An additive manufacturing approach based on electrohydrodynamic printing to fabricate P3HT:PCBM thin films

Additive manufacturing (AM) enables the production of high value and high performance components with applications from aerospace to biomedical fields. We report here on the fabrication of poly(3-hexylthiophene): phenyl-C61-butyric acid methyl ester (P3HT:PCBM) thin films through the electrohydrodynamic atomization (EHDA) process and its integration as absorber layer for organic solar cells. Prior to the film fabrication, the optimization of the process was carried out by developing the operating envelope for the P3HT:PCBM ink to determine the optimal flow rate and the appropriate applied voltage to achieve a stable-cone deposition mode. The EHDA printed thin-film’s topography, morphology and optical properties were systematically analyzed. The root-mean-square roughness was found to vary significantly with the annealing temperature and the flow rate and ranged from 1.938 to 3.345 nm. The estimated film mass and thickness were found between 3.235 and 23.471 mg and 597.5 nm to 1.60 µm, respectively. The films exhibited a broad visible absorption spectrum ranging from ~ 340 to ~ 600 nm, with a maximum peak λmax located at ~ 500 nm. As the annealing temperature and the flow rate were increased, discernible alterations in the PCBM clusters were consequently observed in the blends of the film and the size of the PCBM clusters has decreased by 3% while the distance between them was highly reduced by as much as 82%.

The technology of manufacturing highly efficient, lightweight, low-cost, and stable solar cells has progressed dramatically during the last two decades.One of the highly promising potential manufacturing candidates is the additive manufacturing (AM), which involves the layered production of objects.It is seen as a revolutionizing manufacturing owing to its unique ability to fabricate complex shapes and produce accurate systems 1 .Nowadays, human tissues, microelectromechanical systems (MEMS), biomaterials, food and much more are being produced with the help of AM.Among the different types of AM techniques currently in use in the industry 2 , micro/ nanoscale 3D printing technologies such as electrohydrodynamic printing (EHDP), direct ink writing (DIW), two-photon lithography, and projection micro stereolithography (PμSL) have rapidly attracted the attention of researchers and have been studied in various areas, including solar cells 3 .
OSCs are classified according to their active layer architecture, i.e. bi-layer heterojunction and bulk-heterojunction (BHJ) 4 .Bi-layer heterojunction structures involve stacking donor and acceptor (D/A) materials on top of each other, whereas BHJ structures involve blending D/A materials to form the active layer of OSC.Bi-layer devices are limited by the D/A exciton diffusion length, where excitons are only generated at the interface of the two materials.In contrast, the active layer of BHJ structures contains both D/A materials blended together, providing greater flexibility for exciton diffusion 5 .Indeed, BHJ active layers have drawn increasing attention because of their multiple advantages such as high mechanical flexibility, light weight, semi-transparency and high-throughput fabrication 6,7 .Driven by the rapid development of organic photovoltaic materials and device engineering, OSCs have achieved power conversion efficiencies (PCEs) of over 18% 8 .Despite the great success in PCE enhancement, the practical applications of OSCs are limited by their low operational stability 9 .Indeed, the degradation of photoactive layer morphology under various environmental conditions (i.e., light, oxygen and heat) after long-term exposure remains a grand challenge for the commercialization of OSC technology 10 .More specifically, highly efficient OSCs require an optimized BHJ morphology with abundant electron donor/ electron acceptor (D/A) interfaces for efficient exciton dissociation 11 .However, the intrinsic instability of the blend photoactive layer commonly occurs during prolonged illumination or thermal aging 12 .
In order to meet the requirement of high performance and stability, it is imperative to develop effective strategies to achieve a stable and efficient optimized BHJ morphology 13 .Although the photon-to-charge conversion process is significantly complex and involves several steps, such as exciton diffusion and dissociation, charge transfer and transport, and charge recombination and collection 14 , it is well known that the morphology of BHJ blend film plays a key role in all these processes 15 .Furthermore, the morphology is also tightly correlated with the device lifetime (i.e.stability) which is crucial for determining whether OSCs can get access into the consumer market 16 .
In this regard, great efforts have been particularly devoted to optimize the manufacturing process.Poly(3hexylthiophene) (P3HT) and [6, 6]-phenyl C61 butyric acid methyl ester (PCBM) are two fundamental materials widely utilized in BHJ OSCs.P3HT is a conjugated polymer that functions as the electron donor, whereas PCBM is a fullerene-based polymer that acts as the electron acceptor [17][18][19] .The state-of-the-art conjugated polymer can significantly benefit in fabrication and processing of OSCs.Several wet-process coating methods can be used in the fabrication process of the polymer thin films, such as casting, spin-coating, blading, screen printing, inkjet printing, spray-coating and roll-to-roll technique [20][21][22] .
Spin coating is one of the most common approaches in research studies in solar cell fabrication, because of its capability to produce a thin film and wide range of materials, by fine-tuning of the parameters such as spin speed and spin time etc.This method can produce film ranging from a few nanometers (30 nm-PAni film 23 , 40 nm-P3HT film and 30 nm-PCBM film 24 ) to micrometers (1-1.5 μm AZ3312-photoresist film 25 ).However, the effectiveness of the above techniques is limited due to various factors such as thickness output, vacuum usage, mask application, and material wastage 26 .A promising approach is electrohydrodynamic atomization (EHDA), which employs an electric field to generate charged droplets that can be sprayed onto a substrate in a controlled way.In addition, EHDA is highly sustainable and green process as it is carried out in ambient conditions (i.e.vacuum free) and uses minimal materials with virtually less material wastage 27,28 .
EHD process methods specifically the OSC field and photoelectric converters generally are considered premature processes.In this work, we aim to improve the EHD process and employed the EHDA process to fabricate an optimize the deposition of P3HT:PCBM inks for OSC active layer application.We assessed the compatibility and the jetting of P3HT and PCBM inks materials.We demonstrate that by varying the flow rate and the annealing temperature, significant changes in the mass, PCBM cluster size, the separation distance and the structure variations are triggered accordingly.

