On the origin of the open-circuit voltage of polymer-fullerene solar cells

On the origin of the open-circuit voltage of polymer-fullerene solar cells

(Parte 1 de 3)

ARTICLES PUBLISHEDONLINE:11OCTOBER2009 | DOI:10.1038/NMAT2548

On the origin of the open-circuit voltage of polymer–fullerene solar cells

Koen Vandewal1*, Kristofer Tvingstedt2, Abay Gadisa1, Olle Inganäs2 and Jean V. Manca1

The increasing amount of research on solution-processable, organic donor–acceptor bulk heterojunction photovoltaic systems, based on blends of conjugated polymers and fullerenes has resulted in devices with an overall power-conversion efficiency of 6%. For the best devices, absorbed photon-to-electron quantum efficiencies approaching 100% have been shown. Besides the produced current, the overall efficiency depends critically on the generated photovoltage. Therefore, understanding and optimization of the open-circuit voltage (Voc) of organic solar cells is of high importance. Here, we demonstrate that charge-transfer absorption and emission are shown to be related to each other and Voc in accordance with the assumptions of the detailed balance and quasi-equilibrium theory. We underline the importance of the weak ground-state interaction between thepolymerandthefullereneandweconfirmthatVoc isdeterminedbytheformationofthesestates.Ourworkfurthersuggests alternative pathways to improve Voc of donor–acceptor devices.

The most successful solution-processable organic solar cells use a C60 or C70 fullerene derivative as an electron acceptor blended with a conjugated polymer1–3. In the field, attempts have been made to derive upper limits for the efficiency of this type of polymer–fullerene photovoltaic device, albeit with empirical arguments related to the details of the origin of the open-circuit voltage3–5 (Voc). However, as energy is converted from one form (radiation) to another (electrical), fundamental losses should be taken into account and it should be possible to derive an upper limit for Voc, purely on the basis of thermodynamic considerations. For single absorber materials, this fundamental question was answered in 1961 in a seminal paper by Shockley and Queisser6. Their analysis was based on the detailed balance of absorption and emission events from the solar cell, a ‘grey’ body at the surface of the Earth, illuminated by the Sun, a black body of much higher temperature.

This allowed the derivation of an expression for Voc as a function of the material’s bandgap. It was found that Voc is maximal for the idealcaseinwhichthechargescanrecombineonlyradiatively.

According to this reasoning, it is clear that the Voc of polymer– fullerene devices has not reached its thermodynamic maximal value yet. This value would be reached if the only recombination mechanism at open-circuit conditions is a radiative one6. As a result of the severe luminescence quenching in material blends yielding a substantial charge generation, it is clear that radiative recombination is just a small fraction of the total recombination, and a reduction of the maximum obtainable Voc is expected. In fact, no correlations of Voc with the optical gap of any of the blend constituents, as predicted by Shockley and Queisser6, are observed.

Instead, Voc is found to scale with the difference between the highest occupied molecular orbital energy of the donor and the lowest unoccupied molecular orbital energy of the fullerene acceptor4,7.

This leads to the conclusion that in this type of solar cell, the Voc is determinedbyrecombinationatthedonor/acceptorinterface8–12.

Recently, for some polymer–fullerene blends, radiative interface recombination was observed. The presence of a weak emission signal, redshifted compared to the pure components, was detected in the photoluminescence and electroluminescence spectra and

1IMEC-IMOMEC, vzw, Institute for Materials Research, Hasselt University, Wetenschapspark 1, 3590 Diepenbeek, Belgium, 2Biomolecular and Organic Electronics, Center of Organic Electronics (COE), Department of Physics, Chemistry and Biology, Linköping University, 58183 Linköping, Sweden. *e-mail:koen.vandewal@uhasselt.be.

was assigned to the emission of interface electron–hole pairs or charge-transfer excitons13–17. The signature of this emitting state is also present in the absorption spectrum as a new, weak subgap absorption band in several polymer–fullerene blends used for photovoltaic applications18–20. Such absorption bands are typical for the formation of a ground-state charge-transfer complex (CTC) between the polymer and the fullerene. Furthermore, good correlations between the open-circuit voltage and the spectral position of the charge-transfer absorption20, photoluminescence15 or electroluminescence17 could be made.

