Bottom‐Up Photosynthesis of an Air‐Stable Radical Semiconductor Showing Photoconductivity to Full Solar Spectrum and X‐Ray

Abstract Single‐component semiconductors with photoresponse to full solar spectrum are highly desirable to simplify the device structure of commercial photodetectors and to improve solar conversion or photocatalytic efficiency but remain scarce. This work reports bottom‐up photosynthesis of an air‐stable radical semiconductor using BiI3 and a photochromism‐active benzidine derivative as a photosensitive functional motif. This semiconductor shows photoconductivity to full solar spectrum contributed by radical and non‐radical forms of the benzidine derivative. It has also the potential to detect X‐rays because of strong X‐ray absorption coefficient. This finding opens up a new synthetic method for radical semiconductors and may find applications on extending photoresponsive ranges of perovskites, transition metal sulfides, and other materials.


Introduction
Photoresponsive range is a key parameter of semiconductors for solar conversion, photocatalysis, and photodetection. [1,2]The photoresponsive upper limit of monocrystalline or polycrystalline silicon in commercial solar cells is usually less than 1100 nm, [3] and the rest of the full solar spectrum (≈295-2500 nm) is still not effectively used.As for photocatalysis, the majority of single-component photocatalysts have limited photoresponsive ranges in the visible spectrum.For instance, the photoresponsive range of the well-known photocatalyst TiO 2 is DOI: 10.1002/advs.202302978out of the visible spectrum.The limited range usually leads to low photocatalytic efficiency. [4]To realize photodetection for the full solar spectrum, a general commercial structure is the integration of two detectors with complementary photoresponsive ranges, such as Si (≈200-1100 nm) and In-GaAs (≈900-1700 nm) photodetectors.This structure makes the system very complex and sometimes difficult to maintain.Therefore, the exploration of semiconductive materials with photoresponse to the full solar spectrum has been an important aim in the fields of solar utilization, photocatalysis, and photodetection.
There have been many traditional methods to broaden the photoresponsive ranges of semiconductors, such as doping, [5] surface coating, [6] introducing a component with localized surface plasmon resonance effect, [7] embedding defects, [8] and forming a quantum well structure. [9]Among these methods, doping and surface coating using a photosensitizer are the most common two ones.However, the broadened ranges are far from covering the full solar spectrum.For example, the absorption upper limit of Zn x Cd 1−x S is about 900 nm. [10]Integration of two or more semiconductive components with complementary photoresponsive ranges is a novel and effective method to achieve photoresponse to the full solar spectrum. [11]For instance, Yu and co-workers united three semiconducting sulfides, namely ZnS, CdS, and Cu 2−x S, into a single nanocrystal through a colloidal chemical transformation strategy and realized full harvest of solar energy. [12]One shortage of this multi-component method is the complex preparation process.
Except for the above work, some efforts have been made to achieve single-component semiconductors with intrinsic photoresponsive ranges covering the full solar spectrum.To date, very few of these semiconductors have been found.The representatives include graphene, [13] black phosphorus/phosphide, [14] and crystalline topological insulators (for example, SnTe). [15]hey have some deficiencies need to be addressed before their use in applications.For instance, graphene has low absorption coefficient (<3%) and high dark current; black phosphorus/phosphide is not stable in air; crystalline topological insulators are still limited by poor crystal quality and unstable morphology. [1][18][19][20] They undergo photoinduced electron transfer and yield stable radical products with broadband absorption.For instance, a series of 2D cyanide-bridged semiconductor with infinitely -stacked redox-active N-methyl bipyridinium cations were prepared through layer-directed intercalation approach. [16]heir photoinduced radical products showed photoresponse to the full solar spectrum.Such post-photosynthetic methods are still subject to the low content of photogenerated radicals and limited improvement of photoelectric properties because of the low penetration of common lights inside the crystals.
In this work, we develop an unprecedented bottom-up photosynthetic method to obtain single-component semiconductors with photoresponse to the full solar spectrum.The key is to introduce a photosensitive functional motif with broadband absorption in the solar spectrum.Chopoorian et al. reported   in 1964 that p-phenylenediamine tetraacetic acid absorbed in porous glass showed photochromism. [21]A radical product was observed after irradiation.It was ascribed to photoinduced electron transfer from the organic component to the glass, that is, p-phenylenediamine tetraacetic acid was oxidized.The Gopidas group [22] and the Wang group, [23] respectively, found that oxidation of benzidine and its derivatives by Cu(ClO 4 ) 2 and silver(Ι) salts yielded radical cations with two absorption bands around 470 and 1000 nm.We here found, for the first time, that direct irradiation of N,N,N',N'-tetramethylbenzidine (tmb) and BiI 3 in a mixture of DMF and DMA yielded an air-stable radical semiconductor [tmb  1a).This compound has strong absorption spanning the full solar spectrum.In comparison with BiI 3 , it has a more than 1750 nm redshift of the absorption edge and a conductivity gain of five orders of magnitude.Meanwhile, it shows photoconductivity to the full solar spectrum or X-ray.To our knowledge, only a few single component detectors have been reported to exhibit both infrared and Xray response simultaneously (Table S2, Supporting Information).

