Heterovalent cation substitutional doping for quantum dot homojunction solar cells

Koh W., Koposov A.Y., Stewart J.T., Pal B.N., Robel I., Pietryga J.M., Klimov V.I.

Colloidal nanocrystals (NCs) of lead chalcogenides are a promising class of tunable infrared materials for applications in devices such as photodetectors and solar cells. Such devices typically employ electronic materials in which charge carrier concentrations are manipulated through “doping;” however, persistent electronic doping of these NCs remains a challenge. Here, we demonstrate that heavily doped n-type PbSe and PbS NCs can be realized utilizing ground-state electron transfer from cobaltocene. This allows injecting up to eight electrons per NC into the band-edge state and maintaining the doping level for at least a month at room temperature. Doping is confirmed by inter- and intra-band optical absorption, as well as by carrier dynamics. Finally, FET measurements of doped NC films and the demonstration of a p-n diode provide additional evidence that the developed doping procedure allows for persistent incorporation of electrons into the quantum-confined NC states.

Luther J.M., Pietryga J.M.

A growing body of research indicates that the stoichiometry of compound semiconductor quantum dots (QDs) may offer control over the materials' optoelectronic properties in ways that could be invaluable in electronic devices. Quantum dots have been characterized as having a stoichiometric bulk-like core with a highly reconstructed surface of a more flexible composition, consisting essentially of ligated, weakly bound ions. As such, many efforts toward stoichiometry-based control over material properties have focused on ligand manipulation. In this issue of ACS Nano, Murray and Kagan's groups instead demonstrate control of the conductive properties of QD arrays by altering the stoichiometry via atomic infusion using a thermal evaporation technique. In this work, PbSe and PbS QD films are made to show controlled n- or p-type behavior, which is key to developing optimized QD-based electronics. In this Perspective, we discuss recent developments and the future outlook in using stoichiometry as a tool to further manipulate QD material properties in this context.

Buonsanti R., Milliron D.J.

Synthetic control over inorganic nanocrystals has made dramatic strides so that a great number of binary and a few ternary or more complex compounds can now be prepared with good control over size and physical properties. Recently, chemists have tackled the long-standing challenge of introducing dopant atoms into nanocrystals, and strategies that apply across diverse compositions are beginning to emerge. In this review, we first briefly summarize the array of characterization methods used to assess doping efficacy for reference throughout the discussion. We then enumerate chemical strategies for doping with illustrative examples from the literature. A key concept is that the reactions leading to growth of the host crystal and to deposition of dopant ions must be balanced to succeed in incorporating dopants during crystal growth. This challenge has been met through various chemical strategies, and new methods, such as postsynthetic diffusion of dopant ions, continue to be developed. The opportunity to deliver new functionality by doping nanocrystals is great, particularly as characterization methods and synthetic control over introduction of multiple dopants advance.

Oh S.J., Berry N.E., Choi J., Gaulding E.A., Paik T., Hong S., Murray C.B., Kagan C.R.

We investigate the effects of stoichiometric imbalance on the electronic properties of lead chalcogenide nanocrystal films by introducing excess lead (Pb) or selenium (Se) through thermal evaporation. Hall-effect and capacitance-voltage measurements show that the carrier type, concentration, and Fermi level in nanocrystal solids may be precisely controlled through their stoichiometry. By manipulating only the stoichiometry of the nanocrystal solids, we engineer the characteristics of electronic and optoelectronic devices. Lead chalcogenide nanocrystal field-effect transistors (FETs) are fabricated at room temperature to form ambipolar, unipolar n-type, and unipolar p-type semiconducting channels as-prepared and with excess Pb and Se, respectively. Introducing excess Pb forms nanocrystal FETs with electron mobilities of 10 cm(2)/(V s), which is an order of magnitude higher than previously reported in lead chalcogenide nanocrystal devices. Adding excess Se to semiconductor nanocrystal solids in PbSe Schottky solar cells enhances the power conversion efficiency.

