Band-bending induced passivation: high performance and stable perovskite solar cells using a perhydropoly(silazane) precursor

Surface passivation of the perovskite photo absorber is a key factor to improve the photovoltaic performance. So far robust passivation strategies have not yet been revealed. Here, we demonstrate a successful passivation strategy which controls the Fermi-level of the perovskite surface by improving the surface states. Such Fermi-level control caused band-bending between the surface and bulk of the perovskite, which enhanced the hole-extraction from the absorber bulk to the HTM side. As an added benefit, the inorganic passivation layer improved the device light stability. By depositing a thick protection layer on the complete device, a remarkable waterproofing effect was obtained. As a result, an enhancement of VOC and the conversion efficiency from 20.5% to 22.1% was achieved. We revealed these passivation mechanisms and used perhydropoly(silazane) (PHPS) derived silica to control the perovskite surface states.

Phosphine Oxide Derivative as a Passivating Agent to Enhance the Performance of Perovskite Solar Cells

Aron Joel Huckaba, Cansu Igci, Hiroyuki Kanda, Hobeom Kim, Mounir Driss Mensi, Mohammad Khaja Nazeeruddin, Valentin Ianis Emmanuel Queloz, Albertus Adrian Sutanto

Defects of metal-halide perovskites detrimentally influence the optoelectronic properties of the thin film and, ultimately, the photovoltaic performance of perovskite solar cells (PSCs). Especially, defect-mediated nonradiative recombination that occurs at the perovskite interface significantly limits the power conversion efficiency (PCE) of PSCs. In this regard, interfacial engineering or surface treatment of perovskites has become a viable strategy for reducing the density of surface defects, thereby improving the PCE of PSCs. Here, an organic molecule, tris(5-((tetrahydro-2H-pyran-2-yl)oxy)pentyl) phosphine oxide (THPPO), is synthesized and introduced as a defect passivation agent in PSCs. The P.O terminal group of THPPO, a Lewis base, can passivate perovskite surface defects such as undercoordinated Pb2+. Consequently, improvement of PCEs from 19.87 to 20.70% and from 5.84 to 13.31% are achieved in n-i-p PSCs and hole-transporting layer (HTL)-free PSCs, respectively.

2021

Investigation on the impact of passivation in perovskite solar cells

Erfan Shirzadi

Conversion of sunlight to electricity is the most plausible approach towards the generation of renewable energies. During recent decade lead halide perovskites (LHPs) have proven to be promising candidates for photovoltaic applications such as solar cells. LHPs can be made easily in laboratories around the world via solution process methods and many universities can afford to participate in their development. How-ever, LHPs suffer from instability issues and their efficiencies can be further improved. In this thesis, both instability and efficiency challenges are addressed. We showed by doping the precursor of methylammonium lead iodide (MAPbI3) perovskite with a relatively large cation of 2-Choloroethylammonium (CLEA), perovskites films with CLEA incorporated into their structures are formed. The novel perovskite is able to withstand high temperatures as high as 80°C while achieving efficiencies > 19% in MAPbI3-based perovskite solar cells (PSCs). Various approaches have been employed for the passivation of the defects in perovskite solar cells Vari-ous approaches have been employed to passivate the defects in perovskite films used in perovskite solar cells (PSCs). However, the passivation mechanism is often unclear at a molecular level and the reason for enhanced PSC performance remains elusive. Here, we describe passivation based on the deposition of two dimensional (2D) perovskites (obtained from para-halophenylethylammonium iodide) on the surface of perovskite films. Moreover, the mechanism of passivation was elucidated and was traced to the high work function of the 2D perovskites, which induce band-bending at the surface of perovskite film and therefore reduces the charge carrier recombination at the interface of perovskite and the hole transporting material (HTM). Efficiencies >22% with average voltages >1.15 V were achieved using a typical mesoporous-TiO2 and Spiro-OMeTAD device configuration. This study paves the way for PSCs efficiency improvement by rational design. The employment of 2D perovskites is a promising approach to tackle the stability and voltage issues inherent in perovskite solar cells. It remains unclear however whether other perovskites with different dimensionalities have the same effect on efficiency and stability. Here, we report the use of quasi-3D azetidinium lead iodide (AzPbI3) as a secondary layer on top of the primary 3D perovskite film that results in significant improvements in the photovoltaic parameters. Remarkably, utilization of AzPbI3 leads to the passivation resulting in a power conversion efficiency (PCE) of 22.4 %. The open-circuit voltage obtained is as high as 1.18 V, which is among the highest reported to date for single-junction perovskite solar cells, corresponding to a voltage deficit of 0.37 V for a bandgap of 1.55 eV. Studying the compositional instability of mixed ion perovskites under light illumination is important to understand the mechanisms underlying their efficiency and stability. However, current techniques are limited in resolution and are unable to deconvolute minor ion migration phenomena. Here, we describe a method that enables ion migration to be studied allowing different segregation mechanisms to be elucidated. We applied statistical analysis to cathodoluminescence data to generate compositional distribution histograms. Using these histograms, we identified two different ion migration phenomena, horizontal ion migration (HIM) and vertical ion migration (

