Supporting Information

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1 Supporting Information Semiconductor Nanocrystals as Luminescent Down-Shifting Layers to Enhance the Efficiency of Thin-Film CdTe/CdS and Crystalline Si Solar Cells Sergii Kalytchuk,, Shuchi Gupta, Olga Zhovtiuk, Aleksandar Vaneski, Stephen V. Kershaw, Huiying Fu, Zhiyong Fan, Eric C. H. Kwok, Chiou-Fu Wang, Wey Yang Teoh and Andrey L. Rogach,* Department of Physics and Materials Science and Centre for Functional Photonics (CFP), City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong SAR Clean Energy and Nanotechnology (CLEAN) Laboratory, School of Energy and Environment, City University of Hong Kong, Shatin, N.T., Hong Kong SAR Department of Electronic and Computer Engineering, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong SAR Du Pont Apollo Limited, Hong Kong Science Park, Shatin, N.T., Hong Kong SAR * To whom correspondence should be addressed. andrey.rogach@cityu.edu.hk. 1

2 Performance of two photovoltaic modules, CdTe/CdS and c-si Table S1. Photovoltaic module parameters I SC (ma/cm 2 ) V OC (V) FF η (%) CdTe/CdS c-si Fig. S1. Performance of CdTe/CdS solar cell: (a) current-voltage characteristic; (b) external quantum efficiency. Fig. S2. Performance of c-si photovoltaic module: (a) current-voltage characteristic; (b) normalized external quantum efficiency provided by the manufacturer. 2

3 Chemicals. Cadmium acetate dihydrate (Cd(CH 3 COO) 2, 97.0%), cadmium oxide (99.99%), 1-octadecene (ODE, 95%), octadecylamine (ODA, 97%), sodium borohydride (NaBH 4, >98%), selenium powder (99.5%), sulfur ( %), trioctylphosphine (TOP, 97%), trioctylphosphine oxide (TOPO, 99%), tributylphosphine (TBP, 97%), stearic acid (98.5%) were purchased from Sigma Aldrich, n-propylphosphonic acid (PPA, 95%), sodium tellurite (Na 2 TeO 3, 99%) were purchased from Aldrich, oleic acid (OA, AR), oleylamine (OlAm, 80-90%), thioglycolic acid (TGA, 90%) were purchased from AccuChem, lead(ii) chloride (PbCl 2, 99%) was purchased from Acros Organics, n-octadecylphosphonic acid (ODPA, 97%) was purchased from PlasmaChem. All organic solvents were purchased from Sigma Aldrich. All chemicals were used directly without any further purification. Synthesis of Water-Soluble CdTe QDs. Aqueous based CdTe QDs stabilized by thioglycolic acid were synthesized by the method of Wu et al. 1. At first, 0.4 mmol Cd(CH 3 COO) 2 was dissolved in 50 ml Milli-Q water in a two-necked flask. Then 36 µl TGA was injected into the above solution and the ph was adjusted to using 1M NaOH solution and left stirring for 5 min. After that 0.08 mmol Na 2 TeO 3 was dissolved in 50 ml Milli-Q water and added to the cadmium precursor solution. Then 160 mg of NaBH 4 was added to the main solution and the reaction continued for another 5 min. The Cd 2+ /TGA/TeO 2-3 molar ratio was 1:1:0.2. Further growth of the QDs was carried out under open air conditions upon refluxing at 100 C with a condenser attached and controlled by the duration of the reflux. The PL quantum yield (PLQY) of as-prepared CdTe QDs was 37% and 46% for corresponding QD sizes of 2.5 and 3.0 nm. Fig. S3. Absorption (dashed lines) and PL spectra (solid lines) of as synthesised CdTe QDs in water with two different sizes: 2.5 nm (blue lines) and 3.0 nm (red lines). 3

4 Synthesis of CdSe/CdS Core/Shell Nanocrystals. CdSe/CdS core/shell nanocrystals dispersed in toluene were synthesized by the method of Li et al. 2 Synthesis of CdSe Core Nanocrystals. CdSe nanocrystals were synthesized made by a typical reaction: in a 25 ml three-neck flask the mixture of 0.2 mmol of CdO, 0.8 mmol of stearic acid, and 2 g of ODE was heated to 200 C to obtain a clear solution. After cooling, ODA (1.5 g) and 0.5 g of TOPO were added into the flask and this mixture was heated to 280 C under an argon flow. A selenium solution made by dissolving 2 mmol of Se in g of TBP and further diluting with 1.37 g of ODE was quickly injected at this temperature and the growth temperature was reduced to 250 C. The CdSe nanocrystals were isolated and purified from the reaction mixture by a solvent extraction procedure at room temperature. Preparation of Injection Solutions. Both injection solutions were made under an argon flow. The cadmium injection solution (0.04 M) was prepared by dissolving CdO (0.615 g) in OA (10.83 g) and ODE (108 ml) at 250 C. The sulfur injection solution (0.04 M) was prepared by dissolving sulfur in ODE at 200 C. After clear solutions were obtained, the Cd injection solution was cooled down to 60 C and the sulfur injection solution was cooled to room temperature. For each injection, a calculated amount of a given injection solution was taken with a calibrated syringe using a standard air-free procedure. 2 The PLQY of as-prepared CdSe/CdS core/shell QDs was 36%. Fig. S4. Absorption (dashed line) and PL spectra (solid line) of CdSe/CdS core/shell QDs dispersed in toluene. 4

