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High electrical conductivity is a prerequisite for improving the performance of organic semiconductors for various applications and can be achieved through molecular doping. However, often the conductivity is enhanced only up to a certain optimum doping concentration, beyond which it decreases significantly. We combine analytical work and Monte Carlo simulations to demonstrate that carrier-carrier interactions can cause this conductivity decrease and reduce the maximum conductivity by orders of magnitude, possibly in a broad range of materials. Using Monte Carlo simulations, we disentangle the effect of carrier-carrier interactions from carrier-dopant interactions. Coulomb potentials of ionized dopants are shown to decrease the conductivity, but barely influence the trend of conductivity versus doping concentration. We illustrate these findings using a doped fullerene derivative for which we can correctly estimate the carrier density at which the conductivity maximizes. We use grazing-incidence wide-angle X-ray scattering to show that the decrease of the conductivity cannot be explained by changes to the microstructure. We propose the reduction of carrier-carrier interactions as a strategy to unlock higher-conductivity organic semiconductors.

Emerging nonfullerene acceptors (NFAs) with crystalline domains enable high-performance bulk heterojunction (BHJ) solar cells. Thermal annealing is known to enhance the BHJ photoactive layer morphology and performance. However, the microscopic mechanism of annealing-induced performance enhancement is poorly understood in emerging NFAs, especially regarding competing factors. Here, optimized thermal annealing of model system PBDB-TF:Y6 (Y6 = 2,2'-((2Z,2'Z)-((12,13-bis(2-ethylhexyl)-3,9-diundecyl-12,13-dihydro-[1,2,5]thiadiazolo[3,4-e]thieno[2″,3'':4',5']thieno[2',3':4,5]pyrrolo[3,2-g]thieno[2',3':4,5]-thieno[3,2-b]indole-2,10-diyl)bis(methanylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile) decreases the open circuit voltage (VOC) but increases the short circuit current (JSC) and fill factor (FF) such that the resulting power conversion efficiency (PCE) increases from 14 to 15% in the ambient environment. Here we systematically investigate these thermal annealing effects through in-depth characterizations of carrier mobility, film morphology, charge photogeneration, and recombination using SCLC, GIXRD, AFM, XPS, NEXAFS, R-SoXS, TEM, STEM, fs/ns TA spectroscopy, 2DES, and impedance spectroscopy. Surprisingly, thermal annealing does not alter the film crystallinity, R-SoXS characteristic size scale, relative average phase purity, or TEM-imaged phase separation but rather facilitates Y6 migration to the BHJ film top surface, changes the PBDB-TF/Y6 vertical phase separation and intermixing, and reduces the bottom surface roughness. While these morphology changes increase bimolecular recombination (BR) and lower the free charge (FC) yield, they also increase the average electron and hole mobility by at least 2-fold. Importantly, the increased μh dominates and underlies the increased FF and PCE. Single-crystal X-ray diffraction reveals that Y6 molecules cofacially pack via their end groups/cores, with the shortest π-π distance as close as 3.34 A, clarifying out-of-plane π-face-on molecular orientation in the nanocrystalline BHJ domains. DFT analysis of Y6 crystals reveals hole/electron reorganization energies of as low as 160/150 meV, large intermolecular electronic coupling integrals of 12.1-37.9 meV rationalizing the 3D electron transport, and relatively high μe of 10-4 cm2 V-1 s-1. Taken together, this work clarifies the richness of thermal annealing effects in high-efficiency NFA solar cells and tasks for future materials design.

