Single-step colloidal quantum dot films for infrared solar harvesting

Farmer J.D., Lafond F.

Recently it has become clear that many technologies follow a generalized version of Moore's law, i.e. costs tend to drop exponentially, at different rates that depend on the technology. Here we formulate Moore's law as a correlated geometric random walk with drift, and apply it to historical data on 53 technologies. We derive a closed form expression approximating the distribution of forecast errors as a function of time. Based on hind-casting experiments we show that this works well, making it possible to collapse the forecast errors for many different technologies at different time horizons onto the same universal distribution. This is valuable because it allows us to make forecasts for any given technology with a clear understanding of the quality of the forecasts. As a practical demonstration we make distributional forecasts at different time horizons for solar photovoltaic modules, and show how our method can be used to estimate the probability that a given technology will outperform another technology at a given point in the future.

Lan X., Voznyy O., Kiani A., García de Arquer F.P., Abbas A.S., Kim G., Liu M., Yang Z., Walters G., Xu J., Yuan M., Ning Z., Fan F., Kanjanaboos P., Kramer I., et. al.

A solution-based passivation scheme is developed featuring the use of molecular iodine and PbS colloidal quantum dots (CQDs). The improved passivation translates into a longer carrier diffusion length in the solid film. This allows thicker solar-cell devices to be built while preserving efficient charge collection, leading to a certified power conversion efficiency of 9.9%, which is a new record in CQD solar cells.

Kim G., García de Arquer F.P., Yoon Y.J., Lan X., Liu M., Voznyy O., Yang Z., Fan F., Ip A.H., Kanjanaboos P., Hoogland S., Kim J.Y., Sargent E.H.

The optoelectronic tunability offered by colloidal quantum dots (CQDs) is attractive for photovoltaic applications but demands proper band alignment at electrodes for efficient charge extraction at minimal cost to voltage. With this goal in mind, self-assembled monolayers (SAMs) can be used to modify interface energy levels locally. However, to be effective SAMs must be made robust to treatment using the various solvents and ligands required for to fabricate high quality CQD solids. We report robust self-assembled monolayers (R-SAMs) that enable us to increase the efficiency of CQD photovoltaics. Only by developing a process for secure anchoring of aromatic SAMs, aided by deposition of the SAMs in a water-free deposition environment, were we able to provide an interface modification that was robust against the ensuing chemical treatments needed in the fabrication of CQD solids. The energy alignment at the rectifying interface was tailored by tuning the R-SAM for optimal alignment relative to the CQD quantum-confined electron energy levels. This resulted in a CQD PV record power conversion efficiency (PCE) of 10.7% with enhanced reproducibility relative to controls.

Yang Z., Janmohamed A., Lan X., García de Arquer F.P., Voznyy O., Yassitepe E., Kim G., Ning Z., Gong X., Comin R., Sargent E.H.

Solution-processed quantum dots are a promising material for large-scale, low-cost solar cell applications. New device architectures and improved passivation have been instrumental in increasing the performance of quantum dot photovoltaic devices. Here we report photovoltaic devices based on inks of quantum dot on which we grow thin perovskite shells in solid-state films. Passivation using the perovskite was achieved using a facile solution ligand exchange followed by postannealing. The resulting hybrid nanostructure created a more intrinsic CQD film, which, when incorporated into a photovoltaic device with graded bandstructure, achieved a record solar cell performance for single-step-deposited CQD films, exhibiting an AM1.5 solar power conversion efficiency of 8.95%.

Diroll B.T., Gaulding E.A., Kagan C.R., Murray C.B.

Quantum confinement is the divergence, at small crystallite size, of the electronic structure of semiconductor nanocrystals, or quantum dots, from the properties of larger crystals of the same materials. Although the extinction properties of quantum dots in the dispersed state have been extensively studied, many applications for quantum dots require the formation of a solid material which nonetheless retains a size-dependent electronic structure. The complex index of refraction (or complex dielectric function), including the extinction coefficient, is critical information for interpretation of optoelectronic measurements and use of quantum dot solids in optoelectronic devices. Here, spectroscopic ellipsometry is used to provide an all-optical method to determine the thickness, complex index, and extinction coefficient of thin films made of quantum-confined materials through the visible and near-infrared spectral ranges. The characteristic, size-dependent spectral features in the absorption of monodisperse...

Ip A.H., Kiani A., Kramer I.J., Voznyy O., Movahed H.F., Levina L., Adachi M.M., Hoogland S., Sargent E.H.

