Integrating Down-Shifting and Down-Conversion into MetalâOrganic

Article pubs.acs.org/JPCC

Cite This: J. Phys. Chem. C 2018, 122, 96−104

Integrating Down-Shifting and Down-Conversion into Metal− Organic Frameworks to Enhance the Spectral Conversion for Solar Cells Yi Yu, Lindong Lan, and Huaqiang Cai* Institute of Chemical Materials, China Academy of Engineering Physics, Mianyang, Sichuan 621900, P. R. China S Supporting Information *

ABSTRACT: Luminescent spectral conversion, including down-conversion (DC) and down-shifting (DS) is a potential route to overcome the spectral mismatch between solar photons and the semiconductor energy gap. Here, we introduce metal organic frameworks (MOFs) that integrate DC and DS processes into one system. Three different MOFs (Gd(BTC)(1), Gd(BPT)(2), and Gd(OBT)(3)) were synthesized doped with lanthanide ions couples. The DS− DC synergistic fluorescent processes were observed in all the MOFs. The overall absorption and near-infrared (NIR) emission intensity are dramatically enhanced (by factors of ∼4 800 and 550) due to the DS-sensitized effect for DC. As for DS process, the DC-bridge effect is quantified for the first time by comparing the NIR quantum yield (QYNIR). Benefiting from the bridge effect, QYNIR (5.30%) of Dy0.01Yb0.10-3 is increased around 3 times greater than the system without DC process, and it is close to the highest value reported for the C−H bond containing Yb3+ emitter. Our study demonstrated that MOFs can be regarded as easily prepared, environmentally friendly and efficient spectral conversion materials to improve the photon-response for c-Si solar cells. efficiency potentially more than 100%.6,7 Nonetheless, DC materials have significant drawbacks  narrow and low absorption efficiency due to parity forbidden 4f−4f transitions of lanthanide ions, finite doping quantity as a result of concentration quenching and complicated synthetic method.8 In contrast, DS materials can also shift short wavelength photons (from 280 to 500 nm) into longer wavelength ones.9 Meanwhile, DS materials will not be able to beat the Shockley−Queisser limit, since the absorption of one UV-blue photon can only result in the emission of one lower energy photon following the Stokes law.10 By now, four kinds of DS materialsorganic dyes, quantum dots, inorganic phosphors, and organolanthanide complexeshave been widely studied as potential spectral conversion candidates. In addition, other defects of DS materials are photo bleaching for organic dyes,11 toxicity and photoblinking for quantum dots,12,13 complex synthetic method for inorganic phosphors and relatively low quantum yield (QY) for NIR organolanthanide coordination compounds14 (owing to high lattice oscillation and low energy transfer efficiency (ηET)) respectively. Since DS and DC have complementary properties, scientists have tried to integrate those two process in the same platform to overcome the spectra mismatch and enhance the efficiency

1. INTRODUCTION Although great attention has been paid to new generation solar cells, crystalline silicon (c-Si) solar cells still domain the worldwide market.1 The solar spectrum consists of photons with wide wavelengths ranging from 280 to 2 500 nm (0.5−4.4 eV), but semiconductors are sensitive to a relatively narrow band.2 Thus, taking the spectral mismatch into account, the maximum power conversion efficiency that can be reached is known as the Shockley−Queisser limit.3 For example, the c-Si solar cell works efficiently in the 500−1 100 nm spectral region, while it shows very low spectral response to the wavelengths less than 500 nm and cannot generate electron− hole pairs with the wavelength more than 1100 nm. To overcome the spectral mismatch, three luminescence processes, namely up-conversion (UC), down-conversion (DC), and down-shifting (DS) have been used to convert the solar spectrum to match up with the response of solar cells.4 Among the three processes, extensive attention has been paid to UC process where sub-bandgap (infrared) photons are integrated to produce usable ones for generating electron−hole pairs.5 The DS and DC processes are feasible to convert shortwavelength (from 280−500 nm) photons to near-infrared (NIR) ones which is effective for c-Si solar cells. However, both processes have definite shortcomings. As for the DC process, generally in inorganic lanthanide phosphors, high-energy photons are absorbed and cut into two or more lower-energy photons for photovoltaics, with a conversion © 2017 American Chemical Society

Received: September 14, 2017 Revised: December 9, 2017 Published: December 11, 2017 96