The charge carrier mobility is one of the most critical electronic materials properties that determines the ultimate performance of organic photovoltaic (OPV) cells. However, it is also a property with complex dependencies on the charge carrier density, electric field, lengthscale, and timescale, which can each vary depending on the chemical structure, molecular order and orientation, phase morphology, etc. These issues have made it extremely challenging to develop quantitative structure–property relationships that would allow rational molecular and materials design for next generation OPVs. Using a unique combination of advanced experimental morphology characterization (electron tomography) and recently developed open-source computational tools for morphology analysis and kinetic Monte Carlo charge transport simulations, we investigate how the microstructural features in real bulk heterojunction blends impact charge transport physics. This work demonstrates that simulated charge transport in real morphologies can differ significantly from that found with the commonly used Ising-based model. However, most significantly, there are fundamental differences in the mobility relaxation dynamics between homogeneous neat materials and bulk heterojunction blends. The tortuosity of the bulk heterojunction domain network causes electric-field-induced dispersion that can significantly prolong the mobility relaxation dynamics. These morphological effects must be considered when analyzing experimental mobility results and when choosing the appropriate measurement technique.

The rational design of new high performance materials for organic photovoltaic (OPV) applications is largely inhibited by a lack of design rules for materials that have slow bimolecular charge recombination. Due to the complex device physics present in OPVs, rigorous and reliable measurement techniques for charge transport and charge recombination are needed to construct improved physical models that can guide materials development and discovery. Here, we develop a new technique called impedance-photocurrent device analysis (IPDA) to quantitatively characterize the competition between charge extraction and charge recombination under steady state operational conditions. The measurements are performed on actual lab scale solar cells, have mild equipment requirements, and can be integrated into normal device fabrication and testing workflows. We perform IPDA tests on a broad set of devices with varying polymer:fullerene blend chemistry and processing conditions. Results from the IPDA technique exhibit significantly improved reliability and self-consistency compared to the open-circuit voltage decay technique (OCVD). IPDA measurements also reveal a significant negative electric field dependence of the bimolecular recombination coefficient in high fill factor devices, a finding which is inaccessible to most other common techniques and indicates that many of these techniques may overestimate the value that is most relevant for describing device performance. Future work utilizing IPDA to build structure–property relationships for bimolecular recombination will lead to enhanced design rules for creating efficient OPVs that are suitable for commercialization.

Since the landmark discovery of semiconducting polymers, the development of organic electronic devices has constituted a major international research effort and has evolved into a well-established field of fundamental and applied research. Stemming from these efforts, organic light-emitting diodes (OLEDs), organic photovoltaics, and organic field-effect transistors are the most heavily developed technologies, with OLEDs in particular achieving widespread commercial success in mobile and television displays. Throughout the many development phases of all these devices, simulation and modeling have played an important role in understanding the fundamental mechanisms governing energy and charge transport, as well as how these mechanisms eventually connect to the performance of real devices. Simulation and modeling techniques span a wide range of time and length scales and can have immense variations in complexity, depending on the problem being investigated. In this chapter, we present the latest organic semiconductor (OSC) device simulation and modeling techniques and highlight the major developments and types of problems that have been addressed using these techniques. We divide the discussion into two sections—one covering microscopic techniques (primarily kinetic Monte Carlo simulations) and another covering macroscopic techniques (primarily drift-diffusion simulations). In the end, we also provide an outlook for the future of OSC device simulation and modeling that highlights particularly interesting and impactful areas for further research.

Exciton delocalization has been proposed to have a strong impact on the performance of organic solar cells. For example, large exciton delocalization estimates have promoted the theory of long-range charge transfer as a mechanism for efficient charge separation. Here, two new computational modeling techniques for analyzing femtosecond transient absorption spectroscopy experiments are developed in order to estimate the magnitude of exciton delocalization in semiconducting polymers. The developed techniques are then used to analyze previously published experimental data for regioregular poly(3-hexylthiophene) (P3HT). Based on modeling both the exciton–exciton annihilation behavior in a pure P3HT film and the exciton dissociation dynamics in a P3HT:PCBM blend film, the exciton delocalization radius in regioregular P3HT is estimated to be in the range of 1–2 nm, which is significantly smaller than estimated in a number of previous studies. These results suggest that exciton delocalization is not likely to be ...

