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.

Infrared‐absorbing colloidal quantum dots (IR CQDs) are materials of interest in tandem solar cells to augment perovskite and cSi photovoltaics (PV). Today's best IR CQD solar cells rely on the use of passivation strategies based on lead iodide; however, these fail to passivate the entire surface of IR CQDs. Lead chloride passivated CQDs show improved passivation, but worse charge transport. Lead bromide passivated CQDs have higher charge mobilities, but worse passivation. Here a mixed lead‐halide (MPbX) ligand exchange is introduced that enables thorough surface passivation without compromising transport. MPbX–PbS CQDs exhibit properties that exceed the best features of single lead‐halide PbS CQDs: they show improved passivation (43 ± 5 meV vs 44 ± 4 meV in Stokes shift) together with higher charge transport (4 × 10‐2 ± 3 × 10‐3 cm2 V‐1 s‐1 vs 3 × 10‐2 ± 3 × 10‐3 cm2 V‐1 s‐1 in mobility). This translates into PV devices having a record IR open‐circuit voltage (IR Voc) of 0.46 ± 0.01 V while simultaneously having an external quantum efficiency of 81 ± 1%. They provide a 1.7× improvement in the power conversion efficiency of IR photons (>1.1 µm) relative to the single lead‐halide controls reported herein.

passivation and device architecture have led to improvements in CQD solar cell performance, [19][20][21] and these recently enabled power conversion efficiencies (PCE) above 12% for lead sulfide (PbS) CQDs. [19] The conventional CQD solar cell architecture consists of a transparent cathode, electron transport layer (ETL), a lightabsorbing active layer, a hole transport layer (HTL), and a metal anode. To achieve high-performing devices, the optoelectronic properties of the ETL, HTL, and active layer, which determine the charge absorption and extraction capacity of the devices, require accurate control. A number of excellent studies have illuminated the role of the ETL [22][23][24][25][26][27][28] and the active layer; [29] while the HTL is relatively less examined. Organic p-type semiconductors [30,31] and metal oxides (e.g., MoO 3 , NiO x , etc.) [32] have been explored to replace the thiol-passivated CQD HTL. Non-thiol ligands (e.g., NaHS) [33,34] have also been reported to produce p-type CQD solids; however, to date, device performance has not yet surpassed that of thiolpassivated CQD-based HTLs.State-of-art CQD solar cells employ 1,2-ethanedithiol (EDT) in the process of fabricating the CQD HTL. [19] This EDT HTL has been used in most high-performing CQD solar cells; but EDT has long been suspected of negatively affecting the underlying CQD active layer. In particular, its high reactivity [35] is proposed to be implicated in interfering with efficient charge extraction at the back-junction. [36] Herein we seek experimental evidence of any role of the EDT HTL in performance; and we find, using a new spatial collection efficiency (SCE) technique, [37] that the EDT HTL does indeed cause a rapid drop in the collection efficiency at the interface between HTL and active layer. We then develop an orthogonal CQD HTL that employs malonic acid (MA) instead of EDT. As a result of the lower reactivity of carboxylic acids compared to thiols, [38] the MA HTL substantially preserves the original surface chemistry of the CQD active layer after its deposition, as evidenced by X-ray photoelectron spectroscopy (XPS) analyses.The orthogonality of the MA HTL enables full charge collection at the back interface in CQD solar cells. This advance Colloidal quantum dots (CQDs) are of interest in light of their solutionprocessing and bandgap tuning. Advances in the performance of CQD optoelectronic devices require fine control over the properties of each layer in the device materials stack. This is particularly challenging in the present best CQD solar cells, since these employ a p-type hole-transport layer (HTL) implemented using 1,2-ethanedithiol (EDT) ligand exchange on top of the CQD active layer. It is established that the high reactivity of EDT causes a severe chemical modification to the active layer that deteriorates charge extraction. By combining elemental mapping with the spatial charge collection efficiency in CQD solar cells, the key materials interface dominating the subpar performance of prior CQD PV devices is demonstrat...

Emerging technologies such as autonomous driving and augmented reality rely on light detection and ranging (LiDAR based on time of flight (ToF). [3] This requires sensitive and ultrafast photodetection of infrared light with nanoseconds' resolution. [4] Today, this is achieved in the near-infrared (NIR) using indirect bandgap silicon detectors-limited by silicon's low absorption coefficient-and, at longer wavelengths, using epitaxially grown semiconductors such as III-Vs and Hg 1−x Cd x Te. [5,6] Colloidal quantum dots (CQDs) are of interest given by their low-temperature solution processing, which allows them to be integrated with silicon electronic readout and signal-processing circuitry. [7][8][9][10] Their bandgap is size-tuned over a wide range of wavelengths. PbS, for example, has a widely programmable absorption onset covering the visible and shortwavelength infrared (SWIR); [11,12] however, its high permittivity, stemming from its ionic character-ε r = 180 for bulk PbS [13] slows charge extraction both for bulk [14] and CQD photodiodes [15] due to screening and capacitance effects.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 cm 2 V −1 s −1 ) with a 4× reduction in permittivity compared to PbS CQDs. The resulting photodiodes achieve a response time faster than 2 nsthe fastest photodiode among previously reported CQD photodiodes-combined with an external quantum efficiency (EQE) of 30% at 940 nm.

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.