Sustainable energy devices

We work on developing thin film solar cells based on emerging materials. In the past few years, we have worked with colloidal quantum dot absorbers, polymers, cuprous oxide, as well as ns² compounds predicted to be defect-tolerant. We have developed a wide range of charge transport layers for optimal carrier extraction and to control non-radiative losses at the interfaces. Our work heavily focusses on understanding the processes limiting device performance, such as through optical loss analyses, transient photovoltage measurements, intensity-dependent device measurements and white-light biased external quantum efficiency measurements. In numerous cases, we have devised strategies to achieve improved performance over the state-of-the-art.

Figure 1: Cross-sectional SEM image and performance of BiOI solar cells. Reprinted with permission from Adv. Mater., 2017, 29, 1702176

Further Reading

BiOI solar cells: R. L. Z. Hoye,* et al., Strongly Enhanced Photovoltaic Performance and Defect Physics of Air‐Stable Bismuth Oxyiodide (BiOI). Advanced Materials, 2017, 29(36), 1702176. Press release

Perovskite solar cells with oxide buffer layers: R. D. Raninga,^† R. A. Jagt,^† ..., R. L. Z. Hoye,* Strong performance enhancement in lead-halide perovskite solar cells through rapid, atmospheric deposition of n-type buffer layer oxides. Nano Energy, 2020, 75, 104946.
^†Equal contribution

Colloidal quantum dot solar cells: R. L. Z. Hoye, et al., Improved Open‐Circuit Voltage in ZnO–PbSe Quantum Dot Solar Cells by Understanding and Reducing Losses Arising from the ZnO Conduction Band Tail. Advanced Energy Materials, 2014, 4(8), 1301544.

The Internet of Things (IoT) describes an ecosystem of interconnected devices, creating smart homes and workplaces. It is predicted that by 2025, there will be 75 billion devices part of the IoT ecosystem. Many of these devices will be small, autonomous and located in remote locations. Powering all of these devices with batteries will no longer be practical or sustainable, given the presence of scarce and, in some cases, toxic elements in batteries. Using photovoltaics to harvest indoor light is a promising alternative to not only 'wirelessly' power the IoT device, but also to recharge the energy storage device when lights are on, creating a long-lasting power supply unit.

We demonstrated that lead-free perovskite-inspired materials (PIMs) are promising as indoor photovoltaics (IPV). Recently, we tested two PIMs (BiOI and Cs₃Sb₂(I,Cl)₉) for IPV, and demonstrated efficiencies within the range of commercial technologies. We calculated the optically-limited efficiencies of these materials and other PIMs, showing that fully optimised devices could reach efficiencies of 40-60%, far surpassing devices based on hydrogen-passived amorphous silicon, which is the current industry standard for IPV. 

Figure 2: Illustration of lab-scale perovskite-inspired material PVs powering printed inverters. Reprinted with permission from Adv. Energy Mater., 2021, 11, 2002761.

Further Reading

Y. Peng,^† T. N. Huq,^† J. Mei,^† ..., R. L. Z. Hoye,* V. Pecunia.* Lead-Free Perovskite-Inspired Absorbers for Indoor Photovoltaics. Advanced Energy Materials, 2021, 11, 2002761. 
^†Equal contribution

V. Pecunia,* L. G. Occhipinti, R. L. Z. Hoye.* Emerging Indoor Photovoltaic Technologies for Sustainable Internet of Things. Advanced Energy Materials, 2021, 11(29) 2100698. 

Tandem photovoltaics combine multiple solar cells (Figure 3) to increase the fraction of the solar spectrum converted to electrical energy, allowing single-junction device efficiency limits to be overcome. We have developed tandems between metal-halide perovskite top-cells (visible light absorbers) and silicon bottom cells (near-infrared light absorbers) in a monolithic configuration, achieving high performance and stable device operation. Our focus in this area is in: 1) developing recombination contacts that couple together both sub-cells to achieve voltage addition with minimal parasitic optical losses, 2) developing tandems using industrially-relevant bottom cells, and 3) faster methods to grow the oxide buffer layer protecting the perovskite top cell. For example, in 2016, we developed an ITO/NiO_x recombination contact. In collaboration with Stanford, Arizona State University and others, this led to a 23.6%-efficient two-terminal tandem, which became the first entry for perovskite-silicon tandems on the NREL chart of best research-cell efficiencies. 

