W Hashini K Perera

The development of perovskite solar cells (PSCs) with low recombination losses, at low processing temperatures is an area of growing research interest as it enables compatibility with roll-to-roll processing on flexible substrates as well as with tandem solar cells. The inverted or p-i-n device architecture has emerged as the most promising PSC configuration due to the possibility of using low temperature processable organic hole transport layers and more recently, self-assembled monolayers such as, [4-(3,6-Dimethyl-9H-carbazol-9-yl)butyl]phosphonic Acid (Me-4PACz). However, devices incorporating these interlayers suffer from poor wettability of the precursor leading to pin hole formation and poor device yield. Here, we demonstrate the use of alumina nanoparticles (Al2O3 NPs) for pinning the perovskite precursor on Me-4PACz, thereby improving the device yield. While similar wettability enhancements can also be achieved by using poly[(9,9-bis(3’-((N,N-dimethyl)-N-ethylammonium)-propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)]dibromide (PFN-Br), a widely employed surface modifier, the incorporation of Al2O3 NPs results in significantly enhanced Shockley-Read-Hall recombination lifetimes exceeding 3 μs, which is higher than those on films coated directly on Me-4PACz and on PFN-Br modified Me-4PACz. This translates to a champion power conversion efficiency of 19.9% for PSCs fabricated on Me-4PACz modified with Al2O3, which is a ∽20% improvement compared to the champion device fabricated on PFN-Br modified Me-4PACz.

Over the last decade, 2,2 '',7,7 ''-Tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9 '-spirobifluorene (spiro-OMeTAD) has remained the hole transporting layer (HTL) of choice for producing high efficiency perovskite solar cells (PSCs). However, PSCs incorporating spiro-OMeTAD suffer significantly from dopant induced instability and non-ideal band alignments. Herein, a new approach is presented for tackling these issues using the functionality of organometallocenes to bind to Li+ dopant ions, rendering them immobile and reducing their impact on the degradation of PSCs. Consequently, significant improvements are observed in device stability under elevated temperature and humidity, conditions in which ion migration occurs most readily. Remarkably, PSCs prepared with ferrocene retain 70% of the initial power conversion efficiency (PCE) after a period of 1250 h as compared to only 8% in the control. Synergistically, it is also identified that ferrocene improves the hole extraction yield at the HTL interface and reduces interfacial recombination enabling PCEs to reach 23.45%. This work offers a pathway for producing highly efficient spiro-OMeTAD devices with conventional dopants via addressing the key challenge of dopant induced instability in leading PSCs.