Effect of Morphology on Ultrafast Free Carrier Generation in Polythiophene:Fullerene Organic Solar Cells

Abstract

Despite significant study, the precise mechanisms that dictate the efficiency of organic photovoltaic cells, such as charge separation and recombination, are still debated. Here, we directly observe efficient ultrafast free charge generation in the absence of field in annealed poly(3-hexylthiophene):methanofullerene (P3HT:PCBM). However, we find this process is much less efficient in unannealed and amorphous regiorandom blends, explaining the superior short-circuit current and fill-factor of annealed RR-P3HT:PCBM solar cells. We use transient optical spectroscopy in the visible and near-infrared spectral region covering, but not limited to, the previously unobserved and highly relevant time scale spanning 1 to 100 ns, to directly observe both geminate and nongeminate charge recombination. We find that exciton quenching leads directly (time scale less than 100 fs) to two populations: bound charges and free charges. The former do not lead to photocurrent in a photovoltaic cell; they recombine geminately within 2 ns and are a loss channel. However, the latter can be efficiently extracted in photovoltaic cells. Therefore, we find that the probability of ultrafast free charge formation after exciton quenching directly limits solar cell efficiency. This probability is low in disordered P3HT:PCBM blends but approaches unity in annealed blends.

Role of Structural Order and Excess Energy on Ultrafast Free Charge Generation in Hybrid Polythiophene/Si Photovoltaics Probed in Real Time by Near-Infrared Broadband Transient Absorption

“…In this range, the difference spectrum, given by the green solid curve in Figure 5, does not match the polaron absorption spectrum (blue dots). In agreement with previous investigations, 16,25,29,30 we assign the photoinduced absorption in this spectral range to absorption by singlet excitons. To support this assignment, we performed TA measurements of RR-P3HT in various dilute solutions, where the intermolecular distance is high.…”

“…We now consider the quantitative differences that arise between the various samples. In agreement with previous work, 16,29,33 we find some polaron formation to occur also for the neat P3HT films. Taking the signal magnitude around 660 nm as a measure for the amount of polaron formation, we find the initial yield to decrease in the order aggRR-P3HT, RR-P3HT, and RRa-P3HT.…”

“…The energetic position of the P3HT triplet transition confirms and adds to recent experimental and theoretical investigations. 16,55 In addition to confirming the assignment of the singlet and triplet excitons, this measurement also implies that excitation dissociation does not occur efficiently on a single polymer chain.…”

“…While it became clear that the charge carrier mobility is enhanced in aggregated P3HT chains, 26,27 studies on the role of the aggregated or coiled conformation in the process of charge carrier generation and separation in organic devices have been emerging only recently. 16 For the design and operation of a solar cell, it is therefore of crucial importance to understand the influence of morphology on each of the individual photophysical steps. In organic semiconductors, there is widespread agreement about the photoconversion process.…”

“…16,29,30 There are indications that the charge separation takes place more efficiently for blends of RR-P3HT:PCBM than for RRa-P3HT: PCBM. 16 In contrast, prompt polaron formation during laser excitation was considered for neat P3HT and RRa-P3HT: PCBM. 30,33 For the polymer PCDTBT in combination with PCBM, there are also contradicting interpretations in discussion.…”