Progress on the Synthesis and Application of CuSCN Inorganic ... - MDPI

materials Review

Progress on the Synthesis and Application of CuSCN Inorganic Hole Transport Material in Perovskite Solar Cells Funeka Matebese 1,2 , Raymond Taziwa 1, * 1 2

*

and Dorcas Mutukwa 1,2

Fort Hare Institute of Technology, University of Fort Hare, Alice 5700, South Africa; [email protected] Department of Chemistry, University of Fort Hare, Alice 5700, South Africa; [email protected] Correspondence: [email protected]; Tel.: +27-40-602-23-11-2086

Received: 24 October 2018; Accepted: 17 November 2018; Published: 19 December 2018







Abstract: P-type wide bandgap semiconductor materials such as CuI, NiO, Cu2 O and CuSCN are currently undergoing intense research as viable alternative hole transport materials (HTMs) to the spiro-OMeTAD in perovskite solar cells (PSCs). Despite 23.3% efficiency of PSCs, there are still a number of issues in addition to the toxicology of Pb such as instability and high-cost of the current HTM that needs to be urgently addressed. To that end, copper thiocyanate (CuSCN) HTMs in addition to robustness have high stability, high hole mobility, and suitable energy levels as compared to spiro-OMeTAD HTM. CuSCN HTM layer use affordable materials, require short synthesis routes, require simple synthetic techniques such as spin-coating and doctor-blading, thus offer a viable way of developing cost-effective PSCs. HTMs play a vital role in PSCs as they can enhance the performance of a device by reducing charge recombination processes. In this review paper, we report on the current progress of CuSCN HTMs that have been reported to date in PSCs. CuSCN HTMs have shown enhanced stability when exposed to weather elements as the solar devices retained their initial efficiency by a greater percentage. The efficiency reported to date is greater than 20% and has a potential of increasing, as well as maintaining thermal stability. Keywords: perovskite solar cells; hole transport materials; inorganic hole transport materials; CuSCN

1. Introduction We have recently witnessed the rise of perovskite solar cells (PSCs) based on organic–inorganic halide perovskites as the next generation of thin-film solar cells. They have managed to rise above their predecessors, dye-sensitized solar cells (DSSCs) from an efficiency of 3.8% to 23.3% in less than 10 years of research [1–3]. The keen interest in PSCs and their exceptional performance can be attributed to the excellent properties of the perovskite light absorbers which include long diffusion lengths, defect tolerance, strong absorption coefficient, low recombination rates, ease of fabrication and high charge mobility as well as favorable bandgaps [4–6]. However, commercialization of PSCs has been limited due to stability, carrier lifetime and current-voltage hysteresis issues [7]. Perovskite materials are known to degrade when exposed to moisture, heat, oxygen and UV radiation. Consequently, research efforts are now focused on improving the stability and cell performance issues of PSCs to pave way for commercialization. The PSC is made up of an active perovskite layer, an HTM, an electron transport material (ETM), a back electrode, a transparent electrode and or a mesoporous TiO2 layer. Figure 1 presents the schematic diagrams of the mesoscopic and planar PSC device architecture.

Materials 2018, 11, 2592; doi:10.3390/ma11122592

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(a)

(b)

(c)

(d)

Figure 1. Different PSC device architecture (a) Inverted mesoscopic (p-i-n) device (b) Inverted planar (p-i-n) device (c) Mesoscopic Mesoscopic (n-i-p) (n-i-p) device device (d) (d) Planar Planar (n-i-p) (n-i-p) device. device.

