The Future of Organic Photovoltaics by Katherine A. Mazzio and Christine K. Luscombe, reviews current scientific progress made in the field of organic photovoltaics (OPV), relating directly to:
- Common types of OPV device structures;
- Popular electron donor and acceptor materials;
- Common active layer morphology optimisation techniques.
Here I will present a summary of particular strengths and limitations displayed in the review by Mazzio et al. (2009) .
Can a single layered OPV device demonstrate reasonable efficiency?
Mazzio et al. (2015) presents an informative, chronological summary of device structural characteristics, limitations and strengths . However, a slight discontinuity was reported in the authors’ description of single layered devices, when compared to current research. Mazzio, et al. (2015) suggests that PCEs in the order of only ca. 0.1% have been achieved, to date, for single layered devices, owing to, limitations in device arrangement . The device arrangement for a single layered OPV is displayed in Figure 1 below . Zhang, Q. et al. (2015) recently reported a power conversion efficiency, PCE of ca. 8% for polymer fullerene solar cells . Greater power conversion efficiencies were also reported by Liao, S, H. et al. (2014); a polymer fullerene single cell containing a cathode doped with ZnO, achieved a PCE of ca.10.31% . From the cited material it can be presumed that single layered devices can produce power conversion efficiencies of greater magnitude than those cited by Mazzio. et al. (2015). Recent developments in single junction OPVs clearly discredit the inaccurate information presented by Mazzio. et al. (2015) in this review.
Limitations of the Bulk Heterojunction
Careful selection of device architecture could improve OPV performance [1, 5]. Mazzio. et al. (2015) reported limitations in the stability of Bulk Heterojunction (BHJ) OPVs. The authors notably suggested an alternative device architecture that may overcome these limitations; an Inverted Bulk Heterojunction, as illustrated in Figure 2 [1, 5, 6]. Mazzio. et al. (2015) reported that the acidic nature of PEDOT: PSS could contribute to BHJ device degradation . Mazzio. et al. (2015) suggested that inverted architectures typically replaced PEDOT: PSS with a low work function metal such as zinc oxide, ZnO . However, Hau, S, K. et al. (2008) reported an excellent air-stable inverted BHJ device containing PEDOT:PSS alongside a silver, Ag electrode . Hau, S, K. et al. (2008) suggested that the superb stability displayed in tests is owed to, PEDOT: PSS and Ag limiting device oxidation . In spite of, the suggestion made by Mazzio, et al. (2015); PEDOT: PSS must be removed in order to improve stability, Hau, S, K. et al. demonstrates that the PEDOT: PSS layer can improve device stabilities. This improvement in stability in inverted BHJ OPVs make this device an attractive alternative when compared to regular BHJ device architectures. Mazzio. et al. (2015) highlighted the importance of careful selection of device architectures, in order to minimise limitations.
Are small molecules contenders to polymers?
Limitations of polymer active layers were expressed by Mazzio. et al. (2015) concerning: variations in batch to batch properties, end group variations and dispersity . Small molecules (displayed in Figure 3) are presented as potential future alternatives and contenders to polymers, not only overcoming the mentioned limitations but also exhibiting greater hole and electron mobilities [1, 7].
Small molecules have come a long way, since, Mazzio. et al. (2015) reported their highest PCE’s as ca. 6% with Zhang, Q. et al. (2015) reporting a remarkable efficiency of ca. 9% for a single layered OPVs  and tandem devices recording efficiencies of >12 . Comparatively, polymer based tandem OPVs demonstrate efficiencies of ca. 10% also, PCEs of ca. 9% for a single layered device . From these studies, one can note the improvement in the PCE generated from tandem device architectures, for both small molecule and polymer active layers [3, 9] . From the perspective of power conversion efficiencies alone Mazzio. et al. (2015) is correct to describe small molecules as competitors to polymers, as they have achieved PCEs in a similar range. The reader that is interested in tandem device architectures should read the article by You, J. et al.(2012), where PCES of >10% are recorded for OPVs or Sista, S. et al. (2011) .
