Direct Free Carrier Photogeneration in Single Layer and Stacked Organic Photovoltaic Devices

Abstract

High performance organic photovoltaic devices typically rely on type-II P/N junctions for assisting exciton dissociation. Heremans and co-workers recently reported a high efficiency device with a third organic layer which is spatially separated from the active P/N junction; but still contributes to the carrier generation by passing its energy to the P/N junction via a long-range exciton energy transfer mechanism. In this study the authors show that there is an additional mechanism contributing to the high efficiency. Some bipolar materials (e.g., subnaphthalocyanine chloride (SubNc) and subphthalocyanine chloride (SubPc)) are observed to generate free carriers much more effectively than typical organic semiconductors upon photoexcitation. Single-layer devices with SubNc or SubPc sandwiched between two electrodes can give power conversion efficiencies 30 times higher than those of reported single-layer devices. In addition, internal quantum efficiencies (IQEs) of bilayer devices with opposite stacking sequences (i.e., SubNc/SubPc vs SubPc/SubNc) are found to be the sum of IQEs of single layer devices. These results confirm that SubNc and SubPc can directly generate free carriers upon photoexcitation without assistance from a P/N junction. These allow them to be stacked onto each other with reversible sequence or simply stacking onto another P/N junction and contribute to the photocarrier generation.

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

“…Considering the shape of J – V curve, as well as the current densities appear arbitrary irrelevant to the nature of AIL, we suggest that the very high‐lying LUMO energy level of NPAE and TCTA does not offer many benefits over the LUMO energy level‐matching materials FPhSPFN , NPAAnQ , PhSPFN , and PyPAFN . In addition, SubPc is known to be ambipolar, and thus, it stands to reason that the reduction in dark current of the four narrow‐bandgap AIL materials originates from a reduced electron transport through the AIL. We believe that the electron‐trapping nature of electron‐withdrawing moieties, anthraquinone and fumaronitrile, and the thinness of the AIL are key factors in this regard.…”

Thiadiazole Fused Subporphyrazines as Acceptors in Organic Photovoltaic Cells

“…As known, an exciton-blocking material should carefully be adapted for the specific acceptor/cathode pair to avoid substantial energy losses at the interface. [1][2][3]6,8,18,19,22] An indirect argument for this is the lack of correlation between the value of U oc (Table 1) and the LUMO position in acceptor (Figure 1). This is illustrated in Figure 2(f).…”

“…The rectification in the darkness is usually poor, most likely due to compensating (unwanted) contribution of the interactive 'acceptor/Al' junction. [1][2][3]18,19] Under illumination of 100 mW/cm 2 , a photovoltaic effect arises (Figure 2(a-d)), the corresponding parameters are summarized in Table 1. Significant photovoltage generated by the molecular heterojunction can be expected from the large difference between HOMO energy of donor (SubPcH 12 ) and LUMO energy of acceptor (see Figure 1).…”

“…[8] It was recently shown that even without p/n junction subphthalocyanines give higher yields of free charge carriers upon photoexcitation than other molecular semiconductors. [3] Fusion of 1,2,5-thiadiazole rings strongly enhances the π-electron-deficiency of the porphyrazine macrocycle [9,10] allowing to use tetra(1,2,5-thiadiazolo)porphyrazine and its metal complexes as n-type organic semiconductors in prototypes of the field-effect transistors [11] and in photovoltaic cells as acceptors instead of fullerene. [12] We have synthesized novel heterocyclic subphthalocyanine analogue -subporphyrazine bearing three fused 1,2,5-thiadiazole rings [13] and, very recently, two low-symmetry analogues of perfluorinated subphthalocyanine having 1,2,5-thiadiazole ring(s) instead of one or two benzene rings.…”

High‐Efficient Charge Generation in Single‐Donor‐Component‐Based p‐i‐n Structure Organic Solar Cells

“…A pure fullerene‐based OSC was reported, which can work even without donor material, although PCE, especially J sc , is quite low . Other acceptor material‐based single‐layer OPV devices, such as subnaphthalocyanine chloride (SubNc) and subphthalocyanine chloride (SubPc), can effectively generate free charge carriers without any assistance from a P/N junction; the PCEs of single‐layer SubNc and SubPc devices are as high as 0.3% . These studies reveal that different from those “double‐cable” polymer‐based single‐component OSCs, fullerene, SubNc, and SubPc are special bipolar semiconductors, which can directly generate free carriers upon photoexcitation.…”