Tail state limited photocurrent collection of thick photoactive layers in organic solar cells

One of the key challenges for organic photovoltaics (OPV) is to achieve high device efficiencies with photoactive layers thick enough for scalable printing methodologies, which typically require thicknesses of  300 nm. For laboratory scale spin coated devices, however, the optimum device is typically limited by a photoactive layer thickness of 80 – 100 nm. The reasons of the limited effective thickness remains debated, and the choice of material combinations that can enable efficient thick devices is still very limited. So far, most research on this issue has focused on the drop off of device fill factor FF with thicker photoactive layers, which has mainly been attributed to bimolecular recombination losses during transport. Drop off of device short circuit currents J_sc with increasing thickness has been less studied, and normally considered to result from the same recombination / transport limitation limiting FF.  Herein we focus upon the thickness dependence of J_sc for four representative blends exhibiting very different thickness dependencies. We analyses several possible causes of the drop of J_sc with thickness, and conclude that the key determinant is the density of intraband tail states in the photoactive blend, a factor not previously considered to be important in limiting device thickness. We find a correlation between the tail state density and J_sc vs thickness, and provide both experimental and modelling analyses which indicate that such behaviour can result from space charge layer effects generated in the presence of tail states.

Our study employs four series of organic donor:acceptor bulk heterojunction solar cells with which exhibit maximal photoactive layer thicknesses for efficient photocurrent generation and device performance ranging from 80 to 530 nm. These blends and devices are analyzed by as correct photocurrent data, transient photovoltage and charge extraction of carrier kinetics and densities, ambient photoemission spectroscopies of material energetics, kelvin probe measurements of work function, space charge limited current measurements of material doping, Mott-Schottky analyses of apparent doping density and by device simulations. Two methodologies are employed to quantify bimolecular recombination losses at short circuit, both methodologies conclude that these losses are too small to explain the loss of J_sc for thicker devices. Two further methodologies are employed to estimate the space charge layer depth for the devices studied, from which we conclude that the drop off of J_sc­ with photoactive layer thickness correlates with space charge layer width, with efficient photocurrent generation requiring the material space charge layer width to be greater than the photoactive layer thickness. Analyses of the dark doping density of the absorber layer by work function and SCLC measurements indicate this space charge layer does not result from unintentional material. doping. Instead, we observe that the formation of this space charge layer is correlated with the presence of tail (shallow trap) states in the absorber layer. Device simulation studies quantitatively support this conclusion, indicating that the presence of tail states can, under irradiation, generate a space charge layer which screens the built field in the device for thick absorber layers.  The results and conclusions reported herein helps explain the widely reported observation that many bulk heterojunctions which perform well when spin coated as thin absorber layers, do not perform well when printed with thicker absorber layers. We further demonstrate a simple strategy to address this issue, with a simple polymer purification to remove low molecular weight fractions shown to reduce tail state density and enable the fabrication of efficient devices with thicker photoactive layer thicknesses.