Article pubs.acs.org/JPCC
Tunable Exciton Dissociation and Luminescence Quantum Yield at a Wide Band Gap Nanocrystal/Quasi-Ordered Regioregular Polythiophene interface Seongeun Cho,† Youngjun Kim,† Yujin Park,† Miri Choi,‡ Jun-young Park,§ Jihoon Lee,∥ Sungyoung Park,⊥ Mincheol Chang,# Jiung Cho,*,‡ Insik In,*,∥ and Byoungnam Park*,† †
Department of Materials Science and Engineering, Hongik University 72-1, Sangsu-dong, Mapo-gu, Seoul 121-791, Korea Korea Basic Science Institute, Jukheon-gil, Gangneung 210-702, Gangwon-do, Korea § HMC&Green Research Institute, Department of Nanotechnology and Advanced Materials Engineering, Sejong University, Seoul 143-747, Korea ∥ Department of Polymer Science and Engineering and ⊥Department of Chemical and Biological Engineering/Department of IT Convergence, Korea National University of Transportation, Chungju 380-702, Korea # School of Polymer Science and Engineering, Chonnam National University, Gwangju 61186, Korea ‡
S Supporting Information *
ABSTRACT: A comprehensive understanding of the effect of polymer chain aggregation-induced molecular ordering and the resulting formation of lower excited energy structures in a conjugated polymer on exciton dissociation and recombination at the interface with a wide-bandgap semiconductor is provided through correlation between structural arrangement of the polymer chains and the consequent electrical and optoelectronic properties. A vertical diode-type photovoltaic test probe is combined with a field effect current modulating device and various spectroscopic techniques to isolate the interfacial properties from the bulk properties. Enhanced energy migration in the quasi-ordered (poly(3hexylthiophene)) (P3HT) film, processed through vibration-induced aggregation of polymer chains in solution state, is attributed to the presence of the aggregationinduced interchain species in which excitons are allowed to migrate through low barrier energy sites, enabling efficient iso-energetic charge transfer followed by the downhill energy transfer. We discovered that formation of nonemissive excitons that reduces the photoluminescence quantum yield in the P3HT film deactivates exciton dissociation at the donor (P3HT) close to the acceptor (ZnO) as well as in the P3HT far away from the ZnO. In other words, exciton deactivation in its film state arising from the quasi-ordered structural arrangement of polymer chains in solution is retained at the donor/acceptor interface as well as in the bulk P3HT. Effect of change in the highest occupied molecular orbital level and the resulting energy band bending at the P3HT/ZnO interface on exciton dissociation is also discussed in relation to the presence of vibration-induced aggregates in the P3HT film.
1. INTRODUCTION
In controlling structural arrangement of polymer chains in solution state, the choice of solvent, addition of poor solvent in the solution of conjugated polymer and the concentration of solution have been known to be effective in changing structural ordering by altering interaction between polymer chains, resulting in aggregation by which polymer chain segments are arranged in close proximity to each other sharing their πelectron density.7−9 The choice of solvent resulted in a different degree of crystallinity, for example, by altering the degree of freedom of polymer chains determined by the conformation of the solvent.10,11 Recently, change in the structural arrangement of polymer chains through sonication has been reported,
Controlling structural arrangement of conjugated polymers in solution state has provided new opportunities in tuning electrical and optical properties because the arrangement is retained into the film state after solidification, altering dark and light characters. Particularly, as the size of the electronic devices becomes smaller, the contribution of the interface to the bulk has become significant in flexible electronics such as display and energy conversion/storage devices incorporating the conjugated polymers.1−3 Tuning crystallinity of conjugated polymers in the solution state has, therefore, a profound impact in optimizing performance and acquiring flexibility in structural design by altering relevant charge transfer and transport properties near the interface region in photovoltaics (PVs), light-emitting diodes (LEDs), and flexible batteries.4−6 © 2016 American Chemical Society
Received: August 29, 2016 Revised: October 24, 2016 Published: October 24, 2016 26119
DOI: 10.1021/acs.jpcc.6b08728 J. Phys. Chem. C 2016, 120, 26119−26128
Article
The Journal of Physical Chemistry C
measurements, it is important to note that deactivation of exciton dissociation due to VIA of conjugated polymer chains is found to occur at the conjugated polymer/electron acceptor interface as well as in the bulk, providing insights into interface engineering in LEDs and PV devices. Energy band bending at the interface depending on the formation of aggregates is also discussed.
