KEYWORDS: Interfaces, Copper indium gallium selenide, Crystals, Thin films, Process control, Thin film solar cells, Manufacturing, Thin film manufacturing, Thin film devices, Control systems
Optimizing the buffer layer in manufactured thin-film PV is essential to maximize device efficiency. Here, we describe a combined synthesis, characterization, and theory effort to design optimal buffers based on the (Cd,Zn)(O,S) alloy system for CIGS devices. Optimization of buffer composition and absorber/buffer interface properties in light of several competing requirements for maximum device efficiency were performed, along with process variations to control the film and interface quality. The most relevant buffer properties controlling performance include band gap, conduction band offset with absorber, dopability, interface quality, and film crystallinity. Control of an all-PVD deposition process enabled variation of buffer composition, crystallinity, doping, and quality of the absorber/buffer interface. Analytical electron microscopy was used to characterize the film composition and morphology, while hybrid density functional theory was used to predict optimal compositions and growth parameters based on computed material properties.
Process variations were developed to produce layers with controlled crystallinity, varying from amorphous to fully epitaxial, depending primarily on oxygen content. Elemental intermixing between buffer and absorber, particularly involving Cd and Cu, also is controlled and significantly affects device performance. Secondary phase formation at the interface is observed for some conditions and may be detrimental depending on the morphology. Theoretical calculations suggest optimal composition ranges for the buffer based on a suite of computed properties and drive process optimizations connected with observed film properties.
Prepared by LLNL under Contract DE-AC52-07NA27344.
We report preliminary studies of the nature of hole injection from poly(3,4-ethylenedioxythiophene)/polystyrenesulphonic acid (PEDOT:PSS) into three commercial conjugated light emitting polymers (LEPs). Sumation's LUMATION Green 1300, LUMATION Blue, and Merck's SuperYellow LEPs are studied in combination with interlayers of poly(9,9-dioctyl-fluorene-co-N-(4-butylphenyl)-diphenylamine) (TFB), and poly[9,9-dioctylfluorene-co-(bis-N,N'-(3-carboxyphenyl)-bis-N,N'-phenylbenzidine)] (BFA). Despite the highest occupied molecular orbitals (HOMOs) of the interlayers being close to that of PEDOT:PSS and the LEP, different interlayers have different effects on hole injection and OLED device performance. We use dark injection transient current method to show that interfacial morphology changes results in modulation of hole trap densities that in turn affect hole injection. Depending on the interlayer/LEP combination partial penetration of interlayer into the LEP layer may also occur resulting in additional changes in the bulk transport properties of the LEP. Our results show that it is not the interfacial energy level alignment but the physical morphology changes at the interface which are important for varying hole injection into the device. A combination of either improved or reduced hole injection due to variations in physical contact, intermixing and trapping at the interlayer/LEP boundary dominate device performance.
The transport of charge carriers in polymer-based Organic Light-Emitting Diodes (OLEDs) as determined by the hopping mobility is an important factor influencing both lifetime and performance of OLED devices. It is strongly dependent on the density and energetic distribution of trap states in the polymer material. Especially in multi-component copolymers single functional groups can act as hole or electron traps determining the optical and electrical characteristics of the device. Transient measurements of the charge carrier mobility together with steady-state current-voltage characteristics are used to investigate the behavior of three blue polyspiro-based light-emitting polymers (LEP) with varying compositions. The first material is a simple homopolymer, the second adds a hole transporting component which is copolymerized into the backbone and the third, most complex, additionally includes a blue chromophore. With some of the added components acting as charge carrier traps the electrical behaviour of the diodes changes significantly.
We report for the first time the bipolar transport properties of the LUMATION* 1300 Series green emitting polymer investigated by means of admittance spectroscopy. Analysis of the inductive response in single carrier polymer diodes yields electron and hole mobilities which are in excellent agreement with the results of independent measurements. Admittance measurements in dual injection diodes provide evidence that the dual injection diodes operate in spacecharge-limited regime, indicative of strong recombination within the material. Our results provide strong evidence that the space-charge-related admittance response of dual carrier diodes is dominated by combined electron-hole response, which corresponds to the sum of electron and hole mobilities. This implies that electron and hole mobilities cannot be obtained separately from admittance measurements in space-charge-limited dual carrier devices.
Experimental studies of charge injection and transport of holes and electrons in LUMATIONTM Green 1300 Series Light-Emitting Polymer (LEP) by dark injection space-charge-limited technique are performed. It is found that hole mobility is lower than electron mobility and the former exhibits steeper electric field dependence thus reducing the disbalance between charge mobilities at higher device operating voltages. Electron current is affected by trapping, mainly due to deep traps prevailing at low electric fields and with an estimated concentration of 1016 cm-3. Hole current
is affected both by trapping and injection limitation, with the trapping being approximately independent of electric field and injection efficiency increasing with increasing electric field. Electron trapping is found to be significantly reduced in dual carrier devices, which is believed to be the effect of faster exciton formation and recombination rates, compared to electron trapping processes.
Here we report the observation of ohmic hole injection from a conducting polymer anode into poly(9,9-dioctylfluorene) (PFO) in a polymer light-emitting diode (LED) structure. Although initially non ohmic, the contact can be made locally ohmic by electrically conditioning the device at voltages higher than the electroluminescence (EL) onset voltage. The ohmic nature of the contact in selected regions is confirmed by the appearance of dark injection space-charge-limited transient currents, which yield hole mobilities in good agreement with those measured by the time-of-flight method. The appearance of ohmic injection is discussed within a model that assumes the existence of electron traps near the anode interface. When the sample is conditioned electrons are injected from the cathode and are trapped near the anode inducing an interfacial dipole that reduces the barrier for hole injection.
We study the charge recombination kinetics and photovoltaic performance of composites of poly (9,9-dioctylfluorene-co-bithiophene) polymer with nanocrystalline TiO2. Transient optical spectroscopy confirms that photoexcitation of the polymer leads to electron transfer to the TiO2 and indicates that charge recombination is slow with a half-time of 100 μs to 10ms. Polymer penetration into thick porous TiO2 layers is improved by melt-processing and treatment of the TiO2 surface. We study the photovoltaic characteristics of devices with different layer thickness and interface morphology. Quantum efficiency (QE) of all devices is increased by reducing the TiO2 and polymer layer thickness. Inserting a thin porous TiO2 layer in to a thin bi-layer device increases the QE by a factor of five. The improved device shows peak QE and monochromatic power conversion efficiencies of over 11% and 1% at 440nm respectively. The device produced a short-circuit current density of 300μAcm-2, a fill factor of 0.24 and an open-circuit voltage of 0.8V under AM1.5 illumination. The fill factor is increased from 0.24 to 0.40 by introducing an additional dip-coating layer and overall power conversion efficiency is increased by 50%. However, the device produced degraded current-voltage characteristics. We investigate this using an alternative polymers and different top contact metals.
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