Transparent conducting graphene electrodes for photovoltaic applications

Transcription

1 Transparent conducting graphene electrodes for photovoltaic applications Luca Ortolani 1, Caterina Summonte 1, Rita Rizzoli 1, Meganne Christian 1, Isabella Concina 2,3, Gurpreet S. Selopal 2,3, Riccardo Milan 2,3, Alberto Vomiero 3,4, Vittorio Morandi 1 1. CNR-IMM, Via Gobetti 101, 40129, Bologna, Italy. 2. SENSOR Lab, Department of Information Engineering, University of Brescia, Via Valotti 9, Brescia, Italy. 3. CNR-INO SENSOR Lab, Via Branze 45, Brescia, Italy. 4. Luleå University of Technology, Luleå, Sweden.

2 as transparent conductive electrode is 2D crystal made of sp2 hybridized Carbon atoms. has remarkable physical and chemical properties from flat spectrum transparency, high electrical and thermal conductivity. It is an excellent candidate to replace MO film as transparent conductive electrodes (TCEs) in many technological applications. vs. different TCEs types R 10 2 Ω/ T % [ F. Bonaccorso et al. Nature Photonics 4 (2010), 611 ] [ S. Bae et al. Nature Nanotechnology 5 (2010), 574 ] 2

3 key properties for technological exploitation Flexibility Conductance (ms) Conductance (ms) Bending angle (2θ) [ L.G. de Arco et al. ACS Nano 5 (2010), 2865 ] Bending angle (2θ) High-temperature resistant Chemical Stability (in air) ITO structure changes over 500 C [Thomas Swan Corp. Elicarb graphene ] can sustain high in plane strains without damaging, it can sustain temperature above 800C and its structure provides excellent chemical stability over a wide range of harsh environments. 3

4 CVD synthesis and characterization CVD Growth is grown using CVD over either foils or films of Cu at 1000C using methane as C precursor. Samples max. dim.: 30x60mm 2 (60x100 mm 2 for foils) P regimes: APCVD, LPCVD Gas lines: Ar, N 2, H 2, NH 3, CH 4, C 2 H 2, O 2 LPCVD APCVD Polycrystalline graphene is grown on both substrate types, with typical uniform thickness of 1-4 layers over the whole surface. Grain size is over 10 µm. APCVD samples on Cu films have smaller grain size and higher number of defects. SEM TEM Raman 4

5 electrodes for dyesensitized solar cells. In collaboration with A. Vomiero group, CNR-INO Brescia (IT) & Lulea TU (SE). 5

6 DSSC design Counter electrode: Pt (about 5 nm thick) on TCO glass Electrolyte: iodine redox couple Light harvester Dye: usually a Ru complex (called N719) Photoanode: TiO 2 NPs (transparent: nm NPs); scattering ( nm NPs) TCE must provide adhesion during titania annealing and during wet operation. can provide superior optical properties and improved chemical stability towards electrolyte etching 6

7 DSSC based on graphene-coated front contact Transparent TiO 2 layer (for photogeneration purposes) on glass over a large area (15 25 mm 2 ) Deposition of one single layer of transparent 20 nm-sized TiO 2 nanoparticles (Dyesol) at a time TiO o C for 30 mins for sintering purposes has to maintain conductivity, transparency and adhesion to the substrate after annealing graphene FTO ITO TCE Dye band + TiO2 + TiO2 +Dye -glass electrode shows superior optical properties and the dye could be successfully loaded on the TiO 2 - graphene substrate. [G.Singh Selopal, L.O. et al. Sol. En. Mat. Sol. Cell 135 (2015) ] 7

8 DSSC cells characterization J-V characteristics as a function of the TiO 2 film thickness under simulated sunlight (AM 1.5 G, 100 mw cm -2 ) Optimal Titania layer thickness is reported to be 15 µm rgo TCE in an all solid-state DSSC, PCE at 0.26 % (Wang et al., Nano Lett. 2008) PCE linear increase is related to J SC linear increase. FF and V OC play a minor role in boosting PCE 2% PCE for 4 µm -DSSC higher series resistance (420 ohm) compared to FTO (10 ohm) limits the performances of the final device [G.Singh Selopal, L.O. et al. Sol. En. Mat. Sol. Cell 135 (2015) ] 8

9 as TCE in thirdgeneration PV cells In collaboration with C. Summonte group in CNR IMM-Bologna 9

