Page 237 - A Comprehensive Guide to Solar Energy Systems

240 A COMPREHENSIVE GUIdE TO SOLAR ENERGy SySTEMS

11.3.2 Planar Structure

In planar structure of perovskite solar cells, the mesoporous scaffold layer is removed and
only the perovskite absorber layer is kept between the ETL and HTL. This planar structure
can be considered as an evolution of the mesoporous structure. Its simplified device structure
has attracted the interest of researchers working in the area of thin film PV cells. In the n-i-p
planar structure, the solar cell is illuminated through ETL side whereas in the p-i-n planar
structure, the solar cell is illuminated through HTL side as shown in Fig. 11.3C and d. The first
successful demonstration of a planar structure only had an efficiency of 4% due to the inferior
film quality and inadequate absorption of the perovskite film [63]. Today, the planar struc-
ture shows a similar efficiency performance to the mesoporous structure. The actual solar cell
structure in the two cases, consists of glass/TCO/ETL/perovskite/HTL/metal and glass/TCO/
HTL/perovskite/ETL/metal, respectively [26,43,47,63,64]. The efficiency of planar n-i-p solar
cells has been increasing continuously with new developments and the present day highest
efficiency is about 19% [50,65]. The J–V curves in perovskite solar cells do not coincide while
scanning from forward to reverse and reverse to forward unlike in other regular solar cells
such as CdTe, CGTS, and Si. This behavior is more severe in the n-i-p planar structure.
The inverted p-i-n planar structure resembles typical organic solar cells. The tradi-
tional organic transport layers such as [poly(3,4-ethylenedioxythiophene) polystyrene
sulfonate] (PEdOT:PSS) and fullerence derivative [[6,6]-phenyl-C61-butyric acid methyl
ester (PCBM)] are directly implemented as in the HTL and ETL layers of n-i-p perovskite
solar cells. Similarly, instead of using FTO, tin-doped indium oxide (ITO) is preferred in
the p-i-n structure as shown in Fig. 11.3. The efficiency of the p-i-n planar structure has
been improving with the better selection of fullerence derivatives from the initial 3.9%
efficiency to present day 18.9% [63,66–68]. The commonly used HTLs in the p-i-n structure
are PEdOT: PSS, PTAA (poly-triarylamine), and NiO x and ETLs are PCBM, PC 61 BM, C 60 ,
ZnO, and combinations of them [66,69–72]. The current–density voltage characteristics
with J–V parameters and external quantum efficiency (EQE) data of perovskite solar cells
in planar structure are shown in Fig. 11.5 [67].

11.4 Device Optimization

In the development of these solar cells, device optimization is an ongoing process and
for efficient performance, each layer needs to be optimized. Since some of the layers in
perovskite solar cells are already very well optimized, emphasis has been focused on the
optimization of the three main layers such as the ETL, the perovskite absorber layer, and
the HTL. This has achieved an efficiency of more than 22% over a short time period. This
is a significant gain especially when compared to the progress made in other thin film
technologies. As far as the optimization of the absorber layer is concerned, the focus has
been on controlling the precursor solution, solution and film processing, perovskite com-
position, and the interface properties with the aim of obtaining smooth, pin-hole free
perovskite films consisting of large grains with good crystallinity. The following optimiza-
tion has been done to improve the absorber layer in perovskite solar cells.