Abstract
All-polymer solar cells are herein presented utilizing the PBDTTT-CT donor and the P(NDI2OD-T2) acceptor with 1,8-diiodooctane (DIO) and 1-chloronaphthalene (CN) binary solvent additives. A systematic study of the polymer/polymer bulk heterojunction photovoltaic cells processed from the binary additives revealed that the microstructures and photophysics were quite different from those of a pristine system. The combination of DIO and CN with a DIO/CN ratio of 3:1 (3 vol% DIO, 1 vol% CN and 96 vol% o-DCB) led to suitable penetrating polymer networks, efficient charge generation and balanced charge transport, which were all beneficial to improving the efficiency. This improvement is attributed to increase in power conversion efficiency from 2.81% for a device without additives to 4.39% for a device with the binary processing additives. A detailed investigation indicates that the changes in the polymer:polymer interactions resulted in the formation of a percolating nasnoscale morphology upon processing with the binary additives. Depth profile measurements with a two-dimensional grazing incidence wide-angle X-ray scattering confirm this optimum phase feature. Furthermore impedance spectroscopy also finds evidence for synergistically boosting the device performance.
Highlights
Solution processed thin-film solar cells have the potential to deliver cheap, clean energy by converting incident solar flux into an electrical current
In an energy level diagram, the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels for PBDTTT-CT20 are more than 0.5 eV higher than those of P(NDI2OD-T2)[21], indicating that the energy level positions of the donor and the acceptor are suitable for an efficient charge transfer and separation at the interface between these two polymers[22]
We report on the optical properties of PBDTTT-CT:P(NDI2OD-T2) blend films in order to distinguish the effects arising from the binary additive system
Summary
A higher Jph − Veff characteristics from the 3:1 DIO/CN processed device identifies the effect of the optimal DIO/CN binary additive ratios on the reduction of the bimolecular recombination and more efficiently sweeping out free charges at a low effective voltage, at which the maximum power output condition of the solar cells occurs. The lowest internal series resistance is observed for the device processed with 3:1 DIO/CN additives, which suggests that the thin film morphology of the BHJ composite greatly improved due to the reduction in the phase separation and improvement in the percolating network due to the assistance of the binary solvent processing additives
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