N-functionalized graphene quantum dots with ultra-high quantum yield

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N-functionalized graphene quantum dots with ultra-high quantum yield and large Stokes shift: efficient downconverters for CIGS solar cell Firoz Khan, and Jae Hyun Kim ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.8b01125 • Publication Date (Web): 24 Sep 2018 Downloaded from http://pubs.acs.org on September 25, 2018

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N-functionalized graphene quantum dots with ultra-high quantum yield and large Stokes shift: efficient downconverters for CIGS solar cell Firoz Khan and Jae Hyun Kim* Smart Textile Convergence Research Group, Daegu Gyeongbuk Institute of Science & Technology (DGIST), 333 Techno Jungang-Daero, Hyeonpung-Myeon, Dalseong-Gun, Daegu42988, Republic of Korea E-mail: [email protected]

Keywords: N-doped graphene quantum dots, downconverters, quantum yield, N moieties, CIGS solar cells

Copper indium gallium selenide (CIGS) is the most promising thin film solar cell technology. However, its high performance is hampered by its poor short-wavelength response. The shortwavelength response can be enhanced via photon downconversion using quantum dots. Unfortunately, most graphene quantum dots (GQDs) are not suitable as downconverters in CIGS cells owing to their low photoluminescence quantum yield (PL QY) and/or low Stokes shift. Herein, an ultra-high PL QY (99%) and a large Stokes shift (98 nm) are achieved for N-doped GQDs via a novel method. The performance of a CIGS solar cell is enhanced via photon downconversion and the light-trapping effect using the NGQDs. The effectiveness of the NQGDs is manifested in a conversion efficiency (η) of 15.31%. In addition, improvements in the short circuit current density from 30.69 mA/cm2 to 31.77 mA/cm2 and fill factor from 71.25% to 73.09% are observed. The n, and J0 values are decreased by insertion of NGQDs, indicating a reduction in the recombination losses.

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Cu(In,Ga)Se2 (CIGS)-based solar cells, the most promising thin film technology, are receiving worldwide attention for solar power generation.1 Owing to its high absorption coefficient, only 1~2 µm thickness is sufficient to absorb more than 95% of the useful solar radiation (in the wavelength range of 300 to 1300 nm).2,3 Unfortunately, the high performance of CIGS solar cells is hampered by parasitic absorption losses in the ZnO window and CdS buffer layers.4 ZnO strongly absorbs photons of λ < 390 nm, while CdS absorbs photons of λ < 500 nm. Only a fraction of photons absorbed by CdS is utilized for current generation, whereas the photons absorbed by ZnO are completely lost.5 Thus, CIGS solar cells typically exhibit a poor short-wavelength (λ) response, which reduces their current output. The maximum losses in current density corresponding to absorbed photons in ZnO and CdS are ~ 1.1 mA/cm2 and 7.4 mA/cm2, respectively. These losses can be reduced either by improving the electronic properties of the solar cell or by photon management via downconversion.6–8 The electronic properties of CIGS solar cells can be improved by creating a very narrow junction, or employing low doping levels or very thin window/buffer layers.6 However, it is difficult to implement these steps as they increase the production cost. Additionally, this approach may have detrimental effects on the overall solar cell performance.6 On other hand, in the photon management approach, a luminescent down-shifting (LDS) material is used. The LDS material absorbs short-λ photons and re-emits them at a more favorable λ at which the solar cells exhibit a significantly better response. Since the LDS layer is applied on top of the solar cell, no change occurs in the junction and interfaces of the cell. Therefore, it is a passive approach to enhancing the short-λ response. Moreover, a theoretical efficiency of 33% can be achieved via photon management.9