Results and Discussions
When tmb and BiI 3 was dissolved in the mixture of DMF and DMA, a clear orange solution was obtained (Figure 1a).The color of the solution gradually deepened upon irradiation of a 365 nm LED lamp (1.02 W cm -2 ) in air and reached saturation at 15 min (Figure S1, Supporting Information).Meanwhile, the UV/vis/NIR absorption intensity between 400 and 1600 nm were enhanced with increasing irradiation time (Figure S2, Supporting Information).The BiI 3 solution shows an absorption band ranging from 300 to 600 nm while the tmb solution has an absorption band below 385 nm.We found that, unlike the 365 nm lamp, irradiation using a 420 nm lamp did not result in any color change (Figure S3, Supporting Information).Therefore, photoreaction of the BiI 3 and tmb solution is initiated after photoexcitation of tmb.A control experiment revealed that O 2 was required for this reaction (Figure S4, Supporting Information).Black plate single crystals were obtained after evaporation of the black solution.The characteristic IR peaks of 1 are consistent with those of tmb, which proves the presence of tmb in 1 (Figure S5, Supporting Information).They had a completely different powder X-ray diffraction pattern to that of BiI 3 (Figure 1b) and exhibited a single line signal at 3523 Gauss with g = 2.0030 and a linewidth of 6.30 Gauss in the electron paramagnetic resonance (EPR) spectrum (Figure 1c).This g value is close to 2.0023 for free electrons, which indicates the generation of a radical product.In addition, an X-ray photoelectron spectroscopy (XPS) study (Figure 1d) showed that binding energies of both Bi and I elements are almost the same in BiI 3 and 1 (Figure S6, Supporting Information), but that of the N element changed remarkably.The N 1s core-level spectrum of the tmb molecule had a peak centered at 400.0 eV, while those of the black plate single crystals were split into two peaks at 399.6 and 402.5 eV, respectively.The appearance of the peak at 402.5 eV manifests the oxidation of the tmb molecule.Therefore, the black plate single crystals belong to a species containing the tmb •+ radicals.
Electron absorption band of these crystals covers the full solar spectrum and is more than 1750 nm wider than that of BiI 3 (Figure 1e).It is composed of three main sub-bands at ≈500, 750, and 1200 nm, respectively. [22,23]The 500 nm band is contributed by the tmb •+ radical and the inorganic component.The 750 nm sub-band and the shoulders at around 1500 and 2000 nm in the 1200 nm sub-band are different from those of BiI 3 (Figure 1e) and the reported radical cations of benzidine and its derivatives. [22,23]The clear red-shift nature of the electron absorption spectrum compared with that of the tmb •+ radical and the -stacking structure (described below; Figure 2) indicate the contribution of intervalent charge-transfer transition between the neutral molecule and the tmb •+ radical.The absorption band almost remained when the sample was placed in the dark in air for 4 months (Figure 1e), which suggests a high air stability of the radical product.
In the crystal structure, compound 1 consists of one tetranuclear [Bi 4 I 16 ] 4− cluster and eleven tmb units per molecule.The tetranuclear cluster is built by edge-sharing of four BiI 4 − ions (Figure 2).To satisfy the valence balance, four tmb units should be oxidized to form the tmb •+ cation radical.The XPS data shown in Figure 1d revealed that the area ratio of two N 1s peaks is 0.56, which agrees well the presence of 4 tmb •+ cation radicals and 7  2).The other 8 tmb units, including 5 neutral tmb molecules and 3 tmb •+ radicals, are intercalated between these layers and connected by C-H⋅⋅⋅ interactions (mean separation between centroid and H, 2.84 Å).
