Strategies for controlled electronic doping of colloidal quantum dots

Over the last several years tremendous progressed progress has been made in incorporating Colloidal Quantum Dots (CQDs) as photoactive components in optoelectronic devices. A significant part of that progress is associated with significant advancements made in achievingon controlled electronic doping of the CQDs and thus improving the electronic properties of CQDs solids. Today, a variety of strategies exists towards that purpose and this minireview aims at surveying major published works in this subject. Additional attention is given to the many challenges associated with the task of doping CQDs as well as to the optoelectronic functionalities and applications being realized when successfully achieving light and heavy electronic doping of CQDs.Peer ReviewedPostprint (author's final draft

Suppressing Deep Traps in PbS Colloidal Quantum Dots via Facile Iodide Substitutional Doping for Solar Cells with Efficiency >10%

Surface passivation of PbS colloidal quantum dots (QDs) with iodide has been used in highly efficient solar cells. Iodide passivation is typically achieved by ligand exchange processes on QD films. Complementary to this approach, herein we present a non-intrusive solution-based strategy for doping QDs with iodide to further optimize solar cell performance. The doping step is applied in-situ at the end of the synthesis of the QDs. The optimum precursor I/Pb ratio is found to be in the 1.5-3% range at which iodide substitutes S without excessively altering the dots´ surface chemistry. This allows for band engineering and decreasing the density of deep trap states of the QDs which taken together lead to PbS QD solar cells with efficiency in excess of 10%.Peer ReviewedPostprint (author's final draft

The Role of Surface Passivation for Efficient and Photostable PbS Quantum Dot Solar Cells

Peer ReviewedPostprint (author's final draft

High Open Circuit Voltage Solar Cells based on bright mixed-halide CsPbBrI2 Perovskite Nanocrystals Synthesized in Ambient Air Conditions

Lead halide perovskite nanocrystals (NCs) are currently emerging as one of the most interesting solution processed semiconductors since they possess high photoluminescence quantum yield (PLQY), and colour tunability through anion exchange reactions or quantum confinement. Here, we show efficient solar cells based on mixed halide (CsPbBrI2) NCs obtained via anion exchange reactions in ambient conditions. We performed anion exchange reactions in concentrated NC solutions with I-, thus inducing a PL red-shift up to 676 nm, and obtaining a high PLQY in film (65%). Solar cell devices operating in the wavelength range 350-660 nm were fabricated in air with two different deposition methods. The solar cells display a photo-conversion efficiency of 5.3% and open circuit voltage (Voc) up to 1.31V, among the highest reported for perovskite based solar cells with band gap below 2eV, clearly demonstrating the potential of this material.Peer ReviewedPostprint (author's final draft

Trap-state suppression and improved charge transport in PbS quantum dot solar cells with synergistic mixed ligand treatments

The power conversion efficiency of colloidal PbS‐quantum‐dot (QD)‐based solar cells is significantly hampered by lower‐than‐expected open circuit voltage (VOC). The VOC deficit is considerably higher in QD‐based solar cells compared to other types of existing solar cells due to in‐gap trap‐induced bulk recombination of photogenerated carriers. Here, this study reports a ligand exchange procedure based on a mixture of zinc iodide and 3‐mercaptopropyonic acid to reduce the VOC deficit without compromising the high current density. This layer‐by‐layer solid state ligand exchange treatment enhances the photovoltaic performance from 6.62 to 9.92% with a significant improvement in VOC from 0.58 to 0.66 V. This study further employs optoelectronic characterization, X‐ray photoelectron spectroscopy, and photoluminescence spectroscopy to understand the origin of VOC improvement. The mixed‐ligand treatment reduces the sub‐bandgap traps and significantly reduces bulk recombination in the devices.Peer ReviewedPostprint (author's final draft

Solution-Processed, Solid-State Solar Cells based on Environmentally Friendly AgBiS2 Nanocrystals

