Khan Mamun Reza, Sally Mabrouk, Qiquan Qiao*
Department of Electrical Engineering, Center for Advanced Photovoltaics, South Dakota State University, Brookings, South Dakota 57007, United States
Proceedings of the Nature Research Society, Volume 2, Article Number 02004, 2018
Published Online: 30 March 2018 (Review)
DOI: 10.11605/j.pnrs.201802004
*Corresponding Author. Email: This email address is being protected from spambots. You need JavaScript enabled to view it.

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Abstract

Poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) is the most commonly used hole transport layer (HTL) in inverted (p-i-n) structured perovskite solar cells (PSCs) due to remarkable transparency and conductivity. Despite high transparency and conductivity, PEDOT:PSS has some concealed problems such as acidity, hygroscopic  nature, lower work function than the ionization potential of perovskite, and poor electron blocking capability. All these properties hinder the efficient charge extraction and transport from perovskite absorber to ITO. In addition, acidic and hygroscopic nature of PEDOT:PSS can corrode the ITO electrode and degrade the moisture sensitive PSK layer, respectively. These can degrade the long-term stability and lower the performance of PSCs. Therefore, tailoring PEDOT:PSS HTL is essential for achieving highly efficient PSCs to mitigate these drawbacks. This review article gives an overview of approaches tailoring PEDOT:PSS that can minimize the limitations as HTL to improve the performance of PSCs. These include solvent treatment, composite structure, doping, and bi-layer structure PEDOT:PSS. In addition, the roles of tailored PEDOT:PSS HTL was understood in perovskite solar cells.

Introduction

Metal halide based perovskite (PSK) materials possess excellent structural, optical and electrical properties including low exciton binding energy [1], broad absorption spectrum in visible range [2], large electron and hole diffusion lengths [1, 3], ambipolar charge transport capabilities with high electron hole mobility [4]. Because of these excellent properties, PSCs exhibit enormous advantages in power conversion efficiency (PCE) as it increased from 3.8% in 2009 to 22.7% in 2017 and still have potential of reaching even higher efficiency [5, 6]. Perovskite materials do not require interfaces for exciton separation due to weak exciton binding energy. However, the PCE without HTL (also act as electron blocking layer) is lower than that of PSCs with HTL due to the probability of higher charge recombination [7-9]. Therefore, effective HTLs are still an important issue to achieve higher efficiency of PSCs.

In inverted (p-i-n) structured PSCs, the HTL requires high transparency in addition to conductivity. Among several conductive polymers used as charge transport layers, PEDOT:PSS is the most commonly used HTL [10]. Jeng et al. in 2013 first reported on PEDOT:PSS in inverted planar PSCs with a device structure of ITO/PEDOT:PSS/CH3NH3PbI3/C60/BCP/Al [11]. In addition to high transparency and conductivity, PEDOT:PSS HTL is solution-processable at low temperature, making it compatible for mechanically flexible devices [12, 13]. Nevertheless, PEDOT:PSS is not considered as the most efficient and appropriate HTL.

Electrical conductivity of PEDOT:PSS is significantly lower than that of commonly used metal cathode due to the presence of insulating PSS, which causes energy loss during charge transport, reducing the photocurrent desnity. Moreover, conductivity of PEDOT:PSS is lower compared to that of widely used electron transport layers (ETLs) such as [6,6]- phenyl-C61-butyric acid methyl ester (PC61BM). Unbalanced charge transport from the conductivity difference of ETL and HTL can create charge carrier accumulation, which reduces shut resistance, resulting in large leakage current and low fill factor (FF) [14, 15]. Furthermore, mismatched energy level alignment between the work function (WF) of PEDOT:PSS and the valence band maximum (VBM) of PSK causes inferior open circuit voltage (Voc) due to large potential energy loss at the interface [ 16- 18]. In addition, acidic and hygroscopic nature of PEDOT:PSS can corrode the ITO electrode and degrade the moisture sensitive PSK layer, respectively; these can degrade the long-term stability and lower the performance of PSCs [19]. Inferior crystallization of PSK on PEDOT:PSS is also one of the major reasons behind the poor performance of PSCs [20]. In these circumstances, tailoring PEDOT:PSS can improve the performance of PSCs.

In this review, we focus on various approaches applied to modify the properties of PEDOT:PSS HTL of inverted (p-i-n) structured PSCs. Properties change of HTL due to tailored PEDOT:PSS and the impact on crystallization of PSK layer are also provided. Finally, the factors that improve the performance of PSCs after modification of PEDOT:PSS HTL are also discussed.

2 Approaches tailoring the PEDOT:PSS HTL in perovskite solar cells

2.1 Solvent treated PEDOT:PSS HTL

Conductivity of PEDOT:PSS depends on the arrangement and the amount of PEDOT and PSS. Most reports suggested that solvent additive methods help to rearrange PEDOT and PSS. In addition, solvent post treatment partially removes the insulating PSS from the film resulting in an increased ratio of PEDOT to PSS [21, 22]. Both the procedures help PEDOT to orient themselves leading to high crystallinity and efficient charge transport [23].