Morphology Optimisation Procedures
In order to optimise small molecule and polymer active layers, Mazzio. et al. (2015) provided an extensive description of optimisation procedures that alter active layer morphologies and improve device efficiencies . The authors explore the effect of; solvent annealing, thermal annealing and the addition of solvent additives on the active layer morphology of materials, using peer-reviewed sources . The authors discuss, thermal annealing; the process by which a film can be heated above its glass transition temperature, allowing the material to re-orientate itself . They go on to discuss the effect of different types of annealing; post-annealing and pre-annealing . In post-annealing, the sample is thermally annealed after cathode deposition onto the device, whereas, pre-annealing describes thermal annealing prior to cathode deposition . Mazzio. et al. (2015) suggests that these techniques can be used to improve device efficiencies . This is supported by a study carried out by Yi, Z. (2014) on the pre-annealing of small molecule, that increased the dark current, reduced the open circuit voltage, hence, improved the PCE . An experimental study conducted by Yang, X. et al. (2012) supported this further; post annealing was found to enhance light absorption (as shown in Figure 4) and increase hole mobilities, furthermore, improving PCEs . In addition to demonstrating that device architectures can influence power conversion efficiencies the writers also portray that annealing a sample can also optimise OPV properties.
Importantly, the authors provide an alternative to thermal annealing, for materials such as PCPDTBT that does not respond well to thermal annealing; the addition of alkane dithiols . Mazzio. et al. (2015) report an improvement in the nanomorphology of PCPDTBT/PCBM on the addition of 1, 8-di-iodooctane; through increased PCBM domain development . Albrecht, S et al. (2012) also found that the addition of solvent additives drove phase segregation, in active layers containing PCPDTBT, as displayed in the energy filtered transmission electron microscopy, EFTEM surface images; Figure 5 below .
However, the authors do not report the enhanced device properties, that result from improved nano-morphologies of PCPDTBT; improved mobility , reduced field dependence  improved short circuit current , enhanced optical absorption [13, 14]. Both thermal and solvent additive morphological optimisation techniques were found to drive phase segregation and improve various properties of OPVs [1, 8, 12, 13]. Yet, the discussion of morphology optimisation procedures by Mazzio. et al. (2015) will provide valuable information to new researchers, to the popular field of OPVs, on how power conversion efficiencies can be enhanced.
Conclusively, from the discussion above it is apparent that the review by Mazzio, et al. (2015) demonstrates an informative insight into potential routes of optimisation of OPV efficiencies. The review allows the reader to recognise the influence that components in an OPV can have on the device efficiency. Overall, this review article was mostly consistent but in certain areas the information presented was inaccurate, perhaps, owing to negligence.
Predominantly, Mazzio, et al. (2015) presented a well-structured review suitable for those new to the field. Moreover, for future work I would suggest that the writers present a literature review on the current status and future applications of small molecules suitable to those new to the field, since, these are limited.
By Naeema Ebrahim
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An experimental study of the field dependence of photogeneration in PCPDTBT/PCBM blends have been described in the research paper: ‘Field-Independent Charge Photogeneration in PCPDTBT/PCBM Solar Cells’ written by Fiona, C. Jamieson, Tiziano Agostinelli, Hamed Azimi, Jenny Nelson and James Durrant . Field dependence of photogeneration in the following PCPDTBT/PCBM blends have been analysed using transient absorption spectroscopy (TAS):
- (C-PCPDTBT + octane dithiol (OD)/PCBM have been assessed in this paper .
ODT is a processing agent used to effect the morphology of a polymer blend through phase separation .
The paper is aimed at probing the effect of an applied electric field on photogeneration using TAS decays of PCPDTBT/PCBM blends . Here I will summarise the main arguments, strengths and limitations in the research paper by Jameison, F, C. et al. (2010) will be discussed in this review.
Non-geminate recombination in PCPDTBT/PCBM blends containing octanedithiol (ODT)
Jamieson, F, C. et al. (2010) fitted a power law onto a TAS decay of an active layer blend (C-PCPDTBT/PCBM) to demonstrate that the blend follows a non-geminate (bimolecular) recombination route; figure 1 . Researchers have applied this method to identifying non-geminate recombination in TAS decays .
The transient decay in figure 1 was shown to fit the power law decay, fulfilling the following equation:
Where OD is optical density, t is equal to time, the exponent (alpha) is equal to T/T0, where T is temperature and T0 represents the characteristic temperature .
