WO2016014845A1

WO2016014845A1 - Ultrasensitive solution-processed perovskite hybrid photodetectors

Info

Publication number: WO2016014845A1
Application number: PCT/US2015/041820
Authority: WO
Inventors: Xiong Gong; Xiaowen Hu; Pengcheng DU
Original assignee: The University Of Akron
Priority date: 2014-07-23
Filing date: 2015-07-23
Publication date: 2016-01-28

Abstract

A photodetector utilizing a perovskite active layer is formed using a spin-coating process, whereby in one spin-coating process, a spin-time that remains constant while the acceleration-time is adjusted; and, in another spin-coating process, a spin-time is adjusted, while an acceleration-time is kept constant. Thus, the spin-coating method of the present invention allows the formation of perovskite thin films for photodetector devices, which are uniform and have high surface coverage. In addition, such perovskite thin films have optimized crystallization morphology, which possesses superior optical and electrical properties, which is desirable for photodetector devices.

Description

ULTRASENSITIVE SOLUTION-PROCESSED

PEROVSKITE HYBRID PHOTODETECTORS

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application No.

62/027,825 filed on July 23, 2014, the content of which is incorporated herein by reference.

TECHNICAL FIELD

The present invention relates generally to photodetector devices. Particularly, the present invention is directed to photodetectors that include a perovskite active layer. More particularly, the present invention relates to photodetectors that include a perovskite hybrid active layer that is formed through solution processing. BACKGROUND OF THE INVENTION

Photodetectors or PDs, including photodiodes and solar cells, are among the most ubiquitous types of technology in use today. Their application includes, among others, chemical/biological sensing, environmental monitoring,

daytime/nighttime surveillance, as well as use in remote-control devices, such as television remotes for example. During the evolution of photodetectors, various types of semiconductor materials have been utilized in their design, including ZnO, Si, InGaAs, colloidal quantum dots, graphene, carbon nanotubes and conjugated polymers. Furthermore, it is desirable that the semiconductor materials used in fabricating PDs possess a high-absorption extinction coefficient, which ensures that sufficient light is able to be absorbed by the active layer of the device. This feature is important to ensure that a large charge carrier mobility is provided by the photodetector, so that high photocurrent can be generated, and to ensure that the photodetector is fabricated with a low density of structural defects, so that the dark current density is sufficiently diminished. Given the operating properties desired by photodetector designers, a hybrid organometal halide perovskite material, or perovskite material, has been considered as a promising candidate for use in photodetectors due to its properties. Perovskite materials are direct bandgap semiconductors, which possess a high absorption extinction coefficient within the range of visible light to near infrared light. Moreover, ambipolar transport characteristics of perovskite materials enable both holes and electrons to be transported simultaneously in perovskite-based electronic devices. In addition, the long charge carrier diffusion length of perovskite materials (~1 μηη in CH₃NH3Pbl_3-xClx, -100 nm in CH₃NH₃Pbl₃) results in a low defect density in a perovskite thin film that is formed therefrom, which would be desirable in the fabrication of photodetectors.

Due to the desirable features of perovskite materials, perovskite-based photodetectors have been investigated. However, such efforts have failed to realize a perovskite-based photodetector that has sufficient daytime/nighttime surveillance sensitivity and chemical biological detection sensitivity. In addition, current perovskite-based photodetectors fail to achieve the desired operating features of low-power consumption and high-speed operation. In addition, perovskite PDs of existing designs suffer from decreased performance due to various reasons, including the degradation of the various layers of the detector resulting various from internal and external reactions. Thus, such existing photodetector designs are inherently flawed, giving poor long-term stability. In addition, the availability of low- work-function metal inks, such as aluminum (Al) metal inks, which are needed to manufacture electrodes of PDs based on conventional PSC designs is limited.

Thus, the compatibility of such photodetector designs with continuous, low-cost roll- to-roll manufacturing technology, which requires depositing a large-area Al electrode, also remains problematic.

In addition, because conventional photodetectors are formed of inorganic materials, they require high-temperature processing, and require that the active layer be formed from an expensive metal element, thus making the photodetector a costly device.

In addition, one of the challenges in the fabrication of solution-processed perovskite heterojunction (PHJ) photodetectors is to control the intrinsic morphology of the perovskite films that are used in PHJ photodetectors. Specifically, it has been demonstrated that poor film morphology of perovskite films results in a low shunting resistance and reduced light absorption, which adversely affects the photoelectric conversion process of PHJ photodetectors. Therefore, it is important to precisely control the thin film morphology of crystalized perovskite materials during the film fabrication process to achieve a PHJ photodetector that has a high operating efficiency. For example, it is has been demonstrated that uniform perovskite films with "full coverage", as well as a non-mesoporous layer, can be achieved by primarily controlling the solvent atmosphere and annealing temperature during the preparation of the perovskite thin film. It has also been demonstrated that enhanced crystallization of perovskite films can be obtained by incorporating solvent processing additives with a perovskite precursor solution. It has also been determined that the enhanced crystallization of perovskite films is mainly due to the additive chelating effect, which induces film homogeneous nucleation and modifies the interfacial energy favorability, thereby altering the kinetics of crystal growth. Thus, given this understanding, it would be desirable to utilize a process that is able to precisely control perovskite film formation to ensure favorable thin film

morphology, in order to allow perovskite heterojunction (PHJ) photodetectors to achieve a high operating efficiency.

Therefore, there is a need for a method of controlling the crystallization morphology of solution-processed perovskite thin films. In addition, there is a need for a method of controlling the crystallization morphology of perovskite thin films by tuning the perovskite film fabrication process. Furthermore, there is a need for forming perovskite thin films using a spin-coating process in which different acceleration-times and spin-times are used, so as to allow the perovskite thin film to have a high surface coverage.

SUMMARY OF THE INVENTION

In one aspect of the present invention, a photodetector comprises a first electrode; an electron-extraction layer disposed on the first electrode; a perovskite active layer disposed on the electron-extraction layer, wherein the perovskite active layer is formed by a spin-coating process, wherein the perovskite active layer is disposed onto the electron-extraction layer by a spin-coating process, wherein a spin-time or an acceleration-time of the spin-coating process is adjusted; a hole- extraction layer disposed on the perovskite active layer; and a second electrode; wherein at least one of the first or second electrodes is at least partially transparent to light.

In another aspect of the present invention, a method of preparing a photodetector comprises providing a first electrode that is at least partially transparent to light; disposing an electron-extraction layer on the first electrode; disposing a perovskite light-absorbing layer on the electron-extraction layer by a spin-coating process; adjusting a spin-time or an acceleration-time of the spin- coating process; disposing a hole-extraction layer on the perovskite light-absorbing layer; and disposing a second electrode on the hole-extraction layer.

In yet another aspect of the present invention, a method of preparing a photodetector comprises providing a first electrode that is at least partially transparent to light; disposing a hole-extraction layer on the first electrode;

disposing a perovskite light-absorbing layer on the hole-extraction layer by a spin- coating process; adjusting a spin-time or an acceleration-time of the spin-coating process; disposing an electron-extraction layer on the perovskite light-absorbing layer; and disposing a second electrode on the electron-extraction layer.

In any of the embodiments of the present invention, it is another aspect to provide a photodetector, such that one of a spin-time or an acceleration-time is adjusted, while the other one (time) that is not adjusted is kept constant.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a schematic diagram showing a device structure of one or more embodiments of a hybrid perovskite photodetector in accordance with the concepts of the present invention;

Fig. 2A is a schematic diagram showing a device structure of one or more alternate embodiments of a hybrid perovskite photodetector in accordance with the concepts of the present invention;

Fig. 2B is a graph showing the LUMO (lowest unoccupied molecular orbital) and HOMO (highest occupied molecular orbital) energy levels of T1O2, PC61 BM, CH3NH3PDI3, P3HT (poly(3-hexylthiophene-2,5-diyl), M0O3 and work functions of ITO and Ag of the photodetector of Fig. 2A;

Fig. 3A is a chart showing the J-V characteristics of the hybrid perovskite photodetector of Fig. 2A under dark and under monochromatic illumination at the wavelength of 500 nm with a light intensity of 0.53 mW/cm², whereby the

photodetector of Fig. 2A is structurally configured as:

ITO/TiO₂/CH3NH₃Pbl3/P3HT/MoO3/Ag (PD represented with TiO₂), and structurally configured as: ITO/TiO^PCei BM/CHsNHsPbh/PSHT/MoOs/Ag (PD represented with TiO₂/PC₆i BM);

