Solution-Processable PEDOT:PSS:α-In₂Se₃ with Enhanced Conductivity as a Hole Transport Layer for High-Performance Polymer Solar Cells

Abstract

Two-dimensional (2D) nanosheets have attracted significant attention in photovoltaic devices in recent years owing to their outstanding photoelectric properties. Herein, 2D α-In₂Se₃ nanosheets with high conductivity and suitable work function are synthesized by liquid-phase exfoliation method. To ameliorate the low conductivity of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) (2.21 × 10^-3 S cm^-1), α-In₂Se₃ nanosheets are directly added into PEDOT:PSS to obtain the PEDOT:PSS:α-In₂Se₃ composite film. The composite film exhibits excellent optical transmittance, suitable work function, and enhanced conductivity (1.54 × 10^-2 S cm^-1). To profoundly investigate the mechanism of conductivity improvement, X-ray photoelectron spectroscopy, Raman spectroscopy, electron paramagnetic resonance, and atomic force microscopy are conducted. The results show that the synergistic effect of 2D α-In₂Se₃ nanosheets and isopropyl alcohol/deionized water cosolvent screens the Coulombic attraction among PEDOT and PSS. The screening effect results in the partial removal of PSS and the benzoid-quinoid transition of PEDOT. In addition, α-In₂Se₃ nanosheets may serve as physical linkers for PEDOT chains. Both these effects are beneficial to increase the interfacial contact area between PEDOT chains and form a larger conductive network of PEDOT, leading to an enhanced conductivity. The composite film is first employed as a hole transport layer (HTL) in polymer solar cells (PSCs). The power conversion efficiency (PCE) of the poly[2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-<i>b</i>:4,5-<i>b</i>']dithiophene)-<i>co</i>-(1,3-di(5-thiophene-2-yl)-5,7-bis(2-ethylhexyl)benzo[1,2-<i>c</i>:4,5-<i>c</i>']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-<i>d</i>:2',3'-<i>d</i>']-<i>s</i>-indaceno[1,2-b:5,6-<i>b</i>']dithiophene (ITIC)-based device with composite HTL is 10% higher than that of the unmodified PBDB-T:ITIC-based device, and the maximum PCE of 15.89% is achieved in the (poly[(2,6-(4,8-bis(5-(2-ethylhexyl)-4-fluorothiophen-2-yl)benzo[1,2-<i>b</i>:4,5-<i>b</i>']dithiophene))-<i>co</i>-(1,3-di(5-thiophene-2-yl)-5,7-bis(2-ethylhexyl)-benzo[1,2-<i>c</i>:4,5-<i>c</i>']dithiophene-4,8-dione))] (PM6):(2,2'-((2<i>Z</i>,2<i>Z</i>)-((12,13-bis(2-ethylhexyl)-3,9-diundecyl-12,13-dihydro-[1,2,5]thiadiazolo[3,4-<i>e</i>]thieno[2,″3″:4',50]thieno[2',3':4,5]pyrrolo[3,2-<i>g</i>]thieno[2',3':4,5]thieno[3,2-<i>b</i>]indole-2,10-diyl)bis(methanylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1<i>H</i>-indene-2,1-diylidene))dimalononitrile) (Y6) system. More interestingly, the stability of devices with composite HTL is improved owing to the partial removal of PSS. Thus, the PEDOT:PSS:α-In₂Se₃ composite can be a potential HTL material in PSCs.