Impact of Polymer Molecular Weight on Polymeric Photodiodes

Abstract

tion. [1] Recently, they have gained significant interest primarily due to their cost-effective approach to fabrication, higher throughput additive manufacturing on mechanically flexible substrates, and spectral tunability comparable to that of conventional inorganic photodiodes. [1][2][3] Along with these exciting features, OPDs have been reported for their potential in a number of applications, including pulse oximeters, [4] biometric analysis systems, [5] retinal prosthesis, [6] wearable electronics, [7] and image sensing. [8] As a result, it has recently gained interest amongst commercial manufacturers as well. [9][10][11] An OPD has similar architecture to an organic photovoltaic device, but operates in reverse bias. While photovoltaic solar cells rely strongly on matching to the incident solar spectrum in order to maximize power conversion efficiencies, the OPD characteristics on the other hand are tailored toward lower dark current (i.e., current measured in the device under null illumination), higher detectivity, and longer linear dynamic range (LDR). The active materials of OPDs commonly include evaporated molecules, solution-processable molecules, and polymers as electron-donating materials. In particular, the semiconducting The field of organic photodiodes (OPDs) has witnessed continuous development in the last decade. Although a considerable portion of electron-donating materials are polymers, there has been an existential gap in deciphering the influence of the polymer's molecular-weight on the photodiode performance. We take up OPDs based on 5,5′-[(9,9-Dioctyl-9H-fluorene-2,7-diyl)bis(2,1,3benzothiadiazole-7,4diylmethylidyne)]bis[3-ethyl-2-thioxo-4-thiazolidinone] (FBR) acceptor material blended with three different molecular-weights of defect-free form of a well-known donor polymer poly(3-hexylthiophene-2,5-diyl) (P3HT) are taken up, and their optoelectronic performance along with morphological characteristics are studied. Disparity of up to a decade in key photodetecting characteristics is observed. Further, the tools of near-edge X-ray absorption fine-structure spectroscopy, resonant soft X-ray scattering spectroscopy, atomic force microscopy, and time-delayed collection-field measurements are employed to decipher the difference in the fundamental photo-physical processes and the operating mechanisms of the OPDs. It is concluded that the molecular weight and the resulting morphology of the active layer strongly influence photodiode performance, in particular, dark current, linear dynamic range, and specific-detectivity.

Raman Snapshots of Side-Chain Dependent Polaron Dynamics in PolyThiophene Films

“…The weight-average molecular weight and polydispersity index (as determined by GPC) were found to be 66 kDa; 1.5 for P3HT and 26.7 kDa; 1.3 for P3EHT. Defect-free perfectly region-regular P3HT has a metallic luster as a bulk solid while having exceptional photoconductivity which separates it from P3HT obtained from various commercial sources. Thin films of such defect-free P3HT and P3EHT were cast on the quartz substrate via spin coating.…”

High‐Performance Air‐Stable 2D‐WSe2/P3HT Based Inorganic–Organic Hybrid Photodetector with Broadband Visible to Near‐IR Light Detection

“…The increase in the responsivity and EQE in the hybrid device is primarily due to three reasons: i) the enhanced light absorption by the organic P3HT layer, which has a good absorption in the visible range, [44,45] and ii) effective separation of photogenerated electron-hole pairs at the HJ interface and iii) long-range energy transfer from P3HT to WSe 2 . WSe 2 has good absorption from the visible to NIR range, with a few-layer WSe 2 having a strong excitonic absorption peak at ≈750 nm which gets red-shifted with increasing thickness.…”

Response Speed of Organic Photodiodes as a Function of Incident Optical Intensity

“…To elicit qualitative information about the BMR process, the suppressed recombination factor (𝛽/𝛽 L ) ought to be evaluated. [25] 𝛽 L is the coefficient of Langevin-type BMR coefficient, and it is mathematically proportional to the sum of electrons (μ e ) and hole (μ h ) charge mobility in the device. 𝛽 L can be theoretically calculated using Equation 3,…”

“…where 𝜖 r is the relative permittivity of the organic semiconductors and 𝜖 0 is the permittivity of free space. [25] Resultantly, 𝛽/𝛽 L was evaluated to be 0.07. Considering more than an order of magnitude of suppression with respect to the theoretical upper limit (𝛽 L ), it is appropriate to infer that the PTNT:PC 71 BM OPD presents non-Langevin type of bimolecular recombination.…”

“…We previously found that the lower molecular weight system has a higher bimolecular recombination coefficient. [25] For these two systems, the transient response to a range of fluence was recorded, for similar optical conditions (short green pulse, 532 nm; 20 ns) and under short circuit conditions, as shown in Supplementary Figure S2 (Supporting Information). The charge carrier density (n charges ), fall time, and response speed (f -3dB ) from these transient response results have been calculated and plotted in Figure S3 (Supporting Information).…”