Imperial College London
Abstract: Perovskite solar cells (PSCs) continue to stimulate intense research activity with a multitude of processing modifications, obscure additives and some good old-fashioned chemistry being cited as the underlying reasons for improvements in PSC performance. Of the growing volume of PSC literature a small quantity focuses on minor changes to the processing methodology employed and/or additives that have been cited to be responsible for a variety of measurable improvements in device performance. Here I will discuss several recent case studies where processing modifications have been investigated as a means of controllably varying active layer microstructure and morphology and highlight their impact on structural and electronic defects in these intriguing systems.
First using scanning transmission electron microscopy (STEM) we locate and identify previously unobserved nanoscale defects residing within individual grains of solution‐processed methylammonium lead tri‐iodide (CH3NH3PbI3, MAPI) thin films. The presence of these defects impede device performance and are directly related to the established processing methodology. We identify a facile modification in the processing that eliminates the defects, improves crystallinity within the MAPI active layer. By removing these defects, significant improvements in intragrain crystallographic coherence along the charge-transport direction can be achieved, which results in improved power conversion efficiencies from 13.6% to 17.4%. Our in-situ optoelectronic characterizations link this to an improvement in charge collection efficiency and a reduction in electron–hole recombination at PEDOT:PSS/MAPI interface and an overall decrease of trap state density in our PSCs.
We then demonstrate the impact of active layer crystallinity on the accumulated charge and open circuit voltage (Voc) in solar cells based on MAPI is demonstrated. We show that MAPI crystallinity can be systematically tailored by modulating the stoichiometry of the precursor mix, where small quantities of excess methylammonium iodide (MAI) improve crystallinity, increasing device Voc by ≈200 mV. Using in situ differential charging and transient photovoltage measurements, charge density and charge carrier recombination lifetime are determined under operational conditions. Increased Voc is correlated to improved active layer crystallinity and a reduction in the density of trap states in MAPI. Photoluminescence spectroscopy shows that an increase in trap states correlates with faster carrier trapping and more nonradiative recombination pathways. Fundamental insights into the origin of Voc in perovskite photovoltaics are provided and it is demonstrated why highly crystalline perovskite films are paramount for high-performance devices.
Finally, we focus on the origin of performance enhancements of PSCs observed by incorporating low concentrations of the bulky cation 1‑naphthylmethylamine (NMA) as a processing additive. The addition of 0.25 vol/vol % NMA is increases the open circuit voltage (Voc) of methylammonium lead iodide (MAPbI3) PSCs from 1.06 V to 1.16 V, in addition to increasing their power conversion efficiency (PCE) from 18.7 % to 20.1 %. We show that this addition of NMA with solvent assisted solvent annealing favours the formation of monolithic grains orientated in the charge transport direction. Steady state and transient photoluminescence data indicate NMA is suppressing non-radiative recombination resulting from charge trapping, consistent with passivation of grain boundaries. Further raising the NMA concentrations leads to reductions in device short-circuit current density (Jsc) and PCE in addition to suppressed photoluminescence quenching by the charge transport layers. TO demonstrate the applicability of this approach we show that NMA can also employed to enhance the performance of mixed iodide/bromide wide band gap perovskites, achieving a Voc of 1.22 V with band gap of 1.68 eV.
Biography: Dr Martyn McLachlan is currently a Reader (Associate Professor) in the Department of Materials and a member of the Centre for Plastic Electronics at ICL. He completed his PhD in 2005 at the University of Glasgow before joining ICL as a post-doctoral researcher. In 2007 he was awarded a 5-year Royal Academy of Engineering Research Fellowship and was subsequently appointed as a Lecturer (2013), Senior Lecturer (2015) and to his current position in 2017. He leads a research with active interests spanning inorganic and hybrid material synthesis, thin film and device deposition, microstructural and interfacial characterization – in these areas he has published around 100 peer reviewed journal articles and has 2 patent filings.