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In this work, the structural and optical properties of bulk heterojunctions (BHJ) composed of MEH-PPV and carbon-based non-fullerene nanodiamond (ND) have been investigated. The steady-state absorption, temperature-dependent fluorescence and Raman spectroscopy demonstrate the conjugated length is increased and more ordered crystalline domains are formed in MEH-PPV films with the increment of ND content. The time-resolved fluorescence spectroscopy and 2D microscopy indicate that the BHJs with higher concentrations of ND may enhance the donor/acceptor interfacial contact which favors the dissociation of excitons into free charge carriers. Solar cells based on MEH-PPV:ND BHJs were fabricated to explore the interplay between structural, photophysical properties, and ultimately, device performances. The results show that optimization of blend composition and structures with ND may improve the efficiency of the thin film photovoltaic devices.
© 2017 Optical Society of America
1. Introduction
Solution-processible bulk heterojunction (BHJ) organic solar cells (OSCs) have attracted intense research interest as a kind of renewable energy source. Currently most high-performance BHJs are composed of electron-donating semiconducting polymers and electron-accepting molecules in which the photoinduced charge generation and transfer between the donor and acceptor are involved [1,2]. While diverse polymers have been widely used in BHJs as donors, such as poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV), poly(3-hexylthiophene) (P3HT), and poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b’]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7) [3,4], there is only a small series of acceptor materials suitable for OSCs.
Among the commonly used electron acceptor materials, fullerenes and their derivatives, e.g. 6,6-phenyl C61 or C71 butyric acid methyl ester (PC61BM and PC71BM), are the most successful due to their unique properties such as favorable LUMO energy, large electron affinity, high electron mobility and the quasi spherical shape which may favor the charge transfer in three dimensions [5,6]. However, PCBM is expensive owing to its multistep synthesis, which underscores a key economic advantage of OSCs. Incentive remains to overcome the disadvantages of fullerene derivatives, such as the high cost and difficult band gap tuning by chemical modifications.
Non-fullerenes carbon-based materials are coming to view because of the tunable optical and electrical properties, such as carbon nanotubes, graphene and graphite oxide and nanodiamond (ND) [7–11]. Among them, ND is stable, non-toxic, abundant in nature and can be easily surface-functionalized. ND has been reported as an additive to P3HT-involved photo-active layer [10–12], which enhanced the device performance. MEH-PPV is a conjugated polymer and is widely used as a donor material in OSCs. The power conversion efficiency of MEH-PPV solar cells can be as high as 4.2%, which is comparable to the performance of P3HT:PCBM solar cells. However, the structural and optical properties of MEH-PPV and ND blend films are inadequately studied for photovoltaic applications.
In this work, the impact of ND addition on the conformational structures of MEH-PPV films is characterized by the steady-state absorption, temperature-dependent photoluminescence (PL) and Raman spectra. The kinetic behavior of electron generation and transfer in BHJs of MEH-PPV:ND is investigated by the time-resolved PL spectroscopy. Finally, OSC devices were fabricated and the photovoltaic parameters were analyzed to correlate the underlying physics with the ultimate device performances in the MEH-PPV: ND blends.
2. Experimental
MEH-PPV and NDs were purchased from Sigma-Aldrich and Plasma Chem, respectively. NDs with a diameter below 5 nm were dispersed in chloroform by ultrasonication (50 W, 43 kHz) for 1 h. MEH-PPV was dissolved in chloroform at a concentration of 5mg/ml. The solutions were stirred overnight. Then the ND solution was added to the MEH-PPV solution. The blend was sonicated for 1 h and then stirred for five hours in order to obtain a homogeneous solution. Agglomeration can be effectively prevented by ultrasonication followed by stirring, as reported in previous studies [13]. Thin films of MEH-PPV and NDs blend with a weight ratio of 1:0, 1:0.1, 1:0.2 and 1:0.3 were prepared by spin-coating the blend solutions on pre-cleaned ITO glass at a speed of 1000 rpm for 50s, respectively. All of the blend films were annealed in nitrogen at 120 for 10 min.
