Fluorescence “Giant” Red Edge Effect
7.2 Carbon quantum dots in dye-sensitized solar cells
Dye-sensitized solar cells (DSSCs) have attracted much attention as a viable, promis- ing, cost-effective thin-film technology since it was first reported by Regan and Gr¨atzel in 1991[46]. A typical DSSC consists of titanium dioxide (TiO2) nanoparti- cles coated photoanode, light-absorbing dye molecules which generate photoelec- trons, an electrolyte containing a redox couple for electron transfer, and a counter electrode (CE). The sensitizer or dye plays a crucial role in the overall process.
Therefore, various synthetic dyes as well as natural dyes and quantum dots as sensi- tizer have been explored extensively. At present, most successful DSSCs typically
use ruthenium complex as a photosensitizer with so far maximum power conversion efficiency of 11.9%. However, these dye molecules are very expensive due to their cumbersome synthesis process and use of rare earth metals. This led the scientific community to look for a suitable and environmentally friendly alternative. In this direction, CQDs are one of the premier alternatives. For the first time, Mirtchev et al.
used CQDs as a sensitizer in DSSC[7]. After which, it got wide recognition and was used in every component of the DSSC device, such as a sensitizer, cosensitizer, elec- trolyte, and CE for improved performances which are briefly discussed here.
7.2.1 Carbon quantum dots as sensitizer
CQDs derived from natural extracts have been investigated as sensitizers in DSSC application due to their strong absorbance in the visible light spectrum. CQD-based dye has been studied as a potential replacement of the highly toxic and costlier ruthenium-containing dye. Until now, several studies have reported using CQDs as a sensitizer as well as a cosensitizer in combination with other materials. The theo- retical simulation has been performed by Yan et al. and it is calculated that their highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels were 25.3 and 23.80 eV, respectively [8]. These values indicate that the CQDs have the potential to be used as a sensitizer in DSSCs because it perfectly matches with the energy levels of TiO2and electrolyte.
In a typical quantum dot-based DSSCs, photosensitizer CQDs absorb photon energy and reach to the excited state which is demonstrated in Fig. 7.1. The excited
Figure 7.1 Schematic device structure of CQD-based dye-sensitized solar cell.CQDs, Carbon quantum dots.
sensitizer transfers photoelectron to the external electrode via the semiconductor which is mostly TiO2. On the other hand, the oxidized sensitizer is getting back to the neutral state by accepting one electron from I32/I2electrolyte which is received through the platinum CE (Pt). In 2011, Mirtchev et al. demonstrated first water sol- uble colloidally stable CQDs as a sensitizer in DSSCs with power conversion effi- ciency (PCE) of 0.13% withVOCof 380 mV,JSCof 0.532 mA/cm2, andFFof 64%
as proof of concept. They found that CQDs consist of Sp2 hybridization core capped with hydroxyl, carboxyl, and sulfonate groups through core and surface properties analysis via Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance (NRM), and X-ray photoelectron spectroscopy (XPS) studies.
This makes it function as an electron acceptor/donor, and provides an efficient con- ducting platform for electrons thus functioning as of electron funnel/carrier/bridge [7]. In a pivotal work, Guo et al. synthesized CQDs from bee pollen (B-CQDs), citric acid (C-CQDs), and glucose (G-CQDs) via hydrothermal methods and used as a sensitizer for solar cell application (Fig. 7.2A). The transmission electron microscopy (TEM) images of these three CQDs show spherical morphology with an average particle size of 2.4 nm (B-CQDs), 2.4 nm (C-CQDs), and 5.2 nm (G-CQDs) (Fig. 7.2BE). Fig. 7.2F shows the current densityvoltage (JV) curves of three kinds of CQDs-based DSSC devices along with uncoated pure TiO2. Among three of them, the highest efficiency of around 0.11% was achieved in the case of B-CQDs with higherJSCandVOCcompared to other devices[9]. The observed increase in VOC was due to the smaller size of the B-CQDs which was governed by the quantum size effect and the higher JSC value was due to broad absorbance spectra and strong electron transfer from B-CQDs. But in the case of C-CQD-based device performance is low though it was in similar size because of the higher surface defect and internal recombination which limits the photogener- ated carrier movement.
