To provide orthogonal solvent processable surface modification and improve the device stability of bulk-heterojunction polymer solar cells (PSCs), n-type semiconducting material naphthalene diimide (NDI) was chemically introduced onto the ITO surface as a cathode interlayer (CIL) using 3-bromopropyltrimethoxysilane (BrTMS) as a coupling agent. After modification, the work function of ITO can be decreased from 4.70 to 4.23 eV. The modified ITO cathode was applied in inverted PSCs based on PTB7-Th:PC71BM. With the CIL modification, a champion power conversion efficiency (PCE) of 5.87% was achieved, showing a dramatic improvement compared to that of devices (PCE = 3.58%) without CIL. More importantly, with these chemical bonded interlayers, the stability of inverted PSCs was greatly enhanced. The improved PCE and stability can be attributed to the increased open-circuit voltage and the formation of robust chemical bonds in NDI-TMS films, respectively. This study demonstrated that chemical modification of ITO with n-type semiconducting materials provides an avenue for not only solving the solvent orthogonal problem but also improving the device performance in terms of the PCE and the stability.

A dopant-free hole-transporting material (HTM), with (2-ethylhexyl)-9H-carbazole as core and N,N-di-p-methylthiophenylamine as end groups, termed CMT, has been designed and synthesized by a simple method. For the first time, four methylthiol groups have been introduced, rather than methoxy groups, at the para position of the diphenylamine. Under AM 1.5 illumination at 100 mW cm–2, perovskite solar cells based on CH3NH3PbI3 with pristine CMT as the HTM achieved a power conversion efficiency of 13.05%, with a short-circuit current density of 21.82 mA cm–2, an open-circuit voltage (VOC) of 1.03 V, and a fill factor of 58.23%. The value of VOC is comparable to that of the device based on 2,2′,7,7′-tetrakis(N,N-di-para-methoxy-phenylamino)-9,9′-spirobifluorene, which was 1.02 V.

Different contents of fluorine in side alkyl chains were incorporated into three conjugated polymers (namely, PBDTTT-f13, PBDTTT-f9, and PBDTTT-f5) whose backbones consist of benzodithiophene donors and thienothiophene acceptors. These three fluorinated polymers, in comparison with the well-known analogue PTB7-Th, show comparable energy levels and optical band gaps. However, the fluorination of side alkyl chains significantly changed the surface energy of bulk materials, which leads to distinctly different self-assembly behaviors and phase separations as being mixed with PC71BM. The increased mismatch in surface energies between the polymer and PC71BM causes larger scale phase domains, which makes a sound explanation for the difference in their photovoltaic properties.Topics: Contact angle; Electric properties; Electric transport processes and properties; Electronic structure; Energy level; Heterojunction solar cells; Molecular structure-property relationship; Organic solar cells; Physical and chemical processes; Polymer morphology; Polymers; Quantum mechanics; Quantum mechanics; Self-assembly; Separation science; Spectra; Surface energy; Thin films; Thin films;

•An impressive PCE of 3.64% was achieved in P3HT based-non fullerene PSCs.•The high PCE is insensitive to the active layer thickness in the range of 70–250 nm.•The P3HT:IDTIDT-IC PSCs show much higher temperature tolerance than P3HT:PCBM PSCs.In order to fabricate highly efficient polymer solar cells (PSCs) in industrial scale, one of the key issues is to use thick active layer (>200 nm) in the device module without sacrificing the power conversion efficiency (PCE). In this article, we have studied the blend of the medium-bandgap polymeric donor P3HT and the low-bandgap acceptor IDTIDT-IC as the active layer in non-fullerene PSCs, and successfully maintained the device performance with the thickness of the active layer close to 250 nm. The P3HT:IDTIDT-IC based PSCs with simple thermal annealing exhibits a PCE of 3.49% at a thin active layer, 74 nm. More importantly, the PCE remains almost constant with increasing the thickness of the active layer, and reaches a peak value of 3.64% at 236 nm. This thickness-insensitive photovoltaic performance of the P3HT:IDTIDT-IC system makes them compatible with large-scale roll-to-roll processing. Furthermore, the P3HT:IDTIDT-IC devices show a very high tolerance to temperature, and the PCE keeps nearly unchanged after annealing the active layer at 150 °C for 75 min. All in all, our results show that thickness-tolerable and thermal-stable P3HT:IDTIDT-IC system is more suitable for large-scale industrial manufacturing than the classic P3HT:PCBM formula.Download high-res image (242KB)Download full-size image

