FIELD OF THE INVENTION
[001] The present invention relates to the fabrication of organic solar cells using novel transport material. More specifically, the process involves use of cost effective, robust and solution processable hole transport layer material which can be used for efficient and low cost large area organic solar cell fabrication with 5 simplest device architect under ambient conditions. The aim of the invention is to fabricate cost effective and efficient large area organic solar cells by using hole transport layer which is economic and gives better efficiency as compared to conventional hole transport layer.
KEY WORDS AND ABBREIATIONS USED 10
[002] It may be noted that the abbreviations are not necessarily used commonly. For the facilitation of drafting, several abbreviations may be formulated strictly for this specification describing the present invention.
OSC(s): Organic Solar Cell(s)
HTL(s): Hole Transport Layer(s) 15
ETL(s): Electron Transport Layer(s)
ITO: Indium Tin Oxide
PV: Photo Voltaic
OPV: Organic Photo Voltaic
OPVD(s): Organic Photo Voltaic Device(s) 20
OLED: Organic Light Emitting Device(s)
BHJ: Bulk Hetero Junction
IFL(s): Interface Layer(s)
PEDOT: Poly (3,4-Ethylenedioxythiophene)
PSS: Poly (Styrene Sulfonate) 25
RT: Room Temperature (25-35ºC)
CuS: Copper Sulfide
3
BACKGROUND AND PRIOR ARTS OF THE INVENTION
[003] Organic photovoltaic (OPV) cells have drawn great scientific attention over the last few years, due to its potential to produce flexible, light weight, low cost solar cells using organic materials. However, the power conversion efficiency achieved for these systems is low for extensive implementation of the technology. 5 Among the various photovoltaic technologies, organic/ polymer photovoltaics based on solution processed bulk-heterojunction (BHJ) concept gained significant attention due to the use of inexpensive light-weight materials, exhibiting high mechanical flexibility and compatibility with low temperature roll-to-roll manufacturing techniques. In OPVs, especially in bulk heterojunction organic 10 solar cells consist of many components such as electrodes (anode/ cathode), interface layers (IFLs) and active materials (donor/ acceptor). Each and every component has its own importance and functionality.
[004] Interface layers play a very important role in collection and extraction of the charge carriers, these layers are inserted between electrodes (anode/cathode) 15 and active layer interface. To moderate the charge carrier recombination at the electrodes, various interface layer (IFL) materials have been developed to selectively allow desired charge carriers to pass through and block undesired carriers. Therefore, charge carrier recombination at electrodes can be substantially suppressed and PCEs can be significantly improved for the cells with engineered 20 IFLs. Hole transport layer (HTLs) and electron transport layer (ETLs) are part of Interface layers.
[005] In the field of organic solar cells, conventional as well as inverted structures, Poly (3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS), is a most successfully used solution processed HTL but due to its 25 hygroscopic, acidic and protonation nature of PSS influences the device stability and degradation due to these limitations hole transport layer PEDOT:PSS replaced by the several inorganic materials and organic materials. In inorganic materials, Transition metal oxides were also used, these materials have air stability and high
4
optical transparency but due to insolubility in most of the common solvents, these materials are usually deposited by vacuum deposition technique, which is incompatible with the concept of low-cost OSCs fabrication. To overcome the problem of vacuum deposition of inorganic materials the preference comes to solution processable approach. Several solution processable methods were 5 reported by using different inorganic precursors, nanoparticles, colloidal particles etc. In recent years, more and more solution processable metal oxides, such as CuO
x, MoO3, ReOx, VOx, NiOx, SnOx, WO3 and RuOx etc have been used for stable OSCs. Out of these copper based materials like copper iodide (CuI) and copper thiocyanate (CuSCN) have recently emerged as other effective and robust 10 inorganic hole transport materials for OSCs. CuI and CuSCN are highly transparent and efficient HTL for organic solar cells but these materials required selective solvents to dissolve which is very expensive and bad smelly because of these reasons we required inexpensive, easily soluble in common solvents and stable hole transport material for low cost and efficient OSCs fabrication. 15
[006] Although significant efforts have been devoted for solution processable HTL for fabrication of large area, stable and cost effective solar cells, it is surprising that very scantly of materials have been reported.
