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. 2020 Jun 15;23(7):101272. doi: 10.1016/j.isci.2020.101272

High-Performance, Large-Area, and Ecofriendly Luminescent Solar Concentrators Using Copper-Doped InP Quantum Dots

Sadra Sadeghi 1,6, Houman Bahmani Jalali 2,6, Shashi Bhushan Srivastava 3, Rustamzhon Melikov 3, Isinsu Baylam 4, Alphan Sennaroglu 4,5, Sedat Nizamoglu 1,2,3,7,∗
PMCID: PMC7322176  PMID: 32590328

Summary

Colloidal quantum dots (QDs) are promising building blocks for luminescent solar concentrators (LSCs). For their widespread use, they need to simultaneously satisfy non-toxic material content, low reabsorption, high photoluminescence quantum yield, and large-scale production. Here, copper doping of zinc carboxylate-passivated InP core and nano-engineering of ZnSe shell facilitated high in-device quantum efficiency of QDs over 80%, having well-matched spectral emission profile with the photo-response of silicon solar cells. The optimized QD-LSCs showed an optical quantum efficiency of 37% and an internal concentration factor of 4.7 for a 10 × 10-cm2 device area under solar illumination, which is comparable with the state-of-the-art LSCs based on cadmium-containing QDs and lead-containing perovskites. Synthesis of the copper-doped InP/ZnSe QDs in gram-scale and large-area deposition (3,000 cm2) onto commercial window glasses via doctor-blade technique showed their scalability for mass production. These results position InP-based QDs as a promising alternative for efficient solar energy harvesting.

Subject Areas: Nanoparticles, Energy Resources, Energy Engineering, Energy Materials

Graphical Abstract

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Highlights

  • •

    The luminescent solar concentrators based on copper-doped InP QDs are demonstrated

  • •

    Efficient excitation transfer led to the exceptionally high in-film PLQY of 81.2%

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    The LSCs based on copper-doped QDs showed the optical quantum efficiency of 37%

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    The gram-scale synthesis of QDs led to the fabrication of large-area LSCs (3,000 cm2)

Nanoparticles; Energy Resources; Energy Engineering; Energy Materials

Introduction

The global demand for electrical energy is increasing each year. According to the International Energy Agency (IEA) report on 2018 (International Energy Agency IEA report, 2018), the worldwide electrical energy consumption showed significant increase in the last 10 years from 18,000 TWh in 2008 to approximately 24,000 TWh in 2018. Today the majority of the global electricity production is done by using fossil fuels, which drastically increases the air pollution and greenhouse gas emission with significant side effects (e.g., global warming, extinction of species, and public health). Despite its great advancements during the recent years, the contribution of the solar energy is limited to only 1.8% (450 TWh) (Bergren et al., 2018), which remained far below its great potential as being one of the most abundant form of “green” energy.

Semi-transparent luminescent solar concentrators (LSCs) offer simple and cost-effective solar energy harvesting solutions to boost the solar energy contribution to global electricity production. LSCs are made of fluorescent materials embedded inside transparent optical waveguides. Fluorescent materials absorb the incoming solar radiation impinging on a large area and convert it to longer-wavelength radiation, and the down-converted light is then guided to the small-area solar cells placed at the edges of the waveguide. The concentration of light by luminescence leads to a significant increase of energy production for the same area of solar cells (Meinardi et al., 2014, Meinardi et al., 2017a, Meinardi et al., 2017b, Wu et al., 2018). Therefore, this approach provides a practical energy harvesting solution that can be broadly used as “solar windows” in buildings, vehicles, and roofs (Figure 1).

Figure 1.

Figure 1

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Schematic Representation of Widespread Usability of Luminescent Solar Concentrators (LSCs) in Buildings, Vehicles, and Roofs as “Solar Windows” That Can Boost the Solar Energy Contribution to Global Electricity Production

The fluorescent particles absorb the sunlight and re-emit photons, which are guided through the glass substrate by total internal reflection (TIR) and collected by the solar cells coupled to the edges.

For its wide-spread use, LSC technology needs to be transformed such that it can simultaneously satisfy efficiency, biocompatibility, and feasibility for large-volume production. More specifically, the following challenges need to be simultaneously addressed: (1) high photoluminescence quantum yield (PLQY) of fluorescent material in its synthesis batch, (2) high quantum efficiency (QE) of them in host matrix, (3) low overlap between the absorbance and photoluminescence (i.e., reabsorption) of the fluorophore, (4) well-matched spectral profile between the photoluminescence of the fluorescent material with the photo-response of the solar cells, (5) being free of toxic elements, and (6) scalable production for wide-spread use. Optical quantum efficiency of an LSC is a key efficiency-related figure of merit (Wei et al., 2019) defining the ratio of the output photons, which are extracted from the edges of the LSC to the total number of absorbed photons by the LSC. In order to reach a high optical quantum efficiency level, simultaneous satisfaction of (1)–(3) is required. In addition, (4) allows for efficient conversion of luminescence to electricity, (5) ensures minimum adverse effects of LSCs on environmental sustainability, and (6) shows their ability for large-scale production. Therefore, fulfilling the requirements (1)–(6) can lead to an efficient, ecofriendly, and practical LSC technology for safe and effective solar energy harvesting.