Indene-C60 bisadduct (IC60BA), which can offer a significantly higher open-circuit voltage (Voc) than monoadducts, has become the research focus as electron acceptor materials in polymer solar cells (PSCs) in recent years. However, despite its popularity, IC60BA have always been applied in PSCs as mixture of several regioisomers and the nature of this mixture has never been fully investigated and understood. Herein, for the first time, 12 major regioisomers of IC60BA were isolated and a full investigation was carried out with respect to their structure, abundance, solubility and their corresponding photovoltaic performance. The results show that the PSCs based on these regioisomeric structures present very diverse PCE and their photovoltaic performance was dramatically affected not only by the relative indene positions but also by the steric orientation of the two indene groups. Electrochemical studies further revealed that the effect of energetic disorder inside the IC60BA regioisomers on their photovoltaic performance is insignificant when applied in PSCs. However, the steric structures and solubility of the regioisomers were found to have significant impact on the morphology and bulk properties of the active layer of PSCs, which give rise to very different PCE of devices based on IC60BA regioisomers with different structures.

A methanol-soluble diamine-modified fullerene derivative (denoted as PCBDANI) was applied as an efficient cathode buffer layer (CBL) in planar p-i-n perovskite solar cells (pero-SCs) based on the CH3NH3PbI3–xClx absorber. The device with PCBDANI single CBL exhibited significantly improved performance with a power conversion efficiency (PCE) of 15.45%, which is approximately 17% higher than that of the control device without the CBL. The dramatic improvement in PCE can be attributed to the formation of an interfacial dipole at the PCBM/Al interface originating from the amine functional group and the suppression of interfacial recombination by the PCBDANI interlayer. To further improve the PCE of pero-SCs, PCBDANI/LiF double CBLs were introduced between PCBM and the top Al electrode. An impressive PCE of 15.71% was achieved, which is somewhat higher than that of the devices with LiF or PCBDANI single CBL. Besides the PCE, the long-term stability of the device with PCBDANI/LiF double CBLs is also superior to that of the device with LiF single CBL.

In this study, two fullerenes (C60, C70) and their methano-substitutions (PC61BM, PC71BM), as electron transport materials (ETMs) in perovskite solar cells (Pero-SCs), were systematically studied. As being used as ETMs, methanofullerenes, though with lower electron mobility compared to the counterpart pristine fullerenes, lead to higher power conversion efficiencies (PCEs) of Pero-SCs. The difference is likely caused by the fill-out vacancies and smoother morphology of the interfaces between ETM and perovskite layers, as they were prepared by different methods. In addition, compared to C60 and PC61BM, C70 and PC71BM showed priority in terms of short-circuit current density, which should be attributed to fast free charge extraction abilities.

For planar p–i–n perovskite solar cells (Pero-SCs), the bottom hole transporting layer (HTL) material is crucially important, since it can greatly affect the device performance in two aspects: (1) hole collection and transportation and (2) the crystallinity of the perovskite layer formed on it. Herein, a series of catechol derivatives, L-3,4-dihydroxyphenylalanine (DOPA), norepinephrine (NE) and 3,4-dihydroxybenzhydrazide (DOBD), were employed as dopants in PEDOT:PSS and applied as HTLs, and the influence on performance of p–i–n Pero-SCs was systematically studied. It is found that all these three catechols can improve the power conversion efficiency (PCE) of the Pero-SCs, among which DOBD shows far better performance than the other two. Under optimized conditions, a PCE of 17.46% was achieved for the p–i–n Pero-SCs using DOBD-doped PEDOT:PSS as the HTL. The investigations on morphology, fluorescence and electrochemical impedance spectra indicate that the PCE improvement should be mainly attributed to the facilitated charge collection and transportation due to the doped HTL and the enhanced crystallinity of the perovskite films. This line of research demonstrates that the easily accessible catechols can be employed as an excellent dopant in PEDOT:PSS for application as HTLs in Pero-SCs and opens a novel avenue for further improving the performance of the devices.

