Pauls Stradins

Area-Dependent Switching in Thin Film-Silicon Devices

We report on the area dependence of switching in both Cr/ p + a-Si:H/Ag(Al) and Cr/ p + μc-Si/Ag(Al) filament switches. The doped amorphous (a-Si:H) or microcrystalline (μc-Si) thin Si layers are made by hot-wire chemical vapor deposition. The device active region area (A) is varied over 5 orders of magnitude, from 10 -7 to 10 -2 cm 2 , using photolithographically defined Ag and Al top contacts. Before switching, the resistance of 100-μm 2 devices is normally about 100 kΩ for μc-Si and 10 GΩ for a-Si:H. After switching with applied current ramps, the resistance decreases to a few hundred ohms in all a-Si devices and to a few thousands ohms in μc-Si devices. In both μc-Si and a-Si:H devices, the switching voltage (V sw ) decreases with increasing device area according to V sw ~ V 0 -αln(A/A 0 ) with α=0.3V for a-Si:H and α=0.04V for μc-Si. For both materials, the switching current roughly obeys the power law I sw ∞ A β with β~1. A statistical model is proposed to explain the area scaling of the switching voltage and relate the parameters to the material properties.

Air Passivation of Chalcogen Vacancies in Two‐Dimensional Semiconductors

Defects play important roles in semiconductors (SCs). Unlike those in bulk SCs, defects in two-dimensional (2D) SCs are exposed to the surrounding environment, which can potentially modify their properties/functions. Air is a common environment, yet its impact on the defects in 2D SCs still remains elusive. Here we study the interaction between air and chalcogen vacancies (V(X)), the most typical defects in 2D SCs. Although the interaction is weak for most molecules in air, O2 can be chemisorbed at V(X) with a barrier that correlates with the SC cohesive energy and can be overcome even at room temperature for certain SCs. Importantly, the chemisorbed O2 changes the V(X) from commonly believed harmful carrier-traps to electronically benign sites. This unusual behavior originates from the isovalence between O2 and X when bonded with metal. Based on these findings, a facile approach is proposed to improve the performance of 2D SCs by using air/O2 to passivate the defects.

Nanoscale measurements of local junction breakdown in epitaxial film silicon solar cells

In this contribution, the authors report on near-field scanning optical microscopy measurements of the luminescence emitted from localized junction breakdown in epitaxial silicon solar cells. Our measurements suggest that the observed local reduction in breakdown voltage results from avalanche multiplication assisted by the reinforcing combination of (i) the local enhancement of the electrostatic field at the apex of inverted pyramid pits and (ii) the participation of defect states in the avalanche breakdown. Transmission electron microscopy reveals the microstructure of the defect responsible for the local junction breakdown.

Tritium Induced Defects in Amorphous Silicon

We report the growth of tritium induced defects in tritium doped hydrogenated amorphous silicon (a-Si:H,T) as measured by electron spin resonance (ESR) and photothermal deflection spectroscopy (PDS). The measurements allow one to examine the accumulation of defects in a-Si:H,T where the defect production mechanism is known. Defects produced by tritium decay are found to be much less numerous than the number of decayed tritium atoms and they are metastable like Staebler-Wronski defects. These results provide new insight into the metastable defect creation and the role of hydrogen motion.

600 mV epitaxial crystal silicon solar cells grown on seeded glass

We report progress made at the National Renewable Energy Laboratory (NREL) on crystal silicon solar cells fabricated by epitaxially thickening thin silicon seed layers on glass using hot-wire chemical vapor deposition. Four micron thick devices grown on single-crystal silicon layer transfer seeds on glass achieved open circuit voltages (Voc) over 600 mV and efficiencies over 10%. Other devices were grown on laser crystallized mixed phase solidification (MPS) seeds on glass and e-beam crystallized (EBC) a-Si on SiC coated glass seeds. We discuss the material quality of the various devices on seeds and summarize the prospects for the seed and epitaxy PV approach.

