Investigation of Hole-Transfer Dynamics through Simple EL De-Convolution in Non-Fullerene Organic Solar Cells

Abstract

In conventional fullerene-based organic photovoltaics (OPVs), in which the excited electrons from the donor are transferred to the acceptor, the electron charge transfer state (^eE_CT) that electrons pass through has a great influence on the device’s performance. In a bulk-heterojunction (BHJ) system based on a low bandgap non-fullerene acceptor (NFA), however, a hole charge transfer state (^hE_CT) from the acceptor to the donor has a greater influence on the device’s performance. The accurate determination of ^hE_CT is essential for achieving further enhancement in the performance of non-fullerene organic solar cells. However, the discovery of a method to determine the exact ^hE_CT remains an open challenge. Here, we suggest a simple method to determine the exact ^hE_CT level via deconvolution of the EL spectrum of the BHJ blend (EL_B). To generalize, we have applied our EL_B deconvolution method to nine different BHJ systems consisting of the combination of three donor polymers (PM6, PBDTTPD-HT, PTB7-Th) and three NFAs (Y6, IDIC, IEICO-4F). Under the conditions that (i) absorption of the donor and acceptor are separated sufficiently, and (ii) the onset part of the external quantum efficiency (EQE) is formed solely by the contribution of the acceptor only, EL_B can be deconvoluted into the contribution of the singlet recombination of the acceptor and the radiative recombination via ^hE_CT. Through the deconvolution of EL_B, we have clearly decided which part of the broad EL_B spectrum should be used to apply the Marcus theory. Accurate determination of ^hE_CT is expected to be of great help in fine-tuning the energy level of donor polymers and NFAs by understanding the charge transfer mechanism clearly.

1. Introduction

Organic photovoltaic cells (OPVs) based on the bulk-heterojunction (BHJ) blend of semiconducting polymers and small molecule acceptors are considered the most promising candidate as a future mobile energy source due to their advantages, including light weight, flexibility, and low-cost fabrication process [1,2,3,4,5]. The power conversion efficiency (PCE) of single-junction polymer solar cells has steadily increased and recently exceeded 18%. This was due to the introduction of non-fullerene acceptors (NFAs) [6,7,8]. The major advantage of NFAs is their relatively low energy loss, which can be as low as 0.4–0.5 eV [9,10]. It is generally believed that this energy loss is associated with the small charge transfer offset required to drive charge separation. In other words, charge separation efficiency is directly influenced by charge transfer offsets. Therefore, finding accurate information about the charge transfer (CT) state is essential for improving photovoltaic performance.

In the BHJ system based on a fullerene acceptor (f-BHJ), the electron transfer state (^eE_CT) was determined by Marcus theory, which is based on the mirror image relationship between the photovoltaic external quantum efficiency (EQE_PV) and electroluminescence (EL) emission [11,12]. All the charges that contribute to the onset part of the EQE_PV are initially excited at the donor site, and then electrons are transferred to the acceptor through the ^eE_CT. Thus, if the EQE_PV is measured with a very sensitive tool such as Fourier transform photocurrent spectroscopy EQE (FTPS-EQE), it is expected that the CT-bands are visible in the low-energy part of the EQE_PV (see Figure 1a) [13,14]. In this framework, ^eE_CT was extracted by fitting the CT-band from the EQE_PV and projecting it on the EL spectrum (Figure S1 and Table S1). In the BHJ system based on a low bandgap non-fullerene acceptor (nf-BHJ), however, the electron density excited in the acceptor itself is already high, and the low-energy onset part of EQE_PV is also formed only by the contribution of the acceptor. Therefore, ^eE_CT created near the lowest unoccupied molecular orbital (LUMO) levels of donor and acceptor has little influence on the device performance, and there is no CT-band feature in the EQE_PV onset (see Figure 1b) [13]. Rather, in this non-fullerene acceptor system, the energy offset associated with the hole transfer occurring on the highest occupied molecular orbital (HOMO) side of the donor and acceptor is more important. Accordingly, many previous studies to date have discussed the hole transfer on the HOMO side rather than electron transfer on the LUMO side [15,16,17,18,19]. Of course, there may be a change in photocurrent due to the influence of ^eE_CT in operation under the saturation mode, but the influence of ^eE_CT is negligible because there are no electrons transferring from the donor in the V_OC formation near the EQE onset we consider.

