Interplay of nanoscale domain purity and size on charge transport and recombination dynamics in polymer solar cells

Abstract

Charge transport and bimolecular recombination dynamics were correlated with nanomorphology in polymer solar cells. The morphology of poly(diketopyrrolopyrrole-terthiophene) (PDPP3T) and phenyl-C61-butyric acid methyl ester (PC60BM) blend films was modified using different solvent additives namely 1-chloronaphthalene (CN), 1,8-diiodooctane (DIO) and 1,8-octanedithiol (ODT) and their role on steady state and transient optoelectronic properties was investigated. The energy filtered transmission electron microscopy (EFTEM) images showed that additives (e.g. CN and DIO) improved the domain purity which leads to significantly higher short circuit current densities (Jsc). However when the cells were processed with the ODT additive, the fill factor (FF) and open circuit voltage (Voc) decreased dramatically. Films processed with the ODT additive showed a smaller domain size but were more connected compared to films processed using CN and DIO additives. Transient photocurrent analysis indicates faster charge collection in the case of CN and DIO processed solar cells and the slowest charge collection in ODT processed solar cells. Interestingly devices processed with the ODT additive also showed the longest charge carrier recombination lifetime and lowest bimolecular recombination coefficient. This is attributed to the smaller donor domains that are connected with each other to provide a more interconnected and efficient charge transport matrix but longer pathways in ODT films. Such a matrix helped the charge to escape from the donor-acceptor interfaces and thus reduces the bimolecular recombination, while the longer pathway increases the charge collection time. Further insight is provided into the selection of processing conditions to achieve an ideal active layer morphology consisting of domains with higher polymer purity and optimal size that lead to higher Jsc and FF.

Development of Fourier transform mid-infrared calibrations to predict acetone, β-hydroxybutyrate, and citrate contents in bovine milk through a European dairy network

“…All observations were made using a commercial Agilent 5500 scanning probe microscope. The imaging was done in tapping mode using Si cantilevers (Budget Sensors TAP 300 Al; Innovative Solutions Bulgaria Ltd., Sofia, Bulgaria) with the AFM tip having resonant frequency around 300 kHz (Venkatesan et al, 2014). The membrane sample was mounted on the sample stage by sticking 4 corners of the membrane to the stage with scotch tape.…”

Device and morphological engineering of organic solar cells for enhanced charge transport and photovoltaic performance

“…Venkatesan et.al. 17 compared the effects of several additives on the morphology of the active layer. Results showed that CN and DIO additives gave an optimal morphology, which helped to increase the overall efficiency of BHJ solar cells.…”

“…7. 17 The EFTEM analysis gave deep insight into the local domain of both donor and acceptor phases. The donor and acceptor mapping for each type of film was compared.…”

“…Conjugated polymers based on organic solar cell can be fabricated from low-cost solution processing techniques, including spin-coating, spray-coating, and ink-jet printing. 17,18 The mechanism for converting light into electricity in organic solar cells is governed by the absorption of photons in the active layer, which is followed by exciton formation, exciton diffusion, exciton dissociation, charge transport, and charge collection. Figure 2 shows the mechanism of photogenerated exciton formation upon light absorption, exciton dissociation at the donoracceptor interface, and then transportation of the dissociated charges out of the film.…”

“…The additives induce better nanoscale phase separation [63][64][65] for the same or a similar polymer structure and also lead to higher domain purity or crystallinity in the polymer phase. 17 Investigation has been carried out on material selection, interpretation of the additive interaction with BHJ materials, and variation of morphology at different length scales. The role of solvent additives has been an important resource for further improving the efficiency of organic solar cells.…”