A method for photocatalytic doping of an organic semiconductor

Abstract

The present invention relates to a method for photocatalytic doping of an organic semiconductor, the method comprising the steps of: a) providing a first organic semiconductor; b) subjecting the first organic semiconductor to a solution comprising a solvent, a counter ion salt, a weak dopant and a photocatalyst; c) subjecting the first organic semiconductor and solution in step b) to electromagnetic irradiation having a first dominant peak wavelength in the range from 10 nm to 2000 nm.

Description

[0001] A METHOD FOR PHOTOCATA LYTIC DOPING OF AN ORGANIC

[0002] SEMICONDUCTOR

[0003] TECHNICAL FIELD

[0004] The present invention relates to a method for photocatalytic doping of an organic semiconductor, and a doped organic semiconductor obtained by such a method.

[0005] BACKGROUND OF THE INVENTION

[0006] Chemical doping plays a pivotal role in optimizing the performance of various organic semiconductor-based devices, including light-emitting diodes, organic / perovskite photovoltaics, field-effect transistors, thermoelectrics, and electrochemical devices. In particular, chemical doping is a key approach to manipulate charge carrier concentration and trapping in organic semiconductors, ultimately enhancing their device performance. Numerous doping methods have been developed to achieve effective chemical doping, such as single-electron direct transfer doping, stimulus-activated indirect doping, in-situ doping during polymerization, ground-state charge transfer, transition metal-catalyzed doping, and post-processing techniques involving secondary doping or counterion exchange. However, all these strategies rely on the use of strong oxidizing / reducing agents (dopants) to alter the redox state of organic semiconductors. This often necessitates the use of large amounts of strong dopants, which are consumed during the doping process and result in undesirable chemical residues within the doped semiconductor films. Moreover, these doping methods require harsh processing conditions to activate the dopants, such as vacuum evaporation or high-temperature annealing. Therefore, developing a mild doping method utilizing weak dopants is crucial to enhance compatibility with semiconductor devices.

[0007] Therefore, there is a need for developing an improved method for efficient doping of organic semiconductors with weak dopants under mild conditions.

[0008] SUMMARY OF THE INVENTION

[0009] Considering the above, the present invention aims to solve the problems of the prior art. To this end, the present invention relates to a method for photocatalytic doping of an organic semiconductor, the method comprising the steps of: a) providing a first organic semiconductor; b) subjecting the first organic semiconductor to a solution comprising a solvent, a counter ion salt, a weak dopant and a photocatalyst; c) subjecting the first organic semiconductor and solution in step b) to electromagnetic irradiation having a first dominant peak wavelength in the range from 10 nm to 2000 nm.

[0010] The first organic semiconductor may be a conjugated polymer. Such conjugated polymers may have different ionization potentials (IPs) and hydrophilic / hydrophobic side chains. Examples of conjugated polymers suitable as organic semiconductors in the context of the present invention are poly[2,3-bis(3- octyloxyphenyl)-5,8-quinoxalinediyl]-2,5-thiophenediyl] (PTQ1), poly(3- hexylthiophene-2,5-diyl) (P3HT), polar side chain modificated poly-bithiophene- thiophene (P(g42T-T)) and poly-bithiophene-thienothiophene (P(g42T-TT)), poly[2,5- bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene] (PBTTT) and poly(benzimidazobenzophenanthroline) (BBL) or the like.

[0011] Preferably, the first organic semiconductor is in the form of a thin film arranged on a substrate. Such an embodiment offers the advantage of facilitating recovery of the photocatalyst, as will be described in detail below. The first organic semiconductor may be spin-coated or drop-cast in air and ambient temperature, forming the film having thicknesses from 1 nm to 1 cm, more preferably from 10 nm to 1 pm. The substrate may be a glass or Si / SiC>2 substrate having the SiO2 layer thickness of 10-2000 nm.

[0012] According to step b) of the present invention, the first organic semiconductor is subjected to a solution comprising a solvent, a counter ion salt, a weak dopant and a photocatalyst. By the term “weak dopant” is understood a dopant lacking the ability to directly dope organic semiconductor. The solvent may be any solvent suitable for solubilizing the counter ion salt, weak dopant and the photocatalyst. If first organic semiconductor is in solution rather than as a thin film, the solvent used in step b) should be able to solubilize the first organic semiconductor as well. When the first organic semiconductor is in the form of a thin film, step b) may be performed by immersing the substrate with the thin film arranged on it into the solution comprising the counter ion and the photocatalyst.

[0013] The counter ion salt may be in the form of an organic salt, such as lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), 1-Ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][TFSI]).

[0014] The cation in the organic salt may be selected from a group consisting of 1- ethyl-3-methylimidazolium (EMIM), 1-butyl-3-methylimidazolium (BMIM), 1-allyl-3- methylimidazolium (AMIM), 1-hexyl-3-methylimidazolium (HMIM), butyl-methyl pyrrolidinium (BMP), propyl-methyl pyrrolidinium (PMP), triethyl sulfonium and mixtures thereof. The structures of the cations are shown below.

[0015] According to the present invention, the anion may be selected from a group consisting of chloride (Cl"), bromide (Br), iodide (I-), acetate (OAc), tetrafluoro borate (BFT), hexafluoro phosphate (PFe’), bis-trifluoromethanesulfonimide (TFSI), trifluoromethanesulfonate (OTf-), dicyanamide (DCA-), hydrogen sulphate (HSOT), ethyl sulphate (ESOT), thiocyanate (SON-), tosylate (OTs-), mesylate (OMs-), tetrachloro aluminate (AICU ), diethyl phosphate (DEP-), dimethyl phosphate (DMP-), lactate (La-), L-alanine anion (APP-) and mixtures thereof. The structures of the anions are illustrated below.