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The most suitable LDS material for CIGS solar cells should exhibit: (i) a high photoluminescence quantum yield (PL QY); (ii) a wide absorption band for wavelengths < 500 nm; (iii) a high absorption coefficient; (iv) a narrow emission band for wavelengths > 500 nm; and (v) good separation between the absorption and emission bands to minimize losses due to reabsorption. In addition, they should be inexpensive. A large number of luminescent materials have been investigated for LDS and can be classified into three main categories: inorganic quantum dots (QDs),10 organic dyes,11 and rareearth ions/complexes.12 Inorganic QDs are nano-sized semiconducting crystals, whose absorption and emission bands can be tuned according to their size. They exhibit a wide absorption band, high emission intensity, and relatively good photo-stability.11 On the other hand, they result in high re-absorption losses due to the large overlap of absorption and emission bands, exhibit relatively poor PL QYs, and are generally expensive.13 Organic dyes exhibit relatively high absorption coefficients, close to unity PL QYs, and can be easily incorporated in polymeric matrices. However, their drawbacks are narrow absorption bands and relatively small Stokes shifts, which result in significant re-absorption losses. Further, rare-earth ions exhibit a high PL QYs but have extremely low absorption coefficients.11 Therefore, it is important to develop QDs that strongly absorb over a broad wavelength range of 300–500 nm, yielding a sharp emission at wavelengths > 500 nm (large Stokes shift). Recently, graphene quantum dots (GQDs) are the emerging material for photon downconversion owing to their unique semiconducting properties.14,15 Generally, with increase in the Stokes shift, the PL QY is decreased. Due to extraordinary optical and electrical features, the GQDs are attracted much towards applications in Si heterojunction,14,15 organic/Si hybrid,16,17 and dye-sensitized18,19 photovoltaic (PV) solar cells. Most of the GQDs reported in literature have one or more drawbacks, such as a low absorption, low PL QY, low Stokes shift (emission PL peak position < 3 ACS Paragon Plus Environment

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500 nm), or the overlap of absorption and emission spectra, and are thus not applicable as downconverters in CIGS solar cells.14-19 They have used GQDs of very low PL QYs (< 11%).15,17

To address these issues, we have developed a new method to rapidly produce, nitrogenfunctionalized GQDs (NGQDs) with an ultra-high PL QY (99%) and a large Stokes shift (emission peak > 500 nm) simultaneously. The obtained NGQDs were applied to a CIGS solar cell to enhance the cell performance. First, graphene oxide (GO) powder was dissolved in deionized (DI) water to obtain a concentration of 4.0 mg/mL via sonication. Then, a polyethylenimine (PEI, Mw = 1800, 0.5 g/mL in DI water) solution was added to the GO solution in a PEI/GO volume ratio of 20. The GO/PEI composite was stirred at 90 °C for 4 h to obtain a functionalized GO paste. The functionalized GO was heated at 300 °C for 1 h in a horizontal furnace under ambient Ar to obtain NGQDs along with porous reduced graphene oxide (rGO). The obtained product was dispersed in ethanol by sonication. Finally, the dispersed solution was filtered using a 0.1 PTFE syringe filter to obtain a light yellow solution. The PL QY calculation method is provided in the supporting information. High-resolution TEM (HRTEM) and energy dispersive X-ray spectroscopy (EDS) (HF3300/NB5000/S-4800, Hitachi) were performed to image the atomic structure and elemental analysis, respectively. Surface analyses were conducted using non-conducting atomic force microscopy (AFM; Park Systems, NX10). The Raman spectra were measured at room temperature using a confocal Raman spectrometer (Thermo Fisher Scientific, Nicolet Almeca XRA) with an λex of 532 nm. The absorbance and transmittance spectra were measured using a UV-Vis-NIR spectrophotometer (Agilent Technologies UV–Vis–NIR system, Carry-5000). XPS (Thermo Scientific, ESCALAB 250Xi) measurements were performed to study the elemental 4 ACS Paragon Plus Environment

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composition of the NGQDs using an ultra-high-vacuum apparatus equipped with a monochromatic Al Kα X-ray source (1486.6 eV). All the XPS peaks were calibrated using the C 1s peak. PL spectra were measured at an λex of 405 nm using a PL spectrophotometer (Darsa, PSI Trading Co. Ltd.) with a Xe-lamp. The details of CIGS solar cell fabrication is provided in the Supporting Information. The NGQDs were applied on the top of the CIGS solar cell. The illuminated J–V characteristics of the solar cells (active area ~ 0.43 cm2) were measured under simulated sunlight at an intensity of 100 mW/cm2 in the AM1.5 global solar spectrum using a source meter (Keithley, 2400) and a solar simulator system (Newport, 91192) equipped with a 1-kW Xe-arc lamp (Oriel). The EQEs of the solar cells were acquired using a PV measurement device in the range of 300–1100 nm (PV Measurement Incorporated, G1218a). The illumination intensity was calibrated using a Si reference solar cell (PV Measurement Incorporated). All measurements were performed at 25 °C. The schematic of the NGQD synthesis process is shown in Figure 1(a) and their expected synthesis mechanism is shown in Figure 1(b). The colored photographs of the produced NGQDs in ethanol under white-light and UV-light (365 nm) are shown in the Figure S1 of Supporting Information. The AFM image of the NGQDs is shown in Figure 2(a). The observed height of the NGQDs is 2–4 nm (inset of Figure 2(a)). The large height is possibly owing to the functionalization of GQDs. The transmission electron microscopy (TEM) images of the GQD sample are shown in Figure 2(b). It can be seen that all the GQDs are highly crystalline due to the presence of lattice fringes. The distance between the lattice fringes are 0.346, 0.214, and 0.246 nm corresponding to the basal plane spacing (d002 = 0.334 nm), in-plane lattice spacing (d100 = 0.213 nm), and lattice constant (d = 0.246 nm) of graphite, respectively (Figure 2(c)).20 The d spacing values of the GQDs are slightly higher than those of bulk graphite owing to the presence of functional groups. The Raman spectroscopy results also confirmed the formation of 5 ACS Paragon Plus Environment

