Ultrafast Charge Generation Enhancement in Nanoscale Polymer Solar Cells with DIO Additive.

Tongchao Shi,Tingyuan Jia,Sihao Huang,Qian Xiong,Maojie Zhang,Yuxin Leng,Chunwei Wang,Chenjing Quan,Juan Du,Zeyu Zhang,Zhengzheng Liu,Xia Guo

doi:10.3390/nano10112174

Abstract

We study the ultrafast photoexcitation dynamics in PBDTTT-C-T (P51, poly(4,8-bis(5-(2-ethylhexyl)-thiophene-2-yl)-benzo[1,2-b:4,5-b′]dithiophene-alt-alkylcarbonyl-thieno[3,4-b]thiophene)) film (~100 nm thickness) and PBDTTT-C-T:PC71BM (P51:PC71BM, phenyl-C71-butyric-acid-methyl ester) nanostructured blend (∼100 nm thickness) with/without DIO(1,8-diiodooctane) additives with sub-10 fs transient absorption (TA). It is revealed that hot-exciton dissociation and vibrational relaxation could occur in P51 with a lifetime of ~160 fs and was hardly affected by DIO. However, the introduction of DIO in P51 brings a longer lifetime of polaron pairs, which could make a contribution to photocarrier generation. In P51:PC71BM nanostructured blends, DIO could promote the Charge Transfer (CT) excitons and free charges generation with a ~5% increasement in ~100 fs. Moreover, the dissociation of CT excitons is faster with DIO, showing a ~5% growth within 1 ps. The promotion of CT excitons and free charge generation by DIO additive is closely related with active layer nanomorphology, accounting for Jsc enhancement. These results reveal the effect of DIO on carrier generation and separation, providing an effective route to improve the efficiency of nanoscale polymer solar cells.

Highlights

The polymer solar cells (PSCs) are attracting great commercial and academic interest due to the clean energy demand and the advantages of light weight, mechanical flexibility and low-cost fabrication [1,2]
We used sub-10 fs time-resolved spectroscopy to investigate the effect of DIO additives on the P51 and P51:PC71BM nanostructured blends
DIO barely affects the hot-exciton dissociation into polaron pairs and vibrational relaxation at ~160 fs scale, the longer lifetime (1750 fs) of polaron pairs is shown in P51 with DIO, which could promote to the photocarrier generation rather than recombination

Summary

Introduction

The polymer solar cells (PSCs) are attracting great commercial and academic interest due to the clean energy demand and the advantages of light weight, mechanical flexibility and low-cost fabrication [1,2]. The ultrafast hole transfer (about 3 ps) in the all-polymer blends of J51/N2200 was found to be triggered by a polaron pair-derived hole transfer process, which is different from the EX-dominated channel in the polymer/fullerene systems [16]. These series of works focusing on the ultrafast dynamics in PSCs have shown that the enhanced device performances are closely related to charge dissociation, charge generation rate, and the geminate and bimolecular recombination [16,17,18]. It is expected that the mechanism found in the experiment could provide an alternative route to improve the efficiency of nanoscale polymer solar devices

Material Synthesis and Optical Properties

Ultrafast Transient Absorption System

Results and Discussion

Conclusions