Efficient and stable perovskite solar cells with all solution processed metal oxide transporting layers

Abstract

An opto-electronic device includes a first electrode, a first buffer layer formed on the first electrode, and a perovskite semiconductor active layer formed on the first buffer layer. The opto-electronic device further includes a second buffer layer formed on the perovskite semiconductor active layer, and a second electrode formed on the second buffer layer. The first buffer layer, the second buffer layer, and the perovskite semiconductor active layer each consists essentially of inorganic materials.

Claims

1. An opto-electronic device, comprising: a first electrode; a first buffer layer formed on said first electrode; a perovskite semiconductor active layer formed on said first buffer layer; a second buffer layer formed on said perovskite semiconductor active layer; and a second electrode formed on said second buffer layer, wherein said first buffer layer has a thickness between about 75 nm and about 85 nm, wherein said second buffer layer has a thickness between about 65 nm and about 75 nm, and wherein said first buffer layer is nickel oxide and said second buffer layer is zinc oxide.

2. An opto-electronic device according to claim 1, wherein said first and second electrodes each consists essentially of inorganic materials.

3. An opto-electronic device according to claim 1, wherein said first buffer layer comprises a plurality of nanoparticles.

4. An opto-electronic device according to claim 1, wherein said second buffer layer comprises a plurality of nanoparticles, wherein each of the plurality of nanoparticles has a size that is less than 10 nm.

5. An opto-electronic device according to claim 1, wherein said first buffer layer has a work function between about 5.00 eV and 5.10 eV.

6. An opto-electronic device according to claim 1, wherein said perovskite semiconductor active layer consists essentially of at least one perovskite that satisfies the formula ABX.sub.3, wherein A is at least one of CH.sub.3NH.sub.3 and NH.sub.2CHNH.sub.2, wherein B is at least one of PB and Sn, and wherein X is at least one of Cl, Br and I.

7. An opto-electronic device according to claim 1, wherein said perovskite semiconductor active layer has a thickness between about 315 nm and about 325 nm.

8. An opto-electronic device according to claim 1, wherein said perovskite semiconductor active layer comprises a plurality of crystals, and wherein each of said plurality of crystals has a width greater than 1 m.

9. An opto-electronic device according to claim 1, wherein said perovskite semiconductor active layer comprises a plurality of crystals, and wherein each of said plurality of crystals has a width that is greater than a thickness of said perovskite semiconductor active layer.

10. An opto-electronic device according to claim 1, wherein at least one of said first and second electrodes is a transparent electrode.

11. An opto-electronic device according to claim 1, wherein said opto-electronic device is at least one of a photovoltaic cell, an optical detector, a radiation detector, a light emitting diode (LED), a laser and a memory device.

12. An opto-electronic device according to claim 1, wherein said opto-electronic device has a normalized power conversion efficiency that remains substantially constant over a period of 20 days.

13. A method of producing an opto-electronic device, comprising: providing a substrate; forming a first electrode on said substrate; forming a first buffer layer on said first electrode; forming a perovskite semiconductor active layer on said first buffer layer; forming a second buffer layer on said perovskite semiconductor active layer; and forming a second electrode on said second buffer layer, wherein said first buffer layer has a thickness between about 75 nm and about 85 nm, wherein said second buffer layer has a thickness between about 65 nm and about 75 nm, and wherein said first buffer layer is nickel oxide and said second buffer layer is zinc oxide.

14. A method of producing an opto-electronic device according to claim 13, further comprising treating said perovskite semiconductor active layer with at least one of an exposure to air for less than one hour, exposure to oxygen, exposure to ultraviolet light, moisture exposure and thermal annealing with a temperature less than 120 C.

15. A method of producing an opto-electronic device according to claim 13, wherein forming a perovskite semiconductor active layer on said first buffer layer comprises: forming a layer of PbI.sub.2 on said first buffer layer; coating said PbI.sub.2 layer with a CH.sub.3NH.sub.3I solution; and annealing said CH.sub.3NH.sub.3I solution coated PbI.sub.2 layer to form said perovskite semiconductor active layer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Further objectives and advantages will become apparent from a consideration of the description, drawings, and examples.