Understanding how the complex intermolecular- and nano-structure present in organic semiconductor donor-acceptor blends impacts charge carrier motion, interactions, and recombination behavior is a critical fundamental issue with a particularly major impact on organic photovoltaic applications. In this study, kinetic Monte Carlo (KMC) simulations are used to numerically quantify the complex bimolecular charge carrier recombination behavior in idealized phase-separated blends. Recent KMC simulations have identified how the encounter-limited bimolecular recombination rate in these blends deviates from the often used Langevin model and have been used to construct the new power mean mobility model. Here, we make a challenging but crucial expansion to this work by determining the charge carrier concentration dependence of the encounter-limited bimolecular recombination coefficient. In doing so, we find that an accurate treatment of the long-range electrostatic interactions between charge carriers is critical, and we further argue that many previous KMC simulation studies have used a Coulomb cutoff radius that is too small, which causes a significant overestimation of the recombination rate. To shed more light on this issue, we determine the minimum cutoff radius required to reach an accuracy of less than $\pm10\%$ as a function of the domain size and the charge carrier concentration and then use this knowledge to accurately quantify the charge carrier concentration dependence of the recombination rate. Using these rigorous methods, we finally show that the parameters of the power mean mobility model are determined by a newly identified dimensionless ratio of the domain size to the average charge carrier separation distance., Comment: Main article: 9 pages, 5 figures, Supplementary Information: 3 pages, 1 figure

A strategy for increasing the conversion efficiency of organic photovoltaics has been to increase the VOC by tuning the energy levels of donor and acceptor components. However, this opens up a new loss pathway from an interfacial charge transfer state to a triplet exciton (TE) state called electron back transfer (EBT), which is detrimental to device performance. To test this hypothesis, we study triplet formation in the high performing PTB7:PC\(_{71}\)BM blend system and determine the impact of the morphology-optimizing additive 1,8-diiodoctane (DIO). Using photoluminescence and spin-sensitive optically detected magnetic resonance (ODMR) measurements at low temperature, we find that TEs form on PC\(_{71}\)BM via intersystem crossing from singlet excitons and on PTB7 via EBT mechanism. For DIO blends with smaller fullerene domains, an increased density of PTB7 TEs is observed. The EBT process is found to be significant only at very low temperature. At 300 K, no triplets are detected via ODMR, and electrically detected magnetic resonance on optimized solar cells indicates that TEs are only present on the fullerenes. We conclude that in PTB7:PC\(_{71}\)BM devices, TE formation via EBT is impacted by fullerene domain size at low temperature, but at room temperature, EBT does not represent a dominant loss pathway.

A colloidal lithography method has been developed for patterning nonplanar surfaces. Hexagonal noncontiguously packed (HNCP) colloidal particles 127 nm-2.7 μm in diameter were first formed at the air-water interface and then adsorbed onto a substrate coated with a layer of polymer adhesive ∼17 nm thick. The adhesive layer plays the critical role of securing the order of the particles against the destructive lateral capillary force generated by a thin film of water after the initial transfer of the particles from the air-water interface. The soft lithography method is robust and very simple to carry out. It is applicable to a variety of surface curvatures and for both inorganic and organic colloidal particles.

Thermally stimulated current (TSC) measurements are used to characterize electronic trap states in methylammonium lead iodide perovsite solar cells. Several TSC peaks were observed over the temperature range from 20 K to room temperature. To elucidate the origins of these peaks, devices with various organic charge transport layers and devices without transport layers were tested. Two peaks appear at very low temperatures, indicating shallow trap states that are mainly attributed to the PCBM/C60 electron transport bilayer. However, two additional peaks appear at higher temperatures, that is, they are deeper in energy, and are assigned to the perovskite layer. At around T = 163 K, a sharp peak, also present in the dark TSC measurements, is assigned to the orthorhombic-tetragonal phase transition in the perovskite. However, a peak at around T = 191 K is assigned to trap states with activation energies of around 500 meV but with a rather low concentration of 1 × 10(21) m(-3).

