Core/shell structured NaYF₄:Yb³⁺/Er³⁺/Gd⁺³ nanorods with Au nanoparticles or shells for flexible amorphous silicon solar cells

Upconversion (UC) describes a nonlinear optical process in which a UC material can generate one high-energy photon for every two, three or more low-energy photons, corresponding to two-, three- and more higher-order multi-photon processes. Since the concept of UC was first proposed in 1959, when it was called 'quantum counter action' [1], UC materials have attracted a great deal of attention from researchers because of their potential for optical manipulation in optical devices (such as infrared quantum counter detectors, temperature sensors and compact solid-state lasers), bio-analysis, medical therapy, display technologies and light harvesting in photovoltaic cells. Among all kinds of UC materials, rare-earth-ion-doped phosphors offer a wide range of emission, from the near-infrared (NIR) through the visible to the ultraviolet (UV), and have increasingly moved into the focus of solar cell research in recent years [2–6].

Hexagonal phase NaYF₄ codoped with Yb³⁺/Er³⁺ and Yb³⁺/Tm³⁺ are observed to have the highest UC efficiencies. The NIR excitation light is absorbed by the Yb³⁺ions and the UC luminescence is emitted by Er³⁺ and Tm³⁺, respectively, in these materials [7–10]. Crystalline materials doped with these ion couples, however, normally consist of sub-micrometer to micrometer-sized grains, which do not form the transparent films needed for thin film solar cell applications. UC nanomaterials usually have low emission efficiency because of their structural defects and large surface area with a variety of quenchers [11–13]. Recently, Wang et al reported a lanthanide doping approach for preparing hexagonal phase UC nanoparticles and nanorods [14]. Their work demonstrated that Yb³⁺–Er³⁺–Gd⁺³-codoped sodium yttrium fluoride (NaYF₄:Yb/Er/Gd) nanocrystals can be tuned in size (down to ten nanometers), phase (cubic or hexagonal) and upconversion emission color (green to blue) through the use of trivalent lanthanide dopant ions at precisely defined concentrations.

On the other hand, the spectral properties and UC efficiency of lanthanide doping NaYF₄ nanomaterials can be modulated and enhanced through plasmonic interactions between the upconverters and gold nanostructures [15–18]. Schietinger et al observed a strong UC emission enhancement from a single NaYF₄:Yb³⁺/Er³⁺ nanocrystal coupled with gold spheres by the use of atomic force microscopy [16]. Zhang et al reported the enhancement upconversion emission of hexagonal NaYF₄:Yb³⁺/Tm³⁺ nanocrystals coupled with gold nanoparticles or nanoshells [17]. More recently, Zhang et al reported a fivefold overall enhancement of UC emission in NaYF4:Yb³⁺/Er³⁺ nanocrystals when coupled with gold island films [18]. These results and features triggered our study of the development of photovoltaic cells with unique properties by a combination of UC nanomaterials and metal nanostructures.