[007] Following are the works done so far in the field of solution processable hole transport layer materials for organic solar cells. 20
[008] US 9543537 relates to the organic solar cells and similar optoelectronic devices. Solar energy is most important renewable energy sources to develop cost effective and efficient photovoltaic devices. Organic photovoltaics (OPVs) devices are an important and attractive technology to solve energy problem. In OPVs, Bulk heterojunction (BHJ) solar cells have several advantages such as low 25 cost, light weight, flexible and high throughput manufacturing like as roll-to-roll and other similar techniques. The BHJ solar cells have shown power conversion efficiencies (PCEs) around 4 to 7%.The PCEs of organic solar cells increases around 2.5% to 7.7% in between the years of 2001 to 2010. A benchmark for
5
OPV researchers is to achieve PCE around 10%, which would help to make OPV competitive with other photovoltaic technologies. To achieve the high efficiency and performance of the devices interfaces play a very important role. A method of solution processable metal oxide hole transport layers used in organic photovoltaic devices described. The metal oxide may be resulting from a metal-5 organic precursor enabling solution processing from p-type metal oxide. A solution processable metal oxide thin film as a hole transport layer used in organic photovoltaic devices.
[009] WO 2013123605 relates to the field of organic electronic devices like organic photovoltaic devices (OPVs) and organic light emitting devices (OLEDs). 10 It provides intermediates and materials suitable for organic electronics manufacturing to specific manufacturing method as well as uses. In the field of organic electronics such as OLED s and OPVs, buffer layers are used to improve the device efficiencies. The typically thickness of these layers less than 100nm to retain optical transparency and low series resistance. These buffer layers may 15 consist of WO3 or MoO3, which have deep lying electronic states and are strongly n-doped by oxygen vacancies. Meyer et al. (Adv. Mater . 2008 , 20, 3839—3843 ) reported the efficient hole-injection into organic materials with deep-lying HOMO levels from an ITO electrode covered with a MoO3 or WO3 hole transport layer (HTL) or hole injection layer (HIL). The simplest device structures consist of one 20 or two organic layers. The hole injection layers MoO3 and WO3 are typically deposited by thermal evaporation method under high vacuum, which is major drawback for cost effective and large area fabrication.
[0010] US 8963132 relates to the organic light emitting devices (OLEDs). More especially on device fabrication methods, device containing an organic layer 25 which is a combination of an organic electron donor material and an organic electron acceptor material that forms a layer insoluble in a non-polar solvent, and devices containing the organic layer. In Opto-electronic devices there are several reasons using organic materials such as these materials are relatively inexpensive so these devices are cost effective than inorganic devices. In addition, the 30
6
important property of organic materials such as deposited on flexible substrates so these materials well suited for large area roll to roll fabrication. Examples of organic opto-electronic devices included light emitting devices (OLEDs), organic phototransistors, organic photovoltaic devices and organic photo detectors. In OLEDs, the organic materials may have performance advantages over 5 conventional materials.For example, the wavelength at which an organic emissive layer emits light may generally be readily tuned with appropriate dopants. Methods for fabricating a solution-processed OLED are provided. The methods include depositing an organic layer comprising mixture of an organic electron acceptor and an organic electron donor to form a layer that is insoluble to a non-10 polar solvent. Devices containing the organic layer may demonstrate improved lifetime and have a lower operating voltage while maintaining good luminous efficiency.
[0011] WO 2013142850 discloses small molecule based bulk hetrojunction (SM BHJ) solar cells have become a competitive alternative. Intense investigation into 15 the design and utility of conjugated polymers for light harvesting has provided great insight into the design and implementation of organic semiconductors for OPV technology, to the point where power conversion efficiencies (PCEs) up to 8.4 % have been achieved.
