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Study on intermediate band materials with prevailing radiative carrier recombination for superior solar energy applications

Final Report Summary - SIRACUSA (Study on intermediate band materials with prevailing radiative carrier recombination for superior solar energy applications)

Solar cells (SCs) fall into two general types, large area low cost devices for one sun applications and small area high efficiency devices for concentrator applications. For the latter application, efficiency is a key factor. At present the highest efficiency SCs are triple junction devices with an efficiency ~44% under concentration. However, in such SCs the current in each junction must be matched and if the distribution of solar energy changes e.g. due to clouds obscuring the direct sunlight, the overall efficiency drops and the current is determined by the least efficient of the three junctions. Several proposed third generation SCs seek to overcome this restriction [1], including the intermediate band solar cell (IBSC) concept [2].

The overall objective of this project was to develop an advanced, third generation, solar cell for concentrator applications based on the intermediate band concept [2]. Various approaches to this have been based on insertion of quantum dots (QD) in the intrinsic region of a p-i-n semiconductor, but the absorption of radiation using this method is limited due to the finite number of QDs. Instead our approach is by the insertion of an intermediate band based on deep level impurities [2]. In order to achieve this overall goal, we have grown by molecular beam epitaxy (MBE) GaAs structures containing very large concentrations of deep levels, including Fe, Cr & Co. According to the theory [2], at a sufficiently high concentration the wavefunctions should overlap to form a band within the intrinsic region of the GaAs and thus suppress the non-radiative processes normally associated with mid-gap deep levels. Due to the finite solubility of such impurities at conventional MBE growth temperatures, it is essential to grow the samples at very low temperature, which in turn can introduce deleterious point defects. By carefully adjusting the growth conditions, to minimise the concentration of point defects, we have achieved this objective. We have produced GaAs samples containing ~2 % Fe co-doped with Si, which are both electrically and optically active. SIMS studies show we have successfully incorporated Fe at the expected level (Milestone 1). The electrical properties have been studied by Hall effect measurements. The measurements show that both the conductivity and mobility are influenced by exposure to light. This result shows that we have good evidence for the formation of the intermediate band in GaAs (Milestone 2). Using this knowledge we have also grown and processed complete intermediate band SCs, which are currently being characterised at UPM Madrid (Milestone 4). This work will now be submitted for publication (Milestone 3).

References
1) G Conibeer, Materials Today, 10 (2007) 42
2) A Luque and A Marti, Phys. Rev. Lett., 78 (1997) 5014