Materials optimized for single-junction solar spectral harvesting, such as silicon, perovskites, and large-band-gap colloidal quantum dot solids, fail to absorb the considerable infrared spectral energy that lies below their respective band gap. Here we explore through modeling and experiment the potential for colloidal quantum dots (CQDs) to augment the performance of solar cells by harnessing transmitted light in the infrared. Through detailed balance modeling, we identify the CQD band gap that is best able to augment wafer-based, thin-film, and also solution-processed photovoltaic (PV) materials. The required quantum dots, with an excitonic peak at 1.3 μm, have not previously been studied in depth for solar performance. Using computational studies we find that a new ligand scheme distinct from that employed in better-explored 0.95 μm band gap PbS CQDs is necessary; only via the solution-phase application of a short bromothiol can we prevent dot fusion during ensuing solid-state film treatments and simultaneously offer a high valence band-edge density of states to enhance hole transport. Photoluminescence spectra and transient studies confirm the desired narrowed emission peaks and reduced surface-trap-associated decay. Electronic characterization reveals that only through the use of the bromothiol ligands is strong hole transport retained. The films, when used to make PV devices, achieve the highest AM1.5 power conversion efficiency yet reported in a solution-processed material having a sub-1 eV band gap.

Crisp R.W., Kroupa D.M., Marshall A.R., Miller E.M., Zhang J., Beard M.C., Luther J.M.

We developed a layer-by-layer method of preparing PbE (E = S or Se) quantum dot (QD) solar cells using metal halide (PbI2, PbCl2, CdI2, or CdCl2) salts dissolved in dimethylformamide to displace oleate surface ligands and form conductive QD solids. The resulting QD solids have a significant reduction in the carbon content compared to films treated with thiols and organic halides. We find that the PbI2 treatment is the most successful in removing alkyl surface ligands and also replaces most surface bound Cl- with I-. The treatment protocol results in PbS QD films exhibiting a deeper work function and band positions than other ligand exchanges reported previously. The method developed here produces solar cells that perform well even at film thicknesses approaching a micron, indicating improved carrier transport in the QD films. We demonstrate QD solar cells based on PbI2 with power conversion efficiencies above 7%.

Kramer I.J., Minor J.C., Moreno-Bautista G., Rollny L., Kanjanaboos P., Kopilovic D., Thon S.M., Carey G.H., Chou K.W., Zhitomirsky D., Amassian A., Sargent E.H.

Ning Z., Dong H., Zhang Q., Voznyy O., Sargent E.H.

New inorganic ligands including halide anions have significantly accelerated progress in colloidal quantum dot (CQD) photovoltaics in recent years. All such device reports to date have relied on halide treatment during solid-state ligand exchanges or on co-treatment of long-aliphatic-ligand-capped nanoparticles in the solution phase. Here we report solar cells based on a colloidal quantum dot ink that is capped using halide-based ligands alone. By judicious choice of solvents and ligands, we developed a CQD ink from which a homogeneous and thick colloidal quantum dot solid is applied in a single step. The resultant films display an n-type character, making it suitable as a key component in a solar-converting device. We demonstrate two types of quantum junction devices that exploit these iodide-ligand-based inks. We achieve solar power conversion efficiencies of 6% using this class of colloids.

Norman Z.M., Anderson N.C., Owen J.S.

We report the evolution of electrical transport and grain size during the sintering of thin films spin-cast from soluble phosphine and amine-bound, chloride-terminated cadmium selenide nanocrystals. Sintering of the nanocrystals occurs in three distinct stages as the annealing temperature is increased: (1) reversible desorption of the organic ligands (≤150 °C), (2) irreversible particle fusion (200–300 °C), and (3) ripening of the grains to >5 nm domains (>200 °C). Grain growth occurs at 200 °C in films with 8 atom % Cl–, while films with 3 atom % Cl– resist growth until 300 °C. Fused nanocrystalline thin films (grain size = 4.5–5.5 nm) on thermally grown silicon dioxide gate dielectrics produce field-effect transistors with electron mobilities as high as 25 cm2/(Vs) and on/off ratios of 105 with less than 0.5 V hysteresis in threshold voltage without the addition of indium.

Brown P.R., Kim D., Lunt R.R., Zhao N., Bawendi M.G., Grossman J.C., Bulović V.

The electronic properties of colloidal quantum dots (QDs) are critically dependent on both QD size and surface chemistry. Modification of quantum confinement provides control of the QD bandgap, while ligand-induced surface dipoles present a hitherto underutilized means of control over the absolute energy levels of QDs within electronic devices. Here, we show that the energy levels of lead sulfide QDs, measured by ultraviolet photoelectron spectroscopy, shift by up to 0.9 eV between different chemical ligand treatments. The directions of these energy shifts match the results of atomistic density functional theory simulations and scale with the ligand dipole moment. Trends in the performance of photovoltaic devices employing ligand-modified QD films are consistent with the measured energy level shifts. These results identify surface-chemistry-mediated energy level shifts as a means of predictably controlling the electronic properties of colloidal QD films and as a versatile adjustable parameter in the performance optimization of QD optoelectronic devices.

