Tandem solar cells involving metal-halide perovskite subcells offer routes to power conversion efficiencies (PCEs) that exceed the single-junction limit; however, reported PCE values for tandems have ...so far lain below their potential due to inefficient photon harvesting. Here we increase the optical path length in perovskite films by preserving smooth morphology while increasing thickness using a method we term boosted solvent extraction. Carrier collection in these films - as made - is limited by an insufficient electron diffusion length; however, we further find that adding a Lewis base reduces the trap density and enhances the electron-diffusion length to 2.3 µm, enabling a 19% PCE for 1.63 eV semi-transparent perovskite cells having an average near-infrared transmittance of 85%. The perovskite top cell combined with solution-processed colloidal quantum dot:organic hybrid bottom cell leads to a PCE of 24%; while coupling the perovskite cell with a silicon bottom cell yields a PCE of 28.2%.
Control over carrier type and doping levels in semiconductor materials is key for optoelectronic applications. In colloidal quantum dots (CQDs), these properties can be tuned by surface chemistry ...modification, but this has so far been accomplished at the expense of reduced surface passivation and compromised colloidal solubility; this has precluded the realization of advanced architectures such as CQD bulk homojunction solids. Here we introduce a cascade surface modification scheme that overcomes these limitations. This strategy provides control over doping and solubility and enables n-type and p-type CQD inks that are fully miscible in the same solvent with complete surface passivation. This enables the realization of homogeneous CQD bulk homojunction films that exhibit a 1.5 times increase in carrier diffusion length compared with the previous best CQD films. As a result, we demonstrate the highest power conversion efficiency (13.3%) reported among CQD solar cells.
The best‐performing colloidal‐quantum‐dot (CQD) photovoltaic devices suffer from charge recombination within the quasi‐neutral region near the back hole‐extracting junction. Graded architectures, ...which provide a widened depletion region at the back junction of device, could overcome this challenge. However, since today's best materials are processed using solvents that lack orthogonality, these architectures have not yet been implemented using the best‐performing CQD solids. Here, a new CQD ink that is stable in nonpolar solvents is developed via a neutral donor ligand that functions as a phase‐transfer catalyst. This enables the realization of an efficient graded architecture that, with an engineered band‐alignment at the back junction, improves the built‐in field and charge extraction. As a result, optimized IR CQD solar cells (Eg ≈ 1.3 eV) exhibiting a power conversion efficiency (PCE) of 12.3% are reported. The strategy is applied to small‐bandgap (1 eV) IR CQDs to augment the performance of perovskite and crystalline silicon (cSi) 4‐terminal tandem solar cells. The devices show the highest PCE addition achieved using a solution‐processed active layer: a value of +5% when illuminated through a 1.58 eV bandgap perovskite front filter, providing a pathway to exceed PCEs of 23% in 4T tandem configurations with IR CQD PVs.
Phase‐transfer catalyzed colloidal‐quantum‐dot (CQD) inks are developed with the aid of a neutral donor ligand. This enables graded IR CQD solar cells exhibiting the highest power conversion efficiency (PCE) of 12.3% using large‐bandgap (Eg ≈ 1.3 eV) CQDs. By using small‐Eg (1 eV) CQDs, a new record PCE of +5.0% is demonstrated on top of a perovskite (Eg ≈ 1.58 eV) front filter.
Infrared (IR)‐to‐visible up‐conversion device allows a low‐cost, pixel‐free IR imaging over the conventional expensive compound semiconductor‐based IR image sensors. However, the external quantum ...efficiency has been low due to the integration of an IR photodetector and a light‐emitting diode (LED). Herein, by inducing a strong micro‐cavity effect, a highly efficient top‐emitting IR‐to‐visible up‐conversion device is demonstrated where PbS quantum dots IR‐absorbing layer is integrated with a phosphorescent organic LED. By optimizing the optical cavity length between indium tin oxide (ITO)/thin Ag/ITO anode and semi‐transparent Mg:Ag top cathode, the up‐conversion device yields 15.7% of photon‐to‐photon conversion efficiency from the top‐emission. The high efficiency can be achieved under a low IR transmission through the semi‐reflective anode. Finally, pixel‐free IR imaging is demonstrated using the up‐conversion device, boosting the effect of micro‐cavity on the brightness and the contrast of an IR image.
