Introduction

In OSCs, the carriers are produced by the exciton dissociation at the D/A interface in the active layer, reflecting the weak screening effect and/or strong exciton effect (Frenkel exciton)1. In other words, the carriers are localized at the interface at the initial stage of the photovoltaic process. This makes a sharp contrast with the conventional inorganic solar cells (ISCs): The carriers are produced at every positions of the active layer, reflecting the strong screening effect and/or weak exciton effect (Wannier exciton). Then, the quantitative determination of the branching ratio between the carrier recombination at the interface region and the carrier escape from the region is indispensable for true comprehension of the photovoltaic process of OSCs. The escaped carriers will be collected at the collector electrodes. We propose that we can quantitatively investigate the carrier recombination/escape branching ratio by preparing the instantaneous dense carrier states by photo-excitation with femtosecond light pulses. Especially, in the HJ device, the instantaneous carrier density (n) per D/A interface area can be determined because the interface area is the same as the device area.

BHJ-type OSCs2,3, in which nano-level mixture of electron-donating polymer and electron-accepting fullerene-derivative provides a wide exciton-dissociative region within the active layer, show high PCEs and are promising alternatives to the conventional ISCs. Especially, the development of low band-gap polymer, e.g., PTB7, increases the PCE up to 9%4,5,6 in the last decade7,8. The increase in PCE stimulates extensive time-resolved spectroscopic investigations of the low-band gap OSCs to reveal the carrier formation process9,10,11,12. The femtosecond time-resolved spectroscopy is a powerful tool to reveal the carrier formation process, because the spectroscopy monitors the relative numbers of the photo-created excitons and carriers in the time domain. Significantly, the spectroscopy decouples the carrier formation and transfer processes, because the former process completes within several ps. Actually, the exciton-to-carrier conversion process in PTB7/PC71BM blend film completes in 0.2–0.5 ps9,10. Recently, a spectroscopic method13 was proposed to evaluate the carrier formation efficiency (ΦCF). ΦCF is defined by nformed/nphoton, where nformed (nphoton) is the density of the instantaneously formed carriers (absorbed photons) per unit area of the device. nformed includes the weakly-bound state of the carriers. Absolute magnitude of nformed can be estimated by the combination of the time-resolved and electrochemical spectroscopies. ΦCF is the same as the exciton quenching efficiency, if all the quenched excitons are converted to carriers.

In this paper, we investigated the branching ratio between the carrier recombination at the interface region and the carrier escape from the region in the PTB7/C70 HJ device. By means of the time-resolved spectroscopy, we confirmed that the exciton-to-carrier conversion process takes place within ~1 ps at the D/A interface in the PTB7/C70 HJ device. In the PTB7/C70 HJ device, ΦCF (=0.56) is nearly constant against the excitation pulse energy (Iphoton) per unit area of the device while the internal quantum efficiency (ΦIQ) steeply decreases with increase in Iphoton beyond 0.8 μJ/cm2. We further determined the interrelation between the instantaneous carrier density (n) per D/A interface area and the carrier density (ncollected) collected as photocurrent. We found that the carrier recombination becomes dominant if n exceeds a critical concentration (nc = 0.003 carriers/nm2). A phenomenological rate equation with the carrier recombination term (−αn3) well reproduces the experimental relation between n and ncollected. We confirmed that a similar behavior is observed in the PTB7/PC71BM BHJ device even though nc is considered to be much lower than the value of the HJ device.

Results

Pulse energy dependence of ΦIQ of PTB7-based OSCs

We fabricated the two types of the PTB7-based OSCs: PTB7/C70 HJ solar cell with a structure of indium tin oxide (ITO)/poly-(3,4-ethylenedioxythiophene) (PEDOT): poly-(styrenesulfonate) (PSS) (40 nm)/PTB7 (18 nm)/C70 (25 nm)/bathocuproine (BCP) (5 nm)/MgAg and PTB7/PC71BM BHJ cell with a structure of ITO/PEDOT:PSS (40 nm)/active layer (89 nm)/LiF (1 nm)/Al. We measured current (J)—voltage (V) curves (see Supplementary Fig. S1 online) of the two OSCs. The HJ device exhibits an open circuit voltage (Voc) of 0.68 V, a short circuit current (Jsc) of 5.9 mA/cm2, a fill factor (FF) of 0.68 and a PCE of 2.7%, while BHJ device exhibits a Voc of 0.74 V, a Jsc of 17.7 mA/cm2, a FF of 0.65 and a PCE of 8.5%. The FF value (=0.68) of the HJ device is slightly higher than that (=0.65) of the BHJ device. The suppressed FF value of the BHJ device is ascribed to the efficient carrier recombination during the carrier transfer process, reflecting the nano-level mixing of the D and A domains. We measured incident photon-to-current conversion efficiency (IPCE) spectra (see Supplementary Fig. S2 oneline) of the two OSCs. The IPCE of the HJ device is nearly the half of that of the BHJ device. The suppression is ascribed to the thinner active layer (43 nm) of the HJ device as compared with that (89 nm) of the BHJ device. The thinner active layer is further responsible for the lower PCE and Jsc values of the HJ device.

