Abstract
Fundamentally, organic solar cells (OSCs) with a bulk-heterojunction active layer are made of at least two electronically dissimilar molecules, in which photoabsorption in one (donor) generates Frenkel excitons. The formation of free charge carriers emerge after exciton dissociation at the donor:acceptor interface. In the past decade, most of the progress in enhanced device performance has been steered by the rapid development of novel donor and acceptor materials and on device engineering. Among these donor materials, regioregular poly(3-hexylthiophene) (P3HT) produced better performance despite the mismatch of its absorption coefficient with the solar emission spectrum. Comparatively the donor PBDB-T exhibits an outstanding absorption coefficient with a deeper-lying highest occupied molecular orbital (HOMO) level. Previously most of the efficient acceptors were based on fullerene molecules characterized by limited photoabsorption and stability. In contrast, the recently developed non-fullerene OSCs have a tunable absorption spectrum and exhibit improved stability. In this work, we explore the fundamental sources of the differences in the device performance for different blend compositions made of fullerene derivative (PC71BM) and non-fullerene (ITIC-Th) when paired with the polymer donors P3HT and PBDB-T. The characteristic changes of the optical properties of these blends and their roles in device performance are also investigated. We also studied charge generation where PBDB-T:PC71BM showed the highest maximum exciton generation rate (Gmax) of 3.22 × 1028 s–1 while P3HT: ITIC-Th gave the lowest (0.96 × 1028 s–1). Also noted, PC71BM based counterparts gave better charge transfer capabilities as seen from the lower PL quenching and higher charge carrier dissociation plus collection probability P(E,T) derived from a plot of Jph/Jsat ratio under short-circuit conditions against the effective voltages.
Highlights
In the past decade, bulk heterojunction (BHJ) organic solar cells (OSCs) have been intensively studied for their high potential for realizing environmentally friendly, low cost, noncomplex, and flexible large area devices (Zhou et al, 2012; Liu et al, 2015)
It is seen in the figure that PBDB-T has strong absorption in the range spanning from 500 to 750 nm
The extent of the absorption window within the visible range is much broader for this donor material compared to absorption compared to the fullerene, PC71BM based blends
Summary
Bulk heterojunction (BHJ) organic solar cells (OSCs) have been intensively studied for their high potential for realizing environmentally friendly, low cost, noncomplex, and flexible large area devices (Zhou et al, 2012; Liu et al, 2015). Despite the rapid improvement in the power conversion efficiency in the recent past, the progress toward commercialization of these devices is still hindered by their low carrier mobility, insufficient photoabsorption (Li et al, 2018), and degradation by intrinsic factors such as inter-layer molecular cross diffusion. One strategy to enhance the device performance is to increase the photo generated current through extending the light harvesting capabilities to a broad absorption window (Yang et al, 2019) This has seen the development of numerous donor and acceptor materials with varied band gaps as well as the design of many classical donor/acceptor heterojunction systems (Gao et al, 2017). This is attributed mainly to self-organization of its molecules as laminar structures with crystal properties that enable high hole mobility (>10−2 cm2/Vs)
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