Polymer-Fullerene Bulk-Heterojunction Solar Cells

Polymer-Fullerene Bulk-Heterojunction Solar Cells

(Parte 1 de 5)

Polymer-Fullerene Bulk-Heterojunction Solar Cells By Gilles Dennler, Markus C. Scharber, and Christoph J. Brabec*

Solution-processed bulk-heterojunction photovoltaic cells were first reported in 1995.[1] It took another 3–4 years until the scientific community realized the huge potential of this technology, and suddenly, in 1999, the number of publications in that field started to rise exponentially. Since then, the number of publications on organic semiconductor photovoltaics has increased by about 65% per year. While the best efficiency reported eight years ago barely reached values higher than 1%, efficiencies beyond 5% are achieved today.[2–6]

This article reviews the recent developments that have guided the community and the whole field to the current performance of organic photovoltaic devices (OPVs). We start with reviewing the performance of the currently most prominent material system in OPVs, namely the mixture of poly(3-hexylthiophene):1-(3- methoxycarbonyl)propyl-1-phenyl[6,6]C61 (P3HT:PCBM). In the second part of this article, we discuss new and promising active materials that have already shown promising performances in actual devices, and have the potential to go to significantly higher efficiencies than those achieved by P3HT-based solar cells. The third part is devoted to the recent development of a tandem technology for the organic field. The last two sections go beyond pure advanced material science, and discuss necessary requirements to ensure that OPVs will become a sustainable technology in the market. The first part analyzes the impact of the fundamental, OPV-specific losses on the maximum theoretical efficiency, in a simplified Shockley-Queisser approach. The second part tries to answer the question of what are the minimum efficiency and lifetime a low-cost PV technology needs to demonstrate in order to become competitive for grid-connected energy supply.

Despite the great progress of several different organic/hybrid approaches, like solution-processed or evaporated small molecules, polymer–polymer blends, or organic– inorganic blends, this review will focus exclusively on bulk-heterojunction composites from polymer–fullerene blends.

2. The P3HT:PCBM Blend

2.1. Estimation of the Maximum Expectable Efficiency

For more than 5 years, the P3HT:PCBM blend has been dominating the organic-solar-cell research. Although the material blend is well known and investigated, there are still discussions on the practical efficiency one may expect from that system. Although the device physics of polymer:fullerene bulk heterojunctions has been the object of many recent review articles[7] and book chapters,[8] it is still important to set the efficiency expectations for that material system. Consider a material, say P3HT, that absorbs photons with wavelengths smaller than 675nm (a band-gap energy Eg 1.85eV). Assuming that in a P3HT:PCBM blend the polymer defines the optical gap of the composite, one can calculate the absorbed photon density as well as the power density by combining the absorption spectrum with the sun’s spectrum. The typical spectrum of the light impinging on the surface of the Earth is given by the ASTM Standard G159,[9] and named Air Mass 1.5 (AM1.5). The so-called AM1.5G, the overall reference for solar-cell characterization,[10] cumulates an integrated power density of 1000Wm 2 (100mWcm 2), and an integrated photon flux of 4.31 1021s 1m 2, distributed over a large range of wavelengths (280–4000nm). Under these assumptions, a P3HT:PCBM layer can absorb, at best, 27% of the available photons and 4.3% of the available power, while the ultimate efficiency, as defined by Shockley and Queisser,[1] predicts a value of 34.6% for a semiconductor with a band gap of 1.85eV. This difference arises from the fact that each photon having an energy

En larger than Eg produces only one electronic charge q, extracted at a maximum potential Eg. The external quantum efficiency (EQE) of a device is defined by the ratio of the collected electrons to the incident photons. The

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Konarka Austria GmbH Altenbergerstrasse 69 4040 Linz, Austria E-mail: cbrabec@konarka.com

DOI: 10.1002/adma.200801283

Solution-processed bulk-heterojunction solar cells have gained serious attention during the last few years and are becoming established as one of the future photovoltaic technologies for low-cost power production. This article reviews the highlights of the last few years, and summarizes today’s state-of-the-art performance. An outlook is given on relevant future materials and technologies that have the potential to guide this young photovoltaic technology towards the magic 10% regime. A cost model supplements the technical discussions, with practical aspects any photovoltaic technology needs to fulfil, and answers to the question as to whether low module costs can compensate lower lifetimes and performances.

