Modeling Dark Current Conduction Mechanisms and Mitigation Techniques in Vertically Stacked Amorphous Selenium-Based Photodetectors

Abstract

Amorphous selenium (a-Se) with its single-carrier and non-Markovian, hole impact ionization process can revolutionize low-light detection and emerge to be a solid-state replacement to the vacuum photomultiplier tube (PMT). Although a-Se-based solid-state avalanche detectors can ideally provide gains comparable to PMTs, their development has been severely limited by the irreversible breakdown of inefficient hole blocking layers (HBLs). Thus, understanding of the transport characteristics and ways to control electrical hot spots and, thereby, the breakdown voltage is key to improving the performance of avalanche a-Se devices. Simulations using Atlas, SILVACO, were employed to identify relevant conduction mechanisms in a-Se-based detectors: space-charge-limited current, bulk thermal generation, Schottky emission, Poole–Frenkel activated mobility, and hopping conduction. Simulation parameters were obtained from experimental data and first-principle calculations. The theoretical models were validated by comparing them with experimental steady-state dark current densities in avalanche and nonavalanche a-Se detectors. To maintain bulk thermal generation-limited dark current levels in a-Se detectors, a high-permittivity noninsulating material is required to substantially decrease the electric field at the electrode/hole blocking layer interface, thus preventing injection from the high-voltage electrode. This, in turn, prevents Joule heating from crystallizing the a-Se layer, consequently avoiding early dielectric breakdown of the device.

Vertical Architecture Solution-Processed Quantum Dot Photodetectors with Amorphous Selenium Hole Transport Layer

“…Due to the large electronic bandgap of the materials used in the detector (see SI, section S2 and Table S1), the thermally generated carriers in the intrinsic region of the detector do not contribute to the measured J D . Under light starved conditions, the dominant source limiting J D in a -Se based detectors is Schottky or thermionic hole injection from the high voltage ITO electrode . Thus, we observe even lower levels of J D in Figure a for the CdSe5.0/ a -Se detector with an added CeO 2 CQD HBL with an enhanced bandgap of ∼3.77 eV.…”

“…Moreover, the device J D performance is not significantly improved with the use of a higher work-function bottom electrode (see SI, section S8.3 and Figure S12). , To investigate the dependence of J D on the thickness of the a -Se, we simulate four a -Se layer thicknesses: 1, 5, 10, and 15 μm, as shown in SI section S8.4. From Figure S13, we observe an insignificant dependence of the bulk a -Se thickness on the overall J D due to negligible thermal generation of carriers in the wide-bandgap a -Se layer.…”

“…It was our first demonstration of the coupling of a -Se and solution-processed CQDs. CeO 2 CQD HBL was used as an efficient noninsulating and nearly stoichiometric n -type HBL (permittivity ∼ 30 and an enhanced bandgap ∼ 3.77 eV: limits Schottky injection from the electrodes, prevents Joule heating, and hinders the irreversible breakdown of the device) in the desired p - i - n vertical geometry system, demonstrating an avalanche gain of ∼42 and the lowest measured J D of ∼30 pA/cm 2 at the onset of avalanche. ,, With our first successful coupling of CQDs and a -Se achieving gain, in this work we proceed to utilize a -Se as a HTL with a spectrally tunable solution-processed CdSe CQD as a photon absorption layer (Figure a, device 2). To enable fast and sensitive photodetection, photocarrier recombination needs to be limited during the transport process by physically separating the photogenerated hole and electron carriers, and here we show the design of hybrid photodetectors where optical absorption and charge transport are physically separated, thus enabling increased flexibility for spectral tuning and increased charge extraction .…”

“…Due to a high density of traps (deep trap density ∼ 10 19 cm –3 , see Table S1), the electron mobility in the EBL is extremely small, 10 –5 cm 2 /V·s. This limits the injected electrons from reaching the a -Se layer . The detectors were fabricated on 75 nm ITO coated substrates.…”

“…This limits the injected electrons from reaching the a-Se layer. 20 The detectors were fabricated on 75 nm ITO coated substrates. Figure 1b shows the technology for computer-aided design (TCAD) simulated energy-band diagram of the device under applied bias (see SI, section S2 and Figure S2, for further details on the Silvaco ATLAS simulation framework).…”