# All-visible Light-switchable Thin-film Transistor

Taibin Wang, Paolo Samorì, Lili Hou

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Taibin Wang, Paolo Samorì, Lili Hou. All-visible Light-switchable Thin-film Transistor. Advanced Optical Materials, 2025, 13 (27), 10.1002/adom.202501408. hal-05284153

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# All-visible Light-switchable Thin-film Transistor

Taibin Wang, Paolo Samorì, Lili Hou*

T. Wang, L. Hou

State Key Laboratory of Precision Measurement Technology and Instruments, School of Precision

Instruments and Optoelectronics Engineering, Tianjin University, Tianjin, 300072, China. E-mail:

lilihou@tju.edu.cn

P. Samorì

Université de Strasbourg, ISIS UMR 7006, 8 Allée Gaspard Monge, Strasbourg F-67000, France.

L. Hou

Department of Chemistry and Chemical Engineering, Chalmers University of Technology,

Gothenburg 412 96, Sweden.
ABSTRACT

Stimuli-responsive organic thin-film transistors (TFTs) such as light-switchable TFTs are key components for multifunctional optoelectronics beyond Moore. However, the modulation of the light-switchable TFTs output developed so far requires the use of ultraviolet (UV) light, despite the latter triggers photooxidation and degradation of the molecular materials and hybrids thereof. Herein, we report an all-visible light-switchable TFT whose current output can be reversibly interconverted between two different states by non-coherent and low-power (< 1 mW cm⁻²) visible light. The light-sensitive material was assembled by blending CdS quantum dots (QDs) coated by photochromic diarylethene (DAE) molecules with a semiconducting *p*-type poly[2,5-(2-octyldodecyl)-3,6-diketopyrrolopyrrole-alt-5,5-(2,5-di(thien-2-yl)thieno[3,2-b]thiophene)] (DPP-DTT). For the first time, triplet energy transfer (TET) at the organic-inorganic interface between photochromic molecules and QDs was exploited to drive light-switchable TFTs devices, enabling DAEs reversible photoisomerization when exposed to 405 nm and 515 nm visible light. Significantly, the conversion efficiency of DAEs *via* all-visible-light switching is comparable to that achieved under UV light irradiation, while the light switching fatigue resistance of the devices displayed a radical improvement. Our work provides a new pathway to realize all-visible-light activated devices for future design of advanced digital optoelectronics in the context of next-generation data storage technologies and neuromorphic computing.

**KEYWORDS:** light-switchable thin-film transistors, quantum dots, diarylethene photoswitches, triplet energy transfer, visible-light activation
INTRODUCTION

During the last two decades, organic thin-film transistors (TFTs) have attracted significant attention
as key elements for printed, large-area electronics in view of their high performance, mechanical
flexibility, low-cost, and scalability.[1-5] TFTs are key enablers for the fabrication of logic circuits,
memory storage devices and wearable devices.[6-9] The combination of semiconducting polymers
with functional molecules was demonstrated being an effective strategy for fabricating
multifunctional and stimuli-responsive TFTs, allowing to modulate the device output by means of
external stimuli. These responsive devices revealed widespread and practical applications in smart
sensing, logic circuitries, neurotransmitter functions in electronic devices, etc.[10-15] Blending the
photochromic molecules with the organic semiconducting polymer has been successfully employed
to construct light-switchable TFTs,[16-22] in which light is used as high spatio-temporal and non-
invasive input to remote-control the current output of TFTs.

Blends of organic semiconducting polymers with various UV-light responsive photochromic
molecules such as azobenzenes,[18] spiropyrans[17,23] and diarylethenes (DAEs) [10][16][20-22][24] have
been integrated as active components in light-switchable TFTs to realize flexible non-volatile optical
memories[13,14] and multifunctional light-emitting transistors.[15] In DAE containing blends, the
device's light modulation is based on the reversible photoisomerization between the ring-open and
ring-closed forms triggered by UV and visible light, respectively. In particular, the phototunable
lowest unoccupied molecular orbital (LUMO) or highest occupied molecular orbital (HOMO) levels
enables DAEs to reversibly function as charge trapping and non-trapping centers, to achieve the
light switching of the output current.

