In the past decade, great success has been achieved in bulk heterojunction (BHJ) solution-processed polymer BHJ solar cells. Despite the high PCEs reported for polymer BHJ solar cells, one finds that for a given polymer structure, batch-to-batch variations in solubility, molecular weight, polydispersity and purity can lead to different processing properties and performance. In contrast, solution-processed small molecule BHJ (SM BHJ) solar cells consist of well-defined molecules. However, despite their recent rapid rise in PCE, there have been few detailed efforts to delineate the effect of different morphological parameters on the device performance - a well-known strategy to further optimize the power conversion efficiency. Here we investigate the three-phase morphology and identify the crucial morphological parameters that determine the performance of a highly efficient solution processed small molecule system. The recently introduced 7,7′-(4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b′]dithiophene-2,6-diyl)bis (6-fluoro-4-(5′-hexyl-[2,2′-bithiophen]-5-yl)benzo[c][1,2,5]thiadiazole) (p-DTS(FBTTh2)2) molecule [1] was used as the donor material with phenyl-C71-butyric acid methyl ester (PC71BM) as the acceptor. The current-voltage characteristics of p-DTS(FBTTh2)2:PC71BM solar cell devices is known to be strongly dependent on the device processing condition and so are the degree of bimolecular as well as the voltage-dependent geminate recombination [2, 3]. Utilizing the as-cast and optimized device from a varying solvent additive sequence [3] and a series of annealed devices we create unique morphologies that result in sets of distinctive device parameters. The donor coherence length as revealed by grazing incidence X-ray scattering (GIWAXS) corresponds to smallest length scale of the composition variations revealed by Resonant Soft X-ray Scattering (R-SoXS) and the charge carrier mobilities tracked the GIWAXS coherence lengths of the donor and acceptor phases. An analysis scheme combining GIWAXS and scanning transmission X-ray microscopy (STXM) data was developed to quantify the ratio between edge-on and face-on donor populations with respect to the substrate. The R-SoXS technique being more sensitive to edge-on populations the scattering data were suitably normalized using the donor orientation parameter. The average domain purities extracted from the sample absorption and orientation corrected R-SoXS profiles correlate linearly with the FF measured at different illumination establishing that increasing the phase purity helps diminish voltage dependent geminate recombination as well as bimolecular recombination. The average domain size from R-SoXS as well as the orientation parameter was anticorrelated with the photocurrent. Multipeakfit analysis of R-SoXS profiles enabled identification of the length scale and composition variation of the phase separation primarily responsible in determining the device performance. The results support a morphology paradigm in actual devices in which maximizing relative phase purity, structural order while simultaneously limiting domain size and edge-on orientation of donor molecules with respect to the substrate may be essential for achieving optimal solar cell performance in solution processed small molecule solar cells.
References:
[1] T. S. van der Poll, J. A. Love, T.-Q. Nguyen, G. C. Bazan, Advanced Materials 2012, 24, 3646. [2] C. M. Proctor, S. Albrecht, M. Kuik, D. Neher, T. Q. Nguyen, Advanced Energy Materials 2014, 4, 1400230. [3] S. Mukherjee, C. M. Proctor, J. R. Tumbleston, G. C. Bazan, T. Q. Nguyen, H. Ade, Advanced Materials 2014, 10.1002/adma.201404388.