Photophysics is an important link between the structural properties of the diverse layers in organic solar cells and the ultimate device performance. Usually, the photophysical properties are investigated by steady-state and transient optical spectroscopies. The steady-state photophysics, such as the absorption and reflection, are key criteria to judge whether the plasmonic effects take place [51
]. Normally, the enhancement in the optical absorption and the decrease in reflection are indicators of effective plasmonics due to the far-field scattering and near-field LSPR. It is important to point out at the outset of this part that these optical footprints should be analyzed in association with the electrical properties (e.g., EQE) to unambiguously determine the involvement of plasmonics for boosting the OPV performance.
The steady-state optical spectroscopies have been performed to reveal the fundamental plasmonic mechanisms including various modes such as the scattering, LSPR, SPP, waveguide, and the plasmon-gap mode. In organic solar cells, characteristics like the exciton generation/dissociation and the charge carrier transport/collection, which are crucial to the device performance, can be addressed based on the transient photophysical properties. The charge transfer and transport properties have been studied via monitoring the relaxation kinetics of excitons in excited states with the time-resolved absorption and fluorescence measurements.
Overall, controversial findings have been reported on incorporation effect of the plasmonic metallic nanostructures. Despite the fact that many studies demonstrated the improvement of the photovolatic performance thanks to the introduction of the nanometals, the device performance deterioration shown in some other works should not be ignored. Hence, it is urgently needed to obtain physical insights into the plasmonics from the investigation of the photophysical processes for exciton dissociation into free charge carriers and charge recombination/transport/collection [93
]. In this part, we focus on the photophysical properties in plasmonic photovoltaic structures in correlation with the device configurations and performances.
5.2. Transient Absorption and Photoluminescence Spectroscopies
Paci et al. incorporated Au NPs into the active layer P3HT:PCBM of the organic solar cells, achieving an enhancement of PCE by 42% and improvement in stability. From the PL decay profiles, one can see the fluorescence decay of the polymer is retarded by the addition of Au NPs (Figure 16
). The authors attributed the increased fluorescence lifetime to the interaction between the comparatively long-lived triplet excitons of P3HT as the donor and the Au NPs as the acceptor. Such interactions can strongly quench the triplet state and ameliorate the photo-oxidation of the organic materials. The mitigation mechanism for the device degradation as unveiled by the photophysical characterization is supported by the time resolved energy dispersive X-ray reflectometry (EDXR) study [101
Since the metallic NPs embedded in the active layer may increase the trapping of polarons and recombination of excitons, limiting the amount of free charge carriers, ultrathin layer of oxide can be added to encapsulate the NPs. The femtosecond transient absorption results of P3HT:PCBM and their blends with [email protected]2
at 200 fs and 1 ps delay are displayed in Figure 17
a,b. The films exhibited three vibronic peaks in the wavelength range from 520 to 610 nm, which arise from ground-state bleaching (GSB). The negative peak centered at 660 nm originates from the photoinduced absorption due to polaron pairs according to Korovyanko et al. [102
]. The increase of this peak for the 200 fs and 1 ps delay suggests that polaron pair generation is more efficient in the hybrid system with the capped Ag NPs than in P3HT:PCBM film.
Consistently with the above Au NPs case, the relaxation time of the photoinduced polarons is also prolonged when [email protected]
NPs are hybridized in the active layer. This is caused by the enhanced absorption due to LSPR, which is beneficial for the generations of charge carriers. Hence, the P3HT:PCBM-based organic solar cells with [email protected]2
-ODA nanoprisms in the active layer achieved a dramatically improved PCE of 4.03% with enhanced absorption over a broad spectral range of 400–620 nm [93
PEDOT:PSS is a popular hole transport layer in OPV devices. Metallic nanostructures doped in the HTL or inserted at the HTL-anode interface may contribute plasmonic improvement for the photovoltaic devices. The photophysics of P3HT has been explored in two configurations where Ag NRs were mixed with PEDOT:PSS or covered with PEDOT:PSS prior to the deposition of the P3HT layer. As shown in Figure 18
, the P3HT fluorescence intensity is larger on Ag NRs than that off the NRs. The fluorescence emission is intensified due to the enhanced absorption of P3HT aided by the plasmonic interaction between Ag NRs and P3HT. The PL intensity changes with the varying distance between them. Compared with the case where Ag NRs are inside the HTL, the P3HT-Ag NRs distance is more homogeneous but larger when Ag NRs were covered under the HTL. Therefore, the PL intensity and thus the absorption are comparatively more homogeneous but smaller when the nanometals are between the HTL and anode. Hence, there is a compromise between the homogeneity and strength of the metal-polymer plasmonic interactions. This can serve as experimental evidence to support the theoretical simulation conclusion that the LSPR may diffuse into the active layer for absorption improvement even when the metallic nanostructures are placed outside the active layer. If the enhancement of the optical absorption had only been caused by the far-field scattering effect of the Ag NRs, the amplitude of fluorescence intensity for the two geometries would have been quite similar [94
The transient fluorescence decay profiles are displayed in Figure 18
c,d. There is no clearly visible variation to the fluorescence decay times for P3HT on and off the Ag NRs, suggesting there is no direct charge transfer between the polymer and the Ag NRs. It is different from the case when the nanometals are blended in the P3HT:PCBM active layer for which the charge transfer between the polymer and the metal nanostructures gives rise to distinct change of PL decays.
