Photocurrent Mapping of 3D CdSe/CdTe Windowless Solar Cells Carlos M. Hangarter, †,# Ratan Debnath, †,‡,§,# Jong Y. Ha, †,∥ Mehmet A. Sahiner, ⊥ Christopher J. Reehil, ⊥ William A. Manners, ⊥ and Daniel Josell* ,† † Materials Science and Engineering Division, Material Measurement Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, United States ‡ Department of Materials Science and Engineering, University of Maryland, College Park, Maryland 20742-2115, United States § N5 Sensors Inc., Rockville, Maryland 20852, United States ∥ Institute for Research in Electronics and Applied Physics, University of Maryland, College Park, Maryland 20742, United States ⊥ Department of Physics, Seton Hall University, South Orange, New Jersey 07079, United States 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. KEYWORDS: back contact, CdSe, CdTe, photovoltaic, 3D solar cells, SPCM ■ INTRODUCTION Scanning photocurrent microscopy (SPCM) has been used to evaluate the local performance of solar cells by scanning a finely focused optical beam across the surface while monitoring device response. In contrast to electron beam induced current mapping techniques, SPCM compromises resolution; however, it provides direct translation to operating conditions in which both minority and majority carriers are generated. 1,2 With length scales defined by the light source and optics, SPCM can be used to study transport behavior in thin-film materials exhibiting micrometer-scale diffusion lengths and designs incorporating similar feature size. As such, SPCM has been used in studies exploring the impact of processing conditions, window layer thickness, accelerated aging and back contact treatments, performance and structural inhomogeneities, series resistance, grain boundaries, and pinholes for a mechanistic understanding and process optimization. 3 Recently, it has been used to evaluate photovoltaic performance of thin-film and nanowire-based devices with submicrometer resolution. 4,5 This capability is relevant to efforts aimed at improved efficiency or utilization of more abundant, but typically lower quality, materials with light management strategies and advanced architectures that often include intricate nanoscale concepts. These approaches include plasmonic, quantum dot and nanowire-based devices that, while promising, are empirically challenging for a variety of reasons. 5−9 Contact and heterojunction structures that depart from conventional planar designs to yield improved performance are particularly appealing. 10 Critical to achieving better performance of all these platforms is an improved understanding of electron−hole pair generation in three-dimensional (3D) absorber materials and concomitant charge carrier transport. This work explores a dual back contact geometry that imparts three dimensionality to thin-film CdTe photovoltaic devices. By relocating the front contact to the rear of the device, adjacent to the other contact in an interdigitated fashion, the associated window layer and its absorptive losses are eliminated. While a number of back contact geometry devices have been detailed in the literature for crystalline Si, 11−13 finer pitch electrodes consistent with the shorter carrier recombina- tion lengths of thin-film materials impose a significant departure in terms of design and processing. 14−16 For thin- film devices in particular, the back contact geometry removes the requirement for a transparent conducting oxide contact and the need for a wide band gap n-type junction layer transparent to the illuminating light. The opportunity provided by the latter change is exemplified by the use of CdSe (band gap ≈ 1.75 eV) here and in a previous study of analogous CdTe/CdSe devices. 17 In this study, prepatterned substrates were utilized with site-selective electrodeposition to coat one electrode with CdSe and subsequent blanket deposition of CdTe via pulse laser deposition (PLD) completing the device; in the previous Received: June 27, 2013 Accepted: August 22, 2013 Published: August 22, 2013 Research Article www.acsami.org © 2013 American Chemical Society 9120 dx.doi.org/10.1021/am402507f | ACS Appl. Mater. Interfaces 2013, 5, 9120−9127