Lead sulfide colloidal quantum dot (CQD) solar cells demonstrate extremely high short-circuit currents (Jsc) and are making decent progress in power conversion efficiencies. However, the low fill factors (FF) and open-circuit voltages have to be addressed with urgency to prevent the stalling of efficiency improvements. This paper highlights the importance of improving hole extraction, which received much less attention as compared to the electron-accepting component of the device architecture (e.g., TiO2 or ZnO). Here, we show the use of semiconducting polymer poly(3-hexylthiophene-2,5-diyl) to create efficient CQD devices by improving hole transport, removing interfacial barriers, and minimizing shunt pathways, thus resulting in an overall improvement in device performance stemming from better Jsc and FF.
The band structure of colloidal quantum dot (CQD) bilayer heterojunction solar cells is optimized using a combination of ligand modification and QD band gap control. Solar cells with power conversion efficiencies of up to 9.33 ± 0.50% are demonstrated by aligning the absorber and hole transport layers (HTL). Key to achieving high efficiencies is optimizing the relative position of both the valence band and Fermi energy at the CQD bilayer interface. By comparing different band gap CQDs with different ligands, we find that a smaller band gap CQD HTL in combination with a more p-type-inducing CQD ligand is found to enhance hole extraction and hence device performance. We postulate that the efficiency improvements observed are largely due to the synergistic effects of narrower band gap QDs, causing an upshift of valence band position due to 1,2-ethanedithiol (EDT) ligands and a lowering of the Fermi level due to oxidation.
Given the growing demand for environmentally friendly energy sources, thermoelectric energy conversion has attracted increased interest as a promising CO2-free technology. SnSe single crystals have attracted attention as a next generation thermoelectric material due to outstanding thermoelectric properties arising from ultralow thermal conductivity. For practical applications, on the other hand, polycrystalline SnSe should be also focused because the production cost and the flexibility for applications are important factors, which requires the systematic investigation of the stability of thermoelectric performance under a pseudo operating environment. Here, we report that the physical properties of SnSe crystals with nano to submicron scale are drastically modified by atmospheric annealing. We measured the Seebeck effect while changing the annealing time and found that the large positive thermopower, + 757 μV K−1, was completely suppressed by annealing for only a few minutes and was eventually inverted to be the large negative value, − 427 μV K−1. This result would further accelerate intensive studies on SnSe nanostructures, especially focusing on the realistic device structures and sealing technologies for energy harvesting applications.
Colloidal quantum dot (CQD)-based photovoltaics are an emerging low-cost solar cell technology with power conversion efficiencies exceeding 10%, i.e., high enough to be interesting for commercialization. Well-controlled and understood charge carrier transport through the device stack is required to make the next step in efficiency improvements. In this paper, polymer-wrapped single-walled carbon nanotube (SWNT) films embedded in an insulating poly(methyl methacrylate) (PMMA) matrix and capped by a thermally evaporated Au electrode are investigated as a composite hole transport layer and optical spacer. Employing transient absorption spectroscopy we show that the SWNTs enhance the charge transfer rate from CQD to CQD, ZnO, or SWNT. In order to pinpoint the underlying mechanism for the improvement, we investigate the energetics of the junction by measuring the relative alignment of the band edges, using Kelvin probe and cyclic voltammetry. Measuring the external quantum efficiency and absorption we find that the improvement is not mainly from electronic improvements but from enhanced absorption of the CQD absorber. We demonstrate experimentally and theoretically, by employing a transfer-matrix model, that the transparent PMMA matrix acts as an optical spacer, which leads to an enhanced absorption in the absorber layer. With these electronic and optical enhancements, the efficiency of the PbS CQD solar cells improved from 4.0% to 6.0%.
Our recent work (1) has shown that polymer wrapped carbon nanotubes can be used to transform the properties of the hole transporting layer in a number of different perovskite based solar cell configurations. This presentation will report on two examples where the CNT hole transporter produces enhanced device performance, firstly for tin oxide/perovskite alloy/CNT devices and secondly in PbS colloidal quantum dots (CQD) where improved charge extraction by the CNT is particularly important. Halide perovskite solar cells remain to generate a lot of interest and enthusiasm among researchers, largely due to the prospect of readily transitioning from a purely lab based technology to real-world scales. This expectation is largely based on the rapid increase in efficiency over the past few years to values above 22%. An essential component of this device architecture are the two charge selective contacts. The n-type contact in the majority of all devices is TiO2, either as a thin compact layer, or a mesostructured configuration, but recently, SnO2 has moved into the spotlight as n-type contact with recent studies reporting excellent steady-state performances. The p-type contact on the other side of the absorber has been investigated even more intensively. The best-performing devices still use the “original” hole-transport material, spiro-OMeTAD, which provided the crucial breakthrough for the perovskite solar cell in 2012, but which suffers from the fact that it is not sufficiently conductive for efficient charge-transport, and requires extrinsic doping, typically with Li-TFSI, which has been shown to detrimentally impact the device stability. This presentation will demonstrate that for FA0.83MA0.117Pb(I0.87Br0.17)3 based solar cells we can achieve steady-state efficiencies of up to 18.8%, by using polymer-wrapped single-walled carbon nanotubes (SWNTs) as an inert conductive element in undoped spiro-OMeTAD, exceeding the performance of their fully doped counterparts. In a completely different example we can use the same polymer wrapped SWNTs as a hole collection electrode for PbS based CQD photodetectors which are capable of operating over a spectral range extending beyond one micron. We find that the use of a polymer wrapped CQD combined with PMMA filler more than doubles the current collection in such devices and enables efficiencies of over 7%. In addition the SWNT layer produces a substantial improvement in long term device stability in normal ambient conditions. (1) Habisreutinger, S. N.; Leijtens, T.; Eperon, G. E.; Stranks, S. D.; Nicholas, R. J.; Snaith, H. J. Nano Lett. 2014, 14, 5561–5568.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
customersupport@researchsolutions.com
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.