"Relations between morphology and electro-optical properties of MDMO-PPV: PCBM bulk heterojunction organic solar cells"
At the start of this thesis, it was not yet clear if there even was something to be investigated regarding the morphology of MDMO-PPV:PCBM bulk heterojunctions. Due to the very strong luminescence quenching it was generally agreed that MDMO-PPV (and some other polymers used in bulk heterojunction solar cells) and C60 or PCBM mixed very good together. Due to this good mixing, it would probably not be very interesting to investigate these materials using microscopy since there would be nothing to see.
However, already very soon it became clear that this good mixing of both components did not mean that morphology was not important. After a long search for suitable preparation conditions to prepare thin films that we could investigate in our microscopes, already the first images showed clear phase separation. After the paper from Shaheen et all.26, showing the importance of morphology for a better understanding of solar cell efficiency, our lab was on a head start to further investigate the importance of phase separation. During this research, we made some very interesting observations that will be summarized below.
The morphology of layers of MDMO-PPV:PCBM: If these two materials are mixed together and processed as in today’s solar cells, they do not perfectly mix together, but instead they show clear phase separation. SEM and optical microscopy showed that at high concentrations of PCBM, the two materials phase separate. Whenever more PCBM was added, film quality went down quickly, troubling the preparation of well performing solar cells due to the formation of big chunks of PCBM.
From the SEM and AFM images, it could be shown that phase separation occurred in concentrations starting from around 65% when spincoating from CB and around 60% when spincoating from TOL, always using standard preparation conditions. The phase separation now appeared to occur as a large matrix that contains small particles of another phase. Higher concentrations also show phase separation. The point where phase separation starts is slightly different for the two solvents, and this means that the matrix of films spincoated from CB and TOL, which is totally saturated with PCBM, contains different amounts of PCBM. When spincoating from CB, more PCBM is mixed within the matrix before phase separation occurs than when using TOL. Therefore, the matrix in the phase separated CB spincoated samples does contain more PCBM.
Using TEM, the surface roughness increases that were detected by analyses of AFM measurements could indeed be incontestably linked to phase separation and this statement was proven using various arguments. Phase separation was intensively studied and the differences between chlorobenzene and toluene regarding morphology were explained.
Using KFM, the phase separated areas haven been proven to be on a different local surface potential then the surrounding matrix. This makes the link from morphology to electro-optical properties, and stressing the importance of the understanding of the nano-morphology of the active layer to understand the working principle of these solar cells.
It was proven that the dynamic phase separation behavior during drying of the films could be influenced by altered casting conditions. Dropcasted films clearly have a different morphology then spincoated films, and the drying procedure in the spincoating process also clearly influences the final morphology. Using a slower drying process results in films with larger phase separated areas.
In this way, it was possible to produce films with identical composition but with a different morphology.
Relating morphology with electro-optical properties: After a thorough investigation of the morphology of these solar cells, it was the goal to link these results with observations that could be made when these cells were measured in optical and electrical measurement equipment.
Using optical absorption spectroscopy, the formation of new optical states could be exclude by studying the optical absorption patterns. Fluorescence measurements confirmed the quenching of the fluorescence of MDMO-PPV proving at least minimum mixing of polymer and PCBM on a nanometer scale and confirming a more efficient charge separation than exciton recombination.
Using time-resolved microwave conductivity measurements, more links between morphology and electro-optical characteristics could be obtained.
At WPCBM up to 0.55, The photoconductivity is mainly determined by the positive charge carriers on the MDMO-PPV and is similar for blend films spincoated from both solvents. On going from a WPCBM = 0.55 to WPCBM = 0.75, the photoconductivity has increased by an order of magnitude for both chlorobenzene and toluene solvents. This is explained by the considerably larger electron mobility of the electrons in the PCBM rich aggregates (Μu = ca 40x10-3 cm2/Vs) as compared to mobility of the holes on the polymer chains (Μu = ca 3x10-3 cm2/Vs) or of the electrons in the PCBM poor matrix. Furthermore, phase separation leads to a five-fold increase of the half lifetime, which is attributed to a reduced rate for charge carrier recombination.
The obtained data explain why solar cells based on blends of MDMO-PPV:PCBM spin-coated from CB perform better than the ones prepared from TOL.
Integration into device models: The mean electron diffusion length could be calculated and based on some assumptions, TOL0.60 and TOL0.75 yield a ΛD value of close to 45 nm. This remarkably high value might suggest that phase separation is not essential in highly efficient cells. Future gains in efficiency have to be searched in higher fill factors coming from lower recombination and higher mobilities. Fine control over morphology of the suggested interpenetrated network will be necessary. The twofold function of PCBM (charge separation and electron transport) makes the cell optimization more complex than necessary. A differentiation in materials could possibly be a solution to these problems as explained.
Using modeling, it could be shown that the proposed morphology and device operation is feasible. Using the concept of electron transporting PCBM particles, surrounded by a photo-active layer, the short circuit current could be calculated and using reasonable assumptions it could be calculated to be very close to the actual measured short circuit currents. Moreover, the differences in morphology were proven to be accountable for the measured differences in short circuit current between samples spincoated from chlorobenzene and toluene. Again, the importance of mobility and charge diffusion length could be concluded. It is especially this route that future optimizations could take.