Optimization of Organic Photovoltaics Incorporating P(VDF-TrFE) Nanocrystals Prepared By Reprecipitation Method
Authors: Aita Yoshiki , Takeda Masaki , Matsui Jun , Sariciftci Niyazi Serdar , Stadler Philipp , Denk Patrick John , Sharber Markus Clark , Yoshida Tsukasa , Masuhara Akito , Schwoediauer Reinhard 
Published: February 2020

Organic photovoltaics (OPVs) cells are spotlighted as a one of the candidates for future energy source owing to their flexibility, low-cost and high-performance [ref. 1].
In a relatively short period, the power conversions (PCEs) of OPVs have achieved over 11% so that a variety of the recent researches such as improvement in molecular design and synthesis of new polymers and in improved solar cell device design has been successfully conducted to improve the OPVs performances. However, PCEs of other kinds of solar cells such as silicon solar cell and compound semiconductor solar cell have already reached over 20%.
The reason why PCEs of OPVs is lower than one of the others is a recombination of carriers by Coulomb force, which is one of the biggest issues in OPVs. In many of the most efficient polymer-fullerene OPVs devices, 50% or more of the energy loss is caused by the recombination of carriers [ref. 2].
Recently, several reports to solve this problem have been reported. One of the promising ways is incorporating Ferroelectric co-polymer poly(vinylidenefluoride-trifluoroethylene) (P(VDF-TrFE)) between an active layer and electrode interface as a Langmuir-Blodgett films in OPV [ref. 3]. Ferroelectric polymers show electric field derived from its spontaneous polarization due to difference in electronegativity of fluorine and hydrogen. It is therefore possible to enhance in charge separation efficiency and carrier mobility of electron for suppression of recombination.
In this research, we have successfully prepared P(VDF-TrFE) nanocrystals as s dispersion state by the reprecipitation method so that we could incorporate the nanocrystals in active layer in OPV [ref. 4]. P(VDF-TrFE) nanocrystals prepared by this method are approximately 40 nm. The obtained dispersion was mixed to dissolve with photoactive materials (P3HT : PCBM) at ratio of 15.0 : 10.5 by weight instead of a pure solvent. Moreover, the mixed solution was applied for OPV cells as an active layer.
Short-circuit current (Jsc) and PCEs of OPV incorporating P(VDF-TrFE) nanocrystals was improved in at most 20% and 40%, respectively comparing to the one without P(VDF-TrFE) nanocrystals. The electric field derived from P(VDF-TrFE) nanocrystals and carrier mobility were estimated from J-V curve and calculation, respectively.
Incident photo to current conversion efficiency (IPCE) of OPVs cells which are incorporating P(VDF-TrFE) was three times as high as the one which was not incorporating P(VDF-TrFE) although they had almost same absorbance and peak positions. Therefore, it was confirmed that P(VDF-TrFE) improved in charge separation efficiency and carrier mobility of electron, but not influenced light absorption.
The details in experimental conditions and characteristics of P(VDF-TrFE) nanocrystals and OPVs, morphology and so on will be discussed in this presentation.

References:

[1] Erik J. et al., ACS Nano, 6, 4708-4714 (2013).
[2] Yongbo Yuan. et al., Nat. Mater., 10, 296-302 (2011).
[3] Zhengguo X. et al., Adv. Energy Mater., 3, 1581-1588 (2013).
[4] Nakanishi H. et al., in Single Organic Nanoparticles, eds. Masuhara H. et al., (Springer, Berlin, 2003) Chap. 2.