Dr Pascal Kaienburg

Abstract:

Vacuum-deposited organic solar cells (OSCs) have lagged behind their solution-processed counterparts in achieving high power conversion efficiency (PCE), in particular as result of higher voltage losses. In this study, we demonstrate a bulk heterojunction OSC using SubNc as donor and DCV3T as non-fullerene acceptor, achieving a PCE of 2.6% and a remarkably low total voltage loss of 0.64 V, lower than the typical values exceeding 0.7 V observed in vacuum thermally evaporated fullerene-based systems. The device also exhibits non-radiative voltage losses comparable to leading non-fullerene-acceptor (NFA) based OSCs. Transient absorption spectroscopy confirms efficient Förster resonance energy transfer from DCV3T to SubNc, followed by electron transfer for exciton separation. Morphological Grazing Incidence Wide-Angle X-ray Scattering features suggest both blends have a preferential edge-on orientation of DCV3T molecules, and the blends with higher DCV3T content could suffer from suppressed out-of-plane lamellar crystallinity with possible connection with greater non-radiative losses. Our findings demonstrate the potential of designing low-voltage-loss evaporated OSCs by building on strategies from solution-processed NFA systems, while highlighting the continued need for new evaporable acceptors with optimised optoelectronic and morphological properties.

Abstract:

Monolithic perovskite–organic tandem solar cells (POTSCs) have attracted considerable attention in recent years due to their compatible fabrication routes and advances in single-cell efficiencies. To further boost the performance of POTSCs, reducing the voltage losses that mainly arise from wide bandgap (WBG, >1.7 eV) perovskite subcells and interconnecting layers (ICLs) is critical. Here, a new ICL with a configuration of C60/YbO x /Au/MoO x is demonstrated for constructing the monolithic POTSC. The YbO x -based ICL benefits from an ohmic contact and high transparency, resulting in improved POSTC performance. The champion device presents a PCE of 23.2% owing to a high V OC of 2.11 V (approximately equal to the sum of individual V OC’s of the subcells) without compromising the short-circuit current density and fill factors. This work opens an avenue for developing efficient ICLs in POTSCs.

Abstract:

<jats:p>Substrate heating during co-evaporation of bulk heterojunction organic solar cells aids phase separation and improves performance. While recombination remains unaffected, hole transport improves due to more crystalline donor domains.</jats:p>

Abstract:

Abstract Photovoltaics (PVs) are a critical technology for curbing growing levels of anthropogenic greenhouse gas emissions, and meeting increases in future demand for low-carbon electricity. In order to fulfil ambitions for net-zero carbon dioxide equivalent (CO2eq) emissions worldwide, the global cumulative capacity of solar PVs must increase by an order of magnitude from 0.9 TWp in 2021 to 8.5 TWp by 2050 according to the International Renewable Energy Agency, which is considered to be a highly conservative estimate. In 2020, the Henry Royce Institute brought together the UK PV community to discuss the critical technological and infrastructure challenges that need to be overcome to address the vast challenges in accelerating PV deployment. Herein, we examine the key developments in the global community, especially the progress made in the field since this earlier roadmap, bringing together experts primarily from the UK across the breadth of the photovoltaics community. The focus is both on the challenges in improving the efficiency, stability and levelized cost of electricity of current technologies for utility-scale PVs, as well as the fundamental questions in novel technologies that can have a significant impact on emerging markets, such as indoor PVs, space PVs, and agrivoltaics. We discuss challenges in advanced metrology and computational tools, as well as the growing synergies between PVs and solar fuels, and offer a perspective on the environmental sustainability of the PV industry. Through this roadmap, we emphasize promising pathways forward in both the short- and long-term, and for communities working on technologies across a range of maturity levels to learn from each other.

Abstract:

Free charge generation after photoexcitation of donor or acceptor molecules in organic solar cells generally proceeds via (1) formation of charge transfer states and (2) their dissociation into charge separated states. Research often either focuses on the first component or the combined effect of both processes. Here, we provide evidence that charge transfer state dissociation rather than formation presents a major bottleneck for free charge generation in fullerene-based blends with low energetic offsets between singlet and charge transfer states. We investigate devices based on dilute donor content blends of (fluorinated) ZnPc:C60 and perform density functional theory calculations, device characterization, transient absorption spectroscopy and time-resolved electron paramagnetic resonance measurements. We draw a comprehensive picture of how energies and transitions between singlet, charge transfer, and charge separated states change upon ZnPc fluorination. We find that a significant reduction in photocurrent can be attributed to increasingly inefficient charge transfer state dissociation. With this, our work highlights potential reasons why low offset fullerene systems do not show the high performance of non-fullerene acceptors.