demonstrating crystallinity can be tuned depending on sonication time.12 Surprisingly, the field effect mobility has been improved significantly by orders of magnitude. The effect of aggregation induced interchain species in conjugated polymers, as a result of post fabrication process such as thermal annealing, on the luminescence quantum yield has been extensively researched due to application to display devices including organic light-emitting diodes.9,13 In isolated π-conjugated polymers, polaron−excitons in which photogenerated electrons and holes are localized and bound by lattice distortion comprise primary photoexcitations. Through strong interchain interactions, lower energy excited states compared with isolated polymer chains are created forming interchain species.14−16 The lower energy excited states are not optically coupled to the ground state forming radiationless pathways. The presence of the nonemissive interchain species (so-called aggregates) has been known to degrade the quantum yield detrimental to the application of the LEDs.15,17 In contrast to the study of the luminescence quantum yield depending on the degree of interaction between polymer chains in solution state, however, effect of aggregation-induced interchain species on exciton dissociation both at the donor/ acceptor interface and in the bulk has not been highlighted because of its complexity arising from change in the energy level both in the bulk and at the interface when coupled to the counterpart. Indeed, it has been reported that the highest occupied molecular orbital (HOMO) energy levels of (poly(3hexylthiophene)) (P3HT) films were altered depending on the degree of crystallinity, potentially affecting exciton dissociation when coupled to the electron-accepting materials.18 Especially, considering that charge transport can be tuned by modifying the degree of conjugation resulting in change in the carrier mobility, a comprehensive understanding of the correlation between structural properties and the behavior of excitons is crucial in optimizing PVs and LEDs. To induce strong interchain interactions between polymer chains, irradiation of vibration in solution state has been developed, demonstrating far better charge transport properties as a result of enhanced molecular ordering.12,19 In researching the effect of vibration-induced aggregation (VIA) of polymer chains in solution on exciton dissociation, P3HT and ZnO nanocrystals (NCs) were adopted because they are representative electron donor and acceptor materials, respectively, for hybrid photovoltaic devices. The individual P3HT and ZnO nanocrystal (NC) component layers are model materials in conjugated polymers and nanostructured metal oxide for electron extraction or transport, respectively, for applications to LEDs and PVs. Incorporating these materials in assembling PVs has advantages over other novel and new synthesized materials because these materials are very well-known and are best suited to fundamental understanding of correlation between structure of material and optoelectronic outputs. The present study demonstrates how exciton dissociation under illumination as well as charge transport in the dark can be altered through VIA of conjugated polymer chains in solution state, resulting in the formation of quasi-ordered polymer domains. From field effect transport measurements in combination with photovoltaic devices incorporating donor/ acceptor interface, it is found that quasi-ordered conducting polymers in solution state deactivates both exciton dissociation and emission in its film state, while charge transport is improved by enhanced structural ordering. Through photoinduced field effect transistor (FET) threshold voltage
2. EXPERIMENTAL SECTION ZnO NC Synthesis. Zinc acetate dihydrate (1.097g) was dissolved in 93.75 mL ethanol and the solution was stirred for 1 h at 60 °C. In a separate container, tetramethylammonium hydroxide pentahydrate (TMAH, 0.9687 g) was dissolved in 4 mL ethanol at room temperature with the beaker covered with para-film to prevent solvent evaporation. After stirring for 1 h, the TMAH aqueous solution was added dropwise to the zinc acetate solution, maintaining 60 °C. The injection process lasted for 15 min. After injection process, the solution was maintained at 60 °C for 30 min and then slowly cooled in air. The mother-liquor solution was stored in refrigerator at ∼5 °C for use. For device fabrication, the solution was mixed with 30 mL of hexane and centrifuged at 9000 rpm for 10 min. The entire process was performed in ambient air. Solar Cell and FET Fabrication. A bottom-contact FET was structured to measure charge transport in P3HT layers close to the SiO2 interface and exciton dissociation at the P3HT/ZnO interface through the FET mobility and photoinduced threshold voltage measurements. The source and drain electrodes [Au (80 nm)/Ti (3 nm)] were photolithographically patterned onto a 200 nm SiO2 gate dielectric. Highly doped silicon substrate served as a gate electrode. P3HT solutions with different concentrations ranging between 1.25 and 30 mg/ mL were spin-coated on the SiO2 gate dielectric. To induce molecular ordering in P3HT films through vibration, P3HT solutions were sonicated in a cold water bath for 30 min. For investigation of photoinduced charge transfer at the P3HT/ ZnO interface, ZnO nanoparticle films (∼7 nm) were deposited prior to deposition of P3HT. Fabrication of P3HT/ZnO PV test probe devices, pristine and sonicated P3HT solutions were spin-coated on an indium tin oxide substrate followed by spin-coating of ZnO. Al (80 nm) electrodes were formed onto the ZnO layer. Investigation of the structural properties of P3HT and P3HT on ZnO, atomic force microscopy (AFM) and X-ray diffraction (XRD) measurements were carried out in air. FET and PV measurements were done using HP4145B semiconductor parameter analyzer. To illuminate FET and PV test probe devices, a green laser diode with a wavelength of 520 nm (5 mW/cm2) was used. A green laser diode was used for selective excitation of P3HT films excluding complications associated with ultravioletinduced conductivity enhancement effect in the electron acceptor (ZnO) layer.20 All the electrical measurements were carried out in an argon-filled glovebox. Photoluminescence Decay Measurements. For PL decay lifetime measurement, a light source at a wavelength of 450 nm was used as an excitation sorce of P3HT. The PL intensity was measured at 655 and 705 nm. P3HT films were spin-coated on glass substrate from in its solution state (10 mg/ mL). To fabricate a sonicated P3HT film, the solution was sonicated (1500 rpm for 1 min.) for 30 min in an ice bath. The color of P3HT solutions changed from transparent red to dark wine, resulting from aggregation of polymer chains induced by sonication. To minimize oxidation of the pristine and sonicated 26120
DOI: 10.1021/acs.jpcc.6b08728 J. Phys. Chem. C 2016, 120, 26119−26128
Article
The Journal of Physical Chemistry C P3HT films, the films were encapsulated by a glass slide sealed through a taping method. Energy Band Bending Measurement Using PESA. By measuring the HOMO level depending on P3HT thickness spin-coated on a ZnO substrate, the band bending of the HOMO level at the ZnO/P3HT interface was measured. P3HT thickness was varied between 18 and 160 nm by changing the concentration of P3HT solution in chloroform. To change the P3HT concentration, chloroform was systematically added in a 30 mg/mL P3HT solution. For measurement of the P3HT HOMO level, photoelectron spectroscopy in air (PESA) was used in which an incident radiation energy was scanned between 4.0 and 6.2 eV. PESA enables measurement of the ionization energy by estimating the relative intensity of electrons at different energy levels emitted from the sample. During irradiation, the incident energy was scanned between 4.2 and 6.0 eV, exciting electrons in the ground state above the vacuum energy level. The electrons photoemitted are collected and counted by the anode electrode at a high positive voltage. The number of photoemitted electrons are converted to the photoemission yield at a given photon energy. A weak UV (∼50 nW/cm2) intensity was used to suppress surface charging effect. A ZnO film (∼40 nm) was spin coated onto a clean glass substrate at 1500 rpm for 1 min and then allowed over 30 min for drying in air. The P3HT solutions treated at different concentrations were spin-coated on the ZnO/glass substrate at 1500 rpm for 1 min. All the process was conducted in an Arfilled glovebox. The films were encapsulated for protection from moisture and oxygen. UPS Analysis of ZnO NCs. UPS measurements were carried out in vacuum (10−10 mbar) using monochromatized He I radiation (hν = 21.2 eV) to measure the valence band maximum of ZnO NCs. UPS samples were biased at −10 V for measurements in the secondary electron cut off and the valence region. Mott−Schottky Analysis. To calculate the built-in potential at the interface, Mott−Schottky analysis was performed at a modulation frequency of 1 kHz for ITO/ P3HT (60 nm)/Al and ITO/P3HT (60 nm)/ZnO/Al devices. The plot of 1/C2 as a function of potential was measured at a voltage range between −2 and 2 V. To determine the value of the built-in potential, a linear fit in the reverse bias region was obtained to extrapolate to zero (y-axis) at which the voltage is a built-in potential.
