Carrier Formation Dynamics in Prototypical Organic Solar Cells as Investigated by Transient Absorption Spectroscopy

Moritomo, Yutaka; Yonezawa, Kouhei; Yasuda, Takeshi

doi:https://doi.org/10.1155/2016/9105460

International Journal of Photoenergy

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Review Article | Open Access

Volume 2016 | Article ID 9105460 | https://doi.org/10.1155/2016/9105460

Carrier Formation Dynamics in Prototypical Organic Solar Cells as Investigated by Transient Absorption Spectroscopy

Yutaka Moritomo,^1,2,3Kouhei Yonezawa,¹and Takeshi Yasuda⁴

Academic Editor: Petr S. Sherin

Received16 Apr 2016

Revised12 Jun 2016

Accepted23 Jun 2016

Published08 Aug 2016

Abstract

Subpicosecond transient absorption spectroscopy is a powerful tool used to clarify the exciton and carrier dynamics within the organic solar cells (OSCs). In this review article, we introduce a method to determine the absolute numbers of the excitons and carriers against delay time (t) only from the photoinduced absorption (PIA) and electrochemically induced absorption (EIA) spectra. Application of this method to rr-P3HT-, PTB7-, and SMDPPEH-based OSCs revealed common aspects of the carrier formation dynamics. First, the temporal evolution of the numbers of the excitons and carriers indicates that the late decay component of exciton does not contribute to the carrier formation process. This is probably because the late component has not enough excess energy to separate into the electron and hole across the donor/acceptor (D/A) interface. Secondly, the spectroscopy revealed that the exciton-to-carrier conversion process is insensitive to temperature. This observation, together with the fast carrier formation time in OSCs, is consistent with the hot exciton picture.

1. Introduction

1.1. Exciton-Carrier Conversion in Organic Solar Cells

Organic solar cells (OSCs) with bulk heterojunction (BHJ) [1–4] are promising energy conversion devices with high power conversion efficiency (PCE > 10% [5]), flexibility, and low-cost production process, for example, the roll-to-role process. The BHJ active layer, which consists of phase-separated nanosize domains of the donor (D) and acceptor (A) materials, efficiently absorbs the solar energy and converts it to the electric energy. The BHJ layer is easily prepared by the spin-coating from an organic solvent and the appropriate thermal annealing. In some cases, an additive, for example, diiodooctane (DIO), in the organic solvent is effective in obtaining a fine domain structure [6]. In the actual OSCs, the active layer is sandwiched between an Al cathode and an indium tin oxide (ITO) transparent anode. The ITO electrode is used as an optical window.

Significant feature of the OSCs is that Frenkel-type exciton with a high binding energy is stable even at room temperature, reflecting the small dielectric constant ( = 2-3) [7, 8]. Then, the photoexcitation creates donor exciton () in the donor domain or acceptor exciton () in the acceptor domains. The photovoltaic effect is realized by the conversion process from exciton to carrier. This is in a sharp contrast with inorganic solar cells (ISCs), in which the photoexcitation directly creates free carriers in the active layer. Figure 1 schematically shows the exciton-carrier conversion process around D/A interface. The photoirradiation directly creates excitons in the respective domains [(2) exciton formation]. The excitons migrate to the D/A interface [(3) exciton migration] and dissociate into donor hole () and acceptor electron () [(4) exciton dissociation]. The exciton dissociation process is the most important issue to comprehend the photovoltaic effects in OSCs. Importantly, the exciton migration distance (~10 nm) is very short in OSCs [9]. If the domains size is much larger than the migration distance, most of the excitons recombine before they reach the D/A interface. Therefore, the nanosize domain structure of the BHJ layer is indispensable for the efficient carrier formation process. After the carrier formation, transfers within the D domain to reach the anode, where the hole is collected. transfers within the A domain to reach the cathode, where the electron is collected.