The The PSC PSC consisted The first first solid-state solid-state PSC PSC was was introduced introduced by by Kim Kim etet al.al. [8]. [8]. The consisted of of methylammonium lead iodide (MAPbI ) as a sensitizer, with a spiro-OMeTAD (2,2’,7,7’-tetra-kis methylammonium lead iodide (MAPbI33) as a sensitizer, with a spiro-OMeTAD (2,2’,7,7’-tetra-kis (N,N-di-p-methoxyphenylamine)-9,9’-spirobifluorene) TiO22 as as an an ETM. ETM. (N,N-di-p-methoxyphenylamine)-9,9’-spirobifluorene) as as an an HTM HTM and and nanoporous nanoporous TiO This device architecture had an efficiency of 9.7% and was similar to that of DSSCs, with the liquid This device architecture had an efficiency of 9.7% and was similar to that of DSSCs, with the liquid electrolyte solid spiro-OMeTAD. spiro-OMeTAD. Introduction significant electrolyte replaced replaced by by the the solid Introductionof of aa solid-state solid-state HTM HTM led led to to aa significant increase to better better stability stability as as compared compared to first increase in in the the efficiency efficiency of of PSCs. PSCs. This This might might have have been been due due to to the the first PSC which was introduced by Miyasaka and co-workers [9]. In their work, they employed a liquid PSC which was introduced by Miyasaka and co-workers [9]. In their work, they employed a liquid − /I− − ) employed in DSSCs and achieved an efficiency of 3.8% [9]. electrolyte electrolyte tri-iodide tri-iodideredox redoxcouple couple(I(I33−/I ) employed in DSSCs and achieved an efficiency of 3.8% [9]. The low efficiency was as a result of the The low efficiency was as a result of the perovskite perovskite material material dissolving dissolving in in the the tri-iodide tri-iodide redox redox couple couple electrolyte. Even though the use of spiro-OMeTAD led to improved performance, spiro-OMeTAD electrolyte. Even though the use of spiro-OMeTAD led to improved performance, spiro-OMeTAD 5 −1 2 V−1 s−1 and low electrical conductivity in −5 cm suffers from low lowhole holemobility mobilityofofabout about 6× 10−2V cm suffers from 6×10 s−1 and low electrical conductivity in its pristine its pristine state. In order to improve the performance of spiro-OMeTAD, additives suchlithium as Li-TFSI state. In order to improve the performance of spiro-OMeTAD, additives such as Li-TFSI bis lithium bis (trifluoromethanesulfonyl)imide and silver bis (trifluoromethanesulfonyl)imide (AgTFSI) (trifluoromethanesulfonyl)imide and silver bis (trifluoromethanesulfonyl)imide (AgTFSI) have been have been commonly adopted [10,11]. commonly adopted [10,11]. However, the use theseadditives additives resulted in the degradation of perovskite the perovskite However, the use ofofthese resulted in the degradation of the layer,layer, thus thus leading to long-term instability. Moreover, use of spiro-OMeTAD PSCs leading to long-term PSCs PSCs instability. Moreover, use of spiro-OMeTAD as HTMasinHTM PSCsin present present several limitations as itto is costly to manufacture and the synthesis routes are does tedious several limitations as it is costly manufacture and the synthesis routes are tedious which not which does not augur well for large-scale fabrication, thus making it not viable for commercial augur well for large-scale fabrication, thus making it not viable for commercial production [10,11]. production [10,11]. efforts Therefore, research efforts have been dedicated alternative Therefore, research have been dedicated to finding alternative HTMstooffinding improved stability HTMs of improved stability and hole mobility as they have a significant contribution to the and hole mobility as they have a significant contribution to the performance of PSCs [12]. Since the performance of PSCs [12]. Since the introduction of spiro-OMeTAD, various organic HTMs such introduction of spiro-OMeTAD, various organic HTMs such as PEDOT:PSS (poly(3,4as PEDOT:PSS (poly(3,4-ethylenedioxythiophene)-poly (styrenesulfonate)), PTAA (poly(triarylamine)), ethylenedioxythiophene)-poly (styrenesulfonate)), PTAA (poly(triarylamine)), triphenylaminetriphenylamine-based molecules (TPA), carbazole-based molecules, spiro-OMeTAD derivatives and based molecules (TPA), carbazole-based molecules, spiro-OMeTAD derivatives and thiophene-based thiophene-based molecules been tested in PSCs [13–15]. All these research were molecules have been testedhave in PSCs [13–15]. All these research efforts wereefforts centered atcentered finding at finding affordable HTMs with improved hole mobility, stability, and a feasible synthesis route. affordable HTMs with improved hole mobility, stability, and a feasible synthesis route.

In addition to organic HTMs, inorganic HTMs such as CuI, CuSCN, NiO, and Cu2O which potentially have better stability than organic HTMs have also been tested in PSCs [16–18]. Christians

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In addition to organic HTMs, inorganic HTMs such as CuI, CuSCN, NiO, and Cu2 O which potentially have better stability than organic HTMs have also been tested in PSCs [16–18]. Christians and co-workers [19] were the first to apply inorganic HTM materials in PSCs. They fabricated copper iodide (CuI) on regular-structured PSC and obtained a conversion efficiency of 6% with low open-circuit voltage (VOC ) of 0.79 V that was due to high recombination levels. The PCE of the CuI-based HTM swiftly reached 16.8% in inverted planar architecture [20]. Inherent advantages, such as the ability to reduce the production costs, suitable energy levels, high hole mobility as well as enhancement of its resistance to degradation, make inorganic HTMs a promising class of materials to replace spiro-OMeTAD [21,22]. In addition, the stability of PSCs has been improved by inorganic HTMs and the conversion efficiency has rapidly increased in the past few years [23]. However, it has been reported that most solvents used for inorganic material deposition dissolve the perovskite materials, and this could be the reason why there is a dearth of scientific reports that report on applications of inorganic HTMs in PSCs. One such promising inorganic HTM is CuSCN, which is an inexpensive and abundant metal pseudohalide of singly ionized copper. It has a well-aligned work function and has demonstrated high mobility as well as good thermal stability [24]. Therefore, this review is focused on reporting the progress of CuSCN inorganic HTMs. This review also discusses the various fabrication routes employed up to date to prepare CuSCN-based PSCs. This review paper also evaluates the photovoltaic performance of the tested solar cell devices employing CuSCN HTMs. Lastly, the review paper presents the various architectures employed in the PSCs. The triumphs and challenges of the CuSCN HTMs in PSC are presented here in detail. 2. Roles and Ideal Characteristics of HTM Hole transport materials play different crucial roles, such as: (i) blocking electron transfer to the back-contact metal electrode, (ii) extraction of photo-generated holes from the perovskite and transportation of these extracted charges back to the back-contact metal electrode, and (iii) prevent direct contact between the metal electrode and the perovskite layer [25–29]. An ideal HTM should exhibit properties, such as: (i) high hole mobility, chemical stability and thermal robustness to withstand annealing process during fabrication of the PSC; (ii) the highest occupied molecular orbital (HOMO) energy level should match the valence band energy of the perovskite; (iii) should protect the perovskite layer from air and moisture and be able to prevent the diffusion of external moieties or elements inside the photo-absorber. Moreover, the HTM should have a low annealing temperature and short annealing times to avoid degradation of the underlying layers. Last but not least, the HTM (iv) should show minimum absorption in the visible and near-infrared of the solar spectrum to avoid absorption of photons during the photo-excitation of excitons [29–33]. CuSCN has received significant attention as an HTM in PSC due to the fact that it has a higher maximum hole mobility of about 0.1 cm2 V−1 s−1 which is far larger than of spiro-OMeTAD with a maximum hole mobility of 6 × 10−5 cm2 V−1 s−1 . Moreover, CuSCN HTM has suitable energy levels (Figure 2) as well as affordable and simplified synthesis routes as compared to spiro-OMeTAD which makes it ideal for large-scale applications. In light of these observations, this review paper will focus on the recent progress of CuSCN HTM in PSCs.

