On the Origin of the Quadrant I Semicircle in Intensity-Modulated Photocurrent Spectra of P3HT:PCBM Bulk Heterojunction Solar Cells: Evidence of Degradation-Related Trap-Assisted Recombination

Abstract

Intensity-modulated photocurrent spectroscopy (IMPS) characterizes the charge carrier transport in a solar cell by subjecting it to a sinusoidally varying illumination and measuring the resultant photocurrent. The IMPS spectra of certain P3HT:PCBM bulk heterojunction solar cells exhibit a poorly understood feature: a quadrant I semicircle, which implies that the sinusoidal variation of the photocurrent leads that of the illumination in a certain frequency range. To understand the mechanisms underlying this feature, we have adopted a mathematical framework based on drift-diffusion modeling instead of the conventional equivalent circuit approach. By incorporating the effects of traps into the mathematical model and conducting targeted experiments, we have demonstrated that the quadrant I semicircle arises from degradation-related trap states behaving as recombination centers. Our analysis of the device's intrinsic mechanisms indicates that the trap-assisted recombination can have a time scale that is at least one order of magnitude slower than that of the carrier extraction mechanism; such a mismatch can generate the observed quadrant I semicircle.

Spatially Resolving Ordered and Disordered Conformers and Photocurrent Generation in Intercalated Conjugated Polymer/Fullerene Blend Solar Cells

“…Intensity modulated photocurrent spectroscopy (IMPS) measurements of annealed PBTTT/PCBM devices also display positive phase shifts at low modulation frequencies (i.e., photocurrent leads the modulation frequency), which is a signature of nongeminate charge recombination becoming operative. 26 , 27 IMPS images of these model PBTTT/PCBM solar cells reveal that this process is most prevalent at boundaries of ordered/disordered PBTTT and PCBM aggregates.…”

Loss Mechanisms in High Efficiency Polymer Solar Cells

“…IMPS has been widely applied to study electrochemical systems [ 15 ] such as dye sensitized solar cells, [ 43 ] but rarely to study organic photovoltaics. [44][45][46][47] In this measurement a small optical AC perturbation is superimposed on the background light intensity, causing periodic variations in carrier density. By monitoring the changes in the photocurrent collected at the contacts as a function of light modulation, transient phenomena such as trapping, recombination, and charge transfer at material interfaces can be probed as a function of carrier density.…”

Intensity-Modulated Spectroscopy on Loaded Organic Photovoltaic Cells

“…Typically, a fast LED is modulated to emit a light intensity L + L sin t, L < L, that is projected onto a PV cell, and amplitude and phase of the cell's electrical response are recorded as a function of modulation frequency. IMS has been applied to various types of solar cells, including OPVs, dyesensitised solar cells (DSSCs), and CdTe/CdS cells [1,[5][6][7][8][9][10]. Two modes of IMS are established in the literature, namely intensity-modulated photovoltage spectroscopy (IMPV) [8,9], which records the frequency dependent modulation of a solar cell's open-circuit voltage (V OC ) under modulated L, and intensity-modulated photocurrent spectroscopy (IMPS) [1,[5][6][7], recording the frequency dependent modulation of a solar cell's short-circuit current (I SC ).…”

“…IMS has been applied to various types of solar cells, including OPVs, dyesensitised solar cells (DSSCs), and CdTe/CdS cells [1,[5][6][7][8][9][10]. Two modes of IMS are established in the literature, namely intensity-modulated photovoltage spectroscopy (IMPV) [8,9], which records the frequency dependent modulation of a solar cell's open-circuit voltage (V OC ) under modulated L, and intensity-modulated photocurrent spectroscopy (IMPS) [1,[5][6][7], recording the frequency dependent modulation of a solar cell's short-circuit current (I SC ). IMPS is more common, as for moderate or high L (including AM 1.5), V OC is nearindependent of L, while I SC is proportional to L. Frequency dependency of the amplitude and phase of the PV cell's electrical response may be presented in Bode and/or Nyquist plots, like in EIS, albeit an IMS Nyquist plot does not display electric impedance.…”

“…Also, excluding oxygen and water during manufacture is critical, which is emphatically demonstrated e.g. by the work of Luther et al [1], wherein devices were purposefully exposed to oxygen during manufacture, with significant impact on their performance.…”