Elisa
Palacios-Lidón
*a,
Emin
Istif‡
b,
Ana M.
Benito
b,
Wolfgang K.
Maser
*b and
Jaime
Colchero
*a
aDepartamento Física, Edificio CIOyN (Campus Espinardo), Universidad de Murcia, E-30100 Murcia, Spain. E-mail: elisapl@um.es; colchero@um.es
bInstituto de Carboquímica (ICB-CSIC), E-50018 Zaragoza, Spain. E-mail: wmaser@icb.csic.es
First published on 16th May 2019
The nanoscale aggregate structure of conjugated polymers critically determines the performance of organic thin film optoelectronic devices. Their impact on electronic interface interactions with adjacent layers of graphene, widely reported to improve the device characteristics, yet remains an open issue, which needs to be addressed by an appropriate benchmark system. Here, we prepared discrete ensembles of poly(3-hexylthiophene) nanoparticles and graphene oxide sheets (P3HTNPs–GO) with well-defined aggregate structures of either J- or H-type and imaged their photogenerated charge transfer dynamics across their interface by Kelvin probe force microscopy (KPFM). A distinctive inversion of the sign of the surface potential and surface photovoltage (SPV) demonstrates that J-aggregates are decisive for establishing charge transfer interactions with GO. These enable efficient injection of photogenerated holes from P3HTNPs into GO sheets over a range of tens of nanometers, causing a slow SPV relaxation dynamics, and define their operation as an efficient hole-transport layer (HTL). Conversely, H-type aggregates do not facilitate specific interactions and entrust GO sheets the role of charge-blocking layers (CBLs). The direct effect of the aggregate structure of P3HT on the functional operation of GO as a HTL or CBL thus establishes clear criteria towards the rational design of improved organic optoelectronic devices.
Over the last few years, it has been found that the aggregate structure can be established prior to thin film deposition in a controllable manner by liquid phase self-assembly approaches rendering P3HT in the form of nanofibers and spherical nanoparticles, thus contributing to overcome the dependency on extrinsic processing conditions.11–15 In this context, we have recently shown that the crystalline chain packing of P3HT nanoparticles (P3HTNPs) can be controlled by sheets of graphene oxide (GO).16 Their presence during the liquid phase self-assembly process of P3HTNPs affords a significant change in their aggregate structure from H-type to J-type. Concomitantly, this enables the creation of P3HTNPs–GO charge transfer complexes exhibiting superior optoelectronic properties. The study of individual P3HTNPs–GO ensembles is fundamental to unambiguously determine the influence of the aggregate structure for establishing interface interactions between P3HT and GO sheets and its effect on the charge-transport properties of the photogenerated charge carriers. Such a study would clarify the yet somewhat arbitrarily operational functionality of GO sheets in layered P3HT–GO assemblies, which are reported to act as a hole-transport layer (HTL), electron-blocking layer, electron-transport layer (ETL) or hole-blocking layer.17,18
Kelvin Probe Force Microscopy (KPFM) is a highly versatile technique to elucidate the nanoscale electronic properties.19 Combined with controlled illumination, KPFM allows the study of local photophysical processes, providing important feedback on the design of improved materials to achieve enhanced performance of thin film optoelectronic devices.20 Many types of conjugated polymers have been investigated by KPFM in the dark and under illuminated conditions, revealing the direct impact of blend morphology and nanoscale donor–acceptor interface structures on the photoinduced charge transport processes and conduction pathways.21–25
In this work, we study the optoelectronic properties of individual P3HTNPs–GO nanoscale ensembles deposited on ultra-flat ITO surface. To elucidate the influence of the polymer aggregate structure on the interactions with the GO sheets and the photoinduced charge transport processes between the two components, the surface potential (SP) and surface photovoltage (SPV) have been monitored by KPFM with and without illumination. To gain insight into the relevance of the aggregate structure, we probed P3HTNPs–GO nanoscale ensembles prepared either by an ex situ mixing process (P3HTNPs are first synthesized in solution and then mixed with a GO solution) or by an in situ re-precipitation approach (P3HTNPs are directly synthesized in a GO solution), favouring aggregate structures in the P3HTNPs of either H-type or J-type aggregate, respectively.16 SP values clearly enable to distinguish between H- and J-type aggregates. In addition, an important sign inversion of the SPV indicates the presence of interface interactions between P3HTNPs and GO sheets and the formation of charge transfer complexes established exclusively for J-type aggregates formed in the in situ sample. Here, charge injection over a range of several tens of nanometers is observed causing a slow relaxation process. This endows GO the role of a hole-transport layer upon interaction with P3HT interfaces exhibiting J-type aggregates. In contrast, H-type P3HTNPs do not enable interactions and assign GO the role of a hole-blocking layer. This study underlines the important role of the type of the aggregate structure for defining interactions, photocharge transport, and operational functionality of GO as either a hole-transport layer or hole-blocking layer.
