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White electroluminescence based on PFO:CdSe(ZnS):P3OT hybrid blends
Synthetic Metals 237 (2018) 10–15
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White electroluminescence based on PFO:CdSe(ZnS):P3OT hybrid blends ⁎
T
W. Renzi , N.J.A. Cordeiro, E. Laureto, A. Urbano, P.R.C. da Silva, J.L. Duarte Departament of Physics, State University of Londrina, Londrina, 86057-970, Brazil
A R T I C L E I N F O
A B S T R A C T
Keywords: Optical characterization White electroluminescence Energy transfer PFO:CdSe(ZnS):P3OT Hybrid blend
White light emitting diodes (WOLEDs) were prepared from easily processable materials, well known in the literature (PFO and P3OT), with the intention of exploring the blueshift of the P3OT emission, when in low concentration, in blends with PFO in beta phase. The photoluminescence (PL) study, through donor excitation, of PFO:P3OT blends with different relative concentrations, served as basis for comparing and predicting the behavior of these blends when excited via carrier injection. Despite the good PL results, the white emission of the PFO:P3OT blends with low P3OT concentration was not replicable via electroluminescence (EL), due to the lack of a more intense green component. To circumvent this problem, new PFO:P3OT blends were prepared with the insertion of CdSe(ZnS) quantum dots (QDs), which have green emission. With this new composition, the devices presented white emission (0.33:0.33) at low voltages and current (8 V, 0.250 mA). For the PFO:CdSe(ZnS):P3OT devices, the change in the applied voltage also showed the possibility of tuning the emission of these structures. Therefore, this blend is very attractive to make solution processed, low cost, large area, and flexible lighting panels.
1. Introduction The demand for devices for internal and external lighting has been growing a lot in last years. As a result, the number of researches that seek the manufacture of organic and hybrid based devices began to take prominence. Organic structures present properties different from inorganic ones, such as physical malleability, easy structural manipulation, easy manufacturing processes and large area deposition, associated with low cost of production [1,2]. In relation to lighting and wide-scale applications, the great interest behind organic structures lies in the structures composed mainly of conjugated polymers, whose deposition can be performed in large areas through simple techniques such as roll-to-roll [3,4] and spray coating [5–7]. The structures constituted only by small molecules usually require more controlled and expensive processes of deposition such as evaporation techniques, making these structures not very suitable due to the difficulty of large scale applications and large area depositions, despite their high efficiency and stability. The search for polymer combinations to obtain white emission often results in the study of donor-acceptor blends, which depend, in addition to the involved materials (absorption and emission spectra overlap), on the solvent [8,9] and on the relative proportion of materials in the blends [10–13], making it difficult to work with more than two materials. Another procedure that has often been discussed in recent works is the construction of light emitting devices with hybrid organic⁎
inorganic structures, the HLEDs (Hybrid Light Emitting Diodes) [14,15]. Some hybrid structures that are becoming popular are those containing quantum dots (QDs) as active layers, surrounded by organic layers, acting as charge transporting and/or blocking layers [16–19]. The QDs are nanocrystals of inorganic semiconductors that have well defined emissions due to the confining effects of their carriers, making these materials interesting for applications in light emitting devices [16,20,21]. The insertion of QDs into donor-acceptor polymer matrices is also used in order to complement the emission of the matrix, leading to changes in the emission color of the device [22,23]. Another advantage of these hybrid devices lies in the possibility of tuning the emission of the blends, according to the voltage applied to the device [2,24]. Due to the distribution of the QD levels, they act as radiative traps within the structure, favoring their emission at low voltages, allowing the recreation of a spectral narrow emission, even within the matrix. Studies in the literature [25–27] report the emission of PFO:P3AT across the visible spectrum, due to the emission of P3AT chains with different conjugation lengths. In a previous work with PFO:P3OT blends, Renzi et al. [27] explored a mixture of these two polymers in different relative concentrations and studied their behavior in different solvents, achieving white emission photoluminescence (PL) for blends with low relative concentration of P3OT. Taking into account these results, in this work PFO:P3OT devices with different relative concentrations were developed, as well as devices of this blend with the
Corresponding author. E-mail address: [email protected] (W. Renzi).
https://doi.org/10.1016/j.synthmet.2018.01.005 Received 3 September 2017; Received in revised form 21 November 2017; Accepted 10 January 2018 0379-6779/ © 2018 Elsevier B.V. All rights reserved.
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Fig. 1. Monomer structures of (a) P3OT and (b) PFO.
Fig. 2. Energy level of the produced devices with active layer (a) PFO:P3OT and (b) PFO:CdSe(ZnS):P3OT.
resistance, purchased from Hudson Surface Technology were used. The pre-cleaned substrates were exposed to ozone plasma bath (30 min). In order to produce the multilayer device, first a PEDOT:PSS layer was deposited by spin coating (25 nm thickness), and annealed at 393 K for 15 min. Next, it was deposited a thin layer of poly (9,9-dioctylfluoreny2,7-diyl) −co- (4,4 '- (N- (4-s-butylphenyl) diphenylamine)] (TFB), acting as HTL/EBL. Thermal treatment (343 K for 10 min) was carried out in the TFB, in order to minimize solvent non-orthogonality problems between layers. After HTL/EBL deposition, the active layer was spin casted (80 nm). The active layers used in this work are composed of polymer blends (PFO:P3OT) and hybrid blends (PFO:CdSe (ZnS):P3OT), with different relative concentrations (wt.%). The Al cathode of the devices (Sigma-Aldrich, 99.999%) was deposited by thermal evaporation under high vacuum (5.0 × 10−6 mbar), with 200 nm thickness. In Fig. 2 the energy levels of the produced devices are shown, taking into account the literature values [26,28–30].
