Functionalized Polyurethane as a Hole Transport Layer for Quantum Dot-Sensitized Solar Cells with Higher Open-Circuit Voltage and Fill Factor

Abstract

A functionalized polyurethane-based hole transport layer (HTL) for quantum dot-sensitized solar cells (QDSSCs) has been developed by tailoring the redox behaviour of segmented polyurethane. The successful incorporation of ionic moieties into the hard segment was confirmed by Fourier-transform infrared (FTIR) and Nuclear magnetic resonance (NMR) spectroscopy. Sulfonation significantly improved the polymer’s electrical conductivity, electrochemical activity, and optical properties, thereby enabling efficient hole transport through favorable work-function alignment and reduced interfacial energy barriers between the photoactive layer and the Ag counter electrode. Ultrasmall spherical CuInS₂ quantum dots with an average size of 3.15 nm were synthesized using a capping-assisted method and characterized by X-ray diffraction (XRD), UV-Vis spectroscopy, and Transmission electron microscopy (TEM). The HOMO-LUMO and valence/conduction band energy levels, determined by cyclic voltammetry and UV-vis spectroscopy, revealed favorable energy level alignment for efficient charge separation and hole extraction. QDSSCs were fabricated with the architectures FTO/TiO₂/CuInS₂/SPU-2/Ag and FTO/SnO₂/TiO₂/CuInS₂/SPU-2/Ag. The introduction of an SnO₂ interfacial electron transport layer enhanced electron extraction by improving the energy band alignment with TiO₂, resulting in an increase in photocurrent density from 0.74 to 2.12 mA·cm⁻². The corresponding devices exhibited high open-circuit voltages of 0.89 and 0.75 V, fill factors of 67% and 56%, and power conversion efficiencies of 0.45% and 0.89%, respectively. These results demonstrate that the functionalized polyurethane HTL, combined with a SnO₂/TiO₂ bilayer electron transport layer (ETL), provides an effective strategy to improve charge transport and enhance the photovoltaic performance of CuInS₂-based QDSSCs.

Keywords

QDSSCs / CuInS2 / HTL / ETL / Polyurethane

References

[1] Yu Y, Ma T, Huang H . Semiconducting Quantum Dots for Energy Conversion and Storage. Adv. Funct. Mater. 2023, 33, 2213770. DOI: 10.1002/adfm.202213770

[2] Duan L, Hu L, Guan X, Lin C, Chu D, Huang S, et al. Quantum Dots for Photovoltaics: A Tale of Two Materials. Adv. Energy Mater. 2021, 11, 2100354. DOI: 10.1002/aenm.202100354

[3] Kokilavani S, Selopal GS . Advanced Surface Engineering and Passivation Strategies of Quantum Dots for Breaking Efficiency Barrier of Clean Energy Technologies: A Comprehensive Review. Adv. Funct. Mater. 2026, 36, e12718. DOI: 10.1002/adfm.202512718

[4] Buitrago E, Novello AM, Meyer T . Third Generation Solar Cells: Toxicity and Risk of Exposure. Helv. Chim. Acta. 2020, 103, e2000074. DOI: 10.1002/hlca.202000074

[5] Gidwani B, Sahu V, Shukla SS, Pandey R, Joshi V, Jain VK, et al. Quantum dots: Prospectives, toxicity, advances and applications. J. Drug Deliv. Sci. Technol. 2021, 61, 102308. DOI: 10.1016/j.jddst.2020.102308

[6] Dong C, Liu R, Cao W, Meng W, Wang M . 6.56% Efficient CuInS₂ Superstrate Solar Cells Enabled by an Amorphous Tin Oxide Interlayer . ACS Appl. Energy Mater. 2026, 9, 2884-2891. DOI: 10.1021/acsaem.6c00146

[7] Peng Z, Gu Y, Li X, Lu B, Zhang X, Zhang Y, et al. High Performance CuInS₂ Quantum Dot Sensitized Solar Cells Through I⁻/MPA Dual Ligands Passivation . Chem. Asian J. 2024, 19, e202400836. DOI: 10.1002/asia.202400836

[8] Shishodia S, Rinnert H, Balan L, Jasniewski J, Bruyère S, Medjahdi G, et al. Microwave—assisted synthesis of highly photoluminescent core/shell CuInZnSe/ZnS quantum dots as photovoltaic absorbers. Nanoscale Adv. 2025, 7, 1326-1334. DOI: 10.1039/D4NA00893F