Liu H., Zhitomirsky D., Hoogland S., Tang J., Kramer I.J., Ning Z., Sargent E.H.

The recently reported quantum junction architecture represents a promising approach to building a rectifying photovoltaic device that employs colloidal quantum dot layers on each side of the p-n junction. Here, we report an optimized quantum junction solar cell that leverages an improved aluminum zinc oxide electrode for a stable contact to the n-side of the quantum junction and silver doping of the p-layer that greatly enhances the photocurrent by expanding the depletion region in the n-side of the device. These improvements result in greater stability and a power conversion efficiency of 6.1% under AM1.5 simulated solar illumination.

Zhitomirsky D., Furukawa M., Tang J., Stadler P., Hoogland S., Voznyy O., Liu H., Sargent E.H.

N-type PbS colloidal-quantum-dot (CQD) films are fabricated using a controlled halide chemical treatment, applied in an inert processing ambient environment. The new materials exhibit a mobility of 0.1 cm2 V−1 s−1. The halogen ions serve both as a passivating agent and n-dope the films via substitution at surface chalcogen sites. The majority electron concentration across the range 1016 to 1018 cm−3 is varied systematically.

Voznyy O., Zhitomirsky D., Stadler P., Ning Z., Hoogland S., Sargent E.H.

We present a framework--validated using both modeling and experiment--to predict doping in CQD films. In the ionic semiconductors widely deployed in CQD films, the framework reduces to a simple accounting of the contributions of the oxidation state of each constituent, including both inorganic species and organic ligands. We use density functional theory simulations to confirm that the type of doping can be reliably predicted based on the overall stoichiometry of the CQDs, largely independent of microscopic geometrical bonding configurations. Studies employing field-effect transistors constructed from CQDs that have undergone various chemical treatments, coupled with Rutherford backscattering and X-ray photoelectron spectroscopy to provide compositional analysis, allow us to test and confirm the proposed model in an experimental framework. We investigate both p- and n-type electronic doping spanning a wide range of carrier concentrations from 10(16) cm(-3) to over 10(18) cm(-3), and demonstrate reversible switching between p- and n-type doping by changing the CQD stoichiometry. We show that the summation of the contributions from all cations and anions within the film can be used to predict accurately the majority carrier type. The findings enable predictable control over majority carrier concentration via tuning of the overall stoichiometry.

Tang J., Liu H., Zhitomirsky D., Hoogland S., Wang X., Furukawa M., Levina L., Sargent E.H.

Colloidal quantum dot solids combine convenient solution-processing with quantum size effect tuning, offering avenues to high-efficiency multijunction cells based on a single materials synthesis and processing platform. The highest-performing colloidal quantum dot rectifying devices reported to date have relied on a junction between a quantum-tuned absorber and a bulk material (e.g., TiO2); however, quantum tuning of the absorber then requires complete redesign of the bulk acceptor, compromising the benefits of facile quantum tuning. Here we report rectifying junctions constructed entirely using inherently band-aligned quantum-tuned materials. Realizing these quantum junction diodes relied upon the creation of an n-type quantum dot solid having a clean bandgap. We combine stable, chemically compatible, high-performance n-type and p-type materials to create the first quantum junction solar cells. We present a family of photovoltaic devices having widely tuned bandgaps of 0.6–1.6 eV that excel where conventional quantum-to-bulk devices fail to perform. Devices having optimal single-junction bandgaps exhibit certified AM1.5 solar power conversion efficiencies of 5.4%. Control over doping in quantum solids, and the successful integration of these materials to form stable quantum junctions, offers a powerful new degree of freedom to colloidal quantum dot optoelectronics.

Ip A.H., Thon S.M., Hoogland S., Voznyy O., Zhitomirsky D., Debnath R., Levina L., Rollny L.R., Carey G.H., Fischer A., Kemp K.W., Kramer I.J., Ning Z., Labelle A.J., Chou K.W., et. al.

