Ottimizzazione del processo di produzione di celle solari a perovskite ibrida in ambiente ad umidità controllata

Dipartimento di Chimica

Direttore Ch.ma Prof. Antonella Profumo

Corso di Laurea Magistrale in Chimica

Ottimizzazione del processo di produzione

di celle solari a perovskite ibrida in

ambiente ad umidità controllata

Relatore:

Prof.ssa Giulia Grancini

Correlatore:

Matteo Degani Tesi di Laurea Magistrale di

Tommaso Pasetti

Anno Accademico 2022 / 2023

Summary 1

1.0 Introduzione 10

1.1 Problema energetico 10

1.2 Energia solare 12

1.3 Proprietà dei semiconduttori 14

1.3.1 Modello a bande di energia 14

1.3.2 Proprietà del silicio cristallino 15

1.3.3 Struttura a bande per il silicio cristallino 17

1.3.4 La lacuna 18

1.3.5 Silicio cristallino estrinseco 19

1.4 La giunzione p-n 21

1.4.1 Effetto della luce sui semiconduttori 22

1.5 La cella solare 25

1.5.1 Interazione luce – materia 25

1.5.2 Giunzione p-n in una cella solare 26

1.6 Parametri di merito di una cella solare 30

1.6.1 Efficienza di conversione fotovoltaica (PCE) 30

1.6.2 Altri parametri di merito 33

1.7 Nuove tecnologie fotovoltaiche 35

1.7.1 Celle solari a base di Perovskite 36

1.7.2 Proprietà della perovskite 37

1.7.3 Struttura delle celle solari basate su perovskite 39

1.7.4 Stabilità di una cella fotovoltaica a perovskite 40

1.8 Metodo di deposizione 42

1.8.1 Metodi di deposizione: One step o two steps 44

1.9 Commercializzazione di celle solari a base di perovskite 47

2.0 Metodo sperimentale 49

2.1 Fabbricazione della cella solare 49

2.1.1 Procedura di fabbricazione 50

2.2 Passivazione con Sali di potassio e rubidio 51

2.2.1 Processi di ricombinazione 51

2.2.2 Strategia di passivazione 54

2.3 Caratterizzazione 56

2.3.1 Diffrazione ai Raggi X (XRD) 56

2.3.2 Caratteristica tensione (V) – Corrente (I) 57

2.3.3 External quantum efficiency (EQE) 57

2.3.4 Photoluminescence (PL e TRPL) 59

3.0 Risultati e discussioni 60

3.1 Conversione della parte inorganica e organica nella perovskite

FAPbI 60

3

3.2 Effetto della Passivazione con RbX e KX 62

3.3 Incident Photon to Charge Carrier Efficiency of Solar Cells (IPCE)

63

3.3 Caratterizzazioni ottiche 64

3.3.1 Fotoluminescenza (PL) 64

3.3.2 Fotoluminescenza risolta nel tempo (TRPL) 65

4.0 Conclusioni 66

Bibliografia 68

Summary

The rapid depletion of fossil fuels, which accounts for nearly 80% of

global energy consumption, demands an urgent need for research aimed at

Climate change

finding sustainable and renewable energy alternatives. 1

has become one of the most urgent global issues, threatening the natural

environment and the well-being of people worldwide. One of the main

causes of climate change is the high emission of greenhouse gases, mainly

produced by energy production and consumption, especially through the

use of fossil fuels such as oil, coal, and natural gas. The global

2,3

community is committed to transforming the energy sector towards

cleaner, more sustainable, and renewable sources. Policies and

4

international initiatives have been developed and implemented to promote

the transition to a low-carbon economy. Among these, the Paris Agreement

of 2015 stands out, signed by numerous countries and the European Union,

which aims to limit the global temperature increase to below 2°C

compared to pre-industrial levels. Furthermore, the UN's Agenda 2030,

with its Sustainable Development Goals (SDGs), promotes a holistic

approach to development that considers not only environmental issues but

also social and economic. The SDGs represent a global framework for

addressing crucial challenges such as climate change, poverty, energy

access, and resource sustainability. 4–6

Figure 1 - Sustainable development objectives linked to the environment, economy, and society.