[9] Peng Z, Sun Z, Chen J, Li W, Chen J, Liu Y, et al. Enhanced charge generation and transfer performance of the conical bamboo—like TiO₂ nanotube arrays photo—electrodes in quantum dot sensitized solar cells . Sol. Energy 2020, 205, 161-169. DOI: 10.1016/j.solener.2020.05.026

[10] Rahman A, Jennings JR, Khan MM. CuInS₂ and CuInS₂—based nanostructures as photocatalysts . Mater. Sci. Semicond. Process. 2024, 169, 107930. DOI: 10.1016/j.mssp.2023.107930

[11] Yoon N, Joo O, Chae S, Park E . Recent Advances in CuInS₂—Based Photocathodes for Photoelectrochemical H₂ Evolution . Nanomaterials 2023, 13, 1361. DOI: 10.3390/nano13081361

[12] Shilpa G, Kumar PM, Kumar DK, Deepthi PR, Sadhu V, Sukhdev A, et al. Recent advances in the development of high efficiency quantum dot sensitized solar cells (QDSSCs): A review. Mater. Sci. Energy Technol. 2023, 6, 533-546. DOI: 10.1016/j.mset.2023.05.001

[13] Guan S, Cao M, Jiao S, Ma R, Xi R, Hu C, et al. Dye Sensitized Solar Cells With Quasi Solid State Electrolytes Based on Freeze Dried Hydrogel Porous Structures. ChemistrySelect 2025, 10, e01854. DOI: 10.1002/slct.202501854

[14] Daems K, Yadav P, Dermenci KB, Van Mierlo J, Berecibar M . Advances in inorganic, polymer and composite electrolytes: Mechanisms of Lithium—ion transport and pathways to enhanced performance. Renew. Sustain. Energy Rev. 2024, 191, 114136. DOI: 10.1016/j.rser.2023.114136

[15] Kumar S, Maurya IC, Prakash O, Srivastava P, Das S, Maiti P . Functionalized Thermoplastic Polyurethane as Hole Conductor for Quantum Dot—Sensitized Solar Cell. ACS Appl. Energy Mater. 2018, 1, 4641-4650. DOI: 10.1021/acsaem.8b00783

[16] Kumar S, Maiti P . Review on Functional Electrolyte, Redox Polymers, and Solar Conversions in 3G Emerging Photovoltaic Technologies: Progress and Outlook. Energy Fuels 2023, 37, 14473-14511. DOI: 10.1021/acs.energyfuels.3c01695

[17] Ogundeji TS, Alamu GA, Oyewole OJ, Johnson MJ, Olaoye AO, Vyas S, et al. Review of recent progress in enhanced quantum dot solar cells via plasmonic nanostructures. Next Mater. 2026, 11, 101599. DOI: 10.1016/j.nxmate.2026.101599.

[18] Prakash R, Maiti P . Functionalized Thermoplastic Polyurethane Gel Electrolytes for Cosensitized TiO₂/CdS/CdSe Photoanode Solar Cells with High Efficiency . Energy Fuels 2020, 34, 16847-16857. DOI: 10.1021/acs.energyfuels.0c03250

[19] Li L, Pandey A, Werder DJ, Khanal BP, Pietryga JM, Klimov VI . Efficient Synthesis of Highly Luminescent Copper Indium Sulfide—Based Core/Shell Nanocrystals with Surprisingly Long—Lived Emission. J. Am. Chem. Soc. 2011, 133, 1176-1179. DOI: 10.1021/ja108261h

[20] Wang Z, Zhang X, Xin W, Yao D, Liu Y, Zhang L, et al. Facile Synthesis of Cu In S/ZnS Core/Shell Quantum Dots in 1—Dodecanethiol for Efficient Light—Emitting Diodes with an External Quantum Efficiency of 7.8%. Chem. Mater. 2018, 30, 8939-8947. DOI: 10.1021/acs.chemmater.8b04282

[21] Aldakov D, Sajjad MT, Ivanova V, Bansal AK, Park J, Reiss P, et al. Mercaptophosphonic acids as efficient linkers in quantum dot sensitized solar cells. J. Mater. Chem. A 2015, 3, 19050-19060. DOI: 10.1039/C5TA04021C

[22] Kumagai S, Motokucho S, Yabuki R, Anzai A, Kameda T, Watanabe A, et al. Effects of hard— and soft—segment composition on pyrolysis characteristics of MDI, BD, and PTMG—based polyurethane elastomers. J. Anal. Appl. Pyrolysis 2017, 126, 337-345. DOI: 10.1016/j.jaap.2017.05.012