Figure 1. (a) Optical absorption spectra for pristine and sonicated P3HT films. The concentration of P3HT solution in chloroform is the same at 10 mg/mL. (b) ID−VG curves for pristine and sonicated P3HT FETs measured at the linear regime in transistor operation (drain voltage = −3 V). The inset shows a schematic of the bottom-contact FET.
P3HT film.21 Comparing the absorption spectra from solution (Supporting Information Figure S1) and film states, those of the P3HT film states feature well-resolved peaks with the major peaks being red-shifted, clarifying that those are associated with a larger effective conjugation length.21 Importantly, the presence of the peak at ∼610 nm in the sonicated P3HT solution, not seen in the pristine one, ensures that the individual polymer chains aggregate undergoing disorder− order transformation. This demonstrates that enhanced molecular ordering in the sonicated P3HT film results from the formation of aggregates in its solution state in which increased planarization of the conjugated polymer backbone occurs leading to a larger effective conjugation length. Enhanced molecular ordering observed from the spectral absorption in the sonicated P3HT film is confirmed in out-ofplane grazing incidence X-ray diffraction (GIXD) data in Figure S2. The sonicated P3HT film exhibits (100) peak originated from the lamellar packing in which planar thiophene main chains are stacked forming π-orbital overlap, while the pristine P3HT has a broad peak, demonstrating that structural ordering was enhanced in the sonicated P3HT film with an increased overlap between side chains.22 Importantly, reduced “d”-spacing from ∼17.2 to ∼16.5 Å was observed after sonication, suggesting that the degree of interchain interaction between polymer chains can be tuned in the vibration-induced aggregates.12,22 Enhanced structural ordering in the sonicated P3HT film is reflected in improved charge transport of holes in the film as
3. RESULTS AND DISCUSSION Charge transport and structural properties in a P3HT film as a result of sonication in solution state are clearly compared with those in a pristine P3HT film through optical absorption and FET measurements in Figure 1. From optical absorption spectra of P3HT films in Figure 1a, the spectrum of a sonicated P3HT film is red-shifted from 516 to 522 nm and 553 to 560 nm in comparison with that of the pristine P3HT film, a signature of improved planarization of the conjugated backbone polymer chains in the sonicated P3HT film consistent with the previous result.12,14 The absorption peak at ∼605 nm becomes sharper than that of the pristine P3HT for which absorption is broader and featureless, explaining that vibration-induced interchain interaction between polymer chains in the sonicated P3HT film is stronger than in the pristine P3HT film. These suggest that cofacial π-stacked polymer chains, resulting from increased planarization, are well ordered with a larger effective conjugation length of the polymer chains in the sonicated 26121
DOI: 10.1021/acs.jpcc.6b08728 J. Phys. Chem. C 2016, 120, 26119−26128
Article
The Journal of Physical Chemistry C shown in Figure 1b. For both films incorporated in the bottomcontact FET arrangement in the inset, the drain current increased with increasing negative gate voltage at a low drain voltage, inducing more holes in the channel close to the gate dielectric, SiO2 in the bottom contact FET. Threshold voltage is the minimum required gate voltage to induce mobile carriers in the channel between the source and drain electrodes. Above the threshold voltage VT, the transistor is turned on and the magnitude of the drain current, ID, is proportional to the FET mobility of holes μ, and carrier concentration, Ci(VG−VT), as given by below eq 1 ID =
Z μCi(VG − VT)VD L
(1)
where, Ci is the capacitance per unit area of the interface, Z is the width, and L is the channel length of the device. To measure the FET mobility and the threshold voltage, the gate voltage was plotted as a function of drain current in the linear regime of transistor operation. On the basis of eq 1, the FET hole mobility of a sonicated P3HT film was 4.5 × 10−4 cm2/(V s) while that of a pristine P3HT film was 2.6 × 10−5 cm2/(V s). The far higher mobility is due to enhanced π-orbital overlap arising from stacked lamellar structure, evidenced by the GIXD and optical absorption measurements. Using a Schottky-diode arrangement (ITO/P3HT/Al), charge carrier transport following exciton separation under illumination with a green laser diode (530 nm) in pristine and sonicated P3HT films was compared by measuring the photocurrent in the forward (positive voltage) and reverse (negative) regimes in Figure 2. In the dark, a far larger FET mobility in the sonicated P3HT film [Figure 1b] is consistent with a higher current in the forward bias regime in Figure 2a, assuming that the lowest unoccupied molecular orbital (LUMO) and the HOMO energy levels of the pristine and sonicated P3HT films are identical. Indeed, the assumption is validated by optical absorption and photoelectron spectroscopy in air (PESA) measurements, demonstrating that the HOMO levels and the optical band gap of both P3HT films are similar (see the Supporting Information Figure S3). In the forward bias regime, holes in the sonicated P3HT film have enhanced transport properties both in the dark and under illumination. In the reverse regime (negative voltage region), magnified in Figure 2b, however, the photocurrent density in the pristine one increased by a factor of 5 in comparison with that of the sonicated one. The reversal of the photocurrent in the reverse regime, particularly in the short-circuit condition, indicates that dissociation of excitons into free electrons and holes in the reverse regime is dominant over the carrier transport properties in determining the magnitude of the photocurrent. It is noted that the absorption coefficient is similar at the wavelength of 530 nm in both films, ensuring that the short circuit current difference is not due to difference in optical absorption in both films. Under illumination, a higher threshold voltage shift to a more positive value in the pristine P3HT FET (−9.0 V → 8.0 V, ΔVT = 17 V) than that in the sonicated P3HT FET (2.0 V → 5.3 V, ΔVT = 3.3 V) [Figure 1] is consistent with a higher shortcircuit current in the diode structure, explaining that more excitons in the pristine P3HT film were dissociated according to the equation Δn = CoxΔVT/q. Here, Δn corresponds to the number of carriers dissociated under illumination. More number of dissociated carriers fill the deep traps and cause
Figure 2. (a) Plots of current density as a function of voltage for ITO/ P3HT/Al PV test probes. (b) A magnified view of the short circuit and open circuit bias conditions in (a). The thickness of the P3HT films is 60 nm. A green laser diode (5 mW/cm2, 530 nm) was used as a light source.
