Researchers design transport layers that decouple perovskite thickness from efficiency limitations

Researchers at Germany's Forschungszentrum Jülich have reported a method to fabricate >1-micrometer thick perovskite films by employing hole-transporting bilayers of self-assembled monolayers (SAMs) and poly[bis(4-phenyl) (2,4,6-trimethylphenyl)amine] (PTAA). Recognizing the critical role transport layers play in exacerbating thickness-dependent losses, the team optimized a dual-layer hole transport architecture to reduce resistive losses and recombination. The authors achieved remarkable efficiency retention at over 1 micron thickness.

This work focuses on a solar cell architecture that decouples thickness from efficiency limitations. By sandwiching specialty organic films around the perovskite layer, the authors enabled micron-scale thicknesses without forfeiting peak performance. Their design notably achieves a remarkable 20.2% efficiency at over 1 micron thickness with minimal losses compared to thinner versions.


As lead author Thomas Kirchartz explained: “Realizing highly efficient thicker solar cell films allows covering the small pyramids of textured silicon wafers necessary for high-performance tandem cells”. Previous attempts at thicker perovskite films tended to suffer degraded fill factors and voltages at micron scales. But the researchers’ unique charge transport bilayers curb these declines, demonstrating a path to integrating perovskites into high-efficiency silicon-based tandems.

Thickness tends to impede efficiency because thicker films exacerbate tiny manufacturing imperfections that cumulatively undermine performance. Defect densities increase, interfaces roughen, voltages sag. Separating these intertwined mechanisms has proven very difficult. The key insight of Kirchartz’s team was recognizing transport layers themselves imposed intrinsic limits. Traditional logic held charge collection depended chiefly on absorber properties. However, imbuing transport layers with sufficient conductivity and optimal band alignments relaxes thickness constraints altogether. 

This revelation led the researchers to sandwich two specialty organic films with complementary advantages around the perovskite core. The bottom self-assembled monolayer (SAM) film possesses superb conductivity and hole transport abilities. Meanwhile, the top poly[bis(4-phenyl)]amine (PTAA) film offers close lattice matching and band alignment to the perovskite. Blending compatible SAM variants fine-tuned interfacial properties even further.

Meticulous characterizations revealed the transport bilayers slash resistive losses, facilitate charge extraction, and impede recombination relative to single SAM or PTAA films. Thereby enabling exceptional fill factors and voltages approaching 80% and 1.2V respectively at over 1 micron thickness. Such remarkable retention of peak efficiencies could help advance the commercialization of perovskites.

Theere are also broader impacts of high-performance thick-film perovskite solar cells. Micron scales align well with existing manufacturing tools optimized for silicon, thereby easing integration with existing infrastructures. Enhancing compatibilities between cutting-edge perovskites and incumbent technologies could catalyze their widespread adoption. Additionally, thick perovskite films better conform to the textures of silicon cell pyramids, thereby improving tandem cell geometries targeting 45% combined efficiencies. 

Kirchartz therefore believes “an intermediate step to enable efficient, solution-processed perovskite top cells on textured Si bottom cells for tandem applications is the ability to design cell structures with suitably high band gaps combined with high efficiencies at thickness above 1 µm.”

However, work remains to be done before thick-film perovskites reach commercial readiness. Long-term stability is still lacking while efficiency and performance consistency need improvement. Nonetheless, the exceptional 20% efficiency milestone at micron scales represents a watershed moment. Proving the commercial viability of this emerging photovoltaic technology.

This approach efficiently produces the thick perovskite films needed to cover silicon pyramids for high efficiency tandem cells. thereby helping to fulfill the long-standing promise of economical, scalable hybrid tandem architectures. Additionally, reliably achieving 20%+ efficiency at micron scales largely allays concerns over subpar voltages and fill factors as film thickness increases. This helps in overcoming one of the most stubborn efficiency and performance tradeoffs constraining commercialization efforts.

Building on these advances, further optimization has potential to enhance efficiency limits even beyond initial expectations. The viability of low-cost manufacturing-friendly perovskite solar cells now looks increasingly achievable. Widespread adoption just over the horizon pending incremental scaling up and stability improvements. Though work remains, proving exceptional efficiency retention at 20%+ for micron-thick perovskites constitutes a real breakthrough. 

Posted: Dec 27,2023 by Roni Peleg