Ab initio modeling of excitonic and Charge-transfer states in organic semiconductors: The PTB1/PCBM low band gap system

A detailed quantum chemical simulation of the excitonic and charge-transfer (CT) states of a bulk heterojunction model containing poly(thieno[3,4-b] thiophene benzodithiophene) (PTB1)/[6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) is reported. The largest molecular model contains two stacked PTB1 trimer chains interacting with C₆₀ positioned on top of and lateral to the (PTB1)₃ stack. The calculations were performed using the algebraic diagrammatic construction method to second order (ADC(2)). One main result of the calculations is that the CT states are located below the bright inter-chain excitonic state, directly accessible via internal conversion processes. The other important aspects of the calculations are the formation of discrete bands of CT states originating from the lateral C₆₀'s and the importance of inter-chain charge delocalization for the stability of the CT states. A simple model for the charge separation step is also given, revealing the energetic feasibility of the overall photovoltaic process.