Page 232 - A Comprehensive Guide to Solar Energy Systems

Chapter 11 • Hybrid Organic–Inorganic Metal Halide Perovskite Solar Cells 235

The hybrid organic–inorganic metal halide perovskite-based materials exhibit several
outstanding optical and electrical properties, which are ideal for photovoltaic applica-
5
−1
tions. The absorption coefficient of perovskite (α > 10 cm ) is higher than for existing PV
materials such as CIGS, Si, GaAs, and CdTe with minimum Urbach energy and utilizes all
the radiations higher than the band gap energy and provides higher short circuit current
density (J SC ) even from ∼300 nm thick films [28,29]. The Urbach energy of methylammo-
nium tri-iodide is ∼15 meV, very close to the Urbach energy of high quality GaAs [30]. The
band gap of methylammonium lead tri-iodide is 1.5 eV and can be tuned with the addition
of other halide ions ranging from 1.5 to 2.3 eV [31]. A very high open circuit voltage (V OC ) is
achieved from perovskite solar cells relative to its band gap and also in comparison to oth-
er thin film solar cells [32,33]. Wolf et al. [30] found very sharp absorption edge with neg-
ligible deep defect states and minimum nonradiative recombination loss. The perovskite
materials possess three important properties such as high electron and hole mobilities in
-1 −1
2
the range of 10–60 cm V s [34–36]; long carrier lifetime as high as 1.07 µs [37]; and long
diffusion lengths (>1 µm) [38]. Chen et al. [39] found still longer carrier lifetimes and dif-
fusion lengths with the values of 30 µs and 23 µm for polycrystalline films and up to 3 ms
and 650 µs for single crystals, respectively. All these factors are responsible for the high V OC
values exhibited by perovskite solar cells.
The photo-conversion efficiency achieved by perovskite solar cells is very high, but the
stability of the solar cells in ambient environments is still a serious problem and improve-
ments are relatively very slow. The stability is the main issue preventing perovskite solar
cells from commercialization. Since the organic materials that make up perovskite solar
cells are volatile, the organic species not only can easily escape from perovskite films and
interact with moisture, but they are also not sustainable at even slightly raised tempera-
tures. When completing current–voltage measurement, performance depends on which
direction bias voltage is applied in solar cells. This does not happen in standard inorganic
solar cells such as Si, CdTe, CIGS, and GaAs. Finally, there are environmental concerns
about using hazardous materials such as lead in perovskite solar cells.
This chapter focuses on general information related to hybrid organic–inorganic metal
halide perovskite thin film and solar cells. For a detailed analysis, the readers need to fol-
low several other review papers or journal articles [15,40,41]. This chapter provides discus-
sions on device fabrication, device architecture, progress made so far, and details about
the stability of solar cells.

11.2 Thin Film Fabrication/Formation

The performance of perovskite solar cells or any other solar cells is mainly judged by the
absorber layer film quality even though other layers are also equally important. There are
several factors that determine the fabrication of high quality film and high performance
solar cells such as controlled morphology, thickness uniformity, high surface coverage with
no or minimum pinholes, material phase purity, and high crystallinity. For example, high
crystallinity of the film determines the charge separation efficiency, charge transport, and