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inorganic hybrid solar cells by up-conversion luminescence nanophosphors
Electrochimica Acta 182 (2015) 416–423
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Tailoring solar energy spectrum for efficient organic/inorganic hybrid solar cells by up-conversion luminescence nanophosphors Dongyu Lia,1, Weifu Sunb,1, Lexi Shaoa,* , Shuying Wuc, Zhen Huanga , Xiao Jind,* , Qin Zhangd, Qinghua Lid,* a
Department of Physics, Lingnan Normal College, Zhanjiang 524048, PR China School of Aerospace, Mechanical and Mechatronic Engineering, The University of Sydney, Sydney, NSW 2006, Australia School of Aerospace, Mechanical and Manufacturing Engineering, RMIT University, Melbourne, VIC 3001, Australia d Jiangxi Engineering Laboratory for Optoelectronics Testing Technology, Nanchang Hangkong University, Nanchang 330063, PR China b c
A R T I C L E I N F O
A B S T R A C T
Article history: Received 1 June 2015 Received in revised form 17 July 2015 Accepted 5 September 2015 Available online 28 September 2015
Solar light harvesting ability is one of the key properties in organic/inorganic solar cells. One of the most popular organic materials used is poly(3-hexylthiophene) (P3HT), which can make use of solar spectrum in the range 400-600 nm, however, this polymer is incapable of utilizing low energy photons. Herein, we incorporate erbium ion decorated gadolinium oxymolybdate (GMO:Er) nanophosphors (NPs) into mesoporous acceptor film (TiO2) in an attempt to enhance the light harvest. The nanophosphor can convert the near-infrared solar spectrum to visible region (near 550 nm), in which the energy can be recaptured by P3HT. The results show that the up-conversion proceeds via the two-photon upconversion mechanism. It is found that after the incorporation of GMO:Er NPs into TiO2 at 5 wt%, the charge transfer rate was enhanced from 2.79 to 5.83 109 s1. The device performance of solar cells based on GMO:Er NPs demonstrates a more than 30% improvement compared to their neat TiO2/P3HT analogue and such enhancement can be ascribed to the broader light harvest together with faster photoexcited charge transfer. This platform can be readily implemented by introducing more demanding energy conversion phosphors and allows for the development of optoelectronic applications with tailored optoelectronic properties. ã 2015 Elsevier Ltd. All rights reserved.
Keywords: Organic/inorganic hybrid solar cell Charge-transfer dynamics Two-photon up-conversion mechanism Bulk-heterojunction
1. Introduction Organic/inorganic bulk-heterojunction (BHJ) hybrid solar cells (HSCs) have become one of the relatively newly emerging hotspots due to the potential advantageous properties of both organic polymer and inorganic semiconductor so far [1,2]. Unfortunately, the efficiency of HSCs remains in its infancy. The photoresponse of conducting polymer usually behaves well in the visible range but lots of energies have been lost in infrared and ultraviolet range, thus leading to poor photoelectric conversion performances. For example, conducting polymers, e.g., poly(3-hexylthiophene) (P3HT), poly (3-Octylthiophene) (P3OT), thieno[3,4 b]thiophenealt -benzodithiophene (PTB7), as the most conventionally used ones, have a band-gap of 1.9 eV only and the limited absorption
* Corresponding authors. Tel.: +8615070889816; fax.: +8607918395468 E-mail addresses: [email protected] (L. Shao), [email protected] (X. Jin), [email protected] (Q. Li). 1 These authors contributed equally to this work. http://dx.doi.org/10.1016/j.electacta.2015.09.023 0013-4686/ ã 2015 Elsevier Ltd. All rights reserved.
range up to 700 nm [3,4]. Some efforts have been put to steadily enlarge the absorption range of solar cells by engineering the morphologies of the conducting polymer [5–7]. However, the absorption domains of the polymer still largely mismatch the solar spectrum at present stage of development, thus restricting the further enhancement in device performances. An effective approach to broaden the absorption domain of solar cells is to make use of up-conversion (UC) luminescence by doping rare-earth (RE) ions into inorganic semiconductors [8] because of their intrinsic and unique conversion properties [9]. UC luminescence phenomenon is featured by the conversion of near infrared radiation to visible range emission [10], and has received increasing attention due to the broad potential applications [11– 13]. Since discovered more than four decades ago, most of UC materials have exhibited red, green and blue luminescence emission, which can match the absorption domain of conducting polymer in solar cells. Up to now, the UC luminescence properties of metal oxide families as host materials doped with Er3+ have been widely investigated, such as silicon oxides [14], titanium oxides [15–17], phosphorus oxides [18,19], and molybdenum oxides
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[20,21]. All of these host materials show the excellent UC optical performances, however, so far the systems are largely a subject of fundamental research and lacks wide application. Although the UC luminescence characteristics of Er3+ doped gadolinium oxymolybdate (GMO:Er) polycrystalline were previously reported [22], nonetheless the optical properties of GMO:Er single crystal nanophosphors (NPs) and their applications in HSCs have rarely been reported. Molybdate oxides have many advantages such as chemical inertness, strong photocatalytic performance, long-term photostability, better discharge capacity and low interfacial resistance, which guarantee their wide applications in photovoltaics (PVs) as node materials or buffer layer [23–25]. On the other hand, molybdate based phosphors have shown excellent UC properties, which can potentially make use of near-infrared part of the solar spectrum to broaden the photoresponse of the devices. All these merits motivate us to incorporate GMO:Er NPs into the TiO2 acceptor.
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In an attempt to validate our hypothesis and potentially make use of infrared solar spectrum, herein GMO:Er NPs will be utilized as UC luminescent dopant to improve the photoelectric properties of the organic/inorganic HSCs. Moreover, the enhancement of electron transfer at organic/inorganic BHJ interface that originates from the novel design of incorporating nanophosphor into TiO2 films will be examined. The innate charge-transfer dynamics within HSCs will be deeply explored by the transient absorption photoluminescence (PL) spectroscopy and the corresponding photovoltaic performances of HSC will be demonstrated. 2. Experimental 2.1. Materials All chemical reagents including tetrabutyl titanate, poly(ethylene glycol) (PEG, molecular weight of 20 000), nitric acid,
Fig. 1. The XRD pattern (a), TEM image (b), HR-TEM image (c) and EDS spectrum (d) of GMO:Er NPs. The inserts in (a) and (c) highlight the enlarged view of the maximum peak (221) and the SAED pattern, respectively.
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phosphoric acid, P25 (Degussa), OP emulsifying agent (Triton X100), citric acid, acetonitrile, isopropanol, ammonia, chloroform, Gd(NO3)36H2O, Er(NO3)36H2O and (NH4)6Mo7O24 are all analytic grade and were provided by Sigma-Aldrich, Hongkong, China. P3HT together with poly(3,4-ethylenedioxylenethiophene)-polystylene sulfonic acid (PEDOT:PSS) and fluorine-doped tin oxide glass (FTO, sheet resistance 8 V cm2) were purchased from Aldrich and Hartford Glass Co., U.S, respectively. 2.2. Preparation of GMO:Er NPs The Gd2(MoO4)3: x mol% Er (x = 1, 5, 10, 20, 30) (GMO:x Er) phosphors were synthesized via the following modified procedure [15]. Briefly, Gd(NO3)36H2O, Er(NO3)36H2O and (NH4)6Mo7O24 at precalculated concentrations were firstly dissolved in deionized water under magnetic stirring. Then, citric acid acting as complexing agent was added to the mixed solution at the molar ratio of 1:1.5. Next, the pH value of the solution was adjusted to about 7 by adding ammonia, followed by being vigorously stirred for 1 h, then dried at 130 C for about 20 h at ambient atmosphere until it was transformed into a black bulk. Finally, the black bulk was subjected to sintering in air at 800 C for 2 h and flat disks were made by pressing the as-obtained powders, which would be used spectral characterizations.