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NGQD, as shown in Figure 2(d). The D and G peaks appeared at 1346 cm−1 and 1587 cm−1, respectively. The D peak is related to defects, whereas the G peak is related to the hexagonal lattice of graphene associated with the double-degenerated E2g mode.21 The UV-Vis absorption spectrum of the NGQDs is shown in Figure 2(e), which is a combination of four typical characteristic absorption bands. The absorption band is deconvoluted into four sub-bands with peaks centered at 223, 278, 315, and 353 nm (corresponding to energies of 5.56, 4.46, 3.93, and 3.51 eV, respectively; see Figure S2 of Supporting Information); this resulted in a strong and broad absorption band at λ < 500 nm, which is the most suitable for downconversion application in CIGS solar cells. Generally, the absorption band in the deep UV (< 250 nm) region is attributed to the π→π* transition of C=C (of sp2 C domain in sp3 C matrix) and π→π* transition of C=O or C=N caused by absorption in the UV-Vis region (250 to 500 nm).22-26 The photoluminescence excitation (PLE, at emission wavelength λem = 503 nm) and emission (PL, at excitation wavelength λex = 405 nm) spectra were measured and are shown in Figure 2(f). The PLE and PL spectra are centered at 405 nm and 503 nm, respectively, which resulted in a large Stokes shift of 98 nm. A narrow full width at half maximum (FWHM) of ~66 nm is obtained for PL emission spectrum, which is lower than the value reported by Qu et al.24 Both the PLE and PL spectra are deconvoluted into three sub-bands to get the possible transitions. The deconvoluted PLE sub-bands are corresponding to peak energies of 3.06 eV, 3.15 eV, and 3.27 eV. Whereas, the peak energies of deconvoluted PL spectra are found to be 2.30, 2.46, and 2.60 eV. X-ray photoelectron spectroscopy (XPS) characterization was conducted to investigate the doping configuration and electronic structure of NGQDs. Three predominant peaks of C 1s (~284 eV), N 1s (~399 eV), and O 1s (~531 eV) were observed by elemental survey scan (Figure S3 of 6 ACS Paragon Plus Environment

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Supporting Information). Gaussian-Lorentzian combined peaks were used to fit the highresolution XPS spectra (Figure 2(g–i)). The atomic percentages of C, N, and O were calculated as 69.25, 25.91, and 4.84%, respectively. The C 1s peak is deconvoluted into four peaks corresponding to C-C/C=C (at ~284.02 eV, ~60.58%), C-N (at ~ 285.45 eV, ~29.21%), C=O (at ~286.58 eV, ~8.18%), and O=C-O (at ~287.79 eV, 2.03%).25 The N 1s peak is deconvoluted into four peaks of pyridinic N (at ~ 398.25 eV, ~33.54%), pyrrolic N (at ~399.08 eV, 46.41%), graphitic N (at ~400.02 eV, ~16.21%) and oxidized N (at ~402.01 eV, ~3.84).26,27 Figure 2(i) shows that only the carbonyl O of the C=O bond is present (quinone O is not observed).24 The possible absorption and emission transitions are shown in Figure 3(a). The absorption spectrum in the wavelength range of 270 to 420 nm is assigned to the Cπ→Nπ* transitions of conjugated C=N or Cπ→Oπ* transitions of C=O.28 The PLE peaks can originate from the transitions of Cπ→Cπ*, Cπ→Nπ*, and Cπ→Oπ*,29 whereas the PL emissions arise from the transitions of Cπ*→Cπ, Nπ*→Cπ, and Oπ*→Cπ. The absorption of a high-energy single photon can lead to the emission of more than one photon, which can result in a PL QY higher than 100% (Figure 3(a)). By controlling the doping concentration and N and O moieties, the energy levels of Cπ*, Nπ*, and Oπ* can be tuned to achieve a desirable PL QY and Stokes shift. NGQDs with an ultra-high PL QY of 99%, which is the highest PL QY among all the reported PL QYs of NGQDs in literature, were achieved via thermal treatment of graphene oxide (GO) and polyethylenimine composite.24,30,31 There are two possibilities for the high PL QY: i) a very small Stokes shift or ii) more than one photon emission for a single photon absorption.24 The second phenomenon has a better role in downconversion application in solar cells because of the absence of overlapping between the absorption and emission spectra. The absorption and emission ranges along with the PL QY of our NGQDs are compared with those of the other reported NGQDs (Table S1 of Supporting Information). It can 7 ACS Paragon Plus Environment

