Interface Modification Of PEDOT:PSS For ITO-Free Polymer Solar Cells

During the last decade, polymer solar cells(PSCs) have achieved tremendous progress due to their advantages. Up to now, the power conversion efficiency(PCE) of single cells is over 10%, which is considered the threshold to break through into commercial applications. For the goal of 20% PCE, which is the achievable maximum value, apart from synthesis of new narrow band gap materials for better photon harvesting, interface modification is also crucial. Stability of PSCs becomes critical after realization of high PCE PSCs. Currently, fabricating high-performance devices mostly depend on indium tin oxide(ITO) as transparent electrodes. However, ITO has intrinsic problems such as high mechanical brittleness, scarce indium on earth, poor adhesion to organic and polymeric materials and inferior physical properties for high temperature treatment. Therefore, there is strong demand for new transparent conductivity materials to replace ITO.Conducting polymer poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate)(PEDOT:PSS) has been successfully applied as anode interface(hole transport layer(HTL)) and transparent electrode. However, PEDOT:PSS has a low conductivity, and the corrosion of acid on ITO electrode resulting in low PCE and poor stability of PSCs. The high content of insulating species PSS in pristine PEDOT:PSS always leads to a low conductivity, and influences on hole and charge transport. The conductivity of pristine PEDOT:PSS is too lower to apply as the transparent electrode. Thus, this paper focuses on interface modification of PEDOT:PSS for PSCs, including inducing formation of PEDOT:PSS ordered stack microstructure, enhancing conductivity of the modification films and improving stability of the devices. The application of modified PEDOT:PSS as transparent electrodes is also studied. The paper content is as follows:Polymer solar cells(PSCs) with high short current density(Jsc) have been fabricated through a facile way of using a low-cost polyelectrolyte-modified poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate)(PEDOT:PSS, P VP Al 4083) bilayer film as anode buffer layer. Spin-coating a layer of sulfonate poly(aryl ether sulfone)(SPES) on the surface of PEDOT:PSS hole-transporting layer(HTL) is found to dramatically improve the Jsc value even up to 21.66 mA cm-2. The notable Jsc is demonstrated to be correlated with interaction between the SPES and PEDOT which removes the insulator of PSS, formation of continuous PEDOT domains, consequently leading to the improved conductivity and more imitate interfacial contact. It should be noted that the notable Jsc also partly results from the effect of a second anode, due to the high conductivity of SPES modified PEDOT:PSS. Through systematically investigation on a series of devices with different areas, it can be found that a real effective area of the devices should be carefully addressed to exclude the effect of a second anode, especially when a highly conductive interfacial material is incorporated. More interestingly, apart from the successful application in HTL, SPES also works well as transparent electrode. Compared with the pristine PEDOT:PSS(PH1000) anode, SPES modified PH1000 as transparent anode achieves a dramatically increased performance in the ITO-free PSCs together with overall improved parameters, even equal to the one based on ITO anode. These findings indicate that solution-processed SPES shows a great potential in the fabrication of highly efficient PSCs as well as large-area, flexible printable PSCs.Polymer solar cells(PSCs) with high power conversion efficiency(PCE) and excellent stability have been fabricated using multiple HTL of PEDOT:PSS(P VP Al 4083) modified by MoO3 and liquid-crystalline ionic liquids(LCILs) of 1-hexadecyl-3-methylimidazolium hexafluorophosphate([C16MIm]PF6) or 1-hexadecyl-3-methylimidazolium tetrafluoroborate([C16MIm]BF4). LCILs can effectively induce PEDOT to form microstructure and favorable molecular packing. The solution-processed MoO3 is inserted between ITO and PEDOT layer to avoid corrosion of ITO electrode. Furthermore, the introduction of MoO3 is expected to further improve the stability of the device, and to enhance the charge selective of PEDOT.Meanwhile, the orientated and self-assemble spin-coated LCIL on PEDOT films induces PEDOT microstructure to form ordered stack and improving the conductivity. The PCEs value based on MoO3/P VP Al4083/[C16MIm]PF6 and MoO3/P VP Al4083/[C16MIm]BF4 as HTL improved from 2.3%(P VP Al4083 HTL) to 3.0% and 2.8% in the PSCs with an effective area of 18 mm2 based on PTB7:PC71BM, respectively. In addition the PCE stability of devices based on MoO3/P VP Al4083/LCIL as HTL is higher than those of devices based on MoO3 or MoO3/P VP Al 4083 as HTL. Thus, these novel MoO3/P VP Al4083/LCILs HTL show great potential applications in fabrication of highly efficient and stability PSCs.Indium tin oxide(ITO)-free polymer solar cells(PSCs) with high power conversion efficiency(PCE) have been fabricated by using two types of liquid-crystalline ionic liquids(LCILs) [C16MIm]PF6 and [C16MIm]BF4 modified PEDOT:PSS(PH1000) as transparent anode. LCILs modification can effectively remove the insulating PSS on top surface of the PH1000 and induce the PEDOT with ordered and continuous molecular packing. At the same time, spontaneous orientation of the LCILs with liquid crystalline property can further promote the ordered packing arrangement of both PH1000 and active layer. The conductivity of the PH1000 has been dramatically be enhanced from 0.4 S cm-1 to 1457.7 S cm-1 for PH1000/[C16MIm]PF6 and 1243.8 S cm-1 for PH1000/[C16MIm]BF4. The PCE value based on PH1000 treated by [C16MIm]PF6 and [C16MIm]BF4 as anode is improved to 4.6% and 4.5% in the ITO-free PSC with an effective area of 18 mm2 based on PTB7:PC71BM, respectively, which is comparable or even higher than that obtained from the device with ITO anode(2.4%). The improved PCE is demonstrated to be correlated with high conductivity of LCILs modified PH1000 films, imitate contact between anode and active layer, and improved morphology of both the anode and active layer. In addition, LCILs modification can also make a better energy alignment for charge injection and transport, enabling a simplified device structure without any extra hole transport layer(HTL). Furthermore, these novel PH1000/LCIL anode have an universal application for ITO-free devices based on low-band gaps active layer, showing their great potential applications in the fabrication of highly efficient PSCs as well as large-area, flexible printed PSCs.