Photogenerated charge carrier transport and recombination in polyfluorene/fullerene bilayer and blend photovoltaic devices

doi:10.1016/j.orgel.2008.11.010

Organic Electronics

Volume 10, Issue 3, May 2009, Pages 501-505

https://doi.org/10.1016/j.orgel.2008.11.010 Get rights and content

Abstract

Using extraction of photogenerated charge carriers by linearly increasing voltage (photo-CELIV), we investigated two key transport parameters in photovoltaic materials based on the donor APFO-3 and acceptor PCBM: the mobility and lifetime of photogenerated charge carriers, in bilayers of varying geometry and in blends with various acceptors loading. We find that mobility depends strongly on delay time for shorter delay time in all devices. The observed recombination kinetics is found to be monomolecular. The mean lifetime of charge carriers is 2–3 μs in blends and is slightly greater than 4 μs in bilayer devices. In addition, the implications of mobility and lifetime values on the collection efficiency of the devices are presented.

Introduction

Organic solar cells have become a center of attention because they are considered to be a potential low cost renewable energy sources [1], [2], [3]. There are several challenges to be overcome to realize this potential in practical devices. Among the challenges, increasing power conversion efficiency is paramount. In general the overall efficiency of organic solar cells is influenced by four main processes; absorption (creation of bound electron-hole pairs (excitons)), charge generation (dissociation of exciton to free carriers), recombination and/or collection of carriers to their respective electrodes [1].

Recently conversion efficiency above 6% was reported [4]. This remarkable improvement of polymer solar cells was reached by using conjugated polymer–fullerene heterojunctions (HJ) and improved device architecture in a tandem cell. A doubling of the efficiency of organic solar cells was also achieved by folding two planar cells, similar or spectrally different, towards each other [5]. In HJ solar cells the photogenerated excitons dissociate at the donor/acceptor interface via an ultrafast electron transfer from the donor (polymer) to the acceptor [6]. In order to dissociate, excitons must be created within the exciton diffusion length from the interface. There are two categories of HJ solar cells, bilayer HJ and bulk HJs. Bulk HJ solar cells have interpenetrating donor/acceptor interface, which provides large interface area for exciton dissociation and hence increase free carrier generation compared to bilayer HJ. On the other hand, numerical simulations show that collection efficiency of charge carriers is higher in bilayers [7], while still the overall efficiency is better in bulk heterojunctions.

In studies using transient optical spectroscopy, the different neutral and charged species formed by photoexcitation of bulk heterojunctions of APFO3 and PCBM have been followed during the ps–μs time interval after excitation. A full kinetic model has been proposed, where geminate recombination is an important loss mechanism at long times [8]. It is desirable to follow the fate of these charge carriers as we enter the time slot where electrical transport can be directly followed. This is our ambition in the present study. Collection efficiency and loss through recombination of free carriers are controlled by the mobility of free carriers. Therefore, better knowledge of mobility and carrier lifetime will certainly help further improvement of efficiency of polymer solar cells.

In this work we report the mobility and lifetime of photogenerated carriers in donor/acceptor blends with various compositions, and in donor/acceptor bilayer photovoltaic devices and their significance for conversion efficiency. We used the photo-CELIV technique [9], which enables determination of mobility and recombination rate of free carriers simultaneously. The donor polymer used was APFO-3 (poly [2,7-(9,9-dioctyl-fluorene)-alt-5, 5-(4′,7′-di-2 thienyl-2′,1′,3-benzo-thiadiazole)]). The acceptors are PCBM ([6,6]-phenyl-C₆₁-butyric acid methyl ester) in blends and C₆₀ in bilayer.

Section snippets

Experimental

Devices studied in this work are bilayers and blends with a configuration of ITO/APFO-3/C₆₀/Al and ITO/(APFO-3:PCBM) blend/Al, respectively. Solutions of APFO-3:PCBM blend in chloroform with a PCBM content of 80%, 60%, 40% and 0% (pure APFO-3) with concentrations 75 mg/ml, 40 mg/ml, 25 mg/ml and 15 mg/ml were spin coated on pre cleaned glass substrate coated with ITO. Then aluminum was thermally evaporated on the film in a vacuum chamber at a pressure of 2.6 × 10⁻⁶ mbar, to complete the device

Results

Fig. 1 shows transient curves obtained from photo-CELIV measurements; (a) bilayer and (b) blend of 1:4 weight ratios APFO-3:PCBM. The extraction current decreases as delay time is increased for all devices. The time at which the maximum of extraction is reached also increased with increasing delay time. The mobility calculated using Eq. (1) for various delay times are displayed in Fig. 2a for bilayer and b for blends of different compositions of APFO-3 and PCBM. Calculated concentration of

Discussion

The charge transport mechanism in disordered organic systems depends upon the morphology, order, and molecular structure. In such systems the energy distribution of the HOMO (highest occupied molecular orbital) and the LUMO (lowest unoccupied molecular orbital) levels can be well approximated by a Gaussian distribution [10] as in Eq. (4) where σ is the standard deviation energy levels. $g (E) = \frac{1}{\sqrt{2 π σ}} \exp (- \frac{E^{2}}{2 σ^{2}})$

Charge carriers transport in such system is via hopping transport between localized states,

Conclusion

Photo-CELIV technique has been used to investigate mobility and lifetime of photo generated charge carriers in blend and bilayer structures of photovoltaic devices. It is found that the mobility decreases very fast for shorter delay time and slower for longer delay time in all blends and bilayer devices. This decrease in mobility is ascribed to the relaxation of carriers to lower energy states in the distribution of energy states. Monomolecular recombination is found to be dominant in all

Acknowledgments

We would like to thank Fengling Zhang for her assistance in device fabrication. One of the authors (Bekele Homa) acknowledges the financial support from the International Program in the Physical Sciences (IPPS) of Uppsala University, Sweden. These investigations were financially supported by the Center of Organic Electronics (COE) at Linköping University, Sweden, financed by the Strategic Research Foundation SSF.

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LetterPhotogenerated charge carrier transport and recombination in polyfluorene/fullerene bilayer and blend photovoltaic devices

Abstract

Introduction

Section snippets

Experimental

Results

Discussion

Conclusion

Acknowledgments

Progress in photovoltaic

Res. Appl.

Adv. Funct. Mater.

Science

Science

Appl. Phys. Lett.

Phys. Rev. Lett.

J. App. Phys.

Letter
Photogenerated charge carrier transport and recombination in polyfluorene/fullerene bilayer and blend photovoltaic devices