Session W33: Focus Session: Organic Electronics and Photonics - Organic Photovoltaics II - Efficiency, Stability, and Interfaces

Organic Solar Cell Efficiency Limitations and Pathways to Overcoming Them

Organic solar cell device efficiencies are often limited by a reduced external quantum efficiency, particularly for low band gap materials used in either single- or double-junction devices. This can be attributed to loss mechanisms occurring either at the device-physics scale, in the form of carrier recombination, or at the molecular donor-acceptor scale, in the form of incomplete photo-carrier generation and/or geminate recombination. Here mechanisms at both scales are addressed, utilizing drift-diffusion models of device operation and kinetic models of photo-carrier production. At the device-physics level, the negative impact of dark carriers, commonly derived from defect states in the organic semiconductor, is demonstrated. For dark carrier densities above a typical threshold of $10^{16}$ cm$^{-3}$, depletion at one of the electrodes leads to a field-free region of the device and substantial carrier recombination. At the molecular level, the fundamental impact of the molecular reorganization energy $\lambda$ on device efficiency is considered through use of a Marcus Theory-based kinetic model. It is shown that substantial gains in efficiency, to values approaching 20\%, are possible for hypothetical materials in which $\lambda$ has been reduced to approximately 0.3 eV. Finally, measurements of molecular alignment at interfaces are presented, and implications on the above two mechanisms are explored.   

Tailored exciton diffusion in organic photovoltaic cells for enhanced power conversion efficiency

Organic photovoltaic cells (OPVs) have the potential to become a low-cost source of renewable energy due to their compatibility with high throughput processing techniques and the demonstration of power conversion efficiencies exceeding 10{\%}. In the simplest planar heterojunction OPVs, photoconversion is limited by a short exciton diffusion length (L$_{\mathrm{D}})$ that restricts migration to the dissociating electron donor-acceptor (D-A) interface. Consequently, bulk heterojunctions are often used to realize high efficiency as these structures reduce the distance an exciton must travel to be dissociated. Here, we present an alternate approach that seeks to directly engineer L$_{\mathrm{D}}$ by optimizing the intermolecular separation and consequently, the photophysical parameters responsible for excitonic energy transfer. By diluting the electron donor boron subphthalocyanine chloride (SubPc) into a wide energy gap host material, we optimize the degree of interaction between donor molecules and observe a nearly 50{\%} increase in L$_{\mathrm{D}}$. Using this approach, we construct planar heterojunction OPVs with a power conversion efficiency of 4.4{\%}, \textgreater 30{\%} larger than the case of optimized devices containing an undiluted donor layer. It is worth noting that this efficiency also rivals those realized in optimized, bulk heterojunction OPVs based on SubPc and C$_{\mathrm{60}}$. The underlying correlation between L$_{\mathrm{D}}$ and the degree of molecular interaction has wide implications for the design of both OPV active materials and device architectures.   

\textgreater 1.0{\%} solar cell derived from carbon nanotube excitons

Semiconducting single-walled carbon nanotubes (s-SWCNTs) are promising photoabsorbers for photovoltaics due to their strong optical absorptivity, tunable NIR bandgaps, fast charge transport, and solution processability.~We have previously shown that electrons can be extracted from photogenerated excitons in s-SWCNTS by C$_{60}$ with internal quantum efficiency (QE) over 90{\%}. Here, we demonstrate s-SWCNT/C$_{60}$~heterojunction devices with over 1.0{\%} AM1.5G power conversion efficiency for the first time. We implemented highly monochiral (7,5) s-SWCNTs to optimize exciton diffusivity and tailored the device stack to tune the spectral response. External QE of over 35{\%} and 20{\%} are achieved at the~$E_{11}$~bandgap of the s-SWCNTs at 1055 nm and the~$E_{22}$~transition at 655 nm. More than 50{\%} of the AM1.5G photoresponse is derived from the s-SWCNTs, substantially exceeding previous s-SWCNT hybrid devices in which the photoresponse has mostly originated from the organic phase. This work will lead to solar cells based on s-SWCNT photoabsorbers with higher responsivity across the solar spectrum by tailoring the s-SWCNT film morphology and blending them directly with acceptors.   

