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Role of Near Substrate and Bulk Polymer Morphology on Out-of-Plane Space-Charge Limited Hole Mobility Johnathan Turner, and Abay Gadisa ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b11232 • Publication Date (Web): 09 Nov 2016 Downloaded from http://pubs.acs.org on November 10, 2016

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ACS Applied Materials & Interfaces

Role of Near Substrate and Bulk Polymer Morphology on Out-of-Plane Space-Charge Limited Hole Mobility Johnathan Turner, Abay Gadisa* Department of Physics and Organic and Carbon Electronics Laboratory, North Carolina State University, Raleigh, NC 27695, USA. KEYWORDS. Mobility, Morphology, Transport, Thickness-dependent, Bulk Heterojunction, Solar Cell

ABSTRACT: Charge transport is a central issue in all types of organic electronic devices. In organic films, charge transport is crucially limited by film microstructure and the nature of the substrate/organic interface interactions. In this report, we discuss the influence of active layer thickness on space-charge limited hole transport in pristine polymer and polymer/fullerene bulk heterojunction thin films (~15-300 nm) in a diode structure. According to the results, the out-ofplane hole mobility in pristine polymers is sensitive to the degree of polymer chain aggregation. Blending the polymers with a fullerene molecule does not change the trend of hole mobility if the polymer tends to make an amorphous structure. However, employing an aggregating polymer in a bulk heterojunction blend gives rise to a marked difference in charge carrier transport behavior compared to the pristine polymer and this difference is sensitive to active layer thickness. In aggregating polymer films, the thickness-dependent interchain interaction was found to have direct impact on hole mobility. The thickness-dependent mobility trend was found to correspond well with the trend of fill factors of corresponding bulk heterojunction solar cells.

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This investigation has a vital implication for material design and the development of efficient organic electronic devices, including solar cells and light-emitting diodes. 1. INTRODUCTION Conjugated polymers have unique chemical and structural properties that enable the development of cheap, and flexible opto-electronic devices for energy production, lighting, sensors, and several other applications.1,2 In solid films, most conjugated polymers form amorphous phases characterized by various scales of nano-crystalline domains. For example, the chains of region-regular poly(3-hexylthiophene) (rr-P3HT) can easily organize into semicrystalline

aggregates,3

while

poly(2-methoxy-5-(2′ethyl-hexoxy)-1,4-phenylene-vinylene)

(MEH-PPV) possess relatively negligible chain packing and inhomogeneous phases leading to larger energetic disorders, thus lower hole mobility.4 Moreover, rr-P3HT is known to form ordered stacks of chains defined by edge-on and face-on configurations, wherein the π-π stacking direction lies along the face-on configuration.5 This varying chain orientation leads to an anisotropy in carrier mobility.5 In general, charge carrier mobility in conjugated polymers is limited by several factors including substrate type,6 molecular weight,7 side chain,8,9 and region-regularity.10 Moreover, since organic films comprise amorphous and aggregated structures coexisting together, the interconnecting routes among these different domains play crucial role in creating fast and efficient percolation paths.11,12 These extrinsic and intrinsic effects impose severe restriction on the performance of electronic devices such as polymer solar cells (PSCs) and light emitting diodes (PLEDs), whose functionality is highly limited by charge transport. Spin-casted PSC bulk heterojunction films, in particular, compose vertical stratifications with varying composition and crystallinity, which directly links to device performance.13,14 In organic electronic devices, both the in-plane15 and


