ADVANCED SOLAR CELL CONCEPTS

C. S. Solanki1 and G. Beaucarne2 1Energy System Engineering, IIT Bombay, Powai, Mumbai-400076, India.

2Interuniversity Microelectronics Center (IMECvzw), Leuven, Belgium Phone: +91-22-2576-7895, Fax: +91-22-2572-6875 / 2576-4890, e-mail: chetanss@iitb.ac.in

Abstract In order to ensure the widespread use of solar photovoltaic technology for terrestrial applications, cost per unit watt must be significantly lower than 1$/Watt level. Material limitation of wafer based Si cell technology and efficiency limitation of thin-film solar cell technologies needs to overcome in order to achieve the above-mentioned cost goal. Thermodynamically solar cell efficiencies can be as high as 86%.. Progress in implementation of advanced concepts to achieve high cell efficiencies at moderate cost is reported in this paper. New concepts that strive for better utilization of Sun’s spectrum, hence better cell efficiency are under development. In multi-junction solar cells, better spectrum utilization is obtained by stack several solar cells. A record efficiency of over 39% has been achieved under 236 suns light concentration. Theoretical analysis of Impurity Photovoltaic and Intermediate Band solar cells have shown that achievable cell efficiency is about 63.2% for single impurity level and single intermediate band. Photon shifting of UV photon has been demonstrated by fabricating Si nanocrystals in SiOx and SiNx matrix. Proof of concept has been shown for quantum confinement in Si quantum dots and enhanced photon absorption using metal nanoparticles on solar cell surface. Similarly, most of the advanced concepts are in the initial stage of experimentation. Significant breakthroughs are required before these concepts can contribute in the mainstream PV production. 1. Introduction Solar cell technologies are an attractive option for clean and renewable energy generation in the form of electricity. Several technologies have been developed since 1950s and many of them are now reached to a stage of commercialization. But photovoltaic (PV) solar electricity is still not economical in comparison to the grid power that we use today. The cost of photovoltaic electricity is about 60 $cents/kWh in temperate climate and about 30 $cents/kWh for climates with a high solar insulation. This is substantially higher than the retail price of conventional electricity and much higher than bulk electricity price. The main reason for the high cost of present PV modules is the high cost of the base material, ultra-pure Silicon (Si) wafers, mostly used in today’s solar cell technologies. Si solar cells, which contribute 94% of the world market, are made from single crystalline - and multi-crystalline silicon (sc-Si & mc-Si). Because of the high electronic quality of sc-Si and mc-Si (diffusion lengths in the range of 100s of micrometer) cells with stable and reasonably high efficiencies (ranging from 14 to 25%) can be realized in these materials. But the high cost of ultra-pure Si combined with a large material consumption (200-300 m wafer thickness) results in a high cost of finished PV module. Therefore, much of the research efforts are geared towards solar cell fabrication on very thin substrates and on Si ribbons, which consume much less Si per unit area. Alternative technologies that strive for further reduction in cost with reduced material consumption material are termed as thin film solar cell technologies. These include technologies based on materials like amorphous Si (a-Si), thin-film polycrystalline Si, polycrystalline Copper-Indium-Gallium-Disellenide (CIGS), polycrystalline Cadmium Telluride (CdTe) and organic solar cells. Although thin-film technologies have been considered as promising candidates for low cost PV power for a long time, none of them have so far had a real breakthrough, and bulk crystalline silicon appears likely to dominate further the photovoltaic field for at least one more decade. However, because of the compelling argument of the need for material cost reduction, it appears likely that a thin-film technology eventually will replace the conventional bulk Si technology. While reducing material cost will achieve a large cost reduction, further cost reduction to achieve a cost of photovoltaic electricity below the bulk electricity price will only be possible if the conversion efficiency is increased to much higher values than those of conventional Si solar cells. To achieve this however, more

advanced solar cell structures are needed. In this paper, we present and discuss the advanced solar cell concepts that have been proposed and that are being developed. 2. Fundamental limits to solar energy conversion The maximum efficiency one can reach with the conventional solar cell structure is given by the so-called single material or Schockley-Queisser (Shockley and Queisser, 1961) limit. Using a detailed balance approach and assuming that each photon above bandgap gives rise to just one electron-hole pair, while all photons with energy below the bandgap is lost, one comes to a theoretical maximum energy conversion efficiency of about 30 %. This is relatively low, mainly because of the thermalisation and transmission losses, which account for about 56% loss of photon energy. These losses are the results of the fact that a single energy band gap is used to convert wide range of solar energy photons. This is demonstrated in Fig. 1(a). Red photons having energy lower than the band gap are not absorbed, hence loss of energy, while the energy of the blue photon, having higher than the band gap energy, is only partially utilized. It would be wrong to consider the Shockley-Queisser limit as the ultimate limit one can reach with photovoltaics. The physical limit is actually much higher. Considering the Sun as a black body at 5760 K and solar cell (another black body) at 300 K, the Carnot efficiency limit is 95%. Landsberg and Baruch, (1989) calculated energy conversion limit of 93.3%, in which radiative losses from the solar cell was also considered. Taking into account entropy generation during the energy conversion, one obtains a black body limit of about 86%. Efficiencies of this magnitude can be obtained by using monochromatic filter with an ideal solar cell wherein the cell band gap is matched with the non-filtered frequency (see Fig. 1(b)).

