After decades of significant development in the field of optoelectronics and optoelectronic systems mostly photodetectors. Depending on the applications, photosensors are used everywhere from industrial to commercial applications. However, most of these sensors consist of conventional inflexible inorganic semiconductors which limit their demand in today’s world of wearable and flexible electronics. With the advent of IoT, the demand for economical, flexible, wearable, lightweight, low-power consumption sensors is growing rapidly. To meet all such demands, the cross-disciplinary research field of Organic electronics has paved a way for the following generation in offering simplistic processing and innovative and conformal substrate integration for a variety of applications[1–7].The most studied and investigated devices in Organic optoelectronics are photodetectors which include organic phototransistors(OPT’s) and organic photodiodes(OPD’S). Since organic transistors are considered to be an alternative to inorganic transistors. Notably organic field effect transistors (OFETs) are the prevailing subject of R&D right now due to their remarkable advantages compared to inorganic-based transistors.The OFET-based sensors are anticipated to have widespread applications from flexible displays, optical imaging, artificial skin, ecological monitoring, food safety detection, medication delivery, and medical diagnostics to high-performance biodegradable sensors [8–11].
The organic phototransistor is a combination of a photodetector and an amplifier that has the ability to detect and convert an incident optical signal into an electrical signal, along with the amplification of the signal. As it is the combination of a photodetector and an amplifier it makes a viable applicant to be used in a wide range of optoelectronic applications, especially in the field of imaging where light detection and signal amplification plays a key role. Moreover, Organic phototransistors withfield-effect transistor geometry are extensively studied as they could be integrated into huge area circuits with higher densities as compared to BJT[12],[13]. Like MOSFET, an Organic phototransistor with OFET geometry is a three terminal electrical device with source, drain and gate terminals but here instead of using inorganic semiconductor, we have photoactive organic semiconductor material and its resistance and conductance can controlled by Gate bias and light radiation[8], [12].
Under Dark conditions, on application of lateral external electric field across source-drain terminals, the current flows through the organic semiconductor referred as dark current. However this current can be further modulated by the applied gate voltage only [14].Under illumination conditions, when the energy of irradiance is made equivalent or higher than the forbidden energy gap of photoactive organic semiconductors, there happens to be the generation of electrons and holes with the large electrostatic force of attraction between them. These confined electrons and holes are known as excitons. These excitons will diffuse through the bulk of this organic semiconductor channel and with the application of an external electric field these excitons will undergo dissociation and thus creates free electron and hole carriers in the channel which are then collected at source-drain terminals and thus a significant amount of photocurrent will flow through the organic channel[8],[12],[14].
Upon illumination, the operation of an OPT can be divided into two modes; Photovoltaic mode and photoconductive mode.
In Photovoltaic mode applied VGS<Vth (for P-Type) and OPT is in “ON state”(Accumulation Mode), the photo-generated electrons and holes in presence of lateral electric field gets accumulate under source and drain electrodes respectively which lowers the potential barriers across the electrodes and photoactive semiconductor that effectively reduces the contact resistance and induces a shift in Vth[15]. Thereby a substantial amount of photocurrent will flow through the organic channel. The logarithmic relationship defines the dependence of Photocurrent due to photovoltaic effect and light irradiance is expressed in Eq. (1);
$$I=\frac{AKT}{q}{ln}\left(1+\frac{\eta qPopt}{Idark hc}\right)$$
1
A is a proportionality parameter,k is the Boltzmann constant, η is the photo-generation quantum efficiency, hc/λ is the photon energy[8],[12],[14].
In photoconductive mode applied Vgs > Vth(for p-type) and OPT is in “off state” and works in depletion mode, the photo-generated electrons and holes are responsible for the photocurrent in the device. Here only the light bias is responsible for modulating the current. Moreover, with an increase in optical intensity, the drain current increases linearly. In photoconductive mode, photocurrent is expressed by Eq. (2);
$$I=\left(q{\mu }_{p}pE\right)WD=B{P}_{opt}$$
2
Here B is a proportionality constant.
In addition to the specific OFET characteristics, the most important figures of merit that characterize the performance of an OPT are its Photoresponsivity, photosensitivity, External quantum efficiency, and spectral range. Photosensitivity(P) is defined as the ratio of photo-generated current to dark current and is expressed by Eq. (3)[16].i.e,
$$P=\frac{Ilight-Idark}{Idark}$$
3
Responsivity (R) is the measure of how efficiently a responsive photodetection takes place whenever exposed to light Intensity. It is determined by the ratio of generated photocurrent in the device to the incident power of irradiances given in Eq. (4)[17].i-e,
$$R=\frac{Ilight-Idark}{Popt}$$
4
Where Popt is incident optical power.
And External quantum efficiency (EQE) is the ratio of the number of photo-generated carriers to the number of incident photons in the device. It determines the efficiency associated with both light capturing and light conversion capabilities of the device and is expressed by Eq. (5) [17]. i.e,
$$\text{E}\text{Q}\text{E}=\text{R}\left(\frac{\text{h}\text{c}}{{\lambda }\text{q}}\right)$$
5
Where R is Responsivity.
There are a lot of experimental studies on OPTs in the literature that is mostly concerned with analyzing the functionality of these devices. Different synthesis techniques, different semiconductors, and gate insulators with variable mobilities have all been claimed to increase the performance of existing devices. The organic semiconductor used in OFET-based OPT should have exceptional light-absorption capabilities, great carrier generation efficiency across a broad spectral range, and good mobility[8],[12],[18]. Additionally, the materials of gate insulators and organic semiconductors are crucial to how well they work. Charge buildup and charge transport in the device are mainly influenced by different gate insulator parameters like dielectric constant and thickness[19],[20]P-type photoactive organic semiconductor pentacene is primarily explored since it possesses the necessary characteristics. Moreover, Pentacene's photosensing characteristics gets influenced by the dielectric/semiconductor interfacial traps and it has been observed thinner the gate oxide better the photosensitivity. Pentacene performs better under illumination when its thickness is reduced[18]. From the prespective of device architecture in order to increase the photodetection and sensitivity, several device designs, such as dual gate, vertical Organic Phototransistors, are being researched. Even though layered OPTs have made considerable strides, wide channel lengths prevent many excitons from effectively dissociating and even due to short exciton diffusion lengths which results in poor charge collection and the performance of OPT becomes limited. Since,a novel dual gate phototransistor device idea with a bulk heterojunction was described, in which both gates could accumulate and deplete both types of charge carriers to produce conductive n- and p-type channels at the gate's dielectric-semiconductor interface. As a result, high gain and linear photoresponse may be accomplished concurrently without the need of any additional circuitry[21]. In order to improve the device performance vertical OPTs with shorter channel lengths are employed [22].As the electrical behaviour of an OFET is greatly influenced by the material utilised for the organic semiconductor thin film, channel length, and its thickness.With respect to various architecture configurations, the influence of channel length modification on device performance parameters has been demonstrated using Slivaco TCAD Atlas[23]. Furthermore, a comparative analysis of various architectural designs of OFETs has been investigated. All the above investigations were taken into consideration before carrying out all the optoelectrical simulations.In this work, our main aim is to study and compare the performance of OPT from the perspective of device configuration and provide deeper insight into behaviour of the device from physics perspective and various quantitative standpoints.