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Insufficient harvest of solar light energy is one of the obstacles for current photovoltaic devices to achieve high performance. Especially, conventional organic/inorganic hybrid solar cells (HSCs) based on PTB7 as p-type semiconductor can only utilize 400-800 nm solar spectrum. One effective strategy to overcome this obstacle is the introduction of up-conversion nanophosphors (NPs), in the virtue of utilizing the near infrared region (NIR) of solar radiation. Up-conversion can convert low-energy photons to high-energy ones through multi-photon processes, by which the solar spectrum is tailored to well match the absorptive domain of the absorber. Herein we incorporate erbium-ytterbium co-doped gadolinium molybdate (Gd2(MoO4)3, GMO), denoted as GMO:Yb/Er, into TiO2 acceptor film in HSCs to enhance the light harvest. Here Er3+ acts as activator while Yb-MoO42- is the joint sensitizer. Facts proved that the GMO:Yb/Er single crystal NPs are capable of turning NIR photons to visible photons that can be easily captured by PTB7. Studies on time-resolved photoluminescence demonstrate that electron transfer rate at the interface increases sharply from 0.65 to 1.42 × 109 s−1. As a result, the photoelectric conversion efficiency of the GMO:Yb/Er doped TiO2/PTB7 HSCs reach 3.67%, which is increased by around 25% compared to their neat PTB7/TiO2 counterparts (2.94%). This work may open a hopeful way to take the advantage of those conversional rare-earth ion doped oxides that function in tailoring solar light spectrum for optoelectronic applications.
© 2016 Optical Society of America
1. Introduction
Organic/inorganic hybrid solar cells (HSCs) have intrigued great interest from researchers in recent years because of utilizing advantages of both organic and inorganic materials [1], such as low cost, high dielectric constant and low toxicity for inorganic semiconductors together with low-cost of room-temperature processing, mechanically flexible, lightweight, large-area and ease of fabrication in tuning their electronic structures for organic semiconductors [2, 3]. However, at the current stage of development, the delivered power conversion efficiency (PCE) remains lagged behind the practical applications, ranging from 1% to 3% [4, 5]. One of the important reasons can be ascribed to the insufficient usage of solar spectrum [6–9]. For instance, the conventional HSCs made from PTB7 as light absorber can only use 400-600 nm solar spectrum due to its relatively wide band gap (~1.9 eV) [10, 11].
In a bid to broadening the harvesting of solar spectrum, up-conversion (UC) rare-earth ions, which were introduced by F. Auzel in 1966 [12], have received wide and continuing interest. Among them, erbium-ytterbium (Er-Yb) system remains the key constituent of the UC luminescent materials for red, green and blue (RGB) emissions. Due to the large absorption cross-section for near-fared (NIR) spectrum, Yb3+ cation acts as an ideal sensitizer in the Er-Yb system. What’s more, Yb3+ has only two energy levels, which can effectively suppress those undesired non-radiative relaxations where the energy loss of the absorbed incident light may happen [13]. It has been discovered that the photon energy in NIR region can be efficiently transported from Yb to Er and thus electrons of Er ion can be promoted to the high-lying energy level, followed by UC emission of high photon energy through radiative transitions [14]. Most of UC rare-earth ions can give out luminescence, notwithstanding only host materials with low photon energy are capable of inhibiting intermediate state excitations from relaxing fast to the lower lying levels, thus leading to valuable luminescence for HSCs to use [15]. Hence much attention has been paid to the screening of host materials to reduce the electron-photon coupling.