Materials and ink preparation
Regioregular P3HT (C 10 H 14 S) n (average molecular weight M w = 50-100 K, regioregularity = >90%, semiconductor properties = P-type (mobility = 1E−4 to 1E−1 cm 2 /V s) and PCBM (C 72 H 14 O 2 ) (Molecular weight = 910.88g/ mol, assay = >99.5%,semiconductor properties = N-type (mobility = 0.21cm 2 /V s) were purchased from Sigma Aldrich.The molecular structure of P3HT and PCBM are shown in Figure 1.The inks were prepared by diluting 20 mg of P3HT and 20 mg of PCBM, in 1 ml of chlorobenzene (CB), separately, and stirred at room temperature at 500 rpm using a magnetic stirrer for 45 min.The two ink solutions were then mixed in a 1:1 ratio, and the prepared P3HT:PCBM inks in CB were put onto a shaker for 24 h before use.Corning Plain glass was used as a substrate, which was first cleaned ultrasonically with ethanol, di-water and acetone for 10 min respectively.Before transferring to different solvents, the substrate was oven-dried at 50 °C for 10 min.

P3HT:PCBM and P3HT:PCBM deposition
To assess the printability of the developed inks via EHDA, the jetting produced by each individual ink, namely P3HT, PCBM, and P3HT: PCBM blend was analyzed.The schematic diagram of the fabrication setup is depicted in Fig. 2.An EHDA printer (NanoNC ESDR300HP) equipped with an ink supply system consisting of a syringe pump loader (eS-Pump: ESP200P), a syringe for storing ink (HENKE SASS WOLF, 10 ml (12 ml) NORM-EJECT) with a 27-gauge metallic nozzle from NanoNC is used.The inner and outer diameters of the metallic nozzle were 200 and 400 µm, respectively.Prior to the use, the metallic nozzle was cleaned with acetone in an ultrasonic bath for 30 min to remove any impurities.A high-voltage power supply connected to the metallic nozzle amplifies and creates an electrical force between the nozzle tip and the grounded substrate platform stage.A programmable stage was utilized to control the movement speed of the substrate and camera with a zoom lens to identify the cone-jet transition of the ink.The offset distance between the nozzle and substrate was kept at 20 mm and the flow rate of ink deposition was tested between 10 and 30 µl h −1 , to identify the jetting interval.
P3HT:PCBM inks were sprayed onto a glass substrate via EHDA at a flow rate of 30 µl h −1 and 20 µl h −1 , with a substrate movement speed of 20 mm h −1 .The sprayed ink was printed in 40, 20, and 10 passes.The deposited materials were sintered in a muffle furnace at 150 ℃ and 175 ℃ for 15 min.Table 1 provides an overview of the printing parameters utilized in the production of the P3HT:PCBM thin films.

P3HT:PCBM thin film characterization
The root-mean-square roughness (Rq) and topography of EHDA-printed thin films were measured by atomic force microscopy (AFM, Enviroscope Veeco digital instrument), in a tapping mode, with a scanning size of 4 µm × 4 µm.Surface morphologies and cross-sectional images for thickness estimation, were characterized using scanning electron microscopy (SEM, Tescan vega3 LMH) operated at a very low accelerated voltage of 2 kV.The light absorbance and transmittance spectra of the P3HT:PCBM thin films were obtained with a UV-Vis spectrometer (Perkin Elmer LAMDA 650) in the 300-800 nm range.The deposited materials' weight was measured using an analytical microbalance with a resolution of 1 mg.
