MgCl2 passivated ZnO electron transporting layer to improve PbS quantum dot solar cells

Sn-Doped Zinc Oxide as an Electron Transporting Layer for Enhanced Performance in PbS Quantum Dot Solar Cells

Colloidal PbS quantum dot solar cells (QDSCs) have been primarily demonstrated in n-i-p structures by incorporating a solution-processed ZnO electron transporting layer (ETL). Nevertheless, the inherent energy barrier for the electron extraction at the ZnO/PbS junction along with the defective nature significantly diminishes the performance of the PbS QDSCs. In this study, by employing Sn-doped ZnO (ZTO) ETL, we have tuned the conduction band offset at the junction from spike-type to cliff-type so that the electron extraction barrier can be eliminated and the overall photovoltaic parameters can be enhanced (open-circuit voltage of 0.7 V, fill factor over 70%, and efficiency of 11.3%) as compared with the counterpart with the undoped ZnO ETL. The X-ray photoelectron spectroscopy (XPS) analysis revealed a mitigation of oxygen vacancies in the ZTO ETL of our PbS QDSCs. Our work signifies the importance of Sn doping into the conventional ZnO ETL for the superior electron extraction in PbS QDSCs.

Role of the ZnO electron transport layer in PbS colloidal quantum dot solar cell yield

The development of lead sulfide (PbS) colloidal quantum dot (CQD) solar cells has led to significant power conversion efficiency (PCE) improvements in recent years, with record efficiencies now over 15%. Many of the recent advances in improving PCE have focused on improving the interface between the PbS CQD active layer and the zinc oxide (ZnO) electron transport layer (ETL). Proper optimization of the ZnO ETL also increases yield, or the percentage of functioning devices per fabrication run. Simultaneous improvements in both PCE and yield will be critical as the field approaches commercialization. This review highlights recent advances in the synthesis of ZnO ETLs and discusses the impact and critical role of ZnO synthesis conditions on the PCE and yield of PbS CQD solar cells.

The Proof-of-Concept of Anode-Free Rechargeable Mg Batteries

The desperate pursuit of high gravimetric specific energy leads to the ignorance of volumetric energy density that is one of the basic requirements for batteries. Due to the high volumetric capacity, less-prone formation of dendrite, and low reduction potential of Mg metal, rechargeable Mg batteries are considered with innately high volumetric energy density. Nevertheless, the substantial elevation in energy density is compromised by extremely excessive Mg metal anode. Herein, the proof-of-concept of anode-free Mg₂ Mo₆ S₈ -MgS/Cu batteries is proposed, in which MgS as the premagnesiation additive constantly decomposes to replenish Mg loss by electrolyte corrosion over cycling, while both Mg₂ Mo₆ S₈ and MgS acts as the active material to reversibly provide high capacities. Besides, Mg₂ Mo₆ S₈ shows superior catalytic activity on the decomposition of MgS and provides the strong affinity to polysulfides to restrain their dissolution. Consequently, the anode-free Mg₂ Mo₆ S₈ -MgS/Cu batteries deliver a high reversible capacity of 190 mAh g^-1 with the capacity retention of 92% after 100 cycles, corresponding to the highly competitive energy density of 420 Wh L^-1 . The proposed anode-free Mg battery here spotlights the great promise of Mg batteries in achieving high volumetric energy densities, which will significantly expedite the advances of Mg batteries in practice.

Phase-Transfer Exchange Lead Chalcogenide Colloidal Quantum Dots: Ink Preparation, Film Assembly, and Solar Cell Construction

Solution-processed colloidal quantum dots (CQDs) are promising candidates for the third-generation photovoltaics due to their low cost and spectral tunability. The development of CQD solar cells mainly relies on high-quality CQD ink, smooth and dense film, and charge-extraction-favored device architectures. In particular, advances in the processing of CQDs are essential for high-quality QD solids. The phase transfer exchange (PTE), in contrast with traditional solid-state ligand exchange, has demonstrated to be the most promising approach for high-quality QD solids in terms of charge transport and defect passivation. As a result, the efficiencies of Pb chalcogenide CQD solar cells have been rapidly improved to 14.0%. In this review, the development of the PTE method is briefly reviewed for lead chalcogenide CQD ink preparation, film assembly, and device construction. Particularly, the key roles of lead halides and additional additives are emphasized for defect passivation and charge transport improvement. In the end, several potential directions for future research are proposed.

