Photogenerated charge carrier length scales and optimization for efficient plastic solar cells

Performance of efficient bulk heterojunction solar cell relies on the mobility-lifetime product of the photogenerated charge carriers. A recent technique based on scanning photocurrent microscopy has been useful in estimating diffusion length scales of the photogenerated charge carriers in pristine polymer systems. The method provides an estimate of the decay length scales of both positively and negatively charged carriers and also the degree of ambipolarity. The estimates from these measurements of solar cells consisting of pristine polymers as well as blend systems can be utilized to construct pixilated solar cells of appropriate dimensions. We demonstrate the importance of the geometrical factors of the device and correlate it to the microscopic transport characteristics in order to maximize the efficiency of the system. We also discuss the dependence of the open circuit voltage and short circuit current density on the geometrical factors and the effect of local illumination on the geometry of the cathode.