Low Temperature Atomic Layer Deposition for Flexible Dye Sensitized Solar Cells

Transcription

1 ALD4PV workshop Low Temperature Atomic Layer Deposition for Flexible Dye Sensitized Solar Cells V. Zardetto, D. Garcia-Alonso, A.J.M. Mackus, M.A. Verheijen, T. Brown*, W.M.M. Kessels, M. Creatore * In collaboration with University of Rome Tor Vergata

2 Introduction ALD is an ultra-thin film deposition technique attractive for DSCs In literature: Thermal ALD on glass based devices Blocking layer on the TCO Passivation layer on the TiO 2 mesoporous electron acceptor FLEXIBLE DSCs require high quality thin film layers at low temperature (T< 150 C) PAGE 1

3 Plasma-assisted ALD for Flexible DSC Plasma process Highly reactive species at low thermal budget H. Profijt, Ph.D Thesis Plasma-surface interaction in plasma-assisted atomic layer deposition Extension of ALD temperature window down to 25 C Higher film density and lower impurity levels; Increased choice of precursors; PA- ALD enables the deposition of metals at low temperature PAGE 2

4 Outline Low temperature plasma-assisted ALD for flexible DSC applications 1. Blocking layer via plasma assisted ALD: TiO 2 Dielectric materials Al 2 O 3 and SiO 2 2. Synthesis of platinum nanoparticles via low-t ( C) plasma assisted ALD Use of Pt NPs as counter-electrodes on ITO/PEN. PAGE 3

5 Blocking Layer on TCO Blocking layer on the TCO To prevent back-recombination reactions at TCO /electrolyte interface (2); To allow electron injection from the electron acceptor to the TCO (3); Improvement of V OC under low light illumination* back reaction via TiO 2 back reaction via TCO Injection from electron acceptor to TCO Requirements for Blocking layer on ITO/PEN Thin layer processed at temperature max 150 C * Cameron et ai, J. Phys. Chem. B 2005, 109, PAGE 4

6 Platinum nanoparticles by ALD Platinum NPs on the TCO To catalyze the reduction of I 3 - into I - Distributed NPs ( high surface area) for good catalytic activity Requirements for an efficient platinum layer for back side illuminated FDSC High transmittance in visible range High electro-catalytic activity towards I - 3 species in the electrolyte I 3 + 2e 3I Applied Physics - PmP group PAGE 5

7 Platinum ALD process Novel Pt Plasma process (A) MeCpPtMe 3 dosing (B) O 2 plasma exposure (C) H 2 gas (C*) H 2 plasma Knoops et al., Electrochem.Solid-State Lett. 12, G34 (2009) Mackus et al., Chem. Mat. 25, 1769 (2013) Thermal ALD Pt based on MeCpPtMe 3 and O 2 : C Plasma-assisted ALD Pt based on MeC p PtMe 3 and O 2 plasma: C. Low T processing requires stronger oxidizing agents (O 2 plasma, ozone) and an additional step (C or C*) to convert PtO x to Pt. PAGE 6

8 ALD platinized counterelectrodes 10 Nucleation and growth of ALD Pt on ITO/PEN Thickness (nm) C 150 C Number of ALD cycles 25 C C : island coalescence, followed by layer-by-layer growth, after 300 cycles. 25 C: delayed nucleation. 50 cycles 100 cycles TAILORING of Pt NPs size changing the number of ALD cycles Garcia-Alonso, V. Zardetto et al., Adv. Energy Mat. 4, (2014) PAGE 7

9 ALD platinized counterelectrodes Optical Characterization of Pt NPs on ITO/PEN Charge transfer resistance (R CT ) at Pt NPs/ liquid electrolyte interface C: ALD cycles : transmittance of Pt (T Pt ) > electrodeposited (ED) and sputtered (SP) platinum layers 25 C T Pt close to 99% C: good catalytic performance for 50 cycles. 25 C: at least 300 cycles required to achieve similar catalytic performance. Garcia-Alonso, V. Zardetto et al., Adv. Energy Mat. 4, (2014) PAGE 8

10 ALD platinized counterelectrodes J-V curves and PV parameters of Flexible back-side illuminated DSCs Number of ALD cycles (100 ) η(%) J SC (ma/cm 2 ) V OC (mv) FF (%) C FF is significantly affected for ALD cycles < 50, due to the higher value of R CT. J SC (and V OC ) decreases for ALD cycles > 100, due to the lower Pt layer transmittance (lower light harvesting) At RT, h reaches 3.5% at cycles, influenced by the improvement in R CT. Garcia-Alonso, V. Zardetto et al., Adv. Energy Mat. 4, (2014) PAGE 9

11 ALD platinized counterelectrodes J-V curves and PV parameters of Flexible back-side illuminated DSCs ALD: 100 o C 100 cycles η(%) J SC (ma/cm 2 ) V OC (mv) FF (%) ALD ED SP Higher J SC ( and V OC ) for ALD platinized CE than SP and ED CEs (higher transmittance- light harvesting). Garcia-Alonso, V. Zardetto et al., Adv. Energy Mat. 4, (2014) PAGE 10

12 Conclusions on ALD Pt NPs Low temperature PA-ALD was successfully used to deposit platinum nanoparticles on ITO/PEN for metal based DSCs. Morphological, optical, electrochemical characterizations of Pt NPs on ITO/PEN Electrocatalytic properties depend on Pt content and also NPs surface area ALD-platinized CEs improved efficiency with respect to sputtering (+29%) and electrodeposition (+19%) platinized CEs PAGE 11