1

Solar energy is one of the best options to meet future energy demand as it

is superior in terms of availability, affordability, accessibility, capacity, and

efficiency compared to other renewable energy sources. 7

The transition to solar energy and other renewable sources is crucial to

addressing climate change and ensuring a sustainable future for

Silicon-based photovoltaic technology represents

generations to come. 8

one of the fundamental pillars of solar energy. Despite the enormous uses

and applications of this technology, silicon has limitations that emerging

technologies can overcome such as reduced availability of silicon, initial

costs for implementing a manufacturing plant and lower photovoltaic

conversion efficiency (PCE) compared to new technologies. 9

A new type of technology showed great promise in recent years identifying

new technologies as emerging. These technologies exploit organic

materials and hybrid organic - inorganic materials and offer lower costs

and easier production also from solution techniques. Perovskite solar

10

cells, in recent years, achieved photovoltaic conversion efficiencies of

Originally, perovskite was discovered as a mineral with the

26.1%. 11

chemical formula CaTiO , in 1839 in the Ural Mountains. The organic-

3

inorganic hybrid perovskite has a general chemical formula of ABX ,

3

where A is an organic cation, such as methylammonium (CH NH , MA )

+

3 3

and/or formamidinium (CH(NH ) FA ), B is a bivalent inorganic cation

+

2 2,

such as Pb or Sn , and X is an halide element, such as iodide (I ),

2+ 2+ −

bromide (Br ) or chloride (Cl ).

− − 2

Figure 2 – representation of the ABX perovskite.

3

There is an empirical law that indicates the conditions necessary to form a

stable perovskite structure, in particular, this is formed when the radius of

the elements A, and B form the Goldschmidt factor, t is between 0.9 and

1.0: ý +ý (1.0)

ý �㕋

Ć= +ý )

:2(ý

þ �㕋

Compared with ordinary semiconductors, organic-inorganic hybrid

perovskites exhibit excellent optical and electrical properties such as high

absorption coefficient and tunable band gap. Despite the high

12

photovoltaic conversion efficiency performance (PCE) achieved by solar

cells based on this technology, sensitivity to air, humidity, heat, and UV

light limits the implementation and development of these photovoltaic

systems. 9

The moisture presents in the air solvate the perovskite, causing the

degradation of the active material in the photovoltaic cell with an overall

decrease in photovoltaic performance. For this reason, degradation

13

caused by humidity is one of the main challenges to the development of

this technology. Perovskite solar cell devices were mostly fabricated in

nitrogen glove box, which increases the difficulty of their future

commercial production. 14 3

The architecture of a perovskite-based solar cell consists of different layers

and can be classified according to the placement of the charge extractors:

the electron transport layer (ETL) and the hole transport layer (HTL). We

talk about n-i-p (negative – intrinsic – positive) and p-i-n (positive –

intrinsic – negative) architecture. The layers that form the perovskite solar

cell are deposited using a spin coating technique. 15

Figure 3 - (a) n-i-p structure, (b) p-i-n structure.

Some studies suggested how the importance of the perovskite deposition

method can play a fundamental role in the ambient environment –

fabrication of the solar cell. For this purpose, the two-step method

represents an excellent candidate. In this method, the two perovskite

precursors are deposited consecutively one after the other. Furthermore,

16

it9s not just the humidity that can decrease the performance of photovoltaic

cell; there are also internal factors, such as electron-hole recombination

processes, caused by intrinsic structural defects. These electron-hole

recombination processes include different portions of the perovskite, from

the bulk to the surface interfaces and are considered the main cause that

limits the achievement of high PCEs. 17,18

To solve this problem, there are various strategies such as interface

passivation or bulk passivation. Potassium ions, K , and rubidium, Rb ,

+ +

due to their small size and ionic radius, can infiltrate at the perovskite

interface, decreasing the defect sites and limiting ionic migration, acting

4

as passivant, improving photovoltaic performance. Based on these

19,20

considerations, my thesis project aims to optimize perovskite-based solar

cells in controlled humidity environments, using the two-step deposition

method. I planned three work packages for the thesis project: (i)

optimization of the deposition process of the inorganic (PbI ) and organic

2

phase (FAI) using a two-steps method, (II) optimization of the deposition

environment (dry box) and (III) passivation of the ETL/PbI interface and

2

of the PbI bulk with potassium (KX) and rubidium salts (RbX) where X=

2

Cl , Br , and I . In the first step, to investigate the complete conversion of

- - -

the inorganic precursor through treatment with the organic salt, x-ray

diffraction (XRD) analyzes were performed on glasses on which the

precursors were deposited at different rpm values. (20 uL of PbI at 2000

2

rpm were kept constant while the rpm of the organic salt was varied).