To investigate the electrical behavior of 1, a polycrystalline wafer (thickness, 0.18 mm) was used to construct Ag/1/Ag device (Figure 3a, inset).As shown in Figure 3a, the dark current curve under continuous voltage (I-V) of the device manifested nearly linear relation, revealing its inherent Ohmic characteristics.The Wang group previously found that the benzidine radical cation-OC(CF 3 ) 3 − salt was nonconductive. [23]In contrary, compound 1 has a conductivity of 5.13 × 10 −5 S cm −1 , which is comparable to that of GaAs and is also five orders of magnitude higher than that of the BiI 3 precursor (7.94 × 10 −10 S cm −1 ).Compound 1 is stable after thermal annealing at 110 °C for 1 h.However, it has a phase transition at around 130 °C (Figure S8, Supporting Information), and starts to decompose at ≈200 °C (Figure S9, Supporting Information).The conductivity increased with rising temperature between 20 and 60 °C (Figure 3b) and a linear trend was well fitted using the Arrhenius equation (ln = −E a /k B T + constant), where k B is the Boltzmann constant and T is the absolute temperature, respectively. [24]The large charge density was derived from the low activation energy (E a = 0.20 eV), which was in accordance with its intrinsically high conductivity.Figure 3c,d illustrates photoelectric response of 1 using a Xe lamp and a series of semiconductor lasers for the UV-vis (380, 420, 520, 650 nm) and shortwave infrared (SWIR: 808, 1310, 1550, 1850, 2200 nm) regions, respectively, was recorded to validate photodetection performance in the full solar spectrum.The relative magnitude of the current change between dark (I dark ) and bright (I irr ) conditions, (I irr − I dark )/I dark , was used to deduce photocurrent gain.As can be seen, the device unveiled a photocurrent gain upon irradiation of both the UVvis and the SWIR light.For instance, the gains are about 15% at 650 nm (89.2 mW cm −2 ) and 24% at 1550 nm (324.8 mW cm −2 ).
Except the full solar spectrum, the Ag/1/Ag device shows also photoconductivity to X-rays.The presence of the high atomic number elements Bi and I give 1 both a high density and a strong absorption coefficient to X-ray.The calculated absorption coefficient of 1 is basically larger than that of Si in the 1-100 keV (Figure S10, Supporting Information).As shown in Figure 4a, the device proved an evident response to the tungsten anode X-ray, giving a ON/OFF ratio ranging from 180 to 500 with the dose rate rising from 1.52 to 7.93 mGy s −1 .The sensitivity (S) of a detector reflects the charge collection efficiency under continuous X-ray photons irradiation. [25]As illustrated in Figure 4b,c, the photocurrent density and S value for the Ag/1/Ag device were enhanced with a raised electric field.At an electric field of 10 V mm −1 , the S value was 76.90 C Gy −1 cm −2 , which was higher than that of the commercial Si detector. [26]Furthermore, the mobility-lifetime () product represents the potential of a detector to extract X-ray induced carriers. [27]The calculated  value using the modified Hecht equation is 3.50 × 10 −4 cm 2 V −1 (Figure 4d). [28]This value is comparable to that of MAPbI 3 wafer (3.84 × 10 −4 cm 2 V −1 ). [29,30]ignal-to-noise ratio (SNR), evaluated under different bias, is a standard for detecting the amplitude of noise current and low detection limit (LoD). [31]The minimum dose rate of our X-ray detection system is 0.26 mGy s −1 .At this dose rate, the SNR index for the Ag/1/Ag device was 70.93, 97.44, and 135.91 when applying bias voltages of 1, 5, and 15 V, respectively (Figure S11, Supporting Information).They are far larger than 3, indicating that the actual detection limit of the detector should be much smaller.

