Solar Cell

Abstract

The present invention is related to a solar cell comprising a first electrode; a second electrode; and a stack of layers provided between the first electrode and the second electrode; wherein the stack of layers comprises one light absorbing layer provided with a perovskite crystal structure; and at least one dopant layer, wherein the dopant layer consists of one or more n-dopant material(s); or one or more p-dopant material(s).

Claims

1. Solar cell comprising: a) a first electrode; b) a second electrode; and c) a stack of layers provided between the first electrode and the second electrode; wherein the stack of layers comprises (c1) one light absorbing layer provided with a perovskite crystal structure; and (c2) at least one dopant layer, wherein the at least one dopant layer consists of (i) one or more n-type dopant material(s); or (ii) one or more p-type dopant material(s).

2. Solar cell according to claim 1, wherein the at least one dopant layer is arranged between the first electrode and the light absorbing layer.

3. Solar cell according to claim 1, wherein the at least one dopant layer is arranged between the second electrode and the light absorbing layer.

4. Solar cell according to claim 1, wherein the solar cell comprises two or more layer stacks and optionally at least one interconnecting layer, wherein the interconnecting layer is arranged between two of the different stacks of layers.

5. Solar cell according to claim 1, wherein the p-type dopant material is an organic compound, a metal-organic compound or an organo-metallic compound, wherein the total amount of electron withdrawing groups in the organic compound is from 17 atomic percent to 90 atomic percent.

6. Solar cell according to claim 1, wherein the n-type dopant material is selected from the group consisting of metals, metal salts, metal complexes and mixtures thereof.

7. Solar cell according to claim 6, wherein the metal is selected from the group consisting of alkali metals, alkaline earth metals, transition metals, rare earth metals and mixtures thereof.

8. Solar cell according to claim 7, wherein the metal is selected from Li, Na, K, Rb, Cs, Mg, Ca, Sr, Ba, Sm, Eu, or Yb.

9. Solar cell according to claim 7, wherein the metal is selected from Li, Na, Cs, Mg, Sr, Yb, Eu, or Sm.

10. Solar cell according to claim 6, wherein the metal salt is selected from the group consisting of alkali metal salts, alkaline earth metal salts, rare earth metal salts and mixtures thereof.

11. Solar cell according to claim 10, wherein the alkali metal salt is selected from the group consisting of LiF, LiCl, LiBr, LiI and mixtures thereof.

12. Solar cell according to claim 10, wherein the alkali metal salt is LiF.

13. Solar cell according to claim 6, wherein the metal complex is an organic alkali metal complex.

14. Solar cell according to claim 1, wherein the thickness of the dopant layer is from 0.1 to 25 nm.

15. Solar cell according to claim 1, wherein the dopant layer is a self-assembled monolayer.

16. Solar panel comprising the solar cell according to claim 1.

Description

DESCRIPTION OF EMBODIMENTS

[0104] Following, further aspects are disclosed by referring to Figures. In the figures show:

[0105] FIG. 1A is a schematic representation of a solar cell according to the prior art with p-i-n sequence;

[0106] FIG. 1B is a schematic representation of a solar cell according to the prior art with n-i-p layer sequence;

[0107] FIG. 2 a schematic representation of a solar cell in p-i-i layer sequence;

[0108] FIG. 3 a schematic representation of a solar cell in i-i-n layer sequence;

[0109] FIG. 4 a schematic representation of a solar cell in n-i-i layer sequence;

[0110] FIG. 5 a schematic representation of a solar cell in i-i-p layer sequence;

[0111] FIG. 6 a schematic representation of a solar cell provided with a first stack of layers and a second stack of layers;

[0112] FIG. 7 a schematic representation of a solar cell provided with more than two different stacks of layers; and

[0113] FIG. 8 a schematic representation of a solar cell provided with more than two different stacks of layers which are interconnected by interconnecting layers.

[0114] FIG. 1A and FIG. 1B showa solar cell in p-i-n and n-i-p layer sequence, respectively, in accordance with the prior art. Referring to FIG. 1A the solar cell is provided with a first and a second electrode 120, 140. Between the first and the second electrode 120, 140 a light absorbing layer 100 is provided. The light absorbing layer comprises an absorber compound provided with a perovskite crystal structure. The absorber compound may have a stoichiometry of AMX.sub.3, where A and M are cations and X is an anion. By the first and second electrode 120, 140 an anode and a cathode are implemented, thereby, a n-i-p device and a p-i-n device may be provided.