Unfortunately, all above reported light-switchable TFTs required the use of UV light to trigger the
isomerization process in one direction. UV light is known to cause photo-induced oxidation and decomposition of (bio)molecules, degrading semiconducting polymers, thereby reducing the performance and long-term stability of the devices.[25-27] It also affects the cyclability of DAE, a problem that has been solved *via* the more cumbersome synthesis of fluorinated derivatives that are less prone to oxidation.[28] Moreover, the short penetration depth of UV light limits its effectiveness in triggering the isomerization process throughout the materials in multilayered devices. Therefore, it is highly desired to achieve all-visible or near-infrared (NIR) light activated TFTs. Zhang et al recently designed a NIR light activated TFT based on photoswitchable arylazopyrazole functionalized semiconducting donor-acceptor (D-A) polymer and upconversion nanoparticles (UCNPs).[29] The UCNPs is capable to convert 980 nm NIR light to 316 nm UV light, which can trigger the isomerization of arylazopyrazole from *trans* to *cis*. Meanwhile, the photothermal effect of the conjugated D-A polymer promotes an increase in temperature after NIR irradiation, resulting in the reversible conversion of *cis*-arylazopyrazole into the *trans*-form. However, the upconversion efficiency of UCNPs remains low,[30] and strong and coherent NIR light sources were required, such as a laser with the power of 400 mW cm⁻² for 980 nm and 600 mW cm⁻² for 808 nm. Therefore, it is essential to design light-switchable TFTs that can be efficiently switched with a low-power, incoherent light to achieve all-visible light activation.

Here, we report a novel and simple strategy for developing all-visible light-switchable TFTs, in which the functional film was prepared by mixing the DAE coated CdS quantum dots (QDs) with the p-type poly[2,5-(2-octyldodecyl)-3,6-diketopyrrolopyrrole-alt-5,5-(2,5-di(thien-2-yl)thieno[3,2-b]thiophene)] (DPP-DTT), as shown in Figure 1a.[28,31] The all-visible-light control was achieved *via* the unique triplet energy transfer (TET) mechanism from QDs to DAEs. The DAE
molecules were functionalized with a carboxylic acid end-group to guarantee covalent grafting onto
the surface of CdS QDs. Under 405 nm light irradiation, DAEs undergo photoisomerization from
the open to the closed form both in solution and in the solid state *via* low-lying triplet excited state
through TET from CdS QDs (Figure 1b),[32] thus avoiding the common use of UV light (310 nm).
The back DAE photoisomerization from closed to open form can occur upon 515 nm light
irradiation, thereby realizing all-visible light triggered DAE photoswitches. The light-switchable
functions of TFTs is achieved *via* the engineering of the energy levels of the components, with a
HOMO level of DPP-DTT (-5.2 eV) being located in-between the one of the open (-5.8 eV) and the
closed (-5.1 eV) form of the employed DAE derivative.[33] Specifically, the hole transport in DPP-
DTT is trapped by the DAE-closed isomers during the all-visible activated photoisomerization
processes. Notably, the presence of CdS QDs does not affect hole transport in DPP-DTT because of
its significantly lower HOMO level (-6.7 eV) (Figure 1c).[34] The output current of our light-
switchable TFTs, based on DAEs coated CdS QDs and DPP-DTT, can be gated electrically by
applying a bias voltage and optically upon irradiation with non-coherent and low-power visible light
at 405 nm (0.5 mW cm⁻²) and 515 nm (0.7 mW cm⁻²), with a maximum current photo-modulation
of 63% and enhanced the fatigue resistance.
<figure><img src="image_2.png" /><figcaption>**Figure 1. Device structure, components, energetics and visible light activation mechanism.** (a) Device configuration and chemical structures of *p*-polymer DPP-DTT, DAE-open, DAE-closed and CdS QDs. (b) The visible-light activation mechanism of TET from CdS QDs to DAE-open to trigger the photoisomerization from the open to the closed form *via* triplet excited state upon 405 nm light irradiation, and back photoisomerization upon 515 nm light irradiation *via* singlet excited state. (c) HOMO energy levels of *p*-type polymer DPP-DTT, DAEs and CdS QDs and the principle of charge trapping in our light-switchable TFT.</figcaption></figure>
Results and discussion