Fluorophores inside the transparent medium are the key building blocks of the LSCs. To date, different types of fluorophores have been employed in LSC structures such as organic dyes (Liang et al., 2004), fluorescent proteins (Sadeghi et al., 2019b), lanthanide-based materials (Graffion et al., 2011), and perovskites (Wei et al., 2019). However, the use of each type of these fluorophores is accompanied with major challenges. Organic dyes have low stability and PLQY when integrated into host materials (Zhou et al., 2018). Fluorescent proteins, despite their unique bio-compatibility and even edibility, similarly suffer from the low PLQY in solid state, which leads to high optical losses (Sadeghi et al., 2019b, 2020). Lanthanides show low absorption coefficient and their compounds are expensive, which are not appropriate for their wide-spread use (Graffion et al., 2011; Nolasco et al., 2013). Perovskites show limited operational lifetime for LSCs (Li et al., 2020; Wei et al., 2019).

Alternatively, colloidal quantum dots (QDs) offer exceptional optoelectronic properties for LSCs. They allow the control of the optical properties by tuning their size, shape, and chemical composition (Bahmani Jalali et al., 2019b; Grabolle et al., 2009; Huang et al., 2015; Kagan et al., 2016). By using different inorganic core/shell structures, high PLQY can be reached to decrease the optical losses (Sharma et al., 2017). The ability of precisely tailoring the absorbance and emission of the QDs can enable low reabsorption losses, which is important to efficiently deliver the down-converted light to solar cells (Meinardi et al., 2014). Moreover, the photo/chemical stability of the QDs can be optimized for long-term use (Kim et al., 2016; Regulacio and Han, 2010; Zhao et al., 2014; Zheng et al., 2019). Therefore, QDs are highly promising candidates for LSC applications.

One compelling material design of QDs is Stokes shift engineering that can suppress reabsorption losses, which is important to minimize the propagation losses of the down-converted luminescence inside the waveguide. Different synthetic procedures have been performed to enlarge the Stokes shift of the QDs such as using indirect band-gap materials (Wang et al., 2017), varying the semiconductor compositions (Bailey and Nie, 2003), growing giant shells (Meinardi et al., 2014), and synthesis of quasi-type II QDs (Meinardi et al., 2014; Sadeghi et al., 2018a). However, QDs produced by these approaches have either toxic heavy-metal content (such as cadmium or lead) or low in-device PLQY. An alternative strategy to engineer the Stokes shift of QDs and enhancing the in-device PLQY can be doping with transition-metal ions (Erickson et al., 2014; Sahu et al., 2012; Viswanatha et al., 2011). Incorporation of small amount of metal impurities (0.1–10 atomic %) as dopants into the host semiconductor QDs introduces new electronic states within the bandgap (Erickson et al., 2014; Xie and Peng, 2009). The emission from these states is due to the exchange-mediated energy transfer from the photo-excited host semiconductor (Beaulac et al., 2008). Based on the energy level of the metal activator ion, the reabsorption can be effectively suppressed in doped QDs and leads to the Stokes shift up to hundreds of meV (Ronda, 2007; Bryan and Gamelin, 2005). However, the QE of the doped QDs is typically low owing to the slow radiative recombinations, which can be easily outcompeted with the non-radiative processes (Meinardi et al., 2014). Overall, the quest for finding the promising QD candidates, which simultaneously address the above challenges of (1)–(6), is still continuing.

Indium phosphide (InP) QDs, one of the most promising members of QD family for future optoelectronics, showed exceptional efficiency levels for LEDs (Won et al., 2019), photocatalysis (Yu et al., 2018), and neural interfaces (Bahmani Jalali et al., 2018) and also hold high potential for LSCs. Owing to their large exciton Bohr radius (∼10 nm) (Tamang et al., 2016) InP QDs are one of the most studied colloidal semiconductor nanocrystals, and their photoluminescence can be sensitively tuned within the visible spectrum via quantum confinement effect. Advantageously, owing to its environmentally benign nature, they are more favorable in comparison with its counterparts containing cadmium and lead elements (Won et al., 2019). Although the optoelectronic properties of InP QDs have been investigated extensively (Tamang et al., 2016), their use for LSCs have been limited owing to their high reabsorption and low efficiency in host materials.

