Session W2: Invited Session: Instrumentation and Measurement Science for Energy Research: PV and Batteries

Using Deep Level Transient Spectroscopy (DLTS) to characterize defects in semiconductor devices

Deep Level Transient Spectroscopy (DLTS) is a member of the class of instrumentation methods that utilizes the detection of trapped electronic charge to characterize defects in solids. Such methods detect this charge either directly, e.g. via capacitance measurements, or indirectly, e.g. via the current associated with the release of trapped charge. These types of instrumentation have been widely used since the dawn of solid-state physics, particularly for nonradiative defects in semiconductors and insulators. In the case of semiconductor devices, the highly sensitive capacitive detection of trapped charge in the junction depletion layer makes these methods particularly powerful. The DLTS method introduced the concept of time-domain filtering (the so-called ``rate window'') to create a defect spectrum from the transient response of the device versus temperature. This talk will give an overview of DLTS, with particular emphasis on the correlation between defects and device performance.   

Role of Defects and Their Analysis in Photovoltaics

Defects are intrinsically related to the performance of solar cells. In solar cells the generation and collection of charge carriers determines their efficiency. Effective transport of charge carriers across interfaces and minimization of their recombination in the bulk or at surfaces and interfaces is of utmost importance. In this talk we will discuss the role of surface and bulk defects. First, the role of surface passivation is very important in limiting the rate of carrier of recombination. Here we will combine spectroscopic evaluation of the surface of a Si device with electrical lifetime measurements to ascertain what factors determine the quality of a solar cell passivation. We have also utilized time-resolved photoluminescence (TRPL) to assess the quality of materials grown under varying conditions. TRPL decay is best fit by a biexponential model that includes both the minority carrier lifetime and the rate of trap filling. At low laser powers the trap state recombination dominates and decay times are very short. At higher laser powers the trap states become saturated and we can extract the minority carrier lifetime. By evaluating the relative contributions of the trap-filling and minority carrier lifetimes we can assess the density of traps (defects) as a function of the growth conditions and guide refinement of growth recipes to improve material quality.   

The Integration of Scanning Electron Microscopy, Scanning Probe Microscopy, and Luminescence Spectroscopy in one Platform: New Opportunities and Applications in Photovoltaics

We have recently integrated scanning tunneling microscopy (STM), atomic force microscopy (AFM), and near-field scanning optical microscopy (NSOM) onto the mechanical stage of a scanning electron microscope compatible with operation under high vacuum and the use of cryogenics. This instrument is unique in the sense that is not just the assembly of different microscopes but an integrated platform in which both the electron beam and the ultrasharp tip of the AFM/STM/NSOM can be controlled simultaneously and independently as excitation or sensing elements, providing innovative modes of operation and access to optoelectronic properties in the micro and nanoscale not accessible before. Furthermore, this instrument is equipped with focused laser illumination of the tip and detection of luminescence and can be used to measure cathodoluminescence, scanning tunneling luminescence, photoluminescence, and electroluminescence, all with high resolution. In this contribution, we review the application of these techniques to the development of second- and third-generation photovoltaics (PV) beyond those commercially available today. Among these applications, we present the luminescence and electron transport across single grain boundaries in chalcopyrite and kesterite compounds, the detection of single molecule species using plasmonics, the nanoscale imaging of the exciton transport in organic semiconductors, and the insitu manipulation and measurement of nanowires.   

Li ion nanowire batteries and their \textit{in situ} characterization in the TEM

The ability to measure the morphological, chemical, and transport characteristics with nanoscale resolution in electrochemical energy storage devices is critical for understanding the complex interfacial reactions and phase transformation that accompany cycling of secondary batteries. In this talk I will describe the use of an all-nanowire Li ion battery for \textit{in situ} characterization of charge and discharge reactions. The nanowire batteries (NWBs) consist of a metalized core, a LiCoO$_{2}$ cathode, LiPON solid electrolyte, and a thin film Si anode. Measuring several micrometers in length and several hundred nanometers in diameter, the NWBs can be readily imaged and analyzed in transmission electron microscopes (TEM, STEM). We use focused ion beam milling and electron beam induced deposition to separate the cathode and anode and fabricate Pt contacts to a NWB. \textit{In situ} electrical cycling of NWBs in TEM reveals that the most of the structural changes due to cycling happens in the electrolyte layer especially near the cathode/electrolyte interface. Electrical response from a single NWB was measured in the sub-pA range. For NWBs with the thinnest electrolyte, approximately 100 nm, we observe rapid self-discharge, along with void formation at the electrode/electrolyte interface, indicating electrical and chemical breakdown. The analysis of the NWB's electrical characteristics reveals space-charge limited electronic conduction, which effectively shorts the anode and cathode electrodes. When the electrolyte thickness is increased, the self-discharge rate is reduced substantially and the NWBs maintain a potential above 2 V. Our study illustrates that at reduced dimensions the increase in the electric field can lead to large electronic current in the electrolyte effectively shorting the battery even when the electrolyte layer is uniform and pinhole free. The scaling of this phenomenon provides useful guidelines for design of 3D Li ion batteries.   

In-Situ TEM Electrochemistry of Individual Nanowire and Nanoparticle Electrodes in a Li-Ion Cell

Recently, we created the first Li-ion electrochemical cell inside a transmission electron microscope (TEM) and observed, in real time with atomic scale resolution, the lithiation/delithiation processes. This experiment opened the door for a suite of experimental studies involving in-situ TEM characterization of Li-ion battery materials. In this presentation, I'll first review our latest progress of using the in-situ electrochemical cell setup inside the TEM to reveal the intrinsic electrochemistry of several high energy density anode materials such as SnO$_{2}$, ZnO, Si, Ge, Al nanowires, Si nanoparticles, carbon nanotubes, and graphene. Several electrochemical mechanisms were observed and characterized in real-time, including lithiation induced stress, volume changes, phase transformations, pulverization, cracking, embrittlement, and mechanical failure in anode materials. These results indicate the strong material, size and crystallographic orientation dependent electrochemical behavior and degradation mechanisms that occur in Li-ion battery anodes. In the future, we will need further advancements in in-situ characterization for understanding important processes in Li-ion batteries. For example, liquid cells are required in order to examine the electrochemical reactions between battery materials and the standard battery electrolytes, which are ethylene carbonate-based. Furthermore, in-situ studies need to be correlated with electrochemical studies performed on bulk electrodes. I will present a comparison between our in-situ results and electrochemical studies on conventional battery electrodes and highlight how in-situ studies can have important impact on the design of Li-ion batteries. Finally I will discuss outstanding challenging issues and opportunities in the field of Li-ion battery research. \\[4pt] References: \textbf{Science} 330, 1515 (2010); 330, 1485 (2010); \textbf{Nano Lett}. Doi: 10.1021/nl200412p, 10.1021/nl2024118, 10.1021/nl201684d, 10.1021/nl202088h, \textbf{ACS Nano}, doi: 10.1021/nn200770p, 10.1021/nn202071y; \textbf{PRL} 106, 248302 (2011); \textbf{Eng. Env. Sci.} doi: 10.1039/c1ee01918j