Influence of the indium tin oxide/organic interface on open-circuit voltage, recombination, and cell degradation in organic small-molecule solar cells

Abstract

In this paper we investigate the performance and stability of small-molecule organic solar cells with respect to the indium tin oxide (ITO)/organic interface. Different zinc-phthalocyanine (ZnPc)/fullerene (C 60 ) cell architectures with and without ITO O 2 -plasma treatment are compared and tested with respect to their degradation behavior under illumination in inert atmosphere. Photoelectron spectroscopy (UPS and XPS) shows that the O 2 -plasma treatment increases the ITO work function from 4.3 eV up to 5.6 eV. We find that both the increased ITO work function as well as the introduction of an electron blocking layer between ITO and the mixed donor/acceptor layer increases the open-circuit voltage V oc by more than 200 mV. For both cases our continuum approach device simulation quantitatively relates the increase of V oc to a reduced contact recombination and thus a reduced dark current. For cells built on ozone treated ITO we find a fast cell degradation caused by the UV part of the AM 1.5 spectrum. We identify the degradation, which manifests itself in a decrease of V oc of up to 25%, as a partial reversion of the plasma induced ITO work function increase. Additionally, we demonstrate that the degradation can be reduced by structural changes in the cell architecture, leading to improved cell stability. We present a comprehensive study of the recombination at the ITO/organic interface and its influence on the open-circuit voltage and the cell stability.

Meaning of reaction orders in polymer:fullerene solar cells

“…[56][57][58] In the context of organic solar cells, surface recombination is rarely discussed in much detail, and there are only few theoretical studies on its effects. 9,33,[59][60][61] However, there are extensive studies on cathode interlayers, [62][63][64] electrode work functions, [65][66][67][68] and hole transporting layers [69][70][71] and their effect on the device performance, including the open-circuit voltage. All of these studies implicitly support the theory that, at least in some organic solar cells, recombination at the contact affects the open-circuit voltage.…”

“…Due to the low dielectric constants, and in consequence due to the high exciton binding energies in molecular semiconductors, the situation is less clear in organic solar cells where geminate recombination of excitons or bound charge carriers can be a substantial loss mechanism. 1,2 However, recent studies [3][4][5][6][7][8][9][10][11][12][13] suggest that nongeminate recombination is limiting the efficiency of most well-performing bulk heterojunction solar cells. Experimentally, nongeminate recombination is often studied at the open circuit where, due to the lack of an external current flow, effects of series resistances and charge carrier transport are minimized.…”

Field-dependent exciton dissociation in organic heterojunction solar cells

“…This combination is typical for state-of-the-art organic BHJ solar cells which are not limited by the geminate recombination of CT-states, 63,64 but mainly by nongeminate recombination of free charge carriers. [23][24][25][26] Another proposed mechanism which could explain a weak temperature dependence was a dissociation process involving disorder-assisted tunneling jumps. 65 In contrast to this theory, a weak temperature dependence was found for both strongly and weakly disordered materials.…”

“…Recently, it was demonstrated that such nongeminate recombination losses, which occurred after exciton and CT-state dissociation, could explain the voltage dependence of the photocurrent of several state-of-the-art BHJ solar cells. [23][24][25][26][27] In such a case, the additional application of a field-dependent CT-state dissociation process has to result in overparametrization and ambiguous results.…”

On the stability of a variety of organic photovoltaic devices by IPCE and in situ IPCE analyses – the ISOS-3 inter-laboratory collaboration

“…The IPCE spectra of the IAPP device consist of five clearly distinguished peaks, three for zinc phthalocyanine (ZnPc) and two for the C 60 molecule. 3 The five peaks in the IPCE spectra of the IAPP sample correspond to the ZnPc at 340 nm, 630-640 nm and 680-700 nm, [11][12][13][14] and to the C 60 molecule at 380 nm and 460 nm. 19,20 The ZnPc peaks are in good correspondence with the absorption bands observed in the UV-Vis absorption spectra, called the B (or Soret) band around 300-400 nm in the UV region, and the Q band in the visible region between 400 and 800 nm.…”