We have carefully analyzed several of the highest performing semiconducting polymers used in photovoltaic cells, including P3HT, PBTTT, PCDTBT and PBDTPD, and made many interesting observations. We show how the performance of photovoltaic cells made by blending these polymers with fullerenes depends on factors such as the film thickness, testing conditions, annealing and polymer:fullerene ratio. We present x-ray diffraction data that makes it possible to assess how crystalline the films are and whether or not the fullerenes mix with the polymer at the molecular scale as opposed to phase separating from them. We have made hole-only diodes by using two high work function electrodes and analyzed the space charge limited current to measure charge carrier mobilities.  In some cases we have observed the presence of deep traps. We analyze the internal quantum efficiency of the devices as a function of electric field to determine recombination mechanisms.  We will connect our morphological characterization with the device results to explain why several of the most studied semiconducting polymers behave very differently when used in solar cells. We have also measured how well the solar cells perform over long periods of time and will discuss our current understanding of what causes degradation.

Contrasts and similarities between some of the highest performing photovoltaic polymers

243rd National Meeting (2012)

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