High photon energy losses limit the open‐circuit voltage (VOC) and power conversion efficiency of organic solar cells (OSCs). In this work, an optimization route is presented which increases the VOC by reducing the interfacial area between donor (D) and acceptor (A). This optimization route concerns a cascade device architecture in which the introduction of discontinuous interlayers between alpha‐sexithiophene (α‐6T) (D) and chloroboron subnaphthalocyanine (SubNc) (A) increases the VOC of an α‐6T/SubNc/SubPc fullerene‐free cascade OSC from 0.98 V to 1.16 V. This increase of 0.18 V is attributed solely to the suppression of nonradiative recombination at the D–A interface. By accurately measuring the optical gap (Eopt) and the energy of the charge‐transfer state (ECT) of the studied OSC, a detailed analysis of the overall voltage losses is performed. Eopt – qVOC losses of 0.58 eV, which are among the lowest observed for OSCs, are obtained. Most importantly, for the VOC‐optimized devices, the low‐energy (700 nm) external quantum efficiency (EQE) peak remains high at 79%, despite a minimal driving force for charge separation of less than 10 meV. This work shows that low‐voltage losses can be combined with a high EQE in organic photovoltaic devices.

We present efficient, semitransparent small molecule organic solar cells. The devices employ an indium tin oxide-free top contact, consisting of thin metal films and an additional organic capping layer for enhanced light in/outcoupling. The solar cell encorporates a bulk heterojunction with the donor material Ph2-benz-bodipy, an infrared absorber. Combination of Ph2-benz-bodipy with C60 as acceptor leads to devices with high open circuit voltages of up to 0.81 V and short circuit current densities of 5-6 mA/cm2, resulting in efficiences of 2.2%-2.5%. At the same time, the devices are highly transparent, with an average transmittance in the visible range (400-750 nm) of up to 47.9%, with peaks at 538 nm of up to 64.2% and an average transmittance in the yellow-green range of up to 61.8%.

To further improve the power conversion effi ciencies (PCEs) of OSCs, it is necessary to understand the complicated photon-to-electron conversion processes in detail. One important challenge is to understand the relationship between controlled processing of high-performance OSCs, the resulting nanoscopic morphology of the absorber layer, and subsequent PCE of the completed device. This multiscale problem can be only tackled by thoroughly characterizing the different length scales, from the sub-nanometer to the centimeter scale, and complementary simulations to obtain comprehensive insight into the microsopic processes involved in every step. [5][6][7] At the heart of a bulk heterojunction (BHJ) OSC is a blend of two or more organic materials forming the absorber layer of the solar cell. [ 5,6 ] The advantage of this blend architecture over a bilayer structure lies in the increased interfacial area between donor and acceptor materials, which increases the dissociation probability of the strongly bound photogenerated exciton (Frenkel exciton). Once separated into free charge carriers at the material interface, the morphology of the blend layer must provide closed transport paths to the adjacent layers for charge extraction. Thus, the solar cell efficiency is highly sensitive to the size and connectivity of a phase to its adjacent layer, as well as the material purity of each domain. [ 7,8 ] Overall, the constraints for an optimal morphology are manifold, requiring a suffi ciently fi ne-grained morphology for effi cient exciton dissociation, but still suffi ciently coarse for an unhampered charge carrier transport. Hence, the BHJ of an effi cient OSC requires a morphology balancing these competing demands. A powerful method to adjust the BHJ morphology in vacuum processed small molecule based OSCs is to control the substrate temperature ( T sub ) during deposition of the absorber layer. [9][10][11][12] In this contribution, we study the relationship between T sub , nanoscopic morphology of the BHJ and PCE of the fi nal OSC, with both experimental and theoretical methods.We fabricated and characterized BHJ OSCs, where the intrinsic absorber layer is sandwiched between n-doped and p-doped layers (nip OSC) to ensure an unhampered charge transport to the respective contacts. [ 13 ] The intrinsic BHJ absorber layer of such nip OSCs is composed of the dicyanovinyl-substituted oligothiophene derivative DCV5T-MeThe nanoscale morphology of the bulk heterojunction absorber layer in an organic solar cell (OSC) is of key importance for its effi ciency. The morphology of high performance vacuum-processed, small molecule OSCs based on oligothiophene derivatives (DCV5T-Me) blended with C 60 on various length scales is studied. The analytical electron microscopic techniques such as scanning transmission electron microscopy, energy dispersive X-ray spectroscopy, highly sensitive external quantum effi ciency measurements, and meso and nanoscale simulations are employed. Unique insights into the relation between processing, morphology...

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