[0115] FIG. 2 shows a schematic representation of a solar cell in a p-i-i layer sequence in accordance with the present invention. The solar cell contains between the first and the second electrodes 120, 140 a stack of layers containing a first hole transport layer 150, a p-type dopant layer 180, a second hole transport layer 160, a light absorbing layer 100 and an electron transport layer 170 (forming together a stack of layers). The p-type dopant layer 180 is provided between the first electrode 120 and the light absorbing layer 110. No n-type dopant layer is provided between the light absorbing layer 100 and the second electrode 140.

[0116] FIG. 3 shows another solar cell in a i-i-n layer sequence. In this embodiment, a stack of layers which is formed by a first hole transport layer 150, a light absorbing layer 100, a first electron transport layer 170, a n-type dopant layer 190 and a second electron transport layer 200 is arranged between a first electrode 120 and a second electrode 140. In this embodiment, a n-type dopant layer (consisting of one or more n-type dopant material(s)) is arranged between the light absorbing layer 100 and the second electrode 140. In this embodiment, no p-type dopant layer is formed between the light absorbing layer 100 and the first electrode 120.

[0117] FIG. 4 shows a schematic representation of a solar cell in a n-i-i layer sequence. The solar cell contains a stack of layers comprising a first electron transport layer 170, a n-type dopant layer 190, a second electron transport layer 200, a light absorbing layer 100 and a first hole transport layer 150 which are arranged between a first electrode 120 and a second electrode 140. In this embodiment, the n-type dopant layer 190 is formed between the light absorbing layer 100 and the first electrode 120. No p-type dopant layer is formed in this embodiment between the light absorbing layer 100 and the second electrode 140.

[0118] FIG. 5 shows a further schematic representation of an embodiment of a solar cell in the i-i-p layer sequence. Again, a stack of layers is arranged between a first electrode 120 and a second electrode 140. The stack of layers in this embodiment is formed by a first electron transport layer 170, a light absorbing layer 100, a first hole transport layer 150, a p-type dopant layer 180 and a second hole transport layer 160. The p-type dopant layer 180 is arranged between the light absorbing layer 100 and the second electrode 140. No n-type dopant layer is formed in this embodiment between the light absorbing layer 100 and the first electrode 120.

[0119] FIG. 6 shows a further embodiment of the present invention wherein the solar cell comprises two different stacks of layers 210 and 220. The first stack of layer 210 is provided between and in contact with the first electrode 120 and the second stack of layer 220. The second stack of layers 220 is provided between and in contact with the first stack of layers 210 and the second electrode 140.

[0120] FIG. 7 shows a schematic representation of a solar cell comprising more than two stack of layers. In this embodiment, a variety of different stacks of layers 210, 220, 230 is arranged between the first electrode 120 and the second electrode 140 wherein the stacks of layers 210, 220, 230 are in contact with each other and with the electrodes. Each stack of layers 210, 220, 230 shown in FIG. 6 and in FIG. 7 may independently from each other be one stack of layers as shown in any of the FIGS. 2 to 5.

[0121] FIG. 8 shows a special embodiment of the solar cell shown in FIG. 7. In addition to the elements shown in FIG. 7, the solar cell according to FIG. 8 contains a first interconnecting layer 240 and a second interconnecting layer 250 connecting the first stack of layers 210 and the second stack of layers 220, respectively the second stack of layers 220 and the third stack of layers 230.

EXAMPLES

[0122] Following, experimental results with regard to the different embodiments of the solar cell shown in the Figures are described.

General Procedure for Fabrication of Vacuum-Processed Perovskite Solar Cells

[0123] Solar Cell 1 (p-i-i type) and Solar Cell 2 (n-i-i type) are prepared as follows:

[0124] ITO coated glass substrates are patterned by photolithography to limit the active area of the solar cell and allow for easy contacting of the top electrode. Materials used are: p-type dopant 2,2-(Perfluoronaphthalene-2,6-diylidene) dimalononitrile (F6-TCNNQ), the hole transport material N4,N4,N4,N4-tetra([1,1-biphenyl]-4-yl)-[1,1:4,1-terphenyl]-4,4-diamine (TaTm) and n-type dopant N1,N4-bis(tri-p-tolylphosphoranylidene)benzene-1,4-diamine (PhIm). The electron transport material is fullerene (C60). The precursor materials for the perovskite light absorbing layer are PbI2 and CH3NH3I (MAI).

[0125] With regard to characterization of the embodiments prepared, grazing incident X-ray diffraction (GIXRD) pattern are collected at room temperature on an Empyrean PANanalytical powder diffractometer using the Cu K1 radiation. Typically, three consecutive measurements are collected and averaged into single spectra. The surface morphology of the thin films is analyzed using atomic force microscopy (AFM, Multimode SPM, Veeco, USA). Scanning Electron Microscopy (SEM) images is performed on a Hitachi S-4800 microscope operating at an accelerating voltage of 2 kV over Platinummetallized samples. Absorption spectra are collected using a fiber optics based Avantes Avaspec2048 Spectrometer.

