Crystalline Si Surface Passivation with Nafion for Bulk Defects Detection with Electron Paramagnetic Resonance.

In monocrystalline Si (c-Si) solar cells, identification and mitigation of bulk defects are crucial to achieving a high photoconversion efficiency. To spectroscopically detect defects in the c-Si bulk, it is desirable to passivate the surface defects. Passivation of the c-Si surface with dielectrics such as Al2O3 and SiNx requires deposition at elevated temperatures, which can influence defects in the bulk. Herein, we report on the passivation of different Czochralski (Cz) Si wafer surfaces by an organic copolymer, Nafion. We test the efficacy of the surface passivation at temperatures ranging from 6 to 473 K to detect bulk defects using electron paramagnetic resonance (EPR) spectroscopy. By comparing with state-of-the-art passivation layers, including Al2O3 and liquid HF/HCl, we found that at room temperature, Nafion can provide comparable passivation of n-type Cz Si with an implied open-circuit voltage (iVoc) of 713 mV and a recombination current prefactor J0 of 5 fA/cm2. For p-type Cz Si, we obtained an iVoc of 682 mV with a J0 of 22.4 fA/cm2. Scanning electron microscopy and photoluminescence reveal that Nafion can also be used to passivate the surface of c-Si solar cell fragments scribed from a solar cell module by using a laser. Consistent with previous studies, analysis of the EPR spectroscopy data confirms that the H-terminated surface is necessary, and fixed negative charge in Nafion is responsible for the field-effect passivation. While the surface passivation quality was maintained for almost 24 h, which is sufficient for spectroscopic measurements, the passivation degraded over longer durations, which can be attributed to surface SiOx growth. These results show that Nafion is a promising room-temperature surface passivation technique to study bulk defects in c-Si.

3D and Multimodal X-Ray Microscopy Reveals the Impact of Voids in CIGS Solar Cells.

Small voids in the absorber layer of thin-film solar cells are generally suspected to impair photovoltaic performance. They have been studied on Cu(In,Ga)Se2 cells with conventional laboratory techniques, albeit limited to surface characterization and often affected by sample-preparation artifacts. Here, synchrotron imaging is performed on a fully operational as-deposited solar cell containing a few tens of voids. By measuring operando current and X-ray excited optical luminescence, the local electrical and optical performance in the proximity of the voids are estimated, and via ptychographic tomography, the depth in the absorber of the voids is quantified. Besides, the complex network of material-deficit structures between the absorber and the top electrode is highlighted. Despite certain local impairments, the massive presence of voids in the absorber suggests they only have a limited detrimental impact on performance.

Optical and Structural Properties of High-Efficiency Epitaxial Cu(In,Ga)Se2 Grown on GaAs.

Photovoltaic devices based on Cu(In,Ga)Se2 (CIGS) typically employ polycrystalline thin films as the absorber layer. This is because, to date, the highest conversion efficiencies have been attained with polycrystalline CIGS films. Recently, Nishinaga et al. presented an epitaxial CIGS thin-film solar cell grown on a GaAs (100) substrate with a conversion efficiency of 20.0%. In this contribution, we study the optical and structural properties of this high-efficiency epitaxial film, along with others with different compositions using cathodoluminescence spectrum imaging and transmission electron microscopy. A comparison of the high-efficiency epitaxial film and a traditional polycrystalline film with a similar global composition reveals significant differences in microstructure and uniformity of emission properties despite similar performance. The analysis of epitaxial films with a higher gallium concentration indicates that the emission characteristics and nature of extended defects in epitaxial CIGS films are strongly dependent on the gallium content. The results presented here provide evidence that, with further optimization, photovoltaic conversion efficiencies of epitaxial CIGS films could exceed those of polycrystalline CIGS.

Effect of Surface Texture on Pinhole Formation in SiOx-Based Passivated Contacts for High-Performance Silicon Solar Cells.

