The systematic variations in structural, optical and electrical properties of lead phthalocyanine (PbPc)-based solar cells with organic buffer layers were investigated. Transition of the PbPc crystal from a monoclinic phase to a triclinic one was observed when the buffer layers were changed. The structural properties of triclinic PbPc grown on the sexithiophene (6T) buffer layer was superior to those of PbPc grown on the other organic substrates. Since the crystal growth of organic layers is dominated by the anisotropic intermolecular interactions at organic hetero-interfaces, the highly oriented 6T buffer layer with the atomically flat morphology promotes the growth of well-ordered PbPc layers. The performance of the organic solar cells (OSCs) was in direct correlation with the structural and electrical properties of the PbPc single layers. The formation of the triclinic phase enhances the spectrum sensitivity at around 890 nm, suggesting the enhancement of the current density of OSCs. Thus, the control of both crystal quality and phase of PbPc layers using a proper organic buffer layer is effective in enhancing the device performance of OSCs.Graphical abstractHighlights► Influence of an organic buffer layer on structural properties and performances of PbPc-based solar cells is reported. ► Mechanism of structural variations of the PbPc layer formed on buffer layers is discussed by XRD, synchrotron XRD, and AFM. ► Formation of triclinic PbPc phase enhances the spectrum sensitivity at around 890 nm, suggesting the enhancement of the performance of PbPc-based solar cells.

Deep levels in Cu(In1 − x,Gax)Se2 (CIGS) are studied by transient photocapacitance (TPC) spectroscopy by varying the Ga concentration, x, from 0.38 to 0.7. The TPC spectra of CIGS thin-film solar cells at 140 K exhibited a defect level with an optical transition energy of about 0.8 eV. The spectrum shape in the sub-bandgap region is independent of the Ga concentration. Therefore, the optical transition energy to the defect level is almost constant with about 0.8 eV from the valence band. The TPC signals for defect level are quenched by increasing temperature. The activation energy of thermal quenching is estimated to be about 0.3 eV. The thermal and optical activation processes are explained using configuration coordinate diagram.

The effects of annealing under various atmospheres on the electrical properties of Cu(In,Ga)Se2 (CIGS) films and CdS/CIGS heterostructures were investigated. For CIGS films without CdS, the electrical properties of CIGS degraded under vacuum and O2 annealing, although such degradations were not observed under N2 annealing. For the CdS/CIGS heterostructures, the electrical properties of the junctions improved after annealing under all gas ambients. Therefore, CdS films prevent the chemical reactions at the CIGS surfaces and are necessary for effectively annealing the CIGS film. We observed a distinct correlation between the degradation of the electrical properties and increase in the defect density. Finally, we discussed the origin of the defect states.

Structural control of the organic intrinsic layer (i-layer), which consists of a mixture of p- and n-type organic semiconductor materials, and the organic p–i–n triple-layer structure has been investigated. Significant modification of these structures, which used metal-free phthalocyanine (H2Pc) and 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) as the p- and n-type materials, respectively, was observed by introducing a modification layer deposited on a glass substrate. The orientation of the molecular planes of both H2Pc and PTCDA in the i-layer (H2Pc:PTCDA mixture) and p–i–n structure (H2Pc/i-layer/PTCDA) formed onto a PTCDA modification layer was parallel to the substrate surface. On the other hand, when H2Pc was used as the modification layer, the orientation of the molecular planes in the i-layer was found to be random. These results indicate that the internal structure of the organic mixed-layer is strongly affected by the organic layer-substrate interface. The enhancement in optical absorption observed for the orientation controlled p–i–n structure suggests the importance of the control of molecular orientation for improving the optical properties in organic device structures.