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Cu2O Cu2O

NiO NiO 5.3 5.3

3.2 3.2

5.4 5.4

1.5 1.5

CuSCN CuSCN

5.1 5.1

2.1 2.1

CuI CuI

5.1 5.1

Spiro-OMeTAD Spiro-OMeTAD

5.4 5.4

3.5 3.5

PEDOT: PEDOT: PSS PSS

3.9 3.9

MAPbI3 MAPbI3

4.0 4.0

TiO2 TiO2

Energy/ -eV Energy/ -eV

2.2 2.2

1.8 1.8

5.2 5.2

5.3 5.3

7.0 7.0

Figure 2. thethe energy levels of CuSCN, perovskite material, TiO2 ETM HTMs Figure 2. Schematic Schematicofof energy levels of CuSCN, perovskite material, TiO2 and ETMvarious and various Figure 2. Schematic of the energy levels of CuSCN, perovskite material, TiO 2 ETM and various HTMs [34,35].[34,35]. HTMs [34,35].

3. of CuSCN CuSCN HTM HTM 3. Synthesis Synthesis and and Deposition deposition of 3. Synthesis and deposition of CuSCN HTM Copper Copper thiocyanate thiocyanate is is aa p-type p-type semiconductor semiconductor with with appealing appealing characteristics characteristics such such as as aa wide wide 2 − 1 − 1 Copper thiocyanate is a p-type semiconductor with appealing characteristics such as a wide band mobility of of 0.01–0.1 cmcmV2V−1ss−1, ,high hightransparency, transparency, and and high band gap gap of of3.6 3.6V,V,high highhole hole mobility 0.01–0.1 high chemical chemical −1s−1, high transparency, and high chemical band gap [36]. of 3.6CuSCN V, highexists hole mobility 0.01–0.1 with cm2Vthe stability CuSCN exists in αwith structures stability [36]. in α- and andofβ-phase, β-phase, the structures given given in in Figure Figure 3. 3. It It has has been been stability [36]. CuSCN exists in αand β-phase, with the structures given in Figure 3. It has beenas reported that β-phase is more stable and is easily accessible. The αand β-phase can be classified reported that β-phase is more stable and is easily accessible. The α- and β-phase can be classified as reported that β-phase is more stable and is easily structure, accessible.respectively The α- and β-phase can be classified as orthorhombic and or [37]. other inorganic orthorhombic and hexagonal hexagonal or rhombohedral rhombohedral structure, respectively [37]. Unlike Unlike other inorganic orthorhombic orSnO, rhombohedral respectively [37]. Unlikematerial other inorganic HTMs Cu CuSCN efficient hole injection/extraction as as well as HTMs such suchas asand Cu2hexagonal 2O, O,NiO NiOand and SnO, CuSCNisisanstructure, an efficient hole injection/extraction material well HTMs such as Cu 2 O, NiO and SnO, CuSCN is an efficient hole injection/extraction material as well an blocking material due to due its appropriate electronic electronic levels. The levels. excellent hole transporting as electron an electron blocking material to its appropriate The excellent hole asproperties an electron blocking to its appropriate electronic TheS 3p excellent of properties CuSCN areofmaterial due to Cudue 3d due orbital with hybridization from the orbitals near transporting CuSCN are to Cu 3dsome orbital with some levels. hybridization from thehole S 3p transporting properties ofband CuSCN are due 3d orbital some from the S 3p * antibonding the valence edge and theedge π* antibonding orbital from with the cyanide portion nearportion the conduction orbitals nearband the valence and thetoπCu orbital fromhybridization the cyanide near the * antibonding orbital from the cyanide portion near the orbitals near the valence band edge and the π band edge. Additionally, CuSCN can beCuSCN modifiedcan easily to its quasi-molecular resulting conduction band edge. Additionally, be due modified easily due to itsproperty quasi-molecular conduction band edge. Additionally, CuSCN be modified easily due to CuSCN its choice quasi-molecular in new materials with different properties [38].can This makes CuSCN a material of various property resulting in new materials with different properties [38]. This makes a for material of property resulting in new materials with different properties [38]. This makes CuSCN a material of optoelectronic applications. choice for various optoelectronic applications. choice for various optoelectronic applications.