A hole-transporting material (HTM) based on (2-ethylhexyl)-9H-carbazole as the core and N,N-di-p-methoxyphenylamine as the end group (CMO) has been designed. CMO with a simple structure can be synthesized by a one-step process in a high yield and it shows good solubility in solvents. Steady-state and time-resolved photoluminescence measurements show that CMO has significant charge extraction ability. Planar perovskite solar cells (pero-SCs) based on CMO as the HTM showed a high power conversion efficiency of 15.92%. For reference purposes, pero-SCs based on 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9-9′-spirobifluorene (Spiro-OMeTAD) were fabricated and a PCE of 16.70% was reached. CMO is one of the simplest HTM materials reported, which shows a PCE comparable to that of Spiro-OMeTAD at the same time.

•A new 2D-conjugated molecule DR3TBDTTVT was synthesized, and OSCs based on DR3TBDTTVT:PC71BM achieved a best PCE of 5.71%.•DR3TBDTTVT shows complementary absorption to PTB7-Th & PC71BM, and was introduced as third component to ternary OSCs.•The ternary OSCs displayed improved device performance compared with the binary OSCs based on PTB7-Th:PC71BM.Ternary organic solar cells (OSCs) are burgeoning as one of the effective strategies to achieve high power conversion efficiencies (PCEs) by incorporating a third component with a complementary absorption into the binary blends. In this study, we presented a new two-dimension-conjugated small molecule denoted by DR3TBDTTVT, which alone gave rise to a best PCE of 5.71% with acceptor PC71BM as active layer. Given the complementary absorption with PTB7-Th, DR3TBDTTVT was doped into (PTB7-Th:PC71BM)-based binary blends, and ternary OSCs were developed. The ternary OSCs with 10 wt% of DR3TBDTTVT displayed improved hole-mobility, reduced device resistance and better phase separation of active layer, thus leading to an impressive PCE of 7.77% with open-circuit voltage of 0.77 V, short-circuit density of 14.52 mA cm−2 and fill factor of 70.3%. Ternary OSCs well make up for the light-harvesting insufficiency of binary OSCs, and this research provides a new material for the improvement of PCEs for single-junction OSCs.Download high-res image (328KB)Download full-size image

A non-fullerene electron acceptor bearing a fused 10-heterocyclic ring (indacenodithiopheno-indacenodithiophene) with a narrow band gap (∼1.5 eV) was designed and synthesized. It possesses excellent planarity and enhanced effective conjugation length compared to previously reported fused-ring electron acceptors. When this acceptor was paired with PTB7-Th and applied in polymer solar cells, a power conversion efficiency of 6.5% was achieved with a high open circuit voltage of 0.94 V. More significantly, an energy loss as low as 0.59 eV and an external quantum efficiency as high as 63% were obtained simultaneously.

In this paper, we introduce a room-temperature mixed-solvent-vapor annealing (rtMSVA) method to fabricate high performance perovskite solar cells (pero-SCs) based on MAPbI3−xClx without the need for thermal annealing (TA). An ultra-smooth perovskite thin-film with high crystallinity was obtained by the DMF/CB mixed-solvent (1:20, v/v) vapor annealing at room-temperature without TA and the power conversion efficiency (PCE) of the pero-SCs reached 16.4%. More importantly, the reproducibility of the PCEs is quite good among 40 different devices. Furthermore, large active area pero-SCs were fabricated with the rtMSVA method. The PCEs of the pero-SCs based on ITO and flexible PET/Ag mesh electrodes with an active area of 1.21 cm2 reached 11.01% and 7.5%, respectively. We anticipate that rtMSVA would very possibly become a promising crystallization method for the fabrication of large area pero-SCs in the near future.

