Conventional single-junction solar cells are limited to a maximum theoretical efficiency of around 32%. However, the practical limit of the dominant silicon technologies has been reached at around 25-26%. In order to reduce further the cost of photovoltaic electricity production, new ways of achieving higher efficiencies are required.^[1] In this context, the intermediate band solar cell^[2] (IBSC) is proposed as an appealing alternative, able to achieve efficiencies up to 50%,^[3] in pair with triple-junction solar cells, but with a much simpler structure consisting of only one junction. Such high efficiencies are achieved by reducing the optical losses via the absorption of photons with less energy than the semiconductor’s bandgap. For this, a special semiconductor, an intermediate band (IB) material, is needed. Such materials have a collection of in-gap states, called the IB, which allow harvesting energy from otherwise wasted low-energy photons via a two-step absorption process, pumping electrons first from the valence band (VB) to the IB, and then from the IB to the conduction band (CB).

Quantum-dot-in-perovskite solids^[4] (QDiPs), where inorganic colloidal quantum dots (QDs) are embedded in halide perovskites (HPs) are suggested as an optimum platform for developing high-quality IB materials that enable the fabrication of performant IBSCs.^[3,5] In the QDiP-based IBSC, the IB emerges from the confined electronic levels introduced by the QDs inside the HP host. QDiPs may solve the two main drawbacks encountered so far in the IBSC research, namely weak below-bandgap absorption, and voltage degradation resulting from an increase in non-radiative recombination due to thermal coupling of the IB with the VB or the CB.^[3,6]

We have synthesized QDiP solids, using PbS as the QD material and methylammonium lead iodide (MAPbI₃) as the perovskite host, with excellent morphological, crystalline quality. We will show that the IB introduced by the QDs enables strong below-bandgap absorption, with potential to reach absorption coefficients close to 10⁴ cm^-1, thus solving one major drawback encountered in previous IB materials. Using these QDiP solids, we have fabricated IBSCs. We will show that the introduction of the QDs allows harvesting photons with less energy than the MAPI bandgap, resulting in additional photocurrent. Moreover, we will show that the thermal coupling between the QD levels and the perovskite’s bands is very weak, thus solving the second major drawback found in previous IBSC attempts. This work experimentally confirms QDiPs as a very promising platform for IBSC research able to yield low-cost high-efficiency devices.

References

[1]       M. A. Green, S. P. Bremner, Nature Materials 2017, 16, 23.

[2]       A. Luque, A. Martí, Physical Review Letters 1997, 78, 5014.

[3]       I. Ramiro, A. Martí, Prog Photovolt Res Appl 2021, 29, 705.

[4]       Z. Ning, X. Gong, R. Comin, G. Walters, F. Fan, O. Voznyy, E. Yassitepe, A. Buin, S. Hoogland, E. H. Sargent, Nature 2015, 523, 324.

[5]       M. Alexandre, H. Águas, E. Fortunato, R. Martins, M. J. Mendes, Light Sci Appl 2021, 10, 231.

[6]       Y. Okada, N. J. Ekins-Daukes, T. Kita, R. Tamaki, M. Yoshida, A. Pusch, O. Hess, C. C. Phillips, D. J. Farrell, K. Yoshida, Applied Physics Reviews 2015, 2, 21302.