Soline Boyer-Richard

Hot carrier solar cells are a concept of photovoltaic devices, which offers the opportunity to harvest solar energy beyond the Shockley-Queisser limit. Unlike conventional photovoltaic devices, hot carrier solar cells convert excess kinetic energy into useful electrical power rather than losing it through thermalisation mechanisms. To extract the carriers while they are still “hot”, efficient energy-selective contacts must be developed. In previous studies, the presence of the hot carrier population in a p-i-n solar cell based on a single InGaAsP quantum well on InP substrate at room temperature has been demonstrated by means of complementary optical and electrical measurements, leading to an operating condition for this device beyond the limit for classical device operation. This result allows to design a new generation of devices to increase the hot carrier conversion contribution. In this work, we study InGaAs/AlInAs type II heterojunction as a selective contact for a future hot carrier solar cell device epitaxially grown on (001) oriented InP substrate. Two p-i-n solar cells have been grown by molecular beam epitaxy on InP. The absorber is a 50 nm-thick InGaAs layer surrounded by AlInAs barriers, all lattice-matched to InP. Two architectures are compared, the first with two symmetrical AlInAs barriers and the second with a single InGaAs quantum well in the center of the n-side barrier to allow electron tunneling across the barrier. Electrical characteristics under laser illumination with two different wavelengths have been measured to investigate the effect of the selective contact compared to the barrier. This preliminary study of InGaAs/AlInAs-based selective contacts show that such III–V combination is adapted for a future hot carrier solar cell in the InP technology.

The best solar conversion efficiencies have been reached thanks to multijunction solar based on III-V semiconductors on GaAs or Ge substrates. While displaying high conversion efficiencies, these solar cells suffer from the high cost of such substrates. To benefit from both the low cost and technological maturity of silicon cells, III-V tandem cells on silicon seem a good compromise to overpass the theoretical efficiency limit of the Si single cells. To study the GaP/Si interface effect on the solar cell characteristic, a GaP n-i-p solar cell has been grown on silicon substrate. Two types of electrical contacts configurations have been processed: a top-top configuration in which the current does not see the GaP/Si interface and the top-bottom configuration where the electric current crosses the interface. A comparison of dark I-V, I-V under solar illumination, and EQE measurements on both configurations is performed. The top-bottom contacts configuration shows an EQE a little bit lower than the top-top contact one, likely due to lower carrier diffusion length or recombination at the lower interface. However, the result on the EQE of the top-bottom configuration is encouraging for the future development of the GaP-based/Si tandem solar cells, and any other tandem cell on silicon using GaP as an intermediate selective contact.

Hydrogen produced using artificial photosynthesis, i.e. solar splitting of water, is a promising energy alternative to fossil fuels. Efficient solar water splitting demands a suitable band gap to absorb near full spectrum solar energy and a photoelectrode that is stable in strongly alkaline or acidic electrolytes. In this work, we demonstrate for the first time, a perfectly relaxed GaP_0.67Sb_0.33 monocrystalline alloy grown on a silicon substrate with a direct band gap of 1.65 eV by molecular beam epitaxy (MBE) without any evidence of chemical disorder. Under one Sun illumination, the GaP_0.67Sb_0.33 photoanode with a 20 nm TiO₂ protective layer and 8 nm Ni co-catalyst layer shows a photocurrent density of 4.82 mA cm⁻² at 1.23 V and an onset potential of 0.35 V versus the reversible hydrogen electrode (RHE) in 1.0 M KOH (pH = 14) aqueous solution. The photoanode yields an incident-photon-to-current efficiency (IPCE) of 67.1% over the visible range between wavelengths 400 nm to 650 nm. Moreover, the GaP_0.67Sb_0.33 photoanode was stable over 5 h without degradation of the photocurrent under strong alkaline conditions under continuous illumination at 1 V versus RHE. Importantly, the direct integration of the 1.65 eV GaP_0.67 Sb_0.33 on 1.1 eV silicon may pave the way for an ideal tandem photoelectrochemical system with a theoretical solar to hydrogen efficiency of 27%.

