An Efficient Silicon (Oxide) Based n/n+/p Distributed Bragg Intermediate Reflector For Multi- Junction Solar Cells

Transcription

1 An Efficient Silicon (Oxide) Based n/n+/p Distributed Bragg Intermediate Reflector For Multi- Junction Solar Cells Simon Kirner 1, Andre Hoffmann 2, Max Klingsporn 1, Patrick Krüger 1, Karsten Bittkau 2, Bernd Stannowski 1 and Rutger Schlatmann 1 1 PVcomB, Helmholtz-Zentrum Berlin, Germany 2 IEK-5, Forschungszentrum Jülich, Germany May 9 th, 2016, PVTC, Marseille

2 Outline Current limitation in thin film silicon solar cells Distributed Bragg Intermediate Reflectors (DBIR) Proof-of-concept for a-si / a-si / DBIR / µc-si triple junctions Conclusions 2

3 Advantages of Multi-Junction Solar Cells Reduction of thermalization- and sub-bandgap losses Reduction of ohmic series resistance losses P loss = j mpp ² * R s Hirst et al, PIP, 2011; 19: Silicon is very good, low-cost bottom cell material, improved top cell needed High V mpp > 1.23V allows direct water splitting Thin-film silicon based integrated PV-EC show STH >9.5% Urbain et al, EES, 2016; 9:

4 Thin Film Silicon Multijunction Solar Cells (p 1 ): 20 nm + RL (p 2 ): 20 nm + RL (p 3 ): 30 nm (p 4 ): 30 nm a-si:h µc-si:h µc-sio x :H AR foil/ Glass capannealed HCl etched, ZnO:Al (~800 nm) (i 2 ): 320 nm (n 1 ) : 20 nm (i 1 ) : 70 nm (low Temperature) (n 4 ): 25 nm (i 3 ): 1750 nm (i 4 ): 2800 nm 80 nm AZO / 200 nm Ag (n 2 ): 80 nm + RL (n 3 ): 25 nm + RL back contact / -reflector TF-Si specific advantage: Lower light induced degradation (LID) Trade-off thickness: (thin) low current density vs. (thick) high LID 2nd a-si:h cell limits the current density Kirner et al, Jap. JAP, 2015; 54, 08KB03 Solution: Intermediate reflector layer 4

5 Intermediate Reflector Layers EQE, 1-R Change in current density (ma/cm²) TC BC Rtot IRL tcknss (nm) Top cell Bottom cell ZnO:B ZnO:Al (etched) SnO 2 :F IRL thickness (nm) Example a-si / µc-si double junction Top cell current can be increased At the same time, bottom cell current and R tot is reduced Lower total current Optimum IRL thickness determined by constructive interference Coherent case optimum thickness: t IRL* = l * / ( 4 n IRL ) (depends on light scattering TCO surface) Better / more selective IRL desirable for AZO Distributed Bragg Reflectors Kirner et al, IEEE J-PV, 2013; 4,

6 Distributed Bragg Intermediate Reflectors (DBIR) a-si:h n o High index layer (HIL) n 2 c-si:h n s Total reflection at interface can be enhanced by: combination of constructive interferences of partial reflections Low index layer (LIL), n 1 Example TF-Si, ZnO/SiOx DBIR Reflection dependent on: number of layers N refractive index contrast n o, n 2, n s versus n 1 Optical thickness Light scattering Previous realizations in TF-Si solar cells: ZnO/SiOx (n-type) SiOx/Si (n-type) Myong et al, SOLMAT, 2013; 119, 77. Hoffmann et al, Optics Express, 2014; 22,

7 Approach n/n+/p DBIR for a-si / a-si / µc-si triple cells (n)nc-sio x :H / (n)nc-si:h / (p)nc-sio x :H - (n)nc-sio x :H as low index layer, ~2.2 + (n)nc-si:h as high index layer ~ 3.5 alternatively (n)a-si:h as high index layer ~ 4.0 (p)nc-si:h as second low index layer ~ 2.7 Tunnel recombination junction: adjacent layers have to be highly doped 7

8 Model prediction based on experimental input data angular intensity distribution AID T,Si [a.u.] IR reflectance R IR 0.4 TIR,IR / 30 / / 20 / x 80 / 10 / / 0 / 0 (n) nc-sio x t [nm] a-si:h (p) nc-sio x scattering angle [ ] R IR -> 1 x=60 x=50 x= wavelength l [nm] R IR -> 0 HIL type Light path enhancement calculated rigorously based on angular distribution from AZO topography from AFM n, k data from R, T and PDS measurements Light propagation within full stack calculated coherently by transfer matrix algorithm Optimum thicknesses (n)nc-sio x HIL (p)nc-sio x a-si nc-si

9 Experimental results Energy filtered TEM micrograph: phase separation in nc-sio x visible a-sio x phase: low index Si phase: high conductivity (p) material has lower oxygen content EQE sum: 22.0 ma/cm² 22.2 ma/cm² n+ layer 20 nm (a-si:h) none wavelength (nm) Application in a-si / a-si / µc-si triple junction cell: Proof-of-concept, external quantum efficiency measurements: With inclusion of high index layer: mid cell current increase by 7% total current increases 9

10 Experimental j-v results J SC (ma/cm²) V OC (V) (n)nc-sio x : (p)nc-sio x : nm 65 nm n+ layer nc-si:h a-si:h layer thickness (nm) thin BC FF (%) (%) LID (h) (n)a-si and (n)nc-si layers with various thicknesses have been tested J sc trend like expected FF drops with HIL thickness matching TRJ, nucleation(?) V oc decreases with a-si TRJ, nucleation(?) Best cell a-si / a-si / µc-si (w/o AR) (~1.2 µm thickness) Eta (%) Voc (V) FF (%) Jsc (ma/cm²)

11 Conclusions Reflectivity and selectivity of intermediate reflector can be enhanced by DBR stack Experimental proof-of-concept for n / n+ / p all in-situ PECVD silicon (oxide) based DB- IRL stack in a-si /a-si /µc-si triple junction solar cell (n)nc-si:h works better than (n)a-si:h as high index layer Best cell: Eta = 11.3% and 10.5% after one week of LID (w/o AR) Acknowledgements Technical Support: Matthias Zelt, Khalid Bhatti, Christoph Schultz, Hoora Sarajan, Matteo Werth, Tobias Henschel Funding Helmholtz Association Energie System 2050 Thank you for your Attention! 11