Fig. 3B is a graph showing the external quantum efficiency (EQE) spectra of hybrid perovskite photodetector of Fig. 2A, whereby the structures of the

photodetectors are configured as: ITO/TiO₂/CH₃NH3Pbl3/P3HT/MoO₃/Ag (PD represented as TiO₂), and configured as: ITO/TiO^PCei BM/CHsNHsPbh/PSHT/ MoO₃/Ag (PD represented with TiO₂/PC₆i BM);

Fig. 4A is a graph showing detectivities vs. wavelength of the hybrid perovskite photodetector of Fig. 2A, whereby the structures of the photodetector are configured as: ITO/TiO₂/CH₃NH3Pbl3/P3HT/MoO₃/Ag (PD represented with TiO₂), and configured as: ITO/TiO^PCei BM/CHsNHsPbb/PSHT/MoOs/Ag (PD represented with TiO₂/PC₆i BM);

Fig. 4B is a graph of the linear dynamic range of the photodetector of Fig. 2A with TiO₂/PC₆i BM;

Fig. 5A is an atomic force microscope (AFM) height image of a TiO₂ thin film utilized by the photodetector of Fig. 2A in accordance with the concepts of the present invention;

Fig. 5B is an atomic force microscope (AFM) height image of a TiO₂/PC6i BM thin film in accordance with the concepts of the present invention;

Fig. 5C is an atomic force microscope (AFM) phase image of a TiO₂ thin film in accordance with the concepts of the present invention;

Fig. 5D is an atomic force microscope phase (AFM) image of a TiO₂/PC6i BM thin film in accordance with the concepts of the present invention; Fig. 6 is a graph of the photoluminescence spectra of TiO₂ CH₃NH₃Pbl3 and TiO₂ PC6iBM/CH3NH3Pbl₃ thin films used by the photodetector of Fig. 2A in accordance with the concepts of the present invention;

Fig. 7 is a graph showing Nyquist plots at V ^« V₀c for the hybrid perovskite photodetector of Fig. 2A, whereby the photodetector is structurally configured as: ITO/TiO₂/CH3NH₃Pbl3/P3HT/MoO3/AI (PD represented with TiO₂), and structurally configured as: ITO/TiO₂/PC₆i BM /CHsNHsPbb/PSHT/MoOs/AI (PDs with

TiO₂/PC₆iBM);

Fig. 8 is a graph of the normalized UV (ultra violet) absorption of perovskite (CH3NH3Pbl3-_xCl_x) utilized by the photodetectors of the present invention;

Fig. 9A is a schematic diagram showing the structure of another exemplary perovskite hybrid photodetector in accordance with the concepts of the present invention;

Fig. 9B is a chart showing the energy level alignment of the structural layers of the perovskite hybrid photodetector of Fig. 9A;

Fig. 10 is a graph showing the J-V characteristics of the perovskite hybrid photodetector of Fig. 9A measured under dark conditions and under illuminated conditions; and

Fig. 1 1 is a graph showing the EQE spectra of the perovskite hybrid photodetector of Fig. 9A measured under short-circuit condition using lock-in amplifier technique.

Fig. 12A is a current-voltage characteristic of the perovskite photodetector in which perovskite thin films are processed with different spin-times, including: Device E: spin-time of about 5 seconds; Device F: spin-time of about 10 seconds; Device G: spin-time of about 20 seconds; and Device H: spin-time of about 30 seconds in accordance with the concepts of the present invention;

Fig. 12B is an external quantum efficiency (EQE) spectra of the perovskite photodetector in which perovskite thin films are processed with different spin-times, including: Device E: spin-time of about 5 seconds; Device F: spin-time of about 10 seconds; Device G: spin-time of about 20 seconds; and Device H: spin-time of about 30 seconds in accordance with the concepts of the present invention; Fig. 13A-H are scanning electron microscope (SEM) micrographs of perovskite thin films that are processed with different spin-times, such that the perovskite thin films of Figs. 13A-B have a spin-time of about 5 seconds; the perovskite thin films of Figs. 13C-D have a spin-time of about 10 seconds; the perovskite thin films of Figs. 13E-F have a spin-time of about 20 seconds, and Figs. 13G-H have a spin-time of about 30 seconds in accordance with the concepts of the present invention;

Figs. 14A-D are atomic force microscopy (AFM) images (20 urn x 20 urn) of perovskite thin films that are processed with different spin-times, such that the perovskite thin film of Fig. 14A has a spin-time of about 5 seconds; the perovskite thin film of Fig. 14B has a spin-time of about 10 seconds; the perovskite thin film of Fig. 14C has a spin-time of about 20 seconds; and the perovskite thin film of Fig. 14D has a spin-time of about 30 seconds in accordance with the concepts of the present invention;

Fig. 15 is a graph showing the UV-vis absorption spectra of perovskite thin films that are processed with different spin-times, such that the perovskite thin film E has a spin time of about 5 seconds; the perovskite thin film F has a spin time of about 10 seconds; the perovskite thin film G has a spin time of about 20 seconds; and the perovskite thin film H has a spin time of about 30 seconds in accordance with the concepts of the present invention;

Fig. 16 is a graph showing X-ray diffraction (XRD) patterns of perovskite thin films processed with different spin-times, whereby film E is processed with a spin- time of about 5 seconds; film F is processed with a spin-time of about 10 seconds; film G is processed with a spin-time of about 20 seconds; and film H is processed with a spin-time of about 30 seconds in accordance with the concepts of the present invention; and

Fig. 17 is a graph showing a Nyquist plot at V=V₀c for a perovskite

photodetector processed with different spin-times, whereby Device E is processed with a spin-time of about 5 seconds; Device F is processed with a spin-time of about 10 seconds; Device G is processed with a spin-time of about 20 seconds; and Device H is processed with a spin-time of about 30 seconds in accordance with the concepts of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

A solution-processed perovskite hybrid photodetector, or PD, is generally referred to by numeral 10, as shown in Fig. 1 of the drawings. It should be appreciated that the terms "photodetector", "PD", and "pero-PD", as used herein, are defined as any electronic light-detecting, light-sensing, or light-converting device, including, but not limited to, photodiodes and solar cells, and any other photodetector device (i.e. photovoltaic devices).

Specifically, the perovskite hybrid photodetector 10 comprises a laminated or layered structure that is formed in a manner to be discussed. As such, the photodetector 10 includes an electrically-conductive electrode 20. In one or more embodiments of the photodetector 10, such as in an inverted design, the electrode 20 may be a transparent or partially-transparent electrode. In other embodiments of the photodetector 10, such as in a non-inverted configuration, the first electrode 20 may be a formed of high work-function metal. High work-function metals suitable for use in electrode 20 include, but are not limited to, silver, aluminum and gold.

Positioned adjacent to the electrode 20 is an electron-extraction layer (EEL) 30. In one or more embodiments, the electron-extraction layer 30 may include an electron-extraction component layer 34 and a passivating component layer 36. In other embodiments, the electron-extraction layer 30 includes an electron-extraction component layer 34 without the passivating component layer 36.

Positioned adjacent to the electron-extraction layer 30 is a light-absorbing layer (i.e. active layer) 40, which is formed of perovskite. Positioned adjacent to the perovskite active layer 40 is a hole-extraction layer (HEL) 50. In one or more embodiments, the hole-extraction layer 50 may include one or more layers that are capable of facilitating the extraction of holes from the photodetector 10. In one or more embodiments, the hole-extraction layer 50 comprises a plurality of sub-layers, including a hole-extraction sub-layer 54 and a hole-extraction sub-layer 56. Positioned adjacent to the hole-extraction layer 50 is an electrically- conductive electrode 60. In one or more embodiments, where the photodetector 10 has an inverted configuration, the electrode 60 may be formed of a high work- function metal. High work-function metals suitable for use as electrode 60 include, but are not limited to, silver, aluminum and gold. In other embodiments, where the photodetector 10 has a non-inverted configuration, the electrode 60 may also comprise a transparent or partially-transparent electrode.

As previously discussed, the photodetector 10 includes both a transparent or partially-transparent electrode and an electrode formed from a high work-function metal. That is, one of the electrodes 20 and 60 is formed so as to be transparent or partially transparent, and positioned so that light is able to enter the photodetector 10. For example, in one or more embodiments, where the where the photodetector 10 has an inverted design, the electrode 20 may be a transparent or partially- transparent electrode, and light will enter the photodetector 10 through electrode 20.

