Engineering the Optical Properties of Eco‐Friendly CuGaS2/ZnS and CuGaInS2/ZnS Core/Shell Quantum Dots for High‐Performance Tandem Luminescent Solar Concentrators

Herein, highly luminescent eco‐friendly CuGaS2/ZnS (CGS/ZnS) and CuGaInS2/ZnS (CGIS/ZnS) core/shell quantum dots (QDs) are rationally prepared for luminescent solar concentrator (LSC) application. It is demonstrated that the optical properties of these core/shell QDs can be tailored by engineering the ZnS shell thickness, leading to large Stokes shifts and high‐photoluminescence quantum yields up to 94.6%. As‐synthesized core/shell QDs with optimized optical properties are employed to fabricate LSCs (5 × 5 × 0.5 cm3) using glasses as waveguides, wherein the individual CGS/ZnS and CGIS/ZnS QD‐based LSCs, respectively, exhibit an optical efficiency (η opt) of ≈3.26% and 6.53% under AM1.5G illumination (100 mW cm−2). Remarkably, a tandem QDs‐LSC integrated via vertical stacking of the top yellow‐emitting CGS/ZnS QDs‐LSC and bottom red‐emitting CGIS/ZnS QDs‐LSC delivers an optical efficiency (η opt) as high as 9.94%, which is, respectively, ≈3 and 1.5 times higher than the individual QDs‐LSCs and is comparable to various best‐reported QDs‐LSCs. The results indicate that environment‐benign I–III–VI2 core/shell QDs with engineered optical properties and LSC architectural design are promising to develop future cost‐effective and high‐performing building‐integrated photovoltaics.


Introduction
Harnessing solar energy is promising to address global energy needs and maintain the environmental sustainability by reducing the use of fossil-based fuels. [1]Solar cells can directly convert sunlight into electricity and the power conversion efficiency (PCE) of silicon solar cells has reached over 20% with reasonable long-term durability. [2]However, their large-scale implementation is still hindered due to the expensive fabrication cost of silicon materials and integration technology for solar cell assembly. [3]Fortunately, the production cost of a silicon solar cell can be further lowered by reducing the dimensions of the modules. [4]uminescent solar concentrators (LSCs) are optoelectronic devices made up of fluorophores and can act as solar energy collectors for photovoltaic (PV) cells. [5]pon exposure to sunlight, the fluorophores enable the absorption of solar radiation and reemit photons at longer wavelengths, which are propagated via a waveguide toward the edges of the LSCs via total internal reflection. [6]According to the position of the fluorophores, LSCs can be categorized into different types. [7]n a polymer-based LSC, the fluorophores are embedded in the waveguide constructed by the polymerization of organic monomers. [8]To reduce the optical loss of fluorophores during the polymerization process, liquid LSCs have been developed by using a liquid medium. [9]Moreover, thin film LSCs are formed by depositing the fluorophores on the waveguide, while the fluorophores layer is sandwiched between two waveguides in laminated LSCs. [7,10]erein, highly luminescent eco-friendly CuGaS 2 /ZnS (CGS/ZnS) and CuGaInS 2 / ZnS (CGIS/ZnS) core/shell quantum dots (QDs) are rationally prepared for luminescent solar concentrator (LSC) application.It is demonstrated that the optical properties of these core/shell QDs can be tailored by engineering the ZnS shell thickness, leading to large Stokes shifts and high-photoluminescence quantum yields up to 94.6%.As-synthesized core/shell QDs with optimized optical properties are employed to fabricate LSCs (5 Â 5 Â 0.5 cm 3 ) using glasses as waveguides, wherein the individual CGS/ZnS and CGIS/ZnS QD-based LSCs, respectively, exhibit an optical efficiency (η opt ) of %3.26% and 6.53% under AM1.5G illumination (100 mW cm À2 ).Remarkably, a tandem QDs-LSC integrated via vertical stacking of the top yellow-emitting CGS/ZnS QDs-LSC and bottom redemitting CGIS/ZnS QDs-LSC delivers an optical efficiency (η opt ) as high as 9.94%, which is, respectively, %3 and 1.5 times higher than the individual QDs-LSCs and is comparable to various best-reported QDs-LSCs.The results indicate that environment-benign I-III-VI 2 core/shell QDs with engineered optical properties and LSC architectural design are promising to develop future cost-effective and high-performing building-integrated photovoltaics.
