Defect and Contact Passivation for Perovskite Solar Cells.

TLDR: The focus is on the origin of the various voltage-limiting mechanisms in PSCs, and the effect of such methods on the reduction of hysteresis are described.

Abstract: Metal-halide perovskites are rapidly emerging as an important class of photovoltaic absorbers that may enable high-performance solar cells at affordable cost. Thanks to the appealing optoelectronic properties of these materials, tremendous progress has been reported in the last few years in terms of power conversion efficiencies (PCE) of perovskite solar cells (PSCs), now with record values in excess of 24%. Nevertheless, the crystalline lattice of perovskites often includes defects, such as interstitials, vacancies, and impurities; at the grain boundaries and surfaces, dangling bonds can also be present, which all contribute to nonradiative recombination of photo-carriers. On device level, such recombination undesirably inflates the open-circuit voltage deficit, acting thus as a significant roadblock toward the theoretical efficiency limit of 30%. Herein, the focus is on the origin of the various voltage-limiting mechanisms in PSCs, and possible mitigation strategies are discussed. Contact passivation schemes and the effect of such methods on the reduction of hysteresis are described. Furthermore, several strategies that demonstrate how passivating contacts can increase the stability of PSCs are elucidated. Finally, the remaining key challenges in contact design are prioritized and an outlook on how passivating contacts will contribute to further the progress toward market readiness of high-efficiency PSCs is presented.

Figures

Figure 2. a) The VOC values for the champion laboratory cells. The shaded gray area shows the difference between bandgap and Shockley-Queisser limits for given bandgap value.[37] b) The correlation between E0 and WOC for typical absorber materials. The values in the parenthesis are showing the Eg of the selected absorbers. (Reproduced with permission [2]). b) The dependency of FF on c) VOC and d) WOC for the state-of-the-art PV devices. FF values were taken from the literature for different absorber technologies[38]. For panel c, the thick vertical solid lines shows the Shockley-Queisser limits for the given bandgap. For PSCs, the FF values were taken from a=[39], b=[40], c=[41], d=[42]. All reported PSCs to have contact passivation. Shaded areas represent physically inaccessible parameter space. The dashed lines are only guide to the eye

Figure 3. Schematic illustration of typical PSCs and detailed view of possible surface defects on perovskite crystals, e.g., interstitials, substitutional and vacancies.

Table 1. Survey of some available passivation routes in the literature showing reduced hysteresis increased VOC of PSCs. (*,**)

Figure 9. a) Example of O vacancy and passivation of these defects +3 valency atoms (Reproduced with permission[151]), b) illustration of O vacancy defects on the surface TiO2 (Reproduced with permission[152]), c) Sn interstitial in SnO2 lattice. In blue are represented interstitial atoms, in green nearest-neighbor O atom, in red O, in the gray Sn (Reproduced from [150]). d) Schematic illustration of energy disorder influence on the VOC of a p-i-n device. The purple and blue dashed lines are the electron and hole quasi-Fermi levels of devices with ordered and disorder PCBM layer, respectively (Reproduced with permission[153]).

Table 2. Successful heteroatom doping in literature as defects passivation of TiO2 for PSC applications.

Figure 7. a) Schematic illustration of the self-assembled 2D/3D perovskite film structure after 2D perovskite formation with different chain lengths. (Reproduced with permission[109]) b) Schematic of the iBAI passivation treatment together with JV curves and stability graph of the passivated and non-passivated devices. (Reproduced with permission[108])

Full text

Defect and Contact Passivation for Perovskite Solar Cells
Item Type Article
Authors Aydin, Erkan; de Bastiani, Michele; De Wolf, Stefaan
Citation Aydin E, Bastiani M, Wolf S (2019) Defect and Contact Passivation
for Perovskite Solar Cells. Advanced Materials: 1900428.
Available: http://dx.doi.org/10.1002/adma.201900428.
Eprint version Post-print
DOI 10.1002/adma.201900428
Publisher Wiley
Journal Advanced Materials
Rights Archived with thanks to Advanced Materials
Download date 09/08/2022 23:52:11
Link to Item http://hdl.handle.net/10754/652896

