Nanotechnologies. National Institute for Materials Science (NIMS)

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1 Dye-Sensitized Solar Cells with Nanotechnologies Liyuan Han Advanced Photovoltaics Center National Institute for Materials Science (NIMS)

2 Expectations to PV market 12,000 World mark ket scale (MW) 10,000 8, ,000 4,000 2, ,000 6,150 3,950 2,446 1,870 1,382 1, Source: IEA PVPS 2007

3 Scenario for improving the economic efficiency of PV power generation PV2030

4 Types of solar cells Silicon Bulk Thin film (a-si, uc-si) Single crystalline Multi crystalline Solar cells Compound GaAs CdS,CdTe,CulnGaSeCdT C S 2 New materials Dye-sensitized Thin film (Polymer SC) Quantum Dot

5 Advanced Photovoltaics Center Dye sensitized OPV QD Compound Si thin film TCO TiO 2 Dye Electrolyte CE Light e - I 3 - I - TCO PCBM Light e - Polymer - CE Interband e - e - e - h+ h+ h+ Multi exiton p i n p i n Glass SnO 2 a Si uc Si ZnO Ag Materials, Fundamental, Simulation

6 Dye Sensitized solar cell TCO TiO 2 Dye Electrolyte (I - /I 3- ) CE Light I - I 3 - External circuit electron Merit 1) Low manufacture cost No vacuum process Cheap materials 2) Colorful 3) Flexibility

7 Features of dye-sensitized solar cells - 1 Device Concept: Artificial Photosynthesis TCO TiO 2 Dye Electrolyte e 導電帯 S* Fermi Level CE LHS e - Red Vmax Ox e - RC S e - LHS The principles of DSCs are different from those that conventional solar cells, are more similar to plant photosynthesis, as light absorption (dye) and carrier transportations in both TiO 2 and electrolyte in DSCs occur separately.

8 DSC Equivalent Circuits of DSC 1F 2 F C Element C Main origin 3 1 I R Resistance of TCO R 3 R 1 Rs R 3 I sc Electrolyte Z 2 R sh R h Series Resistance R h R 1 Electrolyte/CE Diffusion in the Diode R 2 TiO 2 /Dye/Electrolyte The experience & knowledge of Si solar cells are useful to DSCs In the DC condition Si Solar Cells I sc R s R sh I V R s = R h + R 1 + R 3 Equivalent circuit of DSC is similar to that t of Si solar cells when working in DC condition. L. Hanet el, Appl. Phys. Lett. 84, 2433(2004)

9 Improvement of efficiency of dye-sensitized solar cells

10 Approaches for improving efficiency Jsc Voc FF 20 Curre ent Densit ty (ma/cm m2) Jsc FF Voc : V Jsc : 18.9 ma/cm 2 FF : : 9.0 % Voc ) Jsc 2) Voc 3) FF Voltage (V) Typical I-V curve of the cell

11 Improvement of Jsc Enhancement of the light harvesting efficiency J SC qf( )(1 r( )) IPCE( ) d energy[mw/(s scm*10nm)] energy[mw/(scm*10nm)] IPCE (%) Wavelength (nm) IPCE (%) h nt Glass ransparen ductive G Tr Cond TiO 2 film Nanocr rystalline Electrolyt te Cou unter Elec ctrode h (1) Improvement of IR-response by development of new dyes (2) Increase of effective light path length

12 (1) Improvement of IR-response by development of new dyes Photocurrent action spectra of Ru-based sensitizers (%) IPCE COO - TBA + HOOC N NCS 40 N HOOC N Ru N NCS COO - TBA + N719 Jsc: 17mA/cm 2 HOOC Jsc: 21mA/cm 2 HOOC OC N HOO N Ru N NCS NCS NCS Black dye Wavelength / nm

13 Photovoltaic Properties of Fph-tfac sensitizer 80 IPCE = 82% IPC CE / % 60 F F 40 N N 20 O F TBAO O F O O Ru N N C S OH Fph-tfac OTBA O J sc = 22.0 ma cm -2 V oc = 059V Wavelength / nm 13

14 STM images of dye/tio 2 surface Low Jsc Absorption of dye without DCA High Jsc Absorption of dye with DCA [001] nm nm 2 Aggregation Isolated LANGMUIR, 24, 8056 (2008)

