Fluorescence “Giant” Red Edge Effect
7.5 Carbon quantum dots in perovskite solar cells
Owing to their inherent structural property, perovskite-based solar cells are one of the most promising third-generation solar cells which have achieved a PCE of 25.6% in single junction just a little more than a decade since its inception [39].
The perovskite solar cell (PSC) comprises an active layer of perovskite material sandwiched between the electron and HTL on a transparent conducting oxide sub- strate with back metal contact at the other end. Depending on the arrangement of layers, PSCs could be of n-i-p or p-i-n configuration. In a typical PSC, incident photon energy absorbed by the perovskite materials generates exciton which gets separated within the perovskite layer or heterojunction interface to form electron and hole followed by transported through ETL and HTL, respectively. To facilitate the charge extraction and transportation processes, various strategies and techniques have been adopted which contribute to the efficient functioning of the overall device. On the other hand, CQDs have been widely used in PSCs as hole extraction material and almost research advancement revolves around it. CQDs, being a p-type semiconductor and its application as a HTL, have contributed significantly to the improved performance of the PSCs. The CQDs increase conductivity, reduce hysteresis, downshift band structure in material, and thus improve device perfor- mances. CQDs play a significant role in the passivating grain boundary, and increase crystallinity and ion mobility. Han et al. reported FTO/cp-TiO2/mp-TiO2/ MAPbI3-CQDs/carbon structure device.JVcurve of the device recorded the vari- ation in PCE from 10.7% to 13.3% with an increasing concentration of CQD.
The CQD not only played a vital part in hole extraction from MAPbI3 layer but also passivated trap states at MAPbI3-CDQs interface film, thus leading to reduced recombination and interface loss, and increased grain size and efficiency [40].
Recently, hydrothermally synthesized CQDs were used to get ITO/HTL/Perovskite/
PCBM/BCP/Ag,p-i-nconfiguration by Kim et al. JVcurves of devices with bare NiO HTL and NiO:CQD HTL exhibited highest PCE of 16.4260.392% at a ratio of 95:5[41]. Forward and reverse scan showed were recorded with an efficiency of 15.34% and 16.03% for bare NiO, and 16.75% and 16.91% for NiO:CQD::95:5, respectively. The hysteresis reduced significantly from 4.5% to less than 1% NiO.
After inclusion of CQDs energy band alignment of NiO down shifts providing a justified orientation to the ITO work function and band edges of the perovskite layer. Electrical characteristics of HTL got improved because of CQDs which assisted in the improved charge conductance and decreased the series resistance (Rs) of the PSCs to 6.12Ωcm2for NiO and 4.25Ωcm2for NiO:CQD. Long-term stability of PCEs was achieved under ambient atmospheric conditions, and the CQD-incorporated NiO-based PSCs retained B70% of its initial efficiency even after 192 hours. In another research by Paulo et al. group from Spain reported methyl ammonium lead iodide(MAPbI3) based perovskite solar devices with CQDs as HTL for improved PCE of 3%. The investigation reveals limitations such as poor perovskite coverage over the mp-TiO2surface lower efficiency as confirmed by environmental scanning electron microscopy (ESEM) analysis [42]. Hao et al.
reported an increase of almost 24.6% in PCE after optimized addition of 10 wt.%
CQDs in ITO/TiO2(CQD)/Perovskite/spiro-OMeTAD/Au cell architecture[43]. The champion device exhibitsVOCof around 1.14 V, aJSCup to 21.36 mA/cm2, a FF of 78%, with high PCE of 18.9%. This significant improvement in efficiency could be accredited to efficient charge carrier extraction and injection in PSCs, especially between the TiO2and perovskite layers. The CQDs increase the electronic coupling between the CH3NH3PbI3-xClxand TiO2ETL interface as well as energy levels that contribute to electron extraction. It was also observed that CQDs/TiO2combination improvesVOCandJSCsimultaneously when compared to standalone TiO2devices.
A theoretical study performed by Matta et al. reveals about efficient charge transfer mechanism between the C-Dot/PbI2 system interfaces when compared to that of CQDs/MAI system interface[44]. The study found that C-dots with functionalized -OH and -COOH moieties which acts as the potential hole transfer entity for PSCs.
It was also found that the bonding position on the C-dot impacts the bandgap and band edge positions. This opens up the avenue to explore and tune the band align- ment of C-dots to get desired value for other solar cells applications. The valance band maximum (VBM) levels for C-dots are found to be more suitable for the pur- pose of efficient hole transportation after considering the minimum driving force that is required for an effective hole transporting material. Zou et al. reported hole- transport material (HTM)-free CQDs incorporated PSCs. The observations revealed the effects of CQDs on the TiO2nanosheet-based and HTM-free PSCs performance.
At CQD content of 0.1% (optimized values) the PSC exhibited 60% higherJSCup to 16.40 mA/cm2and an efficiency of 7.62%. The reason behind this improvement could be due to excellent conductivity resulting from the heterogeneous nuclei of
CQDs in course of perovskite crystallization, which increases the number of perov- skite nuclei and forms a fine perovskite grain, for increased coverage on the sub- strate. Moreover, CQDs help in the efficient transportation of the photo-excited electrons, thus accelerating the separated and mobilized charge carriers[45]. Mat et al. fabricated the device with PCE of 18.24% withholding its initial PCE of around 73.4% after 48 hours of aging under 80% relative humidity dark room tem- perature condition. The results reveal the CQDs passivation mechanism on the grain boundaries and its role as the nucleation site to improve the crystallinity of the perovskite, thus reducing the recombination of charge carrier at the grain bound- aries. Carbonyl functional groups on CQDs could reduce down the perovskite crys- tal growth, resulting in larger size of perovskite grains. Moreover, CQDs can passivate the uncoordinated lead ions at grain boundaries of perovskite by func- tional groups such as hydroxyl and carbonyl to decrease nonradiative recombination [45]. Maxim et al. reported the downconversion effect with the use of CQDs to sub- stantially convert fraction of incident UV light to visible light for optimized photo- voltaic applications. After embedding CQDs on PMMA the PSCs exhibited improved PCE of 17.29% and 17.86% for forward and reverse bias, respectively, especially resulting increased FF and photocurrent density[46].
