Role of Hybrid Charge Transfer States in the Charge Generation at

Subscriber access provided by UNIV OF CAMBRIDGE

Article

The Role of Hybrid Charge Transfer States in the Charge Generation at ZnMgO/P3HT Heterojunctions Moritz Eyer, Johannes Frisch, Sergey Sadofev, Norbert Koch, Emil J.W. List-Kratochvil, and Sylke Blumstengel J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b07293 • Publication Date (Web): 14 Sep 2017 Downloaded from http://pubs.acs.org on September 15, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

The Role of Hybrid Charge Transfer States in the Charge Generation at ZnMgO/P3HT Heterojunctions Moritz Eyer,† Johannes Frisch,† Sergey Sadofev,† Norbert Koch,‡ Emil J.W. List-Kratochvil,¶ and Sylke Blumstengel∗,¶ Institut für Physik, Humboldt-Universität zu Berlin, Institut für Physik & IRIS Adlershof, Humboldt-Universität zu Berlin, and Institut für Physik, Institut für Chemie & IRIS Adlershof, Humboldt-Universität zu Berlin E-mail: [email protected]

Abstract Hybrid charge transfer states (HCTS) have been shown to form at interfaces between ZnMgO and P3HT. Combined analysis of the electronic structure of ZnMgO/P3HT interfaces and the corresponding photovoltaic performance, also as a function of temperature, reveals that the HCTS are constituted of an electron in the metal oxide’s conduction band and a hole in the polymer’s highest occupied molecular orbital. Their formation is thus an intrinsic feature of hybrid interfaces. Radiative recombination of HCTS is observed in the infrared spectral range of the electroluminescence spectrum. Band gap engineering by varying the Mg content in ZnMgO allows tuning of the HCTS energy in order to conduct a systematic study of charge carrier generation and recombination. The results shows that recombination of the HCTS effectively competes with its dissociation and limits the performance of hybrid photovoltaic ∗ To

whom correspondence should be addressed für Physik, Humboldt-Universität zu Berlin, 12489 Berlin, Germany ‡ Institut für Physik & IRIS Adlershof, Humboldt-Universität zu Berlin, 12489 Berlin, Germany ¶ Institut für Physik, Institut für Chemie & IRIS Adlershof, Humboldt-Universität zu Berlin, 12489 Berlin, Germany † Institut

1 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

cells. Evidently, a single charge-separating interface does not suffice to assure efficient photocarrier generation. Further measures have to be taken to funnel the charges away from the interface in order to prevent their recombination. The results of this study provide guidance for optimization of metal oxide/organic hybrid interfaces.

Introduction To realize hybrid photovoltaic cells, an organic conjugated molecule or polymer serving as absorber and electron donor is combined with an inorganic component, commonly a transparent metal oxide, acting as electron acceptor. 1 Especially ZnO has emerged as a prospective candidate 2–5 because of its low cost, ease of preparation, its ability to grow in different kinds of nanostructures enlarging the interface area, and the possibility of tuning the interface electronic properties by introduction of molecular interlayers. 6–9 Due to its large electron affinity, ZnO forms a typeII interface with many conjugated organic molecules. Time-resolved photoluminescence (PL) and photoinduced absorption measurements have shown that excitons generated in the organic layer dissociate with efficiencies beyond 90 % at the hybrid interface. 2,9,10 Owing to the high charge carrier mobility and dielectric constant of ZnO it could be expected that the transferred electrons delocalize quickly in the inorganic bulk leading to highly efficient generation of free carriers. However, this is not the case in practice. Despite the apparent advantages of ZnO-based hybrid cells and despite intensive research over the last years, the photovoltaic performance is not yet viable lagging even behind that of "organic" photovoltaic cells where fullerenes and their derivatives are typically employed as electron acceptors. 11–15 While bulk heterojunction photovoltaic cells based on ZnO nanoparticles and poly(3-hexylthiophene) (P3HT) have power conversion efficiencies of 2 %, PCBM:P3HT devices yield typically 3.5 to 4 %. 14,15 In "organic" photovoltaics, the formation of coulombically bound electron-hole pairs, also called charge transfer states, at the donor-acceptor interface and the always present disorder, which hampers efficient charge transport, are detrimental. In particular, the recombination of the charge transfer state across the donor-acceptor interface is believed to be the cause for the unsatisfactory low power conversion efficiencies. 16–18 It is ar2 ACS Paragon Plus Environment

