Solar cells for energy harvesting A. Kaminski-Cachopo IMEP-LAHC, Grenoble, France 1
Introduction � Solar energy conversion in electricity well established thanks to: - continuous increase of solar cells efficiencies - decrease of photovoltaic cost. � Solar energy is mainly used in outdoor conditions to produce large power. Crystalline silicon solar cells are dominating the market but other materials are also good candidates for photovoltaic conversion. � There is an increasing interest to microenergy harvesting by using photovoltaic technologies to power electronic devices using indoor light. However there is no standard measurement procedures for testing solar cells in indoor conditions. Several studies have compared the performances of solar cells in indoor conditions. Photovoltaic Report, Fraunhofer ISE, 2016 2
Outline � Operation of solar cells � Solar cells technologies and state of the art - Crystalline Si solar cells - Thin films - Multijunctions � Solar cells for energy harvesting 3
I. Operation of solar cells � Incident energy: the sun or indoor light � Absorption of light in the semiconductor � Light absorbing properties (absorption of light and generation of carriers) � Electrical transport properties http://pveducation.org/pvcdrom � Collection of the photogenerated carriers: the solar cell device � Electric field � Diode (P/N junction) � Production of power : modules (solar cells are interconnected and encapsulated in a module) 4
I. Operation of solar cells I d Under dark conditions http://pveducation.org/pvcdrom I d Under illumination V oc : open-circuit voltage Under illumination, the photogenerated current is I sc : short-circuit current subtracted from the forward biased diode current: V m : voltage at maximum output power I = I diode -I photogenerated I m : current at maximum output power 5
Equivalent electrical circuit of a solar cell � The solar cell is generating power and the convention is to invert the current axis. C. Honsberg and S. Bowden, http://pveducation.org/pvcdrom R s : series resistance (semiconductor resistivity, wire � resistivity, metal-semiconductor contact resistivity) R sh : Shunt resistance: leakage in the device � I L : photogenerated current � I 0 , n: saturation current and ideality factor of the diode � 6
Important electrical parameters V oc : open-circuit voltage I sc : short-circuit current P max : maximum output power=I m V m I m V m : voltage at P max I m : current at P m P inc : incident light power η η η : efficiency η � � � V m η = = ��� � � C. Honsberg and S. Bowden, http://pveducation.org/pvcdrom � � The photogenerated current depends on: ��� ��� - intensity of light - absorbing properties of the semiconductor - quality of the semiconductor 7
Solar cells Standard Test Conditions (STC) � Solar spectrum at the Earth's surface : Air mass 1.5 spectrum (AM1.5) � Intensity of 1 kW/m 2 (one-sun illumination) -> if η =20% then P max =20 mW/cm 2 � Cell temperature of 25 °C 3,10 eV 0,8 eV 1,65 eV 1.12eV E (eV) Black body at 6000K Radiation outside atmosphere Sun radiation at Earth surface AM 1.5 (1kW/m2) C. Honsberg and S. Bowden, http://pveducation.org/pvcdrom
Outline � Operation of solar cells � Solar cells technologies and state of the art - Crystalline Si solar cells - Thin films - Multijunctions � Solar cells for energy harvesting 9
II. Solar cells technologies � Bulk silicon � PN junction � Well-known technology, 90% of the industrial production � Thin layers – low cost, flexible � Amorphous silicon, CdTe, CIGS � Organic, quantum dots, DSSC, perovskite… � Multijunctions � Concentration : III-V materials multijunctions… 10
Classification by material Crystalline Si Inorganic thin films Organic thin film Photoelectroche- mical solar cell Monocrystalline Si Polymer Amorphous Si Molecular Amorphous/ Multicrystalline Si µcrystalline Si CIGS CdTe Interpenetrating HIT lattice a-Si/c-Si Ribbon Crystalline thin films III-V compounds CPV, spatial 11
Bulk silicon solar cells � Si: abundant : 26% of the surface of the earth � Well-known material (most used material in microelectronics), reliable � Theoretical maximum efficiency of about 31% � Industrial efficiencies: 18-22% � Si solar cells world production ~ 90% The Shockley-Queisser limit for the efficiency of a single junction solar cell under one-sun illumination. � For single Si junction: maximum efficiency is about 31% � Optimal band gap: 1-1.5eV Shockley W, Queisser HJ, Journal of Applied Physics ,1961, 32:510-519. 12
