A Novel TiO x Protection Film for Organic Solar Cells

Transcription

1 A Novel TiO x Protection Film for Organic Solar Cells Mool C. Gupta 1, John T. Yates, Jr 2 (principle investigators) J. Li 1, Y. Shen 1, S. Kim 2, S. Edington 2, Shinuk Cho 3, Kwanghee Lee 3, and Alan J. Heeger 3 1. Department of Electrical and Computer Engineering, University of Virginia 2. Department of Chemistry, University of Virginia 3. Heeger Center for Advanced Materials, University of California at Santa Barbara Supported by nanostar, University of Virginia

2 Organic Solar Cells Advantages Low cost Solution processable Light weight Flexible geometry Al electrode PCBM/P3HT Organic Active Layer ITO electrode Glass substrate Light

3 Issues with Organic Solar Cells Low efficiency Poor long term stability, short life time

4 Previous Work Thin film of TiO x Enhanced efficiency due to optical spacer Extended lifetime Mechanism is unclear Al TiOx Active layer Reference: Heeger et al.

5 Mechanism Study: Photoactivation of TiO x Photoactivation of TiO x proposed by Yates group UV Light exposure in the presence of O 2 (gas) produces electron hole pairs which evolve to produce adsorbed O 2. The production of O 2 from adsorbed O 2 is coupled to O hole stabilization and subsequent oxidation of the alkoxide moiety. TiO x Reference: S. H. Kim et al

6 Mechanism Study: Protection Mechanism Mechanism Ti OR functionalities in TiO x and Ti OH groups are photooxidized, consuming O 2 (gas) and producing CO 2 (gas) and H 2 O(gas). The photoactivation of these films leads to O 2 scavenging and forms the basis for thin films which remove oxygen when exposed to light,

7 Mechanism Study IR spectrum Gas phase photooxidation products from TiO x film containing isopropoxide functionalities. CO 2 (gas) and H 2 O(gas) are observed Reference: S. H. Kim et al

8 Mechanism Study ESR spectrum after photooxidation After exposure to UV irradiation for 15 minutes, three strong ESR features are observed. The three ESR transitions are assigned to adsorbed O2 Reference: S. H. Kim et al

9 Photovoltaic Device Structure Al (80nm) TiO x (20nm) P3HT/PCBM Layer (120 nm) PEDOT:PSS (40nm) ITO (150nm 200nm) Glass (0.9 mm)

10 Degradation in dark Oxygen effect Stabilization in N 2 Normalized Efficiency TiO x protection in Air Degradation in Air Exposure Time (hours)

11 Degradation in N 2 UV effect With TiO x protection UV:10 mw/cm 2 Normalized Efficiency No TiO x protection Exposure Time (hours)

12 Degradation under combined effect of UV and oxygen exposure With TiO x protection Normalized Efficiency No TiO x protection UV:10 mw/cm 2 Exposure Time (hours)

13 UV intensity effects Degradation in O 2 With TiO x protection under weak UV Normalized Efficiency No TiO x protection under weak UV With TiO x protection under strong UV No TiO x protection under strong UV Exposure Time (hours) (Weak UV:10 mw/cm 2 strong UV: 300 mw/cm 2 )

14 Photovoltaic characterizations Normalized Fill Factor Normalized Short Normalized Open Normalized Circuit Current Circuit Voltage Efficiency Photovoltaic parameters in N 2 Similar results in air Major loss due to lowering of the short circuit current and the fill factor Exposure Time (hours) UV:10 mw/cm 2

15 Conclusions TiO x acts as a protection layer TiO x is activated by UV to absorb O 2 and this process protects polymer from oxygen degradation Demonstrated enhanced stability using TiO x under UV: 30 times improvement

16 Future work New material Optimal TiO x synthesis Doped TiO x material New characterization technique Electrical properties and mobility Temperature dependence of degradation

17 Outcomes/ Funding support Outcomes: Developed collaboration with Professor Alan J. Heeger (Nobel prize winner) at UC Santa Barbara Two scientific publications Submitted J. Li, M. C. Gupta, S. Edington, J. T. Yates, Jr, S. Cho, K. Lee and A. J. Heeger, A Study of Stabilization of P3HT/PCBM Organic Solar Cells by TiO x Layer S. Kim, S. Edington, M. C. Gupta, J. Nedy, J.T. Yates, Jr, S. Cho, K. Lee and A. J. Heeger, Photochemically Activated TiOx Sol Gel Film Observation of Oxygen Gas Removal Using a Built in Organic Scavenger Functionality Graduate student supported J. Li Funding support Proposed to NSF Thank nanostar, the Institute for Nanoscale and Quantum Scientific and Technological Advanced Research at the University of Virginia for their financial support