The current state of research concerning charge generation at the charge transfer (CT) state is summarised in ‘Charge Generation in Polymer-Fullerene Bulk-Heterojunction Solar Cells’ by Feng Gao and Olle Inganäs . They aim their review at providing a balanced discussion of the possible mechanisms which drive charge dissociation at the CT state . Researchers agree that the dissociation of charges occurs at the CT state as illustrated in figure 1 below . Here I will provide an overview of strengths and limitations of the major arguments presented by Gao, F. Inganäs, O. et al. (2014) with respect to the charge generation process at the CT state .
The review written by Gao, F. Inganäs, O. et al. (2014) mainly focuses on the following topics (focusing on polymer:fullerene solar cells only):
- How researchers in the field currently observe charge dissociation process experimentally.
- A discussion of the most likely proposed mechanisms for charge dissociation at the CT state.
- The effect of the electric field, disorder, mobility and temperature on charge dissociation at the CT state.
Does exciton dissociation occur through the Hot CT state or Relaxed CT state?
Hot CT state dissociation relies upon excess energy to dissociate tightly bound excitons into free charge carriers . Whereas, the relaxed CT state does not rely on an energy excess since it is described as the lowest energy emissive interfacial state . A weak argument is present by Gao, F. Inganäs, O. et al. (2014) in support of exciton dissociation at the hot CT state of polymer:fullerene OSCs . Gao, F. Inganäs, O. et al. (2014) and other researchers imply that scientists in the field question whether charge dissociation occurs through one of two mechanisms: the relaxed CT state or hot CT state . Gao, F. Inganäs, O. et al. (2014) cite transient absorption spectroscopy (TA) measurements conducted by Ohkita, H. et al (2008) to support the argument that the exciton dissociation occurs through the hot CT state . Ohkita, H. et al. (2008) demonstrates that excess energy produced after exciton dissociation provides greater kinetic energy for CT state dissociation .Implying that excess energy can help to drive exciton dissociation and further support the hot CT state theory. Gonzalez, J, C. et al. (2011) suggests that the probability of excitons overcoming an energy barrier to form dissociated pairs increases with increasing thermal energy . Deibel, C. et al. (2010) reiterates this; the introduction of excess energy at the donor acceptor interface will lead to the activation of higher vibrational modes resulting in effective charge carrier production . Furthermore, from the cited literature one can assume that excess energy will yield a greater level of dissociation in charge carriers . However, this argument does not necessarily disprove the relaxed CT state mechanism.
Strong experimental evidence is provided to suggest that exciton dissociation occurs through the relaxed CT state in polymer: fullerene OSCs . Gao, F. Inganäs, O. et al. (2014) cites strong experimental work by Vandewal, K. et al. (2014) suggesting that dissociation through the relaxed CT state is more viable. Vandewal, K. et al. (2014) displayed that the internal quantum efficiency (IQE), which can be calculated from the number of minority carriers which contribute to the short circuit current divided by the number of photons entering the cell of the system, remained constant irrespective of the degree of excess energy in the system . Both, Vandewal, K. et al (2014) and Gao, F. Inganäs, O. et al. (2014) felt that this evidence strongly suggests that charge dissociation occurs via the relaxed CT state . To further support this argument, the authors cited experiments performed by Howard, I, A. (2010) whereby TA measurements were recorded for both regiorandom (RRa) and regioregular (RR) forms of P3HT in blends with PCBM . Where regiorandom blends have a regular chain like morphology, whereas regioregular systems have increased inter chain interaction, hence, possess lamellae like structure, as displayed in Figure 2 below . Howard, I, A. (2010) found that the regiorandom polymers had a larger (excess thermal energy) value than the regioregular form, yet displayed a lower free charge carrier generation . Furthermore, the authors provide strong experimental evidence in support of the relaxed CT state. The evidence emphasises that excess energy does not drive charge carrier generation at the CT state; suggesting that charge dissociation does not occur solely through the hot CT state.
Consideration of the driving force of the relaxed CT state
The driving force of the relaxed CT state is not addressed under the segment titled ‘Driving force in the case of dissociation via the relaxed CT state’ of the review written by Gao, F. Inganäs, O. et al. (2014). However, the authors digress from this topic to describe the effect of: electric field, disorder, mobility and temperature on charge dissociation at the CT state (with respect to the Braun Onsager theory, as explained below) . Furthermore, in the conclusion of the review the authors mention that they have considered the driving force of CT charge dissociation in their review, however, this is not the case. The effects of the mentioned parameters on charge dissociation have been displayed instead.
Effect of temperature on charge generation process
In accordance to Gao, F. Inganäs, O. et al. (2014) and other researchers the Braun Onsager model provides an unrealistic, hence, ineffective description of CT state dissociation in a polymer:fullerene solar cell . Gao, F. Inganäs, O. et al. (2014) presents limitations and strengths of the Braun Onsager theory through a balanced appraisal of some parameters (electric field, temperature and mobility) in order to comprehend their role in explaining charge generation at the CT state . Gao, F. Inganäs, O. et al. (2014) argue that one of the limitations of the Braun Onsager model is its dependence on temperature, since, the model displays that a decrease in temperature will result in an exponential reduction in the rate of dissociation at the CT state . The authors present a weak argument in support of the independence of charge dissociation from temperature. However, the authors suggested that at low temperatures a significant photocurrent was observed, using this to confirm the independence of charge generation on temperature . However, the authors cited Chirvase, D. et al (2003, pp 3381) in support of their argument, who experimentally observed that at low temperatures materials only generated a few charge carriers which were not easily transported . Furthermore, the evidence provided in support of the argument did not suitably justify the statement made by the authors. Additionally, the evidence does not disprove the hot dissociation state; furthermore, the argument requires more experimental justification. Yet, researchers such as Gonzalez, J, C. et al. (2011) agree that from an experimental observation charge dissociation at the CT state is independent of temperature . Hence, the assumptions made by the authors can be justified through experimental evidence. Furthermore, thermal induction into a system should yield an increase in charge carrier generation, but, might not necessarily drive the process.
From the review by Gao, F. Inganäs, O. et al. (2014) it seems that the exciton dissociation route through the relaxed CT state seems more viable due to the strong experimental evidence displayed in comparison to that of the hot CT state. The introduction of thermal energy into the system, or the presence of excess energy will result in a greater likelihood of charge dissociation, hence, temperature could be viewed as a relevant parameter in describing charge dissociation .
In order to improve their review Gao, F. Inganäs, O. et al. (2014) could provide more relevant evidence to justify claims made. Further studies on the driving force and charge dissociation mechanism at the relaxed CT state would be useful, in order to potentially confirm its observance at the CT state.
By Naeema Ebrahim
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