Chuang C.M., Brown P.R., Bulović V., Bawendi M.G.

Solution processing is a promising route for the realization of low-cost, large-area, flexible and lightweight photovoltaic devices with short energy payback time and high specific power. However, solar cells based on solution-processed organic, inorganic and hybrid materials reported thus far generally suffer from poor air stability, require an inert-atmosphere processing environment or necessitate high-temperature processing, all of which increase manufacturing complexities and costs. Simultaneously fulfilling the goals of high efficiency, low-temperature fabrication conditions and good atmospheric stability remains a major technical challenge, which may be addressed, as we demonstrate here, with the development of room-temperature solution-processed ZnO/PbS quantum dot solar cells. By engineering the band alignment of the quantum dot layers through the use of different ligand treatments, a certified efficiency of 8.55% has been reached. Furthermore, the performance of unencapsulated devices remains unchanged for over 150 days of storage in air. This material system introduces a new approach towards the goal of high-performance air-stable solar cells compatible with simple solution processes and deposition on flexible substrates.

Weidman M.C., Beck M.E., Hoffman R.S., Prins F., Tisdale W.A.

Despite their technological importance, lead sulfide (PbS) nanocrystals have lagged behind nanocrystals of cadmium selenide (CdSe) and lead selenide (PbSe) in terms of size and energy homogeneity. Here, we show that the ratio of lead to sulfur precursor available during nucleation is a critical parameter affecting subsequent growth and monodispersity of PbS nanocrystal ensembles. Applying this knowledge, we synthesize highly monodisperse (size dispersity

Dirin D.N., Dreyfuss S., Bodnarchuk M.I., Nedelcu G., Papagiorgis P., Itskos G., Kovalenko M.V.

Lead halide perovskites (CH3NH3PbX3, where X = I, Br) and other metal halide complexes (MXn, where M = Pb, Cd, In, Zn, Fe, Bi, Sb) have been studied as inorganic capping ligands for colloidal nanocrystals. We present the methodology for the surface functionalization via ligand-exchange reactions and the effect on the optical properties of IV–VI, II–VI, and III–V semiconductor nanocrystals. In particular, we show that the Lewis acid–base properties of the solvents, in addition to the solvent dielectric constant, must be properly adjusted for successful ligand exchange and colloidal stability. High luminescence quantum efficiencies of 20–30% for near-infrared emitting CH3NH3PbI3-functionalized PbS nanocrystals and 50–65% for red-emitting CH3NH3CdBr3- and (NH4)2ZnCl4-capped CdSe/CdS nanocrystals point to highly efficient electronic passivation of the nanocrystal surface.

Ip A.H., Labelle A.J., Sargent E.H.

Atomic layer deposition was used to encapsulate colloidal quantum dot solar cells. A nanolaminate layer consisting of alternating alumina and zirconia films provided a robust gas permeation barrier which prevented device performance degradation over a period of multiple weeks. Unencapsulated cells stored in ambient and nitrogen environments demonstrated significant performance losses over the same period. The encapsulated cell also exhibited stable performance under constant simulated solar illumination without filtration of harsh ultraviolet photons. This monolithically integrated thin film encapsulation method is promising for roll-to-roll processed high efficiency nanocrystal solar cells.

Yang K., Huang W., Chen C., Tan W., Peng J., Tian J.

Sheikh T., Mir W.J., Alofi A., Skoroterski M., Zhou R., Nematulloev S., Hedhili M.N., Hassine M.B., Khan M.S., Yorov K.E., Hasanov B.E., Liao H., Yang Y., Shamim A., Abulikemu M., et. al.

Wang H., Loi M.A.

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.

Zheng S., Mei X., Chen J., J. Johansson E.M., Zhang X.