In other embodiments, where the photodetector 10 has a non-inverted design, the electrode 60 may be a transparent or partially-transparent electrode, and light will enter the photodetector 10 through electrode 60. Suitable transparent or partially-transparent materials for use as the electrodes 20,60 include those materials that are conductive and transparent to at least one wavelength of light.

An example of a conductive material suitable for use as electrodes includes indium tin oxide (ITO). In certain embodiments, the conductive electrode 20,60 may be formed as a thin film that is applied to a substrate, such as glass or polyethylene terephthalate. Electron-Extraction Layer

The electron-extraction layer (EEL) 30 is a layer that is configured for capturing an electron generated in the perovskite light-absorbing layer 40 and transferring it to electrode 20. Exemplary materials for preparing the electron- extraction layer 30 include, but are not limited to, T1O2 and phenyl-C61 -butyric acid methyl ester (a fullerene derivative, which may be abbreviated asPC₆i BM). In certain embodiments, where the electron-extraction layer 30 includes the extraction component layer 34, which is formed of T1O2, the T1O2 layer may be applied by depositing a ΤΊΟ2 precursor on the PD 10, such as tetrabutyl titanate (TBT), in solution, and then processing the T1O2 precursor to form TiO2_, for example, by thermally annealing the TiO₂ precursor. A TiO₂ layer of any suitable thickness may be used.

In certain embodiments, where the electron-extraction layer 30 includes the passivating component layer 36 of PC61 BM_, the PC61 BM layer may be applied by solution process such as solution casting. A PC₆iBM layer of any suitable thickness may be used. In one or more embodiments, the PC61 BM layer may be from about 5 nm to about 400 nm in thickness, in other embodiments from about 10 nm to about 300 nm, and in still other embodiments from about 100 nm to 250 nm in thickness.

Perovskite Light-Absorbing Active Layer

The perovskite light-absorbing active layer 40 is a layer capable of

generating holes and electrons upon the absorption of light from any suitable light source. In one aspect, the structure of the perovskite material that is utilized by the light-absorbing layer 40 is denoted by the generalized formula AMX₃, where the A cation, the M atom is a metal cation, and X is an anion (O^2", C^1", B^r", , etc.). The metal cation M and the anion X form the MX_C octahedra, where M is located at the center of the octahedral, and X lies in the corner around M. The MX₆ octahedra form an extended three-dimensional (3D) network of an all-corner-connected type.

Suitable the perovskite materials for using in the light-absorbing layer include organometal halide perovskite. In one or more embodiments, an organometal halide perovskite may be defined by the formula RMX₃, where the R organic cation,

M is a metal cation, and each X is individual a halogen atom. In these or other embodiments, the perovskite light-absorbing active layer 40 includes organometal halide perovskite material, which may be defined by the formula CH3NH3Pbl3__x

Cl_x, where x is from 0 to 3. Advantageously, CH3NH3Pbl3__x Cl_x is an

inorganic/organic hybrid material that combines favorable properties of both inorganic and organic materials. In certain embodiments, the perovskite light- absorbing active layer 40 includes perovskite material that may be defined by the formula CH₃NH₃Pbl3.

In one or more embodiments, the perovskite light-absorbing active layer 40 may be applied to the photodetector 10 through a solution process. Although any suitable technique may be used, a suitable method of solution processing the perovskite light-absorbing active layer is a spin-coating process. After the perovskite light-absorbing active layer 40 is applied to the photodetector 10 thermal annealing may be applied to the photodetector 10. In certain embodiments, the perovskite light-absorbing active layer 40 is applied in a two-step process. In these or other embodiments, the perovskite light-absorbing layer 40 may be prepared by separately depositing an organohalide salt layer and a metal halide salt layer. The organohalide salt and a metal halide salt may be applied through a solution process such as depositing through spin coating. In one or more embodiments, the organohalide salt may be applied to the photodetector 10 first. In other

embodiments, the metal halide salt may be applied to the photodetector 10 first. Suitable metal halide salts include, but are not limited to PblCI, Pb or PbC^.

Suitable organohalide salts include, but are not limited to, CH3NH3I or CH3NH3CI.

The perovskite light-absorbing active layer 40 may have any suitable thickness. In one or more embodiments, the perovskite light-absorbing active layer 40 has a thickness of about 100 nm to about 1200 nm, in other embodiments, from abbot 400 nm to about 1000m, and in other embodiments from about 600 nm to about 700 nm in thickness. Hole-Extraction Layer

The hole-extraction layer (HEL) 50 is a layer capable of capturing a hole generated in the perovskite light-absorbing active layer 40 and transferring it to the electrode 60. Exemplary materials for preparing the hole-extraction layer 50 include, but are not limited to, M0O3, P3HT [poly(3-hexylthiophene-2,5-diyl)], and PEDOTPSS [poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)]. As previously discussed, the hole-extraction layer 50 may include one or more sub- layers 54,56 that are capable of capturing a hole generated in the perovskite light- absorbing layer 40. In one aspect the hole-extraction sub-layer 54 may include a layer of M0O₃, while the hole-extraction sub-layer 56 includes a layer of P3HT. In these or other embodiments, the layer 56 of P3HT may be disposed between the perovskite light-absorbing layer 40 and the M0O₃ layer 54.

In certain embodiments where the hole-extraction layer 50 includes the layer 54 of M0O3, the M0O3 may be applied to the photodetector 10 by thermal evaporation. In these or other embodiments, the M0O₃ layer may be from about 4 nm to about 400 nm, in other embodiments from about 6 nm to about 200 nm, and in other embodiments about 8 to about 50 nm in thickness.

In certain embodiments, where the hole-extraction layer 50 includes the layer 56 of poly(3-hexylthiophene-2,5-diyl), the poly(3-hexylthiophene-2,5-diyl) may be applied to the photodetector 10 by dispensing a solution of poly(3-hexylthiophene- 2,5-diyl) to a spinning device. Exemplary conditions for depositing a solution of poly(3-hexylthiophene-2,5-diyl) include preparing a 20 mg/mL solution of poly(3- hexylthiophene-2,5-diyl) in dichlorobenzene (o-DCB) and depositing it onto a device spinning at 1000 RPMs for approximately 55 seconds. A poly(3-hexylthiophene- 2,5-diyl) layer of any suitable thickness may be used.

In certain embodiments where the hole-extraction layer 50 comprises a layer of PEDOTPSS [poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate], the

PEDOTPSS may be applied to the photodetector 10 by casting the PEDOTPSS from an aqueous solution. In these or other embodiments, the PEDOTPSS may be from about 5 nm to about 200 nm, in other embodiments from about 10 to about 100 nm, and in other embodiments from about 20 to about 60 nm in thickness.

Photodetector Properties

The photodetector 10 of the present invention has a desirable external quantum efficiency (EQE). In one or more embodiments, the photodetector 10 of the present invention has an EQE greater than 50%; in other embodiments, greater than 60%; in other embodiments, greater than 70%; in other embodiments, greater than 80%; and in still other embodiments, greater than 85%. In addition, the photodetector 10 of the present invention has a desirable detectivity, which may be obtained from about 375nm to about 800nm. In one or more embodiments, the photodetector 10 has a detectivity greater than 2 X 10¹² Jones, in other embodiments, greater than 2.8 X 10¹² Jones, in other embodiments, greater than 3 X 10¹² Jones, and in still other embodiments, greater than 4 X 10¹² Jones.

Solution-Processed Perovskite Photodetector I

The following discussion presents the structural details of a particular embodiment of the photodetector 10, which is referred to by numeral 1 10, as shown in Fig. 2A. Specifically, the photodetector 1 10 is a solution-processed perovskite hybrid photodetector that is based on a conventional device structure of ITO/T1O₂ (or TiO₂/PC₆iBM)/perovskite/P3HT/MoO₃/Ag. The photodetector 1 10 comprises a laminated or layered structure formed in a manner to be discussed. Photodetector 1 10 includes a transparent or partially-transparent electrically-conductive electrode 120 that is prepared from indium-tin-oxide (ITO), or any other suitable material. In one aspect, the electrically-conductive electrode 120 may be disposed upon a glass substrate (not shown). Positioned adjacent to the electrically-conductive electrode 120 is the electron-extraction layer (EEL) 130. The electron-extraction layer 130 includes an electron-extraction component layer 134 formed of T1O₂ and a passivating component layer 136 formed of PC61 BM. In certain embodiments, the photodetector 1 10 may not include the passivating component layer 136, thereby leaving only the electron-extraction component layer 134. Positioned adjacent to the electron-extraction layer 130 is a light-absorbing active layer 140, which is formed of perovskite material that is defined by the formula CH₃NH₃Pbl. Positioned adjacent to the perovskite active layer 140 is a hole-extraction layer (HEL) 150. The hole-extraction layer 150 includes a hole-extraction component layer 154 that is formed of P3HT [poly(3-hexylthiophene-2,5-diyl] and a hole-extraction component layer 156 formed of M0O3. However, it should be appreciated that the HEL 150 may be formed of any suitable material. Positioned adjacent to the hole-extraction layer 150 is an electrically-conductive electrode 160 formed of any suitable high work-function metal, such as silver (Ag).

