Voltage-matched thin film solar cells in 3-terminal configuration

Abstract

Multijunction solar cells, proven technological route for achievements of highest PV conversion efficiencies, require accurate tuning of the sub-cell absorption to ensure that every cell in the stack delivers the same current density. Even though currents of the sub-cells can be precisely matched for a fixed illumination spectrum, current mismatch cannot be avoided in real terrestrial applications due to variations of the irradiance spectrum. The issue becomes critical when a solar cell has to cover wider range of applications such as a mixture of direct sun / shadow / artificial light – the case for various distributed PV-powered electronics. Furthermore the current matching constrains choice of materials and designs of sub-cells for a tandem device. A straightforward basic configuration with decoupling the sub-cell's currents is a 3-terminal configuration with one additional contact sheared by the top and bottom cell. This concept requires voltage-matching between the top and bottom cell when these cells are integrated in modules. A realistic concept for the voltage-matched 3-terminal cell reported recently includes a wide gap top cell combined with a tandem bottom cell made of 2 sub-cells with lower bandgaps. The concept is a hybrid between 2 and 3-terminal configurations with voltage matching and relaxed current matching constrains. Established thin film silicon solar cell technology provides interesting option to realize the hybrid 3-terminal cell with amorphous Si top cell (VOC ≈ 0.9 V) and two microcrystalline Si cells (VOC ≈ 0.5 V). In this work we present proof of concept of the voltage matched 3-terminal tandem cell prepared with highly transparent and conductive IOH intermediate contact. The efficiency of 10.4% has been achieved made up of independently operating 7.9% efficient top cell and 2.5% efficient bottom tandem cell. The paper summarizes the development and discusses optical losses identified in the 3-T devices.