Antonio Martí

Light management issues in intermediate band solar cells

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,

Intermediate band solar energy conversion in ZnTeO

Energy conversion in solar cells incorporating ZnTeO base layers is presented. The ZnTeO base layers incorporate intermediate electronic states located approximately 0.4eV below the conduction band edge as a result of the substitution of O in Te sites in the ZnTe lattice. Cells with ZnTeO base layers demonstrate optical response at energies lower than the ZnTe bandedge, a feature that is absent in reference cells with ZnTe base layers. Quantum efficiency is significantly improved with the incorporation of ZnSe emitter/window layers and transition from growth on GaAs substrates to GaSb substrates with a near lattice match to ZnTe.

A puzzling solar cell structure: An exercise to get insight on intermediate band solar cells

We introduce one trivial but puzzling solar cell structure. It consists of a high bandgap pn junction (top cell) grown on a substrate of lower bandgap. Let us assume, for example, that the bandgap of the top cell is 1.85 eV (Al0.3Ga0.7As) and the bandgap of the substrate is 1.42 eV (GaAs). Is the open-circuit of the top cell limited to 1.42 V or to 1.85 V? If the answer is “1.85 V” we could then make the mind experiment in which we illuminate the cell with 1.5 eV photons (notice these photons would only be absorbed in the substrate). If we admit that these photons can generate photocurrent, then because we have also admitted that the voltage is limited to 1.85 V, it might be possible that the electron-hole pairs generated by these photons were extracted at 1.6 V for example. However, if we do so, the principles of thermodynamics could be violated because we would be extracting more energy from the photon than the energy it initially had. How can we then solve this puzzle?

Module interconnection for the three-terminal heterojunction bipolar transistor solar cell

In common multijunction solar cells the subcells are connected in series. In this way, achieving a high voltage at module level is straightforward. However, calculations have proven that the annual energy efficiency limit is higher for independently connected subcells, because they are more tolerant to spectral variations throughout the year. We have recently proposed a three-terminal heterojunction bipolar transistor solar cell (HBTSC) with the maximum limiting efficiency of a dual-junction solar cell, but without the need for a tunnel junction and with only three crucial semiconductor layers. In this work, we present the implementation of a two-terminal module prototype including five HBTSCs which provides a high-voltage power output.In common multijunction solar cells the subcells are connected in series. In this way, achieving a high voltage at module level is straightforward. However, calculations have proven that the annual energy efficiency limit is higher for independently connected subcells, because they are more tolerant to spectral variations throughout the year. We have recently proposed a three-terminal heterojunction bipolar transistor solar cell (HBTSC) with the maximum limiting efficiency of a dual-junction solar cell, but without the need for a tunnel junction and with only three crucial semiconductor layers. In this work, we present the implementation of a two-terminal module prototype including five HBTSCs which provides a high-voltage power output.

Nano-imprinted rear-side diffraction gratings for absorption enhancement in solar cells

As wafer-based solar cells become thinner, light-trapping textures for absorption enhancement will gain in importance. In this work, crystalline silicon wafers were textured with wavelength-scale diffraction grating surface textures by nanoimprint lithography using interference lithography as a mastering technology. This technique allows fine-tailored nanostructures to be realized on large areas with high throughput. Solar cell precursors were fabricated, with the surface textures on the rear side, for optical absorption measurements. Large absorption enhancements are observed in the wavelength range in which the silicon wafer absorbs weakly. It is shown experimentally that bi-periodic crossed gratings perform better than uni-periodic linear gratings. Optical simulations have been made of the fabricated structures, allowing the total absorption to be decomposed into useful absorption in the silicon and parasitic absorption in the rear reflector. Using the calculated silicon absorption, promising absorbed photocurrent density enhancements have been calculated for solar cells employing the nano-textures. Finally, first results are presented of a passivation layer deposition technique that planarizes the rear reflector for the purpose of reducing the parasitic absorption.