Charge Photogeneration in Neat Conjugated Polymers

Abstract

The origin and yield of charges in neat conjugated polymers has long been controversial. In this paper, we review the body of literature that has been created over the past three decades of research in this field and provide insight from our own recent work highlighting the importance of polymer microstructure in understanding the photophysics of these materials. We focus primarily on polythiophene, poly(p-phenylene vinylene), and ladder-type poly(p-phenylene) derivatives, as these three prototypical polymer backbone structures have undergone the most complete study. We find compelling evidence that the primary photoexcitations in conjugated polymers include both intrachain excitons and excimers, that charges are produced in a secondary process, primarily from breakup of intrachain excitons, and that the locus of long-lived charge generation is at the interface between amorphous and crystalline domains of the polymer. Interestingly, the existence of interchromophore complexes that we refer to as excimers has largely been ignored in the development of organic photovoltaics based on conjugated polymers. We suggest that the prevalence of this species may help explain certain mysterious features in that body of work, in particular the excess energy offset required for efficient charge separation in donor/acceptor blends and the requirement for intimately mixed phases of the two materials for maximally efficient photocurrent generation.

Crystallinity Effects in Sequentially Processed and Blend-Cast Bulk-Heterojunction Polymer/Fullerene Photovoltaics

“…We note that in polymeric systems, the broadening of X-ray scattering peaks results from disorder in the polymer domains, rather than from the finite size of crystallites as seen in crystalline materials. 16,88,89 Thus, the fwhm or coherence lengths reported here are those derived from the Scherrer equation, even though the size of the crystalline polymer domains are likely smaller. In Table 3, the two P3HT batches show similar fwhm for the (200) peaks.…”

Effect of Systematically Tuning Conjugated Donor Polymer Lowest Unoccupied Molecular Orbital Levels via Cyano Substitution on Organic Photovoltaic Device Performance

“…The positive signals for all polymers are assigned to singlet exciton absorption, in agreement with other TAS studies of conjugated polymers. 51,52 Exponential fitting of the singlet exciton decays therefore allows us to estimate that their lifetimes are as follows: τ P(Ge-DTBT) = 109 ps, τ P(Ge-DTCNBT) = 239 ps, τ P(Ge-DTDCNBT) = 158 ps. It is apparent that the shorter exciton lifetime for P(Ge-DTBT) does not reduce the efficiency of exciton dissociation (as measured above from PLQ to be ~ 100 %), most likely due to significant fine mixing of fullerene into polymer rich domains reducing requirements for polymer exciton diffusion, as is often observed for polymer / fullerene blends.…”

Tracking the coherent generation of polaron pairs in conjugated polymers

“…Such organic π-conjugated semiconductors have striking and rather complex optical properties. Their optical excitation not only results in the formation of excitons, Coulomb bound electron-hole pairs, but—in contrast to, for example, inorganic semiconductors—also in the creation of a wealth of other quasiparticles, namely polaron pairs, polarons, biexcitons and others91011. Polaron pairs, sometimes also called spatially indirect or charge-transfer excitons, are charge-neutral excitations in which spatially separated electrons and holes weakly interact via their Coulomb attraction and are each coupled to their own lattice distortion.…”

“…Polaron pairs, sometimes also called spatially indirect or charge-transfer excitons, are charge-neutral excitations in which spatially separated electrons and holes weakly interact via their Coulomb attraction and are each coupled to their own lattice distortion. They are thought to act as precursors of free charges, which in polymers are also coupled to the lattice and appear in the form of electron and hole polarons1112. Hence, both polaron pairs and polarons are of fundamental importance, for instance, for the efficient light-to-current conversion in polymer-based photovoltaic devices.…”