Figure 4 – X-ray diffraction pattern of FAPbI .

3 5

A small excess of lead iodide is known, from scientific literature, to be

beneficial for an increase of photovoltaic performance by defects

passivation; from the XRD pattern the full conversion of the organic and

inorganic phase in the perovskite was obtained at rpm lower than 2500

rpm. 21

In the second step, photovoltaic devices were manufactured, selecting rpm

values between 1500 and 2500, at different humidity conditions. The

averages of the main figures of merit of a solar cell are reported below.

Figure 5 - performance of photovoltaic devices at different humidity and rpm ranges.

For subsequent experiments, it was decided to maintain a rotation speed

of 2000 rpm for the organic and inorganic parts. For the organic part, a

value of 2000 rpm was chosen despite the photovoltaic performance being

6

better for 2500 rpm, because an immediate degradation of the cell was

observed, just after measuring the cells. The humidity percentage was

settled at less than 5% for the subsequent experiments. Once the rpm and

the deposition environment were fixed, various experiments were

conducted to obtain an optimal concentration for passivation of the

intermediate layer between the ETL and PbI , and an optimal

2

concentration for doping of the bulk PbI . For the experiments, different

2

combinations of KX and RbX were tried. Batches of 60 substrates were

produced in which, for each batch, a single compound (KX or RbX) was

tested at different concentrations (bulk and surface). After the

experiments, a final batch was made with the best combinations, chosen

based on the performance that every single combination had compared to

the references (solar cell with no passivation), given below.

Figure 6 – Box charts with photovoltaic parameters of the manufactured devices (Jsc, Voc, FF,

PCE). 7

We observed that, regarding PbI doping (bulk), the RbBr compound

2

provided the best performances with an optimal concentration of 5

mg/mL. For the passivation layer between the inorganic (PbI ) and tin

2

oxide (SnO ) layer, the best concentration was 20mM. For KCl there were

2

no interesting results regarding the doping of the PbI solution, while for

2

the passivation layer, an optimal concentration of 10mM KCl was defined.

To understand the reason behind the photovoltaic performance

improvement, optoelectronic characterizations were conducted on FAPbI 3

films untreated and treated with the salts that give the best performance.

From the intensity of photoluminescence (PL) qualitative information

about the amount of non-radiative recombination can be obtained. Time-

resolved photoluminescence (TRPL) is a useful measurement for

obtaining information on the charge9s lifetime, proportional to the

diffusion length and, therefore, to the concentration of defects. In both

22

characterizations we had a better response for FAPbI doped with 5 mg/mL

3

of RbBr. The combination of the passivation layer between the inorganic

part and the tin oxide (10 mM KCl) and the doping directly in the bulk of

PbI (5 mg/mL) was also characterized, but an improvement in the PL

2

intensity compared to that with PbI 5 mg/mL doping alone.

2

–

Figure 7 Photoluminescence (PL) measurement, FAPbI reference (light blue-blue), FAPbI

3 3

+ KCl 10mM (black), FAPbI + 5 mg/mL RbBr (red), FAPbI + KCl 10mM + 5 mg/mL RbBr

3 3

(Green). 8

It can be noted that the emission signal is more intense in the film treated

with 5 mg/mL of RbBr compared to the other untreated films, this is an

indication of the presence of fewer trap states (correlate to the amount of

22

non-radiative recombination).

–

Figure 8 Time resolved photoluminescence measurement (TRPL), FAPbI reference (light

3

blue, blue), FAPbI + KCl 10mM (black), FAPbI + 5 mg/mL RbBr (red), FAPbI + KCl

3 3 3

10mM + 5 mg/mL RbBr (Green).