In 2017, Huang et al. [24] added different volume ratios of dimethylsulfoxide (DMSO) into the aqueous solution of PEDOT:PSS, which considerably enhanced charge extraction and photocurrent production of PSCs. The PCE of untreated PEDOT:PSS based devices was 11.8%, which significantly increased to 15.8% with a champion cell at 16.7%. The higher shunt resistance (Rsh) induced by the DMSO treated PEDOT:PSS indicated a lower leakage current while more efficient PL quenching confirmed that the charge transfer between the perovskite and DMSO-treated PEDOS:PSS is more favorable [25, 26]. PEDOT:PSS treatment also affected the morphology and crystallization of the perovskite layer. DMSO added PEDOT:PSS surface showed the enhancement of aggregated PEDOT-rich particles. These aggregated particles acted as the growing sites for crystal nucleus of PSK films during annealing, which resulted in more compact, highly crystalline and larger grain size. Improved morphology of PSK accounted for more efficient pathways of charge carrier transport to the electrodes [27, 28].

Enhanced PCEs of perovskite solar cells were achieved by Chen et al. [29] when PEDOT:PSS was rinsed by Methanol (MeOH) and N,N-dimethylformamide (DMF) in comparison to the reference devices with pristine one. Regardless of higher conductivity, the ethylene glycol (EG) treated devices showed lower efficiency. A small fraction of PSS is actually removed when the film is treated with solvents, leading to increased PEDOT ratio in PEDOT:PSS film. Besides the volume fraction of PEDOT, distance and crystallite size of PEDOT also affects the conductivity. Table 1 shows the summary of the crystallite sizes, stacking distances, and the PEDOT ratios in PEDOT:PSS films treated by different solvents. Solvent-treated devices had higher PCEs than the reference one (PCE = 16.69%). DMF is found to be more appropriate as the highest PCE of 18.02% is found for PEDOT:PSS treated by it. However, reduced open-circuit voltages (VOC) and (FF) were observed for EG-treated devices regardless of the highest conductivity among all the treated samples. This may results from the higher surface roughness of ~2.53 nm after EG treatment compared to ~1.98 nm of pristine PEDOT:PSS films (Figure 1). The rough surface of PEDOT:PSS could create sharp traps, which facilitates surface charge-carrier recombination resulting in lower VOC and FF [27]. The RMS roughness of the DMF-treated sample decreased to ≈0.76 nm (Figure 1b). The smoother surface of PEDOT:PSS obtained from the DMF treatment could facilitate the interfacial contact between the HTL and the PSK layer, resulting in a high VOC and FF [30].

Table 1. Summary of the crystallite sizes, stacking distances, and the PEDOT ratios in PEDOT:PSS films treated by organic solvents.

PEDOT:PSS

PSS

PEDOT

PSS

PEDOT

PEDOT/

treatment

distance (Å)

distance (Å)

Crystal size (Å)

Crystal size (Å)

Total ratio

Pristine

4.99

3.53

16.81

12.96

0.41

MeOH

4.92

3.45

17.08

17.99

0.42

DMF

4.97

3.43

21.48

24.41

0.45

EG

4.81

3.39

16.30

18.90

0.40

 

Figure 1. Atomic force microscopy (AFM) topography images of: (a) pristine, (b) DMF-treated, (c) MeOH-treated, and (d) EG-treated PEDOT:PSS. The RMS roughness values are 1.98, 0.76, 1.32 and 2.53 nm, respectively. Reproduced with permission from Ref. [29]

Figure 1. Atomic force microscopy (AFM) topography images of: (a) pristine, (b) DMF-treated, (c) MeOH-treated, and (d) EG-treated PEDOT:PSS. The RMS roughness values are 1.98, 0.76, 1.32 and 2.53 nm, respectively. Reproduced with permission from Ref. [29] 

2.2 Composite structure of PEDOT:PSS HTL

Despite high transparency and conductivity, PEDOT:PSS has some concealed problems such as acidity, hygroscopic nature, lower work function than the ionization potential of perovskite, and poor electron blocking ability [31-33]. Several groups have developed promising alternatives to PEDOT:PSS [ 31, 34-36]. Recently, graphene based derivatives show potential toward high performance PSCs [19].

Wu et al. [37] employed graphene oxide (GO) as HTL in planar PSC and achieved efficiency more than 12%. However, the performance is highly sensitive to the thickness of HTL due to the insulating property of GO. The homogeneous dispersion and high uniformity with full coverage of ITO is difficult to get. This can result in poor hole transport and collection due to the direct contact of PSK layer with the ITO. Furthermore, ununiformed GO deposited on ITO will lead to poor repeatability and reliability of device performance owing to the insulating property of GO. To overcome this, HTLs were prepared by successive spin-coating of GO and PEDOT:PSS (GO/PEDOT:PSS) [38]. The synergistic effects of GO and PEDOT:PSS help to develop efficient HTL for improved performance of PSCs with high repeatability. Meanwhile, GO/PEDOT:PSS composite results in a smoother surface indicating that sequential deposition of PEDOT:PSS and GO can planarize the rough GO films.

