Low-bandgap Polymer Solar Cells Research Articles

Degradation of electrical characteristics in low-bandgap polymer solar cells associated with light-induced aging

Abstract As the efficiency of organic solar cells has been consistently improved, interest is being raised to understand degradation mechanisms inside the solar cells. Among many factors that affect solar cell deterioration, light is the most crucial factor in the degradation mechanism. Here we report degradation under the continuous light-induced conditions for PTB7:PC70BM organic solar cells during the first 24 h with maintaining the temperature at 25 °C. It revealed 30% of initial performance drop after 24 h of light-induced aging, mostly through a decrease in JSC and FF. The morphological and electrical characteristics of the active layer and devices were investigated by atomic force microscopy, impedance spectroscopy, and temperature-dependent current density–voltage characteristics to figure out the origin of the photo-induced degradation. As a result, the light-induced traps were found to be a primary cause of loss. Furthermore, this trap formation was observed with an energy of 78 meV leading to trapped charge limited conduction properties and electrical degradation of solar cells.

Investigation of low-bandgap nonfullerene acceptor-based polymer solar cells with very low photovoltage loss

Polymer solar cells (PSCs) have seen great progress in recent years, with power conversion efficiencies of over 15%. However, PSCs suffer from larger energy losses than inorganic and perovskite solar cells, leading to lower open-circuit voltage ( V OC). The main factors that hinder the V OC improvements include (i) relatively large nonradiative recombination losses and thus low electroluminescence quantum efficiency (EQEEL) in PSCs and (ii) the existence of a charge transfer state at the interface of donor and acceptor. For efficient charge separation in state-of-the-art PSCs, empirically, the driving force for exciton dissociation is considered to be at least 0.3 eV. The large driving force could lead to large voltage losses and thus hinder the PSC performance. In this study, we report using wide bandgap material PB3T as electron donor and low bandgap material IEICO-4F as electron acceptor for nonfullerene PSCs with very small driving forces, which, however, show a decent maximum external quantum efficiency (EQE) of nearly 40%. Moreover, we demonstrate a nonfullerene PSC with high EQEEL up to 5.1 × 10 − 4, corresponding to very low nonradiative recombination losses of 0.20 eV and overall photovoltage energy losses of 0.46 to 0.52 eV, derived from different bandgap ( E gap) determination methods, which can now be comparable to those in perovskite solar cells and inorganic solar cells.

Cold Crystallization Temperature Correlated Phase Separation, Performance, and Stability of Polymer Solar Cells

Summary Fabrication of non-fullerene (NF)-based polymer solar cells (PSCs) has been an arduous task involving various approaches to optimize the morphology of blend film for superior performance. In this work, unique exothermic peaks can be observed in differential scanning calorimetry first heating scan of the blend films, which are assigned to the cold crystallization temperature (TCC) of NF acceptors when blended with different degree of crystallinity polymers, and TCC contains the phase information at quenched stage of the film development. A linear correlation between the phase separation parameters and TCC was established, which helps to select the appropriate donor for a specific acceptor. Utilizing multiple polymer-NF blend systems, we report an expectation TCC value between 210°C and 230°C, which would promise balanced domain size, purity, and consequentially superior performance. Further investigation revealed that TCC can also be used to evaluate the thermal stability.

Roles of Interfacial Tension in Regulating Internal Organization of Low Bandgap Polymer Bulk Heterojunction Solar Cells by Polymer Additives

AbstractThe role of a tertiary polymer‐based additive is investigated in increasing the efficiency of inverted low bandgap polymer:fullerene bulk heterojunction (BHJ) solar cells. Charge separation in polymer BHJ solar cells relies on the phase separation between electron accepting fullerene derivatives and photoactive polymers. Proper distribution of individual phases of suitable crystallinities within the active layer is a key factor for efficient charge transport/extraction and high photovoltaic performance. Here, it is demonstrated that the minor addition of a tertiary amorphous polymer, polystyrene (PS), with optimized molecular weights can increase the overall photovoltaic efficiency of poly[N‐9′‐heptadecanyl‐2,7‐carbazole‐alt‐5,5‐(4′,7′‐di‐2‐thienyl‐2′,1′,3′‐benzothiadiazole)]:phenyl‐C61butyric acid methyl ester (PCDTBT:PCBM) inverted BHJ solar cells, through the interfacial‐tension‐driven increase in crystallinities of photoactive phases and redistribution of PCBM molecules away from the top hole‐collecting anode interface. Complementary studies correlating polymer interfacial tension, blend internal structure, charge transport, and photovoltaic characteristics show that tertiary, high molecular‐weight polymers can serve as effective additives for improving the performance of low bandgap polymer solar cells.

