2.1.

Semi-Transparent Organic Photovoltaic Cells with Near-Infrared Region Absorption Polymers

A straightforward strategy to fabricate semi-transparent OPVs is to use donor polymers, harvesting most of the photons in the NIR. In Ref. 79, the authors used the as absorber film a blend of PBDTT-DPP and PCBM. PBDTT-DPP is a low-band gap polymer with a strong photosensitivity in the 650 to 850 nm wavelength range, while the absorption of PCBM is located below 400 nm. With a combination of these two materials, the PBDTT-DPP: PCBM photoactive layer has an average transmission of 68% over the visible range (from 400 to 650 nm), but is strongly absorbing in the NIR range (from 650 to 850 nm). This spectral coverage of the PBDTT-DPP:PCBM film ensures harvesting of UV and NIR photons leading to PCEs above 4%. More recently, a PCPDTFBT polymer with a similar spectral response, i.e., a major absorption located at the near infrared region, was used to fabricate semi-transparent OPV cells with a PCE above 5%.⁹³ In the fabricated devices, $\mathrm{PCPDTFBT}:{\mathrm{PC}}_{71}\mathrm{BM}$ was used as active BHJ layer, with a configuration of $\mathrm{ITO}/\mathrm{ZnO}/\mathrm{PCPDTFBT}:{\mathrm{PC}}_{71}\mathrm{BM}/\mathrm{PEDOT}:\mathrm{PSS}/\text{ultra}\text{-}\text{thin}$ Ag. With a 15 nm Ag as the semi-transparent top electrode, the device exhibited an average transmission of 39.4% while for device with a thinner Ag layer of 10 nm the average transmission was increased to 47.3% without significantly compromising its PCE. This finding was associated with the good wettability of Ag atoms on the polar PEDOT:PSS layer, which allowed Ag atoms to grow homogeneously.

Although the results reported above indicate the soundness of the NIR absorption polymer approach to obtain high performance semi-transparent cells, there is a limit linked to the decrease in harvesting capacity when a device incorporates two semi-transparent electrodes. This is the case because the semi-transparency of the electrodes is usually homogenously distributed in the UV-visible-NIR range and the device loses its capacity to trap invisible UV or NIR light as well. To compensate for this effect, the authors of Ref. 84 considered a transparent OPV having a tandem structure using two different polymers with an absorption band in the NIR. The front subcell in the device incorporated the transparent absorber PBDTT-FDPP-C12:PC61BM which exhibits an average visible transmission from 400 to 650 nm of approximately 60% and an IR transmission of 52% from 650 to 800 nm. Therefore, approximately half of the IR energy was not fully captured for energy conversion. The back subcell featuring PBDTT-SeDPP:PC61BM as the absorber exhibited a similar NIR transmission of 53% with an extended NIR response from 650 to 900 nm. By stacking these two transparent absorbers in a tandem structure, NIR transmission dropped to 26%. In other words, the photon absorption efficiency in the NIR range increased nearly twofold and semi-transparent OPV cells exhibiting a PCE above 7% were reported. Recently, an efficiency of 8.02% in a tandem OPV cell with a semi-transparency of 44.90% was achieved using solution-processed graphene as the front electrode and laminated nanowires as the top electrode.⁷¹ In all such tandem devices, the NIR energy was more completely harnessed while approximately half of the visible photons were transmitted.

2.2.

Semi-Transparent Organic Photovoltaic Cells Incorporating One-Dimensional Photonic Crystals or Multilayers

A manipulation of the photon propagation inside the cell can be externally achieved with the use of one-dimensional (1-D) photonic crystals or dielectric multilayers. To enhance trapping of the electromagnetic field at the near UV and NIR wavelengths, one may consider 1-D photonic structures incorporated above the top semi-transparent electrode to reflect the NIR and UV and to transmit the visible spectrum. A combination of a Bragg reflector and an anti-reflective coating (ARC) has been used to increase NIR photon harvesting demonstrating that the efficiency of small molecule OPV cells could be increased from 1.3% to 1.7%.⁴² Similar configurations considered the use of a single Bragg mirror deposited on top of the back metal electrode to reflect the red and NIR wavelengths. This was shown to increase the short-circuit current density ($J_{\mathrm{sc}}$) of OPVs as the number of layers was increased from 2 to 8.⁸⁶

The 1-D photonic crystals or Bragg reflectors are designed to satisfy the Bragg condition to get maximum reflectivity at NIR wavelengths. However, in a photovoltaic device, interference must be optimal at all wavelengths of interest to achieve the highest visible transmission and optimal trapping for UV and IR light. One way to better reach the goal of a broadband photonic control using simple 1-D structures is to increase the degrees of freedom and use a numerical inverse problem solving method. For a semi-transparent OPV cell, there are essentially two parameters that will determine its level of performance: the efficiency in converting light to electricity and the device visible transmission or luminosity. Numerical inverse problem solving must be implemented by removing the periodicity constraint to design a photonic multilayer (cf. Fig. 1) that maximizes the contribution to the $J_{\mathrm{sc}}$ for wavelengths below the near UV and above the NIR while keeping the device’s visible transparency above the desired lower limit value.

