Polymer-based Bulk Heterojunction Solar Cells Research Articles

Submicron Organic–Inorganic Hybrid Radiative Cooling Coatings for Stable, Ultrathin, and Lightweight Solar Cells

Solar cell technology requires materials that are efficient, lightweight, and stable. Organic–inorganic lead halide perovskite- and polymer-based bulk heterojunction solar cells have emerged as highly promising, ultralightweight, flexible, and highly efficient power sources. However, they suffer from limited stability, which is significantly affected by the cell temperature, in addition to other factors like oxidization and moisture in the absence of appropriate encapsulation. Here, by performing a coupled opto-electro-thermal modeling, we report the design of a compatible and novel, ultrathin, submicron, multipurpose organic–inorganic hybrid radiative cooling coating/scheme that, by providing photonic cooling, decreases the cell temperature by up to ∼7.2 K compared to the encapsulated ultrathin solar cell technology, without requiring any external energy input. In addition to the significant temperature reduction, the power conversion efficiency of cells also increases, fulfilling the requirements of high performance at minimal weight, combined with high stability and flexibility, paving the way for next-generation, stable and efficient, ultralightweight solar cells.

Improvement in photovoltaic performance of anthracene-containing PPE–PPV polymer-based bulk heterojunction solar cells with silver nanoparticles

In this study, we investigated the effect of silver nanoparticles (Ag NPs) in the active layer of anthracene-containing poly(p-phenylene-ethynylene)-alt-poly(p-phenylene-vinylene):phenyl-C61-butyric acid methyl ester (AnE-PVstat:PCBM)-based bulk heterojunction solar cells. By incorporating Ag NPs of 6 nm in diameter, the power conversion efficiency of AnE-PVstat:PCBM solar cells was improved to 3.10 % from a value of 2.46 % for the solar cells fabricated without nanoparticles. Moreover, ISOS-L-1 stability test showed that the lifetime of the cells was also significantly improved by doping Ag NPs into the active layer.

Recent Approaches to Controlling the Nanoscale Morphology of Polymer-Based Bulk-Heterojunction Solar Cells

The need for clean, inexpensive and renewable energy has increasingly turned research attention towards polymer photovoltaic cells. However, the performance efficiency of these devices is still low in comparison with silicon-based devices. The recent introduction of new materials and processing techniques has resulted in a remarkable increase in power-conversion efficiency, with a value above 10%. Controlling the interpenetrating network morphology is a key factor in obtaining devices with improved performance. This review focuses on the influence of controlled nanoscale morphology on the overall performance of bulk-heterojunction (BHJ) photovoltaic cells. Strategies such as the use of solvents, solvent annealing, polymer nanowires (NWs), and donor–acceptor (D–A) blend ratios employed to control the active-layer morphologies are all discussed.

Continuous-wave photoinduced absorption study on trapped carriers in bulk-heterojunction solar cells connected to load

By using continuous-wave photoinduced absorption spectroscopy, we investigate hole-transport processes in polymer-based bulk-heterojunction solar cells at various operating points between the open- and short-circuit conditions. The results reveal that the detrapping time constant of holes decreases as the short-circuit condition is approached. This decrease may be explained by an acceleration of detrapping that is caused by an internal electric field. This interpretation is supported by a numerical simulation based on rate equations, considering a low density of a single trap level. Since the detrapping signal disappears at short-circuit, we estimate the associated trap levels to be relatively shallow.

On the Role of Graphene in Polymer-Based Bulk Heterojunction Solar Cells

As a new material, graphene is considered to have great potential in photovoltaic applications, due to its superior physical and electronic properties. In this manuscript, the behavior of graphene nanosheets prepared by different processing methods were investigated in order to probe their applicability in polymer-based bulk heterojunction optoelectronic devices. Raman spectroscopy was employed to study the formation of interfaces between the conjugated polymer and graphene, while photoluminescence quenching was used to investigate charge transfer from P3HT to graphene. The current-voltage characteristics of fabricated cells were investigated to elucidate the role of graphene in their performance. We demonstrate that the addition of small quantities of graphene promotes exciton dissociation and charge transport in P3HT:PCBM BHJ devices, leading to a novel paradigm for organic solar cells.

