Report from August 13: Time Delayed Collection Field and Recombination Currents

Yesterday, Josh Carpenter from Ade group presented this article (subscription required as usual) that describes a variety of pulsed electrical measurements aimed at clarifying recombination effects in organic photovoltaics (OPVs).  The Ade group has been particularly interested in the time delayed collection field (TDCF) approach to measuring recombination processes occuring at different time scales.  I'll describe this below.

Overview:

This article continues our discussion of "loss mechanisms" in OPV's.  To aid in this discussion, we  always seem to need a glossary:

Exciton:  Bound state of a photoexcited electron and the hole it left behind (typically though of as localized on a single molecule or short part of polymer chain)

Charge Transfer Exction:  Bound state across an interface of an electron in the acceptor and hole in the donor

Geminate Recombination:  recombination of an electron and hole that come from the same initial exciton state

Polaron:  Electron or hole in a solid combined with the polarization of the solid surroundings (electron and ion charges)

Bimolecular recombination:  nongeminate recombination of uncorrelated polarons that do not come from the same initial excited state

Josh Carpenter made a nice two column overview of important loss mechansims associated with the conceptual steps in the photocurrent generation process.  I reproduce it here partly in my own words as transcribed from Josh's talk and my notes.  Josh also had pictures.

Step in Photocurrent Generation Process	Important Loss Mechanism
Light absorption to make a neutral exciton	Exciton decay (is this not considered "geminate"?), e.g. by light emission
Diffusion of the exciton to D-A interface to create a charge transfer exciton	Geminate recombination of the CT exciton
Charge Separation of the CT exciton to make polarons	Non-geminate "bimolecular" recombination of polarons
Hopping of polarons to the electrodes to finally make a photocurrent	Surface recombination and/or current leakage

The paper uses TDCF in combination with other related tools to clarify the contribution of geminate and bimolecular recombination to the non-ideality (i.e. FF<1) of the I/V curves of 3 different solar cells.

Methods:

The experimental tools used in this paper are somewhat uncommon in the field.  In part this is the reason for focusing on this paper: Can we start to evaluate these tools and whether we or others should be using them?

I want to focus on only TDCF in detail.  The other two unusual tools of bias-assisted charge extraction (BACE) and photocharge extraction by linearly increasing voltage (photo-CELIV) are too much for a blog post (I'm tending already to a TLDR regime here!).

In TDCF, you apply a "prebias" to your device.  You then hit it with a pulsed laser (Nd:YAG, 650 nm) that creates a population of photoexcitations.  Next, you wait a short delay time before applying a large (reverse) collection bias that sweep all charges out of the device.

The authors state that for delay times of 10 ns or less, the total charge collected at the last step of TDCF reflects only geminate recombination losses (since they are fast compared to non-geminate).  If you then do TDCF for different pre-biases, you get the voltage dependence of geminate recombination.  TDCF at longer times probes bimolecular recombination and the authors have a model (you need to dig  into the past literature to find details unfortunately) for how to extract bimoleuclar recombination rates from the long time TDCF.

The paper reports photo-I/V curves for three solution-processed small molecule solar cells.  They use the D-A combination p-DTS(FBTTH2)2:PC71BM (3:2 by weight) in as-cast from chlorobenzene (CB), annealed at 125 deg C after casting, and cast from CB with diodooctance (DIO) additive.  The DIO additive is widely believed to enhance crystallinity in the OPV's.  Devices cast with a DIO additive show the best OPV performance (PCE =7.1%).

Results:

I think Figure 2B is the key result of the paper.  The voltage dependence of geminate recombination in the 3 different device reflects the identical trend that is seen in their respective I/V curves under AM1.5G illumination.  The DIO dependence is basically flat and this is argued to result in a better Fill factor for this device.

In the other Figures, the authors present long-time TDCF and BACE, CELIV results that establish a role for bimolecular recombination in the devices.  Then finally in Figure, they attmept to show the relative contributions of geminate recombination and bimolecular recombination to the total I/V curve deviation from FF=1.  It is great that they over-plot the I-V curves with the geminate recombination current measured by TDCF at 10ns.  However, it was not clear why they could not also use their direct measurements of bimolecular recombination to separately quantify  its contribution to the I/V curve.  It seems they have simply (plausibly) attributed all deviations from ideality not encompassed by geminate recombination to bimolecular. It would be much better to use data to prove this quantitatively using data and I'd like to understand if there is a reason why this is not possible (comment section anyone?). 

Conclusions:

TDCF can deconvolve different recombination mechanisms contributing to OPV non-ideality.  The good performance of DIO-processed solar cells using p-DTS(FBTTH2)2:PC71BM materials is reflected in a voltage-independent geminate recombination rate.  In addition, bimolecular recombination currents are small. 

At the microscopic level, both of these factors are explained by the improvement in crystallinity afforded by the DIO additive.  Better crystallinity results in more efficient dissociation at the interface that is independent of internal field (flat TDCF at 10 ns).  In addition, it reduces bimolecular recombination by increasing carrier mobility.  This allows polarons to get to the electrode without recombining.

The consensus in the journal club was that it is very hard to evaluate the results and methods in this paper as it stands on its own.  Josh did a lot of digging through other work to find details about TDCF, BACE, and CELIV and associated modeling.  It is certainly not obvious that any of us should drop what we are doing to assemble these tools.  Perhaps the more pressing issue is how to critically evaluate new results coming from such unfamiliar tools. I do not personally feel comfortable with this.