(a) (a)

(b) (b)

Figure Bulk three-dimensional three-dimensional phases phases of CuSCN: (a) α-phase Figure 3. 3. Bulk α-phase (orthorhombic); (orthorhombic); (b) (b) β-phase β-phase Figure 3. Bulk three-dimensional of CuSCN: (a)S; α-phase (orthorhombic); β-phase (hexagonal) where brown sphere Cu; yellow sphere = = C; and blue(b) = (hexagonal) sphere ==phases gray sphere sphere = N. (hexagonal) where brown sphere = Cu; yellow sphere = S; gray sphere = C; and blue sphere = N. Reproduced Reproduced with with permission permission from from Reference Reference [38]. [38]. Wiley Wiley2017. 2017. Reproduced with permission from Reference [38]. Wiley 2017.

CuSCN CuSCN has has been been used used in in quantum quantum dot-based dot-based light-emitting light-emitting diodes diodes [39], [39], organic organic light-emitting light-emitting CuSCN has been used in quantum dot-based light-emitting diodes [39], organic light-emitting diodes [40] and transparent thin-film transistors, and has yielded exceptional results [41]. to diodes [40] and transparent thin-film transistors, and has yielded exceptional results [41]. DueDue to the diodes [40] and transparent thin-film transistors, and has yielded [41].HTMs, Due toPSCs the the simplified, affordable straightforward techniques usedexceptional fabricateresults CuSCN HTMs, simplified, affordable andand straightforward techniques used totofabricate CuSCN PSCs simplified, affordable and straightforward techniques used to fabricate CuSCN HTMs, PSCs

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employing CuSCN markets asas compared to employing CuSCN HTMs HTMshave haveaagreater greateropportunity opportunitytotoreach reachthe thecommercial commercial markets compared PSCs employing spiro-OMeTAD as HTM. to PSCs employing spiro-OMeTAD as HTM. To that such as as doctor doctor blading, blading, spin spin coating, coating, spray spray coating, coating, To that end, end, simple simple deposition deposition techniques techniques such and electrodeposition have been used in depositing CuSCN HTMs [42–44] which augur well and electrodeposition have been used in depositing CuSCN HTMs [42–44] which augur well with with the commercialization of these However, itit has has been been reported reported that that solvent solvent selection selection for for the commercialization of these devices. devices. However, solution-based deposition deposition methods methods plays plays aa crucial crucial part part in in the of solution-based the overall overall photovoltaic photovoltaic performance performance of PSCs employing CuSCN HTMs. Polar solvents, such as ethyl sulfide, dipropyl sulfide, and diethyl PSCs employing CuSCN HTMs. Polar solvents, such as ethyl sulfide, dipropyl sulfide, and diethyl sulfide [45–47] [45–47] have of CuSCN with dipropyl dipropyl sulfide sulfide being sulfide have been been used used in in the the deposition deposition of CuSCN HTMs, HTMs, with being the the most widely used. The use of a wide range of solvents is very limited in case of CuSCN as it is only most widely used. The use of a wide range of solvents is very limited in case of CuSCN as it is only soluble in in polar polar solvents. solvents. However, However, these these polar polar solvents solvents tend tend to to dissolve dissolve the the perovskite perovskite layer layer as as well, well, soluble which thinthin perovskite layers and, consequently, lowers the power efficiency which results resultsininvery very perovskite layers and, consequently, lowers theconversion power conversion (PCE). Qin et al. [48] used doctor blading to deposit CuSCN using dipropyl sulfide solvent. A film efficiency (PCE). Qin et al. [48] used doctor blading to deposit CuSCN using dipropyl sulfide solvent. thickness of 600 nm was reported and there was some evidence of dissolution of the perovskite on A film thickness of 600 nm was reported and there was some evidence of dissolution of the perovskite thethe CuSCN. A PCE of of 12.4% was exhibited byby the and on CuSCN. A PCE 12.4% was exhibited theCuSCN-based CuSCN-basedPSC, PSC,with withaaVVOC OC of of 1.016 1.016 V V and −2 . The number of deposition cycles did not have −2 short-circuit current density (J ) of 19.7 mAcm SC) of 19.7 mAcm . The number of deposition cycles did not have an short-circuit current density (JSC an effect JSC, however, ledantoimproved an improved FF.aIn a move to reduce dissolution of OC and effect on on VOCVand JSC, however, it ledit to FF. In move to reduce the the dissolution of the the perovskite layer in the HTM, Sepalage et al. [49] applied a chlorobenzene protective layer on the perovskite layer in the HTM, Sepalage et al. [49] applied a chlorobenzene protective layer on the perovskite prior The PSC PSC exhibited exhibited aa PCE PCE of of 9.6% 9.6% which which perovskite prior to to deposition deposition of of CuSCN CuSCN by by doctor doctor blading. blading. The to the the best PSC to best of of our our knowledge knowledge is is the the highest highest achieved achieved to to date date in in aa planar planar (n-i-p) (n-i-p) CuSCN-based CuSCN-based PSC device. The doctor blading method involved wetting the perovskite layer with chlorobenzene prior to device. The doctor blading method involved wetting the perovskite layer with chlorobenzene prior depositing thethe CuSCN inin dipropyl sulfide. CuSCN to depositing CuSCN dipropyl sulfide.This Thisresulted resultedininthe theformation formationof ofaa thick thick uniform uniform CuSCN film. Whilst buffer resulted in relatively thinthin filmsfilms as shown here film. Whilst the thedevice devicewithout withoutthe thechlorobenzene chlorobenzene buffer resulted in relatively as shown in Figure 4. Figure 4, clearly shows that dissolution of the perovskite layer occurs when only dipropyl here in Figure 4. Figure 4, clearly shows that dissolution of the perovskite layer occurs when only sulfide is sulfide used forisdeposition of CuSCN. dipropyl used for deposition of CuSCN.