As such, the photodetector 1 10 of the present invention overcomes the problems of conventional photodetector designs by eliminating the strong acidic PEDOTPSS layer, and by substituting the low work-function metal of aluminum (Al) with a high work-function metal electrode of silver (Ag), which can be printed from paste inks. Such a configuration of the photodetector 1 10 dramatically improves the stability of the PD 1 10, as well as its compatibility with large-scale, high-throughput manufacturing techniques, such as roll-to-roll manufacturing. When operated at room temperature, the detectivities (D^*) of the solution-processed photodetector 1 10 is more than about 10¹² Jones for wavelengths from about 375nm to 800nm. The detectivities achieved by the photodetector 1 10 are further enhanced at least four times by modifying the surface of the TiO₂ component layer 134 of the electron extraction layer (EEL) 130 with the solution-processed ΡΟβι ΒΜ component layer 136.

As previously discussed, the solution-processed photodetector 1 10 may be configured so that the electron-extraction layer (EEL) 130 comprises only the TiO₂ component layer 134, or may be configured to comprise both the T1O2 component layer 134 and the component layer 136 formed of TiO2 PC₆i BM, which are fabricated on the ITO substrate 120. The lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) energy levels of T1O2, PCei BM, CH₃NH₃Pbl₃, P3HT, M0O3 and work functions of the ITO and Ag electrodes of the PD 1 10 are shown in Fig. 2B. The LUMO energy levels of P3HT (-3.2 eV) and M0O3 (-2.3 eV) which are higher than that of CH₃NH₃Pbl₃ (-3.9 eV) indicates that separated electrons can be blocked by both P3HT and MoO₃ hole- extraction layers (HEL). The similar values of HOMO energy levels of the HEL 150 and the CH₃NH₃Pbl₃ (perovskite) indicates that separated holes can be efficiently transported through HEL 150 and collected by the Ag electrode (anode) 160. On the other hand, the HOMO energy levels of TiO₂ (-7.4 eV) and PCei BM (-6.0 eV) which are lower than that of CH₃NH₃Pbl₃ (-5.4 eV) (perovskite) indicates that separated holes can be blocked by both the T1O2 and the PC61 BM of the electron- extraction layer (EEL) 130. Efficient electron extraction from the CH₃NH₃Pbl₃ layer 140 to the PC₆i BM/TiO₂ EEL 130 is facilitated due to the -0.3 eV energy offset between the LUMO energy levels of the PC₆i BM/TiO₂ and CH₃NH₃Pbl₃. Based on the band alignment, high photocurrent and low dark current are expected from PD 1 10.

Fig. 3A presents the current density versus voltage (J-V) characteristics of the PD 1 10 with the TiO₂ EEL 130 and the TiO₂/PC₆i BM EEL130 when subjected to both dark conditions and when subjected to monochromatic light illumination at the wavelength (λ) of 500 nm, measured at room temperature. Under dark conditions, the reversed dark-current densities of the PD 1 10 with a TiO₂/PC₆i BM EEL 130 are approximately 10 times smaller than the PD 1 10 with a TiO₂ EEL 130. The low dark-current densities suggest that the PD 1 10 with a TiO₂/PC₆iBM EEL 130 possesses high detectivity. Under the illumination of monochromatic light at a wavelength (λ) of about 500 nm with an illumination intensity of about 0.53 mW/cm², large photocurrent densities are observed from the PD 1 10, which suggest that the PD 1 10 possesses desirable photodiode operation. Moreover, nearly two times larger photocurrent densities are observed from the PD 1 10 having a TiO₂/PC₆i BM EEL 130, as compared with the PD 1 10 having a TiO₂ EEL 130. This demonstrates that PC61 BM is able to boost the charge carrier transport from the perovskite

CH₃NH₃Pbl₃ active layer 140 to the TiO₂ EEL 130, resulting in high photocurrent densities, which is consistent with the band alignment shown in Fig. 2B.

Fig. 3B shows the external quantum efficiencies (EQE) versus wavelength of the PD 1 10 measured under short-circuit conditions and under reverse bias using lock-in amplification techniques, measured at room temperature. At about λ=500 nm, the EQE values achieved were approximately 62% and 84% for the PD 1 10 with a TiO₂ EEL 130 and for the PD 1 10 with a TiO₂/PC₆i BM EEL 130, respectively. The photoresponsivity of the PD 1 10 is calculated according to the following formula: photoresponsivity (R)

where J_Ph is the photocurrent and L|_ight is the incident light intensity. Thus, the photoresponsivity values achieved are 250 mA/W and 339 mA/W for the PD 1 10 with a TiO₂ EEL 130, and for the PD 1 10 with a TiO₂/PC₆i BM EEL 130, respectively. These photoresponsivities (R) are much higher than those from conventional photodetectors.

The detectivities (D*) of the photodetector 1 10 are expressed as D* =

R/(2qJ_d)^{1 2} (Jones, 1 Jones = 1 cm^»Hz^{1 2}/W), where q is the absolute value of electron charge (1 .6 10^~19 Coulombs), and J_d is the dark current density (A/cm²). Accordingly, the detectivities (D*) are calculated to be 1 .4*10¹² Jones, and 4.8x10¹² Jones at about λ = 500 nm, for the PD 1 10 with a TiO₂ EEL 130 and for the PD 1 10 with a TiO₂/PC₆i BM EEL 130, respectively. Based on the EQE spectra of the PD 1 10, the D* versus wavelength are estimated, as shown in Fig. 4A. It is clear that the detectivities D* of the PD 1 10 with a TiO₂/PC₆i BM EEL 130 are notably higher than the PD 1 10 that utilizes the TiO₂ EEL 130. This is the result of the combined function of PC61 BM of simultaneously accelerating the charge carrier transfer at the CH₃NH₃Pbl₃/TiO₂ interface of the EEL 130 and decreasing the dark current densities.

Based on the photocurrent densities versus the incident light intensity of the photodetector 1 10, as shown in Fig. 4B, the linear dynamic range (LDR) or photosensitivity linearity (typically quoted in dB) is calculated according to the equation: LDR=20 log (J*_Ph/Jdark), where J*_Ph is the photocurrent measured at a light intensity of 1 mW/cm². The LDR is over approximately 100 dB for the PD 1 10 with a TiO₂/PC₆i BM EEL 130. This large LDR is comparable to that of silicon (Si) photodetectors (120 dB) and is significantly higher than indium gallium arsenide (InGaAs) photodetectors (66 dB). All of these results demonstrate that the photodetector 1 10 of the present invention is comparable to conventional Si photodetectors and InGaAs photodetectors.

In order to evaluate the detectivities of photodetectors 1 10 with a

TiO₂/PC₆i BM EEL 130, atomic force microscopy (AFM) was used to study the surface morphologies of the TiO₂ thin film and TiO₂/PC6i BM thin film of the EEL 130. Specifically, height AFM images are shown in Figs. 5A and 5B, while AMF phase images are shown in Figs. 5C and 5D. Based on the images, the sol-gel processed TiO₂ thin film shows a rather uneven surface, with a relatively large root mean square roughness (RMS) of about 3.5 nm. Upon passivation of the TiO₂ with PC₆i BM, the surface becomes substantially smoother, with a remarkably reduced RMS of 0.25 nm. The smooth surface of the TiO₂/PC₆i BM EEL 130 produces fewer defects and traps in the interface between the perovskite (i.e. CH₃NH₃Pbl3) and the TiO₂/PC6i BM EEL 130, resulting in small reverse dark current densities. Such structural parameters of the PD 1 10 are in agreement with the J-V characteristics of the PD 1 10 shown in Fig. 3A, thus verifying the dark current densities were suppressed by the passivation of the inhomogeneous ΤΊΟ2 thin film by the PC61 BM layer.

