THE POTENTIAL OF PEROVSKITE SOLAR CELLS IN

33rd European Photovoltaic Solar Energy Conference and Exhibition

THE POTENTIAL OF PEROVSKITE SOLAR CELLS IN MOROCCO Saida LAALIOUI *1,2, Kawtar BELRHITI ALAOUI 1,2, Aziza CHACHDI 1, 3, Houda AITDADS 2, Kassem EL ASSALI2, Bouchra RAHMOUNI 3, Badr IKKEN 1, Abdelkader OUTZOURHIT 2 1Research

Institute for Solar Energy and New Energies (IRESEN), 16, street Amir Sidi Mohamed souissi - Rabat, Morocco Tel: +212 (0) 537 682 236 | Fax : +212 (0) 537 688 852 2 Laboratory of Nanomaterials for Energy and Environnement (LNEE), Faculty of Sciences Semlalia, Cadi Ayyad University, PO Box: 2390, Marrakech 40000, Morocco 3Faculty of Juridical, Economic and Social Sciences, Km 3 road of Casablanca, B.P784, Settat, Morocco *

[email protected] , [email protected] , [email protected] , [email protected], [email protected], [email protected], [email protected], [email protected]

ABSTRACT: The performance of solar cells based on hybrid perovskites has undergone an unprecedented evolution and has stimulated the interest of the scientific and industrial community in the photovoltaics sector. Nevertheless, no study for the potential of this technology in term of industrial integration and research capability has been performed for Morocco. In order to set up a research roadmap in Morocco for perovskite-based PV, a detailed state of the art of this technology was carried out. Furthermore, a market analysis was performed to in order to gage the interest of the Moroccan industries in this technology. The results show that the tandems cells such using stable perovskite top cell and a silicon heterojunction bottom cell has the highest potential in terms of efficiency, stability and industrial integration. 53% of the companies surveyed expressed their interest in this technology. Keywords: Tandem, perovskite, Solar Cell Efficiencies, Heterojunction, PV Market.

1

INTRODUCTION

a detailed state of the art of the technology should be established in order identify best cell configurations and key parameters that should be investigated in local R&D projects as well as technological issues that can be adressed by the Moroccan industries for local production. Morocco has a huge potential for industrial integration of technologies based on thin film solar cells. However, no market analysis has been yet performed to evaluate the real potential of integration of this technology.

Morocco depends heavily on imports to meet its energy needs, since it imported 98% of its primary energy needs in 2009. Given the dynamics that the national economy is undergoing, the demand for primary energy is raising by an average of almost 5% during the past decade. To alleviate this energy dependency, Morocco launched an energy strategy that is axed on renewable energies. The Moroccan energy strategy aims to raise the part of renewables to 42% of installed capacity in 2020, and to 52% in 2030 [1]. To achieve this objective, Morocco will develop between 2016 and 2030, an additional capacity of electricity generation from renewable sources of more than 10,000 MW distributed as follows: 4,500 MW of solar, 4,200 MW of wind, and 1,300 MW of hydro [1]. The “Moroccan solar plan” is one of the corner stones of this strategy. In addition to set targets of solar capacity, the plan includes the development of technical expertise, the establishment of R&D infrastructure and the promotion of an integrated solar industry. The identification of key PV technologies where the R&D community as well as the local industries can actively contribute is therefore necessary. Within a few years, the performance of solar cells based on hybrid perovskites has undergone an unprecedented evolution rising from 3.8% in 2009 to 22.10% in 2016 [2], [3]. This has stimulated the interest of the scientific and industrial community in this sector. Moreover, the abundance and low cost of these materials and their ease of implementation offer a great potential for industrialization to this sector. Furthermore, perovskite cells have demonstrated their suitability to be used other technologies in the tandem configuration, which constitute a strong impetus for improvement solar cell performance [4]. To build the necessary research roadmap in Morocco,

2

METHODOLOGY

2.1 State of the art of perovskite-based solar cells In this work, we aim to establish the state of the art by studying the materials and structures of solar cells based on perovskite, and comparing them to identify the best choices and to develop a device with an excellent suitability for the Moroccan climate. It is well known that tandem structures increases the performances of the cell device and even exceed the theoretical Shockley-Queisser limit of the simple junction [5]. Particularly, tandem cells combining a perovskite and a silicon sub cells are investigated. 2.2 The potential of perovskite solar cells in Morocco Local industries were surveyed in order to find out their interest in this new solar cell technology. Four main families of data collection methods can be used: interviews, secondary data analysis, direct observation and questionnaires [6]. Interviews. The interview takes the shape of a conversation, directed objectively by the interviewer, on the object of the study. The interview doesn’t make it possible to work on a large scale in terms of time and the costs. This is why it is not adapted for a quantitative treatment of the main object of our study. The secondary data are data which were collected or

