“Plastic” Electronics and Optoelectronics Low Cost “Plastic” Solar Cells Light emitting Field Effect Transistor (LEFET) CLEO Baltimore May 7, 2007

Low Cost “Plastic” Solar Cells: A Dream or Reality??? We have an energy problem. Nuclear, hydroelectric, wind etc must all contribute to the solution. Solar cells --- Power from the Sun must be --- and will be a significant contribution. Two problems that must be solved with solar: 1. Cost 2. Area

Printing of Plastic Electronics “inks” ---- with electronic functionality! Plastic Substrate Solar Cells We should remember to explain that “Plastic electronics is based on conducting and semiconducting polymers Much lower capital and materials cost.  The Dream  Functional Ink

“Plastic” Solar Cells Ultrafast charge separation with quantum efficiency approaching Unity ! e - ħ R O 1992 50 femtoseconds!!

Ultrafast electron transfer is important --- Charge transfer is 1000 times faster than any competing process, and Back charge transfer is inhibited Therefore, Quantum efficiency for charge separation approaches unity! Every photon absorbed yields one pair of separated charges! (Route to photovoltaics and photodetectors)

How do we create a material with charge-separating junctions everywhere?? e - ħ R O All must be accomplished at nanometer length scale!

Self-assembled nanoscale material with PCBM P3HT Bulk Heterojunction Material Bicontinuous interpenetrating network Self-assembled nanoscale material with charge-separating junctions everywhere!

“Bulk” D-A Heterojunction Material A self-assembled nanomaterial Electrical Contact Must break the symmetry --- use two different metals with different work functions. Electrons will automatically go toward lower work function contact and holes toward higher work function contact

“Bulk” Heterojunction Material Before 150oC anneal 150oC anneal 30 min 150oC anneal 2 hours P3HT PCBM TEM images of the P3HT/PCBM interpenetrating network

P3HT/PCBM (1:0.8) prior to annealing

After annealing for 10 minutes at 150oC

Spatial Fourier Transform Relevant length scale --- peak corresponds to structure with “periodicity” of 16-20 nm Large scale segregation or slow variation in thickness for distances > 100 nm III. Noise at very short length scales

Spatial Fourier Transform High temperature annealing --- 150o C 1. Stable for long times at High-T 2. 10 minutes is sufficient to lock in the structure

”Period” 160 -200 Å Distance from any point in the material to a charge separating interface  40-50 Å Less than the mean exciton diffusion length --- High charge separation efficiency.

Series resistance decreased from 113  to 7.9  Solar Cell Performance Device structure: ITO/PEDOT/P3HT:PCBM/Al. Eff = 5.0% VOC = 0.625 V FF = 68% w/o anneal 70oC, 30 min 150oC, 30 min Series resistance decreased from 113  to 7.9 

New Device Architecture: Optical Spacer ITO Glass PEDOT Al Active Layer Conventional Device Device with Optical Spacer Light intensity in Dead zone Optical spacer

Device architecture Charge separation layer TiOX Ti OR OH + H2O O   Device architecture   Ti OR OH + H2O TiOX O Glass ITO PEDOT:PSS P3HT:PCBM TiOX Aluminum Charge separation layer

Power Conversion Efficiency Green light (532 nm) Efficiency e = 8.1% → 12.6% AM1.5 (solar spectrum) Efficiency e = 2.3% → 5.0% Optical spacer: 50% improvement !

Polymer Solar cells: Present status in our labs --- 5% - 6% What can we expect to achieve?? --- Eff  15% New Architecture --- optical spacer --- 50% improvement (already included) Devices with eff.  5 - 6% fabricated with P3HT Band gap too large --- missing half the solar spectrum Opportunity: Potential for 50% improvement using polymer with smaller band gap Increase open circuit voltage --- Opportunity: Potential for 50% improvement Tandem Cell --- Opportunity: Potential for > 50% improvement

Semiconducting polymers with smaller band gap? ( ) n O C H 3 a b c Zhengguo Zhu (ZZ50) Absorption and photoresponse out to 900 nm in the IR. Should be capable of 7% in single cell and >10% in a Tandem Cell with P3HT

