# Photoelectron Yield Spectroscopy and Transient Photocurrent Analysis for Triphenylamine-Based Photorefractive Polymer Composites

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## Abstract

**:**

_{p}) and the width of the density of states (DOS) were evaluated using photoelectron yield spectroscopy, and the transient photocurrent was analyzed using a two-trap model. The estimated hole mobility was also rationalized using a Bässler formalism together with the energetic disorder of the width of the DOS and the positional disorder of the scattering situation for carrier hopping.

## 1. Introduction

^{−3}to 10

^{−2}cm

^{2}V

^{−1}s

^{−1}for PTAA due to high hole mobility, the PTAA-based PR polymer has a response time of the order of hundreds of microseconds with a high optical diffraction efficiency of over 50% and a low trap density of the order of 10

^{14}cm

^{−3}and, thus, a very low space-charge field of less than 1 V μm

^{−1}. Based on the theory of the formation and diminishing of space-charge gratings in photoconductive polymers [12], a two-trap model with shallow and deep traps has been developed [13]. The space-charge dynamics for poly(N-vinylcarbazole) (PVK)-based PR polymers [13] and the photocurrent dynamics for poly(phenylene vinylene)-based PR polymers have also been investigated using a two-trap model [14]. Quantitative analysis of the transient photocurrent has been carried out for poly(4-diphenylamino) styrene (PDAS)-based PR polymers using a two-trap model [15]. Transient photocurrents were reported for poly(4-(diphenylamino)benzyl acrylate) (PDAA)-based PR polymers [16,17]. In particular, a significant difference in the photocurrent of two orders of magnitude has been reported for PDAA PR composites with different photoconductive plasticizers, (4-(diphenylamino)phenyl)methanol (TPAOH) and (2,4,6-trimethylphenyl)diphenylamine (TAA) [17]. However, a detailed analysis of the photocurrent in PDAA PR composites has not yet been performed.

## 2. Materials and Methods

^{−2}at an applied electric field of 40 Vμm

^{−1}. Laser source is iFLEX2000, QIOPTIQ. The photocurrent signal was monitored under illumination at 1 s followed by under unillumination at 1 s. Then, repeated measurements of under illumination at 1 s and under unillumination at 1 s were performed 4 times. Total repeated measurements were 5 times.

## 3. Results and Discussion

#### 3.1. Photoelectron Yield Spectroscopy and Energy Diagram

^{−1}. To determine the significant difference in the photocurrent between PDAA/TPAOH/7-DCST/PCBM and PDAA/TAA/7-DCST/PCBM, the role of TPAOH and TAA should be clarified in photorefractive composites. The molecular structures of TPAOH and TAA are shown in Figure 1. The main framework of the molecular structure of TPAOH is almost the same as that of the PDAA monomer. However, TAA has three bulky methyl moieties attached to one phenyl group. Ionization potentials for donors of PDAA, TPAOH, and TAA of 5.69 eV, 5.64 eV, and 5.90 eV, respectively, have been reported previously [8,16]. These ionization potentials (I

_{p}) correspond to the highest occupied molecular orbital (HOMO) level, and, thus, the negative numeral of I

_{p}is a HOMO level for these donors. However, these numerals do not directly tell us how the ionization state (HOMO level state) is formed in the composites. Thus, we need to know the difference in the ionization state (HOMO level state) between PDAA in the presence of TPAOH and in the presence of TAA.

^{1/3}linearly increases above the threshold. The ionization potential and the HOMO level are determined from the threshold at which the photoelectron yield

^{1/3}linearly increases from the baseline. The photoelectron yield

^{1/3}is increased by 5.78 eV (averaged) for PDAA/TPAOH and 5.79 eV (averaged) for PDAA/TAA with increasing photon energy. These numerals are identified with the ionization potential (HOMO level) of the PDAA in the presence of TPAOH and in the presence of TAA. The ionization potential of PDAA/TPAOH is close to that of PDAA/TAA.

^{1/3}is increased by 5.80 eV (averaged) for PDAA/TPAOH/7-DCST/PCBM and by 5.72 eV (averaged) for PDAA/TAA/7-DCST/PCBM with increasing photon energy. Thus, the ionization potential (HOMO level) of PDAA/TPAOH/7-DCST/PCBM is 5.80 eV (−5.80 eV), and that of PDAA/TAA/7-DCST/PCBM is 5.72 eV (−5.72 eV). It is noted that the HOMO level of −5.72 eV for PDAA/TAA/7-DCST/PCBM is higher than that of −5.80 eV for PDAA/TPAOH/7-DCST/PCBM.

