Multi-Functional Optoelectronic Heterostructure Devices Based on Transfer Printing of Nanomaterials

School of Energy and Chemical Engineering (Energy Engineering)Heterostructure devices, combining different electronic properties of semiconductors, offer novel electronic functionalities, which are critically required in emerging applications in high performance and multi-functional electronics. Previously, heterostructure devices have attracted a great attention due to the enhancing performances, adding functionalities and broadening absorption range, through components modulation, resulting in many applications in high electron mobility transistors, non-volatile memory, light emitting diodes, and broadband photodetectors. However, traditional semiconductor heterostructures present significant challenges due to the lattice constant mismatch with other substrates and generation of defects during the direct growth and deposition processes. To address these challenges, a transfer printing was introduced to heterogeneously integrate various nanomaterials onto arbitrary substrates, whereby the bonding at heterointerfaces with a large lattice mismatch is facilitated by van der Waals forces during the transfer printing processes. The transfer printing can provide a freedom of material choice, from zero to three dimensional materials, in the formation of heterostructures without the restriction from lattice mismatch, which enabled various heterostructure devices with unique physical properties. In this thesis, we demonstrate multi-functional optoelectronic heterostructure devices based on transfer printing of nanomaterials. First, in chapter 1, we briefly introduce the research trends in electronic devices and basic concept of transfer printing methods and multi-functional heterostructure devices. In chapter 2, we demonstrate a new type of heterostructure device based on black phosphorus and n-InGaAs nanomembrane semiconductors. The device offers gate-tunable rectification and switching behaviors. In addition, the proposed heterojunction diode can be programed by the modulation of forward current due to the capacitive gating effect. Furthermore, the device is photoresponsive in a spectral range spanning the ultraviolet to near infrared. In chapter 3, we describe the fine patterning technique of silver nanowires on various substrates using vacuum filtration and transfer printing process. This technique provides very simple and cost-effective fabrication for fine patterning of AgNWs electrode for optically transparent and mechanically flexible optoelectronic device applications. This patterning technique can be applied to other nanomaterials such as CNT and graphene and combination of nanomaterials to realize highly flexible and transparent optoelectronic devices. In chapter 4, the large-area MoS2 film and pattering process is demonstrated by shadow mask assisted transfer printing process. The liquid exfoliated MoS2 flakes can be easily patterned by vacuum filtration with polyimide shadow mask. Patterned film is transferred to arbitrary substrate by using transfer printing process for high performance and flexible electronic applications. Therefore, the heterostructure devices made by transfer printing are advantageous in scalability and avoids complicated fabrication process for multi-functional applications.ope

Heterojunction Structures for Photon Detector Applications

The work presented here report findings in (1) infrared detectors based on p-GaAs/AlGaAs heterojunctions, (2) J and H aggregate sensitized heterojunctions for solar cell and photon detection applications, (3) heterojunctions sensitized with quantum dots as low cost solar energy conversion devices and near infrared photodetectors. (1)A GaAs/AlGaAs based structure with a graded AlGaAs barrier is found to demonstrate a photovoltaic responsivity of ~ 30mA/W (~ 450mV/W) at the wavelength of 1.8 mm at 300K. Additionally the graded barrier has enhanced the photoconductive response at 78 K, showing a responsivity of ~ 80mA/W with a D*=1.4×108 Jones under 1V bias at 2.7 mm wavelength. This is an approximately 25 times improvement compared to the flat barrier detector structure, probably due to the improved carrier transport, and low recapture rate in the graded barrier structure. However, these graded barrier devices did not indicate a photoresponse with photoconductive mode at 300K due to high shot noise. Additionally, two generation-recombination noise components and a 1/f noise component were identified. A series of GaAs/AlGaAs multilayer hetero-junction structures were tested as thermal detectors. A superlattice structure containing 57% Al fraction in the barrier and 3 × 1018 cm-3 p-doped GaAs emitter showed the highest responsivity as a thermal detector with a TCR of ~ 4% K-1, at 300K. (2)The photovoltaic properties of heterojunctions with J-/ H- aggregated dye films sandwiched between n– and p-type semiconductors were investigated for potential application as solar cells and IR detectors. Films of cationic dye Rhodamine-B-thiocyanate adsorbed on Cu2O substrate are found to form organized dye layers by self-assembled J- aggregation, resulting in large red-shifts in the photo -response. Additionally, cells sensitized with a pentamethine cyanine dye exhibited a broad spectral response originating from J- and H-aggregates. The photocurrent is produced by exciton transport over relatively long distances with significant hole-mobility as well as direct sensitized injection at the first interface. (3) A ZnO/PbS-QD/Dye heterostructure had enhanced efficiency compared to ZnO/Dye heterostructure as a solar cell. Furthermore, a ZnO/PbS-QD structure has demonstrated UV and NIR responses with 4×105V/W (390 nm) and 5.5×105 V/W (750 nm) under 1V bias at 300K