High efficiency top‐emitting infrared (IR)‐to‐visible up‐conversion device is demonstrated by exploiting microcavity effect. Compared to up‐conversion device using conventional indium tin oxide (ITO) electrode, ITO/Ag/ITO reflective electrode offers a strong optical resonance toward the top side, yielding 15.7 % IR‐to‐photon conversion efficiency. Using the microcavity effect, pixel‐free IR imaging is demonstrated with higher brightness and image contrast.
The electrochemical carbon dioxide reduction reaction (CO2RR) produces diverse chemical species. Cu clusters with a judiciously controlled surface coordination number (CN) provide active sites that ...simultaneously optimize selectivity, activity, and efficiency for CO2RR. Here we report a strategy involving metal–organic framework (MOF)-regulated Cu cluster formation that shifts CO2 electroreduction toward multiple-carbon product generation. Specifically, we promoted undercoordinated sites during the formation of Cu clusters by controlling the structure of the Cu dimer, the precursor for Cu clusters. We distorted the symmetric paddle-wheel Cu dimer secondary building block of HKUST-1 to an asymmetric motif by separating adjacent benzene tricarboxylate moieties using thermal treatment. By varying materials processing conditions, we modulated the asymmetric local atomic structure, oxidation state and bonding strain of Cu dimers. Using electron paramagnetic resonance (EPR) and in situ X-ray absorption spectroscopy (XAS) experiments, we observed the formation of Cu clusters with low CN from distorted Cu dimers in HKUST-1 during CO2 electroreduction. These exhibited 45% C2H4 faradaic efficiency (FE), a record for MOF-derived Cu cluster catalysts. A structure–activity relationship was established wherein the tuning of the Cu–Cu CN in Cu clusters determines the CO2RR selectivity.
Colloidal quantum dots (CQDs) are promising materials for infrared (IR) light detection due to their tunable bandgap and their solution processing; however, to date, the time response of CQD IR ...photodiodes is inferior to that provided by Si and InGaAs. It is reasoned that the high permittivity of II–VI CQDs leads to slow charge extraction due to screening and capacitance, whereas III–Vs—if their surface chemistry can be mastered—offer a low permittivity and thus increase potential for high‐speed operation. In initial studies, it is found that the covalent character in indium arsenide (InAs) leads to imbalanced charge transport, the result of unpassivated surfaces, and uncontrolled heavy doping. Surface management using amphoteric ligand coordination is reported, and it is found that the approach addresses simultaneously the In and As surface dangling bonds. The new InAs CQD solids combine high mobility (0.04 cm2 V−1 s−1) with a 4× reduction in permittivity compared to PbS CQDs. The resulting photodiodes achieve a response time faster than 2 ns—the fastest photodiode among previously reported CQD photodiodes—combined with an external quantum efficiency (EQE) of 30% at 940 nm.
An amphoteric liganding strategy is developed to simultaneously address In and As dangling bonds on InAs colloidal quantum dot (CQD) surfaces, which provides passivation and charge transport for low‐permittivity CQD solids. The resulting photodiodes achieve a response time faster than 2 ns—the fastest photodiode among previously reported CQD photodiodes—combined with an external quantum efficiency of 30% at 940 nm.
