Singlet Exciton Lifetimes in Conjugated Polymer Films for Organic Solar Cells

Polymers

Stoichko D. Dimitrov¹  Bob C. Schroeder²  Christian B. Nielsen¹  Hugo Bronstein³  Zhuping Fei¹  Iain McCulloch¹  Martin Heeney¹  James R. Durrant¹ 

[1]Centre for Plastic Electronics, Department of Chemistry, Imperial College London, Exhibition Road, London SW7 2AZ, UK

[2]Department of Chemical Engineering, Stanford University, Stanford, CA 94305, USA

[3]Department of Chemistry, University College London, London WC1H 0AJ, UK

关键词: excited states;    diffusion;    energy gap law;    non-radiative;    ultrafast transient absorption spectroscopy;   

DOI  :  10.3390/polym8010014

来源: mdpi

【 摘 要 】

The lifetime of singlet excitons in conjugated polymer films is a key factor taken into account during organic solar cell device optimization. It determines the singlet exciton diffusion lengths in polymer films and has a direct impact on the photocurrent generation by organic solar cell devices. However, very little is known about the material properties controlling the lifetimes of singlet excitons, with most of our knowledge originating from studies of small organic molecules. Herein, we provide a brief summary of the nature of the excited states in conjugated polymer films and then present an analysis of the singlet exciton lifetimes of 16 semiconducting polymers. The exciton lifetimes of seven of the studied polymers were measured using ultrafast transient absorption spectroscopy and compared to the lifetimes of seven of the most common photoactive polymers found in the literature. A plot of the logarithm of the rate of exciton decay vs. the polymer optical bandgap reveals a medium correlation between lifetime and bandgap, thus suggesting that the Energy Gap Law may be valid for these systems. This therefore suggests that small bandgap polymers can suffer from short exciton lifetimes, which may limit their performance in organic solar cell devices. In addition, the impact of film crystallinity on the exciton lifetime was assessed for a small bandgap diketopyrrolopyrrole co-polymer. It is observed that the increase of polymer film crystallinity leads to reduction in exciton lifetime and optical bandgap again in agreement with the Energy Gap Law.

【 授权许可】

CC BY   
© 2016 by the authors; licensee MDPI, Basel, Switzerland.