Challenges for developing high-efficiency solar cells


Editor : SolarBee
2014/11/30

Thin-film solar cell technology to get to the milestone stopped


 A large-scale solar technology development program for CIGS thin film solar cells carried out over the past three years as a part of Korean Government’s energy technology development business was completed in recent days. More than 50 billion Won (about 45 million USD) together from government funding and private investment was invested and 11 companies and 8 universities and research institutes have participated.Thechampion module efficiency of 16.0 % at the area of 1.44 ㎡ was achieved and the average module efficiency of 15 % was achieved in the pilot-scale test production. This value of 16.0 %@ 1.44 ㎡ is ahead of the champion module efficiency record of Solar Frontier and TSMC Solar (each 14.7%@ 1.23 ㎡, 15.7 % @ 1.09 ㎡) and the technical objectives were achieved but the final goal of building a 200 MW production line in 2014 is not proceeded so far.


 Since at the technology development project planning stage, in early 2011, the crystalline silicon PV module prices were around $2/W, the program that would accelerate Grid Parity of photovoltaic power by developing thin-film solar cells aiming at solar module prices of $1/W or less was a convincing proposal. But with the deepening of imbalance that over investment is causing for crystalline silicon solar industry conducted mainly in China in 2011, the solar module market price fell down to $ 0.6/WCIGS, the target price of solar module technology development and the main sponsor enterprise has abandoned construction of a 200 MW line.


Solar module efficiency viewed interms of the levelized cost of electricity


 With severe restructuring and cost-cutting efforts of crystalline silicon photovoltaic industry, photovoltaic power system installation costs fell down to around $2/W and in some areas Grid Parity starts to reach and downstream markets of the PV industry is recovering since last year, but at the position of the photovoltaic power investors, photovoltaic power requires heavy initial investment and investment attractiveness is still not higher than that of any other power source.


 In order to compare the economics of photovoltaic power which requires heavy initial investment, we use LCOE(Levelized Cost of Electricity) as a yardstick expressed by the formula below.1



  LCOE : Levelized cost of electricity in Korean Won/kWh
  I0 : Investment expenditures in Won
  At : Annual total costs in Won in year t
  Mt,el : Produced quantity of electricity in the respective year in kWh
  i : Real interest rate in %
  n : Economic operational lifetime in years
  t : Year of lifetime (1, 2, …n)


 The denominator, the total power generation is increased or decreased in accordance with the economic life or long-term performance of the system and by default, is given the greatest impact from the climate conditions, i.e.insolation of the install ation area. The annual operation and main tenance costs, excluding the initial investment from the numerator, the total cost, is 2 % of the system install ation cost and the economic service life is typically 20 years or so, so the total operation and maintenance cost over the lifetime is roughly about 40 % of the initial investment.


 After all in order to reduce the LCOE of photovoltaic power with no fuel, it is the key to reduce the initial system installation investment. Looking installation cost configuration of photovoltaic power generation system, it is divided into module cost and power system installation incidental expenses(BOS, Balance of System) and then BOS cost is divided into the installation area relevant costs (install ation supporter cost, wiring cost, etc.) and the installation area irrelevant costs (an inverter cost, design expense, etc.). The current photovoltaic power system installation cost of about $2/W consists of the average solar module cost of $0.7/W, the installation area relevant BOS of approximately $0.5/W and the remainder accounted for $0.8/W. Therefore, by increasing the efficiency of the photovoltaic modules, not only the effect of lowering the cost per W but also the hidden effect by decreasing the installation area relevant BOS at the same time reducing the footprint can be obtained and increasing the efficiency of the photovoltaic modules is considered a decisive factor in lowering the LCOE of solar photovoltaic power.


Towards the solar module efficiency of more than 30 %


 In order to greet the Grid Parity in all corners of the globe without Feed-in-Tariff, Renew ables Portfolio Standards and Investment Tax Credit, to lower the PVLOCE half now, and solar module efficiency in the long run should exceed 30 %, two times the current. Of course, now also designed to meet a number of the wave length band of the solar spectrum, the record efficiency of a multi-junction solar cell such as InGaP/GaAs/InGaAs tandem solar cell is more than 40 %, but because only produced in a manner similar to the micro processor, it is made at small area and very expensive and applicable to the concentration type which has very limited market potential for installation and spaceship power where the efficiency to weight ratio is more important. The theoretical efficiency limit of the crystalline silicon solar cells is 29 % and the achieving maximum efficiency of 25 % goes close to the limit, so the theoretical efficiency limit of single junction solar cell is 33.7 %, more than the 30 % efficiency in commercial solar modules requires multi-junction solar cell structure.2


 These lection maybe bonded tovarious material shaving different Bandgap to evenly absorb the solar spectrum when designing the junction solar cells tructure, the actual selects acombination in consideration of matching between the current density and the crystal lattice. Crystalline silicon solar cells and the structure for bonding the thin-film solar cell, an organic solar cell and abonding structure for crystalline solar cells, in addition to the structure of connecting the solar cells and organic thin film solar cell by applying aQuantum Dotonhow to adjust thecrystalline silicon Bandgap silicon Quantum Dot design to the junction solar cells can be fabricated. As shown in the graph below, the efficiency of solar cell efficiency in combination with there centsurge in Perovskite cell sand crystalline silicon solar cell sand organic solar cells will also bedesigning the junction solar cell.



Fig. 1. Best research cell efficiencies (Source : NREL NCPV Home Page,
http://www.nrel.gov/ncpv/images/efficiency_chart.jpg)


 Besides bandgap, current density, crystallattice matching design in consideration, a number of problems such as the problem of recombination at the interface, the durability problem of the device, replacement with harmless material should be solved and therefore to develop and mass produce commercial large area multi-junction photo voltaic modules with th eefficiency of more than 30 %, it will require a lot of manpower, a longtime and budget. In the process, as if a high efficient heterojunction solar cell with crystalline silicon and amorphous silicon thin-filmmaterial such as HIT(Heterojunction with Intrinsic Thin layer), became commercially available, multi-junction solar cells will been developed in a variety of materials and structural form sand will attempt to verify it self in the market.




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1http://www.ise.fraunhofer.de/en/publications/veroeffentlichungen-pdf-dateien-en/studien-und-konzeptpapiere/study-levelized-cost-of-electricity-renewable-energies.pdf
2http://en.wikipedia.org/wiki/Shockley%E2%80%93Queisser_limit#Tandem_Cells



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