CQDs are also doped with nitrogen to generate a large of number of active sites which help in fast carrier transportation along with bandgap tunability for better solar cell performance. In another work, Huang et al. have reported DSSCs with PCE of around 0.45% and FF of 60% from nitrogen self-doped CQDs as sensitizer via the one-pot hydrothermal method fromAllium fistulosum[10]. The typicalJV curve of the nitrogen-doped CQDs (NCQDs) sensitized solar cells is shown in Fig. 7.2G. Wang et al. synthesized the green NCQDs via direct pyrolysis of citric acid (CA) and ammonia as shown schematically inFig. 7.3A [11]. They have dem- onstrated that the absorbance spectra of NCQDs can be tuned by controlling the mass ratio of reactants which was confirmed from XPS analysis. They observed that when mass ratio is 1:4 (ammonia:CA), the NCQDs process highest visible absorption which is shown in UVvis spectra of both NCQDs and nitrogen-free CQDs (Fig. 7.3B). It is due to the strong chemical reaction between excess ammo- nia and overheated NCQDs, which eliminates water molecules from the NCQDs surface.
The absorbance spectra of optimal NCQDs solution not only shows a strong peak of absorption at 335 nm but also shows broad absorption in visible spectra extended to 550 nm. In general, 335 nm peak corresponds to nπ transition of
C5O bonds and on the other hand broader absorption peak in visible range is due to presence of amino groups in NCQDs. NCQDs also show blue light under UV lamp (365 nm) irradiation as depicted in the inset ofFig. 7.3B. The enhanced visi- ble light absorbance in NCQDs is beneficial for DSSC application. It can be noted that the presence of N-dopant in NCQD creates additional energy levels betweenπ of carbon andπ of oxygen, which improves in the absorption of visible low energy photons and generates photoexcited charge carrier compared to n-free CQDs illus- trated in Fig. 7.3C. These processes in turn result in more photoexcited electrons that transfer to the conduction band of TiO2 resulting in enhanced photovoltaic
Bee-Pollen
Citric Acid
Glucose
EtOH Hydrothermal 180°C, 4 h
Carbon Quantum Dots
B-CQDs
G-CQDs C-CQDs 0.3
0.2
0.1
0.0
0.0 0.1 0.2 0.3 0.4 0.5 0.0
0.0 0.1 0.2 0.3 0.4
0.5 1.0 1.5
Pure TiO2
Voltage / V
Voltage(V)
Current Density / mA/cm-2 Current Density (mA cm-2)
Jsc=1.74
Voc=0.43 PCE=0.45%
FF=0.60 Sample
Pure TIO2 C-CQD G-CQD B-CQD
Jsc (mA/cm2)
0.0572 0.409 0.4 0.009 0.02 0.0297
0.11 0.414 0.603
0.535 0.726 0.375 0.461 0.082 0.148 0.33
Voc (V) FF PCE (%)
20 nm
5 nm 2 nm
10 nm
10 nm
(A) (B) (C)
(D) (E)
(F) (G)
CQD
100nm
Figure 7.2 (A) Synthesis scheme of CQDs from various sources. (BE) High-resolution TEM images of B-CQDs (B, C), C-CQDs (D), and G-CQDs (E). Scale bar in (B) 20 nm, (C) 10 nm, (D) 5 nm, (E) 2 nm. (F)JVcharacteristics of curves and main device parameters for CQD-sensitized solar cells. (G)JVcharacteristics of NCQDs-sensitized solar cells. Inset shows TEM images of CQDs and fistulosum.CQDs, Carbon quantum dots;TEM, transmission electron microscopy.