earlier turn-on in the FET, shifting the threshold voltage to a more positive value.23,24 To probe exciton dissociation depending on VIA of polymer chains in the presence of an electron acceptor, a conventional hybrid PV device was adopted as a test probe by inserting ZnO NCs between P3HT and Al as seen in the inset of Figure 3a. Far higher short-circuit current in the pristine P3HT is sustained even with the introduction of a ZnO NC film. The valence and conduction band edges of ZnO NC film in our experiments were determined using ultraviolet photoelectron spectroscopy (UPS) from the Supporting Information Figure S4 to complete ITO/P3HT/ZnO/Al energy band diagrams in Figure 3b. Energy band diagrams measured for P3HT and ZnO are displayed in Figure 3b. Considering a far higher FET hole mobility in the sonicated P3HT film [Figure 1], a higher shortcircuit current in the P3HT/ZnO assembly can be attributed to a higher mobile carrier concentration as a result of efficient excition dissociation in the pristine P3HT film. The short circuit current (JSC) in the diode arrangement is represented by the drift current and the diffusion current as given in eqs 2 and 3 JSC = Jh + Je (2) ⎛ dp ⎞ Jh = enhμ pE + kT μ p⎜ ⎟ ⎝ dx ⎠
(3)
, where μp is the hole mobility, dp/dx is the hole concentration gradient, E is the electric field, k is the Boltzmann constant, and T is the temperature. In the PV test probe, ITO/P3HT/ZnO/ Al, the short circuit current is largely determined by the hole 26122
DOI: 10.1021/acs.jpcc.6b08728 J. Phys. Chem. C 2016, 120, 26119−26128
Article
The Journal of Physical Chemistry C
Figure 4. GIXD for sonicated and pristine P3HT films deposited on ZnO or glass substrate. The thickness of P3HT is 60 nm.
Under illumination, higher concentration of mobile carriers in the pristine P3HT film than in the sonicated P3HT film close to the ZnO NC film as a result of exciton dissociation at the P3HT/ZnO interface is consistent with the results from the photoinduced threshold voltage measurement using bottomcontact FETs.24 With a conventional PV cell structure in which component layers are vertically stacked, accurate probe of exciton dissociation at the electron donor/acceptor interface remains challenging because the short circuit current as a device output is determined by a combination of the concentration of dissociated carriers and the mobility in P3HT and ZnO NC layers. Furthermore, localized states (trap sites) at the ITO/ P3HT and ZnO/Al interfaces can alter the magnitude of the short circuit current.25 To probe exciton separation, excluding other complications, photoinduced threshold voltage measurements were carried out using a bottom-contact FET arrangement.24 Threshold voltage is defined as a required gate voltage to induce mobile charge carriers in the active layer. The magnitude of the threshold voltage is independent of the mobility, therefore, reflecting only the number of mobile carriers that can contribute to the drain current that increases linearly with the gate voltage, as demonstrated in the eq 1. To compare the number of carriers dissociated at the P3HT/ZnO interface, independent of the mobility in the P3HT films, threshold voltage difference between in the dark and under illumination was measured. For the FET with a pristine P3HT film deposited on a thin ZnO NC layer, as seen in the inset of Figure 5a, the threshold voltage shift under illumination with a green laser diode was 8.3 V, far larger than that for the sonicated one, 1.9 V, in Figure 5b. More positive threshold voltage shift in the pristine P3HT FETs with ZnO has reproducibly been observed. A schematic in Figure 5c illustrates that threshold voltage shift in P3HT films in the presence of a ZnO NC layer reflects the number of mobile carriers (hole) in the P3HT resulting from exciton dissociation at the P3HT/ZnO interface. Under illumination electron and hole pairs (excitons) in the P3HT film are created. Excitons created in the P3HT in close proximity to the ZnO layer are diffused into the ZnO side and separated through exciton dissociation, producing electron and hole polarons that can contribute to increase in the short-circuit current for the conventional PV device. In the case of a bottom-contact FET structure, as illustrated in Figure 5c, the LUMO and HOMO
Figure 3. (a) J−V curves for PV test probe with P3HT/ZnO interface. The inset shows the PV test probe structure. The probe device was illuminated with a green laser diode of 530 nm wavelength (light intensity: 5 mW/cm2). (b) Energy diagrams representing the LUMO and HOMO levels of P3HT and ZnO NC films. The valence and conduction band edges of the ZnO NC film were determined using UPS and optical absorption spectroscopy.
