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(a) Energy diagram of a conventional IBSC, in which extra photocurrent is produced due to sequential absorption of sub-bandgap photons increasing theoretical limit in the conversion efficiency. (b) Energy diagram of a “photon ratchet” IBSC. The photon ratchet band, ERB , is located at an energy interval below the IB, EIB , and the thermal transition between the IB and the RB is assumed to be so rapid that the occupation of both bands can be described by the single quasi-Fermi level. The charge carriers in the IB will quickly move to the RB where they may have a long lifetime since the RB is optically isolated from the VB. Radiative generation rates are marked as Gi and radiative recombination rates are marked as Ri , where subscripts i = VC, VI and IC indicate the bandgap to which the transition corresponds. The occupation of each band is determined by the corresponding quasi-Fermi levels EF , j , where j = CB, IB and VB.
Efficiency limit of photon ratchet IBSC of various concentration as a function of ΔE. The efficiency is computed by global optimisation for a given concentration and ΔE. Efficiency at ΔE = 0 corresponds to conventional IBSCs.
Efficiency of photon ratchet IBSC (solid black line) and recombination rates RVI (blue dashed line), RIC (red dash-dot line) and RVC (green dotted line) as a function of ΔE at fixed bandgaps (a) and at 1 sun and (b) and at full concentration.
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