insertion of CdSe(ZnS) QDs, seeking the white emission through the use of these nanocrystals, which contribute to enhance the green emission of the device electroluminescence (EL), reinforcing the emission in the intermediate spectral region present in the PFO:P3OT blends. Tests with voltage variation show that an increase in the applied voltage results in changes in the emission color of the device, which are presented through the emission coordinates CIE –1931 (Commission Internationale de l'Eclairage).
2. Sample preparation and experimental methods 2.1. Sample preparation Poly(3-octylthiophene-2,5-diyl (P3OT), (Mw = 20.000–70.000), was purchased from American Dye Source, and poly(9,9-dioctylfluorenyl-2,7-diyl (PFO) (Mw ≥ 20.000) was purchased from Sigma Aldrich. The CdSe(ZnS) quantum dots were obtained from Sigma Aldrich with nominal diameter of 5 nm and size dispersion of 1.9%, and present absorption and emission peaks at 530 nm and 560 nm, respectively. The P3OT and PFO monomer structures are shown in Fig. 1(a) and (b), respectively. Both polymers were weighed, in solid state, and dissolved in toluene, at 8 mg/mL. The solutions were prepared by mechanical agitation for 72 h, protected from light exposure. The quantum dot solution was also prepared in toluene, at 8 mg/mL, in ultrasonic bath, for a better dispersion of the nanocrystals. To prepare the polymer blends, solutions of the donor (PFO) and acceptor (P3OT) materials were mixed in relative concentrations (wt.%) of (97:03), (95:05) and (93:07). The preparation of the hybrid blends was done by the insertion of CdSe (ZnS) QDs in solution of the polymer blends, in different relative concentrations (wt.%). The chosen ratios used for the ternary blends preparation and for the device fabrication were (92: 03: 05) and (94: 02: 04) for PFO, CdSe(ZnS) and P3OT, respectively. The polymer solutions for optical spectroscopy were deposited by spin coating (80 nm) on glass slides, pre-washed in an ultrasonic bath for 30 min (15 min in acetone + 15 min in isopropyl alcohol). The samples were dried for 2 h at room temperature. Films obtained only with the P3OT polymer and only with the PFO were also deposited for characterization. For device preparation, glass/ITO substrates with 20 Ω/sq. sheet
2.2. Experimental methods The absorption spectra were measured with a UV–Vis spectrometer (Shimadzu UV-2600). The PL spectra were obtained using as excitation source a diode laser with emission at 375 nm, and the 488 and 514 nm lines of an Ar+ laser. The emissions (PL and EL) were detected using an Ocean Optics USB2000+ Spectrometer. The OLEDs EL were performed using as excitation source a Keithley 2400 SourceMeter. 3. Results and discussion 3.1. About the blends The materials used in the preparation of the donor-acceptor blends were chosen in such a way that the composition of their emissions covers a wide range of the visible spectrum, and there is also an overlap of the donor emission with the acceptor absorption, allowing energy transfer between them (SI Fig. 1). In a previous work [27] a detailed study of the PFO:P3OT blend behavior was carried out by analyzing the solvent effect on the formation of the PFO crystalline and amorphous phases, and how these phases act on the energy transfer processes. One of the reported results in the cited work was the almost white emission of the PFO:P3OT blend in toluene, with 5% acceptor concentration, 11
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Fig. 3. Photoluminescence of PFO, P3OT and the PFO:P3OT blends with different ratios of D:A (97:03), (95:05) and (93:07). Excitation was done using 375 nm for PFO and blends and 514 nm laser line for P3OT.
Fig. 5. Normalized electroluminescence spectra at 7.0 V for different PFO:P3OT blends with 3, 5 and 7% acceptor ratio.
emission gives place to the emission of P3OT aggregates, in which the energy transfer among the molecules is facilitated, leading to a predominantly red emission. [27]. Analyzing the CIE emission coordinates as a function of the acceptor ratio in the blend, one notes a shift from blue to green/yellow as the amount of P3OT in the blend increases. The use of low percentages (3% and 5%) of P3OT reaches a balance near the ideal white, represented in the CIE coordinates by (0.33:0.33). This fact, however, is only possible due to the appearance of the green emission on blends, caused by the blueshift in the acceptor emission (typically orange/red), providing a complement and a better balance of the emission, resulting in white emission.