Fig. 1

$\mathrm{PTB}7:{\mathrm{PC}}_7\mathrm{BM}$ cells incorporating (a) a periodic one-dimensional (1-D) photonic crystal of six layers, (b) a nonperiodic multilayer of five layers. ${\mathrm{MoO}}_3/{\mathrm{MgF}}_2$ have been used as high/low refractive index materials.

In an application of semi-transparent OPV cells of the inverse solving method, single junction cells using absorber layers of the $\mathrm{PTB}7:{\mathrm{PC}}_{71}\mathrm{BM}$ blend were considered.⁴¹ As shown in Fig. 2, the $J_{\mathrm{sc}}$ obtained following such a procedure increases rapidly when layers are added in the photonic crystal but saturates beyond five layers. For the five-layer structure, the calculated $J_{\mathrm{sc}}$ was 76.3% that of the corresponding opaque cell. On the contrary, for an optimal six-layer periodic structure, the best efficiency that can be reached is 72% that of the opaque cell. The better performance of the nonperiodic structure is attributed to reflectivity, as shown in Fig. 3, that adapts optimally not only to the absorption spectrum of the absorber blend but also to the sun photon flux. As seen in Fig. 3, the reflectivity of the nonperiodic structure is enhanced for the NIR photons at the expense of a reduction for the near UV photons when compared to the reflectivity of the six-layer periodic structure. This result is in correspondence to a larger photon flux in the NIR range relative to the UV. The reflectivity in the visible range is kept low in both cases, ensuring a visible device transparency or luminosity close to or above 30%.

Fig. 2

As a function of the number of layers numerically determined relative to the short-circuit current (green solid dots) and luminosity (green circles) for devices incorporating the nonperiodic multilayer, and the relative short-circuit current (red solid squares) and luminosity (red empty squares) for devices incorporating optimal periodic 1-D photonic crystals of 2 and 3 periods. The short-circuit currents are given relative to the corresponding one from an equivalent opaque cell.

Fig. 3

Reflectivity of the periodic 1-D photonic crystal of four layers (red dashed line), of six layers (red solid line) and a nonperiodic multilayer of five layers (green solid line). All three structures were designed to maximize the performance of the entire organic photovoltaic (OPV) device using the same inverse integration. The constraint of periodicity was removed for the last case.

This type of design has been tested and implemented in cells constructed either in a standard or inverted configuration using the $\mathrm{PTB}7:{\mathrm{PC}}_{71}\mathrm{BM}$ blend as the absorber layer. For the standard configuration, ITO and a 10-nm thick Ag layer were used as electrodes while PEDOT:PSS and thermally evaporated BCP were used as an electron blocking layer (EBL) and hole blocking layer (HBL), respectively. The multilayer structure implemented on top of the Ag electrode combined layers of a low refractive index material such as LiF with layers of a high refractive index material such as ${\mathrm{MoO}}_3$. As shown in Fig. 4(a) where the calculated external quantum efficiency (EQE) of the cell including the multilayer is compared to the EQE of a semi-transparent cell not including the multilayer, one observes that contributions to $J_{\mathrm{sc}}$ from the NIR as well as near UV photons are clearly enhanced. For certain NIR photons, the EQE for the device incorporating the multilayer is close to matching the EQE of an equivalent opaque cell.⁴¹ On the other hand, the contribution from visible photons to the EQE remains similar to the one seen for a bare semi-transparent cell for the same type of photons.

Fig. 4

(a) Experimentally measured external quantum efficiency (EQE) for semi-transparent cells in the standard configuration when no photonic management is incorporated (red solid line), and when a 1-D nonperiodic crystal of five layers is included (green solid line). Numerically predicted EQE for a semi-transparent standard cell incorporating a 1-D nonperiodic crystal of five layers designed ad hoc to optimize visible transparency and power conversion efficiency (PCE) (green dashed line). (b) Same as in (a) but for an inverted configuration.

