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Upconversion and Downconversion

Processes for Photovoltaics

Aruna Ivaturi*, Hari Upadhyaya**
*UNIVERSITY OF STRATHCLYDE, GLASGOW, UNITED KINGDOM; **WOLFSON CENTRE FOR
MATERIALS PROCESSING, INSTITUTE OF MATERIALS AND MANUFACTURING, DEPARTMENT
OF MECHANICAL, AEROSPACE AND CIVIL ENGINEERING, BRUNEL UNIVERSITY, LONDON,
UXBRIDGE, UNITED KINGDOM
hari.upadhyaya@brunel.ac.uk, aruna.ivaturi@strath.ac.uk

13.1 Introduction

In all photovoltaic (PV) devices, apart from carrier recombination and parasitic resistance
related losses, there are primarily two main loss mechanisms arising due to the absorption
threshold of the absorber material [1,2]. All the incident photons with energy less than
this threshold are not absorbed and hence do not significantly contribute to the genera-
tion of electron-hole pairs. These losses are called the sub-bandgap or transmission losses.
For example, in the case of crystalline silicon solar cells, about 20% of the sun’s energy
(AM1.5 solar spectrum) is lost owing to these losses (see Fig. 13.1) [3]. On the other hand,
all the incident photons with energy greater than the absorption threshold give rise to
lattice thermalization losses because of the excess energy that is transformed into heat.
This loss mechanism accounts for approximately 35% of the sun’s energy for a crystalline
silicon device [3]. Besides these two primary losses related to the intrinsic properties of the
absorber material, there are other losses more related to the electronic properties of solar
cells: (1) contact voltage losses, (2) recombination losses due to poor interface or material
quality, (3) junction losses, and (4) reflection losses from interfaces. All these fundamental
losses directly lead to an efficiency limit of ∼30% for single-junction PV devices under
nonconcentrated AM1.5 illumination—this is the so-called Shockley–Queisser theoretical
efficiency limit (S-Q limit) [4].
One generic approach to address the fundamental losses arising from the mismatch be-
tween the incident photon energy and the absorber bandgap (or the absorption threshold)
is via manipulating the sunlight prior to conversion also termed as photon conversion. The
sub-bandgap or transmission losses can be addressed via a process called upconversion (UC)
whereas the lattice thermalization losses can be addressed via downconversion (DC) [1].
This chapter gives a brief overview of the upconversion and downconversion concepts,
materials, and integrated PV devices reported in the literature for performance enhancement.

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