Bifacial n- type crystalline silicon solar cells Bifacial n- type crystalline silicon solar cells Valentin D. Mihailetchi, A. Halm, A. Edler, G. Galbiati, L. J. Koduvelikulathu, R. Valentin D. Mihailetchi, A. Halm, A. Edler, G. Galbiati, L. J. Koduvelikulathu, R. Roescu, C. Comparotto, R. Kopecek, K. Peter, Roescu, C. Comparotto, R. Kopecek, K. Peter, International Solar energy research Center (ISC), Konstanz (Germany) International Solar energy research Center (ISC), Konstanz (Germany) 1 bifiPV workshop, Konstanz, April 2012
Outline Outline Motivation • why bifacial cells/modules? • installation possibilities of bifacial modules • n-type solar cell concepts Fabrication of bifacial n-type solar cells • front and back contacted cells • all back contacted cells (IBC) • all back contacted cells (IBC) • our best cell results (monofacial measurement) Preliminary tests of several small modules • standard cell versus back contact cell • indoor and outdoor tests Summary 2 bifiPV workshop, Konstanz, April 2012
Motivation Motivation Why bifacial cells & modules? A bifacial panel generates electricity from ambient light reflected off surrounding surfaces, and combines with power from the front face of the panel. Advantages: • increase power generation per m 2 • enables owners to generate more kWh per KWp • enables owners to generate more kWh per KWp • … Disadvantages: • more expensive fabrication !? • power output depends on installation site and conditions • difficult to quantify and predict the gain due to bifacial contribution • … with bifacial (€/kWh) < without bifacial (€/kWh) ??? 3 bifiPV workshop, Konstanz, April 2012
Motivation Motivation Installation possibilities of bifacial modules Vertical Slanted Horizontal Direct Direct Direct sunlight sunlight sunlight Direct sunlight Reflected Reflected sunlight sunlight Reflected Reflected Reflected sunlight sunlight sunlight Few application possibilities: Balconies, Bus Shelters, Walkway Covers, Deck & Porch Coverings, Canopies, Carports, Fences, Facades, Siding, Trellises, Tracking Systems, BIPV, Ground Mount, Roof Mount, and more…! 4 bifiPV workshop, Konstanz, April 2012
Motivation Motivation Cell fabrication and module installation Horizontal Slanted Direct sunlight Direct sunlight Direct Direct Reflected Reflected > Reflected Reflected light light light light sunlight sunlight Reflected sunlight Cells (or modules) fabrication: • can have asymmetric front/rear performance • should be optimized based on front side performance Preferred orientation: • South (North) for slanted 5 bifiPV workshop, Konstanz, April 2012
Motivation Motivation Cell fabrication and module installation Vertical Direct Direct sunlight sunlight Front Rear if = sunlight sunlight Reflected Reflected sunlight sunlight Cells (or modules) fabrication: • could have symmetric front/rear performance Preferred orientation: • East – West 6 bifiPV workshop, Konstanz, April 2012
h N-type solar cell concepts N-type solar cell concepts HIT solar cells • front, emitter: a-Si (p) • back, BSF: a-Si (n) • bifacial (Sanyo’s HIT double) • h ≤ 23% (Sanyo) IBC solar cells • FSF/BSF: n + /n ++ diffusion • FSF/BSF: n + /n ++ diffusion • emitter: p + diffusion • h ≤ 24.2% (Sunpower) Standard solar cells • front, emitter: p + diffusion • back, BSF: n + diffusion • bifacial • h ≤ 19.6% (Yingli) 7 bifiPV workshop, Konstanz, April 2012
Outline Outline Motivation • why bifacial cells/modules? • installation possibilities of bifacial modules • n-type solar cell concepts Fabrication of bifacial n-type solar cells • front and back contacted cells • all back contacted cells (IBC) • all back contacted cells (IBC) • our best cell results (monofacial measurement) Preliminary tests of several small modules • standard cell versus back contact cell • indoor and outdoor tests Summary 8 bifiPV workshop, Konstanz, April 2012
Fabrication of bifacial n-type solar cells Fabrication of bifacial n-type solar cells N-type cell process development at ISC-Konstanz Standard solar cells IBC solar cells (ZEBRA) • n-type Cz substrate • n-type Cz substrate • n-type Cz substrate • n-type Cz substrate • 156 × 156 mm 2 area • 156 × 156 mm 2 area • homogeneous diffusion • homogeneous diffusion • front side textured, rear side flat or • front side textured, rear side flat textured • industrial partner: • industrial partner: 9 bifiPV workshop, Konstanz, April 2012