The surface morphology of the thin films was analyzed with atomic force microscopy (AFM, Nano Scope IIIA). Raman spectra were measured by a confocal Raman microscope (LabRam HR800). The optical absorption spectra were recorded with a UV-visible dual-beam spectrophotometer (TU-1900, PG Instruments Ltd., China). Steady-state PL measurements were conducted with a PG2000-Pro-EX spectrometer with an excitation wavelength of 400 nm. Time-resolved fluorescence decay measurements were performed by means of the fluorescence up-conversion technique. Two-dimensional (2D) images of the fluorescence decay lifetimes were obtained by means of confocal optical microscopy (Nanofinder FLEX2. Tokyo Instruments, Inc.) combined with a time-correlated single-photon counting (TCSPC) module (Becker & Hickl, SPC-150).
OSCs were fabricated with layers of glass/ITO/ZnO/MEH-PPV:ND/MoO3/Ag. ITO glasses were cleansed with detergent, deionized water, acetone and ethanol, respectively. They were dried with compressed nitrogen and kept in an oven for 5 min at 110. ZnO solution was spin-coated onto ITO glass at 2500 rpm for 45 s and annealed in a glove box filled with nitrogen at 120 for 10 min. On the electron extraction ZnO layer, the MEH-PPV:ND solution was spin coated at 900 rpm for 45 s. The polymer films were annealed at 130 for 15 min. A MoO3 layer with the thickness of 10 nm and then a 100 nm thick Ag layer were thermally evaporated onto the active layer in vacuum with a pressure <1.0 × 10−4 Pa. The devices were tested right after preparation. Current density vs. voltage (J–V) characteristics of the OSCs were recorded with Keithley 2400 source meter under AM1.5G illumination of 100 mW/cm2 (Sofn Instruments Co., Ltd., China).
3. Results and discussion
MEH-PPV as the donor and ND as the acceptor were blended to form BHJs. The morphological structures of the BHJs play an important role in the charge transfer processes and can be revealed in the AFM images. As shown in Fig. 1, the polymer and ND form penetrating networks, which favors the generation of free charge carriers. The photoinduced excitons in MEH-PPV migrate to the donor/acceptor interfaces and some of them dissociate into free charge carriers. Aggregates are clearly visible in the AFM images of the blend films. The r.m.s. (root-mean-square) roughness is 2.58 nm, 7.33 nm, 7.58 nm and 7.77 nm in Fig. 1(a)-(d) respectively. The solubility of NDs is modest in chloroform, and hence the aggregate size increases as the ND concentration becomes larger. With a rougher surface morphology, the light scattering and absorption may be enhanced. Finer interpenetrating network of the donor and acceptor is formed. It favors the charge transfer between the donor and acceptor materials and thus is beneficial to the photovoltaic performance [14].
Fig. 1 AFM images of dried films of pristine MEH-PPV (a) and its blends with ND at a concentration of 10 (b), 20 (c), and 30 (d) wt.%, respectively.
Conformational structures of conjugated polymer thin films are correlated to the absorption and fluorescence emission properties. The UV-visible absorption spectra exhibit a common strong absorption maximum at 498 nm (2.49 eV) for all the samples, as shown in Fig. 2(a). The band edge for these films appears around 565nm (2.19 eV). The direct transition equation is
where α is the optical absorption coefficient, hν the photon energy, C a constant, and Eg the direct bandgap. In Fig. 2(b), (αhν)2 is plotted as a function of the photon energy hν. By extrapolating the linear portion of the curve to the horizontal zero absorption line, Eg was determined to be 2.20 eV (561 nm), 2.18 (569 nm), 2.17 (571 nm) and 2.19 eV (566 nm) for the samples of pristine MEH-PPV, 10, 20 and 30 wt.% ND:MEH-PPV blends, respectively. The bandgap of the MEH-PPV film decreased when ND was added. This was due to an increase in the conjugation length so that the wave function of excitons was localized less and therefore the bandgap decreased [15, 16].Fig. 2 (a) Normalized absorption of pristine MEH:PPV and MEH-PPV:ND films. (b)Plot of (αhν)2 as a function of photon energy, Eg, for the MEH-PPV:ND samples.
