Vacuum Electrospray Deposition for Face-on Orientation and Interface Preservation in Organic Photovoltaics

doi:10.21203/rs.3.rs-5308209/v1

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Vacuum Electrospray Deposition for Face-on Orientation and Interface Preservation in Organic Photovoltaics

https://doi.org/10.21203/rs.3.rs-5308209/v1

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Despite recent advancements in organic photovoltaics (OPVs), further improvements in power conversion efficiency (PCE) and device lifetime are necessary for commercial viability. Strategies such as optimizing the molecular orientation and minimizing the charge traps of organic films are particularly effective in enhancing photovoltaic performance. In this study, we successfully utilized vacuum electrospray deposition (VESD) to achieve favorable face-on stacking geometries while preserving the integrity of the interfaces in poly(3-hexylthiophene-2,5-diyl) (P3HT):[6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) bulk heterojunction (BHJ) films. Unlike conventional spin-coated (SC) P3HT:PCBM BHJ films, which predominantly exhibit an edge-on orientation, VESD facilitates a beneficial face-on orientation, improving vertical charge transport through enhanced π-π stacking interactions. Furthermore, VESD effectively eliminates residual solvents during film formation, ensuring well-defined interfaces between the layers in the OPV devices. As a result, the VESD OPVs demonstrated enhanced PCE and extended operational lifetimes compared to their SC counterparts. Impedance spectroscopy analysis confirmed that the VESD OPVs possessed significantly higher electron mobility and longer electron lifetimes, indicating reduced charge traps and improved charge dynamics. These results highlight the potential of VESD as a versatile technique for controlling molecular orientation in solution-processable organic semiconductors, enabling the development of highly efficient devices with fewer charge traps without relying on synthetic or epitaxial methods.

Physical sciences/Materials science/Materials for devices

Physical sciences/Materials science/Soft materials/Organic molecules in materials science

vacuum electrospray deposition

organic photovoltaics

face-on stacking

device lifetime

Over the past two decades, organic photovoltaics (OPVs) have attracted considerable attention as promising renewable energy sources owing to advantages such as strong light absorption, short energy payback times, lightweight design, mechanical flexibility, and low-cost manufacturing^1–4. Recently, OPVs utilizing bulk heterojunction (BHJ) architectures with narrow-bandgap materials have achieved power conversion efficiencies (PCEs) exceeding 19%⁵. However, several challenges remain, including further PCE improvements, extension of device lifetimes, and the development of scalable fabrication methods, all of which are essential for the commercial viability of OPVs. Consequently, significant efforts are underway to overcome these hurdles^6–8.

An effective method for enhancing the photovoltaic performance of organic films involves controlling their morphology⁹. The optimal molecular orientation depends on the charge transport direction; a face-on orientation is preferred for vertical transport in OPVs and organic light-emitting diodes, whereas an edge-on orientation is advantageous for lateral transport, as observed in organic field-effect transistors¹⁰. Techniques such as atomic substitution and solvent processing have been employed to manipulate the molecular orientation in OPVs^11,12, resulting in significant improvements in short-circuit current density (J_SC) and fill factor (FF) by boosting exciton generation and charge transport. However, atomic substitution requires extensive synthetic efforts to achieve the desired morphology, and solvent processing may not be suitable for uniform large-scale deposition. Another approach involves the use of a template layer, such as graphene, on the electrode to guide molecular alignment¹³. However, this method can alter the interfacial energy levels of the material by modifying the surface properties of the device, potentially undermining its performance. Therefore, developing alternative approaches for morphology control is crucial for optimizing the device architecture while maintaining the material properties. For instance, modifying the deposition method to adjust the molecular solidification can be an efficient strategy for morphological engineering¹⁴.

Significant efforts have been made to extend the operational lifetime of OPVs^15,16. Various factors contribute to performance instability during electric power generation, and minimizing charge traps is important for improving both the PCE and device longevity^17,18. Defect-induced trap sites hinder charge transport and promote charge recombination, leading to significant performance degradation. These defects may arise from external sources such as oxygen and moisture infiltration through the top electrode or from intrinsic sources such as material impurities or grain boundaries¹⁹. Solution-processed OPVs are particularly susceptible to the formation of charge traps at interfaces where layer mixing may occur. Moreover, solvent residues in the organic film can act as charge trap sites, reducing the carrier mobility and lifetime. Ensuring interface integrity through controlled solvent removal during deposition is a key strategy for significantly enhancing the device's performance.