The need for optoelectronic and chemical compatibility between the layers in colloidal quantum dot (CQD) photovoltaic devices remains a bottleneck in further increasing performance. Conjugated ...polymers are promising candidates as new hole‐transport layer (HTL) materials in CQD solar cells (CQD‐SCs) owing to the highly tunable optoelectronic properties and compatible chemistries. A diketopyrrolopyrrole‐based polymer with benzothiadiazole derivatives (PD2FCT‐29DPP) as an HTL in these devices is reported. The energy level, molecular orientation, and hole mobility of this HTL are manipulated through molecular engineering. By levering the polymer's optical absorption spectrum complementary to that of the CQD active layer, EQE across the visible and near‐infrared regions is maximized. As a result, a PD2FCT‐29DPP‐based device exhibits a fill factor of 70% and approximately 35% efficiency enhancement compared to a PTB7‐based device.
A new DPP‐based alternating D–A copolymer (PD2FCT‐29DPP) is developed for use as a hole‐transport layer. PD2FCT‐29DPP addresses the different requirements for an HTL, offering favorable energetics, near‐infrared absorption, and efficient charge transfer. Therefore, a PD2FCT‐29DPP‐based device achieves a remarkable FF of 70% and the highest PCE of 14.0% among PbS CQD‐SCs.
Charge carrier transport in colloidal quantum dot (CQD) solids is strongly influenced by coupling among CQDs. The shape of as‐synthesized CQDs results in random orientational relationships among ...facets in CQD solids, and this limits the CQD coupling strength and the resultant performance of optoelectronic devices. Here, colloidal‐phase reconstruction of CQD surfaces, which improves facet alignment in CQD solids, is reported. This strategy enables control over CQD faceting and allows demonstration of enhanced coupling in CQD solids. The approach utilizes post‐synthetic resurfacing and unites surface passivation and colloidal stability with a propensity for dots to couple via (100):(100) facets, enabling increased hole mobility. Experimentally, the CQD solids exhibit a 10× increase in measured hole mobility compared to control CQD solids, and enable photodiodes (PDs) exhibiting 70% external quantum efficiency (vs 45% for control devices) and specific detectivity, D* > 1012 Jones, each at 1550 nm. The photodetectors feature a 7 ns response time for a 0.01 mm2 area—the fastest reported for solution‐processed short‐wavelength infrared PDs.
A strategy to achieve enhanced coupling from colloidal quantum dot (CQD) inks is introduced. By using colloidal atomic layer deposition, the surface of CQDs is reconstructed to facilitate coupling along the (100) facets and increase coupling. This increases the hole mobility by one order of magnitude and enables the fabrication of record‐speed solution‐processed short‐wavelength infrared photodiodes.
Emerging semiconducting materials including colloidal quantum dots (CQDs) and organic molecules have unique photovoltaic properties, and their hybridization can result in synergistic effects for high ...performance. For realizing the full potential of CQD/organic hybrid devices, controlling interfacial properties between the CQD and organic matter is crucial. Here, the electronic band between the CQD and the polymer layers is carefully modulated by inserting an interfacial layer treated with several types of ligands. The interfacial layer provides a cascading conduction band offset (ΔEC), and reduces local charge accumulation at CQD/polymer interfaces, thereby suppressing bimolecular recombination; a thin thiol‐treated interfacial layer (≈6 nm) decreases shallow traps, resulting in higher short‐circuit current (JSC) and fill factor of hybrid solar cells. Based on these results, a high performance CQD/polymer hybrid solar cell is introduced that demonstrates a power conversion efficiency of 13.74% under AM 1.5 solar illumination. The hybrid device retains more than 90% of its initial performance after 402 days under ambient conditions.
A new design strategy for exploring colloidal quantum dot (CQD)/polymer interfaces is proposed: an additional interfacial layer is incorporated between the CQD/polymer bilayer structures. The interfacial layer between CQD and polymer reduces the localized charge accumulation, suppressing bimolecular recombination. The optimized hybrid devices show a maximum power conversion efficiency of 13.74% and retains over 90% of its initial performance for 402 days under ambient condition without any treatment.
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