A small-area PV cell attached at one of the edges of LSCs can utilize the concentrated solar light to convert into electricity. [7,11]uilding-integrated photovoltaics (BIPVs) consisting of solarharvesting devices in the form of PV windows in building facades hold great promise to solve the problem of excessive land cost and insufficient high-rise roof space for PV installation in urban cities, thus realizing the construction of net-zero buildings. [12]ompared with traditional light collecting devices, LSCs can absorb scattering light [13] of sunlight with no angle dependence, indicating that there is no need to install a complex sunlight tracking system and the cost of PV systems via replacing the solar cells can be further reduced. [14]8c,10b,10c,16a,16c,18] Construction of heterostructured core/shell QDs (i.e., PbS/CdS, [19] CdSe/CdS, [20] PbSe/PbS, [21] etc.) is able to tune the absorption and emission spectra for a larger Stokes shift, offering efficient surface passivation that reduces the surface traps/defects and significantly improves PLQY.For instance, the growth of a thick CdS shell (%4.2 nm) on CdSe QDs enabled a large Stokes shift owing to the formation of a quasi-type II band alignment.At the same time, there is a tradeoff between the shell thickness and PLQY, and the LSC based on these QDs exhibited an optical efficiency (η opt ) of 1%. [20]Besides, the QDs-LSCs may be used in extreme environmental conditions with high-energy UV radiation or high temperatures.Thus, the stable optical properties of the devices are more favorable. [22]Similarly, the effective surface passivation by constructing core/shell structure can greatly prevent the QDs' surface oxidation or degradation upon exposure to the harsh conditions. [23]n addition to enhancing the optical properties of QDs, the design of LSC device architecture has a substantial impact on their η opt .For example, Wu et al. have demonstrated a tandem LSC based on Mn 2þ :CdZnS/ZnS and CuInSe 2 /ZnS QDs with a η opt of 6.4%, which is %2.5 and 1.2 times higher than the individual QDs-LSCs (η opt of 2.6% and 5.3%). [24]Similarly, a tandem LSC based on carbon dots and CdSe/CdS QDs was reported by Liu et al., showing a η opt as high as 1.4%, which is %2 and 1.1 times higher than the individual LSC devices (η opt of 0.7% and 1.2%). [22]Hence, the rational design of a tandem QDs-LSC holds a great potential to reach a higher device performance than the individual QDs-LSCs.However, the QDs assembled in most of the high-performing LSCs are II-VI/IV-VI group semiconductors composed of highly toxic heavy metals (Cd, Pb, Hg), [12b,18a,25] which are prone to cause environmental and human health issues.
Recently, environment-friendly I-III-VI core/shell QDs, CuGaS 2 /ZnS (CGS/ZnS) core/shell QDs with small reabsorption and high PLQY have been widely used as fluorescent materials in light-emitting diodes (LEDs) and LSCs. [26]26a] Nevertheless, η opt is still far from η opt of Cd/Pb-based QDs-LSCs [e.g., η opt of CdSe/Cd x Pb 1-x S QD-based LSCs is 5.3% (7 Â 3 Â 0.3 cm 3 ) [18a] ]; thus, it is crucial to develop methods for enhancing the η opt of LSC based on such eco-friendly QDs.In this context, tandem architectural design of LSCs in which QDs absorbing different wavelengths of sunlight by adjusting the bandgap are suitable to further boost the overall device efficiency. [24]As the relatively narrow absorption range of CGS/ZnS QDs is favorable for a top LSC to absorb shorter wavelength light, their derivative of CuGaInS 2 /ZnS (CGIS/ZnS) with a lower bandgap and large absorption coefficient is promising to act as a bottom LSC for effective longer wavelength light absorption.Moreover, to the best of our knowledge, there is no report on the tandem LSC based on eco-friendly CGS/ZnS and CGIS/ZnS QDs.
Herein, we synthesized highly emissive, environment-benign CGS/ZnS and CIGS/ZnS core/shell QDs with slight spectral overlaps between absorption/emission spectra (large Stokes shift) and optimized PLQYs up to 65.4% and 94.6%, respectively.As a result, the LSCs with dimensions of 5 Â 5 Â 0.5 cm 3 were fabricated via drop-casting the QDs solution on glasses as waveguides.Under 1 sun illumination (AM1.5G, 100 mW cm À2 ), the assembled single-junction CGS/ZnS and CIGS/ZnS QD-based LSCs exhibited a η opt of %3.26% and 6.53%, respectively.Furthermore, a tandem LSC integrated by stacking a top yellow-emitting CGS/ZnS QDs-LSC and a bottom red-emitting CGIS/ZnS QDs-LSC showed a remarkable η opt as high as 9.94%.

