Shi Wun Tong

Interface Engineering of Layer‐by‐Layer Stacked Graphene Anodes for High‐Performance Organic Solar Cells

The major efforts in solar energy research are currently directed at developing cost-effective systems for energy conversion and storage. [ 1–3 ] The high cost of materials and preparation methods that are required for the fabrication of inorganic solar cells prevent their widespread deployment. Seeking a low-cost alternative in the form of solution-processable or roll-to-roll printable organic solar cells features prominently in the energy research roadmap. The conventional anode of choice for organic solar cells has been indium tin oxide (ITO), which consumes as much as 30% of the fabrication cost in solar cells. High quality ITO is expensive due to the dwindling supplies of indium. ITO also suffers from drawbacks like brittleness, sensitivity to acids and bases during processing, and reactive interface formation with copper indium sulfi de during high-temperature sintering. Graphene fi lms have been proposed as the new generation of multifunctional, transparent, and conducting electrodes. The attractiveness of graphene arises from their low cost, transparency, high electrical conductivity, chemical robustness, and fl exibility, as opposed to the rising cost and brittleness of ITO. [ 4–6 ]

Simple tandem organic photovoltaic cells for improved energy conversion efficiency

We proposed and demonstrated a simple tandem structure of organic photovoltaic (PV) cell for efficient light harvesting. In this device structure, a soluble fullerene derivative of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) is employed simultaneously to form a bilayer heterojunction PV subcell with the underlying copper phthalocyanine (CuPc) and a bulk heterojunction PV subcell with blended poly(3-hexylthiophene-2,5-diyl) (P3HT). In comparison with the conventional tandem structure, the omission of the semitransparent intercellular connection layer reduces the complexity of the device and the light loss. The enhanced short circuit current density (JSC=8.63mA∕cm2) and power conversion efficiency (PCE) (2.79%) of the tandem structure are nearly the sum of those of the stand-alone cells of CuPc/PCBM (JSC=2.09mA∕cm2, PCE=0.43%) and P3HT:PCBM (JSC=6.87mA∕cm2, PCE=2.50%).

Efficient multilayer organic solar cells using the optical interference peak

A multilayer structure of copper phthalocyanine/poly(3-hexylthiophene-2,5-diyl): [6,6]-phenyl-C61-butyric acid methyl ester (CuPc/P3HT:PCBM) is used to extend the light absorption spectrum covering almost the entire visible spectrum. To maximize the light absorption, the total number of excitons created in the multilayer structure as a function of layer thickness of both CuPc and P3HT:PCBM is simulated by using the optical transfer matrix formalism. The solar cells with a device structure of ITO/PEDOT:PSS/CuPc/P3HT:PCBM/Al are fabricated with different layers thicknesses. The optimized solar cell with a high short circuit current density of 12.54mA∕cm2 and power conversion efficiency as high as 4.13% is achieved, owing to the utilization of the second optical interference peak in the multilayer structure for the enhanced light absorption.

Graphene and Graphene-like Molecules: Prospects in Solar Cells

Graphene is constantly hyped as a game-changer for flexible transparent displays. However, to date, no solar cell fabricated on graphene electrodes has out-performed indium tin oxide in power conversion efficiency (PCE). This Perspective covers the enabling roles that graphene can play in solar cells because of its unique properties. Compared to transparent and conducting metal oxides, graphene may not have competitive advantages in terms of its electrical conductivity. The unique strength of graphene lies in its ability to perform various enabling roles in solar cell architectures, leading to overall improvement in PCE. Graphene can serve as an ultrathin and transparent diffusion barrier in solar cell contacts, as an intermediate layer in tandem solar cells, as an electron acceptor, etc. Inspired by the properties of graphene, chemists are also designing graphene-like molecules in which the topology of π-electron array, donor-acceptor structures, and conformation can be tuned to offer a new class of light-harvesting materials.

First-principles study of silicon nanowire approaching the bulk limit.

First-principles density functional theory calculations on hydrogenated silicon nanowires (SiNWs) with diameters up to 7.3 nm are carried out for comparing to experimentally relevant SiNWs and evaluating its radial doping profiles. We show that the direct band gap nature of both the small diameter (110) and (100) SiNWs fades when the diameter reaches beyond about 4 nm, where the difference of direct and indirect band gaps are close, within the experimental measurement uncertainty of ±0.1 eV, suggesting the diameter size where the gap nature transition starts. In addition, we reveal that core-surface boron (B) codoped SiNW forms more preferably at large diameter than that of the surface-surface codoped one, attributing to the lower energy configuration raised by the core B dopant at large diameter SiNW. More importantly, the diameter for such a preferential transition increases as the doping concentration decreases. Our results rationalize photoluminescent measurements and radial doping distributions of SiNWs.