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analyzed by other reliable people or official establishments. They are generally data collected at the time of studies or research; available data via official statistics, or produced by survey institutes, presented in the form of a synthetic description, with or without analysis and interpretation. The direct observation gives access directly to the facts. The data collected through this tool are structured by grids which make it possible to judge and qualify what is observed. On the other hand, this tool is very relevant to study behaviors, attitudes or interactions, which is not paramount for our study. The questionnaire: is used when the respondent is alone to answer the questions asked. It is a fast method to diffuse where the participant answers at his rhythm without the obligation of displacement, which minimizes the time and the cost of the envisaged investigation. It also limits the effects of interviewer’s personalities and adapts to respondent schedules. The present study relied mainly on the questionnaire method.

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The use of a single halide in the fabrication of perovskites in the (Br3 or I3 or Cl3) form gave good efficiencies, and the Bromine-based perovskite is known for its high gap, improved solar cell performance and stability [4]. However, the combination of two halides offered better performance [9, 10]. Indeed, perovskites containing iodide and chloride (Cl) have an efficiency of 12.7%, a high diffusion lengths and improved stability compared to those containing only iodide [9]. On the other hand, perovskites combining iodine (I) and bromine (Br) halides such as methylamunium lead bromide-iodide MAPbIxBr3-x have a maximum efficiency a round 14,9%[4]. 3.1.2 Comparison between solar cells based on perovskite Several combinations of organic and inorganic compounds were investigated by the researchers in this field. The efficiencies of the various perovskite cells are summarized in Table 1 [3], [4], [8], [11]. Table 1: Efficiencis of perovskite solar cells in relation to the absorbers employed

RESULTS

The results of the different studies will be presented in this section, staring by the perovskite study, followed by the tandem cells investigation and finally the results of the survey. 3.1 State of the art of perovskite technology 3.1.1 Choice of the absorber Research has shown that tin-based perovskites are very unstable, due to the instability of the Sn2+ ions which easily transform into Sn4+ [5]. In addition, perovskites containing tin like methylammonium tin halide MASnX3 have lower optical gaps and efficiencies than those of the methylammonium lead halide MAPbX3 [3], [4]. This makes lead-based perovskites best suited for the solar cells [7]. Furthermore, the perovskites containing cesium (Cs) and iodide (I) have a good performances and good stability. However, the perovskites containing cesium and bromide (Br) are not good to use in solar cells because they have high band gap up to 2,25 eV [8]. The best single junction solar cells based on perovskites were made of mixed organic cations (methylammonium (MA) and formamidinium (FA)) and mixed halides (Br and I). For example, the perovskite (MA)x(FA)1-xPbI3 has an efficiency up to 14.9% [4], and the perovskite MA0.17FA0.83Pb(I0.83Br0.17)3 offer an efficiency up to 20.2% [8]. However, Csx(MA)1-xPbI3 has a lower efficiency which is of the order of 7.68% [4], but the mixture of the cations of Cs and FA has reached an efficiency of 16.5% [8]. One of the interesting approaches is to combine cesium Cs with the two organic cations MA and FA to have a triple cation configuration (Cs / MA / FA). This approach has allowed an efficiency up to 21.1% with the combinationCsx(MA0.17FA0.83) (100-x) Pb (I0.83Br0.17)3 [8]. Indeed, this configuration offers a reproducible perovskite films and guaranties a good thermal stability. When MA is replaced by a larger cation such as ethylamonium EA, the structure of the perovskite changes completely, moving from a three-dimensional structure to a two-dimensional structure that is more resistant to solar radiation and humidity [4].