In single junction solar cells a large part of the incident energy ends up as heat which limits their maximum achievable efficiency. Thus the achievement of maximum power conversion efficiencies relies on complex multijunction devices. Here we show the possibility to harvest the available solar energy using hot carrier devices and evidence a positive contribution of the hot carrier effect on photovoltaic performances. We investigated a semiconductor heterostructure based on a single InGaAsP quantum well using quantitative optoelectrical characterization, especially luminance measurements. The quantitative thermodynamic study of the hot carrier population allows us to discuss the hot carrier contribution to the solar cell performance. We demonstrate that voltage and current are enhanced due to the presence of the hot carrier population in the quantum well. These experimental results substantiate the potential of increasing photovoltaic performances in the hot carrier regime. Moreover, by developing a suitable analytic theoretical framework, we show how to obtain separate (hot) temperatures of electrons and holes from photoluminescence spectra analysis. The individual thermalization coefficients of each carrier type are also discussed. The method developed in this article paves the way towards the design of new energy harvesting devices and to the development of advanced characterization tools. Finaly, to increase the PV performance enhancement and reduce the concentration factor, an optimize design is investigated.

Hybrid perovskite materials have emerged over the past five years as absorber layers for new high-efficiency yet low-cost solar cells (PSCs) that combine the advantages of organic and inorganic semiconductors. Here, employing the linear response (DFPT) approach of Density Functional Theory (DFT) and frozen phonon calculations, we reveal strong anharmonic effects in the inorganic CsPbI3 perovskite. We also show through high resolution in-situ synchrotron XRD measurements that CsPbI3 can be undercooled below its transition temperature and temporarily maintained in its perovskite structure down to room temperature, stabilizing a metastable perovskite polytype (black γ-phase) crucial for photovoltaic applications. Among the four phases of CsPbI3, we found that this phase is the only one to behave harmonically around equilibrium. Last, we explain the Rashba effect in these materials as a consequence of the symmetry breaking induced by the double-well instabilities.

Hybrid organic-inorganic perovskites emerged as a new generation of absorber materials for high-efficiency low-cost solar cells in 2009. Very recently, fully inorganic perovskite quantum dots also led to promising efficiencies, making them a potentially stable and efficient alternative to their hybrid cousins. Currently, the record efficiency is obtained with CsPbI₃ whose crystallographical characterization is still limited. Here we show through high resolution in-situ synchrotron XRD measurements that CsPbI₃ can be undercooled below its transition temperature and temporarily maintained in its perovskite structure down to room temperature, stabilizing a metastable perovskite polytype (black γ-phase) crucial for photovoltaic applications. Our analysis of the structural phase transitions reveals a highly anisotropic evolution of the individual lattice parameters versus temperature. Structural, vibrational and electronic properties of all the experimentally observed black phases are further inspected based on several theoretical approaches. While the black γ-phase is shown to behave harmonically around equilibrium, for the tetragonal phase density functional theory reveals the same anharmonic behavior, with a Brillouin zone-centered double-well instability, as for the cubic phase. Using total energy and vibrational entropy calculations, we highlight the competition between all the low-temperature phases of CsPbI₃ (γ, δ, β) and show that avoiding the order-disorder entropy term arising from double-well instabilities is key in order to prevent the formation of the yellow perovskitoid phase. A symmetry-based tight-binding model, validated by self-consistent GW calculations including spin-orbit coupling, affords further insight into their electronic properties, with evidence of Rashba effect for both cubic and tetragonal phases when using the symmetry breaking structures obtained through frozen phonon calculations.

We investigated a semiconductor heterostructure based on InGaAsP multi quantum wells using optical and electrical characterizations in the scope of hot carrier solar cell device. The potential of the investigated quantum well structure to overpass the Schockley Queisser limit is discussed. Population density, temperature and quasi-Fermi level splitting of photogenerated carriers are investigated by fitting the full luminescence spectra using generalized Planck's law. A proper optical study is realized thanks to a detailed description of the absorption of excitons and free carriers in the quantum well. Optical measurements are compared to electrical measurements where the open circuit voltage electrically measured is higher than the minimum absorption threshold. To probe the hot carrier effect in such measurements we look at the changes in thermodynamic properties of carriers in the quantum well and in the barriers when changing the excitation power and the electrical bias.