Vertically Oriented Carbon Nanostructures and Their Application Potential for Polymer-Based Solar Cells

One factor limiting the performance of polymer-based bulk heterojunction solar cells is inefficient charge transport in the disordered donor/acceptor blends. The incorporation of carbon nanotubes (CNTs) into the active layer is considered as a promising concept to improve charge transport toward the electrodes. Whereas disordered ternary blends of polymer, fullerenes and CNTs, were already examined in the past, there is much less work on ordered vertically oriented arrays of nanotubes for solar cell applications. We focused on the fabrication of corresponding arrays with spatial dimensions that are suitable for application in polymer-based solar cells. We demonstrate in this contribution that CNTs can be grown on ITO covered glass at temperatures below 600 °C by static pressure chemical vapor deposition (CVD). Using short growth times, we were able to obtain CNT arrays with a rather uniform length limited to about 200 nm. With a plasma-enhanced CVD process, we were also able to produce wall-like carbon na...

Synthesis of Poly(fluorene-alt-thieno[3,4-b]thiophene) for Photovoltaic Devices

Polymer-based bulk heterojunction solar cells have been fabricated using blended films composed of an electrondonating conjugated polymer and an electron-accepting material such as a fullerene derivative; they are promising low-cost, flexible renewable energy sources. A solar cell’s power conversion efficiency (PCE) is the product of the short-circuit current density (Jsc), open-circuit voltage (Voc), and fill factor (FF) divided by the incident light power density. The Voc of polymer solar cells is generally proportional to the energy difference between the highest occupied molecular orbital (HOMO) of the electron donor and the lowest unoccupied molecular orbital (LUMO) of the electron acceptor, suggesting that polymers with deep HOMO levels are advantageous if conditions are otherwise similar. Photovoltaic cells have had PCEs of ca. 5% reported when a mixture of poly(3-hexylthiophene) (P3HT) and a fullerene derivative were respectively used as the electron donor and acceptor. Interesting low band gap polymers consisting of alternating ester or ketone-substituted thieno[3,4-b]thiophene and alkoxybenzo[1,2-b:4,5-b']dithiophene units have been developed. Among them, a polymer consisting of a fluorinated ketone-substituted thieno[3,4-b]thiophene and alkoxybenzo[1,2-b:4,5-b']dithiophene units, whose band gap and HOMO level were 1.77 eV and −5.22 eV, respectively, exhibited a maximum PCE of 7.73%. We have also synthesized conjugated polymers for photovoltaic applications. Especially, a polymer consisting of alternating carbazole and ester group-substituted thieno[3,4b]thiophene units (PCTT1) showed a band gap of 2.1 eV and HOMO level of −5.52 eV. This polymer, blended with [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM), when used as the photoactive layer of solar cells, yielded a maximum PCE of 2.53%. Polyfluorenes have advantages such as thermal and oxidation stabilities. They can also form films easily and have good hole-transporting properties. The band gaps and HOMO levels of fluorene-containing polymers have been reported to be similar to those of carbazole-containing counterpart polymers, though different PCEs were shown by resulting polymer-based solar cells. Thus poly(fluorenealt-thieno[3,4-b]thiophene polymer) (PFTT) was synthesized as a counterpart of PCTT1. This work describes the synthesis and characterization of the polymer along with preliminary tests of resulting polymer-based photovoltaic devices. Experimental Section

Influence of particle size in hybrid solar cells composed of CdSe nanocrystals and poly(3-hexylthiophene)

Inorganic semiconductor nanoparticles, such as CdSe quantum dots, are considered to be a promising alternative to fullerene derivates for application as electron acceptors in polymer-based bulk heterojunction solar cells. The main potential advantage is the strong light absorption of CdSe nanoparticles with a spectral bandwidth, which can even be tuned, due to the quantum size effect. However, the impact of the particle size on the performance of polymer/CdSe solar cells has remained largely unexplored so far. Therefore, the influence of particle size in hybrid solar cells using a blend of poly(3-hexylthiophene) (P3HT) and quasi-spherical CdSe nanoparticles on relevant cell parameters and the overall solar cell performance is systematically studied in the present work. As the most important result, an increase of the open-circuit voltage (VOC) can be found for smaller nanoparticles and can be explained by an “effective bandgap” model. In contrast, no significant changes of the short-circuit current density with particle size are observed. Smaller particles were found to yield a lower fill factor, compensating the gain in VOC, so that the power conversion efficiency finally turned out to be independent of the particle size in this study. Spectral differences observed in the respective external quantum efficiency spectra of the solar cells can be attributed to size-dependent changes of the particle absorption. Temperature-dependent measurements of the current-voltage (I-V) characteristics suggest that the transport of photogenerated charge carriers in the bulk heterojunction is limited by localized states, with activation energy beyond thermal energy at room temperature.