[0012] However, polymer systems inherently suffer from batch-to-batch 20 variations and limited options for purification of the polymeric materials. Small-molecule semiconductors avoid the drawbacks inherent to polymeric semiconductors, as they are monodisperse in nature and due to having a higher solubility than polymeric analogs, can be purified and characterized using standard organic chemistry protocols. Additionally, modifications to fine-tune 25 properties can be made to small molecules more readily and with fewer complications. Recently, it has been demonstrated that small molecule-based solar cells can achieve efficiencies comparable to that of polymer-based solar cells. A small molecule system with a central electron-rich core, flanked by relatively electron-poor units, and terminated with a π-conjugated end-cap has been 30
7
previously described (Welch et al, J. Materials Chemistry 21(34): 12700-12709 (2011), U.S. Provisional Patent Appl. No. 61/416,251, International Patent Appl. No. PCT/US2011/061963. The success of this system is in large part due to the inclusion of pyridal[2,l,3] thiadiazole (PT) as an acceptor unit. The PT-based compounds have led to fabrication of a SM BHJ solar cell with a PCE of 6.7 % 5 (see Sun et al., Nature Materials, 11:44-48 (2011).One drawback to using PT-based materials in fabrication of small molecule solar cells is that the cells must employ molybdenum oxide as a hole-transport layer (HTL) for maximum efficiency. Molybdenum oxide is thermally evaporated onto devices, which prevents the use of inexpensive solution deposition during roll-to-roll 10 manufacture. It would be preferable to use a solution-processable HTL material, such as poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS), or other doped conjugated polymers. Due to hygroscopic, acidic and protonation nature of PSS influences the device stability and degradation. This is a major cause of device degradation, so there are several alternatives were used such as 15 Graphene, Carbon nanotubes (CNTs), Polyaniline: poly (styrene sulfonate) and small molecules. For high efficiency there is a need of high efficiency small molecule materials, which do not limit manufacturing options, and which do not have sites that react with materials like PEDOT:PSS, other acidic materials. The present invention seeks to address the need for improved light harvesting 20 molecules for bulk hetrojunction devices by using new and efficient materials for organic solar cells.
[0013] US 20130284242 describes the fabrication and characterization of large scale inverted organic solar array by all-spray process. The solar illumination has been demonstrated to improve transparent solar photovoltaic devices. The 25 technology using SAM has a potential to revolute the silicon based photovoltaic technology by providing a complete solution processed manufacturing process. The solar module used for windows and windshields applications due to its semi transparent property. The inventive solar modules are more efficient as compare to the silicon solar cells in artificial light environments; which significantly 30
8
expand their use in indoor applications. Additionally, these modules can be incorporated into soft fabric substances like tents, military back-packs or uniforms, providing a highly portable renewable power supply for deployed military forces. In recent years, the energy consumption has drastically increased due to increased industrial development throughout the world. Due to increased 5 energy consumption we required natural resources, such as fossil fuels, as well as global capacity to handle the byproducts of consuming these resources. In future demands for energy are expected in greatly increase due to increase in population so we required the development of new and clean energy sources which is efficient, cost effective and have minimal impact on the global environment. 10 Photovoltaic technology has been used since 1970s as an alternative to traditional energy sources. Because the photovoltaic cells use existing energy from sunlight, the environmental impact from photovoltaic energy generation is significantly less than traditional energy generation. Most of commercialized photovoltaic cells are inorganic or silicon based like single crystal, polycrystalline and amorphous 15 silicon. Mainly, solar modules fabricated by silicon for different applications like rooftops of buildings. But due to high cost of silicon as well as complicated fabrication process limits its large area commercialization. Silicon wafer based cells are brittle, opaque substance they limit their use such as on window technology where transparency is most important issue. To resolve the problems 20 or drawbacks of inorganic photovoltaic technology organic materials have been investigated for efficient, cost effective and large area flexible solar cells. There are several organic materials are used like organic polymers, small molecules, carbon nanotubes and self assemble monolayers so in this work self assembled monolayers are used. 25
[0014] US 8980677 describe the fabrication technique of organic solar panels with transparent contacts. In this method layer-by-layer spray technique is used for anode layer deposition. This method includes placing the substrate, using photoresist on to the substrate using photolithography, substrate etching, substrate cleaning, spin coating a tuning layer on substrate, active layer combination 30
9
P3HT/PCBM deposited by spin coating method, the modified PEDOT solution was deposited by spray coating on the substrate and then annealed the substrate.