As shown in Fig. 3, PL spectra were obtained at room temperature (RT) for neat MEH-PPV and MEH-PPV:ND blend films. They show two main peaks at c.a. 593 and 642 nm, corresponding to 0-0 and 0-1 transitions respectively according to Frank-Condon model. The relative intensity of 0-1 band compared with the 0-0 band varies noticeably among the samples. While being 0.93 for pristine MEH-PPV thin film, it increased to 0.95 and 0.998 for 10% and 20% ND:MEH-PPV, and dropped down to 0.88 when 30% ND was blended in the films. Spano et al. demonstrated that lower energy fluorescence is associated with more ordered regions in the polymer films [17]. Therefore, the crystalline structure in micro domains of polymer films can be promoted with appropriate compositional addition of ND. Moreover, as illustrated in the inset of Fig. 4, there is a slight shift for the (0-0) transition. The maximum, which is at 592.6 nm for pure MEH-PPV film, red-shifts to 593.1, 593.6 and 592.9 nm for 10%, 20% and 30% ND: MEH-PPV composites. The emission wavelength changes probably because the conjugation length of the polymer backbones changes in the MEH-PPV:ND films. The absorption and PL results both suggest that the conjugation length of MEH-PPV chains is enhanced upon the addition of ND and reaches its maximum at the concentration of 20 wt.% ND in MEH-PPV.
Fig. 3 Steady-state PL spectra of pristine MEH-PPV and composite MEH-PPV:ND thin films at RT. Inset: Peak positions of (0-0) peak around 593 nm for the samples.
Fig. 4 (a) Raman spectra of pristine MEH-PPV and MEH-PPV: ND blend films. The inset is the molecular structure of MEH-PPV. (b) Detailed structure of the strongest Raman band at c.a. 1585 cm−1 of the films.
The change in the conjugation length of the polymer can be further confirmed in the Raman spectra, as shown in Fig. 4. The Raman spectra of all the samples include four bands around 968, 1112, 1310 and 1585 cm−1.The 968 cm−1 peak is from the out-of-plane C-H bending mode of the vinylene group. The peaks at 1112 cm−1 is due to C-H vibration in the phenyl ring. The band at 1310 cm−1 arises from the anti C = C vibration in the phenyl ring. The strongest Raman band around 1585 cm−1 is attributed to the symmetric stretching vibration of the phenyl ring. A shoulder at 1625 cm−1 is correlated to the anti C = C stretching vibration of the vinylene group. The detailed structures, i.e. the positions and widths of the strongest Raman band at c.a. 1585 cm−1 of MEH-PPV upon ND addition, are compared as shown in Fig. 2(d). The frequency downshifts to 1583 cm−1 with increasing ND content of 10% and 20%. Meanwhile the width of this band becomes smaller. The FWHM of the main peak decreases from 21.5 cm−1 for pristine MEH-PPV film to 20.8 cm−1 when 20% ND was added into the blend. It indicates better molecular ordering of the polymer chains in the MEH-PPV:ND blend films [18]. Therefore, the optical and conformational characterizations all suggest that the involvement of ND increases the conjugation length of MEH-PPV.
Figure 5(a) and 5(b) present the steady-state PL spectra of pristine MEH-PPV and blended MEH-PPV:30% ND films measured at various temperatures ranging from 6 to 300 K. The two vibronic peaks are prominent and well resolved at low temperatures. The energy difference between the (0-0) and (0-1) peak is roughly independent of the temperature and resides around 0.16 eV (~52 nm) [19], which can be assigned to the coupling between carbon-carbon stretch vibration and the conjugated backbone. The overall spectrum shifts to higher frequency with increasing temperatures. That is most likely because the conformational disorder can be aggravated due to the thermally induced torsion and vibration modes as the temperature rises. The area of the conjugation segments becomes smaller in the polymer films. Consequently, the delocalization of π-electron is reduced, rendering increased energy of π-π* transition. The inset displays the energy positions of the 0-0 and 0-1 vibronics as a function of temperature. The average shift rate of the peak position is 1.83 × 10−4 and 2.36 × 10−4 eV/K for (0-0) and (0-1) bands in pristine MEH-PPV film. In the composite films, the temperature-dependence of the PL energies resembles the pristine one, but with varied peak positions. The slope of the fitting line is apparently smaller in the composite film with 30% ND than the pristine one (see the inset of Fig. 5(a) and (b)). The shift rate of the peak position is summarized in Table 1 for all the samples under consideration. This value goes down all the way as the concentration of ND increases. The involvement of ND generates more barriers between the polymer chains, which restricts the rotational modes and slows down the process of conformational disorder. As a result, the conjugation lengths and therefore the π-π* energy change more slowly with ND than that without as the temperature increases.