So far, recent studies were most concentrated on fluoride and oxide host materials. Er-Yb co-doped fluorides as the typical fluoride system was investigated mostly due to its relatively low lattice phonon energy [16, 17]. Fluorides have lower photon energy compared to other materials, which make doped rare-earth ions having longer level lifetime, resulting in more stable metastable levels [18–20]. Nevertheless, most fluorides are highly toxic, and have poor chemical stability and poor mechanical strength, thus making them difficult for practical applications. In contrast, oxides, such as silicon oxides [21], titanium oxides [22], phosphorus oxides [23] and molybdenum oxides [24], have an edge over fluorides including their excellent chemical stability, high thermal stability (high melting point) and facile preparation [25], etc. However, the UC efficiency of rear-earth (RE) oxides is poorly low because of their inherent high-energy phonons. In an attempt to mitigate the shortcoming and enhance the UC luminescence of Er3+ in oxide materials, attention has been paid to make use of the synergistic effect of various ions through engineering energy level alignment [26].
Transitional mental (TM) ions bear unique spectroscopic characteristics and electronic structures, and have attracted much attention. Thanks to the exposed d-electrons, TM ions tend to sensitively respond to the external ions and their excited states are prone to be tuned to match the acceptor ions through a high excited state energy transfer (HESET) [14, 27, 28] thus leading to an enhancement of green emission and a restraint of red emission. Although Er3+ and Yb3+ co-doped gadolinium molybdate (Gd2(MoO4)3, GMO) polycrystalline has been reported before [29, 30], to the best of our knowledge, the optical properties of Er3+:Yb-MoO42- single crystal nanophosphors (NPs), the role of them playing in photon-generation dynamics and solar devices have rarely been reported. In order to validate our hypothesis, herein, Er3+ and Yb3+ co-doped GMO NPs were prepared and characterized using various techniques. The electron transfer dynamics at the acceptor/donor interface will be explored using transient absorption photoluminescence (PL) spectroscopy and the role of enhancing photovoltaic performance will be demonstrated in this work.
2. Experimental
2.1 Materials
Tetrabutyl titanate, nitric acid, citric acid, isopropanol, OP emulsifying agent (Triton X-100), poly(ethylene glycol) (PEG, molecular weight of 20 000), phosphoric acid, P25 (Degussa), acetonitrile, ammonia, chloroform, Gd(NO3)3·6H2O, Er(NO3)3·6H2O, Yb(NO3)3·6H2O and (NH4)6Mo7O24 are of analytic purity and were purchased from Sigma-Aldrich, Hongkong. Poly(3,4-ethylenedioxylenethiophene)-polystylene sulfonic acid (PEDOT:PSS) and Poly{4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl-alt-3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4- b]thiophene-4,6-diyl} (PTB7) were provided by Aldrich and the Fluorine-doped tin oxide glass (FTO, sheet resistance 8 Ωcm−2) was obtainedfrom Hartford Glass Co., U.S.
2.2 Preparation of GMO:xYb3+/1Er3+ NPs
The Gd2(MoO4)3:xmol%Yb3+/1mol%Er3+ (x = 1, 3, 5, 7, 9, 12,GMO:xYb3+/1Er3+) phosphors were prepared by a simple sol-gel method. The pre-calculated amounts of Er(NO3)3·6H2O, Gd(NO3)3·6H2O,Yb(NO3)3·6H2O and (NH4)6Mo7O24 were dissolved in deionized water to prepare solutions of particular concentration, following by adding complexing agent citric acid to the solution (molar ratio of cation to citric acid at 1:1.5). Then, the pH value was adjusted to neutral around 7 using ammonia. Prior to further being calcined at 800 °C for 2h in ambinent atmosphere, the resulted solution was subject to stirring for 1h and subsequent drying in air at 130 °C until transforming into a black bulk. At last, the products were pressed to flat disk for further spectral analysis.
2.3 Fabrication of HSCs
TiO2 colloid and NP-doped TiO2 was prepared by the procedures in a similar way as done in the previous works [31]. The TiO2 colloid was spin-coated on FTO glass to obtain NP-doped or bare TiO2 acceptor layer with a particular thickness [32, 33], which was then sintered in air. Then, FTO with TiO2 film was soaked in PTB7 toluene solution (0.15 mM) for loading PTB7 molecules [34]. Next, PEDOT: PSS, layer was spin-coated onto the bulk heterojunction (BHJ). Finally, by using thermal evaporation, Pt electrode of 50 nm thickness was deposited on the top of the PEDOT: PSS layer under vacuum. The active area of the cell is 0.50 cm2.
