Organic semiconductors

Transcription

1 Plastic (Organic) Solar Cells: Accomplishments, Challenges, and Strategies Sumit Chaudhary Assistant Professor Department of Electrical and Computer Engineering Materials Science and Engineering Iowa State University, Ames IA

2 Organic semiconductors Function comes from form!

3 Some representative OPV materials Donors Acceptors Enormous variety of molecules is a great strength of organic photovoltaic But it also brings challenges

4 Conventional solar cells vs. Plastic solar cells Photogenerated electon hole pair (exciton) are tightly bound

5 Heterojunction vs. Bulk-heterojunction Inefficient i t s architecture t State of the art architecture

6 Progress in photovoltaic efficiencies i i

7 OPV reaches 10% efficiency

8 Companies working on OPV (partial list)

9 OPV products & prototype installations

10 OPV products & prototype installations

11 Typical device: Bulk lkh Heterojunction Blend of Donor and acceptor phases Phaseseparation separation ~ exciton diffusion length Should have continuous pathways for electron and hole transport. Anode (ITO ) LUMO Donor + HOMO LUMO Acceptor HOMO Cathode (Al, Mg, Ca) 100 nm nm ~ 100nm Glass Transparent electrode e - h + Metal Electrode Hole transporting material (Donor) Electron transporting material (Acceptor)

12 Major issues (losses) Theoretical efficiency ~ 23 % Realized efficiency ~ 8% Poor absorption of light Efficiency Losses Recombination losses 38% Low charge mobility 17% 45%

13 Poor Absorption High band gap of polymer less absorption of higher wavelength photons Moderate carrier mobility requires thin active layer Absorption length requires thick active layer Incident light Thin blend film Good collection of charges Transmitted light Requires thin layer for absorption due to light trapping Poor absorption!! Solution Textured substrates for Light trapping 13

14 Best Photonic design (FEM) inbuilt ilt in COMSOL MULTIPHYSICS best design: 2 μm pitch-1.5 5μm height show 40% enhancement. Nalwa KS, Chaudhary S, Design of light-trapping microscale-textured surfaces for efficient organic solar cells Optics Express, 2010, 8 (5),

15 Best Practical Design Reduce height to submicron Increase pitch

16 Photovoltaic characteristics Incident light 20 % Recombination losses in valley Micronscale pitch (2 µm) + submicron height (300 nm) with conformal active layer leadsto highest photocurrent. Nalwa KS, JM Park, KM Ho, Chaudhary S, On realizing efficient polymer solar cells using a textured substrate platform Advanced Materials, 23, (2011).

17 Novel transparent electrodes (metal based) Conventional Our structure

18 Issues Theoretical efficiency ~ 23 % Realized efficiency ~ 8% Poor absorption of light Efficiency Losses Recombination losses 38% Low charge mobility 17% 45%

19 A Solution to enhance exciton dissociation Addition of ferroelectric polymer For 3% volume fraction (f) addition E ~ 240 V/µm poly(vinylidenefluoride) PVDF δ+ δ- P Why PVDF-TrFE copolymer? Solution processibility High dielectric constant (~10) Existence in ferroelectric β-phase at room temperature.

20 Photovoltaic performance and cause Light scattering by PVDF (n = 10) Enhanced charge transport Enhanced exciton dissociation 10 % ferroelectric addition gives highest efficiency. 50% improvement in efficiency

21 Enhanced Exciton Dissociation? Count ts % PVDF < > = 100 ps 5% PVDF < > = 95 ps 10% PVDF < > = 73 ps 20% PVDF < > = 122 ps LUMO P3HT HOMO Time (ns) + Indeed!!

22 In summary Organic photovoltaics are at an exciting stage. More challenges hll ahead in terms of efficiency and stability We realized two ways to improve efficiency of organic solar cells: 20% improvement 50% improvement Solution for thin layer OPV Solution for thick layer OPV

23 Acknowledgements STUDENTS Kanwar Nalwa (now at Intel) Rakesh Mahadevapuram John Carr Yuqing Chen Moneim Ismail COLLABORATORS Kai-Ming Ho, Baskar Ganapathysubramanian, Jake Petrich $$$$ Iowa Power Fund from State of Iowa s Office of Energy Interdependence National Science Foundation CAREER Award 2011