achieved due to the emission of P3OT chains with different conjugation lengths, isolated from each other by the PFO matrix. The P3OT emission in the blend is characterized by a broadband with maximum at 575 nm. 3.2. Optical characterization of the polymeric blends Fig. 3 shows the PL spectra of the PFO:P3OT blends with few different acceptor ratios, aiming to find the relative concentration of polymers that provides the partial transfer of energy suitable for white emission. The optical characterization was performed using excitation at 375 nm for PFO and blends, and 514 nm for the P3OT. The CIE coordinates from PL of the blends are presented in Fig. 4 and show the behavior of the emission color with the acceptor ratio. As the films were produced from low-volatile solutions, it should be taken into account, in addition to the vitreous phase, the formation of the beta phase (lower energy) in the PFO, which acts as radiative trap for excitons of the amorphous PFO chains [31]. The presence of the beta phase is also responsible for the formation of PFO clusters, containing a mixture of amorphous and beta phases (with predominance of beta phase), and acceptor material (P3OT) [27]. In these clusters the energy transfer among P3OT chains is hindered, allowing that the emission of this material occurs trough higher energy chains, leading to a blueshift of the emission. However, despite the formation of clusters in the blend, it is possible to observe a decrease in the blueshift with the increase of P3OT ratio in the blend. With this increase in P3OT ratio, the clusters
3.3. Electrical characterization of the polymeric blends In order to study and compare the emission behavior via electrical stimulation, devices were made with blends of the different PFO:P3OT concentrations previously studied. The EL spectra as a function of the applied voltage are shown in SI Fig. 2, and the main ELs of the blends with applied voltage of 7.0 V are presented, normalized, in Fig. 5. Their respective CIE coordinates are shown in Fig. 6. For intermediate voltages, the change in excitation (PL to EL) shifts the PFO:P3OT emissions towards the blue, causing the emission to be further from the white, unlike the PL results. The variations in the
Fig. 4. CIE coordinates for PL emissions of PFO:P3OT blends with different acceptor ratios.
Fig. 6. CIE coordinates for PFO:P3OT with intermediate voltage (∼7 V) at different P3OT ratios.
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relative emission intensity between PFO and P3OT as a function of the P3OT ratio were small for electrical excitation. It was not possible to reach white emission for the devices operating at low voltages (S.I. Fig. 4). This change in the emission behavior is related to changes in the excitation and energy transfer processes between the polymers in the blends. In PL, the excitation of the blend is done at the donor material (PFO), making the emission of the acceptor (P3OT) to occur mainly by exciton migration, through energy transfer processes between the polymers [32]. Thus, there may be recombination before an exciton reaches a lower energy position. On the other hand, in the EL process there is injection of separate carriers (electrons and holes), which tend to migrate to the lowest local energy levels [33], giving priority to the emission by these levels. So, the presence in the blend of PFO in different phases is very important. As in PL, the PFO in the beta phase acts as a potential well for charges, not allowing, for low potentials, that these charges migrate all to P3OT, regulating their emission. Thus, at low voltages, the EL resembles the PL due to the greater confinement of the carriers within the longer chains of PFO (beta phase). With increasing applied voltage on the blends (S.I. Fig. 2) there is a significant increase of the P3OT emission, in comparison to the PFO emission. This relative increase in P3OT emission can be explained as due to two contributions: a) an increase in the number of injected carriers, which causes a saturation of PFO beta phase levels, thus allowing the carriers to reach more effectively the P3OT, and b) by increasing the probability of escape (tunneling and hopping) of the carriers, from the beta phase PFO back to the amorphous PFO, thus allowing the continuation of the migration to other device regions of lower energy. Fig. 7 shows the EL spectrum at 13 V for the 5% P3OT blend. In this case the PFO and P3OT emissions intensities become similar, leading to white emission (0.32:0.35). On the other hand, the increase in applied voltage also leads to an increase in emission intensity at 530 nm, related to PFO degradation (fluorenone formation) [34,35]. Although harmful to the device, the appearance of this well-characterized green emission was essential to reach the balance between the emission intensities of each material of the blend.
Fig. 8. Photoluminescence spectra of the PFO:CdSe(ZnS):P3OT hybrid blends.
incorporated to the polymeric matrix. The hybrid blend ratios among the materials were: (92:03:05) and (94:02:04), for PFO:CdSe (ZnS):P3OT, respectively. The PL spectra of these new blends are shown in Fig. 8. 3.5. Hybrid blends electrical characterization Finally, devices of these hybrid blends were prepared in order to obtain a better balance in the EL emission colors, as discussed in Section 3.3. The EL spectra as a function of the applied voltage are presented in SI Fig. 3, and the main results, with intermediary voltages (∼8 V), are shown in Fig. 9. Although the incorporation of QDs into the blends did not present different emission under optical excitation (PL), for electrical excitation (EL) it presented a major change. The increase in green emission, provided by the QDs, in addition to the broadband emission of P3OT, reaches, for the 92:03:05 blend, the necessary balance for obtaining white emission (0.33:0.33). In this case, the QDs also act as radiative traps for the carriers with an even greater confinement, forcing the emission to occur through this material. Both hybrid blends show (Fig. 9) similar intensity for the blue emission component, while the red/green contribution, which comes from the acceptors, becomes responsible for the change in the emission color of the device. Moreover, the insertion of the QDs and the consequent decrease of the operating voltage required for the white emission prevents the degradation of the PFO, allowing a greater stability of these devices. The CIE coordinates for the ELs are shown in Fig. 10. Fig. 11 shows the different energy levels for HOMO and LUMO of the blend materials, where each block is responsible for the
3.4. Hybrid blends optical characterization As the search for devices with white emission cannot be associated with defects that these structures may present, it was necessary to find another way to reach the white emission. Taking into account that the degradation cannot be reversed, but attenuated, the production of new devices should be based on the emissions of pure materials, disregarding the possible contribution linked to the appearance of defects in it. Thus, new solutions of PFO:P3OT were made with the insertion of CdSe(ZnS) QDs, which has intermediate emission (560 nm),
Fig. 9. Electroluminescence spectra at intermediary voltages (∼8 V) for the hybrid blends.