Results and Discussion
26a] Figure 1a,d shows the transmission electron microscope (TEM) images of both CGS/ZnS and CGIS/ZnS QDs, exhibiting near-spherical morphology with similar average particle sizes of 2.0 AE 0.4 and 2.2 AE 0.4 nm.The size distribution histograms of CGS/ZnS and CGIS/ZnS QDs are displayed in Figure 1b,e, respectively.It is indicated that both CGS/ZnS and CGIS/ZnS QDs undergo analogous reaction kinetics for nucleation and growth during synthesis because similar synthetic conditions for core and shell growth were adopted except for the addition of In precursor in the case of CGIS core QDs.
Moreover, the insets in Figure 1a,d depict the high-resolution TEM (HRTEM) images of QDs, showing lattice fringes with interplanar spacing of %3 Å that are indexed to the (111) plane of ZnS, as a result of comparatively thick ZnS shell coating on CGS and CGIS cores.Selected area electron diffraction (SAED) pattern of CGIS/ZnS core/shell QDs presented in Figure 1c exhibits three typical crystalline planes of ZnS, which is in accordance with the HRTEM results.Consistently, X-ray diffraction (XRD) patterns (Figure 1f ) of CGS/ZnS and CGIS/ZnS QDs both demonstrate the characteristic peaks indexed to (111), (220), and (311) planes of zinc blende phase ZnS, further inferring the successful formation of ZnS shell. [27]etailed chemical states and compositions of as-synthesized CGS/ZnS and CGIS/ZnS QDs were recorded by X-ray photoelectron spectroscopy (XPS) technique.Figure S1, Supporting Information, and Figure 2 depict the survey spectra and high-resolution XPS (HRXPS) spectra of each element in QDs, including Cu, Ga, In, S, and Zn. Figure S1, Supporting Information, shows the XPS survey spectra of CGS/ZnS and CGIS/ZnS QDs, wherein the XPS signals of Zn 2p and S 2p are obviously stronger than that of other elements.Since XPS is a surface-sensitive technique, the higher signal intensities  of Zn 2p and S 2p imply the existence of a ZnS shell on the top of CGS or CGIS core QDs.For HRXPS spectra, the characteristic XPS peaks at 951.6/931.9 and 19.3 eV shown in Figure 2a,b correspond to Cu 2p and Ga 3d core levels, demonstrating the presence of Cu and Ga components in CGS/ZnS QDs. [28]In parallel, the XPS peaks at 951.9/931.9,18.9, and 452.5/444.9eV exhibited in Figure 2e-g, respectively, account for the presence of Cu, Ga, and In components in CGIS/ZnS QDs system. [29]fter ZnS shell coating on CGS and CGIS cores, the characteristic XPS peaks of Zn 2p appeared and are displayed in Figure 2c,h, wherein the 1044.7/1021.7 and 1044.9/1021.929a,30] As for S 2p spectra in Figure 2d,i, the fitted peaks at binding energies of 163.1 and 162.5 eV are in good agreement with the formation of metals sulfides in CGS/ZnS and CGIS/ZnS QDs, respectively.Due to the use of 1-dodecanethiol (DDT) with -SH during QDs' synthesis, thiol molecules can be adsorbed onto the surfaces of the QDs, which interfered with the detection of S 2p in the QD. [14,31]oreover, the slight difference in the peaks of identical elements in different QDs is attributed to the different chemical bonding environment caused by the incorporation of In. [32] Optical properties of QDs were studied, and the results are shown in Figure 3. Figure 3a displays the absorption and PL spectra of CGS/ZnS and CGIS/ZnS QDs, wherein large Stokes shifts of %0.63 and 0.72 eV (Figure S2, Supporting Information) and a minor reabsorption were obtained.More importantly, the CGS/ZnS QDs show a narrow absorption onset of %450 nm, whereas the CGIS/ZnS QDs exhibit a redshifted and extended absorption onset to %620 nm, indicating their promising supplement absorption of the solar spectrum to construct tandem LSC devices.33a,34] After incorporating In in the CGS core, the resulting CIGS/ZnS QDs exhibited a redshifted PL peak at 630 nm (red-emitting) owing to a smaller bandgap of CIGS with respect to CGS. [35] This is confirmed from the estimated bandgap values of 3.1 and 2.7 eV for CGS/ZnS and CGIS/ZnS QDs, respectively, calculated from Tauc plots (Figure 3b,c).
Transient PL decay curves of all QDs in toluene were measured at an excitation wavelength of 400 nm and were fitted using a triexponential model.The average lifetime "τ" can be calculated using the following equation: [36] τ where A x (x = 1, 2, 3) are the fitting coefficients, while the τ x (x = 1, 2, 3) represent the characteristic lifetimes.The detailed fitting parameter and results are presented in Table S1, Supporting Information.As illustrated in Figure 3d,e, the average PL lifetimes of CGS/ZnS and CGIS/ZnS QDs were estimated to be %528 and %516 ns.A direct method using an integration sphere was used to measure the absolute PLQYs of QDs in solution (more details in Supporting Information).