Towards high performance inverted polymer solar cells

Bulk heterojunction polymer solar cells that can be fabricated by solution processing techniques are under intense investigation in both academic institutions and industrial companies because of their potential to enable mass production of flexible and cost-effective alternative to silicon-based electronics. Despite the envisioned advantages and recent technology advances, so far the performance of polymer solar cells is still inferior to inorganic counterparts in terms of the efficiency and stability. There are many factors limiting the performance of polymer solar cells. Among them, the optical and electronic properties of materials in the active layer, device architecture and elimination of PEDOT:PSS are the most determining factors in the overall performance of polymer solar cells. In this presentation, I will present how we approach high performance of polymer solar cells. For example, by developing novel materials, fabrication polymer photovoltaic cells with an inverted device structure and elimination of PEDOT:PSS, we were able to observe over 8.4{\%} power conversion efficiency from inverted polymer solar cells.   

The Science of Making Organic Solar Cells Stable

As organic PV efficiencies exceed 10{\%}, the science of stabilization and lifetime gains importance. We seek the origin of the exponential decrease, or ``burn-in,'' of OPV device efficiency in the first 200 hours of operation. First, we examine an efficient polymer, PCDTBT, and demonstrate a 6.2 year lifetime. For standard PCDTBT devices, burn-in is not caused by reactions at the transport layers; rather, it is caused by photochemical traps. We hypothesize that impurities could play a role. The effect of impurities is investigated in another polymer, PBDTTPD, with an 8.3{\%} PCE. For PBDTTPD we find that degradation correlates to the presence of small, organic impurities. We stabilize PBDTTPD, without diminishing performance, by purifying it further. We also investigate the fullerene's role in degradation using photobleaching experiments, and find that photoactive layer stability correlates with the fullerene's electron affinity. From our conclusions, we outline strategies for improving OPV device stability.   

Influence of MoO$_{\mathrm{x}}$ interlayer on the maximum achievable open-circuit voltage in organic photovoltaic cells

Transition metal oxides including molybdenum oxide (MoO$_{\mathrm{x}})$ are characterized by large work functions and deep energy levels relative to the organic semiconductors used in photovoltaic cells (OPVs). These materials have been used in OPVs as interlayers between the indium-tin-oxide anode and the active layers to increase the open-circuit voltage (V$_{\mathrm{OC}})$ and power conversion efficiency. We examine the role of MoO$_{\mathrm{x}}$ in determining the maximum achievable V$_{\mathrm{OC}}$ in planar heterojunction OPVs based on the donor-acceptor pairing of boron subphthalocyanine chloride (SubPc) and C$_{\mathrm{60}}$. While causing minor changes in V$_{\mathrm{OC}}$ at room temperature, the inclusion of MoO$_{\mathrm{x}}$ significantly changes the temperature dependence of V$_{\mathrm{OC}}$. Devices containing no interlayer show a maximum V$_{\mathrm{OC\thinspace }}$of 1.2 V, while devices containing MoO$_{\mathrm{x}}$ show no saturation in V$_{\mathrm{OC}}$, reaching a value of \textgreater 1.4 V at 110 K. We propose that the MoO$_{\mathrm{x}}$-SubPc interface forms a dissociating Schottky junction that provides an additional contribution to V$_{\mathrm{OC}}$ at low temperature. Separate measurements of photoluminescence confirm that excitons in SubPc can be quenched by MoO$_{\mathrm{x}}$. Charge transfer at this interface is by hole extraction from SubPc to MoO$_{\mathrm{x}}$, and this mechanism favors donors with a deep highest occupied molecular orbital (HOMO) energy level. Consistent with this expectation, the temperature dependence of V$_{\mathrm{OC}}$ for devices constructed using a donor with a shallower HOMO level, e.g. copper phthalocyanine, is independent of the presence of MoO$_{\mathrm{x}}$.   

The effect of interfaces on charge transport and recombination in polymeric solar cells

Charge-carrier transport and recombination in hybrid TiO2/P3HT:PCBM bulk-heterojunction solar cells (BHSCs) have been measured using photo-CELIV. We have fabricated hybrid devices in the form of indium tin oxide/titanium dioxide/P3HT:PCBM/Cu) to clarify the impact of the TiO$_{2}$/P3HT:PCBM interface on the charge transport using the charge extraction by linearly increasing voltage (CELIV) technique. We found that a large equilibrium charge reservoir is accumulated at negative offsets at the TiO$_{2}$/P3HT:PCBM interface leading to space charge limited extraction current (SCLC) transients. We show analytically the SCLC transient response and compare the experimental data to calculated SCLC in a linearly increasing voltage. The theoretical calculations indicate that the large charge reservoir at negative offset voltages is due to thermally generated charges combined with poor hole extraction at the ITO/TiO$_{2}$ contact, due to the hole blocking character of TiO2. In this presentation we will discuss how interfaces, both metal-organic but also organic-organic interfaces affect charge carrier transport and recombination measurements.   