Solution-processed inorganic solar cells are a promising low-cost alternative to firstgeneration solar cells.1,2 Solution processing at low temperatures and the use of nontoxic and abundant elements can help minimize cost and facilitate regulatory acceptance. However, until now there has been no material that exhibits all of these features while demonstrating promising efficiencies. Many of the most promising solution-processed inorganic solar cells contain toxic elements such as lead or cadmium (perovskites,2,3 PbS,4 CdTe,5,6 CdS(Se)7,8) or scarce elements like tellurium or indium (CdTe, CIGS(Se)/CIS9,10). Others require high-temperature processes such as selenization or sintering or rely on vacuum deposition techniques ((Sb2S(Se)3,11–13 SnS,14,15 CZTS(Se)16). Here, we present AgBiS2 nanocrystals as a novel nontoxic,17 earth-abundant18 material for highperformance, solution-processed solar cells fabricated in ambient conditions at low temperatures (≤100°C). The AgBiS2 nanocrystals have favorable properties for solar-cell applications including a near-ideal bandgap and strong, broad absorption. We demonstrate a Newport certified power conversion efficiency of 6.3% with no hysteresis and a remarkably high short-circuit current density of about 22 mA·cm-2 for an active layer thickness of only ~35 nm.Peer ReviewedPostprint (author's final draft

Engineering Vacancies in Bi2S3 yields sub-Bandgap Photoresponse and highly sensitive Short-Wave Infrared Photodetectors

Defects play an important role in tailoring the optoelectronic properties of materials. Here we demonstrate that sulphur vacancies are able to engineer sub-band photoresponse into the short-wave infrared range due to formation of in-gap states in Bi2S3 single crystals supported by density functional (DF) calculations. Sulfurization and subsequent refill of the vacancies results in faster response but limits the spectral range to the near infrared as determined by the bandgap of Bi2S3. A facile chemical treatment is then explored to accelerate the speed of sulphur deficient (SD)-based detectors on the order of 10 ms without sacrificing its spectral coverage into the infrared, while holding a high D* close to 10^15 Jones in the visible-near infrared range and 10^12 Jones at 1.6 um. This work also provides new insights into the role sulphur vacancies play on the electronic structure and, as a result, into sub-bandgap photoresponse enabling ultrasensitive, fast and broadband photodetectors

Understanding light trapping by resonant coupling to guided modes and the importance of the mode profile

We present a simple conceptual model describing the absorption enhancement provided by diffraction gratings due to resonant coupling to guided modes in a multi-layered structure. In doing so, we provide insight into why certain guided modes are more strongly excited than others and demonstrate that the spatial overlap of the mode profile with the grating is important. The model is verified by comparison to optical simulations and experimental measurements. We fabricate metal nanoparticle gratings integrated as back contacts in solution-processed PbS colloidal quantum dot photodiodes. The measured photocurrent at the target wavelength is enhanced by 250%, with reference to planar devices, due to resonant coupling to guided modes with strong spatial overlap with the gratings. In comparison, resonant coupling to weakly overlapping modes results in a 25% increase at the same wavelength.Peer ReviewedPostprint (published version

High-Efficiency Light-Emitting Diodes Based on Formamidinium Lead Bromide Nanocrystals and solution processed transport layers

Perovskite nanocrystal light-emitting diodes (LEDs) employing architecture comprising a ZnO nanoparticles electron-transport layer and a conjugated polymer hole-transport layer have been fabricated. The obtained LEDs demonstrate a maximum external-quantum-efficiency of 6.04%, luminance of 12998 Cd/m2 and stable electroluminescence at 519 nm. Importantly, such high efficiency and bright-ness have been achieved by employing solution processed transport layers, formamidinium lead bromide nanocrystals (CH(NH2)2PbBr3 NCs) synthesized at room-temperature and in air without the use of a Schlenk line, and a procedure based on atomic layer deposition to insolubilize the NC film. The obtained NCs show a photoluminescence quantum yield of 90% that is retained upon film fabrication. The results show that perovskite NC LEDs can achieve high-performance without the use of transport layers deposited through evaporation in ultra-high-vacuum.Peer ReviewedPostprint (author's final draft