Figure 1a shows I against Iphoton of femtosecond light pulse at 400 nm in the PTB7-based HJ and BHJ devices. In both the devices, the magnitude of I linearly increases with Iphoton in the low-Iphoton region below ~0.2 μJ/cm2. However, I becomes nearly constant (~0.03 mA/cm2) above ~2 μJ/cm2. A similar saturation of I against Iphoton is observed in P3HT/PCBM BHJ device14. Figure 1b shows ΦIQ at 400 nm against Iphoton. The magnitudes of ΦIQ were evaluated from I and Iphoton with considering the absorption indexes: 0.61 and 0.84 for HJ and BHJ devices, respectively. In the HJ device (open circles), the ΦIQ value (~0.4) in the low- Iphoton region is nearly the same as the value (=0.59) determined under 400 nm monochromatic radiation of 6.6 mW/cm2. The ΦIQ value exponentially decreases with increase in Iphoton beyond 0.8 μJ/cm2. In the BHJ device (closed circles), the ΦIQ value (~0.6) in the low- Iphoton region is close to the value (=0.79) determined under 400 nm monochromatic radiation of 6.6 mW/cm2. The ΦIQ value exponentially decreases with increase in Iphoton beyond 0.2 μJ/cm2.

Figure 1
figure 1

(a) Photocurrent (I) against energy (Iphoton) of femtosecond light pulse at 400 nm in the PTB7/C70 HJ and PTB7/PC71BM BHJ devices. (b) ΦIQ against Iphoton. The magnitudes of ΦIQ were estimated with considering the absorption indexes: 0.61 and 0.84 for HJ and BHJ devices, respectively. The curves are merely the guides to the eyes.

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Carrier formation dynamics in PTB7/C70 heterojunction

We investigated ΔOD spectra of PTB7/C70 bilayer, PC71BM neat and PTB7 neat films at 400 nm (see Supplementary Fig. S3 online). The ΔOD spectra of the PC71BM (PTB7) neat film show a characteristic photoinduced absorption (PIA), which should be ascribed to the acceptor exciton (A*) [donor exciton (D*)] . The ΔOD spectra of the PTB7/C70 bilayer film show a broad PIA centered at 1150 nm. In the late stage (>10 ps), the profile of the PIA is essentially unchanged. In addition, the spectral profile is similar to that of the doping-induced spectrum of the PTB7 neat film (see Supplementary Fig. S4 online). Therefore, we ascribed the PIA to the photo-created donor carriers (D+). In the early state (<10 ps), however, an additional absorption component is observed around 1500 nm. The additional component should be ascribed to the PIAs due to A* and D*. We decomposed the ΔOD spectra (ϕexp) of the PTB7/C70 bilayer film into the components of A* (ϕA*), D* (ϕD*) and D+ (ϕD+). The ΔOD spectra of the PC71BM (PTB7) neat film at 1 ps was regarded as ϕA* (ϕD*) while the ΔOD spectra of the PTB7/C70 bilayer film at 10 ps was regarded as ϕD+. The spectral weights of the respective components were evaluated by least-squares fitting of the observed spectra (ϕexp) with the linear combination of ϕA*, ϕD* and ϕD+ : ϕcal = CA*ϕA* + CD*ϕD* + CD+ϕD+. The coefficients, CA*, CD* and CD+, were determined so that the evaluation function, , becomes the minimum. Figure 2a show an example of the spectral decomposition at 0.9 ps. We clearly observed that the photo-excitation at 400 nm excites D* in addition to A*.