Ion implanted passivated contacts for interdigitated back contacted solar cells

We describe work towards an interdigitated back contacted (IBC) solar cell utilizing ion implanted, passivated contacts. Formation of electron and hole passivated contacts to n-type CZ wafers using tunneling SiO₂ and ion implanted amorphous silicon (a-Si) are described. P and B were ion implanted into intrinsic amorphous Si films at several doses and energies. A series of post-implant anneals showed that the passivation quality improved with increasing annealing temperatures up to 900 °C. The recombination parameter, J_o, as measured by a Sinton lifetime tester, was J_o ~ 14 fA/cm² for Si:P, and J_o ~ 56 fA/cm² for Si:B contacts. The contact resistivity for the passivated contacts, as measured by TLM patterns, was 14 milliohm-cm² for the n-type contact and 0.6 milliohm-cm² for the p-type contact. These J_o and p_contact values are encouraging for forming IBC cells using ion implantation to spatially define dopants.

Synthesis and characterization of PECVD-grown, silane-terminated silicon quantum dots

Semiconductor quantum dots (QDs) have been the subject of intense research interest due to novel experimentally observed properties, such as tunable bandgap, phonon bottleneck, and a variety of surface effects. The control of these properties makes quantum dots a candidate for revolutionizing a variety of fields, including photovoltaics. Because silicon is such a well characterized PV material in its bulk form, it would be a good choice for QD research for application in solar cells. In addition, there is recent theoretical evidence that its indirect gap may become more direct as size decreases, allowing for a fine-tuning of the absorption characteristics for photovoltaics. We present a method for grafting silanes onto low-temperature-plasma synthesized silicon quantum dots. The resulting solution of dots is characterized with Fourier transform infrared spectroscopy and transmission electron microscopy, and determined to be a colloidal suspension. The silane is attached at a single point on the quantum dot surface to avoid cross-linking and multilayer formation, and photoluminescence spectroscopy shows the colloidal suspension of dots is stable for over two months in air. The hydroxyl-terminated surfaces required for silanization are created by wet chemical etch, which can be used to tune the luminescence of the silicon dots in the green- to red-wavelength range. Unpassivated Si quantum dots show vastly different behaviors in electron paramagnetic resonance than wet-chemically oxidized, silane-functionalized particles. The dangling bond density of unpassivated Si quantum dots is large and changes over time, while the dangling bond density of the silanized dots is unchanged and undetectable. This suggests silanized dots will be better for solution-processed PV devices since transport will not be hindered by dangling bonds. Finally, we perform PL excitation (PLE) spectroscopy on both ensembles of dots, and discuss the way such spectra are represented in the literature, especially in comparison with absorption. This discussion is critical to the success of Si QDs in optoelectronic devices, since absorption and luminescence play critical roles.

Yield analysis and comparison of GaInP/Si and GaInP/GaAs multi-terminal tandem solar cells

We present a yield analysis of tandem devices consisting of GaInP top cells on Si or GaAs bottom cells with different terminal configurations. Inputs are the I-V and external quantum efficiency of the individual subcells and the irradiance-dependent module temperature of the bottom cell. Our model calculates the temperature of the tandem module by taking into account the performance, spectral working range and luminescent coupling of the different tandem devices, enabling an irradiance- and weather-dependent yield analysis for these modules. We apply the model to compare two types of two junction devices, a GaInP/GaAs monolithically grown tandem device, and a GaInP top cell stacked on a Si bottom cell, the present two best dual junction devices. When the subcells are series connected both technologies perform equally well. The performance of the GaInP/Si can be significantly improved relatively by 5.8% using 3-terminal (3T) devices with a back-contacted bottom cell instead of a 2T configuration, showing a...

Study of nickel silicide as a copper diffusion barrier in monocrystalline silicon solar cells

NiSi as a conductive diffusion barrier to silicon has been studied. We demonstrate that the NiSi films formed using the single step annealing process are as good as the two step process using XRD and Raman. Quality of NiSi films formed using e-beam Ni and electroless Ni process has been compared. Incomplete surface coverage and presence of constituents other than Ni are the main challenges with electroless Ni. We also demonstrate that Cu reduces the thermal stability of NiSi films. The detection of Cu has proven to be difficult due to temperature limitations.

Silicon quantum dot optical properties and synthesis: Implications for photovoltaic devices

We study key optical properties for designing an absorber layer with silicon quantum dots (Si-QDs), including the absorptivity of the material, whether the character of the bandgap is direct or indirect, and the relation between absorption and photoluminescence. We report necessary synthesis conditions in order to control size, size distribution, and crystallinity of Si-QDs. This is important for applications of Si-QDs in photovoltaics [1,2], where they excite interest due to their size-tunable bandgap [3], potentially cheap fabrication, and possible enhancement of solar energy conversion efficiency through mechanisms such as multiple exciton generation [4].