Effect of illumination intensity and temperature on open circuit voltage in organic solar cells

Abstract

The effect of illumination intensity and temperature on open circuit voltage ͑V oc ͒ in organic photovoltaic devices has been investigated. V oc is observed to saturate at high illumination intensities. The illuminated J-V characteristics at different intensities intersect the dark characteristic at a single point. This intersection point is shown to be equal to the built-in voltage ͑V bi ͒ in the sample. A reduction in temperature shows increment in saturated V oc . This increment in saturated V oc is attributed to the variation of V bi with temperature. A model has been presented that explains the observed behavior of V bi at different temperatures. © 2009 American Institute of Physics. ͓DOI: 10.1063/1.3129194͔Organic photovoltaic ͑OPV͒ devices can play an important role in generating long-term clean and cheap energy. Power conversion efficiencies and lifetime of these devices are not yet high enough for their commercial viability. Efforts are being made all over the world to understand the physics behind the operation and to improve the performance of these devices. The interpenetrating bulk-heterojunction tandem devices have shown a power conversion efficiency ͑͒ of ϳ6.5%.2 In order to optimize the power conversion efficiency and their reliability in various operating conditions, the complete understanding of device physics is of prime importance. There are various fundamental aspects regarding the physics of OPV devices that require attention. For example, contrary to the Si photovoltaic devices the forward bias current in OPV devices under illumination intersects the dark current and becomes more than the dark current. And another important issue is the origin of V oc . For the design of future solar cells, it is important to understand whether the V oc is a bulk property or an electrode property or a combination of both. It has been suggested that V oc in OPV devices depends either on the difference of energies of highest occupied molecular orbital of the donor and lowest occupied molecular orbital of the acceptor 3,4 or on the difference of the work functions of anode and cathode ͑⌬W͒ used. 5The variation of V oc with temperature 6 and illumination intensity 7,8 cannot be explained by either of these models. Various models have been presented to explain the experimental observations in OPV devices 5,7 yet the physics behind the operation is not well understood and requires more work to be done. Schilinsky et al. 7 extended the standard Si one diode model and a good agreement with the experimental data at different illumination intensities was observed. By self-consistent calculations, Schilinsky et al. 7 showed the existence of a constant electric field in the OPV devices. This finding is an important contribution. The correction due to this electric field was applied to the observed short circuit current ͑J sc ͒. However, the effects of the electric field on the dark currents were not investigated. We have recently presented a model considering the effect of constant electric field on both the dark an...

Temperature dependence of polymer/fullerene organic solar cells

“…Quantum efficiency depends on changes in current as well as absorption. The linear decrease in open circuit voltage with increasing temperature has been discussed in literature, but is not fully theoretically understood [25,26]. This may also be due to changes in energy level positions with temperature as indicated from absorption and photoluminescence data.…”

Temperature Dependence of Open-Circuit Voltage and UPS Study for P3HT:PCBM Organic Solar Cells

“…In this study, P3HT and PCBM were chosen due to the great interest for the fabrication of BHJ) devices with their possible solar cell efficiency of 5% and lowcost fabrication [10], [11]. The temperature dependence studies of different I-V parameters of a small-molecular donor/acceptor system [copper phthalocyanine(CuPc)/fullerene(C 60 )] have been reported [9], [12]. The V oc of the CuPc/C 60 solar cell showed the same decreasing trend when the temperature increased as Bagienski and Gupta [8] found out in their study.…”

Temperature dependence of HOMO-LUMO levels and open circuit voltage for P3HT:PCBM organic solar cells

“…Bagienski and Gupta have reported temperature dependence of P3HT:PCBM solar cells parameters [8]. The temperature dependence studies of different I-V parameters of a small-molecular donor/acceptor system (copper phthalocyanine(CuPc)/fullerene(C 60 )) have been reported [9,10]. The V OC of the CuPc/C 60 solar cell showed the same decreasing trend when the temperature increased as Bagienski et al found out in their study.…”