Page 2 of 23

Page 3 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


gued that delocalization of charges, through the use of well-ordered materials, is essential for an efficient electron-hole pair dissociation and photocurrent generation. 19–21 Recently, the formation of a hybrid charge transfer state (HCTS) at interfaces of ZnO and ZnMgO with conjugated organic materials has been directly proven by the observation of its radiative recombination in electroluminescence (EL) spectra. 22,23 Therefore, the question arises whether the HCTS plays a similar role as the charge transfer state in "organic" photovoltaics to the detriment of device performance. In the literature, the poor performance of hybrid photovoltaic cells is often alternatively assigned to defect states, either intrinsic or extrinsic in nature, present at the oxide surface that trap charges and prevent electron-hole dissociation. 24,25 In order to develop strategies for the improvement of the photovoltaic performance, it is necessary to identify the loss channels at the hybrid interface for photogenerated charges, to clarify the role of the HCTS and to quantify their impact on the device parameters. In this contribution, we investigate hybrid photovoltaic cells based on ZnMgO and the model polymer P3HT. Instead of ZnO, ZnMgO is chosen since its band gap can be tuned by the Mg content x. By doing so, the energy gap ∆EIO between the P3HT highest occupied molecular orbital (HOMO) and the ZnMgO conduction band minimum (CBM) can be systematically varied without the need for additional interlayers, allowing investigation of the properties of intrinsic ZnMgO/P3HT interfaces. Furthermore, the ZnMgO layers are prepared by radical-source molecular beam epitaxy (RS-MBE) assuring single crystalline films with a defined surface termination. 26 The energy offsets between the occupied levels of the organic and the inorganic component as a function of the Mg content are determined by ultraviolet photoemission spectroscopy (UPS). On that basis, the correlation between the interfacial energy level alignment and photovoltaic parameters is established. It is found that an increase of ∆EIO directly translates into an increase of the open circuit voltage VOC . However, there is a substantial offset between the two quantities. The values of qVOC are smaller by about 680 meV than ∆EIO . To elucidate the origin of this loss in the VOC , the impact of the formation of HCTS at the hybrid interface on the photovoltaic performance is analyzed by temperature-dependent EL and current-voltage (J-V ) measurements. The



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

combined results suggest that the HCTS is formed between an electron in the ZnMgO conduction band and a hole in the P3HT HOMO. Its formation is thus an intrinsic feature of ZnMgO/organic hybrid interfaces. The substantial loss in the open circuit voltage is caused by recombination of the HCTS. At room temperature recombination is dominated by non-radiative processes which are facilitated by the ability of the carriers to move along the hybrid interface. The results imply that further engineering of the hybrid interfaces is necessary to assure efficient charge separation prior recombination. A viable strategy for future optimization is suggested, based on these findings.

Methods Sample Preparation. The bottom Ga-doped Zn1−x Mgx O layer and and the nominally undoped Zn1−x Mgx O layer were produced by RS-MBE on a-plane sapphire according to a procedure described previously. 26,27 resulting in O-polar ZnMgO(0001) layers. The Mg content in the doped and nominally undoped ZnMgO layer was uniformly varied between 1 % and 14 %. The samples were then transferred to a metallization chamber (base pressure 10−8 mbar) for Ag deposition, providing an ohmic contact with ZnMgO. P3HT (purchased from Riecke Metals Inc.) layers were spin cast from chlorobenzene solution and, finally, MoO3 /Ag top contacts were deposited. Sample transfer and P3HT layer preparation were performed in HV (10−5 mbar) and an inert N2 atmosphere, respectively, following a defined protocol to assure reproducible results. To contact the sample, the P3HT on top of the Ag bottom contact was removed. The device characterization was performed in an N2 atmosphere. The results presented were obtained evaluating 3 to 5 identical diodes.

Energy Level Alignment. The optical gap was determined by absorption measurements. UPS measurements were performed on pristine Zn1−x Mgx O:Ga/ Zn1−x Mgx O surfaces, and after deposition of a 15-nm-thick P3HT


Page 4 of 23

Page 5 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


layer. Sample preparation and transfer were performed following the same protocol as for device fabrication to obtain consistent results. UPS measurements were carried out using a He discharge lamp (Omicron) as excitation source (Eex (HeI) = 21.22 eV). The light intensity was reduced by a factor of 0.07 using a thick Al filter. The filter also prevented irradiation of the sample with light in the visible range from the HeI excitation source. The positions of both the SECO and the VB region spectra were found to be independent of the excitation intensity. A hemispherical energy analyser Phoibos 100 from SPECS was used as electron analyser.

Diode Characterisation. Current-voltage characteristics were measured at 100 mW/cm2 AM1.5 G simulated sunlight illumination with the UV light blocked by a 395 nm cut-off filter to avoid excitation of ZnMgO. EL spectra were recorded with a combination of a nitrogen-cooled InGaAs detector and an Acton SpectraPro 300i spectrograph. The sensitivity of the combination was calibrated with a black body radiation source. Both illumination and luminescence collection were performed through the sapphire substrate. The EL spectrum is comprised of P3HT emission in the visible spectral range and HCTS emission in the near-infrared. For better visibility, the contribution of the P3HT emission has been subtracted in the spectra of Figure 3b according to the procedure described in Reference. 23

Results and Discussion Design of ZnMgO/P3HT Devices and Photovoltaic Characteristics The design of the ZnMgO/P3HT diodes is depicted in Figure 1a. The devices consist of a bottom Ga-doped Zn1−x Mgx O layer, a nominally undoped Zn1−x Mgx O layer and a spin-cast P3HT layer. a-plane sapphire wafers are used as substrates for the epitaxial growth of the oxide layers. This approach assures single crystalline layers terminated by the O-polar ZnMgO(0001) surface. 26,27



The bottom Ga-doped Zn1−x Mgx O layer serves here as transparent electrode. The carrier concentration in this layer is about 1020 cm−3 and the conductivity 1000 Ω−1 cm−1 . 27 The Mg contents in the doped and nominally undoped Zn1−x Mgx O are the same. Devices with Mg contents ranging from x = 0.01 to x = 0.14 are studied. a)

MoO /Ag 3

Ag

b) 0.2

Current density (mA/cm²)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 23

x = 0.01 x = 0.05 x = 0.14

0.1

0.0

-0.1

-0.50

-0.25

0.00

0.25

0.50

0.75

Voltage (V)