In this study, we demonstrate face-on orientation and interface preservation of poly(3-hexylthiophene-2,5-diyl) (P3HT):[6,6]-phenyl C₆₁ butyric acid methyl ester (PCBM) BHJ films using vacuum electrospray deposition (VESD). This technique enables the deposition of soluble materials without residual solvents, thereby maintaining the interface integrity. Although electrospray deposition under ambient conditions has recently been explored for device fabrication²⁰, a vacuum-integrated system can offer improved solvent removal and compatibility with other industry-standard vacuum-based techniques. The orientations of P3HT and PCBM were analyzed using glazing incidence wide-angle X-ray scattering (GIWAXS) complemented by UV-vis absorption measurements. Cross-sectional transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDS) were employed to examine the interfacial properties of the VESD-prepared P3HT:PCBM BHJ films in OPVs. The improved photovoltaic performance and device lifetime of the VESD OPVs were compared with those of conventional spin-coated (SC) OPVs. Impedance spectroscopy analysis further confirmed the reduced charge traps in the VESD OPVs. Although P3HT:PCBM is a classical combination that exhibits a moderate PCE below state-of-the-art levels, its well-established material properties render it valuable for exploring new deposition methods and assessing the performance based on modified characteristics^21–23. This study aims to develop innovative strategies for enhancing the optoelectronic properties of organic materials using the VESD method, which has potential applicability to a variety of molecules and polymers.

VESD system

The VESD system integrates an electrospray mechanism for a diverse range of material deposition²⁴ in a vacuum chamber. As illustrated in Fig. 1, our home-built VESD system consisted of three differential pumping stages, each featuring pinholes of varying diameters (1 mm for the first stage and 2 mm for the second and third stages). The prepared P3HT:PCBM solution was injected into the inlet pinhole through a capillary during the first stage of the vacuum system. N₂ gas was flowed counter to the electrospray from the concentric pipe outside the inlet, stabilizing the electrospray process. Chlorobenzene (CB) was used as the solvent for P3HT:PCBM deposition because of its low boiling point and viscosity. When the VESD process begins, the injected solution undergoes Coulombic explosion, generating fine droplets that enter the vacuum chamber. The solvent evaporates before reaching the substrate, resulting in the deposition of only dry solutes through a shadow mask that defines the area of the light-absorbing layer. The detailed setup can be found in our previous report²⁵.

P3HT:PCBM BHJ film preparation

Indium tin oxide (ITO)-coated glass substrates (sheet resistance: 15 Ω sq^− 1; AMG, South Korea) were sequentially cleaned with ultrasonication in deionized (DI) water, acetone, methanol, detergent, and DI water again, with each step lasting 10 min. The VESD BHJ films were deposited from a mixed solution of P3HT (regioregular; Sigma Aldrich Inc., United States) and PCBM (purity: 99.5%; Nano-C Inc., United States) at a 1:1 weight ratio, dissolved in CB (purity: 99.99%; Sigma-Aldrich Inc., United States) in a vacuum chamber maintained at a working pressure of 10^− 4 Torr. The capillary bias was set to 2.5 kV, and the total volume of the injected solution was 0.7 mL. The total thickness of the P3HT:PCBM film was estimated to be 200 nm, as determined from ionic current measurements using a Keithley 6485 picoammeter (Tektronix Inc., United States)²⁵ and a surface profiler. For comparison, SC BHJ films were prepared under ambient conditions from the P3HT:PCBM solution at a concentration of 11 mg mL^− 1 using a spin rate of 4000 rpm for 60 s, achieving the same 200 nm thickness. After depositing the P3HT:PCBM layer, the samples were annealed at 150 ℃ for 20 min in the vacuum chamber.

Film characterization

GIWAXS measurements were conducted at the 3C beamline of the Pohang Accelerator Laboratory (South Korea) using a two-dimensional CCD detector (MarCCD-165, Rayonix LLC). An incidence angle of 0.2° was selected to ensure complete penetration of X-rays into the P3HT:PCBM films, employing monochromatic X-ray radiation with a wavelength of 1.165 Å. The UV-vis absorption spectra were measured using a V-650 spectrophotometer (JASCO Co., Japan). The morphology and crystal structure of the P3HT:PCBM films were examined using TEM (Tecnai F20, FEI Co., United States). EDS (Tecnai F20, FEI Co., United States) was performed to assess the chemical composition of the P3HT:PCBM films.

