We have presented a thorough experimental investigation of electroabsorption spectroscopy on quasi-one-dimensional organic molecular crystals such as PTCDA and MePTCDI vapor deposited thin films to clarify the involvement of the charge-transfer exciton in the lowest excited state. By a self-built experimental setup, two kinds of electroabsorption measurements, called "perpendicular" and "parallel" measurements, were conducted at room temperature in ambient air. The crystalline texture of PTCDA and MePTCDI thin film samples are characterized by X-ray diffraction measurements. Current-voltage, capacitance-frequency and capacitance-voltage measurements are performed to clarify the electric field distribution inside organic layers. The results from electrical measurements show that only under certain conditions (electroabsorption measurements with proDC bias), the perpendicular and parallel electroabsorption meaurements can be directly compared. The electroabsorption spectra of MePTCDI and PTCDA thin films can be interpreted by neither pure Frenkel exciton nor pure charge-transfer exciton model. Essential features of electroabsorption spectra of MePTCDI and PTCDA thin films can be understood by the the mixture of Frenkel and charge-transfer exciton model. However, there is still a discrepancy in the directional properties of electroabsorption signals between experimental results and modle calculations. This small discrepancy suggests that a full interpretation of electroabsorption spectra of quasi-one-dimensional organic molecular crystals needs further experimental and theoretical investigations.

We have presented a thorough experimental investigation of electroabsorption spectroscopy on quasi-one-dimensional organic molecular crystals such as PTCDA and MePTCDI vapor deposited thin films to clarify the involvement of the charge-transfer exciton in the lowest excited state. By a self-built experimental setup, two kinds of electroabsorption measurements, called "perpendicular" and "parallel" measurements, were conducted at room temperature in ambient air. The crystalline texture of PTCDA and MePTCDI thin film samples are characterized by X-ray diffraction measurements. Current-voltage, capacitance-frequency and capacitance-voltage measurements are performed to clarify the electric field distribution inside organic layers. The results from electrical measurements show that only under certain conditions (electroabsorption measurements with proDC bias), the perpendicular and parallel electroabsorption meaurements can be directly compared. The electroabsorption spectra of MePTCDI and PTCDA thin films can be interpreted by neither pure Frenkel exciton nor pure charge-transfer exciton model. Essential features of electroabsorption spectra of MePTCDI and PTCDA thin films can be understood by the the mixture of Frenkel and charge-transfer exciton model. However, there is still a discrepancy in the directional properties of electroabsorption signals between experimental results and modle calculations. This small discrepancy suggests that a full interpretation of electroabsorption spectra of quasi-one-dimensional organic molecular crystals needs further experimental and theoretical investigations.