Fig. 5 PL spectra of films of pristine MEH-PPV (a) and 30 wt.% ND:MEH-PPV blend (b) measured at different temperatures. The insets in (a) and (b) show the energy positions of the two main peaks ((0-0) and (0-1) transitions) as a function of temperature. (c) Intensity ratio between (0-1) and (0-0) peaks as a function of temperature.
Table 1. Average values of shift rate of peak positions
Furthermore, the relative intensity of the individual vibronic peaks, that is, I0-1/I0-0, increases monotonically with elevating temperatures from 30 to 150 K (Fig. 5(c)). Conjugated polymers can be treated as an inhomogeneous collection of chain segments with diverse lengths, corresponding to diverse energy sites. As soon as the polymer is excited by the laser, excitons transit to low energy via Förster energy transfer [19,20]. Then the emission occurs when the excitons recombine at the low energy sites. As the temperature increases, the conformational disorders in the MEH-PPV may bring in broader spectral widths. Hence the spectral overlap of donor and acceptor micro-domains could be enhanced, facilitating the transfer of the excitation energy in conjugated polymers from high to low energy segments. Therefore, there are more excitons for the low-energy (0-1) emission, leading to larger I0-1/I0-0 ratio. As the concentration of ND increases in the MEH-PPV films, the intensity ratio I0-1/I0-0 decreases, although this trend stops when the concentration is 30%. It may be associated with the presence of ND that hinders the Förster energy transfer to sites for long-wavelength luminescence emission. Increased I0-1/I0-0 can be observed from 30 to 150 K, but such trend does not sustain at higher temperatures. We tentatively suggest the decrease of the intensity ratio I0-1/I0-0 is associated with the thermally induced “noise” in charge transport, emission and recombination processes. Such thermal noise and the Förster energy transfer are two competing factors to influence the I0-1/I0-0 ratio. As the temperature increases, the thermal noise becomes more dominant than the Förster energy transfer effect. Therefore, the ratio I0-1/I0-0 is not all the way increasing from low to room temperatures.
The un-normalized steady-state PL spectra of MEH-PPV:ND blend films are exhibited in Fig. 6(a). The PL intensity of MEH-PPV is quenched in the presence of ND. The PL quenching phenomenon can be interpreted from two aspects. One is the decrease in the concentration of the fluorescent MEH-PPV in the BHJs. The other possible reason is that the interfacial contact between MEH-PPV and ND enhances exciton dissociation to form free charges and inhibit exciton recombination towards the fluorescence emission.
Fig. 6 (a) PL quenching effect in composite of MEH-PPV and ND. (b) Schematic sketch of experimental setup for the time-resolved fluorescence up-conversion measurement. (c) Fluorescence decay profiles of pure MEH-PPV and MEH-PPV:ND blend films.
To study the charge transfer dynamics in MEH-PPV:ND nanocomposites, time-resolved fluorescence measurements were performed using a fluorescence upconversion technique (see Fig. 6(b)). The excitation wavelength was fixed at 400 nm and the fluorescence decay was detected at 600 nm. The fluorescence decay curves of neat MEH-PPV and MEH-PPV:ND blends are shown in Fig. 6(c). The instrumental response function (IRF) was measured to be 400 fs. The compositional increment of ND leads to a more swift fluorescence decay. The obtained curves were fitted by a multi-exponential decay equation. The fitted data are listed in Table 2, showing the samples possess two lifetime components. The short lifetime component τ1 is around 20 ps, owing to exciton hopping to sites with lower energy. The long lifetime τ2 is on a 100-200 ps scale and can be ascribed to the formation of relaxed intrachain and interchain singlet excitons [21,22]. In contrast to pristine MEH-PPV film, the amplitude of τ1 increases from 48% to 64% in the blends, indicating NDs facilitate the exciton hopping to low energy sites. It is consistent with the steady-state PL results at room temperature. The improved crystallinity of MEH-PPV favors exciton hopping. The time constant of singlet exciton formation, τ2, is lessened from 201 ps to smaller than 150 ps in blends. As a result of the enhanced interfacial contact between the donor MEH-PPV and the acceptor ND, the exciton dissociation and charge transfer is more efficient in the composite BHJs.
