Understanding and suppressing non-radiative losses in methylammonium-free wide-bandgap perovskite solar cells

Energy and Environmental Science Royal Society of Chemistry (2021)

Authors:

Robert DJ Oliver, Pietro Caprioglio, Francisco Peña-Camargo, Leonardo Buizza, Fengshuo Zu, Alexandra J Ramadan, Silvia Motti, Suhas Mahesh, Melissa McCarthy, Jonathan H Warby, Yen-Hung Lin, Norbert Koch, Steve Albrecht, Laura M Herz, Michael B Johnston, Dieter Neher, Martin Stolterfoht, Henry Snaith

Abstract:

With power conversion efficiencies of perovskite-on-silicon and all-perovskite tandem solar cells increasing at rapid pace, wide bandgap (> 1.7 eV) metal-halide perovskites (MHPs) are becoming a major focus of academic and industrial photovoltaic research. Compared to their lower bandgap (< 1.6 eV) counterparts, these types of perovskites suffer from higher levels of non-radiative losses in both the bulk material and in device configurations, constraining their efficiencies far below their thermodynamic potential. In this work, we investigate the energy losses in methylammonium (MA) free high-Br-content widegap perovskites by using a combination of THz spectroscopy, steady-state and time-resolved photoluminescence, coupled with drift-diffusion simulations. The investigation of this system allows us to study charge-carrier recombination in these materials and devices in the absence of halide segregation due to the photostabilty of formamidinium-cesium based lead halide perovskites. We find that these perovskites are characterised by large non-radiative recombination losses in the bulk material and that the interfaces with transport layers in solar cell devices strongly limit their open-circuit voltage. In particular, we discover that the interface with the hole transport layer performs particularly poorly, in contrast to 1.6 eV bandgap MHPs which are generally limited by the interface with the electron-transport layer. To overcome these losses, we incorporate and investigate the recombination mechanisms present with perovskites treated with the ionic additive 1-butyl-1-methylpipiderinium tetrafluoroborate. We find that this additive not only improves the radiative efficiency of the bulk perovskite, but also reduces the non-radiative recombination at both the hole and electron transport layer interfaces of full photovoltaic devices. In addition to unravelling the beneficial effect of this specific treatment, we further optimise our solar cells by introducing an additional LiF interface treatment at the electron transport layer interface. Together these treatments enable MA-free 1.79 eV bandgap perovskite solar cells with open-circuit voltages of 1.22 V and power conversion efficiencies approaching 17 %, which is among the highest reported for this material system.

Low-cost dopant-free carbazole enamine hole-transporting materials for thermally stable perovskite solar cells

Solar RRL Wiley 6:11 (2021) 2100984

Authors:

Suer Zhou, Maryte Daskeviciene, Matas Steponaitis, Giedre Bubniene, Vygintas Jankauskas, Kelly Schutt, Philippe Holzhey, Ashley R Marshall, Pietro Caprioglio, Grey Christoforo, James M Ball, Tadas Malinauskas, Vytautas Getautis, Henry J Snaith

Abstract:

Perovskite solar cells deliver high efficiencies, but are often made from high-cost bespoke chemicals, such as the archetypical hole-conductor, 2,2′,7,7′-tetrakis(N,N-di-p-methoxy-phenylamine)-9-9′-spirobifluorene (spiro-OMeTAD). Herein, new charge-transporting carbazole-based enamine molecules are reported. The new hole conductors do not require chemical oxidation to reach high power conversion efficiencies (PCEs) when employed in n-type-intrinsic-p-type perovskite solar cells; thus, reducing the risk of moisture degrading the perovskite layer through the hydrophilicity of oxidizing additives that are typically used with conventional hole conductors. Devices made with these new undoped carbazole-based enamines achieve comparable PCEs to those employing doped spiro-OMeTAD, and greatly enhanced stability under 85 °C thermal aging; maintaining 83% of their peak efficiency after 1000 h, compared with spiro-OMeTAD-based devices that degrade to 26% of the peak PCE within 24 h. Furthermore, the carbazole-based enamines can be synthesized without the use of organometallic catalysts and complicated purification techniques, lowering the material cost by one order of magnitude compared with spiro-OMeTAD. As a result, we calculate that the overall manufacturing costs of future photovoltaic (PV) modules are reduced, making the levelized cost of electricity competitive with silicon PV modules.