Figure 2
figure 2

(a) ΔOD spectra (open circles) of PTB7/C70 bilayer film at 0.9 ps and at 400 nm together with the spectral decomposition into acceptor exciton (A*: blue curve), donor exciton (D*: green curve) and donor carrier (D+ : red curve) components. The ΔOD spectra of the PC71BM neat, PTB7 neat and PTB7/C70 bilayer films at 1, 1 and 10 ps are regarded as the A*, D* and D+ components, respectively. (b) Absolute number of acceptor exciton (nA*), donor exciton (nD*) and donor carrier (nD+) per an absorbed photon against the delay time. The magnitudes of nA*, nD* and nD+ were evaluated by the spectral decomposition of the ΔOD spectra of the PTB7/C70 bilayer film. The solid curves are results of the least-squares fittings with an exponential function.

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Now, let us evaluate the absolute numbers of the acceptor exciton (nA*), donor exciton (nD*) and donor carrier (nD+) per absorbed photon from the ΔOD spectra of the PTB7/C70 bilayer film. For this purpose, we need the spectral intensities, αA*, αD* and αD+, per unit densities of A*, D* and D+. In order to determine αA* and αD*, we assumed that one absorbed photon creates one A* (D*) in the PC71BM (PTB7) neat film. We determined αA* (αD*) with use of the ΔOD spectra of PC71BM (PTB7) neat film (see Supplementary Fig. S3 online): αA* (αD*) is evaluated to be 0.002 nm2/photons (0.020 nm2/photons) with considering absorption indexes. Then, nA* (nD*) is calculated by (IA*/nphoton)/αA* [(ID*/nphoton)/αD*], where IA* (ID*) and nphoton are the intensity of the (A*) D* component in the PTB7/C70 bilayer film and the absorbed photon number per unit density of the device. The magnitude of αD+ was determined from the electrochemical differential (ΔODEC) spectrum (see Supplementary Fig. S4 online). αD+ is evaluated to be 0.013 nm2/carriers with use of the doped carrier density. Then, nD+ is calculated by (ID+/nphoton)/αD+, where ID+ is the intensity of the D+ component in the PTB7/C70 bilayer film. The magnitudes of IA*, ID* and ID+ were evaluated by the spectral decomposition of the ΔOD spectra into the A*, D* and D+ components, as exemplified in Fig. 2a.

Figure 2b shows the absolute numbers of A* (nA*), D* (nD*) and D+ (nD+) per an absorbed photon against the delay time. The nA* values significantly scatter reflecting the small cross section of A* (αA* = 0.002 nm2/photons). Nevertheless, we observed steep decreases in nA* and nD* with time. The decay times (τdecay) of nA* and nD* are roughly evaluated to be 3.1 and 1.6 ps, respectively. On the other hand, nD+ exponentially increases with the rise time (τrise) of 1.0 ps. The rather slow decay time (τdecay = 3.1 ps) of A* suggests that the late decay component (>1 ps) of A* does not contribute to the carrier formation. Actually, the sum of the initial exciton number, i.e., nA* + nD* (~0.7/photon) is slightly larger than the carrier number (nD+ ~ 0.5/photon). Thus, the exciton-to-carrier conversion process in the PTB7/C70 bilayer film takes place at the D/A interface within ~1 ps. The carrier formation time (=~1 ps) in the PTB7/C70 bilayer film is longer than that (=0.2–0.5 ps) of PTB7/PC71BM blend film10. The longer conversion time is ascribed to be the longer exciton diffusion to the D/A interface in the HJ device than that in the BHJ device.

Pulse energy dependence of ΦCF in PTB7-based OSCs

Next, we investigated pulse energy dependence of the ΔOD spectra at 10 ps of the PTB7/C70 bilayer and PTB7/PC71BM blend films at 400 nm (see Supplementary Fig. S5 online). The magnitude of ΦCF (=αphoton/αD+) can be calculated from the two quantities. αphoton (=ID+/nphoton) is the spectral intensities due to D+ per unit density of absorbed photons. αD+ is reported to be 0.013 nm2/carriers for PTB713. In the PTB7-based OSCs, the carrier formation time is less than 1 ps. Therefore, we evaluated the αD+ values from the ΔOD spectra at 10 ps of the PTB7/C70 bilayer and PTB7/PC71BM blend films with considering the absorption indexes: 0.37 and 0.60 for the bilayer and blend films, respectively. Thus obtained ΦCF values are plotted in Fig. 3 together with ΦIQ against Iphoton. We found that ΦCF (=0.56) is nearly independent of Iphoton in the PTB7/C70 bilayer film even though ΦCF at Iphoton < 2 μJ/cm2 is difficult to determine. This makes a sharp contrast with a steep decrease in ΦIQ with increase in Iphoton beyond 0.8 μW/cm2. A similar behavior of ΦIQ and ΦCF is observed in the PTB7/PC71BM BHJ device. In the high-Iphoton region, most of the carriers recombine and do not contribute to the photocurrent (ΦIQ ≪ 1). We note that the carrier recombination process takes several nanoseconds in both the devices (see Supplementary Fig. S6 online). We tentatively evaluate the decay times (τdecay) by least-squares fittings with exponential functions, i.e., ΔmOD = Aexp(−t/τ): τdecay = 3.4 and 5.8 ns for the HJ and BHJ devices, respectively.

