Comparing Matched Polymer:Fullerene Solar Cells Made by Solution-Sequential Processing and Traditional Blend Casting: Nanoscale Structure and Device Performance

Published

July 8, 2014

Author(s)

Andrew Herzing, Steven A. Hawks, Jordan C. Aguirre, Laura T. Schelhas, Robert Thompson, Rachel Huber, Amy Ferreira, Guangye Zhang, Robert A. Street, Sarah Tolbert, Benjamin Schwartz

Abstract

Efficient bulk heterojunction (BHJ) solar cells based on polymer:fullerene photoactive layers can be created either by traditional blend casting (BC), where the components are mixed together in solution before being cast, or by solution-sequential processing (SqP), where the polymer underlayer is first deposited and then the fullerene overlayer is deposited from a semi-orthogonal solvent. Presently, however, the relative merits of SqP are not well understood due to a lack of detailed comparisons between the two processing techniques. The main reason for this lack of comparison is that the composition of SqP active layers is not accurately known. Thus, in this paper, we present a new technique based upon redissolving the active-layer film and analyzing its dilute-solution absorbance for measuring polymer:fullerene stoichiometry in films. After proving that our method yields accurate film compositions, we find that SqP P3HT:PCBM active layers in optimal photovoltaic devices have a similar composition as their optimized BC counterparts (44-50 wt% PCBM). We then present a thorough comparison of the physical and device properties of thickness- and composition- matched P3HT:PCBM SqP and BC active layers in order to better understand the advantages and drawbacks of both processing approaches. For our matched devices, we see that small-area SqP cells perform better than BC cells due to both superior film quality and enhanced absorption from more crystalline P3HT. The enhanced film quality of SqP active layers also manifests in higher performance and significantly better reproducibility in larger-area devices, indicating that SqP is more amenable to scaling than the traditional BC approach. X-ray diffraction, UV-vis absorption, and energy-filtered transmission electron tomography all show that annealed SqP active layers have a finer-scale blend morphology and more crystalline polymer and fullerene domains when compared to equivalently-processed BC active layers.

Citation

Journal of Physical Chemistry C

Pub Type

Journals

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DOI Link

Keywords

quasi-bilayer, bulk heterojunction, organic photovoltaic, conjugated polymer, diffuse bilayer, layer-evolved bulk heterojunction, sequential processing

Citation

Herzing, A. , Hawks, S. , Aguirre, J. , Schelhas, L. , Thompson, R. , Huber, R. , Ferreira, A. , Zhang, G. , Street, R. , Tolbert, S. and Schwartz, B. (2014), Comparing Matched Polymer:Fullerene Solar Cells Made by Solution-Sequential Processing and Traditional Blend Casting: Nanoscale Structure and Device Performance, Journal of Physical Chemistry C, [online], https://doi.org/10.1021/jp504560r (Accessed May 4, 2024)