PCBM Solar Cells Research Articles

Realization of Large‐Scale Polymer Solar Cells Using Ultrasonic Spray Technique Via Solvent Engineering (Solar RRL 7∕2018)

Reliability and reproducibility remain as main challenges in the realization of large-scale polymer solar cells. In article no. 1800064, Jiang Cheng, Lu Li and co-workers demonstrate a smart method to prototyping high yield and largescale PTB7:PC71BM solar cells using ultrasonic spray coating. Marangoni-effect related coffee-rings and pin holes are successfully overcome via tuning the drying processes. The formation of PTB7-enriched surface, confirmed in solvent engineering, led to an improved PTB7:PC71BM blend film, and the influence on the device characteristic performance was systematically studied. The power conversion efficiency of the device is improved to 8.23%, and the preparation of a large-scale active area solar cell of 18 cm2 with a maximum power output of 74.8 mW is demonstrated promising for industrial applications.

Triptycene as a Supramolecular Additive in PTB7:PCBM Blends and Its Influence on Photovoltaic Properties.

Additives play an important role in modifying the morphology and phase separation of donor and acceptor molecules in bulk heterojunction (BHJ) solar cells. Here, we report triptycene (TPC) as a small-molecule additive for supramolecular control of phase separation and concomitant improvement of the power conversion efficiency (PCE) of PTB7 donor and fullerene acceptor-based BHJ polymer solar cells. An overall 60% improvement in PCE is observed for both PTB7:PC61BM and PTB7:PC71BM blends. The improved photovoltaic (PV) performance can be attributed to three factors: (a) TPC-induced supramolecular interactions with donor:acceptor components in the blends to realize a nanoscale phase-separated morphology; (b) an increase in the charge transfer state energy that lowers the driving force for electron transfer from donor to acceptor molecules; and (c) an increase in the charge carrier mobility. An improvement in efficiency using TPC as a supramolecular additive has also been demonstrated for other BHJ blends such as PBDB-T:PC71BM and P3HT:PCBM, implying the wide applicability of this new additive molecule. A comparison of the photostability of TPC as an additive for PTB7:PCBM solar cells to that of the widely used 1,8-diiodooctane additive shows ∼30% higher retention of PV performance for the TPC-added solar cells after 34 h of AM 1.5G illumination. The results obtained suggest that the approach of using additives that can promote supramolecular interactions to modify the length scale of phase separation between donor and acceptor is very promising and can lead to the development of highly efficient and stable organic photovoltaics.

Tuning the optical properties of luminescent down shifting layers based on organic dyes to increase the efficiency and lifetime of P3HT: PCBM photovoltaic devices

Mixtures of luminescence downshifting (LDS) materials has been used to increase the efficiency of poly(3-hexylthiophene-2,5-diyl):phenyl-C61-butyric acid methyl ester (P3HT:PC61BM) bulk heterojunction solar cell. This layers convert more energetic photons to lower energies that are better matched with wavelength peak of the external quantum efficiency (EQE) of a P3HT:PC61BM solar cell. Experimental studies were used to optimise the optical properties of LDS layers including the maximum of absorbance and the photoluminescence quantum yield (PLQY). To provide the significant improvements, combinations of LDS mixtures were prepared to provide the greatest absorption and PLQY. The approach is shown to simultaneously improve the photocurrent and increase the lifetime of the device by absorbing UV light. By optimising the optical properties of the LDS mixture, a relative increase of about 20% in the photocurrent density produced by the P3HT:PCBM cell could be achieved, which to our knowledge is one of the most significant reported for OPVs.

Impedance spectroscopy and capacitance – voltage measurements analysis: Impact of charge carrier lifetimes and mapping vertical segregation in bulk heterojunction P3HT: PCBM solar cells

Impedance spectroscopy measurement is employed to study the impact of thermal annealing on charge carrier lifetimes in P3HT: PCBM bulk heterojunction solar cells. Upon thermal annealing at 150 °C, a correlation between charge carrier lifetime and device performance is identified. The best power conversion efficiency reported here corresponds to the devices annealed at 150 °C, yielding the longest charge carrier life time. In addition to lateral segregation, thermal annealing promotes accumulation of PCBM molecular aggregates towards the cathode inducing vertical segregation as is evident from the analysis of results obtained from capacitance – voltage measurements. Based on Mott-Schottky relation, the measured values of the flat band potential as a function of thermal annealing is correlated to the concentration of PCBM at cathode interface and a compositional gradient profile induced by thermal annealing is proposed.

