Key Tradeoffs Limiting the Performance of Organic Photovoltaics

Abstract

perspective on the physics underlying the conventional and new systems and their performance limits.The discussion is focused on two key issues, which have for long hindered OPV performances, and their applicability to the new systems. The first is a charge generation-open-circuit voltage (V oc ) tradeoff, which has typically meant a driving force is required for charge transfer. Consequently, high-performance devices have exhibited either a high short-circuit current J sc or a high V oc . [1] The second is that the slow charge extraction and fast recombination mean the optimum device performance is obtained at active layer thicknesses inferior to those required for full absorption. This is manifested in a tradeoff between absorption and fill factor (FF). We first provide a modern outlook on the physics behind these tradeoffs. Both the fundamentals and their application to OPV systems are covered. We introduce the thermodynamics of photovoltaic conversion, which are currently the most powerful tool to describe the V oc of OPVs. We cover the various theories behind charge generation and, to widen the debate, their implications on devices and especially their V oc .This leads us to consider the recent small molecule systems and the exciting possibilities for experimental insights they offer. Some of these systems show record values for their emission yield and very low charge generation-V oc tradeoffs. Also, the role of nonradiative recombination due to molecular vibrations is receiving increasing attention. We examine the role of the tradeoffs in limiting NFA performance and suggest potential explanations for their improved performance as well as some promising avenues for research.

Accurate Determination of the Minimum HOMO Offset for Efficient Charge Generation using Organic Semiconducting Alloys

“…[38][39][40][41][42][43][44] Current research shows that NFA OSCs can provide decent PCEs with ΔE HOMO low to 0 eV. This has become a significant advantage of NFA-based OSCs compared with fullerene-based OSCs, that is, ΔE HOMO modulation may effectively manage the V oc -short-circuit current (J sc ) trade-off.…”

Anode interlayer in organic photovoltaics: Narrow bandgap small molecular materials as exciton‐blocking layer

“…It is noteworthy that, in this study, we do not observe a clear correlation between V OC and dark current, which is sometimes proposed in the literature . In addition, deriving parameters or extracting information by way of utilizing the Shockley equation can lack physical meaning when applied to typically low carrier‐mobility organic materials . Therefore, we suggest that, under illumination, there is little need of using wide‐bandgap AIL for SubPc/C 60 ‐based OSCs due to the efficient D/A interface for exciton dissociation and the effective electron selectivity of BCP on the cathode side.…”

“…[36,37] In addition, deriving parameters or extracting information by way of utilizing the Shockley equation can lack physical meaning when applied to typically low carrier-mobility organic materials. [38,39] Therefore, we suggest that, under illumination, there is little need of using wide-bandgap AIL for SubPc/C 60based OSCs due to the efficient D/A interface for exciton dissociation and the effective electron selectivity of BCP on the cathode side. As long as the AIL is thin and its HOMO energy level aligns well with that of SubPc, narrow-bandgap AILs work well and outperform conventional wide-bandgap AILs.…”

Fluorous Fullerene Acceptors in Vacuum‐Deposited Photovoltaic Cells

“…Top: Drawings of molecular structures of fluorous fullerene acceptors used in this work. Middle: Schlegel diagrams with addition patterns (black circles are located at the sp 3 carbon atoms and yellow field shows edge-sharing hexagons on C 60 core with para-or meta-CF 3 groups). Bottom: E 1/2 values versus C 60 from the literature.…”

“…Organic photovoltaics (OPV) research has reached impressive milestones in the past decade, including an improved performance of 10%–13% power conversion efficiency (PCE) of vacuum‐processed multijunction OPV devices and a remarkable 17.3% PCE of tandem device produced recently by Meng et al with the theoretically predicted limits of 25%–33% yet to be achieved . Continuous effort in fundamental understanding of the photophysics, material science, device physics, and engineering of OPV cells has resulted in the formulation of new research directions, including search for alternative donor and acceptor molecular and polymeric materials, morphology control, exploration of ternary compositions for active layers, and others.…”