Fluorescence “Giant” Red Edge Effect
7.3 Carbon quantum dots in organic solar cells
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.
most successful OSCs are in bulk heterojunction device architecture where acceptor and donor materials are mixed at the nanoscale level and placed between cathode and anode[21,22]. Overall, the operation of the device remains the same in all the device structures. The incident photons are absorbed by the active materials and generate excitons (loosely bound electronhole pairs) which are dissociated at the donoracceptor interface followed by transport to the respective electrode for the generation of electricity. Till date, the major challenges of OSCs have been their efficiency and stability to which incorporation of CQDs has contributed consider- ably. Complex production, a tendency to aggregation, and heterogeneous sizes of the organic molecule have made interfacial layers kinetically unfavorable for charge transportation, thus making most of the organic photovoltaic devices poorly per- forming devices. Despite that, Yang et al. reported a low-cost and low-cytotoxic inverted polymer solar cell with CQDs as the cathode interlayer (CIL) and the photoactive layer of PTB7-Th:PC71BM[23]. The device achieved a PCE of 8.13%, outperforming the control device without CQDs (4.14%). Parameters likeVOCand FFincrease with the incorporation of CIL, thus enhancing electron extraction and hole-blocking. Values of series resistance (RS) and the shunt resistance (RSH) con- firm this speculation and hence justify the results [23]. In another research, Yan et al. reported first-time synthesis of OSCs with graphitically structured fluorescent CQDs via chemical vapor deposition [24]. Synthesized material showed excellent crystallinity and the solar cell was further fabricated using the solution-processed CQDs as electron transport layer (ETL) as depicted in device structure (Fig. 7.6A) with the corresponding energy band structure (Fig. 7.6B). Device exhibited almost similar optimized PCE as that of P3HT:PC61BM, PTB7:PC61BM, and PTB7-TH:
PC71BM. After integration of CQD ETLs, devices reached to optimum PCE of 3.11%, 6.85%, and 8.23%, respectively.Fig. 7.6Cshows theJVcurves of PTB7- Th:PC71BM devices with various ETLs. It is clear from the curves that CQDs incorporated devices exhibited enhanced device performances when compared to the reference device untreated with ETL or methanol whereas its performance was comparable with LiF-based devices. The enhanced device performances are due to the low series resistance in the devices with CQDs, exhibiting a superior interfacial contact between the contact electrode and polymer by taking advantage of this ETL. Their study revealed that CQDs-based devices are thermally stable at 80C for more than 150 hours, which is three times larger than LiF-based devices as shown inFig. 7.6D. As it is quite known that Li2and F2diffuse very quickly into the organic part, but it is very difficult for CQDs to diffuse because of higher molecular dimension. The CQDs could be used as ETL in organic solar devices with superior thermal stability.
Lim et al. reported improved photovoltaic device performance of the inverted polymer solar cells after using hybrid CQDs and absorption polymer materials.
CQDs ease carrier extraction in the PV structures, and thus increase PCE by 30% at around 3.3% on use of 0.05 weight % of carbon compared with that of the reference device in PCE[25]. Cui et al. reported fabrication of polymer solar cell with device architecture ITO/ZnO/P3HT:C-CQDs/MoO3/Al and ITO/ZnO/P3HT:C-CQDs:
PC61BM/MoO3/Al with varying concentration of CQDs. The PCE went up to 0.29% and 3.67% with incorporation of CQD in P3HT:CQDs and P3HT:CQDs:
PC61BM configuration at varying carbon concentration of 5% and 7.5%, respec- tively. This increment could be due to the increased absorption and decreased impedance after incorporation of CQDs[26].
A highly efficient tandem solar device, consisting of hole transport layer (HTL) and polyethyleneimine (PEI) polyelectrolyte, was reported by Kang et al. in single junction. Efficiency up to 9.49% was recorded after CQDs-doped PEI was used as ETL in between ITO and photoactive layer which is almost 10% higher than only pristine PEI-based solar cell [27]. Device structure (schematic) of tandem organic solar device with CQDs is shown inFig. 7.7A, and cross-section TEM of the real tandem solar cell to identify different constituted layers with proper estimation of thickness of each layers is shown inFig. 7.7B. The JV characteristics of single- junction and tandem solar device with and without CQD doping on PEI are shown inFig. 7.7C, and the corresponding EQE spectra of devices is shown inFig. 7.7D.