Figure 3. Schematic of a four-terminal tandem, comprised of a silicon bottom cell (band gap of 1.1 eV) and a wider band gap top-cell

Further Reading

Collaborative work with Stanford, ASU and others: Kevin A. Bush,^† Axel F. Palmstrom,^† J. Yu Zhengshan,^† et al., 23.6%-efficient monolithic perovskite/silicon tandem solar cells with improved stability. Nature Energy, 2017, 2(4), 17009.
^†Equal contribution

Oxide buffer layers: Robert A. Jagt, ..., Robert L. Z. Hoye. Rapid Vapor-Phase Deposition of High-Mobility p-Type Buffer Layers on Perovskite Photovoltaics for Efficient Semitransparent Devices. ACS Energy Letters, 2020, 5, 2456-2465. Press release

Light-emitting diodes are the inverse of photovoltaics: Electrons and holes are injected to an active layer to achieve electroluminescence. Over the past few years, we have worked primarily on metal-halide perovskite emitters. We have developed device structures that reduce non-radiative losses and more effectively confine electrons and holes within the emissive layer to give colour-pure electroluminescence. A particular advantage of metal-halide perovskites is that their low levels of disorder result in sharper electroluminescence peaks than conventional inorganic and organic materials, making the perovskites particularly appealing for ultrahigh definition display applications.

Figure 4. Electroluminescence spectra and photographs of blue and sky-blue perovskite LEDs. Reprinted with permission from ACS Energy Lett, 2019, 4, 1181. Copyright 2019 American Chemical Society

Further Reading

Surface chemistry of perovskite nanocrystals: J. Ye, ..., R. L. Z. Hoye,* Elucidating the Role of Antisolvents on the Surface Chemistry and Optoelectronic Properties of CsPbBr_xI_3-xPerovskite Nanocrystals. Journal of the American Chemical Society, 2022, 144, 12102-12115. 

Review on factors influencing light-emission in perovskites: S. D. Stranks, R. L. Z. Hoye, et al., The physics of light emission in halide perovskite devices. Advanced Materials, 2019, 31(47), 1803336.

Sharp emission in green LEDs: R. L. Z. Hoye, et al., Enhanced performance in fluorene‐free organometal halide perovskite light‐emitting diodes using tunable, low electron affinity oxide electron injectors. Advanced Materials, 2015, 27(8), 1414-1419.

Blue perovskite nanoplatelet LEDs: R. L. Z. Hoye,^† M.-L. Lai,^† et al., Identifying and reducing interfacial losses to enhance color-pure electroluminescence in blue-emitting perovskite nanoplatelet light-emitting diodes. ACS Energy Letters, 2019, 4(5), 1181-1188.
^†Equal contribution

Our primary focus is on optoelectronic materials for photovoltaics and LEDs. But there is strong overlap in the properties of solar absorbers, light-emitting and photocatalysts. The materials we have been developing (e.g., lead-halide perovskites, but also the lead-free materials) have suitable band positions for water reduction and, in some cases, oxidation as well. We work in collaboration with the Reisner Group (Chemistry, University of Cambridge) in the development of these materials for photoelectrochemical cells. In a recent work, we developed a device structure that improved the stability of BiOI photocathodes from a few minutes (BiOI/electrolyte junctions) to a couple of months (device structure shown in Figure 5). Coupled with a BiVO₄ photoanode, we achieved bias-free water and CO₂ splitting.

Figure 5. Structure of BiOI photocathode, along with cross-sectional scanning electron microscopy images of the cross-section of the device and catalyst.

Further Reading

V. Andrei,^† R. A. Jagt,^† M. Rahaman, L. Lari, V. K. Lazarov, J. L. MacManus-Driscoll,* R. L. Z. Hoye,* E. Reisner.* Long-term solar water and CO₂ splitting with photoelectrochemical BiOI-BiVO₄tandems. Nature Materials, 2022, Just Accepted. DOI: 10.1038/s41563-022-01262-w

V. Andrei,^† G. M. Ucoski,^† C. Pornrungroj, C. Uswachoke, Q. Wang, D. S. Achilleos, H. Kasap, K. P. Sokol, R. A. Jagt, H. Lu, T. Lawson, A. Wagner, S. D. Pike, D. S. Wright, R. L. Z. Hoye, J. L. MacManus-Driscoll, H. J. Joyce, R. H. Friend, Erwin Reisner.* Floating perovskite-BiVO₄ devices for scalable solar fuel production. Nature, 2022, Just Accepted. DOI: 10.1038/s41586-022-04978-6