The PLQYs for both yellow-emitting CGS/ZnS and red-emitting CGIS/ZnS QDs were measured to be 65.4% and 94.6%, respectively (Figure 3f ).To study the impact of shell thickness on the optical properties, the core/shell QDs with a thinner ZnS shell thickness were synthesized and named as CGS/ZnS-thin and CGIS/ZnS-thin.It was revealed that decreasing the ZnS shell thickness for both types of QDs leads to no significant change of PL peak positions (Figure S3, Supporting Information).26c,34c] As-synthesized CGS/ZnS and CGIS/ZnS QDs were further applied to fabricate the LSC devices.4b,37] As the yellow-emitting CGS/ZnS QDs can only absorb light the wavelength of 450 nm, thus the LSC based on CGS/ZnS QDs is only active in the shorter-wavelength region of the solar spectrum.In contrast, the red-emitting CGIS/ZnS QDs possess broad light absorbance; therefore, the fabricated CGIS/ZnS QD-based LSC can collect the photons from the longer wavelength (%620 nm) portion of the solar spectrum. [24]The spectral tunable characteristics of the QDs enable the architectural design of stacked multilayered LSCs, which enhanced the performance through spectral splitting of incident sunlight.Figure 4b-d shows the optical photographs of the top CGS/ZnS QDs-LSC layer, bottom CGIS/ZnS QDs-LSC layer, and the integrated tandem QDs-LSC under standard 1 sun illumination (AM1.5G, 100 mW cm À2 ).To evaluate the device performance, the J-V curves of LSC-PV systems were measured, and the η opt was calculated to describe the efficiency of the QDs-LSCs. [38]s shown in Figure 4b,c, S7, Supporting Information, we observed significantly increased photocurrent densities of LSC-PV systems based on CGS/ZnS and CGIS/ZnS QDs with a thicker ZnS shell, which is ascribed to a higher PLQY and a larger Stokes shift of these QDs that are beneficial to generating and re-emitting more photons for solar energy conversion.As a result, the CGS/ZnS and CGIS/ZnS QDs-LSCs (dimension of 5 Â 5 Â 0.5 cm 3 ) delivered a η opt of 3.26% and 6.53%, respectively.Moreover, a tandem QDs-LSC device was fabricated by stacking two layers of yellow-emitting CGS/ZnS and red-emitting CGIS/ZnS QDs-LSCs, which simultaneously absorb different wavelength regions of the solar spectrum to boost the overall solar energy conversion efficiency.10b,10c,22,24,26a,39]

Conclusions
In summary, eco-friendly CGS/ZnS and CGIS/ZnS core/shell QDs with different shell thicknesses were prepared to achieve high-efficiency tandem LSCs.It is revealed that the growth of a thicker ZnS shell on the core QDs can effectively passivate the surface defects/traps and lead to optimized optical properties, resulting in larger Stokes shifts and maximum PLQYs of 65.4% and 94.6% for CGS/ZnS and CGIS/ZnS QDs, respectively.Accordingly, the fabricated LSCs based on yellow-emitting CGS/ZnS and red-emitting CGIS/ZnS QDs delivered high optical efficiencies of 3.26% and 6.53%.Moreover, a tandem QD-LSC constructed by stacking both types of QDs-LSCs exhibited a higher optical efficiency of 9.94% under standard 1 sun illumination (AM1.5G, 100 mW cm À2 ), which is comparable to various best-reported QDs-LSCs.The results indicate that engineering the optical properties of environment-friendly I-III-VI core/shell QDs by composition tuning and shell passivation is promising for realizing high-performance tandem LSC devices.

Figure 1 .
Figure 1.a) TEM image with inset HRTEM image and b) relevant size distribution histograms of CGS/ZnS QDs.c) SAED pattern, d) TEM image with inset HRTEM image, and e) size distribution histograms of CGIS/ZnS QDs.f ) XRD patterns of CGS/ZnS and CGIS/ZnS QDs.

Figure 4 .
Figure 4. a) Schematic diagram of a working mechanism for the CGS/ZnS and CGIS/ZnS QDs-LSCs.b) Performance comparison of η opt for different QD-based LSCs.J-V curves of the c) CGS/ZnS QDs, d) CGIS/ZnS QD-based LSCs, and e) tandem QDs-LSCs with inset optical photographs.