PTB7-Th, PCE10, PBDTTT-EFT

Usage Details

Reference Devices

Reference devices were made on batch M261 to assess the effect of PBDTTT-EFT:PC₇₀BM active layer thickness on OPV efficiency with the below structure. These were fabricated under inert atmosphere before encapsulation and measurement under ambient conditions.

Glass / ITO (100 nm) / PEDOT:PSS (30 nm) / PBDTTT-EFT:PC₇₀BM (1:1.5) / Ca (5 nm) / Al (100 nm)

For generic details please see the fabrication guide and video. For specific details please see the below condensed fabrication report which details the optical modelling and optimisation of the multilayer stack.

The PBDTTT-EFT:PC₇₀BM solution was made in chlorobenzene (CB) at 35 mg/ml before being diluted with 3% diiodooctane (DIO) to promote the correct morphology.

Active layer thicknesses were achieved from spincasting the film at spin speeds of 2000, 2700, 3900 and 6000 rpm for 30s. Additionally, a methanol wash was performed for all devices to help remove the DIO additive. For each of these spin speeds a total of 2 substrates (3 in the case of 2700 rpm) was produced, each with 8 pixels and the data presented below represents a non-subjective (no human intervention) analysis of the best 75% of pixels by PCE (18 pixels for 2700 rpm condition, 12 pixels for each other).

Overall, the average efficiency of 8.30% PCE (9.01% maximum) was found from a 2700 rpm spin speed.

Note on effect of epoxy: Due to the very high solubility of the PBDTTT-EFT it was noted during fabrication that the film changed colour when in contact with the encapsulation epoxy in liquid form for extended periods indicating that there was some miscibility. Inspection of the active areas underneath the top cathode indicated that the epoxy had not seeped into the active area before curing and device metrics indicate that this did not appear to affect performance. However, we would recommend minimising contact time between the epoxy and PBDTTT-EFT films before UV curing.

Condensed Fabrication Routine

Substrates and cleaning

All solvent cleaning was carried out using Ossila Annealing and Cleaning Beakers with Ossila's complimentary Substrate Rack for Cleaning and Storage.

• Edgeless 8 pixel substrates (S211)
• 5 minutes sonication in hot 10% NaOH solution
• 1x boiling deionised water (DI) dump rinse, 1x cold dump rinse
• 5 minutes sonication in hot 1% Hellmanex III
• 2x boiling DI rinses, DI
• 5 minutes sonication in warm isopropyl alcohol (IPA)
• 1x boiling DI dump rinse, 1x cold dump rinse
• N₂ blow dry
• Substrates held on a hotplate at 120°C before spin-coating the hole transport layer (no further cleaning or surface treatment required)

PEDOT:PSS

• AI 4083 PEDOT:PSS filtered through a 0.45 µm PES filter (C2009S1)
• Spin on heated substrates at 6000 rpm for 30s
• Bake at 120°C after spincast
• Cathode strip wipe with cleanroom swab dipped in DI, replaced back on hotplate until transfer to glove box
• Additional bake in the glove box for 30 mins to remove residual moisture

Active Layer Solution

• Fresh stock solutions of PBDTTT-EFT (M261) made at a concentration 14 mg/ml in anhydrous CB and dissolved at 70°C for 1.5 hours
• Mixed with dry Ossila 95% C70 PCBM at a blend ratio of 1:1.5 to make an overall solution concentration of 35 mg/ml
• Mixed in 3% DIO and then heated the solution at 70°C with a stirbar for 2 hours
• Cooled prior to spincasting

Active layer test films

• Test film spun at 2000 rpm for 30s using unfiltered solution with a methanol wash before measuring with a Dektak surface profiler
• Reference film displayed a thickness of 140 nm

Active layers

• Devices spun using 30 µl dynamic dispense for 30s
• Methanol wash was then immediately performed as a secondary spin step, 20 µl at 4000 rpm for 30 seconds
• Cathode wiped with CB

Evaporation

Left in vacuum chamber overnight and evaporated with the below parameters.

• 5 nm Ca at 0.2 Å/s
• 100 nm Al at 1.5 Å/s
• Deposition pressure

Encapsulation

• As standard using Ossila Encapsulation Epoxy (E132) with glass coverslips (C181): 30 mins UV exposure in glove box using UV lamp MEGA LV101

Measurements

• JV sweeps taken with Keithley 237 sourcemeter
• Illumination by Newport Oriel 9225-1000 solar simulator with 100 mW/cm² AM1.5 output
• NREL certified silicon reference cell used to calibrate
• Lamp current: 7.8 A
• Solar output at start of testing: 1.00 suns at 23 °C
• Solar output at end of testing: 1.00 suns at 25 °C
• Air cooled substrates
• Room temperature at start of testing : 25 °C
• Room temperature at end of testing: 25 °C
• No aperture mask, pixel size: 0.4 mm²