[0015] US 20130263916 describe the fabrication of an inverted organic solar photovoltaic cell onto grid or flexible substrates using spray-on technology for various layers deposition. A thin layer of cesium carbonate deposited on Indium 5 tin oxide used as a cathode electrode for inverted cells. A combination of Poly-3(hexylthiophene) and [6,6]-phenyl C61-butyric acid methyl ester having a thickness around 200nm to 600nm used as a active layer combination, facilitates a high level of light transmittal through the cell. A modified PEDOT:PSS layer made by the doping of conductive polymer with dimethylsulfoxide (DMSO) used 10 as an anode electrode. A fabrication method of inverted organic solar cell is described by using gas-propelled spraying for thin layers deposition. After layers deposition the cell is sealed by using vacuum and temperature based annealing and encapsulated with UV-core epoxy.
[0016] CN101673806B discloses a solution processed material for electronic and 15 electro-optic applications. In electro-optic device, this has a first electrode separated from a second electrode, an active layer deposited between the first and second electrode and an interfacial layer in contact with the active layer. The interfacial layer consists of a metal oxide and a second material that at least one material reduces a work function or increases an electrical conductivity of the 20 interfacial layer according to the work. It is necessary a composition for electro-optic devices is a combination of at least one metal oxide and at least one salt in a ratio, by volume, of at least 1:0.1 and less than 1:1.2.
OBJECTS OF THE INVENTION
[0017] The principal object of the present invention is to fabricate cost effective, 25 efficient, solution processable large area organic solar cells.
[0018] Another object of the present invention is to produce solar cells using inexpensive, effective, stable, easily soluble hole transport material.
10
[0019] Yet another object of the present invention is to disclose a hole transport material with appropriate efficacy without inflicting any uncomfort to human being.
SUMMARY OF THE INVENTION
[0020] The present invention describes an organic solar cell, said cell comprising 5 of an Indium Tin Oxide (ITO) coated glass substrate as anode electrode for light incident, Copper Sulfide (CuS) in 7 mg/ ml optimum concentration as hole transport layer (HTL) soluble in common organic solvent such as dimethylformamide (DMF), two (2) different active layer combinations of electron donor material(s) and electron acceptor material(s) wherein said active 10 layers consists of two different active layers consisting of electron donor Poly[N-9″-hepta-decanyl-2,7-carbazole-alt-5,5- (4′,7′-di-2-thienyl-2′,1′,3′- benzothiadiazole) (PCDTBT): electron acceptor [6,6]-phenyl C71-butyric acid methyl ester (PC71BM) (PCDTBT:PC71BM) in 1:4 w/w, and Poly[[4,8-bis[(2-ethylhexyl)oxy]Benzo[1,2-b:4,5-b’]dithiophene-2,6-diyl] [3-fluro-2-[(2-15 ethylhexyl) carbonyl] thieno [3,4-b] thiophenediyl]] (PTB7) : [6,6]-phenyl C71-butyric acid methyl ester (PC71BM) (PCDTBT:PC71BM) in 1:1.5 w/w, and an aluminium (Al) cathode wherein said cathode and anode is sandwiching said active layers and HTL.