Here, we show that the electroluminescence and photovoltaic external quantum efficiency spectra in the low-energy, chargetransfer region are related to each other as predicted by the detailed balance approach. Furthermore, it is shown that at Voc, the photocurrent generated by the absorption of sunlight balances with the recombination current, resulting in emission of photons by the excited CTCs. This confirms previous suggestions10,15,20, that Voc is determined by the CTC formation between the polymer and the fullerene.

To validate the generality of the detailed balance treatment for polymer–fullerene solar cells, blends of five different donor polymers and two fullerene derivatives, that is, [6,6]-phenyl

C61 butyric acid methyl ester (PC61BM) and [6,6]-phenyl C71 butyric acid methyl ester (PC71BM), were investigated. The polymers belong to different conjugated polymer material families, comprising different conjugated backbones. These conjugated polymers are representative of the donor polymers used in polymer–fullerene solar cells explored in the community at present. Their chemical structures are shown in Fig. 1.

Devices based on poly[2-methoxy-5-(30,70-dimethyloctyloxy)- 1,4-phenylene vinylene] (MDMO-PPV) and poly[2,7-(9-di-octylfluorene)-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′ benzothiadiazole)] (APFO3) were prepared using different polymer/fullerene stoichiometries. Optimal devices were obtained using a 1:4 polymer/fullerene weight ratio, resulting in a power conversion efficiency of ∼2% and ∼2.5% respectively. At a lower fullerene content, the photogenerated current becomes lower and the

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ade

Figure 1 | The chemical structures of the donor polymers used. a, MDMO-PPV. b, P3HT. c, PCPDTBT. d, LBPP5. e, APFO3.

Table 1|Measured Jsc,Voc and calculated J0 for all of the devices studied in this work.

J was calculated using the EQE (E) and EQE (E) spectra by means of equation (3). The errors on J and V are experimental errors obtained by measuring different devices. For the errors on J , the variation of J over the spectral range of the CTC was taken into account as well as the experimental error on EQE .

efficiency drops. However, Voc increases as the fullerene content is decreased (see Supplementary Information). APFO3-based devices were prepared with both PC61BM and PC71BM. For the poly[3- hexylthiophene] (P3HT)–PC61BM blends, ordering of the polymer phase, for example, induced by annealing, has been proven to have a major influence on the device performance21. Therefore, in this study, as-prepared and annealed devices were characterized. For this material system, typical conversion efficiencies of 3.5% were reached. However, higher efficiencies for P3HT–PC61BM devices of up to 5% have been reported22. A polymer of particular interest is 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), as it has a low optical gap, close to the optimum as predicted by Shockley and Queisser6. It was shown that the addition of thiols in a PCPDTBT–fullerene solution improves the device performance23.

We obtained for PCPDTBT–PC61BM devices typically a power conversion efficiency of 3%. Efficiencies of 5.5% were reported for

Number of photons (eV

Number of photons (x10

21 eV

P3HT–PC61BM

APFO3–PC61BM

PCPDTBT–PC61BM

MDMO-PPV–PC61BM LBPP5–PC71BM a b

Figure 2 | The EQEPV spectra of polymer–fullerene devices. The devices comprise active layers: P3HT–PC61BM (1:1) (annealed), PCPDTBT–PC61BM (1:2), LBPP5–PC71BM (1:3), MDMO-PPV–PC61BM (1:4) and

APFO3–PC61BM (1:4). a, The spectra on a linear scale. The standard AM1.5G spectrum is shown on the right axis. b, The spectra on a logarithmic scale, to make the weak contribution of the low oscillator strength CTC visible. A charge-transfer band is clearly visible for all five material blends. Depending on the donor polymer, the spectral position of the charge-transfer band varies. On the right axis of b, the emission spectrum of a black body at room temperature is shown.

PCPDTBT–PC71BM devices23. As in this article, there is particular interest in the Voc of the devices; note that the obtained Voc values correspond to what is found in the literature for similar devices.