Fabrication of bifacial n-type solar cells Fabrication of bifacial n-type solar cells N-type cell process development at ISC-Konstanz Standard solar cells IBC solar cells (ZEBRA) BBr 3 diffusion, p + emitter BBr 3 diffusion, p + emitter POCl 3 diffusion, n + BSF POCl 3 diffusion, n + BSF Saw damage etching, alkaline texturization, cleaning steps Saw damage etching, alkaline texturization, cleaning steps SiO 2 /SiN x passivation & ARC SiO 2 /SiN x passivation & ARC PECVD masking PECVD masking Laser ablation Laser ablation Screen printing and firing of p and n fingers, busbars Screen printing and firing of p and n fingers, busbars 10 bifiPV workshop, Konstanz, April 2012
Fabrication of bifacial n-type solar cells Fabrication of bifacial n-type solar cells N-type cell process development at ISC-Konstanz Standard solar cells IBC solar cells (ZEBRA) Both cell concepts are bifacial !!! 11 bifiPV workshop, Konstanz, April 2012
h Fabrication of bifacial n-type solar cells Fabrication of bifacial n-type solar cells Best cell results (monofacial) area J SC V OC FF P max Cell type [ cm 2 ] [ mA/cm 2 ] [ mV ] [ % ] [ % ] [W] Standard (both sides contacted) 241 38.8 643 77.6 19.4 4.66 ZEBRA (back contacted) 238.9 41.2 646 79.1 21.0 5.03 Highlights: 156×156 mm 2 Cz substrates! • • pseudo-FF ≥ 82% → no indication of shunts! • implied Voc ≥ 670 mV → good surface passivation! What is the bifacial performance? 12 bifiPV workshop, Konstanz, April 2012
Fabrication of bifacial n-type solar cells Fabrication of bifacial n-type solar cells Optimization route for best bifacial performance η front (%) η rear / η front Cell type optimized for best: Standard monofacial 19.4 0.75-0.8 Standard bifacial* 18.0** 0.91*** * compared with monofacial cell, it has larger rear side metal finger pitch and higher BSF sheet resistance ** FF of ≤ 72% due to higer series resistance *** ratio different than 1 because of a flat rear side (different Jsc values) 100 100 90 80 70 IQE [%] 60 Monofacial optimized cell : 50 front illumination 40 rear illumination 30 Bifacial optimized cell : 20 front illumination rear illumination 10 Cells with symmetric front & rear performance can be fabricated !! 0 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 Wavelength [µm] 13 bifiPV workshop, Konstanz, April 2012
Outline Outline Motivation • why bifacial cells/modules? • installation possibilities of bifacial modules • n-type solar cell concepts Fabrication of bifacial n-type solar cells • front and back contacted cells • all back contacted cells (IBC) • all back contacted cells (IBC) • our best cell results (monofacial measurement) Preliminary tests of several small modules • standard cell versus back contact cell • indoor and outdoor tests Summary 14 bifiPV workshop, Konstanz, April 2012
Preliminary tests of several small modules Preliminary tests of several small modules One cell module: for bifacial test Back sheet variation: black, white, transparent Indoor measurements (as reference): @ 1 sun and 25 ° C • front versus rear performance ratios η rear / η front (module) η rear / η front (cell) module type back sheet Standard transparent 0.85 0.79 ZEBRA transparent 0.75 not measured 15 bifiPV workshop, Konstanz, April 2012
Preliminary tests of several small modules Preliminary tests of several small modules One cell module: for bifacial test Back sheet variation: black, white, transparent Indoor measurements (as reference): @ 1 sun and 25 ° C • black versus white back sheet module type back sheet gain in Isc Standard white vs. black +0.80% ZEBRA white vs. black +0.81% 16 bifiPV workshop, Konstanz, April 2012
Preliminary tests of several small modules Preliminary tests of several small modules One cell module: outdoor measurements test condition and assumptions: • ground reflectivity → ≈ 70% (300-1200nm) (white back sheet) • orientation and inclination → South, 30 ° • measurement interval → every 5 min. (10 sec. module measure time) • all modules assumed to have the same temperature during an IV scan • front glass taped black outside cell area • front glass taped black outside cell area • one additional module was used to measured the back side light intensity Photograph of the outdoor setup. 17 bifiPV workshop, Konstanz, April 2012
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