Influence of doping on charge carrier collection in normal and inverted geometry polymer:fullerene solar cells

Abstract

While organic semiconductors used in polymer:fullerene photovoltaics are generally not intentionally doped, significant levels of unintentional doping have previously been reported in the literature. Here, we explain the differences in photocurrent collection between standard (transparent anode) and inverted (transparent cathode) low band-gap polymer:fullerene solar cells in terms of unintentional p-type doping. Using capacitance/voltage measurements, we find that the devices exhibit doping levels of order 10 16 cm 23 , resulting in space-charge regions ,100 nm thick at short circuit. As a result, low field regions form in devices thicker than 100 nm. Because more of the light is absorbed in the low field region in standard than in inverted architectures, the losses due to inefficient charge collection are greater in standard architectures. Using optical modelling, we show that the observed trends in photocurrent with device architecture and thickness can be explained if only charge carriers photogenerated in the depletion region contribute to the photocurrent.T he record power conversion efficiency (PCE) achieved by polymer:fullerene solar cells has increased considerably in the past 4 years to a record published value of 9.2% 1 for a single bulk heterojunction and efficiencies of 10.6% for tandem solar cells 2 . This is despite the fact that organic semiconductors are known to be both structurally and electronically disordered, have lower dielectric constants inhibiting separation of the photogenerated excitonic species and have charge carrier mobilities orders of magnitude lower than inorganic semiconductors.Whilst charge mobilities are low in organic semiconductors and collection losses have been shown to limit the fill factor (FF) 3-5 and short circuit current density (J SC ) 6-10 of certain devices, low mobilities do not necessarily prevent devices from performing efficiently. However the lower charge mobilities and diffusion coefficients in organic semiconductors do mean that diffusion alone is insufficient for charge carrier collection and drift must account for a large proportion of the generated photocurrent. Additionally, polymer:fullerene solar cells are not intentionally doped like their inorganic counterparts or like many small molecule solar cells 11 and therefore rely on selective contacts and the difference in work function between electrodes for efficient charge collection. However, several studies have found evidence for unintentional doping [12][13][14][15][16][17][18][19] and discussed the consequences for device behaviour 6,[20][21][22][23][24][25][26][27][28][29][30] . Whilst the origin of this doping is unclear 15 , its effects on photovoltaic performance can be substantial; however many recent analyses of device performance neglect doping 8,[31][32][33] despite the fact that the influence of doping and the electric field on charge carrier collection is well known for a long time 34 and wellstudied for instance in the field of quantum dot photovoltaics 35,36 .In this paper, we address the...

CuO Nanoparticles Based Bulk Heterojunction Solar Cells: Investigations on Morphology and Performance

“…Electronic composition of the device plays a key role in accomplishing this step. Successful injection of electrons into the cathode depends on the magnitude of the conduction band energy level of the acceptor material, compared to the vacuum level, being lower than the work function of the donor material [29]. Similarly, for successful injection of holes into the anode, the magnitude of the conduction band energy level of the acceptor material, compared to the vacuum level, should be higher than the work function of the transparent anode.…”

Effect of doping on the short-circuit current and open-circuit voltage of polymer solar cells

“…The width of the space charge region (depletion region) depends on the doping density and the built-in voltage V BI via 21,22 w…”

“…4(a), N a ¼ 1.6 Â 10 16 cm À3 ). 21,22 The carrier mobilities of the as-fabricated and the thermal-annealed oDCB BHJ layer are measured by the space-charge-limited-current (SCLC) method in an electron-only device and a hole-only device. Both electron mobility and hole mobility of the oDCB layer are in the order of 10 À4 cm 2 /V.s (l n ¼ 2.8 Â 10 À4 cm 2 /V.s, l p ¼ 4.0 Â 10 À4 cm 2 /V.s) before thermal treatment, increasing to 10 À3 cm 2 /V.s (l n ¼ 2.6 Â 10 À3 cm 2 /V.s, l p ¼ 1.5 Â 10 À3 cm 2 /V.s) after thermal annealing (see Figure S3 of the supporting information for the J-V characteristics of the electron-and hole-only device 30 ).…”

“…The simple assumption of 100% collection efficiency in the depletion region and 0% collection efficiency in the neutral region was proved to be valid by George et al 22 We can roughly consider the width of the depletion region to be the carrier collection depth of the device. From Eq.…”

“…Recently, Nelson et al discovered that p-type doping has a dominant influence on the photocurrent of PSCs based on some low bandgap polymers. 21,22 These discoveries suggest that, as in inorganic semiconductor devices, the level of doping also has profound influence in organic semiconductor devices. However, the doping in the polymer-fullerene blends is always unintentional and investigation into the nature of the doping is necessary.…”