Among the exciton-carrier conversion process, (4) exciton dissociation process is still controversial [9–19]. The Frenkel exciton in organic materials has high binding energy (=0.2–0.5 eV) due to the electrostatic potential between them. At the D/A interfaces, the electrons will be transferred to A, provided that the energy gain overcomes the exciton binding energy. This requirement is often satisfied by an energetic offset between the donor and acceptor lowest unoccupied molecular orbitals (LUMOs). After the electron transfer, the resulting electron-hole pairs still feel electrostatic potential between them. Such a bound electron-hole state at D/A interface is known as charge-transfer (CT) state. The CT state was detected by absorption [20–24], photoluminescence [25–27], electroluminescence [26, 27], time-resolved/pulsed electron paramagnetic resonance [28–31], and transient absorption spectroscopy [32–41]. Importantly, the free carrier formation from the CT state is very fast, as demonstrated by transient absorption spectroscopies [32–41]. Hwang et al. [32] reported that free carriers were generated from the CT state within a few picoseconds in P3HT/PCBM blend film. Using sub-20 fs pump-probe spectroscopy, Grancini et al. [41] observed the generation of free carries within 50 fs in the PCPDTBT/PCBM blend film.

There exists a long-lasting debate whether the carriers are generated from the hot CT or relaxed CT state. In the hot CT picture, the excess energy helps to dissociate the CT states directly to the free carriers before they reach the relaxed CT states [42]. In this picture, the primary kinetic competition is between the energy relaxation and the dissociation of the hot CT state. The energy relaxation process is usually several hundred femtoseconds [43]. However, several experimental results on the internal quantum efficiency (IQE) [44, 45] are against the hot CT picture. For example, Vandewal et al. [45] reported that IQE is irrespective of the excitation energy in the MEH-PPV/PCBM OSC. This data strongly indicates that free carrier generation is exclusively from the relaxed CT state rather than from the hot CT state. In this situation, a new spectroscopic approach is desired to deepen the understanding of the exciton dissociation.

1.2. Spectroscopic Determination of Absolute Numbers of Photogenerated Species

The transient absorption spectroscopy is one of the powerful tools used to clarify the dynamics of the exciton, CT state, and carrier. The spectroscopy has been applied to the OSCs with BHJ active layers [42, 43, 46–61]. Figure 2 shows schematic diagram of the transient absorption spectroscopy. The irradiation of the pump light pulse creates , , , and . These species cause the photoinduced absorption (PIA) below the absorption edge, as shown in the right panels of Figure 2. This PIA is monitored by the probe light pulse against delay time (t).

Here, let us introduce a method to determine the absolute numbers of the excitons and carriers against only from the photoinduced absorption (PIA) and electrochemically induced absorption (EIA) spectra. In the latter spectroscopy, the electrochemically doped carrier causes a characteristic EIA in the infrared region. In this method, we only consider the photogenerated exciton and carrier and eliminate material dependent assumptions on the spectral interpretation as much as possible. Then, PIA () is decomposed into the respective PIA components, that is, (donor exciton), (acceptor exciton), and (carriers). () should be the PIA of the donor (acceptor) neat films at the early stage after photoexcitation. should be the PIA of the blend film at the late stage, where photogenerated excitons completely disappear. We note that is dominated by the PIA due to , because the profile of the late component significantly depends on the donor material and resembles that of the EIA of the donor neat film. This suggests that is much weaker than . The CT state inevitably coexists with excitons because its lifetime (<50 fs [41]) is very short. Therefore, it is impossible to determine the PIA () due to the CT state without material dependent assumptions. By the spectral decomposition, the relative numbers of the excitons ( and ) and carrier [ (= )] can be determined against . Here, we define , , and as the numbers of absorbed photons, excitons, and carriers, respectively. ( is the exciton components of spectral change) is evaluated from the PIA of the blend film. is evaluated from the PIA of the neat film, with assuming that one absorbed photon creates one exciton. Then, the absolute number () of the photogenerated excitons per an absorbed photon is expressed as Similarly, ( is the carrier components of spectral change) is evaluated from the PIA of the blend film. On the other hand, is evaluated from the EIA of the neat film. Then, the absolute number () of the photogenerated carriers per an absorbed photon is expressed as

