Arnold B. Tamayo

A low band gap, solution processable oligothiophene with a dialkylated diketopyrrolopyrrole chromophore for use in bulk heterojunction solar cells

Bulk heterojunction solar cells are fabricated from blends of oligothiophene with a dialkylated diketopyrrolopyrrole chromophore:[6,6]-phenyl C71 butyric acid methyl ester. Absorption and photocurrent of the films extend to 800 nm. A power conversion efficiency (PCE) of 3.0% is obtained under simulated 100 mW/cm2 AM1.5 illumination with a 9.2 mA/cm2 short-circuit current density and an open-circuit voltage of 0.75 V. The hole and electron mobilities in the 50:50 blend are fairly balanced, 1.0×10−4 and 4.8×10−4 cm2/V s, respectively. This is the highest PCE reported to date for solar cells using solution processable small molecules.

Small molecule sensitizers for near-infrared absorption in polymer bulk heterojunction solar cells

A low band gap small molecule chromophore has been incorporated into a polymer/fullerene bulk heterojunction (BHJ) solar cell yielding increased carrier generation in the near infrared and increased overall short circuit current. The use of a small concentration of a soluble oligothiophene with a diketopyrrolopyrrole core can extend the absorption and photocurrent of poly(3-hexyl thiophene):[6,6]-phenyl C71 butyric acid methyl ester solar cells to 800 nm. Photocurrent from the dye embedded within the polymer BHJ is demonstrated, and the use of soluble small molecule sensitizers as a path toward high efficiency solar cells is discussed.

Influence of alkyl substituents and thermal annealing on the film morphology and performance of solution processed, diketopyrrolopyrrole-based bulk heterojunction solar cells

A solution processable diketopyrrolopyrrole-containing oligothiophene was blended with a series of methanofullerene acceptors having different alkyl substitutents—[6,6]-phenyl C61 butyric acid methyl (PC6161BM) ester, [6,6]-phenyl C61 butyric acid hexyl ester (PC6161BH) and [6,6]-phenyl C61 butyric acid dodecyl ester (PC6161BD) to study the effect of donor–acceptor interactions on the blend film morphology and device characteristics of small molecule-based bulk heterojunction (BHJ) solar cells. A combination of characterization techniques including X-ray diffraction, atomic force microscopy, conducting atomic force microscopy, and transmission electron microscopy was used to investigate the film morphology and phase separation. The results show that changing the length of the alkyl substituent on the methanofullerene acceptor is a good approach to control the film morphology of blended films and these lead to significant differences on the performance of the as cast and annealed devices. Power conversion efficiencies between 2–3% can be reached by simple variation of the alkyl chain length.

Solution-processed small molecule-based blue light-emitting diodes using conjugated polyelectrolytes as electron injection layers

Organic blue light-emitting diodes were studied using the solution processable small molecule 2,7-dipyrenyl-9,9′-dioctyl-fluorene (DPF) as the light-emitting material. The devices were fabricated in two simple structures: indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS)/DPF/LiF∕Al and ITO/PEDOT:PSS/DPF/PFN-BIm4∕Al, where PFN-BIm4 is poly[9,9′-bis[6″-(N,N,N-trimethylammonium)hexyl]fluorene-alt-co-phenylene] with tetrakis(imidazolyl)borate counterions. The LiF or PFN-BIm4 act as electron injection layers. The ITO/PEDOT:PSS/DPF/PFN-BIm4∕Al device, in which all organic layers are cast from solution, has a turn-on voltage of 3.8V, a luminance of 2000cd∕m2, and an efficiency of 0.6cd∕A. Using the PFN-BIm4 layer shows a significant improvement of the device performance when compared to the LiF layer.

Effect of structural variation on photovoltaic characteristics of phenyl substituted diketopyrrolopyrroles

A comprehensive study has been performed on a series of solution processable phenyl substituted diketopyrrolopyrroles blended with [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) in order to investigate how systematic chemical modifications such as solubilizing groups and conjugation length impact solar cell performance. We find that replacement of linear alkyl chains with bulky ethyl-hexyl groups or the removal of linear alkyl chains on the terminal thiophene units leads to micron scale phase segregation at high donor:acceptor blend ratios. It is found that the conjugation length can be used to simultaneously tune energy levels, solubility, and molecular ordering. We show that over-extending the conjugation length can reduce solubility making film fabrication difficult while decreasing the conjugation length past a critical limit can significantly enhance molecular ordering thereby inducing micron scale phase segregation in blend films. This work shows that a materials potential device performance can be limited by slight chemical modifications which prevent device optimization at high donor:acceptor blend ratios and elevated annealing temperatures where charge mobility is balanced and charge collection is enhanced in the donor and acceptor phase.