#### 3.2. Evaluation of the Transient Photocurrent Determined Using Two-Trap Model

_{ph}is the current density; e is the elementary charge; μ is the mobility of the charge carriers; ρ is the charge carrier density; E is the electric field; D is the diffusion coefficient; ε

_{0}is the dielectric permittivity in space; ε

_{r}is the dielectric constant; N

_{A}, T, and M are the total density of sensitizers, shallow traps, and deep traps, respectively; N

_{A}

^{−}, T

^{+}, and M

^{+}are the density of sensitizer anions, filled shallow traps, and filled deep traps, respectively; s is the photogeneration cross-section; γ

_{T}is the shallow trapping rate; γ

_{M}is the deep trapping rate; γ

_{R}is the recombination rate; β

_{T}is the detrapping rate from the shallow traps; β

_{M}is the detrapping rate from the deep traps; and I is the intensity of the light illumination. The photogeneration cross-section s is given by s = ϕαλ⁄(hcN

_{A}), where ϕ is the quantum efficiency (QE) for photocarrier generation, α is the absorption coefficient, λ is the wavelength of the light, h is the Planck constant, and c is the speed of light.

_{T}, the density of shallow trap T, the detrapping rate from the shallow traps β

_{T}, the deep trapping rate γ

_{M}, the density of deep trap M, the detrapping rate from the deep traps β

_{M}, and the recombination coefficient γ

_{R}. Even though the trapping parameters for the shallow and deep traps are almost fixed, a wide range of QE, hole mobility μ, and recombination coefficient γ

_{R}can be reproduced in the measured transient photocurrent as listed in Table A1 in Appendix A. Thus, QE or hole mobility μ should be first determined by other data. The recombination coefficient γ

_{R}is proportional to hole mobility with the Langevin recombination process.

^{16}cm

^{−3}for the shallow trap and 2.0 × 10

^{16}cm

^{−3}for the deep trap for PDAA/TPAOH/7-DCST/PCBM, and 1.1 × 10

^{16}cm

^{−3}for the shallow trap for PDAA/TAA/7-DCST/PCBM are reasonably evaluated. The total density of traps is reasonably comparable to the photorefractive number density of traps determined using the Kukhtarev model [18], 1.4–3.1 × 10

^{16}cm

^{−3}, reported previously for PDAA PR composites [16]. Therefore, QE is determined, and a reasonable trap density is used; we can reproduce the measured photocurrent with proper hole mobility. QE is determined from the photorefractive response time as follows. The difference in the photorefractive performance of PDAA/TAA/7-DCST/PCBM and PDAA/TPAOH/7-DCST/PCBM in terms of the optical diffraction and the response time are discussed from the aspect of trapping behavior. The photorefractive quantities of diffraction efficiency and response time reported previously [17] are summarized in Table 1. The optical diffraction efficiency for both polymers is comparable, but the response time τ of 8 ms for PDAA/TPAOH/7-DCST/PCBM is faster than that of 67 ms for PDAA/TAA/7-DCST/PCBM.

_{i}is the initial trap density in Schildkraut’s trapping model [12]. Thus, we can estimate the quantum efficiency for the photogeneration of charge carriers ϕ using Equation (7) with the observed response time and the trap density. The QE for carrier photogeneration estimated using Equation (7) with the density of the shallow traps and the response time is given in Table 1. The QE for PDAA/TPAOH/7-DCST/PCBM and PDAA/TAA/7-DCST/PCBM is determined to be 4.3 × 10

^{−3}and 7.0 × 10

^{−4}, respectively.

_{T}should be a little lower or comparable to the trapping rate of the shallow trap (the product of γ

_{T}and T), and the detrapping rate from the deep trap β

_{M}should be much lower than the trapping rate of the deep trap (the product of γ

_{M}and M). Conversely, the transient photocurrent for PDAA/TAA/7-DCST/PCBM is solely governed by the shallow trapping event, and the deep trapping effect should be negligibly small.

