Probing the Energy Level Alignment and the Correlation with Open-Circuit Voltage in Solution-Processed Polymeric Bulk Heterojunction Photovoltaic Devices

Abstract

Energy level alignment at the organic donor and acceptor interface is a key to determine the photovoltaic performance in organic solar cells, but direct probing of such energy alignment is still challenging especially for solution-processed bulk heterojunction (BHJ) thin films. Here we report a systematic investigation on probing the energy level alignment with different approaches in five commonly used polymer:[6,6]-phenyl-C71-butyric acid methyl ester (PCBM) BHJ systems. We find that by tuning the weight ratio of polymer to PCBM the electronic features from both polymer and PCBM can be obtained by photoemission spectroscopy. Using this approach, we find that some of the BHJ blends simply follow vacuum level alignment, but others show strong energy level shifting as a result of Fermi level pinning. Independently, by measuring the temperature-dependent open-circuit voltage (VOC), we find that the effective energy gap (Eeff), the energy difference between the highest occupied molecular orbital of the polymer donor (EHOMO-D) and lowest unoccupied molecular orbital of the PCBM acceptor (ELUMO-A), obtained by photoemission spectroscopy in all polymer:PCBM blends has an excellent agreement with the extrapolated VOC at 0 K. Consequently, the photovoltage loss of various organic BHJ photovoltaic devices at room temperature is in a range of 0.3-0.6 V. It is believed that the demonstrated direct measurement approach of the energy level alignment in solution-processed organic BHJ will bring deeper insight into the origin of the VOC and the corresponding photovoltage loss mechanism in organic photovoltaic cells.

Study of the Contributions of Donor and Acceptor Photoexcitations to Open Circuit Voltage in Bulk Heterojunction Organic Solar Cells

“…The photoexcitation of the donor and acceptor individually produces comparable V OC values from Equations (10) and (15), as shown in Figure 3. Moreover, according to Figure 3, the linear correlation between V OC and E DA /q agrees well with previous studies [20,30,32,33,64]. The third open circuit voltage V DA OC calculated in Equation (18) from the simultaneous excitations of both the donor and acceptor, as given in Table 4, is also comparable with the other two V OC s. All three V OC s thus derived produce comparable results and agree reasonably well with the experimental values (see Tables 2-4).…”

“…This implies that both the donor and acceptor photoexcitations can contribute to the achievable open circuit voltage as well as the photovoltaic performance of BHJ OSCs; this agrees with our earlier results [45] [68], 0.34-0.44 V for a range of donor-acceptor blends [69], 0.38 V for OC 1 C 10 -PPV:PCBM solar cells [12], and 0.30-0.60 V for BHJ OSCs [20]. Other low voltage offsets ( E DA/q − V j OC ) that have been reported recently are 0.25 V for an evaporated bilayer OSC [70], 0.23 V and 0.26 V for diketopyrrolopyrrole-thieno[2,3-f]benzofuran (DTD):PC 60 BM and DTD:naphthalene diimide acceptor-polymer (N2200) systems, respectively [71], and 0.34-0.40 V for BHJ OSCs [72], with which our calculated values also somewhat agree in the range.…”

“…The V OC in Equation (20) depends linearly on ln I 0 with a slope of k B T q , which agrees perfectly well with the plotted V OC as a function of ln I 0 in [17,24].…”

“…The validity of Equation (1) has been established for a number of donor-acceptor (fullerene) blends with a voltage loss ∆E loss ranging from 0.3−0.6 eV [19,20]. The question that may be asked is: what are the possible causes of the voltage loss ∆E loss and how can it be reduced?…”

Relating Frontier Orbital Energies from Voltammetry and Photoelectron Spectroscopy to the Open‐Circuit Voltage of Organic Solar Cells

“…Note that Equation (1) is similar to relations where an effective band gap g eff E ( ) was used instead of E CT , defined as the difference between the electron affinity of the acceptor and the ionization potential of the donor. [5][6][7] The second term on the right-hand side of Equation (1) has initially been found to be approximately constant (≈0.6 eV) for many different donor and acceptor blends when the corresponding cells are measured under AM1.5G (100 mW cm −2 ) conditions at room temperature, such that qV oc ∝ E CT . [5][6][7] The second term on the right-hand side of Equation (1) has initially been found to be approximately constant (≈0.6 eV) for many different donor and acceptor blends when the corresponding cells are measured under AM1.5G (100 mW cm −2 ) conditions at room temperature, such that qV oc ∝ E CT .…”