ImageJ software was used to analyze and measure the PCBM patch size and distribution from the sample SEM images.PCBM sizes were measured by calculating the diameter of each patch.The average cluster size was measured by averaging multiple PCBM patches from the sample SEM image.While PCBM distribution distances were measured by averaging the distance between the PCBM patch with its neighbouring patches.

P3HT, PCBM and P3HT:PCBM sprayability
To optimize the fabricating process and develop an operating envelope for P3HT, PCBM, and P3HT:PCBM films, EHDA was performed at different flow rates ranging from 10 to 30 µl/h, with increments of +5 µl/h (Fig. 3).Different jetting modes were observed at various flow rates and various applied voltages to the nozzle, including dripping, microdripping, cone-jet, unstable cone-jet, precession and multijet modes.Dripping mode occurs when the ink droplets are released from the nozzle with a diameter larger than the nozzle size.This mode was observed for P3HT (Fig. 3a), PCBM (Fig. 3b), and P3HT:PCBM (Fig. 3c) inks at voltage ranges of 0 to ~3.5 kV, 0 to ~2.5 kV and 0 to ~2.8 kV, respectively.Microdripping mode occurs where deposited droplets are smaller than the  nozzle size.This mode was observed for P3HT, PCBM, and P3HT:PCBM inks at voltage ranges of 3.5 to 4.0 kV, 2.4 to ~ 4.0 kV, and 2.5 to 3.3 kV, respectively.Stable cone-jet is the ideal jetting mode for EHDA deposition and was observed for P3HT and P3HT:PCBM ink at voltage ranges of 4.0 to 4.9 kV and 3.3 to 4.0 kV, respectively.However, PCBM ink produced an unstable cone-jet, which alternated between cone-jet and microdripping mode, at voltage ranges of 3.5 to 4.5 kV.Precession jet and multijet were observed from the all three developed inks at voltage ranges of > 4.6 kV, > 4.5 kV and > 4.0 kV, respectively.The precession jet occurs when the meniscus starts to skew, while multijet is a phenomenon where multiple streams, at least two, emanate from the nozzle and the number of points increases with an increase in voltage.Figure 3d illustrates the jetting profile of the inks, dripping, microdripping, unstable-cone jet, stable cone-jet, precision jet and nultijet respectively.
After conducting a sprayability analysis on the inks, it was found that only P3HT ink (Fig. 3a) and P3HT:PCBM (Fig. 3c) ink were capable of generating a stable cone jet mode, which is the optimal mode for creating a thin film via EHDA deposition process.In contrast, PCBM ink (Fig. 3b) was unable to achieve the cone-jet mode.Consequently, the sprayability analysis indicates that the EHDA deposition method is the sole viable option for creating a bulk-heterojunction thin film, rather than utilizing bilayer deposition for organic solar cell applications.

