6/10/13 Conjugated Block Copolymer Photovoltaics with near 3% Efficiency through Microphase SeparationA Fully Integrated Nanosystem of Semiconductor Nanowires for Direct Solar Water Splitting

Paper:

Conjugated Block Copolymer Photovoltaics with near 3% Efficiency through Microphase SeparationA Fully Integrated Nanosystem of Semiconductor Nanowires for Direct Solar Water Splitting

Changhe Guo, Yen-Hao Lin, Matthew D. Witman, Kendall A. Smith, Cheng Wang,
Alexander Hexemer, Joseph Strzalka, Enrique D. Gomez, and Rafael Verduzco

Keywords: Organic solar cells, block copolymer, lamellae, self assembly, P3HT, thermal annealing, Resonant soft X-ray scattering

Dead Body: Some BCP PV have been attempted, but efficiencies are low.  Mostly this is attributed to nonconjugated insulating backbones.  For bulk heterojunction organic PVs, the main difficulty is getting a good bulk heterojunction, so control over the molecular interface is lacking.

Prior work: High open-circuit voltages have indeed been demonstrated for solar cells where the active layer is comprised of blends of P3HT and other dioctylfluorene bisthienyl-benzothiadiazole alternating copolymers, 25 ternary blends composed of similar conjugated block copolymers as P3HT-b-PFTBT with donor and acceptor homopolymers,26 or polymer blends with fluorene benzothiadiazole alternating copolymers as acceptor  molecules.

Solution: Use fully conjugated BCPs where phase separation is on order of 10nm for exciton extration.  Lamellae is a good morphology because it has a large amount of surface area for a small amount of required exciton diffusion. the importance is getting blocks which line up well to transport the charge efficiently and have different enough HOMO and LUMO levels for a voltage separation.

Background: difference between the lowest unoccupied molecular orbital of PFTBT (∼ 3.5 eV)24 and the highest occupied molecular orbital of P3HT (∼ 4.9 eV)25 can yield open-circuit voltages above 1 V.

What are the most interesting results: 

• 3% efficiency, 1.2 Voc, 5mA/cm2 Isc
• higher temperature, and shorter annealing, gave better PV results
• homopolymer blends of the individual BCPs didn't result in very good PV cells
• they didn't test the capacitance, might be interesting to see C-V curve

What are the most interesting discussions:

• Interestingly, an EQE value of 31% was recorded at 400 nm where the exciton generation is mostly  attributed to the optical absorption of PFTBT, suggesting efficient exciton dissociation from photoexcitations in the acceptor domains.
• RSXRS is useful for measurement because PFTBT and P3HT differ by 285.4eV in the core electron transitions.
• They suggest that the 18nm domain spacing means that the individual domains are roughly 9nm, or similar to the exciting diffusion length
• The XRS measurments both say that at a higher annealing temperature, the BCP self-assembles into a perpendicular lamellae, so that is why the lower temp annealing isn't as good for PV
• the face-on stacking of the P3HT enhance hole extraction because higher conduction along pi-pi stacking.
• suggested that the PFTBT assembly helps to orient the P3HT to be face-on instead of head-on
• covalent bonding across the donor−acceptor interface has the potential to control charge separation and charge recombination rates
• choosing bcp blocks with complementary, instead of overlapping, absorption spectra could help performance

Materials used: 

• poly(3-hexylthiophene)−block−poly-((9,9-dioctylfluorene)-2,7-diyl-alt-[4,7-bis(thiophen-5-yl)-2,1,3-benzothiadiazole]-2′,2″-diyl) (P3HT-b-PFTBT)
• 56 wt% P3HT, 29 kg/mol total
• 18nm domain spacing
• 60-70 nm thick from anhydrous chloroform

How this applies to my work:

• could this use templating? if so, how much could it increase the ordering.  Or could it decrease defects which might recombine excitons
• could plasmonic NPs be incorportated to assist in the absorption?
• It makes sense that the lamellae would be short circuiting across both electrodes. Is there a way to ensure that only a single block be on the top and bottom respectively?
• The importance of high chi here...with higher chi, the intersection of the two blocks will be finer and therefore offer a better voltage liley
• couldn't solvent annealing be used? It will probably give btter order
• they didn't image with SEM or TEM.  That can help with testing the order of the film.
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Applications:

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