1. Characterization of organic bulk heterojunction and silicon nanowire solar cells Katherine Song Department of Electrical Engineering, Princeton University, Princeton, NJ 08544 Sponsor/host: Bob Street Electronic Materials and Devices Laboratory, Palo Alto Research Center, Palo Alto, CA 94304 Effect of series resistance on photocurrent analysis Analysis of generation via photocurrent vs. voltage measurements key to understanding recombination in organic solar cells. Experimentally measured photocurrent not necessarily reflective of internal photocurrent. P EXP (V) (normalized experimentally measured photocurrent) at a fixed illumination evaluated by measuring voltage dropped across a load resistor of known value with a lock-in amplifier: Proposed model to correct for series resistance effects: Extract values for dark saturation current J 0, ideality factor n, and series resistance R S from measured dark current J D (V). Since experimentally-measured photocurrent eGP EXP (V) = J D (V) – J L (V), “Corrected” internal photocurrent depends on internal voltage V INT = V – J L (V)R S instead of external voltage V. Applying the model to experimental data: Organic bulk heterojunction (BHJ) solar cells Organic solar cells attractive because of their low cost, low weight, and flexibility. BHJ solar cells: a polymer-fullerene blend spontaneously phase separates to form a vertically-structured cell with nanoscale heterojunctions. Studied cells based on 2 different blends: poly(3-hexylthiophene) (P3HT) / [6,6]-phenyl C61-butyric acid methyl ester (PCBM) Power conversion efficiency (PCE) ~ 4-5%. poly[N-9’-hepta-decanyl-2,7-carbazole-alt-5,5- (4’,7’-di-2-thienyl-2’,1’,3’-benzothiadiazole)] (PCDTBT) / [6,6]-phenyl C 70 -butyric acid methyl ester (PC 70 BM) PCE ~ 6%. Cells fabricated by S. Cowan at the University of Santa Barbara. Typical structure shown in figure on right [1]. Silicon nanowire solar cells Concept: enhance light scattering and absorption by increasing surface area. Advantage: easily incorporated into existing a-Si solar cell fabrication process. Structure [2]: Photocurrent vs. voltage (DC measurement): Shape of curve suggests significant series resistance. Close-up of power-producing quadrant: Spectral response measurements Photoconductivity spectrum useful for extracting interface band gap and band offset (difficult to determine from optical absorption measurements). Band offset provides energy to split an exciton into a separate electron and hole. Experiment: Solar cell illuminated with monochromatic light source chopped at 230 Hz. Voltage dropped across a 9.1kΩ load resistor measured with lock-in amplifier. 100kΩ resistor used instead at low energies to amplify weak signal. 715, 850, and 1000 nm cutoff filters used to suppress scattered light at low energies. Spectrum normalized to incident power, measured by calibrated Si and Ge photodiodes. Optical absorption coefficient, α(ħω) obtained from photocurrent (I PC ) spectrum: I PC (ħω)=I 0 [1-exp(-α(ħω)d)] I 0 : photoconductivity at complete absorption; determined by fitting spectrum to optical absorption measurements (inset in graph below) d: thickness of cell (~100nm) Data: Changes in slope of spectrum indicate location of absorption bands. Bulk optical band gap ~ 1.9 eV for both types of cells. Interface band gap ~ 1.2 eV for P3HT/PCBM cells; ~ 1.4 eV for PCDTBT/PC 70 BM cells. References: 1. Sung Heum Park, Anshuman Roy, Serge Beaupré, Shinuk Cho, Nelson Coates, Ji Sun Moon, Daniel Moses, Mario Leclerc, Kwanghee Lee & Alan J. Heeger, Nature Photonics 3, 297 - 302 (2009). 2. Figure and image from Sourobh Raychaudhuri. 55 nm ITO 15 a-Si P+ 100 nm a-Si Intrinsic 70 nm a-Si N+ Un-doped Si NW band offset donor (polymer) donor (polymer) acceptor (fullerene) acceptor (fullerene) band offset interface band gap bulk band gap