The drawback of this method is that it does not explicitly include the CT state, which is believed to play an essential role in the carrier formation process. Nevertheless, the simple model, which excludes material dependent assumptions, has two advantages over the conventional complicated models. First, the model enables us to determine the absolute numbers of the excitons (carriers) against only from the spectroscopic data. Secondly, the model is applicable to even unknown D/A systems, because we can experimentally determine , , and without any material dependent assumptions.

We note that the temperature effect on the carrier formation dynamics gives us a clue on the carrier formation mechanism. For example, Yonezawa et al. [60] reported that the carrier formation efficiency (), which is defined as the number of the photoinduced carriers per an absorbed photon, is nearly insensitive to temperature in several OSCs. This suggests that the exciton dissociation is treated by quantum-mechanical approach [62–67] and not by the so-called Marcus picture [68]. In the former picture, the exciton dissociation is quantum-mechanically treated by the time-evolution of a wave function. In the latter picture, the charge separation is classically treated by the energy shift induced by displacement of the surrounding molecules.

1.3. Donor Polymers and Small Molecules for OSC

Historically, extensive spectroscopic investigations [32–43, 46–51] have been carried out on the charge formation dynamics in regioregular poly(3-hexylthiophene) (rr-P3HT)/6,6-phenyl C₆₁-butyric acid methyl ester (PCBM) blend film, due to its reproducible power conservation efficiency (PCE > 5% [69–71]). In particular, the regioregularity of P3HT and the annealing procedure have significant effects on the carrier formation dynamics of P3HT/PCBM blend film. The cross-sectional transmission electron microscopy (TEM) of rr-P3HT/PCBM blend film shows characteristic pathway for carrier transport through the film thickness. The length scale of the phase separation was 24 nm [72]. The recent development of the low-band gap donor polymers further increased PCE beyond 5%, which motivated intensive spectroscopic investigation of the low-band gap donor polymers [56–64]. In particular, Yu and coworkers [73, 74] have developed a series of donor polymers based on alternating ester-substituted thieno3,4-b]thiophene and benzodithiophene units (PTBn: n = 1–7). The poly[[4,8-bis [(2-ethylhexyl)oxy] benzo 1,2-b:4,5-b′] dithiophene-2,6-diyl] 3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno 3,4-b] thiophenediyl]] (PTB7)/6,6]-phenyl C₇₁-butyric acid methyl ester (PC₇₁BM) OSC shows a high PCE (~9% [75]). The AFM images [76] of the PCE-optimized PTB7/PC₇₁BM blend films show fiber-like structures (10–50 nm wide and 200–400 nm long), which are ascribed to the donor- or acceptor-rich domains.

The domain structure of the BHJ layer is complicated and its relation to the PCE value is still controversial. By means of the scanning transmission X-ray microscopy (STXM), Collins et al. [77] investigated the domain structure of PTB7/PC₇₁BM blend film prepared without additive. They found that the PTB7-rich domain shows considerable fullerene mixing. By means of atomic force microscopy (AFM) coupled with plasma-ashing technique, Hedley et al. [76] observed a substructure of ~10 nm inside the fullerene domain (~100 nm) of the PTB7/PC₇₁BM blend film prepared without additive. Such a complexity of the domain structure of the BHJ layer may prevent a true understanding of the carrier formation dynamics. In this sense, a planar heterojunction (HJ) OSC with well-defined D/A interface is suitable for detailed investigation on the carrier formation and recombination dynamics [78–80]. For example, Moritomo et al. [80] revealed carrier density effect on the carrier recombination process in PTB7/C₇₀ HJ solar cell.

