Characterization of 3D 

Photovoltaics SEMICONDUCTORS

Our goal is to provide industry with test structures and models of next-generation 

photovoltaics, with an initial focus on cadmium telluride (CdTe) and CuInx

Ga1-xSe2 

(CIS or CIGS) materials. These tools will enable interpretation of measured external 

properties affected by three-dimension (3D) geometry, grain structure, and nanoscale 

phase separation, supporting improved processing and design of Second Generation 

(thin film) and Third Generation (nanostructured) photovoltaic devices.

Objective

Impact and Customers

• The U.S. Photovoltaic Industry Roadmap calls for a 40-fold reduction in 

manufacturing costs by 2020. Thin film technologies such as CdTe and CIGS 

can be used on lower-cost substrates (glass, stainless steel, mylar, fabric) than 

the prevalent Si technology.

• Thin-film photovoltaics, particularly CdTe and CIGS, are the fastest-growing 

segment of the solar industry, with 43 startups in 2007 alone.

• Despite rapid growth and significant materials advantages, thin-film photovoltaics 

manufacturers have been hampered by manufacturing difficulties that stem 

from limited understanding of the detailed processing-microstructure-property 

relationships.

• The issues are compounded in so-called “Third Generation” devices proposed 

to yield enhanced efficiency and/or lower cost through the use of 3D nanoscale 

architectures.

• We are working with researchers in industry, national laboratories, and academia to develop experimental 

test structures and computational models that will lead to a better understanding of these 3D materials and 

microstructures and, ultimately, to improved manufacturability.

Approach

Materials Science and Engineering Laboratory

First Generation (thick layers of crystalline silicon) and Second Generation (thin films 

of CdTe, CIGS, and related materials) PV devices are both well established in the 

marketplace, with well defined techniques for fabrication and characterization. Third 

Generation PV encompasses a wide diversity of materials and nanostructures and 

commercially viable technologies and means of fabrication are yet to be determined. 

Many of the proposed structures have considerable microstructural variability that 

complicates interpretation of macroscopic device measures. The aim of this project is 

to develop idealized, three-dimensionally patterned templates that can serve as test 

structures to enable measurement of critical device and materials properties. 

One of the most attractive properties of CdTe and CIGS for photovoltaic applications is their amenability to a variety of costeffective fabrication methods, including sputtering, printing, and electrodeposition. We are developing three-dimensionally 

patterned photovoltaic test structures as well as fabrication processes including electrochemical deposition to enable the impact of 

geometry and internal defect structures on efficiency and broader photoelectrical properties to be determined. 

To guide and interpret the experimental measurements we are developing models of light and charge transport within arbitrary 

2D and 3D semiconductor and device geometries. These models will ultimately include the full effects of optical refraction and 

reflection that are relevant to all PV devices but play a particularly important role in 3D structures.

855-13-33d
Materials Science and Engineering Laboratory

In FY10 we substantially expanded our nascent 

effort to develop photovoltaics templates 

with 3D microstructures for application to 

3rd Generation Photovoltaics through joint 

funding with NIST’s CNST and EEEL from the 

NIST Director’s Innovation in Measurement 

Science program. A focus of this major new 

effort remains developing structures with 

adjustable, well-defined geometries that 

are applicable to a broad set of materials 

and consistent with insightful measurements 

and interpretation of the properties of 3D 

photovoltaics. 

In collaboration with the CNST we have 

developed the capability to fabricate 

photovoltaic test structures using a set of 

interdigitated back-contact electrodes. 

These electrodes are used to independently 

electrodeposit one or more of the p- and ntype semiconductors that constitute the active 

device. After deposition, the same contacts 

are also used to extract light-generated 

current from the device as well as for electrical 

characterization.

Initial studies focused on devices composed 

solely of CdTe. By independently adjusting 

the potential of each set of electrodes, 

different stoichiometries were obtained, 

allowing the deposition of p- and n-type 

material from the same electrochemical 

bath on the two different electrodes. 

These structures demonstrated the proof 

of concept for more typical devices based 

on two different semiconducting materials 

(heterojunctions). Electrical, optical, and 

structural characterization was performed, in 

concert with the CNST and CSTL. 

More recently we extended the study of these 

structures to include heterojunctions, which 

should lead to better device performance and 

significantly expand the range of industrially 

relevant materials and combinations that 

we can study. Thus far, we have combined 

our extant CdTe electrodeposition capability 

with sputtered indium-tin-oxide (ITO), 

electrodeposited CdS, and sol-gel deposited 

titanium oxide.

To support this experimental effort, we have 

developed a photovoltaic device model that 

gives us complete control over geometry and 

microstructure. We implemented our device 

model in the FiPy partial differential equation 

solver package. This freely available 

package, developed within the Metallurgy 

Division, allows us to easily distribute our 

photovoltaic codes to researchers in industry 

and elsewhere as they are validated. 

Because it was designed to model phase 

transformations, FiPy will also allow us 

to examine the effect of microstructures 

obtained from rigorous thermodynamic and 

kinetic modeling. 

Using an early generation model, we were 

able to create simulations of the external 

quantum efficiency (EQE) that successfully 

captured major trends in the measured 

performance of CdTe devices over a range 

of illuminating wavelengths for three different 

device geometries.

Accomplishments

Daniel Josell

Carlos Beauchamp

Carlos Hangarter

Albert Davydov

Abhishek Motayed (MSEL)

Nikolai Zhitenev

Behrang Hamadani

Henri Lezec

Suyong Jung (CNST)

David Gundlach (EEEL)

Lee Richter, Albert Fahey (CSTL)

Jonathan E. Guyer

(Metallurgy Division)

(301) 975-5329

guyer@nist.gov

www.nist.gov/metallurgy

Learn More Publications

Josell D, Beauchamp CR, Jung S, Hamadani BH, Motayed A, Richter LJ, Williams M, Bonevich 

JE, Shapiro A, Zhitenev N, Moffat TP, Three Dimensionally Structured CdTe Thin-Film Photovoltaic 

Devices with Self-Aligned Back-Contacts: Electrodeposition on Interdigitated Electrodes, Journal of 

the Electrochemical Society 156:H654 (2009)

Guyer JE, Wheeler D, Warren JA, FiPy: PDEs with Python, Computing in Science and Engineering 

11:6 (2009)

The CdTe grains coarsen during annealing 

Comparison of experimental and simulated External Quantum Efficiency for different geometries

A photovoltaic with CdTe on one platinum 

electrode with ITO over the entire device.