Best performance for a single cell architecture ( ) n O C H 3 a b c ZZ50 5.5% Best performance for a single cell architecture

Tandem Cell (Multilayer architecture equivalent to two solar cells in series) Open circuit voltage doubled Efficiency 6.5%

We can do even better ---- Increased performance of each of the two subcells --- Already demonstrated. Goal for coming months: 10%

Air-stable polymer electronic devices Thin layer of amorphous TiOx (x<2) improves performance and Enhances Lifetime of Polymer Based Solar Cells

Bulk Heterojunction Solar Cells Single TiOx passivation layer signficantly enhances the lifetime Factor of 100 ! Goal: Simple inexpensive barrier materials will be sufficient for achieving long lifetime.

Lifetime: Light soak of flex devices Flex OPV with low cost packages pass 1000 hrs @ 65°C, 1 sun Fully flexible devices under 1 sun, 65°C Flexible devices packaged with commercial, low cost films (Konarka)

Lifetime: Damp heat stability Flex OPV with low cost packages pass 500 hrs in damp heat. Cells are fairly stable, though slow degradation observed % Efficiency change under 65C/85%RH -100 -80 -60 -40 -20 20 40 200 400 600 800 1000 1200 Time (hours) % Change of Efficiency Structure 1 Structure 4 Flexible devices packaged with commercial, low cost films

Environmental lifetime (Konarka) Little (no!) degradation observed after ½ year outdoor testing. OPV modules loaded on roof top test in Lowell, MA Reduced efficiency observed in the winter period is not degradation --- efficiency increases with increasing temperature.

Polymer Solar cells: Present status in our labs --- 5 - 6% What can we expect to achieve?? --- Eff  15% New Architecture --- optical spacer --- 50% improvement (already included) Devices with eff.  5 - 6% fabricated with P3HT Band gap too large --- missing half the solar spectrum Opportunity: Potential for 50% improvement using polymer with smaller band gap (ongoing ---) Increase open circuit voltage --- Opportunity: Potential for 50% improvement Tandem Cell --- Opportunity: Potential for > 50% improvement

Active development program at Konarka Technologies Can we achieve Eff > 10% with the polymer “bulk heterojunction” nano-material? Active development program at Konarka Technologies in USA and in Europe.

Goal: Roll-to-roll manufacturing by printing and coating technologies Joint Venture: Konarka - KURZ (Germany)

Low Cost “Plastic” Solar Cells: A Dream Becoming a Reality

Light Emitting FETs Gate-Induced Insulator-to-Metal Transition --- Field induced metallic state in organic FETs Light Emitting FETs and --- the relationship between the two

Field Effect Transistor Drain Source Semiconductor Layer ++++ Gate Insulator Gate Electrode When voltage is applied between gate and source-drain, mobile electronic charge carriers (positive or negative) are induced into the semiconductor. Can we increase the field induced carrier density to levels sufficient to cross the insulator-to-metal boundary???

Can we increase the field induced carrier density to high enough levels sufficient to cross the insulator-to-metal boundary??? Q = CV = (A/d)V N = A(/e)(V/d) nA = (/e)(V/d) electrons (or holes) per unit area Vmax/d = EBreakdown These charges are confined to a thin layer (approx 10 nm) in the semiconductor adjacent to the interface with the gate dielectric. Conclusion: n  1020 cm-3 --- approaching metallic densities

Gated four-probe measurement (P3HT) Material deposition by spin casting Device annealed at 235C Carrier mobilities  0.1 cm2/Vs source L = 16 m drain W = 1000 m SiO2 dielectric, 200 nm gate, Si (n++) We measure the voltage drop inside the transistor channel Typically Rcontact < channel resistance

Field-induced M-I Transition in poly(3-hexyl thiophene) --- PRL (2006) P3HT VG = 150 V Metallic VG = 50V Critical line VG = 40 V Insulator Power law indicates critical line M-I Transition in 2d --- (Zero-T Quantum Phase Transition)