Another candidates for the donor materials are the small molecules, because they are easy to synthesize and purify. This is in sharp contrast with the polymer donors, which suffer from bad synthetic reproducibility and difficult purification procedures [81, 82]. Among small molecular donors, diketopyrrolopyrrole (DPP) pigments were developed in the early 1970s and have been widely used in inks, paints, and plastics [83]. Recently, Nguyen’s group developed an oligothiophene-DPP molecule with ethylhexyl substituents [2,5-di-(2-ethylhexyl)-3,6-bis-(5′′-n-hexy-2,2′,5′,2′′]terthiophen-5-yl)-pyrrolo[3,4-c]pyrrolo-1,4-dione] (SMDPPEH) [83, 84]. The SMDPPE-based OSC shows a high PCE (~3% [85]) with PC₇₁BM among the small molecule-based OSCs. The AFM images [85] of the PCE-optimized SMDPPEH/PC₇₁BM blend films show fiber-like (~20 nm wide and ~50 nm long) and overshaped structures, which are ascribed to the donor- and acceptor-rich domains, respectively.

In this review article, we introduce a method to determine the absolute numbers of the excitons and carriers against only from the PIA and EIA spectra. The method was applied to the prototypical OSCs, that is, (i) PTB7/PC₇₁BM blend, (ii) rr-P3HT/PCBM blend, (iii) PTB7/C₇₀ bilayer, and (iv) SMDPPEH/PC₇₁BM blend films. Figure 3 shows energy level diagram for respective D/A interfaces. Quantitative analyses clarified important aspects of the carrier formation dynamics in the OSCs. First, the late decay component of exciton does not contribute to the carrier formation process, as observed in (i) PTB7/PC₇₁BM blend, (ii) rr-P3HT/PCBM blend, (iii) PTB7/C₇₀ bilayer, and (iv) SMDPPEH/PC₇₁BM blend films. This is probably because the late component has not enough excess energy to separate into electron and hole at D/A interface. Secondly, the exciton-carrier conversion process is insensitive to temperature, as observed in (iii) PTB7/C₇₀ bilayer and (iv) SMDPPEH/PC₇₁BM blend films. This observation, together with the fast carrier formation time in OSCs, is consistent with the hot exciton picture.

2. Experimental Technique

2.1. Transient Absorption Spectroscopy

Figure 4 shows a prototypical setup for the transient absorption spectroscopy. Second harmonics (=400 nm) of the regenerative amplified Ti:sapphire laser are usually used as the pump pulse. In some cases, the wavelength of the pump pulse is converted with use of an optical parametric amplifier (OPA). A white pulse, which is generated by self-phase modulation in a sapphire plate, is used as the probe pulse. The delay stage controls the delay time (t) between the pump and probe pulse. The spectra of the transmitted probe pulse are analyzed with a multichannel detector attached to a spectrometer. The differential absorption spectra (ΔOD) are defined as , where and are the transmitted light intensity with and without pump excitation, respectively. The photogenerated exciton and carrier cause characteristic PIA in the infrared region.

2.2. Electrochemical Differential Absorption Spectroscopy

The electrochemical differential absorption spectra () [61] are defined as , where and are the transmitted spectra of hole-doped and nondoped films, respectively. The electrochemically doped carrier causes a characteristic EIA in the infrared region. The doped carrier density () is evaluated from the current density and doping time. The electrochemical carrier-doping was usually performed in two-pole electrochemical cell with a pair of quartz windows. The electric current is parallel to the light path. The cathode and anode are the donor neat film and a small piece of Li metal. The electrolyte is usually propylene carbonate (PC) solution containing 1 mol/l LiClO₄.

