Journal Club Report from August 6: The Role of Spin of excited states in OPV losses

Masoud Ghasemi, a graduate student who recently joined Ade group, presented this article (subscription required) and included some tutorial information on the nature of singlet and triplet excited states.  This is crucial background: photoexcitation in molecular solids results in bound states between the excited electron and the hole it leaves behind that are called "excitons".  If the spins of the excited electron and the remaining electron (in what used to be the ground state) are opposite, we have a singlet exciton.  If the spins are aligned we have a triplet exciton.  These excited states differ in energy due to exchange interactions that are fundamentally Coulombic in origin (note well:  magnetostatic interactions between spin dipole moments are way too small to matter here).  Photoexcitation dominantly makes singlet excitons (initially) due to dipole selection rules (spin is conserved upon light absorption).

Overview:
The big question in the paper is how to identify and mitigate the important mechanisms of "losses" in organic photovoltaics (OPV's).  A loss mechanism is any way that a photoexcited state ends up not creating a free charge that can be used for electrical power.  The key result from the paper is that the creation of triplet excitons formed by  bimolecular recombination of dissociated carriers (with random spins) is one of these loss mechanisms, but that it can be minimized by "wave function delocalization" that tends to favor dissociated charges.  As an aside, "The Role of Spin..." that appears in the title seems to me like a bit of a red herring.  The spin in the loss channel causes it to be slowly-evolving and helps distinguish it in the experiments, but is not crucial in the final message relating to "wave function delocalization".

This paper touches on some of the perennial questions of the ORaCEL journal club.  Note that the word "polaron" has come up immediately.  When a charge (i.e. an electron or hole) is injected into a solid it must often be considered not as a free charge but rather as a "quasiparticle" that consists of the injected charge modified by the polarization that it induces in the other electrons in the solid and in the positions of nuclei in the solid.  In briefer terms, polaron = (charge + complicated polarization response of the solid environment)

Methods:
The experimental technique used in the paper is an ultrafast pump-probe spectroscopy called Transient Absorption (TA) where an initial laser "pump" pulse creates an excited state and a slightly later "probe" pulse measures the excited state.  By varying the delay between pump and probe people have learned a lot about excited state dynamics in a wide variety of systems.  In the paper, TA is applied to a range of polymer/fullerene bulk heterojunction (BHJ) blends.  The donor polymer "PIDT-PhanQ" is specially chosen to have polaron (specifically "hole polaron") and triplet exciton features that are distinct.  As a result TA evolution of these separate excitations can be observed over the course of nanoseconds can be used to track recombination effects.

Comparison of the triplet and polaron dynamics when PIDT is blended with different fullerene acceptor derivatives provides the evidence used to generate new ideas about the supression of bimolecular recombination to triplets.  We noted that the authors use a special "genetic algorithm" to deconvolve triplet and polaronic contributions to the total TA signal.  This is probably similar to a principle component analysis, though no one in our journal club was familiar with the details.

Key results and details:

The basic result is that TA evolution shows polarons evolving to triplet excitons with different kinetics for different blends. Figure 2 and 4 show the following comparisons:

• PIDT:PCBM (1:3) blends (good OPV performance) :  TA shows only ONE excited state species (has to be polarons, which in all cases are created "within the response time of the instrument")
• PIDT:ICBA or ICMA blends (poor OPV performance) :  TA shows a change in evolution indicating one excited state evolving to another.  Specifically, early-time polarons recombine to form triplet excitons (the triplet is identified based on the fact that it is a long lived excitation at lower energy than the polaron) that form after a nansecond or so.
• PIDT:PCBM (1:1) blends (poor fullerene aggregation OPV performance) :  TA shows polaron to triplet evolution.
• PCPDTBT:PCBM (1:2) blends (modest OPV performance):  TA shows polaron to triplet evolution

Quantitative analysis of deconvolved polaron and triplet evolution in ICBA and ICMA blends is carried out with a kinetic model that supports the simple picture that triplet are created as polarons decay.  In addition, light intensity dependence of the triplet feature is used as the "smoking gun" that bimolecular recombination is the root cause of the creation of these low energy excitations.  This is an argument that I am aware of being made in the OPV literature but that I would like to have clarified (comments anyone?).

The temperature dependence of the kinetics is also argued to be consistent with a bimolecular recombination process.  Charge (i.e. polaron) diffusion is slower at low temperatures and it is observed that the triplet population arises more slowly (Figure 3c) at lower temperatures.

All of the systems that show evolution of polarons to triplet excitons suffer from poor fullerene aggregation.  Thus, it is concluded that fullerene aggregation is important in minimizing this loss mechanism.  The microscopic explanation advanced in the paper is that larger fullerene aggregates are more crystalline and allow better wave function delocalization.  This, in turn, makes it more likely that polarons interacting in a charge transfer exciton state re-dissociate to polarons again rather than recombine to a triplet exciton.
*Note that the direct experimental connection with crystallinity is NOT made in the paper.  This is a weak point and place for further work.

Conclusions:
Polarons are created very rapidly in a BHJ but they can be lost due to the formation of triplet exctions via triplet charge transfer states.  Large fullerene aggregates, with presumably good crystallinity, promote wavefunction delocalization in the charge-transfer state that in turn promotes re-dissociation to polarons rather than formation of triplets.  The practical message is that you need large fullerene aggregates to prevent this loss mechanism.  I would have liked to see another example of large fullerene aggregates with no triplet formation in the TA.  The paper shows multiple cases of polaron-to-triplet recombination but only 1:3 PIDT:PCBM shows the supression of this loss channel.

I hope that the comment section of this blog can be used to help clarify any misunderstandings and further the discussion about the article and related topics.