(2) FIG. 1 illustrates an opto-electronic device according to some embodiments of the invention;

(3) FIG. 2A shows an atomic force microscopy (AFM) image of NiO.sub.x on an ITO substrate;

(4) FIG. 2B shows the surface potential distribution of NiO.sub.x on the ITO substrate;

(5) FIG. 2C shows an atomic force microscopy (AFM) image of ZnO on an ITO substrate;

(6) FIG. 2D shows the surface potential distribution of ZnO on the ITO substrate;

(7) FIG. 3 is a transmission electron microscopy (TEM) image of synthesized ZnO nanoparticles;

(8) FIG. 4 shows transmission spectra of the NiO.sub.x and ZnO on ITO substrates;

(9) FIG. 5A shows an X-ray photoelectron spectroscopy (XPS) peak of Ni 2p.sub.3/2 in NiO.sub.x;

(10) FIG. 5B shows an XPS peak of O 1s in NiO.sub.x;

(11) FIG. 5C illustrates ultraviolet photoelectron spectroscopy of NiO.sub.x and PEDOT:PSS;

(12) FIG. 5D shows an XPS peak of Zn 2p.sub.3/2 in ZnO;

(13) FIG. 5E shows an XPS peak of O 1s in ZnO;

(14) FIG. 5F illustrates ultraviolet photoelectron spectroscopy of ZnO and PCBM;

(15) FIG. 6A shows valence band maximum spectra of NiO.sub.x measured by x-ray photoelectron spectroscopy;

(16) FIG. 6B shows valence band maximum spectra of ZnO measured by x-ray photoelectron spectroscopy;

(17) FIG. 7 shows the photoluminescence of glass/CH.sub.3NH.sub.3PbI.sub.3, glass/CH.sub.3NH.sub.3PbI.sub.3/ZnO, and glass/CH.sub.3NH.sub.3PbI.sub.3/PCBM;

(18) FIG. 8 shows time-resolved photoluminescence (TRPL) of glass/CH.sub.3NH.sub.3PbI.sub.3, glass/CH.sub.3NH.sub.3PbI.sub.3/ZnO, and glass/CH.sub.3NH.sub.3PbI.sub.3/PCBM;

(19) FIG. 9 shows the device structure of metal oxide, Glass/ITO/NiO.sub.x/CH.sub.3NH.sub.3PbI.sub.3/ZnO/Al;

(20) FIG. 10 shows the band alignment of the metal oxide based perovskite solar cells according to the UPS data shown in FIGS. 5C and 5F;

(21) FIG. 11 illustrates an X-ray diffraction pattern of the CH.sub.3NH.sub.3PbI.sub.3 obtained by two-step spin-coating process;

(22) FIG. 12A shows an SEM image of the perovskite growth on an NiOx surface achieved by two step-processing;

(23) FIG. 12B shows an SEM image of perovskite coated by ZnO films;

(24) FIG. 12C shows the cross-section of the metal oxide based perovskite solar cells, wherein the thickness of NiO.sub.x, CH.sub.3NH.sub.3PbI.sub.3, and ZnO are 80 nm, 330 nm, and 70 nm, respectively;

(25) FIG. 13 shows an SEM image of perovskite film on a PEDOT:PSS substrate by a two-step spin-coating process;

(26) FIG. 14A shows how the device performance using NiO.sub.x as hole transport layers depends on thickness based on a device structure of glass/ITO/NiO.sub.x/CH.sub.3NH.sub.3PbI.sub.3/PCBM/Al;

(27) FIG. 14B shows how the device performance using ZnO as electron transport layers depends on thickness based on a device structure of glass/ITO/NiO.sub.x (80 nm)/CH.sub.3NH.sub.3PbI.sub.3/ZnO/Al;

(28) FIG. 14C illustrates the champion device performance of metal oxide based perovskite solar cells;

(29) FIG. 14D shows the external quantum efficiency (EQE) of the metal oxide based perovskite solar cells;

(30) FIG. 15 shows J-V curve of the devices based on organic or inorganic charge transport layers;

(31) FIG. 16A shows the normalized power conversion efficiency of devices with inorganic transport layers (NiO.sub.x as hole transport layer, ZnO as electron transport layer) and organic transport layers (PEDOT:PSS as hole transport layer and PCBM as electron transport layer) as a function of storage time in a nitrogen box;