current (Jh) in the P3HT as given in the eq 3. To account for the higher short-circuit current in ITO/pristine P3HT/Al, therefore, the carrier concentration in the pristine P3HT, nh, resulting from exciton separation at the P3HT/ZnO interface should be far larger than that of the sonicated one, compensating for a lower FET mobility in the pristine P3HT film. It is important to note that, in the P3HT thickness for the above hybrid PV cell the electrical properties of the PV cell are limited by the P3HT/ZnO interfacial properties. In other words, the short circuit current difference found between the P3HT films in Figure 3 mainly results from exciton dissociation in the P3HT near the P3HT/ZnO interface. This is evidenced by the GIXD data in Figure 4 in which the (100) peak intensity of the P3HT film spin-coated on the ZnO NC film substantially increased in comparison with that of the P3HT film without ZnO for the pristine and sonicated P3HT films. This is interpreted that a significant fraction of the P3HT film exhibits interface-induced structural arrangement of the polymer chains. The d-spacing for the pristine P3HT was 17.2 Å and reduced to 16.5 Å after sonication independent of the presence of ZnO NC layer. A reduced d-spacing for the sonicated P3HT film regardless of the presence of ZnO NC layer clarifies that aggregation induced polymer phase is still retained even in the region close to the ZnO NC layer. Indeed, in the electrical measurements for the hybrid PV devices, the short circuit current increased with ZnO thickness increasing (i.e., increasing the series resistance) in Figure S5, demonstrating that the optoelectronic properties of the PV test probe are dominated by the interfacial properties. 26123
DOI: 10.1021/acs.jpcc.6b08728 J. Phys. Chem. C 2016, 120, 26119−26128
Article
The Journal of Physical Chemistry C
Figure 5. (a) ID−VG plots in the dark and under illumination (530 nm green laser diode at 5 mW/cm2) obtained in the linear regime of the transistor operation (VD = −3 V) for (a) pristine and (b) sonicated P3HT films deposited on a ZnO NC layer. The thickness of the P3HT films is 60 nm. The FET mobilities for pristine P3HT/ZnO and sonicated P3HT/ZnO were 1.6 × 10−5 and 7.3 × 10−4 cm2/(V s), respectively, for L = 70 μm Z = 1000 mm. The inset shows a schematic diagram of P3HT/ZnO FET as an exciton dissociation test probe. (c) Energy band diagrams in which exciton dissociation is facilitated under application of a gate electric field. The LUMO and HOMO levels are bent resulting from exciton dissociation. Under illumination, photogenerated electrons in the P3HT are dissociated and transferred to the ZnO accompanied by accumulation of holes in the P3HT close to the ZnO, bending the LUMO and HOMO levels upward. In other words, in the P3HT/ZnO interface hole carriers are induced in the P3HT in the absence of the gate electric field by band bending. Accumulation of holes in the absence of applied gate electric field leads to a positive threshold voltage.
In accounting for the origin of the enhanced exciton dissociation in the pristine P3HT film, energy band bending, that is, the electric field at the P3HT/ZnO interface should be considered because the interfacial energy band bending has been known to play crucial roles in determining the driving force for exciton dissociation.26 Indeed, according to SchottkyMott experiments from ITO/P3HT/Al and ITO/P3HT/ZnO/ Al devices, the built-in potentials for the devices including the pristine P3HT and sonicated P3HT films were distinctly different, implying that the interfacial properties are sensitive to VIA of polymer chains resulting from strong interaction between polymer chains (see the Supporting Information
energy levels of the P3HT are bent upward as a result of photoinduced electron transfer from the P3HT to the ZnO side, accumulating holes in the P3HT close to the ZnO layer. The formation of hole carriers in the P3HT even without applying gate voltage shifts the threshold voltage of the P3HT FET to a more positive value. In other words, under illumination the magnitude of the threshold voltage shift, ΔVT, in the linear regime of transistor operation is proportional to the number of carriers separated at the interface, Δn, by the relation Δn = CiΔVT/q. These clarify that exciton dissociation at the interface with the pristine P3HT is more efficient. 26124
DOI: 10.1021/acs.jpcc.6b08728 J. Phys. Chem. C 2016, 120, 26119−26128
Article
The Journal of Physical Chemistry C Figure S6). To elucidate the possibility, PESA27 was adopted to probe the electric field arising from energy band bending of the HOMO levels at the interface, varying the thickness of the P3HT film onto a ZnO NC layer in Figure 6. Figure 6a shows a
polymer chains.14,28 It has been well-known that the reduced PL intensity as well as red shift at the two PL peaks arises from the presence of the charge-separated interchain species such as “polaron pair” (spatially indirect exciton) quenching photoluminescence.9 According to Bredas et al.,29 in the conjugated polymers, as the distance between polymer chains decreases, the lowest singlet excited state is split into the number of chains in interaction. In other words, as the interchain distance decreases, excitonic splitting is initiated, generating lower energy states that can accommodate more excited electrons in comparison to those from intrachains. This facilitates charge transfer associated with the low-energy sites as a result of VIA of the polymer chains. Importantly, in cofacial configurations of dimers that can represent polythiophene, the lowest excited state arising from intermolecular interactions between chains is not optically coupled to the ground state, decreasing the luminescence quantum yield in comparison with that of isolated chains. Consequently, charge transfer rate to the lowest excited state increases, deactivating the radiative pathways, reducing the luminescence quantum yield. This is supported by our GIXD measurements in which d-spacing decreased after sonication of P3HT film, leading to the formation of the low-energy structure through interchain species in the film. From the GIXD data in Figure 4, VIA of the polymer chains in the sonicated P3HT film is still retained with ZnO from the fact that the crystallinity of the sonicated P3HT with ZnO was enhanced compared with that of the pristine P3HT with ZnO. The other evidence is a far higher FET mobility in the sonicated P3HT with ZnO compared with the pristine one, as seen in Figure 5. In the FET measurement, the FET mobilities of the pristine and sonicated P3HT on ZnO were 1.6 × 10−5 and 7.3 × 10−4 cm2/(V s), respectively. The FET current in P3HT modulated by the gate electric field through the gate dielectric is a sensitive probe of the interface between the gate dielectric and the active layer.30,31 In other words, the FET mobility is a measure of efficiency of charge transport in the P3HT close to the P3HT/ZnO interface. Consequently, a far higher FET mobility in the sonicated P3HT on ZnO demonstrates that enhanced crystallinity in P3HT through VIA of polymer chains is extended into the interface region in contact with ZnO and the presence of interchain species in the P3HT through VIA of the polymer chains is validated even at the close approximity to the ZnO. Reduced short circuit current in a sonicated P3HT film, arising from deactivation of exciton dissociation based on the PV test probe and interfacial energy band bending experiments performed in the earlier section, can be attributed to charge transfer to lower energy barrier sites of the photoexcited carriers resulting from energy splitting for a reduced interchain distance. Right after photon excitation, exciton dissociation competes with spectral relaxation of excitons, including intramolecular vibrational relaxation and self-trapping of excitons, on a time scale of few picoseconds.9,15 As mentioned in the previous section, increase in the splitting into lower energy states due to strong interchain interaction in the sonicated P3HT film facilitates spectral relaxation from the originally excited states to lower energy states induced by aggregation of polymer chains. In the circumstance, downhill energy transfer as seen in Figure 7b,c on the time scale of hundreds of femtosecond occurs followed by iso-energetic transfer which takes place slowly (hundreds of picoseconds). In sonicated P3HT films, more energy states at lower energy levels could be found, causing efficient iso-energetic energy transfer
Figure 6. (a) A schematic diagram of PESA measurement for P3HT/ ZnO/ITO samples. (b) Plots of the HOMO energy levels as a function of P3HT thickness for pristine and sonicated P3HT films.
