24 - 28 October 2016 • Marina Bay Sands Sands Expo and Convention Centre, Singapore
The fabrication of high-efficiency CZTS thin film solar cell (TFSC) with a structure of Al/MGZO/ ZnO/Zn(O, S)/CZTS/Mo have been demonstrated for the first time. Specifically, n-type Mg and Ga doped ZnO (MGZO) thin films with wide band gap energy (3.8 eV) and low electrical resistivity (3.1×10-4 Ωcm) as an alternative TCO were prepared using sputtering method. The influence of different preparative parameters on the properties of MGZO thin films have been investigated. Further, a comparative study on the electrical properties between Al-doped ZnO (AZO) and MGZO window layer deposited on CZTS TFSCs was performed. The preliminary efficiency of 7.4 % with a Jsc of 14.9 mA/cm2 Voc of 376.9 mV, and a FF of 47 % have been obtained for Al/MGZO/Zn(O,S)/CZTS solar cell, although the processing parameters are not yet optimized. The efficiency can be further improved by optimizing the process parameters and controlling the conduction band alignment.
In the CuInSe2 (CIS)-based thin-film photovoltaic (PV) technology, Solar Frontier K.K. is currently a leading global company not only manufacturing with an environmentally-friendly (i.e. Cd- & Pb-free) device structure in annual production volume of around 1 GW, but also selling the products in cumulative sales volume of over 4 GW worldwide after 2007 when Solar Frontier started the sales activity. Accumulated reliability & durability data from the installation sites worldwide has been fed back steadily to the production stage and contributes to improve the product quality. However, it is obvious that the device structure and its manufacturing process employed by Solar Frontier are considered to be a unique exception. In this PV technology, the industry (Solar Frontier) must accelerate the speed transferring their R&D results to the manufacturing stage to make their business profitable. At this moment, First Solar seems to be the only company successfully performing the integration between their own R&D and the production, not CIS-based thin-film PV companies. It is urgently necessary for Solar Frontier to improve the product-based performance for making it competitive enough against the poly-crystalline Si PV technology. However, CdTe thin-film PV technology (First Solar) is surpassing the performance level of poly-crystalline Si PV by preparing the products with a module efficiency of over 16 %. CIS-based thin-film PV (Solar Frontier) is currently behind this level of product-based performance. It would be recognized that this must be one of the necessary conditions for thin-film PV technologies to have a “blue ocean” in a global PV market. At the conference, these issues would be discussed more in detail.
In this talk we will describe our research efforts leading to the development of Zn(O,S) based p-n junctions for CIGSSe cells and modules. The exploratory cell level development work was done in collaboration with NREL where CBD and Sputtering based emitter layers and cells were developed. The CBD Zn(O,S) process was adapted to production and a certified champion module was achieved. The sputtering approach also produced efficient cells when the film composition and thickness were tuned. Finally, we have achieved superior performance for full size modules utilizing ALD Zn(O,S) buffer layers. Output power increased by 5 to 7W compared to the baseline CdS modules. In this talk, we will discuss the scientific underpinnings of the above results, the opportunities for further improvements and the cost comparisons.
We investigated the origin of hysteresis in I-V curves of a planar perovskite cell using different equivalent circuit models. A planar cells showing huge hysteresis with PCE 18.0% on reverse scan and 8.8% on forward scan was used for validating the equivalent circuits. We found that the equivalent circuit model composed of two series connected diodes, two capacitors, two shunt resistances and a series resistance clearly reproduced the hysteretic I-V curves. According to this equivalent circuit model, the computationally simulated I-V curves matched closely with the experimental one. This suggests that perovskite cell has two active interfaces; TiO2/CH3NH3PbI3 and CH3NH3PbI3/spiro-OMeTAD. Hysteresis is essentially caused by carrier accumulation at these active interfaces. The electrical capacitances generated by defects due to the lattice mismatch at the TiO2/CH3NH3PbI3 and CH3NH3PbI3/spiro-OMeTAD interface are truly responsible for the hysteresis in the perovskite solar cells.
