Abstract
Provided here nontoxic CuGaS.sub.2/ZnS core/shell nanocrystals with free-self-reabsorption losses and large Stokes shift synthesized on an industrially gram-scale. The nanocrystals exhibited a typical energy-down-shift that absorbs only ultraviolet light and emits the whole range of visible light with a high photoluminescence-quantum yield. The straightforward application of these energy-down-shift nanocrystals on the front surface of a monocrystalline p-type silicon solar cell significantly enhanced the short-circuit current density and power conversion efficiency. The significant improvement in the external quantum efficiency and that decreasing in the surface reflectance in the ultraviolet region clearly manifest the photovoltaic enhancement. Such promising results together with the simple (one-pot core/shell synthesis), cost-effective, and scalable preparation methods might encourage the manufacturers of solar cells and other optoelectronic applications to apply these energy-down-shift nanocrystals to different broader eco-friendly applications.
Claims
1. A monocrystalline p-type silicon solar cell device comprising: an eco-friendly front layer of CuGaS.sub.2/ZnS core/shell nanocrystals layer, working as typical energy-down-shift layer to absorb only ultraviolet light and emit the whole range of visible light with a high photoluminescence-quantum yield; wherein the energy-down-shift layer has free-self-reabsorption losses and large Stokes; and wherein CuGaS.sub.2/ZnS core/shell nanocrystals have been synthesized on an industrially one-pot gram-scale. the straightforward application of this energy-down-shift layer on the front surface of a monocrystalline p-type silicon solar cell significantly enhanced the short-circuit current density and power conversion efficiency.
2. The CuGaS.sub.2/ZnS core/shell nanocrystals layer of claim 1, wherein the energy-down-shift layer absorbs ultraviolet light of wavelength lower than 407 nm.
3. The CuGaS.sub.2/ZnS core/shell nanocrystals layer of claim 1, wherein the energy-down-shift layer emits the visible light in the wavelength range of 400-800 nm.
4. The CuGaS.sub.2/ZnS core/shell nanocrystals layer of claim 1, wherein the energy-down-shift layer has photoluminescence-quantum yield of 76%.
5. The CuGaS.sub.2/ZnS core/shell nanocrystals layer of claim 1, wherein the energy-down-shift layer has a large Stokes shift greater than 190 nm.
6. The monocrystalline p-type silicon solar cell of claim 1, having a current density improved by 1.64 mA/cm.sup.2 (+4.20%).
7. The monocrystalline p-type silicon solar cell of claim 1, having an efficiency improved by 4.11%.
8. The monocrystalline p-type silicon solar cell of claim 1, having an external quantum efficiency increased by 35.7%.
9. The monocrystalline p-type silicon solar cell of claim 1, having a surface reflectance decreased by 14.1% in the UV region of 300-450 nm.
10. A method of synthesizing CuGaS.sub.2/ZnS Nanocrystals on an industrially one-pot gram-scale comprising the steps of: mixing a first mixture at least gallium iodide, copper iodide, 9-Octadecenylamine, and 1-dodecanethiol and heating to at least 100 degrees Celsius; injecting sulfur at least 160 degrees Celsius into said first mixture, forming a second mixture with a core of CuGaS.sub.2; injecting into said second mixture zinc sterate forming an opaque layer creating a third mixture; depositing said third mixture on a solar cell as the energy-down-shift layer said in claim-1.
11. The method of claim 10, wherein said mixing of said first mixture is carried out at or above 125 degrees Celsius.
12. The method of claim 11, wherein said injecting is at 180 degrees.
13. The method of claim 10, wherein said first mixtur ![text missing or illegible when filed]()
14. ![text missing or illegible when filed]()
e further comprises oleic acid.
15. The method of claim 13, wherein said first mixture further comprises 1-octadecene.
16. The monocrystalline p-type silicon solar cell of claim 1, further comprising a step of preparing said solar cell by immersing a p-type single-crystalline silicon substrate in potassium hydroxide.
17. The method of claim 15, wherein said step of preparing said solar cell further comprises adding phosphoryl chloride to said solar cells forming phosphorous silicate glass.
18. The method of claim 16, wherein an n-type layer is created on said solar cell with an emitter resistance of about 58 ohm per square.