[0021] The present invention further describes a process of preparation of an 20 organic solar cell comprising arranging in a series the glass substrate, Indium Tin Oxide (ITO) coated glass substrate wherein the ITO coated glass substrate was patterned with the help of laser ablation system, cleaned with soap solution followed by cleaning with deionised water after that boiled in different solvents such as acetone, trichloroethylene and isopropanol, CuS as HTL, wherein said 25 CuS is dissolved in DMF solvent 7 mg/ ml as the optimal con active layers comprising of electron donor and electron recipient duo wherein electron donor material PCDTBT and electron acceptor material PC71BM were weighed (1:4 w/w) and dissolved in mixed solvents of chlorobenzene (CB) and 1,2-
11
dichlorobenzene (DCB), and donor-acceptor combination PTB7:PC
71BM were weighed (1:1.5 w/w) and dissolved in chlorobenzene with DIO (ratio 97:3 %), wherein further active layer solutions were stirred for 12 hrs at room temperature, and Aluminium (Al) cathode.
BREIF DESCRIPTION OF ACCOMPANYING DRAWINGS 5
[0022] In the drawings accompanying this specification:
[0023] Fig. 1: Schematic of the Organic Solar cell devices with CuS as a hole transport layer, wherein 1 is sunlight, 2 is Substrate (glass), 3 is Anode (ITO coated glass), 4 is CuS used as HTL, 5 is Active Layer(s) PTB7: PC71BM and PCDTBT: PC71BM, and 6 is Al as Cathode. 10
[0024] Fig. 2: Schematic representation of solar cell fabrication process by using CuS as a HTL, wherein 1: ITO electrode, 1’: Spin Coated HTL (CuS), 2: Hole Transport Layer (HTL), 2’: Active layer deposition, 3: Active layer(s), 3’: Cathode (Al) electrode deposition, 4: Cathode (Al) electrode, and 4’: Device active area. 15
[0025] Fig.3: J-V Characteristics of reference devices with PTB7: PC71BM and PCDTBT : PC71BM combination under dark and illumination conditions.
[0026] Fig. 4: J-V Characteristics with PTB7:PC71BM combination using CuS as a HTL.
[0027] Fig. 5: J-V Characteristics with PCDTBT:PC71BM combination using 20 CuS as a HTL.
[0028] Fig. 6: SEM micrographs showing morphology of CuS in DMF solvent.
DETAILED DESCRIPTION OF THE INVENTION
[0029] The present invention describes an organic solar cell, said cell in series comprising of an outer substrate for light incident, said outer substrate is glass, a 25 Indium Tin Oxide (ITO) coated glass substrate used as of anode electrode, a hole
12
transport layer (HTL) soluble in common organic solvent such as dimethylformamide (DMF), two (2) different active layer combinations of electron donor material(s) and electron acceptor material(s) duo. The active layers may consist of two different active layers consisting of electron donor Poly[N-9″-hepta-decanyl-2,7-carbazole-alt-5,5- (4′,7′-di-2-thienyl-2′,1′,3′- benzothiadiazole) 5 (PCDTBT): electron acceptor [6,6]-phenyl C71-butyric acid methyl ester (PC
71BM) (PCDTBT:PC71BM), and Poly[[4,8-bis[(2-ethylhexyl)oxy]Benzo[1,2-b:4,5-b’]dithiophene-2,6-diyl] [3-fluro-2-[(2-ethylhexyl) carbonyl] thieno [3,4-b] thiophenediyl]] (PTB7) : [6,6]-phenyl C71-butyric acid methyl ester (PC71BM) (PCDTBT:PC71BM), and an aluminium (Al) cathode wherein said cathode and 10 anode is sandwiching said active layers and HTL.
[0030] The present invention further describes a process of preparation of an organic solar cell comprising arranging in a series the glass substrate, ITO coated glass wherein the glass substrate was patterned with the help of laser ablation system, cleaned with soap solution followed by cleaning with deionised water 15 after that boiled in different solvents such as acetone, trichloroethylene and isopropanol. CuS as HTL, wherein said CuS is dissolved in DMF as the optimal con active layers comprising of electron donor and electron recipient duo wherein electron donor material PCDTBT and electron acceptor material PC71BM were weighed and dissolved in mixed solvents of chlorobenzene (CB) and 1,2-20 dichlorobenzene (DCB), and donor-acceptor combination PTB7:PC71BM were weighed and dissolved in chlorobenzene with DIO, wherein further active layer solutions were stirred for 12 hrs at room temperature, and Aluminium (Al) cathode electrode deposited by thermal evaporation.