GO is an insulator due to the presence of oxygen containing functional groups on the carbon basal plane. Graphene can improve the conductivity of PEDOT:PSS [39]. Moreover, improved charge extraction occurs at the interface of perovskite/graphene-PEDOT:PSS. Despite high electrical conductivity and mobility, graphene can shift the work function towards vacuum level and increase the roughness of PEDOT:PSS HTL, which is not favorable for the deposition of PSK layer. As an alternative GO can be used by improving the conductivity using several reduction methods based on thermal [40, 41], photo reduction [42], chemical [43, 44], or electrochemical approaches [45].

Giuri et al. [46] developed PEDOT:PSS+graphene nanocomposite HTL by reducing GO through a green, simple and cost effective way by using UV radiation. UV treatment improves the wettability of the HTL, which influences the morphology of PSK film and, hence, the devices performances. Huang et al. [47] also used environmental friendly thermal reduction method to reduce GO. Both the HTLs reduced by thermal and UV reduction method give rise to increased open circuit voltage, which suggests that energy level of rGO/PEDOT:PSS HTLs is in good alignment with the VBM of PSK.

Synergistic effect of glucose and GO enhances the wettability of PEDOT:PSS while improving the conductivity of the composite [48]. Improved conductivity is mainly due to the addition of glucose in the GO (GGO) suspension facilitating the GO reduction process [49, 50] while the enhanced wettability is due to the presence of hydroxyl group terminations [51]. The improved morphology of the PSK film on the nanocomposite results in photovoltaic (PV) device performances of 12.8%, higher than the reference cells based on unmodified PEDOT:PSS (9.4% PCE) with a very high , reaching values over 1.05 V. This suggests minimal recombination losses.

Figure 2 shows the XRD patterns of GO, compared to that of GO-PEDOT:PSS and GGO-PEDOT:PSS composites before annealing. Peak at is the characteristic peak of GO. This peak disappears after adding GO to PEDOT:PSS indicating that GO sheets do not stack when dispersed (Figure 2a). Some stacked GO sheets are seen from the SEM images (Figure 2c), whereas addition of glucose results in homogeneous film with evenly dispersed GO sheets (Figure 2d). Steric bonding between carbon glucose and carbon of GO assist to disperse GO, facilitating the formation of homogeneous GGO-PEDOT:PSS film.

Figure 2. a) Diffraction XRD patterns of GO, GO-PEDOT, and GGO-PEDOT, b) nanocomposite precursors; SEM images of c) GO PEDOT and d) GGO-PEDOT nanocomposite films and corresponding AFM micrographs of e) GO-PEDOT (scan area 20 × 20 μm2, Z-range 25 nm) and f) GGO-PEDOT (scan area 50 μm × 50 μm, Z-range 25 nm). Reproduced with permission from Ref. [48]

Figure 2. a) Diffraction XRD patterns of GO, GO-PEDOT, and GGO-PEDOT, b) nanocomposite precursors; SEM images of c) GO PEDOT and d) GGO-PEDOT nanocomposite films and corresponding AFM micrographs of e) GO-PEDOT (scan area 20 × 20 μm2, Z-range 25 nm) and f) GGO-PEDOT (scan area 50 μm × 50 μm, Z-range 25 nm). Reproduced with permission from Ref. [48]

The wettability of PEDOT:PSS films with the addition of glucose and GO was measured using contact angle measurements (Figure 3a). GGO reduces the CE from to . Importantly, CE of perovskite precursor solution on the GGO-PEDOT:PSS has the minimum value of (Figure 3b). This suggests superior wettability and compatibility of GGO-PEDOT:PSS substrates with perovskite precursor solution. Figure 3c shows perovskite grown on GGO-PEDOT:PSS has more compact and closer grains with fewer defects compared to that grown on pristine PEDOT:PSS. Higher performance is mainly due to higher and anticipated from improved conductivity of HTL, and minimal recombination loss and very efficient charge extraction due to high quality perovskite film [52-54].

Figure 3. a) Contact angle of pristine PEDOT:PSS, GO-PEDOT, G-PEDOT, and GGO-PEDOT on ITO, b) contact angle of perovskite precursor drops on nanocomposite substrates, and c) SEM of PSK layer deposited on different nanocomposite substrates. Reproduced with permission from Ref. [48]

Figure 3. a) Contact angle of pristine PEDOT:PSS, GO-PEDOT, G-PEDOT, and GGO-PEDOT on ITO, b) contact angle of perovskite precursor drops on nanocomposite substrates, and c) SEM of PSK layer deposited on different nanocomposite substrates. Reproduced with permission from Ref. [48]

Without reduction technique, conductivity of GO can also be improved by chemically doping with silver trifluoromethanesulfonate (AgOTf) inorganic dopant [55]. Both the optical and electrical properties of GO can be controlled by the doping concentration. Thickness and roughness of the GO film increase with the doping concentration while the sheet resistance gradually decreases (Figure 4). Optimized doped GO incorporated with PEDOT:PSS improves the performance of PSCs.

