Optical Studies of Photoexcitations in Polymer/Fullerene Blends for Organic Photovoltaic Applications

Abstract

We used a variety of optical probe techniques including broadband femtosecond transient and continuous wave (cw) photomodulation spectroscopies and electroabsorption for studying the photophysics in two typical π-conjugated polymers, namely regio-regular poly (3-hexyl thiophene) (RR-P3HT) with selforganized π-stacked two-dimensional lamellae, and 2-methoxy-5-(2′-ethylhexyloxy) poly(p-para-phenylene-vinylene) (MEH-PPV) with amorphous nanomorphology; both polymers in pristine and blend with fullerene molecules. In the pristine forms we identified singlet excitons as the primary photoexcitations, having typical photoinduced absorption (PA) band that is correlated with stimulated emission. In contrast, in polymer/fullerene blends the photogenerated excitons quickly decay giving rise to a novel photoexcitation species, and the stimulated emission is absent. For the blends we provide strong evidence for the existence of charge transfer complex (CTC) manifold that is formed inside the optical gap of the polymer and fullerene constituents, which is clearly revealed in the electro-absorption spectrum. Because the lowest energy CTC lies below the optical gap then it is possible to directly generate polarons in the blends without involving intrachain excitons in the polymer phase, when using below gap pump excitation. When using excellent quality materials we present evidence that the CTC states are populated in RR-P3HT/fullerene blend within 20 ps following exciton photogeneration in the polymer chains; but no charge polarons are generated on their expense up to *2 ns. Interestingly the CTC states are photogenerated much faster in D-A blends having smaller domain size such as in regio-random P3HT/PCBM and MEH-PPV/C 60 ; however the CTCs do not easily dissociate in these blends because of the large binding energy. Our findings indicate that the CTC state and film morphology play a

Magnetic field enhancement of organic photovoltaic cells performance

“…Theoretical modelling of polymer based solar cells is usually involved, and approximations such as the unidimensional conjugate polymer character have to be used [32]. Nonetheless, the general framework for the process of photocurrent generation -represented in Figure [1] for a schematic P3HT:PCBM solar cell-can be simplified to a handful of appropriately chosen states.…”

“…Among the most efficient organic photovoltaic (OPV) cells to date are those based on bulk heterojunctions composed of conjugated polymers blended with fullerene derivatives, such as the one presented in Fig. [1] [ [5][6][7]. Numerous approaches have focused on the study of the impact that morphology and interfaces of the blend have in the charge generation and charge transfer processes [8,9].…”

Magnetic fields: a tool for the study of organic solar cells

“…The ubiquitous goal in photovoltaic research is to improve the performance of existing materials and configurations, as nowadays the best polymeric photovoltaic (OPV) cells present an efficiency not surpassing 11% [1][2][3][4]. Typical configurations for the active layer of these devices are conjugated polymers blended with fullerene derivatives [5][6][7], whose quantum efficiency, namely the ratio of absorbed photon to created exciton, raises in these compounds above 0.9 [8,9]. This means that there exists a necessity to bridge the gap existing between the initial high efficiency and the final photocurrent generated, and for that purpose the position of the energy levels -and in particular the charge transfer states-has been proposed to be critical [10].…”