Hybrid organolead trihalide perovskite (OTP) solar cells have developed as a promising candidate in photovoltaics due to their excellent properties including a direct bandgap, strong absorption coefficient, long carrier lifetime, and high mobility. Most recently, formamidinium (NH2CH=NH2+ or FA) lead iodide (FAPbI3) has attracted significant attention due to several advantages: (1) the larger organic FA cation can replace the MA cation and form a more symmetric crystal structure, (2) the smaller bandgap of FAPbI3 allows for near infrared (NIR) absorption, and (3) FAPbI3 has an elevated decomposition temperature and thus potential to improve stability. Single crystals provide an excellent model system to study the intrinsic electrical and optical properties of these materials due to their high purity, which is particularly important to understand the limits of these materials.
In this work, we report the growth of large (~5 millimeter size) single crystal FAPbI3 using a novel liquid based crystallization method. The single crystal FAPbI3 demonstrated a δ-phase to α-phase transition with a color change from yellow to black when heated to 185°C within approximately two minutes. The crystal structures of the two phases were identified and the PL emission peak of the α-phase FAPbI3 (820 nm) shows clear red-shift compared to the FAPbI3 thin film (805 nm). The FAPbI3 single crystal shows a long carrier lifetime of 484 ns, a high carrier mobility of 4.4 cm2·V-1·s-1, and even more interestingly a conductivity of 1.1 × 10-7(ohm·cm)-1, which is approximately one order of magnitude higher than that of the MAPbI3 single crystal. Finally, high performance photoconductivity type photodetectors were successfully demonstrated using the single crystal FAPbI3.
In the current study, the perovskite absorber (CH3NH3PbI3) is processed via one-step deposition employing the small molecule additive, BmPyPhB, which can be dissolved in dimethylformamide along with precursors. Here, 1,3-Bis[3,5-di(pyridin-3-yl)phenyl]benzene (BmPyPhB) functions as the morphology controller to introduce an intermediate phase during perovskite film growth, which allows well-defined and precrystallized domains formed before the annealing treatment. Furthermore, a chloroform solvent wash procedure is applied afterward to remove BmPyPhB from perovskite without damaging the predetermined morphology. Thus, postannealing as low as 100°C for 5 min can achieve the optimal power conversion efficiency of 8% in a planar-structured inverted solar cell.
Tandem solar cells provide an effective way to harvest a broader spectrum of solar radiation by combining two or more
solar cells with different absorption ranges. However, for polymer solar cells (PSCs), the performance of tandem devices
lags behind single-layer cells mainly due to the lack of a high-performance low-bandgap polymer with appropriate
spectral response range. Here, we demonstrate a novel low bandgap conjugated polymer (~1.44 eV) specifically suitable for tandem structure. In the single-layer device, power conversion efficiency (PCE) of 6.5% was achieved. When the polymer was applied to tandem solar cells, we demonstrated a NREL certified PCE of 8.62% . Further optimization on materials and devices of this system has lead to record breaking efficiency of 10.6%. Furthermore, the tandem devices show excellent stability due both to the intrinsic stability of the polymer and the advanced device structure.
Organic solar cells (OSCs) have been extensively studied and significant improvements have been demonstrated in
recent years. Along with the excitement in technology development, the accurate measurement of OSCs has become
critical for the healthy development of this promising technology. The limited absorption and spectral response of
organic based solar cells could lead to significant derivation in solar cell measurement. In this paper, we will discuss
several issues in the measurement of organic solar cells, including spectral mismatch factor, elimination of the
mismatch by proper selection of reference cell, external quantum efficiency testing, device area issue etc. Results on
both polymer based bulk hetero-junction solar cell and small molecule based solar cell will be presented.
Interface and its engineering are critical to achieve efficient organic light-emitting diodes (OLEDs). In this presentation,
we would like to present a new type of electron transport layer (ETL) based on metal oxide between emissive layer and
metal electrode. The metal oxide synthesized by sol-gel process and can be solution processed on top of the emissive
layer. The efficiency of OLEDs based on green polyfluorene polymer has found increased significantly, from 15
lumen/watt to 19 lumen/watt compared with our previous cathode structure of Cs2CO3/Al. The electrochemical
properties and electronic structure of the ETL, and energy alignment at the interface are examined to understand the
function of this ETL and the contribution to the improvements for device performances. We will present the details of
the analysis and the composition of the materials in the presentation.