3. Results and discussion
3.1 Characterization of prepared GMO:xYb3+/1Er3+ NPs
Figure 1(a) depicts the XRD patterns of the prepared GMO:9Yb3+/1Er3+ NPs, which well agrees with the standard diffraction card (JCPDS 019-0476). Unfortunately, standard pattern of JCPDS 019-0476 does not mark the hkl planes, so the corresponding crystal structures are unknown. We note that the standard card JCPDS 026-0655 also matches very well with the XRD results and the standard card JCPDS 019-0476. Besides, the crystal structures are provided in the card. Therefore, we choose JCPDS 026-0655 to identify the crystalline phases. It reveals that the obtained NPs is a single monoclinic phase with the cell parameters of a = 7.53,b = 11.38,c = 11.40 Å and a space group of C2/c. Briefly, Herein, the stronger diffraction peaks located at 34.32°, 28.42° and 29.4° well match those from the standard card, i.e., 34.358°, 28.218° and 29.356°, corresponding to the (2 0 2)/( 0 4), ( 2 1) and (0 2 3) planes, respectively. The TEM image, as shown in Fig. 1(b), further confirms structural information of the NPs, which still fall within nanometer domain. The HR-TEM image shown in Fig. 1(c) reveals its well crystallized structure without noticeable deficiencies at atomic resolution and according to the finger fringe, the estimated distance-spacing of 0.58 nm well agrees with d-space of 0.571 nm corresponding to (2 0 2) crystal plane. More accurate results can be derived from SAED in Fig. 1(d), the calculated d-spacings are d1 = 1.12, d2 = 0.58, d3 = 0.51 nm, which well match with those of 1.078, 0.571 and 0.504 nm and correspond to the indexed crystalline planes of R1 = (0 0 1), R2 = (0 2 0) and R3 = (0 2 1), respectively.
Fig. 1 (a) XRD pattern of GMO:9Yb3+/1Er3+ and JCPDS Card (No.26-0655 and 19-0476) for monoclinic GMO; TEM images at low (b) and high (c) magnifications, and (d) SAED pattern of the prepared GMO:9Yb3+/1Er3+ NPs.
3.2 UC luminescence of GMO:xYb3+/1Er3+ NPs
The UC-PL properties of the GMO:xYb3+/1Er3+ NPs are sensitively dependent on the concentration of the doping RE ions. A power-controllable 976 nm LD (Hi-Tech Optoe-lectronics Co. Ltd. Beijing) with the maximum power output of 500 mW was used to excite the samples. The laser beam was not focused with an irradiance area of about 1cm2. The UC luminescent spectra were collected at room temperature, by using a spectrometer (Jobin Yvon, iHR 550) with a photomultiplier tube (R928, Hamamatsu). The emission spectra of Yb3+/Er3+ co-doped GMO NPs excited by a 976 nm at room temperature are shown in Fig. 2 at different loading concentrations of Yb3+while the Er3+ concentration keeps unchanged at 1 mol%. The observed intense green and weak red emissions located at 531, 552 and 668 nm are arisen from the transitions 2H11/2→ 4I15/2, 4S3/2 → 4I15/2, and 4F9/2 → 4I15/2 transitions of Er3+, respectively (Fig. 3). Moreover, the intensities of both green and red emissions increase with the increase of the concentration of Yb3+ ions and reach a peak 9 mol%, followed by a decrease at higher concentrations exceeding 9 mol%.
Fig. 2 UC-PL luminescent spectra of GMO:xYb3+/1Er3+ (x = 1, 3, 5, 7, 9, 12 mol%) NPs excited at 976 nm using LD.
