Alkyl substituent length dependences of photovoltaic performance of bulk-heterojunction (BHJ) organic solar cells (OSCs) utilizing 1,4,8,11,15,18,22,25-octaalkylphthalocyanine (CnPcH2) mixed in [6,6]-phenyl C71 butyric acid methyl ester have been studied. By shortening the alkyl substituents, stacking of the discotic CnPcH2 columns is probably changed from 2D rectangular lattices to pseudohexagonal structures, and Davydov splitting at the Q-band of CnPcH2 absorbance spectra decreases, which results in the higher hole mobility and the deeper highest occupied molecular orbital energy levels. As a result, the power conversion efficiencies of CnPcH2-based BHJ OSCs are improved from 0.3 to 3.7% by changing the alky substituent length.
The fabrication of small-molecule bulk-heterojunction solar cells utilizing a mixed donor material composed of two types of soluble phthalocyanine derivatives with different substituent length has been studied. The power conversion efficiency (PCE) and short-circuit current density (J sc ) of the solar cells fabricated using the mixed donor material with an optimized mixture ratio reached 3.8% and 9.2 mA/cm 2 , respectively, which were superior to those of organic solar cells utilizing each type of phthalocyanine derivative as a single donor material. The improvement of PCE and J sc has been discussed from the viewpoints of the miscibility and carrier transport properties of the mixed donor material.
Cu(In,Ga)S 2 (CIGS) solar cells were prepared from Cu-deficient absorber films using a lift-off process and then compared with CIGS solar cells prepared without using the lift-off process. The lift-off was achieved through sacrificial molybdenum layers and optimized by different modifications of the latter. We report on the properties of the detached surfaces in terms of their dependence on the composition of the films and compare them to those of the original upper surfaces. This comparison suggests the advantageous properties of the detached surfaces in terms of composition, absence of Cu-deficient phases, and effective doping.
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