AbstractThe effect of the electrolyte's counter‐ion in organic electrochemical transistors is often neglected. the influence of anions (i.e., counter ions) is investigated on organic electrochemical transistors (OECTs) with a PEDOT:PSS‐like semiconductor through device simulations. The study examined the effects of mobile anions on OECT performance under two scenarios: when anions are blocked by the semiconductor and when they can penetrate it. In each case, the OECT's ON and OFF states are analyzed. The findings show that when anions can penetrate the semiconductor, the ON/OFF ratio of the OECT remains unchanged while the transconductance significantly increases. In the ON state, the case of blocked anions is observed that the current is predominantly surface‐current, whereas it becomes volumetric only when anions can penetrate the semiconductor. Furthermore, the extreme case is explored in which anions remain stationary within the electrolyte. In this scenario, achieving a reasonable ON/OFF ratio necessitates an ion density within the electrolyte that is two orders of magnitude higher than the dopant density of the semiconductor. This work underscores the substantial influence of counter anions on OECT performance, highlighting their critical role in shaping device behavior.

Traditionally, new material-based technologies evolve first through chemistry and physics, with device engineering assuming a key role only towards commercialization. The device structure of an OLED pixel in any screen is a testament to the contribution of engineering to the challenge of achieving efficient and stable devices. In the talk, I will discuss two fields where this is yet to happen. The first is the organic electrochemical transistors, and the second is the solution-processed organic solar cells. Using both a detailed 2D chemical-physics semiconductor device model and a rate-equation level model, I will delve into the physics and electrochemistry of OECTs showing the role of electrode reactions and counter ions. In the context of OPVs, I will show that focusing on the performance at the maximum power point, instead of the short and open circuit, allows the design where the device contributes to charge generation and extraction.

Electrical doping of semiconductors is a revolutionary development that enabled many electronic and optoelectronic technologies. While doping of many inorganic and organic semiconductors is well-established, controlled electrical doping of metal halide perovskites (MHPs) is yet to be demonstrated. In this work, efficient n- and p-type electrical doping of MHPs by co-evaporating the perovskite precursors alongside organic dopant molecules is achieved. It is demonstrated that the Fermi level can be shifted by up to 500 meV toward the conduction band and by up to 400 meV toward the valence band by n- and p-doping, respectively, which increases the conductivity of the films. The doped layers are employed in PN and NP diodes, showing opposing trends in rectification. Demonstrating controlled electrical doping by a scalable, industrially relevant deposition method opens the route to developing perovskite devices beyond solar cells, such as thermoelectrics or complementary logic.

The Bernards-Malliaras model, published in 2007, is the primary reference for the operation of organic electrochemical transistors (OECTs). It assumes that, as in most transistors, the electronic transport is drift only. However, in other electrochemical devices, such as batteries, the charge neutrality is accompanied by diffusion-only transport. Using detailed 2D device simulations of the entire structure while accounting for ionic and electronic conduction, we show that high ion density (>1019 cm-3) results in Debye screening of the drain-source bias at the electrodes' interface. Hence, unlike the drift-only current in standard FETs or low ion density OECTs, the current in high ion density OECTs is diffusion only. Also, we show that since in OECTs, the volumetric capacitor and the semiconductor are one, the threshold voltage has a different meaning than that in FETs, where the semiconductor and the gate-oxide capacitor are distinct entities. We use the above insights to derive a new model useful to experimentalists. Lastly, we fabricated PEDOT:PSS fiber-OECTs and used the results to verify the model.

Accounting for the perovskite ionic transport and reactions reveals the importance of the electron blocking (hole transporting) layer in determining device stability.

The roll-to-roll printing production process for hybrid organic–inorganic perovskite solar cells (PSCs) demands thick and high-performance solution-based diffusion blocking layers. Inverted (p-i-n) PSCs usually incorporate solution-processed PC70BM as the electron-transporting layer (ETL), which offers good electron charge extraction and passivation of the perovskite active layer grain boundaries. Thick fullerene diffusion blocking layers could benefit the long-term lifetime performance of inverted PSCs. However, the low conductivity of PC70BM significantly limits the thickness of the PC70BM buffer layer for optimized PSC performance. In this work, we show that by applying just enough N-DMBI doping principle, we can maintain the power conversion efficiency (PCE) of inverted PSCs with a thick (200 nm) PC70BM diffusion blocking layer. To better understand the origin of an optimal doping level, we combined the experimental results with simulations adapted to the PSCs reported here. Importantly, just enough 0.3% wt N-DMBI-doped 200 nm PC70BM diffusion blocking layer-based inverted PCSs retain a high thermal stability at 60 °C of up to 1000 h without sacrificing their PCE photovoltaic parameters.

The mixed ionic-electronic nature of lead halide perovskites makes their performance in solar cells complex in nature. Ion migration is often associated with negative impacts-such as hysteresis or device degradation-leading to significant efforts to suppress ionic movement in perovskite solar cells. In this work, we demonstrate that ion trapping at the perovskite/electron transport layer interface induces band bending, thus increasing the built-in potential and open-circuit voltage of the device. Quantum chemical calculations reveal that iodine interstitials are stabilized at that interface, effectively trapping them at a remarkably high density of ∼1021 cm-3 which causes the band bending. Despite the presence of this high density of ionic defects, the electronic structure calculations show no sub-band-gap states (electronic traps) are formed due to a pronounced perovskite lattice reorganization. Our work demonstrates that ionic traps can have a positive impact on device performance of perovskite solar cells.

