Photo-physics of PTB7, PCBM and ICBA based ternary solar cells

• PTB7, PCBM70 and ICBA based binary and ternary solar cells are studied.
• I-V and EQE measurements confrims loss in photocurrent in ICBA based devices.
• Femtosecond transient absorption spectroscopy and Photo-EIS studies were carried out to understand photo-physics.
• AFM studies and the space charge limited current (SCLC) method are adopted as independent characterization tools.
• Inefficient charge-separation and poor charge transport is responsible for low photocurrent generation.

Indene-C60bisadduct (ICBA) is one of the rare acceptors which can supersede commonly used phenyl-C71-butyric acid methyl ester (PCBM70) in enhancing the performance of bulk heterojunction (BHJ) solar cells owing to its shallower lowest unoccupied molecular orbital (LUMO) level. However, ICBA tends to decrease the photocurrent for most of the low band-gap polymers synthesized to date. Here we examine the interaction of ICBA with the one of the popular low band-gap polymers poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl) carbonyl] thieno[3,4-b]thiophenediyl]] (PTB7), at femtosecond level, to explore key dynamics governing the operation of BHJ cells involving ICBA. The photo-physics of binary and ternary systems based on PTB7 coupled with PCBM70 and/or ICBA are studied by means of transient absorption spectroscopy (TAS) and electrochemical impedance spectroscopy (EIS) and supported by morphology analysis. Our study suggests that both inefficient charge-separation and poor charge transport of ICBA is responsible for relatively low photocurrent generation.

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