An overview of the devices studied in this work and the measured short-circuit current (Jsc) and Voc values are listed in Table 1.

In Fig. 2a, photovoltaic external quantum efficiency (EQEPV) spectra are shown on a linear scale for five devices using the

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N (s

P3HT PCPDTBT LBPP5 MDMO-PPV APFO3

∼exp(qV/kT)

Figure 3 | Electroluminescence emission and corresponding injected current versus voltage curves of polymer–fullerene devices. The active layers of the devices are: P3HT–PC61BM (1:1) (annealed), PCPDTBT–PC61BM (1:2), LBPP5–PC71BM (1:3), MDMO-PPV–PC61BM (1:4) and APFO3–PC61BM (1:4). a, The number of detected photons by a silicon detector versus the applied voltage over the device. The black line represents a curve proportional to exp(qV/kT). An onset proportional to this curve is measurable for all polymer–fullerene devices, except for the annealed P3HT–PC61BM device, because of its low-efficiency electroluminescence. b, The corresponding injection current versus voltage curves.

above conjugated polymers. Spectra for further devices using different preparation conditions are shown in the Supplementary Information. For the P3HT-, MDMO-PPV-, APFO3- and poly[1,4-(2,5-dioctyloxybenzene)-alt-5,5-(5′,8′-di-2-thienyl-2′,3′- di-(3′′-butoxyphenyl)quinoxaline)] (LBPP5)-based devices, the fullerene has the lowest optical gap of the blend constituents

(1.7eV). For the PCPDTBT–PC61BM device however, the lowest optical gap is the polymer bandgap (1.4eV). From these EQEPV spectra, the total photogenerated current can be calculated, by integrating the EQEPV spectrum over the solar spectrum, shown on the right axis of Fig. 2a. From this figure, it is clear that the photocurrent under solar illumination for these five devices is dominated by polymer absorption. In this respect, the use of a C70 fullerene derivative is beneficial, as it aids in absorbing a substantiallygreaterpartofthesunlightthantheC60 derivative24. Polymer–fullerene ground-state material interaction and CTC formation is characterized by the presence of a new absorption band, owing to an optical transition in which an electronic charge is transferred from the donor-conjugated polymer to the fullerene acceptor. The low oscillator strength of this transition and hence low absorption coefficient however, forces us to use specialized techniques to detect charge-transfer bands in the

EQEPV(E) spectra. Therefore, the sensitive detection method Fourier-transform photocurrent spectroscopy20,25 (FTPS) is also used to collect the very low photocurrent signals generated by excitation of the CTCs. These signals become visible in the

EQEPV(E) spectra, shown in Fig. 2b on a logarithmic scale. For all five material combinations shown in the figure and the combinations shown in the Supplementary Information, the lowest energy excitation is due to a charge-transfer optical transition. Dependingonthedonorpolymer,thissubgapcharge-transferband has an onset ranging from 1 to 1.5eV.

In Fig. 3, the total number of photons Nph emitted by electroluminescence and detected by a silicon detector, versus voltage, and the corresponding injected current Jinj(V) for the five material combinations are shown. It can be seen that the

electroluminescenceemissiononsetisproportionaltoexp(qV/kT). Here, k is the Boltzmann constant, T is the absolute room temperature and q is the elementary electron charge. This onset is lower than the voltage onset of the electroluminescence of the pure materials17. At high voltages, the electroluminescence and injected current are space-charge and/or series-resistance limited and deviate from the exponential. Owing to the low quantum efficiency of the charge-transfer emission of annealed

P3HT–PC61BM devices and the limited detection range of the Si photodiode, the exponential part in the electroluminescence versus voltage curves could not be resolved for this device.

To spectrally resolve the electroluminescence spectra with a sufficiently high signal, they were measured with a sensitive set-up using injection currents corresponding to a charge density present in the device comparable to 1–10 sun conditions. As the total number of photons emitted by electroluminescence scales with the injected current, we choose to show the electroluminescence external quantum efficiency (EQEEL), calculated as the number of emitted photons divided by the number of injected electrons.

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