Embodiments: 25
[0031] In one embodiment of the invention, the HTL chosen for study is Copper Sulfide (CuS).
13
[0032] In another embodiment of the invention, the material taken as HTL is selected because of low cost, easily soluble in common organic solvents, stable, efficient and the like.
[0033] In another embodiment of the invention, the process of organic solar fabrication is cost effective. 5
[0034] In another embodiment of the invention, the HTL material used is easily soluble in DMF which is used in device fabrication.
[0035] In further embodiment of the invention, the quantity of HTL material required ranges from 5mg/ml to 15 mg/ml.
[0036] In another embodiment of the invention, the HTL material spin coated on 10 clean patterned ITO coated glass substrates with 3000rpm and annealed at 1000C.
[0037] In another embodiment of the invention, for the study the active layer combination PTB7:PC71BM ratio was kept (1:1.5) in chlorobenzene and diidooctane (DIO).
[0038] In another embodiment of the invention, the solvents for another active 15 layer combination PCDTBT:PC71BM was chlorobenzene and dichlorobenzene.
[0039] In one embodiment of the invention, after HTL active layer combinations were spin coated on it.
[0040] In another embodiment of the invention, the active layer film was annealed at 700C (PCDTBT:PC71BM). 20
[0041] In another embodiment of the invention, after annealing cathode electrode (Al) were deposited by thermal evaporation method with base pressure 10-6 torr.
[0042] In another embodiment of the invention, J-V characteristics of the fabricated devices were characterized by using Keithley source meter having with power intensity 100mW/cm2. 25
14
[0043] In another embodiment of the invention, the HTL coated (CuS) samples were tested by AFM for their surface morphological study.
[0044] Below tables reveal the operating parameters of OSCs based on the CuS as a HTL with different active layer combinations, and comparison of different operating parameters of OSCs based on the PEDOT:PSS and CuS as HTLs with 5 different active layer combinations.
Table 1: Operating parameters of OSCs based on the CuS as a HTL with different active layer combinations
Active Layer(s)
Jsc (mA/cm2)
Voc (V)
FF (%)
PCE (%)
PTB7/ PC71BM
16.2
0.48
39.7
3.12
17.6
0.54
48.6
4.64
PCDTBT/ PC71BM
10.7
0.43
38.2
1.76
9.44
0.46
38.5
1.7
Table 2: Comparison of different operating parameters of OSCs based on the 10 PEDOT:PSS and CuS as HTLs with different active layer combinations
Different HTLs
Active Layer
Jsc(mA/cm2)
Voc (V)
FF (%)
PCE (%)
CuS
PTB7/PC71BM
17.6
0.54
48.6
4.64
PCDTBT/PC71BM
10.7
0.43
38.2
1.76
PEDOT:PSS
PTB7/PC71BM
9.75
0.59
38.7
2.48
PCDTBT/PC71BM
7.83
0.49
30.3
1.41
15
[0045] The following examples are given to illustrate the process of the present invention and should not be construed to limit the scope of the present invention:
Example 1:
Reference photovoltaic devices using solution processed PEDOT: PSS as a hole Transport Layer: 5
[0046] In this example, a well known and widely used solution processed HTL layer material poly (3,4-ethylenedioxythiophene): polystyrene sulfonate (PEDOT:PSS), is used in the fabrication of reference device. These devices were fabricated under same environmental conditions which are used in making devices using CuS as hole transport layer. These reference devices were fabricated for 10 comparing the results with the novel hole transport layer which is used in fabricating devices.