CdS QDs and DAEs were synthesized in compliance to previous reports,[35,36] and the corresponding absorption spectra in toluene are shown in Figure S1. Photochromic DAEs can undergo reversible photoisomerization under 310 nm and 515 nm light irradiation, accompanied by the appearance and disappearance of a typical visible absorption band with the peak around 550 nm. The synthesized CdS QDs, coated with oleic acid as a surfactant ligand, exhibit a sphere morphology with an average size of 3.5-4 nm.[36] Its first absorption peak is at 405 nm, falling in a region characterized by a minimal absorptivity of both the open and closed DAEs. For the preparation of DAE coated CdS QDs, the two components were mixed and stirred for ligand exchange in toluene with a molar ratio of 1:100 for 8-12 h, to enable the covalent grafting *via* the carboxylic acid groups. The non-bonded DAEs in the mixture were then washed away by centrifugation in acetonitrile and acetone (see details in the Supporting Information). In average 9 DAE molecules were found to covalently anchor onto the surface of an individual CdS QD (Figure S2). The absorption spectrum of the DAE coated CdS QDs in dichlorobenzene are shown in Figure 2a. It exhibits the combined characteristics of the individual spectra of DAEs and CdS QDs, indicating the absence of interactions between the two components at the ground state. Upon the 405 nm light irradiation, the DAEs displayed a new broad absorbance feature between 450 nm and 650 nm, confirming the efficient isomerization to the closed form, due to the TET process from CdS QDs to DAEs which we have previously investigated thoroughly.[32] Upon further irradiation with 515 nm light, the DAEs recovered its initial open state, confirming a good reversibility in our all-visible light control. Figure 2b portrays the absorption spectra of the blend of DAE coated CdS QDs with DPP-DTT (25% w/w between DAEs and DPP-DTT) in dichlorobenzene. Upon irradiation at 405 nm and 515 nm, it also exhibits the reversible
photoisomerization from the ring-open and ring-closed form, indicating that the presence of DPP-DTT does not affect the all-visible light activation of DAEs through TET from CdS QDs. Subsequently, the blend solution of DAEs coated CdS QDs and DPP-DTT was spin-coated onto a glass substrate to investigate all-visible photoswitching processes in the solid state. Figure 2c displays the absorption spectra of the blended film upon 405 nm and 515 nm light irradiation. The reversible appearance and disappearance of the typical visible band of DAEs confirms that the all-visible-light activation of the photoisomerization also occurs in the solid state. It is worth noting that the changes in the absorbance of the film in the visible region after 405 nm irradiation is almost the same as that recorded upon exposure to UV light (310 nm) (Figure S3), proving the high conversion of the all-visible light activation through TET from CdS QDs in the solid state. We also performed the control experiments to confirm the presence of both CdS QDs and DAEs are keys to achieve the all-visible light activation. When blending DPP-DTT and CdS QDs without photochromic DAEs, the absorption spectra recorded upon 515 nm and 405 nm light irradiation remains unchanged, as shown in Figure S4a and S4b. When blending DPP-DTT and photochromic DAEs without CdS QDs, new absorption peak in the visible region only appeared upon 310 nm irradiation whereas no change occurred after irradiation at 405 nm (Figure S4c and S4d).

<figure><img src="image_3.png" /><figcaption>**Figure 2.** UV/vis absorption spectra of (a) DAEs anchored on the surface of CdS QDs in the</figcaption></figure>
solution upon 405 nm and 515 nm light irradiation. (b) DAEs coated CdS QDs blended with polymer DPP-DTT in the solution upon 405 nm and 515 nm light irradiation. (c) DPP-DTT/CdS QDs/DAEs blended film upon 405 nm and 515 nm light irradiation.

