A Caltech Library Service

Rational Design of Zinc Phosphide Heterojunction Photovoltaics


Bosco, Jeffrey Paul (2014) Rational Design of Zinc Phosphide Heterojunction Photovoltaics. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/09NG-5E90.


The prospect of terawatt-scale electricity generation using a photovoltaic (PV) device places strict requirements on the active semiconductor optoelectronic properties and elemental abundance. After reviewing the constraints placed on an "earth-abundant" solar absorber, we find zinc phosphide (α-Zn3P2) to be an ideal candidate. In addition to its near-optimal direct band gap of 1.5 eV, high visible-light absorption coefficient (>104 cm-1), and long minority-carrier diffusion length (>5 μm), Zn3P2 is composed of abundant Zn and P elements and has excellent physical properties for scalable thin-film deposition. However, to date, a Zn3P2 device of sufficient efficiency for commercial applications has not been demonstrated. Record efficiencies of 6.0% for multicrystalline and 4.3% for thin-film cells have been reported, respectively. Performance has been limited by the intrinsic p-type conductivity of Zn3P2 which restricts us to Schottky and heterojunction device designs. Due to our poor understanding of Zn3P2 interfaces, an ideal heterojunction partner has not yet been found.

The goal of this thesis is to explore the upper limit of solar conversion efficiency achievable with a Zn3P2 absorber through the design of an optimal heterojunction PV device. To do so, we investigate three key aspects of material growth, interface energetics, and device design. First, the growth of Zn3P2 on GaAs(001) is studied using compound-source molecular-beam epitaxy (MBE). We successfully demonstrate the pseudomorphic growth of Zn3P2 epilayers of controlled orientation and optoelectronic properties. Next, the energy-band alignments of epitaxial Zn3P2 and II-VI and III-V semiconductor interfaces are measured via high-resolution x-ray photoelectron spectroscopy in order to determine the most appropriate heterojunction partner. From this work, we identify ZnSe as a nearly ideal n-type emitter for a Zn3P2 PV device. Finally, various II-VI/Zn3P2 heterojunction solar cells designs are fabricated, including substrate and superstrate architectures, and evaluated based on their solar conversion efficiency.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:Zinc phosphide; solar energy; photovoltaics; earth abundant; semiconductors; molecular-beam epitaxy; x-ray photoelectron spectroscopy; energy-band alignment
Degree Grantor:California Institute of Technology
Division:Chemistry and Chemical Engineering
Major Option:Chemical Engineering
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Atwater, Harry Albert
Thesis Committee:
  • Giapis, Konstantinos P. (chair)
  • Lewis, Nathan Saul
  • Flagan, Richard C.
  • Atwater, Harry Albert
Defense Date:30 May 2014
Non-Caltech Author Email:Jeff.p.bosco (AT)
Funding AgencyGrant Number
Record Number:CaltechTHESIS:06052014-153503097
Persistent URL:
Related URLs:
URLURL TypeDescription growth of zinc phosphide alignment of II-VI/Zn3P2 heterointerfaces alignment of ZnS/Zn3P2 heterojunctions passivation of Zn3P2 surfaces mechanism of Zn3P2 on GaAs(001) of doped ZnS emitters
Bosco, Jeffrey Paul0000-0002-4570-6934
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:8488
Deposited By: Jeffrey Bosco
Deposited On:06 Jun 2014 22:16
Last Modified:08 Nov 2023 00:12

Thesis Files

PDF (Thesis, two-sided) - Final Version
See Usage Policy.

PDF (Thesis, one-sided) - Final Version
See Usage Policy.


Repository Staff Only: item control page