Seongrok Seo

Abstract:

Performance losses in positive–intrinsic–negative architecture perovskite solar cells are dominated by nonradiative recombination at the perovskite/organic electron transport layer interface, which is particularly problematic for wider bandgap perovskites. Large endeavours have been dedicated to the replacement of fullerenes, which are the most commonly used class of electron transport layers, with limited success thus far. In this work, we demonstrate blending the fullerene derivatives [6,6]-phenyl C61 butyric acid methyl ester (PCBM) and indene-C60 bis-adduct (ICBA) as a thin interlayer between 1.77 eV bandgap perovskite and an evaporated C60 layer. By tuning the fullerene blend to a trace 2% by mass of PCBM in ICBA, we remarkably form an interlayer which features improved energetic alignment with the perovskite and the PCBM : ICBA fullerene mixture, together with a stronger molecular ordering and an order of magnitude higher electron mobility than either neat PCBM or ICBA. Additional molecular surface passivation approaches are found to be beneficial in conjunction with this approach, resulting in devices with 19.5% steady state efficiency, a fill factor of 0.85 and an open-circuit voltage of 1.33 V, which is within 10% of the radiative limit of the latter two device parameters for this bandgap. This work highlights the complex nonlinear energetic behaviour with fullerene mixing, and how control of the energetics and crystallinity of these materials is crucial in overcoming the detrimental recombination losses that have historically limited perovskite solar cells.

Abstract:

We show that the use of 1,3-diaminopropane (DAP) as a chemical modifier at the perovskite/electron-transport layer (ETL) interface enhances the power conversion efficiency (PCE) of 1.7 eV band gap mixed-halide perovskite containing formamidinium and Cs single-junction cells, primarily by increasing the open-circuit voltage (VOC) from 1.06 to 1.15 V. We find that adding a postprocessing annealing step after C60 evaporation further improves device performance. Specifically, the fill factor (FF) increases by 20% in the DAP + postannealing devices compared to the control. Using hyperspectral photoluminescence microscopy, we demonstrate that annealing helps improve compositional homogeneity at the electron-transport layer (ETL) and hole-transport layer (HTL) interfaces of the solar cell, which prevents detrimental band gap pinning in the devices and improves C60 adhesion. Using time-of-flight secondary ion mass spectrometry, we show that DAP reacts with formamidinium (FA+) present at the surface of the perovskite structure to form a larger molecular cation, 1,4,5,6-tetrahydropyrimidinium (THP+), which remains at the interface. Combining the use of DAP and annealing the C60 interface, we fabricate Si-perovskite tandems with a PCE of 25.29%, compared to 23.26% for control devices. Our study underscores the critical role of the chemical reactivity of diamines at the surface and the thermal postprocessing of the C60/Lewis-base passivator interface in minimizing device losses and enhancing solar-cell performance of wide-band-gap mixed-cation mixed-halide perovskites for tandem applications.

Abstract:

Perovskite solar cells achieve high power conversion efficiencies but usually rely on vacuum-deposited metallic contacts, leading to high material costs for noble metals and stability issues for more reactive metals. Carbon-based materials offer a cost-effective and potentially more stable alternative. The vast majority of carbon-electrode PSCs use the negative-intrinsic-positive (n-i-p) or “hole-transport-layer-free” architectures. Here, we present a systematic study to assess the compatibility of “inverted”, p-i-n configuration PSC contact layers with carbon top electrodes. We identify incompatibilities between common electron transport layers and the carbon electrode deposition process and previously unobserved semiconducting properties in carbon electrodes with unique implications for charge extraction and electronic behavior. To overcome these issues, we introduce a double-layer atomic layer deposited tin oxide (SnO2) and Poly­(2,3-dihydrothieno-1,4-dioxin)-poly­(styrenesulfonate) (PEDOT:PSS), yielding up to 16.1% PCE and a retained 94% performance after 500 h of outdoor aging. The study is a crucial step forward for printable, metal-electrode-free, and evaporation-free perovskite PV technologies.

Abstract:

In this work we demonstrate that time-resolved photoluminescence data of metal halide perovskites can be effectively evaluated by combining Bayesian inference with a Markov-Chain Monte-Carlo algorithm and a physical model. This approach enables us to infer a high number of parameters which govern the performance of metal halide perovskite-based devices, alongside the probability distributions of those parameters, as well as correlations among all parameters. Via studying a set of "half-stacks’‘, comprising electron and hole transport materials contacting perovskite thin-films, we determine surface recombination velocities at these interfaces with high precision. From the probability distributions of all inferred parameters, we can simulate intensity-dependent photoluminescence quantum efficiency and compare it to the experimental data. Finally, we estimate mobility values for the "vertical’’ charge carrier transport, that perpendicular to the plane of the substrate, for all samples using our approach. Since this mobility estimation is derived from charge carrier diffusion over the length-scale of the film thickness and in the vertical direction, it is highly relevant to transport in photovoltaic and light emitting devices. Our approach of coupling spectroscopic measurements with advanced, computational analysis will help speed up scientific research in the field of optoelectronic materials and devices and exemplifies how carefully constructed computational algorithms can derive valuable plurality of information from simple datasets. We expect that our approach will be expandable to a variety of other analysis techniques and that our method will be applicable to other semiconductors.

Abstract: