Thin-film optoelectronic devices based on polycrystalline organolead-halide perovskites have recently become a topic of intense research. Single crystals of these materials have been grown from solution with electrical properties superior to those of polycrystalline films. In order to enable the development of more complex device architectures based on organolead-halide perovskite single crystals, we developed a process to form epitaxial layers of methylammonium lead iodide (MAPbI) on methylammonium lead bromide (MAPbBr) single crystals. The formation of the MAPbI layer is found to be dominated by the diffusion of halide ions, leading to a shift in the photoluminescence and absorption spectra. X-ray diffraction measurements confirm the single-crystal nature of the MAPbI layer, while carrier transport measurements show that the converted layer retains the high carrier mobility typical of single-crystal perovskite materials. Such heterostructures on perovskite single crystals open possibilities for new types of devices.

It is commonly believed that large dielectric constants are required for efficient charge separation in polymer photovoltaic devices. However, many polymers used in high‐performance solar cells do not possess high dielectric constants. In this work, the effect of polymer–fullerene interactions on the dielectric environment of the active layer blend and the device performance for several donor–acceptor conjugated polymer systems is investigated. It is found that, while none of the high‐performing polymers studied has a dielectric constant value larger than 3, all polymer–fullerene blends have a significantly larger dielectric constant compared to their pristine constituents. Additionally, it is found that the blend dielectric constant reaches a maximum value in fully optimized devices. Using PTB7:PC71BM blends as an example, it is showed that, in addition to a small increase in the dielectric constant, devices fabricated using the optimum processing additive concentration exhibit almost 3X larger excited state polarizability. This large increase in excited state polarizability results in a substantial difference in short‐circuit current and ultimately device performance. The results show that the excited state polarizability critically depends on polymer–fullerene interactions, and can be a leading indicator of device performance for a given material system.

Fluorinated conjugated polymers leading to enhanced photovoltaic device performance has been widely observed in a variety of donor− acceptor copolymers; however, almost all these polymers have fluorine substituents on the acceptor unit. Building upon our previously reported PBnDT-FTAZ, a fluorinated donor−acceptor conjugated polymer with impressive device performance, we set this study to explore the effect of adding the fluorine substituents onto the flanking thiophene units between the donor unit (BnDT) and the acceptor unit (TAZ). We developed new synthetic approaches to control the position of the fluorination (3′ or 4′) on the thiophene unit, and synthesized four additional PBnDT-TAZ polymers incorporating the fluorine-substituted-thiophene (FT) units, 3′-FT-HTAZ, 4′-FT-HTAZ, 3′-FT-FTAZ, and 4′-FT-FTAZ. We discover that relocating the fluorine substituents from the acceptor to the flanking thiophene units have a negligible impact on the device characteristics (short circuit current, open circuit voltage, and fill factor) when comparing 4′-FT-HTAZ with the original FTAZ. Combining these two fluorination approaches together, 4′-FT-FTAZ shows even higher device performance than FTAZ (7.7% vs 6.6%) with active layers over 200 nm in thickness. Furthermore, high values of fill factor ∼70% are all achieved for photovoltaic devices based on 3′-FT-HTAZ, 4′-FT-HTAZ, or 4′-FT-FTAZ, ascribed to the observed high hole mobilities (over 1 × 10 −3 cm 2 /(V s)) in these devices. Our study offers a new approach to utilize the fluorinated thiophene units in developing new conjugated polymers to further improve the device performance of polymer solar cells.

Despite the rapid development of nonfullerene acceptors (NFAs), the fundamental understanding on the relationship between NFA molecular architecture, morphology, and device performance is still lacking. Herein, poly[[4,8-bis[5-(2-ethylhexyl)thiophene-2-yl]benzo[1,2-b:4,5-b0]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]-thieno[3,4-b]thiophenediyl]](PTB7-Th) is used as the donor polymer to compare an NFA with a 3D architecture (SF-PDI4) to a well-studied NFA with a linear acceptor-donoracceptor (A-D-A) architecture (ITIC). The data suggest that the NFA ITIC with a linear molecular structure shows a better device performance due to an increase in short-circuit current ( J sc ) and fill factor (FF) compared to the 3D SF-PDI4. The charge generation dynamics measured by femtosecond transient absorption spectroscopy (TAS) reveals that the exciton dissociation process in the PTB7-Th:ITIC films is highly efficient. In addition, the PTB7-Th:ITIC blend shows a higher electron mobility and lower energetic disorder compared to the PTB7-Th:SF-PDI4 blend, leading to higher values of J sc and FF. The compositional sensitive resonant soft X-ray scattering (R-SoXS) results indicate that ITIC molecules form more pure domains with reduced domain spacing, resulting in more efficient charge transport compared with the SF-PDI4 blend. It is proposed that both the molecular structure and the corresponding morphology of ITIC play a vital role for the good solar cell device performance.

The effects of the oligothiophene length of two thiophene-isoindigo copolymers on film morphology, charge transfer, and photovoltaic device performance are reported. Despite the similarities in their repeat unit structures, the two polymers show distinctly different film morphologies and photovoltaic performance upon blending with PC71BM. We found that there is a significant increase in the dielectric constant of the photoactive film upon blending fullerene with the polymer that exhibits a higher power conversion efficiency. Blend photoluminescence transients revealed a fast dissociation route in the better performing polymer followed by a slower decay. The fast decay in transient PL is attributed to a higher charge transfer efficiency when blending with the fullerene. We suggest that the charge transfer efficiency is determined not only by the microscopic morphology but also whether the polymer can accommodate the fullerene molecules in close proximity to the acceptor moiety to facilitate electronic coupling between the isoindigo acceptor and the fullerene molecule. We propose that the fast decay component seen in transient PL for the better performing polymer, along with the increase in dielectric constant, is a signature of enhanced electronic coupling between the polymer and the fullerene. The enhanced electronic coupling is thought to originate from a polymer chemical structure which allows the fullerene molecules to come to closer proximity for more efficient charge transfer.