Photocurrent Mapping of 3D CdSe/CdTe Windowless Solar Cells

Abstract

This paper details the use of scanning photocurrent microscopy to examine localized current collection efficiency of thin-film photovoltaic devices with in-plane patterning at a submicrometer length scale. The devices are based upon two interdigitated comb electrodes at the micrometer length scale prepatterned on a substrate, with CdSe electrodeposited on one electrode and CdTe deposited over the entire surface of the resulting structure by pulsed laser deposition. Photocurrent maps provide information on what limits the performance of the windowless CdSe/CdTe thin-film photovoltaic devices, revealing "dead zones" particularly above the electrodes contacting the CdTe which is interpreted as recombination over the back contact. Additionally, the impact of ammonium sulfide passivation is examined, which enables device efficiency to reach 4.3% under simulated air mass 1.5 illumination.

Windowless CdSe/CdTe Solar Cells with Differentiated Back Contacts: J–V, EQE, and Photocurrent Mapping

“…CdTe between 0.4 μm and 0.6 μm thick was subsequently deposited over the entire device by pulsed laser deposition for the asymmetric dies. Symmetric dies (W CdSe = 1 μm) such as have been previously studied 8 were fabricated with CdTe thickness between 0.7 μm and 1.1 μm thick using the same process described in the earlier study. After CdTe deposition, all devices were coated with CdCl 2 and annealed at 400 °C for 10 minutes in a nitrogen plus oxygen environment.…”

“…As in the earlier study, 8 the local EQE measurements used a focused 532 nm laser that was scanned over the device using a piezoelectric stage while the short circuit current (I sc ) was monitored. Laser power at the sample was maintained at (1 to 10) nW; total uncertainties on stated power are ± 3 % based on the measured change of beam intensity over the duration of the mapping experiments.…”

“…1–3 Back contact thin film devices have been fabricated more recently. 4–8 The thin film devices require finer pitch electrodes due to shorter carrier recombination lengths. However, the back contact geometry removes the need for a transparent conducting oxide layer and bandgap restrictions imposed on the (n-type) junction layer for optical transparency.…”

“…Because LBIC previously indicated higher performance over and around the CdSe contact lines of back contact CdSe/CdTe devices, 8 the width of these contacts (W CdSe ) is systematically varied in this study; the width of the CdTe contacts is held constant. Contact lines to both semiconductors were equally wide in previous studies of both CdS/CdTe and CdSe/CdTe back contact devices.…”

Photoconductive NSOM for mapping optoelectronic phases in nanostructures

“…Near-field scanning optical microscopy (NSOM) has been by far the most successful among super-resolution optical microscopy techniques in probing and characterizing optical properties of single molecules and nanomaterials [1][2][3][4][5][6][7][8]. The progress in understanding the morphology of Bulk Heterojunction Organic Photovoltaics (BHJ OPVs) with different techniques was reviewed recently [9].…”