The promise of cheap and efficient solar energy motivates a wide range of research efforts, from fundamental molecular design of new materials to sophisticated device engineering. In the last 12 or so years, halide perovskite solar cells (PSCs) have captured the imagination of scientists and entrepreneurs alike. Their appeal lies in efficiencies on par with more established inorganic technologies and low-cost production. Furthermore, PSCs are promising candidates for unprecedented applications such as flexible or transparent solar cells integrated into windows and blinds. Despite these selling points, there are challenges to manufacturing stable PSCs on a large scale and to commercializing them. Notably, these devices suffer from reproducibility problems that are in part due to the instability of the precursor solutions used to prepare them.
A recent report from Jinsong Huang’s group at the University of North Carolina, Chapel Hill, supported by the Center for Hybrid Organic Inorganic Semi-conductors for Energy (CHOISE), addresses this common PSC shortcoming. The authors have shown that the simple addition of a low-cost reductant molecule to perovskite precursor solutions can reverse the damage caused by degradation to regenerate the precursors and improve device reproducibility. The halide perovskites found in solar cells are part of a large class of materials characterized by their ABX3 crystal structure. This formula refers to the ordered arrangement of component molecules (a combination of organic and inorganic ions) in space that endows the material with desirable properties when it comes to converting light energy into electric current.
Even when handled with utmost precaution, the organic halide salts that serve as perovskite precursors are prone to oxidation, leading to defects in the material structure and thus decreased device performance. Hoping to reverse the damage of oxidation, the researchers added benzylhydrazine hydrochloride (BHC), a readily available reductant, to precursor solutions that showed degradation. BHC targets the inactive forms of the halide component of the precursor mixture, iodine, that are produced as solutions age (I2 and I3-) and converts them back to the active ion (I-). As a result of this transformation, the crystalline perovskite films created from solutions treated with BHC not only regained the ordered structures and light-responsive properties of materials prepared with fresh precursors, but even showed improvement in these metrics.
The findings suggest that the simple addition of BHC reductant improves the reproducibility and stability of PSCs. This solution stabilization strategy shows promise for future scalable production of perovskite-based devices and their commercialization, potentially making solar energy technologies more widely available and affordable.
Furthermore, the authors measured a record-setting power conversion efficiency (PCE) of 23.2% for a device prepared with solutions regenerated with BHC. The PCE value represents how much of the light that hits the device is directly transformed into usable electricity. For commercially available silicon panels, PCE is typically in the 15–17% range, while the theoretical limit is around 30%. As device performance reproducibility is a common fault in PSCs, the researchers examined how BHC addition impacted the performance of 244 individual devices. They measured PCE values greater than 22% for 80% of the devices, a significant improvement over devices prepared without BHC treatment that had a lower average performance as well as a much larger proportion of devices that performed poorly. In addition, the presence of excess BHC in the devices further stabilized their function under operating conditions, contributing to high efficiency results over a sustained testing period.