[0126] Characterization of the solar cells is performed as follows: The external quantum efficiency (EQE) is estimated using the cell response at different wavelength (measured with a white light halogen lamp in combination with band-pass filters), where the solar spectrum mismatch is corrected using a calibrated Silicon reference cell (MiniSun simulator by ECN, the Netherlands).

[0127] The current density-voltage (J-V) characteristics are obtained using a Keithley 2400 source measure unit and under white light illumination, and the short circuit current density is corrected taking into account the device EQE. The electrical characterization was validated using a solar simulator by Abet Technologies (Model 10500 with an AM1.5G xenon lamp as the light source). Before each measurement, the exact light intensity is determined using a calibrated Si reference diode equipped with an infrared cut-off filter (KG-3, Schott). The J-V curves are recorded between 0.2 and 1.2 V with 0.01V steps, integrating the signal for 20 ms after a 10 ms delay. This corresponds to a speed of about 0.3 V s-1.

[0128] The device layout used for the solar cells configurations consists in four equal pixels (area of 0.06 cm2, defined as the overlap between the patterned ITO and the top metal contact) measured through a shadow masks with 0.01 cm2 aperture. For hysteresis study, different scan rates (0.1, 0.5 and 1 Vs-1) are used, biasing the device from 0.2 to 1.2 V with 0.01 V steps and vice versa. Light intensity dependence measurements are done by placing 0.1, 1, 10, 20, 50% neutral density filters (LOT-QuantumDesign GmbH) between the light source and the device.

[0129] Further, with regard to device preparation, ITO-coated glass substrates are subsequently cleaned with soap, water and isopropanol in an ultrasonic bath, followed by UV-ozone treatment. They are transferred to a vacuum chamber integrated into a nitrogen-filled glovebox (MBraun, H2O and O2<0.1 ppm) and evacuated to a pressure of 1.Math.10-6 mbar. The vacuum chamber is equipped with six temperature controlled evaporation sources (Creaphys) fitted with ceramic crucibles. The sources are directed upwards with an angle of approximately 90 with respect to the bottom of the evaporator. The substrate holder to evaporation sources distance is approximately 20 cm. Three quartz crystal microbalance (QCM) sensors are used, two monitoring the deposition rate of each evaporation source and a third one close to the substrate holder monitoring the total deposition rate.

[0130] For thickness calibration, firstly the materials TaTm and F6-TCNNQ, C60 and PhIm are individually sublimed. A calibration factor is obtained by comparing the thickness inferred from the QCM sensors with that measured with a mechanical profilometer (Ambios XP1). Then these materials are co-sublimed for Comparative Examples 1 and 2 at temperatures ranging from 135-160 C. for the dopants to 250 C. for the TaTm and C60, and the evaporation rate is controlled by separate QCM sensors and adjusted to obtain the desired doping concentration. In general, the deposition rate for TaTm and C60 is kept constant at 0.8 s-1, while varying the deposition rate of the dopants during co-deposition. Pure dopant (F6-TCNNQ and PhIm) layers (for Solar Cell 1 and 2) and undoped TaTm and C60 layers (for all examples in Table 2) are deposited at a rate of 0.5 s-1.

[0131] Once the deposition on the ITO is completed, the chamber is vented with dry N2 to replace the crucibles with those containing the precursor materials for the perovskite light absorbing layer deposition, PbI2 and CH3NH3I. The vacuum chamber is evacuated again to a pressure of 10-6 mbar, and the perovskite films (light absorbing layer) are then obtained by co-deposition of the two precursors.

[0132] The calibration of the deposition rate for the CH3NH3I is difficult due to non-uniform layers and the soft nature of the material which impedes accurate thickness measurements. Hence, the source temperature of the CH3NH3I is kept constant at 70 C. and the CH3NH3I:PbI2 ratio is controlled off line using grazing incident x-ray diffraction by adjusting the PbI2 deposition temperature. The optimum deposition temperatures are 250 C. for the PbI2 and 70 C. for the CH3NH3I. After deposition of a 500 nm thick perovskite film, the chamber is vented and the crucibles replaced with those containing C60 and PhIm, and evacuated again to a pressure of 10-6 mbar. This process of exchanging crucibles is done to minimize possible cross-contamination between the organic materials and the perovskite precursors.