Figure 3
figure 3

ΦCF and ΦIQ of (a) PTB7/C70 HJ and (b) PTB7/PC71BM BHJ devices against energy (Iphoton) of femtosecond light pulse at 400 nm.

The magnitudes of ΦCF were estimated with considering the absorption indexes: 0.37 and 0.60 for the bilayer and blend films, respectively. The error bars of ΦCF were roughly evaluated from the signal/noise ratio of the femtosecond time-resolved spectra. The magnitudes of ΦIQ were estimated with considering the absorption indexes: 0.61 and 0.84 for HJ and BHJ devices, respectively. The broken lines are merely the guide to the eyes.

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Here, we define the instantaneous carrier density (n) per unit area of the D/A interface. In the HJ device, the magnitude of n is the same as nD+ (=nphoton × ΦCF) because the D/A interface area is the same as the device area. In the BHJ device, however, the magnitude of n is smaller than nD+ because the interface area is much wider than the device area. Under the femtosecond pulse excitation, the carriers are instantaneously produced at the D/A interface before they start migration toward the collector electrodes. Actually, the carriers are generated at the D/A interface within ~1 ps. For example, the local carrier density (n) at the interface of the HJ device is ~0.1 carriers/nm2 at 10 μJ/cm2. Under a conventional continuous wave (CW) excitation, however, the carrier migration effectively reduces the n value. For example, the local carrier density at the interface is ~10−2 carriers/nm2 at 100 mW/cm2 if the carrier escape time from the interface region is 1 ns. On the other hand, it becomes ~1 carriers/nm2 at 100 mW/cm2 with the carrier lifetime of ~1 ms, which is comparable to the value (~0.1 carriers/nm2) under the femtosecond pulse excitation at 10 μJ/cm2. These arguments indicate that the significant carrier recombination efficiency observed in the high-Iphoton region should be ascribed to the local dense carriers at the interface. In other words, the carriers at the interface region are amenable to the recombination while the carriers escaped from the region are free from the recombination.

Discussion

Figure 4 shows the carrier density (ncollected) collected as photocurrent against n (=ΦCF × nphoton) in the HJ device. The n values were evaluated with assuming constant ΦCF (=0.56). In the dilute-n region, the photocurrent is proportional to n. In the dense-n region, however, the photocurrent becomes nearly constant. This suggests that there exists a critical carrier density (nc = 0.003 carriers/nm2) above which the carrier recombination becomes dominant. The nc value (=0.003 carriers/nm2) corresponds to the value, in which one carrier exists every 18 × 18 nm2 squares. To quantitatively analyze the carrier recombination kinetics against n, we adopted a phenomenological rate equation: dn/dt = −αnN − βn. The first and second terms represent the carrier recombination and the carrier escape from the interface region. The first term at N = 2 describes the conventional electron—hole recombination process. The ratio between the coefficients, α and β, should be fixed as β/α = ncN−1, because the first and the second terms become comparable at n = nc. We assume that all the escaped carriers contribute the photocurrent, i.e., ncollected = ∫βndt. The solid and broken curves in Fig. 4 are results of the phenomenological model at N = 3 and 2, respectively. We found that the curve at N = 3 excellently reproduces the experiment data. The deviation from the conventional n2-dependence (N = 2) is probably ascribed to the electric double layer effect at the interface, which may cause a many-body interaction between the carriers. Here, we note that the recombination/escape branching ratio is determined by the ratio not by the magnitudes of the coefficients, α and β. The magnitude determines only the time scale of the branching kinetics. That is, we have no adjustable parameter in this phenomenological mode.