Here, we report LSCs based on synthetically and structurally engineered copper-doped InP QDs that can simultaneously meet the needs of (1)–(6). The synthesized QDs were integrated inside a polydimethylsiloxane (PDMS) polymeric host matrix and deposited on a 10 × 10-cm2 commercial glass via doctor blade deposition. Owing to efficient excitation transfer from the semiconductor host to the emissive copper ions, photoluminescence in these structures occurs in a nearly reabsorption-free mid-gap region, resulting in an exceptionally high in-film PLQY of 81.2% (Table S2), which led to an optical quantum efficiency of 37% and internal concentration factor of 4.7 for 10 × 10 cm2 LSCs under the illumination of AM 1.5G solar spectrum. These efficiency levels are comparable with the best previously reported efficiency levels of cadmium-containing QDs and lead-based perovskite nanostructures (Table S5). We further synthesized QDs at gram-scale and fabricated non-toxic, low-loss, and large-area QD-based LSCs with an area of 3,000 cm2, which can be directly used in buildings and vehicles as the main source of harnessing the solar power to generate electricity. These results position InP-based QDs as a promising alternative for efficient solar energy harvesting.

Results and Discussion

Synthesis Method and Structural Properties of Copper-Doped InP/ZnSe Core/Shell QDs

We aimed at the synthesis of efficient and bio-compatible QDs with large Stokes shift. For that, first, we synthesized surface-passivated InP cores by hot-injection method (Bahmani Jalali et al., 2019a). Passivation of the InP core was performed by removing the surface dangling bonds using zinc carboxylate (Xu et al., 2008). Zinc carboxylates have the advantages of stable valence state, weak reactivity with phosphorous and low affinity for doping of InP lattice structure, low melting point, high thermal stability at reaction temperature, and non-toxicity (Xu et al., 2008). Moreover, they provide a zinc-rich surface on top of the InP core QDs, which facilitates the growth of additional multi-layer ZnSe shells. Unlike the previous study (Xie and Peng, 2009), which employs indium acetate as the indium precursor, we used indium chloride to form an in situ complex with different ligands. Since indium is easily reduced by many Lewis bases, a mixture of hexadecylamine (HDA) and stearic acid (SA) was used to form stable In (III)-ligand complexes. Then, we chemically doped copper ions to the host InP cores by thermal decomposition of copper stearate (Figure 2A). Since the reaction temperature is a key parameter in the successful doping of InP nanocrystals, we added the copper precursor to the reaction at the temperature of 180°C for complete lattice incorporation (Xie and Peng, 2009). Then, for an effective lattice diffusion (Xie and Peng, 2009), the reaction was heated up to 220°C (Figure 2B). The Cu:P precursor ratio was optimized (Figure S4) and the maximum PLQY of 27.6% was obtained at 1% for copper-doped InP core QDs. To decrease the surface defects and increase the PLQY, we choose ZnSe as the shell material since it has lower lattice mismatch with InP (3.4%) (Lim et al., 2011) compared with other commonly used wide-bandgap semiconductors such as ZnS (7.7%) (Lim et al., 2011). By this way, InP can be coated with a thick shell without causing large accumulation of strain. We performed multiple ZnSe shelling up to six monolayers (MLs) by successive ionic layer adsorption and reaction (SILAR) method. The transmission electron microscopy (TEM) analysis indicated that the averaged particle size of InP core (2.5 nm ± 0.2) increased slightly after copper doping (2.7 nm ± 0.2) (Figure 2C). After ZnSe shelling, the QDs were enlarged to 6.3 ± 0.2 (for 3 ML ZnSe) and 8.5 ± 0.3 nm (for 5 ML ZnSe). Since each monolayer of ZnSe shell with cubic crystal structure represents 0.57 nm (Ippen et al., 2012), full monolayer coverage was seen in multiple shelling (Figures 2D and S1–S3 and Table S1).

Figure 2.

Figure 2

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Schematic of the Synthetic Route and Structural Characterization of Copper-Doped InP/ZnSe QDs

(A) After the synthesis of InP core QDs by hot-injection method, core QDs were doped by introducing the copper activator ions. Finally, multiple shelling of ZnSe was performed on the doped InP QDs to increase the PLQY and further suppress the reabsorption.

(B) The synthesis of the InP core QDs was performed by hot injection of the phosphorous precursor (P(TMS)3) to indium precursor (InCl3). After the formation of the InP QDs, the copper precursor was introduced to the reaction, which caused the color change of the solution. Finally, the growth of the ZnSe shell was conducted by injection of the zinc and selenium precursors to the reaction batch via SILAR method.

(C and D) (C) TEM and (D) size distribution of the synthesized QDs. By increasing the shell thickness from 0 to 5 ML, the size of the synthesized QDs increased from 2.5 nm for InP QDs to 8.5 nm for copper-doped InP/5 ML ZnSe QDs. Scale bar for InP and Cu:InP QDs is 5 nm, and scale bar for Cu:InP/3 ML and Cu:InP/5 ML ZnSe QDs is 10 nm. The size distribution of the particles was performed for 400 particles in each TEM image.

