Exploring the Potential of Ternary Blending for Two and Three-Junction RAINBOW Solar Cells

TL;DR

This study investigates the use of ternary blending in RAINBOW multijunction organic solar cells, demonstrating improved efficiencies through device simulations and scalable fabrication methods.

Contribution

It introduces the application of ternary mixing in RAINBOW architectures, achieving higher efficiencies and validating the approach with scalable manufacturing techniques.

Findings

Ternary mixing enhances Voc and overall PCE in RAINBOW solar cells.

Experimental efficiencies increased from 12.9% to 17.3% with 2- and 3-junction devices.

Simulations suggest potential for even higher efficiencies with suitable wide bandgap materials.

Abstract

The efficiency of organic photovoltaics (OPV) has been steadily increasing over the past decade until reaching the 20\% milestone. Multijunction architectures provide a promising approach to further enhance performance. Here we explore the potential of a spectral splitting geometry, referred to as RAINBOW, in which subcells are placed side-by-side and externally connected, thus minimizing the fabrication and current matching challenges found in vertically stacked configurations. First, we tested 7 different binaries with bandgaps spanning from 1.98 to 1.16 eV. The systems with the widest and narrowest gaps suffered greater losses and so we evaluate if ternary mixing could help to overcome these limitations by evaluating 5 different ternaries. Generally speaking, ternary mixing tunes the Voc, and when morphology and energy levels are well aligned, the overall PCE can be boosted in the spectral region where the subcell should absorb, as is the case for PTB7-Th:COTIC-4F:BTP-eC9 when operating as red subcell. Device simulations help to identify the 2-junction and 3-junction configurations with highest PCEs, all of which include ternaries. We fabricate proof-of-concept RAINBOW devices using scalable methods in which the subcells are deposited by meniscus-guided blade coating. The efficiency improves from 12.9\% in single-junction devices to 15.9\% in 2-junction devices (16.4\% in simulations) and 17.3\% in 3-junction devices (17.7\% in simulations), confirming the viability of the RAINBOW architecture for scalable, high-efficiency OPVs. Finally, detailed balance analysis indicates that the potential of this geometry can be very high provided that high efficiency wide bandgap (2-2.5 eV) materials become available.