1. Piezoelectric photothermal (PPT)
spectroscopy of solar cells for hybrid PV-
thermal solar energy applications
Nadal Sarkytbayev
A literature review submitted in fulfilment of the requirements for the degree of Physics and
the Diploma of Imperial College London
30 June 2016

2. Introduction
The solar cell is the essential building block for all solar photovoltaic effects. These
semiconductor devices have been developed for since late 1950 and have various
applications. Photovoltaic solar cell is the key device that converts solar radiation into useful
electrical energy [1]. Solar cells are also associated with fundamental energy losses during
the process of energy conversion. Therefore, understanding what goes on inside the material
is crucial to explain and prevent useful energy dissipation therefore increasing solar cell
efficiency. Currently the maximum efficiency commercially available is 46% for four-
junction solar cells [2a] which means that half of the sun’s radiation is still not converted into
electricity but wasted in the form of heat wave or lattice vibrations know as phonons. This
energy dissipation can be explained by fundamental limitations of photovoltaic solar cells
[2b], impurities inside the material structure or by other non-radiative processes.
In this paper we are looking to investigate these non-radiative recombination processes by
performing piezoelectric photothermal spectroscopy (PPTS) for silicon based solar cells and
quantum wires. Using the setup described in experiment procedure, we will be able to
demonstrate quantitatively the amount of heat loss from the system and qualitatively give
details where and how it has been lost inside the semiconductor solar cell.
Generationand Recombination
All of the semiconductors work on the basis of the idea of band gap (𝐸𝑔 = 𝐸𝑐 − 𝐸𝑣) that is the
region of space in electronic band structure of solid between top most filled of the valance
band (VB) and bottom of unoccupied conduction band (CB). Only photons with energy
greater than bandgap (𝐸 𝑝ℎ𝑜𝑡𝑜𝑛 > 𝐸𝑔 ) can promote electrons from valance band to conduction
band creating electron-hole pair, which conducts electricity. The photons with energy lower
than bandgap pass through the material and are known as below 𝐸𝑔 loss [2b].
There are two types of band gaps that appear inside the material, direct (Figure 1 (a)) and
indirect gaps (Figure 1 (b), (c)). The only distinctions between them is that for indirect gap
Figure 1: Energy momentum (E-k) diagram shows: (a) photon absorption in direct bandgap,
(b) photon absorption in an indirect bandgap semiconductor assisted by phonon absorption
(c) photon absorption in an indirect bandgap semiconductor assisted by phonon emission.
(Image source [3])

3. CB occurs at different momentum k (Figure 1 (a), (b)) with respect to VB and momentum
must be conserved. As the conduction band is not directly above the valance band phonon of
momentum 𝑘 𝑝 is required to promote a carrier through. In Figure 1(b) photon with energy
less than bandgap (𝐸 𝑝ℎ𝑜𝑡𝑜𝑛 < 𝐸𝑔 ) is assisted by simultaneous absorption of phonon which
provides enough energy and momentum for excitation. On the other hand Figure 1(c) photon
that has excess of energy (𝐸 𝑝ℎ𝑜𝑡𝑜𝑛 > 𝐸𝑔 ) promotes electron from valance band to conduction
band quickly undergoes the process of thermalisation where it emits a phonon and creates an
electron-hole pair. This process can be described microscopically as multiple collisions of
electron with the lattice structure passing some of the extra kinetic energy to produce these
lattice vibrations until equilibrium with the surrounding is achieved.
The process of generation is also known as electronic excitation which increases the number
of free carriers when energetic photons are absorbed. Generally followed by the process of
thermal recombination where relaxation event takes place which decreases the number of free
carriers [4]. In contrast to generation which has only one main process there are three
Figure 2: Free carrier recombination process in semiconductors. (Image source [5])
different methods of recombination. All of them are important in photovoltaic devices. These
processes can also be divided up into two groups: avoidable processes which mainly due to
imperfection in material or defects and unavoidable recombination processes which are basic
physical mechanics that naturally arise inside the material [1,4].
For photovoltaic devices one of the most important is radiative recombination (Figure 2:
band-to-band recombination) which basically is spontaneous emission where electron and
hole recombine and release a photon of energy equal to bandgap 𝐸𝑔 . Also there is a process
of non-radiative recombination or trap-assisted recombination which essentially is avoidable
process as normally includes relaxation by way of localised trap state 𝐸𝑡 that appear due to
impurities in the crystal [6]. Third method, Auger recombination, is an example of
unavoidable process which includes 2-part interaction. Normally electrons and a hole
recombine across the bandgap and that energy (𝐸𝑔 ) instead of being released as a photon is
being transferred into other second corresponding carrier increasing its kinetic energy [4].
Auger recombination becomes significant for materials with low band gap semiconductors
with high electron densities.