Since the surface morphology of thin film significantly affects the charge transport in TFT devices, we investigated the surface morphology of the blends of DAEs coated CdS QDs and DPP-DTT films spun onto SiO₂ substrate by means of Atomic Force Microscopy (AFM) and Transmission Electron Microscopy (TEM). Figure 3a and 3b display the AFM images of neat film of DPP-DTT and the blended film over a 10×10 µm² area. They reveal the absence of a significant phase separation within the film of DPP-DTT/CdS QDs/DAEs, demonstrating the good miscibility between DPP-DTT and the DAEs coated CdS QDs, which is a prerequisite for guaranteeing effective physical and energetic interactions between the different components. Such a good miscibility is key to efficiently modulate the current of the corresponding light-switchable TFT devices using all-visible light control (vide infra). The AFM image of the neat spin-coated DPP-DTT film shows the known fibrillar-like structure of DPP-DTT, reflecting a good degree of order at the supramolecular level, which is instrumental towards high mobility TFTs.[37,38] The morphology of blends of DAEs coated CdS QDs with DPP-DTT also exhibits fibrillar-like structures. Interestingly, when compared to the neat DPP-DTT polymer, the addition of DAEs coated CdS QDs reduces the film roughness from a root-mean square roughness ($R_{RMS}$) of 3.70 nm down to 0.33 nm. The reduced $R_{RMS}$, also observed in the neat diluted DPP-DTT film (Figure S5), indicates a less disordered arrangement of the blend compared to the pure DPP-DTT film, which might increase the complexity and uncertainty of the charge transport path, thereby reducing the mobility. TEM was
further utilized to investigate the distribution of CdS QDs in the blends. As shown in Figure 3c, CdS QDs are relatively well dispersed in the polymer region, further supporting the good miscibility, thereby ensuring the optimal physical and energetic interaction between the blended components for the device application.

<figure><img src="image_4.png" /><figcaption>**Figure 3. Morphology of** (a) DPP-DTT neat film. (b) DPP-DTT/CdS QDs/DAEs blend film. (c) TEM image of DPP-DTT/DAEs/CdS QDs blend film.</figcaption></figure>

In order to examine the all-visible-light switching of TFTs using our approach, bottom-contact bottom-gate TFTs were fabricated by spin-coating the polymer or polymer blends onto Au-patterned Si/SiO₂ substrates (see the Supporting Information for the detailed preparation). For comparison, we first investigated the transfer curves of the device based on neat DPP-DTT (Figures S6), which exhibits typical *p*-type characteristics with a mobility of 0.049 cm² V⁻¹ s⁻¹ in line with previous reports.[³⁹] It should be noted that the device based on neat DPP-DTT film shows no current photo-response neither under UV (310 nm, 0.1 mW cm⁻²) nor visible light irradiation (405 nm, 0.5 mW cm⁻² or 515 nm, 0.7 mW cm⁻²) (Figure S6). Subsequently, we fabricated and investigated a device based on the blend of DPP-DTT and DAEs coated CdS QDs, where a 25% DAE weight ratio was employed (being a typical ratio used in DAE/polymer-based TFTs).[¹⁶][²¹] The transfer curves are
