or spin coating almost does not influence the air stability of the PBDB-T:ITIC-based solar cells.

After systematically assessing the potential of using Maobi-coating for the preparation of the organic thin layers of organic solar cells, we move on to further fabricate Maobi coated organic solar modules. An organic solar module is typically formed by interconnecting individual cell stripes in series via overlapping the positive electrode of one cell with the negative electrode of the next cell. Patterning is necessary and important to fabricate the modules.^51–53 Maobi is, in essence, a writing tool made for patterning. Unlike pre-patterning with masks or post-patterning via laser ablation, patterning using Maobi is easy and energy-economical. Based on the optimized cell structure, we fabricated solar modules containing four or eight sub-cells connected in series via Maobi coating. The structure of the fabricated modules is shown in Fig. 3. Each coating of the photoactive layers and electrodes is shifted with an offset to the previous coating that allows for the serial connection between the two different electrodes belonging to two adjacent sub-cells. Fig. 3b shows an optical microscopy image of two adjacent sub-cells. Fig. 3a shows the J–V curves of the fabricated modules. Photovoltaic data are summarized in Table 1. The total active area of the module is about 1 cm². It can be seen that the V_OC values of the four-cell and eight-cell modules are 3.22 V and 6.30 V, which are nearly four times and eight times as large as the V_OC of the reference cell, respectively. Importantly, a high FF exceeding 70% is obtained for both four-cell and eight-cell modules, which indicates the high-quality films of the active layers as well as the polymer electrodes, and that the interconnection is efficient for the recombination of the opposite charges from the adjacent sub-cells. The overall PCE of the modules with both four cells and eight cells retained more than 90% of the PCE of the single cell references if the area loss is not included. The connection space between adjacent cells which does not contribute to the current generation should be minimized to maximize the utilization of the total active area. Here, the geometric fill factor is about 70%, which should be further improved by optimizing the fabrication and employing higher-conductivity electrodes in the future.

2.4 Large-area organic solar cells and modules with automatic Maobi coating

So far, we have demonstrated small-area solar cells and modules (less than 1 cm²) using Maobi coating. To further evaluate the potential of this technique, we built a motor-driven, computer-controlled Maobi coating system (Fig. 4a) to prepare large-area films for organic solar cells. The two videos in the ESI† demonstrate the automatic writing and coating of the organic functional layers that show the potential of patterning and large-area fabrication. Small-area (5 mm²) solar cells as well as large-area (18 cm²) solar modules were fabricated, where the active layers were prepared with the automatic Maobi coating system. The structure of the sub-cells for the module is glass/ITO/ZnO/PBDB-T:ITIC/MoO₃/Ag, where Ag is used as the electrode instead of PEDOT:PSS because the sheet resistance of PEDOT:PSS is too high for large-area solar modules. Fig. S13 (ESI†) shows the J–V characteristic of the small-area (5 mm²) cells where the active layers were fabricated by hand-held Maobi coating and motor-driven computer-controlled Maobi coating, respectively. The comparable photovoltaic performance indicates the potential of large-area cells prepared by motor-driven Maobi coating. Fig. 4b shows the J–V characteristic of the large-area solar module. The inset is the picture of the fabricated solar cells. The module containing 14 cells connected in series exhibits a V_OC of 11.6 V, a J_SC of 0.99 mA cm⁻², a FF of 0.55 and a PCE of 6.3%. The preliminary results show the potential of motor-driven computer-controlled Maobi coating for large-area (18 cm²) solar modules. This technique still needs to be further developed for the production of larger-area (>1 m²) solar cells and modules. The size and shape of the Maobi tool and coating conditions are to be optimized to achieve more uniform larger-area films. Continuous ink feeding for the Maobi coater is to be realized with an injector pump for uniform and high-throughput production.

Fig. 4 (a) Picture of the motor-driven, computer-controlled Maobi coating setup. Two videos are provided in the ESI† to show the automatic writing and coating process of the organic layers with this setup; (b) J–V characteristic of an 18 cm² solar cell, where the active layer was fabricated with motor-driven, computer-controlled Maobi coating. The inset is a picture of a fabricated 18 cm² solar module.

