Scientists in Saudi Arabia Make 27% Efficient n-i-p Perovskite Tandem Cell

Researchers led by Saudi Arabia’s King Abdullah University of Science and Technology (KAUST) have demonstrated a tandem cell based on an n-i-p perovskite stacked on top of a silicon heterojunction cell that achieved 27% efficiency, breaking the previous record of 22%.

Monolithic perovskite/silicon tandem solar cells are currently of interest thanks to their high power conversion efficiency potential at affordable cost. Following several experiments, research devices have reached efficiency levels of around 25%, many of which utilize an n-type, or n-i-p, cell architecture for the perovskite layer.

But the n-i-p architecture is riddled with many issues, due to which research has rather focussed on the p-i-n architecture. Unfortunately, that has excluded tandem devices (that combine perovskites with a silicon bottom cell) from new studies being done on perovskite.

The KUAST scientists have built a device to solve many of these problems and their findings are contained in a paper entitled “Ligand-bridged Charge Extraction and Enhanced Quantum Efficiency Enable Efficient n–i–p Perovskite/Silicon Tandem Solar Cells,” published in the journal Energy & Environmental Science. The paper can be accessed here.

The researchers state that the initial perovskite/silicon tandems were in the n–i–p configuration. However, as in this architecture light enters from the p-side, the application of the typical electron and hole selective contacts of single-junction devices to the tandem configuration resulted in devices with a poor blue response, in particular due to parasitic absorption in the front-contact stack.

Therefore, global tandem research refocused on the inverted structure (p–i–n configuration, with electron collection at the front), and so far, several efficient devices have been reported in this configuration. Increasingly the power conversion efficiency of p–i–n tandems further is constrained by parasitic absorption and recombination active defects, prompting scientists to revisit the n–i–p structure with the aim to benefit from the wide library of efficient hole selective layers in single-junction n–i–p perovskite solar cells.

In their work, the KUAST scholars systematically overcome the persistent challenges of n–i–p tandem devices by developing novel electron and hole selective contact stacks, fully processed on industry-standard textured silicon bottom cells. Their work aims to enable a new device platform and research avenues to further push the efficiency of perovskite-based tandem solar cells.

The researchers state, “Here, we overcome the persistent processing limitations of n–i–p configuration of tandem cells by combining on pyramidal textured SHJ bottom cells a RF-sputtered a-NbOx with conformal C60-anchoring as ESL, micrometer-thick solution-processed perovskites and a highly transparent conformal HSL stack.”

They claim that all these advancements enabled 27% efficient tandem cells in n–i–p configuration, which is a more than 5% absolute increment over the prior state of the art on SHJ bottom cells.

Furthermore, they argue, to overcome the limited long-term operational stability of the perovskite/silicon tandems, 2D-perovskite passivation strategies can be utilized which has been a challenge with p–i–n devices due to energetic alignment problems of the 2D perovskites with the widely used C60 ESLs.

Also, bifacial tandem devices can be more relevant in the n–i–p configuration, since the bottom cell utilizes an n-type contact at its back, high refractive index but lower conductivity TCOs can be utilized.

Lastly, the n–i–p tandem platform enables utilization of tandems as a photocathode in efficient solar-driven water splitting applications for hydrogen generation since this application requires electrons to be extracted from the cathode side with voltage values of in the range of 1.4 to 1.8 V, as well as high current densities.

The authors of the paper include: Erkan Aydin, Jiang Liu, Esma Ugur, Randi Azmi, George T. Harrison, Yi Hou, Bin Chen, Shynggys Zhumagali, Michele De Bastiani, Mingcong Wang, Waseem Raja, Thomas G. Allen, Atteq ur Rehman, Anand S. Subbiah, Maxime Babics, Aslihan Babayigit, Furkan H. Isikgor, Kai Wang, Emmanuel Van Kerschaver, Leonidas Tsetseris, Edward H. Sargent, Frédéric Laquai, and Stefaan De Wolf.