Advances in quantum dot intermediate band solar cells

Abstract

Several groups have reported on intermediate band solar cells (IBSC) fabricated with InAs/GaAs quantum dots (QD) which exhibit quantum efficiencies (QE) for sub-bandgap photon energies. However, this QE is produced by the absorption of photons only through valence band (VB) to intermediate band (IB) transitions. The absorption of photons of that energy in IB to conduction band (CB) transitions is weak and is usually replaced by carrier escape. This mechanism is incompatible with the preservation of the output voltage, and therefore, it cannot lead to the high efficiencies predicted by the IBSC model. In this work, we discuss the contribution of thermal and tunneling mechanisms to IB-CB carrier escape in current QD-IBSCs. It is experimentally demonstrated that in QD-IBSC prototypes where tunnel escape has been eliminated, the sub-bandgap QE is suppressed at sufficiently low temperatures, and when this occurs, the only limit for the open-circuit voltage (VOC) is the fundamental semiconductor bandgap, as stated by the IBSC theoretical model.

Experimental demonstration of the effect of field damping layers in quantum-dot intermediate band solar cells

“…[3]; and SC3 coincides with SI in Ref. [3], Detailed structures and growth conditions for these samples can be found in Refs. [3,6,8].…”

“…SCI coincides with S3 in Ref. [3] and with SB in Ref. [6]; the structure of SC2 is described in Ref.…”

“…[3], Detailed structures and growth conditions for these samples can be found in Refs. [3,6,8]. We indicate here the most relevant characteristic of these samples for the present work.…”

“…Three quasi-Fermi levels (QFLs) are needed to describe the electronic population of the three bands of an IBSC working in accordance to the voltage preservation principle: e Fe , e FIB and % for the conduction band (CB), IB and valence band (VB), respectively [2]. The voltage preservation principle has been verified so far in IBSC prototypes using InAs/GaAs quantum dots (QD) [3,4] or Ti-doped GaAs [5] as the IB material. The demonstration consists in measuring a photo-voltage larger than £ H /e and, in the limit, close to E G /e, where e is the electronic charge.…”

“…For QD-IBSC prototypes, it has been demonstrated that tunnelassisted carrier escape between the IB and the CB must also be eliminated for achieving voltage preservation [3,6]. Tunnel carrier escape is activated by the electric field that exists in the QD-stack, generally placed between p-doped and n-doped emitters.…”