The integration of a UC layer into a solar cell is quite a recent concept in solar cell research. The UC material layer can be preferentially placed on the back side or both sides of a solar cell, where it takes up the sub-bandgap part of the solar spectrum and re-emits visible light, which now is resonant with the bandgap of the solar cell. About 52% of the total solar energy flux is in the infrared range (λ > 700  nm), which suggests that a large performance gain could be achieved with efficient UC. Study of the UC system concepts to reduce the sub-bandgap losses of solar cells has been focused on crystalline solar cell technologies in the past few years [2–6]. For example, in 2003, Green and co-workers reported the application of rare-earth-ion-based UC material to silicon solar cells [3]. The upconverter consisted of Er³⁺-doped NaYF₄ and was located at the rear side of a bifacial cell. This led to a detectable photo-response in a crystalline silicon (c-Si) cell under excitation at 1500  nm. It is found that doping of these rare-earth materials into c-Si solar cells has shown a better absorption of the long wavelength light in the solar spectrum, thus having the potential to enhance the performance of solar cells [4–6]. Recently, the quite substantial progress in the design and development of UC materials brought the concept of UC to photovoltaic systems other than c-Si. In 2010, de Wild et al [19] reported the application of UC phosphors to a thin film solar cell technology. They coupled the NIR–vis upconverter β-NaYF₄:Yb³⁺ (18%)/Er³⁺ (2%) to the back of an amorphous silicon solar cell in combination with a white back reflector and detected its photo-response to IR irradiation. Photocurrent–voltage (I–V) measurements and spectral response measurements yielded a current of 10 µA cm⁻² under illumination with a 980 nm wavelength diode laser (10 mW) [19, 20]. In 2010, Shan and Demopoulos reported the application of a Yb³⁺/Er³⁺-codoped LaF₃–TiO₂ layer for a dye-sensitized solar cell (DSSC). In their investigation, based on illumination with a 980 nm wavelength fiber laser (fiber core diameter 105 µm) with 2.5 W power supply, open-circuit voltage V_OC = 0.40 V and short-circuit current I_SC = 0.036 mA were obtained [21]. The concept of UC is one of the most elegant concepts to generically increase the performance of all classes of solar cells. Some important improvement, including both theoretical analysis and experimental achievements, has been reported in the field of solar UC so far. However, until now, no significant performance enhancement due to UC has been reported for natural solar irradiation (AM1.5). One important reason is that the efficiency of the existing UC materials is still low. More efforts to design and develop high-efficiency UC materials are required.

In this work, we report for the first time the application of UC nanorods in combination with gold nanostructures to a flexible thin film solar cell on the steel substrate. NaYF₄:Yb³⁺/Er³⁺/Gd³⁺ nanorods with core–shell structures using gold nanoparticles or nanoshells have been prepared and applied in flexible hydrogenated amorphous silicon (a-Si:H) solar cells. Pure hexagonal phase NaYF₄:Yb/Er/Gd nanorods were synthesized by a liquid–solid reaction in oleic acid and ethanol solvents. Nonylphenol ethoxylate NP-10 was used as an amphiphilic surfactant to modify the hydrophobic surfaces of the as-deposited NaYF₄:Yb/Er/Gd nanorods. After this surface modification, gold nanoparticles or nanoshells were successfully attached to the surfaces of NaYF₄:Yb/Er/Gd nanorods. The fabricated core–shell structured nanorods were applied to the top transparent electrode of the a-Si:H solar cell. Experiments were performed to verify the enhanced solar cell response in the NIR. It was found that the UC of NIR light led to 16–72 times enhancement of the photocurrent under 980 nm laser illumination compared with an a-Si:H cell without upconverters. The mechanisms of UC emission of the grown upconverters and photocurrent increase of the formed UC a-Si:H solar cells are investigated.

2.1. Synthesis of NaYF₄:Yb/Er/Gd (18/2/30 mol%) nanorods

2.2. Attachment of gold nanoparticles

2.3. Growth of gold shells

2.4. Preparation of 980 nm laser-driven flexible a-Si thin film solar cells

2.5. Characterization and photoelectron measurements

The microstructures, morphologies and compositions of NaYF₄:Yb/Er/Gd nanorods without or with Au nanostructures were characterized by TEM studies. The as-prepared UC nanorods have a hexagonal structure with uniform size of 70–80 nm in diameter and approximately 300–500 nm in length (figure 1s in supplementary material available at stacks.iop.org/Nano/23/025402/mmedia). These results are well consistent with those of NaYF₄:Yb/Er/Gd (18/2/30 mol%) samples reported in [14]. Figure 1 shows TEM images of NaYF₄:Yb/Er/Gd nanorods with gold nanoparticles after 24 h Au attachment process and shells after 5 min Au shell reaction, respectively. It was found that, with the increased processing time for attachment of gold nanoparticles, the number of darker specks increased. Each dark speck corresponded to a gold nanoparticle (4–8 nm in size) on the nanorod surface (figure 1(a)). During the growth of Au shells, these Au nanoparticles functioned as the seeds for the nucleation of gold on the nanorod surface. As the reaction proceeded, these nanoseeds grew rather quickly and eventually merged together to form a continuous shell (figure 1(b)).

