Time-resolved correlative optical microscopy of charge-carrier transport, recombination, and space-charge fields in CdTe heterostructures

Abstract

From time- and spatially resolved optical measurements, we show that extended defects can have a large effect on the charge-carrier recombination in II–VI semiconductors. In CdTe double heterostructures grown by molecular beam epitaxy on the InSb (100)-orientation substrates, we characterized the extended defects and found that near stacking faults the space-charge field extends by 2–5 μm. Charge carriers drift (with the space-charge field strength of 730–1,360 V cm−1) and diffuse (with the mobility of 260 ± 30 cm2 V−1 s−1) toward the extended defects, where the minority-carrier lifetime is reduced from 560 ns to 0.25 ns. Therefore, the extended defects are nonradiative recombination sinks that affect areas significantly larger than the typical crystalline grains in II–VI solar cells. From the correlative time-resolved photoluminescence and second-harmonic generation microscopy data, we developed a band-diagram model that can be used to analyze the impact of extended defects on solar cells and other electronic devices.

Identification of Recombination Losses in CdSe/CdTe Solar Cells from Spectroscopic and Microscopic Time‐Resolved Photoluminescence

“…[ 14–16 ] When carrier diffusion length exceeds dimensions of the focused laser beam, carrier dynamics can also be impacted by the lateral drift and diffusion. [ 17–20 ] As a result, currently there are no accepted analytical methods for microscopic carrier lifetime analysis in solar cells. [ 19 ] Here, we apply spectroscopic and microscopic measurements to the same samples and establish conditions when microscopic and spectroscopic characterization can produce comparable results, thus validating microscopic carrier lifetime analysis for thin‐film solar cells.…”

Surface versus Bulk: Charge Carriers Play by Different Rules

“…In particular, for the {110} and {111} facets, they estimated diffusion coefficients that were approximately four orders higher than those reported for the bulk. 10 The {111} facet provided smaller diffusion length and diffusion components than {110}; however, these were still substantially higher than those in the bulk. In stark contrast, the scenario was found to be very different for the {211} facet, which was characterized by trapping dynamics rather than facile diffusion of the photogenerated charge carriers as a result of its greater propensity to become oxidized and generate surface trapping sites.…”

Impact of Individual Structural Defects in GaAs Solar Cells: A Correlative and In Operando Investigation of Signatures, Structures, and Effects

“…[5] Many characterization techniques are available to probe individual defects for both structure and properties, and are sometimes applied correlatively. [5,[8][9][10][11][12][13][14][15][16][17][18] However, defect studies have typically been conducted in a parallel mode where the wafer is divided into multiple pieces for independent optical and structural characterization, and separate device fabrication and evaluation. Moreover, it is often highly challenging or even impossible to carry out operando characterization in devices at the individual defect level.…”