Organic solar cells lag behind their inorganic counterparts in efficiency due largely to low open‐circuit voltages (Voc). In this work, a comprehensive framework for understanding and improving the open‐circuit voltage of organic solar cells is developed based on equilibrium between charge transfer (CT) states and free carriers. It is first shown that the ubiquitous reduced Langevin recombination observed in organic solar cells implies equilibrium and then statistical mechanics is used to calculate the CT state population density at each voltage. This general result permits the quantitative assignment of Voc losses to a combination of interfacial energetic disorder, non‐negligible CT state binding energies, large degrees of mixing, and sub‐ns recombination at the donor/acceptor interface. To quantify the impact of energetic disorder, a new temperature‐dependent CT state absorption measurement is developed. By analyzing how the apparent CT energy varies with temperature, the interfacial disorder can be directly extracted. 63–104 meV of disorder is found in five systems, contributing 75–210 mV of Voc loss. This work provides an intuitive explanation for why qVoc is almost always 500–700 meV below the energy of the CT state and shows how the voltage can be improved.

Charge generation in champion organic solar cells is highly efficient in spite of low bulk charge-carrier mobilities and short geminate-pair lifetimes. In this work, kinetic Monte Carlo simulations are used to understand efficient charge generation in terms of experimentally measured high local charge-carrier mobilities and energy cascades due to molecular mixing.

To increase the efficiency of bulk heterojunction (BHJ) solar cells beyond 15%, 300 nm thick devices with 0.8 fill factor (FF) and external quantum efficiency (EQE) >90% are likely needed. This work demonstrates that numerical device simulators are a powerful tool for investigating charge‐carrier transport in BHJ devices and are useful for rapidly determining what semiconductor pro­perties are needed to reach these performance milestones. The electron and hole mobility in a BHJ must be ≈10−2 cm2 V−1 s−1 in order to attain a 0.8 FF in a 300 nm thick device with the recombination rate constant of poly(3‐hexyl­thiophene):[6,6]‐phenyl‐C61‐butyric acid methyl ester (P3HT:PCBM). Thus, the hole mobility of donor polymers needs to increase from ≈10−4 to ≈10−2 cm2 V−1 s−1 in order to significantly improve device performance. Furthermore, the charge‐carrier mobility required for high FF is directly proportional to the BHJ recombination rate constant, which demonstrates that decreasing the recombination rate constant could dramatically improve the efficiency of optically thick devices. These findings suggest that researchers should prioritize improving charge‐carrier mobility when synthesizing new materials for BHJ solar cells and highlight that they should aim to understand what factors affect the recombination rate constant in these devices.

Theoretical and experimental studies suggest that energetic offsets between the charge transport energy levels in different morphological phases of polymer:fullerene bulk heterojunctions may improve charge separation and reduce recombination in polymer solar cells (PSCs). In this work, we use cyclic voltammetry, UV-vis absorption, and ultraviolet photoelectron spectroscopy to characterize hole energy levels in the polymer phases of polymer:fullerene bulk heterojunctions. We observe an energetic offset of up to 150 meV between amorphous and crystalline polymer due to bandgap widening associated primarily with changes in polymer conjugation length. We also observe an energetic offset of up to 350 meV associated with polymer:fullerene intermolecular interactions. The first effect has been widely observed, but the second effect is not always considered despite being larger in magnitude for some systems. These energy level shifts may play a major role in PSC performance and must be thoroughly characterized for a complete understanding of PSC function.