Stability enhancement of ITO-free non-inverted PTB7:PC71BM solar cell using two-step post-treated PEDOT:PSS

The conductivity and stability of specially treated PEDOT:PSS thin films are investigated. Based on the proposed treatment method, ITO-free PTB7:PC71BM organic solar cells are fabricated and the electrical properties of the cells are analyzed. It is shown that by applying a two-step post-treatment method using methanol and ethylene glycol, the conductivity of the PEDOT:PSS thin film increases from 0.9 to 1448 S/cm, and at the same time, a significant improvement of the stability of the layer over time is achieved. It is shown that after 30 days of aging under high humidity condition, the conductivity remains above 70% of its initial value, which is a remarkable result compared with the results reported in the literature. In this paper, important factors affecting the conductivity and stability of the treated layer are studied in detail. In addition, the effect of immersion time in methanol on the conductivity of the layer is also investigated and it is found that dipping times less than 3 min have no appreciable effects on the improvement of the conductivity. An ITO-free non-inverted PTB7:PC71BM solar cell is also fabricated using the proposed post-treated PEDOT:PSS thin films as the transparent anode. The power conversion efficiency of the resulting cell is 5.8%. The stability of the fabricated ITO-free cells is considerably better than the stability of the non-treated ITO-free cells or the cells made using ITO as anode.

Polymer-fullerene based bulk heterojunction P3HT:PCBM solar cell: The influence of PTU as a chemical additive on photovoltaic performance

Polymer-fullerene based bulk heterojunction P3HT:PCBM solar cell: The influence of PTU as a chemical additive on photovoltaic performance

A wide-bandgap polymer based on alkylthio-naphthyl- substituted benzo[1,2-b:4,5-b′]dithiophene units for efficient fullerene-based and fullerene-free polymer solar cells

A wide-bandgap polymer PBDTNS-TPD based on alkylthio-naphthyl-substituted benzo[1,2-b:4,5-b′]dithiophene (BDT) units was designed and synthesized. Benefiting from the prolonged conjugation of the conjugated side groups on BDT units, the PBDTNS-TPD exhibited better light-harvesting properties and charge transport properties than that of alkylthio-phenyl-substituted counterpart PBDTBS-TPD. The PBDTNS-TPD:PC71BM solar cell devices yield a power conversion efficiency (PCE) of 9.09%, which exhibits an increase of 29% in comparison with that of PBDTBS-TPD-based PSCs (PCE 7.02%). Moreover, the PBDTNS-TPD:ITIC based PSCs also showed a higher PCE of 8.19% compared to the PBDTBS-TPD:ITIC based PSCs (PCE 5.93%). This work demonstrates that the replacement of the alkylthio-phenyl moiety with alkylthio-naphthyl unit on BDT is a useful method to design high-performance wide-bandgap polymers for fullerene-based and fullerene-free PSCs.

Realization of Large‐Scale Polymer Solar Cells Using Ultrasonic Spray Technique Via Solvent Engineering

Reliability and reproducibility remain as main challenges in the realization of large‐scale polymer solar cells. An easy and smart method to prototyping a high yield and large‐scale PTB7:PC71BM solar cell is demonstrated by modified ultrasonic spray coating method, using a novel precursor solution containing a portion of a lower‐toxicity trimethylbenzene. The formation of Marangoni‐effect related defects such as coffee‐rings and pin holes is successfully overcome via tuning the drying processes with optimization of the solvent ratio. XPS analysis confirms that the PTB7 enriched surface resulted in a vertical phase separated structure, and led to higher mobility of charge carriers to transport from the film to the buffer layer. The power conversion efficiency of the solar cell is improved to 8.23%. As well, the preparation of a large‐scale active area solar cell of 18 cm2 with a maximum power output of 74.8 mW is demonstrated promising for industrial applications.

Well-aligned Vertically Oriented ZnO Nanorod Arrays and their Application in Inverted Small Molecule Solar Cells.