Figure 1: a) Device layout of P3HT/ZnMgO diodes. The layer thicknesses are: Zn1−x Mgx O:Ga (150 nm), Zn1−x Mgx O (100 nm), P3HT (240 nm), MoO3 (15 nm). b) Comparison of J-V curves in the dark (dashed lines) and under illumination (solid lines) of Zn1-x Mgx O/P3HT devices with different Mg content x. Representative J-V characteristics measured at room temperature in the dark and under illumination of 100 mW/cm2 AM1.5 G simulated sunlight with the UV light blocked by a 395 nm cut-off filter (to avoid excitation of ZnMgO) are presented in Figure 1b. All evaluated devices show a very pronounced rectification behaviour in the dark over at least 5 orders of magnitude (see Figure S1 in the Supporting Information). The VOC increases by ca. 200 mV as the Mg content is increased from x = 0.01 to x = 0.14 (see Figure 1b). An increase of the VOC is expected since the hybrid 6 ACS Paragon Plus Environment

Page 7 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


energy gap ∆EIO should become larger with increasing Mg content. The exact correlation between the two physical quantities is established in the following paragraph. The fill factor of 55 % is independent of the Mg content. The short-circuit current JSC decreases with increasing x, so that altogether the overall power conversion efficiency decreases by ca. 40 % over that range. The VOC in the device with the lowest Mg content x = 0.01 is comparable to values reported for ZnO/P3HT bilayer devices while JSC in the present devices is lower by a factor 3 to 4. 33 This is expected considering the very smooth surface morphology of the epitaxial ZnMgO layer leading to a smaller interface area compared to devices employing rather rough wet-chemically produced ZnO layers. Additionally, the thicknesses of P3HT and MoO3 have not been optimized for maximal device performance.

Energy Level Alignment at ZnMgO/P3HT Interfaces The energy level alignment at Zn1−x Mgx O/P3HT interfaces is determined by photoemission spectroscopy for structures with minimal (x = 0.01) and maximal (x = 0.14) magnesium content. As the ionization potential and electron affinity of ZnMgO scale linearly with the Mg content, 28 two data points are sufficient to derive the energy offsets at Zn1−x Mgx O/P3HT interfaces also for intermediate x. The incorporation of 1 % of Mg increases the band gap of ZnO only slightly by 50 meV while 14 % of Mg lead to a substantial increase of 300 meV according to absorption measurements (data not shown). Also the width of the band gap has been found to be a linear function of the magnesium content up to x = 0.4. 26 The results of the UPS measurements are summarized in Figure 2a-d. The valence band onsets of pristine Zn1−x Mgx O are found at 3.39 eV (x = 0.01) and 3.63 eV (x = 0.14) below the Fermi energy EF . The secondary electron cut off (SECO) yields work functions φ = 3.92 eV (x = 0.01) and φ = 3.85 eV (x = 0.14). Upon deposition of 15 nm of P3HT on ZnMgO, pronounced features of the polymer’s HOMO become visible. The onsets are positioned at 0.81 eV (x = 0.01) and 1.01 eV (x = 0.14) below EF . The work functions of both surfaces are slightly reduced [100 meV (x = 0.01) and 150 meV (x = 0.14)] due to the deposition of P3HT. Such small changes have been observed previously for vacuum-deposited small molecules 7 ACS Paragon Plus Environment


b)

e)

Intensity (arb. units)


a)

3.8 eV

(x5)

0.80 eV 3.4 eV

3.9 eV 3

4

5

6

Work function (eV) c)

5

4

3

2

1

0

Binding energy (eV)

d)

f)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

3.7 eV

(x5)

1.00 eV

3.65 eV

3.85 eV 3

4

5

Work function (eV)

6

5

4

3

2

1

0

Binding energy (eV)

Figure 2: UPS of Zn1−x Mgx O/P3HT interfaces. a) SECO and b) valence band spectra of the pristine Zn0.99 Mg0.01 O surface (blue) and the corresponding Zn0.99 Mg0.01 O/P3HT interface (red). c) SECO and d) valence band spectra of the pristine Zn0.86 Mg0.14 O surface (blue) and the corresponding hybrid interface (red). The values are rounded to 50 meV to account for the uncertainty of the UPS measurements. The P3HT layer thickness is 15 nm. Energy level alignment at Zn1−x Mgx O/P3HT interfaces with x = 0.01 (e) and x = 0.14 (f). All values are given in eV and rounded to 50 meV to account for the uncertainty of the UPS measurements. The black arrows mark the values derived from UPS, the red arrows indicate the transport gap energies (see main text). The energy gap ∆EIO is marked by blue arrows.