DOI: 10.1021/acs.jpcc.7b09184 J. Phys. Chem. C 2018, 122, 96−104

Article

The Journal of Physical Chemistry C

the frameworks. It should be pointed out that the preparation method is more energy saving and environmentally friendly than inorganic phosphors40 as DC materials or QDs12 as DS. 2.2. Broadband Absorption and Excitation Spectrum. In order to study the DC process in MOFs and the mechanism of the DS sensitization, a series of MOF samples were prepared with 1 mol % Tb3+/Dy3+ and various Yb3+ (0, 5, 10, and 15 mol %) concentrations. In Figure S3, the UV−vis absorption spectrum was shown for three kinds of MOFs with the same doping content (Yb3+, 10%; Tb3+, 1%). The broadband absorption nearly from 280 to 440 nm in all the three samples indicates the transition in organic ligands (from the singlet ground state (S0) to the lowest excited singlet state (S1)) and Gd3+ ions (from 8S7/2 to 6P7/2). Since the absorption of Gd3+ ions is narrow band with low intensity (due to much lower cross-section than organic ligands),41 the absorption from 280 to 440 nm could be largely attributes to the absorption of ligands. Figure 1a shows the excitation spectrum of Tb0.01Yb0.10-3 (λem = 980 nm). There is a broadband excitation in 280−440

of solar cell. Up to now, this strategy have been achieved in Ce3+ (Eu2+, Bi3+ as well)/ Ln3+−Yb3+ inorganic phosphor15,16 and quantum dots/Ln3+−Yb3+ inorganic phosphor heterostructure systems.17,18 In it, Ce3+ ions (or quantum dots), acting as harvesting media, absorb a high energy photon at first, then the energy transfer (ET) from absorber to Ln3+ ion, finally DC processes were performed by lanthanide ion couples to split one photon into two photons. However, the ηET in DS process (absorbing media to Ln3+) are low, which affects the whole spectral conversion efficiency. Metal−organic frameworks (MOFs), stable and easily synthesized, are a generation of luminescent materials.19−23 Although MOFs have been utilized to enhanced the performance of solar cells,24−26 there are few literatures about luminescent MOFs applied to photovoltaic devices On the one hand, in lanthanides-based MOFs, the main ET path is that organic ligand absorbs a photon followed by intersystem crossing from first singlet state to triplet state (S1 → T1), then T1 → Ln3+ migration, finally remarkably enhanced 4f-4f transitions happens. The luminescence procedure, known as “antenna effect”,27 is like DS process with strong emission based on overcoming parity-forbidden transition and assuring high ηET.28−30 On the other hand, as supermolecule materials, it is attainable for lanthanide ions to be codoped in MOFs with ET between lanthanide couples.31,32 So the DC process might be observed in MOFs. With these in mind, MOFs could be utilized as a platform to subtly integrate DS and DC procedures for the development of spectral conversion. Here, we aim to test the reliability of a new strategy to enhance the spectral conversion based on integration of DS and DC using in MOFs. Owing to the advantages of structural diversity, three MOFsGd(BTC)33,34(1) (as called MOF76), Gd(BPT)35 (2), and Gd(OBT)36 (3)with different topologies and pore sizes were chosen as the host lattice to investigate the integration process, respectively. The choice was motivated by their NIR emission before.35,37,38 In addition, we will focus on the Tb3+−Yb3+ and Dy3+−Yb3+ DC couples due to high efficiency of ET from organic ligand to both Tb3+ and Dy3+ ions.36,39 The major objective of our present work is to explore the relationship between the two process efficiency and the internal structure in MOFs. Finally, it was found that the two process benefited from each other to enhance the conversion efficiency.

2. RESULTS AND DISCUSSION 2.1. Preparation Method and Structure Refinement. Followed by a modification of former literature,34−36 1, 2, and 3 were synthesized via thermal reactions at 100 °C for ≤48 h (see Supporting Information for further details). All three kinds of MOFs are three-dimensional (3D) porous coordination polymers exhibiting one-dimensional channels along the caxis. The purity of samples was verified through elemental analysis and X-ray powder diffraction (XRPD) measurements in Figure S1. Inductively coupled plasma optical emission spectrometer (ICP-OES) was conducted to determine the actual proportion of lanthanide ions (Table S1). In order to avoid the quenching effect of oscillators from water and other micromolecules around the lanthanide ions, as-synthesized crystals were heated in high vacuum to remove the guest molecules and terminal solvents before photophysical measurements. Thermogravimetric analysis (TGA, Figure S2) shows no weight loss happens for three compounds. Thus, after heating, there is no guest molecules and terminal solvents in all