3. PTB7/PC₇₁BM Blend Film: A Prototypical Example

3.1. Decomposition of the PIA Spectra

First of all, let us demonstrate how to decompose the PIA into the respective components, that is, , , and . Figure 5(a) shows ΔOD spectra of the PTB7/PC₇₁BM blend film. The PIA spectra in the late stage (>1 ps) show broad PIA around 1150 nm. The PIA is originated from the photoinduced carriers () of PTB7 [57–60]. Actually, the spectral profile is similar to that of the electrochemical differential absorption spectra of the PTB7 neat film (vide infra). Therefore, the PIA in the late stage can be used as . We note that the photoexcitation of the BHJ layer creates the excitons, not the carriers. Therefore, the PIA in the early state (<1 ps) is overlapped by PIA due to and . Figure 5(b) shows ΔOD spectra of PC₇₁BM neat film. The PIA due to is very flat and the spectral shape is independent of . Then, the PIA can be used as . Figure 5(c) shows ΔOD spectra of PTB7 neat film. PIA due to shows broad peak around 1200 nm and the spectral shape is nearly independent of t (<10 ps). Then, the PIA (<10 ps) can be used as .

(a)

(b)

(c)

Now, we can decompose of PTB7/PC₇₁BM blend film into , , and . The spectral weights of the respective components were determined so that they minimize the following trial function: . The PIA of the blend film at 3 ps was used as . The PIA of PC₇₁BM neat film at 1 ps was used as . The PIA of PTB7 neat film at 1 ps was used as . Figure 6 shows examples of the decomposition of the PIA spectra into , , and . Strictly speaking, the cross sections for the respective species may differ between the neat and blend films. The difference is considered to be negligible because the PIA is measured in the infrared region below the optical gap.

(a)

(b)

3.2. Absolute Numbers of the Elementary Excitations

In order to spectroscopically evaluate , , and per an absorbed photon, we need the absolute intensity of the PIA per unit densities of , , and . The absolute intensity of the PIA due to is easily evaluated by the electrochemical differential absorption spectroscopy [61]. Figure 6 shows examples of the electrochemical differential spectra (). The shoulder-like structure in spectrum [Figure 7(a)] of rr-P3HT neat film is analogous to the PIA spectrum of rr-P3HT/PCBM blend film. Similarly, the broad peak structure in spectrum [Figure 7(b)] of the PTB7 neat film is analogous to the PIA spectrum of the PTB7/PC₇₁BM blend film. The absolute intensity of the PIA can be evaluated from spectrum of PTB7 neat film with considering the electrochemically doped carrier number per unit area. Concerning (), it is reasonable to assume that one absorbed photon creates one () in the donor (acceptor) neat film. Then, the absolute intensity was evaluated from ΔOD spectrum of PTB7 (PC₇₁BM) neat film with considering the absorption photon number per unit area.

(a)

(b)

3.3. Carrier Formation Dynamics

Figure 8 shows , , and per an absorbed photon in PTB7/PC₇₁BM blend film against . The solid curves are results of least-squares fittings with exponential functions. The carrier formation time ( = 0.3 ps) is very fast. Fast is ascribed to the molecular mixing [77] as well as the nanosize domain structure [76] of the BHJ. Here, we emphasized that the excitation pulse at 400 mn was dominantly absorbed by the acceptor fullerene, not by PTB7 [see upper panels of Figures 5(a) and 5(b)]. Consequently, the photoexcitation only creates . It is interesting that exciton decay time ( = 1.5 ps) is much slower than . This indicates that the late decay component of does not contribute to the carrier formation process.

There are two possibilities to why the late component of does not dissociate. One possibility is that the exciton recombines before it reaches D/A interface. This scenario, however, cannot explain fast (=1.5 ps) of , as follows. Without D/A interface, the decay channel of exciton is (i) radiative recombination, (ii) triplet exciton formation, and (iii) exciton-exciton annihilation. Among them, the former two channels cannot explain fast (=1.5 ps) of , because typical times for these channels are on the order of nanoseconds. On the other hands, of neat PC₇₁BM film is 120 ps [54], which can be ascribed to the (iii) exciton-exciton annihilation in bulk. The value, however, is much longer than the observed (=1.5 ps). Another possibility is that the exciton dissociation efficiency decreases with . This is plausible because the excess energy of should decrease with . Then, late component has not enough excess energy to separate into electron and hole across D/A interface. Fast in the blend film is probably ascribed to the extra recombination process at D/A interface, such as quench on free carrier or some structural defect. By means of a time-evolution simulation of a wave packet, Iizuka and Nakayama [67] theoretically investigated exciton dissociation at D/A interface. They demonstrated that the dissociation probability increases for the hot excitons compared with the ground-state exciton owing to their small binding energies and large diameters. The present method does not explicitly include the CT state, which is believed to play an essential role on the carrier formation process. Nevertheless, if is nearly the same as , the experimental results suggest that excess energy is needed even for the formation of the CT state.