2.3. Fabrication of HSCs The GMO:Er NPs-doped TiO2 colloid as n-type acceptor was prepared according to the slightly modified method reported in previous works [26,27]. Briefly, the approximately 250 nmthickness acceptor layer film was prepared by sintering at 450 C for 30 min at ambient atmosphere, followed by spincoating the colloid on FTO glass. Subsequently, organic/inorganic BHJ was fabricated by soaking the film in a 0.15 M P3HT solution in chloroform for 12 h to uptake p-type polymer, followed by being dried under nitrogen flow. Finally, PEDOT:PSS layer and Pt electrodes were deposited on the BHJ by spin-coating and thermal evaporation under vacuum, respectively. 2.4. Characterizations The phase of prepared NPs was characterized using an X-ray diffractometry (XRD) on a MSAL XD-2/3 powder diffractometer with graphite monochromatized Cu Ka radiation (l = 0.1540596 nm) at a scanning rate of 1 deg/min. The microstructures and morphologies of GMO:Er NPs were characterized by high-resolution transmission electron microscopy (HR-TEM, JEM2010, JEOL Ltd.) at 200 kV and a field emission scanning electron microscope (FE-SEM, Hitachi S4800). HR-TEM specimen was prepared by placing a small volume of ultrasonically dispersed
Fig. 2. The full survey (a), Gd 4d and Er 4d (b), Mo 3d (c) and O 1s (d) XPS spectra of Gd1.54Er0.46(MoO4)3.
D. Li et al. / Electrochimica Acta 182 (2015) 416–423
suspension on carbon-enhanced copper grids and then drying in air. Energy dispersive X-ray spectroscopy (EDS) spectra were recorded on an FE-SEM that is equipped with an EDS detector (OXFORD 7021). An Al Ka X-ray source was used to perform the Xray photoelectron spectra (XPS) measurements and the data were collected by a imaging electron spectrometer (Thermo ESCALAB 250XI, USA). The UC-photoluminescence (UC-PL) spectra were collected on a HR550 spectrofluorometer (Jobin Yvon) using excitation from a laser diode emitting at 976 nm. The PL spectrum was obtained using a spectrophotometer (FLS920, Edinburgh) equipped with a xenon lamp and a photomultiplier tube (R955, Hamamatsu) acting as excitation source and fluorescence detector. Time-resolved PL spectrum was measured on a spectrometer (Bruker Optics 250IS/SM) with an intensified charge coupled device detector (Andor, IStar740) and a time resolution of 60 ps and the samples were excited by 120 fs laser pulses at 400 nm and at a repetition rate of 10 Hz. The photocurrent-voltage (J-V) characteristics were recorded using an Electrochemical Workstation (Xe Lamp Oriel Sol3ATM Class AAA Solar Simulators 9,023A, USA) under an irradiation of a simulated solar light from a 100 W xenon arc lamp at ambient atmosphere.
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3. Results and discussion 3.1. Characterization of Prepared GMO:Er Nanophosphors As observed from Fig. 1a, all diffraction peaks of GMO: xEr (x = 10, 20, 30 mol%) nanocrystals with u values ranging from 10 to 70 can be well indexed as a monoclinic cell with a space group of C2/c, and agree well with the standard diffraction of bulk GMO:Er (JCPDS card No. 26-0655). In detail, the strong diffraction peaks located at 28.42 and 29.4 correspond to the (221) and (0 2 3) planes, respectively. A slight shift of the maximum diffraction peak position (221), depending on the initial loading amounts of Er ions, is observed from the close-up of the peak as shown in the insert of Fig. 1a. The (221) position shifts toward higher 2u values with the Er ions concentration increasing from 0 to 30 mol%. These results indicate that the host lattice shrinks as the Er ions concentration increases up to 30 mol%. It is worth mentioning that trivalent Gd and Er ions have the similar electronic structures (s2p6) as well as similar effective ionic radius (93.8 pm for Gd3+, 89.0 pm for Er3+). Therefore, Er cations could enter into Gd2(MoO4)3 lattice, thus leading to a substitution of Er cation for Gd ions. Substitution of the Gd ions by the smaller Er ions can cause the host lattice to shrink. However, this cation substitution will not induce significant
Fig. 3. (a) UC-PL spectra in GMO:x Er (x = 1, 5, 10, 20, 30 mol%) NCs under a 976-nm LD excitation and the inset highlights the red emissions of the GMO:x Er NPs; (b) Dependence of the green and red UC emissions intensity on the excitation power of GMO:Er NPs and (c) Energy level diagrams and possible UC mechanisms for the GMO:Er NPs under a 976 nm LD excitation. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.).
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change in the host structure because of their similar ion radii and equal electric charges of Gd3+ and Er3+ dopants [22]. TEM image shown in Fig. 1b illustrates that the prepared uniform crystals are in the nanometer domain. The d =-spacings of 0.32 and 0.28 nm estimated from the lattice fringes in Fig. 1c, respectively, agree well those of d1 = 0.316 and d 2 = 0.286 nm, which correspond to (221) and (0 4 0) planes of monoclinic GMO:Er NPs, respectively, according to the standard JCPDS card. Besides, the values of d -spacing of d 1 = 0.33 and d 2 = 0.29 nm obtained from SAED (as shown in the insert of Fig. 1c) are consistent with those obtained from the standard card and TEM lattice fringes. To further confirm the existed element in the NPs, the EDS spectrum of GMO:Er NPs was measured and shown in Fig. 1d, which clearly indicates the existence of Er as well as the elements from GMO and confirms that Er ions have been successfully penetrated in GMO matrix lattices. Besides, elemental analysis was conducted by using Genesis EDS software to quantify the stoichiometry of the NPs. Fig. 1d shows the EDS spectra of the GMO:20 mol% Er3+ nanophosphor and the elemental analysis results. The atomic concentrations of O, Mo, Gd and Er are 74.88%, 18.87%, 8.84% and 3.65%, respectively. The atomic ratio of (Gd + Er):Mo:O is determined to be 2:3.02:11.99. Therefore, the chemical formula can be determined as Gd2-xErx(MoO4)3, where x depends on the initial loading amounts of Er3+. Finally, the chemical formulae of the 20 mol% Er3+ nanophosphor is derived, i.e., Gd1.54Er0.46(MoO4)3. Similarly, the chemical formulae of the GMO:10 mol% Er3+ and GMO:30 mol% Er3+ are determined to be Gd1.78Er0.22(MoO4)3 and Gd1.49Er0.51(MoO4)3, respectively. EDS analysis reveals that the final atom concentration of Er3+ is low. This may further provide some insight into above mentioned nonobvious lattice change. XPS measurements were performed to identify the chemical states for each elements present in the nanophosphors. Fig. 2a shows the full XPS spectrum of Gd1.54Er0.46(MoO4)3, in which Gd, Er, Mo and O core binding energy peaks are clearly revealed. As shown in Fig. 2b, there are three obvious peaks located at 142.91, 148.42 eV and 170.73 eV, corresponding to the spin-orbit doublet Gd 4d5/2, Gd 4d3/2 and Er 4d, respectively. The peak positions are in good agreement with the characteristics for the oxidation state of Gd3+ and Er3+ [28,29]. The above XRD and XPS results demonstrate that Er3+ ions were doped into the Gd2(MoO4)3 host matrix successfully. The XPS spectrum confined to the Mo 3d window (Fig. 2c) exhibits two peaks located at 232.76 and 236.02 eV, which correspond to the Mo6+ 3d5/2 and Mo6+ 3d3/ 2 binding energies, respectively [30]. The O 1s region, as shown in Fig. 2d, reveals a single peak located at 530.78 eV, which is assignable to the form of oxide state O2.