Figure 1. (a) Excitation spectrum of Tb0.01Yb0.10-3 (λem= 980 nm). Inset: wavelength range from 480 to 500 nm (7F6 → 5D4 transitions of Tb3+ ion), magnified by a factor of 1000. (b). Absorption (dashed black curve) and excitation spectra (solid red line) for Dy0.01Yb0.10-1 demonstrating down-conversion. Each spectrum is normalized to the 6 F5/2 peak. The excitation spectrum monitored Yb3+ emission at 980 nm and was corrected for wavelength-dependent instrument response.

nm due to S0 → S1 transition of the organic ligand. While a weak narrowband was observed near 489 nm, which could be attributed to the 5D4 → 7F6 transitions of Tb3+ ion. The result might indicates that NIR emission can be achieved through exciting ligands following the ET process or the DC process in which one photon absorbed by the Tb3+ and cut into two photons by Yb3+ ions. The integrated spectral response of the 97


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The Journal of Physical Chemistry C Table 1. Cross Sections (ε) and Molar Extinction Coefficients (σ) of Three Ligands and Tb3+ Ion BTC −1

ε (cm ·mol ) σ (cm2) 2

BPT

1.2 × 10 8.2 × 10−17 7

2.4 × 10 4.0 × 10−17

broadband is around 4 800 times stronger than that of the narrowband, which suggests spectral conversion efficiency can be strongly enhanced by MOFs than inorganic phosphor system with only Ln3+ absorbing photons. To gain a mechanistic understanding of the difference, the cross sections of the three ligands were measured and shown in Table 1. To our knowledge, the cross-section of lanthanide ions is extremely low (typically on the order of 10−21 cm2) arising from parity-forbidden 4f−4f transitions. On the contrary, in Table 1, all the ligands have high cross-section up to 10−17 cm2, which contributes to more intense absorption response.5 As mentioned above, because of the relatively complicated energy transfer (ET) processes in the system, it is quite challenging to decouple the DS and DC processes and study DS-sensitized effect by studying the adsorption spectra along. Instead, the ET efficiency (ηET) and NIR emission intensity will be discussed in the following result. Referring to previous literature6 and noting the DC process of Dy3+, we see that absorption to 6FJ will result in emission of one 980 nm photon; absorption of 4I15/2 or 4F9/2 levels will be followed by splitting one high energy photon into two photons around 980 nm. To verify the existence of DC, in Figure 1b, both absorption and excitation (λem = 980 nm) spectra of Dy0.01Yb0.10-1 were measured in the region of 6H15/2 → 4I15/2 and 6H15/2 → 4F9/2 transitions (430−500 nm) and 6H15/2 → 6 F5/2 and 6H15/2 →6F7/2 (700−850 nm) transitions. In the absorption spectrum (in dashed black), the average ratio of the integrated area of the 6H15/2 → 4I15/2 and 6H15/2 → 4F9/2 transition relative to that of the 6H15/2 → 6FJ ones is 0.26 (2). While in the corrected excitation spectrum (in solid red), the ratio is 0.49 (3). Considering experimental error, the ratio is twice the value of that of the absorption strengths. Therefore, it is proven that the DC process occurs in MOFs where every photon excited into the 4I15/2 or 4F9/2 levels, then two photons in NIR are generated. 2.3. Emission through DS−DC. Figure 2a shows the emission spectra of the 1, 2 and 3 respectively codoped with Tb0.01−Ybx (x = 0, 0.05, 0.10, 0.15) couples, while Figure 2b presents the spectra from the ones with Dy0.01-Ybx (x = 0, 0.05, 0.10, 0.15) couples. Their excitation peaks around 353, 364, and 350 nm respectively are assigned to the absorption of BTC, BPT and OBT ligands. In agreement with emission spectra of lanthanide coordination polymers,28−30 the ET process was observed in which the delivery occurs from the linker triplet excited state (T1) to the Ln-emissive state. However, the transfer efficiencies are fairly different among three kinds of organic ligands: on the one hand, Tb0.01Ybx-2, Dy0.01Ybx-3, and Tb0.01Ybx-3 present efficient ET process with strong emission based on 4f-4f transitions; on the other hand, in systems of Dy0.01Ybx-1, Tb0.01Ybx-1, and Dy0.01Ybx-2, additional residual emission signals arising from the organic ligands is observed, indicating incomplete ETs. According to Dexter’s theory,42 the suitable energy gap between an Ln3+ ion and the T1 state of ligands is crucial for efficient ET. Table 2 shows the energy differences (ΔE(T1-Ln3+)) between Ln3+ ions and the T1 state (the T1 energy levels of BTC and OBT were obtained from the literature,37,43 while the level of BPT