This manuscript describes how to design and fabricate efficient inverted solar cells, which are based on a two-dimensional conjugated small molecule (SMPV1) and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM), by utilizing ZnO nanorods (NRs) grown on a high quality Al-doped ZnO (AZO) seed layer. The inverted SMPV1:PC71BM solar cells with ZnO NRs that grew on both a sputtered and sol-gel processed AZO seed layer are fabricated. Compared with the AZO thin film prepared by the sol-gel method, the sputtered AZO thin film exhibits better crystallization and lower surface roughness, according to X-ray diffraction (XRD) and atomic force microscope (AFM) measurements. The orientation of the ZnO NRs grown on a sputtered AZO seed layer shows better vertical alignment, which is beneficial for the deposition of the subsequent active layer, forming better surface morphologies. Generally, the surface morphology of the active layer mainly dominates the fill factor (FF) of the devices. Consequently, the well-aligned ZnO NRs can be used to improve the carrier collection of the active layer and to increase the FF of the solar cells. Moreover, as an anti-reflection structure, it can also be utilized to enhance the light harvesting of the absorption layer, with the power conversion efficiency (PCE) of solar cells reaching 6.01%, higher than the sol-gel based solar cells with an efficiency of 4.74%.

A Thieno[3,2‐c]Isoquinolin‐5(4H)‐One Building Block for Efficient Thick‐Film Solar Cells

AbstractAn aromatic lactam acceptor unit, thieno[3,2‐c]isoquinolin‐5(4H)‐one (TIQ), is developed. Compared with its analogues, dithieno[3,2‐b:2′,3′‐d]pyridin‐5(4H)‐one (DTP) and phenanthridin‐6(5H)‐one (PN), TIQ shows its advantage in constructing donor–acceptor (D–A) copolymers for efficient solar cells. TIQ‐based D–A copolymer PTIQ4TFBT delivers a power conversion efficiency (PCE) of 10.16% in polymer:fullerene solar cells, while those based on DTP and PN copolymers, PDTP4TFBT and PPN4TFBT, afford PCEs around 8.5%. The higher performance of PTIQ4TFBT:PC71BM solar cells originates from enhanced short‐circuit current density (Jsc) and fill factor (FF), because of favorable morphology, less bimolecular recombination, and balanced charge transport in the active layer. Moreover, the performance for PTIQ4TFBT:PC71BM solar cells is less sensitive to active layer thickness than PDTP4TFBT:PC71BM and PPN4TFBT:PC71BM solar cells. Over 8% PCEs can be obtained from PTIQ4TFBT:PC71BM solar cells when the active layer thickness is over 500 nm.

Transient Photocurrent Elucidating Carrier Dynamics and Potential of Bulk Heterojunction Solar Cells Fabricated by Thermal Precursor Approach

Transient photocurrents are measured to clarify carrier dynamics in a series of bulk heterojunction solar cells that are composed of [6,6]‐phenyl‐C61‐butyric acid methyl ester (PCBM) and diketopyrrolopyrrole‐tetrabenzoporphyrin conjugates substituted by straight alkyl groups (Cn‐DPP‐BP), of which the device parameters are largely dependent on the length of the substituted alkyl groups (Cn). The time profile of the observed transients indicates the presence of both unimolecular and bimolecular charge recombination processes in the active layer of the cells. Analysis of the photocurrent decays detected at various bias voltages using a drift model that considers charge extraction by electrodes and charge recombination in the active layer enables elucidation of the charge drift mobility, the initial photocarrier density, and the charge recombination rate constants. The bimolecular recombination rate constant is correlated with Cn and this is the main factor that determines the power conversion efficiency (η) of the Cn‐DPP‐BP:PCBM solar cells. The Cn‐dependent fill factor can be interpreted in terms of the bias voltage dependence of the ratio between the extraction and recombination charges (Qex/Qre). Comparison of η and Qex/Qre at the short‐circuit condition predicts that η for the Cn‐DPP‐BP:PCBM solar cell can be potentially improved up to 6%.

Fabrication of Flexible and Semitransparent PTB7:PC<sub>71</sub>BM Organic Solar Cells

Semitransparent bulk-heterojunction (BHJ) solar cells have attracted a strong attention due to the possible use in new photovoltaic applications. However, semitransparent BHJ solar cells on flexible substrates have not yet been developed. In this work, flexible BHJ solar cells were fabricated on an ITO/PET substrate using an inverted PET/ITO/PFN/PTB7:PC71BM/MoO3/Au device structure. The power conversion efficiency (PCE) of the device was 3.4 %. Fabrication of semitransparent solar cells was further demonstrated utilizing a MoO3/Au/MoO3 transparent anode. The semitransparent solar cell showed bifacial energy generation when illuminated from both front and backside. The PCEs with illumination from the ITO or Au/MoO3 side were 2.7 % and 2.1 %, respectively. These results showed that a semitransparent PTB7:PC71BM solar cell was successfully fabricated on flexible PET substrates.