3.2. UC luminescence of GMO:Er NPs Since UC emissions of GMO:Er systems are sensitively dependent on the concentrations of Er ion doping, in order to obtain the optimum concentration of doping ions in GMO host lattices, the Er3 + concentration-dependent UC-PL spectra, excited at 976 nm at room temperature, are shown in Fig. 3a; meanwhile, the inset of Fig. 3a highlights the red emission. The green (at 531 and 562 nm) and red (at 668 nm) emission peaks are observed, which are attributed to the 2H11/2 ! 4I15/2,4S3/2 ! 4I15/2, and 4F9/2 ! 4I15/2 transitions of Er3+, respectively [31]. The intensities of the green and red emissions increase with the increase of Er ions from 1 mol% to 20 mol%, but then decrease at much higher concentration (exceeding 20 mol%) of Er ion-doped GMO host and this could be attributed to the exciton’s quenching [11]. Compared with 1 mol% Er3+ doped GMO, the intensities of the green and red emissions are boosted by a factor of about 6 and 2, respectively, for 20 mol% Er doping. Therefore, the optimum concentration of Er ions at 20 mol % will be chosen for the further investigation except indicated otherwise. Moreover, in a bid to comprehend the UC processes in GMO:Er NPs, the green and red emissions of GMO:Er NPs under the excitation of 976 nm were studied to understand the excitation power dependence. In general, the quantity of photons that are necessary to populate the upper emitting level under unsaturated circumstance can be estimated by the relationshipIup / InNIR , where Iup is the fluorescence intensity, INIR is the pump laser power, and n is the number of laser photons required as determined by the slope of the curve. The ln–ln relationships of green and red emissions, i.e., the relationship between PL intensity and excitation power, are shown in Fig. 3b, in which the slope values are determined to be 1.8 and 1.3, respectively. This illustrates that the emissions of green and red are two-photon processes, thus leading to high efficiencies of UC emissions [32]. The energy level diagram of GMO:Er NPs excited at l = 976 nm and the possible UC mechanism were shown in Fig. 3c. Two potential UC mechanisms of populating the 4F7/2 level of Er3+ are discussed. One is that the Er3 + ion is excited from the ground state 4I15/2 to the 4I11/2 level via ground state absorption (GSA1), and then further excited from the 4 I11/2 level to the 4F7/2 level via excited state absorption (ESA1). The other one is supposed to be a high excited state energy transfer (HESET) from the Er3+–MoO42 dimer complex to the Er3+ ion. The MoO42 complex is a closed shell configuration with an 1A1 ground state and four excited states 3T1, 3T2, 1T1 and 1T2, respectively [15,33].
Fig. 4. (a) Steady-state and (b) time resolved PL spectra of neat P3HT, TiO2/P3HT and Gd1.54Er0.46(MoO4)3-TiO2/P3HT films under 580-nm excitation.
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The Er3+–MoO42 dimer ground state is represented by |4I15/2, A1>, the intermediate excited state by |4I11/2, 1A1>, and the relevant higher excited states by |4I15/2, T1>, |4I15/2, 3T2>, |4I15/2, 1 T1> and |4I15/2, 1T2>, respectively. The Er3+–MoO42 dimer complex is excited from the ground state |4I15/2, 1A1> to the intermediate state via GSA2, under a 976 nm LD excitation. Subsequently, the |4I15/2, 1T1> state is further excited from the intermediate state via ESA3, and then decays nonradiatively to the |4I15/2, 3T2> state of Er3+–MoO42 dimer complex. The HESET from the |4I15/2, 3T2> state to the 4F7/2 level of Er3+ is effective. The populated 4F7/2 level can relax nonradiatively by a fast multiphonon decay process to the 2H11/2/4S3/2 level with the excess energy dissipated by the lattice, and the intense green emission (2H11/2/4S3/2 ! 4I15/2) is then observed. There are also two paths for the population of 4F9/2 level (Er3+) of the GMO:Er NPs: one is the excitation from the 4I13/2 level via ESA2, and the other is the nonradiative relaxation of 2H11/2/4S3/2 ! 4F9/2. The electrons in the 4F9/2 level mostly relaxed radiatively to the ground state 4I15/2 level, which generates the red emission. Consequently, the nearinfrared irradiation can be absorbed by the P3HT in the hybrid HSCs via the UC-processed. In other words, the light absorption range of donor polymer was widened, which may also prompt donor to generate much more delocalized charges. Thus, this approach can allow better utilization of infrared solar light and contribute to the photovoltaic properties of HSCs. 1
3.3. Charge-transfer dynamics of organic/inorganic BHJ architecture Apart from the excellent optical properties of GMO:Er NPs, the efficient charge-transfer properties in BHJs are also important to the device performance [34,35]. It is widely accepted that photoexcited charge transfer efficiency and rates can be roughly evaluated by the steady-state and transient PL measurements of P3HT layer when interfaced with an appropriate electron- or holeextraction layer. Therefore we performed the comparative studies on P3HT neat film, TiO2/P3HT and GMO:Er NPs:TiO2/P3HT blend films to explore the role of GMO:Er NPs in the charge-transfer dynamics. Fig. 4a shows the steady-state PL spectra of the different films. The PL intensity of GMO:Er NPs:TiO2 film was further quenched by a factor of about 1.3 compared with its counterpart TiO2/P3HT film, suggesting that an enhanced charge transfer efficiency is achieved for the BHJ incorporated with NPs [34]. To further reveal the photoexcited charge-transport dynamics in organic/inorganic (D/A) interfaces, the time-resolved singlephoton counting PL spectrum was performed. Fig. 4b shows the transient PL spectra for P3HT, TiO2/P3HT and Gd1.54Er0.46(MoO4)3TiO2/P3HT BHJ, respectively. Neat P3HT film exhibits a long tail (from 500 to 3000 ps) whereas after coupled with acceptor layers, both of the PL emissions are greatly quenched after about 500 ps, indicating that strong charge transfer occurs at D/A interfaces in this time region. In BHJ architecture, the excitation of P3HT donor absorbed on the porous structures of acceptor films (i.e., GMO:ErTiO2) leads to electron(e)-hole(h) separation (reaction (1)), and subsequent either recombination of the electron carriers (reaction (2)) or an additional deactivation pathway for the bleaching recovery, by which the photogenerated electrons are injected into the inorganic crystals (reaction (3)): P3HT + hn ! P3HT (h + e)
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The bleaching revovery usually occurs with a lot of heterogeneous kinetics but can be fitted to a sum of two-exponential function, which was found to be satisfactory in determination of emission lifetimes: IðtÞ ¼ ½a1 expð1=t 1 Þ þ a2 expð1=t 2 Þ FðtÞ
ð4Þ
where F(t) is the impulse response (IRF) function, a is the amplitude, and t is the time constant. F(t) is related with half width at half-maximum (HWHM) of the pulse duration and then fitted to the data points in order to numerically simulate the measured fluorescence dynamics. These valus were then used to estimate the D E average lifetime t of P3HT emission decay using Eq. (5): E a t2 þ a2 t22 ð5Þ t ¼ 1 1 a1 t1 þ a2 t 2 The charge transfer rate constant can be obtained from Eq. (6) as follows: ket ¼
1
tP3HTþGMO:ErTiO2
1
ð6Þ
tP3HT
D E The average fluorescence life time t of the neat P3HT film was determined to be 634 ps as listed in Table 1. Correspondingly, the fitted lifetimes t heterojuction were estimated to be 229 and 135 ps for TiO2/P3HT and Gd1.54Er0.46(MoO4)3-TiO2/P3HT respectively. According to the Eq. (6), the charge transfer rate kct can be estimated to be 2.79 and 5.83 109 s1 for TiO2/P3HT and Gd1.54Er0.46(MoO4)3-TiO2/P3HT BHJs, respectively. These results indicate that much faster charge-transfer occurs at the Gd1.54Er0.46(MoO4)3-TiO2/P3HT interface than TiO2/P3HT interface. 3.4. Photovoltaic Performances of HSCs With the purpose of exploring the doping effect on the photoelectric properties of HSCs, TiO2/P3HT neat film and Gd1.54Er0.46(MoO4)3-TiO2/P3HT blend film have been fabricated for comparison. The morphology of Gd1.54Er0.46(MoO4)3 NPsdoped TiO2 was characterized by SEM. As observed from Fig. 5a, the diameter of TiO2 crystals is uniform and in the range of about 10-30 nm while the diameter of Gd1.54Er0.46(MoO4)3 NPs is much larger but still in the nano domain. The surface of the Gd1.54Er0.46(MoO4)3 NPs-decorated TiO2 film obviously exhibits porous structure, which makes for excellent p-n contact in BHJ as well as the adsorption of P3HT solution by trapping the donor P3HT molecules in the microporo-acceptor [36–39]. To confirm the intimately contacting interface of BHJ, the cross-sectional SEM images in Fig. 5c and d highlight the obvious differences between the film morphologies before and after soaking processes. Compared to the bare TiO2 film, the pores become almost invisible in the BHJ, suggesting that the polymers have been permeated well into the porous structure of the acceptor film to form an intimate interface contact. The individual thickness of BHJ including Gd1.54Er0.46(MoO4)3-TiO2 and P3HT can be visibly distinguished and is estimated to be about 250 nm. The intimate contact between donor and acceptor is crucial for the robust charge transport and separation because of the short diffusion length of the phototoexcited exiton in polymer, usually
(1) Table 1 Kinetic parameters obtained from fits to time-resolved PL decays.