19. The method of claim 10, wherein said depositing comprises depositing CuGaS.sub.2/ZnS nanocrystals solution by weight percentage between 0.3% and 0.5% on the front surface of said solar cell.
20. The method of claim 17, wherein said nanocrystals solution is CuGaS.sub.2/ZnS nanocrystals dispersed in an organic solvent.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
[0042] FIG. 1 is a schematic illustration of (a) energy losses in typical monocrystalline p-type silicon solar cells having no energy-down-shift quantum-dot. layer; and (b) energy-down-shift quantum-dot layer implemented on textured pyramid-like surface of monocrystalline p-type silicon solar cell.
[0043] FIG. 2 is a schematic illustration of the one-pot core/shell synthesis methods of CuGaS.sub.2/ZnS QDs.
[0044] FIG. 3 is the structural and morphological characterizations of as-synthesized CuGaS.sub.2/ZnS QDs. (a) X-ray diffraction (XRD) pattern (standard JCPDS files for zinc blende ZnS and Chalcopyrite CuGaS.sub.2 are shown on top and bottom, respectively) and corresponding selected area electron diffraction (SAED) pattern (inserted image). (b) High-resolution transmission electron microscopy (HRTEM) image with a scale bar of 5 nm, 2 nm (inset, upper right), and size distribution (inset, upper left).
[0045] FIG. 4 is the energy-down-shift mechanism of CuGaS.sub.2/ZnS QDs. (a) Absorption and photoluminescence spectra of CuGaS.sub.2/ZnS QDs, CIExy 1931 (inset, under PL spectra), and emitting light photography under UV-lamp of 365 nm (inset, right). (b) Energy-bandgap-alignment diagram of CuGaS.sub.2/ZnS QDs.
[0046] FIG. 5 is the photovoltaic performance of Si solar cells implemented with energy-down-shift layer of CuGaS.sub.2/ZnS QDs. (a) Short-circuit current density (J.sub.SC) and open-circuit voltage (V.sub.OC) curves. (b) Fill factor (FF) and power conversion efficiency (PCE).
[0047] FIG. 6 shows (a) the cross-sectional TEM image of EDS-QD layer implemented on mc-p-Si solar cell (scale bar=100 nm); and (b) the zoomed-in HRTEM image of EDS-QD layer in scale bar of 5 nm.
[0048] FIG. 7 is the short-circuit current density vs. open-circuit voltage (J-V) characteristics comparison between the solar cell sample coated with the optimal QD concentration (0.4 wt %) and bare sample.
[0049] FIG. 8 shows (a) the external quantum efficiency (EQE) of Si solar cells implemented with energy-down-shift layer of CuGaS.sub.2/ZnS QDs, and surface reflectance (inset) of 0.4 wt % QD concentration sample; and (b) the change ratio in the integrated UV region (300-450 nm) of EQE as a function of QD concentrations (0, 0.1, 0.2, 0.3, 0.4, 0.5 and 0.7 wt %).
[0050] FIG. 9 is the surface reflectance (SR) of Si solar cells implemented with energy-down-shift (EDS) layer of CuGaS.sub.2/ZnS QDs. (a) SR spectra in wavelength-range of 300-1100 nm and (b) integrated area of SR over a range of 300-450 nm (UV light) and 450-800 nm (visible light) as a function of QD-wt % concentrations.
[0051] FIG. 10 is the gram-scale synthesis of CuGaS.sub.2/ZnS QDs. (a) 11 gram of purified QD powder on a digital scale under UV-365 nm lamp. (b) Absorption and photoluminescence (PL) of QDs in chloroform as shown in the inset image of vial under UV-365 nm lamp. PL was excited by 325 nm-laser source.