Optical Properties of Copper-Doped InP/ZnSe QDs

To determine the nanostructure with low reabsorption and high efficiency that is suitable for LSC applications, we investigated the optical properties of the synthesized QDs. We performed time-resolved photoluminescence (TRPL) decay measurements of the InP core and copper-doped InP/5 ML ZnSe. The PL decay of the InP core is described by a biexponential decay due to the insufficient surface passivation. The contribution of the fast decay components was decreased by multiple ZnSe shelling, which supports the decrease of the non-radiative components (Figure 3E) (Sharma et al., 2017). In addition, we observed that, after copper doping, InP QDs experienced a significant PL redshift of 0.72 eV that leads to a substantial decrease of the reabsorption (Figures 3A and 3B). Furthermore, the presence of copper dopant centers located in the interior of the copper-doped InP core upon lattice diffusion leads to higher PLQY in comparison with the intrinsic InP cores (Xie and Peng, 2009). Via the growth of ZnSe shell, the emission and absorption bands were further separated and led to the significant Stokes shift up to 650 meV (the difference between the PL peak at 1.62 eV and the absorption broad shoulder at 2.27 eV) and suppression of reabsorption (Figure 3A). In addition to the decrease of reabsorption, the shell growth isolates the surface traps and effectively confines the electron-hole wave functions inside the nanostructure, which leads to the increase of the PLQY. To select the appropriate nanostructure for LSC, the quenching factor (i.e., reabsorption × (1-ФPL)) was calculated for different ZnSe shell thicknesses (Figure 3C) and the minimum of the quenching factor was observed for five MLs. The maximum PLQY of QDs in hexane solution was observed as 87% ± 4 for five monolayers of ZnSe shell formation (Figures 3D and S5). Hence, considering the quenching factor and PLQY, we selected copper-doped InP/5 ML ZnSe as the optimum sample for LSC fabrication. To investigate the photo-stability of the QDs in-film, their quantum efficiency was measured by accelerated-aging test under 365 nm illumination (see Transparent Methods) and showed only 5.8% decrease (from 81.2% to 76.5%) in 50 days (Figure 3F).

Figure 3.

Figure 3

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The Optical Properties of Copper-Doped InP/ZnSe QDs

(A) The evolution of the absorbance and photoluminescence spectra of the core InP, copper-doped InP, and copper-doped InP QDs with different ZnSe shell thicknesses ranging from 1 to 6 ML. The absorbance range for each panel was from 0 to 0.1.

(B) The photograph of the synthesized QDs under 365 nm UV irradiation.

(C) Normalized quenching factor of the copper-doped InP QDs calculated by multiplication of reabsorption (R) and (1-ФPL).

(D) The evolution of the PLQY by increasing ZnSe shell thickness from 0 to 6 ML. The average PLQY increased from 3.4% for InP QDs to 87% for copper-doped InP/5 ML ZnSe QDs in hexane (circles). The similar behavior was also observed in the PLQY of the QDs when embedded inside the polymeric matrix of PDMS (squares) as the average QE of the QDs was increased from 1.5% for InP to 81.2% for copper-doped InP/5 ML ZnSe QDs. Owing to the low reabsorption and high QE, the copper-doped InP/5 ML ZnSe is an appropriate nanostructure for LSCs (red dashed rectangular in (C) and (D)). (The error bars were calculated by the standard error of mean of three samples.)

(E) Time-resolved PL measurements of the InP core and copper-doped InP/5 ML ZnSe QDs in hexane. The copper doping increased the fluorescent lifetime decay significantly from 45 ns for InP to 292 ns for copper-doped InP/5 ML ZnSe owing to the introduction of the localized states by the copper ions.

(F) The accelerated-aging test performed by the excitation of the copper-doped InP/5 ML ZnSe QD film at 365 nm UV irradiation.

The Non-linear Absorption Spectroscopy of the Copper-Doped QDs

To further investigate the nature of the Stokes shift in the synthesized copper-doped QDs and understand the emission mechanisms (Figure 4A), the non-linear absorption and ultrafast decay properties of the copper-doped and undoped InP/5 ML ZnSe QDs were further investigated by using a femtosecond pump-probe spectrometer (Baylam et al., 2019). QD solutions were excited at 400 nm at a fluence of 135 μJ.cm−2, and the difference between the absorbance of the pumped and unpumped sample (ΔA = Apumped-Aunpumped) was probed by using a femtosecond white light continuum ranging from 450 to 800 nm (Figures 4B and S16). The ΔA peak wavelength of the undoped QDs remains almost identical to the absorbance shoulder of the QDs at 556 nm (2.23 eV), showing that the interband transitions are responsible for the transitions in the undoped samples (see Figure S17). The ultrafast transient evolution of the nonlinear absorbance (Figure 4C) was analyzed by employing a biexponential decay fit consisting of decay times τ1 and τ2. Similar to what was reported in earlier investigations (Dutta et al., 2018; Peng et al., 2010), the decay times τ1 and τ2 describe fast decay due to intermediate trapping states. In addition, as can be seen from Figure 4B, ΔA remains non-zero beyond a probe delay of 700 ps, due to inter-band electron-hole recombination that occurs over nanosecond time scales. The latter transition rates are determined from luminescence decay measurements described in Figure 3E. The measured ultrafast decay times of copper-doped QDs (τ1 = 0.9 ps, τ2 = 6.3 ps) at the probe wavelength of 580 nm (Figure 4C) are considerably shorter than those for undoped QDs (τ1 = 2.4 ps, τ2 = 25.5 ps, probe wavelength of 500 nm, see inset of Figure 4B) owing to copper-induced mid-gap states. In addition, the peak wavelength of the ΔA spectrum of the doped QDs shifted from 566 (2.19 eV) to 591 nm (2.09 eV) over a delay of 1.5 ps (Figure 4D). The pump fluence was relatively low (the estimated average number of generated electron-hole pairs corresponded to <N>∼0.5) for the observation of a Stark effect-induced spectral change, and at the same fluence level no spectral variation was observed for the undoped QDs. Hence, we attributed that the copper dopants in InP/5 ML ZnSe QDs play the role as an “extra” hole acceptor level (Knowles et al., 2016).