Non-geminate recombination pathways, in all blends, were identified through fitting power laws to transient decay signals; a method supported by various researchers . Yet, the authors mention that they were unable to quantify non-geminate recombination losses in the PCPDTBT/PCBM blends, as the blends were irradiated to produce a greater polaron density than solar irradiation would produce . Hence, non-geminate recombination losses could not be estimated for these blends . Quantifying the degree of non-geminate recombination in PCPDTBT/PCBM blends might have been useful for the authors to determine the effect of ODT addition and understand the increase in power conversion efficiencies, short circuit currents and fill factors of PCPDTBT/PCBM blends containing ODT .
ODT introduction in PCPDTBT/PCBM blends, studied by Agostinelli, T. et al. (2011), promoted faster non-geminate recombination as well as a higher degree of photogeneration, when compared to C-PCPDTBT/PCBM blends . Yet, this faster non-geminate recombination was shown to inhibit the blend from reaching its ideal power conversion efficiency . Furthermore, without quantifying recombination in these blends, Jamieson, F, C. et al. (2010) found that ODT improved the efficiency of the solar cell . Whilst being ignorant to the fact that ODT increased recombination, reducing the efficiency of the solar cell . From the results depicted by Agostinelli, T. et al. (2011), it is evident that quantifying loss mechanisms such as non-geminate recombination is important to understanding the efficiency of a solar cell .
Studying field dependence
Within the paper by Jamieson, F, C. et al. (2010), the authors reason that all analysed PCPDTBT/PCBM blends are independent of an applied bias, since, their initial absorptions, across all applied biases, were within the signal to noise ratio limits of 10% (as shown in figure 2) . From figure 2, the authors deduced that blends with greater initial absorption magnitudes, were shown to possess greater power conversion efficiencies . The authors found that the initial absorption magnitude represents the initial yield of dissociated charge carriers . Furthermore, the authors were able to demonstrate that the initial yield of dissociated polarons (charge generation) is independent of an applied bias . The data displayed in figure 2 was an effective means of comparing blend efficiencies and demonstrating that all blends were independent of an applied bias. It can be clearly noted, from figure 2, that blends containing ODT demonstrate higher charge generation efficiencies, as demonstrated by Agostinelli, T. et al. (2011) .
The addition of a processing agent was also found to enhance the charge generation efficiency in C-PCPDTBT/PCBM studied by Albrecht, S. et al. (2012) . A PCPDTBT/PCBM blend containing a processing agent, diooctane, was shown to possess a greater power conversion efficiency and short circuit current . Yet, unlike, Jamieson, F, C. et al. (2010), Albrecht, S. et al. (2012) studied the direct correlation between an applied bias and charges extracted from electrodes over the course of an experiment: free charge generation efficiency, Q(TOT), as shown in figure 3 below . From figure 3, the free charge generation efficiency, Q(TOT) and therefore, device efficiency increases with increasing internal field . Moreover, an increase in internal field yields a greater collected charge density, hence, a more efficient solar cell. Furthermore, Albrecht, S. et al. (2012) was able to directly correlate the efficiency of a polymer/fullerene blend to an applied bias . Jamieson, F, C. et al. (2010) were unable to demonstrate a clear correlation between device efficiency and applied field as demonstrated in figure 3.
To conclude, the aims of the research paper by Fiona, C. Jamieson and Tiziano Agostinelli were met; photogeneration in PCPDTBT/PCBM solar cells were found to demonstrate independence from an applied bias. Moreover, Figure 2 was effective means of demonstrating this, as well as, comparing blend efficiencies.
However, in avoiding the quantification of recombination the researchers were unable to understand the full effect of adding the processing agent, ODT, on the efficiency of the solar cell. Furthermore, the authors could be advised to carry out further research to investigate the influence of the processing agent on recombination kinetics (which should be quantified).
By Naeema Ebrahim
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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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I will be using this platform to provide monthly reviews on journals relating to organic solar cells. I will be writing these reviews as part of an academic course which will be assessed.
I would truly encourage and appreciate your critique, regardless of whether or not you may be an expert in the field.
Thank you very much,