Tb3+

OBT 4.9 × 10 2.0 × 10−17

7

5.1 × 102 8.5 × 10−22

7

Figure 2. (a) Photoluminescence (PL) spectra of Tb0.01Ybx-1 (1), Tb0.01Ybx-2 (2), and Tb0.01Ybx-3 (3) with different doping content of Yb3+ ions. (b) PL spectra of Dy0.01Ybx-1 (4), Dy0.01Ybx-2 (5), and Dy0.01Ybx-3 (6). (x = 0, 0.05, 0.10, 0.15;λex = 353(for 1), 364 (for 2) and 350 (for 3)).

Table 2. Energy Differences between T1 of Ligands and Ln3+ (ΔE) ΔE (cm−1)a

a

ligand

Tb

3+

Dy3+

BTC BPT OBT

4600 1600 2100

4100 1100 1600

ΔE = T1 (ligand) − Ln3+

was calculated by measuring the phosphorescence spectrum of 2 at 77 K upon excitation at 364 nm in Figure S4). Feasible energy gaps were found in Tb0.01Ybx-2, Dy0.01Ybx-3, and Tb0.01Ybx-3 systems. Nonetheless, in Dy0.01Ybx-1 and Tb0.01Ybx-1, the energy gap is too wide to sensitize the forbidden transitions. In contrast, the gap between the T1 state of BPT and Dy3+ is relatively narrow, which could result in energy back-transfer from the Dy3+ to BPT linker.44 Ligand design is a practical method to tune the photophysical property of optical functional MOFs.45 The energy of the T1 state can be tuned through changing the functional groups to better match the Ln3+ energy level for efficient ET. Thus, it is 98


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Figure 3. Single crystal X-ray structure and Representation of the Metallic Surrounding of One Considered Metallic Center (purple polyhedrons) in 1 (a), 2 (b), and 3 (c): four Yb3+ ions nearer than 10 Å along the Ln3+−carboxylate chain on the c-axis for all the three structures, the distances between center metal ion and Yb3+ ions in other directions are shown in the figures.

advantageous to undergo DS first before DC process to achieve a higher light harvesting and band broadening. To study the DC process in MOFs, the emission spectra are shown with various Yb3+ concentrations. In Figure 2a, the visible (vis) spectra of three samples show characteristic green emission resulting from the 5D4 → 7FJ (J = 6, 5, 4, 3) transitions of Tb3+ ions (484, 544, 584, 620 nm). In the NIR region, the emission band peaked at around 980 nm is assigned to the 2F5/2 → 2F5/7 transition of Yb3+. As for codoped Dy−Yb samples, the similar ET process can be found in Figure 2b. In contrast to Tb3+, not only the characteristic visible emission but also the weak NIR emission at about 930, 1010, and 1175 nm is attributed to the transitions of Dy3+.With the increase of the Yb3+ content, visible emissions from Tb3+ (or Dy3+) are significantly weakened indicating the presence of ET from Tb3+ (or Dy3+) to Yb3+, which is consistent with the changes in DC inorganic phosphors.46 As the increase of the Yb3+ loading, the emission intensity arising first and later decreases as a result of concentration quenching. It is noted that the most intense Yb3+ emission is obtained from 5 mol % Yb3+ in LnYb-1 samples, while the Yb3+ ratio with most strong NIR emission is 10 mol % in LnYb-2 and LnYb-3 samples. It has been studied that the distance of the involved metallic centers plays a crucial role in the concentration quenching effect, and the quenching effect is low when the Ln3+ ions are more than 10 Å from each other.47 Therefore, we came to a realization that MOF structure might plays a key role in the difference of concentration quenching effect among three MOFs. In order to investigate the structural difference among the three samples, single crystal structures are shown in Figure 3. The analysis of these structures reveals that, in all the structures along the Ln3+-carboxylate chain in c-axis, there are four Yb3+ ions nearer than 10 Å from a considered central Yb3+ ion (purple polyhedrons). While only in LnYb-1, four other Yb3+ ions are near the central Yb3+ with the distance about or less than 10 Å as well. Therefore, with the increase of Yb3+ content, concentration quenching effect is more inclined to occur in the structure of LnYb-1. This study shows MOF is a feasible candidate for DC material to avoid finite doping quantity due to concentration quench-ing, since the distance between the metal ions can be controlled through topological design and ligand change.48,49 Thus, MOFs is a feasible candidate for DC material to avoid finite doping quantity due to concentration quenching 2.4. Transient Spectra and Energy Transfer Efficiencies. To further confirm the occurrence of DC and estimate its efficiency, we recorded luminescent decay curves of samples. The samples were excited in their own excitation peaks and recorded at 544 nm for Tb3+-doped samples and at 575 nm for