For perovskite solar cells (Pero-SCs), one of the key issues with respect to the power conversion efficiency (PCE) is the morphology control of the perovskite thin-films. In this study, an easily-accessible additive polyethylenimine (PEI) is utilized to tune the morphology of CH3NH3PbI3−xClx. With addition of 1.00 wt% of PEI, the smoothness and crystallinity of the perovskite were greatly improved, which were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). A summit PCE of 14.07% was achieved for the p-i-n type Pero-SC, indicating a 26% increase compared to those of the devices without the additive. Both photoluminescence (PL) and alternating current impedance spectroscopy (ACIS) analyses confirm the efficiency results after the addition of PEI. This study provides a low-cost polymer additive candidate for tuning the morphology of perovskite thin-films, and might be a new clue for the mass production of Pero-SCs.

TiO2 is widely used in perovskite solar cells (Pero-SCs), but its low electrical conductivity remains a drawback for application in electron transport layer (ETL). To overcome this problem, an easily accessible hydroxylated fullerene, fullerenol, was employed herein as ETL modified on ITO in n-i-p type (ITO as cathode) Pero-SCs for the first time. The results showed that the insertion of a single layer of fullerenol between perovskite and TiO2 dramatically facilitates the charge transportation and decreases the interfacial resistance. As a consequence, the device performance was greatly improved, and a higher power conversion efficiency of 14.69% was achieved, which is ∼17.5% enhancement compared with that (12.50%) of the control device without the fullerenol interlayer. This work provides a new candidate of interfacial engineering for facilitating the electron transportation in Pero-SCs.

•To solve the solvent orthogonal problem, buffer layers based on AOB were chemically modified on ITO through BrTMS.•This buffer layers can efficiently lower the work function of the ITO, and greatly improve the performance of PSCs.•After insertion of this interlayer, the PCE of PSCs based on PBDTTT-C-T:PCBM was enhanced from 4.10% to 7.56%.In order to avoid an interpenetration of the buffer and the photoactive layers during preparation of polymer solar cells (PSCs), solvent-resistant buffer films were chemically modified on indium tin oxide (ITO) surface. The conjugated aromatics acridine orange base (AOB) was introduced into the films using 3-bromopropyltrimethoxysilane (BrTMS) as coupling agent. Upon ITO surface modification, the respective work functions show a significant decrease. The modified ITO substrates were implemented in inverted PSCs based on PBDTTT-C-T:PC71BM. With the modification, the power conversion efficiency (PCE) was improved significantly from 4.10% (for the inverted PSC without this buffer layer) to 7.56%. The PCE enhancement is mainly caused by the increase of the open-circuit voltage (43%). These results indicate that the solvent-resistant film is able to facilitate electron collection and transportation, thus providing a novel route to high efficient PSCs by surface engineering.

•Two novel copolymers, with priorities in light absorption and hole mobility, were applied as HTL materials in perovskite solar cells.•To balance the charge transportation, pristine C60 was employed as interlayer at the cathode side.•With the two above-mentioned interlayers, a PCE as high as 15.83% was achieved.The interlayers, including hole transporting layer (HTL) and electron transporting layer (ETL), segregating photoactive layer and the electrodes play an important role in charge extraction and transportation in perovskite solar cells (pero-SCs). Two novel copolymers, PDTSTTz and PDTSTTz-4, for the first time were applied as HTL in the n-i-p type pero-SCs, with the device structure of ITO/compact TiO2/CH3NH3PbI3-xClx/HTL/MoO3/Ag. The highest occupied molecular orbitals (HOMO) levels of PDTSTTz and PDTSTTz-4 exhibit a suitable band alignment with the valence band edge of the perovskite. Both of them lead to improved device performances compared with reference pero-SCs based on P3HT as HTL. To further balance the charge extraction and the diffusion length of charge carriers, pristine C60 was introduced at the cathode side of the pero-SCs, working together with TiO2 as ETL. With insertion of both the HTL and ETL, the performance of pero-SCs was greatly enhanced. The optimized devices exhibited impressive PCEs of 14.4% and 15.8% for devices based on PDTSTTz and PDTSTTz-4. The improved performance is attributed to better light harvest ability, decreased interface resistance and faster decay time due to the introduction of the interlayers.

