Solar light induced opacity of MIND cells

Abstract

Multi-interface novel devices (MIND) exhibit a dramatically low UV-and blue-spectrum photovoltaic (PV) performance. A paradox could even be observed, the better the electronic passivation the poorer the PV performance. The paradox appears under relatively low excitations in comparison with intense laser fluxes usually at its origin. The effect can be explained by solar light induced opacity, which reduces considerably or even totally the photon penetration into deeper layers, from which exclusively the photocarrier collection is possible. This opacity results from a feedback occasioned by the free-carrier absorption: better surface passivation, higher free-carrier density, stronger surface dead zone absorptance. The total energy of the incident short wavelength beam can be absorbed before a carrier collection limit buried in the emitter. This limit acts simultaneously on the electronic performance, blocking free-carriers, and on the optical performance, being at the origin of an enhancement of the surface absorptance. As a consequence, a thin surface zone dominates the optical functions of MIND cells through the free-carrier gas confined inside it. In this work we report specific effects concerning the solar-light induced opacity in MIND cells. The investigation allows modification of the free-carrier confinement using different device architectures. The main characterization methods were reflectivity and spectral response with a varying incident beam. The results prove the domination of the free-carrier optical functions on the MIND PV conversion.

Current-voltage curves of PV metamaterial based on the nanostructured Si

“…The specific effects can only take place within the emitter, because: i) electron multiplication can appear inside the buried nanoscale Si-layered system [5,7], ii) the free path of excess carriers under low excitation is relatively larger due to lower density of the photogenerated/additional population than under high excitations, iii) of the lower screening effect. Under high light excitations (0.3-1.0 sun) and large reverse polarizations the relative modification of the I-V curve is practically negligible, because the density of the free-carrier population is relatively high and the specific current reducing effects taking place within the emitter are: i) the light-induced voltage drop (within the upper emitter), which is directed opposite to the open-circuit voltage, V oc , ii) a screening effect of the electric charge confined in the buried nanoscale Si-layered system, iii) low electron mobility (low free path) in dense gas, iv) light-induced opacity [8], The conversion improvement, due to amplified carrier injection into the upper emitter (due to primary photogeneration and secondary low-energy generation from the nanoscale Si-layered system [11]), is totally neutralized. Time-resolved I-V curves confirm a charge storage within the only possible place which is the emitter (see Figures 3 and 4) [12].…”

“…The first MIND generation [5] had poor collection efficiencies, which contrasts with proven additional low-energy generation and multiplication [7]. One previously found explanation has to do with a light-induced opacity [8]. We have investigated the electron transport phenomena through their integral form as are I-V curves.…”

PV conversion of energetic photons of the solar spectrum

“…The incident solar-like spectrum has been strongly attenuated to avoid a light-induced opacity [29] which totally hides specific effects occurring within the upper emitter. Because of the double light absorption nonlinearity (with wavelength and penetration depth) one can easily determine the UV photon penetration depth.…”

“…When comparing the dark characteristics with a calculated curve using the parameters found from the illuminated curve, and vice-versa, the discrepancy becomes obvious due to the nonlinearity of optical functions (as shown in FIG. 1) The high photogeneration regime in the presence of the free-carrier confinement produces a light-induced opacity [29] hiding optoelectronic effects which can appear within the upper emitter. Previous theoretical and experimental investigations [17,30] taking into account a multistage PV conversion with two basic mechanisms, photon absorption and low-energy generation with several stages [34], suggest extremely large densities for the carrier population under a 1-sun intensity light beam.…”

“…The most energetic photons are, of course, absorbed within the superficial nanostratum. When a carrier collection limit (CCL) [27][28] is present in the emitter, the cascade-like process of secondary generation, involving a large number of excess electrons, produces a light-induced opacity due to the free-carrier confinement within the upper emitter [29]. The upper emitter represents a potential well filled with a carrier population totally excluded from collection.…”