[0016] The photocatalyst may be an acridinium-based photocatalyst, such as 10- methylacridinium perchlorate (Acr-Me), 9-mesityl-10-methylacridinium tetrafluoroborate (Mes-Acr-Me), 9-mesityl-3,6-di-tert-butyl-10-phenylacridinium tetrafluoroborate (Mes-Acr-Ph). Further, the photocatalyst may be alkylated perylene- diimide (PDI-CeC?), or 2',4',5',7'-tetrabromofluorescein disodium salt (Eosin Y).

[0017] Mes^Ac^r-Me PDI-C6C7

[0018] In particular, the solvent may be a mixture of butyl acetate and acetonitrile. This solvent mixture exhibits orthogonality to PBTTT and good solubility of the photocatalyst and the counter ion.

[0019] Concentration of the photocatalyst in step b) may be in the range from 10'8to 5 M. Concentration of the counter ion in step b) may be in the range from 10'8to 5 M.

[0020] During step c) of the method according to the present invention, the first organic semiconductor, the counter ion and the photocatalyst are subjected to electromagnetic irradiation having a first dominant peak wavelength in the range from 10 nm to 2000 nm in the presence of a dopant.

[0021] Photocatalysis is a widely employed technique in organic synthesis due to its exceptional selectivity and efficiency in facilitating redox-based reactions. Weak oxidants, such as oxygen in air, or weak reductants, like triethylamine or diisopropylethylamine, can effectively oxidize or reduce aromatic compounds through photoactivation or photoinduced electron transfer.

[0022] Thus, intriguing questions are whether weak dopants can be photocatalytically activated to controllably oxidize (p-doping) / reduce (n-doping) the TT- electron system of organic semiconductors and whether this can have technological implications.

[0023] The inventors surprisingly found that the method of the present invention allows using weak dopants under mild conditions for doping organic semiconductors. As will be shown in detail below, weak dopants or ground state photocatalysts cannot dope organic semiconductors, while excited state photocatalysts obtained by electromagnetic irradiation can dope organic semiconductor and allow regeneration of the photocatalyst by the weak dopant.

[0024] As weak p-dopant, oxygen or 1 ,2-diphenyldisulfane (DPDS) may be used. As weak n-dopant, triethylamine (EtsN) may be used.

[0025] The electromagnetic irradiation may be visible blue light having a dominant peak wavelength in the range from 380 to 500 nm, or UV light having a dominant peak wavelength in the range from 10 to 380 nm.

[0026] The first organic semiconductor may be a p-type organic semiconductor and the weak dopant may be a p-type dopant. Alternatively, the first organic semiconductor may be an n-type organic semiconductor and the weak dopant may be a n-type dopant.

[0027] Duration of step c) may be from 0.001 s to 1 h.

[0028] The method of the present invention further allows for coupled photocatalytic p-doping and n-doping. A method according to such an embodiment further comprises the steps of: a') providing a second organic semiconductor; b') subjecting the second organic semiconductor to the solution comprising a solvent, an organic salt and a photocatalyst; c') subjecting the second organic semiconductor to electromagnetic irradiation having a second dominant peak wavelength in the range from 10 nm to 2000 nm. wherein the first dominant peak wavelength is different from the second dominant peak wavelength.

[0029] Preferably, the second organic semiconductor is in the form of a thin film arranged on a substrate. Such an embodiment offers the advantage of facilitating recovery of the photocatalyst, as will be described in detail below. The second organic semiconductor may be spin-coated or drop-cast in air and ambient temperature, forming the film having thicknesses from 1 nm to 1 cm, more preferably from 10 nm to 1 pm. The substrate may be a glass or Si / SiC>2 substrate having the SiC>2 layer thickness of 200 nm.

[0030] In particular, the first organic semiconductor may be a p-type organic semiconductor and the second organic semiconductor may be an n-type organic semiconductor. Alternatively, the first organic semiconductor may be an n-type organic semiconductor and the second organic semiconductor is a p-type organic semiconductor.

[0031] The first dominant peak wavelength may be in the range from 380 to 500 nm, and the second dominant peak wavelength may be in the range from 10 to 380 nm.

[0032] In order to improve cost efficiency of the method according to the present invention even further, the method according to the present invention may further comprise a step of: d) recovering the photocatalyst and the counter ion.

[0033] The present invention further relates to a doped organic semiconductor manufactured by the method as described above. Such a doped organic semiconductor exhibits high electrical conductivity in the order of 1 to 1000 S / cm and even up to 3000 S / cm.

[0034] The general overview of the method of the present invention may be summarized as follows:

[0035]

[0036] As shown in the proposed catalytic cycle, this photocatalytic doping process is happening between also the photocatalyst solution and organic semiconductor thin films, and suitable counter ions are also involved, so that the molecular packing of organic semiconductors can be preserved after doping.

[0037] The present invention relates to a novel concept of photocatalytic doping for organic semiconductors, which offers a simple and efficient solution-based process at room temperature. The doping level can be easily controlled by adjusting the light irradiation dose. Compared to conventional doping methods that rely on highly reactive dopants, which are consumed during the doping process, photocatalytic doping employs recyclable and air-stable photocatalysts and consumes only salt and weak dopants such as air. This method exhibits a certain degree of universality and can achieve both p-doping, n-doping, and even coupled p-doping / n-doping. Photocatalyically p-doped organic semiconductors with electrical conductivity exceeding 3000 S cm-1and n-doped organic semiconductors with electrical conductivity over 1 S cm-1were obtained. This demonstrates the great application potential of photocatalytic doping in organic semiconductor doping and organic semiconductor devices.

[0038] All these results suggest that photocatalytic doping can lead to microstructures that are favourable for carrier transport, because suitable counterions can be selected and the photocatalysts do not remain in the doped organic semiconductor thin films.

[0039] The present invention shows a novel concept of photocatalytic doping of organic semiconductors, utilizing air as a weak oxidant (p-dopant) and operating at room temperature. This approach demonstrates universality across various organic semiconducting polymers and photocatalysts, yielding electrical conductivities that exceed 3000 S cm-1. Additionally, successful photocatalytic reduction (n-doping) and one-pot p-doping / n-doping of organic semiconductors were demonstrated, where the only chemical to be consumed is salt. The photocatalytic doping method of the present invention offers significant potential for advancing organic semiconductor doping and developing next-generation organic electronic devices.