A low-temperature solution-processed indium incorporated zinc oxide electron transport layer for high-efficiency lead sulfide colloidal quantum dot solar cells

Colloidal quantum dot solar cells (CQDSCs) have achieved remarkable progress recently in terms of mainly surface passivation and composition-matching matrices on CQDs, while improving the overall photoelectric conversion efficiency (PCE) through electron transport layer (ETL) modifications is less explored. We report a low-temperature solution route to synthesize donor (Al³⁺/Ga³⁺/In³⁺) incorporated zinc oxide (AZO/GZO/IZO) ETL films for PbS CQDSCs. Spectroscopic characterization studies indicate that the IZO ETL fabricated with 150 °C annealing can increase the bandgap the most from 3.56 eV to 3.74 eV, possesses enhanced light transmission (∼94%) and finer particle sizes, and importantly shows the most suitable band alignment and charge transfer ability. Well-dispersed PbS CQDs of around 3 nm are synthesized by a N₂-protected reflux method and are surface exchanged with 1-ethyl-3-methylimidazolium iodide (EMII) to allow I^- grafting and ethanedithiol (EDT) for the active layer and hole transport layer, respectively. The IZO based PbS CQDSC, with a device architecture of ITO/IZO/PbS-EMII/PbS-EDT/Au, shows an enhanced PCE of 11.1% (comparatively 18% higher than that of the ZnO ETL), a V_OC value of 0.64 V, and a J_SC of 25.8 mA cm^-2. The improved performances benefit from the higher recombination resistance and constrained photoluminescence emission with the utilization of the IZO ETL that provides a superior charge transfer property.

Photoexcited carrier dynamics in colloidal quantum dot solar cells: insights into individual quantum dots, quantum dot solid films and devices

The certified power conversion efficiency (PCE) record of colloidal quantum dot solar cells (QDSCs) has considerably improved from below 4% to 16.6% in the last few years. However, the record PCE value of QDSCs is still substantially lower than the theoretical efficiency. So far, there have been several reviews on recent and significant achievements in QDSCs, but reviews on photoexcited carrier dynamics in QDSCs are scarce. The photovoltaic performances of QDSCs are still limited by the photovoltage, photocurrent and fill factor that are mainly determined by the photoexcited carrier dynamics, including carrier (or exciton) generation, carrier extraction or transfer, and the carrier recombination process, in the devices. In this review, the photoexcited carrier dynamics in the whole QDSCs, originating from individual quantum dots (QDs) to the entire device as well as the characterization methods used for analyzing the photoexcited carrier dynamics are summarized and discussed. The recent research including photoexcited multiple exciton generation (MEG), hot electron extraction, and carrier transfer between adjacent QDs, as well as carrier injection and recombination at each interface of QDSCs are discussed in detail herein. The influence of photoexcited carrier dynamics on the physiochemical properties of QDs and photovoltaic performances of QDSC devices is also discussed.

Enhancing PbS Colloidal Quantum Dot Tandem Solar Cell Performance by Graded Band Alignment

Colloidal quantum dot solids are attractive candidates for tandem solar cells because of their widely tunable bandgaps. However, the development of the quantum dot tandem solar cell has lagged far behind that of its single-junction counterpart. One of the fundamental problems with colloidal quantum dot solar cells is the relatively small diffusion length, which limits the quantum dot absorbing layer thickness and hence the power conversion efficiency. In this research, guided by optical modeling and utilizing a graded band alignment strategy, a two-terminal monolithic solution-processed quantum dot tandem solar cell has been successfully fabricated and a power conversion efficiency of 6.8% has been achieved. The band grading approach utilized the complementary tuning of work functions and band alignment through judicious choices of the nanoparticle surface chemistry and quantum dot confined size. This work demonstrates a general approach to improving the efficiency for tandem thin-film solar cells.

Quantum Dot Donor-Polymer Acceptor Architecture for a FRET-Enabled Solar Cell

Forster resonance energy-transfer (FRET)-based solution-processed solar cell is fabricated with cadmium sulfide (CdS) as the energy donor and poly[ N-9'-heptadecanyl-2,7-carbazole- alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (PCDTBT) as the energy acceptor. Carbon dots (C-dots) deposited on carbon fabric are applied as a counter electrode. Although electron injection from CdS to PCDTBT is energetically disfavored, evidences for energy transfer between the two components of the cell are obtained in terms of FRET parameters with the relative quantum yield of donor CdS quantum dots (QDs) being ∼0.3, a Forster radius of ∼3.7 nm, and an energy-transfer efficiency of ∼55%. Power conversion efficiency (PCE) of the TiO₂/PCDTBT cell without the donor is 0.23% and when coupled with donor CdS QDs, the ensuing TiO₂/PCDTBT/CdS cell experiences a 23 time increment in PCE, reaching 5.3%. The complete FRET cell: TiO₂/PCDTBT/CdS/ZnS-S^2--C-dots/C-fabric produces a PCE of 7.42%, under 1 sun illumination. External quantum efficiency studies reveal an enhanced spectral response spanning from 300 to 670 nm, with 300 and 175% increases attained for the FRET-enabled TiO₂/PCDTBT/CdS/ZnS photoanode compared with the TiO₂/PCDTBT photoanode over the blue and green-red portions of the solar spectrum.