Yannan Zhang

Room-Temperature Processed Nb2O5 as the Electron-Transporting Layer for Efficient Planar Perovskite Solar Cells

In this work, we demonstrate high-efficiency planar perovskite solar cells (PSCs), using room-temperature sputtered niobium oxide (Nb2O5) as the electron-transporting layer (ETL). Widely spread ETL-like TiO2 often requires high-temperature (>450 °C) sintering, which is not desired for the fabrication of flexible devices. The amorphous Nb2O5 (labeled as a-Nb2O5) ETL, without any heat treatment, can give a best power conversion efficiency (PCE) of 17.1% for planar PSCs. Interestingly, the crystalline Nb2O5 (labeled as c-Nb2O5), with high-temperature (500 °C) annealing, results in a very similar PCE of 17.2%, indicating the great advantage of a-Nb2O5 in energy saving. We thus carried out a systematical investigation on the properties of the a-Nb2O5 film. The Hall effect measurements indicate both high mobility and conductivity of the a-Nb2O5 film. Kelvin probe force microscopy measurements define the Fermi levels of a-Nb2O5 and c-Nb2O5 as -4.31 and -4.02 eV, respectively, which allow efficient electron extraction at the Nb2O5/perovskite interface, regardless of the additional heat treatment on Nb2O5 film. Benefitting from the low-temperature process, we further demonstrated flexible PSCs based on a-Nb2O5, with a considerable PCE of 12.1%. The room-temperature processing and relatively high device performance of a-Nb2O5 suggest a great potential for its application in optoelectrical devices.

Efficient PbS quantum dot solar cells employing a conventional structure

New-generation solar cells based on colloidal lead chalcogenide (PbX) quantum dots (CQDs) are promising low-cost solution-processed photovoltaics. However, current state-of-the art CQDs are all using an inverted device architecture. The performance gap between CQD solar cells with conventional and inverted structures is much larger than that for other solution-processed photovoltaics such as organic and perovskite solar cells, which may restrict the future development of CQD solar cells. Here, we reported a record-high power conversion efficiency of 8.45% for conventionally structured PbS QD solar cells by the introduction of a unique conjugated polymer PDTPBT as the anode buffer layer. With the modification of the anode, the device performance was largely improved through a dramatic enhancement in open circuit voltage (Voc), which can be attributed to the enhanced hole extraction to the anode after PDTPBT modification. Meanwhile, the polymer layer can also efficiently improve charge separation and reduce interfacial charge recombination as well as reverse saturation current density, which result in significantly enhanced Voc. More importantly, our results proposed a new conventional architecture for QD solar cells which can avoid the complex processing of metal oxides and is free of light-soaking. This new device structure may offer more flexibility in future device design and show potential advantages in large-scale manufacturing by simplifying the fabrication process.

Widely Applicable n-Type Molecular Doping for Enhanced Photovoltaic Performance of All-Polymer Solar Cells

A widely applicable doping design for emerging nonfullerene solar cells would be an efficient strategy in order to further improve device photovoltaic performance. Herein, a family of compound TBAX (TBA= tetrabutylammonium, X = F, Cl, Br, or I, containing Lewis base anions are considered as efficient n-dopants for improving polymer-polymer solar cells (all-PSCs) performance. In all cases, significantly increased fill factor (FF) and slightly increased short-circuit current density (Jsc) are observed, leading to a best PCE of 7.0% for all-PSCs compared to that of 5.8% in undoped devices. The improvement may be attributed to interaction between different anions X- (X = F, Cl, Br, and I) in TBAX with the polymer acceptor. We reveal that adding TBAX at relatively low content does not have a significantly impact on blend morphology, while it can reduce the work function (WF) of the electron acceptor. We find this simple and solution processable n-type doping can efficiently restrain charge recombination in all-polymer solar cell devices, resulting in improved FF and Jsc. More importantly, our findings may provide new protocles and insights using n-type molecular dopants in improving the performance of current polymer-polymer solar cells.

Alkenyl Carboxylic Acid: Engineering the Nanomorphology in Polymer–Polymer Solar Cells as Solvent Additive

We have investigated a series of commercially available alkenyl carboxylic acids with different alkenyl chain lengths (trans-2-hexenoic acid (CA-6), trans-2-decenoic acid (CA-10), 9-tetradecenoic acid (CA-14)) for use as solvent additives in polymer-polymer non-fullerene solar cells. We systematically investigated their effect on the film absorption, morphology, carrier generation, transport, and recombination in all-polymer solar cells. We revealed that these additives have a significant impact on the aggregation of polymer acceptor, leading to improved phase segregation in the blend film. This in-depth understanding of the additives effect on the nanomorphology in all-polymer solar cell can help further boost the device performance. By using CA-10 with the optimal alkenyl chain length, we achieved fine phase separation, balanced charge transport, and suppressed recombination in all-polymer solar cells. As a result, an optimal power conversion efficiency (PCE) of 5.71% was demonstrated which is over 50% higher than that of the as-cast device (PCE = 3.71%) and slightly higher than that of devices with DIO treatment (PCE = 5.68%). Compared with widely used DIO, these halogen-free alkenyl carboxylic acids have a more sustainable processing as well as better performance, which may make them more promising candidates for use as processing additives in organic non-fullerene solar cells.

Synthesis of cesium-doped ZnO nanoparticles as an electron extraction layer for efficient PbS colloidal quantum dot solar cells

Colloidal quantum dot (CQD) solar cells based on lead sulfide (PbS) have attracted tremendous interest due to their strong near-infrared absorption and air-stable photovoltaic performance. To improve the electron transporting layer in PbS CQD solar cells, we have invented a new synthesis protocol to achieve uniformly Cs-doped ZnO nanoparticles (NPs) with finely controlled doping concentrations. An optimal amount of Cs doping can effectively passivate the ZnO defects, resulting in excellent transparency, ideal energy levels, high nanoparticle crystallinity, large conductivity, and smooth film morphology. These Cs-doped ZnO NPs can be used as the low-temperature solution-processed electron transporting layer in PbS CQD solar cells. Consequently, the PbS CQD solar cells adopting 5% Cs-doped ZnO achieved the best PCE of 10.43%, while the devices using pristine ZnO NPs only exhibit a PCE of 9.20%, which can be attributed to the reduced interfacial recombination and improved charge extraction at the PbS/ZnO interface. In addition, the devices using Cs–ZnO show extremely stable performance over three months storage under ambient conditions. Moreover, compared to reported Cs-doping of ZnO using sol–gel processing, our doped ZnO NPs demonstrate superior properties without high-temperature annealing, which makes them excellent candidates for use as the ETL in future flexible photovoltaics.