Photonic structuring and transport for bulk heterojuction polymer solar cells Rene Lopez Department of Physics and Astronomy University of North Carolina at Chapel Hill

Talk Outline Introduction and Motivation Photonics Photonic crystal design Nanopatterning using P.R.I.N.T. Resonant mode enhancements Electro-optical improvements Transport Connection to optical profile Scaling of photocurrent with light intensity Hole transport limited for P3HT:PCBM Conclusions and Outlook

Motivation 3 TW 10% efficient solar cells

Mitsubishi Chemical 9.2% Just this april 2011 Exciting time for OPV 6.8% efficiency 6.0% efficiency Scharber et al., Adv. Mater. (2006). Servaites et al., Appl. Phys. Lett. (2009).

9.2% Efficiencies of Photovoltaic Cells Multijunction ~ 50% ~11%

1.Absorption of photon, α -1 2.Charge separation 3.Carriers transport through drift and diffusion, L c The Photovoltaic device

Anode Cathode EFEF EFEF (2) Donor Acceptor - (3) (4) (5) (1) (4) (5) (2) (3) + - Diffusion length (5~10nm) LUMO HOMO LUMO HOMO (1) Light absorption for exciton generation (2) Exciton diffusion (3) Exciton transfer for hole-electron separation (4) Carrier transport towards the electrodes (5) Charge collection at the respective electrodes OMe O PCBM P3HT Physics on Bulk Heterojunction Solar Cells

1. Lc/α -1 ratio ~ peak wavelengths 2. Over MOST of their absorption Lc/α -1 < 1, devices are thus losing great number of absorbed photons into lost carriers Find better materials? Yes, 999,999 nerds are working on that…but… What can we do? it possible to enhance optical absorption while also improving electrical performance ? Key elements: Lc/α -1 are intrinsic, but absorption and carrier travel distances no! How do BHJSCs and DSSCs stand in terms of characteristic lengths?

Designs to Enhance Light Absorption Kim et al., Adv. Mater. (2006). Tvingstedt et al., Appl. Phys. Lett. (2007). Ray Optics 50% increase in J sc Wave Optics θ 1 mm 1 μm

Wave Optics Approach via Periodic Nanostructures Na et al., Adv. Funct. Mater. (2008). Yu et al., Proc. Natl. Acad. Sci. (2010). Tikhodeev et al., Phys. Rev. B (2002). Natural Inspiration Diffraction Grating Absorption enhancement L = 1200 nm 80 nm 60 nm 5 nm 150 X enhancement compared to 4n 2 for randomly roughened surface Complex Nanostructures 1.8 μm

A Look at Nature Vikusic et al. 1.8 μm

Resonant Mode Excitations n 1 = i n 2 (pillars) = 2.5 Absorption in increased even with reduction in volume of absorbing material Absorption spikes depend on physical dimensions, so desired region of solar spectrum can be targeted (e.g. near band edge)

Absorption Enhancement Prediction J.R. Tumbleston et al., Appl. Phys. Lett. (2009). 2-D periodic1-D periodicPlanar 17% increase13% increase 1-D periodic 2-D periodic key

Optical Optimization J.R. Tumbleston et al., Opt. Express (2009). 2-D periodic1-D periodicPlanar nc-ZnO index of refraction

Detailed Optical Model J.R. Tumbleston et al., Opt. Express (2009). Flash Nanostructure local absorption profile important

The chosen pattern ZnO nc 400 nm 220 nm Top View 175 nm post height 22% increase in absorption Blend

Photonic Crystal Solar Cell Fabrication Nano Lett. (2009). J. Phys. Chem. C (2010). Spin coat nc-ZnO Aluminum Polymer blend PEDOT:PSS ITO Glass substrate Pressure + Heat PFPE mold Si master same substrate Polymer blend

Robust Material: TDPTD Nano Lett. (2009). Nanopatterning over large area Photonic crystal and planar cells fabricated on same substrate Nanopattern periodicity on order of wavelength of light (400 nm) Refractive index contrast between BHJ and nc-ZnO

Nano Lett. (2009). Encouraging Results for TDPTD:PCBM

Angular Dependence of IPCE and Optical Properties s-pol p-pol θ = 15°

P3HT:PCBM Planar cell Flash layer Nano- pattern layer Photoactive layer (Planar) PC cell

The good news and the so-so news Nanofabrication optimization PlanarPatterned Jsc6.36 mA/cm28.93 mA/cm2 Voc0.58 V0.61 V FF Efficiency1.89 %2.92 %

What about electrical processes? Well understood and well modeled optical enhancements Very nice nanopatterning technique 400 nm short long carrier density Distance from Anode E-field We wanted a steady-state measurement of carrier transport for functional devices using photoexcited carriers that are extracted from device. e - /h + What effect does pattern have on charge creation and transport?

single device 100 nm → 10 nm Light From ITO – “electron restricted” From Al – “hole restricted” θ = 0° λ = 473 nm E-field h+h+ e-e- 225 nm J.R. Tumbleston et al., Phys. Rev. B (2010).

100 nm → 10 nm Light θ = 0° λ = 473 nm 90, 225 nm D e (nm)D h (nm) 225 nm from ITO nm from Al nm from ITO nm from Al nm active layer225 nm active layer

Summary Photonic cell Light absorption enhancement 3. Nano Lett. 9, 2742 (2009). 4. J. Phys. Chem. C (2011). 5. Soft Matter (2011). 1. Opt. Express 17, 7670 (2009). 2. Appl. Phys. Lett. 94, (2009). 6. Phys. Rev. B 82, (2010). 7. J. Appl. Phys. 108, (2010). 8. J. Appl Phys. 108, (2010). nanofabrication, instrumentation, device characterization electro-optical measurements, transport lengths in active cells

Working towards model of PC device performance Oxide P3HT:PCBM Lateral distance (m) Height (m) Anode Cathode Log (E-field) Anode Cathode Oxide P3HT:PCBM Lateral distance (m)

Toward the actual butterfly structure Forest of Silicon pine treesArray if Silicon oxide/ Silicon nitride xmas trees

Acknowledgments: Students: John tumbleston (patterned organic PV) Kristen Alexander (gold SERS) Emily Ray (metamaterials work) Yingchi Liu (Cu 2 O-ZnO work) Rudresh Ghosh (Naobia DSSCs) Posdocs: Mukti Aryal Abay Dinku Doo-Hyun Ko from Ed Samulski’s group Meredith Hampton from Joe Desimone group Kyle Brenneman from Tom Meyer group Christoph Kirsh from Sorin Mitran group Lopez group Main collaborators And last but certainly no least: the current sponsors of the Lopez lab projects: Currently looking for more good people, ask me !