Source: (AF) Reproduced with permission from X. Guo, H. Zhang, H. Sun, M.O. Tade, S.
Wang, Green synthesis of carbon quantum dots for sensitized solar cells, ChemPhotoChem (1) (2017) 116119. (G) Reproduced with permission from P. Huang, S. Xu, M. Zhang, W.
Zhong, Z. Xiao, Y. Luo, Green allium fistulosum derived nitrogen self-doped carbon dots for quantum dot-sensitized solar cells, Materials Chemistry and Physicsater (240) (2020) 122158.
performance which is reflected in the JV curve of NCQDs-based DSSC device under 100 mW/cm2AM 1.5 G illumination intensity shown inFig. 7.3D. The over- all PCE of around 0.79% is measured with aVOCof 0.47 V, JSCof 2.65 mA/cm2, and FF of 62.5%. Fig. 7.3E depicts 34% incident photon-to-electron conversion efficiency (IPCE) of NCQD-based DSSCs with over 10% between 400 and 550 nm.
Further, Zhang et al. improved the efficiency of CQD-based DSSCs to 0.87% by in situ growing of CQDs of 26 nm size on a TiO2photoanode for effective charge transfer between CQDs and TiO2[12].
It has been noted that DSSCs with only CQDs as sensitizers exhibit poor PCE values as a consequence of narrow spectral response and weaker affinity amongst CQDs and mesoscopic TiO2. On the other hand, state-of-art ruthenium complex (N719 dye) also shows narrow spectral absorption and serious electronhole recombination issue. To overcome this issue, different research groups have explored combining CQDs with organic dye as a cosensitizer which ultimately
200 °C 3 h Pyrolysis
Citric acid Ammonia NCQDs
NCQDs
NCQDs
N-free CQDs
TiO2
VB CB
HOMO LUMO Amino Traps
300 400 500 600
Wavelength (nm)
Wavelength (nm) Voltage (V)
3.0
2.0 2.5
1.5 1.0 0.5
0.00.0 0.1 0.2 0.3 0.4 0.5 0
350 450 550 650
10 20 30 40 50
Absorbance (a.u.)Current Density (mAcm-2) IPCE (%)
η= 0.79%
V = 0.47 V J = 2.65 mA cm-2
FF= 62.5%
sc oc
(A)
(B) (C)
(D) (E)
Figure 7.3 (A) Preparation of NCQDs using citric acid and ammonia via direct pyrolysis method. (B) UVvis spectra of NCQDs and N-free CQDs. Insets showing the photographs of NCDs aqueous solution photographs under UV lamp irradiation (right) and day light (left).
(C) Schematic illustration of photoinduced transfer of electron between CQDs and TiO2. (D) JV curve and (E) IPCE spectra of nitrogen-doped CQDs (NCQDs)-sensitized solar cells [11].CQDs, Carbon quantum dots;IPCE, incident photon-to-electron conversion efficiency;
NCQDs, nitrogen-doped carbon quantum dots.
Source: Published under creative commons CC BY.
widened the absorption of light ranging from visible to near-infrared regions along with fast photogenerated carrier extraction as illustrated in Fig. 7.4A. Zhu et al.