P3HT (h + e) ! P3HT + hn’
P3HT (e) + GMO:Er-TiO2 ! P3HT + GMO:Er-TiO2 (e)
(2)
(3)
t 1(ps)
t 2(ps)
(ps) ket*(109 s1)
Kinetic parameters
a1
P3HT TiO2/P3HT GMO:Er NPs:TiO2/ P3HT
0.26 0.75 105 11 880 26 634 2.79 0.24 0.77 86 7 321 13 229 0.25 0.74 89 7 205 12 135 5.83
a2
422
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Fig. 5. (a) SEM image of GMO:Er NPs:TiO2 acceptor film; (b) Photocurrent- voltage (J-V) characteristics of HSCs with GMO:Er NPs at different contents and cross-sectional SEM images of the BHJ architecture before (c) and after (d) soaking P3HT molecules.
Table 2 Photovoltaic performances of the HSCs. GMO:Er NPs (wt%)
Voc (V)
Jsc (mA cm2)
FF
ha(%)
0 1 3 5 7
0.61 0.62 0.65 0.71 0.67
7.04 7.62 7.70 8.58 8.29
0.54 0.54 0.55 0.56 0.55
2.32 2.55 2.75 3.41 3.05
a
h = JscVocFF/Pin, where Pin = 100 mW cm2 (AM 1.5).
within 12 nm [40]. Fig. 5b displays the photovoltaic (J-V) performances of the different BHJ HSCs made from different concentrations of Gd1.54Er0.46(MoO4)3 NPs (ranging from 0 to 7 wt
%) and the photovoltaic parameters are summarized in Table 2. It is clear that under the same experimental conditions, the HSCs with Gd1.54Er0.46(MoO4)3-TiO2 BHJ exhibit excellent photovoltaic performance, which is better than most of the other organic/inorganic HSCs so far [34,41,42]. With increasing Gd1.54Er0.46(MoO4)3 NPs doping, the short-circuit current density JSC increases from 7.04 to 8.58 mA cm2 and the open-circuit voltage Voc increases from 0.61 to 0.71 V whereas the filling factor (FF) remains almost unchanged, thus resulting in a steady improvement from 2.32% for pure TiO2 to 3.41% for 5 wt% Gd1.54Er0.46(MoO4)3 NPs doping TiO2. This can be attributed to the improved photoexcited charge transfer and charge collection efficiency [43]. After doping Gd1.54Er0.46(MoO4)3 NPs, both the charge transfer lifetimes and the light harvestings of HSCs have
Fig. 6. (a) Absorption spectrum of P3HT and (b) EQE spectra of Gd1.54Er0.46(MoO4)3-TiO2/P3HT and TiO2/P3HT HSCs.
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been improved. However, after further increasing the doping amount of Gd1.54Er0.46(MoO4)3 NPs, i.e., >5.0 wt %, a flat BHJ film becomes difficult to be formed. In order to identify individual contribution of solar spectra, external quantum efficiency (EQE) measurements for the HSCs with and without nanophosphors were provided in Fig. 6. Overally speaking, the EQE spectra closely match the corresponding absorption spectra of P3HT. However, the broad EQE band covering the spectrum from 350 to 600 nm is enhanced significantly by Gd1.54Er0.46(MoO4)3 doping. This improvement originates from the faster charge transfer induced by the doping effects of GMO:Er phosphors. We calculated JSC value for each device by integrating the quantum efficiency over the entire solar energy spectrum with the relationship [44]: Z q Jsc ¼ EQEðlÞl EAM1:5G ðlÞdl ð7Þ hc where q is the elementary charge, h is Planck’s constant, c is the speed of light in a vacuum, and EAM1.5G is the spectral irradiance of AM 1.5 G. The calculated JSC are 7.63 and 6.42 mA/cm2 for Gd1.54Er0.46(MoO4)3-TiO2/P3HT and TiO2/P3HT HSCs, respectively. After introducing Gd1.54Er0.46(MoO4)3 phosphors, the calculated JSC is enhanced by 19%, which is close to enhancement estimated from the J-V measured results of about 22%. 4. Conclusion In summary, we have introduced GMO:Er NPs as a solar energy conversion platform for inorganic solution-processed solar cells. GMO:Er NPs demonstrate a two-photon UC mechanism, thus tailoring the near-infrared photons to visible photons that can be recaptured efficiently by P3HT. The solar cell based on 5 wt% Gd1.54Er0.46(MoO4)3 NPs has delivered a higher PCE of 3.41% as compared to the conventional TiO2/P3HT device, i.e., 2.32%. This improvement, on one hand, can be ascribed to the enhanced harvest of the low energy photons by broadening absorption range; on the other hand, the unique electronic activity of the Gd1.54Er0.46(MoO4)3 NPs enhance the photoexcited charge rate, i.e., more than 2 times faster than its counterpart bare TiO2. This work provides compelling evidences that the promising potential of up-conversion luminescence nanophosphor dopants in obtaining efficient HSCs, and also inspires the development of robust rare-earth phosphor to promise applications in photovoltaics. Acknowledgments This work was financially supported by the Natural Science Foundation of China (61366003, 11404283, 11564026), National Natural Science Foundation of Guangdong Province, China (2014A030307028), Natural Science Foundation of Jiangxi Province (20151BAB212001), Science and Technology Project of the education department of Jiangxi Province, China (GJJ12449, GJJ14533), and Distinguished Young Talents in Higher Education of Guangdong, China (2013LYM 0053).