_{A}

^{−}, T

^{+}, M

^{+}, and the photocurrent and decaying photocurrent after blocking illumination for PDAA/TPAOH/7-DCST/PCBM is plotted as a function of the logarithmic timescale in Figure 5a,b, respectively. The same type of simulation is shown in Figure 6a,b for PDAA/TAA/7-DCST/PCBM.

#### 3.3. Analysis of the Trapping Behavior and Transient Density for Filled Traps

^{16}cm

^{−3}, a shallow trapping rate γ

_{T}of 1.6–1.7 × 10

^{−13}cm

^{3}s

^{−1}, and a recombination coefficient γ

_{R}of 1.4–2.7 × 10

^{−13}cm

^{3}s

^{−1}. The total density of the deep traps M is 2.0 × 10

^{16}cm

^{−3}for PDAA/TPAOH/7-DCST/PCBM. The total density of the traps is comparable to the photorefractive number density of traps determined using the Kukhtarev model [18], 1.4–3.1 × 10

^{16}cm

^{−3}, reported previously for PDAA PR composites [16].

_{T}and T) for PDAA/TPAOH/7-DCST/PCBM of 1920 s

^{−1}is comparable with that obtained for PDAA/TAA/7-DCST/PCBM, 1870 s

^{−1}. On the other hand, the detrapping rate β

_{T}for PDAA/TPAOH/7-DCST/PCBM of 200 s

^{−1}is much faster than that obtained for PDAA/TAA/7-DCST/PCBM, 0.2 s

^{−1}. A faster tapping rate of 1870 s

^{−1}and a slower detrapping rate β

_{T}of 0.2 s

^{−1}contributed to the almost flat and plateau photocurrent measured in the time range of 0.001 to 10 s for PDAA/TAA/7-DCST/PCBM.

_{M}and M) of 4.0 s

^{−1}and the detrapping rate β

_{M}from the deep trap of 0.01 s

^{−1}significantly contributed to the large decrease in the transient photocurrent in the time range beyond 0.1 s for PDAA/TPAOH/7-DCST/PCBM. On the other hand, the contribution of the deep trapping event to the transient photocurrent is negligibly small for PDAA/TAA/7-DCST/PCBM. In other words, the transient photocurrent for PDAA/TAA/7-DCST/PCBM can be described by the one-trap model.

^{−5}to 10

^{−1}s, which is followed by the detrapping of filled traps from the deep trap in the time range beyond 10

^{−1}s for PDAA/TPAOH/7-DCST/PCBM. In contrast, the detrapping of the hole carriers from the shallow trap contributes to the transient photocurrent decay for PDAA/TAA/7-DCST/PCBM.

#### 3.4. Relationship between Trapping Behavior and Photorefractive Response

^{15}cm

^{−3}at a response time of 8 ms, whereas the density of the filled shallow traps for PDAA/TAA/7-DCST/PCBM is 2.43 × 10

^{15}cm

^{−3}at a response time of 67 ms. These results explain that the response time for optical diffraction is given by the time taken to fill a sufficient density of shallow traps to form the space-charge field. In other words, these filled shallow traps work as effective photorefractive traps.

^{−6}cm

^{2}V

^{−1}s

^{−1}with QE = 4.3 × 10

^{−3}and that for PDAA/TAA/7-DCST/PCBM is determined to be 3.9 × 10

^{−7}cm

^{2}V

^{−1}s

^{−1}with QE = 7.0 × 10

^{−4}. PDAA/TAA/7-DCST/PCBM shows a hole mobility of 3.9 × 10

^{−7}that is one order slower than that of 3.3 × 10

^{−6}cm

^{2}V

^{−1}s

^{−1}obtained for PDAA/TPAOH/7-DCST/PCBM.

#### 3.5. Estimation of Value for Trap State

_{tr}. The trap residing time is expressed as

#### 3.6. DOS Width and Hole Mobility

_{0}is the prefactor mobility, and C is an empirical constant [20]. Equation (10) is valid for a high electric field on the order of a few tens of V μm

^{−1}and T

_{g}> T > T

_{c}, where T

_{g}is the glass transition temperature, and T

_{c}is the nondispersive-to-dispersive transition temperature [21].