EHDA deposited P3HT:PCBM thin film
Figure 4, illustrates the energy level of P3HT:PCBM blend and proposed device architecture for future works.In this current work, we have utilized EHD printing to prepare the P3HT:PCBM blend thin film.The highest occupied molecular orbital (HOMO) of P3HT is typically higher than the HOMO of PCBM, and the lowest unoccupied molecular orbital (LUMO) of PCBM is generally lower than the LUMO of P3HT.As specified by the manufacturer HOMO and LUMO for P3HT, 5.0 eV and 3.0 eV respectively, and PCBM, 6.1 eV and 3.7 eV respectively.
Figure 5 shows the AFM surface topography results for EHD-printed P3HT: PCBM films with varying printing parameters and annealing temperatures.Rq is used as a parameter to quantitatively characterize the surface roughness of the fabricated films.Table 2 shows that the films annealed at 150 °C (PP1, PP3, and PP5) exhibit lower Rq values-ranging from 1.938 to 2.352 nm, compared to those annealed at a higher temperature of 175 °C (PP2, PP4, PP6, and PP7), which show Rq values ranging from 2.416 to 3.910 nm.Thicker films annealed at lower temperature exhibit lower Rq values.However, the 10-layer film (PP8) annealed at 150 °C exhibits a higher Rq of 4.316 nm, which might be due to the presence of voids or pinholes in the deposited films caused by material's evaporation during the printing or annealing process.Furthermore, the comparison of fabricated films at flow rates of 30 and 20 µl/h (specifically, the PP1 & PP3, PP2 & PP4 and PP7 & PP6 samples) did not demonstrate a significant impact on the Rq (Rq was mainly affected by the number of printing passes and by the annealing temperature).EHDA printing flow rate has been found to have a significant effect on the distribution of the PCBM cluster, as demonstrated in Table 2. Specifically, at the same annealing temperature, the cluster average size was reduced by 82% (PP1 & PP3), 68% (PP2 & PP4) and 59% (PP7 & PP6).The size of PCBM cluster distribution in the fabricated films is significantly reduced with higher annealing temperatures, as a result of PCBM segregation from P3HT 29 .The SEM images in Fig. 6 illustrates the impact of annealing temperature on EHDA-deposited PCBM.At lower annealing temperatures, PCBM clusters have an average size between 2.888 to 6.408 µm 2 , forming a flakes-like shape.However, at a higher annealing temperature, the size of the cluster was reduced by 82% and formed a circular shape.Figure 7a, b show the spatial distribution of the PCBM cluster as measured from the center of the cluster to its neighboring.Table 2 shows the results of the EHD printed thin films.Higher annealing temperature shows PCBM cluster has an average size of up to 0.13 µm 2 , results show the distribution of PCBM cluster approximately 1.50 to 3.60 µm.Whereas lower annealing temperature shows a large PCBM size of up to 6.40 µm 2 and distributed approximately ~ 3.60 to 8.20 µm apart from other PCBM cluster patch.In sum, the distribution of the PCBM cluster is mainly resulting from the  Additionally, Table 2 shows the measured mass and calculated thickness of the thin films.Increasing the annealing temperature has resulted in a significant reduction in the film' mass, due to its evaporation.A reduction in mass of 48%, 73%, and 83% associated to printing parameters of 30 µl h −1 @20layers, 20 µl h −1 @20layers and 30 µl h −1 @40layers, respectively, was observed.This in turns has decreased the thickness of the film.The thickness measured from the side profile with PP5 films having the thickest film at 1.60 µm (40 layers), and PP3 films having the thinnest film at 597.5 nm (20 layers).The thickness of the film was impacted by the slow substrate movement speed and offset distance of the printing.It should be noted printing parameters, including flow rate, substrate movement speed, offset distance, and annealing temperature, complement each other for producing a high quality thin film.
The absorption spectra of the EHD-printed P3HT:PCBM thin film at different parameter settings were then examined and results were displayed in Fig. 8a, b. Figure 8a illustrates the absorption spectra for film annealed at 150 °C and Fig. 8b shows the absorption spectra film annealed at 175 °C.The films were found to exhibit a broad visible spectral band spanning from 300 to 650 nm.In a typical PCBM film, there is an absorption peak located at 335 nm, and the primary absorption band of the P3HT material is located at 515 nm, with two shoulders at 558 nm and 609 nm 30 .The absorption of the films was proportional to the number of layer passes.The 40-pass films (PP5 and PP7) showed absorbance peaks (λ max ) at 340 nm and broad features in the 500 nm region.When printed at a higher flow rate, the PP7 film has an absorbance peak at 340 nm and λ max at ~ 480 nm.The PP5 film printed at a lower flow rate exhibited a good absorbance spectrum up to 600 nm, with a blueshift observed at ~ 320 nm and λ max at ~ 520 nm.The λ max of the PP5 film also showed a red shift and two shoulders located at ~ 590 and ~ 610 nm.Similar trends were found for the P3HT:PCBM films which were annealed at 150 °C.The presence of a slight peak shoulder indicated a strong interchain interaction between the P3HT chain and the PCBM in the blend 31 .Increasing the annealing temperature of the composite film resulted in red-shifted absorption spectra that matched the absorption spectrum of a P3HT film 29 .Results from UV-Vis in Fig. 8 and the study of the morphology of P3HT:PCBM thin film in Table 2 show a higher annealing post-process of the deposited materials increases the surface roughness of the thin film.A correlation effect between the surface roughness and the absorbance performance has been studied, it has shown higher surface roughness and non-uniform films help to increase the light absorption of the thin film with minimal reflection 32,33 .Previous researchers have reported that the distribution of P3HT and PCBM in the active layer can improve the power conversion efficiency of the OSCs.This can be achieved by shortening the path of electrons and holes transport between the cathode and the anode [34][35][36] .