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Tailoring solar energy spectrum for efficient organic/inorganic hybrid solar cells by up-conversion luminescence nanophosphors Dongyu Lia,1, Weifu Sunb,1, Lexi Shaoa,* , Shuying Wuc, Zhen Huanga , Xiao Jind,* , Qin Zhangd, Qinghua Lid,* a
Department of Physics, Lingnan Normal College, Zhanjiang 524048, PR China School of Aerospace, Mechanical and Mechatronic Engineering, The University of Sydney, Sydney, NSW 2006, Australia School of Aerospace, Mechanical and Manufacturing Engineering, RMIT University, Melbourne, VIC 3001, Australia d Jiangxi Engineering Laboratory for Optoelectronics Testing Technology, Nanchang Hangkong University, Nanchang 330063, PR China b c
A R T I C L E I N F O
A B S T R A C T
Article history: Received 1 June 2015 Received in revised form 17 July 2015 Accepted 5 September 2015 Available online 28 September 2015
Solar light harvesting ability is one of the key properties in organic/inorganic solar cells. One of the most popular organic materials used is poly(3-hexylthiophene) (P3HT), which can make use of solar spectrum in the range 400-600 nm, however, this polymer is incapable of utilizing low energy photons. Herein, we incorporate erbium ion decorated gadolinium oxymolybdate (GMO:Er) nanophosphors (NPs) into mesoporous acceptor film (TiO2) in an attempt to enhance the light harvest. The nanophosphor can convert the near-infrared solar spectrum to visible region (near 550 nm), in which the energy can be recaptured by P3HT. The results show that the up-conversion proceeds via the two-photon upconversion mechanism. It is found that after the incorporation of GMO:Er NPs into TiO2 at 5 wt%, the charge transfer rate was enhanced from 2.79 to 5.83 109 s1. The device performance of solar cells based on GMO:Er NPs demonstrates a more than 30% improvement compared to their neat TiO2/P3HT analogue and such enhancement can be ascribed to the broader light harvest together with faster photoexcited charge transfer. This platform can be readily implemented by introducing more demanding energy conversion phosphors and allows for the development of optoelectronic applications with tailored optoelectronic properties. ã 2015 Elsevier Ltd. All rights reserved.
Keywords: Organic/inorganic hybrid solar cell Charge-transfer dynamics Two-photon up-conversion mechanism Bulk-heterojunction
1. Introduction Organic/inorganic bulk-heterojunction (BHJ) hybrid solar cells (HSCs) have become one of the relatively newly emerging hotspots due to the potential advantageous properties of both organic polymer and inorganic semiconductor so far [1,2]. Unfortunately, the efficiency of HSCs remains in its infancy. The photoresponse of conducting polymer usually behaves well in the visible range but lots of energies have been lost in infrared and ultraviolet range, thus leading to poor photoelectric conversion performances. For example, conducting polymers, e.g., poly(3-hexylthiophene) (P3HT), poly (3-Octylthiophene) (P3OT), thieno[3,4 b]thiophenealt -benzodithiophene (PTB7), as the most conventionally used ones, have a band-gap of 1.9 eV only and the limited absorption
* Corresponding authors. Tel.: +8615070889816; fax.: +8607918395468 E-mail addresses: [email protected] (L. Shao), [email protected] (X. Jin), [email protected] (Q. Li). 1 These authors contributed equally to this work. http://dx.doi.org/10.1016/j.electacta.2015.09.023 0013-4686/ ã 2015 Elsevier Ltd. All rights reserved.
range up to 700 nm [3,4]. Some efforts have been put to steadily enlarge the absorption range of solar cells by engineering the morphologies of the conducting polymer [5–7]. However, the absorption domains of the polymer still largely mismatch the solar spectrum at present stage of development, thus restricting the further enhancement in device performances. An effective approach to broaden the absorption domain of solar cells is to make use of up-conversion (UC) luminescence by doping rare-earth (RE) ions into inorganic semiconductors [8] because of their intrinsic and unique conversion properties [9]. UC luminescence phenomenon is featured by the conversion of near infrared radiation to visible range emission [10], and has received increasing attention due to the broad potential applications [11– 13]. Since discovered more than four decades ago, most of UC materials have exhibited red, green and blue luminescence emission, which can match the absorption domain of conducting polymer in solar cells. Up to now, the UC luminescence properties of metal oxide families as host materials doped with Er3+ have been widely investigated, such as silicon oxides [14], titanium oxides [15–17], phosphorus oxides [18,19], and molybdenum oxides
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[20,21]. All of these host materials show the excellent UC optical performances, however, so far the systems are largely a subject of fundamental research and lacks wide application. Although the UC luminescence characteristics of Er3+ doped gadolinium oxymolybdate (GMO:Er) polycrystalline were previously reported [22], nonetheless the optical properties of GMO:Er single crystal nanophosphors (NPs) and their applications in HSCs have rarely been reported. Molybdate oxides have many advantages such as chemical inertness, strong photocatalytic performance, long-term photostability, better discharge capacity and low interfacial resistance, which guarantee their wide applications in photovoltaics (PVs) as node materials or buffer layer [23–25]. On the other hand, molybdate based phosphors have shown excellent UC properties, which can potentially make use of near-infrared part of the solar spectrum to broaden the photoresponse of the devices. All these merits motivate us to incorporate GMO:Er NPs into the TiO2 acceptor.
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In an attempt to validate our hypothesis and potentially make use of infrared solar spectrum, herein GMO:Er NPs will be utilized as UC luminescent dopant to improve the photoelectric properties of the organic/inorganic HSCs. Moreover, the enhancement of electron transfer at organic/inorganic BHJ interface that originates from the novel design of incorporating nanophosphor into TiO2 films will be examined. The innate charge-transfer dynamics within HSCs will be deeply explored by the transient absorption photoluminescence (PL) spectroscopy and the corresponding photovoltaic performances of HSC will be demonstrated. 2. Experimental 2.1. Materials All chemical reagents including tetrabutyl titanate, poly(ethylene glycol) (PEG, molecular weight of 20 000), nitric acid,
Fig. 1. The XRD pattern (a), TEM image (b), HR-TEM image (c) and EDS spectrum (d) of GMO:Er NPs. The inserts in (a) and (c) highlight the enlarged view of the maximum peak (221) and the SAED pattern, respectively.
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phosphoric acid, P25 (Degussa), OP emulsifying agent (Triton X100), citric acid, acetonitrile, isopropanol, ammonia, chloroform, Gd(NO3)36H2O, Er(NO3)36H2O and (NH4)6Mo7O24 are all analytic grade and were provided by Sigma-Aldrich, Hongkong, China. P3HT together with poly(3,4-ethylenedioxylenethiophene)-polystylene sulfonic acid (PEDOT:PSS) and fluorine-doped tin oxide glass (FTO, sheet resistance 8 V cm2) were purchased from Aldrich and Hartford Glass Co., U.S, respectively. 2.2. Preparation of GMO:Er NPs The Gd2(MoO4)3: x mol% Er (x = 1, 5, 10, 20, 30) (GMO:x Er) phosphors were synthesized via the following modified procedure [15]. Briefly, Gd(NO3)36H2O, Er(NO3)36H2O and (NH4)6Mo7O24 at precalculated concentrations were firstly dissolved in deionized water under magnetic stirring. Then, citric acid acting as complexing agent was added to the mixed solution at the molar ratio of 1:1.5. Next, the pH value of the solution was adjusted to about 7 by adding ammonia, followed by being vigorously stirred for 1 h, then dried at 130 C for about 20 h at ambient atmosphere until it was transformed into a black bulk. Finally, the black bulk was subjected to sintering in air at 800 C for 2 h and flat disks were made by pressing the as-obtained powders, which would be used spectral characterizations.
