2, Institute for Solar Energy Research Hamelin, Emmerthal, , Germany
In order to improve substantially on the efficiency of c-Si solar cells, we have been developing stacked GaInP/Si tandem cells, recently attaining efficiencies above 32% in four-terminal configuration. However, while four-terminal devices allow maximum power extraction and easy integration of two separately processed sub-cells, highly doped layers or grids are required above and below each sub-cell, which leads to optical losses and contributes to processing complexity and thus cost. By rewiring a four-terminal (4T) device in two- and three-terminal (2T, 3T) configuration, we demonstrate that while a 2T configuration is less efficient, a 3T configuration can yield exactly the same overall efficiency as four terminals. This finding has been confirmed through Sentaurus simulations and, with respect to the Si cell, by experimental investigations on isolated 3T Si bottom cells. The 3T configuration used consists of two circuits that share a terminal on the back of the Si cell: a conventional, 2T tandem circuit, and a Si IBC (interdigitated back contact) circuit.
We subsequently prepared a fully integrated three-terminal device. The cells were integrated by bonding an inverted GaInP cell stack onto a Si IBC cell with a full-area front surface field on a planar front-side using a transparent conductive adhesive. Both cells were coated with an optimized ITO layer before bonding to improve optical coupling between the cells. The GaInP stack is subsequently processed on top of the Si IBC cell (contact formation, mesa isolation). We show that the conductive adhesive, which consists of conductive spheres embedded in epoxy, does not result in appreciable series resistance losses while maintaining acceptable transparency and protecting the Si cell during GaInP processing. Furthermore, a voltage-voltage mapping approach to study the full operating space of the cell with its two interacting circuits is presented.
In a 2T measurement (keeping the IBC circuit open), we achieved Voc=2.09V, Jsc=13.45 mA/cm2, FF=85.6%, and an efficiency of 24.1%, which increased to a 3T efficiency of 25.2% when power was collected in both circuits. This demonstrates the added benefit of a third terminal, and based on PVLighthouse simulations it is expected that when using textured Si cells, this benefit will be more pronounced, and higher 3T tandem cell efficiencies will be achieved.