Effect of interfacial modification of organophosphonate-based self-assembled monolayers on the performance of inverted hybrid ZnO:P3HT photovoltaic devices

Hybrid organic-inorganic photovoltaics have not lived up to their promise because of our poor handle of the exciton dissociation interface. Interfacial modification based on self-assembled monolayer (SAM) adsorption provides a way of improving device performance. Here, we provide the first examples of a stepwise functionalization methodology that allows binding of phosphonic acid derivatives to ZnO nanowire arrays with minimal surface degradation and etching. We examined different adsorption methods; SAM adsorption via tethering-by-aggregation-and-growth (T-BAG) yields the most robust surface-bound monolayers. Poly(3-hexylthiophene), P3HT, infiltrated in surface modified ZnO nanowire arrays yielded functional hybrid solar cells with power conversion efficiencies as high as 2.1{\%} due to improvements in both the short-circuit current density (Jsc) and the open-circuit voltage (Voc). The increase in Jsc can be attributed to enhanced charge transfer with surface passivation of ZnO, while the increase in Voc is attributed to the interfacial dipole introduced and improved P3HT wettability on ZnO with SAM adsorption.   

Anomalous charge storage exponents of organic bulk heterojunction solar cells.

Organic bulk heterojunction (BHJ) devices are increasingly being researched for low cost solar energy conversion. The efficiency of such solar cells is dictated by various recombination processes involved. While it is well known that the ideality factor and hence the charge storage exponents of conventional PN junction diodes are influenced by the recombination processes, the same aspects are not so well understood for organic solar cells. While dark currents of such devices typically show an ideality factor of 1 (after correcting for shunt resistance effects, if any), surprisingly, a wide range of charge storage exponents for such devices are reported in literature alluding to apparent concentration dependence for bi-molecular recombination rates. In this manuscript we critically analyze the role of bi-molecular recombination processes on charge storage exponents of organic solar cells. Our results indicate that the charge storage exponents are fundamentally influenced by the electrostatics and recombination processes and can be correlated to the dark current ideality factors. We believe that our findings are novel, and advance the state-of the art understanding on various recombination processes that dictate the performance limits of organic solar cells.   

Physical Processes in Organic Photovoltaic Devices Tuned by Ionic Double Layer Doping

We have recently found that Organic Photovoltaic (OPV) performance can be improved by creating p-i-n structures via doping by double layer charging. We have designed a hybrid device; an OPV attached to a supercapacitor via a common transparent carbon nanotube (CNT) electrode. We've demonstrated that photoexcitation of this hybrid results in double layer capacitive doping of the upper organic layers in the OPV and the CNT electrode. This device can also be viewed as an electrochemically gated CNT/OPV which is ionically reconfigurable either upon photoexcitation or upon application of a voltage bias to the gate electrode. We have demonstrated a two fold increase in the short circuit current and filling factor of our initial test device; an inverted P3HT:PCBM bulk heterojunction cell attached to an electrochemical microcell with a CNT anode laminated on top of the OPV functioning as a common anode. The physical processes in this ionically tuned OPV are discussed in terms of better ohmic contact with CNT electrode and formation of p-i junction in P3HT chains which contribute to better separation of photogenerated carriers and their improved collection. Optical studies of the bleaching effects in both in CNT and in P3HT independently confirm the DLC ionic doping.   

Correlation between magneto-photocurrent and power conversion efficiency in organic solar cells

In order to investigate the effect of spin 1/2 radical on the photocurrent in organic solar cells, we studied magneto-photocurrent (MPC) and power conversion efficiency (PCE) of ``standard'' P3HT:PCBM (1.2:1) device at various Galvinoxyl radical wt{\%}. The MPC measurements were performed to understand the increase in $J$sc and hence PCE of the OPV device with Galvinoxyl wt{\%}. We found that the MPC reduction with Galvinoxyl wt{\%} follows the same trend as that of the PCE enhancement. We propose that MPC in OPV blends is due to spin-mixing mechanism related with the manifold of the charge transfer (CT) state at the donor-acceptor interfaces. Our results thus demonstrate that the Galvinoxyl spin 1/2 radical additives act as spin flip initiator within this exciton manifold. This process is unraveled via MPC of the doped devices. Supported by the NSF-MRSEC program at the UoU.