[0040] BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Embodiments of the invention will now be described by way of example with reference to the accompanying drawings, of which:

[0042] Figs. 1a-1c show schematics of the photocatalytic p-doping process: weak dopant cannot dope organic semiconductor (a); ground state photocatalyst (PC) cannot dope organic semiconductor (b); excited state PC can dope organic semiconductor and the PC can be regenerated by weak dopant (c).

[0043] Fig. 2a shows schematics of the photocatalytic p-doping condition and process of PBTTT ;

[0044] Fig. 2b depicts absorption spectra of undoped and doped PBTTT thin films;

[0045] Fig. 2c illustrates X-ray photoelectron spectroscopy (XPS) analysis of undoped and doped PBTTT thin films;

[0046] Fig. 2c'a illustrates out-of-plane GIWAXS line cuts of undoped and doped PBTTT thin films; Fig. 2c'b depicts in-plane GIWAXS line cuts of undoped and doped PBTTT thin films;

[0047] Fig. 2d shows S(2p) and F(1s) XPS analysis of undoped and doped PBTTT films;

[0048] Fig. 2d'a shows XPS analysis of undoped PBTTT and doped PBTTT (photocatalyzed by Acr-Me) thin films under different 455 nm light irradiation time;

[0049] Fig. 2d'b depicts N(1s) XPS spectra undoped PBTTT and doped PBTTT thin films under different light irradiation time. XPS spectra shows increased TFSI- intensity in PBTTT films after light irradiation, indicating effective photocatalytic doping;

[0050] Fig. 2e illustrates Ultraviolet photoelectron spectroscopy (UPS) of undoped and photodoped PBTTT thin films;

[0051] Fig. 2f depicts electrical conductivity of undoped and photodoped PBTTT thin films, demonstrating photocatalytic doping can happen only when both photocatalyst and light are present;

[0052] Fig. 2f'a shows electrical conductivity of doped PBTTT (photocatalyzed by Acr-Me) thin films in various thickness;

[0053] Fig. 2f'b shows absorption spectra of undoped and doped PBTTT thin films in various thickness;

[0054] Fig. 2f"a illustrates electrical conductivity of doped PBTTT (photocatalyzed by Acr-Me) thin films under different light exposure power, plotting in light irradiation time;

[0055] Fig. 2f"b illustrates electrical conductivity of doped PBTTT (photocatalyzed by Acr-Me) thin films under different light exposure power, plotting in light irradiation dose (b);

[0056] Fig. 3a depicts schematic diagram of the transition of ground state, excited state, reduced state and excited reduced state of photocatalyst Mes-Acr-Me, by laser pump pulse exciting or reacting with optically transparent n-dopant (triethylamine) under 455 nm light irradiation or regenerating in air; Figs 3b-d illustrate In-situ 2D transient absorption spectra (TAS) of PC (Mes- Acr-Me) + 10 equivalent weak n-dopant (triethylamine) solution, as the function of detector wavelength and delay time: initial state in nitrogen atmosphere (b), after 455 nm light irradiation for 120 s in nitrogen (c), and PC regenerated in air (d).

[0057] Fig. 3e shows energy levels: electron affinity (EA) of traditional single-electron transfer p-dopants, ionisation potential (IP) of organic semiconductors, and EA of photocatalyst in both ground state and excited state;

[0058] Fig. 3e'a-e illustrate absorption spectra of undoped and doped (photocatalyzed by Acr-Me) PTQ1 (a), PBTTT (b), P3HT (c), P(g42T-TT) (d), and P(g42T-T) (e) thin flims;

[0059] Fig. 3e'f illustrate absorption spectra of absorption spectra of undoped and doped P(g42T-T) in various thickness;

[0060] Fig. 3f illustrates electrical conductivity of photocatalytically doped P(g42T-T) thin films by different photocatalysts, showing the maxima electrical conductivity over 3000 S cm’1;

[0061] Fig. 3f a depicts energy barrier (EAphotocatatiyst—IP organic semiconductor) of singleelectron transfer from organic semiconductor to ground / excited photocatalyst;

[0062] Fig. 3f'b shows maximum electrical conductivity of undoped and catalytically doped PTQ1 , PBTTT, P3HT, P(g42T-TT), and P(g42T-T), by Eosin Y, PDI-C6C7, Mes- Acr-Ph, Mes-Acr-Me, and Acr-Me;

[0063] Fig. 3f'c-f illustrate electrical conductivity of organic semiconductor PTQ1 (c), P3HT (d) PBTTT (e) and P(g42T-TT) (f), the thin films were photocatalytically doped by different photocatalysts;

[0064] Fig. 3f"a shows electrical conductivity of doped P(g42T-T) thin films in the absence of p-dopant (diphenyl disulfide, DPDS) or photocatalyst (Acr-Me), showing that both the p-dopant and photocatalyst are necessary for p-doping;

[0065] Fig. 3f"b illustrate electrical conductivity of doped P(g42T-T) thin films in the presence of DPDS with low or high photocatalyst concentration, showing that the doped P(g42T-T) shows a consistent trend in the maximum conductivity with a different photocatalyst concentration used. High photocatalyst concentration leads to faster doping;

[0066] Fig. 3f"c illustrate electrical conductivity of doped P(g42T-T) thin films in the presence or absence of O2, presenting that p-dopant is necessary for doping, and 0.1 M DPDS is stronger that air concentration O2 at low photocatalyst concentration;

[0067] Fig 3f"d depicts comparison of electrical conductivity of doped P(g42T-T) thin films with different p-dopant (DPDS, O2), showing that O2 and DPDS show very similar effect in photocatalytic doping at high photocatalyst concentration;

[0068] Fig. 3f'"a illustrates out-of-plane GIWAXS line cuts of undoped and doped P(g42T-T) thin films;