^{−1}using Equation (10). For PDAA/TPAOH/7DCST/PCBM, μ is evaluated to be 3.4 × 10

^{−6}cm

^{2}V

^{−1}s

^{−1}with a DOS width and parameters of C = 5.3 × 10

^{−4}cm

^{1/2}V

^{−1/2}, μ

_{0}= 0.01 cm

^{2}V

^{−1}s

^{−1}, and Ʃ = 3.8; for PDAA/TAA/7DCST/PCBM, μ is evaluated to be 4.0 × 10

^{−7}cm

^{2}V

^{−1}s

^{−1}with a DOS width and parameters of C = 5.3 × 10

^{−4}cm

^{1/2}V

^{−1/2}, μ

_{0}= 0.01 cm

^{2}V

^{−1}s

^{−1}, and Ʃ = 4.32, as listed in Table 3. These parameters are reasonable for photorefractive polymers [22]. In addition to the higher energetic disorder of the larger width of DOS, the broader positional disorder of the larger Ʃ value is also evaluated for PDAA/TAA/7-DCST/PCBM compared with PDAA/TPAOH/7-DCST/PCBM. Namely, lower hole mobility for the hole transport for PDAA/TAA/7-DCST/PCBM is significantly related to the more energetic disorder of the broader width of DOS and the scattering situation of the positional disorder. As shown in Figure 1, bulky methyl moieties attached to the phenyl group may hinder the molecular packing preferencing hole carrier hopping, and this hindrance leads to the scattering situation.

## 4. Conclusions

^{−6}cm

^{2}V

^{−1}s

^{−1}with a QE = 4.3 × 10

^{−3}and 3.9 × 10

^{−7}cm

^{2}V

^{−1}s

^{−1}with a QE = 7.0 × 10

^{−4}, respectively. The density of the shallow traps is 1.1–1.2 × 10

^{16}cm

^{−3}for both polymer systems and that for the deep traps is 2.0 × 10

^{16}cm

^{−3}for PDAA/TPAOH/7-DCST/PCBM. No significant difference in trap density is evaluated for either system. These values are comparable to the photorefractive number density of traps, 1.4–3.1 × 10

^{16}cm

^{−3}, as previously reported for PDAA composites [16]. The initial photocurrent for PDAA/TPAOH/7-DCST/PCBM is simulated to be mainly governed by the transient density of the filled shallow trap, which is replaced by the transient density of the filled deep trap at a later time. However, the entire photocurrent for PDAA/TAA/7-DCST/PCBM is governed by the transient density of the shallow trap. The width of the DOS was evaluated for both polymer systems using PYS measurements. The width of the DOS for PDAA/TPAOH/7-DCST/PCBM and PDAA/TAA/7-DCST/PCBM is determined to be 0.138 eV and 0.153 eV, respectively, which represents only a small difference for both polymer composite systems. The Bässler formalism, together with the energetic and positional disorders, was used to evaluate the hole mobility for both systems. Lower hole mobility for PDAA/TAA/7-DCST/PCBM is attributed to both the energetic disorder of the broader width of DOS and the positional disorder of the scattering situation for the carrier hopping. The latter is caused by the hindrance of molecular packing due to bulky methyl moieties attached to the phenyl group.

## Author Contributions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Data Availability Statement

## Conflicts of Interest

## Appendix A

PDAA/TPAOH/7-DCST/PCBM (35/35/30/0.6) | |||||||||
---|---|---|---|---|---|---|---|---|---|

Type | QE/α (cm^{−1}) | μ (cm ^{2} V^{−1} s^{−1}) | γ_{T}(cm ^{3} s^{−1}) | T (cm ^{−3}) | β_{T}(s ^{−1}) | γ_{M}(cm ^{3} s^{−1}) | M (cm ^{−3}) | β_{M}(s ^{−1}) | γ_{R}(cm ^{3} s^{−1}) |

No. 1 | 1.6 × 10^{−2}/59 | 9.0 × 10^{−7} | 1.6 × 10^{−13} | 1.2 × 10^{16} | 120 | 1.5 × 10^{−16} | 2 × 10^{16} | 0.01 | 1.0 × 10^{−13} |

No. 2 | 6.0 × 10^{−3}/59 | 2.3 × 10^{−6} | 1.6 × 10^{−13} | 1.2 × 10^{16} | 200 | 2.0 × 10^{−16} | 2 × 10^{16} | 0.01 | 2.0 × 10^{−13} |