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Figure 2 shows UC emission spectra of as-synthesized and core–shell structured NaYF₄:Yb/Er/Gd nanorods. The emission spectra were collected under 980 nm laser excitation with a power of 500 mW at room temperature. For the as-synthesized NaYF₄:Yb/Er/Gd nanorods, the spectrum exhibits two strong emission bands at 540 and 660 nm, and a weak emission peak at 524 nm, corresponding to ⁴S_3/2 → ⁴I_15/2, ⁴F_9/2 → ⁴I_15/2 and ²H_11/2 → ⁴I_15/2 transitions of Er³⁺, respectively [11, 16]. The emission spectrum of the UC nanorods with Au nanoparticles or shells shows a significant increase in emission intensity. Compared with the case of the as-synthesized NaYF₄:Yb/Er/Gd nanorods without Au nanostructures, the emission intensity of UC nanorods with Au nanoparticles at 540 nm increases by a factor of 3.8 while the emission intensity at 660 nm has an enhancement factor of 4.0. Nanorods with Au shells show a UC emission enhancement factor of 3.2 and 9.6 at 540 nm and 660 nm, respectively. We suggest the enhancement effect from the gold nanoparticles or shells may be attributed to an increase of the excitation rate by local field enhancement at the location of the UC nanorods because of metal nanoparticle plasmonic resonances and by surface plasmon-coupled emission because of the coupling of the UC emission with the nanoparticle plasmonic resonances [15–18].

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It is well known for a-Si:H that sub-bandgap absorption arises due to a continuous density of localized states. As a result, a part of the NIR radiation transmitted through the upconverting layer is absorbed by the cell. To distinguish the response to upconverted light from the primary response due to absorption of sub-bandgap radiation, the response to NIR radiation of the solar cell without the upconverter but with otherwise the same cell structure as the reference was measured as well. Figures 3 and 4 show the image of an as-fabricated a-Si:H cell and the typical SEM image of a UC NaYF₄:Yb/Er/Gd nanorod thin film on the front surface of a mini-cell, respectively. From figure 4, the UC nanorods were very sparsely distributed over the whole front surface of the mini-cell. Figure 5 shows I–V characteristics of the as-fabricated a-Si:H mini-cell (denoted as the reference) and mini-cells covered with different kinds of upconverters under the irradiation of a 980 nm laser with an excitation power of 1100 mW. The reference cell shows a short-circuit current (I_sc) of 0.016 mA. There is a 16- to 72-fold improvement due to UC, leading to a short-circuit current I_sc of 0.26 mA, 0.44 mA and 1.16 mA in the cell covered by bare NaYF₄:Yb/Er/Gd nanorods, and UC nanorods with Au shells and nanoparticles, respectively. The Au shell case had a much lower I_sc than the Au nanoparticle case. This result can be explained by the following consideration. UC nanorods with Au shells show a strong increase in emission intensity at 660 nm, but a decrease in the green band, as shown in figure 2. Moreover, the quantum efficiency (QE) of a-Si cell in the red band (30% at 600 nm) is much lower than that in the green band (62% at 540 nm) (see figure S6 available at stacks.iop.org/Nano/23/025402/mmedia). As a result, the I_sc value for the Au shell case is much lower. We also found that the cell without and with different kinds of upconverters shows similar behaviors under AM 1.5 illumination (100 mW cm⁻²) (figure S7 available at stacks.iop.org/Nano/23/025402/mmedia). However, the short-circuit current and conversion efficiency (η) were higher for the latter case under simultaneous illumination of AM1.5 and 980 nm laser. For example, I_sc and η values of the cell with Au-nanoparticle- attached UC nanorods increased by 3.4% and 3.0%, respectively, compared to the reference cell without converters.