Dy3+ ones. Figure 4 and Figure S5 show the rapid decreases of lifetimes (τ) with the increase of Yb3+ content. The decline of

Figure 4. (a) Luminescence decay curves of the emission in Tb0.01Ybx-2 where x = 0, 0.05, 0.10, and 0.15. The decay of the Tb3+5D4 emission at 544 nm as a function of time is shown for different Yb3+ concentrations under excitation at 364 nm. (b) Luminescence decay curves of the emission in Dy0.01Ybx-3 with the same Yb3+ concentration ranges excited at 350 nm.

lifetime arises from extra decay pathway by Yb3+: for codoped Tb−Yb samples, DC process occurs from Tb3+ to Yb3+ by cross relaxation from 5D4 → 2F5/2 + 2F5/2; as for the other one, ET happened from Dy3+ to Yb3+ by cross relaxation: Dy3+(4F9/2 → 6H5/2)−Yb3+(2F5/2 → 2F7/2) followed by Dy3+(6H5/2 → 6H15/2)−Yb3+(2F5/2 → 2F7/2). The DC ET efficiency (ηETDC), is defined as the ratio of donors (Tb3+ or Dy3+) depopulated by ET to the acceptors (Yb3+) over the total number of donors being excited in the DC process. The ηETDC is calculated by the following equation:50 ηETDC = 1 − τYbx /τYbo 99

(1) DOI: 10.1021/acs.jpcc.7b09184 J. Phys. Chem. C 2018, 122, 96−104

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Table 3. (a) Lifetimes (τ), DC Energy Transfer Efficiency (ηETDC) and Quantum Efficiency in NIR (ηNIR) through the DC Process in Tb−Yb Couples and (b) τ, ηNIR in Dy−Yb Couples part a Tb0.01Ybx-1

Tb0.01Ybx-2

Yb3+ content

τ (μs)

ηETDC (%)a

ηNIR (%)b

τ (μs)

0 0.05 0.10 0.15

1372(3) 1241(1) 1134(9) 1041(5)

− 9.64 17.4 24.1

− 19.2 34.8 48.2

Yb3+ content

τ (μs)

ηETDC (%)a

ηNIR (%)b

τ (μs)

0 0.05 0.10 0.15

55.26(7) 39.11(4) 25.47(6) 22.31(1)

− 29.2 53.9 59.6

− 58.4 107.8 119.2

37.07(5) 22.52(8) 14.30(3) 8.467(2)

ηNIR (%)b

τ (μs)

ηETDC (%)a

ηNIR (%)b

− 33.8 51.8 59.9

− 67.6 103.6 119.8

1600(5) 1454(3) 1270(2) 1189(1)

− 9.11 20.6 25.7

− 18.2 41.2 51.4

ηETDC (%)a

ηNIR (%)b

τ (μs)

ηETDC (%)a

ηNIR (%)b

− 39.2 61.4 77.2

− 78.4 122.8 154.4

160.4 (3) 101.2(6) 72.0(1) 56.3(5)

− 36.9 55.1 64.9

− 73.8 110.2 129.8

387.3(7) 256.7(4) 186.6(5) 155.4(2) part b

Dy0.01Ybx-1

Tb0.01Ybx-3

ηETDC (%)a

Dy0.01Ybx-2

Dy0.01Ybx-3

ηETDC = 1 − τYbx/τYb0. bηNIR = QE − (1 − ηETDC) = 2 × ηETDC.