Fig. 7. Electroluminescence at higher voltage (13 V) of PFO:P3OT with 5% P3OT.
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that the PFO:P3OT blend presented white PL motivated the use of these materials to make devices with white EL. However, the PFO:P3OT devices were not able to achieve, without degradation, white EL only by changing the relative concentration of the materials. It was observed that the problem for obtaining white emission was related to the lack of a more intense green emission, which was supplied with the addition of CdSe(ZnS) QDs, with emission around 560 nm. The PFO:CdSe (ZnS):P3OT device with 92:03:05 concentration was characterized and presented white emission, with CIE coordinates (0.33:0.33), operating at low power. The possibility of making white OLEDs by solution process using the studied blends makes them very attractive for low cost, large area, and flexible lighting panels. Acknowledgements This work was supported by the Brazilian agencies Coordination for the Improvement of Higher Education Personal (CAPES), National Council for Scientific and Technological Development (CNPq), Fundação Araucária, and National Institute for Science and Technology on Organic Electronics (INEO). The authors gratefully acknowledge the ESPEC Laboratory-Central Multiusuário-PROPPG-UEL.
Fig. 10. CIE diagram for the electroluminescence emission of the hybrid blends at intermediary voltage (∼8 V).
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Fig. 11. Schematic representation of charge carriers and of their migration processes in HOMO and LUMO of the blend PFO:CdSe(ZnS):P3OT working as active layer of the device.
representation of the material levels, with violet for PFO in amorphous phase, blue for PFO in beta phase, green for QDs and P3OT molecules with low conjugation, and in red, P3OT agglomerates. The increase in the value of the applied voltage leads to an increase in the number of carriers injected into the blends and also gives carriers a greater ability to move between the levels of the different materials, regulating the role of beta phase as radiative traps in certain regions of the blends. The emission of the PFO occurs through the beta phase (blue blocks), and not by the amorphous phase (purple blocks), what is characterized by luminescence at longer wavelengths [27]. In Fig. 11 the electrons are presented as black spheres and the holes as white spheres and, in addition, the dashed circles mark the initial position of the carriers before their migration through the materials, with some of these migrations represented by the arrows. Finally, the dashed lines between HOMO and LUMO indicate the radiative recombination processes of the carriers. This figure schematizes how may occur the distribution of the carriers along the blend, trying to show the migration processes of the charges, originally injected in amorphous PFO (purple), to the materials with lower energies, such as PFO beta phase, CdSe (ZnS) and P3OT, through which recombination and light emission occurs. 4. Conclusions In this work we prepared devices with white emission, based on the PFO:P3OT blends, and PFO:CdSe(ZnS):P3OT hybrid blends. The fact 14
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White electroluminescence based on PFO:CdSe(ZnS):P3OT hybrid blends ⁎
T
W. Renzi , N.J.A. Cordeiro, E. Laureto, A. Urbano, P.R.C. da Silva, J.L. Duarte Departament of Physics, State University of Londrina, Londrina, 86057-970, Brazil
A R T I C L E I N F O
A B S T R A C T
Keywords: Optical characterization White electroluminescence Energy transfer PFO:CdSe(ZnS):P3OT Hybrid blend
White light emitting diodes (WOLEDs) were prepared from easily processable materials, well known in the literature (PFO and P3OT), with the intention of exploring the blueshift of the P3OT emission, when in low concentration, in blends with PFO in beta phase. The photoluminescence (PL) study, through donor excitation, of PFO:P3OT blends with different relative concentrations, served as basis for comparing and predicting the behavior of these blends when excited via carrier injection. Despite the good PL results, the white emission of the PFO:P3OT blends with low P3OT concentration was not replicable via electroluminescence (EL), due to the lack of a more intense green component. To circumvent this problem, new PFO:P3OT blends were prepared with the insertion of CdSe(ZnS) quantum dots (QDs), which have green emission. With this new composition, the devices presented white emission (0.33:0.33) at low voltages and current (8 V, 0.250 mA). For the PFO:CdSe(ZnS):P3OT devices, the change in the applied voltage also showed the possibility of tuning the emission of these structures. Therefore, this blend is very attractive to make solution processed, low cost, large area, and flexible lighting panels.