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In this work, the short-circuit current can be calculated as [22–24]

$$\begin{eqnarray} &&{I}_{\mathrm{sc}}=B k{P}_{\mathrm{exc}}^{n}, \end{eqnarray} \tag{ 1 }$$

or

$$\begin{eqnarray} &&\log ({I}_{\mathrm{sc}})=n\log ({P}_{\mathrm{exc}})+\log (B k) \end{eqnarray} \tag{ 2 }$$

where P_exc is the power of the 980 nm light intensity, n is the number of 980 nm photons required to produce one visible photon via the UC process, B is a constant for a fixed device and k is another constant depending on the crystallinity of the upconversion materials and the absorption efficiency of the incident light [24]. From equation (2), log(I_sc) is linearly dependent on log(P_exc). The dependence between the short-circuit current and the excitation power of the 980 nm laser was examined. Figure 6(a) shows short-circuit current I_sc as a function of excitation power (P_exc) for devices covered by different kinds of UC nanorods. The I_sc value was found to increase with the increase of P_exc for a fixed cell. The short-circuit current I_sc for the device based on nanorods with Au nanoparticles increases from 18 µA to 1.16 mA when the excitation power P_exc increases from 200 to 1100 mW. In order to further understand the relationship between the excited short-circuit current and the excitation power, the short-circuit current versus the excitation power was plotted in log–log scale in figure 6(b). The experimental I_sc data for the device covered by each kind of UC nanorods were fitted with a straight line. The n values can be easily calculated from the slopes of the linear fits. The n values were calculated to be 1.67, 1.78 and 2.58 for devices covered by bare upconverters and nanorods with Au shells and nanoparticles, respectively. They are consistent with two- or three-photon absorption processes [25, 26]. Non-integer n values are commonly observed in the intermediate power region, and the reduced slope or deviation from integers (2 or 3) is determined by the competition between linear decay and upconversion for the depletion of the intermediate excited states in NaYF₄:Yb/Er/Gd nanorods [27, 28].

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Different mechanisms are responsible for UC luminescence. The dominant UC mechanism in NaYF₄:Yb/Er/Gd nanorods is energy transfer upconversion. The mechanism of the upconverted emission of Er³⁺ has been well established in the literature [18, 23, 24, 29]. A typical energy-level diagram for the upconverted emission from NaYF₄ codoped with Yb³⁺, Er³⁺ and Gd³⁺ ions under infrared excitation is shown in figure 7. Er³⁺ ions (acceptors) are excited by the energy transfer from the Yb³⁺ (donor) that are excited directly (²F_5/2 → ²F_7/2) under 980 nm excitation. Part of the ⁴I_11/2 excited ion relaxes nonradiatively to the ⁴I_13/2 level and from here relaxes to the ground state. Then part was promoted to ⁴F_7/2 by the energy transfer from the relaxation of another excited Yb or Er (⁴I_11/2 → ⁴I_15/2) ion. The ⁴F_7/2 level decays nonradiatively to ²H_11/2 and ⁴S_3/2 due to phonon energy. From here, the population decay to the ground state producing green emissions centered at 524 and 540 nm, respectively. Also part decays nonradiatively to ⁴F_9/2 and finally decays to the ground state (⁴F_9/2 → ⁴I_15/2) producing the red emission centered at 660 nm. It is well known for an unsaturated mechanism that the intensity of the upconverted luminescence is proportional to some power n of the excitation intensity, where n = 2,3,..., is the number of pump photons required to populate the emitting state and is also determined from the slope of the line of the graph of intensity versus pump power in a log–log plot.

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The experimental I_sc data for devices covered by bare UC nanorods have been fitted with a straight line with a slope of 1.67, which confirms the two-photon absorption process. As shown in figure 6(b), the slope is the integration result of the green and red emissions. The experimental I_sc data for the device covered by nanorods with nanoparticles have been fitted with a straight line with a slope of 2.58, which confirms the three-photon absorption process. The two-photon process (figure 7(a)) is the main upconversion mechanism for bare NaYF₄:Yb/Er/Gd nanorods. However, the three-photon process (figure 7(b)) has been included or enhanced partly due to the presence of Au nanoparticles on NaYF₄:Yb/Er/Gd nanorod surfaces. For the cell using NaYF₄:Yb/Er/Gd nanorods with Au shell structure, the n decreases to 1.78, slightly smaller than 2, showing that the two-photon upconversion process dominates again. When the emitter is localized near the metal particle, the noble metallic nanostructure usually displays a plasmon resonance arising from the collective oscillation of migrated electrons on its surfaces. The quantum field near the emitter is dramatically altered through coupling with the metal plasmon resonance to cause a change of the excitation properties and impact on a more-photon upconversion involved in the UC process, resulting in an enhancement of UC fluorescence. When this UC structure is applied in the solar cells, the short-circuit current would be enhanced significantly. The slope n varies for the devices covered by NaYF₄:Yb/Er/Gd nanorods with Au nanoparticles and the Au shell nanostructure suggests that the size, shape and the configuration of gold present on the nanorod surfaces play an important role in modulating the excitation process, which may enhance the upconversion process involving more photons.

