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We studied the photoluminescence (PL) and photovoltaic current-voltage characteristics of the three-junction InGaP/InGaAs/Ge solar cells by depositing Au nanoclusters on the cell surface. The increases of the PL intensity and short-circuit current after incorporation of Au nanoclusters are evident. An increase of 15.3 % in energy conversion efficiency (from 19.6 to 22.6 %) is obtained for the three-junction solar cells in which Au nanoclusters have been incorporated. We suggest that the increased light trapping due to radiative scattering from Au nanoclusters is responsible for improving the performance of the three-junction solar cells.
©2008 Optical Society of America
1. Introduction
Multi-junction tandem solar cells consisting of III-V compound semiconductors have attracted considerable attention due to their very high conversion efficiencies and potential for space applications [1–3]. Very recently, conversion efficiencies as high as 40 % have been demonstrated in the three-junction InGaP/InGaAs/Ge cells [4]. To implement higher efficiency, it is important to improve absorption properties in the active layers. A well-known method for improving the optical absorption of the photovoltaic devices is to trap the incident light in the absorbing region by metal nanoparticles. The surface plasmons or/and interband transitions in metal nanoparticles can couple with incident light, producing strong scattering and absorption of incident light and hence improving performance in optoelectronic devices. For example, an enhancement in photocurrent response from photodetectors due to plasmon excitation of metal nanoparticles has been demonstrated [5–6]. In addition, incorporating the metal nanoparticles on solar cells has led to an increase of light absorption in the active layers, which resulted in an increase of the photovoltaic conversion efficiency [7–10].
So far most of the metal nanoparticles mentioned above have dimensions of particles about 5–100 nm. However, when the metal nanoparticles become smaller than ~2 nm, they exhibit unique structures and properties. For instance, the metal nanoclusters display a strong luminescence from visible to near-infrared range, which may make them useful for applications in light emitting sources, chemical sensing, and biological labeling. The luminescence from the metal nanoclusters has been viewed as a radiative recombination of Fermi level electrons and d-band holes or an excited-state highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) transition [11–13]. In this article, we study the influence of photoluminescence (PL) and photovoltaic current-voltage characteristics by incorporation of Au nanoclusters on the three-junction InGaP/InGaAs/Ge solar cells. It was found that Au nanoclusters enhance PL intensity and short-circuit current, hence significantly improving energy conversion efficiency for the three-junction InGaP/InGaAs/Ge solar cells.
2. Experiment
The samples investigated were composed of monolithic cascade-type InGaP/InGaAs/Ge three junctions connected in series. The In0.5Ga0.5P top subcell, the In0.01Ga0.99As middle subcell, and Ge bottom subcell were all lattice-matched and grown on a p-type Ge substrate. The InGaP subcell was connected to the InGaAs subcell by a p-AlGaAs/n-InGaP tunnel junction. The InGaAs subcell was connected to the Ge subcell by a p-GaAs/n-GaAs tunnel junction. The details of the sample structure are described elsewhere [14]. The Au nanoclusters investigated were prepared according to a modified Peng reaction [15]. In brief, the gold precursor solution was prepared by dissolving AuCl3 in the DDAB (Didodecyldimethylammonium bromide) solution. Decanoic acid was then combined with TBAB (Tetrabutylammonium borohydride) in toluence, followed by the gold precursor solution. The reduction of Au ions was realized quickly under vigorous stirring, leading to the formation of a dark-red Au colloid. The Au nanoparticles were further fragmented by adding the gold precursor solution and DDT (dodecanethiol) in toluene, leading to formation of the luminescent Au nanoclusters. The PL measurements were performed using a diode laser (470 nm) as the excitation source. The collected luminescence was dispersed by a grating spectrometer and detected with a photomultiplier (PMT) tube.
3. Results and discussion
The size of the Au nanoclusters was determined by high-resolution transmission electron microscopy (HRTEM). Figure 1 shows the HRTEM micrograph obtained for the Au nanoclusters deposited on an electron microscope grid from colloidal solution. An uniform distribution of well-dispersed nanoclusters is seen. The statistics of the size distribution as determined from the above and other micrographs obtained from different regions indicate an average size of 1.9±0.2 nm.
PL technique has been applicable to solar cell development because it not only conveniently gives insight into nature of the materials in the active layer, but also provides information for monitoring the fabrication of solar cells. Since the PL intensity of the active layer is strongly correlated to the open-circuit voltage in solar cells [16], the increase of the PL intensity reveal the increase of the conversion efficiency in solar cells. Figure 2(a) shows the PL spectrum of the three-junction InGaP/InGaAs/Ge solar cells. The PL peak energy is about 1.81 eV at room temperature, indicating the InGaP layers have a smaller band gap than that of the completely disordered InGaP (~2.005 eV) and are partially ordered [17]. To study the influence of Au nanoclusters on PL properties of the three-junction solar cells, the synthesized Au nanoclusters were deposited by placing a drop of Au colloidal solution onto the surface of the solar cells. Figure 2(b) shows the PL intensity after incorporation of the Au nanoclusters, indicating an increase of the PL intensity and thus a possible increase of the energy conversion efficiency. The enhancement ratio IAu/Iwithout of the three-junction InGaP/InGaAs/Ge solar cells is displayed as the open circle in Fig. 3, where IAu (Iwithout) denotes the luminescence intensity with (without) the introduction of Au nanoclusters.
