Abstract
Five important topics in organic solar cells are included in this dissertation. First, a new method which does not require presumed current–voltage functional form is proposed for the determination of device-level parameters. The method was applied to analyze a bulk heterojunction organic solar cell (OSC), multi-crystal (m-Si) and amorphous silicon (a-Si) solar cell. It was found that the extracted intrinsic current–voltage characteristic of the OSC clearly exhibits a linear hopping current component and a quadratic space-charge limited current component. A 120mV/dec exponential component in the extracted intrinsic I–V, which may result from trap-assisted photo-carrier recombination is also observed. Furthermore, the reconstructed dark current–voltage curves of both OSC and a-Si solar cell are found to differ significantly from the measured dark current–voltage curve, revealing the importance of electric field in the operation of bulk heterojunction OSC and a-Si solar cells. Second, a multi-layered polymer solar cell (PSC) with a graded compositional profile to reduce space-charge limited (SCL) transport was designed and analyzed in details. We found that by increasing the effective carrier lifetime for the slower photo-carriers, SCL effects that degrade the fill factor and the power conversion efficiency can be alleviated in PSCs with a thick active layer. The reduction of SCL effects was evidenced by a diminished dependence of the incident photon-to-electron conversion efficiency (IPCE) on light bias, which also indicates the importance of light bias in characterizing the operating IPCE of PSCs with thick active layers which operate in SCL region. Based on our results, device architecture to enhance FF was proposed. Third, we developed an easy and accurate measurement method for incident photons-to-electrons conversion efficiency（IPCE） of space-charge-limited (SCL) photovoltaics (PV), which show non-linear relationship between current and incident light. An additional white light bias was shown to effect identically as monochromatic light in the relevant spectrum and was necessary in the IPCE measurement of both conventional and inverted structured SCL-PVs. We also demonstrated that the SCL effect appears more significant during the degradation process of the devices. Forth, a model which simulates the effect of the resistance of the transparent electrode on the device performance is proposed. We also used the model to investigate the effects of metal grid design. In the last topic, inverted organic solar cells were fabricated and analyzed. It was found that a combination of an ultra-thin Molybdenum oxide (MoO3) layer and thin gold film can be used as the top transparent anode and the inclusion of MoO3 layer improves significantly the power conversion efficiency (PCE) as well as the optical transmittance. Besides, thinner Au film leads to higher PCE in the case of Schottky contact. We also found that the PCE of the organic solar cells was found to be dominated by the optical transparency of incident electrode, indicating no structural preference of the bulk active layer for charge transport. With a MoO3/10 nm Au anode, semi-transparent inverted organic solar cells with PCE of 2.8% and optical transmittance over 50% at spectral region beyond 650 nm can be achieved.