shown in Figure 4a and 4b. When DAEs are in the open form, the mobility of the TFT amounts to 0.012 cm² V⁻¹ s⁻¹ with an Ion/Ioff of 3.2×10². Compared to the neat DPP-DTT device, the decreased mobility can be ascribed to the reduced crystallinity of the DPP-DTT blends. However, this mobility is comparable to other DPP-DTT blended TFTs,[¹⁴,²³] and further improving the mobility can be achieved either by reducing the ratio of the DAEs coated CdS QDs (vide infra) or introducing self-assembled monolayers (SAMs) of organosilane compound to heal silanol traps.[⁴⁰] When DAEs are in the closed form, which is triggered upon exposure to 405 nm visible light for 3 min, the drain-source current (I<sub>ds</sub>) of the device decreased dramatically due to the significant hole trapping by DAE molecules in their closed form. In this case, the mobility of the device drops to 3.8×10⁻³ cm² V⁻¹ s⁻¹ with a light triggered current modulation degree of 63% (I<sub>405</sub>/I<sub>ini</sub>, where the I<sub>405</sub> is the output current value at V<sub>g</sub> = -60 V, V<sub>ds</sub> = -30 V after 405 nm light irradiation, and I<sub>ini</sub> is initial output current value at V<sub>g</sub> = -60 V, V<sub>ds</sub> = -30 V). This device was further irradiated with 515 nm light to convert DAEs back to the open form, resulting in the complete recovery of the transfer curve to its original state as displayed in Figure 4b, which indicates the good reversibility of our device under all-visible light switching. The dynamic response of designed TFT under 405 nm and 515 nm light irradiation is shown in Figure S7, showing that as the duration of 405 nm (515 nm) light exposure increases, the current gradually decreases (increases). The corresponding output characteristics (I<sub>ds</sub>-V<sub>ds</sub>) of this device after 405 nm and 515 nm light irradiation were also determined as shown in Figure S8, which also exhibit light switching upon alternating exposure to 405 nm and 515 nm light. The isomerization of the DAEs to the closed form, which is obtained upon exposure to 405 nm visible light for 3 min, is accompanied by a significant decrease in the TFT's drain-source current (I<sub>ds</sub>) due to the significant hole trapping by DAE molecules in their
closed form. The more favorable hole trapping of DPP-DTT by the closed DAEs compared to the open form is also evidenced by the femtosecond transient absorption spectroscopy (Figure S9). In order to verify that the presence of both CdS QDs and DAEs is essential for achieving the all-visible light-switchable TFT, control devices were fabricated and tested. The output curves of TFTs based on DPP-DTT and 25% w/w DAEs without CdS QDs showed a current decrease solely upon irradiation at 310 nm and a recovery at 515 nm illumination as illustrated in Figure S10a. In other words, the current of this device remains constant when exposed to 405 nm irradiation, providing unambiguous evidence that in absence of CdS QDs the DAEs can only undergo photoisomerization through the high-lying singlet excited state upon UV light irradiation. Another control device was fabricated by blending CdS QDs with DPP-DTT in the absence of DAEs. Such TFTs exhibited no changes in the output current when exposed to 405 nm or 310 nm irradiation (Figure S10b). The above control experiments indicates that the current of light-switchable TFTs can be only modulated by all-visible light control when both the DAEs and CdS QDs components are present in DPP-DTT matrix.

