Ashida Laboratory Keita Miyagawa 1 Carrier dynamics evaluation in the photoelectric device from the terahertz electromagnetic wave

1. Introduction Photoelectric device Conventional methods 2. Terahertz electromagnetic waves 3. Previous work Polycrystalline silicon solar cell 4. My work 2 Contents

Solar cell  Thermalization ~ 100 ps  Charge separation ~ ps  Nonradiative recombination ~ 100 ps  Charge collection Ultrafast carrier dynamics is important for the design of the photoelectric devices. 3 Photoelectric device hν > E g     P N Nonradiative recombination : 非輻射再結合

Two-dimensional mapping of the current Short time 4 Conventional methods (EBIC, LBIC) * electron/ laser beam induced current (EBIC/ LBIC) Laser beam induced current : レーザー光線誘起電流 photovoltaics/equipment/light-beam-induced-current/

excited state excitation luminescence ground state Excitation method ・ light -> PL ・ electron beam -> CL ・ electric field/ current -> EL 5 Conventional methods (PL, EL) * Photoluminescence (PL), electroluminescence (EL) mechatronics.co.jp/products/pv/eltester.html

Motivation good time resolution Selective detection of carrier in depletion layer -> THz wave generation pn junction 6 Feasibility of conventional methods CharacteristicsLBICPLEL Signal generationPhoto induced current Radiative recombination Excitation sourceCW laserLightCurrent injection ContactNecessaryNot necessaryNecessary Time resolutionpoorgood Electric fieldgoodpoor

Electronics THz waves Photonics Frequency : 0.1 and 10 THz Wavelength : 3 mm and 30 µ m Energy : 0.41 and 41 meV 7 Terahertz (THz) electromagnetic waves spectroscopic-imaging-systems/about-terahertz-waves

Optical pulse Transient current THz pulse Time (ps) 8 THz waves radiation mechanism P N

Sample : Commercially available polycrystalline silicon solar cell emitter n-Si depletion region base p-Si EL PL (LED with 850-nm wavelength) LBIC (laser diode with 808-nm wavelength) LTEM 9 Experiment * laser terahertz emission microscope (LTEM) Laser terahertz emission microscope : レーザーテラヘルツ放射顕微鏡

Mode-locked Ti:sapphire laser ・ center wavelength : 800 nm ・ pulse width : 100 fs ・ repetition rate : 80 MHz Incidence angle of 45 ° ・ laser spot size : 50 µ m ・ laser power : 50 mW 10 Experimental Setup (LTEM) H.Nakanishi, S.Fujiwara, K.Takayama, I.Kawayama, H.Murakami, and M.Tonouchi, Appl. Phys. Express 5, (2012).

11 Images of the polycrystalline silicon solar cell (a) Optical image (b) LTEM image (c) EL image(d) PL image(e) LBIC image

The THz emission at point 2 has higher intensity than that at point 1,3 and 4. imperfections in crystals The strong electric field at point 2 was generated by defects or dislocations. 12 Time-domain THz waveforms

I dark V photocurrent 0 bright pn junction forward bias reverse bias 13 THz emissions at various conditions

The depletion layer influences the properties of solar cell, and it can therefore be concluded that LTEM can be used as a complementary tool to the conventional evaluation methods for solar cell inspection. 14 Summary CharacteristicsLBICPLELLTEM Signal generation Photo induced current Radiative recombination Current modulation Excitation source CW laserLightCurrent injection Ultrashort pulsed laser ContactNecessaryNot necessaryNecessaryNot necessary Time resolution poorgood Excellent Electric fieldgoodpoor Excellent

15 My work GaAsTHz Pulse Probe Pump tt Δt electron hole