Thermodynamic Efficiency Limit of Molecular Donor‐Acceptor Solar Cells and its Application to Diindenoperylene/C₆₀‐Based Planar Heterojunction Devices

Abstract

In organic photovoltaic (PV) cells, the well‐established donor‐acceptor (D/A) concept enabling photo‐induced charge transfer between two partners with suitable energy level alignment has proven extremely successful. Nevertheless, the introduction of such a heterojunction is accompanied with additional energy losses as compared to an inorganic homojunction cell, owing to the presence of a charge‐transfer (CT) state at the D/A interface. Based on the principle of detailed balance, a modified Shockley‐Queisser theory is developed including the essential effects of interfacial CT states, that allows for a quantitative assessment of the thermodynamic efficiency limits of molecular D/A solar cells. Key parameters, apart from the optical gap of the absorber material, entering the model are the energy (ECT) and relative absorption strength (αCT) of the CT state. It is demonstrated how the open‐circuit voltage (VOC) and thus the power conversion efficiency are affected by different parameter values. Furthermore, it is shown that temperature dependent device characteristics can serve to determine the CT energy, and thus the upper limit of VOC for a given D/A combination, as well as to quantify non‐radiative recombination losses. The model is applied to diindenoperylene (DIP)‐based photovoltaic devices, with open‐circuit voltages between 0.9 and 1.4 V, depending on the partner, that have recently been reported.

Role of Polymer Fractionation in Energetic Losses and Charge Carrier Lifetimes of Polymer: Fullerene Solar Cells

“…[5][6][7][8][9][10] In a practical solar cell both V oc and the LED quantum efficiency, Q LED , are reduced relative to their thermodynamic limits by non-radiative recombination pathways, the degree of which can be quantified via a reciprocity theorem. [11][12][13][14][15][16][17] Therefore, it is of high interest in the field of organic photovoltaics to reduce these non-radiative recombination losses, thereby bringing the V oc closer to the radiative limit. Recently, indacenodithiophene (IDT) copolymers have shown promising field-effect transistor mobilities and solar cell performance.…”

Multiple state representation scheme for organic bulk heterojunction solar cells: A novel analysis perspective

“…[7,10] However, because of both the low carrier mobility and the interpenetrated nature of typical BHJ blends, there is a non-negligible probability that dissociated free carriers recombine again at the large D-A interface (nongeminate recombination) before being collected at the electrodes. [7,11] These nonradiative recombinations can be a major loss mechanism p-1 that strongly reduces the power conversion efficiency in BHJ solar cells, [12][13][14][15][16] and are mainly influenced by the energy difference between the highest occupied molecular level ε D1 of D (called HOMO of D) and the lowest unoccupied level ε A2 of A (called LUMO of A). Since it was experimentally found for several donor-acceptor material combinations [5,[17][18][19] that the open circuit voltage U oc is proportional to this effective energy gap, the nonradiative recombination losses can be traced back to a drop of U oc .…”

Energy Losses in Small‐Molecule Organic Photovoltaics

“…Reproduced with permission. [13] Copyright 2012, Wiley-VCH. be used to calculate an upper limit for the PCE of an excitonic solar cell.…”

“…be used to calculate an upper limit for the PCE of an excitonic solar cell. [12,13] Therein, the single step profile with an onset at the energy gap E g (which has to be identified with the optical gap of the organic semiconductor having the smaller gap) and unity absorbance α g 1 has to be extended by a second step corresponding to the CT state, which is characterized by two parameters E CT and α CT . E CT is lower than E g by the driving force ΔE CT and the absorption strength α CT typically is of the order of 10 −3 times the fundamental absorption across the optical gap.…”