Zhenhua Lin

A simple and efficient solar cell parameter extraction method from a single current-voltage curve

In this work, a simple and efficient method for the extraction of all the parameters of a solar cell from a single current-voltage (I-V) curve under the constant illumination level is proposed. With the help of the Lambert W function, the explicit analytic expression for I is obtained. By reducing the number of the parameters, the expression for I only depends on the ideality factor n, the series resistance Rs, and the shunt resistance Rsh. This analytic expression is directly used to fit the experimental data and extract the device parameters. This simple solar cell parameter extraction method can be directly applied for all kinds of solar cells whose I-V characteristics follow the single-diode model. The parameters of various solar devices including silicon solar cells, silicon solar modules, dye-sensitized solar cells, and organic solar cells with standalone, tandem, and multi-junction structures have been successfully extracted by using our proposed method.

Development of Inverted Organic Solar Cells with TiO2 Interface Layer by Using Low-Temperature Atomic Layer Deposition

Organic solar cells (OSCs) with inverted structure have attracted much attention in recent years because of their improved device air stability due to the use of stable materials for electrodes and interface layers. In this work, TiO(2) films, fabricated using low temperature (e.g., 130-170 °C) atomic layer deposition (ALD) on ITO substrates, are used as electron selective interface layers to investigate inverted OSCs. It is found that though the as-deposited TiO(2) films are high resistive due to the presence of oxygen defects, the defects can be significantly reduced by light soaking. PV cells with 15-nm-thick amorphous-TiO(2) layers fabricated at low temperature show better performance than those with poly crystal TiO(2) with same thickness deposited at 250 °C. The low temperature ALD-grown TiO(2) films are dense, stable and robust with capability of conformal coating on nanostructural surfaces, showing a promising interface layer for achieving air-stable plastic OSCs with roll-to-roll mass production potential.

Solution-processed LiF-doped ZnO films for high performance low temperature field effect transistors and inverted solar cells.

This paper reports that high performance metal oxide thin film transistors (TFTs) can be achieved by using LiF-doped ZnO thin films processed from aqueous solution. It was found that LiF doping at an appropriate amount enhanced the oxide film carrier concentration. The TFTs based on the 10 mol % LiF-doped ZnO thin films annealed at 300 °C revealed a good device performance with an average electron mobility of 8.9 cm(2) V(-1) s(-1) and a high on/off current ratio of 4 × 10(7), superior to the devices based on the nondoped ZnO TFTs (1.6 cm(2) V(-1) s(-1)). Even when annealed at 150 °C, the device still showed good transistor operation with an electron mobility of 0.54 cm(2) V(-1) s(-1). The inverted bulk heterojunction solar cells based on P3HT:PCBM blend system fabricated using 10 mol % LiF doped ZnO as electron selective layer showed higher power conversion efficiency (η = 3.3%) than that from undoped ZnO thin films (η = 2.94%) due to enhanced short circuit current (Jsc = 10.55 mA/cm(2)). Our results suggest that LiF incorporation can be a useful technique to produce high performance and low temperature solution-processed oxide TFTs and interface layer for solar cells.

Design and synthesis of conjugated polymers containing Pt(II) complexes in the side-chain and their application in polymer memory devices

We have synthesized conjugated polymers containing Pt(II) complexes in the side-chain with different main-chains via a Suzuki coupling reaction. These polymers exhibit bistable properties and can be applied in memory devices, in which charge transfer and traps are responsible for the conductance switching behavior. The devices could be defined as resistive random-access memory (ReRAM) with a high ON/OFF current ratio, excellent stability and high read cycles (107). Furthermore, through the study of the electrochemical properties and theoretical calculations of the polymers, we investigated the significant effect of the polymer main-chain on the memory device performances. The device based on the polymer with a polycarbazole main-chain exhibited a lower threshold voltage and a higher ON/OFF current ratio than the device based on the polymer with a polyfluorene main-chain. Our preliminary results indicate that this kind of material offers promising opportunities for the development of polymer memory devices.

Inverted Organic Photovoltaic Cells with Solution-Processed Zinc Oxide as Electron Collecting Layer

In this work, inverted polymer:fullerene organic photovoltaic (OPV) cells with solution-processed zinc oxide (ZnO) as the electron collecting layer are investigated. ZnO films are prepared simply by the spin-casting of a zinc acetate dehydrate precursor solution, followed by sintering under ambient conditions. The performance of the fabricated inverted OPV cells shows a clear dependence on precursor concentration and sintering conditions. With the ZnO film derived from a sol–gel concentration of 0.1 M and sintered at 350 °C for 10 min, the inverted OPV cell shows optimum performance.