•A series of novel dihydrobenzofuran-C60 bisadducts were synthesized by a facile method and utilized as acceptor materials in polymer solar cells.•Device performances were significantly improved by using the alkyl chain substituted dihydrobenzofuran-C60 bisadducts (Cn-BFCBA) as acceptors.•When n = 4, i.e. tert-butyl as substituent (C4-BFCBA), PCE of the corresponding device reached 3.40%, which is comparable to that of the device based on PC61BM.A series of novel dihydrobenzofuran-C60 bisadducts, denoted by BFCBA (without side chain substituent) and Cn-BFCBA (n represents the number of carbon atoms on side chain substituent, and varies as 1–5), were synthesized by a facile method and utilized as acceptor materials in polymer solar cells (PSCs). These fullerene bisadducts were found to possess similar optical, electrochemical properties but the PSCs based on them as acceptors present very diverse photovoltaic performance as the variation of n. Compared to the PSC based on BFCBA, device performances were significantly improved by the application of alkyl chain substituted dihydrobenzofuran-C60 bisadducts (Cn-BFCBA). In particular, when n = 4, i.e. tert-butyl used as substituent, PCE of the corresponding device reached to a summit of 3.40%. This result is comparable to that of the device based on PC61BM as acceptor. The morphology studied by AFM and electron mobility investigated by the space charge limited current (SCLC) method further revealed that C4-BFCBA shows a reasonably well miscibility with P3HT and a significantly higher electron mobility. These properties might account for its optimal photovoltaic performance among these series of the fullerene bisadducts. For comparison, dihydrobenzofuran-C60 monoadduct and trisadduct based on tert-butyl substituents were also synthesized. The corresponding PSC device test results showed that C4-BFCBA present much better photovoltaic performance than its corresponding monoadduct (C4-BFCMA) and trisadduct (C4-BFCTA) as well as BFCBA.

An alcohol-soluble fullerene derivative functionalized with a crown-ether end group in its side chain (denoted as PCBC) was synthesized and applied as a cathode buffer layer in planar p–i–n perovskite solar cells and bulk-heterojunction polymer solar cells. It is found that the introduction of the PCBC cathode buffer layer can greatly improve the photovoltaic performance of the planar p–i–n perovskite solar cells based on CH3NH3PbI3−xClx with power conversion efficiency (PCE) reaching 15.08%. In addition, the bulk-heterojunction polymer solar cells based on PBDTTT-C-T:PC70BM with the PCBC cathode buffer layer also showed a higher PCE of 7.67%, which is improved in comparison with the traditional device with the Ca/Al cathode. This work indicates that PCBC is a promising solution-processable cathode buffer layer material for application in both planar p–i–n perovskite solar cells and bulk-heterojunction polymer solar cells.

In this paper, triple cathode buffer layers (CBLs) composed of phenyl-C61-butyric acid methyl ester (PCBM), C60, and LiF layers were introduced into the planar p–i–n perovskite solar cells (p–i–n PSCs) with a device structure of ITO/PEDOT:PSS/CH3NH3PbI3–xClx/CBLs/Al. For comparison, a single CBL of PCBM and a double CBL of PCBM/LiF were also investigated in the p–i–n PSCs. On the basis of the PCBM buffer layer, the addition of a thin LiF layer facilitated the charge collection process and led to the dramatic improvement of the power conversion efficiency (PCE) of the PSCs up to 14.69% under an illumination of AM 1.5G, 100 mW/cm2, which is to date one of the highest efficiencies of the p–i–n PSCs. By further insertion of a C60 layer between PCBM and LiF in the triple CBLs, a PCE of 14.24% was obtained, and more importantly, the PCBM/C60/LiF triple CBLs are very helpful for improving the stability of the devices and making the LiF layer less thickness-sensitive for achieving high performances of the p–i–n PSCs.Keywords: cathode buffer layers; CH3NH3PbI3−xClx; PCBM/C60/LiF triple CBLs; planar perovskite solar cells; stability