5 Synthesis of CdSe/CdS Tetrapods. CdSe/CdS tetrapods dispersed in toluene were synthesized according to Talapin et al. 3 Solutions for synthesis of CdSe/CdS tetrapods were prepared in a 25 ml three-neck flask by the mixing at 120 C for 30 min under vacuum of g (1.61 mmol) of CdO, 1.08 g (3.23 mmol) of n- octadecylphosphonic acid, g (0.12 mmol) of PPA, and 3.35 g (9.18 mmol) of TOPO. The mixture was heated to 320 C under flowing nitrogen to obtain a clear solution and afterwards the solution was cooled to 120 C and put under vacuum for 2 h, and again was heated to 300 C under flowing nitrogen. Then 1.5 g (4.05 mmol) of TOP was injected. Synthesis of the CdS/CdSe tetrapods. Trioctylphosphine sulfide (TOPS) was prepared by reacting equimolar amounts of TOP and elemental sulfur at 50 C under inert atmosphere g of TOPS was injected into the prepared Cd/ODPA/PPA/TOPO/TOP solution at 300 C, followed after 40 s by the injection of zinc blende-cdse seeds dissolved in 0.50 g of TOP. The reaction temperature was increased to 315 C and the growth of the CdS arm continued for min and was stopped by cooling the reaction mixture to room temperature. CdSe/CdS tetrapods were precipitated by centrifugation, dried under a stream of nitrogen and redissolved in toluene. The PLQY of as-prepared CdSe/CdS tetrapods QDs was 28%. Fig. S5. Absorption (dashed line) and PL spectra (solid line) of CdSe/CdS tetrapods dispersed in toluene. 5

6 Synthesis of PbS QDs. PbS QDs dispersed in tetrachloroethylene were synthesized following Moreels et al. 4 In a round bottom three neck flask, 5 g (17.97 mmol) of PbCl 2 was mixed with 50 ml OlAm. This solution was degassed for 30 min under nitrogen and simultaneously heated to 125 C. Later PbCl 2 solution was cooled to 80 C. In another three-neck flask, g (9.94 mmol) of sulfur was mixed with 30 ml of OlAm and 2.04 ml of TOP. It was stirred at room temperature and degassed for 15 min. 15 ml of S-precursor was rapidly injected into the PbCl 2 solution. The reaction was quenched in 30 seconds by adding 30 ml of methanol. PbS quantum dots were precipitated by centrifugation and dried under a stream of nitrogen. The QDs were then redissolved in 10 ml of tetrachloroethylene. The PLQY of as-prepared PbS QDs was 25%. Fig. S6. Absorption (dashed line) and PL spectra (solid line) of PbS QDs dispersed in tetrachloroethylene. 6

7 Transmission spectra of polymers: polyvinyl alcohol (PVA) and poly(methyl methacrylate) (PMMA) Fig. S7. Transmission spectra of PVA (green line) and PMMA (blue line) with film thickness of ~0.1 mm deposited onto glass substrates by doctor blading. 7

8 Absorption and photoluminescence spectra of Rhodamine 6G, Rhodamine 800, IR-26, Q Switch 5 dyes used for figure of merit values calculation Fig. S8. Normalized absorption (dashed line) and PL spectra (solid line) of Rhodamine 6G dye dispersed in ethanol. Fig. S9. Normalized absorption (dashed line) and PL spectra (solid line) of Rhodamine 800 dye dispersed in ethanol. 8

9 Fig. S10. Normalized absorption (dashed line) and PL spectra (solid line) of IR-26 dye dispersed in 1,2-dichloroethane. Fig. S11. Normalized absorption (dashed line) and PL spectra (solid line) of Q Switch 5 dye dispersed in trichlorotrifluoroethane. 1. Wu, S.; Dou, J.; Zhang, J.; Zhang, S., A simple and economical one-pot method to synthesize high-quality water soluble CdTe QDs. J. Mater. Chem. 2012, 22 (29), Li, J. J.; Wang, Y. A.; Guo, W.; Keay, J. C.; Mishima, T. D.; Peng, M. B. J., Large-Scale Synthesis of Nearly Monodisperse CdSe/CdS Core/Shell Nanocrystals Using Air-Stable Reagents via Successive Ion Layer Adsorption and Reaction. J. Am. Chem. Soc. 2003, 125, Talapin, D. V.; Nelson, J. H.; Shevchenko, E. V.; Aloni, S.; Sadtler, B.; Alivisatos, A. P., Seeded Growth of Highly Luminescent CdSe/CdS Nanoheterostructures with Rod and Tetrapod Morphologies. Nano Lett. 2007, 7 (10), Moreels, I.; Justo, Y.; De Geyter, B.; Haustraete, K.; Martins, J. C.; Hens, Z., Size-Tunable, Bright, and Stable PbS Quantum Dots: A Surface Chemistry Study. ACS Nano 2011, 5 (3),