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short-circuit current density Jsc is expressed by:

Jsc ¼ hc q where h is Plank’s constant, c is the speed of light in vacuum, and l1 and l2 are the limits of the active spectrum of the device. In the case of the P3HT:PCBM blend, and for an EQE of 100%, the maximum possible Jsc is about 18.7mAcm 2. If the average EQE is only 50%, Jsc would then be only about 9.35mAcm 2. More information about expected efficiencies and accuracy of mea- surement can be found in the literature.[10,12]

In a real device, the absorption in the photoactive blend cannot be 100%, because the active layer (AL) is embedded within a stack of several layers, which have different complex refractive indexes. Thus, absorption can occur in some layer located between the incident medium and the AL, and reflection can happen at any interface located before the bulk of the active layer. In order to precisely quantify the amount of light absorbed within the active layer, one needs first to calculate the 1D distribution of the optical electromagnetic field E(x) across the device in any of the layers involved. This is usually solved by the so-called transfer-matrix formalism (TMF), which incorporates both the absorption and the reflection events in each subsequent layer.[13–15]

Figure1summarizes the numberof photons(Nph)absorbed in the P3HT:PCBM layer versus the thickness of this layer for an organic solar cell having the following structure: glass (1mm)/ indium tin oxide (ITO, 140nm)/poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate)(PEDOT:PSS, 50nm)/P3HT:PCBM (x nm)/ Al (100nm). The refractiveindexesused for this calculationcan be found elsewhere.[16] It appears, in this figure, that Nph generally increases with increasing thickness, but not monotonically. If the thickness of the layers is smaller than the coherence of the light, interference occurs, because the light is reflected by the opaque electrode. About 9.5 1016photonss 1cm 2 are absorbed in an AL of 5mm. Assuming an average internal quantum efficiency

(IQE) of 100%, this represents a Jsc value of 15.2mAcm 2,o r approximately 20% less than in the theoretical consideration.

In the case of an AL that has a more realistic thickness of

400nm, the maximum Jsc (IQE¼100%) is 12.8mAcm 2. If the average IQE is lower than 100%, Jsc is further reduced. At 80% average IQE, Jsc should be around 10.2mAcm 2. Thus, despite the fact that the theoretical short-current density of a

Gilles Dennler received his Engineering and Masters Degrees at the National Institute for Applied Sciences, Lyon, France, in 1999. He obtained a first Ph.D. in plasma physics at the University of Toulouse, France, and a second in Experimental Physics at Ecole Polytechnique of Montreal, Canada. In 2003, he moved to the Linz Institute for Organic Solar Cells (Austria), where he was appointed Assistant Professor. He joined Konarka in September 2006, where he is currently Director of European Research.

Markus Scharber received an Applied Physics B.Sc. degree from Napier University Edinburgh, Scotland, a Masters Degree from the Johannes Kepler University Linz, Austria, and a Ph.D. at the Linz Institute for Organic Solar Cells. He joined the company Quantum Solar Energy Linz (QSEL) in 2002, which was acquired by Konarka Technologies Inc. in 2003. Over the last 5 years, his main research activities have been new materials for efficient plastic solar cells and their efficiency limitations.

Christoph J. Brabec is the CTO of Konarka technologies Inc. He received his PhD in physical chemistry in 1995 from Linz university, joined the group of Prof Alan Heeger at UCSB for a sabbatical, and continued to work on organic semiconductors as assistant professor at Linz university with Prof. Serdar Sariciftci. He joined the SIEMENS research labs as project leader for organic optoelectronic devices in 2001 and finally joined Konarka in 2004.