Fig. 3 Energy level diagrams and possible UC mechanisms for the GMO:Yb3+/Er3+ NPs under a 976 nm laser excitation.
In order to have a comprehension of the UC mechanism of GMO:Yb3+/Er3+ NPs excited at 976 nm, the effect of excitation power on the red and green emissions was examined using GMO:9Yb3+/1Er3+ NPs shown in Fig. 4. It is known that there exists a relationship to describe the number of required photons to populate the unsaturated upper emitting level. That is the PL intensity IPL is proportional to the pump laser power INIR as described by the ln–ln relationship. The slope of curve, i.e., approximately n = 2 is deemed to be the number of required laser photons for both the green and red emissions, indicating that a two-photon excitation process is associated with GMO:xYb3+/1Er3+ NPs.
Fig. 4 Dependence of the green and red emission intensities of GMO:9Yb3+/1Er3+ NPs on the excitation power.
The MoO42− complex is a closed shell configuration with an 1A1 ground state and the four excited states 3T1, 3T2, 1T1 and 1T2 [35, 36]. Upon the addition of Yb3+ ions, the Yb3+-MoO42− dimer complex can be formed, thus enabling both ground state absorption (GSA,|2F7/2, 1A1> → |2F5/2, 1A1>) and subsequent excited state absorption (ESA, |2F5/2, 1A1> → |2F7/2, 1T1>). The ground state, intermediate excited state of Yb3+-MoO42− dimer are denoted by |2F7/2, 1A1> and |2F5/2, 1A1>, respectively. Apart from these, there still exist the relevant higher excited states as depicted by |2F7/2, T1>, |2F7/2, 3T2>, |2F7/2, 1T1> and |2F7/2, 1T2>. Due to the effective energy transfer (ET) between the sensitizer Yb3+-MoO42− dimer and the activator Er3+, the high excited state ET (HESET) from the |2F7/2, 3T2> state of the Yb3+-MoO42− dimer to the 4F7/2 level of Er3+ ion plays a dominant role. Subsequently, the three lower emitting levels (2H11/2, 4S3/2 and 4F9/2 levels) are then populated via multiphonon relaxation, followed by the red and green emissions. In detail, the 4F9/2 level is generated via nonradiative decay from the 4S3/2 level and several phonons are emitted to bridge the ~3000 cm–1 gap between these two levels. And it is expected that the 4F9/2 level is lower than the 4S3/2 level, which can explain the relatively strong green emission but weak red emission. Such HESET mechanism can effectively mitigate the disadvantages of the exchange enhancement in the sensitization complex and lattice phonon quenching processes, thus resulting in the bright green and red emission. Consequently, the absorption range was widened and the NIR spectrum can be used by HSCs for the red emissions. Besides, intensified green emission caused by HESET is also conducive to the photovoltaic performances of solar devices, which will be explored next.
