Control of Nanomorphology in Fullerene-Free Organic Solar Cells by Lewis Acid Doping with Enhanced Photovoltaic Efficiency

Application of n-Type or p-Type Dopants in Organic Photovoltaics

Compared to inorganic semiconductors, organic semiconductors (OSCs) exhibit lower permittivity and carrier mobility. This is primarily attributed to their weaker van der Waals forces and the significant structural and energetic disorder, ultimately impeding the commercial application of organic photovoltaics (OPVs). However, the introduction of n-type or p-type dopants offers a solution. These dopants effectively eliminate intrinsic traps in OSCs through trap-filling techniques, elevating carrier concentration and mobility, and consequently enhancing overall performance. This article delves into the systematic exploration of n-type and p-type dopant applications in OPVs. It encompasses doping mechanisms, commonly used n-type and p-type dopants, doping methodologies, the strategic distribution of dopants and the effect of doping on device performance. Ultimately, this concept strives to offer invaluable insights and guidance for advancing OPV performance via doping techniques.

What We have Learnt from PM6:Y6

Over the past three years, remarkable advancements in organic solar cells (OSCs) have emerged, propelled by the introduction of Y6-an innovative A-DA'D-A type small molecule non-fullerene acceptor (NFA). This review provides a critical discussion of the current knowledge about the structural and physical properties of the PM6:Y6 material combination in relation to its photovoltaic performance. The design principles of PM6 and Y6 are discussed, covering charge transfer, transport, and recombination mechanisms. Then, the authors delve into blend morphology and degradation mechanisms before considering commercialization. The current state of the art is presented, while also discussing unresolved contentious issues, such as the blend energetics, the pathways of free charge generation, and the role of triplet states in recombination. As such, this review aims to provide a comprehensive understanding of the PM6:Y6 material combination and its potential for further development in the field of organic solar cells. By addressing both the successes and challenges associated with this system, this review contributes to the ongoing research efforts toward achieving more efficient and stable organic solar cells.

Highly Efficient and Stable ITO-Free Organic Solar Cells Based on Squaraine N-Doped Quaternary Bulk Heterojunction

Simultaneously achieving high efficiency and robust device stability remains a significant challenge for organic solar cells (OSCs). Solving this challenge is highly dependent on the film morphology of the bulk heterojunction (BHJ) photoactive blends; however, there is a lack of rational control strategy. Herein, it is shown that the molecular crystallinity and nanomorphology of nonfullerene-based BHJ can be effectively controlled by a squaraine-based doping strategy, leading to an increase in device efficiency from 17.26% to 18.5% when doping 2 wt% squaraine into the PBDB-TF:BTP-eC9:PC71 BM ternary BHJ. The efficiency is further improved to 19.11% (certified 19.06%) using an indium-tin-oxide-free column-patterned microcavity (CPM) architecture. Combined with interfacial modification, CPM quaternary OSC excitingly shows an extrapolated lifetime of ≈23 years based on accelerated aging test, with the mechanism behind enhanced stability well studied. Furthermore, a flexible OSC module with a high and stable efficiency of 15.2% and an overall area of 5 cm2 is successfully fabricated, exhibiting a high average output power for wearable electronics. This work demonstrates that OSCs with new design of BHJ and device architecture are highly promising to be practical relevance with excellent performance and stability.

Understanding the Role of Removable Solid Additives: Selective Interaction Contributes to Vertical Component Distributions

