Efficient Homojunction Tin Perovskite Solar Cells Enabled by Gradient Germanium Doping.

P-type self-doping is known to hamper tin-based perovskites for developing high-performance solar cells by increasing the background current density and carrier recombination processes. In this work, we propose a gradient homojunction structure with germanium doping that generates an internal electric field across the perovskite film to deplete the charge carriers. This structure reduces the dark current density of perovskite by over 2 orders of magnitude and trap density by an order of magnitude. The resultant tin-based perovskite solar cells exhibit a higher power conversion efficiency of 13.3% and excellent stability, maintaining 95% and 85% of their initial efficiencies after 250 min of continuous illumination and 3800 h of storage, respectively. We reveal the homojunction formation mechanism using density functional theory calculations and molecular level characterizations. Our work provides a reliable strategy for controlling the spatial energy levels in tin perovskite films and offers insights into designing intriguing lead-free perovskite optoelectronics.

Understanding the Interfacial Reactions and Band Alignment for Efficient and Stable Perovskite Solar Cells Built on Metal Substrates with Reduced Upscaling Losses.

Conventional perovskite solar cells (PSC) built on transparent conductive oxide (TCO) glass face a fundamental challenge to retain fill factor (FF) for large-area upscaling due to series resistance loss. Building a perovskite solar cell on metal has the potential to reduce this FF loss and is promising for flexible applications. However, their efficiency and stability lag far behind their TCO counterparts. Herein, findings on the complex chemical reactions and degradation-promoting processes at different perovskite/metal (Cu, Au, Ag, and Mo) interfaces, which are closely linked with the inherent stability; and the interlayer engineering for perovskite/metal interface's band alignment, which plays an essential role in achieving high efficiency, are reported. Leveraging these findings, 21% power conversion efficiency (PCE) is achieved for 1 cm2 perovskite solar cells using a p-i-n top-illumination structure on a molybdenum substrate, the highest reported for a PSC built on metal. Notably, the FF and PCE losses due to area upscaling are remarkably reduced by one order of magnitude relative to the counterparts on conventional TCO glass, highlighting an alternative pathway for PSC upscaling and module design.

Reducing nonradiative recombination in perovskite solar cells with a porous insulator contact.

Inserting an ultrathin low-conductivity interlayer between the absorber and transport layer has emerged as an important strategy for reducing surface recombination in the best perovskite solar cells. However, a challenge with this approach is a trade-off between the open-circuit voltage (Voc) and the fill factor (FF). Here, we overcame this challenge by introducing a thick (about 100 nanometers) insulator layer with random nanoscale openings. We performed drift-diffusion simulations for cells with this porous insulator contact (PIC) and realized it using a solution process by controlling the growth mode of alumina nanoplates. Leveraging a PIC with an approximately 25% reduced contact area, we achieved an efficiency of up to 25.5% (certified steady-state efficiency 24.7%) in p-i-n devices. The product of Voc × FF was 87.9% of the Shockley-Queisser limit. The surface recombination velocity at the p-type contact was reduced from 64.2 to 9.2 centimeters per second. The bulk recombination lifetime was increased from 1.2 to 6.0 microseconds because of improvements in the perovskite crystallinity. The improved wettability of the perovskite precursor solution allowed us to demonstrate a 23.3% efficient 1-square-centimeter p-i-n cell. We demonstrate here its broad applicability for different p-type contacts and perovskite compositions.

Dual Graphitic-N Doping in a Six-Membered C-Ring of Graphene-Analogous Particles Enables an Efficient Electrocatalyst for the Hydrogen Evolution Reaction.

Graphene-based materials still exhibit poor electrocatalytic activities for the hydrogen evolution reaction (HER) although they are considered to be the most promising electrocatalysts. We fabricated a graphene-analogous material displaying exceptional activity towards the HER under acidic conditions with an overpotential (57 mV at 10 mA cm-2 ) and Tafel slope (44.6 mV dec-1 ) superior to previously reported graphene-based materials, and even comparable to the state-of-the art Pt/C catalyst. X-ray absorption near-edge structure (XANES) and solid-state NMR studies reveal that the distinct feature of its structure is dual graphitic-N doping in a six-membered carbon ring. Density functional theory (DFT) calculations show that the unique doped structure is beneficial for the activation of C-H bonds and to make the carbon atom bonded to two graphitic N atoms an active site for the HER.

Incorporation of Cu-Nx cofactors into graphene encapsulated Co as biomimetic electrocatalysts for efficient oxygen reduction.

Unlike metals with incomplete d-shells such as Pt and Fe, copper (Cu) with a filled d-electron shell is generally regarded as a sluggish oxygen reduction reaction (ORR) electrocatalyst. However, laccase and other copper enzymes could catalyze the ORR efficiently in nature. Inspired by this, we incorporated Cu-Nx cofactors (Cu-N2 and Cu-N4) into graphene encapsulated Co frameworks by direct annealing of MOFs with a post etching process. The bioinspired electrocatalyst exhibits excellent performance and stability for ORR which is comparable to or even better than Pt/C. Meanwhile, it also illustrates a fantabulous performance in a zinc-air battery device. The excellent performance can be ascribed to the abundant atomically dispersed Cu-Nx cofactors in the graphene frameworks confirmed by aberration corrected HAADF-STEM and XAFS analyses. Density functional theory calculations suggest that when Cu atoms are coordinated with the surrounding N atoms, the valence electrons of Cu atoms will transfer to nitrogen atoms, simultaneously tuning the d electronic states near the Fermi level to realize fast ORR kinetics.

O-, N-Atoms-Coordinated Mn Cofactors within a Graphene Framework as Bioinspired Oxygen Reduction Reaction Electrocatalysts.

Manganese (Mn) is generally regarded as not being sufficiently active for the oxygen reduction reaction (ORR) compared to other transition metals such as Fe and Co. However, in biology, manganese-containing enzymes can catalyze oxygen-evolving reactions efficiently with a relative low onset potential. Here, atomically dispersed O and N atoms coordinated Mn active sites are incorporated within graphene frameworks to emulate both the structure and function of Mn cofactors in heme-copper oxidases superfamily. Unlike previous single-metal catalysts with general M-N-C structures, here, it is proved that a coordinated O atom can also play a significant role in tuning the intrinsic catalytic activities of transition metals. The biomimetic electrocatalyst exhibits superior performance for the ORR and zinc-air batteries under alkaline conditions, which is even better than that of commercial Pt/C. The excellent performance can be ascribed to the abundant atomically dispersed Mn cofactors in the graphene frameworks, confirmed by various characterization methods. Theoretical calculations reveal that the intrinsic catalytic activity of metal Mn can be significantly improved via changing local geometry of nearest coordinated O and N atoms. Especially, graphene frameworks containing the Mn-N3 O1 cofactor demonstrate the fastest ORR kinetics due to the tuning of the d electronic states to a reasonable state.