For the tandem solar cell, the PCE rose as high as 12.13% using CQDs-doped PEI as intermediate tunnel junction connection layer between two sub cells, which is approximately 15% higher than only pristine PEI as a layer. The reason behind the
Al C-CQDs
Active layer PEDOT
:PSS ITO
Substrate
Current density J(mA/cm )
no ETL methol LiF 0.05 mg/mL C-CQDs 0.05 mg/mL H-CQDs 0.1 mg/mL H-CQDs 0.1 mg/mL C-CQDs 16
8
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0.4 0 20 40 60 80 100 120 140 160 180
Nor-PCE
Bias V (V) Time / h
Lif methol 0.05 mg/mL C-CQDs 0.1 mg/mL C-CQDs -4.8
-5.10 -5.15 -5.38
-5.91-6.10
-7.00 -5.0
-3.0-3.31 -3.81
-4.01 -4.20
-4.2 -3.84
Al
Energy level (eV) PEDOT:PSS
ITO
P3HT PTB7 PTB7-Th C-CQDs
PC61BM PC71BM
(A) (B)
(C) (D)
Figure 7.6 (A) Structure of device and (B) energy level alignment diagram of the solar device with CQDs as ETLs and P3HT:PC61BM, PTB7:PC61BM, or PTB7-Th:PC71BM as active layers. (C)JVcurves of PTB7-Th:PC71BM-based devices with different ETL. (D) Normalized PCE degradation of P3HT:PC61BM devices with various ETLs at the temperature of 80C.CQDs, Carbon quantum dots;ETL, electron transport layer;PCE, power conversion efficiency.
Source:Reproduced with permission from L. Yan, Y. Yang, C.-Q. Ma, X. Liu, H. Wang, B.
Xu, Synthesis of carbon quantum dots by chemical vapor deposition approach for use in polymer solar cell as the electrode buffer layer, Carbon (109) (2016) 598607.
increment is due to electron extraction properties in single-junction solar devices and improved series connection in tandem devices. In another research, Liu et al.
reported improved energy transfer and charge transport property with CQDs in polymer solar cells in the inverted configuration. The PCE of doped devices increased to 7.05%, an improvement of around 28.2% compared with that of con- trast devices. Significant enhancement in FF and a slight improvement inJSCwere observed after the use of CQDs[29].
Wang et al. synthesized N, S-doped CQDs (N,S-CQDs) and used them for photo- voltaic application with ZnO ETL and got higher power conversion efficiency of around 9.31% without S-shape kink in the current density2voltage curves as of in reference device. The efficient surface modification for ZnO is to downplay light soaking effect in inverted OSCs. After placing of the N,S-CQDs on ZnO, roughness and surface energy reduce, thereby facilitating the transport and collection of photo- generated carriers[30]. In another research, an increase in PCE was reported by Lim et al. after incorporation of CQD/polyethylenimineethoxylated (PEIE) composites at interfacial layer with an electron extraction property in PTB7:PC71BM-based solar cell
EQE(%)
0 80
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0 3x104
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Current density(mA/cm2) PL Intensity(a.u.) -Z′(Ω)
PCE=7.59%
PCE=9.64% Reference
Reference Reference
6000rpm
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EEL
Voltage(V) Wavelength(nm) Z′(Ω)
J (mA/cm2)
(A) (C) (D)
(E) (F) (G)
(B)
Figure 7.7 (A) Tandem cell architecture. (B) TEM image of tandem cell with CQD-doped PEI cross sectional view. (C)JVcurves of single-junction and tunnel-junction tandem cell with and without CQD-doped PEI. (D) Corresponding EQE spectra of devices[27]. (E)JV plots of organic solar device in inverted structure with and without with CQD EEL on ZnO layer. Inset shows illustration of typical device structure. (F) Photoluminescence spectra for samples with layer structure of ITO/EEL(40 nm)/PTB7(50 nm)/MoO3(5 nm)/Al(10 nm) under photoexcitation of 500 nm at room temperature. (G) Nyquist plot of the CQD-based devices and reference devices[28].CQD, Carbon quantum dot;EEL, electron extraction layer;EQE, external quantum efficiency;TEM, transmission electron microscopy.
Source:(AD) R. Kang, S. Park, Y.K. Jung, D.C. Lim, M.J. Cha, J.H. Seo, et al., High- efficiency polymer homo-tandem solar cells with carbon quantum-dot-doped tunnel junction intermediate layer, Advanced Energy Materials (8) (2018) 1702165. (EG) Reproduced with permission from R. Zhang, M. Zhao, Z. Wang, Z. Wang, B. Zhao, Y. Miao, et al., Solution- processable ZnO/carbon quantum dots electron extraction layer for highly efficient polymer solar cells, ACS Applied Materials & Interfaces (10) (2018) 48954903.
got PCE of 8.34%[31]. They have reported that the significant improvement in device performance is due to the strong ultraviolet (UV) and visible range (300780 nm) light absorbance due to CQDs reflected in the external quantum efficiency measure- ment. Inverted polymer solar cell with PCE of 9.64% has been reported by Zhang et al. applying CQDs electron extraction layer (EEL) on ZnO layer, which is 27%
higher than control device as shown inFig. 7.7E; inset shows typical device structure [28]. The contribution of increment is due to the incorporation of bilayer ZnO/CQDs EEL which suppresses the exciton quenching by ZnO passivation of the surface defects in EEL leading to an improved dissociation of an exciton, reduced recombina- tion of charge as manifested in the PL spectra ofFig. 7.7F, and thus resulting in higher JSCin CQDs-based devices. It was further verified by the electric impedance spectros- copy measurement through Nyquist plots displayed in Fig. 7.7G at zero bias under dark conditions. In comparison with the reference device, OSCs with ZnO/CQDs layer display a smaller diameter as compared to the reference device, indicating a very low contact resistance and lower transport resistance which results to an efficient photo- generated carrier extraction probability.