[23] Santra A, Prakash R, Maity S, Nilawar S, Chatterjee K, Maiti P . Core Shell Structure of Photopolymer—Grafted Polyurethane as a Controlled Drug Delivery Vehicle for Biomedical Application. ACS Appl. Mater. Interfaces 2024, 16, 17193-17207. DOI: 10.1021/acsami.3c19155

[24] Pramanik S, Chakraborty R, Maurya PP, Suman S, Swaminathan P, Pal BN . Oxide Memristor of Ag/Ag⁺ Doped V₂O₅ and SnO₂ Bilayer Thin Film for Energy Efficient Memory, Logic and Neuromorphic Device Application . Adv. Funct. Mater. 2026, 36, e14732. DOI: 10.1002/adfm.202514732

[25] Peng S, Cheng F, Liang J, Tao Z, Chen J . Facile solution—controlled growth of CuInS₂ thin films on FTO and TiO₂/FTO glass substrates for photovoltaic application . J. Alloys Compd. 2009, 481, 786-791. DOI: 10.1016/j.jallcom.2009.03.084

[26] Parcheta P, Głowińska E, Datta J. Effect of bio—based components on the chemical structure, thermal stability and mechanical properties of green thermoplastic polyurethane elastomers. Eur. Polym. J. 2020, 123, 109422. DOI: 10.1016/j.eurpolymj.2019.109422

[27] Polo ML, Spontón ME, Jaramillo F, Estenoz DA, Meira GR . Linear segmented polyurethanes: I. A kinetics study. J. Appl. Polym. Sci. 2018, 135, 45747. DOI: 10.1002/app.45747

[28] Tang Q, Gao K . Structure analysis of polyether—based thermoplastic polyurethane elastomers by FTIR,¹H NMR and¹³C NMR . Int. J. Polym. Anal. Charact. 2017, 22, 569-574. DOI: 10.1080/1023666X.2017.1312754

[29] Stefanović IS, Špírková M, Poręba R, Steinhart M, Ostojić S, Tešević V, et al. Study of the Properties of Urethane Siloxane Copolymers Based on Poly(propylene oxide)—b—poly(dimethylsiloxane)—b—poly(propylene oxide) Soft Segments. Ind. Eng. Chem. Res. 2016, 55, 3960-3973. DOI: 10.1021/acs.iecr.5b04975

[30] Xiao Y, Huang H, Peng X . Synthesis of self—healing waterborne polyurethanes containing sulphonate groups. RSC Adv. 2017, 7, 20093-20100. DOI: 10.1039/C6RA28416G

[31] Bauri S, Tripathi S, Garg KA, Maiti P . Modulation of Drug Release: Exploring the Influence of Diisocyanate Variants on Polyurethane Matrix. ACS Appl. Polym. Mater. 2024, 6, 9499-9511. DOI: 10.1021/acsapm.4c01231

[32] Prakash R, Maurya IC, Srivastava P, Mondal S, Ray B, Maiti P . Functionalized polyurethane composite gel electrolyte with cosensitized photoanode for higher solar cell efficiency using a passivation layer. Nanoscale Adv. 2022, 4, 1199-1212. DOI: 10.1039/D1NA00801C

[33] Zhang M, Song X, Zhou C, Liu L, Wu G . Sulphonated hyperbranched wholly bio—based waterborne polyurethane sizing agents for strengthening the wettability and mechanical properties of carbon fibres. Polymer 2025, 330, 128510. DOI: 10.1016/j.polymer.2025.128510

[34] Prakash R, Das S, Maiti P . Non—toxic CuInS2 quantum dot sensitized solar cell with functionalized thermoplast polyurethane gel electrolytes. Polymer 2023, 269, 125708. DOI: 10.1016/j.polymer.2023.125708

[35] Erekath S, Kalpathy SK . Red—shifted optical absorption in films of azo—polyurea—polystyrene blends: Structural correlations and implications. Opt. Mater. 2022, 126, 112155. DOI: 10.1016/j.optmat.2022.112155

[36] Liu G, Zhang S, Xu L, Hu Y, Li X, Zhang S, et al. Recent advances of eco—friendly quantum dots light—emitting diodes for display. Prog. Quantum Electron. 2022, 86, 100415. DOI: 10.1016/j.pquantelec.2022.100415

