LEGACY

Photovoltaic (PV) energy offers a beautiful chance to greenify the world. However, the vast power potential of the sun (8000x humankind’s needs) is barely exploited due to the limited efficiency of commercial PV panels. Today’s PV market is dominated by the 1^st generation solar cells based on silicon, but their power conversion efficiency (maximum 26.1 %) is reaching a standstill, due to Auger recombination.

Unlike Si, the stable chalcogenide semiconductor Cu(In,Ga)Se₂ (CIGS) is unaffected by Auger because its direct bandgap requires thinner films to absorb sunlight in 2^nd generation PV. Hence, silicon’s competitive advantage will be eroded by CIGS when this will demonstrate higher efficiencies.

The bandgap of CIGS is tunable from ca. 1.0 eV (CuInSe2, CIS) to ca. 1.7 eV (CuGaSe₂, CGS), matching precisely the Shockley-Queisser requirements for efficient solar energy conversion in both single and double junctions. CIGS cells based on Ga-poor compositions display the highest efficiency of all commercial polycrystalline thin films: 23.3 %. Conversely, Ga-rich CIGS is stuck at 12 %. This is unfortunate, because efficient CuGaSe₂ could enable dual junction PV exceeding 40 % efficiency.

The CIGS technology benefits from 30+ years of R&D, where extrinsic doping and band offset engineering have played key roles in the enhancement of bulk and interface optoelectronic properties, respectively. In analogy, LEGACY targets the enhancement of bulk and interface optoelectronic properties of CuGaSe₂ with the aim of improving CuGaSe₂ solar cell efficiency.

In 2018, the PI (UniGe) has shown that sodium can enhance the interdiffusion of In and Ga in CIGS, unlike previously accepted for 20 years. The discovery was followed by atomistic models describing the effect of extrinsic dopants on the migration barriers of point defects in the semiconductor. LEGACY builds on the PI’s breakthrough atomistic model and combinatorial doping strategies to engineer the mobility and location of detrimental point defects in CuGaSe₂, thus healing the CuGaSe₂ bulk optoelectronic weakness.

In 2017, Larsson et al. obtained a new world record CuGaSe₂ solar cell by adopting a (Zn,Sn)O_x buffer which appears to improve the band offset. Unfortunately, (Zn,Sn)O_x atomic layer deposition can hardly be scaled industrially. More recently, solar cell capacitance simulations by Liu et al. have suggested TiO₂ buffers. TiO₂ can be easily deposited from more scalable and environmentally friendly routes. LEGACY investigates experimentally TiO₂ buffer layers for CuGaSe₂ solar cells, building on the device fabrication expertise of Stefano Rampino (CNR), thus healing the CuGaSe₂ device interface weakness.

The two experimental activities are aided theoretically by Riccardo Freccero (UniGe) for an exploration of the chemical bonding of extrinsic doping, and by Giovanna Sozzi (UniPr) for a deeper solar cell device modelling, thus consolidating our understanding of the technology.