For the inverted solar cells, the architecture considered was similar except that a thermally evaporated ${\mathrm{MoO}}_3$ layer was used as EBL while a ZnO layer was used as HBL. The ZnO layer was grown by sol-gel where the precursor solution was prepared according to Ref. 94. In order to make the inverted devices semi-transparent, similar to the standard configuration, the back silver contact was made 10 times thinner than that for the opaque cells, i.e., a 10 nm instead of 100 nm thickness. To enhance the performance of the semi-transparent cells, a five-layer structure based on ${\mathrm{MoO}}_3$ (high refractive index material) and ${\mathrm{MgF}}_2$ (low refractive index material) was incorporated. The EQE shown in Fig. 4(b) shows a similar redistribution of photon harvesting as the one found for the standard configuration devices. In both cases, as seen in Fig. 4, the agreement between the experimentally measured EQE and the numerical design is remarkable.

2.3.

Semi-Transparent Organic Photovoltaic Cells with Light Trapping Metal Cavities

Light trapping by using two metal electrodes has been considered in several OPV opaque cell configurations. Recently, OPV devices with an ITO-free microcavity structure that reached high PCEs of 8.5% on both glass and flexible plastic substrates have been reported.⁹¹ This corresponds to $\sim 20\%$ improvement in PCE when compared to the equivalent ITO-based devices. The significantly enhanced performance was ascribed to the substantially improved photon collection by the resonant microcavity structure, which contributed to an improved photocurrent compared with devices built on ITO-coated substrates.

Photon trapping in between the two metal electrodes can also be applied to semi-transparent OPV cells. In that event, both electrodes in the device are kept thin to ensure a sufficiently high luminosity. In a recent implementation of this configuration, to increase light trapping an ARC was deposited on top of the front metal contact while a nonperiodic multilayer was inserted in between the back metal contact and the substrate. As for the configuration considered in the previous section, the optimal layer distribution was specifically designed for the cell architecture used. With a device architecture such as the one shown schematically in Fig. 5, semi-transparent cells whose PCE was 5.3%, corresponding to 90% of the PCE of the corresponding opaque cell, were reported.⁹² The visible transparency of such cells differed little from the semi-transparent cell which did not include the multilayer, while the EQE closely matched that of the opaque cell as seen in Fig. 6. The opaque cell was in an inverted configuration with the following architecture: As the active material, a thin layer of $\mathrm{PTB}7:{\mathrm{PC}}_7\mathrm{BM}$ blend was used. The bottom electrode was an opaque layer of 120 nm of Au and the top electrode was a semi-transparent layer of 10 nm of Ag. ZnO and ${\mathrm{MoO}}_3$ were used as HBL and EBL, respectively. On top of the Ag electrode, a two-layer ARC made of ${\mathrm{MoO}}_3$ and LiF was deposited. For the semi-transparent devices, the same exact architecture was used except that the Au electrode was thinned down to 13 nm. As seen in Fig. 5, in between the Au electrode and the substrate, a six-layer 1-D multilayer made of alternating ${\mathrm{TiO}}_2$ and ${\mathrm{SiO}}_2$ was incorporated. Following an inverse integration procedure as discussed above, such a structure was numerically designed to maximize the current while keeping the luminosity of the solar cell above 20%.

Fig. 5

Schematic picture of a $\mathrm{PTB}7:{\mathrm{PC}}_{71}\mathrm{BM}$ cell in a metal cavity configuration incorporating a periodic 1-D photonic crystal of six layers and an anti-reflection coating.

Fig. 6

Experimentally measured EQEs of an opaque cell (black solid line), of a bare semi-transparent cell (red solid line) and a semi-transparent cell in a metal cavity configuration (green solid line), such as the one shown in Fig. 5, incorporating a periodic 1-D photonic crystal of six layers and an anti-reflection coating.

The conclusion was that when the OPV architecture included two thin metallic electrodes with one of them being assisted with a 1-D multilayer to enhance reflectivity for the case of semi-transparent cells, one may obtain a broadband photon trapping capacity sufficient to match the performance of semi-transparent cells to opaque ones. It was demonstrated that it is the combined effect of such a 1-D multilayer and a thin-metal layer that prevents, to a large extent, the loss in photon harvesting capacity exhibited by the majority of semi-transparent cells. Indeed, the $J_{\mathrm{sc}}$ for a cell device incorporating such a cavity configuration which exhibited a 21% luminosity amounted to 96.4% the $J_{\mathrm{sc}}$ of the corresponding opaque cell.