Study of impedance spectra parameters of PCDTBT:PC70BM solar cells as a function of the deposition solution concentration

In this contribution, we apply impedance spectroscopy (IS) to study a series of bulk-heterojunction PCDTBT:PC70BM solar cells deposited by spin-coating from dichlorobenzene solution of different concentrations. The samples are deposited on ITO covered glass and have the structure PEDOT:PSS / PCDTBT:PC70BM / Me where Me is Ag or Al deposited by either magnetron sputtering or thermal evaporation. The impedance spectra are measured upon one sun illumination at short-circuit conditions as a function of the concentration of the solution used for the deposition of the active layer. For one of the samples, measurements are made as a function of the applied bias as well. The measured spectra are fitted using an equivalent circuit. The dependences of the parameters derived from the fits on the concentration of the solution and on the bias voltage are presented and discussed in correlation with the quality of the solar cells.

Fabrication of flexible indium tin oxide-free polymer solar cells with silver nanowire transparent electrode

Flexible indium tin oxide (ITO)-free poly(3-hexylthiophene):[6,6]-phenyl C61-butyric acid methyl ester (P3HT:PC61BM) solar cells with a spin-coated silver nanowire transparent electrode are demonstrated. The solution-processed silver nanowire thin film not only exhibits high transmission (∼87%), but also shows low sheet resistance Rs (∼25 Ω/sq). For solar cells with a conventional structure, the power conversion efficiency (PCE) of devices based on silver nanowires can reach around 2.29%. For the inverted structure, the PCE of devices can reach 3.39%. Conventional and inverted flexible ITO-based P3HT:PC61BM solar cells are also fabricated as a reference for comparison. For both types of solar cells, the PCE of ITO-free devices is very close that of an ITO-based polymer solar cell.

Improving Photovoltaic Performance of a Fused-Ring Azepinedione Copolymer via a D-A-A Design.

Two conjugated copolymer donors, PTTABDT and PBTTABDT, based on a fused-ring azepinedione acceptor unit, 5-(2-octyldodecyl)-4H-thieno[2',3':4,5]thieno[3,2-c]thieno[2',3':4,5]thieno[2,3-e]azepine-4,6(5H)-dione (TTA), are prepared. PTTABDT possesses a conventional donor-acceptor (D-A) structure with one TTA in the repeat unit, while PBTTABDT has a D-A-A structure with two TTAs in the repeat unit. Compared with PTTABDT, PBTTABDT shows a deeper highest occupied molecular orbital (HOMO) level, a narrower bandgap, and a higher hole mobility, and exhibits better performance in bulk heterojunction solar cells. Power conversion efficiencies of 6.18% and 7.81% are achieved from PTTABDT:PC71 BM and PBTTABDT:PC71 BM solar cells, respectively. The higher performance of PBTTABDT:PC71 BM solar cells results from the enhanced open-circuit voltage (V oc ) and short-circuit current density ( J sc ).

Aluminium Nanoparticles Prepared by Ultrasonic Ablation Technique inside the Active Layer of Organic Solar Cell

In the present work, chemical-free aluminium nanoparticles (Al NPs) have been incorporated inside active layer of organic solar cell (OSC) using a new ultrasonic ablation technique, by which the Al NPs have been physically prepared in 1,2-dichlorobenzene (DCB) as a solvent for preparing solar cell active layer. The concentration of Al NPs inside active layer can be increased by increasing the volume of DCB-Al NPs and, also, by increasing the ultrasonication time applied upon DCB-Al NPs mixture. The prepared Al NPs are incorporated inside the common P3HT:PCBM active layer without accumulation and with low distribution. Optical absorption of the P3HT:PCBM active layer is improved through plasmon resonance induced by Al NPs inside active layer. The fill factor (FF) is, also, improved by the presence of Al NPs which facilitate charge carrier transfer in the P3HT:PCBM solar cell. The FF can be reduced due to presence of Al bulk pieces inside active layer. However, these Al bulk pieces are dissociated under the effect of ultrasonic irradiation for longer time. Optical absorption as well as charge carrier transfer induced by Al NPs are the responsible to increase performance parameters of the P3HT:PCBM solar cell under increasing the concentration of Al NPs inside OSC. As a conclusion, the incorporation of Al NPs inside P3HT:PCBM solar cell induces a significant improvement of the FF by 35% and consequently, the PCE is improved with about 39%.