Colloidal quantum dot (CQD) films allow large-area solution processing and bandgap tuning through the quantum size effect. However, the high ratio of surface area to volume makes CQD films prone to high trap state densities if surfaces are imperfectly passivated, promoting recombination of charge carriers that is detrimental to device performance. Recent advances have replaced the long insulating ligands that enable colloidal stability following synthesis with shorter organic linkers or halide anions, leading to improved passivation and higher packing densities. Although this substitution has been performed using solid-state ligand exchange, a solution-based approach is preferable because it enables increased control over the balance of charges on the surface of the quantum dot, which is essential for eliminating midgap trap states. Furthermore, the solution-based approach leverages recent progress in metal:chalcogen chemistry in the liquid phase. Here, we quantify the density of midgap trap states in CQD solids and show that the performance of CQD-based photovoltaics is now limited by electron-hole recombination due to these states. Next, using density functional theory and optoelectronic device modelling, we show that to improve this performance it is essential to bind a suitable ligand to each potential trap site on the surface of the quantum dot. We then develop a robust hybrid passivation scheme that involves introducing halide anions during the end stages of the synthesis process, which can passivate trap sites that are inaccessible to much larger organic ligands. An organic crosslinking strategy is then used to form the film. Finally, we use our hybrid passivated CQD solid to fabricate a solar cell with a certified efficiency of 7.0%, which is a record for a CQD photovoltaic device.

Sahu A., Kang M.S., Kompch A., Notthoff C., Wills A.W., Deng D., Winterer M., Frisbie C.D., Norris D.J.

We dope CdSe nanocrystals with Ag impurities and investigate their optical and electrical properties. Doping leads not only to dramatic changes but surprising complexity. The addition of just a few Ag atoms per nanocrystal causes a large enhancement in the fluorescence, reaching efficiencies comparable to core-shell nanocrystals. While Ag was expected to be a substitutional acceptor, nonmonotonic trends in the fluorescence and Fermi level suggest that Ag changes from an interstitial (n-type) to a substitutional (p-type) impurity with increased doping.

Sargent E.H.

Kramer I.J., Sargent E.H.

Colloidal quantum dots (CQDs) offer a path toward high-efficiency photovoltaics based on low-cost materials and processes. Spectral tunability via the quantum size effect facilitates absorption of specific wavelengths from across the sun's broad spectrum. CQD materials' ease of processing derives from their synthesis, storage, and processing in solution. Rapid advances have brought colloidal quantum dot photovoltaic solar power conversion efficiencies of 6% in the latest reports. These achievements represent important first steps toward commercially compelling performance. Here we review advances in device architecture and materials science. We diagnose the principal phenomenon-electronic states within the CQD film band gap that limit both current and voltage in devices-that must be cured for CQD PV devices to fulfill their promise. We close with a prescription, expressed as bounds on the density and energy of electronic states within the CQD film band gap, that should allow device efficiencies to rise to those required for the future of the solar energy field.

Martinez L., Bernechea M., de Arquer F.P., Konstantatos G.

Bi2S3 nanocrystals are employed in a polymer/nanocrystal solar cell as a non-toxic inorganic electron acceptor with a high absorption coefficient and a bandgap of 1.3 eV optimal for single-junction solar harnessing. The reported solar cell yields a power conversion efficiency of 0.46% in a bilayer structure and shows an internal quantum efficiency in excess of 70% as a result of efficient exciton dissociation at the interface of Bi2S3 and P3HT.

Gao J., Perkins C.L., Luther J.M., Hanna M.C., Chen H., Semonin O.E., Nozik A.J., Ellingson R.J., Beard M.C.