Figure 4
figure 4

Carrier density (ncollected) collected as photocurrent against instantaneous carrier density (n) per unit area of the D/A interface in PTB7/C70 HJ device.

The n values were evaluated with assuming constant ΦCF (=0.56). The solid and broken curves are drown by a phenomenological model (see text).

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A similar relation between n and ncollected is observed in the PTB7/PC71BM BHJ device even though we cannot evaluate the magnitude of n. Alternatively, we estimated the critical carrier density (nc’ = 0.002 carriers/nm2) per unit area of the device, above which the carrier recombination becomes dominant. We note that nc (per unit area of the interface) is much smaller than nc’ (per unit area of the device) because the D/A interface area is much wider than that of the device area. That is, nc of the BHJ device is much lower than nc (=0.003 carriers/nm2) of the HJ device. We ascribe the suppressed nc of the BHJ device to the unevenness of the D/A interface due to the to the nano-level mixing of the D and A domains15,16. Hedley et al.15 reported that the domain (100–200 nm) of PTB7/PC71BM blend film consists of small fullerene spheres (20–60 nm) inside the domain. The resultant inhomogeneity of the interface activates only a portion of the interface to dissociate the excitons. Then, the interface inhomogeneity effectively reduces the interface area and increases n (per unit area of the interface). In addition, the inhomogeneity causes the carrier trapping at the interface region. The carrier trapping should enhance the carrier recombination probability. On the other hand, the interface of HJ device, which is prepared by vacuum evaporation process, is considered to be much smoother. Actually, the root-mean-square (RMS) of the PTB7 film is 0.32 nm (see Supplementary Fig. S7 online). Such a smooth interface is free from carrier trapping and is advantageous for the high branching ratio of the carrier escape.

Summary

In summary, we investigated the interrelation between the instantaneous carrier density (n) per D/A interface area and the carrier density (ncollected) collected as photocurrent in PTB7/C70 HJ device. By means of the time-resolved spectroscopy, we confirmed that the exciton-to-carrier conversion process takes place within ~1 ps at the D/A interface of the PTB7/C70 HJ device. We further determined the absolute magnitude of n by combination of the time-resolved and electrochemical spectroscopies. We found that the carrier recombination becomes dominant if n exceeds a critical concentration (nc = 0.003 carriers/nm2). A phenomenological rate equation with carrier recombination term (−αn3) well reproduces the experimental relation between n and ncollected. We confirmed that a similar behavior is observed in the PTB7/PC71BM BHJ devices even though nc is much lower than the value of the HJ device. Our quantitative investigation based on the HJ device demonstrates that the fast carrier escape from the interface region is indispensable for high PCE, because the carrier accumulation nonlinearly accelerates the carrier recombination process.

Method

Fabrication and characterization of the OSCs

PTB7/C70 HJ solar cell was fabricated with a structure of indium tin oxide (ITO)/poly-(3,4-ethylenedioxythiophene) (PEDOT): poly-(styrenesulfonate) (PSS) (40 nm)/PTB7 (18 nm)/C70 (25 nm)/bathocuproine (BCP) (5 nm)/MgAg. The patterned ITO (conductivity: 10 Ω/sq) glass was pre-cleaned in an ultrasonic bath of acetone and ethanol and then treated in an ultraviolet-ozone chamber. A thin layer of PEDOT:PSS (40 nm) was spin-coated onto the ITO and dried at 110 °C for 10 min on a hot plate in air. A neat PTB7 film was spin-coated from an o-dichlorobenzene (o-DCB) solution. PTB7 was purchased from Sigma-Aldrich and used as received. The atomic force microscope (AFM) image of the PTB7 film is flat and consists of small grains less than 100 nm. Then, C70 (25 nm) was deposited by vacuum evaporation. Finally, BCP and MgAg were deposited onto the active layer by conventional thermal evaporation at a chamber pressure lower than 5 × 10−4 Pa, which provided the devices with an active area of 2 × 5 mm2.

