EVS 25 Shenzhen , China , Nov 5-9 , 2010 Flexible Thin Film Solar Cells Using in the Car

In recent years, renewable energy technologies are being explored to meet the challenges of energy security and climate change. Solar energy is a clean and green renewable source of energy. It is an inevitable option to use photovoltaic effect to directly convert sunlight into electricity. CuIn1-xGaxSe2 (CIGS) thin films were formed from an electrodeposited CuInSe2 (CIS) precursor by thermal processing in vacuum in which the film stoichiometry was adjusted by adding In, Ga and Se. The structure, composition, morphology and opto-electronic properties of the as-deposited and selenized CIS precursors were characterized by various techniques. A 17.2% CIGS based thin film solar cell was developed using the electrodeposited and processed film. The cell structure consisted of Mo/CIGS/CdS/ZnO/MgF2. The cell parameters such as Jsc, Voc, FF and η were determined from I-V characterization of the cell. The solar electric vehicle is made using flexible thin film solar cells. With the solar cell the full charge endurance of SEV can be increased about 35% substantially compared with no solar cells.


Introduction
Industrialized nations face an uncertain energy future.Threats such as global warming and anticipated scarcity of hydrocarbon fuels present a problem to long-term planning efforts.Renewable energy has been recognized as part of a solution; wind, solar, tidal, wave, and biomass technologies are receiving steadily increasing investment, though their contribution to global energy supply remains small.Transportation systems in particular are almost exclusively powered by nonrenewable energy sources [1].
As a new and environmental vehicle, the electric vehicle has advantages of zero discharge, low noise and wide source for energy supplement [2].In the solar car, the photovoltaic solar cell plate transforms the solar energy into electric power, then the power is stored in the batteries, and then the batteries supplies the power to the electric vehicle.The use of flexible plastic substrates is becoming an issue of great interest in thin film solar cells technology, as they allow cost reduction in the production process by using roll-to-roll deposition systems and facilitate monolithic interconnection of the cells to produce a photovoltaic module [3,4].Furthermore, those lightweight, reduced fragility and flexibility of the modules can reduce the storage and transportation costs.Plastic substrates solar cells can also well used in the solar car because of those characteristic [5].One of the advantages of Cu(In, Ga)Se 2 (CIGS) solar cells is their ability to use very thin photoabsorber layers with thickness on the order of several micrometers [6].This feature offers not only a reduction of consumption of raw materials, but also leads to a wide variety of applications of solar cells that take advantages of lightweight and flexible devices.Applications of flexible CIGS solar cells include mobile use which has a strict weight requirement as well as for use as construction materials and car roofs by application to curved surfaces.In addition to the properties of low weight and flexibility, the advantages of a low energy pay back time and strong radiation hardness of flexible CIGS solar cells are attractive [7].Solar cars have gained the public attention through several races organized across the world including the Australian World solar Challenge, the Swiss Tour de Sol, and the GM Sunrayce [8,9].While the entrants in these races are, for the most part, very lightweight, unsafe vehicles, the concept of a safe, practical commuting vehicle which derives its energy primarily from the sun has been little explored.While such a vehicle is expected to have very limited range (20-50 miles), the possibility of being able to drive a car to work, recharge the batteries with solar energy during the day, and drive back home at night with enough stored energy to get back to work the next day, offers a very attractive motive option.In the project described here we set out to design and fabricate flexible solar films to be used in the solar car.Despite the significant advancement made in solar cell technology in the past decade, the purchase and installation cost of solar energy system is still not competitive in the energy industry.Ideally, mass-produced solar cells that cost $0.06 per kilowatt-hour or less are required for solar energy to be as competitive as energy from hydrocarbon sources [3].Because of its favorable electrical, optical, and semiconducting properties and its feasibility of large-scale fabrication, CuInSe 2 (CIS) or CIGS-based thin films have attracted many research interests [4].The energy bandwidth of about 1.1 eV and absorption coefficient of 3×10 4 cm -1 make CIS most suitable for photovoltaic applications [5].One of the important advantages of CIGS solar cells is their ability to use very thin photoabsorber layers with a thickness on the order of several micrometers [10].This feature offers not only a reduction of consumption of raw materials, but also leads to a wide variety of applications of thin film solar cells that take advantages of lightweight and flexible devices.Applications of flexible CIGS thin film solar cells include mobile use which has a strict weight requirement as well as for use as construction materials and car roofs by application to curved surfaces [11].In addition to the properties of low weight and flexibility, the advantages of high efficiency and strong radiation hardness of flexible CIGS thin film solar cells are attractive.The most commonly used substrate for rigid CIGS thin film solar cells and modules is soda-lime glass (SLG) at present [12].For flexible CIGS cells, on the other hand, a wide variety of substrates such as stainless steel (SS), Ti, Mo, Cu, Al and polyimide (PI) have been examined to date.Among these, SS (magnetic) and Ti foils are relatively suitable and commonly used because these metal foils meet the demands required for the CIGS flexible substrate [13].The demands on the CIGS substrate include, for example, vacuum compatibility, thermal stability, thermal expansion, chemical inertness, the ability to act as a sufficient humidity barrier, surface smoothness, cost, and weight.Using SS or Ti substrates, conversion efficiencies over 17% have been demonstrated [14].were used as Alkali-silacate glass thin layers (ASTL) [15].SLGTF with a thickness of 100-120 nm were deposited by RF-magnetron sputtering on substrates prior to the deposition of the Mo back contact layer.After SLGTF deposition, 800nm thick Mo layers were deposited by sputtering.CIS, CIGS and CGS absorber layers were grown by the three-stage process.Before sputtering the CIGS film, the substrate was cleaned in de-ionized water in an ultrasonic bath.The substrate temperature during the first stage of growth was 350 and the temperature during the ℃ second stage and the third stage was 550 .The ℃ [Ga] / [In + Ga] com-position ratio of the CIGS layers was about 0.4.The thickness of CIS and CIGS was about 2 μm, and the thick-ness of CGS was 1μm.The composition ratio and thick-ness were determined by electron probe microanalyzer (EPMA) measurements and scanning electron microscopy (SEM), respectively.After CIS, CIGS and CGS growth, chemical bath deposition of CdS buffer layers was performed in a water bath [16].Highly resistive intrinsic ZnO and conductive ZnO: Al layers were then deposited by sputtering in a sputtering chamber.Finally, Al grid electrodes were formed by thermal evaporation.No anti-reflection coating was used.A completed flexible thin film solar cell fabricated on a zirconia sheet is shown in Fig. 2.

