Karan Suranaab,
R. M. Mehraa,
Saurabh S. Soni
b and
Bhaskar Bhattacharya*ac
aCentre of Excellence on Solar Cells & Renewable Energy, School of Basic Sciences and Research, Sharda University, Greater Noida – 201310, U.P., India
bDepartment of Chemistry, Sardar Patel University, Vallabh Vidyanagar, Anand – 388120, Gujarat, India
cDepartment of Physics, Mahila Mahavidyalaya, Banaras Hindu University, Varanasi – 221005, U.P., India. E-mail: bhaskar.phys@bhu.ac.in
First published on 6th January 2022
The need for replacing conventional sources of energy with renewable ones has been on a swift rise since the last couple of decades. In this context, the progress in third-generation solar cells has taken a good leap in the last couple of years with increasing prospects of high efficiency, stability, and lifetime. Quite recently, a new form of carbon has been discovered accidentally in the form of carbon quantum dots (C QD), which is being pursued actively owing to its chemical stability and luminescent properties. In the current work, we report highly luminescent C QD prepared via a simple hydrothermal route. Transmission electron microscopy revealed an average particle size of 3.4 nm. The prepared C QD were used in a co-sensitized solar cell, where an improvement in the device characteristics was observed. The enhancement in the device characteristics is supported by impedance and electron life-time analysis. Further, the time-dependent analysis of the current and voltage revealed the functioning of the solar cell in real-time condition.
Carbon has been the epitome of smart materials since the discovery of graphene.10 Quite recently, a new colourful form of carbon has been discovered accidently in the form of carbon quantum dots (C QD).11 These carbon dots have been pursued intensively by scientists and fellow researchers chiefly due to their chemical stability, biocompatibility, low toxicity, and luminescent properties, which have led to its application in optical bioimaging, light emitting diodes (LED), sensors, photocatalysis, electrocatalysis, and solar energy conversion.12–21 Interestingly enough, the photoluminescence properties of these C QD does not arise from the quantum confinement effect. Rather, it is the result of the surface passivation or the dopant material(s). For instance, Liu et al. introduced S and N dopants in lotus root powder before subjecting them to hydrothermal treatment, which resulted in yellow and blue luminescent particles, respectively.22 Lim et al. used ethylenediamine as the N source with citric acid to prepare C QD by microwave irradiation for tuning the band-gap from blue to green.23
In the current work, we have synthesized aqueous carbon QD via a simple hydrothermal route, exhibiting excellent luminescence under long-range UV light. High-resolution transmission electron microscopy (HRTEM) images revealed an average particle size of 3.4 nm. The prepared C QD were used in a co-sensitized type solar cell, where an improvement in the solar cell characteristics was observed. The enhancement in the cell characteristics is supported by impedance and electron life-time analysis. Furthermore, the time-dependent analysis of the current and voltage were carried out using electrochemical station and also using an external load, which revealed the solar cell functioning in real-time condition.
Fig. 2 shows the QD photographed under long-range UV light (365 nm). Since all of the QD possessed the same transparency under daylight, only one of the samples is shown. Unarguably, the luminescence of the QD increases with an increase in nitrogen doping. As can be observed, CQ1 fails to show prominent luminescence. However, the change in colour from transparent (under daylight) to indigo is visible, and it gradually progresses to brighter light blue or aqua in the rest of the QD.
Fluorescence (FL) spectroscopy of the C QD was carried out with a PerkinElmer LS-55 instrument using an excitation wavelength of 290 nm, and the obtained result is shown in Fig. 3. CQ1, which showed minimal UV-vis absorption, gives two uneven emission peaks in FL spectroscopy. In the rest of the QD, a small initial emission peak below 400 nm appeared. In order to properly assess the emission peaks, Gaussian fitting for multiple peaks was applied using Origin 7.0 software. The obtained two distinct emission peaks of each QD are shown in Fig. 4, while the peak positions are tabulated in Table 1.
FL spectra | Gaussian fitted FL spectra | |||
---|---|---|---|---|
Peak 1 | Peak 2 | Peak 1 | Peak 2 | |
CQ1 | 403.42 | 458.67 | 403.57 ± 3.93 | 468.26 ± 2.28 |
CQ3 | 386.58 | 468.42 | 386.35 ± 0.42 | 470.95 ± 0.12 |
CQ5 | 382.80 | 461.92 | 385.20 ± 0.55 | 462.94 ± 0.23 |
CQ7 | 374.70 | 458.67 | 383.77 ± 0.88 | 458.10 ± 0.24 |
CQ9 | 369.26 | 457.61 | 367.36 ± 0.36 | 461.66 ± 0.12 |
The reason for the strong emission of these QD could be attributed to the presence of surface states owing to the conjugated bonds. The presence of various functional groups leads to different surface state energy levels, which are responsible for the emission characteristics of the C QD.32 The emission peaks of CQ3 to CQ9 are quite uniform in nature, which is suggestive of consistency in the structure of the QD. The full width at half maximum (FWHM) of the peaks are rather wide, which suggests a wider distribution in the particle size of the QD. Overall, the emission peaks suggest the formation of high-quality C QD. As can be observed from Table 1, the emission peaks estimated from Fig. 3 are pretty close to the ones obtained from Gaussian fitting. The peak 1 position in the FL spectra, as well as the Gaussian fitted counterpart, are found to shift gradually towards the lower wavelength, which could be due to the decrease in size of the QD or increase in their surface states. At this stage, either of the conditions shall result in shifting of the emission peaks towards higher energy. However, with the exception of CQ1, the peak 2 of FL spectra also showed a gradual blue shift.
