With the recent record power conversion efficiency of 17.3% , organic solar cells have become serious contenders among third generation photovoltaics. This success is related amongst other things to efforts in overcoming the drawbacks of organic semiconductors, such as their low exciton-dissociation yield and charge-carrier mobility, as they limit device performance.
Studying light-induced charge-carrier dynamics under working conditions can help develop new ideas for improving device operation.
As the transport in organic semiconductors occurs by hopping of charge carriers from and to localized states, the spin degree of freedom plays a role in the allowed hops according to the Pauli principle. Thus the spin provides a probe for the identification of transport-limiting processes and the type of spin species they involve. At this stage, the spin-sensitive techniques Electron Paramagnetic Resonance (EPR) and Electrically Detected Magnetic Resonance (EDMR) spectroscopy become conducive. The detection of light-induced EPR signals in a time-resolved mode allows us to follow the evolution of photo-generated charge carriers. By simultaneously measuring time-resolved EDMR signals, the contribution of the detected spin species to the photocurrent is elucidated. This enables us to correlate the state after photo-excitation, being a charge-transfer state or free charge carriers, and its influence on the photocurrent. Furthermore, we show that the biasing conditions (charge-carrier injection or extraction) directly affect the dynamics of the current-influencing paramagnetic species.
We present results obtained from bias-dependent transient (tr)EPR and trEDMR measurements on poly(4,4-dioctyldithieno(3,2-b:2’,3‘-d)silole)-2,6-diyl-alt-(2,1,3-benzothiadiazole)-4,7-diyl (PSBTBT-8) blended with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) bulk-heterojunction solar cells and show that the resonant signals observed at low-temperature, attributed to positive polarons in the polymer and negative polarons in the fullerene phase, are involved in different spin-dependent processes. We will report on the possible spin-dependent mechanisms the polarons undergo at specific times after optical excitation.
 L. Meng et al., Science, 2018, 361, 1094.