Figure 4. 4. Scanning Electron Microscopy (SEM) cross-sectional images 2 /CH of 3FTO/cScanning Electron Microscopy (SEM) cross-sectional images of FTO/c-TiO NH3 TiO /CH3NH3PbI 3/CuSCNthat assemblies that wereby fabricated by doctor blading aofsolution in PbI32/CuSCN assemblies were fabricated doctor blading a solution CuSCNofinCuSCN dipropyl 2 S) and (c, d) on a dipropyl sulfide (a, b) on a chlorobenzene pre-wetted perovskite layer (CBZ→Pr sulfide (a,b) on a chlorobenzene pre-wetted perovskite layer (CBZ→Pr2 S) and (c,d) on a dry perovskite 2S only).electron SE: secondary images; BS: backscattered electron images. dry layer layerperovskite (Pr2 S only). SE: (Pr secondary images; electron BS: backscattered electron images. Reproduced with Reproduced withReference permission from Reference permission from [49]. Elsevier 2017.[49]. Elsevier 2017.

Doctor blading blading is is aa simple, and low-cost low-cost thin thin film film deposition deposition method method that that is is Doctor simple, solution-based solution-based and suitable for roll-to-roll (R2R) device fabrication. It is one of the common deposition methods used suitable for roll-to-roll (R2R) device fabrication. It is one of the common deposition methods used for for depositing CuSCN HTMs. However, Yang [42]reported reportedthat thatdamage damageof ofthe the perovskite perovskite layer layer depositing CuSCN HTMs. However, Yang et et al.al. [42] induced by by doctor doctor blading blading of of CuSCN CuSCN HTM HTM in in planar planar (n-i-p) (n-i-p) PSC PSC devices devices may may be be the the reason reason for for low low induced

performance of planar (n-i-p) devices as compared to inverted (p-i-n) devices. Yang and associates reported an economical simple spray deposition method employing a homemade spray equipment.

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performance of planar (n-i-p) devices as compared to inverted (p-i-n) devices. Yang and associates reported an economical simple spray deposition method employing a homemade spray equipment. The The deposition The CuSCN CuSCN was was dissolved dissolved in in dilute dilute dipropyl dipropyl sulfide. sulfide. The deposition process process involved involved spraying spraying ◦ C for 5 s. The thickness of the HTM CuSCN at a distance of about 6 cm for 2 s and drying at 80 CuSCN at a distance of about 6 cm for 2 seconds and drying at 80 °C for 5 seconds. The thickness of layer was layer controlled by the number deposition cycles. Spray deposition resulted inresulted the formation the HTM was controlled by theofnumber of deposition cycles. Spray deposition in the of a uniform layer with alayer thickness 50 nm. There 50 wasnm. no significant to the formation of CuSCN a uniform CuSCN with of a about thickness of about There was damage no significant perovskite layer which was attributed to the minimum contact of the perovskite layer to the dipropyl damage to the perovskite layer which was attributed to the minimum contact of the perovskite layer sulfide solvent. A PCE ofsolvent. 17% wasAreported for a was Au/CuSCN/perovskite/TiO VOC 2 /FTO device and a to the dipropyl sulfide PCE of 17% reported for a Au/CuSCN/perovskite/TiO 2/FTO of about 1 V which is typical of organic HTM-based PSCs without additives. Spray coating is a simple device and a VOC of about 1 V which is typical of organic HTM-based PSCs without additives. Spray deposition is compatible with large-scale fabrication. However,fabrication. MadhavanHowever, et al. [44] coating is amethod simple which deposition method which is compatible with large-scale proved CuSCN by doctor blading can still attain high PCEscan in mesoscopic (n-i-p)-based Madhavan et al.deposited [44] proved CuSCN deposited by doctor blading still attain high PCEs in PSC devices. In their work, they reported a PCE of 16.6% for a doctor-bladed CuSCN-based PSCs mesoscopic (n-i-p)-based PSC devices. In their work, they reported a PCE of 16.6% for a doctorwhile the spin-coated CuSCN-based device had a 15.4%. The difference in PCE was attributed to bladed CuSCN-based PSCs while the spin-coated CuSCN-based device had a 15.4%. The difference different HTM film thickness, with the doctor CuSCN-based film having thickness of about in PCE was attributed to different HTM film bladed thickness, with the doctor bladedaCuSCN-based film 500 nm while the spin-coated CuSCN-based had a thickness of about 30 nm as shown in Figure 5. having a thickness of about 500 nm while the spin-coated CuSCN-based had a thickness of about 30 The spin-coated CuSCN5.thin were uniform to effectively extract holesto even though nm as shown in Figure Thefilms spin-coated CuSCNand thinmanaged films were uniform and managed effectively the films were limited to only 30 nm. When compared to doctor blading, spin coating is not cost extract holes even though the films were limited to only 30 nm. When compared to doctor blading, effective and is not compatible with a large-scale manufacture where high-throughput fabrication spin coating is not cost effective and is not compatible with a large-scale manufacture where highis required. fabrication The non-cost effectiveness spin coating can be of attributed to the requirement ofto large throughput is required. The of non-cost effectiveness spin coating can be attributed the volumes of solvents and raw materials used in spinning coating [50]. requirement of large volumes of solvents and raw materials used in spinning coating [50].

