A Critical Study of the Field Dependence of Charge Generation and Recombination in Polymer Fullerene Solar Cells

Introduction

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 [1]. Field dependence of photogeneration in the following PCPDTBT/PCBM blends have been analysed using transient absorption spectroscopy (TAS):

  • C-PCPDTBT/PCBM
  • (C-PCPDTBT + octane dithiol (OD)/PCBM have been assessed in this paper [1].
  • Si-PCPDTBT/PCBM

ODT is a processing agent used to effect the morphology of  a polymer blend through phase separation [2].

The paper is aimed at probing the effect of an applied electric field on photogeneration using TAS decays of PCPDTBT/PCBM blends [1]. 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 [1]. Researchers have applied this method to identifying non-geminate recombination in TAS decays [3][4].

The transient decay in figure 1 was shown to fit the power law decay, fulfilling the following equation:

Where OD is optical density, is equal to time, the exponent (alpha) is equal to T/T0, where T is temperature and  T0 represents the characteristic temperature [5].

Figure 1: (a) displays a transient decay signal for a C-PCPDTBT/PCBM (ODT) blend (b) displays a typical TAS decay of C-PCPDTBT/PCBM (ODT) where the decay is shown to fit the power law (red) [1].
Figure 1: (a) displays a transient decay signal for a C-PCPDTBT/PCBM (ODT) blend (b) displays a typical TAS decay of C-PCPDTBT/PCBM (ODT) where the decay is shown to fit the power law (red) [1].
Non-geminate recombination pathways, in all blends, were identified through fitting power laws to transient decay signals; a method supported by various researchers [1][3][4]. 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 [1].  Hence, non-geminate recombination losses could not be estimated for these blends [1]. 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 efficienciesshort circuit currents and fill factors of PCPDTBT/PCBM blends containing ODT [1][2].

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 [2]. Yet, this faster non-geminate recombination was shown to inhibit the blend from reaching its ideal power conversion efficiency [2]. Furthermore, without quantifying recombination in these blends, Jamieson, F, C. et al. (2010) found that ODT improved the efficiency of the solar cell [1]. Whilst being ignorant to the fact that ODT increased recombination, reducing the efficiency of the solar cell [2]. 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 [2][6].

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) [1]. From figure 2, the authors deduced that blends with greater initial absorption magnitudes, were shown to possess greater power conversion efficiencies [1]. The authors found that the initial absorption magnitude represents the initial yield of dissociated charge carriers [1]. Furthermore, the authors were able to demonstrate that the initial yield of dissociated polarons (charge generation) is independent of an applied bias [1]. 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) [2].

Figure 2: displays photo induced absorption transients for all three blends, at various applied biases [1].
Figure 2: displays photo induced absorption transients for all three blends, at various applied biases [1].
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) [7]. A PCPDTBT/PCBM blend containing a processing agent, diooctane, was shown to possess a greater power conversion efficiency and short circuit current [7]. 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 [7]. From figure 3, the free charge generation efficiency, Q(TOT) and therefore, device efficiency increases with increasing internal field [7]. 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 [7]. Jamieson, F, C. et al. (2010) were unable to demonstrate a clear correlation between device efficiency and applied field as demonstrated in figure 3.

Figure 3: displays I/V curve of the C-PCPDTBT/PCBM blend related to Q_TOT [6].
Figure 3: displays I/V curve of the C-PCPDTBT/PCBM blend related to Q_TOT [6].
Conclusion

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

References

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