schematic of PESA operation in which the incident photon energy produces photocurrent as a result of emission of electrons in the P3HT film. In Figure 6b, surprisingly, for the sonicated and pristine P3HT films, energy band bending of P3HT close to the ZnO exhibited distinctly different features while the HOMO levels in the bulk region far away from the ZnO were similar for both films. In the pristine P3HT film, upward band bending was observed while downward band bending was observed in the sonicated one. On the basis of the direction of the electric field at the interface, exciton dissociation at the sonicated P3HT/ZnO interface is supposed to be more favorable, expecting a higher short circuit current. A higher short circuit current in the pristine P3HT/ZnO interface even in the unfavorable electric field at the interface indicates that the mechanism that causes more efficient exciton dissociation in the bulk pristine P3HT for the ITO/P3HT/Al structure, as demonstrated in Figure 2, still dominates in separating excitons into mobile carriers. The mechanism will be discussed later. Photoluminescence (PL) measurement in Figure 7a provides insight into understanding of how a particular arrangement of polymer chains in the films can determine the efficiency of exciton dissociation. The PL intensity from a sonicated P3HT film is far lower than that from a pristine P3HT film, indicating that the quantum luminescence yield from the sonicated P3HT film is far lower. Importantly, the red shifts of the two PL peaks for emitting species from 662 (713) to 653 nm (705 nm), as seen in the blue (sonicated P3HT) and red (pristine P3HT) lines in Figure 7a, reflect enhanced interactions between 26125
DOI: 10.1021/acs.jpcc.6b08728 J. Phys. Chem. C 2016, 120, 26119−26128
Article
The Journal of Physical Chemistry C
Figure 7. (a) PL spectra for pristine (60 nm) and sonicated P3HT (60 nm) films. The excitation wavelength is 450 nm. Schematic diagrams representing downhill and iso-energetic transfer in the (b) sonicated and (c) the pristine P3HT films. Energy migration time scale of downhill transfer is hundreds of femtoseconds, whereas that of iso-energetic transfer is hundreds of picoseconds. In the sonicated P3HT film more available pathways for energy migration exist. (d) PL decay measurements for pristine and sonicated P3HT films. The excitation wavelength is 450 nm and the PL intensity was measured at 705 nm. (e) A schematic diagram for comparison of energy barrier for energy migration over trapping sites in the pristine and sonicated P3HT films.
[Figure 7b], while for pristine P3HT films the distance between the energy sites is substantial, suppressing efficient energy migration over the polymer chains [Figure 7c]. In other words, energy transfer in the sonicated P3HT film is more efficient due to more available pathways for energy migration, supporting lower quantum yield as seen in the PL measurements in Figure 7a. Excitons in a sonicated P3HT film can migrate over a substantial distance through iso-energetic energy transfer depending on the degree of aggregation, being either trapped deactivating exciton dissociation (dark character) or dissociated generating mobile charge carriers increasing the photocurrent. However, reduced photocurrent in the sonicated P3HT film, as seen in Figure 2 and 3, indicates that excitons are easily deactivated from dissociation. Time-resolved PL decay measurements for both films in Figure 7d show that the lifetime of exciton is longer in the pristine one (fast component, 735 ps; slow component, 1240
ps) than in the sonicated P3HT one (fast component, 360 ps; slow component, 670 ps), ensuring that more efficient isoenergetic energy transfer pathways exist in the sonicated P3HT film depopulating both emission and dissociation of excitons trapped in the low barrier sites through nonradiative processes, consistent with the description in Figure 7b. For both films, the PL decay curves matched well with the second-order exponential decay curves, ID(t) = I1 exp(−t/τ1) + I2 exp(−t/ τ2), as summarized in Table 1. This indicates that excitons migrate over “trap” sites at the multiple energy levels. It can be Table 1. Fitting Parametersa sonicated P3HT pristine P3HT a
26126
τ1 (ps)
τ2 (ps)
I1
I2
360 735
670 1242
1.1 × 106 1.38 × 105
1.1 × 105 5337
Excitation at 470 nm; emission detected at 705 nm. DOI: 10.1021/acs.jpcc.6b08728 J. Phys. Chem. C 2016, 120, 26119−26128
Article
The Journal of Physical Chemistry C
exciton dissociation under illumination is dominant in determining the value of the open circuit voltage.