The n-type transition metal oxides (TMO) consisting of molybdenum oxide (MoO(x)) and vanadium oxide (V(2)O(x)) are used as an efficient hole extraction layer (HEL) in heterojunction ZnO/PbS quantum dot solar cells (QDSC). A 4.4% NREL-certified device based on the MoO(x) HEL is reported with Al as the back contact material, representing a more than 65% efficiency improvement compared with the case of Au contacting the PbS quantum dot (QD) layer directly. We find the acting mechanism of the hole extraction layer to be a dipole formed at the MoO(x) and PbS interface enhancing band bending to allow efficient hole extraction from the valence band of the PbS layer by MoO(x). The carrier transport to the metal anode is likely enhanced through shallow gap states in the MoO(x) layer.

Mocatta D., Cohen G., Schattner J., Millo O., Rabani E., Banin U.

Impurities can be added into semiconductor nanoparticles to control their electronic and optical properties.

Mukai K., Sato Y., Yoshikuni N., Ikeda S.

Zhao M., Zhao Y., Wang J., Liu J., Zha X., Quan Y., Wang G., Liu M.

AbstractSpintronic devices represent a promising advancement in information storage, sensors, RF/microwave tunable devices, and other applications. Recently, researchers have developed a novel approach to energy‐efficiently manipulate spin states using photovoltaic (PV) thin‐film. However, optimization strategies for this method are relatively scarce. Here, a PV/magnetic thin film heterojunction featuring a perovskite quantum dots (PQDs) composite layer is presented with a hybrid interfacial architecture consisting of PCBM/PCBM@CsPbI3 QDs/CsPbI3 QDs/ PTB7‐Th heterojunction. The heterostructure facilitates more injection of photoelectrons into the ferromagnetic layer through an energy cascade mechanism model, resulting in greater magnetic changes compared to the PTB7‐Th: PC71 BM system. Under 100 mW cm−2 sunlight illumination, the out‐of‐plane ferromagnetic resonance shift increases by 626% (from −19 to −138 Oe), owing to improved photo‐induced electron doping. Additionally, the fluctuation of saturation magnetization (MS) is magnified by 200% (from 9% to 27%) as well. These findings demonstrate that the efficient photovoltaic layer plays a critical role in optimizing magnetic manipulation and lays the groundwork for the next generation of solar‐driven spintronic devices.

Barman C., Lavadiya S., Nayak S.K., Soma V.R., Raavi S.S.

Hu L., Wan T., Guan X., Li Z., Mei T., Dong B., Gao L., Chen C., Li X., Lin C., Li M., Chen F., Su D., Han Z., Xu H., et. al.

AbstractInfrared PbS quantum dot (QD) photodiodes play a vital role in various applications, including photovoltaics, light‐emitting diodes, lasers, and photodetectors. Despite their superior potential, high‐performance all‐QD homojunction photodiodes with bifacial structures have yet to be reported. Here, post‐treatment ligand engineering is successfully employed to precisely tune the doping dipoles of PbS QDs, transitioning them from n‐type, through intrinsic, to p‐type. All‐QD homojunction photodiodes solar cells with a n‐i‐p architecture are constructed by integrating three types of PbS QD layers of 1.37 eV bandgaps with controllable doping dipoles, which delivers a power conversion efficiency of 10.0%, among the highest values reported in PbS all‐QD homojunction solar cells so far. Owing to symmetry all‐QD architecture, bifacial PbS all‐QDs photodiodes, using 1.37 eV bandgap PbS QDs as both n‐type and p‐type charge transport layers and 0.90 eV bandgap PbS QDs as intrinsic light absorber layers, achieved an almost ideal bifactor approaching 93% and decent detectivities of 1.63 × 1011 Jones from ITO illumination and 1.86 × 1011 Jones from silver nanowire (Ag NW) illumination at 1370 nm. Therefore, this work provides a facile approach for the design of bifacial all‐QD homojunction photodiodes, broadening their potential applications in advanced QD optoelectronic systems.

Jiang Y., Zhang Y., Zheng J., Gao Y., Xiang C., Dong B., Lin C., Chen F., Guan X., Li X., Wan T., Mei T., Huang S., Hu L., Chu D.

