Abstract
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.
Highlights
Organic solar cells (OSC) have power conversion efficiencies that are surpassing 11% which is a five-fold increase from the efficiency of the first reported OSC device 30 years ago [1,2,3]
The exciton lifetimes were received by exponential fitting of the singlet exciton absorption decays presented in the SI and extracted from the NIR transient absorption spectra of the polymers
In order to expand the number of data points and compare our results to previously published values, the lifetimes of seven additional polymers were taken from the literature [42,44,45,57,58,59,60,61,62,63,64,65,66,67,68,69]. These include some of the most widely studied polymers to date: P3HT, PCDTBT, PCPDTBT, PFO, PTB7, and MEH-PPV
Summary
Organic solar cells (OSC) have power conversion efficiencies that are surpassing 11% which is a five-fold increase from the efficiency of the first reported OSC device 30 years ago [1,2,3]. Conjugated polymers are the most widely used light harvesting materials for OSC. In the device active layer, they are normally blended with a soluble derivative of the fullerene C60 or C70 or with a high electron affinity aromatic molecule to create an electron donor-acceptor (D-A) pair that has an energy landscape that favors photocurrent generation. The absorption of light by conjugated polymers does not lead to the Polymers 2016, 8, 14; doi:10.3390/polym8010014 www.mdpi.com/journal/polymers. Polymers 2016, 8, 14 direct generation of free charges, but instead coulombically-bound pairs of electrons and holes are created [4]. These so called excitons have relatively short lifetimes and limited diffusivities [5,6,7]
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