Photocatalysis is an extensively applied process that employs light energy to drive chemical reactions in various fields, including environmental remediation, water splitting for energy production, and organic synthesis. Achieving effective photocatalyst films necessitates careful considerations such as film thickness, uniformity, stability, and light-harvesting efficiency. Langmuir-Blodgett (LB) technology, a technique enabling precise control over molecular structure and layer thickness, proves advantageous in creating ultrathin devices with excellent electrical and optical properties, particularly using conjugated polymers like polythiophene.
Poly(3-hexylthiophene) (P3HT), a commonly used polymer in organic photovoltaics, encounters challenges related to monolayer aggregation and limited homogeneity. To address these issues, P3HT is often combined with Phenyl-C61-butyric acid methyl ester (PCBM), resulting in improved film properties and enhanced charge transfer pathways. Integrating PCBM into P3HT films produces a composite film with increased interfacial area, efficient charge separation, and enhanced light absorption, thereby enhancing photocatalytic performance.
In the P3HT/PCBM heterojunction, structure, arrangement, and contact between P3HT and PCBM greatly affect the electrical property. In this study, we demonstrate two different deposition techniques, including LB and spin-coating (SC) techniques. The LB technique facilitates the creation of a precise P3HT/PCBM heterojunction by accurately depositing P3HT and PCBM layers, enabling efficient charge transfer and collection. Meanwhile, the SC technique arranges PCBM within the active layer bulk, leading to substantial photocurrent generation. LB technique results in a ribbon-like structure, having P3HT nano fiber crystal and PCBM nanoparticle. On the other hand, the SC technique results in a domain structure. The effect of ratio and annealing temperature have been addressed. A ratio of 1:4 for P3HT and PCBM, respectively is found as the optimized ratio for LB and SC techniques. Meanwhile, the optimized annealing temperatures of LB and SC are 160 and 200°C, respectively. The thin film obtained from LB has a smaller band gap than the thin film obtained from SC. However, the photocurrent of the LB thin film has a lower photocurrent than that of the SC thin film, due to its inhomogeneous film distribution. This work can give guidance to obtain desired thin film P3HT/PCBM for photoelectrical application.

1. Oklobia, O. and T. Shafai, A study of donor/acceptor interfaces in a blend of P3HT/PCBM solar cell: Effects of annealing and PCBM loading on optical and electrical properties. Solid-state electronics, 2013. 87: p. 64-68.
2. Chen, X., S. Shen, L. Guo, and S.S. Mao, Semiconductor-based photocatalytic hydrogen generation. Chemical reviews, 2010. 110(11): p. 6503-6570.
3. Hisatomi, T., J. Kubota, and K. Domen, Recent advances in semiconductors for photocatalytic and photoelectrochemical water splitting. Chemical Society Reviews, 2014. 43(22): p. 7520-7535.
4. Falsetti, P.H.E., F.C. Soares, G.N. Rodrigues, D.M.S. Del Duque, W.R. de Oliveira, B.F. Gianelli, and V.R. de Mendonça, Synthesis and photocatalytic performance of Bi2O3 thin films obtained in a homemade spin coater. Materials Today Communications, 2021. 27: p. 102214.
5. Yang, S., L. Fan, and S. Yang, Langmuir−Blodgett Films of Poly(3-hexylthiophene) Doped with the Endohedral Metallofullerene Dy@C82:  Preparation, Characterization, and Application in Photoelectrochemical Cells. The Journal of Physical Chemistry B, 2004. 108(14): p. 4394-4404.
6. Park, J.Y., Nanomaterials fabrication and characterization using Langmuir monolayer and Langmuir-Blodgett films. 2009: University of Houston.
7. Wang, K.-H., W.-P. Hsu, L.-H. Chen, W.-D. Lin, and Y.-L. Lee, Extensibility effect of poly (3-hexylthiophene) on the glucose-sensing performance of mixed poly (3-hexylthiophene)/octadecylamine/glucose oxidase Langmuir-Blodgett films. Colloids and Surfaces B: Biointerfaces, 2017. 155: p. 104-110.
8. Kalonga, G., Optimization and characterization of Organic Solar cells based on Regio-P3HT and C66-PCBM blends. 2012. p. 1-99.
9. Zhang, L., K. Zhao, H. Li, T. Zhang, D. Liu, and Y. Han, Liquid Crystal Ordering on Conjugated Polymers Film Morphology for High Performance. Journal of Polymer Science Part B: Polymer Physics, 2019. 57(23): p. 1572-1591.
