Research Highlights

The InGaN compound semiconductors (alloys of Indium-Gallium and Nitrogen) are a class of materials with exceptional properties and promises for the development of a new generation of solar cells. Their direct energy bandgap spans the range from 3.4 eV (GaN) to 0.65 eV (InN) covering almost entirely the solar spectrum. However, despite the intense international research effort, up to now, the obtained solar cell devices’ efficiency characteristics, fabricated out of InGaN/GaN junctions, have been poor. 
 
In a recent publication in IEEE Journal of Photovoltaics, the group of Prof. Eleftherios Iliopoulos, studied the underlying physical mechanisms of carrier generation, recombination and transport in InGaN/GaN heterostructure devices and identified the limiting factors of their performance.  They have shown that in the commonly employed device design (top p-GaN layer), the polarization fields present due to the strong spontaneous and piezoelectric properties of the materials, oppose the efficient photogenerated carriers’ collection and strongly reduced the device efficiency. Furthermore, they have shown that by employing a new design, based on polarization engineering, the efficiency of single InGaN/GaN heterojunction devices can be boosted by an order of magnitude. 
 
By clarifying the role of polarization fields and introducing a novel approach to efficient device design, this work is expected to lead to the development of new generations of solar cells, based on InGaN heterostructures, with high conversion efficiencies and high ratio of efficiency to production costs. 
 

Picture: Energy band and major recombination mechanisms’ rates diagrams for conventional (p-i-n) -left- and novel (n-p) polarization-engineered -right- approaches of InGaN/GaN heterostructure based photovoltaic devices.

Article: “Polarization-Engineered InGaN/GaN Solar Cells: Realistic Expectations for Single Heterojunctions", S.A.Kazazis, E. Papadomanolaki and E. Iliopoulos, IEEE J. Photov. 8, 118 (2018).