Insights into ultrafast charge-pair dynamics in P3HT:PCBM devices under the influence of static electric fields

Polymer-fullerene blends based on poly(3-hexylthiophene-2,5-diyl) (P3HT) and phenyl-C61-butyric-acid methyl ester (PCBM) have been extensively studied as promising bulk heterojunction materials for organic semiconductor devices with improved performance. In these donor–acceptor systems where the bulk morphology plays a crucial role, the generation and subsequent decay mechanisms of photoexcitation species are still not completely understood. In this work, we use femtosecond transient absorption spectroscopy to investigate P3HT:PCBM diodes under the influence of applied static electric fields in comparison to P3HT:PCBM thin films. At the same time, we try to present a detailed overview about work already done on these donor–acceptor systems. The excited state dynamics obtained at 638 nm from P3HT:PCBM thin films are found to be similar to those observed earlier in neat P3HT films, while those obtained in the P3HT:PCBM devices are affected by field-induced exciton dissociation, resulting not only in comparatively slower decay dynamics, but also in bimolecular deactivation processes. External electric fields are expected to enhance charge generation in the investigated P3HT:PCBM devices by dissociating excitons and loosely bound intermediate species like polaron pairs (PPs) and charge transfer (CT) excitons, which can already dissociate only due to the intrinsic fields at the donor–acceptor interfaces. Our results clearly establish the formation of PP-like transient species different from CT excitons in the P3HT:PCBM devices as a result of a field-induced diffusion-controlled exciton dissociation process. We find that the loosely bound transient species formed in this way also are reduced in part via a bimolecular annihilation process resulting in charge loss in typical donor–acceptor P3HT:PCBM bulk heterojunction semiconductor devices, which is a rather interesting finding important for a better understanding of the performance of these devices

Ultrafast polaron-pair dynamics in a poly(3-hexylthiophene-2,5-diyl) device influenced by a static electric field: insights into electric-field-related charge loss

The generation and decay mechanisms of polaron pairs in organic semiconductor-based optoelectronic devices under operational conditions are relevant for a better understanding of photophysical processes affecting the device performance, since the possible occurrence of a polaron pair introduces an intermediate step in exciton dissociation into fully separated charge carriers. The role played by static electric fields in polaron-pair dynamics is important but poorly understood or not investigated in detail. In this work, insights into the polaron-pair dynamics in neat poly(3-hexylthiophene-2,5-diyl) (P3HT) thin films and P3HT films sandwiched between electrical contacts with an applied external static electric field are probed using femtosecond pump–probe transient absorption spectroscopy. Asymmetric contacts result in P3HT devices with application-related diode characteristics. Consistent with the electric field-induced dissociation of oppositely charged species, we show that polaron-pair dissociation into charge carriers occurs in the P3HT device more significantly with increasing reverse bias, and that this process follows an initial instantaneous polaron-pair photoabsorption quenching due to a pronounced immediate loss of primary photoexcitation species (hot excitons). Furthermore, we show that the net-electric field present in the P3HT diode (including built-in-potential at 0 V bias) results in a more complex dynamics with new findings as compared to the neat-P3HT thin film case. Indeed, besides polaron pairs directly originating from hot excitons, we experimentally observe polaron-pair formation during exciton dissociation via a field-mediated generation process, resulting in a slower contribution to the overall decay dynamics. Moreover, unlike in the external electric field-free P3HT film, bimolecular annihilation processes clearly appear as an additional loss channel when a field is applied and hence have an impact on carrier generation performance in a working device

Tailoring of enhanced interfacial polarization in WO3_3 nanorods grown over reduced graphene oxide synthesized by a one-step hydrothermal method

In the present report, well-defined WO3 nanorods (NRs) and a rGO–WO$_3$ composite were successfully synthesized using a one-pot hydrothermal method. The crystal phase, structural morphology, shape, and size of the as-synthesized samples were studied using X-ray diffraction (XRD) and transmission electron microscopy (TEM) measurements. The optical properties of the synthesized samples were investigated by Raman, ultraviolet-visible (UV-Vis) and photoluminescence (PL) spectroscopy. Raman spectroscopy and TEM results validate the formation of WO$_3$ (NRs) on the rGO sheet. The value of the dielectric constant (ε′) of WO3 NRs and rGO–WO$_3$ composite is decreased with an increase in frequency. At low frequency (2.5 to 3.5 Hz), the value of ε′ for the rGO–WO3 composite is greater than that of pure WO$_3$ NRs. This could be due to the fact that the induced charges follow the ac signal. However, at higher frequency (3.4 to 6.0), the value of ε′ for the rGO–WO$_3$ composite is less compared to that of the pure WO3 NRs. The overall decrease in the value of ε′ could be due to the occurrence of a polarization process at the interface of the rGO sheet and WO3 NRs. Enhanced interfacial polarization in the rGO–WO$_3$ composite is observed, which may be attributed to the presence of polar functional groups on the rGO sheet. These functional groups trap charge carriers at the interface, resulting in an enhancement of the interfacial polarization. The value of the dielectric modulus is also calculated to further confirm this enhancement. The values of the ac conductivity of the WO$_3$ NRs and rGO–WO$_3$ composite were calculated as a function of the frequency. The greater value of the ac conductivity in the rGO–WO$_3$ composite compared to that of the WO$_3$ NRs confirms the restoration of the sp:$^{++}$ network during the in situ synthesis of the rGO–WO$_3$ composite, which is well supported by the results obtained by Raman spectroscopy