Letter High resolution saturation current density imaging at grain boundaries by lock-in thermography S. Rißland n , O. Breitenstein Max Planck Institute of Microstructure Physics, Weinberg 2, 06120 Halle, Germany article info Article history: Received 29 February 2012 Received in revised form 3 May 2012 Accepted 9 May 2012 Keywords:: Lock-in thermography Modeling IV characteristics Deconvolution Local analysis Grain boundary abstract The electronic properties of multicrystalline solar silicon materials are dominated by low-lifetime defect regions containing recombination-active grain boundaries and dislocations. Besides reducing the carrier collection probability, these regions increase the dark saturation current density J 01 , which governs the open circuit voltage. By applying lock-in thermography with spatial deconvolution it is shown that the dominant contribution to J 01 comes from recombination-active grain boundaries and to a lower degree from intra-grain defects like dislocations. & 2012 Elsevier B.V. All rights reserved. 1. Article To raise the performance of multicrystalline solar cells it is necessary to understand the recombination effects lowering the efficiency. Crystal defects such as dislocations or grain boundaries (GBs) act as recombination centers and thus lower the local carrier lifetime and increase the local diffusion current described by J 01 . Due to the macroscopic dimension of grain boundaries, their recombination activity becomes visible in beam-injection methods like electron beam induced current, light beam induced current, and cathodoluminescence imaging. These methods were used to measure the recombination velocity at GBs quantitatively [1–3]. More recently electroluminescence (EL) imaging is also used to image recombination-active defects in solar cells [4]. On the other hand, dark lock-in thermography (DLIT) enables a direct quantitative measurement of the dark current density [5]. Earlier investigations have demonstrated a clear correlation between EL and DLIT in low-lifetime regions, but the J 01 images obtained from both methods disagreed quantitatively [6]. Since DLIT shows a considerably worse spatial resolution than EL, a direct comparison on microscopic scale was not possible yet. In this letter, we demonstrate that it is possible to measure the local diffusion current caused by GBs with considerably improved spatial resolution, comparable to that of EL imaging, by dark lock-in thermography (DLIT) in combination with spatial deconvolution. By applying a model proposed by Lax [7] the results are evaluated quantitatively for measuring the recombination velocity at the GBs. The DLIT measurements are performed using an infrared camera system by Thermosensorik GmbH [8]. The investigated sample is a 3  3 cm 2 sized part of an industrial multicrystalline solar cell (15.6  15.6 cm 2 , J sc ¼ 32.9 A/cm 2 , V oc ¼ 618 mV, FF ¼ 78.3%), which contains a high concentration of GBs. The cell is acid textured and possesses screen printed contacts, an alumi- num back surface field, and a silicon nitride antireflection layer. The small piece is contacted on the busbar using a four-probe contacting scheme. The used lock-in frequency of 10 Hz leads to a thermal diffusion length of 1.6 mm in the 200 mm thick silicon solar cell. To guarantee a homogeneous emissivity, the surface of the cell was covered with black paint. By measuring DLIT-images at different biases it is possible to separate the different dark current contributions due to their specific current–voltage char- acteristic by using an algorithm and software code called ‘‘Local- IV’’ [9]. Assuming an ideality factor of two for the depletion region recombination current and neglecting ohmic shunt components, the investigation is based on two DLIT images measured at 550 mV and 600 mV with a spatial resolution of 50 mm per pixel at 25 1C. Though it is well-known that the lifetime in multi- crystalline material may depend on the injection level [10], here we assume a constant lifetime in simplest approximation. If the lifetime should be injection-dependent, the DLIT investiga- tions have to be made at an injection level comparable to the Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/solmat Solar Energy Materials & Solar Cells 0927-0248/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.solmat.2012.05.011 n Corresponding author. Tel.: þ49 345 5582692; fax: þ49 345 5511223. E-mail address: rissland@mpi-halle.mpg.de (S. Rißland). Solar Energy Materials & Solar Cells 104 (2012) 121–124