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out-of-plane16,17 carrier transport shows strong correlation with the vertical morphology of the active films. Such thickness-dependent vertical material stratification clearly determines the quality of carrier percolation networks and hence may lead to thickness-dependent mobility as previously observed in field effect transistors (FET),12,18,19 time-of-flight (ToF), and charge extraction by linearly increasing voltage (CELIV) measurement techniques.20,21 Herein we aim at measuring thickness-dependent hole mobility in singe-carrier diodes with both pristine polymer and polymer/fullerene blends. This investigation specifically aims at unveiling limitations of out-of-plane carrier transport close to the organic/substrate interface in various types of films processed from dilute and concentrated solutions. FETs measure exclusively inplane carrier transport, which usually does not match well with the out-of-plane carrier transport measured in diodes. On the other hand, thickness-dependent mobilities of pristine polymer layers measured by photo-CELIV method were found to correspond well with ToF measurements.20,21 This is expected since both methods probe the mobility of the fastest charge carriers with an exception of a rare observation of two transit peaks attributed to mobility distribution. Here, we exclusively study hole transport in steady state and in single-carrier device architectures, which have direct relevance for actual PSCs and PLEDs. Our investigation spans two material systems, namely P3HT and MEH-PPV, as well as their blends with the fullerene molecule phenyl-C61butyric-acid-methyl-ester (PCBM). P3HT is the most highly investigated bench-mark material.22 Moreover, compared to other materials which currently give high efficiency in PSCs, P3HT shows a significantly better promise for commercialization of PSCs due to its high relative scalability and stability. This was recently demonstrated by I. McCulloch et al. who reported efficient and highly air-stable P3HT/non-fullerene acceptor-based solar cells.23


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Hole mobilities extracted from the space-charge limited current-voltage (JV) characteristics of hole-only diodes, with varying organic layer thickness (~15-300 nm), show distinct trends based on material type and film thickness. In thin films (50 nm), hole mobility in pristine P3HT exceeds that of the blend material by nearly up to an order of magnitude, while this difference narrows down to a factor of 2 in thinner films (See Figure 2(b)). This is an interesting result that needs further explanations. The thickness-dependent FET, ToF and CELIV mobilities of pristine P3HT were mainly attributed to the preferential orientations of the polymer in the bulk of the film and near the substrate interface.12,21 Specifically, P3HT forms an edge-on orientation near substrates while the percentage of face-on orientation increases away from the substrate.12,21 Selective orientation of polymer chains are attained in many ways including changing solution concentration,29 substrate modification,12 region-regularity5 and solvent types.12 The face-on orientation favors out-of-plane carrier transport while the edge-on orientation facilitates in-plane transport as seen in FETs. The surface morphology of some of the films investigated here were characterized by atomic force microscopy (AFM) and the images show notable differences for P3HT films with different thickness as displayed in Figure 3. Inferring from the AFM phase images of 40 nm and 240 nm thick films of P3HT, we observe that the thinner films are characterized by shorter domains comprising bundled fiber-like structures whereas the thicker films are characterized by longer and highly interconnected fiberlike structures. This is also evident in the height images where the thick film shows greater roughness (see Fig. S1 in supporting information). Figure 3 also shows the AFM phase images of the P3HT/PCBM blend films, which do not show clear correlation with film thickness.


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The thickness-dependent hole transport assumes a completely different form in MEH-PPV and MEH-PPV/PCBM blend films. Compared to P3HT, hole transport in MEH-PPV films is lower by several order of magnitude (~10-7 cm2/Vs).30,31 The JV characteristics of hole-only diodes comprising MEH-PPV and the corresponding mobilities of both the pristine polymer and its blend with PCBM are depicted in Figure 4, while the JV characteristics of hole-only diodes with MEH-PPV/PCBM blends are shown in Figure S2 (Supporting information). The hole mobility is strongly field dependent for the pristine polymer while for the blend device the field dependence was observed only in thicker films as shown in Figure S3 in supporting information. In thin films, the mobility is weakly dependent on field for both device types, which seemingly correlates with the observed slight increase in mobility. At large, unlike the P3HT systems, the hole transport does not show significant variation with thickness. This indicates that the morphology of MEH-PPV and its blend with PCBM is most probably uniform throughout the film thickness with negligible preferential chain orientation. It is known that in solid films MEHPPV does not possess well defined micro-structures, and its electronic structure is mainly limited by the conformational disorder along its individual chains.4 In general, the overall results indicate that the organization of the organic materials at the organic/electrode interface and in the bulk may play a major role in the performance of electronic devices. Optical absorption and Photoluminescence Spectra. We have characterized the optical properties of P3HT and P3HT/PCBM films to find a correlation with film morphology and the measured charge transport characteristics discussed above. The microstructural changes of P3HT films show vibrionic features in optical absorption spectra. Figure 5 (a) shows the absorption spectra of P3HT films with varying thicknesses, while the absorption spectra of the blend film (see Figure S4, supporting information) show relatively weaker vibrionic peaks. As inferred from


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Figure 5 (a), aggregate-induced vibrionic peaks of P3HT are clearly visible regardless of film thickness, while the spectra of thinner films are slightly red-shifted, indicating morphological changes.8,32 According to Spano’s theoretical model,33 the degree of excitonic (intermolecular) coupling of interchain interactions can be estimated from the intensity ratio of the first and second vibrionic transitions using the following expression,33,34

≈

" "

#

$%.&'(/*+

$%.,-(/*+

&

.