The functional group analysis of the QD was carried out using Agilent Cary 630 Fourier Transform Infrared (FTIR) spectroscopy technique within the range of 600–4000 cm−1. For this a drop of the sample was made to come in contact with the probe. Since the QD are in aqueous state, the presence of the O–H stretch at ∼3350 cm−1 was inevitable, as shown in Fig. 5. It also suggests the presence of the N–H stretch. The sharp peak at ∼1640 cm−1 denotes the CC stretch, N–H bend and C
N stretching. The structures of citric acid and uric acid are shown in Fig. 6.
There is no hint of the presence of any carboxylic acid or any ring type formation of the C QD. The characteristic peaks of citric acid appearing at ∼1720 cm−1, corresponding to CO, and at ∼1100 and 1210 cm−1, corresponding to C–O, are not present in Fig. 5. Furthermore, the C–N stretch and bending peaks characteristic of uric acid found at ∼1028, 1124 and 1310 cm−1 are also absent. Hence, this clearly eliminates the possibility of the precursors being present in their raw form, and confirms the presence of only nitrogen-doped C QD.
The Raman spectrum of the C QD was measured using micro-Raman model STR 500 with an excitation laser source of 532 nm. Since the FTIR analysis showed almost identical result for the different C QD, the Raman analysis was carried out for the highly fluorescent CQ9 only. The obtained result is shown in Fig. S4 (ESI).† It may also be noted that the intensity of the laser source was reduced to 0.1% for obtaining the result, owing to the extremely high fluorescence emission. Moreover, as reported previously, the used laser source has an impact on the band's positions of the carbon materials.33 A broad D band is observed at 1369 cm−1 with a shoulder 2D band appearing at ∼3014 cm−1. Such broad bands originated from the presence of defects (surface states in the case of QD). As reported previously, when the number of defects in a material are large, it starts behaving like an amorphous one.34 Fig. 5 indicates no presence of any ring type/graphitic peak. Hence, the G band might be completely absent. In such a scenario, the QD has only surface defects instead of any defined geometry, thereby leading to the occurrence of only defect bands in the Raman analysis.
The CV was carried out in three conditions – dark, 1 sun condition (light), and long-range UV (365 nm), with 9 sweep segments in each. The CV of CQ5 is shown in Fig. 7 with an expanded region in the inset. The CV of the rest of the QD is given in the ESI (Fig. S1–S3†), except CQ1 since it did not give any observable pattern.
Under dark condition, the segments are perfectly overlapping with one another, with no change in the maximum value of current at −2 V or +2 V. However, under UV and light condition, the QD show its photon responsive nature, owing to the maximum value of the current increasing after each segment. The obtained maximum value of the current is higher in the light condition compared to that under UV light, which affirms its responsiveness to visible light. The redox potential of the QD is almost overlapping in all three conditions, which confirms the absence of any permanent physiochemical change in CQ5 because of light exposure. The first oxidation peak is obtained at −0.65 V, followed by a miniature one at 0.56 V. The corresponding reduction peaks were obtained at −0.25 V and 0.44 V. It is expected that the obtained wide electrochemical window should serve well for its application in solar cells. The CVs of CQ3, CQ5, and CQ9 shown in Fig. S1, S2, and S3,† respectively, depict similar responses under the given conditions.
Solar cells | Voc (mV) | Jsc (mA cm−2) | FF (%) | η (%) | Rs (Ω) | Rt (Ω) | Rc (Ω) | τ (ms) |
---|---|---|---|---|---|---|---|---|
S1 | 784 | 0.61 | 45.1 | 0.22 | 20.96 | 48.89 | 493.80 | 5.1 |
S2 | 820 | 1.10 | 59.8 | 0.54 | 33.19 | 62.16 | 513.60 | 8.2 |
To further check the stability of the cells, their J–V characteristics were analysed after an inactive period of 5 months, and the obtained characteristics are shown in Fig. 9. A significant improvement in the Voc of the two cells can be seen, which would have most probably happened in the initial 72 h (electrode formation period in polymer-based cells).24,37 Since the cell was used after a dormant period of 5 months, the formation of a strong polarization cloud over the electrolyte was unavoidable and the same is visible in the J–V pattern. The maxima in current were observed at much higher potential, and almost two times higher than the Jsc values. In fact, this maximum value of current should be extrapolated to the 0 V and noted as Jsc, which eventually shall give much higher FF and efficiency. However, to avoid any confusion, we have used the measured value of Jsc and only indicated the possible Jsc values by a dotted line in the figure. Nevertheless, the cell was active with almost similar efficiency even after a dormant period of 5 months.
