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By combining X-ray absorption fine structure and X-ray diffraction measurements with density functional and molecular dynamics simulations, we study the structure of a set of AgxBi1−xS2 nanoparticles, a materials system of considerable current interest for photovoltaics. An apparent contradiction between the evidence provided by X-ray absorption and diffraction measurements is solved by means of the simulations. We find that disorder in the cation sublattice induces strong local distortions, leading to the appearance of short Ag–S bonds, the overall lattice symmetry remaining close to hexagonal, Peer Reviewed, Postprint (published version)

We report on a nanomechanical engineering method to monitor matter growth in real time via e-beam electromechanical coupling. This method relies on the exceptional mass sensing capabilities of nanomechanical resonators. Focused electron beam-induced deposition (FEBID) is employed to selectively grow platinum particles at the free end of singly clamped nanotube cantilevers. The electron beam has two functions: it allows both to grow material on the nanotube and to track in real time the deposited mass by probing the noise-driven mechanical resonance of the nanotube. On the one hand, this detection method is highly effective as it can resolve mass deposition with a resolution in the zeptogram range; on the other hand, this method is simple to use and readily available to a wide range of potential users because it can be operated in existing commercial FEBID systems without making any modification. The presented method allows one to engineer hybrid nanomechanical resonators with precisely tailored functionalities. It also appears as a new tool for studying the growth dynamics of ultrathin nanostructures, opening new opportunities for investigating so far out-of-reach physics of FEBID and related methods.

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.

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 Reviewed, Postprint (author's final draft)

Colloidal quantum dot (CQD) light-emitting diodes (LEDs) deliver a compelling performance in the visible, yet infrared CQD LEDs underperform their visible-emitting counterparts, largely due to their low photoluminescence quantum efficiency. Here we employ a ternary blend of CQD thin film that comprises a binary host matrix that serves to electronically passivate as well as to cater for an efficient and balanced carrier supply to the emitting quantum dot species. In doing so, we report infrared PbS CQD LEDs with an external quantum efficiency of ~7.9% and a power conversion efficiency of ~9.3%, thanks to their very low density of trap states, on the order of 1014 cm−3, and very high photoluminescence quantum efficiency in electrically conductive quantum dot solids of more than 60%. When these blend devices operate as solar cells they deliver an open circuit voltage that approaches their radiative limit thanks to the synergistic effect of the reduced trap-state density and the density of state modification in the nanocomposite., Peer Reviewed, Postprint (author's final draft)

Two-terminal tandem cell architectures are believed to be an effective way to further improve the power conversion efficiency in solution processed photovoltaics. To design an efficient tandem solar cell, two key issues need to be considered. First, subcells with well-matched currents and complementary absorption characteristics are a prerequisite for high efficiency. Second, identifying the appropriate intermediate layer (IML) to connect the subcells is necessary to minimize the optical and electronic losses. PbS colloidal quantum dots (CQDs) are a notable choice for the subcells due to their low cost, solution processability, and remarkable wide range band gap tunability. Single-layer graphene (Gr) has been proposed to be a promising IML due to its high transparency and conductivity. Here, as a proof of concept, we demonstrate a solution-processed, two-terminal PbS CQDs tandem solar cell employing chemical vapor deposited Gr as the IML. In doing so, we report a PbS CQD cell comprising subcells with bandgaps of 1.4 and 0.95 eV that delivers power conversion efficiency in excess of 7%, substantially higher than that of previously reported CQD tandem cells., Peer Reviewed, Postprint (author's final draft)

Developing low‐cost photovoltaic absorbers that can harvest the short‐wave infrared (SWIR) part of the solar spectrum, which remains unharnessed by current Si‐based and perovskite photovoltaic technologies, is a prerequisite for making high‐efficiency, low‐cost tandem solar cells. Here, infrared PbS colloidal quantum dot (CQD) solar cells employing a hybrid inorganic–organic ligand exchange process that results in an external quantum efficiency of 80% at 1.35 µm are reported, leading to a short‐circuit current density of 34 mA cm−2 and a power conversion efficiency (PCE) up to 7.9%, which is a current record for SWIR CQD solar cells. When this cell is placed at the back of an MAPbI3 perovskite film, it delivers an extra 3.3% PCE by harnessing light beyond 750 nm., Peer Reviewed, Postprint (author's final draft)

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 Reviewed, Postprint (author's final draft)

To realize the full potential of colloidal quantum dot (CQD) based solar cells, it is important to address the issue of large open-circuit voltage (VOC) deficit which is a major roadblock in reaching higher efficiencies. The origin of the VOC deficit in these solar cells lies primarily in the presence of sub-bandgap trap states of the QDs. Here, we present a synergistic engineering framework to passivate these sub-bandgap states in PbS QDs through chemical surface passivation and remote passivation exploiting ligand and architecture engineering. In particular, we form bulk nanoheterojunctions (BNH) by mixing PbS QDs with ZnO nanocrystals in conjunction with mixed ligand treatments to passivate surface traps. We employ the mixed ligand system of zinc iodide and 3-mercatopropyonic acid to leverage the benefits of both organic and inorganic ligands for surface passivation and improved charge transport. This mixed ligand treatment in BNH architectures leads to record low VOC deficit for PbS QDs of 0.4–0.55 V compared to previously reported 0.6–0.8 V for the range of 1.1–1.35 eV bandgap PbS QDs., Peer Reviewed, Postprint (author's final draft)

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 Reviewed, Postprint (author's final draft)

The performance of ZnO/PbS colloidal quantum dot (CQD)-based heterojunction solar cells is hindered by charge carrier recombination at the heterojunction interface. Reducing interfacial recombination can improve charge collection and the photocurrent of the device. Here we report the use of a mixed nanocrystal (MNC) buffer layer comprising zinc oxide nanocrystals and lead sulfide quantum dots at the respective heterojunction interface. Remote trap passivation of the PbS CQDs taking place within this MNC layer reduces interfacial recombination and electron back transfer which in turn improves charge collection efficiency. Upon the addition of the MNC layer, the overall power conversion efficiency increases from 9.11 to 10.16% and Short-circuit current density (JSC) increases from 23.54 to 25.23 mA/cm2. Optoelectronic characterization of the solar cells confirms that the effects underlying device improvement are reduced trap density and improved charge collection efficiency due to the presence of the MNC buffer layer., Peer Reviewed, Postprint (author's final draft)

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 Reviewed, Postprint (author's final draft)

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 Reviewed, Postprint (author's final draft)

Peer Reviewed, Postprint (author's final draft)

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 Reviewed, Postprint (published version)

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 Reviewed, Postprint (author's final draft)

Doping colloidal quantum dots (CQDs) with aliovalent cations is a promising, yet underexplored, approach to control the optoelectronic properties in CQDs. In CQD doping, kinetics determine whether a dopant element will incorporate into the host crystal structure, while thermodynamics dictate the mechanism of dopant incorporation. Here, we show that those mechanisms can be readily monitored by simple optical measurements and XRD studies in CQD ensembles. Based on this, we outline the critical role of dopant solubility limit in CQD doping, bridging the gap between nanocrystalline and bulk semiconductors. Finally, we present a combined simulation and X-ray absorption fine structure (XAFS) data study to shed new insights on the origin of charge compensation upon doping in CQD materials that has, thus far, limited high doping efficacy, even under efficient dopant incorporation schemes.