Purcell effect and luminescent downshifting in silicon nanocrystals coated back-contact solar cells F. Sgrignuoli a,n , P. Ingenhoven a , G. Pucker b , V.D. Mihailetchi c , E. Froner a , Y. Jestin b , E. Moser a , G. Sànchez d , L. Pavesi a a Nanoscience Laboratory, Department of Physics, University of Trento, Via Sommarive 14, 38123 Povo, Trento, Italy b Advanced Photonics and Photovoltaics Group, Center for Materials and Microsystems, Bruno Kessler Foundation, Via Sommarive 18, 38123 Povo, Trento, Italy c International Solar Energy Research Center (ISC), Rudolf-Diesel-Str. 15, D-78467 Konstanz, Germany d Universidad Politécnica de Valencia, NTC, B 8F, 21 Camino de Vera s/n, 46022, Valencia, Spain article info Article history: Received 10 June 2014 Accepted 7 September 2014 Keywords: Silicon nanocrystals Luminescent downshifting effect Local density of optical states Interdigitated back contact silicon solar cell abstract Silicon nanocrystals show a significant shift between the strong absorption in the blue–ultraviolet region and their characteristic red–near-infrared emission as well as space separated-quantum cutting when short wavelength photons are absorbed. These two effects can be used to increase the efficiency of crystalline silicon solar cells. We fabricated high quality interdigitated back-contact crystalline silicon solar cells in an industrial pilot line and coated them with optimized silicon nanocrystals layers in a cost effective way. Here we demonstrate an increase of 0.8% of the power conversion efficiency of the interdigitated back-contact cell by the silicon nanocrystals layer. In addition, we prove that this increase is due to a combination of a better surface passivation, a better optical coating, and of the luminescent downshifting effect. Moreover we demonstrated that the engineering of the local density of photon states, thanks to the Purcell effect, is instrumental in order to exploit this effect. & 2014 Elsevier B.V. All rights reserved. 1. Introduction Production of photovoltaic cells is dominated by single junction devices based on crystalline silicon [1], which account for about 90% of the world total photovoltaic cell production. Considerable progresses in increasing the efficiency of crystalline silicon solar cells have been made by minimizing photon, carrier and electrical losses through new cell structures and processes, such as inter- digitated back-contact cells (IBC) or heterojunction cells with intrinsic thin layer [2]. At the same time, enormous interest raised in novel approaches–third generation photovoltaic–aimed at increasing the efficiency with new concepts [3]: improved light harvesting [4–7], minimization of hot carrier losses by promoting fast and non-dissipative recombination mechanisms [8–12], and modification of the solar spectrum through photon conversion [13,14]. In this context the exploitation of luminescent down- shifting effect (LDS) and space separated-quantum cutting (SSQC) [12] is of special interest, allowing the combination of different concepts to improve the cell efficiency with practical production technologies. The basic idea of LDS is to move short-wavelength photons to long wavelength photons [13]: a range where silicon solar cell has almost 100% internal quantum efficiency (IQE). In this way, the short circuit current density (J sc ) is increased while the open circuit voltage (V oc ) is barely affected [13]. Up to now, LDS has not been engineered in an effective way. A few successful reports demonstrated industrialization of LDS by using phosphor doped ethylene vinyl acetate with an increase of 0.3% in module effi- ciency [15] or by using inkjet printing with a relative increase in the cell efficiency of 2% [16]. Interestingly, many reports address the use of silicon nanocrystals (Si-NCs) as suitable candidates for LDS [16–19]: these nanoparticles are stable, bright, show a significant red-shift between emission and absorption [20], and can be manufactured by using standard deposition techniques. In addition, SSQC in Si-NCs has also been reported [8–12]. This process involves the transformation of a high-energy photon into two or more photons of lower energy, hence “cutting” the energy quantum [12]. Ideally, the down-converted photons are in a suitable range of the spectrum and can be further used without Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/solmat Solar Energy Materials & Solar Cells http://dx.doi.org/10.1016/j.solmat.2014.09.007 0927-0248/& 2014 Elsevier B.V. All rights reserved. n Correspondence to: Department of Physics, University of Florence, and European Laboratory for Non-Linear Spectroscopy (LENS), Via Nello Carrrara 1, 50019 Sesto Fiorentino (Firenze), Italy. E-mail address: sgrignuoli@lens.unifi.it (F. Sgrignuoli). Solar Energy Materials & Solar Cells 132 (2015) 267–274