4. P3HT/PCBM Blend Film: Classical but Complicated System

4.1. PIA Spectra and Analyses

As mentioned in the introduction, there already exists extensive spectroscopic investigation on rr-P3HT/PCBM blend film in literatures [32–43, 46–51]. Nevertheless, we reanalyzed the PIA spectra of rr-P3HT/PCBM blend film [46] and tried to determine , , and per an absorbed photon against . Systematic analyses of the PIA spectra in the different BHJ systems may reveal common aspects of the carrier formation dynamics in the OSCs.

Figure 9(a) shows ΔOD spectra of rr-P3HT/PCBM blend film. The PIA spectra in the late stage (>10 ps) show a shoulder-like structure, which is analogous to spectrum [Figure 7(a)] of rr-P3HT neat film. Therefore, the PIA in the late stage can be used as . is obtained from ΔOD spectra [Figure 9(b)] of PCBM neat film, while is obtained from ΔOD spectra [Figure 9(c)] of rr-P3HT neat film.

(a)

(b)

(c)

By means of the least-squares fitting, were decomposed into , , and . The PIA of the blend film at 3 ps was used as . The PIA of the PCBM neat film at 1 ps was used as . The PIA of rr-P3HT neat film at 1 ps was used as . Figure 10 shows examples of the decomposition of the PIA spectra of rr-P3HT/PCBM blend film into , , and . In order to spectroscopically evaluate , , and per an absorbed photon, we need the absolute intensity of the PIA per unit densities of , , and . The absolute intensity of the PIA due to is evaluated by the electrochemical differential spectroscopy [Figure 7(a)]. The absolute intensity of the PIA due to () is evaluated from ΔOD spectrum of rr-P3HT (PCBM) neat film.

(a)

(b)

4.2. Carrier Formation Dynamics

Figure 11 shows , , and per an absorbed photon in rr-P3HT/PCBM blend film against . The solid curves are results of least-squares fittings with exponential functions. The present analysis fails to observe the carrier formation process, because monotonously decreases with decay time of 4.4 ps. One possible reason for this unexpected result is that the carrier formation time is too fast to observe. Consistently, Hwang et al. [32] proposed fast formation (<0.25 ps) of the interfacial charge-transfer states. Another possible reason may be the variation of the cross section of the carrier. The present method assumes constant cross section of the carriers. The extended electronic state of the CT state, however, may enhance the cross section as compared with that of the small polaron state (free carrier). If the cross section of the CT state is higher than that of free carrier, value is overestimated in the early stage. Consistently, Takahashi et al. [86] reported intense PIA due to the CT state at α-sexithiophene (6T)/C₆₀ interface below ~10 ps. This observation implies high oscillator strength of the CT state.

On the other hand, slowly decreases with the decay time ( = 2.9 ps). This indicates that the late decay component of does not contribute to the carrier formation process, similar to the case of PTB7/PC₇₁BM blend film.

5. PTB7/C₇₀ Bilayer Film: A HJ System

5.1. PIA Spectra and Analyses

Figure 12(a) shows ΔOD spectra of PTB7 (100 nm)/C₇₀ (25 nm) bilayer film. At 300 K, the PIA spectra in the late stage (>10 ps) show broad PIA around 1150 nm. The PIA is originated from the photoinduced carriers () of PTB7 [57–60]. In the early stage (=1 ps), additional absorption component is observed around 1500 nm. The additional components are reasonably ascribed to the PIA due to and . is obtained from ΔOD spectra [Figure 12(b)] of C₇₀ neat film, while is obtained from ΔOD spectra [Figure 12(c)] of PTB7 neat film.