1
DOI: 10.1002/ ((please add manuscript number))
Article type: Review
Defect and Contact Passivation for Perovskite Solar Cells
Erkan Aydin, Michele De Bastiani, and Stefaan De Wolf*
King Abdullah University of Science and Technology (KAUST), KAUST Solar Center (KSC),
Physical Sciences and Engineering Division (PSE), Thuwal 23955-6900, Kingdom of Saudi
Arabia
E-mail: stefaan.dewolf@kaust.edu.sa
Keywords: solar cells, perovskites, contacts, recombination, passivation, hysteresis, stability
Metal-halide perovskites are rapidly emerging as an important class of photovoltaic absorbers that
may enable high-performance solar cells at affordable cost. Thanks to the appealing optoelectronic
properties of these materials, tremendous progress has been reported in the last few years in terms
of power conversion efficiencies of perovskite solar cells (PSCs), now with record values in excess
of 23%. Nevertheless, the crystalline lattice of perovskites often includes defects, such as
interstitials, vacancies, and impurities; at the grain boundaries and surfaces, dangling bonds can
also be present, which all contribute to non-radiative recombination of photo-carriers. On device
level, such recombination undesirably inflates the open-circuit voltage deficit, acting thus as a
significant roadblock towards the theoretical efficiency limit of 30%. In this review, we focus on
the origin of various voltage-limiting mechanisms in PSCs and discuss possible mitigation
strategies. We describe contact passivation schemes and the effect of such methods on the
reduction of hysteresis. Furthermore, we elucidate several strategies that demonstrate how
passivating contacts can increase the stability of PSCs. Finally, we prioritize the remaining key

2
challenges in contact design and present an outlook on how passivating contacts will contribute to
further the progress towards market readiness of high-efficiency PSCs.
1. Introduction
Perovskite solar cells (PSCs) have gained rapid widespread attention for its promise as a high-
efficiency photovoltaic (PV) technology; devices now already achieve impressive power
conversion efficiencies (PCEs) of more than 23%.
[1]
Such performance can largely be attributed
to the remarkable optoelectronic properties of perovskites which combine, for instance, a high
absorption coefficient with low Urbach energy (E
0
).
[2]
On device level, a high absorption
coefficient enables photocurrents close to the theoretical maximum without the need for
complicated light-trapping schemes, whereas a low E
0
is essential to obtain a low open-circuit-
voltage deficit W
OC
= E
g
/q – V
OC
, where
E
g
is the bandgap, q is the elementary charge, and V
OC
is
the open circuit voltage.
[3]
Thus far, one of the highest reported V
OC
is 1.24 V
[4]
(for a 1.6 eV band
gap perovskite absorber), which is below the theoretical value of 1.32 V, obtained when only
considering radiative recombination.
[5]
Similar to other thin-film PV materials such as gallium-
arsenide (GaAs), copper-indium-gallium-selenide (CIGS), cadmium-telluride (CdTe), perovskite
films often feature intrinsic defects, such as interstitials and vacancies, as well as impurities and
non-coordinated ions at their grain boundaries and surfaces. Such defects can result in gap states
that induce non-radiative recombination of photo-generated carriers. Such recombination
undesirably lowers the operating voltages under open circuit as well as maximum power point
(MPP) conditions. The latter detrimentally affects the fill factor (FF) of devices and thus their
overall power output. The presence of such defects also contributes to hysteresis in the current-
voltage characteristics. To translate the attractive optoelectronic properties of perovskites into
higher device performance and to continue progress towards realizing their theoretical PCE limit