10 Third generation photovoltaics with nanostructured absorbers Third Generation Photovoltaics goal is to achieve higher efficiency using thin film technology as in Second Generation PV (p-i-n a-si:h thin film solar cells): use of abundant and low cost material (as a-si:h) better use of the solar spectrum reduction of all optical losses NASCEnT: SILICON NANODOTS FOR SOLAR CELL TANDEM Fabrication of Si nanocrystals by means of high-t thermal treatment of Si rich SiO 2 /SiC Transparent Conductive Electrode should sustain processes up to 1100 C 10

11 Issues on the introduction of Si-NCs within a photovoltaic (PV) device Attempts of introducing Si-NCs within a PV devices have been reported in the literature. Perez-Wulf, APL 95 (2009) Conibeer Prog Photov 19 (2011) 813 Perez-Wulf SEMSC 2012 The problem of the TCE has been circumvented by modifying the design, exposing the TCE to low-t annealing Wu, SEMSC 128 (2014) 435 Löper APL 102 (2013) Löper Adv. Mater 24 (2012) 3124 However, all reported devices are useful for material characterization, but the introduced constraints make At the moment none of the approaches succeeded in producing a working device Song SEMSC 92 (2008) 474 Janz 28thEPVSEC Paris

12 Test process to check high-t resistance of graphene TCE 1. transfer on quartz Quartz 2. Protective capping of graphene a-si Testing of graphene TCE resistance to high-temperature annealing 1. PECVD deposition of 40 nm a-si:h capping 3. Anneal a 1100C in N 2 Quartz 1100C 2. Annealing at different T in flowing N 2 3. Capping removal in 2% diluted tetramethyl ammonium hydroxide (TMAH) 4. Test optical and electrical properties 4. Capping removal in TMAH Quartz The capping is needed to prevent graphene burning due to residual oxygen in the annealing atmosphere during the annealing step for NPs formation 12

13 Thermal treatments: optical and electrical stability Annealed 600C 2h N 2 Best samples maintain their optical properties up to 1100C a-si residues reduced transparency Residues from a-si capping affect few samples optical performances. A spread is observed after G-sheet transfer on fused silica substrates due to the transfer procedure After deposition and removal of the capping layer and after thermal treatment, the spread does not increase Compatibility of graphene based TCE with high temperature processes (1100C) confirmed 13

14 Introduction of annealed G in p-i-n devices: results and comparison with ITO PiN full processed device with graphene electrode Silver contact n-asi:h asi:h adsorber p-asi:h Fused Silica 1000 C, 30' annealed graphene has been introduced as TCE within a standard amorphous silicon p-i-n device (process temperature: 250 C) Measurement under simulated sunlight (AM1.5G) show nice devices. a-si residues Higher Sheet Resistance Increased V OC suggests improved band alignment between graphene and p-type selective contact. [Fujii 28th EPVSEC p.2694] A moderate decrease of J sc is observed for G after 1000 C, probably due to incomplete a-si capping removal. Improved band alignment High series resistance observed: better sheet resistance required! 14

15 Introduction of G in an actual n-i-p structure including Si-NCs: issues and open points PiN full processed nanostructured device with graphene electrode Silver contact p-asi:h a-sic:h / a-src n-a-si:h Process sequence: PECVD deposition, through a mask of the precursor of the Si-nc material (400 nm) Furnace annealing in fluent N C 4h ( hydrogen evolution needed to avoid explosive evolution during crystallization) 900 C C 30 Fused Silica 30 min Silver contact Strain due to H desorption and recrystallization and delamination of the multilayer film 1100C n-asi:h Si NC in SiC matrix p-a-si:h Fused Silica 200 µm 15

16 Conclusions can be applied in operating DSSCs even after high temperature sintering, with a very good PCE (about 2 %) Successful dye loading on G-coated electrode without detaching problems up to high TiO 2 thickness layer. It will possible to further increase TiO 2 layer thickens to improve the optical density of the film (known result: optimum thickness about 15 mm). Measured PCE is almost 8 times higher than previous achievements in comparable devices. Compatibility of graphene based TCE with high temperature processes has been proven TCE can sustain processes up to 1100 C without significant degradation in terms of sheet resistance and transmittance (contrary to ITO, which deteriorates above 900 C). sheet-resistance in TCE must be improved Doping of graphene, Transfer of multiple graphene membranes, at the cost of worsening T%, improvement of the synthesis, with larger domains, reduced defects and impurities. 16

17 Thank you all for your attention Luca Ortolani CNR IMM- 17