10. Zhao, Y., S. Shao, Z. Xie, Y. Geng, and L. Wang, Effect of poly (3-hexylthiophene) nanofibrils on charge separation and transport in polymer bulk heterojunction photovoltaic cells. The Journal of Physical Chemistry C, 2009. 113(39): p. 17235-17239.
11. Tsoi, W.C., S.J. Spencer, L. Yang, A.M. Ballantyne, P.G. Nicholson, A. Turnbull, A.G. Shard, C.E. Murphy, D.D.C. Bradley, J. Nelson, and J.-S. Kim, Effect of Crystallization on the Electronic Energy Levels and Thin Film Morphology of P3HT:PCBM Blends. Macromolecules, 2011. 44(8): p. 2944-2952.
12. Roberts, G., Langmuir-blodgett films. 2013: Springer Science & Business Media.
13. Pockels, A., On the relative contamination of the water-surface by equal quantities of different substances. Nature, 1892. 46(1192): p. 418-419.
14. Langmuir, I., The constitution and fundamental properties of solids and liquids. II. Liquids. Journal of the American chemical society, 1917. 39(9): p. 1848-1906.
15. Xu, G., Syntheses and Analyses of Materials at Interfaces: From Organic Soft Materials to Inorganic Hard Materials. 2000, Princeton University.
16. Fainerman, V.B. and R. Miller, 2. Thermodynamics of adsorption of surfactants at the fluid interfaces, in Studies in Interface Science, V.B. Fainerman, D. Möbius, and R. Miller, Editors. 2001, Elsevier. p. 99-188.
17. Kohoutek, T., M. Parchine, M. Bardosova, H. Fudouzi, and M. Pemble, Large-area flexible colloidal photonic crystal film stickers for light trapping applications. Optical Materials Express, 2018. 8(4): p. 960-967.
18. Ferreira, M., A. Riul, K. Wohnrath, F.J. Fonseca, O.N. Oliveira, and L.H.C. Mattoso, High-Performance Taste Sensor Made from Langmuir−Blodgett Films of Conducting Polymers and a Ruthenium Complex. Analytical Chemistry, 2003. 75(4): p. 953-955.
19. Riul, A., D.S. dos Santos, K. Wohnrath, R. Di Tommazo, A.C.P.L.F. Carvalho, F.J. Fonseca, O.N. Oliveira, D.M. Taylor, and L.H.C. Mattoso, Artificial Taste Sensor:  Efficient Combination of Sensors Made from Langmuir−Blodgett Films of Conducting Polymers and a Ruthenium Complex and Self-Assembled Films of an Azobenzene-Containing Polymer. Langmuir, 2002. 18(1): p. 239-245.
20. Medina-Plaza, C., J.A. de Saja, J.A. Fernández-Escudero, E. Barajas, G. Medrano, and M.L. Rodriguez-Mendez, Array of biosensors for discrimination of grapes according to grape variety, vintage and ripeness. Anal Chim Acta, 2016. 947: p. 16-22.
21. Spano, F.C. and C. Silva, H-and J-aggregate behavior in polymeric semiconductors. Annual review of physical chemistry, 2014. 65: p. 477-500.
22. Jo, G., J. Jung, and M. Chang, Controlled self-assembly of conjugated polymers via a solvent vapor pre-treatment for use in organic field-effect transistors. Polymers, 2019. 11(2): p. 332.
23. Kroon, R., D.A. Mengistie, D. Kiefer, J. Hynynen, J.D. Ryan, L. Yu, and C. Müller, Thermoelectric plastics: from design to synthesis, processing and structure–property relationships. Chemical Society Reviews, 2016. 45(22): p. 6147-6164.
24. Kleinschmidt, A.T., S.E. Root, and D.J. Lipomi, Poly (3-hexylthiophene)(P3HT): fruit fly or outlier in organic solar cell research? Journal of Materials Chemistry A, 2017. 5(23): p. 11396-11400.
25. Rahimi, K., I. Botiz, J.O. Agumba, S. Motamen, N. Stingelin, and G. Reiter, Light absorption of poly (3-hexylthiophene) single crystals. Rsc Advances, 2014. 4(22): p. 11121-11123.
26. Zheng, L., J. Liu, Y. Ding, and Y. Han, Morphology Evolution and Structural Transformation of Solution-Processed Methanofullerene Thin Film under Thermal Annealing. The Journal of Physical Chemistry B, 2011. 115(25): p. 8071-8077.
27. Yang, X., J.K.J. van Duren, M.T. Rispens, J.C. Hummelen, R.A.J. Janssen, M.A.J. Michels, and J. Loos, Crystalline Organization of a Methanofullerene as Used for Plastic Solar-Cell Applications. Advanced Materials, 2004. 16(9-10): p. 802-806.
