Session K4: Materials VIII: Photovoltaic Materials

Controlled Synthesis of Hollow Carbon Nanostructures by Using ZnO Nanostructures as Templates

A rich variety of morphologies of ZnO nanostructures have been synthesized. Those nanostructures include particles, wires, sheets, belts, flowers, etc. Three dimensional hierarchical ZnO nanoarchitectures have been fabricated as well. It is hypothesized that these ZnO nanostructures can be used as templates to produce derivative nanostructures that may possess unique physicochemical properties. We recently developed a simple method to fabricate hollow carbon nanostructures by using the ZnO nanostructures as templates. By decomposing ethanol or other types of selected molecules onto the surfaces of the ZnO nanostructures, conformal deposition of carbonaceous materials can be obtained. The ZnO template can be removed by a high temperature reduction process, resulting in hollow carbon nanostructures. The morphology as well as the wall thickness of the carbon hollow nanostructures can be manipulated or controlled. Further high temperature annealing can convert the hollow carbon nanostructures to graphitic or graphene-like structures with better electrochemical properties for energy storage applications such as suppercapacitors or batteries. Examples of hollow carbon nanofibers and nanosheets will be discussed.   

\textbf{Band Alignment of PEALD Al2O3 and SiO2 Dielectric Passivation Layers on Zinc Oxide}

Zinc Oxide (ZnO) is an efficient photocatalyst for water splitting and cleaning organic waste despite its UV bandgap due to a high absorption coefficient. However, ZnO undergoes photocorrosion under UV illumination in electrochemical solutions, which hinders its application. This research is focused on measuring the band alignment of dielectric layers on ZnO which can allow carrier tunneling to the surface while passivating the surface to limit photocorrosion. In this study, we prepared plasma enhanced atomic layer deposited (PEALD) 2 nm Al2O3 and SiO2 films on O-face ZnO. The band gaps of ZnO, Al2O3 and SiO2 are 3.4 eV, 6.5 eV and 8.9 eV, respectively. Prior to deposition the ZnO surfaces were cleaned with a mixed He and O2 plasma at 310 C. In situ XPS and UPS were used to determine the Al2O3/ZnO and SiO2/ZnO band alignment. The PEALD process results in excess oxygen and negative charges in the dielectric layer, which are removed by an in situ annealing in N2$_{\, }$following PEALD deposition. Upward band bending in ZnO was observed after deposition of Al2O3 and SiO2; however, after annealing a flat band structure was achieved in ZnO. The Al2O3/ZnO and SiO2/ZnO measured valence band offset is 1.1 eV and 1.0 eV and conduction band offset is 2.2 eV and 3.2 eV, respectively.   

Metal Hydroxide Electron-selective Interlayers for Solution-processable Bulk Heterojunction Solar Cells

We show here how highly efficient inverted bulk heterojunction organic solar cells can be fully solution processed using highly efficient electron extraction interlayers composed of inexpensive metal hydroxides (e.g., LiOH, NaOH, KOH and Ba(OH))We show that these solution processed alkali hydroxides successfully reduce the work function of the bottom contact and are further proven to effectively passivate defect states at the contact interface. We use photoelectron spectroscopy (X-ray and UV) to elucidate the chemical and energetic aspects of the contacts and their energetic alignment with a prototypical electron transport material (i.e., C60 molecules appear to be negatively charged (reduced) at the near surface region due to Fermi level equilibration with the low work function metal hydroxide contacts, providing for beneficial energy level alignment for electron extraction in solar cell devices. If next generation semiconductors (e.g., organic and hybrid materials) are going to become a viable option for solar energy technologies, the materials and fabrication procedures need to be significantly less expensive compared to established high efficiency technologies (e.g., Si.) Since these next generation materials are solution processable, roll to roll fabrication technologies can be utilized to reduce production cost by offering the ability to significantly increase module throughput and decrease processing temperatures. The air stable, solution processable and inexpensive alkalihydroxide interface modifiers presented here provide an enabling technology to afford environmentally stable, inexpensive low work function contacts for next generation solar cells.   

Determination of Heterointerface Band Alignments in nBn Photodetectors Using Off-Axis Electron Holography

The zero valence-band offset between the barrier and absorber of a nBn photodetector is challenging to achieve practically, which could lead to different electrical characteristics. Long-wave (Sample A) and mid-wave (Sample B) infrared nBn photodetectors with absorbers consisting of InAs/InAsSb SLs and barriers consisting of InAs/AlGaSb(As) SLs, were grown using MBE. Distinctively different electrical characteristics suggested the possibility of different types of band alignments between the barrier and absorber for these two devices, which was attributed to the difference in Ga composition in the barrier layer. Examination of the barrier layers using off-axis electron holography showed the presence of positive charge with an estimated density of 1.8\texttimes 10\textasciicircum 17/cm\textasciicircum 3 in Sample B as a result of a type-II band alignment, whereas negligible charge was detected in Sample A, consistent with a type-I band alignment. This staggered type-II alignment of Sample B caused the significant increase in its dark current when strong bias is applied, because electrons from the valence band of the barrier layer can tunnel to the conduction band of the absorber layer.   

TCAD design and simulation of InGaN-based high temperature solar cells

To improve the efficiency of concentrated solar power hybrid system, a photovoltaic (PV) solar cell with high efficiency and operated at high temperatures is needed.~ In that regard, InGaN material system provides a platform for high temperature PV solar cells since nitride based optoelectronics are demonstrated to operate at high temperatures (\textgreater 400 ${^\circ})$ and, thus, became the basis for high power, high temperature electronics. The direct and tunable band gap of InGaN semiconductor offers a unique opportunity to develop high efficiency solar cells. ~Considering these advantages, this work involves TCAD simulation and optimization for InGaN solar cell at high temperature. Monolithic and mechanical multi-junction solar cell designs are investigated, and showing promising efficiency under light trapping. Theoretical conversion efficiency of the best devices are larger than 26{\%} at 450 ${^\circ}$ with an incident solar radiation concentration of 200 suns. Thus, we demonstrate that 2J tandem solar cells made in InGaN material system are very suitable for concentrated solar power hybrid system.