Temperature-dependent Current Density-voltage Characteristics Research Articles

Effect of Diffusion on Charge Transport in Organic Photovoltaic Blends

Charge transport is one of the key factors in the operation of organic photovoltaic devices. Non-fullerene acceptor materials have recently attracted significant attention in organic photovoltaics due to their great potential to realize high power conversion efficiencies. In this paper, we investigate the effect of diffusion on charge transport in a variety of binary and ternary organic photovoltaic materials, including both polymer: fullerene and polymer:non-fullerene blends. It is shown that the temperature dependent current density-voltage characteristics from the drift-diffusion simulations incorporating the extended Gaussian disorder model (EGDM) are more consistent with experimental data in comparison with those obtained from the only drift model for all these material systems. Furthermore, it is found that the effect of diffusion on charge transport is more pronounced at low voltages and seems to be negligible when the applied voltage exceeds 1 V. The deviation of calculated curves from experimental measurements gradually increases with increasing temperature. It is of great importance to the influence of diffusion effect on charge transport in both polymer:fullerene and polymer:non-fullerene blends.

Phase evolution during annealing of low-temperature co-evaporated precursors for CZTSe solar cell absorbers

Systematic investigations into the phase evolution during reactive annealing of copper–zinc–tin–selenide (CZTSe) precursors for the fabrication of kesterite solar cell absorber layers have been paramount in understanding and suppressing the formation of secondary phases that deteriorate device performance. In this study, the phase evolution during annealing of low-temperature co-evaporated CZTSe precursors is investigated. A detailed analysis of films selenized at different temperatures is used to reveal the possible reaction pathway of CZTSe formation. Utilizing a combination of x-ray diffraction, Raman spectroscopy, scanning electron microscopy, transmission electron microscopy, and energy-dispersive x-ray spectroscopy, it is shown that CZTSe formation starts by Cu out-diffusion to the surface and Cu–Se phase formation at a temperature of 350 °C. An intimate mixing of binaries and ternaries during low-temperature selenization is observed. On the contrary, only binaries are observed at high-temperature selenization. This suggests that the CZTSe formation pathway involves reaction schemes where (i) a competition between binary and ternary phases dominates at low-temperature and (ii) binary reactions dominate the process at high temperatures. However, the number of binary phases decreases with increasing selenization temperature until they become undetectable by XRD and Raman spectroscopy at a temperature of 540 °C (selenization time 10 min). Utilizing the presented selenization conditions, prototype solar cells with an efficiency of up to 7.5%, an open-circuit voltage of 407 mV, and a fill factor of 59%, could be demonstrated. The temperature-dependent current density–voltage characteristics indicate that the performance of the prototype devices is limited by bulk Schottky–Read–Hall recombination.

Organic heterojunctions of phthalocyanine-reduced graphene oxide above percolation threshold for photovoltaic application

In this article, non-fullerene, cost effective, and highly soluble zinc tetra-tert-butyl phthalocyanine-reduced graphene oxide (ZnTTBPc-RGO) heterojunction have been used as an active layer in organic photovoltaic devices. The RGO content in the active material is varied from 0.25–0.5 volume fractions which are above the percolation threshold of 0.167 volume fraction of RGO. Temperature dependent current density–voltage characteristics show a transition from Ohmic conduction to space charge limited current (SCLC) conduction at higher applied bias voltages (≥1 V). This enables us to extract charge carrier mobility values of the heterojunction at different temperatures using a trap-free SCLC model. A donor–acceptor solution processable photovoltaic device with 0.33 vol.% RGO content of active layer show the highest power conversion efficiency of 1.02%, highest recombination resistance of 48.65 kΩ, and maximum carrier mobility of 4.32 × 10−4 cm2V−1s−1 among the three devices studied. 80 h exposure to ambient conditions see a ∼40% decay of power conversion efficiency. Quantitative analysis on the density of trap states indicates that the best power conversion efficiency and the highest charge carrier mobility correlate well with a minimum of the trap density and a maximum of recombination resistance for these devices.

Temperature and dopant dependence of hole transport in a green light emitting polyspirobifluorene polymer

In the present work, hole transport in a green light emitting polyspirobiflorene polymer has been studied. We have realized better hole injection by using tetra-fluoro-tetracyano-quinodimethane hole injection layer in the polymer. The temperature dependent current density–voltage characteristics of polyspirobifluorene thin films have been investigated. Hole mobility has been calculated by direct current space charge limited conduction method. Further, the charge transport of the interface modified polymer has been considered as a function of doping concentrations. The current density has been found to increase with the doping concentrations. The conductivity of the polyspirobifluorene has also been calculated at room temperature and found to be increased with the dopant concentrations. The present study opens a new way to improve the optoelectronic devices.