A series of fulleropyrrolidine derivatives (FPx, x = 1–8) with alternating N-phenyl or N-methyl group were prepared as acceptors for polymer solar cells (PSCs) with the purpose of investigating the effect of N-substitutions on the photovoltaic properties of fullerene materials. More importantly, the morphology studies by means of atomic force microscopy (AFM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and space charge limited current (SCLC) measurements revealed that FP1 with N-phenyl group possessed not only appropriate miscibility with P3HT but also high electron mobility, which may account for its optimal photovoltaic properties.

In this paper, we introduce a room-temperature mixed-solvent-vapor annealing (rtMSVA) method to fabricate high performance perovskite solar cells (pero-SCs) based on MAPbI3−xClx without the need for thermal annealing (TA). An ultra-smooth perovskite thin-film with high crystallinity was obtained by the DMF/CB mixed-solvent (1:20, v/v) vapor annealing at room-temperature without TA and the power conversion efficiency (PCE) of the pero-SCs reached 16.4%. More importantly, the reproducibility of the PCEs is quite good among 40 different devices. Furthermore, large active area pero-SCs were fabricated with the rtMSVA method. The PCEs of the pero-SCs based on ITO and flexible PET/Ag mesh electrodes with an active area of 1.21 cm2 reached 11.01% and 7.5%, respectively. We anticipate that rtMSVA would very possibly become a promising crystallization method for the fabrication of large area pero-SCs in the near future.

An alcohol-soluble fullerene derivative functionalized with a crown-ether end group in its side chain (denoted as PCBC) was synthesized and applied as a cathode buffer layer in planar p–i–n perovskite solar cells and bulk-heterojunction polymer solar cells. It is found that the introduction of the PCBC cathode buffer layer can greatly improve the photovoltaic performance of the planar p–i–n perovskite solar cells based on CH3NH3PbI3−xClx with power conversion efficiency (PCE) reaching 15.08%. In addition, the bulk-heterojunction polymer solar cells based on PBDTTT-C-T:PC70BM with the PCBC cathode buffer layer also showed a higher PCE of 7.67%, which is improved in comparison with the traditional device with the Ca/Al cathode. This work indicates that PCBC is a promising solution-processable cathode buffer layer material for application in both planar p–i–n perovskite solar cells and bulk-heterojunction polymer solar cells.

A non-fullerene electron acceptor bearing a fused 10-heterocyclic ring (indacenodithiopheno-indacenodithiophene) with a narrow band gap (∼1.5 eV) was designed and synthesized. It possesses excellent planarity and enhanced effective conjugation length compared to previously reported fused-ring electron acceptors. When this acceptor was paired with PTB7-Th and applied in polymer solar cells, a power conversion efficiency of 6.5% was achieved with a high open circuit voltage of 0.94 V. More significantly, an energy loss as low as 0.59 eV and an external quantum efficiency as high as 63% were obtained simultaneously.

Indene-C60 bisadduct (IC60BA), which can offer a significantly higher open-circuit voltage (Voc) than monoadducts, has become the research focus as electron acceptor materials in polymer solar cells (PSCs) in recent years. However, despite its popularity, IC60BA have always been applied in PSCs as mixture of several regioisomers and the nature of this mixture has never been fully investigated and understood. Herein, for the first time, 12 major regioisomers of IC60BA were isolated and a full investigation was carried out with respect to their structure, abundance, solubility and their corresponding photovoltaic performance. The results show that the PSCs based on these regioisomeric structures present very diverse PCE and their photovoltaic performance was dramatically affected not only by the relative indene positions but also by the steric orientation of the two indene groups. Electrochemical studies further revealed that the effect of energetic disorder inside the IC60BA regioisomers on their photovoltaic performance is insignificant when applied in PSCs. However, the steric structures and solubility of the regioisomers were found to have significant impact on the morphology and bulk properties of the active layer of PSCs, which give rise to very different PCE of devices based on IC60BA regioisomers with different structures.