3
of 30%, further device engineering is therefore essential. In the development of crystalline silicon
(c-Si) solar cells, the use of high-quality silicon wafers, combined with effective surface- and
contact-passivation strategies pushed experimental operating voltages close to their theoretical
limit,
[6]
and device efficiencies as high as 26.7%.
[7]
The performance of other PV technologies,
such as CdTe, CIGS and CZTS thin-film solar cells, also benefits from the use of surface-
passivation strategies.
[8]
Taking inspiration from these improvements, we argue that a deeper
understanding of the specific defect physics of perovskites and the use of passivation strategies,
combined will lead to performance improvements in PSCs.
In this review, we first consider how unintentional bulk defects may be eliminated by increasing
the quality of perovskite crystals. Next, we describe available extrinsic defect-passivation methods
to minimize interfacial and grain-boundary (GB) recombination losses. To improve device
performance, we focus on surface-passivating materials that can be integrated into the contact
stacks of devices without jeopardizing carrier extraction, with an in-depth discussion of their
underlying passivation mechanisms. Finally, we correlate passivation schemes with their effect on
minimized hysteresis, increased PCE (focusing on V
OC
improvement) and enhanced stability. We
conclude by highlighting the remaining challenges in contact design that need to be solved and by
providing an outlook on how passivating contacts will bring forward the market readiness of high-
efficiency PSCs.
2. Role of Defects in Voltage and Hysteresis in Perovskite Solar Cells
The V
OC
of a solar cell is directly related to the splitting of the electron and hole quasi-Fermi levels
in its semiconducting absorber (and thus its excess-charge carrier densities, n and p) excited
under specific illumination conditions, usually at one sun. In turn, n (usually, n  p) directly

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depends on the effective free-carrier lifetime, 
eff
, through 
eff
= n/U
eff
, where U
eff
is the effective
recombination rate. Notably, 
eff
is a parameter that is fairly easily experimentally extracted and
used to assess the electronic quality of a semiconductor. The fact that 
eff
can strongly depend on
n underlines the importance of measuring 
eff
under excitation conditions that are representative
for actual device operation, as well as always reporting the n value at which 
eff
was measured.
As recombination rates are additive in semiconductors, 
eff
depends in perovskite absorbers on
their bulk and surface recombination, according to:
[9]
(1)
In this, the bulk carrier lifetime, 
bulk
, depends on the trap- or defect-induced (also referred to as
Shockley-Read-Hall, τ
SRH
) recombination, band-to-band radiative recombination (which is the
reciprocal process of photon absorption, τ
rad
) and Auger recombination (τ
Auger
), as follows:
[10]
(2)
Figure 1a is a representative plot of these different contributions for a perovskite material as a
function of n, using data taken from
[10]
. It is seen that SRH recombination dominates 
bulk
at low
carrier injection (Δn < ~10
15
cm
-3
), whereas Auger recombination dominates at high injection (Δn
> ~10
18
cm
-3
). In PSCs, the V
OC
conditions under 1-sun illumination reportedly correspond to Δn
~ 10
16
cm
-3
.
[5, 11]
This Δn value is comparable to that in high-quality, well passivated, c-Si solar
cells under similar operating conditions, such as silicon heterojunction solar cells.
[12]
We remark
that Δn also strongly depends on the thickness of the absorber; c-Si solar cells usually have an
absorber thickness >100 m, compared to at most a few hundreds of nm for perovskites. Therefore,
the similar Δn value for the two technologies may be surprising, but is explained by silicon solar

Frequently asked questions

Q1. What contributions have the authors mentioned in the paper "Defect and contact passivation for perovskite solar cells" ?

In this paper, the authors focus on surface-passivating materials that can be integrated into the contact stacks of devices without jeopardizing carrier extraction, with an in-depth discussion of their underlying passivation mechanisms. 

Q2. What is the effect of dangling bonds on the surface of ETLs?

The presence of dangling bonds on the surface of ETLs or HTLs generates trap states at their interface with the perovskite, causing recombination and reducing the PCE. 