Organic bulk heterojunction (BHJ) solar cells require energetic offsets between the donor and acceptor to obtain high short-circuit currents (J(SC)) and fill factors (FF). However, it is necessary to reduce the energetic offsets to achieve high open-circuit voltages (V-OC). Recently, reports have highlighted BHJ blends that are pushing at the accepted limits of energetic offsets necessary for high efficiency. Unfortunately, most of these BHJs have modest FF values. How the energetic offset impacts the solar cell characteristics thus remains poorly understood. Here, a comprehensive characterization of the losses in a polymer:fullerene BHJ blend, PIPCP:phenyl-C61-butyric acid methyl ester (PC61BM), that achieves a high V-OC (0.9 V) with very low energy losses (E-loss = 0.52 eV) from the energy of absorbed photons, a respectable J(SC) (13 mA cm(-2)), but a limited FF (54%) is reported. Despite the low energetic offset, the system does not suffer from field-dependent generation and instead it is characterized by very fast nongeminate recombination and the presence of shallow traps. The charge-carrier losses are attributed to suboptimal morphology due to high miscibility between PIPCP and PC61BM. These results hold promise that given the appropriate morphology, the J(SC), V-OC, and FF can all be improved, even with very low energetic offsets.

Over the last 5 years, research on the synthesis, device engineering, and device physics of solution-processed small molecule solar cells (SMSCs) has rapidly expanded. Improvements in molecular design and emergent device processing techniques have helped solution-processed SMSCs overcome earlier difficulties in controlling active layer morphology, such that many systems are now at—or approaching—10% power conversion efficiency. In this review, details of the highest performing blend systems are presented in order to identify key trends and provide perspective on current progress in the field. Among the best systems, a planarized molecular structure is prevalent, which can be achieved using large fused-ring moieties, intermolecular non-bonding interactions, and side chain engineering. To obtain efficient devices, the highest performing systems have been optimized through the careful combination of thermal and solvent annealing procedures. Even without additional processing, some systems have been able to obtain interconnected morphologies and efficient charge generation and charge transport. Ultimately, the design of more efficient materials also requires additional understanding of the device physics and loss mechanisms. After highlighting what is known to date on processes limiting device efficiency, an outlook on the most important challenges remaining to the field is provided.

The theoretical effects of phase separation on encounter-limited charge carrier recombination in organic semiconductor blends are investigated using kinetic Monte Carlo (KMC) simulations of pump-probe experiments. Using model bulk heterojunction morphologies, the dependence of the recombination rate on domain size and charge carrier mobility are quantified. Unifying competing models and simulation results, we show that the mobility dependence of the recombination rate can be described using the power mean of the electron and hole mobilities with a domain size dependent exponent. Additionally, for domain sizes typical of organic photovoltaic devices, we find that phase separation reduces the recombination rate by less than one order of magnitude compared to the Langevin model and that the mobility dependence can be approximated by the geometric mean., Comment: Main article: 6 pages, 4 figures, Supplementary Information: 3 pages, 1 figure

Kinetic Monte Carlo (KMC) simulations have been previously used to model and understand a wide range of behaviors in bulk heterojunction (BHJ) organic photovoltaic devices, from fundamental mechanisms to full device performance. One particularly unique and valuable aspect of this type of modeling technique is the ability to explicitly implement models for the bicontinuous nanostructured morphology present in these devices. For this purpose, an Ising-based method for creating model BHJ morphologies has become prevalent. However, this technique can be computationally expensive, and a detailed characterization of this method has not yet been published. Here, we perform a thorough characterization of this method and describe how to efficiently generate controlled model BHJ morphologies. We show how the interaction energy affects the tortuosity of the interconnected domains and the resulting charge transport behavior in KMC simulations. We also demonstrate how to dramatically reduce calculation time by several orders of magnitude without detrimentally affecting the resulting morphologies. In the end, we propose standard conditions for generating model morphologies and introduce a new open-source software tool. These developments to the Ising method provide a strong foundation for future simulation and modeling of BHJ organic photovoltaic devices that will lead to a more detailed understanding of the important link between morphological features and device performance., Comment: Main article: 9 pages, 6 figures, Supplementary Information: 6 pages, 6 figures