Figure 4.

Figure 4

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Band Diagram and Ultrafast Non-linear Absorption Spectroscopy of the Copper-Doped InP/5 ML ZnSe QDs

(A) The energy band diagram of the undoped (upper panel) and copper-doped (lower panel) InP/ZnSe QDs. In undoped QDs, the photon is absorbed and interband recombination takes place. When QDs are doped with copper, mid-gap states are introduced, which are responsible for the red shift in emission.

(B) The contour plot showing the measured ΔA of the copper-doped InP/ZnSe QDs as a function of probe delay and wavelength.

(C) Time-resolved ΔA response of the copper-doped InP/ZnSe QDs at the probe wavelength of 580 nm. The decay lifetimes of the copper-doped QDs were determined as 0.9 and 6.3 ps. Inset: Time-resolved ΔA response of the undoped InP/ZnSe QDs at the probe wavelength of 500 nm with the decay lifetimes 2.4 and 25.5 ps.

(D) The measured ΔA spectra of the QDs at eight selected probe delays ranging from −4 ps to 700 ps. The excitation wavelength was 400 nm.

Low-Loss InP-Based QD-LSCs

We fabricated copper-doped InP/ZnSe QD-based LSCs by doctor-blade deposition of QD-polymer mixture (d = 100 μm) onto the glass substrates (D = 2 mm) (Figure 5A). We selected PDMS elastomer as the polymeric host matrix that is highly transparent both in the visible and near-IR region (Wang et al., 2016) and offers inexpensive large-scale production (Sackmann et al., 2014; Sadeghi et al., 2018c, 2019a). Despite the favorable properties of PDMS, in fact, the initiator radicals of such polymers can interact with QDs and increase the non-radiative channels (Sadeghi et al., 2018a). This has the risk of significantly decreasing the QE of QDs. For example, in previous reports, a QE decrease of 50% is observed (Sadeghi et al., 2018b), which can be solved by special methods such as cell casting (Meinardi et al., 2014). Noticeably, without any special treatment of PDMS, we observed strong photoluminescence of QDs in regular PDMS films with only a ∼6% QE decrease with respect to the solution due to the strong confinement of photo-generated charge carriers via the sufficiently thick ZnSe shell (Figures 3D and 5C). The total internal reflection of the photoluminescence was especially visible at the edges of the waveguide (Figure 5C). The QD layer in the fabricated QD-LSC showed a small PL peak wavelength change from 767 nm in solution to 772 nm in film, respectively (Figure 5D). Advantageously, the thin film deposition of QD-polymer layer preserves the transparency of the fabricated QD-LSCs, which is critical for indoor and outdoor solar window applications (Figure 5B). The absorbance of the QD-LSC at the first excitonic peak was measured as 0.093 (Figure 5D), equal to the high transmission level of 81%, which was also observable in the photograph of QD-LSC slab under ambient light (Figure 5B). The small offset (0.05) in the absorbance of the device (dashed line in Figure 5D) is due to the light reflection from the LSC (equal to ∼11% reflectivity).

Figure 5.

Figure 5

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Copper-Doped InP/ZnSe QD-Based LSCs

(A) Schematic representation of the fabrication process. After mixing the QD solution with PDMS elastomer and degassing, the mixture was poured onto a glass substrate. Then, by using the doctor blade, a thin film with the thickness of 100 μm was formed on the glass.

(B) The transparency of the fabricated LSC with the loading concentration of 26 mg.mL−1 was shown under ambient light.

(C) Photograph of the QD-based LSC under 365 nm UV irradiation. The total internal reflection is visible on the edges as they seemed brighter (scale bar, 1 cm).

(D) Absorbance and PL of the QDs in the film (black dashed line) with the loading concentration of 26 mg.mL−1 and in solution (red line).

(E) The optical output intensity of the 10 × 10 cm2 QD-based LSC by changing the illumination area from 0 to 100 cm2. Inset: Schematic of the setup to change the portion of the illuminated area of LSC under UV radiation.