Figure 8 shows how the FAPbI 5 mg/mL of RbBr has a longer charge

3

carrier lifetime compared to the reference. Furthermore, KCl also has a

positive, but less pronounced effect. This means that doping with RbBr

causes a decrease in trap states, as also demonstrated by the PL

measurement, the increase of the charge carrier9s lifetime, and the

consequent photovoltaic performance improvement. 9

1.0 Introduzione

1.1 Problema energetico

Uno dei problemi maggiormente discussi negli ultimi anni è il

cambiamento climatico, il quale mette in pericolo l9ambiente che ci

circonda, il nostro benessere e quello delle future generazioni. 2

Tra le cause responsabili del cambiamento climatico, la produzione di

energia contribuisce con l987% alle emissioni globali di gas serra. Il modo

più semplice per produrre energia è utilizzando combustibili fossili (legno,

carbone, petrolio e gas naturale). Si prevede che la domanda totale di

23

combustibili fossili raggiungerà il picco entro il 2030 ed è previsto che

copriranno dal 36% al 66% della domanda globale di energia entro il

2050. Inoltre, si ipotizza che i combustibili fossili, dopo aver raggiunto

24

il picco, saranno parte integrante del mix energetico mondiale nei decenni

a venire. Pertanto, la necessità di trovare fonti energetiche alternative,

24

pulite e più sostenibili, è uno degli obiettivi che la società globale si è posta

entro il 2050. 2

Figura 9 - Scenario domanda combustibili fossili entro il 2050. 10

Spinti da queste considerazioni, negli ultimi anni, molti paesi hanno

iniziato a vedere l9energia come parte del progresso umano, sostenibile.

Sulle basi di questo, gli SDGs (obiettivi di sviluppo sostenibile)

rappresentano il cuore dell9agenda 2030 dell9ONU (Organizzazione delle

Nazioni Unite) per lo sviluppo sostenibile. In questo documento sono

4,5

esplicate le politiche di sviluppo di ciascun Paese in un9ottica di

sostenibilità ambientale ed economica. Il documento propone di orientare

5

le scelte strategiche dei Paesi firmatari dell9accordo sia nel contesto della

loro politica nazionale che a livello di cooperazione internazionale,

riconoscendo lo stretto legame tra il benessere umano, la salute dei sistemi

naturali e le sfide comuni a tutti i paesi. Gli obiettivi sono stati approvati

6

dall9Assemblea delle Nazioni Unite nel settembre 2015, insieme ai paesi

che l9hanno firmato (169), e devono essere affrontati entro il 2030. 4

L9agenda si basa su tre pilastri:

I. Sviluppo sostenibile

II. Sviluppo ambientale

III. Sviluppo sociale ed economico

Figura 10 – Obiettivi per lo sviluppo sostenibile legati ad ambiente, economia, società. 11

Il 28 novembre 2019, il parlamento europeo ha adottato una politica rigida

sull9argomento, chiedendo all9unione europea (UE) il raggiungimento

della neutralità climatica entro il 2050, come obiettivo a lungo termine, e

la riduzione delle emissioni fino al 55% entro il 2030. Inoltre, l9accordo

di Parigi, firmato da 194 paesi e dall9UE, mira a limitare il riscaldamento

globale al disotto di 2°C e a proseguire gli sforzi per circoscriverlo a 1,5°C

al fine di evitare le conseguenze catastrofiche del cambiamento climatico.

Per raggiungere gli obiettivi dell'accordo di Parigi, ogni cinque anni, i

paesi devono fissare dei traguardi per i loro sforzi in materia di clima.

Questi obiettivi sono noti come contributi determinati a livello nazionale

e verranno riesaminati nel 2025. 6

1.2 Energia solare

L'energia solare è considerata una fonte di energia non inquinante,

affidabile e pulita. Negli ultimi decenni, vi è stato un aumento importante

7

della domanda di risorse energetiche più pulite. Si prevede che le energie

3

rinnovabili continueranno la loro rapida crescita grazie alla domanda

sempre maggiore e alla competitività economica. Le fonti rinnovabili

24

forniranno tra il 45% e il 50% della produzione globale entro il 2030, e tra

il 65-85% entro il 2050. In tutti gli scenari, il solare è il maggiore

24

contribuente delle energie rinnovabili, seguito dall9eolico. In particolare,

il sole rappresenta una delle principali fonti di energia libera, inesauribile,

per il pianeta terra. L9energia solare ha il più alto potenziale di

7

applicazione globale; la quantità media di energia solare ricevuta

nell'atmosfera terrestre è di circa 342 Wm , di cui circa il 30% viene

−2

disperso o riflesso nello spazio, lasciando utilizzabile il 70% (239 Wm ).

−2

L9irradianza effettiva annua varia dai 60 Wm ai 250 Wm a seconda

−2 −2

della posizione geografica. La figura 11 mostra l9intensità media

7,8

annuale della radiazione solare sulla superficie terrestre. Sono indicate,

25 12

con dei punti neri (Black delle specifiche aree nelle quali, se riempite

dot),

con pannelli