inferred, therefore, that more efficient exciton deactivation through multiple exciton trapping sites (i.e., lower energy barrier sites), failing to dissociate into mobile charge carriers, occurs in the sonicated P3HT film and this explains the reduced short circuit current in the PV test probe. The presence of low-energy barrier sites in sonicated P3HT films, as illustrated in Figure 7e, enabling efficient energy migration between polymer chains, is also evidenced by distinctly different features in the PL emission spectra [Figure 7] in the both P3HT films that disappear in the absorption spectra [Figure 1]. Absorption spectra reflect signals from all polymer chain segments while PL detects signals from the lowest vibration state, that is, the most conjugated and wellordered polymer segments. The red shifts of the two PL peaks for emitting species in the sonicated P3HT film in comparison to those in the pristine one confirms the presence of interchain species in the sonicated P3HT film as a result of aggregates of more ordered polymer chains with longer conjugation lengths. On the other hand, it is worth noting that far higher FET mobility in the sonicated P3HT [Figure 1] can also enhance the exciton mobility, driving efficient energy migration between the low-energy barrier sites by increasing the probability to find the sites. In the presence of an electron-accepting layer (ZnO) in contact with the electron donor (P3HT), ZnO as well as the low-energy barrier sites in the P3HT film serves as “exciton dissociation sites” forming photogenerated charge pairs (polarons). It is found that increased exciton dissociation sites in both P3HT films in contact with the exciton quenching layer ZnO mitigate effect of energy migration through interchain aggregate induced states on exciton dissociation. Indeed, with a ZnO layer the short circuit current difference between the pristine and sonicated P3HT PV test probe devices decreased in comparison with that without ZnO. It is also demonstrated that aggregate induced electronic states can alter not only the short circuit current but also the open circuit voltage. Higher open-circuit voltage in the pristine P3HT is attributed to more carrier concentrations n and p due to efficient exciton dissociation. It has been accepted that the open circuit voltage is determined by the difference between the HOMO of the donor and the LUMO of the acceptor. According to eq 4 loss in the open circuit voltage is determined by disorder induced traps states (second term on the right side) and electron (n) and hole (p) carrier concentrations. ⎛N N ⎞ σ − − kBT ln⎜ A D ⎟ kBT ⎝ np ⎠
4. CONCLUSIONS To summarize, solution treated quasi-ordered structure exhibits two different features in the dark and under illumination. In the dark, charge transport was significantly improved due to enhanced molecular ordering resulting in a larger effective conjugation length. Under illumination, efficient energy migration (iso-energetic energy transfer) through aggregationinduced low-energy barrier sites was facilitated, decreasing both the luminescence quantum yield and efficiency of exciton dissociation. The effect of energy migration on exciton dissociation was separated from the change in the energy barrier through probing energy band bending at the electron donor/acceptor interface. In assembling conjugate polymers for optoelectronic devices, exciton quenching under illumination as well as charge transport in the dark should be controlled by varying the degree of aggregation, thereby finding an optimum point in which charge transport and the behavior of excitons are in concert for maximum performance. Our findings contrast with the prediction in which preventing luminescence quenching in conjugated polymer would be beneficial for exciton dissociation in that it can prolong the lifetime of excitons such that more excitons are allowed to dissociate into free carriers, contributing to photocurrent. To tune exciton dissociation in various modern optoelectronic devices, instead it should be considered that the charge transport resulting from crystallinity and the number of mobile carriers dissociated at the electron/donor−acceptor interface are counterbalanced by challenging control of the structural arrangement of polymers.
■
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b08728. Synthesis of ZnO NCs, fabrication of field effect transistors and photovoltaic devices, structural and optical characterizations of P3HT and ZnO NCs including PL and PESA measurements (PDF) .
■
2
qVoc = ΔE DA
ASSOCIATED CONTENT
AUTHOR INFORMATION
Corresponding Authors
*(J.C.) E-mail:
[email protected]. *(I.I.) E-mail:
[email protected]. *(B.P.) E-mail:
[email protected]. Tel: 82-2-320-1631.
(4)
Here, ΔEDA is the difference between the HOMO of the donor and the LUMO of the acceptor. σ is the width of the Gaussian density of states (DOS). Disorder induced trap states within the band gap of organic semiconducting materials are characterized by tail states in the Gaussian or exponential type with widths of ∼0.2 eV. According to PESA measurement in Figure S3, a maximum obtainable open-circuit voltage is similar for both P3HT films in the bulk without ZnO. In the presence of ZnO, considering the HOMO level of the pristine P3HT film close to the ZnO lies far above that of the sonicated P3HT film [Figure 6], the maximum obtainable open circuit voltage is larger in the sonicated P3HT. A higher open circuit voltage, regardless of the presence of the electron-accepting layer (ZnO) in the photovoltaic test probe, clarifies that the difference in the mobile carrier concentrations as a result of
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (2014R1A1A1002636), and by the Ministry of Education (2015R1A6A1A03031833). This work was also supported by the 2015 Hongik Faculty Research Support Fund. This work was also supported by the Korea Foundation for the Advancement of Science & Creativity (KOFAC), and funded by the Korean Government (MOE). 26127
DOI: 10.1021/acs.jpcc.6b08728 J. Phys. Chem. C 2016, 120, 26119−26128
Article
The Journal of Physical Chemistry C
■
Interactions on the Absorption and Emission of Poly(3-hexylthiophene). Phys. Rev. B: Condens. Matter Mater. Phys. 2003, 67, 064203. (22) Prosa, T. J.; Winokur, M. J.; Moulton, J.; Smith, P.; Heeger, A. J. X-ray Structural Studies of Poly(3-alkylthiophenes): An Example of an Inverse Comb. Macromolecules 1992, 25, 4364−4372. (23) Park, B.; Whitham, K.; Cho, J.; Reichmanis, E. Creating and Optimizing Interfaces for Electric-Field and Photon-Induced Charge Transfer. ACS Nano 2012, 6, 9466−9474. (24) Park, B.; Paoprasert, P.; In, I.; Zwickey, J.; Colavita, P. E.; Hamers, R. J.; Gopalan, P.; Evans, P. G. Functional Self-Assembled Monolayers for Optimized Photoinduced Charge Transfer in Organic Field Effect Transistors. Adv. Mater. 2007, 19, 4353−4357. (25) Graetzel, M.; Janssen, R. A. J.; Mitzi, D. B.; Sargent, E. H. Materials Interface Engineering for Solution-Processed Photovoltaics. Nature 2012, 488, 304−312. (26) Deibel, C.; Strobel, T.; Dyakonov, V. Role of the Charge Transfer State in Organic Donor-Acceptor Solar Cells. Adv. Mater. 2010, 22, 4097−4111. (27) Jasieniak, J.; Califano, M.; Watkins, S. E. Size-Dependent Valence and Conduction Band-Edge Energies of Semiconductor Nanocrystals. ACS Nano 2011, 5, 5888−5902. (28) Ruseckas, A.; Namdas, E.; Theander, M.; Svensson, M.; Yartsev, A.; Zigmantas, D.; Andersson, M. R.; Inganas, O.; Sundstrom, V. Luminescence from Inter-Chain Aggregates in Polythiophene Films. Synth. Met. 2001, 119, 603−604. (29) Cornil, J.; dos Santos, D. A.; Crispin, X.; Silbey, R.; Brédas, J. L. Influence of Interchain Interactions on the Absorption and Luminescence of Conjugated Oligomers and Polymers: A QuantumChemical Characterization. J. Am. Chem. Soc. 1998, 120, 1289−1299. (30) Park, B. N.; Soonjoo, S.; Paul, G. E. Channel Formation in Single-Monolayer Pentacene Thin Film Transistors. J. Phys. D: Appl. Phys. 2007, 40, 3506. (31) Park, B.; Aiyar, A.; Hong, J. I.; Reichmanis, E. Electrical Contact Properties between the Accumulation Layer and Metal Electrodes in Ultrathin Poly(3-hexylthiophene)(P3HT) Field Effect Transistors. ACS Appl. Mater. Interfaces 2011, 3, 1574−1580.