Based on above experience and knowledge, we also examined to evaluate the cell performance at low light intensity condition. Very surprisingly, due to the charge / discharge property with internal capacitance, we found the limitation to define the cell performance from the I-V curve because of the fake current. To solve this issue, we newly propose the Maximum Power Point Tracking (MPPT) technique to define the most accurate cell performance of the hysteric device.
The photovoltaic (PV) solar energy conversion is expected to become the major clean energy source because further installation of nuclear energy in the world is presumed to be very difficult as a result of the most recent crisis of the Fukushima nuclear power plant in Japan. We will have to contribute to creation of clean energy society for the future by using solar energy. The concentrator PV have great potential for very large-scale integration of PV as well as high performance crystalline Si PV.
This paper summarizes fundamental physics of high-efficiency III-V compound semiconductor and multi-junction (MJ) solar cells and their key technologies for realizing higher efficiency. This paper also reviews Japanese research activities for III-V MJ and concentrator solar cells. Concentrator 4-junction or 5-junction solar cells have great potential for realizing super high efficiencies of over 50%. Lattice-mismatched and III-V-N are thought to be promising materials for realizing more than 50% efficiency. Improvement in the 1-sun efficiency of triple-junction solar cells is also possible. Recently, high efficiency (37.9%) at 1-sun (AM1.5G) has been realized with inverted epitaxially-grown InGaP/GaAs/InGaAs 3-junction cells by Sharp. 44.4% efficiency has also been demonstrated with InGaP/GaAs/InGaAs 3-junction solar cells by Sharp under the Europe-Japan Collaborative Project on Concentrator PV (NGCPV Project). Major results attained under the NGCPV Project are also presented. It is clear that Japanese group has greatly contributed to development of high efficiency cells.
III-V/Si tandem solar cells have also great potential of high efficiency and low cost. We will also have to contribute to creation of mobility society by using solar energy throughout further development of high-efficiency, low-cost and highly reliable PV.
Advanced light trapping concept is vital for high efficiency silicon thin film solar cell. Multiscale architecture such as nano, micro and macro sized texturing is required for increasing the saturation current density (Jsc) of the tandem cell. The use of multi-scale architecture increases the haze ratio as the light gets diffracted in all directions. The high aspect ratio of the multiscale architecure increases the surface area and hence the improvement in the saturation current density. All wavelength angular distribution function is improved to a great extent. The increase in saturation current density is due to the efficient collection of photons injected into the cell. The solar cell was fabricated using multi-scale architecture and the best cell yielded an open circuit voltage of 1.405 V, saturation current density of 15.36 mA/cm2, fill factor of 71.5% and efficiency of 15.38%
Thin-film silicon (TF-Si) solar-cell technology is a mature PV technology that can deliver low-cost solar electricity. A drawback of this technology is a low efficiency of commercial modules that varies between 6 and 11%. In last years this drawback has significantly reduced a number of companies that were manufacturing TF Si modules for large-scale generation of electricity. However, meanwhile still a lot of effort has been spent on improving light management, materials, and interface properties in TF-Si solar cells that has resulted single- and multi- junction solar cells with record high performances. The possibility to manufacture lightweight, flexible, and customized TF Si modules offers a variety of built-integrated applications. Examples of flexible TF-Si module applications will be presented.
TF Si layers proved to play an important role in improving performance of other solar cell technologies. Crystalline silicon (c-Si) wafer-based PV technology is an excellent example. Applying TF-Si layers in c-Si solar cells has strongly contributed to reaching record efficiencies. C-Si solar cell structures containing TF-Si layers such as heterojunction or TOPcon structures will be presented and the role of TF-Si layers explained. By developing and applying a stack of a tunneling dielectric layer with TF-Si-based passivating contacts a flat IBC c-Si solar cell was fabricated at TU Delft with an efficiency above 21%.
Since the band gap of TF Si-based layers can be varied in a broad range from 1.1 eV to 2.0 eV they can be readily used as absorber materials in hybrid multi-junction solar cells. Hybrid multi-junction solar cells are considered as a strong candidate for demonstrating efficiency beyond the efficiencies of best c-Si solar cells. Different hybrid multi-junction solar cells containing TF Si absorbers will be presented and the advantages of these cells discussed.