A study by J. Zhang et al. [20] showed that the minimum HOMO offset needed to achieve the most efficient exciton dissociation and photovoltaic performance was ~40 meV, and the method for relative comparison of HOMO offset energy was demonstrated by Y. Xie et al. [21]. However, the former was just an estimate of the minimum value obtained through a simple comparison of the HOMO level difference and photovoltaic performance [20], and the latter was a relative comparison of estimated values obtained by determining EQE_EL difference [21]. Thus, the discovery of a method to determine hole transport energy offsets by accurately measuring the E_CT on the HOMO side (^hE_CT) remains an open challenge. In recent research by J. Wu et al., it was pointed out that the key factor of the high efficiency of the PM6:Y6 system was because of the low energetic disorder due to the low energy offset [22]. Therefore, the accurate determination of ^hE_CT is essential for achieving further enhancements in the non-fullerene organic solar cell since it can be a guideline for the design of a higher-performing novel material.

In this work, we suggest a simple method to determine the exact ^hE_CT level via deconvolution of the EL spectrum of the BHJ blend (EL_B). In general, the onset part of the EQE of the BHJ blend with NFA is formed solely by the contribution of the acceptor itself. In an EL measurement, electrons are injected directly into the acceptor LUMO, and holes are initially injected into the HOMO of the donor and then transferred to the acceptor through ^hE_CT. Thus, EL_B has constraints on both the singlet recombination of the acceptor and the radiative recombination via ^hE_CT. Therefore, by deconvolution of EL_B, it is possible to extract out the part of CT-band contribution. Through this, we can clearly decide which part of the broad EL_B spectrum should be used to apply the Marcus theory. To generalize, we have applied our EL_B deconvolution method to nine different BHJ systems consisting of the combination of three donor polymers (PM6, PBDTTPD-HT, PTB7-Th) and three NFAs (Y6, IDIC, IEICO-4F).

2. Materials and Methods

2.1. Device Fabrication

All organic solar cell devices were fabricated with an inverted structure of ITO/ZnO/BHJ/MoO₃/Ag. Patterned indium tin oxide (ITO) on glass was was purchased from AMG (Seoul, Republic of Korea). It was cleaned in an ultrasonic bath of detergent, acetone (99.9%), and isopropyl alcohol (IPA, 99.9%) for 10 min each and UV-ozone treated for 60 min. A 0.75 M ZnO sol-gel was prepared with 3.28 g of zinc acetate dihydrate (Zn(CH₃COO)₂∙2H₂O, 98%), 1 mL of ethanolamine (NH₂CH₂CH₂OH, 99.5%), and 20 mL of 2-methoxyethanol (CH₃OCH₂CH₂OH, 99.9%). The ZnO sol-gel was spin-coated on ITO glass at 5000 rpm for 30 s under ambient conditions and thermally annealed at 200 °C. The employed BHJs are as follows: PM6: Y6 (D:A = 1:1.2, 16.5 mg/mL in chloroform (CF, 99%) with 0.5 v/v% of chloronaphthalene (CN, ≥90%) additive), PM6: IDIC (D:A = 1:1, 18 mg/mL in CF with 0.5 v/v% 1,8-diiodooctane (DIO, 98%) additive), PM6: IEICO-4F (D:A = 1:1.5, 18 mg/mL in CF with 0.5 v/v% of CN additive), PBDTTPD-HT: Y6 (D:A = 1:1.4, 20 mg/mL in CF with 0.5 v/v% of DIO additive), PBDTTPD-HT: IDIC (D:A = 1:1.4, 18 mg/mL in CF with 0.8 v/v% of DIO additive), PBDTTPD-HT: IEICO-4F (D:A = 1:1.5, 20 mg/mL in CF with 0.5 v/v% of CN additive), PTB7-Th: Y6 (D:A = 1:1.5, 20 mg/mL in CF with 0.5 v/v% of CN additive), PTB7-Th: IDIC (D:A = 1:1.5, 20 mg/mL in CF with 0.5 v/v% of DIO additive), and PTB7-Th: IEICO-4F (D:A = 1:1.5, 20 mg/mL in CF with 3 v/v% of CN additive).The BHJs were spin-coated at 5000 rpm for 30 s on the ZnO layer and moved to a vacuum chamber. The hole-transport layer (MoO₃, 99.9%, 5.5 nm) and metal electrode (Ag, 99.99%, 100 nm) were deposited via thermal evaporation under a vacuum of 2 × 10⁻⁶ Torr. All materials except donor and acceptor materials were purchased from Sigma Aldrich (St. Louis, MO, USA). Detailed information on PM6, PBDTTPD-HT, and Y6 can be found in our previous works [23,24]. PTB7-Th, IDIC, and IEICO-4F were procured from 1-Material Inc. (Dorval, Quebec, Canada). The device area was 0.13 cm².

