Search

We report a trilayer hybrid polymer-quantum-dot light-emitting diode fabricated by sandwiching a CdSe-ZnS core–shell quantum-dot (QD) layer, a few monolayers thick, between films of polyvinylcarbazole (PVK) and an oxadiazole derivative (butyl-PBD). All the layers have been deposited by a controlled spin-coating technique. Stable aqueous dispersion of QDs has been prepared to make possible the spin coating of multiple layers without affecting the layer underneath. Our device shows 20 times the quantum efficiency (0.2%) and less than half the threshold voltage (5 V) of a single-layer device made from the PVK-QD-PBD blend. This is attributed to balanced carrier conduction, enhanced recombination, and reduced quenching of emission due to a better electro-optical device design.

DTS-(FBTTh2)2 is a prominent solution-processable small-molecule donor for donor-acceptor bulk-heterojunction organic photovoltaics. Power conversion efficiency of DTS-(FBTTh2)2 based photovoltaic devices exceeds 8%. This paper reports on the distribution of sub-bandgap trap states in DTS-(FBTTh2)2. Trap states were probed using admittance spectroscopy and low-frequency capacitance-voltage profiling and analyzed using established theoretical models. Three distributions were revealed in the trap density of states energy spectra. Key observations were (1) thicker solution-processed films with higher drying time had 55% less traps than thinner films that dried relative faster (2) blending of DTS-(FBTTh2)2 with the acceptor PC70BM introduced traps at the center of the donor-acceptor interfacial bandgap. Charge carrier dynamics in DTS-(FBTTh2)2 based thin-films was also characterized using impedance spectroscopy.

Luminescence of amorphous Si quantum dots (Si QDs) in a hydrogenated silicon nitride (SiNx:H) matrix was examined over a broad range of stoichiometries from Si3N2.08 to Si3N4.14, to optimize light emission. Plasma-enhanced chemical vapor deposition was used to deposit hydrogenated SiNx films with excess Si on Si (001) substrates, with stoichiometry controlled by variation of the gas flow rates of SiH4 and NH3 gases. The compositional and optical properties were analyzed by Rutherford backscattering spectroscopy, elastic recoil detection, spectroscopic ellipsometry, photoluminescence (PL), time-resolved PL, and energy-filtered transmission electron microscopy. Ultraviolet-laser-excited PL spectra show multiple emission bands from 400 nm (3.1 eV) to 850 nm (1.45 eV) for different Si3Nx compositions. There is a red-shift of the measured peaks from ∼2.3 eV to ∼1.45 eV as Si content increases, which provides evidence for quantum confinement. Higher N content samples show additional peaks in their PL spectra at...

Organic photovoltaics (OPVs) are regarded as promising for solar-electric conversion with steadily improving power conversion efficiencies. For further progress, it is crucial to understand and mitigate defect states (traps) residing in the band-gap of OPV materials. In this work, using capacitancemeasurements, we analyzed two major bands in the density of states (DOS) energy spectra of defects in poly(3-hexylthiophene) (P3HT); regio-regular and regio-random species of P3HT were compared to elucidate the role of morphological disorder. To accurately interpret the obtained DOS profile, trap emission prefactors and activation energy were extracted from temperature dependent capacitance-frequency measurements, while doping, Fermi energy, built-in voltage, and energy levels of the defects were extracted from capacitance-voltage measurements. We identified that the lower energy band—misinterpreted in literature as a defect distribution—stems from free carrier response. The higher energy defect distribution band for regio-random P3HT was an order of magnitude higher than region-regular P3HT, thus stemming from morphological disorder. Impedance spectroscopy was also employed for further comparison of the two P3HT species.

