The fate of electron–hole pairs in polymer:fullerene blends for organic photovoltaics

Abstract

There has been long-standing debate on how free charges are generated in donor:acceptor blends that are used in organic solar cells, and which are generally comprised of a complex phase morphology, where intermixed and neat phases of the donor and acceptor material co-exist. Here we resolve this question, basing our conclusions on Stark effect spectroscopy data obtained in the absence and presence of externally applied electric fields. Reconciling opposing views found in literature, we unambiguously demonstrate that the fate of photogenerated electron–hole pairs—whether they will dissociate to free charges or geminately recombine—is determined at ultrafast times, despite the fact that their actual spatial separation can be much slower. Our insights are important to further develop rational approaches towards material design and processing of organic solar cells, assisting to realize their purported promise as lead-free, third-generation energy technology that can reach efficiencies over 10%.

Unveiling the Nature of Charge Carrier Interactions by Electroabsorption Spectroscopy: An Illustration with Lead-Halide Perovskites

“…EAS and TREAS have been successfully applied to scrutinize the dynamics of photocarriers in small molecule-and conjugated polymer-based organic photovoltaic (OPV) systems. [1][2][3] They are currently believed to constitute very powerful tools for the study of another moment and polarizability changes upon the optical transition of interest (0 → k).…”

“…[4] The term perovskite originally designated calcium titanate CaTiO 3 and is now generalized to all compounds that exhibit an ABX 3 crystal structure in the form of corner-sharing BX 6 [5][6][7][8][9][10][11][12][13][14][15][16] Such a wide range of applications implies a wide range of shapes and structure, and, indeed, although mostly exploited in the form of semiconductor thin-films, lead-halide perovskites can be found as nanocrystals or colloidal nanoparticles, retaining their 3D geometry, as well as quantum-confined, 2D or quasi-2D layers.…”

Charge separation and carrier dynamics in donor-acceptor heterojunction photovoltaic systems

“…One hypothesis that has been suggested is that excitons are highly delocalized for about 100–200 fs after photoexcitation, so that they sample a larger extent of the BHJ, and that their dissociation takes place before the excited state relaxation to a more localized species 48–50 . Subsequently, it was shown, using femtosecond transient absorption (TA) studies on polymers such as poly[2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene] (PBTTT), poly[[5-(2-ethylhexyl)-5,6-dihydro-4,6-dioxo-4H-thieno[3,4-c]pyrrole-1,3-diyl][4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl]] (PBDTTPD) and poly(3-hexylthiophene-2,5-diyl) (P3HT), that an even more important parameter leading to ultrafast exciton dissociation in polymer:fullerene blends is the presence of an intermixed polymer:fullerne phase 51–55 . Prompt (∼100 fs) exciton dissociation was found to occur predominantly in the regions where the polymer and fullerene are molecularly intermixed, so that no exciton diffusion is necessary.…”

Pushing the knowledge of interfaces

“…Light absorption by the donor or acceptor gives rise to the formation of excitons (bound electron-hole pairs), which dissociate to charges at the interface. Those positive and negative charges are either initially still bound in an interfacial charge-transfer state, or are already long-range separated at ultrafast times, according to opposing views that have fuelled a rich scientific debate about the mechanism of charge separation in organic solar cells [5][6][7][8] .…”

“…In femtosecond pump-probe spectroscopy, this leads to the appearance of an electroabsorption (EA) signal, defined as the difference between the absorption spectrum with and without the field 9 . The phenomenon has been used to probe the local environment of the charges, and to estimate the electron-hole separation with high time resolution 5,6 . In pump-push spectroscopy, the sample is excited with a visible femtosecond pump pulse, and then the photogenerated species are re-excited to slightly higher-lying states using a nearinfrared push pulse.…”