Elucidating the spatial arrangement of emitter molecules in organic light-emitting diode films

The effect of varying the emitter concentration on the structural properties of an archetypal phosphorescent blend consisting of 4,4′-bis(N-carbazolyl)biphenyl and tris(2-phenylpyridyl)iridium(III) has been investigated using non-equilibrium molecular dynamics (MD) simulations that mimic the process of vacuum deposition. By comparison with reflectometry measurements, we show that the simulations provide an accurate model of the average density of such films. The emitter molecules were found not to be evenly distributed throughout film, but rather they can form networks that provide charge and/or energy migration pathways, even at emitter concentrations as low as ≈5 weight percent. At slightly higher concentrations, percolated networks form that span the entire system. While such networks would give improved charge transport, they could also lead to more non-radiative pathways for the emissive state and a resultant loss of efficiency

Hole-transporting materials for low donor content organic solar cells: charge transport and device performance

Low donor content solar cells are an intriguing class of photovoltaic device about which there is still considerable discussion with respect to their mode of operation. We have synthesized a series of triphenylamine-based materials for use in low donor content devices with the electron accepting [6,6]-phenyl-C71-butyric acid methyl ester (PC0BM). The triphenylamine-based materials absorb light in the near UV enabling the PC0BM to be be the main light absorbing organic semiconducting material in the solar cell. It was found that the devices did not operate as classical Schottky junctions but rather photocurrent was generated by hole transfer from the photoexcited PC0BM to the triphenylamine-based donors. We found that replacing the methoxy surface groups with methyl groups on the donor material led to a decrease in hole mobility for the neat films, which was due to the methyl substituted materials having the propensity to aggregate. The thermodynamic drive to aggregate was advantageous for the performance of the low donor content (6 wt%) films. It was found that the 6 wt% donor devices generally gave higher performance than devices containing 50 wt% of the donor

Thiophene dendrimer-based low donor content solar cells

Low donor content solar cells containing polymeric and non-polymeric donors blended with fullerenes have been reported to give rise to efficient devices. In this letter, we report that a dendrimeric donor can also be used in solution-processed low donor content devices when blended with a fullerene. A third generation dendrimer containing 42 thiophene units (42T) was found to give power conversion efficiencies of up to 3.5% when blended with PC70BM in optimized devices. The best efficiency was measured with 10 mole percent (mol. %) of 42T in PC70BM and X-ray reflectometry showed that the blends were uniform. Importantly, while 42T comprised 10 mol. % of the film, it made up 31% of the film by volume. Finally, it was found that solvent annealing was required to achieve the largest open circuit voltage and highest device efficiencies

Flexible ITO-free organic photovoltaics on ultra-thin flexible glass substrates with high efficiency and improved stability

The ability for organic solar cells to be conformable and bendable enables them to be used in a broad range of applications. Indium tin oxide (ITO) or PEDOT:PSS on plastic substrates such as poly(ethylene terephthalate) or poly(ethylene napthalate) (PET or PEN) have been used for transparent conductive electrodes (TCEs) for flexible devices. However, ITO is brittle and when used on flexible substrates is prone to cracking, and the acidity of PEDOT:PSS can lead to corrosion of the ester-based plastic substrates and cause device degradation. In this work, TCEs based on modified high-conductivity PEDOT:PSS on 100 mu m-thick flexible glass substrates are used as the anode for organic solar cells. The optimized PEDOT:PSS TCE anode on flexible glass has a sheet resistance of approximate to 30 Omega/sq and a transmission of approximate to 77% at 550nm, with a broad transmission window between 300 and 800nm. The best PEDOT:PSS on flexible glass-based organic solar cell has a power conversion efficiency (PCE) of 8.0%, which is higher than devices comprising ITO or PEDOT:PSS on PEN, which have PCEs of 6.4% and 5.8%, respectively. It is also found that the ultra-thin glass devices can be scaled, with 1.6cm(2) flexible cells having an efficiency of 5.2%