Conclusion: We can reach metallic densities in organic FETs (gate–induced through field effect) n  1020 cm-3 Question: Can we reach metallic densities (field-induced) for both electrons and holes in the same device? Answer: Yes! Light-emitting Field Effect Transistor (LEFET)

Polymer Light Emitting Field-Effect Transistor hv Drain (Ag) Conjugated Polymer Source (Ca) + + + + + + + + + + + + + + - - - - - - - - - - - - - - - - - - Gate Dielectric Gate Qualitative description of device operation: Gate controlled “p-n junction” Light emission from the overlap recombination zone

Gate voltage controls the electron current, the hole current, and the brightness!

Maximum efficiency at crossover point where electron and hole currents are equal.

Device physics analyzed in detail by Smith and Ruden experiment theory D. L. Smith and P. P. Ruden, Appl Phys Lett 2006, 89, 233519

|Ids|1/2 vs gate voltage, (where Ids is the total current between the Au and Ca electodes). µh = 2.1 x10-5 cm2/V/s µe = 1.6 x10-5 cm2/V/s Vth(holes) = 150V -103V = 47 V Large threshold voltages imply disorder and a high density of traps at the semiconductor-dielectric interface. Vth(electrons) = 65 V

and holes in the -band only for a narrow range of gate voltages |Ids|1/2 vs gate voltage, (where Ids is the total current between the Au and Ca electodes). The LEFET operates in the ambipolar regime with electrons in the *-band and holes in the -band only for a narrow range of gate voltages 65 V < VG < 103 V.

Calculated curves (Smith and Ruden): (b) Calculated curves (Smith and Ruden): Gate-to-Channel potential and Carrier Densities (electrons and holes) vs x Gate to channel potential  0 between the hole and electron accumulation regions. This location defines the center of the light emission zone (and the center of the charge recombination zone). D. L. Smith and P. P. Ruden, Appl Phys Lett 2006, 89, 233519

Definitive demonstration of LEFET Location of emission zone controlled by gate voltage Good agreement with Smith and Ruden VG = 87 V VG = 93 V VG = 99 V Definitive demonstration of LEFET

Spatial Resolution of the Emission Zone (EZ) Diagram of a confocal microscrope (Prof. S. Burrato, UCSB) Images of the emission zone collected as EZ is moved across the channel  Cross section plots of emission intensity vs. lateral position Full width at half maximum approx 2 m (determines recombination rate, Smith & Ruden)

Emission at the extremes --- VG  0 and VG  150 V Occurs adjacent to the Ca and the Au electrodes: PLEDs with electron (hole) accumulation layer extending all the way across the channel as electrode with hole (electron) injection by tunneling Hole accumulation layer acts as low resisitance cotnact across the channel Electron accumulation layer acts as low resistance contact across the channel

Conclusions At the voltage extremes, the electron (hole) density extends all the way across the 16 m channel length such that the accumulation layer functions as the electrode for an LED with opposite carrier injection by tunneling. In these two limits the carrier densities (electrons in the *- band or holes in the - band) are sufficiently high that the accumulation layer functions as a low resistance contact, implying near metallic transport. These very long distances for electron and hole accumulation, respectively, more than 10,000 repeat units on the semiconducting polymer chain and more than 10,000 interchain spacings imply the existence of well-defined - and *- bands with a very small density of deep traps within the band gap.

The LEFET as the route to injection lasers fabricated from luminescent semiconducting polymers The data imply inverted populations: High density of electrons in *-band High density of holes in - band 2. Minimal loss from the source and drain electrodes 3. Minimal loss from ITO gate electrode 4. Electron & hole densities confined to narrow region near the gate dielectric

Light emitting FETs The high gate induced carrier densities imply inverted population in the LEFET  High density of electrons in the *–band and  High density of holes in the –band. LEFET Injection Laser: Important Opportunity

Acknowledgement Professor Kwanghee Lee ---UCSB and GIST (Korea) Dr. Jin Young Kim Christoph Brabec and colleagues at Konarka James Swensen (LEFET)