2.2. Device Characterization

The power conversion efficiencies of the devices were measured by current density-voltage (J-V) curves using a Keithley 2401 (Keithley instruments, Cleveland, OH, USA) source measurement unit under AM 1.5 G (100 mW/cm²) illumination from a solar simulator (McScience, Suwon, Republic of Korea). The simulated light intensity was calibrated using a standard Si solar cell. External quantum efficiency (EQE) was measured using a solar cell spectral response/QE/IPCE (IQE-200B, Newport Co., Irvine, CA, USA). The light intensity at each wavelength was calibrated using a standard Si solar cell.

The absorption spectra of each BHJ layer were measured by using a UV-Vis spectrophotometer (Cary 5000, Agilent Technologies Inc., Santa Clara, CA, USA). Electroluminescence (EL) signals passed through the integral sphere and were recorded by using a high-sensitivity spectrophotometer (MAYA 2000 Pro, Ocean Insight Inc., Orlando, FL, USA). The wavelength of the PL excitation source was 632 nm. The integral sphere and spectrophotometer were calibrated to collect the radiated photon flux from the device by using a standard halogen calibration light source (HL-3-plus-INT-CAL, Ocean Insight Inc., FL, USA).

Fourier transform photocurrent spectroscopy (FTPS) was used to perform measurements in an FTPS setup built in-house, which consisted of a Fourier-transform infrared spectrometer (INVENIO-R, Bruker Co., Billerica, MA, USA) equipped with quartz beam splitter. The photocurrent produced by the solar cell under illumination was amplified using a low-noise preamplifier (SR570, Stanford Research Systems, Sunnyvale, CA, USA) and fed back into the external detector port of the FTIR.

The steady-state photoluminescence (PL) spectra were obtained using a multichannel spectrophotometer (QEPro, Ocean Insight Inc., FL, USA). In PL measurements, acceptors and BHJs were excited from active layer side using a continuous wave diode laser (Nd-YAG tuned at 532 nm). To calculate the hole-transfer yields, PL spectra were divided by the absorption intensity at the 532 nm. Femtosecond transient absorption (TA) spectra were obtained using a homemade TA measurement system comprising a femtosecond Ti:sapphire regenerative amplifier system (Hurricane, Spectra-Physics Inc., Milpitas, CA, USA) with a 1 kHz repetition rate, an optical parametric amplifier (OPA-800CF, Spectra-Physics Inc., CA, USA), and multichannel spectrometers (QEPro and NIRQUEST, Ocean Insight Inc., FL, USA). The pump pulse wavelength was 630 nm with a power density of ~1 µJ/cm².

3. Result and Discussion

3.1. Y6 Acceptor Based nf-BHJ Systems

3.1.1. Background of ^hE_CT Determination

We first analyzed the BHJ systems based on the representative non-fullerene acceptor Y6 (Figure 2a) with three donor polymers: PM6, PBDTTPD-HT, and PTB7-Th (Figure S2). Figure 2b shows the current density (J)–voltage (V) characteristics of OPVs. The parameters related to the performance of these OPVs are listed in Table S2. The acceptor Y6 has a lower bandgap and higher absorption coefficient than those of the three donors we used, as shown in Figure 2c. Thus, in the low-energy onset part of EQE spectra, there is no photocurrent contribution from the donor. All the contributions originated from Y6 only (Figure S4), which means that all photoexcited electrons are created in the acceptor and then extracted to the cathode directly. In the case of holes, they would be transferred to the donor through the ^hE_CT level and then extracted to the anode electrode (Figure 2d). Therefore, as similar to the FTPS-EQE onset of the f-BHJ system in which electrons generated from the donor are transferred to the acceptor has the information ^eE_CT, the FTPS-EQE onset of nf-BHJ system will have information on ^hE_CT. In the case of the f-BHJ system, since the ^eE_CT bands are quite visible in the low-energy part of the FTPS-EQE spectrum, after determining the reorganization energy (λ) by fitting the ^eE_CT signal of the EQE, the fitting can be applied to the EL spectrum (Figure S1). However, in the case of the nf-BHJ systems, since there is no CT band feature in FTPS-EQE onset, we designed a way to secure the information of the ^hE_CT band from the EL spectrum and reflect it to FTPS-EQE.