a

where x represents the Yb3+ ions concentration. τYbx is the lifetimes with different doping contents. The corresponding ηETDC is also calculated as shown in the Table 3 as well as the lifetimes. Table 3a and 3b show the ηETDC increases as the ratio of Yb3+ increases. Besides, for all the three chosen MOFs, the ηETDC doped with Dy−Yb couple is higher than the one with Tb−Yb couple, which is consistent with the result of DC process in inorganic crystals.6 This phenomenon is attributed to the fact that the Tb3+ has no intermediate energy level at 10 000 cm−1.50 Accordingly, ET from the 5D4 level of Tb3+ to Yb3+ could only proceed through a second-order cooperative ET process, which results in low DC probability. So far, we have demonstrated the existence of the DS−DC process in MOFs. Possible energy migration path in the platform, specifically in the case of the Dy−Yb codoped sample, is proposed in Figure 5. DS procedure includes the following:

F5/2 followed by 6H5/2 → 2F5/2, which brings about two excited Yb3+ ions and the emission of two NIR photons. 2.5. The DS-Sensitized Effect. The total quantum efficiency (QE), defined as the ratio of the photons emitted to the photons absorbed, is commonly used in the literatures to evaluate DC materials.51 Nonetheless, c-Si solar cells are most sensitive to the NIR photon with the wavelength around 1 000 nm. By separating the NIR photons emitted with Vis photon in DC process, the quantum efficiency in NIR (ηNIR) is obtained as a function of the Yb3+ ions concentration. in the following equation: 2

ηNIR = QE − (1‐ηETDC) = 2·ηETDC

(2)

the ηNIR of the six kinds of sample with various Yb ratios were calculated and shown in Table 3. The ηNIR of Tb0.01Yb0.10-2 and Dy 0.01Yb0.10-3 is still higher than 100% without concentration quenching. By now, our study has shown that ET are both efficient in both DC and DS processes in Tb0.01Yb0.10-2 and Dy0.01Yb0.10-3 systems. intensities were compared. The first method is by DS-sensitized process and the second one is by exciting Ln3+ (Tb3+ and Dy3+) directly. The NIR emission intensities of Tb 0.01 Yb 0.10 -2 and Dy0.01Yb0.10-2 are compared in Figure 6a and 6b respectively (Figure S6 shows the result of Dy0.01Yb0.10-3). The strong emission by DS-sensitized effect is obvious, while the emission by exciting straightforward is simply invisible on this scale. The integrated spectral response in the 900−1 100 nm excited through DS-sensitized way is 560 times stronger than the one upon direct excitation for Tb0.01Yb0.10-2 (Figure 6a), and the former is 535 times stronger than the latter for Dy0.01Yb0.10-3 system (Figure 6b). Combined with the excitation comparison above, these results clearly show that the DS-sensitized method can highly boost the DC process, which is as a result of enhanced and broadened absorptivity and efficient ET in both pathways.5 Nevertheless, the response in NIR through DSsensitized way is merely 25 times stronger than the direct one without sensitized effect. This is due to the fact that the ET procedure is incomplete in Dy 0.01Ybx-2, arising from efficiencies of the ET from ligand to donor ions (Dy3+ or Tb3+). 2.6. The DC-Bridge Effect. One kind of the DS material, namely NIR organolanthanide complexes, is regarded to have relatively low QY (usually below 2%) due to the high lattice 3+

Figure 5. Schematic energy diagram showing the integrate of DS and DC procedures in MOFs.

first, the excitation happens from So to S1 as an organic ligand absorbs a photon from the sun light; second, the electron transfers from S1 to T1 through intersystem crossing; finally, the delivery occurs from the linker triplet excited state to the Ln-emissive state. And DC is realized by depopulating the 4 F9/2 excited state through two relaxation pathway between Dy3+ and Yb3+. 6H5/2 acts as the intermediate level: 4F9/2 → 100


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Figure 7. (a) PL emission spectra of TbxYb0.10-2 excited at 364 nm (x = 0, 0.003, 0.01, 0.03). (b) PL emission spectra of DyxYb0.10-3 excited at 350 nm with the same donor ion content range.

