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Fabrication and Characterization Fabrication and Characterization of Organic Semiconductors f for Use in Photovoltaics U i Ph t lt i Department of Chemical Engineering D t t f Ch i l E i i Eric Bonaventure Chris Carach – Mentor Prof. Michael Gordon – P.I. *Funded by*

Why Are Organic Semiconductors Important? Important? Organic Solar cell Silicon Solar Cell • Easy to mass produce…much cheaper • Silicon needs to be purified to p 99.9999% (no impurities/defects) • Materials defects/impurities are tolerable • Time and energy to produce Si is costly is costly • Flexible substrates, large area solar cells • Payback time can take many years

Why Are Organic Semiconductors Important? Important? Organic Solar cell Silicon Solar Cell • Easy to mass produce…much cheaper • Silicon needs to be purified to p 99.9999% (no impurities/defects) • Materials defects/impurities are tolerable • Time and energy to produce Si is costly is costly • Flexible substrates, large area solar cells However, efficiencies are low… • Payback time can take many processing really affects film morphology years Understanding organic semiconductors at multiple length scales (device-level to nano) is a key to increasing efficiency

What is my role in the research? OMe Electrode hv O e ‐ P3HT P3HT donor PCBM acceptor S Semiconductor n Transparent Conductor Glass • Fabricate different varieties of solar cells • Develop a “protocol” to create the cells • Test cells and analyze their respective IV curve • Test cells and analyze their respective IV curve. (Current vs. Voltage)

“Dark Current” Voltage V lt Voltage V V Source Source *Should see current current* Positive Negative Solar cell Solar cell Negative Negative Positive Positive I I *Should not Ammeter see current* see current Ammeter Ammeter Ideal Diode Curve • Conduct a voltage sweep g p • Does current move both directions? • Does the solar cell work? • Does the solar cell work? Forward Reverse bias bias

Photocurrent Test With M Monochromator h • Monochromator turns white V lt Voltage V light (all colors) into one color Source • Can test what wavelength • Can test what wavelength (color) of light is most efficient Negative Solar cell Positive Positive Light A A Ammeter Fiber optic cable cable

Photocurrent Results IV Curve 40 [uA] 20 Current [ 0 ‐ 1 1 ‐ 0.5 0.5 0 0 0.5 0.5 1 1 C ‐ 20 Dark Current Dark Current ‐ 40 40 100 Percent Intensity ‐ 60 Voltage [V]

Photocurrent Results IV Curve Useable energy created from light created from light 40 uA] 20 urrent [u 0 ‐ 1 1 ‐ 0.5 0 5 0 0 0 5 0.5 1 1 C ‐ 20 Dark Current Dark Current ‐ 40 100 Percent Intensity ‐ 60 Voltage [V]

Photocurrent Results IV Curve 40 increasing light [uA] intensity 20 Current [ 0 ‐ 1 1 ‐ 0.5 0.5 0 0 0.5 0.5 1 1 C ‐ 20 Dark Current ‐ 40 40 10 Percent Intensity 25 Percent Intensity ‐ 60 50 Percent Intensity 100 Percent Intensity Voltage [V]

Conclusions • Polymer dissolution was critical in spinning high quality films quality films • Slowing down solvent evaporation during and after spin coating fosters polymer chain ft i ti f t l h i organization → increases charge transport efficiency • P3HT/PCBM device with PCBM overlayer gave the best performance best performance • Oxidative damage to the conjugated polymer inhibited charge transport inhibited charge transport

Future Research ~ Nanoscale Characterization~ N l Ch t i ti Topographical analysis with Atomic Force Microscopy

Future Research ~ Nanoscale Characterization~ N l Ch t i ti Topographical analysis with Atomic Force Microscopy Topography of semiconducting film 0nm 750nm

Future Research ~ Nanoscale Characterization~ N l Ch t i ti Topographical analysis with Atomic Force Microscopy Topography of semiconducting film PCBM crystal Material Material heterogeneity at nano/micro scale is observable! Well-mixed 0nm 750nm

Future Research ~ Nanoscale Characterization~ N l Ch t i ti Analyze point ‐ by ‐ point current reading with Tunneling AFM (TUNA) Nanoscale IV measurement…at every point!

