Controlling the Texture and Crystallinity of Evaporated Lead Phthalocyanine Thin Films for Near-Infrared Sensitive Solar Cells Karolien Vasseur, †,‡ Katharina Broch, § Alexander L. Ayzner, ⊥ Barry P. Rand, † David Cheyns,* ,† Christian Frank, § Frank Schreiber, § Michael F. Toney, ⊥ Ludo Froyen, ‡ and Paul Heremans †,# † imec, Kapeldreef 75, 3001 Heverlee, Belgium ‡ Department of Metallurgy and Materials Engineering, KU Leuven, Kasteelpark Arenberg 44, 3001 Heverlee, Belgium § Institut fü r Angewandte Physik and LISA +, Universitä t Tü bingen, Auf der Morgenstelle 10, Tü bingen 72076, Germany ⊥ Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, Menlo Park, California 94025, United States # Department of Electrical Engineering, KU Leuven, Kasteelpark Arenberg 10, B-3001 Heverlee, Belgium * S Supporting Information ABSTRACT: To achieve organic solar cells with a broadened spectral absorption, we aim to promote the growth of the near- infrared (NIR)-active polymorph of lead phthalocyanine (PbPc) on a relevant electrode for solar cell applications. We studied the effect of different substrate modification layers on PbPc thin film structure as a function of thickness and deposition rate (r dep ). We characterized crystallinity and orientation by grazing incidence X-ray diffraction (GIXD) and in situ X-ray reflectivity (XRR) and correlated these data to the performance of bilayer solar cells. When deposited onto a self-assembled monolayer (SAM) or a molybdenum oxide (MoO 3 ) buffer layer, the crystallinity of the PbPc films improves with thickness. The transition from a partially crystalline layer close to the substrate to a more crystalline film with a higher content of the NIR-active phase is enhanced at low r dep , thereby leading to solar cells that exhibit a higher maximum in short circuit current density (J SC ) for thinner donor layers. The insertion of a CuI layer induces the formation of strongly textured, crystalline PbPc layers with a vertically homogeneous structure. Solar cells based on these templated donor layers show a variation of J SC with thickness that is independent of r dep . Consequently, without decreasing r dep we could achieve J SC = 10 mA/cm 2 , yielding a bilayer solar cell with a peak external quantum efficiency (EQE) of 35% at 900 nm, and an overall power conversion efficiency (PCE) of 2.9%. KEYWORDS: structural templating, molecular orientation, polymorphism, copper iodide (CuI), self-assembled monolayer (SAM), planar heterojunction 1. INTRODUCTION Continuous progress in designing new organic semiconduc- tors, 1-3 optimizing device architectures, 4,5 and controlling active layer morphology 6-10 has allowed organic solar cell efficiencies to improve dramatically, with record efficiencies now exceeding 10%. 11 Further improvement of efficiency requires an increased overlap with the solar spectrum in order to maximize photocurrent generation, and in particular, for use with complementary absorbing tandem cells. 12 Since an important part of the photon flux is situated above λ = 800 nm, it is key to incorporate organic solar cells into the tandem structure that exhibit a sensitivity into the near-infrared (NIR) region while simultaneously ensuring efficient photon-to- current conversion. Among small-molecular-weight donor materials, phthalocya- nines (Pcs) are an archetypal material class due to their high optical absorption coefficients in the visible region and their tunable functional properties by incorporating different metal ions in the Pc ring. 13 Introducing a large metal ion, a metal- halide or metal-oxo moiety into the Pc ring disrupts its planarity; the three-dimensionality of the resultant molecule causes its optical and electrical properties in the solid state to be strongly influenced by molecular packing. 14-19 Nonplanar Pcs such as titanyl (TiOPc), 4,20-22 vanadyl (VOPc) 23 and lead Pc (PbPc) 24,25 are reported to crystallize as a variety of polymorphs, most of them exhibiting either monoclinic or triclinic symmetry. The triclinic polymorph of these com- pounds displays a broadened and red-shifted absorption spectrum due to the increased intermolecular interactions in this close packing arrangement. 10 For PbPc, the phase transition from monoclinic to triclinic is accompanied by a distinct shift of the dominant peak in the absorption spectrum from λ = 740 nm to λ = 900 nm. 26-28 The characteristic absorption peak of triclinic PbPc is positioned at even longer Received: May 21, 2013 Accepted: August 1, 2013 Published: August 1, 2013 Research Article www.acsami.org © 2013 American Chemical Society 8505 dx.doi.org/10.1021/am401933d | ACS Appl. Mater. Interfaces 2013, 5, 8505-8515