Organic Cocrystal Photovoltaic Behavior: A Model System to Study Charge Recombination of C₆₀ and C₇₀ at the Molecular Level

Abstract

regions shows an improved PCE, indicating that cocrystallinity could play an important role in photovoltaic properties. [21][22][23] Here, we demonstrate this concept by studying the photovoltaic behavior of individual organic segregated-packing cocrystal (Figure 1 d).D-A system is important for the nucleation and growth of cocrystals, [24][25][26][27][28][29][30] forming mixed-and segregated-packing motifs, and the latter provides not only a molecular-scale phase layer for excitons diffusion but also a bicontinuous layer-by-layer donoracceptor composite with large interlayer area for exciton dissociation. And hence such packing motif is optimum for photovoltaic study. For example, sulfur-bridged annulene DPTTA and C 60 system form nice donor-acceptor segregated-packing cocrystals, [ 25 ] and here we use devices of individual cocrystal of DPTTA-C 60 to study the photovoltaic behavior of such D-A segregated-packing structure. It should be the fi rst cocrystal prototype photovoltaic device with ideal segregated arrangement in the active layer.Figure 2 a shows chemical structure of DPTTA, C 60 , and C 70 . C 60 /C 70 -DPTTA cocrystals were prepared by a facile drop-casting method. Blend solution of DPTTA and C 60 /C 70 dissolved in chlorobenzene was poured over clean Si/SiO 2 (300 nm) substrate at room temperature in a glovebox. After solvent evaporation, nanosheet cocrystals were obtained. Before fabrication of devices, residual solvent was removed by annealing at 80 °C for 1 h under vacuum condition. Figure 2 b,c represents the single crystal morphology of DPTTA, C 60 , C 60 -DPTTA, and C 70 -DPTTA. According to X-ray crystallography and X-ray diffraction, donor and acceptor molecules alternatively stack layer by layer on Si/SiO 2 substrate for both C 60 -DPTTA and C 70 -DPTTA nanosheets. [ 25 ] Figure 2 d indicates that DPTTA and C 60 /C 70 have several absorption peaks at the visible range. Relatively, the absorbance ability is much stronger for DPTTA with respect to the fullerenes. Hence, most of excitons are generated in the donor layers. UV-vis absorptions for C 60 , C 70 , DPTTA, DPTTA/C 60 , and DPTTA/C 70 fi lm were also performed, as shown in Figure S2 (Supporting Information).

Insights into the Control of Optoelectronic Properties in Mixed‐Stacking Charge‐Transfer Complexes

“…The photoresponsive behavior was further studied by exploring the photocurrent, and the corresponding phototransistor schematic is depicted in Figure a. Under white‐light illumination, significantly enhanced generation of photocarriers occurred in the TMCA‐based device with photocurrent on/off ratio [ P =( I light − I dark )/ I dark ] greater than 2×10 3 ( V G =16 V, Figure e), which is among the highest values for all reported organic cocrystals, and shifting of the onset voltage V on to the positive indicates a photovoltaic effect . In comparison, the TMFA‐based device exhibits much inferior photoresponse with decreased P value (≈500), whereas TMTQ shows loss of the photoresponse (Figure b and h), which is also evidenced by the output curves (Figure S24 in the Supporting Information).…”

“…Considering this, Zhu, Hu, and co‐workers have made some preliminary attempts in which, in analogy with bulk heterojunctions, a segregated donor (D) and acceptor (A) packing motif with interdigitated network may be beneficial for exciton separation and for photoconductivity and photovoltaic applications. On this basis, they synthesized segregated‐packing cocrystals of sulfur‐bridged annulene DPTTA‐C 60 /C 70 with light responsivity of 300 A W −1 and power conversion efficiency of 0.27 % . However, the nature of the photoconductivity is still unclear and they almost completely ignored mixed‐stacking cocrystals .…”

“…In addition, another intriguing feature of cocrystallization is that it has offered ap erfect platform to uncover the inherent mechanisms of the optoelectronic properties, due to the longrange order and controllable molecular p-n junctionsi nc ocrystals.C onsidering this, Zhu, Hu, and co-workers have made some preliminarya ttempts in which, in analogyw ith bulk heterojunctions, as egregated donor (D) and acceptor( A) packing motif with interdigitated network may be beneficial for exciton separation andf or photoconductivity and photovoltaic applications.O nt his basis, they synthesized segregated-packing cocrystalso fs ulfur-bridged annulene DPTTA-C 60 /C 70 with light responsivity of 300 AW À1 and power conversion efficiency of 0.27 %. [13,23] However,t he nature of the photoconductivity is still unclear and they almostc ompletely ignoredm ixed-stacking cocrystals. [21,24] Thus, it is highly justified to put more efforts (including more comprehensive insights into the structure-property relationship of rationally co-assembled systems) to elucidate the underlying mechanismso ft his specific optoelectronic property in cocrystals.…”

Organic Semiconductor Single Crystals for Electronics and Photonics

“…Organic cocrystals are single‐phase materials composed of two or more molecular and/or ionic compounds in a stoichiometric ratio, with the possibility to be used for the construction of molecular heterojunctions with functions of 1 + 1 = 2 and/or 1 + 1 > 2 effects . Among organic cocrystals, CT interactions are an important type of interaction and play an important role in directing the supramolecular coassembly of the donor and acceptor components, which can form via various methods, such as vapor‐phase methods, solid‐phase methods under mechanochemical force and liquid‐phase methods with different solution‐processing conditions .…”

2D–Organic Hybrid Heterostructures for Optoelectronic Applications

“…Most importantly, from the viewpoint of hybridization with organic semiconductors, the atomically flat and dangling‐bond‐free surface of 2D vdW materials not only provides an ideal interface for high‐efficiency charge separation, transfer, and transport but also plays the important role of a template for the epitaxial growth of organic semiconductors . As the result, 2D/organic hybrid heterostructure generally exhibits enhanced electrical and optical performances compared to heterostructures consisting only of organic or inorganic materials . In the next section of this review, we present the fabrication procedures and physical properties of 2D–organic hybrid structures.…”