Sequentially deposited organic solar cells (SD-OSCs) have attracted great attention owing to their ability in achieving a more favorable, vertically phase-separated morphology to avoid the accumulation of counter charges at absorber/transporting layer interfaces. However, the processing of SD-OSCs is still quite challenging in preventing the penetration of small-molecule acceptors into the polymer donor layer via erosion or swelling. Herein, solid additives (SAs) with varied electrostatic potential distributions and steric hinderance are introduced into SD-OSCs to investigate the effect of evaporation dynamics and selective interaction on vertical component distribution. Multiple modelings indicate that the π-π interaction dominates the interactions between aromatic SAs and active layer components. Among them, p-dibromobenzene shows a stronger interaction with the donor while 2-chloronaphthalene (2-CN) interacts more preferably with acceptor. Combining the depth-dependent morphological study aided by multiple X-ray scattering methods, it is concluded that the evaporation of SAs can drive the stronger-interaction component upward to the surface, while having minor impact on the overall molecular packing. Ultimately, the 2-CN-treated devices with reduced acceptor concentration at the bottom surface deliver a high power conversion efficiency of 19.2%, demonstrating the effectiveness of applying selective interactions to improve the vertical morphology of OSCs by using SAs with proper structure.

Doping Approaches for Organic Semiconductors

Electronic doping in organic materials has remained an elusive concept for several decades. It drew considerable attention in the early days in the quest for organic materials with high electrical conductivity, paving the way for the pioneering work on pristine organic semiconductors (OSCs) and their eventual use in a plethora of applications. Despite this early trend, however, recent strides in the field of organic electronics have been made hand in hand with the development and use of dopants to the point that are now ubiquitous. Here, we give an overview of all important advances in the area of doping of organic semiconductors and their applications. We first review the relevant literature with particular focus on the physical processes involved, discussing established mechanisms but also newly proposed theories. We then continue with a comprehensive summary of the most widely studied dopants to date, placing particular emphasis on the chemical strategies toward the synthesis of molecules with improved functionality. The processing routes toward doped organic films and the important doping-processing-nanostructure relationships, are also discussed. We conclude the review by highlighting how doping can enhance the operating characteristics of various organic devices.

Novel semi-analytical optoelectronic modeling based on homogenization theory for realistic plasmonic polymer solar cells

Numerical-based simulations of plasmonic polymer solar cells (PSCs) incorporating a disordered array of non-uniform sized plasmonic nanoparticles (NPs) impose a prohibitively long-time and complex computational demand. To surmount this limitation, we present a novel semi-analytical modeling, which dramatically reduces computational time and resource consumption and yet is acceptably accurate. For this purpose, the optical modeling of active layer-incorporated plasmonic metal NPs, which is described by a homogenization theory based on a modified Maxwell-Garnett-Mie theory, is inputted in the electrical modeling based on the coupled equations of Poisson, continuity, and drift-diffusion. Besides, our modeling considers the effects of absorption in the non-active layers, interference induced by electrodes, and scattered light escaping from the PSC. The modeling results satisfactorily reproduce a series of experimental data for photovoltaic parameters of plasmonic PSCs, demonstrating the validity of our modeling approach. According to this, we implement the semi-analytical modeling to propose a new high-efficiency plasmonic PSC based on the PM6:Y6 PSC, having the highest reported power conversion efficiency (PCE) to date. The results show that the incorporation of plasmonic NPs into PM6:Y6 active layer leads to the PCE over 18%.

Probe and Control of the Tiny Amounts of Dopants in BHJ Film Enable Higher Performance of Polymer Solar Cells

To achieve efficient doping in polymer solar cells (PSCs), the dopant needs to be selectively located in the binary components of a bulk heterojunction (BHJ) film according to its polarity. The rarely studied n-type dopant is thoroughly examined in a simplified planar heterojunction (PHJ) device to address its favored location in the active layer. Results show that the n-dopant distribution in the acceptor layer or at the donor/acceptor interface produces enhanced device performance, whereas it harms the device when located in the donor layer. Based on the results, the benefit of n-type doping is then transferred to the highly efficient BHJ devices via a sequential coating procedure. The performance improvement is closely linked to the variations in the dopant's location in the BHJ film, which is carefully examined by the synchrotron techniques with delicate chemical sensitivity. More interestingly, the sequential coating procedure can be easily extended to the p-doped device only by changing the dopant's polarity in the middle layer. These findings pave the way for ambipolar doping in PSCs and enable performance improvement by molecular doping within the expectations.