Electronic Properties of Polymer-Fullerene Solar Cells (original) (raw)
Related papers
2001
We report on the temperature dependence of various photovoltaic device parameters of solar cells, fabricated from interpenetrating networks of conjugated polymers with fullerenes, in the wide temperature range of their possible operating conditions ͑25-60°C͒. The open-circuit voltage was found to decrease linearly with increasing temperature. For the short-circuit current, we observed a monotonic increase with increasing temperature, followed by a saturation region. The rate of this increase ͑coupled to a corresponding increase for the fill factor͒ was found to overtake the corresponding rate of decrease in voltage, resulting in an overall increase of the energy conversion efficiency. The efficiency was observed to reach a maximum value in the approximate range 47-60°C. The results are discussed with respect to possible mechanisms for photovoltage generation and charge carrier transport in the conjugated polymer-fullerene composite, and in particular, thermally activated charge carrier mobility.
Spectroscopy on polymer-fullerene photovoltaic cells
Conference Record of the Twenty-Eighth IEEE Photovoltaic Specialists Conference - 2000 (Cat. No.00CH37036), 2000
We investigate the electrical transport properties of ITOlconjugated polymer-fullerene/Al photovoltaic cells and the role of defect states with current-voltage studies, admittance spectroscopy, and electron spin resonance technique. In the temperature range 29340K, the characteristic step in the admittance spectrum can be observed originating from the electrically active acceptor level. The activation energy determined from the Arrhenius plot is 34meV. The diode capacitance as a function of the reverse bias is different from the Schottky diode behavior. We found a bias independent capacitance under reverse bias. This indicates that the devices are either fully depleted, or the space charge region exceeds the device thickness. We can clearly follow the formation of photogenerated electron hole pairs under illumination of the device absorber by using the electron spin resonance technique. Important for the cell performance is that photogenerated electron-hole pairs remain in the composites even after the photoexcitation is off, implying the presence of defect induced trap states.
Organic Photovoltaics, 2001
Bulk donor-acceptor heterojunctions between conjugated polymers and fullerene derivatives have been utilized successfully for photovoltaic devices showing monochromatic efficiencies above 1 %. The present paper reports the temperature and irradiance dependencies of full-spectrum photovoltaic parameters for such devices. The measurements were performed under real sun conditions and under a solar simulator. The sun provided a light source stable in intensity to within ±1% and closely approximating a true AM1.5 spectrum, whereas the simulator enabled the light intensity to be varied in the range 80-600 W m-2. The most interesting feature that was observed for these devices is that above a cell temperature of 20 o C the positive temperature coefficient observed for the short-circuit current exceeds in magnitude the negative temperature coefficient that was found for the open-circuit voltage. This means that, unlike the situation for conventional PV devices, these cells actually exhibit an increase in efficiency with increasing temperature (reaching a value of 0.63 % at 40 o C). We suggest that the observed behavior originates from the temperature dependence of the conductivity of the conjugated polymers-fullerene composite. This hypothesis is confirmed by the irradiance-resolved measurements performed at different cell temperatures. We observe a linear increase in the short-circuit current with light intensity over the whole ranges of irradiances and temperatures but maximum temperature influence is observed at highest light intensity.
Light intensity dependence of open-circuit voltage of polymer:fullerene solar cells
Applied Physics Letters, 2005
The open-circuit voltage V oc of polymer:fullerene bulk heterojunction solar cells is investigated as a function of light intensity for different temperatures. Devices consisted of a blend of a poly ͑p-phenylene vinylene͒ derivative as the hole conductor and 6,6-phenyl C 61 -butyric acid methyl ester as the electron conductor. The observed photogenerated current and V oc are at variance with classical p -n junction-based models. The influence of light intensity and recombination strength on V oc is consistently explained by a model based on the notion that the quasi-Fermi levels are constant throughout the device, including both drift and diffusion of charge carriers.
Advanced Energy Materials, 2014
conjugated polymer or small molecule as the donor material and a fullerene derivative as the electron acceptor. Efforts to raise the power conversion effi ciency by increasing the open-circuit voltage (V OC) have primarily focused on fi nding donor materials with lower-lying highest occupied molecular orbital (HOMO) levels [ 4 ] or fullerene derivatives with higher-lying lowest unoccupied molecular orbital (LUMO) levels. [ 5 ] Several research groups [ 6 ] have shown that a maximum of approximately 1.0 V for the V OC exists for effi cient OPV devices using fullerene derivatives as the electron acceptor. This limit is due to the inability to effi ciently split excitons on the fullerene molecule when the energy of the charge transfer (CT) state is less than 0.15 eV below the fullerene singlet excited state energy of 1.7 eV. In polymerfullerene systems where the fullerene has the smaller singlet energy, excitons formed in the polymer can reach the fullerenes via energy transfer, and ineffi cient hole transfer from the fullerenes to the polymer results in a large current loss in devices with V OC values exceeding 1.0 V. In order to relax this ceiling on the V OC and fi nd effi cient devices with voltages that can yield high effi ciencies in both single and multi-junction devices, new electron acceptors are needed with higher energy singlet excited states. [ 3b , 6d , 7 ] In addition to this restriction on the V OC , fullerene derivatives are relatively expensive [ 8 ] and C 60 derivatives do not absorb light well. One study [ 8a ] has shown that the PC 60 BM commonly used in bulk heterojunction (BHJ) organic solar cells could account for 12% of the overall OPV module cost and that C 70-based derivatives would be even more expensive. Current research into new electron acceptors [ 9 ] has covered an array of polymers [ 10 ] and small molecules. [ 11 ] While most devices prepared with these acceptors have effi ciencies near or below 2%, a few, including those based on evaporated devices incorporating halogenated boron subphthalocyanine molecules, [ 11p ] dimeric perylene diimide small molecules, [ 11r ] and solution-processed all-polymer devices based on naphthalene diimide [ 10a ] have achieved effi ciencies greater than 4%. V OC values approaching and even exceeding 1.0 V have been achieved in a few of these devices [ 11b , 11h ] but the typical values for the short-circuit current (J SC) and fi ll factor are There is a need to fi nd electron acceptors for organic photovoltaics that are not based on fullerene derivatives since fullerenes have a small band gap that limits the open-circuit voltage (V OC), do not absorb strongly and are expensive. Here, a phenylimide-based acceptor molecule, 4,7-bis(4-(N-hexyl-phthalimide)vinyl)benzo[c]1,2,5-thiadiazole (HPI-BT), that can be used to make solar cells with V OC values up to 1.11 V and power conversion effi ciencies up to 3.7% with two thiophene polymers is demonstrated. An internal quantum effi ciency of 56%, compared to 75-90% for polymer-fullerene devices, results from less effi cient separation of geminate charge pairs. While favorable energetic offsets in the polymer-fullerene devices due to the formation of a disordered mixed phase are thought to improve charge separation, the low miscibility (<5 wt%) of HPI-BT in polymers is hypothesized to prevent the mixed phase and energetic offsets from forming, thus reducing the driving force for charges to separate into the pure donor and acceptor phases where they can be collected.
On the origin of the open-circuit voltage of polymer–fullerene solar cells
Nature Materials, 2009
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 (V oc) 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 V oc in accordance with the assumptions of the detailed balance and quasi-equilibrium theory. We underline the importance of the weak ground-state interaction between the polymer and the fullerene and we confirm that V oc is determined by the formation of these states. Our work further suggests alternative pathways to improve V oc of donor-acceptor devices.