Page 8 of 23

Page 9 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


and have been attributed to the electron push-back effect. 10,29–31 The energetic positions of the unoccupied levels of ZnMgO are deduced from optical absorption measurements adding the exciton binding energy. The HOMO-LUMO gap of P3HT is derived from direct and inverse photoemission spectroscopy data reported in the literature. 32 The resulting energy level schemes are depicted in Figure 2e, f. The Fermi energy at the surface is at the CBM of ZnMgO. The doping level of the nominally undoped ZnMgO layers is about 1017 cm−3 placing the Fermi level in the bulk ca. 100 meV below the conduction band minimum. Thus, the bands are slightly bent downwards at the surface. The ionization potential of P3HT (4.65 ± 0.05) eV is in agreement with previously reported values. 32 The energy offset between the P3HT LUMO and the ZnMgO CBM, providing the driving force for the charge separation process, is very large (1.70 eV for x = 0.01 and 1.50 eV x = 0.14). It exceeds by far the exciton binding energy of P3HT which is ca. 600 meV. Thus, about 1 eV will be lost in the light-to-electrical energy conversion process. Most relevant in the context of the present work is that the hybrid energy gap ∆EIO widens with the Mg content in the Zn1−x Mgx O layer from (800 ± 50) meV to (1000 ± 50) meV, increasing x from 0.01 to 0.14. That means that about 80 % of the ZnMgO band gap change goes into the relevant energy offset between the unoccupied levels. As shown in Figure 1b, also the VOC increases by 200 mV going x = 0.01 to x = 0.14 proving that the open circuit voltage is indeed correlated to the hybrid energy gap. However, the absolute values of qVOC are considerably smaller than ∆EIO . The orgin of this loss in VOC is elucidated in the following.

Electroluminescence of HCTS Driving the diodes in forward direction, the injected charges recombine across the hybrid interface as schematically depicted in the Figure 3a. Upon recombination, light is emitted in the nearinfrared spectral region. Exemplary HCTS EL spectra of a diode with intermediate Mg content x = 0.05 are shown in Figure 3b. The line-shape is Gaussian with a variance σ ≈ 135 meV. From the previously reported weak temperature-dependence of the line width, the contribution of inhomogeneous broadening to the variance σ is estimated to be ca. 100 meV. 23 Such inhomogenous 9 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

broadening is explained by the formation of domains with differently orientated polymer chains and, thus, different ionization potentials. 34 An additional contribution to inhomogeneous broadening originates in the alloy broadening at higher Mg content (see Figure S2 in the Supporting Information). Close inspection reveals that the HCTS EL spectral position shifts with increasing voltage to higher energies. As simultaneously also the current through the device increases, this shift might be either related to the change in current or voltage. Upon increasing the current, the filling of localized states may cause a blue-shift accompanied by a broadening of the spectrum. As a line broadening is not observed (see Figure S3 in the Supporting Information), the spectral shift is more likely induced by the voltage: An increasing applied voltage steepens the confining potential for the electron and hole at the hybrid interface as schematically depicted in the Figure 3c. This picture is valid under the assumption that the carriers are delocalized over several nanometers in the direction perpendicular to the interface. The ground state energy in a potential well with applied electric field F rises with F 2/3 . Such field-dependence of the HCTS EL peak energies is indeed observed, not only at room-temperature but also at lower temperatures (see Figure 3d). The overlap of data points recorded at different temperatures indicates that the temperature-dependence of the HCTS transition energy is negligible compared to the electric field-dependence. Further evidence that the EL maximum shifts with the applied electric field is obtained by analyzing diodes with different P3HT layer thicknesses (see Figure S4 in the Supporting Information). Voltage-dependent EL measurements have been performed for diodes with different Mg content x (see Figure S4 and table S1 in the Supporting Information). From these experiments we can infer that the EL maximum shifts by 200 meV to higher energies upon increasing the Mg content from 1% to 14% in accordance to the widening of the hybrid energy gap (see Figure 3e). The recombination of the HCTS represents a loss channel of photocurrent and photovoltage in a photovoltaic cell. To gain information on the recombination process, the voltage- and temperaturedependence of the relative EL quantum yield ηEL is investigated (see Figure 3f). While no correlation with the applied voltage (current) is found, the HCTS EL yield increases strongly with cool-


Page 10 of 23

Page 11 of 23

EL intensity (a. u.)

b)

a)

LUMO CBM

HOMO

VBM ZnMgO

2.25 V 2.60 V 2.90 V

0.8

P3HT

0.9

1.0

1.1

1.2

Photon energy (eV)

d)

CB

LUMO

VB

HOMO

ZnMgO

EL maximum (eV)

c)

1.2

77 K 110 K

1.1

150 K 190 K

1.0 225 K

0.9

260 K

0.8

0.00

P3HT

e)

298 K

0.84 eV

0.05

F

2/3

0.10

0.15

2/3

((V/nm)

)

f) 1.1 EL Maximum

1.0

E

0

10

IO

EL,rel

Energy (eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-1

10

0.9

0.8

E

-2

10

A

= 70

10 meV

0.7 3.3

3.4

E

gap

3.5

3.6

4

(eV)

6

8

10

T (K

1000/

12

14

-1

)

Figure 3: a) Schematic depiction of the recombination of injected charges at ZnMgO/P3HT interfaces. b) HCTS EL spectra of a diode with a Mg content x = 0.05 driven at different voltages. c) Energy diagram of the ZnMgO/P3HT interface under forward bias. d) Electric field F dependence of the position of the HCTS EL maximum recorded at different temperatures. F is calculated taking into account the built-in voltage and assuming that the voltage drop occurs mostly in the polymer layer. The position of the EL maximum for F → 0 is indicated. e) Energies of the HCTS EL maxima and the hybrid gaps ∆EIO derived from UPS plotted as a function of the ZnMgO optical gap Egap . The positions of the HCTS EL maxima correspond to the values obtained for F → 0 (see Figure S4 and table S1 in the Supporting Information). f) Temperature-dependence of the relative EL quantum yield ηEL,rel = ηEL (T )/ηEL (T = 77K) of the diode with x = 0.05. The different data points for a fixed temperature correspond to different applied voltages. The red line is a fit taking into account a thermally activated non-radiative recombination channel with an activation energy EA . No correlation of the EL yield to the applied voltage is found.