The topography of the P3HTNPs (Fig. 1c) shows spherical nanoparticles ranging from 50 to 100 nm that tend to agglomerate, probably during the deposition drying process (low magnification images in Fig. S3†). The P3HTNPs present mostly a bright SP contrast (Fig. 1d) with respect to the ITO substrate. The SP of the P3HTNPs is not homogeneous: at high resolution, it is possible to resolve a SP nanostructure within the nanoparticle not correlated with topographic features. The SP difference between high and low SP regions is as high as +250 mV, consistent with the existence of different types of polymer chain aggregates. The high SP regions correspond to more disordered or non-aggregated entangled polymer chains, while the low SP regions can be ascribed to molecularly ordered polymer chain aggregates of H-type characteristic, in agreement with recent findings.11 In these aggregates, the P3HT chains adopt a face-to-face orientation enabling effective π–π inter-chain stacking interactions, which leads to a lowering of the highest occupied molecular energy level, and a corresponding increase of the work function. Results from UV-Vis spectroscopy confirm the co-existence of ordered domains of H-aggregates, exhibiting weakly coupled excitonic interchain interactions and regions of non-aggregated polymer chains in the P3HTNPs.16
To access the optoelectronic properties on a nanometer scale and, in particular, the SPV, KPFM experiments are performed without and with illumination at λ = 535 nm. This wavelength ensures the excitation of the P3HTNPs systems and the involvement of their fundamental vibronic transitions relevant for the transport properties, as outlined in Fig. S4.† As discussed in more detail in the ESI,† in addition to the acquisition of whole SP images in darkness (SPdark), a “two pass” illumination method is applied, where each image line is scanned with and without illumination.26 From this “two pass” method, a pair of SP images (SPon, SPoff) is obtained. These two images allow the classification of the total SPV = SPon − SPdark into a fast contribution SPVfast = SPon − SPoff and a slow contribution SPVslow = SPoff − SPdark. SPV experiments carried out on the pristine materials show that both the GO and the ITO substrate present a null SPV (SPdark = SPon), as expected due to the negligible absorption of these materials at λ = 535 nm (results not shown). In contrast, under green illumination, the SPon of the P3HTNPs is shifted by about −50 mV (Fig. 1f), which is entirely due to slow optoelectronic processes. This well-known behavior is characteristic of the P3HT/ITO interface: holes are transferred to the ITO substrate and trapped at the interface, while electrons are pushed to the external surface.30,31 This explains the slow characteristic of the relaxation process (SPV ≈ SPVslow).