Designing a ternary photovoltaic cell for indoor light harvesting with a power conversion efficiency exceeding 20%

A PCDTBT:PC71BM solar cell adopted by a ternary strategy with PDTSTPD achieves an overall PCE of 20.8% under indoor illumination.

Performance of Cu Nanoparticles Incorporated P3HT:PCBM Solar cell Prepared via Spin and Bar coating

Performance of Cu Nanoparticles Incorporated P3HT:PCBM Solar cell Prepared via Spin and Bar coating

Effect of regioregularity on recombination dynamics in inverted bulk heterojunction organic solar cells

The effect of polymer regioregularity on the charge transport properties and bimolecular recombination rates of polymer-based solar cells is studied in detail using transient photovoltaic techniques. We compare organic solar cells fabricated with an ITO/ZnO/PEIE/P3HT:PCBM/MoO3/Ag structure using either 100% regioregular poly(3-hexylthiophene) (DF-P3HT) yielding an average power conversion efficiency (PCE) of 3.8 ± 0.3% or 92% regioregular P3HT (rr-P3HT) that yields an average PCE of 3.28 ± 0.4%. Transient photocurrent measurements reveal the presence of less mobile photoinduced charges in rr-P3HT:PCBM cells when compared to DF-P3HT:PCBM solar cells. Transient photovoltage measurements are used to establish the relationship between regioregularity and bimolecular recombination rate constant (k) finding that under 1 Sun, devices with high regioregularity have a longer τ despite having a higher k. The high value of k for the DF-P3HT:PCBM system as compared to the rr-P3HT:PCBM system is attributed to enhanced mobility and better charge transport of mobile charges in the DF-P3HT:PCBM system, consistent with enhanced fibrillar order in DF-P3HT films observed with transmission electron microscopy. We also note a slight decrease in cell open circuit voltage with increase in polymer regioregularity, which is due to the increase in k. Other recombination mechanisms such as trap-assisted recombination are found to be important in the lower regioregular P3HT device compounded by the reduced mobility and poor inter-chain ordering.

Balancing High Open Circuit Voltage over 1.0 V and High Short Circuit Current in Benzodithiophene‐Based Polymer Solar Cells with Low Energy Loss: A Synergistic Effect of Fluorination and Alkylthiolation

AbstractBased on the most recently significant progress within the last one year in organic photovoltaic research from either alkylthiolation or fluorination on benzo[1,2‐b:4,5‐b′]dithiophene moiety for high efficiency polymer solar cells (PSCs), two novel simultaneously fluorinated and alkylthiolated benzo[1,2‐b:4,5‐b′] dithiophene (BDT)‐based donor–acceptor (D–A) polymers, poly(4,8‐bis(5′‐((2″‐ethylhexyl)thio)‐4′‐fluorothiophen‐2′‐yl)benzo[1,2‐b:4,5‐b′]dithiophene‐2,6‐diyl)‐alt‐2′‐ethylhexyl‐3‐fluorothieno[3,4‐b]thiophene‐2‐carboxylate (PBDTT‐SF‐TT) and poly(4,8‐bis(5′‐((2″‐ethylhexyl)thio)‐4′‐fluorothiophen‐2′‐yl)benzo[1,2‐b:4,5‐b′]dithiophene‐2,6‐diyl)‐alt‐1,3‐bis(thiophen‐2‐yl)‐5,7‐bis(2‐ethylhexyl)benzo[1,2‐c:4,5‐c′]dithiophene‐4,8‐dione (PBDTT‐SF‐BDD), namely, via an advantageous and synthetically economic route for the key monomer are reported herein. Synergistic effects of fluorination and alkylthiolation on BDT moieties are discussed in detail, which is based on the superior balance between high Voc and large Jsc when PBDTT‐SF‐TT/PC71BM and PBDTT‐SF‐BDD/PC71BM solar cells present their high Voc as 1.00 and 0.97 V (associated with their deep highest occupied molecular orbital level of −5.54 and −5.61 eV), a moderately high Jsc of 14.79 and 14.70 mA cm−2, and thus result a high power conversion efficiency of 9.07% and 9.72%, respectively. Meanwhile, for PBDTT‐SF‐TT, a very low energy loss of 0.59 eV is pronounced, leading to the promisingly high voltage, and furthermore performance study and morphological results declare an additive‐free PSC from PBDTT‐SF‐TT, which is beneficial to practical applications.