The impedance analysis was also carried out after a period of five months, and an increase in both Rt and Rc can be observed. The long inactive period would have led to the shrinkage of the polymer chain in the device, thereby leading to the loss of flexibility, resulting in increased junction resistance. However, the impedance pattern stayed the same, which suggests no permanent physical or chemical alternation in the device.
The Bode plot was derived from the impedance data, and is shown in Fig. 11. The electron lifetime could be estimated by the following formula (1).
τn = 1/2πfmax | (1) |
The calculated electron lifetime for the two solar cells S1 and S2 was 5.1 ms and 8.2 ms, respectively. The increased electron lifetime in S2 could be attributed to the decrease in recombination with an additional photon trapping layer of C QD, which not only helped in enhancing the band-alignment, but also accelerated the electron transfer process.
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Fig. 12 (A and B) Current vs. time of S1 and S2 in light condition (inset – dark condition) (C and D) voltage vs. time of S1 and S2 in light condition ((inset – dark condition)). |
As discussed previously, S2 has an additional layer of only C QD compared to S1. In the current vs. time analysis (Fig. 12A and B), both cells start showing a similar trend where the initial high current drops within a matter of seconds, followed by an increase. The time taken by S1 to achieve the peak current is ∼135 s, following which, it declines a little before the current saturates. On the other hand, the time taken by S2 to achieve peak current is ∼55 s, following which, it declines before saturating. The minor rise and fall in current value between time 75 s to 300 s may be due to the small fluctuations in the halogen lamp intensity. However, the same effect is not visible in S1, which suggests a higher level of radiation sensitivity of S2.
Furthermore, in the case of the dark condition of the two cells shown in the inset of Fig. 12A and B, both cells show a fast decrease in current initially, following which, S1 shows a gradual smooth decline. However, S2 shows a minor rise in current before progressing towards saturation. Owing to the presence of QD, the decrease in the value of the current with time is not as smooth compared to S1. This is due to the distinct ability of the QD to harness the infra-red (IR) radiation.38,39
In the case of time-dependent voltage analysis, as shown in Fig. 12C and D, S1 shows an initial fast rise in voltage before gradually progressing towards a peak, following which, it drops down slowly to the peak voltage obtained during the initial few seconds. S2, on the other hand, shows a gradual increase in voltage with time. Beyond 175 s, the fluctuation is again present due to the slight change in halogen intensity, which if neglected, then we can see that the voltage is almost saturated. Similar to the current analysis, the voltage of S1 under dark condition shows a gradual decrease before saturating down to a few mV. On the other hand, the voltage does not show a smooth decrease in the case of S2 owing to the presence of QD. Another important thing to note is the starting voltage and current under dark condition in both cells. S2 is much more sensitive to radiation, and therefore starts at much lower voltage and current. The presence of voltage and current even in dark condition was conventionally not desired for solar cells. However, at present, cells with such properties are welcomed and explored for the possibility of use at lower light conditions.
The cell parameters (current and voltage) were monitored with time with the intention to understand if any permanent effect due to polarization had occurred during the exposure and/or measurements. The experimental setup for the measurement is shown in Fig. 13. An external circuit was prepared on a bread board using a carbon resistor of 2.4 kΩ, aluminium wires and two multi-meters. No other sophisticated instrument was employed for measurement.
The current and voltage at different time intervals were measured using a multimeter under continuous irradiation with a halogen lamp at 1 sun condition with no external biasing. The peak temperature attained for the surrounding air was 56 °C. For avoiding any rise in temperature, a cooling fan was adjusted near the setup for blowing normal air around the setup. The obtained results for S1 and S2 are shown in Fig. 14A and B, respectively. After an almost instantaneous rise in the current and voltage, a small drop is noticed, followed by a rise, which ended to a stable value. This real-time activity of the solar cells is quite helpful in understanding their working.
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Fig. 14 Current density and voltage measurement with rest to time under real-time conditions for (A) S1 and (B) S2. |
The initial rise that appears as a sharp peak in the figures is due to the obvious photo-response of the cells upon illumination. The subsequent drop in current is attributed to the electrode formation, which is a well-known phenomenon, especially in polymer electrolyte-based devices, and has been observed in solar cells as well.40 The gradual increase to a stable value is the actual time-dependent performance of the cells. This increase in current includes the effects of an increase in the ambient temperature upon continuous exposure of light, and also due to the stabilized folding–unfolding of the polymer chain. Once all of these activities get stabilized, the current attains a stable value. The voltage across the cells has also been noted and plotted against time (shown in the same Fig. 14 A and B). A similar explanation stands for the photo-voltage also. As can be seen from Fig. 14, around 60 min is required for the device to start giving stable output values. The values of the current and voltage are almost comparable in both S1 and S2.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/d1ra07634e |
This journal is © The Royal Society of Chemistry 2022 |