EPFL2021

Molecularly Engineered Functional Materials for High Performance Perovskite Solar Cells

Cansu Igci

Over the last few years, thanks to rapid development of molecular engineering and perovskite composition, it has allowed extremely promising solar energy conversion to achieve a certified power conversion efficiency (PCE) of over 25% for perovskite solar cells (PSCs). However, there is always room for improvement in research for the long-term stability and high cost of PSCs, which are the major obstacles to the commercialization process, which can also be caused by organic materials and dopants. Thus, the work presented in this thesis is focused on synthesis, profound characterization and use of novel organic functional materials for PSCs. In addition, I aim to find synthetic guidelines that show how thoughtful molecular design, such as different atoms in molecular structure, different functional groups can have different effects on photovoltaic performance and long-term stability of PSCs.In this work, I introduced three novel hole-transporting materials (HTMs) with donor-bridge-acceptor design principle. Triazatruxene-based donor units were functionalized with terthiophene conjugated arms, then differentiated into CI-B1, CI-B2 and CI-B3 HTMs by linking strong electron-accepting units. All three HTMs showed intermolecular aggregation, especially CI-B3 molecules are packed in favor of well-ordered edge-on stacking. PSCs with triple cation perovskite in n-i-p architecture with dopant-free HTM CI-B3 showed an impressive PCE of 17.54% with a small hysteresis and improved long-term stability.Following up this work, the novel dopant-free HTM, CI-TTIN-2F, based on donor-bridge-acceptor, have been demonstrated for all-inorganic CsPbI3 PSCs. CI-TTIN-2F features intermolecular aggregation formed both face-on and edge-on orientation. Molecular dynamics (MD) calculations revealed the passivation effect of CI-TTIN-2F with Pb on the perovskite surface with different contact types. All-inorganic CsPbI3 PSCs with dopant-free CI-TTIN-2F HTM demonstrate a high PCE of 15.9%, along with 86% efficiency retention after 1000 hours. Moreover, the largest all-inorganic perovskite solar module was fabricated using the CI-TTIN-2F HTM, and exhibited PCE of 11.0% with an area of 27 cm2.As the next part of our work, three benzodipyrrole-based organic small-molecules functionalized with 4-methoxyphenyl (CB-1), 3-fluorophenyl (CB-2), and 3-trifluoromethylphenyl (CB-3) have been introduced as HTMs for PSCs. PL characteristics and MD simulations revealed the passivation effect of the CB-2 and CB-3 molecules on top of perovskite surface. The PSC using the highly planar CB-2 achieved the highest PCE of 18.23% with excellent long-term storage stability in the air without encapsulation, and the molecular hydrophobicity of the fluorinated HTMs ensured that the devices showed no degradation of their PCEs over 6 months. Finally, a novel passivation molecule with a phosphine oxide group providing a Lewis base, tris(5-((tetrahydro-2Hpyran-2-yl)oxy)pentyl)phosphine oxide (THPPO), has been employed at the HTM/perovskite interface. THPPO effectively passivated the undercoordinated Pb defect on the surface of the perovskite layer by forming a coordinate bond with P=O functional Lewis base. The HTM-free devices have confirmed significant increase in PCE from 5.84% to 13.31% with a significant improvement of the stability. Further, THPPO boosted PCE of the devices from 19.87% to 20.70% when combining the device with spiro-OMeTAD as an HTM.