Low magnification images show that in the ex situ sample, the P3HTNPs preserve their original size, while in the in situ sample, they tend to form larger aggregates. In both samples, from the topography (Fig. 2a and d) and the SPdark (Fig. 2b and e) images, we distinguish between two types of P3HTNPs exhibiting a very different SPdark value: those situated above and those localized between the GO sheets (marked with yellow and black circles, respectively, in Fig. 2). For both ex situ and in situ P3HTNPs, those placed on top of the GO sheet always present a large bright SPdark contrast. For the ex situ P3HTNPs (Fig. 2b), the ΔSP value with respect to the GO sheet is about +220 ± 50 mV. This value is similar to the one observed in the P3HTNPs directly deposited on ITO, indicating that the polymer chain aggregates within the P3HTNPs are unaltered during the ex situ mixing and confirming the previous spectroscopic results.16 We thus conclude that the properties of the ex situ P3HTNPs–GO ensembles can be completely understood knowing the individual properties of the P3HTNPs on the one hand and the GO on the other, as deduced from the data shown in Fig. 1.
The in situ P3HTNPs, in contrast, behave very differently. The SPdark value is reduced to +100 ± 50 mV (Fig. 2e). Taking into account the fact that the SPdark value is mainly related to the chain aggregates within the P3HTNPs that determine its local work function, the lower SPdark value suggests the existence of a higher number of J-type aggregates (crystalline regions formed by lamellae of folded P3HT chains in a head-to-tail conformation) in the in situ nanoparticles.12,14 Furthermore, the interpretation of lower SPdark values in terms of a higher number of J-type aggregates is fully consistent with the spectroscopic UV-Vis data of in situ P3HTNPs–GO samples.16 These reveal significantly reduced exciton-coupling constants, characteristic of J-aggregates with their highly planar P3HT chains, enabled through π–π interface interactions with GO during the in situ re-precipitation self-assembly process.
The SPdark difference between ex situ and in situ samples is even more drastic in the P3HTNPs covered by GO sheets. The ex situ nanoparticles show a slightly bright SPdark contrast due to the screening effect of the high permittivity GO sheet that reduces the apparent SP of the nanoparticle.32 However, the in situ P3HTNPs present an apparent dark contrast with respect to the GO. As will be discussed below, this SPdark contrast inversion cannot be explained assuming that the GO plays a passive role, acting just as a screening layer, but we should assume that the polymer chain aggregation is modified and that an electron transfer to the GO sheet is taking place. This fully agrees with the results from Raman, FTIR and XPS spectroscopy,16 thus confirming the formation of a ground-state charge-transfer complex on the scale of individual P3HTNPs.
SPV experiments (Fig. 2c and f) also reveal remarkable differences between the in situ and ex situ photoresponses of the P3HTNPs. In the ex situ samples, independently of the nanoparticle position (above or below GO sheets), the P3HTNPs present a positive SPV of about +100 mV. This behaviour is completely opposed to the one of in situ samples, where the P3HTNPs show a negative SPV of about −150 mV that is even more pronounced for the particles covered by GO sheets. Moreover, comparing the SPV image together with its corresponding SPVfast and SPVslow components, we find that the ex situ P3HTNPs almost recover their initial SPdark value instantly (in our experimental time scale <2 s), therefore the SPV ≈ SPVfast. In contrast, for the in situ P3HTNPs, we find SPV ≈ SPVslow, i.e. the photogenerated charges do not have enough time to relax during the “off” part of the two-pass method.33 It should be mentioned that in the in situ sample, a few particles present an ex situ-like behaviour (marked with a red arrow in Fig. 2(d), (f) and (g)). We attribute this performance to P3HTNPs that during the in situ self-assembly process could not establish interactions with GO and thus co-exist as a clearly discernable minor fraction in the in situ P3HTNPs–GO sample.