Efficient CsF interlayer for high and low bandgap polymer solar cell

Low bandgap polymer solar cells have a great deal of importance in flexible photovoltaic market to absorb sun light more efficiently. Efficient wide bandgap solar cells are always available in nature to absorb visible photons. The development and incorporation of infrared photovoltaics (IR PV) with wide bandgap solar cells can improve overall solar device performance. Here, we have developed an efficient low bandgap polymer solar cell with CsF as interfacial layer in regular structure. Polymer solar cell devices with CsF shows enhanced performance than Ca as interfacial layer. The power conversion efficiency of 4.5% has been obtained for PDPP3T based polymer solar cell with CsF as interlayer. Finally, an optimal thickness with CsF as interfacial layer has been found to improve the efficiency in low bandgap polymer solar cells.

Enhanced efficiency and stability in low band-gap polymer solar cells by introducing nickel acetate as anode interlayer

Abstract Nickel acetate (Ni(Ac)2) was successfully introduced to the low band-gap system for the first time and incorporated as the anode interlayer in conventional polymer solar cells (PSCs). For PBDT-TS1/PC71BM system, the device based on as-prepared Ni(Ac)2 performs a lower power conversion efficiency (PCE) of 6.26% in comparison to that of PEDOT:PSS-based device (6.56%). After appropriate pre-bias treatment, the device efficiency rapidly increases to 7.27%, which is 11% and 16% higher than that of PEDOT:PSS- and as-prepared Ni(Ac)2-based device, respectively, due to the suppressed charge recombination and enhanced hole transport at anode interface. More importantly, device with pre-bias treated Ni(Ac)2 as anode interlayer exhibits significantly improved lifetime in N2 atmosphere. The Ni(Ac)2-based PSCs could maintain almost 85% of its original PCE after 200 h, while PEDOT:PSS-based PSCs only retains 60% of its original efficiency. This work demonstrates that the low-cost Ni(Ac)2 is an ideal anode interlayer material, which could simultaneously improve the device efficiency and stability, to replace the widely used and high-cost PEDOT:PSS.

Kinetic Monte Carlo modeling on organic solar cells: Domain size, donor-acceptor ratio and thickness

Organic solar cells have received a lot of attention in the last few decades. Extensive experimental work has been conducted to improve organic solar cell performance. However, a much more realistic and accurate model needs to be developed that can simulate and predict photovoltaic performance for organic solar cells. In this work, a Kinetic Monte Carlo (KMC) model was developed to simulate the morphological variation of organic solar cells and its effects on photovoltaic parameters. This model is currently based on P3HT: PCBM system and can be easily extended to any low bandgap polymer solar cells. The novelty and advancement of this work is that this new KMC simulation model can simulate three different parameters including domain size, donor-acceptor ratio and active layer thickness in the same model and predict the efficiency of organic solar cells on the variation of these parameters. This simulation model has been validated by photovoltaic performance from fabricated devices. The optimized parameters of simulation and fabrication are correlated and the simulation results are in agreement with the experimental results. With the assistance from this model, researchers may be able to simplify the complex fabrication processing by identifying optimal conditions such as domain size, donor-acceptor ratio and active layer thickness via this new simulation model.