Figure 1. Numberof photons(Nph) absorbed inthe active layer (AL)under AM1.5G calculated by TMF, for a device having the following structure:

glass (1mm)/ITO (140nm)/PEDOT:PSS (50nm)/P3HT:PCBM (x nm)/Al (100nm). The right axis represents the corresponding short-circuit current density Jsc at various IQE, indicated in the graph.

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P3HT:PCBM blend could be close to 19mAcm 2, the practically achievable Jsc of real devices will be in the range of 10–12mAcm 2.

2.2. Review of Experimental Results

The first years of OPVs were dominated by poly[2-methoxy,5-

(20-ethyl-hexyloxy)-p-phenylene vinylene) (MEH-PPV)/C60 composites, which were later on substituted by the better-processable combination of poly[2-methoxy-5-(30,70-dimethyloctyloxy)-1,4- phenylenevinylene](MDMO-PPV)/1-(3-methoxycarbonyl)propyl-1- phenyl[6,6]C61 (PCBM).[1,17–21] Because of the rather large gap and low mobility of the PPV-type polymers, efficiencies remained at 3% at best,[2,23] and the general interest in this class of material faded.

During the last five years, research efforts have focused on poly(alkyl-thiophenes), and in particular on P3HT. In 2002, the first encouraging results for P3HT:PCBM solar cells with a weight ratio of 1:3 were published.[24] At that time, the short-circuit current density was the largest ever observed in an organic solar cell (8.7mAcm 2), and resulted from an EQE that showed a maximum of 76% at 550nm. This paper appeared to be a starting point for a rapid development for the P3HT:PCBM blend, followed by the first explicit reports on efficiency enhancement in P3HT/PCBM cells as a result of thermal annealing.[25] The main development over the last years has consisted in understanding and optimizing the processing of the active layer and, especially, the device annealing conditions, which, until recently, appeared to be mandatory to achieve high efficiencies. Table 1 gives a nonexhaustive survey of reports that deal with efficient photovoltaic cells based on a P3HT:PCBM blend.[2,26–35]

Controlling the morphology of the bulk heterojunction in order to ensure maximum exciton dissociation at the interface between the donor and the acceptor, in parallel to an efficient charge-carrier extraction, was found to be the key for high performance. The optimum P3HT:PCBM weight ratio for that is about 1:1, and the two best-suited solvents for this blend are chlorobenzene (CB) and ortho-dichlorobenzene (oDCB). Upon annealing, the open-circuit voltage (Voc) was usually found to decrease slightly, while both the Jsc and the fill factor (F) increased significantly. Figure 2 illustrates a typical enhancement of the EQE upon thermal annealing, as reported by Yang et al.[27] This phenomenon is attributed mainly to an enhancement of the charge-carrier transport, by a larger hole mobility,[36,37] a reduced dispersivity,[38] and a reduced recombination kinetics.[39,40] X-Ray investigations allowed the development of a microscopic picture of the annealing process,[41] as depicted in Figure 3. Several detailed morphological studies revealed that the organization of the P3HT:PCBM is modified upon annealing,[27,32,36] with fibrillar-like P3HT crystals embedded in a matrix believed to comprise mostly PCBM nanocrystals and amorphous P3HT.[27]

The influence of the molecular weight (Mw) on the performance of P3HT:PCBM was quickly addressed once the annealing process was understood.[42] Too-short molecular-weight fractionswere shown to have inferiormobility, most likely because of main-chain defects and low mobility.[43] Furthermore,the role of smallerMw fractions was found to initiate or facilitate the growth of crystalline fibrils during the annealing step, leading to a large

Table 1. Nonexhaustive survey of reports focusing on photovoltaic devices based on P3HT:PCBM blends.

