Byeongseop Song

Effect of Mixed Layer Crystallinity on the Performance of Mixed Heterojunction Organic Photovoltaic Cells

Organic vapor-phase deposition (OVPD) is used to grow tetraphenyldibenzoperiflanthen (DBP):C70 mixed heterojunction photovoltaic devices. Compared with vacuum thermal evaporation (VTE), the OVPD-grown film develops nanocrystalline domains of C70. Optimized OVPD-grown OPVs have a 61% fill factor for a 100 nm active layer thickness, whereas VTE-grown devices have a 47% fill factor at the same thickness.

Exciton-blocking phosphonic acid-treated anode buffer layers for organic photovoltaics

We demonstrate significant improvements in power conversion efficiency of bilayer organic photovoltaics by replacing the exciton-quenching MoO3 anode buffer layer with an exciton-blocking benzylphosphonic acid (BPA)-treated MoO3 or NiO layer. We show that the phosphonic acid treatment creates buffers that block up to 70% of excitons without sacrificing the hole extraction efficiency. Compared to untreated MoO3 anode buffers, BPA-treated NiO buffers exhibit a ∼ 25% increase in the near-infrared spectral response in diphenylanilo functionalized squaraine (DPSQ)/C60-based bilayer devices, increasing the power conversion efficiency under 1 sun AM1.5G simulated solar illumination from 4.8 ± 0.2% to 5.4 ± 0.3%. The efficiency can be further increased to 5.9 ± 0.3% by incorporating a highly conductive exciton blocking bathophenanthroline (BPhen):C60 cathode buffer. We find similar increases in efficiency in two other small-molecule photovoltaic systems, indicating the generality of the phosphonic acid-treated buffer approach to enhance exciton blocking.

Surprisingly High Conductivity and Efficient Exciton Blocking in Fullerene/Wide-Energy-Gap Small Molecule Mixtures

We find that mixtures of C60 with the wide energy gap, small molecular weight semiconductor bathophenanthroline (BPhen) exhibit a combination of surprisingly high electron conductivity and efficient exciton blocking when employed as buffer layers in organic photovoltaic cells. Photoluminescence quenching measurements show that a 1:1 BPhen/C60 mixed layer has an exciton blocking efficiency of 84 ± 5% compared to that of 100% for a neat BPhen layer. This high blocking efficiency is accompanied by a 100-fold increase in electron conductivity compared with neat BPhen. Transient photocurrent measurements show that charge transport through a neat BPhen buffer is dispersive, in contrast to nondispersive transport in the compound buffer. Interestingly, although the conductivity is high, there is no clearly defined insulating-to-conducting phase transition with increased insulating BPhen fraction. Thus, we infer that C60 undergoes nanoscale (<10 nm domain size) phase segregation even at very high (>80%) BPhen fractions.

Morphology control in fullerene-based buffer layer by organic vapor phase deposition

The morphology of small molecular weight organic thin film mixtures used in organic photovoltaics (OPV) is precisely controlled through growth conditions used in organic vapor phase deposition (OVPD). We found that the crystallinity of C60 in 3, 5, 3, 5-tetra(m-pyrid-3-yl)phenyl[1,1]biphenyl (BP4mPy):C60 buffer layers undergoes changes as a function of chamber growth pressure. The chamber growth pressure is optimized at 0.28 torr, resulting in the largest C60 crystalline domains. The power conversion efficiency of organic vapor phase deposited tetraphenyldibenzoperiflanthene (DBP):C70 planar mixed heterojunction OPVs using the optimized BP4mPy:C60 buffer layer is (8.0±0.2)% compared to (6.6±0.2)% using0 amorphous buffers grown by vacuum thermal evaporation.