1. Introduction The demand for devices for internal and external lighting has been growing a lot in last years. As a result, the number of researches that seek the manufacture of organic and hybrid based devices began to take prominence. Organic structures present properties different from inorganic ones, such as physical malleability, easy structural manipulation, easy manufacturing processes and large area deposition, associated with low cost of production [1,2]. In relation to lighting and wide-scale applications, the great interest behind organic structures lies in the structures composed mainly of conjugated polymers, whose deposition can be performed in large areas through simple techniques such as roll-to-roll [3,4] and spray coating [5–7]. The structures constituted only by small molecules usually require more controlled and expensive processes of deposition such as evaporation techniques, making these structures not very suitable due to the difficulty of large scale applications and large area depositions, despite their high efficiency and stability. The search for polymer combinations to obtain white emission often results in the study of donor-acceptor blends, which depend, in addition to the involved materials (absorption and emission spectra overlap), on the solvent [8,9] and on the relative proportion of materials in the blends [10–13], making it difficult to work with more than two materials. Another procedure that has often been discussed in recent works is the construction of light emitting devices with hybrid organic⁎
inorganic structures, the HLEDs (Hybrid Light Emitting Diodes) [14,15]. Some hybrid structures that are becoming popular are those containing quantum dots (QDs) as active layers, surrounded by organic layers, acting as charge transporting and/or blocking layers [16–19]. The QDs are nanocrystals of inorganic semiconductors that have well defined emissions due to the confining effects of their carriers, making these materials interesting for applications in light emitting devices [16,20,21]. The insertion of QDs into donor-acceptor polymer matrices is also used in order to complement the emission of the matrix, leading to changes in the emission color of the device [22,23]. Another advantage of these hybrid devices lies in the possibility of tuning the emission of the blends, according to the voltage applied to the device [2,24]. Due to the distribution of the QD levels, they act as radiative traps within the structure, favoring their emission at low voltages, allowing the recreation of a spectral narrow emission, even within the matrix. Studies in the literature [25–27] report the emission of PFO:P3AT across the visible spectrum, due to the emission of P3AT chains with different conjugation lengths. In a previous work with PFO:P3OT blends, Renzi et al. [27] explored a mixture of these two polymers in different relative concentrations and studied their behavior in different solvents, achieving white emission photoluminescence (PL) for blends with low relative concentration of P3OT. Taking into account these results, in this work PFO:P3OT devices with different relative concentrations were developed, as well as devices of this blend with the
Corresponding author. E-mail address: [email protected] (W. Renzi).
https://doi.org/10.1016/j.synthmet.2018.01.005 Received 3 September 2017; Received in revised form 21 November 2017; Accepted 10 January 2018 0379-6779/ © 2018 Elsevier B.V. All rights reserved.
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Fig. 1. Monomer structures of (a) P3OT and (b) PFO.
Fig. 2. Energy level of the produced devices with active layer (a) PFO:P3OT and (b) PFO:CdSe(ZnS):P3OT.
resistance, purchased from Hudson Surface Technology were used. The pre-cleaned substrates were exposed to ozone plasma bath (30 min). In order to produce the multilayer device, first a PEDOT:PSS layer was deposited by spin coating (25 nm thickness), and annealed at 393 K for 15 min. Next, it was deposited a thin layer of poly (9,9-dioctylfluoreny2,7-diyl) −co- (4,4 '- (N- (4-s-butylphenyl) diphenylamine)] (TFB), acting as HTL/EBL. Thermal treatment (343 K for 10 min) was carried out in the TFB, in order to minimize solvent non-orthogonality problems between layers. After HTL/EBL deposition, the active layer was spin casted (80 nm). The active layers used in this work are composed of polymer blends (PFO:P3OT) and hybrid blends (PFO:CdSe (ZnS):P3OT), with different relative concentrations (wt.%). The Al cathode of the devices (Sigma-Aldrich, 99.999%) was deposited by thermal evaporation under high vacuum (5.0 × 10−6 mbar), with 200 nm thickness. In Fig. 2 the energy levels of the produced devices are shown, taking into account the literature values [26,28–30].
insertion of CdSe(ZnS) QDs, seeking the white emission through the use of these nanocrystals, which contribute to enhance the green emission of the device electroluminescence (EL), reinforcing the emission in the intermediate spectral region present in the PFO:P3OT blends. Tests with voltage variation show that an increase in the applied voltage results in changes in the emission color of the device, which are presented through the emission coordinates CIE –1931 (Commission Internationale de l'Eclairage).
2. Sample preparation and experimental methods 2.1. Sample preparation Poly(3-octylthiophene-2,5-diyl (P3OT), (Mw = 20.000–70.000), was purchased from American Dye Source, and poly(9,9-dioctylfluorenyl-2,7-diyl (PFO) (Mw ≥ 20.000) was purchased from Sigma Aldrich. The CdSe(ZnS) quantum dots were obtained from Sigma Aldrich with nominal diameter of 5 nm and size dispersion of 1.9%, and present absorption and emission peaks at 530 nm and 560 nm, respectively. The P3OT and PFO monomer structures are shown in Fig. 1(a) and (b), respectively. Both polymers were weighed, in solid state, and dissolved in toluene, at 8 mg/mL. The solutions were prepared by mechanical agitation for 72 h, protected from light exposure. The quantum dot solution was also prepared in toluene, at 8 mg/mL, in ultrasonic bath, for a better dispersion of the nanocrystals. To prepare the polymer blends, solutions of the donor (PFO) and acceptor (P3OT) materials were mixed in relative concentrations (wt.%) of (97:03), (95:05) and (93:07). The preparation of the hybrid blends was done by the insertion of CdSe (ZnS) QDs in solution of the polymer blends, in different relative concentrations (wt.%). The chosen ratios used for the ternary blends preparation and for the device fabrication were (92: 03: 05) and (94: 02: 04) for PFO, CdSe(ZnS) and P3OT, respectively. The polymer solutions for optical spectroscopy were deposited by spin coating (80 nm) on glass slides, pre-washed in an ultrasonic bath for 30 min (15 min in acetone + 15 min in isopropyl alcohol). The samples were dried for 2 h at room temperature. Films obtained only with the P3OT polymer and only with the PFO were also deposited for characterization. For device preparation, glass/ITO substrates with 20 Ω/sq. sheet
2.2. Experimental methods The absorption spectra were measured with a UV–Vis spectrometer (Shimadzu UV-2600). The PL spectra were obtained using as excitation source a diode laser with emission at 375 nm, and the 488 and 514 nm lines of an Ar+ laser. The emissions (PL and EL) were detected using an Ocean Optics USB2000+ Spectrometer. The OLEDs EL were performed using as excitation source a Keithley 2400 SourceMeter. 3. Results and discussion 3.1. About the blends The materials used in the preparation of the donor-acceptor blends were chosen in such a way that the composition of their emissions covers a wide range of the visible spectrum, and there is also an overlap of the donor emission with the acceptor absorption, allowing energy transfer between them (SI Fig. 1). In a previous work [27] a detailed study of the PFO:P3OT blend behavior was carried out by analyzing the solvent effect on the formation of the PFO crystalline and amorphous phases, and how these phases act on the energy transfer processes. One of the reported results in the cited work was the almost white emission of the PFO:P3OT blend in toluene, with 5% acceptor concentration, 11
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Fig. 3. Photoluminescence of PFO, P3OT and the PFO:P3OT blends with different ratios of D:A (97:03), (95:05) and (93:07). Excitation was done using 375 nm for PFO and blends and 514 nm laser line for P3OT.