Taking into account that the SPV is sensitive not only to the polymer chain aggregation determining the photoexciton generation and charge separation within the particle but also to the nanoparticle/GO interface controlling the charge-injection from the P3HTNPs to the GO,34 we can extract several important conclusions: in the ex situ sample, the SPV sign and the time scale is opposite to the SPV of the P3HTNPs directly deposited on ITO (Fig. 1f). Thus, the SPV differences should be due to the presence of low conducting GO sheets in between the ITO substrate and the nanoparticle. In this situation, the GO acts as a blocking-charge layer, whereby the photogenerated charges do not leave the nanoparticle but are redistributed within it. In this situation, as schematically represented in Fig. 3a, the SPV possibly is related to the formation of an effective positive dipole. However, its origin is not easy to elucidate and it may arise from a number of possible processes.34 The photogenerated charges remain in proximity to the nanoparticle and can quickly recombine during the dark periods of the “two pass” method leaving only a small SPslow component. In contrast, in the in situ sample, the SPV behaviour is qualitatively similar to the one observed in the pristine P3HTNPs sample; however, a larger SPV effect is observed since the SPV value changes from −40 mV to −150 mV. This means that holes are more effectively transferred to the GO sheet, acting as hole-acceptor, while the particle remains negatively charged (Fig. 3b).17
In this situation, the photogenerated charges are far apart and the equilibrium is slowly restored once the sample is brought again into darkness. The resulting charge-separation of the photogenerated charges in the P3HTNPs–GO hybrid system should thus be responsible for the enhanced photocurrents observed in thin film devices.
Ex situ samples are characterized by P3HTNPs not interacting with GO sheets, leading to a shielding effect under SPdark conditions and to quickly relaxing effective dipoles under illuminated conditions. In situ samples are characterized by donor–acceptor interactions leading to a P3HT–GO charge-transfer complex with reduced SP dark values and transfer of photogenerated holes to GO under illuminated conditions.
The fact that the SPV sign and the time scale of the relaxation processes differ depending on the ex situ or in situ P3HTNPs–GO prepared samples provides clear evidence that during the in situ preparation process P3HTNPs–GO charge-transfer complexes are formed. These are based on favourable donor–acceptor interface interactions involving J-type aggregates with a more planar chain conformation as evidenced by the SPdark. This results in significantly reduced exciton-coupling constants of the more planar P3HT chains interacting with GO enabling the transfer of photoexcited holes to GO sheets and thus the effective separation of the photogenerated charges (see also sections S1.1 and S4 in the ESI†).
To further understand the in situ P3HTNPs–GO nanohybrids formation, and how it determines the photogenerated charge transport through the P3HTNPs–GO interface, we will focus on the P3HTNPs situated below the GO sheet, where the interface is directly exposed to the SFM tip, compared to the nanoparticles located above the GO sheet, where the interface is hidden by the particle itself. A detailed inspection of the in situ P3HTNPs–GO interface by means of high-resolution images (Fig. 4) reveals that the dark SPdark of the P3HTNPs below the GO sheet has a SP sub-structure that clearly differs from the surrounding GO. Here, large low SP domains comprising most of the P3HTNPs coexist with smaller high SP regions (Fig. 4b). The existence of the low SP domains, related to higher local electronegativity, indicates that at these regions, a negative charge-transfer from the P3HTNPs to the GO is taking place, modifying the local GO work function, confirming the formation of donor–acceptor P3HTNPs–GO charge-transfer complexes, as discussed above.
Under illumination (Fig. 4c), the entire P3HTNP becomes more electronegative (SPon is downshifted). Although a certain SPon particle sub-structure is also observed, there is no direct correlation with the original SPdark one, as further evidenced by the SPV image (Fig. 4d). Moreover, small positively charged patches appear at the surrounding GO sheet confirming that the photogenerated holes are effectively injected from the P3HTNPs to the GO sheets over distances of several tens of nanometers, demonstrating the role of the interacting GO sheets as a HTL. The successful charge-separation of the photogenerated charges disclosed in this KPFM study explains well the findings of enhanced photocurrents in films of P3HTNPs–GO donor–acceptor ensembles and thus further highlights their value as a photoactive layer.16
Footnotes |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c9nr01491h |
‡ Present address: Univ. Bordeaux, CNRS, Bordeaux-INP, LCPO, UMR 5629, F-33600 Pessac, France. |
This journal is © The Royal Society of Chemistry 2019 |