Three-Phase Morphology Evolution in Sequentially Solution-Processed Polymer Photodetector: Toward Low Dark Current and High Photodetectivity

Abstract

Sequentially solution-processed polymer photodetectors (SSP PPDs) based on poly(3-hexylthiophene-2,5-diyl) (P3HT)/[6,6]-phenyl C-butyric acid methyl ester (PCBM) are fabricated by depositing the top layers of PCBM from an appropriate cosolvent of 2-chlorophenol (2-CP)/o-dichlorobenzene (ODCB) onto the predeposited bottom layers of P3HT. By adjusting the ratio of 2-CP/ODCB in the top PCBM layers, the resulting SSP PPD shows a decreased dark current and an increased photocurrent, leading to a maximum detectivity of 1.23 × 10 Jones at a wavelength of 550 nm. This value is 5.3-fold higher than that of the conventional bulk heterojunction PPD. Morphology studies reveal that the PCBM partially penetrates the predeposited P3HT layer during the spin-coating process, resulting in an optimal three-phase morphology with one well-mixed interdiffusion P3HT/PCBM phase in the middle of the bulk and two pure phases of P3HT and PCBM at the two electrode sides. We show that the pure phases form high Schottky barriers (>2.0 eV) at the active layer/electrodes interface and efficiently block unfavorable reverse charge carrier injection by significantly decreasing the dark current. The interdiffussion phase enlarges the donor-acceptor interfacial area leading to a large photocurrent. We also reveal that the improved performance of SSP PPDs is also due to the enhanced optical absorption, improved P3HT crystallinity, increased charge carrier mobilities, and suppressed bimolecular recombination.

Effect of Crystallinity Modulation between Electron Transport Layer and Photo‐Generation Materials on ZnO‐Based Polymer Solar Cells

“…For PffBT4T‐2OD, the crystalline intensity of photo‐generation materials processed on NPs ZnO is stronger than that processed on sol‐gel ZnO. These results are in accordance with the formation of a cross‐linked D/A network, resulting in the improvement of J SC and FF . ZnO nanoparticles would play a role of inoculating crystal, inducing the crystallinity of photo‐generated layer.…”

Organic Photodetectors and their Application in Large Area and Flexible Image Sensors: The Role of Dark Current

“…Planar heterojunction (PHJ) OPDs made via sequential deposition of individual donor and acceptor layers are widely investigated . Alternatively, sequentially solution‐processed (SSP) OPDs can lead to a partial donor‐acceptor interpenetration, resulting in a well‐mixed interdiffusion phase in the middle of the bulk. Both approaches can lead to the formation of pure phases at the electrodes, which can effectively block unfavorable charge carrier injection under reverse bias.…”

“…However, the OPD with the lowest J d (≈ 10 −8 mA cm −2 ) also showed a relatively low EQE of 0.024 at 500 nm, mainly attributed to the high series resistance of the thick PC 61 BM layer and the inefficient separation of photogenerated excitons due to insufficient interdiffusion at the P3HT‐PC 61 BM interface. In the work of Wang et al, optimal interdiffusion of at the D‐A interface led to high EQE of 0.69, but J d did not decrease below 10 −4 mA cm −2 at F = 6 × 10 6 V m −1 . Thus, the trade‐off between EQE and dark current density depending on the degree of D‐A vertical segregation is the main limitation of SSP OPDs.…”

Low‐Bandgap Polymers for High‐Performance Photodiodes with Maximal EQE near 1200 nm and Broad Spectral Response from 300 to 1700 nm

“…Polymer photodiodes can meet the demands of low cost, simple structures, solution fabrication, tunability of structures and properties, light weight, abundant material selectivity, mechanical flexibility, and room temperature operation, which are usually difficult for inorganic semiconductor photodiodes to match . Moreover, polymer photodiodes can be used for large‐area detection and can be adopted in an upconversion device for near‐infrared (NIR) or UV–vis–NIR sensing . Therefore, polymer photodiodes with broad spectral response are much needed, yet challenging, for NIR and UV–vis–NIR full‐spectrum imaging.…”