Impact of Fullerene Intercalation on Structural and Thermal Properties of Organic Photovoltaic Blends

Abstract

The performance of organic photovoltaic blend devices is critically dependent on the polymer:fullerene interface. These interfaces are expected to impact the structural and thermal properties of the polymer with regards to the conjugated backbone planarity and transition temperatures during annealing/cooling processes. Here, we report the impact of fullerene intercalation on structural and thermal properties of poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene (PBTTT), a highly stable material known to exhibit liquid crystalline behavior. We undertake a detailed systematic study of the extent of intercalation in the PBTTT:fullerene blend, considering the use of four different fullerene derivatives and also varying the loading ratios. Resonant Raman spectroscopy allows direct observation of the interface morphology in situ during controlled heating and cooling. We find that small fullerene molecules readily intercalate into PBTTT crystallites, resulting in a planarization of the polymer backbone, but high fullerene loading ratios or larger fullerenes result in nonintercalated domains. During cooling from melt, nonintercalated blend films are found to return to their original morphology and reproduce all thermal transitions on cooling with minimal hysteresis. Intercalated blend films show significant hysteresis on cooling due to the crystallized fullerene attempting to reintercalate. The strongest hysteresis is for intercalated blend films with excess fullerene loading ratio, which form a distinct nanoribbon morphology and exhibit a reduced geminate recombination rate. These results reveal that careful consideration should be taken during device fabrication, as postdeposition thermal treatments significantly impact the charge generation and recombination dynamics

Irradiation-induced degradation of PTB7 investigated by valence band and S 2pphotoelectron spectroscopy

“…The curve decreases towards longer wavelengths of incident photons indicating that the higher the photon energy is the stronger the degradation effects are. This observation is in line with earlier studies where organic compounds are damaged by radiation exposure [23,[26][27][28]. The radiant exposure (the amount of energy exposed to the irradiated spot on the sample surface due to the incident photons) plays only a secondary role for the occurrence of the thin film changes.…”

Intercalated vs Nonintercalated Morphologies in Donor–Acceptor Bulk Heterojunction Solar Cells: PBTTT:Fullerene Charge Generation and Recombination Revisited

“…In contrast, the ICTA blend does not intercalate and therefore constitutes a model of an abrupt interface between pure phases, which we denote as a domain heterojunction. We have further confirmed these results using resonant Raman spectroscopy …”

“…We have further confirmed these results using resonant Raman spectroscopy. 32 In order to assess how the nature of the heterojunction affects the efficiency of charge generation, we performed simulations of the free energy ΔG(r) of the electron−hole pair. As we have previously described, 33 the electron and hole are initially simulated as a charge-transfer (CT) state with the hole in the PBTTT and the electron in the acceptor (PC 70 BM or ICTA).…”

Identifying the Molecular Origins of High-Performance in Organic Photodetectors Based on Highly Intermixed Bulk Heterojunction Blends

“…To investigate the mechanics of IDDSe structural changes, in situ temperature-dependent Raman is performed on neat IDDSe and IDDSe:C 60 blend as shown in Figure c (a well-established method reported on several polymers and small molecules blends). − Here, both neat IDDSe and IDDSe:C 60 blend show strong selective increases of the peaks related to the core and the end groups of IDDSe upon thermal stress, a clear indication of thermally induced structural reorganization of these groups (note that neat IDDSe shows much stronger changes than blends where the surrounding matrix of C 60 causes physical hindrance). Figure d shows the thermal evolution of peak D (end groups) and peak E (core groups), where critical transition temperatures exist at which the respective structural reorganizations of IDDSe are initiated.…”