Influence of Fullerene Acceptor on the Performance, Microstructure, and Photophysics of Low Bandgap Polymer Solar Cells

The morphology, photophysics, and device performance of solar cells based on the low bandgap polymer poly[[2,6′‐4,8‐di(5‐ethylhexylthienyl)benzo[1,2‐b;3,3‐b]dithiophene]3‐fluoro‐2[(2‐ethylhexyl)carbonyl]thieno[3,4‐b]thiophenediyl (PBDTTT‐EFT) (also known as PTB7‐Th) blended with different fullerene acceptors: Phenyl‐C61‐butyric acid methyl ester (PC61BM), phenyl‐C71 ‐butyric acid methyl ester (PC71BM), or indene‐C60 bisadduct (ICBA) are correlated. Compared to PC71 BM‐based cells – which achieve a power conversion efficiency (PCE) of 9.4% – cells using ICBA achieve a higher open‐circuit voltage (VOC) of 1.0 V albeit with a lower PCE of 7.1%. To understand the origin of this lower PCE, the morphology and photophysics have been thoroughly characterized. Hard and soft X‐ray scattering measurements reveal that the PBDTTT‐EFT:ICBA blend has a lower crystallinity, lower domain purity, and smaller domain size compared to the PBDTTT‐EFT:PC71BM blend. Incomplete photoluminescence quenching is also found in the ICBA blend with transient absorption measurements showing faster recombination dynamics at short timescales. Transient photovoltage measurements highlight further differences in recombination at longer timeframes due to the more intermixed morphology of the ICBA blend. Interestingly, a mild thermal treatment improves the performance of PBDTTT‐EFT:ICBA cells which is exploited in the fabrication of a homo PBDTTT‐EFT:ICBA tandem solar cell with PCE of 9.0% and VOC of 1.93 V.

High-Performance Low-Bandgap Polymer Solar Cells With Optical Microcavity Employing Ultrathin Ag Film Electrode

In this paper, the performance of indium-tin-oxide (ITO) free microcavity polymer solar cells based on the thieno[3,4-b]thiophene/benzodithiophene:[6,6]-phenyl C71-butyric acid methyl ester (PTB7:PC71 BM) blend and transparent Ag electrode is systematically investigated by optical simulation. Here, the device has a structure of glass/opaque Ag/FPI-PEIE/PTB7:PC71BM/MoO3/ultrathin Ag/TeO 2, where the fulleropyrrolidine ethoxylated polyethylenimine (FPI-PEIE) and MoO3 act as charge transport layers and optical spacers, and the TeO2 serves as a transmittance enhancement layer. Compared to ITO-based devices, the microcavity device shows an obviously increased optical electric field intensity for the incident light wavelength from 600 to 750 nm, which agrees well with the better light absorption and lower reflectivity at relatively long wavelength. Further, enhanced performance is obtained from an optimizing top-illuminated device. It is noted that a remarkably enhanced short-circuit current density of 19.50 mA/cm2 is achieved for the microcavity device at optimized layer thicknesses, which is 11% higher than that of ITO-based devices. The improved performance of the microcavity device could be attributed to optically confined and reinforced incident light between two Ag reflective electrodes induced by the coherent interference, which boosts the light harvesting in the active layer, especially the long-wavelength incident photons. The results could provide a promising optical design for low-bandgap polymer solar cells.

Dramatically enhanced performances and ideally controlled nano-morphology via co-solvent processing in low bandgap polymer solar cells

The device performance of photovoltaics with a polymer:fullerene bulk heterojunction (BHJ) structure, consisting of DT-PDPP2T-TT donor polymer and poly(3-hexylthiophene):[6,6]phenyl-C61-butyric acid methyl ester (PC61BM) acceptor compound, was investigated as a function of co-solvent composition. An enhancement of the photocurrent density and fill factor is observed in diodes made by spin-coating with chloroform mixed with ortho-dichlorobenzene, which allows a significantly higher device efficiency of 5.55% compared to diodes made from neat chloroform (efficiency = 3.61%). To clarify the role of the co-solvent, we investigated the nanoscale morphology with AFM, TEM and 2D-GIWAXS techniques and also the free-charge carrier mobility via space-charge limited current theory. We obtained the result that, under such supersaturated conditions, co-solvents induce increased polymer crystalline aggregation into a 3D phase structure and boost charge-carrier transport characteristics. This provides a rational basis for the development of ideally-controlled BHJ films that yield efficient DT-PDPP2T-TT:PCBM solar cells. Therefore, carefully selecting solvent mixtures provides an approach toward efficient low bandgap polymer solar cells.