High-efficiency silicon solar cells rely on some form of passivating contact structure to reduce recombination losses at the crystalline silicon surface and losses at the metal/Si contact interface. One such structure is polycrystalline silicon (poly-Si) on oxide, where heavily doped poly-Si is deposited on a SiOx layer grown directly on the crystalline silicon (c-Si) wafer. Depending on the thickness of the SiOx layer, the charge carriers can cross this layer by tunneling (<2 nm SiOx thickness) or by direct conduction through disruptions in the SiOx, often referred to as pinholes, in thicker SiOx layers (>2 nm). In this work, we study structures with tunneling- or pinhole-like SiOx contacts grown on pyramidally textured c-Si wafers and expose variations in the SiOx layer properties related to surface morphology using electron-beam-induced current (EBIC) imaging. Using EBIC, we identify and mark regions with potential pinholes in the SiOx layer. We further perform high-resolution transmission electron microscopy on the same areas, thus allowing us to directly correlate locally enhanced carrier collection with variations in the structure of the SiOx layer. Our results show that the pinholes in the SiOx layer preferentially form in different locations based on the annealing conditions used to form the device. With greater understanding of these processes and by controlling the surface texture geometry, there is potential to control the size and spatial distribution of oxide disruptions in silicon solar cells with poly-Si on oxide-type contacts; usually, this is a random phenomenon on polished or planar surfaces. Such control will enable us to consistently produce high-efficiency devices with low recombination currents and low junction resistances using this contact structure.

Directly Linking Low-Angle Grain Boundary Misorientation to Device Functionality for GaAs Grown on Flexible Metal Substrates.

A new growth method to make highly oriented GaAs thin films on flexible metal substrates has been developed, enabling roll-to-roll manufacturing of flexible semiconductor devices. The grains are oriented in the <001> direction with <1° misorientations between them, and they have a comparable mobility to single-crystalline GaAs at high doping concentrations. At the moment, the role of low-angle grain boundaries (LAGBs) on device performance is unknown. A series of electron backscatter diffraction (EBSD) and cathodoluminesence (CL) studies reveal that increased doping concentrations decrease the grain size and increase the LAGB misorientation. Cross-sectional scanning transmission electron microscopy (STEM) reveals the complex dislocation structures within LAGBs. Most importantly, a correlative EBSD/electron beam-induced current (EBIC) experiment reveals that LAGBs are carrier recombination centers and that the magnitude of recombination is dependent on the degree of misorientation. The presented results directly link increased LAGB misorientation to degraded device performance, and therefore, strategies to reduce LAGB misorientations and densities would improve highly oriented semiconductor devices.

Effect of Crystallographic Orientation and Nanoscale Surface Morphology on Poly-Si/SiOx Contacts for Silicon Solar Cells.

High-efficiency crystalline silicon (Si) solar cells require textured surfaces for efficient light trapping. However, passivation of a textured surface to reduce carrier recombination is difficult. Here, we relate the electrical properties of cells fabricated on a KOH-etched, random pyramidal-textured Si surface to the nanostructure of the passivated contact and the textured surface morphology. The effects of both microscopic pyramidal morphology and nanoscale surface roughness on passivated contacts consisting of polycrystalline Si (poly-Si) deposited on top of an ultrathin, 1.5-2.2 nm, SiOx layer are investigated. Using atomic force microscopy, we show a pyramid face, which is predominantly a Si(111) plane to be significantly rougher than a polished Si(111) surface. This roughness results in a nonuniform SiOx layer as determined by transmission electron microscopy of a poly-Si/SiOx contact. Our device measurements also show an overall more resistive and hence a thicker SiOx layer over the pyramidal surface as compared to a polished Si(111) surface, which we relate to increased surface roughness. Using electron-beam-induced current measurements of poly-Si/SiOx contacts, we further show that the SiOx layer near the pyramid valleys is preferentially more conducting and hence likely thinner than over pyramid tips, edges, and faces. Hence, both the microscopic pyramidal morphology and nanoscale roughness lead to a nonuniform SiOx layer, thus leading to poor poly-Si/SiOx contact passivation. Finally, we report >21% efficient and ≥80% fill-factor front/back poly-Si/SiOx solar cells on both single-side and double-side textured wafers without the use of transparent conductive oxide layers, and show that the poorer contact passivation on a textured surface is limited to boron-doped poly-Si/SiOx contacts.