OPV fabrication and characterization

The OPVs were fabricated with a conventional structure of ITO/MoO₃ (10 nm)/P3HT:PCBM (200 nm)/LiF (0.4 nm)/Al (200 nm), where ITO served as the anode and Al as the cathode. The MoO₃ (purity: 99.99+%; Sigma-Aldrich Inc., United States) hole extraction layer was deposited onto a cleaned ITO glass substrate via thermal evaporation in a vacuum chamber at a base pressure of 2 × 10^− 7 Torr and a deposition rate of 0.8 nm s^− 1. After depositing MoO₃, a P3HT:PCBM layer was deposited using either VESD or SC. The VESD film was deposited under vacuum without breaking the vacuum, whereas the SC film was deposited under ambient conditions after exposure to air. Subsequently, the LiF (purity: ≥ 99.99%; Sigma-Aldrich Inc., United States) electron extraction layer at a rate of 0.01 nm s^− 1 and the Al (purity: 99.999%; Taewon Scientific Co., South Korea) cathode at rates > 0.1 nm s^− 1 were deposited onto the sample in the vacuum chamber. The deposition rates and total thicknesses of the thermally evaporated materials were monitored using a quartz-crystal microbalance. The fabricated OPVs were annealed at 150°C for 20 min in a vacuum chamber. The device area was 0.2 × 0.2 cm². The current density–voltage (J–V) characteristics were measured using a Keithley 2400 source measure unit (Tektronix Inc., United States) under AM 1.5G, 1-sun illumination using a K401 solar simulator (McScience Inc., South Korea). Impedance spectroscopy was performed using a Solartron SI 1260 impedance/gain-phase analyzer (AMETEK Inc., United States).

GIWAXS 2D diffraction patterns were collected to compare the molecular orientations of the SC and VESD P3HT:PCBM films. Figure 2 shows the GIWAXS results for both the SC and VESD P3HT:PCBM films before and after annealing. P3HT typically exhibits two primary orientations: face-on and edge-on²⁶. In Fig. 2(a), the unannealed SC film displays (h00) diffraction peaks attributed to the lamellar structure with a spacing of ~ 1.6 nm in the out-of-plane direction. These peaks are located at q_z ~ 0.38 and 0.75 Å⁻¹, corresponding to the primary (100) and secondary (200) reflections of P3HT^27,28. This finding suggests that P3HT adopts a thermodynamically favorable edge-on orientation early in the SC process^29,30. Additionally, a broad, diffuse symmetric ring was observed at q ~ 1.4 Å⁻¹, indicating the presence of randomly oriented nanoscale PCBM domains^31,32. Ring patterns at q ~ 1.5, 2.2, and 2.5 Å⁻¹ are attributed to the ITO substrate³³. After annealing [Figure 2(b)], the diffraction patterns sharpened, reflecting improved crystallinity. A weak (010) π-π stacking peak in the out-of-plane, with a spacing of ~ 3.7 Å and located at q_z ~ 1.7 Å⁻¹, also appears. Consequently, the annealed SC film exhibits a predominantly edge-on orientation in the mixed phase.

In contrast, the unannealed VESD film showed no significant diffraction patterns in either the out-of-plane or in-plane directions [Figure 2(c)]. However, after annealing [Figure 2(d)], there was a remarkable improvement in both P3HT crystallinity and the formation of nanoscale PCBM domains. Notably, distinct (010) π-π stacking in the out-of-plane and (h00) lamellar stacking in the in-plane diffraction patterns emerge. The prominent (010) π-π stacking peak indicates a substantial increase in face-on orientation, which is likely to enhance vertical charge transport due to stronger π-π interactions. These results clearly demonstrate that VESD, when combined with annealing, yields better crystallinity and a more pronounced face-on orientation compared to SC. The rapid solvent drying characteristics of VESD may have contributed to this difference.