Conclusion
Additive manufacturing EHDA technique was developed to fabricate a P3HT:PCBM thin film for OSCs application.The optimized fabrication process allowed generating a stable cone jet under various operating conditions for P3HT:PCBM in a chlorobenzene ink.The effect of flow rates on the distribution of PCBM clusters in the P3HT:PCBM blends ink at different thicknesses and annealing temperatures was also investigated.The printing flow rate was observed to reduce the mean size of the PCBM cluster up to ~ 82% as the flow rate increased.The annealing temperature also played a critical role in the post-processing of the film in terms of Rq roughness measurement.Indeed, the EHDA thin film had an Rq roughness measured at ~ 1.938 to 4.316 nm, at different printing and post-process.The mass of the deposited P3HT:PCBM has decreased from ~ 50% to 83% with respect to the annealing temperature.The film thickness was measured at approximately 597.5 nm to 1.60 µm, depending on the printing parameters.The optical properties of the fabricated films show a broad absorption covering the entire visible region which is very relevant for OSC application.The peaks at 330 nm and λ max ~ 500 nm, with two shoulders located at ~ 590 and 610 nm, indicated the absorption bands of PCBM and P3HT, respectively.Ongoing work is currently focusing on the EHDA fabrication of full OSCs and the effect of the PCBM clusters distribution in the photoactive layer on the device performance.

Figure 4 .
Figure 4. Energy levels alignment and the future proposed device architecture.