The effect of side-chains on the molecular weight and the optical and electrical property of a low band gap copolymer
was studied. The decyloxy side-chains help to increase molecular weight (Mw = 115,000) and decrease the band gap
(1.78 eV) as well as the oxidation potential (-5.4 eV). Zero-field mobility of 2×10-5 cm2/Vs is measured in hole-only devices. Photovoltaic devices based on PF-co-DTB/fullerene bulk-heterojunction show power conversion efficiency of
up to 1.6% under air mass 1.5G, 100 mW/cm2 illumination. Side-chains effect on the photovoltaic devices studies show
the trade-off between short circuit current increase and open-circuit voltage drop. Thermal annealing on device
performance is also discussed.
To increase the absorption of sunlight in polymer solar cells a large active layer thickness is desired. This, however, is
limited by the short charge carrier diffusion lengths in the active organic materials. Efficient light harvesting can be
achieved in organic solar cells by using a tandem structure. However, fabricating a tandem structure for polymer solar
cells presents its own difficulties. Since the polymer film is solution processed, spin-coating multiple layers in tandem
can result in significant damage to the underlying layers. This problem can be overcome by fabricating separate PV cells
and stacking them in tandem. Here, we report a multiple-device stacked structure where two polymer photovoltaic cells
are stacked together with the help of a multi-layer semi-transparent electrode, made of lithium fluoride (LiF) / aluminum
(Al) / gold (Au) metal layers. The semi-transparent electrode is used as the top contact in the bottom cell to efficiently
transmit the unabsorbed photons to the upper cell. Maximum transparency of up to 80% is achieved for the semitransparent
cathode. In the stacked structure, the open circuit voltage and the short circuit current are twice those of a
single cell. As a result, power conversion efficiency of up to 2.6% is achieved, which is double than that of a single cell.
The concept of tandem organic light-emitting devices (OLEDs) provides a pathway for developing highly stable and efficient OLEDs. The connecting structure that bridges adjacent light-emitting units, substantially affects the device performance of tandem OLEDs. In this letter, we introduce an effective connecting structure in which an ultrathin middle metal layer is sandwiched between efficient electron- and hole-injection layers for the tandem OLEDs, which in essence, avoids the use of reactive metals during fabrication. Two-unit tandem OLEDs with such connecting structure exhibit less than double the driving voltage, yet more than double the efficiency, more saturated emission color, and longer operational lifetime compared to those of single-unit devices. A model based on a hypothesis of energy level pinning effect has been proposed as the mechanism of the connecting structure in the tandem devices. This model is also consistent with the results obtained from the photovoltaic effect measurements in tandem OLEDs.
Recently conjugated polymers and conjugated organic molecules have drawn a great deal of attention, since they are uniquely suited for thin film, large area, mechanically flexible devices. On the other hand, polymer/inorganic nanocomposite have also been pursued to deliver unique electronic properties in various device applications such as organic light-emitting diodes, organic thin film transistors, and solar cells. Here we demonstrate a nanocomposite based on polyaniline nanofibers decorated with gold nanoparticles and apply this composite into memory devices. The electronic property shows an electric bistable effect in a two terminal sandwiched structure. These two bistable states have different conductivities by three orders of magnitude. The mechanism is likely involving electric-field induced charge transfer between the polymer and nanoparticles. This nanocomposite material provides a unique functionality and possibility to open a new direction for future organic electronics.
A capacitor that basically consists of two parallel metal plates with dielectric materials in between, can store charge at the two inner surfaces of the metal plates when a voltage bias is applied. As the thickness of the metal-plate electrodes decrease to nanometers range, we find an interesting physical phenomenon, i.e., the stored charges can modify the physical properties of the semiconductor layer coated on top of it. This discovery leads us to demonstrate a whole-new concept field-effect transistor, a vertical organic transistor with a novel device structure by stacking gate-source-drain vertically. This vertical stack consists of an active cell (drain/organics/source) on top of a capacitor cell (source/dielectrics/gate). When the gate (capacitor) is biased, charges are stored in the capacitor cell. As the thickness of the source electrode is thin enough, typically in the nanometer range, the active cell can also sense the stored charges within the capacitor cell, and, subsequently, modulate the charge injection from the source into the organics. This unique device structure provides an extremely short "channel length" and large channel conduction area. As a result, we achieved organic transistors with low working voltage (less than 5 V), high current output (up to 10 mA or 4 A/cm2), and high ON/OFF ratio (up to 4×106). This device solves two long-standing issues with organic transistors, high working voltages and low current output. This novel device with its enhanced operating characteristics opens new directions for organic transistors and their applications. The device operation mechanism is different from traditional field effect transistors, where we proposed an injection-controlled mechanism, which can basically explain the observed electrical phenomenon of our vertical transistors.