Figure 5.5.(a) (a)SEM SEM cross-section of spiro-OMeTAD-based (b) SEM cross-section of HTM cross-section of spiro-OMeTAD-based HTM;HTM; (b) SEM cross-section of HTM deposited by doctor-blade technique; (c)technique; SEM cross-section HTM deposited spin-coating technique; (d) SEM deposited by doctor-blade (c) SEMofcross-section of by HTM deposited by spin-coating cross-section PSC without HTM. Reproduced with Reproduced permission from Reference [44]. technique; (d)of SEM cross-section of PSC without HTM. with permission from American Reference Chemical Society 2016. Society 2016. [44]. American Chemical

Another method has been deposition of CuSCN HTM in PSCs HTM is electrodeposition. Another methodwhich which has used been for used for deposition of CuSCN in PSCs is Electrodeposition is a simple method which is low cost and is compatible with large-scale electrodeposition. Electrodeposition is a simple method which is low cost and is compatible with fabrication fabrication [51]. Shlenskaya et al. [52]etreported an electrodeposited CuSCNCuSCN blocking layer layer from large-scale [51]. Shlenskaya al. [52] reported an electrodeposited blocking a solution of CuSO Na2 EDTA, NaSCN a three-electrode cell. The electrodeposition mechanism from a solution of4 ,CuSO 4, Na2and EDTA, and in NaSCN in a three-electrode cell. The electrodeposition proceeded via nucleation and growth of the resulting CuSCN thin films. Formation of a porous mechanism proceeded via nucleation and growth of the resulting CuSCN thin films. Formation of a CuSCNCuSCN involved spin coating the sacrificial polystyrene template on b-CuSCN/ITO substrates from porous involved spin coating the sacrificial polystyrene template on b-CuSCN/ITO substrates a water/ethanol suspension in the presence a surfactant. was followed electrodeposition from a water/ethanol suspension in the ofpresence of aThis surfactant. Thisbywas followed by of CuSCN into the template at − 0.1 V and with the thickness of the deposited film dependentfilm on electrodeposition of CuSCN into the template at −0.1 V and with the thickness of the deposited the potentiostatic time.deposition They obtained film thickness which ranged from which 400 to dependent on thedeposition potentiostatic time.a CuSCN They obtained a CuSCN film thickness 1400 nm for 0.5–3 layers of CuSCN. Porous CuSCN can be applied in mesoscopic (p-i-n) PSC devices. ranged from 400 to 1400 nm for 0.5–3 layers of CuSCN. Porous CuSCN can be applied in mesoscopic Nevertheless, Shlenskaya and associates did notand report on the optical, as well as the (p-i-n) PSC devices. Nevertheless, Shlenskaya associates did notstability report properties on the optical, stability

properties as well as the photovoltaic performance of porous CuSCN HTM-based PSCs. Hence, there is a need to conduct more research to evaluate the optical stability properties and photovoltaic performance of electrodeposited porous CuSCN-based PSC.

As we have mentioned earlier, finding an ideal solvent for deposition CuSCN HTMs has been a challenge. Research efforts have been dedicated to finding an ideal solvent which can dissolve CuSCN without damaging the perovskite layer. Recently, Murugadoss et al. [53] investigated the suitability of various solvent mixtures in dissolving CuSCN prior to deposition. The solvents were Materials 2018, 11, 2592 7 of 19 mixed in the following ratios; dipropyl sulfide and chlorobenzene (1:1); isopropanol and methylammonium iodide (MAI) (10 mg/ml); dipropyl sulfide, isopropanol and MAI ((1:2) +10 mg/ml) photovoltaic performance porous HTM-based PSCs. Hence,dipropyl there is a need to conduct more which were assigned S2, S3of and S4,CuSCN respectively. The pristine sulfide solvent was assigned research to evaluate the optical stability properties and photovoltaic performance of electrodeposited S1. The photovoltaic parameters of the PSCs fabricated with different solvent ratios are shown here porous CuSCN-based PSC. in Table 1. As we have mentioned earlier, finding an ideal solvent for deposition CuSCN HTMs has been a challenge. Research efforts have been dedicated to finding an ideal solvent which can dissolve CuSCN Table 1. Thethe photovoltaic parameters ofMurugadoss the PSCs fabricated different ration. without damaging perovskite layer. Recently, et al. [53] with investigated thesolvent suitability of various solvent mixtures in dissolving CuSCN prior to deposition. The solvents were mixed in the Solvent Sampleand methylammonium JSC VOC following ratios; dipropyl sulfide and chlorobenzene (1:1); isopropanol iodide FF Mixture Codes (mAcm ) assigned (V) (%) (MAI) (10 mg/mL); dipropyl sulfide, isopropanol and MAI ((1:2) +10 mg/mL) which -2were S2, S3 and S4, respectively. The pristine S1. The photovoltaic pristine dipropyl sulfidedipropyl sulfide solvent was S1 assigned 18.76 0.92 56.0 parameters of the PSCs fabricated with different solvent ratios are shown here in Table 1. dipropyl sulfide and chlorobenzene (1:1) S2 18.93 0.95 49.0