Perovskite MASnX 3

Efficiency 6%

MAPbX 3

19.7 %

MAPbI3

19.7 %

FAPbI3

16,60%

MAPbIxBr 3-x

14,90%

MAPbIxCl 3-x

12,30%

CsPbBr 3

5,95%

Csx(MA)1-xPbI 3

7,68%

Csx(FA)1-xPbI 3

16,50%

(MA)x(FA)1-xPbI3

14,90%

MA0.17 FA0.83 Pb(I0.83 Br0.17 )3

20,20%

Csx(MA0.17 FA0.83 )(100-x)Pb(I0.83 Br0.17 )3

21,10%

3.1.3 Fabrication techniques Several deposition techniques were used to fabricate op the hybrid perovskite layers. The spin coating is the most widely used technique since it is simple, fast and low cost [12], [3],[13]. The thin film deposition can be carried out either in one or two steps followed by a heat treatment at 100 °C. Thermal evaporation was also used for the fabrication of thin layers of hybrid perovskites [12] in one or two steps. This technique guaranties smooth and uniform films with better quality than spin-coating [3], since it avoids the formation of holes in the film due to the evaporation of the solvents [14], [15]. Another method that has been used is vapor-assisted solution deposition (VASP) where a thin film of the first precursor is deposited on a substrate; the latter is then exposed to the vapor of the second precursor [3]. High efficiencies of up to 20.1% were obtained with this technique [13], [12]. 3.1.6 Stability: The stability of the solar cell based on perovskites depend on several factors: climatic conditions (temperature, humidity, irradiation), the nature of the perovskite absorber, the hole transport layer, electron transport layer and the back contact. As discussed above, the partial substitution of iodine by Br in MAPb (I1-x, Brx)3 improves the stability of perovskite [16]. On the other hand, Mckeee et al. [17] showed that the combination of cesium ions with formamidinium (FA) cations in mixed bromide-iodide halide perovskite cells; FA0.83Cs0.17Pb (I0.6Br0.4)3,

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enhances the photo-stability and thermal stability of cells made with this perovskite. Recently, a new two-dimensional perovskite family has been implemented to overcome stability problems. The two-dimensional perovskites (2D) are derived from the 3D structure APbX3 by the incorporation of voluminous organic cations which play the role of intercalary between the different MX6 octahedra layers [18] The pathways to enhance the stability of perovskite cells include the adequate choice of the materials, the structure of the cells and the development of a good encapsulation that protects de device from the precursors of the degradation. So far, the best single junction solar cell containing mixed cations and halide, reached an efficiency of 21, 1% and showed a good stability of up to 1000 hours at 85°C and 85% relative humidity.[19]

wafer is passivated with thin layers of intrinsic of hydrogenated amorphous silicon (a-Si: H). A p-type layer of a-Si: H forms is then used to form the heterojunction with n-type crystalline silicon [24]. The efficiency of (SHJ) keeps improving from one year to the next, as it evolved from a value of 23.7% in 2011 to 26.3% in March 2017 [28], [29], [30]. This value is close to the maximum theoretical efficiency a simple junction, which is of the order of 29.4% for the silicon technology [31].

3.2 State of the art of tandem cells based on perovskites 3.2.1 Configuration of tandem cells Generally, the tandem cells are either monolithic or four-terminal. Other configurations are less used because they are more bulky [20]. In the four-terminal configuration, the sub-cells are manufactured independently and are mechanically stacked. Whereas in the monolithic configuration the upper sub-cell is deposited directly on the lower sub-cell thus forming a single block [19], [21]. [22] [2] [23], [24] . The development of monolithic tandem cells is more demanding than that of four-terminal cells. Indeed, the two subcells must be optimized in such a way as to produce the same current [19]. On the other hand, the upper sub-cell must be able to be deposited at low temperatures in order to avoid damage to the lower subcell [19]. The rear contacts must be replaced by a highly transparent conductive layer [19]. The coupling between the two subcells can be carried out by a tunnel junction of the P + N+ type or by an oxide layer of very small thickness, allowing the passage of the carriers by tunneling.

Figure 1: Evolution of performances of SHJ It is expected that the coupling of the silicon heterojunction (SHJ) cells with the perovskite cells will improve the performance of the resulting devices at a reasonable cost. The efficiencies reported in the literature for this family of tandem cells are summarized in the table 3 [19], [26], [27]. These results suggested that there is still plenty of room for improvement in this technology, as both the pervoskite top cells and the coupling between the two sub cells require further optimization. . Table 3: Performances of the best tandem cells combining Perovskite/ and SJH Cell structure (MA)x(FA)1-xPbIxBr3-x / SnO2/ SHIJ