Device fabrication procedure-
[0047] The reference devices were fabricated with simplest device geometry ITO/PEDOT:PSS/active layer/Al. Prior to use, ITO coated glass (used as anode 15 electrode) substrates were patterned with the help of laser ablation system after that cleaned with soap solution followed by deionized water. After that these substrates were boiled in different solvents like acetone, trichloroethylene and isopropanol respectively. For drying the substrates put inside the heating oven after drying the substrates a thin film of PEDOT: PSS was spin coated at 1500 20 rpm for 60seconds.The resulting PEDOT: PSS films were annealed at 120oC for 20 minutes. PTB7:PC71BM active layer combination were weighed (1:1.5 w/w) and dissolved in chlorobenzene with DIO (ratio 97:3 %). Other donor-acceptor combination PCDTBT:PC71BM were weighed (1:4 w/w) and dissolved in mixed solvents of chlorobenzene (CB) and 1,2-dichlorobenzene (DCB). Both active 25 layer solutions were stirred for 12 hrs at room temperature. After annealing the active layer solutions (PTB7:PC71BM and PCDTBT:PC7IBM) were spin coated at 1000rpm for 90 seconds and the resulting substrates were further annealed at
16
70
0C for 10 minutes. Finally devices were completed by the deposition of Al as a cathode electrode at a base pressure 10-6 torr.
Device characterization-
[0048] All device measurements were performed in ambient conditions. The current-voltage (J-V) characteristics were measured using computer controlled 5 keithley 2400 source meter. The devices were illuminated from the transparent ITO anode electrode side using a solar simulator with AM 1.5G and incident power is 100 mW/cm2.From J-V measurements, we found that the resulting reference devices show PCE with PTB7:PC71BM combination is 2.48%, open circuit voltage (Voc) 0.59 V, short circuit current (Jsc) 9.75 mA/cm2 and fill factor 10 (FF) 38.7%. With PCDTBT:PC71BM combination PCE is 1.41 %, Voc 0.59 V, Jsc 9.75 mA/cm2 and FF 38.7%, respectively (shown in figure 3 and Table 2).
Example 2:
Solutions preparation of Hole Transport Material (CuS) and Active Layer Materials (PTB7:PC71BM & PCDTBT: PC71BM): 15
[0049] For this study, CuS was used as a solution processable hole transport layer. Different inorganic materials such as transition metal oxide, Copper based materials are used as a solution proceesed hole transport layer but due to solubility issue, required selective solvents to dissolve, which is very expensive and bad smell. Due to these reasons we have selected CuS material as a solution 20 processable hole transport layer for efficient organic solar cells, which is inexpensive, stable, environment friendly, good film quality and easily soluble in common solvents like DMF. Firstly, CuS dissolve in a DMF solvent in different weight ratios and the optimized concentration of CuS is 7mg/ml. For active layer solution, two well studied active layer combinations were used for device 25 fabrication. Electron donor material PCDTBT and electron acceptor material PC71BM were weighed (1:4 w/w) and dissolved in mixed solvents of chlorobenzene (CB) and 1,2-dichlorobenzene (DCB).Other donor-acceptor
17
combination PTB7:PC
71BM were weighed (1:1.5 w/w) and dissolved in chlorobenzene with DIO (ratio 97:3 %). Both active layer solutions were stirred for 12 hrs at room temperature. The optimized concentration of CuS was weighed out and dissolved in DMF solvent.
Example 3: 5
Process of device fabrication:
[0050] All devices were fabricated on ∼20 Ω/cm2 ITO coated glass substrates. ITO coated substrates were patterned using laser ablation technique. The patterned ITO coated substrates were cleaned in sequential with acetone, methanol and isopropanol, followed by drying for 20 min. After that the CuS HTL solution in 10 DMF solvent was spin coated on cleaned ITO substrates at 3000 rpm, followed by baking on a hot plate at 100°C for 15 minutes and then drying for 1 hour at room temperature. The active layer solutions were spin-coated onto the HTL layer (CuS layer) with spin speed of 1000 rpm and annealed for 10 min at 70°C on a hot plate. Finally, the devices were completed by thermally deposited Al as cathode 15 electrode at base pressure of 10-6Torr. The completed devices were then transferred for the characterization.