Fig. 5. Normalized electroluminescence spectra at 7.0 V for different PFO:P3OT blends with 3, 5 and 7% acceptor ratio.
emission gives place to the emission of P3OT aggregates, in which the energy transfer among the molecules is facilitated, leading to a predominantly red emission. [27]. Analyzing the CIE emission coordinates as a function of the acceptor ratio in the blend, one notes a shift from blue to green/yellow as the amount of P3OT in the blend increases. The use of low percentages (3% and 5%) of P3OT reaches a balance near the ideal white, represented in the CIE coordinates by (0.33:0.33). This fact, however, is only possible due to the appearance of the green emission on blends, caused by the blueshift in the acceptor emission (typically orange/red), providing a complement and a better balance of the emission, resulting in white emission.
achieved due to the emission of P3OT chains with different conjugation lengths, isolated from each other by the PFO matrix. The P3OT emission in the blend is characterized by a broadband with maximum at 575 nm. 3.2. Optical characterization of the polymeric blends Fig. 3 shows the PL spectra of the PFO:P3OT blends with few different acceptor ratios, aiming to find the relative concentration of polymers that provides the partial transfer of energy suitable for white emission. The optical characterization was performed using excitation at 375 nm for PFO and blends, and 514 nm for the P3OT. The CIE coordinates from PL of the blends are presented in Fig. 4 and show the behavior of the emission color with the acceptor ratio. As the films were produced from low-volatile solutions, it should be taken into account, in addition to the vitreous phase, the formation of the beta phase (lower energy) in the PFO, which acts as radiative trap for excitons of the amorphous PFO chains [31]. The presence of the beta phase is also responsible for the formation of PFO clusters, containing a mixture of amorphous and beta phases (with predominance of beta phase), and acceptor material (P3OT) [27]. In these clusters the energy transfer among P3OT chains is hindered, allowing that the emission of this material occurs trough higher energy chains, leading to a blueshift of the emission. However, despite the formation of clusters in the blend, it is possible to observe a decrease in the blueshift with the increase of P3OT ratio in the blend. With this increase in P3OT ratio, the clusters
3.3. Electrical characterization of the polymeric blends In order to study and compare the emission behavior via electrical stimulation, devices were made with blends of the different PFO:P3OT concentrations previously studied. The EL spectra as a function of the applied voltage are shown in SI Fig. 2, and the main ELs of the blends with applied voltage of 7.0 V are presented, normalized, in Fig. 5. Their respective CIE coordinates are shown in Fig. 6. For intermediate voltages, the change in excitation (PL to EL) shifts the PFO:P3OT emissions towards the blue, causing the emission to be further from the white, unlike the PL results. The variations in the
Fig. 4. CIE coordinates for PL emissions of PFO:P3OT blends with different acceptor ratios.
Fig. 6. CIE coordinates for PFO:P3OT with intermediate voltage (∼7 V) at different P3OT ratios.