3.3 Charge-transfer dynamics of organic/inorganic BHJ architecture
A highly efficient HSC also needs an efficient charge-transfer in BHJs apart from optical properties of photovoltaic material [4, 37]. Therefore, aiming at exploring the photoexcited electron transfer mechanism, the transient PL spectra were utilized to estimate the photo-generated electron transfer dynamics. Hence, time-resolved PL spectra of PTB7 neat film, TiO2/PTB7 and (GMO:9Yb3+/1Er3+):TiO2/PTB7 blend films have been collected to comparatively probe into the influence of GMO:9Yb3+/1Er3+ NPs-doping on the charge-transfer dynamics as shown in Fig. 5. Compared to neat PTB7 films, when interfaced with bare or NP-doped TiO2 acceptors, both TiO2/PTB7 and (GMO:9Yb3+/1Er3+):TiO2/PTB7 films decays much faster than neat PTB7 film; And the intensities of PL emissions were quenched significantly after 1000 ps for these two blend films, indicating that a strong charge transfer happened at donor-acceptor (D-A) interfaces over this time span. In BHJ architecture, the electron(e)-hole(h) separation appears when the excited PTB7 donor adsorbed on the porous structures of TiO2 acceptor films (reaction 1), followed by either charge recombination (reaction 2) or an additional deactivation pathway for the bleaching recovery, i.e., the injection of photo-generated electrons into TiO2 acceptor(reaction 3):
The decay curves can be fitted to a single-exponential function Eq. (4) to derive the fluresecent lifetime given by:where δ(t) is the electronic hyperpolarizability, F(t) is the impulse response function (IRF) which is determined by half width at half-maximum (HWHM) of the pulse duration, A is the amplitude, and is PL lifetime. The electron transfer rate constant ket can be obtained by comparing the PL decays of the pure donor and the donor-acceptor blend, taking the form ofThe fluorescence life times of the PTB7 neat film, TiO2/PTB7 and GMO:Yb3+/Er3+ NPs:TiO2/PTB7 blend films were determined to be 1770, 821 and 504 ps, respectively (Table 1). Therefore, as per Eq. (5), the electron transfer rate ket can be estimated to be 0.65 and 1.42 × 109 s−1 for TiO2/PTB7 and (GMO:9Yb3+/1Er3+):TiO2/PTB7 BHJs, respectively. Obviously, after the incorporation of NP into TiO2 acceptor, the electron transfer rate has been significantly enhanced by 100% comparing with bare TiO2 and much faster charge-transfer at the (GMO:9Yb3+/1Er3+):TiO2/PTB7 interface can outstrip charge recombination, thus giving rise to better charge collections. An efficient PL quenching for NPs:TiO2/PTB7 blend film can be ascribed to the redundant space provided or pores formed by the incorporation of nanoparticles, thus favoring the physical adsorptions of PTB7 molecules in virtue of intermolecular attractive forces and forming intimate contact between PTB7 molecules and acceptor. After incorporating nanoparticles into TiO2, the interface contact between donor and acceptor has been improved, thus benefiting efficient charge transfer in BHJ.Fig. 5 PL decays of neat PTB7, TiO2/PTB7 and (GMO:9Yb3+/1Er3+):TiO2/PTB7 films under 580-nm excitation.
Table 1. Kinetic parameters obtained from fits to the time-resolved PL decays
3.4 Photovoltaic performances of HSCs
External quantum efficiency (EQE) measurements for the HSCs with and without nanophosphors were provided in Fig. 6. Overall, the EQE spectra closely match the corresponding absorption spectra PTB7. The broad EQE band covering the spectrum from 350 to 800 nm is enhanced significantly by GMO:Yb3+/Er3+ doping. The low EQE for TiO2/PTB7 throughout the absorption spectrum is consistent with a slow charge transfer as shown above. Unfortunately, due to the wavelength limitation of EQE instrument in the NIR range, we cannot provide the clean-cut evidence of the nanophosphors induced improvement in the NIR spectrum. However, the UC properties of the nanophosphors that can convert the NIR photons into the red or green ones, together with the good light utilization ability of the PTB7 in the visible range, may guarantee the better performance of the NPs:TiO2/PTB7 HSC.
In order to demonstrate the effect that GMO:9Yb3+/1Er3+ NPs have on the device performance of HSCs, TiO2/PTB7 and GMO:9Yb3+/1Er3+ NPs doped TiO2/PTB7 BHJs films at different loading amounts of GMO:9Yb3+/1Er3+ NPs (ranging from 0% to 10%) were fabricated for comparison. Figure 7(a) reveals that under the identical experimental conditions, HSCs with GMO:9Yb3+/1Er3+ NPs exhibit better photovoltaic performance than HSCs with pure TiO2. The photovoltaic parameters are summarized in Table 2. The JSC increases from 8.38 at 0wt% to 10.29 mA cm−2 at 7.5wt% of GMO:9Yb3+/1Er3+ NPs whereas both the Voc and filling factor (FF) remain almost constant, thus leading to an improvement by almost 34% than its counterpart without doping, i.e., from 2.94% at 0wt% to 3.67% at 7.5wt% of GMO:9Yb3+/1Er3+ NPs. Such enhancement can be explained by the dual role of NPs: the broader light harvest and more efficient photoexcited charge transfer. However, note that the loading amount of GMO:9Yb3+/1Er3+ NPs is adverse to exceed 7.5wt%, because it will become hard to form a flat film and thus damage the intimate interfacial contact within BHJ device.
