Nanoscale Morphology of Conjugated Polymer/Fullerene‐Based Bulk‐ Heterojunction Solar Cells

Abstract

The relation between the nanoscale morphology and associated device properties in conjugated polymer/fullerene bulk‐heterojunction “plastic solar cells” is investigated. We perform complementary measurements on solid‐state blends of poly[2‐methoxy‐5‐(3,7‐dimethyloctyloxy)]‐1,4‐phenylenevinylene (MDMO‐PPV) and the soluble fullerene C60 derivative 1‐(3‐methoxycarbonyl) propyl‐1‐phenyl [6,6]C61 (PCBM), spin‐cast from either toluene or chlorobenzene solutions. The characterization of the nanomorphology is carried out via scanning electron microscopy (SEM) and atomic force microscopy (AFM), while solar‐cell devices were characterized by means of current–voltage (I–V) and spectral photocurrent measurements. In addition, the morphology is manipulated via annealing, to increase the extent of phase separation in the thin‐film blends and to identify the distribution of materials. Photoluminescence measurements confirm the demixing of the materials under thermal treatment. Furthermore the photoluminescence of PCBM clusters with sizes of up to a few hundred nanometers indicates a photocurrent loss in films of the coarser phase‐separated blends cast from toluene. For toluene‐cast films the scale of phase separation depends strongly on the ratio of MDMO‐PPV to PCBM, as well as on the total concentration of the casting solution. Finally we observe small beads of 20–30 nm diameter, attributed to MDMO‐PPV, in blend films cast from both toluene and chlorobenzene.

Influence of blend microstructure on bulk heterojunction organic photovoltaic performance

“…A representative case is the blend of MDMO-PPV with PCBM. 44 Upon changing from toluene to chlorobenzene (CB), the PV power conversion efficiency increased significantly (from 0.9% to 2.5%, with a concurrent improvement in fill factor and J sc ). Further studies using transmission electron microscopy (TEM) and atomic force microscopy (AFM) verified the strong influence of solvent on the size of the phase separated PCBM-rich domains in spin-coated MDMO-PPV:PCBM films.…”

Luminescent and photovoltaic properties of poly(9,9-dioctylfluorene-co-bithiophene) in organic electronic devices

“…The main PSC parameters are summarized in Table 2. The V oc of cells with a co-polymer as the electron donor stayed in the range from 0.8 to 0.85 V, which was decreased by doping with P3HT and was larger than that of P3HT : PCBM (0.6-0.67 V) [27]. The underlying reasons may be: (1) the connection based on F8T2 : PCBM and P3HT : PCBM sub-cells, and (2) the energy transfer between F8T2 and P3HT, which is supported by the overlap between the emission spectra of F8T2 and absorption spectra of P3HT as shown in Figure 4.…”

“…The PL spectra of F8T2, F8T2 : P3HT, PCBM and their thin films blends were measured and are shown in Figure 4 The PL spectrum of F8T2 shows one stronger emission peak at 570 nm and a shoulder emission peak at 605 nm under excitation by 450 nm light. The PCBM films show a much weaker emission, which is mainly attributed to a less efficient emission resulting from symmetry-forbidden singlet radiative recombination as well as efficient intersystem crossing to the triplet state [27]. The PL intensity of F8T2 was extensively quenched by doping with PCBM, which is attributed to effective exciton dissociation at the interfaces between F8T2 and PCBM.…”

“…The J sc of cells with different Bphen thicknesses is relatively small, which is attributed to its large bandgap (2.4 eV). Hoope et al pointed out that only 30% of the photons in the whole solar spectrum have energy higher than 1.9 eV [27]. Therefore, F8T2 absorbs only a small fraction of irradiant light.…”

Controlled Assembly of Hybrid Bulk−Heterojunction Solar Cells by Sequential Deposition

“…Nanocomposite bulk-heterojunctions constitute the most efficient class of polymer-based solar cells; however, their superior performance is contingent upon on a complex architecture that is extremely difficult to control [5][6][7][8][9][10] . The ideal nanocomposite solar cell architecture allows the small domain sizes and large interfacial area required for optimal charge separation while still providing directed pathways for rapid carrier conduction to a set of carrier-selective electrodes.…”