Bulk silicon solar cells Al-BSF structure (Aluminum Back Surface Field): the most commercialized Antireflection coating and passivation layer Texturation ≈ 200µm Field effect passivation (BSF) Jan Krügener and Nils-Peter Harder, Energy Procedia 38 (2013 ) 108 – 113 Optimisation by reduction of : - Optical losses (metal shadowing, reflection…) - Recombination in the volume (purification, defects, grain boundaries…) and at the surfaces - Series resistance (due to metallisation, material, contacts…) 13
High-efficiency concepts of crystalline silicon (c-Si) wafer based solar cells � Improved surface passivation and light trapping and low Rs ���� ����������� ������� ���� ������� �������������������������� ���������� M.A. Green, Prog. Photovolt: Res. Appl. 2009; 17:183–189 ������� ���������� ���������� Jan Krügener and Nils-Peter Harder, Energy Procedia 38 ( 2013 ) 108 – 113
High-efficiency concepts of crystalline silicon (c-Si) wafer based solar cells HIT (Heterojunction with thin intrinsic layer) Passivation of c-Si surface by a-Si: reduction of recombinations ��������� 15 �������� Rear contact solar cell Reduction of front contact shading 15
High-efficiency concepts of crystalline silicon (c-Si) wafer based solar cells � Rear contact heterojonction solar cell : record efficiency on c-Si Masuko K, et al IEEE Journal of Photovoltaics 2014; 4: 1433–1435. 16
International Technology Roadmap for Photovoltaic (ITRPV 2016) : Worldwide market share for different cell technologies 17
Expected average stabilized efficiencies State-of-the-art mass production lines for double-sided contact (BSF, PERC, PERT) and rear-contact cells on multicristalline (mc) and monocrystalline (mono) silicon. 18 ITRPV 2016
II. Solar cells technologies � Bulk silicon � PN junction � Well-known technology, 90% of the industrial production � Thin layers – low cost, flexible � Amorphous silicon, CdTe, CIGS � Organic, quantum dots, DSSC, perovskite… � Multijunctions � Concentration : III-V materials multijunctions… 19
Thin film solar cells Drawbacks with crystalline silicon: � Thick wafers are necessary to absorb most of the sunlight � Good quality material is required � Material cost is significant in the total cost of the module � Idea: to use thinner layer of semiconductor with better absorbing properties: 1-10 � � m � � - Commercialised: a-Si, CIGS, CdTe - Under development with some products for sale: organic, DSSC, perovskite,… � Flexible solar cells are possible � Lightweight solar cells � Ratio: material cost / efficiency Miasolé 20
a-Si solar cell Thin film solar cells Interconnexion Glass Encapsulation Rear contact CdTe Calyxo - QCells CdS TCO Glass Panasonic � Three technologies dominate the thin film area: - CdTe and CIGS solar cells have efficiency just behind c-Si - a-Si presents the lowest efficiency but improvements have been obtained with a-Si/µc-Si Flisom 21
Technological Roadmap, Solar Photovoltaic Energy,2014 c-Si CdTe a-si CIGS Expected commercial efficiencies improvements: - 19% (2017) and 22% (2025) for CdTe and CIGS - 12% (2017) and 16% (2025) for a-Si/µc-Si 22
Dye sensitized solar cells (DSSC) Organic solar cells Best efficiencies: about 12% � Advantages: Easy to elaborate, flexible � Advantages: simple technology, semi-transparent � Drawbacks: Diffusion length ≈ 10 nm, Low � Main issues: the electrolyte, the price of the dye efficiency, Unstable materials (oxidation,…), limited solar cell lifetime B. O'Regan, M. Grätzel (1991). Nature. 353, 737–740. � � ��� �������� � � � ����� ���� ���������! � � ��������� �� � ��� ��������� � ���" � � ��# � ��� � � �$# ������� � �������! In a Bulk heterojunction BHJ , the donor and acceptor materials are mixed together. Regions of each material in the device are separated by only several nanometers, a distance optimized for carrier diffusion. ��
Perovskites ABX 3 X: Br, I, Cl Hole transporting material � High absorption, high diffusion length, easy to fabricate, high efficiency. Au � Drawbacks: stability, reproducibility on large area, FTO Pb toxicity. ���� �������������������� ������ Small, Volume 11, Issue 1, pages 10-25, 30 (2014) 24
http://www.nrel.gov/ncpv/images/efficiency_chart.jpg 25
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