Fig. 2. PL spectra of the three-junction InGaP/InGaAs/Ge solar cells: (a) before and (b) after incorporation of Au nanoclusters
Fig. 3. Enhancement ratio of luminescence as a function of photon energy. The open circle (square) shows the measured data of the three-junction solar cells (InGaN/GaN LEDs). The solid line displays the calculated result using Eq. (3).
To find out mechanism of the luminescence enhancement, the photon energy dependence of the enhancement ratio has been investigated. Two InGaN/GaN light emitting diodes (LEDs) with different emission energy have been taken as the reference samples. The enhancement ratio IAu/Iwithout for these two reference samples after incorporation of Au nanoclusters has also been measured and plotted in Fig. 3. It is found the enhancement ratio depends on the emission energy. In previous studies the enhancement of the absorption or luminescence intensity for incorporation of metal nanoparticles has been related to light trapping by radiative scattering from nanoparticles [5, 10]. Here, based on the radiative scattering from the Au nanoclusters we try to explain the dependence of enhancement ratio on emission energy. For small particles in the quasistatic limit, the radiative scattering efficiency can be estimated from the absorption and scattering cross-sections Cabs and Crad of the individual particles: [5,10]
where α is the polarizability of the particle, determined by
for a small spherical particle in vacuum, where V o is the volume of the particle. ε(ω) is the dielectric function of the metal nanoclusters, which depends on the bulk plasmon frequency, the rate of scattering from the particle surface, and the interband transition characteristics of the nanoclusters [18]. The radiative scattering efficiency Qrad, given by
represents the fraction of the extincted energy that is reradiated. According to Eqs. (1)–(3), Q rad versus photon energy can be calculated, as displayed in the solid line of Fig. 3. As can be seen, the calculated curve agrees closely with the experimental results. Thus, the PL enhancement in Fig. 2 can be interpreted by the increased light trapping due to radiative scattering from Au nanoclusters.
To investigate how Au nanoclusters influence the photovoltaic effect in the three-junction solar cells, the photovoltaic current-voltage (I-V) characteristics were measured under AM1.5 global solar spectrum with 1 sun total intensity (100 mW cm-2). Figure 4 shows the photovoltaic I-V characteristics of the three-junction InGaP/InGaAs/Ge solar cells before and after the incorporation of Au nanoclusters. The device parameters for the investigated cell without (with) Au nanoclusters on the cell surface are Isc=2.64 (2.99) mA, Voc=2.4 (2.41) V, and FF=0.79 (0.8), where Isc, Voc, and FF are short-circuit current, open-circuit voltage, and fill factor, respectively. The incorporation of the Au nanoclusters yields a 13.3 % increase in short-circuit current for the three-junction InGaP/InGaAs/Ge solar cells. This observation indicates the absorption intensity of incident light in the active layers is enhanced, resulting in an increased photocurrent. We suggest that Au nanoclusters provide light-trapping effect and produce an increase of light scattering into the active layers in the solar cells, leading to the absorption enhancement of the incident light. The increase of Isc leads to a 15.3 % increase in energy conversion efficiency (from 19.6 to 22.6 %) for the three-junction InGaP/InGaAs/Ge cells. To our knowledge, the 15.3 % increase in energy conversion efficiency is better than previous reports on similar increase in energy conversion efficiency by using the large-size metal nanoparticles [8–9].
Fig. 4. I-V characteristics of the three-junction InGaP/InGaAs/Ge solar cells: (a) without and (b) with incorporation of Au nanoclusters.
4. Conclusion
The PL and photovoltaic current-voltage characteristics of the three-junction InGaP/InGaAs/Ge solar cells with Au nanoclusters on the cell surface were investigated. The PL intensity was found to increase after incorporation of Au nanoclusters. This observation was explained by the increased light trapping due to radiative scattering from Au nanoclusters. A 13.3 % increase in short-circuit current (from 2.64 to 2.99 mA) and a 15.3 % increase in energy conversion efficiency (from19.6 to 22.6 %) are obtained for the three-junction InGaP/InGaAs/Ge solar cells where Au nanoclusters have been incorporated.
Acknowledgment
This project was supported in part by the National Science Council under the grant numbers NSC97-2112-M-033-004-MY3 and NSC 97-2627-B-033 -002, and the Institute of Nuclear Energy Research under the grant number 962001INER0041.
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