2.3. Fabrication of HSCs The GMO:Er NPs-doped TiO2 colloid as n-type acceptor was prepared according to the slightly modified method reported in previous works [26,27]. Briefly, the approximately 250 nmthickness acceptor layer film was prepared by sintering at 450 C for 30 min at ambient atmosphere, followed by spincoating the colloid on FTO glass. Subsequently, organic/inorganic BHJ was fabricated by soaking the film in a 0.15 M P3HT solution in chloroform for 12 h to uptake p-type polymer, followed by being dried under nitrogen flow. Finally, PEDOT:PSS layer and Pt electrodes were deposited on the BHJ by spin-coating and thermal evaporation under vacuum, respectively. 2.4. Characterizations The phase of prepared NPs was characterized using an X-ray diffractometry (XRD) on a MSAL XD-2/3 powder diffractometer with graphite monochromatized Cu Ka radiation (l = 0.1540596 nm) at a scanning rate of 1 deg/min. The microstructures and morphologies of GMO:Er NPs were characterized by high-resolution transmission electron microscopy (HR-TEM, JEM2010, JEOL Ltd.) at 200 kV and a field emission scanning electron microscope (FE-SEM, Hitachi S4800). HR-TEM specimen was prepared by placing a small volume of ultrasonically dispersed
Fig. 2. The full survey (a), Gd 4d and Er 4d (b), Mo 3d (c) and O 1s (d) XPS spectra of Gd1.54Er0.46(MoO4)3.
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suspension on carbon-enhanced copper grids and then drying in air. Energy dispersive X-ray spectroscopy (EDS) spectra were recorded on an FE-SEM that is equipped with an EDS detector (OXFORD 7021). An Al Ka X-ray source was used to perform the Xray photoelectron spectra (XPS) measurements and the data were collected by a imaging electron spectrometer (Thermo ESCALAB 250XI, USA). The UC-photoluminescence (UC-PL) spectra were collected on a HR550 spectrofluorometer (Jobin Yvon) using excitation from a laser diode emitting at 976 nm. The PL spectrum was obtained using a spectrophotometer (FLS920, Edinburgh) equipped with a xenon lamp and a photomultiplier tube (R955, Hamamatsu) acting as excitation source and fluorescence detector. Time-resolved PL spectrum was measured on a spectrometer (Bruker Optics 250IS/SM) with an intensified charge coupled device detector (Andor, IStar740) and a time resolution of 60 ps and the samples were excited by 120 fs laser pulses at 400 nm and at a repetition rate of 10 Hz. The photocurrent-voltage (J-V) characteristics were recorded using an Electrochemical Workstation (Xe Lamp Oriel Sol3ATM Class AAA Solar Simulators 9,023A, USA) under an irradiation of a simulated solar light from a 100 W xenon arc lamp at ambient atmosphere.
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3. Results and discussion 3.1. Characterization of Prepared GMO:Er Nanophosphors As observed from Fig. 1a, all diffraction peaks of GMO: xEr (x = 10, 20, 30 mol%) nanocrystals with u values ranging from 10 to 70 can be well indexed as a monoclinic cell with a space group of C2/c, and agree well with the standard diffraction of bulk GMO:Er (JCPDS card No. 26-0655). In detail, the strong diffraction peaks located at 28.42 and 29.4 correspond to the (221) and (0 2 3) planes, respectively. A slight shift of the maximum diffraction peak position (221), depending on the initial loading amounts of Er ions, is observed from the close-up of the peak as shown in the insert of Fig. 1a. The (221) position shifts toward higher 2u values with the Er ions concentration increasing from 0 to 30 mol%. These results indicate that the host lattice shrinks as the Er ions concentration increases up to 30 mol%. It is worth mentioning that trivalent Gd and Er ions have the similar electronic structures (s2p6) as well as similar effective ionic radius (93.8 pm for Gd3+, 89.0 pm for Er3+). Therefore, Er cations could enter into Gd2(MoO4)3 lattice, thus leading to a substitution of Er cation for Gd ions. Substitution of the Gd ions by the smaller Er ions can cause the host lattice to shrink. However, this cation substitution will not induce significant
Fig. 3. (a) UC-PL spectra in GMO:x Er (x = 1, 5, 10, 20, 30 mol%) NCs under a 976-nm LD excitation and the inset highlights the red emissions of the GMO:x Er NPs; (b) Dependence of the green and red UC emissions intensity on the excitation power of GMO:Er NPs and (c) Energy level diagrams and possible UC mechanisms for the GMO:Er NPs under a 976 nm LD excitation. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.).
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change in the host structure because of their similar ion radii and equal electric charges of Gd3+ and Er3+ dopants [22]. TEM image shown in Fig. 1b illustrates that the prepared uniform crystals are in the nanometer domain. The d =-spacings of 0.32 and 0.28 nm estimated from the lattice fringes in Fig. 1c, respectively, agree well those of d1 = 0.316 and d 2 = 0.286 nm, which correspond to (221) and (0 4 0) planes of monoclinic GMO:Er NPs, respectively, according to the standard JCPDS card. Besides, the values of d -spacing of d 1 = 0.33 and d 2 = 0.29 nm obtained from SAED (as shown in the insert of Fig. 1c) are consistent with those obtained from the standard card and TEM lattice fringes. To further confirm the existed element in the NPs, the EDS spectrum of GMO:Er NPs was measured and shown in Fig. 1d, which clearly indicates the existence of Er as well as the elements from GMO and confirms that Er ions have been successfully penetrated in GMO matrix lattices. Besides, elemental analysis was conducted by using Genesis EDS software to quantify the stoichiometry of the NPs. Fig. 1d shows the EDS spectra of the GMO:20 mol% Er3+ nanophosphor and the elemental analysis results. The atomic concentrations of O, Mo, Gd and Er are 74.88%, 18.87%, 8.84% and 3.65%, respectively. The atomic ratio of (Gd + Er):Mo:O is determined to be 2:3.02:11.99. Therefore, the chemical formula can be determined as Gd2-xErx(MoO4)3, where x depends on the initial loading amounts of Er3+. Finally, the chemical formulae of the 20 mol% Er3+ nanophosphor is derived, i.e., Gd1.54Er0.46(MoO4)3. Similarly, the chemical formulae of the GMO:10 mol% Er3+ and GMO:30 mol% Er3+ are determined to be Gd1.78Er0.22(MoO4)3 and Gd1.49Er0.51(MoO4)3, respectively. EDS analysis reveals that the final atom concentration of Er3+ is low. This may further provide some insight into above mentioned nonobvious lattice change. XPS measurements were performed to identify the chemical states for each elements present in the nanophosphors. Fig. 2a shows the full XPS spectrum of Gd1.54Er0.46(MoO4)3, in which Gd, Er, Mo and O core binding energy peaks are clearly revealed. As shown in Fig. 2b, there are three obvious peaks located at 142.91, 148.42 eV and 170.73 eV, corresponding to the spin-orbit doublet Gd 4d5/2, Gd 4d3/2 and Er 4d, respectively. The peak positions are in good agreement with the characteristics for the oxidation state of Gd3+ and Er3+ [28,29]. The above XRD and XPS results demonstrate that Er3+ ions were doped into the Gd2(MoO4)3 host matrix successfully. The XPS spectrum confined to the Mo 3d window (Fig. 2c) exhibits two peaks located at 232.76 and 236.02 eV, which correspond to the Mo6+ 3d5/2 and Mo6+ 3d3/ 2 binding energies, respectively [30]. The O 1s region, as shown in Fig. 2d, reveals a single peak located at 530.78 eV, which is assignable to the form of oxide state O2.