(F) The output emission spectra of the LSC, when illumination area increased as in (E). Inset: Normalized output spectra at the optical distances of 0 and 10 cm from the collection edge.

(G) The spectrally integrated output of QD-based LSC under the illumination by a UV laser beam at 365 nm wavelength. Inset: Laser beam excitation was perpendicular to the surface with different optical distances and the light was collected at the slab edge.

(H) The output emission spectra of the LSC, when the optical distance from the excitation point and the collection edge was increased as in (G). (The error bars were calculated by the standard error of mean for three measurements.)

To evaluate the suitability of the copper-doped InP/5 ML ZnSe QDs in LSC panels, the optical properties of the fabricated QD-based LSC with the dimensions of 10 × 10 cm2 were investigated. To simulate the situations in which the solar window panels are illuminated partially by sunlight irradiation on the different hours of the day, the portion of the illumination area under UV light was altered (Figure 5E inset). As illumination area increased from 0 to 100 cm2 (equal to the full length of LSC at 10 cm), the optical output intensity increased with a linear slope owing to the effective suppression of the reabsorption by QDs (Figure 5E). At the same time, the emission spectra of the QDs at different illumination area showed almost no significant spectral peak position change (Δλ = 4 nm), demonstrating the low reabsorption of the LSC under different areal illumination conditions (Figure 5F), and the emission spectrum also matched well with the spectral response of silicon photovoltaic cells for efficient light harvesting (Figure S7). For direct evaluation of the reabsorption in the fabricated QD-LSC, laser light at 365 nm was used to excite the face of QD film and the output intensity was recorded from the edge at different optical distances from the excitation spot (Figure 5G inset). The normalized integrated PL intensity decreased by 21% by increasing the optical distance up to 10 cm (Figures 5G and 5H). This trend mainly originates from the reabsorption and scattering losses. To figure out the scattering losses, we investigated experimentally by propagating a laser beam through the edge of the fabricated QD-based LSC at the wavelength of 780 nm, where QDs have negligible absorption (Figure 5D). The output light was collected at different optical distances from the illumination edge by using an optical fiber (Figure S9). By increasing the light collection distance up to 10 cm, the output intensity corresponds to a scattering loss of 0.016 cm−1 (Figure S9B). By including the scattering term in the PL decay (Figure 5G), the effective reabsorption coefficient corresponds to 0.004 cm−1 (Figure S8), showing that the reabsorption was significantly suppressed.

We characterized and analyzed the efficiency of the copper-doped QD-based LSCs. To investigate the effect of LSC size, we fabricated QD-LSCs with the dimensions of 5 × 5, 10 × 10, and 20 × 20 cm2 having loading concentration of 26 mg.mL−1. At the same time, we fabricated two separate LSCs with the same dimension of 10 × 10 cm2 having different loading concentrations of 13 and 52 mg.mL−1 (Figure S7). QD-LSCs were illuminated orthogonal to the front surface by a calibrated AM 1.5G solar spectrum (Figure S10), and the photoluminescence generated by the QDs was coupled to the silicon solar cell positioned on the collection edge (see Transparent Methods).

In order to measure the optical quantum efficiency of the fabricated LSCs, the expression ηOQE=(JLSCAPV)/(JPVALSCηabsqLSCηcoupling) (Wei et al., 2019) was used, where JLSC is the short-circuit current of the solar cell coupled to the LSC when LSC is illuminated by the solar simulator, JPV is the short-circuit current of the solar cell when the solar cell is directly illuminated by the solar simulator without LSC, qLSC is the reshaping factor equal to 1.73, ηabs is the absorption efficiency dependent on the loading concentration of the QDs (Figure 6B), and ηcoupling is the PL coupling efficiency from the LSC to the solar cell, which is calculated as 75.2% (see Supplemental Information) (Wu et al., 2018). Based on the electro-optical characterizations, JLSC/JPV values measured as 0.24, 0.44, and 0.74 for the loading concentration of 13, 26, and 52 mg.mL−1, respectively (Figure 6A), which resulted in the optical quantum efficiency of QD-LSCs of 40.7%–39.9% and 37.2% for a 10 × 10 cm2 area, respectively (Figure 6C). When the side length of LSC changed to 5 and 20 cm at 26 mg.mL−1, JLSC/JPV values of the QD-LSCs were shifted 0.24 and 0.73, respectively (Figure S15). But, at the same time, since the G factor increased from 6.25 to 25, the optical quantum efficiency of the QD-LSC with different LSC lengths led to 44.3% and 32.7%, respectively (Figure 6D and Table S4). The observed optical quantum efficiency levels are as a result of high in-film QE of the QDs, suppressed reabsorption due to the separation of absorption and emission centers (Stokes shift > 600 meV), and the high optical quality of the fabricated films mitigating the propagation-loss problem. To the best of our knowledge, the optical quantum efficiencies of the more environmentally benign InP-based LSCs have comparable efficiency levels with the best Cd- and Pb-based LSCs (Table S5). Table S5 shows that Si and CuInS QDs have high potential for future LSCs as well.