Example 4:
Thin film and device characterization:
[0051] The surface morphology of CuS film in DMF solvent on ITO substrates 20 was acquired by using atomic force microscopy (AFM) NT-MDT Solver Pro. To study the surface morphology of CuS HTL layer because smoother surface allow the formation of a better contact between the HTL and active layer and improve device performance. To examine the effectiveness of CuS as a solution processable HTL material for OSCs, we have fabricated devices by using simplest 25 device structure ITO/HTL/active layer/Al. Two most studied low band gap donor polymers PTB7 and PCDTBT blended with PC71BM were used as active layer for device fabrication. The current density–voltage (J–V) characteristics of fabricated
18
devices were measured using a computer controlled Keithley 2400 source meter under dark and illumination conditions. The devices were illuminated from the transparent ITO anode electrode side using a solar simulator with AM 1.5G and incident power is 100 mW/cm
2. From J-V characteristics we have calculated the device parameters summarized in table 1 and the power conversion efficiency of 5 fabricated devices with PTB7:PC71BM combination 3.12 and 4.6 % in DMF solvent. However with PCDTBT: PC71BM combination the device efficiencies are 1.76 and 1.7 %.

We Claim:
1. An organic solar cell, said cell comprising of:
a) Indium Tin Oxide (ITO) coated glass substrate used as an anode electrode for light incident;
b) a solution processed hole transport layer (HTL) soluble in common 5 organic solvent such as dimethylformamide (DMF),
c) two different active layer combinations of electron donor material(s) and electron acceptor material(s), and
d) metal is used as cathode electrode, wherein
active layer and HTL layer sandwiching in between anode and cathode 10 electrode.
2. The organic solar cell as claimed in claim 1, wherein said outer substrate is glass.
3. The organic solar cell as claimed in claim 1, wherein said anode is ITO coated glass substrate. 15
4. The organic cell as claimed in claim 1, wherein said HTL is Copper Sulfide (CuS) in 7 mg/ ml optimum concentration.
5. The organic cell as claimed in claim 1, wherein said active layers consists of two different active layers combination of electron donor Poly [N-9″-hepta-decanyl-2,7-carbazole-alt-5,5 (4′,7′-di-2-thienyl-2′,1′,3′- 20 benzothiadiazole) (PCDTBT): electron acceptor [6,6]-phenyl C71-butyric acid methyl ester (PC71BM) (PCDTBT:PC71BM) in 1:4 w/w, and Poly[[4,8-bis[(2-ethylhexyl)oxy]Benzo[1,2-b:4,5-b’]dithiophene-2,6-diyl] [3-fluro-2-[(2-ethylhexyl) carbonyl] thieno [3,4-b] thiophenediyl]] (PTB7) : [6,6]-phenyl C71-butyric acid methyl ester (PC71BM) 25 (PCDTBT:PC71BM) in 1:1.5 w/w.
6. The organic cell as claimed in claim 1, wherein said cathode is Aluminium (Al).
7. A process of preparation of an organic solar cell comprising arranging in a series the glass substrate, Indium Tin Oxide (ITO) coated glass as anode, CuS as HTL, active layers consisting of electron donor and electron 5 recipient pairs, and Aluminium (Al) cathode.
8. The process as claimed in claim 7, wherein ITO coated glass substrate was patterned with the help of laser ablation system, cleaned with soap solution followed by cleaning with deionised water, and finally boiled in acetone, trichloroethylene and isopropanol. 10
9. The process as claimed in claim 7, wherein electron donor material PCDTBT and electron acceptor material PC71BM were weighed (1:4 w/w) and dissolved in mixed solvents of chlorobenzene (CB) and 1,2-dichlorobenzene (DCB), and donor-acceptor combination PTB7:PC71BM were weighed (1:1.5 w/w) and dissolved in chlorobenzene with DIO (ratio 15 97:3 %), wherein further active layer solutions were stirred for 12 hrs at room temperature.
10. The process as claimed in claim 1, wherein the CuS as a solution processed HTL material was weighed out and dissolved in common solvent DMF in 7 mg/ ml as the optimal concentration.