4. The main purpose of this research will be based on investigating non-radiative recombination
using PPTS [6,7,9] and is detected by piezoelectric transducer (PZT) that senses temperature
changes and generates voltage [8]. In the next section we will describe the experimental set
up and techniques implemented for our investigation of solar cells.
Experimental Procedure
Piezoelectric photothermal spectroscopy allows us to evaluate the thermal waves (phonons)
produced as a result of non-radiative recombination inside the solar cells [6,7,9]. These
generated phonons are detected and measured by piezoelectric transducer (PZT) that is
typically attached to the back of the solar cell [8]. Two methods have been investigated in the
early paper Ikari et al (1987) where they used two type of detector geometries: ring-shaped
and disc-shaped PZT. According to [ref 10] signal acquired by the disc-shaped PZT is more
than ten times larger than ring shape due to contact area being greater. The PZT is glued onto
the rear side of the sample using silver conducting paint as it has good thermal properties and
can easily be removed for the next sample analysis. Electrodes that are attached to the sample
Figure 3:
Experimental setup: PZT detector attached to the sample. (Image source [10])
will produce voltage fluctuation in response to the non-radiative recombination process
[8,11,12]. The whole system is isolated inside cryostat to remove any temperature
disturbances and light absorption from the surrounding. The experiment is carried out at
various temperatures to understand samples performance at different scales [9]. The
wavelength of light covering the range of solar spectrum determines the quantity of photons
absorbed and transformed into phonons. From PPTS we will have useful data about
photovoltaic conversion efficiency of solar cells and detail analysis of non-radiative
recombination process. Previous work Imai et al (2004) suggests significant advantage of
PPTS that enables direct observation and heat detection of non-radiative recombination using
PZT. Since our experiment requires high sensitivity for very thin sample thickness accurate
setup [8]and isolation [12] is essential for achieving reliable and precise data. In addition,
silicon based solar cells haven’t been investigated in great depth using PPTS to describe
phonons therefore some new ideas and results are anticipated throughout the research.

5. References:
[1] Jenny Nelson “The Physics of Solar Cells” Imperial College Press, UK 2002.
[2a] Fraunhofer-Institut für Solare Energiesysteme ISE “Press Release” 2014
[2b] L. C. Hirst, N. J. Ekins-Daukes. “Fundamental Losses In Solar Cells”
[3] Image source: http://ecee.colorado.edu/~bart/book/book/chapter4/ch4_6.htm#fig4_6_2
[4] S.M. Sze “Semiconductor Devices Physics and Technology” 2nd Edition 2001.
[5] Image source: http://ecee.colorado.edu/~bart/book/recomb.htm
[6] Ping Wang, Kentaro Sakai, Atsuhiko Fukuyama, and Tetsuo Ikari “Investigation of
Carrier Recombination Processes in GaAs/AlAs Multiple Quantum Wells Using
Piezoelectric Photothermal and Surface Photovoltage Techniques” Japanese Journal of
Applied Physics 48 (2009) 07GB01
[7] Eiki Kawano, Yuki Uchibori, Takashi Shimohara, Hironori Komaki, Ryuji Katayama,
Kentaro Onabe, Atsuhiko Fukuyama and Tetsuo Ikari “Piezoelectric Photothermal and
Photoreflectance Spectra of InxGa1-xN Grown by Radio-Frequency Molecular Beam
Epitaxy” Japanese Journal of Applied Physics Vol. 45, No. 5B, 2006, pp. 4601–4603
[8] Ping Wang, Masaki Tada, Masashi Ohta, Kentaro Sakai, Atsuhiko Fukuyama and Tetsuo
Ikari “Piezoelectric Photothermal Study of the Optical Absorption Spectra of
Microcrystalline Silicon” Japanese Journal of Applied Physics Vol. 43, No. 5B, 2004,
pp. 2965–2968
[9] Taketo Aihara, Atsuhiko Fukuyama, Yuki Yokoyama, Michiya Kojima, Hidetoshi
Suzuki, Masakazu Sugiyama, Yoshiaki Nakano, and Tetsuo Ikari “Detection of
miniband formation in strain-balanced InGaAs/GaAsP quantum well solar cells by using
a piezoelectric photothermal spectroscopy” Journal of Applied Physics 116, 044509
(2014)
[10] T. Ikari, S. Shigetomi, Y. Koga, H. Nishimura,H. Yayama and A. Tomokiyo “Low-
temperature photoacoustic spectra of Bii3 single crystals” Physical Review B (1987)
[11] Kenji Imai, Shin-ichi Fukushima, Tetsuo Ikari and Masahiko Kondow “Investigation of
the Electron Nonradiative Transition in Extremely Thin GaInNAs/GaAs Single Quantum
Well by Using a Piezoelectric Photothermal Spectroscopy” Japanese Journal of Applied
Physics Vol. 43, No. 5B, 2004, pp. 2942–2945
[12] Atsuhiko Fukuyama, Takahiro Kuroki, Kentaro Sakai, Tomohisa Iwamoto2, Shoji
Furukawa, and Tetsuo Ikari “Nonradiative Investigation of Photodecomposition of
Poly(di-n-hexylsilane) Thin Films Using Piezoelectric Photothermal Spectroscopy”
Japanese Journal of Applied Physics 48 (2009) 07GB02