3.2.2 Tandem cells record efficiencies According to the principle of the detailed balance, the optimal bandgap is 1.7 eV for the upper cell and 1.1 eV for the lower cell. Several configurations of tandem cells have been developed combining perovskite, CIGS and silicon. The efficiencies of the best combinations are present in the table below [19] [23] [25] [26] [27]:

surface (cm²)

Scan

0,25

Inverted

1.785

14

79.5

19.9

Direct

1.759

14

77.3

19.1

Inverted

1,701

16,1

70,1

19,2

Direct

1,703

16,1

70,9

19,5

Inverted

1,69

15,9

77,6

20,9

Direct

1,692

15,8

79,9

21,4

18,1

79

23,6

1,22 CH3NH3PbI3/ PCBM/PEIE/SHIJ

Cs0,17FA0,83Pb(Br0,17I0,83)3/ NiO/SHIJ

0,17

1

Voc (V)

unknown 1,65

Jsc (mA/cm²)

FF (%)

Ƞ (%)

Table 2: Efficiencies of the best tandem cells Tandem

Configuration

Efficiency (%)

Perovskite/Perovskite

Monolithic

17

Perovskite/Perovskite

Four-terminal

20,3

Perovskite/ CIGS

Four-terminal

19,5

perovskite/ SHJ

Monolithic

18,1

perovskite/ SHJ

Monolithic

21,2

perovskite/ SHJ

Monolithic

23,6

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MARKET ANALYSIS

The comapgnies that were surveyed are located in the big cities of Morocco (figure 2). 89% are located in the administrative (Rabat) and economic (Casablanca) capitals of Morocco, whereas 11% are situated Marrakech. The dynamics stimulated by the Moroccan Energy Strategy and in particular the Moroccan Solar Plan, has stimulated the interest of companies working in the photovoltaic sector to invest in new technologies. As shown in Figure 3, 60% of the actors with activities not related to the photovoltaic sector envisage possible reorientation towards the photovoltaic sector.

The perovskite / silicon heterojunction tandem cell has a significant interest because silicon technology is more mature and has achieved efficiencies close to its theoretical limit through the heterojunction structure (SHJ). In these cells, the surfaces of an n-type silicon

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Figure 2: Geographical location of the companies interviewed

Figure 4: Purchase price allowed for the perovskite cells

This observation is a clear indication of how the photovoltaic industry can be rapidly developed in the coming years in Morocco.

Perovskite / Silicon Tandem Technology is now major candidate for the production of cheap photovoltaic panels. 9 out of 10 of the surveyed companies are for the adoption of the Tandem technology.

Figure 5: Potential adoption of perovskite/ silicon tandem technology

Figure 3: Possibility of the companies to change theirs activity toward the photovoltaic sector

Regarding the tolerated purchase price for the more efficient Tandem technology, 63% of the companies can readily accept prices that are higher those of the actual silicon technology, while 37% are likely to accept the same price.

The rapid advances in the perovskite technology in terms of efficiency, stability and lower production costs can be strong impetus and may play a prominent role as alternative or a complement to the photovoltaic silicon solar cells[32] The cost remains a key factor. As shown in figure 3, 79% of the surveyed companies will opt for this alternative only if it’s less expensive than the traditional silicon technology.

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Figure 6: Purchase price allowed for perovskite/ silicon tandem technology

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Most of the actors operating in the photovoltaic sector are currently aware that the efficiency of the solar cells of new technologies compete today with those of the traditional silicon cells. According to this survey, 79% of the companies are ready to invest in production units if they are accompanied to produce at an attractive price / quality ratio.

Figure 7: National industries interested by providing a part of the technology from a national reaserch center 5

CONCLUSION

The hybrid perovskite technology is undergoing a continuous development and may be regarded as an attractive alternative to the other technologies and a way to improve their performances. Indeed, the hybrid perovskite solar cell achieved an efficiency exceeding 22 % while the best tandem cells containing perovskites reached 23, 6%. However, several pathways for improving this technology still need to explore in order to achieve the 46% the theoretical limit of the tandem structures [33]. The overwhelming majority of companies interviewed for the market analysis expressed their interest to opt for this technology either as simple junction or in tandem configurations. Furthermore, the majority are motivated by the integration of more expensive technologies in there production line instead of conventional silicon solar cells. The hybrid Perovskite solar cells have a great potential to become a key player in the Moroccan photovoltaic market provided that the pending stability issues are resolved.

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