Nowadays, organic solar cells have the interest of engineers for manufacturing flexible and low cost devices. The considerable progress of this nanotechnology area presents the possibility of investigating new effects from a fundamental science point of view. In this letter we highlight the influence of the concentration of fullerene molecules on the ultrafast transport properties of charged electrons and polarons in P3HT/PCBM blended materials which are crucial for the development of organic solar cells. Especially, we report on the femtosecond dynamics of localized (P2at 1.45 eV) and delocalized (DP2at 1.76 eV) polaron states of P3HT matrix with the addition of fullerene molecules as well as the free-electron relaxation dynamics of PCBM-related states. Our study shows that as PCBM concentration increases, the amplified exciton dissociation at bulk heterojunctions leads to increased polaron lifetimes. However, the increase in PCBM concentration can be directly related to the localization of polarons, creating thus two competing trends within the material. Our methodology shows that the effect of changes in structure and/or composition can be monitored at the fundamental level toward optimization of device efficiency.
KeywordsUltrafast Composites Fullerenes Polarons
The conversion of solar energy into electrical energy using thin film organic photovoltaics has showed great potential as a renewable energy source [1, 2]. Typical organic solar cells are based on the dissociation of photogenerated excitons (electron–hole pairs) by the sunlight to charged carriers and polarons (carriers coupled with the induced polarized electric field) at the vicinity of bulk heterojunctions formed within blends of organic semiconductors . Nowadays, there is good progress toward efficient polymer-based solar cells, and efficiencies of approximately 5% have already been demonstrated . Considerable attention has been focused on high solar efficiency blend materials such as π-conjugated poly-3-hexyl thiophene (P3HT) and fullerene derivatives such as [6,6]-phenyl-C61 butyric acid methyl ester (PCBM). Recently, localized and delocalized polaron transitions inside the gap of P3HT matrix were investigated using spectroscopic measurements . Although recent studies on P3HT/PCBM composites have revealed the effect of structural changes on the device efficiency , spectroscopic studies of ultrafast electron transfer in these donor–acceptor systems remain a challenge [6, 7].
In this letter, we have investigated the influence of PCBM concentration on the ultrafast photoinduced absorption (PA) of P3HT/PCBM blends after excitation with photon energies large enough to induce excitons in both materials. Our study elucidates the ultrafast polaron dynamics at localized and delocalized polaron transitions of P3HT before and after the dissociation of bound excitons at bulk P3HT/PCBM heterojunctions. Importantly, our ultrafast study also reveals information about the influence of coupling coefficients and the carrier density present in the localized and delocalized polaron states for materials with different PCBM concentrations. We also present the dynamics of excited states formed at PCBM network chains. We have found that these ultrafast carrier dynamics play the key role in the optimization of carrier transport in these organic solar cells.
The experimental data for the 1 wt% PCBM blend show that photoexcited P2 and DP2 polarons have a very short relaxation time (within ~100 ps). Similar spectra behavior and relaxation times have also been observed for the pure P3HT polymer matrix  which is reasonable since the PCBM concentration in our sample is very low. These PA bands remain at the same energies for all delay times except for a small energy shift (indicated with the horizontal arrows in Fig. 2a) of PA bands between 0 and 1 ps. This is a trend that does not appear in the data for any of the other composites we studied. When the ratio of absorption amplitudes for the P2 and DP2 bands is examined as a function of PCBM concentration, an interesting trend is observed. At 1 wt% PCBM the DP2 transition is stronger with a DP2 to P2 ratio of (3:2). This ratio is maintained for all time frames until these polarons relax. With increasing the PCBM concentration to 10 wt%, both absorption amplitudes increase and the DP2 to P2 ratio changes to (1:1). At the highest PCBM concentration composite (50 wt%), the absorption amplitudes increase considerably compared to the 1 wt% PCBM blend: P2 transition becomes ~5.6 times higher while the DP2 transition increases only by ~1.76 times. As a result the DP2 to P2 ratio reduces further to (1:2). The progressive reduction in the DP2 to P2 ratio suggests that as the PCBM concentration increases, the P3HT regions with long range order become less, giving rise to disordered regions. Therefore, the introduction of PCBM within the P3HT matrix interrupts P3HT crystallinity which is reasonable consequence of blending the two materials. However, these results illustrate that the average hole diffusion length will decrease with an expected negative knock on effect on device efficiency.