15 (2) Increase of effective light path length in TiO 2 Transparent TiO 2 film L Externa IPCE (% al %) QE Optical path length = L External quantum efficiency (QE) spectrum 1 Scattered film TCO TiO 2 Electrolyte Transparent film 0 Scattered TiO 2 film Wavelength (nm) CE J SC qf( )(1 r( )) IPCE( ) d Effective path length >> L Light scattering effect

16 How to estimate the light scattering effect? T0 TCO TCO Haze of TCO substrates has been used as a useful index of light trapping effects in a- Si solar cells. Ti Tj Ti A. Loffl et al., EUPVSEC-14, (1998) H i ( Ti TT i T ) Incident light T0 j i, j TiO 2 Tj Ti Tj

17 Effect of haze on IPCE and Jsc QE (%) (%) External IPCE Q Haze Haze 60% Haze 53% Haze 36% Curre ent density (ma/cm Wavelength (nm) Haze 800nm (%) Y. Chiba et al. Jpn. J. Appl. Phys. 45 L638 (2006) 17 Haze of TiO 2 is a useful index for improvement of Jsc.

18 2) Improvement of Voc Suppression of recombination TCO TiO 2 Electrolyte CE I 3 - I - I - Regeneration I 3 - e - Injection e - e - I - Electrolyte Recombination TiO 2 e - I q( V IR ) V I ph I0 exp 1 nkt R S IR S Sh

19 Effect of TBP Improvement of Voc COOH with TBP SCN SCN Ru SCN Ru COO- COO- e - Jsc (ma/cm m2) CH 3 TiO 2 0 H 3 C C N CH 3 TBP without TBP Voltage (V) Voc was improved by absorption of TBP

20 3) Improvement of FF Improvement of FF L.Han et el, Appl. Phys. Lett. 84, 2433 (2004) R CE R ELE R TCO I TCO TiO 2 Electrolyte CE I - I 3 sc R sh I - Series Resistance: R s R s = R CE + R ELE + R TCO Reductions of R CE, R ELE and R TCO were investigated.

21 Elevating of Voc without decrease of Jsc Curren nt Density (ma/cm m 2 ) No additive Adding of THF Voltage (V) A. Fukui, L. Han et al., Sol. Energy Mater. Sol. Cells, 90, 649 (2006).

22 Reduction of R CE ( cm 2 ) Roughness factor TCO TiO 2 Electrolyte CE RF = S total /S proj S total RCE 1 0 I - 3 I /RF S proj measured by AFM RF: increase R CE : decrease R CE decreased by using high roughness factor of CE. L.Han et al. Appl. Phys. Lett (2005)

23 Reduction of R ELE 8 Thickness 6 TCO TiO 2 Electrolyte CE 2 RELE R ( cm ) cm 2 I - 3 I Thickness of electrolyte layer ( m) Thickness: decrease R ELE : decrease R ELE 0.7 cm 2

24 Optimization of series resistance of DSC ( cm 2 ) Resist tance optimize sheet resistance and transmittance of TCO Increase surface roughness of CE Decrease thickness of electrolyte layer Befo ore optim mization Rh R1 R3 Total R TCO R CE R ELE FF : Afte er optimiz zation

25 Results Jsc : 18.9 ma/cm 2 Jsc : 20.9 ma/cm 2 Voc : V Voc : 0.73 V FF : F.F. : 0.72 The I-V curve of the DSC was independently measured by the public test center (AIST, JAPAN).

26 The Highest Efficiency for single cell Effi : 11.1% Jsc : 20.9 ma/cm 2 Voc : V FF : (AIST: National Institute of Advanced Industrial Science and Technology)

27 How to further improve the efficiency? Efficiency of DSC > 11% in small cell Cell Effi.(%) Practical module Effi. (%) Single crystal Si Poly crystal Si Thin film Si ) Improvement of cell structure 2) Understanding of mechanism 3) Development of new materials

28 1) Improvement of cell structure Absorbance of TCO T % counter electrode (CE) Electrolyte Back Contact Dye-sensitized Solar Cells (BCDSCs) Back contact electrode(bce) Counter electrode(ce) Electrolyte 40 TiO Dye TCO TiO 2 10 h Merit Glass 0 Low Cost High Transmittance Wavelength(nm) Chem. Mater., Chemistry of Materials, 20, 4974 (2008) J. Appl. Phys., 104, (2008) hν =7.1% : 8.9%

29 Thank you for your kind attention Acknowledgements A part of this work was carried out in SHARP Corp. I acknowledge the colleagues.