Healing wide-gap Chalcopyrite
  active project [2023-2025]
Project Leader: Prof. Diego Colombara (diego.colombara@unige.it) - Dipartimento di chimica e chimica industriale - Università di Genova
Head of IMEM activities: Stefano Rampino (stefano.rampino@cnr.it)

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 1st 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 2nd 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)Ox buffer which appears to improve the band offset. Unfortunately, (Zn,Sn)Ox 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.