Conclusion
In summary, this work develops an unprecedented bottom-up photochemical method to prepare radical semiconductors using photosensitive organic components.With the photochemical reaction of BiI 3 and the photochromism-active benzidine derivative, an air-stable single-component radical semiconductor was successfully synthesized.In comparison with BiI 3 , the electron absorption band and conductivity are significantly widened and enhanced, respectively.Especially, this radical semiconductor shows photoresponse to full solar spectrum and X-ray.It may find applications for broadband photodetection, solar conversion or photocatalysis.

Experimental Section
Measurements: Powder X-ray diffraction (PXRD) patterns were collected on a Rigaku Desktop MiniFlex II diffractometer under a range from 5°to 50°, using Cu K  ( = 1.5406Å) as radiation source powered at 30 kV and 15 mA.Analysis spectra of C, H, and N contents were recorded on an Elementar Vario MICRO microanalyzer.Thermogravimetric analysis (TGA) data were taken on a METTLER TOLEDO apparatus from 25 to 1000 °C under N 2 at a heating rate of 10 °C min −1 .Electronic absorption (UV/vis/NIR) spectra were recorded in the diffuse reflectance mode on a PerkinElmer Lambda 900 UV/vis/NIR spectrophotometer with a BaSO 4 plate as the reference.FT-IR spectra were determined by using a Perkin-Elmer spectrum instrument in the range of 4000−400 cm −1 .Electron paramagnetic resonance (EPR) signals at the X band were recorded in a Bruker-BioSpin E500 spectrometer at room temperature.X-ray photoelectron spectroscopy (XPS) spectra were obtained with a ThermoFisher ESCALAB 250Xi X-ray photoelectron spectrometer using Al K  radiation ( = 8.357 Å).X-ray electricity studies were conducted on a Keithley 2400 instrument with a tungsten target continuous X-ray spectrum light source.Routine electricity tests were executed in a Keithley 4200-SCS semiconductor parameter analyzer with a PLS-SXE300D 50-W xenon lamp and a multi-wavelength laser system.All pellet samples for tests were

Figure 1 .
Figure 1.Photosynthesis and radical feature of 1. a) The photosynthetic process.b) Powder X-ray diffraction (PXRD) patterns of 1 and BiI 3 .c) EPR spectrum of 1. d) N 1s XPS data of 1 and tmb.e) UV/vis/NIR absorption spectra of BiI 3 and 1 before and after standing in air for 4 months.

Figure 2 .
Figure 2. Crystal structure of 1. C-H⋅⋅⋅ and - interactions are shown in green and purple dash lines, respectively.Only H atoms included in the C-H⋅⋅⋅ interactions are shown.All possible tmb •+ radicals are noted.

Figure 3 .
Figure 3. Electrical properties of 1. a) Current-voltage (I-V) plot.Inset: Ag/1-wafer/Ag device.The plot for BiI 3 is also shown for comparison.b) Correlation between conductivity and temperature, fitted by the Arrhenius equation.c,d) Wavelength-dependent photoelectric response using a Xe lamp (bias, 15 V) and a series of semiconductor lasers (bias, 5 V) for the UV-vis and SWIR regions, respectively.Note: the relative magnitude of the current change is not comparable because of the different powder densities of light.

Figure 4 .
Figure 4. X-ray detection performance of 1. a) Photocurrent gain at various dose rates with an electric field of 10 V mm −1 .b) Photocurrent density at different X-ray dose rate and electric field.c) Sensitivity under different electric fields.d) Bias-dependent photoconductivity.