Enhancement in open circuit voltage induced by deep interface hole traps in polymer-fullerene bulk heterojunction solar cells

A significant increase in open circuit voltage (VOC) is obtained in the polymer-fullerene bulk heterojunction solar cell by using the e-beam deposited Al cathode. Compared with the device with the thermal evaporated Al cathode, an obvious enhancement of VOC from 596 to 664 mV is obtained, which makes the overall device power conversion efficiency improved by 12.4% (from 3.79% to 4.26%). Electrical characterizations suggest that the energetic particles in the e-beam deposition induce deep interface hole traps in the poly(3-hexylthiophene-2,5-diyl) (P3HT), while leaving the fullerene unaffected. The deep trapped holes near the P3HT/cathode interface can induce the image negative charges in the cathode and thus form “dipoles.” These dipoles lead to the lowering of the Al effective work function and cause the enhancement of VOC.

Origin of Different Dependences of Open-Circuit Voltage on the Electrodes in Layered and Bulk Heterojunction Organic Photovoltaic Cells

Experimental results show that the V OC of layered heterojunction (HJ) organic photovoltaic (PV) cells behaves with a very weak dependence on the electrodes. However, the V OC of bulk HJ PV cells behaves with a strong dependence on the electrodes. In this paper, an explanation for the different behaviors of V OC on the electrodes is proposed. It is found that the V OC of the two types of PV cells follows the same mechanism and is mainly determined by the light-injected carriers at the donor/acceptor (D/A) interface and the electrodes. However, the distinct device structures make the boundary conditions in layered and bulk HJ PV cells different, which leads to the different dependences of V OC on the electrodes. The layered HJ PV cells have geometrically ¿flat¿ D/A and metal/organic (M/O) interfaces (the interface near the electrode), which makes the effective thickness from the D/A interface to the M/O interface large. Thus, there is a low electric field at the M/O interface and, then, a very small barrier lowering. Under this condition, the light-injected carriers at the D/A interface tend to ¿pin¿ the Fermi level of the electrodes. As a result, V OC shows only a very weak dependence on the work function of the electrodes. However, the formation of the interpenetrating network in bulk HJ PV cells greatly decreases the D and A domain dimensions and induces the ambipolar carrier distribution in the blend layer. This will cause very large barrier lowering at the M/O interface when there is a high barrier. Under this condition, the light-injected carriers at the D/A interface can no longer ¿pin¿ the electrode Fermi level. Thus, a strong dependence of V OC on the electrodes for bulk HJ PV cells is observed.

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.

The use of thermal initiator to make organic bulk heterojunction solar cells with a good percolation path

A simple method is developed to make an interpenetrating network of poly(3-hexylthiophene-2,5-diyl) (P3HT) and fullerene (C60) by mixing P3HT solution with a thermal initiator 2,2′-azobis(isobutyronitrile) (AIBN). After mild annealing, the release of nitrogen from AIBN increases the roughness of P3HT dramatically. Significant photoluminescence quenching between the roughened donor P3HT and overlaying acceptor C60 is related to the significant increment of donor-acceptor interfacial areas. Based on this interpenetrated network of P3HT/C60, more than threefold increase in the photovoltaic efficiency of devices is achieved compared with bilayer structure. Fill factor is also improved, implying good percolation path in this heterojunction structure.

Chemically doped radial junction characteristics in silicon nanowires.

We evaluate the boron (B) and phosphorus (P) core-surface codoped radial p-n junction characteristics in silicon nanowires (SiNWs) using density functional theory calculations. We find that the formation of radial p-n junction is energetically favorable. The stability depends on the diameter of SiNWs and the dopant concentration. Generally, a higher concentration of B-P pair dopants results in a more stable nanowire. More importantly, we predict that the radial p-n junction can evolve into a Schottky-like junction in relatively highly doped SiNWs when the diameter increases, attributing to the change of the core p-doping characteristic, that is, the core p-junction becomes metallic, while the n-junction near the surface remains semiconducting. The interfacial contact between the junctions is found to be the key for such change. Our calculated results support an experimental observation in SiNW solar cells.