DETAILED DESCRIPTION
[0052] To avoid excessive energy losses of UV photons by the surface scattering and reflection in solar cells (FIG. 1), an eco-friendly EDS-QD layer has been provided to be employed on the surface of the fabricated mc-p-Si solar cells as an efficient UV-harvesting down-converter to the whole range of visible light (FIG. 1). The Cd-free CuGaS.sub.2/ZnS QDs were prepared via the conventional wet colloidal synthesis using a simple one-pot core/shell method at low growth temperature ranging from 180 C. for the inner core to 250 C. for the outer shell under N.sub.2 atmosphere (FIG. 2). To investigate the structural and morphological characterizations of as-synthesized CuGaS.sub.2/ZnS QDs, X-ray diffraction (XRD), selected area electron diffraction (SAED), and transmission electron microscopy (TEM) were carried out (FIG. 3). In the wide-angle diffraction region (FIG. 3a), the XRD pattern revealed that the QDs possess a single-crystal chalcopyrite structure (referred to as the tetragonal phase). The characteristic (112), (220)/(204), and (116)/(312) planes of the chalcopyrite structure were observed with a slight shift toward lower 2 angles to be centered at 28.4, 47.9, and 56.5, respectively, approaching those 2 angles of the (111), (220), and (311) planes of the zinc blende ZnS structure (referred to as the cubic phase). This shift is highly characteristic of the successful formation of the outer ZnS shell. The SAED image (the inset of FIG. 3) also confirms the presence of the ZnS shell since the distinct rings clearly originated from (111), (220), and (311). The broad diffractive peaks observed in the XRD pattern are indicative of the small particle size of the as-synthesized QDs. The average crystallite size was estimated from the dominant (111) peak using Scherrer's formula to be 3.04 nm. It is apparent from the TEM micrograph (FIG. 3) that the well-dispersed and crystallized QDs have a spherical-like shape with a fairly uniform size of about 3.250.62 nm (the left inset of FIG. 3), which is in reasonable agreement with the estimated XRD result. The right inset high-resolution TEM (HRTEM) image (scale bar =2 nm) shows that the lattice interplanar spacing of an individual QD is 3.1 , which is slightly higher than the d.sub.112-spacing of CuGsS.sub.2 (3.0655 ) but much closer to the d.sub.111-spacing of ZnS (3.1261 ) with an interplanar angle of 71 providing further evidence of the core/shell structure (the right inset of FIG. 3).
[0053] The optical properties of the as-prepared CuGaS.sub.2/ZnS QDs were systematically characterized and optimized as EDS-QDs to harvest the wasted UV light from the sun and re-emit it as visible light for solar cell applications. The absorption and PL spectra of the CuGaS.sub.2/ZnS QDs, dispersed in chloroform, show that they absorb only UV light (<407 nm) and emit a wide range of visible light (400-800 nm) peaked at 544 nm (FIG. 4). The International Commission Illumination (CIE) color coordinates and emitting light photography of these QDs, excited by 325 and 365 nm, respectively, reveal that the QDs can emit in a bright saturated white color making these QDs also very attractive for optoelectronic applications such as LEDs and displays (the insets of FIG. 4). The successful growth of the inorganic ZnS shell was essential to passivate the outer surface of the CuGaS.sub.2 core to remove the surface defects (such as surface imperfections, trapping states, and dangling bonds); therefore, enhancing the optical properties of QDs, especially for the exciton radiative recombination. In addition, this inorganic shell acts as an effective energy barrier to confine the photo-excited excitons in the core due to type-I band offset alignment between the core and the shell; then limit the sensitivity of the core to lower-energy photons, as shown in the energy bandgap diagram (FIG. 4). The optical bandgap of the QDs was estimated from the direct-bandgap absorption using Tauc equation of the transformed Kubelka-Munk function..sup.[31] It is obvious that the ZnS shell (bandgap of 4.12 eV) has higher conduction band (CB) and lower valence band (VB) than that of the CuGaS.sub.2 core (bandgap of 3.1 eV); therefore, the shell layer enhances PLQY to 76%. More importantly, coherently with the above-mentioned absorption and PL properties, these QDs exhibit free-self-reabsorption losses due to the large Stokes shift (>190 nm). These results make the as-prepared QDs comparable to the recently reported Mn-doped Cd.sub.0.5Zn.sub.0.5S/ZnS QDs that had the phenomenon of zero self-reabsorption used for enhancing the efficiency of Si solar cells..sup.[11a, 12a] Accordingly, these CuGaS.sub.2/ZnS QDs can function as an effective EDS layer for solar cells.