Another important trend revealed by our data is the increase in lifetime of polarons in P2 and DP2 states with increasing the PCBM concentration. The transient absorption spectra of the composite with the highest PCBM concentration show that after 200 ps a similar amount of polarons are still available as in the 1 wt% PCBM composite immediately after (0 ps) the absorption of the pump pulse. In this comparison we also probe an opposite behavior of relative amplitudes for the P2 and DP2 polarons between the two samples. Assuming that the fundamental polaron relaxation lifetime for P3HT does not change with increasing PCBM concentration, this trend can be explained by the independent or combined action of increased production of polarons and/or reduced availability of electrons for recombination. The dissociation of excitons formed at the P3HT–PCBM interface is more likely than in bulk P3HT due to the existence of a build-in electric field at the heterojunction. Therefore, as the PCBM concentration increases so does the proportion of excitons that dissociate, resulting in a progressively increasing number of P2 and DP2 polarons immediately after the absorption of the pump pulse as can be seen from the relative amplitudes of the spectra in Fig. 2. In addition to increasing the exciton dissociation rate, electron capture by PCBM also minimizes recombination . Therefore, the rate of recombination loss of polarons will decrease as seen in Fig. 2.
Figure 2also shows that when the PCBM concentration increases, the population of localized polarons (P2) increases at the expense of the delocalized ones (DP2). This trend can be attributed directly to the disruption in the long range order of P3HT chains as the PCBM regions increase in size and number. This finding has immediate relevance to the efficiency of P3HT/PCBM solar cells. Our results show that on one hand the population of polarons increases considerably with the addition of PCBM but, on the other, the relative amount of mobile (delocalized) polarons decreases. Therefore, in terms of device efficiency there will be an upper limit in PCBM concentration with further improvements possible only if long range order in P3HT is maintained.
In Fig. 2 we have also observed the existence of a photobleaching (PB) band at 2.25 eV for blends with 1 and 10 wt% PCBM concentration where state filling (SF) effect plays the dominant role. This probing energy corresponds to the first vibronic sideband E1 of the P3HT material where there is a significant density of states [7, 12]. The transient absorption decay profile of the PB band is also shown in the Fig. 3. From the transient absorption spectra we conclude that the relaxation dynamics of this energy state appear to be governed by two recombination mechanisms (fast and slow component). Upon addition of PCBM molecules, the secondary excitations of the mobile electrons (see PA3 arrow in Fig. 1b) contribute to the absorption signal giving positive absorption changes within the first few ps (two times higher absorption at 3.8 eV of the highest PCBM concentration sample). As a result, the existence of the PA in the highest PCBM concentration composite (Fig. 2c) at 2.25 eV probing energy is attributed to the secondary re-excitations of electrons from the lower unoccupied molecular orbital (LUMO) of PCBM to higher energy states. At this probing energy of 2.25 eV and after the first 10 ps, we have also the ability to detect the PB band (at the first vibronic sideband of P3HT matrix) where the density of states of P3HT seems to play a dominant role in the carrier dynamics . An additional experimental evident of this free-electron re-excitation can be extracted comparing the sign of the absorption change at 3.8 and 3.1 eV  excitation using the same probing energy of 2.25 eV.
Our study shows that there is indeed very close correlation between the structure of the blends and carrier dynamics. As the PCBM concentration increases, so does the availability of polarons in the P3HT matrix. This is expected since exciton dissociation is expected to take place at the P3HT/PCBM heterojunctions. However, we directly probe the gradual decrease in the relative amount of delocalized polarons as the PCBM concentration increases. We would expect that in such devices, as PCBM concentration increases the increased number of polarons find it progressively more difficult to diffuse within the blends and reach the electrodes. Device annealing has recently been proven to optimize the blend microstructure and improve the device efficiency. Therefore, we can anticipate that by providing direct fundamental information on carrier dynamics our methodology can be used to monitor the effect of blend fabrication steps or post-formation annealing.
1All our films were prepared from solutions that contain the same concentration of total polymer (P3HT + PCBM) and the same amount of 250 μL of solution was drop cast on identical quartz disks. Therefore, by keeping the mass content and thickness of our films the same through our samples, differences in the absolute value of transient absorption in Fig. 2can be taken as indication of polaron densities within the material