GB/Z 46984.3-2026Photovoltaic cells - Part 3: Measurement of current-voltage characteristics of bifacial photovoltaic cells (English PDF)

3.1 Bifacial standard test condition; BSTC

A test condition that simultaneously meets the spectral distribution of an atmospheric quality of AM1.5, a cell temperature of 25 °C, a front irradiance of 1,000 W - m^-², and a rear irradiance of 135 W - m^-².

3.2 Low reflection background

A low reflection background refers to the use of anti-reflective materials or/and design to reduce the influence of ambient light or light reflected from the test environment on the irradiance of the cell under test.

NOTE: for bifacial cells under standard test condition (STC), with one side exposed to light and the unexposed side is either shaded or unshaded, if the difference in short-circuit current is less than 0.3%, then, the background is considered low reflection background.

3.3 Bifaciality

The ratio of the I-V characteristics of the rear to the front of the bifacial cells.

NOTE 1: bifaciality is calculated by default using the I-V characteristics under rear STC and front STC.

NOTE 2: the main bifaciality factors include short-circuit current bifaciality phiI, open-circuit voltage bifaciality phiV, and power bifaciality phiP.

NOTE 3: the bifaciality definition is consistent with IEC TS 60904-1-2.

NOTE 4: the subscripts "f" and "r" respectively represent the front and rear tests.

3.4 Bifacial reference cell

A reference cell with a bifacial structure similar to the cell under test.

NOTE: the electrical performance of bifacial reference cells is usually certified and calibrated by a third-party laboratory to calibrate the light intensity of the solar simulator. The specific requirements for bifacial reference cells depend on the test method used for the cell under test.

3.5 Bifacial I-V characteristics

The I-V characteristics of a bifacial cell under the bifacial standard test condition (BSTC), including the contributions of front and rear irradiation.

NOTE 1: the main bifacial I-V characteristics include bifacial short-circuit current IscBiFi, bifacial open-circuit voltage VocBiFi, and bifacial maximum power PmaxBiFi.

NOTE 2: bifacial I-V characteristics are obtained by direct test or equivalent calculation and can be used to evaluate the power generation capacity of bifacial cells under bifacial illumination.

3.6 Bifacial power gain coefficient

BiFirel The gain of maximum power PmaxBiFi under BSTC relative to maximum power Pmax under front STC. It is defined as. BiFirel = ( -1) x 100%.

4.1 Overview

In most cases, bifacial photovoltaic (PV) modules operate outdoors under bifacial irradiation conditions. Compared to mono-facial photovoltaic modules, the additional power gain of bifacial photovoltaic modules comes from the rear irradiation.

To effectively evaluate the actual power generation capacity of bifacial cells, the bifacial I-V characteristics are recommended to be tested in accordance with the requirements of 3.11 in IEC 61215-1:2021, under bifacial nameplate irradiance (BNPI), i.e., front irradiance Gf = 1,000 W - m^-², and meanwhile, rear irradiance Gr = 135 W - m^-². In reality, there are permissible deviations (temperature and irradiance) between actual test conditions and standard test conditions. In such cases, corrections should be made in accordance with IEC 60891, and uncertainty calculations should be considered.

Depending on the equipment configuration, bifacial cells can be tested using one of the following three methods. Following the steps and requirements described in this document, the three methods can yield consistent results.

a) Bifacial synchronous illumination method;

b) Equivalent illumination method;

c) Sequential illumination method.

The bifacial synchronous illumination method requires specific equipment to provide bifacial irradiation for the bifacial cell. The mono-facial and bifacial I-V characteristics of the cell under test can be obtained at the same test position, with the same probe contact, and in the same test cycle.

Optionally, conventional equipment for testing mono-facial cells may use the equivalent illumination method to test bifacial cells. The equivalent light intensity GE is determined by the bifaciality phiI using Formula (1).

When test time permits (typically for laboratory applications), phiI is determined by testing the I-V characteristics of the cell under both front and rear STC. When test time is limited (typically for mass production line applications), phiI can be determined by a bifacial reference cell, representative of all cells under test. If the phiI distribution of the mass-produced cells is within ±3% relative to the bifacial reference cell, the test error generated by this method is acceptable.

The sequential illumination method can also be performed using conventional equipment, requiring continuous testing of the front and rear of the bifacial cell, with the cell pieces flipped during the process. The obtained front STC, rear STC, and (optionally) rear low irradiation I-V characteristics can be used to calculate the bifacial I-V characteristics. The calculation steps are as per Appendix A.

4.2 Bifacial Synchronous Illumination Method

Place the bifacial cell under test in a low radiation background environment. Measurements are taken by applying illumination only to the front STC, only to the rear STC, and simultaneously to the front and rear BSTC to obtain the I-V characteristics under the front STC, rear STC, and bifacial BSTC. […]