Revealing charge carrier dynamics in squaraine:[6, 6]-phenyl-C 71-butyric acid methyl ester based organic solar cells

Aniket Rana,Deepak D Prabhu,Chhavi Sharma,Mahesh Kumar,Suresh Das,Yoosaf Karuvath,Rajiv K Singh,Suresh Chand

doi:10.1063/1.5018253

Abstract

Ultrafast charge carrier dynamics as well as the generation of polaron pair in squaraine (SQ) and squaraine:[6,6]-phenyl-C 71-butyric acid methyl ester (SQ:PCBM71) have been studied using ultrafast transient absorption spectroscopy (UTAS). The current study reveals that the pure SQ exhibits the creation of singlet and triplet states; however, incorporation of PCBM71 in SQ results in the formation of polaron pairs with ∼550ps lifetime, which in turn leads to the creation of free electrons in the device. We show that the considerable increment in monomolecular and bimolecular recombination in SQ:PCBM71 compared to pure SQ which describes the interfacial compatibility of SQ and PCBMC71 molecules. The present work not only provides the information about the carrier generation in SQ and SQ:PCBM71 but also gives the facts relating to the effect of PCBM71 mixing into the SQ which is very significant because the SQ has donor-acceptor-donor (D-A-D) structure and mixing one more acceptor can introduce more complex recombinations in the blend. These findings have been complimented by the charge transport study in the device using impedance spectroscopy. The various important transport parameters are transit time (τt), diffusion constant (Dn), global mobility (μ) and carrier lifetime (τr). The values of these parameters are 26.38 μs, 4.64x10-6 cm2s-1, 6.12x10-6 cm2V-1s-1 and 399 μs, respectively. To the best of our knowledge such study related to SQ is not present in the literature comprehensively.

Highlights

Organic solar cell (OSC) is a fast emerging technology as an effective alternative to the conventional inorganic photovoltaic due to its inherent advantages such as cost effectiveness, flexibility, large area possibility, ease of fabrication, etc.[1,2,3] Recently Heliatek has achieved 13.2% efficiency in the multi-junction solar cell.[4]
Thermally and optically stable, absorption spectrum ranging from visible to near-infrared spectral region,[6,7,8] etc. Even though it is being used in the fabrication of OSC devices, yet the literature lack studies on its photophysical and carrier dynamics which is an important aspect to enhance further the efficiency and stability of these devices
We have established the concept of charge carrier dynamics in organic solar cells based on SQ:PCBM71 donor-acceptor combination by ultrafast transient absorption and impedance

Summary

INTRODUCTION

Organic solar cell (OSC) is a fast emerging technology as an effective alternative to the conventional inorganic photovoltaic due to its inherent advantages such as cost effectiveness, flexibility, large area possibility, ease of fabrication, etc.[1,2,3] Recently Heliatek has achieved 13.2% efficiency in the multi-junction solar cell.[4] maintaining high stability and efficiency together is still a great challenge for the commercialization of OSC To achieve this goal, it is essential to understand the physics of charge carrier dynamics at femtosecond timescale. Even though it is being used in the fabrication of OSC devices, yet the literature lack studies on its photophysical and carrier dynamics which is an important aspect to enhance further the efficiency and stability of these devices Keeping this in view we have performed electro-optical and electrical measurements such as ultrafast transient absorption spectroscopy, impedance spectroscopy and current-voltage (J-V) characteristics in pure SQ films as well as SQ:PCBM71 blends. These important studies will be proved immensely useful in designing efficient and stable SQ based OSCs

EXPERIMENTAL METHODS

AND DISCUSSION

Findings

CONCLUSION