Figure 4. Roughness, thickness and sheet resistance of AgOTf-doped GO layers as a function of doping concentration. Reproduced with permission from Ref. [55]

Figure 4. Roughness, thickness and sheet resistance of AgOTf-doped GO layers as a function of doping concentration. Reproduced with permission from Ref. [55]

PEDOT:PSS-GeO2 composite HTLs are developed by incorporating GeO2 NPs [56] and GeO2 aqueous solution [57] into the PEDOT:PSS aqueous dispersion. GeO2 particles affect the morphology of the PSK layer. Island-like particles are perceived on the composite film (Figure 5b). These particles act as growing sites of crystal nucleus of the perovskites. PSK films on pristine PEDOT:PSS contain interconnected nanoscale domains with lot of pin-holes (Figure 5c). These obstructs charge transport and induces carrier recombination, which results in lower performance. Superior quality of PSK film with large-scale domains (Figure 5d) and high crystallinity (Figure 5f) was obtained by the seed-mediation of GeO2 particles. Further improvement of PSK film and enhancement of the performance might be achievable if the voids (Figure 5d) could be eliminated.

Figure 5. SEM images of (a) PEDOT:PSS film, (b) PEDOT:PSS-GeO2 (4:1) film, crystalline PSK films deposited on (c) PEDOT:PSS, and (d) PEDOT:PSS-GeO2, XRD patters of crystalline PSK films deposited on (e) PEDOT:PSS and (f) PEDOT:PSS-GeO2. Reproduced with permission from Ref. [57]

Figure 5. SEM images of (a) PEDOT:PSS film, (b) PEDOT:PSS-GeO2 (4:1) film, crystalline PSK films deposited on (c) PEDOT:PSS, and (d) PEDOT:PSS-GeO2, XRD patters of crystalline PSK films deposited on (e) PEDOT:PSS and (f) PEDOT:PSS-GeO2. Reproduced with permission from Ref. [57]

2.3 Doped PEDOT:PSS HTL

Additives were introduced to improve the conductivity of PEDOT:PSS HTL [58]. As additives reside in the layer, they also affect the morphology of PEDOT:PSS and PSK layer. It also has influence on the work function of PEDOT:PSS layer.

Huang et al. [59] employed highly conductive polyethylene oxide (PEO) doped PEDOT:PSS as HTL to improve the performance of PSCs. Enhanced conductivity of PEO-doped PEDOT:PSS facilitates the hole extraction and transport to the ITO anode from the active layer. Improved conductivity originates from the tuned film morphology of the doped HTL and increased ratios of PEDOT in the bipolaron states. Morphology of PEDOT:PSS with different doping concentrations of PEO are shown in the AFM topography images (Figure 6a-e). RMS roughness values calculated from the height images are ≈1.38, ≈1.25, ≈0.88, ≈1.43 and ≈1.73 nm for the PEO doping concentration of 0, 0.3, 0.5, 0.8 and 1.2 vol%, respectively.

Figure 6. AFM topography images of PEDOT:PSS films doped at different concentrations of PEO. Reproduced with permission from Ref. [59]

Figure 6. AFM topography images of PEDOT:PSS films doped at different concentrations of PEO. Reproduced with permission from Ref. [59]

Rough surfaces of pristine PEDOT:PSS or PEO-doped PEDOT:PSS at higher doping concentration create trap states at the interface of PSK and HTL [60]. Fiber-like PEDOT domains at higher doping concentration (over 0.8 vol%) introduce extra pin-holes, resulting in trap-assisted charge carrier recombination [61]. The highest PCE was observed at 0.5 vol% doping concentration despite higher conductivity at higher doping concentrations (Figure 7a). PEO-doped PEDOT:PSS with optimized doping concentration (0.5 vol%) was used to adjust the thickness of the HTL and 50 nm (similar to pristine PEDOT:PSS) offered the highest efficiency of 16.52% compared to 11.43% from pristine PEDOT:PSS HTL (Figure 7b).