Colloidal quantum dot (CQD) shows great potential for application in infrared solar cells due to the simple synthesis techniques, tunable infrared absorption spectrum, and high stability and solution-processability. Thanks to significant efforts made on the surface chemistry of CQDs, device structure optimization, and device physics of CQD solar cells (CQDSCs), remarkable breakthroughs are achieved to boost the infrared photovoltaic performance and stability of CQDSCs. In particular, the CQDSC with a high power conversion efficiency of ~ 14% and good stability is reported, which is very promising for infrared-absorbing solar cells. In this review, we highlight the unique optoelectronic properties of CQDs for the development of infrared-absorbing solar cells. Meanwhile, the latest advances in finely controlling surface properties of CQDs are comprehensively summarized and discussed. Moreover, the device operation of CQDSCs is discussed in-depth to highlight the impact of the device structure optimization of CQDSCs on their photovoltaic performance, and the emerging novel types of CQDSCs, such as semitransparent, flexible, and lightweight CQDSCs, are also demonstrated. The device stability of CQDSCs is also highlighted from the viewpoint of practical applications. Finally, the conclusions and possible challenges and opportunities are presented to promote the development steps of the CQDSCs with higher infrared photovoltaic performance and robust stability.

Zhu J., Lu K., Li J., Liu Z., Ma W.

Establishing tandem photovoltaic device structures to achieve full-spectrum utilization of solar energy is a vital pathway to maximizing power conversion efficiency (PCE). The dominant photovoltaic materials currently available (including silicon,...

Sheikh T., Mir W.J., Nematulloev S., Maity P., Yorov K.E., Hedhili M.N., Emwas A., Khan M.S., Abulikemu M., Mohammed O.F., Bakr O.M.

Aftab S., Iqbal M.Z., Hussain S., Kabir F., Al‐Kahtani A.A., Hegazy H.H.

AbstractNanocrystals, called semiconductor quantum dots (QDs), contain excitons that are three‐dimensionally bound. QDs exhibit a discontinuous electronic energy level structure that is similar to that of atoms and exhibit a distinct quantum confinement effect. As a result, QDs have unique electrical, optical, and physical characteristics that can be used in a variety of optoelectronic device applications, including solar cells. In this review article, the stable and controllable synthesis of QD materials is outlined for upscaling solar cells, including material development and device performance enhancement. It includes a systematic variety of device structures for the fabrication of solar cells, such as QD, hybrid QD/organic, hybrid QD/inorganic, perovskite QD, and hybrid 2D MXene QD/perovskite. The mechanisms for the improvement of stability by QD treatment are examined. For example, the 2D MXene QD and/or Cu1.8S nanocrystal doping significantly increases the long‐term light and ambient stability of perovskite solar cells, resulting from improved perovskite crystallization, reduced hole transport layer (HTL) aggregation and crystallization of films, and reduced UV‐induced photocatalytic activity of the electron transport layer (ETL). For the advancement of QD solar cells and their interaction with various materials, the conclusions from this review are crucial. Finally, future prospects for the development of QD solar cells as well as current challenges are discussed.

Kim Y., Choi M., Choi J.

Colloidal quantum dots (CQDs) are promising semiconducting materials, which can be used as a photoactive layer in various optoelectronic applications, because of their size-tunable bandgap energy, solution processability, and excellent optical and optoelectronic properties. In particular, these features have generated great interest in the development of CQD solar cells and led to a rapid increase in their power conversion efficiency. These improvements were enabled by many innovative approaches in terms of CQD's surface chemistry and device architecture optimizations. In this review, a critical overview of the research progress in CQD solar cells is presented with a focus on the strategies adopted for achieving record efficiency in CQD solar cells. These strategies include the use of organic/inorganic surface ligands, pre- and post-treatment of CQDs, and solid-state/solution-phase ligand exchange. Additionally, we provide an understanding of the research history to inspire the rational design of next-generation CQD optoelectronic devices, such as solar cells, light-emitting diodes, and photodetectors. Recent research on the development of infrared CQD solar cells as complementary platforms to other solar cell technologies is also critically discussed to provide another perspective on CQD technologies.

Yu L., Tian P., Tang L., Zuo W., Zhong H., Hao Q., Teng K.S., Zhao G., Su R., Gong X., Yuan J.

Lead sulfide colloidal quantum dots (PbS CQDs) are promising optoelectronic materials due to their unique properties, such as tunable band gap and strong absorption, which are of immense interest for application in photodetectors and solar cells. However, the tunable band gap of PbS CQDs would only cover visible short-wave infrared; the ability to detect longer wavelengths, such as mid- and long-wave infrared, is limited because they are restricted by the band gap of the bulk material. In this paper, a novel photodetector based on the synergistic effect of PbS CQDs and bismuth telluride (Bi2Te3) was developed for the detection of a mid-wave infrared band at room temperature. The device demonstrated good performance in the visible-near infrared band (i.e., between 660 and 850 nm) with detectivity of 1.6 × 1010 Jones at room temperature. It also exhibited photoelectric response in the mid-wave infrared band (i.e., between 4.6 and 5.1 μm). The facile fabrication process and excellent performance (with a response of up to 5.1 μm) of the hybrid Bi2Te3/PbS CQDS photodetector are highly attractive for many important applications that require high sensitivity and broadband light detection.