No. 3 | 4.3 × 10^{−3}/59 | 3.3 × 10^{−6} | 1.6 × 10^{−13} | 1.2 × 10^{16} | 200 | 2.0 × 10^{−16} | 2 × 10^{16} | 0.01 | 2.7 × 10^{−13} |

No. 4 | 3.8 × 10^{−3}/59 | 3.7 × 10^{−6} | 1.6 × 10^{−13} | 1.2 × 10^{16} | 200 | 2.2 × 10^{−16} | 2 × 10^{16} | 0.01 | 2.9 × 10^{−13} |

PDAA/TAA/7-DCST/PCBM (35/35/30/0.6) | |||||||||

Type | QE/α (cm^{−1}) | μ(cm^{2} V^{−1} s^{−1}) | γ_{T}(cm^{3} s^{−1}) | T(cm^{−3}) | β_{T}(s^{−1}) | γ_{M}(cm^{3} s^{−1}) | M(cm^{−3}) | β_{M}(s^{−1}) | γ_{R}(cm^{3} s^{−1}) |

No. 1 | 7.0 × 10^{−4}/45 | 3.9 × 10^{−7} | 1.7 × 10^{−13} | 1.1 × 10^{16} | 0.2 | 1.6 × 10^{−19} | 1 × 10^{16} | 0.001 | 1.4 × 10^{−13} |

No. 2 | 6.4 × 10^{−4}/45 | 3.7 × 10^{−7} | 1.5 × 10^{−13} | 1.1 × 10^{16} | 0.2 | 1.0 × 10^{−19} | 1 × 10^{16} | 0.001 | 1.2 × 10^{−13} |

No. 3 | 4.0 × 10^{−4}/45 | 6.2 × 10^{−7} | 1.5 × 10^{−13} | 1.1 × 10^{16} | 0.2 | 1.0 × 10^{−19} | 1 × 10^{16} | 0.001 | 1.3 × 10^{−13} |

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**Figure 1.**Molecular structures of TPAOH and TAA energetically stabilized using a molecular orbital simulation. Theoretical calculations with the Guassian09 package software, using the functional/basis set RB3LYP, were applied.

**Figure 2.**Plots of photoelectron yield as a function of incidence photon energy for (

**a**) PDAA/TPAOH and (

**b**) PDAA/TAA. Three times measurements were performed at different illumination positions for each sample. Each measurement is separately shown by black, red, and blue plots. Black arrows and numerals indicate the ionization potential for each sample.

**Figure 3.**Plots of photoelectron yield as a function of incidence photon energy (

**a**) for PDAA/TPAOH/7-DCST/PCBM and (

**b**) for PDAA/TAA/7-DCST/PCBM. Three times measurements were performed at different illumination positions for each sample. Each measurement is separately shown by black, red, and blue plots. Black arrows and numerals indicate the ionization potential for each sample.

**Figure 4.**Energy diagram (HOMO and LUMO level) for (

**a**) PDAA/TPAOH/7-DCST/PCBM and (

**b**) PDAA/TAA/7-DCST/PCBM.

**Figure 5.**(

**a**) Rising transient photocurrent (black plots) and the simulated photocurrent (pale purple curve) for PDAA/TPAOH/7-DCST/PCBM: red curve, transient density for the sensitizer anion N

_{A}

^{−}; blue curve, transient density for filled shallow traps T

^{+}; green curve, transient density for filled deep traps M

^{+}; (

**b**) decay profile.

**Figure 6.**(

**a**) Rising transient photocurrent (black plots) and the simulated photocurrent (pale purple curve) for PDAA/TAA/7-DCST/PCBM: red curve, transient density for the sensitizer anion N

_{A}

^{−}; blue curve, transient density for filled shallow traps T

^{+}; (

**b**) decay profile.

**Figure 7.**DOS curves are plotted as a function of photon energy for (

**a**) PDAA/TPAOH/7-DCST/PCBM and (

**b**) PDAA/TAA/7-DCST/PCBM: black solid curve, measured DOS curve; red curve, separated Gaussian curve at low photon energy region.

**Table 1.**Summary of the photorefractive parameters of diffraction efficiency, response time, the absorption coefficient, and the evaluated QE for different plasticizers.

