We investigate the influence of the built-in voltage on the performance of organic bulk heterojuction solar cells that are based on a p-i-n structure. Electrical doping in the hole and the electron transport layer allows to tune their work function and hence to adjust the built-in voltage: Changing the doping concentration from 0.5 to 32 wt% induces a shift of the work function towards the transport levels and increases the built-in voltage. To determine the built-in voltage, we use electroabsorption spectroscopy which is based on an evaluation of the spectra caused by a change in absorption due to an electric field (Stark effect). For a model system with a bulk heterojunction of BF-DPB and C60, we show that higher doping concentrations in both the electron and the hole transport layer increase the built-in voltage, leading to an enhanced short circuit current and solar cell performance.

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.