(32) FIG. 16B shows the normalized open circuit voltage of devices with inorganic transport layers (NiO.sub.x as hole transport layer, ZnO as electron transport layer) and organic transport layers (PEDOT:PSS as hole transport layer and PCBM as electron transport layer) as a function of storage time in a nitrogen box;

(33) FIG. 16C shows the normalized short circuit current of devices with inorganic transport layers (NiO.sub.x as hole transport layer, ZnO as electron transport layer) and organic transport layers (PEDOT:PSS as hole transport layer and PCBM as electron transport layer) as a function of storage time in a nitrogen box;

(34) FIG. 16D shows the normalized fill factor of devices with inorganic transport layers (NiO.sub.x as hole transport layer, ZnO as electron transport layer) and organic transport layers (PEDOT:PSS as hole transport layer and PCBM as electron transport layer) as a function of storage time in a nitrogen box;

(35) FIG. 17A shows a J-V curve of the devices using metal oxide charge transport layers stored in nitrogen box for 20 days;

(36) FIG. 17B shows a J-V curve of the devices using organic charge transport layers stored in nitrogen box for 20 days; and

(37) FIG. 18 shows a J-V curve of the devices under reverse (1.2 V.fwdarw.0 V) and forward (0V.fwdarw.1.2 V) scan measurement, wherein the voltage step is 0.02 V, and the delay time is 200 ms.

DETAILED DESCRIPTION

(38) Some embodiments of the current invention are discussed in detail below. In describing embodiments, specific terminology is employed for the sake of clarity. However, the invention is not intended to be limited to the specific terminology so selected. A person skilled in the relevant art will recognize that other equivalent components can be employed and other methods developed without departing from the broad concepts of the current invention. All references cited anywhere in this specification, including the Background and Detailed Description sections, are incorporated by reference as if each had been individually incorporated.

(39) The terms light, optics, etc. are not limited to only visible regions of the electromagnetic spectrum, but are intended to include nonvisible regions such as, but not limited to, infrared, far infrared, and ultraviolet, in addition to visible regions.

(40) The term transparent is intended to mean that a sufficient amount of light passes through to function for the intended purpose.

(41) The term about when referring to a measurable value such as an amount, a temporal duration, and the like, is meant to encompass variations of 20% or 10%, more preferably 5%, even more preferably 1%, and still more preferably 0.1% from the specified value, as such variations are appropriate to perform the disclosed methods.

(42) Traditionally, organic materials have been used as charge transport layers in efficient perovskite solar cells, thus leading to less stable devices. Recently, several groups have tried to replace these organic transport layers by inorganic materials, e.g. CuSCN,.sup.18, 19 CuI,.sup.20NiO.sub.x.sup.21-24 as hole transport layer, ZnO.sup.25 and TiO.sup.28-11 as n-type transport layer. It is known that the metal oxides demonstrate much higher stability than the organic materials. In addition, metal oxides can be easily obtained by solution processing from their corresponding precursors or nanoparticles at low temperature. Lead halide perovskite solar cells have attracted tremendous attention and have shown excellent photovoltaic performance. However, obtaining stable devices is still a great challenge due to the instability of both the perovskite material itself and/or the charge transport layers.

(43) Some embodiments of the current invention provide solution processed metal oxide materials as charge transport layers in perovskite solar cells, i.e. NiO.sub.x as the hole transport layer, ZnO as the electron transport layer, and a p-i-n inverted structure of glass/ITO/NiOx/perovskite/ZnO/Al. We demonstrated all metal oxide based perovskite solar cells with maximum power conversion efficiency of 16.1% and excellent stability, which shows the great promise of efficient and stable perovskite solar cells.

(44) An opto-electronic device according to some embodiments of the invention is shown in FIG. 1. The opto-electronic device 100 includes a first electrode 102, a first buffer layer 104 formed on the first electrode 102, and a perovskite semiconductor active layer 106 formed on the first buffer layer. The opto-electronic device 100 further includes a second buffer layer 108 formed on the perovskite semiconductor active layer 106, and a second electrode 110 formed on the second buffer layer 108. The first buffer layer 102, the second buffer layer 108, and the perovskite semiconductor active layer 106 each consists essentially of inorganic materials.