To confirm that the TiO₂ layer of the EEL 130 is passivated by the PC₆iBM layer, a photoluminescence (PL) analysis was performed to inspect the charge carrier generation at the TiO2/CH₃NH₃Pbl3 (i.e. perovskite) and the

TiO2 PC6iBM CH₃NH₃Pbl3 interfaces. Fig. 6 shows the photoluminescence spectra of the TiO₂/CH₃NH₃Pbl₃ and the TiO₂/PC₆i BM/CH₃NH₃Pbl₃ thin films used by the photodetector 1 10. Thus, it was found that a more strikingly quenching effect is observed in the TiO₂/PC₆i BM/CH₃NH₃Pbl₃ than in that of TiO₂/CH₃NH₃Pbl₃. This indicates that a more efficient electron transport has occurred at the

PC₆i BM/CH₃NH₃Pbl₃ (perovskite) interfaces over that of the TiO₂/CH₃NH₃Pbl₃ (perovskite) interfaces, confirming the role of higher electrically conductive PC61 BM (~10^"7S/cm) over the TiO₂ (~10^"11S/cm) for favoring the electron extraction at the EEL 130/CH₃NH₃Pbl₃ 140 material interfaces, resulting in high photocurrents in the PD 1 10 with TiO₂/PC₆i BM EEL 130.

To further evaluate the charge carrier transport at the EEL 130/CH₃NH₃Pbl₃ 140 interfaces, AC impedance spectroscopy (IS) was performed, which provides detailed electrical properties of the PD 1 10 that cannot be determined through direct current measurement. Fig. 7 presents the IS spectra of the PD 1 10 using either a TiO₂ or a TiO₂/PC₆i BM EEL 130. The internal series resistance (R_s) is the sum of the sheet resistance (RSH) of the electrodes and the charge-transfer resistance (RCT) inside the perovskite thin film and at perovskite material/EEL (HEL) interfaces. Since all the PDs 1 10 possess the same device structure, the RSH is assumed to be the same. The only difference is the R_CT, which arises from the different electron transport at the EEL/CH₃NH₃Pbl₃ interface. Upon the modification with the PC61 BM layer, the R_s of the PD 1 10 significantly decreased from about 976 Ω to about 750 Ω, further confirming the role of ΡΟβι ΒΜ in favoring the electron transfer from the CH₃NH₃Pbl₃ to the cathode electrode 120.

In order to evaluate photodetector 1 10 of the present invention, the various components thereof were prepared in the manner discussed below. However, the following discussion should not be viewed as limiting the scope of the invention.

Materials

The TiO₂ precursor, tetrabutyl titanate (TBT) and PC₆i BM were purchased from Sigma-Aldrich and Nano-C Inc., respectively, and used as received without further purification. Lead iodine (Pb ) was purchased from Alfa Aesar.

Methylammonium iodide (CH3NH3I, MAI) was synthesized using the method reported in Z. Xiao, et al., Energy Environ. Sci. 2014, 7, 2619, which is incorporated herein by reference. The perovskite precursor solution was prepared, whereby the Pb and the CH3NH3I were dissolved in dimethylformamide (DMF) and ethanol with a concentration of about 400 mg/nnL for Pb , and about 35 mg/mL for CH₃NH₃I, respectively. All the solutions were heated at about 100 °C for approximately 10 minutes to make sure both the MAI and Pb^ are fully dissolved.

Thin Film Characterizations

Surface morphologies of T1O2 and PC61 BM were measured by tapping-mode atomic force microscopy (AFM) imaging using a NanoScope NS3A system (Digital Instrument). Photoluminescence (PL) spectra were obtained with a 532 nm pulsed laser as excitation source at a frequency of 9.743 MHz.

Pero-PDs Fabrication and Characterization

The compact T1O2 layer was deposited on a pre-cleaned ITO substrate from tetrabutyl titanate (TBT) isoproponal solution (concentration 3 vol%) followed by thermal annealing at about 90 °C for approximately 60 min in an ambient

atmosphere. Next, PC₆i BM layer was casted on the top of the compact TiO₂ layer formed from dichlorobenzene (o-DCB) solution with a concentration of 20 mg/mL, at 1000 RPM for 35 seconds. For the PHJ (perovskite hybrid junction) PD (photodetector) fabrication, the Pb layer was spin-coated from a 400 mg/mL DMF solution at 3000 RPM for about 35 seconds, on the top of thePCeiBM layer, then the film was dried at about 70°C for approximately five minutes. After the film cooled to room temperature, MAI layer was spin-coated on the top of Pbl₂ layer from a 35 mg/mL ethanol solution at 3000 RPM for about 35 seconds, followed by transferring to the hot plate (100 °C) immediately. After thermal annealing at 100 °C for about two hours, the poly(3-hexylthiophene-2,5-diyl) P3HT layer was deposited from a 20 mg/mL o-DCB solution at 1000 RPM for about 55 seconds. Lastly, the pero-HSCs (perovskite hybrid photodetectors) were finished by thermally evaporating M0O3 (8 nm) and aluminum (Ag) (100 nm). The device area is defined to be about 0.16 cm².

The current density-voltage (J-V) characteristics of the PD 1 10 were measured using a Keithley 2400 source-power unit. The PD was characterized using a solar simulator at a wavelength of about 500 nm with an irradiation intensity of approximately 2.61 mW/cm². The external quantum efficiency (EQE) was measured through the incident photon to charge carrier efficiency (IPCE)

measurement setup in use at European Solar Test Installation (ESTI) for cells and mini-modules. A 300 W steady-state xenon lamp provides the source light. Up to 64 filters (8 to 20 nm width, range from 300 to 1200 nm) are available on four filter- wheels to produce the monochromatic input, which is chopped at 75 Hz,

superimposed on the bias light and measured via the usual lock-in technique.

The impedance spectroscopy (IS) was obtained using a HP 4194A

impedance/gain-phase analyzer, under the illumination of white light with the light intensity of about 100 mW/cm², with an oscillating voltage of 50 mV and frequency of 5 Hz to 13 MHz.

Thin Film Characterizations

Surface morphologies of the T1O2 and the PC61 BM were measured by tapping-mode atomic force microscopy (AFM) imaging using a NanoScope NS3A system (Digital Instrument). Photoluminescence (PL) spectra were obtained with a 532 nm pulsed laser as an excitation source at a frequency of about 9.743 MHz. Solution-Processed Perovskite Photodetector II

The following discussion presents the structural details of another

embodiment of the PD 10 discussed above, which is referred to by numeral 210 shown in Fig. 9A. Specifically, the photodetector 210 comprises a laminated or layered structure formed in a manner to be discussed. Photodetector 210 includes a transparent or partially-transparent, electrically-conductive electrode 260 that is prepared from indium-tin-oxide (ITO) or another suitable material, and is disposed upon a suitable glass substrate 270. Positioned adjacent to the electrically- conductive electrode 260 is a hole-extraction layer (HEL) 250 formed of poly(3,4- ethylenedioxythiophene):poly(styrenesulfonate) (i.e. PEDOTPSS). Positioned adjacent to the hole-extraction layer 250 is a light-absorbing active layer 240, which is formed of perovskite, which is defined by the formula CH3NH3Pbl3__x Cl_x, where x is from 0 to 3. Positioned adjacent to the perovskite active layer 240 is an electron-extraction layer (EEL) 230 formed of ΡΟβιΒΜ. Positioned adjacent to the electron-extraction layer 230 is an electrically-conductive electrode 220 formed of aluminum (Al).

In one aspect, the CH3NH₃Pbl3_-x Cl_x active layer 240 has a thickness of about 650 nm and is solution-processed upon an about 40 nm thick poly(3,4- ethylenedioxythiophene):poly(styrenesulfonate) (i.e. PEDOTPSS) layer 250. The electron extraction layer 230 of phenyl-C61 -butyric methyl ester (PC61 BM) has a thickness of about 200 nm and is followed by thermal deposition of an about 100 nm aluminum (Al) electrode layer 220. Fig. 9B depicts the energy level diagram of CH₃NH3Pbl_3-x Clx, PC61 BM and workfunctions of PEDOTPSS and aluminum that comprise the photodetector 210. The LUMO offset between the CH₃NH3Pbl_3-x Clx and the PC61 BM is much larger than 0.3 eV, indicating the charge transfer between CH₃NH₃Pbl_3-x Cl_x and PC61 BM is efficient. Furthermore, both the anode and cathode electrodes 260,220 are small enough to ensure an efficient photo-induced charge transfer from the BHS active layer 240 to the respective electrodes 220,260. In addition, the surface roughness of the perovskite active layer 240 is large enough to form a planar heteroj unction with the PC61 BM layer 230, making good contact for electron transfer. Fig. 8 shows the UV-vis absorption spectra of the CH3NH₃Pbl3_-x Clx utilized by the photodetector 210. The light extinction coefficient is 3.4 x 10^"3 at about 780 nm. Moreover, by tuning the composition of the CH3NH3Pbl3_-x Cl_x perovskite material, the absorption spectra can be extended to the near-infrared region.