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relative emission intensity between PFO and P3OT as a function of the P3OT ratio were small for electrical excitation. It was not possible to reach white emission for the devices operating at low voltages (S.I. Fig. 4). This change in the emission behavior is related to changes in the excitation and energy transfer processes between the polymers in the blends. In PL, the excitation of the blend is done at the donor material (PFO), making the emission of the acceptor (P3OT) to occur mainly by exciton migration, through energy transfer processes between the polymers [32]. Thus, there may be recombination before an exciton reaches a lower energy position. On the other hand, in the EL process there is injection of separate carriers (electrons and holes), which tend to migrate to the lowest local energy levels [33], giving priority to the emission by these levels. So, the presence in the blend of PFO in different phases is very important. As in PL, the PFO in the beta phase acts as a potential well for charges, not allowing, for low potentials, that these charges migrate all to P3OT, regulating their emission. Thus, at low voltages, the EL resembles the PL due to the greater confinement of the carriers within the longer chains of PFO (beta phase). With increasing applied voltage on the blends (S.I. Fig. 2) there is a significant increase of the P3OT emission, in comparison to the PFO emission. This relative increase in P3OT emission can be explained as due to two contributions: a) an increase in the number of injected carriers, which causes a saturation of PFO beta phase levels, thus allowing the carriers to reach more effectively the P3OT, and b) by increasing the probability of escape (tunneling and hopping) of the carriers, from the beta phase PFO back to the amorphous PFO, thus allowing the continuation of the migration to other device regions of lower energy. Fig. 7 shows the EL spectrum at 13 V for the 5% P3OT blend. In this case the PFO and P3OT emissions intensities become similar, leading to white emission (0.32:0.35). On the other hand, the increase in applied voltage also leads to an increase in emission intensity at 530 nm, related to PFO degradation (fluorenone formation) [34,35]. Although harmful to the device, the appearance of this well-characterized green emission was essential to reach the balance between the emission intensities of each material of the blend.
Fig. 8. Photoluminescence spectra of the PFO:CdSe(ZnS):P3OT hybrid blends.
incorporated to the polymeric matrix. The hybrid blend ratios among the materials were: (92:03:05) and (94:02:04), for PFO:CdSe (ZnS):P3OT, respectively. The PL spectra of these new blends are shown in Fig. 8. 3.5. Hybrid blends electrical characterization Finally, devices of these hybrid blends were prepared in order to obtain a better balance in the EL emission colors, as discussed in Section 3.3. The EL spectra as a function of the applied voltage are presented in SI Fig. 3, and the main results, with intermediary voltages (∼8 V), are shown in Fig. 9. Although the incorporation of QDs into the blends did not present different emission under optical excitation (PL), for electrical excitation (EL) it presented a major change. The increase in green emission, provided by the QDs, in addition to the broadband emission of P3OT, reaches, for the 92:03:05 blend, the necessary balance for obtaining white emission (0.33:0.33). In this case, the QDs also act as radiative traps for the carriers with an even greater confinement, forcing the emission to occur through this material. Both hybrid blends show (Fig. 9) similar intensity for the blue emission component, while the red/green contribution, which comes from the acceptors, becomes responsible for the change in the emission color of the device. Moreover, the insertion of the QDs and the consequent decrease of the operating voltage required for the white emission prevents the degradation of the PFO, allowing a greater stability of these devices. The CIE coordinates for the ELs are shown in Fig. 10. Fig. 11 shows the different energy levels for HOMO and LUMO of the blend materials, where each block is responsible for the
3.4. Hybrid blends optical characterization As the search for devices with white emission cannot be associated with defects that these structures may present, it was necessary to find another way to reach the white emission. Taking into account that the degradation cannot be reversed, but attenuated, the production of new devices should be based on the emissions of pure materials, disregarding the possible contribution linked to the appearance of defects in it. Thus, new solutions of PFO:P3OT were made with the insertion of CdSe(ZnS) QDs, which has intermediate emission (560 nm),
Fig. 9. Electroluminescence spectra at intermediary voltages (∼8 V) for the hybrid blends.
Fig. 7. Electroluminescence at higher voltage (13 V) of PFO:P3OT with 5% P3OT.
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that the PFO:P3OT blend presented white PL motivated the use of these materials to make devices with white EL. However, the PFO:P3OT devices were not able to achieve, without degradation, white EL only by changing the relative concentration of the materials. It was observed that the problem for obtaining white emission was related to the lack of a more intense green emission, which was supplied with the addition of CdSe(ZnS) QDs, with emission around 560 nm. The PFO:CdSe (ZnS):P3OT device with 92:03:05 concentration was characterized and presented white emission, with CIE coordinates (0.33:0.33), operating at low power. The possibility of making white OLEDs by solution process using the studied blends makes them very attractive for low cost, large area, and flexible lighting panels. Acknowledgements This work was supported by the Brazilian agencies Coordination for the Improvement of Higher Education Personal (CAPES), National Council for Scientific and Technological Development (CNPq), Fundação Araucária, and National Institute for Science and Technology on Organic Electronics (INEO). The authors gratefully acknowledge the ESPEC Laboratory-Central Multiusuário-PROPPG-UEL.
Fig. 10. CIE diagram for the electroluminescence emission of the hybrid blends at intermediary voltage (∼8 V).