A work-function tunable polyelectrolyte complex (PEI:PSS) as a cathode interfacial layer for inverted organic solar cells

High-efficiency inverted polymer solar cells (PSCs) with a polyelectrolyte complex, polyethylenimine:poly(styrenesulfonate) (PEI:PSS), as the cathode interfacial layer for efficient electron extraction are demonstrated. By introducing the negatively charged PSS− as the counter ions into PEI, the imine protonation could be tuned, leading to tunable work-function of the PEI:PSS coated ITO. The incorporation of PSS in PEI enhances the photocurrent and power conversion efficiency (PCE) of the devices, due to an improved electron extraction at the PEI:PSS–active-layer interfaces. Furthermore, a TiOx–PEI:PSS combined interfacial layer further enhances the cell performance and eliminates the need for light-soaking treatment for TiOx, owing to the improved hole-blocking and surface-state passivation in the interfacial layer. The achieved high cell performance, better stability, low-cost materials, and low-temperature solution processes of the TiOx–PEI:PSS interfacial layers demonstrate a promising cathode configuration for realizing efficient and long lifetime PSCs.

Solution processed F doped ZnO (ZnO:F) for thin film transistors and improved stability through co-doping with alkali metals

This paper reports solution-processed metal oxide semiconductor thin film transistors (TFTs), which were produced using fluorine (F) doped ZnO-based aqueous solution. It was found that doping F into the ZnO film improves thin film transparency and TFT performance with an ultrahigh on/off ratio of 108. The F doped ZnO TFT devices showed no improvement in shelf-life stability but improved bias stress stability. Moreover, when the ZnO:F was co-doped with alkali metals like Li, Na, and K, the co-doped ZnO TFT devices exhibited much higher electron mobility, in comparison with ZnO or the ZnO:F TFTs. In addition, the co-doped TFT device exhibited excellent shelf-life stability and bias stress stability. These results suggest that F and alkali metal co-doping can be a useful technique to produce more reliable and low temperature solution-processed ZnO semiconductors for TFTs and their applications.

Enhanced inverted organic solar cell performance by post-treatments of solution-processed ZnO buffer layers

The effects of post-treatments (150 °C-thermal, humidity, and vacuum) on the cathode buffer layer of aqueous-solution-processed zinc oxide (ZnO) films on the performance of the inverted organic solar cells (OSCs) are investigated, based on poly(3-hexylthiophene)/[6,6]-phenyl-C61-butyric acid methyl ester (P3HT/PC61BM) as the active layers. The devices with the ZnO buffer layers that underwent thermal and vacuum post-treatments exhibited 17% and 15% increments in the power conversion efficiency (PCE) as compared to that of the cell without post-treatment on the ZnO layer, mainly due to the increases of the short circuit current (Jsc). It was found that the thermal and vacuum post-treatments reduced the defects and increased the electron mobility in the ZnO buffer layers, improving the electron extractions in the inverted OSCs. Both the 150 °C-thermal and vacuum post-treatments are compatible with plastic substrates, showing a potential way to further improve the film properties of the low-temperature processed ZnO buffer layers.

Effects of Cathode Confinement on the Performance of Polymer/Fullerene Photovoltaic Cells in the Thermal Treatment

Polymeric photovoltaic (PV) cells based on poly(3-hexylthiophene-2,5-diyl):[6, 6]-phenyl C61 butyric acid methyl ester (P3HT:PCBM) with the cathode confinement in the thermal treatment show better performance than the PV cells without the cathode confinement in the thermal treatment. The functions of the cathode confinement are investigated in this paper by using X-ray photoelectron spectroscopy, atomic force microscopy, optical absorption analysis, and X-ray diffraction analysis. It is found that the cathode confinement in the thermal treatment strengthens the contact between the active layer and the cathode by forming Al-O-C bonds and P3HT-A1 complexes. The improved contact effectively improves the device charge collection ability. More importantly, it is found that the cathode confinement in the thermal treatment greatly improves the active layer morphology. The capped cathode effectively prevents the overgrowth of the PCBM molecules and, at the same time, increases the crystallization of P3HT during the thermal treatment. Thus, a better bicontinuous interpenetrating network is formed, which greatly reduces the exciton loss and improves the charge transport capability. Meanwhile, the enhanced crystallites of P3HT improve the absorption property of the active layer. All these aforementioned effects together lead to the great performance improvement of polymeric PV cells.

TiOx/Al bilayer as cathode buffer layer for inverted organic solar cell

Titanium oxide (TiOx) modified with a thin layer of Al was used as an electron transporting layer in an inverted organic solar cell based on the P3HT/PCBM blend. The thin Al layer was shown to improve the TiOx surface properties, decreased the work-function of TiOx, increased the built-in voltage, and facilitated electron extraction. As a result, good device performance with power conversion efficiency of 3.6%, open circuit voltage of 0.60 V, short circuit current of 9.13 mA/cm2, and fill factor of 0.66 was achieved. The light soaking problem was eliminated compared to the TiOx only device.