[37] Ismoilov K, Akram W, Chauhan S, Ergasheva K, Artikboeva R, Islomova Z, et al. Synthesis and Evaluation of Properties of a Novel Cationic Waterborne Polyurethane Finishing Agent. J. Chem. Eng. Process Technol. 2019, 10, 398. DOI: 10.35248/2157—7048.19.10.398

[38] Luo YL, Gao WT, Luo ZY, Li CH . Toward mechanically robust self—healing polyurethanes using dynamics chemistry. Mater. Chem. Front. 2024, 8, 1767-1791. DOI: 10.1039/D3QM01251D

[39] Wu HC, Shao SR, Dong SH, Wang A, Li Z, Han XL, et al. Influence of Hydrogen Bonding on the Temperature—Accelerated Hydrolysis of Silicone Based Polyetherurethane. Chin. J. Polym. Sci. 2023, 41, 1230-1237. DOI: 10.1007/s10118—023—2957—8

[40] Mandal SS, Choudhury AM, Gupta A, Maiti P . An injectable cyclodextrin extended polyurethane/carboxymethyl cellulose hydrogel for controlled release of insulin: In—vitro and in—vivo diabetic animal model study. Carbohydr. Polym. 2025, 356, 123396. DOI: 10.1016/j.carbpol.2025.123396

[41] Puskás I, Szemjonov A, Fenyvesi É, Malanga M, Szente L . Aspects of determining the molecular weight of cyclodextrin polymers and oligomers by static light scattering. Carbohydr. Polym. 2013, 94, 124-128. DOI: 10.1016/j.carbpol.2013.01.025

[42] Zhang J, Lv S, Zhao X, Ma S, Zhou F . Functional Zwitterionic Polyurethanes: State of the Art Review. Macromol. Rapid Commun. 2024, 45, 2300606. DOI: 10.1002/marc.202300606

[43] Zhang J, Lv S, Zhao X, Ma S, Zhou F . Surface functionalization of polyurethanes: A critical review. Adv. Colloid Interface Sci. 2024, 325, 103100. DOI: 10.1016/j.cis.2024.103100

[44] Lu SH, Chen TF, Wang AJ, Zheng D, Li YL, Wang YS . Modulation of structures of CuInS₂ nanoparticles grown with ethylxanthate salts . Mater. Sci. Eng. B 2016, 203, 19-26. DOI: 10.1016/j.mseb.2015.09.001

[45] Dong X, Ren J, Li T, Wang Y . Synthesis, characterization and application of red—emitting CuInS₂/ZnS quantum dots for warm white light—emitting diodes . Dye. Pigment. 2019, 165, 273-278. DOI: 10.1016/j.dyepig.2019.02.035

[46] Nam DE, Song WS, Yang H . Noninjection, one—pot synthesis of Cu—deficient CuInS₂/ZnS core/shell quantum dots and their fluorescent properties . J. Colloid Interface Sci. 2011, 361, 491-496. DOI: 10.1016/j.jcis.2011.05.058

[47] Richardson A, Alster J, Khoroshyy P, Psencik J, Valenta J, Tuma R, et al. Direct Synthesis and Characterization of Hydrophilic Cu—Deficient Copper Indium Sulfide Quantum Dots. ACS Omega 2024, 9, 17114-17124. DOI: 10.1021/acsomega.3c09531

[48] Wu Q, Cai C, Zhai L, Wang J, Kong F, Yang Y, et al. Zinc dopant inspired enhancement of electron injection for CuInS₂ quantum dot—sensitized solar cells . RSC Adv. 2017, 7, 39443-39451. DOI: 10.1039/C7RA06659G

[49] Alayeto I, Gutierrez—Gonzalez P, Dongmei Q, Pallarés Vilar A, Hamraoui K, Civera C, et al. Ion—Mediated Photoluminescence Enhancement in CuInS₂ Quantum Dots and Its Use as Heavy Metal Ion Sensor . ACS Appl. Nano Mater. 2025, 8, 14461-14469. DOI: 10.1021/acsanm.5c02927

[50] Yan R, Zhang W, Wu W, Dong X, Wang Q, Fan J. Optical spectroscopy reveals transition of CuInS₂/ZnS to Cu_xZn_1−xInS₂/ZnS:Cu alloyed quantum dots with resultant double—defect luminescence . APL Mater. 2016, 4, 126101. DOI: 10.1063/1.4971353