ing from room temperature to 77 K, indicating the presence of a thermally-activated non-radiative recombination process with an activation energy of (70 ± 10) meV. As incoherent transport processes are typically thermally activated, a plausible conclusion is that the HCTS moves along the hybrid interface to find sites for non-radiative recombination. The measured activation energy is thus associated to the HCTS migration. The underlying transport mechanism, i.e. the question if the electron and hole motion occurs in a correlated or uncorrelated fashion, remains, however, to be elucidated. It should be noted that nanoscale transport of electron-hole pairs was recently also invoked at purely organic donor/acceptor heterointerfaces. 35 A lower bound of the fraction of HCTS recombining at room-temperature via this thermally-activated non-radiative channel can be estimated to ηth ≥ 1 − (ηEL (290K)/ηEL (77K)) ≈ 0.99. It is thus the dominating recombination mechanism.

Temperature Dependence of the Open-Circuit Voltage

1.2

x

OC

(V)

1.0

E

HCTS

(eV)

0.01

1.06 ± 0.07

0.05

1.20 ± 0.07

0.14

1.33 ± 0.10

0.8

V

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

0.6

x = 0.01 x = 0.05 x = 0.14

0.4

100

150

200

250

300

Temperature (K)

Figure 4: Temperature dependence of the VOC of Zn1−x Mgx O/P3HT diodes with different Mg content x measured at 100 mW/cm2 AM1.5 G illumination. The dotted lines correspond to linear fits taking only data points from 200 K to 300 K. Further information on the magnitude of the loss of the open-circuit voltage VOC and the underlying mechanism is gained from temperature-dependent J-V measurements under illumination. Figure 4 summarizes the VOC vs. temperature of the three different devices presented in Figure 1. 12 ACS Paragon Plus Environment

Page 12 of 23

Page 13 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The VOC increases with an almost constant slope between 298 K and 200 K. At lower temperatures the slope diminishes. The temperature-dependence of VOC can be understood on the basis of the Shockley equation whereby, under illumination, the photocurrent Jph is added to the diode dark current Jd . At open circuit, the net current J = Jph + Jd vanishes, yielding the open-circuit voltage

JSC qVOC ≈ kB T ln . J0 Here it is assumed that JSC is equal to Jph and

JSC J0

(1)

1. J0 is the reverse saturation current which is

related to the recombination of carriers in the diode. For radiative recombination, detailed balance between recombination and generation yields 36

J0r

Z

=q

αA ΦBB dE

r ≈ J00 · exp

(E0 − σ 2 /(2kB T )) . − kB T

(2)

where αA is the absorption coefficient of the HCTS at the interface, and ΦBB is the black body spectrum at the temperature T of the cell. Though not measured, mirror symmetry between the absorption and the emission spectra suggests also a Gaussian line shape of αA with a maximum at E0 and a variance σ . The evaluation of the integral yields the activation energy of the reverse saturation current E0 − σ 2 /(2kB T ) which can be assigned to the HCTS transition energy EHCTS . For a homogeneously broadened transition, σ 2 /(2kB T ) = λ with λ being the reorganization energy 37 while for an inhomogeneously broadened transition, σ 2 /(2kB T ) represents the transport r is a material specific energy if thermal equilibrium is reached. 38 The proportionality constant J00

constant, being a measure of the coupling strength between the electron and hole states at the heterointerface. As has been shown above, the recombination of the HCTS is dominated by nonradiative processes. Introducing the total recombination current being the sum of the radiative and non-radiative recombination currents J0 = J0r + J0nr = J0r /η finally yields 36 r J00 . qVOC ≈ EHCTS − kB T · ln η · JSC

(3)

The equation shows that qVOC is smaller than the HCTS energy. It equals actually the splitting of 13 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

the quasi-Fermi levels of the electrons in the acceptor and the holes in the donor. The difference between the HCTS energy, i.e. the hybrid energy gap, and the quasi-Fermi level splitting is due to recombination of the carriers forming the HCTS. Therefore, the second term in Eq. 3 represents the recombination losses of qVOC . To estimate the magnitude of these losses, the slope of the VOC vs. T curve in the linear range, i.e. in the temperature range between 298 K and 200 K, is extended to T → 0 (see Figure 4). In this temperature range, JSC has been found to be temperature-independent under the present illumination conditions (see Figure S5 in the Supporting Information). To obtain finally qVOC,max = EHCTS for which the recombination term vanishes, the temperature dependence of η has to be taken into account which yields a correction term of the magnitude of the activation energy of the non-radiative recombination. The derived values for VOC,max are reported in the table of Figure 4 and discussed in the following section. One notices that the values are larger than the ∆EIO derived from the UPS measurements by about 300 meV. This difference is explained by the fact that UPS measures the ionization potential of P3HT at the surface, and not that at the interface with ZnMgO. It is plausible that the polymer chains close to the interface assume preferentially a face-on orientation featuring a larger ionization potential than that of the polymer chains in the bulk, where a more random orientation is expected. The difference in the ionisation potential between edge-on and face-on orientation is reported to be about 400 meV. 34

Conclusions Figure 5 summarizes the qVOC,max derived from the analysis of the temperature-dependence of the illuminated J-V curves, the measured room-temperature values of qVOC and the positions of the HCTS EL maxima at zero electric field conditions for diodes with different Mg content. The three quantities increase linearly with the optical gap energy of ZnMgO with a slope identical to that of the hybrid energy gap ∆EIO obtained from UPS. This demonstrates clearly their correlation. According to the above discussion, qVOC,max is equal to EHCTS . HCTS absorption and emission spectra are expected to lie mirror symmetric around EHCTS as depicted in Figure 5. The HCTS


Page 14 of 23

E

HCTS

1.4

absorption

1.2 HCTS energy

1.0

680 meV

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


HCTS

0.8

EL max.