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be seen that because of either a low PL QY or an unsuitable absorption/emission range (also Stoke shift), the other reported NQGDs are unsuitable as downconverters in CIGS solar cells.24,30,31 The synthesized NGQDs were applied to a CIGS solar cell for application as downconverters (Figure 3(b)). In this solar cell, photons of wavelength < 500 nm were absorbed by the NGQD top layer and photons of wavelength > 500 nm were emitted, which were easily absorbed in the CIGS layer and utilized for current generation. To the best of our knowledge, we are the first to establish a new method to obtain efficient downconversion with ultra-high-PL QY and large-Stokes-shift NGQDs. The current density-voltage (J–V) characteristics of the bare and NGQD (10, 20, and 30 µL/cm2 which will be referred as NGQD10, NGQD20 and NGQD30, respectively)-coated CIGS solar cells are shown in Figure 3(c). Jsc increased up to a NGQD concentration of 20 µL/cm2 owing to light trapping (Figure S4 of Supporting Information) and photon downconversion management. The reflectance values significantly reduced in the wavelength regions of < 400 nm and >1000 nm for NGQD concentrations of 20 µL/cm2 and 30 µL/cm2. For a NGQD concentration of 30 µL/cm2, the photons of λ > 500 nm are also absorbed; thus, the Jsc reduces (Figure S5 of Supporting Information). It can be seen that the transmittance of NGQD30 is higher than NGQD20 in the wavelength region of < 400 nm, whereas in long-λ region (400 ≤ λ ≤ 800 nm), the transmittance is decreased. The refractive index and thickness of NGQD layer govern the reflectance of the front surface NGQD layer coated on the substrate. With the increase in the NGQD concentration, the compactness of NGQD layer is increased. Thus, both the refractive index of the NGQD layer and NGQD layer thickness are increased with the increase in the concentration of NGQD. Which results for lower reflection in the wavelength region of < 400 nm. However, the observed reduction in the transmittance in long-λ region is due to increase in light absorption by thicker layer of NGQD. To achieve maximum output current, the NGQD 8 ACS Paragon Plus Environment

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layer must exhibit high transmittance for wavelength of > 500 nm. With the increase in the concentration of NGQD (> 30 µL/cm2), a huge transmission loss has been observed for wavelength > 500 nm due to scattering effect (not shown here). Therefore, an optimum thickness is required for maximum solar cell performance. The PV performance parameters of the solar cells containing various concentrations of NGQDs were compared with those of a bare cell (see Table S2 of Supporting Information). The best Jsc, Voc, FF, and η values of the solar cell containing the NGQDs (20 µL/cm2) were 31.77 mA/cm2, 695.46 mV, 73.09%, and 15.31%, respectively, which demonstrate an improvement of ~ 3.5% in Jsc and ~ 6.6% in η over those of the bare cell (Jsc = 30.69 mA/cm2, Voc = 656.80 mV, FF = 71.25%, and η = 14.36%). Further, the performance parameters obtained in this work were compared with previously published results (for other types of QDs) (see Table S3 of Supporting Information).7,8 The external quantum efficiency (EQE) of both the bare and NGQD-coated solar cells are shown in Figure 3(d). It clearly shows that the main improvement in EQE is achieved for the short-wavelength region (< 500 nm) owing to photon downconversion, while the EQE slightly improved in the long-wavelength region (> 1000 nm) due to the light-trapping effect via the insertion of a NGQD layer. The theoretical values of Jsc due to light trapping are calculated for 1000 ≤ λ ≤ 1300 nm, and are found to be 10.88 mA/cm2 and 10.93 mA/cm2 for bare and NGQD20 solar cells, respectively (see the Supporting Information)32. Which shows a small enhancement in Jsc (~ 0.05 mA/cm2) is obtained due to light trapping. It is a clear evidence that the improvement in Jsc is mainly due to the photon management. Due to the creation of a NGQD conductive layer on the top surface of the cell, the contact resistance reduced, which resulted in a slight enhancement in the FF. However, the FF decreases upon insertion of CdSe/CdZnS QDs on the top surface of the CIGS solar cells.8 The cross-