Figure 7. (a) Electrical conductivities of the PEO-doped PEDOT:PSS layer and PCEs of PSCs versus doping concentrations of PEO in the layer; (b) J-V characteristics of PSCs incorporated with the pristine and PEO-doped PEDOT:PSS HTL at different thickness. Reproduced with permission from Ref. [59]

Figure 7. (a) Electrical conductivities of the PEO-doped PEDOT:PSS layer and PCEs of PSCs versus doping concentrations of PEO in the layer; (b) J-V characteristics of PSCs incorporated with the pristine and PEO-doped PEDOT:PSS HTL at different thickness. Reproduced with permission from Ref. [59]

Qian et al. [62] prepared Ag-NPs doped PEDOT:PSS HTL and observed that the performance of PSCs is influenced by the doping concentration of Ag-NPs. Ag-NPs improves the conductivity of PEDOT:PSS and the crystalline properties of PSK films. Besides these, it has minimal influence on the energy level alignment and absorption due to Plasmon resonance effect. Liu et al. [63] improved the morphology of PSK films (Figure 8) deposited on core/shell NPs doped PEDOT:PSS and perceive the only reason for improved performance of PSCs.

Figure 8. SEM images of PSK films deposited on (a) pristine and (b) 20% NPs doped PEDOT:PSS layers, and (c, d) their corresponding AFM images. Reproduced with permission from Ref. [63]

Figure 8. Figure 8. SEM images of PSK films deposited on (a) pristine and (b) 20% NPs doped PEDOT:PSS layers, and (c, d) their corresponding AFM images. Reproduced with permission from Ref. [63]

Proper energy level alignment of HTL with PSK is reported by several research groups by doping PEDOT:PSS. Doping PEDOT:PSS film with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) [64] increases the work function of the HTL from 5.08 eV to 5.18 eV (Figure 9a), whereas Huang et al. [65] achieved similar outcome using dopamine (DA) (Figure 9b). The work function of PEDOT:PSS was tuned by Lim et al. [32] using self-organized HEL (SOHEL), which was composed of PEDOT:PSS and tetrafluoroethylene-perfluoro-3,6-dioxa-4-methyl-7-octene-sulfonic acid copolymer (Figure 9c).

Figure 9. Energy level diagram of the cell with (a) F4-TCNQ doped, (b) DA, and (c) SOHEL doped PEDOT:PSS HTLs. Reproduced with permission from Ref. [64, 65, 32]

Figure 9. Energy level diagram of the cell with (a) F4-TCNQ doped, (b) DA, and (c) SOHEL doped PEDOT:PSS HTLs. Reproduced with permission from Ref. [ 64, 65, 32]

2.4 Bi-layer structure of PEDOT:PSS HTL

Appropriate energy level tailoring can improve device performance by increasing. This also facilitates efficient charge extraction and transfer which contribute to increasing Jsc and FF [66]. Suitable interfacial layer can modify the WF of ITO/PEDOT:PSS HTL to reduce the potential energy loss at the junction formed by PSK and PEDOT:PSS.

Malinkiewicz et al. [52] and Lin et al. [67] reduced the energy gap between WF of HTLs and VBM of PSK employing PEDOT:PSS/poly-TPD and PEDOT:PSS/conjugated polymers (DPP-DTT, PCDTBT, P3HT and PCPDTBT) double layers, respectively. Due to poor wetting of DMF or DMSO on the hydrophobic polymer surface, these structures are only compatible for vacuum-deposited PSCs.

Xue et al. [16] solved the above problem introducing two new alcohol-soluble conjugated copolymers PTPAFSONa (HSL1) and PTPADCF3FSONa (HSL2) (Figure 10a) with tailored energy levels (Figure 10b) and hydrophilic property (Figure 10c-f). Deeper HOMO levels of HSLs provide efficient hole transfer and the high LUMO energy levels block the electron preventing charge recombination at the anode interface. They achieved remarkable improvements in PCEs (16.6%) and (1.07 V) of PSCs by using HSL2 compared to 14.2% and 0.98 V of pristine PEDOT:PSS based devices. Similarly, cross-linked electron blocking interlayers made from QUPD and OTPD can improve the performance of PSCs by reducing the electron-hole recombination [68].

Figure 10. (a)  Molecular structure of HSL1 and HSL2, (b) energy level diagrams of PSCs using HSL1 and HSL2 as HTLs, isopropanol contact angle measurements of (c) PEDOT:PSS, (d) HSL1, (e) HSL2, and (f) PSK film. Reproduced with permission from Ref. [16]

Figure 10. (a) Molecular structure of HSL1 and HSL2, (b) energy level diagrams of PSCs using HSL1 and HSL2 as HTLs, isopropanol contact angle measurements of (c) PEDOT:PSS, (d) HSL1, (e) HSL2, and (f) PSK film. Reproduced with permission from Ref. [16]

GO and PEDOT:PSS hybrid bi-layer HTLs can remarkably suppress leakage current and electron-hole recombination. Li et al. introduced GO layer in between ITO and PEDOT:PSS [69]. In contrast, Feng et al. [70] added GO on top of the PEDOT:PSS layer. The work function of PEDOT:PSS-GO film is found to be 5.19 eV, which is higher than that of pristine PEDOT:PSS (5.10 eV). The work function is further improved to 5.37 eV by introducing ammonia modified GO layer (PEDOT:PSS-GO:NH3). The performance of PSCs depends on the doping concentration of ammonia into GO and the highest efficiency of 16.11% was achieved for the doping ratio of 1:03 (GO:NH3). Performance improvements were caused by increased quality of PSK films (Figure 11a-c) and enhanced optical absorption (Figure 11d). PSK films deposited on PEDOT:PSS-GO have more regular and larger grain size with some pores compared to that atop pristine PEDOT:PSS. Higher ordered structure of PSK with complete coverage was achievable by depositing on PEDOT:PSS-GO:NH3 film. This was caused by the surface and electronic property changes due to the GO:NH3 film.

