Tailoring Cu x S semiconductor-sensitized SnO 2 solar cells Auttasit Tubtimtae a,n , Yi-Chung Wu b , Yung-Chou Chen b , Jen-Bin Shi c , Ming-Way Lee b,n Q1 a Department of Physics, Faculty of Liberal Arts and Science, Kasetsart University, Kamphaeng Saen Campus, Nakhon Pathom 73140, Thailand b Institute of Nanoscience and Department of Physics, National Chung Hsing University, Taichung 40227, Taiwan c Department of Electronic Engineering, Feng Chia University, Taichung 40724, Taiwan article info Article history: Received 29 December 2014 Accepted 3 February 2015 Keywords: Copper sulfide Tin oxide Successive ionic layer adsorption and reaction Semiconductor sensitizer Solar cell Quantum-dot abstract The Cu x S SSCs were fabricated on mesoporous SnO 2 electrodes using the successive ionic layer adsorption and reaction (SILAR) process. The optimal Cu x S SSC yielded the highest efficiency of 0.31% under 100 mW/cm 2 (AM 1.5G) after various passivation treatments with a current density J sc of 10.99 mA/cm 2 , an open-circuit voltage V oc of 0.12 V and a fill factor FF of 23.6%. The external quantum efficiency (EQE) of 35% and the maximal internal quantum efficiency (IQE) of 61% at λ ¼500 nm with an average IQE of 43% over the entire spectrum were revealed. The results show that the localized 3d Cu states in chalcogenide play a principal role for the photoinduced effect, leading to the enhanced polarizability in the cell. Thus, Cu 1.97 S can be a broad-band solar absorber due to changes in the optical properties in the material. & 2015 Published by Elsevier B.V. 1. Introduction Semiconductor-sensitized solar cells (SSCs) have recently become one of the most attractive research subjects for renewable photo- voltaic resources because they provide several advantages such as tunable absorption bands due to the quantum-size effect [1,2], high extinction coefficient [3], and multi-exciton generation by a single incident photon [4]. In addition, inorganic absorbers also provide better long-term photostability. Many binary metal chalcogenides, including CdTe [5], Sb 2 S 3 [6], SnS [7], and Ag 2 S [8] have been employed as sensitizers for SSCs. One metal chalcogenide has been interested, i.e., the Cu ions-incorporated chalcogenide materials, playing a principal role for the photoinduced effect based on the enhanced polarizability due to the localized 3d Cu states [9]. Alternatively, copper sulfide (Cu x S) as a non toxic material, is a p-type semiconductor for solar cells had been performed by several groups. For example, Page et al. observed an efficiency of  0.06% in solid-state extremely thin absorber (ETA) Cu x S solar cells. They found that the performance was strongly affected by the carrier recombination at the TiO 2 /Cu x S interfaces [10]. Isac et al. observed an extremely small short-circuit current density of  0.1 mA/cm 2 in three-dimenstional Cu x S solar cells [11]. Lin et al. investigated liquid- junction quantum-dot Cu x S as a broadband sensitizer for solar cells [12]. Both Page and Lin's Cu x S solar cells were fabricated on a TiO 2 electrode. In this report, we investigated the photovoltaic performance of Cu x S liquid-junction SSCs fabricated on a new electrode-SnO 2 film, using the successive layer adsorption and reaction (SILAR) process to investigate the photovoltaic performance of the Cu x S solar cells as a function of SILAR cycles. The performance was improved by applying various passivation treatments and annealing effect on the solar cells. 2. Experimental details A TiO 2 compact layer was coated on the FTO glass by spin-coating a 0.2 M titanium isopropoxide (TIP) solution onto a clean FTO glass substrate, followed by heating at 450 1C for 30 min. Then, the SnO 2 paste wa Q2 s prepared and spread onto a FTO glass substrate (15 Ω/□, Nippon Sheet glass) with an active area of  0.3  0.3 cm 2 using the doctor blade technique as reported previously [12]. A scattering layer (SCL, particle size E350–450 nm, WER4-O, Dyesol) was coated on top of the SnO 2 layer. The thickness of the SnO 2 and SnO 2 /SCL photoelectrodes were determined to be  6 and 10 μm, respectively by an alpha-step profiler. Cu x S nanoparticles were synthesized using the SILAR process by dipping a SnO 2 electrode into the 25 1C, 0.02 M Cu(NO 3 ) 2 ethanol solution for 1 min, rinsed with ethanol, then heated at 75 1C for Cu 2 þ cations and dipped into a 25 1C, 0.08 M Na 2 S methanol solution for 1 min, washed with methanol, then heated at 75 1C for S 2 anions. This two-step dipping process formed one SILAR cycle. After the SILAR process, the prepared Cu x S QDs were annealed at 200 1C in flowing N 2 gas for three minutes. A ZnS passivation layer 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/matlet Materials Letters http://dx.doi.org/10.1016/j.matlet.2015.02.007 0167-577X/& 2015 Published by Elsevier B.V. n Corresponding authors. E-mail addresses: tubtimtae@gmail.com (A. Tubtimtae), mwl@phys.nchu.edu.tw (M.-W. Lee). Please cite this article as: Tubtimtae A, et al. Tailoring Cu x S semiconductor-sensitized SnO 2 solar cells. Mater Lett (2015), http://dx.doi. org/10.1016/j.matlet.2015.02.007i Materials Letters ∎ (∎∎∎∎) ∎∎∎–∎∎∎