0.6

Energy (eV)

Page 15 of 23

0.4 3.3

3.4

E

gap

3.5

3.6

(eV)

Figure 5: qVOC,max derived from the analysis of the VOC (T )-measurements (blue solid triangles), the position of the HCTS EL maximum under zero electric field (red dots) and qVOC measured at room-temperature (black squares) plotted vs. the optical gap energy of ZnMgO. The dashed lines correspond to the slope of ∆EIO of the UPS measurements. The arrow indicates the losses in qVOC due to radiative and non-radiative recombination of the HCTS. On the left side, the HCTS energy EHCTS with respect to the HCTS absorption and emission spectra is schematically depicted. For a homogeneously broadened transition, σ 2 /(2kB T ) corresponds to the reorganization energy λ . emission maximum is found at about 350 meV below the HCTS transition energy. This redshift corresponds to the expected value of σ 2 /(2kB T ), indicating that the recombination occurs between an electron in the ZnO conduction band and a hole in the P3HT HOMO. Consequently, the formation of the HCTS is intrinsically occurring at the metal oxide/organic interface. The difference between qVOC,max and the measured room-temperature values of qVOC corresponds to the losses due to radiative and non-radiative recombination of the HCTS at the hybrid interface. These losses amount to about (680 ± 80) meV and are independent of the magnitude of the hybrid energy gap. This large value indicates that recombination of the HCTS is very effective. Radiative recombination is inherent to photovoltaic cells, as generation of carriers by illumination and their radiative recombination are two sides of the same coin. However, the non-radiative losses can in principle avoided. To reduce the losses in the VOC due to HCTS recombination, measures need to be taken to assure its efficient dissociation. In this case, a lesson may be learned from nature: In a photosynthetic reaction centre, charge separation occurs with unity efficiency because the electron transfer



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

proceeds in a multi-step process. Therefore, introduction of an electron transfer staircase – which could be readily achieved by band gap engineering of the metal oxide – could similarly help to funnel the electron efficiently away from the hybrid interface, and thus prevent HCTS recombination. As the energy offset driving initial Frenkel exciton separation is very large, in the present system such a charge transfer cascade could be easily realized without introducing further energy losses. Supporting Information Available J −V characteristics of three ZnMgO/P3HT diodes with Mg contents of x = 0.01, x = 0.05 and x = 0.14; linewidth of the HCTS EL spectra as function of the Mg content; electric field dependence of HCTS EL of diodes with different P3HT thickness and Mg content; temperature dependence of the short circuit current. Acknowledgements Financial support by the DFG in the framework of CRC 951 is gratefully acknowledged. The authors thank D. Neher for helpful discussions.

References (1) Wright, M.; Uddin, A. Organic-Inorganic Hybrid Solar Cells: A Comparative Review. Sol. Energy Mater. Sol. Cells 2012, 107, 87-111. (2) Hoye, R. L. Z.; Muñoz-Rojas, D.; Iza, D. C.; Musselman, K. P.; MacManus-Driscoll, J. L. High Performance Inverted Bulk Heterojunction Solar Cells by Incorporation of Dense, Thin ZnO Layers Made using Atmospheric Atomic Layer Deposition. Sol. Energ. Mater. Sol. Cells 2013, 116, 197-202. (3) Oosterhout,S. D.; Koster, L. J. A.; v. Bavel,S. S.; Loos, J.; Stenzel, O.; Thiedmann, R.; Schmidt, V.; Campo, B.; Cleij, T. J.; Lutzen, L.; Vanderzande, D.; Wienk, M. M.; Janssen, R. A. J. Controlling the Morphology and Efficiency of Hybrid ZnO:Polythiophene Solar Cells via Side Chain Functionalization. Adv. Energy Mater. 2011, 1, 90-96. (4) Baeten,L.; Conings, B.; Boyen, H.-G.; D’Haen, J.; Hardy, A.; D’Olieslaeger, M.; Manca, J. V.; 16 ACS Paragon Plus Environment