The reversibility of the devices was further assessed over 5 cycles of alternative 405 nm and 515 nm visible light irradiation. The output current of the device can be reversibly switched between a low mobility state and its initial state without noticeable fatigue as shown in Figure 4c. It should be noted that the DAE photoswitches employed here was chosen in view of its intrinsic fatigue when exposed to UV light irradiation.[41] In fact, not surprisingly, upon exposure to UV light the output current reduction (Figure S11) is similar to the one observed upon irradiation at 405 nm, yet cyclability studies revealed a notable fatigue with just 85% of the initial current being recovered after 5 irradiation cycles (see black line in Figure 4c). These results clearly demonstrate our
approach of using all-visible light can effectively improve the fatigue resistance of light-switchable TFTs.

We further investigated the light-switchable performance of the blend TFTs when incorporating various weight ratios of DAE (coating the CdS QDs) in DPP-DTT under all-visible light control. The DAE/CdS QDs were added into DPP-DTT at weight ratios of 1%, 5%, 10%, and 25% for the DAE content, respectively. The mobility and light-switchable behaviors of these TFTs were investigated and summarized in Figure 4e and 4f (transfer curves of devices are shown in Figure S12). All these TFTs demonstrated light-switchable output current under alternating irradiation at 405 nm and 515 nm. When DAEs are in the open form, the hole mobility decreases with increasing DAE content, with the values of 0.045, 0.041, 0.025, and 0.012 cm² V⁻¹ s⁻¹ for 1%, 5%, 10%, and 25% DAE content, respectively. The decrease in the field-effect mobility measured in the TFTs upon increasing the content of DAEs coated CdS QDs can be attributed to the following reasons: 1. The DAE molecules and QDs act as scattering centers for charge carriers, and higher weight ratio in the blend further reduce the TFT mobility. 2. The increased amount of DAEs coated CdS QDs determines a disruption of the molecular packing of DPP-DTT, leading to a more disordered blend arrangement and reduce the device's mobility. A negligible effect on the mobility of DPP-DTT was observed when the DAE content (coated on CdS QDs) within the blend is ≤5%. In contrast, the current modulation degree of TFTs under all-visible light activation increases with the DAE content, reaching values of 12%, 13%, 37%, and 63% for blends containing 1%, 5%, 10%, and 25% DAE, respectively. These devices with varied weight ratios of DAEs coated CdS QDs all exhibit excellent fatigue resistance over irradiation cycles of alternating 405 nm and 515 nm visible light as evidenced in Figure 4f.
<figure><img src="image_5.png" /><figcaption>**Figure 4.** Light switching of TFTs based on DPP-DTT and DAEs coated CdS QDs upon visible light irradiation. (a) Transfer curves of the device based on DPP-DTT/25%CdS QDs/DAEs blend film before and after 405 nm light irradiation ($V_{ds}$ = -30 V). (b) Transfer curves of the device based on DPP-DTT/25%CdS QDs/DAEs blend film subsequently irradiation after 405 nm and 515 nm light ($V_{ds}$ = -30 V). (c) Reversible modulation of drain-source current of DPP-DTT/25%CdS QDs/DAEs blend-based device under alternatively 405 nm and 515 nm light irradiation compared to under 310 nm and 515 nm light irradiation. (d) The mobility of devices of various weight ratio of DAEs coated CdS QDs in DPP-DTT under 405 nm and 515 nm light irradiation. (e) The modulation degree of devices based on various weight ratio of DPP-DTT/CdS QDs/DAEs film. (f) Reversible modulation of drain-source current of various weight ratio of DPP-DTT/CdS QDs/DAEs blend-based devices under 405 nm and 515 nm light irradiation.</figcaption></figure>

CONCLUSION
In summary, all-visible light-switchable TFTs was demonstrated upon blending DAEs coated CdS QDs with DPP-DTT polymer as ruled for the first time by a unique TET mechanism. The all-visible light activated photoisomerization of DAEs was confirmed both in the solution and solid-state by UV-visible absorption spectroscopy. AFM and TEM measurements revealed the good miscibility of three components and even dispersion of CdS QDs in the blended films. The TFTs displayed an all-visible light response upon exposure to low-power and non-coherent irradiation. The output current of TFTs can be reversibly modulated under alternating exposure to 405 nm and 515 nm light, with the mobility and light-induced modulation degree of the devices depending on the ratio of the DAEs coated CdS QDs within the blended film. Our all-visible light-switchable TFTs represent a significant advancement in the development of light-responsive devices in view of the modest invasiveness of the selected wavelengths, paving the way towards advanced technologies in digital electronics, smart sensing and neuromorphic computing.
## ASSOCIATED CONTENT

**Supporting Information.** The Supporting Information is available free of charge at http://dx.doi.org/****-****-*.

## AUTHOR INFORMATION

### Corresponding Author

Lili Hou -ª State Key Laboratory of Precision Measurement Technology and Instruments, School of Precision Instruments and Optoelectronics Engineering, Tianjin University, Tianjin, 300072, China. E-mail: lilihou@tju.edu.cn. ᵇDepartment of Chemistry and Chemical Engineering, Chalmers University of Technology, Gothenburg 412 96, Sweden.

### Authors

#### Author Contributions

L.H. conceived and designed the experiments. T.W. performed the experiments and the electrical testing and analysis. T.W. drafted the manuscript, L.H. and P.S. revised the manuscript, and all authors commented on the manuscript.

#### Notes

The authors declare no completing financial interest.

#### Acknowledgements

This work was financially supported by Key Technologies Research and Development Program of China (2023YFB3609300), National Natural Science Foundation of China (12274320, 62205240). PS acknowledges financial support from the ANR through the Interdisciplinary Thematic Institute SysChem via the IdEx Unistra (ANR-10-IDEX-0002) within the program Investissement d'Avenir, the Foundation Jean-Marie Lehn and the Institut Universitaire de France
(IUF). We thank Ms. Lin Xi for the femtosecond transient absorption measurement.

**Data Availability Statement**

The data that support the findings of this study are available from the corresponding author upon
reasonable request.
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