Florian Lentz

Capacity based nondestructive readout for complementary resistive switches.

Complementary resistive switches (CRS) were recently suggested to solve the sneak path problem of larger passive memory arrays. CRS cells consist of an antiserial setup of two bipolar resistive switching cells. The conventional destructive readout for CRS cells is based on a current measurement which makes a considerable call on the switching endurance. Here, we report a new approach for a nondestructive readout (NDRO) based on a capacity measurement. We suggest a concept of an alternative setup of a CRS cell in which both resistive switching cells have similar switching properties but are distinguishable by different capacities. The new approach has the potential of an energy saving and fast readout procedure without decreasing cycling performance and is not limited by the switching kinetics for integrated passive memory arrays.

Current Compliance-Dependent Nonlinearity in

Nonvolatile redox-based resistive RAM (ReRAM) is considered to be a promising candidate for passive nanocrossbar integration. For this application, a high degree of nonlinearity in I-V characteristics of the ReRAM device is required. In this letter, the nonlinearity parameter as a function of forming/SET current compliance in a MOSFET-integrated TiN/TiO2/Ti/Pt ReRAM device is investigated. The nonlinearity parameter in the ReRAM device improves at the lower SET current compliance. This is due to scaling down the conductive filaments during the forming and the SET process. The nonlinearity is further increased by scaling down the oxide thickness that is accompanied by a reduction of the switching current.

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Highly doped p-type Si thin films are hardly etched by alkaline etchants unlike i- and n-type Si films. In addition, the use of a HNO₃/HF/H₂O mixture to etch p-type Si films can lead to inhomogeneous etching of p-type Si films and consequently result in excessive surface roughness and inferior surface passivation. For these reasons, we have implemented a dry etching method to selectively pattern p-type Si films for interdigitated back contact (IBC) silicon heterojunction (SHJ) solar cells using a SiO<italic>_x</italic> masking layer. In this way, the underlying passivation layer and the Si surfaces can be undamaged. In this study, the etch rates of all Si films that make up IBC-SHJ solar cells have been investigated in order to obtain high etch selectivity and establish proper patterning steps. Furthermore, we have fabricated IBC-SHJ solar cells on planar wafers by using dry and wet patterning methods developed in this study. As a result, a conversion efficiency of 15.7% is obtained with <italic>V</italic>_oc of 682 mV, <italic>J</italic>_sc of 33.8 mA/cm², and FF of 0.68.

ReRAM

Light trapping in crystalline silicon (c-Si) solar cells is an essential building block for high efficiency solar cells targeting low material consumption and low costs. In this study, we present the successful implementation of highly efficient light-trapping back contacts, subsequent to the passivation of Si heterojunction solar cells. The back contacts are realized by texturing an amorphous silicon layer with a refractive index close to the one of crystalline silicon at the back side of the silicon wafer. As a result, decoupling of optically active and electrically active layers is introduced. In the long run, the presented concept has the potential to improve light trapping in monolithic Si multijunction solar cells as well as solar cell configurations where texturing of the Si absorber surfaces usually results in a deterioration of the electrical properties. As part of this study, different light-trapping textures were applied to prototype silicon heterojunction solar cells. The best path length enhancement factors, at high passivation quality, were obtained with light-trapping textures based on randomly distributed craters. Comparing a planar reference solar cell with an absorber thickness of 280 μm and additional anti-reflection coating, the short-circuit current density (JSC) improves for a similar solar cell with light-trapping back contact. Due to the light trapping back contact, the JSC is enhanced around 1.8 mA cm-2 to 38.5 mA cm-2 due to light trapping in the wavelength range between 1000 nm and 1150 nm.

Selective Dry Etching of p-Type Si Films for Photolithography Processing of Interdigitated Back Contact Silicon Heterojunction Solar Cells

Wide gap n-type microcrystalline silicon carbide [µc-SiC:H(n)] is highly suitable as window layer material for silicon heterojunction (SHJ) solar cells due to its high optical transparency combined with high electrical conductivity. However, the hot wire chemical vapor deposition (HWCVD) of highly crystalline µc-SiC:H(n) requires a high hydrogen radical density in the gas phase that gives rise to strong deterioration of the intrinsic amorphous silicon oxide [a-SiO x :H(i)] surface passivation. Introducing an n-type microcrystalline silicon oxide [µc-SiO x :H(n)] protection layer between the µc-SiC:H(n) and the a-SiO x :H(i) prevents the deterioration of the passivation by providing an etch resistance and by blocking the diffusion of hydrogen radicals. We fabricated solar cells with µc-SiC:H(n)/µc-SiO x :H(n)/a-SiO x :H(i) stack for the front side and varied the µc-SiO x :H(n) material properties by changing the microstructure of the µc-SiO x :H(n) to evaluate the potential of such stack implemented in SHJ solar cells and to identify the limiting parameters of the protection layer in the device. With this approach we achieved a maximum open circuit voltage of 677 mV and a maximum energy conversion efficiency of 18.9% for a planar solar cell.

Post passivation light trapping back contacts for silicon heterojunction solar cells

The growth of aluminum-doped zinc oxide (ZnO:Al) on textured substrates is challenging due to growth disturbances that deteriorate ZnO:Al conductivity. This contribution demonstrates that highly conductive ZnO:Al films on textured substrates can be obtained by the use of proper substrate morphologies. A model was developed to predict the suitability of textured surfaces for the growth of ZnO:Al. Besides the number of growth disturbances within the ZnO:Al films, their distribution is an important parameter determining the conductivity.

Wide gap microcrystalline silicon carbide emitter for amorphous silicon oxide passivated heterojunction solar cells

Transparent passivated contacts (TPCs) using a wide band gap microcrystalline silicon carbide (μc-SiC:H(n)), silicon tunnel oxide (SiO2) stack are an alternative to amorphous silicon-based contacts for the front side of silicon heterojunction solar cells. In a systematic study of the μc-SiC:H(n)/SiO2/c-Si contact, we investigated selected wet-chemical oxidation methods for the formation of ultrathin SiO2, in order to passivate the silicon surface while ensuring a low contact resistivity. By tuning the SiO2 properties, implied open-circuit voltages of 714 mV and contact resistivities of 32 mΩ cm2 were achieved using μc-SiC:H(n)/SiO2/c-Si as transparent passivated contacts.

ZnO:Al on rough substrates: From surface texture to conductivity prediction

In the present work, we investigate light-management concepts applied subsequent to the passivation of the Si wafer of planar Si heterojunction solar cells. As a first concept, we apply amorphous silicon based nanophotonic textures at the back side of the Si wafer to realize efficient light trapping. As a second concept, we use a nanoimprint lithography based front side anti-reflection coating to improve the incoupling of light into the solar cell absorber. Both concepts allow for efficient improvements in the short-circuit current densities without degrading the planar passivation layers. As the highlight of this work, we demonstrate planar silicon heterojunction solar cells (thickness ~ 280 μm) with high passivation quality as well as a short-circuit current density of 38.8 mA/cm2.