3. Conclusions

In this work, we demonstrate a novel solution-processed technique to fabricate organic solar cells and modules based on the “Maobi” tool (also called “Chinese ink brush”) inspired by an old Chinese calligraphy tradition. The film thickness of the Maobi-coated active layers and polymer electrodes can be tuned over a broad range from a few tens of nanometers to one micrometer, which meets the critical requirement for the film thickness of these layers to achieve optimized photovoltaic performance. Organic solar cells with a Maobi-coated PBDB-T:ITIC layer exhibit a power conversion efficiency of 10.1%, which is comparable to that of devices with a spin-coated active layer. Furthermore, after the optimization of the polymer electrode and the addition of the surface modifier into the active layer solution, we realized the organic solar cells via a two-step Maobi coating process and further solar modules. The demonstrated Maobi-coated solar modules containing 8 sub-cells exhibit a V_OC of 6.30 V and a high FF of 0.71. Large-area solar modules (18 cm²) fabricated via motor-driven, computer-controlled automatic Maobi coating display a V_OC of up to 11.6 V and an efficiency of 6.3%. The coating technique based on Maobi is low-cost, easy to pattern, large-area scalable and has a high utilization rate of solutions. This technique is promising for the application of organic solar cells as well as other printed electronics.

4. Experimental section

4.1 Materials

The Maobi is made from weasel hair and the handle is made of “White Bamboo” (A-61, Shuangxi Hubi) produced in Huzhou, China. P3HT (M_w = 60–80 kDa), ICBA and PCBM were received from Luminescence Technology Corp. PTB7-Th (M_w = 50 kDa) was purchased from 1-Material. PBDB-T (M_w = 11.5 kDa) and ITIC were purchased from Solarmer Energy Inc. PH1000 was purchased from Heraeus. PEI (M_w = 25 000 kDa) was purchased from Sigma-Aldrich. Silver slug (99.999%) and molybdenum oxide (99.9%) for evaporation were received from Alfa Aesar. All materials were used without further purification.

The PEDOT:PSS solution is prepared by mixing PH1000, ethylene glycol and surfactant PEG-TmDD as we reported earlier.³⁷ The P3HT and ICBA blend film was prepared from a chlorobenzene solution of P3HT : ICBA (1 : 1, wt%) with a total concentration of 40 mg ml⁻¹. The BHJ of PTB7-Th : PC₇₁BM (1 : 1.5, wt%) was prepared from a chlorobenzene/1,8-diiodoctane (97 : 3 by volume) solution (a total concentration of 25 mg ml⁻¹). As for the active layer of PBDB-T:ITIC, 10 mg PBDB-T and 10 mg ITIC were dissolved in 1 ml chlorobenzene/1,8-diiodoctane (99.5 : 0.5, by volume). When mixed with P3HT:ICBA, PEI was first diluted to 0.1 wt% in 2-methoxyethanol (Sigma-Aldrich, anhydrous, 99.8%) and then mixed at a 10 : 90 (PEI : BHJ) volume ratio. The ZnO precursor solution was synthesized according to the literature.⁵⁴

4.2 Fabrication of OSCs and modules

The P3HT:ICBA, PTB7-Th:PC₇₁BM, PBDB-T:ITIC and PEDOT:PSS films were Maobi-coated at a speed of 0.5 cm s⁻¹, 1.5 cm s⁻¹ and 2.5 cm s⁻¹, respectively. The substrate (ITO glass) temperature was changed from 30 to 100°. Film thicknesses that varied from 30 nm to 1 μm were obtained. Solar cells based on Maobi-coated PTB7-Th:PC₇₁BM and PBDB-T:ITIC are fabricated with the structure ITO/ZnO/BHJ/MoO₃/Ag. ZnO is spin-coated on pre-cleaned ITO glass at 2500 rpm and heated at 150 °C in air. Then the ZnO coated ITO glass is transferred into a glovebox and deposited with BHJ either by spin-coating or Maobi coating. The top electrode of 7 nm MoO₃ and 60 nm Ag was prepared on a thermal evaporator (Mini-Spectros, Kurt J. Lesker). The device area is 10 mm². For the cells with the Maobi-coated P3HT:ICBA active layer, transfer-printed PEDOT:PSS³⁸ was used as the top electrode with a device structure of ITO/PEI/P3HT:ICBA/PEDOT:PSS.

