The supplementary materials for the article "A highly stretchable, transparent, and conductive polymer" provide detailed information on the selection and effects of small molecule ionic additives (STECs) on the properties of PEDOT:PSS films. The key points include: 1. **Selection of STEC Enhancers**: - STECs are effective enhancers for PEDOT:PSS, enhancing both stretchability and conductivity. - Additives with bulky, long hydrocarbon side chains do not significantly plasticize PEDOT:PSS due to high hydrophobicity. - Anions like TFMSI or triflate are highly acidic and serve as effective dopants for PEDOT. - STEC1, 2, and 3 were chosen for further investigation due to their high conductivity (over 100 S/cm) and maximum tensile strain (above 100%). 2. **Mechanical Characterization of Bulk Freestanding Films**: - STEC additives enhance the maximum tensile strain of PEDOT films, with the most stretchable films exhibiting fibrous morphology. - Mechanical tests show hysteresis and viscoplastic behavior, indicating the film's solid-like nature despite high STEC content. - Stress relaxation, strain cycling, and frequency sweep experiments confirm the film's mechanical properties. 3. **Effect of STEC on PEDOT:PSS**: - STECs induce phase separation, enhancing the crystallinity of PEDOT-rich domains. - The combination of phase separation and high crystallinity is crucial for achieving high conductivity. - GIWAXS analysis shows improved crystallinity in PEDOT/STEC films with high conductivity and stretchability. 4. **Morphology and Electrical Properties**: - SEM and AFM analyses show that PEDOT/STEC films have a solid and densely packed interior. - Conductivity is influenced by processing methods, with additional STEC solution soaking enhancing conductivity. - XPS and AFM studies confirm the chemical and structural changes in the films. 5. **Low-Temperature Measurements and FoM for Transparent Conductors**: - Temperature-dependent conductivity and first- and second-order temperature coefficients are provided. - The figure of merit (FoM) for transparent conductors is above 100, surpassing previous records. 6. **Cycling Stability and Mixed Ion-Electron Conductivity**: - Cycling stability tests show that PEDOT/STEC films maintain their conductivity over multiple cycles. - Electrochemical impedance spectra indicate mixed ion-electron conductivity in some samples. 7. **PEDOT/STEC as Interconnects for FET Arrays**: - Finite element simulations and experimental results show that PEDOT/STEC interconnects can support FET arrays under various strains, including multi-dimensional stretching on curved surfaces. These supplementary materials provide a comprehensive overview of the experimental and theoretical aspects of the study, supporting the main findingsThe supplementary materials for the article "A highly stretchable, transparent, and conductive polymer" provide detailed information on the selection and effects of small molecule ionic additives (STECs) on the properties of PEDOT:PSS films. The key points include: 1. **Selection of STEC Enhancers**: - STECs are effective enhancers for PEDOT:PSS, enhancing both stretchability and conductivity. - Additives with bulky, long hydrocarbon side chains do not significantly plasticize PEDOT:PSS due to high hydrophobicity. - Anions like TFMSI or triflate are highly acidic and serve as effective dopants for PEDOT. - STEC1, 2, and 3 were chosen for further investigation due to their high conductivity (over 100 S/cm) and maximum tensile strain (above 100%). 2. **Mechanical Characterization of Bulk Freestanding Films**: - STEC additives enhance the maximum tensile strain of PEDOT films, with the most stretchable films exhibiting fibrous morphology. - Mechanical tests show hysteresis and viscoplastic behavior, indicating the film's solid-like nature despite high STEC content. - Stress relaxation, strain cycling, and frequency sweep experiments confirm the film's mechanical properties. 3. **Effect of STEC on PEDOT:PSS**: - STECs induce phase separation, enhancing the crystallinity of PEDOT-rich domains. - The combination of phase separation and high crystallinity is crucial for achieving high conductivity. - GIWAXS analysis shows improved crystallinity in PEDOT/STEC films with high conductivity and stretchability. 4. **Morphology and Electrical Properties**: - SEM and AFM analyses show that PEDOT/STEC films have a solid and densely packed interior. - Conductivity is influenced by processing methods, with additional STEC solution soaking enhancing conductivity. - XPS and AFM studies confirm the chemical and structural changes in the films. 5. **Low-Temperature Measurements and FoM for Transparent Conductors**: - Temperature-dependent conductivity and first- and second-order temperature coefficients are provided. - The figure of merit (FoM) for transparent conductors is above 100, surpassing previous records. 6. **Cycling Stability and Mixed Ion-Electron Conductivity**: - Cycling stability tests show that PEDOT/STEC films maintain their conductivity over multiple cycles. - Electrochemical impedance spectra indicate mixed ion-electron conductivity in some samples. 7. **PEDOT/STEC as Interconnects for FET Arrays**: - Finite element simulations and experimental results show that PEDOT/STEC interconnects can support FET arrays under various strains, including multi-dimensional stretching on curved surfaces. These supplementary materials provide a comprehensive overview of the experimental and theoretical aspects of the study, supporting the main findings