[0069] Fig. 3f'"b illustrates in-plane GIWAXS line cuts of undoped and doped P(g42T-T) thin films;

[0070] Fig. 3g depicts electrical conductivity enhancement of organic semiconductors (compared to undoped thin films) versus energy barrier of singleelectron transfer from organic semiconductors to excited photocatalysts;

[0071] Fig. 4a shows schematics of the photocatalytic n-doping condition and process;

[0072] Fig. 4b shows proposed catalytic cycle for coupled photocatalytic p-doping and n-doping of organic semiconductors;

[0073] Fig. 4c shows differential absorption spectra of BBL thin films after photocatalytic n-doping (by Mes-Acr-Me) with triethylamine or P(g42T-T);

[0074] Fig. 4c'a shows absorption spectra of undoped BBL, triethylamine and P(g42T-T) doped (photocatalyzed by Mes-Acr-Me) BBL thin films;

[0075] Fig. 4c'b depicts absorption spectra of undoped P(g42T-T), BBL doped (photocatalyzed by Mes-Acr-Me) and oxygen doped P(g42T-T) thin films;

[0076] Fig. 4d depicts electrical conductivity of undoped and triethylamine n-doped BBL thin films;

[0077] Fig. 4e illustrates electrical conductivity of coupled photocatalytic p-doped P(g42T-T) and n-doped BBL; Fig. 4e'a shows electrical conductivity of triethylamine n-doped BBL thin films, under both 390 nm and 455 nm light irradiation or only 390 nm light irradiation;

[0078] Fig. 4e'b illustrates electrical conductivity of coupled photocatalytically p- doped P(g42T-T) and n-doped BBL, under both 390 nm and 455 nm light irradiation or only 390 nm light irradiation.

[0079] DETAILED DESCRIPTION OF THE INVENTION

[0080] Methods

[0081] Materials. 10-methylacridinium perchlorate (Acr-Me) is purchased from TCI (www.tcichemicals.com). 9-mesityl-10-methylacridinium tetrafluoroborate (Mes-Acr- Me), 9-mesityl-3,6-di-terf-butyl-10-phenylacridinium tetrafluoroborate (Mes-Acr-Ph), 2',4',5',7'-tetrabromofluorescein disodium salt (Eosin Y), , lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), 1-Ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][TFSI]), 1 ,2-diphenyldisulfane (DPDS), n- butyl acetate (BuOAc), acetonitrile (CH3CN), ether (Et20), triethylamine (EtsN), 1 ,2- dichlorobenzene (ODCB), methanesulfonic acid (MSA), and dimethylformamide (DMF) were purchased from Sigma-Aldrich (www.sigmaaldrich.com). Poly[2,3-bis(3- octyloxyphenyl)-5,8-quinoxalinediyl]-2,5-thiophenediyl] (PTQ1) and Poly(3- hexylthiophene-2,5-diyl) (P3HT) were purchased from Ossila (www.ossila.com). CH3CN were dried with 3 molecular sieve before use. Acr-Me, Mes-Acr-Me, Mes- Acr-Ph were recrystallized using CH3CN / Et2O mixed solvent before use. Other materials were used as received. Poly[2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2- b]thiophene] (PBTTT), Alkylated perylene-diimide (PDI-CeC?), polar side chain modificated poly-bithiophene-thiophene (P(g42T-T)) and poly-bithiophene- thienothiophene (P(g42T-TT)), and poly(benzimidazobenzophenanthroline) (BBL, r) = 6.3 dL g-1in MSA at 30 °C, Mw= 35 kDa) were synthesized following the procedure known in the art.

[0082] Sample preparation. PTQ1 , PBTTT, P3HT, P(g42T-TT), and P(g42T-T) were dissolved in ODCB to obtain corresponding organic semiconductor solutions. PTQ1 , PBTTT, P3HT, P(g42T-TT), and P(g42T-T) thin films were obtained by spin-coating the corresponding organic semiconductor solutions to substrates. BBL was dissolved in MSA and spin-coated to substrates, then the substrates were immersed in water to remove residual MSA and dried with nitrogen flow to obtain BBL thin films. Unless otherwise stated, the typical photocatalyst solution recipes: (1) for p-doping with O2 as p-dopant: BuOAc:CHsCN = 3:1 (volume content), CLITFSI = 0.1 M, Cphotocataiyst = 0.01 M; (2) for p-doping with DPDS as p-dopant: BuOAc:CHsCN = 3:1 , CLITFSI = 0.1 M, CDPDS = 0.1 M, Cphotocataiyst = 0.01 M; (3) for n-doping with triethylamine as n-dopant: BuOAC.CHsCN = 3.1 , C[EMEI][TFSI] = 0.1 M, Ctriethylamine = 0.1 M, Cphotocataiyst = 0.01 M, (4) for coupled p-type and n-type doping: BuOAc:CHsCN = 3:1 , C[EMEI][TFSI] = 0.1 M, Cphotocataiyst = 0.01 M. Typical solution configuration process: Acr-Me, Mes-Acr-Me, Mes-Acr-Ph, PDI-CeC?, or Eosin Y were dissolve in dried CH3CN, then BuOAc was added to reach BuOAc: CH3CN = 3:1 in in volume content. For Eosin Y, 1% volume content of DMF was involved to help dissolving. Then counter ions LiTFSI (or [EMIM][TFSI]) was then added, and DPDS or Et3N are also added for recipe (2) or (3). For recipe (1), the solution was saturated with air before use.

[0083] Photocatalytic p-doping. Organic semiconductor thin films were covered with photocatalyst solution and irradiated with 455 nm blue light. Unless otherwise stated, the light irradiation intensity to organic semiconductor thin films is 50 mW cm’2. A heat sink was placed under the organic semiconductor thin films to avoid the thermal effect of light irradiation and to slow down solvent evaporation. For using O2 as weak p-dopant, the photocatalytic doping is processed in air. For using DPDS as weak p-dopant, the photocatalytic doping is processed in nitrogen atmosphere. After light irradiation, the photocatalyst solution were recovered and organic semiconductor thin films were washed with clean BuOAc:CH3CN = 3:1 mixed solvent, and dried with nitrogen flow.