Figure 11. SEM images of PSK films on (a) PEDOT:PSS, (b) PEDOT:PSS-GO, (c) PEDOT:PSS-GO:NH3 and (d) UV-vis
absorption spectra of PSK films on three different substrates. Reproduced with permission from Ref. [70]

Figure 11. Figure 11. SEM images of PSK films on (a) PEDOT:PSS, (b) PEDOT:PSS-GO, (c) PEDOT:PSS-GO:NH3 and (d) UV-vis absorption spectra of PSK films on three different substrates. Reproduced with permission from Ref. [70]

Analogous consequence was also observed when perylene was deposited atop PEDOT:PSS [71]. Depending on varied concentrations of perylene, the morphology of perylene layer and corresponding PSK layers changes (Figure 12a-h). In comparison with the reference devices (only PEDOT:PSS HTL), PSCs having perylene layer with a concentration of 4 mg/mL shows the best performance and improved PCE from 12.67% to 17.06%, from 0.91 V to 0.98 V, from 19.61 mA/cm2 to 22.61 mA/cm2, and from 71% to 77%. The maximum PCE is mainly ascribed to the enhanced and in comparison with the other perylene-based devices. The fabric feature of the PSK film (Figure 12g) deposited on branch-shaped perylene under layer (Figure 12c) was found to be the most favorable morphology.

Figure 12. SEM images of perylene films deposited on PEDOT:PSS from varied concentrations of (a) 2 mg/mL, (b) 3 mg/mL, (c) 4 mg/ mL, and (d) 5 mg/mL in chloroform; (e-h) SEM images of PSK films deposited on the perylene under layers corresponding to (a-d). Reproduced with permission from Ref. [71]

Figure 12. SEM images of perylene films deposited on PEDOT:PSS from varied concentrations of (a) 2 mg/mL, (b) 3 mg/mL, (c) 4 mg/ mL, and (d) 5 mg/mL in chloroform; (e-h) SEM images of PSK films deposited on the perylene under layers corresponding to (a-d). Reproduced with permission from Ref. [71]

3 Roles of tailored PEDOT:PSS HTL in perovskite solar cells

As a HTL, PEDOT:PSS has some limitations. To improve the performance of PSCs, these drawbacks should be properly addressed. Tailoring PEDOT:PSS can improve the morphology of HTL and perovskite layer deposited atop it. Among several roles of tailored PEDOT:PSS, we will discuss on the contributions to electrical conductivity, energy level alignment, trap passivation and long-term stability.

3.1 Electrical conductivity of tailored PEDOT:PSS HTLs

Charge accumulation at the interface occurs due to the difference in electrical conductivity between the PSK and the charge transport layer. This can cause the formation of space charges that can inhibit efficient charge transport. Therefore, high electrical conductivity of charge transport layers is important to ensure efficient charge transport to desired electrode.

Conductivity of PEDOT:PSS can easily be modulated by the removal of PSSH chains [72-74]. Removal of PSSH chains results in conductive PEDOT rich film. Furthermore, the removal of PSSH chains can induce the conformational change of PEDOT chains from a coil structure to an extended coil structure [75].

Conductivity variation of PEDOT:PSS affects the performance of PSCs. Sin et al. [76] varied the concentration of PSS and added small amount of DMSO before spin casting PEDOT:PSS, which improved the conductivity of PEDOT:PSS HTL from to without significant variations in their energy levels. With conductivity, grain size of PEDOT:PSS also increased with fibril structures. In contrast, no significant morphological differences were observed between the PSK films deposited on different conductive PEDOT:PSS films, confirming that the performance of PSCs are affected solely by the properties of PEDOT:PSS films. Incident light power (ILP) dependent photocurrent of PSCs with low and high conductive PEDOT:PSS (PEDOT-L and PEDOT-H, respectively) was measured to investigate charge collection balance or charge accumulation (Figure 13a,b). At effective applied voltage far from open circuit condition (3 V) with minimum charge carrier recombination, the S parameter (approaches a unity for ideally fabricated device) for the PEDOT-H-based PSCs is 0.969. Whereas, for the PEDOT-L-based PSCs, S parameter is 0.904, which represents that highly conductive PEDOT:PSS HTL reduces hole accumulation. S parameter at low effective applied voltage (0.4 V) also shows much higher value (0.953 compared to 0.893) for PEDOT-H based device, which means reduced nongeminate recombination with increased conductivity. Higher conductivity of HTL reduces charge accumulation and recombination loss, which result in enhanced short circuit current density (Jsc) and. Facilitation of hole transport through the highly conductive PEDOT:PSS HTL finally leads to 25% higher PCE in PSCs.