Figure 6.

Figure 6

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Large-Area Copper-Doped InP/ZnSe QD-Based LSCs and Evaluation of Optical Quantum Efficiency of Fabricated Devices

(A) The J-V curves of the silicon solar cells coupled to the QD-LSCs with the LSC length of 10 cm at different loading concentrations of 13, 26, and 52 mg.mL−1 under the illumination of AM 1.5G solar simulator. The J-V curve of the silicon solar cell under direct illumination of the solar simulator was also shown (black line).

(B) The absorption efficiency values for different loading concentration of the QDs. The dashed line and the points represent the simulated and experimental results, respectively.

(C and D) (C) The simulated (dashed line) and experimental (symbols) values of optical and external quantum efficiency for the QD-LSCs with different loading concentration of the QDs ranging from 13, 26, and 52 mg.mL−1 and (D) different side lengths of 5, 10, and 20 cm.

(E) The simulation of the optical quantum efficiency of QD-LSCs having different QE of QDs ranging from 10%, 50%, 81.2% (QE of this study) and unity at different LSC lengths ranging from 1 to 100 cm and different loading concentration between 2.6 and 520 mg.mL−1.

(F) Up-scaled amount of copper-doped InP/ZnSe core/shell QDs after single synthesis.

(G) The fabricated large-area QD-based LSC under sunlight irradiation with the dimensions of 100 × 30 cm2.

(H) The large-area LSC under weak UV illumination (scale bar, 5 cm).

Besides, the optical quantum efficiency of the InP QD-LSC having 52 mg.mL−1 led to a high internal concentration factor calculated by CFint=G×ηOQE (Wei et al., 2019), which corresponds to 4.7 for the LSC panels with the dimensions of 10 × 10 cm2. The external quantum efficiency of the LSC is defined as the ratio of the number of the photons reaching to the coupled solar cell to the total number of the input photons. As a result of electro-optical measurement, the concentration of 52 mg.mL−1 achieved the highest external quantum efficiency of 5.9% owing to high absorption efficiency in comparison with the other LSCs having different concentration levels (Figures 6C and 6D). These external quantum efficiencies are also in agreement with the simulations based on ηEQE=ηabsηOQEqLSCηcoupling (Figures S11–S13 and Table S3). Moreover, the solar-to-electrical power conversion efficiency (i.e., PCELSC) corresponds to 0.9% by using a stand-alone solar cell that has an efficiency of 14.5%, respectively (Figure S14).

To understand the ultimate efficiency levels achievable by the QD-LSCs, we performed the simulation of the optical quantum efficiency by considering QEs of 10%, 50%, 81.2% (this study), and 100%, with the LSC length and loading concentration ranging from 1 to 100 cm and from 2.6 to 520 mg.mL−1, respectively (Figure 6E). By increasing the QE of the QDs to unity, the ultimate optical quantum efficiency levels of QD-LSCs can reach up to 70% for an area of 10 × 10 cm2, while keeping sufficient level of transmittance more than 50% in the eye sensitivity peak (λ = 555 nm). Thus, the optical quantum efficiency of 70% can push up the external quantum efficiency to the maximum value of ∼14% while a refractive index-matching polymer between LSC and PV module is used and a moderate absorption efficiency of 20% is assumed. If a more efficient solar cell module (having 23.7%) (Wu et al., 2018) would be used for optical quantum efficiency of 70%, the power conversion efficiency could be further increased up to ∼3.3% under the standard AM 1.5G solar illumination. In addition, integration of lower-band gap QDs emitting at 900–1,000 nm matching with the responsivity of the silicon solar cells can significantly boost the power conversion efficiency levels. Furthermore, the use of reflectors on the bottom of the LSC can increase the power conversion efficiency though decreasing the transparency.

Scalability of Copper-Doped InP QDs for Large-Area and Wide-Spread-Usable LSCs

In order to fabricate large-area QD-based LSCs, the amount of QDs needs to be scaled up to a sufficient level. For that, we increased the reaction yield up to 4.28 g of the copper-doped InP/5 ML ZnSe QDs in one reaction batch (Figure 6F) (see Transparent Methods for detailed amounts of precursors and synthetic procedure). To assess the applicability of the fabricated LSCs to large-area solar window applications, the synthesized copper-doped QDs with the concentration of 26 mg.mL−1 were mixed with the polymer and deposited on 100 × 30 cm2 glass substrate by using doctor-blade technique (Figures 6G and S6). Regardless of the size and the high loading concentration of the QDs in the large-area device (26 mg.mL−1), the fabricated devices are highly transparent under sunlight irradiation (Figure 6G). At the same time, the UV illumination of the large-area QD-based LSC led to strong emission from all parts of the fabricated device, indicating the feasibility for large-area LSC operation and fabrication (Figure 6H).