The large difference between the Shockley-Queisser and the thermodynamic limit arises from the fact that a single material is characterized by only two energy levels, whereas the solar spectrum contains photons with a wide range of energies. Devices that rely on a single transition between energy levels are intrinsically ill-suited for the broad spectrum of sunlight. What is needed is to make more effective use of solar spectrum. Different concepts have been proposed to overcome the Shockley-Queisser limit of light conversion efficiency. The concepts are based on splitting of sun’s spectrum to be absorbed by many cells (e.g. multijunction solar cells), adopting solar spectrum for one host material (e.g. up and down-conversion of photon energy) and reduction in thermalisation losses (e.g. hot carrier cells). 3. Multijunction solar cells The first concept is multijunction solar cells or tandem solar cells. Here, multiple solar cells are stacked on each other in decreasing energy band gap. The first material absorbs the high energy photons, but transmits photons with lower energies, which then get absorbed in the cells below.

Single band

Solar cell

Filter Monochromatic filter

Black body absorber

Solar cell

(a) (b)

Figure 1: (a) Loses in a single band gap solar cell due to absorption of broad Sun’s spectrum, (b) monochromatic filter with ideally suited solar cell can achieve the energy conversion efficiency as high as 86%.

red blue green

Multi-band gap III-V triple junction solar cells have been successfully developed (Fig. 2). In this case the top cell is made up of GaInP2 (Eg - 1.89eV), middle cell is GaAs (Eg – 1.42 eV) and bottom cell is Ge (Eg - 0.67 eV). In the two contact multi-band gap solar cell the design challenge is to match current from each cell. Fig. 2 demonstrates the use of three solar cells to absorb broad Sun’s spectrum. The highest efficiency achieved so far is over 39.0% with triple junction GaInP/GaInAs/Ge solar cell under high light concentration (236 Suns) (King et al, 2005). These solar cells were originally developed for space applications, where efficiency is of prime importance whereas requirements in terms of cost less stringent. There is now a large interest in implementing this technology for terrestrial applications, using concentrating systems (Frideman et al, 1995, and King et al, 2005). Here, light is concentrated onto a small area tandem solar cell. Thanks to light concentration, the cell cost can be kept low in spite of the expensive materials needed to make it, because only a small area is needed. However, this is at the expense of increased system complexity. Indeed, because of the large energy fluxes, the solar cell needs to be effectively cooled. Moreover, a tracking system has to ensure that the cell is constantly directed towards the sun.

To be able to make flat plate 1-sun high efficiency photovoltaic modules based on tandem solar cells, cheaper materials should be used. Silicon can act as an excellent material for the bottom cell in dual junction solar cells, but it is not obvious which material can be used for the top cell that is potentially low-cost. Some researchers investigate silicon quantum dots in a dielectric matrix with the aim of creating an artificial wide bandgap Si-based material. (De Torre et al, 2006, Conibeer et al, 2006) The band gap tenability of Si quantum dots in oxide matrix has been demonstrated by showing photo-luminescence (PL) energy shift as a function of dot size (2 to 7 nm diameter) (Conibeer et al, 2006). The main issue here is to achieve sufficient regularity of the quantum dots in order to reach the transport properties required. 4. Impurity photovoltaic effect, intermediate band solar cells and quantum well solar cells The impurity photovoltaic (IPV) effect is the idea of exploiting two-step generation via impurity states within the band gap to utilise sub-band gap photons and therefore enhance solar cell performance (see Fig. 3(a)). Three transitions available in that system, in principle, enables a better matching to the solar spectrum. High efficiencies have been predicted for IPV solar cells. The challenge is to find a suitable host wide band gap semiconductor combined with a sufficiently radiatively efficient impurity (Beaucarne et al, 2002). A related concept is the so-called intermediate-band (IB) solar cell, characterized by the existence of a narrow band within the main band gap (see Fig. 3(b)). For this concept, experimental work has been carried out using the InAs quantum dots embedded in GaAs (Luque et al, 2005). Evidence of generation from sub-band gap was found. However, the concept is still far from enabling an efficiency increase above conventional cells. The limiting solar cell efficiency for single impurity level and single intermediate band gap is above 63%(Luque and Marti, 1997, Green et al, 2002) which is same as the limiting efficiency of stacked solar cells (three). Through

Figure 2 : Triple junction GaAs solar cell structure with internal quantum efficiency of three cell covering the entire Sun’s spectrum.

the advantage in IPV and IB concepts is that the same material is used throughout and the interconnection between cells occurs automatically. A quantum well (QW) (Barnham and Duggan, 1990) solar cell is a multiple-band gap device with intermediate properties between heterojunction cells and tandem solar cell. In a heterojunction solar cell, the total current is sum of the currents generated in the different materials but voltage is controlled by the lowest of the two band gaps and while in a tandem cells, the total voltage is sum of the voltages but current is determined by the worst of the two sub-cells. Marginal efficiency enhancement of 2% is demonstrated in GaAsP/InGaAs (barrier/well) QW cell (Mazzer et al, 2006). The increase in efficiency is attributed to absorption edge enhancement in QW structure.