Fig. 10 displays the J-V characteristics of the PD 210 that is measured under dark conditions and under illuminated conditions. In the dark, the PD 210 shows a rectification ratio of about 10³, demonstrating good photodiode properties and operation. Under illumination of about 1 .23 mW/cm² at approximately λ=500 nm, the reverse current was largely enhanced by photo-generated charge carriers, while the forward current remained almost the same. J_Ph was demonstrated to be orders of magnitude higher than J_d at the reversed bias, which implies an efficient exciton dissociation and ultrafast photo-induced charge transfer in the CHsNHsPbb-x Clx /PC61 BM bilayer. However, smaller J_d may be achieved by interfacial engineering of the PD 210 or modification of the interfaces between perovskite/ PC61 BM and metal/perovskite of the photodetector 210.

The spectra response of the photodetector 210 was measured under short- circuit condition using lock-in amplifier, and presented in Fig. 1 1 . This data indicates that photons absorbed in the visible to NIR range by the CHsNHsPbb-x Clx perovskite do contribute to the photocurrent. At about λ= 500 nm, the EQE is approximately 66% electron-per-photon, and the corresponding responsivity (R) is calculated to be about 264 mA W, which is significantly larger than the values reported before.

D* is one of the most important figures of merits (FOM) for evaluating performance of a photodetector, and is expressed as

^V2 , where Light is the incident light intensity and q is the electron charge. D* is calculated to be 2.85 x 10¹² Jones at λ=500 nm with light intensity of 1 .23 mW/cm², shown in Table 1 for the photodetector 210. Table 1. Parameters for perovskite-based PD 210.

As such, the high-charge carrier mobility, large light-extinction coefficient and large film thickness of the perovskite material makes it an excellent light absorber in the photodetector 10, 1 10, and 210 of the present invention. Additionally, the solution-processed perovskite photodetectors of the present invention exhibit a wide and strong response ranging from UV (ultraviolet) to the NIR (near infrared), with a high detectivity (D^*) of 2.85 x 10¹² Jones at wavelength of about 500 nm and an enhanced device stability.

In one aspect, the perovskite light-absorbing active layer or film 40 used by the photodetector 10 may be formed using a spin-coating process to be discussed in detail below. Furthermore, while the perovskite spin-coating coating process and the performance characteristics of the resultant perovskite thin film are discussed in detail below with regard to the fabrication of photodetectors, it should be

appreciated that the perovskite spin-coating process may be utilized in the fabrication of any type of perovskite-based photodetection device, including but not limited to solar cells.

To evaluate the influence on photodetector performance that is caused by the morphology of perovskite films, which are formed using the process to be discussed, a conventional p-i-n PHJ device architecture of indium-tin-oxide

(ITOyPEDOTPSS/CHsNHsPbls-xClx/PCeiBM/Aluminum (Al) was used, such as that of device 10, shown in Fig. 1 . The thickness of the PEDOTPSS layer 50 was ~35nm, the CHsNHsPbb-xClx (perovskite) layer 40 was ~600nm, and the PC₆iBM layer 30 was ~100nm. The PEDOTPSS layer 50 and the PC₆iBM layer 30 in the p- i-n PHJ device 10 operates as the hole-extraction layer (HEL) and electron extraction layer (EEL), respectively. As such, the hole-extraction layer (HEL) 50 and the electron extraction layer (EEL) 30 function to dissociate the excitons generated in the active layer 40 into free charge carriers, whereupon these free charge carriers are extracted by their respective electrodes 60 and 20.

Specifically, the present invention provides two methods for processing or forming the perovskite thin film layer 40 using a spin-coating technique. In a first spin-coating process, the spin-time is kept constant (i.e. fixed amount of time), but the acceleration-time is tuned or adjusted from between about 4 seconds to 16 seconds for example. In a second spin-coating process, the acceleration-time was kept constant (i.e. fixed amount of time), but the spin-time is tuned or adjusted from about 5 seconds to 30 seconds for example. Table 2, shown below, summarizes the performance of various devices 10, denoted as Devices A-H, in which the perovskite film 40 was formed using the two different spin-coating processing methods. It should be appreciated that the spin-time and the acceleration time may be set to any desired time quantity or value. Table 2. Perovskite thin film characteristics and perovskite photodetector erformance

^*Spin speed is 2500 RPM

Specifically, all perovskite thin films 40 that were processed by the first method (spin-time: constant; acceleration-time: adjusted) achieved the same thickness. As such, perovskite photodetectors, denoted as Devices A to D in Table 2, possess the same open circuit voltage (V_oc) of approximately 0.85V, but have different short circuit current densities (J_sc) and fill factors (FF), consequently with different power conversion efficiencies (PCE). The best PCE of about 1 1 .43% was observed from the perovskite photodetectors of Device B, which were processed with the acceleration-time of about 8 seconds and a spin-time of about 20 seconds. In addition, prolonging the acceleration-time to 12 seconds and 6 seconds, however, resulted in PCEs with a downward trend, with decreasing PCEs of 10.12% and 9.99% respectively.

All perovskite thin films 40, which are processed by the second method have different thicknesses. Consequently, Devices E to H that are formed using the second process possess significantly different device performance, as shown in Table 2. Furthermore, Fig. 12 presents the current-density versus voltage (J-V) characteristics of p-i-n PHJ perovskite photodetectors 10, denoted as Devices E to H, which are processed by the second method. Device E, which was processed with the acceleration-time of about 8 seconds and a spin-time of about 5 seconds, exhibits a J_Sc of about 18.56 mA/cm², a V₀c of approximately 0.75 V, a FF (fill factor) of about 57.5% and a corresponding PCE of approximately 8.00%. Device F, which was spin-cast with an acceleration-time of about 8 seconds and a spin- time of 10 seconds, exhibited a significantly enhanced J_sc of about 22.59 mA cm², Voc of about 0.90 V, a FF of approximately 69.5% and a remarkably enhanced PCE of about 14.13%. However, by further increasing the spin-time, the thickness of the perovskite thin film 40 is kept almost the same, but the corresponding Devices G and H show decreased Jsc, FF and PCE.

The external quantum efficiency (EQE) spectra of devices 10, which are denoted as Devices E to H in Table 2, are presented in Fig. 12B. The onset of the photocurrents that are observed from Fig. 12B for all of the devices are at 800nm, which is in good agreement with the bandgap of CH3NH3Pbl3_-xCl_x observed from Fig. 15. Based on the EQE spectra, the J_sc of the perovskite photodetectors 10 can be estimated by integrating the EQE spectrum from 375 nm to 800 nm. The estimated Jsc from the EQE are about 17.92 mA cm², about 21 .14 mA/cm², about 19.77 mA/cm² and about 19.36 mA/cm² for Devices E to H, respectively. These estimated Jsc values are consistent with those observed from the J-V

characteristics shown in Fig. 12A. These results demonstrate that the device performance of perovskite photodetectors 10 is very sensitive to the spin-coating processing procedures that are utilized for fabrication of perovskite thin films.

Thus, device performance of PHJ perovskite photodetectors 10 is

substantially determined by the crystallization morphology of the resultant perovskite absorbing layer 40 because the defects in the perovskite crystals deleteriously impact the efficiency of charge dissociation, transport and

recombination. To this end, scanning electron microscopy (SEM) was carried out to investigate the morphological properties of perovskite thin films that are processed by the second procedure (Table 2). Figs. 13A-H show SEM images of the perovskite thin films 40 that are coated on top of PEDOT:PSS/ITO/glass with the constant acceleration-time, but different spin-times. It is clear that there are many pinholes and voids in the perovskite film 40, as shown in Fig. 13A, which was continue to evolve to form continuous crystal islands as shown in Fig. 13B. The crevices that remain in the crystal boundaries of the perovskite film 40 result in incomplete surface coverage and non-uniform perovskite thin films due to de- wetting or agglomeration by such a short spin-time. This poor quality perovskite thin film results in a low PCE for perovskite photodetectors 10, such as Device E.