Defect levels play a significant role in altering organic photovoltaic (OPV) performance, affecting device aspects such as recombination, carrier transport, and Fermi-level pinning. In the ongoing effort to optimize the promising OPV technology, the identification, characterization, and potential mitigation or enhancement of such defect states remain important regions of interest. Herein, low frequency admittance spectroscopy is coupled with a high frequency, point-by-point capacitance versus voltage measurement to reveal a previously unknown deep-defect distribution in poly(3-hexylthiophene) based OPVs. The capacitance models of Cohen and Lang, Walter et al. and Kimmerling are employed alongside a trap-free dark current model to give good characterization and substantiation to the discovered band. Repetitions of the measurements on devices with and without a fullerene acceptor show the measured distribution to contain acceptor-like traps spatially located in the polymer bulk. The findings presented here are important for the understanding and optimization of organic solar cells and we expect the presented methods to be generally applicable to other OPV material sets.

Related Articles Morphological characterization of plastic solar cells using polarized neutron reflectivity Appl. Phys. Lett. 102, 083305 (2013) Morphological characterization of plastic solar cells using polarized neutron reflectivity APL: Org. Electron. Photonics 6, 40 (2013) A multiscale modeling study of loss processes in block-copolymer-based solar cell nanodevices J. Chem. Phys. 138, 094901 (2013) Highly efficient inverted polymer solar cells with a solution-processable dendrimer as the electron-collection interlayer APL: Org. Electron. Photonics 6, 37 (2013) Highly efficient inverted polymer solar cells with a solution-processable dendrimer as the electron-collection interlayer Appl. Phys. Lett. 102, 083302 (2013)

The bulk heterojunction (BHJ) nanomorphology in organic solar cells strongly affects the final efficiency of the device. Progress in experimental techniques now allows visualization of the complex 3D BHJ morphology. It is, therefore, important to characterize the topological properties of the morphology in order to quantify the link between morphology features and performance. Here, we introduce a suite of morphology descriptors which encode the complex nature of the multi-stage photovoltaic process in the BHJ. These morphology descriptors are easily determined using an approach based on converting the morphology into an equivalent weighted, labeled, undirected graph. We show how these descriptors can be used to interrogate BHJ morphologies, allow identification of bottlenecks in the photovoltaic process, and conduct quantitative comparison between morphologies with respect to each sub-process in the photovoltaic phenomena. This framework provides a simple and easy-to-use characterization tool that can be used to unravel the impact of morphology on complex transport phenomena.

Capacitance measurements, widely used to characterize numerous semiconductor properties, have been recently adopted to characterize organic photovoltaic (OPV) devices. It is known that certain challenges are associated with capacitance measurements. Of upmost importance is the employment of a proper measurement model (series or parallel). Owing to larger capacitive impedances and low series resistances, the parallel model is typically employed in inorganics. However, we find that for characteristically thinner organic films, a hybrid model should be used. We highlight the inconsistencies in OPV literature due to indiscriminate usage of parallel model and show how proper model selection can rectify any artifacts.

Charge carrier recombination due to carrier trapping is not often considered in polymer based solar cells, except in those using non-fullerene acceptors or new donor polymers with limited short-range order. However, we show that even for the canonical poly(3-hexylthiophene): phenyl-C61-butyric acid methyl ester (P3HT:PCBM) system, relative strengths of bimolecular and trap-assisted recombination are strongly dependent on processing conditions. For slow-grown active-layers, bimolecular recombination is indeed the major loss mechanism under one sun illumination. However, for fast-grown active-layers, trap-assisted recombination dominates over bimolecular recombination by an order of magnitude, and recombination strength at short-circuit condition is 3-4 times higher, leading to loss of photocurrent and lowering of fill factor.

To understand the effect of processing conditions such as spin coating speed and drying rate on the density of defects; poly(3-hexylthiophene):fullerene-derivative solar cells A, B, and C were fabricated with solvent drying times of ∼40 min, 7 min, and 1 min, respectively. We show that slowest grown device A has one order of magnitude less subband gap traps than device C. The open circuit voltage and its light intensity dependence was strongly affected by interfacial recombination of carriers at subgap defect states. The losses due to trap-assisted recombination can even dominate over bimolecular recombination, depending on the density of defect states