Under the EL measurement mode, the electrons are directly injected into the LUMO of the acceptor, while the holes are injected into the HOMO of the donor and transferred to the acceptor through ^hE_CT. Thus, the hole injection can be hindered because the HOMO offset acts as a potential barrier. In EL_B, of course, the EL emission from the singlet of the acceptor will be dominant. However, since there is the possibility of radiative recombination between electrons at LUMO of acceptor and holes at the ^hE_CT, both radiative charge recombination from singlet excitons and ^hE_CT states are superimposed in the EL_B spectrum. Our first goal is to extract the contribution of the ^hE_CT in EL_B to apply the Marcus theory from EL to FTPS-EQE. To achieve this, the exact position of ^hE_CT emission should be addressed. To estimate the underlying contribution from ^hE_CT, we have simply subtracted the acceptor EL (EL_A) spectrum from EL_B. Figure 2e–g shows the difference in the EL spectrum obtained from PM6:Y6, PBDTTPD-HT:Y6, and PTB7-Th:Y6, respectively. The difference in the EL spectrum (shaded blue area) represents the EL contribution from ^hE_CT.

3.1.2. EL Deconvolution

To determine the exact peak position, Gaussian fittings for EL_B were performed based on the information about the components obtained from EL_B-EL_A. In the case of PM6:Y6 EL_B (Figure 2h), the spectrum was deconvoluted into three components. The peak in the center (1.372 eV) is obviously the singlet radiative recombination between the LUMO and HOMO of the acceptor. The peaks on both the low- and high-energy side are caused by the effective ^hE_CT band. It is likely that ^hE_CT does not exist at a finite level. It has a broad distribution near the acceptor’s HOMO. The ^hE_CT levels near the acceptor’s HOMO are like a degenerate state, so it will be indistinguishable. Therefore, the ^hE_CT band can be divided into three regions (Figure 2d). What we are interested in is the energy range that is lower than the acceptor’s HOMO (higher effective ^hE_CT), which affects energy loss. The ^hE_CT levels in the energy region that is higher than the acceptor’s HOMO (lower effective ^hE_CT) will be excluded from the analysis because this part has no effect on the energy loss of the solar cells. Moreover, there is some overlap with the tail of the donor’s EL (EL_D). The results of the same Gaussian fittings for PBDTTPD-HT and PTB7-Th are shown in Figure 2i,j, respectively. In the case of the PBDTTPD-HT:Y6 blend, EL spectrum features are similar to the PM6:Y6 blend due to the similar energy band configurations. However, in the case of the PTB7-Th:Y6 system, it was determined that the contribution of singlet excitons was lower than that of ^hE_CT. Simply put, this means that the energy loss caused by ^hE_CT is greater in the PTB7-Th:Y6 blend.

3.1.3. Marcus Theory and Energy Loss Model

Now, we can apply the Marcus theory (Appendix A) to determine the exact Δ^hE_CT (=E_g^PV − ^hE_CT), where E_g^PV is the photovoltaic gap determined from the derivatives of EQE_PV [25]. Based on the ^hE_CT position obtained through EL_B deconvolution, it required as much reorganization energy (λ) to match the onset part of FTPS-EQE (Figure 3). The Δ^hE_CT values obtained in this analysis were 0.030 eV for PM6:Y6, 0.058 eV for PBDTTPD-HT:Y6, and 0.044 eV for PTB7-Th:Y6. In many previous studies, E_CT determination was performed using the maximum peak of EL_B [26,27,28]. However, as confirmed in the previous discussion, the maximum peak region of the EL_B contains a lot of the singlet contribution of the acceptor. Although a Gaussian fit can be matched to the FTPS-EQE spectrum, even if the CT state is analyzed based on the maximum of EL_B, the extracted CT state value was nearly the same as or even larger than the bandgap of the Y6 acceptor (see Figure S5 and Table S3). Furthermore, many researchers simply considered E_CT to be the same as qV_OC in the Shockley–Queisser (SQ) limit (qV_OC^SQ). However, these two values have different theoretical backgrounds [29,30,31,32]. In our study, we compared the qV_OC^SQ values obtained through energy loss analysis with ^hE_CT (Figure S6). The results clearly showed that ^hE_CT and qV_OC^SQ have different values (Table S4). The results of the total energy loss analysis indicate that PM6:Y6 forms a large V_OC compared to PBDTTPD-HT:Y6, which has similar energy band configuration due to the low Δ^hE_CT, radiative energy loss (ΔE₂ = qV_OC^SQ − qV_OC^rad, where qV_OC^rad is qV_OC in the radiative limit), and non-radiative energy loss (ΔE₃ = qV_OC^rad − qV_OC^nonrad, where qV_OC^nonrad is qV_OC in the non-radiative limit; details are presented in Appendix B). In the case of PTB7-Th:Y6, although hole transfer is slightly better than the PBDTTPD-HT:Y6 due to the lower Δ^hE_CT, PTB7-Th:Y6 has a large V_OC loss due to relatively large values of ΔE₂ and ΔE₃.

