Bilayer heterojunction devices

Although the quantum efficiency for photoinduced chaige separation is close to unity for a D/A pair, the conversion efficiency in a bilayer heterojunction device is limited ... 

The spectral dependence of the photoresponse of these bilayer heterojunction devices, illuminated from the 1TO side, is displayed in Figure 15-22. The onset of photocurrent at hv— 1.7 cV follows the absorption of the fullerene, indicating a symmetric hole transfer from the excited fullerene to the MEH-PPV. The minimum in the photocurrent at /iv=2.5 eV corresponds to the photon energy of maximum absorption of MEH-PPV. The MEH-PPV layer, therefore, acts as a filter, which reduces the number of photons reaching the MEH-PPV/C()0 interlace. Thus, the thickness of the MEH-PPV layer determines the anlibatic spectral be-... 

Bilayer Heterojunction Devices Bulk Heterojunction Devices... 

Efficiency of planar heterojunction devices is limited by low exciton diffusion lengths in organic senuconductors that typically stay below 20 nm." This means that excitons generated more than 10-20 nm away from the interface between the p- and n-type materials do not contribute to the charge generation. As a consequence of this drawback, the power conversion efficiencies of the planar bilayer heterojunction devices typically stay in the range of 1.0-1.5% and approach 2-3% only in rare examples." " ... 

Bilayer heterojunction devices consisting of aromatic diamine hole transporting layers and 8-hydroxyquinoline aluminium (Alq) [8] or dye-doped Alq [9] emissive layers sandwiched between indium tin oxide (ITO) and Mg/Ag electrodes have been found to exhibit high external efficiency, luminous efficiency and brightness. 

In the bilayer heterojunction devices, the donor-acceptor phases are separated from each other and can selectively contact the anode and cathode, whereas in the bulk heterojunction both phases are intimately mixed and there is no preferred direction for the internal fields of separated charges. Fig. 20. The electrons and holes are thus created within the volume having concentration gradient (diffusion) as driving force. The separated charges require percolated pathways and the donor-acceptor phases form bicontinuous interpenetrating network [123]. Bulk heterojunction devices are sensitive to the morphology in the blend [124]. Majority of... 

FIGURE 8.13 Schematic diagram of (a) a bulk heterojunction device and (b) an MEH-PPV/ Ti02 hilayer device, as a typical bilayer device. 

D.S. Yu, et al. Soluble P3HT-grafted graphene for efficient bilayer-heterojunction photovoltaic devices. ACS Nano, 2010. 4(10) p. 5633-5640. 

With the development of organic semicondnctors which support electron or hole transport (in analogy of p- and n-type inorganic semiconductors) bilayer type p-n heterojunction devices have been constructed. In the case when organic p- and n-type materials are deposited consecutively in two layers we get lateral heterojunction devices which mimic classical p-n junction junction solar cells based on silicon. The first such cell was designed by Tang and pnb-lished in 1986 (Fignre 3)." Copper phthalocyanine was utilized as electron donor (p-type) material, whereas pery-lene derivative was nsed as electron acceptor connterpart (n-type material). 

A special type of solar cells as a hybrid combination of planar bilayer and bulk heterojunction devices has been... 

Attempts have been made to deposit TIPS-pentacene from solution as the functional layer in a pentacene/C60 bilayer photovoltaic device. Careful optimization of deposition conditions, optimal concentration of mobile ion dopants, thermal postfabrication annealing, and the addition of an exciton-blocking layer yielded a device with a moderate white-light PCE of 0.52% [41]. Since TIPS-pentacene derivatives rapidly undergo a Diels-Alder reaction with fiillerene, the assembly of potentially more efficient bulk-heterojunction photovoltaic devices from TIPS-pentacene and fiillerene derivatives were not possible [42]. The energy levels of the TIPS-pentacene-PCBM adduct (PCBM is [6,6]-phenyl C61-butyric acid methyl ester) ineffectively supports the photoinduced charge transfer. 

Fig. 20 a Bilayer Device fabrication and b Bulk heterojunction device fabrication. Reprinted with permission from ACS, S. Gunes, H. Neugebauer, and N. S. Sariciftci, Chem. Rev. 2007, 107, 1324-1338... 

In particular, the introduction of fullerene showed its ability as electron acceptor for performance polymer solar cells. The single-layer device based on conducting polymer was showing the 0.001-0.1 % PCE [22, 23]. Since the discovery of ultrafast photoinduced electron transfer from MDMO-PPV to fullerene, the bilayer cell of conducting polymer/C6o was investigated [24, 25]. The heterojunction device showed the rectification ratios on the order of 10", while energy conversion efficiency was not high. 

Yoshino and co-workers also reported the optical response of a heterojunction device comprising a P30T and C o bilayer [120]. The photoresponse of these devices shows a broad excitation profile ranging from 750 nm into the UV... 

Tokue H, Oyaizu K, Sukegawa T, Nishide H (2014) TEMPO/viologen electrochemical heterojunction for diffusion-controlled redox mediation a highly rectifying bilayer-sandwiched device based on cross-reaction at the interface between dissimilar redox polymers. ACS Appl Mater Interfaces 6 4043 049... 

In a bulk heteroj unction, the D and A components are physically mixed so that they can interact through the entire device s volume (therefore called bulk heterojunction). Indeed, this approach enhances the process of photoinduced charge carrier generation and, thus, short circuit current and power conversion efficiency [1-7, 31,32]. The driving forces relevant for the operation of both D/A bilayer and D A bulk-heterojunction devices are discussed in Section 8.3. 

The generation of photoexcited species at a particular position in the film structure has been shown in (6.19) and (6.20) to be proportional to the product of the modulus squared of the electric field, the refractive index, and the absorption coefficient. The optical electric field is strongly influenced by the mirror electrode. In order to illustrate the difference between single (ITO/polymer/Al) and bilayer (ITO/polymer/Ceo/Al) devices, hypothetical distributions of the optical field inside the device are indicated by the gray dashed line in Fig. 6.1. Simulation of a bilayer diode (Fig. 6.1b) clearly demonstrates that geometries may now be chosen to optimize the device, by moving the dissociation region from the node at the metal contact to the heterojunction. Since the exciton dissociation in bilayer devices occurs near the interface of the photoactive materials with distinct electroaffinity values, the boundary condition imposed by the mirror electrode can be used to maximize the optical electric field E 2 at this interface [17]. 

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