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Abstract:
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For more than 20 years , scientists have studied solar cells made from organic semiconductors . Throughout this time , device structures have evolved from bilayer devices to bulk heterojunction (BHJ ) devices and even though efficiencies are
approaching 10 % , scientists still know relatively little about the transport of charge carriers and recombination mechanisms in these materials . Novel structures , based on lateral BHJ solar cells , have proven to be versatile tools to study transport and recombination mechanisms . In addition , these structures can easily be employed by researchers and solar cell manufacturers to determine the quality and measure the improvement of their materials . For these studies , poly (3 -hexylthiophene ) (P3HT ) :[6 ,6] -phenyl C61 -butyric acid methyl ester (PCBM ) has been employed due to its wide use among researchers as well as potential for commercialization .
DC photocurrent measurements as a function of device length have yielded the mobility -lifetime product and the generation rate of free carriers within these BHJ
devices . In addition to these parameters , the recombination rate as a function of light intensity provides information about the mechanisms of recombination . For example , by measuring the recombination rate as a function of applied electric field and light intensity we have found that recombination is unimolecular in nature and shifts to bimolecular
at increased electric field strengths . Additionally , the mobility -lifetime product ,
generation rate , and recombination mechanism have been studied as a function of applied electric field , illumination spectrum , illumination intensity , etc . This information has provided much insight on physics of the P3HT :PCBM material system which did not exist before these studies . |