(a)

(b)

(c)

By means of the least-squares fitting, were decomposed into , , and . The PIA of the blend film at 10 ps was used as . The PIA of C₇₀ neat film at 1 ps was used as . The PIA of PTB7 neat film at 1 ps was used as . Figure 13 shows examples of the decomposition of the PIA spectra of PTB7/C₇₀ bilayer film into , , and . The absolute intensity of the PIA due to is evaluated by the electrochemical differential spectroscopy [Figure 7(a)]. The absolute intensity of the PIA due to () is evaluated from ΔOD spectrum of PTB7 (C₇₀) neat film. Strictly speaking, the cross sections for the respective species may depend on temperature. In this sense, one should be careful when he discusses temperature effect on the absolute number of , , and . On the other hand, the temporal behaviors of , , and are independent of the cross section.

(a)

(b)

5.2. Carrier Formation Dynamics

Figure 14 shows , , and per an absorbed photon in PTB7 (100 nm)/C₇₀ (25 nm) bilayer film at (a) 300 K and (b) 80 K against . The solid curves are results of least-squares fittings with exponential functions. At 300 K [Figure 14(a)], the carrier formation time ( = 1.1 ps) is rather slow as compared with that (=0.3 ps) in the BHJ system. Slower is ascribed to thicker domain in the heterojunction system and resultant long journey of the excitons to D/A interface. Decay time ( = 1.2 ps) of is comparable to (=1.1 ps), indicating an efficient conversion process from to the carrier. Decay time ( = 3.5 ps) of , however, is much longer than (=1.1 ps). This indicates that the late decay component of does not contribute to the carrier formation process, similar to the case of PTB7/PC₇₁BM blend film.

(a)

(b)

Overall carrier formation dynamics at 80 K [Figure 14(b)] is similar to that at 300 K [Figure 14(a)]. In particular, the carrier formation efficiency (=0.5 carriers/absorbed photon) at 80 K is almost the same as that at 300 K. Thus, the transient absorption spectroscopy revealed that the exciton-carrier conversion process is insensitive to temperature. We emphasize that PCE of OCS steeply decreases with decrease in temperature, because carrier transport process is significantly sensitive to temperature. This observation, together with the fast carrier formation time in OSCs [32, 41], is consistent with the hot exciton picture. The low temperature effect is the elongation of carrier formation ( = 1.5 ps) and exciton decay ( = 1.8 ps and = 5.1 ps) times.

6. SMDPPEH/PC₇₁BM Blend Film: Small Molecule System

6.1. PIA Spectra and Analyses

Figure 15(a) shows ΔOD spectra of SMDPPEH/PC₇₁BM blend film. At 300 K, the PIA spectra show a broad absorption band in the infrared region. The peak position shifts show a red-shift from 1100 nm at 1 ps to 1200 nm at 10 ps. The red-shift disappears above 10 ps and the spectral profile becomes independent of . Therefore, the PIA in the late stage can be used as . is obtained from ΔOD spectra [Figure 15(b)] of PC₇₁BM neat film, while is obtained from ΔOD spectra [Figure 15(c)] of SMDPPEH neat film.

(a)

(b)

(c)

By means of the least-squares fitting, were decomposed into , , and . The PIA of the blend film at 10 ps was used as . The PIA of PC₇₁BM neat film at 1 ps was used as . The PIA of SMDPPEH neat film at 1 ps was used as . Figure 16 shows examples of the decomposition of the PIA spectra of SMDPPEH/PC₇₁BM blend film into , , and . The absolute intensity of the PIA due to () is evaluated from ΔOD spectrum of SMDPPEH (PC₇₁BM) neat film. It was difficult to obtain a quantitative electrochemical differential spectra, because the SMDPPEH molecule is solvable to the organic electrolyte.