Colloidal Quantum Dots and J-Aggregating Cyanine Dyes for Infrared Photodetection

The emergence of nanostructured semiconducting materials enables new approaches toward the realization of photodetectors that operate in the technologically important near- and short-wave infrared (NIR and SWIR) spectral ranges. In particular, organic semiconductors and colloidal quantum dots (QDs) possess numerous advantages over conventional crystalline semiconductors including highly tunable optical and electronic characteristics, the prospect for low-temperature processing, and compatibility with inexpensive and exible substrates. Photodetectors based on such materials may lead to low-cost IR focal plane arrays for night-vision imaging as well as a multitude of novel applications in biological and chemical sensing. This thesis describes the development and detailed characterization of several photodetectors that incorporate colloidal QD and organic semiconductor thin films as active layers. The electronic properties of PbS QDs are thoroughly investigated in a field effect transistor (FET) geometry and several QD-based photoconductive structures exhibiting SWIR photosensitivity are demonstrated. We describe a novel QD-sensitized lateral heterojunction photoconductor in which the functions of optical absorption and charge transport are dissociated into different physical layers that can be independently optimized. This structure is advantageous because it obviates the need for aggressive chemical treatments to the QD film that may compromise the quality of QD surface passivation. Photovoltaic device architectures are addressed, noting their advantages of being operable without an external applied bias and at fast response speeds. We present detailed experimental and theoretical characterization of a photovoltaic structure that is sensitized at NIR wavelengths by a J-aggregating cyanine dye. The high absorption coefficient of the J-aggregate film, combined with the use of a reflective anode and optical spacer layer, enables an external quantum efficiency (EQE) of $16.1\pm0.1\%(\lambda = 756 nm)$ to be achieved at zero-bias in a device that incorporates an $8.1\pm0.3 nm$-thick dye film. The merits and drawbacks of the various device architectures and nanostructured material systems are discussed and the outlook for nanostructured photodetectors that exhibit infrared sensitivity is presented.Engineering and Applied Science

Two-Dimensional Electronics - Prospects and Challenges

During the past 10 years, two-dimensional materials have found incredible attention in the scientific community. The first two-dimensional material studied in detail was graphene, and many groups explored its potential for electronic applications. Meanwhile, researchers have extended their work to two-dimensional materials beyond graphene. At present, several hundred of these materials are known and part of them is considered to be useful for electronic applications. Rapid progress has been made in research concerning two-dimensional electronics, and a variety of transistors of different two-dimensional materials, including graphene, transition metal dichalcogenides, e.g., MoS2 and WS2, and phosphorene, have been reported. Other areas where two-dimensional materials are considered promising are sensors, transparent electrodes, or displays, to name just a few. This Special Issue of Electronics is devoted to all aspects of two-dimensional materials for electronic applications, including material preparation and analysis, device fabrication and characterization, device physics, modeling and simulation, and circuits. The devices of interest include, but are not limited to transistors (both field-effect transistors and alternative transistor concepts), sensors, optoelectronics devices, MEMS and NEMS, and displays

Advanced AFM techniques to characterize all-oxide solar cells at the nanoscale

In this work, hydrothermally grown nanowires of n-type oxides have been conformally covered by p-type oxides by different deposition methods, like ALD, CVD and sputtering. Cu2O/ZnO, Co3O4/ZnO, and Co3O4/TiO2 all-oxide heterojunctions have been investigated by means of structural, morphological, optical and electrical techniques. C-AFM was used to study the local current distribution at the nanoscale

