PV-3: Nanostructured Absorbers and Electrodes

Solar Energy Research Center for India and the United States (SERIIUS)

Activity:

Earth-abundant photovoltaics and advanced processing

Objective:

To demonstrate that controlled nanostructures can significantly improve the performance (efficiency, stability) of dye-sensitized solar cells (DSSCs) and a variety of hybrid PV devices.

Project Milestones:

P8: Develop an 8% DSSC-based device on a controlled nanocarpet of TiO2 or hybrid TiO2/ZnO (24 months).

P9: Demonstrate an 8%-DSSC-based device with new BODIPY-based dyes (48 months).

P10: Demonstrate a >10% nanostructured Si-based device (52 months).

P11: Demonstrate a functioning, proof-of-concept prototype Si/Ge nanostructure-based multijunction solar cell (36 months).

Task 1: Nanostructured acceptors

  • Washington University in St. Louis
  • Indian Institute of Technology Bombay (IITB)
  • Indian Institute of Science (IISc)

The dye-sensitized solar cell, by its very nature, is a nanostructured device. Over the past few years, the controlled synthesis of nanostructures and hybrid nanostructures has dramatically improved. In this task, we employ a set of these synthesis approaches to produce unique nanostructures with the high potential to improve the performance and stability of DSSC and sensitized devices. This includes work on improving the inherent nanostructure of the TiO2, looking at 3D junctions, and nano-composites. These structures will be grown by scalable solution-based or electrochemically based techniques.

Task 2: Donors for sensitized solar cells

  • Indian Institute of Science (IISc)
  • Washington University in St. Louis
  • Indian Association for the Cultivation of Science (IACS)
  • Arizona State University (ASU)
  • Indian Institute of Technology Bombay (IITB)

This task is coupled and uses the materials from Task 1. The focus is on improving the materials for the light harvesting and initial electron injection into the nanostructured electrode. Historically, the field has focused predominately on a variety of Ru-based dyes in this role. More recently, the ability to develop controlled semiconductor nanostructures with desired opto-electronic properties and surface chemistries has enabled potential, new donor materials. This task creates a model describing the interface and the relationship of the absorbers and defect structure, leading to the development of new materials for both 1) photon harvesting and 2) minimizing recombination through new hybrid donor systems. Coupling of Tasks 1 and 2 is designed to lead to a new generation of sensitized solar cells.

Task 3: Nanostructured Si and Si/Ge/III-V solar cells

  • Stanford University
  • Indian Institute of Technology Bombay (IITB)
  • Colorado School of Mines (CSM)

To achieve the goal of under $0.50/W and levelized cost of energy of $0.05/kWh prescribed by the U.S. DOE, there are basically two tactics: 1) increase the efficiency, and 2) decrease the manufacturing cost of energy generation.

There are two major approaches in this task:

Approach 1: To design and implement Si wire solar cells that offer several potential advantages, including i) low thickness of Si due to light trapping by wire arrays, ii) lower-quality Si (cost) since the wire diameters and total thicknesses do not exceed a few microns, iii) cell processing follows the conventional Si cell production techniques (minor modifications), and iv) anticipated reliability is that of wafer-based Si approaches. Critical design aspects—including optimum values of wire diameter, spacing, height, distribution, junction depth, carrier lifetime, and surface recombination velocity—form the critical-design and modeling components.

Approach 2: Multijunction solar cells have higher efficiency than the pervasive crystalline silicon cells. However, they have traditionally been limited by very high cost. To meet the DOE goals, we are developing disruptive Ge/III-V solar cells fabricated on a reusable Si substrate with efficiencies approaching that of the best III-V multijunction cells and with a manufacturing cost substantially below the conventional crystalline-Si PV technology. This scheme further reduces the cost through the process of transferring the absorber layers from Si substrate to less expensive glass or flexible substrates. The Si substrate is then reused to fabricate the next cell. Multijunction cells are today usually fabricated on costly Ge substrates. Thus, compared to the current Ge bottom-cell paradigm, our technique eliminates this dependency on the Ge feedstock, ingot, and wafer-supply chain. Active cell area is made through the use of plentiful, less expensive Si, Ge, and III-V source gases, as opposed to the bulk Ge wafers.