Ma Y., Wang Y., Wen J., Li A., Li X., Leng M., Zhao Y., Lu Z.

Colloidal quantum dots (CQDs) are of great interest for photovoltaic (PV) technologies as they possess the benefits of solution-processability, size-tunability, and roll-to-roll manufacturability, as well as unique capabilities to harvest near-infrared (NIR) radiation. During the last decade, lab-scale CQD solar cells have achieved rapid improvement in the power conversion efficiency (PCE) from ∼1% to 18%, which will potentially exceed 20% in the next few years and approach the performance of other PV technologies, such as perovskite solar cells and organic solar cells. In the meanwhile, CQD solar cells exhibit long lifetimes either under shelf storage or continuous operation, making them highly attractive to industry. However, in order to meet the industrial requirements, mass production techniques are necessary to scale up the fabrication of those lab devices into large-area PV modules, such as roll-to-toll coating. This paper reviews the recent developments of large-area CQD solar cells with a focus on various fabrication methods and their principles. It covers the progress of typical large-area coating techniques, including spray coating, blade coating, dip coating, and slot-die coating. It also discusses next steps and new strategies to accomplish the ultimate goal of the low-cost large-area fabrication of CQD solar cells and emphasizes how artificial intelligence or machine learning could facilitate the developments of CQD solar cell research.

huang K., Liu J., Yuan J., Zhao W., Zhao K., Zhou Z.

Solution-processed solar cells have witnessed unparalleled progress in the past decade, owing to their great potential in countering global warming and high competitiveness in light and flexible electronics. Perovskite solar...

Wang H., Nakao S., Miyashita N., Oteki Y., Giteau M., Okada Y., Takamoto T., Saito H., Magaino S., Takagi K., Hasegawa T., Kubo T., Kinoshita T., Nakazaki J., Segawa H.

Muchahary D., Bhattarai S., Mahato A.K., Maity S.

In this chapter, an overview of different novel materials used in solar cell is presented. The inside physics and most common materials used in a variety of solar cells are described. Material is the heart of a solar cell device and selection and development of proper material are a crucial task for device development. Amongst different type hybrid, perovskite and organic solar cells are cheap and provide comparable power conversion efficiency to the conventional silicon based solar cell. Superior element for a hybrid solar cell is PEDOT:PSS blend and Si heterojunction. Different additives to improve the efficiency in such solar cell are discussed. Moreover, different organo-metallic perovskite materials with ferroelectric characteristics are discussed in this chapter. The ferroelectricity uplifts the open circuit voltage of the solar cell beyond its energy band gap. The analytical description of device performance in such under different charge carrier transport materials are presented as well. Both organic and inorganic materials are suitable for electron and hole transporting layer. The perovskite materials are leading its way to lower dimensional versions which are suitable for both solar cell and other optoelectronic devices. The stability of lower dimensional perovskite leads to Dion-Jacobson (DJ) and Ruddlesden-Popper (RP) phases and are discussed in this chapter. Moreover, working physics and materials used in organic solar cell are also a part of this chapter.

Khrebtov A.I., Kulagina A.S., Dragunova A.S., Reznik R.R., Cirlin G.E., Danilov V.V.

We investigate the photoluminescence of a film obtained by the uniform deposition of a colloidal solution of CdSe/ZnS quantum dots capped by trioctylphosphine oxide on an array of isolated InP/InAsP/InP nanowires and subsequently removed from the substrate. The photoluminescence spectrum of the film shows emission bands corresponding to InAsP nano-insertions (1.25–1.5 μm) and quantum wells (1.0–1.2 μm). We find that the dependence of the photoluminescence intensity on the excitation intensity has a nonlinear character, which we interpret as a manifestation of the light quenching effect, and that the photodynamics of excitation of the nano-insertions and quantum wells differ from each other. We analyse the light quenching effect taking into account the significant increase in the luminescence intensity observed in such hybrid film as compared to an array of isolated InP/InAsP/InP nanowires. We hypothesize the possible suppression of Auger relaxation due to multistep nonradiative transfer of excitation. • We measure the photoluminescence intensity as a function of excitation intensity in hybrid films. • We find evidence of the light quenching effect. • We suggest that suppression of Auger recombination in the hybrid film is due to the multistep nature of the excitation transfer through higher exciton states. • We discuss the role of back-tranfer of excitation from traps to higher exciton states.