3.2. UC luminescence of GMO:Er NPs Since UC emissions of GMO:Er systems are sensitively dependent on the concentrations of Er ion doping, in order to obtain the optimum concentration of doping ions in GMO host lattices, the Er3 + concentration-dependent UC-PL spectra, excited at 976 nm at room temperature, are shown in Fig. 3a; meanwhile, the inset of Fig. 3a highlights the red emission. The green (at 531 and 562 nm) and red (at 668 nm) emission peaks are observed, which are attributed to the 2H11/2 ! 4I15/2,4S3/2 ! 4I15/2, and 4F9/2 ! 4I15/2 transitions of Er3+, respectively [31]. The intensities of the green and red emissions increase with the increase of Er ions from 1 mol% to 20 mol%, but then decrease at much higher concentration (exceeding 20 mol%) of Er ion-doped GMO host and this could be attributed to the exciton’s quenching [11]. Compared with 1 mol% Er3+ doped GMO, the intensities of the green and red emissions are boosted by a factor of about 6 and 2, respectively, for 20 mol% Er doping. Therefore, the optimum concentration of Er ions at 20 mol % will be chosen for the further investigation except indicated otherwise. Moreover, in a bid to comprehend the UC processes in GMO:Er NPs, the green and red emissions of GMO:Er NPs under the excitation of 976 nm were studied to understand the excitation power dependence. In general, the quantity of photons that are necessary to populate the upper emitting level under unsaturated circumstance can be estimated by the relationshipIup / InNIR , where Iup is the fluorescence intensity, INIR is the pump laser power, and n is the number of laser photons required as determined by the slope of the curve. The ln–ln relationships of green and red emissions, i.e., the relationship between PL intensity and excitation power, are shown in Fig. 3b, in which the slope values are determined to be 1.8 and 1.3, respectively. This illustrates that the emissions of green and red are two-photon processes, thus leading to high efficiencies of UC emissions [32]. The energy level diagram of GMO:Er NPs excited at l = 976 nm and the possible UC mechanism were shown in Fig. 3c. Two potential UC mechanisms of populating the 4F7/2 level of Er3+ are discussed. One is that the Er3 + ion is excited from the ground state 4I15/2 to the 4I11/2 level via ground state absorption (GSA1), and then further excited from the 4 I11/2 level to the 4F7/2 level via excited state absorption (ESA1). The other one is supposed to be a high excited state energy transfer (HESET) from the Er3+–MoO42 dimer complex to the Er3+ ion. The MoO42 complex is a closed shell configuration with an 1A1 ground state and four excited states 3T1, 3T2, 1T1 and 1T2, respectively [15,33].
Fig. 4. (a) Steady-state and (b) time resolved PL spectra of neat P3HT, TiO2/P3HT and Gd1.54Er0.46(MoO4)3-TiO2/P3HT films under 580-nm excitation.
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The Er3+–MoO42 dimer ground state is represented by |4I15/2, A1>, the intermediate excited state by |4I11/2, 1A1>, and the relevant higher excited states by |4I15/2, T1>, |4I15/2, 3T2>, |4I15/2, 1 T1> and |4I15/2, 1T2>, respectively. The Er3+–MoO42 dimer complex is excited from the ground state |4I15/2, 1A1> to the intermediate state via GSA2, under a 976 nm LD excitation. Subsequently, the |4I15/2, 1T1> state is further excited from the intermediate state via ESA3, and then decays nonradiatively to the |4I15/2, 3T2> state of Er3+–MoO42 dimer complex. The HESET from the |4I15/2, 3T2> state to the 4F7/2 level of Er3+ is effective. The populated 4F7/2 level can relax nonradiatively by a fast multiphonon decay process to the 2H11/2/4S3/2 level with the excess energy dissipated by the lattice, and the intense green emission (2H11/2/4S3/2 ! 4I15/2) is then observed. There are also two paths for the population of 4F9/2 level (Er3+) of the GMO:Er NPs: one is the excitation from the 4I13/2 level via ESA2, and the other is the nonradiative relaxation of 2H11/2/4S3/2 ! 4F9/2. The electrons in the 4F9/2 level mostly relaxed radiatively to the ground state 4I15/2 level, which generates the red emission. Consequently, the nearinfrared irradiation can be absorbed by the P3HT in the hybrid HSCs via the UC-processed. In other words, the light absorption range of donor polymer was widened, which may also prompt donor to generate much more delocalized charges. Thus, this approach can allow better utilization of infrared solar light and contribute to the photovoltaic properties of HSCs. 1
3.3. Charge-transfer dynamics of organic/inorganic BHJ architecture Apart from the excellent optical properties of GMO:Er NPs, the efficient charge-transfer properties in BHJs are also important to the device performance [34,35]. It is widely accepted that photoexcited charge transfer efficiency and rates can be roughly evaluated by the steady-state and transient PL measurements of P3HT layer when interfaced with an appropriate electron- or holeextraction layer. Therefore we performed the comparative studies on P3HT neat film, TiO2/P3HT and GMO:Er NPs:TiO2/P3HT blend films to explore the role of GMO:Er NPs in the charge-transfer dynamics. Fig. 4a shows the steady-state PL spectra of the different films. The PL intensity of GMO:Er NPs:TiO2 film was further quenched by a factor of about 1.3 compared with its counterpart TiO2/P3HT film, suggesting that an enhanced charge transfer efficiency is achieved for the BHJ incorporated with NPs [34]. To further reveal the photoexcited charge-transport dynamics in organic/inorganic (D/A) interfaces, the time-resolved singlephoton counting PL spectrum was performed. Fig. 4b shows the transient PL spectra for P3HT, TiO2/P3HT and Gd1.54Er0.46(MoO4)3TiO2/P3HT BHJ, respectively. Neat P3HT film exhibits a long tail (from 500 to 3000 ps) whereas after coupled with acceptor layers, both of the PL emissions are greatly quenched after about 500 ps, indicating that strong charge transfer occurs at D/A interfaces in this time region. In BHJ architecture, the excitation of P3HT donor absorbed on the porous structures of acceptor films (i.e., GMO:ErTiO2) leads to electron(e)-hole(h) separation (reaction (1)), and subsequent either recombination of the electron carriers (reaction (2)) or an additional deactivation pathway for the bleaching recovery, by which the photogenerated electrons are injected into the inorganic crystals (reaction (3)): P3HT + hn ! P3HT (h + e)
421
The bleaching revovery usually occurs with a lot of heterogeneous kinetics but can be fitted to a sum of two-exponential function, which was found to be satisfactory in determination of emission lifetimes: IðtÞ ¼ ½a1 expð1=t 1 Þ þ a2 expð1=t 2 Þ FðtÞ
ð4Þ
where F(t) is the impulse response (IRF) function, a is the amplitude, and t is the time constant. F(t) is related with half width at half-maximum (HWHM) of the pulse duration and then fitted to the data points in order to numerically simulate the measured fluorescence dynamics. These valus were then used to estimate the D E average lifetime t of P3HT emission decay using Eq. (5): E a t2 þ a2 t22 ð5Þ t ¼ 1 1 a1 t1 þ a2 t 2 The charge transfer rate constant can be obtained from Eq. (6) as follows: ket ¼
1
tP3HTþGMO:ErTiO2
1
ð6Þ
tP3HT
D E The average fluorescence life time t of the neat P3HT film was determined to be 634 ps as listed in Table 1. Correspondingly, the fitted lifetimes t heterojuction were estimated to be 229 and 135 ps for TiO2/P3HT and Gd1.54Er0.46(MoO4)3-TiO2/P3HT respectively. According to the Eq. (6), the charge transfer rate kct can be estimated to be 2.79 and 5.83 109 s1 for TiO2/P3HT and Gd1.54Er0.46(MoO4)3-TiO2/P3HT BHJs, respectively. These results indicate that much faster charge-transfer occurs at the Gd1.54Er0.46(MoO4)3-TiO2/P3HT interface than TiO2/P3HT interface. 3.4. Photovoltaic Performances of HSCs With the purpose of exploring the doping effect on the photoelectric properties of HSCs, TiO2/P3HT neat film and Gd1.54Er0.46(MoO4)3-TiO2/P3HT blend film have been fabricated for comparison. The morphology of Gd1.54Er0.46(MoO4)3 NPsdoped TiO2 was characterized by SEM. As observed from Fig. 5a, the diameter of TiO2 crystals is uniform and in the range of about 10-30 nm while the diameter of Gd1.54Er0.46(MoO4)3 NPs is much larger but still in the nano domain. The surface of the Gd1.54Er0.46(MoO4)3 NPs-decorated TiO2 film obviously exhibits porous structure, which makes for excellent p-n contact in BHJ as well as the adsorption of P3HT solution by trapping the donor P3HT molecules in the microporo-acceptor [36–39]. To confirm the intimately contacting interface of BHJ, the cross-sectional SEM images in Fig. 5c and d highlight the obvious differences between the film morphologies before and after soaking processes. Compared to the bare TiO2 film, the pores become almost invisible in the BHJ, suggesting that the polymers have been permeated well into the porous structure of the acceptor film to form an intimate interface contact. The individual thickness of BHJ including Gd1.54Er0.46(MoO4)3-TiO2 and P3HT can be visibly distinguished and is estimated to be about 250 nm. The intimate contact between donor and acceptor is crucial for the robust charge transport and separation because of the short diffusion length of the phototoexcited exiton in polymer, usually
(1) Table 1 Kinetic parameters obtained from fits to time-resolved PL decays.