In general, two methods are mainly utilized to determine ΔE₃. When qV_OC^SQ and qV_OC^rad are derived theoretically based on measured J_SC and FTPS-EQE, ΔE₃ is determined by simply subtracting the qV_OC^rad from the V_OC of the solar cells. Alternatively, ΔE₃ can also be calculated using the theoretical approach given by ΔE₃ = −k_BTln(EQE_EL), where k_B is the Boltzmann constant, T is temperature, and EQE_EL is the external radiative efficiency. Considering that the EL_B of the nf-BHJ system consists of EL from the acceptor’s single recombination and EL from ^hE_CT, once EQE_EL obtained based on EL_B is used, both the contribution of the acceptor and the contribution of ^hE_CT are already included in the ΔE₃ calculation. The fact that the non-radiative voltage loss can be further split into these two contributions was already recognized and reported in previous studies [20]. However, the problem is to accurately extract the contribution of the acceptor from only EL_B. It is expected to be very difficult to extract only the contribution of acceptors in a general situation where EL_A and ^hE_CT bands overlap each other. In particular, since there is also a degeneracy part between the ^hE_CT band and acceptor HOMO, it is currently impossible to completely separate the two components. Therefore, to obtain the exact Δ^hE_CT, the application of Marcus theory through our EL deconvolution is the most accurate method so far. The effect of ^hE_CT on energy loss is already included in recombination loss (ΔE₃) if EL_B is used to extract EQE_EL. On the other hand, the effect of ^eE_CT on energy loss is mainly reflected in ΔE₁. ΔE₁ should be selected with the larger of ^eE_CT and qV_OC^SQ (see Figure 1a). Of course, since there is a possibility of recombination between an electron at the ^eE_CT level and a hole at the HOMO of the donor, the effect of ^eE_CT is also included in ΔE₃ if EL_B is used to extract EQE_EL.

3.2. IDIC-Acceptor-Based nf-BHJ Systems (Larger HOMO Offset)

Next, we applied the EL deconvolution method to another NFA system, a BHJ blend with IDIC. The chemical structure and J-V characteristics of the IDIC system are shown in Figure 4a,b. The HOMO level of IDIC is similar to that of Y6, but it has a larger bandgap. Therefore, the absorption of IDIC largely overlaps with the donor polymers (Figure 4c). In particular, the absorption of IDIC completely overlaps with PTB7-Th absorption. These characteristics are directly reflected in the composition of EQE (Figure S7). For PM6:IDIC and PBDTTPD-HT:IDIC, though, the onset of EQE_PV is based on the photo-carriers created by the absorption of the acceptors. In the case of the PTB7-Th:IDIC, both donor and acceptor appear to be involved. Figure 4d–f shows the difference between EL_B and EL_A. The peculiar point is that the contribution of the effective ^hE_CT was symmetric with respect to the HOMO of the acceptor in the case of Y6, whereas the IDIC system has an asymmetric shape where the lower-energy side is dominant. Thus, the effective ^hE_CT region is formed only on the upper side of the CT-band. In the case of PM6:IDIC and PBDTTPD-HT:IDIC, the maximum peak of EL_B was the same as that of EL_A, but in the case of PTB7-Th:IDIC, the maximum part of EL_B was completely different from that of EL_A. It can be seen that the CT-band contribution is much larger in PTB7-Th:IDIC only by a simple comparison of these EL deconvolutions. With this simple EL deconvolution analysis, it can be simply confirmed that the ^hE_CT band contribution in PTB7-Th:IDIC is much larger compared to PM6:IDIC and PBDTTPD-HT:IDIC. Exact Δ^hE_CT obtained through accurate Gaussian fitting (Figure 4g–i) and Marcus theory (Figure 4j–l) for PM6:IDIC, PBDTTPD-HT:IDIC are shown in

Table 1. Extracted ^hE_CT values of each BHJ system obtained using EL_B deconvolution method.