Page 16 of 23

Page 17 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Bael, M. K. V. Towards Efficient Hybrid Solar Cells Based on Fully Polymer Infiltrated ZnO Nanorod Arrays. Adv. Mater. 2011, 23, 2802-2805. (5) Moghaddam, R. S.; Huettner, S.; Vaynzof, Y.; Ducati, C.; Divitini, G.; Lohwasser, R. H.; Musselman, K. P.; Sepe, A.; Scherer, M. R. J.; Thelakkat, M.; Steiner, U.; Friend, R. H. Polymer Crystallization as a Tool To Pattern Hybrid Nanostructures: Growth of 12 nm ZnO Arrays in Poly(3-hexylthiophene). Nano Lett. 2013, 13, 4499-4504. (6) Kedem, N.; Blumstengel, S.; Henneberger, F.; Cohen, H.; Hodes, G.; Cahen, D. Morphology-, Synthesis- and Doping-Independent Tuning of ZnO Work Function using Phenylphosphonates, Phys. Chem. Chem. Phys. 2014, 16, 8310-8319. (7) Lange, I.; Reiter, S.; Pätzel, M.; Zykov, A.; Nefedov, A.; Hildebrandt, J.; Hecht, S.; Kowarik, S.; Wöll, C.; Heimel, G.; Neher, D. Tuning the Work Function of Polar Zinc Oxide Surfaces using Modified Phosphonic Acid Self-Assembled Monolayers. Adv. Funct. Mater. 2014, 24, 7014-7024. (8) Schlesinger, R.; Xu, Y.; Hofmann, O. T.; Winkler, S.; Frisch, J.; Niederhausen, J.; Vollmer, A.; Blumstengel, S.; Henneberger, F.; Rinke, P.; Scheffler, M.; Koch, N. Controlling the Work Function of ZnO and the Energy-Level Alignment at the Interface to Organic Semiconductors with a Molecular Electron Acceptor. Phys. Rev. B 2013, 87, 155311. (9) Schlesinger, R.; Bianchi, F.; Blumstengel, S.; Christodoulou, C.; Ovsyannikov, R.; Kobin, B.; Moudgil, K.; Barlow, S.; Hecht, S.; Marder, S.R.; Henneberger, F.; Koch, N. Efficient Light Emission from Inorganic and Organic Semiconductor Hybrid Structures by Energy-Level Tuning. Nature Commun. 2015, 6, 6754. (10) Blumstengel, S.; Sadofev, S.; Xu, C.; Puls, J.; Johnson, R. L.; Glowatzki, H.; Koch, N.; Henneberger, F. Electronic Coupling in Organic-Inorganic Semiconductor Hybrid Structures with Type-II Energy Level Alignment. Phys. Rev. B 2008, 77, 085323.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(11) Huang, J.; Yin, Z.; Zheng, Q. Applications of ZnO in Organic and Hybrid Solar Cells. Energy Environ. Sci. 2011, 4, 3861-3877. (12) Hewlett, R. M.; McLachlan, M. A. Surface Structure Modification of ZnO and the Impact on Electronic Properties. Adv. Mater. 2016, 28, 3893-3921. (13) Sung, Y.-H.; Liao, W.-P.; Chen, D.-W.; Wu, C.-T.; Chang, G.-J.; Wu, J.-J. Room-Temperature Tailoring of Vertical ZnO Nanoarchitecture Morphology for Efficient Hybrid Polymer Solar Cells. Adv. Funct. Mater. 2012, 22, 3808-3814. (14) Oosterhout, S. D.; Wienk, M. M.; van Bavel, S. S.; Thiedmann, R.; Koster, L. J. A.; Gilot, J.; Loos, J.; Schmidt, V.; Janssen, R. A. J. The Effect of Three-Dimensional Morphology on the Efficiency of Hybrid Polymer Solar Cells. Nat. Mater. 2009, 8, 818-824. (15) Dang, M. T.; Hirsch, L.; Wantz, G. P3HT:PCBM, Best Seller in Polymer Photovoltaic Research. Adv. Mater. 2011, 23, 3597-3602. (16) Vandewal, K.; Tvingstedt, K.; Gadisa, A.; Inganäs, O.; Manca, J. V. On the Origin of the Open-Circuit Voltage of Polymer-Fullerene Solar Cells. Nat. Mater. 2009, 8, 904-909. (17) Vandewal, K.; Gadisa, A.; Oosterbaan, W. D.; Bertho, S.; Banishoeib, F.; Van Severen, I.; Lutsen, L.; Cleij, T. J.; Vanderzande, D.; Manca, J. V. The Relation Between Open-Circuit Voltage and the Onset of Photocurrent Generation by Charge-Transfer Absorption in Polymer:Fullerene Bulk Heterojunction Solar Cells. Adv. Funct. Mater. 2008, 18, 2064-2070. (18) Hörmann, U.; Kraus, J.; Gruber, M.; Schuhmair, C.; Linderl, T.; Grob, S.; Kapfinger, S.; Klein, K.; Stutzman, M.; Krenner, H. J.; Brütting, W. Quantification of Energy Losses in Organic Solar Cells from Temperature-Dependent Device Characteristics. Phys. Rev. B 2013, 88, 235307. (19) Gélinas, S.; Rao, A.; Kumar, A.; Smith, S. L.; Chin, A.W.; Clark, J.; van der Poll, T. S.;


