Recently, the development of technologies for green energy harvesting and generation has become a priority. Organic thermoelectric devices meet the requirement using a temperature gradient that is converted into electricity and vice versa. To achieve na optimal performance, a complementary P-type (hole transporting) and N-type (electron transporting) architecture is required. However, while high performance P-type organic systems are already well-stablished by having achieved comparable values of conductivity and Seebeck comparable to the inorganic semiconductors within the same application, high performing electron transporting materials (N-type) are still scarce, hindering the advance of thermoelectric devices.
Throughout this work, it was possible to tune the electrical conductivity of the N-type organic semiconductor N2200 by means of solution doping introducing the (NDMBI)2.
The EPR and the UV-vis ensured the doping process under the conditions adopted. The appearance of an intense EPR signal demonstrated the occurrence of doping after the dimer incorporation to the host (N2200), this addition provoked the formation of delocalized charge carriers, also known as polarons, that in practical ways are the unpaired electrons detected by the EPR; the more the dimer content was increased, the more intese was the signal for the 10%, 20% and 30% doped samples, highlighting the last one. The UV-vis showed a peak growth around 550 and 820 nm, in solution and film state, corresponding to polaron transition peaks; the 20% doped sample showed the most well defined polaron peaks.
The two-point probe station revealed an average conductivity of 6 . 10-3 S/cm for the 20% doped sample. This reveals a lower electrical conductivity than the one of the N2200-NDMBI system, contradicting the expectation of better performance due to the injection of two electrons with the dimer instead of only one as in the monomer, but the N2200-dimer still exhibits a 4 orders improvement when compared to N2200 neat material and its slightly doped samples, praising the success of the doping with the intention of tuning the electrical conductivity of the N-type semiconductor N2200.
The TFA provided the Thermoelectrical parameters such as electrical conductivity, thermal conductivity and Seebeck coefficient for the 10% and 20% doped samples, all these parameters as a function of the Temperature (ºC). As expected, the 20% doped sample exhibited higher values of thermal and electrical conductivity in comparison with the 10% doped sample because of the higher charge carrier density
of the first. The electrical conductivity increases when the temperature increases, but the thermal conductivity reamins unchanged. In addition, the electrical conductivity holds the same value as the average obtained with the two-point probe station 6 . 10
-3 S/cm. The modulus of the Seebeck coefficient measured for the 10% doped sample, -80 uV/K, is higher than the one for the 20%, -66 uV/K. This can be explained by the anticorrelation between Seebeck and electrical conductivity, and thus, the charge carrier density.
Finally, the AFM and KPM were successfully used to confirm and justify the poor performance of the samples with a dimer content above 20%, elucidating the morphological aspects of the N2200-(NDMBI)2 system as proposed at the beginning of this work. The dopant islands were identified through different signals: height, phase and surface potential difference. The aggregation reflects the poor miscibility of the dopant in the matrix, being the cause of the lower performance when compared to the system N2200-NDMBI.
5.3 Future Outlook
The findings exposed here craft promising paths for future enhancements in the thermoelectric performance of N-type organic semiconductor materials. In the future, the influence of additives such as compatibilizers and antioxidants need to be focused in order to improve the electrical conductivity by increasing the stability and the doping efficiency. Additionally, a rational synthetic modification of the side chains substituting the alkyl side chains for glycolated side chains could also provide a better miscibility of the dopant in the matrix.
With the data presented in this thesis, it is still necessary the Transmission Electron Microscopy in order to comprehend the effect of the doping in the microstructure within the film, below the surface, so it would be possible to explain deeply the organization attributed as an effect of the doping process.
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