BHJ	Eg	hECT	ΔhECT
PM6: Y6	1.418	1.388	0.030
PBDTTPD-HT: Y6	1.431	1.373	0.058
PTB7-Th: Y6	1.401	1.357	0.044
PM6: IDIC	1.601	1.570	0.031
PBDTTPD-HT: IDIC	1.573	1.527	0.046
PTB7-Th: IDIC	1.611	1.566	0.045
PM6: IEICO-4F	1.323	n/a	n/a
PBDTTPD-HT: IEICO-4F	1.333	n/a	n/a
PTB7-Th: IEICO-4F	1.330	1.358	n/a (−0.028)

Unit: eV.

. The overall energy loss analysis results for the IDIC system are summarized in Figure S8 and Table S5. Note that it is impossible to distinguish whether it is Δ^hE_CT or Δ^eE_CT only with the current method in the case of PTB7-Th:IDIC blend. Of course, this discernment is also not possible with the conventional method (Gaussian fitting of FTPS-EQE and then matching to the maximum of EL_B). The conventional method also gave unacceptable values (Figure S9 and Table S6).

3.3. IEICO-4F-Acceptor-Based BHJ Systems (Smaller HOMO Offset)

What if the acceptor’s HOMO is lower than the donor’s HOMO while having a low bandgap? We selected IEICO-4F as a material that satisfies these conditions (Supplementary Figure S3). The chemical structures and J-V characteristics of the IEICO-4F system are shown in Figure 5a,b. Since IEICO-4F has a lower bandgap than Y6, there is no overlap with that of absorption of the donor like Y6 (Figure 5c), and the EL_B and EL_D are also separated. In the IEICO-4F system, the onset of EQE is formed by only the contribution from the acceptor. Figure 5d–f shows the difference between EL_B and EL_A. In the case of IEICO-4F, unlike IDIC, the difference between EL_B and EL_A does not appear at low energy but is formed in a higher-energy region (Figure 5d–f). The low-energy part of EL_B exactly overlaps with the EL_A. Thus, the effective ^hE_CT region is formed only on the lower side of the CT band. The EL_B is perfectly simulated with the EL_A spectrum and one additional Gaussian (Figure 5g–i). The absence of the low-energy CT-band leads to a complete overlap of EQE fitting and EL fitting in Marcus theory analysis for PM6:IEICO-4F and PBDTTPD-HT:IEICO-4F, as shown in Figure 5j,k. The lower effective ^hE_CT than the HOMO of the acceptor indicates an insufficient HOMO offset for charge separation. The low J_SC in PM6:IEICO-4F and PBDTTPD-HT:IEICO-4F are attributed to this charge-separation problem. Only PTB7-Th:IEICO-4F has a proper condition for ^hE_CT formation. As a result, PTB7-Th:IEICO-4F shows the best photovoltaic performance. In fact, the values of J_SC are quite predictable by simple comparison of the lower effective ^hE_CT size. For PM6:IEICO-4F, which has a relatively large size of lower effective ^hE_CT, the smallest J_SC was shown, and in PTB7-Th:IEICO-4F, which has a relatively small size of lower effective ^hE_CT, the largest J_SC was formed. Sufficient hole transfer seems to be made through the degeneracy region. It is possible that in nf-BHJ, a certain level of hole offset may not be a prerequisite. Lastly, the overall energy loss composition of PTB7-Th:IEICO-4F was similar to that of PM6:Y6. Nevertheless, the lower photovoltaic performance of PTB7-Th:IEICO-4F relative to that of PM6:Y6 is attributed to the fact that the absorption coefficient of IEICO-4F itself is lower than that of Y6.

3.4. Hole Decay and Transfer Time at ^hE_CT

The effect of the configuration change of the hole-transfer state on the hole-transfer time was investigated through transient absorption (TA) measurement. Figure 6 shows the decay profiling of the hole-transfer time obtained from the TA measurements for acceptors and BHJ films used in this study. Detailed values of the hole-transfer time and PL-quenching efficiency are listed in

Table 2. Hole-transfer time and PL-quenching efficiency of BHJ used in this study.