[0084] Photocatalytic n-doping. All the n-doping photocatalytic n-doping and coupled p-type and n-type doping were processed in glove box filled with nitrogen. Organic semiconductor thin films were covered with photocatalyst solution and irradiated with both 455 nm blue light and 390 nm UV light. The light irradiation intensity for both blue light and UV light are 50 mW cm-2. A heat sink was placed under the organic semiconductor thin films to avoid the thermal effect of light irradiation and to slow down solvent evaporation. After light irradiation, the photocatalyst solution were recovered and organic semiconductor thin films were washed with clean BUOAC:CH3CN = 3:1 mixed solvent, and dried with nitrogen flow.

[0085] UV-Vis-NIR absorption spectra. Organic semiconductor thin films were prepared on glass substrates. For air-sensitive samples, like n-doped thin films, samples were placed in sealed spectral cell filled with nitrogen. UV-Vis-NIR absorption spectra of the films were measured with Perkin Elmer Lambda 900 with a resolution of 2 nm.

[0086] UPS and XPS. Organic semiconductor thin films were prepared on high conductivity ITO glass substrates. XPS experiment was carried out in a Scienta ESCA 200 system with a base pressure of 2x1O-10mbar equipped with an SES 200 electron analyzer, a monochromatic Al Ka X-ray source (1486.6 eV), and a helium discharge lamp (21 .22 eV) for XPS and UPS, respectively. All spectra were collected at normal emission and were calibrated by a sputter-cleaned Au film with the Fermi level at 0 eV and the Au(4 ) peak at 84.0 eV. The work function was extracted from the edge of the secondary electron cutoff in UPS while applying a -3 V bias on the sample.

[0087] Electrical characterization. High electrical conductivity samples (> 1 S cm’1) were measured with four-probe conductivity measurements, the channel length / width is 0.5 mm / 10 mm. Low electrical conductivity samples (< 1 S cm-1) measured with two-probe conductivity measurements, the channel length / width is 0.5 mm / 10 mm or 6 pm / 200 pm. For channel length / width of 0.5 mm / 10 mm substrates, 5 nm of chromium as adhesive layer and 50 nm of gold were thermally evaporated on cleaned glass substrates, and the electrodes was patterned by photolithography. Organic semiconductor thin films were spin-coated and patterned by plastic tape. For channel length / width of 6 pm / 200 pm substrates, 5 nm of chromium as adhesive layer and 50 nm of gold were thermally evaporated on cleaned glass substrates, and the electrodes was patterned by photolithography. Double-layer parylene technique was used to pattern organic semiconductor52. A first-layer of parylene C (1 pm) was deposited in the presence of 3-(trimethoxysilyl)propyl methacrylate (A-174 Silane) to increase adhesion. An antiadhesive layer of industrial surfactant (2% Micro-90) was spin-coated, and a second layer of parylene C (sacrificial layer, 2 pm) was deposited. A 5-pm-thick AZ10XT520CP positive photoresist was then spin-coated on the second parylene C layer. This protects the parylene C layers from the following plasma reactive ion etching (RIE) step (150 W, 500 seem 02, 100 seem CF4, 380 s). Another photolithographic patterning was performed to define the contact pads and channel, and the AZ developer was applied to the photoresist. A plasma RIE was carried out to remove the organic materials (photoresist and parylene C), exposing the channel area and the contact pads. The remaining surface remained covered with layers of parylene C. Organic semiconductor thin film was spin-coated, and the sacrificial parylene layer was then peeled off to remove the unwanted organic semiconductor film outside the channel area. Electrical measurements were performed using a Keithley 4200-SCS semiconductor characterization system. For n-doped organic semiconductors, the electrical conductivity measurements were performed inside a nitrogen-filled glovebox.

[0088] Cyclic voltammetry. Cyclic voltammetry measurements were performed on Potentiostat BioLogic SP-200. A 0.1 M n-Bu4NPFe in anhydrous BuOAc:CH3CN (3:1) solution electrolyte (bubbling with nitrogen for 1 min to remove the dissolved oxygen before measures) was used for Acr-Me, Mes-Acr-Me, Mes-Acr-Ph or PDI-CeC? and standard ferrocene sample. For Eosin Y, 1% volume content of DMF was add to the above electrolyte for helping dissolving. The photocatalysts or ferrocene were dissolved in the electrolyte, the working electrode (WE) was glassy carbon, the counter electrode (CE) was a platinum mesh and reference electrode (RE) is Ag / AgCI (saturated). The scan rate was 50 mV s-1.

[0089] Photoluminescence spectroscopy. The samples were excited at 3.1 eV (400 nm) using the second harmonic of a mode-locked Thsapphire laser (Mira 900, coherent) at a repetition rate of 76 MHz. The excitation beam was spatially limited by an iris and focused with a lens of 150 mm focal length. The fluence was adjusted using neutral density filters to 76 nJ cm-2. The excitation fluence per pulse was estimated assuming the diameter of the focused beam dfocus is equal to 1.27xflensM2 / D, where flens is the focal length, M the quality factor (assumed to be 1), and D the incident diameter of the excitation beam. The spectra were taken in reflection geometry. The photoluminescence was collected using an achromatic doublet, where 425 nm and 435 nm long pass filters were used to block the scattered laser light. Steady-state spectra were recorded with a Hamamatsu EM-CCD camera that was spectrally calibrated with a spectrograph equipped with a 50 l / mm grating. Time-resolved traces were taken with a Hamamatsu streak camera working in single sweep mode. A pulse picker was used to vary the repetition rate of the exciting pulses. The absolute photoluminescence quantum yields (PLQYs) were measured using a Horiba Scientific Jobin Yvon spectrometer equipped with a Quanta-cp Integrating Sphere.