Sample | η (%) | τ (ms) | α_{532}/α_{640} | QE |
---|---|---|---|---|

PDAA/TPAOH/7-DCST/PCBM (35/35/30/0.6) | 39 ± 1 | 8 ± 0.8 | 200/59 | 4.3 × 10^{−3} |

PDAA/TAA/7-DCST/PCBM (35/35/30/0.6) | 75 ± 0.8 | 67 ± 0.6 | 134/45 | 7.0 × 10^{−4} |

^{−1}; wavelength, 532 nm; laser power, 650 mW cm

^{−2}[16].

**Table 2.**Summary of the simulated parameters of quantum efficiency (QE) for photocarrier generation, hole mobility, trapping, and recombination parameters for PDAA/TPAOH/7-DCST/PCBM (35/35/30/0.6) and PDAA/TAA/7-DCST/PCBM (35/35/30/0.6).

PDAA/TPAOH/7-DCST/PCBM (35/35/30/0.6) | ||||||||
---|---|---|---|---|---|---|---|---|

QE/α (cm^{−1}) | μ (cm ^{2} V^{−1} s^{−1}) | γ_{T}(cm ^{3} s^{−1}) | T (cm ^{−3}) | β_{T}(s ^{−1}) | γ_{M}(cm ^{3} s^{−1}) | M (cm ^{−3}) | β_{M}(s ^{−1}) | γ_{R}(cm ^{3} s^{−1}) |

4.3 × 10^{−3}/59 | 3.3 × 10^{−6} | 1.6 × 10^{−13} | 1.2 × 10^{16} | 200 | 2.0 × 10^{−16} | 2.0 × 10^{16} | 0.01 | 2.7 × 10^{−13} |

PDAA/TAA/7-DCST/PCBM (35/35/30/0.6) | ||||||||

QE/α (cm^{−1}) | μ(cm^{2} V^{−1} s^{−1}) | γ_{T}(cm^{3} s^{−1}) | T(cm^{−3}) | β_{T}(s^{−1}) | γ_{M}(cm^{3} s^{−1}) | M(cm^{−3}) | β_{M}(s^{−1}) | γ_{R}(cm^{3} s^{−1}) |

7.0 × 10^{−4}/45 | 3.9 × 10^{−7} | 1.7 × 10^{−13} | 1.1 × 10^{16} | 0.2 | - | - | - | 1.4 × 10^{−13} |

_{A}= (PCBM) = 4.76 × 10

^{18}cm

^{−3}; E = 40 V μm

^{−1}; wavelength, 640 nm; laser power, 400 mW cm

^{−2}.

DOS Width (eV) | μ^{1}(cm ^{2} V^{−1} s^{−1}) | μ^{2}(cm ^{2} V^{−1} s^{−1}) | |
---|---|---|---|

PDAA/TPAOH/7DCST/PCBM (35/35/30/0.6) | 0.138 | 3.4 × 10^{−6} | 3.3 × 10^{−6} |

PDAA/TAA/7DCST/PCBM (35/35/30/0.6) | 0.153 | 4.0 × 10^{−7} | 3.9 × 10^{−7} |

^{1}Hole mobility was evaluated using Equation (10).

^{2}Hole mobility was evaluated from the transient photocurrent.

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**MDPI and ACS Style**

Tsutsumi, N.; Mizuno, Y.; Jackin, B.J.; Kinashi, K.; Sassa, T.; Giang, H.N.; Sakai, W.
Photoelectron Yield Spectroscopy and Transient Photocurrent Analysis for Triphenylamine-Based Photorefractive Polymer Composites. *Photonics* **2022**, *9*, 996.
https://doi.org/10.3390/photonics9120996

**AMA Style**

Tsutsumi N, Mizuno Y, Jackin BJ, Kinashi K, Sassa T, Giang HN, Sakai W.
Photoelectron Yield Spectroscopy and Transient Photocurrent Analysis for Triphenylamine-Based Photorefractive Polymer Composites. *Photonics*. 2022; 9(12):996.
https://doi.org/10.3390/photonics9120996

**Chicago/Turabian Style**

Tsutsumi, Naoto, Yusuke Mizuno, Boaz Jessie Jackin, Kenji Kinashi, Takafumi Sassa, Ha Ngoc Giang, and Wataru Sakai.
2022. "Photoelectron Yield Spectroscopy and Transient Photocurrent Analysis for Triphenylamine-Based Photorefractive Polymer Composites" *Photonics* 9, no. 12: 996.
https://doi.org/10.3390/photonics9120996