[0133] The solar cell devices for Comparative Example 1 are further processed by depositing a film of pure C60 and one of the n-type doped C60 layer (C60:PhIm), with thicknesses of 10 and 40 nm, respectively. Processing of Solar Cell 1 involves the sequential deposition of C60 and BCP, with thickness of 25 and 8 nm, respectively. For Solar Cell 2, the devices are processed by depositing a film of pure TaTm and one of MoO3, with thicknesses of 15 and 10 nm, respectively. Deposition of the MoO3 layout is performed in a separate vacuum chamber from an alumina-coated aluminum crucible at a deposition rate of 0.5 s-1.

[0134] In a single evaporation run five substrates (3 by 3 cm) are prepared, each substrate containing four cells. Generally, one substrate is reserved for a reference configuration. Finally the substrates are transferred to a second vacuum chamber where the metal electrode (100 nm thick) is deposited. For n-i-p and n-i-i devices, the same procedure as described before is used in the inverted order.

[0135] Layer stack details are given in Table 4.

[0136] The details of the layer stack in the solar cell devices are given as follows: A slash / separates individual layers. Layer thicknesses are given in squared brackets [ . . . ].

Technical Effect of the Invention

[0137] The solar cell devices according to the invention show improved efficiency and lifetime when compared to the solar cells of the prior art.

TABLE-US-00003 TABLE 3 List of compounds used Compound IUPAC Name name Reference TaTm N4,N4,N4,N4-tetra([1,1- WO2011134458A1 biphenyl]-4-yl)-[1,1:4,1- terphenyl]-4,4-diamine (TaTm) CAS 952431-34-4 F6- 2,2-(Perfluoronaphthalene-2,6- US2005121667A1 TCNNQ diylidene) dimalononitrile (F6- US2005139810A1 TCNNQ) CAS 912482-15-6 MAPI MAPbI3 prepared in-situ from precursor materials PbI.sub.2 and CH.sub.3NH.sub.3I (MAI) PhIm N1,N4-bis(tri-p-tolylphosphoranyl- WO2012175219A1 idene)benzene-1,4-diamine (PhIm) CAS 51870-56-5 C60 Fullerene-C60 Reed, Bolskar, CAS 99685-96-8 Chem. Rev. 100, 1075 (2000) BCP 2,9-dimethyl-4,7-diphenyl- 1,10-phenanthroline (BCP) CAS 4733-39-5

TABLE-US-00004 TABLE 4 Overview of layer stack of inventive Solar Cell 1, Solar Cell 2, Comparative Example 1 Cell name Cell Type Layer Stack Solar p-i-i ITO/F6- Cell 1 TCNNQ[2.5 nm]/TaTm[10 nm]/ MAPI[500 nm]/C60[25 nm]/BCP [8 nm]/Ag Solar n-i-i ITO/PhIm [2 nm]/C60 [10 nm]/ Cell 2 MAPI[500 nm]/TaTm [15 nm]/MoO3 [10 nm]/Au Comparative p-i-n ITO/TaTm:F6-TCNNQ[40 nm]/ Example 1 TaTm[10 nm]/MAPI [500 nm]/C60[10 nm]/ C60:PhIm [40 nm]/Ag

TABLE-US-00005 TABLE 5 Performance of inventive Solar Cell 1 and Solar Cell 2 in comparison to prior art solar cell (Comparative Example 1) LT PCE Voc J.sub.SC FF PCE after 250 h (mV) (mA cm.sup.2) (%) (%) (%) Solar 1059 21.3 80.4 18.2 94 Cell 1 Solar 1128 21.2 74.8 17.9 Cell 2 Comparative 1076 20.8 72.5 16.2 62 Example 1

[0138] For comparison of the performance of different solar cells four parameters are selected, which are defined as follows (source: www.pveducation.org):

1) Open circuit voltage (Voc) in mVmaximum voltage available from a solar cell, and this occurs at zero current
2) Short circuit current (Jsc) in mA cm.sup.2current through the solar cell when the voltage across the solar cell is zero (i.e., when the solar cell is short circuited). This is the largest current which may be drawn from the solar cell.
3) At both of these operating points, Voc and Jsc, the power from the solar cell is zero. The fill factor (FF) in %, is a parameter which, in conjunction with Voc and Isc, determines the maximum power from a solar cell. The FF is defined as the ratio of the maximum power from the solar cell to the product of Voc and Isc. Graphically, the FF is a measure of the squareness of the solar cell and is also the area of the largest rectangle which will fit in the IV curve.
4) Power conversion efficiency (PCE) in %ratio of energy output from the solar cell to input energy from the sun. PCE=Voc*Jsc*FF

[0139] The features disclosed in this specification, the figures and/or the claims may be material for the realization of various embodiments, taken in isolation or in various combinations thereof.