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sectional TEM image of CIGS solar cell coated with NGQDs is shown in Figure 4. The corresponding EDS elemental mapping is shown in Figure S6 of Supporting Information). The dependency of performance parameters (Jsc, Voc, FF and η) on NGQDs is shown in Figure 5(a,b). These performance parameters are governed by PV cell parameters (Shunt resistance Rsh, series resistance Rs, diode ideality factor n, and reversed saturation current density J0). The values of PV cell parameters were analytically predicted using previously reported analytical method.33

Rsh = Rsc  J 0 =  J sc − Voc  Rsc  n=

(1) 

V



  − nVoc   e T   

(2)

(Vm + Roc J m − Voc )  Jm    VT . ln  J sc − V m − J m  − ln J sc − V oc + R sc R sc    J sc − V oc R sc 

(

)

(

)

    

 Voc  −  nVT  nVT  Rs = Roc − e J0

(3)

(4)

where Rsc and Roc are inverse of slopes (dJ/dV)-1 at short circuit (V = 0, J = -Jsc) and open circuit (V = Voc, J = 0) conditions, Jm is the current density at maximum power, Vm is the voltage at the maximum power point, and VT = kT/q (k being the Boltzmann’s constant, and q the elementary electronic charge). The values of PV cell parameters are shown in Figure 5 (c,d). The Rsh value enhanced to 1222.64 Ωcm2 for NGQD20 from its value (900.91 Ωcm2) of bare cell. Owing to downconversion and the light-trapping effect, the number of photons absorbed in the CIGS were increased. Thus, number of charge carriers generated in the CIGS are increased, which results for enhancement in Rsh due to reduction in the leakage current across the junction.34 While the value of Rs is slightly 10 ACS Paragon Plus Environment

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reduced to 1.20 Ωcm2 for NGQD20 from 1.21 Ωcm2 (bare cell). This decrease in Rs may be attributed to increase in the conductivity of the active layer by insertion of NGQD downconverters.35 Both the n, and J0 values of the NGQD20 were reduced to 1.39, and 2.95 x 1010

A/cm2 form their values of 1.44, and 5.73 x 10-10 A/cm2 for bare cell, respectively. The values

of both n and J0 are indicative of recombination at interface/surface, in the space charge region, and in the bulk.32,33 Owing to low photon absorption at CIGS/CdS interface, the recombination in the space charge region is dominated in the bare CIGS solar cell. The number of photons absorbed at CIGS/CdS interface and in the bulk are increased due insertion of NGQDs down converters on the top of the CIGS solar cell. Thus, the FF and Voc values are increased for NGQD20. The present work highlights the important parameters of QDs than can be applied for downconversion in CIGS solar cells. NGQDs with an ultra-high PL QY and a large Stokes shift were synthesized by a newly developed method by controlling the doping concentration and the N and O moieties. It has been demonstrated that the NGQDs could function as downconverters for CIGS solar cells. The conversion efficiency of the CIGS solar cell increased to 15.31% via downconversion and the light-trapping effect induced by employing the NGQDs. This technology can be used for the fabrication of high-efficiency and cost-effective CIGS solar cells. Supporting Information Supporting Information is available from Online Library or from the author. Acknowledgements This research was supported by the Energy Technology Development Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP, Grant No. 20163010012570), Republic of Korea. Conflict of Interest 11 ACS Paragon Plus Environment

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Figure 1. a) Schematic illustration of synthesis process of NGQDs. b) Proposed reaction mechanism for NGQD synthesis.


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Figure 2. a) AFM image. b) TEM image. c) HRTEM image. d) Raman spectrum. e) Absorbance spectrum. f) PLE and PL spectra. g) XPS C 1s spectra. h) XPS N 1s spectra. i) XPS O 1s spectrum.


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Figure 3. a) Jablonski diagram (top) representing the energy levels of NGQDs along with associated absorption, PLE, and PL spectra (bottom). b) Schematic illustration of photon downconversion process using NGQDs. c) Illuminated J–V characteristics of bare and NGQDcoated CIGS solar cells. d) EQEs of bare and NGQD-coated solar cells, and the related change in EQE after insertion of NGQDs.


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Figure 4. Cross-sectional view of TEM (inset HRTEM) image of NGQDs coated CIGS solar cell.


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Figure 5. Values of a) Jsc and Voc. b) FF and η. c) Rsh and Rs. d) n and J0 of bare and NGQDcoated solar cells.


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N-functionalized graphene quantum dots with ultra-high quantum yield

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