In‐Plane Alignment in Organic Solar Cells to Probe the Morphological Dependence of Charge Recombination

Abstract

Bulk heterojunction (BHJ) organic solar cells are fabricated with the polymer semiconductor aligned in the plane of the film to probe charge recombination losses associated with aggregates characterized by varying degrees of local order. 100% uniaxial strain is applied on ductile poly(3‐hexylthiophene):phenyl‐C61‐butyric acid methyl ester (P3HT:PCBM) BHJ films and characterize the resulting morphology with ultraviolet‐visible absorption spectroscopy and grazing incidence X‐ray diffraction. It is found that the strained films result in strong alignment of the highly ordered polymer aggregates. Polymer aggregates with lower order and amorphous regions also align but with a much broader orientation distribution. The solar cells are then tested under linearly polarized light where the light is selectively absorbed by the appropriately oriented polymer, while maintaining a common local environment for the sweep out of photogenerated charge carriers. Results show that charge collection losses associated with a disordered BHJ film are circumvented, and the internal quantum efficiency is independent of P3HT local aggregate order near the heterojunction interface. Uniquely, this experimental approach allows for selective excitation of distinct morphological features of a conjugated polymer within a single BHJ film, providing insight into the morphological origin of recombination losses.

Panchromatic All‐Polymer Photodetector with Tunable Polarization Sensitivity

“…There may be other losses associated with increased charge recombination in the strain‐oriented device that may contribute to the lower responsivity that would require further analysis. However, previous analysis on strain‐oriented P3HT:PCBM devices showed that internal quantum efficiency only slightly reduced compared to control spun‐cast films, suggesting that the primary difference in EQE was due to absorption differences . Despite the reduced R (and EQE) in strain‐oriented films, the performance metrics still compare favorably to other high‐performing all‐polymer photodetector demonstrations.…”

Intrinsic coincident linear polarimetry using stacked organic photovoltaics

“…From Eq. (24), it is also notable that there is a small (average 3%) S 3 response in the second and third OPVs. This is due to the stress birefringence contained within the strained polymer film.…”

“…A detailed analysis of the polarized OPV cell performance has shown that the internal quantum efficiency of the device remains similar to an unstrained device and is independent of the incident polarization state [24]. Thus the strained devices have a similar efficiency of collecting photogenerated carriers as an archetypal processed counterpart and the anisotropic performance is attributed primarily to the difference in light absorption in the cell.…”

Macroscopic alignment of poly(3‐hexylthiophene) for enhanced long‐range collection of photogenerated carriers

“…Large‐area alignment can be achieved using casting techniques such as directional solidification, solution shearing, and microchannel confinement, yielding anisotropic, and in some cases substantially improved, carrier mobilities . Mechanical post‐processing techniques such as stamping, rubbing, and straining have also been shown to induce polymer chains to adopt highly aligned configurations . Molecular design of polymers exhibiting liquid crystalline behavior has also been used to supplement the effectiveness of alignment techniques …”

“…10,[15][16][17] Mechanical post-processing techniques such as stamping, rubbing, and straining have also been shown to induce polymer chains to adopt highly aligned configurations. [18][19][20][21][22] Molecular design of polymers exhibiting liquid crystalline behavior has also been used to supplement the effectiveness of alignment techniques. 16,23 The aforementioned macroscopic alignment techniques focus on in-plane alignment, but for some applications, such as organic photovoltaics, transport along the out-of-plane or "vertical" direction is of greater interest.…”