[0090] Transient absorption spectroscopy. The differential transmission was measured by exciting the samples with an ultrafast laser source (100-fs laser pulses at 390 nm) obtained from a Thsapphire regenerative amplifier (Coherent Libra) with a repetition rate of 1 kHz and an optical parametric amplifier (Topas 800, Light Conversion). The pump pulses had a fluency of the order of 100 pJ cm-2adjusted in order to have the best signal-to-noise ratio while avoiding sample degradation. DT / T measurements were performed by probing the samples with fs supercontinuum white pulses (with a spectral range from 400 to 800 nm) at different time delays with respect to the excitation (pump) pulses, in a pump-probe configuration in which a transmitted beam and a reference beam are acquired with two CMOS grating spectrometers with 1 nm of spectral resolution (Ultrafast Systems Helios). The overall temporal resolution was always of the same order of magnitude of the excitation pulses (120 fs, FWHM)

[0091] GIWAXS. Organic semiconductor thin films were prepared on Si / SiO2 substrates, the SiO2 layer thickness is 200 nm. GIWAXS experiments were performed at Beamline 9A at the Pohang Accelerator Laboratory in South Korea. The X-ray energy was 11.09 eV and the incidence angle was 0.12°. Samples were measured in vacuum and total exposure time was 10 s. The scattered X-rays were recorded by a CCD detector located 222.1186 mm from the sample. All samples for GIWAXS measurements had similar thickness of around 60 nm.

[0092] As mentioned above, the photocatalytic doping process in a quaternary system comprising weak dopants like oxygen in air, acridinium-based photocatalysts (PCs), salt, and various conjugated polymers is presented. It has been shown that neither the weak dopants nor the PC in the ground state can extract / donate electrons from / to the organic semiconductors (Fig. 1a-b). However, when excited by blue (or even UV) light, the PC can oxidize / reduce the organic semiconductor and subsequently be regenerated by the weak dopants (Fig. 1c). Acridinium is used because of its commercial availability, air stability, and strong oxidizing / reducing character in their excited / radical reducing states. The inventors choose different acridinium derivatives (Acr-Me, Mes-Acr-Me, Mes-Acr-Ph, see above and other well- known air-stable organic PCs, such as perylene diimide (PDI-CeC?) and Eosin Y, to carry out the photocatalytic doping study. Salts, such as LiTFSI and [EMIM][TFSI], are used for counter ions to stabilize charges on the doped organic semiconductors. Weak p-dopants like oxygen in air and disulfide (DPDS) are utilized to regenerate the PC.

[0093] The photocatalytic p-doping methodology is straightforward and efficient, as demonstrated in Fig. 2 for the case-study oxidation of PBTTT by the photocatalyst Acr- Me. Acr-Me (0.01 M) was dissolved in a mixture of n-butyl acetate and acetonitrile, together with the salt LiTFSI (0.1 M). This solvent mixture was chosen for its orthogonality to PBTTT and good solubility of Acr-Me and LiTFSI. PBTTT thin films were immersed in the Acr-Me:LiTFSI solution and irradiated with a 455 nm blue light to photoactivate Acr-Me to its excited state (Acr-Me*) in the presence of oxygen (air) acting as a weak p-dopant. The Acr-Me: LiTFSI solution was then recovered, and the p-doped PBTTT films were washed and dried as described in Fig. 2a. The UV-Vis-NIR absorption spectra of PBTTT films show that oxygen in the air cannot dope PBTTT in the absence of Acr-Me or with Acr-Me in the dark, but a strong polaronic absorption extending in the NIR region was observed upon blue light irradiation (Fig. 2b).

[0094] X-ray photoelectron spectroscopy (XPS) shows that TFSI- anions did not penetrate the PBTTT layer when not irradiated. However, upon light irradiation, strong F(1s) and 0(1 s) signals relative to TFSI- were observed, as shown in Fig. 2c. Further analysis of the S(1s), F(1s), N(1s), and C(1s) peaks (Fig. 2d and Fig. 2d'), indicates an increase in TFSI- anion content with increasing the irradiation time, leading to a positively charged PBTTT polymer backbone. Grazing-incidence wide-angle X-ray scattering (GIWAXS) measurements reveal that penetration of TFSI- anions does not disrupt the orientation of the PBTTT chains, but instead leads to a reduction in the TT- TT stacking distance and an increase in the lamellar packing distance which is consistent with oxidation of the PBTTT polymer backbone (Fig. 2c'). This also suggests that TFSI- anions are mainly located within the side chain region. In addition, ultraviolet photoelectron spectroscopy (UPS) analysis reveals that the Fermi level of PBTTT shifted towards its highest occupied molecular orbital (HOMO) after light irradiation, confirming the effective occurrence of p-doping (Fig. 2c).

[0095] The electrical conductivity of PBTTT dramatically increased from 10-5S cm-1to over 700 S cm-1after 10 min of light irradiation, while no changes in electrical conductivity were observed either without Acr-Me or with Acr-Me but no irradiation (Fig 2f). Notably, the observed conductivity values are typical for molecularly p-doped PBTTT. It was also found that PBTTT films with thicknesses ranging from 16 nm to 60 nm exhibited similar maximum conductivity and polaronic absorption intensity, with thinner films being doped faster (Fig. 2f'). Doped PBTTT in various thickness shows consistent maximum conductivity and similar polaronic absorption intensity, indicating photocatalytic doping is bulk doping. These results suggest that doping occurs throughout the entire bulk of the organic semiconductor layer. Moreover, the degree of photodoping is dependent on the light irradiation dose, with the doping level increasing as the irradiation dose increases (Fig. 2f") Photocatalytic doping shows good light irradiation dose dependency. To understand the charge transfer mechanisms involved in the photocatalytic p-doping process, transient absorption spectroscopy (TAS) experiments were performed. The results obtained from undoped PBTTT films reveal the presence of photobleaching peaks in the range of 553-574 nm, along with a photoinduced absorption peak at 638 nm. By monitoring the decaying of these features associated with excited state absorption / bleaching, a relatively short excited state lifetime of 46- 60 ps undoped PBTTT was observed. Upon photocatalytic doped with Acr-Me, an even shorter excited state lifetime (19-24 ps) for doped PBTTT was observed. In contrast, Acr-Me exhibited a considerably longer excited state lifetime of 6-9 ns, which is 100 times longer than that of PBTTT. This significant difference in excited state lifetimes is also reflected in their fluorescence lifetimes, where Acr-Me shows a much longer fluorescence lifetime of 18 ns compared to PBTTT (0.9 ns). Note that all PCs show systematically longer fluorescence lifetime than typical organic semiconductors. This result suggests that the photoinduced charge transfer (PET) process is mainly dominated by the PC’s excited state, with the PET predominately occurring from the ground state of the organic semiconductor to the excited state of the PC.

