Big step towards stable, lead-free perovskite solar cells
Perovskite’s fascinating structure and properties has propelled it into the forefront of materials research for a wide range of applications. Metal-halide perovskites – a subset of hybrid organic-inorganic perovskites containing halide ions such as iodide or bromide – are especially prominent for their unprecedented potential to convert sunlight into electricity. Since their discovery in 2009, efficiencies for these fledgling materials have continued to climb, moving from 3.8 % to over 25 %. Their use is tantalising for several reasons. The ingredients are abundant, and researchers can easily combine them into thin films that have a highly crystalline structure similar to that achieved in silicon wafers after costly, high-temperature processing. Thin and flexible rolls of transparent perovskite film could one day be rapidly spooled from a 3D printer to make lightweight, colourful solar sheets integrated into windows and building facades – a hard feat for silicon solar cells to emulate. Unfortunately, metal-halide perovskites can degrade rapidly because they are sensitive to moisture and heat. Besides stability, lead toxicity has been one of the most vexing issues that have stymied perovskite path to commercialisation. The Marie Skłodowska-Curie-funded TinPSC project was established to address these challenges.
Lead-free double perovskite structures
“We focused on developing lead-free double perovskites, an entirely new generation of compounds in which a monovalent metal and a trivalent metal replace the divalent lead. We capitalised on earlier successful endeavours that resulted in a highly stable device with excellent electronic properties and diffusion lengths exceeding 100 nm. Such high values indicate the material’s quality and suitability for optoelectronics use,” notes Feng Wang, TinPSC coordinator. Project activities largely focused on effective strategies to narrow the band gap of lead‐free double perovskites – a common factor that limits effective light absorption. The beauty of perovskites is that it allows researchers to adjust the energy gap at will by tweaking the mix of ingredients, raising the prospect of increasing absorption efficiency. “We successfully broadened the absorption edges of Cs2AgBiBr6 to the near-infrared through Cu doping. Results indicated Cu doping weakly affects the valence and conduction bands of the host structure, but introduces intermediate band-gap states that strongly encourage infrared absorption. More interestingly, the sub-band-gap state can generate considerable band carriers through near-infrared excitation,” explains Wang. The researchers also devised a crystal engineering strategy to modify the material band gap. “By simply controlling the crystal growth temperature and speed, we narrowed the band gap to a record 1.72 eV. This is the smallest band gap ever reported for Cs2AgBiBr6 under ambient conditions,” adds Wang. The band gap narrowing was confirmed by both absorption and photoluminescence measurements.
Enhancing thermal and moisture stability
Just like any solar cell device architecture, a perovskite thin film is sandwiched between two charge extraction layers. Once it is exposed to light, the holes and electrons generated in the perovskite lattice move towards the outer layers, creating an electric current. The electron and hole transport layers play a key role in suppressing recombination losses in the interfaces, which derail the electron journey to the exit of the cell. The transport layer stability largely determines the whole device stability. The project team devised a unique scheme to protect the hole transport layer by moisture and heat but are keeping most of the details under wraps. The team claims the resulting solar cell perovskite devices will be much more stable compared to state of the art.
Keywords
TinPSC, perovskite, solar cell, stability, lead-free, moisture, metal-halide perovskites, silicon, hole transport layer