Blend	Hole-Transfer Time (ps)	${\mathit{\eta }}_{\mathbf{PL}}$* (%)
PM6:Y6	2.8 ± 0.3	93
PBDTTPD-HT:Y6	5.1 ± 0.5	81
PTB7-Th:Y6	7.0 ± 0.4	78
PM6:IDIC	2.4 ± 0.7	75
PBDTTPD-HT:IDIC	1.8 ± 0.4	81
PTB7-Th:IDIC	6.0 ± 0.4	54
PM6:IEICO-4F	13.0 ± 1.7	72
PBDTTPD-HT:IEICO-4F	7.8 ± 0.7	83
PTB7-Th:IEICO-4F	5.2 ± 0.8	96

*${\eta }_{\mathrm{PL}}$: PL-quenching efficiency.

. In the case of the BHJ system with a Y6 acceptor, the measured hole-transfer time was much faster than the TA decay time of the Y6 film (~211 ps). The hole-transfer time was the fastest in PM6:Y6 (~2.8 ps), became slower in PBDTTPD-HT:Y6 (~5.1 ps), and showed the lowest value in PTB7-Th:Y6 (~7.0 ps). This order of hole-transfer time is well correlated with the order of the ^hE_CT area size obtained in EL deconvolution (see Figure 2h–j). The fast hole-transfer property delivers a lower recombination rate. Thus, the PL-quenching efficiency was the highest in PM6:Y6 (93%), and it was the lowest in PTB7-Th:Y6 (78%), which has the slowest hole-transfer rate (Figure S18). In the BHJ system with the IDIC acceptor (Figure 6e–h), the PBDTTPD-HT:IDIC exhibited the fastest hole-transfer time of ~1.8 ps, even faster than PM6:Y6. This is probably because the upper ^hE_CT region acting as an obstruction region of hole transfer is smaller. However, the overall PCE was lower than that of Y6-based devices. This is because of the decreased J_SC due to the large bandgap characteristics of IDIC. The longest hole-transfer time and the lowest PL-quenching efficiency were observed in PTB7-Th:IDIC, with the largest upper ^hE_CT region. For the BHJ systems with IEICO-4F, a prolonged hole-transfer time of ~13.0 ps was observed for PM6:IEICO-4F, and even the hole-transfer time of pristine IEICO-4F was remarkably faster (~71 ps) than other acceptors (Figure 6i). This is because of the relatively large upper ^hE_CT and the small degeneracy region, as predicted by the EL deconvolution result. Since most of the hole-transfer levels were created on the upper ^hE_CT region that causes hole-transfer retardation, the hole-transfer characteristics were significantly degraded compared to other acceptor systems. However, PTB7-Th: IEICO-4F, with a relatively small upper ^hE_CT region, showed a fast hole-transfer time and high hole-quenching efficiency. Thereby, PTB7-Th: IEICO-4F exhibited relatively higher performance within BHJ systems based on the IEICO-4F acceptor.

4. Conclusions

In summary, we have presented a more accurate analysis method that can obtain the exact value of Δ^hE_CT in NFA-based OPV by extracting information on ^hE_CT through EL_B deconvolution. We have applied our EL_B deconvolution method on nine BHJ systems consisting of a combination of three donor polymers (PM6, PBDTTPD-HT, PTB7-Th) and three NFAs (Y6, IDIC, IEICO-4F). To extract the exact Δ^hE_CT, the following conditions must be satisfied to apply our EL_B deconvolution method. (i) Absorption of the donor and acceptor must be separated sufficiently. Accordingly, (ii) the composition of EQE should be clearly divided into donor and acceptor contribution parts. In particular, the initial EQE_PV onset of an OPV with NFA should be formed only by the contribution of the acceptor. If these conditions are satisfied, through EL_B deconvolution, it is possible to specify exactly which part of the broad EL_B should be used to apply the Marcus theory, and based on the information on ^hE_CT, we were able to obtain a fairly accurate Δ^hE_CT. Accurate determination of ^hE_CT is very important to understand the overall charge dynamics of NFAs-based OPVs. Since improved performance and stability are required for the commercialization of OPVs, it is expected that our method can be implemented for the development of better photoactive layer materials with improved performance in the future.