(2)

where /%0 is the real part of the refractive index at 0 − 2 peak, and 34 is the energy of the vibration coupled to the electronic transition (0.18 eV for P3HT). The ratio

" "

was assumed to

be 1 (0.97 for P3HT),34–36 and 5 is the excitonic coupling bandwidth, which is inversely related

to conjugation length.34–37 The excitonic coupling bandwidth 5 is depicted in Figure 5 (b). In light of the interpretation of W, thinner films studied here are characterized by strongly aggregated chains with higher conjugation length, while thicker films comprised of lessaggregated chains defined by strong interchain interactions. The thickness dependence of 5 with device type seems to have a direct link with the observed change in mobility. The monotonic increase of 5 with P3HT film thickness was previously reported to be indicating an increase in face-on orientation, which facilitate charge transport in the out-of-plane direction.12 However, the decrease in mobility in thinner films could be due to dominating edge-on orientation near the substrate12 and/or lack of adequate domain interconnections as observed in the AFM images.11 The 5 of P3HT/PCBM film approaches that of P3HT in thinner films, while clear differences emerge as film thickness tends to increase, indicating evolution of morphological changes. This is an interesting optical-probe-based observation probably indicating preferential phase


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separation of the bulk heterojunction blend in thin films (near the substrate interface), whereby the P3HT chain conformation and interconnections are similar regardless of the presence of PCBM in the blend film.14 In fact, the photoluminescence quenching of the blend film was found to be thickness-dependent as shown in Figure S5 in the supporting information. In thin films, exciton quenching was dramatically reduced indicating a large polymer-fullerene phase separation. Currently, we do not have any evidence of the type of phase separation (lateral versus vertical), and this should be a subject of future investigation. However, this result indicates that the kinetics of material organization under spin-casting condition may be highly influenced by the concentration of the casting solution (see experimental section for processing details).29,38 Based on these results, it is observed that films spin-casted from less concentrated solution (90%. Adv. Energy Mater. 2015, 5 (15), 1500577.

(45)

Gasparini, N.; Jiao, X.; Heumueller, T.; Baran, D.; Matt, G. J.; Fladischer, S.; Spiecker, E.; Ade, H.; Brabec, C. J. Designing Ternary Blend Bulk Heterojunction Solar Cells with Reduced Carrier Recombination and a Fill Factor of 77%. Nat. Energy 2016, 1 (9), 16118(1)‒ 16118(9).


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Figure Captions Figure 1. The current‒voltage characteristics of hole-only diodes with (a) P3HT, and (b) P3HT/PCBM films. The lines are fits generated using the space charge limited charge transport model described in equation 1. Figure 2. (a) Thickness-dependent hole mobility of P3HT and P3HT/PCBM films. (b) The ratio of the hole mobility of P3HT against that of P3HT/PCBM blend. Figure 3. AFM phase images of (a) 40, and (b) 240 nm thick P3HT films. The corresponding phase images of P3HT/PCBM blend films of 35 nm and 270 nm are displayed in (c), and (d), respectively. Figure 4. (a) The current-voltage characteristics of hole-only diodes with different thicknesses of MEH-PPV solid films. The symbols represent experimental data and the lines are fits generated by the space-charge limited charge transport model. (b) The hole mobilities in the pristine MEHPPV polymer and its blend with PCBM. Figure 5. (a) Absorption spectra of P3HT, and (b) the exciton bandwidth of P3HT and P3HT/PCBM films are displayed as a function of film thickness. Figure 6. (a) Current-voltage characteristics of P3HT/PCBM BHJ solar cells measured under an A.M. 1.5 solar light illumination (intensity 100 mW/cm2), and (b) device fill factor as a function of film thickness.


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Figure 1

Figure 2


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Figure 3

Figure 4


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Figure 5

Figure 6


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Role of Near Substrate and Bulk Polymer ... - ACS Publications

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