(a)

(b)

6.2. Carrier Formation Dynamics

Figure 17 shows , , and per an absorbed photon in SMDPPEH/PC₇₁BM blend film at (a) 300 K and (b) 80 K against . The solid curves are results of least-squares fittings with exponential functions. At 300 K [Figure 16(a)], carrier formation time ( = 0.4 ps) is very fast. Fast is ascribed to the nanosize domain structure of the BHJ. Decay time ( = 0.4 ps) of is comparable to (=0.4 ps), indicating an efficient conversion process from to the carrier. Decay time ( = 2.6 ps) of , however, is much longer than (=0.4 ps). This indicates that the late decay component of does not contribute to the carrier formation process, similar to the case of PTB7/PC₇₁BM blend film.

(a)

(b)

Overall carrier formation dynamics at 80 K [Figure 17(b)] is similar to that at 300 K [Figure 17(a)], similar to the case of PTB7/C₇₀ bilayer film. The low temperature effect is the elongation of decay time ( = 1.0) of .

7. Summary

We applied a new method, which determines the absolute numbers of the excitons and carriers only from the PIA and EIA spectra, to (i) PTB7/PC₇₁BM blend, (ii) rr-P3HT/PCBM blend, (iii) PTB7/C₇₀ bilayer, and (iv) SMDPPEH/PC₇₁BM blend films. The analyses revealed important common features on the carrier formation dynamics in the OSCs. First, the late decay component of exciton does not contribute to the carrier formation process, as observed in the (i) PTB7/PC₇₁BM blend, (ii) rr-P3HT/PCBM blend, (iii) PTB7/C₇₀ bilayer, and (iv) SMDPPEH/PC₇₁BM blend films. This is probably because the late component has not enough excess energy to separate into electron and hole at D/A interface. Secondly, entire carrier formation dynamics is insensitive to temperature, as observed in (iii) PTB7/C₇₀ bilayer and (iv) SMDPPEH/PC₇₁BM blend films. This observation, together with the fast carrier formation time in OSCs [32, 41], is consistent with the hot exciton picture.

Competing Interests

The authors declare that they have no competing interests.

Acknowledgments

This work was supported by Futaba Electronics Memorial Foundation and a Grant-in-Aid for Young Scientists (B) (22750176) from Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology, Japan.

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Copyright © 2016 Yutaka Moritomo et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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International Journal of Photoenergy

Carrier Formation Dynamics in Prototypical Organic Solar Cells as Investigated by Transient Absorption Spectroscopy

Abstract

1. Introduction

1.1. Exciton-Carrier Conversion in Organic Solar Cells

1.2. Spectroscopic Determination of Absolute Numbers of Photogenerated Species

1.3. Donor Polymers and Small Molecules for OSC

2. Experimental Technique

2.1. Transient Absorption Spectroscopy

2.2. Electrochemical Differential Absorption Spectroscopy

3. PTB7/PC71BM Blend Film: A Prototypical Example

3.1. Decomposition of the PIA Spectra

3.2. Absolute Numbers of the Elementary Excitations

3.3. Carrier Formation Dynamics

4. P3HT/PCBM Blend Film: Classical but Complicated System

4.1. PIA Spectra and Analyses

4.2. Carrier Formation Dynamics

5. PTB7/C70 Bilayer Film: A HJ System

5.1. PIA Spectra and Analyses

5.2. Carrier Formation Dynamics

6. SMDPPEH/PC71BM Blend Film: Small Molecule System

6.1. PIA Spectra and Analyses

6.2. Carrier Formation Dynamics

7. Summary

Competing Interests

Acknowledgments

References

Copyright

3. PTB7/PC₇₁BM Blend Film: A Prototypical Example

5. PTB7/C₇₀ Bilayer Film: A HJ System

6. SMDPPEH/PC₇₁BM Blend Film: Small Molecule System