For the Maobi-coated solar cells, the structure of the cells is glass/ITO/P3HT:ICBA:PEI/PEDOT:PSS. P3HT:ICBA:PEI was coated at 1.5 cm s⁻¹ in a N₂-filled glove box and the substrate temperature was 50 °C, and the strip-shaped PEDOT:PSS electrode was then Maobi-coated onto the active layer at 50 °C. Before deposition of the aqueous PEDOT:PSS formulations, a flash (3 s) of plasma treatment (PDC-002, Harrick) was performed on the hydrophobic surface of the BHJ layer. The device area of the Maobi-coated solar cells is about 8 mm². For all-plastic fully Maobi-coated solar cells, the cell structure is PES/PEDOT:PSS/P3HT:ICBA:PEI/PEDOT:PSS, where PES/PEDOT:PSS was used to replace the previous glass/ITO substrates. For the Maobi-coated solar modules, the ITO electrode (a stripe width of 6 or 3 mm and a gap of 0.2 mm) was prepared by processing using laser ablation (KW FIB 20w, Kewei laser). Each Maobi-coating of the active layers and electrodes is shifted with an offset (about 0.5 mm) in relation to the previous coating that allows for the serial connection between the two different electrodes belonging to two adjacent sub-cells. The active area of the module (dead area not included) is about 1 cm². For the large-area (18 cm²) solar modules, the structure of the sub-cells is glass/ITO/ZnO/PBDB-T:ITIC/MoO₃/Ag, where Ag is used as the electrode instead of PEDOT:PSS to minimize the resistance effect because PEDOT:PSS is too resistive for large-area solar modules. The active layer of PBDB-T:ITIC was fabricated with a motor-driven, computer-controlled Maobi coating system in air. The area of the solar modules is 18 cm².

4.3 Characterization

Current density–voltage (J–V) characteristics were measured inside a N₂-filled glove box using a source meter (2400, Keithley Instruments) controlled by a LabVIEW program under AM 1.5G (100 mW cm⁻²) and a Newport solar simulator. Optical microscopic images were captured using an optical microscope (DM4000 M, Leica). The thickness values of the samples were recorded by a DEKTAK XT profilometer. The P3HT:ICBA thin films for the XRD measurements were prepared on quartz to avoid unnecessary diffraction peaks. The XRD measurements were performed using a Philips diffractometer (X’pert PRO MRD) equipped with a Cu Kα source. The absorbance spectra of the P3HT:ICBA films were recorded with a UV-Vis-NIR (near infrared) spectrophotometer (UV-3600, Shimadzu Scientific Instruments). The external quantum efficiency (EQE) spectra were measured by a standard system using a 150 W xenon lamp (Oriel) with a monochromator (Cornerstone 74004) as the monochromatic light source.

Author contributions

YHZ and LM conceived the project. LM, BWL and LLS worked on the setup of the motor-driven, computer-controlled Maobi coating system, and fabricated the large-area solar cells and modules. LM, JCF, FQ and LH worked on the small-area Maobi-coated solar cells and modules. SXX studied the morphology of the active layer using Maobi coating at different temperatures. YYJ and ZFL optimized the formulations of PEDOT:PSS for Maobi coating. LM and YHZ prepared the manuscript. All authors revised the manuscript and approved the submission.

Conflicts of interest

There area no conflicts to declare.

Acknowledgements

This work is supported by the Recruitment Program of Global Youth Experts, the National Natural Science Foundation of China (Grant No. 21474035 and 51773072), the Vinnova Marie Curie incoming project (2016-04112), the HUST Key Innovation Team for Interdisciplinary Promotion (Grant No. 2016JCTD111), the Open Fund of the State Key Laboratory of Luminescent Materials and Devices (South China University of Technology), and the China Postdoctoral Science Foundation funded project (2016M602289).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7mh00559h

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