Time-resolved photoluminescence of Cu(In,Ga)(Se,S)2 thin films and temperature dependent current density-voltage characteristics of their solar cells on surface treatment effect

Influences of the surface treatments of Cu(In,Ga)(Se,S)2 (CIGSSe) thin films, which are KCN, HCl, or thiuorea treatments, were investigated by time-resolved photoluminescence (TRPL) and temperature dependent current density-voltage (J-V) characteristics of their solar cells. It is demonstrated that the KCN treatment optimized under 1 wt% leads to the significant increase in conversion efficiency (η) up to 19.21%. On the other hand, the η of the CIGSSe solar cells is in ranges of 13.70–15.51% and 9.86–10.70% with the HCl treatments (0.3–0.7 mol/L), and thiuorea treatments (0.5–1.5 mol/L), respectively, which are lower than 16.66% that of the reference solar cell without the surface treatment. According to TRPL measurements, the quality of near-surface CIGSSe is improved with the KCN treatment (1 wt%) owing to enhanced TRPL lifetimes, whereas that is deteriorated with the HCl and thiuorea treatments due to decreased TRPL lifetimes. In addition, according to the temperature-dependent J-V measurement, the interface recombination of the CIGSSe solar cell is decreased with the KCN treatment, while that of the CIGSSe solar cells is increased with the HCl and thiuorea treatments. Ultimately, 19.21%-efficient CIGSSe solar cell with the KCN treatment (1 wt%) at room temperature with the increased VOC of 0.692 V was obtained, which is around 15.3% relatively higher η than that of the solar cell without the surface treatment.

Origin of the open-circuit voltage in small-molecular organic photovoltaic devices: Effects of deposition rate control of donor material

Small-molecular organic photovoltaic devices with different open-circuit voltage (VOC) were fabricated by simply controlling the deposition rate of donor material. Higher VOC was obtained when the donor deposited at a higher rate and vice versa. The origin of the improved VOC was studied by means of morphological change and temperature-dependent current density–voltage characteristics. The presence of pinholes as shown in atomic force microscopic images indicated the strong molecular interaction at the lower donor deposition rate, resulting in the severe leakage current from acceptor to anode as observed in the dark current. Equivalent circuit model and temperature-dependent dark currents were utilized to realize the effect of reverse saturation current on VOC. The higher barrier height at the donor–acceptor interface was attributed to the improved VOC for the device with higher donor deposition rate.

Enhanced Hole Injection in Phosphorescent Organic Light-Emitting Diodes by Thermally Evaporating a Thin Indium Trichloride Layer

An ultrathin layer of indium trichloride (InCl(3)) is thermally evaporated on the indium tin oxide (ITO) anode to enhance the hole injection in simplified phosphorescent organic light-emitting diodes (PHOLEDs). Comparing with the device with ultraviolet (UV)-ozone treatment, the device modified by InCl(3) exhibits a maximum current efficiency of 82.2 cd/A measured at about 2000 cd/cm(2) and 36% improvement in power efficiency measured at 20 mA/cm(2). More importantly, more than three times improvement in half lifetime estimated at an initial luminance of 1000 cd/cm(2) is achieved. The investigations using ultraviolet photoelectron spectroscopy, X-ray photoelectron spectroscopy, and the bias- and temperature-dependent current density-voltage characteristics in the related hole-dominated devices have revealed that the improved device performance is mainly attributed to the enhanced hole injection resulting from the lowered hole injection barrier height in the InCl(3)-modified devices.

Numerical and experimental indication of thermally activated tunneling transport in CIS monograin layer solar cells

The purpose of this work was to build a numerical model of thermally activated tunneling transport by upgrading the drift-diffusion transport model in one-dimensional semiconductor simulator ASPIN, and to identify the current limiting mechanisms of the CIS monograin layer solar cells. The superposition of the classical drift-diffusion transport and the semi-classical thermionic-field transport is applied across the whole solar cell structure. The implemented model correctly predicts the shapes of temperature dependent current density–voltage characteristics and the temperature dependence of the photogenerated current density at the short-circuit condition in the range from 320 K down to 160 K.

Charge transport in polymer light-emitting diodes

The transport of electrons and holes in poly(dialkoxy-p-phenylene vinylene) (PPV) has been investigated using temperature dependent current density-voltage characteristics and admittance spectroscopy. From admittance spectroscopy, both the electron and hole mobility of PPV can be obtained from a single polymer light-emitting diode (PLED). These experiments reveal that at room temperature the electron transport is strongly reduced with regard to the hole transport. Due to the smaller activation energy of the electron transport the difference between electron and hole transport in PPV diminshes with decreasing temperature.

Charge transport in poly(p-phenylene vinylene) light-emitting diodes

Abstract Since the discovery of electroluminescence in conjugated polymers it has been recognized that charge transport is a key ingredient for the efficiency of the polymer light-emitting diodes (PLEDs). This review focuses on the charge transport properties of these materials. From temperature dependent current density–voltage characteristics it has been obtained that the hole transport in poly(dialkoxy-p-phenylene vinylene) (PPV) is governed by a combination of space-charge effects and a field- and temperature-dependent mobility. The origin of the hole mobility, which seems to be generic for a large class of disordered materials, arises from hopping in a system with both energetic and structural disorder. The response time of PPV-based PLEDs is governed by the dispersive transport of holes towards the cathode. Based on the results of the electron- and hole-transport a device model for PLEDs is proposed in which the light generation is due to bimolecular recombination between the injected electrons and holes. The unbalanced electron and hole transport gives rise to a bias dependent efficiency. By comparison with experiment it is found that the bimolecular recombination process is of the Langevin-type, in which the rate-limiting step is the diffusion of electrons and holes towards each other. The occurrence of Langevin recombination explains why the conversion efficiency of current into light of a PLED is temperature independent. The understanding of the device operation of PLEDs indicates directions for further improvement of the performance.