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EETE NOV 2013

Fig. 2: Multi-junction solar cells mechanically stacked with terminals for each cell, versus monolithically integrated twoterminal multi-junction cells. In previous research, Pieter highlighted that by mechanically C stacking cells connected in parallel rather than using a monolithic M cell stack suffers less from varying light conditions (which can Y affect differently the bandgaps and will turn some of them CM into loads in a series configuration). In fact, based on device performance measurements and climatological information, the MY researcher was able to calculate an overall annual energy yield CY increase of up to 25% for mechanically stacked cells, simply because this type of cells is less sensitive to spectral variations caused by time of day changes and atmospheric conditions – see figure 3. In comparison, the performance of currentmatched build with the different bandgaps all in series, the overall CMY K Energy democratization will come: it’s called multi-junction photovoltaics By Julien Happich The mod was very optimistic at the 28th European photovoltaic solar energy conference (EU PVSEC) taking place in Paris early October. The scientific opening session brought to light very promising results and new cell concepts enabled by III-V material combinations grown into nanowire arrays. The conference was also an opportunity for delegates of the European Commission’s Joint Research Center to present their PV Status report for 2013, and discuss the strategies needed to increase the role of photovoltaics in the European energy landscape. Different ways to stack multi-junction solar cells In his keynote presentation, head of the division “Materials – Solar Cells and Technology” at Fraunhofer ISE, Dr. Andreas W. Bett unveiled state-of-the-art results in solar cell efficiency, claiming a world record at 44.7% of efficiency with a four-junction structure (four subcells). This record efficiency was measured at a concentration of 297 suns, the cell being able to convert 44.7% of the solar spectrum’s energy from ultraviolet through to the infrared (each subcell absorbing different wavelength ranges, see figure 1). In solar plants, using glass or plastic optics to concentrate the sun’s ray on smaller stacked cells also comes out cheaper than spending silicon on larger areas. W. Bett said on a side line that optical spectrum splitting, although feasible, was not cost efficient, hence the cell-stacking option. Instead of taking the standard monolithic layered approach of growing lattice-matched layers of III-V compound semiconductors, W. Bett looked at detailed simulation results to find the most efficient stack of energy band gaps. His lab then relied on metalorganic vapour phase epitaxy (MOVPE) to reach the right material combinations and optimum bandgaps, using a novel metamorphic growth concept whereby one builds buffer layers of progressive compositions between stacks of slightly mismatched lattice constants. According to W. Bett, this strategy gives more flexibility in the choice of materials, hence the higher photovoltaic conversion efficiency achieved. Fig. 1: Fraunhofer ISE’s four-junction solar cell and its external quantum efficiency as measured by the Fraunhofer ISE CalLab. Not all would agree with the overall cost efficiency of this approach, at least not Philip Pieter, Development Director at imec. According to Pieter, building multi-junction cells is better done through the mechanical bonding of different cells (see figure 2), each chosen for their efficiency rather than for their crystal lattices or current matching. monolithically grown multi-junction cells can be limited by a top underperforming junction in the early morning and late afternoon due to red-rich spectra (at sun rise and sun set). Fig. 3: Comparing the overall annual energy yield of mechanically stacked versus monolithically integrated multijunction cells, based on calculations and climatological data. Enhancing light absorption Maybe still exotic at this stage, the research presented by Lars Samuelson, Professor of Semiconductor Physics at Lund University (Sweden), is an attempt to bring down the cost of single crystal III-V solar cells to that of thin films solar cells. Samuelson has recently published 13.8% efficiency results for an InP nanowire array solar cell, based on resonant lighttrapping in 180-nm diameter nanowires grown to feature junctions and heterostructures (axially and radially) – see figure 4. Grown by MOVPE using patterning by nano imprint lithography, 6 Electronic Engineering Times Europe November 2013 www.electronics-eetimes.com


EETE NOV 2013
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