Letter Nanostructured broadband antireﬂection coatings on AlInP fabricated by nanoimprint lithography J. Tommila n , V. Poloj¨ arvi, A. Aho, A. Tukiainen, J. Viheri¨ al¨ a, J. Salmi, A. Schramm, J.M. Kontio, A. Turtiainen, T. Niemi, M. Guina Optoelectronics Research Centre, Tampere University of Technology, Korkeakoulunkatu 3, 33720, Tampere, P.O. Box 692, 33101 Tampere, Finland article info Article history: Received 13 April 2010 Accepted 27 May 2010 Available online 9 June 2010 Keywords: Nanoimprint lithography Multijunction photovoltaic cell Antireﬂection coating Moth-eye abstract We report the fabrication of moth-eye antireﬂection nanostructures on AlInP compound commonly used as a window layer in high-efﬁciency multijunction solar cells. The broadband antireﬂective nanostructures were fabricated by nanoimprint lithography directly on molecular beam epitaxy grown AlInP/GaAs surface. At normal incidence, the structures exhibited an average reﬂectivity of 2.7% measured in a spectral range 450–1650 nm. Photoluminescence measurements of the emission from GaAs substrate suggest that the optical losses associated with the moth-eye pattern are low. Nanoimprint lithography offers a cost-effective approach to fabricate broadband antireﬂection coatings required in III–V high-efﬁciency multijunction solar cells. & 2010 Elsevier B.V. All rights reserved. 1. Introduction The deployment of high-efﬁciency multijunction III–V semi- conductor solar cells (SCs) in terrestrial solar power plants has attracted increasing interest in the past few years, owing to the development of advanced solar concentrator systems that are able to meet grid parity. Conversion efﬁciencies of more than 40% have been demonstrated for three-junction III–V cells using concen- trators [1,2] and higher efﬁciencies are predicted for cells with more than three junctions [3]. Due to the exploitation of multiple absorption bands, such devices exhibit a wide absorption spectrum ranging from 300 nm to beyond 1800 nm. To fully exploit the potential of multijunction cells broadband antireﬂec- tion (AR) coatings should be employed to mitigate the high discontinuity of refractive index at the semiconductor–air inter- face. In addition to the requirement for broader AR coatings, multijunction solar cells are more sensitive than single-junction cells to variation of the reﬂectivity because of current matching requirements; deviation of the coating from the optimum reﬂectance results in increased current mismatch. The optimiza- tion of widely used dielectric multilayer interference structures for use as AR coatings in high-efﬁciency III–V solar cells is therefore challenging [1,4–7]. A more recent approach to fabricate broadband AR coatings is to exploit the so-called moth-eye concept and fabricate sub-wavelength nanostructures on the surface of the SC [8,9]. This type of nanostructure provides a graded transition of the effective refractive index between the air and semiconductor surface, and thus, decreases effectively the amount of reﬂected light in a broad wavelength range and at all angles [8,9]. Moth-eye AR structures have been fabricated on silicon, GaAs, and GaSb substrates [9–12]; however, no estima- tions of the losses in these structures were reported. In this letter, we demonstrate moth-eye antireﬂection coatings fabricated by high-resolution low-cost nanoimprint lithography (NIL) on molecular beam epitaxy (MBE) grown AlInP/GaAs structure. AlInP has a very large band gap and high transparency. Therefore, it is usually added as a front window on III–V multijunction SC in order to passivate the emitter [1]. We show that the reﬂection from the AlInP/air interface can be strongly suppressed by using nanopatterned surfaces. Additionally, we present a method to estimate the losses caused by the surface pattern. Minimizing the losses in the nanostructure is essential in order to reach high-efﬁciencies by maximizing the transmission of light to the absorbing layers. 2. Material and methods The semiconductor layers were deposited by molecular beam epitaxy (MBE). First, a 100 nm GaAs buffer was grown on an n-type GaAs(1 0 0) substrate at a growth temperature of Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/solmat Solar Energy Materials & Solar Cells 0927-0248/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2010.05.053 n Corresponding author. Tel.: + 358 40 198 1052; mobile: + 358 50 573 0984; fax: + 358 3 3115 3400. E-mail addresses: Juha.Tommila@tut.ﬁ (J. Tommila), Ville.Polojarvi@tut.ﬁ (V. Poloj ¨ arvi), Arto.j.Aho@tut.ﬁ (A. Aho), Antti.Tukiainen@tut.ﬁ (A. Tukiainen), Jukka.Viheriala@tut.ﬁ (J. Viheri ¨ al¨ a), Joel.Salmi@tut.ﬁ (J. Salmi), Andreas.Schramm@tut.ﬁ (A. Schramm), Juha.Kontio@tut.ﬁ (J.M. Kontio), Antti.Turtiainen@iki.ﬁ (A. Turtiainen), Tapio.k.Niemi@tut.ﬁ (T. Niemi), Mircea.Guina@tut.ﬁ (M. Guina). Solar Energy Materials & Solar Cells 94 (2010) 1845–1848