Unified Study of Recombination in Polymer:Fullerene Solar Cells Using Transient Absorption and Charge-Extraction Measurements (original) (raw)

2013, The Journal of Physical Chemistry Letter

Recombination in the well-performing bulk heterojunction solar cell blend between the conjugated polymer TQ-1 and the substituted fullerene PCBM has been investigated with pump−probe transient absorption and charge extraction of photogenerated carriers (photo-CELIV). Both methods are shown to generate identical and overlapping data under appropriate experimental conditions. The dominant type of recombination is bimolecular with a rate constant of 7 × 10 −12 cm −3 s −1 . This recombination rate is shown to be fully consistent with solar cell performance. Deviations from an ideal bimolecular recombination process, in this material system only observable at high pump fluences, are explained with a time-dependent charge-carrier mobility, and the implications of such a behavior for device development are discussed. SECTION: Energy Conversion and Storage; Energy and Charge Transport R ecombination is a collective label to a wide range of mechanisms that returns an excited system to its ground state. All opto-electronic devices are influenced by recombination. In some cases it is a desirable process, such as for lightemitting diodes, while in others, such as solar cells, it is a major loss mechanism. A given system usually expresses a collection of different recombination mechanisms, of which one or more can be significant. Examples of mechanisms frequently encountered in bulk heterojunction solar cells are: recombination of excitons before reaching an interface, recombination of coloumbically bound charge pairs at interfaces, recombination between trapped charges and free charges, and recombination between free charges. Among these examples, the first two are first-order processes, the third can be of either first or second order depending on conditions, and the last one is a secondorder process. In studying recombination, it is perhaps of greater importance to identify the nature of the dominant recombination process(es) than to quantify, for example, carrier lifetimes, because different processes demand different means of manipulation. Material development and device engineering can benefit greatly from knowledge about the dominant recombination pathways. Obtaining such information, however, is not always trivial. Because time scales, measurement conditions, and sample geometries tend to differ significantly, both for different recombination measurement techniques as well as between recombination measurements and operational devices, the measurement results, and consequently the conclusions therefrom, vary significantly between different studies even on the same material systems. 1−6 Here data from two of the most common and straightforward techniques for recombination studies, pump−probe transient absorption (TA) 2 and charge extraction of photogenerated carriers by linearly increasing voltage (photo-CELIV), 5 are correlated with each other. The results obtained with the two methods are shown to be mutually fully consistent, but viewed separately, the data also illustrate the risk of incomplete or even disagreeing interpretations regarding the underlying recombination mechanism due to differences in the experimentally accessible time range. Both the rate and dominating type of the recombination process in the test system based on poly[2,3-bis-(3-octyloxyphenyl)quinoxaline-5,8-diyl-alt-thiophene-2,5-diyl] (TQ-1) 7 and [6,6]-phenyl-C71-butyric acid methyl ester ([70]PCBM) are consistent with solar cell device data. Chemical structures can be found in the Supporting Information.