28. Cates, N.C., R. Gysel, J.E.P. Dahl, A. Sellinger, and M.D. McGehee, Effects of Intercalation on the Hole Mobility of Amorphous Semiconducting Polymer Blends. Chemistry of Materials, 2010. 22(11): p. 3543-3548.
29. Sun, Y., B. Onwona-Agyeman, and T. Miyasato, Properties of Charge Carrier Transport in Au/Phenyl C61 Butyric Acid Methyl Ester/Au Structure. Japanese Journal of Applied Physics, 2011. 50(3R): p. 031601.
30. Cates, N.C., R. Gysel, Z. Beiley, C.E. Miller, M.F. Toney, M. Heeney, I. McCulloch, and M.D. McGehee, Tuning the Properties of Polymer Bulk Heterojunction Solar Cells by Adjusting Fullerene Size to Control Intercalation. Nano Letters, 2009. 9(12): p. 4153-4157.
31. Deng, L.-L., S.-L. Xie, C. Yuan, R.-F. Liu, J. Feng, L.-C. Sun, X. Lu, S.-Y. Xie, R.-B. Huang, and L.-S. Zheng, High LUMO energy level C60(OCH3)4 derivatives: Electronic acceptors for photovoltaic cells with higher open-circuit voltage. Solar Energy Materials and Solar Cells, 2013. 111: p. 193-199.
32. Ngo, T.T., D.N. Nguyen, and V.T. Nguyen, Glass transition of PCBM, P3HT and their blends in quenched state. Advances in Natural Sciences: Nanoscience and Nanotechnology, 2012. 3(4): p. 045001.
33. Baek, W.-H., H. Yang, T.-S. Yoon, C. Kang, H.H. Lee, and Y.-S. Kim, Effect of P3HT: PCBM concentration in solvent on performances of organic solar cells. Solar Energy Materials and Solar Cells, 2009. 93(8): p. 1263-1267.
34. Xin, Y., Z. Wang, L. Xu, X. Xu, Y. Liu, and F. Zhang, UV-ozone treatment on Cs2 CO3 interfacial layer for the improvement of inverted polymer solar cells. Journal of nanomaterials, 2013. 2013.
35. Zhao, J., A. Swinnen, G. Van Assche, J. Manca, D. Vanderzande, and B.V. Mele, Phase diagram of P3HT/PCBM blends and its implication for the stability of morphology. The Journal of Physical Chemistry B, 2009. 113(6): p. 1587-1591.
36. Gevaerts, V.S., L.J.A. Koster, M.M. Wienk, and R.A.J. Janssen, Discriminating between Bilayer and Bulk Heterojunction Polymer:Fullerene Solar Cells Using the External Quantum Efficiency. ACS Applied Materials & Interfaces, 2011. 3(9): p. 3252-3255.
37. Fan, X., G.J. Fang, P.L. Qin, F. Cheng, and X.Z. Zhao, Rapid phase segregation of P3HT:PCBM composites by thermal annealing for high-performance bulk-heterojunction solar cells. Applied Physics A, 2011. 105(4): p. 1003-1009.
38. Treat, N.D., M.A. Brady, G. Smith, M.F. Toney, E.J. Kramer, C.J. Hawker, and M.L. Chabinyc, Interdiffusion of PCBM and P3HT Reveals Miscibility in a Photovoltaically Active Blend. Advanced Energy Materials, 2011. 1(1): p. 82-89.
39. Pearson, A.J., T. Wang, R.A.L. Jones, D.G. Lidzey, P.A. Staniec, P.E. Hopkinson, and A.M. Donald, Rationalizing Phase Transitions with Thermal Annealing Temperatures for P3HT:PCBM Organic Photovoltaic Devices. Macromolecules, 2012. 45(3): p. 1499-1508.
40. Hajduk, B., H. Bednarski, B. Jarząbek, P. Nitschke, and H. Janeczek, Phase diagram of P3HT:PC70BM thin films based on variable-temperature spectroscopic ellipsometry. Polymer Testing, 2020. 84: p. 106383.
41. Hajduk, B., H. Bednarski, M. Domański, B. Jarząbek, and B. Trzebicka, Thermal transitions in P3HT: PC60BM films based on electrical resistance measurements. Polymers, 2020. 12(7): p. 1458.
42. Agostinelli, T., S. Lilliu, J.G. Labram, M. Campoy‐Quiles, M. Hampton, E. Pires, J. Rawle, O. Bikondoa, D.D. Bradley, and T.D. Anthopoulos, Real‐time investigation of crystallization and phase‐segregation dynamics in P3HT: PCBM solar cells during thermal annealing. Advanced Functional Materials, 2011. 21(9): p. 1701-1708.