As the spin-time of the perovskite film 40 was increased to about 10 seconds, the crevices began to vanish, and a uniform perovskite thin film with few voids, as well as high surface coverage was observed, as shown in Figs. 13C and 13D. Moreover, the enlarged surface coverage increases the contact areas between the perovskite thin film 40 with both PEDOTPSS hole-extraction layer 50 and PC61 BM electron extraction layer 30, minimizing the shunt-path and reducing the leakage-current. Enlarged surface coverage also enables perovskite thin films to harvest more incident light illumination, resulting in high photocurrent. As such, the reduction of the shunt-path and leakage-current would therefore result in a larger V₀c in such devices 10, such as the perovskite photodetectors of Devices E and F of Table 2. In addition, further prolonging the spin-time of the spin-coating process does not produce more favorable perovskite films (Figs. 13E-F).

Specifically, additional small pores and coarsened perovskite crystals emerge in the perovskite thin films, resulting in markedly degraded device performance, as in Devices G and H. In other to further understand the underlying device

performance, atomic force microscopy (AFM) was used to study the surface morphology of perovskite thin films processed with different spin-time. The height of the AFM images is shown in Figs. 14A-D. It is clear that there are dramatic differences in the surface morphologies of perovskite thin films 40 that are processed with different spin-times.

In the perovskite thin film of Fig. 14A, which is processed with a spin-time of about 5 seconds, there are many crevices. However, densely interconnected crystalline grains are presented in the perovskite thin films 40 shown in Fig. 14B, which is processed with the spin-time of about 10 seconds. Aggregated clusters of perovskite crystalline grains with pores, which coarsen the perovskite crystallinity appear in the perovskite thin films 40 processed with the spin-time of approximately 20 seconds and 30 seconds, respectively. All these observations are consistent with those observed from the SEM (scanning electron microscope) images. Thus, the perovskite thin film processed with a spin-time of about 10 seconds possesses higher surface coverage as compared to the perovskite thin films processed with other conditions. As a result, the corresponding perovskite photodetectors 10, such as Device F, have the best performance among all the perovskite photodetectors that are processed with different conditions. These results demonstrate that the formation of perovskite thin films 40 during solvent evaporation plays a critical role in the creation of favorable film morphologies of perovskite crystallinity towards high efficiency photodetector devices.

The crystallization morphologies of perovskite thin films 40 are further investigated by absorption spectra, which are shown in Fig. 15. Under the same conditions, a perovskite film 40 that is processed with a spin-time of about 10 seconds exhibits increased light absorption co-efficiency, as compared to that of other films, which are processed with a spin-time of about 5 seconds, 20 seconds, and 30 seconds. These observations are consistent with the results from the EQE spectra. In addition all thin films 40 processed with different spin-times possess similar thickness (Table 1 ). Thus, the increased absorption co-efficiencies are solely originated from improved surface coverage and uniform crystal formation in the perovskite thin films.

The crystallinity of perovskite thin films 40 is further evidenced by X-ray diffraction (XRD) patterns in Fig. 16, which show the XRD patterns of perovskite thin films 40 that are processed with different spin-times. The crystal structure of perovskite thin films is maintained the same, but the degrees of perovskite crystallinity is significantly altered by different spin-times. The main diffraction peaks at 14.1 °, 28.4°, 31 .8°, 40.6° and 43.2°, which are assigned to the (1 10), (220), and (310), (224), (314) faces, are in identical positions for all the films processed with different spin-times. This indicates that the halide perovskite possesses an orthorhombic crystal structure. However, the intensities of the diffraction peak at 14.1 ° are different. The film 40 processed with the spin-time of about 10 seconds exhibits the strongest diffraction intensity among all of the thin films. The stronger the difference in diffraction intensity, the higher the level of crystallinity. Therefore, the film processed with the spin-time of 10 seconds possesses high charge carrier mobility and consequently high PCE.

Four-probe measurements were performed to preliminarily investigate the electrical conductivities of perovskite thin films 40 that were processed with different spin- times, the results of which are summarized in Table 2. Under the same conditions, the films 40 that were processed with the spin-time of about 10 seconds exhibited the highest electrical conductivity as compared with other films. High electrical conductivity suggests that the charge carriers generated in perovskite thin films 40 can be effectively transported to the PEDOTPSS/perovskite interface and to the perovskite/PC6iBM interface, and then extracted by their respective electrodes, resulting in high photocurrent. These results are consistent with those observed from J-V characteristics, as shown in Fig. 12A and Table 1 .

In order to further confirm the electrical conductivities of perovskite thin films 40 produced using the process of the present invention, the internal series resistances (R_s) of the perovskite photodetectors 10 were investigated by AC impedance spectroscopy (IS). The IS analysis enables the monitoring of the detailed electrical properties of perovskite photodetectors 10 that cannot be determined by direct current measurements. The R_s is composed of the sheet resistance (RSH) of the electrodes, the charge-transfer resistance (RCT) inside the perovskite thin films and at PEDOTPSS/perovskite and perovskite/PC6i BM interfaces. Since all perovskite photodetectors 10 have the same device structure, only the perovskite films processed with different conditions, the R_SH can be assumed the same. The Nyquist plots for all of the perovskite photodetectors are shown in Fig.17. The semicircles in the Nyquist plot identify the homogeneous transport pathways in these devices. R_s of 10.3 ±0.5 kQ was observed from Device E, which is processed with a spin-time of 5 seconds, however, with increasing spin- time of 10 seconds, 20 seconds, and 30 seconds, R_s of 6.6 ±0.2 kQ, R_s of 7.4 ±0.2 kQ R_s of 7.8 ±0.3 kQ are observed from Devices F, G and H, respectively. These results indicate that electrical conductivities of perovskite thin films 40 are affected by crystallization morphologies of perovskite thin films 40 that are processed with different spin-times. Low R_s from device F indicates high electrical conductivities of perovskite thin film. As a result, a high PCE from Device F is achieved, whereby the perovskite thin film is processed with a spin time of about 10 seconds.

Experimental Section

Materials: PEDOTPSS and ΡΟβι ΒΜ were purchased from Clevius and 1 - Material Inc., respectively, and used as received without further purification. Lead chloride (PbC^) was purchased from Alfa Aesar. Methylammonium iodide

(CH3NH3I) was synthesized using known techniques. The perovskite precursor solution was prepared as follows: CH3NH3I and PbC^ powder were mixed together in anhydrous dimethylformamide (Aldrich) with a molar ratio of 3:1 . In particular, the concentrations of CH3NH3I and PbC^ were 2.64 mol/L and 0.88 mol/L. The solution was stirred at approximately 60° for about 6 hours and then placed standstill overnight before being used for preparation of any thin films.

Characterization of perovskite thin films: A field emission scanning electron microscope (JEOL-7401 ) was used to acquire SEM images. To acquire images of moisture-sensitive perovskite films 40, samples were kept in nitrogen atmosphere until imaged. Bruker AXS Dimension D8 X-Ray System was used to investigate the XRD patterns of perovskite films 40 that are coated on polyethylene terephthalate (PET) substrates. The tapping-mode AFM (Nano-Scope NS3A system) was used to observed the surface morphologies of perovskite films. The thickness of the perovskite films, PEDOTPSS films and ΡΟβιΒΜ films were measured by AFM. The absorption spectra of perovskite films 40 coated on

PEDOT:PSS/ITO/Glass were measured by an HP 8453 UV-vis spectrophotometer.

Electrical conductivities of perovskite thin films: A four-probe method was utilized to measure the electrical conductivity of perovskite thin films. The measurement was conducted using a homemade conductivity apparatus that was equipped with tow Keithley 2400 Source Meters. The electrical conductivities of the perovskite thin films 40 were calculated by σ =— , where σ is the conductivity of the

R_st

thin films, R_s is the sheet resistance, and t is the thickness of the films.

