Influence of the Bridging Atom on the Performance of a Low‐Bandgap Bulk Heterojunction Solar Cell

Abstract

Bulk heterojunction solar cells have attracted considerable attention over the past several years due to their potential for low-cost photovoltaic technology. The possibility of manufacturing modules via a standard printing/coating method in a roll-to-roll process in combination with the use of low-cost materials will lead to a watt-peak price of less than 1 US$ within the next few years. [1] Despite the low-cost potential, the power conversion efficiency of bulk heterojunction devices is low compared to inorganic solar cells. Efficiencies in the range of 5-6% have been certified at NREL and AIST usually on devices with small active areas.[2]The current understanding of bulk heterojunction solar cells suggests that the maximum efficiency is in the range of 10-12%.[3] Several reasons for the power conversion efficiency limitation have been identified.[1] Some of the prerequisites for achieving highest efficiencies are donor and acceptor materials with optimized energy levels [highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO)], efficient charge transport in the donor-acceptor blend, efficient charge generation and limited recombination losses. Power conversion efficiency is strongly dependent on charge transport and charge generation, which are dominated by the phase behavior of the donor and acceptor molecules. The resulting, and often unfavorable, nanomorphology of this two-component blend limits the power conversion efficiency of bulk heterojunction solar cells. Precise control of the nanomorphology is very difficult and has been achieved only for a few systems. [4][5][6] The relation between the chemical structure of donor and acceptor materials and the nanomorphology that they form when they are blended is currently not well understood, and as will be shown in this paper, minor changes in the chemical structure can cause major changes in the performance of the materials in organic solar cells.

On the morphology of polymer‐based photovoltaics

“…16(b,c)]. 99,100 The higher crystallinity of the polymer is thought to be responsible for the better charge transport and reduced bimolecular recombination. Thermal annealing was demonstrated to be effective to increase PCE from 3.8 to 5.6%.…”

A low band gap co-polymer of dithienogermole and 2,1,3-benzothiadiazole by Suzuki polycondensation and its application in transistor and photovoltaic cells

“…Several studies have demonstrated that the electronic properties of the polymer 20 backbone can be modified by interaction of the low lying * orbitals of the silicon atom with * orbitals of the conjugated system, primarily resulting in stabilisation of the polymer LUMO in comparison to the carbon bridged analogues. 1 Furthermore studies by Yang 2, 3 , Morana 4 and Scharber 5 et al 25 have directly compared the properties of a silole-containing polymer, poly{ [4,4'- fig 1). They found significant differences in the crystallinity and thin film morphology of the two polymers, which were rationalised on the basis of detailed modelling that the longer carbon-silicon bond in PSBTBT resulted in a 35 reduction of steric hindrance between the solubilising alkyl groups and adjacent thiophene rings.…”

“…The SiO 2 layer was treated with octadecyltrichlorosilane (OTS) before use, and the Au source drain electrodes were treated with pentafluorobenzene thiol before use to afford a reliable work function. Devices were 5 fabricated by spin coating hot 1,2-dichlorobenzene solutions followed by annealing of the devices at 140°C. The transfer and output characteristics of a typical BG device are shown in fig.…”

Charge carrier transport properties in ternary Si-PCPDTBT:P3HT:PCBM solar cells

“…Maximum efficiencies were obtained from the blends consisting of the 40% donor loading of Si-PCPDTBT, i. e. a ratio of 0.4:0.6:1 for Si-PCPDTBT:P3HT:PCBM, respectively. An increase in V oc is observed with increasing the Si-PCPDTBT content of the blends, which could be originating from the lower lying HOMO levels of Si-PCPDTBT with respect to P3HT [10]. Such a trend of increasing open circuit voltages has been reported in analogous ternary blends consisting of Si-PCPDTBT [8].…”