Fig. 7 (a) J-V characteristics of BHJs with different contents of NPs under the illumination of a AM1.5 G solar simulator (100 mW/cm2); SEM image of (b) bare TiO2, (c) and (d) cross-sectional SEM images of BHJ made from NP-doped TiO2 and PTB7/NPs:TiO2 (GMO:9Yb3+/1Er3+, 7.5 wt%).
Table 2. Photovoltaic performances of the HSCs
The morphology and structure of the synthesized TiO2 was characterized using SEM. As observed from Fig. 7(b), the TiO2 layer exhibits porous structure, which can facilitate the intake of PTB7 molecules and form an intimate interfacial contact by virtue of intermolecular forces [38–40]; and the TiO2 nanoparticles is about 20 nm in diameter. With a view to distinguish the individual thickness of different layers and interfacial structure of BHJ, the cross-sectional SEM images of TiO2 films before and after PTB7 soaking process are shown in Fig. 7(c) and 6(d), and obvious differences can be clearly seen. The GMO:9Yb3+/1Er3+ NPs-doped TiO2 film still presents a loose structure, which is beneficial to the uptake and penetration of PTB7 molecules into the porous acceptor through the porosity [34,41, 42] and form an intimate p-n contact of the BHJ layer. This can be confirmed from Fig. 7(d), compared to bare TiO2 film, the BHJ film becomes much smoother and there exist almost no visible pores, indicating that the PTB7 polymers have been well permeated into the porous structure of the acceptor film and intimate interfacial contact has been formed. The GMO:9Yb3+/1Er3+ NPs, TiO2, and PTB7 can be clearly distinguished. Especially, the thickness of the BHJ layer including all above films was estimated to be about 350 nm. The suitable interface thickness and the intimate contact of BHJ make for charge transport in the D-A interface, in particular for short diffusion length of the phototoexcited exciton in polymer [43].
4. Conclusion
In summary, the GMO:xYb3+/1Er3+ NPs have been successfully doped into TiO2 acceptor film, and the UC luminescent properties under 976 nm excitation was investigated. The energy was transferred from the Yb3+-MoO42- sensitizers to Er3+ activators and the excited Er3+ ions give out the green and red UC emissions. In particular, the UC luminescence intensity can be intensified by the HESET mechanism, which can partly avoid the lattice phonon quenching processes at lower energy levels of Er3+. Besides, because of the GMO:9Yb3+/1Er3+ NPs doping and its two-photon UC mechanism, the NIR photons can be tailored to visible photons that can be recaptured efficiently by PTB7, thus leading to a broader light harvest and an improved PCE of the HSCs from 2.94% for conventional TiO2/PTB7 device to 3.67% for GMO:9Yb3+/1Er3+-doped TiO2/PTB7 device at 7.5wt%. Apart from the enhanced harvest of the low-energy photons, an enhanced photo-excited charge rate also favors a higher efficiency. The electron transport lifetime was shortened by more than 200% from 1770 to 504 ps and correspondingly, the electron transfer rate has been enhanced by more than 100%. This work provides the evidence that GMO:xYb3+/1Er3+ NPs as up-conversion phosphors materials have versatile and promising applications in obtaining efficient HSCs, and also inspires the development of robust RE phosphor in applications for solar cells.
Funding
Natural Science Foundation of China (11564026, 61366003, 11404283); Science and Technology Project of the Education Department of Jiangxi Province, China (GJJ150727); Natural Science Foundation of Jiangxi Province (20142BAB213015).
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