Stable Inverted Low-Bandgap Polymer Solar Cells with Aqueous Solution Processed Low-Temperature ZnO Buffer Layers

Efficient inverted low-bandgap polymer solar cells with an aqueous solution processed low-temperature ZnO buffer layer have been investigated. The low-bandgap material PTB-7 is employed so that more solar light can be efficiently harvested, and the aqueous solution processed ZnO electron transport buffer layer is prepared at 150°C so that it can be compatible with the roll-to-roll process. Power conversion efficiency (PCE) of the inverted device reaches 7.12%, which is near the control conventional device. More importantly, the inverted device shows a better stability, keeping more than 90% of its original PCE after being stored for 625 hours, while PCE of the conventional device is only 75% of what it was. In addition, it is found that the ZnO thin film annealed in N2can obviously increase PCE of the inverted device further to 7.26%.

Femtosecond transient absorption dynamics in low bandgap polymer solar cell materials including poly(thienylenevinylene) derivative and benzothiadiazole moiety

We report dynamics of photo-generated charge carriers in (E)-poly[2,2′-(1,2-ethenediyl)bisthiophene-alt-4,7-(2,1,3-benzothiadiazole)] (PEBTBT) donor/acceptor copolymer, and in PEBTBT:fullerene bulk heterojunctions (BHJs) blended with [6,6]-phenyl-C61-butylic acid methyl ester (PCBM) and with [6,6]-phenyl-C71-butylic acid methyl ester (PC70BM) by using femtosecond transient absorption (TA) spectroscopy. The TA decay dynamics appeared that both photo-induced bleaching and absorption of the PEBTBT/fullerene BHJs exhibited slower lifetimes compared to the decay of the pristine copolymer. This can be an evidence of photo induced charge separation at PEBTBT/fullerene interface. When the pump fluence exceeded over the threshold for exciton–carrier (and exciton–exciton) annihilation, within short time delays, the PEBTBT:PC70BM BHJ exhibited a bit faster decay by bimolecular process than the PEBTBT:PCBM BHJ. With regard to the decay property of interfacial charge transfer states, slower geminate recombination was expected in the PC70BM blended film compared with PCBM blended film.

Highly efficient low-bandgap polymer solar cells with solution-processed and annealing-free phosphomolybdic acid as hole-transport layers.

We demonstrate a novel solution-processed method to fabricate a stable anode buffer layer without any annealing process. As we know, buffer layers in polymer solar cells (PSCs) are always prepared by the traditional high-vacuum thermal evaporation or annealing-treated spin-coating methods, but the fabricating processes are complicated and time-consuming. Here, a solution method without any annealing to fabricate phosphomolybdic acid (PMA) as anode buffers is presented, which brings an obvious improvement of power conversion efficiency (PCE) from 1.75% to 6.57% by optimizing the PMA concentrations and interface pretreatment with device structure shown as ITO/TiO2/PCDTBT:PC70BM/PMA/Ag. The improvement is ascribed to the fine energy-level matching and perfect surface modification. This annealing-free method greatly simplifies the device fabrication process and supplies a wide way to achieve a large area fabrication for PSCs.

The role of Au nanorods in highly efficient inverted low bandgap polymer solar cells

Abstract

Efficient Low Bandgap Polymer Solar Cell with Ordered Heterojunction Defined by Nanoimprint Lithography

In this work, we demonstrate the feasibility of using nanoimprint lithography (NIL) to make efficient low bandgap polymer solar cells with well-ordered heterojunction. High quality low bandgap conjugated polymer poly[2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b']-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT) nanogratings are fabricated using this technique for the first time. The geometry effect of PCPDTBT nanostructures on the solar cell performance is investigated by making PCPDTBT/C70 solar cells with different feature sizes of PCPDTBT nanogratings. It is found that the power conversion efficiency (PCE) increases with increasing nanograting height, PCPDTBT/C70 junction area, and decreasing nanograting width. We also find that NIL makes PCPDTBT chains interact more strongly and form an improved structural ordering. Solar cells made on the highest aspect ratio PCPDTBT nanostructures are among the best reported devices using the same material with a PCE of 5.5%.

Light trapping design for low band-gap polymer solar cells.