Fig 3: Schematic diagram showing absorption of above band gap and sub-band gap photons through, (a) impurity level, (b) intermediate band. 5. Hot carrier solar cells Hot carrier solar cell are designed to collect electron-hole pair before they thermalise to (gets cooled to) their respective band edges. This requires both an absorber with slowed carrier cooling properties and collection of carriers over a limited range of energies, such that cold carriers in the external contacts do not cool the hot carriers to be extracted (Conibeer et al, 2006). In this case the design of contacts is a major challenge. Contacts based on resonant tunneling using quantum dots have been proposed (Jiang et al, 2004). 6. Enhanced absorption through spectral conversion and plasmonic effects The concepts presented in section 4 and 5 require major change in the structure of the device, but there are a few concepts that could possibly be implemented by just adding layers on top or at the bottom of conventional cells. This is attractive because one can keep the assets of the existing technology, whereas the implementation of a completely new structure and material sets often leads to performance loss. Up-conversion or down-conversion concepts in which the Sun spectrum is converted into a smaller energy range by converting two or more low energy photons into a high energy photon or converting one high energy photon into two or more low energy photons. Photon shifting can also be used, where by a luminescence process, a high energy photon is converted into a single lower energy photon. Energetically, the latter process is not favourable, but there might still be some gain at solar cell level for industrial cells, which feature a rather poor response for short wavelength light. Up-conversion has been demonstrated by putting NaYF4:20%Er3+ at the rear of the cell (Shalav et al, 2005) and photon down-shifting has been demonstrated by putting Si nanocrystals in spin-on-glass matrix on the front of solar cells (Svrcek et al, 2004). Photon conversion to lower energies has also been demonstrated (see Fig. 4) by fabricating silicon nanocrystals in SiOx and SiNx matrix to be put on the top of cell to work as anti-reflective coating which additionally will provide quantum efficiency enhancement in UV range (De Torre et al, 2006 ). In one approach, metal nanoparticles (Ag or Au, with particles sizes in the range of 20 to 100 nm) are deposited on the solar cell surface such that the surface plasmon can be excited in the nanoparticles layer. Such surface plasmon excitation results in large electromagnetic field enhancement near the metal surface where the

Sub-band gap photons

Impurity level

sub-band gap photons

Intermediate band

(a) (b)

absorption of photon increases (Schaadt D. M. et al, 2005, Stenzel O. et al, 1995). Schematic diagram of the device is given in Fig. 5(a). Nanoparticles can be varied in shape and size to enhance the absorption of a photon of given wavelength. This principle is particularly suited for absorption of long wavelength photons (> 700 nm) in thin layer of Si, which otherwise requires very thick material. At or near the plasmon resonance wavelengths, increases in photocurrent response of 50%–80% or more relative to that of the Si p-n junction diode has been recently demonstrated by D. M. Schaadt et al (2005) (refer to Fig. 5(b). Considerably larger increases may be possible provided nanoparticles size isoptimised and large density of particles can be deposited.

Figure 4: The photocurrent spectrum obtained from the 3 to 5 nm Si nanocrystals dispersed in SiOx matrix showing the increase of absorption at high excitation energy compared to the reference sample (without SiOx layer)( De Torre et al, 2006) .

Figure 5: (a) Schematic representation of use of metal nanoparticles for surface plasmon excitation to enhance absorption, (b) Photocurrent enhancement as a function of wavelength for different metal nanoparticles sizes (Schaadt D. M. et al, 2005). 7. Conclusion There are several new solar cell concepts that aim at making better use of the solar spectrum and doing so achieve much higher energy conversion efficiencies than the present conventional solar cells. The only concept that has demonstrated higher efficiency in practice so far is the multijunction solar cell based on InGaP/GaAs/Ge solar cells, which is presently used for space application and is also being introduced for terrestrial concentration system. The other concepts are presently at the stage of fundamental research, but might lead to very high solar cell efficiency at lower cost. 8. References Beaucarne G., A. S. Brown, M. J. Keevers, R. Corkish, 2002. M. A. Green, Prog. Phot. Res. Appl. 10, 345-353. Barnham K.W.J., G. Duggan, 1990. A new approach to high efficiency multi-band-gap solar cells, J. Appl. Phys. 67, 3490-3493.

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