REVIEW: “AIR-STABLE ALL-INORGANIC NANOCRYSTAL SOLAR CELLS PROCESSED FROM SOLUTION” I. GUR, N. FROMER, M. GEIER, A.P. ALIVISATOS. SCIENCE, OCT EE 235 Presentation #1 S. Swisher 3/2/09

Motivation: Existing technologies Broadband absorption Superior transport properties  High cost of materials, processing, & installation Potential cost reduction Scalable, controlled synthesis Processed in solution  Low carrier mobility  Limited spectral absorption  Environmental stability problems Inorganic bulk solar cellsOrganic solar cells

Experimental data, Part 1: Bilayer cells  Spin-cast rod shaped CdSe & CdTe NCs  Strong photoconductive effect suggests limited free carriers in dark  Depletion region with built-in field unlikely mechanism for charge separation  Rigid band model more accurate than band bending  Carrier extraction by directed diffusion: electrons and holes both go to lower energy states!

Part 2: Single material & blended cells  Bilayer vs. CdSe or CdTe only, and mixed blends  No rectification in single material  no Schottky contact  Blends performed better than single material; implies D-A junction rather than conventional P-N  Bilayer cell = highest efficiency  Very little confinement along length of rod. Free carriers move throughout NC film.

Part 3: Anneal & sinter Best Device: I SC = 13.2mA/cm 2 V OC = 0.45V FF = 0.49 Eff = 2.9%  Minimize surface traps by annealing at 400C for 15 minutes  Increase photoconductivity 2 orders of magnitude  No photo-oxidation

Colloidal Semiconductor Nanocrystals, in summary…  Ultra-thin, solution processed, stable in ambient environment  Diffusion-assisted D-A heterojunctions  Sintering enhances performance  Key development toward stable and low-cost solar energy conversion

Questions??