(45) According to some embodiments, the first and second electrodes each consists essentially of inorganic materials. The first and second buffer layers can each consist essentially of at least one metal oxide. According to some embodiments, one of the first and second buffer layers is a p-type buffer layer and the other one of the first and second buffer layers is an n-type buffer layer, wherein the p-type buffer layer comprises at least one of NiO.sub.x, MoO.sub.3, V.sub.2O.sub.5, and WO.sub.3, and wherein the n-type buffer layers comprises at least one of TiO.sub.2, ZnO, and Nb.sub.2O.sub.5. According to some embodiments, the first buffer layer can be NiO.sub.x and the second buffer layer can be ZnO.

(46) According to some embodiments, the first buffer layer can have a thickness between about 75 nm and about 85 nm, and the second buffer layer can have a thickness between about 65 nm and about 75 nm. According to some embodiments, the first buffer layer can include a plurality of nanoparticles, wherein each of the plurality of nanoparticles has a size between about 50 and about 100 nm. The second buffer layer can include a plurality of nanoparticles, wherein each of the plurality of nanoparticles has a size that is less than 10 nm. According to some embodiments, the first buffer layer has a work function between about 5.00 eV and 5.10 eV.

(47) According to some embodiments, the perovskite semiconductor active layer consists essentially of at least one perovskite that satisfies the formula ABX3, wherein A is at least one of CH3NH3 and NH2CHNH2, wherein B is at least one of PB and Sn, and wherein X is at least one of Cl, Br and I. According to some embodiments, the perovskite semiconductor active layer can have a thickness between about 300 nm and about 340 nm. According to some embodiments, the perovskite semiconductor active layer can have a thickness between about 315 nm and about 325 nm.

(48) According to some embodiments, the perovskite semiconductor active layer includes a plurality of crystals, and wherein each of the plurality of crystals has a size greater than 1 m. According to some embodiments, the plurality of crystals has a size that is greater than a thickness of the perovskite layer. At least one of the first and second electrodes can be a transparent electrode.

(49) According to some embodiments, the opto-electronic device is at least one of a photovoltaic cell, an optical detector, a radiation detector, a light emitting diode (LED), a laser and a memory device. The opto-electronic device can have a normalized power conversion efficiency that remains substantially constant over a period of 20 days.

(50) According to some embodiments, a method of producing an opto-electronic device includes providing a substrate, forming a first electrode on the substrate, forming a first buffer layer on the first electrode, and forming a perovskite semiconductor active layer on the first buffer layer. The method further includes forming a second buffer layer on the perovskite semiconductor active layer, and forming a second electrode on the second buffer layer. The first buffer layer, the second buffer layer and the perovskite semiconductor active layer each consists essentially of inorganic materials.

(51) According to some embodiments, the method further comprises treating the perovskite semiconductor active layer with at least one of an exposure to air for less than one hour, exposure to oxygen, exposure to ultraviolet light, moisture exposure and thermal annealing with a temperature less than 120 C. According to some embodiments, forming a perovskite semiconductor active layer on the first buffer layer comprises forming a layer of PbI.sub.2 on the first buffer layer, coating the PbI.sub.2 layer with a CH.sub.3NH.sub.3I solution, and annealing the CH.sub.3NH.sub.3I solution coated PbI.sub.2 layer to form the perovskite semiconductor active layer.

(52) Some embodiments of the current invention provide perovskite solar cells employing all solution processed metal oxide layers as the charge transport layers. Specifically, we utilize a p-type NiO.sub.x and n-type ZnO nanoparticles films as the hole and electron transport layers, respectively. We have successfully demonstrated perovskite solar cells based on all metal oxide charge transport layers with 16.1% efficiency and excellent stability.

(53) FIGS. 2A and 2C show the atomic force microscopy (AFM) images of about 80 nm of NiO.sub.x film and 70 nm of ZnO film on the ITO substrates, respectively. The NiO.sub.x film consisted of uniform nanoparticles, with particle size ranging from 50-100 nm, which can be compared with the corresponding films annealed at higher temperature such as 500 C..sup.24 The NiO.sub.x film is also dense and can act as a blocking layer. The high quality film obtained is due to the introduction of nickel nitride precursor, which is much different from previous reports utilizing nickel formate dehydrate.sup.21, 26 or nickel acetate tetrahydrate..sup.27 It has been established that the nitride salt precursor can grow high quality metal oxide films..sup.28 The AFM image of the ZnO film from spin-coated nanoparticles solution is shown in FIG. 2C. The ZnO nanoparticles film is very smooth and continuous, and the particles size is less than 10 nm. FIG. 3 is a TEM image showing that the synthesized ZnO nanoparticle size is around 5 nm. The average roughness of the ZnO film is less than 2 nm. This will guarantee good coverage of the ZnO on the perovskite film.