Results and discussion
The final configuration of photovoltaic device formed was Mo/CuIn 1-x Ga x Se 2 /CdS/ZnO/MgF 2 .Once the device was formed, it was characterized by I-V measurements in which case the intensity of illumination, solar spectrum distribution and test temperature was 1000 W/m 2 , AM1.5 and 25 °C, respectively.The results obtained from the I-V characterization of the solar cell are displayed in Fig. 3. which is a plot of current density (J) versus applied voltage (V) for the cell in darkness and under illumination.The cell parameters were calculated from this plot and are the following results: Jsc (current density) = -26.55mA/cm 2 , Voc (output voltage) = 550 mV, FF (fill factor) = 67.885%and (conversion efficiency) η = 17.2%.It is indicated a high efficiency thin film solar cell which can be used in the solar electric car is developed.There is no clear blow-up on the top of car.It is very beautiful that SEV is installed with the solar cells.In good sunny conditions, thin film solar cells convert sunlight into electricity continuously and the electricity is stored in batteries by the controller.The full charge endurance of SEV can be increased about 35% substantially compared with no flexible thin film solar cells.

Conclusion
In the paper flexible CIGS thin films were formed using a thermal processing in vacuum.According to process of developing the thin film solar cells, the film stoichiometry was adjusted by adding In, Ga and Se.The structure, composition, morphology and opto-electronic properties of the asdeposited and selenized CIS were characterized by various analysis techniques.A 17.2% CIGS based thin film solar cell was developed using the electrodeposited and processed film.The parameters of solar cell such as Jsc, Voc, FF and η were determined from the I-V characterization of the solar cell.Using the flexible thin film solar cells, a solar electric vehicle is developed.The full charge endurance can be increased about 35% substantially compared with no solar cells.

Figure 1 :
Figure 1: Schematic illustration of the device structure Fig.1.shows the structure of the flexible CIGS thin film solar cell fabricated on zirconia sheets in this study.A 50μm thick flexible zirconia sheet was used as substrate.SLG thin films (SLGTF), one of the most familiar and inexpensive alkalisilicates consisting chiefly of Na 2 O. CaO.5SiO 2 , were used as Alkali-silacate glass thin layers (ASTL)[15].SLGTF with a thickness of 100-120 nm were deposited by RF-magnetron sputtering on substrates prior to the deposition of the Mo back contact layer.After SLGTF deposition, 800nm thick Mo layers were deposited by sputtering.CIS, CIGS and CGS absorber layers were grown by the three-stage process.Before sputtering the CIGS film, the substrate was cleaned in de-ionized water in an ultrasonic bath.The substrate temperature during the first stage of growth was 350 and the temperature during the ℃ second stage and the third stage was 550 .The ℃ [Ga] / [In + Ga] com-position ratio of the CIGS

Figure 2 :
Figure 2: Picture of a solar cell fabricated on a flexible substrate

Figure 3 :Figure 4 :
Figure 3: The j-V characteristics for CIGS thin film solar cell in dark and under illumination

Figure 5 :
Figure 5: View of PV-assisted electric vehicle from the front side