[0096] To further support the notion that PET solely involves the excitation of PCs, the optically transparent reducing molecule triethylamine (EtsN) for an in-situ study of the PET process was used (Fig 3a). Mes-Acr-Me and Mes-Acr-Ph were selected as representative PCs due to their excellent stability in the presence of basic Et3N. Absorption spectra revealed that PET between Mes-Acr-Me (or Mes-Acr-Ph) and Et3N induces a reduced state of Mes-Acr-Me (or Mes-Acr-Ph). Notably, this photo-reduced state by Et3N matches the single-electron-transfer reduced state induced by the strong reducing agent tetrakis(dimethylamino)ethylene (TDAE). Subsequently, Mes-Acr-Me (or Mes-Acr-Ph) can be regenerated through oxidation with oxygen in air or the weak oxidant DPDS. In-situ photoluminescence studies clearly demonstrate the reduction and regeneration of Mes-Acr-Me and Mes-Acr-Ph. Moreover, in-situ TAS spectra captured during the PET reduction / oxidation-regeneration process reveal that Mes- Acr-Me initially forms an intramolecular-charge-transfer (ICT) excited state under light irradiation (Fig. 3a-b). Subsequently, in the presence of Et3N, Mes-Acr-Me transitions to the reduced state, accompanied by a significant photoinduced absorption at 658 nm, indicating the formation of an excited reduced state of Mes-Acr-Me under laser pump pulse (Fig. 3c). Upon oxidation in air, Mes-Acr-Me is fully regenerated (Fig. 3d). This result strongly supports the notion that the PET process solely requires the excitation of PCs and that PCs can be fully regenerated in air, thereby closing the catalytic cycle shown above.

[0097] The IP values of the semiconducting polymers and the EA values of the PCs are reported in Fig. 3e. For comparative analysis, the IPs of conventional molecular p- dopants such as F4TCNQ, FeTCNNQ, magic blue, CN6-CP, and NOPFe, are also reported in Fig. 3e. At the ground state, all the PCs exhibit weak EA values ranging from 3.6 to 4.2 eV, which are insufficient to thermodynamically drive the p-doping of the organic semiconductors with IP ranging from 4.3 to 5.7 eV. However, upon photoactivation, the PCs’ excited states possess energy values ranging from 5.9 to 6.9 eV, thereby enabling the p-doping of the aforementioned semiconducting polymers by lowering the energy barrier for electron transfer from the conjugated polymer to the PC in the excited state. The absorption spectra of the Acr-Me catalytically doped polymer films (Fig. 3e') demonstrate that all five semiconducting polymers investigated in this study, including the high-IP polymer PTQ1 , can be effectively catalytically doped by Acr-Me. This indicates that Acr-Me’s p-doping ability under light activation is comparable to that of Mo(tfd)s (EA ~ 5.5 eV).

[0098] The electrical conductivity of the photodoped polymeric thin films was investigated for the different PCs (Fig. 3f) and as a function of the light irradiation time (Fig. 3f'). PBTTT, P(g42T-T), and P(g42T-T) were doped by all five PCs in the presence of oxygen, with P(g42T-T) (IP = 4.3 eV*) exhibiting high electrical conductivity of up to 3000 S cm-1, among the highest values reported for molecularly p-doped polymers. Similar results were obtained using DPDS as the weak oxidant to regenerate the photocatalyst (Fig. 3f"). Similar to what was observed for PBTTT, the photocatalytic doping of P(g42T-T) is a bulk process (Fig. 3e'), causing the appearance of a strong TT-TT stacking peak and an increase in the lamellar packing distance of the doped films as compared to the undoped films (Fig. 3f"'). P3HT (IP = 4.7 eV) was photocatalytically doped by Acr-Me, Mes-Acr-Me, Mes-Acr-Ph, and PDI-CeC?, while the high-IP conjugated polymer PTQ1 (IP = 5.7 eV) was photodoped by Acr-Me, Mes- Acr-Me, and Mes-Acr-Ph. The electrical conductivity enhancement for the five organic semiconductor films strongly depends on the energy difference between the semiconductor’s IP and the photocatalyst’s excited state EA. As the energy difference decreases, the electrical conductivity enhancement increases by orders of magnitude (Fig. 3g). Finally, the photocatalytic reduction (n-doping) of organic semiconductors and the feasibility of achieving one-pot photocatalytic p-doping and n-doping of p- / n- type semiconducting polymers were investigated (Fig. 4a). To achieve this, the ground-state PC is photoactivated using a 455 nm visible light to generate an excited- state PC that can oxidize weak n-dopants. After reduction, the PC is converted to its reduced state form, which can be photoactivated using 390 nm UV light to obtain an excited reduced state (Fig. 3a, d) capable of n-doping organic semiconductors (Fig. 4b). In this process, the n-type organic semiconductor accepts an electron from the excited reduced state of the PC, facilitating the regeneration of the ground-state PC.