High efficiency of around 22.77%, has been attained by Hui et al. by the use of inexpensive carboxylic acid and red CQDs (hydroxyl-rich) (RCQs)-doped SnO2. This could be accredited to the doped ETL which has highest electron mobility for modified SnO2, i.e., 1.7331022 cm2/V/s. They have fabricated Cs0.05(MA0.17FA0.83)0.95Pb (I0.83Br0.17)3, a planar-type PSCs on both the SnO2and SnO2-RCQs ETLs with ITO/
SnO2/perovskite/spiro-OMeTAD/MoO3/Au structure which is depicted in Fig. 7.9A.
The device upon optimization displayed superior performance, withVOCof 1.14 V,JSC
of 24.1 mA/cm2, and FF of 83%, resulting is an excellent PCE reaching 22.77% as compared to 19.15% PCE of the control device PSC based on the SnO2at 1 vol.% of SnO2-RCQs ETL as shown in the typicalJVcurve inFig. 7.9B. The common prob- lem in the PSCs is hysteresis behavior in theJVcurve.Fig. 7.9CshowsJVhystere- sis of PSCs based upon SnO2-RCQs with a significant reduction within 1% as compared to pure SnO2ETL-based devices. This improvement is much likely due to improved electron mobility of RCQs-SiO2 ETL. The stabilized output (steady-state) with PCE of 21.50% andJscof 21.50 mA/cm2of SnO2-RCQs-based devices at which maximum power point (Vmax 1 V) was achieved for 120 hours of testing is shown inFig. 7.9D. The enhancement in EQE spectra of SnO2-RCQs-based PSC devices was reported over the entire visible range with integratedJSCof 22.7 mA/cm2as shown in Fig. 7.9E. The device shows environmental stability without encapsulation, retaining up to 95% of initial PCE at 25C after 1000 hours, when exposed to lesser relative humidity of 40%60% as compared to SnO2-based device which holding 80% as depicted inFig. 7.9F. The improved performance of SnO2-RCQs-based device is due to RCQs which reduces Gibbs free energy of SnO2surface and acts as nucleation cen- ter for perovskite crystal development. Alongside, RCQs can efficiently bond with incompletely coordinated Pb21 ions of the perovskites to support high-quality forma- tion of film which is depicted in Fig. 7.9G. The passivation conclusively leads to improved crystalline phase purity over large areas with enhanced uniformity and
reduced traps/defects at ETL/perovskite interface attributed to higher performances [47]. PCE of 8.29% was recorded by Zhou et al. after sensitization of cesium lead bro- mide (CsPbBr3) inverse opal PSC with CQD. The inorganic CsPbBr3I shows a slow photon effect from tunable bandgaps exhibiting optical responses novel in nature as it has a broad light absorption range, higher charge transfer rate, and facilitates electro- nhole extraction, thus resulting in improved PCE[48]. Lio et al. reported an increase in PEC of 12.08% and 28.7% after integration of CQDs and CQDs with red phosphorus quantum dots (RPQDs), respectively. This increase could be due to the setting up of intermediate energy levels around TiO2/CsPbBr3 and CsPbBr3/carbon interfaces with
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Figure 7.9 (A) Schematic of device structure used in this study. (B) Current density vs voltage (JV) plot of the champion perovskite devices based on SnO2and SnO2-RCQs ETLs under the illumination of 1 sun AM 1.5 G. (C)JVplot for forward and reverse scans in case of both the champion devices. (D) Photocurrent and efficiency of the PSCs on the SnO2-RCQs ETL at the at 1.0 V (maximum power point). (E) EQE and current density plot of both of the champion perovskite solar cells. (F) PCE (normalized) of PSCs based on two ETLs without encapsulation as function of time at 25C in ambient environment (dark) with relative humidity of 40%60%. (G) Perovskite crystal growth mechanism in RCQs doped SnO2(ETLs) precursor solution.EQE, External quantum efficiency;ETL, electron transport layer;PCE, power conversion efficiency;PSC, perovskite solar cell.
Source:Reproduced with permission from W. Hui, Y. Yang, Q. Xu, H. Gu, S. Feng, Z. Su, et al., Red-carbon-quantum-dot-doped SnO2composite with enhanced electron mobility for efficient and stable perovskite solar cells, Advanced Materials (32) (2020) 1906374.
CQDs and RPQDs, respectively. The interface modification increases charge extraction and passivation effect contributing to a highest PCE of around 8.20% and 7.14% for FTO/c-TiO2/m-TiO2/CQDs/CsPbB3/RPQDs/carbon configurations and FTO/c-TiO2/m- TiO2/CQDs/CsPbBr3/carbon configurations, respectively [49]. On the other hand, these all-inorganic PSCs are very stable more than 1056 hours and 80% relative humidity, which indicates a good environmental stability toward future commercialization.