On the other hand, the external quantum efficiency (QE_up) under 980 nm laser illumination is related to I_sc by the equation (see supplementary material available at stacks.iop.org/Nano/23/025402/mmedia)

$$\begin{eqnarray} &&Q{E}_{\mathrm{up}}=\frac{{I}_{\mathrm{sc}}}{{P}_{\mathrm{exc}}q/h v}, \end{eqnarray} \tag{ 3 }$$

where q is the electron charge and hv the energy of the photon. Figure 8 shows QE_up as a function of excitation power P_exc for the devices based on bare NaYF₄:Yb/Er/Gd nanorods and nanorods with Au nanoparticles and nanoshells. The QE_up of the device based on nanorods with Au nanoparticles increases more strongly with increase in P_exc than that of the device based on bare nanorods or nanorods with nanoshells. A maximum external quantum efficiency value of 0.14% was achieved for 980 nm diode laser at a power of 1100 mW. This external quantum efficiency result is much higher than that reported by de Wild et al [19, 20]. They applied the UC phosphor NaYF₄/Yb³⁺ (18%)/Er³⁺ (2%) on the back of an amorphous silicon solar cell in combination with a white back reflector. In their work, a maximum current enhancement of 6.2 µA was obtained on illumination with a 980 nm diode laser, corresponding to a maximum external quantum efficiency of 0.03%.

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In this work, a new method to integrate UC material layers into flexible thin film solar cells has been proposed and reported. The NaYF₄:Yb/Er/Gd nanorods modified with Au nanostructures were placed on the front side of the flexible a-Si:H solar cell on a steel substrate, where they took up the sub-bandgap part of the solar spectrum and in situ re-emits visible light, which was resonant with the bandgap of the solar cell. Compared with the reference cell without UC nanomaterial, the solar cells based on NaYF₄:Yb/Er/Gd nanorods with different Au nanostructures show significant improvement of short-circuit current. The device based on NaYF₄:Yb/Er/Gd nanorods with Au nanoparticles shows the best NIR photovoltaic performance, displaying a short-circuit current of 1.16 mA and an external quantum efficiency of 0.14% under the irradiation of 980 nm laser. There is no need to reform or destroy the intrinsic structure of the solar cells or change the solar cell fabrication processes in this approach. The prepared UC materials can be applied and added to any solar cell technology: crystalline, thin film and organic/hybrid solar cells, allowing an increase in the efficiency of the photo-conversion process in the red region, a range notoriously poor for current commercial solar cells [30, 31]. The grown UC nanorods can be applied and placed on both sides of a solar cell, because of their high transmission in the visible wavelength range.

We have reported a simple approach for the preparation of upconversion NaYF₄:Yb/Er/Gd (18/2/30 mol%) nanorods coated with Au nanoparticles or Au shell nanostructures. The prepared UC nanorods were applied on the front of a-Si:H solar cells. The cell with the upconverters showed a 16- to 72-fold improvement of the photocurrent under 980 nm light, compared with the cell without an upconverter. The device using NaYF₄:Yb/Er/Gd nanorods with Au nanoparticles achieved a photocurrent of 1.16 mA and an external quantum efficiency of 0.14% under the irradiation of 980 nm laser with a excitation power of 1100 mW. Plasmon-enhanced multi-photon luminescence in NaYF₄:Yb/Er/Gd nanorods with Au nanostructures were responsible for the NIR photovoltaic performance. It is expected that the unique properties and functions offered by these core–shell structured NaYF₄:Yb/Er/Gd nanorods will enable its wide implementation in photovoltaic applications.

This work was supported by the National Natural Science Foundation of China (no. 10774046), the Shanghai Municipal Science and Technology Commission Foundation (no. 09JC1404600) and the Defense Industrial Technology Development Program (no. B0320110005).