Transient photoconductivity measurements of transport and recombination in organic solar cells

The processes of electronic transport and recombination in bulk heterojunction (BHJ) organic solar cells determine many aspects of the performance of the cell, including the shape of the current-voltage characteristics.  The time of flight technique (i.e., transient photoconductivity) is often used to obtain the carrier mobility and recombination lifetimes in low mobility materials.  In BHJ cells the measurement is complicated because the response of electrons and holes cannot be separated since the films are optically thin.  In addition, carrier mobility is expected to be dispersive in these disordered materials.

We report measurements of the transient photoconductivity response as a function of bias voltage in two different solar cells made from P3HT/PCBM and PCDTBT/PCBM.  A model is developed to analyze the data and to separate the effects of the two carriers, and we show that the mobility and recombination lifetime for both carriers can be deduced with reasonable accuracy.  The transport is characterized by different mobility of the two carriers, with the faster and slower carrier mobility differing by about a factor 10.  The recombination lifetimes for the two carriers differ correspondingly. The transport properties are related to the dc performance of the solar cells.  In particular, the shape of the current voltage characteristics is consistent with the mobility and lifetime data. 

We also demonstrate that the transient photoconductivity data can be used to distinguish between geminate and non-geminate recombination, through measurements of the free carrier concentration at short times after excitation.  Measurements show that non-geminate recombination dominates in the solar cells that are studied.