(54) Scanning kelvin probe microscopy (SKPM) was employed to examine electrical properties of the metal oxide films. The SKPM has been used to determine the surface potential difference in the thin film, which helps to reveal the electrical uniformity of the film..sup.29 The SKPM images of NiO.sub.x and ZnO films are shown in FIGS. 2B and 2D, and the average roughness of the surface potential for both of NiO.sub.x and ZnO are less than 5 mV, indicating that the films have excellent continuous electrical properties. The absorption of the NiO.sub.x and ZnO film are shown in FIG. 4. The NiO.sub.x film showed good transparency from 300-900 nm except for a little bit of absorption in the visible region. This might be from the defect-related absorption..sup.20-24 The ZnO film also showed good transparency. The transparency of the metal oxide, especially for the NiO.sub.x, will guarantee that most of the light shining from the substrate can be transmitted into the perovskite layer and be absorbed.

(55) X-ray spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) measurements were carried out to understand the chemical compositions and band structures of the solution-processed NiO.sub.x and ZnO films. FIG. 5a shows the XPS spectrum of the Ni 2p3/2 in NiO.sub.x, which can be separated into three peaks. This is consistent with previous reports by several groups where the NiO.sub.x is obtained via sol-gel processes or sputtering..sup.24, 27 The peak centered at a binding energy of 853.8 eV has been defined as Ni2+ in the standard NiO octahedral bonding configuration in cubic rocksalt NiO.sub.x..sup.27 The peak centered at 855.5 eV was ascribed to the Ni2+ vacancy-induced Ni3+ ion, and the broad peak centered at 861.1 eV was defined as a shakeup process..sup.27

(56) FIG. 5B shows the XPS spectrum of the O 1s in NiO.sub.x. The peak centered at 529.7 eV confirms the NiO octahedral bonding in NiO.sub.x..sup.27 The peak at 531.6 eV might be ascribed to nickel hydroxides, including defective nickel oxide with hydroxyl groups adsorbed on the surface..sup.27 Generally, the NiO.sub.x obtained by different processing usually shows the non-stoichiometric properties, strictly, it should be NiO.sub.x,.sup.24, 27 which is the reason for p-type conductivity of the NiO.sub.x thin film.

(57) The difference between the valence band and Fermi level of NiO.sub.x is about 0.2 eV based on the valence band spectra shown in FIG. 6A, confirming that the NiO.sub.x is a p-type metal oxide. The work function of NiO.sub.x is 5.05 eV determined by ultraviolet photoelectron spectroscopy (UPS) (FIG. 5C), which is higher than that of the conventionally used organic hole transport layer of PEDOT:PSS (4.9 eV, also in FIG. 5C). The work function of NiO.sub.x is closer to the perovskite than that of PEDOT:PSS, inferring that higher open circuit voltage could be obtained for the devices (the device data will be shown later).

(58) The Zn 2p3/2 and O 1s XPS spectra in ZnO are shown in FIGS. 5D and 5E, respectively. The Zn 2p3/2 peak is located at 1021.5 eV. The O 1s XPS spectrum in ZnO also exhibits asymmetric line shapes..sup.29 The peak with lower binding energy (530.1 eV) corresponds to O atoms in a ZnO crystal. The second peak, at 531.7 eV, is attributed to an oxygen-deficient component such as zinc hydroxide..sup.30 The maximum valence band spectrum (FIG. 6B) confirms that the ZnO is an n-type metal oxide. The work function of the ZnO is determined to be 4.2 eV by UPS shown in FIG. 5F. For comparison, the UPS of PCBM is also included in FIG. 5F. It can be found that ZnO and PCBM show similar work functions; both of them are close to the conduction band of perovskite of CH.sub.3NH.sub.3PbI.sub.3..sup.3