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.synthmet.2018.01.005. References [1] C. Zhong, C. Duan, F. Huang, H. Wu, Y. Cao, Materials and devices toward fully solution processable organic light-emitting diodes, Chem. Mater. 23 (2011) 326–340, http://dx.doi.org/10.1021/cm101937p. [2] S.H. Cho, E.H. Kim, B. Jeong, J.H. Lee, G. Song, I. Hwang, H. Cho, K.L. Kim, S. Yu, R.H. Kim, S.W. Lee, T.-W. Lee, C. Park, Solution-processed electron-only tandem polymer light-emitting diodes for broad wavelength light emission, J. Mater. Chem. C 5 (2017) 110–117, http://dx.doi.org/10.1039/C6TC04229E. [3] R.R. Sondergaard, M. Hösel, F.C. Krebs, Roll-to-roll fabrication of large area functional organic materials, J. Polym. Sci. Part B Polym. Phys. 51 (2013) 16–34, http:// dx.doi.org/10.1002/polb.23192. [4] X. Gu, Y. Zhou, K. Gu, T. Kurosawa, Y. Guo, Y. Li, H. Lin, B.C. Schroeder, H. Yan, F. Molina-Lopez, C.J. Tassone, C. Wang, S.C.B. Mannsfeld, H. Yan, D. Zhao, M.F. Toney, Z. Bao, Roll-to-roll printed large-area all-polymer solar cells with 5% efficiency based on a low crystallinity conjugated polymer blend, Adv. Energy Mater. (2017) 1–13, http://dx.doi.org/10.1002/aenm.201602742 201602742. [5] S.Y. Park, Y.J. Kang, S. Lee, D.G. Kim, J.K. Kim, J.H. Kim, J.W. Kang, Spray-coated organic solar cells with large-area of 12.25 cm2, Sol. Energy Mater. Sol. Cells 95 (2011) 852–855, http://dx.doi.org/10.1016/j.solmat.2010.10.033. [6] M. Habibi, A. Rahimzadeh, I. Bennouna, M. Eslamian, Defect-free large-area (25 cm2) light absorbing perovskite thin films made by spray coating, Coatings 7 (42) (2017), http://dx.doi.org/10.3390/coatings7030042. [7] Y. Xia, R.H. Friend, Phase separation of polyfluorene-based blend films and its influence on device operations, Adv. Mater. 18 (2006) 1371–1376, http://dx.doi.org/ 10.1002/adma.200502338. [8] V. Bulović, R. Deshpande, M. Thompson, S. Forrest, Tuning the color emission of thin film molecular organic light emitting devices by the solid state solvation effect, Chem. Phys. Lett. 308 (1999) 317–322, http://dx.doi.org/10.1016/S00092614(99)00580-1. [9] C. Reichardt, Solvents and Solvent Effects in Organic Chemistry, (2003), http://dx. doi.org/10.1002/3527601791. [10] A. Prakash, M. Katiyar, White polymer light emitting diode using blend of fluorescent polymers, 16th Int. Work. Phys. Semicond. Devices, IWPSD 2011 8549 (2012) 1–6, http://dx.doi.org/10.1117/12.928008. [11] G.K. Ho, H.F. Meng, S.C. Lin, S.F. Homg, C.S. Hsu, L.C. Chen, S.M. Chang, Efficient white light emission in conjugated polymer homojunctions, Appl. Phys. Lett. 85 (2004) 4576–4578, http://dx.doi.org/10.1063/1.1813643. [12] J. Huang, G. Li, E. Wu, Q. Xu, Y. Yang, Achieving high-efficiency polymer whitelight-emitting devices, Adv. Mater. 18 (2006) 114–117, http://dx.doi.org/10.1002/ adma.200501105. [13] Y. Ma, S. Liu, H. Yang, Y. Zeng, P. She, N. Zhu, C.L. Ho, Q. Zhao, W. Huang, W.Y. Wong, Luminescence color tuning by regulating electrostatic interaction in light-emitting devices and two-photon excited information decryption, Inorg. Chem. 56 (2017) 2409–2416, http://dx.doi.org/10.1021/acs.inorgchem.6b02319. [14] A.K. Bansal, F. Antolini, S. Zhang, L. Stroea, L. Ortolani, M. Lanzi, E. Serra, S. Allard, U. Scherf, I.D.W. Samuel, Highly luminescent colloidal CdS quantum dots with efficient near-infrared electroluminescence in light-emitting diodes, J. Phys. Chem. C 120 (2016) 1871–1880, http://dx.doi.org/10.1021/acs.jpcc.5b09109. [15] Z. Xiao, R.A. Kerner, L. Zhao, N.L. Tran, K.M. Lee, T.-W. Koh, G.D. Scholes,
Fig. 11. Schematic representation of charge carriers and of their migration processes in HOMO and LUMO of the blend PFO:CdSe(ZnS):P3OT working as active layer of the device.
representation of the material levels, with violet for PFO in amorphous phase, blue for PFO in beta phase, green for QDs and P3OT molecules with low conjugation, and in red, P3OT agglomerates. The increase in the value of the applied voltage leads to an increase in the number of carriers injected into the blends and also gives carriers a greater ability to move between the levels of the different materials, regulating the role of beta phase as radiative traps in certain regions of the blends. The emission of the PFO occurs through the beta phase (blue blocks), and not by the amorphous phase (purple blocks), what is characterized by luminescence at longer wavelengths [27]. In Fig. 11 the electrons are presented as black spheres and the holes as white spheres and, in addition, the dashed circles mark the initial position of the carriers before their migration through the materials, with some of these migrations represented by the arrows. Finally, the dashed lines between HOMO and LUMO indicate the radiative recombination processes of the carriers. This figure schematizes how may occur the distribution of the carriers along the blend, trying to show the migration processes of the charges, originally injected in amorphous PFO (purple), to the materials with lower energies, such as PFO beta phase, CdSe (ZnS) and P3OT, through which recombination and light emission occurs. 4. Conclusions In this work we prepared devices with white emission, based on the PFO:P3OT blends, and PFO:CdSe(ZnS):P3OT hybrid blends. The fact 14
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[16]
[17]
[18]
[19]
[20]
[21]
[22]
[23]
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