Polymer solar cells open circuit voltage

Scharber et al. have proposed a relation between the HOMO of the polymer and the open circuit voltage Foo which was used to estimate the maximum efficiency of bulk-heterojunction solar cells for a number of material combinations, including P3HT and phenyl-C61-butyric acid methyl ester (PCBM) [36]. As an alternative to energy level determination with electrochemistry, the method of photoelectron spectroscopy can be used Kahn and coworkers used a combination of direct and inverse photoemission spectroscopy complemented by near edge X-ray absorption fine structure (NEXAFS) for determination of the HOMO and LUMO energy levels and the energy gap between the LUMO of the acceptor and the HOMO of the donor [37]. 

X. Gong, C. H. Li, Z. Lu, G. W. Li, Q. Mei, T. Fang and Z. S. Bo, Anthracene-Containing Wide-Band-Gap Conjugated Polymers for High-Open-Circuit-Voltage Polymer Solar Cells, Macromol Rapid Commun., 2013, 34(14), 1163-1168. 

Kymakis E, Amaratunga GAJ (2003). Photovoltaic cells based on dye-sensitisation of single-wall carbon nanotubes in a polymer matrix. Solar Ener. Mater. Solar Cells 80 465 172.Kymakis E, Alexandrou I, Amaratunga GAJ (2003). High open-circuit voltage photovoltaic devices from carbon-nanotube-polymer composites. J. Appl. Phys. 93 1764-1768. 

He, I. Zhong, C. Huang, X. Wong, W.-Y. Wu, H. Chen, L. Su, S. Cao, Y., Simultaneous enhancement of open-circuit voltage, short-circuit current density, and fill factor in polymer solar cells. Adv. Mater. 2011, 23, 4636-4643. 

Lenes M, Wetzelaer G, Kooistra FB, Veenstra SC, Hummelen JC, Blom PWM (2008) Fullerene bisadducts for enhanced open-circuit voltages and efficiencies in polymer solar cells. Adv Mater 20 2116... 

Vandewal K, Tvingstedt K, Gadisa A, Inganas O, Manca JV (2009) On the origin of the open-circuit voltage of polymer-fullerene solar cells. Nat Meter 8 904... 

The experimentally observed Voc of PSC cannot be explained by the MIM picture alone. For typical devices, based on ITO/conjugated polymer fullerene/Al, values of Voc can be observed in a range of 800 mV and higher for several polymer/fullerene mixtures, in contrast to the 400 mV expected from the MIM picture. The origin of the open-circuit voltage in plastic solar cells will be discussed and explained in Sect. 5.3.4. 

V.D. Mihailetchi, P.W.M. Blom, J.C. Hummelen, M.T. Rispens, Cathode dependence of the open-circuit voltage of polymer fullerene bulk heterojunction solar cells,./. Appl. Phys. 94 (2003) 6849-6854. 

Fig. 17 Simple relationship of open circuit voltage Vqc for drift-current dominated bulk heterojunction polymer solar cells. The first limitation arises from the molecular energy levels (Voci) secondly, improper match with the contact work function may further reduce the achievable voltage to 002- (Reprinted with permission from [105], 2003, American Institute of Physics)...

Katz et al. investigated the performance of the polymer solar cells under elevated temperatures in the range of 25-60 °C, which represents real operating conditions due to heating under solar irradiation [122]. While the open circuit voltage ( oc) decreased linearly with temperature, the short circuit current (Jsc) and the fill factor (FF) increased up to about 50 °C, followed by a saturation region (Fig. 28). These effects overcompensated the dropping 7oc and thus the efficiency was maximal for a 50 °C cell temperature [122]. 

Fig. 28 Solar cell parameters for MDMO-PPV PCBM polymer solar cells under slightly elevated temperatures, as expected for realistic operation conditions. Interestingly, the short circuit photocurrent increases with temperature, while the open circuit voltage drops. As a result the power conversion efficiency is maximized for temperatures of 50 (Reprinted with permission from [122], 2001, American Institute of Physics)...

Polymer-polymer solar cells employ two different polymers as donor and acceptor components in the photoactive layer. These two polymers require a molecular energy level offset between their HOMO and LUMO levels to enable a photoinduced charge transfer. Due to the close vicinity of the respective molecular energy levels, polymer-polymer solar cells allow high open circuit voltages to be reached. 

Fig. 44 Light intensity dependence of short circuit photocurrent (filled circles) and open circuit voltage of laminated POPT (MEH-)CN-PPV diffuse bilayer polymer solar cells. The scaling factor of the current calculates as 1.02. (Reprinted with permission from [32], 1998, Macmillan Publishers Ltd)...

In these cases bimolecular recombination limited device efficiencies at higher light intensities nonetheless, up to 1.1% power conversion efficiency was reached under full AM 1.5 solar irradiation [222]. Interestingly, the authors observed an open circuit voltage exceeding the work function difference of the respective electrodes by more than a factor of 2 for various acceptor polymers. The origin of this will be discussed on the basis of polyflu-orene based polymer-polymer solar cells later in this section. 

Kietzke et al. have shown for bilayer solar cells based on M3EH-PPV and several acceptor polymers with varying electron affinities and the fullerene derivative PCBM that the open circuit voltage is linearly related to the respective LUMO levels [225]. While CN-PPV-PPE acceptors resulted in an increased open circuit voltage of about 1.5 V, the fill factor and photocurrent were smaller than those for CN-ether-PPV [225]. 

A logarithmic relationship between light intensity and open circuit voltage had been shown for bilayer polymer solar cells by Ramsdale et al. [125]. This and the observation of an overpotential of the open circuit voltage with respect to the work function difference of the two electrodes—as inferred... 

Roster LJA, Mihailetchi VD, Ramaker R, Blom PWM (2005) Light intensity dependence of open-circuit voltage of polymer fullerene solar cells. Appl Phys Lett 86 123509... 

Finally, conjugated materials 40 based on poly(phenylene thiophene) and poly (fluorene thiophene) main chain polymers functionalized with pendant trithiocyanato ruthenium terpyridine complexes were synthesized by the Suzuki coupling reaction. Heterojunction photovoltaic cells with the simple structure ITO/polymer/C-60/Al were fabricated. Under simulated AM1.5 solar light illumination, the short circuit currents, open circuit voltages, and power conversion efficiencies of the photovoltaic cells were measured to be 1.53-2.58 mAcm 2, 0.12-0.24 V, and 0.084-0.12%, respectively [77]. 

Furthermore, the production is expected to be easily scalable. This technology is currently developed by many researchers around the world, but has not yet reached the marketplace. In order for polymer solar cells to become economic their efficiency must be improved. The power conversion efficiency of a solar cell is dictated by three factors (i) the fraction of sunlight that can be absorbed, (ii) the fraction of absorbed photons that lead to extracted charges ( internal quantum efficiency ), and (iii) the energy that is retained by the extracted charges (ideally close to the open-circuit voltage ). In this review we will refer often to factors (ii) and (ui). Their interplay is not well understood and at present these have not both been optimized simultaneously even in state-of-the-art organic solar cells. 

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