Photoinduced Carrier Generation and Decay Dynamics in Intercalated and Non-intercalated Polymer:Fullerene Bulk Heterojunctions

Abstract

The dependence of photoinduced carrier generation and decay on donor-acceptor nanomorphology is reported as a function of composition for blends of the polymer poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene) (pBTTT-C(14)) with two electron-accepting fullerenes: phenyl-C(71)-butyric acid methyl ester (PC(71)BM) or the bisadduct of phenyl-C(61)-butyric acid methyl ester (bis-PC(61)BM). The formation of partially or fully intercalated bimolecular crystals at weight ratios up to 1:1 for pBTTT-C(14):PC(71)BM blends leads to efficient exciton quenching due to a combination of static and dynamic mechanisms. At higher fullerene loadings, pure PC(71)BM domains are formed that result in an enhanced free carrier lifetime, as a consequence of spatial separation of the electron and hole into different phases, and the dominant contribution to the photoconductance comes from the high-frequency electron mobility in the fullerene clusters. In the pBTTT-C(14):bis-PC(61)BM system, phase separation results in a non-intercalated structure, independent of composition, which is characterized by exciton quenching that is dominated by a dynamic process, an enhanced carrier lifetime and a hole-dominated photoconductance signal. The results indicate that intercalation of fullerene into crystalline polymer domains is not detrimental to the density of long-lived carriers, suggesting that efficient organic photovoltaic devices could be fabricated that incorporate intercalated structures, provided that an additional pure fullerene phase is present for charge extraction.

A Close Look at Charge Generation in Polymer:Fullerene Blends with Microstructure Control

“…20,22,24,28,62 This is consistent with the low solar cell efficiency of this system. Here, we directly observe the geminate charge recombination (gCR) as a significant decay of all TA features with a relatively short 211 ps time constant ( Figure 5A, dynamics of the 605 nm GSB and 850 nm charge absorption in Figure 5D, amplitude spectra in Figure S6, Supporting Information).…”

“…48 Only a small fraction of free charges is formed, leading to a weakly pronounced longlived TA spectrum at our longest time delay of 1 ns. In the 1:4 blend, the presence of PCBM clusters largely suppresses gCR, 20,22,28,62 so that the decay of the TA features on the 1 ns time scale is notably reduced (Figure 5C,D). This can be explained by favored spatial separation of charge pairs formed in the co-crystalline regions, driven by an energy cascade to neat fullerene domains with higher electron affinity.…”

The influence of microstructure on charge separation dynamics in organic bulk heterojunction materials for solar cell applications

“…[19][20][21][22][23] This has strong implications on the photophysics of the materials. [24][25][26][27][28][29][30] In the present contribution, we demonstrate that the dynamics of charge generation in polymer:fullerene blends is largely determined by the microstructure. To that aim, we have used femtosecond transient absorption (TA) spectroscopy to investigate CS in several systems with well-characterized BHJ structure.…”

“…This is in excellent agreement with higher solar cell efficiency in the 1 : 4 blend, 22 and with previous TA, PIA and microwave conductivity results, mostly recorded at much longer times than the femtosecond results presented here. 26,28,30,40 The high electron mobility in the pure fullerene domains and increased electron affinity compared to intercalated PCBM provide a driving force for the spatial separation of charges formed by exciton dissociation in the co-crystals. 26,28 Finally, we will consider what happens in the 1 : 4 blend with 390 nm excitation (Fig.…”

On the morphology of polymer‐based photovoltaics

“…Rance et al studied the photoinduced carrier generation and decay physics of PBTTT-PCBM blends and their result shows that the intercalated PBTTT-PCBM crystals are not detrimental to the density of long-lived carriers, indicating these structures can be used to fabricate efficient organic photovoltaics in a bi-continuous network blends. 79 Although important progress has been made in understanding the physical properties and device function of the mixing phase in BHJ blends, this topic still needs much more effort to further explore.…”