Role of Electronic States and Their Coupling on Radiative Losses of Open-Circuit Voltage in Organic Photovoltaics.

ACS applied materials & interfaces 13:50 (2021) 60279-60287

Authors:

Nakul Jain, Ramakant Sharma, Suhas Mahesh, Dhanashree Moghe, Henry J Snaith, Seunghyup Yoo, Dinesh Kabra

Abstract:

Voltage losses (ΔVOC) are a crucial limitation for the performance of excitonic organic solar cells (OSCs) and can be estimated by two approaches─the radiative limit and the Marcus charge-transfer (MCT) model. In this work, we show that combining the radiative limit and MCT models for voltage loss calculations provides useful insights into the physics of emerging efficient OSCs. We studied nine different donor-acceptor systems, wherein the power conversion efficiency ranges from 4.4 to 14.1% and ΔVOC varies from 0.55 to 0.95 V. For these state-of-the-art devices, we calculated the ΔVOC using the radiative limit and the MCT model. Furthermore, we combined both models to derive new insights on the origin of radiative voltage losses (ΔVrad) in OSCs. We quantified the contribution in ΔVrad due to the bulk intramolecular (S1) disorder and interfacial intermolecular (CT) disorder by revisiting the spectral regions of interest for OSCs. Our findings are in agreement with the expected relationship of VOC with Urbach energy (EU), which suggests that the low EU is beneficial for reduced losses. However, unprecedentedly, we also identify a universal, almost linear relationship between the interfacial disorder (λ) and ΔVrad. We believe that these results can be exploited by the organic photovoltaic (OPV) community for the design of new molecules and a combination of donor-acceptors to further improve OSCs.

Phase segregation in mixed-halide perovskites affects charge-carrier dynamics while preserving mobility.

Nature communications Springer Science and Business Media LLC 12:1 (2021) ARTN 6955

Authors:

Silvia G Motti, Jay B Patel, Robert DJ Oliver, Henry J Snaith, Michael B Johnston, Laura M Herz

Abstract:

Mixed halide perovskites can provide optimal bandgaps for tandem solar cells which are key to improved cost-efficiencies, but can still suffer from detrimental illumination-induced phase segregation. Here we employ optical-pump terahertz-probe spectroscopy to investigate the impact of halide segregation on the charge-carrier dynamics and transport properties of mixed halide perovskite films. We reveal that, surprisingly, halide segregation results in negligible impact to the THz charge-carrier mobilities, and that charge carriers within the I-rich phase are not strongly localised. We further demonstrate enhanced lattice anharmonicity in the segregated I-rich domains, which is likely to support ionic migration. These phonon anharmonicity effects also serve as evidence of a remarkably fast, picosecond charge funnelling into the narrow-bandgap I-rich domains. Our analysis demonstrates how minimal structural transformations during phase segregation have a dramatic effect on the charge-carrier dynamics as a result of charge funnelling. We suggest that because such enhanced recombination is radiative, performance losses may be mitigated by deployment of careful light management strategies in solar cells.

In situ cadmium surface passivation of perovskite nanocrystals for blue LEDs

JOURNAL OF MATERIALS CHEMISTRY A (2021)

Authors:

Woo Hyeon Jeong, Zhongkai Yu, Luca Gregori, Jonghee Yang, Su Ryong Ha, Ji Won Jang, Hochan Song, Jong Hyun Park, Eui Dae Jung, Myoung Hoon Song, Sung Heum Park, Henry J Snaith, Alberto Boretti, Filippo De Angelis, Daniele Meggiolaro, Jeongjae Lee, Hyosung Choi, Bo Ram Lee