Figure 3 shows the UV-vis absorption spectra of the P3HT:PCBM films deposited via SC (blue) and VESD (red) before (dashed line) and after (solid line) annealing. All four spectra display absorption features originating from PCBM at approximately 345 nm and P3HT at approximately 500 and 600 nm. For both the SC and VESD films, annealing enhanced the absorption intensity of P3HT. The shoulder around 600 nm, known as interchain absorption and attributed to ordered P3HT domains^34,35, is accompanied by a red shift in the absorption feature near 500 nm. The intensity of the absorption at 600 nm served as an indicator of P3HT ordering and crystallization. This feature behaves differently depending on the film preparation method (SC or VESD). Before annealing, the SC film exhibited a small but distinct shoulder at 600 nm, suggesting the crystallization of P3HT during the SC process. In contrast, the VESD film exhibited no such signal, indicating that it was amorphous. After annealing, the VESD film demonstrated a higher overall absorption, including the shoulder at 600 nm, than the SC film. These results are in good agreement with the GIWAXS results, suggesting a higher crystallinity and molecular ordering in the VESD film.

Based on the GIWAXS and UV-vis results, the initial state of the VESD film before annealing appeared to be almost amorphous, with highly mobile P3HT chains. These P3HT chains likely exhibit stronger intermolecular interactions than interactions with the substrate, adopting a face-on orientation after annealing. Further studies are required to understand the fundamental mechanisms underlying the molecular orientation of organic films prepared using VESD.

As mentioned previously, the VESD method effectively eliminates the solvent from the injected solution droplets through differential pumping before they reach the substrate, thereby helping to preserve the interface. To evaluate the effect of the residual solvent, cross-sectional TEM images were obtained for both the anode/light-absorbing layer and the light-absorbing layer/cathode interfaces. Figure 4 shows TEM images of the ITO/MoO₃/P3HT:PCBM (anode) and P3HT:PCBM/LiF/Al (cathode) interfaces. In the SC sample [Figure 4(a)], interfacial mixing between the MoO₃ and P3HT:PCBM layers is evident as a darker region, which is probably caused by the interaction of the residual solvent with the underlying MoO₃ layer. Similarly, at the cathode interface, the evaporation of the residual solvent through the Al layer results in damage, which is also manifested as a darker region. In contrast, the VESD method deposits a P3HT:PCBM layer onto MoO₃ without residual solvent, thereby maintaining an intact interface, as shown in Fig. 4(b). The same is true for the cathode interface.

To determine the elemental composition of the interfacial region, EDS measurements were conducted in two areas: near the anode (labeled 1) and near the cathode (labeled 2). As shown in Fig. 4(c), the Mo and Al contents were significantly higher in the SC sample than in the VESD sample. This increase can be attributed to the residual solvent, which facilitates the diffusion of materials from the hole extraction and cathode layers into the light-absorbing layer. Such diffusion hinders charge transport and reduces the operational lifetime because these mixed-element regions act as charge trap sites, accelerating device degradation.

To investigate the effects of face-on orientation and interface preservation of the VESD film on the OPV performance, devices with an ITO/MoO₃/P3HT:PCBM/LiF/Al structure were fabricated by incorporating either the VESD or SC light-absorbing layer. The J–V characteristics under AM 1.5G, 1-sun illumination are presented in Fig. 5, and the corresponding photovoltaic parameters are summarized in Table 1. The VESD OPV exhibited a higher J_SC of 8.51 mA cm^− 2 and a FF of 60.0% compared to the SC OPV, which had a J_SC of 7.94 mA cm^− 2 and a FF of 57.9%. Both devices exhibited the same open-circuit voltage (V_OC) of 0.61 V. Consequently, the VESD OPV achieved a higher PCE of 3.11% compared to 2.80% for the SC OPV. Based on the measured J–V characteristics, the series resistance (R_S) of the VESD OPV was calculated to be 8.84 Ω cm², which is lower than that of the SC OPV (10.31 Ω cm²). This reduction in R_S is primarily attributed to the improved charge transport facilitated by the face-on stacking geometry and the intact P3HT:PCBM/electrode interfaces with minimal trap sites. These advantages of the VESD film contributed to the higher J_SC and FF values observed for the VESD OPV compared to the SC OPV.

The device lifetimes of SC and VESD OPVs were also compared. The OPVs were stored under ambient conditions without encapsulation, and their PCEs were measured after 3, 6, 9, and 12 days. As shown in Fig. 5(b), the PCE of the VESD OPV degraded much slower than that of the SC OPV. This difference in device lifetime can be attributed to the interfacial diffusion of Mo and Al, as observed in the TEM images. The diffusion of Mo and Al atoms into the SC BHJ film creates charge-trap sites, accelerating the degradation of the light-absorbing layer. In contrast, the VESD-BHJ film maintains an intact interface with fewer trap sites, resulting in a significantly longer device lifetime.