REFERENCES
(1) Arango, A. C.; Johnson, L. R.; Bliznyuk, V. N.; Schlesinger, Z.; Carter, S. A.; Horhold, H. H. Efficient Titanium Oxide/Conjugated Polymer Photovoltaics for Solar Energy Conversion. Adv. Mater. 2000, 12, 1689−1692. (2) Gunes, S.; Neugebauer, H.; Sariciftci, N. S. Conjugated PolymerBased Organic Solar Cells. Chem. Rev. 2007, 107, 1324−1338. (3) Sirringhaus, H.; Tessler, N.; Friend, R. H. Integrated Optoelectronic Devices Based on Conjugated Polymers. Science 1998, 280, 1741−1744. (4) Cheng, Y. J.; Yang, S. H.; Hsu, C. S. Synthesis of Conjugated Polymers for Organic Solar Cell Applications. Chem. Rev. 2009, 109, 5868−5923. (5) Coakley, K. M.; McGehee, M. D. Conjugated Polymer Photovoltaic Cells. Chem. Mater. 2004, 16, 4533−4542. (6) Facchetti, A. pi-Conjugated Polymers for Organic Electronics and Photovoltaic Cell Applications. Chem. Mater. 2011, 23, 733−758. (7) Li, Y. C.; Chen, K. B.; Chen, H. L.; Hsu, C. S.; Tsao, C. S.; Chen, J. H.; Chen, S. A. Fractal Aggregates of Conjugated Polymer in Solution State. Langmuir 2006, 22, 11009−11015. (8) Nguyen, T.-Q.; Doan, V.; Schwartz, B. J. Conjugated Polymer Aggregates in Solution: Control of Interchain Interactions. J. Chem. Phys. 1999, 110, 4068−4078. (9) Schwartz, B. J. Conjugated Polymers as Molecular Materials: How Chain Conformation and Film Morphology Influence Energy Transfer and Interchain Interactions. Annu. Rev. Phys. Chem. 2003, 54, 141−172. (10) Chang, J. F.; Sun, B. Q.; Breiby, D. W.; Nielsen, M. M.; Solling, T. I.; Giles, M.; McCulloch, I.; Sirringhaus, H. Enhanced Mobility of Poly(3-hexylthiophene) Transistors by Spin-Coating from HighBoiling-Point Solvents. Chem. Mater. 2004, 16, 4772−4776. (11) Liu, X. L.; Huettner, S.; Rong, Z. X.; Sommer, M.; Friend, R. H. Solvent Additive Control of Morphology and Crystallization in Semiconducting Polymer Blends. Adv. Mater. 2012, 24, 669−674. (12) Aiyar, A. R.; Hong, J. I.; Nambiar, R.; Collard, D. M.; Reichmanis, E. Tunable Crystallinity in Regioregular Poly(3Hexylthiophene) Thin Films and Its Impact on Field Effect Mobility. Adv. Funct. Mater. 2011, 21, 2652−2659. (13) Jakubiak, R.; Collison, C. J.; Wan, W. C.; Rothberg, L. J.; Hsieh, B. R. Aggregation Quenching of Luminescence in Electroluminescent Conjugated Polymers. J. Phys. Chem. A 1999, 103, 2394−2398. (14) Brown, P. J.; Thomas, D. S.; Köhler, A.; Wilson, J. S.; Kim, J.-S.; Ramsdale, C. M.; Sirringhaus, H.; Friend, R. H. Effect of Interchain Interactions on the Absorption and Emission of Poly(3-hexylthiophene). Phys. Rev. B: Condens. Matter Mater. Phys. 2003, 67, 064203. (15) Scheblykin, I. G.; Yartsev, A.; Pullerits, T.; Gulbinas, V.; Sundström, V. Excited State and Charge Photogeneration Dynamics in Conjugated Polymers. J. Phys. Chem. B 2007, 111, 6303−6321. (16) Beljonne, D.; Cornil, J.; Sirringhaus, H.; Brown, P. J.; Shkunov, M.; Friend, R. H.; Bredas, J. L. Optical Signature of Delocalized Polarons in Conjugated Polymers. Adv. Funct. Mater. 2001, 11, 229− 234. (17) Nguyen, T. Q.; Kwong, R. C.; Thompson, M. E.; Schwartz, B. J. Improving the Performance of Conjugated Polymer-Based Devices by Control of Interchain Interactions and Polymer Film Morphology. Appl. Phys. Lett. 2000, 76, 2454−2456. (18) Tsoi, W. C.; Spencer, S. J.; Yang, L.; Ballantyne, A. M.; Nicholson, P. G.; Turnbull, A.; Shard, A. G.; Murphy, C. E.; Bradley, D. D. C.; Nelson, J.; Kim, J. S. Effect of Crystallization on the Electronic Energy Levels and Thin Film Morphology of P3HT:PCBM Blends. Macromolecules 2011, 44, 2944−2952. (19) Park, B.; Ko, D. H. Charge Transport in Ordered and Disordered Regions in Pristine and Sonicated-Poly(3-hexylthiophene) Films. J. Phys. Chem. C 2014, 118, 1746−1752. (20) Jin, Y.; Wang, J.; Sun, B.; Blakesley, J. C.; Greenham, N. C. Solution-Processed Ultraviolet Photodetectors Based on Colloidal ZnO Nanoparticles. Nano Lett. 2008, 8, 1649−1653. (21) Brown, P. J.; Thomas, D. S.; Kohler, A.; Wilson, J. S.; Kim, J. S.; Ramsdale, C. M.; Sirringhaus, H.; Friend, R. H. Effect of Interchain 26128
DOI: 10.1021/acs.jpcc.6b08728 J. Phys. Chem. C 2016, 120, 26119−26128