Device lifetime improvement of polymer-based bulk heterojunction solar cells by incorporating copper oxide layer at Al cathode

Organic solar cells are commonly susceptible to degradation in air. We present that insertion of a thin layer of thermally evaporated copper oxide (CuOx) between the organic active layer and the Al cathode can greatly extend the lifetime of P3HT:PCBM based bulk heterojunction solar cells. The performance can be further improved by applying an interfacial bilayer of CuOx/LiF. Our results suggest that the CuOx functions not only as a charge transport layer but also as a protection layer, which prevents formation of thick organic-Al interdiffusion area. This leads to a more air-resistive cathode/organic interface.

Influence of processing additives to nano-morphology and efficiency of bulk-heterojunction solar cells: A comparative review

Research and development towards high efficiency plastic solar cells have been accelerating in recent years. Polymer-based bulk heterojunction solar cells are offering an attractive and inexpensive concept for large scale production by solution processing as well as advantageous flexible and aesthetic form factors. The thin film nano-morphology of bulk-heterojunction solar cells has been shown to dramatically influence the photovoltaic performance of the devices. This article reviews the different methods used to control the film nano-morphology of bulk-heterojunction solar cells focussing on the chemical additives during solution processing. All power conversion efficiency limiting mechanisms of bulk-heterojunction solar cells are discussed in detail. It is shown, how the formation of optimal percolation pathways between donor and acceptor influences the photovoltaic device performance. It is explained how the film nano-morphology relates to light absorption, free charge carrier generation as well as charge transport to the electrodes.

Elementary processes and limiting factors in hybrid polymer/nanoparticle solar cells

Semiconductor nanoparticles are promising for use as electron acceptors in polymer-based bulk heterojunction solar cells. Potential advantages over fullerene derivates that are widely used in organic photovoltaics are tuneable absorption properties and the possibility to use elongated nanoparticles for more efficient electron transport. Despite these advantages, efficiencies obtained with hybrid polymer/nanoparticle solar cells are still below those of state-of-the-art polymer/fullerene solar cells. This Perspective summarises the achievements in the field of hybrid solar cells, compares the knowledge on elementary processes in hybrid and organic systems and points out the most recent trends in research. The design of the polymer nanoparticle/interface by the choice of capping ligands and development of appropriate surface treatments for the nanoparticles plays an important role, and recent progress opens new perspectives for the future improvement of hybrid solar cells.

Donor−Acceptor Molecule as the Acceptor for Polymer-Based Bulk Heterojunction Solar Cells

We fabricated polymer-based bulk heterojunction (BHJ) solar cells using a donor−acceptor (D-A) molecule N-propyl-3,6-bis[2-(3-dicyanomethylene-5,5-dimethylcyclohex-1-enyl)vinyl]carbazole (PDHC) as the acceptor. The strong photoluminescence (PL) quenching of poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) when mixed with PDHC means that efficient charge transfer has happened between MEH-PPV and PDHC. We used MEH-PPV and poly (3-hexylthiophene) (P3HT) as the donors and PDHC as the acceptor to fabricate BHJ solar cells. The cells with the structure of ITO/PEDOT/polymer:PDHC/LiF/Al exhibited a power conversion efficiency (PCE) about 0.2% under 100 mW/cm2 white light illumination. The open-circuit voltages (Vocs) of 1.14 V of the cells based on MEH-PPV:PDHC and about 0.76 V of the cells based on P3HT:PDHC were achieved, which are higher than the Vocs of the corresponding solar cells with phenyl-C-butyric acid methyl ester (PCBM) as the acceptor. In comparison with the cells based on PCBM...

Methods of Measuring Energy Conversion Efficiency in Dye-sensitized Solar Cells

The current–voltage characteristics of dye-sensitized solar cells (DSCs) were measured and compared with those of crystalline silicon solar cells. It was found that the energy conversion efficiency of DSCs is dependent on voltage sweep direction and sampling delay time (Td). Measurement of the transient photocurrent revealed that this dependence is due to the longer time constant of DSCs. This dependence was also confirmed in a simulation of current–voltage curves based on an equivalent circuit model of DSCs. Analysis of the current–voltage characteristics of polymer-based bulk heterojunction solar cells (BHSCs) and simulated measurements showed that the longer time constant is due to slow movement of ions in the electrolyte. To improve accuracy, the I–V measurement should be carried out from short circuit to open circuit with Td of 100 ms or longer.