In order to commercialize polymer solar cells, the fast initial performance losses present in many high efficiency materials will have to be managed. This burn-in degradation is caused by light-induced traps and its characteristics depend on which polymer is used. We show that

different effect on device performance depending on the morphology of the active layer. We investigate this infl uence of morphology in the context of two fundamental mechanisms that are able to cause a V oc loss. First, an increased trap-assisted recombination rate that lowers the charge carrier density [ 2,3 ] and second a broadening of the density of states that reduces V oc [ 4,5 ] with the same charge carrier density and unchanged recombination dynamics. Answering this question is important not only for making more stable solar cells, but also for understanding in general how traps reduce the open-circuit voltage in organic solar cells. Aging is a convenient way to add traps into an organic solar cell and see their effect on recombination and V oc without introducing additional materials into the active layer.We illuminate BHJ solar cells of various materials in inert conditions for three days. Using transient photovoltage (TPV), [ 6 ] charge extraction (CE) [ 7 ] and temperature dependent measurements of the opencircuit voltage, we compare the recombination dynamics and charge carrier densities of fresh and aged solar cells and relate them to the observed V oc losses. We find that in degraded solar cells of amorphous materials such as poly[9′hepta-decanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′benzothiadiazole)] (PCDTBT), the recombination dynamics are basically unchanged. Instead, the observed open-circuit voltage losses appear to result from increased energetic disorder that causes a change in the relevant low-energy portion of the density of states such that for the same charge carrier density a lower quasi Fermi level splitting is obtained. We observe that solar cells made from crystalline materials do not show V oc losses and have a higher charge carrier density than solar cells made from amorphous materials. Apparently crystalline materials are not affected by a moderate increase of energetic disorder, most likely due to a higher charge carrier density.Besides reduced open-circuit voltages, also losses of fi ll factor [ 1,8 ] and short-circuit current [ 9,10 ] are observed upon burnin. Often a convolution of degradation mechanisms at material interfaces and within the bulk of the absorbing layer is present. Recent work suggests that a decrease in fi ll factor during degradation is related to interface defects [ 1,8 ] which are most likely related to injection or extraction barriers at the electrodes. [11][12][13] The photoinduced open-circuit voltage ( V oc ) loss commonly observed in bulk heterojunction organic solar cells made from amorphous polymers is investigated. It is observed that the total charge carrier density and, importantly, the recombination dynamics are unchanged by photoinduced burn-in. Charge extraction is used to monitor changes in the density of states (DOS) during degradation of the solar cells, and a broadening over time is observed. It is proposed that the V oc losses observed during burn-in are caused by a redistribution of charge carriers in a broader DOS. The tempe...

We present the effect of donor-acceptor phase separation, controlled by the donor-acceptor mixing ratio, on the charge generation and recombination dynamics in pBTTT-C14:PC 70 BM bulk heterojunction photovoltaic blends. Transient absorption (TA) spectroscopy spanning the dynamic range from pico-to microseconds in the visible and near-infrared spectral regions reveals that in a 1:1 blend exciton dissociation is ultrafast, however, charges cannot entirely escape their mutual Coulomb attraction and thus predominantly recombine geminately on a sub-ns timescale. In contrast, a polymer:fullerene mixing ratio of 1:4 facilitates the formation a Supporting Information ((bold)) is available online from the Wiley Online Library or from the author.-2 -of spatially-separated, that is free, charges and reduces substantially the fraction of geminate charge recombination, in turn leading to much more efficient photovoltaic devices. This illustrates that spatially-extended donor or acceptor domains are required for the separation of charges on an ultrafast timescale (<100 fs), indicating that they are not only important for efficient charge transport and extraction, but also critically influence the initial stages of free charge carrier formation.

Energy levels in the mixed polymer:fullerene phase of three-phase bulk heterojunction solar cells are significantly shifted from their values in the pure materials. These shifts provide an important driving force for separating charge carriers. Through cyclic voltammetry, we measure a gradual shift in the polymer HOMO and fullerene LUMO as a function of blend composition in a model bulk heterojunction (BHJ) system that only contains amorphous polymer. The effective band gap of the polymer:fullerene blend varies by up to 300 meV with varying blend composition. The shifts in polymer HOMO and fullerene LUMO can be quantitatively accounted for by the electrostatic potential generated by induced dipoles at the polymer:fullerene interfaces. Remarkably, however, the measured charge transfer state energy and open-circuit voltage shift far less.