Year P3HTProvider

Mw [gmol 1] Ratio to PCBM(weight) Layer thickness [nm]

Solvent Annealing time [min]

Annealing

Temp. [8C] Max EQE [%] Voc[V]

Figure 2. EQE of different P3HT:PCBM devices reported in the literature. Adapted from [27,2].

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number of small crystals, while higher Mw P3HT stays amorphous.[43] On the other hand, too-high molecular weights produced highly entangled polymer networks, rendering annealing either impossible or requiring higher temperatures and/or longer annealing times.[4] The ideal morphology appears to be formed for P3HTwith an average Mw in the range 30 0–70 0, and a rather high polydispersity of around 2, which gives a good mix of highly crystalline regions formed by low-Mw P3HT embedded in and interconnected by a high-Mw P3HT matrix.[45]

Like the effect of Mw, the influence of the polymer’s regioregularity (R) (defined as the percentage of monomers adopting a head-to-tail configuration, rather than a head-to-head) is critical. A specific threshold for R (about 95%) seems to be necessary to give the highest Jsc and F,[3] mainly because of the better transport properties of highly R P3HT.[46]

2.3. Towards a Better Control of Morphology

As described above, P3HT:PCBM blends require thermal annealing in order to self-organize into a conformation that ensures optimum charge- carrier creation and extraction. But other ways of controlling the morphology have been proposed, and proven to be highly effective.

Slow drying was reported as one of the methods to improve the order in P3HT blends with PCBM.[30] The improved order[47] was reflected in a higher hole mobility,[48] higher FFs, and a reduced series resistance.

Additives were reported as an alternative method to create better order in blends of P3HT and PCBM. Oleic acids and alkylthiols of different lengths,[49] like n-hexylthiol, n-octylthiol, or n-dodecylthiol,[50] were added to P3HT/PCBM solutions, and allowed the formation of thin films with slightly enhanced hole mobility and significantly enhanced charge-carrier lifetimes, because of enlarged P3HT domains with higher crystallinity. Nevertheless, some thermal annealing was still necessary to give the highest possible performance.

This approach is actually very similar to a technique that employs miniemulsions, described earlier and in detail by others.[51,52] In that approach, a mixture of P3HT in water, surfactants, and a solvent was rigorously sonicated, before allowing the solvent evaporate. Such dispersions typically have a particle distribution between 70–200nm, and give homogeneous films[53] upon spin coating. Field-effect-transistor mobilities for such nanoparticular films were found to be on the order of 10 4–10 3cm2V 1s 1. Solar-cell fabrication was more difficult, because there are no known well-performing, water-soluble fullerenes. Thus, only investigations of bilayer devices were performed, which exhibited moderate performances.[54]

A third, quite similar approach to control the nanomorphology of P3HT/PCBM blends requires the addition of ‘nonsolvents’ into the P3HT/PCBM solution (Fig. 4).[5,56] This phenomenon is attributed to the aggregation of the polymer into nanoparticulates, similar to the miniemulsion mentioned above. Addition of nitrobenzene (NtB) to a P3HT/PCBM solution in chlorobenzene allows an increase in the volume fraction of P3HT aggregates from some 60% to up to 100% with increasing NtB content. Photovoltaic devices from P3HT/PCBM mixtures with NtB as additive allowed the manufacture of devices with efficiencies as highas4%withoutthermalannealing.Theseexperimentsproved that a good part of the thin-film morphology can already be introduced on the solution level.

Creating order in the P3HT phase is the key to high performance.[57] The most recent approach grew fibers[57,58] by slow cooling of P3HT solutions, with the crystalline fibers being isolated from the amorphous material by centrifugation and filtration. The fibers were reformulated in dispersions with PCBM, and used for solar-cell processing. The best results (efficiency up to 3.6% under 100mWcm 2) were obtained for a mixture of 75% P3HT fibers and 25% disorganized P3HT, the latter being suspected necessary to fill the gaps present in the nanostructure layer, and to ensure intimate contact between the donor fibers and the PCBM domains (Fig. 5).[57]

(Parte 1 de 5)

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