Table 1

Photovoltaic parameters of SC and VESD OPVs.
	J_SC (mA cm^− 2)	V_OC (V)	FF (%)	PCE (%)	R_S (Ω cm²)
SC	7.94	0.61	57.9	2.80	10.31
VESD	8.51	0.61	60.0	3.11	8.84

The impact of traps on the charge dynamics was further investigated using impedance spectroscopy to assess the electron lifetime, which is a key factor in determining the current loss in photovoltaic devices. Figures 6(a) and 6(b) show the Nyquist plots of the SC and VESD OPVs under forward bias in the dark. The electron lifetime values were extracted by modeling an equivalent circuit that incorporated the diffusion-recombination mechanism, as shown in Fig. 6(c)³⁶. The equivalent circuit comprised several components: contact and wiring resistance (R_c), electron transport resistance (r_t), electron recombination resistance (r_rec), chemical capacitance due to excess electrons under bias (c_n), and geometrical capacitance (C_g). The fitting lines generated by the equivalent circuit closely match the experimental data plotted in Figs. 6(a) and 6(b).

From the impedance spectroscopy results, the electron diffusion time was calculated as the product of r_t and c_n, whereas the electron lifetime was derived from the product of r_rec and c_n. Using the Nernst-Einstein relationship, the electron mobility of the SC OPV was determined to be 2.1 × 10^− 3 cm² V^− 1 s^− 1, aligning with values reported in the literature^36,37. In contrast, the VESD OPV exhibited significantly higher electron mobility of 6.2 × 10^− 3 cm² V^− 1 s^− 1, approximately three times greater than that of the SC OPV. Furthermore, as depicted in Fig. 6(d), the electron lifetime of the SC OPV was calculated to be 0.20 ms at 0.50 V, in agreement with previous reports^36,37. This value decreases with increasing bias voltage. In comparison, the electron lifetime of the VESD OPV was 0.66 ms at 0.50 V, more than three times higher than that of the SC OPV and consistently higher across all measured bias regions. These results can be attributed to the reduced charge traps, as observed in the TEM images (Fig. 4), and they correlate well with the PCE and device lifetime results, as shown in Fig. 5. Collectively, all the data clearly demonstrate the superior performance of the VESD OPV compared with that of the SC OPV.

In this study, we presented a VESD method that effectively controlled molecular orientation and protected electrode interfaces. While SC P3HT:PCBM films exhibited an edge-on-dominant orientation, VESD P3HT:PCBM films displayed a face-on-dominant orientation, enhancing vertical charge transport through increased π-π stacking interactions. The GIWAXS and UV-vis results indicated that the initial state of the VESD films was amorphous, which may account for the observed morphological differences. TEM and EDS analyses revealed that the VESD method eliminated the residual solvent during deposition, preserving distinct interfaces between the layers in the OPV devices. In contrast, significant interfacial mixing and material diffusion were evident in SC OPVs. Consequently, the OPVs incorporating the VESD P3HT:PCBM layer exhibited higher PCEs and longer lifetimes compared to their SC counterparts. Impedance spectroscopy revealed that VESD OPVs have significantly higher electron mobilities and longer electron lifetimes compared to SC OPVs, confirming the reduced charge traps and improved charge dynamics that contribute to the superior performance of the VESD OPVs. Our findings clearly demonstrate that the VESD technique can effectively engineer the material properties required for OPVs and establish it as a promising method for various solution-processable organic semiconductors for future device fabrication.

Author Contribution

Y.L. performed the experiments and wrote the main manuscript text. J. J, K. J, J.L, Y.Youn, and S.P. assisted the analysis of the data. H.L. and Y.Yi supervised the project. All authors reviewed the manuscript.

Acknowledgements

This work was supported by the research projects of the National Research Foundation of Korea (Grant Nos. 2018R1A6A1A03025582, 2021R1A2C1009324, 2021M3H4A6A02050351, RS-2024-00335481, and RS-2024-00449682), Korea Institute for Advancement of Technology (KIAT) grant funded by the Korea Government (Ministry of Education) (PP0028088, Semiconductor-Specialized University), Samsung Display Company, and LG Display Company.

Data Availability

The datasets used and/or analyzed during the current study available from the corresponding author on reasonable request.

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Vacuum Electrospray Deposition for Face-on Orientation and Interface Preservation in Organic Photovoltaics

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