A high-performance organic diode is demonstrated by using C60 sandwiched between a cathode and an anode using metals with different diffusivity and donor ability. In this manuscript,copper (Cu)and aluminum (Al)are selected as the cathode and anode, respectively. C60 is used as the organic electron-acceptor for its high stability and high carrier mobility. The as-prepared diode shows poor performance.However,after heat
treatment, the Cu/C60 interface becomes an Ohmic contact through Cu diffusion and charge-transfer processes,allowing highly efficient electron injection from the Cu electrode. On the other hand, a rectified C60/Al contact is formed, prohibiting efficient electron injection from the Al electrode into C60. Hence,a high-performance organic diode is formed through a heat treatment process, not by the selection of metals with
different work functions. Due to the high mobility of C60, the device shows megahertz frequency response, and it can also handle rather high current density (363 A/cm2 at 2.4V). This opens the way for the formation of high-performance organic electronic devices.
In this manuscript, we report on the successful fabrication of high performance polymer light emitting diodes (PLEDs) using a low temperature, plastic lamination process. Blue- and red-emitting PLEDs were fabricated by laminating different luminescent polymers and organic compounds together to form the active media. This unique approach eliminates the issue of organic solvent compatibility with the organic layers for fabricating multi-layer PLEDs. In addition, a template activated surface process (TAS) has been successfully applied to generate an optimum interface for the low temperature lamination process. The atomic force microscopy analysis reveals a distinct difference in the surfaces created by the TAS and the spin-coating process. This observation coupled with the device data confirms the importance of the activated interface in the lamination process.
We present a successful demonstration of controllable patterning of dual-color polymer light-emitting pixels using a hybrid inkjet printing technique. In this demonstration, the polymer buffer layer is a wide bandgap, blue emitting semiconducting polymer (PPP-NRt3+), prepared by the spin-casting technique. The inkjet printed layer is a red-orange semiconductor polymer, (MPS-PPV) which was printed onto the buffer layer.When a proper solvent was selected, MPS-PPV diffused into the buffer layer and efficient energy transfer took place from the PPP-NEt3+ to the MPS-PPV generating a red-orange photoluminescence and electroluminescence from the inkjet printed sites. Based on this principle, blue and orange-red dual-color polymer light-emitting pixels were fabricated on the same substrate. The use of this concept represents an entirely new technology for fabricating polymer multicolor displays with high-resolution, lateral patterning capability.
Ink-jet printing (IJP) technology is a popular technology for desktop publishing. Since some of the conducting polymers are solution processable, IJP technology becomes an ideal method for printing polymer light-emitting diodes with high resolution. In this manuscript, we present the first successful demonstration of patterning the polymer electroluminescent devices using the IJP technology. Unfortunately due to the dot form printing by the IJP, the polymer film printed from an ink-jet printer consists of pin-holes. This makes it unsuitable for fabricating high quality polymer electronic devices, particularly for devices in the sandwich structure. In this paper, we submit a hybrid structure, which consists of an ink-jet printed layer in conjunction with another uniform spin coated polymer layer, as an alternative to the regular ink-jet printed structure. The uniform spin coated polymer layer, as an alternative to the regular ink-jet printed structure. The uniform layer serves as a buffer layer to seal the pin hoe.s and the IJP layer is the layer consisting of the desired pattern, for example the red-green-blue dots for a multicolor display. To demonstrate, we applied this hybrid technology to fabricate efficient and large area polymer light-emitting logos. The use of this concept represents a whole new technology of fabricating polymer electronic device with lateral patterning capability.
An array of polymer grid triodes (PGTs) connected through a common grid functions as a 'plastic retina' which provides local contrast gain control for image enhancement. This device, made from layers of conducting polymers, functions as an active resistive network that performs center-surround filtering. The PGT array with common grid is a continuous analog of the discrete approach of Mead, with a variety of fabrication advantages and with a significant saving of 'real estate' within the unit cell of each pixel.