However, it is possible to induce charge/energy transfer between the donor in the active layer and the nanometals in CTL. Chi et al. reported the photophysical process for metallic NPs in ETL as shown in Figure 19
. The size of the NPs was manipulated to vary from 16 to 72 nm. The absorption of the plasmonic structures is enhanced due to the scattering and LSPR effects. Although the optical absorption of the whole solar cell reached its maximum with Au NPs of 16 and 25 nm, the device PCE had the highest value, i.e., 7.86%, when Au NPs with diameters of 41 nm were doped in the ZnO ETL. Apart from the absorption, the efficiency of exciton dissociation into free charge carriers is another vital influencing factor to the device performance, which can be explored by time-resolved PL decay characterizations. The PL from the plasmonic samples decays faster than the reference structure, indicating Au NPs improve the exciton dissociation into free holes and electrons. The fluorescence lifetimes have been attained by a bi-exponential fitting, whose dependence on the particle size is presented as the inset of Figure 19
c. Therefore, Au NPs with appropriate sizes are able to increase the absorption and exciton separation, giving rise to improved Jsc
and PCE. Too small NPs cannot convert the absorbed photons efficiently into free charge carriers. Too large NPs may induce a high roughness of the layer morphology, deteriorating the OPV devices.
In addition, metallic nanostructures can be located between neighboring layers, and the photophysical results can well explain the device performance enhancement. Yang et al. analyzed the steady-state and time-resolved PL curves of P3HT for samples with Ag NRs at ITO/ETL, ETL/active layer, and active layer/HTL interfaces, respectively, as exhibited in Figure 20
. From the steady-state PL spectra in Figure 20
b, the Huang-Rhys factor (S
) is obtained as an indicator of the conjugation length of polymers. The S value is the smallest when Ag NRs are between ITO and ETL. The optical absorption is amplified to the greatest extent with the same plasmonic architecture. The transient PL profiles presented faster decays for plasmonic samples compared with the reference one (see Figure 20
c), demonstrating strong coupling of the excitons and plasmonic field. The photophysical characterizations manifest that the exciton dissociation efficiency, the electron delocalization, and the absorption are improved by the Ag NRs, in particular when they are at ITO/ETL interface. With this optimized plasmonic structure, Jsc
is increased and Voc
and FF remain almost unchanged. The PCE of P3HT:PCBM solar cells is increased from 3.10% for the control device to 4.05% in the plasmonic device with Ag NRs inserted at the ETL/cathode interface [79
Additionally, organic-inorganic hybrid nanostructures comprised of polymer semiconductors and plasmonic metal have been newly developed for future photocatalytic and photovoltaic applications [95
]. The nanostructures hold the polymer and metal in close proximity, facilitating the charge/energy transfer and the exciton dissociations that are beneficial to charge collection in OPV devices. For instance, [email protected]
nanocomposites and Au-P3HT NRs have been fabricated, and the time-resolved PL profiles are illustrated in Figure 21
. The mean decay time of [email protected]
composite is shorter than that of pristine P3HT. Analogously, Au-P3HT NRs possess notably decreased fluorescence lifetimes compared to neat P3HT NRs [85
]. The increased fluorescence decay rate of semiconductor polymers in the presence of plasmonic metals is associated with the energy transfer from the excited state of P3HT to the SPR state of gold NPs [103
As illustrated in Figure 22
, the transient absorption spectra from P3HT on a flat Ag film and on Ag gratings were compared. Whereas the GSB signals distribute from 500 to 620 nm, the positive peak around 660 nm is attributed to the polaron pair absorption. From the comparison of the peak at 660 nm, one can see that much more polaron pairs are generated via E-E annihilation on samples with Ag gratings, which may be correlated to the enhanced density of the singlet excitons. Owing to the involvement of periodic Ag gratings, the absorption in P3HT is enhanced, and hence more singlet excitons are generated. In Figure 22
d, the normalized bleaching recovery of P3HT decays faster when it is on Ag grating, suggesting that the E-E annihilation is enhanced due to the improved absorption in P3HT [104