(59) To study the charge transport and collection efficiency between the perovskite materials with the corresponding metal oxide layers, the photoluminescence (PL) of the perovskite with the metal oxide layers has been studied. It was shown that the crystallinity of perovskite on different surfaces is totally different..sup.31 Thus, it is hard to define the PL quench effect between the perovskite with the bottom of buffer layer (NiO.sub.x). Here, we focus on the charge transport layer study between perovskite layers with the top layer (ZnO). The steady-state photoluminescence (PL) and time-resolved photoluminescence (TRPL) of the CH.sub.3NH.sub.3PbI.sub.3 with and without the ZnO capping layer are shown in FIG. 7. The PL intensity has been quenched a lot, and as a result, the life time of perovskite has been decreased from 145 ns to 21 ns, indicating that the ZnO was also a good charge transport layer for the perovskite (FIG. 8). The PL quench for perovskite/PCBM has also been studied. It seems that the PCBM is a better quencher for the CH.sub.3NH.sub.3PbI.sub.3 from the steady and time-resolve photoluminescence..sup.32, 33 However, the devices shown below indicate that the ZnO can still give similar or even better device performance than that of the PCBM buffer layer. It was shown that the NiO.sub.x is an efficient hole transport layer in organic solar cells.sup.26, 27 and also in perovskite solar cells,.sup.20-24, 34 and we believe that the NiO.sub.x films achieved in this study can act as an effective p-type charge transport layer. The device structure of a metal oxide based perovskite solar cell according to some embodiments is shown in FIG. 9, i.e. the glass/ITO/NiO.sub.x/perovskite/ZnO/Al, where the NiO.sub.x and ZnO were used as the hole and electron transport layers, respectively. The band alignment of the devices are shown in FIG. 10 based on the work function of the NiO.sub.x and ZnO determined by the UPS shown in FIG. 5C, and also the band structure of perovskite (CH.sub.3NH.sub.3PbI.sub.3)..sup.3 Based on this device structure, when the device is exposed to light, an electron-hole will be generated in the perovskite layer, move to the interface between the perovskite and metal oxides, and then be collected by the electrodes.

(60) The X-ray diffraction pattern of perovskite formed on a NiO.sub.x substrate is shown in FIG. 11. All of the diffraction peaks come from CH.sub.3NH.sub.3PbI.sub.3 except for the small peak at 12.6, indicating that the layer on NiO.sub.x surface is mainly CH.sub.3NH.sub.3PbI.sub.3, with a small amount of residual PbI.sub.2. The scanning electron microscopy image of the perovskite film growth on NiO.sub.x is shown in FIG. 12A. The film showed the large crystal size above 1 m, and is almost free of pinholes. For comparison, a perovskite film grown on PEDOT:PSS is shown in FIG. 13. The crystal size is about 300-500 nm, indicating the NiO.sub.x surface can give better crystallinity of perovskite film. This improvement of crystallinity of perovskite on an NiO.sub.x surface could be due to induced crystal growth by the crystallized substrate or rougher surface. The top image of perovskite film capped by ZnO is shown in FIG. 12B. The perovskite film has been fully covered by the ZnO nanoparticles. As a result, the subsequently deposited electrode such as Al or Ag will not react with the perovskite, and will also keep the interface stabile. The cross section of the device (not including the top electrode) is shown in FIG. 12C. The sandwich structure of NiO.sub.x/perovskite/ZnO can be easily distinguished, and the thicknesses of the layers are 80 nm, 320 nm and 70 nm, respectively. It can also be found that the NiO.sub.x layer is dense, and the formation of the perovskite layer is almost free of pin-holes and is smooth. It must be noted that the crystal size (>1 m) of perovskite is larger than the thickness (300 nm), indicating that the charge could transport and collect in one crystal, which is helpful for achieving high device performance..sup.35

(61) The device performance using NiO.sub.x and ZnO as charge transport layers was systematically studied. The dependence of device performance on NiO.sub.x thickness was investigated based on the device structure of glass/ITO/NiO.sub.x/CH.sub.3NH.sub.3PbI.sub.3/PCBM/Al. The results are shown in FIG. 14A and also summarized in Table I. It was found that the device can show optimized performance when the thickness of the NiO.sub.x is sufficient. When the NiO.sub.x layer is thin, it cannot fully cover the ITO surface, resulting in the perovskite layer being in direct contact with the ITO substrate, thus leading to leakage or bad band alignment. On the other hand, the series resistance will be increased if the NiO.sub.x layer is too thick. It was found that the optimal thickness of NiO.sub.x layer for the devices is around 80 nm because the NiO.sub.x layer will guarantee the coverage and also the electronic charge transport.

