PEDOT:PSS

Abstract: reported under AM1.5 (AM: air mass) illumination, this efficiency is not sufficient to meet realistic specifications for commercialization. The need to improve the light-to-electricity conversion efficiency requires the implementation of new materials and the exploration of new device architectures. Polymer-based photovoltaic cells are thin-film devices fabricated in the metal-insulator-metal configuration sketched in Figure 1a. The absorbing and charge-separating bulk-heterojunction layer with a thickness of approximately 100 nm is sandwiched between two charge-selective electrodes; a transparent bilayer electrode comprising poly(3,4-ethylenedioxylenethiophene):polystyrene sulfonic acid (PEDOT:PSS) on indium tin oxide (ITO) glass for collecting the holes and a lower-work-function metal (here, Al) for collecting the electrons. The work-function difference between the two electrodes provides a built-in potential that breaks the symmetry, thereby providing a driving force for the photogenerated electrons and holes toward their respective electrodes. Because of optical interference between the incident (from the ITO side) and back-reflected light, the intensity of the light is zero at the metallic (Al) electrode; Figure 1a shows a schematic representation of the spatial distribution of the squared optical electric-field strength. [9–11] Thus, a relatively large fraction of the active layer is in a dead-zone in which the photogeneration of carriers is significantly reduced. Moreover, this effect causes more electron–hole pairs to be produced near the ITO/PEDOT:PSS electrode, a distribution which is known to reduce the photovoltaic conversion efficiency. [12,13] This “optical interference effect” is especially important for thin-film structures where layer thicknesses are comparable to the absorption depth and the wavelength of the incident light, as is the case for photovoltaic cells fabricated from semiconducting polymers. In order to overcome these problems, one might simply increase the thickness of the active layer to absorb more light. Because of the low mobility of the charge carriers in the polymer:C60 composites, however, the increased internal resistance of thicker films will inevitably lead to a reduced fill factor. An alternative approach is to change the device architecture with the goal of spatially redistributing the light intensity inside the device by introducing an optical spacer between the active layer and the Al electrode as sketched in Figure 1a. [11] Although this revised architecture would appear to solve the problem, the prerequisites for an ideal optical spacer limit the choice of materials: the layer must be a good acceptor and an electron-transport material with a conduction band edge lower in energy than that of the lowest unoccupied molecular orbital (LUMO) of C60; the LUMO must be above (or close to) the Fermi energy of the collecting metal electrode; and it must be transparent to light with wavelengths within the solar spectrum.

Abstract: Perovskite compounds have attracted recently great attention in photovoltaic research. The devices are typically fabricated using condensed or mesoporous TiO2 as the electron transport layer and 2,2′7,7′-tetrakis-(N,N-dip-methoxyphenylamine)9,9′-spirobifluorene as the hole transport layer. However, the high-temperature processing (450 °C) requirement of the TiO2 layer could hinder the widespread adoption of the technology. In this report, we adopted a low-temperature processing technique to attain high-efficiency devices in both rigid and flexible substrates, using device structure substrate/ITO/PEDOT:PSS/CH3NH3PbI3–xClx/PCBM/Al, where PEDOT:PSS and PCBM are used as hole and electron transport layers, respectively. Mixed halide perovskite, CH3NH3PbI3–xClx, was used due to its long carrier lifetime and good electrical properties. All of these layers are solution-processed under 120 °C. Based on the proposed device structure, power conversion efficiency (PCE) of 11.5% is obtained in rigid substrates (glass/...

Abstract: The conversion efficiency of heat to electricity in thermoelectric materials depends on both their thermopower and electrical conductivity. It is now reported that, unlike their inorganic counterparts, organic thermoelectric materials show an improvement in both these parameters when the volume of dopant elements is minimized; furthermore, a high conversion efficiency is achieved in PEDOT:PSS blends.

Abstract: Highly conductive poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) films as stand-alone electrodes for organic solar cells have been optimized using a solvent post-treatment method. The treated PEDOT:PSS films show enhanced conductivities up to 1418 S cm−1, accompanied by structural and chemical changes. The effect of the solvent treatment on PEDOT:PSS has been investigated in detail and is shown to cause a reduction of insulating PSS in the conductive polymer layer. Using these optimized electrodes, ITO-free, small molecule organic solar cells with a zinc phthalocyanine (ZnPc):fullerene C60 bulk heterojunction have been produced on glass and PET substrates. The system was further improved by pre-heating the PEDOT:PSS electrodes, which enhanced the power conversion efficiency to the values obtained for solar cells on ITO electrodes. The results show that optimized PEDOT:PSS with solvent and thermal post-treatment can be a very promising electrode material for highly efficient flexible ITO-free organic solar cells.

Abstract: The authors report on a bulk heterojunction photovoltaic cell in which an isomeric mixt. of C70 derivs. is used as an electron acceptor in combination with poly[2-methoxy-5-(3',7'-dimethyloctyloxy)-p-phenylenevinylene] (MDMO-PPV). [70]PCBM, in this case a mixt. of isomeric [6,6]-phenyl- C71- butyric acid Me esters, is the higher fullerene analog of [60]PCBM, and displays improved light absorption in the visible region. Consequently, when this material is used in a photovoltaic cell instead of [60]PCBM, 50 % higher current densities are obtained. The synthesis of [70]PCBM was performed and the monoadduct fraction was isolated from the higher adducts and unreacted C70 by column chromatog. 1H and 13C NMR were performed on the [70]PCBM, as was UV/VIS absorption in toluene soln. Spin-coated composite films were made with MDMO-PPV, and photoinduced absorption measurements give direct spectral evidence for photoinduced charge sepn., not only upon excitation of the polymer, but also after selective excitation of [70]PCBM at 630 nm. Time-resolved photoluminescence measurements ater excitation at 488 mn, obsd. at 570 nm indicate near quant. charge generation upon polymer excitation, but at 720 nm this quenching depends on the processing solvent, chlorobenzene, o-dichlorobenzene, vs. o-xylene. At. force microscopy reveals a difference in film roughness dependent upon spinning solvent. Photovoltaic cells were made by sandwiching the photoactive mixt., consisting of [70]PCBM and MDMO-PPV in a 4.6:1 (wt./wt.) ratio, between charge-selective electrodes of ITO/PEDOT:PSS (ITO: indium tin oxide; PEDOT: poly(3,4-ethylenedioxythiophene); PSS: poly(styrenesulfonate)) and LiF/Al. The external quantum efficiency (EQE) from a 12V, 50 W halogen lamp varied from a max. of 0.2 for chlorobenzene- processed films, to 0.68 for the o-dichlorobenzene- processed films, compared to 0.5 for equiv. films made with [60]PCBM. Photovoltage-photocurrent behavior were also measured. Overall power conversion efficiency was about 3.0%. [on SciFinder (R)]