Radiation induced photocurrent in the active volume of P3HT/PCBM BHJ photodiodes

Hupman, Michael A.; Valitova, Irina; Hill, Ian G.; Syme, Alasdair

doi:10.1016/j.orgel.2020.105890

Cited by 8 publications

(13 citation statements)

References 17 publications

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“…This is in part because the diode, which has a decreased response with energy in the kV range, will contribute a more significant portion of the measured signal. 14 The dose gradient across the larger scintillators at these energies may also contribute to the enhanced energy dependence. The data suggest that it may be possible to find an optimal scintillator thickness to minimize the energy dependence of the device.…”

Section: Discussionmentioning

confidence: 99%

“…Organic photodiodes should perform well due to their ability to absorb a high proportion of the light emitted from the scintillator while also not creating much signal due to direct interactions with the irradiation beam. [12][13][14] Kingsley et al measured a reduction of~5% in sensitivity with 0.5 kGy dose of a P3HT/PCBM photodiode device and a reduction of~30% over 4 kGy when irradiated with 15 MV photon beam. 15 The SPSD was characterized with respect to linearity with dose and reproducibility.…”

Section: Introductionmentioning

confidence: 99%

“…Although future work will minimize the extraneous signal in the device, in the current work, a correction method was used to permit the characterization of the scintillation signal as measured by the photodiodes. 14…”

Section: Introductionmentioning

confidence: 99%

“…This eliminates the need for an optical fiber to carry the signal from the scintillator, thus eliminating the need for a stem correction while preserving the well‐documented benefits of PSDs. Organic photodiodes should perform well due to their ability to absorb a high proportion of the light emitted from the scintillator while also not creating much signal due to direct interactions with the irradiation beam 12–14 . Kingsley et al measured a reduction of ~5% in sensitivity with 0.5 kGy dose of a P3HT/PCBM photodiode device and a reduction of ~30% over 4 kGy when irradiated with 15 MV photon beam 15 …”

Section: Introductionmentioning

confidence: 99%

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Fabrication and characterization of a stemless plastic scintillation detector

et al. 2020

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Purpose To fabricate a stemless plastic scintillation detector (SPSD) and characterize its linearity and reproducibility, and its dependence on energy and dose per pulse; and to apply it to clinical PDD and output factor measurements. Methods An organic bulk heterojunction photodiode was fabricated by spin coating a blend of P3HT and PCBM onto an ITO‐coated glass substrate and depositing aluminum top contacts. Eljen scintillators (~5 × 5 × 5 mm3; EJ‐204, EJ‐208, and EJ‐260) or Saint‐Gobain scintillators (~3 × 3 × 2 mm3; BC‐400 and BC‐412) were placed on the opposite side of the glass using a silicone grease (optical coupling agent) creating the SPSD. Energy dependence was measured by using 100, 180, and 300 kVp photon beams from an orthovoltage treatment unit (Xstrahl 300) and 6 and 10 MV photons from a Varian TrueBeam linear accelerator. Linearity, dose per pulse dependence, output factors, and PDDs were measured using a 6 MV photon beam. PDDs and output factors were compared to ion chamber measurements. A control device was fabricated by substituting polystyrene (PS) for the P3HT/PCBM layer. No photocurrent should be generated in the control device and so any current measured is due to Compton current in the electrodes, wires, and surroundings from the irradiation. Output factors were corrected by subtracting the signal measured using the control device from the photodiode measured signal to yield the photocurrent. Results Each SPSD had excellent linearity with dose having an r2 of 1 and sensitivities of 1.07 nC/cGy, 1.04 nC/cGy, 1.00 nC/cGy and 0.10 nC/cGy, and 0.10 nC/cGy for EJ‐204, EJ‐208, EJ‐260 (5 × 5 × 5 mm3 volumes), BC‐400, and BC‐412 (3 × 3 × 2 mm3 volumes), respectively. No significant dose per pulse dependence was measured. Output factors matched within 1% for the large scintillators for field sizes of 5 × 5 cm2 to 25 × 25 cm2, but there was a large under‐response at field sizes below 3 × 3 cm2. After correcting the signal of the small scintillators by subtracting the current measured using the PS control, the output factors agreed with the ion chamber measurements within 1% from field sizes 1 × 1 cm2 to 20 × 20 cm2. The impact of Cerenkov emissions in the scintillator was effectively corrected with a simple reflective coating on the scintillator. In comparison to a 6 MV photon beam, the large scintillator SPSDs exhibited 37%, 52%, and 73% of the response at energies 100 kVp, 180 kVp and 300 kVp, respectively. Conclusion The principle of the SPSD was demonstrated. Devices had excellent linearity, reproducibility, and no significant dose per pulse dependence, and a simple reflective coating was sufficient to correct for Cerenkov emissions from within the scintillator. The devices demonstrated similar energy dependence to other scintillator detectors used in a radiotherapy setting.