This contribution highlights the importance of device architecture design using a comprehensive semiconductor device model. In the context of organic PV, we show that the mobility balance is not an issue and that the only important guideline is to ensure that the lowest mobility is above 4x10(-4)cm(2)V(-1)s(-1). With this out of the way, researchers could focus on more cost-effective challenges. In the context of perovskite cells, we show that since the presence of ions means that electrochemistry is at play, the traditional semiconductor device models that exclude electrochemistry are incomplete.

Non-radiative recombination in the perovskite bulk and at its interfaces prohibits the photovoltaic performance from reaching the Shockley-Queisser limit. While interfacial recombination has been widely discussed and demonstrated, bulk recombination and especially the influence of grain boundaries remain under debate. Most studies explore the role of grain boundaries on perovskite films rather than devices, making it difficult to link the film properties with those of the devices. Here, we systematically investigate the effects of grain boundaries on the performance of perovskite solar cells by two different methods. By combining experimental characterization with theoretical device simulations, we find that the recombination at grain boundaries is diffusion limited and hence is inversely proportional to the grain area to the power of 3/2. Consequently, the prevalence of small grains-which act as recombination hot spots-across the perovskite active layer dictates the photovoltaic performance of the perovskite solar cells.

Ion diffusion into the BLs changes the device from symmetric-intrinsic to asymmetric-doped with all internal parameters changed.

The polycrystalline nature of perovskite thin films suggests that the nonradiative recombination losses may be grain size dependent. We use measured grain size distributions of methylammonium lead triiodide layers to describe the macroscopic solar cell as composed of multiple single grain size cells operating in parallel. Using a model, we show that the grain size distribution results in spatial dispersion of the local open-circuit voltage (0.9-1.15 V), fill factor (50-82%), and power conversion efficiency (7.5-19%). When the device is held at open-circuit voltage, there is significant current exchange between large and small grains. Smaller grains are a "parasitic shunt" for the larger grains.

Defect chemistry plays a major role in the operation of perovskites cells. On one hand they are behind the remarkable self-healing,[1] while on the other they dictate the level of traps, non-radiative recombination,[2] as well as promote ion migration.[3] Moreover, once ions reach the electrodes the device degradation is accelerated.[4, 5] Combining the various reports it seems that if the perovskite would not lose its constituents it would self-heal and if it do lose them it is game-over. We claim that ion migration into the blocking layer is the most important electrochemical process standing in the way of long-term stability. As we will show at the presentation, a significant part of the electrochemistry behind ion leakage and consequently electrode reactions, is captured by a semiconductor device model that accounts for mixed electronic-ionic conduction.[6] While the mere transport of ions introduces no device degradation, introducing oxidation/reduction reactions at the electrodes reproduces the effect of light & bias on device stability. Lastly, we’ll discuss if it is possible to design the device to mitigate this ion leakage out of the perovskite layer. References [1] D. R. Ceratti et al., "Self-Healing Inside APbBr3 Halide Perovskite Crystals," Advanced Materials, vol. 30, no. 10, p. 1706273, 2018, doi: 10.1002/adma.201706273. [2] J. M. Azpiroz, E. Mosconi, J. Bisquert, and F. De Angelis, "Defect migration in methylammonium lead iodide and its role in perovskite solar cell operation," Energy Environ. Sci., 10.1039/C5EE01265A vol. 8, no. 7, pp. 2118-2127, 2015, doi: 10.1039/C5EE01265A. [3] D. Meggiolaro, E. Mosconi, and F. De Angelis, "Formation of Surface Defects Dominates Ion Migration in Lead-Halide Perovskites," ACS Energy Letters, vol. 4, no. 3, pp. 779-785, 2019/03/08 2019, doi: 10.1021/acsenergylett.9b00247. [4] F. Galatopoulos, I. T. Papadas, G. S. Armatas, and S. A. Choulis, "Long Thermal Stability of Inverted Perovskite Photovoltaics Incorporating Fullerene-Based Diffusion Blocking Layer," Advanced Materials Interfaces, vol. 5, no. 20, p. 1800280, 2018/10/01 2018, doi: 10.1002/admi.201800280. [5] B. Rivkin, P. Fassl, Q. Sun, A. D. Taylor, Z. Chen, and Y. Vaynzof, "Effect of Ion Migration-Induced Electrode Degradation on the Operational Stability of Perovskite Solar Cells," ACS Omega, vol. 3, no. 8, pp. 10042-10047, 2018/08/31 2018, doi: 10.1021/acsomega.8b01626. [6] S. Bitton and N. Tessler, "Electronic-ionic coupling in perovskite based solar cells: Implications for device stability," Applied Physics Letters, vol. 117, no. 13, p. 133904, 2020, doi: 10.1063/5.0023902.

Ion migration into blocking layers toward the metallic electrodes is studied within a semiconductor device model framework. We find that ion leakage into the blocking layers and their accumulation at the electrode interface are significantly affected by the electronic injection barrier at the contact. Specifically, we find that if the device structure promotes, under light, hole (electron) accumulation within the perovskite layer, these excess holes (electrons) would release an almost equivalent number of cations (anions) into the transport layers toward the contacts. Our analysis suggests that it would be beneficial to include intentional doping of the blocking layers and that it should follow the “just enough” strategy.