reported enhanced PCE of 8.19% using cosensitizer of polyethylene glycol (PEG)- modified CQDs (PEG-m-CQDs) and N719 dye[14]. They have further pushed the efficiency to 9.89% with the same cosensitizer with transparent metal selenide CE modified with green light-emitting long persistent phosphors (LPPs). The main pur- pose of LPPs is to harvest scattered light from the surrounding and emit green photofluorescence for several hours at dark-light conditions. In another research, Zhao et al. demonstrated DSSCs with cosensitizer NCQDs and N719 dye.Fig. 7.4B shows the schematic device of cosensitized solar device, and the corresponding energy band diagram is shown in Fig. 7.4C which reflects proper energy levels alignment for fast photocarrier generation and better charge transport. Fig. 7.4D validates the optimalJVcharacteristic of N719 dye and also cosensitizer NCQDs/
N719 dye. They have reported DSSCs with PCE of around 9.29% using cosensitizer
CQDs Dye
Absorbance
Wavelength
N719 N300-CQDs/N719 20
15
10
5
00.0 0.2 0.4 0.6 0.8
Voltage (V) Current density (mA cm-2)
Vacuum Level (eV)
0 -1 -2 -3 -4 -5 -6 -7 -8
FTO
CB
LUMO
HOMO
VB TiO2 N719
CQDs I-/I3- N300-
e e
e
- -
-
-4.5 -4.2
-7.5 -5.45 -3.85
-3.12
-5.28 -4.9
h+ h+
(A) (B)
(C) (D)
Figure 7.4 (A) Schematic illustration of cosensitizer of CQDs with dye. (B) Schematic of the N300-CQDs and N719 cosensitized DSSCs device with a m-TiO2/LPP photoanode. (C) Corresponding energy level diagram to illustrate charge transfer process, distribution and charge transfer processes. (D)JVcurves of characteristics of optimized DSSCs with N719 dye and N300-CQDs/N719 under solar irradiation (100 mW/cm2, AM 1.5G).DSSCs, Dye- sensitized solar cells.LPP, long persistent phosphors.
Source: Reproduced with permission from Y. Zhao, J. Duan, B. He, Z. Jiao, Q. Tang, Improved charge extraction with N-doped carbon quantum dots in dye-sensitized solar cells, Electrochimica Acta (282) (2018) 255262.
of NCQDs with N719 dye. Whereas DSSCs with only N719 dye exhibit PCE of around 8.09% [13]. This significant enhancement is due to upconversion and fast hole extraction properties of NCQDs. Recently, Shejale et al. reported PCE of 8.78% using a cophotoactive layer of NCQDs and N719 dye. They have prepared photoanode by incorporating NCQDs in TiO2after that N719 dye was absorbed. In this process, NCQDs are efficiently anchored to TiO2via a large number of carbox- ylic group sites which enhances the photogenerated carrier transport[15]. The detail performances of photovoltaic devices with CQDs as sensitizer and cosensitizer are shown inTable 7.1.
7.2.2 Carbon quantum dots as counter electrode
The CE is another major component in DSSCs which especially contributes to the electrocatalytic performances. The standard CE utilized in high-performing DSSCs is platinum (Pt) which is deposited onto transparent conductive oxide coated glass sub- strate due to high catalytic activity toward iodine reduction. Even so, Pt is a very expensive material, so researchers are looking for alternative substitutes which are cheap and easily processable. However, CQDs have been extensively used as a pho- tosensitizer in DSSCs, due to their interesting electronic properties which make them a favorable candidate as additives with other materials in CE. Lee et al. synthesized exceptionally porous polyaniline (PANI) using carbon nanodots (CNDs) as a nucleat- ing agent and demonstrated their use as CE in DSSCs[18]. High number of porous sites increases the reactant diffusion for improved electrocatalytic activity. CNDs are caped with highly active aniline which ultimately increases the surface area, facili- tates the generation of head-to-tail dimers, and improves the degree of para-coupling Table 7.1 Performance of photovoltaic devices with carbon quantum dots as sensitizer and cosensitizer.