Q3. What is the main reason for the improved performance of the perovskite?

Reduced carrier recombination at the GBs as well at the interfaces the perovskite shares with the ETL and HTL is likely the main reason for such improved performance.[61] 

Q4. What is the main limiting mechanism for eff of polycrystalline perovskite films?

with the obtained S values,surface recombination remains one of the main limiting mechanism for eff of such polycrystalline perovskite films, which mandates the search for more effective surface and contact passivation strategies. 

Q5. What is the effect of surface recombination on the performance of a solar cell?

As recombination rates are additive in semiconductors, eff depends in perovskite absorbers on their bulk and surface recombination, according to:[9] 

Q6. Why is the capture cross section of minority carriers an open challenge?

due to the intrinsic nature of perovskites, quantifying the capture cross section of minority carriers remains an open challenge. 

Q7. Why are organic ETLs unsuitable for n-i-p PSCs?

organic ETLs are usually unsuited for the realization of n-i-p PSCs due to the high solubility of organic compounds in most of the commonly used perovskite solvents such as dimethylformamide (DMF) and dimethyl sulfoxide (DMSO). 

Q8. What is the effect of the passivation of perovskite surfaces?

Similar to the passivation of perovskite surfaces, ultrathin metal oxides can also be used for passivation of ETL and HTL surfaces. 

Q9. What are the main limiting factors for the development of passivation layers?

The main limiting factors for the development of passivation layers are given by the (limited) choice of solvents that is compatible with the perovskite solvents and by the maximum annealing temperature that is compatible with the device fabrication. 

Q10. What is the origin of the low electronic quality of metal halide perovskites?

The origin of such a high electronic quality may be speculated to stem from strong coupling between cation Pb lone-pair s orbitals and anion p orbitals and the large atomic size of constitute cation atoms;[24] the s-orbital lone pair represents a pair of valence electrons in the outermost shell of atoms which is not used in any bond between atoms. 

Q11. What is the main cause of the rapid rise of p-i-n PSCs?

The lack of solvent-compatible polymers and efficient p-type metal oxides has been are arguably the main causes inhibiting the rapid rise of p-i-n PSCs; for several years the HTL was essentially limited to PEDOT:PSS. 

Q12. What is the interesting modification of the m-TiO2 layer?

Another interesting modification of the m-TiO2 layer consists of incorporating cesium bromide (CsBr) in the dense scaffold of TiO2 nanoparticles. [166] 

Q13. What is the main reason for the improved performance of the perovskite layer?

In this regard, a controlled excess of lead iodide (PbI2) in the perovskite layer has been argued to form a shell around the individual perovskite crystals in the films, improving device performance (Figure 5a).[60] 

Q14. What are the main criteria to be considered for effective scaling-up of perovskite?

Besides this, scaling-up of perovskite solar cells requires large-area compatible deposition techniques; vacuum-based deposition techniques can here be counted to be particularly attractive. 

Q15. What can be explained by the fact that defects and impurities at GBs can cause?

This can be explained by the fact that defects and impurities at GBs can induce electrostatic potential barriers, which can cause the spatial separation of photogenerated electrons and holes,[20] decreasing carrierrecombination at GBs. 

Q16. What is the main reason for the inclusion of foreign additives in PSCs?

Foreign additives are far from being limited to these examples, suggesting that additive engineering may become one of the key approaches to enhance the performance of PSCs. 

Q17. What are the main criteria to be considered for passivating contact stacks?

To fulfil these, engineering of the optical properties of the passivating contact stack will be of extreme importance, especially for ultra-high efficiency devices such as perovskite-based tandem cells. 

Q18. What is the way to approach the PCE limits of the PSCs?

Millimeterscale grains or single-crystal thin films may thus not be mandatory to approach the practical PCE limits of the PSCs, under the condition that effective surface and GB passivation is implemented.