PTB7/PC71BM BHJ solar cell was fabricated with a structure of ITO/PEDOT:PSS (40 nm)/active layer (140 nm)/LiF (1 nm)/Al. The patterned ITO glass was pre-cleaned in an ultrasonic bath of acetone and ethanol and then treated in an ultraviolet-ozone chamber. A thin layer of PEDOT:PSS (40 nm) was spin-coated onto the ITO and dried in air at 110 °C for 10 min on a hot plate. The substrate was then transferred to an N2 glove box and dried again at 110 °C for 10 min on a hot plate. An o-DCB/1.8-diiodooctane (DIO) solution of PTB7 : PC71BM with a ratio of 2 : 3 by weight (8 : 12 mg/mL) was subsequently spin-coated onto the PEDOT:PSS surface to form an active layer. Finally, LiF (1 nm) and Al (80 nm) were deposited onto the active layer by conventional thermal evaporation at a chamber pressure lower than 5 × 10−4 Pa. The active area of the OSCs is 2 × 5 mm2.

The J − V curves were measured using a voltage—current source/monitor under AM 1.5 solar-simulated light irradiation of 100 mW/cm2 (Bunkou-keiki, OTENTO-SUN III). The IPCE spectra was measured using a SM-250 system (Bunkou-keiki). The magnitudes of ΦIQ at 400 nm were evaluated from the IPCE spectra with considering the absorption indexes: 0.61 and 0.84 for the HJ and BHJ devices, respectively.

Pulse energy dependence of ΦIQ of the OSCs

The pulse energy dependences of ΦIQ of the OSCs were measured in a N2-filled box. For this experiment, the active area of the OSCs is 2 × 3 mm2. The excitation light pulse at 400 nm was generated as the second harmonics of a regenerative amplified Ti: sapphire laser in a β-BaB2O4 (BBO) crystal. The pulse width and repetition rate were 100 fs and 500 Hz, respectively. The maximum excitation intensity was 27 μJ/cm2. The photocurrents (I) from the OCSs were measured against the excitation pulse energy (Iphoton) per unit area of the device. The magnitudes of ΦIQ were evaluated by I and Iphoton, with considering the absorption indexes: 0.61 and 0.84 for the HJ and BHJ devices, respectively.

Femtosecond time-resolved spectroscopy

The time-resolved spectroscopy was performed in a pump-probe configuration. In order to reduce the irradiation damage, the blend films were placed in N2 atmosphere. The pump pulse at 400 nm was generated as the second harmonics of a regenerative amplified Ti: sapphire laser in a β-BaB2O4 (BBO) crystal. The pulse width, repetition rate and pulse energy were 100 fs, 1000 Hz and 27 μJ/cm2 respectively. The frequency of the pump pulse was decreased by half (500 Hz) to provide “pump-on” and “pump-off” conditions. A white probe pulse, generated by self-phase modulation in a sapphire plate was focused on the sample with the pump pulse. The spot sizes of the pump and probe pulses were 4.0 and 2.1 mm in diameter, respectively. The differential absorption (ΔOD) spectrum is expressed as -log(Ion/Ioff) , where Ion and Ioff are the transmission spectra under the pump-on and pump-off conditions, respectively.

Films prepared on quartz substrates were used in the time-resolved spectroscopies. PTB7/C70 bilayer film was prepared as follows. First, PTB7 neat film (18 nm) was spin-coated on quartz substrate from an o-DCB solution. The spin-coated film was dried in an inert N2 atmosphere. Then, C70 (25 nm) was deposited by vacuum evaporation. PTB7/PC71BM blend film was spin-coated on quartz substrate from a mixed solvent of o-DCB /DIO (97.5 : 2.5 vol %) of PTB7 : PC71BM with a ratio of 2 : 3 by weight (8 : 12 mg/mL). PTB7 neat film was spin-coated on quartz substrate from o-DCB solution. The spin-coated films were dried in an inert N2 atmosphere. The thicknesses of the PTB7 and PTB7/PC71BM blend films were 100 and 140 nm, respectively.

Pulse energy dependence of ΦCF

The magnitude of ΦCF (=αphoton/αD+) can be calculated from the two quantities. αD+ (αphoton) is the spectral intensity due to D+ per unit density of D+ (absorbed photons). αD+ is reported to be 0.013 nm2/carriers for PTB713. In the PTB7-based OSCs, the carrier formation time is less than 1 ps. Therefore, we evaluated the αD+ values from the ΔOD spectra at 10 ps of the PTB7/C70 bilayer and PTB7/PC71BM blend films with considering the absorption indexes: 0.37 and 0.60 for the bilayer and blend films, respectively.

Additional Information

How to cite this article: Moritomo, Y. et al. Carrier density effect on recombination in PTB7-based solar cell. Sci. Rep. 5, 13648; doi: 10.1038/srep13648 (2015).