(62) TABLE-US-00001 TABLE I Voc Jsc FF PCE t.sub.NiOx (nm) (V) (mA/cm.sup.2) (%) (%) 20 0.94 19.4 59.0 10.8 40 1.00 19.0 67.8 12.9 80 0.99 19.5 76.6 14.8 120 1.01 19.3 67.0 13.0

(63) The optimized condition of the ZnO on the perovskite layer were also investigated based on the device configuration of glass/ITO/NiO.sub.x (80 nm)/CH.sub.3NH.sub.3PbI.sub.3/ZnO (x nm)/Al. The device performance with different thicknesses of ZnO electron transport layers is shown in FIG. 14B and summarized in Table II. It was found that the optimized thickness of ZnO is around 70 nm. The best device performance based on the optimized conditions are shown in FIG. 14C. The VOC is 1.01 V, JSC is 21.0 mA/cm.sup.2, and FF is 0.76, leading to an efficiency as high as 16.1%.

(64) TABLE-US-00002 TABLE II Voc Jsc FF PCE t.sub.ZnO (nm) (V) (mA/cm.sup.2) (%) (%) 30 1.01 21.7 64.8 14.2 70 0.98 21.3 71.8 15.0 70 1.01 21.0 76.0 16.1* 120 0.98 20.1 46.6 9.2

(65) The all metal oxide charge transport layers based perovskite devices showed promising performance, even better than that of the devices using all organic transport layers (FIG. 15). The NiO.sub.x layer can give higher VOC while the ZnO layer can give higher J.sub.SC. The higher V.sub.OC from the NiO.sub.x based device could be from two factors: band alignment between the hole transport layer and the perovskite layer, and better crystallinity of the perovskite layer on the NiO.sub.x substrate (FIGS. 12A-12C and FIG. 13). This will decrease the recombination and increase V.sub.OC. The higher J.sub.SC from the ZnO based device could be attributed to the strong hole blocking effect due to the deep valence band of ZnO..sup.36 The external quantum efficiency of the all metal oxide based devices is shown in FIG. 14D. The integrated short circuit current is consistent with the value measured from the solar simulator.

(66) Lastly, the stability of the devices using inorganic or organic charge transport layers were monitored, and the results are shown in FIG. 16A-16D. The metal oxide based devices (labelled as inorganic charge transport layers) retain their original power conversion efficiency after 20 days stored in the nitrogen glove box (FIG. 16A, J-V curve of the devices are shown in FIGS. 17A and 17B). As shown in FIG. 16A, the normalized power conversion efficiency remains substantially constant over a period of 20 days, decreasing by less than 10% of the initial value. However, for the devices based on organic charge transport layers, such as PEDOT:PSS (HTL) and PCBM (ETL), the power conversion efficiency dropped significantly. The efficiency was degraded about 40% after 20 days in storage. The degradation of the detailed parameters such as J.sub.SC, V.sub.OC and FF vs. time for the devices using two series of charge transport layers (inorganic or organic) are shown in FIGS. 16B-16D. It can be found that the degradation of organic buffer layer based devices mainly from the FF, which is highly possible from the interface deterioration. While introducing the metal oxide charge transport layers, the device showed significant improvement on stability, the FF remained nearly the same after 20 days storage, indicating the improvement mainly from oxide layer has been greatly improved.

(67) Materials and Methods According to Some Embodiments

(68) According to some embodiments of the invention, Nickel (II) nitrate hexahydrate (Ni(NO.sub.3).sub.2.6H.sub.2O) (sigma aldrich) was dissolved in ethylene glycol solution containing 1 M Nickel(II) nitrate hexahydrate with ethylenediamine (Aldrich). The solution was spun-cast on glass/ITO substrate at a spin-speed ranging from 1000-4000 rpm for 90 seconds for controlling the NiO.sub.x thickness. Then, the substrate was post-annealed at 300 C. in ambient air for 60 minutes.