Page 18 of 23

Page 19 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Bazan, G. C.; Friend, R. H. Ultrafast Long-Range Charge Separation in Organic Semiconductor Photovoltaic Diodes. Science 2014, 343, 512-516. (20) Bernardo, B.; Cheyns, D.; Verreet, B.; Schaller, R. D.; Rand, B. P.; Giebink, N. C. Delocalization and Dielectric Screening of Charge Transfer States in Organic Photovoltaic Cells. Nat. Commun. 2014, 5, 3245. (21) Yang, B.; Yi, Y.; Zhang, C.-R.; Aziz, S. G.; Coropceanu, V.; Brédas, J.-L. Impact of Electron Delocalization on the Nature of the Charge-Transfer States in Model Pentacene/C60 Interfaces: A Density Functional Theory Study. J. Phys. Chem. C 2014, 118, 27648-27656. (22) Piersimoni, F.; Schlesinger, R.; Benduhn, J.; Spoltore, D.; Reiter, S.; Lange, I.; Koch, N.; Vandewal, K.; Neher, D. Charge Transfer Absorption and Emission at ZnO/Organic Interfaces. J. Phys. Chem. Lett. 2015, 6, 500-504. (23) Eyer, M.; Sadofev, S.; Puls, J.; Blumstengel, S. Charge Transfer Excitons at ZnMgO/P3HT Heterojunctions: Relation to Photovoltaic Performance. Appl. Phys. Lett. 2015, 107, 221602. (24) Li, H.; Brédas, J.-L. Comparison of the Impact of Zinc Vacancies on Charge Separation and Charge Transfer at ZnO/Sexithienyl and ZnO/Fullerene Interfaces. Adv. Mater. 2016, 28, 3928-3936. (25) Sevinchan, Y.; Hopkinson, P. E.; Bakulin, A. A.; Herz, J.; Motzkus, M.; Vaynzof, Y. Improving Charge Separation across a Hybrid Oxide/Polymer Interface by Cs Doping of the Metal Oxide. Adv. Mater. Interfaces 2016, 3, 1500616. (26) Sadofev, S.; Blumstengel, S.; Cui, J.; Puls, J.; Rogaschewski, S.; Schäfer, P.; Sadofyev, Yu. G.; Henneberger, F. Growth of High-quality ZnMgO Epilayers and ZnO/ZnMgO Quantum Well Structures by Radical-Source Molecular-Beam Epitaxy on Sapphire. Appl. Phys. Lett. 2005, 87, 091903.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(27) Sadofev, S.; Kalusniak, S.; Schäfer, P.; Henneberger, F. Molecular Beam Epitaxy of nZn(Mg)O as a Low-Damping Plasmonic Material at Telecommunication Wavelengths. Appl. Phys. Lett. 2013, 102, 181905. (28) Zhang, H. H.; Pan, X. H.; Lu, B.; Huang, J. Y.; Ding, P.; Chen, W.; He, H. P.; Lu, J. G.; Chen, S. S.; Ye, Z. Z. Mg Composition Dependent Band Offsets of Zn(1-x)Mg(x)O/ZnO Heterojunctions. Phys. Chem. Chem. Phys. 2013, 15, 11231-11235. (29) Ishii, H.; Sugiyama, K.; Ito, E.; Seki, K. Energy Level Alignment and Interfacial Electronic Structures at Organic/Metal and Organic/Organic Interfaces. Adv. Mater. 1999, 11, 605-625. (30) Kahn, A.; Koch, N.; Gao, W. Y. Electronic Structure and Electrical Properties of Interfaces between Metals and π-Conjugated Molecular Films. J. Polym. Sci. Part B 2003, 41, 2529-2548. (31) Greiner, M. T.; Helander, M. G.; Tang, W.-M.; Wang, Z.-B.; Qiu, J.; Lu, Z.-H. Universal Energy-Level Alignment of Molecules on Metal Oxides. Nat. Mater. 2012, 11, 76-81. (32) Guan, Z.-L.; Kim, J. B.; Loo, Y.-L.; Kahn, A. Electronic Structure of the Poly(3hexylthiophene):Indene-C60 Bisadduct Bulk Heterojunction. J. Appl. Phys. 2011, 110, 043719. (33) Spoerke, E. D.; Lloyd, M. T.; McCready, E. M.; Olson, D. C.; Lee, Y. J.; Hsu, J. W. P. Improved Performance of Poly(3-hexylthiophene)/Zinc Oxide Hybrid Photovoltaics Modified with Interfacial Nanocrystalline Cadmium Sulfide. Appl. Phys. Lett. 2009, 95, 213506. (34) Poelking, C.; Tietze, M.; Elschner, C.; Olthof, S.; Hertel, D.; Baumeier, B.; Würthner, F.; Meerholz, K.; Leo, K.; Andrienko, D. Impact of Mesoscale Order on Open-Circuit Voltage in Organic Solar Cells. Nature Mater. 2015, 14, 434-439. (35) Deotare, P. B.; Chang, W.; Hontz, E.; Congreve, D. N.; Shi, L.; Reusswig, P. D.; Modtland, B.; Bahlke, M. E.; Lee, C. K.; Willard, A. P.; Bulovic, V.; Van Voorhis, T.; Baldo, M.


Page 20 of 23

Page 21 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


A. Nanoscale Transport of Charge-Transfer States in Organic Donor-Acceptor Blends. Nat. Mater. 2015, 14, 1130-1134. (36) Rau, U. Reciprocity Relation between Photovoltaic Quantum Efficiency and Electroluminescent Emission of Solar Cells. Phys. Rev. B 2007, 76, 085303. (37) Marcus, R. A. Relation between Charge Transfer Absorption and Fluorescence Spectra and the Inverted Region. J. Phys. Chem. 1989, 93, 3078-3086. (38) Baessler, H.; Schweitzer, B. Site-Selective Fluorescence Spectroscopy of Conjugated Polymers and Oligomers. Acc. Chem. Res. 1999, 32, 173-182.


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy (eV)


Page 22 of 23

EHCTS

VOC Inorganic gap energy (eV)

TOC Graphic.


Page 23 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


For Table of Contents use only 430x214mm (300 x 300 DPI)

ACS Paragon Plus Environment

Role of Hybrid Charge Transfer States in the Charge Generation at

Recommend Documents