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 SiO_x layer grown directly on the crystalline silicon (c-Si) wafer. Depending on the thickness of the SiO_x layer, the charge carriers can cross this layer by tunneling (<2 nm SiO_x thickness) or by direct conduction through disruptions in the SiO_x, often referred to as pinholes, in thicker SiO_x layers (>2 nm). In this work, we study structures with tunneling- or pinhole-like SiO_x contacts grown on pyramidally textured c-Si wafers and expose variations in the SiO_x 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 SiO_x 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 SiO_x layer. Our results show that the pinholes in the SiO_x 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.

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 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, SiO_x 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 SiO_x layer as determined by transmission electron microscopy of a poly-Si/SiO_x contact. Our device measurements also show an overall more resistive and hence a thicker SiO_x 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/SiO_x contacts, we further show that the SiO_x 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 SiO_x layer, thus leading to poor poly-Si/SiO_x contact passivation. Finally, we report >21% efficient and ≥80% fill-factor front/back poly-Si/SiO_x 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/SiO_x contacts.

The Effect of Ga Content on the Recombination Behavior of Grain Boundaries in Cu(In,Ga)Se2 Solar Cells

The highest efficiency CuIn1-xGaxSe2 (CIGS) based solar cells have been produced from films with x∼0.3 which gives a value of Eg around 1.1-1.2eV. Increasing the Ga content of the CIGS absorber provides an increase in Voc, allows tuning of the band gap that can enhance performance under actual operating conditions, and potentially makes it possible to use CIGS films in multi-junction devices. However, champion cells have not yet been produced for values of x significantly greater than 0.3. This work focuses on how increased Ga content in CIGS films affects the recombination behavior of grain boundaries. Cathodoluminescence spectral imaging (CLSI) measurements on fully processed devices allow us to compare device properties with recombination behavior and optical properties of grain boundaries in films with different Ga content. Our data suggests that grain boundaries in high efficiency films with x∼0.3 exhibit a significant red shift in the CL spectra whereas grain boundaries in films with higher Ga content typically show either a small shift or none at all. This shift indicates band bending near the boundaries which could enhance charge separation and subsequent collection of carriers generated near grain boundaries. This is investigated statistically to identify trends in different regions of the films.