P3HT (h + e) ! P3HT + hn’
P3HT (e) + GMO:Er-TiO2 ! P3HT + GMO:Er-TiO2 (e)
(2)
(3)
t 1(ps)
t 2(ps)
(ps) ket*(109 s1)
Kinetic parameters
a1
P3HT TiO2/P3HT GMO:Er NPs:TiO2/ P3HT
0.26 0.75 105 11 880 26 634 2.79 0.24 0.77 86 7 321 13 229 0.25 0.74 89 7 205 12 135 5.83
a2
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D. Li et al. / Electrochimica Acta 182 (2015) 416–423
Fig. 5. (a) SEM image of GMO:Er NPs:TiO2 acceptor film; (b) Photocurrent- voltage (J-V) characteristics of HSCs with GMO:Er NPs at different contents and cross-sectional SEM images of the BHJ architecture before (c) and after (d) soaking P3HT molecules.
Table 2 Photovoltaic performances of the HSCs. GMO:Er NPs (wt%)
Voc (V)
Jsc (mA cm2)
FF
ha(%)
0 1 3 5 7
0.61 0.62 0.65 0.71 0.67
7.04 7.62 7.70 8.58 8.29
0.54 0.54 0.55 0.56 0.55
2.32 2.55 2.75 3.41 3.05
a
h = JscVocFF/Pin, where Pin = 100 mW cm2 (AM 1.5).
within 12 nm [40]. Fig. 5b displays the photovoltaic (J-V) performances of the different BHJ HSCs made from different concentrations of Gd1.54Er0.46(MoO4)3 NPs (ranging from 0 to 7 wt
%) and the photovoltaic parameters are summarized in Table 2. It is clear that under the same experimental conditions, the HSCs with Gd1.54Er0.46(MoO4)3-TiO2 BHJ exhibit excellent photovoltaic performance, which is better than most of the other organic/inorganic HSCs so far [34,41,42]. With increasing Gd1.54Er0.46(MoO4)3 NPs doping, the short-circuit current density JSC increases from 7.04 to 8.58 mA cm2 and the open-circuit voltage Voc increases from 0.61 to 0.71 V whereas the filling factor (FF) remains almost unchanged, thus resulting in a steady improvement from 2.32% for pure TiO2 to 3.41% for 5 wt% Gd1.54Er0.46(MoO4)3 NPs doping TiO2. This can be attributed to the improved photoexcited charge transfer and charge collection efficiency [43]. After doping Gd1.54Er0.46(MoO4)3 NPs, both the charge transfer lifetimes and the light harvestings of HSCs have
Fig. 6. (a) Absorption spectrum of P3HT and (b) EQE spectra of Gd1.54Er0.46(MoO4)3-TiO2/P3HT and TiO2/P3HT HSCs.
D. Li et al. / Electrochimica Acta 182 (2015) 416–423
been improved. However, after further increasing the doping amount of Gd1.54Er0.46(MoO4)3 NPs, i.e., >5.0 wt %, a flat BHJ film becomes difficult to be formed. In order to identify individual contribution of solar spectra, external quantum efficiency (EQE) measurements for the HSCs with and without nanophosphors were provided in Fig. 6. Overally speaking, the EQE spectra closely match the corresponding absorption spectra of P3HT. However, the broad EQE band covering the spectrum from 350 to 600 nm is enhanced significantly by Gd1.54Er0.46(MoO4)3 doping. This improvement originates from the faster charge transfer induced by the doping effects of GMO:Er phosphors. We calculated JSC value for each device by integrating the quantum efficiency over the entire solar energy spectrum with the relationship [44]: Z q Jsc ¼ EQEðlÞl EAM1:5G ðlÞdl ð7Þ hc where q is the elementary charge, h is Planck’s constant, c is the speed of light in a vacuum, and EAM1.5G is the spectral irradiance of AM 1.5 G. The calculated JSC are 7.63 and 6.42 mA/cm2 for Gd1.54Er0.46(MoO4)3-TiO2/P3HT and TiO2/P3HT HSCs, respectively. After introducing Gd1.54Er0.46(MoO4)3 phosphors, the calculated JSC is enhanced by 19%, which is close to enhancement estimated from the J-V measured results of about 22%. 4. Conclusion In summary, we have introduced GMO:Er NPs as a solar energy conversion platform for inorganic solution-processed solar cells. GMO:Er NPs demonstrate a two-photon UC mechanism, thus tailoring the near-infrared photons to visible photons that can be recaptured efficiently by P3HT. The solar cell based on 5 wt% Gd1.54Er0.46(MoO4)3 NPs has delivered a higher PCE of 3.41% as compared to the conventional TiO2/P3HT device, i.e., 2.32%. This improvement, on one hand, can be ascribed to the enhanced harvest of the low energy photons by broadening absorption range; on the other hand, the unique electronic activity of the Gd1.54Er0.46(MoO4)3 NPs enhance the photoexcited charge rate, i.e., more than 2 times faster than its counterpart bare TiO2. This work provides compelling evidences that the promising potential of up-conversion luminescence nanophosphor dopants in obtaining efficient HSCs, and also inspires the development of robust rare-earth phosphor to promise applications in photovoltaics. Acknowledgments This work was financially supported by the Natural Science Foundation of China (61366003, 11404283, 11564026), National Natural Science Foundation of Guangdong Province, China (2014A030307028), Natural Science Foundation of Jiangxi Province (20151BAB212001), Science and Technology Project of the education department of Jiangxi Province, China (GJJ12449, GJJ14533), and Distinguished Young Talents in Higher Education of Guangdong, China (2013LYM 0053).
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