Nonthermal Site Occupation at the Donor-Acceptor Interface of Organic Solar Cells

We explore the nature of occupation and relaxation within the charge transfer density of states (CT DOS) for bulk heterojunction organic solar cells consisting of the donor boron subphthalocyanine chloride and the acceptor ${\mathrm{C}}_{60}$. We observe relaxation of geminate CT states on a sub-ns timescale via an approximately 70-meV dynamic redshift in their photoluminescence, whereas free carrier relaxation at longer times leads to the formation of nongeminate CT states at even lower energy. In steady state, we find that thermalization within the DOS is incomplete, resulting in a Boltzmann-like CT state distribution characterized by an effective temperature above that of the ambient. These results confirm that electron and hole populations can be far from equilibrium in organic solar cells and may prompt a reassessment of analyses that assume the same temperature for their charge-carrier distributions in the dark and under illumination.

Figure 1

Schematic showing charge separation, relaxation, and recombination pathways within the disorder-broadened CT and free carrier density of states distributions in a typical OPV D-A blend. Once formed, CT states may relax geminately through correlated motion of the electron and hole toward lower-energy sites (Process 1, shown by the solid arrows in the inset) or they may dissociate into free carriers that relax independently and recombine to form nongeminate CT states (Process 2, shown by the dashed arrows).

Figure 2

(a) Streak camera image showing the CT PL decay of a 1:1 blend SubPc:${\mathrm{C}}_{60}$ film. The dashed line shows the peak PL energy at the start of the decay. (b) Photoluminescence spectra integrated over different time intervals in the streak image, showing a redshift of the emission peak within the first few nanoseconds of the decay. The solid lines are Gaussian fits to the data and the vertical dashed line denotes the PL peak energy immediately following the excitation pulse.

Figure 3

(a) Redshift of the CT PL emission peak as a function of time for different SubPc:${\mathrm{C}}_{60}$ blend ratios. The corresponding color dashed lines indicate the full shift to steady state. (b) FWHM broadening of the same CT PL spectra over time. The peak energy and broadening are extracted from Gaussian fits to the emission spectra as shown in Fig. 2.

Figure 4

Early time CT PL spectra collected from a 1:1 SubPc:${\mathrm{C}}_{60}$ blend within the first 0.5 ns following excitation (a) and at steady state under cw illumination (b) for different sample temperatures. Dashed black lines in (a) and (b) denote the emission peak energy of each data set at room temperature. (c)–(e) Summary of the variation in early time and steady-state CT PL peak energies as a function of temperature for each SubPc:${\mathrm{C}}_{60}$ blend (full spectra provided in the Supplemental Material [29]).

Figure 5

CT PL decay collected from a 1:1 SubPc:${\mathrm{C}}_{60}$ film at a substrate temperature of 102 K (black line). The data exhibit two clear regimes: monoexponential decay ($\propto e^{-t/\tau }$, with $\tau =3.9\mathrm{ns}$) at early times <10 ns that is consistent with geminate CT recombination, and a power law decay ($\propto t^{-\alpha }$, with $\alpha =1.6$) at longer times that is characteristic of nongeminate recombination in an energetically disordered system [30, 31, 32]. The red and blue lines overlaid on the data show these individual functional dependences.

Figure 6

(a) Early time CT PL spectra collected within the first 0.5 ns following excitation from a 1:1 SubPc:${\mathrm{C}}_{60}$ blend solar cell at different reverse bias values. (b) Steady-state emission from the same device collected under cw illumination over the same bias range. The vertical dashed line in each panel marks the peak of the emission spectrum at open circuit. (c) Integrated intensity of the early time and steady-state CT PL spectra in (a) and (b) as a function of reverse bias, normalized to that at open circuit.

Figure 7

(a) Log ratios of the temperature-dependent CT PL spectra shown in Fig. 4, using the room-temperature spectrum ($T_1=293\mathrm{K}$) as the reference. Light solid lines show the experimental log ratios, dark solid lines show linear fits to the data, and the dashed lines denote the linear slopes that would occur in each case if the luminescence temperature corresponded to the actual sample temperature. (b) Summary of the effective luminescent temperatures extracted from the log PL ratios of each SubPc:${\mathrm{C}}_{60}$ blend as well as that from a $\mathrm{Ga}\mathrm{As}$ wafer for reference. The pink dashed line marks thermal luminescence with a temperature equal to that of the substrate.

Figure 8

(a) Steady-state EL and PL spectra collected from a 1:1 SubPc:${\mathrm{C}}_{60}$ solar cell at room temperature. (b) The same data measured from a $\mathrm{Ga}\mathrm{As}$ solar cell (EL) and wafer (PL). The EL drive current densities for the OPV and $\mathrm{Ga}\mathrm{As}$ cell were 300 mA/cm² and 20 mA/cm², respectively. (c) PL-to-EL log ratios of each device. Assuming the EL to be at room temperature in each case, the SubPc:${\mathrm{C}}_{60}$ slope indicates an effective PL temperature elevated by approximately 35 K, whereas the EL and PL temperatures of $\mathrm{Ga}\mathrm{As}$ differ only negligibly.