10.1.2 Measurement techniques
The drift distance of charge carriers photogenerated anywhere within the active layer of the solar cell is given by ld = ц x т x E, where ц is the mobility, т is charge carrier lifetime and E is the electric field. This equation assumes that the charge carriers are electric field driven . Due to the rather low mobility of organic materials, high concentrations of photogenerated charge carriers are required to reach a short circuit current density of ~ 10 mAcm-2, e. g, n ~ 1016cm-3 if ц = 10-4 cm2 V-1s-1. High charge carrier concentration generally leads to increased bimolecular recombination resulting in short lifetimes (т(t) = [вn(t)]-1 andconse – quently, short drift and diffusion distances.
Charge carrier mobility and the lifetime of the charge carriers are not independent in most organic materials. The bimolecular recombination coefficient of a Langevin type recombination typical for low mobility organic materials is written as pL = є(цє + ць)/е£0, where цє (ць) is the electron (hole) mobility and e, £, £o are the elementary charge and dielectric constants of the material and vacuum, respectively. The important questions are, therefore, how to measure charge carrier mobility (ц) and lifetime (т) in bulk heterojunction solar cells, and how ц and т are correlated within the interpenetrating network of the electron donor and electron acceptor materials.
First it will be demonstrated that improving the charge carrier mobility of conjugated polymers is a viable way to improve the power conversion efficiency of bulk heterojunction solar cells. The charge transport properties of regioregular MDMO-PPV is compared to its regio – random counterpart, and the importance of molecular structure-morphology-charge carrier mobility is emphasized. Next, the principally different techniques of time of flight (ToF) and charge carrier extraction by linearly increasing voltage (CELIV) are compared in samples of regioregular poly(3-hexylthiophene). Finally, it will be demonstrated how to determine the charge carrier mobility and lifetime of the charge carriers simultaneously in bulk heterojunction solar cells using the novel photo-CELIV technique.