Ion-damage-free planarization or shallow angle sectioning of solar cells for mapping grain orientation and nanoscale photovoltaic properties

Abstract

Ion beam milling is the most common modern method for preparing specific features for microscopic analysis, even though concomitant ion implantation and amorphization remain persistent challenges, particularly as they often modify materials properties of interest. Atomic force microscopy (AFM), on the other hand, can mechanically mill specific nanoscale regions in plan-view without chemical or high energy ion damage, due to its resolution, directionality, and fine load control. As an example, AFM-nanomilling (AFM-NM) is implemented for top-down planarization of polycrystalline CdTe thin film solar cells, with a resulting decrease in the root mean square (RMS) roughness by an order of magnitude, even better than for a low incidence FIB polished surface. Subsequent AFM-based property maps reveal a substantially stronger contrast, in this case of the short-circuit current or open circuit voltage during light exposure. Electron back scattering diffraction (EBSD) imaging also becomes possible upon AFM-NM, enabling direct correlations between the local materials properties and the polycrystalline microstructure. Smooth shallow-angle cross-sections are demonstrated as well, based on targeted oblique milling. As expected, this reveals a gradual decrease in the average short-circuit current and maximum power as the underlying CdS and electrode layers are approached, but a relatively consistent open-circuit voltage through the diminishing thickness of the CdTe absorber. AFM-based nanomilling is therefore a powerful tool for material characterization, uniquely providing ion-damage free, selective area, planar smoothing or low-angle sectioning of specimens while preserving their functionality. This enables novel, co-located advanced AFM measurements, EBSD analysis, and investigations by related techniques that are otherwise hindered by surface morphology or surface damage.

Thickness scaling of ferroelectricity in BiFeO 3 by tomographic atomic force microscopy

“…The thickness dependence of the local piezoresponse acquired using TAFM has been analyzed in the context of Landau–Ginzburg–Devonshire phenomenological theory, which reveals the thickness dependence of the spontaneous polarization and critical switching thickness in BiFeO 3 . Additionally, TAFM provides an experimental platform for thickness-dependent and defined-geometry experiments with site selectivity (20, 21). This capability is leveraged to perform polarization switching experiments on a programmed thickness gradient in BiFeO 3 following TAFM, and report localized, direct measurements of the electric field required for nucleation of ferroelectric domains as a function of film thickness.…”

Direct AFM-based nanoscale mapping and tomography of open-circuit voltages for photovoltaics

“…Specific settings include a load of ca. 1 µN, a line rate of 0.5 Hz, and a low-deflection feedback gain producing near “open loop” scanning and hence an essentially planar surface milling [ 8 ]. Approximately 15 nm in depth are practically removed per image frame, leading to effective 30 nm resolution in the z -direction between consecutive pairs of I SC * and V OC * maps throughout the polycrystalline film thickness.…”

“…Routines to test for such associations are therefore increasingly employed [ 1 , 24 ], allowing scientists to focus on or ignore such regions depending on whether they are true local variations or experimental artifacts. In any case, in order to minimize the influence of such topography, our specimens are first partially planarized [ 8 ]. This provides a surface morphology with slopes gradually transitioning ±5° over several micrometers according to the true topography [ 25 ] ( Fig.…”

“…There is a particular need for such efficient direct property-mapping routines for computed tomographic AFM (CT-AFM), in which images are serially acquired during progressive surface milling [ 6 , 8 ]. For instance, to investigate the nearly 50% reduction in efficiency for CdTe solar cells compared to their theoretical limits [ 9 – 15 ], it would be beneficial to have through-thickness V OC maps with high spatial and energy resolution.…”

Phase mode nanomachining on ultra-thin films with atomic force microscopy

“…Atomic Force Microscopy (AFM), using a micro probe with a nano tip as force sensor and machining tool, can achieve both topography imaging and material removing [4,5]. The imaging and machining ability of AFM has been improved by various advanced methods recently [6][7][8]. Taking the advantage of high resolution, flexibility and low cost, the AFM nanomachining has been widely employed to fabricate nanostructures with sub-nanometer accuracy on various materials including ultra-thin films [9].…”