Lead sulfide (PbS) quantum dots (QDs) and lead halide perovskites (LHPs) have emerged as highly promising materials for high‐efficiency photovoltaics. PbS QDs offer size‐dependent bandgaps in the infrared region and the potential for multiple exciton generation, while LHPs feature tunable bandgaps, high absorption coefficients, and long carrier diffusion lengths in the visible spectrum. This review focuses on two primary approaches to breaking the Shockley–Queisser (S–Q) limit based on the combinations of these two semiconducting materials: 1) monolithic 2‐terminal tandem photovoltaics with complementary spectral absorption; and 2) intermediate‐band solar cells (IBSCs) leveraging PbS QDs within a LHP matrix. Due to the ideally complementary spectrum of PbS and LHPs, emphasis is placed on the prevailing strategies for enhancing efficiency, addressing the major challenges in rational materials designs and device optimizations. Then, key obstacles including surface passivation, solvent compatibility, and the limited performance of small‐bandgap PbS QD solar cells are analyzed, along with various potential solutions for tandem cells. For IBSCs, the evolution of materials and device architecture and the unique advantages of their combination are outlined in detail. Finally, this review provides a comprehensive outlook on future research directions to develop efficient tandem and IBSC devices for breaking the S–Q limit.

Tong Y., Zhang D., Li Z., Hu G., Zou Q., Xiao L., Wu W., Huang L., Liang S., Duan H., Hu J., Hou H., Zhang J., Yang J.

Ghosh B., Yamada H., Nemoto K., Jevasuwan W., Fukata N., Sun H., Shirahata N.

Semiconductor p‐n homojunction is a requisite building block of operating transistors and diodes which make up the modern electronic circuits and optoelectronic applications. However, it has been so far limited to bulk form of single crystals such as silicon (Si) or gallium arsenide. Herein, a brand‐new method of constructing p‐n homojunction architectures that breaks through the limitation is presented. Colloidal inks of p‐type and n‐type Si quantum dots (QDs) are synthesized by thermal disproportionation of (HSiO1.5)n doped with boron or phosphorus, followed by surface ligand engineering. Analysis combining UV photoelectron spectroscopy, electron spin resonance, and current–voltage characteristics confirms that an orthogonal solvent trick makes clean interfaces between n‐type and p‐type SiQD layers without disruption on film formation. The forward and reverse current–voltage characteristics of the diode, along with various spectroscopic characterizations, demonstrate the formation of the first p‐n homojunction of SiQDs. The self‐powered photodiode provides a tunable response specific to the wavelength.

Liu Y., Tan L., Fu Y., Tang H., Dai G., Tan L.

Yee P.Y., Brittman S., Cunningham P.D., Stroud R.M., Burgess K.D., Erwin S.C., Lyons J.L., Stewart M.H., Marcheschi B.A., Durant B., Ellis C.T., Boercker J.E.

Mawaddah F.A., Bisri S.Z.

Colloidal quantum dots (CQDs) show unique properties that distinguish them from their bulk form, the so-called quantum confinement effects. This feature manifests in tunable size-dependent band gaps and discrete energy levels, resulting in distinct optical and electronic properties. The investigation direction of colloidal quantum dots (CQDs) materials has started switching from high-performing materials based on Pb and Cd, which raise concerns regarding their toxicity, to more environmentally friendly compounds, such as AgBiS2. After the first breakthrough in solar cell application in 2016, the development of AgBiS2 QDs has been relatively slow, and many of the fundamental physical and chemical properties of this material are still unknown. Investigating the growth of AgBiS2 QDs is essential to understanding the fundamental properties that can improve this material’s performance. This review comprehensively summarizes the synthesis strategies, ligand choice, and solar cell fabrication of AgBiS2 QDs. The development of PbS QDs is also highlighted as the foundation for improving the quality and performance of AgBiS2 QD. Furthermore, we prospectively discuss the future direction of AgBiS2 QD and its use for solar cell applications.