[51] Yu Y, Zeng Z, Gao X, Xiong C, Zhu H, Cen H, et al. A Maximization of the Proton Conductivity of Sulfonated Poly(Ether Ether Ketone) with Grafted Segments Containing Multiple, Flexible Propanesulfonic Acid Groups. Macromol. Rapid Commun. 2023, 44, 2200926. DOI: 10.1002/marc.202200926

[52] Li X, He B, Li P, Tang S . In Situ—Doped Sulfonated Schiff—Base Networks in SPEEK Composite Membranes with Enhanced Proton Conductivity. ACS Appl. Mater. Interfaces 2023, 15, 25584-25593. DOI: 10.1021/acsami.3c03885

[53] Tithito T, Kondee S, Seekaew Y, Traiwatcharanon P, Chaloeipote G, Seesaard T, et al. CuInS₂ quantum dots—based electrochemical sensor for indole detection . J. Alloys Compd. 2026, 1062, 187552. DOI: 10.1016/j.jallcom.2026.187552

[54] Milinković E, Cvetanović K, Bošković MV, Conić N, Jovanov V, Stanisavljev D, et al. The Comprehensive Study of TiO₂ Blocking Layer with Complementary Electrochemical and SPM Methods for the Application in Photovoltaics . Inorganics 2025, 13, 270. DOI: 10.3390/inorganics13080270

[55] Raj DV, Ponpandian N, Viswanathan C . Electrodeposition of SnO₂ nanostructures onto copper substrates and their electrochemical properties . Mater. Sci. Technol. 2023, 39, 1414-1424. DOI: 10.1080/02670836.2023.2171766

[56] Bertens K, Fan JZ, Biondi M, Rasouli AS, Lee S, Li P, et al. Colloidal Quantum Dot Solar Cell Band Alignment using Two—Step Ionic Doping. ACS Mater. Lett. 2020, 2, 1583-1589. DOI: 10.1021/acsmaterialslett.0c00379

[57] Deo M, Möllmann A, Haddad J, Ünlü F, Kulkarni A, Liu M, et al. Tantalum Oxide as an Efficient Alternative Electron Transporting Layer for Perovskite Solar Cells. Nanomaterials 2022, 12, 780. DOI: 10.3390/nano12050780

[58] Krishna BG, Ghosh DS, Tiwari S . Hole and electron transport materials: A review on recent progress in organic charge transport materials for efficient, stable, and scalable perovskite solar cells. Chem. Inorg. Mater. 2023, 1, 100026. DOI: 10.1016/j.cinorg.2023.100026

[59] Patel U, Kumar A, Pramanik S, Devi A, Pal BN, Maiti P . SnO₂ as Electron Transport Layer in CsPbBr₃ Perovskite Solar Cells: Experimental and Simulation Approaches . ACS Appl. Energy Mater. 2026, 9, 593-605. DOI: 10.1021/acsaem.5c03357

[60] Liu M, Yan N, Zhang C, Li L . Integrated solar cells with non toxic inorganic nanocrystals and polymer bulk heterojunction. Appl. Surf. Sci. Adv. 2021, 3, 100052. DOI: 10.1016/j.apsadv.2020.100052

[61] Yue W, Han S, Peng R, Shen W, Geng H, Wu F, et al. CuInS₂ quantum dots synthesized by a solvothermal route and their application as effective electron acceptors for hybrid solar cells. J. Mater. Chem. 2010, 20, 7570. DOI: 10.1039/c0jm00611d

[62] Karkhaneh A, Marandi M . Modified synthesis of CuInS₂ nanoparticles for enhancement of the efficiency of corresponding quantum dot—sensitized solar cells: A simplified aqueous approach . Emergent Mater. 2025, 8, 3529-3543. DOI: 10.1007/s42247—024—00987—5

[63] Du Z, Ma D . Recent progress in I—III—VI colloidal quantum dots—integrated solar cells. Curr. Opin. Colloid Interface Sci. 2025, 75, 101890. DOI: 10.1016/j.cocis.2024.101890

[64] Du B, He K, Tian G, Che X, Song L. Robust electron transport layers of SnO₂ for efficient perovskite solar cells: Recent advances and perspectives . J. Mater. Chem. C 2023, 11, 13625-13646. DOI: 10.1039/D3TC02445H

[65] Cai X, Hu T, Hou H, Zhu P, Liu R, Peng J, et al. A review for nickel oxide hole transport layer and its application in halide perovskite solar cells. Mater. Today Sustain. 2023, 23, 100438. DOI: 10.1016/j.mtsust.2023.100438