(69) According to some embodiments of the invention, the chemical synthesis of ZnO nanoparticles was performed using the method outlined by our previous results.sup.37. The difference is that methanol has been totally removed to prevent the residual methanol solvent from decomposing the perovskite layer. It has been confirmed that some solvents such as ethanol and methanol can degrade the perovskite layer. After drying the ZnO nanoparticles, the powder is then re-dispersed in chlorobenzene with a concentration of 2%. The thicknesses of the ZnO layer was controlled by the spin-speed.

(70) Device fabrication and measurement according to some embodiments of the invention is now described. PEDOT:PSS was spin-cast on an ITO surface at 4000 rpm, and then annealed at 120 C. for 15 min in ambient air. The NiO.sub.x precursor was spin-coated on ITO and annealed at 300 C. for 60 min in ambient air. Then the substrates were transferred into a nitrogen glove box for coating of the perovskite layer. It was found that a one-step solution process may not form a sufficiently high quality perovskite film on NiO.sub.x,.sup.21, 24, 34 which could be due to the surface roughness of NiO.sub.x induced over the crystallinity of the perovskite. According to some embodiments, a two-step process can be adopted to obtain a high quality perovskite layer..sup.14 The PbI.sub.2 layer was spin-coated firstly using the 460 mg/ml solution, and then dried at 70 C. for 10 min. Then, 50 mg/ml of CH.sub.3NH.sub.3I solution was coated on the PbI.sub.2 layer. After that, the film was taken out for annealing in ambient air (it was confirmed that a certain level of moisture can induce higher perovskite crystallinity.sup.13) at 100 C. for 2 hours..sup.14 For PCBM coating, a 2% PCBM in chlorobenzene solution was coated onto the perovskite layer at 1000 rpm. For ZnO, different thicknesses of ZnO were coated on the perovskite surface by controlling the spin-coating speed. Finally, the device was transferred to a vacuum chamber for Al electrode evaporation. The device's area is 0.1 cm.sup.2, though the embodiments of the invention are not limited to this size, and can be larger or smaller. J-V characteristics of photovoltaic cells were taken using a Keithley 2400 source measure unit under a simulated AM1.5G spectrum. With an Oriel 9600 solar simulator, the light intensity was calibrated by a KG-5 Si diode. The J-V measurements were carried out in a nitrogen glove box. The measurement details can be found in FIG. 18, and a summary of the device performance is given in Table III. The devices were taken out for external quantum efficiency (EQE) measurement, and were measured by an Enli tech (Taiwan) EQE measurement system.

(71) TABLE-US-00003 TABLE III Voc Jsc FF PCE (V) (mA/cm.sup.2) (%) (%) Reverse Scan 0.99 20.7 75.6 15.5 Forward Scan 0.97 20.2 73.4 14.4

(72) The embodiments of the invention are not limited to the materials described above. For example, the p-type buffer layer can include a metal oxide, for example, one or more of NiO.sub.x, MoO.sub.3, V.sub.2O.sub.5, and WO.sub.3. The n-type buffer layer can also include a metal oxide, for example, one or more of TiO.sub.2, ZnO, and Nb.sub.2O.sub.5. The perovskite semiconductor active layer can consist essentially of at least one perovskite that satisfies the formula ABX.sub.3, wherein A is at least one of CH.sub.3NH.sub.3 and NH.sub.2CHNH.sub.2, wherein B is at least one of PB and Sn, and wherein X is at least one of Cl, Br and I.

(73) In conclusion, we have successfully demonstrated perovskite solar cells employing all solution processed metal oxide layers as the charge transport layers. The device has greater than 16% power conversion efficiency. The devices based on the metal oxide charge transport layers also showed significant improvement in stability.

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(75) The embodiments illustrated and discussed in this specification are intended only to teach those skilled in the art how to make and use the invention. In describing embodiments of the invention, specific terminology is employed for the sake of clarity. However, the invention is not intended to be limited to the specific terminology so selected. The above-described embodiments of the invention may be modified or varied, without departing from the invention, as appreciated by those skilled in the art in light of the above teachings. It is therefore to be understood that, within the scope of the claims and their equivalents, the invention may be practiced otherwise than as specifically described.