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Solar & Alternative Energy
Enhancing solar cell efficiency by tailoring the spectrum
Silicon nanocrystals can shape incident solar radiation and ‘cut’ high-energy photons into multiple low-energy ones to increase photovoltaic energy conversion.
5 March 2012, SPIE Newsroom. DOI: 10.1117/2.1201202.004136
Silicon (Si) is the most common material for contemporary photovoltaic devices due to its suitable physical and technological properties. However, the quantum efficiency of Si modules is limited, mainly as a consequence of the mismatch between the solar spectrum and the device's absorption characteristics. To overcome this, ‘spectral shapers’ can convert the solar spectrum to better match the device's ability to absorb photons. Quantum dots can be used for this purpose, and in particular Si nanocrystals (NCs) are the high-potential candidates. In addition to being relatively cheap and easy to incorporate in current configurations—for example, NCs can be embedded in a (quartz) coating layer—they feature very desirable physical properties that could facilitate efficient conversion.
At nanometer size, the energy structure can be manipulated through quantum confinement. It is well known that excitonic emission from Si NCs is blue-shifted for smaller sizes and increases in intensity.1 These effects reflect the widening of the indirect bandgap and enhancement in the radiative recombination rate of electron-hole pairs, as momentum conservation is gradually relaxed due to Heisenberg's uncertainty principle. Figure 1 displays emission from Si NCs of different diameters between 2.5 and 5.5nm, dispersed in an SiO2 matrix (red curves). The photoluminescence (PL) maxima shift from λPL=750nm (light red) for the smallest NCs to λPL=990nm (dark red), while the width of the PL bands reflects the NC size distribution. To demonstrate the suitability of Si NCs for photovoltaics, the conversion efficiency of a conventional Si-based solar cell is also displayed in Figure 1 (blue curve). The trace range is 400–1200nm. Evidently, the emission of the Si NCs overlaps very well with the maximum energy-conversion range of the solar device.
Figure 1. Normalized photoluminescence spectra of silicon (Si) nanocrystals (NCs) embedded in an SiO2matrix for different average diameters of NCs (right axis). Emission of these sizes of Si NCs covers the best conversion range of the solar spectrum. The blue curve represents the conversion efficiency characteristics of a conventional Si solar cell (left axis).
In addition, Si NCs exhibit a unique carrier multiplication (CM) process.2 CM—the generation of multiple excitons on absorption of a single (high-energy) photon—can increase conversion efficiency of a solar device by using energy that is normally lost to heat, and so is very attractive for photovoltaics. At present, CM has been found in different kinds of nanocrystalline materials, such as cadmium selenide,3 lead telluride,4 lead sulfide,5 lead selenide,6 and also in (colloidal) Si NCs.7 In contrast to these materials—where the extra excitons are generated in one and the same NC—the multiple excitons in the high-density dispersion of Si NCs in SiO2 appear in adjacent NCs. As a result, their lifetimes are microseconds rather than the picoseconds characteristic of the ‘conventional’ CM process.
We obtained this remarkable finding in a PL experiment, where we determined the absolute quantum efficiency (QE)—the ratio between the number of absorbed and emitted photons—as a function of the energy of the incident photons. As can be seen in Figure 2(a), the QE has a constant value up to a certain threshold, after which it increases. The threshold energy can be as low as roughly twice the NC optical bandgap. The formation of two and then three excitons per absorption of a single high-energy photon causes the steplike behavior, which has been theoretically modeled in the past but previously observed only for carbon nanotubes.9
Figure 2. Photoluminescence quantum efficiency (QE) vs. excitation photon. (a) Ratio of emitted vs. absorbed photons for the sample with Si NCs embedded in an SiO2 matrix. After a plateau (with value ∼6:5%), the QE increases for photon energies exceeding Eexc > 2Egapto a value of ∼13%. For even higher energies, the QE grows again until it reaches about three times its initial value. The black dotted line is a guide to the eye. (b) Induced absorption traces for a solid-state sample with Si NCs with an average diameter of dNC=3nm. IA transient obtained for high photon energy (Eexc > 2Egap, black) has about double the intensity of the low photon energy excitation transient (red). Normalizing the red trace on top of the black reveals great similarity between the dynamics for both excitations. The inset shows that within the time resolution of the experiment (τresolution≈100fs), the buildup of the IA signal for both excitation conditions is identical.
We carried out induced absorption (IA) experiments to further investigate the increased carrier generation rate responsible for the QE enhancement.10 This technique enables tracking of excitons on a femtosecond timescale, which could also give insights into the mechanism behind the CM process. Figure 2(b) shows the IA traces of 3nm-sized NCs for above (black) and below (red) CM-threshold excitations, in a 1ns time window. The two traces show striking similarity (most evident when normalized: see the red dashed line), with no evidence of Auger interaction of multiple carriers within the same NC. The difference is found in the amplitude of the IA signal (factor ∼2), when scaled for the same number of absorbed photons. Since IA intensity depends on the concentration of generated carriers, we conclude that for the high-energy excitation (Eexc>2Egap), where Eexc is the excitation energy and Egap is the size of the band gap, more excitons are created per photon than in the low-energy range Eexc<2Egap. Combined with the absence of an Auger-related component for the above CM-threshold pumping, this indicates that the multiple excitons are separated into different NCs before they can interact. The separation mechanism must be very fast and efficient, since no difference can be found in the buildup characteristics of the IA signals obtained for the two excitation conditions: see the inset to Figure 2(b).
Figure 3 summarizes the enhancement of the solar conversion efficiency that might be achieved with Si NCs. As can be seen, in the high-energy region Si NCs can double the number of photons by CM. This is the ‘photon cutting’ process. Advantage is also obtained for the visible part of the solar spectrum, where Si NCs provide conversion of photons into the range for which the solar device has the highest efficiency. At this stage of research, we estimate that with Si NCs a total increase of the number of photons in the converted solar spectrum of up to ∼8% might be possible.8 We now plan to explore whether Si NCs might also be used for harvesting IR photons, which so far remain mostly outside the reach of photovoltaic devices.
Figure 3. Solar spectrum divided into different ranges: for the high photon energies, Si NCs can ‘cut’ photons and convert them into two (or more) low-energy photons. In the visible regime, photons can be converted to the range where the Si-based solar cell has its maximum conversion efficiency. The inset presents a schematic illustration of a solar device with an additional top layer of Si NCs that functions as a spectral convertor.
Wieteke de Boer, Tom Gregorkiewicz
University of Amsterdam
Amsterdam, The Netherlands
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