Solar Energy Materials & Solar Cells 91 (2007) 85–90 Letter Current routes in polycrystalline CuInSe 2 and Cu(In,Ga)Se 2 films Doron Azulay a , Oded Millo a,Ã , Isaac Balberg a , Hans-Werner Schock b , Iris Visoly-Fisher c , David Cahen c,Ã a Racah Institute of Physics, The Hebrew University of Jerusalem, Jerusalem 91904, Israel b Hahn-Meitner-Institut, Berlin, Germany c Materials and Interfaces Department, Weizmann Institute of Science, Rehovoth 76100, Israel Received 24 July 2006; received in revised form 14 August 2006; accepted 16 August 2006 Abstract Local electrical transport measurements with scanning probe microscopy on polycrystalline (PX) p-CuInSe 2 and p-Cu(In,Ga)Se 2 films show that the photovoltaic and dark currents for bias voltages smaller than 1 V flow mainly through grain boundaries (GBs), indicating inversion at the GBs. Photocurrent for higher bias flows mainly via the grains. Based on these results and our finding of 100 meV GB band bending we deduce the potential landscape around the GBs. We suggest that high grain material quality, leading to large carrier mobilities, and electron–hole separation at the GBs, by chemical and electrical potential gradients, result in the high performance of these PX solar cells. r 2006 Elsevier B.V. All rights reserved. Keywords: Solar cell; Grain boundaries; Conductive AFM; CIS; CIGS 1. Introduction Several types of thin-film polycrystalline (PX) solar cells with p-CdTe and chalcopyrites (p-CuInSe 2 ; CIS, and p-Cu(In 1x Ga x )Se 2 ; CIGS, with xo0.3) as absorbers exhibit cell efficiencies that surpass those of the corre- sponding single-crystal-based devices. This is remarkable as the electrical behavior of PX electronic materials is often interpreted in terms of defects and impurities, which are thought to segregate at the grain boundaries (GBs) and create in-gap localized states that serve as traps. Free charge carriers are then trapped in these GB states, creating a depleted space charge region next to the boundaries, and a potential barrier for electronic transport between adjacent grains [1]. However, for example for CIS, the highest reported single crystal efficiencies are 11–12% [2], compared to empirically optimized 15% for PX cells (without Ga) [3]. A similar situation holds for CdTe/CdS cells, for which we showed [4,5] that the band bending is strong enough for the GBs to become inverted. As a result, the spatial separation of photo-generated e–h pairs is helped and the minority carriers in the bulk of the grains (electrons here) are channeled along the continuous network of GBs as majority carriers, with minimal e–h recombination [4–7]. These and other possible PX solar cell configurations [6] were considered theoretically already more than 20 years ago [8]. Models, based on GB electrical potential barriers have also been suggested to apply to CIS and CIGS, based on experimental results from various spatially resolved techniques [9–13]. Also a ‘‘structural’’ model for the formation of a hole barrier at the GBs was suggested [14], by assuming that GBs behave as the materials’ surfaces, which were earlier [15] found to be Cu-poor (cf. also ref. 12). The presence of Cu vacancies increases the ionization potential, yielding a wider surface bandgap [16], thus forming a barrier for hole transport. Because, in this model, a GB is taken as two adjacent free surfaces, it implies a GB hole barrier, if electrostatic effects are considered [1]. In this case the barrier does not result from electrical charges and was suggested [14] to be beneficial for charge transport, as electrically active defects are usually ARTICLE IN PRESS www.elsevier.com/locate/solmat 0927-0248/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2006.08.006 Ã Corresponding authors. Fax: +972 8934 4138. E-mail addresses: milode@vms.huji.ac.il (O. Millo), david.cahen@weizmann.ac.il (D. Cahen).