Figure 6. (a) Emission spectra of Tb0.01Yb0.10-2 excited at 364 (integration of DC and DS) and 489 nm (only DC process) respectively. Inset: emission intensity by exciting Tb3+ straightforward magnified by a factor of 500. (b) Emission spectra of Dy0.01Yb0.10-2 excited at 364 nm (integration of DC and DS, solid line) and 430 nm (only DC process, dashed line) respectively, emission intensity by exciting Dy3+ straightforward magnified by a factor of 20.

Yb3+. In addition, the ηNIR is over 100%. So the overall ET of DS−DC process can be more efficiency than the one with only DS process. The result indicates that Dy3+ (or Tb3+) acts as a bridge to modulate the ΔE and enhance the ηET. The DCbridge effect has been mentioned in previous literature.18 However, there has been no research quantifying the effect. Therefore, we introduce QY to explore the quantified impact. The QY is the one of the most significant parameters in luminescent materials. For NIR luminescence coordination compounds under ligand excitation, QYNIR, is defined by the following formula in eq 4:53

oscillation and inefficient ET. In our DS−DC system, both ET processes are efficient in which the ηET is more than 100%. To further test the bridge effect of DC process for DS materials, we chose TbxYb0.10-2 and DyxYb0.10-3 as our model systems with different Ln concentrations (x = 0, 0.003, 0.01, 0.03) and studied the intensity of NIR emission. The reason why those two systems were chosen is that they can allow efficient ETs to occur without concentration quenching. Figure 7 shows the emission spectra of two kinds of samples from 900 to 1 200 nm. For both systems, as the increase of the concentration, the Yb3+ emission intensity is enhanced at first and then weakened. Without doping donor ions, the energy gaphigher than 10 000 cm−1affect the ηET52 (ligands → Yb3+), known as sensitization efficiency (ηsens) as well. After doping donor ions, the ET process has been turned into DS−DC. When a few donor ions doping, the total ηET increases due to the suitable energy gap between ligands and donor ions (Tb3+ or Dy3+). Continuing to increase the donor content gives rise to the highest intensity value because the overall efficiency of both ET processes reach maximum. Afterward, the ηETDC deceases as the increase of the doping ratio, which result in lower NIR emission intensity. Therefore, in the DS−DC process, the overall ηET in NIR range is the product of the two independent ET efficiencies in the following equation: ηET = ηETDS × ηNIR = ηETDS × 2ηETDC (3)

QYNIR = ηsens × QYYb = ηISC × ηET × τobs/τrad Yb

(4)

where is the intrinsic quantum yield; ηsens reflects the Ln sensitization efficiency and is calculated as the product of the intersystem crossing (ηISC) and ηET; τobs and τrad are defined as the observed and radiative lifetimes, of which the quotient is the QYYb Yb. At first, the τobs of two kinds of samples in different donor doping content is recorded at 980 nm. With the variation of donor concentration, the lifetimes remain unchanged (shown in Figure S7). Meanwhile, in our systems, the τrad is deemed invariable because of constant coordination environment and phonon energy.54 In addition, the ηISCs stay identical as a consequence of constant temperature and molecule conformation.55−57 Because of the ηET proportional to the QYNIR, we can quantify the bridge effect through analyzing the QYNIR. The QYNIR of samples above were measured and summarized in Table 4. Hence, we demonstrate that, as a result of the bridge effect, the ηET with donor ions in MOFs is around 3 times of the one without them. It is noted that the QYNIR (5.30%) of Dy0.01Yb0.10-3 is close to the highest value5.88%among Yb3+ coordination compounds containing C−H bonds.53 However, the QYs is still low compared QYYb Yb

To our knowledge, similarly impacted by the ΔE men-tioned above, ηETDS between the ligand and the bridge ion (Dy3+ or Tb3+) is larger than the one with ET only from the ligand to 101


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The Journal of Physical Chemistry C Table 4. Quantum Yield over the NIR range (QYNIR) with Different Bridge Ion Contents (Tb3+ as Bridge ion for 2 as well as Dy3+ for 3) bridge ion content

TbxYb0.10-2

DyxYb0.10-3

0 0.003 0.01 0.03

0.89(3) 1.44(3) 2.61(5) 1.26(2)