Conclusions

In this study, we showed that indium phosphide (InP)-based QDs are a promising candidate for future LSC technology that can simultaneously fulfill the requirements of the efficiency, eco-compatibility, and large-scale production. The Stokes shift of the InP QDs was realized by copper doping (650 meV), and the PLQY of the synthesized QDs was boosted up to 87% in the “near-infrared window” by passivation of surface traps via zinc carboxylation and strong confinement of electron-hole pairs inside the nanostructure via growing lattice-matched ZnSe shell. Moreover, the high PLQY over 80% is also maintained in solid-state form owing to the thick ZnSe shell. The high PLQY and low reabsorption facilitated high optical quantum efficiencies over 30%, comparable with the best reported Cd and Pb-containing LSCs. By using the doctor-blade technique, a large-area QD-based LSC (3,000 cm2) was fabricated, which indicates the applicability for “solar window” applications. Moreover, incorporation of QDs having QE of 100% into LSCs can lead to power conversion efficiency levels above 3%. Collectively, this work demonstrates a significant step forward toward the realization of ecofriendly, efficient, and large-area LSCs, which show remarkable potential to be widely used for solar-powered buildings and vehicles in the near future.

Limitations of the Study

In this study, we report LSCs based on synthetically and structurally engineered copper-doped InP QDs that can simultaneously meet the needs of high efficiency, being free of toxic elements and scalability. The synthesized QDs were integrated inside a polydimethylsiloxane (PDMS) polymeric host matrix and deposited on a 10 × 10 cm2 commercial glass via doctor blade deposition. Owing to efficient excitation transfer from the semiconductor host to the emissive copper ions, photoluminescence in these structures occurs in a nearly reabsorption-free spectral region. Moreover, they have high in-film PLQY of 81.2%, which led to an optical quantum efficiency of 37% and internal concentration factor of 4.7 for 10 × 10 cm2 LSCs under the illumination of AM 1.5G solar spectrum. The current study, however, falls short to reach the external quantum efficiency more than 10% owing to the low efficiency of the silicon solar cells used for the electro-optical measurements. Moreover, the optical stability of the fabricated QD-LSCs has only been tested under UV light. For better accuracy, it is better to evaluate them under sun irradiation or other broadband light sources under different humidity levels. In addition, the distribution of copper dopant inside the core/shell structure can provide more insight about the structural properties. Furthermore, the cost of the PDMS and QD synthesis chemicals may also limit large-scale production. Hence, further studies and optimizations that can address the above issues can improve the understanding of QDs and LSC performance.

Resource Availability

Lead Contact

Further information and requests for resources and reagents should be directed to and will be fulfilled by the Lead Contact, Sedat Nizamoglu (snizamoglu@ku.edu.tr).

Materials Availability

This study did not generate new unique reagents.

Data and Code Availability

The published article includes all datasets/code generated or analyzed during this study.

Methods

All methods can be found in the accompanying Transparent Methods supplemental file.

Acknowledgments

S.N. acknowledges the support by the European Research Council (ERC) under the European Union's Horizon 2020 Research and Innovation Programme (Grant agreement no. 639846). S.N. acknowledges the support by the Turkish Academy of Sciences (TÜBA-GEBİP; The Young Scientist Award Program) and the Science Academy of Turkey (BAGEP; The Young Scientist Award Program). The authors thank M. Han for the assistance in the QD-LSC fabrication and measurements. The authors thank the Koç University Tüpraş Energy Center (KUTEM) and Koç University Surface Science and Technology Center (KUYTAM) for the structural analysis of QDs. The auhtors would like to thank Prof. E. Şenses for his helpful discussions on optical measurements of QD-LSCs.

Author Contributions

S.N and S.S developed the idea and designed the experiments.. S.S. executed the quantum dot and LSC experiments. H.B.J. synthesized QDs and optimized their optical properties. R.M. and S.S. measured and analyzed the optical performances of LSCs. S.S. and H.B.J. performed the QD characterization. S.S. and H.B.J. performed time-resolved PL and analyzed the data. S.B.S. fabricated the QD-based LSCs with different dimensions and performed the electro-optical measurements. I.B. and A.S. performed the non-linear absorption spectroscopy and wrote the related discussion. I.B. assisted S.S. in the optical measurements of QD-LSCs. All authors wrote the manuscript and have given approval to the final version of the manuscript.

Declaration of Interests

The authors declare no competing interest.

Published: July 24, 2020

Footnotes

Supplemental Information can be found online at https://doi.org/10.1016/j.isci.2020.101272.

Supplemental Information

Document S1. Transparent Methods, Figures S1–S17, and Tables S1–S5
mmc1.pdf (2MB, pdf)

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

Document S1. Transparent Methods, Figures S1–S17, and Tables S1–S5
mmc1.pdf (2MB, pdf)

Data Availability Statement

The published article includes all datasets/code generated or analyzed during this study.

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