Figure 13. Incident light power dependent photocurrent of the perovskite solar cells with (a) PEDOT-L and (b) PEDOT-H at two different effective applied voltages. Reproduced with permission from Ref. [76]

Figure 13. Incident light power dependent photocurrent of the perovskite solar cells with (a) PEDOT-L and (b) PEDOT-H at two different effective applied voltages. Reproduced with permission from Ref. [76]

3.2. Energy level tailoring

Loss in built-in potential due to the energy level difference between PSK and HTL can lead to decrease in performance of PSCs. Energy level tailoring helps to reduce the charge accumulation at the interface decreasing charge recombination probability which results in enhanced and . Moreover, proper alignment between the energy level of HTL with the VBM of PSK materials increases the due to the increase in quasi-fermi level difference between the ETL and HTL.

Xue et al. [16] calculated charge recombination lifetime from TPV measurement of PSCs for CH3NH3PbIxCl3-x (I) and CH3NH3Pb(I0.3Br0.7)xCl3-x (Br) using PEDOT:PSS/PTPAFSONa (HSL1) and PEDOT:PSS/PTPADCF3FSONa (HSL2) double layers as HTLs (Figure 14a). Charge recombination lifetime increases for all the cells using HSL1 and HSL2 (HSL1-I: 6.2 µs, HSL2-I: 6.3 µs, HSL2-Br: 4.9 µs) compared to the cells with pristine PEDOT:PSS (None-I: 1.8 µs, None-Br: 1.5 µs), indicating lower charge recombination rate. These suggest that proper energy level alignment effectively suppresses charge recombination.

Figure 14. TPV measurements for PSCs using PEDOT:PSS (pristine), HSL1 and HSL2 HTLs; (b) J-V curve of PSCs using different polymer interlayers; (c) schematic illustration of the energy levels of CH3NH3PbI3 perovskite, PC60BM ‘electron affinity’ and ionization potential of different polymer interlayers. Reproduced with permission from Ref. [16, 67]

Figure 14. TPV measurements for PSCs using PEDOT:PSS (pristine), HSL1 and HSL2 HTLs; (b) J-V curve of PSCs using different polymer interlayers; (c) schematic illustration of the energy levels of CH3NH3PbI3 perovskite, PC60BM ‘electron affinity’ and ionization potential of different polymer interlayers. Reproduced with permission from Ref. [16, 67]

Lin et al. [67] introduced different polymeric organic semiconductors to ITO/PEDOT:PSS HTLs and observed higher (Figure 14b) with the devices having polymers of higher ionization energy (IEs), Figure 14c. Devices with PCDTBT interlayer show the highest average of 1.03 V, which is higher than those with P3HT (0.7 V).

3.3. Passivation of traps

Increased conductivity of the PEDOT:PSS films does not assure higher performance of PSCs all the time. Quality films of PSK and PEDOT:PSS after treatment also play significant role. Surface properties of PSK films depends on the surface morphology of PEDOT:PSS . Unfavorable surface morphology of PEDOT:PSS induces defects on the PSK layer, which can increase leakage current with lowering shunt resistance. Defects such as grain boundaries or atomic vacancies act as trap sites where charge carriers recombine [77, 78]. However, these problems are rarely addressed, where people found lower performance of PSCs after the treatment of PEDOT:PSS [79-81]. It is obvious that surface roughness, wettability, additives and composite materials of tailored PEDOT:PSS can impact the growth of PSK layer. A majority of groups reported superior quality of PSK films deposited on treated PEDOT:PSS with fewer traps, leading to improved performance of PSCs [ 16, 38, 48, 59, 71].

Adam et al. [82] developed pinhole free thin film PSK layers on highly conductive PEDOT:PSS (Clevios PH1000) using DMSO and Zonyl as additives. They found that PEDOT:PSS after treatment with optimum additives helps to grow better quality of PSK layer, which is the key reason for enhanced performance with good operational stability and moderate hysteresis (Figure 15). Additives reduce the number and size of defects in the PSK films (Figure 15c-f) compared to that deposited on ITO (Figure 15a) or pristine PEDOT:PSS (Figure 15b). Optimized concentration of additives (0.7% v/v Zonyl and 5% v/v DMSO) produces PSK film with no visible gaps and defects (Figure 15f). Devices fabricated from this PSK film show low leakage currents.