Role Materials VOC
(V)
JSC
(mA/cm2) FF (%)
PCE (%)
References
Sensitizer CQDs 0.38 0.53 64 0.13 [7]
CQDs 0.461 0.33 72 0.11 [9]
NCQDs 0.43 1.74 60 0.45 [10]
NCQDs 0.47 2.65 63 0.79 [11]
CQDs 0.43 6.47 31 0.87 [12]
N719 0.708 17.3 64.9 8.09 [13]
Cosensitizer CQDs/N719 0.721 17.01 68 8.19 [14]
Cubic CQDs/N719
0.721 16.60 73.5 8.68 [16]
NCQDs/N719 0.736 18.6 67.9 9.29 [13]
S-CQDs/metal selenide CE
Sulfer-CQDa 0.715 0.771 16.6 9.15 [17]
in the molecular structure of PANI. Consequently, PANICND films show high electrical conductivity of 774 S/cm. The PANICND fabricated CEs in DSSCs exhibited PCE as high as 7.45% than those with only platinum (η57.37%) and pris- tine PANI CEs (η55.60%). Dao et al. reported the use of carbon dot-Au nanorasp- berries (Cdot-Au NRs) as CE in ZnO nanowire/CdS/CdSe(QDs) solar cell with 5.4%
PCE, whereas Au-sputtered CE exhibited PCE of 3.6% and C dot exhibited remark- ably low PCE of 0.18%. Such improved performance of the QDSCs with C dot-Au NR-based CE is accredited to its more larger surface area than that of Au-sputtered CE, resulting in an increased in number of the electrocatalytic active sites with improved charge transfer and reduced series resistance[19].
On the other hand, bifacial DSSCs have been fabricated to harvest solar energy from both sides of the device which is a major attraction to the scientific commu- nity because of optimum utilization of solar radiation within the same fabrication cost. In this case, semitransparent CE is required upon shining by sunlight; how- ever, the preferred Pt electrode has transparency below 40% in general, yielding an efficiency lower up to 3%4% in final solar cell devices. For bifacial DSSCs, CQDs can be an important player. Zhu et al. demonstrated that improvement in the bifacial DSSCs by the integration of CQDs with transparent CoSe is an effective strategy to enhance the catalytic activity of a CE and rear efficiency of correspond- ing bifacial DSSCs[20].Fig. 7.5Ashows the schematic device diagram of bifacial DSSCs, and the corresponding energy band diagram is shown inFig. 7.5B. Upon illumination through the front side, the photons that enter from the fluorine-doped tin oxide (FTO) glass are absorbed by organic N719 dye molecules to release elec- trons to TiO2. These exciting dyes go back to the ground state after accepting an electron from I2/I32 electrolyte. When light enters through the rear side of the device, the photons pass through the transparent CE and excite the CQDs according to the same mechanism to transfer an electron to LUMO level of alloy CE. Overall, the CQDs perform three functions: first UV and near-infrared light absorption for downconversion and upconversion channel respectively; second, it helps CE for better electron collection from the external circuit; and third it serves as the better electron donor to accelerate the reduction from electrolyte in the oxidation state. To achieve this, high transmittance of rear CE is very much critical. As presented in Fig. 7.5C, both CQDs-CoSe and CoSe-only CEs have outstanding optical transpar- encies (.60%) at 4001000 nm and are almost slimmer. Fig. 7.5D shows JV curves of DSSCs devices with different CEs under dark and light illumination con- ditions from both sides of the devices with simulated sunlight irradiation. Using only CoSe tailed as CE, a front efficiency of 8.06% and a rear efficiency of 5.18%
were reported for the CoSe tailored solar cell, which are both at a high level for bifacial DSSCs. By using the CQDs-CoSe CE, the front efficiency is increased to 9.08% whereas rear efficiency went up to 7.01%. The enhancement in efficacy in the front is due to the reabsorption of unabsorbed visible light across the photoa- node and electrolyte by the CQDs, maximizing electron concentration at the CoSe electrode. Interestingly, about 35% enhancement of PCE in rear-side illumination of bifacial DSSC was observed due to excellent catalytic activity of the CQDs- CoSe CE. Zhang et al. also performed surface functionalization of CQDs to obtain
S-CQDs to enhance the electron density and used as CE in bifacial DSSCs with the front and rear sides PCE of 9.15% and 6.26%, respectively[17]. CQDs also used as an electrolyte to replace the volatile electrolyte have been looked upon so that its application for wider application is not hampered.