Interplay between efficiency and device architecture for small molecule organic solar cells

Abstract

Small molecule organic solar cells (OSCs) have experienced a resurgence of interest over their polymer solar cell counterparts, owing to their improved batch-to-batch (thus, cell-to-cell) reliability. In this systematic study on OSC device architecture, we investigate five different small molecule OSC structures, including the simple planar heterojunction (PHJ) and bulk heterojunction (BHJ), as well as several planar-mixed structures. The different OSC structures are studied over a wide range of donor:acceptor mixing concentrations to gain a comprehensive understanding of their charge transport behavior. Transient photocurrent decay measurements provide crucial information regarding the interplay between charge sweep-out and charge recombination, and ultimately hint toward space charge effects in planar-mixed structures. Results show that the BHJ/acceptor architecture, comprising a BHJ layer with high C60 acceptor content, generates OSCs with the highest performance by balancing charge generation with charge collection. The performance of other device architectures is largely limited by hole transport, with associated hole accumulation and space charge effects.

Vacuum deposited ternary mixture organic solar cells

“…The ClInPc:C 60 and SubPc:C 60 OSCs maximize their performance at 25% donor content with peak PCE values of 2.7% and 3.1% respectively. At lower donor content, the OSCs experience a reduction in J sc (due to the lower donor content, which is the primary light absorber) and a reduction in FF (essentially due to hole trapping and recombination effects) [47]. We also observe an increase in V oc with decreasing donor content due to Schottky band bending effects, as has been established previously [43,47,48].…”

“…The thickness of the BHJ was chosen to be 40 nm since it provides optimal performance for binary metal phthalocyanine:C 60 BHJ OSCs, as established in our previous work [44]. It should also be noted that a neat layer of C 60 in between the BHJ and the BCP layer could be employed in these devices to further bolster the power conversion efficiency (PCE) of the OSCs [47]. However, this architecture would also make data analysis, particularly with the external quantum efficiency (EQE) spectra, more complicated than necessary.…”

“…The latter UV band comprises contributions from both C 60 and the metal phthalocyanine B bands. The C 60 aggregate band is only present at low donor concentrations (conversely, high C 60 concentrations) [43,47], and is best observed at P75% C 60 within the mixed layer. By balancing the C 60 aggregate photocurrent with the metal phthalocyanine Q band photocurrent, the EQE is optimized at 25% donor content, which corresponds to the peak J sc from Fig.…”

“…Finally, we observe that at very high donor content device performance decreases due to reductions in J sc and FF, largely associated with the poor electron transport properties of the donor-rich BHJ layer (and in consideration of the associated charge recombination effects) [44,47]. It is worth noting that SubPc:C 60 OSCs are generally more sensitive to varying donor content (with poorer performance at >50% donor content), suggesting that there may be some slight differences in the charge transport properties in the two materials systems.…”

“…At lower donor content, the OSCs experience a reduction in J sc (due to the lower donor content, which is the primary light absorber) and a reduction in FF (essentially due to hole trapping and recombination effects) [47]. We also observe an increase in V oc with decreasing donor content due to Schottky band bending effects, as has been established previously [43,47,48]. At 50% donor content, where Schottky band bending effects are not notably present, the V oc of the SubPc:C 60 OSC is $970 mV and the V oc of the ClInPc:C 60 OSC is $820 mV.…”