Figure 15. SEM images of perovskite films coated on top of (a) Glass/ITO; (b) PEDOT:PSS; PEDOT:PSS processed with (c) 5% v/v DMSO, (d) 0.7% v/v Zonyl FS-300, (e) 0.7% v/v Zonyl, FS-300 and 2.5% v/v DMSO, (f) 0.7% v/v Zonyl and 5% v/v DMSO. Reproduced with permission from Ref. [82]

Figure 15. SEM images of perovskite films coated on top of (a) Glass/ITO; (b) PEDOT:PSS; PEDOT:PSS processed with (c) 5% v/v DMSO, (d) 0.7% v/v Zonyl FS-300, (e) 0.7% v/v Zonyl, FS-300 and 2.5% v/v DMSO, (f) 0.7% v/v Zonyl and 5% v/v DMSO. Reproduced with permission from Ref. [82]

Traps in PSK absorbers has been considered as the major cause of photocurrent hysteresis of PSCs, which hinders the measurement of truthful performance and stability of devices [ 83- 87]. It may also originate from interface and surface-trap defects [ 67, 88]. Tailored PEDOT:PSS based devices can minimize hysteresis by reducing trap states and charge accumulation [ 38, 58, 59, 64, 65, 89, 90].

3.4 Enhancement in stability

The acidic and hygroscopic properties of PEDOT:PSS HTL lead to poor stabilities of the PSCs owing to the acid corrosion of ITO by PEDOT:PSS and conductivity degradation of PEDOT:PSS films by absorbing water, respectively [67]. PSS is a strong acid that can react with In2O3 dissociating indium ions which can easily diffuse into active layer, deteriorating the stability of the devices [91]. A common way to neutralize acidic properties of PEDOT:PSS is to add strong bases such as NaOH [92], KOH [93] and guanidine [94]. However, the neutralization using strong base badly affects the conductivity of PEDOT:PSS, which results in low device performance [92].

Wang et al. used [95] mild base imidazole to improve the long-term stability of PSCs. Easily miscible imidazole into PEDOT:PSS can neutralize acidity with minimal effect of conductivity. Acidic PEDOT:PSS (pristine) keeps 38% of its PCE after two weeks, whereas basic-PEDOT:PSS (imidazole doped) maintains 75% of that. PCEs of all the devices show faster decrease of with time. Basic-PEDOT:PSS apparently inhibits the indium contamination, leading to excellent ambient stability. Similarly, dopamine doped PEDOT:PSS improves the stability by reducing the acidity, which in turn inhibits the impurity diffusion from ITO. In parallel, it makes the PEDOT:PSS layer less hygroscopic, which reduces the decomposition of PSK layers [65]. Erosion of ITO by the acidic PEDOT:PSS can be effectively prevented by incorporating MoO3 layer between ITO and PEDOT:PSS. After 10 days, PCE of the devices having MoO3 degraded only 7%, whereas without MoO3 failed completely [96].

Device stability also depends on the crystallinity of perovskite structure. Highly ordered perovskite structure deposited on the GO:NH3 layer shows improved device stability of PSCs compared to that with lower quality of PSK film deposited on pristine PEDOT:PSS [70]. In addition, graphene has a great potential for bridging between the PEDOT:PSS polymer segments, favoring stability and integrity of thin films [39].

4 Conclusions and outlook

In this review paper, we discussed the progresses of PEDOT:PSS HTLs in PSCs. Tailored PEDOT:PSS HTLs have crucial roles in improving PCE and stability of PSCs. Higher conductivity can improve charge transport properties; whereas the proper alignment between the WF of PEDOT:PSS HTL and VBM of PSK reduces the energy offset, and thereby facilitates charge extraction. Surface modification of tailored PEDOT:PSS can passivate traps in PSK films and at the interface, which can eliminate hysteresis and lessen the leakage current. Acidity and hydrophilicity of PEDOT:PSS HTL can also be controlled to reduce the corrosion and degradation of moisture sensitive PSK active layer. On the basis of above optimizations of PEDOT:PSS HTLs, further improvement in PCE and long-term stability of PSCs can be achieved.

Inverted (p-i-n) structured PSCs are more promising due to low cost and possibility of further simplification of the structure. Higher processing temperature of TiO2 ETL and the hysteresis problem originated from FTO and TiO2 compact layer can be minimized by inverted structure. PEDOT:PSS, a most widely used HTL in inverted structure, is solution processable and flexible. This makes it applicable to low cost solution processed flexible devices. It is also possible to achieve high conductivity, improved morphology and a wide range of WF from tailored PEDOT:PSS. WF of PEDOT:PSS can be varied to make it compatible with PSK absorber of different bandgap and VBM. Specially, in case of tandem structure, WF of PEDOT:PSS can be adjustable with the high and low band gap absorber. PEDOT:PSS with enhanced conductivity can replace expensive and brittle metal oxide electrodes. However, the major limitation of PEDOT:PSS is the stability, which can also be partially solved by reducing the acidity and hygroscopicity.

The attempts on exploring the mechanisms of tailoring PEDOT:PSS may help and guide future researchers to find more efficient way to improve the optical, electrical and structural properties of PEDOT:PSS. This can improve the performance of PSCs and will be applicable to more feasible applications in the future.

Notes

The authors declare no competing financial interest.

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