We demonstrate numerically a 2-D nanostructured design for light trapping in a low band-gap polymer solar cell. Finite element method simulations are used to study the effect of varying nanostructure periodicity, height, and shape on active layer absorption. Maintaining a constant active layer thickness of 100 nm we observe an enhancement in solar absorption of almost 40% relative to a planar cell. Improvements of this magnitude enable single-junction, low-band-gap cells to achieve power conversion efficiencies of 11.2% and perform competitively with even state-of-the-art tandem cells. Our design is also shown to significantly outperform tandem cells at off-normal angles of incidence.

Optimization of molecular organization and nanoscale morphology for high performance low bandgap polymer solar cells

Rational design and synthesis of low bandgap (LBG) polymers with judiciously tailored HOMO and LUMO levels have emerged as a viable route to high performance polymer solar cells with power conversion efficiencies (PCEs) exceeding 10%. In addition to engineering the energy-level of LBG polymers, the photovoltaic performance of LBG polymer-based solar cells also relies on the device architecture, in particular the fine morphology of the photoactive layer. The nanoscale interpenetrating networks composed of nanostructured donor and acceptor phases are the key to providing a large donor-acceptor interfacial area for maximizing the exciton dissociation and offering a continuous pathway for charge transport. In this Review Article, we summarize recent strategies for tuning the molecular organization and nanoscale morphology toward an enhanced photovoltaic performance of LBG polymer-based solar cells.

Dye Sensitization in the Visible Region for Low-Bandgap Polymer Solar Cells

We have fabricated ternary blend solar cells by incorporating a wide-bandgap molecule, tris[4-(5-dicyanomethylidenemethyl-2-thienyl)phenyl]amine (TDCV-TPA), into a binary blend of a low-bandgap polymer, poly[2,3-bis-(3-octyloxyphenyl)quinoxaline-5,8-diyl-alt-thiophene-2,5-diyl] (PTQ1), and a fullerene derivative (PCBM) in order to enhance exciton generation and hence charge generation in the visible region. The ternary blend solar cells exhibited large external quantum efficiency (EQE) in the visible range as well as in the near-IR range. As a result, the short-circuit current density (JSC) was improved by more than 20%.

Morphologic improvement of the PBDTTT-C and PC71BM blend film with mixed solvent for high-performance inverted polymer solar cells

Tracing the evolution of the bulk heterojunction structure, a dramatic promotion in the efficiency of polymer solar cells has been obtained in recent years. The active layer morphology of low-bandgap polymer solar cells is one of the critical factors for high-efficiency performance. In the past, the relationship between morphology improvement and the device’s characteristics (such as efficiency, fill factor and short-circuit current) in low-bandgap polymer solar cells has been studied intensively with regards to the conventional structure. Here we demonstrate the morphologic improvement of the poly[(4,8-bis-(2-ethylhexyloxy)-benzo[1,2-b;4,5-b′]dithiophene)-2,6-diyl-alt-(4-(2-ethylhexanoyl)-thieno[3,4-b]thiopene)-2,6-diyl]/[6,6]-phenyl C71 butyric acid methyl ester (PBDTTT-C/PC71BM) blend film for inverted solar cells. By utilizing a mixed solvent of dichlorobenzene/chlorobenzene with (1,8-diiodooctane) additives, the device efficiency can be significantly enhanced, from 0.92% to 4.43%. This enhancement is attributed to active layer morphologic improvement promoting carrier transport. Furthermore, the thickness optimization of the active layer and the electron blocking layer MoO3 further contributes to efficiency. The device performance could be achieved with an efficiency as high as 5.35%, an open-circuit voltage of 0.70 V, a short-circuit current density of 13.5 mA cm−2, and a fill factor of 57%.

Enhanced Power Conversion Efficiency of Low Band-Gap Polymer Solar Cells by Insertion of Optimized Binary Processing Additives

Low band-gap polymer solar cells with enhanced efficiency are fabricated by insertion of optimized binary processing additives. Binary processing additives improve phase separation with finer nanomorphology at the BHJ films, resulting in increased short circuit current density, JSC. Moreover, the device with binary processing additives exhibits a reduced series resistance compared to the device without additives.