isopropanolTable and1.methylammonium iodide (MAI) The photovoltaic parameters of the PSCs fabricated with S3 different solvent 18.31ration. 0.84 (10 mg/ml) Solvent Mixture Sample Codes VOC (V) FF (%) PCE (%) JSC (mAcm−2 ) isopropanol andsulfide MAI ((1:2) +10S1mg/ml) 18.76 S4 pristine dipropyl 0.92 56.019.42 9.790.92 dipropyl sulfide and chlorobenzene (1:1) isopropanol and methylammonium iodide (MAI) (10 mg/mL) isopropanol and MAI ((1:2) +10 mg/mL)

S2 S3 S4

18.93

0.95

49.0

18.31

0.84

55.0

8.53

19.42

0.92

56.0

10.07

FF= Fill factor.

PCE (%) 9.79 8.97

55.0

8.53

56.0

10.07

8.97

Murugadoss et al. [53] found that the peak due to PbI2 was of low intensity for S1 and FF = XRD Fill factor. higher for other solvents, as illustrated in Figure 6. Figure 6 shows PbI2 peaks of higher intensities for Murugadoss et al. [53] found that the XRD peak due to PbI2 was of low intensity for S1 and higher S2 and S3. This signified the high dissolution/decomposition rates of the perovskite structure in the for other solvents, as illustrated in Figure 6. Figure 6 shows PbI2 peaks of higher intensities for S2 presence of other solvents. the resulting efficiency of the fabricated wasinlower and S3. This signified theHence, high dissolution/decomposition rates of the perovskitePSCs structure the as shown in Table 1. Table 2 gives a summary different solvents, used in presence of other solvents. Hence, theof resulting efficiency of themethods fabricatedand PSCsfabrication was lower asconditions shown in Table 1. Table 2 gives a summary of different solvents, methods and fabrication conditions used in the fabrication of CuSCN thin films. the fabrication of CuSCN thin films.

Figure 6. XRD patterns of FTO/bl-TiO /m-TiO /CH3NH3PbI3/CuSCN film, CuSCN was prepared

2 Figure 6. XRD patterns of FTO/bl-TiO22/m-TiO 2/CH3NH3PbI3/CuSCN film, CuSCN was prepared using using different solvents, S1, S2, S3 and S4, deposited by doctor blading method under 80 ◦ C at air different solvents, S2,corresponding S3 and S4,PbI2 deposited by are doctor blading method atmosphere. The S1, peaks and MAPbI3 labelled by asterisk the (*)under and (♦),80 °C at air and MAPbI 3 are labelled by asterisk the (*) and ( ◊ ), atmosphere. The peaks corresponding PbI2Reference respectively. Reproduced with permission from [53]. Elsevier 2017. respectively. Reproduced with permission from Reference [53]. Elsevier 2017.

Materials 2018, 11, 2592

8 of 19

Table 2. Fabrication of CuSCN films using different starting material and solvents. Starting Material

Solvent Used

Temp

Additives

CuSO4 , KSCN CuSO4 , KSCN

DI water

Duration

RT

EDTA

Electrochemical

Deposition

DI water

RT

DEA

Electrochemical

RT

TEA, EDTA, CDTA, NTA

Electrochemical

80

[56]

300 ~400 13 ~30 ~500

[57] [58] [59]

450

[53]

10–40 3–5

[60] [61]

CuSO4 , KSCN

DI water

CuSCN CuSCN CuSCN

Dipropyl sulfide Dipropyl sulfide Dipropyl sulfide

4h Overnight 5h

RT RT RT

CuSCN

Dipropyl sulfide

Overnight

RT

CuSCN

DMSO Dipropyl sulfide; Dipropyl sulfide + Chlorobenzene; Isopropanol + MAI; Dipropyl sulfide + isopropanol + MAI Diethyl sulfide Diethyl sulfide, Ammonia

2h

RT

Spin-coating Doctor-blading Spin-coating Spin-coating Doctor-blading Spin-coating

Overnight

RT

Doctor-blading

1h

RT 50 ◦ C

CuSCN CuSCN CuSCN

Spin-coating Spin-coating

Thickness (nm)

Ref.

70–90

[54] [55]

[44] [47]

RT = room temperature; Temp = temperature; DI = deionized DMSO = dimethyl sulfoxide.