[0054] To demonstrate the feasible application of these Cd-free CuGaS.sub.2/ZnS QDs and consequently the effect of this EDS-QD layer on the PV performance of solar cells, we first optimized the concentration of the EDS-QD layer to increase the absorption of solar cells. Six different concentrations of the optimized EDS-QDs (0.1, 0.2, 0.3, 0.4, 0.5, and 0.7 wt %) in chloroform were prepared to be coated on Si solar cells. The concentration of EDS-QDs has a considerable effect on the efficiency of solar cells, according to previous studies..sup.[11a, 12a] These six prepared QD solutions were deposited subsequently on the front surface of textured mc-p-Si solar cells, fabricated with a SiN.sub.x anti-reflective surface, using the doctor blade casting technique.
[0055] FIG. 5 illustrates the PV performance characteristics, open-circuit voltage (V.sub.OC), fill factor (FF), short-circuit current density (J.sub.SC), and PCE of the mc-p-Si solar cells implemented with eco-friendly EDS-QDs as a function of QD concentration. The current density-voltage (J-V) measurements were carried out with the AM1.5G solar simulator under standard test conditions at room temperature and irradiance of 100 mW/cm.sup.2. The J-V measurements were carried out for all solar cell samples before applying the EDS-QD layer to simplify the performance comparison. It was found that the solar cells coated with the EDS-QD layer with 0.4 wt % QD concentration had the highest J.sub.SC. Specifically, the J.sub.SC increased gradually from 39.07 to 40.71 mA/cm.sup.2 with the QD concentration to peak at 0.4 wt % then decreased rapidly to reach 38.51 mA/cm.sup.2 at 0.7 wt %. This optimal 0.4 wt % concentration, which formed an EDS-QD layer thickness of 70 nm of well-dispersed QDs, deposited in a concaved structure at the bottom to decrease gradually to 5 nm at the sides of pyramids on the surface of mc-p-Si solar cell (FIG. 6). This led to enhance the J.sub.SC by +4.20% compared to the bare solar cell (FIG. 5). The PCE results showed, interestingly, a very similar tendency to that of J.sub.SC. In particular, the PCE progressively increased from 16.88 to 17.57% with the QD concentration to record a peak at 0.4 wt %, presenting a PCE enhancement of +4.11% (+0.69% p) then negatively decreased with further increase in QD concentration, such as 16.65% at 0.7 wt % (FIG. 5). The increment in PCE of our QDs was exceeding the theoretical limit (0.6% p) which is comparable to those published recently for Mn-doped Cd.sub.xZn.sub.1-xS/ZnS QDs (0.5% p)..sup.[12a] It should be noted that all solar cell samples coated with QD concentrations lower than 0.6 wt % exhibited better PV performance compared to the bare samples. However, a higher wt % causes a dramatic decrease in the performance of solar cells. FIG. 7 shows the J-V characteristics comparison between the solar cell sample coated with the optimal QD concentration (0.4 wt %) and bare sample. In contrast to the QD concentration dependency of Jsc and PCE, the Voc and FF showed almost no response to the wt % concentration of QDs (FIG. 5) indicating that the coating of the EDS-QD layer on the solar cells has almost no effect on V.sub.OC and FF but on J.sub.SC and PCE. The significant enhancements in J.sub.SC and PCE can be primarily explained due to the effective EDS mechanism incorporated with the high PLQY (76%) and free-self-reabsorption within the CuGaS.sub.2/ZnS QDs, resulting in an excess energy of visible photons available for solar cells. Therefore, these observations clearly indicate the potential benefits and effectiveness of this EDS-QD layer for the development of eco-friendly solar cell applications.