Annual Report 2020 - Institute of Ion Beam Physics and Materials Research

As for everybody else also for the Institute of Ion Beam Physics and Materials Research (IIM), the COVID-19 pandemic overshadowed the usual scientific life in 2020. Starting in March, home office became the preferred working environment and the typical institute life was disrupted. After a little relaxation during summer and early fall, the situation became again more serious and in early December we had to severely restrict laboratory activities and the user operation of the Ion Beam Center (IBC). For the most part of 2020, user visits were impossible and the services delivered had to be performed hands-off. This led to a significant additional work load on the IBC staff. Thank you very much for your commitment during this difficult period. By now user operation has restarted, but we are still far from business as usual. Most lessons learnt deal with video conference systems, and everybody now has extensive experience in skype, teams, webex, zoom, or any other solution available. Conferences were cancelled, workshops postponed, and seminar or colloquia talks delivered online. Since experimental work was also impeded, maybe 2020 was a good year for writing publications and applying for external funding. In total, 204 articles have been published with an average impact factor of about 7.0, which both mark an all-time high for the Institute. 13 publications from last year are highlighted in this Annual Report to illustrate the wide scientific spectrum of our institute. In addition, 20 new projects funded by EU, DFG, BMWi/AiF and SAB with a total budget of about 5.7 M€ have started. Thank you very much for making this possible. Also, in 2020 there have been a few personalia to be reported. Prof. Dr. Sibylle Gemming has left the HZDR and accepted a professor position at TU Chemnitz. Congratulations! The hence vacant position as the head of department was taken over by PD Dr. Artur Erbe by Oct. 1st. Simultaneously, the department has been renamed to “Nanoelectronics”. Dr. Alina Deac has left the institute in order to dedicate herself to new opportunities at the Dresden High Magnetic Field Laboratory. Dr. Matthias Posselt went to retirement after 36 years at the institute. We thank Matthias for his engagement and wish him all the best for the upcoming period of his life. However, also new equipment has been setup and new laboratories founded. A new 100 kV accelerator is integrated into our low energy ion nanoengineering facility and complements our ion beam technology in the lower energy regime. This setup is particularly suited to perform ion implantation into 2D materials and medium energy ion scattering (MEIS). Finally, we would like to cordially thank all partners, friends, and organizations who supported our progress in 2020. First and foremost we thank the Executive Board of the Helmholtz-Zentrum Dresden-Rossendorf, the Minister of Science and Arts of the Free State of Saxony, and the Ministers of Education and Research, and of Economic Affairs and Energy of the Federal Government of Germany. Many partners from univer¬sities, industry and research institutes all around the world contributed essentially, and play a crucial role for the further development of the institute. Last but not least, the directors would like to thank all members of our institute for their efforts in these very special times and excellent contributions in 2020

Hybrid Organic/Inorganic Optoelectronics.

Traditional inorganic semiconductors are the foundation of modern electronics. These materials are widely used in computing and optical devices (such as lasers, LEDs, cameras, etc.), but they are limited in functionality by their particular set of material properties. In particular, traditional crystalline inorganic semiconductors are not well suited for large area or flexible electronics applications. This limitation has driven the development of a new generation of thin-film semiconductor materials. Thin-film semiconductor materials are ideal for solar energy conversion because they have the potential for cost effective large area fabrication. Promising next generation photovoltaic devices have been demonstrated using organic photovoltaics (OPVs) and dye-sensitized solar cells (DSSCs). OPVs are a promising technology because they are inexpensive, strong absorbers. However, it is challenging to produce efficient OPVs due to the excitonic nature and poor charge transport characteristic of these materials. DSSCs are hybrid devices that combine the strong absorption of organic semiconductors with the good charge transport of inorganic metal-oxide semiconductors. Devices based on thin-film, low mobility, excitonic materials are governed by fundamentally different physical processes than traditional inorganic devices. In this thesis, we develop physical models to analyze the performance of devices based on organic/organic and hybrid organic/inorganic heterojunctions (HJs). These models are based on interface dynamics at the HJ and are used to identify the physical processes that limit device performance. Specifically, we extend the interface model to understand: 1) reciprocal carrier collection in OPVs, 2) photoconductivity in OPVs, 3) the adaptation of traditional depletion models to thin-film devices, and 4) space-charge effects in hybrid devices. To model hybrid devices, we introduce a theory that bridges the gap between traditional semiconductor theory and models developed to explain thin-film excitonic systems. Once the important physical processes are understood, we can proceed to design optimized devices. In the last section we consider the application of organic and hybrid devices for flexible or non-planar devices. Here, we develop technologies to enable the fabrication of high-performance devices and high-density circuits on flexible and non-planar substrates. We then demonstrate an integrated passive pixel photodetector array and discuss the extension to a high-performance hybrid sensor array.PHDApplied PhysicsUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/107245/1/krenshaw_1.pd