Hepp A.F., Raffaelle R.P.

Marjit K., Das A., Ghosh D., Patra A.

AbstractMetal halide perovskite (MHPs) nanocrystals (NCs) have recently emerged as promising materials for optoelectronic applications due to their exceptional photophysical properties. A comprehensive understanding of the underlying principles and key challenges associated with controlling the hot carrier (HC) cooling dynamics in MHPs is pivotal for the design of efficient perovskite‐based optoelectronic applications. This review highlights the general carrier cooling mechanism and identifies influencing crucial controlling parameters for carrier dynamics. The discussion includes strategies for manipulating HC cooling dynamics through carrier‐phonon and carrier‐carrier interactions. Notably, recent advancements in HC extraction with suitable acceptors are explored, as efficient HC extraction is imperative for the advancement of optoelectronic applications. The objective of this review is to outline potential pathways for strategically modifying carrier cooling processes to align with specific optoelectronic applications. A profound comprehension of charge carrier dynamics in MHPs NCs will pave the way for the development of highly efficient optoelectronic applications.

Ponomarenko Vladimir P., Popov Victor, Shuklov Ivan, Ivanov Victor V., Razumov Vladimir F.

Photosensing based on colloidal quantum dots (CQDs) is a rapidly developing area of infrared photoelectronics. The use of colloidal quantum dots markedly simplifies the manufacture, decreases the restrictions to the pixel pitch of the photosensitive elements, and reduces the production cost, which facilitates the wide use of IR sensors in various technological systems. This paper is the first exhaustive overeview of the architectures, methods of manufacturing and basic properties of photonic sensors based on colloidal quantum dots of compounds of Group II, IV and VI elements. Characteristic features of the synthesis and roles of the ligands and CQD morphology in the design of photosensors are considered in detail. The structures of photoresistive, photodiode and phototransistor elements based on HgTe, HgSe, PbS and PbSe CQDs, which are sensitive in various spectral ranges, are described. The main parameters of the most advanced optoelectronic devices based on colloidal quantum dot structures are presented. The key trends in the development of this area are analyzed.The bibliography includes 361 references.

Dolai S., Basu S., Paul A.

We report successful doping of Mn2+ ions within zinc mediated assembly of gold nanoclusters (Zn Au NCs). This resulted in the emergence of luminescence attributed to the doped Mn2+ ions,...

Karmakar G., Tyagi A., Shah A.Y.

Group IV metal chalcogenides (group IV MCs) have garnered significant attention from scientific fraternity owing to their distinct structural features and interesting electronic properties that could be exploited for diverse applications including energy conversion and storage, optoelectronic devices and sensors. However, to make group IV MCs commercially viable, it is imperative to evolve efficient, cost effective and scalable methods for their synthesis. Single source molecular precursor (SSP) mediated synthesis of group IV MCs is one such route which enjoys tremendous advantages over conventional solid state or dual precursor routes. Most importantly, SSP serve as a viable tool to afford phase pure group IV MCs with high reproducibility and excellent control over size and morphology in presence of suitable capping agents. This timely review provides a comprehensive overview of SSPs employed for accessing group IV MCs nanomaterials as well as thin films. Effects of various ligands on SSP performance have been rationalized. Additionally, precursors which can afford selective synthesis of different compositions or phase of group IV MCs by the choice of solvent, temperature and mode of decomposition have been critically assessed. Strength, limitations and opportunities associated with SSP approach are critically evaluated to provide directions for the development of new SSPs. Furthermore, the role of capping agents and fundamentals of viable synthetic strategies in general, for materials synthesis and deposition of thin film have been summarised in this account. Finally, the conclusion and future prospects of SSPs have also been included in this review. It is expected that this review will provide library of precursors and optimized conditions to synthesize group IV MCs and further catch the attention of researchers to explore the SSP mediated route in making size- and shape-tunable nanostructures with improved functionalities.