Table 2. Fluorescence decay parameters of pristine MEH-PPV and MEH-PPV/ND thin films.
Whereas the kinetic emission measurements were performed on individual points of the thin films via the fluorescence up-conversion technique, at wo-dimensional (2D) overview of the fluorescence lifetime distribution can be depicted in the time-resolved fluorescence images [23]. The excitation laser is at 500 nm and the detection wavelength is 600 nm. With the spatial resolution of 0.5, the four images in Fig. 7suggest NDs of different concentrations are all homogenously dispersed in MEH-PPV, leading to almost uniform fluorescence lifetimes across each film. As the lifetime range for the scale bar is set to the same for Fig. 7(a)-(d), the 2D fluorescence dynamics can be directly visualized from the four images. The fluorescence lifetime diminishes with increasing ND concentration, which is in good agreement with the above fluorescence upconversion results.
Fig. 7 Time-resolved fluorescence images of thin films composed of MEH-PPV and ND at concentrations of 0 (a), 10 (b), 20 (c) and 30 (d)wt.%. The color bar corresponds to the range of 370-410 ps for all the four images.
The morphological and conformational structures of the materials, together with the charge transfer properties, play important roles in the device performances. As illustrated in Fig. 8(a), sandwich-structured BHJ solar cells have been fabricated with pristine MEH-PPV and MEH-PPV:ND blends as the photoactive layers. The size of the device is 3 mm ×3 mm for all the solar cells. The energy levels of the different layers in the solar cells are shown in Fig. 8(b). MEH-PPV is the electron donor material and NDs serve as the acceptor. The device performances have been characterized under AM1.5G illumination with a power density of 100 mW/cm2. The characteristic I-V curves of the solar cells comprising pristine MEH-PPV or MEH-PPV:ND blends are plotted in Fig. 8(c). The device parameters are listed in Table 3. Whereas the open-circuit voltage (VOC) does not fluctuate dramatically as the ND concentration varies from 10 to 30%, the short-circuit current density (JSC) and power conversion efficiency (PCE) are conspicuously influenced by the ND concentration. JSC and PCE are enhanced when 10 and 20% ND is blended, with the optimal values achieved at the latter concentration. The enhancement in PCE is correlated with the improved crystallinity, conjugation length, and more efficient charge dissociation into free charges at MEH-PPV:ND interfaces. Nevertheless, as the ND concentration increases further to 30%, the device performance is deteriorated because excessive amount of ND leads to worse morphology and crystalline structures. The photovoltaic performance of the MEH-PPV:ND remains to be further promoted in the future work, e.g. by optimizing the energy levels and the solubility of NDs in the organic solvent by chemical tailoring techniques.
Fig. 8 (a) Schematic diagram of OSC. (b)Energy level diagram of the device. (c) J-V curves of the solar cells with active layers composed of pristine MEH-PPV and blends of MEH-PPV:ND with concentrations of ND at 10, 20 and 30 wt.% respectively.
Table 3. Device parameters of MEH-PPV/ND BHJ solar cells.
4. Conclusions
MEH-PPV:ND BHJs with various ND concentrations have been investigated in terms of structural and photophysical properties. With appropriate ND concentration in MEH-PPV films, more ordered crystalline domains, increased conjugation length and optimized morphology can be obtained, according to the absorption, temperature-dependent photoluminescence, Raman and AFM characterizations. Blends with higher concentration of ND possess enhanced interfacial contact between the donor conjugated polymer and the acceptor carbon-based ND, facilitating the dissociation of excitons into free charge carriers. OSC devices were fabricated based on MEH-PPV:ND BHJs to study the comprehensive effect of ND involvement, showing optimization of blended structures with ND may improve the performance of the thin film photovoltaic devices.
Funding
National Natural Science Foundation of China (11404190, 11574181); Doctoral Program of Higher Education (20130131110004); the Fundamental Research Funds of Shandong University, and the ‘National Young 1000 Talents’ Program of China.
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