Mapping Local Photocurrents in Polymer/Fullerene Solar Cells with Photoconductive Atomic Force Microscopy

Abstract

The performance of organic solar cells is highly dependent on film morphology. However, directly correlating local film structures with device performance remains challenging. We demonstrate that photoconductive atomic force microscopy (pcAFM) can be used to map local photocurrents with 20 nm resolution in donor/acceptor blend solar cells of the conjugated polymer poly[2-methoxy-5-(3',7'-dimethyloctyl-oxy)-1,4-phenylene vinylene] (MDMO-PPV) with the fullerene (6,6)-phenyl-C61-butyric acid methyl ester (PCBM) spin-coated from various solvents. We present photocurrent maps under short-circuit conditions (zero applied bias) as well as under various applied voltages. We find significant variation in the short-circuit current between regions that appear identical in AFM topography. These variations occur from one domain to another as well as on larger length scales incorporating multiple domains. These results suggest that the performance of polymer-fullerene blends can still be improved through better control of morphology.

The effect of morphology upon mobility: Implications for bulk heterojunction solar cells with nonuniform blend morphology

“…14,15,17 Indeed, there is experimental evidence, which shows that the blend morphology can significantly affect mobility in the manner we propose, for example, 28,29 and further data that local morphology affects local charge transport. 8 Drift-diffusion modeling has shown that changes in the local mobility of one order of magnitude can give rise to significant changes in SCL diode and PV device performance. It is shown that the sequence of the layers with different mobilities makes a profound difference to the current the SCL diode carries, and in turn, the effective mobility derived.…”

“…5 However, while these techniques broadly control the domain size, crystallinity, or packing of the components of the BHJ, these characteristics of the blend morphology are not uniform throughout the film. Indeed, studies of the structure of polymer-polymer, 3,6,7 polymer-fullerene, [8][9][10] and polymernanocrystal 11 blends show hierarchies of phase separation and vertical variations in the blend morphology. How this local variation in blend morphology affects the local mobility of carriers, and in particular the PV performance, is largely unknown.…”

Advanced Scanning Probe Microscopy of Graphene and Other 2D Materials

“…Current can in this way flow through the material, from the biased lateral contact to the movable metal-coated scanning probe tip [19]. In a similar configuration, named Photoconductive Atomic Force Microscopy (PC-AFM) [69], a light source is additionally used to excite the sample, so that the resulting photocurrent is measured by the AFM probe. For electrical measurements, preventing probe-induced artifacts is very important.…”

Preparation and processing of electrochemically deposited TiO2 film into a heterojunction photovoltaic device

“…Furthermore, it is apparently clear that the production cost can be severely reduced by the utilization of the abundant inorganic materials and a low production cost film fabrication techniques such as spin casting, electrochemical deposition and screen printings unlike the use of expensive and complex film forming techniques such as CVD, electron beam and MOCVDs [9]. Since the solution processable organic solar cells are extensively studied and highly promising next generation photovoltaics, however, the generation of Frenkel type excitons at the donor/acceptor interface needed a larger surface area and excellent contacts at the film interfaces [10]. Moreover, in organic solar cells the effective harvesting of generated geminate pairs at electrodes also posing limitations to the higher efficiencies due to slow charge carrier mobility's (~10 -4 cm 2 V -1 c -1 ) of organic layers towards the electrode surface, however inorganic type solar cells possess strong chemical stability, high mobility and strong intrinsic charge carriers which make them choice for large scale manufacturing [11].…”