Fabrication of perovskite photodetectors: The photodetectors 10 were fabricated in the configuration of ITO/PEDOT:PSS/perovskite/PC₆iBM/AI. 35nm of PEDOTPSS was spin-coated on pre-cleaned ITO glass substrates followed with thermal annealing at 150° C for 10 minutes. The perovskite films 40 were spin- coated on top of the ITO/PEDOTPSS to form a perovskite precursor solution in a glovebox that has a nitrogen atmosphere. The perovskite thin film was processed using the two different two-step processes discussed above. In the first process, the spin-time was kept constant at about 20 seconds with a spin-speed of about 2500 RPM (revolutions per minute), but the acceleration-time was tuned from 4 seconds to 16 seconds. In the second process, the acceleration-time was kept at about 8 seconds, but the spin-time was tuned from 5 seconds to 30 seconds with the same spin-speed of 2500 RPM. Detailed processing conditions are

summarized in Table 2. The wet perovskite thin films 40 were placed in a petri-dish without cover for 30 minutes and then annealed at 90° C for about 3 hours.

Afterward, about 100 nm or PC₆iBM 30 was spin-coated on the top of the perovskite thin films 40. Aluminum electrode 20 with a thickness of 120 nm was then finally evaporated under a high vacuum (2x10⁶ mbar) through a shade mask. The device area is defined as 4.5mm². Characterization of perovskite solar photodetectors: The J-V

characteristics of perovskite photodetectors 10 were recorded using a Keithley 2400 Source Meter. The device photocurrent was measured under AM 1 .5 illumination at the light intensity of about 100 mW/cm². The light intensity was accurately calibrated by a standard Si (silicon) photodiode. The EQEs of perovskite

photodetectors were performed by a commercial photomodulation spectroscopic setup (DSR100UV-B). The IS was obtained using a HP 4194A impedance/gain- phase analyzer, under the illumination of white light with the light intensity of 100 mW/cm², with an oscillating voltage of 50 mV and a frequency of 5 Hz to 13 MHz.

Therefore, one advantage of the present invention is that a high-performance solution-processed planar heterojunction perovskite photodetector utilizes a spin- coating technique in which a uniform perovskite thin film with high surface coverage is able to be produced. Still another advantage of the present invention is that a high-performance solution-processed planar heterojunction perovskite

photodetector utilizes a spin-coating technique that allows optimized crystallization morphology to be achieved, which possess superior optical and electrical properties. Still another advantage of the present invention is that a high- performance solution-processed planar heterojunction perovskite photodetector utilizes a spin-coating technique in which a uniform perovskite thin film is formed, which allows the photodetector to achieve a PCE efficiency of over 14%.

Thus, it can be seen that the objects of the present invention have been satisfied by the structure and its method for use presented above. While in accordance with the Patent Statutes, only the best mode and preferred

embodiments have been presented and described in detail, with it being understood that the present invention is not limited thereto or thereby. Accordingly, for an appreciation of the true scope and breadth of the invention, reference should be made to the following claims.

Claims

CLAIMS What is claimed is:

1 . A photodetector comprising:

a first electrode;

an electron-extraction layer disposed on the first electrode;

a perovskite active layer disposed on the electron-extraction layer, wherein the perovskite active layer is formed by a spin-coating process, wherein the perovskite active layer is disposed onto the electron-extraction layer by a spin- coating process, wherein a spin-time or an acceleration-time of the spin-coating process is adjusted;

a hole-extraction layer disposed on the perovskite active layer; and a second electrode;

wherein at least one of the first or second electrodes is at least partially transparent to light.

2. The photodetector of claim 1 , wherein the perovskite active layer comprises organometal halide perovskite.

3. The photodetector of claim 2, wherein the organometal halide is defined by the formula CH3NH3Pbl3__x Cl_x, where x is from 0 to 3.

4. The photodetector of claim 3, wherein the organometal halide is defined by the formula CH₃NH₃Pbl3.

5. The photodetector of claim 4, wherein the electron-extraction layer comprises TiO₂.

6. The photodetector of claim 3, wherein the T1O2 is passivated by

[6, 6]-phenyl-C61 -butyric acid methyl ester.

7. The photodetector of claim 4, wherein the photodetector includes a first hole- extraction layer and a second hole-extraction layer.

8. The photodetector of claim 7, wherein the first hole-extraction layer comprises MoO₃ and the second hole-extraction layer comprises poly(3- hexylthiophene-2,5-diyl).

9. The photodetector of claim 3, wherein the electron-extraction layer comprises [6, 6]-phenyl-C61 -butyric acid methyl ester.

10. The photodetector of claim 3, wherein the hole-extraction layer comprises poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate).

1 1 . The photodetector of claim 1 , wherein the external quantum efficiency is greater than 50%.

12. The photodetector of claim 1 , wherein detectivities greater than 2.8

X 10¹² Jones can be obtained for at least one wavelength between 375 nm to 800 nm.

13. The photodetector of claim 12, wherein the detectivities greater than 2.8 X 10¹² Jones can be obtained for the wavelengths between 375 nm to 800 nm.

14. A method of preparing a photodetector comprising:

providing a first electrode that is at least partially transparent to light;

disposing an electron-extraction layer on the first electrode;

disposing a perovskite light-absorbing layer on the electron-extraction layer by a spin-coating process;

adjusting a spin-time or an acceleration-time of the spin-coating process; disposing a hole-extraction layer on the perovskite light-absorbing layer; and disposing a second electrode on the hole-extraction layer.

15. The method of claim 14, wherein the step of disposing the perovskite light absorbing is performed by first disposing a layer comprising a metal halide salt on the electron-extraction layer and then disposing an organohalide salt on the layer comprising a metal halide salt.

16. The method of claim 15, wherein the metal halide salt layer is PblCI, Pbl₂ or PbCI₂.

17. The method of claim 15, wherein the organohalide salt layer is CH3NH3I or CH3NH3CI.

18. The method of claim 14, wherein the electron-extraction layer comprises TiO₂.

19. The method of claim 14, wherein the electron-extraction layer comprises TiO₂ formed by depositing a TiO₂ precursor and then processing the TiO₂ precursor to form TiO₂.

20. The method of claim 18, wherein the TiO₂ is passivated by depositing a layer comprising phenyl-C61 -butyric acid methyl ester on the TiO₂.

21 . The method of claim 14, wherein the hole-extraction layer comprises a material selected from M0O3, poly(3-hexylthiophene-2,5-diyl), poly(3,4- ethylenedioxythiophene):poly(styrenesulfonate), and combinations thereof.

22. The method of claim 14, wherein the hole-extraction layer is includes a layer comprising poly(3-hexylthiophene-2,5-diyl) and a layer comprising M0O3.

23. A method of preparing a photodetector comprising:

providing a first electrode that is at least partially transparent to light;

disposing a hole-extraction layer on the first electrode;

disposing a perovskite light-absorbing layer on the hole-extraction layer by a spin-coating process; adjusting a spin-time or an acceleration-time of the spin-coating process; disposing an electron-extraction layer on the perovskite light-absorbing layer; and

disposing a second electrode on the electron-extraction layer.

24. The method of claim 23, wherein the step of disposing the perovskite light- absorbing layer is performed by first disposing a layer comprising a metal halide salt on the hole-extraction layer and then disposing an organohalide salt on the layer comprising a metal halide salt .

25. The method of claim 24, wherein the metal halide salt layer is PblCI, Pb^ or PbCI₂.

26. The method of claim 24, wherein the organohalide salt is CH3NH3I or CH3NH3CI.

27. The method of claim 23, wherein the electron-extraction layer comprises TiO₂.

28. The method of claim 23, wherein the electron-extraction layer comprises

T1O2 formed by depositing a T1O2 precursor and then processing the T1O2 precursor to form T1O2.

29. The method of claim 27, wherein the T1O2 is passivated by depositing a layer comprising phenyl-C61 -butyric acid methyl ester on the T1O2.

30. The method of claim 23, wherein the hole-extraction layer comprises a material selected from M0O3, poly(3-hexylthiophene-2,5-diyl), poly(3,4- ethylenedioxythiophene):poly(styrenesulfonate), and combinations thereof.

31 . The method of claim 30, wherein the hole-extraction layer includes a layer comprising poly(3-hexylthiophene-2,5-diyl) and a layer comprising M0O₃.

32. The photodetector of claim 1 , wherein one of the spin-time or the acceleration- time is adjusted, and the other one that is not adjusted is kept constant.

33. The method of claim 14, wherein at the adjusting step, one of the spin-time or the acceleration-time is adjusted, and the other one that is not adjusted is kept constant.

34. The method of claim 23, wherein at the adjusting step, one of the spin-time or the acceleration-time is adjusted, and the other one that is not adjusted is kept constant.