Future Research ~ Nanoscale Characterization~ N l Ch t i ti Analyze point ‐ by ‐ point current reading with Tunneling AFM (TUNA) Nanoscale IV measurement…at every point! +30V Tip Bias Tunneling Current Current 10 μ m 850pA 150pA

Perspective Perspective Macroscopic testing shows us a good vs bad solar Macroscopic testing shows us a good vs. bad solar cell…but not the why

Perspective Perspective Macroscopic testing shows us a good vs bad solar Macroscopic testing shows us a good vs. bad solar cell…but not the why We need to understand more about nanoscale interactions to produce higher efficiencies

Perspective Perspective Macroscopic testing shows us a good vs bad solar Macroscopic testing shows us a good vs. bad solar cell…but not the why We need to understand more about nanoscale interactions to produce higher efficiencies Correlating macroscale performance with nanoscale Correlating macroscale performance with nanoscale morphology is necessary to understand and engineer better organic semiconducting materials and solar cells

Acknowledgements Chris Carach Prof. Michael Gordon *All the Gordon Group Gophers* F Funding from di f

Dark Current vs Photocurrent Diode Curve Diode Curve Photocurrent Photocurrent (Dark Current) (Just from light)

PV Stack PV Stack Electrode C 60 P3HT/C 60 PEDOT/PSS ITO Glass

Efficiency Efficiency = 100 mW/cm 2 Power = I * V Power = Current * Voltage Power Current Voltage = Efficiency = Efficiency =

The Monochromator

Set up for photocurrent test

The solar cell The solar cell

Split exciton Pair Exciton Pair Shine light to excite an to induce current electron electron flow LUMO (P3HT) LUMO (PCBM) ITO ALUMINIUM e ‐ + HOMO (P3HT) HOMO (P3HT) HOMO (PCBM)

Current Density vs Voltage 100 0 [uA/cm 2 ] -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 -100 -200 t Denisty [ -300 -400 -500 Dark Current Curren -600 600 Photocurrent -700 -800 -900 900 Voltage [V]

Power Overview Power Overview IV Curve Power = Current * Voltage g 20 A] 10 10 rrent [uA 0 Current equals ‐ 1 ‐ 1 0 0 1 1 Cur zero ‐ 10 Dark Current ‐ 20 Photocurrent Photocurrent ‐ 30 ‐ 40 40 Voltage [V]

Power Overview Power Overview IV Curve Power = Current * Voltage g 20 A] 10 10 rrent [uA 0 ‐ 1 ‐ 1 0 0 1 1 Cur ‐ 10 Dark Current ‐ 20 Voltage equals g q Photocurrent Photocurrent zero ‐ 30 ‐ 40 40 Voltage [V]

Power Overview Power Overview IV Curve Power = Current * Voltage g 20 Intentisty graph 10 10 [mA] 0 ‐ 1 ‐ 1 0 0 1 1 M Maximum i Current [ ‐ 10 power!!! Dark Current ‐ 20 C Photocurrent Photocurrent ‐ 30 ‐ 40 40 Voltage [V]

Analyzing Results IV Curve IV Curve 40 100 0 A] 20 20 u] rrent [nA ent [mu ‐ 1.00 0.50 ‐ 100 0 ‐ 200 ‐ 1 ‐ 1 0 0 1 1 Cur Curr ‐ 20 ‐ 300 ‐ 400 ‐ 40 ‐ 500 Dark Current Dark Current ‐ 60 ‐ 600 Photocurrent Photocurrent ‐ 80 80 ‐ 700 Voltage [V] Voltage [V]