[0099] To test this approach, poly(benzimidazobenzophenanthroline) (BBL) was selected as the n-type organic semiconductor due to its high EA (4.15 eV), high electrical conductivity upon doping (>1 S / cm), and good ion permeability. Triethylamine (EtsN) was used as the weak n-dopant, while [EMIM][TFSI] was used as the counterions to stabilize the negative polarons on the n-doped organic semiconductor. The results show that without light and photocatalyst, Et3N is unable n-dope BBL (Fig. 4c, Fig. 4c'). However, when combined with light and photocatalyst, Et3N significantly n-dope BBL, resulting in strong negative polaron absorption peaks at 400 nm, 720 nm, and 865 nm, similar to those observed in tetrakis(dimethylamino)ethylene (TDAE), poly(ethyleneimine) (PEI), and electrochemically n-doped BBL thin films. Upon photocatalytic n-doping, the electrical conductivity of BBL increases by more than 5 orders of magnitude, going from less than 10-5S cm-1for undoped BBL to >1 S cm-1after only 2 min of light irradiation (Fig. 4d).

[0100] By replacing Et3N with the low-IP polymer P(g42T-T), photocatalytic p-doping and n-doping of P(g42T-T) and BBL, respectively, could be coupled. Both semiconductor thin films were immersed in the same Mes-Acr-Me solution and irradiated with 455 nm blue light (PC ground state activation) and 390 nm UV light (PC reduced state activation) to activate PC. The resulting UV-Vis spectra showed clear polaronic absorptions in both P(g42T-T) and BBL thin film (Fig. 4c andFig. 4c’). Photocatalytic p-doped P(g42T-T) displayed a maximum electrical conductivity over 200 S cm-1, while photocatalytically n-doped BBL showed maximum electrical conductivity over 0.1 S cm-1(Fig. 4e). It is worth noting that doping cannot occur with counterions alone, in the absence of PC (Fig. 4e). Although the achieved electrical conductivity values were one order of magnitude lower than those obtained through separate photocatalytic p-doping and n-doping processes, they still demonstrate the efficacy of the photocatalytic doping approach. PC acts as a redox shuttle, transferring electrons from P(g42T-T) to BBL, with only the salt in the electrolyte being consumed. Compared with ground-state electron transfer occurring spontaneously between P(g42T-T) and BBL, it is possible to achieve entirely separate p-doped and n-doped P(g42T-T) and BBL films through the photocatalytic process described here. It was also observed that using a single 390 nm light source, photocatalytic n-doping and combined p / n doping can be achived (Fig. 4e') because both Et3N and P(g42T-T) can reduce PC at this wavelength. Both 390 nm and 455 nm exposure powers are 50 mW cm-2. The photocatalytic n-doping and coupled p / n doping can be obtained under single 390 nm light source, since 390 nm light can also activate the photocatalyst from the ground state to the excited state. The doping with single 390 nm light source is a little slower than with both 390 nm and 455 nm light source, since the total exposure dose is lower.

[0101] In summary, a one-pot method for the synthesis of highly conductive n-type polymer inks using vitamin E as a catalyst has been developed. The method does not require post-processing. The achieved conductivity was measured to be as high as 1000 S / cm.

[0102] Although the present invention has been described with reference to various embodiments, those skilled in the art will recognize that changes may be made without departing from the scope of the invention. It is intended that the detailed description be regarded as illustrative and that the appended claims including all the equivalents are intended to define the scope of the invention.

Claims

CLAIMS1 . A method for photocatalytic doping of an organic semiconductor, said method comprising the steps of: a) providing a first organic semiconductor; b) subjecting said first organic semiconductor to a solution comprising a solvent, a counter ion salt, a weak dopant and a photocatalyst; c) subjecting said first organic semiconductor and solution in step b) to electromagnetic irradiation having a first dominant peak wavelength in the range from 10 nm to 2000 nm.

2. The method according to claim 1 , said method further comprising the step of: a') providing a second organic semiconductor; b') subjecting said second organic semiconductor to said solution comprising a solvent, an organic salt and a photocatalyst; c') subjecting said second organic semiconductor to electromagnetic irradiation having a second dominant peak wavelength in the range from 10 nm to 2000 nm. wherein said first wavelength is different from said second wavelength.

3. The method according to claim 1 or 2, wherein said first organic semiconductor is a p-type organic semiconductor and dopant is a p-type dopant or wherein said first organic semiconductor is an n-type organic semiconductor and said dopant is a n-type dopant.

4. The method according to claim 2 or 3, wherein said first organic semiconductor is a p-type organic semiconductor and said second organic semiconductor is an n-type organic semiconductor, or wherein said first organic semiconductor is an n-type organic semiconductor and said second organic semiconductor is a p-type organic semiconductor.

5. The method according to any one of claims 2-4, wherein said first organic semiconductor and / or said second organic semiconductor is in the form of a thin film arranged on a substrate.

6. The method according to any one of the preceding claims, wherein said solvent is a mixture of butyl acetate and acetonitrile.

7. The method according to any one of the preceding claims, wherein said photocatalyst is an acridinium-based photocatalyst.

8. The method according to any one of the preceding claims, wherein said first electromagnetic irradiation is visible blue light having a dominant peak wavelength in the range from 380 to 500 nm.

9. The method according to any one of claims 2-8, wherein said second electromagnetic irradiation is UV light having a dominant peak wavelength in the range from 10 to 380 nm.

10. The method according to any one of the preceding claims, wherein said weak dopant is oxygen or triethylamine.

11. The method according to any one of the preceding claims, wherein said method further comprises the step of: d) recovering said photocatalyst and said counter ion.

12. The method according to any one of the preceding claims, wherein concentration of said photocatalyst in step b) is in the range from 10'8to 5 M.

13. The method according to any one of the preceding claims, wherein concentration of said counter ion in step b) is in the range from 10'8to 5 M.

14. The method according to any one of the preceding claims, wherein duration of step c) is from 0.1 s to 1 h.

15. A doped organic semiconductor manufactured by the method according to any one of the preceding claims.

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