1.58(2) 2.02(5) 5.30(3) 2.54(4)

with QDs and inorganic phosphors due to the high lattice oscillation. This could be overcome by using deuteration weaken the effect of phonon vibrations.58 Notwithstanding its limitation, the introduction of DC is a reliable strategy for the optimization to improve the ηET of NIR emitter. So far, we have assumed that the DC−ET efficiency in NIR (ηNIR) plays the key role in the bridge effect. Because of inefficient DC−ET of Tb0.01Yb0.10-3, to verify our hypothesis, the Yb3+ emission intensities and QYNIR of Tb0.01Yb0.10-3 and Yb0.10-3 were compared separately. Figure 8 shows the two

Figure 9. Spectral conversion design for solar cell applications, involving the AM 1.5 solar spectrum, spectral response of c-Si, and the PLE and PL spectra of Dy0.01Yb0.10-3.

referring to previous literature.60 As for Dy0.01Yb0.10-3 under 350 nm illumination, the result shows, in ideal condition, a 5.17-fold increase in power conversion efficiency occurs with the DS−DC spectral conversion device. The detailed calculative process of the ratio is shown in the Supporting Information.

3. CONCLUSIONS By introducing MOFs for spectral conversion, we have successfully realized the integration of DC and DS processes to enhance the broadband photoresponse of solar cells. In our works, the absorption, energy transfer and emission properties can be modulated on command by varying energy differences between T1 of ligands and Ln3+ (ΔE), the distance among Ln3+ ions and doping concentration. In addition, The overall absorption and NIR emission intensity are dramatically enhanced (by factors of ∼4 800 and 550) by the DS-sensitized effect. Meanwhile, presented for the first time is the quantified DC-bridge effect which facilitates the energy transfer efficiency (ηET). Arising from the approximate 3-fold raise of the ηET by the bridge effect, the NIR quantum yield (5.30(4)%) is close to the highest value among the Yb3+ emitter containing C−H bond. In a word, these breakthroughs are achieved by making proper use of the diversity and designability of MOF materials.48,49 Thus, the integration overcomes the disadvantages of two processes and makes it more possible to enhance the spectral conversion for c-Si solar cells.

Figure 8. PL emission spectra of Tb0.01Yb0.10-3 and Yb0.10-3 excited at 350 nm. Inset: NIR quantum yield of Tb0.01Yb0.10-3 and Yb0.10-3.

NIR emission spectra with the same excitation condition. Different from the systems of TbxYb0.10-2 and DyxYb0.10-3, the emission intensity of Tb0.01Yb0.10-3 is slightly enhanced than the one of Yb0.10-3, which indicates the weak bridge effect in it, and the calculated QYNIR (inset) show the consistence with the emission intensity. Therefore, the importance of DC−ET efficiency on the bridge effect has been confirmed. 2.7. Application to Solar Cells. Figure 9 illustrates a scheme for spectral conversion design of the DS−DC phosphor MOFs. According to the normalized spectral response of c-Si solar cells, the wavelengths between 950 and 1 100 nm can be utilized most effectively.4 However, the high energy photons (wavelengths from 280 to 500 nm) cannot be conversed with high efficiency due to absorption and reflection by the antireflection coatings and increase emitter recombination in semiconductors.59 In codoped MOFs, as the sunlight falls, the high energy photon are first absorbed by the organic ligands. And then, the energy was transferred from ligands to the bridge ions (Dy3+ or Tb3+) through the DS process. Last, in the DC procedure, the NIR photons is emitted by Yb3+ ions with high spectral response. The quantum efficiency in NIR (ηNIR) can be up to 100%. While the main disadvantage is the relatively low overall quantum yield. To verify the improvement of the DS−DC synergistic process, the conversion efficiency of the spectral conversion sheet coated solar cell was theoretically compared with the one of uncoated solar cell

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b09184. Experimental details, theoretical calculations, the UV− vis spectrum (Figure S1), X-ray power diffraction patterns (Figure S2), TGA traces (Figure S3), phosphorescence spectra at low-temperature (Figure S4), transient state spectra (Figures S5 and S7), and emission spectra excited at different wavelengths (Figure S6) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*(H.C.) E-mail: [email protected]. 102


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The Journal of Physical Chemistry C ORCID

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Huaqiang Cai: 0000-0003-3032-9256 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is financially supported by the National Natural Science Foundation of China (51402268, 21401178).

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