Abstract
AbstractSolution‐processed conducting polymer thin films are key components in organic and flexible electronic and optoelectronic devices. An archetypal conducting polymer is poly(3,4‐ethylenedioxythiophene)‐poly(styrenesulfonate) (PEDOT:PSS), which can feature a high work function and thus helps achieving Ohmic contacts for holes with many semiconductors. However, it is known that residual water in PEDOT:PSS films lowers their work function and is detrimental for device lifetime. Our photoelectron spectroscopy experiments reveal that the work function of PEDOT:PSS films containing residual water shows the same trend as function of temperature as does the dielectric constant (ε) of water, in the range between 25 °C and ‐100 °C. Consistently, it is found from impedance spectroscopy measurements that ε of residual water containing PEDOT:PSS films increases with decreasing temperature. After removal of residual water from PEDOT:PSS films by annealing in ultrahigh vacuum, the work function of thin films is much higher than before (reaching 6.1 eV) and, notably, independent of temperature. In contrast, no indication is found that the presence of residual water has any impact on the electrical conductivity. For a nominally water‐free molecularly doped conjugated donor/acceptor copolymer films, a correlation between sample work function and temperature similar to those seen for PEDOT:PSS is found.
Highlights
Solution-processed conducting polymer thin films are key components in organic and flexible electronic and optoelectronic devices
This was attributed to both compositional changes[33,35] and wateraided dielectric screening of the local electric dipoles between the positively charged PEDOT and PSS anions.[33,36]. These earlier studies on the effect of residual water on the work function of PEDOT:PSS have focused on the effect of residual water at room temperature and its removal upon annealing. This is in line with the conventional stability studies of organic electronic devices, in which the device performance is monitored at room temperature or at elevated temperatures
As benchmark to assess the effect of water desorption from the films in ultrahigh vacuum (UHV) at RT on the measured work function, a HIL1.3 sample was measured at RT after 1 h and after 20 h residing in UHV
Summary
We investigated two classes of conductive polymer systems: i) PEDOT of different formulations, namely, AI4083, HIL1.3, and HTL Solar 3, and ii) molecularly doped polymers with typically employed higher-end dopant molecules (D) to polymer monomer units (M) D:M ratio, namely, 7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane (F4TCNQ):P3HT (1:10),[9] F4TCNQ:poly [2,6-(4,4-bis[2-ethylhexyl]-4H-cyclopenta[2,1-b;3,4-b′] dithiophene)-alt-4,7(2,1,3-benzothiadiazole)] (PCPDTBT) (1:5),[47] and BCF:PCPDTBT (1:5).[46]. The activation energy values change for AI4083 from 38 to 45 meV, for HIL1.3 changes from 24 to 26 meV, and for HTL Solar 3 it stays at 33 meV This indicates that residual water and its physical state in the bulk of the polymer has no apparent influence on the electrical conductivity of PEDOT:PSS films. The hole injection barrier at the interface of high ionization energy organic semiconductors with PEDOT:PSS can be minimized if the latter is annealed in UHV prior to deposition or the semiconductor, to assure an effective removal of residual water and high work function
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