Two-step charge photogeneration dynamics in polymer/fullerene blends for photovoltaic applications

We measured the picoseconds (ps) transient dynamics of photoexcitations in blends of poly(3-hexyl-thiophene) (P3HT; donors-D) and fullerene [6,6]-phenyl-C61-butyric acid methyl ester (PCBM; acceptor-A), using the transient pump/probe photomodulation technique in an unprecedented broad spectral range from 0.25 to 2.5 eV, and compared the results with organic solar cell performance based on the same blends. In D-A blends with maximum domain separation such as regio-regular P3HT/PCBM with (1.2:1) weight ratio having solar cell power conversion efficiency of ∼4$\%$, we found that, although the photogenerated intrachain excitons in the polymer nano-domains decay within ∼10 ps, no charge polarons are generated on their expense up to ∼2 ns. Instead, there is a buildup of charge transfer (CT) excitons at the D-A interfaces having the same kinetics as the exciton decay, which dissociate into separate polarons in the D and A domains at a much later time ($\gg 1$ ns). This two-step charge photogeneration process may be typical in organic bulk heterojunction cells. Although the CT excitons are photogenerated on the exciton expense much faster in D-A blends having smaller domain size such as in regio-random P3HT/PCBM, their dissociation is less efficient because of larger binding energy. This explains the poor solar cell power conversion efficiency (<0.1$\%$) based on this blend. Our results support the two-step charge photogeneration mechanism in polymer/fullerene blends and emphasize the important role of the CT binding energy in generating free charge polarons in organic solar cells.

Figure 1

(a) The transient photomodulation spectrum of a pristine RR-P3HT film at $t=0$ and 100 ps, respectively; the exciton bands PA${}_1$, SE, and PB are indicated. The right inset shows the transient decay of PA${}_1$ and PB bands up to $t=160$ ps; the left inset shows the polymer backbone chain. (b) The transient photomodulation spectrum of a PCBM film at $t=0$; the exciton PA bands EX${}_1$ and EX${}_2$, and CT exciton band are indicated. The right inset shows the $\Delta T/T$(0) spectrum of isolated PCBM molecules in polystyrene (weight ratio of 1:100) that lacks the CT exciton band. The left inset shows the PCBM molecular structure.

Figure 2

The XRD pattern from RRa-P3HT/PCBM (red) and RRa-P3HT/PCBM (blue), where the P3HT bands [100], [200], and [300] and PCBM band [311] are assigned; the inset focuses on the PCBM band.

Figure 3

(a) The transient photomodulation spectrum of an RR-P3HT/PCBM blend film at $t$ $=$ 0 and 300 ps, respectively; the exciton band PA${}_1$ and CT exciton bands CT${}_1$ and CT${}_2$ are indicated. The green circles and line represent the background (BG) PA spectrum measured at $t=-5$ ps. The inset shows the doping-induced absorption of a pristine RR-P3HT film, where the polaron bands P${}_1$ and P${}_2$ are assigned. (b) The transient decay of PA${}_1$, buildup dynamics of CT${}_2$, and the PB decay up to 180 ps. The line through the data points is a fit using the FRET mechanism (see text); the same function also fits the CT${}_2$ buildup dynamics.

Figure 4

(a) The transient photomodulation spectrum of an RR-P3HT/PCBM blend film at $t=30$ ps excited at 3.1 eV, normalized and subtracted from the spectrum at $t=0$, that shows the two newly formed CT${}_1$ and CT${}_2$ bands. (b) Same as in (a) but at $t=0$ and excited at 1.55 eV, which is below the gap of both polymer and fullerene constituents.

Figure 5

The transient photomodulation spectrum of a pristine RRa-P3HT film at $t=0$ and 200 ps, respectively; the exciton bands PA${}_1$ and SE are indicated. The right inset shows the transient decay of PA${}_1$ and SE bands up to $t=150$ ps; the left inset shows the polymer backbone chain.

Figure 6

(a) The transient photomodulation spectrum of an RRa-P3HT/PCBM blend at $t=0$ (black squares) and 10 ps (red circles), respectively, excited at 3.1 eV; the PA bands PA1, CT1, and CT2 are assigned. The background PA is also shown (green circles) up to 1 eV. (b) The transient decay of PA${}_1$ and buildup dynamics of CT${}_1$ and CT${}_2$ up to 15 ps.

Figure 7

The I-V characteristic of two solar cells based on the PCBM blend with RR-P3HT (black) and RRa-P3HT (red) donor polymers under solarlike illumination of AM 1.5. The inserted table gives the device photovoltaic characteristic parameters such as short-circuit current density $J_{\mathrm{sc}}$, open-circuit voltage $V_{\mathrm{oc}}$, fill-factor FF, and the power conversion efficiency PCE in$\%$.

Figure 8

(a) Schematic diagram of the polymer grain of radius $R$, where $r$ is the exciton distance from the center. (b) The decay of the PA${}_1$ band in the RR-P3HT/PCBM blend, measured at 1 eV probe photon energy. The decay is fit using Eq. (A6) with the parameters given in the Appendix.