Organic Semiconductor Detector for Large Area Digital Imaging

Organic semiconductor technology has gained attention in both the sensor and display markets due to its low cost and simple fabrication techniques. The ability to fabricate organic semiconductor devices such as photodetectors and transistors on a flexible, lightweight substrate makes them less fragile and ideal candidates for portable large-area imaging applications. The use of organic semiconductor technology in large-area medical imaging can bring about a new generation of flexible and lightweight indirect X-ray imagers. These imagers are immune to mechanical shock and should be ideal for portable intraoral X-ray radiology. In order to realize these organic flexible imagers and their use in large-area medical imaging, many challenges associated with the device performance and fabrication need to be overcome. Among these challenges, one of the greatest is to improve the dark current performance of the organic semiconductor photodetectors (key for imager performance) with a high-photo to-dark current ratio. Low dark current is needed to improve the sensitivity of the imager, whereas a large photo-to-dark current ratio reduces noise in the extracted image. Numerous techniques have been reported to improve the dark current performance in vertical organic photodetector design; however, lateral photodetectors still lack research attention. This thesis presents a lateral multilayer photodetector design and a simplified technique to improve the dark current performance of lateral organic semiconductor photodetectors. Our technique allows us to apply a large bias voltage while maintaining a low dark current, high photo-to-dark current ratio, and improves detector speed; thus, the overall sensitivity of the detector is improved. We further show the integration of an organic photodetector with an organic backplane readout circuit to form a flexible large-area imager. This imager can be used for large-area digital imaging applications such as in medical radiology.4 month

Energy Transport in Organic Photovoltaics.

Organic photovoltaics (OPV) have the potential to be a flexible and low-cost form of carbon-neutral energy production. However, many of the underlying physical mechanisms that dictate the behavior of OPVs remain frustratingly obscure in comparison to the well-understood physics of inorganic semiconductors. This dissertation centers around the development of new techniques to characterize the behavior of excitons in organic semiconductors, both in the bulk and at interfaces. We first examine the method of spectrally-resolved photoluminescence quenching (SR-PLQ), the most convenient and powerful current technique for measuring the exciton diffusion length (LD) of organic semiconductors, and extend it to work with optically thin films. This allows for its application to a much wider range of materials and physical systems. The second part of the dissertation presents a further extension of the method of PL quenching to characterize non-ideal interfaces, those which block or quench only a fraction of incident excitons. This is used to understand the operation of a novel fullerene:wide energy gap material buffer in OPVs. In combination with charge transport and morphological studies, it is shown that the mixed buffer shows disproportionate benefits from the two materials; blocking excitons superlinearly with wide energy gap material concentration and still conducting charges efficiently even at very small (10%) fullerene concentration. Finally, we extend the principles of PL quenching to characterize arbitrary interfaces, including those between materials with identical energy levels but different LD and exciton lifetime, and those between materials with small (~20 meV) energy offsets. These techniques allow us to finally resolve the ambiguity in the spin state of the exciton which serves as the primary source of photocurrent in C60, one of the most important materials in current efficient OPVs.PhDPhysicsUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/113547/1/kjberg_1.pd

Investigation Of The Spatial Dependence Of Carrier Dynamics In Semiconductor Optoelectronic Devices

Optoelectronic devices, such as solar cells and photodetectors, rely on separating and transporting charge through semiconductors that have excess electron-hole pairs created by photon absorption. This talk will focus of several different semiconductor materials of various dimensionality, such as lead halide perovskites, wide bandgap SiC, graphene, semiconductor nanowires (NWs), and quantum dots (QDs). The results of Chapter 2 demonstrate the dependence of the perovskite/hole transport layer on carrier transport of perovskite films for solar cells. Efficient charge separation at the interfaces of the perovskite with the carrier transport layers is crucial for perovskite solar cells to achieve high power conversion efficiency. A systematic experimental study on the hole injection dynamics from MAPbI3 perovskite to three typical hole transport materials (HTMs) is discussed Graphene layers grown epitaxially on SiC substrates are attractive for a variety of sensing and optoelectronic applications because the graphene acts as a transparent, conductive, and chemically responsive layer that is mated to a wide-bandgap semiconductor with large breakdown voltage. Recent advances in control of epitaxial growth and doping of SiC epilayers have increased the range of electronic device architectures that are accessible with this system. In particular, a recently introduced Schottky-emitter bipolar phototransistor (SEPT) based on an epitaxial graphene (EG) emitter grown on a p-SiC base epilayer has been found to exhibit a maximum common emitter current gain of 113 and a UV responsivity of 7.1 A W−1. In Chapter 3, the sub- bandgap performance of the device is addressed, and a visible rejection ratio is calculated. Additionally, scanning photocurrent microscopy (SPCM) shows the localized effects of the photocurrent and the presence of an 8H- stacking fault. A new device fabricated with a thinner base region and SiF4 mediated EG growth process will be studied in Chapter 4. The spatial response of the photocurrent allows for determination of the visible rejection ratio, as well as a model of how generated carriers interact within the device. Nanoscale optoelectronic devices of semiconductor CdS nanowires (NWs) and PbS quantum dots (QDs) are investigated in Chapter 5. The fabrication techniques, responsivities, on/off ratio, and spatial dependence of the devices will be discussed