Light management issues in intermediate band solar cells
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1101-KK06-02
Light management issues in intermediate band solar cells Antonio Martí, Elisa Antolín, Enrique Cánovas, Pablo García Linares, and Antonio Luque Instituto de Energía Solar - Universidad Politécnica de Madrid, ETSI Telecomunicación, Avda Complutense 30, Madrid, 28040, Spain
ABSTRACT This paper discusses several topics related to light management that improve our understanding of the performance and potential of the intermediate band solar cell (IBSC). These topics are photon recycling, photon selectivity and light confinement. It is found that neglecting photon recycling leads to underestimate the limiting efficiency of the IBSC in 7 points (56.1 % vs 63.2 %). Light trapping allows to effectively absorbing photons whose energy is associated to the weakest of the optical transitions in the IBSC, allowing also for higher efficiencies with lower device thickness. The impact of photon selectivity on the cell performance is also discussed. INTRODUCTION The intermediate band solar cell (IBSC) is a novel type of solar cell conceived to effectively use the energy of below bandgap energy photons [1, 2]. To this end, it requires the existence of an intermediate band (IB) located within the semiconductor bandgap (Fig. 1). This band divides the total bandgap of the semiconductor, EG, into two sub−bandgaps, EL and EH. The IB is separated from the conduction and valence band (CB and VB) by a null density of states. Thanks to this band, two below bandgap energy photons, as those labelled “1” and “2” in Fig. 1 can create one electron−hole pair by pumping an electron from the VB to the IB (photon “1”) and an electron from the IB to the CB (photon “2”). To this end, in the classical IBSC model, the IB should be half−filled with electrons in order to provide both empty states to receive electrons from the VB as well as electrons to supply to the CB. The IB material is sandwiched between conventional p and n type semiconductors. The region of IB material of interest is assumed to lay beyond the space charge region created by the junctions. When a voltage is applied, the null density of states between the IB and the CB and VB makes possible the appearance of three distinguished quasi-Fermi levels (EFC, EFI and EFV, related each one to the conduction, intermediate and valence semiconductor bands respectively). The p and n regions (emitters) act as selective contacts and determine the position of the electron and hole quasi-Fermi levels (EFC and EFV). A sufficiently high density of states in the IB is assumed as to be able to keep EFI at its equilibrium position [3]. As sketched in Fig. 1, the output voltage, V, determined by the quasi-Fermi level split between electrons and holes (EFC−EFV = eV) is still limited by the high bandgap EG. The complete details of the theory have been explained in detail in previous works [1, 2, 4, 5]. Experimental results related to the verification of the existence of three distinguished quasi-Fermi levels and absorption of two below bandgap energy photons can be found in Refs. [6, 7]. In this
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