[0056] To further understand the significant increase in J.sub.SC after the coating of an effective EDS-QD layer and the reason behind the dramatic degradation of performance when the QD concentration exceeds 0.6 wt %, all samples were subjected to EQE and SR measurements. The EQE is mainly for investigating the former (the increase in J.sub.SC) while the SR for the latter (the degradation of performance). The EQE and SR measurements were taken for the bare mc-p-Si solar-cell samples before adding the EDS-QD layer. These measurements were carried out again after applying the EDS-QD layer. FIG. 8 shows the EQE and SR (inset) results of the optimal concentration (0.4 wt %) presenting the highest J.sub.SC. It reveals that the EQE increased by 35.7% and, simultaneously, the SR decreased by 14.1% in the UV region between 300 and 450 nm after the coating of the functional Cd-free QDs, which reflect the clear presence of the EDS function rule in the QD layer that led to the enhancement in J.sub.SC. The corresponding calculations of the change in EQE (AEQE) in percentages (FIG. 8) showed similarity to the tendency of J.sub.SC. In particular, AEQE in the UV region ranging from 300 to 450 nm increased gradually from 7.3 to 35.7% for the QD concentration from 0.1 to 0.4 wt %, respectively, then further concentration led to more reduction in EQE. In general, all QD concentrations, except 0.7 wt %, displayed an increase in the EQE in the UV region with no noticeable degradation in the visible wavelength region (450-800 nm). On the other hand, 0.7 wt % showed a contradictory result in that EQE deceased in both UV and visible regions and negative performance in J-V measurements compared to its reference sample (without QDs). Thus, as a conclusion, the increase in EQE in the UV region verifies the effectiveness of the EDS-CuGaS.sub.2/ZnS QD layer as an energy-down converter that leads to direct enhancement in both J.sub.SC and PCE.
[0057] In addition to EQE characterizations, the SR was investigated using UV-visible light spectrometry for further understanding of the underlying mechanism behind the observed tendency in J.sub.SC. Previous works on EDS Cd-based QDs for solar cell applications used SR data, along with EQE, to explain the enhancements in solar cell performance as a function of QD concentration or thickness of EDS-QD layer..sup.[11a, 11b, 15-16] FIG. 9 depicts the SR results of the mc-p-Si solar cells implemented with the EDS-QD layer at various QD concentrations, as mentioned above. It was observed that the decrease in SR in the UV region (300-450 nm) with QD concentrations is accompanied by an increase in the visible region (450-800 nm) (FIG. 9). The decrease in the UV region clearly explains the beneficial effect of the EDS-QD layer in harvesting incident UV photons on the surface of solar cells. In other words, the higher the QD concentration, the higher absorption then lower reflectance in the UV region. This enhancement was limited by the accompanying increase in SR in the visible region (450-800 nm) due to the scattering of visible light that increases with QD concentration in the EDS-QD layer. FIG. 9 shows that a lower concentration than 0.4 wt % leads to a higher SR in the UV region while a higher concentration leads to a higher SR in the visible region. Therefore, the QD concentration should be considered carefully for scale-up applications of the EDS-CuGaS.sub.2/ZnS-QD layer on any type of solar cells. The high PLQY, free-reabsorption and nontoxicity features of our CuGaS.sub.2/ZnS QDs indicate that they are promising as a Cd-free EDS-QD layer for clean, eco-friendly, and highly efficient future solar cells.
[0058] The EDS-QD layer can be readily implemented to currently used PV modules to enhance their PCE with no costly replacements of the whole modules via a straightforward coating. The successful marketing and industrial applicability of this EDS-QD layer for commercial PV modules depend essentially on their economic feasibility and capability for industrial productivity. To investigate their economic feasibility, we conducted a bill of material-system (BoM-S) analysis.sup.[12a] to calculate and examine the BoM-S cost of the Cd-free EDS-QD layer regarding its PV enhancements. We found that the EDS-CuGaS.sub.2/ZnS QD layer with an optimal concentration of 0.4 wt % can effectively reduce the price of the commercial 248.4-watt Q.PLUS L-G4.1 335 mc-Si module by 2.62% due to the enhanced PCE (+4.11%), which facilitates the reduction in the usage area and the number of PV cells.
[0059] Furthermore, the up-scaling production of Cd-free CuGaS.sub.2/ZnS QDs was carried out for the first time in a facile one-pot core/shell synthesis for industrial producibility. This was achieved using an industrial-sized 2000-mL three-neck flask to produce 11 g of CuGaS.sub.2/ZnS QD powder at high-quality and very good reproducibility (FIG. 10). The optical characterizations of the QD powder dispersed in chloroform show that they have an optical bandgap of 3.05 eV and a wide range of PL emission peaked at 465 nm with a high PLQY of 73-76% (FIG. 10). It is important to mention that the purification process is essential for this QDs. The more cycles of purification, the higher PLQY can be achieved. These QDs show high bright white emission when they illuminated by 365 nm light under UV-lamp in both the powder form (FIG. 10) and the solution form (the inset of FIG. 10).