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Section: Discussionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Fabrication and characterization of a stemless plastic scintillation detector

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“…Fabrication of the organic P3HT (Molecular weight 50–80 kg/mol, Brilliant Matters, Quebec City, Canada)/PCBM (Solenne BV, Groningen, Netherlands) BHJ diodes followed established methods from the photovoltaic literature [1] , [2] , [3] , [4] . ITO-coated glass substrates were patterned by etching with hydrochloric acid for approximately 5 min.…”

Section: Device Fabricationmentioning

confidence: 99%

Method for the differentiation of radiation-induced photocurrent from total measured current in P3HT/PCBM BHJ photodiodes

et al. 2020

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Thin film radiation-detecting diodes fabricated in the laboratory, such as an organic bulk heterojunction, often contain conductive leads, indium tin oxide traces and metallic interconnects which are exposed to the high-energy photon beam during operation. These components generate extraneous radiation-induced currents, that, if not accounted for, will erroneously be attributed to the detector. In commercial devices, these contributions are mitigated by minimizing the size of these components, an approach that is often not feasible in a research lab. Here we demonstrate a method to measure these extraneous signals, and by subtraction, correct the gross signal to accurately reflect the signal generated in the active volume of the diode itself. The method can effectively correct the extraneous signal. The method showed promise over a range of photon beam energies, dose rates, and field sizes.

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Measuring small field profiles and output factors with a stemless plastic scintillator array

2021

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To fabricate a 1D stemless plastic scintillation detector (SPSD) array using organic photodiodes and to use the detector to measure small field profiles and output factors. Methods: An organic photodiode array was fabricated by spin coating a mixture of P3HT and PCBM organic semiconductors onto an ITO-coated glass substrate and depositing aluminum top contacts. Four bulk scintillators of various dimensions were placed on top of the photodiode array. A fifth scintillator was used that had been segmented by laser etching and the septa filled with black paint. Each detector array was first calibrated using a reference field of 95 cm SSD, 5 cm depth, and 10 × 10 cm 2 field size for a 6 MV photon beam. After calibration, profiles were measured for three small field sizes: 0.5 × 0.5 cm 2 , 1 × 1 cm 2 , and 2 × 2 cm 2 . Using the central pixel of the array, output factors were measured for field sizes of 0.5 × 0.5 cm 2 to 25 × 25 cm 2 . Small field profiles were compared to film measurements and output factors compared to ion chamber measurements. Results:The segmented scintillator measured profiles that were in good agreement with film for all three field sizes. Output factors agreed to within 1.2% of ion chamber over the field size range of 1 × 1 cm 2 to 25 × 25 cm 2 . At 0.5 × 0.5 cm 2 the segmented scintillator underestimated the output factor compared to film and a microDiamond detector. Bulk scintillators failed to produce a good agreement with film for measured profiles and deviations from ion chamber for output factors were apparent at field sizes below 5 × 5 cm 2 . In comparison to a bulk scintillator of dimensions 5 × 5 × 0.5 cm 3 the etched scintillator saw a reduction of 5.1, 7.1, and 10.5 times the signal for field sizes of 0.5 × 0.5 cm 2 , 1 × 1 cm 2 , and 2 × 2 cm 2 , respectively. The reduction of signal comes from reduced cross-talk that was present in all of the bulk scintillator geometries to various degrees. Conclusion: A 1D SPSD array was demonstrated with various scintillator designs. The etched scintillator array demonstrated excellent small field profile measurements when compared to film and output factors (down to 1 × 1 cm 2 field size) when compared to micro ion chamber and diamond detector measurements.

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Radiation induced photocurrent in the active volume of P3HT/PCBM BHJ photodiodes

Cited by 8 publications

References 17 publications

Fabrication and characterization of a stemless plastic scintillation detector

Fabrication and characterization of a stemless plastic scintillation detector

Method for the differentiation of radiation-induced photocurrent from total measured current in P3HT/PCBM BHJ photodiodes

Measuring small field profiles and output factors with a stemless plastic scintillator array

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