4. Architectures Used for CuSCN-Based PSCs Device structure also plays an important role in the stability and efficiency of PSCs. The mesoscopic device architecture commonly demonstrates higher PCE as compared to planar device architecture. This can be attributed to the greater contact area with perovskite as well as decreased carrier transport distance in mesoscopic PSCs [62]. Having dipropyl sulfide as the solvent of choice during deposition of CuSCN limits the PSC device architecture which can be used. This results in inverted device architecture being the device structure of choice when it comes to CuSCN-based PSCs. However, to the best of our knowledge, there are no reports on mesoscopic (p-i-n) CuSCN-based PSCs. CuSCN films were first fabricated on the mesoporous structure by Ito et al. [63] and the structure proposed was FTO/compact TiO2 /mesoporous TiO2 /CH3 NH3 PbI3 /CuSCN/Au which achieved a low PCE of 4.85%. Even though mesoscopic structure-based PSCs with spiro-OMeTAD HTM have exhibited high efficiency of 22.1%, they require high temperature during fabrication of mesoporous TiO2 layer which is not good for flexible PSCs [64] and does not bode well with characteristics of an ideal HTM. Hence, regular and inverted planar architectures were taken into consideration. The first inverted planar architecture dates back to 2013; it was reported by Chen et al. [27] where it was applied in PSCs using PEDOT:PSS as the HTM. Unfortunately, their device degraded abruptly due to the acidic and hygroscopic nature of PEDOT:PSS. The inverted planar architecture was also introduced in inorganic HTM-based PSCs and has shown promising results as it requires low-temperature processing conditions. This makes it compatible with flexible substrates and moreover, inverted planar devices exhibit high FF [65,66]. Inverted planar architecture offers an easy device structure which is compatible with large-scale fabrication. Furthermore, it has comparable conversion efficiency with a mesoporous device structure, and negligible hysteresis effect made this structure the best so far [57,58]. The regular planar-based CuSCN PSCs, on the one hand, have exhibited high conversion efficiency and stability. Additionally, the planar structure is characterized by low-temperature processing requirements, low-cost, tunable device performance and an architecture that is capable of employing different interfacial materials [67,68]. In the following sub-sections, we discuss some of the regular planar and inverted planar-based CuSCN films structures and their device performances that have been reported to date. 4.1. n-i-p Architecture of CuSCN-Based PSCs In 2014, Chavhan et al. [69] worked on organo-metal halide PSCs with CuSCN as the inorganic hole selective content and glass/FTO/TiO2 /CH3 NH3 PbI3−x Clx /CuSCN/Au as the device architecture. The fabricated planar PSCs were annealed at different temperatures; thereafter, solar cell characterization was performed. In their work, they discovered that PSC device annealed at 110 ◦ C showed improved performance with a PCE of 6.4%. The photovoltaic performance parameters

In 2014, Chavhan et al. [69] worked on organo-metal halide PSCs with CuSCN as the inorganic hole selective content and glass/FTO/TiO2/CH3NH3PbI3-xClx/CuSCN/Au as the device architecture. The fabricated planar PSCs were annealed at different temperatures; thereafter, solar cell characterization was performed. In their work, they discovered that PSC device annealed at 110 °C showed improved performance with a PCE of 6.4%. The photovoltaic performance parameters of Materials 2018, 11, 2592 9 of 19 other PSCs with planar structure glass/FTO/TiO2/CH3NH3PbI3-xClx/CuSCN/Au annealed at different temperatures are shown here in Table 3. of other PSCs with planar structure glass/FTO/TiO2 /CH3 NH3 PbI3−x Clx /CuSCN/Au annealed at Table 3. temperatures Photovoltaic performance of 3. glass/FTO/TiO2/CH3NH3PbI3-X/CuSCN/Au device different are shown parameters here in Table annealed at different temperatures. Table 3. Photovoltaic performance parameters -2of glass/FTO/TiO2 /CH3 NH3 PbI3−X /CuSCN/Au Annealing Temp. (°C) Jsc (mAcm ) Voc (V) FF (%) PCE (%) device annealed at different temperatures.

90 100 (◦ C) Annealing Temp. 90 110 100 120 110 120

13.04 14.27−2 ) Jsc (mAcm 14.4 13.04 11.1 14.27

0.49 0.67 Voc (V) 0.73 0.49 0.45 0.67

14.4 temperature. 0.73 Temp.= 11.1 0.45

49.0 3.1 48.1 FF (%)4.5 61.7 49.0 6.4 53.8 48.1 2.7 61.7 53.8

PCE (%) 3.1 4.5 6.4 2.7

Temp. =7temperature. It is clearly evident from Table 3 and Figure that annealing had a great impact on solar cell performance as it has exhibited different photovoltaic performances at different annealing It is clearly evident from Table 3 and Figure 7 that annealing had a great impact on solar cell temperatures. It is also evident from Table 3 that the planar PSC structure yielded impressive results performance as itahas exhibited different photovoltaic at differentThe annealing temperatures. and even achieving good FF of 61.7% which signifies performances good carrier selectivity. efficiency of the It is also evident from Table 3 that the planar PSC structure yielded impressive results and even CuSCN-based PSC was comparable to the first spiro-OMeTAD-based PSC with an efficiency of 9.7% good FFofofthe 61.7% which signifies good carrier selectivity. The efficiency of the CuSCN-based [8]. achieving However,athe Voc planar PSC structures was a major setback, with a reported low value of PSC was comparable to the first spiro-OMeTAD-based PSC with an efficiency of 9.7% [8]. However, 0.45 V which is indicative of short diffusion length of