Carbon-based/PEDOT:PSS Nano-composite Transparent Electrode For Polymer Solar Cells

Indium tin oxide(ITO) as transparent electrodes is often used in high-performance devices. ITO has intrinsic problems such as high mechanical brittleness, exorbitant price, not compatible with solution process, which can not use roll-to-roll printing technology. Therefore, there is strong demand for new transparent conductivity materials to replace ITO.Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)(PEDOT:PSS) with a high work function(5.2 eV) has been most commonly used as a hole transport layer(HTL) in PSCs to improve hole collection at the anode. However, PEDOT:PSS has several problems, including high acidity, hygroscopic properties and inhomogeneous electrical properties, resulting in poor long-term stability. Besides modified PEDOT:PSS with high conductivity offers new opportunity for TCEs, and which is compatible with printing because of its excellent coating properties. Carbon materials with excellent electrical and optical, good durability and flexibility have showed great potential in photoelectric materials. However, their intrinsic problem is the insolubility in most of the solvents. The paper content is as follows:Sulfonated carbon nanotubes(S-CNTs) and sulfonated graphene(S-Gra) with superior dispersibility were successfully prepared to modify poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate)(PEDOT:PSS) as hole transport layer(HTL) and transparent electrode for applications in organic optoelectronics. Due to the synergetic effect between S-CNTs/S-Gra and PEDOT:PSS, it could remove excess insulating PSS chains due leading to an obvious phase separation between the PEDOT and PSS chains, which allows the enrichment of PEDOT on the surface of the film. Thus, the PEDOT:PSS:S-CNTs with well-matched work function, favorable morphology, optimized hydrophobicity and superior hole mobility is demonstrated to be an excellent HTL for polymer solar cells(PSCs). After changing the content of the PEDOT:PSS:S-CNTs buffer layer, an increasement of the power conversion efficiency is obtained from 3.15% to 3.91%. Then PEDOT:PSS(PH 1000) modified with S-Gra shows high conductivity, because the sulfonated graphene lamellae contributes to the connection between the insulator and conductive PEDOT islands and improves the charge conduction. As a result, a solution processed, PEDOT:PSS:S-Gra electrode with a high transmittance of 85.5% at 550 nm and a sheet resistance of 45.51 ? sq-1 is obtained.Beside, the 4-halobenzoics(XBA) including 4-fluorobenzoic acid(FBA), 4-chlorobenzoic acid(CBA), 4-bromobenzoic acid(BBA) and 4-iodobenzoic acid(IBA) have been applied to modify poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate)(PEDOT:PSS) by simply spin-coating solvent treatment. A universal and significant improvement in the conductivity of PEDOT:PSS(from ~1 S/cm to more than 3300 S/cm) has been achieved by XBA modification, which results from the acid XBA-induced phase segregation, depletion of PSS chains and the conformational change of the conductive PEDOT chains.Especially, the oxidation CBA treated PEDOT:PSS exhibits low sheet resistance of 43 U/sq and transparency of over 80% in the visible range, which are comparable to those of indium tin oxide(ITO). The modified PEDOT:PSS has been facilely applied as the transparent anode for high-efficiency ITO-free organic solar cell device without hole transport layer.Moreover, synergistic action once has again been employed in a ternary PEDOT:PSS/S-CNTs@Gra bilayer composite electrode. The interpenetrating networks S-CNTs@Gra film can achieve the rearrangement of the PEDOT chains with more expended conformation via the ?-? interaction between functional carbon materials and PEDOT. As a result, PEDOT:PSS/S-CNTs@Gra electrode presents a high conductivity of 1962.53 S cm-1 with a high transmittance over 88% at 550 nm, and ITO-free PSCs with PEDOT:PSS/S-CNTs@Gra electrode achieves a PCE of 5.8% with high stability, which based on the TE/PTB7:PC71BM:Al device structure. Combined the flexible and conductive composite carbon TE with scalable roll-to-roll process, we anticipate that the commercial production of large-scale transparent electrode, replacing ITO, will be realized in the near future.