The slow decay of charge carriers in polymer-fullerene blends measured in transient studies has raised a number of questions about the mechanisms of nongeminate recombination in these systems. In an attempt to understand this behavior, we have applied a combination of steady-state and transient photoinduced absorption measurements to compare nongeminate recombination behavior in films of neat poly(3-hexyl thiophene) (P3HT) and P3HT blended with [6,6]-phenyl-C61 butyric acid methyl ester (PCBM). Transient measurements show that carrier recombination in the neat P3HT film exhibits second-order decay with a recombination rate coefficient that is similar to that predicted by Langevin theory. In addition, temperature dependent measurements indicate that neat films exhibit recombination behavior consistent with the Gaussian disorder model. In contrast, the P3HT:PCBM blend films are characterized by a strongly reduced recombination rate and an apparent recombination order greater than two. We then assess a number of previously proposed explanations for this behavior, including phase separation, carrier concentration dependent mobility, non-encounter limited recombination, and interfacial states. In the end, we propose a model in which pure domains with a Gaussian density of states are separated by a mixed phase with an exponential density of states. We find that such a model can explain both the reduced magnitude of the recombination rate and the high order recombination kinetics and, based on the current state of knowledge, is the most consistent with experimental observations., Comment: 9 pages, 4 figures; corrected a few minor typos and grammatical errors

Triplet exciton formation in neat 7,7-(4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b′] dithiophene-2,6-diyl)bis(6-fluoro-4-(5′-hexyl-[2,2′-bithiophen]-5-yl)benzo[c][1,2,5]thiadiazole) (p-DTS(FBTTh2)2) and blends with [6,6]-Phenyl C70 butyric acid methyl ester (PC70BM), with and without the selective solvent additive 1,8-diiodooctane, is investigated by means of spin sensitive photoluminescence measurements. For all three material systems, a significant amount of long living triplet excitons is detected, situated on the p-DTS(FBTTh2)2 molecules. The characteristic zero-field splitting parameters for this state are identified to be D = 42 mT (1177 MHz) and E = 5 mT (140 MHz). However, no triplet excitons located on PC70BM are detectable. Using electrically detected spin resonance, the presence of these triplet excitons is confirmed even at room temperature, highlighting that triplet excitons form during solar cell operation and influence the photocurrent and photovoltage. Surprisingly, the superior performing blend is found to have the largest triplet population. It is concluded, that the formation of triplet excitons from charge transfer states via electron back transfer has no crucial impact on device performance in p-DTS(FBTTh2)2:PC70BM based solar cells.

Open circuit voltage decay measurements are performed on methylammonium lead iodide (CH3NH3PbI3) perovskite solar cells to investigate the charge carrier recombination dynamics. The measurements are compared to the two reference polymer-fullerene bulk heterojunction solar cells based on P3HT:PC60BM and PTB7:PC70BM blends. In the perovskite devices, two very different time domains of the voltage decay are found, with a first drop on a short time scale that is similar to the organic solar cells. However, two major differences are also observed. 65-70% of the maximum photovoltage persists on much longer timescales, and the recombination dynamics are dependent on the illumination intensity., 5 pages, 3 figures

A general dynamic Monte Carlo model for exciton dissociation at a donor-acceptor interface that includes exciton delocalization and hot charge separation is developed to model the experimental behavior observed for the poly(3-hexylthiophene):fullerene system and predict the theoretical performance of future materials systems. The presence of delocalized excitons and the direct formation of separated charge pairs has been recently measured by transient photo-induced absorption experiments and has been proposed to facilitate charge separation. The excess energy of the exciton dissociation process has also been observed to have a strong correlation with the charge separation yield for a series of thiophene based polymer:fullerene systems, suggesting that a hot charge separation process is also occurring. Hot charge separation has been previously theorized as a cause for highly efficient charge separation. However, a detailed model for this process has not been implemented and tested. Here, both conceptual models are implemented into a dynamic Monte Carlo simulation and tested using a simple bilayer donor-acceptor system. We find that exciton delocalization can account for a significant reduction in geminate recombination when compared to the traditional, bound polaron pair model. In addition, the hot charge separation process could further reduce the geminate recombination, but only if the hot charge mobility is several orders of magnitude larger than the standard charge mobility.